WO2023053837A1 - Rectangular steel pipe and method for manufacturing same, hot-rolled steel sheet and method for manufacturing same, and building structure - Google Patents

Rectangular steel pipe and method for manufacturing same, hot-rolled steel sheet and method for manufacturing same, and building structure Download PDF

Info

Publication number
WO2023053837A1
WO2023053837A1 PCT/JP2022/032953 JP2022032953W WO2023053837A1 WO 2023053837 A1 WO2023053837 A1 WO 2023053837A1 JP 2022032953 W JP2022032953 W JP 2022032953W WO 2023053837 A1 WO2023053837 A1 WO 2023053837A1
Authority
WO
WIPO (PCT)
Prior art keywords
less
hot
steel pipe
bainite
present
Prior art date
Application number
PCT/JP2022/032953
Other languages
French (fr)
Japanese (ja)
Inventor
直道 岩田
晃英 松本
信介 井手
Original Assignee
Ｊｆｅスチール株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to KR1020247009566A priority Critical patent/KR20240053606A/en
Priority to CN202280063910.7A priority patent/CN117980519A/en
Priority to JP2022573502A priority patent/JPWO2023053837A1/ja
Publication of WO2023053837A1 publication Critical patent/WO2023053837A1/en

Links

Images

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
    • B21C37/15—Making tubes of special shape; Making tube fittings
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a square steel pipe having high strength, a low yield ratio and excellent low-temperature toughness, which is particularly suitable for building structural members of large buildings, a method for producing the same, and a hot-rolled steel plate used as a raw material for the square steel pipe. and its manufacturing method, and a building structure using this square steel pipe.
  • building structural members used in large-scale buildings such as factories, warehouses, and commercial facilities have been increasing in strength in order to reduce construction costs by reducing weight.
  • buildings large-scale buildings
  • square steel pipe square column
  • corner portions which is used as a column material for buildings
  • high strength is required for the flat plate portion.
  • square steel pipes used for building structural members are also required to have both high plastic deformability and excellent low-temperature toughness. In order to meet these requirements, it is necessary to select an appropriate square steel pipe material.
  • Square steel pipes are generally made from hot-rolled steel sheets (hot-rolled steel strips) or thick steel sheets, and are manufactured by cold-forming this material.
  • Cold forming methods include a cold press bending method and a cold roll forming method.
  • a square steel pipe manufactured by roll-forming a raw material (hereinafter sometimes referred to as a roll-formed square steel pipe) is made by cold-roll-forming a hot-rolled steel plate into a cylindrical open pipe, and Sewing welding (sometimes called electric resistance welding) is performed. After that, the cylindrical round steel pipe is drawn by several percent in the axial direction by rolls arranged on the top, bottom, right and left of the round steel pipe, and then formed into a square shape to produce a square steel pipe.
  • square steel pipes manufactured by press-bending materials (hereinafter sometimes referred to as press-formed square steel pipes) are made by cold press-bending thick steel plates to form square cross-sectional shapes. shape) and the butt part is joined by submerged arc welding, or two members with a U-shaped cross section are butted and joined by submerged arc welding. may be manufactured.
  • the method of manufacturing roll-formed square steel pipes has the advantage of being highly productive and capable of being manufactured in a short period of time compared to the method of manufacturing press-formed square steel pipes.
  • press-formed square steel pipes only the corners are work-hardened without being cold-formed on the flat plate part
  • roll-formed square steel pipes when the pipe is cold-formed into a cylindrical shape in particular, the entire circumference of the pipe is hardened. A large working strain is introduced in the axial direction. Therefore, the roll-formed square steel pipe has a high yield ratio in the tube axial direction not only at the corners but also at the flat portions, and has a problem of low low-temperature toughness.
  • the thicker the roll-formed square steel pipe the greater the work hardening during roll forming, so the yield ratio becomes higher and the toughness decreases. Therefore, especially when a thick-walled roll-formed square steel pipe is manufactured, it is necessary to select an appropriate material in consideration of changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll-forming.
  • Patent Document 1 proposes a square steel pipe in which the area fraction of bainite structure is 40% or more in the microstructure of the flat plate portion.
  • Patent document 2 proposes a square steel pipe with excellent weldability and plastic deformation capability of cold-worked parts, with the steel composition and cleanliness within a predetermined range.
  • Patent Document 3 proposes a square steel pipe having a low yield ratio and high toughness by subjecting the whole pipe to strain relief annealing after pipemaking by cold forming.
  • Patent Document 4 when the steel composition is within a predetermined range and the crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the average circle equivalent diameter of the crystal grain is 7.0 ⁇ m.
  • a square steel pipe has been proposed in which the total volume fraction of the crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more is 30% or less of the entire steel structure at the 1/4t position.
  • Patent Document 5 the steel composition is set to a predetermined range, and the steel structure at a position of 1/4 t of the plate thickness t from the outer surface of the steel pipe has a ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite of 2.0.
  • a square steel pipe has been proposed in which the ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 or more and 20.0 or less.
  • Patent Document 6 in terms of % by mass, C ⁇ 0.02%, Si ⁇ 1.0%, Mn: 0.05-2.0%, S ⁇ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.25%, Ti ⁇ 0.2%, B ⁇ 0.0020%, and one or more of Ni, Cr, Sn, and Cu in a total amount of 0.00 02% or more and 0.3% or less, the balance being Fe and unavoidable impurities, and the Nb amount satisfying Nb ⁇ 0.05 + 7.75C - 1.98Ti + 6.64N + 0.000035 / (B + 0.0004),
  • the ferrite phase in the metal structure has a volume fraction of 70% or more, the ferrite grain size is 10.5 or more and 15 or less in grain size number, and the yield ratio at room temperature is 70% or less, so that toughness is excellent.
  • a hot-rolled steel sheet for fire resistance with a low yield ratio is disclosed.
  • Patent Document 7 in mass%, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, composition consisting of the balance Fe and inevitable impurities, ferrite as the main phase, and pearlite or pearlite and bainite as the second phase.
  • the second phase frequency defined by the predetermined formula is 0.20 to 0.42, and the average grain size including the main phase and the second phase is 7 to 15 ⁇ m.
  • a thick hot-rolled steel plate for square steel pipes for building structural members is disclosed.
  • Patent Document 8 C: 0.06 to 0.12% (meaning % by mass, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.06%, P: 0.025% or less (not including 0%), S: 0.01% or less (not including 0%), Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009%, and B: 0.0005 to 0.003%, and the carbon equivalent Ceq defined by the predetermined formula is 0 .40% or less, with the balance consisting of iron and unavoidable impurities, consisting of a structure mainly composed of the bainite phase, and adjacent When a region surrounded by large-angle grain boundaries with a crystal misorientation of 15° or more is defined as a crystal grain, the average equivalent circle diameter D A of the crystal grain measured by an electron backscatter diffraction pattern method is 10 ⁇ m or less, and The grain size of the crystal grains measured by the electron backsc
  • Patent Document 9 contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150% and Ti: 0.003 to 0.050% by weight, and is calculated by a predetermined formula A steel with a carbon equivalent (Ceq.) of 0.10 to 0.45%, a pearlite phase in an area fraction of 5 to 20%, and an average grain size in the steel of 1 to 20%.
  • a high-strength hot-rolled steel sheet with excellent uniform elongation after cold working that is, a low yield ratio
  • Patent Document 10 discloses that the carbon equivalent Ceq calculated from the steel composition (% by mass) is 0.33% or more and 0.43% or less, the weld crack sensitivity composition PCM is 0.15% or more and 0.24% or less, and the welding A thick steel plate for cold press-forming square steel pipe is disclosed, which is made of steel having a composition in which the heat affected zone toughness index f HAZ is 0.30% or more and 0.47% or less.
  • the thick steel plate for cold press-formed square steel pipes of Patent Document 10 has a steel structure composed of ferrite and the balance of bainite or pearlite.
  • Patent Document 11 in mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, Al: 0.003 to 0 0.06%, Ti: 0.005 to 0.050%, the balance being Fe and impurities, and a steel material satisfying a Ceq defined by a predetermined formula of 0.34 or more is heated to 900 to 1200 ° C. After heating, rolling is started, and after rolling is completed at Ar 3 point or higher, water cooling is performed from Ar 3 point or lower to Ar 3 point ⁇ 400° C. or lower, and then tempering is performed at 500° C. or lower. is disclosed.
  • the steel plate for square steel pipes of Patent Document 11 has a steel structure composed of soft ferrite and hard bainite or martensite.
  • Patent Document 12 discloses that the steel composition is within a predetermined range, and the steel structure at a position of 1/2 t of the plate thickness t from the steel plate surface has a volume fraction of more than 30% ferrite and 10% or more bainite.
  • the sum of ferrite and bainite is 70% or more and 95% or less of the entire steel structure at the 1/2t position, and the balance is one or more selected from pearlite, martensite, and austenite, and is adjacent to each other
  • the crystal grain has an average equivalent circle diameter of less than 7.0 ⁇ m and an equivalent circle diameter of 40.0 ⁇ m or more.
  • a hot-rolled steel sheet has been proposed in which the total grain content is 30% or less by volume with respect to the entire steel structure at the 1/2t position.
  • Patent Documents 1 and 2 are based on the premise of manufacturing square steel pipes by press bending. Therefore, when applying the techniques described in Patent Documents 1 and 2 to roll-formed square steel pipes whose mechanical properties are significantly deteriorated during cold forming, there is a problem that the yield ratio and toughness cannot be achieved at the same time.
  • the techniques described in Patent Documents 1 and 2 only evaluate the Charpy absorbed energy (vE 0 ) at 0°C, and the results of evaluating toughness at low temperatures below 0°C are not described. No mention is made of whether it can be used in the environment.
  • Patent Document 3 With the technology described in Patent Document 3, in order to obtain a low yield ratio and high toughness, it is necessary to heat-treat square steel pipes after pipemaking. Therefore, the manufacturing cost is very high as compared with the square steel pipe as cold-worked.
  • Patent Document 5 in the production of a steel material, the number of times of resting for 30 seconds or more in a state where the plate thickness center temperature is 1000 ° C. or more until the rough rolling process of hot rolling is completed is 1 or more times. It was necessary to control to 5 times or less, and there was a problem regarding productivity.
  • the average crystal grain size including the main phase and the second phase is 7 to 15 ⁇ m. Within this average crystal grain size range, there is a problem that a tensile strength of 520 MPa or more cannot be obtained after roll forming.
  • the bainite phase is the main component (70 area% or more). Since the area ratio of hard bainite is high, there is a problem that the yield ratio of the steel sheet exceeds 0.75.
  • Patent Document 9 is a composite structure steel of soft ferrite and hard pearlite. For this reason, the yield ratio is low, but the toughness is poor, so there is a problem that the toughness required for square steel pipes cannot be secured.
  • Patent Document 11 The steel sheet manufactured by the above manufacturing method of Patent Document 11 requires tempering after hot rolling and subsequent cooling in order to make the yield ratio 80% or less. Therefore, it is disadvantageous in terms of manufacturing cost.
  • the present invention has been made in view of the above circumstances, and provides a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness suitable for building structural members, a method for producing the same, and a material for the square steel pipe.
  • An object of the present invention is to provide a hot-rolled steel sheet, a method for manufacturing the same, and a building structure using the square steel pipe.
  • the present invention is particularly suitable for application to thick square steel pipes and thick hot-rolled steel sheets used for thick square steel pipes.
  • high strength of the square steel pipe means that the yield strength of the flat plate portion of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll formed square steel pipe) is It refers to having a strength of 385 MPa or more and a tensile strength of the flat plate portion of 520 MPa or more.
  • excellent in low-temperature toughness of the square steel pipe means that the flat plate portion of the square steel pipe has a Charpy absorbed energy of 110 J or more at -20°C.
  • high strength refers to the hot-rolled steel sheet that is the raw material of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll-forming square steel pipe).
  • hot-rolled steel sheet for square steel pipe has a yield strength of 330 MPa or more and a tensile strength of 520 MPa or more.
  • the hot-rolled steel sheet of the present invention “excellent in low-temperature toughness” means that the material has a Charpy absorbed energy of 180 J or more at -20°C.
  • the term “thick” as used in the present invention means that the thickness and plate thickness are more than 5 mm and less than 26 mm.
  • the hot-rolled steel sheet as the raw material includes a hot-rolled steel strip.
  • the wall thickness referred to in the present invention refers to the thickness of the square steel pipe
  • the plate thickness refers to the thickness of the hot-rolled steel plate.
  • the rectangular steel pipe manufactured by cold roll forming should have a flat plate portion with a yield strength of 385 MPa or more, a flat plate portion with a tensile strength of 520 MPa or more, high plastic deformability, and excellent toughness.
  • the C content should be 0.04% by mass or more, and the main structure of the steel sheet should be a mixed structure of ferrite and bainite. It is necessary to
  • the residual structure of the steel sheet may be one or more selected from hard pearlite, martensite, and austenite. is necessary.
  • the C content must be 0.04% by mass or more.
  • the main structure at a depth of 1/4t (surface layer) of the wall thickness t from the outer surface of the square steel pipe must be a mixed structure of ferrite and bainite.
  • a square steel pipe having a flat plate portion and corner portions The component composition of the flat plate portion is % by mass, C: 0.04% or more and 0.45% or less, Si: 1.8% or less, Mn: 0.5% or more and 2.5% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.012% or more and 0.100% or less, with the remainder consisting of Fe and unavoidable impurities, The content of Nb and Ti satisfies the following formula (1), When the thickness of the flat plate portion is t, the steel structure of the flat plate portion at a depth of 1/4t of the wall thickness t from the outer surface of the pipe is The volume fraction is more than 30% ferrite and 10% or more bainite, The total of the ferrite and the
  • the number of crystal grains of .0 or more is 30 / mm 2 or less
  • 1.20 ⁇ %Nb ⁇ %Ti (1)
  • %Nb and %Ti are contents (% by mass) of each element.
  • the yield strength of the flat plate portion is 385 MPa or more
  • the tensile strength of the flat plate portion is 520 MPa or more
  • the yield ratio of the flat plate portion is 0.90 or less
  • the Charpy absorbed energy of the flat plate portion at -20 ° C. is 110 J or more.
  • the component composition is mass%, C: 0.04% or more and 0.45% or less, Si: 1.8% or less, Mn: 0.5% or more and 2.5% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.012% or more and 0.100% or less, with the remainder consisting of Fe and unavoidable impurities,
  • the content of Nb and Ti satisfies the following formula (1)
  • the steel structure at the 1/4t position of the plate thickness t from the steel plate surface is The volume fraction is more than 30% ferrite and 10% or more bainite, The total of the ferrite and the bainite is 75% or more and 95% or less,
  • the balance consists of one or more selected from pearlite, martensite, and austenite, When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between
  • the number of crystal grains of .0 or more is 30 / mm 2 or less, A hot-rolled steel sheet containing 20% or less by volume of crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more. 1.20 ⁇ %Nb ⁇ %Ti (1) Here, %Nb and %Ti are contents (% by mass) of each element. [8] The hot-rolled steel sheet according to [7], which has a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more.
  • V 0.01% or more and 0.15% or less
  • Cr 0.01% or more and 1.0% or less
  • Mo 0.01% or more and 1.0% or less
  • Cu 0.01% or more and 0.5% or less
  • Ni 0.01% or more and 0.3% or less
  • Ca 0.0005% or more and 0.010% or less
  • B 0.0003% or more and 0.010% or less
  • the rough rolling finish temperature is 850° C. or higher and 1150° C. or lower
  • finish rolling is finished.
  • cooling stop temperature A method for manufacturing a hot-rolled steel sheet, comprising cooling at 450° C. or higher and 650° C. or lower and winding at 440° C. or higher and 650° C. or lower.
  • a hot-rolled steel sheet having high strength, a low yield ratio and excellent low temperature toughness and a method for producing the same and a steel sheet having high strength, a low yield ratio and excellent low temperature toughness.
  • a square steel pipe and a method for manufacturing the same can be provided.
  • FIG. 1 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention.
  • FIG. 2 is a schematic diagram showing the sampling positions of flat plate portion tensile test pieces of square steel pipes carried out in the present invention.
  • FIG. 3 is a schematic diagram showing the sampling positions of the Charpy test piece of the square steel pipe implemented in the present invention.
  • FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. of a square steel pipe and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more.
  • FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. of a hot-rolled steel sheet and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more.
  • the present invention relates to a square steel pipe having a flat portion and corner portions and a hot-rolled steel sheet used as a material for the square steel pipe, wherein the chemical composition of the flat portion and the hot-rolled steel sheet of the square steel pipe is 0.04% by mass and C: 0.04%.
  • Nb and Ti satisfies the formula (1), and the thickness t (meaning the wall thickness t and the plate thickness t; the same applies hereinafter) from the outer surface of the pipe and the surface of the steel plate 1/4t depth position
  • the steel structure in the volume fraction is more than 30% ferrite and 10% or more bainite, the total of the ferrite and the bainite is 75% or more and 95% or less, and the balance is pearlite, martensite, and austenite.
  • a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals
  • the major axis is 50 ⁇ m or more
  • the ratio of the major axis to the minor axis (major axis) / (minor axis)) is 4.0 or more
  • the number of crystal grains is 30/mm2 or less in the steel structure
  • the volume fraction of crystal grains with an equivalent circle diameter of 40.0 ⁇ m or more is 20%. It is below. 1.20 ⁇ %Nb ⁇ %Ti (1)
  • %Nb and %Ti are contents (% by mass) of each element.
  • % indicating steel composition is “% by mass” unless otherwise specified. Since the square steel pipe of the present invention is manufactured by cold roll forming a hot-rolled steel plate, the flat plate portion and the corner portions are made of the same hot-rolled steel plate, and the flat plate portion and the corner portions have the same chemical composition. is. On the other hand, since the welded portion is heated to a high temperature during welding, it reacts with oxygen in the atmosphere and is oxidized, so there is a possibility that the component composition is different from that of the flat portion and the corner portion. Since the volume of the weld zone in the total volume of the square steel pipe is small, the chemical composition of the weld zone has little effect on the characteristics of the square steel pipe. , either is fine.
  • C 0.04% to 0.45%
  • C is an element that increases the strength of steel by solid solution strengthening.
  • C is an element that promotes the formation of pearlite, improves hardenability, and contributes to the formation of bainite.
  • it is necessary to contain 0.04% or more of C.
  • the C content should be 0.04% or more and 0.45% or less.
  • the C content is preferably 0.08% or more, more preferably over 0.12%, and even more preferably 0.14% or more. Also, the C content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.22% or less.
  • Si 1.8% or less
  • Si is an element that increases the strength of steel by solid-solution strengthening, and can be contained as necessary. In order to obtain such effects, it is desirable to contain 0.01% or more of Si. However, if the Si content exceeds 1.8%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. In addition, the toughness of the base metal portion other than the electric resistance welded portion is also lowered. Therefore, the Si content is set to 1.8% or less.
  • the Si content is preferably 0.01% or more, more preferably 0.10% or more. Also, the Si content is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.
  • Mn 0.5% to 2.5%
  • Mn is an element that increases the strength of steel through solid solution strengthening. Moreover, Mn is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the strength and structure targeted in the present invention, it is necessary to contain 0.5% or more of Mn. However, when the Mn content exceeds 2.5%, the yield ratio of the flat plate portion of the square steel pipe exceeds 0.90 due to an excessive amount of bainite structure, and the desired yield ratio cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. Therefore, the Mn content should be 0.5% or more and 2.5% or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, and even more preferably 1.0% or more. Also, the Mn content is preferably 2.0% or less.
  • P 0.10% or less P segregates at grain boundaries and causes nonhomogeneity of the material. Therefore, it is preferable to reduce P as an unavoidable impurity as much as possible, but a content of 0.10% or less is acceptable. Therefore, the P content should be within the range of 0.10% or less.
  • the P content is preferably 0.03% or less, more preferably 0.020% or less, and even more preferably 0.015% or less.
  • the lower limit of P is not specified, it is preferable to set P to 0.002% or more because excessive reduction leads to a rise in smelting costs.
  • S 0.05% or less S usually exists as MnS in steel, but MnS is thinly drawn in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.05% or less is permissible. Therefore, the S content should be 0.05% or less.
  • the S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. Although the lower limit of S is not specified, excessive reduction leads to an increase in smelting costs, so S is preferably 0.0002% or more.
  • Al 0.005% to 0.100%
  • Al is an element that acts as a strong deoxidizing agent. In order to obtain such effects, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates and the amount of alumina-based inclusions increases, resulting in deterioration of the surface properties. Also, the toughness of the weld zone is reduced. Therefore, the Al content is set to 0.005% or more and 0.100% or less.
  • the Al content is preferably 0.010% or more, more preferably 0.015% or more. Also, the Al content is preferably 0.070% or less, more preferably 0.050% or less.
  • N 0.010% or less
  • N is an unavoidable impurity, and is an element that has the effect of lowering the toughness by firmly fixing the movement of dislocations.
  • the N content can be allowed up to 0.010%. Therefore, the N content is set to 0.010% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. Since an excessive reduction causes a rise in smelting costs, the N content is preferably 0.0010% or more, more preferably 0.0015% or more.
  • Nb 0.005% or more and 0.050% or less
  • Nb is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. In order to obtain such effects, it is necessary to contain 0.005% or more. However, when the Nb content exceeds 0.050%, coarse carbides and nitrides are formed, and the formation of crystal grains with a large ratio of major axis to minor axis as described later is promoted, resulting in a decrease in toughness. may lead to Therefore, the Nb content should be 0.005% or more and 0.050% or less.
  • the Nb content is preferably 0.006% or more, more preferably 0.007% or more, and even more preferably 0.008% or more. Also, the Nb content is preferably 0.045% or less, more preferably 0.035% or less.
  • Ti 0.012% or more and 0.100% or less
  • Ti is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. Moreover, when Ti is added in an appropriate amount, it is possible to improve the strength without promoting the formation of coarse crystal grains, and it is one of the most important elements in the present invention. In order to obtain such effects, it is necessary to contain 0.012% or more. However, when the Ti content exceeds 0.100%, coarse carbides and nitrides are formed, which may lead to a decrease in toughness. Therefore, the Ti content should be 0.012% or more and 0.100% or less. The Ti content is preferably 0.015% or more, more preferably 0.017% or more, and even more preferably 0.018% or more. Also, the Ti content is preferably 0.090% or less, more preferably 0.070% or less.
  • %Nb and %Ti are contents (% by mass) of each element. In the present invention, it is necessary that the contents of Nb and Ti be within the above ranges and that 1.20 ⁇ %Nb ⁇ %Ti is satisfied.
  • the low temperature toughness is lowered.
  • 1.50 ⁇ %Nb ⁇ %Ti more preferably 2.30 ⁇ %Nb ⁇ %Ti.
  • the balance is Fe and unavoidable impurities.
  • the O content may be 0.005% or less.
  • V less than 0.01%
  • Cr less than 0.01%
  • Mo less than 0.01%
  • Cu less than 0.01%
  • Ni less than 0.01%
  • Ca less than 0.0005%
  • B Less than 0.0003% can be included among the inevitable impurities.
  • the above ingredients are the basic ingredient composition of the square steel pipe in the present invention. Although the properties aimed at in the present invention can be obtained with the essential elements described above, the following elements can be contained as necessary.
  • V 0.01% to 0.15%
  • Cr 0.01% to 1.0%
  • Mo 0.01% to 1.0%
  • Cu 0.01% to 0.5%
  • Ni 0.01% or more and 0.3% or less
  • Ca 0.0005% or more and 0.010% or less
  • B 0.0003% or more and 0.010% or less
  • V 0.01% to 0.15%
  • Cr 0.01% to 1.0%
  • Mo 0.01% to 1.0%
  • V, Cr, and Mo are used for quenching steel It is an element that enhances the strength of steel and can be contained as necessary. In order to obtain the above effects, when V, Cr, and Mo are contained, it is preferable that V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively.
  • V 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more.
  • an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when V, Cr, and Mo are contained, it is preferable that V: 0.15% or less, Cr: 1.0% or less, and Mo: 1.0% or less, respectively. More preferably, V: 0.10% or less, Cr: 0.50% or less, and Mo: 0.50% or less.
  • Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and are contained as necessary. be able to.
  • Cu when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferably, Cu: 0.10% or more and Ni: 0.10% or more.
  • an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. More preferably, Cu: 0.40% or less and Ni: 0.20% or less.
  • Ca 0.0005% or more and 0.010% or less
  • Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process. can be contained.
  • the Ca content is preferably 0.010% or less. More preferably, the Ca content is 0.0050% or less.
  • B 0.0003% or more and 0.010% or less
  • B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature.
  • the B content is 0.0005% or more.
  • the yield ratio may increase. Therefore, when B is contained, it is preferably 0.010% or less. More preferably, the B content is 0.0050% or less.
  • the steel structure at a depth position of 1/4t of the thickness t from the pipe outer surface of the steel pipe and the surface of the steel plate has a volume fraction of more than 30% ferrite and 10% or more bainite. and the sum of the ferrite and the bainite is 75% or more and 95% or less of the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the steel plate, and the balance is pearlite and marten It consists of one or more selected from site and austenite.
  • the number of crystal grains of 0.0 or more is 30/mm2 or less, and the crystal grains with an equivalent circle diameter (crystal grain size) of 40.0 ⁇ m or more are 1/4 t of the thickness t from the outer surface of the pipe and the surface of the steel plate. It is 20% or less in volume ratio with respect to the entire steel structure at the depth position.
  • the equivalent circle diameter is the diameter of a circle having the same area as the target crystal grain.
  • the steel structure of the square steel pipe is at a depth of 1/4t of the wall thickness t from the outer surface of the flat plate portion of the square steel pipe, excluding the electric resistance welded portion.
  • both the corner portion and the flat plate portion have the same steel structure at a depth of 1/4t of the wall thickness t from the outer surface of the pipe. Therefore, the steel structure of the flat plate portion is specified here.
  • the steel structure of the hot-rolled steel sheet is at a depth of 1/4t of the thickness t from the surface of the steel sheet.
  • volume fraction of ferrite more than 30%
  • volume fraction of bainite 10% or more
  • total volume fraction of ferrite and bainite to the steel structure 75% or more and 95% or less
  • the volume fraction of ferrite must exceed 30%.
  • the volume fraction of ferrite is preferably 40% or more, more preferably 43% or more, and even more preferably 45% or more.
  • the volume fraction of ferrite is preferably less than 75%, more preferably less than 70%, and still more preferably 60% or less in order to ensure a desired yield ratio. .
  • Bainite is a structure with intermediate hardness and increases the strength of steel. Since the yield strength and tensile strength targeted in the present invention cannot be obtained with the ferrite alone, the volume fraction of bainite must be 10% or more.
  • the volume fraction of bainite is preferably 15% or more, more preferably 20% or more, and still more preferably 25% or more. Although no particular upper limit is specified, the volume fraction of bainite is preferably 55% or less, more preferably 50% or less, and still more preferably 45% or less in order to ensure a desired yield ratio. , even more preferably less than 40%.
  • the total volume fraction of ferrite and bainite must be 75% or more and 95% or less. Preferably, it is 78% or more, preferably 93% or less. More preferably, it is 80% or more, and more preferably 90% or less.
  • the sum of the volume fractions of pearlite, martensite, and austenite should be 5% or more and 25% or less. Preferably, it is 7% or more, preferably 23% or less. More preferably, it is 10% or more, and more preferably 20% or less.
  • volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.
  • the steel structure is a steel in which a soft structure and a hard structure are mixed (hereinafter referred to as "composite structure steel") in order to obtain the low yield ratio, yield strength, and tensile strength aimed at in the present invention.
  • the ratio of the major axis to the minor axis of the crystal grains with the major axis of 50 ⁇ m or more exceeds 30 / mm2 , or the circle equivalent diameter is
  • crystal grains of 40.0 ⁇ m or more exceed 20% in volume ratio with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the hot-rolled steel sheet, the desired low temperature toughness cannot be obtained. .
  • the number of crystal grains with a major axis to minor axis ratio of 4.0 or more with a major axis of 50 ⁇ m or more is 30 / mm 2 or less, and a crystal grain with an equivalent circle diameter of 40.0 ⁇ m or more
  • the low-temperature toughness aimed at in the present invention can be ensured by setting the volume ratio to 20% or less with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the surface of the hot-rolled steel sheet.
  • the number of crystal grains having a ratio of major axis to minor axis of 4.0 or more is preferably 28/mm 2 or less, more preferably 26/mm 2 or less.
  • the volume fraction of crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more is preferably 18% or less, more preferably 16% or less.
  • Bainite does not grow across boundaries with large misorientation (austenite grain boundaries and sub-boundaries formed by accumulation of dislocations). Therefore, in order to suppress the formation of coarse bainite as described above, finish rolling in hot rolling is performed at a temperature as low as possible to introduce a large amount of dislocations into austenite to increase the area of sub-boundaries, resulting in a fine sub-grain structure. (hereinafter also referred to as “miniaturization”) is particularly effective.
  • the crystal misorientation, the average crystal grain size, and the volume fraction of crystal grains with a crystal grain size of 40.0 ⁇ m or more can be measured by the SEM/EBSD method. Here, it can be measured by the method described in Examples described later.
  • the “steel structure at a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate” means a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate. It means that the above steel structure exists in any of the range of ⁇ 1.0 mm in the thickness direction centered on the position.
  • the method for producing a square steel pipe of the present invention includes, for example, heating a steel material having the above-described chemical composition to a heating temperature of 1100° C. or higher and 1300° C. or lower, followed by finishing rolling at a rough rolling end temperature of 850° C. or higher and 1150° C. or lower. Finishing temperature: 750° C. or higher and 850° C. or lower and total rolling reduction: 40% or higher and 63% or lower at 930° C. or lower. Next, cooling is performed at the sheet thickness center temperature at an average cooling rate of 2 ° C./s or more and 27 ° C./s or less, cooling stop temperature: 450 ° C. or more and 650 ° C.
  • the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming, and the butted portions are electric resistance welded, and then formed into a square shape to obtain a square steel pipe.
  • °C indicates the surface temperature of the steel material or steel plate (hot-rolled steel plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the steel plate cross section by heat transfer analysis and correcting the result with the surface temperature of the steel plate.
  • hot-rolled steel sheet includes hot-rolled steel sheet and hot-rolled steel strip.
  • the method of melting the steel material is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is suitable.
  • the casting method is also not particularly limited, but the desired dimensions are manufactured by a known casting method such as a continuous casting method. It should be noted that there is no problem even if an ingot casting-slabbing rolling method is applied instead of the continuous casting method.
  • the molten steel may be further subjected to secondary refining such as ladle refining.
  • the obtained steel material (steel slab) is heated to a heating temperature of 1100° C. or more and 1300° C. or less, and then subjected to rough rolling at a rough rolling end temperature of 850° C. or more and 1150° C. or less. Finish rolling is performed at 750° C. or higher and 850° C. or lower, and hot rolling is performed such that the total rolling reduction at 930° C. or lower is 40% or higher and 63% or lower to obtain a hot rolled steel sheet.
  • Heating temperature 1100° C. or higher and 1300° C. or lower If the heating temperature is lower than 1100° C., the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling and finish rolling). It becomes difficult to ensure the crystal grain size. In addition, it becomes difficult to suppress the formation of coarse bainite, and it is difficult to control the volume fraction of crystal grains having a grain size of 40.0 ⁇ m or more within the target range of the present invention. Therefore, the heating temperature in the hot rolling process is set to 1100° C. or higher and 1300° C. or lower. It is preferably 1120° C. or higher and preferably 1280° C. or lower.
  • Rough rolling finish temperature 850° C. or more and 1150° C. or less
  • the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during the subsequent finish rolling, and a large amount of ferrite is generated, and bainite is formed.
  • the volume ratio becomes less than 10%.
  • the rough rolling finish temperature exceeds 1150° C., the rolling reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the steel structure of the square steel pipe, which is the object of the present invention, cannot be obtained.
  • the finish temperature of rough rolling is set to 850° C. or higher and 1150° C. or lower. It is preferably 860° C. or higher, more preferably 870° C. or higher. It is preferably 1000° C. or lower, more preferably 980° C. or lower.
  • Finish rolling finish temperature 750° C. or higher and 850° C. or lower
  • the finishing temperature of finish rolling is set to 750° C. or more and 850° C. or less. It is preferably 770° C. or higher, more preferably 780° C. or higher. It is preferably 830° C. or lower, more preferably 820° C. or lower.
  • Total rolling reduction at 930° C. or less 40% or more and 63% or less
  • subgrains in austenite are refined in the hot rolling process, so that ferrite, bainite and the remainder generated in the subsequent cooling process and coiling process
  • the steel structure of the square steel pipe having the strength and toughness targeted by the present invention can be obtained by refining the structure.
  • the total rolling reduction exceeds 63%, crystal grains with a large major axis to minor axis ratio tend to form, resulting in a decrease in toughness.
  • the total rolling reduction from 930° C. or lower to the finish rolling finish temperature is set to 63% or lower. It is preferably 61% or less, more preferably 60% or less. If the total rolling reduction is less than 40% up to the finish rolling finish temperature of 930° C. or lower, the grain size of ferrite and bainite increases, leading to a decrease in toughness. Therefore, the total rolling reduction from 930° C. or lower to the finish rolling finish temperature was set to 40% or higher. It is preferably 42% or more, more preferably 45% or more.
  • the reason why the temperature is set to 930°C or less is that if it exceeds 930°C, the austenite recrystallizes in the rolling process, dislocations introduced by rolling disappear, and refined austenite cannot be obtained.
  • the above-mentioned total rolling reduction refers to the total rolling reduction of each rolling pass in the temperature range from 930°C to the finish rolling end temperature.
  • hot rolling may be performed with a total reduction rate of 40% or more and 63% or less to the finish rolling end temperature of 930°C or less in both the rough rolling and the finish rolling described above.
  • only finish rolling may be performed by hot rolling with a total rolling reduction of 40% or more and 63% or less until the finish rolling finish temperature of 930° C. or less.
  • the slab is cooled during rough rolling to reduce the temperature to 930°C or less. After that, the total rolling reduction from 930° C. or lower to finish rolling finish temperature in both rough rolling and finish rolling is set to 40% or more and 63% or less.
  • the upper limit of the finished plate thickness (the thickness of the hot-rolled steel plate after finish rolling) is not particularly specified, but from the viewpoint of ensuring the required rolling reduction and steel plate temperature control, the finished plate thickness is more than 5 mm and less than 26 mm. is preferred.
  • the hot rolled steel sheet is subjected to a cooling process.
  • cooling is performed at an average cooling rate to the cooling stop temperature: 2°C/s or more and 27°C/s or less, and the cooling stop temperature: 450°C or more and 650°C or less.
  • the average cooling rate exceeds 27° C./s, a large amount of martensite or bainite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the steel structure of the obtained square steel pipe, and ferrite and bainite are generated. is less than 75%.
  • the average cooling rate is preferably 4°C/s or higher, more preferably 6°C/s or higher. It is preferably 25° C./s or less, more preferably 20° C./s or less.
  • Cooling stop temperature 450° C. or higher and 650° C. or lower
  • the cooling stop temperature is less than 450° C. at the thickness center temperature of the hot-rolled steel sheet
  • a large amount of martensite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the pipe of the steel structure, and the total volume fraction of ferrite and bainite may be less than 75%.
  • the volume fraction of ferrite may be 30% or less.
  • the cooling stop temperature is preferably 460°C or higher, more preferably 470°C or higher. It is preferably 620° C. or lower, more preferably 600° C. or lower.
  • the average cooling rate is a value obtained by ((thickness center temperature of hot-rolled steel sheet before cooling-thickness center temperature of hot-rolled steel sheet after cooling)/cooling time).
  • Cooling methods include, but are not limited to, water cooling such as water injection from nozzles, cooling by cooling gas injection, and the like.
  • both sides of the hot-rolled steel sheet are preferably cooled (treated) so that both sides of the hot-rolled steel sheet are cooled under the same conditions.
  • the hot-rolled steel sheet is coiled, and then subjected to a coiling process of standing to cool.
  • the steel sheet is coiled at a coiling temperature of 440° C. or higher and 650° C. or lower from the viewpoint of building the steel sheet structure. If the coiling temperature is less than 440°C, a large amount of martensite may be generated and the total volume fraction of ferrite and bainite may be less than 75%. Moreover, the volume fraction of ferrite may be 30% or less.
  • the winding temperature is preferably 450°C or higher, more preferably 460°C or higher. It is preferably 620° C. or lower, more preferably 590° C. or lower.
  • the hot-rolled steel sheet of the present invention is produced.
  • a hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more can be obtained.
  • a pipe-making process is applied.
  • a hot-rolled steel plate is roll-formed into a cylindrical open pipe (round steel pipe), and the butt portions are electric resistance welded.
  • the round steel pipe is drawn in the axial direction by several percent by rolls arranged vertically and horizontally with respect to the round steel pipe, and formed into a square shape to obtain a square steel pipe.
  • the square steel pipe in the present invention is not limited to a square steel pipe in which each side length is equal (the value of (long side length/short side length) is 1.0), (long side length/short side length) value of more than 1.0 is also included.
  • the value of (long side length/short side length) of the square steel pipe is preferably 1.0 or more and 2.5 or less.
  • the value of (long side length/short side length) is more preferably 1.0 or more and 2.0 or less.
  • the square steel pipe of the present invention is manufactured.
  • the yield strength of the flat plate portion is 385 MPa or more
  • the tensile strength of the flat plate portion is 520 MPa or more
  • the yield ratio of the flat plate portion is 0.90 or less
  • the Charpy absorbed energy of the flat plate portion at ⁇ 20° C. is 110 J or more.
  • the present invention can be suitably used particularly for thick square steel pipes.
  • the term "thickness” as used herein means that the thickness of the flat plate portion of the square steel pipe is more than 5 mm and less than 26 mm.
  • Fig. 1 schematically shows an example of a building structure using the square steel pipe of the present invention described above.
  • a plurality of rectangular steel pipes 1 of the present invention are erected and used as pillars.
  • a plurality of girders 4 made of steel such as H-shaped steel are installed between the adjacent square steel pipes 1 .
  • a plurality of small beams 5 made of steel such as H-shaped steel are installed between adjacent large beams 4 .
  • the square steel pipes 1 and the H-section steel forming the large girders 4 are welded together via through-diaphragms 6, so that the large beams 4 made of steel such as H-section steel are constructed between the adjacent square steel pipes 1.
  • studs 7 are provided as necessary for attachment to a wall or the like.
  • the square steel pipe of the present invention has excellent strength and low-temperature toughness, so even when used in large buildings, it is possible to sufficiently ensure the deformation performance of the entire structure. Therefore, the building structure of the present invention exhibits better earthquake resistance performance than building structures using conventional rectangular steel pipes. In addition, even when used in a building in a low-temperature environment such as a cold region, the above-mentioned excellent earthquake resistance performance can be exhibited.
  • a molten steel having the chemical composition shown in Table 1 was cast into a slab.
  • the obtained slabs were subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2 to obtain hot rolled steel sheets for square steel pipes. After the winding process, the following pipe-making process was performed.
  • the obtained hot-rolled steel sheets for square steel pipes were formed into cylindrical round steel pipes by roll forming, and the butt portions were electric resistance welded.
  • the round steel pipe was formed into a square shape by rolls arranged on the top, bottom, left, and right sides of the round steel pipe to obtain a roll-formed square steel pipe having a side length (mm) and a wall thickness (mm) shown in Table 2.
  • Test pieces were taken from the obtained square steel pipes (roll-formed square steel pipes) and hot-rolled steel sheets, and the following structural observations, tensile tests, and Charpy impact tests were performed.
  • test piece for observing the structure of the square steel pipe was measured from the flat plate portion next to the side including the welded portion of the square steel pipe (the 3 o'clock side when the welded portion is in the 12 o'clock direction). The sample was taken from the tube axial direction section and the depth position of 1/4t of the wall thickness t from the tube outer surface, polished, and then nital corroded.
  • a test piece for observing the structure of the hot-rolled steel sheet was taken from the central portion in the width direction of the hot-rolled steel sheet and at a depth of 1/4t of the sheet thickness t. The observation surface was made to be a cross section in the rolling direction at the time of hot rolling, and after polishing, it was produced by nital corrosion.
  • Microstructural observation is performed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) from the outer surface of the flat plate portion of the square steel pipe and the surface of the hot rolled steel plate to 1/ of the thickness t.
  • the tissue at the 4t depth position was observed and imaged.
  • the area ratios of ferrite, pearlite, bainite and residual structures were obtained from the obtained optical microscope images and SEM images.
  • the area ratio of each tissue was calculated as the average value of the values obtained in each field of view after observing five or more fields of view using a test piece taken from one representative flat plate.
  • the area ratio obtained by tissue observation was used as the volume ratio of each tissue.
  • ferrite is a product of diffusion transformation, and has a low dislocation density and exhibits a nearly recovered structure.
  • Pearlite is a structure in which cementite and ferrite are arranged in layers.
  • bainite is a lath-like multi-phase structure of ferrite and cementite with a high dislocation density.
  • the volume fraction of austenite was measured by X-ray diffraction.
  • the test piece for structural observation was ground so that the diffractive surface was at a depth of 1/4 t of the thickness t from the outer surface of the flat plate portion of the steel pipe and the surface of the hot-rolled steel plate, and then chemically polished to form a surface processed layer.
  • the K ⁇ ray of Mo was used for the measurement, and the volume fraction of austenite was obtained from the integrated intensities of the (200), (220) and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.
  • the average equivalent circle diameter (average crystal grain size) and the volume fraction of crystal grains having an equivalent circle diameter (crystal grain size) of 40.0 ⁇ m or more were measured using the SEM/EBSD method.
  • the measurement area was 500 ⁇ m ⁇ 1000 ⁇ m, and the measurement step size was 0.5 ⁇ m.
  • the crystal grain size was obtained by determining the orientation difference between adjacent crystal grains, and measuring the boundary with the orientation difference of 15° or more as the crystal grain boundary.
  • the average grain size was obtained by calculating the arithmetic mean of grain sizes from the obtained grain boundaries.
  • the number of crystal grains with a ratio of major axis to minor axis of 4.0 or more is measured, and divided by the area of the measurement area (0.5 mm 2 ).
  • the number (particles/mm 2 ) of crystal grains with a ratio of 4.0 or more was calculated.
  • crystal grain sizes of 2.0 ⁇ m or less are excluded from the analysis as measurement noise, and the area ratio obtained in the crystal grain size analysis is equal to the volume ratio.
  • FIG. 2 is a schematic diagram showing the sampling positions of the tensile test pieces of the flat plate portion of the square steel pipe.
  • a JIS No. 5 tensile test piece was taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axial direction.
  • a JIS No. 5 tensile test piece was taken so that the tensile direction was parallel to the rolling direction.
  • a tensile test was performed on the collected tensile test pieces in accordance with the provisions of JIS Z 2241, the yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength) / (tensile strength) was calculated. bottom.
  • the tensile test piece of the flat plate portion of the square steel pipe was taken from the position of the width center of the flat plate portion (see FIG. 2) on the side of 3 o'clock when the welded portion of the square steel pipe is in the 12 o'clock direction. .
  • the number of test pieces was set to two for each, and the average values thereof were calculated to obtain YS, TS, and yield ratio.
  • FIG. 3 is a schematic diagram showing the sampling positions of Charpy test pieces of square steel pipes.
  • the longitudinal direction of the test piece was taken parallel to the pipe axis direction at a depth of 1/4t of the wall thickness t from the outer surface of the square steel pipe.
  • a V-notch standard test piece conforming to JIS Z 2242 was used.
  • the test piece was sampled from a depth of 1/4 t in the thickness of the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was parallel to the rolling direction.
  • a V-notch standard specimen was used.
  • a Charpy impact test was carried out at a test temperature of -20°C in accordance with JIS Z 2242 to determine absorbed energy (J). Incidentally, the number of test pieces was three, and the average value was calculated to obtain the absorbed energy (J).
  • Tables 3-1 and 3-2 show the results for the obtained square steel pipes, and Tables 4-1 and 4-2 show the results for the hot-rolled steel sheets.
  • Steel No. in Table 1 steel plate Nos. in Tables 2 and 4; and steel pipe No. in Table 3. correspond to each other, and the same No.
  • a hot-rolled steel plate is manufactured from the steel of 1998, and a square steel pipe is manufactured from the hot-rolled steel plate.
  • steel pipe No. 1 to 22 are examples of the present invention.
  • 23 to 46 are comparative examples.
  • the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is A crystal grain having an equivalent circle diameter of 40.0 ⁇ m or more when a region surrounded by a boundary with a misorientation of 15° or more is made of one or more selected from pearlite, martensite, and austenite.
  • the flat plate portion had a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.90 or less, and a Charpy absorbed energy of -20°C of the flat plate portion of 110 J or more.
  • Comparative steel pipe No. 26 since the Si content exceeded the range of the present invention, the yield strength was excessively increased due to solid solution strengthening without refinement of the structure. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. 32 Comparative steel pipe No. In No. 32, the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. 41 Comparative steel pipe No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more was outside the range of the present invention. As a result, the tensile strength of the flat plate portion and the Charpy absorbed energy at -20°C did not reach the desired values.
  • the volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C of the flat plate part did not reach the desired values.
  • Comparative steel pipe No. 43 Comparative steel pipe No. In No. 43, the total reduction ratio at 930 ° C. or lower exceeds the range of the present invention, the formation of coarse bainite cannot be suppressed, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more is outside the range of the present invention. became. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 ⁇ m or more.
  • the Charpy absorbed energy at ⁇ 20° C. is 110 J or more, and excellent low temperature toughness is exhibited.
  • the Charpy absorbed energy at -20°C was less than 110J.
  • steel plate No. Steel sheets Nos. 1 to 22 are examples of the present invention.
  • 23 to 46 are comparative examples.
  • the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is is composed of one or more selected from pearlite, martensite, and austenite, and a crystal grain having an equivalent circle diameter of 40.0 ⁇ m or more when a region surrounded by a boundary with a misorientation of 15° or more is defined as a crystal grain.
  • the volume ratio of 20% or less and the major axis is 50 ⁇ m or more
  • these mechanical properties were such that the yield strength was 330 MPa or more, the tensile strength was 520 MPa or more, the yield ratio was 0.75 or less, and the Charpy absorbed energy at -20°C was 180 J or more.
  • Comparative example steel plate No. 28 the P content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 29 the S content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 32 the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 35 Comparative example steel plate No. In No. 35, the Cr content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 38 the Ni content exceeded the range of the present invention, so the yield ratio and the Charpy absorbed energy at -20°C did not reach the desired values.
  • Comparative example steel plate No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 41 Comparative example steel plate No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more was outside the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • the volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
  • Comparative example steel plate No. 44 since the average cooling rate was below the range of the present invention, the volume fraction of bainite was less than 10%, and fell outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
  • FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 ⁇ m or more.
  • the Charpy absorbed energy at ⁇ 20° C. is 180 J or more, and excellent low temperature toughness is exhibited.
  • the Charpy absorbed energy at -20°C was less than 180J.

Abstract

Provided are a rectangular steel pipe and a method for manufacturing the same, and a hot-rolled steel sheet and a method for manufacturing the same. This rectangular steel pipe and hot-rolled steel sheet have a prescribed component composition, the steel structure in a position at a depth of 1/4t, where t is the wall thickness from the pipe outer surface and the steel sheet surface, comprises in terms of volume fraction, more than 30% ferrite and at least 10% bainite, the total of ferrite and bainite being 75-95%, and the remainder being one or more selected from pearlite, martensite, and austenite, the number of crystal grains having a long axis of 50 µm or greater and a ratio of long axis and short axis (= (long axis)/(short axis)) of 4.0 or greater is 30/mm2 or fewer, where a crystal grain is a region surrounded by a boundary where the orientation difference between adjacent crystals is at least 15°, and the volume fraction of crystal grains having an equivalent circle diameter of 40.0 µm or greater is 20% or less.

Description

角形鋼管およびその製造方法、熱延鋼板およびその製造方法、並びに建築構造物Rectangular steel pipe and manufacturing method thereof, hot-rolled steel plate and manufacturing method thereof, and building structure
　本発明は、特に大型建築物の建築構造部材に好適に用いられる、高強度と低降伏比を有し、低温靱性に優れた角形鋼管およびその製造方法、角形鋼管の素材として用いられる熱延鋼板及びその製造方法、並びにこの角形鋼管を使用した建築構造物に関する。 The present invention relates to a square steel pipe having high strength, a low yield ratio and excellent low-temperature toughness, which is particularly suitable for building structural members of large buildings, a method for producing the same, and a hot-rolled steel plate used as a raw material for the square steel pipe. and its manufacturing method, and a building structure using this square steel pipe.
　近年、例えば工場、倉庫、商業施設などの大型建築物（以下、建築物と称する）に用いられる建築構造部材は、軽量化による施工コスト削減のため、高強度化が進んでいる。特に建築物の柱材として用いられる平板部と角部を有する角形鋼管（角コラム）では、平板部に高い強度が求められている。同時に、建築構造部材に用いられる角形鋼管は、耐震性の観点から、高い塑性変形能と優れた低温靱性を同時に備えることも求められる。これらの要求を実現するため、適切な角形鋼管の素材を選択する必要がある。 In recent years, building structural members used in large-scale buildings (hereinafter referred to as buildings) such as factories, warehouses, and commercial facilities have been increasing in strength in order to reduce construction costs by reducing weight. In particular, in a square steel pipe (square column) having a flat plate portion and corner portions, which is used as a column material for buildings, high strength is required for the flat plate portion. At the same time, from the viewpoint of earthquake resistance, square steel pipes used for building structural members are also required to have both high plastic deformability and excellent low-temperature toughness. In order to meet these requirements, it is necessary to select an appropriate square steel pipe material.
　角形鋼管は、一般に熱延鋼板（熱延鋼帯）または厚鋼板を素材とし、この素材を冷間で成形することにより製造される。冷間で成形する方法としては、冷間でプレス曲げ成形する方法あるいは冷間でロール成形する方法がある。 Square steel pipes are generally made from hot-rolled steel sheets (hot-rolled steel strips) or thick steel sheets, and are manufactured by cold-forming this material. Cold forming methods include a cold press bending method and a cold roll forming method.
　素材をロール成形して製造される角形鋼管（以下、ロール成形角形鋼管と称する場合もある。）は、熱延鋼板を冷間でロール成形して円筒状のオープン管とし、その突合せ部分を電縫溶接（電気抵抗溶接と称する場合もある。）する。その後、丸形鋼管の上下左右に配置されたロールにより、円筒状の丸形鋼管に対して管軸方向に数％の絞りを加え、続けて角形に成形して角形鋼管を製造する。一方、素材をプレス曲げ成形して製造される角形鋼管（以下、プレス成形角形鋼管と称する場合もある。）は、厚鋼板を冷間でプレス曲げ成形し、断面形状をロの字型（四角形状）にして突合せ部をサブマージアーク溶接により接合して製造する場合や、断面形状をコの字型（Ｕ字形状）にした対となる２つの部材を突き合わせこれらをサブマージアーク溶接により接合して製造する場合がある。 A square steel pipe manufactured by roll-forming a raw material (hereinafter sometimes referred to as a roll-formed square steel pipe) is made by cold-roll-forming a hot-rolled steel plate into a cylindrical open pipe, and Sewing welding (sometimes called electric resistance welding) is performed. After that, the cylindrical round steel pipe is drawn by several percent in the axial direction by rolls arranged on the top, bottom, right and left of the round steel pipe, and then formed into a square shape to produce a square steel pipe. On the other hand, square steel pipes manufactured by press-bending materials (hereinafter sometimes referred to as press-formed square steel pipes) are made by cold press-bending thick steel plates to form square cross-sectional shapes. shape) and the butt part is joined by submerged arc welding, or two members with a U-shaped cross section are butted and joined by submerged arc welding. may be manufactured.
　ロール成形角形鋼管の製造方法は、プレス成形角形鋼管の製造方法と比較して生産性が高く、短期間での製造が可能であるという利点がある。しかし、プレス成形角形鋼管では、平板部には冷間成形が加わらず角部のみが加工硬化するのに対し、ロール成形角形鋼管では、特に円筒状に冷間成形する際に鋼管全周にわたり管軸方向に大きな加工ひずみが導入される。そのため、ロール成形角形鋼管は角部だけでなく平板部においても管軸方向の降伏比が高く、低温靱性が低いという問題がある。 The method of manufacturing roll-formed square steel pipes has the advantage of being highly productive and capable of being manufactured in a short period of time compared to the method of manufacturing press-formed square steel pipes. However, in press-formed square steel pipes, only the corners are work-hardened without being cold-formed on the flat plate part, whereas in roll-formed square steel pipes, when the pipe is cold-formed into a cylindrical shape in particular, the entire circumference of the pipe is hardened. A large working strain is introduced in the axial direction. Therefore, the roll-formed square steel pipe has a high yield ratio in the tube axial direction not only at the corners but also at the flat portions, and has a problem of low low-temperature toughness.
　さらに、ロール成形角形鋼管は、肉厚が大きいほどロール成形時の加工硬化が大きくなるため、降伏比はより高くなり、靱性はより低下する。そのため、特に厚肉のロール成形角形鋼管を製造する場合には、ロール成形による降伏比の上昇や靱性の低下といった機械特性の変化を考慮して適切な素材を選択する必要がある。 Furthermore, the thicker the roll-formed square steel pipe, the greater the work hardening during roll forming, so the yield ratio becomes higher and the toughness decreases. Therefore, especially when a thick-walled roll-formed square steel pipe is manufactured, it is necessary to select an appropriate material in consideration of changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll-forming.
　このような要求に対し、例えば、特許文献１には、平板部のミクロ組織において、ベイナイト組織の面積分率を４０％以上とする角形鋼管が提案されている。 In response to such demands, Patent Document 1, for example, proposes a square steel pipe in which the area fraction of bainite structure is 40% or more in the microstructure of the flat plate portion.
　特許文献２には、鋼成分および清浄度を所定の範囲内とした溶接性および冷間加工部の塑性変形能力に優れた角形鋼管が提案されている。 Patent document 2 proposes a square steel pipe with excellent weldability and plastic deformation capability of cold-worked parts, with the steel composition and cleanliness within a predetermined range.
　特許文献３には、冷間成形により造管した後に全管ひずみ取り焼鈍を施すことで、低降伏比および高靱性を有する角形鋼管が提案されている。 Patent Document 3 proposes a square steel pipe having a low yield ratio and high toughness by subjecting the whole pipe to strain relief annealing after pipemaking by cold forming.
　特許文献４には、鋼成分を所定の範囲とし、隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、該結晶粒の平均円相当径が７．０μｍ未満であり、かつ円相当径で４０．０μｍ以上の該結晶粒の合計が１／４ｔ位置における鋼組織全体に対して体積率で３０％以下である角形鋼管が提案されている。 In Patent Document 4, when the steel composition is within a predetermined range and the crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the average circle equivalent diameter of the crystal grain is 7.0 μm. A square steel pipe has been proposed in which the total volume fraction of the crystal grains having an equivalent circle diameter of 40.0 μm or more is 30% or less of the entire steel structure at the 1/4t position.
　特許文献５には、鋼成分を所定の範囲とし、鋼管の外表面から板厚ｔの１／４ｔ位置における鋼組織は、フェライトの面積率に対するベイナイトとパーライトの面積率の合計の割合が２．０以上２０．０以下で、かつパーライトの面積率に対するベイナイトの面積率の割合が５．０以上２０．０以下である角形鋼管が提案されている。 In Patent Document 5, the steel composition is set to a predetermined range, and the steel structure at a position of 1/4 t of the plate thickness t from the outer surface of the steel pipe has a ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite of 2.0. A square steel pipe has been proposed in which the ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 or more and 20.0 or less.
　特許文献６には、質量％で、Ｃ≦０．０２％、Ｓｉ≦１．０％、Ｍｎ：０．０５～２．０％、Ｓ≦０．０２％、Ａｌ：０．０１～０．１％、Ｎｂ：０．０８～０．２５％、Ｔｉ≦０．２％、Ｂ≦０．００２０％を含み、かつＮｉ，Ｃｒ，Ｓｎ，Ｃｕの１種または２種以上を総量で０．０２％以上かつ０．３％以下含み、残部がＦｅ及び不可避的不純物からなり、かつＮｂ量がＮｂ≧０．０５＋７．７５Ｃ－１．９８Ｔｉ＋６．６４Ｎ＋０．００００３５／（Ｂ＋０．０００４）を満足し、その金属組織がフェライト相が体積率７０％以上で、かつフェライト粒径が粒度番号で１０．５番以上１５番以下であり、常温での降伏比が７０％以下とすることで、靭性に優れた低降伏比耐火用熱延鋼板が開示されている。 In Patent Document 6, in terms of % by mass, C≦0.02%, Si≦1.0%, Mn: 0.05-2.0%, S≦0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.25%, Ti ≤ 0.2%, B ≤ 0.0020%, and one or more of Ni, Cr, Sn, and Cu in a total amount of 0.00 02% or more and 0.3% or less, the balance being Fe and unavoidable impurities, and the Nb amount satisfying Nb ≥ 0.05 + 7.75C - 1.98Ti + 6.64N + 0.000035 / (B + 0.0004), The ferrite phase in the metal structure has a volume fraction of 70% or more, the ferrite grain size is 10.5 or more and 15 or less in grain size number, and the yield ratio at room temperature is 70% or less, so that toughness is excellent. A hot-rolled steel sheet for fire resistance with a low yield ratio is disclosed.
　特許文献７には、質量％で、Ｃ：０．０７～０．１８％、Ｍｎ：０．３～１．５％、Ｐ：０．０３％以下、Ｓ：０．０１５％以下、Ａｌ：０．０１～０．０６％、Ｎ：０．００６％以下を含み、残部Ｆｅおよび不可避的不純物からなる組成と、フェライトを主相とし、第二相として、パーライト、または、パーライトおよびベイナイトを有し、所定の式で定義される第二相頻度が０．２０～０．４２であり、主相と第二相とを含む平均結晶粒径が７～１５μｍである組織を有することで、靱性を改善した建築構造部材向け角形鋼管用厚肉熱延鋼板が開示されている。 In Patent Document 7, in mass%, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, composition consisting of the balance Fe and inevitable impurities, ferrite as the main phase, and pearlite or pearlite and bainite as the second phase. However, the second phase frequency defined by the predetermined formula is 0.20 to 0.42, and the average grain size including the main phase and the second phase is 7 to 15 μm. A thick hot-rolled steel plate for square steel pipes for building structural members is disclosed.
　特許文献８には、Ｃ：０．０６～０．１２％(質量％の意味、以下同じ)、Ｓｉ：０．０５～０．５％、Ｍｎ：１．０～１．８％、Ａｌ：０．０１～０．０６％、Ｐ：０．０２５％以下（０％を含まない）、Ｓ：０．０１％以下（０％を含まない）、Ｎｂ：０．００５～０．０２５％、Ｔｉ：０．００５～０．０３％、Ｎ：０．００２～０．００９％およびＢ：０．０００５～０．００３％を夫々含有すると共に、所定の式で規定される炭素当量Ｃｅｑが０．４０％以下であり、残部が鉄および不可避不純物からなり、ベイナイト相を主体とする組織からなり、表面から深さｔ／４（ｔは板厚を表す、以下同じ）の位置において、隣り合う結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、当該結晶粒を電子後方散乱回折像法によって測定した平均円相当径ＤＡが１０μｍ以下であると共に、前記電子後方散乱回折像法によって測定した前記結晶粒の粒径を、所定の式に基づく極値統計法によって算出した予測最大粒径ＤＭが、８０μｍ以下とすることで、母材低温靱性に優れた大入熱溶接用高張力鋼板が開示されている。 In Patent Document 8, C: 0.06 to 0.12% (meaning % by mass, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.06%, P: 0.025% or less (not including 0%), S: 0.01% or less (not including 0%), Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009%, and B: 0.0005 to 0.003%, and the carbon equivalent Ceq defined by the predetermined formula is 0 .40% or less, with the balance consisting of iron and unavoidable impurities, consisting of a structure mainly composed of the bainite phase, and adjacent When a region surrounded by large-angle grain boundaries with a crystal misorientation of 15° or more is defined as a crystal grain, the average equivalent circle diameter D A of the crystal grain measured by an electron backscatter diffraction pattern method is 10 μm or less, and The grain size of the crystal grains measured by the electron backscatter diffraction pattern method is calculated by the extreme value statistical method based on a predetermined formula, and the predicted maximum grain size D M is 80 μm or less. An excellent high-strength steel sheet for large heat input welding is disclosed.
　特許文献９には、重量でＣ：０．０４～０．２５％、Ｎ：０．００５０～０．０１５０％およびＴｉ：０．００３～０．０５０％を含有し、かつ所定の式で求められる炭素当量（Ｃｅｑ．）が０．１０～０．４５％の鋼であって、かつパーライト相が面積分率で５～２０％の範囲にあり、さらに鋼中に粒径の平均が１～３０μｍのＴｉＮが重量で０．０００８～０．０１５％の割合で分散させることで、冷間加工後の一様伸びの優れた（すなわち低降伏比である）高強度熱延鋼板が開示されている。 Patent Document 9 contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150% and Ti: 0.003 to 0.050% by weight, and is calculated by a predetermined formula A steel with a carbon equivalent (Ceq.) of 0.10 to 0.45%, a pearlite phase in an area fraction of 5 to 20%, and an average grain size in the steel of 1 to 20%. Disclosed is a high-strength hot-rolled steel sheet with excellent uniform elongation after cold working (that is, a low yield ratio) by dispersing 30 μm TiN at a rate of 0.0008 to 0.015% by weight. there is
　特許文献１０には、鋼成分（質量％）から計算される炭素当量Ｃｅｑが０．３３％以上０．４３％以下、溶接割れ感受性組成ＰＣＭが０．１５％以上０．２４％以下、溶接熱影響部靭性指標ｆＨＡＺが０．３０％以上０．４７％以下の組成を有する鋼からなる冷間プレス成形角形鋼管用厚鋼板が開示されている。特許文献１０の冷間プレス成形角形鋼管用厚鋼板は、鋼組織がフェライトおよび残部ベイナイトまたはパーライトから構成される。 Patent Document 10 discloses that the carbon equivalent Ceq calculated from the steel composition (% by mass) is 0.33% or more and 0.43% or less, the weld crack sensitivity composition PCM is 0.15% or more and 0.24% or less, and the welding A thick steel plate for cold press-forming square steel pipe is disclosed, which is made of steel having a composition in which the heat affected zone toughness index f HAZ is 0.30% or more and 0.47% or less. The thick steel plate for cold press-formed square steel pipes of Patent Document 10 has a steel structure composed of ferrite and the balance of bainite or pearlite.
　特許文献１１には、質量％で、Ｃ：０．０５～０．２０％、Ｓｉ：０．１０～０．４０％、Ｍｎ：１．２０～１．５０％、Ａｌ：０．００３～０．０６％、Ｔｉ：０．００５～０．０５０％を含有し、残部がＦｅおよび不純物からなり、かつ所定の式で定義されるＣｅｑが０．３４以上を満たす鋼素材を９００～１２００℃に加熱した後、圧延を開始し、Ａｒ３点以上で圧延終了後、Ａｒ３点以下からＡｒ３点－４００℃以下まで水冷し、その後、５００℃以下での焼戻しする角形鋼管用鋼板の製造方法が開示されている。特許文献１１の角形鋼管用鋼板は、鋼組織が軟質なフェライトと硬質なベイナイトまたはマルテンサイトから構成される。 In Patent Document 11, in mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, Al: 0.003 to 0 0.06%, Ti: 0.005 to 0.050%, the balance being Fe and impurities, and a steel material satisfying a Ceq defined by a predetermined formula of 0.34 or more is heated to 900 to 1200 ° C. After heating, rolling is started, and after rolling is completed at Ar 3 point or higher, water cooling is performed from Ar 3 point or lower to Ar 3 point −400° C. or lower, and then tempering is performed at 500° C. or lower. is disclosed. The steel plate for square steel pipes of Patent Document 11 has a steel structure composed of soft ferrite and hard bainite or martensite.
　特許文献１２には、鋼成分を所定の範囲とし、鋼板表面からの板厚ｔの１／２ｔ位置における鋼組織は、体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、該フェライトおよび該ベイナイトの合計が１／２ｔ位置における鋼組織全体に対して７０％以上９５％以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、該結晶粒の平均円相当径が７．０μｍ未満であり、かつ円相当径で４０．０μｍ以上の該結晶粒の合計が１／２ｔ位置における鋼組織全体に対して体積率で３０％以下である熱延鋼板が提案されている。 Patent Document 12 discloses that the steel composition is within a predetermined range, and the steel structure at a position of 1/2 t of the plate thickness t from the steel plate surface has a volume fraction of more than 30% ferrite and 10% or more bainite. The sum of ferrite and bainite is 70% or more and 95% or less of the entire steel structure at the 1/2t position, and the balance is one or more selected from pearlite, martensite, and austenite, and is adjacent to each other When a region surrounded by a boundary with a crystal orientation difference of 15° or more is defined as a crystal grain, the crystal grain has an average equivalent circle diameter of less than 7.0 μm and an equivalent circle diameter of 40.0 μm or more. A hot-rolled steel sheet has been proposed in which the total grain content is 30% or less by volume with respect to the entire steel structure at the 1/2t position.
特許第５３８５７６０号公報Japanese Patent No. 5385760 特許第４６１１２５０号公報Japanese Patent No. 4611250 特許第４９５７６７１号公報Japanese Patent No. 4957671 特許第６６９３６０６号公報Japanese Patent No. 6693606 特許第６８１３１４１号公報Japanese Patent No. 6813141 特許第４２７６３２４号公報Japanese Patent No. 4276324 特許第５５８９８８５号公報Japanese Patent No. 5589885 特許第５０９６０８７号公報Japanese Patent No. 5096087 特開平７－２２４３５１号公報JP-A-7-224351 特開２０１６－１１４３９号公報JP 2016-11439 A 特許第５６５５７２５号公報Japanese Patent No. 5655725 特許第６６９３６０７号公報Japanese Patent No. 6693607
　しかしながら、特許文献１および２に記載の技術は、プレス曲げ成形による角形鋼管の製造を前提としたものである。そのため、冷間成形時に機械的特性が大きく劣化するロール成形角形鋼管に特許文献１および２に記載の技術を適用する場合には、降伏比と靱性を同時に達成できないという問題がある。また、特許文献１および２に記載の技術は、０℃でのシャルピー吸収エネルギー（ｖＥ０）のみを評価しており、０℃未満の低温で靱性を評価した結果は記載されておらず、低温環境での使用可否に関しては言及されていない。 However, the techniques described in Patent Documents 1 and 2 are based on the premise of manufacturing square steel pipes by press bending. Therefore, when applying the techniques described in Patent Documents 1 and 2 to roll-formed square steel pipes whose mechanical properties are significantly deteriorated during cold forming, there is a problem that the yield ratio and toughness cannot be achieved at the same time. In addition, the techniques described in Patent Documents 1 and 2 only evaluate the Charpy absorbed energy (vE 0 ) at 0°C, and the results of evaluating toughness at low temperatures below 0°C are not described. No mention is made of whether it can be used in the environment.
　特許文献３に記載の技術では、低降伏比および高靱性を得るためには、造管後に角形鋼管に熱処理を施す必要がある。そのため、冷間加工ままの角形鋼管と比較して製造コストが非常に高くなる。特許文献５に記載の技術は、鋼素材の製造において、熱間圧延の粗圧延工程が終了するまでの間に板厚中心温度が１０００℃以上の状態で３０秒以上静止する回数を１回以上５回以下に制御することが必要であり、生産性に関して課題があった。 With the technology described in Patent Document 3, in order to obtain a low yield ratio and high toughness, it is necessary to heat-treat square steel pipes after pipemaking. Therefore, the manufacturing cost is very high as compared with the square steel pipe as cold-worked. In the technique described in Patent Document 5, in the production of a steel material, the number of times of resting for 30 seconds or more in a state where the plate thickness center temperature is 1000 ° C. or more until the rough rolling process of hot rolling is completed is 1 or more times. It was necessary to control to 5 times or less, and there was a problem regarding productivity.
　特許文献４に記載の技術では、低降伏比および高靱性を得るためには、熱延鋼板を製造する際の合計圧下率６５％以上および板厚中心の平均冷却速度を１０℃／ｓ以上３０℃／ｓ以下にする必要が有り、高い圧下率と大きな冷却速度が必要であるため、製造可能な設備に制約がある、という課題があった。 In the technique described in Patent Document 4, in order to obtain a low yield ratio and high toughness, the total rolling reduction when producing a hot-rolled steel sheet is 65% or more and the average cooling rate at the center of the thickness is 10 ° C./s or more 30 C./s or less is required, and a high reduction rate and a high cooling rate are required, so there is a problem that there are restrictions on the equipment that can be manufactured.
　特許文献６の技術では、鋼の高強度化に大きく寄与する元素であるＣ含有量が０．０２重量％以下に抑えられている。このため、ロール成形後の降伏強度を安定的に３８５ＭＰａ以上とすることが困難であるという問題があった。 In the technique of Patent Document 6, the C content, which is an element that greatly contributes to increasing the strength of steel, is suppressed to 0.02% by weight or less. Therefore, there is a problem that it is difficult to stably set the yield strength after roll forming to 385 MPa or more.
　特許文献７の技術では、主相と第二相とを含む平均結晶粒径が７～１５μｍである。この平均結晶粒径の範囲では、ロール成形後に引張強度５２０ＭＰａ以上の強度を得ることはできないという問題があった。 With the technique of Patent Document 7, the average crystal grain size including the main phase and the second phase is 7 to 15 μm. Within this average crystal grain size range, there is a problem that a tensile strength of 520 MPa or more cannot be obtained after roll forming.
　特許文献８の技術では、ベイナイト相を主体（７０面積％以上）とする。硬質なベイナイトの面積率が高いため、鋼板の降伏比が０．７５超になるという問題があった。 In the technique of Patent Document 8, the bainite phase is the main component (70 area% or more). Since the area ratio of hard bainite is high, there is a problem that the yield ratio of the steel sheet exceeds 0.75.
　特許文献９の技術では、軟質なフェライトと硬質なパーライトの複合組織鋼である。このため、降伏比は低いが靱性は悪いため、角形鋼管に必要な靱性を確保できないという問題があった。 The technology of Patent Document 9 is a composite structure steel of soft ferrite and hard pearlite. For this reason, the yield ratio is low, but the toughness is poor, so there is a problem that the toughness required for square steel pipes cannot be secured.
　特許文献１０の技術で得られる冷間プレス成形角形鋼管用厚鋼板を冷間ロール成形角形鋼管の素材に適用した場合、冷間ロール成形時に管軸方向に導入される加工ひずみのために靱性が低下する。このため、角形鋼管に必要な靱性を確保できないという問題があった。 When the thick steel plate for cold press-formed square steel pipe obtained by the technique of Patent Document 10 is applied as a raw material for cold roll-formed square steel pipe, toughness is reduced due to processing strain introduced in the pipe axial direction during cold roll forming. descend. Therefore, there is a problem that the toughness required for the square steel pipe cannot be ensured.
　特許文献１１の上記製造方法により製造される鋼板は、降伏比を８０％以下とするために、熱間圧延とそれに続く冷却を施した後に焼戻し処理を必要とする。このため、製造コストの面で不利であった。 The steel sheet manufactured by the above manufacturing method of Patent Document 11 requires tempering after hot rolling and subsequent cooling in order to make the yield ratio 80% or less. Therefore, it is disadvantageous in terms of manufacturing cost.
　特許文献１２に記載の技術では、低降伏比および高靱性を得るためには、熱延鋼板を製造する際の合計圧下率６５％以上および板厚中心の平均冷却速度を１０℃／ｓ以上３０℃／ｓ以下にする必要が有り、高い圧下率と大きな冷却速度が必要であるため、製造可能な設備に制約がある、という課題があった。 In the technique described in Patent Document 12, in order to obtain a low yield ratio and high toughness, the total rolling reduction when producing a hot rolled steel sheet is 65% or more and the average cooling rate at the center of the thickness is 10 ° C./s or more 30 C./s or less is required, and a high reduction rate and a high cooling rate are required, so there is a problem that there are restrictions on the equipment that can be manufactured.
　本発明は、上記の事情を鑑みてなされたものであって、建築構造部材に好適な高強度と低降伏比を有し低温靱性に優れた角形鋼管およびその製造方法、角形鋼管の素材である熱延鋼板及びその製造方法、並びにこの角形鋼管を使用した建築構造物を提供することを目的とする。また、本発明は特に厚肉の角形鋼管および厚肉の角形鋼管に用いられる厚肉の熱延鋼板に適用することが好適である。 The present invention has been made in view of the above circumstances, and provides a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness suitable for building structural members, a method for producing the same, and a material for the square steel pipe. An object of the present invention is to provide a hot-rolled steel sheet, a method for manufacturing the same, and a building structure using the square steel pipe. Moreover, the present invention is particularly suitable for application to thick square steel pipes and thick hot-rolled steel sheets used for thick square steel pipes.
　なお、本発明でいう角形鋼管について「高強度」とは、冷間でロール成形して製造される角形鋼管（以下、冷間ロール成形角形鋼管と称する場合もある）の平板部の降伏強度が３８５ＭＰa以上、かつ、平板部の引張強度が５２０ＭＰａ以上の強度を有することを指す。また、本発明でいう角形鋼管について「低降伏比」とは、平板部の降伏比（＝降伏強度／引張強度）が０．９０以下であることを指す。また、本発明でいう角形鋼管について「低温靱性に優れた」とは、上記角形鋼管の平板部の－２０℃におけるシャルピー吸収エネルギーが１１０J以上であることを指す。さらに、本発明でいう熱延鋼板について「高強度」とは、冷間ロール成形して製造される角形鋼管（以下、冷間ロール成形角形鋼管と称する場合もある）の素材である熱延鋼板（角形鋼管用の熱延鋼板）の降伏強度が３３０ＭＰa以上、引張強度が５２０ＭＰａ以上の強度を有することを指す。また、本発明でいう熱延鋼板について「低降伏比」とは、上記熱延鋼板の降伏比（＝降伏強度／引張強度）が０．７５以下であることを指す。また、本発明でいう熱延鋼板について「低温靱性に優れた」とは、上記素材の－２０℃におけるシャルピー吸収エネルギーが１８０Ｊ以上であることを指す。また、本発明でいう「厚肉」とは、肉厚および板厚が５ｍｍ超２６ｍｍ未満であることを指す。なお、本発明では、上記素材の熱延鋼板には熱延鋼帯を含むものとする。また、本発明でいう肉厚は角形鋼管の厚さを指し、板厚は熱延鋼板の厚さを指す。 In the present invention, "high strength" of the square steel pipe means that the yield strength of the flat plate portion of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll formed square steel pipe) is It refers to having a strength of 385 MPa or more and a tensile strength of the flat plate portion of 520 MPa or more. In addition, the term "low yield ratio" in the square steel pipe referred to in the present invention means that the flat plate portion has a yield ratio (=yield strength/tensile strength) of 0.90 or less. Further, in the present invention, "excellent in low-temperature toughness" of the square steel pipe means that the flat plate portion of the square steel pipe has a Charpy absorbed energy of 110 J or more at -20°C. Furthermore, with respect to the hot-rolled steel sheet referred to in the present invention, "high strength" refers to the hot-rolled steel sheet that is the raw material of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll-forming square steel pipe). (Hot-rolled steel sheet for square steel pipe) has a yield strength of 330 MPa or more and a tensile strength of 520 MPa or more. In addition, the "low yield ratio" of the hot-rolled steel sheet referred to in the present invention means that the yield ratio (=yield strength/tensile strength) of the hot-rolled steel sheet is 0.75 or less. In addition, with respect to the hot-rolled steel sheet of the present invention, "excellent in low-temperature toughness" means that the material has a Charpy absorbed energy of 180 J or more at -20°C. In addition, the term "thick" as used in the present invention means that the thickness and plate thickness are more than 5 mm and less than 26 mm. In addition, in the present invention, the hot-rolled steel sheet as the raw material includes a hot-rolled steel strip. Further, the wall thickness referred to in the present invention refers to the thickness of the square steel pipe, and the plate thickness refers to the thickness of the hot-rolled steel plate.
　上述の通り、ロール成形を施す熱延鋼板は、ロール成形による降伏比の上昇や靱性の低下といった機械特性の変化を考慮して適切に選択することが必要である。本発明では、まず、冷間ロール成形して製造された後の角形鋼管における平板部の降伏強度を３８５ＭＰａ以上、平板部の引張強度を５２０ＭＰａ以上、かつ高い塑性変形能と優れた靱性を備えることができる熱延鋼板について検討した。その結果、冷間ロール成形角形鋼管用の素材である熱延鋼板の機械特性は、具体的に、降伏強度が３３０ＭＰa以上、引張強度が５２０ＭＰa以上、降伏比（＝降伏強度／引張強度）が０．７５以下、－２０℃におけるシャルピー吸収エネルギーが１８０Ｊ以上であればよいことを見出した。 As mentioned above, it is necessary to appropriately select the hot-rolled steel sheet to be roll-formed, taking into account changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll-forming. In the present invention, first, the rectangular steel pipe manufactured by cold roll forming should have a flat plate portion with a yield strength of 385 MPa or more, a flat plate portion with a tensile strength of 520 MPa or more, high plastic deformability, and excellent toughness. A hot-rolled steel sheet that can be As a result, the mechanical properties of the hot-rolled steel sheet, which is the raw material for cold roll-forming square steel pipes, are specifically that the yield strength is 330 MPa or more, the tensile strength is 520 MPa or more, and the yield ratio (= yield strength/tensile strength) is 0. .75 or less and the Charpy absorbed energy at -20° C. is 180 J or more.
　そして、上記した機械特性を満足する冷間ロール成形角形鋼管用の熱延鋼板について、更に検討した結果、以下の知見（１）～（３）を得た。 As a result of further studies on hot-rolled steel sheets for cold roll-forming square steel pipes that satisfy the mechanical properties described above, the following findings (1) to (3) were obtained.
（１）熱延鋼板が、本発明で目的とする降伏強度および引張強度を満足するためには、Ｃ含有量を０．０４質量％以上とし、さらに鋼板の主体組織をフェライトとベイナイトの混合組織とすることが必要である。 (1) In order for the hot-rolled steel sheet to satisfy the yield strength and tensile strength aimed at in the present invention, the C content should be 0.04% by mass or more, and the main structure of the steel sheet should be a mixed structure of ferrite and bainite. It is necessary to
（２）熱延鋼板が、本発明で目的とする降伏比を満足するためには、鋼板の残部組織を硬質なパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上とすることが必要である。 (2) In order for the hot-rolled steel sheet to satisfy the yield ratio targeted by the present invention, the residual structure of the steel sheet may be one or more selected from hard pearlite, martensite, and austenite. is necessary.
（３）熱延鋼板が上記した（１）および（２）の両方を満足する鋼組織において、さらに本発明で目的とする靱性を備えるためには、隣り合う結晶の方位差１５°以上の境界によって囲まれる領域を結晶粒としたとき、円相当径４０．０μｍ以上の結晶粒の体積率を２０％以下とすることが必要である。 (3) In a steel structure in which the hot-rolled steel sheet satisfies both of (1) and (2) above, in order to further provide the toughness targeted by the present invention, a boundary with an orientation difference of 15° or more between adjacent crystals When the region surrounded by is defined as crystal grains, it is necessary that the volume ratio of crystal grains having an equivalent circle diameter of 40.0 μm or more is 20% or less.
　さらに、本発明者らは角形鋼管について鋭意検討を行った結果、以下の知見（４）～（６）を得た。 Furthermore, as a result of diligent studies on square steel pipes, the inventors obtained the following findings (4) to (6).
（４）角形鋼管が、本発明で目的とする平板部の降伏強度および引張強度を満足するためには、Ｃの含有量を０．０４質量％以上とする必要がある。さらに、角形鋼管の管外面から肉厚ｔの１／４ｔ深さ位置（表層部）における主体組織をフェライトとベイナイトの混合組織とする必要がある。 (4) In order for the rectangular steel pipe to satisfy the yield strength and tensile strength of the flat plate portion targeted by the present invention, the C content must be 0.04% by mass or more. Furthermore, the main structure at a depth of 1/4t (surface layer) of the wall thickness t from the outer surface of the square steel pipe must be a mixed structure of ferrite and bainite.
（５）角形鋼管が、上記（４）を満足する鋼組織において、さらに本発明で目的とする平板部の低温靱性を得るためには、上記（４）に加えて、角形鋼管の管外面から肉厚ｔの１／４ｔ深さ位置（表層部）の鋼組織において、隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数を３０個／ｍｍ２以下とし、円相当径４０．０μｍ以上の結晶粒の体積率を２０％以下とする必要がある。 (5) In order for the square steel pipe to obtain the low temperature toughness of the flat plate portion, which is the object of the present invention, in the steel structure that satisfies the above (4), in addition to the above (4), from the outer surface of the square steel pipe In the steel structure at a depth position of 1/4t of the thickness t (surface layer), when the crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and The number of crystal grains with a ratio of major axis to minor axis (= (long axis) / (short axis)) of 4.0 or more is 30 / mm 2 or less, and the volume ratio of crystal grains with an equivalent circle diameter of 40.0 μm or more is It should be 20% or less.
（６）上記の（４）および（５）の鋼組織を得るためには、成分組成を適切な範囲に調整し、且つ、ＮｂとＴｉの含有量を特定の範囲に制御することが必要である。 (6) In order to obtain the steel structures of (4) and (5) above, it is necessary to adjust the chemical composition to an appropriate range and to control the contents of Nb and Ti within a specific range. be.
　本発明は、これらの知見に基づいて完成されたものであり、下記の要旨からなる。
［１］　平板部と角部を有する角形鋼管であって、
　平板部の成分組成が、質量％で、
Ｃ　：０．０４％以上０．４５％以下、
Ｓｉ：１．８％以下、
Ｍｎ：０．５％以上２．５％以下、
Ｐ　：０．１０％以下、
Ｓ　：０．０５％以下、
Ａｌ：０．００５％以上０．１００％以下、
Ｎ　：０．０１０％以下、
Ｎｂ：０．００５％以上０．０５０％以下、
Ｔｉ：０．０１２％以上０．１００％以下、
を含み、残部がＦｅおよび不可避的不純物からなり、
ＮｂとＴｉの含有量が下記（１）式を満足し、
　前記平板部の肉厚をｔとしたとき、管外面から肉厚ｔの１／４ｔ深さ位置における平板部の鋼組織は、
体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、
該フェライトおよび該ベイナイトの合計が、７５％以上９５％以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、
隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であり、
円相当径で４０．０μｍ以上の結晶粒が体積率で２０％以下である、角形鋼管。
１．２０×％Ｎｂ≦％Ｔｉ　　　　　　・・・（１）
ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。
［２］　平板部の降伏強度が３８５ＭＰa以上、かつ、平板部の引張強度が５２０ＭＰａ以上、平板部の降伏比が０．９０以下、平板部の－２０℃におけるシャルピー吸収エネルギーが１１０J以上である、［１］に記載の角形鋼管。
［３］　平板部は、前記成分組成に加えてさらに、質量％で、下記のうちから選ばれた１種または２種以上を含有する、［１］または［２］に記載の角形鋼管。
Ｖ：０．０１％以上０．１５％以下、
Ｃｒ：０．０１％以上１．０％以下、
Ｍｏ：０．０１％以上１．０％以下、
Ｎｉ：０．０１％以上０．３％以下、
Ｃａ：０．０００５％以上０．０１０％以下、
Ｂ：０．０００３％以上０．０１０％以下、
Ｃｕ：０．０１％以上０．５％以下
［４］　前記鋼組織は、体積率で、ベイナイトが１０％以上４０％未満である、［１］～［３］のいずれかに記載の角形鋼管。
［５］　［１］または［３］に記載の成分組成を有する鋼素材を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下、仕上圧延終了温度：７５０℃以上８５０℃以下、かつ９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延を施し、次いで、板厚中心温度で平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却を施し、
　次いで、４４０℃以上６５０℃以下で巻取り熱延鋼板とし、
　次いで、冷間ロール成形により、前記熱延鋼板を円筒状に成形し、突き合わせ部を電縫溶接した後、角形状に成形して角形の鋼管とする造管工程を施す、角形鋼管の製造方法。
［６］　［１］～［４］のいずれかに記載の角形鋼管が、柱材として使用されている、建築構造物。
［７］　成分組成は、質量％で、
Ｃ　：０．０４％以上０．４５％以下、
Ｓｉ：１．８％以下、
Ｍｎ：０．５％以上２．５％以下、
Ｐ　：０．１０％以下、
Ｓ　：０．０５％以下、
Ａｌ：０．００５％以上０．１００％以下、
Ｎ　：０．０１０％以下、
Ｎｂ：０．００５％以上０．０５０％以下、
Ｔｉ：０．０１２％以上０．１００％以下、
を含み、残部がＦｅおよび不可避的不純物からなり、
ＮｂとＴｉの含有量が下記（１）式を満足し、
　鋼板表面から板厚ｔの１／４ｔ位置における鋼組織は、
体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、
該フェライトおよび該ベイナイトの合計が、７５％以上９５％以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、
隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であり、
円相当径で４０．０μｍ以上の結晶粒が体積率で２０％以下である、熱延鋼板。
１．２０×％Ｎｂ≦％Ｔｉ　　　　　　・・・（１）
ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。
［８］　降伏強度が３３０ＭＰa以上、かつ、引張強度が５２０ＭＰａ以上、降伏比が０．７５以下、－２０℃におけるシャルピー吸収エネルギーが１８０J以上である、［７］に記載の熱延鋼板。
［９］　前記成分組成に加えてさらに、質量％で、下記のうちから選ばれた１種または２種以上を含有する、［７］または［８］に記載の熱延鋼板。
Ｖ：０．０１％以上０．１５％以下、
Ｃｒ：０．０１％以上１．０％以下、
Ｍｏ：０．０１％以上１．０％以下、
Ｃｕ：０．０１％以上０．５％以下、
Ｎｉ：０．０１％以上０．３％以下、
Ｃａ：０．０００５％以上０．０１０％以下、
Ｂ：０．０００３％以上０．０１０％以下
［１０］　前記鋼組織は、体積率で、ベイナイトが１０％以上４０％未満である、［７］～［９］のいずれかに記載の熱延鋼板。
［１１］　［７］または［９］に記載の成分組成を有する鋼素材を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下、仕上圧延終了温度：７５０℃以上８５０℃以下、かつ９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延を施し、次いで、板厚中心温度で平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却を施し、４４０℃以上６５０℃以下で巻取る、熱延鋼板の製造方法。 The present invention has been completed based on these findings, and consists of the following gists.
[1] A square steel pipe having a flat plate portion and corner portions,
The component composition of the flat plate portion is % by mass,
C: 0.04% or more and 0.45% or less,
Si: 1.8% or less,
Mn: 0.5% or more and 2.5% or less,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.010% or less,
Nb: 0.005% or more and 0.050% or less,
Ti: 0.012% or more and 0.100% or less,
with the remainder consisting of Fe and unavoidable impurities,
The content of Nb and Ti satisfies the following formula (1),
When the thickness of the flat plate portion is t, the steel structure of the flat plate portion at a depth of 1/4t of the wall thickness t from the outer surface of the pipe is
The volume fraction is more than 30% ferrite and 10% or more bainite,
The total of the ferrite and the bainite is 75% or more and 95% or less,
The balance consists of one or more selected from pearlite, martensite, and austenite,
When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis)) is 4. The number of crystal grains of .0 or more is 30 / mm 2 or less,
A square steel pipe in which crystal grains having an equivalent circle diameter of 40.0 μm or more account for 20% or less by volume.
1.20×%Nb≦%Ti (1)
Here, %Nb and %Ti are contents (% by mass) of each element.
[2] The yield strength of the flat plate portion is 385 MPa or more, the tensile strength of the flat plate portion is 520 MPa or more, the yield ratio of the flat plate portion is 0.90 or less, and the Charpy absorbed energy of the flat plate portion at -20 ° C. is 110 J or more. The square steel pipe according to [1].
[3] The square steel pipe according to [1] or [2], wherein the flat plate portion further contains, in % by mass, one or two or more selected from the following in addition to the chemical composition.
V: 0.01% or more and 0.15% or less,
Cr: 0.01% or more and 1.0% or less,
Mo: 0.01% or more and 1.0% or less,
Ni: 0.01% or more and 0.3% or less,
Ca: 0.0005% or more and 0.010% or less,
B: 0.0003% or more and 0.010% or less,
Cu: 0.01% or more and 0.5% or less [4] The square steel pipe according to any one of [1] to [3], wherein the steel structure has a volume fraction of bainite of 10% or more and less than 40%. .
[5] After heating the steel material having the chemical composition described in [1] or [3] to a heating temperature of 1100° C. or higher and 1300° C. or lower, the finish rolling is completed at a rough rolling finish temperature of 850° C. or higher and 1150° C. or lower. Temperature: 750 ° C. or higher and 850 ° C. or lower and total rolling reduction at 930 ° C. or lower: hot rolling with 40% or higher and 63% or lower, then average cooling rate at plate thickness center temperature: 2 ° C./s or higher 27 ° C./s or less, cooling stop temperature: 450 ° C. or higher and 650 ° C. or lower,
Next, the hot-rolled steel sheet is coiled at 440° C. or higher and 650° C. or lower,
Then, the hot-rolled steel plate is formed into a cylindrical shape by cold roll forming, and the butt joints are electric resistance welded, and then formed into a square shape to obtain a square steel pipe. .
[6] A building structure in which the square steel pipe according to any one of [1] to [4] is used as a pillar material.
[7] The component composition is mass%,
C: 0.04% or more and 0.45% or less,
Si: 1.8% or less,
Mn: 0.5% or more and 2.5% or less,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.010% or less,
Nb: 0.005% or more and 0.050% or less,
Ti: 0.012% or more and 0.100% or less,
with the remainder consisting of Fe and unavoidable impurities,
The content of Nb and Ti satisfies the following formula (1),
The steel structure at the 1/4t position of the plate thickness t from the steel plate surface is
The volume fraction is more than 30% ferrite and 10% or more bainite,
The total of the ferrite and the bainite is 75% or more and 95% or less,
The balance consists of one or more selected from pearlite, martensite, and austenite,
When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis)) is 4. The number of crystal grains of .0 or more is 30 / mm 2 or less,
A hot-rolled steel sheet containing 20% or less by volume of crystal grains having an equivalent circle diameter of 40.0 μm or more.
1.20×%Nb≦%Ti (1)
Here, %Nb and %Ti are contents (% by mass) of each element.
[8] The hot-rolled steel sheet according to [7], which has a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more.
[9] The hot-rolled steel sheet according to [7] or [8], which further contains, in % by mass, one or more selected from the following in addition to the chemical composition.
V: 0.01% or more and 0.15% or less,
Cr: 0.01% or more and 1.0% or less,
Mo: 0.01% or more and 1.0% or less,
Cu: 0.01% or more and 0.5% or less,
Ni: 0.01% or more and 0.3% or less,
Ca: 0.0005% or more and 0.010% or less,
B: 0.0003% or more and 0.010% or less [10] The hot rolling according to any one of [7] to [9], wherein the steel structure has a volume fraction of bainite of 10% or more and less than 40%. steel plate.
[11] After heating the steel material having the chemical composition described in [7] or [9] to a heating temperature of 1100° C. or higher and 1300° C. or lower, the rough rolling finish temperature is 850° C. or higher and 1150° C. or lower, and finish rolling is finished. Temperature: 750 ° C. or higher and 850 ° C. or lower and total rolling reduction at 930 ° C. or lower: hot rolling with 40% or higher and 63% or lower, then average cooling rate at plate thickness center temperature: 2 ° C./s or higher 27 C/s or less, cooling stop temperature: A method for manufacturing a hot-rolled steel sheet, comprising cooling at 450° C. or higher and 650° C. or lower and winding at 440° C. or higher and 650° C. or lower.
　本発明によれば、高強度および低降伏比を備え、低温靱性に優れた熱延鋼板およびその製造方法を提供することができ、また、高強度および低降伏比を備え、低温靱性に優れた角形鋼管およびその製造方法を提供することができる。 According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, a low yield ratio and excellent low temperature toughness and a method for producing the same, and a steel sheet having high strength, a low yield ratio and excellent low temperature toughness. A square steel pipe and a method for manufacturing the same can be provided.
図１は、本発明の角形鋼管を使用した建築構造物の一例を模式的に示す斜視図である。FIG. 1 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention. 図２は、本発明で実施した角形鋼管の平板部引張試験片の採取位置を示す概略図である。FIG. 2 is a schematic diagram showing the sampling positions of flat plate portion tensile test pieces of square steel pipes carried out in the present invention. 図３は、本発明で実施した角形鋼管のシャルピー試験片の採取位置を示す概略図である。FIG. 3 is a schematic diagram showing the sampling positions of the Charpy test piece of the square steel pipe implemented in the present invention. 図４は、角形鋼管の－２０℃におけるシャルピー吸収エネルギーと結晶粒の長径と短径の比が４．０以上の結晶粒の個数との関係を示すグラフである。FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at −20° C. of a square steel pipe and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more. 図５は、熱延鋼板の－２０℃におけるシャルピー吸収エネルギーと結晶粒の長径と短径の比が４．０以上の結晶粒の個数との関係を示すグラフである。FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at −20° C. of a hot-rolled steel sheet and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more.
　以下、本発明について詳細に説明する。 The present invention will be described in detail below.
　本発明は、平板部と角部を有する角形鋼管およびその素材として使用する熱延鋼板であって、角形鋼管の平板部および熱延鋼板の成分組成が、質量％で、Ｃ：０．０４％以上０．４５％以下、Ｓｉ：１．８％以下、Ｍｎ：０．５％以上２．５％以下、Ｐ：０．１０％以下、Ｓ：０．０５％以下、Ａｌ：０.００５％以上０．１００％以下、Ｎ：０．０１０％以下、Ｎｂ：０．００５％以上０．０５０％以下、Ｔｉ：０．０１２％以上０．１００％以下を含み、残部がＦｅおよび不可避的不純物からなり、ＮｂとＴｉの含有量が（１）式を満足し、管外面および鋼板表面から厚さｔ（肉厚ｔおよび板厚ｔを意味する。以下同じ。）の１／４ｔ深さ位置における鋼組織は、体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、該フェライトおよび該ベイナイトの合計が、７５％以上９５％以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が鋼組織において３０個／ｍｍ２以下であり、円相当径で４０．０μｍ以上の結晶粒が体積率で２０％以下である。
１．２０×％Ｎｂ≦％Ｔｉ　　　　　　・・・（１）
ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。 The present invention relates to a square steel pipe having a flat portion and corner portions and a hot-rolled steel sheet used as a material for the square steel pipe, wherein the chemical composition of the flat portion and the hot-rolled steel sheet of the square steel pipe is 0.04% by mass and C: 0.04%. 0.45% or less, Si: 1.8% or less, Mn: 0.5% or more and 2.5% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% 0.100% or less, N: 0.010% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.012% or more and 0.100% or less, the balance being Fe and unavoidable impurities The content of Nb and Ti satisfies the formula (1), and the thickness t (meaning the wall thickness t and the plate thickness t; the same applies hereinafter) from the outer surface of the pipe and the surface of the steel plate 1/4t depth position The steel structure in the volume fraction is more than 30% ferrite and 10% or more bainite, the total of the ferrite and the bainite is 75% or more and 95% or less, and the balance is pearlite, martensite, and austenite. When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis ( = (major axis) / (minor axis)) is 4.0 or more, the number of crystal grains is 30/mm2 or less in the steel structure, and the volume fraction of crystal grains with an equivalent circle diameter of 40.0 μm or more is 20%. It is below.
1.20×%Nb≦%Ti (1)
Here, %Nb and %Ti are contents (% by mass) of each element.
　以下に、本発明の角形鋼管およびその製造方法、並びに熱延鋼板およびその製造方法について説明する。 The square steel pipe of the present invention and its manufacturing method and the hot-rolled steel sheet and its manufacturing method will be described below.
　本発明において、角形鋼管および熱延鋼板の成分組成を限定した理由について説明する。本明細書において、特に断りがない限り、鋼組成を示す「％」は「質量％」である。なお、本発明の角形鋼管は、熱延鋼板を冷間でロール成形する方法で製造されるため、平板部と角部は同一の熱延鋼板から成り、平板部と角部の成分組成は同一である。一方、溶接部は溶接時に高温まで加熱されるため、大気中の酸素と反応して酸化が生じ、平板部や角部とは成分組成が異なる可能性が有る。角形鋼管全体の体積に占める溶接部の体積は少ないため、溶接部の成分組成が角形鋼管の特性に及ぼす影響は小さく、溶接部の成分組成は平板部の成分組成と同一でも、異なっていても、いずれでも良い。 The reason for limiting the composition of the square steel pipe and the hot-rolled steel plate in the present invention will be explained. In this specification, "%" indicating steel composition is "% by mass" unless otherwise specified. Since the square steel pipe of the present invention is manufactured by cold roll forming a hot-rolled steel plate, the flat plate portion and the corner portions are made of the same hot-rolled steel plate, and the flat plate portion and the corner portions have the same chemical composition. is. On the other hand, since the welded portion is heated to a high temperature during welding, it reacts with oxygen in the atmosphere and is oxidized, so there is a possibility that the component composition is different from that of the flat portion and the corner portion. Since the volume of the weld zone in the total volume of the square steel pipe is small, the chemical composition of the weld zone has little effect on the characteristics of the square steel pipe. , either is fine.
　Ｃ：０．０４％以上０．４５％以下
　Ｃは固溶強化により鋼の強度を上昇させる元素である。また、Ｃは、パーライトの生成を促進し、焼入れ性を高めてベイナイトの生成に寄与する元素である。本発明で目的とする強度および降伏比を確保するためには、０．０４％以上のＣを含有することが必要である。しかしながら、Ｃ含有量が０．４５％を超えると、硬質相の割合が高くなり靱性が低下し、また角形鋼管の平板部の降伏比が０．９０を超え所望の降伏比が得られなくなる。また、溶接性も悪化する。このため、Ｃ含有量は０．０４％以上０．４５％以下とする。Ｃ含有量は、好ましくは０．０８％以上であり、より好ましくは０．１２％超であり、より一層好ましくは０．１４％以上ある。また、Ｃ含有量は、好ましくは０．３０％以下であり、より好ましくは０．２５％以下であり、より一層好ましくは０．２２％以下である。 C: 0.04% to 0.45% C is an element that increases the strength of steel by solid solution strengthening. Also, C is an element that promotes the formation of pearlite, improves hardenability, and contributes to the formation of bainite. In order to secure the strength and yield ratio targeted in the present invention, it is necessary to contain 0.04% or more of C. However, if the C content exceeds 0.45%, the proportion of the hard phase increases and the toughness decreases, and the yield ratio of the flat plate portion of the square steel pipe exceeds 0.90, making it impossible to obtain the desired yield ratio. Weldability also deteriorates. Therefore, the C content should be 0.04% or more and 0.45% or less. The C content is preferably 0.08% or more, more preferably over 0.12%, and even more preferably 0.14% or more. Also, the C content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.22% or less.
　Ｓｉ：１．８％以下
　Ｓｉは固溶強化により鋼の強度を上昇させる元素であり、必要に応じて含有することができる。このような効果を得るためには、０．０１％以上のＳｉを含有することが望ましい。しかし、Ｓｉ含有量が１．８％を超えると、電縫溶接部に酸化物が生成しやすくなり、溶接部特性が低下する。また電縫溶接部以外の母材部の靱性も低下する。このため、Ｓｉ含有量は１．８％以下とする。Ｓｉ含有量は、好ましくは０．０１％以上であり、より好ましくは０．１０％以上である。また、Ｓｉ含有量は、好ましくは０．５％以下であり、より好ましくは０．４％以下であり、より一層好ましくは０．３％以下である。 Si: 1.8% or less Si is an element that increases the strength of steel by solid-solution strengthening, and can be contained as necessary. In order to obtain such effects, it is desirable to contain 0.01% or more of Si. However, if the Si content exceeds 1.8%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. In addition, the toughness of the base metal portion other than the electric resistance welded portion is also lowered. Therefore, the Si content is set to 1.8% or less. The Si content is preferably 0.01% or more, more preferably 0.10% or more. Also, the Si content is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.
　Ｍｎ：０．５％以上２．５％以下
　Ｍｎは固溶強化により鋼の強度を上昇させる元素である。また、Ｍｎはフェライト変態開始温度を低下させることで組織の微細化に寄与する元素である。本発明で目的とする強度および組織を確保するためには、０．５％以上のＭｎを含有することが必要である。しかしながら、Ｍｎ含有量が２．５％を超えると、ベイナイト組織の生成量が多くなりすぎることで角形鋼管の平板部の降伏比が０．９０を超え、所望の降伏比が得られなくなる。また、Ｍｎ含有量が２．５％を超えると、電縫溶接部に酸化物が生成しやすくなり、溶接部特性が低下する。このため、Ｍｎ含有量は０．５％以上２．５％以下とする。Ｍｎ含有量は、好ましくは０．７％以上であり、より好ましくは０．９％以上であり、より一層好ましくは１．０％以上である。また、Ｍｎ含有量は、好ましくは２．０％以下である。 Mn: 0.5% to 2.5% Mn is an element that increases the strength of steel through solid solution strengthening. Moreover, Mn is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the strength and structure targeted in the present invention, it is necessary to contain 0.5% or more of Mn. However, when the Mn content exceeds 2.5%, the yield ratio of the flat plate portion of the square steel pipe exceeds 0.90 due to an excessive amount of bainite structure, and the desired yield ratio cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. Therefore, the Mn content should be 0.5% or more and 2.5% or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, and even more preferably 1.0% or more. Also, the Mn content is preferably 2.0% or less.
　Ｐ：０．１０％以下
　Ｐは、粒界に偏析し材料の不均質を招くため、不可避的不純物としてできるだけ低減することが好ましいが、０．１０％以下の含有量までは許容できる。このため、Ｐ含有量は０．１０％以下の範囲内とする。Ｐ含有量は、好ましくは０．０３％以下であり、より好ましくは０．０２０％以下であり、より一層好ましくは０．０１５％以下である。なお、特にＰの下限は規定しないが、過度の低減は製錬コストの高騰を招くため、Ｐは０．００２％以上とすることが好ましい。 P: 0.10% or less P segregates at grain boundaries and causes nonhomogeneity of the material. Therefore, it is preferable to reduce P as an unavoidable impurity as much as possible, but a content of 0.10% or less is acceptable. Therefore, the P content should be within the range of 0.10% or less. The P content is preferably 0.03% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Although the lower limit of P is not specified, it is preferable to set P to 0.002% or more because excessive reduction leads to a rise in smelting costs.
　Ｓ：０．０５％以下
　Ｓは、鋼中では通常、ＭｎＳとして存在するが、ＭｎＳは、熱間圧延工程で薄く延伸され、延性に悪影響を及ぼす。このため、本発明ではＳをできるだけ低減することが好ましいが、０．０５％以下の含有量までは許容できる。このため、Ｓ含有量は０．０５％以下とする。Ｓ含有量は、好ましくは０．０１５％以下であり、より好ましくは０．０１０％以下であり、より一層好ましくは０．００８％以下である。なお、特にＳの下限は規定しないが、過度の低減は製錬コストの高騰を招くため、Ｓは０．０００２％以上とすることが好ましい。 S: 0.05% or less S usually exists as MnS in steel, but MnS is thinly drawn in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.05% or less is permissible. Therefore, the S content should be 0.05% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. Although the lower limit of S is not specified, excessive reduction leads to an increase in smelting costs, so S is preferably 0.0002% or more.
　Ａｌ：０．００５%以上０．１００％以下
　Ａｌは、強力な脱酸剤として作用する元素である。このような効果を得るためには、０．００５％以上のＡｌを含有することが必要である。しかし、Ａｌ含有量が０．１００％を超えると溶接性が悪化するとともに、アルミナ系介在物が多くなり、表面性状が悪化する。また溶接部の靱性も低下する。このため、Ａｌ含有量は０．００５％以上０．１００％以下とする。Ａｌ含有量は、好ましくは０．０１０％以上であり、より好ましくは０．０１５％以上である。また、Ａｌ含有量は、好ましくは０．０７０％以下であり、より好ましくは０．０５０％以下である。 Al: 0.005% to 0.100% Al is an element that acts as a strong deoxidizing agent. In order to obtain such effects, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates and the amount of alumina-based inclusions increases, resulting in deterioration of the surface properties. Also, the toughness of the weld zone is reduced. Therefore, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, more preferably 0.015% or more. Also, the Al content is preferably 0.070% or less, more preferably 0.050% or less.
　Ｎ：０．０１０％以下
　Ｎは、不可避的不純物であり、転位の運動を強固に固着することで靭性を低下させる作用を有する元素である。本発明では、Ｎは不純物としてできるだけ低減することが望ましいが、Ｎの含有量は０．０１０％までは許容できる。このため、Ｎ含有量は０．０１０％以下とする。Ｎ含有量は、好ましくは０．００８０％以下であり、より好ましくは０．００４０％以下であり、より一層好ましくは０．００３５％以下である。なお、過度の低減は製錬コストの高騰を招くため、Ｎ含有量は０．００１０％以上とすることが好ましく、０．００１５％以上とすることがより好ましい。 N: 0.010% or less N is an unavoidable impurity, and is an element that has the effect of lowering the toughness by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the N content can be allowed up to 0.010%. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. Since an excessive reduction causes a rise in smelting costs, the N content is preferably 0.0010% or more, more preferably 0.0015% or more.
　Ｎｂ：０．００５％以上０．０５０％以下
　Ｎｂは、鋼中で微細な炭化物、窒化物を形成し、析出強化を通じて鋼の強度向上に寄与する元素である。このような効果を得るため、０．００５％以上含有することが必要である。しかしながら、Ｎｂの含有量が０．０５０％を超えると、粗大な炭化物、窒化物が形成され、また、後述するような長径と短径の比が大きい結晶粒の形成が促進され、靱性の低下を招く恐れがある。このため、Ｎｂ含有量は０．００５％以上０．０５０％以下とする。Ｎｂ含有量は、好ましくは０．００６％以上であり、より好ましくは０．００７％以上であり、より一層好ましくは０．００８％以上である。また、Ｎｂ含有量は、好ましくは０．０４５％以下であり、より好ましくは０．０３５％以下である。 Nb: 0.005% or more and 0.050% or less Nb is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. In order to obtain such effects, it is necessary to contain 0.005% or more. However, when the Nb content exceeds 0.050%, coarse carbides and nitrides are formed, and the formation of crystal grains with a large ratio of major axis to minor axis as described later is promoted, resulting in a decrease in toughness. may lead to Therefore, the Nb content should be 0.005% or more and 0.050% or less. The Nb content is preferably 0.006% or more, more preferably 0.007% or more, and even more preferably 0.008% or more. Also, the Nb content is preferably 0.045% or less, more preferably 0.035% or less.
　Ｔｉ：０．０１２％以上０．１００％以下
　Ｔｉは、鋼中で微細な炭化物、窒化物を形成し、析出強化を通じて鋼の強度向上に寄与する元素である。また、Ｔｉを適切な量添加すると粗大な結晶粒の生成を促進せずに強度を向上させることが可能であり、本発明において最も重要な元素の一つである。このような効果を得るため、０．０１２％以上含有することが必要である。しかしながら、Ｔｉの含有量が０．１００％を超えると、粗大な炭化物、窒化物が形成され靱性の低下を招く恐れがある。このため、Ｔｉ含有量は０．０１２％以上０．１００％以下とする。Ｔｉ含有量は、好ましくは０．０１５％以上であり、より好ましくは０．０１７％以上であり、より一層好ましくは０．０１８％以上である。また、Ｔｉ含有量は、好ましくは０．０９０％以下であり、より好ましくは０．０７０％以下である。 Ti: 0.012% or more and 0.100% or less Ti is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. Moreover, when Ti is added in an appropriate amount, it is possible to improve the strength without promoting the formation of coarse crystal grains, and it is one of the most important elements in the present invention. In order to obtain such effects, it is necessary to contain 0.012% or more. However, when the Ti content exceeds 0.100%, coarse carbides and nitrides are formed, which may lead to a decrease in toughness. Therefore, the Ti content should be 0.012% or more and 0.100% or less. The Ti content is preferably 0.015% or more, more preferably 0.017% or more, and even more preferably 0.018% or more. Also, the Ti content is preferably 0.090% or less, more preferably 0.070% or less.
１．２０×％Ｎｂ≦％Ｔｉ
　ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。
　本発明では、ＮｂとＴｉの含有量を上記した範囲とし、さらに１．２０×％Ｎｂ≦％Ｔｉを満足することが必要である。本関係式を満足することで、後述するような、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下の金属組織を得ることが可能となる。一方、１．２０×％Ｎｂ＞％Ｔｉの場合には、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２を超えるため、低温靱性の低下を招く。好ましくは１．５０×％Ｎｂ≦％Ｔｉであり、より好ましくは２．３０×％Ｎｂ≦％Ｔｉである。 1.20×%Nb≤%Ti
Here, %Nb and %Ti are contents (% by mass) of each element.
In the present invention, it is necessary that the contents of Nb and Ti be within the above ranges and that 1.20×%Nb≦%Ti is satisfied. By satisfying this relational expression, the number of crystal grains having a major axis of 50 μm or more and a ratio of the major axis to the minor axis (=(major axis)/(minor axis)) of 4.0 or more is 30. It becomes possible to obtain a metallographic structure of particles/mm 2 or less. On the other hand, when 1.20×%Nb>%Ti, the number of crystal grains having a major axis of 50 μm or more and a ratio of the major axis to the minor axis (=(major axis)/(minor axis)) of 4.0 or more exceeds 30/mm 2 , the low temperature toughness is lowered. Preferably 1.50×%Nb≦%Ti, more preferably 2.30×%Nb≦%Ti.
　残部はＦｅおよび不可避的不純物である。ただし、本発明の効果を損なわない範囲においては、Ｏを０．００５％以下含有することを拒むものではない。Ｖ：０．０１％未満、Ｃｒ：０．０１％未満、Ｍｏ：０．０１％未満、Ｃｕ：０．０１％未満、Ｎｉ：０．０１％未満、Ｃａ：０．０００５％未満、Ｂ：０．０００３％未満を不可避的不純物の中に含めることができる。 The balance is Fe and unavoidable impurities. However, as long as the effect of the present invention is not impaired, the O content may be 0.005% or less. V: less than 0.01%, Cr: less than 0.01%, Mo: less than 0.01%, Cu: less than 0.01%, Ni: less than 0.01%, Ca: less than 0.0005%, B: Less than 0.0003% can be included among the inevitable impurities.
　上記の成分が本発明における角形鋼管の基本の成分組成である。上記した必須元素で本発明で目的とする特性は得られるが、必要に応じて下記の元素を含有することができる。 The above ingredients are the basic ingredient composition of the square steel pipe in the present invention. Although the properties aimed at in the present invention can be obtained with the essential elements described above, the following elements can be contained as necessary.
　Ｖ：０．０１％以上０．１５％以下、Ｃｒ：０．０１％以上１.０％以下、Ｍｏ：０．０１％以上１．０％以下、Ｃｕ：０．０１％以上０．５％以下、Ｎｉ：０．０１％以上０．３％以下、Ｃａ：０．０００５％以上０．０１０％以下、Ｂ：０．０００３％以上０．０１０％以下のうちから選ばれた１種または２種以上
　Ｖ：０．０１％以上０．１５％以下、Ｃｒ：０．０１％以上１．０％以下、Ｍｏ：０．０１％以上１．０％以下
　Ｖ、Ｃｒ、Ｍｏは、鋼の焼入れ性を高め、鋼の強度を上昇させる元素であり、必要に応じて含有することができる。上記した効果を得るため、Ｖ、Ｃｒ、Ｍｏを含有する場合には、それぞれＶ：０．０１％以上、Ｃｒ：０．０１％以上、Ｍｏ：０．０１％以上とすることが好ましい。より好ましくは、それぞれＶ：０．０２％以上、Ｃｒ：０．１０％以上、Ｍｏ：０．１０％以上である。一方、過度の含有は、靱性の低下および溶接性の悪化を招く恐れがある。よって、Ｖ、Ｃｒ、Ｍｏを含有する場合には、それぞれＶ：０．１５％以下、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下とすることが好ましい。より好ましくは、Ｖ：０．１０％以下、Ｃｒ：０．５０％以下、Ｍｏ：０．５０％以下である。 V: 0.01% to 0.15%, Cr: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Cu: 0.01% to 0.5% Below, Ni: 0.01% or more and 0.3% or less, Ca: 0.0005% or more and 0.010% or less, B: 0.0003% or more and 0.010% or less One or two selected from V: 0.01% to 0.15%, Cr: 0.01% to 1.0%, Mo: 0.01% to 1.0% V, Cr, and Mo are used for quenching steel It is an element that enhances the strength of steel and can be contained as necessary. In order to obtain the above effects, when V, Cr, and Mo are contained, it is preferable that V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. More preferably, V: 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more. On the other hand, an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when V, Cr, and Mo are contained, it is preferable that V: 0.15% or less, Cr: 1.0% or less, and Mo: 1.0% or less, respectively. More preferably, V: 0.10% or less, Cr: 0.50% or less, and Mo: 0.50% or less.
　Ｃｕ：０．０１％以上０．５％以下、Ｎｉ：０．０１％以上０．３％以下
　Ｃｕ、Ｎｉは、固溶強化により鋼の強度を上昇させる元素であり、必要に応じて含有することができる。上記した効果を得るため、Ｃｕ、Ｎｉを含有する場合には、それぞれＣｕ：０．０１％以上、Ｎｉ：０．０１％以上とすることが好ましい。より好ましくは、それぞれＣｕ：０．１０％以上、Ｎｉ：０．１０％以上である。一方、過度の含有は、靱性の低下および溶接性の悪化を招く恐れがある。よって、Ｃｕ、Ｎｉを含有する場合には、それぞれＣｕ：０．５％以下、Ｎｉ：０．３％以下とすることが好ましい。より好ましくは、それぞれＣｕ：０．４０％以下、Ｎｉ：０．２０％以下である。 Cu: 0.01% or more and 0.5% or less, Ni: 0.01% or more and 0.3% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and are contained as necessary. be able to. In order to obtain the above effects, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferably, Cu: 0.10% or more and Ni: 0.10% or more. On the other hand, an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. More preferably, Cu: 0.40% or less and Ni: 0.20% or less.
　Ｃａ：０．０００５％以上０．０１０％以下
　Ｃａは、熱間圧延工程で薄く延伸されるＭｎＳ等の硫化物を球状化することで鋼の靱性向上に寄与する元素であり、必要に応じて含有できる。このような効果を得るため、Ｃａを含有する場合は、０．０００５％以上のＣａを含有することが好ましい。より好ましくは、Ｃａ含有量は０．００１０％以上である。しかし、Ｃａ含有量が０．０１０％を超えると、鋼中にＣａ酸化物クラスターが形成され、靱性が悪化する場合がある。このため、Ｃａを含有する場合は、Ｃａ含有量は０．０１０％以下とすることが好ましい。より好ましくは、Ｃａ含有量は０．００５０％以下である。 Ca: 0.0005% or more and 0.010% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process. can be contained. In order to obtain such an effect, when Ca is contained, it is preferable to contain 0.0005% or more of Ca. More preferably, the Ca content is 0.0010% or more. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, which may deteriorate the toughness. Therefore, when Ca is contained, the Ca content is preferably 0.010% or less. More preferably, the Ca content is 0.0050% or less.
　Ｂ：０．０００３％以上０．０１０％以下
　Ｂは、フェライト変態開始温度を低下させることで組織の微細化に寄与する元素である。このような効果を得るため、Ｂを含有する場合は、０．０００３％以上のＢを含有することが好ましい。より好ましくは、Ｂ含有量は０．０００５％以上である。しかし、Ｂ含有量が０．０１０％を超えると、降伏比が上昇する場合がある。このため、Ｂを含有する場合は、０．０１０％以下とすることが好ましい。より好ましくは、Ｂ含有量は０．００５０％以下である。 B: 0.0003% or more and 0.010% or less B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, when containing B, it is preferable to contain 0.0003% or more of B. More preferably, the B content is 0.0005% or more. However, when the B content exceeds 0.010%, the yield ratio may increase. Therefore, when B is contained, it is preferably 0.010% or less. More preferably, the B content is 0.0050% or less.
　次に、本発明の角形鋼管および熱延鋼板の鋼組織を限定した理由について説明する。 Next, the reason for limiting the steel structure of the square steel pipe and hot-rolled steel sheet of the present invention will be explained.
　本発明の角形鋼管および熱延鋼板における、鋼管の管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置における鋼組織は、体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、該フェライトおよび該ベイナイトの合計が、管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置における鋼組織全体に対して７５％以上９５％以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなる。隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であり、かつ円相当径（結晶粒径）で４０．０μｍ以上の結晶粒が管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置における鋼組織全体に対して体積率で２０％以下である。 In the square steel pipe and hot-rolled steel plate of the present invention, the steel structure at a depth position of 1/4t of the thickness t from the pipe outer surface of the steel pipe and the surface of the steel plate has a volume fraction of more than 30% ferrite and 10% or more bainite. and the sum of the ferrite and the bainite is 75% or more and 95% or less of the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the steel plate, and the balance is pearlite and marten It consists of one or more selected from site and austenite. When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis)) is 4. The number of crystal grains of 0.0 or more is 30/mm2 or less, and the crystal grains with an equivalent circle diameter (crystal grain size) of 40.0 μm or more are 1/4 t of the thickness t from the outer surface of the pipe and the surface of the steel plate. It is 20% or less in volume ratio with respect to the entire steel structure at the depth position.
　なお、本発明において、円相当径（結晶粒径）とは、対象となる結晶粒と面積が等しい円の直径とする。また、角形鋼管の鋼組織は、電縫溶接部を除く、角形鋼管の平板部の管外面から肉厚ｔの１／４ｔ深さ位置におけるものとする。一般的に、熱延鋼板を素材とするロール成形角形鋼管では、角部および平板部ともに管外面から肉厚ｔの１／４ｔ深さ位置の鋼組織が同じとなる。そのため、ここでは平板部の鋼組織について規定している。また、熱延鋼板の鋼組織は、鋼板の表面から板厚ｔの１／４ｔ深さ位置におけるものとする。 In the present invention, the equivalent circle diameter (crystal grain size) is the diameter of a circle having the same area as the target crystal grain. Also, the steel structure of the square steel pipe is at a depth of 1/4t of the wall thickness t from the outer surface of the flat plate portion of the square steel pipe, excluding the electric resistance welded portion. In general, in a roll-formed square steel pipe made of hot-rolled steel plate, both the corner portion and the flat plate portion have the same steel structure at a depth of 1/4t of the wall thickness t from the outer surface of the pipe. Therefore, the steel structure of the flat plate portion is specified here. The steel structure of the hot-rolled steel sheet is at a depth of 1/4t of the thickness t from the surface of the steel sheet.
　フェライトの体積率：３０％超、ベイナイトの体積率：１０％以上、鋼組織に対するフェライトおよびベイナイトの体積率の合計：７５％以上９５％以下
　フェライトは軟質な組織であり、他の硬質な組織と混合させることで、鋼の降伏比を低くする。このような効果により本発明で目的とする低降伏比を得るためには、フェライトの体積率は３０％を超える必要がある。フェライトの体積率は、好ましくは４０％以上であり、より好ましくは４３％以上であり、より一層好ましくは４５％以上である。なお、特に上限は規定しないが、所望の降伏比を確保するため、フェライトの体積率は、好ましくは７５％未満であり、より好ましくは７０％未満であり、より一層好ましくは６０％以下である。 Volume fraction of ferrite: more than 30%, volume fraction of bainite: 10% or more, total volume fraction of ferrite and bainite to the steel structure: 75% or more and 95% or less Mixing lowers the yield ratio of the steel. In order to obtain the low yield ratio aimed at by the present invention due to such effects, the volume fraction of ferrite must exceed 30%. The volume fraction of ferrite is preferably 40% or more, more preferably 43% or more, and even more preferably 45% or more. Although no particular upper limit is specified, the volume fraction of ferrite is preferably less than 75%, more preferably less than 70%, and still more preferably 60% or less in order to ensure a desired yield ratio. .
　ベイナイトは中間的な硬さを有する組織であり、鋼の強度を上昇させる。上記したフェライトだけでは本発明で目的とする降伏強度および引張強度が得られないため、ベイナイトの体積率は１０％以上とすることが必要である。ベイナイトの体積率は、好ましくは１５％以上であり、より好ましくは２０％以上であり、より一層好ましくは２５％以上である。なお、特に上限は規定しないが、所望の降伏比を確保するため、ベイナイトの体積率は、好ましくは５５％以下であり、より好ましくは５０％以下であり、より一層好ましくは４５％以下であり、さらに一層好ましくは４０％未満である。 Bainite is a structure with intermediate hardness and increases the strength of steel. Since the yield strength and tensile strength targeted in the present invention cannot be obtained with the ferrite alone, the volume fraction of bainite must be 10% or more. The volume fraction of bainite is preferably 15% or more, more preferably 20% or more, and still more preferably 25% or more. Although no particular upper limit is specified, the volume fraction of bainite is preferably 55% or less, more preferably 50% or less, and still more preferably 45% or less in order to ensure a desired yield ratio. , even more preferably less than 40%.
　なお、フェライトとベイナイトの体積率の合計が７５％未満であると、本発明で目的とする降伏比またはシャルピー吸収エネルギーが得られない。一方、フェライトとベイナイトの体積率の合計が９５％を超えると、本発明で目的とする降伏強度および降伏比が得られない。このため、上記した条件に加えて、フェライトとベイナイトの体積率の合計を７５％以上９５％以下とすることが必要である。好ましくは、７８％以上であり、好ましくは９３％以下である。より好ましくは、８０％以上であり、より好ましくは９０％以下である。 If the total volume fraction of ferrite and bainite is less than 75%, the yield ratio or Charpy absorbed energy targeted by the present invention cannot be obtained. On the other hand, if the total volume fraction of ferrite and bainite exceeds 95%, the yield strength and yield ratio aimed at in the present invention cannot be obtained. Therefore, in addition to the above conditions, the total volume fraction of ferrite and bainite must be 75% or more and 95% or less. Preferably, it is 78% or more, preferably 93% or less. More preferably, it is 80% or more, and more preferably 90% or less.
　残部：パーライト、マルテンサイト、オーステナイトから選択される１種または２種以上
　パーライト、マルテンサイト、およびオーステナイトは硬質な組織であり、特に鋼の引張強度を上昇させるとともに、軟質なフェライトと混合させることで鋼の降伏比が低くなる。このような効果を得るためには、残部は、パーライト、マルテンサイト、およびオーステナイトの各体積率の合計が５％以上２５％以下とする。好ましくは、７％以上であり、好ましくは２３％以下である。より好ましくは、１０％以上であり、より好ましくは２０％以下である。 Balance: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite are hard structures that increase the tensile strength of steel in particular. The yield ratio of steel becomes lower. In order to obtain such an effect, the sum of the volume fractions of pearlite, martensite, and austenite should be 5% or more and 25% or less. Preferably, it is 7% or more, preferably 23% or less. More preferably, it is 10% or more, and more preferably 20% or less.
　なお、フェライト、ベイナイト、パーライト、マルテンサイト、およびオーステナイトの体積率は、後述する実施例に記載の方法で測定することができる。 The volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.
　隣り合う結晶の方位差（結晶方位差）が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下、結晶粒径（円相当径）で４０．０μｍ以上の結晶粒の体積率：２０％以下
　上述のとおり、本発明の鋼組織は、本発明で目的とする低降伏比、降伏強度、および引張強度を得るために、軟質組織と硬質組織を混合させた鋼（以下、「複合組織鋼」と称する）とする。しかし、複合組織鋼は、単一組織鋼と比較して靱性が悪い。そこで、本発明では、上記の機械特性と優れた靱性を両立するため、結晶方位差が１５°以上の境界によって囲まれた領域を結晶粒としたとき、長径が５０μｍ以上の結晶粒の長径と短径の比（＝（長径）／（短径））および粗大な結晶粒の体積率を規定する。長径が５０μｍ以上の結晶粒の長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２を超える、または、円相当径が４０．０μｍ以上の結晶粒が管外面および熱延鋼板表面から厚さｔの１／４ｔ深さ位置における鋼組織全体に対して体積率で２０％を超えると、所望の低温靱性が得られない。
　このため、長径が５０μｍ以上の結晶粒の長径と短径の比が４．０以上の結晶粒の個数を３０個／ｍｍ２以下とし、円相当径が４０．０μｍ以上の結晶粒が管外面および熱延鋼板表面から厚さｔの１／４ｔ深さ位置における鋼組織全体に対して体積率で２０％以下とすることにより本発明で目的とする低温靱性を確保できる。結晶粒の長径と短径の比が４．０以上の結晶粒の個数は、好ましくは２８個／ｍｍ２以下であり、より好ましくは２６個／ｍｍ２以下である。また、円相当径が４０．０μｍ以上の結晶粒の体積率は、好ましくは１８％以下であり、より好ましくは１６％以下である。 When a crystal grain is a region surrounded by a boundary with an orientation difference (crystal orientation difference) of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis) The number of crystal grains with a diameter) of 4.0 or more is 30/mm 2 or less, and the volume ratio of crystal grains with a crystal grain size (equivalent circle diameter) of 40.0 μm or more: 20% or less As described above, the present invention The steel structure is a steel in which a soft structure and a hard structure are mixed (hereinafter referred to as "composite structure steel") in order to obtain the low yield ratio, yield strength, and tensile strength aimed at in the present invention. However, composite steels have poor toughness compared to single structure steels. Therefore, in the present invention, in order to achieve both the above mechanical properties and excellent toughness, when a region surrounded by boundaries with a crystal orientation difference of 15 ° or more is defined as a crystal grain, the major axis of the crystal grain having a major axis of 50 μm or more It defines the minor axis ratio (=(major axis)/(minor axis)) and the volume fraction of coarse grains. The ratio of the major axis to the minor axis of the crystal grains with the major axis of 50 μm or more (= (major axis) / (minor axis)) exceeds 30 / mm2 , or the circle equivalent diameter is When crystal grains of 40.0 µm or more exceed 20% in volume ratio with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the hot-rolled steel sheet, the desired low temperature toughness cannot be obtained. .
For this reason, the number of crystal grains with a major axis to minor axis ratio of 4.0 or more with a major axis of 50 μm or more is 30 / mm 2 or less, and a crystal grain with an equivalent circle diameter of 40.0 μm or more And the low-temperature toughness aimed at in the present invention can be ensured by setting the volume ratio to 20% or less with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the surface of the hot-rolled steel sheet. The number of crystal grains having a ratio of major axis to minor axis of 4.0 or more is preferably 28/mm 2 or less, more preferably 26/mm 2 or less. The volume fraction of crystal grains having an equivalent circle diameter of 40.0 μm or more is preferably 18% or less, more preferably 16% or less.
　ベイナイトは、方位差の大きい境界（オーステナイト粒界や、転位の集積により形成されたサブバウンダリー）を超えて成長しない。そのため、上記の粗大なベイナイトの生成を抑制するには、熱間圧延における仕上圧延をできるだけ低温で行い、オーステナイトに多量の転位を導入してサブバウンダリーの面積を増加させ、微細なサブグレイン構造（以下、「微細化」とも呼ぶ。）を形成することが特に有効である。 Bainite does not grow across boundaries with large misorientation (austenite grain boundaries and sub-boundaries formed by accumulation of dislocations). Therefore, in order to suppress the formation of coarse bainite as described above, finish rolling in hot rolling is performed at a temperature as low as possible to introduce a large amount of dislocations into austenite to increase the area of sub-boundaries, resulting in a fine sub-grain structure. (hereinafter also referred to as “miniaturization”) is particularly effective.
　なお、結晶方位差、平均結晶粒径、および結晶粒径が４０．０μｍ以上の結晶粒の体積率は、ＳＥＭ／ＥＢＳＤ法によって測定することが可能である。ここでは、後述する実施例に記載の方法で測定することができる。 The crystal misorientation, the average crystal grain size, and the volume fraction of crystal grains with a crystal grain size of 40.0 μm or more can be measured by the SEM/EBSD method. Here, it can be measured by the method described in Examples described later.
　本発明では、鋼管の管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置を中心として肉厚方向に±１．０ｍｍの範囲内に、上述の鋼組織が存在していても同様に上述の効果は得られる。そのため、本発明において「鋼管の管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置における鋼組織」とは、鋼管の管外面および鋼板の表面から厚さｔの１／４ｔ深さ位置を中心として肉厚方向に±１．０ｍｍの範囲のいずれかにおいて、上述の鋼組織が存在していることを意味する。また、本発明で目的とする低降伏比、高強度および高靱性を得るためには、角形鋼管の平板部において上記した鋼組織を満足することが必要である。一方、角部の鋼組織は上記した鋼組織を満足しても、満足していなくても角形鋼管の特性に及ぼす影響は小さく、角部の鋼組織は問わない。 In the present invention, even if the above-described steel structure exists within a range of ±1.0 mm in the thickness direction centering on the 1/4t depth position of the thickness t from the outer surface of the steel pipe and the surface of the steel plate. The above-mentioned effect can be obtained. Therefore, in the present invention, the “steel structure at a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate” means a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate. It means that the above steel structure exists in any of the range of ±1.0 mm in the thickness direction centered on the position. Further, in order to obtain the low yield ratio, high strength and high toughness which are the objectives of the present invention, it is necessary for the flat plate portion of the square steel pipe to satisfy the above steel structure. On the other hand, even if the steel structure of the corners satisfies the steel structure described above, the effect on the characteristics of the square steel pipe is small, and the steel structure of the corners does not matter.
　次に、本発明の一実施形態における熱延鋼板および角形鋼管の製造方法を説明する。 Next, a method for manufacturing a hot-rolled steel plate and a square steel pipe according to one embodiment of the present invention will be described.
　本発明の角形鋼管の製造方法は、例えば、上記した成分組成を有する鋼素材を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下、仕上圧延終了温度：７５０℃以上８５０℃以下、かつ９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延を施す。次いで、板厚中心温度で平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却を施し、次いで、４４０℃以上６５０℃以下で巻取り熱延鋼板とする。次いで、冷間ロール成形により、熱延鋼板を円筒状に成形し、突き合わせ部を電縫溶接した後、角形状に成形して角形の鋼管とする造管工程を施す。 The method for producing a square steel pipe of the present invention includes, for example, heating a steel material having the above-described chemical composition to a heating temperature of 1100° C. or higher and 1300° C. or lower, followed by finishing rolling at a rough rolling end temperature of 850° C. or higher and 1150° C. or lower. Finishing temperature: 750° C. or higher and 850° C. or lower and total rolling reduction: 40% or higher and 63% or lower at 930° C. or lower. Next, cooling is performed at the sheet thickness center temperature at an average cooling rate of 2 ° C./s or more and 27 ° C./s or less, cooling stop temperature: 450 ° C. or more and 650 ° C. or less, and then coiled and hot rolled at 440 ° C. or more and 650 ° C. or less. Steel plate. Next, the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming, and the butted portions are electric resistance welded, and then formed into a square shape to obtain a square steel pipe.
　なお、以下の製造方法の説明において、温度に関する「℃」表示は、特に断らない限り、鋼素材や鋼板（熱延鋼板）の表面温度とする。これらの表面温度は、放射温度計等で測定することができる。また、鋼板板厚中心の温度は、鋼板断面内の温度分布を伝熱解析により計算し、その結果を鋼板の表面温度によって補正することで求めることができる。また、「熱延鋼板」には、熱延鋼板、熱延鋼帯を含むものとする。 In addition, in the following description of the manufacturing method, "°C" regarding temperature indicates the surface temperature of the steel material or steel plate (hot-rolled steel plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the steel plate cross section by heat transfer analysis and correcting the result with the surface temperature of the steel plate. In addition, "hot-rolled steel sheet" includes hot-rolled steel sheet and hot-rolled steel strip.
　本発明において、鋼素材（鋼スラブ）の溶製方法は特に限定されず、転炉、電気炉、真空溶解炉等の公知の溶製方法のいずれもが適合する。鋳造方法も特に限定されないが、連続鋳造法等の公知の鋳造方法により、所望寸法に製造される。なお、連続鋳造法に代えて、造塊－分塊圧延法を適用しても何ら問題はない。溶鋼にはさらに、取鍋精錬等の二次精錬を施してもよい。 In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is suitable. The casting method is also not particularly limited, but the desired dimensions are manufactured by a known casting method such as a continuous casting method. It should be noted that there is no problem even if an ingot casting-slabbing rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.
　次いで、得られた鋼素材（鋼スラブ）を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下とする粗圧延を施し、仕上圧延終了温度：７５０℃以上８５０℃以下とする仕上げ圧延を施し、かつ、９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延工程を施して熱延鋼板とする。 Next, the obtained steel material (steel slab) is heated to a heating temperature of 1100° C. or more and 1300° C. or less, and then subjected to rough rolling at a rough rolling end temperature of 850° C. or more and 1150° C. or less. Finish rolling is performed at 750° C. or higher and 850° C. or lower, and hot rolling is performed such that the total rolling reduction at 930° C. or lower is 40% or higher and 63% or lower to obtain a hot rolled steel sheet.
　加熱温度：１１００℃以上１３００℃以下
　加熱温度が１１００℃未満である場合、被圧延材の変形抵抗が大きくなり圧延が困難となる。一方、加熱温度が１３００℃を超えると、オーステナイト粒が粗大化し、後の圧延（粗圧延、仕上圧延）において微細なオーステナイト粒が得られず、本発明で目的とする角形鋼管の鋼組織の平均結晶粒径を確保することが困難となる。また、粗大なベイナイトの生成を抑制することが困難となり、結晶粒径が４０．０μｍ以上の結晶粒の体積率を、本発明で目的とする範囲に制御することが難しい。このため、熱間圧延工程における加熱温度は、１１００℃以上１３００℃以下とする。好ましくは１１２０℃以上であり、好ましくは１２８０℃以下である。 Heating temperature: 1100° C. or higher and 1300° C. or lower If the heating temperature is lower than 1100° C., the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling and finish rolling). It becomes difficult to ensure the crystal grain size. In addition, it becomes difficult to suppress the formation of coarse bainite, and it is difficult to control the volume fraction of crystal grains having a grain size of 40.0 μm or more within the target range of the present invention. Therefore, the heating temperature in the hot rolling process is set to 1100° C. or higher and 1300° C. or lower. It is preferably 1120° C. or higher and preferably 1280° C. or lower.
　なお、本発明では、鋼スラブ（スラブ）を製造した後、一旦室温まで冷却し、その後再度加熱する従来法に加え、室温まで冷却しないで、温片のままで加熱炉に装入する、あるいは、わずかの保熱を行った後に直ちに圧延する、これらの直送圧延の省エネルギープロセスも問題なく適用できる。 In addition, in the present invention, in addition to the conventional method of manufacturing a steel slab (slab), cooling it to room temperature and then heating it again, it is charged into a heating furnace as a hot piece without cooling to room temperature, or These direct rolling energy-saving processes, such as rolling immediately after a slight heat retention, can also be applied without problems.
　粗圧延終了温度：８５０℃以上１１５０℃以下
　粗圧延終了温度が８５０℃未満である場合、後の仕上圧延中に鋼板表面温度がフェライト変態開始温度以下になり、多量のフェライトが生成し、ベイナイトの体積率が１０％未満となる。一方、粗圧延終了温度が１１５０℃を超えると、オーステナイト未再結晶温度域での圧下量が不足し、微細なオーステナイト粒が得られない。その結果、本発明で目的とする角形鋼管の鋼組織が得られず、隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、該結晶粒の長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下、および、円相当径で４０．０μｍ以上の該結晶粒が１／４ｔ深さ位置における平板部の鋼組織全体に対して体積率で２０％以下の鋼組織を得ることが困難となる。また、粗大なベイナイトの生成を抑制することが困難となる。このため、粗圧延終了温度は、８５０℃以上１１５０℃以下とする。好ましくは８６０℃以上であり、より好ましくは８７０℃以上である。好ましくは１０００℃以下であり、より好ましくは９８０℃以下である。 Rough rolling finish temperature: 850° C. or more and 1150° C. or less When the rough rolling finish temperature is lower than 850° C., the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during the subsequent finish rolling, and a large amount of ferrite is generated, and bainite is formed. The volume ratio becomes less than 10%. On the other hand, if the rough rolling finish temperature exceeds 1150° C., the rolling reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the steel structure of the square steel pipe, which is the object of the present invention, cannot be obtained. In addition, the number of crystal grains having a ratio of the major axis to the minor axis (= (major axis) / (minor axis)) of 4.0 or more is 30 / mm 2 or less, and the circle equivalent diameter is 40.0 μm or more It is difficult to obtain a steel structure in which the volume fraction of the crystal grains is 20% or less with respect to the entire steel structure of the flat plate portion at the 1/4t depth position. Moreover, it becomes difficult to suppress the generation of coarse bainite. Therefore, the finish temperature of rough rolling is set to 850° C. or higher and 1150° C. or lower. It is preferably 860° C. or higher, more preferably 870° C. or higher. It is preferably 1000° C. or lower, more preferably 980° C. or lower.
　仕上圧延終了温度：７５０℃以上８５０℃以下
　仕上圧延終了温度が７５０℃未満である場合、仕上圧延中に鋼板表面温度がフェライト変態開始温度以下になり、多量のフェライトが生成し、ベイナイトの体積率が１０％未満となる。一方、仕上圧延終了温度が８５０℃を超えると、オーステナイト未再結晶温度域での圧下量が不足し、微細なオーステナイト粒が得られない。その結果、結晶粒が粗大になり、本発明で目的とする強度の確保が困難となる。また、粗大なベイナイトの生成を抑制することが困難となる。このため、仕上圧延終了温度は、７５０℃以上８５０℃以下とする。好ましくは７７０℃以上であり、より好ましくは７８０℃以上である。好ましくは８３０℃以下であり、より好ましくは８２０℃以下である。 Finish rolling finish temperature: 750° C. or higher and 850° C. or lower When the finish rolling finish temperature is lower than 750° C., the steel sheet surface temperature becomes lower than the ferrite transformation start temperature during finish rolling, a large amount of ferrite is generated, and the volume ratio of bainite is low. is less than 10%. On the other hand, when the finishing temperature of finish rolling exceeds 850°C, the reduction amount in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the crystal grains become coarse, making it difficult to secure the strength aimed at in the present invention. Moreover, it becomes difficult to suppress the generation of coarse bainite. Therefore, the finishing temperature of finish rolling is set to 750° C. or more and 850° C. or less. It is preferably 770° C. or higher, more preferably 780° C. or higher. It is preferably 830° C. or lower, more preferably 820° C. or lower.
　９３０℃以下の合計圧下率：４０％以上６３％以下
　本発明では、熱間圧延工程においてオーステナイト中のサブグレインを微細化することで、続く冷却工程、巻取工程で生成するフェライト、ベイナイトおよび残部組織を微細化し、本発明で目的とする強度および靱性を有する角形鋼管の鋼組織を得られる。熱間圧延工程においてオーステナイト中のサブグレインを微細化するためには、オーステナイト未再結晶温度域での圧下率を高くし、十分な加工ひずみを導入する必要がある。しかしながら、合計圧下率が６３％を超えると、長径と短径の比が大きな結晶粒が生成し易く、靱性の低下を招く。そのため、本発明では、９３０℃以下仕上圧延終了温度までの合計圧下率を６３％以下とした。好ましくは６１％以下であり、より好ましくは６０％以下である。９３０℃以下仕上圧延終了温度までの合計圧下率が４０％未満となると、フェライトやベイナイトの結晶粒径が大きくなり、靱性の低下を招く。そのため、９３０℃以下仕上圧延終了温度までの合計圧下率は４０％以上とした。好ましくは４２％以上であり、より好ましくは４５％以上である。 Total rolling reduction at 930° C. or less: 40% or more and 63% or less In the present invention, subgrains in austenite are refined in the hot rolling process, so that ferrite, bainite and the remainder generated in the subsequent cooling process and coiling process The steel structure of the square steel pipe having the strength and toughness targeted by the present invention can be obtained by refining the structure. In order to refine the subgrains in the austenite in the hot rolling process, it is necessary to increase the rolling reduction in the austenite non-recrystallization temperature range and introduce sufficient working strain. However, if the total rolling reduction exceeds 63%, crystal grains with a large major axis to minor axis ratio tend to form, resulting in a decrease in toughness. Therefore, in the present invention, the total rolling reduction from 930° C. or lower to the finish rolling finish temperature is set to 63% or lower. It is preferably 61% or less, more preferably 60% or less. If the total rolling reduction is less than 40% up to the finish rolling finish temperature of 930° C. or lower, the grain size of ferrite and bainite increases, leading to a decrease in toughness. Therefore, the total rolling reduction from 930° C. or lower to the finish rolling finish temperature was set to 40% or higher. It is preferably 42% or more, more preferably 45% or more.
　なお、９３０℃以下としたのは、９３０℃超では圧延工程においてオーステナイトが再結晶し、圧延により導入された転位が消失してしまい、微細化したオーステナイトが得られないためである。 The reason why the temperature is set to 930°C or less is that if it exceeds 930°C, the austenite recrystallizes in the rolling process, dislocations introduced by rolling disappear, and refined austenite cannot be obtained.
　上記した合計圧下率とは、９３０℃以下仕上圧延終了温度までの温度域における各圧延パスの圧下率の合計をさす。 The above-mentioned total rolling reduction refers to the total rolling reduction of each rolling pass in the temperature range from 930°C to the finish rolling end temperature.
　なお、スラブを熱間圧延するに際し、上記した粗圧延および仕上圧延の両方において９３０℃以下仕上圧延終了温度までの合計圧下率を４０％以上６３％以下とする熱間圧延としても良い。あるいは、仕上圧延のみで９３０℃以下仕上圧延終了温度までの合計圧下率を４０％以上６３％以下とする熱間圧延としても良い。後者において、仕上圧延のみで９３０℃以下仕上圧延終了温度までの合計圧下率を４０％以上６３％以下とすることができない場合には、粗圧延の途中でスラブを冷却して温度を９３０℃以下とした後、粗圧延と仕上圧延の両方における９３０℃以下仕上圧延終了温度までの合計圧下率を４０％以上６３％以下とする。 In addition, when hot rolling the slab, hot rolling may be performed with a total reduction rate of 40% or more and 63% or less to the finish rolling end temperature of 930°C or less in both the rough rolling and the finish rolling described above. Alternatively, only finish rolling may be performed by hot rolling with a total rolling reduction of 40% or more and 63% or less until the finish rolling finish temperature of 930° C. or less. In the latter case, if the total rolling reduction to the finish rolling finish temperature of 930°C or less cannot be made 40% or more and 63% or less only by finish rolling, the slab is cooled during rough rolling to reduce the temperature to 930°C or less. After that, the total rolling reduction from 930° C. or lower to finish rolling finish temperature in both rough rolling and finish rolling is set to 40% or more and 63% or less.
　本発明では、仕上板厚（仕上圧延後の熱延鋼板の板厚）の上限は特に規定しないが、必要圧下率の確保や鋼板温度管理の観点より、仕上板厚は５ｍｍ超２６ｍｍ未満とすることが好ましい。 In the present invention, the upper limit of the finished plate thickness (the thickness of the hot-rolled steel plate after finish rolling) is not particularly specified, but from the viewpoint of ensuring the required rolling reduction and steel plate temperature control, the finished plate thickness is more than 5 mm and less than 26 mm. is preferred.
　熱間圧延工程後、熱延鋼板に冷却工程を施す。冷却工程では、冷却停止温度までの平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却する。 After the hot rolling process, the hot rolled steel sheet is subjected to a cooling process. In the cooling step, cooling is performed at an average cooling rate to the cooling stop temperature: 2°C/s or more and 27°C/s or less, and the cooling stop temperature: 450°C or more and 650°C or less.
　冷却開始から冷却停止（冷却終了）までの平均冷却速度：２℃／ｓ以上２７℃／ｓ以下
　熱延鋼板の板厚中心温度で、冷却開始から後述する冷却停止までの温度域における平均冷却速度が２℃／ｓ未満では、フェライトの核生成頻度が減少し、フェライト粒が粗大化するため、所望の強度を得られない。また、本発明で目的とする結晶粒径が４０．０μｍ以上の体積率の範囲に制御することが困難である。一方で、平均冷却速度が２７℃／ｓを超えると、得られる角形鋼管の鋼組織の管外面から肉厚ｔの１／４ｔ深さ位置において多量のマルテンサイトまたはベイナイトが生成し、フェライトとベイナイトの体積率の合計が７５％未満となる。平均冷却速度は、好ましくは４℃／ｓ以上であり、より好ましくは６℃／ｓ以上である。好ましくは２５℃／ｓ以下であり、より好ましくは２０℃／ｓ以下である。 Average cooling rate from cooling start to cooling stop (cooling end): 2 ° C / s or more and 27 ° C / s or less Average cooling rate in the temperature range from the start of cooling to the stop of cooling described later at the plate thickness center temperature of the hot rolled steel sheet is less than 2° C./s, the ferrite nucleation frequency decreases and the ferrite grains become coarse, making it impossible to obtain the desired strength. In addition, it is difficult to control the crystal grain size in the range of 40.0 μm or more by volume, which is the object of the present invention. On the other hand, when the average cooling rate exceeds 27° C./s, a large amount of martensite or bainite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the steel structure of the obtained square steel pipe, and ferrite and bainite are generated. is less than 75%. The average cooling rate is preferably 4°C/s or higher, more preferably 6°C/s or higher. It is preferably 25° C./s or less, more preferably 20° C./s or less.
　なお、本発明では、冷却前の鋼板表面におけるフェライト生成抑制の観点より、仕上圧延終了後直ちに冷却を開始することが好ましい。 In addition, in the present invention, from the viewpoint of suppressing ferrite formation on the surface of the steel sheet before cooling, it is preferable to start cooling immediately after finish rolling.
　冷却停止温度：４５０℃以上６５０℃以下
　熱延鋼板の板厚中心温度で、冷却停止温度が４５０℃未満では、得られる熱延鋼板表面から板厚ｔの１／４ｔ深さ位置および角形鋼管の鋼組織の管外面から肉厚ｔの１／４ｔ深さ位置において多量のマルテンサイトが生成し、フェライトとベイナイトの体積率の合計が７５％未満となる場合がある。また、フェライトの体積率が３０％以下となる場合がある。一方で、冷却停止温度が６５０℃を超えると、フェライトの核生成頻度が減少し、フェライト粒が粗大化するとともに、ベイナイト変態開始温度を上回るためベイナイトの体積率を１０％以上とすることができない。冷却停止温度は、好ましくは４６０℃以上であり、より好ましくは４７０℃以上である。好ましくは６２０℃以下であり、より好ましくは６００℃以下である。 Cooling stop temperature: 450° C. or higher and 650° C. or lower When the cooling stop temperature is less than 450° C. at the thickness center temperature of the hot-rolled steel sheet, the depth position of 1/4 t of the plate thickness t from the surface of the obtained hot-rolled steel plate and the square steel pipe A large amount of martensite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the pipe of the steel structure, and the total volume fraction of ferrite and bainite may be less than 75%. Moreover, the volume fraction of ferrite may be 30% or less. On the other hand, when the cooling stop temperature exceeds 650° C., the ferrite nucleation frequency decreases, the ferrite grains become coarse, and the bainite transformation start temperature is exceeded, so the volume fraction of bainite cannot be 10% or more. . The cooling stop temperature is preferably 460°C or higher, more preferably 470°C or higher. It is preferably 620° C. or lower, more preferably 600° C. or lower.
　なお、本発明において、平均冷却速度は、特に断らない限り、（（冷却前の熱延鋼板の板厚中心温度－冷却後の熱延鋼板の板厚中心温度）／冷却時間）で求められる値（冷却速度）とする。冷却方法は、ノズルからの水の噴射等の水冷や、冷却ガスの噴射による冷却等が挙げられるが、これらに限定しない。本発明では、熱延鋼板の両面が同条件で冷却されるように、熱延鋼板両面に冷却操作（処理）を施すことが好ましい。 In the present invention, unless otherwise specified, the average cooling rate is a value obtained by ((thickness center temperature of hot-rolled steel sheet before cooling-thickness center temperature of hot-rolled steel sheet after cooling)/cooling time). (cooling rate). Cooling methods include, but are not limited to, water cooling such as water injection from nozzles, cooling by cooling gas injection, and the like. In the present invention, both sides of the hot-rolled steel sheet are preferably cooled (treated) so that both sides of the hot-rolled steel sheet are cooled under the same conditions.
　冷却工程後に、熱延鋼板を巻取り、その後放冷する巻取工程を施す。
巻取工程では、鋼板組織の作り込みの観点より、巻取温度：４４０℃以上６５０℃以下で巻取る。
巻取温度が４４０℃未満では、多量のマルテンサイトが生成し、フェライトとベイナイトの体積率の合計が７５％未満となる場合がある。また、フェライトの体積率が３０％以下となる場合がある。巻取温度が６５０℃超えでは、フェライトの核生成頻度が減少し、フェライト粒が粗大化するとともに、ベイナイト変態開始温度を上回るためベイナイトの体積率を１０％以上とすることができない場合がある。巻取温度は、好ましくは４５０℃以上であり、より好ましくは４６０℃以上である。好ましくは６２０℃以下であり、より好ましくは５９０℃以下である。 After the cooling process, the hot-rolled steel sheet is coiled, and then subjected to a coiling process of standing to cool.
In the coiling step, the steel sheet is coiled at a coiling temperature of 440° C. or higher and 650° C. or lower from the viewpoint of building the steel sheet structure.
If the coiling temperature is less than 440°C, a large amount of martensite may be generated and the total volume fraction of ferrite and bainite may be less than 75%. Moreover, the volume fraction of ferrite may be 30% or less. If the coiling temperature exceeds 650° C., the ferrite nucleation frequency decreases, the ferrite grains become coarse, and the bainite volume fraction cannot be increased to 10% or more because the temperature exceeds the bainite transformation start temperature. The winding temperature is preferably 450°C or higher, more preferably 460°C or higher. It is preferably 620° C. or lower, more preferably 590° C. or lower.
　以上により、本発明の熱延鋼板が製造される。本発明によれば、降伏強度が３３０ＭＰa以上、引張強度が５２０ＭＰa以上、降伏比が０．７５以下、－２０℃におけるシャルピー吸収エネルギーが１８０J以上である、熱延鋼板が得られる。 Through the above steps, the hot-rolled steel sheet of the present invention is produced. According to the present invention, a hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more can be obtained.
　さらに、巻取工程後に、造管工程を施す。造管工程では、熱延鋼板をロール成形により円筒状のオープン管（丸形鋼管）とし、その突合せ部分を電縫溶接する。その後、丸形鋼管に対して上下左右に配置されたロールにより、円筒状のまま管軸方向に数％の絞りを加え、角形状に成形して角形鋼管を得る。 Furthermore, after the winding process, a pipe-making process is applied. In the pipemaking process, a hot-rolled steel plate is roll-formed into a cylindrical open pipe (round steel pipe), and the butt portions are electric resistance welded. Thereafter, the round steel pipe is drawn in the axial direction by several percent by rolls arranged vertically and horizontally with respect to the round steel pipe, and formed into a square shape to obtain a square steel pipe.
　なお、本発明における角形鋼管には、各々の辺長がすべて等しい（（長辺長さ／短辺長さ）の値が１．０）角形鋼管に限られず、（長辺長さ／短辺長さ）の値が１．０超の角形鋼管も含まれる。ただし、角形鋼管の（長辺長さ／短辺長さ）の値が２．５を超えると、長辺側で局部座屈が生じやすくなり管軸方向の圧縮強度が低下する。そのため、角形鋼管の（長辺長さ／短辺長さ）の値は、１．０以上２．５以下とするのが好ましい。（長辺長さ／短辺長さ）の値は、より好ましくは１．０以上２．０以下である。 In addition, the square steel pipe in the present invention is not limited to a square steel pipe in which each side length is equal (the value of (long side length/short side length) is 1.0), (long side length/short side length length) value of more than 1.0 is also included. However, if the value of (length of long side/length of short side) of the square steel pipe exceeds 2.5, local buckling tends to occur on the long side, and compressive strength in the direction of the pipe axis decreases. Therefore, the value of (long side length/short side length) of the square steel pipe is preferably 1.0 or more and 2.5 or less. The value of (long side length/short side length) is more preferably 1.0 or more and 2.0 or less.
　以上により、本発明の角形鋼管が製造される。本発明によれば、平板部の降伏強度が３８５ＭＰa以上、平板部の引張強度が５２０ＭＰa以上、平板部の降伏比が０．９０以下、平板部の－２０℃におけるシャルピー吸収エネルギーが１１０J以上である、角形鋼管を得られる。これにより、冷間プレス曲げ成形と比較して、生産性が高く短納期（短期間）で、高強度ロール成形角形鋼管を製造することが可能となる。このロール成形角形鋼管は、特に工場、倉庫、商業施設などの大型建築物の建築部材に好適に用いることができるため、施工コスト削減に大きく貢献することができる。また、低温靱性にも優れているため、寒冷地の建築物等、低温環境下で使用される建築物にも適用が可能である。 As described above, the square steel pipe of the present invention is manufactured. According to the present invention, the yield strength of the flat plate portion is 385 MPa or more, the tensile strength of the flat plate portion is 520 MPa or more, the yield ratio of the flat plate portion is 0.90 or less, and the Charpy absorbed energy of the flat plate portion at −20° C. is 110 J or more. , a square steel pipe can be obtained. This makes it possible to manufacture high-strength roll-formed square steel pipes with high productivity and short delivery times (short period of time) compared to cold press bending. Since this roll-formed square steel pipe can be suitably used as building members for large-scale buildings such as factories, warehouses, and commercial facilities, it can greatly contribute to the reduction of construction costs. In addition, since it is also excellent in low-temperature toughness, it can be applied to buildings used in low-temperature environments such as buildings in cold regions.
　このため、本発明は、特に厚肉の角形鋼管に好適に用いることができる。なお、ここでいう「厚肉」とは、角形鋼管の平板部の肉厚が５ｍｍ超２６ｍｍ未満であることを指す。 Therefore, the present invention can be suitably used particularly for thick square steel pipes. The term "thickness" as used herein means that the thickness of the flat plate portion of the square steel pipe is more than 5 mm and less than 26 mm.
　次に、本発明の一実施形態における角形鋼管を使用した建築構造物を説明する。 Next, a building structure using square steel pipes in one embodiment of the present invention will be described.
　図１には、上述した本発明の角形鋼管を使用した建築構造物の一例を模式的に示す。図１に示すように、本実施形態の建築構造物は、本発明の角形鋼管１が複数立設され、柱材として用いられている。隣り合う角形鋼管１の間には、Ｈ形鋼等の鋼材からなる大梁４が複数架設されている。また、隣り合う大梁４の間には、Ｈ形鋼等の鋼材からなる小梁５が複数架設されている。角形鋼管１と大梁４となるＨ形鋼は、通しダイアフラム６を介して溶接接合することによって、隣り合う角形鋼管１の間にＨ形鋼等の鋼材からなる大梁４が架設されている。また、壁等の取り付けのため、必要に応じて間柱７が設けられる。 Fig. 1 schematically shows an example of a building structure using the square steel pipe of the present invention described above. As shown in FIG. 1, in the building structure of the present embodiment, a plurality of rectangular steel pipes 1 of the present invention are erected and used as pillars. A plurality of girders 4 made of steel such as H-shaped steel are installed between the adjacent square steel pipes 1 . A plurality of small beams 5 made of steel such as H-shaped steel are installed between adjacent large beams 4 . The square steel pipes 1 and the H-section steel forming the large girders 4 are welded together via through-diaphragms 6, so that the large beams 4 made of steel such as H-section steel are constructed between the adjacent square steel pipes 1. Further, studs 7 are provided as necessary for attachment to a wall or the like.
　本発明の角形鋼管は、強度および低温靱性に優れているため、大型の建築物に使用した場合でも構造物全体の変形性能を十分に確保することができる。そのため、本発明の建築構造物は、従来の角形鋼管を使用した建築構造物と比べて、より優れた耐震性能を発揮する。また、寒冷地等の低温環境下の建築物で使用される場合にも、上記の優れた耐震性能を発揮することができる。 The square steel pipe of the present invention has excellent strength and low-temperature toughness, so even when used in large buildings, it is possible to sufficiently ensure the deformation performance of the entire structure. Therefore, the building structure of the present invention exhibits better earthquake resistance performance than building structures using conventional rectangular steel pipes. In addition, even when used in a building in a low-temperature environment such as a cold region, the above-mentioned excellent earthquake resistance performance can be exhibited.
　以下、実施例に基づいてさらに本発明を詳細に説明する。なお、本発明は以下の実施例に限定されない。 Hereinafter, the present invention will be described in further detail based on examples. In addition, the present invention is not limited to the following examples.
　表１に示す成分組成を有する溶鋼を鋳造してスラブとした。得られたスラブを表２に示す条件の熱間圧延工程、冷却工程、巻取工程を施して、角形鋼管用熱延鋼板とした。巻取工程後、以下に示す造管工程を行った。 A molten steel having the chemical composition shown in Table 1 was cast into a slab. The obtained slabs were subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2 to obtain hot rolled steel sheets for square steel pipes. After the winding process, the following pipe-making process was performed.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　得られた角形鋼管用熱延鋼板を、ロール成形により円筒状の丸形鋼管に成形し、その突合せ部分を電縫溶接した。その後、丸形鋼管の上下左右に配置したロールにより角形状に成形し、表２に示す辺長（ｍｍ）および肉厚（ｍｍ）のロール成形角形鋼管を得た。 The obtained hot-rolled steel sheets for square steel pipes were formed into cylindrical round steel pipes by roll forming, and the butt portions were electric resistance welded. After that, the round steel pipe was formed into a square shape by rolls arranged on the top, bottom, left, and right sides of the round steel pipe to obtain a roll-formed square steel pipe having a side length (mm) and a wall thickness (mm) shown in Table 2.
　得られた角形鋼管（ロール成形角形鋼管）および熱延鋼板から試験片を採取して、以下に示す組織観察、引張試験、シャルピー衝撃試験を実施した。 Test pieces were taken from the obtained square steel pipes (roll-formed square steel pipes) and hot-rolled steel sheets, and the following structural observations, tensile tests, and Charpy impact tests were performed.
　［組織観察］
　角形鋼管の組織観察用の試験片は、角形鋼管の溶接部を含む辺部の隣の辺部（溶接部を１２時方向としたときの３時の辺部）の平板部から、観察面が管軸方向断面かつ管外面から肉厚ｔの１／４ｔ深さ位置となるように採取し、研磨した後、ナイタール腐食して作製した。熱延鋼板の組織観察用の試験片は、熱延鋼板の幅方向中央部かつ板厚ｔの１／４ｔ深さ位置から採取した。観察面が熱間圧延時の圧延方向断面となるようにし、研磨した後、ナイタール腐食して作製した。 [Tissue observation]
The test piece for observing the structure of the square steel pipe was measured from the flat plate portion next to the side including the welded portion of the square steel pipe (the 3 o'clock side when the welded portion is in the 12 o'clock direction). The sample was taken from the tube axial direction section and the depth position of 1/4t of the wall thickness t from the tube outer surface, polished, and then nital corroded. A test piece for observing the structure of the hot-rolled steel sheet was taken from the central portion in the width direction of the hot-rolled steel sheet and at a depth of 1/4t of the sheet thickness t. The observation surface was made to be a cross section in the rolling direction at the time of hot rolling, and after polishing, it was produced by nital corrosion.
　組織観察は、光学顕微鏡（倍率：１０００倍）または走査型電子顕微鏡（ＳＥＭ、倍率：１０００倍）を用いて、角形鋼管の平板部の管外面および熱延鋼板の表面から厚さｔの１／４ｔ深さ位置における組織を観察し、撮像した。得られた光学顕微鏡像およびＳＥＭ像から、フェライト、パーライト、ベイナイトおよび残部組織の面積率を求めた。 Microstructural observation is performed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) from the outer surface of the flat plate portion of the square steel pipe and the surface of the hot rolled steel plate to 1/ of the thickness t. The tissue at the 4t depth position was observed and imaged. The area ratios of ferrite, pearlite, bainite and residual structures were obtained from the obtained optical microscope images and SEM images.
　各組織の面積率は、代表の１つの平板部から採取した試験片を用いて５視野以上で観察を行い、各視野で得られた値の平均値として算出した。ここでは、組織観察により得られた面積率を、各組織の体積率とした。 　The area ratio of each tissue was calculated as the average value of the values obtained in each field of view after observing five or more fields of view using a test piece taken from one representative flat plate. Here, the area ratio obtained by tissue observation was used as the volume ratio of each tissue.
　ここで、フェライトは拡散変態による生成物のことであり、転位密度が低くほぼ回復した組織を呈する。ポリゴナルフェライトおよび擬ポリゴナルフェライトがこれに含まれる。パーライトは、セメンタイトとフェライトが層状に並んだ組織である。また、ベイナイトは転位密度が高いラス状のフェライトとセメンタイトの複相組織である。 Here, ferrite is a product of diffusion transformation, and has a low dislocation density and exhibits a nearly recovered structure. This includes polygonal ferrite and quasi-polygonal ferrite. Pearlite is a structure in which cementite and ferrite are arranged in layers. Also, bainite is a lath-like multi-phase structure of ferrite and cementite with a high dislocation density.
　なお、光学顕微鏡像およびＳＥＭ像ではマルテンサイトとオーステナイトの識別が難しい。このため、得られたＳＥＭ像からマルテンサイトあるいはオーステナイトとして観察された組織の面積率を測定し、それから後述する方法で測定したオーステナイトの体積率を差し引いた値をマルテンサイトの体積率とした。 It should be noted that it is difficult to distinguish between martensite and austenite in optical microscope images and SEM images. Therefore, the area ratio of the structure observed as martensite or austenite was measured from the obtained SEM image, and the value obtained by subtracting the volume ratio of austenite measured by the method described later was taken as the volume ratio of martensite.
　オーステナイトの体積率の測定は、Ｘ線回折により行った。組織観察用の試験片は、回折面が鋼管平板部の管外面および熱延鋼板の表面から厚さｔの１／４ｔ深さ位置となるように研削した後、化学研磨をして表面加工層を除去して作製した。測定にはＭｏのＫα線を使用し、ｆｃｃ鉄の（２００）、（２２０）、（３１１）面とｂｃｃ鉄の（２００）、（２１１）面の積分強度からオーステナイトの体積率を求めた。 The volume fraction of austenite was measured by X-ray diffraction. The test piece for structural observation was ground so that the diffractive surface was at a depth of 1/4 t of the thickness t from the outer surface of the flat plate portion of the steel pipe and the surface of the hot-rolled steel plate, and then chemically polished to form a surface processed layer. was prepared by removing The Kα ray of Mo was used for the measurement, and the volume fraction of austenite was obtained from the integrated intensities of the (200), (220) and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.
　また、平均円相当径（平均結晶粒径）および円相当径（結晶粒径）が４０．０μｍ以上の結晶粒の体積率は、ＳＥＭ／ＥＢＳＤ法を用いて測定した。測定領域は５００μｍ×１０００μｍ、測定ステップサイズは０．５μｍとした。結晶粒径は、隣接する結晶粒の間の方位差を求め、方位差が１５°以上の境界を結晶粒界として測定した。得られた結晶粒界から粒径の算術平均を求めて、平均結晶粒径とした。また、結晶粒の長径と短径はＪＩＳ　Ｒ　１６７０（２００６）に記載の方法で測定し、長径と短径の比（＝（長径）／（短径））を算出した。長径が５０μｍ以上の結晶粒について、長径と短径の比が４．０以上の結晶粒の数を測定し、測定領域の面積（０．５ｍｍ2）で除することで、長径と短径の比が４．０以上の結晶粒の個数（個／ｍｍ2）を算出した。なお、結晶粒径解析および結晶粒個数の測定においては、結晶粒径が２．０μｍ以下のものは測定ノイズとして解析対象から除外し、結晶粒径解析では得られた面積率が体積率と等しいとした。 Also, the average equivalent circle diameter (average crystal grain size) and the volume fraction of crystal grains having an equivalent circle diameter (crystal grain size) of 40.0 μm or more were measured using the SEM/EBSD method. The measurement area was 500 μm×1000 μm, and the measurement step size was 0.5 μm. The crystal grain size was obtained by determining the orientation difference between adjacent crystal grains, and measuring the boundary with the orientation difference of 15° or more as the crystal grain boundary. The average grain size was obtained by calculating the arithmetic mean of grain sizes from the obtained grain boundaries. Also, the major axis and minor axis of the crystal grain were measured by the method described in JIS R 1670 (2006), and the ratio of major axis to minor axis (=(major axis)/(minor axis)) was calculated. For crystal grains with a major axis of 50 μm or more, the number of crystal grains with a ratio of major axis to minor axis of 4.0 or more is measured, and divided by the area of the measurement area (0.5 mm 2 ). The number (particles/mm 2 ) of crystal grains with a ratio of 4.0 or more was calculated. In the crystal grain size analysis and the measurement of the number of crystal grains, crystal grain sizes of 2.0 μm or less are excluded from the analysis as measurement noise, and the area ratio obtained in the crystal grain size analysis is equal to the volume ratio. and
　［引張試験］
　図２は、角形鋼管の平板部の引張試験片の採取位置を示す概略図である。
　引張試験は、図２に示すように、引張方向が管軸方向と平行になるように、角形鋼管の平板部からＪＩＳ５号引張試験片を採取した。熱延鋼板の引張試験においては、引張方向が圧延方向と平行になるように、ＪＩＳ５号引張試験片を採取した。採取した引張試験片について、ＪＩＳ　Ｚ　２２４１の規定に準拠して引張試験を実施し、降伏強度ＹＳ、引張強度ＴＳを測定し、（降伏強度）／（引張強度）で定義される降伏比を算出した。なお、角形鋼管の平板部の引張試験片は、角形鋼管の溶接部を１２時方向としたときの３時の辺部における、平板部の幅中央部の位置（図２を参照）から採取した。なお、試験片本数は各２本とし、それらの平均値を算出してＹＳ、ＴＳ、降伏比を求めた。 [Tensile test]
FIG. 2 is a schematic diagram showing the sampling positions of the tensile test pieces of the flat plate portion of the square steel pipe.
For the tensile test, as shown in FIG. 2, a JIS No. 5 tensile test piece was taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axial direction. In the tensile test of the hot-rolled steel sheet, a JIS No. 5 tensile test piece was taken so that the tensile direction was parallel to the rolling direction. A tensile test was performed on the collected tensile test pieces in accordance with the provisions of JIS Z 2241, the yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength) / (tensile strength) was calculated. bottom. The tensile test piece of the flat plate portion of the square steel pipe was taken from the position of the width center of the flat plate portion (see FIG. 2) on the side of 3 o'clock when the welded portion of the square steel pipe is in the 12 o'clock direction. . In addition, the number of test pieces was set to two for each, and the average values thereof were calculated to obtain YS, TS, and yield ratio.
　［シャルピー衝撃試験］
　図３は、角形鋼管のシャルピー試験片の採取位置を示す概略図である。
　角形鋼管のシャルピー衝撃試験は、図３に示すように、角形鋼管の管外面から肉厚ｔの１／４ｔ深さ位置において、試験片長手方向が管軸方向と平行となるように採取した、ＪＩＳ　Ｚ　２２４２の規定に準拠したＶノッチ標準試験片を用いた。熱延鋼板のシャルピー衝撃試験は、得られた熱延鋼板の板厚１／４ｔ深さ位置から、試験片長手方向が圧延方向と平行となるように採取した、ＪＩＳ　Ｚ　２２４２の規定に準拠したＶノッチ標準試験片を用いた。
　ＪＩＳ　Ｚ　２２４２の規定に準拠して、試験温度：－２０℃でシャルピー衝撃試験を実施し、吸収エネルギー（Ｊ）を求めた。なお、試験片本数は各３本とし、それらの平均値を算出して吸収エネルギー（Ｊ）を求めた。 [Charpy impact test]
FIG. 3 is a schematic diagram showing the sampling positions of Charpy test pieces of square steel pipes.
In the Charpy impact test of the square steel pipe, as shown in FIG. 3, the longitudinal direction of the test piece was taken parallel to the pipe axis direction at a depth of 1/4t of the wall thickness t from the outer surface of the square steel pipe. A V-notch standard test piece conforming to JIS Z 2242 was used. In the Charpy impact test of the hot-rolled steel sheet, the test piece was sampled from a depth of 1/4 t in the thickness of the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was parallel to the rolling direction. A V-notch standard specimen was used.
A Charpy impact test was carried out at a test temperature of -20°C in accordance with JIS Z 2242 to determine absorbed energy (J). Incidentally, the number of test pieces was three, and the average value was calculated to obtain the absorbed energy (J).
　得られた角形鋼管についての結果を表３－１および表３－２に、熱延鋼板についての結果を表４－１および表４－２に示す。 Tables 3-1 and 3-2 show the results for the obtained square steel pipes, and Tables 4-1 and 4-2 show the results for the hot-rolled steel sheets.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
　表１中の鋼Ｎｏ．、表２及び表４中の鋼板Ｎｏ．および表３中の鋼管Ｎｏ．はそれぞれ対応しており、同一のＮｏ．の鋼から熱延鋼板が製造され、その熱延鋼板から角形鋼管が製造されている。 Steel No. in Table 1 , steel plate Nos. in Tables 2 and 4; and steel pipe No. in Table 3. correspond to each other, and the same No. A hot-rolled steel plate is manufactured from the steel of 1998, and a square steel pipe is manufactured from the hot-rolled steel plate.
　表３中、鋼管Ｎｏ．１～２２は本発明例であり、鋼管Ｎｏ．２３～４６は比較例である。 In Table 3, steel pipe No. 1 to 22 are examples of the present invention. 23 to 46 are comparative examples.
　本発明例の角形鋼管は、いずれも鋼組織が体積率で３０％超のフェライト、１０％以上のベイナイトを含み、フェライトとベイナイトの体積率の合計が７５％以上９５％以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、かつ方位差１５°以上の境界によって囲まれる領域を結晶粒としたとき、円相当径が４０．０μｍ以上の結晶粒の体積率が２０％以下であり、かつ長径が５０μｍ以上の結晶粒について、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であった。さらに、平板部における降伏強度が３８５ＭＰa以上、平板部における引張強度が５２０ＭＰa以上、平板部における降伏比が０．９０以下、平板部の－２０℃におけるシャルピー吸収エネルギーが１１０J以上であった。 In the square steel pipes of the present invention examples, the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is A crystal grain having an equivalent circle diameter of 40.0 μm or more when a region surrounded by a boundary with a misorientation of 15° or more is made of one or more selected from pearlite, martensite, and austenite. For crystal grains having a volume fraction of 20% or less and a major axis of 50 μm or more, the number of crystal grains having a ratio of major axis to minor axis (=(major axis)/(minor axis)) of 4.0 or more is 30/ mm2 or less. Further, the flat plate portion had a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.90 or less, and a Charpy absorbed energy of -20°C of the flat plate portion of 110 J or more.
　比較例の鋼管Ｎｏ．２３、２４は、１．２０×Ｎｂ≦Ｔｉの範囲外となっていたため、長径が５０μｍ以上の結晶粒について、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２超となり、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel pipe No. 23 and 24 were out of the range of 1.20 × Nb ≤ Ti, so the ratio of the major axis to the minor axis (= (long axis) / (short axis)) was 4.0 for crystal grains with a major axis of 50 μm or more. The number of the above crystal grains exceeded 30/mm 2 , and the Charpy absorbed energy at −20° C. of the flat portion did not reach the desired value.
　比較例の鋼管Ｎｏ．２５は、Ｃの含有量が本発明の範囲を上回っていたため、平板部の降伏比が本発明の範囲外となった。 Comparative steel pipe No. In No. 25, the C content exceeded the range of the present invention, so the yield ratio of the flat plate portion was outside the range of the present invention.
　比較例の鋼管Ｎｏ．２６は、Ｓｉの含有量が本発明の範囲を上回ったため、組織の微細化を伴わずに、固溶強化により降伏強度が過度に上昇した。その結果、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 26, since the Si content exceeded the range of the present invention, the yield strength was excessively increased due to solid solution strengthening without refinement of the structure. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．２７は、Ｍｎの含有量が本発明の範囲を上回ったため、固溶強化により降伏強度が過度に上昇した。その結果、平板部の降伏比が所望の値に達しなかった。 Comparative steel pipe No. In No. 27, the Mn content exceeded the range of the present invention, so the yield strength increased excessively due to solid-solution strengthening. As a result, the yield ratio of the flat plate did not reach the desired value.
　比較例の鋼管Ｎｏ．２８は、Ｐの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 28, the P content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．２９は、Ｓの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 29, the S content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３０は、Ａｌの含有量が本発明の範囲を上回ったため、アルミナ系介在物が多くなったと考えられる。その結果、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 30, the content of Al exceeded the range of the present invention, so it is considered that the amount of alumina-based inclusions increased. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３１は、Ｎｂの含有量が本発明の範囲を上回り、Ｔｉ／Ｎｂが本発明の範囲外となった。その結果、長径と短径の比が４．０以上の結晶粒の個数が本発明の範囲外となり、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 31, the Nb content exceeded the range of the present invention, and the Ti/Nb ratio was outside the range of the present invention. As a result, the number of crystal grains having a major axis to minor axis ratio of 4.0 or more fell outside the scope of the present invention, and the Charpy absorbed energy at -20°C of the flat plate part did not reach the desired value.
　比較例の鋼管Ｎｏ．３２は、Ｔｉの含有量が本発明の範囲を上回ったため、粗大な炭化物や窒化物が形成されたと考えられる。その結果、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 32, the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３３は、Ｔｉ／Ｎｂが本発明の範囲外となった。その結果、長径と短径の比が４．０以上の結晶粒の個数が本発明の範囲外となり、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 33, Ti/Nb is outside the scope of the present invention. As a result, the number of crystal grains having a major axis to minor axis ratio of 4.0 or more fell outside the scope of the present invention, and the Charpy absorbed energy at -20°C of the flat plate part did not reach the desired value.
　比較例の鋼管Ｎｏ．３４は、Ｖの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 34, the V content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３５は、Ｃｒの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 35, the Cr content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３６は、Ｍｏの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 36, the Mo content exceeded the range of the present invention, so the Charpy absorbed energy at −20° C. of the flat portion did not reach the desired value.
　比較例の鋼管Ｎｏ．３７は、Ｃｕの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 37, the Cu content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３８は、Ｎｉの含有量が本発明の範囲を上回ったため、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 38, the Ni content exceeded the range of the present invention, so the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．３９は、Ｃａの含有量が本発明の範囲を上回ったため、Ｃａ酸化物クラスターが形成されたと考えられる。その結果、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．４０は、Ｂの含有量が本発明の範囲を上回ったため、平板部の降伏比が本発明の範囲外となった。 Comparative steel pipe No. In No. 40, the content of B exceeded the range of the present invention, so the yield ratio of the flat portion was outside the range of the present invention.
　比較例の鋼管Ｎｏ．４１は、スラブ加熱温度が本発明の範囲を上回っており、結晶粒が粗大化し、結晶粒径４０．０μｍ以上の結晶粒の体積率が、本発明の範囲外となった。その結果、平板部の引張強度および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 μm or more was outside the range of the present invention. As a result, the tensile strength of the flat plate portion and the Charpy absorbed energy at -20°C did not reach the desired values.
　比較例の鋼管Ｎｏ．４２は、仕上圧延終了温度が本発明の範囲を上回ったため、９３０℃以下での合計圧下率が本発明の範囲を下回り、粗大なベイナイトの生成を抑制できず、結晶粒径４０．０μｍ以上の結晶粒の体積率が本発明の範囲外となった。その結果、平板部の降伏強度、引張強度および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 42, the finish rolling finishing temperature exceeded the range of the present invention, so the total rolling reduction at 930 ° C. or less was below the range of the present invention, and the formation of coarse bainite could not be suppressed, and the grain size was 40.0 µm or more. The volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C of the flat plate part did not reach the desired values.
　比較例の鋼管Ｎｏ．４３は、９３０℃以下での合計圧下率が本発明の範囲を上回っており、粗大なベイナイトの生成を抑制できず、結晶粒径４０．０μｍ以上の結晶粒の体積率が本発明の範囲外となった。その結果、平板部の－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 43, the total reduction ratio at 930 ° C. or lower exceeds the range of the present invention, the formation of coarse bainite cannot be suppressed, and the volume ratio of crystal grains with a grain size of 40.0 μm or more is outside the range of the present invention. became. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
　比較例の鋼管Ｎｏ．４４は、平均冷却速度が本発明の範囲を下回ったため、ベイナイトの体積率が１０％未満となり、本発明の範囲外となった。その結果、平板部の降伏強度、引張強度および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative steel pipe No. In No. 44, since the average cooling rate was below the range of the present invention, the volume fraction of bainite was less than 10%, and fell outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C of the flat plate part did not reach the desired values.
　比較例の鋼管Ｎｏ．４５は、冷却停止温度が本発明の範囲を上回ったため、ベイナイトの体積率が本発明の範囲外となった。その結果、平板部の降伏強度および引張強度が所望の値に達しなかった。 Comparative steel pipe No. In No. 45, the cooling stop temperature exceeded the range of the present invention, so the volume ratio of bainite was outside the range of the present invention. As a result, the yield strength and tensile strength of the flat plate did not reach the desired values.
　比較例の鋼管Ｎｏ．４６は、冷却停止温度および巻取温度が本発明の範囲を上回ったため、ベイナイトの体積率が本発明の範囲外となった。その結果、平板部の降伏強度および引張強度が所望の値に達しなかった。
　また、図４は－２０℃におけるシャルピー吸収エネルギーと長径が５０μｍ以上の結晶粒における長径／短径の比が４．０以上の結晶粒の個数の関係を示すグラフである。本発明に従う、長径／短径の比が４．０以上の結晶粒の個数が３０個／ｍｍ2以下の範囲では、－２０℃におけるシャルピー吸収エネルギーが１１０Ｊ以上であり、優れた低温靱性を示す。一方、本発明の範囲を外れる場合には、－２０℃におけるシャルピー吸収エネルギーが１１０Ｊ未満であった。 Comparative example steel pipe No. In No. 46, the cooling stop temperature and the coiling temperature exceeded the range of the present invention, so the volume fraction of bainite was outside the range of the present invention. As a result, the yield strength and tensile strength of the flat plate did not reach the desired values.
FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at −20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 μm or more. According to the present invention, when the number of crystal grains having a major axis/minor axis ratio of 4.0 or more is 30/mm 2 or less, the Charpy absorbed energy at −20° C. is 110 J or more, and excellent low temperature toughness is exhibited. . On the other hand, in cases outside the scope of the present invention, the Charpy absorbed energy at -20°C was less than 110J.
　表４中、鋼板Ｎｏ．１～２２は本発明例であり、鋼板Ｎｏ．２３～４６は比較例である。 In Table 4, steel plate No. Steel sheets Nos. 1 to 22 are examples of the present invention. 23 to 46 are comparative examples.
　本発明例の熱延鋼板は、いずれも鋼組織が体積率で３０％超のフェライト、１０％以上のベイナイトを含み、フェライトとベイナイトの体積率の合計が７５％以上９５％以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、かつ方位差１５°以上の境界によって囲まれる領域を結晶粒としたとき、円相当径が４０．０μｍ以上の結晶粒の体積率が２０％以下であり、かつ長径が５０μｍ以上の結晶粒について、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であった。さらに、これらの機械的特性は、降伏強度が３３０ＭＰa以上、引張強度が５２０ＭＰa以上、降伏比が０．７５以下、－２０℃におけるシャルピー吸収エネルギーが１８０J以上であった。 In the hot-rolled steel sheets of the present invention examples, the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is is composed of one or more selected from pearlite, martensite, and austenite, and a crystal grain having an equivalent circle diameter of 40.0 μm or more when a region surrounded by a boundary with a misorientation of 15° or more is defined as a crystal grain. volume ratio of 20% or less and the major axis is 50 μm or more, the number of crystal grains with a ratio of major axis to minor axis (= (long axis) / (short axis)) of 4.0 or more is 30 / mm 2 or less. Furthermore, these mechanical properties were such that the yield strength was 330 MPa or more, the tensile strength was 520 MPa or more, the yield ratio was 0.75 or less, and the Charpy absorbed energy at -20°C was 180 J or more.
　比較例の鋼板Ｎｏ．２３、２４は、１．２０×Ｎｂ≦Ｔｉの範囲外となっていたため、降伏比が本発明の範囲外となった。長径が５０μｍ以上の結晶粒について、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２超となり、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Steel plate No. of the comparative example. Nos. 23 and 24 were out of the range of 1.20×Nb≦Ti, so the yield ratio was out of the range of the present invention. For crystal grains with a major axis of 50 μm or more, the number of crystal grains with a ratio of major axis to minor axis (= (major axis) / (minor axis)) of 4.0 or more is more than 30 / mm 2 , Charpy at -20 ° C. The absorbed energy did not reach the desired value.
　比較例の鋼板Ｎｏ．２５は、Ｃの含有量が本発明の範囲を上回っていたため、降伏比および－２０℃におけるシャルピー吸収エネルギーが本発明の範囲外となった。 Comparative example steel plate No. In No. 25, the C content exceeded the scope of the present invention, so the yield ratio and the Charpy absorbed energy at -20°C were outside the scope of the present invention.
　比較例の鋼板Ｎｏ．２６は、Ｓｉの含有量が本発明の範囲を上回ったため、組織の微細化を伴わずに、固溶強化により降伏強度が過度に上昇した。その結果、降伏比および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 26, since the Si content exceeded the range of the present invention, the yield strength was excessively increased due to solid solution strengthening without refinement of the structure. As a result, the yield ratio and the Charpy absorbed energy at -20°C did not reach the desired values.
　比較例の鋼板Ｎｏ．２７は、Ｍｎの含有量が本発明の範囲を上回ったため、固溶強化により降伏強度が過度に上昇した。その結果、降伏比が所望の値に達しなかった。 Comparative example steel plate No. In No. 27, the Mn content exceeded the range of the present invention, so the yield strength increased excessively due to solid-solution strengthening. As a result, the yield ratio did not reach the desired value.
　比較例の鋼板Ｎｏ．２８は、Ｐの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 28, the P content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．２９は、Ｓの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 29, the S content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３０は、Ａｌの含有量が本発明の範囲を上回ったため、アルミナ系介在物が多くなったと考えられる。その結果、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 30, the content of Al exceeded the range of the present invention, so it is considered that the amount of alumina-based inclusions increased. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３１は、Ｎｂの含有量が本発明の範囲を上回り、Ｔｉ／Ｎｂが本発明の範囲外となった。その結果、長径と短径の比が４．０以上の結晶粒の個数が本発明の範囲外となり、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 31, the Nb content exceeded the range of the present invention, and the Ti/Nb ratio was outside the range of the present invention. As a result, the number of crystal grains having a major axis to minor axis ratio of 4.0 or more was outside the scope of the present invention, and the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３２は、Ｔｉの含有量が本発明の範囲を上回ったため、粗大な炭化物や窒化物が形成されたと考えられる。その結果、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 32, the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３３は、Ｔｉ／Ｎｂが本発明の範囲外となった。その結果、長径と短径の比が４．０以上の結晶粒の個数が本発明の範囲外となり、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 33, Ti/Nb is outside the scope of the present invention. As a result, the number of crystal grains having a major axis to minor axis ratio of 4.0 or more was outside the scope of the present invention, and the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３４は、Ｖの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 34, the Charpy absorbed energy at -20°C did not reach the desired value because the V content exceeded the range of the present invention.
　比較例の鋼板Ｎｏ．３５は、Ｃｒの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 35, the Cr content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３６は、Ｍｏの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 36, the Mo content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３７は、Ｃｕの含有量が本発明の範囲を上回ったため、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 37, the Cu content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．３８は、Ｎｉの含有量が本発明の範囲を上回ったため、降伏比および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 38, the Ni content exceeded the range of the present invention, so the yield ratio and the Charpy absorbed energy at -20°C did not reach the desired values.
　比較例の鋼板Ｎｏ．３９は、Ｃａの含有量が本発明の範囲を上回ったため、Ｃａ酸化物クラスターが形成されたと考えられる。その結果、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．４０は、Ｂの含有量が本発明の範囲を上回ったため、フェライトの体積率が本発明の範囲外となった。
その結果、降伏比が本発明の範囲外となった。 Steel plate No. of the comparative example. In No. 40, the content of B exceeded the range of the present invention, so the volume fraction of ferrite was outside the range of the present invention.
As a result, the yield ratio fell outside the scope of the present invention.
　比較例の鋼板Ｎｏ．４１は、スラブ加熱温度が本発明の範囲を上回っており、結晶粒が粗大化し、結晶粒径４０．０μｍ以上の結晶粒の体積率が、本発明の範囲外となった。その結果、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 μm or more was outside the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．４２は、仕上圧延終了温度が本発明の範囲を上回ったため、９３０℃以下での合計圧下率が本発明の範囲を下回り、粗大なベイナイトの生成を抑制できず、結晶粒径４０．０μｍ以上の結晶粒の体積率が本発明の範囲外となった。その結果、降伏強度、引張強度および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 42, the finish rolling finishing temperature exceeded the range of the present invention, so the total rolling reduction at 930 ° C. or less was below the range of the present invention, and the formation of coarse bainite could not be suppressed, and the grain size was 40.0 µm or more. The volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
　比較例の鋼板Ｎｏ．４３は、９３０℃以下での合計圧下率が本発明の範囲を上回っており、加工組織の影響を受けて粗大なベイナイトが生成したため、結晶粒径４０．０μｍ以上の結晶粒の体積率が本発明の範囲外となった。その結果、－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 43, the total rolling reduction at 930 ° C. or lower exceeded the range of the present invention, and coarse bainite was generated due to the influence of the worked structure, so the volume ratio of crystal grains with a grain size of 40.0 µm or more was reduced. outside the scope of the invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
　比較例の鋼板Ｎｏ．４４は、平均冷却速度が本発明の範囲を下回ったため、ベイナイトの体積率が１０％未満となり、本発明の範囲外となった。その結果、降伏強度、引張強度および－２０℃におけるシャルピー吸収エネルギーが所望の値に達しなかった。 Comparative example steel plate No. In No. 44, since the average cooling rate was below the range of the present invention, the volume fraction of bainite was less than 10%, and fell outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
　比較例の鋼板Ｎｏ．４５は、冷却停止温度が本発明の範囲を上回ったため、ベイナイトの体積率が本発明の範囲外となった。その結果、降伏強度および引張強度が所望の値に達しなかった。 Comparative example steel plate No. In No. 45, the cooling stop temperature exceeded the range of the present invention, so the volume ratio of bainite was outside the range of the present invention. As a result, the yield strength and tensile strength did not reach the desired values.
　比較例の鋼板Ｎｏ．４６は、冷却停止温度および巻取温度が本発明の範囲を上回ったため、ベイナイトの体積率が本発明の範囲外となった。その結果、降伏強度および引張強度が所望の値に達しなかった。
　また、図５は－２０℃におけるシャルピー吸収エネルギーと長径が５０μｍ以上の結晶粒における長径／短径の比が４．０以上の結晶粒の個数の関係を示すグラフである。本発明に従う、長径／短径の比が４．０以上の結晶粒の個数が３０個／ｍｍ2以下の範囲では、－２０℃におけるシャルピー吸収エネルギーが１８０Ｊ以上であり、優れた低温靱性を示す。一方、本発明の範囲を外れる場合には、－２０℃におけるシャルピー吸収エネルギーが１８０Ｊ未満であった。 Steel plate No. of the comparative example. In No. 46, the cooling stop temperature and the coiling temperature exceeded the range of the present invention, so the volume fraction of bainite was outside the range of the present invention. As a result, the yield strength and tensile strength did not reach the desired values.
FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at −20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 μm or more. According to the present invention, when the number of crystal grains having a major axis/minor axis ratio of 4.0 or more is 30/mm 2 or less, the Charpy absorbed energy at −20° C. is 180 J or more, and excellent low temperature toughness is exhibited. . On the other hand, in cases outside the scope of the present invention, the Charpy absorbed energy at -20°C was less than 180J.
　１　　　角形鋼管
　４　　　大梁
　５　　　小梁
　６　　　ダイアフラム
　７　　　間柱 1 square steel pipe 4 large beam 5 small beam 6 diaphragm 7 stud

Claims (11)

  1. 　平板部と角部を有する角形鋼管であって、
    　平板部の成分組成が、質量％で、
    Ｃ　：０．０４％以上０．４５％以下、
    Ｓｉ：１．８％以下、
    Ｍｎ：０．５％以上２．５％以下、
    Ｐ　：０．１０％以下、
    Ｓ　：０．０５％以下、
    Ａｌ：０．００５％以上０．１００％以下、
    Ｎ　：０．０１０％以下、
    Ｎｂ：０．００５％以上０．０５０％以下、
    Ｔｉ：０．０１２％以上０．１００％以下、
    を含み、残部がＦｅおよび不可避的不純物からなり、
    ＮｂとＴｉの含有量が下記（１）式を満足し、
    　前記平板部の肉厚をｔとしたとき、管外面から肉厚ｔの１／４ｔ深さ位置における平板部の鋼組織は、
    体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、
    該フェライトおよび該ベイナイトの合計が、７５％以上９５％以下であり、
    残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、
    隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であり、
    円相当径で４０．０μｍ以上の結晶粒が体積率で２０％以下である、角形鋼管。
    １．２０×％Ｎｂ≦％Ｔｉ　　　　　　・・・（１）
    ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。     A square steel pipe having a flat plate portion and a corner portion,
    The component composition of the flat plate portion is % by mass,
    C: 0.04% or more and 0.45% or less,
    Si: 1.8% or less,
    Mn: 0.5% or more and 2.5% or less,
    P: 0.10% or less,
    S: 0.05% or less,
    Al: 0.005% or more and 0.100% or less,
    N: 0.010% or less,
    Nb: 0.005% or more and 0.050% or less,
    Ti: 0.012% or more and 0.100% or less,
    with the remainder consisting of Fe and unavoidable impurities,
    The content of Nb and Ti satisfies the following formula (1),
    When the thickness of the flat plate portion is t, the steel structure of the flat plate portion at a depth of 1/4t of the wall thickness t from the outer surface of the pipe is
    The volume fraction is more than 30% ferrite and 10% or more bainite,
    The total of the ferrite and the bainite is 75% or more and 95% or less,
    The balance consists of one or more selected from pearlite, martensite, and austenite,
    When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis)) is 4. The number of crystal grains of .0 or more is 30 / mm 2 or less,
    A square steel pipe in which crystal grains having an equivalent circle diameter of 40.0 μm or more account for 20% or less by volume.
    1.20×%Nb≦%Ti (1)
    Here, %Nb and %Ti are contents (% by mass) of each element.
  2. 　平板部の降伏強度が３８５ＭＰa以上、かつ、平板部の引張強度が５２０ＭＰａ以上、平板部の降伏比が０．９０以下、平板部の－２０℃におけるシャルピー吸収エネルギーが１１０J以上である、請求項１に記載の角形鋼管。 Claim 1, wherein the flat plate portion has a yield strength of 385 MPa or more, a tensile strength of the flat plate portion of 520 MPa or more, a yield ratio of the flat plate portion of 0.90 or less, and a Charpy absorbed energy of the flat plate portion of -20°C of 110 J or more. The square steel pipe described in .
  3. 　平板部は、前記成分組成に加えてさらに、質量％で、下記のうちから選ばれた１種または２種以上を含有する、請求項１または請求項２に記載の角形鋼管。
    Ｖ：０．０１％以上０．１５％以下、
    Ｃｒ：０．０１％以上１．０％以下、
    Ｍｏ：０．０１％以上１．０％以下、
    Ｎｉ：０．０１％以上０．３％以下、
    Ｃａ：０．０００５％以上０．０１０％以下、
    Ｂ：０．０００３％以上０．０１０％以下、
    Ｃｕ：０．０１％以上０．５％以下     3. The square steel pipe according to claim 1, wherein the flat plate portion further contains, in % by mass, one or more selected from the following in addition to the chemical composition.
    V: 0.01% or more and 0.15% or less,
    Cr: 0.01% or more and 1.0% or less,
    Mo: 0.01% or more and 1.0% or less,
    Ni: 0.01% or more and 0.3% or less,
    Ca: 0.0005% or more and 0.010% or less,
    B: 0.0003% or more and 0.010% or less,
    Cu: 0.01% or more and 0.5% or less
  4. 　前記鋼組織は、体積率で、ベイナイトが１０％以上４０％未満である、請求項１～３のいずれかに記載の角形鋼管。 The square steel pipe according to any one of claims 1 to 3, wherein the steel structure has a volume fraction of bainite of 10% or more and less than 40%.
  5. 　請求項１または請求項３に記載の成分組成を有する鋼素材を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下、仕上圧延終了温度：７５０℃以上８５０℃以下、かつ９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延を施し、次いで、板厚中心温度で平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却を施し、
    　次いで、４４０℃以上６５０℃以下で巻取り熱延鋼板とし、
    　次いで、冷間ロール成形により、前記熱延鋼板を円筒状に成形し、突き合わせ部を電縫溶接した後、角形状に成形して角形の鋼管とする造管工程を施す、角形鋼管の製造方法。     After heating the steel material having the chemical composition according to claim 1 or claim 3 to a heating temperature of 1100° C. or higher and 1300° C. or lower, the rough rolling finish temperature is 850° C. or higher and 1150° C. or lower, and the finish rolling finish temperature is 750. Total rolling reduction at ℃ to 850 ℃ and 930 ℃ or less: hot rolling with 40% to 63%, then average cooling rate at plate thickness center temperature: 2 ℃ / s or more and 27 ℃ / s Below, cooling stop temperature: cooling at 450 ° C. or higher and 650 ° C. or lower,
    Next, the hot-rolled steel sheet is coiled at 440° C. or higher and 650° C. or lower,
    Then, the hot-rolled steel plate is formed into a cylindrical shape by cold roll forming, and the butt joints are electric resistance welded, and then formed into a square shape to obtain a square steel pipe. .
  6. 　請求項１～４のいずれかに記載の角形鋼管が、柱材として使用されている、建築構造物。 A building structure in which the square steel pipe according to any one of claims 1 to 4 is used as a pillar material.
  7. 　成分組成は、質量％で、
    Ｃ　：０．０４％以上０．４５％以下、
    Ｓｉ：１．８％以下、
    Ｍｎ：０．５％以上２．５％以下、
    Ｐ　：０．１０％以下、
    Ｓ　：０．０５％以下、
    Ａｌ：０．００５％以上０．１００％以下、
    Ｎ　：０．０１０％以下、
    Ｎｂ：０．００５％以上０．０５０％以下、
    Ｔｉ：０．０１２％以上０．１００％以下、
    を含み、残部がＦｅおよび不可避的不純物からなり、
    ＮｂとＴｉの含有量が下記（１）式を満足し、
    　鋼板表面から板厚ｔの１／４ｔ位置における鋼組織は、
    体積率で、フェライトが３０％超、ベイナイトが１０％以上であり、
    該フェライトおよび該ベイナイトの合計が、７５％以上９５％以下であり、
    残部がパーライト、マルテンサイト、オーステナイトから選択される１種または２種以上からなり、
    隣り合う結晶の方位差が１５°以上の境界で囲まれた領域を結晶粒としたとき、長径が５０μｍ以上、且つ、長径と短径の比（＝（長径）／（短径））が４．０以上の結晶粒の個数が３０個／ｍｍ２以下であり、
    円相当径で４０．０μｍ以上の結晶粒が体積率で２０％以下である、熱延鋼板。
    １．２０×％Ｎｂ≦％Ｔｉ　　　　　　・・・（１）
    ここで、％Ｎｂ、％Ｔｉは各元素の含有量（質量％）である。     The component composition is in mass %,
    C: 0.04% or more and 0.45% or less,
    Si: 1.8% or less,
    Mn: 0.5% or more and 2.5% or less,
    P: 0.10% or less,
    S: 0.05% or less,
    Al: 0.005% or more and 0.100% or less,
    N: 0.010% or less,
    Nb: 0.005% or more and 0.050% or less,
    Ti: 0.012% or more and 0.100% or less,
    with the remainder consisting of Fe and unavoidable impurities,
    The content of Nb and Ti satisfies the following formula (1),
    The steel structure at the 1/4t position of the plate thickness t from the steel plate surface is
    The volume fraction is more than 30% ferrite and 10% or more bainite,
    The total of the ferrite and the bainite is 75% or more and 95% or less,
    The balance consists of one or more selected from pearlite, martensite, and austenite,
    When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the major axis is 50 μm or more, and the ratio of the major axis to the minor axis (=(major axis)/(minor axis)) is 4. The number of crystal grains of .0 or more is 30 / mm 2 or less,
    A hot-rolled steel sheet containing 20% or less by volume of crystal grains having an equivalent circle diameter of 40.0 μm or more.
    1.20×%Nb≦%Ti (1)
    Here, %Nb and %Ti are contents (% by mass) of each element.
  8. 　降伏強度が３３０ＭＰa以上、かつ、引張強度が５２０ＭＰａ以上、降伏比が０．７５以下、－２０℃におけるシャルピー吸収エネルギーが１８０J以上である、請求項７に記載の熱延鋼板。 The hot-rolled steel sheet according to claim 7, which has a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more.
  9. 　前記成分組成に加えてさらに、質量％で、下記のうちから選ばれた１種または２種以上を含有する、請求項７または請求項８に記載の熱延鋼板。
    Ｖ：０．０１％以上０．１５％以下、
    Ｃｒ：０．０１％以上１．０％以下、
    Ｍｏ：０．０１％以上１．０％以下、
    Ｃｕ：０．０１％以上０．５％以下、
    Ｎｉ：０．０１％以上０．３％以下、
    Ｃａ：０．０００５％以上０．０１０％以下、
    Ｂ：０．０００３％以上０．０１０％以下     The hot-rolled steel sheet according to claim 7 or 8, further comprising, in % by mass, one or more selected from the following in addition to the chemical composition.
    V: 0.01% or more and 0.15% or less,
    Cr: 0.01% or more and 1.0% or less,
    Mo: 0.01% or more and 1.0% or less,
    Cu: 0.01% or more and 0.5% or less,
    Ni: 0.01% or more and 0.3% or less,
    Ca: 0.0005% or more and 0.010% or less,
    B: 0.0003% or more and 0.010% or less
  10. 　前記鋼組織は、体積率で、ベイナイトが１０％以上４０％未満である、請求項７～９のいずれかに記載の熱延鋼板。 The hot-rolled steel sheet according to any one of claims 7 to 9, wherein the steel structure has a bainite content of 10% or more and less than 40% by volume.
  11. 　請求項７または請求項９に記載の成分組成を有する鋼素材を、加熱温度：１１００℃以上１３００℃以下に加熱した後、粗圧延終了温度：８５０℃以上１１５０℃以下、仕上圧延終了温度：７５０℃以上８５０℃以下、かつ９３０℃以下での合計圧下率：４０％以上６３％以下である熱間圧延を施し、次いで、板厚中心温度で平均冷却速度：２℃／ｓ以上２７℃／ｓ以下、冷却停止温度：４５０℃以上６５０℃以下で冷却を施し、４４０℃以上６５０℃以下で巻取る、熱延鋼板の製造方法。 After heating the steel material having the chemical composition according to claim 7 or claim 9 to a heating temperature of 1100° C. or higher and 1300° C. or lower, the rough rolling finish temperature is 850° C. or higher and 1150° C. or lower, and the finish rolling finish temperature is 750. Total rolling reduction at ℃ to 850 ℃ and 930 ℃ or less: hot rolling with 40% to 63%, then average cooling rate at plate thickness center temperature: 2 ℃ / s or more and 27 ℃ / s Cooling stop temperature: A method for manufacturing a hot-rolled steel sheet, comprising cooling at 450°C or higher and 650°C or lower and winding at 440°C or higher and 650°C or lower.
PCT/JP2022/032953 2021-09-29 2022-09-01 Rectangular steel pipe and method for manufacturing same, hot-rolled steel sheet and method for manufacturing same, and building structure WO2023053837A1 (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
KR1020247009566A KR20240053606A (en) 2021-09-29 2022-09-01 Rectangular steel pipe and its manufacturing method, hot rolled steel sheet and its manufacturing method, and building structures
CN202280063910.7A CN117980519A (en) 2021-09-29 2022-09-01 Square steel pipe and method for producing same, hot-rolled steel plate and method for producing same, and building structure
JP2022573502A JPWO2023053837A1 (en) 2021-09-29 2022-09-01

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2021-159272 2021-09-29
JP2021159272 2021-09-29

Publications (1)

Publication Number Publication Date
WO2023053837A1 true WO2023053837A1 (en) 2023-04-06

Family

ID=85780628

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2022/032953 WO2023053837A1 (en) 2021-09-29 2022-09-01 Rectangular steel pipe and method for manufacturing same, hot-rolled steel sheet and method for manufacturing same, and building structure

Country Status (5)

Country Link
JP (1) JPWO2023053837A1 (en)
KR (1) KR20240053606A (en)
CN (1) CN117980519A (en)
TW (1) TWI825985B (en)
WO (1) WO2023053837A1 (en)

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2000144316A (en) * 1998-11-10 2000-05-26 Kawasaki Steel Corp Hot rolled steel sheet for working having superfine grain
JP2012132088A (en) * 2010-11-30 2012-07-12 Jfe Steel Corp Thick hot-rolled steel sheet for square steel tube for building structure member, and method for manufacturing the same
WO2018235244A1 (en) * 2017-06-22 2018-12-27 新日鐵住金株式会社 As-roll electric resistance-welded steel pipe for line pipe, and hot-rolled steel sheet
US20200255920A1 (en) * 2017-11-03 2020-08-13 Posco Steel material for welding steel pipe having excellent low-temperature toughness, steel material that has undergone weld heat treatment, and method for manufacturing same
WO2020209060A1 (en) * 2019-04-08 2020-10-15 Ｊｆｅスチール株式会社 Square steel tube, method for manufacturing same, and building structure
JP2021147630A (en) * 2020-03-17 2021-09-27 日本製鉄株式会社 Hot rolled steel sheet, square steel pipe and method for manufacturing them

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3686303B1 (en) * 2017-09-19 2021-12-29 Nippon Steel Corporation Steel pipe and steel plate
WO2020039980A1 (en) * 2018-08-23 2020-02-27 Ｊｆｅスチール株式会社 Square steel pipe, manufacturing method thereof, and building structure

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2000144316A (en) * 1998-11-10 2000-05-26 Kawasaki Steel Corp Hot rolled steel sheet for working having superfine grain
JP2012132088A (en) * 2010-11-30 2012-07-12 Jfe Steel Corp Thick hot-rolled steel sheet for square steel tube for building structure member, and method for manufacturing the same
WO2018235244A1 (en) * 2017-06-22 2018-12-27 新日鐵住金株式会社 As-roll electric resistance-welded steel pipe for line pipe, and hot-rolled steel sheet
US20200255920A1 (en) * 2017-11-03 2020-08-13 Posco Steel material for welding steel pipe having excellent low-temperature toughness, steel material that has undergone weld heat treatment, and method for manufacturing same
WO2020209060A1 (en) * 2019-04-08 2020-10-15 Ｊｆｅスチール株式会社 Square steel tube, method for manufacturing same, and building structure
JP2021147630A (en) * 2020-03-17 2021-09-27 日本製鉄株式会社 Hot rolled steel sheet, square steel pipe and method for manufacturing them

Also Published As

Publication number Publication date
TW202314003A (en) 2023-04-01
CN117980519A (en) 2024-05-03
TWI825985B (en) 2023-12-11
KR20240053606A (en) 2024-04-24
JPWO2023053837A1 (en) 2023-04-06

Similar Documents

Publication Publication Date Title
JP6693606B1 (en) Square steel pipe, manufacturing method thereof, and building structure
JP6874913B2 (en) Square steel pipe and its manufacturing method and building structure
KR102498956B1 (en) Hot-rolled steel sheet and its manufacturing method
TWI762881B (en) Electric-welded steel pipe, method for manufacturing the same, and steel pipe pile
JP6690787B1 (en) ERW steel pipe, its manufacturing method, and steel pipe pile
WO2023053837A1 (en) Rectangular steel pipe and method for manufacturing same, hot-rolled steel sheet and method for manufacturing same, and building structure
JP7424551B1 (en) Hot-rolled steel plates, square steel pipes, their manufacturing methods and architectural structures
JP7396552B1 (en) Hot-rolled steel plates, square steel pipes, their manufacturing methods, and architectural structures
WO2024062686A1 (en) Hot-rolled steel sheet, square steel tube, methods for producing same, and building structure
JP2002363642A (en) Method for producing rolled wide flange shape having low yield ratio and excellent toughness
JP7276641B1 (en) Electric resistance welded steel pipe and its manufacturing method
CN114729426B (en) Hot-rolled steel sheet for resistance-welded steel pipe, method for producing same, line pipe, and building structure
WO2024100939A1 (en) Hot-rolled steel sheet, electric resistance welded steel pipe, rectangular steel pipe, line pipe, and building structure
TW202413663A (en) Hot-rolled steel plates, square steel pipes and their manufacturing methods and building structures
KR20230059820A (en) Rectangular steel pipe, its manufacturing method and building structure

Legal Events

Date Code Title Description
WWE Wipo information: entry into national phase

Ref document number: 2022573502

Country of ref document: JP

121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 22875709

Country of ref document: EP

Kind code of ref document: A1

ENP Entry into the national phase

Ref document number: 20247009566

Country of ref document: KR

Kind code of ref document: A