WO2011065479A1 - High-strength ultra-thick h shape steel and process for production thereof - Google Patents

High-strength ultra-thick h shape steel and process for production thereof Download PDF

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WO2011065479A1
WO2011065479A1 PCT/JP2010/071125 JP2010071125W WO2011065479A1 WO 2011065479 A1 WO2011065479 A1 WO 2011065479A1 JP 2010071125 W JP2010071125 W JP 2010071125W WO 2011065479 A1 WO2011065479 A1 WO 2011065479A1
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strength
steel
toughness
rolling
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PCT/JP2010/071125
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French (fr)
Japanese (ja)
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卓 吉田
裕史 北
晃央 奥村
博一 杉山
輝行 若月
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新日本製鐵株式会社
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Priority to JP2011516589A priority Critical patent/JP4855553B2/en
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    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a high-strength H-section steel excellent in toughness and weldability and a method for producing the same, and more particularly to a high-strength ultrathick H-section steel used for a structural member of a building and the method for producing the same.
  • H-shaped steel with a wall thickness of 40 mm or more is used for buildings, especially for super high-rise buildings.
  • an H-section steel having a thickness of 40 mm or more is referred to as an extremely thick H-section steel.
  • extra-thick H-section steels are required to have higher performance such as improved toughness in addition to higher strength.
  • rolled steel has been proposed in which a large amount of Cu and Nb, V, and Mo are added to suppress generation of island martensite (see, for example, Patent Document 1).
  • the H-section steel has a unique shape, rolling conditions (temperature, rolling reduction) are limited when the H-section steel is rolled by universal rolling. Therefore, in particular, differences in the rolling finishing temperature, reduction rate, and cooling rate at each part of the web, flange, and fillet of the ultra-thick H-shaped steel are likely to occur at each part. As a result, variations in strength, ductility, and toughness occur in the extremely thick H-section steel. For this reason, depending on the part of the ultra-thick H-section steel, the strength, ductility, and toughness may not meet the standards for rolled steel for welded structure (JIS G3106).
  • the extra-thick H-shaped steel requires not only the toughness of the base metal but also the toughness of the weld heat affected zone (hereinafter referred to as HAZ).
  • HAZ weld heat affected zone
  • the present invention provides a high-strength ultra-thick H-section steel excellent in strength and toughness, and further in weldability, and a method for producing the same.
  • the high-strength ultrathick H-shaped steel of the present invention uses fine Ti oxides to reduce the grain size during rolling and heating, while reducing the coarse Ti-containing oxides by vacuum degassing. Is. Furthermore, the high-strength ultra-thick H-shaped steel of the present invention controls the content of C, Si, Al, and B to suppress the formation of island martensite in order to increase the toughness of the base material and HAZ. .
  • the gist of the present invention is as follows.
  • the present invention it is possible to produce a high-strength ultra-thick H-shaped steel excellent in toughness and weldability as it is without being subjected to a tempering heat treatment.
  • the construction cost can be reduced and the cost can be greatly reduced by shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without sacrificing economy.
  • the present inventors performed further vacuum degassing treatment after adding Ti to remove coarse inclusions.
  • Ti is added after preliminary deoxidation so that coarse inclusions do not remain at a high density, and further vacuum degassing treatment is performed to remove coarse inclusions in the molten steel. It is necessary to take sufficient measures.
  • standard of the size of the oxide and the distribution number density which should be removed in order to ensure the toughness which becomes the foundation of this invention is demonstrated.
  • the conditions for vacuum degassing for removing coarse oxides will be described.
  • the present inventors pre-deoxidized steel having various components, added Ti, and further changed the vacuum degassing time. And melted. Steel pieces obtained by melting and casting steel were hot-rolled to produce steel plates.
  • HAZ weld heat affected zone
  • the heat treatment is performed at a heating rate of 10 ° C./s to 1400 ° C., maintained for 1 s, and then cooled at a cooling rate from 800 ° C. to 500 ° C. at 15 ° C./s.
  • a V-notch test piece was collected from the small piece and subjected to a Charpy impact test at 0 ° C. in accordance with JIS Z 2242. Furthermore, the fracture surface and the metal structure were observed with a scanning electron microscope (SEM), and the size and density of the oxide affecting the toughness were examined.
  • SEM scanning electron microscope
  • FIG. 1 shows the relationship between the density of the Ti-containing oxide exceeding 10 ⁇ m and the toughness of the test piece. From FIG. 1, it was found that if the density of the Ti-containing oxide exceeding 10 ⁇ m is 10 pieces / mm 2 or less, the Charpy absorbed energy at 0 ° C. can be 50 J or more, although there is variation.
  • FIG. 2 shows the relationship between the density of the Ti-containing oxide exceeding 10 ⁇ m and the vacuum degassing time after adding Ti.
  • the degree of vacuum in FIG. 2 is 20 Torr or less. From FIG. 2, it was found that the vacuum degassing time was required to be 30 minutes or more in order to make the density of the Ti-containing oxide exceeding 10 ⁇ m 10 pieces / mm 2 or less. Furthermore, it was found that the Ti-containing oxide having a particle diameter of more than 10 ⁇ m can be surely reduced to 10 pieces / mm 2 or less by setting the vacuum degassing treatment time to 35 minutes or more.
  • the crystal grain size becomes coarse by heating to 1400 ° C., and further, the formation of a hard phase is promoted by rapid cooling after heating. Therefore, in HAZ, due to the coarsening of the particle size of the Ti-containing oxide and the generation of the hard phase, the toughness is significantly reduced.
  • the fine Ti-containing oxide which does not become a solution even if it heats to 1400 degreeC is disperse
  • the fine Ti-containing oxide is very effective not only for HAZ, but also for refining the grain size contained in H-section steel. This is because with H-section steel, it is not possible to secure the amount of processing by hot rolling until the final product is manufactured from the steel slab, which is the raw material. Fine graining using recrystallization by hot working Is difficult. Therefore, the pinning effect of the crystal grain boundaries by the fine Ti-containing oxide, which is effective for refining the microstructure of the steel slab, is extremely important. In order to disperse a large number of fine Ti-containing oxides in steel, appropriate deoxidation treatment and degassing treatment should be performed in the steelmaking process for melting steel to adjust the dissolved oxygen concentration before Ti addition. is required.
  • the C is an element effective for strengthening steel, and in order to obtain the strength required for structural steel, the lower limit of the C content is 0.01% or more. On the other hand, if the amount of C exceeds 0.05%, island-shaped martensite is generated, and in particular, the toughness is lowered. Therefore, the upper limit of the amount of C is set to 0.05% or less. In order to improve the toughness and weld crack resistance of the base material and HAZ, the upper limit of the amount of C is preferably set to 0.04% or less.
  • the Si is an element useful for securing the strength of the base metal and preliminary deoxidation of the molten steel.
  • the lower limit of the amount of Si is 0.05% or more.
  • the upper limit of the Si content is 0.50% or less.
  • the lower limit of the amount of Si is preferably 0.1% or more.
  • the upper limit of the amount of Si is preferably 0.45% or less, more preferably 0.2% or less.
  • Mn needs to be added in an amount of 0.8% or more in order to ensure the strength of the base material.
  • the amount of Mn is preferably 1.0% or more, more preferably 1.3% or more.
  • the upper limit of the amount of Mn is set to 2.0% or less.
  • Cu is an element that contributes to strengthening of the base material by precipitation hardening.
  • a Cu phase precipitates on the dislocations of the ferrite and is increased in strength due to holding and slow cooling in a temperature range where ferrite is generated during rolling.
  • the precipitation strengthening is saturated even if more than 1.2% of Cu is added. Therefore, the Cu content is set to 0.3 to 1.2%.
  • a more preferable content of Cu is 0.4 to 1.0%.
  • Ni is an extremely effective element for increasing the strength and toughness of the base material.
  • the Ni content is set to 0.1% or more.
  • the effect is saturated even if Ni exceeding 1.0% is added. Therefore, the upper limit of the Ni content is 1.0% or less. A more preferable upper limit of the Ni content is 0.8% or less.
  • Ti is an extremely important element in the present embodiment in order to improve the toughness of the base material and the HAZ.
  • Ti forms a fine Ti-containing oxide and contributes to refinement of the crystal grain size, so 0.005% or more is added.
  • the fine TiN formed by adding Ti contributes to the refinement of the crystal grain size.
  • Ti fixes solute N as TiN, it is also effective in suppressing the formation of island martensite. Therefore, it is preferable to add Ti by 0.010% or more.
  • the amount of Ti exceeds 0.025%, the particle size of the Ti-containing oxide becomes coarse, and the toughness of the base material and the weld heat affected zone is impaired. Therefore, the upper limit of the amount of Ti is set to 0.025% or less.
  • the upper limit of the amount of Ti is preferably set to 0.020% or less.
  • Nb is an element that increases hardenability, and it is necessary to add 0.01% or more.
  • the Nb content is preferably 0.02% or more.
  • the upper limit of the Nb content is 0.25% or less.
  • the Nb content is preferably 0.08% or less, and more preferably 0.04% or less.
  • N is added in an amount of 0.001% or more in order to refine crystal grains with fine TiN.
  • N forms a solid solution in the steel, island martensite is generated in the upper bainite structure and the toughness is deteriorated. Therefore, when the amount of N exceeds 0.009%, if Ti is added to fix solid solution N, TiN having a coarse particle diameter is generated and the toughness is lowered. Therefore, the upper limit of the amount of N is set to 0.009% or less.
  • the amount of N is preferably 0.005% or less.
  • the amount of N is preferably 0.004% or less.
  • O is an extremely important element in the present embodiment, and forms a Ti-containing oxide and contributes to refinement of crystal grains.
  • the amount of O is preferably set to 0.0008% or more.
  • the upper limit of the O content is less than 0.0020%.
  • the O content is preferably 0.0015% or less.
  • a more preferable upper limit of the O amount is 0.0012% or less, and a most preferable upper limit of the O amount is 0.0009% or less.
  • Al is a deoxidizing element, but in the present embodiment, in order to generate a fine Ti-containing oxide, the upper limit of the amount of Al is limited to 0.025% or less. Further, the reduction of the amount of Al is effective for suppressing the formation of island martensite, and the upper limit of the amount of Al is preferably set to 0.010% or less.
  • B is an element that increases the hardenability by adding a small amount, but in this embodiment, the B content is limited to promote the formation of island martensite.
  • the B content is limited to less than 0.0003%. More preferably, the amount of B is limited to 0.0001% or less.
  • the content of P and S contained as inevitable impurities is not particularly limited. In addition, since P and S cause weld cracking due to solidification segregation and a decrease in toughness, they should be reduced as much as possible.
  • the amount of P is preferably limited to 0.02% or less, and a more preferable upper limit is 0.002% or less.
  • the content of S is preferably limited to 0.002% or less.
  • one or more of V, Mo, Cr, and Mg may be contained.
  • V contributes to refinement of the structure and precipitation strengthening by V carbonitride. However, when V is added excessively, the toughness may be impaired. Therefore, the upper limit of the amount of V is preferably set to 0.1%.
  • Mo is an element that dissolves in steel and enhances hardenability, and contributes to improvement in strength. However, even if 0.3% or more of Mo is contained, Mo carbide (Mo 2 C) is precipitated, and the effect of improving the hardenability by solute Mo is saturated. Therefore, the upper limit of the amount of Mo is preferably less than 0.3%. The upper limit of the amount of Mo is more preferably 0.2% or less, and still more preferably 0.1% or less. In addition, Mo is an element that is essentially unnecessary in the component design of the present invention, so it is not necessary to add it actively. If the strength can be sufficiently secured by adding other elements, Mo is not added. May be.
  • the upper limit of the Cr amount is preferably 1.5%.
  • a more preferable upper limit of Cr is 1.2%, and a more preferable upper limit is 0.8%.
  • Mg is a deoxidizing element, and the Mg-based oxide contained in the steel becomes fine. Mg is particularly effective in suppressing the coarsening of crystal grains in the HAZ structure. Therefore, in order to improve HAZ toughness, it is preferable to contain 0.001% or more. In addition, since the oxide of Mg easily floats in the molten steel, the upper limit of Mg contained in the steel is 0.005% or less, preferably 0.003% or less.
  • the Mg-based oxide forms a composite oxide with Ti, Al, Ca, and the like. When adding Mg, Mg alloys such as Si—Mg—Al and Ni—Mg are preferably used.
  • Zr and Hf are deoxidizing elements and generate nitrides in the molten steel, which is effective in reducing the amount of solute N in the steel.
  • REM and Ca are deoxidizing elements and contribute to the control of the form of sulfide.
  • Zr, Hf, REM, and Ca are all strong deoxidizing elements, and the addition of these elements may coarsen the Ti-containing oxide. Therefore, it is better not to positively add Zr, Hf, REM and Ca.
  • the microstructure of the H-section steel of this embodiment consists of bainite, island martensite, and ferrite pearlite.
  • Bainite contributes to increased strength and refinement of the structure.
  • the area ratio of bainite is less than 10%, the strength is insufficient.
  • the area ratio of bainite exceeds 40%, the toughness decreases. Therefore, the area ratio of bainite is 10 to 40%, preferably 20 to 40%.
  • Island-like martensite is the starting point of fracture and reduces toughness. Therefore, the area ratio of island martensite is limited to 0.5% or less, preferably 0.3% or less, and more preferably 0.2% or less. The remainder of bainite and island martensite is ferrite pearlite.
  • control of the particle size and density of the Ti-containing oxide is extremely important for improving toughness by refining the base material and HAZ crystal grains.
  • the Ti-containing oxide also functions as a nitride nucleation, promotes the fixation of N by a nitride generated at a high temperature, such as TiN, and suppresses the precipitation of NbN and BN. As a result, the effect of improving the hardenability by Nb and B can be maximized, so that the Ti-containing oxide indirectly contributes to the improvement of strength.
  • the Ti-containing oxide is confirmed to contain Ti and O at the same time by an X-ray microanalyzer (EPMA), and 50% by mass or more of elements other than oxygen contained in one solid is Ti.
  • EPMA X-ray microanalyzer
  • the Ti-containing oxide is preferably such that 70% by mass or more of an element other than oxygen contained in one solid is Ti, more preferably 80% by mass or more is Ti.
  • Ti-based oxides such as TiO, TiO 2 , and Ti 2 O 3 , composite oxides of these Ti-based oxides and oxides other than Ti-based oxides, and these Ti-based oxides It is a general term for composite inclusions of composite oxides and sulfides.
  • the particle size and density of the Ti-containing oxide can be measured by observing the metal structure by SEM and identifying the elements contained in the oxide by EDX.
  • the particle size and density of the Ti-containing oxide may be measured by detecting inclusions containing Ti and O with an X-ray microanalyzer (EPMA) and collating them with image analysis and structural photographs.
  • An average particle diameter and a particle number density of about 50 particles are obtained in a field of view of 0.5 mm ⁇ 0.5 mm or more.
  • the particle diameter of the Ti-containing oxide is the maximum diameter in the structure photograph.
  • the Ti-containing oxide having a particle size of 0.05 ⁇ m or more and 10 ⁇ m or less pins the crystal grain boundary, delays the grain growth, and contributes to refinement of the base material and HAZ crystal grains. If the particle size of the Ti-containing oxide is less than 0.05 ⁇ m, the pinning effect cannot be obtained, but in particular, it does not cause a decrease in toughness.
  • the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m is less than 30 / mm 2 , the pinning effect is insufficient.
  • the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m exceeds 300 pieces / mm 2 , it becomes a path of crack growth, so that toughness decreases. Therefore, in order to improve the toughness of HAZ, it is necessary to set the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m to 30 to 300 / mm 2 .
  • the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m can be controlled by adjusting the amount of dissolved oxygen and the amount of Ti added before adding Ti.
  • the lower limit of the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m is preferably 50 pieces / mm 2 or more, more preferably 60 pieces / mm 2 or more, still more preferably 100 pieces / mm 2 or more.
  • the upper limit of the density of the Ti-containing oxide having a particle size of 0.05 to 10 ⁇ m is preferably 200 pieces / mm 2 or less, more preferably 160 pieces / mm 2 or less.
  • the particle size of the Ti-containing oxide exceeds 10 ⁇ m, as described above, it becomes a starting point of fracture, and when the density exceeds 10 pieces / mm 2 , the toughness of the base material and the HAZ decreases. Therefore, it is necessary to limit the density of the Ti-containing oxide having a particle size of more than 10 ⁇ m to 10 pieces / mm 2 or less, preferably 7 pieces / mm 2 or less, more preferably 5 pieces / mm 2 or less.
  • the density of the Ti-containing oxide having a particle size of more than 10 ⁇ m can be controlled by the time of vacuum degassing treatment.
  • the plate thickness of the H-section steel of this embodiment is 40 mm or more. This is because a H-shaped steel having a plate thickness of 40 mm or more is employed as the H-shaped steel used for a column such as a super high-rise building.
  • the upper limit of the thickness of the H-section steel of the present embodiment is set to 150 mm or less in order to ensure toughness industrially due to the limitation of the thickness of the slab.
  • the target values of mechanical properties when using H-shaped steel as a structural member are the yield point at room temperature or 0.2% proof stress 450 MPa or more, and tensile strength 550 MPa or more (equivalent to ASTM standard grade 65). More preferably, the yield point at normal temperature or the 0.2% proof stress is 345 MPa or more and the tensile strength is 450 MPa or more (equivalent to ASTM standard grade 50). Further, the Charpy impact absorption energy at 0 ° C. is the base material portion 47J or more and the HAZ portion 47J or more.
  • H-shaped steel it is difficult for H-shaped steel to ensure strength and toughness compared to the case of manufacturing a steel plate. This is because when manufacturing H-section steel from a slab or beam blank-shaped material, it is difficult to ensure the processing amount of not only the flange but also the fillet portion (portion where the flange and the web are bonded). .
  • a steelmaking process for melting steel is extremely important.
  • deoxidation is important, and it is necessary to control the amount of dissolved oxygen before adding Ti to an appropriate range and to perform vacuum degassing treatment under appropriate conditions after adding Ti.
  • the amount of dissolved oxygen before Ti addition can be controlled by the amount of deoxidizing elements such as Si and Mn, and selectively added Al and Mg. If the dissolved oxygen before addition of Ti is less than 0.003% by mass, the amount of Ti-containing oxide having a particle size of 10 ⁇ m or less becomes insufficient. On the other hand, if the dissolved oxygen before addition of Ti exceeds 0.015%, coarse Ti-containing oxides having a particle size exceeding 10 ⁇ m increase.
  • vacuum degassing treatment is performed.
  • the degree of vacuum in the vacuum degassing process is set to 20 Torr or less, preferably 5 Torr or less.
  • the upper limit of the processing time is preferably 60 minutes or less in order to suppress an increase in manufacturing cost.
  • the casting is preferably continuous casting from the viewpoint of productivity.
  • the thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.
  • the heating temperature of the steel slab is in the range of 1100 to 1350 ° C.
  • the heating temperature is less than 1100 ° C.
  • the lower limit of the reheating temperature is preferably set to 1150 ° C. or higher.
  • the heating temperature is higher than 1350 ° C.
  • the scale of the surface of the steel slab, which is the raw material, is liquefied and the inside of the furnace is damaged, so that the economic merit is reduced. Therefore, the upper limit of the heating temperature for hot working is set to 1350 ° C.
  • the upper limit of the heating temperature is preferably 1300 ° C. or lower.
  • Control rolling and controlled cooling are manufacturing methods for controlling the rolling temperature, the rolling reduction, and the cooling rate.
  • finish rolling it is preferable to perform one or more passes of rolling that is water-cooled between passes.
  • the rolling process in which water cooling is performed between passes is a manufacturing method in which water cooling is performed and rolling is performed in a recuperation process. More preferably, heat treatment is performed after finish rolling.
  • a so-called two-heat rolling process may be employed in which primary rolling is performed to cool to 500 ° C. or lower and then heating is performed again to 1100 to 1350 ° C. to perform secondary rolling. In the two-heat rolling, the amount of plastic deformation in the hot rolling is small, and the temperature drop in the rolling process is also small, so that the heating temperature can be lowered.
  • the cumulative rolling reduction at 1000 ° C. or lower is 10% or higher after heating the steel slab. This is because hot rolling promotes work recrystallization, refines austenite, and improves toughness and strength. Depending on the thickness of the steel slab and the thickness of the product, rough rolling may be performed before finish rolling. In the case of adopting two-heat rolling, it is necessary to set the cumulative rolling reduction at 1000 ° C. or lower in secondary rolling to 10% or more.
  • Control cooling is performed after controlled rolling.
  • the average cooling rate up to a temperature range of 400 to 700 ° C. is set to 0.1 to 5 ° C./s.
  • the area ratio of bainite is 10 to 40% and the area ratio of island martensite is 0.5% or less, and the strength and toughness can be improved.
  • the cooling stop temperature exceeds 700 ° C. or when the cooling rate is less than 0.1 ° C./s, crystal grains may grow or the area ratio of bainite may decrease.
  • the cooling stop temperature is less than 400 ° C. or when the cooling rate exceeds 10 ° C./s, the area ratio of bainite exceeds 40% and the area ratio of island martensite is 0.5%. And toughness may be reduced.
  • Rolling that is water-cooled between passes is a method of rolling by giving a temperature difference between the surface layer portion and the inside of the flange by water-cooling between passes of rolling.
  • processing strain can be introduced to the inside of the plate thickness. Further, productivity is improved by lowering the rolling temperature in a short time by water cooling.
  • Rolling with water cooling between passes is a method in which the surface temperature of the flange is cooled to 700 ° C. or lower and then rolled in the reheating process, and quenching and hardening of the surface can be suppressed.
  • the processing is performed in a temperature range in which austenite and ferrite coexist ( ⁇ / ⁇ two-phase coexistence temperature range).
  • ⁇ / ⁇ two-phase coexistence temperature range As a result, a mixed structure of finely divided austenite and processed fine ferrite is formed.
  • the hardenability of the surface layer portion can be remarkably reduced, and hardening of the surface layer caused by accelerated cooling can be prevented.
  • the heating temperature is preferably 400 ° C. or higher and the holding time is preferably 15 minutes or longer.
  • the upper limit of the heating temperature and the upper limit of the holding time are not particularly defined, it is preferable that the heating temperature is 500 ° C. or less and the holding time is 5 hours or less from the viewpoint of manufacturing cost.
  • Reheating after cooling can be performed in a heat treatment furnace.
  • the cooling rate after reheating is not particularly limited, and may be rapidly cooled or allowed to cool.
  • Steel having the composition shown in Table 1 was melted, and a steel piece having a thickness of 240 to 300 mm was produced by continuous casting.
  • Steel melting is performed in a converter, primary deoxidation is performed, alloy components are added, and after adjusting the dissolved oxygen concentration as shown in Table 2, Ti deoxidation treatment is performed, and vacuum degassing is further performed. Processed. The degree of vacuum during the vacuum degassing process was set to 10 Torr or less.
  • the obtained steel slab was heated, hot-rolled and cooled. Although illustration of the rough rolling process is omitted, an H-section steel was manufactured by performing hot rolling with a universal rolling apparatus row shown in FIG.
  • water cooling between the rolling passes is performed by using water cooling devices 2a provided before and after the intermediate universal rolling mill 1, and spray cooling and reverse of the outer surface of the flange.
  • Control cooling after controlled rolling was performed by cooling the outer surface of the flange with a cooling device 2b installed on the rear surface after finishing rolling by the finishing universal rolling mill 3.
  • the production conditions are shown in Table 3.
  • the results are shown in Table 4.
  • the target values for mechanical properties are: yield point at room temperature or 0.2% proof stress of 450 MPa or higher, tensile strength of 550 MPa or higher (equivalent to ASTM standard grade 65), and Charpy impact absorption energy at 0 ° C. at 47 J As mentioned above, it is 47J or more in the HAZ part.
  • the surface of the 1/4 F test piece cut out in FIG. 4 was observed at a magnification of 10,000 times or more using EPMA, and a Ti-containing oxide containing Ti and O was detected.
  • the density of each of the Ti-containing oxides having a maximum diameter of 0.05 ⁇ m to 10 ⁇ m and a particle having a maximum diameter of more than 10 ⁇ m is obtained. Asked. The density was calculated by measuring 50 particles. The results are shown in Table 2.
  • the chemical composition of inclusions on the surface of the test piece was measured under the following conditions, and a Ti-containing oxide having a Ti concentration of 50% or more was used.
  • a Ti-containing oxide having a Ti concentration of 50% or more was used as an observation condition.
  • an observation visual field area of 25 ⁇ 10 ⁇ 2 mm 2 is observed with 5 or more visual fields, the number of particles to be analyzed is about 50, and the central part of the particle is analyzed by wavelength dispersion spectroscopy of characteristic X-rays.
  • the component composition was quantitatively analyzed.
  • the analysis target elements are Ti, Si, Al, Mg, Mn, and O (oxygen), and the relationship between the electron beam intensity and the element concentration of each element is obtained in advance using a known substance as a calibration curve. From the electron beam intensity obtained from the above and the calibration curve, the elemental concentration of the particles was quantified.
  • the production No. of the present invention As shown in Table 4, the production No. of the present invention. In Nos. 1 to 12, the 0.2% yield strength and the tensile strength at room temperature satisfy the target lower limit values of 450 MPa and 550 MPa, respectively. Furthermore, the Charpy impact absorption energy at 0 ° C. is 47 J or more in the base material portion and the HAZ portion, and thus sufficiently satisfies the target.
  • production No. which is a comparative example.
  • one or both of the dissolved oxygen concentration and the vacuum degassing treatment before the addition of Ti are outside the scope of the present invention, and the toughness is lowered due to the increase in the size of Ti-containing oxides and the increase in the size.
  • production No. in No. 17 the cooling rate of the controlled cooling is slow, bainite is reduced, and the strength is reduced.
  • Production No. No. 18 has a low cumulative rolling reduction at 1000 ° C. or lower, and has reduced strength and toughness.
  • Manufacturing No. Nos. 19 to 29 have component compositions outside the scope of the present invention.
  • Production No. No. 19 has a small amount of C.
  • No. 22 has a small amount of Mn.
  • No. 24 is an example in which the amount of Cu is small and the strength is lowered.
  • Production No. No. 20 is an example in which the amount of C is large, the strength is increased, the number of island martensites is increased, and the toughness is lowered.
  • Production No. No. 21 is an example in which the amount of Si is large, island martensite is increased, and toughness is lowered.
  • Production No. No. 23 is an example in which the amount of Mn is large, the strength is increased, and the toughness is lowered.
  • Manufacturing No. No. 25 has a small amount of Ti.
  • No. 29 is an example in which since the amount of Al is large, the fine Ti-containing oxide is reduced and the toughness is lowered.
  • Production No. No. 26 is an example in which the amount of Ti is large, the coarse Ti-containing oxide is increased, and the toughness is lowered.
  • Production No. 27 is an example in which the amount of N is large, bainite and island martensite are increased, and the toughness is lowered.
  • Production No. No. 28 is an example in which the amount of B is large, the number of island martensite is increased, and the toughness is lowered.
  • the present invention makes it possible to produce a high-strength, extremely thick H-section steel excellent in toughness and weldability as it is without being subjected to a tempering heat treatment.
  • the construction cost can be reduced and the cost can be greatly reduced by shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without sacrificing economy.

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Abstract

A high-strength ultra-thick H shape steel having such a chemical composition that 0.01 to 0.05 mass% of C, 0.05 to 0.50 mass% of Si, 0.005 to 0.025 mass% of Ti and 0.0005 to 0.0035 mass% of O are contained, the contents of Al and B are limited to 0.025 mass% or less and less than 0.0003 mass%, respectively, and the contents of Mn, Cu, Ni, Nb and N are defined within proper ranges, wherein the areal ratio of bainite is limited to 10 to 40%, the areal ratio of island-shaped martensite is limited to 0.5% or less, the density of Ti-containing oxide particles having particle diameters of 0.05 to 10 μm is 30 to 300 particles/mm2, the density of Ti-containing oxide particles having particle diameters of larger than 10 μm is 10 particles/mm2 or less, and the web thickness or the flange thickness is 40 to 150 mm.

Description

高強度極厚Ｈ形鋼及びその製造方法High-strength ultra-thick H-section steel and its manufacturing method
　本発明は、靱性、溶接性に優れた高強度Ｈ形鋼及びその製造方法に関し、より詳しくは建造物の構造部材などに用いられる高強度極厚Ｈ形鋼及びその製造方法に関する。
　本願は、２００９年１１月２７日に、日本に出願された特願２００９－２７０５４１号に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a high-strength H-section steel excellent in toughness and weldability and a method for producing the same, and more particularly to a high-strength ultrathick H-section steel used for a structural member of a building and the method for producing the same.
This application claims priority based on Japanese Patent Application No. 2009-270541 filed in Japan on November 27, 2009, the contents of which are incorporated herein by reference.
　建築物、特に、超高層化された建築物には、肉厚が４０ｍｍ以上のＨ形鋼が使用されている。本明細書では、肉厚が４０ｍｍ以上のＨ形鋼を極厚Ｈ形鋼という。建築物の安全基準の厳格化などに伴って、極厚Ｈ形鋼には、高強度化に加えて、靭性の向上などの高性能化が要求されている。これまでに、多量のＣｕと、Ｎｂ、Ｖ及びＭｏとを添加し、島状マルテンサイトの生成を抑制した、圧延形鋼が提案されている（例えば、特許文献１、参照）。 H-shaped steel with a wall thickness of 40 mm or more is used for buildings, especially for super high-rise buildings. In the present specification, an H-section steel having a thickness of 40 mm or more is referred to as an extremely thick H-section steel. Along with stricter safety standards for buildings, extra-thick H-section steels are required to have higher performance such as improved toughness in addition to higher strength. So far, rolled steel has been proposed in which a large amount of Cu and Nb, V, and Mo are added to suppress generation of island martensite (see, for example, Patent Document 1).
　また、Ｈ形鋼は形状が特異であるため、Ｈ形鋼をユニバーサル圧延で圧延する際には、圧延条件（温度、圧下率）が制限される。そのため、特に、極厚Ｈ形鋼のウェブ、フランジ、フィレットの各部位における圧延仕上げ温度、圧下率、冷却速度は、各部位毎に差が生じ易くなる。その結果、極厚Ｈ形鋼では、強度、延性、靱性のバラツキが発生する。このため、極厚Ｈ形鋼の部位によっては、強度、延性、靱性が溶接構造用圧延鋼材（ＪＩＳ Ｇ３１０６）等の規準に満たない場合がある。 Also, since the H-section steel has a unique shape, rolling conditions (temperature, rolling reduction) are limited when the H-section steel is rolled by universal rolling. Therefore, in particular, differences in the rolling finishing temperature, reduction rate, and cooling rate at each part of the web, flange, and fillet of the ultra-thick H-shaped steel are likely to occur at each part. As a result, variations in strength, ductility, and toughness occur in the extremely thick H-section steel. For this reason, depending on the part of the ultra-thick H-section steel, the strength, ductility, and toughness may not meet the standards for rolled steel for welded structure (JIS G3106).
　特に、連続鋳造によって得られた鋳片を熱間圧延して極厚Ｈ形鋼を製造する場合には、結晶粒の微細化によって靭性を確保することが困難になる。これは、連続鋳造設備で製造可能な鋳片の最大厚みに限界があり、圧延の圧下比が不足するためである。更に、圧延によって製品の寸法精度を高めるために高温で圧延を施すと、板厚の厚いフランジ部では圧延温度が高くなり、冷却速度は遅くなる。その結果、フランジ部では、結晶粒が粗大化し、特に、靱性が低下し易い。 In particular, when an extremely thick H-section steel is manufactured by hot rolling a slab obtained by continuous casting, it becomes difficult to ensure toughness by refining crystal grains. This is because there is a limit to the maximum thickness of the slab that can be produced by the continuous casting equipment, and the rolling reduction ratio is insufficient. Further, when rolling is performed at a high temperature in order to increase the dimensional accuracy of the product by rolling, the rolling temperature becomes high and the cooling rate becomes slow in the flange portion having a large plate thickness. As a result, in the flange portion, the crystal grains are coarsened, and in particular, the toughness tends to decrease.
　このような問題に対して、Ｔｉ系酸化物を鋼中に分散させて、粒内フェライトを生成させて結晶粒を微細化する方法が提案されている（例えば、特許文献２、参照）。更に、Ｔｉ酸化物及びＴｉＮの微細分散に加え、温度制御圧延及び加速冷却によって高強度で靭性に優れた圧延形鋼を製造する方法が提案されている（例えば、特許文献３～５、参照）。 In response to such a problem, a method has been proposed in which Ti-based oxides are dispersed in steel to generate intragranular ferrite to refine crystal grains (for example, see Patent Document 2). Furthermore, in addition to fine dispersion of Ti oxide and TiN, a method for producing a rolled section steel having high strength and excellent toughness by temperature-controlled rolling and accelerated cooling has been proposed (see, for example, Patent Documents 3 to 5). .
特開平９－１９４９８５号公報Japanese Patent Laid-Open No. 9-194985 特開平５－２６３１８２号公報Japanese Patent Laid-Open No. 5-263182 特開平１０－１４７８３５号公報Japanese Patent Laid-Open No. 10-147835 特開２０００－５４０６０号公報JP 2000-54060 A 特開２００１－３１３６号公報Japanese Patent Laid-Open No. 2001-3136
　しかし、極厚Ｈ形鋼では、母材の靱性だけでなく、溶接熱影響部（以下、ＨＡＺという）の靭性も要求される。特に、ウェブまたはフランジの肉厚が増加すると、溶接の入熱が大きくなり、冷却速度が低下するため、ＨＡＺの靭性の確保は難しくなる。本発明は、強度及び靭性、更には溶接性にも優れた高強度極厚Ｈ形鋼及びその製造方法を提供するものである。 However, the extra-thick H-shaped steel requires not only the toughness of the base metal but also the toughness of the weld heat affected zone (hereinafter referred to as HAZ). In particular, when the thickness of the web or flange increases, the heat input of welding increases and the cooling rate decreases, so it becomes difficult to ensure the toughness of the HAZ. The present invention provides a high-strength ultra-thick H-section steel excellent in strength and toughness, and further in weldability, and a method for producing the same.
　本発明の高強度極厚Ｈ形鋼は、圧延加熱時の粒径を細粒化するため、微細Ｔｉの酸化物を利用し、一方、真空脱ガスによって粗大な含Ｔｉ酸化物を減少させたものである。更に、本発明の高強度極厚Ｈ形鋼は、母材及びＨＡＺの靭性を高めるために、Ｃ、Ｓｉ、Ａｌ、Ｂの含有量を制御し、島状マルテンサイトの生成を抑制している。本発明の要旨は以下のとおりである。 The high-strength ultrathick H-shaped steel of the present invention uses fine Ti oxides to reduce the grain size during rolling and heating, while reducing the coarse Ti-containing oxides by vacuum degassing. Is. Furthermore, the high-strength ultra-thick H-shaped steel of the present invention controls the content of C, Si, Al, and B to suppress the formation of island martensite in order to increase the toughness of the base material and HAZ. . The gist of the present invention is as follows.
（１）　質量％で、
Ｃ：０．０１～０．０５％、
Ｓｉ：０．０５～０．５０％、
Ｍｎ：０．８～２．０％、
Ｃｕ：０．３～１．２％、
Ｎｉ：０．１～１．０％、
Ｔｉ：０．００５～０．０２５％、
Ｎｂ：０．０１～０．２５％、
Ｎ：０．００１～０．００９％、
Ｏ：０．０００５～０．００２０％未満
を含有し、
Ａｌ：０．０２５％以下、
Ｂ：０．０００３％未満
に制限し、残部がＦｅ及び不可避不純物からなり、ミクロ組織中のベイナイトの面積率が１０～４０％であり、島状マルテンサイトの面積率を０．５％以下に制限し、残部がフェライト・パーライトからなり、粒子径が０．０５～１０μｍの含Ｔｉ酸化物の密度が３０～３００個／ｍｍ２であり、粒子径１０μｍ超の含Ｔｉ酸化物の密度が１０個／ｍｍ２以下であり、ウェブ厚又はフランジ厚が４０～１５０ｍｍである高強度極厚Ｈ形鋼。 (1) In mass%,
C: 0.01 to 0.05%,
Si: 0.05 to 0.50%,
Mn: 0.8 to 2.0%,
Cu: 0.3 to 1.2%,
Ni: 0.1 to 1.0%,
Ti: 0.005 to 0.025%,
Nb: 0.01 to 0.25%,
N: 0.001 to 0.009%,
O: 0.0005 to less than 0.0020%,
Al: 0.025% or less,
B: Restricted to less than 0.0003%, the balance is Fe and inevitable impurities, the area ratio of bainite in the microstructure is 10 to 40%, and the area ratio of island martensite is 0.5% or less The density of the Ti-containing oxide having a particle diameter of 0.05 to 10 μm is 30 to 300 / mm 2 and the density of the Ti-containing oxide having a particle diameter of more than 10 μm is 10 A high-strength, ultra-thick H-section steel having a piece thickness / mm 2 or less and a web thickness or flange thickness of 40 to 150 mm.
（２）質量％で、更に、
Ｖ：０．１％以下、
Ｍｏ：０．３％未満、
Ｃｒ：１．５％以下の１種又は２種以上を含有することを特徴とする上記（１）に記載の高強度極厚Ｈ形鋼。
（３）質量％で、更に、
Ｍｇ:０．００５％以下
を含有することを特徴とする上記（１）又は（２）に記載の高強度極厚Ｈ形鋼。 (2) In mass%,
V: 0.1% or less,
Mo: less than 0.3%,
Cr: The high-strength ultra-thick H-section steel according to (1) above, which contains one or more of 1.5% or less.
(3) In mass%,
Mg: 0.005% or less, The high-strength ultrathick H-section steel according to (1) or (2) above.
（４）上記（１）～（３）の何れか１項に記載の成分からなる鋼を溶製する際に、予備脱酸処理によって溶存酸素を０．００３～０．０１５質量％に調整した後、Ｔｉを添加し、更に真空脱ガス処理を３０分以上施し、溶製後、連続鋳造し、得られた鋳片を１１００～１３５０℃に加熱し、１０００℃以下での累積圧下率が１０％以上である仕上圧延を行って、ウェブ厚又はフランジ厚を４０～１５０ｍｍとし、０．１～５℃／ｓの範囲内の冷却速度で４００～７００℃の温度域まで冷却することを特徴とする高強度極厚Ｈ形鋼の製造方法。 (4) When melting the steel composed of the component described in any one of (1) to (3) above, the dissolved oxygen was adjusted to 0.003 to 0.015 mass% by preliminary deoxidation treatment. Thereafter, Ti is added, and vacuum degassing treatment is further performed for 30 minutes or more. After melting, continuous casting is performed. The obtained slab is heated to 1100 to 1350 ° C., and the cumulative rolling reduction at 1000 ° C. or less is 10 %, The web thickness or the flange thickness is 40 to 150 mm, and cooling is performed to a temperature range of 400 to 700 ° C. at a cooling rate within a range of 0.1 to 5 ° C./s. The manufacturing method of the high strength extra-thick H-section steel.
（５）仕上圧延で、表面温度が７００℃以下になるように水冷し、復熱過程で圧延するパス間水冷圧延加工を１パス以上行うことを特徴とする上記（４）に記載の高強度極厚Ｈ形鋼の製造方法。
（６）冷却後、４００～５００℃の温度域に加熱し、１５分～５時間保持し、再度冷却することを特徴とする上記（４）に記載の高強度極厚Ｈ形鋼の製造方法。
（７）冷却後、４００～５００℃の温度域に加熱し、１５分～５時間保持し、再度冷却することを特徴とする上記（５）に記載の高強度極厚Ｈ形鋼の製造方法。 (5) The high strength according to (4) above, wherein water cooling is performed so that the surface temperature is 700 ° C. or lower in finish rolling, and the water-cooled rolling process between passes is performed in the reheating process for one or more passes. Manufacturing method for extra-thick H-section steel.
(6) The method for producing a high-strength ultrathick H-section steel according to (4) above, wherein after cooling, heated to a temperature range of 400 to 500 ° C., held for 15 minutes to 5 hours, and cooled again .
(7) After cooling, heating to a temperature range of 400 to 500 ° C., holding for 15 minutes to 5 hours, and cooling again, The method for producing a high-strength ultrathick H-section steel according to (5) above .
　本発明によれば、靭性及び溶接性に優れた高強度極厚Ｈ形鋼を、調質熱処理を施すことなく、圧延ままで製造することが可能になる。その結果、施工コスト低減、工期の短縮による大幅なコスト削減を図ることができる。したがって、経済性を損なうことなく、大型建造物の信頼性が向上するなど、本発明は、産業上の貢献が極めて顕著である。 According to the present invention, it is possible to produce a high-strength ultra-thick H-shaped steel excellent in toughness and weldability as it is without being subjected to a tempering heat treatment. As a result, the construction cost can be reduced and the cost can be greatly reduced by shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without sacrificing economy.
ＨＡＺ靭性に及ぼす粗大酸化物個数密度の影響を示す図である。It is a figure which shows the influence of the coarse oxide number density which acts on HAZ toughness. 真空脱ガス処理と粗大酸化物個数密度との関係を示す図である。It is a figure which shows the relationship between a vacuum degassing process and coarse oxide number density. 本発明法を実施するＨ形鋼製造プロセスの概略を示す図である。It is a figure which shows the outline of the H-section steel manufacturing process which enforces this invention method. Ｈ形鋼の断面形状及び機械試験片の採取位置を示す図である。It is a figure which shows the cross-sectional shape of H-section steel, and the collection position of a mechanical test piece.
　引張強度が５５０ＭＰａ以上であり、肉厚が４０ｍｍ以上である本発明の高強度極厚Ｈ形鋼（以下、Ｈ形鋼という）は、靭性を向上させるために、微細な含Ｔｉ酸化物を利用し、結晶粒を微細化したものである。一方、鋼中に存在する酸化物が粗大であると、破壊の起点となり、靭性が低下する原因になる。本発明者らは、特に、粗大介在物を起点とした破壊機構による靭性低下が著しいことに着目した。まず、靭性確保のために除去すべき酸化物のサイズ、分布数密度の基準を明らかにした。次に、粗大な酸化物の除去方法について検討を行った。 The high-strength ultra-thick H-shaped steel (hereinafter referred to as H-shaped steel) of the present invention having a tensile strength of 550 MPa or more and a wall thickness of 40 mm or more uses a fine Ti-containing oxide to improve toughness. The crystal grains are made finer. On the other hand, if the oxide present in the steel is coarse, it becomes a starting point of fracture and causes toughness to decrease. The inventors of the present invention particularly focused on the remarkable decrease in toughness due to the fracture mechanism starting from coarse inclusions. First, the criteria for the size and distribution number density of oxides to be removed to ensure toughness were clarified. Next, a method for removing coarse oxides was examined.
　その結果、本発明者らは、高強度を有する厚鋼材、特に、極厚Ｈ形鋼の靭性を確保するには、Ｔｉを添加した後に、更に真空脱ガス処理を施し、粗大な介在物を減少させることが極めて有効であることを見出した。したがって、本発明においては、粗大な介在物が高密度で残存しないように、予備脱酸後、Ｔｉを添加し、更に、真空脱ガス処理を施して、溶鋼中の粗大な介在物を除去する対策を十分に施すことが必要である。 As a result, in order to ensure the toughness of thick steel materials having high strength, in particular, extra-thick H-shaped steel, the present inventors performed further vacuum degassing treatment after adding Ti to remove coarse inclusions. We have found that reducing is extremely effective. Therefore, in the present invention, Ti is added after preliminary deoxidation so that coarse inclusions do not remain at a high density, and further vacuum degassing treatment is performed to remove coarse inclusions in the molten steel. It is necessary to take sufficient measures.
　以下に、本発明の基礎となる靭性確保のために除去すべき酸化物のサイズ、分布数密度の基準について説明する。また、粗大な酸化物を除去するための真空脱ガスの条件について説明する。
　本発明者らは、鋼中のＴｉを含む酸化物のサイズや密度を変化させるために、種々の成分を有する鋼を、予備脱酸し、Ｔｉを添加し、更に真空脱ガスの時間を変化させて溶製した。鋼を溶製し、鋳造して得られた鋼片を熱間圧延し、鋼板を製造した。 Below, the reference | standard of the size of the oxide and the distribution number density which should be removed in order to ensure the toughness which becomes the foundation of this invention is demonstrated. The conditions for vacuum degassing for removing coarse oxides will be described.
In order to change the size and density of the oxide containing Ti in the steel, the present inventors pre-deoxidized steel having various components, added Ti, and further changed the vacuum degassing time. And melted. Steel pieces obtained by melting and casting steel were hot-rolled to produce steel plates.
　得られた鋼板から小片を採取して、溶接熱影響部（以下、ＨＡＺという。）の熱履歴を模擬した熱処理を施した。熱処理の条件は、昇温速度を１０℃／ｓとして１４００℃に加熱し、１ｓ保持した後、８００℃から５００℃までの冷却速度を１５℃／ｓとして冷却するものである。熱処理後、小片からＶノッチ試験片を採取し、ＪＩＳ　Ｚ　２２４２に準拠して０℃でシャルピー衝撃試験を行った。更に、破面及び金属組織を走査型電子顕微鏡（ＳＥＭ）で観察し、靭性に影響を及ぼす酸化物のサイズと密度について検討を行った。 Small pieces were collected from the obtained steel plate and subjected to heat treatment simulating the heat history of the weld heat affected zone (hereinafter referred to as HAZ). The heat treatment is performed at a heating rate of 10 ° C./s to 1400 ° C., maintained for 1 s, and then cooled at a cooling rate from 800 ° C. to 500 ° C. at 15 ° C./s. After the heat treatment, a V-notch test piece was collected from the small piece and subjected to a Charpy impact test at 0 ° C. in accordance with JIS Z 2242. Furthermore, the fracture surface and the metal structure were observed with a scanning electron microscope (SEM), and the size and density of the oxide affecting the toughness were examined.
　その結果、靭性が著しく低下した試験片の破面には、１０μｍ超の介在物が存在することがわかった。なお、ＳＥＭに付属するエネルギー分散型Ｘ線装置（ＥＤＸ）によって分析を行い、１０μｍ超の介在物が、Ｔｉを含有する酸化物であることを確認した。更に、金属組織のＳＥＭ写真から、１０μｍ超の含Ｔｉ酸化物の密度を測定した。図１に、１０μｍ超の含Ｔｉ酸化物の密度と、試験片の靭性との関係を示す。図１から、１０μｍ超の含Ｔｉ酸化物の密度を１０個／ｍｍ2以下にすれば、ばらつきはあるものの、０℃におけるシャルピー吸収エネルギーを５０Ｊ以上にすることができることがわかった。 As a result, it was found that inclusions exceeding 10 μm were present on the fracture surface of the test piece in which the toughness was significantly reduced. In addition, it analyzed by the energy dispersive X-ray apparatus (EDX) attached to SEM, and confirmed that the inclusion over 10 micrometers was an oxide containing Ti. Furthermore, from the SEM photograph of the metal structure, the density of the Ti-containing oxide exceeding 10 μm was measured. FIG. 1 shows the relationship between the density of the Ti-containing oxide exceeding 10 μm and the toughness of the test piece. From FIG. 1, it was found that if the density of the Ti-containing oxide exceeding 10 μm is 10 pieces / mm 2 or less, the Charpy absorbed energy at 0 ° C. can be 50 J or more, although there is variation.
　更に、１０μｍ超の含Ｔｉ酸化物の密度と、Ｔｉを添加した後の真空脱ガス時間との関係を図２に示す。図２における真空度は２０Ｔｏｒｒ以下である。図２から、１０μｍ超の含Ｔｉ酸化物の密度を１０個／ｍｍ2以下にするためには、真空脱ガス時間を３０分以上にすることが必要であることがわかった。更に、真空脱ガス処理の時間を３５分以上にすれば、粒径１０μｍ超の含Ｔｉ酸化物は、確実に１０個／ｍｍ2以下にすることができることがわかった。 Further, FIG. 2 shows the relationship between the density of the Ti-containing oxide exceeding 10 μm and the vacuum degassing time after adding Ti. The degree of vacuum in FIG. 2 is 20 Torr or less. From FIG. 2, it was found that the vacuum degassing time was required to be 30 minutes or more in order to make the density of the Ti-containing oxide exceeding 10 μm 10 pieces / mm 2 or less. Furthermore, it was found that the Ti-containing oxide having a particle diameter of more than 10 μm can be surely reduced to 10 pieces / mm 2 or less by setting the vacuum degassing treatment time to 35 minutes or more.
　また、Ｈ形鋼を溶接する際には、厚みの増加に伴って、溶接の入熱量を増加させる必要がある。そのため、特に、ＨＡＺでは、１４００℃への加熱によって結晶粒径が粗大化し、更に、加熱後の急冷によって硬質相の生成が促進される。したがって、ＨＡＺでは、含Ｔｉ酸化物の粒径の粗大化と硬質相の生成とに起因して、靭性の低下が顕著になる。これに対し、本発明では、１４００℃に加熱されても溶体化しない微細な含Ｔｉ酸化物を分散させているので、加熱による粒径の粗大化が抑制される。即ち、微細な含Ｔｉ酸化物は、溶接熱サイクルでの最高到達温度においてもピンニング効果を発現し、結晶粒の成長を抑制するので、ＨＡＺにおける結晶粒の粒径の粗大化が防止される。 Also, when welding H-section steel, it is necessary to increase the heat input of welding as the thickness increases. Therefore, in particular, in HAZ, the crystal grain size becomes coarse by heating to 1400 ° C., and further, the formation of a hard phase is promoted by rapid cooling after heating. Therefore, in HAZ, due to the coarsening of the particle size of the Ti-containing oxide and the generation of the hard phase, the toughness is significantly reduced. On the other hand, in this invention, since the fine Ti-containing oxide which does not become a solution even if it heats to 1400 degreeC is disperse | distributed, the coarsening of the particle size by heating is suppressed. That is, the fine Ti-containing oxide exhibits a pinning effect even at the highest temperature reached in the welding heat cycle and suppresses the growth of crystal grains, so that the grain size of the crystal grains in the HAZ is prevented from becoming coarse.
　微細な含Ｔｉ酸化物は、ＨＡＺだけでなく、Ｈ形鋼に含まれる結晶粒粒径の微細化にも極めて有効である。これは、Ｈ形鋼では、素材である鋼片から最終製品を製造するまでの間に、熱間圧延での加工量を確保できないためであり、熱間加工による再結晶を利用した細粒化が難しい。したがって、鋼片のミクロ組織の細粒化にも有効な、微細な含Ｔｉ酸化物による結晶粒界のピンニング効果は、極めて重要である。なお、鋼中に多数の微細な含Ｔｉ酸化物を分散させるには、鋼を溶製する製鋼工程において適正な脱酸処理、脱ガス処理を行い、Ｔｉ添加前の溶存酸素濃度を調整することが必要である。 The fine Ti-containing oxide is very effective not only for HAZ, but also for refining the grain size contained in H-section steel. This is because with H-section steel, it is not possible to secure the amount of processing by hot rolling until the final product is manufactured from the steel slab, which is the raw material. Fine graining using recrystallization by hot working Is difficult. Therefore, the pinning effect of the crystal grain boundaries by the fine Ti-containing oxide, which is effective for refining the microstructure of the steel slab, is extremely important. In order to disperse a large number of fine Ti-containing oxides in steel, appropriate deoxidation treatment and degassing treatment should be performed in the steelmaking process for melting steel to adjust the dissolved oxygen concentration before Ti addition. is required.
　以下、本発明の実施形態について順次説明する。
　まず、本実施形態のＨ形鋼の成分範囲の限定理由について述べる。 Hereinafter, embodiments of the present invention will be sequentially described.
First, the reason for limiting the component range of the H-section steel of this embodiment will be described.
　Ｃは、鋼の強化に有効な元素であり、構造用鋼として必要な強度を得るために、Ｃの含有量の下限値を０．０１％以上とする。一方、Ｃの量が０．０５％を超えると、島状マルテンサイトが生成し、特に、靭性が低下するため、Ｃの量の上限を０．０５％以下とする。母材及びＨＡＺの靱性、耐溶接割れ性を向上させるためには、Ｃの量の上限を０．０４％以下にすることが好ましい。 C is an element effective for strengthening steel, and in order to obtain the strength required for structural steel, the lower limit of the C content is 0.01% or more. On the other hand, if the amount of C exceeds 0.05%, island-shaped martensite is generated, and in particular, the toughness is lowered. Therefore, the upper limit of the amount of C is set to 0.05% or less. In order to improve the toughness and weld crack resistance of the base material and HAZ, the upper limit of the amount of C is preferably set to 0.04% or less.
　Ｓｉは、母材の強度確保、溶鋼の予備脱酸などに有用な元素であり、本実施形態では、Ｓｉの量の下限を０．０５％以上とする。しかし、Ｓｉの量が０．５０％を超えると、母材及びＨＡＺに島状マルテンサイトを生成し、母材及び溶接部の靱性が著しく低下する。したがって、Ｓｉの含有量の上限は、０．５０％以下とする。Ｓｉの量の下限は、好ましくは０．１％以上である。Ｓｉの量の上限は、好ましくは０．４５％以下、より好ましくは０．２％以下である。 Si is an element useful for securing the strength of the base metal and preliminary deoxidation of the molten steel. In this embodiment, the lower limit of the amount of Si is 0.05% or more. However, when the amount of Si exceeds 0.50%, island martensite is generated in the base material and the HAZ, and the toughness of the base material and the welded portion is significantly reduced. Therefore, the upper limit of the Si content is 0.50% or less. The lower limit of the amount of Si is preferably 0.1% or more. The upper limit of the amount of Si is preferably 0.45% or less, more preferably 0.2% or less.
　Ｍｎは、母材の強度を確保するため、０．８％以上の添加が必要である。母材の強度を高めるには、Ｍｎの量を１．０％以上にすることが好ましく、１．３％以上が更に好ましい。一方、２．０％を超えるＭｎを添加すると、母材及び溶接部の靱性、割れ性などを損なう。したがって、Ｍｎの量の上限を２．０％以下とする。 Mn needs to be added in an amount of 0.8% or more in order to ensure the strength of the base material. In order to increase the strength of the base material, the amount of Mn is preferably 1.0% or more, more preferably 1.3% or more. On the other hand, when Mn exceeding 2.0% is added, the toughness and cracking properties of the base material and the welded portion are impaired. Therefore, the upper limit of the amount of Mn is set to 2.0% or less.
　Ｃｕは、析出硬化によって母材の強化に寄与する元素である。０．３％以上のＣｕを添加すると、圧延時、フェライトが生成する温度域での保持及び緩冷却により、フェライトの転位上にＣｕ相が析出し、強度が上昇する。一方、１．２％超のＣｕを添加しても、析出強化は飽和する。したがって、Ｃｕの含有量を０．３～１．２％とする。Ｃｕのより好ましい含有量は０．４～１．０％である。 Cu is an element that contributes to strengthening of the base material by precipitation hardening. When 0.3% or more of Cu is added, a Cu phase precipitates on the dislocations of the ferrite and is increased in strength due to holding and slow cooling in a temperature range where ferrite is generated during rolling. On the other hand, the precipitation strengthening is saturated even if more than 1.2% of Cu is added. Therefore, the Cu content is set to 0.3 to 1.2%. A more preferable content of Cu is 0.4 to 1.0%.
　Ｎｉは、母材の強度及び靭性を高めるために、極めて有効な元素である。特に、靭性を高めるために、本実施形態では、Ｎｉの含有量を０．１％以上とする。一方、１．０％を超えるＮｉを添加しても効果が飽和する。したがって、Ｎｉの含有量の上限を１．０％以下とする。より好ましいＮｉの含有量の上限は０．８％以下である。 Ni is an extremely effective element for increasing the strength and toughness of the base material. In particular, in order to increase toughness, in this embodiment, the Ni content is set to 0.1% or more. On the other hand, the effect is saturated even if Ni exceeding 1.0% is added. Therefore, the upper limit of the Ni content is 1.0% or less. A more preferable upper limit of the Ni content is 0.8% or less.
　Ｔｉは、母材及びＨＡＺの靭性を向上させるために、本実施形態では極めて重要な元素である。Ｔｉは、微細な含Ｔｉ酸化物を形成して、結晶粒径の微細化に寄与するため、０．００５％以上を添加する。また、Ｔｉの添加によって形成される微細なＴｉＮは、結晶粒径の微細化にも寄与する。更に、Ｔｉは、固溶ＮをＴｉＮとして固定するため、島状マルテンサイトの生成の抑制にも有効である。したがって、Ｔｉを０．０１０％以上添加することが好ましい。一方、Ｔｉの量が０．０２５％を超えると、含Ｔｉ酸化物の粒径が粗大化し、母材及び溶接熱影響部の靱性を損なう。したがって、Ｔｉの量の上限を０．０２５％以下とする。また、ＴｉＣの析出を抑制し、析出硬化による靭性の低下を抑制するために、Ｔｉの量の上限を０．０２０％以下にすることが好ましい。 Ti is an extremely important element in the present embodiment in order to improve the toughness of the base material and the HAZ. Ti forms a fine Ti-containing oxide and contributes to refinement of the crystal grain size, so 0.005% or more is added. Further, the fine TiN formed by adding Ti contributes to the refinement of the crystal grain size. Furthermore, since Ti fixes solute N as TiN, it is also effective in suppressing the formation of island martensite. Therefore, it is preferable to add Ti by 0.010% or more. On the other hand, if the amount of Ti exceeds 0.025%, the particle size of the Ti-containing oxide becomes coarse, and the toughness of the base material and the weld heat affected zone is impaired. Therefore, the upper limit of the amount of Ti is set to 0.025% or less. Moreover, in order to suppress precipitation of TiC and suppress a decrease in toughness due to precipitation hardening, the upper limit of the amount of Ti is preferably set to 0.020% or less.
　Ｎｂは、焼入性を上昇させる元素であり、０．０１％以上を添加することが必要である。強度を向上させるためには、Ｎｂの含有量を０．０２％以上にすることが好ましい。一方、０．２５％を超えるＮｂを添加しても、Ｎｂの炭窒化物が析出し、固溶Ｎｂは増加せず、靭性を損なうことがある。したがって、Ｎｂの含有量の上限は０．２５％以下とする。靭性を高めるためには、Ｎｂの含有量を０．０８％以下にすることが好ましく、０．０４％以下が更に好ましい。 Nb is an element that increases hardenability, and it is necessary to add 0.01% or more. In order to improve the strength, the Nb content is preferably 0.02% or more. On the other hand, even if Nb exceeding 0.25% is added, Nb carbonitride precipitates, so that solid solution Nb does not increase, and the toughness may be impaired. Therefore, the upper limit of the Nb content is 0.25% or less. In order to increase toughness, the Nb content is preferably 0.08% or less, and more preferably 0.04% or less.
　Ｎは、微細なＴｉＮによって結晶粒を微細化するために、０．００１％以上を添加する。一方、Ｎが鋼中に固溶すると、上部ベイナイト組織において島状マルテンサイトを生成し、靱性を劣化させる。そのため、Ｎの量が０．００９％を超えた場合、固溶Ｎを固定するためにＴｉを添加すると、粗大な粒径のＴｉＮを生じて靭性が低下する。したがって、Ｎの量の上限を０．００９％以下とする。靭性を高めるには、Ｎの量を０．００５％以下にすることが好ましい。また、Ｎの量は好ましくは０．００４％以下がよい。 N is added in an amount of 0.001% or more in order to refine crystal grains with fine TiN. On the other hand, when N forms a solid solution in the steel, island martensite is generated in the upper bainite structure and the toughness is deteriorated. Therefore, when the amount of N exceeds 0.009%, if Ti is added to fix solid solution N, TiN having a coarse particle diameter is generated and the toughness is lowered. Therefore, the upper limit of the amount of N is set to 0.009% or less. In order to increase toughness, the amount of N is preferably 0.005% or less. The amount of N is preferably 0.004% or less.
　Ｏは、本実施形態では極めて重要な元素であり、含Ｔｉ酸化物を生成し、結晶粒の微細化に寄与する。しかし、Ｏ量が０．０００５％未満では、微細な含Ｔｉ酸化物によるピンニング効果が不十分であるため、Ｏの量の下限を０．０００５％以上とする。ＨＡＺにおける結晶粒の粒径の粗大化を抑制するには、Ｏの量を０．０００８％以上にすることが好ましい。一方、Ｏの量が０．００２０以上になると粗大な粒径の含Ｔｉ酸化物が増加し、ＨＡＺ靭性を損なう。したがって、Ｏの含有量の上限を０．００２０％未満とする。ＨＡＺ靭性を向上させるには、Ｏ量を０．００１５％以下にすることが好ましい。更に好ましいＯ量の上限は０．００１２％以下であり、最も好ましいＯ量の上限は０．０００９％以下である。 O is an extremely important element in the present embodiment, and forms a Ti-containing oxide and contributes to refinement of crystal grains. However, if the amount of O is less than 0.0005%, the pinning effect due to the fine Ti-containing oxide is insufficient, so the lower limit of the amount of O is made 0.0005% or more. In order to suppress the coarsening of the crystal grain size in the HAZ, the amount of O is preferably set to 0.0008% or more. On the other hand, when the amount of O becomes 0.0020 or more, the Ti-containing oxide having a coarse particle diameter increases, and the HAZ toughness is impaired. Therefore, the upper limit of the O content is less than 0.0020%. In order to improve the HAZ toughness, the O content is preferably 0.0015% or less. A more preferable upper limit of the O amount is 0.0012% or less, and a most preferable upper limit of the O amount is 0.0009% or less.
　Ａｌは、脱酸元素であるが、本実施形態では、微細な含Ｔｉ酸化物を生成させるために、Ａｌの量の上限を０．０２５％以下に制限する。また、Ａｌの量の低減は、島状マルテンサイトの生成の抑制にも有効であり、Ａｌの量の上限を０．０１０％以下にすることが好ましい。 Al is a deoxidizing element, but in the present embodiment, in order to generate a fine Ti-containing oxide, the upper limit of the amount of Al is limited to 0.025% or less. Further, the reduction of the amount of Al is effective for suppressing the formation of island martensite, and the upper limit of the amount of Al is preferably set to 0.010% or less.
　Ｂは、微量の添加で焼入性を上昇させる元素であるが、島状マルテンサイトの生成を促進するため、本実施形態では、Ｂの含有量を制限する。０．０００３％以上のＢを含有すると、上部ベイナイト組織中に島状マルテンサイトを生成し、靱性が著しく低下する。したがって、Ｂの含有量を０．０００３％未満に制限する。Ｂの量は、０．０００１％以下に制限することが更に好ましい。 B is an element that increases the hardenability by adding a small amount, but in this embodiment, the B content is limited to promote the formation of island martensite. When 0.0003% or more of B is contained, island martensite is generated in the upper bainite structure, and the toughness is remarkably lowered. Therefore, the B content is limited to less than 0.0003%. More preferably, the amount of B is limited to 0.0001% or less.
　不可避不純物として含有するＰ、Ｓについては、含有量を特に限定しない。なお、Ｐ、Ｓは、凝固偏析による溶接割れ、靱性低下の原因となるので、極力低減すべきである。Ｐの量は０．０２％以下に制限することが好ましく、更に好ましい上限は０．００２％以下である。また、Ｓの量の含有量は、０．００２％以下に制限することが好ましい。 The content of P and S contained as inevitable impurities is not particularly limited. In addition, since P and S cause weld cracking due to solidification segregation and a decrease in toughness, they should be reduced as much as possible. The amount of P is preferably limited to 0.02% or less, and a more preferable upper limit is 0.002% or less. The content of S is preferably limited to 0.002% or less.
　更に、強度及び靱性の向上や、介在物の形態の制御を目的として、Ｖ、Ｍｏ、Ｃｒ、Ｍｇのうちの１種又は２種以上を含有させてもよい。 Furthermore, for the purpose of improving strength and toughness and controlling the form of inclusions, one or more of V, Mo, Cr, and Mg may be contained.
　Ｖは、組織の微細化及びＶの炭窒化物による析出強化に寄与する。しかし、Ｖを過剰に添加すると、靭性を損なうことがある。したがって、Ｖの量の上限を０．１％とすることが好ましい。 V contributes to refinement of the structure and precipitation strengthening by V carbonitride. However, when V is added excessively, the toughness may be impaired. Therefore, the upper limit of the amount of V is preferably set to 0.1%.
　Ｍｏは、鋼中に固溶して焼入れ性を高める元素であり、強度の向上に寄与する。しかし、０．３％以上のＭｏを含有させても、Ｍｏの炭化物（Ｍｏ2Ｃ）を析出し、固溶Ｍｏによる焼入性の向上の効果は飽和する。したがって、Ｍｏの量の上限は、０．３％未満にすることが好ましい。Ｍｏの量の上限はより好ましくは０．２％以下であり、更に好ましくは０．１％以下である。なお、Ｍｏは本発明の成分設計においては本質的に不要な元素であるため、積極に添加する必要はなく、他の元素の添加によって強度が十分に確保できるのであれば、Ｍｏは添加しなくても良い。 Mo is an element that dissolves in steel and enhances hardenability, and contributes to improvement in strength. However, even if 0.3% or more of Mo is contained, Mo carbide (Mo 2 C) is precipitated, and the effect of improving the hardenability by solute Mo is saturated. Therefore, the upper limit of the amount of Mo is preferably less than 0.3%. The upper limit of the amount of Mo is more preferably 0.2% or less, and still more preferably 0.1% or less. In addition, Mo is an element that is essentially unnecessary in the component design of the present invention, so it is not necessary to add it actively. If the strength can be sufficiently secured by adding other elements, Mo is not added. May be.
　Ｃｒは、焼入性を向上させる元素であり、強化に寄与する。しかし、Ｃｒを過剰に添加すると、靱性を損なうことがある。したがって、Ｃｒの量の上限は、１．５％とすることが好ましい。Ｃｒのより好ましい上限は１．２％であり、更に好ましい上限は０．８％である。 Cr is an element that improves hardenability and contributes to strengthening. However, excessive addition of Cr may impair toughness. Therefore, the upper limit of the Cr amount is preferably 1.5%. A more preferable upper limit of Cr is 1.2%, and a more preferable upper limit is 0.8%.
　Ｍｇは、脱酸元素であり、鋼中に含まれるＭｇ系酸化物は微細になる。Ｍｇは、特に、ＨＡＺ組織における結晶粒の粗大化の抑制に有効である。したがって、ＨＡＺ靭性を向上させるために、０．００１％以上を含有させることが好ましい。なお、Ｍｇの酸化物は溶鋼中で容易に浮上するため、鋼中に含有されるＭｇの上限は、０．００５％以下であり、好ましくは０．００３％以下である。また、Ｍｇ系酸化物は、Ｔｉ、Ａｌ、Ｃａなどとの複合酸化物を形成する。Ｍｇを添加する際には、Ｓｉ－Ｍｇ－Ａｌ及びＮｉ－ＭｇなどのＭｇ合金を用いることが好ましい。 Mg is a deoxidizing element, and the Mg-based oxide contained in the steel becomes fine. Mg is particularly effective in suppressing the coarsening of crystal grains in the HAZ structure. Therefore, in order to improve HAZ toughness, it is preferable to contain 0.001% or more. In addition, since the oxide of Mg easily floats in the molten steel, the upper limit of Mg contained in the steel is 0.005% or less, preferably 0.003% or less. The Mg-based oxide forms a composite oxide with Ti, Al, Ca, and the like. When adding Mg, Mg alloys such as Si—Mg—Al and Ni—Mg are preferably used.
　Ｚｒ、Ｈｆは脱酸元素であるとともに、溶鋼中で窒化物を生成し、鋼中の固溶Ｎ量の低減に有効である。固溶Ｎの低減により、上部ベイナイト組織において島状マルテンサイトの生成を抑制することができる。
　また、ＲＥＭ、Ｃａは、脱酸元素であり、硫化物の形態の制御にも寄与する。　しかし、Ｚｒ、Ｈｆ、ＲＥＭ及びＣａはいずれも強い脱酸元素であり、これらの元素の添加によって、含Ｔｉ酸化物を粗大化させるおそれがある。従って、Ｚｒ、Ｈｆ、ＲＥＭ及びＣａは積極的に添加しないほうがよい。 Zr and Hf are deoxidizing elements and generate nitrides in the molten steel, which is effective in reducing the amount of solute N in the steel. By reducing the solid solution N, generation of island martensite in the upper bainite structure can be suppressed.
REM and Ca are deoxidizing elements and contribute to the control of the form of sulfide. However, Zr, Hf, REM, and Ca are all strong deoxidizing elements, and the addition of these elements may coarsen the Ti-containing oxide. Therefore, it is better not to positively add Zr, Hf, REM and Ca.
　次に、本実施形態のＨ形鋼のミクロ組織について説明する。本実施形態のＨ形鋼のミクロ組織は、ベイナイト、島状マルテンサイト、フェライト・パーライトからなる。 Next, the microstructure of the H-section steel of this embodiment will be described. The microstructure of the H-section steel of this embodiment consists of bainite, island martensite, and ferrite pearlite.
　ベイナイトは、強度の上昇及び組織の微細化に寄与する。しかし、ベイナイトの面積率が１０％未満では、強度が不十分になる。一方、ベイナイトの面積率が４０％を超えると靭性が低下する。したがって、ベイナイトの面積率は、１０～４０％、好ましくは２０～４０％とする。 Bainite contributes to increased strength and refinement of the structure. However, when the area ratio of bainite is less than 10%, the strength is insufficient. On the other hand, if the area ratio of bainite exceeds 40%, the toughness decreases. Therefore, the area ratio of bainite is 10 to 40%, preferably 20 to 40%.
　島状マルテンサイトは、破壊の起点となり、靭性を低下させる。したがって、島状マルテンサイトの面積率は、０．５％以下、好ましくは０．３％以下、更に好ましくは０．２％以下に制限する。なお、ベイナイト、島状マルテンサイトの残部はフェライト・パーライトある。 Island-like martensite is the starting point of fracture and reduces toughness. Therefore, the area ratio of island martensite is limited to 0.5% or less, preferably 0.3% or less, and more preferably 0.2% or less. The remainder of bainite and island martensite is ferrite pearlite.
　次に、含Ｔｉ酸化物について説明する。本実施形態において、含Ｔｉ酸化物の粒径及び密度の制御は、母材及びＨＡＺの結晶粒の微細化による靭性の向上のために、極めて重要である。また、含Ｔｉ酸化物は、窒化物の生成核としても機能し、ＴｉＮなど、高温で生成する窒化物によるＮの固定を促進し、ＮｂＮやＢＮの析出を抑制する。その結果、Ｎｂ、Ｂによる焼入れ性の向上効果を最大限に発揮させることが可能となるため、含Ｔｉ酸化物は、強度の向上にも間接的に寄与する。 Next, the Ti-containing oxide will be described. In the present embodiment, control of the particle size and density of the Ti-containing oxide is extremely important for improving toughness by refining the base material and HAZ crystal grains. The Ti-containing oxide also functions as a nitride nucleation, promotes the fixation of N by a nitride generated at a high temperature, such as TiN, and suppresses the precipitation of NbN and BN. As a result, the effect of improving the hardenability by Nb and B can be maximized, so that the Ti-containing oxide indirectly contributes to the improvement of strength.
　本実施形態において、含Ｔｉ酸化物とは、Ｘ線マイクロアナライザー（ＥＰＭＡ）によってＴｉとＯを同時に含むことが確認され、かつ１個体中に含まれる酸素以外の元素の５０質量％以上がＴｉであるものをいう。含Ｔｉ酸化物は、好ましくは１個体中に含まれる酸素以外の元素の７０質量％以上がＴｉであるものがよく、更に好ましくは８０質量％以上がＴｉであるものがよい。具体的には、ＴｉＯ、ＴｉＯ2、Ｔｉ2Ｏ3などのＴｉ系酸化物、及びこれらのＴｉ系酸化物とＴｉ系酸化物以外の酸化物との複合酸化物、さらにこれらのＴｉ系酸化物や複合酸化物と硫化物との複合介在物の総称である。Ｔｉ以外の元素を含む酸化物には、ＳｉＯ2などのＳｉ系酸化物、Ａｌ2Ｏ3などのＡｌ系酸化物、その他、Ｍｇ系酸化物、Ｃａ系酸化物などを挙げることができる。なお、Ｔｉ系酸化物とＳｉ系酸化物、Ａｌ系酸化物、Ｍｇ系酸化物、Ｃａ系酸化物などとの複合酸化物や、Ｔｉ系酸化物を生成核として析出するＭｎＳなどの硫化物を伴う複合介在物は、１個体として取り扱うものとする。 In this embodiment, the Ti-containing oxide is confirmed to contain Ti and O at the same time by an X-ray microanalyzer (EPMA), and 50% by mass or more of elements other than oxygen contained in one solid is Ti. Say something. The Ti-containing oxide is preferably such that 70% by mass or more of an element other than oxygen contained in one solid is Ti, more preferably 80% by mass or more is Ti. Specifically, Ti-based oxides such as TiO, TiO 2 , and Ti 2 O 3 , composite oxides of these Ti-based oxides and oxides other than Ti-based oxides, and these Ti-based oxides It is a general term for composite inclusions of composite oxides and sulfides. Examples of oxides containing elements other than Ti include Si-based oxides such as SiO 2 , Al-based oxides such as Al 2 O 3 , Mg-based oxides, and Ca-based oxides. It should be noted that composite oxides of Ti-based oxides and Si-based oxides, Al-based oxides, Mg-based oxides, Ca-based oxides, and sulfides such as MnS that are deposited using Ti-based oxides as production nuclei. The accompanying complex inclusions shall be handled as one individual.
　含Ｔｉ酸化物は、金属組織をＳＥＭによって観察し、ＥＤＸによって酸化物に含まれる元素を同定することによって、粒径及び密度を測定することができる。また、Ｘ線マイクロアナライザー（ＥＰＭＡ）によってＴｉとＯを含む介在物を検出し、画像解析や組織写真との照合を行うことにより、含Ｔｉ酸化物の粒径及び密度を測定しても良い。０．５ｍｍ×０．５ｍｍの範囲、又はそれ以上の視野で、かつ５０粒子程度の粒子の平均粒径および粒子数密度を求める。なお、含Ｔｉ酸化物の粒径は、組織写真における最大の径である。 The particle size and density of the Ti-containing oxide can be measured by observing the metal structure by SEM and identifying the elements contained in the oxide by EDX. Alternatively, the particle size and density of the Ti-containing oxide may be measured by detecting inclusions containing Ti and O with an X-ray microanalyzer (EPMA) and collating them with image analysis and structural photographs. An average particle diameter and a particle number density of about 50 particles are obtained in a field of view of 0.5 mm × 0.5 mm or more. The particle diameter of the Ti-containing oxide is the maximum diameter in the structure photograph.
　粒径が０．０５μｍ以上、１０μｍ以下の含Ｔｉ酸化物は、上述のように、結晶粒界をピンニングして粒成長を遅延させ、母材及びＨＡＺの結晶粒の微細化に寄与する。含Ｔｉ酸化物の粒径が０．０５μｍ未満では、ピンニング効果は得られないが、特に、靭性を低下させる原因にはならない。 As described above, the Ti-containing oxide having a particle size of 0.05 μm or more and 10 μm or less pins the crystal grain boundary, delays the grain growth, and contributes to refinement of the base material and HAZ crystal grains. If the particle size of the Ti-containing oxide is less than 0.05 μm, the pinning effect cannot be obtained, but in particular, it does not cause a decrease in toughness.
　粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度は、３０個／ｍｍ2未満ではピンニング効果が不十分である。一方、粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度が３００個／ｍｍ2を超えると、亀裂の進展の経路になるので、靭性が低下する。したがって、ＨＡＺの靭性を向上させるには、粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度を３０～３００個／ｍｍ2とすることが必要である。粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度は、Ｔｉを添加する前の溶存酸素量、Ｔｉの添加量を調整することによって、制御することができる。粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度の下限は、好ましくは５０個／ｍｍ2以上、より好ましくは６０個／ｍｍ2以上、更に好ましくは１００個／ｍｍ2以上である。また、粒径が０．０５～１０μｍの含Ｔｉ酸化物の密度の上限は、好ましくは２００個／ｍｍ2以下、より好ましくは１６０個／ｍｍ2以下である。 If the density of the Ti-containing oxide having a particle size of 0.05 to 10 μm is less than 30 / mm 2 , the pinning effect is insufficient. On the other hand, if the density of the Ti-containing oxide having a particle size of 0.05 to 10 μm exceeds 300 pieces / mm 2 , it becomes a path of crack growth, so that toughness decreases. Therefore, in order to improve the toughness of HAZ, it is necessary to set the density of the Ti-containing oxide having a particle size of 0.05 to 10 μm to 30 to 300 / mm 2 . The density of the Ti-containing oxide having a particle size of 0.05 to 10 μm can be controlled by adjusting the amount of dissolved oxygen and the amount of Ti added before adding Ti. The lower limit of the density of the Ti-containing oxide having a particle size of 0.05 to 10 μm is preferably 50 pieces / mm 2 or more, more preferably 60 pieces / mm 2 or more, still more preferably 100 pieces / mm 2 or more. The upper limit of the density of the Ti-containing oxide having a particle size of 0.05 to 10 μm is preferably 200 pieces / mm 2 or less, more preferably 160 pieces / mm 2 or less.
　一方、含Ｔｉ酸化物の粒径が１０μｍを超えると、上述のように、破壊の起点となり、密度が１０個／ｍｍ2を超えると母材及びＨＡＺの靭性が低下する。したがって、粒径が１０μｍ超の含Ｔｉ酸化物の密度を１０個／ｍｍ2以下、好ましくは７個／ｍｍ2以下、更に好ましくは５個／ｍｍ2以下に制限することが必要である。粒径が１０μｍ超である含Ｔｉ酸化物の密度は、真空脱ガス処理の時間によって、制御することができる。 On the other hand, when the particle size of the Ti-containing oxide exceeds 10 μm, as described above, it becomes a starting point of fracture, and when the density exceeds 10 pieces / mm 2 , the toughness of the base material and the HAZ decreases. Therefore, it is necessary to limit the density of the Ti-containing oxide having a particle size of more than 10 μm to 10 pieces / mm 2 or less, preferably 7 pieces / mm 2 or less, more preferably 5 pieces / mm 2 or less. The density of the Ti-containing oxide having a particle size of more than 10 μm can be controlled by the time of vacuum degassing treatment.
　本実施形態のＨ形鋼の板厚は、４０ｍｍ以上とする。これは、超高層建築物などの柱に用いられるＨ形鋼には、板厚が４０ｍｍ以上の大きなサイズのＨ形鋼が採用されるためである。一方、鋳片の肉厚の制限などから、工業的に靭性を確保するため、本実施形態のＨ形鋼の板厚の上限は、１５０ｍｍ以下とする。 The plate thickness of the H-section steel of this embodiment is 40 mm or more. This is because a H-shaped steel having a plate thickness of 40 mm or more is employed as the H-shaped steel used for a column such as a super high-rise building. On the other hand, the upper limit of the thickness of the H-section steel of the present embodiment is set to 150 mm or less in order to ensure toughness industrially due to the limitation of the thickness of the slab.
　Ｈ形鋼を構造部材として用いる際の機械特性の目標値は、常温の降伏点もしくは０．２％耐力４５０ＭＰａ以上、引張強度５５０ＭＰａ以上（ＡＳＴＭ規格グレード６５相当）である。さらに好ましくは、常温の降伏点もしくは０．２％耐力３４５ＭＰａ以上、引張強度４５０ＭＰａ以上（ＡＳＴＭ規格グレード５０相当）である。また、０℃でのシャルピー衝撃吸収エネルギーは、母材部４７Ｊ以上、ＨＡＺ部４７Ｊ以上である。 The target values of mechanical properties when using H-shaped steel as a structural member are the yield point at room temperature or 0.2% proof stress 450 MPa or more, and tensile strength 550 MPa or more (equivalent to ASTM standard grade 65). More preferably, the yield point at normal temperature or the 0.2% proof stress is 345 MPa or more and the tensile strength is 450 MPa or more (equivalent to ASTM standard grade 50). Further, the Charpy impact absorption energy at 0 ° C. is the base material portion 47J or more and the HAZ portion 47J or more.
　特に、Ｈ形鋼は、鋼板を製造する場合よりも強度、靭性を確保することが難しい。これは、スラブまたはビームブランク形状の素材からＨ形鋼を製造する際に、フランジのみならず、フィレット部（フランジとウェブが結合している部位）の加工量を確保することが難しいためである。 In particular, it is difficult for H-shaped steel to ensure strength and toughness compared to the case of manufacturing a steel plate. This is because when manufacturing H-section steel from a slab or beam blank-shaped material, it is difficult to ensure the processing amount of not only the flange but also the fillet portion (portion where the flange and the web are bonded). .
　なお、フランジ／ウェブの板厚比に関してはＨ形鋼を熱間圧延で製造する場合を想定して、０．５～２．０とすることが好ましい。フランジ／ウェブの板厚比が２．０を超えると、ウェブが波打ち状の形状に変形することがある。一方、フランジ／ウェブの板厚比が０．５未満の場合は、フランジが波打ち状の形状に変形することがある。 The flange / web thickness ratio is preferably set to 0.5 to 2.0 assuming that the H-shaped steel is manufactured by hot rolling. If the flange / web thickness ratio exceeds 2.0, the web may be deformed into a wavy shape. On the other hand, when the flange / web plate thickness ratio is less than 0.5, the flange may be deformed into a wavy shape.
　次に、本実施形態のＨ形鋼の製造方法について説明する。 Next, the manufacturing method of the H-section steel of this embodiment will be described.
　本実施形態では、微細な含Ｔｉ酸化物を生成させ、粗大な含Ｔｉ酸化物の生成を抑制するために、鋼を溶製する製鋼工程が極めて重要である。特に、脱酸は重要であり、Ｔｉ添加前の溶存酸素量を適正な範囲に制御し、Ｔｉ添加後、真空脱ガス処理を適正な条件で行うことが必要である。 In this embodiment, in order to produce fine Ti-containing oxides and suppress the production of coarse Ti-containing oxides, a steelmaking process for melting steel is extremely important. In particular, deoxidation is important, and it is necessary to control the amount of dissolved oxygen before adding Ti to an appropriate range and to perform vacuum degassing treatment under appropriate conditions after adding Ti.
　まず、微細な含Ｔｉ酸化物を生成させるためには、Ｔｉ添加前の溶存酸素の量を制御することが重要である。Ｔｉ添加前の溶存酸素量は、Ｓｉ、Ｍｎなどの脱酸元素や、選択的に添加されるＡｌ、Ｍｇの添加量によって制御することができる。Ｔｉ添加前の溶存酸素が、質量％で、０．００３％未満であると、粒径が１０μｍ以下の含Ｔｉ酸化物の生成量が不十分になる。一方、Ｔｉ添加前の溶存酸素が０．０１５％超であると、粒径が１０μｍを超える粗大な含Ｔｉ酸化物が増加する。 First, in order to generate a fine Ti-containing oxide, it is important to control the amount of dissolved oxygen before Ti addition. The amount of dissolved oxygen prior to the addition of Ti can be controlled by the amount of deoxidizing elements such as Si and Mn, and selectively added Al and Mg. If the dissolved oxygen before addition of Ti is less than 0.003% by mass, the amount of Ti-containing oxide having a particle size of 10 μm or less becomes insufficient. On the other hand, if the dissolved oxygen before addition of Ti exceeds 0.015%, coarse Ti-containing oxides having a particle size exceeding 10 μm increase.
　また、溶存酸素が多いと、後に続く真空脱ガス処理を行う際に、粗大酸化物を十分に低減させるのに必要な処理時間が長くなる。そのため、溶存酸素が多いと、製造コストが高くなるだけでなく、粒径が１０μｍ以下の含Ｔｉ酸化物の密度も低下する。したがって、Ｔｉを添加する前に、予備脱酸処理によって溶存酸素を０．００３～０．０１５質量％に調整することが必要である。 Also, if there is a large amount of dissolved oxygen, the processing time required to sufficiently reduce the coarse oxide will be prolonged when the subsequent vacuum degassing process is performed. For this reason, when the amount of dissolved oxygen is large, not only the production cost is increased, but also the density of the Ti-containing oxide having a particle size of 10 μm or less is lowered. Therefore, it is necessary to adjust the dissolved oxygen to 0.003 to 0.015 mass% by preliminary deoxidation treatment before adding Ti.
　製鋼工程では、上述のように、適正な条件でＴｉを添加し、溶鋼の化学成分を調整した後、真空脱ガス処理を行う。上述のように、粒径が１０μｍ以下の含Ｔｉ酸化物の密度を１０個／ｍｍ2以下にするためには、真空脱ガス処理の時間を３０分以上にすることが必要である。また、効率良く、粗大な含Ｔｉ酸化物を減少させるには、真空脱ガス処理の真空度を２０Ｔｏｒｒ以下、好ましくは５Ｔｏｒｒ以下にするとよい。さらに、靭性を向上させるためには、真空脱ガス処理を真空度５Ｔｏｒｒ以下で３５分以上行うことが望ましい。なお、処理時間の上限は、製造コストの上昇を抑えるために６０分以下とすることが好ましい。 In the steel making process, as described above, after adding Ti under appropriate conditions and adjusting the chemical components of the molten steel, vacuum degassing treatment is performed. As described above, in order to make the density of the Ti-containing oxide having a particle size of 10 μm or less 10 pieces / mm 2 or less, it is necessary to set the vacuum degassing treatment time to 30 minutes or more. Further, in order to efficiently reduce the coarse Ti-containing oxide, the degree of vacuum in the vacuum degassing process is set to 20 Torr or less, preferably 5 Torr or less. Furthermore, in order to improve toughness, it is desirable to perform the vacuum degassing treatment at a degree of vacuum of 5 Torr or less for 35 minutes or more. The upper limit of the processing time is preferably 60 minutes or less in order to suppress an increase in manufacturing cost.
　鋼を溶製した後、鋳造し、鋼片を得る。鋳造は、生産性の観点から、連続鋳造が好ましい。また、鋼片の厚みは、生産性の観点から、２００ｍｍ以上とすることが好ましく、偏析の低減や、熱間圧延における加熱温度の均質性などを考慮すると、３５０ｍｍ以下が好ましい。 After melting the steel, it is cast to obtain a steel piece. The casting is preferably continuous casting from the viewpoint of productivity. The thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.
　次に、鋼片を加熱し、熱間圧延を行う。鋼片の加熱温度は１１００～１３５０℃の範囲内とする。加熱温度が１１００℃未満であると、変形抵抗が高くなる。Ｎｂなど、炭化物、窒化物を形成する元素を十分に固溶させるため、再加熱温度の下限を１１５０℃以上とすることが好ましい。特に、板厚が薄い場合は、累積圧下率が大きくなるため、１２００℃以上に加熱することが好ましい。一方、加熱温度が１３５０℃よりも高温である場合は、素材である鋼片の表面のスケールが液体化して炉内が損傷することにより、経済的なメリットが薄れてしまう。そのため、熱間加工の加熱温度の上限は１３５０℃とする。組織の粗大化を抑制するためには、加熱温度の上限を１３００℃以下にすることが好ましい。 Next, the steel slab is heated and hot rolled. The heating temperature of the steel slab is in the range of 1100 to 1350 ° C. When the heating temperature is less than 1100 ° C., deformation resistance increases. In order to sufficiently dissolve elements that form carbides and nitrides such as Nb, the lower limit of the reheating temperature is preferably set to 1150 ° C. or higher. In particular, when the plate thickness is thin, the cumulative rolling reduction increases, so heating to 1200 ° C. or higher is preferable. On the other hand, when the heating temperature is higher than 1350 ° C., the scale of the surface of the steel slab, which is the raw material, is liquefied and the inside of the furnace is damaged, so that the economic merit is reduced. Therefore, the upper limit of the heating temperature for hot working is set to 1350 ° C. In order to suppress the coarsening of the structure, the upper limit of the heating temperature is preferably 1300 ° C. or lower.
　熱間圧延の仕上圧延では、制御圧延及び制御冷却を行う。制御圧延及び制御冷却は、圧延温度、圧下率、冷却速度を制御する製造方法である。仕上圧延では、パス間で水冷する圧延加工を１パス以上施すことが好ましい。パス間で水冷する圧延加工は、水冷し、復熱過程で圧延する製造方法である。仕上圧延後、熱処理を施すことが更に好ましい。また、一次圧延して５００℃以下に冷却した後、再度、１１００～１３５０℃に加熱し、二次圧延を行う製造するプロセス、いわゆる２ヒート圧延を採用してもよい。２ヒート圧延では、熱間圧延での塑性変形量が少なく、圧延工程での温度の低下も小さくなるため、加熱温度を低めにすることができる。 In the finish rolling of hot rolling, controlled rolling and controlled cooling are performed. Control rolling and controlled cooling are manufacturing methods for controlling the rolling temperature, the rolling reduction, and the cooling rate. In finish rolling, it is preferable to perform one or more passes of rolling that is water-cooled between passes. The rolling process in which water cooling is performed between passes is a manufacturing method in which water cooling is performed and rolling is performed in a recuperation process. More preferably, heat treatment is performed after finish rolling. Alternatively, a so-called two-heat rolling process may be employed in which primary rolling is performed to cool to 500 ° C. or lower and then heating is performed again to 1100 to 1350 ° C. to perform secondary rolling. In the two-heat rolling, the amount of plastic deformation in the hot rolling is small, and the temperature drop in the rolling process is also small, so that the heating temperature can be lowered.
　熱間圧延の仕上圧延は、鋼片を加熱した後、１０００℃以下での累積圧下率が１０％以上となるように制御圧延を行うことが必要である。これは、熱間圧延で、加工再結晶を促進させ、オーステナイトを細粒化し、靭性と強度を向上させるためである。なお、鋼片の厚みと製品の厚みに応じて、仕上圧延の前に粗圧延を行っても良い。２ヒート圧延を採用する場合は、二次圧延の１０００℃以下での累積圧下率を１０％以上とすることが必要である。 In the finish rolling of hot rolling, it is necessary to perform controlled rolling so that the cumulative rolling reduction at 1000 ° C. or lower is 10% or higher after heating the steel slab. This is because hot rolling promotes work recrystallization, refines austenite, and improves toughness and strength. Depending on the thickness of the steel slab and the thickness of the product, rough rolling may be performed before finish rolling. In the case of adopting two-heat rolling, it is necessary to set the cumulative rolling reduction at 1000 ° C. or lower in secondary rolling to 10% or more.
　制御圧延後、制御冷却を行う。制御冷却は、４００～７００℃の温度域までの平均冷却速度を０．１～５℃／ｓとする。この制御冷却により、ベイナイトの面積率が１０～４０％、島状マルテンサイトの面積率が０．５％以下となり、強度及び靭性を向上させることができる。冷却停止温度が７００℃を超えた場合や、冷却速度が０．１℃／ｓ未満である場合は、結晶粒が成長したり、ベイナイトの面積率が低下することがある。一方、冷却停止温度が４００℃未満である場合や、冷却速度が１０℃／ｓを超えた場合は、ベイナイトの面積率が４０％を超え、島状マルテンサイトの面積率が０．５％を超え、靱性が低下することがある。 Control cooling is performed after controlled rolling. In the controlled cooling, the average cooling rate up to a temperature range of 400 to 700 ° C. is set to 0.1 to 5 ° C./s. By this controlled cooling, the area ratio of bainite is 10 to 40% and the area ratio of island martensite is 0.5% or less, and the strength and toughness can be improved. When the cooling stop temperature exceeds 700 ° C. or when the cooling rate is less than 0.1 ° C./s, crystal grains may grow or the area ratio of bainite may decrease. On the other hand, when the cooling stop temperature is less than 400 ° C. or when the cooling rate exceeds 10 ° C./s, the area ratio of bainite exceeds 40% and the area ratio of island martensite is 0.5%. And toughness may be reduced.
　仕上圧延のうち、１パス以上をパス間で水冷する圧延とすることが好ましい。パス間で水冷する圧延は、圧延のパス間で水冷することにより、フランジの表層部と内部とに温度差を付与して、圧延する方法である。パス間で水冷する圧延では、圧下率が小さい場合でも、板厚の内部まで加工歪みを導入することができる。また、水冷により圧延温度を短時間で低下させることによって、生産性も向上する。 Among finish rolling, it is preferable to perform rolling in which one or more passes are water-cooled between passes. Rolling that is water-cooled between passes is a method of rolling by giving a temperature difference between the surface layer portion and the inside of the flange by water-cooling between passes of rolling. In rolling with water cooling between passes, even when the rolling reduction is small, processing strain can be introduced to the inside of the plate thickness. Further, productivity is improved by lowering the rolling temperature in a short time by water cooling.
　パス間で水冷する圧延は、フランジの表面温度を７００℃以下に冷却した後、復熱過程で圧延する方法であり、表面の焼入れ硬化を抑制することができる。フランジの表面温度を７００℃以下に冷却し、複熱させると、オーステナイトとフェライトとが共存する温度域（γ／α二相共存温度域）での加工となる。その結果、細粒化されたオーステナイトと加工された微細なフェライトとの混合組織を形成する。これにより、表層部の焼入性を著しく低減でき、加速冷却により生じる表面層の硬化を防止できる。 Rolling with water cooling between passes is a method in which the surface temperature of the flange is cooled to 700 ° C. or lower and then rolled in the reheating process, and quenching and hardening of the surface can be suppressed. When the surface temperature of the flange is cooled to 700 ° C. or lower and double-heated, the processing is performed in a temperature range in which austenite and ferrite coexist (γ / α two-phase coexistence temperature range). As a result, a mixed structure of finely divided austenite and processed fine ferrite is formed. Thereby, the hardenability of the surface layer portion can be remarkably reduced, and hardening of the surface layer caused by accelerated cooling can be prevented.
　フランジの平均温度を４００℃以下まで冷却した後、４００～５００℃の温度域まで再び加熱してもよい。４００～５００℃に再加熱すると、圧延ままの状態でミクロ組織中に存在する島状マルテンサイトを分解させることができる。島状マルテンサイト中のＣをマトリクス中へ拡散させるためには、加熱温度を４００℃以上、保持時間を１５分以上にすることが好ましい。加熱温度の上限、保持時間の上限は特に規定しないが、製造コストの観点から、加熱温度を５００℃以下、保持時間を５時間以下にすることが好ましい。冷却後の再加熱は、熱処理炉で実施することができる。再加熱後の冷却の速度は特に限定されるものではなく、急冷してもよいし、放冷してもよい。 ¡After cooling the average temperature of the flange to 400 ° C or lower, it may be heated again to a temperature range of 400 to 500 ° C. When reheated to 400 to 500 ° C., the island-like martensite present in the microstructure can be decomposed as it is rolled. In order to diffuse C in the island martensite into the matrix, the heating temperature is preferably 400 ° C. or higher and the holding time is preferably 15 minutes or longer. Although the upper limit of the heating temperature and the upper limit of the holding time are not particularly defined, it is preferable that the heating temperature is 500 ° C. or less and the holding time is 5 hours or less from the viewpoint of manufacturing cost. Reheating after cooling can be performed in a heat treatment furnace. The cooling rate after reheating is not particularly limited, and may be rapidly cooled or allowed to cool.
　表１に示す成分組成を有する鋼を溶製し、連続鋳造により、厚みが２４０～３００ｍｍの鋼片を製造した。鋼の溶製は転炉で行い、一次脱酸し、合金成分を添加して、表２に示すように、溶存酸素濃度を調整した後、Ｔｉ脱酸処理を施して、更に、真空脱ガス処理を行った。真空脱ガス処理時の真空度は１０Ｔｏｒｒ以下とした。得られた鋼片を加熱し、熱間圧延を行い、冷却した。粗圧延工程の図示は省略するが、図３に示す、ユニバーサル圧延装置列で熱間圧延を行い、Ｈ形鋼を製造した。 Steel having the composition shown in Table 1 was melted, and a steel piece having a thickness of 240 to 300 mm was produced by continuous casting. Steel melting is performed in a converter, primary deoxidation is performed, alloy components are added, and after adjusting the dissolved oxygen concentration as shown in Table 2, Ti deoxidation treatment is performed, and vacuum degassing is further performed. Processed. The degree of vacuum during the vacuum degassing process was set to 10 Torr or less. The obtained steel slab was heated, hot-rolled and cooled. Although illustration of the rough rolling process is omitted, an H-section steel was manufactured by performing hot rolling with a universal rolling apparatus row shown in FIG.
　熱間圧延をパス間で水冷する圧延とする場合、圧延のパス間での水冷には、中間ユニバーサル圧延機１の前後に設けた水冷装置２ａを用い、フランジの外側の面のスプレー冷却とリバース圧延を行う。制御圧延後の制御冷却は、仕上ユニバーサル圧延機３で仕上圧延の終了後、後面に設置した冷却装置２ｂにより、フランジの外側の面を水冷して行った。製造条件を表３に示す。 When the hot rolling is rolling with water cooling between passes, water cooling between the rolling passes is performed by using water cooling devices 2a provided before and after the intermediate universal rolling mill 1, and spray cooling and reverse of the outer surface of the flange. Roll. Control cooling after controlled rolling was performed by cooling the outer surface of the flange with a cooling device 2b installed on the rear surface after finishing rolling by the finishing universal rolling mill 3. The production conditions are shown in Table 3.
　図４に示すように、Ｈ形鋼４のフランジ５の板厚ｔ2の中心部（１／２ｔ2）でフランジ幅の全長（Ｂ）の１／４　（１／４Ｂ）から、試験片を採取し、機械特性を測定した（図４の１／４Ｆ）。なお、これらの箇所の特性を求めたのは、図４のフランジ１／４Ｆ部が、Ｈ形鋼の平均的な機械特性を示すと判断したためである。引張試験は、ＪＩＳ　Ｚ　２２４１に準拠して行い、シャルピー衝撃試験は、ＪＩＳ　Ｚ　２２４２に準拠して０℃で行った。また、ＨＡＺの靭性は、溶接入熱量を約４００００Ｊ／ｃｍとして溶接を行い、ＨＡＺから試験片を採取して評価した。 As shown in FIG. 4, from 1/4 (1 / 4B) of the total length of the flange width at the center of the plate thickness t 2 of the flange 5 of the H-beams 4 (1 / 2t 2) ( B), a test piece Samples were collected and measured for mechanical properties (1 / 4F in FIG. 4). Note that the characteristics of these portions were obtained because it was determined that the flange 1 / 4F portion in FIG. 4 exhibited the average mechanical characteristics of the H-section steel. The tensile test was performed according to JIS Z 2241, and the Charpy impact test was performed at 0 ° C. according to JIS Z 2242. Further, the toughness of HAZ was evaluated by performing welding with a welding heat input of about 40,000 J / cm and collecting a test piece from the HAZ.
　結果を表４に示す。機械特性の目標値は、常温の降伏点又は０．２％耐力が４５０ＭＰａ以上、引張強度が５５０ＭＰａ以上（ＡＳＴＭ規格グレード６５相当）、かつ、０℃でのシャルピー衝撃吸収エネルギーが母材部で４７Ｊ以上、ＨＡＺ部で４７Ｊ以上である。 The results are shown in Table 4. The target values for mechanical properties are: yield point at room temperature or 0.2% proof stress of 450 MPa or higher, tensile strength of 550 MPa or higher (equivalent to ASTM standard grade 65), and Charpy impact absorption energy at 0 ° C. at 47 J As mentioned above, it is 47J or more in the HAZ part.
また、先に切り出した図４の１／４Ｆの試験片の表面を、ＥＰＭＡを用いて１００００倍以上の倍率で観察し、ＴｉとＯを含む含Ｔｉ酸化物を検出した。含Ｔｉ酸化物の検出結果と、組織写真との照合を行うことにより、含Ｔｉ酸化物のうち、最大径が０．０５μｍ～１０μｍの粒子と、最大径が１０μｍ超の粒子とについてそれぞれ密度を求めた。密度は５０個の粒子を計測対象として算出した。結果を表２に示す。 Moreover, the surface of the 1/4 F test piece cut out in FIG. 4 was observed at a magnification of 10,000 times or more using EPMA, and a Ti-containing oxide containing Ti and O was detected. By comparing the detection result of the Ti-containing oxide with the structure photograph, the density of each of the Ti-containing oxides having a maximum diameter of 0.05 μm to 10 μm and a particle having a maximum diameter of more than 10 μm is obtained. Asked. The density was calculated by measuring 50 particles. The results are shown in Table 2.
　なお、試験片の表面における介在物の化学組成は次の条件で測定し、Ｔｉ濃度が５０％以上のものを含Ｔｉ酸化物とした。まず、観察条件として、観察視野面積を２５×１０－２ｍｍ2， を５視野以上で観察を行い、粒子の分析個数を５０個程度とし、特性Ｘ線の波長分散分光により粒子中央部での成分組成を定量分析した。分析対象元素は、Ｔｉ、Ｓｉ、Ａｌ、Ｍｇ、ＭｎおよびＯ（酸素）とし、既知物質を用いて各元素の電子線強度と元素濃度の関係を予め検量線として求めておき、次いで、上記粒子から得られた電子線強度と予め前記検量線からその粒子の元素濃度を定量した。 The chemical composition of inclusions on the surface of the test piece was measured under the following conditions, and a Ti-containing oxide having a Ti concentration of 50% or more was used. First, as an observation condition, an observation visual field area of 25 × 10 −2 mm 2 is observed with 5 or more visual fields, the number of particles to be analyzed is about 50, and the central part of the particle is analyzed by wavelength dispersion spectroscopy of characteristic X-rays. The component composition was quantitatively analyzed. The analysis target elements are Ti, Si, Al, Mg, Mn, and O (oxygen), and the relationship between the electron beam intensity and the element concentration of each element is obtained in advance using a known substance as a calibration curve. From the electron beam intensity obtained from the above and the calibration curve, the elemental concentration of the particles was quantified.
　得られた定量結果のうちＴｉが５０％以上で酸素が５％以上の粒子を含Ｔｉ酸化物とし、単独酸化物として質量換算したものを平均したものを含Ｔｉ酸化物の平均組成とした。含Ｔｉ酸化物のＴｉ濃度を表２に示す。 Among the obtained quantitative results, particles having Ti of 50% or more and oxygen of 5% or more were regarded as Ti-containing oxides, and the average composition of those converted into mass as a single oxide was defined as the average composition of Ti-containing oxides. Table 2 shows the Ti concentration of the Ti-containing oxide.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　表４に示すように、本発明の製造Ｎｏ．１～１２は、常温の０．２％耐力及び引張強度が、それぞれ、目標の下限値である４５０ＭＰａ及び５５０ＭＰａ以上を満足している。さらに、０℃でのシャルピー衝撃吸収エネルギーは、母材部及びＨＡＺ部で４７Ｊ以上であることから、目標を十分に満たしている。 As shown in Table 4, the production No. of the present invention. In Nos. 1 to 12, the 0.2% yield strength and the tensile strength at room temperature satisfy the target lower limit values of 450 MPa and 550 MPa, respectively. Furthermore, the Charpy impact absorption energy at 0 ° C. is 47 J or more in the base material portion and the HAZ portion, and thus sufficiently satisfies the target.
　一方、比較例である製造Ｎｏ．１３～１６は、Ｔｉ添加前の溶存酸素濃度、真空脱ガス処理の一方又は双方が本発明の範囲外であり、含Ｔｉ酸化物の増加、粗大化によって靭性が低下している。また、製造Ｎｏ．１７は、制御冷却の冷却速度が遅く、ベイナイトが減少し、強度が低下している。製造Ｎｏ．１８は、１０００℃以下での累積圧下率が低く、強度及び靭性が低下している。 On the other hand, production No. which is a comparative example. In Nos. 13 to 16, one or both of the dissolved oxygen concentration and the vacuum degassing treatment before the addition of Ti are outside the scope of the present invention, and the toughness is lowered due to the increase in the size of Ti-containing oxides and the increase in the size. In addition, production No. In No. 17, the cooling rate of the controlled cooling is slow, bainite is reduced, and the strength is reduced. Production No. No. 18 has a low cumulative rolling reduction at 1000 ° C. or lower, and has reduced strength and toughness.
　製造Ｎｏ．１９～２９は、成分組成が本発明の範囲外である。製造Ｎｏ．１９はＣ量が少なく、製造Ｎｏ．２２はＭｎ量が少なく、製造Ｎｏ．２４はＣｕ量が少なく、強度が低下した例である。製造Ｎｏ．２０はＣ量が多く、強度が上昇し、また、島状マルテンサイトが増加して、靭性が低下した例である。製造Ｎｏ．２１は、Ｓｉ量が多く、島状マルテンサイトが増加して、靭性が低下した例である。製造Ｎｏ．２３はＭｎ量が多く、強度が上昇して、靭性が低下した例である。 Manufacturing No. Nos. 19 to 29 have component compositions outside the scope of the present invention. Production No. No. 19 has a small amount of C. No. 22 has a small amount of Mn. No. 24 is an example in which the amount of Cu is small and the strength is lowered. Production No. No. 20 is an example in which the amount of C is large, the strength is increased, the number of island martensites is increased, and the toughness is lowered. Production No. No. 21 is an example in which the amount of Si is large, island martensite is increased, and toughness is lowered. Production No. No. 23 is an example in which the amount of Mn is large, the strength is increased, and the toughness is lowered.
　製造Ｎｏ．２５はＴｉ量が少なく、製造Ｎｏ．２９はＡｌ量が多いため、微細な含Ｔｉ酸化物が減少し、靭性が低下した例である。製造Ｎｏ．２６はＴｉ量が多く、粗大な含Ｔｉ酸化物が増加し、靭性が低下した例である。製造Ｎｏ．２７は、Ｎ量が多く、ベイナイト及び島状マルテンサイトが増加して、靭性が低下した例である。製造Ｎｏ．２８はＢ量が多く、島状マルテンサイトが増加して、靭性が低下した例である。 Manufacturing No. No. 25 has a small amount of Ti. No. 29 is an example in which since the amount of Al is large, the fine Ti-containing oxide is reduced and the toughness is lowered. Production No. No. 26 is an example in which the amount of Ti is large, the coarse Ti-containing oxide is increased, and the toughness is lowered. Production No. 27 is an example in which the amount of N is large, bainite and island martensite are increased, and the toughness is lowered. Production No. No. 28 is an example in which the amount of B is large, the number of island martensite is increased, and the toughness is lowered.
　本発明は、靭性及び溶接性に優れた高強度な極厚のＨ形鋼を、調質熱処理を施すことなく、圧延ままで製造することが可能になる。その結果、施工コスト低減、工期の短縮による大幅なコスト削減を図ることができる。したがって、経済性を損なうことなく、大型建造物の信頼性が向上するなど、本発明は、産業上の貢献が極めて顕著である。 The present invention makes it possible to produce a high-strength, extremely thick H-section steel excellent in toughness and weldability as it is without being subjected to a tempering heat treatment. As a result, the construction cost can be reduced and the cost can be greatly reduced by shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without sacrificing economy.
　１　　中間圧延機
　２ａ　　中間圧延機前後面の水冷装置
　２ｂ　　仕上げ圧延機後面冷却装置
　３　　仕上げ圧延機
　４　　Ｈ形鋼
　５　　フランジ
　６　　ウェブ DESCRIPTION OF SYMBOLS 1 Intermediate rolling mill 2a Water cooling device of the front and rear surfaces of the intermediate rolling mill 2b Finishing rolling mill rear surface cooling device 3 Finishing rolling mill 4 H-section steel 5 Flange 6 Web

Claims (7)

  1. 　質量％で、
    Ｃ：０．０１～０．０５％、
    Ｓｉ：０．０５～０．５０％、
    Ｍｎ：０．８～２．０％、
    Ｃｕ：０．３～１．２％、
    Ｎｉ：０．１～１．０％、
    Ｔｉ：０．００５～０．０２５％、
    Ｎｂ：０．０１～０．２５％、
    Ｎ：０．００１～０．００９％、
    Ｏ：０．０００５～０．００２０％未満
    を含有し、
    Ａｌ：０．０２５％以下、
    Ｂ：０．０００３％未満
    に制限し、残部がＦｅ及び不可避不純物からなり、ミクロ組織中のベイナイトの面積率が１０～４０％であり、島状マルテンサイトの面積率を０．５％以下に制限し、残部がフェライト・パーライトからなり、粒子径が０．０５～１０μｍの含Ｔｉ酸化物の密度が３０～３００個／ｍｍ2であり、粒子径１０μｍ超の含Ｔｉ酸化物の密度が１０個／ｍｍ2以下であり、ウェブ厚又はフランジ厚が４０～１５０ｍｍであることを特徴とする高強度極厚Ｈ形鋼。     % By mass
    C: 0.01 to 0.05%,
    Si: 0.05 to 0.50%,
    Mn: 0.8 to 2.0%,
    Cu: 0.3 to 1.2%,
    Ni: 0.1 to 1.0%,
    Ti: 0.005 to 0.025%,
    Nb: 0.01 to 0.25%,
    N: 0.001 to 0.009%,
    O: 0.0005 to less than 0.0020%,
    Al: 0.025% or less,
    B: Restricted to less than 0.0003%, the balance is Fe and inevitable impurities, the area ratio of bainite in the microstructure is 10 to 40%, and the area ratio of island martensite is 0.5% or less The density of the Ti-containing oxide having a particle diameter of 0.05 to 10 μm is 30 to 300 / mm 2 and the density of the Ti-containing oxide having a particle diameter of more than 10 μm is 10 A high-strength, ultra-thick H-section steel having a thickness of 40/150 mm or less per piece / mm 2 .
  2. 　質量％で、更に、
    Ｖ：０．１％以下、
    Ｍｏ：０．３％未満、
    Ｃｒ：１．５％以下
    の１種又は２種以上を含有することを特徴とする請求項１に記載の高強度極厚Ｈ形鋼。     In mass%,
    V: 0.1% or less,
    Mo: less than 0.3%,
    The high-strength ultra-thick H-section steel according to claim 1, characterized by containing one or more of Cr: 1.5% or less.
  3. 　質量％で、更に、
    Ｍｇ:０．００５％以下
    を含有することを特徴とする請求項１又は２に記載の高強度極厚Ｈ形鋼。     In mass%,
    Mg: 0.005% or less is contained, The high intensity | strength extra heavy H-section steel of Claim 1 or 2 characterized by the above-mentioned.
  4. 　請求項１～３の何れか１項に記載の成分からなる鋼を溶製する際に、予備脱酸処理によって溶存酸素を０．００３～０．０１５質量％に調整した後、Ｔｉを添加し、更に真空脱ガス処理を３０分以上施し、溶製後、連続鋳造し、得られた鋳片を１１００～１３５０℃に加熱し、１０００℃以下での累積圧下率が１０％以上である仕上圧延を行って、ウェブ厚又はフランジ厚を４０～１５０ｍｍとし、０．１～５℃／ｓの範囲内の冷却速度で４００～７００℃の温度域まで冷却することを特徴とする高強度極厚Ｈ形鋼の製造方法。 When the steel comprising the component according to any one of claims 1 to 3 is melted, the dissolved oxygen is adjusted to 0.003 to 0.015 mass% by preliminary deoxidation treatment, and then Ti is added. Further, vacuum degassing treatment is performed for 30 minutes or more, and after casting, continuous casting is performed. The obtained slab is heated to 1100 to 1350 ° C., and finish rolling at a cumulative reduction ratio of 10% or more at 1000 ° C. or less. And a web thickness or flange thickness of 40 to 150 mm, and cooling to a temperature range of 400 to 700 ° C. at a cooling rate within a range of 0.1 to 5 ° C./s. A method of manufacturing shape steel.
  5. 　仕上圧延で、表面温度が７００℃以下になるように水冷し、復熱過程で圧延するパス間水冷圧延加工を１パス以上行うことを特徴とする請求項４に記載の高強度極厚Ｈ形鋼の製造方法。 5. The high-strength ultra-thick H-shape according to claim 4, wherein in the finish rolling, the water-cooling is performed so that the surface temperature is 700 ° C. or less, and the inter-pass water-cooled rolling process is performed for one or more passes. Steel manufacturing method.
  6. 　冷却後、４００～５００℃の温度域に加熱し、１５分～５時間保持し、再度冷却することを特徴とする請求項４に記載の高強度極厚Ｈ形鋼の製造方法。 5. The method for producing a high-strength ultrathick H-section steel according to claim 4, wherein after cooling, it is heated to a temperature range of 400 to 500 ° C., held for 15 minutes to 5 hours, and then cooled again.
  7. 　冷却後、４００～５００℃の温度域に加熱し、１５分～５時間保持し、再度冷却することを特徴とする請求項５に記載の高強度極厚Ｈ形鋼の製造方法。 6. The method for producing a high-strength ultra-thick H-section steel according to claim 5, wherein after cooling, it is heated to a temperature range of 400 to 500 ° C., held for 15 minutes to 5 hours, and cooled again.
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