WO2015141203A1 - Steel material for welding - Google Patents

Steel material for welding Download PDF

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Publication number
WO2015141203A1
WO2015141203A1 PCT/JP2015/001417 JP2015001417W WO2015141203A1 WO 2015141203 A1 WO2015141203 A1 WO 2015141203A1 JP 2015001417 W JP2015001417 W JP 2015001417W WO 2015141203 A1 WO2015141203 A1 WO 2015141203A1
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WO
WIPO (PCT)
Prior art keywords
steel
haz
less
toughness
welding
Prior art date
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PCT/JP2015/001417
Other languages
French (fr)
Japanese (ja)
Inventor
亮 荒尾
善明 村上
長谷　和邦
遠藤　茂
Original Assignee
Ｊｆｅスチール株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to KR1020167024001A priority Critical patent/KR20160117536A/en
Priority to JP2016508528A priority patent/JP6128276B2/en
Priority to CN201580003317.3A priority patent/CN105899702B/en
Publication of WO2015141203A1 publication Critical patent/WO2015141203A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—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
    • C21D8/0231—Warm rolling
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D2211/004—Dispersions; Precipitations
    • 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

Definitions

  • the present invention is a steel material for welding having a yield stress of 460 MPa or more, which is used for various steel structures in the fields of ships, construction, civil engineering, etc., and in particular, high heat input welding with a welding heat input exceeding 200 kJ / cm.
  • the present invention relates to a steel material having excellent weld toughness and joint strength even when the high heat input welding is performed.
  • Steel structures used in the fields of ships, marine structures, architecture, steel pipes, etc. are generally finished in a desired shape by welding. Therefore, from the viewpoint of ensuring safety, these structures are required to have excellent toughness of welds in addition to ensuring the base material characteristics of the steel materials used, that is, strength and toughness. Yes.
  • HAZ structure when high heat input welding is performed.
  • a portion in contact with the weld metal in the HAZ is generally called a “bond portion”.
  • HAZ in the vicinity of the bond part is exposed to high temperatures in the vicinity of the melting point, particularly in the heat-affected zone, so that the crystal grains become coarse and the toughness tends to be remarkably lowered.
  • the crystal grains become a fine grain region, so that a softened region is formed, which is a main cause of a decrease in joint strength.
  • the toughness of HAZ is reduced, but many countermeasures have been studied for this reduction in the HAZ toughness.
  • a technique for finely dispersing TiN in steel to suppress coarsening of austenite grains or to use it as a ferrite transformation nucleus has already been put into practical use.
  • a technique aiming at the effect of suppressing the coarsening of austenite grains similar to the above by dispersing Ti oxide has been developed.
  • Patent Document 3 discloses a Ca used for sulfide morphology control in order to improve the toughness of a weld heat affected zone subjected to a high heat input welding exceeding 200 kJ / cm.
  • a technique is disclosed in which CaS is crystallized by optimizing the addition amount of Ca and effectively used as ferrite transformation nuclei.
  • the above CaS crystallizes at a lower temperature than the oxide, it can be finely dispersed in the steel. Further, when the steel sheet is cooled, it is used as a core for MnS, TiN, BN. Since ferrite transformation nuclei such as the above are finely dispersed, the structure of the weld heat-affected zone can be a fine ferrite-pearlite structure, and high toughness can be achieved. Therefore, the technique of Patent Document 3 has made it possible to suppress the HAZ toughness reduction accompanying high heat input welding to some extent.
  • Patent Document 4 discloses that in addition to reducing the content of C and Si, it is effective to reduce the content of P. Yes.
  • Patent Document 5 when Mn is positively added and P is reduced as much as possible, the martensite in the vicinity of the bond part HAZ is reduced, and a steel material having a yield stress of 460 MPa grade with excellent toughness can be obtained. Has been.
  • Patent Document 6 describes that HAZ softening is reduced by increasing the amount of C, reducing Si and Mn, and containing Nb and V.
  • Patent Document 7 in order to improve the hardenability by B, HAZ softening is suppressed by defining a component formula so as to contain a large amount of Ti, B and Nb with respect to the N amount.
  • the hardenability improvement by B is aimed at by suppressing the amount of solute B, and HAZ softening suppression is aimed at.
  • Patent Document 3 is a technique for improving the toughness of the bond part, particularly when high heat input welding is performed on a steel material having a yield stress of 390 MPa grade. It may not be possible to sufficiently cope with high heat input HAZ toughness and HAZ softening for steel materials with higher yield strength, for example, yield stress: 460 MPa grade.
  • Patent Document 4 is intended for steel materials with a yield stress of 460 MPa grade, and reduces the content of C, Si, and P to reduce HAZ island martensite near the bond portion, and , Ca is added in an appropriate amount to finely disperse ferrite transformation nuclei to ensure HAZ toughness, but there is no description for softening of HAZ, and addition of Ni is essential, so the alloy cost is high. There is a problem that it may become.
  • Patent Document 5 is intended for steel materials with a yield stress of 460 MPa grade, and it is said that island-shaped martensite is reduced by actively using Mn, and the required steel materials can be obtained at low cost.
  • Patent Document 4 there is no description regarding softening of the HAZ.
  • Patent Document 6 has a high C content and uses a precipitation strengthening element such as Nb or V to take sufficient measures against softening of the HAZ.
  • a precipitation strengthening element such as Nb or V to take sufficient measures against softening of the HAZ.
  • patent document 7 and 8 is a technique which suppresses HAZ softening using the hardenability of B, especially patent document 7 presupposes addition of a lot of Ti, B, and N.
  • the technique described in Patent Document 8 is premised on Nb-free, and when a steel material with a yield stress of 460 MPa is targeted, it has been difficult to ensure joint strength.
  • An object of the present invention is to provide an inexpensive steel material for high heat input welding having a high yield HAZ toughness and a yield stress of 460 MPa or more.
  • the inventors have made a HAZ near the bond part when high heat input welding with a heat input exceeding 200 kJ / cm is applied to a high-strength steel material having a yield stress of 460 MPa or more.
  • the influence of the structure factor on the toughness and the hardness of the HAZ softest zone was investigated in detail.
  • the HAZ toughness in the vicinity of the bond part the presence of island martensite has an adverse effect on the toughness even in a small amount, whereas the hardness of the softest zone is formed by island martensite. I found out that it improves by doing.
  • the inventors examined a method for increasing the amount of island martensite generated in the softest region after suppressing the generation of island martensite in the HAZ near the bond portion.
  • the amount of C, Si, and P is suppressed to a low level, and Mn is actively included to compensate for the decrease in the strength of the base material that is a concern due to the reduction in the C amount. It has been found that the base metal strength can be effectively increased without generating island martensite which adversely affects the HAZ toughness as much as possible.
  • the inventors first performed rolling in the austenite recrystallization temperature range after performing rolling in the austenite recrystallization temperature range, followed by accelerated cooling to the austenite non-recrystallization temperature range.
  • austenite non-recrystallization temperature range rolling and then performing accelerated cooling again, the precipitation of B nitride is suppressed as much as possible, and excellent HAZ characteristics are obtained by combining with the optimization of the components described above.
  • the present invention has been developed.
  • the gist configuration of the present invention is as follows. 1. By mass%, C: 0.030 to 0.080%, Si: 0.01 to 0.10%, Mn: 1.80 to 2.40%, P: 0.010% or less, S: 0.0005 To 0.0040%, Al: 0.005 to 0.100%, Nb: 0.003 to 0.030%, Ti: 0.010 to 0.050%, N: 0.0030 to 0.0120%, and B: 0.0005 to 0.0025% is contained, and the mass% ratio (Ti / N) of Ti and N is 2.0 or more and less than 4.0, and is defined by the following formula (1).
  • the A value is 3 or more and 25 or less
  • the Ceq defined by the following formula (2) is in the range of 0.38 to 0.43
  • the balance is composed of Fe and inevitable impurities
  • the yield stress is 460 MPa or more.
  • the amount of solute B is 5 ppm by mass or more
  • the amount of welding heat input is 200 kJ / cm or more.
  • V 0.20% or less
  • Cu 0.30% or less
  • Ni 0.30% or less
  • Cr 0.40% or less
  • Mo 0.40% or less.
  • the present invention when high heat input welding is performed, a steel material having both good joint strength and weld heat-affected zone toughness can be obtained, so that high heat input welding such as submerged arc welding and electroslag welding is achieved.
  • high heat input welding such as submerged arc welding and electroslag welding is achieved.
  • the place that contributes to the quality improvement of ships and large-scale structures constructed by this method is significant.
  • the present invention when the present invention is applied to a steel material having a thickness of more than 50 mm, the present invention has a more significant advantage in terms of both the joint strength of welding and the toughness of the heat affected zone compared to the steel materials according to the prior art. .
  • the steel material made into object by this invention means the steel material manufactured by hot rolling.
  • the volume fraction of island martensite in the heat affected zone which is a feature of the steel material of the present invention, will be described.
  • Island-like martensite in the structure in the vicinity of the bond part in the heat-affected zone is 1 vol% or less Among the welding heat-affected zone (HAZ), austenite is coarsened when exposed to high temperatures, By suppressing the generation, the toughness in the high heat input weld can be improved. In order to obtain such an effect, it is necessary to suppress the volume fraction of island martensite in the vicinity of the bond portion to 1 vol% or less.
  • the lower limit of the volume fraction of the island martensite is not particularly limited, and may be 0 vol%.
  • the vicinity of the bond part in the heat-affected zone refers to a weld heat-affected zone in a range of 600 ⁇ m or less from the bond zone, and the structure is mainly acicular ferrite or bainite outside the island martensite. And a known structure including ferrite and pearlite.
  • the welded martensite in the heat-affected zone is 5 vol% or more.
  • Yield stress A joint welded with a steel material of 460 MPa or more requires a tensile strength equivalent to that of the base material, that is, a tensile strength of 570 MPa or more. It is said.
  • the factors affecting the tensile strength of the joint mainly include the strength of the weld metal, the plate thickness, and the hardness of the HAZ most softened area. In particular, the influence of the structure of the softened area in the heat affected zone. Is big.
  • region in the steel materials whose yield stress is 460 Mpa or more is a ferrite and a 2nd phase structure
  • by generating the island-like martensite of a volume fraction of 5 vol% or more as a 2nd phase structure A desired joint strength can be obtained.
  • the softened region in the heat-affected zone refers to a heat-affected zone in which austenite becomes fine grains after heating by welding, which is about 10 mm away from the bond portion.
  • the softest zone in the heat-affected zone is a Vickers hardness HV (JIS Z 2244 (1998)) measured in a lattice pattern at regular intervals of about 1 mm, and centered on the point showing the minimum hardness. , And refers to the area connecting the four nearest measurement points.
  • HV Vickers hardness
  • the component composition which a steel material should have is prescribed
  • the% display regarding the component composition of steel means mass%.
  • C exceeds 0.080% MA in the HAZ near the bond portion is easily generated, so the upper limit is made 0.080%.
  • Si 0.01 to 0.10%
  • Si is an element added as a deoxidizer when melting steel, and it is necessary to add 0.01% or more.
  • Si is set in the range of 0.01 to 0.10%.
  • Mn 1.80 to 2.40% Mn, like C, is an element that increases the strength, is cheaper than alloy elements such as Mo and V, and does not promote the formation of MA in the HAZ near the bond portion, so it is actively added in the present invention. In order to secure the required strength and obtain the above effect, 1.80% or more of addition is necessary, 1.90% or more of addition is more preferable, and 2.00% or more of addition is more preferable. . On the other hand, if it is excessively contained, the toughness of the welded portion is impaired, so 2.40% or less is necessary, more preferably 2.20% or less, and even more preferably 2.10% or less. .
  • P 0.010% or less
  • P is a kind of element contained as an impurity.
  • the amount of P is limited to 0.010% or less. Preferably, it is 0.008% or less.
  • S 0.0005 to 0.0040% S is an element necessary for forming MnS or CaS that acts as a nucleation site for ferrite. For this reason, 0.0005% or more is added. However, excessive addition causes a decrease in the base material toughness, so the upper limit is made 0.0040%.
  • Al 0.005 to 0.100%
  • Al is an element added for deoxidation of steel, and it is necessary to contain 0.005% or more.
  • the content exceeds 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, Al is in the range of 0.005 to 0.100%. Preferably it is 0.010 to 0.100% of range.
  • Nb 0.003 to 0.030%
  • Nb is an element effective for ensuring the strength of the base material and the hardness of the HAZ softened portion, and consequently the weld joint strength.
  • the content is less than 0.003%, the above effect is small.
  • the content exceeds 0.030%, MA is generated in the HAZ near the bond portion and the toughness is lowered. Therefore, Nb is set in the range of 0.003 to 0.030%.
  • Ti 0.010 to 0.050% Ti precipitates as TiN during solidification and suppresses coarsening of austenite grains in the vicinity of the bond portion HAZ. In addition, Ti becomes a transformation nucleus of ferrite and contributes to increasing the toughness of HAZ. At the same time, by reducing N that can be combined with B and securing solid solution B, the hardness of the HAZ softened part is increased. It works effectively in securing the weld joint strength. In order to acquire such an effect, 0.010% or more of addition is required, and it is preferable to add 0.015% or more. On the other hand, if the content exceeds 0.050%, the precipitated TiN becomes coarse and the above effect cannot be obtained. Therefore, Ti is set to a range of 0.010 to 0.050%.
  • N 0.0030 to 0.0120% N forms TiN during solidification and contributes to the suppression of the coarsening of HAZ austenite grains in the vicinity of the bond portion.
  • N forms BN, and the BN acts as a ferrite transformation nucleus so that the HAZ structure in the vicinity of the bond portion is formed.
  • the upper limit is made 0.0120% or less. Preferably it is 0.0100% or less.
  • B 0.0005 to 0.0025%
  • B is an element that improves the hardenability of steel, and by reducing the transformation temperature of austenite, it promotes the formation of hard structures such as bainite and martensite, and contributes to increasing the strength of the base steel sheet.
  • the formation of ferrite, which is a soft phase is also suppressed in the HAZ softened part, and the strength of the HAZ softened part is improved.
  • B is in the range of 0.0005 to 0.0025%.
  • the amount of solid solution B is 5 mass ppm or more.
  • the amount of dissolved B in the steel material is less than 5 ppm, the effect of improving the hardenability of untransformed austenite at the time of forming the structure of the HAZ softened region is insufficient, and the amount of island martensite for obtaining the desired hardness You won't get.
  • Ti / N Mass% ratio of Ti and N (Ti / N): 2.0 or more and less than 4.0
  • Ti / N is an important requirement in the present invention, together with the definition of the A value described later.
  • Ti / N greatly affects the fine dispersion state of TiN and the toughness deterioration due to solid solution N in the HAZ bond portion, and thus needs to be appropriately controlled. That is, BN does not precipitate when Ti / N is 4.0 or more, and HAZ toughness is significantly reduced by precipitation of Ti borocarbide and the like, while HAZ toughness due to solute N is less than 2.0.
  • the value of Ti / N is set to 2.0 or more and less than 4.0. Preferably, it exists in the range of 2.5 or more and 3.5 or less.
  • the A value defined by the following formula (1) is one of the most important items in the present invention.
  • the A value is 3 or more after satisfying the amount of addition of the steel materials described above with respect to Ti, N, and B.
  • the A value exceeds 25, the hardenability of the steel material becomes excessive and adversely affects the toughness of the HAZ. Therefore, in the present invention, the A value is 3 or more and 25 or less. Preferably it is in the range of 6-15.
  • A 2256 ⁇ Ti-7716 ⁇ N + 10000 ⁇ B (1)
  • each element symbol (Ti, N, B) indicates the content (% by mass) of each element in steel.
  • C eq When the C eq is less than 0.38, the hardenability is insufficient and the hardness of the softest region is remarkably lowered, so that the desired weld joint strength cannot be ensured. On the other hand, if C eq exceeds 0.43, the hardenability becomes excessive, the formation of ferrite in the vicinity of the bond portion is suppressed, and the formation of island martensite is promoted, so that sufficient toughness can be ensured. become unable.
  • a preferred C eq is in the range of 0.39 to 0.42.
  • the above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities.
  • an inevitable impurity for example, O is acceptable if it is 0.0050% or less.
  • the steel material of the present invention may further contain at least one selected from V, Cu, Ni, Cr and Mo as a selective element in the following range for the purpose of improving the strength and the like. it can.
  • V 0.20% or less
  • Cu 0.30% or less
  • Ni 0.30% or less
  • Cr 0.40% or less
  • Mo 0.40% or less
  • V, Cu, Ni, Cr and Mo are It is an element effective for increasing the strength of the base material, and in order to obtain the effect, it is preferable to add 0.05% or more of V, Cu and Ni and 0.02% or more of Cr and Mo.
  • V is 0.20% or less and Cu is 0.30. % Or less, Ni is 0.30% or less, and Cr and Mo are preferably 0.40% or less.
  • the steel material of the present invention can contain one or more selected from Ca, Mg, Zr and REM as selective elements in the following range.
  • Ca can be contained in order to obtain an effect of improving toughness by fixing S and dispersing oxides and sulfides. In order to acquire the said effect, it is preferable to contain at least 0.0005%. However, even if added over 0.0050%, the above effect is only saturated. Therefore, when it contains Ca, it is preferable to set it as 0.0005 to 0.0050% of range.
  • Mg, Zr, and REM are all elements having an effect of improving toughness due to oxide dispersion. In order to exhibit such an effect, it is preferable to contain 0.0005% or more of Mg and 0.0010% or more of Zr and REM. On the other hand, even if Mg exceeds 0.0050% and Zr and REM add more than 0.0200%, the effect is only saturated. Therefore, when it contains these elements, it is preferable to set it as the said range.
  • Manufacturing method Steel having the above-mentioned composition is melted using a conventional welding method such as a converter or an electric furnace, and a slab for manufacturing a steel sheet by a conventional method such as a continuous casting method or an ingot forming method. It is preferable to use a raw material.
  • a conventional welding method such as a converter or an electric furnace
  • a slab for manufacturing a steel sheet by a conventional method such as a continuous casting method or an ingot forming method. It is preferable to use a raw material.
  • preferable steel plate manufacturing conditions applied to the present invention will be described.
  • Heating temperature 1050-1200 ° C
  • the heating temperature of the steel material is preferably set to 1050 ° C. or higher.
  • the upper limit is preferably set to 1200 ° C.
  • Rolling in the austenite recrystallization temperature range has the effect of refining austenite grains during heating to some extent, and it is desirable to perform at least one pass, preferably a cumulative reduction of 20% or more.
  • the lower limit temperature of the austenite recrystallization temperature range is in the range of approximately 900 to 1000 ° C.
  • the solid solution B amount capable of improving the hardenability of the structure in the HAZ softened region corresponds to the solid solution B amount ensured in the state at the time of manufacturing the steel sheet. Therefore, when a large amount of B nitride is precipitated during the production of the steel sheet, the solid solution B for securing the hardenability is insufficient, and sufficient hardness may not be obtained in the HAZ softened region.
  • the cooling rate from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range is as high as possible.
  • this process is air-cooled as the temperature reduction standby time of hot rolling, but in the present invention, the time to the control rolling process, which is the next process, is increased by performing accelerated cooling having a cooling rate larger than that of air cooling. While shortening, the decrease of the solid solution B by precipitation of B nitride can be prevented.
  • accelerated cooling performed after rolling in the austenite recrystallization temperature range is referred to as primary cooling.
  • primary cooling it is preferable to achieve a cooling rate higher than that of air cooling by using an accelerated cooling facility by water cooling or a so-called deske facility that removes the scale generated on the surface of the steel sheet during rolling. Specifically, a cooling rate of 3 ° C./second or more is preferable.
  • the secondary cooling is cooling aimed at transforming the austenite structure processed by controlled rolling. is there. And since it is necessary to cool to the temperature range below 550 degreeC in order to complete the phase transformation of a steel structure, 550 degreeC is preferable as the minimum of the completion
  • the cooling rate in the secondary cooling requires a cooling rate larger than that of air cooling, and strong cooling of 5 ° C./second or more is preferable. More preferably, it is strong cooling of 10 ° C./second or more.
  • the cooling method is not particularly limited, cooling by water cooling is desirable.
  • a steel having the composition shown in Table 1 was melted in a converter and then made into a slab by a continuous casting method, and a steel sheet having a thickness of 40 to 80 mm was manufactured under the controlled rolling and controlled cooling conditions shown in Table 2.
  • the branch numbers shown in Table 2 indicate that the steel components are the same and the manufacturing conditions are different.
  • primary cooling is implemented with the water cooling equipment installed in the exit side of the rolling mill, and it has confirmed that the average cooling rate during cooling is 3 degreeC or more.
  • a tensile test piece with a parallel part of 14 mm ⁇ is taken from the position in the plate thickness direction 1/4, and a tensile test is performed in accordance with the provisions of JIS Z 2241 (1998).
  • the 0.2% yield strength (YS) and the tensile strength (TS) were determined.
  • a Charpy impact test piece having a V-notch standard dimension was taken from the position in the plate thickness direction 1/4 according to JIS Z 2202 (1998), and the impact test was performed according to JIS Z 2242 (1998).
  • the fracture surface transition temperature (vTrs) was determined.
  • the target value of vTrs was set to ⁇ 60 ° C. or lower.
  • a small size of 3 mm ⁇ ⁇ 10 mm from the 1/4 position in the plate thickness direction A test piece was collected and heated to 900 ° C., which is a temperature just above the transformation point, and then subjected to a heat treatment of cooling between 800 and 500 ° C. in 390 seconds.
  • the Vickers hardness HV (JIS Z 2244 (1998)) of the small test piece after these treatments was measured in a grid pattern at intervals of about 1 mm, and the lowest hardness was taken as the softest part hardness.
  • the target value of the softest part hardness was 160 or more.
  • the HAZ most softened area is an area where the closest measurement points are connected to each other with the point showing the lowest hardness as the center.
  • the structure corresponding to the above-mentioned softened area of the HAZ was etched with nital to reveal the structure.
  • Three-view tissue photographs were taken at 1000 times using SEM, and the images were analyzed to determine the average area fraction of MA, which was defined as the MA volume fraction (vol%) of the HAZ softest area. .
  • the target value was an average absorbed energy (vE -40 ° C) at -40 ° C of 50 J or more.
  • the area fraction of MA in the test piece cross section after thermal history provision was evaluated similarly to the above.
  • Table 3 shows the measurement results of the base material characteristics, the HAZ characteristics, and the MA volume fraction (vol%) in the HAZ evaluated in the above procedure.
  • the steel plate composition No. In the steel plates with branch numbers B in 1-4 the requirements of the present invention are not satisfied due to the influence of manufacturing conditions, and the base material characteristics and the HAZ characteristics are inferior.

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Abstract

The purpose of the present invention is to obtain a steel material which has a yield stress of at least 460 MPa, and which exhibits excellent weld-section toughness and joint strength, even in cases when the welding heat input is 200 kJ/cm or more. This steel material is provided with a prescribed component composition, and is configured such that: the mass percentage ratio of Ti to N is at least 2.0, but less than 4.0; the value of A specified in formula (1), namely A=2256×Ti-7716×N+10000×B, is at least 10, but not more than 25, with the caveat that each of the element symbols indicates the amount (in mass%) of each element included in the steel; Ceq specified in formula (2), namely Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, is 0.38-0.43, with the caveat that each of the element symbols indicates the amount (in mass%) of each element included in the steel; and the amount of B dissolved in the steel material is at least 5 ppm by mass.

Description

溶接用鋼材Steel for welding
　本発明は、船舶や建築・土木等の分野における各種鋼構造物に使用される、降伏応力が４６０ＭＰａ以上の溶接用鋼材であって、特に、溶接入熱量が２００ｋＪ／ｃｍを超える大入熱溶接に適した鋼材に関し、詳しくは、上記大入熱溶接を施した場合においても、優れた溶接部靭性および継手強度を有する鋼材に関する。 The present invention is a steel material for welding having a yield stress of 460 MPa or more, which is used for various steel structures in the fields of ships, construction, civil engineering, etc., and in particular, high heat input welding with a welding heat input exceeding 200 kJ / cm. Specifically, the present invention relates to a steel material having excellent weld toughness and joint strength even when the high heat input welding is performed.
　船舶、海洋構造物、建築および鋼管等の分野で使用される鋼構造物は、溶接接合によって所望の形状の構造物に仕上げられるのが一般的である。したがって、これらの構造物は、安全性を確保する観点から、使用される鋼材の母材特性、すなわち、強度・靱性の確保に加えて、溶接部の靱性にも優れていることが要請されている。 Steel structures used in the fields of ships, marine structures, architecture, steel pipes, etc. are generally finished in a desired shape by welding. Therefore, from the viewpoint of ensuring safety, these structures are required to have excellent toughness of welds in addition to ensuring the base material characteristics of the steel materials used, that is, strength and toughness. Yes.
　近年では、上記船舶などの鋼構造物はますます大型化し、使用される鋼材に対しては、高強度化および厚肉化が積極的に進められている。それに伴って、溶接施工には、サブマージアーク溶接や、エレクトロガス溶接およびエレクトロスラグ溶接などの高能率で大入熱の溶接方法が適用されるようになってきているが、このような大入熱溶接によって溶接施工した場合においても、溶接部の靱性に優れる鋼材が必要である。 In recent years, steel structures such as the above ships have become increasingly larger, and the strength and thickness of the steel materials being used are being actively promoted. Along with that, high efficiency and high heat input welding methods such as submerged arc welding, electrogas welding and electroslag welding have come to be applied to welding construction. Even when welding is performed by welding, a steel material that is excellent in the toughness of the welded portion is required.
　しかしながら、高強度鋼や厚肉鋼板において、母材の機械的特性（特に低温靭性）と溶接熱影響部（Ｈｅａｔ　Ａｆｆｅｃｔｅｄ　Ｚｏｎｅ；以下、ＨＡＺと記載）の低温靭性との両立が困難であるという報告が散見される。
　この報告に対して、例えば特許文献１および特許文献２に記載のように、制御圧延や制御冷却を利用して、母材の低温靭性とＨＡＺの低温靭性との両立を達成する技術が開示されている。 However, in high-strength steel and thick steel plates, it is difficult to achieve both the mechanical properties of the base metal (particularly low-temperature toughness) and the low-temperature toughness of the weld heat affected zone (hereinafter referred to as HAZ). Is occasionally seen.
In response to this report, for example, as described in Patent Document 1 and Patent Document 2, a technique for achieving both low temperature toughness of a base material and low temperature toughness of HAZ using controlled rolling and controlled cooling is disclosed. ing.
　次に大入熱溶接を施した際のＨＡＺ組織について説明する。ＨＡＺにおいて溶接金属に接する部分は、一般に「ボンド部」と称されている。ボンド部近傍のＨＡＺは、熱影響部の中でも特に、溶融点付近の高温に曝されるため、結晶粒が粗大化して靭性が著しく低下してしまう傾向にある。一方、ボンド部からやや離れたところでは、結晶粒が細粒域となるため軟化領域を形成し、継手強度の低下の主因となる。 Next, the HAZ structure when high heat input welding is performed will be described. A portion in contact with the weld metal in the HAZ is generally called a “bond portion”. HAZ in the vicinity of the bond part is exposed to high temperatures in the vicinity of the melting point, particularly in the heat-affected zone, so that the crystal grains become coarse and the toughness tends to be remarkably lowered. On the other hand, at a distance from the bond portion, the crystal grains become a fine grain region, so that a softened region is formed, which is a main cause of a decrease in joint strength.
　上述したように、大入熱溶接では、ＨＡＺの靱性低下が起こるが、このＨＡＺ靱性低下に対し、これまでにも多くの対策が検討されてきた。例えば、ＴｉＮを鋼中に微細分散させて、オーステナイト粒の粗大化を抑制したり、フェライト変態核として利用したりする技術が既に実用化されている。また、Ｔｉの酸化物を分散させることで、上記と同様のオーステナイト粒の粗大化抑制効果を狙った技術も開発されている。 As described above, in high heat input welding, the toughness of HAZ is reduced, but many countermeasures have been studied for this reduction in the HAZ toughness. For example, a technique for finely dispersing TiN in steel to suppress coarsening of austenite grains or to use it as a ferrite transformation nucleus has already been put into practical use. Further, a technique aiming at the effect of suppressing the coarsening of austenite grains similar to the above by dispersing Ti oxide has been developed.
　しかしながら、ＴｉＮを活用する上記技術は、大入熱溶接を受けた際に、溶接熱影響部がＴｉＮの溶解温度域まで加熱されるため、ＴｉＮが分解して上記分散効果が消失したり、ＴｉＮの分解によって生成した固溶Ｔｉおよび固溶Ｎによって鋼の地組織が脆化したりして、溶接熱影響部の靱性が著しく低下するという問題を抱えている。 However, in the above technique using TiN, when the heat input welding is performed, the heat affected zone is heated to the melting temperature range of TiN, so that TiN decomposes and the above dispersion effect disappears, or TiN The solid structure of steel is embrittled by solid solution Ti and solid solution N generated by decomposition of the steel, and the toughness of the weld heat affected zone is significantly reduced.
　また、Ｔｉ酸化物を活用する技術は、酸化物を均一微細に分散させることが難しいという問題がある。このような問題を解決する技術として、例えば、特許文献３には、２００ｋＪ／ｃｍを超える大入熱溶接を施した溶接熱影響部の靱性を向上させるために、硫化物の形態制御に用いるＣａの添加量を適正化して、ＣａＳを晶出させ、これをフェライト変態核として有効に活用する技術が開示されている。 Also, the technology using Ti oxide has a problem that it is difficult to disperse the oxide uniformly and finely. As a technique for solving such a problem, for example, Patent Document 3 discloses a Ca used for sulfide morphology control in order to improve the toughness of a weld heat affected zone subjected to a high heat input welding exceeding 200 kJ / cm. A technique is disclosed in which CaS is crystallized by optimizing the addition amount of Ca and effectively used as ferrite transformation nuclei.
　上記ＣａＳは、酸化物に比べて低温で晶出するために、鋼中に微細分散させることが可能であり、さらに、鋼板が冷却される際には、これを核として、ＭｎＳやＴｉＮ、ＢＮ等のフェライト変態生成核が微細に分散するので、溶接熱影響部の組織を微細なフェライト－パーライト組織とすることができ、高靱性化を達成することができる。故に、特許文献３の技術によって、大入熱溶接に伴うＨＡＺ靭性低下は、ある程度抑制できるようになった。 Since the above CaS crystallizes at a lower temperature than the oxide, it can be finely dispersed in the steel. Further, when the steel sheet is cooled, it is used as a core for MnS, TiN, BN. Since ferrite transformation nuclei such as the above are finely dispersed, the structure of the weld heat-affected zone can be a fine ferrite-pearlite structure, and high toughness can be achieved. Therefore, the technique of Patent Document 3 has made it possible to suppress the HAZ toughness reduction accompanying high heat input welding to some extent.
　ところが、その後の研究によって、鋼板の降伏応力が４６０ＭＰａ以上と高強度化され、比較的多量のＣや合金元素が添加された鋼の場合においては、溶接入熱量が２００ｋＪ／ｃｍを超える大入熱溶接を施すと、ボンド部近傍のＨＡＺに島状マルテンサイト（以下、本発明では、ＭＡとも記す）と呼ばれる硬質の脆化組織が数体積％形成され、この脆化組織が溶接部の靭性のさらなる向上を阻むことが分かってきた。 However, in the case of steel in which the yield stress of the steel sheet has been increased to 460 MPa or more and steels to which a relatively large amount of C and alloy elements have been added have been obtained through subsequent research, a large heat input with a welding heat input exceeding 200 kJ / cm. When welding is performed, a hard embrittled structure called island martensite (hereinafter also referred to as MA in the present invention) is formed in the HAZ in the vicinity of the bond part by several volume%, and this embrittled structure becomes the toughness of the welded part. It has been found to prevent further improvement.
　すなわち、このような高強度鋼の大入熱溶接部におけるボンド部近傍のＨＡＺ靭性改善には、オーステナイト粒粗大化抑制やフェライト変態核の微細分散、固溶Ｎの低減に加えて、さらに、島状マルテンサイトの生成を抑制する必要があることを知見した。 That is, in order to improve the HAZ toughness in the vicinity of the bond portion in the high heat input weld of such high strength steel, in addition to suppressing austenite grain coarsening, fine dispersion of ferrite transformation nuclei, and reduction of solute N, further, It was found that it is necessary to suppress the formation of glassy martensite.
　ＨＡＺ部の島状マルテンサイトを低減する技術については、例えば、特許文献４に、Ｃ、Ｓｉの含有量を低減することの他に、Ｐの含有量の低減が有効であることが開示されている。 Regarding the technology for reducing island martensite in the HAZ part, for example, Patent Document 4 discloses that in addition to reducing the content of C and Si, it is effective to reduce the content of P. Yes.
　さらに特許文献５では、Ｍｎを積極的に添加し、なおかつＰを極力低減することで、ボンド部近傍ＨＡＺの島状マルテンサイトを低減し、靭性の優れた降伏応力４６０ＭＰａグレードの鋼材が得られるとされている。 Further, in Patent Document 5, when Mn is positively added and P is reduced as much as possible, the martensite in the vicinity of the bond part HAZ is reduced, and a steel material having a yield stress of 460 MPa grade with excellent toughness can be obtained. Has been.
　他方、大入熱溶接に伴うＨＡＺ軟化を抑制する技術に関しては、ＨＡＺ靱性対策ほど多く開示されていない。上掲した特許文献３、４および５のいずれにおいてもＨＡＺ軟化に関する記述はない。これは、もともと大入熱溶接用鋼の設計にあたっては、継手強度が確保できることを前提としていたためであると思われる。 On the other hand, the technology for suppressing the HAZ softening associated with the high heat input welding is not disclosed as much as the HAZ toughness countermeasure. None of Patent Documents 3, 4 and 5 listed above describes HAZ softening. This seems to be because the design of the steel for high heat input welding originally assumed that joint strength could be secured.
　そこで、ＨＡＺ軟化の抑制に関し、すでに開示されている技術について説明する。
　これらの技術には、ＮｂやＶなどの析出強化元素を利用する技術と、Ｂによる焼入性の向上効果を用いる技術がある。 Then, the technique already disclosed regarding suppression of HAZ softening is demonstrated.
These techniques include a technique using precipitation strengthening elements such as Nb and V, and a technique using the effect of improving hardenability by B.
　例えば特許文献６では、Ｃ量を高めるとともにＳｉ、Ｍｎを低減し、ＮｂやＶを含有することでＨＡＺ軟化が低減されるとしている。
　一方、特許文献７では、Ｂによる焼入性向上を図るために、Ｎ量に対してＴｉ、ＢおよびＮｂを多く含有するよう成分式を規定することで、ＨＡＺ軟化抑制を図っている。
　また、特許文献８では、固溶Ｂ量を規定することで、Ｂによる焼入性向上を図り、ＨＡＺ軟化抑制を図っている。 For example, Patent Document 6 describes that HAZ softening is reduced by increasing the amount of C, reducing Si and Mn, and containing Nb and V.
On the other hand, in Patent Document 7, in order to improve the hardenability by B, HAZ softening is suppressed by defining a component formula so as to contain a large amount of Ti, B and Nb with respect to the N amount.
Moreover, in patent document 8, the hardenability improvement by B is aimed at by suppressing the amount of solute B, and HAZ softening suppression is aimed at.
特開昭５７－１３４５１８号公報JP 57-134518 A 特開昭５９－８３７２２号公報JP 59-83722 A 特許第３５４６３０８号公報Japanese Patent No. 3546308 特開２００８－１６３４４６号公報JP 2008-163446 A 特開２０１１－６７７２号公報JP 2011-6772 A 特開昭６０－６７６２２号公報JP-A-60-67622 特開２００７－１７７３２７号公報JP 2007-177327 A 特許第４２３３０３３号公報Japanese Patent No. 4233033
　しかしながら、上述したように、特許文献３に記載の技術は、特に、降伏応力が３９０ＭＰａグレードの鋼材に対して大入熱溶接を施した際の、ボンド部の靱性を改善する技術であるが、それよりも降伏強度が高い、例えば、降伏応力：４６０ＭＰａグレードの鋼材に対する大入熱ＨＡＺ靱性およびＨＡＺ軟化に対しては、十分に対処することができない場合がある。 However, as described above, the technique described in Patent Document 3 is a technique for improving the toughness of the bond part, particularly when high heat input welding is performed on a steel material having a yield stress of 390 MPa grade. It may not be possible to sufficiently cope with high heat input HAZ toughness and HAZ softening for steel materials with higher yield strength, for example, yield stress: 460 MPa grade.
　また、特許文献４に記載の技術は、降伏応力が４６０ＭＰａグレードの鋼材を対象とし、Ｃ、ＳｉおよびＰの含有量を低減することでボンド部近傍のＨＡＺの島状マルテンサイトを低減し、かつ、Ｃａを適正量添加してフェライト変態核を微細に分散させてＨＡＺ靱性の確保を図っているが、ＨＡＺ軟化に対しては記述がなく、またＮｉの添加を必須としているため合金コストが高くなる可能性があるという問題がある。 In addition, the technique described in Patent Document 4 is intended for steel materials with a yield stress of 460 MPa grade, and reduces the content of C, Si, and P to reduce HAZ island martensite near the bond portion, and , Ca is added in an appropriate amount to finely disperse ferrite transformation nuclei to ensure HAZ toughness, but there is no description for softening of HAZ, and addition of Ni is essential, so the alloy cost is high. There is a problem that it may become.
　さらに、特許文献５に記載の技術は、降伏応力が４６０ＭＰａグレードの鋼材を対象とし、Ｍｎを積極的に利用することで島状マルテンサイトを低減し、安価に所要の鋼材が得られるとしているが、これもまた特許文献４と同様にＨＡＺ軟化に関する記述がない。 Furthermore, the technique described in Patent Document 5 is intended for steel materials with a yield stress of 460 MPa grade, and it is said that island-shaped martensite is reduced by actively using Mn, and the required steel materials can be obtained at low cost. Similarly to Patent Document 4, there is no description regarding softening of the HAZ.
　他方、特許文献６に記載の技術は、Ｃ量が高く、ＮｂやＶなどの析出強化元素を利用してＨＡＺ軟化に対する十分な対処を採っているが、大入熱溶接時にボンド部近傍ＨＡＺに多量の島状マルテンサイトを形成することで、ボンド部近傍のＨＡＺの靭性を顕著に低下させるという懸念がある。 On the other hand, the technique described in Patent Document 6 has a high C content and uses a precipitation strengthening element such as Nb or V to take sufficient measures against softening of the HAZ. By forming a large amount of island-like martensite, there is a concern that the HAZ toughness in the vicinity of the bond portion is significantly reduced.
　また、特許文献７および８に記載の技術は、Ｂの焼入性を用いてＨＡＺ軟化を抑制する技術であるが、中でも特許文献７は、多量のＴｉ、ＢおよびＮの添加を前提としていて、製造性に問題があるとともに、ボンド部近傍のＴｉＮが溶ける領域においては、固溶ＮによるＨＡＺの靭性の低下が懸念される。
　加えて、特許文献８に記載の技術は、Ｎｂフリーを前提としており、降伏応力：４６０ＭＰａグレードの鋼材を対象とした場合、継手強度の確保が困難であるという問題を残していた。 Moreover, although the technique of patent document 7 and 8 is a technique which suppresses HAZ softening using the hardenability of B, especially patent document 7 presupposes addition of a lot of Ti, B, and N. In addition, there is a problem in manufacturability, and in the region where TiN in the vicinity of the bond portion is melted, there is a concern that the toughness of the HAZ is lowered due to the solid solution N.
In addition, the technique described in Patent Document 8 is premised on Nb-free, and when a steel material with a yield stress of 460 MPa is targeted, it has been difficult to ensure joint strength.
　本発明は、上記した現状に鑑み開発されたもので、溶接入熱量が２００ｋＪ／ｃｍを超える大入熱溶接を施しても、溶接熱影響部の硬度が低下しない耐軟化性と、ボンド部近傍のＨＡＺ靭性に優れる、降伏応力が４６０ＭＰａ以上の大入熱溶接用鋼材を安価に提供することを目的とする。 The present invention has been developed in view of the above-described present situation, and even when high heat input welding with a heat input of welding exceeding 200 kJ / cm is applied, the hardness of the weld heat affected zone does not decrease, and the vicinity of the bond portion. An object of the present invention is to provide an inexpensive steel material for high heat input welding having a high yield HAZ toughness and a yield stress of 460 MPa or more.
　発明者らは、上記した課題を解決するために、降伏応力が４６０ＭＰａ以上の高強度鋼材に対して溶接入熱量が２００ｋＪ／ｃｍを超える大入熱溶接を施した際の、ボンド部近傍のＨＡＺ靭性とＨＡＺ最軟化部域の硬度に及ぼす組織因子の影響について詳細に調査した。その結果、ボンド部近傍のＨＡＺ靭性に関しては、少量であっても島状マルテンサイトの存在が靭性に対して悪影響を及ぼすのに対して、最軟化部域の硬度は、島状マルテンサイトが生成することで向上することを知見した。 In order to solve the above-mentioned problems, the inventors have made a HAZ near the bond part when high heat input welding with a heat input exceeding 200 kJ / cm is applied to a high-strength steel material having a yield stress of 460 MPa or more. The influence of the structure factor on the toughness and the hardness of the HAZ softest zone was investigated in detail. As a result, regarding the HAZ toughness in the vicinity of the bond part, the presence of island martensite has an adverse effect on the toughness even in a small amount, whereas the hardness of the softest zone is formed by island martensite. I found out that it improves by doing.
　そこで、発明者らは、ボンド部近傍のＨＡＺにおける島状マルテンサイトの生成を抑制した上で、最軟化部域の島状マルテンサイトの生成量を高めるための方策について検討した。その結果、ボンド部近傍のＨＡＺにおいては、Ｃ、Ｓｉ、Ｐ量を低く抑えるとともに、Ｃ量低減により懸念される母材強度低下を補うためＭｎを積極的に含有させることによって、ボンド部近傍のＨＡＺ靭性に悪影響を及ぼす島状マルテンサイトを極力生成させることなく、母材強度を効果的に高めることができることが分かった。 Therefore, the inventors examined a method for increasing the amount of island martensite generated in the softest region after suppressing the generation of island martensite in the HAZ near the bond portion. As a result, in the HAZ in the vicinity of the bond portion, the amount of C, Si, and P is suppressed to a low level, and Mn is actively included to compensate for the decrease in the strength of the base material that is a concern due to the reduction in the C amount. It has been found that the base metal strength can be effectively increased without generating island martensite which adversely affects the HAZ toughness as much as possible.
　また、最軟化部域においてはＴｉ、ＮおよびＢを適正範囲に制御することでＢの焼入性向上効果を活用することでボンド部近傍ＨＡＺの島状マルテンサイトを増加させることなく、最軟化部域における島状マルテンサイトの形成を促進させることができることを知見した。
　すなわちＢは、溶融点付近の高温に曝されるボンド部近傍のＨＡＺにおいては、上部ベイナイトの生成や、成長に伴う粒界からの移動が起こらずに、ベイナイトラス間に残留した未変態オーステナイトの焼入性を上げることがない一方で、熱影響による温度上昇が比較的小さいＨＡＺ軟化領域においては、フェライト変態に伴って拡散し、未変態オーステナイトの粒界に偏析することでその焼入性を向上させ、島状マルテンサイトの形成を促進する効果があることが分った。 In the softest zone, Ti, N, and B are controlled within the proper range, making use of the effect of improving the hardenability of B, thereby increasing the softness without increasing island martensite near the bond zone. It was found that the formation of island martensite in the region can be promoted.
That is, in the HAZ in the vicinity of the bond portion exposed to a high temperature near the melting point, the formation of upper bainite and migration from the grain boundary accompanying the growth do not occur, and the untransformed austenite remaining between the bainite laths. While the hardenability is not increased, in the HAZ softened region where the temperature rise due to thermal effects is relatively small, the hardenability is increased by diffusing with the ferrite transformation and segregating at the grain boundaries of untransformed austenite. It has been found that there is an effect of improving and promoting the formation of island martensite.
　ただし、ＨＡＺ軟化領域は、熱影響による温度上昇が比較的小さいため、析出物の溶解がほとんど起こらないことから、焼入性に寄与するＢの存在量は製造工程時の状態に依存する。そして、Ｂは、母材製造工程の制御圧延および冷却の各段階で、製造条件によっては窒化物を形成する場合があり、この場合、その焼入性向上効果は発揮されない。 However, in the HAZ softened region, since the temperature rise due to the heat effect is relatively small, the dissolution of precipitates hardly occurs, so the amount of B that contributes to hardenability depends on the state during the manufacturing process. And B may form a nitride depending on manufacturing conditions at each stage of controlled rolling and cooling in the base material manufacturing process. In this case, the effect of improving hardenability is not exhibited.
　そこで、発明者らは、さらに検討を重ねた結果、鋼板製造工程において、まず初めにオーステナイト再結晶温度域にて圧延を実施した後にオーステナイト未再結晶温度域まで加速冷却を実施し、引続いてオーステナイト未再結晶温度域圧延を行い、その後、再度、加速冷却を実施することによって、Ｂ窒化物の析出を可能な限り抑制し、前述した成分の最適化と併せることで優れたＨＡＺ特性を得られることを見出し、本発明を開発するに至った。 Therefore, as a result of further studies, the inventors first performed rolling in the austenite recrystallization temperature range after performing rolling in the austenite recrystallization temperature range, followed by accelerated cooling to the austenite non-recrystallization temperature range. By carrying out austenite non-recrystallization temperature range rolling and then performing accelerated cooling again, the precipitation of B nitride is suppressed as much as possible, and excellent HAZ characteristics are obtained by combining with the optimization of the components described above. As a result, the present invention has been developed.
　すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、Ｃ：０．０３０～０．０８０％、Ｓｉ：０．０１～０．１０％、Ｍｎ：１．８０～２．４０％、Ｐ：０．０１０％以下、Ｓ：０．０００５～０．００４０％、Ａｌ：０．００５～０．１００％、Ｎｂ：０．００３～０．０３０％、Ｔｉ：０．０１０～０．０５０％、Ｎ：０．００３０～０．０１２０％およびＢ：０．０００５～０．００２５％を含有し、さらにＴｉとＮの質量％比（Ｔｉ／Ｎ）が２．０以上４．０未満であって、以下の（１）式で規定されるＡ値が３以上２５以下、以下の（２）式で規定されるＣｅｑが０．３８～０．４３の範囲で、残部がＦｅおよび不可避的不純物の成分組成からなり、降伏応力が４６０ＭＰａ以上であって、かつ、固溶Ｂ量が５質量ｐｐｍ以上であり、溶接入熱量：２００ｋＪ／ｃｍ以上の入熱溶接を施した際の、熱影響部におけるボンド部近傍の組織中の島状マルテンサイトが１vol％以下で、かつ熱影響部における最軟化部域の組織中の島状マルテンサイトが５vol％以上である溶接用鋼材。
Ａ＝２２５６×Ｔｉ－７７１６×Ｎ＋１００００×Ｂ　　・・・（１）
Ｃeq＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５　……（２）
但し、各元素記号は各元素の鋼中含有量（質量％）を示す。 That is, the gist configuration of the present invention is as follows.
1. By mass%, C: 0.030 to 0.080%, Si: 0.01 to 0.10%, Mn: 1.80 to 2.40%, P: 0.010% or less, S: 0.0005 To 0.0040%, Al: 0.005 to 0.100%, Nb: 0.003 to 0.030%, Ti: 0.010 to 0.050%, N: 0.0030 to 0.0120%, and B: 0.0005 to 0.0025% is contained, and the mass% ratio (Ti / N) of Ti and N is 2.0 or more and less than 4.0, and is defined by the following formula (1). The A value is 3 or more and 25 or less, the Ceq defined by the following formula (2) is in the range of 0.38 to 0.43, the balance is composed of Fe and inevitable impurities, and the yield stress is 460 MPa or more. And the amount of solute B is 5 ppm by mass or more, and the amount of welding heat input is 200 kJ / cm or more. When heat welding is performed, the island-like martensite in the structure in the vicinity of the bond portion in the heat-affected zone is 1 vol% or less, and the island-like martensite in the structure in the softest zone in the heat-affected zone is 5 vol% or more. Steel material.
A = 2256 × Ti-7716 × N + 10000 × B (1)
C eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (2)
However, each element symbol indicates the content (mass%) of each element in steel.
２．上記成分組成に、さらに、質量％で、Ｖ：０．２０％以下、Ｃｕ：０．３０％以下、Ｎｉ：０．３０％以下、Ｃｒ：０．４０％以下およびＭｏ：０．４０％以下のうちから選んだ１種以上を含有する前記１に記載の溶接用鋼材。 2. In addition to the above-described component composition, V: 0.20% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.40% or less, and Mo: 0.40% or less. 2. The steel material for welding as described in 1 above, which contains one or more selected from the above.
３．上記成分組成に、さらに、質量％で、Ｃａ：０．０００５～０．００５０％、Ｍｇ：０．０００５～０．００５０％、Ｚｒ：０．００１０～０．０２００％、ＲＥＭ：０．００１０～０．０２００％のうちから選んだ１種以上を含有する前記１または２に記載の溶接用鋼材。 3. In addition to the above-described component composition, by mass%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, REM: 0.0010 to 3. The steel material for welding as described in 1 or 2 above, which contains one or more selected from 0.0200%.
　本発明によれば、大入熱溶接を施した際に、良好な継手強度と溶接熱影響部靭性とを兼ね備えた鋼材を得ることができるため、サブマージアーク溶接やエレクトロスラグ溶接といった大入熱溶接により施工される、船舶や大型構造物の品質向上に寄与するところが大である。特に、板厚：５０ｍｍを超える鋼材に対して本発明を適用すると、従来技術に係る鋼材に比して、溶接の継手強度と溶接熱影響部の靭性の両立に関して、より顕著な優位性を示す。 According to the present invention, when high heat input welding is performed, a steel material having both good joint strength and weld heat-affected zone toughness can be obtained, so that high heat input welding such as submerged arc welding and electroslag welding is achieved. The place that contributes to the quality improvement of ships and large-scale structures constructed by this method is significant. In particular, when the present invention is applied to a steel material having a thickness of more than 50 mm, the present invention has a more significant advantage in terms of both the joint strength of welding and the toughness of the heat affected zone compared to the steel materials according to the prior art. .
　以下、本発明を具体的に説明する。なお、本発明で対象とする鋼材とは熱間圧延で製造された鋼材をいう。
　本発明では、鋼材の、成分組成と、強度と、２００ｋＪ／ｃｍを超える大入熱溶接によって形成される溶接熱影響部の軟化領域のうち最小の硬度（以下、ＨＡＺ最軟化部域の硬度ともいう）とを、それぞれ制御することが重要である。 Hereinafter, the present invention will be specifically described. In addition, the steel material made into object by this invention means the steel material manufactured by hot rolling.
In the present invention, the minimum hardness (hereinafter referred to as the HAZ most softened area) of the softened area of the weld heat affected zone formed by high heat input welding exceeding 200 kJ / cm in the component composition, strength, and steel. It is important to control each of them.
　まず、本発明の鋼材の特徴である熱影響部の島状マルテンサイトの体積分率について説明する。
熱影響部におけるボンド部近傍の組織中の島状マルテンサイトが１vol％以下
　溶接熱影響部（ＨＡＺ）の中でも、高温に曝されオーステナイトが粗大化する、熱影響部におけるボンド部近傍の島状マルテンサイトの生成を抑制することによって、大入熱溶接部における靭性を向上させることができる。斯かる効果を得るためには、上記ボンド部近傍の島状マルテンサイトの体積分率を１vol％以下に抑える必要がある。なお、上記島状マルテンサイトの体積分率の下限値は特に限定しない、０vol％であっても良い。また、本発明において熱影響部におけるボンド部近傍とは、ボンド部から600μm以内の範囲の溶接熱影響部を指し、その組織は、上記島状マルテンサイトの外は、アシキュラーフェライトやベイナイトを主とし、フェライトやパーライトを含む公知の組織である。 First, the volume fraction of island martensite in the heat affected zone, which is a feature of the steel material of the present invention, will be described.
Island-like martensite in the structure in the vicinity of the bond part in the heat-affected zone is 1 vol% or less Among the welding heat-affected zone (HAZ), austenite is coarsened when exposed to high temperatures, By suppressing the generation, the toughness in the high heat input weld can be improved. In order to obtain such an effect, it is necessary to suppress the volume fraction of island martensite in the vicinity of the bond portion to 1 vol% or less. In addition, the lower limit of the volume fraction of the island martensite is not particularly limited, and may be 0 vol%. Further, in the present invention, the vicinity of the bond part in the heat-affected zone refers to a weld heat-affected zone in a range of 600 μm or less from the bond zone, and the structure is mainly acicular ferrite or bainite outside the island martensite. And a known structure including ferrite and pearlite.
熱影響部における最軟化部域の島状マルテンサイトが５vol％以上
　降伏応力：４６０ＭＰａ以上の鋼材を溶接した継手には、母材と同等の引張強さ、すなわち引張強さにして５７０ＭＰａ以上が必要とされる。ここで、継手の引張強さに影響する因子としては、おもに溶接金属の強度や、板厚、ＨＡＺ最軟化部域の硬度などがあるが、特に熱影響部における最軟化部域の組織の影響が大きい。そして、降伏応力が４６０ＭＰａ以上の鋼材における軟化領域の組織は、フェライトおよび第二相組織であるが、第二相組織として、５vol％以上の体積分率の島状マルテンサイトを生成させることで、所望の継手強度を得ることが可能となる。
　なお、本発明において、熱影響部における軟化領域とは、ボンド部から１０ｍｍ前後離れた、溶接による加熱後に、オーステナイトが細粒となる熱影響部を指す。 The welded martensite in the heat-affected zone is 5 vol% or more. Yield stress: A joint welded with a steel material of 460 MPa or more requires a tensile strength equivalent to that of the base material, that is, a tensile strength of 570 MPa or more. It is said. Here, the factors affecting the tensile strength of the joint mainly include the strength of the weld metal, the plate thickness, and the hardness of the HAZ most softened area. In particular, the influence of the structure of the softened area in the heat affected zone. Is big. And although the structure | tissue of the softening area | region in the steel materials whose yield stress is 460 Mpa or more is a ferrite and a 2nd phase structure, by generating the island-like martensite of a volume fraction of 5 vol% or more as a 2nd phase structure, A desired joint strength can be obtained.
In the present invention, the softened region in the heat-affected zone refers to a heat-affected zone in which austenite becomes fine grains after heating by welding, which is about 10 mm away from the bond portion.
　また、本発明において、熱影響部における最軟化部域とは、１ｍｍ程度の等間隔で格子状にビッカース硬度ＨＶ（ＪＩＳ　Ｚ　２２４４（１９９８））を測定し、最低硬度を示した点を中心として、そこから最も近い４つの測定点同士を結んだ領域を指す。 In the present invention, the softest zone in the heat-affected zone is a Vickers hardness HV (JIS Z 2244 (1998)) measured in a lattice pattern at regular intervals of about 1 mm, and centered on the point showing the minimum hardness. , And refers to the area connecting the four nearest measurement points.
　本発明では、ＨＡＺの最軟化部域の組織を上記とおりに制御すると共に、高強度を達成するために、鋼材が有すべき成分組成を以下のとおり規定する。なお、以下、鋼材の成分組成に関する％表示は質量％を意味している。
Ｃ：０．０３０～０．０８０％
　Ｃは、鋼材の強度を高める元素であり、構造用鋼として必要な強度を確保するためには、０．０３０％以上含有させる必要がある。一方、Ｃが０．０８０％を超えると、ボンド部近傍のＨＡＺにおけるＭＡが生成し易くなるため、上限は０．０８０％とする。 In this invention, while controlling the structure | tissue of the softest part area of HAZ as mentioned above, in order to achieve high intensity | strength, the component composition which a steel material should have is prescribed | regulated as follows. In addition, hereinafter, the% display regarding the component composition of steel means mass%.
C: 0.030 to 0.080%
C is an element that increases the strength of the steel material, and in order to ensure the strength necessary for structural steel, it is necessary to contain 0.030% or more. On the other hand, if C exceeds 0.080%, MA in the HAZ near the bond portion is easily generated, so the upper limit is made 0.080%.
Ｓｉ：０．０１～０．１０％
　Ｓｉは、鋼を溶製する際の脱酸剤として添加される元素であり、０．０１％以上の添加が必要である。一方、０．１０％を超えると、母材の靱性が低下するほか、大入熱溶接したボンド部近傍ＨＡＺにＭＡが生成して、靱性の低下を招きやすくなる。よって、Ｓｉは０．０１～０．１０％の範囲とする。 Si: 0.01 to 0.10%
Si is an element added as a deoxidizer when melting steel, and it is necessary to add 0.01% or more. On the other hand, if it exceeds 0.10%, the toughness of the base material is lowered, and MA is generated in the vicinity of the bond portion HAZ subjected to high heat input welding, and the toughness is easily lowered. Therefore, Si is set in the range of 0.01 to 0.10%.
Ｍｎ：１．８０～２．４０％
　Ｍｎは、Ｃと同じく強度を高める元素であり、ＭｏやＶといった合金元素よりも安価であり、かつボンド部近傍のＨＡＺでのＭＡ生成を促進しないことから、本発明では積極的に添加する。そして、所要の強度を確保し、上記効果を得るためには、１．８０％以上の添加が必要であり、１．９０％以上の添加がより好ましく、２．００％以上の添加がさらに好ましい。一方、過剰に含有すると溶接部靭性を損なうことから、２．４０％以下であることが必要であり、２．２０％以下であることがより好ましく、２．１０％以下であることがさらに好ましい。 Mn: 1.80 to 2.40%
Mn, like C, is an element that increases the strength, is cheaper than alloy elements such as Mo and V, and does not promote the formation of MA in the HAZ near the bond portion, so it is actively added in the present invention. In order to secure the required strength and obtain the above effect, 1.80% or more of addition is necessary, 1.90% or more of addition is more preferable, and 2.00% or more of addition is more preferable. . On the other hand, if it is excessively contained, the toughness of the welded portion is impaired, so 2.40% or less is necessary, more preferably 2.20% or less, and even more preferably 2.10% or less. .
Ｐ：０．０１０％以下
　Ｐは、不純物として含有される元素の一種であるが、鋼板母材およびＨＡＺの靭性を低下させるため、素材溶製時の経済性を考慮した上で可能な範囲で低減することが好ましい。このため、Ｐ量は０．０１０％以下に制限する。好ましくは、０．００８％以下である。 P: 0.010% or less P is a kind of element contained as an impurity. However, in order to reduce the toughness of the steel plate base material and the HAZ, it is within a possible range in consideration of economics at the time of material melting. It is preferable to reduce. For this reason, the amount of P is limited to 0.010% or less. Preferably, it is 0.008% or less.
Ｓ：０．０００５～０．００４０％
　Ｓは、フェライトの核生成サイトとして作用するＭｎＳあるいはＣａＳを形成するために必要な元素である。このため０．０００５％以上を添加する。しかしながら過度に添加すると母材靭性の低下を招くため、上限は０．００４０％とする。 S: 0.0005 to 0.0040%
S is an element necessary for forming MnS or CaS that acts as a nucleation site for ferrite. For this reason, 0.0005% or more is added. However, excessive addition causes a decrease in the base material toughness, so the upper limit is made 0.0040%.
Ａｌ：０．００５～０．１００％
　Ａｌは、鋼の脱酸のために添加される元素であり、０．００５％以上含有させる必要がある。一方、０．１００％を超えて含有すると、母材の靱性のみならず、溶接金属の靱性をも低下させる。よって、Ａｌは０．００５～０．１００％の範囲とする。好ましくは０．０１０～０．１００％の範囲である。 Al: 0.005 to 0.100%
Al is an element added for deoxidation of steel, and it is necessary to contain 0.005% or more. On the other hand, if the content exceeds 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, Al is in the range of 0.005 to 0.100%. Preferably it is 0.010 to 0.100% of range.
Ｎｂ：０．００３～０．０３０％
　Ｎｂは、母材強度およびＨＡＺ最軟化部硬度、ひいては溶接継手強度を確保するのに有効な元素である。しかし、０．００３％未満の添加では、上記効果が小さい一方で、０．０３０％を超えて含有すると、ボンド部近傍のＨＡＺにＭＡが生成して靱性を低下させるようになる。よって、Ｎｂは０．００３～０．０３０％の範囲とする。 Nb: 0.003 to 0.030%
Nb is an element effective for ensuring the strength of the base material and the hardness of the HAZ softened portion, and consequently the weld joint strength. However, when the content is less than 0.003%, the above effect is small. However, when the content exceeds 0.030%, MA is generated in the HAZ near the bond portion and the toughness is lowered. Therefore, Nb is set in the range of 0.003 to 0.030%.
Ｔｉ：０．０１０～０．０５０％
　Ｔｉは、凝固時にＴｉＮとなって析出し、ボンド部近傍ＨＡＺのオーステナイト粒の粗大化を抑制する。また、Ｔｉは、フェライトの変態核となって、ＨＡＺの高靱性化に寄与すると同時に、Ｂと結合し得るＮを低減して、固溶Ｂを確保することにより、ＨＡＺ 最軟化部硬度、ひいては溶接継手強度を確保する上で、有効に作用する。斯かる効果を得るためには、０．０１０％以上の添加が必要であり、０．０１５％以上添加することが好ましい。一方、０．０５０％を超えて含有すると、析出したＴｉＮが粗大化し、上記効果が得られなくなる。よって、Ｔｉは、０．０１０～０．０５０％の範囲とする。 Ti: 0.010 to 0.050%
Ti precipitates as TiN during solidification and suppresses coarsening of austenite grains in the vicinity of the bond portion HAZ. In addition, Ti becomes a transformation nucleus of ferrite and contributes to increasing the toughness of HAZ. At the same time, by reducing N that can be combined with B and securing solid solution B, the hardness of the HAZ softened part is increased. It works effectively in securing the weld joint strength. In order to acquire such an effect, 0.010% or more of addition is required, and it is preferable to add 0.015% or more. On the other hand, if the content exceeds 0.050%, the precipitated TiN becomes coarse and the above effect cannot be obtained. Therefore, Ti is set to a range of 0.010 to 0.050%.
Ｎ：０．００３０～０．０１２０％
　Ｎは、凝固時にＴｉＮを生成し、ボンド部近傍のＨＡＺのオーステナイト粒の粗大化抑制に寄与すると同時に、ＢＮを生成し、当該ＢＮがフェライト変態核として作用することでボンド部近傍のＨＡＺの組織を微細化し、鋼材の高靭化に寄与する。そして、このようなＴｉＮを必要量確保するには、Ｎを０．００３０％以上含有することが必要であり、０．００５０％以上含有することが好ましい。さらに好ましくは０．００７０％以上である。一方、過度に含有すると、溶接入熱条件によってはＴｉＮが溶解する領域で固溶Ｎ量が増加し、ＨＡＺの靭性を低下させることがある。このことから上限を０．０１２０％以下とする。好ましくは０．０１００％以下とする。 N: 0.0030 to 0.0120%
N forms TiN during solidification and contributes to the suppression of the coarsening of HAZ austenite grains in the vicinity of the bond portion. At the same time, N forms BN, and the BN acts as a ferrite transformation nucleus so that the HAZ structure in the vicinity of the bond portion is formed. Contributes to the toughening of steel materials. And in order to ensure the required amount of such TiN, it is necessary to contain N 0.0030% or more, and it is preferable to contain 0.0050% or more. More preferably, it is 0.0070% or more. On the other hand, when it contains excessively, depending on welding heat input conditions, the amount of solid solution N may increase in the area | region where TiN melt | dissolves, and the toughness of HAZ may be reduced. Therefore, the upper limit is made 0.0120% or less. Preferably it is 0.0100% or less.
Ｂ：０．０００５～０．００２５％
　Ｂは、鋼の焼入性を向上させる元素であり、オーステナイトの変態温度を低下させることで、ベイナイトやマルテンサイトといった硬質な組織の生成を促進し、母材鋼板の高強度化に寄与する。同様に、ＨＡＺ軟化部においても軟質相であるフェライトの生成を抑制し、ＨＡＺ軟化部の強度を向上させる。これらの効果を得るには、Ｂを０．０００５％以上含有する必要がある。一方、Ｂを０．００２５％超含有すると、焼入性が過剰に高まり、母材鋼板およびＨＡＺの靱性低下を招く。このため、Ｂは０．０００５～０．００２５％の範囲とする。 B: 0.0005 to 0.0025%
B is an element that improves the hardenability of steel, and by reducing the transformation temperature of austenite, it promotes the formation of hard structures such as bainite and martensite, and contributes to increasing the strength of the base steel sheet. Similarly, the formation of ferrite, which is a soft phase, is also suppressed in the HAZ softened part, and the strength of the HAZ softened part is improved. In order to acquire these effects, it is necessary to contain B 0.0005% or more. On the other hand, when B is contained in excess of 0.0025%, the hardenability is excessively increased and the toughness of the base steel plate and HAZ is reduced. Therefore, B is in the range of 0.0005 to 0.0025%.
固溶Ｂ量を５質量ｐｐｍ以上
　本発明において、鋼材中の固溶Ｂ量は、５質量ｐｐｍ以上とする。鋼材中の固溶Ｂ量が５ｐｐｍに満たない場合、ＨＡＺ軟化領域の組織形成時に未変態オーステナイトの焼入性を向上させる効果が不十分であり、所望する硬度を得るための島状マルテンサイト量を得られなくなる。 In the present invention, the amount of solid solution B is 5 mass ppm or more. When the amount of dissolved B in the steel material is less than 5 ppm, the effect of improving the hardenability of untransformed austenite at the time of forming the structure of the HAZ softened region is insufficient, and the amount of island martensite for obtaining the desired hardness You won't get.
ＴｉとＮの質量％比（Ｔｉ／Ｎ）：２．０以上４．０未満
　Ｔｉ／Ｎは、後述のＡ値の規定とともに、本発明において、重要な要件である。Ｔｉ／Ｎは、ＨＡＺのボンド部において、ＴｉＮの微細分散状況および固溶Ｎによる靭性劣化に大きく影響するため、適切に制御する必要がある。すなわち、Ｔｉ／Ｎが４．０以上になるとＢＮが析出せず、またＴｉの硼炭化物などが析出することでＨＡＺ靭性が大きく低下する一方で、２．０を下回ると固溶ＮによるＨＡＺ靭性の低下、およびＨＡＺにおけるＢＮ析出によって、Ｂの焼入性が確保できずに所要のＨＡＺ最軟化部硬度の確保が困難となる。従って、Ｔｉ／Ｎの値は、２．０以上４．０未満とする。好ましくは、２．５以上３．５以下の範囲内である。 Mass% ratio of Ti and N (Ti / N): 2.0 or more and less than 4.0 Ti / N is an important requirement in the present invention, together with the definition of the A value described later. Ti / N greatly affects the fine dispersion state of TiN and the toughness deterioration due to solid solution N in the HAZ bond portion, and thus needs to be appropriately controlled. That is, BN does not precipitate when Ti / N is 4.0 or more, and HAZ toughness is significantly reduced by precipitation of Ti borocarbide and the like, while HAZ toughness due to solute N is less than 2.0. As a result of the decrease in B and the precipitation of BN in the HAZ, the hardenability of B cannot be ensured, and it becomes difficult to ensure the required HAZ softest part hardness. Therefore, the value of Ti / N is set to 2.0 or more and less than 4.0. Preferably, it exists in the range of 2.5 or more and 3.5 or less.
Ａ値：３以上２５以下
　以下に示す（１）式で規定されるＡ値は、本発明において最も重要な項目の一つである。鋼材が大入熱溶接の熱影響部に相当する熱履歴を受けた際に、ＴｉＮやＢＮなどの生成反応が平衡論的に進行しない場合においても、固溶Ｂによる焼入性向上効果が発揮されるためには、Ｔｉ、Ｎ、およびＢに関して前述した鋼材の添加量を満足した上で、さらにＡ値が３以上である必要がある。ただし、Ａ値が２５を超えると鋼材の焼入性が過剰となりＨＡＺの靭性に悪影響を及ぼす。したがって本発明では、Ａ値は３以上２５以下とする。好ましくは６～１５の範囲である。
　　Ａ＝２２５６×Ｔｉ－７７１６×Ｎ＋１００００×Ｂ　　・・・（１）
　　但し、各元素記号（Ｔｉ、Ｎ、Ｂ）は各元素の鋼中含有量（質量％）を示す。 A value: 3 or more and 25 or less The A value defined by the following formula (1) is one of the most important items in the present invention. When steel material receives a thermal history corresponding to the heat-affected zone of high heat input welding, even if the formation reaction of TiN, BN, etc. does not proceed in equilibrium, the effect of improving hardenability by solute B is demonstrated. In order to achieve this, it is necessary that the A value is 3 or more after satisfying the amount of addition of the steel materials described above with respect to Ti, N, and B. However, if the A value exceeds 25, the hardenability of the steel material becomes excessive and adversely affects the toughness of the HAZ. Therefore, in the present invention, the A value is 3 or more and 25 or less. Preferably it is in the range of 6-15.
A = 2256 × Ti-7716 × N + 10000 × B (1)
However, each element symbol (Ti, N, B) indicates the content (% by mass) of each element in steel.
Ｃeq：０．３８～０．４３
　本発明の大入熱溶接用鋼材は、溶接時の入熱により、母材製造時に施されたＴＭＣＰ等の組織制御の効果が全て無効となってしまう。そのため、溶接時の加熱・冷却下においても溶接継手の強度と靭性を両立させる必要があることから、焼入性の指標である炭素当量Ｃeqを適正範囲に制御する必要がある。
　具体的には、以下の（２）式で定義される炭素当量Ｃeqが０．３８～０．４３の範囲となるよう各成分の組成を制御する必要がある。上記Ｃeqが０．３８未満では、焼入性が不足し、最軟化部域の硬さが著しく低下するため、所望の溶接継手の強度を確保することができない。一方、Ｃeqが０．４３を超えると、焼入性が過剰となり、ボンド部近傍におけるフェライトの生成が抑制され、島状マルテンサイトの生成が促進されるため、十分な靭性を確保することができなくなる。好ましいＣeqは０．３９～０．４２の範囲である。
Ｃeq＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５　……（２）
　ここで、上記式中の各元素記号は、それぞれの元素（Ｃ、Ｍｎ、Ｃｒ、Ｍｏ、Ｖ、Ｃｕ、Ｎｉ）の含有量（ｍａｓｓ％）を示す。 C eq : 0.38 to 0.43
In the steel material for large heat input welding of the present invention, the effects of the structure control such as TMCP applied at the time of manufacturing the base material become invalid due to the heat input during welding. Therefore, since it is necessary to make the strength and toughness of the welded joint compatible even under heating and cooling during welding, it is necessary to control the carbon equivalent C eq , which is an index of hardenability, within an appropriate range.
Specifically, it is necessary to control the composition of each component so that the carbon equivalent C eq defined by the following formula (2) is in the range of 0.38 to 0.43. When the C eq is less than 0.38, the hardenability is insufficient and the hardness of the softest region is remarkably lowered, so that the desired weld joint strength cannot be ensured. On the other hand, if C eq exceeds 0.43, the hardenability becomes excessive, the formation of ferrite in the vicinity of the bond portion is suppressed, and the formation of island martensite is promoted, so that sufficient toughness can be ensured. become unable. A preferred C eq is in the range of 0.39 to 0.42.
C eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (2)
Here, each element symbol in the above formula indicates the content (mass%) of each element (C, Mn, Cr, Mo, V, Cu, Ni).
　以上が本発明の基本成分組成で、残部は、Ｆｅおよび不可避的不純物である。なお、不可避的不純物として、例えば、Ｏは０．００５０％以下であれば許容できる。 The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities. As an inevitable impurity, for example, O is acceptable if it is 0.0050% or less.
　本発明の鋼材は、上記成分に加えてさらに、強度向上などを目的として、Ｖ、Ｃｕ、Ｎｉ、ＣｒおよびＭｏの中から選ばれる１種以上を選択的元素として下記の範囲で含有することができる。 In addition to the above components, the steel material of the present invention may further contain at least one selected from V, Cu, Ni, Cr and Mo as a selective element in the following range for the purpose of improving the strength and the like. it can.
Ｖ：０．２０％以下、Ｃｕ：０．３０％以下、Ｎｉ：０．３０％以下、Ｃｒ：０．４０％以下およびＭｏ：０．４０％以下
　Ｖ、Ｃｕ、Ｎｉ、ＣｒおよびＭｏは、母材の高強度化に有効な元素であって、その効果を得るためには、Ｖ、ＣｕおよびＮｉは０．０５％以上、ＣｒおよびＭｏは０．０２％以上の添加が好ましい。しかし、いずれの元素も多量に添加すると、靱性に悪影響を及ぼすため、また、Ｎｉは、合金コスト増加にもつながるため、含有する場合には、Ｖは０．２０％以下、Ｃｕは０．３０％以下、Ｎｉは０．３０％以下、ＣｒおよびＭｏは０．４０％以下とするのが望ましい。 V: 0.20% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.40% or less and Mo: 0.40% or less V, Cu, Ni, Cr and Mo are It is an element effective for increasing the strength of the base material, and in order to obtain the effect, it is preferable to add 0.05% or more of V, Cu and Ni and 0.02% or more of Cr and Mo. However, if any of these elements is added in a large amount, the toughness is adversely affected. Also, since Ni leads to an increase in alloy cost, when it is contained, V is 0.20% or less and Cu is 0.30. % Or less, Ni is 0.30% or less, and Cr and Mo are preferably 0.40% or less.
　さらに、本発明の鋼材は、上記成分に加えて、Ｃａ、Ｍｇ、ＺｒおよびＲＥＭの中から選ばれる１種以上を選択的元素として下記の範囲で含有することができる。
　Ｃａ：０．０００５～０．００５０％
　Ｃａは、Ｓの固定や、酸化物、硫化物の分散による靱性改善効果を得るために含有することができる。上記効果を得るには、少なくとも０．０００５％を含有することが好ましい。しかし、０．００５０％を超えて添加しても、上記効果は飽和するだけである。よって、Ｃａを含有する場合は、０．０００５～０．００５０％の範囲とするのが好ましい。 Furthermore, in addition to the above components, the steel material of the present invention can contain one or more selected from Ca, Mg, Zr and REM as selective elements in the following range.
Ca: 0.0005 to 0.0050%
Ca can be contained in order to obtain an effect of improving toughness by fixing S and dispersing oxides and sulfides. In order to acquire the said effect, it is preferable to contain at least 0.0005%. However, even if added over 0.0050%, the above effect is only saturated. Therefore, when it contains Ca, it is preferable to set it as 0.0005 to 0.0050% of range.
Ｍｇ：０．０００５～０．００５０％、Ｚｒ：０．００１０～０．０２００％、ＲＥＭ：０．００１０～０．０２００％
　Ｍｇ、ＺｒおよびＲＥＭはいずれも、酸化物の分散による靱性改善効果を有する元素である。このような効果を発現させるには、Ｍｇは０．０００５％以上、ＺｒおよびＲＥＭは０．００１０％以上含有させることが好ましい。一方、Ｍｇは０．００５０％超、ＺｒおよびＲＥＭは０．０２００％超を添加しても、その効果は飽和するだけである。よって、これらの元素を含有する場合は、上記範囲とするのが好ましい。 Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, REM: 0.0010 to 0.0200%
Mg, Zr, and REM are all elements having an effect of improving toughness due to oxide dispersion. In order to exhibit such an effect, it is preferable to contain 0.0005% or more of Mg and 0.0010% or more of Zr and REM. On the other hand, even if Mg exceeds 0.0050% and Zr and REM add more than 0.0200%, the effect is only saturated. Therefore, when it contains these elements, it is preferable to set it as the said range.
製造方法
　上記した成分組成を有する鋼を、転炉あるいは電気炉等の常法の溶接方法を用いて溶製し、連続鋳造方あるいは造塊法等の常法の工程により鋼板製造のためのスラブ素材とすることが好ましい。以下、本発明に適用して好ましい、鋼板製造条件について説明する。 Manufacturing method Steel having the above-mentioned composition is melted using a conventional welding method such as a converter or an electric furnace, and a slab for manufacturing a steel sheet by a conventional method such as a continuous casting method or an ingot forming method. It is preferable to use a raw material. Hereinafter, preferable steel plate manufacturing conditions applied to the present invention will be described.
加熱温度：１０５０～１２００℃
　鋼素材中のＮｂ炭窒化物を完全に固溶させるため、鋼素材の加熱温度を１０５０℃以上とすることが好ましい。一方、加熱温度が１２００℃を超えると、加熱時にオーステナイト粒径の粗大化が起こり母材靱性に悪影響を及ぼすため、上限を１２００℃とすることが好ましい。 Heating temperature: 1050-1200 ° C
In order to completely dissolve Nb carbonitride in the steel material, the heating temperature of the steel material is preferably set to 1050 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grain size becomes coarse during heating and adversely affects the toughness of the base material. Therefore, the upper limit is preferably set to 1200 ° C.
オーステナイト再結晶温度域における圧延
　オーステナイト再結晶温度域における圧延は、加熱時のオーステナイト粒をある程度微細化する効果があり、最低１パス以上、好ましくは累積圧下率２０％以上を行うのが望ましい。上記成分範囲の鋼であれば、オーステナイト再結晶温度域の下限温度はおよそ９００～１０００℃の範囲にある。 Rolling in the austenite recrystallization temperature range Rolling in the austenite recrystallization temperature range has the effect of refining austenite grains during heating to some extent, and it is desirable to perform at least one pass, preferably a cumulative reduction of 20% or more. In the case of steel having the above component range, the lower limit temperature of the austenite recrystallization temperature range is in the range of approximately 900 to 1000 ° C.
オーステナイト再結晶温度域からオーステナイト未再結晶温度域までの一次冷却
　本工程は、製造工程の中では、最も重要な項目の一つである。上述したようにＨＡＺ軟化領域において組織の焼入性を向上させることのできる固溶Ｂ量は、鋼板製造時の状態で確保されている固溶Ｂ量に相当する。
　従って、鋼板製造時にＢ窒化物が大量に析出した場合、焼入性を確保するための固溶Ｂが不足し、ＨＡＺ軟化領域において十分な硬度が得られなくなる場合がある。 Primary cooling from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range This step is one of the most important items in the manufacturing process. As described above, the solid solution B amount capable of improving the hardenability of the structure in the HAZ softened region corresponds to the solid solution B amount ensured in the state at the time of manufacturing the steel sheet.
Therefore, when a large amount of B nitride is precipitated during the production of the steel sheet, the solid solution B for securing the hardenability is insufficient, and sufficient hardness may not be obtained in the HAZ softened region.
　また、鋼板製造時の冷却過程においてＢ窒化物が生成する温度域に相当する、オーステナイト再結晶温度域からオーステナイト未再結晶温度域までの冷却速度を、可能な限り速くすることが望ましい。通常、この工程は熱間圧延の温度低下待機時間として空冷されるが、本発明においては、空冷よりも大きい冷却速度を有する加速冷却を実施することで次工程である制御圧延工程までの時間を短縮するとともに、Ｂ窒化物の析出による固溶Ｂの減少を防止することができる。なお、この加速冷却は、特に１０００℃から６００℃の温度範囲で実施することが有効である。 Also, it is desirable that the cooling rate from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range, which corresponds to the temperature range in which B nitride is generated in the cooling process at the time of steel plate production, is as high as possible. Normally, this process is air-cooled as the temperature reduction standby time of hot rolling, but in the present invention, the time to the control rolling process, which is the next process, is increased by performing accelerated cooling having a cooling rate larger than that of air cooling. While shortening, the decrease of the solid solution B by precipitation of B nitride can be prevented. In addition, it is effective to implement this accelerated cooling in the temperature range of 1000 to 600 degreeC especially.
　本発明においては後述するオーステナイト未再結晶温度域における圧延後の冷却と区別するため、オーステナイト再結晶温度域での圧延に続いて実施する加速冷却を一次冷却と称する。この一次冷却においては、水冷による加速冷却設備、あるいは圧延中に鋼板表面に発生するスケールを除去する、いわゆるデスケ設備等により、空冷より大きい冷却速度を達成することが好ましい。具体的には、３℃／秒以上の冷却速度が好ましい。 In the present invention, in order to distinguish from cooling after rolling in the austenite non-recrystallization temperature range described later, accelerated cooling performed after rolling in the austenite recrystallization temperature range is referred to as primary cooling. In this primary cooling, it is preferable to achieve a cooling rate higher than that of air cooling by using an accelerated cooling facility by water cooling or a so-called deske facility that removes the scale generated on the surface of the steel sheet during rolling. Specifically, a cooling rate of 3 ° C./second or more is preferable.
オーステナイト未再結晶温度域における累積圧下率４０％以上の圧延
　上記加速冷却に引続き、オーステナイト未再結晶温度域にて制御圧延を実施する。この制御圧延において累積圧下率が小さい場合、所定の母材靭性を得ることが難しくなる。このため、累積圧下率の下限を４０％とする。累積圧下率は高い方が望ましいが、工業的には８０％程度が上限となる場合があるため、好ましくは５０～８０％である。 Rolling with a cumulative reduction ratio of 40% or more in the austenite non-recrystallization temperature region Following the accelerated cooling, controlled rolling is performed in the austenite non-recrystallization temperature region. When the cumulative rolling reduction is small in this controlled rolling, it becomes difficult to obtain a predetermined base metal toughness. For this reason, the lower limit of the cumulative rolling reduction is set to 40%. A higher cumulative rolling reduction is desirable, but about 80% may be the upper limit industrially, and therefore it is preferably 50 to 80%.
オーステナイト未再結晶温度域圧延後、Ａｒ3変態点以上の温度から５５０℃以下の温度域に二次冷却
　二次冷却は、制御圧延により加工されたオーステナイト組織を変態させることを目的とする冷却である。そして、鋼組織の相変態を完了させるためには５５０℃以下の温度域まで冷却する必要があることから冷却終了温度の下限は５５０℃が好ましい。二次冷却における冷却速度は、空冷よりも大きい冷却速度が必要であり、５℃／秒以上の強冷却が好ましい。さらに好ましくは、１０℃／秒以上の強冷却である。冷却方法は特に限定しないが、水冷による冷却が望ましい。 After austenite non-recrystallization temperature range rolling, secondary cooling from the temperature above the Ar 3 transformation point to a temperature range below 550 ° C. The secondary cooling is cooling aimed at transforming the austenite structure processed by controlled rolling. is there. And since it is necessary to cool to the temperature range below 550 degreeC in order to complete the phase transformation of a steel structure, 550 degreeC is preferable as the minimum of the completion | finish temperature of cooling. The cooling rate in the secondary cooling requires a cooling rate larger than that of air cooling, and strong cooling of 5 ° C./second or more is preferable. More preferably, it is strong cooling of 10 ° C./second or more. Although the cooling method is not particularly limited, cooling by water cooling is desirable.
　ここで、本発明における鋼材温度は鋼材の表面温度と板厚中心部の温度との平均温度を示している。Ａｒ3変態点は鋼材の組成によって異なるため、簡易的に下式によって求めることができる。なお、下式において、各元素記号は各元素の鋼中含有量（質量％）を示す。含有されない場合は０とする。
Ａｒ3（℃）＝９１０－２７３Ｃ－７４Ｍｎ－５６Ｎｉ－１６Ｃｒ－９Ｍｏ－５Ｃｕ Here, the steel material temperature in this invention has shown the average temperature of the surface temperature of steel materials, and the temperature of plate thickness center part. Since the Ar 3 transformation point varies depending on the composition of the steel material, it can be obtained simply by the following equation. In the following formula, each element symbol indicates the content (mass%) of each element in steel. When it is not contained, 0 is set.
Ar 3 (° C.) = 910-273C-74Mn-56Ni-16Cr-9Mo-5Cu
　表１に示す組成の鋼を転炉で溶製後、連続鋳造法でスラブとし、表２に示す制御圧延、制御冷却条件により４０～８０ｍｍ厚の鋼板を製造した。表２に示す枝番は、鋼成分が同じで、製造条件が違うことを示している。なお、一次冷却は圧延機の出側に設置した水冷設備により実施し、冷却中の平均冷却速度が３℃以上であることを確認している。 A steel having the composition shown in Table 1 was melted in a converter and then made into a slab by a continuous casting method, and a steel sheet having a thickness of 40 to 80 mm was manufactured under the controlled rolling and controlled cooling conditions shown in Table 2. The branch numbers shown in Table 2 indicate that the steel components are the same and the manufacturing conditions are different. In addition, primary cooling is implemented with the water cooling equipment installed in the exit side of the rolling mill, and it has confirmed that the average cooling rate during cooling is 3 degreeC or more.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　上記組成および製造工程を経て製造された厚鋼板について、板厚方向１／４の位置から平行部１４ｍｍΦの引張試験片を採取し、ＪＩＳ　Ｚ　２２４１（１９９８）の規定に準拠して引張試験を実施し、０．２％耐力（ＹＳ）および引張強さ（ＴＳ）を求めた。 For the thick steel plate manufactured through the above composition and manufacturing process, a tensile test piece with a parallel part of 14 mmΦ is taken from the position in the plate thickness direction 1/4, and a tensile test is performed in accordance with the provisions of JIS Z 2241 (1998). The 0.2% yield strength (YS) and the tensile strength (TS) were determined.
　また、板厚方向１／４の位置からＪＩＳ　Ｚ　２２０２（１９９８）の規定に準拠してＶノッチ標準寸法のシャルピー衝撃試験片を採取し、ＪＩＳ　Ｚ　２２４２（１９９８）の規定に準拠して衝撃試験を実施し、破面遷移温度（ｖＴｒｓ）を求めた。
ここで、ｖＴｒｓの目標値は－６０℃以下とした。 In addition, a Charpy impact test piece having a V-notch standard dimension was taken from the position in the plate thickness direction 1/4 according to JIS Z 2202 (1998), and the impact test was performed according to JIS Z 2242 (1998). The fracture surface transition temperature (vTrs) was determined.
Here, the target value of vTrs was set to −60 ° C. or lower.
　また、溶接入熱量：２００ｋＪ／ｃｍ以上の入熱溶接を施した際の、熱影響部における最軟化部域の硬度を評価するために、板厚方向１／４位置から、３ｍｍΦ×１０ｍｍの小型試験片を採取し、変態点直上に相当する温度の９００℃に加熱後、８００～５００℃の間を３９０秒で冷却する熱処理を行った。これらの処理を行った後の小型試験片のビッカース硬度ＨＶ（ＪＩＳ　Ｚ　２２４４（１９９８））を１ｍｍ程度の間隔で格子状に測定し、そのうち最も低い硬度を最軟化部硬度とした。最軟化部硬度の目標値は１６０以上とした。また、ＨＡＺ最軟化部域を、最も低い硬度を示した点を中心として、そこから最も近い測定点同士を結んだ領域とした。 Also, in order to evaluate the hardness of the softest zone in the heat affected zone when heat input welding of 200 kJ / cm or more is applied, a small size of 3 mmΦ × 10 mm from the 1/4 position in the plate thickness direction A test piece was collected and heated to 900 ° C., which is a temperature just above the transformation point, and then subjected to a heat treatment of cooling between 800 and 500 ° C. in 390 seconds. The Vickers hardness HV (JIS Z 2244 (1998)) of the small test piece after these treatments was measured in a grid pattern at intervals of about 1 mm, and the lowest hardness was taken as the softest part hardness. The target value of the softest part hardness was 160 or more. In addition, the HAZ most softened area is an area where the closest measurement points are connected to each other with the point showing the lowest hardness as the center.
　また、上記ＨＡＺ最軟化部域に対応する部位をナイタールでエッチングして組織を現出した。ＳＥＭを用いて１０００倍で３視野の組織写真を撮影し、それらを画像解析して、ＭＡの平均面積分率を求め、これをＨＡＺ最軟化部域のＭＡ体積分率（vol％）とした。 In addition, the structure corresponding to the above-mentioned softened area of the HAZ was etched with nital to reveal the structure. Three-view tissue photographs were taken at 1000 times using SEM, and the images were analyzed to determine the average area fraction of MA, which was defined as the MA volume fraction (vol%) of the HAZ softest area. .
　溶接入熱量：２００ｋＪ／ｃｍ以上の入熱溶接を施した際の、熱影響部におけるボンド部近傍部の靭性を評価するために、上記厚鋼板から幅：８０ｍｍ×長さ：８０ｍｍ×厚さ：１５ｍｍの試験片を採取し、１４５０℃に加熱後、８００～５００℃の間を３９０秒で冷却した後、２ｍｍＶノッチシャルピー試験片を採取し、上記と同様にシャルピー衝撃試験を行った衝撃試験温度は－４０℃とし、３本の試験の平均値により評価した。目標値は－４０℃における平均吸収エネルギー（ｖＥ-40℃）で５０Ｊ以上とした。また、上記と同様に、熱履歴付与後の試験片断面におけるＭＡの面積分率を評価した。 In order to evaluate the toughness of the vicinity of the bond portion in the heat affected zone when heat input welding of 200 kJ / cm or more is applied, the width: 80 mm × length: 80 mm × thickness: A 15 mm test piece was collected, heated to 1450 ° C., cooled between 800 and 500 ° C. in 390 seconds, a 2 mm V notch Charpy test piece was taken, and the Charpy impact test was performed in the same manner as described above. Was −40 ° C. and was evaluated by the average of three tests. The target value was an average absorbed energy (vE -40 ° C) at -40 ° C of 50 J or more. Moreover, the area fraction of MA in the test piece cross section after thermal history provision was evaluated similarly to the above.
　表３に、上記手順にて評価を行った鋼材の母材特性、ＨＡＺ特性およびＨＡＺ中のＭＡ体積分率（vol％）の測定結果を示す。 Table 3 shows the measurement results of the base material characteristics, the HAZ characteristics, and the MA volume fraction (vol%) in the HAZ evaluated in the above procedure.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　同表より、鋼板組成Ｎｏ．１～１２のおける枝番Ａの例では、母材ならびにＨＡＺにおいて優れた特性が得られていることが分かる。これに対して鋼板組成Ｎｏ．１～４における枝番Ｂの鋼板においては、製造条件の影響により、本発明の要件を満足せず、母材特性およびＨＡＺ特性が劣っている。また、鋼板組成Ｎｏ．１３～２７においては、化学成分が本発明の定める範囲を外れているために、枝番Ａであっても、ＨＡＺ特性に劣っていることが分かる。 From the table, steel plate composition No. In the example of branch number A in 1 to 12, it can be seen that excellent characteristics are obtained in the base material and HAZ. On the other hand, the steel plate composition No. In the steel plates with branch numbers B in 1-4, the requirements of the present invention are not satisfied due to the influence of manufacturing conditions, and the base material characteristics and the HAZ characteristics are inferior. Steel plate composition No. In Nos. 13 to 27, since the chemical component is outside the range defined by the present invention, it is understood that even the branch number A is inferior in the HAZ characteristics.

Claims (3)

  1. 　質量％で、Ｃ：０．０３０～０．０８０％、Ｓｉ：０．０１～０．１０％、Ｍｎ：１．８０～２．４０％、Ｐ：０．０１０％以下、Ｓ：０．０００５～０．００４０％、Ａｌ：０．００５～０．１００％、Ｎｂ：０．００３～０．０３０％、Ｔｉ：０．０１０～０．０５０％、Ｎ：０．００３０～０．０１２０％およびＢ：０．０００５～０．００２５％を含有し、さらにＴｉとＮの質量％比（Ｔｉ／Ｎ）が２．０以上４．０未満であって、以下の（１）式で規定されるＡ値が３以上２５以下、以下の（２）式で規定されるＣｅｑが０．３８～０．４３の範囲で、残部がＦｅおよび不可避的不純物の成分組成からなり、降伏応力が４６０ＭＰａ以上であって、かつ、固溶Ｂ量が５質量ｐｐｍ以上であり、溶接入熱量：２００ｋＪ／ｃｍ以上の入熱溶接を施した際の、熱影響部におけるボンド部近傍の組織中の島状マルテンサイトが１vol％以下で、かつ熱影響部における最軟化部域の組織中の島状マルテンサイトが５vol％以上である溶接用鋼材。
    Ａ＝２２５６×Ｔｉ－７７１６×Ｎ＋１００００×Ｂ　　・・・（１）
    Ｃeq＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５　……（２）
    但し、各元素記号は各元素の鋼中含有量（質量％）を示す。     By mass%, C: 0.030 to 0.080%, Si: 0.01 to 0.10%, Mn: 1.80 to 2.40%, P: 0.010% or less, S: 0.0005 To 0.0040%, Al: 0.005 to 0.100%, Nb: 0.003 to 0.030%, Ti: 0.010 to 0.050%, N: 0.0030 to 0.0120%, and B: 0.0005 to 0.0025% is contained, and the mass% ratio (Ti / N) of Ti and N is 2.0 or more and less than 4.0, and is defined by the following formula (1). The A value is 3 or more and 25 or less, the Ceq defined by the following formula (2) is in the range of 0.38 to 0.43, the balance is composed of Fe and inevitable impurities, and the yield stress is 460 MPa or more. And the amount of solute B is 5 ppm by mass or more, and the amount of welding heat input is 200 kJ / cm or more. When heat welding is performed, the island-like martensite in the structure in the vicinity of the bond portion in the heat-affected zone is 1 vol% or less, and the island-like martensite in the structure in the softest zone in the heat-affected zone is 5 vol% or more. Steel material.
    A = 2256 × Ti-7716 × N + 10000 × B (1)
    C eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (2)
    However, each element symbol indicates the content (mass%) of each element in steel.
  2. 　上記成分組成に、さらに、質量％で、Ｖ：０．２０％以下、Ｃｕ：０．３０％以下、Ｎｉ：０．２０％以下、Ｃｒ：０．４０％以下およびＭｏ：０．４０％以下のうちから選んだ１種以上を含有する請求項１に記載の溶接用鋼材。 In addition to the above component composition, further, in mass%, V: 0.20% or less, Cu: 0.30% or less, Ni: 0.20% or less, Cr: 0.40% or less, and Mo: 0.40% or less The steel material for welding according to claim 1, comprising one or more selected from among the above.
  3. 　上記成分組成に、さらに、質量％で、Ｃａ：０．０００５～０．００５０％、Ｍｇ：０．０００５～０．００５０％、Ｚｒ：０．００１０～０．０２００％、ＲＥＭ：０．００１０～０．０２００％のうちから選んだ１種以上を含有する請求項１または２に記載の溶接用鋼材。 In addition to the above-described component composition, by mass%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, REM: 0.0010 to The steel for welding according to claim 1 or 2, comprising one or more selected from 0.0200%.
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JP2011074403A (en) * 2009-09-16 2011-04-14 Jfe Steel Corp Steel for high heat input welding
JP2012162801A (en) * 2011-01-18 2012-08-30 Jfe Steel Corp Method for producing taper plate
JP2014029019A (en) * 2012-07-03 2014-02-13 Jfe Steel Corp Method for producing steel sheet for large heat input welding excellent in brittle crack arrest property
JP2014043642A (en) * 2012-07-30 2014-03-13 Jfe Steel Corp Method of manufacturing steel plate for high heat input welding
JP2014031544A (en) * 2012-08-03 2014-02-20 Jfe Steel Corp Steel material for large heat input welding

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WO2016060141A1 (en) * 2014-10-17 2016-04-21 Ｊｆｅスチール株式会社 Steel for high-energy-input welding
JPWO2016060141A1 (en) * 2014-10-17 2017-04-27 Ｊｆｅスチール株式会社 Steel material for large heat input welding
JPWO2022070873A1 (en) * 2020-09-30 2022-04-07
JP7272471B2 (en) 2020-09-30 2023-05-12 Ｊｆｅスチール株式会社 steel plate

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