WO2022054898A1 - Thick steel sheet and manufacturing method for same - Google Patents

Thick steel sheet and manufacturing method for same Download PDF

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Publication number
WO2022054898A1
WO2022054898A1 PCT/JP2021/033277 JP2021033277W WO2022054898A1 WO 2022054898 A1 WO2022054898 A1 WO 2022054898A1 JP 2021033277 W JP2021033277 W JP 2021033277W WO 2022054898 A1 WO2022054898 A1 WO 2022054898A1
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less
phase
steel sheet
content
thick steel
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PCT/JP2021/033277
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French (fr)
Japanese (ja)
Inventor
義浩 兵藤
智之 横田
仁 末吉
昇輝 藤田
進一 三浦
善明 村上
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Ｊｆｅスチール株式会社
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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 KR1020237005688A priority Critical patent/KR20230041060A/en
Priority to JP2021573411A priority patent/JP7070814B1/en
Priority to CN202180051128.9A priority patent/CN115989327A/en
Publication of WO2022054898A1 publication Critical patent/WO2022054898A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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

Definitions

  • the present invention relates to a thick steel sheet and a method for manufacturing the same, and more particularly to a thick steel sheet having excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at the total thickness and a method for producing the same.
  • the thick steel plate of the present invention is strongly required to have structural safety such as ships, marine structures, bridges, buildings, tanks, etc., and can be suitably used for welded structures.
  • Thick steel plates are widely used in structures such as ships, marine structures, bridges, buildings, and tanks. Such a thick steel sheet is required to have excellent fatigue characteristics in addition to excellent mechanical properties such as strength and toughness and weldability.
  • the thick steel sheet is required to have fatigue characteristics that can ensure the safety of the structure even when such a repetitive load is applied.
  • Fatigue fracture is a phenomenon that follows the stage where fine cracks (fatigue cracks) first occur and then the cracks spread (progress).
  • fatigue cracks generally occur from the weld and propagate through the steel material, leading to fracture in many cases. It is said that this is due to the fact that the welded portion tends to be a stress concentration portion due to its shape, and in addition, residual tensile stress is generated after welding. Therefore, as a means for suppressing the generation of cracks from the welded portion, a technique of introducing the residual stress of compression by peening or the like is widely known.
  • Patent Document 1 in a method for manufacturing a thick steel sheet having a plate thickness of 20 mm or less, the amount of C added is reduced to control Ceq (carbon equivalent) within a specific range, and the cooling shutdown temperature is lowered to achieve elongation.
  • Ceq carbon equivalent
  • Patent Document 2 describes a method for producing a thick steel sheet having a small anisotropy of crack propagation characteristics by combining heating, rolling, accelerated cooling and heat treatment according to the yield stress.
  • Patent Document 3 a duplex stainless steel having a microstructure composed of bainite and ferrite having an area ratio of 38 to 52% is used, and the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase are controlled. By doing so, the fatigue crack propagation characteristics are improved.
  • the microstructure in the range from the surface to 100 ⁇ m below the surface in the plate thickness direction has an area ratio of 80% or more.
  • the microstructure in the range from 100 ⁇ m below the surface to the plate thickness 1/2 position contains a ferrite phase with an area ratio of 80% or less, and the balance is a pearlite phase, a bainite phase, or a pearlite phase and a bainite phase.
  • a thick steel plate composed of a mixed phase of is proposed.
  • Japanese Unexamined Patent Publication No. 2010-196109 Japanese Unexamined Patent Publication No. 2007-332402
  • Japanese Unexamined Patent Publication No. 08-225882 Japanese Unexamined Patent Publication No. 2019-026927
  • Patent Documents 1 to 4 have the following problems.
  • Patent Document 3 As in Patent Document 3, as in Patent Document 1, a thick steel sheet is manufactured by an online process by rolling and accelerated cooling control. Therefore, especially in a thin material having a plate thickness of 20 mm or less, a temperature deviation at the tip and tail of the steel sheet is likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. There is a problem that it cannot be obtained.
  • Patent Document 4 the reheated hot-rolled plate is cooled and hardened at an average cooling rate of 7.7 to 16.9 ° C./s.
  • the cooling rate is high, the bainite phase is predominantly generated over the pearlite phase, and since island-like martensite is present in the bainite phase, the toughness value deteriorates.
  • the conventional manufacturing method has a problem that it is not possible to manufacture a thick steel sheet having all of elongation (also referred to as total thickness elongation) characteristics, fatigue crack propagation characteristics and toughness at the total thickness.
  • the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel sheet having high strength and excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at the total thickness, and a method for producing the same. do.
  • the thick steel sheet after hot rolling is completed and cooled has a structure variation due to a cooling deviation.
  • the structure variation should be reheated to a temperature in the two-phase region or higher. Can be resolved by.
  • the toughness value can be improved by producing more pearlite phase than bainite phase.
  • the present invention has been made based on the above findings, and its gist structure is as follows.
  • C 0.05 to 0.20%
  • Si 0.01-0.50%
  • Mn 0.50 to 2.00%
  • P 0.05% or less
  • S Contains 0.02% or less
  • the microstructure contains a ferrite phase with an area ratio of 80% or more in the range from the surface to 100 ⁇ m below the surface in the plate thickness direction. In the plate thickness direction, in the range from 100 ⁇ m below the surface to the plate thickness 1/4 position.
  • a ferrite phase with an area ratio of 80% or less A thick steel plate in which the balance is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
  • the composition of the components is further increased by mass%.
  • the steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
  • the heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate. Cool the hot rolled plate and Then, it was reheated to a reheating temperature of 950 ° C. or higher and equal to or higher than the Ac1 transformation point.
  • the steel sheet reheated to a temperature above the Ac1 transformation point and below 950 ° C. is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
  • a method for manufacturing a thick steel sheet in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
  • the steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
  • the heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
  • the steel sheet cooled to a temperature above the Ar1 transformation point and below the Ar3 transformation point is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
  • a method for manufacturing a thick steel sheet in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
  • the present invention it is possible to obtain a thick steel sheet having high strength and excellent elongation characteristics, fatigue crack propagation characteristics and toughness at the total thickness.
  • the thick steel sheet of the present invention even if fatigue cracks occur over time from stress-concentrated portions, welded portions, etc., the subsequent propagation of cracks is suppressed, so that the safety of the entire steel structure is enhanced. Is possible.
  • the thick steel plate of the present invention for structures such as bridges, ships, building structures, and construction industrial machinery, it is possible to reduce the maintenance cost and the life cycle cost of such structures. , Extremely useful industrially.
  • FIG. 1 is a schematic diagram of a one-sided notch simple tensile type fatigue test piece used in the fatigue crack propagation test.
  • C 0.05 to 0.20% C is an element having an effect of increasing the hardness of the matrix phase and improving the strength.
  • the C content is preferably 0.08% or more, more preferably 0.10% or more, and further preferably 0.12% or more.
  • the C content is set to 0.20% or less.
  • the C content is preferably 0.18% or less, more preferably 0.16% or less, still more preferably 0.14% or less.
  • Si 0.01-0.50% Si is an element that acts as a deoxidizing agent and dissolves in steel to increase the hardness of the matrix phase by solid solution strengthening.
  • the Si content needs to be 0.01% or more.
  • the Si content is preferably 0.05% or more, more preferably 0.1% or more, still more preferably 0.15% or more, and most preferably 0.20% or more.
  • the Si content is set to 0.50% or less.
  • the Si content is preferably 0.45% or less, more preferably 0.40% or less, still more preferably 0.35% or less, and most preferably 0.30% or less.
  • Mn 0.50 to 2.00%
  • Mn is an element having the effect of increasing the hardness of the matrix phase and improving the strength.
  • the Mn content needs to be 0.50% or more.
  • the Mn content is preferably 0.60% or more, more preferably 0.70% or more, still more preferably 0.80% or more, and most preferably 1.00% or more.
  • the Mn content is set to 2.00% or less.
  • the Mn content is preferably 1.85% or less, more preferably 1.70% or less, still more preferably 1.55% or less, and most preferably 1.40% or less.
  • P 0.05% or less
  • P is an element contained in steel as an unavoidable impurity. P is preferably reduced as much as possible because it segregates at the grain boundaries and has an adverse effect such as lowering the toughness of the base metal and the welded portion. However, a content of 0.05% or less is acceptable. Therefore, the P content is set to 0.05% or less.
  • the P content is preferably 0.04% or less, more preferably 0.03% or less.
  • the lower limit of the P content is not limited, it is preferable to set the P content to 0.001% or more because an excessive reduction causes an increase in the refining cost.
  • the P content is preferably 0.002% or more, more preferably 0.003% or more.
  • S 0.02% or less
  • S is an element contained in steel as an unavoidable impurity. S is present in steel as a sulfide-based inclusion such as MnS and becomes a starting point of brittle fracture and deteriorates toughness. Therefore, it is preferable to reduce S as much as possible, but a content of 0.02% or less is acceptable. Therefore, the S content is 0.02% or less. The S content is preferably 0.01% or less.
  • the lower limit of the S content is not limited, it is preferable to set the S content to 0.0005% or more because an excessive reduction causes an increase in the refining cost.
  • the rest consists of Fe and unavoidable impurities. If the content of oxygen (O) contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet increases, so that cracks occur starting from the inclusions. It will be easier. Therefore, the O content is preferably 0.0050% or less. Similarly, when the content of N contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet becomes large, so that cracks are likely to occur starting from the inclusions. .. Therefore, the N content is preferably 0.0050% or less. The N content is more preferably 0.0040% or less. Similarly, sol., Which is contained as an unavoidable impurity.
  • the Al content is preferably 0.060% or less. If the Al content exceeds 0.060%, Al is mixed into the weld metal portion during welding, and the toughness of the weld portion deteriorates. Therefore, sol.
  • the Al content is preferably 0.060% or less. sol.
  • the Al content is more preferably 0.050% or less, and further preferably 0.040% or less.
  • Cr 0.01 to 1.00%
  • Cu 0.01 to 2.00%
  • Ni 0.01 to 2.00%
  • Mo 0.01 to 1.00%
  • Co 0.01 to 1.00%
  • Sn 0.005 to 0.500%
  • Sb 0.005 to 0.200%
  • Nb 0.005 to 0.200%
  • V 0.005 to 0.200%
  • Ti 0.005 to 0.050%
  • B 0.0001 to 0.0050%
  • Zr 0.005 to 0.100%
  • Ca 0.0001 to 0.020%
  • Mg It can optionally contain one or more selected from: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
  • Cr 0.01-1.00%
  • Cr is an element having an effect of further improving the strength.
  • Cr is an element that promotes the formation of cementite, and promotes the formation of a pearlite phase that is advantageous in fatigue resistance characteristics.
  • the Cr content is set to 0.01% or more in order to obtain the above effect. It is preferably 0.10% or more.
  • the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, when Cr is contained, it is set to 1.00% or less.
  • the Cr content is preferably 0.80% or less, more preferably 0.50% or less.
  • Cu 0.01-2.00%
  • Cu is an element whose strength is further increased by solid solution.
  • the Cu content is set to 0.01% or more in order to obtain the above effect.
  • the Cu content is preferably 0.05% or more, more preferably 0.10% or more.
  • the Cu content exceeds 1.00%, the weldability is impaired and defects are likely to occur during the production of the thick steel sheet. Therefore, when Cu is contained, the content is 2.00% or less.
  • the Cu content is preferably 0.70% or less, more preferably 0.60% or less, still more preferably 0.50% or less.
  • Ni 0.01-2.00%
  • Ni is an element having an effect of improving low temperature toughness, and Ni improves hot brittleness when Cu is contained.
  • the Ni content is set to 0.01% or more in order to obtain the above effect.
  • the Ni content is preferably 0.05% or more.
  • the Ni content exceeds 1.00%, the weldability is impaired and the steel material cost increases. Therefore, when Ni is contained, it should be 1.00% or less.
  • the Ni content is preferably 0.70% or less, more preferably 0.40% or less.
  • Mo 0.01-1.00%
  • Mo is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties.
  • the Mo content is set to 0.01% or more in order to obtain this effect.
  • the Mo content is preferably 0.05% or more.
  • the Mo content is set to 1.00% or less.
  • the Mo content is preferably 0.80% or less, more preferably 0.70% or less.
  • Co 0.01-1.00%
  • Co is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties.
  • the Co content is set to 0.01% or more. It is preferably 0.10% or more, more preferably 0.20% or more, still more preferably 0.35% or more.
  • the Co content is set to 1.00% or less.
  • the Co content is preferably 0.50% or less.
  • Sn 0.005 to 0.500%
  • Sn is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. In order to sufficiently obtain such an effect, when Sn is contained, the content is 0.005% or more. It is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more. On the other hand, if the Sn content exceeds 0.500%, the ductility and toughness of the steel are deteriorated. Therefore, when it is contained, it should be 0.500% or less. It is preferably 0.300% or less, more preferably 0.200% or less, and further preferably 0.100% or less.
  • Sb 0.005 to 0.200%
  • Sb is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. In order to sufficiently obtain such an effect, when Sb is contained, the content should be 0.005% or more.
  • the Sb content is preferably 0.010% or more, more preferably 0.020% or more.
  • the Sb content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, when it is contained, the Sb content is 0.200% or less. It is preferably 0.150% or less, more preferably 0.100% or less, still more preferably 0.080% or less, and most preferably 0.050% or less.
  • Nb 0.005 to 0.200%
  • Nb is an element having an effect of suppressing recrystallization of austenite during hot rolling and finely granulating the finally obtained crystal grains. Further, Nb is deposited during air cooling after accelerated cooling to further improve the strength.
  • the Nb content is set to 0.005% or more in order to obtain the above effect.
  • the Nb content is preferably 0.007% or more, more preferably 0.010% or more.
  • the Nb content exceeds 0.200%, the hardenability becomes excessive and bainite is excessively produced, so that a desired structure cannot be obtained and the toughness is lowered. Therefore, when Nb is contained, the Nb content is 0.200% or less.
  • the Nb content is preferably 0.070% or less, more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.030% or less.
  • V 0.005 to 0.200% Similar to Nb, V is an element having the effect of suppressing recrystallization of austenite during hot rolling to make it finer and precipitating in the air cooling process after hot rolling to increase the strength, which is desired. It can be arbitrarily contained depending on the characteristics to be rolled. In order to obtain the above effect, when V is contained, the V content is set to 0.005% or more. The V content is preferably 0.010%, more preferably 0.020% or more, and even more preferably 0.030% or more. However, if the V content exceeds 0.200%, a large amount of VC is deposited and the toughness is impaired. Therefore, when V is contained, the V content is set to 0.200% or less. The V content is preferably 0.150% or less, more preferably 0.100% or less, and even more preferably 0.070% or less.
  • Ti 0.005 to 0.050% Ti has a strong tendency to form a nitride and fixes N to reduce the solid solution N, so that it has an effect of improving the toughness of the base metal and the welded portion. Further, when B is contained, by including Ti together, it is possible to prevent Ti from fixing N and B from precipitating as BN. As a result, the hardenability improving effect of B can be promoted, and the strength can be further improved. Therefore, it can be arbitrarily contained depending on the desired characteristics. In order to obtain the above effect, when Ti is contained, the content is 0.005% or more. The Ti content is preferably 0.007% or more, more preferably 0.010% or more.
  • the Ti content is set to 0.050% or less.
  • the Ti content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less.
  • B 0.0001 to 0.0050%
  • B is an element having an effect of significantly improving hardenability and increasing strength even when contained in a small amount, and can be contained according to desired properties.
  • the content is 0.0001% or more.
  • the B content is preferably 0.0005% or more, and more preferably 0.001% or more.
  • the B content is set to 0.0050% or less.
  • the B content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less.
  • Zr 0.005 to 0.100%
  • Zr is an element having the effect of further increasing the strength.
  • the Zr content is set to 0.005% or more.
  • the Zr content is preferably 0.010% or more, more preferably 0.030% or more, and even more preferably 0.050% or more.
  • the Zr content exceeds 0.100%, the strength improving effect is saturated. Therefore, when Zr is contained, the Zr content is set to 0.100% or less.
  • Ca 0.0001 to 0.020%
  • Ca binds to S, suppresses the formation of MnS and the like that extend long in the rolling direction, controls the morphology of the sulfide-based inclusions so as to have a spherical shape, and contributes to the improvement of the toughness of the welded portion, which is desired. It can be contained according to the characteristics.
  • the Ca content is set to 0.0001% or more in order to obtain this effect.
  • the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.
  • the Ca content is set to 0.020% or less.
  • the Ca content is preferably 0.010% or less, more preferably 0.006% or less, and even more preferably 0.002% or less.
  • Mg 0.0001 to 0.020%
  • Mg is an element having an effect of improving toughness through the miniaturization of crystal grains.
  • the Mg content is set to 0.0001% or more in order to obtain the above effect.
  • the Mg content is preferably 0.0003% or more, more preferably 0.0005% or more.
  • the Mg content exceeds 0.020%, the effect is saturated. Therefore, when Mg is contained, the Mg content is 0.020% or less.
  • the Mg content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.
  • REM 0.0001 to 0.020% REM (rare earth metal) is an element that has the effect of improving toughness.
  • the REM content is set to 0.0001% or more in order to obtain the above effect.
  • the REM content is preferably 0.0003% or more.
  • the REM content exceeds 0.020%, the effect is saturated. Therefore, when REM is added, the REM content is 0.020% or less.
  • the REM content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less.
  • the reason for limiting the microstructure of the thick steel sheet will be described.
  • “%” in the description of the microstructure shall indicate the area ratio unless otherwise specified.
  • the "tip” of the thick steel sheet in the following description is defined as a position 100 mm from the tip of the steel sheet in the rolling direction to the tail end side.
  • the "tail end” of a thick steel sheet is defined as a position 100 mm from the tail end in the rolling direction of the steel sheet to the tip end side.
  • the "center” of the thick steel sheet is defined as the position at the center of the steel sheet in the rolling direction (longitudinal direction).
  • the microstructure in the range from the surface to 100 ⁇ m below the surface (hereinafter, may be simply referred to as “surface layer portion”) in the plate thickness direction is assumed to contain a ferrite phase having an area ratio of 80% or more. ..
  • a surface decarburization reaction occurs, and 80% or more of ferrite is generated in the surface layer to soften the surface layer of the thick steel sheet, resulting in elongation characteristics at full thickness. Can be significantly improved.
  • This surface decarburization reaction occurs by passing through or retaining the two-phase region in the reheating process.
  • the area ratio of the ferrite phase in the surface layer portion is less than 80%, a large amount of a hard residual structure composed of a bainite phase, a pearlite phase, a martensite phase, or a mixed phase thereof is present.
  • the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness.
  • the tensile strength may become excessive.
  • the area ratio of the ferrite phase in the surface layer portion refers to the average value of the area ratio of the ferrite phase in the range from the surface to 100 ⁇ m below the surface of the thick steel sheet.
  • the microstructure in the surface layer portion refers to the microstructure of the surface layer portion at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the average value of the area ratio of the ferrite phase in the range from the surface to 100 ⁇ m below the surface at the tip, center and tail end in the rolling direction of the thick steel sheet is 80% or more.
  • the thick steel sheet of the present invention has an area ratio of the ferrite phase in the surface layer portion of 80% or more over the entire length in the rolling direction. That is, in the present invention, the area ratio of the ferrite phase in the surface layer portion is 80% or more, which means that the area ratio of the ferrite phase in the surface layer portion is 80% or more at any of the tip, the center, and the tail end over the entire length in the rolling direction. Means that is obtained.
  • the rest of the microstructure of the surface layer other than the ferrite phase is preferably composed of a pearlite phase or a mixed phase of a pearlite phase and a pearlite phase, but the pearlite phase contains island-like martensite and deteriorates toughness. Is preferable, and it is more preferable to use only the pearlite phase.
  • the microstructure in the range from 100 ⁇ m below the surface to 1/4 of the plate thickness (plate thickness internal structure)
  • the microstructure in the range from 100 ⁇ m below the surface to the 1/4 position of the plate thickness (hereinafter, may be simply referred to as “inside the plate thickness”) in the plate thickness direction is 80% or less in area ratio. It shall contain the ferrite phase of.
  • the area ratio of the ferrite phase inside the plate thickness refers to the average value of the area ratio of the ferrite phase in the range from 100 ⁇ m below the surface to the 1/4 position of the plate thickness of the thick steel plate.
  • the microstructure inside the plate thickness refers to the microstructure inside the plate thickness at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the microstructure in the range from 100 ⁇ m below the surface to the 1/4 position of the plate thickness satisfies the above conditions at the tip, center and tail end of the thick steel sheet in the rolling direction.
  • the microstructure inside the plate thickness at the tip, center and tail end satisfies the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. .. Therefore, in the thick steel sheet of the present invention, it can be said that the microstructure inside the plate thickness is a ferrite phase having an area ratio of 80% or less over the entire length in the rolling direction.
  • the remainder in the microstructure inside the plate thickness is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase. ..
  • the bainite phase contains island-like martensite and deteriorates toughness. Therefore, the desired toughness can be obtained by increasing the surface integral of the pearlite phase to be larger than the surface integral of the bainite phase.
  • the surface integral of the bainite phase is preferably 15% or less. It is more preferably 13% or less, still more preferably 11% or less.
  • the remaining portion of the thick steel plate of the present invention refers to the remaining portion inside the surface layer portion and the plate thickness at the tip, center and tail end. That is, over the entire length of the thick steel plate in the rolling direction, the balance of the microstructure is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
  • microstructure inside the surface layer and the plate thickness can be evaluated by the method described in the examples.
  • the total thickness elongation of the thick steel sheet is not particularly limited, but is preferably 19% or more when the plate thickness exceeds 16 mm and 15% or more when the plate thickness is 16 mm or less.
  • the tensile strength (TS) of the thick steel sheet is not particularly limited, but is preferably 490 MPa or more.
  • the upper limit of TS is not particularly limited, but for example, in the case of 490 MPa (50 kgf / mm 2 ) class in JIS, TS may be 610 MPa or less.
  • the upper and lower limits of TS may be set to 570 MPa and 720 MPa, respectively.
  • it is preferable that the above TS conditions are satisfied at the tip, center and tail end of the thick steel sheet in the rolling direction. Normally, if the tip, center and tail end satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Further, TS can be measured by the method described in Examples.
  • the thick steel sheet of the present invention has excellent toughness as a result of having the above-mentioned composition and microstructure.
  • the toughness of the thick steel sheet of the present invention is not particularly limited, but when the test piece thickness is 10 mm, the Charpy absorption energy vE 0 at 0 ° C., which is one of the indicators of toughness, is preferably 100 J or more, preferably 130 J or more. More preferably, it is more preferably 150 J or more, and most preferably 200 J or more.
  • the upper limit of vE 0 is not limited, but may be, for example, 400 J or less, 300 J or less, or 270 J or less.
  • the Charpy absorption energy vE 0 at 0 ° C. is 50 J or more.
  • the upper limit of vE 0 is not limited, but may be, for example, 200 J or less, 150 J or less, or 135 J or less.
  • vE 0 can be measured by the method described in the Example.
  • the thick steel sheet of the present invention can have excellent fatigue crack propagation characteristics.
  • the fatigue crack propagation velocity (da / dN) can be used.
  • the "thick steel sheet” in the present invention refers to a steel sheet having a thickness of 6 mm or more according to the usual definition in the present technical field.
  • the upper limit of the plate thickness of the thick steel plate in the present invention is not particularly limited and can be any value.
  • the thickness of the thick steel plate is preferably 25 mm or less, more preferably 20 mm or less.
  • the thick steel sheet of the present invention is a method in which a steel material having the above-mentioned composition is sequentially subjected to heating, hot rolling, cooling, reheating, cooling, and quenching, or heating, hot rolling, cooling, and quenching. It can be obtained by a method of sequentially performing treatment. First, a method of sequentially performing heating, hot rolling, cooling, reheating, cooling, and quenching will be described.
  • any steel material having the above-mentioned composition and capable of hot rolling can be used, but usually a steel slab may be used.
  • molten steel having the above-mentioned composition can be melted by means such as a converter and used as a steel material such as a slab by a casting method such as a continuous casting method.
  • a steel material such as a slab can be used by the ingot-decomposition rolling method.
  • Heating A steel material having the above composition is heated to 900 to 1200 ° C. If the heating temperature is less than 900 ° C., the deformation resistance of the steel material in the next hot rolling step increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. The heating temperature is preferably 950 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the toughness decreases. Therefore, the heating temperature is set to 1200 ° C. or lower. The heating temperature is preferably 1150 ° C. or lower.
  • the slab When a steel material (slab) is manufactured by a method such as continuous casting, the slab may be directly subjected to the above heating step without being cooled, or may be subjected to the above heating step after being cooled.
  • the heating method is not particularly limited, but for example, heating can be performed in a heating furnace according to a conventional method.
  • the heated steel material is hot-rolled to obtain a hot-rolled plate.
  • the cumulative reduction rate is set to 50% or more.
  • the ferrite grains inside the plate thickness become coarse and a region with low brittleness is locally generated, brittle cracks are likely to occur, and the toughness deteriorates.
  • Other conditions relating to the hot rolling process are not particularly limited.
  • first cooling step the steel sheet after hot rolling is cooled.
  • the cooling can be performed by any method, for example, air cooling or accelerated cooling. Further, the cooling conditions are not particularly limited.
  • the cooled steel sheet is reheated to 950 ° C. or higher at the Ac1 transformation point or higher.
  • the reheating temperature is preferably less than the Ac3 transformation point.
  • the reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, the decarburization reaction peculiar to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer portion can be 80% or more.
  • the reheating temperature is equal to or higher than the Ac3 transformation point and 950 ° C. or lower, the ferrite phase in the surface layer portion generated by the surface decarburization reaction when passing through the two-phase region is formed by shortening the holding time at the reheating temperature. The reaction of reverse transformation to the austenite phase is suppressed, and the area ratio of the ferrite phase in the surface layer portion can be 80% or more.
  • the reheating temperature exceeds 950 ° C.
  • the reaction in which the ferrite phase in the surface layer portion generated by the surface decarburization reaction when passing through the two-phase region reversely transforms into the austenite phase is promoted, and the area ratio of the ferrite phase in the surface layer portion increases. It will be less than 80%. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness.
  • the reheating temperature is above the Ac3 transformation point and below 950 ° C.
  • the crystal grain size of the austenite phase inside the plate thickness becomes coarser than when the reheating temperature is below the Ac3 transformation point. It was found that the toughness was not excessively deteriorated.
  • the rate at which the reverse transformation to the austenite phase inside the plate thickness proceeds increases. Therefore, since the desired matrix structure is obtained in a short heating time, the number of thick steel sheets that can be manufactured in a predetermined time increases, and the productivity is improved.
  • the temperature exceeds 950 ° C. the austenite phase reverse-transformed inside the plate thickness grows and becomes coarse, and as a result, a region having low toughness locally is generated and the toughness decreases.
  • the reheating temperature is lower than the Ac1 transformation point, the reaction of reverse transformation to the austenite phase does not occur, and the ferrite phase, the pearlite phase and the bainite phase inside the plate thickness after cooling do not have the desired area ratio. As a result, fatigue characteristics (crack propagation characteristics) deteriorate. Further, it is not possible to eliminate the variation in mechanical properties due to the cooling deviation in the cooling process after hot rolling.
  • the Ac1 transformation point can be obtained, for example, by the following equation (1).
  • Ac1 (° C.) 723 + 29.1 x Si-10.7 x Mn-16.9 x Ni + 16.9 x Cr ...
  • the Ac3 transformation point can be obtained by, for example, the following equation (2).
  • Ac3 (° C.) 961.6-311.9 x C + 49.5 x Si-36.4 x Mn + 438.1 x P-2818 x S + 12.7 x Al-51 x Cu-29 x Ni-8.7 x Cr + 13 .5 x Mo + 308.1 x Nb-140 x V + 318.9 x Ti + 611.2 x B-969 x N ...
  • the element symbol in the above equations (1) and (2) means the content (mass%) of each element, and is set to zero when the element is not contained.
  • the holding time is preferably 10 minutes or more.
  • the reheating temperature is equal to or higher than the Ac3 transformation point and lower than 950 ° C.
  • the austenite phase grows and becomes coarse when the holding time exceeds 30 minutes. Therefore, the holding time is preferably 30 minutes or less.
  • the steel sheet reheated in the above reheating step or the hot-rolled steel sheet is cooled to a cooling shutdown temperature of 350 to 600 ° C. (second cooling step).
  • the average cooling rate is 2 to 7 ° C./s.
  • a lower average cooling rate is preferable in terms of improving toughness because pearlite transformation is promoted more.
  • the average cooling rate is set to 2 ° C./s or higher.
  • the average cooling rate exceeds 7 ° C./s, the pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and the bainite transformation and the martensitic transformation are likely to proceed. In this case, since the fractions of the bainite phase and the martensite phase increase, the elongation characteristics and toughness at the total thickness deteriorate. Therefore, the average cooling rate is set to 7 ° C./s or less.
  • the average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
  • the cooling shutdown temperature is set to 350 ° C. or higher.
  • the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.
  • the steel sheet cooled to the above cooling shutdown temperature is quenched. Therefore, the quenching temperature is in the range of 350 to 600 ° C. Quenching can be performed under any conditions without particular limitation, but it is preferably water-cooled to a temperature of Ms point or lower, preferably 200 ° C. or lower.
  • the Ms point can be obtained by, for example, the following equation (3).
  • Ms (° C.) 517-300 x C-11 x Si-33 x Mn-17 x Ni-22 x Cr-11 x Mo ... (3)
  • the element symbol in the above formula (3) means the content (mass%) of each element, and is set to zero when the element is not contained.
  • the same steel material as that described above is used.
  • the heating and hot rolling can be carried out in the same manner as the heating and hot rolling described above.
  • the temperature is cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point, and then the average cooling is 2 to 7 ° C./s from the temperature above the Ar1 transformation point and below the Ar3 transformation point (cooling start temperature). Cool to a cooling stop temperature of 350-600 ° C. at a rate.
  • the reason why the cooling start temperature is set to the temperature above the Ar1 transformation point and below the Ar3 transformation point (two-phase region) is that the decarburization reaction peculiar to the two-phase region proceeds and the area ratio of the ferrite phase in the surface layer portion is 80% or more. Because it can be done. Further, the reasons for setting the average cooling rate after that to 2 to 7 ° C./s include the following reasons. If the average cooling rate is less than 2 ° C./s, the grain growth of ferrite becomes excessive and coarse-grained, resulting in deterioration of toughness. Therefore, the average cooling rate is set to 2 ° C./s or more.
  • the average cooling rate is set to 7 ° C./s or less.
  • the average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
  • the reasons for setting the cooling shutdown temperature to 350 to 600 ° C. are as follows.
  • the cooling shutdown temperature is set to 350 ° C. or higher.
  • the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower. Subsequent quenching can be carried out in the same manner as the quenching described above.
  • the Ar1 transformation point can be obtained, for example, by the following equation (4).
  • Ar1 712-17.8 x C-19.1 x Ni + 20.1 x Si + 11.9 x Cr + 9.8 x Mo ...
  • the Ar3 transformation point can be obtained by, for example, the following equation (5).
  • Ar3 910-310 x C-80 x Mn-20 x Cu-15 x Cr-55 x Ni-80 x Mo ... (5)
  • the element symbol in the above formulas (4) to (5) means the content (mass%) of each element, and is set to zero when the element is not contained.
  • the molten steel having the composition shown in Table 1 was melted and used as a steel material (slab).
  • the values of Ac1, Ac3, Ms, Ar1, and Ar3 shown in Table 1 are the above-mentioned equations (1), (2), (3), (4), and (5), respectively. This is the calculated value.
  • the obtained slab was heated and hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled plate having a total length of 20 m and a plate thickness shown in Table 2. Then, the hot-rolled sheet was cooled to room temperature by the cooling method shown in Table 2, reheated to the reheating temperature shown in Table 2, and held for 30 minutes or more. Next, cooling water was sprayed on both sides of the steel sheet, cooled to the cooling stop temperature at the average cooling rate shown in Table 2, and then quenched. In the quenching treatment, it was water-cooled to 150 ° C. or lower.
  • quenching was performed immediately after reheating without cooling to satisfy the conditions of the present invention.
  • the quenching conditions in this comparative example were an average cooling rate of 44.0 ° C./s and a cooling shutdown temperature of 110 ° C.
  • the obtained thick steel sheets were evaluated for (1) microstructure, (2) total thickness elongation, (3) tensile strength (TS), (4) fatigue crack propagation characteristics, and (5) toughness.
  • the test pieces were taken from each of the tip, center, and tail end of the thick steel plate in the rolling direction.
  • the test method is as follows. The test pieces at the tip and the tail end were taken from a position 100 mm from the end of the steel sheet in the rolling direction.
  • the phase was identified by image analysis, (a) the average value of the area ratio of the ferrite phase in the range from the surface to 100 ⁇ m below the surface of the thick steel plate, and (b) the plate thickness from 100 ⁇ m below the surface.
  • the average value of the area ratio of the ferrite phase in the range up to the 1/4 position and (c) the area ratio of the pearlite phase and the bainite phase in the range from 100 ⁇ m below the surface to the 1/4 position of the plate thickness were obtained.
  • Table 3 shows the measurement results of the microstructure.
  • a Charpy impact test piece was taken from the center of the thick steel plate in parallel with the rolling direction (L direction).
  • the test piece thickness was 10 mm when the plate thickness was 10 mm or more, and 5 mm when the plate thickness was less than 10 mm.
  • the test was carried out in accordance with JIS Z 2202 by performing a Charpy impact test at 0 ° C., and the absorbed energy vE 0 was measured.
  • a test piece having a thickness of 10 mm was accepted as having an absorption energy of 100 J or more.
  • a test piece with a thickness of 5 mm was accepted as having an absorption energy of 50 J or more.

Abstract

The purpose of the present invention is to provide a thick steel sheet that has high strength, superior elongation and fatigue crack propagation properties throughout the entire thickness thereof, and superior toughness, as well as a manufacturing method for the same. This thick steel sheet has a component composition including, by mass%, C: 0.05-0.20%, Si: 0.01-0.50%, Mn: 0.50-2.00%, P: 0.05% or less, and S: 0.02% or less, the remainder consisting of Fe and unavoidable impurities. The microstructure includes, by area ratio, 80% or more of a ferrite phase in the range from a surface to 100 μm below the surface in the sheet thickness direction, and includes, by area ratio, 80% or less of a ferrite phase in the range from 100 μm below the surface to a location at 1/4 of the sheet thickness in the sheet thickness direction, the remainder consisting of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, where the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

Description

厚鋼板およびその製造方法Thick steel plate and its manufacturing method
　本発明は、厚鋼板およびその製造方法に関し、特に、全厚での伸び特性および疲労き裂伝播特性ならびに靭性に優れた厚鋼板およびその製造方法に関する。本発明の厚鋼板は、船舶、海洋構造物、橋梁、建築物、タンクなど、構造安全性が強く求められ、溶接構造物に好適に用いることができる。 The present invention relates to a thick steel sheet and a method for manufacturing the same, and more particularly to a thick steel sheet having excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at the total thickness and a method for producing the same. The thick steel plate of the present invention is strongly required to have structural safety such as ships, marine structures, bridges, buildings, tanks, etc., and can be suitably used for welded structures.
　厚鋼板は、船舶、海洋構造物、橋梁、建築物、タンクなどの構造物に広く用いられている。このような厚鋼板には、強度、靭性などの機械的特性および溶接性が優れることに加え、疲労特性に優れることが求められる。 Thick steel plates are widely used in structures such as ships, marine structures, bridges, buildings, and tanks. Such a thick steel sheet is required to have excellent fatigue characteristics in addition to excellent mechanical properties such as strength and toughness and weldability.
　上述したような構造物を使用する際には、該構造物に対して、風や地震による振動など、繰返し荷重がかかる。そのため、厚鋼板には、そのような繰返し荷重が負荷された場合でも構造物の安全性を確保できる疲労特性が求められる。 When using a structure as described above, a repetitive load such as vibration due to wind or earthquake is applied to the structure. Therefore, the thick steel sheet is required to have fatigue characteristics that can ensure the safety of the structure even when such a repetitive load is applied.
　疲労破壊とは、最初に微細なき裂（疲労き裂）が発生し、次にそのき裂が広がっていく（進展）という段階をたどる現象である。疲労破壊は、一般的には溶接部から疲労き裂が発生し、鋼材中を伝播して破壊に至るケースが多い。これは、溶接部がその形状から応力集中部となりやすいこと、加えて溶接後に引張の残留応力が生じることなどに起因するとされている。このため、溶接部からのき裂発生を抑制する手段として、ピーニングなどで圧縮の残留応力を導入する技術などが広く知られている。 Fatigue fracture is a phenomenon that follows the stage where fine cracks (fatigue cracks) first occur and then the cracks spread (progress). In fatigue fracture, fatigue cracks generally occur from the weld and propagate through the steel material, leading to fracture in many cases. It is said that this is due to the fact that the welded portion tends to be a stress concentration portion due to its shape, and in addition, residual tensile stress is generated after welding. Therefore, as a means for suppressing the generation of cracks from the welded portion, a technique of introducing the residual stress of compression by peening or the like is widely known.
　しかしながら、構造物内に多数存在する溶接部全てにこのような処理を施すことは、作業性および製造コストの面からも現実的ではない。そのため、仮に溶接部などから疲労き裂が発生したとしても、その後の鋼材中のき裂伝播を遅延させることで溶接構造物としての疲労寿命を延命させることが重要であり、鋼材自身の耐疲労き裂伝播特性を向上させることが望まれている。 However, it is not realistic from the viewpoint of workability and manufacturing cost to apply such treatment to all the welded parts existing in a large number in the structure. Therefore, even if fatigue cracks occur from the welded part, it is important to extend the fatigue life of the welded structure by delaying the subsequent crack propagation in the steel material, and the fatigue resistance of the steel material itself is important. It is desired to improve the crack propagation characteristics.
　例えば、特許文献１には、板厚２０ｍｍ以下の厚鋼板の製造方法において、Ｃ添加量を低くしてＣｅｑ（炭素当量）を特定の範囲に制御するとともに、冷却停止温度を低くすることで伸びと耐疲労き裂伝播特性を両立させた厚鋼板が記載されている。 For example, in Patent Document 1, in a method for manufacturing a thick steel sheet having a plate thickness of 20 mm or less, the amount of C added is reduced to control Ceq (carbon equivalent) within a specific range, and the cooling shutdown temperature is lowered to achieve elongation. A thick steel sheet that achieves both fatigue resistance and crack propagation characteristics is described.
　また、特許文献２には、加熱、圧延、加速冷却および熱処理を降伏応力に応じて組み合わせることにより、き裂伝播特性の異方性が小さい厚鋼板を製造する方法が記載されている。 Further, Patent Document 2 describes a method for producing a thick steel sheet having a small anisotropy of crack propagation characteristics by combining heating, rolling, accelerated cooling and heat treatment according to the yield stress.
　特許文献３では、ベイナイトと、面積率で３８～５２％のフェライトとからなるミクロ組織を有する二相鋼とし、フェライト相部分のビッカース硬さと、フェライト相とベイナイト相の間の境界の密度を制御することで疲労き裂伝播特性を向上させている。 In Patent Document 3, a duplex stainless steel having a microstructure composed of bainite and ferrite having an area ratio of 38 to 52% is used, and the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase are controlled. By doing so, the fatigue crack propagation characteristics are improved.
　特許文献４では、優れた耐疲労き裂伝播特性と全厚での伸び特性を向上させるために、板厚方向に、表面から表面下１００μｍまでの範囲におけるミクロ組織が、面積率で８０％以上のフェライト相を含み、表面下１００μｍから板厚１／２位置の範囲におけるミクロ組織が、面積率で８０％以下のフェライト相を含み、残部がパーライト相、ベイナイト相、またはパーライト相とベイナイト相との混合相からなる厚鋼板が提案されている。 In Patent Document 4, in order to improve excellent fatigue resistance crack propagation characteristics and elongation characteristics at total thickness, the microstructure in the range from the surface to 100 μm below the surface in the plate thickness direction has an area ratio of 80% or more. The microstructure in the range from 100 μm below the surface to the plate thickness 1/2 position contains a ferrite phase with an area ratio of 80% or less, and the balance is a pearlite phase, a bainite phase, or a pearlite phase and a bainite phase. A thick steel plate composed of a mixed phase of is proposed.
特開２０１０－１９６１０９号公報Japanese Unexamined Patent Publication No. 2010-196109 特開２００７－３３２４０２号公報Japanese Unexamined Patent Publication No. 2007-332402 特開平０８－２２５８８２号公報Japanese Unexamined Patent Publication No. 08-225882 特開２０１９－０２６９２７号公報Japanese Unexamined Patent Publication No. 2019-026927
　しかし、特許文献１～４に記載されているような従来の技術には、以下のような問題がある。 However, the conventional techniques described in Patent Documents 1 to 4 have the following problems.
　特許文献１に記載された方法では、圧延と加速冷却制御によるオンラインプロセスにより厚鋼板が製造されている。そのため、特に、板厚が２０ｍｍ以下であるような薄物においては、熱間圧延時および加速冷却時において、鋼板先尾端での温度偏差が生じやすくなり、全長に亘って安定的な機械特性を得ることができない。 In the method described in Patent Document 1, thick steel sheets are manufactured by an online process of rolling and accelerated cooling control. Therefore, especially in a thin material having a plate thickness of 20 mm or less, a temperature deviation at the tip and tail of the steel sheet is likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. I can't get it.
　また、特許文献２に記載された方法では、二相域再加熱後に即焼入れを行うと、変態収縮に伴い厚鋼板の形状が悪化し、また、厚鋼板の最表層が焼入れにより微細化され、硬化することで全厚での伸び特性が劣化する。これらの傾向は、特に、板厚が薄い場合に顕著である。 Further, in the method described in Patent Document 2, when immediate quenching is performed after reheating in the two-phase region, the shape of the thick steel sheet deteriorates due to transformation shrinkage, and the outermost layer of the thick steel sheet is miniaturized by quenching. Curing deteriorates the elongation characteristics at full thickness. These tendencies are particularly noticeable when the plate thickness is thin.
　特許文献３に記載された方法では、特許文献１と同様に、圧延と加速冷却制御によるオンラインプロセスにより厚鋼板が製造されている。そのため、特に、板厚が２０ｍｍ以下であるような薄物においては、熱間圧延時および加速冷却時において、鋼板先尾端での温度偏差が生じやすくなり、全長に亘って安定的な機械特性を得ることができないという問題がある。 In the method described in Patent Document 3, as in Patent Document 1, a thick steel sheet is manufactured by an online process by rolling and accelerated cooling control. Therefore, especially in a thin material having a plate thickness of 20 mm or less, a temperature deviation at the tip and tail of the steel sheet is likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. There is a problem that it cannot be obtained.
　特許文献４では、再加熱された熱延板は平均冷却速度７．７～１６．９℃／ｓで冷却され、焼入れされている。この方法では冷却速度が高いため、パーライト相よりベイナイト相が優位に生成し、しかもベイナイト相中には島状マルテンサイトが存在するため、靭性値が悪化する。 In Patent Document 4, the reheated hot-rolled plate is cooled and hardened at an average cooling rate of 7.7 to 16.9 ° C./s. In this method, since the cooling rate is high, the bainite phase is predominantly generated over the pearlite phase, and since island-like martensite is present in the bainite phase, the toughness value deteriorates.
　このように、従来の製造方法では、全厚での伸び（全厚伸びとも称する）特性、疲労き裂伝播特性および靭性の全てを兼ね備えた厚鋼板を製造することができないという問題があった。 As described above, the conventional manufacturing method has a problem that it is not possible to manufacture a thick steel sheet having all of elongation (also referred to as total thickness elongation) characteristics, fatigue crack propagation characteristics and toughness at the total thickness.
　本発明は、上記事情に鑑みてなされたものであり、高強度であり、全厚での伸び特性および疲労き裂伝播特性ならびに靭性に優れた厚鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel sheet having high strength and excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at the total thickness, and a method for producing the same. do.
　本発明者らは、上記課題を解決するために検討を行った結果、以下の知見を得た。
（１）熱間圧延が終了し、冷却された後の厚鋼板には、冷却偏差に起因する組織のバラツキが存在するが、この組織のバラツキは、２相域の温度以上に再加熱することによって解消できる。
（２）板厚が薄い場合であっても、再加熱熱処理後の冷却パターンを制御することにより、全長に亘って全厚での伸び特性と耐疲労き裂伝播特性を両立できる。
（３）さらに、パーライト相をベイナイト相よりも多く生成させることによって、靭性値を改善できる。
（４）熱間圧延が終了し、冷却する過程において、冷却速度を適切に制御することにより、組織のバラツキが解消され、全長に亘って高い強度を確保しながら、全厚での伸び特性と耐疲労き裂伝播特性を両立できる。 As a result of studies to solve the above problems, the present inventors have obtained the following findings.
(1) The thick steel sheet after hot rolling is completed and cooled has a structure variation due to a cooling deviation. The structure variation should be reheated to a temperature in the two-phase region or higher. Can be resolved by.
(2) Even when the plate thickness is thin, by controlling the cooling pattern after the reheating heat treatment, it is possible to achieve both the elongation characteristics at the total thickness and the fatigue crack propagation characteristics over the entire length.
(3) Further, the toughness value can be improved by producing more pearlite phase than bainite phase.
(4) In the process of finishing hot rolling and cooling, by appropriately controlling the cooling rate, the variation of the structure is eliminated, and while ensuring high strength over the entire length, the elongation characteristics at the total thickness are achieved. Achieves both fatigue-resistant crack propagation characteristics.
　本発明は上記知見に基づいてなされたものであり、その要旨構成は次のとおりである。
［１］　質量％で、
Ｃ：０．０５～０．２０％、
Ｓｉ：０．０１～０．５０％、
Ｍｎ：０．５０～２．００％、
Ｐ：０．０５％以下、
Ｓ：０．０２％以下を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、
ミクロ組織は、板厚方向に、表面から表面下１００μｍまでの範囲において、面積率で８０％以上のフェライト相を含み、
板厚方向に、表面下１００μｍから板厚１／４位置の範囲において、
面積率で８０％以下のフェライト相を含み、
残部がパーライト相、またはパーライト相とベイナイト相との混合相からなり、かつ前記パーライト相の面積率が前記ベイナイト相の面積率よりも多い厚鋼板。
［２］　前記成分組成が、さらに、質量％で、
Ｃｒ：０．０１～１．００％、
Ｃｕ：０．０１～２．００％、
Ｎｉ：０．０１～２．００％、
Ｍｏ：０．０１～１．００％、
Ｃｏ：０．０１～１．００％、
Ｓｎ：０．００５～０．５００％、
Ｓｂ：０．００５～０．２００％、
Ｎｂ：０．００５～０．２００％、
Ｖ：０．００５～０．２００％、
Ｔｉ：０．００５～０．０５０％、
Ｂ：０．０００１～０．００５０％、
Ｚｒ：０．００５～０．１００％、
Ｃａ：０．０００１～０．０２０％、
Ｍｇ：０．０００１～０．０２０％、および
ＲＥＭ：０．０００１～０．０２０％のうちから選ばれる１種または２種以上を含有する、［１］に記載の厚鋼板。
［３］　前記［１］または［２］に記載の成分組成を有する鋼素材を９００～１２００℃に加熱し、
加熱された前記鋼素材に累積圧下率５０％以上の熱間圧延を施して熱延板とし、
前記熱延板を冷却し、
次いで、Ａｃ１変態点以上、９５０℃以下の再加熱温度に再加熱し、
前記Ａｃ１変態点以上、９５０℃以下の温度に再加熱された鋼板を２～７℃／ｓの平均冷却速度で３５０～６００℃の冷却停止温度まで冷却し、
前記３５０～６００℃の冷却停止温度まで冷却された鋼板に焼入れを施す、厚鋼板の製造方法。
［４］　前記［１］または［２］に記載の成分組成を有する鋼素材を９００～１２００℃に加熱し、
加熱された前記鋼素材に累積圧下率５０％以上の熱間圧延を施して熱延板とし、
次いで、
Ａｒ１変態点以上Ａｒ３変態点以下の温度まで冷却された鋼板を２～７℃／ｓの平均冷却速度で３５０～６００℃の冷却停止温度まで冷却し、
前記３５０～６００℃の冷却停止温度まで冷却された鋼板に焼入れを施す、厚鋼板の製造方法。 The present invention has been made based on the above findings, and its gist structure is as follows.
[1] By mass%,
C: 0.05 to 0.20%,
Si: 0.01-0.50%,
Mn: 0.50 to 2.00%,
P: 0.05% or less,
S: Contains 0.02% or less, has a component composition consisting of the balance Fe and unavoidable impurities, and has a component composition.
The microstructure contains a ferrite phase with an area ratio of 80% or more in the range from the surface to 100 μm below the surface in the plate thickness direction.
In the plate thickness direction, in the range from 100 μm below the surface to the plate thickness 1/4 position.
Contains a ferrite phase with an area ratio of 80% or less,
A thick steel plate in which the balance is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
[2] The composition of the components is further increased by mass%.
Cr: 0.01-1.00%,
Cu: 0.01-2.00%,
Ni: 0.01-2.00%,
Mo: 0.01-1.00%,
Co: 0.01-1.00%,
Sn: 0.005 to 0.500%,
Sb: 0.005 to 0.200%,
Nb: 0.005 to 0.200%,
V: 0.005 to 0.200%,
Ti: 0.005 to 0.050%,
B: 0.0001 to 0.0050%,
Zr: 0.005 to 0.100%,
Ca: 0.0001 to 0.020%,
The thick steel sheet according to [1], which contains one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
[3] The steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Cool the hot rolled plate and
Then, it was reheated to a reheating temperature of 950 ° C. or higher and equal to or higher than the Ac1 transformation point.
The steel sheet reheated to a temperature above the Ac1 transformation point and below 950 ° C. is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
[4] The steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Then
The steel sheet cooled to a temperature above the Ar1 transformation point and below the Ar3 transformation point is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
　本発明によれば、高強度であり、全厚での伸び特性および疲労き裂伝播特性ならびに靭性に優れた厚鋼板を得ることができる。本発明の厚鋼板では、仮に応力集中部や溶接部等から疲労き裂が経年的に発生したとしても、その後のき裂の伝播が抑制されるため、鋼構造物全体の安全性を高めることが可能である。また、本発明の厚鋼板を橋梁・船舶・建築構造物、建設産業機械などの構造物に好適に用いることにより、かような構造物のメンテナンスコスト、ひいてはライフサイクルコストを低減することが可能となり、産業上極めて有用である。 According to the present invention, it is possible to obtain a thick steel sheet having high strength and excellent elongation characteristics, fatigue crack propagation characteristics and toughness at the total thickness. In the thick steel sheet of the present invention, even if fatigue cracks occur over time from stress-concentrated portions, welded portions, etc., the subsequent propagation of cracks is suppressed, so that the safety of the entire steel structure is enhanced. Is possible. Further, by suitably using the thick steel plate of the present invention for structures such as bridges, ships, building structures, and construction industrial machinery, it is possible to reduce the maintenance cost and the life cycle cost of such structures. , Extremely useful industrially.
図１は、疲労き裂伝搬試験に用いた、片側切欠単純引張型疲労試験片の模式図である。FIG. 1 is a schematic diagram of a one-sided notch simple tensile type fatigue test piece used in the fatigue crack propagation test.
　次に、本発明を実施する方法について具体的に説明する。なお、以下の説明は、本発明の好適な実施態様を示すものであり、本発明は以下の説明によって何ら限定されるものではない。 Next, the method for carrying out the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description.
　［成分組成］
　本発明の厚鋼板の成分組成について、その限定理由を以下に説明する。なお、以下の説明における「％」は、特に断らない限り「質量％」を表すものとする。 [Ingredient composition]
The reasons for limiting the composition of the thick steel sheet of the present invention will be described below. In addition, "%" in the following description shall represent "mass%" unless otherwise specified.
　Ｃ：０．０５～０．２０％
　Ｃは、基地相（マトリクス）硬さを増加させ、強度を向上させる効果を有する元素である。また、セメンタイト相の集合であるパーライト相を生成させる効果があるため、耐疲労特性が高まる。このような効果を得るためには、Ｃ含有量を０．０５％以上とすることが必要である。Ｃ含有量は、好ましくは０．０８％以上であり、より好ましくは０．１０％以上であり、さらに好ましくは０．１２％以上である。一方、Ｃ含有量が０．２０％を超えると、基地相の硬度が過度に上昇し、全厚での伸びが劣化する。このため、Ｃ含有量は０．２０％以下とする。Ｃ含有量は、好ましくは０．１８％以下であり、より好ましくは０．１６％以下であり、さらに好ましくは０．１４％以下である。 C: 0.05 to 0.20%
C is an element having an effect of increasing the hardness of the matrix phase and improving the strength. In addition, since it has the effect of generating a pearlite phase, which is a set of cementite phases, fatigue resistance is enhanced. In order to obtain such an effect, it is necessary to set the C content to 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more, and further preferably 0.12% or more. On the other hand, when the C content exceeds 0.20%, the hardness of the matrix phase increases excessively, and the elongation at the total thickness deteriorates. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, more preferably 0.16% or less, still more preferably 0.14% or less.
　Ｓｉ：０．０１～０．５０％
　Ｓｉは、脱酸剤として作用するとともに、鋼中に固溶して固溶強化により基地相の硬さを増加させる元素である。このような効果を得るためには、Ｓｉ含有量を０．０１％以上とする必要がある。Ｓｉ含有量は、好ましくは０．０５％以上であり、より好ましくは０．１％以上であり、さらに好ましくは０．１５％以上であり、もっとも好ましくは０．２０％以上である。一方、Ｓｉ含有量が０．５０％を超えると、全厚での伸び、靭性が低下する。このため、Ｓｉ含有量は０．５０％以下とする。Ｓｉ含有量は、好ましくは０．４５％以下であり、より好ましくは０．４０％以下であり、さらに好ましくは０．３５％以下であり、もっとも好ましくは０．３０％以下である。 Si: 0.01-0.50%
Si is an element that acts as a deoxidizing agent and dissolves in steel to increase the hardness of the matrix phase by solid solution strengthening. In order to obtain such an effect, the Si content needs to be 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.1% or more, still more preferably 0.15% or more, and most preferably 0.20% or more. On the other hand, when the Si content exceeds 0.50%, the elongation and toughness at the total thickness decrease. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less, still more preferably 0.35% or less, and most preferably 0.30% or less.
　Ｍｎ：０．５０～２．００％
　Ｍｎは、基地相の硬さを増加させ、強度を向上させる効果を有する元素である。このような効果を得るためには、Ｍｎ含有量を０．５０％以上とする必要がある。Ｍｎ含有量は、好ましくは０．６０％以上であり、より好ましくは０．７０％以上であり、さらに好ましくは０．８０％以上であり、もっとも好ましくは１．００％以上である。一方、Ｍｎ含有量が２．００％を超えると、溶接性が低下することに加えて、介在物であるＭｎＳが過剰に偏析し靭性が悪化する。このため、Ｍｎ含有量は２．００％以下とする。Ｍｎ含有量は、好ましくは１．８５％以下であり、より好ましくは１．７０％以下であり、さらに好ましくは１．５５％以下であり、もっとも好ましくは１．４０％以下である。 Mn: 0.50 to 2.00%
Mn is an element having the effect of increasing the hardness of the matrix phase and improving the strength. In order to obtain such an effect, the Mn content needs to be 0.50% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more, still more preferably 0.80% or more, and most preferably 1.00% or more. On the other hand, when the Mn content exceeds 2.00%, the weldability is lowered, and MnS, which is an inclusion, is excessively segregated and the toughness is deteriorated. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.85% or less, more preferably 1.70% or less, still more preferably 1.55% or less, and most preferably 1.40% or less.
　Ｐ：０．０５％以下
　Ｐは、不可避的不純物として鋼に含まれる元素である。Ｐは、粒界に偏析し、母材および溶接部の靱性を低下させるなど、悪影響を及ぼすため、できるだけ低減することが好ましいが、０．０５％以下の含有は許容できる。このため、Ｐ含有量は０．０５％以下とする。Ｐ含有量は、好ましくは０．０４％以下であり、より好ましくは０．０３％以下である。一方、Ｐ含有量の下限は限定されないが、過度の低減は精錬コストの高騰を招くため、Ｐ含有量を０．００１％以上とすることが好ましい。Ｐ含有量は、好ましくは０．００２％以上であり、より好ましくは０．００３％以上である。 P: 0.05% or less P is an element contained in steel as an unavoidable impurity. P is preferably reduced as much as possible because it segregates at the grain boundaries and has an adverse effect such as lowering the toughness of the base metal and the welded portion. However, a content of 0.05% or less is acceptable. Therefore, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less. On the other hand, although the lower limit of the P content is not limited, it is preferable to set the P content to 0.001% or more because an excessive reduction causes an increase in the refining cost. The P content is preferably 0.002% or more, more preferably 0.003% or more.
　Ｓ：０．０２％以下
　Ｓは、不可避的不純物として鋼に含まれる元素である。Ｓは、ＭｎＳ等の硫化物系介在物として鋼中に存在し、脆性破壊の発生起点となり靭性が劣化するため、できるだけ低減することが好ましいが、０．０２％以下の含有は許容できる。このため、Ｓ含有量は０．０２％以下とする。Ｓ含有量は０．０１％以下とすることが好ましい。一方、Ｓ含有量の下限は限定されないが、過度の低減は精錬コストの高騰を招くため、Ｓ含有量を０．０００５％以上とすることが好ましい。 S: 0.02% or less S is an element contained in steel as an unavoidable impurity. S is present in steel as a sulfide-based inclusion such as MnS and becomes a starting point of brittle fracture and deteriorates toughness. Therefore, it is preferable to reduce S as much as possible, but a content of 0.02% or less is acceptable. Therefore, the S content is 0.02% or less. The S content is preferably 0.01% or less. On the other hand, although the lower limit of the S content is not limited, it is preferable to set the S content to 0.0005% or more because an excessive reduction causes an increase in the refining cost.
　残部はＦｅおよび不可避的不純物からなる。なお、不可避的不純物として含有される酸素（Ｏ）の含有量が０．００５０％を超えると、鋼板表面での介在物の存在割合が大きくなるため、介在物を起点としたき裂発生が生じやすくなる。そのため、Ｏ含有量は０．００５０％以下とすることが好ましい。同様に、不可避的不純物として含有されるＮの含有量が０．００５０％を超えると、鋼板表面での介在物の存在割合が大きくなるため、介在物を起点としたき裂発生が生じやすくなる。そのため、Ｎ含有量は０．００５０％以下とすることが好ましい。Ｎ含有量は０．００４０％以下とすることがより好ましい。同様に、不可避的不純物として含有されるｓｏｌ．Ａｌの含有量が０．０６０％を超えると、溶接時に溶接金属部にＡｌが混入して、溶接部の靭性が劣化する。そのため、ｓｏｌ．Ａｌ含有量は０．０６０％以下とすることが好ましい。ｓｏｌ．Ａｌ含有量は、０．０５０％以下とすることがより好ましく、０．０４０％以下とすることがさらに好ましい。 The rest consists of Fe and unavoidable impurities. If the content of oxygen (O) contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet increases, so that cracks occur starting from the inclusions. It will be easier. Therefore, the O content is preferably 0.0050% or less. Similarly, when the content of N contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet becomes large, so that cracks are likely to occur starting from the inclusions. .. Therefore, the N content is preferably 0.0050% or less. The N content is more preferably 0.0040% or less. Similarly, sol., Which is contained as an unavoidable impurity. If the Al content exceeds 0.060%, Al is mixed into the weld metal portion during welding, and the toughness of the weld portion deteriorates. Therefore, sol. The Al content is preferably 0.060% or less. sol. The Al content is more preferably 0.050% or less, and further preferably 0.040% or less.
　さらに、本発明において、Ｃｒ：０．０１～１．００％、Ｃｕ：０．０１～２．００％、Ｎｉ：０．０１～２．００％、Ｍｏ：０．０１～１．００％、Ｃｏ：０．０１～１．００％、Ｓｎ：０．００５～０．５００％、Ｓｂ：０．００５～０．２００％、Ｎｂ：０．００５～０．２００％、Ｖ：０．００５～０．２００％、Ｔｉ：０．００５～０．０５０％、Ｂ：０．０００１～０．００５０％、Ｚｒ：０．００５～０．１００％、Ｃａ：０．０００１～０．０２０％、Ｍｇ：０．０００１～０．０２０％、およびＲＥＭ：０．０００１～０．０２０％のうちから選ばれる１種または２種以上を任意に含有することができる。 Further, in the present invention, Cr: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Co: 0.01 to 1.00%, Sn: 0.005 to 0.500%, Sb: 0.005 to 0.200%, Nb: 0.005 to 0.200%, V: 0.005 to 0.200%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0050%, Zr: 0.005 to 0.100%, Ca: 0.0001 to 0.020%, Mg It can optionally contain one or more selected from: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
　Ｃｒ：０．０１～１．００％
　Ｃｒは、強度をさらに向上させる効果を有する元素である。また、Ｃｒはセメンタイト生成を促進する元素であり、耐疲労特性に有利なパーライト相の生成を促進する。Ｃｒを含有する場合、前記効果を得るために、Ｃｒ含有量を０．０１％以上とする。好ましくは０．１０％以上とする。一方、Ｃｒ含有量が１．００％を超えると溶接性と靭性が損なわれる。そのため、Ｃｒを含有する場合は、１．００％以下とする。Ｃｒ含有量は、好ましくは０．８０％以下、より好ましくは、０．５０％以下とする。 Cr: 0.01-1.00%
Cr is an element having an effect of further improving the strength. Further, Cr is an element that promotes the formation of cementite, and promotes the formation of a pearlite phase that is advantageous in fatigue resistance characteristics. When Cr is contained, the Cr content is set to 0.01% or more in order to obtain the above effect. It is preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, when Cr is contained, it is set to 1.00% or less. The Cr content is preferably 0.80% or less, more preferably 0.50% or less.
　Ｃｕ：０．０１～２．００％
　Ｃｕは、固溶により強度をさらに上昇させる元素である。Ｃｕを含有する場合、前記効果を得るため、Ｃｕ含有量を０．０１％以上とする。Ｃｕ含有量を０．０５％以上とすることが好ましく、０．１０％以上とすることがより好ましい。一方、Ｃｕ含有量が１．００％を超えると、溶接性が損なわれ、また、厚鋼板の製造時に疵が生じやすくなる。そのため、Ｃｕを含有する場合、２．００％以下とする。Ｃｕ含有量は、好ましくは０．７０％以下、より好ましくは０．６０％以下、さらに好ましくは０．５０％以下とする。 Cu: 0.01-2.00%
Cu is an element whose strength is further increased by solid solution. When Cu is contained, the Cu content is set to 0.01% or more in order to obtain the above effect. The Cu content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the Cu content exceeds 1.00%, the weldability is impaired and defects are likely to occur during the production of the thick steel sheet. Therefore, when Cu is contained, the content is 2.00% or less. The Cu content is preferably 0.70% or less, more preferably 0.60% or less, still more preferably 0.50% or less.
　Ｎｉ：０．０１～２．００％
　Ｎｉは、低温靭性を向上させる効果を有する元素である、また、Ｎｉは、Ｃｕを含有した場合の熱間脆性を改善する。Ｎｉを含有する場合、前記効果を得るために、Ｎｉ含有量を０．０１％以上とする。Ｎｉ含有量を０．０５％以上とすることが好ましい。一方、Ｎｉ含有量が１．００％を超えると溶接性が損なわれ、鋼材コストが上昇する。そのため、Ｎｉを含有する場合、１．００％以下とする。Ｎｉ含有量は好ましくは０．７０％以下、より好ましくは０．４０％以下とする。 Ni: 0.01-2.00%
Ni is an element having an effect of improving low temperature toughness, and Ni improves hot brittleness when Cu is contained. When Ni is contained, the Ni content is set to 0.01% or more in order to obtain the above effect. The Ni content is preferably 0.05% or more. On the other hand, if the Ni content exceeds 1.00%, the weldability is impaired and the steel material cost increases. Therefore, when Ni is contained, it should be 1.00% or less. The Ni content is preferably 0.70% or less, more preferably 0.40% or less.
　Ｍｏ：０．０１～１．００％
　Ｍｏは、基地相の硬さを増加させる効果を有する元素であり、所望する特性に応じて任意に含有することができる。Ｍｏを含有する場合、この効果を得るために、Ｍｏ含有量を０．０１％以上とする。Ｍｏ含有量を０．０５％以上とすることが好ましい。しかし、Ｍｏ含有量が１．００％を超えると溶接性と靭性が損なわれるので、含有する場合は、Ｍｏ含有量を１．００％以下とする。Ｍｏ含有量を０．８０％以下とすることが好ましく、０．７０％以下とすることがより好ましい。 Mo: 0.01-1.00%
Mo is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. When Mo is contained, the Mo content is set to 0.01% or more in order to obtain this effect. The Mo content is preferably 0.05% or more. However, if the Mo content exceeds 1.00%, weldability and toughness are impaired. Therefore, when the Mo content is contained, the Mo content is set to 1.00% or less. The Mo content is preferably 0.80% or less, more preferably 0.70% or less.
　Ｃｏ：０．０１～１．００％
　Ｃｏは、基地相の硬さを増加させる効果を有する元素であり、所望する特性に応じて任意に含有することができる。この効果を得るために、Ｃｏを含有する場合、Ｃｏ含有量を０．０１％以上とする。好ましくは０．１０％以上であり、より好ましくは０．２０％以上であり、さらに好ましくは０．３５％以上である。一方、Ｃｏ含有量が１．００％を超えても効果が飽和することに加え、合金コストが増大する。このため、含有する場合は、Ｃｏ含有量は１．００％以下とする。Ｃｏ含有量は好ましくは０．５０％以下とする。 Co: 0.01-1.00%
Co is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. In order to obtain this effect, when Co is contained, the Co content is set to 0.01% or more. It is preferably 0.10% or more, more preferably 0.20% or more, still more preferably 0.35% or more. On the other hand, even if the Co content exceeds 1.00%, the effect is saturated and the alloy cost increases. Therefore, when it is contained, the Co content is set to 1.00% or less. The Co content is preferably 0.50% or less.
　Ｓｎ：０．００５～０．５００％
　Ｓｎは、基地相の硬さを増加させる効果を有する元素であり、所望する特性に応じて任意に含有することができる。このような効果を十分に得るためには、Ｓｎを含有する場合は、０．００５％以上とする。好ましくは０．０１０％以上、より好ましくは０．０２０％以上、さらに好ましくは０．０３０％以上とする。一方、Ｓｎ含有量が０．５００％を超えると、鋼の延性や靭性の劣化を招く。このため、含有する場合は、０．５００％以下とする。好ましくは、０．３００％以下、より好ましくは０．２００％以下であり、さらに好ましくは０．１００％以下である。 Sn: 0.005 to 0.500%
Sn is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. In order to sufficiently obtain such an effect, when Sn is contained, the content is 0.005% or more. It is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more. On the other hand, if the Sn content exceeds 0.500%, the ductility and toughness of the steel are deteriorated. Therefore, when it is contained, it should be 0.500% or less. It is preferably 0.300% or less, more preferably 0.200% or less, and further preferably 0.100% or less.
　Ｓｂ：０．００５～０．２００％
　Ｓｂは、基地相の硬さを増加させる効果を有する元素であり、所望する特性に応じて任意に含有することができる。このような効果を十分に得るためには、Ｓｂを含有する場合は、０．００５％以上とする。Ｓｂ含有量は、好ましくは０．０１０％以上、より好ましくは０．０２０％以上である。一方、Ｓｂ含有量が０．２００％を超えると、鋼の延性や靭性の劣化を招く。このため、含有する場合は、Ｓｂ含有量は０．２００％以下とする。好ましくは０．１５０％以下、より好ましくは０．１００％以下、さらに好ましくは０．０８０％以下、もっとも好ましくは０．０５０％以下である。 Sb: 0.005 to 0.200%
Sb is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily contained depending on the desired properties. In order to sufficiently obtain such an effect, when Sb is contained, the content should be 0.005% or more. The Sb content is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if the Sb content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, when it is contained, the Sb content is 0.200% or less. It is preferably 0.150% or less, more preferably 0.100% or less, still more preferably 0.080% or less, and most preferably 0.050% or less.
　Ｎｂ：０．００５～０．２００％
　Ｎｂは、熱間圧延時のオーステナイトの再結晶を抑制し、最終的に得られる結晶粒を細粒化する効果を有する元素である。また、Ｎｂは、加速冷却後の空冷時に析出し、強度をさらに向上させる。Ｎｂを含有する場合、前記効果を得るために、Ｎｂ含有量を０．００５％以上とする。Ｎｂ含有量は０．００７％以上とすることが好ましく、０．０１０％以上とすることがより好ましい。一方、Ｎｂ含有量が０．２００％を超えると、焼入れ性が過剰となり、ベイナイトが過剰に生成するため所望の組織が得られなくなり、靭性が低下する。そのため、Ｎｂを含有する場合、Ｎｂ含有量は０．２００％以下とする。Ｎｂ含有量は、好ましくは０．０７０％以下、より好ましくは０．０５０％以下、さらに好ましくは０．０４０％以下、もっとも好ましくは０．０３０％以下とする。 Nb: 0.005 to 0.200%
Nb is an element having an effect of suppressing recrystallization of austenite during hot rolling and finely granulating the finally obtained crystal grains. Further, Nb is deposited during air cooling after accelerated cooling to further improve the strength. When Nb is contained, the Nb content is set to 0.005% or more in order to obtain the above effect. The Nb content is preferably 0.007% or more, more preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.200%, the hardenability becomes excessive and bainite is excessively produced, so that a desired structure cannot be obtained and the toughness is lowered. Therefore, when Nb is contained, the Nb content is 0.200% or less. The Nb content is preferably 0.070% or less, more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.030% or less.
　Ｖ：０．００５～０．２００％
　Ｖは、Ｎｂと同様、熱間圧延時におけるオーステナイトの再結晶を抑制して細粒化するとともに、熱間圧延後の空冷過程において析出することで強度を上昇させる効果を有する元素であり、所望する特性に応じて任意に含有することができる。前記効果を得るために、Ｖを含有する場合、Ｖ含有量を０．００５％以上とする。Ｖ含有量は、０．０１０％とすることが好ましく、０．０２０％以上とすることがより好ましく、０．０３０％以上とすることがさらに好ましい。しかし、Ｖ含有量が０．２００％を超えるとＶＣが多量に析出し、靭性が損なわれる。そのため、Ｖを含有する場合は、Ｖ含有量を０．２００％以下とする。Ｖ含有量は、０．１５０％以下とすることが好ましく、０．１００％以下とすることがより好ましく、０．０７０％以下とすることがさらに好ましい。 V: 0.005 to 0.200%
Similar to Nb, V is an element having the effect of suppressing recrystallization of austenite during hot rolling to make it finer and precipitating in the air cooling process after hot rolling to increase the strength, which is desired. It can be arbitrarily contained depending on the characteristics to be rolled. In order to obtain the above effect, when V is contained, the V content is set to 0.005% or more. The V content is preferably 0.010%, more preferably 0.020% or more, and even more preferably 0.030% or more. However, if the V content exceeds 0.200%, a large amount of VC is deposited and the toughness is impaired. Therefore, when V is contained, the V content is set to 0.200% or less. The V content is preferably 0.150% or less, more preferably 0.100% or less, and even more preferably 0.070% or less.
　Ｔｉ：０．００５～０．０５０％
　Ｔｉは、窒化物形成傾向が強く、Ｎを固定して固溶Ｎを低減するため、母材および溶接部の靭性を向上させる効果を有する。また、Ｂを含有する場合には、Ｔｉを合わせて含有することにより、ＴｉがＮを固定し、ＢがＢＮとして析出してしまうことを抑制できる。その結果、Ｂの焼入れ性向上効果を助長して、強度をさらに向上させることができる。そのため、所望する特性に応じて任意に含有することができる。前記効果を得るために、Ｔｉを含有する場合、０．００５％以上とする。Ｔｉ含有量は、０．００７％以上とすることが好ましく、０．０１０％以上とすることがより好ましい。しかし、Ｔｉ含有量が０．０５０％を超えるとＴｉＣが多量に析出し、靭性が損なわれる。そのため、Ｔｉを含有する場合は、Ｔｉ含有量を０．０５０％以下とする。Ｔｉ含有量は、０．０４０％以下とすることが好ましく、０．０３０％以下とすることがより好ましく、０．０２０％以下とすることがさらに好ましい。 Ti: 0.005 to 0.050%
Ti has a strong tendency to form a nitride and fixes N to reduce the solid solution N, so that it has an effect of improving the toughness of the base metal and the welded portion. Further, when B is contained, by including Ti together, it is possible to prevent Ti from fixing N and B from precipitating as BN. As a result, the hardenability improving effect of B can be promoted, and the strength can be further improved. Therefore, it can be arbitrarily contained depending on the desired characteristics. In order to obtain the above effect, when Ti is contained, the content is 0.005% or more. The Ti content is preferably 0.007% or more, more preferably 0.010% or more. However, if the Ti content exceeds 0.050%, a large amount of TiC is deposited and the toughness is impaired. Therefore, when Ti is contained, the Ti content is set to 0.050% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less.
　Ｂ：０．０００１～０．００５０％
　Ｂは、微量の含有でも焼入れ性を著しく向上させ、強度を上昇させる効果を有する元素であり、所望する特性に応じて含有することができる。前記効果を得るために、Ｂを含有する場合、０．０００１％以上とする。Ｂ含有量は、０．０００５％以上とすることが好ましく、０．００１％以上とすることがより好ましい。しかし、Ｂ含有量が０．００５０％を超えるとその効果が飽和するだけでなく、溶接性を低下させるため、Ｂを含有する場合は、Ｂ含有量を０．００５０％以下とする。Ｂ含有量は、０．００４０％以下とすることが好ましく、０．００３０％以下とすることがより好ましく、０．００２０％以下とすることがさらに好ましい。 B: 0.0001 to 0.0050%
B is an element having an effect of significantly improving hardenability and increasing strength even when contained in a small amount, and can be contained according to desired properties. In order to obtain the above effect, when B is contained, the content is 0.0001% or more. The B content is preferably 0.0005% or more, and more preferably 0.001% or more. However, if the B content exceeds 0.0050%, the effect is not only saturated but also the weldability is deteriorated. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less.
　Ｚｒ：０．００５～０．１００％
　Ｚｒは、強度をさらに高める効果を有する元素である。前記効果を十分に得るためには、Ｚｒを含有する場合、Ｚｒ含有量を０．００５％以上とする。Ｚｒ含有量が、０．０１０％以上とすることが好ましく、０．０３０％以上とすることがより好ましく、０．０５０％以上とすることがさらに好ましい。一方、Ｚｒ含有量が０．１００％を超えると、その強度向上効果が飽和する。そのため、Ｚｒを含有する場合、Ｚｒ含有量は０．１００％以下とする。 Zr: 0.005 to 0.100%
Zr is an element having the effect of further increasing the strength. In order to obtain the above-mentioned effect sufficiently, when Zr is contained, the Zr content is set to 0.005% or more. The Zr content is preferably 0.010% or more, more preferably 0.030% or more, and even more preferably 0.050% or more. On the other hand, when the Zr content exceeds 0.100%, the strength improving effect is saturated. Therefore, when Zr is contained, the Zr content is set to 0.100% or less.
　Ｃａ：０．０００１～０．０２０％
　Ｃａは、Ｓと結合し、圧延方向に長く伸びるＭｎＳ等の形成を抑制して、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性向上に寄与するため、所望する特性に応じて含有することができる。Ｃａを含有する場合、この効果を得るために、Ｃａ含有量を０．０００１％以上とする。Ｃａ含有量が、０．０００５％以上とすることが好ましく、０．００１０％以上とすることがより好ましい。しかし、Ｃａ含有量が０．０２０％を超えるとその効果が飽和するだけでなく、鋼の清浄度が低下し、表面疵が多発し表面性状が低下する。このため、Ｃａを含有する場合は、Ｃａ含有量を０．０２０％以下とする。Ｃａ含有量は、０．０１０％以下とすることが好ましく、０．００６％以下とすることがより好ましく、０．００２％以下とすることがさらに好ましい。 Ca: 0.0001 to 0.020%
Ca binds to S, suppresses the formation of MnS and the like that extend long in the rolling direction, controls the morphology of the sulfide-based inclusions so as to have a spherical shape, and contributes to the improvement of the toughness of the welded portion, which is desired. It can be contained according to the characteristics. When Ca is contained, the Ca content is set to 0.0001% or more in order to obtain this effect. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. However, when the Ca content exceeds 0.020%, not only the effect is saturated, but also the cleanliness of the steel is lowered, surface defects occur frequently, and the surface texture is deteriorated. Therefore, when Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably 0.010% or less, more preferably 0.006% or less, and even more preferably 0.002% or less.
　Ｍｇ：０．０００１～０．０２０％
　Ｍｇは、結晶粒の微細化を介して靭性を向上させる効果を有する元素である。Ｍｇを含有する場合、前記効果を得るために、Ｍｇ含有量を０．０００１％以上とする。Ｍｇ含有量は、０．０００３％以上とすることが好ましく、０．０００５％以上とすることがより好ましい。一方、Ｍｇ含有量が０．０２０％を超えると、その効果が飽和する。そのため、Ｍｇを含有する場合、Ｍｇ含有量は０．０２０％以下とする。Ｍｇ含有量は、０．０１５％以下とすることが好ましく、０．０１０％以下とすることがより好ましく、０．００５％以下とすることがさらに好ましい。 Mg: 0.0001 to 0.020%
Mg is an element having an effect of improving toughness through the miniaturization of crystal grains. When Mg is contained, the Mg content is set to 0.0001% or more in order to obtain the above effect. The Mg content is preferably 0.0003% or more, more preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.020%, the effect is saturated. Therefore, when Mg is contained, the Mg content is 0.020% or less. The Mg content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.
　ＲＥＭ：０．０００１～０．０２０％
　ＲＥＭ（希土類金属）は、靭性を向上させる効果を有する元素である。ＲＥＭを添加する場合、前記効果を得るために、ＲＥＭ含有量を０．０００１％以上とする。ＲＥＭ含有量は、０．０００３％以上とすることが好ましい。一方、ＲＥＭ含有量が０．０２０％を超えると、その効果が飽和する。そのため、ＲＥＭを添加する場合、ＲＥＭ含有量は０．０２０％以下とする。ＲＥＭ含有量は、０．０１０％以下とすることが好ましく、０．００５％以下とすることがより好ましく、０．００１％以下とすることがさらに好ましい。 REM: 0.0001 to 0.020%
REM (rare earth metal) is an element that has the effect of improving toughness. When REM is added, the REM content is set to 0.0001% or more in order to obtain the above effect. The REM content is preferably 0.0003% or more. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, when REM is added, the REM content is 0.020% or less. The REM content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less.
　［ミクロ組織］
　次に、厚鋼板のミクロ組織の限定理由について説明する。なお、ミクロ組織の説明における「％」は、特に断らない限り面積率を指すものとする。また、以下の説明における厚鋼板の「先端」とは、鋼板の圧延方向先端より尾端側へ１００ｍｍ入った位置と定義する。同様に、厚鋼板の「尾端」とは、鋼板の圧延方向尾端より先端側へ１００ｍｍ入った位置と定義する。また、厚鋼板の「中央」とは、鋼板の圧延方向（長手方向）中央の位置と定義する。 [Micro organization]
Next, the reason for limiting the microstructure of the thick steel sheet will be described. In addition, "%" in the description of the microstructure shall indicate the area ratio unless otherwise specified. Further, the "tip" of the thick steel sheet in the following description is defined as a position 100 mm from the tip of the steel sheet in the rolling direction to the tail end side. Similarly, the "tail end" of a thick steel sheet is defined as a position 100 mm from the tail end in the rolling direction of the steel sheet to the tip end side. Further, the "center" of the thick steel sheet is defined as the position at the center of the steel sheet in the rolling direction (longitudinal direction).
　表面から表面下１００μｍまでの範囲の組織（表層部組織）
　本発明の厚鋼板における、板厚方向に、表面から表面下１００μｍまでの範囲（以下、単に「表層部」という場合がある）におけるミクロ組織を、面積率で８０％以上のフェライト相を含むものとする。Ａｃ１変態点以上Ａｃ３変態点未満とする二相域では、表層脱炭反応が起き、表層部に８０％以上のフェライトを生成させて厚鋼板の表層を軟化させることにより、全厚での伸び特性を顕著に向上させることができる。
この表層脱炭反応は、再加熱過程で二相域を通過もしくは二相域に保持することで起きる。
表層部におけるフェライト相の面積率が８０％未満であると、ベイナイト相、パーライト相、マルテンサイト相、またはそれらの混合相からなる硬質な残部組織が多く存在することになる。その結果、表層部の硬度が増大して所望の全厚での伸び特性を得ることができない。また、引張強さが過大となる場合がある。 Tissue in the range from the surface to 100 μm below the surface (surface structure)
In the thick steel sheet of the present invention, the microstructure in the range from the surface to 100 μm below the surface (hereinafter, may be simply referred to as “surface layer portion”) in the plate thickness direction is assumed to contain a ferrite phase having an area ratio of 80% or more. .. In the two-phase region where the Ac1 transformation point or more and less than the Ac3 transformation point, a surface decarburization reaction occurs, and 80% or more of ferrite is generated in the surface layer to soften the surface layer of the thick steel sheet, resulting in elongation characteristics at full thickness. Can be significantly improved.
This surface decarburization reaction occurs by passing through or retaining the two-phase region in the reheating process.
When the area ratio of the ferrite phase in the surface layer portion is less than 80%, a large amount of a hard residual structure composed of a bainite phase, a pearlite phase, a martensite phase, or a mixed phase thereof is present. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness. In addition, the tensile strength may become excessive.
　なお、ここで表層部におけるフェライト相の面積率は、厚鋼板の、表面から表面下１００μｍまでの範囲におけるフェライト相の面積率の平均値を指すものとする。また、表層部におけるミクロ組織は、厚鋼板の圧延方向における先端、中央および尾端における表層部のミクロ組織を指すものとする。したがって、本発明の厚鋼板は、厚鋼板の圧延方向における先端、中央および尾端において、表面から表面下１００μｍまでの範囲におけるフェライト相の面積率の平均値が８０％以上である。なお、通常は、先端、中央および尾端における表層部のミクロ組織が上記条件を満たしていれば、厚鋼板の圧延方向全長に亘って前記条件を満たしている。したがって、本発明の厚鋼板は、圧延方向の全長に亘って、表層部のフェライト相の面積率が８０％以上であるといえる。すなわち、本発明において、表層部におけるフェライト相の面積率が８０％以上というのは、圧延方向の全長にわたって、先端、中央、尾端のどこにおいても、表層部におけるフェライト相の面積率８０％以上が得られていることを意味する。 Here, the area ratio of the ferrite phase in the surface layer portion refers to the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel sheet. Further, the microstructure in the surface layer portion refers to the microstructure of the surface layer portion at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface at the tip, center and tail end in the rolling direction of the thick steel sheet is 80% or more. Normally, if the microstructure of the surface layer portion at the tip, center and tail end satisfies the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Therefore, it can be said that the thick steel sheet of the present invention has an area ratio of the ferrite phase in the surface layer portion of 80% or more over the entire length in the rolling direction. That is, in the present invention, the area ratio of the ferrite phase in the surface layer portion is 80% or more, which means that the area ratio of the ferrite phase in the surface layer portion is 80% or more at any of the tip, the center, and the tail end over the entire length in the rolling direction. Means that is obtained.
　表層部のミクロ組織におけるフェライト相以外の残部は、パーライト相、またはベイナイト相とパーライト相との混合相からなることが好ましいが、ベイナイト相は島状マルテンサイトを含有し靭性悪化させるため、ベイナイト相は少ないほど好ましく、パーライト相のみとすることがより好ましい。 The rest of the microstructure of the surface layer other than the ferrite phase is preferably composed of a pearlite phase or a mixed phase of a pearlite phase and a pearlite phase, but the pearlite phase contains island-like martensite and deteriorates toughness. Is preferable, and it is more preferable to use only the pearlite phase.
　表面下１００μｍから板厚１／４位置の範囲の組織（板厚内部組織）
　本発明の厚鋼板における、板厚方向に、表面下１００μｍから板厚１／４位置までの範囲（以下、単に「板厚内部」という場合がある）におけるミクロ組織を、面積率で８０％以下のフェライト相を含むものとする。板厚内部のミクロ組織が前記条件を満たすことにより、所望の強度および耐疲労き裂伝播特性を得ることができる。 Structure in the range from 100 μm below the surface to 1/4 of the plate thickness (plate thickness internal structure)
In the thick steel plate of the present invention, the microstructure in the range from 100 μm below the surface to the 1/4 position of the plate thickness (hereinafter, may be simply referred to as “inside the plate thickness”) in the plate thickness direction is 80% or less in area ratio. It shall contain the ferrite phase of. When the microstructure inside the plate thickness satisfies the above conditions, desired strength and fatigue-resistant crack propagation characteristics can be obtained.
　なお、ここで板厚内部におけるフェライト相の面積率は、厚鋼板の、表面下１００μｍから板厚１／４位置までの範囲におけるフェライト相の面積率の平均値を指すものとする。また、ここで板厚内部におけるミクロ組織は、厚鋼板の圧延方向における先端、中央および尾端における板厚内部のミクロ組織を指すものとする。したがって、本発明の厚鋼板は、厚鋼板の圧延方向における先端、中央および尾端において、表面下１００μｍから板厚１／４位置までの範囲におけるミクロ組織が上記条件を満たす。なお、表層部の組織と同様に、通常は、先端、中央および尾端における板厚内部のミクロ組織が上記条件を満たしていれば、厚鋼板の圧延方向全長に亘って前記条件を満たしている。したがって、本発明の厚鋼板は、圧延方向の全長に亘って、板厚内部のミクロ組織が、面積率で８０％以下のフェライト相であるといえる。 Here, the area ratio of the ferrite phase inside the plate thickness refers to the average value of the area ratio of the ferrite phase in the range from 100 μm below the surface to the 1/4 position of the plate thickness of the thick steel plate. Further, here, the microstructure inside the plate thickness refers to the microstructure inside the plate thickness at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the microstructure in the range from 100 μm below the surface to the 1/4 position of the plate thickness satisfies the above conditions at the tip, center and tail end of the thick steel sheet in the rolling direction. As with the structure of the surface layer portion, normally, if the microstructure inside the plate thickness at the tip, center and tail end satisfies the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. .. Therefore, in the thick steel sheet of the present invention, it can be said that the microstructure inside the plate thickness is a ferrite phase having an area ratio of 80% or less over the entire length in the rolling direction.
　本発明において、板厚内部のミクロ組織における残部は、パーライト相、またはパーライト相とベイナイト相との混合相からなり、かつパーライト相の面積率がベイナイト相の面積率よりも多いことを特徴とする。ベイナイト相は島状マルテンサイトを含有し靭性を悪化させる。このため、パーライト相の面積分率をベイナイト相の面積分率よりも多くすることで、所望の靭性を得ることができる。ベイナイト相の面積分率は、１５％以下とすることが好ましい。より好ましくは１３％以下であり、さらに好ましくは１１％以下である。
なお、本発明の厚鋼板の残部は、先端、中央および尾端における、表層部および板厚内部の残部を指す。すなわち、厚鋼板の圧延方向全長に亘って、ミクロ組織の残部は、パーライト相、またはパーライト相とベイナイト相との混合相からなり、かつパーライト相の面積率がベイナイト相の面積率よりも多い。 In the present invention, the remainder in the microstructure inside the plate thickness is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase. .. The bainite phase contains island-like martensite and deteriorates toughness. Therefore, the desired toughness can be obtained by increasing the surface integral of the pearlite phase to be larger than the surface integral of the bainite phase. The surface integral of the bainite phase is preferably 15% or less. It is more preferably 13% or less, still more preferably 11% or less.
The remaining portion of the thick steel plate of the present invention refers to the remaining portion inside the surface layer portion and the plate thickness at the tip, center and tail end. That is, over the entire length of the thick steel plate in the rolling direction, the balance of the microstructure is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
　なお、表層部および板厚内部におけるミクロ組織は、実施例に記載した方法で評価することができる。 The microstructure inside the surface layer and the plate thickness can be evaluated by the method described in the examples.
　［全厚伸び］
　厚鋼板の全厚伸びは、特に限定されないが、板厚１６ｍｍ超えの場合１９％以上、板厚１６ｍｍ以下の場合、１５％以上であることが好ましい。本発明においては、厚鋼板の圧延方向における先端、中央および尾端において、上記全厚伸びの条件を満たすことが好ましい。なお、通常は、先端、中央および尾端が前記条件を満たしていれば、厚鋼板の圧延方向全長に亘って前記条件を満たしている。また、全厚伸びは、実施例に記載の方法で測定することができる。 [Total thickness growth]
The total thickness elongation of the thick steel sheet is not particularly limited, but is preferably 19% or more when the plate thickness exceeds 16 mm and 15% or more when the plate thickness is 16 mm or less. In the present invention, it is preferable that the above-mentioned total thickness elongation condition is satisfied at the tip, center and tail end of the thick steel sheet in the rolling direction. Normally, if the tip, center and tail end satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Further, the total thickness elongation can be measured by the method described in Examples.
　［引張強度］
　厚鋼板の引張強度（ＴＳ）は、特に限定されないが、４９０ＭＰａ以上であることが好ましい。また、ＴＳの上限も特に限定されないが、例えば、ＪＩＳにおける４９０ＭＰａ（５０ｋｇｆ／ｍｍ２）級とする場合には、ＴＳを６１０ＭＰａ以下とすればよい。また、ＪＩＳにおける５７０ＭＰａ（６０ｋｇｆ／ｍｍ２）級とする場合には、ＴＳの上下限をそれぞれ５７０ＭＰａおよび７２０ＭＰａとすればよい。本発明においては、厚鋼板の圧延方向における先端、中央および尾端において、上記ＴＳの条件を満たすことが好ましい。なお、通常は、先端、中央および尾端が前記条件を満たしていれば、厚鋼板の圧延方向全長に亘って前記条件を満たしている。また、ＴＳは、実施例に記載の方法で測定することができる。 [Tensile strength]
The tensile strength (TS) of the thick steel sheet is not particularly limited, but is preferably 490 MPa or more. Further, the upper limit of TS is not particularly limited, but for example, in the case of 490 MPa (50 kgf / mm 2 ) class in JIS, TS may be 610 MPa or less. Further, in the case of 570 MPa (60 kgf / mm 2 ) class in JIS, the upper and lower limits of TS may be set to 570 MPa and 720 MPa, respectively. In the present invention, it is preferable that the above TS conditions are satisfied at the tip, center and tail end of the thick steel sheet in the rolling direction. Normally, if the tip, center and tail end satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Further, TS can be measured by the method described in Examples.
　［靭性］
　本発明の厚鋼板は、上記成分組成とミクロ組織を有する結果、優れた靭性を備える。本発明の厚鋼板の靭性はとくに限定されないが、試験片厚１０ｍｍの場合、靭性の指標の一つである、０℃におけるシャルピー吸収エネルギーｖＥ０を１００Ｊ以上とすることが好ましく、１３０Ｊ以上とすることがより好ましく、１５０Ｊ以上とすることがさらに好ましく、２００Ｊ以上とすることが最も好ましい。一方、ｖＥ０の上限についても限定されないが、例えば、４００Ｊ以下であってよく、３００Ｊ以下であってよく、２７０Ｊ以下であってよい。試験片厚５ｍｍの場合、０℃におけるシャルピー吸収エネルギーｖＥ０を５０Ｊ以上とすることが好ましい。一方、ｖＥ０の上限についても限定されないが、例えば、２００Ｊ以下であってよく、１５０Ｊ以下であってよく、１３５Ｊ以下であってよい。なお、ｖＥ０は実施例に記載した方法で測定することができる。 [Toughness]
The thick steel sheet of the present invention has excellent toughness as a result of having the above-mentioned composition and microstructure. The toughness of the thick steel sheet of the present invention is not particularly limited, but when the test piece thickness is 10 mm, the Charpy absorption energy vE 0 at 0 ° C., which is one of the indicators of toughness, is preferably 100 J or more, preferably 130 J or more. More preferably, it is more preferably 150 J or more, and most preferably 200 J or more. On the other hand, the upper limit of vE 0 is not limited, but may be, for example, 400 J or less, 300 J or less, or 270 J or less. When the test piece thickness is 5 mm, it is preferable that the Charpy absorption energy vE 0 at 0 ° C. is 50 J or more. On the other hand, the upper limit of vE 0 is not limited, but may be, for example, 200 J or less, 150 J or less, or 135 J or less. In addition, vE 0 can be measured by the method described in the Example.
　［疲労き裂伝播特性］
　本発明の厚鋼板は、上記成分組成とミクロ組織を有する結果、優れた疲労き裂伝播特性を備えることができる。疲労き裂伝播特性の指標としては、疲労き裂伝播速度（ｄａ／ｄＮ）を用いることができる。疲労き裂伝播速度の値はとくに限定されないが、本発明においては、ΔＫ＝２５ＭＰａ√ｍでの疲労き裂伝播速度４．２５×１０－８ｍ／ｃｙｃｌｅ以下が好ましい。 [Fatigue crack propagation characteristics]
As a result of having the above-mentioned composition and microstructure, the thick steel sheet of the present invention can have excellent fatigue crack propagation characteristics. As an index of the fatigue crack propagation characteristic, the fatigue crack propagation velocity (da / dN) can be used. The value of the fatigue crack propagation velocity is not particularly limited, but in the present invention, the fatigue crack propagation velocity at ΔK = 25 MPa√m is preferably 4.25 × 10-8 m / cycle or less.
　［板厚］
　本発明における「厚鋼板」とは、本技術分野における通常の定義に従い、厚さ６ｍｍ以上の鋼板を指すものとする。一方、本発明における厚鋼板の板厚の上限は特に限定されず、任意の値とすることができる。しかし、先に述べたように鋼板先尾端での温度偏差が大きくなりやすく、また全厚での伸び特性が優れることが求められる薄物において、本発明の効果は特に顕著となる。そのため、厚鋼板の板厚は、２５ｍｍ以下とすることが好ましく、２０ｍｍ以下とすることがより好ましい。 [Plate thickness]
The "thick steel sheet" in the present invention refers to a steel sheet having a thickness of 6 mm or more according to the usual definition in the present technical field. On the other hand, the upper limit of the plate thickness of the thick steel plate in the present invention is not particularly limited and can be any value. However, as described above, the effect of the present invention is particularly remarkable in a thin material in which the temperature deviation at the tail end of the steel sheet tends to be large and the elongation characteristics at the total thickness are required to be excellent. Therefore, the thickness of the thick steel plate is preferably 25 mm or less, more preferably 20 mm or less.
　［製造方法］
　本発明の厚鋼板は、上述した成分組成を有する鋼素材に対し、加熱、熱間圧延、冷却、再加熱、冷却、焼入れの処理を順次施す方法、あるいは加熱、熱間圧延、冷却、焼入れの処理を順次施す方法によって得ることができる。まず、加熱、熱間圧延、冷却、再加熱、冷却、焼入れの処理を順次施す方法について説明する。 [Production method]
The thick steel sheet of the present invention is a method in which a steel material having the above-mentioned composition is sequentially subjected to heating, hot rolling, cooling, reheating, cooling, and quenching, or heating, hot rolling, cooling, and quenching. It can be obtained by a method of sequentially performing treatment. First, a method of sequentially performing heating, hot rolling, cooling, reheating, cooling, and quenching will be described.
　鋼素材
　本発明の鋼素材としては、上記成分組成を有し、熱間圧延が可能なものであれば任意のものを用いることができるが、通常は鋼スラブとすればよい。例えば、上記の成分組成を有する溶鋼を、転炉等の手段により溶製し、連続鋳造法等の鋳造方法で、スラブ等の鋼素材とすることができる。また、造塊－分解圧延法によりスラブ等の鋼素材とすることもできる。 Steel Material As the steel material of the present invention, any steel material having the above-mentioned composition and capable of hot rolling can be used, but usually a steel slab may be used. For example, molten steel having the above-mentioned composition can be melted by means such as a converter and used as a steel material such as a slab by a casting method such as a continuous casting method. Further, a steel material such as a slab can be used by the ingot-decomposition rolling method.
　加熱
　上記成分組成を有する鋼素材を、９００～１２００℃に加熱する。加熱温度が９００℃未満であると、次の熱間圧延工程における鋼素材の変形抵抗が高くなり、熱間圧延機への負荷が増大し、熱間圧延が困難になる。そのため、加熱温度は９００℃以上とする。加熱温度は９５０℃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、靭性が低下する。そのため、加熱温度は１２００℃以下とする。加熱温度は１１５０℃以下とすることが好ましい。 Heating A steel material having the above composition is heated to 900 to 1200 ° C. If the heating temperature is less than 900 ° C., the deformation resistance of the steel material in the next hot rolling step increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. The heating temperature is preferably 950 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the toughness decreases. Therefore, the heating temperature is set to 1200 ° C. or lower. The heating temperature is preferably 1150 ° C. or lower.
　なお、連続鋳造などの方法によって鋼素材（スラブ）を製造した場合、当該スラブは、冷却することなく直接上記加熱工程に供してもよく、冷却したのちに上記加熱工程に供してもよい。また、加熱方法は特に限定されないが、例えば、常法にしたがい、加熱炉で加熱することができる。 When a steel material (slab) is manufactured by a method such as continuous casting, the slab may be directly subjected to the above heating step without being cooled, or may be subjected to the above heating step after being cooled. The heating method is not particularly limited, but for example, heating can be performed in a heating furnace according to a conventional method.
　熱間圧延
　次いで、加熱された鋼素材を熱間圧延して熱延板とする。その際、製品鋼板の基本性能である靭性を確保するため、累積圧下率を５０％以上とする。累積圧下率が５０％未満の場合は、板厚内部のフェライト粒が粗大化して局所的に脆性が低い領域が発生し、脆性き裂が発生しやすくなり靭性が悪化する。熱間圧延工程に関する他の条件は特に限定されない。 Hot rolling Next, the heated steel material is hot-rolled to obtain a hot-rolled plate. At that time, in order to secure the toughness which is the basic performance of the product steel sheet, the cumulative reduction rate is set to 50% or more. When the cumulative reduction rate is less than 50%, the ferrite grains inside the plate thickness become coarse and a region with low brittleness is locally generated, brittle cracks are likely to occur, and the toughness deteriorates. Other conditions relating to the hot rolling process are not particularly limited.
　冷却
　次に、熱間圧延終了後の鋼板を冷却する（第１の冷却工程）。冷却工程では、再加熱を行う場合は、室温まで冷却することが好ましい。なお、冷却は、任意の方法、例えば、空冷または加速冷却により行うことができる。また、冷却条件については特段制限されない。 Cooling Next, the steel sheet after hot rolling is cooled (first cooling step). In the cooling step, when reheating is performed, it is preferable to cool to room temperature. The cooling can be performed by any method, for example, air cooling or accelerated cooling. Further, the cooling conditions are not particularly limited.
　再加熱
　次いで、冷却後の鋼板を、Ａｃ１変態点以上９５０℃以下に再加熱する。再加熱温度は、好ましくはＡｃ３変態点未満とする。このようにＡｃ１変態点以上９５０℃以下のオーステナイト相を含む温度域に加熱することにより、冷却偏差に起因するミクロ組織のバラツキを解消することができ、その結果、機械的特性のバラツキを解消することができる。再加熱温度がＡｃ３変態点未満であれば、再加熱前の組織を損なうことがないため、好ましい。
再加熱温度がＡｃ１変態点以上Ａｃ３変態点未満の場合、二相域に特有の脱炭反応が進行し、表層部におけるフェライト相の面積率を８０％以上とすることができる。一方、再加熱温度がＡｃ３変態点以上９５０℃以下の場合、再加熱温度での保持時間を短時間とすることで、二相域通過時の表層脱炭反応により生成した表層部におけるフェライト相がオーステナイト相に逆変態する反応が抑制され、表層部におけるフェライト相の面積率を８０％以上とすることができる。
再加熱温度が９５０℃超えであると、二相域通過時の表層脱炭反応により生成した表層部におけるフェライト相がオーステナイト相に逆変態する反応が促進され、表層部におけるフェライト相の面積率が８０％未満となる。その結果、表層部の硬度が増大して所望の全厚での伸び特性を得ることができない。
また、再加熱温度がＡｃ３変態点以上、９５０℃以下である場合は、板厚内部のオーステナイト相の結晶粒径は、再加熱温度がＡｃ３変態点未満である場合に比べ、粗大化するものの、靭性を過度に劣化させないことがわかった。さらに、この温度域では板厚内部のオーステナイト相への逆変態が進行する速度が上昇する。このため、短い加熱時間で所望の母相組織となるため、所定時間に製造可能な厚鋼板の枚数が増加するので生産性が向上する。
一方で、９５０℃を超えると、板厚内部で逆変態したオーステナイト相が成長して粗大化し、その結果、局所的に靭性が低い領域が発生して靭性が低下する。
一方、再加熱温度がＡｃ１変態点未満であると、オーステナイト相に逆変態する反応が起きず、冷却後の板厚内部のフェライト相、パーライト相およびベイナイト相が所望の面積率とならない。その結果、疲労特性（き裂伝播特性）が悪化する。さらに、熱間圧延後の冷却工程における冷却偏差に伴う機械的特性のバラツキを解消することができない。 Reheating Next, the cooled steel sheet is reheated to 950 ° C. or higher at the Ac1 transformation point or higher. The reheating temperature is preferably less than the Ac3 transformation point. By heating to a temperature range including the austenite phase of the Ac1 transformation point or more and 950 ° C. or less in this way, the variation in the microstructure due to the cooling deviation can be eliminated, and as a result, the variation in the mechanical properties is eliminated. be able to. When the reheating temperature is lower than the Ac3 transformation point, the structure before reheating is not damaged, which is preferable.
When the reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, the decarburization reaction peculiar to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer portion can be 80% or more. On the other hand, when the reheating temperature is equal to or higher than the Ac3 transformation point and 950 ° C. or lower, the ferrite phase in the surface layer portion generated by the surface decarburization reaction when passing through the two-phase region is formed by shortening the holding time at the reheating temperature. The reaction of reverse transformation to the austenite phase is suppressed, and the area ratio of the ferrite phase in the surface layer portion can be 80% or more.
When the reheating temperature exceeds 950 ° C., the reaction in which the ferrite phase in the surface layer portion generated by the surface decarburization reaction when passing through the two-phase region reversely transforms into the austenite phase is promoted, and the area ratio of the ferrite phase in the surface layer portion increases. It will be less than 80%. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness.
When the reheating temperature is above the Ac3 transformation point and below 950 ° C., the crystal grain size of the austenite phase inside the plate thickness becomes coarser than when the reheating temperature is below the Ac3 transformation point. It was found that the toughness was not excessively deteriorated. Furthermore, in this temperature range, the rate at which the reverse transformation to the austenite phase inside the plate thickness proceeds increases. Therefore, since the desired matrix structure is obtained in a short heating time, the number of thick steel sheets that can be manufactured in a predetermined time increases, and the productivity is improved.
On the other hand, when the temperature exceeds 950 ° C., the austenite phase reverse-transformed inside the plate thickness grows and becomes coarse, and as a result, a region having low toughness locally is generated and the toughness decreases.
On the other hand, when the reheating temperature is lower than the Ac1 transformation point, the reaction of reverse transformation to the austenite phase does not occur, and the ferrite phase, the pearlite phase and the bainite phase inside the plate thickness after cooling do not have the desired area ratio. As a result, fatigue characteristics (crack propagation characteristics) deteriorate. Further, it is not possible to eliminate the variation in mechanical properties due to the cooling deviation in the cooling process after hot rolling.
　なお、Ａｃ１変態点は、例えば、下記（１）式により求めることができる。
Ａｃ１（℃）＝７２３＋２９．１×Ｓｉ－１０．７×Ｍｎ－１６．９×Ｎｉ＋１６．９×Ｃｒ…（１）
また、Ａｃ３変態点は、例えば、下記（２）式により求めることができる。
Ａｃ３（℃）＝９６１．６－３１１．９×Ｃ＋４９．５×Ｓｉ－３６．４×Ｍｎ＋４３８．１×Ｐ－２８１８×Ｓ＋１２．７×Ａｌ－５１×Ｃｕ－２９×Ｎｉ－８．７×Ｃｒ＋１３．５×Ｍｏ＋３０８．１×Ｎｂ－１４０×Ｖ＋３１８．９×Ｔｉ＋６１１．２×Ｂ－９６９×Ｎ…（２）
ここで、上記（１）～（２）式における元素記号は、各元素の含有量（質量％）を意味し、当該元素が含有されていない場合にはゼロとする。 The Ac1 transformation point can be obtained, for example, by the following equation (1).
Ac1 (° C.) = 723 + 29.1 x Si-10.7 x Mn-16.9 x Ni + 16.9 x Cr ... (1)
Further, the Ac3 transformation point can be obtained by, for example, the following equation (2).
Ac3 (° C.) = 961.6-311.9 x C + 49.5 x Si-36.4 x Mn + 438.1 x P-2818 x S + 12.7 x Al-51 x Cu-29 x Ni-8.7 x Cr + 13 .5 x Mo + 308.1 x Nb-140 x V + 318.9 x Ti + 611.2 x B-969 x N ... (2)
Here, the element symbol in the above equations (1) and (2) means the content (mass%) of each element, and is set to zero when the element is not contained.
　なお、上記再加熱処理においては、再加熱温度まで加熱した後、当該温度に保持することが好ましい。再加熱温度がＡｃ１変態点以上、Ａｃ３変態点未満の場合、保持時間が１０分未満であると、オーステナイト相への逆変態が鋼板全長に亘って開始されず、一部の領域で焼入性が著しく低下する場合がある。そのため、保持時間は１０分以上とすることが好ましい。一方、再加熱温度がＡｃ３変態点以上、９５０℃以下の場合、保持時間が３０分を超えると、オーステナイト相が成長して粗大化する。そのため、保持時間は３０分以下とすることが好ましい。 In the above reheating treatment, it is preferable to heat the temperature to the reheating temperature and then maintain the temperature. When the reheating temperature is above the Ac1 transformation point and below the Ac3 transformation point, if the holding time is less than 10 minutes, the reverse transformation to the austenite phase is not started over the entire length of the steel sheet, and it is hardenable in some regions. May decrease significantly. Therefore, the holding time is preferably 10 minutes or more. On the other hand, when the reheating temperature is equal to or higher than the Ac3 transformation point and lower than 950 ° C., the austenite phase grows and becomes coarse when the holding time exceeds 30 minutes. Therefore, the holding time is preferably 30 minutes or less.
　冷却
　上記再加熱工程で再加熱された鋼板を、もしくは熱間圧延された鋼板を、３５０～６００℃の冷却停止温度まで冷却する（第２の冷却工程）。その際、平均冷却速度を２～７℃／ｓとする。平均冷却速度は低い方がよりパーライト変態が促進されるため靭性改善の点で好ましい。しかし、平均冷却速度が２℃／ｓ未満であると、フェライトの粒成長が過剰となり、粗粒化するため、靭性が悪化する。そのため、平均冷却速度は、２℃／ｓ以上とする。一方、平均冷却速度が７℃／ｓを超える場合、鋼板内部のミクロ組織においてパーライト変態が十分に進行せず、ベイナイト変態やマルテンサイト変態が進行しやすくなる。この場合はベイナイト相やマルテンサイト相の分率が多くなるため、全厚での伸び特性および靭性が悪化する。そのため、平均冷却速度は、７℃／ｓ以下とする。平均冷却速度は、好ましくは５℃／ｓ以下、より好ましくは４℃／ｓ以下、さらに好ましくは３℃／ｓ未満とする。
また、冷却停止温度が３５０℃未満の場合は、板厚内部においてフェライトが過剰に生成するため鋼板全体が軟質化し、所望の引張強度を得ることが出来ない。そのため、冷却停止温度は３５０℃以上とする。一方、冷却停止温度が６００℃を超える場合、未変態オーステナイトが多量に残留したまま焼き入れられるので、硬質なベイナイトやマルテンサイトが過剰に生成する。その結果、全厚での伸び特性が低下し、靭性も悪化する。そのため、冷却停止温度は６００℃以下とする。 Cooling The steel sheet reheated in the above reheating step or the hot-rolled steel sheet is cooled to a cooling shutdown temperature of 350 to 600 ° C. (second cooling step). At that time, the average cooling rate is 2 to 7 ° C./s. A lower average cooling rate is preferable in terms of improving toughness because pearlite transformation is promoted more. However, if the average cooling rate is less than 2 ° C./s, the grain growth of ferrite becomes excessive and coarse-grained, resulting in deterioration of toughness. Therefore, the average cooling rate is set to 2 ° C./s or higher. On the other hand, when the average cooling rate exceeds 7 ° C./s, the pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and the bainite transformation and the martensitic transformation are likely to proceed. In this case, since the fractions of the bainite phase and the martensite phase increase, the elongation characteristics and toughness at the total thickness deteriorate. Therefore, the average cooling rate is set to 7 ° C./s or less. The average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
Further, when the cooling shutdown temperature is less than 350 ° C., ferrite is excessively generated inside the plate thickness, so that the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling shutdown temperature is set to 350 ° C. or higher. On the other hand, when the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.
　焼入れ
　上記冷却停止温度まで冷却された鋼板に焼入れを施す。したがって、焼入れ温度は、３５０～６００℃の範囲となる。焼入れは、特に限定されることなく、任意の条件で行うことができるが、Ｍｓ点以下の温度、好ましくは２００℃以下まで水冷することが好ましい。なお、Ｍｓ点は、例えば、下記（３）式により求めることができる。
Ｍｓ（℃）＝５１７－３００×Ｃ－１１×Ｓｉ－３３×Ｍｎ－１７×Ｎｉ－２２×Ｃｒ－１１×Ｍｏ…（３）
ここで、上記（３）式における元素記号は、各元素の含有量（質量％）を意味し、当該元素が含有されていない場合にはゼロとする。 Quenching The steel sheet cooled to the above cooling shutdown temperature is quenched. Therefore, the quenching temperature is in the range of 350 to 600 ° C. Quenching can be performed under any conditions without particular limitation, but it is preferably water-cooled to a temperature of Ms point or lower, preferably 200 ° C. or lower. The Ms point can be obtained by, for example, the following equation (3).
Ms (° C.) = 517-300 x C-11 x Si-33 x Mn-17 x Ni-22 x Cr-11 x Mo ... (3)
Here, the element symbol in the above formula (3) means the content (mass%) of each element, and is set to zero when the element is not contained.
　次に、加熱、熱間圧延、冷却、焼入れの処理を順次施す方法について説明する。 Next, a method of sequentially performing heating, hot rolling, cooling, and quenching will be described.
　鋼素材は、上記で説明した鋼素材と同様のものを用いる。加熱と熱間圧延は、上記で説明した加熱と熱間圧延と同じ方法で実施できる。
熱間圧延後の冷却は、まず、Ａｒ１変態点以上Ａｒ３変態点以下の温度まで冷却し、Ａｒ１変態点以上Ａｒ３変態点以下の温度（冷却開始温度）から、２～７℃／ｓの平均冷却速度で３５０～６００℃の冷却停止温度まで冷却する。冷却開始温度をＡｒ１変態点以上Ａｒ３変態点以下（二相域）の温度とする理由は、二相域に特有の脱炭反応が進行し、表層部におけるフェライト相の面積率を８０％以上とすることができるためである。
また、その後の平均冷却速度を２～７℃／ｓとする理由としては以下のような理由が挙げられる。平均冷却速度が２℃／ｓ未満であると、フェライトの粒成長が過剰となり、粗粒化するため、靭性が悪化する。そのため、平均冷却速度は、２℃／ｓ以上とする。一方、平均冷却速度が７℃／ｓを超える場合、鋼板内部のミクロ組織においてパーライト変態が十分に進行せず、ベイナイト変態やマルテンサイト変態が進行しやすくなる。この場合はベイナイト相やマルテンサイト相の分率が多くなるため、全厚での伸び特性および靭性が悪化する。そのため、平均冷却速度は、７℃／ｓ以下とする。平均冷却速度は、好ましくは５℃／ｓ以下、より好ましくは４℃／ｓ以下、さらに好ましくは３℃／ｓ未満とする。
また、冷却停止温度を３５０～６００℃とする理由は以下の理由が挙げられる。冷却停止温度が３５０℃未満の場合は、板厚内部においてフェライトが過剰に生成するため鋼板全体が軟質化し、所望の引張強度を得ることが出来ない。そのため、冷却停止温度は３５０℃以上とする。一方、冷却停止温度が６００℃を超える場合、未変態オーステナイトが多量に残留したまま焼き入れられるので、硬質なベイナイトやマルテンサイトが過剰に生成する。その結果、全厚での伸び特性が低下し、靭性も悪化する。そのため、冷却停止温度は６００℃以下とする。
以降の焼入れについては、上記で説明した焼入れと同じ方法で実施することができる。
なお、Ａｒ１変態点は、例えば、下記（４）式により求めることができる。
Ａｒ１＝７１２－１７．８×Ｃ－１９．１×Ｎｉ＋２０．１×Ｓｉ＋１１．９×Ｃｒ＋９．８×Ｍｏ…（４）
また、Ａｒ３変態点は、例えば、下記（５）式により求めることができる。
Ａｒ３＝９１０－３１０×Ｃ－８０×Ｍｎ－２０×Ｃｕ－１５×Ｃｒ－５５×Ｎｉ－８０×Ｍｏ…（５）
ここで、上記（４）～（５）式における元素記号は、各元素の含有量（質量％）を意味し、当該元素が含有されていない場合にはゼロとする。 As the steel material, the same steel material as that described above is used. The heating and hot rolling can be carried out in the same manner as the heating and hot rolling described above.
For cooling after hot rolling, first, the temperature is cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point, and then the average cooling is 2 to 7 ° C./s from the temperature above the Ar1 transformation point and below the Ar3 transformation point (cooling start temperature). Cool to a cooling stop temperature of 350-600 ° C. at a rate. The reason why the cooling start temperature is set to the temperature above the Ar1 transformation point and below the Ar3 transformation point (two-phase region) is that the decarburization reaction peculiar to the two-phase region proceeds and the area ratio of the ferrite phase in the surface layer portion is 80% or more. Because it can be done.
Further, the reasons for setting the average cooling rate after that to 2 to 7 ° C./s include the following reasons. If the average cooling rate is less than 2 ° C./s, the grain growth of ferrite becomes excessive and coarse-grained, resulting in deterioration of toughness. Therefore, the average cooling rate is set to 2 ° C./s or more. On the other hand, when the average cooling rate exceeds 7 ° C./s, the pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and the bainite transformation and the martensitic transformation are likely to proceed. In this case, since the fractions of the bainite phase and the martensite phase increase, the elongation characteristics and toughness at the total thickness deteriorate. Therefore, the average cooling rate is set to 7 ° C./s or less. The average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
The reasons for setting the cooling shutdown temperature to 350 to 600 ° C. are as follows. When the cooling shutdown temperature is less than 350 ° C., ferrite is excessively generated inside the plate thickness, so that the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling shutdown temperature is set to 350 ° C. or higher. On the other hand, when the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.
Subsequent quenching can be carried out in the same manner as the quenching described above.
The Ar1 transformation point can be obtained, for example, by the following equation (4).
Ar1 = 712-17.8 x C-19.1 x Ni + 20.1 x Si + 11.9 x Cr + 9.8 x Mo ... (4)
Further, the Ar3 transformation point can be obtained by, for example, the following equation (5).
Ar3 = 910-310 x C-80 x Mn-20 x Cu-15 x Cr-55 x Ni-80 x Mo ... (5)
Here, the element symbol in the above formulas (4) to (5) means the content (mass%) of each element, and is set to zero when the element is not contained.
　以下、本発明の効果を実施例に基づいて具体的に説明するが、本発明はこれら実施例に限定されるものではない。 Hereinafter, the effects of the present invention will be specifically described based on examples, but the present invention is not limited to these examples.
　表１に示す組成の溶鋼を溶製し、鋼素材（スラブ）とした。なお、表１に示したＡｃ１点、Ａｃ３点、Ｍｓ点、Ａｒ１点、およびＡｒ３点の値は、それぞれ上述した（１）、（２）、（３）、（４）、（５）式で求めた値である。 The molten steel having the composition shown in Table 1 was melted and used as a steel material (slab). The values of Ac1, Ac3, Ms, Ar1, and Ar3 shown in Table 1 are the above-mentioned equations (1), (2), (3), (4), and (5), respectively. This is the calculated value.
　次に、得られたスラブに対し、表２に示す条件で加熱および熱間圧延を施し、全長２０ｍで、表２に示した板厚の熱延板とした。その後、熱延板を表２に記載の冷却方法にて室温まで冷却し、表２に示した再加熱温度まで再加熱し、３０分間以上保持した。次いで、鋼板の両面に冷却水をスプレーし、表２に示した平均冷却速度で冷却停止温度まで冷却後、焼入れ処理を施した。焼入れ処理では１５０℃以下まで水冷した。 Next, the obtained slab was heated and hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled plate having a total length of 20 m and a plate thickness shown in Table 2. Then, the hot-rolled sheet was cooled to room temperature by the cooling method shown in Table 2, reheated to the reheating temperature shown in Table 2, and held for 30 minutes or more. Next, cooling water was sprayed on both sides of the steel sheet, cooled to the cooling stop temperature at the average cooling rate shown in Table 2, and then quenched. In the quenching treatment, it was water-cooled to 150 ° C. or lower.
　なお、比較のため、一部の比較例（表２のＮｏ．２４）では再加熱後に本発明の条件を満たす冷却を行うこと無く、すぐに焼入れを行った。この比較例における焼入れ条件は、平均冷却速度４４．０℃／ｓ、冷却停止温度１１０℃とした。 For comparison, in some comparative examples (No. 24 in Table 2), quenching was performed immediately after reheating without cooling to satisfy the conditions of the present invention. The quenching conditions in this comparative example were an average cooling rate of 44.0 ° C./s and a cooling shutdown temperature of 110 ° C.
　得られた厚鋼板について、（１）ミクロ組織、（２）全厚伸び、（３）引張強度（ＴＳ）、（４）疲労き裂伝搬特性、（５）靭性について、それぞれ評価した。厚鋼板全長での特性を評価するため、試験片は厚鋼板の圧延方向における先端、中央、および尾端のそれぞれから採取した。試験方法は次の通りである。なお、先端および尾端における試験片は、鋼板の圧延方向端部より１００ｍｍ入った位置から採取した。 The obtained thick steel sheets were evaluated for (1) microstructure, (2) total thickness elongation, (3) tensile strength (TS), (4) fatigue crack propagation characteristics, and (5) toughness. In order to evaluate the characteristics over the total length of the thick steel plate, the test pieces were taken from each of the tip, center, and tail end of the thick steel plate in the rolling direction. The test method is as follows. The test pieces at the tip and the tail end were taken from a position 100 mm from the end of the steel sheet in the rolling direction.
　（１）ミクロ組織観察
　以下の手順でミクロ組織を観察した。 (1) Observation of microstructure The microstructure was observed by the following procedure.
　表層部におけるフェライト相の面積率、板厚内部におけるフェライト相の面積率、板厚内部におけるパーライト相およびベイナイト相の面積率
　まず、得られた厚鋼板から、観察面が圧延方向に垂直な断面（板厚方向断面）となるように組織観察用試験片を採取し、鏡面となるまで研磨した後、腐食液（硝酸メタノール溶液）で腐食し、光学顕微鏡（倍率：４００倍）を用いて、鋼板表面から板厚方向に板厚１／４位置まで観察し、画面が連続するように撮像した。得られた組織写真を用い、画像解析により相を同定し、（ａ）厚鋼板の、表面から表面下１００μｍまでの範囲におけるフェライト相の面積率の平均値、（ｂ）表面下１００μｍから板厚１／４位置までの範囲におけるフェライト相の面積率の平均値、および（ｃ）表面下１００μｍから板厚１／４位置までの範囲におけるパーライト相およびベイナイト相の面積率を求めた。 Area ratio of ferrite phase in the surface layer, area ratio of ferrite phase inside the plate thickness, area ratio of pearlite phase and bainite phase inside the plate thickness First, from the obtained thick steel plate, the cross section whose observation surface is perpendicular to the rolling direction ( A test piece for microstructure observation is collected so as to have a cross section in the plate thickness direction, polished to a mirror surface, corroded with a corrosive solution (methanol nitrate solution), and then a steel plate is used with an optical microscope (magnification: 400 times). Observation was performed from the surface to the plate thickness 1/4 position in the plate thickness direction, and images were taken so that the screen was continuous. Using the obtained microstructure photograph, the phase was identified by image analysis, (a) the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel plate, and (b) the plate thickness from 100 μm below the surface. The average value of the area ratio of the ferrite phase in the range up to the 1/4 position and (c) the area ratio of the pearlite phase and the bainite phase in the range from 100 μm below the surface to the 1/4 position of the plate thickness were obtained.
　ミクロ組織の測定結果を表３に示す。 Table 3 shows the measurement results of the microstructure.
　（２）引張試験
　厚鋼板の幅中央部から板幅方向が引張方向と一致するように採取したＪＩＳ　Ｚ　２２０１　１Ａ号の全厚試験片を用いて引張試験を実施し、引張強度（ＴＳ）および全厚伸びを求めた。引張強度は４９０ＭＰａ以上を合格とした。伸び特性は板厚１６ｍｍ以下の場合は１５％以上、板厚１６ｍｍを超える場合は１９％以上を合格とした。 (2) Tensile test A tensile test was carried out using a full-thickness test piece of JIS Z 2201 1A collected so that the plate width direction coincided with the tensile direction from the center of the width of the thick steel plate, and the tensile strength (TS) and We asked for full-thickness growth. The tensile strength of 490 MPa or more was accepted. The elongation characteristics were accepted as 15% or more when the plate thickness was 16 mm or less, and 19% or more when the plate thickness exceeded 16 mm.
　（３）疲労き裂伝搬試験
　図１に示す片側切欠単純引張型疲労試験片を用いて疲労き裂伝搬試験を行い、板厚方向にき裂が進展する時の疲労き裂伝播挙動を評価した。試験条件は、ＡＳＴＭ　Ｅ６４７に準拠し、応力比０．１、周波数１０Ｈｚとし、室温大気中で実施した。本発明では溶接構造物において溶接部などから発生したき裂が鋼材中を進展するときの伝播速度を低減することが目的であるため、このような状況を想定し、応力拡大係数範囲（ΔＫ）が１０～３０ＭＰａ√ｍの範囲で試験を行った。ΔＫ＝２５ＭＰａ√ｍでの疲労き裂伝播速度４．２５×１０－８ｍ／ｃｙｃｌｅ以下を合格とした。 (3) Fatigue crack propagation test A fatigue crack propagation test was conducted using the one-sided notch simple tensile type fatigue test piece shown in Fig. 1, and the fatigue crack propagation behavior when the crack propagated in the plate thickness direction was evaluated. .. The test conditions were based on ASTM E647, a stress ratio of 0.1, a frequency of 10 Hz, and the test was carried out in room temperature atmosphere. Since the object of the present invention is to reduce the propagation speed when cracks generated from a welded portion or the like propagate in a steel material in a welded structure, the stress intensity factor range (ΔK) is assumed in such a situation. Was tested in the range of 10 to 30 MPa√m. Fatigue crack propagation speeds of 4.25 × 10-8 m / cycle or less at ΔK = 25 MPa√m were accepted.
　（４）靭性
　厚鋼板の板厚中心部から、圧延方向（Ｌ方向）に平行にシャルピー衝撃試験片を採取した。試験片厚は、板厚１０ｍｍ以上の場合は試験片厚１０ｍｍとし、板厚１０ｍｍ未満の場合は試験片厚５ｍｍとした。試験はＪＩＳ　Ｚ　２２０２に準拠してシャルピー衝撃試験を０℃で行い、吸収エネルギーｖＥ０を測定した。試験片厚１０ｍｍの試験片は吸収エネルギーが１００Ｊ以上を合格とした。試験片厚５ｍｍの試験片は吸収エネルギーが５０Ｊ以上を合格とした。 (4) Toughness A Charpy impact test piece was taken from the center of the thick steel plate in parallel with the rolling direction (L direction). The test piece thickness was 10 mm when the plate thickness was 10 mm or more, and 5 mm when the plate thickness was less than 10 mm. The test was carried out in accordance with JIS Z 2202 by performing a Charpy impact test at 0 ° C., and the absorbed energy vE 0 was measured. A test piece having a thickness of 10 mm was accepted as having an absorption energy of 100 J or more. A test piece with a thickness of 5 mm was accepted as having an absorption energy of 50 J or more.
　測定結果を表４に示す。この結果から分かるように、本発明の条件を満たす実施例においては、全厚での伸び特性と耐疲労き裂伝播特性と靭性を具備した厚鋼板が得られている。一方、本発明の条件を満たさない比較例では、鋼板の先端、尾端の少なくとも一つの位置において、全厚での伸び特性か、疲労き裂伝播速度か、靭性かの少なくとも一つが劣っている。 The measurement results are shown in Table 4. As can be seen from this result, in the examples satisfying the conditions of the present invention, a thick steel sheet having elongation characteristics at full thickness, fatigue crack propagation characteristics and toughness is obtained. On the other hand, in the comparative example that does not satisfy the conditions of the present invention, at least one of the elongation characteristics at the total thickness, the fatigue crack propagation rate, and the toughness is inferior at at least one position of the tip and the tail end of the steel sheet. ..
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
  
Figure JPOXMLDOC01-appb-T000004
  

Claims (4)

  1. 　質量％で、
    Ｃ：０．０５～０．２０％、
    Ｓｉ：０．０１～０．５０％、
    Ｍｎ：０．５０～２．００％、
    Ｐ：０．０５％以下、
    Ｓ：０．０２％以下を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、
    ミクロ組織は、板厚方向に、表面から表面下１００μｍまでの範囲において、面積率で８０％以上のフェライト相を含み、
    板厚方向に、表面下１００μｍから板厚１／４位置の範囲において、
    面積率で８０％以下のフェライト相を含み、
    残部がパーライト相、またはパーライト相とベイナイト相との混合相からなり、かつ前記パーライト相の面積率が前記ベイナイト相の面積率よりも多い厚鋼板。     By mass%,
    C: 0.05 to 0.20%,
    Si: 0.01-0.50%,
    Mn: 0.50 to 2.00%,
    P: 0.05% or less,
    S: Contains 0.02% or less, has a component composition consisting of the balance Fe and unavoidable impurities, and has a component composition.
    The microstructure contains a ferrite phase with an area ratio of 80% or more in the range from the surface to 100 μm below the surface in the plate thickness direction.
    In the plate thickness direction, in the range from 100 μm below the surface to the plate thickness 1/4 position.
    Contains a ferrite phase with an area ratio of 80% or less,
    A thick steel plate in which the balance is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
  2. 　前記成分組成が、さらに、質量％で、
    Ｃｒ：０．０１～１．００％、
    Ｃｕ：０．０１～２．００％、
    Ｎｉ：０．０１～２．００％、
    Ｍｏ：０．０１～１．００％、
    Ｃｏ：０．０１～１．００％、
    Ｓｎ：０．００５～０．５００％、
    Ｓｂ：０．００５～０．２００％、
    Ｎｂ：０．００５～０．２００％、
    Ｖ：０．００５～０．２００％、
    Ｔｉ：０．００５～０．０５０％、
    Ｂ：０．０００１～０．００５０％、
    Ｚｒ：０．００５～０．１００％、
    Ｃａ：０．０００１～０．０２０％、
    Ｍｇ：０．０００１～０．０２０％、および
    ＲＥＭ：０．０００１～０．０２０％のうちから選ばれる１種または２種以上を含有する、請求項１に記載の厚鋼板。     The component composition is further increased by mass%.
    Cr: 0.01-1.00%,
    Cu: 0.01-2.00%,
    Ni: 0.01-2.00%,
    Mo: 0.01-1.00%,
    Co: 0.01-1.00%,
    Sn: 0.005 to 0.500%,
    Sb: 0.005 to 0.200%,
    Nb: 0.005 to 0.200%,
    V: 0.005 to 0.200%,
    Ti: 0.005 to 0.050%,
    B: 0.0001 to 0.0050%,
    Zr: 0.005 to 0.100%,
    Ca: 0.0001 to 0.020%,
    The thick steel sheet according to claim 1, which contains one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
  3. 　請求項１または２に記載の成分組成を有する鋼素材を９００～１２００℃に加熱し、
    加熱された前記鋼素材に累積圧下率５０％以上の熱間圧延を施して熱延板とし、
    前記熱延板を冷却し、
    次いで、Ａｃ１変態点以上、９５０℃以下の再加熱温度に再加熱し、
    前記Ａｃ１変態点以上、９５０℃以下の温度に再加熱された鋼板を２～７℃／ｓの平均冷却速度で３５０～６００℃の冷却停止温度まで冷却し、
    前記３５０～６００℃の冷却停止温度まで冷却された鋼板に焼入れを施す、厚鋼板の製造方法。     The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200 ° C.
    The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
    Cool the hot-rolled sheet and
    Then, it was reheated to a reheating temperature of 950 ° C. or higher and equal to or higher than the Ac1 transformation point.
    The steel sheet reheated to a temperature above the Ac1 transformation point and below 950 ° C. is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
    A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
  4. 　請求項１または２に記載の成分組成を有する鋼素材を９００～１２００℃に加熱し、
    加熱された前記鋼素材に累積圧下率５０％以上の熱間圧延を施して熱延板とし、
    次いで、
    Ａｒ１変態点以上Ａｒ３変態点以下の温度まで冷却された鋼板を２～７℃／ｓの平均冷却速度で３５０～６００℃の冷却停止温度まで冷却し、
    前記３５０～６００℃の冷却停止温度まで冷却された鋼板に焼入れを施す、厚鋼板の製造方法。

          The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200 ° C.
    The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
    Then
    The steel sheet cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
    A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
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