WO2022054898A1 - Tôle d'acier épaisse et son procédé de fabrication - Google Patents

Tôle d'acier épaisse et son procédé de fabrication 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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English (en)
Japanese (ja)
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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 JP2021573411A priority Critical patent/JP7070814B1/ja
Priority to KR1020237005688A priority patent/KR20230041060A/ko
Priority to CN202180051128.9A priority patent/CN115989327A/zh
Publication of WO2022054898A1 publication Critical patent/WO2022054898A1/fr

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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.

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Abstract

La présente invention a pour objet de fournir une tôle d'acier épaisse qui présente une résistance élevée, un allongement supérieur et des propriétés de propagation de fissure de fatigue supérieures dans toute l'épaisseur de celle-ci, et une ténacité supérieure, ainsi que son procédé de fabrication. Cette tôle d'acier épaisse présente une composition de composants comprenant, en % en masse, C : de 0,05 à 0,20%, Si : de 0,01 à 0,50 %, Mn : de 0,50 à 2,00 %, P : 0,05% ou moins, et S : 0,02 % ou moins, le reste se composant de Fe et d'impuretés inévitables. La microstructure comprend, par rapport de surface, 80 % ou plus d'une phase de ferrite dans la plage allant d'une surface à 100 µm en dessous de la surface dans le sens de l'épaisseur de la tôle, et comprend, par rapport de surface, 80 % ou moins d'une phase de ferrite dans la plage de 100 µm en dessous de la surface jusqu'à un emplacement à 1/4 de l'épaisseur de la tôle dans le sens de l'épaisseur de la tôle, le reste étant composé d'une phase de perlite ou d'une phase mixte d'une phase de perlite et d'une phase de bainite, le rapport de surface de la phase de perlite étant supérieur au rapport de surface de la phase de bainite.
PCT/JP2021/033277 2020-09-14 2021-09-10 Tôle d'acier épaisse et son procédé de fabrication WO2022054898A1 (fr)

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WO2023233980A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud, tube d'acier carré, procédés de fabrication de ceux-ci et structure de bâtiment

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JP2013095928A (ja) * 2011-10-28 2013-05-20 Nippon Steel & Sumitomo Metal Corp 靭性に優れた高張力鋼板およびその製造方法
JP2016125077A (ja) * 2014-12-26 2016-07-11 新日鐵住金株式会社 高強度高延性厚板鋼板とその製造方法
JP2019026927A (ja) * 2017-02-16 2019-02-21 Ｊｆｅスチール株式会社 厚鋼板および厚鋼板の製造方法

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JP3489243B2 (ja) 1995-02-16 2004-01-19 住友金属工業株式会社 フェライト・ベイナイト二相鋼
JP5070744B2 (ja) 2006-06-13 2012-11-14 Ｊｆｅスチール株式会社 耐疲労亀裂伝播特性に優れる鋼材の製造方法
JP5470904B2 (ja) 2009-02-25 2014-04-16 Ｊｆｅスチール株式会社 ＴＳが５７０ＭＰａ以上、全伸びが２５％以上、ΔＫ＝１５ＭＰａ√ｍでの疲労き裂伝播速度８．７５ｘ１０−９ｍ／ｃｙｃｌｅ以下の、全伸びと疲労き裂伝播抵抗性に優れた板厚２０ｍｍ以下の厚鋼板の製造方法

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JP2013095928A (ja) * 2011-10-28 2013-05-20 Nippon Steel & Sumitomo Metal Corp 靭性に優れた高張力鋼板およびその製造方法
JP2016125077A (ja) * 2014-12-26 2016-07-11 新日鐵住金株式会社 高強度高延性厚板鋼板とその製造方法
JP2019026927A (ja) * 2017-02-16 2019-02-21 Ｊｆｅスチール株式会社 厚鋼板および厚鋼板の製造方法

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2023233980A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud, tube d'acier carré, procédés de fabrication de ceux-ci et structure de bâtiment

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