Steel sheet and method for producing same

Abstract

To provide a steel sheet which has high tensile strength, is a thick steel sheet, has excellent corrosion resistance in a chloride-containing environment, and has improved lamellar tear resistance, and to provide a method for producing the same.SOLUTION: The steel sheet has a chemical composition comprising, by mass%, C:0.06 to 0.15%, Si:0.05 to 1.20%, Mn:1.00 to 2.50%, P:0.015% or less, S:0.0024% or less, Ni:0.1 to 2.0%, Mo:0.01 to 0.75%, V:0.005 to 0.150%, Nb:0.003 to 0.040%, Ti:0.003 to 0.040%, Al:0.010 to 0.080%, Sn:0.020 to 0.400%, N:0.0010 to 0.0070%, O:0.0005 to 0.0040%, Ca:0.0001 to 0.0080%, and a balance of Fe and impurities, Ceq is 0.37 to 0.51, SnEQ is 0.10 or more, SC is 0.3 to 15.0, MV is 0.20 to 0.80, a sheet thickness of the steel sheet is 75 to 105mm, a tensile strength at a sheet thickness 1 / 4 position of the steel sheet is 570MPa or more, and in a Sn segregated portion present in a sheet thickness central portion of a C cross section of the steel sheet, a Vickers hardness is 350HV or less, a mean grain size of prior γ grains is 40 μm or less, and a mean value of aspect ratios of the prior γ grains is 2.0 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the same. [Background technology]

[0002] In recent years, the demand for larger and longer-lasting bridges and other structures has led to an increasing demand for steel plates with high tensile strength. Furthermore, bridges and other structures constructed in coastal areas also require excellent corrosion resistance in the severely corrosive chloride environment.

[0003] Generally, in environments where chloride corrosion is a problem, steel materials are painted to prevent corrosion, and regular inspections are conducted to check the progress of corrosion. If corrosion exceeds the control standard, a new coat of paint is applied. However, in the case of bridges and other structures, it is necessary to use gondolas for high-altitude work or to set up scaffolding, and the cost of repainting the work is enormous. Furthermore, because painting has an environmental impact, it is desirable to minimize the use of paint.

[0004] As a high-strength thick steel plate with excellent corrosion resistance in such chloride environments, for example, Patent Document 1 discloses a steel plate containing Sn and having controlled Sn concentrations at the grain boundaries and within the grains, which is used for large structures such as bridges. Also, Patent Document 2 discloses a steel plate for marine structures containing Sn and consisting of ferrite and a hard second phase. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 116520 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-144799 Summary of the Invention [Problem to be solved by the invention]

[0006] In cross joints, T-joints, corner joints, and the like, tensile stress is generated in the steel plate in the plate thickness direction due to volumetric shrinkage of the weld metal caused by cooling after welding. This tensile stress in the plate thickness direction causes cracks to form inside the steel plate. This type of crack is called lamellar tearing. Steel plates used for bridges and the like have beams and other components welded to their surfaces, making their lamellar tear resistance problematic. However, the steel plates disclosed in Patent Documents 1 and 2 still have room for improvement in terms of lamellar tear resistance.

[0007] The present invention aims to solve the above problems and to provide a steel plate having high tensile strength, which is a thick steel plate and has excellent corrosion resistance in chloride-containing environments and improved lamellar tear resistance, and a manufacturing method thereof. [Means for solving the problem]

[0008] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: a steel sheet and a method for manufacturing the same.

[0009] (1) The chemical composition of the steel sheet is, in mass%, C: 0.06~0.15%, Si: 0.05 to 1.20% Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 2.0% Mo: 0.01 to 0.75% V: 0.005~0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010~0.080%, Sn: 0.020~0.400%, N: 0.0010~0.0070%, O: 0.0005 to 0.0040%, Ca: 0.0001 to 0.0080%, and The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, The thickness of the steel plate is 75 to 105 mm, The tensile strength of the steel plate at a 1 / 4 position in the plate thickness direction is 570 MPa or more, In the Sn segregation portion present in the center of the plate thickness in a cross section perpendicular to the rolling direction of the steel plate, Vickers hardness is 350HV0.01 or less, The average grain size of prior austenite grains is 40 μm or less, The average aspect ratio of the prior austenite grains is 2.0 or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (i) SnEQ=Sn+W+Ni / 10+Mo / 4 (ii) SC=S / Ca (iii) MV = Mo + 4 × V (iv) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0010] (2) The chemical composition of the steel plate is, in mass%, C: 0.06~0.15%, Si: 0.05 to 1.20% Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 2.0% Mo: 0.01 to 0.75% V: 0.005~0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010~0.080%, Sn: 0.020~0.400%, N: 0.0010~0.0070%, O: 0.0005 to 0.0040%, Ca: 0.0001 to 0.0080%, and further containing one or more selected from the group consisting of the following groups A, B, and C: The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, The thickness of the steel plate is 75 to 105 mm, The tensile strength of the steel plate at a 1 / 4 position in the plate thickness direction is 570 MPa or more, In the Sn segregation portion present in the center of the plate thickness in a cross section perpendicular to the rolling direction of the steel plate, Vickers hardness is 350HV0.01 or less, The average grain size of prior austenite grains is 40 μm or less, The average aspect ratio of the prior austenite grains is 2.0 or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (i) SnEQ=Sn+W+Ni / 10+Mo / 4 (ii) SC=S / Ca (iii) MV = Mo + 4 × V (iv) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained. [Group A] One or more selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.05% or less [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less

[0011] (3) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from Group A.

[0012] (4) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from the B group.

[0013] (5) The steel sheet according to (2) above, wherein the chemical composition contains one or more elements selected from the C group.

[0014] (6) In a cross section perpendicular to the rolling direction of the steel plate, the maximum defect length at the center of the thickness of the steel plate is 0.50 mm or less; The steel sheet according to any one of (1) to (5) above.

[0015] (7) A method for producing a steel sheet according to (1) above, a soaking process in which the slab is heated and then soaked; a hot rolling step of hot rolling the slab to form a steel plate; a quenching step of quenching the steel plate; a tempering step of tempering the steel plate after quenching, The chemical composition of the cast piece is, in mass%, C: 0.06~0.15%, Si: 0.05 to 1.20% Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 2.0% Mo: 0.01 to 0.75% V: 0.005~0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010~0.080%, Sn: 0.020~0.400%, N: 0.0010~0.0070%, O: 0.0005 to 0.0040%, Ca: 0.0001 to 0.0080%, and The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, In the quenching step, the steel plate is reheated in a temperature range of 850 to 950°C and then quenched. In the tempering step, tempering is performed in a temperature range of 570 to 700 ° C. Steel plate manufacturing method. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (i) SnEQ=Sn+W+Ni / 10+Mo / 4 (ii) SC=S / Ca (iii) MV = Mo + 4 × V (iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained.

[0016] (8) A method for producing a steel sheet according to (2) above, a soaking process in which the slab is heated and then soaked; a hot rolling step of hot rolling the slab to form a steel plate; a quenching step of quenching the steel plate; a tempering step of tempering the steel plate after quenching, The chemical composition of the cast piece is, in mass%, C: 0.06~0.15%, Si: 0.05 to 1.20% Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 2.0% Mo: 0.01 to 0.75% V: 0.005~0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010~0.080%, Sn: 0.020~0.400%, N: 0.0010~0.0070%, O: 0.0005 to 0.0040%, Ca: 0.0001 to 0.0080%, and further containing one or more selected from the group consisting of the following groups A, B, and C: The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, In the quenching step, the steel plate is reheated in a temperature range of 850 to 950°C and then quenched. In the tempering step, tempering is performed in a temperature range of 570 to 700 ° C. Steel plate manufacturing method. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (i) SnEQ=Sn+W+Ni / 10+Mo / 4 (ii) SC=S / Ca (iii) MV = Mo + 4 × V (iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained. [Group A] One or more selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.05% or less [Group B] One or more selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less

[0017] (9) The method for producing a steel sheet according to (8) above, wherein the chemical composition contains one or more elements selected from Group A.

[0018] (10) The method for producing a steel sheet according to (8) above, wherein the chemical composition contains one or more elements selected from the B group.

[0019] (11) The method for producing a steel sheet according to (8) above, wherein the chemical composition contains one or more elements selected from the C group.

[0020] (12) A method for producing a steel sheet according to (6) above, In the soaking step, the slab is soaked at 1050 to 1250°C, In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%. A method for manufacturing a steel sheet according to any one of (7) to (11) above. [Effects of the Invention]

[0021] According to the present invention, a thick steel plate having high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance can be obtained. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram illustrating a method for preparing a test piece for evaluating lamellar tear resistance. DETAILED DESCRIPTION OF THE INVENTION

[0023] The inventors have conducted a detailed study into the effects of chemical composition and metal structure on the tensile strength, corrosion resistance, and lamellar tear resistance of thick steel plates with a thickness of 75 to 105 mm used in structures such as bridges, and have obtained the following findings.

[0024] 1. Tensile strength To increase tensile strength, it is effective to add alloying elements to improve hardenability. Therefore, the value of Ceq, defined as C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, must be set to 0.37 to 0.51.

[0025] 2. Corrosion resistance Steel plate corrosion occurs when iron ions are eluted from water adhering to the steel plate surface and combine with oxygen in the air to form iron oxide, causing rust. The rust then hydrolyzes in the adhering water, generating hydrogen ions, creating an acidic aqueous solution. However, in an environment containing chlorides, the adhering water contains chloride ions, creating a highly acidic aqueous solution. This accelerates the progression of corrosion of the steel plate.

[0026] Therefore, it is necessary to include Sn as an essential additive element and to ensure that the value of SnEQ, defined as SnEQ = Sn + W + Ni / 10 + Mo / 4, is 0.10 or greater. When W, Ni, and Mo are added to steel sheets in addition to Sn, the elution of iron ions is suppressed, slowing down the rate of rust formation. As a result, it has been discovered that excellent corrosion resistance can be achieved even in environments containing chlorides.

[0027] 3. Lamellar tear resistance However, Sn, which is added to improve corrosion resistance, has a small solid-liquid distribution coefficient. Therefore, during slab casting, Sn easily migrates to the liquid phase as the slab solidifies from the surface. As a result, Sn concentrates in the center of the slab, which in turn promotes the concentration of other elements such as Mn, lowering the melting point of the slab center. In particular, in the central segregation zone, which is the final solidification zone, the melting point is significantly reduced due to the concentration of alloying elements, especially in the area where elements such as Sn are segregated (hereinafter referred to as the "Sn-segregated zone"). Therefore, the Sn-segregated zone promotes the formation of casting defects such as porosity and promotes hardening due to its high hardenability. As a result, in a lamellar tear resistance evaluation test, it was found that cracks originated from the Sn-segregated zone.

[0028] Thus, when it is necessary to improve tensile strength and corrosion resistance while adding alloying elements, ensuring lamellar tearing resistance becomes difficult. In particular, thick steel plates contain a large amount of alloying elements to obtain high tensile strength, and Sn is added to obtain corrosion resistance, which promotes the formation of casting defects in Sn segregated areas and hardening due to the concentration of alloying elements. Therefore, ensuring lamellar tearing resistance becomes extremely difficult. Therefore, the present inventors conducted further detailed studies to improve lamellar tearing resistance while maintaining the tensile strength and corrosion resistance of thick steel plates, and as a result, the following findings (a) to (c) were obtained.

[0029] (a) The impurity element S has a particularly small solid-liquid distribution coefficient and is an element that tends to concentrate, along with Sn, in the center of the steel plate in the thickness direction (hereinafter referred to as the "center of the plate thickness"). S is an element that significantly lowers the melting point, which can contribute to casting defects such as porosity. In addition, S combines with Mn to form MnS. MnS is stretched during rolling and behaves similarly to unbonded porosity, thereby degrading lamellar tear resistance.

[0030] Therefore, we found that calcium is an essential additive element and that the SC value, defined as SC = S / Ca, must be in the range of 0.30 to 15.00. Ca is an element that reduces the S content in the liquid phase by forming sulfides during the solidification process. Therefore, it is possible to suppress the concentration of S in the center of the plate thickness and suppress the generation of casting defects, thereby improving lamellar tear resistance.

[0031] (b) As mentioned above, Sn-segregated areas have high hardenability, which causes hardening during reheating and quenching. Therefore, it has been found that tempering in the temperature range of 570 to 700°C is effective in reducing the Vickers hardness of the Sn-segregated areas to 350 HV0.01 or less. On the other hand, tempering in the temperature range of 570 to 700°C not only softens the Sn-segregated areas but also reduces the strength of the entire steel sheet. Therefore, to suppress the overall softening of the steel sheet at this tempering temperature, Mo and V, which are temper softening-resistant elements, are added under conditions where the MV value, defined as MV = Mo + 4 × V, is in the range of 0.20 to 0.80. This softens the Sn-segregated areas and improves lamellar tear resistance while maintaining the strength characteristics of the steel sheet itself. Note that Mo and V also concentrate in the Sn-segregated areas. However, in the Sn-segregated areas, Sn is concentrated several tens of times more than in areas other than the Sn-segregated areas, while Mo and V are concentrated only 1.1 to 1.5 times more. Therefore, the effect of softening resistance caused by the concentration of Mo and V in the Sn-segregated areas is small, and it is thought that the softening effect associated with dislocation recovery due to tempering is greater in the Sn-segregated areas.

[0032] (c) It was found that cracks initiated within the Sn segregation area propagated along the grain boundaries of prior austenite grains (hereafter referred to as "prior γ grains"). Furthermore, rolling flattens the prior γ grains, i.e., elongates them in the rolling direction, increasing the aspect ratio of the prior γ grains and increasing the surface area of ​​the prior γ grains as viewed through the thickness direction. As a result, when tensile stress acts in the thickness direction, stress concentrates at the tip of the unbonded porosity, causing cracks to initiate. The cracks propagate along the boundaries of the flat prior γ grains, resulting in a significant deterioration of lamellar tearing resistance.

[0033] Therefore, reheating in the temperature range of 850 to 950°C re-austenitizes the material, thereby refining and equiaxing the grains that were flattened during hot rolling. Specifically, lamellar tearing resistance can be improved by reducing the average grain size of prior γ grains to 40 μm or less and the average aspect ratio of prior γ grains to 2.0 or less. Refining prior γ grains increases the number of grain boundaries, thereby increasing the fracture stress that can withstand stress concentrations that occur at the unbonded porosity areas and the tips of coarse inclusions, thereby suppressing the initiation and propagation of cracks.

[0034] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0035] (A) Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0036] C: 0.06 to 0.15% C is an element necessary to ensure the strength of steel sheets. However, excessive C content increases the hardness of Sn-segregated areas, reducing lamellar tear resistance and significantly reducing weldability. Furthermore, as the C content increases, the amount of cementite produced, which acts as a cathode and promotes corrosion in a pH-decreasing environment, decreases, resulting in reduced corrosion resistance. Therefore, the C content is set to 0.06 to 0.15%. The C content is preferably 0.07% or more, more preferably 0.08% or more. The C content is preferably 0.13% or less, more preferably 0.12% or less.

[0037] Si: 0.05 to 1.20% Si is added for the purpose of deoxidation. It is also an element necessary for ensuring strength because it has the effect of suppressing temper softening. However, excessive Si content impairs the toughness of the base material and welded joints. Therefore, the Si content is set to 0.05 to 1.20%. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. The Si content is preferably 0.90% or less, and more preferably 0.70% or less.

[0038] Mn: 1.00 to 2.50% Mn is an element that enhances the hardenability and thereby the strength of the steel sheet. However, Mn is an element that easily concentrates in the liquid phase during casting, and if it is added in excess, the Mn concentration in the center segregation region becomes excessive. As a result, the hardenability becomes excessive, and the hardness increases, resulting in a deterioration in lamellar tear resistance. Therefore, the Mn content is set to 1.00 to 2.50%. The Mn content is preferably 1.10% or more, and more preferably 1.20% or more. Furthermore, the Mn content is preferably 2.30% or less, and more preferably 2.00% or less.

[0039] P:0.015% or less P is an element present as an impurity in steel sheets. P reduces acid resistance and reduces corrosion resistance in chloride corrosion environments where the pH of the corrosion interface decreases. Furthermore, P is an element that easily segregates at grain boundaries, and an increase in the amount of grain boundary segregation reduces the lamellar tear resistance and toughness of the steel sheet, so the lower the P content, the better. Therefore, the P content is set to 0.015% or less. The P content is preferably 0.013% or less, and more preferably 0.011% or less.

[0040] S: 0.0024% or less S is an element present as an impurity in steel sheets. S has an extremely small solid-liquid distribution coefficient, and when concentrated in the liquid phase during casting, it forms coarse MnS in the central segregation region, which deteriorates toughness. Furthermore, MnS is stretched during rolling, spreading out in a planar shape in the rolling direction, significantly deteriorating lamellar tear resistance. Therefore, it is preferable to reduce the S content as much as possible. To ensure toughness and lamellar tear resistance, the S content is set to 0.0024% or less. The S content is preferably 0.0020% or less, and more preferably 0.0018% or less.

[0041] Ni: 0.1 to 2.0% Ni improves strength by increasing hardenability and improves the toughness of the matrix structure. Ni also has the effect of improving low-temperature toughness. It is also an element that improves corrosion resistance by suppressing the anodic dissolution of steel in a low-pH environment. However, Ni is an expensive element, and even if it is contained in an amount exceeding 2.0%, not only does the effect saturate but also leads to a significant increase in cost. Therefore, the Ni content is set to 0.1 to 2.0%. The Ni content is preferably 0.2% or more, and more preferably 0.3% or more. Furthermore, the Ni content is preferably 1.7% or less, and more preferably 1.4% or less.

[0042] Mo: 0.01 to 0.75% Mo is an element that has the effect of increasing the strength of steel sheets without reducing lamellar tear resistance. Specifically, Mo improves strength by precipitating Mo carbides during tempering. It also dissolves and forms oxyanion MoO4 2- Mo is an element that adsorbs to rust in the form of Mo and has the effect of inhibiting the permeation of chloride ions through the rust layer. However, if it is contained in excess, not only does the effect saturate, but the strength of the steel sheet becomes excessive, resulting in a deterioration of toughness. Therefore, the Mo content is set to 0.01 to 0.75%. The Mo content is preferably 0.03% or more, and more preferably 0.05% or more. Furthermore, the Mo content is preferably 0.60% or less, and more preferably 0.40% or less.

[0043] V: 0.005 to 0.150% V is an element that improves hardenability and increases the strength of steel sheets. Specifically, V improves strength by precipitating V carbonitrides during tempering. Furthermore, like Mo, V dissolves and exists in the form of oxyanions, inhibiting the permeation of chloride ions through rust layers. However, excessive V content not only saturates the effect, but also increases the alloying cost because V is an expensive alloying element. Therefore, the V content is set to 0.005 to 0.150%. The V content is preferably 0.020% or more, and more preferably 0.030% or more. The V content is preferably 0.130% or less, and more preferably 0.110% or less.

[0044] Nb: 0.003 to 0.040% Nb is an element that improves hardenability and strength. Furthermore, Nb has the effect of widening the unrecrystallized region, so rolling in that temperature range can introduce high-density dislocations. Furthermore, increasing the number of transformation nucleation sites can refine the structure of the steel sheet. As a result, toughness can be improved. However, excessive Nb content increases hardenability by concentrating in the center segregation region during casting, and also forms coarse Nb carbides, deteriorating lamellar tear resistance. Therefore, the Nb content is set to 0.003 to 0.040%. The Nb content is preferably 0.008% or more, more preferably 0.012% or more. The Nb content is preferably 0.035% or less, more preferably 0.030% or less.

[0045] Ti: 0.003 to 0.040% Ti contributes to increasing strength. It also combines with N to form TiN, which acts as pinning particles to inhibit austenite grain growth during reheating and quenching. This refines the austenite grains, which is also effective in improving toughness. However, excessive Ti content leads to the formation of coarse TiN in the central segregation region, deteriorating lamellar tear resistance. Therefore, the Ti content is limited to 0.003 to 0.040%. The Ti content is preferably 0.005% or more, more preferably 0.008% or more. The Ti content is preferably 0.035% or less, more preferably 0.030% or less.

[0046] Al: 0.010 to 0.080% Al is an effective element for deoxidizing steel. However, excessive Al content not only reduces corrosion resistance in low pH environments, thereby reducing corrosion resistance in chloride corrosion environments, but also causes nitrides to coarsen, resulting in a decrease in toughness. Therefore, the Al content is set to 0.010 to 0.080%. The Al content is preferably 0.015% or more, more preferably 0.020% or more. Furthermore, the Al content is preferably 0.070% or less, more preferably 0.060% or less.

[0047] Sn: 0.020 to 0.400% Sn is an element that improves the corrosion resistance of steel. In addition, the inclusion of Sn in steel sheet makes it possible to form a Sn oxide layer on the surface of the steel sheet in advance. The Sn oxide layer significantly suppresses the anodic dissolution reaction and hydrogen evolution reaction of steel in low-pH chloride environments, thereby significantly improving corrosion resistance in chloride corrosive environments.

[0048] However, even if Sn is added in excess, not only does the above effect saturate, but as mentioned above, Sn itself not only segregates during casting but also promotes the segregation of other alloying elements, thereby reducing lamellar tear resistance. Furthermore, the toughness of the base material also decreases. Therefore, the Sn content is set to 0.020 to 0.400%. The Sn content is preferably 0.030% or more, and more preferably 0.080% or more. Furthermore, the Sn content is preferably 0.350% or less, and more preferably 0.300% or less.

[0049] N: 0.0010~0.0070% N forms nitrides with Ti, which inhibits grain coarsening during reheating and quenching, thereby contributing to improved toughness. N also dissolves as ammonia, which dissolves in the Fe 3+ N has the effect of improving the corrosion resistance of steel sheets in chloride corrosion environments by suppressing the decrease in pH due to hydrolysis of N. However, if N is added in excess, not only does this effect saturate, but coarse AlN and TiN are formed, reducing the toughness of the steel sheet. Therefore, the N content is set to 0.0010 to 0.0070%. The N content is preferably 0.0020% or more, and more preferably 0.0025% or more. Furthermore, the N content is preferably 0.0060% or less, and more preferably 0.0050% or less.

[0050] O: 0.0005 to 0.0040% O is added to remove impurities during the refining process, increasing the cleanliness of the steel sheet, and is necessary for ensuring the toughness of the steel sheet. However, O forms oxides such as SnO and SnO2. Therefore, if the O content is excessive, the Sn content in the steel cannot be sufficiently ensured. Furthermore, these oxides act as starting points for corrosion, reducing the corrosion resistance of the steel sheet. Therefore, the O content is set to 0.0005 to 0.0040%. The O content is preferably 0.0007% or more, and more preferably 0.0009% or more. Furthermore, the O content is preferably 0.0030% or less, and more preferably 0.0025% or less.

[0051] Ca: 0.0001 to 0.0080% Ca forms sulfides during the casting process of slabs, suppressing the concentration of S in the center of the plate thickness, thereby preventing a decrease in melting point. As a result, Ca suppresses casting defects such as porosity and inhibits hardening of Sn segregated areas, thereby improving lamellar tear resistance. Ca also exists in the form of oxides in steel, suppressing a decrease in pH at the interface in corrosion reaction areas and suppressing the acceleration of corrosion. However, excessive Ca content causes coarsening of oxides and reduces toughness. Therefore, the Ca content is limited to 0.0001 to 0.0080%. The Ca content is preferably 0.0005% or more, more preferably 0.0009% or more. The Ca content is preferably 0.0070% or less, more preferably 0.0060% or less.

[0052] The steel sheet according to the present invention has the above-mentioned chemical composition, with the balance being Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, during the industrial production of steel sheet, and are acceptable within a range that does not adversely affect the present invention.

[0053] Ceq: 0.37~0.51 As described above, in order to improve hardenability and tensile strength, Ceq, defined by the following formula (i), is set to 0.37 or more. On the other hand, if Ceq exceeds 0.51, not only toughness and ductility but also weldability deteriorate. Furthermore, as Ceq increases, the concentration of alloying elements in the Sn segregated region also increases, which causes excessive hardenability in the Sn segregated region, resulting in hardening. As a result, the Vickers hardness exceeds 350HV0.01, resulting in reduced lamellar tear resistance. Therefore, Ceq is set to 0.37 to 0.51. Ceq is preferably 0.39 or more and 0.45 or less.

[0054] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0055] SnEQ: 0.10 or higher By adding W, Ni, and Mo to a steel sheet in addition to Sn, the elution of iron ions is suppressed, thereby improving corrosion resistance in chloride-containing environments. Therefore, SnEQ, defined by the following formula (ii), is set to 0.10 or more. SnEQ is preferably 0.13 or more. There is no particular upper limit for SnEQ, but in the chemical composition of the present invention, the practical upper limit for SnEQ is 1.59. SnEQ is preferably 1.00 or less. Furthermore, since the effect of improving corrosion resistance saturates when SnEQ exceeds 0.40, it is unnecessary to include any more alloying elements for the purpose of corrosion resistance. Therefore, SnEQ is preferably 0.40 or less. However, when SnEQ is added for the purpose of ensuring tensile strength and low-temperature toughness, there is no problem even if SnEQ exceeds 0.40.

[0056] SnEQ=Sn+W+Ni / 10+Mo / 4 (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0057] SC: 0.30~15.00 As mentioned above, Ca forms sulfides during the solidification process and can improve lamellar tear resistance by suppressing the concentration of S in the center of the plate thickness. However, an excessive reduction in the S content leads to increased steelmaking costs. Therefore, the SC, defined as the ratio of the S content to the Ca content, as shown in the following formula (iii), is set to 0.30 or more. On the other hand, if the SC exceeds 15.00, the S content relative to the Ca content is too high, so even if Ca is added, the concentration of S in the center of the plate thickness cannot be controlled, and lamellar tear resistance decreases. Therefore, the SC is set to 0.30 to 15.00. The SC is preferably 0.50 or more, preferably 10.00 or less, and more preferably 6.00 or less.

[0058] SC=S / Ca (iii) In the above formula, each element symbol represents the content (mass %) of each element contained in the steel sheet.

[0059] MV: 0.20~0.80 As mentioned above, the Sn-segregated region has high hardenability, and therefore hardens during reheating and quenching. However, tempering adjusts the hardness of the Sn-segregated region. On the other hand, tempering softens the Sn-segregated region while also reducing the strength of the entire steel sheet. By setting the MV, defined by the following formula (iv), to 0.20 or more, the precipitation strengthening of Mo and V during the tempering process suppresses the softening of the steel sheet itself and ensures strength. On the other hand, if the MV exceeds 0.80, excessive precipitation strengthening prevents the Sn-segregated region from being softened, resulting in a deterioration in lamellar tear resistance. Furthermore, the coarsening of Mo and V carbonitrides deteriorates toughness. Therefore, the MV is set to 0.20 to 0.80. The MV is preferably 0.25 or more and 0.60 or less.

[0060] MV = Mo + 4 × V (iv) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0061] In order to improve strength, the chemical composition of the steel sheet of the present invention may further contain one or more elements selected from the following Group A (Cu: 0.80% or less, B: 0.005% or less, Zr: 0.05% or less, Ta: 0.05% or less) within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0062] Cu:0.80% or less Cu is an element that enhances the hardenability of steel sheets. It can be added as needed because it inhibits the anodic dissolution of steel in low-pH environments, thereby improving corrosion resistance. However, excessive addition not only saturates the effect, but can also cause embrittlement. Therefore, the Cu content is set to 0.80% or less. The Cu content is preferably 0.50% or less, and more preferably 0.30% or less. To stably obtain the above effects, the Cu content is preferably 0.05% or more, and more preferably 0.10% or more.

[0063] B: 0.0050% or less B is an element that improves hardenability and increases strength, so it can be added as needed. However, if it is added in excess, the strength-enhancing effect saturates and the toughness of both the base metal and the HAZ tends to decrease significantly. Therefore, the B content is set to 0.0050% or less. To stably obtain the above effects, it is preferable that the B content be 0.0003% or more.

[0064] Zr: 0.05% or less Like Ti, Zr is an element that forms oxides and contributes to refining crystal grains and improving strength, so it may be added as needed. However, if Zr is added in excess, the oxides become coarse and mechanical properties deteriorate. Therefore, the Zr content is set to 0.05% or less. The Zr content is preferably set to 0.03% or less. To ensure the above effects, the Zr content is preferably set to 0.001% or more, and more preferably 0.005% or more.

[0065] Ta: 0.05% or less Ta is an element that contributes to improving strength and, although the mechanism is not entirely clear, also contributes to improving corrosion resistance, so it may be added as needed. However, Ta is an expensive element, and adding a large amount increases steelmaking costs. Therefore, the Ta content is set to 0.05% or less. The Ta content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. To more reliably obtain the above effects, the Ta content is preferably 0.001% or more, and more preferably 0.005% or more.

[0066] In order to improve corrosion resistance, the chemical composition of the steel sheet of the present invention may further contain one or more elements selected from the following Group B (Cr: 0.20% or less, W: 0.80% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Sb: 0.10% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, Hf: 0.05% or less) within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0067] Cr:0.20% or less Cr is generally an element that improves corrosion resistance and has the effect of increasing hardenability and improving strength, so it can be added as needed. However, Cr reduces acid resistance, which reduces the corrosion resistance in chloride-rich environments, which is the problem that the present invention aims to solve. On the other hand, since a content of 0.20% or less does not result in a decrease in acid resistance, the Cr content is set to 0.20% or less. The Cr content is preferably 0.15% or less, and more preferably 0.12% or less. In order to stably obtain the above effects, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more.

[0068] W: 0.80% or less W, like Mo, dissolves and forms oxyanion WO42- W exists in the form of , and is an element that has the effect of suppressing the permeation of chloride ions through the rust layer, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel sheet will also increase significantly. Therefore, the W content is set to 0.80% or less. The W content is preferably 0.60% or less, and more preferably 0.40% or less. In order to stably obtain the above effect, the W content is preferably 0.01% or more, and more preferably 0.02% or more.

[0069] Sb: 0.10% or less Sb forms sulfides with S and is an element effective in improving corrosion resistance in acid corrosion environments, so it may be contained as needed. However, if Sb is contained in excess, toughness decreases. Therefore, the Sb content is set to 0.10% or less. The Sb content is preferably 0.08% or less, and more preferably 0.05% or less. To more reliably obtain the above effects, the Sb content is preferably 0.005% or more, and more preferably 0.010% or more.

[0070] As: 0.05% or less Although As does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be added as needed. However, if As is contained in excess, toughness decreases. Therefore, the As content is set to 0.05% or less. The As content is preferably 0.04% or less, and more preferably 0.03% or less. To ensure the above effects, the As content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0071] Bi:0.05% or less Although Bi does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be added as needed. However, if Bi is added in excess, toughness decreases. Therefore, the Bi content is set to 0.05% or less. The Bi content is preferably 0.04% or less, and more preferably 0.03% or less. To ensure the above effects, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0072] Se: 0.05% or less Although Se does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as needed. However, if Se is contained in excess, toughness decreases. Therefore, the Se content is set to 0.05% or less. The Se content is preferably 0.04% or less, and more preferably 0.03% or less. To ensure the above effects, the Se content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0073] Te: 0.05% or less Although Te does not have a significant effect compared to Sb, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be added as needed. However, if Te is added in excess, toughness decreases. Therefore, the Te content is set to 0.05% or less. The Te content is preferably 0.04% or less, and more preferably 0.03% or less. To ensure the above effects, the Te content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0074] Zn: 0.05% or less Ga: 0.05% or less Zn and Ga form sulfides with S and are effective elements for improving corrosion resistance in acid corrosion environments, so they may be included as needed. However, excessive Zn and Ga content reduces toughness. Therefore, the Zn and Ga contents are each set to 0.05% or less. The Zn and Ga contents are each preferably set to 0.04% or less, and more preferably set to 0.03% or less. To ensure the above effects, the Zn and Ga contents are each set to 0.005% or more, and more preferably set to 0.010% or more.

[0075] Ge: 0.05% or less Ge forms sulfides with S and is an element effective in improving corrosion resistance in acid corrosion environments, so it may be contained as needed. However, if Ge is contained in excess, toughness decreases. Therefore, the Ge content is set to 0.05% or less. The Ge content is preferably 0.04% or less, and more preferably 0.03% or less. To more reliably obtain the above effects, the Ge content is preferably 0.005% or more, and more preferably 0.010% or more.

[0076] Co:0.50% or less Co is an element that forms oxides and improves corrosion resistance, so it may be added as needed. However, excessive Co content reduces economic efficiency. Therefore, the Co content is set to 0.50% or less. The Co content is preferably 0.30% or less, and more preferably 0.20% or less. To ensure the above effects, the Co content is preferably 0.05% or more, more preferably 0.08% or more, and even more preferably 0.10% or more.

[0077] Hf: 0.05% or less Hf is an element that forms oxides and improves corrosion resistance, so it may be added as needed. However, excessive Hf content reduces economic efficiency. Therefore, the Hf content is set to 0.05% or less. The Hf content is preferably 0.04% or less, and more preferably 0.03% or less. To ensure the above effects, the Hf content is preferably 0.002% or more, and more preferably 0.005% or more.

[0078] In the chemical composition of the steel sheet of the present invention, for the purpose of controlling the cleanliness of the steel sheet and inclusions, one or more elements selected from the following Group C (Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, REM: 0.010% or less) may be further contained within the ranges shown below. Note that these elements are not necessarily essential for the steel sheet of the present invention, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0079] Mg: 0.010% or less Mg can be added as needed to suppress a decrease in pH at the interface in the corrosion reaction zone. However, if added in excess, the effect will saturate. Therefore, the Mg content should be 0.010% or less. The Mg content should preferably be 0.005% or less. To stably obtain the above effect, the Mg content should preferably be 0.0002% or more, and more preferably 0.0005% or more.

[0080] Sr: 0.010% or less Ba: 0.010% or less Sr and Ba may be added as needed to form fine oxides. However, excessive addition of Sr and Ba increases steelmaking costs. Therefore, the Sr and Ba contents are each set to 0.010% or less. The Sr and Ba contents are each preferably 0.008% or less, and more preferably 0.005% or less. To more reliably obtain the above effects, the Sr and Ba contents are each preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0081] REM: 0.010% or less REM (rare earth elements) have the effect of improving the weldability of steel, so they can be added as needed. However, if they are added in excess, the effect saturates, so the REM content is set to 0.010% or less. The REM content is preferably set to 0.005% or less. To stably obtain the above effect, the REM content is preferably set to 0.0002% or more, and more preferably 0.0005% or more.

[0082] Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc, and one or more of these elements can be contained. Note that the REM content means the total content of these elements.

[0083] (B) Plate thickness The thickness of the steel plate according to the present invention is set to 75 to 105 mm in order to be used for structures such as bridges.

[0084] (C) Vickers hardness The steel sheet according to the present invention has a Vickers hardness of 350 HV0.01 or less in a Sn segregated region present in the center of the sheet thickness in a cross section perpendicular to the rolling direction of the steel sheet (hereinafter referred to as "C cross section"). Here, "HV0.01" refers to the "hardness symbol" when micro Vickers hardness is measured with a test force of 0.09807 N (10 gf).

[0085] In the present invention, the "center portion of the sheet thickness" refers to the center portion of the steel sheet in the sheet thickness direction, which is the portion of the steel sheet where elements such as Sn tend to concentrate. Specifically, when the sheet thickness of the steel sheet is t, this is the region of 9 / 20t to 11 / 20t.

[0086] In the present invention, the "Sn segregation region" refers to a region in the central segregation region where elements such as Sn are segregated, and is observed as a band-like or block-like black distorted metal structure compared to the surrounding metal structure when observing the metal structure at the center of the plate thickness after nital etching, as described below. When it is difficult to distinguish from the metal structure, it can be distinguished by also using Mn mapping measurement by EPMA.

[0087] As described above, in a through-thickness tensile test to evaluate lamellar tear resistance, cracks originate from casting defects such as porosity or unbonded pressure in the Sn-segregated area. The cracks are brittle fractures, and the higher the hardness of the Sn-segregated area, the more likely brittle fractures occur. When the relationship between lamellar tear resistance and Vickers hardness was investigated, it was found that the lower the Vickers hardness of the Sn-segregated area, the higher the lamellar tear resistance. When the Vickers hardness of the Sn-segregated area is 350 HV0.01 or less, the deterioration of lamellar tear resistance is suppressed. Furthermore, the Vickers hardness of the Sn-segregated area is preferably 330 HV0.01 or less. Furthermore, the Vickers hardness of the Sn-segregated area is not particularly limited, but is preferably 200 HV0.01 or more.

[0088] Vickers hardness is measured as follows. A specimen for observation is taken from the center of the steel plate, at a C-section, with the width of the steel plate being W, so that the observation surface is located between 1 / 4W and 3 / 4W. The observation surface is then mirror-polished and then subjected to nital etching. The center of the plate is observed under an optical microscope at 500x magnification. Sn segregation is visually identified as a band-like or clumpy, dark distorted area compared to the surrounding area. Sn segregation areas have high hardenability due to the enrichment of alloying elements, and undergo large distortion due to transformation at a lower temperature compared to the surrounding area. Therefore, they are observed as a band-like or clumpy, dark distorted metal structure compared to the surrounding metal structure. Furthermore, if identification is difficult, Mn mapping measurement using an EPMA can be used in combination to identify Sn segregation areas. While various alloying elements segregate in Sn segregation areas, Mn is the most susceptible to segregation, and an evaluation method for this is well established.

[0089] Then, the Vickers hardness of the Sn segregation area and its surroundings is measured at lattice points at a pitch of 25 μm over an area of ​​1000 μm in the sheet width direction and 500 μm in the sheet thickness direction, centered on the Sn segregation area, using a test force of 0.09807 N (10 gf). The average of the top 20 Vickers hardness data points measured in this way is taken as the Vickers hardness of the Sn segregation area.

[0090] (D) Metal structure (D-1) Average grain size and aspect ratio of prior γ grains In the steel sheet according to the present invention, the average grain size of prior γ grains in the Sn segregated portion present in the C cross section of the steel sheet is 40 μm or less. Also, in the Sn segregated portion present in the C cross section of the steel sheet, the average aspect ratio of the prior γ grains is 2.0 or less. The reasons for limiting each requirement will be explained.

[0091] Average particle size of prior γ grains: 40 μm or less As mentioned above, the brittle fracture resistance stress increases by refining prior γ grains. Specifically, in a tensile test in the Z direction, if plastic deformation occurs before the stress concentration at the tip of a casting defect, such as unbonded porosity in the Sn segregation area, reaches the brittle fracture stress, stress relaxation occurs, resulting in ductile fracture, improving lamellar tearing resistance. Furthermore, even if brittle fracture occurs at the tip of a casting defect, if the prior γ grains are refined, crack propagation is suppressed, so there is no significant decrease in lamellar tearing resistance.

[0092] To suppress crack propagation and ensure lamellar tear resistance, the average grain size of the prior γ grains is set to 40 μm or less. The average grain size of the prior γ grains is preferably 35 μm or less. On the other hand, there is no particular lower limit for the average grain size of the prior γ grains, but under the manufacturing conditions of the present invention, the average grain size of the prior γ grains is 15 μm or more.

[0093] The grain size of prior γ grains is measured using the following method. Observation specimens are prepared in the same manner as for identifying Sn segregation areas in Vickers hardness tests. Specifically, a specimen is taken from the center of the steel plate's thickness so that the C-section serves as the observation surface. The observation surface is mirror-polished, then subjected to nital etching, and the center of the plate is observed under an optical microscope at 500x magnification. Areas that appear darker and distorted than the surrounding area are visually identified as Sn segregation areas. If this is difficult, Mn mapping measurements using an EPMA are also used to identify the Sn segregation areas.

[0094] The specimen is then mirror-polished again, and then etched in accordance with the polishing and etching procedures described in JIS G 0551: 2022 using an etching solution based on a saturated aqueous solution of picric acid to reveal the prior-γ grain boundaries. The Sn segregation areas are then observed to measure the prior-γ grain size.

[0095] The prior γ grain size is measured using the intercept method. A test line is drawn at the longest position in the Sn segregation area in the direction perpendicular to the sheet thickness direction. Then, let L be the length between the intersections of the prior γ grain boundaries at the left and right ends of the Sn segregation area on the test line, and let P be the number of intersections between the test line and the prior γ grain boundaries within the Sn segregation area. Let L / (P-1) be the average grain size of the prior γ grains in the Sn segregation area. If the number of prior γ grains in the test line is less than 40, a test line is drawn in the Sn segregation area at a position 50 μm away from the test line in the sheet thickness direction, or in another Sn segregation area, to calculate the prior γ grain size. This process is repeated until the total number of prior γ grains reaches 40 or more, and the average grain size of the prior γ grains is measured.

[0096] Average aspect ratio of prior γ grains: 2.0 or less As described above, by changing the shape of the prior γ grains from flat to spherical, the surface area of ​​each prior γ grain as viewed in the thickness direction is reduced. As a result, the stress strain generated at each prior γ grain boundary during a tensile test in the thickness direction is dispersed and reduced, suppressing the occurrence of cracks in Sn segregated areas. Therefore, the average aspect ratio of the prior γ grains is set to 2.0 or less. The average aspect ratio of the prior γ grains is preferably 1.8 or less.

[0097] The aspect ratio of prior γ grains is measured using the following method. A test piece for observation is taken from the center of the steel plate thickness so that the C-section of the steel plate serves as the observation surface. Then, after polishing and etching are performed in the same manner as for revealing the prior γ grain boundaries for the prior γ grain size measurement described above, the prior γ grains in the Sn segregation area are photographed using an optical microscope. Based on the photographed structure, the maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction are measured for each prior γ grain, and the ratio (maximum major axis length / maximum minor axis length) is calculated. The average value is then used as the average aspect ratio of the prior γ grains.

[0098] (D-2) Maximum defect length In the steel sheet according to the present invention, it is preferable that the maximum defect length at the center of the thickness of the steel sheet in the C cross section of the steel sheet is 0.50 mm or less. The reason for this limitation will be explained.

[0099] Normally, central segregation occurs in the center of a slab, but the liquid phase remaining in the center at the end of solidification during casting is subjected to tensile strain due to the shrinkage of the surrounding solid phase as the temperature drops. As a result, the liquid phase opens up, forming voids and ultimately casting defects. Elements such as Sn have a small solid-liquid distribution coefficient, so they significantly concentrate in the liquid phase as solidification progresses, lowering its melting point. As the melting point of the liquid phase decreases, the time from the start to the end of solidification increases, and as the solidification time increases, the shrinkage strain of the solid phase surrounding the liquid phase increases, resulting in larger casting defects.

[0100] These casting defects disappear because they are stretched and pressed in the subsequent hot rolling process, but if the size of the casting defect before rolling is large, or if the required steel plate thickness is large and a sufficient reduction ratio cannot be ensured, complete pressing may not be possible, and unpressed defects may remain. Normally, if this defect is small, it does not affect the tensile properties and Charpy impact properties, but in tensile tests in the plate thickness direction, stress concentration may occur at the tip of the casting defect, causing fracture.

[0101] As will be described later, it is desirable to heat the slab so that the soaking temperature is 1050 to 1250°C, and to roll the slab so that the cumulative reduction in the temperature range of 900°C or higher in the hot rolling process is 52 to 70%, thereby reducing the size of casting defects. However, as the plate thickness increases and as the tensile strength of the steel plate increases, it becomes more difficult to reduce casting defects by rolling, but the maximum defect length is preferably 0.50 mm or less. The maximum defect length is more preferably 0.30 mm or less.

[0102] The maximum defect length is measured using the following method. Test specimens for observation are taken from the center of the plate thickness at each of the width positions of 1 / 4W, 1 / 2W, and 3 / 4W, so that the C-section of the steel plate becomes the observation surface. The observation surface is then mirror-polished, and the observation surface is continuously photographed in the width direction at 10 to 15 mm intervals using an optical microscope. Based on the photographed structure, areas that appear black are determined to be casting defects, and each casting defect is magnified and photographed to measure its maximum length. The largest casting defect among these is taken as the maximum defect length.

[0103] (E) Mechanical properties The steel plate of the present invention has a tensile strength of 570 MPa or more at the 1 / 4 position of the plate thickness. The use of a steel plate with a tensile strength of 570 MPa or more enables designs with increased load capacity, making it easier to increase the size of buildings. Furthermore, the higher the tensile strength of the steel plate of the present invention, the larger the size of buildings that can be built, so there is no upper limit to the tensile strength. However, excessive tensile strength results in a decrease in toughness and lamellar tear resistance, so the tensile strength is preferably 760 MPa or less. In the following description, when the plate thickness is t, the 1 / 4 position of the plate thickness is referred to as "1 / 4t."

[0104] The tensile strength is measured based on JIS Z 2241:2022 using a No. 4 round bar tensile test piece taken from a 1 / 4t plate so that the plate width direction coincides with the longitudinal direction of the test piece.

[0105] (F) Anticorrosion coating The steel sheet described above exhibits good corrosion resistance even when used as is. However, when the surface is subjected to a corrosion prevention treatment, specifically when the surface is covered with a corrosion-resistant coating made of an organic resin or metal, the durability of the corrosion-resistant coating improves compared to conventional steel sheets, and the corrosion resistance is further improved.

[0106] Examples of corrosion-resistant coatings made of organic resins include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Examples of corrosion-resistant coatings made of metals include plated coatings of Zn, Al, Zn-Al, etc., and thermally sprayed coatings of Zn, Al, Al-Mg, etc.

[0107] The improved durability of the corrosion-protective coating is believed to be due to the fact that corrosion of the underlying steel sheet of the present invention is significantly suppressed, thereby suppressing swelling or peeling of the corrosion-protective coating due to corrosion of the underlying steel sheet from defects in the corrosion-protective coating.

[0108] (G) Manufacturing method The method for producing a steel plate according to the present invention includes a soaking step of soaking a slab having the above-described chemical composition, a hot rolling step of hot rolling the slab to form a steel plate, a quenching step of quenching the steel plate, and a tempering step of tempering the quenched steel plate. Each step will be described in detail below.

[0109] <Soaking process> In the soaking process, the slab having the above-mentioned chemical composition is soaked. The soaking temperature is preferably 1050 to 1250°C. By bringing the entire slab to the soaking temperature before starting rolling, the temperature at the center of the steel sheet remains high during rolling, which is advantageous for crimping of casting defects. The soaking time varies depending on the operating conditions and is not particularly limited, but it is preferably 200 minutes or more after the slab is placed in the heating furnace. Here, "soaking" means that the temperature at the thickness center of the slab reaches the soaking temperature and then is maintained isothermally. The temperature at the thickness center of the slab is calculated from the temperature inside the heating furnace by performing a heat conduction calculation simulation of the slab.

[0110] <Hot rolling process> In the hot rolling process, the slab is hot rolled to produce a steel plate. In the hot rolling, the cumulative reduction in the temperature range of 900°C or higher is preferably 52 to 70%. By increasing the reduction in the high temperature range, the maximum defect length can be reduced.

[0111] <Quenching process> In the quenching process, the steel sheet is quenched. The heating temperature before quenching is in the temperature range of 850 to 950°C. Flat prior γ grains are formed in the metal structure after the hot rolling process. Therefore, by reheating to the austenite temperature range of 850 to 950°C, the prior γ grains are spheroidized, and the average aspect ratio of the prior γ grains can be set to 2.0 or less. Furthermore, quenching produces a steel sheet with the above-mentioned tensile strength.

[0112] If the heating temperature is less than 850°C, the heating temperature is insufficient for γ transformation, and the γ grain size is small, so quenching does not occur during cooling, and the required tensile strength cannot be obtained. On the other hand, if the heating temperature is more than 950°C, the prior γ grain size becomes coarse, resulting in a decrease in toughness and lamellar tear resistance. Note that the heating time must be adjusted according to the plate thickness. Since it is necessary to heat the center of the steel plate until it reaches the same temperature as the heating temperature, a heating time of 180 to 240 minutes is preferable.

[0113] <Tempering process> In the tempering process, the steel plate is tempered after quenching. The tempering temperature is in the range of 570 to 700°C. The strength of the entire steel plate is ensured by the previous reheating and quenching process, but the Sn segregated areas have high hardenability due to the significant concentration of alloying elements, and quenching causes localized hardening as the structure becomes primarily martensite. When this is tempered, the Sn segregated areas become tempered martensite, causing significant softening.

[0114] On the other hand, although the entire steel sheet also softens during tempering, adding appropriate amounts of Mo and V, which precipitate in this temperature range and suppress the decrease in strength, can suppress the decrease in strength of the entire steel sheet and ensure the required strength. If the tempering temperature is less than 570°C, the Sn segregated areas will not soften sufficiently. On the other hand, if the heating temperature is more than 700°C, the strength will decrease throughout the steel sheet, and the required tensile strength will not be obtained.

[0115] The treatment for covering with the above-mentioned anticorrosion coating may be carried out by a conventional method. Furthermore, it is not necessarily required to apply an anticorrosion coating to the entire surface of the steel sheet, but it is sufficient to apply an anticorrosion treatment to only one side of the steel sheet that is exposed to a corrosive environment, or to only the outer or inner surface in the case of a steel pipe, i.e., only at least a part of the steel sheet surface.

[0116] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0117] Steels having the chemical compositions shown in Tables 1 and 2 were melted and soaked at the heating temperatures shown in Tables 3 and 4, then rough rolled at the cumulative reductions shown in Tables 3 and 4, and finish rolled at 850 to 950°C to obtain the required thickness, followed by water cooling to obtain steel plates. Subsequently, reheating, quenching, and tempering were performed at the temperatures shown in Tables 3 and 4 to obtain steel plates. In Tables 1 and 2, underlines indicate deviations from the specifications of the present invention, and blanks indicate intentional non-inclusion. In Tables 3 and 4, "cumulative reduction" refers to "cumulative reduction in a temperature range of 900°C or higher."

[0118] [Table 1]

[0119] [Table 2]

[0120] [Table 3]

[0121] [Table 4]

[0122] <Vickers hardness measurement method> Vickers hardness was measured using the following method. A specimen was taken from the center of the steel plate so that the C-section served as the observation surface. The observation surface was then mirror-polished and subjected to nital etching. The center of the plate was then observed under an optical microscope at 500x magnification. Areas that appeared dark and distorted, either band-like or clump-like, compared to the surrounding area were visually identified as Sn segregation areas. Sn segregation areas exhibit high hardenability due to the enrichment of alloying elements, and undergo significant distortion due to transformation at lower temperatures compared to the surrounding area. Therefore, they appear as dark and distorted band-like or clump-like areas compared to the surrounding metallographic structure. In cases where discrimination was difficult, Mn mapping measurements using an EPMA were also used to identify Sn segregation areas. While various alloying elements segregate in Sn segregation areas, Mn is the most susceptible to segregation, and an evaluation method for this is well established.

[0123] The Vickers hardness of the Sn segregation area and its surroundings was then measured at lattice points at a pitch of 25 μm over an area of ​​1000 μm in the width direction and 500 μm in the thickness direction, centered on the Sn segregation area, using a test force of 0.09807 N (10 gf). The average of the top 20 Vickers hardness data points measured in this way was taken as the Vickers hardness of the Sn segregation area.

[0124] <Method for measuring the average particle size of prior γ grains> The particle size of prior γ grains was measured using the following method. Observation specimens were prepared in the same manner as for identifying Sn segregation areas in Vickers hardness measurements. Specifically, a specimen was taken from the center of the steel plate thickness so that the C-section of the steel plate served as the observation surface. The observation surface was mirror-polished, then subjected to nital etching, and the center of the plate thickness was observed under an optical microscope at 500x magnification. Areas that appeared darker and distorted than the surrounding area were visually identified as Sn segregation areas. When this was difficult to distinguish, Mn mapping measurements using an EPMA were also used to identify the Sn segregation areas.

[0125] The specimen was then mirror-polished again, and then etched to reveal the prior-γ grain boundaries using an etching solution based on a saturated aqueous solution of picric acid, in accordance with the polishing and etching procedures described in JIS G 0551: 2022. The Sn segregated areas were then observed, and the prior-γ grain size was measured.

[0126] The prior-γ grain size was measured using the intercept method. A test line was drawn at the longest position in the Sn segregation area in the direction perpendicular to the sheet thickness direction. The length between the intersections of the prior-γ grain boundaries at the left and right ends of the Sn segregation area on the test line was defined as L, and the number of intersections between the test line and the prior-γ grain boundaries within the Sn segregation area was defined as P. The average grain size of the prior-γ grains in the Sn segregation area was calculated as L / (P-1). If the number of prior-γ grains in the test line was less than 40, a test line was drawn at a position 50 μm away from the test line in the sheet thickness direction or at another Sn segregation area to calculate the prior-γ grain size. This measurement was repeated until the total number of prior-γ grains reached 40 or more.

[0127] <Method for measuring the average aspect ratio of prior γ grains> The aspect ratio of prior γ grains was measured using the following method. A specimen for observation was taken from the center of the steel plate thickness, with the C-section of the steel plate serving as the observation surface. Then, after polishing and corrosion were performed in the same manner as for revealing the prior γ grain boundaries for the prior γ grain size measurement described above, the prior γ grains in the Sn segregation region were photographed using an optical microscope. Based on the photographed structure, the maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction were measured for each prior γ grain, and the ratio (maximum major axis length / maximum minor axis length) was calculated. The average value was then used as the average aspect ratio of the prior γ grains.

[0128] <Method for measuring maximum defect length> The maximum defect length was measured using the following method. Test specimens for observation were taken from the center of the plate thickness at each of the width positions of 1 / 4W, 1 / 2W, and 3 / 4W so that the C-section of the steel plate was the observation surface. The observation surface was then mirror-polished, and the observation surface was continuously photographed in the width direction at 10 to 15 mm using an optical microscope. Based on the photographed structure, areas that appeared black were determined to be casting defects, and each casting defect was magnified and photographed to measure its maximum length. The largest casting defect among these was taken as the maximum defect length.

[0129] <Evaluation of tensile strength> Tensile strength was measured using No. 4 round bar tensile test specimens taken from 1 / 4t in accordance with JIS Z 2241:2022, with the width direction of the test specimen aligned with the longitudinal direction. Test specimens with a tensile strength of 570 MPa or more were deemed to have passed the test.

[0130] <Evaluation of corrosion resistance> Corrosion resistance was evaluated using each test material. The corrosion test was conducted according to the SAE (Society of Automotive Engineers) J2334 test. The J2334 test is an accelerated test consisting of 6 hours of wet (50°C, 100% RH), 0.25 hours of salt exposure (immersion in a 0.5% NaCl, 0.1% CaCl2, 0.075% NaHCO3 aqueous solution), and 17.75 hours of dry (60°C, 50% RH) conditions (total 24 hours). The corrosion pattern is said to be similar to that in corrosive environments containing chlorides (Hiroo Nagano, Masato Yamashita, and Hitoshi Uchida, Environmental Materials Science, Kyoritsu Shuppan (2004), p. 74).

[0131] After 40 cycles of the J2334 test, the rust layer on the surface of each test piece was removed and the thickness loss (mm) was measured. In the present invention, if the thickness loss in the test was 0.15 mm or less, it was judged as "○: excellent corrosion resistance," and if it was more than 0.15 mm, it was judged as "×: poor corrosion resistance."

[0132] <Evaluation of lamellar tear resistance> Lamellar tear resistance was evaluated using the following method. Figure 1 illustrates a method for preparing a test piece 10 for evaluating lamellar tear resistance. The test piece 10 was prepared by processing in the order shown in Figures 1(a) to 1(f). First, as shown in (a), a section 1 for evaluating lamellar tear resistance was cut from the 1 / 4W width position of the steel plate so as to include the plate thickness Z (see (b)). Then, as shown in (c), blocks 2 were pressed onto one side and the other side of the section 1 in the plate thickness direction to prepare a blank 3 for evaluating lamellar tear resistance. A round bar 4 for evaluating lamellar tear resistance was then cut from the blank 3 (see (d) and (e)), which was further processed to prepare the test piece 10. Specifically, as shown in (f), the gripping portion 5 was threaded, and the length Lc of the parallel portion 6 was set equal to the plate thickness Z. The diameter D0 of the parallel part was set to 10 mm, and the length of the test piece 10 in the longitudinal direction was set to 200 to 300 mm.

[0133] A tensile test was carried out using the test piece 10 in accordance with JIS G 3199:2021, and the reduction of area (%) was calculated. A reduction of area of ​​35% or more was determined to be acceptable.

[0134] Tables 5 and 6 summarize the measurement results of Vickers hardness, average grain size of prior γ grains, average aspect ratio of prior γ grains, and maximum defect length, as well as the evaluation results of tensile strength, corrosion resistance, and lamellar tear resistance.

[0135] [Table 5]

[0136] [Table 6]

[0137] As shown in Tables 5 and 6, Test Nos. 1 to 25, 37 to 63, 69, and 70, which satisfied the requirements of the present invention, were good in tensile strength, corrosion resistance, and reduction of area. On the other hand, Test Nos. 26 to 36 and 64 to 68, which did not satisfy the requirements of the present invention, were poor in at least one of tensile strength, corrosion resistance, and reduction of area. [Industrial Applicability]

[0138] According to the present invention, a thick steel plate having high tensile strength, excellent corrosion resistance in chloride-containing environments, and improved lamellar tear resistance can be obtained. [Explanation of symbols]

[0139] 1 section 2 blocks 3. Materials 4 round bars 5 Grip 6 Parallel section 10 test specimens

Claims

1. The chemical composition of the steel plate is, in mass%, C: 0.06-0.15%, Si: 0.05-1.20%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1-2.0%, Mo: 0.01-0.75%, V: 0.005-0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010-0.080%, Sn: 0.020-0.400%, N: 0.0010-0.0070%, O: 0.0005-0.0040%, Ca: 0.0001 to 0.0080%, and The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, The SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, The thickness of the steel plate is 75 to 105 mm, The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more, In a Sn segregation portion present in the center of the plate thickness in a cross section perpendicular to the rolling direction of the steel plate, Vickers hardness is 350 HV 0.01 or less, The average grain size of prior austenite grains is 40 μm or less, The average aspect ratio of the prior austenite grains is 2.0 or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15...(i) SnEQ=Sn+W+Ni / 10+Mo / 4...(ii) SC=S / Ca...(iii) MV=Mo+4×V...(iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

2. The chemical composition of the steel plate is, in mass%, C: 0.06-0.15%, Si: 0.05-1.20%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1-2.0%, Mo: 0.01-0.75%, V: 0.005-0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010-0.080%, Sn: 0.020-0.400%, N: 0.0010-0.0070%, O: 0.0005-0.0040%, Ca: 0.0001-0.0080%, and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C: The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, The SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, The thickness of the steel plate is 75 to 105 mm, The tensile strength at a quarter-thickness position of the steel plate is 570 MPa or more, In a Sn segregation portion present in the center of the plate thickness in a cross section perpendicular to the rolling direction of the steel plate, Vickers hardness is 350 HV 0.01 or less, The average grain size of prior austenite grains is 40 μm or less, The average aspect ratio of the prior austenite grains is 2.0 or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15...(i) SnEQ=Sn+W+Ni / 10+Mo / 4...(ii) SC=S / Ca...(iii) MV=Mo+4×V...(iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained. [Group A] One or more selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.05% or less. [Group B] One or more elements selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less.

3. The steel sheet according to claim 2 , wherein the chemical composition contains one or more elements selected from Group A.

4. The steel sheet according to claim 2 , wherein the chemical composition contains one or more elements selected from the B group.

5. The steel sheet according to claim 2 , wherein the chemical composition contains one or more elements selected from the C group.

6. In a cross section perpendicular to the rolling direction of the steel plate, the maximum defect length at the center of the thickness of the steel plate is 0.50 mm or less. The steel sheet according to any one of claims 1 to 5.

7. The method for producing a steel sheet according to claim 1, a soaking process in which the slab is heated and then soaked; a hot rolling step of hot rolling the slab to form a steel plate; a quenching step of quenching the steel plate; a tempering step of tempering the steel plate after quenching, The chemical composition of the cast piece is, in mass%, C: 0.06-0.15%, Si: 0.05-1.20%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1-2.0%, Mo: 0.01-0.75%, V: 0.005-0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010-0.080%, Sn: 0.020-0.400%, N: 0.0010-0.0070%, O: 0.0005-0.0040%, Ca: 0.0001 to 0.0080%, and The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, The SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, In the quenching step, the steel plate is reheated in a temperature range of 850 to 950°C and then quenched. In the tempering step, tempering is performed in a temperature range of 570 to 700 ° C. Steel plate manufacturing method. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15...(i) SnEQ=Sn+W+Ni / 10+Mo / 4...(ii) SC=S / Ca...(iii) MV=Mo+4×V...(iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained.

8. The method for producing a steel sheet according to claim 2, a soaking process in which the slab is heated and then soaked; a hot rolling step of hot rolling the slab to form a steel plate; a quenching step of quenching the steel plate; a tempering step of tempering the steel plate after quenching, The chemical composition of the cast piece is, in mass%, C: 0.06-0.15%, Si: 0.05-1.20%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1-2.0%, Mo: 0.01-0.75%, V: 0.005-0.150%, Nb: 0.003 to 0.040%, Ti: 0.003 to 0.040%, Al: 0.010-0.080%, Sn: 0.020-0.400%, N: 0.0010-0.0070%, O: 0.0005-0.0040%, Ca: 0.0001-0.0080%, and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C: The balance is Fe and impurities. Ceq represented by the following formula (i) is 0.37 to 0.51, The SnEQ represented by the following formula (ii) is 0.10 or more, The SC represented by the following formula (iii) is 0.30 to 15.00, MV represented by the following formula (iv) is 0.20 to 0.80, In the quenching step, the steel plate is reheated in a temperature range of 850 to 950°C and then quenched. In the tempering step, tempering is performed in a temperature range of 570 to 700 ° C. Steel plate manufacturing method. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15...(i) SnEQ=Sn+W+Ni / 10+Mo / 4...(ii) SC=S / Ca...(iii) MV=Mo+4×V...(iv) In the above formula, each element symbol represents the content (mass%) of each element contained in the slab, and 0 is substituted if the element is not contained. [Group A] One or more selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.05% or less. [Group B] One or more elements selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.05% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less [Group C] One or more selected from the group consisting of Mg: 0.010% or less, Sr: 0.010% or less, Ba: 0.010% or less, and REM: 0.010% or less.

9. The method for producing a steel sheet according to claim 8 , wherein the chemical composition contains one or more elements selected from Group A.

10. The method for producing a steel sheet according to claim 8 , wherein the chemical composition contains one or more elements selected from the B group.

11. The method for producing a steel sheet according to claim 8 , wherein the chemical composition contains one or more elements selected from the C group.

12. The method for producing a steel sheet according to claim 6, In the soaking step, the cast slab is soaked at 1050 to 1250°C, In the hot rolling step, the cumulative reduction rate in a temperature range of 900°C or higher is set to 52 to 70%. The method for manufacturing a steel sheet according to any one of claims 7 to 11.

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