Steel plate and its manufacturing method

Abstract

The present invention provides a high-strength steel sheet with excellent ammonia stress corrosion cracking resistance and low-temperature toughness, with minimal change in mechanical properties due to PWHT treatment, and a method for manufacturing the same. It contains specific contents of C, Si, Mn, P, S, Al, Ti, Mo, Ca, N, and O in mass%, satisfies 0.05≦2×Cr+Mo+V+W≦0.60, with the remainder being Fe and unavoidable impurities, The steel plate has a steel structure consisting of ferrite and hard structure other than the ferrite at a position 1 / 4 of the plate thickness from the surface, the volume fraction of ferrite is 60% or more and 90% or less, the average crystal grain size of ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure is 250 HV0.01 or more and 350 HV0.01 or less, in terms of volume fraction, 60% or more of the hard structure is adjacent to ferrite but not adjacent to the hard structure, and the yield strength is 325 MPa or more and 440 MPa or less.

Description

[Technical Field]

[0001] The present invention relates to a steel plate having excellent low-temperature toughness and resistance to stress corrosion cracking caused by ammonia, and which is used for tanks for carrying liquefied petroleum gas (hereinafter referred to as LPG) or liquefied ammonia, and a method for manufacturing the same. [Background technology]

[0002] Toward the realization of a low-carbon society, ammonia, which does not emit carbon dioxide when burned, is attracting attention as a new energy source. Hydrogen, which also does not emit carbon dioxide when burned, is considered a candidate for clean energy, and ammonia is also seen as a promising form for stable transportation of such hydrogen. It is expected that international transportation of liquefied ammonia will mainly be by ship. In the medium term, as there will be demand for fossil fuels as an energy source, it is thought that cargoes such as LPG, liquefied butane, dimethyl ether, and butene will be switched or transported together with liquefied ammonia.

[0003] LPG and ammonia are stored in tanks as liquefied gases at low temperatures, so steel materials must have excellent low-temperature toughness. In addition, liquefied ammonia is known to cause stress corrosion cracking of steel materials. Therefore, to ensure the safety of tanks, measures must be taken to prevent stress corrosion cracking caused by liquefied ammonia. To prevent this, the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), an international standard for ships and equipment transporting liquefied gases, stipulates that the yield point (yield strength, hereafter referred to as YS) of carbon-manganese steel for tanks must be 440 MPa or less. It also stipulates that tanks used to liquefy ammonia under pressure must undergo post-weld heat treatment (PWHT) during the manufacturing process.

[0004] On the other hand, tanks are being made larger to improve transport efficiency, and the steel materials used are required to have high tensile strength (hereinafter referred to as TS). Specifically, steel plates for LPG and liquefied ammonia tanks are required to have mechanical properties of YS of 325 to 440 MPa and TS of 440 to 560 MPa, with toughness of a ductile-brittle fracture transition temperature vTrs of -60°C or less. Furthermore, there is also a demand for steel plates with equivalent toughness and YS of 355 to 440 MPa and TS of 490 to 610 MPa.

[0005] Patent Documents 1 to 3 describe techniques for providing the low-temperature toughness required for liquefied gas tanks as described above, and for satisfying the restrictions on YS and the demand for high TS. The technology described in Patent Document 1 aims to control the morphology of ferrite by hot rolling and accelerated cooling, thereby suppressing the yield ratio of the steel sheet.

[0006] Furthermore, the technology described in Patent Document 2 achieves a low yield ratio for steel sheets by controlling the grain size distribution of ferrite through hot rolling and multiple accelerated cooling processes.

[0007] In addition, Patent Document 3 discloses a technique for distributing hard structures by hot rolling and accelerated cooling, thereby creating a difference in hardness with ferrite, thereby reducing the yield ratio. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-89830 [Patent Document 2] Japanese Patent Application Publication No. 2019-214752 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-90406 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, PWHT is sometimes performed in the construction of liquefied ammonia tanks. PWHT is a method of heating steel plates to a temperature below their transformation point and holding them there for a certain period of time to remove residual stresses caused by welding. It is generally known that the mechanical properties of steel plates change when they are heat treated compared to when they were manufactured. Specifically, YS and TS decrease, vTrs increases, and toughness deteriorates. Steel plates that have undergone accelerated cooling show particularly large changes in mechanical properties. If PWHT causes steel plates to no longer meet the required mechanical properties, it becomes difficult for them to meet the required strength for the tank structure, and they may become unusable.

[0010] The present invention aims to solve the above problems and to provide a steel plate that satisfies the YS standard and has excellent ammonia stress corrosion cracking resistance, excellent low-temperature toughness, little change in mechanical properties after PWHT treatment, and high strength, for use in tanks of liquefied gas carriers or for storing liquefied gas, and a manufacturing method thereof.

[0011] Here, excellent resistance to ammonia stress corrosion cracking means that the yield strength YS is 325 MPa or more and 440 MPa or less, as measured by a method conforming to JIS Z2241 (2022). Furthermore, "high strength" refers to a tensile strength TS of 440 MPa or more and 610 MPa or less, measured by a method conforming to JIS Z2241 (2022). Furthermore, excellent low-temperature toughness means that the brittle-ductile fracture transition temperature vTrs is −60° C. or lower in a Charpy impact test in accordance with JIS Z2242 (2023). Furthermore, small changes in mechanical properties after PWHT treatment means that the difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate before and after heating at 630°C for 10 hours is 20HV0.01 or less. [Means for solving the problem]

[0012] To achieve the above object, the inventors investigated methods for improving TS and suppressing changes in mechanical properties due to heat treatment while maintaining the low-temperature toughness of the steel plate, so that YS does not exceed 440 MPa. As a result, they found that it is effective to include elements such as C, Si, Mn, Ti, Cr, Mo, V, and W within predetermined ranges, to set the volume fraction and average grain size of ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate within predetermined ranges, and to control the average hardness of the hard structure and the structure adjacent to the hard structure within predetermined ranges.

[0013] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows.

[0014] [1] In mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P:0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, O: Contains 0.0040% or less, The following formula (1) is satisfied: The remainder is Fe and unavoidable impurities, At a position 1 / 4 of the plate thickness from the surface of the steel plate, The steel has a steel structure consisting of ferrite and a hard structure other than the ferrite, The volume fraction of the ferrite is 60% or more and 90% or less, The average grain size of the ferrite is 3 μm or more and 15 μm or less, The average hardness of the hard tissue is 250 HV0.01 or more and 350 HV0.01 or less, In terms of volume fraction, 60% or more of the hard structure is not adjacent to a hard structure but is adjacent to ferrite, A steel plate having a yield strength of 325 MPa or more and 440 MPa or less. 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each component, and are set to 0 if the component is not contained.

[0015] [2] The component composition is, in mass%, Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Nb: 0.05% or less The steel sheet according to [1] above, containing one or more selected from the following:

[0016] [3] The steel sheet according to [1] or [2], wherein the difference in the average Vickers hardness of the hard structure at a position 1 / 4 of the sheet thickness from the surface of the steel sheet before and after heating at 630°C for 10 hours is 20HV0.01 or less.

[0017] [4] In mass percent, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P:0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, O: Contains 0.0040% or less, The following formula (1) is satisfied: A steel material having a composition consisting of the balance Fe and unavoidable impurities, Heat to a temperature of 1000°C or higher and 1250°C or lower, The cumulative rolling reduction in the non-recrystallization temperature region is 30% or more and 70% or less, Hot rolling is performed with the hot rolling end temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate being 750°C or higher, Cooling is started at a cooling start temperature of 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate, Define the temperature T as between 600°C and 750°C. At a position 1 / 4 of the plate thickness from the surface of the steel plate, The average cooling rate from the cooling start temperature to the temperature T is 2.0°C / s or more and 10.0°C / s or less, Next, the steel sheet is cooled from the temperature T to a cooling stop temperature of 300°C or higher and 550°C or lower at an average cooling rate of 20°C / s or higher and 100°C / s or lower. 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each component, and are set to 0 if the component is not contained.

[0018] [5] The component composition is, in mass%, Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Nb: 0.05% or less The method for producing a steel sheet according to [4] above, wherein the steel sheet contains one or more selected from the following: [Effects of the Invention]

[0019] According to the present invention, a high-strength steel sheet can be obtained which exhibits little change in mechanical properties after PWHT treatment, and is excellent in low-temperature toughness and ammonia stress corrosion cracking resistance. The steel plate of the present invention is suitably used for tanks that are used in environments at low temperatures and in corrosive atmospheres caused by liquefied ammonia. DETAILED DESCRIPTION OF THE INVENTION

[0020] Next, the steel sheet of the present invention will be specifically described. The steel sheet of the present invention contains, by mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, and O: 0.0040% or less, and satisfies the following formula (1), with the balance being Fe and and unavoidable impurities, and the steel plate has a steel structure consisting of ferrite and hard structure other than the ferrite at a position 1 / 4 of the plate thickness from the surface, the volume fraction of ferrite is 60% or more and 90% or less, the average crystal grain size of ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure is 250 HV0.01 or more and 350 HV0.01 or less, in terms of volume fraction, 60% or more of the hard structure is adjacent to ferrite but not adjacent to the hard structure, and the yield strength is 325 MPa or more and 440 MPa or less. 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 if the element is not contained. In the present invention, it is important that the steel sheet and the steel material used for its production have the above-mentioned chemical composition. Therefore, the reason for limiting the chemical composition of the steel sheet in the present invention to the above-mentioned will first be explained. Note that "%" regarding the chemical composition means "mass %" unless otherwise specified.

[0021] [Component composition] C: 0.03% or more and 0.14% or less C is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that must be contained to achieve high tensile strength (TS). To achieve this effect, the C content is set to 0.03% or more. Furthermore, from the perspective of reducing the content of other alloy elements and achieving lower production costs, it is preferable that the C content be 0.05% or more. On the other hand, if the C content exceeds 0.14%, toughness and weldability decrease. Therefore, the C content is set to 0.14% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness and weldability, the C content is preferably set to 0.10% or less.

[0022] Si: 0.10% or more and 0.50% or less Si is an element that acts as a deoxidizer. If the Si content is less than 0.10%, a large amount of oxides will form in the steel, reducing toughness. Therefore, the Si content is set to 0.10% or more. On the other hand, since Si is an element that reduces toughness and weldability, the Si content is set to 0.50% or less. In particular, if the Si content exceeds 0.30%, island martensite may be formed in the HAZ formed during welding with a large heat input, which may reduce toughness, so the Si content is preferably set to 0.30% or less.

[0023] Mn: 0.70% or more and 1.70% or less Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that must be contained to achieve high tensile strength (TS). To achieve this effect, the Mn content is set to 0.70% or more. Furthermore, from the viewpoint of reducing the contents of other alloy elements and producing steel sheets at lower costs, the Mn content is preferably set to 0.90% or more, and more preferably 1.20% or more. On the other hand, if the Mn content exceeds 1.70%, not only will the toughness and weldability decrease, but the alloy cost will also become excessively high. Therefore, the Mn content is set to 1.70% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness and weldability, the Mn content is preferably set to 1.60% or less, and more preferably 1.50% or less.

[0024] P:0.030% or less P is an element contained as an unavoidable impurity, and its segregation at grain boundaries has adverse effects such as reducing toughness and weldability. Therefore, it is desirable to keep the P content as low as possible, but a P content of 0.030% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%, but since P is an element that is usually inevitably contained in steel as an impurity, industrially it may be more than 0%. Excessive reduction of P leads to an increase in refining costs, so the P content is preferably 0.0005% or more.

[0025] S: 0.0030% or less S is an element contained as an unavoidable impurity, and exists in steel as sulfide-based inclusions such as MnS. It acts as a starting point for brittle cracks and reduces toughness. Therefore, it is desirable to keep the S content as low as possible, to 0.0030% or less. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element that is inevitably contained in steel as an impurity, industrially, the S content may be greater than 0%. Since excessive reduction of S leads to an increase in refining costs, from the viewpoint of cost, it is preferable that the S content be 0.0005% or more.

[0026] Al: 0.010% or more and 0.100% or less Al is an element that acts as a deoxidizer, refines crystal grains, and improves toughness. To obtain these effects, the Al content is set to 0.010% or more. To further enhance these effects, the Al content is preferably set to 0.020% or more. On the other hand, if the Al content exceeds 0.100%, oxide-based inclusions increase, reducing the cleanliness and toughness of the steel sheet. The reduction in cleanliness leads to a deterioration in surface quality due to an increase in surface defects and a reduction in bending workability. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.060% or less.

[0027] Ti: 0.010% or more and 0.030% or less Ti has a strong tendency to form nitrides, and precipitates as TiN during solidification. This inhibits austenite coarsening during the heating process of the steel material and acts as a ferrite transformation nucleus, thereby contributing to improving the YS of the steel sheet. To achieve this effect, the Ti content is set to 0.010% or more. Furthermore, if the Ti content is 0.012% or more, a large amount of TiN precipitates, significantly inhibiting the coarsening of austenite. Therefore, the Ti content is preferably set to 0.012% or more. On the other hand, if the Ti content exceeds 0.030%, TiN particles become coarse and act as brittle fracture initiation points, reducing toughness, so the Ti content is set to 0.030% or less, and preferably 0.020% or less.

[0028] Mo: 0.02% or more and 0.10% or less Mo is an element that dissolves in ferrite in a structure consisting of ferrite and hard structure, and improves the YS and TS of steel sheets. It also has a strong tendency to form fine carbides during heating in PWHT, and the presence of these fine carbides in ferrite suppresses a decrease in the hardness of the ferrite. This suppresses a decrease in YS and TS after PWHT. To achieve this effect, the Mo content is set to 0.02% or more and 0.10% or less. If the Mo content is less than 0.02%, the hardness of ferrite decreases due to PWHT, and the YS and TS after PWHT decrease.The Mo content is preferably 0.04% or more. On the other hand, if the Mo content exceeds 0.10%, the YS becomes excessively high. Therefore, the Mo content is set to 0.10% or less, and preferably 0.07% or less.

[0029] Ca: 0.0005% or more and 0.0030% or less Ca is an element that bonds with S and has the effect of suppressing the formation of MnS and other compounds that elongate in the rolling direction. Therefore, by including a predetermined amount of Ca, the morphology of sulfide-based inclusions can be controlled so that they are spherical, thereby improving the toughness of the steel plate. If the Ca content is less than 0.0005%, the toughness of the steel sheet will be inferior. Therefore, the Ca content is set to 0.0005% or more. Furthermore, the Ca content is preferably set to 0.0015% or more. On the other hand, if the Ca content exceeds 0.0030%, the cleanliness of the steel decreases. The decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, the Ca content is set to 0.0030% or less. The Ca content is preferably 0.0025% or less.

[0030] N: 0.0010% or more and 0.0070% or less N combines with Ti to precipitate as TiN, contributing to the refinement of the microstructure and improving the YS and toughness. To achieve this effect, the N content is set to 0.0010% or more. The N content is preferably 0.0030% or more, and more preferably 0.0050% or more. On the other hand, if the N content exceeds 0.0070%, the amount of solute N increases, leading to a decrease in toughness. Furthermore, if the N content exceeds 0.0070%, weldability decreases. Therefore, from the viewpoint of suppressing the decrease in toughness and weldability, the N content is set to 0.0070% or less. It is preferable that the N content is set to 0.0060% or less.

[0031] O: 0.0040% or less O is an element contained as an unavoidable impurity, and since it forms oxides and acts as a starting point for brittle cracks, reducing toughness, the O content is limited to 0.0040% or less, and preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%, but since O is an element that is usually inevitably contained in steel as an impurity, industrially it may be more than 0%. In other words, excessive reduction of O content leads to a rise in refining costs, so from the viewpoint of cost, it is preferable that the O content be 0.0020% or more.

[0032] 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 if the element is not contained. Cr is an element that dissolves in the hard structure of a structure consisting of ferrite and hard structure, suppressing a decrease in the hardness of the hard structure during PWHT. Similarly, V and W, like Mo, are elements that dissolve in ferrite and suppress the hardness of ferrite during PWHT. The combination of Mo and these elements suppresses a decrease in YS and TS after PWHT. To achieve the desired mechanical properties after PWHT, it is essential that the ratio 2×Cr+Mo+V+W be 0.05≦2×Cr+Mo+V+W≦0.60. If 2×Cr+Mo+V+W is less than 0.05, the hardness of the ferrite and hard structure decreases during PWHT, significantly reducing YS and TS after PWHT. Therefore, 2×Cr+Mo+V+W should be 0.05 or greater. 2×Cr+Mo+V+W is preferably 0.10 or greater, and more preferably 0.20 or greater. On the other hand, if 2×Cr+Mo+V+W exceeds 0.60, the ferrite and hard structure will become excessively hard, the TS will become excessively high, and the toughness will deteriorate. Therefore, 2×Cr+Mo+V+W is set to 0.60 or less. 2×Cr+Mo+V+W is preferably 0.50 or less, and more preferably 0.30 or less.

[0033] The basic composition of the present invention contains the above components, with the balance being Fe and unavoidable impurities. This composition may optionally further contain one or more elements selected from the group consisting of Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Nb: 0.05% or less, for the purpose of improving strength characteristics or toughness.

[0034] Cr:0.30% or less Cr is an element that dissolves in the hard structure of a structure consisting of ferrite and hard structure, and suppresses a decrease in hardness of the hard structure during heating in PWHT. The Cr content is preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.30%, it may cause a deterioration in toughness. Therefore, when Cr is contained, the Cr content is set to 0.30% or less, and more preferably, the Cr content is set to 0.20% or less.

[0035] V:0.10% or less V is an element that dissolves in ferrite in a structure consisting of ferrite and hard structure, and improves the YS and TS of steel sheets. It also has a strong tendency to form carbides during PWHT heating, and its presence in the ferrite as fine carbides suppresses a decrease in the hardness of the ferrite. The V content is preferably 0.03% or more. On the other hand, if the V content exceeds 0.10%, the YS may become excessively high. Therefore, when V is contained, the V content is set to 0.10% or less, and more preferably, the V content is 0.05% or less.

[0036] W: 0.10% or less W is an element that dissolves in ferrite in a structure consisting of ferrite and hard structure, and improves the YS and TS of steel sheets. It also has a strong tendency to form carbides during PWHT heating, and its presence in ferrite as fine carbides suppresses a decrease in the hardness of ferrite. The W content is preferably 0.02% or more. On the other hand, if the W content exceeds 0.10%, a large amount of highly hard carbides may be formed, resulting in excessive TS. Therefore, when W is contained, the W content is set to 0.10% or less. The W content is more preferably 0.04% or less.

[0037] Cu:1.00% or less Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet, and can be added as desired. The Cu content is preferably 0.15% or more. On the other hand, if the Cu content exceeds 1.00%, it may lead to a deterioration in toughness and an increase in alloy costs. Therefore, if Cu is contained, the Cu content is set to 1.00% or less. The Cu content is more preferably 0.50% or less.

[0038] Ni: 1.00% or less Ni is an element that has the effect of improving the strength of the steel sheet, similar to Cu, and can be added as desired. The Ni content is preferably 0.15% or more. On the other hand, if the Ni content exceeds 1.00%, it may lead to deterioration of weldability and an increase in alloy costs. Therefore, when Ni is contained, the Ni content is set to 1.00% or less, and more preferably, the Ni content is 0.50% or less.

[0039] Co: 1.00% or less Co is an element that has the effect of improving the strength of the steel sheet, similar to Cu, and can be added as desired. The Co content is preferably 0.20% or more. On the other hand, if the Co content exceeds 1.00%, it may lead to deterioration of weldability and an increase in alloy costs. Therefore, when Co is contained, the Co content is set to 1.00% or less, and more preferably, the Co content is 0.60% or less.

[0040] Nb: 0.05% or less Nb is an element that reduces the prior austenite grain size by precipitating as carbonitrides, contributes to the refinement of the structure by hot rolling, and improves YS and TS. The Nb content is preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.05%, the structure becomes excessively refined, causing the YS to exceed 440 MPa. Therefore, the Nb content is set to 0.05% or less, and more preferably, the Nb content is set to 0.030% or less.

[0041] In addition to having the above-mentioned chemical composition, the steel sheet of the present invention has a microstructure in which, at a position from the surface of the steel sheet to 1 / 4 of the sheet thickness, the volume fraction of ferrite is 60% to 90%, the average grain size of the ferrite is 3 μm to 15 μm, the average hardness of hard structures other than ferrite is 250 HV0.01 to 350 HV0.01, and 60% or more of the hard structures are adjacent to ferrite but not adjacent to hard structures. The reasons for limiting the microstructure as above will be explained below.

[0042] [Microstructure] The microstructure of the steel sheet of the present invention will be described. [The volume fraction of ferrite is 60% or more and 90% or less at a position 1 / 4 of the plate thickness from the surface of the steel plate] In the steel sheet of the present invention, the volume fraction of ferrite at a position from the surface of the steel sheet to 1 / 4 of the sheet thickness is 60% to 90%. If the volume fraction of ferrite exceeds 90%, the hard structure will be insufficient and the desired TS and YS will not be obtained. On the other hand, if the volume fraction of ferrite is less than 60%, the remaining structure will contain more hard structures such as bainite and martensite, causing TS and YS to exceed the specified ranges and reducing toughness. The volume fraction of ferrite is preferably 60% or more and 70% or less.

[0043] The remaining structure may be a mixture of hard structures such as bainite, pearlite, austenite, and martensite. The volume fraction of each structure in the remaining structure does not need to be particularly limited, but from the viewpoint of toughness, it is preferable that the volume fraction of pearlite or bainite in the remaining structure is the second highest after ferrite. The volume fractions of various microstructures can be measured by the method described in the Examples below.

[0044] [Average grain size of ferrite is 3 μm or more and 15 μm or less] In the steel sheet of the present invention, the average grain size of ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet is 3 μm or more and 15 μm or less. If the average grain size of ferrite exceeds 15 μm, the YS does not reach the predetermined range and the toughness decreases. Therefore, the average grain size of ferrite is set to 15 μm or less, preferably 12 μm or less, and more preferably 9 μm or less. On the other hand, if the average grain size of ferrite is less than 3 μm, the YS exceeds a predetermined range, and the occurrence of stress corrosion cracking due to ammonia cannot be prevented. Therefore, the average grain size of ferrite is set to 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more. The grain size of ferrite refers to the equivalent circle diameter, which is the diameter of a circle with the same area as one ferrite grain projected onto a two-dimensional plane. The equivalent circle diameter is calculated by performing image analysis on an image of the microstructure.

[0045] [Average hardness of hard tissue is 250HV0.01 or more and 350HV0.01 or less] In the steel sheet of the present invention, the average hardness of the hard structure other than ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet is 250 HV0.01 or more and 350 HV0.01 or less. If the average hardness of the hard structure exceeds 350 HV0.01, the TS exceeds a predetermined range and the toughness decreases. Therefore, the average hardness of the hard structure is set to 350 HV0.01 or less, preferably 330 HV0.01 or less, and more preferably 320 HV0.01 or less. On the other hand, if the average hardness of the hard tissue is less than 250 HV0.01, the TS will not reach the predetermined range. Therefore, the average hardness of the hard tissue is set to 250 HV0.01 or more, preferably 270 HV0.01 or more, and more preferably 290 HV0.01 or more. The average hardness of the hard tissue can be measured by the method described in the Examples below.

[0046] [More than 60% of the hard structure by volume is adjacent to ferrite and not adjacent to hard structure] PWHT generates carbides within the hard structure. When hard structures are adjacent to each other, carbon diffuses between the hard structures, generating larger carbides. This reduces the hardness of the hard structure and the TS of the steel plate. It also reduces toughness. When hard structures with a volume fraction of 60% or more are adjacent to ferrite and not adjacent to other hard structures, coarsening of carbides is suppressed, the decrease in YS and TS before and after PWHT is small, and the required mechanical properties are satisfied. On the other hand, if the volume fraction of the hard structure adjacent to ferrite but not adjacent to the hard structure is less than 60%, the YS and TS decrease significantly due to PWHT, and the specified mechanical properties are not satisfied. Therefore, it is necessary that 60% or more of the hard structure is adjacent to ferrite but not adjacent to the hard structure. Therefore, the volume fraction of the hard structure adjacent to ferrite but not adjacent to the hard structure is 60% or more, preferably 70% or more, and more preferably 80% or more. The upper limit of the volume fraction of hard tissue that is not adjacent to hard tissue but adjacent to ferrite is not particularly limited, and may be 100%, or the volume fraction of hard tissue that is not adjacent to hard tissue but adjacent to ferrite may be 90% or less. The volume fraction of the hard tissue can be measured by the method described in the Examples below.

[0047] [Difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate before and after heating at 630°C for 10 hours: 20HV0.01 or less] PWHT generates carbides inside the hard structure other than ferrite. When the carbon dissolved in the hard structure becomes carbide, the strain that had formed inside the hard structure due to elemental solid solution disappears, reducing the hardness of the hard structure and the TS of the steel sheet. If the difference in Vickers hardness (hardness before heating minus hardness after heating) before and after heating at 630°C for 10 hours is 20HV0.01 or less, the decrease in TS before and after PWHT will be small and the required mechanical properties will be satisfied. On the other hand, if the decrease in Vickers hardness of the hard structure after PWHT exceeds 20 HV0.01, the decrease in TS due to PWHT is significant, and even if the required mechanical properties are met before PWHT, the TS will be insufficient after PWHT. Alternatively, if the TS is met after PWHT, the TS before PWHT will be excessively high. Therefore, the difference in Vickers hardness of the hard structure before and after PWHT, i.e., the difference in the average Vickers hardness of the hard structure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel sheet before and after heating at 630°C for 10 hours, must be 20 HV0.01 or less. Therefore, the difference in the average Vickers hardness of the hard structure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel sheet before and after heating at 630°C for 10 hours must be 20 HV0.01 or less, preferably 15 HV0.01 or less, and more preferably 10 HV0.01 or less. The lower limit is not particularly limited, and the difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate may be 0 HV0.01 or 5 HV0.01 or more. The average hardness of the hard tissue can be measured by the method described in the Examples below. The difference in average Vickers hardness can be defined as "(the average Vickers hardness before heating at 630°C for 10 hours) - (the average Vickers hardness after heating at 630°C for 10 hours)." Furthermore, even if the PWHT conditions are different, the same effect can be expected as long as the hardness difference requirement is met under the heating conditions described above.

[0048] Next, a method for producing a steel sheet according to the present invention will be described. A steel material having the above-mentioned component composition is heated and hot-rolled to form a hot-rolled steel sheet, which is then cooled under the conditions described below to form a steel sheet. Each manufacturing condition will be explained in detail below.

[0049] The method for producing the steel material does not need to be particularly limited, but it is preferable to produce molten steel having the above-mentioned composition by a known melting method such as a converter, and then form the molten steel into a steel material such as a slab of a predetermined size by a known casting method such as a continuous casting method. Note that there is no problem with producing a steel material such as a slab of a predetermined size by an ingot casting-decomposition rolling method.

[0050] The steel material thus obtained is either directly hot rolled without cooling, or is once cooled and then heated before being subjected to hot rolling. In this hot rolling, the steel plate is heated to a temperature of 1000°C to 1250°C, and then the cumulative reduction in the austenite non-recrystallization temperature region is set to 30% to 70%, and the hot rolling is completed at 750°C or higher at a position 1 / 4 of the plate thickness from the surface of the steel plate. Next, cooling is initiated at a cooling start temperature of 680°C to 900°C at a position 1 / 4 of the plate thickness from the surface of the steel plate, and for a temperature T defined in the range of 600°C to 750°C, the average cooling rate from the cooling start temperature to T at a position 1 / 4 of the plate thickness from the surface of the steel plate is 2.0°C / s to 10.0°C / s, and the average cooling rate from temperature T to a cooling stop temperature of 300°C to 550°C is 20°C / s to 100°C / s.

[0051] (a) Heating temperature of steel material: 1000℃ or higher and 1250℃ or lower If the heating temperature of the steel material is less than 1000°C, the heating temperature is too low, resulting in high deformation resistance of the steel material, which increases the load on the hot rolling mill and may make hot rolling difficult. On the other hand, if the heating temperature of the steel material exceeds 1250°C, oxidation of the material surface becomes significant, which may increase oxidation loss of the material and reduce yield. For these reasons, the heating temperature is set to 1000°C or higher and 1250°C or lower. The heating temperature is preferably 1050°C or higher. Furthermore, the heating temperature is preferably 1150°C or lower.

[0052] (b) Cumulative rolling reduction in the non-recrystallization temperature range: 30% to 70% If the cumulative reduction rate is less than 30% in the non-recrystallization temperature region (which in the present invention means a region where the steel material temperature is below the Ar3 transformation point + 150°C), the austenite is not sufficiently worked during hot rolling. If the austenite is not sufficiently worked, there are fewer ferrite nucleation sites in the cooling process described below, the ferrite becomes coarse, and the average grain size exceeds 15 μm. This causes the YS to fall short of the specified range and the toughness to decrease. On the other hand, if the cumulative reduction in the non-recrystallization temperature region exceeds 70%, the number of ferrite nucleation sites during the cooling process becomes excessive, the average grain size of ferrite becomes less than 3 μm, and the YS exceeds the specified range. Therefore, the cumulative reduction in the non-recrystallization temperature region is specified to be 30% or more and 70% or less. The cumulative reduction in the non-recrystallization temperature region is preferably 40% or more, and more preferably 50% or more. Furthermore, the cumulative reduction in the non-recrystallization temperature region is preferably 67% or less, and more preferably 60% or less.

[0053] Here, the Ar3 transformation point (°C) can be calculated, for example, by the following formula. Ar3 transformation point (℃) = 910 - 273 × C - 74 × Mn - 57 × Ni - 16 × Cr - 9 × Mo - 5 × Cu Each element symbol indicates the content (mass%) of the element in the steel.

[0054] (c) Hot rolling finish temperature: 750°C or higher at a position 1 / 4 of the plate thickness from the surface of the steel plate Hot rolling is completed at a temperature of 750°C or higher. If the hot rolling completion temperature is lower than 750°C, the ferrite generated during hot rolling will be hardened by the rolling process, causing the YS to exceed the specified range and reducing the toughness. Furthermore, the load on the hot rolling mill will increase. Therefore, the hot rolling completion temperature is set to 750°C or higher. The hot rolling completion temperature is preferably 780°C or higher. There is no particular upper limit to the hot rolling end temperature, but if it exceeds 950°C, the ferrite may become coarse and the toughness may deteriorate, so it is preferably 950°C or less.

[0055] (d) Cooling start temperature: 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate If the cooling start temperature of the steel plate is less than 680°C, a large amount of ferrite will be generated before the start of cooling, and the volume fraction of ferrite will exceed 90% after cooling is complete. If the volume fraction of ferrite exceeds 90%, the hard structure will be insufficient and the specified YS and TS will not be obtained. Therefore, the cooling start temperature is set to 680°C or higher. The cooling start temperature is preferably 750°C or higher. On the other hand, if the cooling start temperature of the steel plate exceeds 900°C, the volume fraction of ferrite will be less than 60%, TS and YS will exceed the specified range, and toughness will decrease. Therefore, the cooling start temperature is set to 900°C or less. The cooling start temperature is preferably 830°C or less.

[0056] (e) At a position 1 / 4 of the plate thickness from the surface of the steel plate, the average cooling rate from the cooling start temperature to a temperature T defined as a range of 600°C to 750°C (first average cooling rate): 2.0°C / s to 10.0°C / s The average cooling rate (first average cooling rate) in the temperature range from the cooling start temperature at a position 1 / 4 of the plate thickness from the surface to a temperature T (defined as the range of 600°C to 750°C) is 2.0°C / s or more and 10.0°C / s or less. Ferrite forms from austenite grain boundaries before the start of cooling or during the cooling process, and the distribution of hard structure and ferrite is determined by cooling to temperature T. If temperature T exceeds 750°C or if the first average cooling rate at a position 1 / 4 of the plate thickness from the surface of the steel plate exceeds 10.0°C / s, ferrite formation from austenite grain boundaries is insufficient, resulting in the formation of a structure in which hard structure is adjacent to each other. Specifically, the volume fraction of hard structure surrounded by ferrite becomes less than 60% of the total hard structure, resulting in a significant decrease in hardness after PWHT. On the other hand, if the temperature T is less than 600°C or the first average cooling rate is less than 2.0°C / s, excessive ferrite is generated during this process, the ferrite volume fraction exceeds 90%, and YS and TS decrease. The first average cooling rate in the above temperature range is preferably 4°C / s or more. Also, the first average cooling rate in the above temperature range is preferably 8°C / s or less. Here, the first average cooling rate (°C / s) is obtained by (cooling start temperature-temperature T) (°C) / (cooling time (s) from cooling start temperature to temperature T).

[0057] (f) The average cooling rate from temperature T to the cooling stop temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate (second average cooling rate): 20°C / s or more and 100°C / s or less The average cooling rate (second average cooling rate) in the temperature range from temperature T to the cooling stop temperature is 20°C / s or more and 100°C / s or less. During this process, ferrite grains grow, and the final volume fraction of the structure is determined. If the second average cooling rate in the temperature range exceeds 100°C / s, the volume fraction of ferrite becomes less than 60%, and YS and TS exceed the specified ranges. Furthermore, the formation of a large amount of island martensite increases the hardness of the hard structure and reduces its toughness. On the other hand, if the second average cooling rate is less than 20°C / s, ferrite growth progresses, the ferrite volume fraction exceeds 90%, and YS and TS decrease. The second average cooling rate in the above temperature range is preferably 30°C / s or more. Also, the second average cooling rate in the above temperature range is preferably 70°C / s or less. Here, the second average cooling rate (°C / s) is obtained by (temperature T-cooling stop temperature) (°C) / (cooling time (s) from temperature T to cooling stop temperature).

[0058] (g) Cooling stop temperature at 1 / 4 of the plate thickness from the surface of the steel plate: 300°C or higher and 550°C or lower The cooling stop temperature is set to 300° C. or higher and 550° C. or lower. If the cooling stop temperature exceeds 550° C., the hardness of the hard structure will be less than 250HV0.01, and TS will decrease. On the other hand, if the cooling stop temperature is less than 300°C, the hard structure becomes excessively hard, which increases TS and reduces toughness. The cooling stop temperature is 350°C or higher. The cooling stop temperature is preferably 500°C or lower.

[0059] A steel sheet having the above-described structure (steel sheet according to the present invention) can be obtained by manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing conditions. The steel sheet according to the present invention thus obtained has excellent strength properties and low-temperature toughness. Here, excellent strength properties mean that the YS (upper yield point when there is a yield point, or 0.2% proof stress when there is no yield point) is 325 MPa or more and 440 MPa or less, and the TS is 440 MPa or more and 610 MPa or less. Additionally, the International Maritime Organization's IMO Gas Code and classification regulations stipulate that the yield point of steel plate must be 440 MPa or less to minimize the risk of ammonia stress corrosion cracking. Therefore, if the YS is 440 MPa or less, it can be said that the steel has excellent ammonia stress corrosion cracking resistance.

[0060] Basically, the higher the TS of a steel sheet, the better; however, steel sheets with a TS exceeding 610 MPa are more likely to crack during processing or welding. Alternatively, a large amount of alloy will need to be added, which is likely to increase costs. Furthermore, since it is difficult to achieve both the aim of controlling the YS to 440 MPa or less to ensure ammonia stress corrosion cracking resistance and the improvement of TS, it is desirable for the TS of the steel sheet to be 610 MPa or less. The TS of the steel sheet obtained in the present invention is substantially 610 MPa or less.

[0061] Although not particularly limited, the thickness of the steel sheet of the present invention is preferably 12 mm or more, more preferably 20 mm or more, and is preferably 50 mm or less, more preferably 40 mm or less.

[0062] In the steel sheet and the manufacturing method thereof according to the present invention, any items not described in this specification can be made in accordance with the specifications and conventional methods for known steel sheets. [Example]

[0063] Molten steel having the chemical composition shown in Table 1 was produced as steel materials (slabs). These steel materials (slabs) were hot-rolled and cooled under the conditions shown in Table 2 to obtain steel plates. Blank elements in the table indicate that they were not intentionally added.

[0064] The obtained steel sheets were subjected to a heat treatment simulating PWHT. Using a heating furnace, the steel sheets were heated to a temperature of 630°C at a position 1 / 4 of the sheet thickness from the surface, and this temperature was maintained for 10 hours. The effects of the heat treatment temperature and the holding time at the heat treatment temperature on the mechanical properties of steel materials are summarized in the tempering parameter P below. P=T{log(t)+20}×10 -3 Here, T is the heat treatment temperature (K), and t is the holding time at the heat treatment temperature (hours). The tempering parameter P in the heat treatment simulating the PWHT was 18.963. Since the smaller the P, the smaller the effect on mechanical properties, the inventive examples in the examples of this application are expected to exhibit effects equal to or greater than those achieved under PWHT conditions where P≦18.963.

[0065] The resulting steel sheets were subjected to measurements of the microstructure volume fraction, ferrite grain size distribution, and hardness of the hard structure at a position 1 / 4 of the sheet thickness from the surface, and evaluations of tensile properties and toughness were carried out. Furthermore, the hardness of the steel sheets after heat treatment simulating PWHT was investigated at a position 1 / 4 of the sheet thickness from the surface. The test methods are as follows:

[0066] [Measurement of microstructure volume fraction and ferrite grain size distribution] Observation samples were taken from each of the obtained steel sheets so that their centers were located at a position 1 / 4 of the sheet thickness from the surface. The surfaces of the samples were mirror-polished and further etched with nital, and then photographed using a scanning electron microscope (SEM) (magnification: 1000x). The photographed area was 4 mm x 3 mm. The photographed images were analyzed using an image analyzer to determine the area fraction of the microstructure. Since the steel sheets of the present invention have small anisotropy of the microstructure and the two-dimensional information obtained by the analysis is universal, the area fraction of the microstructure was considered to be equivalent to the volume fraction, and the area fraction was taken as the volume fraction.

[0067] When determining the volume fraction of the microstructure, the distinction between the various structures was made as follows. The ferrite was a structure that did not contain isotropically grown carbides and was surrounded by white lines, while the pearlite was a lumpy or flat structure with dark ferrite and white carbides appearing in a striped pattern. Bainite was defined as a structure with an elongated, lath-shaped ferrite structure containing carbides with an equivalent circle diameter of 0.05 μm or more, while martensite was defined as a structure with an elongated, lath-shaped ferrite structure similar to bainite, containing no carbides with an equivalent circle diameter of 0.05 μm or more. Note that carbides were defined as appearing as white dots. Furthermore, austenite was defined as a structure that is not a carbide and has an equivalent circle diameter of 0.50 μm or more, and that exists between lath-shaped ferrite structures of bainite or martensite.

[0068] The circle-equivalent diameter of ferrite was determined by counting the number of pixels within the area surrounded by grain boundaries as a single crystal in an image taken with a microscope (SEM), and then converting the number of pixels into an actual length to calculate the circle-equivalent diameter. Insufficient resolution in the image to be analyzed or noise due to lens misfocus during photography can be mistakenly judged as a single crystal grain, resulting in an erroneous calculation that indicates the presence of a vast amount of minute ferrite. Therefore, the particle size distribution was obtained by randomly selecting more than 4,000 ferrite particles with a size that can be visually determined to be crystals, i.e., a circular equivalent diameter of 2 μm or more. The average grain size of ferrite was determined by dividing the sum of these grain sizes (circle equivalent diameters) by the number of grains.

[0069] The volume fraction of hard structure adjacent only to ferrite was determined by calculating the ratio of the area of ​​the microstructure adjacent to the hard structure that was determined to be only ferrite to the total area of ​​the hard structure confirmed in the image taken with a microscope (SEM). Because the two-dimensional information obtained by the analysis is universal, the area fraction of the microstructure was considered to be equivalent to the volume fraction, and the obtained area fraction was used as the volume fraction.

[0070] [Hardness of hard tissue (average hardness)] The hardness (average hardness) of the hard structure was measured by hardness measurement using a microindenter. A sample was taken from a position 1 / 4 of the plate thickness from the surface of the steel plate, the surface was polished, and then corroded with a nital solution. After that, the sample was observed using an optical microscope to determine the hard structure to be measured for hardness. A square pyramidal indenter was pressed into the sample with a load of 10 gf, and the Vickers hardness was measured from the indentation dimensions. The hardness of 20 hard structures was measured, and the average value was taken as the average hardness of the hard structure. The hardness measurement method was the same for the obtained steel plate and the steel plate after heat treatment simulating PWHT. In Table 2, the change in hardness before and after PWHT (difference in Vickers hardness) indicates "(average Vickers hardness before heating at 630°C for 10 hours) - (average Vickers hardness after heating at 630°C for 10 hours)."

[0071] [Strength characteristics] Tensile test specimens conforming to JIS Z2241 (2022) were prepared from each steel plate in a direction perpendicular to the rolling direction, i.e., so that the plate width direction and the longitudinal direction of the tensile test specimen were aligned. Tensile tests were performed according to JIS Z2241 (2022), and YS and TS were measured. Yield strength (YS) is closely related to ammonia stress corrosion cracking resistance. Because the risk of ammonia stress corrosion cracking must be minimized for structural components of liquefied gas bulk carriers, the IMO Gas Code and classification regulations stipulate that the yield strength of steel plates must be 440 MPa or less. Therefore, in this example, steel plates with a YS of 325 MPa or more and 440 MPa or less were evaluated as having excellent ammonia stress corrosion cracking resistance. Furthermore, steel plates with a TS of 440 MPa or more and 610 MPa or less were evaluated as having excellent tensile strength.

[0072] [Toughness] Test specimens conforming to JIS Z2242 (2023) were taken from a section 0.5 mm removed from the surface of each steel plate, in a direction perpendicular to the rolling direction, i.e., so that the plate width direction and the longitudinal direction of the tensile test specimen were aligned. Charpy impact tests were then conducted according to the procedures in JIS Z2242 (2023), and the brittle-to-ductile fracture transition temperature (vTrs) was measured. Steel plates with a vTrs of -60°C or less were evaluated as having excellent low-temperature toughness. The evaluation results thus obtained are shown in Table 2.

[0073] [Table 1]

[0074] [Table 2]

[0075] As can be seen from Tables 1 and 2, all of the examples of the invention have a yield strength YS of 325 MPa or more and 440 MPa or less, a tensile strength TS of 440 MPa or more and 610 MPa or less, a brittle-to-ductile fracture transition temperature vTrs of -60°C or less, excellent toughness and ammonia stress corrosion cracking resistance at low temperatures, and high-strength steel plates with little change in mechanical properties before and after PWHT treatment.

[0076] On the other hand, steel plates Nos. 4, 5, 7, 8, 9, 11, 13, and 14, which correspond to comparative examples, have ferrite volume fractions or average ferrite grain sizes outside the ranges of the present invention, and are inferior to the invention examples in at least one of yield strength YS, tensile strength TS, and toughness. Furthermore, the steel plate No. 6, which corresponds to a comparative example, has a rolling finish temperature outside the range of the present invention, and is inferior in yield strength YS and toughness to the invention examples. In steel sheets Nos. 10 and 12, the volume fraction of hard structures adjacent only to ferrite is outside the range of the present invention, and the change in hardness before and after PWHT is large. Furthermore, steel plates Nos. 15 and 16 have average hardness of hard structures outside the range of the present invention, and have inferior TS compared to the invention examples, and No. 15 also has inferior toughness. Furthermore, steel plates Nos. 22 to 41 differ from the invention examples in the contents of various elements and are inferior to the invention examples in at least one of yield strength YS, tensile strength TS, toughness, and change in mechanical properties before and after PWHT treatment.

Claims

1. In mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, O: Contains 0.0040% or less, The following formula (1) is satisfied: The remainder is Fe and unavoidable impurities, At a position 1 / 4 of the plate thickness from the surface of the steel plate, The steel has a steel structure consisting of ferrite and a hard structure other than the ferrite, The volume fraction of the ferrite is 60% or more and 90% or less, The average grain size of the ferrite is 3 μm or more and 15 μm or less, The average hardness of the hard tissue is 250 HV0.01 or more and 350 HV0.01 or less, In terms of volume fraction, 60% or more of the hard structure is not adjacent to a hard structure but adjacent to ferrite, A steel plate having a yield strength of 325 MPa or more and 440 MPa or less. 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 when no element is contained.

2. The component composition is, in mass%, Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Nb: 0.05% or less The steel sheet according to claim 1, comprising one or more selected from the following:

3. 3. The steel sheet according to claim 1, wherein a difference in average Vickers hardness of the hard structure at a position 1 / 4 of the sheet thickness from the surface of the steel sheet before and after heating at 630°C for 10 hours is 20 HV0.01 or less.

4. In mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, O: Contains 0.0040% or less, The following formula (1) is satisfied: A steel material having a composition consisting of the balance Fe and unavoidable impurities, Heating to a temperature of 1000°C or higher and 1250°C or lower, The cumulative rolling reduction in the non-recrystallization temperature region is 30% or more and 70% or less, Hot rolling is performed with the hot rolling finish temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate being 750°C or higher, Cooling is started at a cooling start temperature of 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate, Define the temperature T in the range of 600°C or more and 750°C or less, At a position 1 / 4 of the plate thickness from the surface of the steel plate an average cooling rate from the cooling start temperature to the temperature T is 2.0°C / s or more and 10.0°C / s or less; Next, cooling is performed from the temperature T to a cooling stop temperature of 300°C or higher and 550°C or lower at an average cooling rate of 20°C / s or higher and 100°C / s or lower. At a position 1 / 4 of the plate thickness from the surface of the steel plate, The steel has a steel structure consisting of ferrite and a hard structure other than the ferrite, The volume fraction of the ferrite is 60% or more and 90% or less, The average grain size of the ferrite is 3 μm or more and 15 μm or less, The average hardness of the hard tissue is 250 HV0.01 or more and 350 HV0.01 or less, In terms of volume fraction, 60% or more of the hard structure is not adjacent to a hard structure but adjacent to ferrite, A method for manufacturing a steel plate having a yield strength of 325 MPa or more and 440 MPa or less. 0.05≦2×Cr+Mo+V+W≦0.60...Formula (1) The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 when no element is contained.

5. The component composition is, in mass%, Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Nb: 0.05% or less The method for producing a steel sheet according to claim 4, wherein the steel sheet contains one or more selected from the following: