Steel sheet and method for manufacturing same

Abstract

Provided are a steel sheet having excellent ammonia stress corrosion cracking resistance, excellent low-temperature toughness, small change in mechanical characteristics due to PWHT treatment, and high strength, and a method for producing the steel sheet.　The steel sheet contains, by mass%, a specific content of C, Si, Mn, P, S, Al, Ti, Mo, Ca, N, and O, and satisfies 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60, the remainder being Fe and unavoidable impurities. The steel sheet has a steel structure composed of ferrite and a hard structure other than the ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet, the volume fraction of ferrite is 60-90%, the average crystal grain size of the ferrite is 3-15 μm, the average hardness of the hard structure is 250 HV0.01 to 350 HV-0.01, 60% or more of the hard structure in terms of volume fraction is adjacent to ferrite without being adjacent to the hard structure, and the yield strength is 325-440 MPa.

Description

Steel plate and its manufacturing method

[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.

[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 of stable transportation of such hydrogen. It is expected that international transportation of liquefied ammonia will mainly be by ship. Furthermore, in the medium term, as there will be demand for fossil fuels as an energy source, it is expected that cargoes such as LPG, liquefied butane, dimethyl ether, and butene will be switched between liquefied ammonia and other fuels, or that they will be transported together.

[0003] Since LPG and ammonia are loaded into tanks as liquefied gases at low temperatures, steel materials are required to have excellent low-temperature toughness. In addition, liquefied ammonia is known to cause stress corrosion cracking of steel materials. Therefore, in order to ensure the safety of the tanks, measures must be taken to prevent stress corrosion cracking caused by liquefied ammonia. To prevent stress corrosion cracking caused by liquefied ammonia, 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, specifies that the yield point (hereinafter referred to as yield strength, YS) of carbon-manganese steel for tanks must be 440 MPa or less. Furthermore, it is specified that tanks that pressurize and liquefy ammonia must undergo post-weld heat treatment (hereinafter referred to as PWHT) during the manufacturing process.

[0004] On the other hand, efforts are being made to increase the size of tanks in order to improve transportation 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 toughness with a ductile-brittle fracture transition temperature vTrs of -60°C or less, and mechanical properties of YS of 325 to 440 MPa and TS of 440 to 560 MPa. 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] Technologies for providing the low-temperature toughness required for liquefied gas tanks as described above and satisfying the restrictions on YS and the demand for high TS are described in Patent Documents 1 to 3. The technology described in Patent Document 1 aims to suppress the yield ratio of the steel plate by controlling the morphology of ferrite through hot rolling and accelerated cooling.

[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.

[0008] JP 2006-89830 A JP 2019-214752 A JP 2010-90406 A

[0009] As mentioned above, PWHT is sometimes performed in the construction of liquefied ammonia tanks. PWHT is a technique in which the steel plate is heated to a temperature below the transformation point and held for a certain period of time to remove residual stress imparted by welding. It is generally known that the mechanical properties of steel plate change when heat treated compared to those at the time of manufacture. Specifically, the YS and TS decrease, vTrs increases, and toughness deteriorates. In particular, steel plate that has undergone accelerated cooling experiences significant changes in mechanical properties. If the steel plate no longer satisfies the required mechanical properties due to PWHT, it becomes difficult to achieve the specified strength for the tank structure, and it 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, small 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 ammonia stress corrosion cracking resistance" refers to a yield strength YS of 325 MPa or more and 440 MPa or less, measured by a method conforming to JIS Z2241 (2022). "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). "Excellent low-temperature toughness" refers to a brittle-to-ductile fracture transition temperature vTrs of -60°C or less, measured by a Charpy impact test conforming to JIS Z2242 (2023). "Small change in mechanical properties after PWHT treatment" refers to a 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, of 20HV0.01 or less.

[0012] In order to achieve the above object, the present inventors have investigated a method 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 have found that it is effective to contain 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] A steel sheet comprising, 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, and O: 0.0040% or less, the steel sheet satisfying the following formula (1), with the balance being Fe and unavoidable impurities, and the steel sheet having a steel structure consisting of ferrite and a hard structure other than the ferrite at a position of 1 / 4 of the sheet thickness from the surface thereof, A steel sheet having a volume fraction of the ferrite of 60% or more and 90% or less, an average crystal grain size of the ferrite of 3 μm or more and 15 μm or less, an average hardness of the hard structure of 250 HV0.01 or more and 350 HV0.01 or less, 60% or more of the hard structure being adjacent to the ferrite but not adjacent to the hard structure in volume fraction, and 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 if the element is not contained.

[0015] [2] The steel sheet according to [1], wherein the chemical composition contains, in mass%, one or more selected from 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.

[0016] [3] The steel plate according to [1] or [2], wherein the difference in average Vickers hardness of the hard structure 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 20 HV 0.01 or less.

[0017] [4] A steel material having a component composition containing, 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, and O: 0.0040% or less, which satisfies the following formula (1), with the balance being Fe and unavoidable impurities, is heated to a temperature of 1000°C or more and 1250°C or less, a cumulative reduction rate in the non-recrystallization temperature region of 30% to 70%; hot rolling is performed with a hot rolling finish temperature of 750°C or higher at a position one-quarter of the plate thickness from the surface of the steel plate; cooling is initiated at a cooling start temperature of 680°C to 900°C at a position one-quarter of the plate thickness from the surface of the steel plate; temperature T is defined in the range of 600°C to 750°C; at a position one-quarter of the plate thickness from the surface of the steel plate, an average cooling rate from the cooling start temperature to temperature T is 2.0°C / s to 10.0°C / s; and subsequently cooling is performed at an average cooling rate of 20°C / s to 100°C / s from temperature T to a cooling stop temperature of 300°C to 550°C. 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 not contained.

[0018] [5] The method for manufacturing a steel sheet according to [4], wherein the chemical composition contains, in mass%, one or more selected from 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.

[0019] According to the present invention, a high-strength steel plate can be obtained that 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 suitable for use in tanks used in low-temperature environments where the tanks are corrosive due to liquefied ammonia.

[0020] The steel sheet of the present invention contains, by mass%, C: 0.03% to 0.14%, Si: 0.10% to 0.50%, Mn: 0.70% to 1.70%, P: 0.030% to 0.14%, S: 0.0030% to 0.10%, Al: 0.010% to 0.100%, Ti: 0.010% to 0.030%, Mo: 0.02% to 0.10%, Ca: 0.0005% to 0.0030%, N: 0.0010% to 0.0070%, and O: 0.0040% or less, satisfying the following formula (1), with the balance being Fe and and unavoidable impurities, and a steel structure consisting of ferrite and hard structures other than the ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate, wherein 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 the hard structures 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, and the yield strength is 325 MPa to 440 MPa. 0.05≦2×Cr+Mo+V+W≦0.60 ...Equation (1) The element symbols in Equation (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 plate and the steel material used for its production have the above-mentioned elemental composition. Therefore, first, the reason for limiting the elemental composition of the steel plate in the present invention as described above will be explained. Unless otherwise specified, "%" in the composition of a component means "% by mass."

[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 viewpoint of reducing the content of other alloying elements and manufacturing at lower cost, the C content is preferably set to 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, resulting in a decrease in 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 result in a decrease in toughness. Therefore, 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 increases 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 content of other alloying elements and producing steel sheets at lower cost, the Mn content is preferably set to 0.90% or more, and more preferably set to 1.20% or more. On the other hand, if the Mn content exceeds 1.70%, not only will toughness and weldability decrease, but alloy costs will also become excessively high. Therefore, the Mn content is set to 1.70% or less. Furthermore, from the viewpoint of suppressing deterioration in toughness and weldability, the Mn content is preferably set to 1.60% or less, and more preferably set to 1.50% or less.

[0024] P: 0.030% or less P is an element contained as an unavoidable impurity, and by segregating at grain boundaries, it has adverse effects such as reducing toughness and weldability. Therefore, it is desirable to keep the P content as low as possible, but 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 usually an element that is inevitably contained in steel as an impurity, it may be industrially greater than 0%. Since excessive reduction of P leads to an increase in refining costs, the P content is preferably 0.0005% or more.

[0025] S: 0.0030% or less S is an element contained as an inevitable impurity, and exists in steel as sulfide-based inclusions such as MnS. It is an element that acts as a starting point for brittle cracks and reduces toughness. Therefore, it is desirable to reduce the S content as much as possible, and set it 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 it may be more than 0%. Since excessive reduction of S leads to an increase in refining costs, from a cost perspective, it is preferable to set the S content to 0.0005% or more.

[0026] Al: 0.010% or more and 0.100% or less Al acts as a deoxidizer and also has the effects of refining crystal grains and improving toughness. To achieve 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. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease 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 suppresses 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 suppressing the austenite coarsening effect. 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 coarsen, becoming brittle fracture initiation sites and reducing toughness. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably set to 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 the steel sheet. It also has a strong tendency to form fine carbides during heating in PWHT, and the presence of fine carbides in ferrite suppresses a decrease in ferrite hardness. 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 during PWHT, resulting in a decrease in YS and TS after PWHT. 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. The Mo content is 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 suppresses the formation of MnS and other elements that elongate in the rolling direction. Therefore, by including a predetermined amount of Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of the steel sheet. 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 will decrease. A decrease in cleanliness will lead 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, resulting in 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. Note that the N content is preferably set to 0.0060% or less.

[0031] O: 0.0040% or less O is an element contained as an unavoidable impurity, and forms oxides that act as initiation points for brittle cracks, reducing toughness. Therefore, the O content is limited to 0.0040% or less. The O content is 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 usually an element that is unavoidably contained in steel as an impurity, it may be industrially greater than 0%. In other words, excessive reduction leads to an increase in refining costs, so from a cost perspective, it is preferable to set the O content to 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 if not present, 0 is used. Cr is an element that dissolves in a hard structure in a structure consisting of ferrite and a hard structure, and suppresses a decrease in hardness of the hard structure during PWHT heating. Similarly to Mo, V and W are elements that dissolve in ferrite and suppress the hardness of ferrite during PWHT heating. The combination of Mo and the above elements suppresses a decrease in YS and TS after PWHT. To achieve the required mechanical properties after PWHT, it is essential that 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 due to PWHT, resulting in a significant decrease in YS and TS after PWHT. Therefore, 2×Cr+Mo+V+W is set to 0.05 or more. 2×Cr+Mo+V+W is preferably 0.10 or more, more preferably 0.20 or more. On the other hand, if 2×Cr+Mo+V+W exceeds 0.60, the ferrite and hard structure will be excessively hardened, TS will be excessively high, and toughness will be deteriorated. Therefore, 2×Cr+Mo+V+W is set to 0.60 or less. 2×Cr+Mo+V+W is preferably 0.50 or less, 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 a hard structure in a structure consisting of ferrite and a 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. The Cr content is more preferably 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 a hard structure, and improves the YS and TS of the steel sheet. Furthermore, V has a strong tendency to form carbides during heating in PWHT, and its presence in ferrite as fine carbides suppresses a decrease in the hardness of 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. The V content is more preferably 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 the steel sheet. W also has a strong tendency to form carbides during heating in PWHT, 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 contained 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, when 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, like Cu, has the effect of improving the strength of the steel sheet and can be contained arbitrarily. 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 cost. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is more preferably 0.50% or less.

[0039] Co: 1.00% or less Co, like Cu, is an element that has the effect of improving the strength of the steel sheet and can be contained arbitrarily. 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 cost. Therefore, when Co is contained, the Co content is set to 1.00% or less. The Co content is more preferably 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 the 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 is excessively refined, causing the YS to exceed 440 MPa. Therefore, the Nb content is set to 0.05% or less. The Nb content is more preferably 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 structure is adjacent to ferrite but not adjacent to a hard structure. The reasons for limiting the microstructure as above will be explained below.

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

[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 toughness decreases. Therefore, the average grain size of ferrite is 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 the predetermined range, and the occurrence of stress corrosion cracking due to ammonia cannot be prevented. Therefore, the average grain size of ferrite is 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more. The grain size of ferrite refers to the circle equivalent diameter, which is the diameter of a circle having the same area as one ferrite grain projected onto a two-dimensional plane. The circle equivalent diameter is calculated by performing image analysis on an image of the microstructure.

[0045] [Average hardness of hard structure is 250 HV0.01 or more and 350 HV0.01 or less] In the steel plate of the present invention, the average hardness of the hard structure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate is 250 HV0.01 or more and 350 HV0.01 or less. If the average hardness of the hard structure exceeds 350 HV0.01, TS exceeds the predetermined range and toughness decreases. Therefore, the average hardness of the hard structure is 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 structure is less than 250 HV0.01, TS does not reach the predetermined range. Therefore, the average hardness of the hard structure is 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] [60% or more by volume of hard structure is adjacent to ferrite but not adjacent to hard structure] Carbides are generated within the hard structure by PWHT. When hard structures are adjacent to each other, carbon diffuses between the hard structures, generating coarser carbides. This reduces the hardness of the hard structure and the TS of the steel plate. It also reduces toughness. When 60% or more by volume of hard structure is adjacent to ferrite but not adjacent to hard structure, coarsening of carbides is suppressed, and the decreases in YS and TS before and after PWHT are small, satisfying the required mechanical properties. On the other hand, when the volume fraction of hard structure adjacent to ferrite but not adjacent to hard structure is less than 60%, the decreases in YS and TS due to PWHT are significant, and the required 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 hard structure. Therefore, the volume fraction of the hard tissue adjacent to ferrite and not adjacent to the hard tissue is 60% or more, preferably 70% or more, and more preferably 80% or more. The upper limit of the volume fraction of the hard tissue adjacent to ferrite and not adjacent to the hard tissue is not particularly limited, and may be 100%, or the volume fraction of the hard tissue adjacent to ferrite and not adjacent to the hard tissue 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: 20 HV0.01 or less] PWHT generates carbides within hard structures other than ferrite. When C dissolved in the hard structure becomes carbide, the strain formed within the hard structure due to elemental solid solution disappears, reducing the hardness of the hard structure and the TS of the steel plate. If the difference in Vickers hardness (pre-heating hardness minus post-heating hardness) before and after heating at 630°C for 10 hours is 20 HV0.01 or less, the decrease in TS before and after PWHT is small, satisfying the required mechanical properties. 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 large, and even if the required mechanical properties are satisfied before PWHT, the TS after PWHT is insufficient. Alternatively, if the TS is satisfied after PWHT, the TS becomes excessively high before PWHT. 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 plate 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 plate 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 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 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 calculated 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)." 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 manufacturing a steel sheet according to the present invention will be described. A steel material having the above-described chemical composition is heated and hot-rolled to obtain a hot-rolled steel sheet, which is then cooled under the conditions described below to obtain a steel sheet. Each manufacturing condition will be described 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. This hot rolling involves heating to a temperature of 1000°C to 1250°C, followed by a cumulative reduction of 30% to 70% in the austenite non-recrystallization temperature range, and finishing at 750°C or higher at a position ¼ of the plate thickness from the surface of the steel plate. Next, cooling is started at a cooling start temperature of 680°C to 900°C at a position ¼ of the plate thickness from the surface of the steel plate, and cooling is performed at a temperature T defined as the range of 600°C to 750°C at an average cooling rate of 2.0°C / s to 10.0°C / s from the cooling start temperature to T at a position ¼ of the plate thickness from the surface of the steel plate, and at an average cooling rate of 20°C / s to 100°C / s from temperature T to a cooling stop temperature of 300°C to 550°C.

[0051] (a) Heating temperature of steel material: 1000°C or higher and 1250°C or lower If the heating temperature of the steel material is lower 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 is higher than 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 region: 30% or more and 70% or less Non-recrystallization temperature region (in the present invention, the steel material is Ar ３ If the cumulative reduction rate in the non-recrystallization temperature region (meaning a temperature region below the transformation point + 150°C) is less than 30%, austenite will not be sufficiently worked during hot rolling. If austenite is not sufficiently worked, there will be fewer ferrite nucleation sites during the cooling process described below, the ferrite will coarsen, and the average grain size will exceed 15 μm. As a result, the YS will not reach the specified range and the toughness will decrease. On the other hand, if the cumulative reduction rate in the non-recrystallization temperature region exceeds 70%, there will be an excess of ferrite nucleation sites during the cooling process, the average ferrite grain size will be less than 3 μm, and the YS will exceed the specified range. Therefore, the cumulative reduction rate in the non-recrystallization temperature region is specified to be 30% or more and 70% or less. The cumulative reduction rate in the non-recrystallization temperature region is preferably 40% or more, and more preferably 50% or more. Furthermore, the cumulative reduction rate in the non-recrystallization temperature region is preferably 67% or less, and more preferably 60% or less.

[0053] Here, Ar ３ The transformation point (°C) can be calculated, for example, by the following formula: Ar ３ Transformation point (°C) = 910 - 273 x C - 74 x Mn - 57 x Ni - 16 x Cr - 9 x Mo - 5 x Cu where each element symbol indicates the content (mass%) of the element in the steel.

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

[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 is generated before the start of cooling, and the volume fraction of ferrite after cooling is completed exceeds 90%. If the ferrite volume fraction exceeds 90%, the hard structure is insufficient and the specified YS and TS cannot 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 ferrite volume fraction becomes less than 60%, TS and YS exceed the specified range, and toughness decreases. Therefore, the cooling start temperature is set to 900°C or lower. The cooling start temperature is preferably 830°C or lower.

[0056] (e) Average cooling rate (first average cooling rate) from the cooling start temperature to a temperature T defined as a range of 600°C to 750°C at a position from the surface of the steel plate to 1 / 4 of the plate thickness: 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 to a temperature T defined as a range of 600°C to 750°C at a position from the surface of the steel plate to 1 / 4 of the plate thickness is 2.0°C / s to 10.0°C / s. Before the start of cooling or during the cooling process, ferrite is generated from the grain boundaries of austenite, and the distribution state of the hard structure and ferrite is determined by cooling to temperature T. When temperature T exceeds 750°C or when the first average cooling rate at a position from the surface of the steel plate to 1 / 4 of the plate thickness exceeds 10.0°C / s, ferrite generation from the austenite grain boundaries is insufficient, and a structure in which hard structures are adjacent to each other is formed. Specifically, the volume fraction of the hard structure surrounded by ferrite becomes less than 60% of the entire 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 within the temperature range is preferably 4°C / s or more. Furthermore, the first average cooling rate within the 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 the cooling start temperature to temperature T).

[0057] (f) Average cooling rate (second 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: 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 above temperature range exceeds 100°C / s, the ferrite volume fraction becomes less than 60%, and YS and TS exceed the specified range. Furthermore, the formation of a large amount of island martensite increases the hardness of the hard structure and reduces 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. The second average cooling rate in the temperature range is preferably 70°C / s or less, where the second average cooling rate (°C / s) is calculated by (temperature T - cooling stop temperature) (°C) / (cooling time (s) from temperature T to cooling stop temperature).

[0058] (g) Cooling stop temperature at a position 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 300°C or higher and 550°C or lower. If the cooling stop temperature exceeds 550°C, the hardness of the hard structure becomes less than 250HV0.01, and TS decreases. On the other hand, if the cooling stop temperature is lower than 300°C, the hardness of the hard structure becomes excessive, and in addition to increasing TS, toughness decreases. The cooling stop temperature is 350°C or higher. Furthermore, the cooling stop temperature is preferably 500°C or lower.

[0059] By manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing conditions, a steel plate (steel plate according to the present invention) having the above-described structure can be obtained. The steel plate 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 a yield point exists, or 0.2% proof stress when a yield point does not exist) is 325 MPa or more and 440 MPa or less, and the TS is 440 MPa or more and 610 MPa or less. Furthermore, the IMO Gas Code and classification rules of the International Maritime Organization stipulate that the yield point of a steel plate should 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 plate has excellent ammonia stress corrosion cracking resistance.

[0060] Basically, the higher the TS of a steel sheet, the better. However, a steel sheet with a TS exceeding 610 MPa is 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 in order 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 plate 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.

[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 the temperature was maintained for 10 hours. The influence of the heat treatment temperature and the holding time at the heat treatment temperature on the mechanical properties of the steel material is expressed by the following tempering parameter P: P = T{log(t) + 20} x 10 －３ Here, T is the heat treatment temperature (K), and t is the holding time (hours) at the heat treatment temperature. The tempering parameter P in the heat treatment simulating PWHT was 18.963. Since the smaller the P, the smaller the effect on mechanical properties. Therefore, the inventive examples in the present application examples are expected to exhibit the same or better effects 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 of the steel sheet, and evaluation of the tensile properties and toughness. 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 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, each structure was distinguished as follows. Ferrite was defined as a structure that did not contain isotropically grown carbides and was surrounded by white lines. Pearlite was defined as a structure that was blocky or flat and had a striped pattern of dark ferrite and white carbides. Bainite was defined as a structure that had an elongated, lath-shaped ferrite structure and contained carbides with an equivalent circle diameter of 0.05 μm or more. Martensite was defined as a structure that had an elongated, lath-shaped ferrite structure similar to bainite and did not contain carbides with an equivalent circle diameter of 0.05 μm or more. Carbides were defined as appearing as white dots. Furthermore, austenite was defined as a structure that was present between the lath-shaped ferrite structures of bainite or martensite and did not contain carbides with an equivalent circle diameter of 0.50 μm or more.

[0068] The equivalent circle 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). The number of pixels was then converted to actual length, and the equivalent circle diameter was calculated. Insufficient resolution of the image being analyzed or noise due to lens focus during imaging can lead to incorrectly determining a single crystal grain, resulting in an incorrect calculation that suggests the presence of a large number of tiny ferrite particles. Therefore, the particle size distribution was obtained by randomly selecting more than 4,000 ferrite particles with a size that could be visually determined to be crystals, i.e., a circle equivalent diameter of 2 μm or more. The average crystal grain size of ferrite was calculated by dividing the sum of these particle sizes (equivalent circle diameter) by the number of particles.

[0069] The volume fraction of the hard structure adjacent only to ferrite was determined by calculating the ratio of the area of ​​the microstructure adjacent to the hard structure determined to be only ferrite to the total area of ​​the hard structure confirmed in the image taken by 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 (average hardness) of hard structure] 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 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) refers to "(average Vickers hardness before heating at 630 ° C for 10 hours) - (average Vickers hardness after heating at 630 ° C for 10 hours)".

[0071] [Strength Properties] Tensile test specimens according 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. Since it is necessary to minimize the risk of ammonia stress corrosion cracking as a structural component of a liquefied gas bulk carrier, the IMO Gas Code and classification rules stipulate that the yield point of a steel plate be 440 MPa or less. For this reason, 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 according to JIS Z2242 (2023) were taken from a portion 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 coincided. Then, a Charpy impact test was performed according to the procedure of 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 also shown in Table 2.

[0073]

[0074]

[0075] As can be seen from Tables 1 and 2, all of the invention examples 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-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 sheets 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 at least one of yield strength YS, tensile strength TS, and toughness is inferior to the invention examples. Steel sheet No. 6, which corresponds to a comparative example, has a rolling finish temperature outside the range of the present invention, and has yield strength YS and toughness inferior to the invention examples. Steel sheets Nos. 10 and 12 have a volume fraction of hard structures adjacent only to ferrite outside the range of the present invention, and exhibit a large change in hardness before and after PWHT. Steel sheets Nos. 15 and 16 have average hardnesses outside the range of the present invention, and have inferior TS compared to the invention examples, and No. 15 also exhibits inferior toughness. Steel sheet No. 10 and 12 have a volume fraction of hard structures adjacent only to ferrite outside the range of the present invention, and exhibit a large change in hardness before and after PWHT. Steel sheets Nos. 15 and 16 also have average hardnesses outside the range of the present invention, and have inferior TS compared to the invention examples. 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. A steel sheet comprising, 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, the steel sheet satisfies the following formula (1), with the balance being Fe and unavoidable impurities; and the steel sheet has a steel structure consisting of ferrite and a hard structure other than the ferrite at a position 1 / 4 of the sheet thickness from the surface thereof, A steel sheet having a volume fraction of the ferrite of 60% or more and 90% or less, an average crystal grain size of the ferrite of 3 μm or more and 15 μm or less, an average hardness of the hard structure of 250 HV0.01 or more and 350 HV0.01 or less, 60% or more of the hard structure being adjacent to the ferrite but not adjacent to the hard structure in volume fraction, and 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 if the element is not contained.

2. The steel plate according to claim 1, wherein the chemical composition contains, in mass %, one or more elements selected from the following: 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.

3. A steel plate according to claim 1 or claim 2, wherein the difference in average Vickers hardness of the hard structure 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.

4. A steel material containing, 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, the steel material having a composition that satisfies the following formula (1), the balance being Fe and unavoidable impurities, is heated to a temperature of 1000°C or more and 1250°C or less, a cumulative reduction rate in the non-recrystallization temperature region of 30% to 70%; hot rolling is performed with a hot rolling finish temperature of 750°C or higher at a position one-quarter of the plate thickness from the surface of the steel plate; cooling is initiated at a cooling start temperature of 680°C to 900°C at a position one-quarter of the plate thickness from the surface of the steel plate; temperature T is defined in the range of 600°C to 750°C; an average cooling rate from the cooling start temperature to temperature T at a position one-quarter of the plate thickness from the surface of the steel plate is 2.0°C / s to 10.0°C / s; and subsequently cooling is performed at an average cooling rate of 20°C / s to 100°C / s from temperature T to a cooling stop temperature of 300°C to 550°C. 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.

5. A method for manufacturing a steel plate according to claim 4, wherein the chemical composition contains, in mass %, one or more selected from the following: 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.

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