Steel sheet and method for manufacturing the same

Abstract

The present invention provides a steel sheet having excellent resistance to ammonia stress corrosion cracking, excellent low-temperature toughness, small change in mechanical properties caused by PWHT treatment, and high strength, and a method for manufacturing the same. The steel sheet contains C, Si, Mn, P, S, Al, Ti, Mo, Ca, N, and O in specified amounts, satisfies 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60, and the remainder is composed of Fe and inevitable impurities, and has a steel structure composed of ferrite and hard structures other than the ferrite at a position 1 / 4 of the thickness from the surface of the steel sheet, a volume fraction of the ferrite is 60% to 90%, an average crystal grain size of the ferrite is 3 μm to 15 μm, an average hardness of the hard structures is 250 HV0.01 to 350 HV0.01, 60% or more of the hard structures are adjacent to the ferrite in terms of volume fraction, the yield strength is 325 MPa to 440 MPa.

Description

Technical Field

[0001] This invention relates to a steel plate with excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, and a method for manufacturing the same, for use in tanks for loading liquefied petroleum gas (hereinafter referred to as LPG) or liquefied ammonia. Background Technology

[0002] To achieve a low-carbon society, ammonia, which does not emit carbon dioxide when burned, has attracted attention as a new energy source. Additionally, hydrogen also does not emit carbon dioxide when burned, and is therefore considered a candidate for clean energy; ammonia also holds promise as a stable form for transporting hydrogen.

[0003] Here, the international transport of liquefied ammonia is primarily envisioned to be carried out by ships. Furthermore, in the medium term, there is still demand for fossil fuels as an energy source, therefore it is anticipated that there will be switching or mixed transport of cargoes such as LPG, liquefied butane, dimethyl ether, and butene with liquefied ammonia.

[0004] LPG and ammonia are liquefied gases stored in tanks at low temperatures, thus requiring steel with excellent low-temperature toughness. Furthermore, liquefied ammonia is known to cause stress corrosion cracking in steel materials.

[0005] Therefore, from the perspective of ensuring tank safety, it is necessary to take measures to suppress stress corrosion cracking caused by liquefied ammonia. To suppress this stress corrosion cracking, the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) stipulates that the yield strength (YS) of carbon manganese steel used for tanks should be below 440 MPa. Furthermore, it stipulates that tanks for pressurized ammonia liquefaction undergo post-weld heat treatment (PWHT) during the manufacturing process.

[0006] On the other hand, in order to improve transportation efficiency, there is also a desire to increase the size of tanks, and there is an urgent need for the applicable steel to have high tensile strength (hereinafter referred to as TS).

[0007] Specifically, steel plates used in LPG and liquefied ammonia tanks are required to have toughness with a ductile-brittle section transition temperature (vTrs) below -60°C, and mechanical properties of YS of 325–440 MPa and TS of 440–560 MPa. In addition, there is also a demand for steel plates with the same toughness but with YS of 355–440 MPa and TS of 490–610 MPa.

[0008] Patent documents 1-3 describe technologies that, as described above, possess the low-temperature toughness required for liquefied gas tanks and meet YS restrictions and high TS requirements.

[0009] The technology described in Patent Document 1 controls the morphology of ferrite through hot rolling and accelerated cooling, thereby suppressing the yield strength ratio of the steel plate.

[0010] In addition, the technology described in Patent Document 2 controls the grain size distribution of ferrite through hot rolling and multiple accelerated cooling, thereby achieving a low yield ratio in the steel plate.

[0011] In addition, Patent Document 3 discloses a technique that distributes hard microstructure through hot rolling and accelerated cooling, thereby creating a hardness difference with ferrite and reducing the yield ratio.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2006-89830

[0015] Patent Document 2: Japanese Patent Application Publication No. 2019-214752

[0016] Patent Document 3: Japanese Patent Application Publication No. 2010-90406 Summary of the Invention

[0017] As mentioned above, PWHT (Potentially Heat-Treated Steel) is sometimes performed during the construction of liquefied ammonia tanks. PWHT is a method of eliminating residual stresses introduced by welding by heating the steel sheet to a temperature below its phase transformation point and holding it for a certain period of time. Generally, it is known that if the steel sheet is heat-treated, its mechanical properties will change compared to when it was manufactured. Specifically, YS and TS will decrease, or vTrs will increase, leading to a deterioration in toughness. In particular, the mechanical properties of steel sheets subjected to accelerated cooling change significantly. If the steel sheet cannot meet the required mechanical properties due to PWHT, it will be difficult to meet the predetermined strength required for the tank structure, and sometimes it may even be unusable.

[0018] The purpose of this invention is to solve the above-mentioned problems and provide a steel plate and its manufacturing method, which is intended for use in tanks of liquefied gas transport ships or for liquefied gas storage, ensuring the specified YS, thereby exhibiting excellent resistance to ammonia stress corrosion cracking, excellent low-temperature toughness, minimal change in mechanical properties after PWHT treatment, and high strength.

[0019] Here, "excellent resistance to ammonia stress corrosion cracking" means that, according to the test method specified in JIS Z 2241 (2022), the yield strength YS is 325MPa~440MPa.

[0020] In addition, "high strength" means that the tensile strength TS is 440MPa to 610MPa according to the test method specified in JIS Z 2241 (2022).

[0021] In addition, "excellent low-temperature toughness" means that, according to the Charpy impact test specified in JIS Z 2242 (2023), the brittle-ductile section transition temperature vTrs is below -60℃.

[0022] In addition, "small change in mechanical properties after PWHT treatment" means that the average Vickers hardness difference of the hard tissue other than ferrite at a position 1 / 4 of the thickness from the surface of the steel plate before and after heating at 630℃ for 10 hours is less than 20HV0.01.

[0023] To achieve the above objectives, the inventors investigated a method for improving TS while maintaining the low-temperature toughness of the steel plate, ensuring that YS does not exceed 440 MPa, and suppressing changes in mechanical properties caused by heat treatment. The results showed that the following methods are effective: containing elements such as C, Si, Mn, Ti, Cr, Mo, V, and W within a specified range; controlling 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 a specified range; and controlling the average hardness of the hard microstructure and the microstructure adjacent to the hard microstructure within a specified range.

[0024] This invention was completed based on this insight and further research.

[0025] That is, the main idea of ​​this invention is as follows.

[0026] [1] A steel plate having the following composition:

[0027] It contains, by mass%, C: 0.03%–0.14%, Si: 0.10%–0.50%, Mn: 0.70%–1.70%, P: less than 0.030%, S: less than 0.0030%, Al: 0.010%–0.100%, Ti: 0.010%–0.030%, Mo: 0.02%–0.10%, Ca: 0.0005%–0.0030%, N: 0.0010%–0.0070%, and O: less than 0.0040%, and satisfies the following formula (1), with the remainder consisting of Fe and unavoidable impurities.

[0028] Furthermore, at a position one-quarter of the thickness of the steel plate from the surface, there is a steel microstructure consisting of ferrite and other hard microstructures besides ferrite.

[0029] The volume fraction of ferrite mentioned above is 60%–90%.

[0030] The average crystal grain size of the aforementioned ferrite is 3 μm to 15 μm.

[0031] The average hardness of the aforementioned hard tissue is 250 HV0.01 to 350 HV0.01.

[0032] By volume fraction, more than 60% of the aforementioned hard tissue is adjacent to ferrite and not adjacent to hard tissue.

[0033] The yield strength is 325 MPa to 440 MPa.

[0034] 0.05≤2×Cr+Mo+V+W≤0.60...Equation (1)

[0035] The element symbols in equation (1) represent the content (mass%) of each component, and are set to 0 when they do not contain any.

[0036] [2] The steel plate according to [1] above, wherein the composition contains, by mass%, one or more of 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.

[0037] [3] According to the steel plate described in [1] or [2] above, the difference in average Vickers hardness of the hard structure at a position 1 / 4 of the thickness of the steel plate surface before and after heating at 630°C for 10 hours is less than 20HV0.01.

[0038] [4] A method for manufacturing a steel plate, wherein,

[0039] A steel billet with the following composition is heated to a temperature of 1000℃~1250℃.

[0040] Next, hot rolling is performed with the cumulative reduction rate in the non-recrystallization temperature region set to 30%–70%, and the hot rolling end temperature at 1 / 4 of the plate thickness from the surface of the steel plate set to above 750°C.

[0041] Cooling begins at a starting temperature of 680°C to 900°C, located at a point 1 / 4 of the plate thickness from the surface of the steel plate.

[0042] Temperature T is defined as the range of 600℃ to 750℃.

[0043] At a position 1 / 4 of the plate thickness away from the surface of the steel plate, the average cooling rate from the aforementioned cooling start temperature to the aforementioned temperature T is set to 2.0℃ / s to 10.0℃ / s.

[0044] Next, the average cooling rate from the above temperature T to the cooling stop temperature of 300℃~550℃ is set to 20℃ / s~100℃ / s, and cooling is carried out.

[0045] The above composition, by mass%, contains C: 0.03%–0.14%, Si: 0.10%–0.50%, Mn: 0.70%–1.70%, P: less than 0.030%, S: less than 0.0030%, Al: 0.010%–0.100%, Ti: 0.010%–0.030%, Mo: 0.02%–0.10%, Ca: 0.0005%–0.0030%, N: less than 0.0010%–0.0070%, and O: less than 0.0040%, and satisfies the following formula (1). The remaining portion consists of Fe and unavoidable impurities.

[0046] 0.05≤2×Cr+Mo+V+W≤0.60...Equation (1)

[0047] The element symbols in equation (1) represent the content (mass%) of each component, and are set to 0 when they do not contain any.

[0048] [5] According to the steel plate manufacturing method described in [4] above, the above-mentioned composition contains, by mass %, one or more of 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.

[0049] According to the present invention, a high-strength steel plate with minimal changes in mechanical properties after PWHT treatment, excellent low-temperature toughness, and resistance to ammonia stress corrosion cracking can be obtained.

[0050] The steel plate of the present invention is suitable for use in tanks in environments with low temperatures and corrosive atmospheres caused by liquefied ammonia. Detailed Implementation

[0051] Next, the steel plate of the present invention will be described in detail.

[0052] The steel plate of the present invention has the following composition: by mass % C: 0.03% to 0.14%, Si: 0.10% to 0.50%, Mn: 0.70% to 1.70%, P: less than 0.030%, S: less than 0.0030%, 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%, O: less than 0.0040%. And satisfy the following formula (1), the remaining part is composed of Fe and unavoidable impurities; at a position 1 / 4 of the thickness of the steel plate from the surface, there is a steel structure composed of ferrite and hard structure other than ferrite, the volume fraction of ferrite is 60% to 90%, the average crystal grain size of ferrite is 3μm to 15μm, the average hardness of hard structure is 250HV0.01 to 350HV0.01, in terms of volume fraction, more than 60% of hard structure is adjacent to ferrite and not adjacent to hard structure, and the yield strength is 325MPa to 440MPa.

[0053] 0.05≤2×Cr+Mo+V+W≤0.60...Equation (1)

[0054] The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 when they do not contain any element.

[0055] In this invention, it is important that the steel plate and the steel billet used for manufacturing therefrom have the above-described composition. Therefore, the reasons for limiting the composition of the steel plate in this invention to the above-described manner will be explained first. It should be noted that, unless otherwise specified, the "%" in the composition refers to "mass %".

[0056] [Composition]

[0057] C: 0.03%~0.14%

[0058] Carbon (C) is an element that increases the hardenability of steel and is one of the essential elements required to achieve high tensile strength (TS). To obtain the above 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 a lower cost, the C content is preferably 0.05% or more.

[0059] On the other hand, if the carbon content exceeds 0.14%, toughness and weldability decrease. Therefore, the carbon content is set to 0.14% or less. From the viewpoint of suppressing the decrease in toughness and weldability, the carbon content is preferably 0.10% or less.

[0060] Si: 0.10%~0.50%

[0061] Si is an element that acts as a deoxidizer. When the Si content is less than 0.10%, more oxides are formed in the steel, resulting in reduced toughness. Therefore, the Si content is set to be above 0.10%.

[0062] On the other hand, Si is an element that reduces toughness and weldability, so the Si content is set to 0.50% or less. In particular, if the Si content exceeds 0.30%, island-like martensite may form in the HAZ formed during high heat input welding, which will degrade the toughness. Therefore, the Si content is preferably 0.30% or less.

[0063] Mn: 0.70%~1.70%

[0064] Mn is an element that increases the hardenability of steel and is one of the important elements required to achieve high tensile strength (TS). To obtain the above effect, the Mn content is set to 0.70% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing steel sheets at a lower cost, the Mn content is preferably 0.90% or more, and more preferably 1.20% or more.

[0065] On the other hand, if the Mn content exceeds 1.70%, not only will toughness and weldability decrease, but the alloy cost will also increase excessively. Therefore, the Mn content is set to 1.70% or less. From the viewpoint of suppressing the decrease in toughness and weldability, the Mn content is preferably 1.60% or less, and more preferably 1.50% or less.

[0066] P: below 0.030%

[0067] P is an element present as an unavoidable impurity, and its segregation at grain boundaries can cause adverse effects such as reduced toughness and weldability. Therefore, it is preferable to minimize the P content as much as possible, but it is acceptable as long as it is below 0.030%.

[0068] It should be noted that there is no specific lower limit for the phosphorus (P) content; it can be 0%. However, since P is usually an element that is inevitably included in steel as an impurity, it can exceed 0% in industrial applications. Excessively reducing P will lead to a significant increase in refining costs; therefore, the P content is preferably 0.0005% or higher.

[0069] S: below 0.0030%

[0070] Sulfur (S) is an unavoidable impurity element in steel, existing as sulfide inclusions such as MnS. It serves as the initiation point for brittle cracks and reduces toughness. Therefore, it is preferable to minimize the S content, setting it below 0.0030%.

[0071] It should be noted that there is no specific lower limit for the sulfur (S) content; it can be 0%. Generally, S is an element that is unavoidably included in steel as an impurity, therefore, industrially it can be above 0%. Excessively reducing S content will lead to a significant increase in refining costs; therefore, from a cost perspective, the S content is preferably above 0.0005%.

[0072] Al: 0.010%~0.100%

[0073] Al not only acts as a deoxidizer, but also refines grains and improves toughness. To achieve these effects, the Al content is set to 0.010% or more. To obtain these effects to a greater extent, the Al content is preferably 0.020% or more.

[0074] On the other hand, if the Al content exceeds 0.100%, oxide inclusions increase, reducing the cleanliness and toughness of the steel sheet. Reduced cleanliness leads to increased surface defects, resulting in deteriorated surface properties and decreased bending workability. Therefore, the Al content is set to 0.100% or less. It should be noted that the Al content is preferably 0.060% or less.

[0075] Ti: 0.010%~0.030%

[0076] Ti has a strong tendency to form nitrides and precipitates as TiN during solidification. It suppresses austenite coarsening during the billet heating process or acts as a ferrite transformation nucleus, thus contributing to improved γS content in steel sheets. 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 enhancing the austenite coarsening suppression effect; therefore, the Ti content is preferably 0.012% or more.

[0077] On the other hand, if the Ti content exceeds 0.030%, the TiN particles become coarse, becoming the starting point for brittle fracture and reducing toughness. Therefore, the Ti content is set to 0.030% or less. It should be noted that the Ti content is preferably 0.020% or less.

[0078] Mo: 0.02%~0.10%

[0079] Mo is an element dissolved in ferrite within a microstructure composed of ferrite and hard materials, and it contributes to improving the chromatic alumina (YS) and chromatic saturation (TS) of steel sheets. Furthermore, it exhibits a strong tendency to form fine carbides during the PWHT heating process. These fine carbides, present within the ferrite, suppress the decrease in ferrite hardness. This, in turn, inhibits the reduction in YS and TS after PWHT. To achieve this effect, the Mo content is set at 0.02%–0.10%.

[0080] When the Mo content is below 0.02%, PWHT will reduce the hardness of ferrite, and the YS and TS will decrease after PWHT. The preferred Mo content is 0.04% or higher.

[0081] On the other hand, if the Mo content exceeds 0.10%, the YS content becomes too high. Therefore, the Mo content is set to 0.10% or less. The Mo content is preferably 0.07% or less.

[0082] Ca: 0.0005%~0.0030%

[0083] Ca is an element that combines with S and inhibits the formation of MnS and other inclusions that extend along the rolling direction. Therefore, by including a specified amount of Ca, morphology control can be achieved to make sulfide inclusions spherical, thereby improving the toughness of the steel plate.

[0084] When the Ca content is less than 0.0005%, the steel plate has poor toughness. Therefore, the Ca content is set to 0.0005% or more. Furthermore, the Ca content is more preferably 0.0015% or more.

[0085] On the other hand, if the Ca content exceeds 0.0030%, the cleanliness of the steel decreases. Decreased cleanliness leads to increased surface defects, resulting in deteriorated surface properties and reduced bending workability. Therefore, the Ca content is set to 0.0030% or less. Preferably, the Ca content is 0.0025% or less.

[0086] N: 0.0010%~0.0070%

[0087] Ni combines with Ti to precipitate as TiN, which helps refine the microstructure and improve YS and toughness. To achieve this effect, the Ni content is set to 0.0010% or more. The Ni content is preferably 0.0030% or more, and more preferably 0.0050% or more.

[0088] On the other hand, if the nitrogen content exceeds 0.0070%, the amount of dissolved nitrogen increases, leading to a decrease in toughness. Furthermore, if the nitrogen content exceeds 0.0070%, weldability decreases. Therefore, from the viewpoint of suppressing the decrease in toughness and weldability, the nitrogen content is set to 0.0070% or less. It should be noted that the nitrogen content is preferably 0.0060% or less.

[0089] O: Below 0.0040%

[0090] O is an element that is unavoidably present as an impurity. It forms oxides, becomes the starting point for brittle cracks, and reduces toughness. Therefore, the O content is limited to 0.0040% or less. The preferred O content is 0.0030% or less.

[0091] On the other hand, there is no particular lower limit for the O content, which can be 0%. However, since O is usually an element that is inevitably included in steel as an impurity, it can be above 0% in industrial applications. That is, excessive reduction will lead to a significant increase in refining costs. Therefore, from a cost point of view, the O content is preferably 0.0020% or higher.

[0092] 0.05≤2×Cr+Mo+V+W≤0.60...Equation (1)

[0093] The element symbols in formula (1) represent the content (mass%) of each element, and are set to 0 when they do not contain any element.

[0094] Cr is an element that is dissolved in the hard tissue within the structure composed of ferrite and hard tissue, and it suppresses the decrease in hardness of the hard tissue during PWHT heating. Additionally, V, W, and Mo are also elements that are dissolved in ferrite, and they suppress the decrease in ferrite hardness during PWHT heating. The combination of Mo with the above elements can suppress the decrease in YS and TS after PWHT. To meet the specified mechanical properties after PWHT, it is important that 0.05 ≤ 2×Cr+Mo+V+W ≤ 0.60. If 2×Cr+Mo+V+W is less than 0.05, PWHT will decrease the hardness of both ferrite and hard tissue, and the YS and TS after PWHT will decrease significantly. Therefore, 2×Cr+Mo+V+W is set to 0.05 or more. Preferably, 2×Cr+Mo+V+W is 0.10 or more, and more preferably 0.20 or more.

[0095] On the other hand, if 2×Cr+Mo+V+W exceeds 0.60, the ferrite and hard tissue become excessively hardened, the total ferrite strength (TS) becomes too high, and the toughness deteriorates. Therefore, 2×Cr+Mo+V+W is set to 0.60 or less. Preferably, 2×Cr+Mo+V+W is 0.50 or less, and more preferably 0.30 or less.

[0096] The basic composition of this invention comprises the above-mentioned components, with the remainder consisting of Fe and unavoidable impurities. This composition may, in order to further improve strength properties or toughness, optionally contain one or more components 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.

[0097] Cr: less than 0.30%

[0098] Cr is an element that is dissolved in the hard tissue within a structure composed of ferrite and hard tissue, and it inhibits the decrease in hardness of the hard tissue during the heating of PWHT. The Cr content is preferably 0.05% or more.

[0099] On the other hand, if the Cr content exceeds 0.30%, it can sometimes lead to a deterioration in toughness. Therefore, in cases where Cr is present, the Cr content is set to 0.30% or less. More preferably, the Cr content is 0.20% or less.

[0100] V: Below 0.10%

[0101] V is an element dissolved in ferrite within a microstructure composed of ferrite and hard materials, and it contributes to improving the YS and TS of steel sheets. Furthermore, it has a strong tendency to form carbides during PWHT heating, and its presence as fine carbides within ferrite helps suppress the decrease in ferrite hardness. The V content is preferably 0.03% or higher.

[0102] On the other hand, if the V content exceeds 0.10%, the YS content may become too high. Therefore, when V is present, the V content is set to 0.10% or less. More preferably, the V content is 0.05% or less.

[0103] W: Below 0.10%

[0104] W is an element dissolved in ferrite within a microstructure composed of ferrite and hard materials, and it is responsible for improving the YS and TS of steel sheets. Furthermore, it has a strong tendency to form carbides during the heating of PWHT, and its presence as fine carbides within ferrite can suppress the decrease in ferrite hardness. The preferred W content is 0.02% or more.

[0105] On the other hand, if the W content exceeds 0.10%, high-hardness carbides may sometimes be generated in large quantities, leading to excessively high total strength (TS). Therefore, when W is present, the W content is set to 0.10% or less. More preferably, the W content is 0.04% or less.

[0106] Cu: below 1.00%

[0107] Cu is an element that increases the hardenability of steel, thereby improving the strength of steel plates, and can be contained in any quantity. The preferred Cu content is 0.15% or more.

[0108] On the other hand, if the Cu content exceeds 1.00%, it can sometimes lead to a deterioration in toughness and an increase in alloy costs. Therefore, in the case of Cu, the Cu content is set to 1.00% or less. More preferably, the Cu content is 0.50% or less.

[0109] Ni: below 1.00%

[0110] Like Cu, Ni is an element that can improve the strength of steel plates and can be included in any quantity. The preferred Ni content is 0.15% or more.

[0111] On the other hand, if the Ni content exceeds 1.00%, it can sometimes lead to deterioration in weldability and increased alloy costs. Therefore, in the case of Ni, the Ni content is set to 1.00% or less. More preferably, the Ni content is 0.50% or less.

[0112] Co: below 1.00%

[0113] Co, like Cu, is an element that can improve the strength of steel plates and can be included in any quantity. The preferred Co content is 0.20% or more.

[0114] On the other hand, if the Co content exceeds 1.00%, it can sometimes lead to deterioration of weldability and increased alloy costs. Therefore, in the case of Co content, the Co content is set to 1.00% or less. More preferably, the Co content is 0.60% or less.

[0115] Nb: below 0.05%

[0116] Nb refines the original austenite grain size by precipitating as carbonitrides, which helps to refine the microstructure brought about by hot rolling and is an element that improves YS and TS. The Nb content is preferably 0.020% or more.

[0117] On the other hand, if the Nb content exceeds 0.05%, the YS will exceed 440 MPa due to excessive tissue refinement. Therefore, the Nb content is set to 0.05% or less. More preferably, the Nb content is 0.030% or less.

[0118] In addition to the above-described composition, the steel plate of the present invention also has a microstructure at a position 1 / 4 of the plate thickness from the surface of the steel plate, comprising a ferrite volume fraction of 60% to 90%, an average ferrite grain size of 3 μm to 15 μm, an average hardness of 250 HV0.01 to 350 HV0.01 for the hard tissue other than ferrite, and wherein at least 60% of the hard tissue is adjacent to the ferrite but not to the hard tissue by volume fraction. The reasons for limiting the microstructure to the above-described manner will be explained below.

[0119] [Microstructure]

[0120] The microstructure of the steel plate of the present invention will be described.

[0121] [At a position one-quarter of the plate thickness from the surface, the volume fraction of ferrite is 60%–90%.]

[0122] In the steel plate of the present invention, the volume fraction of ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate is 60% to 90%. If the volume fraction of ferrite exceeds 90%, the hard structure is insufficient, and the specified TS and YS cannot be obtained.

[0123] On the other hand, if the ferrite volume fraction is less than 60%, the remaining microstructure will contain more hard structures such as bainite and martensite, and the TS and YS content will exceed the specified range, resulting in reduced toughness. The preferred ferrite volume fraction is 60% to 70%.

[0124] The remaining microstructure may contain a mixture of bainite, pearlite, austenite, martensite, and other microstructures as a hard microstructure. The volume fraction of each microstructure in the remaining microstructure is not particularly limited, but from a toughness perspective, the volume fraction of pearlite or bainite is preferably second only to ferrite. It should be noted that the volume fraction of various microstructures can be determined according to the methods described in the examples below.

[0125] [The average crystal grain size of ferrite is 3 μm to 15 μm]

[0126] In the steel plate of the present invention, the average ferrite grain size at a position 1 / 4 of the plate thickness from the surface of the steel plate is 3 μm to 15 μm. If the average ferrite grain size exceeds 15 μm, the YS (Yellow Strength) will not reach the specified range, and the toughness will decrease. Therefore, the average ferrite grain size is set to 15 μm or less, preferably 12 μm or less, and more preferably 9 μm or less.

[0127] On the other hand, if the average ferrite grain size is less than 3 μm, then YS exceeds the specified range and cannot prevent stress corrosion cracking caused by ammonia. Therefore, the average ferrite grain size is set to 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more.

[0128] Ferrite crystal grain size refers to the diameter of a circle with the same area as a ferrite grain projected onto a two-dimensional plane, i.e., the equivalent circle diameter. The equivalent circle diameter is calculated by image analysis processing of images of the microstructure taken.

[0129] [The average hardness of hard tissues is 250 HV0.01 to 350 HV0.01]

[0130] In the steel plate of the present invention, the average hardness of the hard microstructure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate is 250HV0.01 to 350HV0.01. If the average hardness of the hard microstructure exceeds 350HV0.01, the toughness (TS) exceeds the specified range, and the toughness decreases. Therefore, the average hardness of the hard microstructure is set to be 350HV0.01 or less, preferably 330HV0.01 or less, and more preferably 320HV0.01 or less.

[0131] On the other hand, when the average hardness of the hard tissue is less than 250HV0.01, the TS does not reach the specified range. Therefore, the average hardness of the hard tissue is set to 250HV0.01 or more, preferably 270HV0.01 or more, and more preferably 290HV0.01 or more.

[0132] It should be noted that the average hardness of hard tissues can be measured using the methods described in the examples below.

[0133] [Hard tissue comprising more than 60% by volume is adjacent to ferrite, but not adjacent to other hard tissue.]

[0134] PWHT generates carbides within the hard microstructure. When the hard microstructures are adjacent to each other, carbon diffuses between them, forming coarser carbides. This reduces the hardness of the hard microstructure and lowers the steel plate's total steel strength (TS). Furthermore, toughness also decreases.

[0135] When more than 60% of the hard microstructure by volume fraction is adjacent to ferrite but not adjacent to the hard microstructure, the coarsening of carbides is suppressed, the reduction of YS and TS before and after PWHT is small, and the required mechanical properties are met.

[0136] On the other hand, if the volume fraction of hard tissue adjacent to ferrite but not adjacent to hard tissue is less than 60%, the reduction in YS and TS due to PWHT is significant, failing to meet the specified mechanical properties. Therefore, it is necessary that at least 60% of the hard tissue is adjacent to ferrite but not adjacent to hard tissue. Thus, the volume fraction of hard tissue adjacent to ferrite but not adjacent to hard tissue is set to 60% or more, preferably 70% or more, and more preferably 80% or more.

[0137] There is no particular upper limit to the volume fraction of hard tissue adjacent to ferrite but not adjacent to hard tissue; it can be 100% or less.

[0138] It should be noted that the volume fraction of hard tissue can be determined using the methods described in the examples described later.

[0139] [The difference in average Vickers hardness of the hard microstructure 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℃ for 10 hours: less than 20HV 0.01]

[0140] PWHT generates carbides within the hard microstructure outside of ferrite. When carbon dissolved in the hard microstructure becomes carbides, the strain formed within the hard microstructure due to elemental dissolution disappears, thus reducing the hardness of the hard microstructure and simultaneously lowering the steel plate's total strain (TS).

[0141] When the difference between the Vickers hardness obtained by subtracting the hardness after heating from the hardness before heating to the hardness after heating is less than 20HV0.01 before and after heating at 630℃ for 10 hours, the decrease in TS before and after PWHT is small, which meets the required mechanical properties.

[0142] On the other hand, when the Vickers hardness of the hard microstructure decreases by more than 20 HV0.01 after PWHT, the reduction in total strength (TS) due to PWHT is significant. Even if the required mechanical properties are met before PWHT, the TS after PWHT will be insufficient. Alternatively, when the TS after PWHT meets the requirements, the TS before PWHT will become excessively high. Therefore, the difference in Vickers hardness between the hard microstructure before and after PWHT, i.e., the average difference in Vickers hardness of the hard microstructure other than ferrite at a position 1 / 4 of the thickness from the surface of the steel plate before and after heating at 630°C for 10 hours, needs to be less than 20 HV0.01. Therefore, the average difference in Vickers hardness of the hard microstructure other than ferrite at a position 1 / 4 of the thickness from the surface of the steel plate before and after heating at 630°C for 10 hours is set to less than 20 HV0.01, preferably less than 15 HV0.01, and more preferably less than 10 HV0.01.

[0143] There is no specific lower limit. The difference in average Vickers hardness between the hard tissues other than ferrite at a position 1 / 4 of the thickness from the surface of the steel plate can be 0HV0.01 or higher.

[0144] It should be noted that the average hardness of the hard tissue can be measured according to the method described in the examples below. In addition, the above-mentioned average Vickers hardness difference can be expressed as "(the above-mentioned average Vickers hardness before heating at 630°C for 10 hours) - (the above-mentioned average Vickers hardness after heating at 630°C for 10 hours)".

[0145] Furthermore, even if the PWHT conditions are different, as long as the hardness difference requirement under the above heating conditions is met, the same effect can be expected.

[0146] Next, the method for manufacturing the steel plate of the present invention will be described.

[0147] A steel billet with the above-mentioned composition is heated, hot-rolled into a hot-rolled steel plate, and then cooled under the conditions described below to produce a steel plate.

[0148] The manufacturing conditions are described in detail below.

[0149] There are no particular limitations on the manufacturing method of steel billets, but it is preferable to melt molten steel with the above-mentioned composition through a known smelting method such as a converter, and then produce steel billets such as slabs of specified dimensions through a known casting method such as continuous casting. It should be noted that there is no problem in producing steel billets such as slabs of specified dimensions through ingot casting-rolling.

[0150] The steel billet obtained in this way can be hot rolled directly without cooling, or it can be cooled first, then heated and then hot rolled.

[0151] After hot rolling to a temperature of 1000℃~1250℃, the cumulative reduction rate in the austenite non-recrystallization temperature range is set to 30%~70%, and hot rolling is completed at a point above 750℃ at a position one-quarter of the plate thickness from the surface of the steel plate. Next, cooling begins at a cooling start temperature of 680℃~900℃ at a position one-quarter of the plate thickness from the surface of the steel plate. For a temperature T defined in the range of 600℃~750℃, the average cooling rate from the cooling start temperature to T at a position one-quarter of the plate thickness from the cooling start temperature is set to 2.0℃ / s~10.0℃ / s, and the average cooling rate from temperature T to the cooling stop temperature of 300℃~550℃ is set to 20℃ / s~100℃ / s.

[0152] (a) Heating temperature of steel billet: 1000℃~1250℃

[0153] If the heating temperature of the steel billet is below 1000℃, the temperature is too low, resulting in high deformation resistance and increased load on the hot rolling mill, sometimes making hot rolling difficult. On the other hand, if the heating temperature of the steel billet exceeds 1250℃, significant oxidation occurs on the billet surface, potentially reducing the yield due to increased oxidation losses. For these reasons, the heating temperature is set between 1000℃ and 1250℃. A heating temperature of 1050℃ or higher is preferred. Furthermore, a heating temperature of 1150℃ or lower is also preferred.

[0154] (b) Cumulative reduction rate in the non-recrystallization temperature region: 30%–70%

[0155] In the non-recrystallization temperature region (in this invention, the temperature at which the billet is below the Ar3 phase transformation point + 150°C), if the cumulative reduction is less than 30%, the austenite is not sufficiently processed during hot rolling. If the austenite is not sufficiently processed, the number of ferrite nucleation sites decreases during the cooling process described later, the ferrite coarsens, and the average grain size exceeds 15 μm. Consequently, YS does not reach the specified range, and toughness decreases.

[0156] On the other hand, if the cumulative reduction rate in the non-recrystallization temperature region exceeds 70%, there are too many nucleation sites for ferrite during the cooling process, the average ferrite grain size is less than 3 μm, and YS exceeds the specified range. Therefore, the cumulative reduction rate in the non-recrystallization temperature region is specified to be 30% to 70%. The cumulative reduction rate in the non-recrystallization temperature region is preferably 40% or more, more preferably 50% or more. Furthermore, the cumulative reduction rate in the non-recrystallization temperature region is preferably 67% or less, more preferably 60% or less.

[0157] Here, the Ar3 phase transition point (°C) can be calculated, for example, by the following formula.

[0158] Ar3 phase transition point (°C) = 910 - 273 × C - 74 × Mn - 57 × Ni - 16 × Cr - 9 × Mo - 5 × Cu

[0159] The symbol for each element indicates its content (mass%) in the steel.

[0160] (c) Finishing temperature of hot rolling: above 750℃ at a position 1 / 4 of the thickness from the surface of the steel plate.

[0161] Hot rolling should be completed at a temperature above 750°C. If the hot rolling end temperature is below 750°C, the ferrite formed during hot rolling will harden due to the rolling process, the γS will exceed the specified range, and the toughness will decrease. Furthermore, the load on the hot rolling mill will increase. Therefore, the hot rolling end temperature is set to 750°C or higher. Preferably, the hot rolling end temperature is 780°C or higher.

[0162] There is no particular upper limit to the hot rolling end temperature, but if it exceeds 950°C, the ferrite will coarsen and the toughness may deteriorate. Therefore, it is preferable to be below 950°C.

[0163] (d) Cooling start temperature: 680℃~900℃ at 1 / 4 of the plate thickness from the surface of the steel plate.

[0164] If the initial cooling temperature of the steel plate is less than 680°C, a large amount of ferrite will be generated before cooling begins, and the volume fraction of ferrite after cooling will exceed 90%. If the volume fraction of ferrite exceeds 90%, the hard structure will be insufficient, and the specified YS and TS cannot be obtained. Therefore, the initial cooling temperature is set to 680°C or higher. Preferably, the initial cooling temperature is 750°C or higher.

[0165] 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 below. Preferably, the cooling start temperature is 830°C or below.

[0166] (e) Average cooling rate (first average cooling rate) from the initial cooling temperature to a temperature T defined in the range of 600°C to 750°C at a position 1 / 4 of the plate thickness from the surface of the steel plate: 2.0°C / s to 10.0°C / s

[0167] At a position 1 / 4 of the plate thickness from the surface of the steel plate, the average cooling rate (first average cooling rate) within the temperature range defined as 600℃~750℃ from the cooling start temperature is 2.0℃ / s~10.0℃ / s. Before or during cooling, ferrite forms from the austenite grain boundaries. The distribution of hard microstructure and ferrite is determined by cooling to temperature T. If temperature T exceeds 750℃, or the first average cooling rate at 1 / 4 of the plate thickness from the surface of the steel plate exceeds 10.0℃ / s, ferrite formation at the austenite grain boundaries is insufficient, resulting in a microstructure where hard microstructures are adjacent to each other. Specifically, the volume fraction of hard microstructures surrounded by ferrite is less than 60% of the total hard microstructure, leading to a greater decrease in hardness after PWHT.

[0168] 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 formation occurs during the process, the ferrite volume fraction exceeds 90%, and YS and TS decrease. The first average cooling rate within the above temperature range is preferably 4°C / s or more. Furthermore, the first average cooling rate within the above temperature range is preferably 8°C / s or less.

[0169] 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).

[0170] (f) Average cooling rate (second average cooling rate) from temperature T to cooling stop temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate: 20℃ / s~100℃ / s

[0171] The average cooling rate (second average cooling rate) within the temperature range from temperature T to the cooling stop temperature is 20℃ / s to 100℃ / s. During this process, ferrite grains grow, and the final volume fraction of the microstructure is determined. If the second average cooling rate within the above temperature range exceeds 100℃ / s, the volume fraction of ferrite is less than 60%, and YS and TS exceed the specified range. In addition, a large amount of island-like martensite is generated, which increases the hardness of the hard microstructure and reduces its toughness.

[0172] On the other hand, if the second average cooling rate is less than 20°C / s, ferrite growth occurs, the ferrite volume fraction exceeds 90%, and YS and TS decrease. The second average cooling rate within the above temperature range is preferably 30°C / s or more. Furthermore, the second average cooling rate within the above temperature range is preferably 70°C / s or less.

[0173] Here, the second average cooling rate (°C / s) is obtained by (temperature T - cooling stop temperature) (°C) / (cooling time from temperature T to cooling stop temperature (s)).

[0174] (g) Cooling stop temperature at 1 / 4 of the plate thickness from the surface of the steel plate: 300℃~550℃

[0175] The cooling stop temperature is set to 300℃~550℃. If the cooling stop temperature exceeds 550℃, the hardness of the hard tissue will be less than 250HV0.01, and the TS will decrease.

[0176] On the other hand, if the cooling stop temperature is below 300°C, the hardness of the hard tissue becomes excessively high, leading to an increase in total strength (TS) and a decrease in toughness. The cooling stop temperature should be above 350°C. Furthermore, the cooling stop temperature is preferably below 500°C.

[0177] By manufacturing a steel billet having the above-described composition according to the above-described manufacturing conditions, a steel plate having the above-described structure (the steel plate according to the invention) can be obtained. The steel plate according to the invention thus obtained possesses excellent strength properties and low-temperature toughness. Here, excellent strength properties refer to YS (upper yield point when there is a yield point, and 0.2% yield strength when there is no yield point) being 325 MPa to 440 MPa, and TS being 440 MPa to 610 MPa.

[0178] Furthermore, the IMO Gas Code of the International Maritime Organization and classification society regulations stipulate that the yield strength of steel plates should be below 440 MPa to minimize the risk of ammonia stress corrosion cracking. Therefore, as long as YS is below 440 MPa, it can be said to have excellent resistance to ammonia stress corrosion cracking.

[0179] Generally, a higher TS (steel plate stress corrosion cracking) is better, but steel plates with TS exceeding 610 MPa are more likely to crack during processing or welding. Alternatively, a large amount of alloying may be required, potentially increasing costs. Furthermore, balancing the goal of controlling YS (yetal stress corrosion cracking) below 440 MPa with an increased TS is difficult to achieve while ensuring resistance to ammonia stress corrosion cracking; therefore, a TS of 610 MPa or less is preferred for the steel plate. It should be noted that the TS of the steel plate obtained in this invention is substantially 610 MPa or less.

[0180] While not particularly limited, the thickness of the steel plate in this invention is preferably 12 mm or more, more preferably 20 mm or more. Furthermore, the thickness of the steel plate in this invention is preferably 50 mm or less, more preferably 40 mm or less.

[0181] In the steel plate and its manufacturing method according to the present invention, for items not described in this specification, well-known steel plate-related regulations and conventional methods may be used.

[0182] Example

[0183] Molten steel with the composition shown in Table 1 is smelted to produce steel billets (slabs). These steel billets (slabs) are then hot-rolled and cooled under the conditions shown in Table 2 to obtain steel plates. Elements in blank columns in the tables are not intentionally added.

[0184] The obtained steel plates underwent simulated PWHT heat treatment. Using a heating furnace, the temperature at a point 1 / 4 of the plate thickness from the surface was raised to 630°C and held for 10 hours. It should be noted that the effects of heat treatment temperature and holding time on the mechanical properties of the steel material are summarized using the following tempering parameter P.

[0185] P=T{log(t)+20}×10⁻³

[0186] Where T is the heat treatment temperature (K) and t is the holding time at the heat treatment temperature (hour).

[0187] The tempering parameter P in the simulated PWHT heat treatment is 18.963. Since a smaller P value has less impact on mechanical properties, the inventive examples in this application are expected to achieve equivalent or better results under PWHT conditions where P ≤ 18.963.

[0188] The volume fraction of the microstructure, ferrite grain size distribution, and hardness of the hard structure at a position one-quarter of the plate thickness from the surface were measured on the obtained steel plates. Tensile properties and toughness were also evaluated. Additionally, the hardness at a position one-quarter of the plate thickness from the surface was investigated on steel plates subjected to simulated PWHT heat treatment. The test methods are as follows.

[0189] [Determination of volume fraction and ferrite grain size distribution of microstructure]

[0190] Samples were collected and observed with the distance from the surface of the steel plate to one-quarter of its thickness centered. The surfaces of these samples were mirror-polished, then etched with nitric acid and ethanol, and photographed using a scanning electron microscope (SEM) (magnification: 1000x). The imaging area was 4mm × 3mm. The images were analyzed using an image analysis device to determine the area fraction of the microstructure. Since the anisotropy of the steel plate microstructure in this invention is small, the obtained two-dimensional information is universal; therefore, the area fraction of the microstructure is considered equivalent to the volume fraction, and the area fraction is used as the volume fraction.

[0191] The discrimination of each tissue when determining the volume fraction of microstructure is as follows.

[0192] Ferrite is a structure consisting of white lines surrounding a carbide that does not grow isotropically; pearlite is a massive or flattened structure in which dark ferrite and white carbide can be observed in stripes (bands).

[0193] Bainite is defined as a structure consisting of elongated lath ferrite with a diameter of 0.05 μm or more. Martensite is defined as a structure consisting of elongated lath ferrite similar to bainite, but without carbides having a diameter of 0.05 μm or more. It should be noted that carbides are considered to appear as white dots.

[0194] Furthermore, austenite is defined as a structure that exists between lath ferrite structures of bainite or martensite and is not a carbide with an equivalent circle diameter of 0.50 μm or more.

[0195] The equivalent circle diameter of ferrite is calculated by counting the number of pixels inside the region enclosed by the grain boundaries of a crystal in an image taken by a microscope (SEM), converting the number of pixels into the actual length, and then calculating the equivalent circle diameter.

[0196] Noise caused by insufficient image resolution or lens focus shift during shooting may be misinterpreted as individual grains, leading to an incorrect calculation of the presence of a large number of tiny ferrite particles. Therefore, more than 4,000 ferrite particles that can be visually identified as crystals, i.e., those with an equivalent circle diameter of 2 μm or more, are randomly selected to obtain the grain size distribution.

[0197] The average grain size of ferrite is obtained by dividing the sum of these grain sizes (equivalent circle diameters) by the number of grains.

[0198] The volume fraction of hard tissue adjacent only to ferrite is determined by calculating the area ratio of microstructures identified as adjacent only to ferrite within the total area of ​​hard tissue confirmed in images taken by a microscope (SEM). Since the two-dimensional information obtained from the analysis is universal, the area fraction of the microstructure is considered equivalent to the volume fraction, and the obtained area fraction is used as the volume fraction.

[0199] [Hardness of hard tissues (average hardness)]

[0200] The hardness (average hardness) of the hard tissue was determined using a micro-indentation hardness tester. Samples were collected from a position 1 / 4 of the steel plate thickness above the surface. After surface grinding, the samples were etched with a nitric acid-ethanol solution and then observed using an optical microscope to identify the hard tissue to be used for hardness testing. A pyramidal indenter was then pressed under a load of 10 gf, and the Vickers hardness was determined by the indentation size. The hardness of 20 hard tissue samples was measured, and their average value was taken as the average hardness of the hard tissue. The hardness measurement method was the same for the obtained steel plate and the steel plate after simulated PWHT heat treatment.

[0201] It should be noted that the hardness change (Vickers hardness difference) before and after PWHT in Table 2 represents "(average Vickers hardness before heating at 630℃ for 10 hours) - (average Vickers hardness after heating at 630℃ for 10 hours)".

[0202] [Strength Characteristics]

[0203] For each steel plate, tensile test specimens were collected according to JIS Z 2241 (2022) in a direction perpendicular to the rolling direction, i.e., the width direction of the plate was aligned with the length direction of the tensile test specimen. Tensile tests were performed according to the guidelines of JIS Z 2241 (2022) to determine YS and TS. Yield strength YS is closely related to resistance to ammonia stress corrosion cracking. As structural components of liquefied gas bulk carriers, the risk of ammonia stress corrosion cracking needs to be minimized. Therefore, the IMO Gas Code and classification society regulations stipulate that the yield point of steel plates should be below 440 MPa. Thus, in this embodiment, steel plates with YS of 325 MPa to 440 MPa are evaluated as having excellent resistance to ammonia stress corrosion cracking. Furthermore, steel plates with TS of 440 MPa to 610 MPa are evaluated as having excellent tensile strength.

[0204] [toughness]

[0205] After removing 0.5 mm from the surface of each steel plate, test specimens were collected according to JIS Z 2242 (2023) in a direction perpendicular to the rolling direction, i.e., the width direction of the plate was aligned with the length direction of the tensile test specimen. Then, Charpy impact tests were performed according to the guidelines of JIS Z2242 (2023) to determine the brittle-ductile section transition temperature vTrs. Steel plates with vTrs below -60°C were evaluated as having excellent low-temperature toughness.

[0206] The evaluation results obtained in this way are recorded in Table 2.

[0207]

[0208] As shown in Tables 1 and 2, the invention examples all have a yield strength YS of 325MPa to 440MPa and a tensile strength TS of 440MPa to 610MPa, a brittle-ductile section transition temperature vTrs of less than -60℃, excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, and high-strength steel plates with small changes in mechanical properties before and after PWHT treatment.

[0209] On the other hand, the ferrite volume fraction or average ferrite grain size of steel plates No. 4, 5, 7, 8, 9, 11, 13, and 14, which are equivalent to the comparative examples, are outside the scope of the present invention, and at least one of the yield strength YS, tensile strength TS, and toughness is worse than that of the inventive examples.

[0210] Furthermore, the rolling end temperature of steel plate No. 6, which corresponds to the comparative example, is outside the scope of the present invention, and its yield strength YS and toughness are worse than those of the inventive example.

[0211] In steel plates No. 10 and 12, the volume fraction of hard tissue adjacent only to ferrite is outside the scope of this invention, and the hardness changes significantly before and after PWHT.

[0212] In addition, the average hardness of the hard tissue of steel plates No.15 and 16 is outside the scope of this invention, TS is worse than the inventive example, and No.15 also has poor toughness.

[0213] In addition, the content of various elements in steel plates No. 22 to 41 is different from that in the invention example, and at least one of the following is worse than that in the invention example: yield strength YS, tensile strength TS, toughness, and changes in mechanical properties before and after PWHT treatment.

Claims

1. A steel plate comprising the following components: It contains, by mass%, C: 0.03%–0.14%, Si: 0.10%–0.50%, Mn: 0.70%–1.70%, P: less than 0.030%, S: less than 0.0030%, Al: 0.010%–0.100%, Ti: 0.010%–0.030%, Mo: 0.02%–0.10%, Ca: 0.0005%–0.0030%, N: 0.0010%–0.0070%, and O: less than 0.0040%, and satisfies the following formula (1), with the remainder consisting of Fe and unavoidable impurities. Furthermore, at a position one-quarter of the thickness of the steel plate from the surface, there is a steel microstructure consisting of ferrite and other hard microstructures besides ferrite. The volume fraction of ferrite is 60%–90%. The average crystal grain size of the ferrite is 3 μm to 15 μm. The average hardness of the hard tissue is 250 HV0.01 to 350 HV0.

01. By volume fraction, more than 60% of the hard tissue is adjacent to ferrite and not adjacent to the hard tissue. The yield strength is 325 MPa to 440 MPa. The element symbols in formula (1) represent the content of each element by mass % and are set to 0 when excluding the mass.

2. The steel plate according to claim 1, wherein, The composition, by mass%, contains one or more of the following: Cr: less than 0.30%, V: less than 0.10%, W: less than 0.10%, Cu: less than 1.00%, Ni: less than 1.00%, Co: less than 1.00%, and Nb: less than 0.05%.

3. The steel plate according to claim 1 or 2, wherein, Before and after heating at 630°C for 10 hours, the average Vickers hardness difference of the hard microstructure at a position 1 / 4 of the thickness from the surface of the steel plate was less than 20HV0.

01.

4. A method for manufacturing a steel plate, wherein, A steel billet with the following composition is heated to a temperature of 1000℃~1250℃. Next, hot rolling is performed with the cumulative reduction rate in the non-recrystallization temperature region set to 30%–70%, and the hot rolling end temperature at 1 / 4 of the plate thickness from the surface of the steel plate set to above 750°C. Cooling begins at a starting temperature of 680°C to 900°C, located at a point 1 / 4 of the plate thickness from the surface of the steel plate. Temperature T is defined as the range of 600℃ to 750℃. At a position 1 / 4 of the steel plate thickness from the surface, the average cooling rate from the cooling start temperature to the temperature T is set to 2.0℃ / s to 10.0℃ / s. Next, the average cooling rate from the temperature T to the cooling stop temperature of 300°C to 550°C is set to 20°C / s to 100°C / s, and cooling is carried out. The composition, by mass%, contains C: 0.03%–0.14%, Si: 0.10%–0.50%, Mn: 0.70%–1.70%, P: less than 0.030%, S: less than 0.0030%, Al: 0.010%–0.100%, Ti: 0.010%–0.030%, Mo: 0.02%–0.10%, Ca: 0.0005%–0.0030%, N: 0.0010%–0.0070%, and O: less than 0.0040%, and satisfies the following formula (1), with the remainder consisting of Fe and unavoidable impurities. The element symbols in formula (1) represent the content of each element, and the unit is mass%, which is set to 0 when excluding mass%.

5. The method for manufacturing a steel plate according to claim 4, wherein, The composition, by mass%, contains one or more of the following: Cr: less than 0.30%, V: less than 0.10%, W: less than 0.10%, Cu: less than 1.00%, Ni: less than 1.00%, Co: less than 1.00%, and Nb: less than 0.05%.

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