Steel sheet, ammonia tank, and line pipe

Abstract

A steel sheet according to the present disclosure which has the chemical composition described in the description, wherein: the bainite area ratio is 70% or more, the aspect ratio As of crystal grains HAGB is 4.00 or more, and the maximum value HVmax of the Vickers hardness is 240HV or less in a surface layer region extending from the surface of the steel sheet to a depth of 0.5mm; the bainite area ratio is 50% or more and the minor axis of bainite crystal grains HAGB is 30.0μm or less in a t / 4 region; and the aspect ratio As of the crystal grains HAGB in the surface layer region and the aspect ratio At / 4 of the crystal grains HAGB in the t / 4 region satisfy formula (1).　(1): 1.00≤As / At / 4≤5.00

Description

Steel plate, ammonia tank, and line pipe

[0001] The present disclosure relates to a steel plate, an ammonia tank composed of the steel plate, and a line pipe.

[0002] Ammonia is used as a raw material for nitrogen fertilizers such as urea, ammonium nitrate, and ammonium sulfate, or as a raw material for synthetic fibers and resins such as nylon, polyurethane, and acrylonitrile. Ammonia is further ２ attracting attention as an alternative fuel for coal-fired power generation effective for CO reduction and as a carrier of hydrogen.

[0003] Ammonia is liquefied and transported in the state of liquid ammonia by ships, pipelines, tank trucks, etc. Also, the transported liquid ammonia is stored in an ammonia tank. Liquid ammonia is transported and stored at a low temperature of -33°C. Therefore, the steel plate used as a material for the line pipe constituting the pipeline for transportation and the ammonia tank for storage requires excellent strength and low-temperature toughness.

[0004] For the steel plate used as a material for the line pipe and the ammonia tank, further, sufficient suppression of the occurrence of stress corrosion cracking (ammonia SCC) in the liquid ammonia environment, that is, excellent ammonia SCC resistance is required.

[0005] In International Publication No. 2023 / 162522 (Patent Document 1) and International Publication No. 2023 / 162571 (Patent Document 2), steel plates having high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance have been proposed.

[0006] The steel sheet of Patent Document 1 contains, in mass %, C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and inevitable impurities. At a position 0.5 mm deep from the surface of the steel sheet, the average hardness is Hv210 or less, and the variation in the average hardness is Hv50 or less. The volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more. At the 1 / 2 position of the plate thickness of the steel sheet, the volume fraction of the bainite structure is 20% or more, and the total volume fraction of the ferrite structure and the bainite structure is 60% or more.

[0007] The steel sheet of Patent Document 2 contains, in mass %, C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.060%, N: 0.0010% or more and 0.0100% or less, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and inevitable impurities. At a position 0.5 mm deep from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the variation in hardness is 30 HV0.1 or less, and the maximum value of the hardness in the plate thickness direction is at a position 1.0 mm or more and 1 / 4 or less of the plate thickness from the surface of the steel sheet, and the variation in the hardness in the plate thickness direction is 70 HV1 or less. The volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more.

[0008] It is described that high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance can be obtained with the steel sheets disclosed in Patent Documents 1 and 2.

[0009] International Publication No. 2023 / 162522, International Publication No. 2023 / 162571

[0010] Incidentally, in steel sheets used in a liquid ammonia environment, it is preferable that the propagation of fine cracks can be suppressed if they occur. Therefore, steel sheets used in the aforementioned liquid ammonia environment require not only high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance, but also excellent arrestability. Patent documents 1 and 2 do not address the issue of arrestability.

[0011] The purpose of this disclosure is to provide steel plates, ammonia tanks, and line pipes that offer high strength, excellent low-temperature toughness, excellent ammonia SCC resistance, and excellent arrestability.

[0012] The steel sheet disclosed herein has a chemical composition in mass percent of: C: 0.02-0.12%, Si: 0.01-0.30%, Mn: 0.50-2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001-0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001-0.100%, Cu: 0-1%. It contains 00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface of the steel plate to a depth of 0.5 mm, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB is 4.00 or more, the maximum Vickers hardness HVmax is 240 HV or less. Let t be the thickness of the steel plate. In the t / 4 region, which is the region from the surface of the steel plate to a depth of t / 4, the bainite area ratio is 50% or more, the minor axis of the bainite crystal grain HAGB is 30.0 μm or less, and the aspect ratio As of the bainite crystal grain HAGB in the surface region is equal to the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0013] The ammonia tank of this disclosure comprises an outer shell, an inner shell, and an insulating material disposed between the outer shell and the inner shell. The inner shell includes a plurality of base metal parts and a weld metal part. The weld metal part is disposed between the plurality of base metal parts and bonded to the base metal parts. The base metal parts include a heat-affected zone and a normal part other than the heat-affected zone. The chemical composition of the normal part is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.0%. It contains 0%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the HAGB grains with large-angle grain boundaries where the crystal orientation difference of bainite is 15° or more is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less. With the plate thickness of the normal portion being t, in the t / 4 region, which is the region from the surface of the normal portion to a depth of t / 4, the bainite area ratio is 50% or more, and the minor axis of the bainite crystal grain HAGB is 30.0 μm or less. The aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0014] The line pipe of this disclosure includes a base material portion and a weld metal portion extending in the axial direction of the pipe. The base material portion includes a heat-affected zone and a normal portion other than the heat-affected zone. The chemical composition of the normal portion is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.0%. It contains 0%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the HAGB grains with large-angle grain boundaries where the crystal orientation difference of bainite is 15° or more is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less. With the plate thickness of the normal portion being t, in the t / 4 region, which is the region at a depth of t / 4 from the surface of the normal portion, the bainite area ratio is 50% or more, and the minor axis of the bainite crystal grain HAGB is 30.0 μm or less. The aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0015] The steel plates, ammonia tanks, and line pipes of this disclosure provide high strength, excellent low-temperature toughness, excellent ammonia SCC resistance, and excellent arrestability.

[0016] The inventors investigated steel sheets that can obtain high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance from the viewpoint of chemical composition. As a result, the chemical composition, in mass%, was found to be C: 0.02-0.12%, Si: 0.01-0.30%, Mn: 0.50-2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001-0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001-0.100%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, M We considered that a steel sheet containing o: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities, could potentially provide high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance.

[0017] Therefore, we investigated methods for obtaining high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance in steel sheets with the above-mentioned chemical composition, from the viewpoint of microstructure. As a result, the inventors obtained the following findings.

[0018] By increasing the bainite area ratio in the surface region up to a depth of 0.5 mm from the surface of the steel sheet, and in the t / 4 region at a depth of t / 4 from the surface, and further refining the HAGB (grain with High Angle Grain Boundary) grains with large-angle grain boundaries where the crystal orientation difference in the bainite is 15° or more in the t / 4 region, the strength and low-temperature toughness of the steel sheet are increased. Furthermore, by keeping the Vickers hardness of the surface region low, the ammonia SCC resistance is improved.

[0019] Based on the above findings, the inventors of the present invention further conducted studies. As a result, by setting the bainite area ratio in the surface layer region from the surface of the steel sheet to a depth of 0.5 mm to 70% or more, the bainite area ratio in the t / 4 region, which is the region at a depth of t / 4 from the surface, to 50% or more, further setting the minor diameter of the grain size HAGB of bainite in the t / 4 region to 30.0 μm or less, and further setting the maximum value HVmax of the Vickers hardness in the surface layer region to 240 HV or less, the inventors found that high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance can be obtained in the steel sheet.

[0020] However, even in the case of a steel sheet satisfying the above configuration, sufficient arrestability may not be obtained. Therefore, the inventors of the present invention further studied means for enhancing the arrestability in a steel sheet satisfying the above-described configuration. As a result, the inventors obtained the following findings.

[0021] When a crack occurs in the steel sheet, the crack propagates in the thickness direction of the steel sheet. If the grain HAGB of bainite in the surface layer region is flat, that is, if the aspect ratio As of the grain HAGB of bainite in the surface layer region is large, the crack is less likely to propagate in the thickness direction.

[0022] However, even if the aspect ratio As of the grain HAGB of bainite in the surface layer region is large, if the aspect ratio A ｔ／４ of the grain HAGB of bainite in the t / 4 region, which is a region deeper than the surface layer region, is small, once the crack propagates through the grain HAGB in the surface layer region, the crack easily propagates into the interior of the steel sheet. In this case, sufficient arrestability cannot be obtained. Therefore, if the aspect ratio A ｔ／４ of the grain HAGB of bainite in the t / 4 region is not excessively smaller than the aspect ratio As of the grain HAGB of bainite in the surface layer region, the propagation of the crack in the thickness direction can be sufficiently suppressed, and excellent arrestability can be obtained in the steel sheet.

[0023] Based on the above findings, the inventors conducted further investigations. As a result, in the steel material having the above-described structure, the aspect ratio As of the bainite crystal grain HAGB in the surface region is further set to 4.00 or higher, and further, the aspect ratio As of the crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region ｔ／４ The inventors have found that by adjusting the ratio to satisfy equation (1), not only high strength, excellent low-temperature toughness, and excellent ammonia SCC resistance, but also excellent arrestability can be obtained. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0024] The steel plate of this embodiment was completed based on the above technical concept and has the following configuration.

[0025] The steel sheet of this embodiment has a chemical composition in mass percent of: C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1 It contains 0.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface of the steel plate to a depth of 0.5 mm, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB is 4.00 or more, the maximum Vickers hardness HVmax is 240 HV or less. Let t be the thickness of the steel plate. In the t / 4 region, which is the region from the surface of the steel plate to a depth of t / 4, the bainite area ratio is 50% or more, the minor axis of the bainite crystal grain HAGB is 30.0 μm or less, and the aspect ratio As of the bainite crystal grain HAGB in the surface region is equal to the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0026] The steel sheet of this embodiment may contain, in mass percent, one or more elements selected from the group consisting of Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050%.

[0027] The ammonia tank of this embodiment comprises an outer shell, an inner shell, and an insulating material disposed between the outer shell and the inner shell. The inner shell includes a plurality of base metal parts and a weld metal part. The weld metal part is disposed between the plurality of base metal parts and bonded to the base metal parts. The base metal part includes a heat-affected zone and a normal part other than the heat-affected zone. The chemical composition of the normal part is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.0%. It contains 0%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the HAGB grains with large-angle grain boundaries where the crystal orientation difference of bainite is 15° or more is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less. With the plate thickness of the normal portion being t, in the t / 4 region, which is the region from the surface of the normal portion to a depth of t / 4, the bainite area ratio is 50% or more, and the minor axis of the bainite crystal grain HAGB is 30.0 μm or less. The aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0028] The ammonia tank of this embodiment may contain, in mass percent, one or more elements selected from the group consisting of Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050% in its normal chemical composition.

[0029] The line pipe of this embodiment includes a base material portion and a weld metal portion extending in the direction of the pipe axis. The base material portion includes a heat-affected zone and a normal portion other than the heat-affected zone. The chemical composition of the normal portion is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.0%. It contains 0%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the HAGB grains with large-angle grain boundaries where the crystal orientation difference of bainite is 15° or more is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less. With the plate thickness of the normal portion being t, in the t / 4 region, which is the region at a depth of t / 4 from the surface of the normal portion, the bainite area ratio is 50% or more, and the minor axis of the bainite crystal grain HAGB is 30.0 μm or less. The aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / Aｔ／４ ≤ 5.00 (1)

[0030] The line pipe of this embodiment may contain, in mass%, one or more elements selected from the group consisting of Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050% in its normal chemical composition.

[0031] The steel plate, ammonia tank, and line pipe of this embodiment will be described below. In the following description, unless otherwise specified, "%" for elements refers to mass percentage.

[0032] [Features of the steel sheet of this embodiment] The steel sheet of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.00%. It contains Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. (Feature 2) In the surface region from the surface of the steel plate to a depth of 0.5 mm, the following features 2(1) to 2(3) are satisfied. Feature 2(1): The bainite area ratio is 70% or more. Feature 2(2): The aspect ratio As of the bainite crystal grain HAGB is 4.00 or more. Feature 2(3): The maximum Vickers hardness HVmax is 240HV or less. (Feature 3) With the thickness of the steel plate as t, the t / 4 region, which is the region from the surface of the steel plate to a depth of t / 4, satisfies the following features 3(1) and 3(2). Feature 3(1): The bainite area ratio is 50% or more. Feature 3(2): The minor axis of the bainite crystal grain HAGB is 30.0 μm or less. (Feature 4) The aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤5.00 (1) Here, HAGB (grain with High Angle Grain Boundary) refers to a crystal grain having a large-angle grain boundary with a crystal orientation difference of 15° or more. Features 1 to 4 are described below.

[0033] [(Feature 1) Regarding the chemical composition] The chemical composition of the steel sheet of this embodiment contains the following elements.

[0034] C: 0.02-0.12% Carbon (C) increases the strength of the steel sheet. If the C content is less than 0.02%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.12%, the toughness and weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.02-0.12%. The preferred lower limit of the C content is 0.03%, more preferably 0.04%, and even more preferably 0.05%. The preferred upper limit of the C content is 0.11%, more preferably 0.10%, and even more preferably 0.09%.

[0035] Si: 0.01 to 0.30% Silicon (Si) deoxidizes the steel during the steelmaking process in the manufacturing of steel sheets. If the Si content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.30%, the toughness and weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.01 to 0.30%. The preferred lower limit of the Si content is 0.02%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Si content is 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0036] Mn: 0.50-2.00% Manganese (Mn) increases the strength of steel plates by improving the hardenability of the steel. If the Mn content is less than 0.50%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the toughness and weldability of the steel plate will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.50-2.00%. The preferred lower limit of the Mn content is 0.60%, more preferably 0.70%, and still more preferably 0.80%. The preferred upper limit of the Mn content is 1.80%, more preferably 1.60%, and still more preferably 1.40%.

[0037] P: 0.025% or less. Phosphorus (P) segregates at the grain boundaries in the steel sheet. If the P content exceeds 0.025%, P will excessively segregate at the grain boundaries of the steel sheet. In this case, even if the content of other elements is within the range of this embodiment, the toughness and weldability of the steel sheet will decrease. Therefore, the P content is 0.025% or less. It is preferable that the P content be as low as possible. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the P content is 0.020%, more preferably 0.017%, and still more preferably 0.015%.

[0038] S: 0.0250% or less. Sulfur (S) forms sulfides, represented by MnS. If the S content exceeds 0.0250%, coarse sulfides are formed. Coarse sulfides reduce the toughness of the steel sheet. Therefore, the S content is 0.0250% or less. The preferred lower limit of the S content is greater than 0%, more preferably 0.0001%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the S content is 0.0200%, more preferably 0.0100%, even more preferably 0.0050%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0039] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes the steel during the steelmaking process in the manufacturing of steel sheets. If the Al content is less than 0.001%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, the cleanliness of the steel will be excessively reduced, and the toughness of the steel sheet will decrease. Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.002%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.080%, more preferably 0.050%, and even more preferably 0.040%.

[0040] N: 0.0100% or less. Nitrogen (N) forms nitrides, which refine the crystal grains and increase the toughness of the steel sheet. Even if only a small amount of N is present, the above effect can be obtained. However, if the N content exceeds 0.0100%, the toughness of the steel sheet will actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.0100% or less. The preferred lower limit of the N content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

[0041] O: 0.0100% or less. Oxygen (O) generates oxides. If the O content exceeds 0.0100%, coarse oxides are formed. Coarse oxides become the starting point for cracks and reduce the toughness of the steel sheet. Therefore, the O content should be 0.0100% or less. It is preferable to have as low an O content as possible. However, excessive reduction of the O content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0008%. The preferred upper limit of the O content is 0.0080%, more preferably 0.0050%, and more preferably 0.0040%.

[0042] Nb: 0.001 to 0.100% Niobium (Nb) generates Nb precipitates such as carbides or carbonitrides. Nb precipitates refine the crystal grains and increase the toughness of the steel sheet. If the Nb content is less than 0.001%, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, coarse Nb precipitates are formed, and the toughness of the steel sheet actually decreases. Therefore, the Nb content is 0.001 to 0.100%. The preferred lower limit of the Nb content is 0.005%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the Nb content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0043] The remainder of the chemical composition of the steel sheet in this embodiment contains Fe and impurities. Preferably, the remainder of the steel sheet in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet in this embodiment. For example, the impurities are one or more selected from the group consisting of Co: 0 to 0.10%, Zr: 0 to 0.05%, and W: 0 to 0.10%.

[0044] [Regarding Optional Elements] The chemical composition of the steel sheet in this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%. All of these elements are optional elements. Each element will be described below.

[0045] [Group 1 (Cu, Ni, Cr, Mo, and B)] The chemical composition of the steel sheet in this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, and B in place of a portion of Fe. All of these elements increase the strength of the steel sheet. Each element will be described below.

[0046] Cu: 0-1.00% Copper (Cu) is an optional element and may not be included. In other words, the Cu content may be 0%. If it is included, that is, if the Cu content is greater than 0%, Cu increases the strength of the steel sheet. Even if only a small amount of Cu is included, the above effect can be obtained to some extent. However, if the Cu content exceeds 1.00%, the toughness and weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0-1.00%. The preferred lower limit of the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Cu content is 0.90%, more preferably 0.80%, and still more preferably 0.50%.

[0047] Ni: 0-1.00% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. If Ni is included, that is, if the Ni content is greater than 0%, Ni increases the strength of the steel sheet. Ni also improves the low-temperature toughness of the steel sheet. Even if only a small amount of Ni is included, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.00%, the weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0-1.00%. The preferred lower limit of the Ni content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Ni content is 0.90%, more preferably 0.80%, and still more preferably 0.50%.

[0048] Cr: 0-1.00% Chromium (Cr) is an optional element and may not be included. In other words, the Cr content may be 0%. If it is included, that is, if the Cr content is greater than 0%, Cr increases the strength of the steel sheet. Even if only a small amount of Cr is included, the above effect can be obtained to some extent. However, if the Cr content exceeds 1.00%, the toughness and weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 0-1.00%. The preferred lower limit of the Cr content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Cr content is 0.90%, more preferably 0.80%, and still more preferably 0.50%.

[0049] Mo: 0-1.00% Molybdenum (Mo) is an optional element and may not be included. In other words, the Mo content may be 0%. If it is included, that is, if the Mo content is greater than 0%, Mo increases the strength of the steel sheet. Even if only a small amount of Mo is included, the above effect can be obtained to some extent. On the other hand, if the Mo content exceeds 1.00%, the toughness and weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mo content is 0-1.00%. The preferred lower limit of the Mo content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Mo content is 0.90%, more preferably 0.80%, and still more preferably 0.50%.

[0050] B: 0 to 0.0050% Boron (B) is an optional element and may not be included. In other words, the B content may be 0%. If it is included, that is, if the B content is greater than 0%, B increases the strength of the steel plate by increasing the hardenability of the steel. The above effect can be obtained even if only a small amount of B is included. However, if the B content exceeds 0.0050%, the weldability of the steel plate will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%.

[0051] [Second Group (V and Ti)] The chemical composition of the steel sheet in this embodiment may further contain one or more elements selected from the group consisting of V and Ti in place of a portion of Fe. All of these elements enhance the toughness of the steel sheet. Each element will be described below.

[0052] V: 0 to 0.100% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V will produce V precipitates such as carbides or nitrides. V precipitates refine the crystal grains and increase the toughness of the steel sheet. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.100%, the weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the V content is 0 to 0.100%. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, more preferably 0.005%, and still more preferably 0.010%. The preferred upper limit of the V content is 0.090%, more preferably 0.070%, and still more preferably 0.050%.

[0053] Ti: 0 to 0.100% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. If Ti is included, that is, if the Ti content is greater than 0%, Ti will produce Ti precipitates such as carbides or nitrides. Ti precipitates refine the crystal grains and increase the toughness of the steel sheet. Even if only a small amount of Ti is included, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.100%, the weldability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ti content is 0 to 0.100%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.003%, more preferably 0.005%, and still more preferably 0.010%. The preferred upper limit of the Ti content is 0.090%, more preferably 0.070%, and still more preferably 0.050%.

[0054] [Group 3 (Ca and Rare Earth Elements (REM))] The chemical composition of the steel sheet in this embodiment may further contain one or more elements selected from the group consisting of Ca and rare earth elements (REM) in place of a portion of Fe. All of these elements improve the hot workability of the steel sheet and increase the toughness of the welded joint after welding. The following describes each element.

[0055] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If it is present, that is, if the Ca content is greater than 0%, the Ca spheroidizes sulfides. This improves the hot workability of the steel plate and increases the toughness of the welded joint after welding. The above effects can be obtained even if only a small amount of Ca is present. However, if the Ca content exceeds 0.0100%, the cleanliness of the steel decreases and the toughness of the steel plate decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. The preferred upper limit of the Ca content is 0.0050%, more preferably 0.0040%, and still more preferably 0.0030%.

[0056] Rare Earth Elements: 0 to 0.0100% Rare Earth Metals (REM) are optional elements and do not need to be included. In other words, the REM content may be 0%. If REM is included, that is, if the REM content is greater than 0%, REM spheroidizes sulfides. This improves the hot workability and ductility of the steel sheet. It also improves the toughness of the welded joint after welding. The above effects can be obtained even if only a small amount of REM is included. However, if the REM content exceeds 0.0100%, the cleanliness of the steel decreases and the toughness of the steel sheet decreases, even if the content of other elements is within the range of this embodiment. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. The preferred upper limit for the REM content is 0.0050%, more preferably 0.0040%, and even more preferably 0.0030%.

[0057] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content refers to the total content of these elements.

[0058] [Group 4 (Sn and Sb)] The chemical composition of the steel sheet in this embodiment may further contain one or more elements selected from the group consisting of Sn and Sb in place of a portion of Fe. All of these elements enhance the corrosion resistance of the steel sheet. Each element will be described below.

[0059] Sn: 0-0.10% Tin (Sn) is an optional element and may not be present. In other words, the Sn content may be 0%. If Sn is present, that is, if the Sn content is greater than 0%, Sn improves the corrosion resistance of the steel sheet. The above effect can be obtained even if only a small amount of Sn is present. However, if the Sn content exceeds 0.10%, the workability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0-0.10%. The preferred lower limit of the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Sn content is 0.09%, more preferably 0.08%, even more preferably 0.07%, and even more preferably 0.05%.

[0060] Sb: 0 to 0.050% Antimony (Sb) is an optional element and may not be present. In other words, the Sb content may be 0%. If it is present, that is, if the Sb content is greater than 0%, Sb enhances the corrosion resistance of the steel sheet. The above effect can be obtained even if only a small amount of Sb is present. However, if the Sb content exceeds 0.050%, the workability of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0 to 0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Sb content is 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0061] [(Feature 2) Surface Region of Steel Plate] In the steel plate of this embodiment, the region from the surface of the steel plate to a depth of 0.5 mm is defined as the "surface region". The steel plate of this embodiment satisfies the following features 2 (1) to (3) in the surface region. Feature 2 (1): The bainite area ratio is 70% or more. Feature 2 (2): The aspect ratio As of the bainite grain HAGB is 4.00 or more. Feature 2 (3): The maximum value of Vickers hardness HVmax is 240 HV or less. Here, grain HAGB (grain with High Angle Grain Boundary) means a grain having a large-angle grain boundary with a crystal orientation difference of 15° or more. Features 2 (1) to (3) will be explained below.

[0062] [(Feature 2 (1)) Regarding the bainite area ratio in the surface region] In this embodiment, the bainite area ratio in the surface region is 70% or more. Bainite enhances the strength and low-temperature toughness of the steel sheet. If the bainite area ratio in the surface region is less than 70%, sufficient strength and sufficient low-temperature toughness cannot be obtained in the steel sheet. Therefore, the bainite area ratio in the surface region is 70% or more.

[0063] The preferred lower limit for the bainite area ratio is 72%, more preferably 74%, more preferably 76%, more preferably 78%, more preferably 80%, more preferably 82%, more preferably 84%, more preferably 86%, more preferably 88%, and more preferably 90%. The bainite area ratio in the surface region may be 100%. The remainder other than bainite consists of one or more materials selected from the group consisting of ferrite, pearlite, cementite (carbide), and inclusions.

[0064] [Method for measuring the bainite area ratio in the surface region] The bainite area ratio in the surface region of a steel sheet is measured by the following method: A test piece is taken that has a cross-section parallel to the rolling direction and the thickness direction of the steel sheet. The cross-section of the test piece that includes both the rolling direction and the thickness direction is used as the observation surface.

[0065] After mirror-polishing the observation surface of the test specimen, it is further polished to a depth of approximately 20 μm by electropolishing. On the observation surface after electropolishing, three arbitrary rectangular regions are defined as the observation field of view, within the surface layer up to a depth of 0.5 mm from the steel plate surface in the thickness direction, with dimensions of 400 μm in the rolling direction and 400 μm in the thickness direction.

[0066] The bainite area ratio is measured by performing EBSD analysis on the observed field of view using an EBSD analyzer equipped with a thermal field emission scanning electron microscope and an EBSD detector. For example, the thermal field emission scanning electron microscope used is the JSM-IT800SHL manufactured by JEOL Ltd. For example, the EBSD detector used is the VelocityEBSD detector manufactured by EDAX.

[0067] The measurement magnification for EBSD analysis shall be 200x. The measurement pitch for a 400 μm x 400 μm observation field shall be 0.4 μm. The electron beam diameter shall be 0.4 μm or less. The Confidence Index (CI) shall be 0.1 or higher.

[0068] The crystal orientation information obtained from EBSD analysis includes the measurement coordinates of the measurement points in the observation field and the crystal orientation measured at those points. Using the obtained crystal orientation information, the regions determined to have a body-centered cubic structure (bcc) (bcc regions) are identified using the EBSD analysis software product name: OIM Analysis.

[0069] Within the bcc region, areas enclosed by boundaries where the crystal orientation difference between adjacent measurement points is 15° or more (i.e., large-angle grain boundaries) are identified as grain HAGBs. For each grain HAGB, the GAM value (Grain Average Misorientation) is calculated. The GAM value is an index defined in OIM Analysis. Within the bcc region, areas with a GAM value of 0.5 or less are identified as ferrite and pearlite. On the other hand, areas with a GAM value greater than 0.5 are identified as bainite.

[0070] The bainite area percentage (%) of the surface region is calculated based on the total bainite area in the three observation fields and the total area of ​​the three observation fields. The bainite area percentage is rounded to the nearest integer.

[0071] Furthermore, the coordinates of all bainite regions identified in each observation field should be recorded. Also, since the above method for identifying microstructures cannot strictly distinguish between bainite and martensite, regions with a GAM value greater than 0.5 should be considered bainite.

[0072] [(Feature 2 (2)) Aspect ratio As of the crystal grain HAGB of bainite in the surface region] As described above, in bainite in the surface region, a closed region surrounded by large-angle grain boundaries with an orientation difference of 15° or more is defined as a crystal grain HAGB. In this embodiment, in bainite in the surface region, the aspect ratio As of the crystal grain HAGB is 4.00 or more.

[0073] If the aspect ratio As of the HAGB crystal grains in the bainite in the surface region is large enough (4.00 or higher), the HAGB crystal grains are sufficiently extended in the rolling direction of the steel sheet. In this case, even if fine cracks occur on the surface of the steel sheet, the propagation of the cracks in the thickness direction can be sufficiently suppressed. As a result, the arrestability of the steel sheet is enhanced. Therefore, in the surface region, the aspect ratio As of the HAGB crystal grains of bainite is 4.00 or higher.

[0074] The preferred lower limit of the aspect ratio As of the surface grain HAGB is 4.20, more preferably 4.50, and even more preferably 5.00. The upper limit of the aspect ratio As of the surface grain HAGB is not particularly limited. For example, the upper limit of the aspect ratio As of the surface grain HAGB is 20.00, and for example, 18.00.

[0075] [Method for measuring the aspect ratio As of bainite grain HAGB in the surface region] The aspect ratio As of bainite grain HAGB in the surface region of a steel sheet is determined by the following method. All bainite grain HAGBs in all bainite regions identified in the three observation fields described in the above-mentioned [Method for measuring the bainite area ratio in the surface region] are to be measured. The major and minor axes of each bainite grain HAGB to be measured are determined by the following method. At the grain boundary (large-angle grain boundary) of the bainite grain HAGB, draw two parallel line segments that are parallel to each other and each touches an arbitrary point on the grain boundary. The maximum distance between the two parallel line segments is taken as the major axis (μm) of the bainite grain HAGB. Furthermore, the distance between two parallel line segments that are parallel to the major axis and each touches the grain boundary of the bainite grain HAGB is taken as the minor axis (μm) of the bainite grain HAGB. Based on the obtained major and minor axes, the aspect ratio of the HAGB crystal grain is determined using the following formula. The aspect ratio of the HAGB crystal grain is rounded to two decimal places, obtained by rounding the third decimal place of the value obtained using the following formula: Aspect Ratio = Major Axis / Minor Axis

[0076] Based on the total area of ​​all bainite in the three observation fields and the area of ​​each grain HAGB, the area percentage (%) of each grain HAGB is determined. The aspect ratio As of the bainite grain HAGB in the surface region is the weighted average of the aspect ratios of each grain HAGB, weighted by the area percentage of the grain HAGB. The aspect ratio As of the bainite grain HAGB is rounded to two decimal places by rounding the third decimal place of the calculated weighted average.

[0077] [(Feature 2 (3)) Vickers hardness in the surface region] In the surface region, the maximum value of Vickers hardness HVmax is 240 HV or less. If the Vickers hardness in the surface region is too high, the ammonia SCC resistance of the steel plate will decrease. If the maximum value of Vickers hardness HVmax in the surface region is 240 HV or less, excellent ammonia SCC resistance can be obtained in the steel plate.

[0078] The preferred upper limit for the maximum Vickers hardness HVmax in the surface region is 235 HV, more preferably 230 HV, and even more preferably 225 HV. The minimum Vickers hardness in the surface region is not particularly limited, but for example, the minimum Vickers hardness can be expressed as a ratio to HVmax, which is 0.80 HVmax, or for example, 0.82 HVmax. 0.80 HVmax is the value obtained by multiplying the HVmax value by 0.80.

[0079] [Method for measuring the maximum Vickers hardness HVmax in the surface region] The maximum Vickers hardness HVmax (HV) in the surface region is determined by the following method.

[0080] A test specimen is taken from the steel plate, having a cross-section parallel to both the rolling direction and the thickness direction. The cross-section of the test specimen that includes both the rolling direction and the thickness direction is designated as the measurement surface. On the measurement surface, ten arbitrary measurement points are selected in the surface region from the surface of the steel plate to a depth of 0.5 mm in the thickness direction, at a depth of 0.25 mm from the surface in the thickness direction. The interval between each measurement point is 0.5 mm in the rolling direction. At each measurement point, a Vickers hardness test is performed at each measurement position in accordance with JIS Z 2244-1:2020. The test force is 9.8 N (1 kgf). Of the ten Vickers hardness values ​​obtained, the maximum value is designated as HVmax (HV). The maximum Vickers hardness value HVmax is an integer value obtained by rounding the first decimal place of the obtained value.

[0081] [(Feature 3) Regarding the t / 4 region of the steel plate] In the steel plate of this embodiment, the region from the surface of the steel plate to a depth of t / 4 is defined as the "t / 4 region," where t is the thickness of the steel plate. More specifically, the region from the t / 8 depth position to the 3t / 8 depth position in the thickness direction, starting from the surface of the steel plate, is defined as the "t / 4 region." In the t / 4 region, the steel plate of this embodiment satisfies the following features 3(1) and (2). Feature 3(1): The bainite area ratio is 50% or more. Feature 3(2): The minor axis of the bainite crystal grain HAGB is 30.0 μm or less. Features 3(1) and (2) will be explained below.

[0082] [Regarding the bainite area ratio in the t / 4 region (Feature 3(1))] In the t / 4 region of the steel sheet of this embodiment, the bainite area ratio is 50% or more. If the bainite area ratio in the t / 4 region is less than 50%, sufficient strength and sufficient low-temperature toughness cannot be obtained in the steel sheet. Therefore, the bainite area ratio in the t / 4 region is 50% or more.

[0083] The preferred lower limit for the bainite area ratio is 52%, more preferably 54%, more preferably 56%, more preferably 58%, more preferably 60%, more preferably 62%, more preferably 64%, and more preferably 66%. The bainite area ratio in the t / 4 region may be 100%. The remainder other than bainite consists of one or more materials selected from the group consisting of ferrite, pearlite, cementite (carbide), and inclusions.

[0084] [Method for measuring the bainite area ratio in the t / 4 region] The bainite area ratio in the t / 4 region of a steel sheet is measured by the following method: A test piece is taken that has a cross-section parallel to the rolling direction and the thickness direction of the steel sheet. The cross-section of the test piece that includes both the rolling direction and the thickness direction is used as the observation surface.

[0085] After mirror-polishing the observation surface of the test specimen, it is further polished to a depth of approximately 20 μm by electropolishing. On the observation surface after electropolishing, the observation field is defined as any three rectangular regions ranging from a depth of t / 8 to 3t / 8 in the thickness direction, starting from the steel plate surface (i.e., the t / 4 region), with a width of 400 μm in the rolling direction and 400 μm in the thickness direction.

[0086] For each observation field, EBSD analysis is performed in accordance with the method described in [Method for Measuring the Bainite Area Ratio of the Surface Region] to identify the bainite region. Based on the total bainite area in the three observation fields and the total area of ​​the three observation fields, the bainite area ratio (%) of the t / 4 region is calculated. The bainite area ratio is rounded to the nearest integer. The coordinates of all bainite regions identified in each observation field are recorded.

[0087] [(Feature 3 (2)) Regarding the minor axis of grain HAGB in bainite in the t / 4 region] In bainite in the t / 4 region, a closed region surrounded by large-angle grain boundaries with an orientation difference of 15° or more is defined as grain HAGB. In this embodiment, in bainite in the t / 4 region, the minor axis of grain HAGB is 30.0 μm or less.

[0088] In the t / 4 region, the minor axis direction of the bainite grain HAGB aligns with the thickness direction of the steel sheet. If the minor axis of the bainite grain HAGB in the t / 4 region is short, such as 30.0 μm or less, the strength and low-temperature toughness of the steel sheet in the t / 4 region will increase. Therefore, in bainite in the t / 4 region, the minor axis of the grain HAGB is 30.0 μm or less.

[0089] The preferred lower limit for the minor axis of the HAGB grains in the t / 4 region is not particularly limited, but is, for example, 1.0 μm and, for example, 2.0 μm. The preferred upper limit for the minor axis of the HAGB grains in the t / 4 region is 28.0 μm, more preferably 26.0 μm, and even more preferably 24.0 μm.

[0090] [Short axis and aspect ratio A of bainite grain HAGB in the t / 4 region] ｔ／４ [Measurement Method] Short axis and aspect ratio A of bainite grain HAGB in the t / 4 region of the steel plate. ｔ／４ The following method is used to determine the aspect ratio of each HAGB grain in the bainite region. All HAGB grains in all bainite regions identified in the three observation fields described above in the [Method for Measuring the Bainite Area Ratio in the t / 4 Region] are to be measured. The major and minor axes of each HAGB grain to be measured are determined by the following method. Two parallel line segments are drawn at the grain boundary (large-angle grain boundary) of the HAGB grain, parallel to each other and each tangent to an arbitrary point on the grain boundary. The maximum distance between the two parallel line segments is taken as the major axis (μm) of the HAGB grain. Furthermore, the distance between two parallel line segments that are parallel to the major axis and tangent to the grain boundary of the HAGB grain is taken as the minor axis (μm) of the HAGB grain. Based on the obtained major and minor axes, the aspect ratio of the HAGB grain is determined by the following formula. The aspect ratio of the HAGB crystal grain is the value obtained by the following formula, rounded to two decimal places: Aspect ratio = Major axis / Minor axis

[0091] Based on the total area of ​​all bainite in the three observation fields and the area of ​​each grain HAGB, the area percentage (%) of each grain HAGB is determined. The minor axis (μm) of the grain HAGB of bainite in the t / 4 region is the weighted average of the minor axes of each grain HAGB, weighted by the area percentage of the grain HAGB. The minor axis (μm) of the grain HAGB is rounded to one decimal place by rounding the second decimal place of the calculated weighted average. The aspect ratio A of the grain HAGB of bainite in the t / 4 region is also determined. ｔ／４ Aspect ratio A of each grain HAGB is defined as the weighted average of the aspect ratios of each grain HAGB, weighted by the area fraction of the grain HAGB. ｔ／４ This value is obtained by rounding the weighted average value to two decimal places, with the third decimal place being rounded off.

[0092] [(Feature 4) Aspect ratio As in the surface region and aspect ratio A in the t / 4 region] ｔ／４ Regarding the ratio of the two: In the steel sheet of this embodiment, the aspect ratio As of the bainite crystal grain HAGB in the surface region and the aspect ratio A of the bainite crystal grain HAGB in the t / 4 region ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0093] As mentioned above, when a crack occurs in a steel plate, the crack propagates in the thickness direction of the steel plate. If the aspect ratio As of the HAGB crystal grains of bainite in the surface region is 4.00 or higher, the crack is less likely to propagate in the thickness direction. However, even if the aspect ratio As of bainite in the surface region is large, the aspect ratio A of bainite in the t / 4 region, which is deeper than the surface region... ｔ／４ If the crack is small, once it propagates through the HAGB grains of bainite in the surface layer, the crack will easily propagate into the interior of the steel plate. In this case, sufficient arrestability cannot be obtained.

[0094] F1 is defined as follows: F1 = As / A ｔ／４ If F1 is between 1.00 and 5.00, the aspect ratio A of bainite in the 4 / t region is ｔ／４However, this value is sufficiently close to the aspect ratio As of the bainite in the surface region. Therefore, the propagation of cracks in the thickness direction can be sufficiently suppressed, and excellent arrestability can be obtained in steel sheets.

[0095] A preferred upper limit for F1 is 4.80, more preferably 4.60, and even more preferably 4.40.

[0096] [How to determine F1] F1 is determined by the following method. Determine the aspect ratio As of the surface region by the method described in [Method for measuring the aspect ratio As of the HAGB crystal grains of bainite in the surface region] above. Furthermore, determine the minor axis and aspect ratio A of the HAGB crystal grains of bainite in the t / 4 region above. ｔ／４ The aspect ratio A in the t / 4 region is measured by the method described in [Measurement Method]. ｔ／４ Determine the aspect ratio As and A obtained. ｔ／４ Based on this, F1 is calculated. F1 is the value obtained by rounding the third decimal place of the result to two decimal places.

[0097] [Effects of the steel sheet of this embodiment] The steel sheet of this embodiment satisfies the above-described features 1 to 4. Therefore, the steel sheet of this embodiment provides high strength, excellent low-temperature toughness, excellent ammonia SCC resistance, and excellent arrestability. The mechanical properties will be described below.

[0098] [Regarding the strength of the steel plate] In this embodiment, by satisfying features 1 to 4, the yield strength YS of the steel plate becomes 360 MPa or more, and the tensile strength TS becomes 450 MPa or more. The upper limit of the yield strength YS is not particularly limited, but for example it is 600 MPa. Similarly, the upper limit of the tensile strength TS is not particularly limited, but for example it is 700 MPa.

[0099] [Method for Measuring the Strength of Steel Plates] The strength of a steel plate is determined by the following method. A JIS No. 5 test specimen, as specified in JIS Z 2201:2022, is taken from the steel plate. The gauge length is set to 50 mm, and the thickness of the test specimen is set to the plate thickness. The test specimen is taken so that its longitudinal direction is perpendicular to the rolling direction of the steel plate and perpendicular to the thickness direction of the steel plate. Using the taken test specimen, a tensile test is performed at room temperature in air in accordance with JIS Z 2241:2011 to obtain a stress-strain curve. Based on the obtained stress-strain curve, the yield strength YS (MPa) and tensile strength TS (MPa) are determined. The yield strength YS is set to the 0.2% proof stress.

[0100] [Regarding the low-temperature toughness of steel sheets] In this embodiment, the low-temperature toughness of the steel sheet is enhanced by satisfying features 1 to 4. Specifically, the fracture transition temperature vTrs (°C) obtained by the Charpy impact test is -60°C or lower.

[0101] [Method for Evaluating Low-Temperature Toughness of Steel Plates] The low-temperature toughness of steel plates shall be evaluated by the following method. V-notch test specimens shall be taken in accordance with JIS Z 2242:2018 by the following method. The V-notch test specimen shall be 10 mm thick x 10 mm wide x 55 mm long. The longitudinal direction of the V-notch test specimen shall be the rolling direction of the steel plate. The V-notch shall be formed at the center of the longitudinal direction of the V-notch test specimen. The extension direction of the V-notch shall be the thickness direction of the steel plate, and the depth direction of the V-notch shall be the width direction of the steel plate.

[0102] If the steel plate thickness is 22 mm or more, a V-notch specimen is taken so that the center of the V-notch specimen's thickness is at a depth of t / 4 from the surface of the steel plate. If the steel plate thickness is 16 mm or more and less than 22 mm, a V-notch specimen is taken so that the center of the V-notch specimen's thickness is at a depth of 7 mm from the surface of the steel plate. A Charpy impact test is performed on the prepared V-notch specimens in accordance with JIS Z 2242:2018. At this time, the test temperature is varied in increments of 20°C within the range of -120 to 20°C. Three specimens are used for each test temperature to perform the Charpy impact test. Under the above conditions, the brittle fracture rate (%) of the V-notch specimens after testing at each temperature is determined. The test temperature (°C) and the obtained brittle fracture rate (%) are plotted, and an approximate curve is obtained by regression analysis. From the obtained approximation curve, the temperature (°C) at which the brittle fracture surface ratio becomes 50% is determined, and this temperature is defined as the fracture transition temperature vTrs (°C). If the fracture transition temperature vTrs is -60°C or lower, it is determined that excellent low-temperature toughness has been obtained.

[0103] [Regarding the ammonia SCC resistance of steel plates] In this embodiment, by satisfying features 1 to 4, the steel plates can be made to have excellent ammonia SCC resistance. Here, "excellent ammonia SCC resistance can be made" means that no cracks are observed in the test specimens in the ammonia SCC resistance evaluation method described in the next section.

[0104] [Ammonia SCC Resistance Evaluation Test] The ammonia SCC resistance of steel plates is evaluated by the following method. Four-point bending test specimens are taken from the steel plate. The four-point bending test specimens shall be 1.5 mm thick, 10 mm wide, and 70 mm long. The thickness direction of the four-point bending test specimens shall be the same as the thickness direction of the steel plate. More specifically, four-point bending test specimens shall be taken from the surface of the steel plate with a thickness of 1.5 mm in the thickness direction, 70 mm in the rolling direction, and 10 mm in the width direction. In other words, the four-point bending test specimens shall include the surface and surface layer region of the steel plate.

[0105] A strain gauge is attached to the center of the surface of the four-point bending test specimen (corresponding to the surface of the steel plate). The four-point bending test specimen with the strain gauge attached is mounted in a four-point bending jig. At this time, bending stress is applied to the four-point bending test specimen so that the amount of strain applied to the outer (tensile) surface (corresponding to the surface of the steel plate) of the four-point bending test specimen corresponds to the amount of strain corresponding to the yield stress YS. The four-point bending test specimen mounted in the four-point bending jig and 50 g of ammonium carbamate are placed in an autoclave. After evacuating the autoclave containing the four-point bending test specimen, the air is replaced with nitrogen. After replacing with nitrogen, 1000 g of liquid ammonia is introduced into the autoclave. The autoclave is immersed in a constant temperature bath, and while maintaining the temperature of the liquid ammonia inside the autoclave at 20°C, oxygen is introduced into the autoclave so that its partial pressure is 10% of the ammonia vapor pressure. Subsequently, a potentiostat is used to apply a potential of +1V (vs Pt) to the four-point bending test specimen, and it is held for 240 hours. After 240 hours, the cross-section of the four-point bending test specimen is observed with an optical microscope to check for the presence or absence of cracks.

[0106] In this embodiment, excellent ammonia SCC resistance means that no cracks are observed after 240 hours in the ammonia SCC resistance evaluation test described above. In this specification, "no cracks observed" means that no cracks are observed when the test specimen is observed using an optical microscope up to 500x magnification after the test.

[0107] [Regarding the Arrestability of Steel Sheets] In this embodiment, excellent arrestability can be obtained by satisfying features 1 to 4. Specifically, in this embodiment, excellent arrestability is determined to be obtained when the following three requirements are met: (Requirement 1) The fracture transition temperature vTrs is -60°C or lower. (Requirement 2) The arrest toughness value K ｃａ－３３℃ However, 6000 N / mm １．５ That concludes the explanation. (Requirement 3) The NDT (Nil-Ductility Transition Temperature) temperature is -70°C or lower. Requirement 1 is obtained by the low-temperature toughness evaluation method described above. Requirements 2 and 3 will be explained below.

[0108] [(Requirement 2) Arrest toughness value K] ｃａ－３３℃ [Measurement Test] Arrest Toughness Value K ｃａ－３３℃ The following method is used to determine the arrest toughness value K, in accordance with the "Test Method for Brittle Crack Arrest Toughness" specified in WES 2815 (2014). ｃａ－３３℃ The following measurements will be taken.

[0109] A plate-shaped test specimen is taken from the steel plate. The size of the test specimen shall be 500 mm in length, 500 mm in width, and the thickness shall be the thickness of the steel plate. The length direction of the test specimen shall be the rolling direction of the steel plate. The shape of the test specimen shall conform to Figure 5 on page 7 of WES 2815 (2014). A notch with a depth of 29 mm shall be formed at the center of the length direction of the test specimen. The notch shall be V-shaped with an angle of 30°. The depth direction of the notch shall be the width direction of the steel plate. In addition, a 0.2 mm wide slit shall be formed at the tip of the notch. The shape of the notch shall conform to Figure 6(a) on page 8 of WES 2815 (2014).

[0110] A temperature gradient is applied to the test specimen. Specifically, the temperature distribution is controlled so that the temperature gradient near the center of the specimen is 0.25 to 0.35 °C / mm, with the notched end of the specimen being at a lower temperature and the other end at a higher temperature. The test specimen with the temperature gradient is held under a predetermined tensile stress, and an impact load is applied to the notch of the specimen via a wedge to induce a brittle crack. The shape of the wedge conforms to Figure 3 on page 5 of WES 2815 (2014).

[0111] After the test is completed, the fracture surface is exposed to identify the crack arrest point and record the length of the arrest crack. Based on the test stress and the length of the arrested crack, K ｃａ Determine the value (arrest fracture toughness value).

[0112] The test was conducted at least four times, varying the temperature gradient conditions applied to the specimen and the applied stress conditions, and the crack initiation temperature and K ｃａ Determine the relationship between the values. From the obtained relationship, determine the K at -33°C. ｃａ The value is the arrest toughness value K ｃａ－３３℃ (N / mm １．５ )

[0113] [(Requirement 3) NDT Temperature Measurement Test] Determine the NDT temperature (°C) in accordance with the NRL drop test method specified in ASTM E208-06 (2012).

[0114] Specifically, a test specimen is taken from a steel plate. The size of the test specimen shall be 130 mm in length, 50 mm in width, and 16 mm in thickness. The center of the test specimen's thickness shall be the center of the steel plate's thickness, and the length direction of the test specimen shall be the rolling direction of the steel plate. If the thickness of the steel plate is 34 mm or more, the test specimen shall be taken so that the center of the test specimen's thickness is at a depth of t / 4 from the surface of the steel plate. If the thickness of the steel plate is 16 mm or more and less than 34 mm, the test specimen shall be taken so that the center of the test specimen's thickness is at a depth of t / 2 from the surface of the steel plate.

[0115] A brittle weld bead is formed by shielded metal arc welding (SMAW) at the center of the width of the specimen surface, running lengthwise. A welding rod NRL-S (5.0 mm diameter) for brittle weld beads is used to form the brittle weld bead. The brittle weld bead has a height of 4.5 ± 0.5 mm, a width of 14 ± 2 mm, and a length of 65 ± 5 mm. The brittle weld bead is formed so that its longitudinal center aligns with the longitudinal center of the specimen surface. A notch with a width of 1.5 mm is introduced in the depth direction from the surface of the brittle weld bead.

[0116] A test specimen, adjusted to a predetermined temperature, is placed on the test stand of the drop-weight test machine. At this time, the test specimen is positioned so that the embrittlement weld bead faces downwards. A drop weight of a predetermined weight is placed at a predetermined height so that the drop energy is 350 J. The drop weight is allowed to free-fall and impact the test specimen.

[0117] After dropping the weight, observe whether the crack originating from the embrittlement weld bead reaches at least one end in the width direction of the specimen. If the crack reaches one end in the width direction, it is judged as "Break". If the crack does not reach the width end, it is judged as "No-Break". The drop weight test is conducted using two specimens, starting at -100°C and changing the test temperature in 5°C increments. Specifically, if both specimens are judged as No-Break, the test is repeated at a 5°C lower temperature. On the other hand, if at least one of the two specimens is judged as Break, the test is repeated at a 5°C higher temperature. The non-ductility transition temperature (NDT temperature) (°C) is defined as a temperature 5°C lower than the lowest test temperature at which both specimens were judged as No-Break.

[0118] [Applications of the steel plate of this embodiment] The steel plate of this embodiment is suitable for use in low-temperature ammonia environments. For example, it is suitable for use in ammonia tanks for storing liquid ammonia and in line pipes that make up pipelines for transporting liquid ammonia. However, the steel plate of this embodiment can also be applied to applications other than those mentioned above.

[0119] [Preferred plate thickness of steel plate in this embodiment] The plate thickness of the steel plate in this embodiment is not particularly limited. For example, the plate thickness of the steel plate in this embodiment is 16 to 70 mm. The preferred lower limit of the plate thickness is 17 mm. The preferred upper limit of the plate thickness is 60 mm, more preferably 50 mm, even more preferably 45 mm, and even more preferably 40 mm.

[0120] [Manufacturing Method] An example of a manufacturing method for the steel sheet of this embodiment will be described. Note that the manufacturing method for the steel sheet of this embodiment is not limited to the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the steel sheet of this embodiment. An example of a manufacturing method for the steel sheet of this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Hot rough rolling step (Step 3) Hot finish rolling step Each step will be described below.

[0121] [(Step 1) Material Preparation Process] In the material preparation process, a slab with a chemical composition that satisfies characteristic 1 is prepared. The slab is manufactured by, for example, the following method: Molten steel with a chemical composition that satisfies characteristic 1 is manufactured. The slab is manufactured by a casting method using the above molten steel. For example, the slab is manufactured by a well-known continuous casting method using the above molten steel.

[0122] [(Process 2) Hot Rough Rolling Process] In the hot rough rolling process, a reverse-type rough rolling mill is used to perform hot rolling (rough rolling) on ​​the prepared slab to produce an intermediate steel plate. The heating temperature of the slab in the hot rough rolling process is, for example, 1100 to 1250°C. The time the slab stays in the heating furnace is 30 minutes or more, preferably 60 minutes or more. There is no particular upper limit to the time the slab stays in the furnace, but for example it is 240 minutes. In a reverse-type rough rolling mill, one pass is defined as the time the slab passes through the rough rolling mill once and receives external force (reduction) from the pair of work rolls of the rough rolling mill. In a reverse-type rough rolling mill, there is a forward pass in which the slab is transported and reduced from upstream to downstream of the rolling line, and a reverse pass in which the slab is transported and reduced from downstream to upstream of the rolling line.

[0123] [(Process 3) Hot Finish Rolling Process] In the hot finish rolling process, the intermediate steel sheet is further rolled (finish rolled) using a reverse-type finish rolling mill to produce a steel sheet (hot-rolled steel sheet). In the reverse-type finish rolling mill, as with the rough rolling mill, one pass is defined as the time it takes for the steel sheet to pass through the mill once and receive external force (reduction) from the pair of work rolls of the finish rolling mill. In the finish rolling mill, there are also forward passes and reverse passes.

[0124] [Manufacturing conditions in the hot rough rolling and hot finish rolling processes] In the hot rough rolling process, the following conditions must be met: (Condition 1) The surface temperature T1 of the steel sheet at the entrance of the rough rolling mill in the final pass must be 900°C or higher. (Condition 2) The average reduction ratio R1 per pass must be 7.0% or higher. (Condition 3) The number of passes PN1 with a reduction ratio of 9.5% or higher must be 3 or more. In the hot finish rolling process, the following conditions must be met: (Condition 4) The surface temperature T2 of the steel sheet at the entrance of the finish rolling mill in the final pass must be 755°C or higher. (Condition 5) The average reduction ratio R2 per pass must be 7.0% or higher. (Condition 6) The number of passes PN2 with a reduction ratio of 8.0% or higher must be 4 or more. (Condition 7) The cooling rate CR after finish rolling must be 7 to 40°C / second. Conditions 1 to 7 will be explained below.

[0125] [Regarding Condition 1] In the hot rough rolling process, the surface temperature T1 of the steel sheet at the entry side of the rough rolling mill in the final pass shall be 900°C or higher. Surface temperature T1 refers to the surface temperature (°C) of the steel sheet measured by a thermometer placed at the entry side of the rough rolling mill when the final pass is performed. If the surface temperature T1 is less than 900°C, dislocations introduced by rolling will remain in the surface region of the manufactured steel sheet without sufficient recovery. As a result, the hardness of the surface region of the steel sheet after cooling will be excessively high. Specifically, the maximum value HVmax of the Vickers hardness in the surface region will exceed 240HV. Therefore, the surface temperature T1 shall be 900°C or higher.

[0126] [Regarding Condition 2] The average reduction ratio R1 per pass in the hot rough rolling process shall be 7.0% or more. If the average reduction ratio R1 is less than 7.0%, the reduction per pass is insufficient. In this case, the minor axis of the crystal grain HAGB in the t / 4 region of the manufactured steel sheet exceeds 30.0 μm.

[0127] [Regarding Condition 3] In the hot rough rolling process, the number of passes PN1 with a reduction ratio of 9.5% or more shall be 3 or more. This ensures sufficient penetration into the interior of the steel sheet. If the number of passes PN1 with a reduction ratio of 9.5% or more is 2 or less, the manufactured steel sheet shall have F1 (= As / A ｔ／４ ) may exceed 5.00. Therefore, the number of passes PN1 with a reduction ratio of 9.5% or higher should be 3 or more.

[0128] [Regarding Condition 4] In the hot finish rolling process, the surface temperature T2 of the steel sheet at the entrance of the finish rolling mill in the final pass shall be 755°C or higher. Surface temperature T2 refers to the surface temperature (°C) of the steel sheet measured by a thermometer placed at the entrance of the finish rolling mill when the final pass is performed. If the surface temperature T2 is less than 755°C, rolling may occur in the two-phase region. In this case, the bainite area ratio in the surface region of the manufactured steel sheet may be less than 70%. Therefore, the surface temperature T2 shall be 755°C or higher.

[0129] [Regarding Condition 5] The average reduction ratio R2 per pass in the hot finishing rolling process shall be 7.0% or more. If the average reduction ratio R2 is less than 7.0%, the reduction per pass is insufficient. In this case, in the manufactured steel sheet, F1 (= As / A ｔ／４ ) may exceed 5.00. Therefore, the average reduction ratio R2 should be 7.0% or higher.

[0130] [Regarding Condition 6] In the hot finishing rolling process, the number of passes PN2 with a reduction ratio of 8.0% or more shall be 4 or more. This ensures sufficient penetration into the interior of the steel sheet. If the number of passes PN2 with a reduction ratio of 8.0% or more is 3 or less, the manufactured steel sheet shall have F1 (= As / A ｔ／４ ) may exceed 5.00. Therefore, the number of passes PN2 with a reduction ratio of 8.0% or higher should be four or more.

[0131] [Regarding Condition 7] In the hot finish rolling process, the cooling rate CR after finish rolling shall be 7 to 40°C / second. If the cooling rate CR is less than 7°C / second, the short axis of the crystal grain HAGB in the t / 4 region of the manufactured steel sheet will exceed 30.0 μm, or F1 (= As / A ｔ／４ ) may exceed 5.00. On the other hand, if the cooling rate CR exceeds 40°C / second, the maximum Vickers hardness HVmax in the surface region of the manufactured steel sheet may exceed 240HV. Therefore, the cooling rate CR should be set to 7 to 40°C / second.

[0132] The cooling rate CR is determined by the following method: The difference between the surface temperature of the steel plate at the start of cooling (°C) and the surface temperature of the steel plate at the end of cooling (°C) is divided by the cooling time (seconds). The integer value obtained by rounding the result to the first decimal place is the cooling rate CR (°C / second). The surface temperature of the steel plate at the start of cooling (°C) is the surface temperature of the steel plate when cooling is started in a cooling device that performs forced water cooling. This surface temperature is measured by a radiation thermometer placed near the cooling device. Similarly, the surface temperature of the steel plate at the end of cooling (°C) is the surface temperature of the steel plate when cooling in the cooling device is terminated. This surface temperature is measured by a radiation thermometer placed near the cooling device.

[0133] The steel sheet of this embodiment is manufactured through the above manufacturing process. As stated above, the steel sheet of this embodiment may be manufactured by a manufacturing method other than the one described above.

[0134] [About the ammonia tank of this embodiment] The ammonia tank of this embodiment comprises an outer shell, an inner shell, and an insulating material. The outer shell and insulating material have a well-known configuration. The inner shell is made of steel plate of this embodiment. Specifically, the inner shell includes a plurality of base metal parts and a welded metal part. The welded metal part is arranged between the plurality of base metal parts and is joined to the base metal parts. The base metal parts include a heat-affected zone and a normal part other than the heat-affected zone.

[0135] The normal portion satisfies the same characteristics as the steel sheet of the embodiment described above. Specifically, the chemical composition of the normal portion is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0% or less It contains 1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB is 4.00 or more, the maximum Vickers hardness HVmax is 240 HV or less. Let t be the thickness of the normal part of the plate. In the t / 4 region, which is the region at a depth of t / 4 from the surface of the normal part, the bainite area ratio is 50% or more, the minor axis of the bainite crystal grains HAGB is 30.0 μm or less, and the aspect ratio As of the bainite crystal grains HAGB in the surface region is the same as the aspect ratio A of the bainite crystal grains HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0136] In the ammonia tank of this embodiment, the normal portion of the base material of the inner shell exhibits high strength, excellent low-temperature toughness, excellent ammonia SCC resistance, and excellent arrestability, similar to the steel plate of this embodiment.

[0137] [Regarding the line pipe of this embodiment] The line pipe of this embodiment is made of the steel plate of this embodiment. Specifically, the line pipe includes a base material portion and a welded metal portion extending in the direction of the pipe axis. The base material portion includes a heat-affected zone and a normal portion other than the heat-affected zone.

[0138] The normal portion satisfies the same characteristics as the steel sheet of the embodiment described above. Specifically, the chemical composition of the normal portion is, in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0% or less It contains 1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities. In the surface region from the surface to a depth of 0.5 mm in the normal part, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB is 4.00 or more, the maximum Vickers hardness HVmax is 240 HV or less. Let t be the thickness of the normal part of the plate. In the t / 4 region, which is the region at a depth of t / 4 from the surface of the normal part, the bainite area ratio is 50% or more, the minor axis of the bainite crystal grains HAGB is 30.0 μm or less, and the aspect ratio As of the bainite crystal grains HAGB in the surface region is the same as the aspect ratio A of the bainite crystal grains HAGB in the t / 4 region. ｔ／４ The equation (1) is satisfied. 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1)

[0139] In the line pipe of this embodiment, high strength, excellent low-temperature toughness, excellent ammonia SCC resistance, and excellent arrestability can be obtained in the normal portion of the base material of the welded steel pipe constituting the line pipe, similar to the steel plate of this embodiment.

[0140] The effects of one embodiment of the steel sheet of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this one example of conditions.

[0141] Hot-rolled steel sheets having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0142]

[0143]

[0144] In Table 1, a "-" indicates that the corresponding element content was intentionally omitted.

[0145] Specifically, slabs were manufactured by continuous casting of molten steel. Hot rough rolling and hot finish rolling processes were performed on the slabs. In the hot rough rolling process, the slabs were heated at 1100 to 1250°C for 60 to 120 minutes. After heating, the slabs were rolled in a rough rolling mill to produce intermediate steel plates. Furthermore, the intermediate steel plates were rolled using a finish rolling mill to produce steel plates. The surface temperature T1 (°C) (Condition 1), average reduction ratio R1 (%) (Condition 2), number of passes PN1 (Condition 3), surface temperature T2 (°C) (Condition 4), average reduction ratio R2 (%) (Condition 5), number of passes PN2 (Condition 6), and cooling rate CR (°C / sec) (Condition 7) are as shown in Table 2. The thickness of the steel plates was 16 to 40 mm.

[0146]

[0147] [Evaluation Tests] The following evaluation tests were performed on the hot-rolled steel sheets for each test number: (Test 1) Measurement of bainite area ratio in the surface region (Test 2) Measurement of aspect ratio As of HAGB crystal grains in the surface region (Test 3) Measurement of Vickers hardness in the surface region (Test 4) Measurement of bainite area ratio in the t / 4 region (Test 5) Measurement of minor axis and aspect ratio A of HAGB crystal grains in the t / 4 region ｔ／４ Measurement Test (Test 6) Strength Measurement Test (Test 7) ​​Low Temperature Toughness Evaluation Test (Test 8) Ammonia Resistance SCC Evaluation Test (Test 9) Arrestability Evaluation Test Tests 1 to 9 will be explained below.

[0148] [(Test 1) Measurement Test of Bainite Area Ratio in the Surface Region] Based on the method described in [Method for Measuring Bainite Area Ratio in the Surface Region] above, the bainite area ratio (%) of the surface region of the steel plate for each test number was determined. The results obtained are shown in the "Bainite Area Ratio (%)" column of the "Surface Region" column in Table 3.

[0149]

[0150] [(Test 2) Measurement Test of Aspect Ratio As of Surface Grain HAGB] Based on the method described in [Method for Measuring Aspect Ratio As of Surface Grain HAGB of Bainite] above, the aspect ratio As of the surface grain HAGB of the steel plate for each test number was determined. The obtained aspect ratio As of the surface grain HAGB is shown in the "Aspect Ratio As" column of the "Surface Region" column in Table 3.

[0151] [(Test 3) Vickers hardness measurement test of the surface region] Based on the method described in [Method for measuring the maximum value HVmax of Vickers hardness in the surface region] above, the maximum value HVmax (HV) of Vickers hardness in the surface region of the steel plate for each test number was determined. The obtained results are shown in the "HVmax (HV)" column of the "Surface Region" column in Table 3.

[0152] [(Test 4) Measurement Test of Bainite Area Ratio in the t / 4 Region] Based on the method described in [Method for Measuring Bainite Area Ratio in the t / 4 Region] above, the bainite area ratio (%) in the t / 4 region of the steel plate for each test number was determined. The results obtained are shown in the "Bainite Area Ratio (%)" column of the "t / 4 Region" column in Table 3.

[0153] [(Test 5) Short axis and aspect ratio A of crystal grain HAGB in the t / 4 region ｔ／４ [Measurement Test] The above-mentioned [short axis and aspect ratio A of the bainite grain HAGB in the t / 4 region] ｔ／４ Based on the method described in [Measurement Method], the minor axis and aspect ratio A of the crystal grain HAGB in the surface region of the steel plate for each test number are measured. ｔ／４ The obtained minor axis (μm) of the crystal grain HAGB is shown in the "Minor axis (μm) of crystal grain HAGB" column in the "t / 4 region" column of Table 3, and the aspect ratio A ｔ／４ In Table 3, in the "t / 4 region" column, under "Aspect Ratio A" ｔ／４ This is shown in the " " column. Furthermore, the F1 value (= As / A) is shown in the "F1" column. ｔ／４ ) indicates.

[0154] [(Test 6) Strength Measurement Test] Based on the method described in [Method for Measuring the Strength of Steel Plates] above, the yield strength YS (MPa) and tensile strength TS (MPa) of the steel plates for each test number were determined. The results obtained are shown in the "Yield Strength YS (MPa)" and "Tensile Strength TS (MPa)" columns of Table 4.

[0155]

[0156] [(Test 7) ​​Low-Temperature Toughness Evaluation Test] Based on the method described in [Method for Evaluating Low-Temperature Toughness of Steel Sheets] above, the fracture transition temperature vTrs (°C) of the steel sheet for each test number was determined. The obtained fracture transition temperatures vTrs (°C) are shown in the "vTrs (°C)" column of Table 4.

[0157] [(Test 8) Ammonia SCC Resistance Evaluation Test] Based on the method described in the [Ammonia SCC Resistance Evaluation Test] above, the ammonia SCC resistance of the steel plates for each test number was evaluated. The evaluation results are shown in Table 4. In the "Ammonia SCC Resistance" column of Table 4, "pass" indicates that no cracks were observed in the test piece as a result of the ammonia SCC resistance evaluation test, and excellent ammonia SCC resistance was obtained. "fail" indicates that cracks were confirmed in the test piece as a result of the ammonia SCC resistance evaluation test, and sufficient ammonia SCC resistance was not obtained.

[0158] [(Test 9) Arrestability Evaluation Test] As mentioned above [(Requirement 2) Arrestability Toughness Value K] ｃａ－３３℃ Based on the measurement test, the arrest toughness value K ｃａ－３３℃ (N / mm １．５ ) was determined. Furthermore, the NDT temperature (°C) was determined based on the above-mentioned [(Requirement 3) NDT temperature measurement test]. The obtained results are shown in Table 4, "K ｃａ－３３℃ (N / mm １．５ This is shown in the ")" column and the "NDT (°C)" column.

[0159] [Evaluation Results] Referring to Tables 1 to 3, test numbers 1 to 18 satisfied features 1 to 4. Therefore, the yield strength YS of the steel plates for these test numbers was 360 MPa or higher, and the tensile strength TS was 450 MPa or higher. Furthermore, the fracture transition temperature vTrs (°C) was -60°C or lower, indicating excellent low-temperature toughness. In addition, no cracks were observed in the ammonia SCC resistance evaluation test, indicating excellent ammonia SCC resistance. Furthermore, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ Furthermore, the NDT temperature was below -70°C, and excellent arrest properties were obtained.

[0160] On the other hand, in test numbers 19 and 20, the surface temperature T1 was too low. As a result, the maximum Vickers hardness HVmax in the surface region exceeded 240 HV. Consequently, the fracture transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness could not be obtained. Furthermore, cracking was observed in the ammonia SCC resistance evaluation test, and sufficient ammonia SCC resistance could not be obtained. In addition, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The value was less than -70°C, and the NDT temperature exceeded -70°C, resulting in insufficient arrest capability.

[0161] In tests 21 and 22, the average reduction ratio R1 was less than 7.0%. Therefore, in the manufactured steel sheets, the minor axis of the HAGB grains in the t / 4 region exceeded 30.0 μm. As a result, the fracture transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness could not be obtained. Furthermore, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The result was less than [a certain value], and sufficient arrest power was not achieved.

[0162] In tests 23 and 24, the number of passes PN1 was less than 3. Therefore, in the manufactured steel plates, F1 (= As / A ｔ／４ The value exceeded 5.00. As a result, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The result was less than [a certain value], and sufficient arrest power was not achieved.

[0163] In tests 25 and 26, the surface temperature T2 was less than 755°C. Therefore, the bainite area ratio in the surface region of the manufactured steel sheet was less than 70%. Furthermore, the maximum Vickers hardness HVmax in the surface region exceeded 240HV. As a result, cracking was observed in the ammonia SCC resistance evaluation test, and sufficient ammonia SCC resistance was not obtained. Furthermore, the arrest toughness value K... ｃａ－３３℃ 6000 N / mm １．５ The value was less than -70°C, and the NDT temperature exceeded -70°C, resulting in insufficient arrest capability.

[0164] In tests 27 and 28, the average reduction ratio R2 was less than 7.0%. Therefore, in the manufactured steel plates, F1 (= As / A ｔ／４ The value exceeded 5.00. As a result, the fracture surface transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness could not be obtained. Furthermore, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The result was less than [a certain value], and sufficient arrest power was not achieved.

[0165] In test numbers 29 and 30, the number of passes PN2 was less than 4. Therefore, in the manufactured steel plate, F1 (= As / A ｔ／４ The value exceeded 5.00. As a result, the fracture surface transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness could not be obtained. Furthermore, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The result was less than [a certain value], and sufficient arrest power was not achieved.

[0166] In tests 31 and 32, the cooling rate CR was too slow. As a result, in test 31, the minor axis of the crystal grain HAGB exceeded 30.0 μm, and in test 32, F1 exceeded 5.00. Consequently, the fracture surface transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness could not be obtained. Furthermore, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The result was less than [a certain value], and sufficient arrest power was not achieved.

[0167] In tests 33 and 34, the cooling rate CR was too fast. As a result, the maximum Vickers hardness HVmax in the surface region exceeded 240 HV. Consequently, the fracture transition temperature vTrs (°C) was higher than -60°C, and sufficient low-temperature toughness was not obtained. Furthermore, cracking was observed in the ammonia SCC resistance evaluation test, and sufficient ammonia SCC resistance was not obtained. In addition, the arrest toughness value K ｃａ－３３℃ 6000 N / mm １．５ The value was less than -70°C, and the NDT temperature exceeded -70°C, resulting in insufficient arrest capability.

[0168] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Steel plate, with a chemical composition in mass percent of: C: 0.02-0.12%, Si: 0.01-0.30%, Mn: 0.50-2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001-0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001-0.100%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100% It contains rare earth elements: 0 to 0.0100%, Sn: 0 to 0.10%, and Sb: 0 to 0.050%, with the remainder being Fe and impurities, and in the surface region up to a depth of 0.5 mm from the surface of the steel plate, the bainite area ratio is 70% or more, and the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB, where the crystal orientation difference of the bainite is 15° or more, is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less, and in the t / 4 region, which is the region from the surface of the steel plate to a depth of t / 4, where the thickness of the steel plate is t, the bainite area ratio is 50% or more, and the minor axis of the bainite grains HAGB is 30.0 μm or less. The aspect ratio As of the crystal grain HAGB in the surface region and the aspect ratio A of the crystal grain HAGB in the t / 4 region. ｔ／４ A steel plate that satisfies equation (1). 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1) 2. A steel sheet according to claim 1, wherein the chemical composition contains, by mass%, one or more elements selected from the group consisting of: Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050%.

3. The material comprises an outer shell, an inner shell, and a heat insulating material disposed between the outer shell and the inner shell, the inner shell includes a plurality of base material parts, and a weld metal part disposed between the plurality of base material parts and bonded to the base material parts, the base material parts include a heat-affected zone and a normal part other than the heat-affected zone, the normal part having a chemical composition in mass%, C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.00%, Ni: 0 to 1.00%. It contains Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities, and in the surface region from the surface of the normal part to a depth of 0.5 mm, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB is 4.00 or more, and the maximum Vickers hardness HVmax is 240 HV or less. Let t be the thickness of the normal portion. In the t / 4 region, which is the region at a depth of t / 4 from the surface of the normal portion, the bainite area ratio is 50% or more, the minor axis of the bainite grain HAGB is 30.0 μm or less, and the aspect ratio As of the bainite grain HAGB in the surface region is equal to the aspect ratio A of the bainite grain HAGB in the t / 4 region. ｔ／４ An ammonia tank that satisfies equation (1). 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1) 4. An ammonia tank according to claim 3, wherein the chemical composition contains one or more elements selected from the group consisting of, by mass%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050%.

5. The pipe comprises a base metal portion and a weld metal portion extending in the axial direction of the pipe, the base metal portion comprises a heat-affected zone and a normal portion other than the heat-affected zone, the normal portion having a chemical composition in mass percent of: C: 0.02 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, B: 0 to 0.0050% It contains V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.10%, and Sb: 0-0.050%, with the remainder being Fe and impurities, and in the surface region from the surface of the normal part to a depth of 0.5 mm, the bainite area ratio is 70% or more, the aspect ratio As of the large-angle grain boundaries of the bainite grains HAGB having a crystal orientation difference of 15° or more is 4.00 or more, the maximum Vickers hardness HVmax is 240HV or less, and, with the plate thickness of the normal part being t, in the t / 4 region which is the region from the surface of the normal part to a depth of t / 4, the bainite area ratio is 50% or more. The minor axis of the HAGB crystal grains of the bainite is 30.0 μm or less, and the aspect ratio As of the HAGB crystal grains in the surface region and the aspect ratio A of the HAGB crystal grains in the t / 4 region are... ｔ／４ A line pipe that satisfies equation (1). 1.00 ≤ As / A ｔ／４ ≤ 5.00 (1) 6. A line pipe according to claim 5, wherein the chemical composition contains one or more elements selected from the group consisting of, by mass%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0050%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Ca: 0.0001 to 0.0100%, rare earth elements: 0.0001 to 0.0100%, Sn: 0.01 to 0.10%, and Sb: 0.001 to 0.050%.

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