steel plate

Abstract

To provide a steel sheet with high strength, excellent low-temperature toughness, corrosion resistance, and lamellar tear resistance. [Solution] The steel sheet of this disclosure has the chemical composition described in the specification, a Ceq defined by formula (1) described in the specification of 0.44 to 0.50, a SnEQ defined by formula (2) of 0.10 or more, an MV defined by formula (3) of 0.30 to 0.90, an SC defined by formula (4) of 0.20 to 15.00, a plate thickness of more than 105 to 125 mm, a tensile strength of 570 MPa or more, and the average circular equivalent diameter D of the prior austenite grains at the center of the plate thickness of the L cross section. Lave The particle size is 40 μm or less, and the aspect ratio is AR L The average equivalent diameter D of the prior austenite grains is 2.0 or less, and at the center of the plate thickness of the C section, Cave The particle size is 40 μm or less, and the aspect ratio is AR C The Vickers hardness in the segregation region is 250 HV or less, and the maximum length of porosity is 0.5 mm or less.

Description

[Technical Field]

[0001] This disclosure relates to steel plates. [Background technology]

[0002] In recent years, with the increasing size and longer lifespan of structures such as bridges and offshore structures, there has been a growing demand for steel plates with high strength and excellent low-temperature toughness. Furthermore, for the aforementioned structures constructed in coastal areas or at sea, excellent corrosion resistance in chloride-corrosive environments caused by seawater and other elements is also required.

[0003] Technologies for improving the strength, low-temperature toughness, and corrosion resistance of steel plates are proposed in International Publication No. 2019 / 116520 (Patent Document 1) and Japanese Patent Application Publication No. 2012-144799 (Patent Document 2).

[0004] The steel sheet disclosed in Patent Document 1 has a chemical composition in mass percent of: C: 0.01~0.20%, Si: 0.01~1.00%, Mn: 0.05~3.00%, P: 0~0.050%, S: 0~0.0100%, Sn: 0.05~0.25%, Al: 0~0.100%, N: 0.0005~0.0100%, O: 0.0001~0.0100%, Ti: 0~0.050%, Nb: 0~0.050%, V: 0~0.050%, W: 0~0.050%, Mo: 0~0.05 The composition is 0%, Cu:0~0.10%, Ni:0~0.05%, Cr:0~0.10%, Sb:0~0.05%, B:0~0.0010%, Ca:0~0.0100%, Mg:0~0.0100%, REM:0~0.0100%, and the remainder is Fe and impurities, with an Sn ratio [a / b], which is the ratio of Sn concentration at grain boundaries [a] to Sn concentration within grains [b], of 1.2 or less. Patent Document 1 states that by setting the Sn ratio to 1.2 or less in this steel sheet, it is possible to achieve both excellent low-temperature toughness and excellent corrosion resistance in a high-strength steel sheet.

[0005] The steel sheet disclosed in Patent Document 2 contains, by mass%, C: 0.02~0.1%, Si: 0.03~0.5%, Mn: 0.5~2.0%, Al: 0.002~0.08%, N: 0.001~0.008%, Nb: 0.003~0.05%, Ti: 0.003~0.05%, and Sn: 0.03~0.50%, with the remainder being Fe and impurities. The microstructure of the steel sheet consists of ferrite and a hard secondary phase transformed from unrecrystallized austenite, with a ferrite grain size of 2~15 μm and an aspect ratio of less than 10 for the hard secondary phase. Patent Document 2 states that by including 0.03~0.50% Sn in this steel sheet, excellent corrosion resistance can be obtained in a high-strength steel sheet. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 116520 [Patent Document 2] Japanese Patent Publication No. 2012-144799 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, due to the increasing size of the structures mentioned above, there is now a demand for extremely thick steel plates exceeding 105 mm in thickness, which had not been considered previously. When such extremely thick steel plates are used in large structures, the plates are joined together by welding. When joints such as cross joints, T-joints, and square joints are formed by welding steel plates, tensile stress in the thickness direction is generated in the steel plate due to the volume contraction of the weld metal as it cools after welding. This causes cracks to occur inside the steel plate in a direction parallel to the surface of the steel plate. Such cracks are called lamellar tears. The thicker the plate, the more likely lamellar tears are to occur. Therefore, steel plates used in the aforementioned structures are required not only to have high strength, excellent low-temperature toughness, and excellent corrosion resistance, but also to have excellent lamellar tear resistance, which is a characteristic that suppresses the occurrence of lamellar tears. Patent documents 1 and 2 mentioned above do not consider the lamellar tear resistance of steel plates.

[0008] The purpose of this disclosure is to provide a steel sheet that has high strength, excellent low-temperature toughness, excellent corrosion resistance, and excellent lamellar tear resistance. [Means for solving the problem]

[0009] The steel sheet disclosed herein has a chemical composition in mass percent of: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, Sn: 0.02~0.4% It contains 0%, N: 0.0010-0.0070%, O: 0.0005-0.0040%, and Ca: 0.0001-0.0080%, with the remainder being Fe and impurities, with a Ceq defined by formula (1) of 0.44-0.50, a SnEQ defined by formula (2) of 0.10 or higher, an MV defined by formula (3) of 0.30-0.90, and an SC defined by formula (4) of 0.20-15.00. The above steel plate has a thickness of over 105 to 125 mm and a tensile strength of 570 MPa or more. At the center of the thickness of the L-shaped cross section parallel to the rolling direction and thickness direction of the steel plate, the average equivalent circular diameter D of the prior austenite grains is... Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L The average equivalent diameter D of the prior austenite grains is 2.0 or less. Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C The value is 2.0 or less. The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less, and the maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) MV = Mo + 4V (3) SC = S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol.

[0010] The steel sheet disclosed herein has a chemical composition in mass percent of: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, Sn: 0.02~0.40%, N: 0.0010~0.00 It contains 70% O: 0.0005~0.0040% and Ca: 0.0001~0.0080%, and further contains one or more selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities, with a Ceq defined by formula (1) of 0.44~0.50, a SnEQ defined by formula (2) of 0.10 or higher, an MV defined by formula (3) of 0.30~0.90, and an SC defined by formula (4) of 0.20~15.00. The above steel plate has a thickness of over 105 to 125 mm and a tensile strength of 570 MPa or more. At the center of the thickness of the L-shaped cross section parallel to the rolling direction and thickness direction of the steel plate, the average equivalent circular diameter D of the prior austenite grains is... Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L The average equivalent diameter D of the prior austenite grains is 2.0 or less. Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C The value is 2.0 or less. The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less, and the maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) MV = Mo + 4V (3) SC = S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol. [Group 1] One or more elements selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.10% or less. [Group 2] One or more elements selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.10% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group 3] One or more elements selected from the group consisting of Mg: 0.010% or less, Sr: 0.020% or less, Ba: 0.010% or less, and rare earth elements: 0.020% or less. [Effects of the Invention]

[0011] The steel sheet disclosed herein has high strength, excellent low-temperature toughness, and excellent corrosion resistance, and furthermore, excellent lamellar tear resistance. [Brief explanation of the drawing]

[0012] [Figure 1A] Figure 1A is a schematic diagram illustrating the method for preparing test specimens for evaluating lamellar tear resistance. [Figure 1B] Figure 1B is a schematic diagram following Figure 1A. [Figure 1C] Figure 1C is a schematic diagram following Figure 1B. [Figure 1D] Figure 1D is a schematic diagram following Figure 1C. [Figure 1E] Figure 1E is a schematic diagram following Figure 1D. [Modes for carrying out the invention]

[0013] The inventors initially investigated steel sheets possessing high strength, excellent low-temperature toughness, excellent corrosion resistance, and excellent lamellar tear resistance from the perspective of chemical composition. As a result, the inventors found that the composition of the materials in mass percent is as follows: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, N: 0.0010~0.0070%, O: 0.0005~0.0040%, Cu: 0~0.80%, B: 0~0.0050%, Zr: 0~0.05%, Ta: 0~0.10%, Cr: 0~0.20%, It was considered that a steel sheet having a chemical composition containing W: 0-0.80%, Sb: 0-0.10%, As: 0-0.05%, Bi: 0-0.10%, Se: 0-0.05%, Te: 0-0.05%, Zn: 0-0.05%, Ga: 0-0.05%, Ge: 0-0.05%, Co: 0-0.50%, Hf: 0-0.05%, Mg: 0-0.010%, Sr: 0-0.020%, Ba: 0-0.010%, and rare earth elements: 0-0.020%, with the remainder being Fe and impurities, would have high strength, excellent low-temperature toughness, excellent corrosion resistance, and potentially excellent lamellar tear resistance.

[0014] Therefore, the inventors further investigated means to obtain high strength, excellent low-temperature toughness, excellent corrosion resistance, and excellent lamellar tear resistance in steel sheets with the above-mentioned chemical composition. As a result, the following findings were obtained.

[0015] To increase the strength of steel plates, it is effective to improve their hardenability. Ceq, defined in equation (1), is an index related to the hardenability of steel plates. If Ceq is between 0.44 and 0.50, the hardenability of the steel plate is sufficiently high. Therefore, the strength of the steel plate increases. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1)

[0016] The corrosion of steel plates progresses through the following mechanism: When water adheres to the surface of the steel plate, Fe ions dissolve. The dissolved Fe ions combine with oxygen in the air to form iron oxide, causing rust. Within the adhering water, the rust undergoes hydrolysis to generate hydrogen ions, forming an acidic aqueous solution. In chloride-corrosive environments containing chlorides, such as coastal areas and the sea, chloride ions are further added to the adhering water. As a result, the acidity of the formed acidic aqueous solution becomes even stronger. Consequently, the corrosion resistance of steel plates is further reduced in chloride-corrosive environments.

[0017] To enhance the corrosion resistance of steel sheets in chloride-corrosive environments, an additional 0.02-0.40% of Sn is added to the above-mentioned chemical composition. Furthermore, the SnEQ, as defined by formula (2), is set to 0.10 or higher. SnEQ is an index of the corrosion resistance of steel sheets in chloride-corrosive environments. Sn, W, Ni, and Mo in the chemical composition of the steel sheet suppress the elution of Fe ions, thereby enhancing the corrosion resistance of the steel sheet in chloride-corrosive environments. By including Sn and further setting the SnEQ to 0.10 or higher, the corrosion resistance of the steel sheet in chloride-corrosive environments is further enhanced. SnEQ = Sn + W + Ni / 10 + Mo / 4 (2)

[0018] However, Sn has a small solid-liquid partition coefficient. Therefore, during the casting of slabs in the steel sheet manufacturing process, Sn moves from the solid phase to the liquid phase. As a result, Sn becomes concentrated in the central segregation region of the slab during casting, and the melting point of the liquid phase in the central segregation region decreases. Consequently, segregation of alloying elements in the central segregation region is further promoted. Therefore, the hardness of the central segregation region in the manufactured steel sheet becomes excessively high. Furthermore, if the melting point of the liquid phase in the central segregation region decreases, porosity is more likely to form in the central segregation region. As described above, when Sn is included, the hardness of the segregation region in the center of the steel sheet thickness tends to become excessively high, and porosity is easily formed in the segregation region. Therefore, lamellar tears are more likely to occur starting from the segregation region.

[0019] Furthermore, when using extremely thick steel plates exceeding 105 mm in thickness, sufficient strength may not be obtained.

[0020] Based on the above findings, the inventors further investigated methods to obtain high strength even in extremely thick steel plates, and to obtain excellent lamellar tear resistance even when Sn is included. As a result, the following findings (A) to (C) were obtained.

[0021] (A) In the case of a steel plate having the above chemical composition, if the plate thickness exceeds 105 mm, the MV defined by formula (3) shall be 0.30 to 0.90. MV = Mo + 4V (3)

[0022] Mo and V increase the strength of steel plates through precipitation strengthening. If MV is between 0.30 and 0.90, high strength can be ensured even for steel plates with a thickness exceeding 105 mm. Furthermore, Mo and V, like Sn, are concentrated in the segregation region. However, while the concentration of Sn in the segregation region is several tens of times higher than the concentration of Sn in other regions, the concentrations of Mo and V in the segregation region are less than 1.5 times higher than the concentrations of Mo and V in other regions. Therefore, even if Mo and V are concentrated in the segregation region, it does not significantly affect the hardness in that region.

[0023] (B) Furthermore, to improve lamellar tear resistance, it is effective to suppress the concentration of alloying elements in the segregation region. S, an impurity element, has a small solid-liquid partition coefficient, similar to Sn, and is prone to concentration in the segregation region at the center of the steel sheet thickness. Therefore, S, like Sn, significantly lowers the melting point of the liquid phase in the central segregation region during solidification. S also combines with Mn to form Mn sulfides. Mn sulfides are stretched during rolling. Therefore, similar to porosity, they become the starting point for cracking (lamellar tears).

[0024] Therefore, the above chemical composition includes Ca as an essential element, and furthermore, the SC defined by formula (4) is set to 0.20 to 15.00. During the solidification process of steel when slab casting, Ca combines with S to form sulfides. This suppresses the concentration of S in segregation regions and the formation of Mn sulfides. As a result, the lamellar tear resistance of the steel sheet can be improved. SC = S / Ca (4)

[0025] (C) The cracks generated in the segregation area of the steel plate propagate along the grain boundaries of the prior austenite grains. When the number of branches of the prior austenite grain boundaries increases, the propagation is suppressed. When the grain size of the prior austenite grains is large or the aspect ratio is large, the branching of the prior austenite grains decreases, and the cracks are likely to propagate. As a result, the lamellar tearing resistance of the steel plate decreases.

[0026] Therefore, the prior austenite grains in the center of the plate thickness are made fine and equiaxed. Specifically, in the center of the plate thickness of the L cross-section parallel to the rolling direction and the plate thickness direction of the steel plate, the average circle equivalent diameter D Lave of the prior austenite grains is set to 40 μm or less, and the aspect ratio AR L of the prior austenite grains is set to 2.0 or less. Further, in the center of the plate thickness of the C cross-section perpendicular to the rolling direction of the steel plate, the average circle equivalent diameter D Cave of the prior austenite grains is set to 40 μm or less, and the aspect ratio AR C of the prior austenite grains is set to 2.0 or less. In this case, the prior austenite grains are sufficiently small. Therefore, the fracture stress of the prior austenite grain boundaries increases, and the durability of the prior austenite grain boundaries against the stress concentration generated at the tip of the porosity increases. Therefore, the generation and propagation of cracks are suppressed, and the lamellar tearing resistance of the steel plate increases.

[0027] The steel plate of the present embodiment is completed based on the above technical idea and has the following configuration.

[0028] The steel sheet of the first composition has a chemical composition in mass percent of: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, Sn: 0.02~0. It contains 40% N: 0.0010-0.0070%, O: 0.0005-0.0040%, and Ca: 0.0001-0.0080%, with the remainder being Fe and impurities. The Ceq defined by formula (1) is 0.44-0.50, the SnEQ defined by formula (2) is 0.10 or higher, the MV defined by formula (3) is 0.30-0.90, and the SC defined by formula (4) is 0.20-15.00. The above steel plate has a thickness of over 105 to 125 mm and a tensile strength of 570 MPa or more. At the center of the thickness of the L-shaped cross section parallel to the rolling direction and thickness direction of the steel plate, the average equivalent circular diameter D of the prior austenite grains is... Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L The average equivalent diameter D of the prior austenite grains is 2.0 or less. Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C The value is 2.0 or less. The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less, and the maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) MV = Mo + 4V (3) SC = S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol.

[0029] The steel sheet of the second composition has a chemical composition in mass percent of: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, Sn: 0.02~0.40%, N: 0.0010~0.0 It contains 0.70%, O: 0.0005~0.0040%, and Ca: 0.0001~0.0080%, and further contains one or more selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities, with a Ceq defined by formula (1) of 0.44~0.50, a SnEQ defined by formula (2) of 0.10 or higher, an MV defined by formula (3) of 0.30~0.90, and an SC defined by formula (4) of 0.20~15.00. The above steel plate has a thickness of over 105 to 125 mm and a tensile strength of 570 MPa or more. At the center of the thickness of the L-shaped cross section parallel to the rolling direction and thickness direction of the steel plate, the average equivalent circular diameter D of the prior austenite grains is... Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L The average equivalent diameter D of the prior austenite grains is 2.0 or less. Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C The value is 2.0 or less. The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less, and the maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) MV = Mo + 4V (3) SC = S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol. [Group 1] One or more elements selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.10% or less. [Group 2] One or more elements selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.10% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group 3] One or more elements selected from the group consisting of Mg: 0.010% or less, Sr: 0.020% or less, Ba: 0.010% or less, and rare earth elements: 0.020% or less.

[0030] The third steel sheet is the same as the second steel sheet, and its chemical composition contains the first group.

[0031] The fourth steel sheet is a steel sheet having the second or third composition, and its chemical composition contains the second group.

[0032] The fifth configuration steel sheet is a steel sheet having one of the second to fourth configurations, and its chemical composition contains the third group.

[0033] The steel plate of this embodiment will be described in detail below. Unless otherwise specified, percentages in the elemental descriptions refer to mass percentages.

[0034] [Features of the steel plate of this embodiment] The steel plate of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.06~0.15%, Si: 0.05~0.80%, Mn: 1.00~2.50%, P: ≤0.015%, S: ≤0.0024%, Ni: 0.1~1.0%, Mo: 0.01~0.60%, V: 0.001~0.200%, Nb: 0.005~0.050%, Ti: 0.005~0.050%, Al: 0.010~0.080%, Sn: 0.02~0.40%, N: 0.0010~0.0070%, O: 0.0005~0.0040%, Ca: 0.0001~0.0080%, Cu: 0~0.80%. It contains B: 0-0.0050%, Zr: 0-0.05%, Ta: 0-0.10%, Cr: 0-0.20%, W: 0-0.80%, Sb: 0-0.10%, As: 0-0.05%, Bi: 0-0.10%, Se: 0-0.05%, Te: 0-0.05%, Zn: 0-0.05%, Ga: 0-0.05%, Ge: 0-0.05%, Co: 0-0.50%, Hf: 0-0.05%, Mg: 0-0.010%, Sr: 0-0.020%, Ba: 0-0.010%, and rare earth elements: 0-0.020%, with the remainder being Fe and impurities. (Feature 2) In the above chemical composition, the Ceq defined by formula (1) is between 0.44 and 0.50. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) (Feature 3) In the above chemical composition, the SnEQ defined by formula (2) is 0.10 or higher. SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) (Feature 4) In the above chemical composition, the MV defined by formula (3) is between 0.30 and 0.90. MV = Mo + 4V (3) (Feature 5) In the above chemical composition, SC, as defined by formula (4), is between 0.20 and 15.00. SC = S / Ca (4) In equations (1) to (4), the mass percentage content of the corresponding element is substituted for each element symbol in each equation. If the corresponding element is not present, "0" is substituted for that element symbol. (Feature 6) It has a plate thickness of over 105 to 125 mm. (Feature 7) The tensile strength is 570 MPa or higher. (Feature 8) In the center of the thickness of the L-shaped cross-section of the steel plate, parallel to the rolling direction and thickness direction, the average equivalent circular diameter D of the prior austenite grains Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L The average equivalent circle diameter D of the prior austenite grains is 2.0 or less, and the average equivalent circle diameter D of the prior austenite grains is at the center of the thickness of the C section perpendicular to the rolling direction of the steel plate. Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C It is 2.0 or less. (Feature 9) The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less. (Feature 10) The maximum length of porosity in the C section is 0.5 mm or less. Features 1 through 10 are explained below.

[0035] [(Feature 1) Regarding chemical composition] The chemical composition of the steel sheet in this embodiment contains the following elements:

[0036] C: 0.06~0.15% Carbon (C) increases the strength of steel plates. If the C content is less than 0.06%, 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 carbon content exceeds 0.15%, the hardness of the segregation region in the center of the steel sheet thickness becomes excessively high. In this case, the lamellar tear resistance of the steel sheet decreases. Furthermore, if the carbon content exceeds 0.15%, excessive cementite is formed. Cementite acts as a cathode in chloride corrosion environments and promotes corrosion. Therefore, the corrosion resistance of the steel sheet decreases. Therefore, the C content is 0.06-0.15%. The preferred lower limit for the C content is 0.07%, more preferably 0.08%, and even more preferably 0.09%. The preferred upper limit for the C content is 0.14%, more preferably 0.13%, and even more preferably 0.12%.

[0037] Si: 0.05~0.80% Silicon (Si) deoxidizes steel during the steelmaking process in the manufacturing of steel sheets. Si further increases the tempering softening resistance of the steel sheet and enhances its strength. If the Si content is less than 0.05%, 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 0.80%, the toughness of the steel plate and the toughness of the welded joint when the steel plate is welded will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.05-0.80%. The preferred lower limit of the Si content is 0.10%, more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit for the Si content is 0.75%, more preferably 0.70%, even more preferably 0.65%, and even more preferably 0.60%.

[0038] Mn: 1.00~2.50% Manganese (Mn) enhances the hardenability of steel sheets, thereby increasing their strength. If the Mn content is less than 1.00%, 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 2.50%, Mn will excessively segregate in the segregation region at the center of the steel sheet's thickness. In this case, the hardness of the segregation region becomes excessively high. Therefore, even if the content of other elements is within the range of this embodiment, the lamellar tear resistance of the steel sheet will decrease. Therefore, the Mn content is 1.00 to 2.50%. The preferred lower limit of the Mn content is 1.05%, more preferably 1.10%, even more preferably 1.30%, and even more preferably 1.50%. The preferred upper limit for the Mn content is 2.45%, more preferably 2.40%, more preferably 2.30%, and still more preferably 2.00%.

[0039] P:0.015% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.015%, the corrosion resistance of the steel sheet will decrease in a chloride corrosion environment, even if the content of other elements is within the range of this embodiment. Furthermore, if the P content exceeds 0.015%, P will excessively segregate at the grain boundaries. In this case, even if the content of other elements is within the range of this embodiment, the lamellar tear resistance of the steel sheet will decrease. Therefore, the P content is 0.015% or less. A low phosphorus (P) content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the P content is 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0040] S: 0.0024% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. The solid-liquid partition coefficient of S is remarkably small. Therefore, if the S content exceeds 0.0024%, coarse Mn sulfides are excessively formed in the segregation region at the center of the steel sheet thickness. In this case, even if the content of other elements is within the range of this embodiment, the toughness of the steel sheet decreases. Furthermore, the aforementioned coarse Mn sulfides are stretched in a planar manner in the rolling direction of the steel sheet. Therefore, the lamellar tear resistance of the steel sheet decreases. Therefore, the sulfur content is 0.0024% or less. A low sulfur (S) content is preferable. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the S content is 0.0023%, more preferably 0.0022%, and even more preferably 0.0020%.

[0041] Ni: 0.1~1.0% Nickel (Ni) enhances the hardenability and strength of steel sheets. Ni further improves the low-temperature toughness of steel sheets. Ni further improves the corrosion resistance of steel sheets in chloride-corrosive environments. If the Ni content is less than 0.1%, 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 Ni content exceeds 1.0%, the above effects saturate, and manufacturing costs also increase. Therefore, the Ni content is 0.1-1.0%. The preferred lower limit for the Ni content is 0.2%, more preferably 0.3%, and even more preferably 0.4%. The preferred upper limit for the Ni content is 0.9%, more preferably 0.8%, and even more preferably 0.7%.

[0042] Mo: 0.01~0.60% Molybdenum (Mo) enhances the hardenability and strength of steel sheets. Furthermore, Mo forms Mo precipitates, increasing the strength of the steel sheet through precipitation strengthening. Mo also dissolves to form oxygen ions (MoO4). 2- It adsorbs to rust and suppresses the permeation of chloride ions in the rust layer. Therefore, it improves the corrosion resistance of steel plates in chloride corrosion environments. If the Mo content is less than 0.01%, 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 Mo content exceeds 0.60%, the strength of the steel plate increases excessively, while the toughness of the steel plate decreases. Therefore, the Mo content is 0.01-0.60%. The preferred lower limit for the Mo content is 0.05%, more preferably 0.07%, and even more preferably 0.10%. The preferred upper limit for the Mo content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0043] V: 0.001~0.200% Vanadium (V) enhances the hardenability and strength of steel sheets. Furthermore, V forms V precipitates, increasing the strength of the steel sheet through precipitation strengthening. If the V 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 V content exceeds 0.200%, the strength of the steel plate will increase excessively, and the toughness of the steel plate will decrease. Therefore, the V content is between 0.001% and 0.200%. The preferred lower limit for the V content is 0.005%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the V content is 0.180%, more preferably 0.150%, and even more preferably 0.120%.

[0044] Nb: 0.005~0.050% Niobium (Nb) enhances the hardenability and strength of steel sheets. Furthermore, Nb expands the non-recrystallization temperature range. Therefore, in the steel sheet manufacturing process, high-density dislocations can be introduced in the non-recrystallization temperature range. These high-density dislocations become transformation nucleation sites. As a result, the microstructure of the steel sheet is refined, and the toughness of the steel sheet is increased. If the Nb content is less than 0.005%, 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 Nb content exceeds 0.050%, Nb segregates in the segregation region at the center of the plate thickness, causing the hardness of the segregation region to become excessively high. Furthermore, coarse Nb precipitates are formed. As a result, the lamellar tear resistance of the steel plate decreases. Therefore, the Nb content is 0.005-0.050%. The preferred lower limit for the Nb content is 0.008%, more preferably 0.010%, and even more preferably 0.015%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0045] Ti: 0.005~0.050% Titanium (Ti) forms Ti precipitates, thereby increasing the strength of the steel sheet. Furthermore, the Ti precipitates refine the crystal grains through a pinning effect, increasing the toughness of the steel sheet. If the Ti content is less than 0.005%, 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 Ti content exceeds 0.050%, coarse Ti precipitates are formed in the segregation region at the center of the plate thickness. As a result, even if the content of other elements is within the range of this embodiment, the lamellar tear resistance of the steel plate decreases. Therefore, the Ti content is 0.005 to 0.050%. The preferred lower limit of the Ti content is 0.006%, more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.030%.

[0046] Al: 0.010~0.080% Aluminum (Al) deoxidizes steel during the steelmaking process in the manufacturing of steel sheets. If the Al content is less than 0.010%, 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 Al content exceeds 0.080%, the corrosion resistance of the steel sheet in a chloride corrosion environment decreases. Furthermore, if the Al content exceeds 0.080%, coarse Al nitrides are formed, reducing the toughness of the steel sheet. Therefore, the Al content is 0.010-0.080%. The preferred lower limit for the Al content is 0.012%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit for the Al content is 0.075%, more preferably 0.070%, and even more preferably 0.065%.

[0047] Sn: 0.02~0.40% Tin (Sn) enhances the corrosion resistance of steel sheets. Specifically, Sn forms a Sn oxide film on the surface of the steel sheet. This Sn oxide film suppresses the anodic reaction and hydrogen evolution reaction of steel in chloride corrosion environments. Therefore, Sn enhances the corrosion resistance of steel sheets in chloride corrosion environments. If the Sn content is less than 0.02%, 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 Sn content exceeds 0.40%, segregation is promoted in the segregation region at the center of the plate thickness. As a result, even if the content of other elements is within the range of this embodiment, the lamellar tear resistance of the steel plate decreases. Therefore, the Sn content is 0.02-0.40%. The preferred lower limit for the Sn content is 0.04%, more preferably 0.06%, and even more preferably 0.08%. The preferred upper limit for the Sn content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0048] N: 0.0010~0.0070% Nitrogen (N) forms nitrides, which refine the crystal grains. This increases the toughness of the steel sheet. Furthermore, N forms and dissolves ammonia, improving the corrosion resistance of the steel sheet in chloride-corrosive environments. If the N content is less than 0.0010%, 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 N content exceeds 0.0070%, coarse nitrides are formed. As a result, the toughness of the steel sheet actually decreases. Therefore, the N content is between 0.0010% and 0.0070%. The preferred lower limit for the N content is 0.0015%, more preferably 0.0020%, and even more preferably 0.0025%. The preferred upper limit for the N content is 0.0065%, more preferably 0.0060%, and even more preferably 0.0055%.

[0049] O: 0.0005~0.0040% Oxygen (O) removes impurities from molten steel during the steelmaking process in the manufacturing of steel sheets, thereby increasing the cleanliness of the steel sheets. As a result, the toughness of the steel sheets is increased. If the O content is less than 0.0005%, 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 O content exceeds 0.0040%, an excess of oxides will be generated. Since oxides are the starting point for corrosion, the corrosion resistance of the steel plate will decrease. Therefore, the O content is between 0.0005% and 0.0040%. The preferred lower limit for the O content is 0.0007%, and more preferably 0.0010%. The preferred upper limit for the O content is 0.0035%, more preferably 0.0030%, and even more preferably 0.0025%.

[0050] Ca: 0.0001~0.0080% Calcium (Ca) forms sulfides, suppressing sulfur segregation in the center of the plate thickness. Therefore, Ca suppresses the formation of porosity and reduces the hardness of the segregated region. As a result, the lamellar tear resistance of the steel plate decreases. Furthermore, Ca forms Ca oxides, suppressing the decrease in pH in the corrosive region. Therefore, the corrosion resistance of the steel plate increases. If the Ca content is less than 0.0001%, 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 Ca content exceeds 0.0080%, coarse oxides are produced in excess. As a result, the toughness of the steel sheet decreases. Therefore, the Ca content is between 0.0001% and 0.0080%. The preferred lower limit of the Ca content is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%. The preferred upper limit for the Ca content is 0.0075%, more preferably 0.0070%, even more preferably 0.0065%, and even more preferably 0.0060%.

[0051] The remainder of the chemical composition of the steel sheet in this embodiment consists of Fe and impurities. Here, impurities 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 according to this embodiment.

[0052] [About Optional Elements] The chemical composition of the steel sheet in this embodiment may further contain one or more elements selected from the groups consisting of Group 1 to Group 3, in place of a portion of Fe. Any of these elements are arbitrary elements. [Group 1] One or more elements selected from the group consisting of Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, and Ta: 0.10% or less. [Group 2] One or more elements selected from the group consisting of Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.10% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, and Hf: 0.05% or less. [Group 3] One or more elements selected from the group consisting of Mg: 0.010% or less, Sr: 0.020% or less, Ba: 0.010% or less, and rare earth elements: 0.020% or less. The following describes each element.

[0053] [Group 1 (Cu, B, Zr, and Ta)] The chemical composition of the steel sheet in this embodiment may include one or more elements selected from the group consisting of Cu, B, Zr, and Ta, instead of a portion of Fe. All of these elements increase the strength of the steel sheet. Each element will be described below.

[0054] Cu: 0.80% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When copper is present, i.e., when the copper content is greater than 0%, it enhances the hardenability of the steel sheet, thereby increasing its strength. Furthermore, copper improves the corrosion resistance of the steel sheet. Even a small amount of copper will provide some of the above effects. However, if the Cu content exceeds 0.80%, the steel sheet becomes brittle, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is between 0 and 0.80%, and if present, the Cu content is 0.80% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Cu content is 0.75%, more preferably 0.70%, and even more preferably 0.65%.

[0055] B: 0.0050% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If B is present, that is, if the B content is greater than 0%, B enhances the hardenability of the steel plate and increases its strength. Even if only a small amount of B is present, the above effect can be obtained to some extent. However, if the B content exceeds 0.0050%, even if the content of other elements is within the range of this embodiment, the toughness of the steel plate and the heat-affected zone of the weld will be significantly reduced. Therefore, the B content is 0-0.0050%, and if present, the B content is 0.0050% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0056] Zr: 0.05% or less Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. When Zr is present, that is, when the Zr content is greater than 0%, Zr forms oxides, which refines the crystal grains. As a result, the strength and toughness of the steel sheet increase. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.05%, coarse oxides are excessively generated. As a result, the toughness of the steel sheet actually decreases. Therefore, the Zr content is 0-0.05%, and if present, the Zr content is 0.05% or less. The preferred lower limit for the Zr content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Zr content is 0.04%, and more preferably 0.03%.

[0057] Ta: 0.10% or less Tantalum (Ta) is an optional element and does not need to be included. In other words, the Ta content may be 0%. When present, i.e., when the Ta content is greater than 0%, Ta increases the strength of the steel plate. Ta also improves the corrosion resistance of the steel plate. Even a small amount of Ta will provide some of the above effects. However, if the Ta content exceeds 0.10%, its effect saturates, and manufacturing costs also increase. Therefore, the Ta content is 0-0.10%, and if present, the Ta content is 0.10% or less. The preferred lower limit for the Ta content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Ta content is 0.08%, and more preferably 0.05%.

[0058] [Group 2 (Cr, W, Sb, As, Bi, Se, Te, Zn, Ga, Ge, Co, and Hf)] The chemical composition of the steel sheet in this embodiment may contain one or more elements selected from the group consisting of Cr, W, Sb, As, Bi, Se, Te, Zn, Ga, Ge, Co, and Hf, instead of a portion of Fe. All of these elements enhance the corrosion resistance of the steel sheet. Each element will be described below.

[0059] Cr:0.20% or less Chromium (Cr) is an optional element and does not need to be included. In other words, the Cr content may be 0%. When present, i.e., when the chromium content is greater than 0%, chromium enhances the corrosion resistance of the steel sheet. Furthermore, chromium increases the strength of the steel sheet. Even a small amount of chromium will provide some of the above effects. However, if the Cr content exceeds 0.20%, even if the content of other elements is within the range of this embodiment, the corrosion resistance of the steel sheet in a chloride corrosion environment will actually decrease. Therefore, the Cr content is 0-0.20%, and if present, the Cr content is 0.20% or less. The preferred lower limit for the Cr content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Cr content is 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0060] W: 0.80% or less Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. If present, i.e., if the W content is greater than 0%, W dissolves into oxygen ions WO4. 2- It adsorbs to rust and suppresses the permeation of chloride ions in the rust layer. As a result, the corrosion resistance of the steel plate is improved. Even if only a small amount of W is present, the above effect can be obtained to some extent. However, if the W content exceeds 0.80%, the above effects will saturate, and manufacturing costs will also increase. Therefore, the W content is 0-0.80%, and if present, the W content is 0.80% or less. The preferred lower limit of the W content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the W content is 0.75%, more preferably 0.70%, and even more preferably 0.65%.

[0061] Sb: 0.10% or less Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. If Sb is present, that is, if the Sb content is greater than 0%, Sb forms sulfides, which improve the corrosion resistance of the steel sheet. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.10%, the toughness 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 0-0.10%, and if present, the Sb content is 0.10% or less. The preferred lower limit for the Sb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Sb content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0062] As: 0.05% or less Arsenic (As) is an optional element and does not need to be included. In other words, the As content may be 0%. When present, i.e., when the As content is greater than 0%, As enhances the corrosion resistance of steel plates. Even a small amount of As can provide some degree of the above effect. However, if the As content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the As content is 0-0.05%, and if present, the As content is 0.05% or less. The preferred lower limit for the As content is 0.01%, and more preferably 0.02%. The preferred upper limit for the As content is 0.04%, and more preferably 0.03%.

[0063] Bi:0.10% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. When present, i.e., when the Bi content is greater than 0%, Bi enhances the corrosion resistance of steel plates. Even a small amount of Bi will provide some degree of the above effect. However, if the Bi content exceeds 0.10%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is 0-0.10%, and if present, the Bi content is 0.10% or less. The preferred lower limit for the Bi content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Bi content is 0.07%, and more preferably 0.05%.

[0064] Se: 0.05% or less Selenium (Se) is an optional element and does not need to be present. In other words, the Se content may be 0%. When present, i.e., when the Se content is greater than 0%, Se enhances the corrosion resistance of steel plates. Even a small amount of Se will provide some degree of the above effect. However, if the Se content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Se content is 0-0.05%, and if present, the Se content is 0.05% or less. The preferred lower limit for the Se content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Se content is 0.04%, and more preferably 0.03%.

[0065] Te: 0.05% or less Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0. When present, i.e., when the Te content is greater than 0%, Te enhances the corrosion resistance of steel plates. Even a small amount of Te will provide some degree of the above effect. However, if the Te content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is 0-0.05%, and if present, the Te content is 0.05% or less. The preferred lower limit for the Te content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Te content is 0.04%, and more preferably 0.03%.

[0066] Zn: 0.05% or less Zinc (Zn) is an optional element and does not need to be included. In other words, the Zn content may be 0%. When Zn is present, that is, when the Zn content is greater than 0%, Zn enhances the corrosion resistance of steel sheets. Even a small amount of Zn will provide some degree of the above effect. However, if the Zn content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Zn content is 0-0.05%, and if present, the Zn content is 0.05% or less. The preferred lower limit for the Zn content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Zn content is 0.04%, and more preferably 0.03%.

[0067] Ga: 0.05% or less Gallium (Ga) is an optional element and does not need to be included. In other words, the Ga content may be 0%. When present, i.e., when the Ga content is greater than 0%, Ga enhances the corrosion resistance of steel plates. Even a small amount of Ga will provide some degree of the above effect. However, if the Ga content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ga content is 0-0.05%, and if present, the Ga content is 0.05% or less. The preferred lower limit for the Ga content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Ga content is 0.04%, and more preferably 0.03%.

[0068] Ge: 0.05% or less Germanium (Ge) is an optional element and does not need to be included. In other words, the Ge content may be 0%. If present, i.e., if the Ge content is greater than 0%, Ge combines with S to form sulfides, improving the corrosion resistance of the steel sheet. Even a small amount of Ge will provide some degree of the above effect. However, if the Ge content exceeds 0.05%, the toughness of the steel sheet will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ge content is 0-0.05%, and if present, the Ge content is 0.05% or less. The preferred lower limit for the Ge content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Ge content is 0.04%, and more preferably 0.03%.

[0069] Co:0.50% or less Cobalt (Co) is an optional element and does not need to be included. In other words, the Co content may be 0%. If Co is present, that is, if the Co content is greater than 0%, Co forms oxides that enhance the corrosion resistance of the steel sheet. Even if only a small amount of Co is present, the above effect can be obtained to some extent. However, if the CO content exceeds 0.50%, the manufacturing cost will increase. Therefore, the Co content is between 0 and 0.50%, and if present, the Co content is 0.50% or less. The preferred lower limit of the Co content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Co content is 0.45%, more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.10%.

[0070] Hf: 0.05% or less Hafnium (Hf) is an optional element and does not need to be included. In other words, the Hf content may be 0%. If Hf is present, that is, if the Hf content is greater than 0%, Hf forms oxides that enhance the corrosion resistance of the steel sheet. Even if only a small amount of Hf is present, the above effect can be obtained to some extent. However, if the Hf content exceeds 0.05%, the manufacturing cost will increase. Therefore, the Hf content is 0-0.05%, and if present, the Hf content is 0.05% or less. The preferred lower limit for the Hf content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Hf content is 0.04%, and more preferably 0.03%.

[0071] [Group 3 (Mg, Sr, Ba, and rare earth elements)] The chemical composition of the steel sheet in this embodiment may contain, in place of some of the Fe, one or more elements selected from the group consisting of Mg, Sr, Ba, and rare earth elements. All of these elements deoxidize or desulfurize the steel during the steelmaking process in the manufacturing of the steel sheet, thereby increasing the cleanliness of the steel sheet.

[0072] Mg: 0.010% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. If magnesium is present, that is, if the magnesium content is greater than 0%, magnesium desulfurizes the steel and improves the cleanliness of the steel sheet. Even if only a small amount of magnesium is present, the above effect can be obtained to some extent. However, the effect saturates once the Mg content exceeds 0.010%. Therefore, the Mg content is 0-0.010%, and if present, the Mg content is 0.010% or less. The preferred lower limit of the Mg content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Mg content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0073] Sr: 0.020% or less Strontium (Sr) is an optional element and does not need to be included. In other words, the Sr content may be 0%. If present, i.e., if the Sr content is greater than 0%, Sr deoxidizes the steel and improves the cleanliness of the steel sheet. Even if only a small amount of Sr is present, the above effect can be obtained to some extent. However, if the Sr content exceeds 0.020%, the manufacturing cost will increase. Therefore, the Sr content is 0-0.020%, and if present, the Sr content is 0.020% or less. The preferred lower limit of the Sr content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Sr content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0074] Ba: 0.010% or less Barium (Ba) is an optional element and does not need to be included. In other words, the Ba content may be 0%. If present, i.e., if the Ba content is greater than 0%, Ba deoxidizes and desulfurizes the steel, improving the cleanliness of the steel plate. Even a small amount of Ba will provide some degree of the above effect. However, if the Ba content exceeds 0.010%, the manufacturing cost will increase. Therefore, the Ba content is 0-0.010%, and if present, the Ba content is 0.010% or less. The preferred lower limit of the Ba content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Ba content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0075] Rare earth elements: 0.020% or less Rare earth elements (REMs) are optional elements and do not need to be included. In other words, the REM content may be 0%. If REM is present, that is, if the REM content is greater than 0%, REM deoxidizes and desulfurizes the steel, improving the cleanliness of the steel sheet. Even if only a small amount of REM is present, the above effect can be obtained to some extent. However, the above effect saturates when the REM content exceeds 0.020%. Therefore, the REM content is 0-0.020%, and if present, the REM content is 0.020% or less. The preferred lower limit for the REM content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the REM content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0076] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content of these elements.

[0077] [Regarding (Feature 2) Equation (1)] The chemical composition of the steel sheet in this embodiment is further characterized by a Ceq of 0.44 to 0.50, as defined by formula (1). Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is substituted for that element symbol.

[0078] Ceq is an index related to hardenability. Even if the chemical composition of the steel sheet satisfies characteristic 1, if the Ceq is less than 0.44, sufficient hardenability cannot be obtained. Therefore, sufficient strength cannot be obtained in the steel sheet. On the other hand, if the Ceq exceeds 0.50, the hardenability becomes excessively high. In this case, the low-temperature toughness of the steel sheet decreases. Therefore, Ceq is between 0.44 and 0.50. A preferred lower limit for Ceq is 0.45, and more preferably 0.46. A preferred upper limit for Ceq is 0.49, and more preferably 0.48.

[0079] [Regarding (Feature 3) Equation (2)] The chemical composition of the steel sheet in this embodiment is further characterized by a SnEQ of 0.10 or higher, as defined by formula (2). SnEQ = Sn + W + Ni / 10 + Mo / 4 (2) Here, each element symbol in equation (2) is substituted with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is substituted for that element symbol.

[0080] SnEQ is an index of the corrosion resistance of steel sheets in chloride-corrosive environments. The Sn, W, Ni, and Mo in the chemical composition of steel sheets suppress the elution of Fe ions, thereby improving the corrosion resistance of steel sheets in chloride-corrosive environments. Even if the chemical composition of the steel plate satisfies characteristic 1, if the SnEQ is less than 0.10, sufficient corrosion resistance cannot be obtained in a chloride corrosion environment. Therefore, the SnEQ must be 0.10 or higher. A preferred lower limit for SnEQ is 0.11, more preferably 0.13, and even more preferably 0.15. The upper limit of SnEQ is not particularly limited. If the chemical composition of the steel sheet satisfies characteristic 1, the upper limit of SnEQ is, for example, 1.45. A preferred upper limit of SnEQ is 1.20, more preferably 1.00, even more preferably 0.80, even more preferably 0.60, and even more preferably 0.40.

[0081] [(Feature 4) Regarding Equation (3)] The chemical composition of the steel sheet in this embodiment is further characterized by an MV of 0.30 to 0.90, as defined by formula (3). MV = Mo + 4V (3) Here, each element symbol in equation (3) is substituted with the mass percentage content of the corresponding element.

[0082] MV is an index used to adjust the strength of steel sheets. More specifically, MV is an index of the amount of Mo precipitates and V precipitates. Even if the chemical composition of the steel sheet satisfies characteristic 1, if the MV is less than 0.30, sufficient Mo precipitates and V precipitates will not form, and sufficient precipitation strengthening will not be obtained. Therefore, sufficient strength cannot be obtained in the steel sheet. On the other hand, if the MV exceeds 0.90, excessive Mo precipitates and V precipitates will be formed. In this case, the low-temperature toughness of the steel sheet will decrease. Therefore, the MV is between 0.30 and 0.90. A preferred lower limit for MV is 0.32, more preferably 0.35, even more preferably 0.38, and even more preferably 0.42. A preferred upper limit for MV is 0.88, more preferably 0.86, even more preferably 0.84, and even more preferably 0.82.

[0083] [Regarding (Feature 5) Equation (4)] The chemical composition of the steel sheet in this embodiment is further such that SC, as defined by formula (4), is between 0.20 and 15.00. SC = S / Ca (4) Here, each element symbol in equation (4) is substituted with the mass percentage content of the corresponding element.

[0084] SC is an index indicating the degree of fixation of sulfur by calcium. Calcium combines with sulfur to form sulfides, suppressing segregation of sulfur in the center of the plate thickness. As a result, the low-temperature toughness and lamellar tear resistance of the steel plate are improved. Even if the chemical composition of the steel sheet satisfies characteristic 1, if the SC is less than 0.20, the Ca content is excessively high relative to the S content. In this case, excess Ca oxide is generated. As a result, sufficient low-temperature toughness cannot be obtained. On the other hand, if SC exceeds 15.00, the sulfur content is excessively high relative to the calcium content in the chemical composition of the steel sheet. In this case, sulfur segregates excessively in the center of the sheet thickness. As a result, the low-temperature toughness and lamellar tear resistance of the steel sheet decrease. Therefore, the SC content is between 0.20 and 15.00. A preferred lower limit for SC is 0.30, and more preferably 0.40. The preferred upper limit of SC is 10.00, more preferably 6.00, and even more preferably 1.50.

[0085] [(Feature 6) Regarding plate thickness] The steel plate of this embodiment has a thickness of over 105 to 125 mm. The thickness of the steel plate of this embodiment is remarkably thick. Therefore, it can be used as a steel plate for structural applications such as bridges and offshore structures, where high strength, excellent low-temperature toughness, and excellent corrosion resistance are required. The preferred lower limit for the thickness of the steel plate is 106 mm, more preferably 107 mm, even more preferably 108 mm, and even more preferably 110 mm. The preferred upper limit for the thickness of the steel plate is 123 mm, more preferably 120 mm, and even more preferably 118 mm.

[0086] [(Feature 7) Regarding Tensile Strength] In this embodiment, the steel plate has a tensile strength of 570 MPa or higher. A tensile strength of 570 MPa or higher is sufficient to provide adequate load-bearing capacity in the steel plate. Therefore, it becomes possible to increase the size of structures such as bridges and offshore structures. The preferred lower limit of the tensile strength is 575 MPa, more preferably 580 MPa, even more preferably 585 MPa, and even more preferably 590 MPa. The preferred upper limit of the tensile strength is 760 MPa, more preferably 755 MPa, and even more preferably 750 MPa.

[0087] [Method for measuring tensile strength] The tensile strength of a steel plate can be determined by the following method. Let the thickness of the steel plate be t (mm). A No. 4 test specimen, in accordance with JIS Z 2241 (2022), is taken from the surface of the steel plate at a depth of t / 4 in the thickness direction of the steel plate. The longitudinal direction of the No. 4 test specimen shall be parallel to the direction perpendicular to the rolling direction and thickness direction of the steel plate (i.e., the width direction). Using the collected specimen No. 4, a tensile test will be conducted at room temperature and in air in accordance with JIS Z 2241 (2022). The tensile strength (MPa) will be determined from the obtained stress-strain curve. The tensile strength will be rounded to the nearest integer.

[0088] [(Feature 8) Regarding prior austenite grains in the center of the plate thickness of L-section and C-section] In the steel sheet of this embodiment, the prior austenite grains further satisfy the following two requirements. (Requirement 1) In the center of the thickness of the L-shaped cross-section of the steel plate, parallel to the rolling direction and thickness direction, the average equivalent circular diameter D of the prior austenite grains Lave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is L It is 2.0 or less. (Requirement 2) In the center of the thickness of the steel plate in a C-section perpendicular to the rolling direction, the average equivalent circular diameter D of the prior austenite grains Cave The particle size is 40 μm or less, and the aspect ratio AR of the prior austenite grains is C It is 2.0 or less. Here, "center of the plate thickness" refers to the region from the surface of the steel plate at a depth of 9t / 20 to 11t / 20, where t (mm) is the thickness of the steel plate. The following explains Feature 8.

[0089] In the center of the thickness of the steel plate in this embodiment, as shown in requirements 1 and 2, the prior austenite grains are fine and approximately equiaxed.

[0090] [Equivalent diameter D] Lave and D Cave [About] If uncompressed porosity exists in the center of the plate thickness, stress concentration will occur at the tip of the porosity when tensile stress is applied in the thickness direction (Z direction) of the steel plate. If the prior austenite grains in the center of the plate thickness are fine, the brittle fracture stress will be high. Therefore, plastic deformation is more likely to occur before the stress concentration at the tip of the porosity reaches the brittle fracture stress. As a result, brittle fracture is avoided and ductile fracture is more likely to occur. Furthermore, if the prior austenite grains are fine, even if brittle fracture occurs, crack propagation is suppressed. As a result, the low-temperature toughness and lamellar tear resistance of the steel plate are improved.

[0091] Equivalent diameter D of prior austenite grains in L section Lave The diameter is 40 μm or less, and the equivalent circular diameter D of the prior austenite grains in the C section. Cave If the particle size is 40 μm or less, the prior austenite grains are sufficiently fine. Therefore, excellent low-temperature toughness and excellent lamellar tear resistance can be obtained in the steel sheet.

[0092] Equivalent diameter D of prior austenite grains in L section Lave The preferred upper limit is 38 μm, more preferably 35 μm, even more preferably 33 μm, and even more preferably 31 μm. Equivalent diameter D of prior austenite grains in section C Cave The preferred upper limit is 38 μm, more preferably 35 μm, even more preferably 33 μm, and even more preferably 31 μm.

[0093] Equivalent diameter D Lave and equivalent diameter D Cave The lower limit is not particularly limited. Equivalent diameter D of prior austenite grains in L section LaveA preferred lower limit is, for example, 10 μm, more preferably 12 μm, more preferably 15 μm, more preferably 18 μm, and still more preferably 20 μm. Equivalent diameter D of prior austenite grains in section C Cave A preferred lower limit is, for example, 10 μm, more preferably 12 μm, more preferably 15 μm, more preferably 18 μm, and still more preferably 20 μm.

[0094] [Aspect Ratio AR] L and AR C [About] If the prior austenite grains are equiaxed, the area of ​​prior austenite grain boundaries, which provides crack propagation resistance, increases compared to when they are flattened. In this case, even if brittle fracture occurs when tensile stress is applied in the thickness direction of the steel plate, crack propagation is suppressed. As a result, the lamellar tear resistance of the steel plate is improved.

[0095] Aspect ratio AR of prior austenite grains in L section L The aspect ratio AR of the prior austenite grains in the C section is 2.0 or less. C If the value is 2.0 or less, the prior austenite grains are sufficiently equiaxed. Therefore, excellent lamellar tear resistance can be obtained in steel sheets.

[0096] Aspect ratio AR of prior austenite grains in L section L A preferred upper limit is 1.9, more preferably 1.8, even more preferably 1.7, and even more preferably 1.6. Aspect ratio AR of prior austenite grains in the C section C A preferred upper limit is 1.9, more preferably 1.8, even more preferably 1.7, and even more preferably 1.6.

[0097] Aspect Ratio AR L and AR C The lower limit is not particularly limited. Aspect ratio AR of prior austenite grains in L section LThe lower limit is, for example, 1.1, for example, 1.2, for example, 1.3. Aspect ratio AR of prior austenite grains in the C section C The lower limit is, for example, 1.1, for example, 1.2, for example, 1.3.

[0098] [Equivalent diameter D] Lave and D Cave Aspect ratio AR L and AR C [Measurement Method] Equivalent circular diameter D of the former austenite grains at the center of the plate thickness Lave , D Cave and aspect ratio AR L AR C It can be found using the following method.

[0099] First, the equivalent circular diameter D of the prior austenite grains at the center of the L-section plate thickness. Lave and aspect ratio AR L This can be found using the following method.

[0100] An L-shaped cross-section test specimen is taken from the center of the width of the steel plate, and includes the center of the thickness (the range from a depth of 9t / 20 to 11t / 20 from the surface of the steel plate in the thickness direction), having an L-shaped cross-section parallel to the rolling direction and the thickness direction.

[0101] For the L-section test specimen, the central part of the L-section (the range from a depth of 9t / 20 to 11t / 20 in the thickness direction from the steel plate surface) is designated as the L-observation surface. The length of the L-observation surface in the rolling direction is 5 mm.

[0102] The L observation surface is mirror-polished. In accordance with JIS G 0551 (2022), etching is performed on the mirror-polished L observation surface using a saturated picric acid aqueous solution as an etching solution. After etching, the prior austenite grain size of the L observation surface is determined by the cutting method. A test line is drawn on the L observation surface in a direction perpendicular to the thickness direction of the steel plate (i.e., the rolling direction). The length of the test line is set so that 10 or more prior austenite grains cross the test line. The distance between the intersection of the grain boundaries of the prior austenite grains closest to one end of the test line and the intersection of the grain boundaries of the prior austenite grains closest to the other end is measured as L. L Let (μm) be the distance. Furthermore, P is the number of intersections between the test line and the prior austenite grain boundary. L Let's assume that the prior austenite grain D Lave (μm) is calculated using the following formula. D Lave =L L / (P L -1)

[0103] Furthermore, the major and minor axes of each prior austenite grain contained within the L observation plane are determined by the following method. First, each prior austenite grain to be measured is assumed to be entirely contained within the L observation plane, and prior austenite grains in which a portion of the crystal grain extends outside the L observation plane are excluded from measurement. In other words, all prior austenite grains that are entirely contained within the L observation plane without any portion extending outside are included in the measurement.

[0104] For each of the prior austenite grains being measured, two parallel line segments are placed, each tangent to the interface (i.e., grain boundary) of the prior austenite grain. The value at which the distance between these two line segments is maximized is defined as the major axis (μm) of the prior austenite grain. Here, the direction perpendicular to the parallel line segments from which the major axis was determined is defined as the major axis direction. Furthermore, two parallel line segments are placed, each parallel to the major axis direction and tangent to the interface of the prior austenite grain. The distance between these two line segments is defined as the minor axis (μm) of the prior austenite grain. The ratio of the obtained major axis to the minor axis (= major axis / minor axis) is defined as the aspect ratio of the prior austenite grain. The arithmetic mean of the aspect ratios of the prior austenite grains being measured is defined as the aspect ratio AR. L Let's assume the aspect ratio is AR.L This is the value of the first decimal place obtained by rounding the second decimal place of the obtained arithmetic mean.

[0105] Next, the equivalent circular diameter D of the prior austenite grains at the center of the plate thickness in section C. Cave and aspect ratio AR C This can be found using the following method.

[0106] A test specimen is taken that includes a cross section (C section) perpendicular to the rolling direction of the steel plate. Of the C section of the test specimen, the "C observation surface" is defined as the area from the W / 4 position to the 3W / 4 position in the width direction, starting from one end of the steel plate in the width direction, which is the center of the thickness of the steel plate (the range from a depth of 9t / 20 to a depth of 11t / 20 from the surface of the steel plate in the thickness direction), and when the width of the steel plate is W (mm).

[0107] Surface C is mirror-polished. In accordance with JIS G 0551 (2022), etching is performed on the mirror-polished surface C using a saturated picric acid aqueous solution as an etching solution. After etching, the prior austenite grain size of the surface L is determined by the cutting method. A test line is drawn on the surface C in a direction perpendicular to the thickness direction of the steel plate (i.e., in the width direction). The distance between the intersection of the grain boundaries of the prior austenite grains closest to one end of the test line and the intersection of the grain boundaries of the prior austenite grains closest to the other end is measured by L. C Let (μm) be the distance. Furthermore, P is the number of intersections between the test line and the prior austenite grain boundary. C Let's assume that the prior austenite grain D Cave (μm) is calculated using the following formula. D Cave =L C / (P C -1)

[0108] Furthermore, the major and minor axes of each prior austenite grain contained within the C observation plane are determined by the following method. First, each prior austenite grain to be measured is assumed to be entirely contained within the C observation plane, and prior austenite grains in which a portion of the crystal grain extends outside the L observation plane are excluded from measurement. In other words, all prior austenite grains that are entirely contained within the C observation plane without any portion extending beyond it are included in the measurement.

[0109] For each of the prior austenite grains being measured, two parallel line segments are placed, each tangent to the interface (i.e., grain boundary) of the prior austenite grain. The value at which the distance between these two line segments is maximized is defined as the major axis (μm) of the prior austenite grain. The direction perpendicular to the parallel line segments from which the major axis was determined is defined as the major axis direction. Furthermore, two parallel line segments are placed, each parallel to the major axis direction and tangent to the interface of the prior austenite grain. The distance between these two line segments is defined as the minor axis (μm) of the prior austenite grain. The ratio of the obtained major axis to the minor axis (= major axis / minor axis) is defined as the aspect ratio of the prior austenite grain. The arithmetic mean of the aspect ratios of the prior austenite grains being measured is defined as the aspect ratio AR. C Let's assume the aspect ratio is AR. C This value is the first decimal place obtained by rounding the second decimal place of the obtained arithmetic mean.

[0110] [(Feature 9) Vickers hardness in the segregation region at the center of the plate thickness of the C section] In this embodiment, the steel plate further exhibits a Vickers hardness of 250 HV or less in the segregation region at the center of the plate thickness of the C cross-section. Here, the segregation region refers to the area within the center of the plate thickness of the C cross-section where Sn and Mn are concentrated and segregated. The method for identifying the segregation region will be described later.

[0111] In the center of the plate thickness, uncompressed porosity exists in the segregation region. As described above, when tensile stress is applied in the thickness direction of the steel plate, stress concentration is likely to occur at the tips of porosity in the segregation region. If the Vickers hardness of the segregation region is low, brittle fracture at the tips of the porosity is easily suppressed. If the Vickers hardness of the segregation region is 250 HV or less, the hardness in the segregation region is sufficiently low. As a result, excellent lamellar tear resistance can be obtained in the steel plate.

[0112] A preferred upper limit for Vickers hardness in the segregation region is 245 HV, more preferably 240 HV, even more preferably 235 HV, and even more preferably 230 HV. There is no particular lower limit to the Vickers hardness in the segregation region. A preferred lower limit for the Vickers hardness in the segregation region is 180 HV, more preferably 200 HV, even more preferably 210 HV, and even more preferably 220 HV.

[0113] [Method for measuring Vickers hardness in segregation regions] The Vickers hardness in the segregation region at the center of the plate thickness of the C section can be determined by the following method.

[0114] A test specimen is taken that includes a cross section (C section) perpendicular to the rolling direction of the steel plate. Of the C section of the test specimen, the "observation surface" is defined as the area from the W / 4 position to the 3W / 4 position in the width direction, starting from one end of the steel plate in the width direction, which is the center of the thickness of the steel plate (the range from a depth of 9t / 20 to a depth of 11t / 20 from the surface of the steel plate in the thickness direction), and when the width of the steel plate is W (mm).

[0115] The observation surface is polished to a mirror finish. After polishing, the observation surface is etched with Nital solution to reveal the metal structure. After the metal structure is revealed, the observation surface is observed with a 500x optical microscope.

[0116] In the segregation region, Sn and Mn are concentrated. Therefore, the hardenability of the segregation region is higher compared to other regions. Consequently, more strain is introduced in the segregation region during the manufacturing process compared to other regions. As a result, in the observation surface after etching, the region containing the segregation region is observed as a darker black region (hereinafter referred to as the black region) compared to other regions.

[0117] Select multiple black regions from the observation surface. Then, identify the segregation regions within the selected black regions using the following method.

[0118] For the black areas, surface analysis will be performed using an electron probe microanalyzer (EPMA) to create an elemental distribution map of Mn concentration (mass%). In the EPMA setup, the acceleration voltage will be 15kV, the irradiation current 0.1μA, and the irradiation time 50ms. The measurement interval will be 1μm pitch, and the beam diameter 0.5μm.

[0119] The Mn content (mass%) in the chemical composition of the steel sheet is set to a standard value [Mn]. ref Based on the created elemental distribution map of Mn concentration (mass%), the elemental distribution map is divided into the following two areas. Area 1: Mn concentration is 1.5 × [Mn] ref The realm of transcendence Area 2: Mn concentration is 1.5 × [Mn] ref The following is the domain The area corresponding to Area 1 is designated as the segregation region.

[0120] From the identified segregation regions, five arbitrary segregation regions are selected. Within each segregation region, the hardness measurement range is defined as a 1000 μm area in the plate width direction and a 500 μm area in the plate thickness direction, centered on the segregation region. Within the hardness measurement range, measurement points (861 points) are set at 25 μm intervals in both the plate width and plate thickness directions. At each set measurement point, a Vickers hardness test is performed in accordance with JIS Z 2244-1 (2024) to determine the Vickers hardness of each measurement point. In the Vickers hardness test, the test force is set to 0.09807 N (10 gf).

[0121] The arithmetic mean of the top 20 Vickers hardness values ​​obtained from five hardness measurement ranges is calculated. This obtained Vickers hardness is defined as the Vickers hardness (HV) in the segregation region. The Vickers hardness is rounded to the nearest integer by rounding the first decimal place of the obtained arithmetic mean.

[0122] [(Feature 10) Regarding the maximum length of porosity in the C section] In the steel plate of this embodiment, the maximum length of porosity in the C section is 0.5 mm or less.

[0123] A segregation region (central segregation region) is formed in the center of the slab, which is the material for steel plates. During the slab casting process, alloying elements become concentrated in the central segregation region towards the end of solidification. Therefore, at the end of solidification, while the areas outside the central segregation region are in a solid phase, the central segregation region is in a liquid phase state. By applying light pressure to the slab in accordance with the solidification shrinkage of the liquid phase in the central segregation region, porosity formation can be suppressed. However, since the amount of solidification shrinkage is not uniform within the central segregation region, it is difficult to completely eliminate porosity.

[0124] The steel sheet of this embodiment contains Sn. The solid-liquid partition coefficient of Sn is small. Therefore, as solidification occurs, Sn becomes significantly concentrated in the liquid phase. As a result, Sn becomes concentrated in the central segregation region, lowering the melting point of the liquid phase in the central segregation region. The lowering of the melting point of the liquid phase prolongs the time it takes for solidification to be completed in the central segregation region. In this case, the amount of solidification shrinkage at various points within the central segregation region becomes significantly non-uniform, and the size of the porosity formed tends to be large.

[0125] As described above, if the steel plate is thin, the porosity is compressed during the hot rolling process after the casting process. However, if the size of the porosity is large, or if the steel plate is extremely thick, such as the steel plate in this embodiment with a thickness of more than 105 to 125 mm, the porosity is more likely to remain. If the maximum length of the remaining porosity is long, when tensile stress is applied in the thickness direction of the steel plate, stress concentration occurs at the tip of the porosity, causing brittle fracture and reducing the lamellar tear resistance of the steel plate.

[0126] In this embodiment, although the steel plate has a thickness of more than 105 to 125 mm, the maximum length of porosity in the C section is 0.5 mm or less. Therefore, when tensile stress is applied in the thickness direction of the steel plate, stress concentration at the tip of the porosity can be sufficiently suppressed. As a result, excellent lamellar tear resistance is obtained in the steel plate.

[0127] The preferred upper limit for the maximum length of porosity in the C section is 0.4 mm, more preferably 0.3 mm, even more preferably 0.2 mm, and even more preferably 0.1 mm.

[0128] [Method for measuring the maximum length of porosity in a C-section] The maximum length of porosity in section C can be determined by the following method. A test specimen is taken that includes a cross section (C section) perpendicular to the rolling direction of the steel plate. Of the C section of the test specimen, the "observation surface" is defined as the area from the W / 4 position to the 3W / 4 position in the width direction, starting from one end of the steel plate in the width direction, which is the center of the thickness of the steel plate (the range from a depth of 9t / 20 to a depth of 11t / 20 from the surface of the steel plate in the thickness direction), and when the width of the steel plate is W (mm). The observation surface is polished to a mirror finish. After polishing, the observation surface is observed with a 500x optical microscope, and a photographic image of the entire observation surface is generated. Alternatively, the observation surface may be divided into multiple regions, and photographic images of each region may be generated.

[0129] Identify porosity from the entire observation surface. Porosity can be easily identified in photographic images from an optical microscope by those skilled in the art. Determine the maximum length of all identified porosity by the following method: At each identified porosity, place two parallel line segments that are tangent to the outer edge of the porosity. The value at which the distance between the two line segments is maximized is taken as the length of that porosity (μm). The maximum length is rounded to the first decimal place by rounding to the second decimal place. The maximum length of all obtained porosity is defined as the maximum porosity length (μm).

[0130] [Regarding the effects of the steel plate in this embodiment] The steel plate of this embodiment satisfies features 1 to 10. Therefore, the steel plate of this embodiment has high strength, excellent low-temperature toughness, and excellent corrosion resistance in chloride corrosion environments. Furthermore, despite its thickness of over 105 to 125 mm, it achieves excellent lamellar tear resistance.

[0131] [Applications of the steel plate in this embodiment] The steel plate of this embodiment is widely applicable to applications requiring high strength, excellent low-temperature toughness, excellent corrosion resistance, and excellent lamellar tear resistance. The steel plate of this embodiment is particularly suitable for use in large structures such as bridges and offshore structures. However, the steel plate of this embodiment can also be applied to other applications besides large structures.

[0132] [Regarding the microstructure of the steel sheet in this embodiment] In the microstructure of the steel sheet of this embodiment, the total area ratio of bainite and martensite is 90% or more. Hereinafter, bainite and martensite will be referred to as the hard structure. In the microstructure of the ko steel sheet of this embodiment, the structure other than the hard structure is ferrite.

[0133] [Method for measuring the total area ratio of bainite and martensite in the microstructure of steel sheets] The total area ratio of bainite and martensite in the microstructure of a steel sheet is determined by the following method. A test specimen is taken that includes a cross section (C section) perpendicular to the rolling direction of the steel plate. Of the C section of the test specimen, the "observation surface" is defined as the area from the W / 4 position to the 3W / 4 position in the width direction, starting from one end of the steel plate in the width direction, which is the center of the thickness of the steel plate (the range from a depth of 9t / 20 to a depth of 11t / 20 from the surface of the steel plate in the thickness direction), and when the width of the steel plate is W (mm).

[0134] The observation surface is polished to a mirror finish. After mirror polishing, the observation surface is etched with Nital solution to reveal the metallic structure. After the metallic structure is revealed, the observation surface is observed with a 500x optical microscope, and ferrite and hard structures (bainite and martensite) are identified based on the contrast. The total area percentage (%) of the hard structures (bainite and martensite) is determined based on the area of ​​the identified hard structures and the total area of ​​the observation surface. In this embodiment, it is not necessary to distinguish between bainite and martensite in the steel sheet; bainite and martensite can be identified as the same hard structure.

[0135] [Other configurations of the steel sheet of the present embodiment] The steel sheet of the present embodiment may further include a corrosion prevention coating on its surface. When a corrosion prevention coating is formed on the surface of the steel sheet, the corrosion resistance of the steel sheet is further enhanced. The corrosion prevention coating includes, for example, a metal coating and / or a resin coating. The metal coating is, for example, one or more selected from the group consisting of a metal plating coating and a thermal spraying coating. The metal plating coating is, for example, one or more selected from the group consisting of a Zn plating coating, an Al plating coating, or an alloy plating coating containing Zn and Al. The thermal spraying coating is, for example, composed of one or more selected from the group consisting of a Zn thermal spraying coating, an Al thermal spraying coating, and an alloy thermal spraying coating containing Al and Mg. The resin coating is, for example, composed of one or more selected from the group consisting of a vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based coatings.

[0136] When the above-described corrosion prevention coating is formed on the steel sheet of the present embodiment, even better corrosion resistance can be obtained in the steel sheet. Further, when a corrosion prevention coating is formed on the steel sheet, local corrosion is less likely to occur in the steel sheet, and swelling and peeling of the corrosion prevention coating associated with locally occurring corrosion are suppressed. As a result, the durability of the corrosion prevention coating is also enhanced.

[0137] The corrosion prevention coating may be formed on at least a part of the surface of the steel sheet. For example, the corrosion prevention coating may be formed on one side of the steel sheet or on both sides.

[0138] [Regarding the manufacturing method of the steel sheet of the present embodiment] An example of the manufacturing method of the steel sheet of the present embodiment will be described. The steel sheet of the present embodiment satisfying the above-described features 1 to 10 may be manufactured by other manufacturing methods than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the manufacturing method of the steel sheet of the present embodiment.

[0139] An example of the manufacturing method of the steel sheet of the present embodiment includes the following steps. (Step 1) Slab manufacturing step (Step 2) Soaking step (Process 3) Hot rolling process (Process 4) Quenching process The following describes each process.

[0140] [(Process 1) Slab manufacturing process] In the slab manufacturing process, first, a well-known refining method is carried out to produce molten steel having a chemical composition satisfying Features 1 to 5. For example, refining (primary refining) in a converter is carried out on molten iron manufactured by a well-known method. Secondary refining, which is well-known, is carried out on the molten steel discharged from the converter. By the above processes, molten steel having a chemical composition satisfying Feature 1 is produced. Further, using the produced molten steel, a slab is manufactured by a continuous casting method.

[0141] [(Process 2) Soaking process] In the soaking process, the manufactured slab is soaked at a predetermined soaking temperature. The soaking of the slab may be carried out in a heating furnace or may be carried out in a batch-type soaking furnace. The conditions of the soaking process will be described later.

[0142] [(Process 3) Hot rolling process] In the hot rolling process, the slab heated by the soaking process is hot-rolled to manufacture an intermediate steel plate. The hot rolling process includes a rough rolling process and a finish rolling process. In the rough rolling process, the slab is hot-rolled using a reverse-type No. 1 rolling mill. The finish rolling process uses a reverse-type No. 2 rolling mill to further hot-roll the slab rolled by the No. 1 rolling mill to manufacture an intermediate steel plate. The intermediate steel plate is cooled to room temperature. The conditions of the hot rolling process will be described later.

[0143] [(Process 4) Quenching process] In the quenching process, quenching is carried out on the intermediate steel plate. In the quenching process, the intermediate steel plate is reheated and then rapidly cooled. By reheating the intermediate steel plate, reverse transformation to austenite occurs, and the austenite grains tend to become fine and equiaxed. The conditions of the quenching process will be described later.

[0144] [Regarding the conditions during the manufacturing process] The manufacturing method of this embodiment satisfies the following conditions. (Condition 1) In the soaking process, the soaking temperature is set to 1050-1140°C. (Condition 2) In the hot rolling process (rough rolling and finish rolling), the maximum reduction ratio in a single pass shall be 10% or more in the passes after the plate thickness reaches 300 mm or less. In other words, at least one pass with a maximum reduction ratio of 10% or more shall be performed after the plate thickness reaches 300 mm or less. (Condition 3) In the quenching process, the quenching temperature is set to 850-950°C.

[0145] [(Condition 1) Regarding the soaking temperature] If the soaking temperature is below 1050°C, porosity will not adhere well in the subsequent hot rolling process. On the other hand, if the soaking temperature exceeds 1140°C, the austenite grains will become excessively coarse. As a result, the old austenite grains after the quenching process may not become sufficiently fine. Therefore, the soaking temperature should be set between 1050°C and 1140°C. Here, the soaking temperature (°C) refers to the furnace temperature (°C) in the soaking zone in the case of a heating furnace, and to the furnace temperature (°C) of the soaking furnace in the case of a batch-type soaking furnace. The holding time at the soaking temperature (soaking time) is, for example, 200 to 400 minutes.

[0146] [(Condition 2) Regarding the maximum reduction ratio in one pass] As mentioned above, a reverse-type rolling mill is used in the hot rolling process (rough rolling and finish rolling). In a reverse-type rolling mill, one pass is defined as the time it takes for the slab to pass through the mill and receive external force (reduction) from the pair of work rolls of the mill. In a reverse-type rolling mill, there are forward passes, in which the slab is transported and reduced from upstream to downstream of the rolling line, and reverse passes, in which the slab is transported and reduced from downstream to upstream of the rolling line. Here, the reduction ratio (%) in one pass is defined by the following formula. Reduction ratio = (1 - (Slab thickness after 1 pass / Slab thickness before 1 pass)) × 100 If the maximum reduction ratio in a single pass is less than 10% in the passes after the slab thickness is 300 mm or less, then the reduction in a single pass is insufficient. In this case, the porosity may not be sufficiently compressed. Therefore, in passes after the slab thickness reaches 300 mm or less, the maximum reduction ratio in a single pass should be 10% or more. Furthermore, if at least one pass with a maximum reduction ratio of 10% or more is performed in passes after the slab thickness reaches 300 mm or less, sufficient porosity will be bonded.

[0147] The thickness of the slab is not particularly limited. For example, the thickness of the slab may be three times or more the thickness of the steel plate in this embodiment, and 400 mm or less.

[0148] [(Condition 3) Regarding the quenching temperature] In the quenching process, if the quenching temperature is below 850°C, sufficient reverse transformation to austenite does not occur, and coarse prior austenite grains remain. On the other hand, if the quenching temperature exceeds 950°C, the prior austenite grains become excessively coarse. Therefore, the quenching temperature should be set between 850°C and 950°C. Here, the quenching temperature (°C) refers to the furnace temperature (°C) of the heat treatment furnace used to maintain the steel plate at the quenching temperature. The holding time (in minutes) at the quenching temperature is, for example, 180 to 240 minutes.

[0149] The steel plate of this embodiment is manufactured through the above manufacturing process. [Examples]

[0150] The effects 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.

[0151] Steel sheets having the chemical compositions shown in Table 1 (Tables 1A to 1F) were manufactured. Table 1 also shows Ceq, SnEQ, MV, and SC as defined by formulas (1) to (4).

[0152]

Table 1A

[0153]

Table 1B

[0154]

Table 1C

[0155]

Table 1D

[0156]

Table 1E

[0157]

Table 1F

[0158] Specifically, a slab was produced by a continuous casting method. A soaking process was carried out on the slab. The soaking temperature (°C) (Condition 1) was as shown in Table 2 (Table 2A and Table 2B). The soaking time at the soaking temperature was 200 to 400 minutes. After the soaking process, a hot rolling process was carried out on the slab to produce an intermediate steel plate, and the intermediate steel plate was water-cooled to room temperature. The maximum reduction ratio (%) in one pass after the slab thickness became 300 mm or less (Condition 2) was as shown in Table 2 (Table 2A and Table 2B).

[0159]

Table 2A

[0160]

Table 2B

[0161] A quenching process was performed on the intermediate steel sheets after the hot rolling process. The quenching temperature (°C) (Condition 3) was as shown in Table 2 (Tables 2A and 2B). The holding time at the quenching temperature was 180 to 240 minutes. Steel sheets for each test number were manufactured using the above manufacturing process. The thickness of the steel sheets for each test number was as shown in Table 3 (Tables 3A and 3B). For each steel sheet for each test number, the total area ratio (%) of bainite and martensite in the microstructure of the steel sheet was determined using a method in accordance with [Method for measuring the total area ratio of bainite and martensite in the microstructure of steel sheets]. As a result, in all test numbers, the total area ratio of martensite and bainite was 90% or more.

[0162] [Table 3A]

[0163] [Table 3B]

[0164] [About the evaluation test] The following evaluation tests were performed on each steel plate with the specified test number that was manufactured. (Test 1) Tensile strength test (Test 2) Equivalent diameter D Lave , D Cave Aspect ratio AR L AR C Measurement test (Test 3) Vickers hardness evaluation test of the segregation region in the center of the plate thickness (Test 4) Evaluation test for the maximum length of porosity in section C (Test 5) Corrosion resistance test (Test 6) Toughness evaluation test (Test 7) ​​Lamellar tear resistance evaluation test The following describes each test.

[0165] [(Test 1) Tensile Strength Test] Based on the method described in the above [Method for Measuring Tensile Strength], the tensile strength (MPa) of the steel plates for each test number was determined. The obtained results are shown in Table 3 (Table 3A and Table 3B).

[0166] [(Test 2) Equivalent Circle Diameter D Lave , D Cave , Aspect Ratio AR L , AR C Measurement Test] Based on the method described in the above [Equivalent Circle Diameter D Lave and D Cave , Aspect Ratio AR L and AR C Measurement Method], the equivalent circle diameter D Lave (μm) of the prior austenite grains at the center of the plate thickness of the L-section of the steel plates for each test number, the aspect ratio AR L , and the equivalent circle diameter D Cave (μm) of the prior austenite grains at the center of the plate thickness of the C-section, and the aspect ratio AR C were determined. The obtained results are shown in Table 3 (Table 3A and Table 3B).

[0167] [(Test 3) Vickers Hardness Evaluation Test of the Segregation Region at the Center of the Plate Thickness] Based on the method described in the above [Method for Measuring Vickers Hardness of the Segregation Region], the Vickers hardness (HV) of the segregation region at the center of the plate thickness of the C-section of the steel plates for each test number was determined. The obtained results are shown in Table 3 (Table 3A and Table 3B).

[0168] [(Test 4) Maximum Length Evaluation Test of Porosity in the C-section] Based on the method described in the above [Method for Measuring the Maximum Length of Porosity in the C-section], the maximum length (mm) of the porosity at the center of the plate thickness of the C-section of the steel plates for each test number was determined. The obtained results are shown in Table 3 (Table 3A and Table 3B).

[0169] [(Test 5) Corrosion Resistance Test] Test pieces were taken at the center position of the plate width and at the center position of the plate thickness of the steel plates for each test number. The test pieces were 100 mm in the rolling direction, 30 mm in the plate width direction, and 20 mm in the plate thickness direction. Among the test pieces, the length in the plate thickness direction of the steel plate was taken as the thickness of the test piece.

[0170] Using the collected test specimens, a combined cycle corrosion test (CCT) was performed in accordance with SAE (Society of Automotive Engineers) J2334 Method C. Specifically, the test consisted of 40 cycles, with each cycle comprising steps 1 through 3. Step 1: Keep the test specimen in a humid environment at 50°C and 100% RH for 6 hours. Step 2: Immerse the specimen from Step 1 in an aqueous solution containing 0.5% by mass of NaCl, 0.1% of CaCl2, and 0.075% of NaHCO3 for 0.25 hours. Step 3: The test specimens from Step 2 are kept in a dry environment at 60°C and 50% RH for 17.75 hours.

[0171] After 40 cycles, the rust layer on the surface of the specimen was removed with a wire brush. The thickness of the specimen (steel plate) after the rust layer was removed was measured at five arbitrary points, and the thickness reduction at each measurement point (thickness before testing - measured thickness) was calculated. The arithmetic mean of the obtained thickness reductions was defined as the thickness reduction (mm) of the specimen.

[0172] If the thickness reduction of the test specimen was 0.15 mm or less, it was determined that excellent corrosion resistance was obtained in a chloride corrosion environment (indicated as "pass" in Table 3). On the other hand, if the thickness reduction of the test specimen exceeded 0.15 mm, it was determined that sufficient corrosion resistance was not obtained in a chloride corrosion environment (indicated as "fail" in Table 3).

[0173] [(Test 6) Toughness Evaluation Test] A Charpy impact test was performed on each steel plate according to JIS Z 2242 (2023). Specifically, a V-notch specimen was taken from the center of the plate width and the center of the plate thickness of each steel plate. The V-notch specimen had a notch surface perpendicular to the plate thickness direction, and the longitudinal direction of the V-notch specimen was parallel to the plate width direction. The size of the V-notch specimen was 10 mm × 10 mm × 55 mm.

[0174] A Charpy impact test was performed on V-notch specimens in accordance with JIS Z 2242 (2023). The test temperature was set at 11 levels (-50°C, -40°C, -30°C, -20°C, -10°C, 0°C, 10°C, 20°C, 30°C, 40°C, and 50°C) by varying the temperature by 10°C increments. Two specimens were used for each test temperature, and the Charpy impact test was performed. Under these conditions, the brittle fracture surface ratio (%) of the specimens after testing at each temperature was determined. The test temperature (°C) and the obtained brittle fracture surface ratio (%) were plotted, and an approximation curve was obtained by linear regression. From the obtained approximation curve, the temperature (°C) at which the brittle fracture surface ratio becomes 50% was determined and defined as the fracture transition temperature vTrs (°C). The obtained fracture transition temperatures vTrs (°C) are shown in Table 3 (Tables 3A and 3B). If the fracture transition temperature vTrs is less than 0°C, it was determined that excellent low-temperature toughness was achieved.

[0175] [(Test 7) ​​Lamellar tear resistance evaluation test] The lamellar tear resistance of the steel plates for each test number was evaluated by the following tests. First, test specimens 10 were prepared by processing the steel plates in the order shown in Figures 1A to 1E. Specifically, as shown in Figure 1A, when the width of the steel plate was W (mm), a rectangular parallelepiped section 1 was taken from one end of the steel plate in the width direction at a position W / 4 in the width direction (Figure 1B). The length of the longitudinal side of section 1 was the thickness of the steel plate t (mm).

[0176] As shown in Figure 1C, a material 3 was prepared by pressing a pair of rectangular blocks 2 onto both ends of the longitudinal direction of section 1. A round bar 4 was cut from material 3 as shown in Figure 1D. The round bar 4 was further processed to prepare a test piece 10 as shown in Figure 1E. The test piece 10 had a pair of gripping parts 5 and a parallel part 6 positioned between the pair of gripping parts 5. The gripping parts 5 were threaded. The length Lc of the parallel part 6 was the same as the thickness t of the steel plate. The diameter D0 of the parallel part was 10 mm. The length Lt of the test piece 10 was 200 to 300 mm.

[0177] A tensile test was performed on test specimen 10 in accordance with JIS G 3199 (2021) at room temperature and in air to determine the reduction of area (%). The obtained reduction of area (%) is shown in Table 3. If the reduction of area exceeded 50%, it was determined that excellent lamellar tear resistance had been achieved.

[0178] [Evaluation Results] Referring to Tables 1 to 3, in test numbers 1 to 36, the steel plates met characteristics 1 to 10. Therefore, high strength was obtained in these steel plates. Furthermore, in the corrosion resistance test, the thickness reduction was 0.15 mm or less, indicating excellent corrosion resistance. Furthermore, in the toughness evaluation test, the fracture transition temperature vTrs was less than 0°C, indicating excellent low-temperature toughness. Furthermore, in the lamellar tear resistance evaluation test, the reduction in area was over 50%, indicating excellent lamellar tear resistance.

[0179] On the other hand, in test numbers 37 and 38, the Ceq was less than 0.44. As a result, the tensile strength was less than 570 MPa, and high strength could not be obtained.

[0180] In tests 39 and 40, the Ceq exceeded 0.50. As a result, the Vickers hardness in the segregation region exceeded 250 HV. Consequently, the fracture transition temperature vTrs was above 0°C, and sufficient low-temperature toughness could not be obtained.

[0181] In tests 41 and 42, the SnEQ was less than 0.10. As a result, the thickness reduction in the corrosion resistance test exceeded 0.15 mm, and sufficient corrosion resistance could not be obtained.

[0182] In tests 43 and 44, the MV was less than 0.30. As a result, the tensile strength was less than 570 MPa, and sufficient strength was not obtained.

[0183] In tests 45 and 46, the MV exceeded 0.90. As a result, the Vickers hardness exceeded 250 HV. Consequently, the fracture transition temperature vTrs was above 0°C, and sufficient low-temperature toughness could not be obtained.

[0184] In tests 47 and 48, the SC value was less than 0.20. As a result, the fracture transition temperature vTrs was 0°C or higher, and sufficient low-temperature toughness could not be obtained.

[0185] In tests 49 and 50, the SC value exceeded 15.00. As a result, the fracture transition temperature vTrs was above 0°C, and sufficient low-temperature toughness could not be obtained. Furthermore, in the lamellar tear resistance evaluation test, the reduction in area was 50% or less, indicating that sufficient lamellar tear resistance could not be obtained.

[0186] In tests 51 and 52, during the hot rolling process, the maximum reduction ratio per pass was less than 10% in the passes after the plate thickness reached 300 mm or less. As a result, the maximum length of porosity exceeded 0.5 mm. Consequently, the reduction in area in the lamellar tear resistance evaluation test was 50% or less, indicating that sufficient lamellar tear resistance was not achieved.

[0187] In tests 53 and 54, the quenching temperature was too high. As a result, the average equivalent circular diameter D of the prior austenite grains in the L section was incorrect. Lave The average equivalent circular diameter D of the prior austenite grains in the C section exceeds 40 μm. Cave The thickness exceeded 40 μm. As a result, the fracture transition temperature vTrs was above 0°C, and sufficient low-temperature toughness could not be obtained. Furthermore, in the lamellar tear resistance evaluation test, the reduction in area was 50% or less, and sufficient lamellar tear resistance could not be obtained.

[0188] In tests 55 and 56, the quenching temperature was too low. As a result, the aspect ratio AR of the prior austenite grains in the L section was L Furthermore, the aspect ratio AR of the prior austenite grains in the C cross-section exceeds 2.0. C The value exceeded 2.0. As a result, the aperture value in the lamellar tear resistance evaluation test was 50% or less, and sufficient lamellar tear resistance was not obtained.

[0189] 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. It is a steel plate, The chemical composition is expressed in mass percent. C: 0.06-0.15%, Si: 0.05-0.80%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 1.0%, Mo: 0.01 to 0.60%, V: 0.001-0.200%, Nb: 0.005-0.050%, Ti: 0.005 to 0.050%, Al: 0.010-0.080%, Sn: 0.02-0.40%, N: 0.0010-0.0070%, O: 0.0005 to 0.0040%, and, It contains Ca: 0.0001 to 0.0080%, The remainder consists of Fe and impurities. The Ceq defined by equation (1) is between 0.44 and 0.

50. The SnEQ defined by equation (2) is 0.10 or greater, The MV defined by equation (3) is between 0.30 and 0.

90. The SC defined in equation (4) is between 0.20 and 15.00, Having a plate thickness of over 105 to 125 mm, The tensile strength is 570 MPa or more. In the center of the thickness of the L-shaped cross-section of the steel plate, parallel to the rolling direction and the thickness direction, Average circular equivalent diameter D of old austenite grains Ｌａｖｅ It is 40 μm or less, Aspect ratio AR of the aforementioned prior austenite grains Ｌ It is 2.0 or less, In the central part of the thickness of the C cross-section of the steel plate perpendicular to the rolling direction, Average circular equivalent diameter D of old austenite grains Ｃａｖｅ It is 40 μm or less, Aspect ratio AR of the aforementioned prior austenite grains Ｃ It is 2.0 or less, The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less. The maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ=Sn+W+Ni / 10+Mo / 4 (2) MV=Mo+4V (3) SC=S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol.

2. It is a steel plate, The chemical composition is expressed in mass percent. C: 0.06-0.15%, Si: 0.05-0.80%, Mn: 1.00-2.50%, P: 0.015% or less, S: 0.0024% or less, Ni: 0.1 to 1.0%, Mo: 0.01 to 0.60%, V: 0.001-0.200%, Nb: 0.005-0.050%, Ti: 0.005 to 0.050%, Al: 0.010-0.080%, Sn: 0.02-0.40%, N: 0.0010-0.0070%, O: 0.0005 to 0.0040%, and, It contains Ca: 0.0001 to 0.0080%, Furthermore, it contains one or more selected from the groups consisting of Group 1 to Group 3, The remainder consists of Fe and impurities. The Ceq defined by equation (1) is between 0.44 and 0.

50. The SnEQ defined by equation (2) is 0.10 or greater, The MV defined by equation (3) is between 0.30 and 0.

90. The SC defined in equation (4) is between 0.20 and 15.00, Having a plate thickness of over 105 to 125 mm, The tensile strength is 570 MPa or more. In the center of the thickness of the L-shaped cross-section of the steel plate, parallel to the rolling direction and the thickness direction, Average circular equivalent diameter D of old austenite grains Ｌａｖｅ It is 40 μm or less, Aspect ratio AR of the aforementioned prior austenite grains Ｌ It is 2.0 or less, In the central part of the thickness of the C cross-section of the steel plate perpendicular to the rolling direction, Average circular equivalent diameter D of old austenite grains Ｃａｖｅ It is 40 μm or less, Aspect ratio AR of the aforementioned prior austenite grains Ｃ It is 2.0 or less, The Vickers hardness in the segregation region at the center of the plate thickness of the C section is 250 HV or less. The maximum length of porosity at the center of the plate thickness of the C section is 0.5 mm or less. steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (1) SnEQ=Sn+W+Ni / 10+Mo / 4 (2) MV=Mo+4V (3) SC=S / Ca (4) Here, each element symbol in each formula is replaced with the mass percentage content of the corresponding element. If the corresponding element is not present, "0" is replaced with that element symbol. [Group 1] Cu: 0.80% or less, B: 0.0050% or less, Zr: 0.05% or less, Ta: One or more selected from the group consisting of 0.10% or less. [Group 2] Cr: 0.20% or less, W: 0.80% or less, Sb: 0.10% or less, As: 0.05% or less, Bi: 0.10% or less, Se: 0.05% or less, Te: 0.05% or less, Zn: 0.05% or less, Ga: 0.05% or less, Ge: 0.05% or less, Co: 0.50% or less, One or more selected from the group consisting of Hf: 0.05% or less. [Group 3] Mg: 0.010% or less, Sr: 0.020% or less, Ba: 0.010% or less, Rare earth elements: One or more selected from the group consisting of 0.020% or less.

3. The steel plate according to claim 2, The aforementioned chemical composition contains the first group, steel plate.

4. The steel plate according to claim 2, The aforementioned chemical composition contains the second group, steel plate.

5. The steel plate according to claim 2, The aforementioned chemical composition contains the third group, steel plate.

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