Steel plate

Abstract

This steel plate has a given chemical composition and has an α value of 1.00-1.50 mass%, a β value of 10.0-15.0, a γ value of 0.70-1.50 mass%, a Ceq value of 0.550-0.620 mass%, a yield strength of 670-870 N / mm2, a tensile strength of 780-940 N / mm2, and a Charpy absorption energy at -65°C of 100 J or greater. When the steel plate is examined for hardness distribution at a pitch of 0.05 mm with respect to 121 portions of 1 mm × 1 mm located at 1 / 4 the plate thickness, the average hardness is 265-290 Hv and the standard deviation is 20 or less. The steel plate has a thickness of 10-60 mm.

Description

steel plate

[0001] This application claims priority from Japanese Patent Application No. 2022-157410, filed on September 30, 2022, the contents of which are incorporated herein by reference.

[0002] In recent years, there has been a strong demand for reducing greenhouse gases as a measure against climate change. Carbon dioxide (CO ２ Carbon dioxide capture and storage (CCS) is a technology that captures and stores CO2 from refineries, power plants, chemical plants, etc. ２ CO emitted from the emission source ２ CO is separated and collected, then injected and stored in a deep underground reservoir. ２ and a CO ２ If the storage facility for injecting and storing CO into an underground reservoir is far away, ２ It must be transported between these facilities by pipelines, ships, etc.

[0003] CO by ship ２ When transporting CO2, liquefied CO2 is stored in a transport tank on board the ship. ２ This allows CO ２ However, the CO in the transport tank ２ In order to prevent CO from solidifying (turning into dry ice), it is necessary to transport it while maintaining a pressure of about 2 MPa. ２ To maintain the liquid state, CO ２ It is necessary to keep the temperature at around -35°C. Furthermore, in order to reduce the weight of ships, there is a demand to make the thickness of transport tanks as thin as possible by increasing the strength of the steel plates used.

[0004] Therefore, the steel plate used as the material for the transport tank is required to have high strength and excellent low-temperature toughness. ２A tensile strength of at least 1000 kJ / cm is required. Furthermore, for low-temperature toughness, although this depends on the plate thickness, excellent low-temperature toughness is required as evaluated by a Charpy test at minus 65°C (-65°C) as the most stringent condition for plate thicknesses of 20 to 60 mm used in transport tanks. The reason the test temperature for the Charpy test is minus 65°C is that the Charpy test is a small-scale test, and is generally evaluated at a temperature that is lower than the service temperature by a certain amount depending on the plate thickness.

[0005] Furthermore, in large welded structures such as transport tanks, stress relief annealing is sometimes performed on the welds to further reduce the possibility of fracture. Stress relief annealing is a heat treatment method in which the welded part of a welded structure is heated to a temperature below the Ac1 transformation point and then slowly cooled, with the aim of reducing residual stress caused by welding. However, if the tensile strength is 780 N / mm ２ When stress relief annealing is applied to the above-mentioned high-tensile steels, alloy carbides are selectively precipitated at the grain boundaries, which causes grain boundary embrittlement, resulting in a significant decrease in the toughness of the stress-relief annealed area. This phenomenon is generally called SR (Stress Relieving) embrittlement. SR embrittlement is particularly likely to occur in high-tensile steels that contain B and are manufactured by quenching and tempering. In such high-tensile steels, not only the base metal but also the heat-affected zone (HAZ) obtained when a welded joint is made using this high-tensile steel undergoes significant embrittlement.

[0006] Therefore, in order to ensure high safety in transport tanks manufactured using such high-tensile steel, it is preferable that the base material and welded parts (especially the weld heat-affected zone) have excellent low-temperature toughness even after stress relief annealing is performed.

[0007] From the above viewpoint, several technical proposals have been made in the past. For example, Patent Document 1 discloses a high-strength steel plate characterized by adjusting the chemical composition and setting the average crystal grain size to 15 μm or less. However, the steel plate described in Patent Document 1 has not been evaluated for low-temperature toughness at minus 65°C, and there is room for further improvement in low-temperature toughness.

[0008] Japanese Patent No. 5590271

[0009] The present invention has been made in view of the above circumstances, and provides a liquefied CO2-based alloy having excellent strength of the base material and low-temperature toughness of the base material and weld heat-affected zone, and further excellent strength of the base material after stress relief annealing and low-temperature toughness of the base material and weld heat-affected zone. ２ The objective is to provide a steel plate suitable for use in a transport tank.

[0010] [Correction based on Rule 91 29.11.2024] In order to solve the above problems, the present invention employs the following configuration: [1] A steel sheet according to one aspect of the present invention has a chemical composition, in mass %, of C: 0.07-0.11%, Si: 0.10-0.15%, Mn: 0.70-1.20%, Ni: 1.00-2.50%, Cr: 0.20-0.80%, Mo: 0.20-0.80%, V: 0.005-0.070%, Al: 0.010-0.100%, B: 0.0005-0.0030%, N: 0.0015-0.0050%, P: 0.006% or less, S: 0.005-0.070 ...005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0.0050%, Mn: 0.005-0. : 0.0030% or less, Cu: 0 to 1.00%, Nb: 0 to 0.030%, Ti: 0 to 0.010%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, O: 0.0040% or less, balance: Fe and impurities, an α value defined by the following formula (1) is 1.00 to 1.50 mass%, a β value defined by the following formula (2) is 10.0 to 15.0, a γ value defined by the following formula (3) is 0.70 to 1.50 mass%, a Ceq value defined by the following formula (4) is 0.550 to 0.620 mass%, and a yield strength of 670 to 870 N / mm ２ , tensile strength is 780 to 940 N / mm ２ , Charpy absorbed energy at -65°C is 100J or more, In hardness distribution measurement at 1 / 4 thickness position with 1mm x 1mm and 0.05mm pitch, the average hardness at 121 measurement positions is 265Hv to 290Hv, with a standard deviation of 20 or less, and the plate thickness is 10 to 60mm. α = [C] + 6 x [Si] + 100 x [P] ... (1) β = 0.65 x [C] １／２× (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) … (2) γ = [Mn] + 20 × [Nb] + 36 × [Ti] … (3) Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5...(4) However, in formulas (1) to (4), [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] are the contents (mass%) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, and elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0. [2] In the steel sheet described in [1], [fB] calculated by the following formulas (A) to (E) may be 0.0003 mass% or more. [fB] = [B] - 0.77 x [fN] ... (A) [fN] = [N] - 0.29 x [fTi] - 0.52 x [fAl] ... (B) [fTi] = [Ti] - 2 x [fO] ... (C) [fAl] = [Al] - 1.125 x [fO] …(D) [fO]=[O]-0.4×[Ca]-0.66×[Mg]-0.11×[REM]…(E) In the formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, and 0 is substituted for elements that are not contained, including the amount of elements mixed as impurities, and 0 is substituted for calculated values ​​of [fN], [fTi], [fAl], and [fO] when the calculated values ​​are less than 0%. [3] In the steel sheet according to [1] or [2], when regions surrounded by grain boundaries having a crystal orientation difference of 15° or more determined by performing crystal orientation analysis using electron beam backscatter diffraction pattern analysis are defined as crystal grains, the circle-equivalent grain diameters of the crystal grains are defined as crystal grain diameters, and a value calculated as an area-weighted average weighted by the area of ​​each crystal grain is defined as an average crystal grain diameter, the average crystal grain diameter at the ¼ thickness position may be 15.0 μm or less.[4] In the steel sheet according to any one of [1] to [3], when stress relief annealing is performed on the steel sheet at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a heating rate of 55°C / hr or less in a temperature range of 425°C or more, the yield strength of the part subjected to the stress relief annealing is 670 to 870 N / mm. ２ and a tensile strength of 780 to 940 N / mm ２ and the Charpy absorbed energy at −40° C. may be 27 J or more.

[0011] According to the above aspect of the present invention, it is possible to provide a steel plate which is excellent in the strength of the base material and the low-temperature toughness of the base material and the weld heat affected zone, and further excellent in the strength of the base material and the low-temperature toughness of the base material and the weld heat affected zone after stress relief annealing. ２ Suitable for transport tanks.

[0012] Hereinafter, a steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) will be described in detail. Unless otherwise specified, "stress relief annealing" in this embodiment refers to stress relief annealing in accordance with the contents specified in JIS Z 3700:2022 "Post-weld heat treatment method." Unless otherwise specified, "welding" in this embodiment refers to welding with a welding heat input of 1.1 to 4.5 kJ / mm. These conditions are general conditions in the technical field to which the present invention belongs. However, even if stress relief annealing or welding is performed under conditions different from the above-mentioned conditions, the same effect as stress relief annealing or welding performed under the above-mentioned conditions can be obtained. Therefore, the steel sheet according to this embodiment may be subjected to stress relief annealing or welding under conditions different from the above-mentioned conditions.

[0013] First, the content of each element constituting the chemical composition of the steel sheet according to this embodiment and the reasons for limiting it will be described. Hereinafter, unless otherwise specified, "%" regarding the content of an element means % by mass.

[0014] (C: 0.07 to 0.11%) C is an element that improves the strength of the base material. In order for the steel plate according to this embodiment to achieve the desired strength, the C content is set to 0.07% or more. The C content is preferably 0.08% or more. On the other hand, if a large amount of C is contained, the hardness of the weld heat affected zone increases and at the same time its toughness decreases, so the C content is set to 0.11% or less. The C content is preferably 0.10% or less, and more preferably less than 0.10%.

[0015] (Si: 0.10 to 0.15%) Si is an element that is generally often contained in steel as a deoxidizing element. In order to contain Si for the purpose of deoxidation, the Si content is set to 0.10% or more. On the other hand, Si is an element that reduces the toughness of steel after stress relief annealing. Furthermore, in order to suppress a decrease in the toughness of the weld heat affected zone after stress relief annealing (SR), a low Si content is preferable. Therefore, in the steel plate according to this embodiment, the Si content is set to 0.15% or less. The Si content is preferably 0.14% or less, more preferably 0.13% or less, and even more preferably 0.12% or less.

[0016] (Mn: 0.70 to 1.20%) Mn is an element effective for deoxidation and also improves the strength of steel. Therefore, the Mn content is set to 0.70% or more. The Mn content is preferably set to 0.90% or more. On the other hand, if Mn is added in excess, there is a risk that the toughness of the steel after stress relief annealing will be impaired due to temper embrittlement. Therefore, the Mn content is set to 1.20% or less. The Mn content is preferably set to 1.10% or less.

[0017] (Ni: 1.00 to 2.50%) Ni is an element effective for improving the hardenability and toughness of steel. Therefore, the Ni content is set to 1.00% or more. The Ni content is preferably set to 1.20% or more. On the other hand, if Ni is contained in excess, there is a risk that the toughness of the steel after stress relief annealing will decrease. In addition, there is a risk that the toughness of the weld heat affected zone after stress relief annealing will deteriorate. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably set to 2.00% or less.

[0018] (Cr: 0.20 to 0.80%) Cr is an element effective for improving the hardenability of steel and for improving the strength of steel by precipitation strengthening during tempering. Therefore, the Cr content is set to 0.20% or more. The Cr content is preferably set to 0.40% or more. On the other hand, if Cr is contained in excess, there is a risk of a decrease in the toughness of the base metal and the weld heat-affected zone after stress relief annealing. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably set to 0.70% or less.

[0019] (Mo: 0.20 to 0.80%) Like Cr, Mo is an effective element for improving hardenability and improving the strength of steel through precipitation strengthening during tempering. Therefore, the Mo content is set to 0.20% or more. The Mo content is preferably set to 0.30% or more, more preferably 0.35% or more, and even more preferably 0.40% or more. On the other hand, if Mo is contained in excess, there is a risk that Mo carbides will precipitate at grain boundaries after stress relief annealing, reducing the toughness of the base material and the weld heat affected zone, and this has a particularly large effect on the weld heat affected zone. Therefore, the Mo content is set to 0.80% or less. The Mo content is preferably set to 0.60% or less.

[0020] (V: 0.005 to 0.070%) Like Cr and Mo, V is an element effective for improving hardenability and improving the strength of steel through precipitation strengthening during tempering. Therefore, the V content is set to 0.005% or more. The V content is preferably set to 0.010% or more. On the other hand, if V is contained in excess, there is a risk that the toughness of the base material and the weld heat-affected zone will decrease after stress relief annealing. Therefore, the V content is set to 0.070% or less. The V content is preferably set to 0.050% or less.

[0021] (Al: 0.010 to 0.100%) Al is an element useful for deoxidation and also refines the grain size during quenching by forming nitrides. Furthermore, by forming nitrides, it is also an essential element for ensuring [fB]. Therefore, in the steel plate according to this embodiment, the Al content is set to 0.010% or more. When the N content is high, the Al content is preferably 0.030% or more, more preferably 0.040% or more, in order to fix N and ensure fB. On the other hand, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the base material and the weld heat-affected zone. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less.

[0022] (B: 0.0005 to 0.0030%) In the steel plate according to this embodiment, B is an element that improves the hardenability of the steel when contained in a small amount. Therefore, the B content is set to 0.0005% or more. The B content may be set to 0.0006% or more, 0.0008% or more, or 0.0010% or more. On the other hand, if B is contained in excess, B may form coarse nitrides and / or carbides, which may reduce the toughness of the base material. Therefore, the B content is set to 0.0030% or less. The B content may be set to 0.0020% or less, or 0.0010% or less.

[0023] (N: 0.0015 to 0.0050%) N is an element that forms nitrides to refine the crystal grain size of the base material and improve toughness. Therefore, the N content is set to 0.0015% or more. The N content may be set to 0.0030% or more or 0.0035% or more. On the other hand, if N is contained in excess, the nitrides become coarse and the toughness of the weld heat affected zone in the as-welded state decreases. Therefore, the N content is set to 0.0050% or less.

[0024] (P: 0.006% or less) (S: 0.0030% or less) P and S are impurity elements contained in steel, and the lower their contents are the better. Therefore, the lower limits of the P content and S content are 0%. In the steel plate according to this embodiment, in order to improve the base material toughness and the toughness of the base material and welded joints after stress relief annealing, the P content is set to 0.006% or less and the S content is set to 0.0030% or less. The P content is preferably set to 0.005% or less. The S content may also be set to 0.0020% or less.

[0025] (Cu: 0 to 1.00%) Cu is not an essential element in the steel sheet according to this embodiment, so the lower limit of the Cu content is 0%. However, Cu has the effect of improving the strength of the steel, so it can be contained as needed. When Cu is contained, in order to utilize this effect, the Cu content is preferably 0.10% or more, more preferably 0.20% or more. If needed, the Cu content may be 0.25% or more or 0.30% or more. On the other hand, if Cu is contained in excess, there is a risk of cracking on the steel sheet surface and Cu precipitation, which may reduce the toughness of the base material. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.80% or less. If needed, the Cu content may be set to 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0026] (Nb: 0 to 0.030%) Nb is not an essential element in the steel plate according to this embodiment, so the lower limit of the Nb content is 0%. However, since Nb is an element that refines crystal grains during quenching, it can be added as needed. When Nb is added, in order to utilize its effect, the Nb content is preferably 0.001% or more. On the other hand, if Nb is added in excess, Nb may form coarse carbonitrides, which may reduce the toughness of the base material. Therefore, the Nb content is set to 0.030% or less. Since a lower Nb content improves the toughness of the weld heat-affected zone, the Nb content may be set to 0.020% or less, 0.010% or less, or 0.005% or less.

[0027] (Ti: 0 to 0.010%) Ti is not an essential element in the steel plate according to this embodiment, so the lower limit of the Ti content is 0%. However, Ti may refine the crystal grains when the steel is heated to a high temperature by slab heating or the like, so it can be added as needed. When Ti is added, in order to utilize its effect, the Ti content is preferably 0.001% or more. On the other hand, if Ti is added in excess, there is a risk that Ti will form coarse carbonitrides, similar to Nb, and reduce the toughness of the base material. Therefore, the Ti content is set to 0.010% or less. If necessary, the Ti content may be set to 0.005% or less or 0.002% or less.

[0028] (Ca: 0 to 0.0030%) (Mg: 0 to 0.0030%) (REM: 0 to 0.0030%) The steel sheet according to this embodiment may contain one or more of Ca, Mg, and REM. Since Ca, Mg, and REM are not essential elements, the lower limits of the contents of Ca, Mg, and REM are all 0%.

[0029] Ca has the effect of spheroidizing sulfides in the steel sheet, thereby reducing the effect of MnS, which reduces the toughness of the steel sheet. To achieve this effect, the Ca content may be set to 0.0001% or more. On the other hand, if a large amount of Ca is contained, the weldability of the steel may be impaired, so the Ca content is set to 0.0030% or less. If necessary, the Ca content may be set to 0.0015% or less, 0.0010% or less, 0.0005% or less, or 0.0002% or less.

[0030] Mg and REM form oxides to improve the toughness of the weld heat-affected zone. To achieve this effect, the Mg and REM contents may each be 0.0001% or more. However, excessive Mg and REM content may result in the formation of coarse oxides, potentially reducing the toughness of the steel. Therefore, the Mg and REM contents are each set to 0.0030% or less. If necessary, the Mg and REM contents may be set to 0.015% or less, 0.010% or less, 0.005% or less, or 0.002% or less, or less than 0.0015%, respectively. REM is a collective term for rare earth metals, including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Compared to other additive elements, REM is characterized by its strong deoxidizing properties and forms stable oxides in steel.

[0031] (O: 0.0040% or less) Oxygen (O) is an impurity element contained in steel, and in many cases, it forms oxides of several μm to several tens of μm in size together with Ca, Mg, REM, Al, Ti, and other elements with strong deoxidizing power in steel. When coarse oxides are contained or when the number density of oxides is high, they can become the starting point of brittle fracture, so the lower the O content, the better. For this reason, the lower limit of the O content is 0%. In the steel plate according to this embodiment, the O content is set to 0.0040% or less, preferably 0.0030% or less, in order to improve the toughness of the weld.

[0032] (Balance: Fe and impurities) The steel sheet according to this embodiment contains the above-mentioned components as well as the balance being Fe and impurities. Here, the impurities refer to components that are mixed in due to raw materials such as ore or scrap, or various factors in the manufacturing process, when industrially manufacturing steel, and are acceptable within a range that does not adversely affect the present invention.

[0033] Furthermore, in the steel sheet according to this embodiment, it is preferable that [fB] calculated by the following formulas (A) to (E) is 0.0003 mass% or more. [fB] represents the amount of B dissolved in the steel. By setting [fB] to 0.0003 mass% or more, a yield strength of 670 to 870 N / mm ２ , tensile strength 780 to 940 N / mm ２In high-tensile steels, the hardenability of the steel can be improved. B easily forms nitrides in steel. Ti and Al also easily form nitrides and oxides. Therefore, the amount of B dissolved in steel [fB] is calculated using the following formulas (A) to (E). [fB] may be 0.0005 mass% or more, or 0.0015 mass% or more. [fB] may be 0.0025 mass% or less, or 0.0018 mass% or less.

[0034] [fB]=[B]-0.77×[fN]…(A) [fN]=[N]-0.29×[fTi]-0.52×[fAl]…(B) [fTi]=[Ti]-2×[fO]…(C) [fAl]=[Al]-1.125×[fO]…(D) [fO]=[O]-0.4×[Ca]-0.66×[Mg]-0.11×[REM]…(E)

[0035] In the formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM in the steel sheet, respectively. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.

[0036] In addition to limiting the content of each element, the steel sheet according to this embodiment limits the ranges of the α value, β value, and γ value calculated from the content of each element as follows.

[0037] (α value: 1.00 to 1.50 mass %) The α value is expressed by the following formula (1): α = [C] + 6 × [Si] + 100 × [P] (1) where [C], [Si], and [P] are the contents (mass %) of C, Si, and P in the steel sheet, respectively.

[0038] In this embodiment, the α value is set to 1.50 mass% or less. This is a necessary condition for improving the toughness of the base material and the coarse-grained portion of the weld heat-affected zone after stress relief annealing, and the C, Si, and P contents must be adjusted to satisfy this condition. After SR treatment, the grain boundary segregation concentration of P increases, making brittle fracture at the grain boundaries more likely to occur, but P, ​​C, and Si can control this brittle fracture. P is inevitably contained in steel considering the manufacturing process, and since grain boundary segregation significantly reduces grain boundary strength, it is a typical element that causes SR embrittlement and has the highest coefficient. C and Si are also elements inevitably contained in steel, and if these elements increase, they can cause embrittlement due to cementite formed at the grain boundaries. While it is desirable to reduce each element, a certain amount may be contained for characteristics or specifications. To improve toughness after SR, the α value is preferably set to 1.40 mass% or less. The α value is 1.00 mass% or more. This lower limit is a range determined by component restrictions in specifications for the field of application, limits on element control in manufacturing, and the like, and is calculated by substituting the above-mentioned lower limits of the C, Si, and P contents and the minimum values ​​realistic for manufacturing into formula (1). A preferable lower limit of the α value can be calculated from the preferable lower limits of the C, Si, and P contents. The α value may be more than 1.10% by mass or may be 1.30% by mass or greater.

[0039] (β value: 10.0 to 15.0) The β value is calculated by the following formula (2): β = 0.65 × [C] １／２ × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) ... (2) where [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel.

[0040] In the steel plate according to this embodiment, the β value is set to a range of 10.0 to 15.0. The β value is an index showing the hardenability of a steel material, and the higher the β value, the more reliably the formation of an upper bainite structure, which has a poor balance of strength and toughness, can be avoided. However, if the β value is too high, the strength of the steel material increases, resulting in a deterioration in toughness. In other words, the β value also serves as an index showing the target range of the content of alloy elements necessary to improve the toughness of the weld heat-affected zone in the as-welded state. If necessary, the β value may be set to 11.0 or more. At the same time, the β value may be set to 14.0 or less.

[0041] (γ value: 0.70 to 1.50 mass%) The γ value is calculated by the following formula (3): γ = [Mn] + 20 × [Nb] + 36 × [Ti] (3), where [Mn], [Nb], and [Ti] are the contents (mass%) of Mn, Nb, and Ti in the steel sheet. In the steel sheet according to this embodiment, the γ value is set to a range of 0.70 to 1.50 mass%. Mn, Nb, and Ti are all elements that promote grain boundary embrittlement after stress relief annealing. By setting the γ value to 1.50 mass% or less, it is possible to suppress a decrease in the toughness of the base metal and weld heat-affected zone after stress relief annealing. There are several possible mechanisms by which these elements promote grain boundary embrittlement, including a decrease in grain boundary strength due to grain boundary segregation and embrittlement due to carbonitrides formed at the grain boundaries. The γ value may be 1.40 mass% or less. On the other hand, in order to ensure a certain level of hardenability and obtain a microstructure that is advantageous in terms of strength and toughness balance, it is preferable to add certain amounts of Mn, Nb, and Ti, and the γ value is set to 0.70 mass% or more. The γ value may be 0.75 mass% or more.

[0042] By satisfying the numerical ranges for the α value, β value, and γ value, it is possible to provide a steel having excellent low-temperature toughness in the weld (weld heat-affected zone) both as-welded and after stress-relief annealing in a wider range of fields. While it is possible to produce a steel having excellent low-temperature toughness in the weld both as-welded and after stress-relief annealing by controlling the α value and β value within a certain range, depending on the application, the α value cannot be freely set due to the chemical composition requirements of the applicable standards, and there are cases where the α value must be increased. In contrast, in the steel plate according to this embodiment, the allowable range of the α value can be broadened by newly setting the γ value, and therefore, it is possible to provide a steel having excellent low-temperature toughness in the weld both as-welded and after stress-relief annealing, even in fields where the chemical composition requirements of the applicable standards are strict, as described above.

[0043] In addition, in the steel plate according to this embodiment, the carbon equivalent Ceq, which is an index showing the hardenability of steel and is calculated by the following formula (4), is set to 0.550 to 0.620 mass %.

[0044] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (4) In formula (4), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] are the contents (mass%) of C, Mn, Cu, Ni, Cr, Mo, and V in the steel plate, respectively.

[0045] If Ceq is less than 0.550 mass%, the strength of the steel plate may be insufficient. If necessary, Ceq may be set to 0.570 mass% or more, or 0.600 mass% or more. If Ceq exceeds 0.620 mass%, the toughness of the steel plate may be reduced. If necessary, Ceq may be set to 0.600 mass% or less.

[0046] (Yield strength: 670-870N / mm ２ ) (Tensile strength: 780 to 940 N / mm ２ In the steel plate according to this embodiment, the yield strength is 670 to 870 N / mm ２ The tensile strength of the steel plate is 780 to 940 N / mm ２ Liquefied CO ２In order to reduce the weight of large welded structures such as transport tanks for ships, a steel plate that can ensure the strength of the structure even if the plate thickness is thin is required. Steel plates that are usually selected for such applications have the above-mentioned yield strength and tensile strength, so the yield strength and tensile strength of the steel plate according to this embodiment are also set to the above-mentioned ranges. If necessary, the yield strength may be increased to 690 N / mm ２ ~830N / mm ２ The tensile strength may be 800 N / mm ２ ~900N / mm ２ It may also be possible to use the following.

[0047] (Charpy absorbed energy of 100 J or more at -65°C) Furthermore, in order to ensure high toughness, the steel plate according to this embodiment needs to have a Charpy absorbed energy of 100 J or more at -65°C. This makes it possible to ensure the safety of transport tanks made of the steel plate according to this embodiment. The Charpy absorbed energy at -65°C is a value measured at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface (sometimes referred to as the t / 4 position or the 1 / 4 thickness position).

[0048] (The δ value in the CTOD test at -35°C is 0.10 mm or more.) Furthermore, in order to ensure the safety of welded structures such as transportation tanks, fracture mechanical evaluation methods have recently been used to evaluate the fracture resistance characteristics of welded structures and incorporate this into their designs. Specifically, as a characteristic of brittle fracture initiation, a crack opening displacement (hereinafter abbreviated as δc) called the CTOD value is determined as a fracture mechanical parameter by a CTOD test (Crack Tip Opening Displacement test) specified by the Japan Welding Engineering Society standard WES1108, etc., and it is often evaluated whether δc satisfies the design criteria.

[0049] In order to improve the δc of a material, it is necessary to improve the material's properties from a different perspective than conventional methods. Conventionally, the Charpy impact test has been used as a method for evaluating the brittle fracture resistance of a material. The value obtained from the Charpy impact test represents the average toughness of the evaluation target region. However, in a CTOD test, even if the average toughness of the evaluation target region is good, the presence of even a small amount of brittleness within the evaluation target region will be reflected in the δc. Because of this nature of δc, in particular in regions where the microstructure of the steel material changes in a nonuniform and complex manner, such as weld heat-affected zones, it is necessary to minimize localized embrittlement regions in order to obtain a high δc value. In order to ensure high toughness, the steel plate according to this embodiment preferably has a δ value of 0.10 mm or more in a CTOD test at −35°C. In this case, the safety of a transport tank made of the steel plate according to this embodiment is further improved.

[0050] (Average Hardness and Standard Deviation) In a hardness distribution measurement at 121 measurement points in a 0.5 mm × 0.5 mm area at the quarter thickness position with a 0.05 mm pitch, the steel plate according to this embodiment must have an average hardness of 265 Hv to 290 Hv and a standard deviation of 20 or less. The structure of the steel plate according to this embodiment is preferably a mixed structure of martensite and lower bainite, which has a superior strength-toughness balance. However, variations in local γ grain size and microsegregation may partially reduce hardenability, resulting in the formation of an upper martensite structure with a poor strength-toughness balance. The presence of an upper bainite structure may result in an uneven hardness distribution, which may deteriorate the toughness of the base material. If the average hardness is less than 265 Hv or the standard deviation exceeds 20, the upper bainite structure may be present, making it difficult to ensure the toughness of the base material. On the other hand, if the average hardness exceeds 290 Hv, the strength may become too high, which may reduce toughness.

[0051] The hardness distribution measurement described above is performed by collecting a micro sample from a surface (L cross section) parallel to the rolling direction and the thickness direction of the steel sheet as the observation surface, and measuring using a micro Vickers hardness tester. The measurement area is a 0.5 mm x 0.5 mm range centered at an arbitrary t / 4 position within the micro observation surface, with a measurement pitch of 0.05 mm and a measurement load of 25 gf, and measurements are performed at 11 vertical points x 11 horizontal points, for a total of 121 points. The average and standard deviation are calculated from the obtained measurements.

[0052] (Plate thickness: 10 to 60 mm) When welding steel plates with a thickness of less than 10 mm, stress relief annealing (SR) is generally not required. However, the steel plate according to this embodiment is intended for steel plates that require SR, so the plate thickness is 10 mm or more. The plate thickness is preferably 25 mm or more. On the other hand, steel plates with a thickness of more than 60 mm are not preferred because they make little contribution to reducing the weight of the transport tank to which they are applied. Therefore, the plate thickness of the steel plate according to this embodiment is 60 mm or less.

[0053] Furthermore, the steel sheet according to this embodiment may have the following configuration.

[0054] In order to satisfy the above-mentioned average value and standard deviation of the 121-point hardness, the steel sheet according to this embodiment preferably has a structure at a quarter-thickness position of the cross section in the sheet thickness direction that is a mixed structure of martensite and lower bainite. The martensite and lower bainite structures may occupy 85% or more by area in total.

[0055] (Average grain size at the 1 / 4 thickness position of the steel plate: 15.0 μm or less) In the steel plate according to this embodiment, the average grain size at the 1 / 4 thickness position may be 15.0 μm or less. In order to improve the base material toughness and the base material toughness after SR, the average grain size may be 14.5 μm or less, or 14.0 μm or less, as necessary. Since a smaller average grain size at the 1 / 4 thickness position of the steel plate is preferable, there is no need to specify a lower limit. Typically, the smallest average grain size is about 10.0 μm.

[0056] The average grain size is defined as follows. A sample in which the L cross section of the steel sheet can be observed is prepared, and the 1 / 4 thickness position of the L cross section is used as the observation area. Crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope is performed at a 0.5 μm pitch within a range of 200 μm in the sheet thickness direction and 250 μm in the rolling direction. From the results of the crystal orientation analysis, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more is defined as a crystal grain, the circle-equivalent grain size of the crystal grain is defined as the crystal grain size, and the value calculated as an area-weighted average weighted by the area of ​​each crystal grain is the average crystal grain size.

[0057] (Charpy absorbed energy of 27 J or more at -40°C after stress relief annealing) In order to prevent fracture, the steel plate according to this embodiment is subjected to stress relief annealing at the welded portion after assembly into a transport tank. During this process, not only the welded portion but also the base material is heated. When the base material is heated, the toughness of the base material tends to decrease. Although the cause is not clear, it is presumed that the diffusion of P (phosphorus) to grain boundaries and the growth or aggregation of inclusions in the structure cause a decrease in brittleness and a decrease in toughness. Therefore, it is preferable that the steel plate according to this embodiment has a Charpy absorbed energy of 27 J or more at -40°C after stress relief annealing. In this case, safety can be further improved.

[0058] The Charpy absorbed energy at −40° C. after stress relief annealing is measured at a location where stress relief annealing has been performed, when stress relief annealing is performed on a steel sheet at a holding temperature of 600° C., a holding time of 2 hours, and a heating rate and cooling rate of 55° C. / hr or less in a temperature range of 425° C. or more.

[0059] (Yield strength after stress relief annealing is 670 to 870 N / mm ２ , tensile strength is 780 to 940 N / mm ２ The steel sheet according to this embodiment has a yield strength of 670 to 870 N / mm after stress relief annealing. ２ , tensile strength is 780 to 940 N / mm ２ This makes it possible to obtain a liquid CO 2 alloy that has been subjected to stress relief annealing. ２This ensures sufficient strength for transport tanks.

[0060] Furthermore, the steel sheet according to this embodiment preferably has a δ value of 0.10 mm or more in a CTOD test at −35° C. even after stress relief annealing. In this case, safety is further improved.

[0061] The steel plate according to this embodiment has the above-described configuration, and therefore is also excellent in the toughness of the weld heat affected zone (as welded and after stress relief annealing). The toughness of the weld heat affected zone is not limited, but as a target value, in the as-welded state, the Charpy absorbed energy at -65°C is preferably 70 J or more, and in the case after stress relief annealing, the Charpy absorbed energy at -65°C is preferably 70 J or more. Furthermore, it is more preferable that the weld heat affected zone has a δ value of 0.10 mm or more in a CTOD test at -35°C, whether in the as-welded state or after stress relief annealing.

[0062] Next, a method for manufacturing a steel sheet according to this embodiment will be described below. As long as the steel sheet according to this embodiment has the above-mentioned characteristics, the effects thereof can be obtained regardless of the manufacturing method. However, the method described below is preferred because it allows stable manufacturing.

[0063] To produce steel having the above-described chemical composition as steel plate, a commonly used method for producing steel products can be used. For example, steel produced by a converter or electric furnace process and refined in secondary refining equipment is formed into slabs by continuous casting or ingot casting. The slab thickness is sufficient as long as it can reduce segregation and improve the material properties by reducing porosity, and a slab thickness of 150 mm or more is preferred. There is no particular upper limit on the slab thickness, but for example, the slab thickness may be 600 mm or less or 400 mm or less. The slab is then heated to approximately 950 to 1250°C in a slab heating furnace and then hot-rolled to a predetermined thickness under the conditions described below to produce a steel plate. This steel plate is then quenched and tempered to obtain a steel plate (final steel plate) with the desired properties.

[0064] In the steel sheet according to this embodiment, the P content needs to be reduced to 0.006% or less. There are cases where the P content cannot be reduced to 0.006% or less by a normal dephosphorization method, but in such cases, measures such as extending the dephosphorization treatment time may be taken.

[0065] During hot rolling, it is desirable to set the cumulative reduction to 50% or more within the rolling temperature range of 1150 to 900° C. There is no particular need to specify an upper limit to the cumulative reduction within the above temperature range, but the cumulative reduction may be 80% or less, or 70% or less.

[0066] Regarding quenching and tempering, when the plate thickness is 50 mm or less or less than 50 mm, direct quenching, in which the plate is immediately water-cooled after hot rolling, may be performed, thereby omitting the reheating and quenching process described below. When performing direct quenching, the cooling start temperature is set to Ar3 point or higher, and water cooling is performed to 300°C or lower. The average cooling rate during water cooling is preferably 5°C / sec or higher in the range of 700°C to 300°C in the cooling temperature history of the front and back surfaces of the steel plate. There is no particular upper limit to the average cooling rate, but the average cooling rate may be, for example, 100°C / sec or lower, 50°C / sec or lower, or 20°C / sec or lower. Furthermore, reheating and further quenching may be performed after direct quenching. The Ar3 point is calculated using the following formula: Ar3 = 910 - 310 x [C] - 8 x [Mn] - 20 x [Cu] - 15 x [Cr] - 55 x [Ni] - 80 x [Mo] + 0.35 x (t - 8) In the formula, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo in the steel plate, respectively, in mass%, and t is the thickness of the steel plate in mm.

[0067] When the plate thickness is 50 mm or more, it is preferable to perform quenching by once cooling the steel plate after rolling and then reheating it. When the plate thickness is 50 mm or more, if reheating and quenching is performed, direct quenching after hot rolling may be omitted or direct quenching may be performed.

[0068] When reheating is performed, the heating temperature during quenching (i.e., quenching temperature) is preferably 925°C or lower, but may also be 920°C or lower, 915°C or lower, or 910°C or lower. This is because thick steel plates may not have sufficiently refined metallurgical structures after rolling. If the quenching temperature exceeds 925°C for steel plates with insufficiently refined metallurgical structures, the reverse-transformed γ structure formed during heating will become coarse, and the average grain size of the final structure after γ / α transformation by subsequent cooling will also become coarse. On the other hand, a lower limit of the quenching temperature is not preferable if it is slightly higher than the Ac3 point (e.g., within a temperature range of the Ac3 point or higher and the Ac3 point + 20°C or lower), because this may result in insufficient hardenability due to variations in the reverse-transformed γ grain size and insufficient solid solution of B-containing carbides. Therefore, the quenching temperature is preferably 880°C or higher, more preferably 890°C or higher. In the above description of the quenching treatment conditions, it is assumed that the thickness of the steel plate is 50 mm or more, but these quenching treatment conditions also apply when reheating and quenching a steel plate with a thickness of less than 50 mm.

[0069] In this embodiment, tempering is performed after quenching (after direct quenching or reheating quenching, or after reheating quenching if both are performed). The heating temperature during tempering (i.e., tempering temperature) is desirably 660°C or lower. If the tempering temperature exceeds 660°C, the tempering effect becomes excessive, making it difficult to ensure the yield stress and tensile strength, and toughness may decrease. The tempering temperature is 500°C or higher, preferably 600°C or higher. If the tempering temperature is too low, tempering becomes insufficient, making it difficult to ensure the desired yield stress and tensile strength.

[0070] When cooling is performed after reheating and quenching or tempering, it is desirable to cool the steel sheet by water cooling (accelerated cooling) rather than air cooling in order to prevent a decrease in the toughness of the base material due to temper embrittlement. In this case, it is preferable to set the average cooling rate to 300°C to 0.1°C / sec or more or 0.5°C / sec or more.

[0071] The steel sheet according to this embodiment is ２The steel plate is suitable for use as a transport tank. For example, it can be used as a transport tank mounted on a ship. ２ When transporting CO2, liquefied CO2 is stored in a transport tank on board the ship. ２ However, CO2 in the transport tank ２ In order to prevent CO from solidifying (becoming dry ice), it is preferable to transport it while maintaining a pressure of about 2 MPa. ２ To maintain the liquid state, CO ２ It is preferable to keep the temperature at about -35° C. The steel sheet according to this embodiment can be suitably used for such applications.

[0072] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0073] The molten iron after the blast furnace treatment was tapped into a hot metal ladle and subjected to pretreatment such as desulfurization, and then the molten iron was transferred to a converter, where it was subjected to dephosphorization treatment to adjust the P content to 0.006% or less.

[0074] After further adjustment of the components of the dephosphorized molten steel, slabs having the chemical compositions shown in Tables 1A and 1B were cast. The slabs were then heated in a heating furnace to the heating temperatures shown in the tables and hot-rolled to the specified thickness to obtain steel plates. These steel plates were then quenched and tempered to obtain steel plates (final steel plates) with the specified properties. Table 2 shows the heating temperature before rolling, the cumulative reduction rate of hot rolling from 1150 to 900°C, the thickness after rolling, the quenching temperature, and the tempering temperature. Cooling after reheating and quenching or tempering was performed by water cooling, with an average cooling rate to 300°C of 0.1°C / second or more. Some steel plates were also subjected to direct quenching, in which the plates were immediately water-cooled after hot rolling. The cooling start temperature, cooling end temperature, and average cooling rate for these steel plates are shown in the table.

[0075] Tables 1A and 1B show the chemical composition, α value, β value, γ value, fB value, and carbon equivalent Ceq of the steel sheet. In addition, the column before SR in the base material properties of Table 3A shows the average value of base material hardness (average Hv), average grain size (EBSD grain size), yield strength (MPa), tensile strength (MPa), yield ratio, Charpy absorbed energy (J) at -65 ° C, and δ value (mm) of a CTOD test at -35 ° C. The EBSD grain size was measured by preparing a sample in which the L cross section of the steel sheet could be observed, and the observation area was a quarter-thickness position of the L cross section. Crystal orientation analysis was performed using an electron beam backscatter diffraction pattern analysis method (EBSD method) using a scanning electron microscope in a range of 200 μm in the sheet thickness direction and 250 μm in the rolling direction, at a 0.5 μm pitch. From the results of the crystal orientation analysis, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more was defined as a crystal grain, the circle-equivalent diameter of the crystal grain was defined as the crystal grain size, and the value calculated as the area-weighted average weighted by the area of ​​each crystal grain was defined as the average crystal grain size.

[0076] The tensile test was conducted in accordance with JIS Z 2241:2011 using JIS No. 4 round bar test pieces with a parallel section of 14 mm in diameter taken from the quarter-thickness position in the C direction. The yield strength and tensile strength were each the average values ​​of two test pieces. The yield strength was taken as the 0.2% proof stress. The yield ratio is the ratio of the yield strength YS to the tensile strength TS, and is expressed as a percentage, i.e., 100 x (YS / TS). The yield ratio is expressed in %.

[0077] For the hardness distribution measurement, a micro sample was taken from the L cross section parallel to the rolling direction and thickness direction of the steel sheet as the observation surface, and the observation surface was wet polished. After that, the mirror surface was polished with 1.0 μm diamond particles and buffed to a polished finish. The measurement was carried out using a micro Vickers hardness tester. The measurement area was randomly selected within a 0.5 mm × 0.5 mm range centered on the 1 / 4 t position within the micro observation surface, and a measurement pitch of 0.05 mm was used. The measurement load was 25 gf. A total of 121 points were measured, 11 vertical points × 11 horizontal points. The average value and standard deviation were calculated from the obtained measurements.

[0078] In addition, semi-automatic welded joints with weld lines parallel to the rolling direction were prepared and evaluated. ２ A welded joint was produced by multi-layer gas shielded arc welding (GMAW) using argon gas containing HCl as the shielding gas, welding wire YM-69F manufactured by Nippon Steel Welding Industries Co., Ltd. as the welding wire, heat input of 2.0 kJ / mm, and preheating at 100°C.

[0079] After revealing the microstructure of the welded joint from the weld (As weld) at the C-section, Charpy test specimens were taken from a position centered 6.5 mm below the surface (referred to as "surface sample" in the table) and a position centered at the plate thickness center (referred to as "t / 2 sample" in the table), with the front I-side fusion line (FL) positioned at the center of each notch. A Charpy test was performed on these specimens at -65°C to determine the absorbed energy. The results are shown in the Asweld column of the joint properties in Table 3B. Furthermore, after revealing the microstructure of the weld, full-thickness CTOD test specimens were taken with the front I-side fusion line positioned at the center of each notch, and a CTOD test was performed at -35°C to determine the δ value. The results are shown in the Asweld column of the joint properties in Table 3B.

[0080] Thereafter, the base metal and the welded portion were subjected to stress relief annealing (SR), which was performed at a holding temperature of 600°C for 2 hours, with a temperature increase rate and a temperature decrease rate of 55°C / hr or less in a temperature range of 425°C or higher.

[0081] The yield strength and tensile strength of the base material after SR were determined in the same manner as before SR. Also, Charpy tests were performed at -40°C on test pieces taken in the C direction at the t / 4 position of the base material after SR to determine the Charpy absorbed energy. Also, CTOD tests were performed at -35°C to determine the δ value. These results are shown in the "After SR" column of the base material properties in Table 3A.

[0082] Furthermore, after revealing the microstructure of the welded joint at the C-section after SR, Charpy test specimens were taken with the I-side fusion line on the front surface at the center of each notch, and the centers were set at 6.5 mm below the surface and at the t / 2 position. The Charpy absorbed energy at -40°C obtained in the test is shown. Furthermore, after revealing the microstructure of the welded joint after SR, full-thickness CTOD test specimens were taken with the I-side fusion line on the front surface at the center of each notch, and the δ value of the CTOD test at -35°C obtained in the test is shown. These results are shown in the "After SR" column of joint properties in Table 3B.

[0083] The Charpy absorbed energy of the base metal and the weld was measured by taking three V-notch test pieces from each of the base metal and the weld, and performing a Charpy impact test at a predetermined temperature. The V-notch test pieces were full-size test pieces specified in JIS Z 2242:2005, taken from each plate thickness position in the C direction. The Charpy impact test was performed in accordance with JIS Z 2242:2005.

[0084] [Correction based on Rule 91, 29.11.2024] The delta value (δc) of the CTOD test was measured in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997). For the base metal, evaluation was performed in the C direction (width direction), where the longitudinal direction of the test specimen was perpendicular to the rolling direction. For the welded joints, gas-shielded arc welding was performed with a heat input of 35 kJ / mm on the butt joints of the steel plates with a K-groove. The tip of the fatigue notch of the CTOD test specimen at the weld was positioned at the center of the plate thickness at the I-side fusion line (FL) of the weld, and the CTOD test was performed at the specified temperature. For the welded joints, evaluation was performed only in the L direction (rolling direction). In the evaluation of the CTOD of welded joints, test specimens were taken so that the tip of the fatigue crack corresponded to the weld bond. Three tests were conducted at each test temperature, and the lowest value of the obtained measurement data was taken as the δ value of the CTOD test. The unit of CTOD shown in Tables 3A and 3B is mm.

[0085] As shown in Tables 1A to 3B, all of the inventive examples Nos. 1 to 14 had excellent strength and toughness. In particular, they exhibited excellent low-temperature toughness even after SR treatment. In addition, the yield strength after SR treatment was 670 to 870 N / mm ２ and a tensile strength of 780 to 940 N / mm ２ Furthermore, for Nos. 1 to 14, the Charpy absorbed energy in the weld heat affected zone of the welded joint exceeded 70 J both before SR treatment (-65°C) and after SR treatment (-40°C), demonstrating good low-temperature toughness.

[0086] On the other hand, as shown in Tables 1A to 3B, in the comparative examples Nos. 15 to 45 and 55, the chemical compositions of the steel plates (element contents or α value, β value, γ value, Ceq.) were outside the ranges specified in the present invention, and therefore the toughness of at least either the base metal or the weld heat-affected zone deteriorated.

[0087] In addition, Nos. 46 to 54 had chemical compositions that satisfied the component ranges of the present invention, but the manufacturing conditions did not satisfy the preferred manufacturing conditions. As a result, toughness was deteriorated. That is, the Charpy absorbed energy at -65°C at least at the t / 4 position was less than 100 J, and in some examples, other toughness properties were also inferior.

[0088]

[0089]

[0090]

[0091]

[0092]

[0093] According to the present invention, it is possible to provide a steel sheet which is excellent in strength and low-temperature toughness, and further excellent in strength and low-temperature toughness after stress relief annealing. ２ The present invention is suitable for use as a transportation tank and has high industrial applicability.

Claims

1. Chemical composition, by mass%, is: C: 0.07-0.11%, Si: 0.10-0.15%, Mn: 0.70-1.20%, Ni: 1.00-2.50%, Cr: 0.20-0.80%, Mo: 0.20-0.80%, V: 0.005-0.070%, Al: 0.010-0.100%, B: 0.0005-0.0030%, N: 0.0015-0.0050%, P: 0.006% or less, S: 0.0030% or less, Cu: 0-1.00%, Nb: 0-0.030%, Ti: 0-0.010%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, O: 0.0040% or less, balance: Fe and impurities, an α value defined by the following formula (1) of 1.00 to 1.50 mass%, a β value defined by the following formula (2) of 10.0 to 15.0, a γ value defined by the following formula (3) of 0.70 to 1.50 mass%, a Ceq value defined by the following formula (4) of 0.550 to 0.620 mass%, and a yield strength of 670 to 870 N / mm ２ Tensile strength is 780 to 940 N / mm ２ A steel plate having a Charpy absorbed energy of 100 J or more at -65°C, an average hardness of 265 Hv to 290 Hv and a standard deviation of 20 or less at 121 measurement positions in a hardness distribution measurement of 1 mm x 1 mm at 0.05 mm pitch at the 1 / 4 thickness position, and a plate thickness of 10 to 60 mm. α = [C] + 6 x [Si] + 100 x [P] ... (1) β = 0.65 x [C] １／２ ×(1+0.64×[Si])×(1+4.10×[Mn])×(1+0.27×[Cu])×(1+0.52×[Ni])×(1+2.33×[Cr])×(1+3.14×[Mo])…(2) γ=[Mn]+20×[Nb]+36×[Ti]…(3) Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5...(4) In the formulas (1) to (4), [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] represent the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, and elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0.

2. The steel sheet according to claim 1, wherein [fB] calculated by the following formulas (A) to (E) is 0.0003 mass% or more: [fB] = [B] - 0.77 x [fN] ... (A) [fN] = [N] - 0.29 x [fTi] - 0.52 x [fAl] ... (B) [fTi] = [Ti] - 2 x [fO] ... (C) [fAl] = [Al] - 1.125 x [fO] ... (D) [fO] = [O] - 0.4 x [Ca] - 0.66 x [Mg] - 0.11 x [REM] ... (E). In the formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. Elements that are not contained, including the amount of elements mixed in as impurities, are substituted with 0, and when the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.

3. [Correction under Rule 91 29.11.2024] The steel sheet according to claim 1 or 2, wherein a region surrounded by grain boundaries having a crystal orientation difference of 15° or more, as determined by performing a crystal orientation analysis using electron beam backscatter diffraction pattern analysis, is defined as a crystal grain, the circle equivalent diameter of the crystal grain is defined as a crystal grain diameter, and a value calculated as an area-weighted average weighted by the area of ​​each crystal grain is defined as an average crystal grain diameter, the average crystal grain diameter at the 1 / 4 thickness position is 15.0 μm or less.

4. When the steel plate is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C / hr or less in a temperature range of 425°C or more, the yield strength of the part subjected to the stress relief annealing is 670 to 870 N / mm ２ and a tensile strength of 780 to 940 N / mm ２ and a Charpy absorbed energy at −40° C. of 27 J or more.

5. When the steel plate is subjected to stress relief annealing at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C / hr or less in a temperature range of 425°C or more, the yield strength of the part subjected to the stress relief annealing is 670 to 870 N / mm ２ and a tensile strength of 780 to 940 N / mm ２ and a Charpy absorbed energy at −40° C. of 27 J or more.

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