Steel plate

Abstract

A steel sheet having a prescribed chemical composition, an alpha value of 1.00-1.50 mass%, a beta value of 10.0-15.0, a gamma 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, a Charpy impact absorption energy at-65 DEG C of 100 J or more, an average value of hardness at a measurement position at 121 points of 265-290 Hv, a standard deviation of 20 or less, and a sheet thickness of 10-60 mm in a hardness distribution measurement at a pitch of 0.05 mm of 1 mm * 1 mm at a 1 / 4 thickness position.

Description

Technical Field

[0001] The present invention relates to steel sheets.

[0002] This application claims priority based on patent application No. 2022-157410 filed in Japan on September 30, 2022, the contents of which are cited here. Background Art

[0003] In recent years, as a countermeasure to the problem of climate change, there is a strong demand for the reduction of greenhouse gases. Among them, as a technology to achieve carbon neutrality, the technology of recovering and storing carbon dioxide (hereinafter referred to as CO2), namely CCS (Carbondioxide Capture and Storage), has attracted attention. In CCS, CO2 emitted from CO2 emission sources such as refineries, power plants, and chemical plant equipment is separated and recovered, compressed and stored in a storage layer deep underground. In the case where the recovery facilities for separating and recovering CO2 and the storage facilities for compressing and storing CO2 in the underground storage layer are far apart, it is necessary to transport the separated and recovered CO2 between these facilities by pipelines, ships, etc.

[0004] When transporting CO2 by ship, the transport tank equipped on the ship is filled with liquefied CO2 for transport. This can improve the transport efficiency of CO2. However, in order to prevent the solidification (dry ice) of CO2 in the transport tank, it is necessary to transport it while maintaining a pressure of about 2MPa. In addition, in order to maintain CO2 in a liquid state at a pressure of about 2MPa, CO2 needs to be maintained at about minus 35°C. Moreover, in order to reduce the weight of the ship, it is also hoped to reduce the wall thickness of the transport tank as much as possible by increasing the strength of the steel plate used.

[0005] Therefore, the steel plate used as the raw material of the transport tank is required to have high strength and excellent low temperature toughness. For example, the strength is required to be 780N / mm 2 In addition, although low-temperature toughness also depends on the plate thickness, in the case of a plate thickness of 20 to 60 mm used as a conveying tank, the most stringent condition requires excellent low-temperature toughness evaluated in a Charpy impact test at minus 65°C (-65°C). Here, the test temperature of the Charpy impact test is minus 65°C because the Charpy impact test is a small-scale test and is usually evaluated at a temperature that is somewhat lower than the operating temperature according to the plate thickness.

[0006] Moreover, in large welded structures such as transport tanks, stress relief annealing is sometimes performed on the welded parts to further reduce the possibility of damage. The so-called stress relief annealing is a heat treatment method that heats the welded parts of the welded structure to a temperature below the Ac1 phase transformation point and then slowly cools them for the purpose of reducing the residual stress caused by welding. However, if the tensile strength is 780N / mm 2 When stress relief annealing is applied to the above high-strength steel, alloy carbides selectively precipitate at the grain boundaries, and the alloy carbides cause grain boundary embrittlement, thereby extremely reducing the toughness of the part where the stress relief annealing is implemented. This phenomenon is generally called SR (Stress Relieving) embrittlement. In particular, in high-strength steels containing B and manufactured by quenching and tempering, there is a strong tendency to produce SR embrittlement. In such high-strength steels, not only the embrittlement of the base material, but also the embrittlement of the weld heat affected zone obtained when the high-strength steel is used to make a welded joint is significant.

[0007] Therefore, in order to ensure high safety in a transport tank manufactured using such high-strength steel, it is preferred that the base material and the weld (particularly the weld heat affected zone) have excellent low-temperature toughness even after stress relief annealing.

[0008] From the above-mentioned viewpoints, several technical solutions have been proposed in the past. For example, Patent Document 1 shows a high-strength steel plate, which is characterized by adjusting the chemical composition and making the average crystal grain size less than 15 μm. 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.

[0009] Prior Art Literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent No. 5590271 Summary of the invention

[0012] The present invention has been completed in view of the above situation, and its object is to provide a steel plate suitable for liquefied CO2 transport tanks, which has excellent strength of the base material and the low-temperature toughness of the base material and the weld heat affected zone, and also has excellent strength of the base material and the low-temperature toughness of the base material and the weld heat affected zone after stress relief annealing.

[0013] In order to solve the above-mentioned problems, the present invention adopts the following means.

[0014] [1] A steel sheet according to one embodiment of the present invention comprises, in terms of mass%, 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: 0.000 5~0.0030%, 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: 0~0.0030%, Mg: 0~0.0030%, REM: 0~0.0030%, O: 0.0040% or less, the balance is Fe and impurities,

[0015] The α value defined by the following formula (1) is 1.00 to 1.50 mass %,

[0016] The β value defined by the following formula (2) is 10.0 to 15.0,

[0017] The γ value defined by the following formula (3) is 0.70 to 1.50 mass %,

[0018] The Ceq value defined by the following formula (4) is 0.550 to 0.620 mass %,

[0019] Yield strength is 670~870N / mm 2 ,

[0020] Tensile strength is 780~940N / mm 2 ,

[0021] The Charpy impact absorption energy at -65°C is more than 100J.

[0022] In the hardness distribution measurement of 1mm×1mm and 0.05mm pitch at the 1 / 4 thickness position, the average hardness at 121 measurement points was 265Hv to 290Hv, and the standard deviation was less than 20.

[0023] The plate thickness is 10~60mm,

[0024] α=[C]+6×[Si]+100×[P]…(1)

[0025] β=0.65×[C] 1 / 2 ×(1+0.64×[Si])×(1+4.10×[Mn])×(1+0.27×[Cu])×(1+0.52×[Ni])×

[0026] (1+2.33×[Cr])×(1+3.14×[Mo])…(2)

[0027] γ=[Mn]+20×[Nb]+36×[Ti]…(3)

[0028] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5…(4)

[0029] Among them, [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti] and [V] in formulas (1) to (4) are the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti and V, respectively, and also include the amounts of elements mixed as impurities, and elements not contained are substituted with 0.

[0030] [2] According to the steel sheet described in [1], [fB] calculated by the following formulas (A) to (E) may be 0.0003 mass % or more,

[0031] [fB]=[B]-0.77×[fN]…(A)

[0032] [fN]=[N]-0.29×[fTi]-0.52×[fAl]…(B)

[0033] [fTi]=[Ti]-2×[fO]…(C)

[0034] [fAl]=[Al]-1.125×[fO]…(D)

[0035] [fO]=[O]-0.4×[Ca]-0.66×[Mg]-0.11×[REM]…(E)

[0036] Among them, [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] in formulas (A) to (E) are the contents (mass %) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, and also include the amounts of elements mixed as impurities. Elements not contained are substituted with 0. In addition, [fN], [fTi], [fAl], and [fO] are substituted with 0 when their calculated values ​​are less than 0%.

[0037] [3] According to the steel plate described in [1] or [2], when the area surrounded by grain boundaries with a crystal orientation difference of 15° or more determined by crystal orientation analysis using electron backscatter diffraction pattern analysis is defined as grains, the equivalent circular grain size of the grains is defined as the crystal grain size, and the value calculated by taking the area-weighted average value after weighting the area of ​​each grain is defined as the average crystal grain size, the average crystal grain size at the 1 / 4 thickness position can be 15.0 μm or less.

[0038] [4] According to any one of [1] to [3], when the steel plate is subjected to stress relief annealing in which the temperature is maintained at 600°C for 2 hours and the heating rate and cooling rate are 55°C / hour or less in a temperature range of 425°C or higher, the yield strength of the portion subjected to the stress relief annealing can be 670 to 870 N / mm 2 , the tensile strength can be 780~940N / mm 2 , the Charpy impact absorption energy at -40°C can be above 27J.

[0039] According to the above-mentioned aspect of the present invention, a steel plate having excellent strength of a base material and low temperature toughness of a base material and a weld heat affected zone, and having excellent strength of a base material and low temperature toughness of a base material and a weld heat affected zone after stress relief annealing can be provided. The steel plate is suitable for use in a liquefied CO2 transport tank. DETAILED DESCRIPTION

[0040] Hereinafter, a steel sheet according to an embodiment of the present invention (steel sheet according to the present embodiment) will be described in detail.

[0041] "Stress relief annealing" in the present embodiment, unless otherwise specified, means stress relief annealing based on the contents specified in JIS Z 3700:2022 "Post-welding heat treatment method". "Welding" in the present embodiment, unless otherwise specified, means welding with a welding line energy 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 conditions, the same effect as the stress relief annealing or welding performed under the above conditions can be obtained. Therefore, the steel plate of the present embodiment can also be subjected to stress relief annealing or welding under conditions different from the above conditions.

[0042] First, the content of each element constituting the chemical composition of the steel sheet of the present embodiment and the reasons for its limitation will be described. Hereinafter, "%" related to the content of an element means mass % unless otherwise specified.

[0043] (C: 0.07~0.11%)

[0044] C is an element that improves the strength of the base material. In order to achieve the target strength of the steel plate of the present embodiment, the C content is set to 0.07% or more. The C content is preferably 0.08% or more.

[0045] On the other hand, when a large amount of C is contained, the hardness of the weld heat affected zone increases and the toughness decreases, so the C content is made 0.11% or less. The C content is preferably 0.10% or less, and more preferably less than 0.10%.

[0046] (Si: 0.10~0.15%)

[0047] Si is an element that is generally 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.

[0048] On the other hand, Si is an element that reduces the toughness of steel after stress relief annealing. In addition, in order to suppress the reduction in toughness of the weld heat affected zone after stress relief annealing (SR), it is also preferable that the Si content is low. Therefore, in the steel plate of 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 further preferably 0.12% or less.

[0049] (Mn: 0.70~1.20%)

[0050] Mn is an element effective for deoxidation and is an element that improves the strength of steel. Therefore, the Mn content is set to 0.70% or more. The Mn content is preferably 0.90% or more.

[0051] On the other hand, if Mn is contained excessively, the toughness of the steel after stress relief annealing may be impaired due to temper embrittlement. Therefore, the Mn content is set to 1.20% or less. The Mn content is preferably 1.10% or less.

[0052] (Ni: 1.00~2.50%)

[0053] 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 1.20% or more.

[0054] On the other hand, if Ni is contained excessively, the toughness of the steel after stress relief annealing may be reduced. In addition, the toughness of the weld heat affected zone after stress relief annealing may be deteriorated. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably 2.00% or less.

[0055] (Cr: 0.20~0.80%)

[0056] Cr is an element effective for improving the hardenability of steel and 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 0.40% or more.

[0057] On the other hand, if Cr is contained excessively, the toughness of the base material and the weld heat affected zone after stress relief annealing may be reduced. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less.

[0058] (Mo: 0.20~0.80%)

[0059] Mo is an element effective for improving hardenability and improving strength of steel by precipitation strengthening during tempering, similarly to Cr. Therefore, the Mo content is set to 0.20% or more. The Mo content is preferably 0.30% or more, more preferably 0.35% or more, and even more preferably 0.40% or more.

[0060] On the other hand, if Mo is contained excessively, Mo carbides will precipitate at the grain boundaries after stress relief annealing, which may reduce the toughness of the base material and the weld heat affected zone, and the weld heat affected zone will be particularly affected. Therefore, the Mo content is set to 0.80% or less. The Mo content is preferably 0.60% or less.

[0061] (V: 0.005~0.070%)

[0062] V is an element effective for improving hardenability and improving strength of steel by precipitation strengthening during tempering, like Cr and Mo. Therefore, the V content is set to 0.005% or more. The V content is preferably 0.010% or more.

[0063] On the other hand, if V is contained excessively, the toughness of the base material and the toughness of the weld heat affected zone may be reduced after stress relief annealing. Therefore, the V content is set to 0.070% or less. The V content is preferably 0.050% or less.

[0064] (Al: 0.010~0.100%)

[0065] Al is an element useful for deoxidation, and an element that refines the crystal grain size during quenching by forming nitrides. In addition, since nitrides are generated, it is also an indispensable element for ensuring [fB]. Therefore, in the steel plate of this embodiment, the Al content is set to 0.010% or more. When the N content is high, in order to fix N and ensure fB, the Al content is preferably 0.030% or more, and more preferably 0.040% or more.

[0066] On the other hand, if Al is contained excessively, Al may form coarse nitrides and 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.

[0067] (B: 0.0005~0.0030%)

[0068] B is an element that improves the hardenability of steel by being contained in a trace amount in the steel sheet of this embodiment. Therefore, the B content is set to 0.0005% or more. The B content may be 0.0006% or more, 0.0008% or more, or 0.0010% or more.

[0069] On the other hand, if B is contained excessively, B forms 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 also be 0.0020% or less or 0.0010% or less.

[0070] (N: 0.0015~0.0050%)

[0071] 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 also be 0.0030% or more or 0.0035% or more.

[0072] On the other hand, if N is contained excessively, nitrides coarsen and the toughness of the heat-affected zone in the as-welded state (As weld) decreases. Therefore, the N content is made 0.0050% or less.

[0073] (P: 0.006% or less)

[0074] (S: 0.0030% or less)

[0075] P and S are impurity elements contained in steel, and the lower the content, the better. Therefore, the lower limits of the P content and the S content are 0%. In the steel plate of this embodiment, in order to improve the toughness of the base material, the toughness of the base material and the weld 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 0.005% or less. The S content may also be 0.0020% or less.

[0076] (Cu: 0~1.00%)

[0077] Cu is not an essential element in the steel sheet of this embodiment, so the lower limit of the Cu content is 0%. However, Cu has the effect of improving the strength of steel, so it can be contained as needed. In the case of containing Cu, in order to utilize its effect, the Cu content is preferably 0.10% or more, more preferably 0.20% or more. The Cu content can also be set to 0.25% or more or 0.30% or more as needed.

[0078] On the other hand, if Cu is contained excessively, it is possible that cracks on the steel plate surface and precipitation of Cu may reduce the toughness of the base material. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less. The Cu content may also be set to 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less as required.

[0079] (Nb: 0~0.030%)

[0080] Nb is not an essential element in the steel sheet of this embodiment, so the lower limit of the Nb content is 0%. However, Nb is an element that refines the crystal grains during quenching, so it can be contained as needed. When Nb is contained, in order to utilize its effect, the Nb content is preferably 0.001% or more.

[0081] On the other hand, if Nb is contained excessively, Nb may form coarse carbonitrides and reduce the toughness of the base material. Therefore, the Nb content is set to 0.030% or less. When Nb is small, the toughness of the weld heat affected zone is improved, so the Nb content can also be set to 0.020% or less, 0.010% or less, or 0.005% or less.

[0082] (Ti: 0~0.010%)

[0083] Ti is not an essential element in the steel plate of the present embodiment, so the lower limit of the Ti content is 0%. However, Ti sometimes refines the grains when the steel is heated to a high temperature by slab heating, etc., so it can be contained as needed. When Ti is contained, in order to utilize its effect, it is preferable to set the Ti content to 0.001% or more.

[0084] On the other hand, if Ti is excessively contained, similarly to Nb, Ti may form coarse carbonitrides and reduce the toughness of the base material. Therefore, the Ti content is set to 0.010% or less. The Ti content may also be set to 0.005% or less or 0.002% or less as required.

[0085] (Ca: 0~0.0030%)

[0086] (Mg: 0~0.0030%)

[0087] (REM: 0~0.0030%)

[0088] The steel sheet of the present embodiment may contain one or more of Ca, Mg, and REM. Ca, Mg, and REM are not essential elements, and therefore the lower limits of the contents of Ca, Mg, and REM are all 0%.

[0089] Ca has the effect of reducing the influence of MnS that reduces the toughness of the steel sheet by spheroidizing the sulfides in the steel sheet. In order to obtain this effect, the Ca content may be set to 0.0001% or more.

[0090] On the other hand, if Ca is contained in a large amount, the weldability of the steel may be impaired, so the Ca content is set to 0.0030% or less. The Ca content may be set to 0.0015% or less, 0.0010% or less, 0.0005% or less, or 0.0002% or less as necessary.

[0091] Mg and REM form oxides to improve the toughness of the weld heat affected zone. To obtain this effect, the contents of Mg and REM may each be set to 0.0001% or more.

[0092] On the other hand, if Mg and REM are contained in large amounts, coarse oxides are formed, and the toughness of the steel may be reduced. Therefore, the Mg content and the REM content are set to 0.0030% or less, respectively. The Mg content and the REM content may also be set to 0.015% or less, 0.010% or less, 0.005% or less, 0.002% or less, or less than 0.0015%, respectively, as needed. REM is a general term for rare earth metals including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It is characterized by its strong deoxidizing property compared to other added elements, and forms stable oxides in steel.

[0093] (O: 0.0040% or less)

[0094] Oxygen (O) is an impurity element contained in steel. When there is a large amount of oxygen in steel, it forms oxides with a size of several μm to tens of μm together with Ca, Mg, REM, Al, Ti, etc., which have strong deoxidizing power. In the case of containing coarse oxides or when the number density of oxides is high, it may become the starting point of brittle fracture, so the less the content of O, the better. Therefore, the lower limit of the content of O is 0%. In the steel plate of this embodiment, in order to improve the toughness of the weld, the content of O is set to 0.0040% or less, preferably 0.0030% or less.

[0095] (Balone: ​​Fe and impurities)

[0096] The steel plate of this embodiment, in addition to the above-mentioned components, the balance is composed of Fe and impurities. Here, the so-called impurities refer to components that are mixed in due to various factors of raw materials such as ore or scrap or the manufacturing process when industrially manufacturing steel materials, and are allowed within the range that does not adversely affect the present invention.

[0097] Furthermore, in the steel plate of the present embodiment, it is preferred that [fB] obtained 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, the yield strength is 670 to 870 N / mm 2 , tensile strength 780~940N / mm 2 For high-strength steel, it can improve the hardenability of steel. B easily forms nitrides in steel. In addition, Ti and Al easily form nitrides and oxides. Therefore, the amount of B [fB] dissolved in steel is calculated by the following formulas (A) to (E). [fB] can be greater than 0.0005 mass%, and can be greater than 0.0015 mass%. In addition, [fB] can be less than 0.0025 mass%, and can be less than 0.0018 mass%.

[0098] [fB]=[B]-0.77×[fN]…(A)

[0099] [fN]=[N]-0.29×[fTi]-0.52×[fAl]…(B)

[0100] [fTi]=[Ti]-2×[fO]…(C)

[0101] [fAl]=[Al]-1.125×[fO]…(D)

[0102] [fO]=[O]-0.4×[Ca]-0.66×[Mg]-0.11×[REM]…(E)

[0103] Among them, [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] in formulas (A) to (E) are the contents (mass %) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, and also include the amounts of elements mixed as impurities. Elements not contained are substituted with 0. In addition, [fN], [fTi], [fAl], and [fO] are substituted with 0 when their calculated values ​​are less than 0%.

[0104] In addition, in the steel sheet of the present embodiment, not only the content of each element is limited, but also the ranges of the α value, β value, and γ value calculated from the content of each element are limited as follows.

[0105] (α value: 1.00 to 1.50 mass %)

[0106] The α value is represented by the following formula (1).

[0107] α=[C]+6×[Si]+100×[P]…(1)

[0108] Here, [C], [Si] and [P] are the contents (mass %) of C, Si and P in the steel sheet, respectively.

[0109] In the present embodiment, the α value is set to 1.50 mass % or less. This is a condition required to improve the toughness of the coarse-grained part of the base material and the weld heat affected zone after stress relief annealing, and C, Si, and P need to be adjusted within the range that satisfies this condition. After SR treatment, the grain boundary segregation concentration of P increases, so brittle failure at the grain boundary is easily generated, but brittle failure can be controlled by P, C, and Si. Regarding P, if the manufacturing process is considered, it must be contained in the steel, and the grain boundary strength is significantly reduced by grain boundary segregation. Therefore, it is a representative element that causes SR embrittlement and has the highest coefficient. C and Si are also elements that must be contained in steel. If these elements become more, embrittlement caused by cementite generated at the grain boundary will be caused. Although it is hoped that any of these elements will be reduced, sometimes a certain amount is contained in the characteristics or specifications. In order to improve the toughness after SR, it is preferred to set the α value to 1.40 mass % or less. The α value is 1.00 mass % or more. This lower limit is a range determined based on the component constraints in the specification of the application field, the limit of element control in manufacturing, etc., and is calculated by substituting the lower limit values ​​of the contents of C, Si and P and the actual minimum values ​​in manufacturing into formula (1). The preferred lower limit value of the α value can be calculated based on the preferred lower limit values ​​of the contents of C, Si and P. The α value may exceed 1.10% by mass, and may be 1.30% by mass or more.

[0110] (β value: 10.0~15.0)

[0111] The β value is calculated by the following formula (2).

[0112] β=0.65×[C] 1 / 2 ×(1+0.64×[Si])×(1+4.10×[Mn])×(1+0.27×[Cu])×(1+0.52×[Ni])×

[0113] (1+2.33×[Cr])×(1+3.14×[Mo])…(2)

[0114] Among them, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass %) of C, Si, Mn, Cu, Ni, Cr and Mo in steel.

[0115] In the steel plate of the present embodiment, the range of β value is set to 10.0 to 15.0. β value is an index indicating the hardenability of steel. The higher the β value, the more stable the formation of upper bainite structure with poor strength-toughness balance can be avoided. However, if the β value is too high, the strength of the steel increases, thereby deteriorating the toughness. That is, it also becomes an index indicating the target range of the content of alloy elements required to improve the toughness of the weld heat affected zone in the welded state. The β value can also be set to 11.0 or more as needed. At the same time, the β value can also be set to 14.0 or less.

[0116] (γ value: 0.70 to 1.50 mass %)

[0117] The γ value is calculated by the following formula (3).

[0118] γ=[Mn]+20×[Nb]+36×[Ti]…(3)

[0119] [Mn], [Nb], and [Ti] are the contents (mass %) of Mn, Nb, and Ti in the steel plate.

[0120] In the steel plate of the present embodiment, the range of the γ value is set to 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, the reduction in toughness of the base material and the weld heat affected zone after stress relief annealing can be suppressed. Considering the mechanism by which these elements promote grain boundary embrittlement, the reduction in grain boundary strength caused by grain boundary segregation and the embrittlement caused by carbonitrides generated at the grain boundaries can be considered. The γ value can also be 1.40 mass% or less.

[0121] On the other hand, in order to ensure a certain hardenability and obtain a microstructure with an excellent balance between strength and toughness, it is preferred to add a certain amount of Mn, Nb and Ti to set the γ value to 0.70 mass % or more. The γ value may also be 0.75 mass % or more.

[0122] By satisfying the numerical ranges for the α value, the β value, and the γ value, the field of steels having excellent low-temperature toughness of the welded portion (weld heat affected zone) even in the as-welded state and after stress relief annealing can be expanded.

[0123] Originally, even if the α value and the β value are controlled within a certain range, it is possible to manufacture steel having excellent low temperature toughness of the welded portion even in the welded state and after stress relief annealing thereof, but depending on the application, the α value may not be freely set according to the chemical composition regulations of the application specification, and the α value has to be increased. In contrast, in the steel plate of the present embodiment, by newly setting the γ value, the allowable range of the α value can be expanded, so even in the field where the chemical composition regulations of the application specification are strict as described above, it is possible to provide steel having excellent low temperature toughness of the welded portion even in the welded state and after stress relief annealing thereof.

[0124] In addition, in the steel plate of the present embodiment, the carbon equivalent Ceq, which is calculated by the following formula (4) and is an index showing the hardenability of steel, is set to 0.550 to 0.620 mass %.

[0125] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5…(4)

[0126] In the formula (4), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] respectively represent the contents (mass %) of C, Mn, Cu, Ni, Cr, Mo, and V in the steel plate.

[0127] When Ceq is less than 0.550 mass%, the strength of the steel plate may be insufficient. Ceq may be set to 0.570 mass% or more, or 0.600 mass% or more as required. In addition, when Ceq exceeds 0.620 mass%, the toughness of the steel plate may be reduced. Ceq may be set to 0.600 mass% or less as required.

[0128] (Yield strength: 670~870N / mm 2 )

[0129] (Tensile strength: 780~940N / mm 2 )

[0130] In the steel plate of this embodiment, the yield strength is set to 670 to 870 N / mm 2 , the tensile strength of the steel plate is set to 780~940N / mm 2 . In order to reduce the weight of large welded structures such as transport tanks for liquefied CO2, it is necessary to have a steel plate that can ensure the strength of the structure even if the plate thickness is thin. Usually, the steel plate selected as a steel plate for use in such an application is a steel plate having the above-mentioned yield strength and tensile strength, so the yield strength and tensile strength of the steel plate in this embodiment are also set to the above-mentioned range. The yield strength can also be set to 690N / mm as needed. 2 ~830N / mm 2 Alternatively, the tensile strength can be set to 800 N / mm 2 ~900N / mm 2 .

[0131] (Charpy impact absorption energy at -65°C is more than 100J)

[0132] In addition, in order to ensure high toughness, the steel plate of this embodiment needs to have a Charpy impact absorption energy of 100 J or more at -65°C. Thus, the safety of the transport tank formed of the steel plate of this embodiment can be ensured. The Charpy impact absorption energy at -65°C is a value measured at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction (sometimes referred to as t / 4 position or 1 / 4 thickness position).

[0133] (The delta value of the CTOD test at -35°C is 0.10 mm or more)

[0134] In addition, in order to ensure the safety of welded structures such as transport tanks, the fracture resistance characteristics of welded structures have recently been evaluated using fracture mechanics evaluation methods and incorporated into the design. Specifically, as the occurrence characteristics of brittle fracture, the crack opening displacement (hereinafter referred to as δc) called the CTOD value is obtained by the CTOD test (Crack Tip Opening Displacement test) specified in the Japan Welding Association Standard WES1108 as a fracture mechanics parameter, and there are many cases in which δc is evaluated to see whether it can meet the design criteria.

[0135] In order to improve the δc of the material, it is necessary to improve the characteristics of the material from a different perspective than before. In the past, the Charpy impact test was used as a method for evaluating the brittle fracture resistance of the material. The value obtained by the Charpy impact test represents the average toughness of the evaluation object area. However, in the CTOD test, even if the average toughness of the evaluation object area is good, if there is even a little bit of fragile part in the evaluation object area, its existence is also reflected in δc. Since δc has such a property, in order to obtain a high δc value, especially in areas where the microstructure of the steel is uneven and complex, such as the heat affected zone of welding, it is necessary to reduce the local brittle area as much as possible.

[0136] In order to ensure high toughness, the steel plate of the present 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 the transport tank formed of the steel plate of the present embodiment is further improved.

[0137] (Average value and standard deviation of hardness)

[0138] The steel plate of this embodiment requires an average value of the hardness at 121 measurement positions of 265Hv to 290Hv and a standard deviation of 20 or less in the hardness distribution measurement within the range of 0.5mm×0.5mm at the 1 / 4 thickness position and at a spacing of 0.05mm. The structure of the steel plate of this embodiment is preferably a mixed structure of a martensite structure and a lower bainite structure with excellent strength-toughness balance. Since the hardenability is locally reduced due to the deviation of the local γ grain size and microsegregation, an upper martensite structure with a poor strength-toughness balance is sometimes formed. If an upper bainite structure exists, the distribution of hardness becomes uneven, and the toughness of the base material may deteriorate. If the average value of the hardness is less than 265Hv or the standard deviation exceeds 20, the upper bainite structure may be contained, and the toughness of the base material cannot be ensured. On the other hand, if the average value exceeds 290Hv, the strength becomes too high, and the toughness may be reduced.

[0139] The above hardness distribution measurement is performed by preparing a microscopic sample with the surface (L section) parallel to the rolling direction of the steel plate and parallel to the plate thickness direction as the observation surface, and measuring it using a micro Vickers hardness tester. The measurement area is set to a range of 0.5mm×0.5mm centered on an arbitrary t / 4 position in the microscopic observation surface, the measurement interval is 0.05mm, the measurement load is 25gf, and a total of 121 points of 11 vertical points×11 horizontal points are measured. The average value and standard deviation are calculated based on the obtained measurement values.

[0140] (Board thickness: 10~60mm)

[0141] When welding a steel plate with a plate thickness of less than 10 mm, stress relief annealing (SR) is generally not required. However, the steel plate of this embodiment is for a steel plate that requires SR, so the plate thickness is set to 10 mm or more. The plate thickness is preferably 25 mm or more. On the other hand, a steel plate with a plate thickness exceeding 60 mm contributes little to the weight reduction of the transport tank to which it is applied, so it is not preferred. Therefore, the plate thickness of the steel plate of this embodiment is set to 60 mm or less.

[0142] Furthermore, the steel plate of the present embodiment may have the structure described below.

[0143] (organize)

[0144] In order to satisfy the above-mentioned average and standard deviation of the hardness at 121 points, the steel plate of this embodiment preferably has a structure of a mixed structure of martensite and lower bainite at a 1 / 4 thickness position of the cross section in the plate thickness direction. The total of martensite and lower bainite is preferably 85% by area or more.

[0145] (Average grain size at 1 / 4 thickness of steel plate: 15.0 μm or less)

[0146] In the steel plate of the present embodiment, the average grain size at the 1 / 4 thick position may also be set to 15.0 μm or less. In order to improve the toughness of the base material and the toughness of the base material after SR, the average grain size may also be set to 14.5 μm or less, or 14.0 μm or less as required. The average grain size at the 1 / 4 thick position of the steel plate is preferably small, so there is no need to specify its lower limit. Usually, the average grain size is about 10.0 μm at the minimum.

[0147] The average crystal grain size is defined as follows.

[0148] A sample capable of observing the L-section of the steel plate was prepared, and the 1 / 4-thick position of the L-section was used as the observation portion. A scanning electron microscope was used to analyze the crystal orientation at a spacing of 0.5 μm in the range of 200 μm in the plate thickness direction and 250 μm in the rolling direction using the electron backscatter diffraction pattern analysis method (EBSD method). According to the results of the crystal orientation analysis, the area surrounded by the grain boundary with a crystal orientation difference of 15° or more was defined as the crystal grain size, and the equivalent circular grain size of the crystal grain was defined as the crystal grain size, and the value calculated by the area weighted average after weighting the area of ​​each crystal grain was used as the average crystal grain size.

[0149] (Charpy impact absorption energy at -40°C after stress relief annealing is more than 27J)

[0150] The steel plate of this embodiment is intended to prevent damage before it occurs. After being assembled into a transport tank, the weld is subjected to stress relief annealing. However, at this time, not only the weld but also the base material is heated. If the base material is heated, the toughness of the base material tends to decrease. The reason is not clear, but it is speculated that: P (phosphorus) diffuses at the grain boundary, and the growth or coagulation of inclusions occurs in the structure, which reduces the brittleness and the toughness. Therefore, the steel plate of this embodiment preferably has a Charpy impact absorption energy of 27J or more at -40°C after stress relief annealing. In this case, safety can be further improved.

[0151] The Charpy impact absorbed energy at -40°C after stress relief annealing is measured at a portion where the stress relief annealing was performed on a steel plate at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C / hour or less in a temperature range of 425°C or higher.

[0152] (The yield strength after stress relief annealing is 670~870N / mm 2 , tensile strength is 780~940N / mm 2 )

[0153] The steel plate of this embodiment preferably has a yield strength of 670 to 870 N / mm after stress relief annealing. 2 , tensile strength is 780~940N / mm 2 Thus, in the liquefied CO 2 transport tank subjected to stress relief annealing, sufficient strength can be ensured.

[0154] In addition, the steel sheet of the present 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.

[0155] The steel plate of the present embodiment has the above-mentioned structure, and thus the toughness of the heat affected zone (as-welded, after stress relief annealing) is also excellent.

[0156] The toughness of the weld heat affected zone is not limited, but as a target value, the Charpy impact absorbed energy at -65°C is preferably 70 J or more in the welded state, and the Charpy impact absorbed energy at -65°C is preferably 70 J or more after stress relief annealing.

[0157] Furthermore, it is more preferred that the δ value of the heat affected zone in the CTO D test at -35°C is 0.10 mm or more, regardless of whether it is in the welded state or after stress relief annealing.

[0158] Next, a method for producing a steel plate according to the present embodiment will be described below.

[0159] The steel sheet of the present embodiment can obtain the effects regardless of the manufacturing method if it has the above-mentioned characteristics, but it is preferable to use the method described below because it can be stably manufactured.

[0160] In order to manufacture the steel having the above chemical composition as a steel plate, it is sufficient to use a commonly used method for manufacturing steel products. That is, for example, steel manufactured by a converter method or an electric furnace method and refined by a secondary refining device is made into a slab by continuous casting or ingot blooming. As for the slab thickness, as long as the segregation can be reduced and the material quality can be improved by loosening and pressing, the slab thickness is preferably 150 mm or more. There is no particular upper limit on the slab thickness, for example, the slab thickness can be 600 mm or less, or 400 mm or less.

[0161] The slab is then preferably heated to about 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. The steel plate is then quenched and tempered to obtain a steel plate (final steel plate) having predetermined properties.

[0162] The steel sheet of this embodiment needs to reduce the P content to 0.006% or less. In a conventional dephosphorization method, the P content may not be reduced to 0.006% or less, but in this case, countermeasures such as extending the dephosphorization treatment time may be taken.

[0163] During hot rolling, the cumulative reduction ratio is preferably 50% or more within the rolling temperature range of 1150 to 900° C. The upper limit of the cumulative reduction ratio in the above temperature range does not need to be particularly specified, but the cumulative reduction ratio may be 80% or less, or 70% or less.

[0164] Regarding quenching and tempering, when the plate thickness is less than 50 mm or less than 50 mm, the reheating quenching treatment described later can be omitted by implementing a direct quenching treatment of direct water cooling after hot rolling. In the case of direct quenching, the cooling start temperature is set to above the Ar3 point, and water cooling is performed to below 300°C. The average cooling rate during water cooling is preferably 5°C / second or more in the range of 700°C to 300°C in the temperature history during cooling of the surface and back of the steel plate. There is no particular limit on the upper limit of the average cooling rate, but the average cooling rate can be, for example, less than 100°C / second, less than 50°C / second, or less than 20°C / second. In addition, further quenching can be performed by reheating after direct quenching.

[0165] The Ar3 point is calculated using the following formula.

[0166] Ar3=910-310×[C]-8×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]+0.35×(t-8)

[0167] Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] in the formula respectively represent the contents of C, Mn, Cu, Cr, Ni, and Mo in the steel plate in mass %, and t represents the thickness of the steel plate in mm.

[0168] When the plate thickness is 50 mm or more, it is preferred to perform quenching by temporarily cooling the steel plate after rolling and then reheating it. When the plate thickness is 50 mm or more, if reheating quenching is performed, direct quenching after hot rolling can be omitted, or direct quenching can be performed.

[0169] When reheating is performed, the heating temperature (i.e., quenching temperature) during quenching treatment is preferably set to 925°C or less, and may be 920°C or less, 915°C or less, or 910°C or less. The reason is that the metal structure of a thick steel plate is sometimes not sufficiently refined after rolling. If the quenching temperature of a steel plate whose metal structure is not sufficiently refined exceeds 925°C, the reverse transformation γ structure formed by heating becomes coarse, and the average grain size of the final structure after the γ / α phase transformation by subsequent cooling also becomes coarse.

[0170] On the other hand, if the lower limit of the quenching temperature is a temperature slightly higher than the Ac3 point (for example, within a temperature range of above the Ac3 point and below the Ac3 point + 20°C), the reverse transformation γ grain size is uneven, the solid solution of carbides containing B is insufficient, and sometimes the hardenability is insufficient, so it is not preferred. Therefore, the quenching temperature is preferably above 880°C, and more preferably above 890°C. In the above description of the quenching treatment conditions, it is assumed that the plate thickness of the steel plate is 50 mm or more, but the quenching treatment conditions are also applicable to the case of reheating and quenching a steel plate with a plate thickness of less than 50 mm.

[0171] In the present embodiment, tempering is performed after quenching (i.e., after direct quenching or reheating quenching, and in the case of performing both, after reheating quenching). The heating temperature during tempering (i.e., the tempering temperature) is preferably set to 660°C or less. If the tempering temperature exceeds 660°C, the tempering effect becomes excessive, and it is sometimes difficult to ensure the yield stress and tensile strength, or the toughness decreases. The tempering temperature is set to 500°C or more, preferably 600°C or more. If the tempering temperature is too low, tempering becomes insufficient, and it is difficult to ensure the specified yield stress and tensile strength.

[0172] When cooling is performed after reheating quenching or tempering, in order to prevent the toughness of the base material from decreasing due to temper embrittlement, it is desirable to cool the steel plate by water cooling rather than air cooling (accelerated cooling). In this case, the average cooling rate to 300°C is preferably set to 0.1°C / sec or more or 0.5°C / sec or more.

[0173] The steel plate of the present embodiment is suitable as a steel plate for a liquefied CO2 transport tank. For example, it can be used as a transport tank mounted on a ship. When CO2 is transported by ship, the transport tank equipped on the ship is filled with liquefied CO2 for transport, but in order to prevent the solidification (dry ice) of the CO2 in the transport tank, it is preferably transported while maintaining a pressure of about 2 MPa. In addition, in order to maintain CO2 in a liquid state at a pressure of about 2 MPa, it is preferred to maintain CO2 at about minus 35°C. The steel plate of the present embodiment can be well used for such purposes.

[0174] Example

[0175] Next, the embodiments of the present invention are described, but the conditions in the embodiments are one condition example adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. As long as it does not deviate from the gist of the present invention and can achieve the purpose of the present invention, the present invention can adopt various conditions.

[0176] The molten iron after the blast furnace treatment is discharged into a molten iron tank, and after the molten iron pretreatment such as desulfurization, the molten iron is charged into a converter. Next, the molten iron is dephosphorized in the converter to adjust the phosphorus content to 0.006% or less.

[0177] The dephosphorized molten steel was further subjected to component adjustment, and slabs having the chemical compositions shown in Table 1A and Table 1B were cast.

[0178] Then, the slab was heated to the heating temperature shown in the table by a heating furnace, and then hot-rolled to a predetermined thickness to produce a steel plate.

[0179] Furthermore, the steel plate is quenched and tempered to obtain a steel plate (final steel plate) having specified characteristics. Table 2 shows the heating temperature before rolling, the cumulative reduction rate of hot rolling in the temperature range of 1150 to 900°C, the plate thickness after rolling, the quenching temperature and the tempering temperature. Cooling after reheating quenching or tempering is carried out by water cooling, and the average cooling rate until 300°C is set to be above 0.1°C / second. In addition, for some steel plates, direct quenching treatment is implemented by direct water cooling after hot rolling. The cooling start temperature, cooling end temperature and average cooling rate in this case are shown in the table.

[0180] Tables 1A and 1B show the chemical composition, α value, β value, γ value, fB value, and carbon equivalent Ceq of the steel plate. In addition, the column before SR of the parent material properties in Table 3A shows the average value (average Hv) of the parent material hardness at 121 measurement positions, the average grain size (EBSD grain size), the yield strength (MPa), the tensile strength (MPa), the yield ratio, the Charpy impact absorption energy (J) at -65°C, and the delta value (mm) of the CTOD test at -35°C.

[0181] The EBSD grain size was determined as follows: a sample capable of observing the L section of the steel plate was prepared, and the 1 / 4 thick position of the L section was used as the observation portion. A scanning electron microscope was used, and the electron backscatter diffraction pattern analysis method (EBSD method) was used to analyze the crystal orientation at a spacing of 0.5 μm in the range of 200 μm in the plate thickness direction and 250 μm in the rolling direction. According to the results of the crystal orientation analysis, the region surrounded by the grain boundary with a crystal orientation difference of 15° or more was defined as the grain, the equivalent circular grain size of the grain was defined as the crystal grain size, and the value calculated by the area weighted average after weighting the area of ​​each grain was used as the average crystal grain size.

[0182] The tensile test was conducted in accordance with JIS Z 2241:2011. The parallel section was made in the C direction from the 1 / 4 thickness position. The yield strength and tensile strength are the average values ​​of the two specimens. The yield strength is the conditional yield strength σ 0.2 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×(YS / TS). The unit of the yield ratio is %.

[0183] For the hardness distribution measurement, a microscopic sample was prepared by taking the L section parallel to the rolling direction and the thickness direction of the steel plate as the observation surface, and the observation surface was wet-polished, and then polished and polished with 1.0 μm diamond particles to obtain a mirror surface, which was then measured using a micro-Vickers hardness tester. The measurement area was randomly selected in a range of 0.5 mm × 0.5 mm centered on the 1 / 4t position in the microscopic observation surface, the measurement interval was set to 0.05 mm, the measurement load was set to 25 gf, and a total of 121 points were measured, 11 points in the vertical direction × 11 points in the horizontal direction. The average value and standard deviation were calculated based on the obtained measurement values.

[0184] In addition, semi-automatic welded joints with a welding line parallel to the rolling direction were produced and evaluated. Specifically, a K groove was produced, and a gas shielded arc welding (GMAW) of a multi-layer weld was performed using argon containing 20% ​​CO2 as a shielding gas, a welding wire YM-69F manufactured by Nippon Steel Welding Co., Ltd., a wire energy of 2.0 kJ / mm, and a preheating temperature of 100°C to produce a welded joint.

[0185] After the microstructure was visualized from the welded portion of the welded joint (as welded (As weld)) in C section, the surface side I side fusion line (FL) was used as the position of the notch center of each specimen, and Charpy impact specimens were prepared from the position centered on the position 6.5 mm below the surface (recorded as surface preparation in the table) and the center of the plate thickness (recorded as t / 2 preparation in the table).

[0186] The sample was subjected to a Charpy impact test at -65°C to determine the absorbed energy. The results are shown in the column of Aswelld of joint properties in Table 3B.

[0187] After the microstructure of the weld was revealed, a full-thickness CTOD specimen was prepared with the surface side I side fusion line as the center of the notch of each specimen, 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.

[0188] Then, the base material and the weld were subjected to stress relief annealing (SR) under the following conditions: the holding temperature was set to 600°C, the holding time was set to 2 hours, and the heating rate and cooling rate were set to 55°C / hour or less in the temperature range above 425°C.

[0189] The yield strength and tensile strength of the base material after SR were determined in the same manner as before SR.

[0190] In addition, a Charpy impact test was performed at -40°C on a sample taken in the C direction at the t / 4 position of the base material after SR to obtain the Charpy impact absorbed energy. In addition, a CTOD test was performed at -35°C to obtain the δ value.

[0191] These results are shown in the column after SR of base material properties in Table 3A.

[0192] Furthermore, after revealing the microstructure of the welded portion after SR in a C-section, the surface side I-side fusion line was used as the position of the notch center of each sample, and Charpy impact specimens centered at the position of 6.5 mm below the surface and the t / 2 position were prepared, and the Charpy impact absorbed energy at -40°C obtained in the test is shown. In addition, after revealing the microstructure of the welded portion after SR, the surface side I-side fusion line was used as the position of the notch center of each sample, and the full-thickness CTOD specimens were prepared, and the δ value of the CTOD test at -35°C obtained in the test is shown.

[0193] These results are shown in the column after SR of joint properties in Table 3B.

[0194] Regarding the Charpy impact absorption energy of the base metal and the weld, three V-notch specimens were prepared from each base metal and weld, and the Charpy impact test was performed at a specified temperature to measure the absorption energy. Regarding the V-notch specimen, a full-size specimen described in JIS Z 2242:2005 was prepared from each plate thickness position in the C direction. In addition, the Charpy impact test was performed in accordance with JIS Z 2242:2005.

[0195] The δ value (δc) of the CTOD test is measured in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997).

[0196] Regarding the base material, the evaluation was performed in the C direction (sheet width direction) in which the longitudinal direction of the sample was perpendicular to the rolling direction.

[0197] Regarding the welded joint, the butt joint of the steel plate processed with a K-shaped groove was subjected to gas shielded arc welding at a line energy of 35 kJ / mm, and the tip of the fatigue notch of the CTOD specimen of the welded part was processed in such a way that the tip of the fatigue notch was located in the middle of the plate thickness of the fusion line (FL) on the I side of the welded part, and the CTOD test was carried out at a specified temperature. Regarding the welded joint, only the L direction (rolling direction) was evaluated. In the evaluation of the CTOD of the welded joint, the specimen was prepared in such a way that the tip of the fatigue crack was equivalent to the fusion line. Three specimens were tested at each test temperature, and the lowest value of the obtained measurement data was used as the δ value of the CTOD test. The unit of CDOD shown in Table 3A and Table 3B is mm.

[0198] As shown in Table 1A to Table 3B, No. 1 to 14 of the present invention all have excellent strength and toughness. In particular, even after SR treatment, they show excellent low-temperature toughness. In addition, the yield strength after SR treatment is 670 to 870 N / mm 2 , tensile strength is 780~940N / mm 2 , showing good values.

[0199] In addition, the Charpy impact absorbed energy of the weld heat affected zone of No. 1 to No. 14 exceeded 70 J both before SR treatment (-65° C.) and after SR treatment (-40° C.), and the low-temperature toughness was good.

[0200] On the other hand, as shown in Tables 1A to 3B, in Nos. 15 to 45 and 55 as comparative examples, the chemical composition (element content or α value, β value, γ value, Ceq.) of the steel plates deviates from the range specified in the present invention, so the toughness of at least one of the base material and the weld heat affected zone deteriorates.

[0201] In addition, No. 46 to No. 54, the chemical composition meets the component range of the present invention, but the manufacturing conditions do not meet the ideal manufacturing conditions. Therefore, the toughness is deteriorated. That is, the Charpy impact absorption energy at -65°C at least at the t / 4 position is less than 100 J, and for some examples, the toughness of other examples is also inferior.

[0202] Table 1A

[0203]

[0204] A blank column means that it was not intentionally added.

[0205] Underline means outside the scope of the present invention.

[0206] Table 1E

[0207]

[0208] A blank column means that it was not intentionally added.

[0209] Underline means outside the scope of the present invention.

[0210] Table 2

[0211]

[0212] Table 3A

[0213]

[0214] Underline means outside the scope of the present invention.

[0215] Table 3B

[0216]

[0217] Industrial Applicability

[0218] According to the present invention, a steel plate having excellent strength and low temperature toughness and excellent strength and low temperature toughness after stress relief annealing can be provided. The steel plate is suitable for use in liquefied CO2 transport tanks and has high industrial applicability.

Claims

1. A steel plate, the chemical composition of which is expressed in mass % 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: 0~0.0030%, Mg: 0~0.0030%, REM: 0~0.0030%, O: 0.0040% or less, The balance is Fe and impurities. The α value defined by the following formula (1) is 1.00 to 1.50 mass %, The β value defined by the following formula (2) is 10.0 to 15.0, The γ value defined by the following formula (3) is 0.70 to 1.50 mass %, The Ceq value defined by the following formula (4) is 0.550 to 0.620 mass %, Yield strength is 670~870N / mm 2 , Tensile strength is 780~940N / mm 2 , The Charpy impact absorption energy at -65°C is more than 100J. In the hardness distribution measurement of 1mm×1mm and 0.05mm pitch at the 1 / 4 thickness position, the average hardness at 121 measurement points was 265Hv to 290Hv, and the standard deviation was less than 20. The plate thickness is 10~60mm, α=[C]+6×[Si]+100×[P]…(1) 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) γ=[Mn]+20×[Nb]+36×[Ti]…(3) Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5…(4) in, In formulas (1) to (4), [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti] and [V] are the contents of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti and V, respectively, in mass %, and also include the amounts of elements mixed as impurities, and elements not contained are substituted with 0.

2. The steel plate according to claim 1, [fB] determined by the following formulas (A) to (E) is 0.0003 mass % or more, [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) in, In formulas (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] are the contents of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, in mass %, and also include the amounts of elements mixed as impurities. Elements not contained are substituted with 0. In addition, [fN], [fTi], [fAl], and [fO] are substituted with 0 when their calculated values ​​are less than 0%.

3. The steel plate according to claim 1 or 2, When the area surrounded by grain boundaries with a crystal orientation difference of 15° or more, which is determined by performing crystal orientation analysis using an electron backscatter diffraction pattern analysis method, is defined as a grain, the equivalent circular grain size of the grain is defined as the crystal grain size, and the value calculated by taking the area-weighted average value after weighting the area of ​​each grain is defined as the average crystal grain size, the average crystal grain size at a 1 / 4 thickness position is less than 15.0 μm.

4. The steel plate according to claim 1 or 2, When the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C for 2 hours and a heating rate and a cooling rate of 55°C / hour or less in a temperature range of 425°C or higher, the yield strength of the portion subjected to the stress relief annealing is 670 to 870 N / mm 2 , and the tensile strength is 780~940N / mm 2 The Charpy impact absorption energy at -40°C is above 27J.

5. The steel plate according to claim 3, When the steel sheet is subjected to stress relief annealing at a holding temperature of 600°C for 2 hours and a heating rate and a cooling rate of 55°C / hour or less in a temperature range of 425°C or higher, the yield strength of the portion subjected to the stress relief annealing is 670 to 870 N / mm 2 , and the tensile strength is 780~940N / mm 2 The Charpy impact absorption energy at -40°C is above 27J.

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