Steel sheet and container for liquefied co2

Abstract

This steel sheet is for liquefied CO2 transport tanks, has a prescribed chemical composition, and has an α value of 1.00-1.50 mass%, a β value of 10.00-15.00, a γ value of 0.70-1.50 mass%, a Ceq value of 0.550-0.620 mass%, a yield strength of 670-870 MPa, a tensile strength of 780-940 MPa, a thickness of 25-60 mm, an average hardness at the t / 4 position of 265 Hv-290 Hv with a standard deviation of 20 or less, and a maximum hardness HVmax in the central segregation area of 400 Hv or less. The segregation degree in the center of the plate thickness satisfies a prescribed range. Of inclusion particles found in a cross section in the sheet thickness direction, in rectangular regions with a side length of 4 mm, the rectangular regions each being centered at the t / 4 and t / 2 positions, the inclusion particles having a circular equivalent diameter of 0.5 μm or more and having Ti at 20 mass% or more, 99% or more of the inclusion particles by number have a circular equivalent diameter of 4.0 μm or less. The average crystal grain diameter at the center of the plate thickness is 15.0 μm or less.

Description

Steel plate and liquefied CO2 container

[0001] The present invention relates to a steel sheet, particularly a steel sheet made of liquefied CO ２ Steel plates for transport tanks and liquefied CO ２ This application claims priority from Japanese Patent Application No. 2024-004182, filed on January 15, 2024, 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. In this situation, carbon dioxide (CO ) is being developed as a technology to achieve carbon neutrality. 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 the solidification of CO (solid carbon dioxide), it is necessary to transport the gas 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, there is a demand to make the wall thickness of transport tanks as thin as possible in order to reduce the weight of ships.

[0004] Therefore, the steel plate used as the material for transport tanks is required to have a tensile strength of 780 MPa or more and excellent low-temperature toughness at -65°C.

[0005] 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 them into designs. Specifically, as a brittle fracture initiation characteristic, a crack opening displacement (hereinafter abbreviated as δc) called the CTOD value is determined as a fracture mechanical parameter by a CTOD (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.

[0006] In order to improve the δc of a material, it is necessary to improve the material's properties from a different perspective than before. Traditionally, 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 area. However, in the CTOD test, even if the average toughness of the evaluation target area is good, the presence of even a small amount of brittleness within the evaluation target area will be reflected in the δc. Because of this property of δc, in order to obtain a high δc value, especially in areas where the microstructure of the steel material changes in a non-uniform and complex manner, such as weld heat-affected zones, it is necessary to minimize localized embrittlement areas.

[0007] Furthermore, in large welded structures such as transport tanks, stress relief annealing is sometimes performed on welds to reduce the possibility of fracture. Stress relief annealing is a heat treatment method in which the welded portion of a welded structure is heated to a temperature below the Ac1 transformation point and then slowly cooled in order to reduce residual stress caused by welding. However, when stress relief annealing is applied to high-tensile steels with a tensile strength of 780 MPa or more, alloy carbides selectively precipitate at grain boundaries, which cause grain boundary embrittlement, significantly reducing the toughness of the stress relief annealed portion. This phenomenon is generally referred to as SR (stress relieving) embrittlement. High-tensile steels containing B and manufactured by quenching and tempering are particularly prone to SR embrittlement. In such high-tensile steels, not only is the base material embrittlement severe, but the weld heat-affected zone (HAZ) embrittlement obtained when welded joints are made using this high-tensile steel is also significantly embrittled.

[0008] Therefore, in order to obtain a high δc value and ensure high safety in transport tanks manufactured using such high-tensile steel, it is necessary to develop a high-tensile steel that maintains high toughness of the base metal even after stress relief annealing.

[0009] 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 −65° C., and there is room for further improvement in low-temperature toughness.

[0010] Japanese Patent No. 5590271

[0011] The present invention has been made in view of the above circumstances, and provides a liquefied CO2-based steel sheet which is excellent in strength and low-temperature toughness as well as in low-temperature toughness after stress relief annealing, and which, when made into a welded joint, is also excellent in low-temperature toughness before and after stress relief annealing. ２ Steel plates for transport tanks and liquefied CO containing the steel plates ２ The object of the present invention is to provide a container for

[0012] In order to solve the above problems, the present invention employs the following configuration: [1] A steel sheet according to one embodiment of the present invention is a steel sheet manufactured by using liquefied CO ２A steel plate for a transport tank, having a chemical composition, in mass %, of C: 0.070 to 0.110%, Si: 0.10 to 0.15%, Mn: 0.70 to 1.20%, Ni: 1.00 to 2.50%, Cr: 0.20 to 0.80%, Mo: 0.20 to 0.80%, V: 0.005 to 0.070%, Al: 0.030 to 0.100%, B: 0.0005 to 0.0030%, N: 0.0015 to 0.0050%, P: 0.006% or less, S: 0.005 to 0.070 ... : 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, the balance: 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 10.00 to 15.00, the γ value defined by the following formula (3) is 0.70 to 1.50 mass%, and Ceq defined by the following formula (4) is 0.550 to 0.620 mass%, the yield strength is 670 to 870 MPa, the tensile strength is 780 to 940 MPa, and when the plate thickness is t, the t is 25 to 60 mm, and in a hardness distribution measurement of 0.5 mm x 0.5 mm with a 0.05 mm pitch at the t / 4 position, the average hardness at 121 measurement positions is 265 Hv to 290 Hv and the standard deviation is 20 or less, and the maximum hardness of the center segregation portion HVmax the hardness is 400 HV or less, the degree of segregation in the sheet thickness center portion satisfies all of the following formulas (5) to (8), the equivalent circle diameter in a rectangular region of 4 mm on a side having centers at the t / 4 position and the t / 2 position in a cross section in the sheet thickness direction is 0.5 μm or more, and among inclusion particles containing 20 mass % or more of Ti, 99% or more of the inclusion particles have equivalent circle diameters of 4.0 μm or less, and when regions surrounded by grain boundaries having a crystal orientation difference of 15° or more as determined by crystal orientation analysis using electron beam backscatter diffraction pattern analysis are defined as crystal grains, the equivalent circle diameters of the crystal grains are defined as crystal grain sizes, and a value calculated as an area-weighted average weighted by the area of ​​each crystal grain is defined as an average crystal grain size, the average crystal grain size in the sheet thickness center portion of the steel sheet is 15.0 μm or less. 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) [Si]max / [Si]≦1.9...(5) [P]max / [P]≦20.0...(6) [Cu]max / [Cu]≦2.5 …(7) [Ni]max / [Ni]≦2.0…(8) Here, in formulas (1) to (8), [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 include the amounts of elements mixed in as impurities, with 0 being substituted for elements that are not contained. In addition, [Si]max, [P]max, [Cu]max, and [Ni]max in formulas (5) to (8) are each the average concentration values ​​of each element in a tertiary region, where a rectangular secondary region with a side length of 1 mm where the concentration of each element is maximum based on the results of EPMA line analysis of each element is selected from a primary region centered on the t / 2 position in the thickness direction of the steel sheet and extending ±5 mm in the thickness direction and 10 mm in the rolling direction, and then a rectangular tertiary region with a side length of 20 μm where the concentration of each element is maximum based on the results of EPMA area analysis of each element is selected from the secondary region. The tertiary region is referred to as the central segregation region. [2] The steel sheet described in [1] above may have [fB] calculated by the following formulas (A) to (E) of 0.0003% 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) Here, [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. 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] The steel plate according to the above [1] or [2], when subjected to stress relief annealing at a holding temperature in the range of 600 to 620°C for a holding time in the range of 2.0 to 2.8 hours and at a temperature decreasing rate in the range of 55 to 100°C / h in a temperature range of 425°C or higher, may have a yield strength of 670 to 870 MPa, a tensile strength of 780 to 940 MPa, and a Charpy absorbed energy at -45°C of 40 J or more at a location subjected to the stress relief annealing. ２ The container includes the steel plate according to any one of [1] to [3].

[0013] According to the above aspect of the present invention, the strength and low-temperature toughness of the steel sheet and the low-temperature toughness after stress relief annealing are excellent, and when a welded joint is formed, the low-temperature toughness after stress relief annealing is also excellent. ２ Steel plates for transport tanks and liquefied CO ２ Containers can be provided.

[0014] FIG. 1 is an explanatory diagram illustrating a state in which a metallographic structure observation sample is processed at a position including the plate thickness center (t / 2 position). FIG. 2 is an explanatory diagram illustrating a state in which a portion with the highest average Mn concentration is identified centered on the t / 2 position, and a 1 mm x 1 mm field of view region (secondary region) is identified. FIG. 3 is an explanatory diagram illustrating a state in which a 20 x 20 μm square portion of a 1 mm x 1 mm square field of view region (secondary region) is scanned lengthwise and widthwise (rolling direction and plate thickness direction), and average values ​​of the region are determined from the mass percentages of Si, P, Cu, and Ni at measurement points included within the square region at each position. FIG. 4 is an explanatory diagram illustrating an example of a square region (tertiary region) where the average values ​​of Si, P, Cu, and Ni are maximum. FIG. 5 is a diagram illustrating an example of a microstructure photograph revealed by nital etching and observed with an optical microscope.

[0015] Hereinafter, a steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) and a liquefied CO ２ A vessel for welding will be described in detail. In this embodiment, unless otherwise specified, "stress relief annealing" refers to stress relief annealing in accordance with the provisions of JIS Z 3700:2009 "Post-weld heat treatment method." In this embodiment, "welding" refers to welding with a welding heat input of 1.1 to 4.5 kJ / mm, unless otherwise specified. These conditions are typical conditions in the technical field to which the present invention pertains. However, even if stress relief annealing or welding is performed under conditions different from those described above, the same effects as those obtained by stress relief annealing or welding performed under the above conditions can be obtained. Therefore, the steel plate according to this embodiment may be subjected to stress relief annealing or welding under conditions different from those described above. In this embodiment, when the thickness of a steel plate is t, the position at 1 / 2 of the plate thickness from the surface in the plate thickness direction (t / 2 position) is referred to as the t / 2 position, and the position at 1 / 4 of the plate thickness from the surface in the plate thickness direction (t / 4 position) is referred to as the t / 4 position.

[0016] <Steel Sheet> First, the ranges of the contents of elements constituting the chemical composition of the steel sheet according to this embodiment and the reasons for limiting these ranges will be described. Hereinafter, unless otherwise specified, "%" means "mass %."

[0017] (C: 0.070 to 0.110%) 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 0.070% or more. The C content is preferably 0.080% 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 0.110% or less. The C content is preferably 0.100% or less, and more preferably less than 0.100%.

[0018] (Si: 0.10 to 0.15%) Si is generally contained in steel as a deoxidizing element. However, in this embodiment, Si is an element that reduces the toughness of steel after stress relief annealing. Therefore, the Si content is set to 0.15% or less. The Si content is preferably set to 0.14% or less, 0.13% or less, or 0.12% or less. Furthermore, in order to suppress a decrease in the toughness of the weld heat affected zone after stress relief annealing, a low Si content is preferable. On the other hand, in order to contain Si for the purpose of deoxidation, the Si content is set to 0.10% or more.

[0019] (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.

[0020] (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.

[0021] (Cr: 0.20 to 0.80%) 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 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.

[0022] (Mo: 0.20 to 0.80%) Like Cr, Mo is an element effective for improving hardenability and improving the strength of steel by 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. 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.

[0023] (V: 0.005 to 0.070%) Like Cr and Mo, V is an element effective for improving hardenability and improving the strength of steel by 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 toughness of 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.

[0024] (Al: 0.030 to 0.100%) Al is an element useful for deoxidation, and also an element that refines the crystal grain size during quenching by forming nitrides. In the steel plate according to this embodiment, the Al content is set to 0.030% or more. The Al content is preferably set to 0.040% or more. 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 set to 0.080% or less.

[0025] (B: 0.0005 to 0.0030%) In this embodiment, B is an element that improves the hardenability of 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.

[0026] (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.

[0027] (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%. If the P content exceeds 0.006% or the S content exceeds 0.0030%, the adverse effect on toughness becomes significant. Therefore, in this embodiment, in order to improve the toughness of 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 set to 0.005% or less. Furthermore, the S content may be set to 0.0020% or less, if necessary.

[0028] (Cu: 0 to 1.00%) Cu is not an essential element in 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. When Cu is contained, in order to utilize this effect, the Cu content is preferably 0.10% or more, and may be 0.20% or more. If necessary, the Cu content may be 0.15% 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 0.80% or less. If necessary, the upper limit of the Cu content may be 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0029] (Nb: 0 to 0.030%) Nb is not an essential element in 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, it is preferable to add 0.001% or more of Nb. 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.

[0030] (Ti: 0 to 0.010%) Ti is not an essential element in 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, etc., 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, Ti, like Nb, may form coarse carbonitrides, which may 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.

[0031] (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 Ca, Mg, and REM contents are all 0%.

[0032] Ca is an element that 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.

[0033] Mg and REM are elements that form oxides and improve the toughness of the weld heat-affected zone. To achieve this effect, the Mg content and REM content may each be 0.0001% or more. On the other hand, if Mg and REM are added in large amounts, coarse oxides may be formed, which may reduce the toughness of the steel. Therefore, the Mg content and REM content are each set to 0.0030% or less. If necessary, the Mg content and REM content 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, Tb, and Lu. REM is characterized by its strong deoxidizing properties compared to other additive elements and forms stable oxides in steel.

[0034] (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, the oxides can become the initiation points of brittle fracture. Therefore, the lower the O content, the better. Therefore, the lower limit of the O content is 0%. In this embodiment, the O content is set to 0.0040% or less in order to improve the toughness of the weld. The O content is preferably set to 0.0030% or less.

[0035] (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 the steel sheet, and are acceptable within a range that does not adversely affect the present invention.

[0036] The content of each element can be measured by the following method. For elements such as C, Si, Mn, P, S, Nb, V, Ni, Cu, Cr, Mo, Ti, Al, Ca, B, Mg, and REM, inductively coupled plasma optical emission spectroscopy (ICP-OES) using an ICP optical emission analyzer or the like is applied. For trace amounts of S, O, and N, such as on the ppm order, infrared absorption spectroscopy or thermal conductivity analysis using a CS analyzer, an ON analyzer, or the like can be applied. Other analytical methods, such as inductively coupled plasma mass spectroscopy (ICP-MS) and atomic absorption spectroscopy (AAS), may also be applied as necessary. Since the chemical composition of the steel sheet according to this embodiment remains almost unchanged during the manufacturing process, if the chemical composition measured using a sample taken from the molten steel in the tundish during the steelmaking stage is known, that chemical composition may be considered to be the chemical composition of the steel sheet according to this embodiment.

[0037] Furthermore, in the steel sheet according to this embodiment, while satisfying the above chemical composition, it is preferable that [fB] calculated by the following formulas (A) to (E) be 0.0003% or more. [fB] represents the amount of B dissolved in the steel. By setting [fB] to 0.0003% or more, the hardenability of high-tensile steel having a yield strength of 670 to 870 MPa and a tensile strength of 780 to 940 MPa can be improved. B easily forms nitrides in steel. Furthermore, Ti and Al easily form nitrides and oxides. Therefore, the amount of B dissolved in the steel, [fB], is calculated by the following formulas (A) to (E). [fB] may be 0.0005% or more, or 0.0015% or more. Furthermore, [fB] may be 0.0025% or less, or 0.0018% or less.

[0038] [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)

[0039] Here, [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] in formulas (A) to (E) represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, and elements not contained, including the amount of elements mixed in as impurities, are substituted with 0. Furthermore, if the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.

[0040] Furthermore, in the steel sheet according to this embodiment, in addition to the limitations on the chemical composition (contents of individual elements) described above, the ranges of the α value, β value, and γ value are limited as follows (the contents of individual elements are limited so that the α value, β value, and γ value are within predetermined ranges).

[0041] (α value: 1.00 to 1.50 mass %) The α value is expressed by the following formula (1).

[0042] α=[C]+6×[Si]+100×[P] (1) Here, [C], [Si], and [P] in formula (1) are the contents (mass%) of C, Si, and P in the steel, and include the amounts of elements mixed in as impurities, with 0 being substituted for elements that are not contained.

[0043] 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 coarse-grained portion of the weld heat-affected zone after stress relief annealing, and the C, Si, and P contents must be adjusted within a range that satisfies this condition. After stress relief annealing, 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 due to the 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 that are inevitably contained in steel, and if these elements increase in amount, they can cause embrittlement due to cementite formed at grain boundaries. While it is desirable to reduce each element, a certain amount may be contained for characteristics or specifications. If necessary, the α value may be set to 1.40 mass% or less to improve the toughness after stress relief annealing. On the other hand, the α value is 1.00 mass% or more. This lower limit (1.00% by mass) is determined by the component restrictions in the specifications for the field of application and the limits of element control in manufacturing, and is a value calculated by substituting the above-mentioned lower limits of the C, Si, and P contents and the realistic minimum values ​​in 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.

[0044] (β value: 10.00 to 15.00) The β value is calculated by the following formula (2).

[0045] 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) Here, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] in formula (2) represent the amounts (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel, and include the amounts of elements present as impurities, with 0 being substituted for elements that are not present.

[0046] In the steel plate according to this embodiment, the β value is set to a range of 10.00 to 15.00. The β value is an index showing the hardenability of the steel plate, 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. On the other hand, if the β value is too high, the strength of the steel plate increases, which leads to a deterioration in toughness. In other words, it 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 11.00 or more. Alternatively, the β value may be set to 14.00 or less.

[0047] (γ value: 0.70 to 1.50 mass %) The γ value is calculated by the following formula (3).

[0048] γ=[Mn]+20×[Nb]+36×[Ti] (3) Here, [Mn], [Nb], and [Ti] in formula (3) represent the contents (mass%) of Mn, Nb, and Ti in the steel, and include the amounts of elements mixed in as impurities, with 0 being substituted for elements that are not contained.

[0049] In the steel sheet according to this embodiment, the γ value ranges from 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 decrease in toughness after stress relief annealing can be suppressed. 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. On the other hand, in order to ensure a certain level of hardenability and obtain a microstructure with an advantageous strength-toughness balance, it is preferable to include certain amounts of Mn, Nb, and Ti, with a γ value of 0.70 mass% or more. The γ value may be 0.75 mass% or more or 1.40 mass% or less.

[0050] By satisfying the numerical ranges for the α value, β value, and γ value, it is possible to provide a steel that has excellent low-temperature toughness in the welded portion both as welded and after stress relief annealing.

[0051] 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 %.

[0052] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (4) Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] in formula (4) represent the contents (mass%) of C, Mn, Cu, Ni, Cr, Mo, and V in the steel, and include the amounts of elements mixed in as impurities, with 0 being substituted for elements that are not contained.

[0053] If Ceq is less than 0.550 mass%, the strength of the steel plate may be insufficient. Therefore, Ceq is set to 0.550 mass% or more. If necessary, Ceq may be set to 0.570 mass% or more, or 0.600 mass% or more. Furthermore, if Ceq exceeds 0.620 mass%, the toughness of the steel plate may be reduced. Therefore, Ceq is set to 0.620 mass% or less. If necessary, the upper limit of Ceq may be set to 0.600 mass% or less.

[0054] In the steel sheet according to this embodiment, the chemical composition, Ceq, α value, β value, and γ value are within predetermined ranges, and preferably, [fB] is further controlled within a predetermined range. The chemical composition, Ceq, fB, α value, β value, and γ value are each controlled by the content of an element. However, since the discharge or addition of a specific element during the refining process may also change the amount of other elements, it is not easy to change only a single element. Furthermore, since multiple elements may form compounds in the molten steel and the steel material, the expected effect may not be obtained unless the addition balance with other components is kept within a target range.

[0055] (Segregation Degree) Next, the segregation of Si, P, Cu, and Ni in the steel sheet will be described. In the steel sheet according to this embodiment, the segregation degree at the center of the sheet thickness (within a range of ±5 mm in the sheet thickness direction including the t / 2 position) needs to satisfy all of the following formulas (5) to (8).

[0056] [Si]max / [Si]≦1.9…(5) [P]max / [P]≦20.0…(6) [Cu]max / [Cu]≦2.5…(7) [Ni]max / [Ni]≦2.0…(8)

[0057] Here, in formulas (5) to (8), [Si], [P], [Cu], and [Ni] respectively represent the contents (mass%) of Si, P, Cu, and Ni in the above-mentioned steel, including the amounts of elements mixed in as impurities, and elements that are not contained are substituted with 0. Furthermore, in formulas (5) to (8), [Si]max, [P]max, [Cu]max, and [Ni]max are respectively the average concentration (content) value of each element in a tertiary region when a rectangular secondary region with a side length of 1 mm where the concentration of each element is maximum based on the results of EPMA line analysis of each element is selected from a primary region of a cross section of the steel sheet in the sheet thickness direction, centered at the t / 2 position and measuring ±5 mm in the sheet thickness direction and 10 mm in the rolling direction, and a rectangular tertiary region with a side length of 20 μm where the concentration of each element is maximum based on the results of EPMA area analysis of each element is selected from the secondary region. In addition, the tertiary region is measured in the region at the half-width position, which is the center position in the plate width direction, and at the quarter-width position, which is 1 / 4 of the plate width from the end in the plate width direction, and the higher value is used as the representative value for the steel plate.

[0058] The center of the steel plate thickness has a high concentration of alloying elements, which locally increases hardenability and results in a hard structure, resulting in reduced toughness. Furthermore, when impurity elements such as P are concentrated, the toughness after stress relief annealing decreases significantly. In order to ensure the toughness of the base material, it is necessary to satisfy all of the above formulas (5) to (8).

[0059] The segregation degree of each element is expressed as [Si]max / [Si] in the case of Si, for example. The segregation degree of each element can be determined by EPMA (Electron Probe Micro Analysis) measurement according to the following steps (a), (b), (c), and (d).

[0060] (a) First, as shown in FIG. 1 , a cross section (TD plane) of a steel sheet 10 parallel to the rolling direction and the thickness direction is machined at positions including the t / 2 position, with the sheet width direction positions being set at the 1 / 2-width position and the 1 / 4-width position. Then, as shown in FIG. 2 , on the TD plane appearing on the metallographic observation sample 11, linear analysis is performed by EPMA measurement with an analysis length of 10 mm, centered on the t / 2 position and at 50 μm pitches in the thickness direction, in a range 12 (primary region) of ±5 mm from the center of the thickness, to measure the mass% of Mn. Then, the average Mn concentrations of each linear analysis are averaged over a range of ±0.5 mm in the thickness direction to identify a thickness position range (region) 13 with the highest average Mn concentration. Within this thickness position range (region) 13, a range of ±0.5 mm in the rolling direction with the highest average Mn concentration is determined, and a 1 mm × 1 mm field of view region 14 (secondary region) is identified as follows:

[0061] (b) That is, on the TD surface appearing in the metallographic structure observation sample 11, the center in the sheet thickness direction of the region 13 where the average Mn concentration is high is set as the longitudinal center position, and a 1 mm x 1 mm square field of view 14 is identified as the secondary region, with the rolling direction center of the region 13 where the average Mn concentration is high within a range of ±0.5 mm in the rolling direction being the horizontal center. Then, in this field of view 14, area analysis is performed by EPMA measurement at a pitch of 2 μm in both the longitudinal and lateral directions (rolling direction and sheet thickness direction), and the mass percentages of Si, P, Cu, and Ni are measured.

[0062] (c) Next, as shown in Figure 3, in a 1 mm x 1 mm square field of view 14 (secondary region), a 20 x 20 µm square portion 15 is scanned lengthwise and widthwise (rolling direction and plate thickness direction), and the average values ​​of the contents of Si, P, Cu, and Ni at the measurement points included in the square portion 15 at each position are determined from the respective contents (mass%) of the elements. The square portion 15 with the largest average value is then identified as the tertiary region, and the average values ​​of the contents of Si, P, Cu, and Ni in this tertiary region are defined as their respective maximum values ​​([Si]max, [P]max, [Cu]max, [Ni]max). The third region is defined as a central segregation region.

[0063] For example, as shown in FIG. 4, if the average Si value determined from the mass percent Si content at the measurement points included in the square region 15-1 is the maximum, the average Si content determined from the mass percent Si content at the measurement points included in the square region 15-1 (tertiary region) will be the maximum value [Si]max. Similarly, if the average P value determined from the mass percent P content at the measurement points included in the square region 15-3 is the maximum, the average P value determined from the mass percent P content at the measurement points included in the square region 15-3 (tertiary region) will be the maximum value [P]max. Similarly, if the average Cu value determined from the mass percent Cu content at the measurement points included in the square region 15-4 is the maximum, the average Cu value determined from the mass percent Cu content at the measurement points included in the square region 15-4 (tertiary region) will be the maximum value [Cu]max. Similarly, if the average value of Ni determined from the Ni content (mass%) of each fixed point included in the area of ​​square portion 15-5 is the maximum value, the average value of Ni determined from the Ni content (mass%) of each measurement point included in the area of ​​square portion 15-5 (tertiary region) will be the maximum value [Ni]max. In this way, the tertiary region may be specified for each element.

[0064] (d) The respective maximum values ​​([Si]max, [P]max, [Cu]max, [Ni]max) divided by the content ([Si], [P], [Cu], [Ni]) of each element in the steel sheet ([Si], [P], [Cu], [Ni]) is taken as the segregation degree of each element at the center of the sheet thickness ([Si]max / [Si], [P]max / [P], [Cu]max / [Cu], [Ni]max / [Ni]). The measurement conditions for EPMA analysis are as follows: an EPMA device with an LaB6 electron gun is used, the acceleration voltage is 15 kV, the probe current is 100 nA, the beam diameter is the same as the measurement pitch, and the measurement time is 50 ms. For line analysis and area analysis with a 50 μm pitch, a short measurement time of 20 ms is acceptable.

[0065] (The equivalent circle diameter of 99% or more of the inclusion particles by number is 4.0 μm or less) In the steel sheet according to this embodiment, when inclusion particles having an equivalent circle diameter of 0.5 μm or more and containing 20 mass% or more of Ti are identified in a rectangular region with sides of 4 mm and centered at the t / 4 position and the t / 2 position of the cross section in the sheet thickness direction, the equivalent circle diameter of 99% or more of the inclusion particles by number must be 4.0 μm or less. In other words, in the particle size distribution of the equivalent circle diameters of the identified inclusion particles, the equivalent circle diameter corresponding to a cumulative distribution function of 99% must be 4.0 μm or less. If the equivalent circle diameter of 99% or more of the inclusion particles by number exceeds 4.0 μm, the deterioration of toughness after stress relief annealing becomes significant. The equivalent circle diameter of 99% or more of the inclusion particles by number may be 3.0 μm or less or 2.5 μm or less.

[0066] The circle-equivalent diameter of 99% or more of the inclusion particles by number is measured by the following method. To measure the circle-equivalent diameter of 99% or more of the inclusion particles by number, rectangular regions with sides of 4 mm and centers at the t / 4 and t / 2 positions on the thickness direction cross section of a mirror-finished steel sheet are randomly selected on the observation surface, and the regions are observed using an SEM equipped with an EDS analyzer or the like at an acceleration voltage of 3 to 30 kV. The field of view within the region is scanned, and inclusion particles with a contrast different from that of the observed parent phase are extracted. The area is determined by image analysis, and EDS point analysis is performed near the center of the particle. The circle-equivalent diameter is calculated by √(4A / π), where A is the area of ​​the inclusion particle. Among the inclusion particles, those having an equivalent circle diameter of 0.5 μm or more and containing 20 mass % or more of Ti are identified, and in the particle size distribution of the equivalent circle diameters of these inclusion particles, the equivalent circle diameter circle corresponding to 99% of the cumulative distribution function is determined.

[0067] (Yield strength: 670 to 870 MPa) (Tensile strength: 780 to 940 MPa) In this embodiment, the yield strength of the steel plate is set to 670 to 870 MPa, and the tensile strength of the steel plate is set to 780 to 940 MPa. 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 with a thin plate thickness is required. Typically, steel plates selected for such applications have the above-mentioned yield strength and tensile strength, and therefore the steel plate of this embodiment is also manufactured to have the above-mentioned yield strength and tensile strength. If necessary, the yield strength may be set to 690 MPa or more and 830 MPa or less. The tensile strength may be set to 800 MPa or more and 900 MPa or less.

[0068] (Average Hardness and Standard Deviation) In a hardness distribution measurement at the t / 4 position using a 0.5 mm × 0.5 mm x 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 at 121 measurement points. If the hardness distribution is uneven, the toughness of the base material may be degraded. If the average hardness is less than 265 Hv and the standard deviation exceeds 20, the base material toughness cannot be ensured. On the other hand, if the average hardness exceeds 290 Hv, the strength may become too high, which may result in a decrease in toughness. The structure of the steel plate according to this embodiment is preferably a mixed structure primarily consisting of martensite and lower bainite, which has an advantageous strength-toughness balance. When the standard deviation and average hardness values ​​are within the above ranges, it is considered that the structure primarily contains martensite and lower bainite. If upper bainite is present, there is a concern that the average hardness may be less than 265 Hv or the standard deviation may exceed 20. Upper bainite may be formed when local variations in γ grain size and microsegregation cause a local decrease in hardenability.

[0069] The hardness standard deviation (distribution) is measured by taking a micro sample from the TD of the steel plate as the observation surface and 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, and measurements are taken at 11 vertical points x 11 horizontal points (total of 121 points) with a measurement pitch of 0.05 mm and a measurement load of 25 gf. The average value and standard deviation are calculated from the obtained measurements.

[0070] (Maximum hardness HVmax of central segregation: 400 HV or less) The harder the base material, the lower the toughness after stress relief annealing. In order to maintain toughness after stress relief annealing, the maximum hardness at the measurement position of the segregation degree [M]max / [M] needs to be 400 HV or less. That is, within the above-mentioned tertiary region (central segregation region), optical microscope observation is performed randomly on, for example, five fields of view, and the hardness at the center of each field of view is measured using a Vickers test with a load of 100 g, and the maximum hardness is defined as the maximum hardness HVmax of the central segregation region. When the tertiary regions are different for Si, P, Cu, and Ni, the maximum hardness in the Ni tertiary region (measurement position of the segregation degree [Ni]max / [Ni]) is 400 HV or less.

[0071] (Plate thickness: 25 to 60 mm) When welding steel plates with a thickness of less than 25 mm, stress relief annealing is generally not required. However, the steel plate according to this embodiment is intended for steel plates that require stress relief annealing, so the plate thickness is set to 25 mm or more. On the other hand, steel plates with a thickness of more than 60 mm are not preferred because they increase the weight of the transport tank. Therefore, the plate thickness of the steel plate according to this embodiment is set to 60 mm or less.

[0072] Furthermore, the steel sheet according to this embodiment may have the following properties.

[0073] (Structure) In the steel sheet according to the present embodiment, the structure at the t / 4 position of the cross section in the sheet thickness direction is preferably a mixed structure of martensite and lower bainite. The martensite and lower bainite structures may occupy 85.0 area % or more in total. In this case, the standard deviation and average hardness described above are likely to be satisfied.

[0074] The area ratios of the martensite structure and the lower bainite structure are measured by the following method. The area ratios of each metal structure are measured on the L cross section of the steel sheet, i.e., on a plane parallel to the rolling direction RD of the steel sheet and perpendicular to the sheet surface. The area of ​​the observation field is 40,000 μm ２ For example, a dimension along the surface of the steel plate is 250 μm, a dimension along the thickness direction of the steel plate is 200 μm, and an area is 50,000 μm ２The metallographic structure is revealed by nital etching. The area fraction of the metallographic structure in each observation region is obtained by observing the structure with an optical microscope and determining the structure as follows. Upper bainite is a massive, acicular, or amorphous (mainly massive with curved grain boundaries) structure in which one or more cementite and austenite-martensite composites are observed as dots or granules with clear black contrast within the ferrite matrix grains, which are observed with white contrast. The cementite and austenite-martensite composites observed in upper bainite may form in rows along the ferrite laths, or may appear randomly arranged. The lath structure of upper bainite has a lath width of 1.0 μm or more, with multiple lath structures arranged in parallel, although the lath spacing and orientation may be less uniform. Prior austenite grain boundaries may be difficult to distinguish due to unclear contrast or an uneven saw blade appearance. For example, the left-hand structural photograph in Figure 5 is a structural photograph of an upper bainite structure. The mixed martensite and lower bainite structure is a structure in which fine laths with clear black contrast are observed within the grains of the parent phase, which are observed with white contrast. In this embodiment, the lath width is 1.0 to 3.0 μm, and numerous cementite particles with black contrast are observed within the laths and at the lath boundaries. However, because the cementite observed in tempered martensite is fine and has a high precipitation density, it may be observed under an optical microscope as countless white granular contrast or as a cloud-like contrast consisting of fine particles. Prior austenite grain boundaries are clearly visible as linear black contrast, but they are sometimes difficult to distinguish from intragranular laths and can be difficult to distinguish. For example, the right-hand structural photograph in Figure 5 is a structural photograph of a mixed martensite and lower bainite structure.

[0075] (Average grain size at the center of the steel plate thickness: 15.0 μm or less) In this embodiment, in order to ensure a predetermined toughness, the average grain size at the center of the steel plate thickness is set to 15.0 μm or less. In order to improve the base material toughness before and after stress relief annealing, the upper limit of the average grain size may be set to 14.5 μm or less, or 14.0 μm or less, as necessary. Since a smaller average grain size at the center of the steel plate thickness is preferable, there is no need to specify a lower limit. Usually, the smallest average grain size is about 10.0 μm.

[0076] The average crystal grain size is measured by the following method. A sample measuring 10 to 20 mm in the longitudinal direction and 10 to 30 mm in the thickness direction is cut out from the steel sheet so that the L cross section can be observed. This sample is polished with colloidal silica. A 500 x 500 μm area of ​​the target cross section is measured in 1.0 μm steps using an EBSD device (manufactured by TSL or Ametek-EDAX) using the electron beam backscatter diffraction pattern analysis (EBSD method). The acceleration voltage is 10 to 30 kV. The obtained crystal orientations were analyzed using crystal orientation analysis software (TSL OIM Analysis 7 x64). The regions surrounded by grain boundaries with a crystal orientation misorientation of 15° or more were defined as crystal grains, the circle-equivalent diameters of the crystal grains were defined as crystal grain sizes, and the average crystal grain size was calculated as an area-weighted average weighted by the area of ​​each crystal grain. The crystal orientation analysis software records crystal orientation data for each hexagonal pixel created on the observation surface. Therefore, each of these pixels has an adjacent pixel, and a crystal misorientation degree is defined for each boundary (one side of the hexagon) between adjacent pixels. If the crystal orientation misorientation between adjacent pixels is 15° or more (hereinafter referred to as a 15° boundary), the pixel boundary is considered to potentially correspond to a crystal grain boundary and is retained on the map as a grain boundary candidate. This misorientation calculation is performed for all pixel boundaries. If adjacent grain boundary candidates are continuous, they are connected to form a closed curve (broken line) within the observation (data collection) area. The area enclosed by this closed curve is defined as a single grain. If a 15° boundary does not form a closed curve and has an end within the observation area, it is not considered a grain boundary (it is ignored as a subgrain boundary). Furthermore, if a 15° boundary intersects with the edge of the observation area, this means that the grain extends beyond the observation area. Since this area does not represent the area of ​​a single grain, it is ignored as a half-area. In this way, only areas completely enclosed by 15° boundaries within the observation area are considered to correspond to grains, and the area of ​​each grain is determined by multiplying the number of such boundaries by the area of ​​one pixel (a hexagon with one side equal to the step size).Finally, the diameter of a circle with the same area as each closed region is calculated and defined as the diameter of one crystal grain. Even if the 15° boundary forms a closed curve within the observation region, if the number of pixels within it is one or less, it is considered noise and is not used in calculating the grain size.

[0077] (Charpy absorbed energy of 40 J or more at -45°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, the steel plate according to this embodiment preferably has a Charpy absorbed energy of 40 J or more at -45°C after stress relief annealing. This can further improve safety.

[0078] The Charpy absorbed energy at −45° 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 to 620° C., a holding time of 2.0 to 2.8 hours, and a temperature decreasing rate in the range of 55 to 100° C. / h (° C. / hour) in a temperature range of 425° C. or higher.

[0079] (δ value in CTOD test at -35°C is 0.05 mm or more) The steel plate according to this embodiment has excellent toughness due to the above-mentioned characteristics, but from the viewpoint of ensuring the safety of a transport tank made of the steel plate according to this embodiment, it is preferable that the δ value in CTOD test at -35°C is 0.05 mm or more.

[0080] (Charpy absorbed energy of 50 J or more at -70°C) Furthermore, the steel plate according to this embodiment preferably has a Charpy absorbed energy of 50 J or more at -70°C. This makes it possible to ensure the safety of a transport tank that includes or is made of the steel plate according to this embodiment. The Charpy absorbed energy at -70°C is a value measured at the t / 4 position. Furthermore, in this embodiment, a Charpy absorbed energy of 50 J or more means that when measured at three different measurement positions at the t / 4 position, the minimum value is 50 J or more.

[0081] (Yield strength after stress relief annealing is 670 to 870 MPa, tensile strength is 780 to 940 MPa) The steel sheet according to this embodiment preferably has a yield strength after stress relief annealing of 670 to 870 MPa and a tensile strength of 780 to 940 MPa. In this case, the liquefied CO 2 that has been subjected to stress relief annealing ２ This ensures sufficient strength for transport tanks.

[0082] <liquefied CO ２ Container for liquefied CO according to this embodiment ２ The container for liquefied CO2 according to this embodiment is formed by processing and welding the steel plate according to this embodiment. ２ The vessel for welding includes the steel plate according to the present embodiment. The vessel for welding may essentially consist of the steel plate according to the present embodiment (base material portion) and a weld formed by melting and re-solidifying the steel plate according to the present embodiment and the welding material.

[0083] <Manufacturing Method> Next, the steel sheet and liquefied CO ２A method for manufacturing a container for a steel vessel is described below. The steel plate according to this embodiment exhibits the above-described effects regardless of the manufacturing method, as long as it has the above-described characteristics. However, the manufacturing method described below allows for stable manufacturing. To manufacture a steel plate having the above-described composition, the steel is produced by a converter or electric furnace process and refined in secondary refining equipment, and then formed into a slab by continuous casting or blooming. The slab is then preferably heated to approximately 950 to 1250°C in a slab heating furnace and then hot-rolled to a predetermined thickness to obtain a steel plate. This steel plate is then quenched and tempered to obtain a steel plate (final steel plate) with the desired properties. Preferred conditions for each process are described below. (Secondary Refining Process) The secondary refining process can be performed using a known method. The steel plate according to this embodiment requires a P content reduced to 0.006% or less. Conventional dephosphorization methods may not be able to reduce the P content to 0.006% or less. In such cases, measures such as extending the dephosphorization treatment time may be taken.

[0084] (Casting Process) In the casting process, a slab is obtained. In order to control the equivalent circle diameter of 99% or more of the inclusion particles by number to 4.0 μm or less, the inclusions are floated and separated in the tundish, the inclusions are floated and separated by an electromagnetic brake on the molten steel flow in the early stage of solidification in the mold, and center segregation is reduced by soft reduction during casting. The soft reduction may be, for example, a reduction rate of 3% or less, 2% or less, or 1% or less. The soft reduction is preferably performed in the final stage of solidification.

[0085] (Hot Rolling Process) Next, the hot rolling process will be described. If the heating temperature before rolling exceeds 1250°C, the average crystal grain size will become coarse. Therefore, it is preferable to set the heating temperature before rolling to 1250°C or less. Furthermore, if the heating temperature before rolling is below 950°C, low-temperature rolling will occur during rolling, the reduction amount per pass will be small, and sufficient reduction effect will not be obtained near the center of the plate thickness. Therefore, it is preferable to set the heating temperature before rolling to 950°C or more.

[0086] During rolling, it is desirable to set the cumulative reduction rate at a rolling temperature in the range of 1150 to 900° C. to 50% or more. When direct quenching in which the steel sheet is immediately water-cooled after hot rolling is performed, it is desirable to set the cumulative reduction rate at a rolling temperature in the range of 1150 to 900° C. to 50% or more. There is no particular need to specify an upper limit for the cumulative reduction rate, but the cumulative reduction rate may be, for example, 80% or less, or 70% or less.

[0087] (Quenching process) After hot rolling, either or both of direct quenching and reheating quenching are performed, in which the steel sheet is once cooled after hot rolling and then reheated. When both are performed, reheating quenching is performed after direct quenching. Direct quenching is a quenching process in which the steel sheet is immediately water-cooled after hot rolling. Reheating quenching is a quenching process in which the steel sheet is once cooled after hot rolling and then reheated.

[0088] The conditions for the direct quenching treatment are that the cooling start temperature is equal to or higher than the Ar3 point, and water cooling is performed to 300°C or lower. The average cooling rate during water cooling is preferably 5°C / sec or higher. There is no particular upper limit to the average cooling rate, but it may be, for example, 50°C / sec or lower, 20°C / sec or lower, or 15°C / sec or lower.

[0089] The conditions for the reheating and quenching treatment are as follows. 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 slightly above the Ac3 point (e.g., within a temperature range of the Ac3 point or higher and the Ac3 point + 20°C or lower) is not preferable because it 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 lower limit of the quenching temperature is preferably 880°C or higher, more preferably 890°C or higher.

[0090] The Ar3 point can be determined by the following method: Ar3 (°C) = 910 - 310 x C - 80 x Mn - 20 x Cu - 15 x Cr - 55 x Ni - 80 x Mo + 0.35 x (t - 8) (A) In the above formula (A), C, Mn, Cu, Cr, Ni, and Mo are the contents (mass%) of each element in the steel, and t is the thickness (mm) of the steel plate after hot rolling. Furthermore, the slab temperature means the surface temperature of the slab.

[0091] (Tempering step) In the steel sheet manufacturing method according to this embodiment, the steel sheet after the quenching step is further tempered. The heating temperature for tempering (i.e., tempering temperature) is set to 660°C or lower. If the tempering temperature exceeds 660°C, the tempering effect becomes excessive, making it difficult to ensure the yield strength and tensile strength. On the other hand, if the tempering temperature is too low, the tempering becomes insufficient, making it difficult to ensure the yield strength and tensile strength. Therefore, the tempering temperature is set to 500°C or higher. The tempering temperature is preferably set to 600°C or higher.

[0092] When cooling is performed after reheating and quenching or tempering, it is desirable to cool the steel plate 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, the average cooling rate to 300°C is preferably 0.1°C / sec or more or 0.5°C / sec or more. The average cooling rate may be 5°C / sec or more.

[0093] The steel sheet according to this embodiment is ２ Transport tank (liquefied CO ２ 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 the solidification of CO, it is preferable to transport the CO while maintaining the pressure at 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.

[0094] Liquefied CO according to this embodiment ２ The vessel is formed by cutting and processing the steel plate according to this embodiment into a predetermined shape, and joining a plurality of steel plates by welding. After welding, stress relief annealing may be performed. The welding may be performed under general conditions with a welding heat input of 1.1 to 4.5 kJ / mm. The stress relief annealing may be performed in accordance with the specifications of JIS Z 3700:2009 "Post-weld heat treatment method."

[0095] 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.

[0096] The molten iron that had undergone blast furnace treatment was tapped into a hot metal ladle, and after preliminary treatment such as desulfurization was carried out, the molten iron was introduced into a converter, and the composition of the molten steel in the converter was adjusted.

[0097] This molten steel was cast by continuous casting to obtain slabs having the chemical compositions shown in Tables 1A to 3B. In the casting process, inclusions were floated and separated in the tundish, and the molten steel flow was electromagnetically braked in the initial solidification stage in the mold to float and separate the inclusions. Furthermore, except for some examples, center segregation was reduced by light reduction during casting (reduction rate: 1% or less). The slabs were then heated in a heating furnace to the heating temperatures shown in Tables 4A and 4B, and then hot-rolled to a predetermined thickness to obtain steel plates. These steel plates were then quenched and tempered to obtain steel plates (final steel plates) having the desired properties. Tables 4A and 4B show the presence or absence of soft reduction during casting, the heating temperature before rolling, the cumulative reduction rate of hot rolling from 1150 to 900°C, the plate thickness after rolling, the conditions of direct quenching (cooling start temperature, cooling end temperature, average cooling rate), the conditions of reheating and quenching (quenching temperature), and the conditions of tempering (tempering temperature). Cooling after reheating and quenching and tempering was performed by water cooling, and the average cooling rate to 300°C was 0.1°C / second or more.

[0098] Tables 1A to 3B show the chemical composition, α value, β value, γ value, fB value, and carbon equivalent Ceq of the steel sheets. Tables 5A to 6B show the average value of the base material hardness (average Hv), its standard deviation, maximum hardness HVmax, the structure fraction (%) of martensite and lower bainite, the average grain size (EBSD grain size), yield strength (MPa), tensile strength (MPa), yield ratio, Charpy absorbed energy (J) at -70°C at the t / 4 position and the t / 2 position, and the δ value (mm) of the CTOD test at -35°C at 121 measurement positions. Furthermore, Tables 5A and 5B show whether the segregation degrees ([Si]max / [Si], [P]max / [P], [Cu]max / [Cu], [Ni]max / [Ni]) at the center of the sheet thickness satisfy predetermined ranges, and whether the circle-equivalent diameters of 99% or more of the inclusion particles were 4.0 μm or less ("◯" if 4.0 μm or less, and "×" if more than 4.0 μm).

[0099] The methods for measuring the segregation ratios ([Si]max / [Si], [P]max / [P], [Cu]max / [Cu], and [Ni]max / [Ni]) at the center of the sheet thickness and the average grain size were as described above.

[0100] The tensile test was conducted using two test pieces in accordance with JIS Z 2241:2023. The yield strength (0.2% proof stress) and tensile strength are each the average values ​​of the two test pieces. 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 %.

[0101] For hardness measurements, a micro sample was taken with the surface parallel to the rolling direction of the steel as the observation surface, and measurements were performed using a micro Vickers hardness tester. The measurement area was a 0.5 mm x 0.5 mm range centered at an arbitrary t / 4 position within the micro observation surface, and measurements were performed at 11 vertical points x 11 horizontal points (121 points in total) with a measurement pitch of 0.05 mm and a measurement load of 25 gf. The average value and standard deviation were calculated from the obtained measurements.

[0102] Furthermore, within the tertiary region identified when measuring the degree of segregation in the center of the plate thickness, for example, five fields of view were randomly photographed under an optical microscope, and the hardness in the center of each field of view was measured using a Vickers test with a load of 100 g, and the maximum hardness was defined as the maximum hardness HVmax of the central segregation portion.

[0103] The structure fractions of the martensite structure and the lower bainite structure were determined by SEM observation of the same cross section as that used for the hardness distribution measurement, identifying the martensite structure and the lower bainite structure, and calculating the total area fraction (structure fraction). The method for measuring the structure fraction was as described above.

[0104] In addition, welded joints were prepared and evaluated. ２ A multi-layer gas-shielded arc welding process was performed to produce a welded joint, using argon gas containing α-methyl-2-pyrrolidone as the shielding gas, YM-69F welding wire manufactured by Nippon Steel Welding Industries Co., Ltd., with a heat input of 2.0 kJ / mm and preheating at 100°C. The base metal and weld were then subjected to stress relief annealing (SR). The stress relief annealing was performed at a holding temperature of 600 to 620°C, for a holding time of 2.0 to 2.8 hours (2 hours and 48 minutes), and with a cooling rate of 55 to 100°C / h in a temperature range of 425°C or higher.

[0105] Tables 6A and 6B show the yield strength and tensile strength of the base material after SR.

[0106] Tables 6A and 6B show the Charpy absorbed energy at -45°C after SR at the t / 4 position of the base material and the δ value of the CTOD test at -35°C after SR.

[0107] Furthermore, Tables 7A and 7B show the Charpy absorbed energy at -70°C and the δ value of the CTOD test at -35°C at the surface layer (I-side FL) and t / 2 position of the base metal on the front side of the weld adjacent to the weld before SR.

[0108] Furthermore, Tables 7A and 7B show the Charpy absorbed energy at -45°C and the δ value of the CTOD test at -35°C in the surface layer (I-side FL) and t / 2 position of the base metal on the front side of the weld adjacent to the weld after SR.

[0109] The Charpy absorbed energy of the base material and the welded joint was measured by taking three V-notch test pieces from each of the base material and the welded joint, conducting a Charpy impact test at a specified temperature, and measuring the absorbed energy (vE －６５ ) was measured. The V-notch test specimen was prepared in accordance with the V-notch test specimen described in JIS Z 2242:2023. The Charpy impact test was also carried out in accordance with JIS Z 2242:2023. The Charpy absorbed energy at -70°C was recorded as the minimum value measured at three different measurement positions at t / 4 positions.

[0110] The σ value of the CTOD test is (δc ａｔ－１０℃ ) was measured in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997). Specifically, gas-shielded arc welding was performed at a heat input of 35 kJ / mm on the butt joint of steel plates with a K-groove, and the weld was processed so that the tip of the fatigue notch of the CTOD test specimen was located in the center of the plate thickness of the I-side fusion line of the weld, and a CTOD test was performed at a predetermined temperature. For the base material, evaluation was performed in the C direction (plate width direction), in which the longitudinal direction of the test specimen was perpendicular to the rolling direction. For the welded joint, evaluation was performed only in the L direction (rolling direction). In evaluating the CTOD of the welded joint, test specimens were taken so that the tip of the fatigue crack corresponded to the weld bond. Three tests were carried out 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 6A to 7B is mm.

[0111] The test was deemed to have passed if the minimum Charpy absorbed energy of the base material at -70°C was 50 J or more, the CTOD of the base material at -35°C was 0.05 mm or more, and the CTOD of the joint at -35°C before and after SR was 0.05 mm or more.

[0112] As shown in Tables 1A to 7B, the base metals of all of the inventive examples Nos. 1 to 14 exhibited excellent toughness at low temperatures both before and after SR. In particular, the δ value of the CTOD test at -35°C after SR at the t / 4 position was 0.05 mm or greater, demonstrating excellent brittle fracture resistance at low temperatures even after SR treatment. Furthermore, the yield strength after SR treatment was 670 to 870 MPa and the tensile strength was 780 to 940 MPa, both of which were favorable values. Furthermore, for Nos. 1 to 14, the δ value of the welded joints in the CTOD test at -35°C was 0.05 mm or greater both before and after SR, demonstrating excellent brittle fracture resistance at low temperatures.

[0113] On the other hand, as shown in Tables 1A to 7B, the comparative examples Nos. 15 to 44 and 58 had chemical compositions outside the range specified in the present invention, resulting in inferior low-temperature toughness. That is, the minimum Charpy absorbed energy of the base metal at −70°C was less than 50 J, the CTOD of the base metal at −35°C was less than 0.05 mm, or the CTOD of the welded joint at −35°C before or after SR was less than 0.05 mm.

[0114] In addition, Nos. 45 to 57 had steel compositions that satisfied the composition ranges of the present invention, but the manufacturing conditions did not satisfy the preferred manufacturing conditions, resulting in inferior low-temperature toughness. That is, the minimum Charpy absorbed energy of the base metal at −70°C was less than 50 J, the CTOD of the base metal at −35°C was less than 0.05 mm, or the CTOD of the welded joint at −35°C before or after SR was less than 0.05 mm.

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[0129] According to the present invention, a liquefied CO2-based steel sheet is excellent in strength and low-temperature toughness, and in low-temperature toughness after stress relief annealing, and when it is made into a welded joint, it is also excellent in low-temperature toughness after stress relief annealing. ２ Steel plates for transport tanks and liquefied CO ２ Therefore, the present invention has high industrial applicability.

[0130] 10 Steel plate 11 Metal structure observation sample

Claims

1. Liquefied CO ２ A steel plate for a transport tank, wherein the chemical components are in mass %, C: 0.070 to 0.110%, Si: 0.10 to 0.15%, Mn: 0.70 to 1.20%, Ni: 1.00 to 2.50%, Cr: 0.20 to 0.80%, Mo: 0.20 to 0.80%, V: 0.005 to 0.070%, Al: 0.030 to 0.100%, B: 0.0005 to 0.0030%, N: 0.0015 to 0.0050%, P: 0.006% or less, S: 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, the balance being 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 10.00 to 15.00, the γ value defined by the following formula (3) is 0.70 to 1.50 mass %, the Ceq defined by the following formula (4) is 0.550 to 0.620 mass %, the yield strength is 670 to 870 MPa, the tensile strength is 780 to 940 MPa, when the plate thickness is t, the t is 25 to 60 mm, in the hardness distribution measurement of 0.5 mm × 0.5 mm, 0.05 mm pitch at the t / 4 position, the average value of the hardness at 121 measurement positions is 265 Hv to 290 Hv, and the standard deviation is 20 or less, the maximum hardness HVmax of the central segregation part is 400 HV or less, the segregation degree of the center part of the plate thickness satisfies all of the following formulas (5) to (8), the equivalent circle diameter of the inclusions having an equivalent circle diameter of 0.5 μm or more and containing 20 mass % or more of Ti in the rectangular region with a side of 4 mm having the t / 4 position and the t / 2 position as the center positions in the cross section in the plate thickness direction, the number ratio of 99% or more of the inclusions is 4.0 μm or less, a region surrounded by grain boundaries with a crystal orientation difference of 15° or more, which is determined by performing crystal orientation analysis using the electron backscatter diffraction pattern analysis method, is defined as a crystal grain, the equivalent circle diameter of the crystal grain is defined as the crystal grain size, and when a value calculated by area-weighted average weighted by the area of each crystal grain is defined as the average crystal grain size, the average crystal grain size at the center part of the plate thickness of the steel plate is 15.0 μm or less, steel plate.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) [Si]max / [Si] ≤ 1.9 …(5) [P]max / [P] ≤ 20.0 …(6) [Cu]max / [Cu] ≤ 2.5 …(7) [Ni]max / [Ni] ≤ 2.0 …(8) Here, [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti], and [V] in formulas (1) to (8) are the contents (mass %) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti, and V, respectively, including the amounts of elements mixed in as impurities. For elements not contained, 0 is substituted. Also, [Si]max, [P]max, [Cu]max, and [Ni]max in formulas (5) to (8) are, respectively, in the cross-section in the thickness direction of the steel plate, within a primary region of ±5 mm in the thickness direction and 10 mm in the rolling direction centered at the t / 2 position. Based on the EPMA line analysis results of each element, a secondary region of a rectangle with a side length of 1 mm where the concentration of each element is the maximum is selected. Further, from the secondary region, based on the EPMA surface analysis results of each element, when a tertiary region of a rectangle with a side length of 20 μm where the concentration of each element is the maximum is selected, they are the average concentration values of each element within the tertiary region. Also, the tertiary region is the center segregation part.

2. The steel sheet according to claim 1, wherein [fB] obtained by the following formulas (A) to (E) is 0.0003% 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) Here, [B], [N], [Ti], [Al], [O], [Ca], [Mg], [REM] in formulas (A) to (E) are the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, REM respectively. For elements not contained, 0 is substituted, including the amount of elements mixed as impurities. Also, when the calculated values of [fN], [fTi], [fAl], [fO] are less than 0%, 0 is substituted.

3. When stress relief annealing is performed on the steel sheet, where the holding temperature is in the range of 600 to 620 °C, the holding time is in the range of 2.0 to 2.8 hours, and the cooling rate is in the range of 55 to 100 °C / h in the temperature range of 425 °C or higher, the yield strength at the location where the stress relief annealing is performed is 670 to 870 MPa, the tensile strength is 780 to 940 MPa, and the Charpy impact energy at -45 °C is 40 J or more. The steel sheet according to claim 1 or claim 2.

4. A liquefied CO container containing the steel sheet according to claim 1 or claim 2. ２ container.

5. A liquefied CO container comprising the steel sheet according to claim 3 ２ ​

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