Steel sheet and container for liquefied co2

Abstract

This steel plate is used for liquefied CO2 transport tanks. It has a specified chemical composition: α value of 1.00~1.50 wt%, β value of 10.00~15.00%, γ value of 0.70~1.50 wt%, Ceq value of 0.550~0.620 wt%, yield strength of 670~870 MPa, tensile strength of 780~940 MPa, plate thickness of 25~60 mm, and average hardness at position t / 4 of tonne of 265 Hv~290 Hv with a standard deviation of less than 20. The maximum hardness HVmax of the central segregation region is 400HV or less, the segregation degree of the central part of the plate thickness meets the specified range, and in the rectangular area with the t / 4 position and t / 2 position of the cross section in the plate thickness direction as the center position, the equivalent circle diameter of the inclusion particles is 0.5μm or more and contains more than 20% by mass Ti, and the equivalent circle diameter of more than 99% of the inclusion particles is 4.0μm or less, and the average crystal grain size of the central part of the plate thickness is 15.0μm or less.

Description

Technical Field

[0001] This invention relates to steel plates, particularly steel plates for liquefied CO2 transport tanks and containers for liquefied CO2.

[0002] This application claims priority based on Japanese Patent Application No. 2024-004182 filed in Japan on January 15, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, as a countermeasure to climate change, there has been a strong demand for the reduction of greenhouse gases. In this context, CCS (Carbon Dioxide Capture and Storage), a technology for achieving carbon neutrality and for the recovery and storage of carbon dioxide (hereinafter referred to as CO2), has attracted attention. CCS separates and recovers CO2 emitted from CO2 emission sources such as oil refineries, power plants, and chemical plants, and then pressurizes and stores it in deep underground storage layers. When the recovery facilities for separating and recovering CO2 and the storage facilities for pressing and storing CO2 underground are geographically separated, the separated and recovered CO2 needs to be transported between these facilities via pipelines, ships, etc.

[0004] When transporting CO2 by ship, liquefied CO2 is filled into transport tanks equipped on the ship and then transported. This improves the efficiency of CO2 transportation. However, to prevent the CO2 inside the transport tank from solidifying (solidifying carbon dioxide), transportation needs to be carried out at a pressure of approximately 2 MPa. Furthermore, to maintain the CO2 in a liquid state at a pressure of approximately 2 MPa, the CO2 needs to be kept at approximately -35°C. Consequently, to achieve ship weight reduction, there is also a need to minimize the wall thickness of the transport tanks.

[0005] Therefore, steel plates used as raw materials for transport tanks are required to have a tensile strength of over 780 MPa and excellent low-temperature toughness at -65°C.

[0006] Furthermore, to ensure the safety of welded structures such as transport tanks, destructive mechanics evaluation methods have recently been used to assess the destructive resistance of welded structures and have been incorporated into the design. Specifically, as a characteristic of brittle fracture, the crack tip opening displacement (hereinafter referred to as δc) known as the CTOD value is determined through the CTOD test (Crack Tip Opening Displacement test) specified by the Japan Welding Society standard WES1108, etc., and is used as a parameter of destructive mechanics to evaluate whether δc can meet the design criteria.

[0007] To improve the δc of a material, a different approach is needed to enhance its properties. Previously, the Charpy impact test was used to evaluate a material's resistance to brittle fracture. The value obtained from the Charpy impact test represents the average toughness of the evaluation area. However, in the CTOD test, even if the average toughness of the evaluation area is good, the presence of a few weak points within that area is also reflected in δc. Because δc has this property, especially in regions of steel where the microstructure is uneven and complex, such as the weld heat-affected zone, it is necessary to minimize localized embrittlement areas to obtain a high δc value.

[0008] Furthermore, in large welded structures such as transport tanks, stress-relief annealing is sometimes performed on the welded parts to further reduce the possibility of failure. Stress-relief annealing is a heat treatment method that aims to reduce residual stress generated by welding by heating the welded part of the structure to a temperature below the Ac1 phase transformation point, followed by slow cooling. However, when stress-relief annealing is applied to high-tensile steel with a tensile strength of 780 MPa or higher, alloy carbides selectively precipitate at grain boundaries, causing grain boundary embrittlement. This results in a significant reduction in the toughness of the stress-relief annealed area. This phenomenon is generally referred to as SR (stress relief) embrittlement. SR embrittlement is particularly prevalent in high-tensile steels containing boron and manufactured through quenching and tempering. In such high-tensile steels, not only is the base material embrittled, but the heat-affected zone of the weld joint is also significantly embrittled when using this high-tensile steel.

[0009] Therefore, in order to obtain high δc values ​​and ensure high safety in transport tanks made using such high-tensile steel, it is necessary to develop high-tensile steels that maintain the toughness of the base material even after stress-relief annealing.

[0010] Based on the aforementioned viewpoints, several technical solutions have been proposed in the past. For example, Patent Document 1 discloses a high-strength steel plate characterized by adjusting the chemical composition to achieve an average crystal grain size of less than 15 μm. However, the steel plate described in Patent Document 1 does not evaluate its low-temperature toughness at -65°C, leaving room for further improvement in low-temperature toughness.

[0011] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 5590271 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] The present invention was made in view of the above circumstances, and its object is to provide a steel plate for a liquefied CO2 transport tank with excellent strength and low-temperature toughness, as well as excellent low-temperature toughness after stress-relief annealing, and excellent low-temperature toughness before and after stress-relief annealing when making a welded joint, and a container for liquefied CO2 containing the steel plate.

[0014] means for solving problems

[0015] To address the aforementioned issues, the present invention employs the following configuration.

[0016] [1] In one embodiment of the present invention, the steel plate is a steel plate for a liquefied CO2 transport tank, wherein the chemical composition is as follows (by mass%): C: 0.070~0.110%, 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: 0.030~0.100%, B: 0.0005~0.0030%, N: 0.0015~0.0050%, P: less than 0.006%, S: 0.003 Below 0%, 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: below 0.0040%, balance: Fe and impurities, α value defined by equation (1) below is 1.00~1.50% by mass, β value defined by equation (2) below is 10.00~15.00%, γ value defined by equation (3) below is 0.70~1.50% by mass, Ceq defined by equation (4) below is 0.550~0.620% by mass, yield strength is The plate thickness is 670~870MPa, the tensile strength is 780~940MPa, and the plate thickness is set as t, where t is 25~60mm. In the hardness distribution measurement at the t / 4 position with a spacing of 0.5mm×0.5mm and 0.05mm, the average hardness at the measurement position of point 121 is 265Hv~290Hv, the standard deviation is less than 20, the maximum hardness HVmax of the central segregation part is less than 400HV, and the segregation degree of the central part of the plate thickness satisfies the following equations (5)~(8). The t / 4 position and t / 2 position of the section in the plate thickness direction are respectively taken as a rectangle with a side of 4mm of the central position. In a region containing inclusion particles with an equivalent circular diameter of 0.5 μm or more and containing 20% ​​by mass or more Ti, 99% or more of the inclusion particles have an equivalent circular diameter of 4.0 μm or less. A region surrounded by grain boundaries with a crystal orientation difference of 15° or more, as determined by crystal orientation analysis using electron beam backscatter diffraction pattern analysis, is defined as a grain. The equivalent circular grain size of the grain is defined as the grain size. When the average grain size is defined as the area-weighted average calculated by weighting the area of ​​each grain, the average grain size at the center of the thickness of the steel plate 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, [C], [Si], [P], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [Ti] and [V] in equations (1) to (8) are the contents (mass%) of C, Si, P, Mn, Cu, Ni, Cr, Mo, Nb, Ti and V, respectively, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted with 0. Furthermore, in equations (5) to (8), [Si]max, [P]max, [Cu]max, and [Ni]max are, respectively, the average concentration values ​​of each element in the three regions of the steel plate cross section in the thickness direction. These are: a primary region centered at position t / 2 with a thickness of ±5 mm in the thickness direction and a rolling direction of 10 mm; a secondary region where the highest concentration of each element is selected based on EPMA line analysis results; and a tertiary region where the highest concentration of each element is selected based on EPMA surface analysis results. The tertiary region is then designated as the central segregation region.

[0017] [2] The steel plate mentioned above [1] can be such that [fB] calculated 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, in equations (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. They also include the amount of elements mixed in as impurities. Elements that are not present are substituted with 0. In addition, if the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%, they are substituted with 0.

[0018] [3] The steel plate described in [1] or [2] above may be a steel plate that has undergone stress-relief annealing with a holding temperature of 600~620℃, a holding time of 2.0~2.8 hours, and a cooling rate of 425℃ or higher in a temperature range of 55~100℃ / h. The yield strength of the stress-relief annealed part is 670~870MPa, the tensile strength is 780~940MPa, and the Charpy absorption energy at -45℃ is 40J or higher.

[0019] [4] Another embodiment of the present invention provides a container for liquefying CO2 comprising any one of the steel plates described in [1] to [3].

[0020] Invention Effects

[0021] According to the above-described solution of the present invention, a steel plate for a liquefied CO2 transport tank and a container for liquefied CO2 can be provided, which have excellent strength and low-temperature toughness of steel plate and excellent low-temperature toughness after stress-relief annealing when the welded joint is made. Attached Figure Description

[0022] Figure 1 This is an explanatory diagram showing the state of a metal structure specimen processed at a location including the center of the plate thickness (t / 2 position).

[0023] Figure 2 This is an explanatory diagram of the state of a specific 1mm×1mm field of view (secondary region) centered at the t / 2 position, where the average Mn concentration is the highest.

[0024] Figure 3 This is an explanatory diagram showing how a 20×20μm square portion is scanned along the longitudinal and transverse directions (rolling direction and plate thickness direction) within a 1mm×1mm square field of view (secondary region). The average values ​​of Si, P, Cu, and Ni at each measurement point within the square portion are determined at each location.

[0025] Figure 4 This is an explanatory diagram illustrating the regions (cubic regions) where the average values ​​of Si, P, Cu, and Ni are at their maximum values, representing square areas.

[0026] Figure 5 The image shows an example of a tissue photograph that has been exposed by ethanol etching and observed under an optical microscope. Detailed Implementation

[0027] Hereinafter, a steel plate (the steel plate of this embodiment) and a container for liquefied CO2 according to one embodiment of the present invention will be described in detail.

[0028] Unless otherwise specified, "stress-relief annealing" in this embodiment refers to stress-relief annealing as specified in JIS Z 3700:2009 "Post-weld heat treatment methods".

[0029] Unless otherwise specified, "welding" in this embodiment refers to welding with a welding heat input of 1.1 to 4.5 kJ / mm. These conditions are general conditions in the technical field to which this invention pertains. However, even when stress-relief annealing or welding is performed under conditions different from those described, the same effect as stress-relief annealing or welding performed under the described conditions can be obtained. Therefore, the steel plate of this embodiment can be stress-relief annealed or welded under conditions different from those described.

[0030] In this embodiment, the position where the thickness of the steel plate is t is defined as 1 / 2 of the plate thickness from the surface in the thickness direction (the t / 2 position), and the position where the thickness of the plate is 1 / 4 of the plate thickness from the surface in the thickness direction (the t / 4 position) is defined as the t / 4 position.

[0031] <steel plate>

[0032] First, the range of the content of elements constituting the chemical composition of the steel plate of this embodiment and the reasons for these limitations will be explained. Hereinafter, unless otherwise specified, "%" refers to mass%.

[0033] (C: 0.070~0.110%) Carbon (C) is an element that improves the strength of the base material. To ensure the steel plate of this embodiment achieves the target strength, the C content is set to 0.070% or more. Preferably, the C content is 0.080% or more. On the other hand, with a high C content, the hardness of the weld heat-affected zone increases while its toughness decreases; therefore, the C content is set to 0.110% or less. Preferably, the C content is 0.100% or less, and more preferably less than 0.100%.

[0034] (Si: 0.10~0.15%) Si is typically found in steel as a deoxidizing element. However, in this embodiment, Si is an element that reduces the toughness of the steel after stress-relief annealing. Therefore, the Si content is set to 0.15% or less. Preferably, the Si content is 0.14%, 0.13%, or 0.12% or less. Furthermore, a low Si content is preferred to suppress the reduction in toughness of the weld heat-affected zone after stress-relief annealing. On the other hand, in order to contain Si for the purpose of deoxygenation, the Si content is set to be 0.10% or more.

[0035] (Mn: 0.70~1.20%) Mn is an effective element for deoxidation and also improves the strength of steel. Therefore, the Mn content is set to 0.70% or more. Preferably, the Mn content is set to 0.90% or more. On the other hand, excessive Mn content may impair the toughness of stress-relieved steel due to temper embrittlement. Therefore, the Mn content is set to 1.20% or less. Preferably, the Mn content is set to 1.10% or less.

[0036] (Ni: 1.00~2.50%) Ni is an effective element for improving the hardenability and toughness of steel. Therefore, the Ni content is set to 1.00% or more. Preferably, the Ni content is 1.20% or more. On the other hand, excessive Ni content may reduce the toughness of the steel after stress-relief annealing. Furthermore, it may deteriorate the toughness of the heat-affected zone after stress-relief annealing. Therefore, the Ni content is set to 2.50% or less. Preferably, the Ni content is set to 2.00% or less.

[0037] (Cr: 0.20~0.80%) Cr is an effective element for improving the hardenability of steel and enhancing its strength through precipitation strengthening during tempering. Therefore, the Cr content is set at 0.20% or more. Preferably, the Cr content is set at 0.40% or more. On the other hand, excessive Cr content may reduce the toughness of the base metal and the heat-affected zone after stress-relief annealing. Therefore, the Cr content is set to 0.80% or less. Preferably, the Cr content is set to 0.70% or less.

[0038] (Mo: 0.20~0.80%) Like Cr, Mo is an effective element for improving hardenability and enhancing steel strength through precipitation strengthening during tempering. Therefore, the Mo content is set to 0.20% or more. Preferably, the Mo content is set to 0.30% or more, more preferably 0.35% or more, and even more preferably 0.40% or more. On the other hand, excessive Mo content may lead to the precipitation of Mo carbides at grain boundaries after stress-relief annealing, potentially reducing the toughness of the base material and the weld heat-affected zone. The impact on the weld heat-affected zone is particularly significant. Therefore, the Mo content is set to 0.80% or less. Preferably, the Mo content is set to 0.60% or less.

[0039] (V: 0.005~0.070%) Like Cr and Mo, V is an effective element for improving hardenability and enhancing steel strength through precipitation strengthening during tempering. Therefore, the V content is set to 0.005% or more. Preferably, the V content is set to 0.010% or more. On the other hand, excessive V content may reduce the toughness of the base material and the weld heat-affected zone after stress-relief annealing. Therefore, the V content is set to 0.070% or less. Preferably, the V content is set to 0.050% or less.

[0040] (Al: 0.030~0.100%) Al is an element useful for deoxidation and also contributes to grain refinement during quenching by forming nitrides. In the steel sheet of this embodiment, the Al content is 0.030% or more. Preferably, the Al content is 0.040% or more. On the other hand, excessive Al content can lead to the formation of coarse Al nitrides, which may reduce the toughness of the base material and the heat-affected zone of the weld. Therefore, the Al content is set to 0.100% or less. Preferably, the Al content is set to 0.080% or less.

[0041] (B: 0.0005~0.0030%) In this embodiment, B is an element that improves the hardenability of steel by being present in trace amounts. Therefore, the B content is set to 0.0005% or more. The B content can be set to 0.0006% or more, 0.0008% or more, or 0.0010% or more. On the other hand, when boron (B) is present in excess, it forms coarse nitrides and / or carbides, which can sometimes reduce the toughness of the base material. Therefore, the B content is set to 0.0030% or less. The B content can also be set to 0.0020% or less, or 0.0010% or less.

[0042] (N: 0.0015~0.0050%) Nitrogen (N) is an element that forms nitrides, thereby refining the grain size of the base material and improving its toughness. Therefore, the N content is 0.0015% or higher. The N content can be set to 0.0030% or higher, or 0.0035% or higher. On the other hand, when nitrogen is present in excessive amounts, nitrides become coarser, and the toughness of the heat-affected zone in the weld state (as weld) decreases. Therefore, the nitrogen content is set to 0.0050% or less.

[0043] (P: below 0.006%) (S: below 0.0030%) P and S are impurity elements contained in steel, and lower content is preferred. Therefore, the lower limit for P and S content is 0%. When 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. Alternatively, the S content can be set to 0.0020% or less as needed.

[0044] (Cu: 0~1.00%) Cu is not an essential element in this embodiment, therefore the lower limit of Cu content is 0%. However, Cu has the effect of improving the strength of steel, so it can be included as needed. When Cu is included, in order to utilize this effect, the Cu content is preferably set to 0.10% or more, or 0.20% or more. Depending on the needs, the Cu content can also be set to 0.15% or more, or 0.30% or more. On the other hand, excessive Cu content may lead to reduced toughness of the base material due to surface cracking and Cu precipitation. Therefore, the Cu content is set to 1.00% or less. Preferably, the Cu content is 0.80% or less. Depending on the requirements, the upper limit of the Cu content can be set to 0.70%, 0.60%, 0.50%, or 0.40% or less.

[0045] (Nb: 0~0.030%) In this embodiment, Nb is not an essential element, therefore the lower limit for Nb content is 0%. However, Nb is an element that refines grains during quenching, so it can be included as needed. When Nb is included, it is preferable to contain 0.001% or more of Nb to take advantage of this effect. On the other hand, excessive Nb content can lead to the formation of coarse carbonitrides, potentially reducing the toughness of the base material. Therefore, the Nb content is set to 0.030% or less. Lower Nb content improves the toughness of the weld heat-affected zone, so the Nb content can also be set to 0.020%, 0.010%, or 0.005% or less.

[0046] (Ti: 0~0.010%) Ti is not an essential element in this embodiment, therefore the lower limit of Ti content is 0%. However, when steel reaches high temperatures due to slab heating, Ti can sometimes cause grain refinement, so it can be included as needed. When Ti is included, in order to take advantage of this effect, the Ti content is preferably set to 0.001% or more. On the other hand, when Ti is present in excessive amounts, similar to Nb, there is a risk of Ti forming coarse carbonitrides, which could reduce the toughness of the base material. Therefore, the Ti content is set to 0.010% or less. Depending on the requirements, the Ti content can also be set to 0.005% or less, or 0.002% or less.

[0047] (Ca: 0~0.0030%) (Mg: 0~0.0030%) (REM: 0~0.0030%) The steel plate of this embodiment may contain one or more of Ca, Mg, and REM. Ca, Mg, and REM are not essential elements, therefore the lower limit for the content of Ca, Mg, and REM is 0%. Ca, by spheroidizing sulfides in steel, is an element that mitigates the effects of MnS, which reduces the toughness of steel. To achieve this effect, the Ca content can be set to 0.0001% or higher. On the other hand, a high Ca content may compromise the weldability of the steel; therefore, the Ca content is set to 0.0030% or less. Depending on the requirements, the Ca content may also be set to 0.0015%, 0.0010%, 0.0005%, or 0.0002% or less. 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 can be 0.0001% or more. On the other hand, when Mg and REM contain large amounts, coarse oxides may form, potentially reducing the toughness of the steel. Therefore, the Mg and REM contents are set to 0.0030% or less. Depending on the requirements, the Mg and REM contents can also be set to 0.015%, 0.010%, 0.005%, or 0.002% or less than 0.0015%, respectively. 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, Tb, and Lu. Compared to other additive elements, it is characterized by strong deoxidizing properties, forming stable oxides in steel.

[0048] (O: below 0.0040%) Oxygen (O) is an impurity element in steel. When present in large quantities, it forms oxides with strong deoxidizing agents such as Ca, Mg, REM, Al, and Ti, ranging in size from several μm to tens of μm. In cases containing coarse oxides or with high oxide density, these oxides can become the starting point for brittle fracture. Therefore, a lower O content is preferred. Thus, the lower limit for O content is 0%. In this embodiment, to improve the toughness of the weld, the O content is set to 0.0040% or less. Preferably, the O content is set to 0.0030% or less.

[0049] (Balance: Fe and impurities) In addition to the components described above, the steel plate of this embodiment contains Fe and impurities in the remainder. Here, "impurities" refers to components that may be introduced during the industrial manufacturing of steel plates due to various reasons related to raw materials such as ore or waste, or during manufacturing processes, and are permitted to the extent that they do not adversely affect the present invention.

[0050] The content of each element can be determined by the following methods.

[0051] Elements such as C, Si, Mn, P, S, Nb, V, Ni, Cu, Cr, Mo, Ti, Al, Ca, B, Mg, and REM are analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES), employing an ICP-based luminescence analyzer. For trace amounts of S, O, and N (at the ppm level), infrared absorption spectrometry and thermal conductivity methods using CS and ON analyzers can be applied. Other analytical methods, such as inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS), can also be used as needed.

[0052] The chemical composition of the steel plate in this embodiment remains almost unchanged during the manufacturing process. Therefore, if the chemical composition is determined by a sample taken from the molten steel in the tundish during the steelmaking stage, its chemical composition can be regarded as the chemical composition of the steel plate in this embodiment.

[0053] Furthermore, in this embodiment, the steel plate, while satisfying the above-mentioned chemical composition, preferably has a [fB] content of 0.0003% or more, as determined by equations (A) to (E) below. [fB] represents the amount of B dissolved in the steel. By setting [fB] to 0.0003% or more, the hardenability of the steel can be improved in high-tensile steels with a yield strength of 670-870 MPa and a tensile strength of 780-940 MPa. B readily forms nitrides in steel. Additionally, Ti and Al readily form nitrides and oxides. Therefore, the amount of B dissolved in the steel [fB] is determined by equations (A) to (E) below. [fB] can be 0.0005% or more, or 0.0015% or more. Alternatively, [fB] can be 0.0025% or less, or 0.0018% or less. [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, in equations (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the content (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively, including the amount of elements mixed in as impurities. Elements not present are substituted with 0. Additionally, 0 is substituted when the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%.

[0054] In addition, in the steel plate of this embodiment, besides the above-mentioned limitation on chemical composition (content of each element), the range of α value, β value and γ value is also limited as follows (the content of each element is limited in such a way that the α value, β value and γ value are within a specified range).

[0055] (α value: 1.00~1.50% by mass)

[0056] The value of α is represented by the following equation (1). α=[C]+6×[Si]+100×[P]…(1) Here, in equation (1), [C], [Si] and [P] are the contents (mass%) of C, Si and P in the steel, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted into 0.

[0057] In this embodiment, the α value is set to 1.50% by mass or less. This is a condition required to improve the toughness of the coarse-grained portion of the heat-affected zone after stress-relief annealing, and C, Si, and P need to be adjusted within this range. After stress-relief annealing, the concentration of P grain boundary segregation increases, thus making brittle fracture at the grain boundaries more likely, but this brittle fracture can be controlled by P, C, and Si. P is necessarily included in the steel in the process, and due to grain boundary segregation, the grain boundary strength is significantly reduced, making it a representative element causing SR embrittlement, with the highest coefficient. C and Si are elements that are necessarily present in steel, and if these elements increase, embrittlement caused by cementite generated at the grain boundaries will occur. It is preferable to reduce the amount of any one element, but sometimes a certain amount is included due to characteristics or specifications. To improve the toughness after stress-relief annealing, if necessary, the α value can also be set to 1.40% by mass or less.

[0058] On the other hand, the α value is 1.00% by mass or more. This lower limit (1.00% by mass) is determined by the component constraints in the application field, the limits 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 equation (1). The preferred lower limit value of α can be calculated from the preferred lower limit values ​​of the contents of C, Si, and P. The α value can exceed 1.10% by mass or be 1.30% by mass or more.

[0059] (β value: 10.00~15.00)

[0060] The value of β is calculated by the following equation (2). β = 0.65 × [C] 1 / 2 ×(1+0.64×[Si])×(1+4.10×[Mn])×(1+0.27×[Cu])×(1+0.52×[Ni])×(1+2.33×[Cr])×(1+3.14×[Mo])…(2) Here, in formula (2), [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted into 0.

[0061] In the steel sheet of this embodiment, the β value is set to a range of 10.00 to 15.00. The β value is an indicator of the hardenability of the steel sheet. The higher the β value, the more stably the formation of upper bainite structure, which leads to a deterioration in the balance between strength and toughness, can be avoided. On the other hand, when the β value is too high, the strength of the steel sheet increases, thereby resulting in a deterioration in toughness. That is, it also serves as an indicator of the target range of alloying element content required to improve the toughness of the weld state of the weld heat-affected zone.

[0062] The β value can be set to 11.00 or higher, depending on the needs. Alternatively, the β value can be set to 14.00 or lower.

[0063] (γ value: 0.70~1.50% by mass)

[0064] The value of γ is calculated by the following equation (3). γ=[Mn]+20×[Nb]+36×[Ti]…(3) Here, in equation (3), [Mn], [Nb], and [Ti] are the contents (mass%) of Mn, Nb, and Ti in the steel, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted into 0.

[0065] In the steel sheet of this embodiment, the γ value is set to a range of 0.70 to 1.50% by mass. Mn, Nb, and Ti are all elements that promote grain boundary embrittlement after stress-relief annealing. By keeping the γ value below 1.50% by mass, the decrease in toughness after stress-relief annealing can be suppressed. Several mechanisms are considered to promote grain boundary embrittlement by these elements, but the reduction in grain boundary strength caused by grain boundary segregation and the embrittlement caused by carbonitrides formed at grain boundaries are also considered.

[0066] On the other hand, in order to ensure a certain hardenability and obtain a microstructure with excellent strength and toughness balance, it is preferable to contain a certain amount of Mn, Nb, and Ti, so that the γ value is 0.70% by mass or more. The γ value can be 0.75% by mass or more, or it can be 1.40% by mass or less.

[0067] By satisfying the numerical ranges related to α, β, and γ values, it is possible to provide steels with excellent weld conditions and low-temperature toughness of the welded parts after stress-relief annealing.

[0068] In addition, in the steel plate of this embodiment, the carbon equivalent Ceq, which is an indicator of the hardening property of steel and is calculated by the following formula (4), is set to 0.550~0.620 mass. Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5…(4) Here, in formula (4), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] are the contents (mass%) of C, Mn, Cu, Ni, Cr, Mo, and V in the steel, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted with 0.

[0069] When Ceq is less than 0.550% by mass, the steel plate sometimes lacks sufficient strength. Therefore, Ceq is set to 0.550% by mass or higher. Depending on the requirements, Ceq can be set to 0.570% by mass or higher, or 0.600% by mass or higher. Conversely, when Ceq exceeds 0.620% by mass, the steel plate sometimes exhibits reduced toughness. Therefore, Ceq is set to 0.620% by mass or lower. Depending on the requirements, the upper limit of Ceq can be set to 0.600% by mass or lower.

[0070] In the steel plate of this embodiment, the chemical composition, Ceq, α value, β value, and γ value are within a specified range, and preferably [fB] is further controlled within a specified range.

[0071] Chemical composition, Ceq, fB, α value, β value, and γ value are controlled by the content of each element. However, when specific elements are removed or added during the smelting process, the amounts of other elements may also change. Therefore, it is not easy to change only a single element. In addition, sometimes multiple elements form compounds in molten steel and steel products. Therefore, if the addition of other components is not included in the target range, the desired effect may not be obtained.

[0072] (Degree of segregation)

[0073] Next, the segregation of Si, P, Cu and Ni in the steel plate will be explained.

[0074] In this embodiment, the segregation of the steel plate at the center of the plate thickness (within the range of ±5mm in the plate thickness direction including the t / 2 position) must satisfy all of the following equations (5) to (8). [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 equations (5) to (8), [Si], [P], [Cu], and [Ni] are the contents (mass%) of Si, P, Cu, and Ni in the steel, respectively, and also include the amount of elements mixed in as impurities. Elements that are not present are substituted with 0.

[0075] Furthermore, in equations (5) to (8), [Si]max, [P]max, [Cu]max, and [Ni]max are, respectively, the average concentration (content) values ​​of each element in the three regions within the cross-section of the steel plate along the thickness direction. These are: a primary region centered at t / 2 with a thickness of ±5 mm in the thickness direction and a rolling direction of 10 mm; a secondary region selected based on EPMA line analysis results of each element; and a tertiary region selected based on EPMA surface analysis results of each element, with a side of 20 μm where the concentration of each element is highest. Additionally, measurements are taken at the center position (1 / 2 width) and the position from the end of the plate width to 1 / 4 width, and the higher value is taken as the representative value for the steel plate.

[0076] The center of the steel plate has high hardenability due to the enrichment of alloying elements, resulting in a hard structure and reduced toughness. Furthermore, when impurity elements, represented by phosphorus, are enriched, the reduction in toughness after stress-relief annealing becomes significant. To ensure the toughness of the base material, all of the above equations (5) to (8) must be satisfied.

[0077] The segregation of each element is expressed, for example, in the case of Si, as [Si]max / [Si]. The segregation of each element can be determined by EPMA (Electron Probe Micro Analysis) according to the following steps (a), (b), (c), and (d).

[0078] (a) First, such as Figure 1 As shown, in a cross-section (TD plane) parallel to the rolling direction and the thickness direction of the steel plate 10, the positions in the width direction are set at 1 / 2 width and 1 / 4 width. Based on this, the metal structure observation specimen 11 is processed at the position including the t / 2 position. Then, as... Figure 2 As shown, in the TD plane appearing in the metal structure observation sample 11, a line analysis with a length of 10 mm was performed by EPMA at intervals of 50 μm in the thickness direction, centered at the t / 2 position, within a range 12 (primary region) of ±5 mm from the center of the thickness, to determine the mass percentage of Mn. Then, the thickness position range (region) 13 with the highest average Mn concentration after averaging the Mn concentration of each line analysis within ±0.5 mm in the thickness direction was identified. Within its thickness position range (region) 13, the range with the highest average Mn concentration within ±0.5 mm in the rolling direction was determined, as shown in the following 1 mm × 1 mm field of view region 14 (secondary region).

[0079] (b) That is, in the TD surface appearing in the metal structure observation sample 11, the center of the plate thickness direction of the region 13 with a high average Mn concentration is taken as the longitudinal center position, and the center of the rolling direction of the region with a high average Mn concentration within ±0.5 mm in the rolling direction in region 13 is taken as the transverse center, forming a 1 mm × 1 mm square field of view 14 as a secondary region. Then, in this field of view 14, surface analysis based on EPMA measurement is performed at 2 μm intervals in both the longitudinal and transverse directions (rolling direction and plate thickness direction) to measure the mass of Si, P, Cu, and Ni.

[0080] (c) Next, as Figure 3 As shown, within a 1mm × 1mm square field of view 14 (secondary region), a 20 × 20μm square portion 15 is scanned along the longitudinal and transverse directions (rolling direction and plate thickness direction). The average content (mass%) of Si, P, Cu, and Ni at each measurement point within the square portion 15 at each location is used to determine the average content of each component in that region. Then, the square portion 15 with the largest average content is designated as a tertiary region, and the average content of Si, P, Cu, and Ni in this tertiary region is set as its respective maximum value ([Si]max, [P]max, [Cu]max, [Ni]max). This third region is designated as the central segregation region. For example, Figure 4 As shown, when the average value of Si, determined by the mass percentage of Si at each measurement point within the square region 15-1, becomes the maximum value, the average value of Si content, determined by the mass percentage of Si at each measurement point within the cubic region 15-1, becomes the maximum value [Si]max. Similarly, if the average value of P, determined by the mass percentage of P at each measurement point within the square region 15-3, becomes the maximum value, the average value of P, determined by the mass percentage of P at each measurement point within the cubic region 15-3, becomes the maximum value [P]max. Likewise, if the average value of Cu, determined by the mass percentage of Cu at each measurement point within the square region 15-4, becomes the maximum value, the average value of Cu, determined by the mass percentage of Cu at each measurement point within the cubic region 15-4, becomes the maximum value [Cu]max. Similarly, if the average value of Ni, determined by the Ni content (mass%) at each measurement point within the square region 15-5, is the maximum value, then the average value of Ni, determined by the Ni content (mass%) at each measurement point within the square region 15-5 (cubic region), becomes the maximum value [Ni]max. Thus, a specific cubic region can also be defined for each element.

[0081] (d) The values ​​obtained by dividing the maximum values ​​of each element ([Si]max, [P]max, [Cu]max, [Ni]max) by the content of each element in the steel plate ([Si], [P], [Cu], [Ni]) are used as the segregation of each element in the center of the plate thickness.

[0082] The measurement conditions for EPMA analysis are as follows: using an EPMA apparatus with a LaB6 electron gun, the accelerating voltage is set to 15 kV, the irradiation current to 100 nA, the beam diameter to the same value as the measurement spacing, and the measurement time to 50 ms. For line and surface analyses with a 50 μm spacing, a shorter measurement time of 20 ms is also acceptable.

[0083] (Based on the proportion of the number of inclusion particles, more than 99% have an equivalent circle diameter of less than 4.0 μm.)

[0084] In this embodiment, when the steel plate contains inclusion particles with an equivalent circular diameter of 0.5 μm or more and containing 20% ​​by mass or more Ti, located within a rectangular region 4 mm wide with the t / 4 and t / 2 positions of the cross-section along the thickness direction as centers, at least 99% of these inclusion particles must have an equivalent circular diameter of 4.0 μm or less. In other words, in the particle size distribution of the equivalent circular diameter of the specified inclusion particles, the equivalent circular diameter corresponding to 99% of the cumulative distribution function must be 4.0 μm or less. When the equivalent circular diameter of at least 99% of the inclusion particles exceeds 4.0 μm in terms of number of particles, the decrease in toughness after stress-relief annealing becomes significant. The equivalent circular diameter of at least 99% of the inclusion particles can be 3.0 μm or less, or 2.5 μm or less.

[0085] The equivalent circle diameter of more than 99% of the inclusion particles, based on their number proportions, is determined by the following method.

[0086] For the equivalent circle diameter of inclusion particles that constitute 99% or more of the total number of inclusions, a rectangular region with sides of 4 mm was randomly selected relative to the observation surface of a mirror-finished steel plate. The center positions were t / 4 and t / 2 of the plate thickness in the thickness direction. This region was observed using a SEM equipped with an EDS analysis device, with an accelerating voltage of 3–30 kV. The field of view was scanned within this region, and inclusion particles with contrast different from the observed parent phase were extracted. The area was calculated through image analysis, and EDS point analysis was performed near the center of each particle. The equivalent circle diameter was calculated as √(4A / π) when the area of ​​the inclusion particle was set to A. Among the inclusion particles, those with a specific equivalent circle diameter of 0.5 μm or more and containing 20% ​​by mass or more Ti were identified. The equivalent circle diameter corresponding to 99% of the cumulative distribution function was calculated from the particle size distribution of the equivalent circle diameter of these inclusion particles.

[0087] (Yield strength: 670~870MPa) (Tensile strength: 780~940MPa)

[0088] In this embodiment, the yield strength of the steel plate is set to 670-870 MPa, and the tensile strength is set to 780-940 MPa. To reduce the weight of large welded structures such as transport tanks for liquefied CO2, steel plates that can ensure structural strength even with relatively thin plates are required. Typically, steel plates with the aforementioned yield strength and tensile strength are selected for use in such applications; therefore, this embodiment is also manufactured with the aforementioned yield strength and tensile strength. The yield strength can be set to 690 MPa or more, or 830 MPa or less, depending on the requirements. The tensile strength can be set to 800 MPa or more, or 900 MPa or less.

[0089] (Average hardness and standard deviation)

[0090] In this embodiment, the hardness distribution measurement of the steel plate at the t / 4 position with a spacing of 0.5mm×0.5mm and 0.05mm requires an average hardness of 265Hv~290Hv and a standard deviation of less than 20 at the measurement positions of 121 points.

[0091] When the hardness distribution is uneven, the toughness of the base material may deteriorate. When the average hardness is less than 265 Hv and the standard deviation exceeds 20, the toughness of the base material cannot be guaranteed.

[0092] On the other hand, when the average value exceeds 290 Hv, the strength becomes too high, which may lead to a decrease in toughness.

[0093] The microstructure of the steel plate in this embodiment is preferably a mixed microstructure of martensite and lower bainite, which provides an excellent balance between strength and toughness. A microstructure primarily containing martensite and lower bainite is considered to exist when the standard deviation and average hardness are within the aforementioned ranges. In cases where upper bainite is present, it is desirable that the average hardness is less than 265 Hv or the standard deviation exceeds 20. Upper bainite can sometimes form due to localized deviations in γ-grain size and microsegregation, leading to localized reductions in hardenability.

[0094] The standard deviation (distribution) of hardness was determined using microscopic samples collected from the TD (dimension tolerance) of the steel plate as the observation surface, and measured using a micro Vickers hardness tester. The measurement area was a 0.5 mm × 0.5 mm range centered at any t / 4 position within the microscopic observation surface, with a measurement interval of 0.05 mm and a measurement load of 25 gf. A total of 121 measurements were taken, with 11 points vertically and 11 points horizontally. The average value and standard deviation were calculated from the obtained measurement values.

[0095] (Maximum hardness HVmax of the central segregation zone: below 400 HV)

[0096] The harder the base material, the lower the toughness after stress-relief annealing. To maintain the toughness after stress-relief annealing, the highest hardness at the measurement location of segregation [M]max / [M] needs to be below 400 HV. That is, within the aforementioned three regions (central segregation zone), optical microscopy observations are taken at five random fields of view, and the hardness at the center of each field of view is measured by a Vickers hardness test with a load of 100g. The maximum hardness is set as the maximum hardness HVmax of the central segregation zone. For Si, P, Cu, and Ni, under different conditions in the three regions, the highest hardness in the Ni three regions (measurement location of segregation [Ni]max / [Ni]) is set below 400 HV.

[0097] (Plate thickness: 25~60mm)

[0098] When welding steel plates with a thickness of less than 25 mm, stress-relief annealing is usually not required. However, the steel plate in this embodiment is designed for steel plates that require stress-relief annealing, therefore the plate thickness is set to 25 mm or more.

[0099] On the other hand, steel plates with a thickness exceeding 60mm increase the weight of the transport tank, which is not preferable. Therefore, the steel plate thickness in this embodiment is 60mm or less.

[0100] Furthermore, the steel plate of this embodiment may also possess the characteristics described below.

[0101] (organize)

[0102] In this embodiment, the steel plate preferably has a mixed microstructure of martensite and lower bainite at the t / 4 position of the cross-section in the thickness direction. The total area percentage of martensite and lower bainite can be 85.0% or more. In this case, the standard deviation and average hardness of the aforementioned hardness are easily satisfied.

[0103] The area ratios of martensite and lower bainite were determined by the following method.

[0104] The area ratio of each metallic structure was determined using a plane parallel to the L-section of the steel plate, i.e., the rolling direction RD of the steel plate, and perpendicular to the plate surface. The area of ​​the observation field was set to 40000 μm. 2 The above. For example, an area of ​​50,000 μm with a dimension of 250 μm along the surface of the steel plate and a dimension of 200 μm along the thickness direction of the steel plate. 2 It has a rectangular shape.

[0105] The metallic microstructure was revealed by etching with nitric acid and ethanol. The area fraction of the metallic microstructure in each observation area was obtained by microscopic observation and microstructural determination based on the following criteria.

[0106] Upper bainite is a microstructure consisting of one or more blocky, acicular, or amorphous (primarily with curved grain boundaries and blocky) aggregates of cementite and austenite-martensite observed against a clear black contrast within ferrite-based grains viewed against a white background. The cementite and austenite-martensite aggregates observed in upper bainite sometimes form rows along ferrite laths, but sometimes are observed in a random arrangement. The lath structure of upper bainite is characterized by multiple parallel laths with a lath width of 1.0 μm or more, and the spacing and orientation of the laths are almost irregular. Old austenite grain boundaries may be difficult to distinguish due to indistinct contrast or a jagged, uneven morphology. For example... Figure 5 The tissue photograph on the left is a tissue photograph of upper bainite.

[0107] A mixed martensite and lower bainite structure is a microstructure in which fine laths with clear black contrast are observed within the grains of the parent phase when viewed against a white background. In this embodiment, the laths are 1.0 to 3.0 μm wide, and multiple cementites with black contrast are observed within and at the lath boundaries. However, the cementite observed in tempered martensite is fine and has a high precipitation density, so in an optical microscope, numerous white granular contrasts and cloud-like contrasts formed by the aggregation of fine particles are sometimes observed. Linear black contrasts can be clearly observed at the old austenite grain boundaries, but they cannot be distinguished from the laths within the grains, making them difficult to identify.

[0108] For example Figure 5 The tissue photograph on the right is a tissue photograph of a mixture of martensite and lower bainite.

[0109] (Average grain size of the steel plate at the center of the plate thickness: below 15.0 μm)

[0110] In this embodiment, to ensure the specified toughness, the average grain size at the center of the steel plate thickness is set to 15.0 μm or less. To improve the toughness of the base material before and after stress-relief annealing, the upper limit of the average grain size can also be set to 14.5 μm or less, or 14.0 μm or less, as needed. Preferably, the average grain size at the center of the steel plate thickness is small, so it is not necessary to specify its lower limit. Typically, the average grain size is approximately 10.0 μm in the minimum case.

[0111] The average crystal grain size was determined by the following method.

[0112] To observe the L-section, a 10-30 mm sample was cut from the steel plate along its length in the direction of 10-20 × plate thickness. The sample was then ground using colloidal silica. For the 500 × 500 μm range of the target cross-section, crystal orientation was determined using electron backscatter diffraction (EBSD) pattern analysis in a 1.0 μm step using an EBSD apparatus (TSL or Ametek-EDAX). The accelerating voltage was 10-30 kV.

[0113] The crystal orientation obtained by analyzing the crystal orientation using the crystal orientation analysis software (TSL OIM Analysis 7 x64) defines the region surrounded by grain boundaries with a crystal orientation difference of 15° or more as a grain, the equivalent circular grain size of the grain as the crystal grain size, and the average crystal grain size is calculated by the area-weighted average of each grain area.

[0114] In crystal orientation analysis software, the orientation data of crystals is recorded in each pixel of a hexagon created on the observation surface. Therefore, there are adjacent pixels in each of these pixels, and the crystal orientation difference (degree) is defined at the boundary (one side of the hexagon) between adjacent pixels.

[0115] When the crystal orientation difference between adjacent pixels is greater than 15° (the 15° boundary is defined below), the pixel boundary is maintained as a potential grain boundary in the mapping, as it may be equivalent to a grain boundary. This orientation difference is calculated for all pixel boundaries, and when adjacent potential grain boundaries are continuous, they are connected to form a closed curve (broken line) within the observation (data acquisition) area. The region enclosed by this closed curve is defined as a grain.

[0116] When a 15° boundary does not form a closed curve but has an end within the observation region, it is not considered a grain boundary (but is ignored as a subgrain boundary). Furthermore, when a 15° boundary intersects with an edge of the observation region, it means that the grain also extends outwards from the observation region; this region does not reflect the area of ​​a single grain and is therefore ignored as a half-region. Thus, within the observation region, only regions completely closed by 15° boundaries are considered equivalent to grains, and the area of ​​the grain is calculated by multiplying its number by the area of ​​a hexagon with a stride of 1 pixel. Finally, the diameter of a circle with the same area for each closed region is determined and defined as the diameter of a grain.

[0117] Even when a 15° boundary forms a closed curve within the observation area, if the number of pixels within it is less than one, it is considered noise and is not used for particle size calculation.

[0118] (The Charpy absorption energy at -45℃ after stress-relief annealing is over 40J)

[0119] The steel plate of this embodiment aims to prevent damage by performing stress-relief annealing on the welded parts after assembly into the transport tank. However, during this process, not only the welded parts but also the base material is heated. When the base material is heated, its toughness tends to decrease. The reason is not yet clear, but it is speculated that phosphorus diffuses at grain boundaries, and that inclusions grow or agglomerate within the microstructure, thereby reducing brittleness and toughness. Therefore, the steel plate of this embodiment preferably has a Charpy absorption energy of 40 J or more at -45°C after stress-relief annealing. This further improves safety.

[0120] The Charpy energy absorbed at -45℃ after stress-relief annealing was measured at the stress-relief annealed area under the condition that the steel plate was subjected to stress-relief annealing with a holding temperature of 600~620℃, a holding time of 2.0~2.8 hours, and a cooling rate of 55~100℃ / h (℃ / hour) above 425℃.

[0121] (The δ value of the CTOD test at -35℃ is above 0.05mm)

[0122] The steel plate of this embodiment can achieve excellent toughness through the above-described characteristics, but from the viewpoint of ensuring the safety of the transport tank made of the steel plate of this embodiment, it is preferable that the δ value of the CTOD test at -35°C is 0.05 mm or more.

[0123] (Charpy absorbs more than 50J of energy at -70℃)

[0124] Furthermore, the steel plate of this embodiment preferably has a Charpy absorption energy of 50 J or more at -70°C. This ensures the safety of transport tanks containing the steel plate of this embodiment or composed of the steel plate of this embodiment.

[0125] The Charpy absorption energy at -70°C is a value measured at position t / 4. Furthermore, in this embodiment, a Charpy absorption energy of 50 J or more means that the minimum value is 50 J or more when measured at three different positions at position t / 4.

[0126] (The yield strength after stress-relief annealing is 670~870MPa, and the tensile strength is 780~940MPa.)

[0127] In this embodiment, the steel plate, after stress-relief annealing, preferably has a yield strength of 670-870 MPa and a tensile strength of 780-940 MPa. Under these conditions, sufficient strength can be ensured in transport tanks for liquefied CO2 that have undergone stress-relief annealing.

[0128] <Containers for liquefied CO2>

[0129] The liquefied CO2 container of this embodiment is formed by processing and welding the steel plate of this embodiment. Therefore, the liquefied CO2 container of this embodiment includes the steel plate of this embodiment. In essence, it may also consist of the steel plate (base material) of this embodiment and the welded part formed by melting and then solidifying the steel plate and welding material of this embodiment.

[0130] <Manufacturing Method>

[0131] Next, the manufacturing method of the steel plate and the container for liquefied CO2 according to this embodiment will be described. The steel plate of this embodiment can achieve its effect by having the above-mentioned characteristics regardless of the manufacturing method, but it can be manufactured stably according to the manufacturing method shown below.

[0132] To manufacture steel plates from steel with the aforementioned composition, steel produced by a converter or electric furnace process and then smelted in a secondary smelting facility is continuously cast or ingot-cast to produce slabs. The slabs are then preferably heated to approximately 950–1250°C in a slab heating furnace and hot-rolled to a specified thickness to produce steel plates. These steel plates are then quenched and tempered to obtain steel plates (final steel plates) with specified properties.

[0133] The optimal conditions for each process are explained.

[0134] (Secondary smelting process)

[0135] The secondary smelting process can be carried out using known methods. The steel plate in this embodiment needs to have its phosphorus content reduced to below 0.006%. In conventional dephosphorization methods, it is sometimes impossible to reduce the phosphorus content to below 0.006%, but in such cases, this can be addressed by extending the dephosphorization treatment time.

[0136] (Casting process)

[0137] In the casting process, a slab is obtained. At this point, in order to control the equivalent circular diameter of more than 99% of the inclusion particles to below 4.0 μm, the following measures are taken: buoyancy separation of inclusions in the tundish; buoyancy separation of inclusions caused by electromagnetic braking of the molten steel flow during the initial solidification stage in the mold; and reduction of center segregation caused by light reduction during casting. Light reduction can be, for example, a reduction rate of 3% or less, 2% or less, or 1% or less. Light reduction is preferably performed at the end of solidification.

[0138] (Hot rolling process)

[0139] Next, the hot rolling process will be explained.

[0140] When the pre-rolling heating temperature exceeds 1250°C, it leads to coarsening of the average crystal grain size. Therefore, the pre-rolling heating temperature is preferably set to 1250°C or lower. Furthermore, when the pre-rolling heating temperature is below 950°C, rolling becomes low-temperature rolling, resulting in a smaller reduction per pass and insufficient reduction near the center of the plate thickness. Therefore, the pre-rolling heating temperature is preferably set to 950°C or higher.

[0141] During rolling, it is preferable to set the cumulative reduction rate to 50% or more within the rolling temperature range of 1150~900℃. When the steel plate is immediately subjected to direct quenching with water cooling after hot rolling, it is preferable to set the cumulative reduction rate to 50% or more within the rolling temperature range of 1150~900℃. The upper limit of the cumulative reduction rate does not need to be specifically specified; for example, it can be 80% or less, or 70% or less.

[0142] (Quenching process)

[0143] The process involves either direct quenching after hot rolling or reheat quenching, where the steel plate is temporarily cooled and then reheated after hot rolling. In the case of both, reheat quenching is performed after direct quenching. Direct quenching is water-cooling quenching immediately after hot rolling. Reheat quenching is quenching where the steel plate is temporarily cooled and then reheated after hot rolling.

[0144] The conditions for direct quenching are to set the cooling start temperature above Ar3 and water cool to below 300°C. The average cooling rate during water cooling is preferably set to 5°C / second or higher. There is no particular upper limit to the average cooling rate; for example, it can be below 50°C / second, below 20°C / second, or below 15°C / second.

[0145] The conditions for reheat quenching are as follows.

[0146] The heating temperature during quenching (i.e., the quenching temperature) is preferably below 925°C, but can be below 920°C, 915°C, or 910°C. This is because the microstructure of thick steel plates is sometimes not sufficiently refined after rolling. This is because when the quenching temperature exceeds 925°C for steel plates with insufficiently refined microstructure, the inverse phase transformation γ-structure formed during heating becomes coarse, and the average grain size of the final microstructure after the γ / α phase transformation also becomes coarse upon subsequent cooling. On the other hand, temperatures slightly above the Ac3 point (e.g., above Ac3 point and below Ac3 point + 20°C) result in deviations in the γ-structure grain size of the inverse phase transformation, insufficient solid solution of carbides containing boron, and sometimes insufficient hardenability, which is therefore undesirable. Therefore, the lower limit of the quenching temperature is preferably 880°C or higher, and more preferably 890°C or higher.

[0147] Point Ar3 can be obtained using the following method. Ar3 (℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo+0.35×(t-8)…(A) In equation (A) above, 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 hot-rolled steel plate. Additionally, the steel sheet temperature refers to the surface temperature of the steel sheet.

[0148] (Tempering process)

[0149] In the steel plate manufacturing method of this embodiment, the steel plate after the quenching process is further tempered. The tempering heating temperature (i.e., tempering temperature) is set to 660°C or below. When the tempering temperature exceeds 660°C, the tempering effect becomes excessive, making it difficult to ensure yield strength and tensile strength.

[0150] On the other hand, if the tempering temperature is too low, the tempering becomes insufficient, making it difficult to ensure yield strength and tensile strength. Therefore, the tempering temperature is 500°C or higher. Preferably, the tempering temperature is 600°C or higher.

[0151] When cooling is performed after reheating and quenching or tempering, to prevent the reduction in toughness of the base material caused by temper embrittlement, it is preferable to cool the steel plate by water cooling (implementing accelerated cooling) rather than air cooling. In this case, the average cooling rate up to 300°C is preferably set to 0.1°C / second or more, or 0.5°C / second or more. The average cooling rate can be 5°C / second or more.

[0152] The steel plate of this embodiment is suitable for use as a liquefied CO2 transport tank (a container for liquefied CO2). For example, it can be used as a transport tank mounted on a ship. When transporting CO2 by ship, liquefied CO2 is filled into a transport tank mounted on the ship and transported, but in order to prevent the CO2 inside the transport tank from solidifying, it is preferable to transport it under a pressure of about 2 MPa. In addition, in order to keep the CO2 in a liquid state at a pressure of about 2 MPa, it is preferable to keep the CO2 at about -35°C. The steel plate of this embodiment is suitable for such applications.

[0153] The liquefied CO2 container of this embodiment is formed by cutting and processing the steel plate of this embodiment into a specified shape, and then joining multiple steel plates together by welding. Alternatively, stress-relief annealing can be performed after welding.

[0154] During welding, the welding heat input is 1.1~4.5kJ / mm, which can be carried out under normal conditions.

[0155] Stress-relief annealing can be performed in accordance with the content specified in JIS Z 3700:2009 "Post-weld heat treatment method".

[0156] Example

[0157] Next, embodiments of the present invention will be described. However, the conditions in the embodiments are merely examples used to confirm the feasibility and effects of implementing the present invention, and the present invention is not limited to these single examples. Various conditions can be used to achieve the purpose of the present invention as long as they do not depart from its spirit.

[0158] After the blast furnace treatment, the molten iron is tapped into a ladle for desulfurization and other pretreatments. Then, the molten iron is inserted into a converter to adjust the composition of the molten steel.

[0159] The molten steel is cast by continuous casting to obtain a slab with the chemical composition shown in Tables 1A to 3B. In the casting process, inclusions in the tundish are floated and separated, and inclusions are floated and separated by electromagnetic braking of the molten steel flow during the initial solidification of the mold. In addition, except in some examples, the center segregation caused by light reduction during casting (reduction rate: 1% or less) is reduced.

[0160] Then, the slab is heated in a heating furnace to the heating temperatures shown in Tables 4A and 4B, and then hot-rolled to the specified thickness to produce a steel plate.

[0161] Then, the steel plate is quenched and tempered to obtain a steel plate (final steel plate) with specified properties.

[0162] Tables 4A and 4B show the presence or absence of light reduction during casting, the heating temperature before rolling, the cumulative reduction rate of hot rolling at 1150~900℃, the plate thickness after rolling, the conditions for direct quenching (cooling start temperature, cooling end temperature, average cooling rate), the conditions for reheat quenching (quenching temperature), and the conditions for tempering (tempering temperature).

[0163] Cooling after reheating, quenching, and tempering is carried out by water cooling, with the average cooling rate to 300℃ set to be above 0.1℃ / second.

[0164] Tables 1A to 3B show the chemical composition, α, β, γ, fB, and carbon equivalent (Ceq) of the steel plates. Tables 5A to 6B show the average hardness (mean Hv), standard deviation, maximum hardness HVmax, martensite and lower bainite fractions (%), average grain size (EBSD grain size), yield strength (MPa), tensile strength (MPa), yield ratio, Charpy absorbed energy (J) at -70°C at t / 4 and t / 2 positions, and δ values ​​(mm) from the CTOD test at -35°C. In addition, Tables 5A and 5B show whether the segregation ([Si]max / [Si], [P]max / [P], [Cu]max / [Cu], [Ni]max / [Ni]) at the center of the plate thickness meets the specified range, and whether the equivalent circle diameter of inclusion particles with a number ratio of more than 99% is less than 4.0 μm ("0" if less than 4.0 μm, "×" if more than 4.0 μm).

[0165] The methods for determining the segregation degree ([Si]max / [Si], [P]max / [P], [Cu]max / [Cu], [Ni]max / [Ni]) and the method for determining the average crystal grain size in the center of the plate thickness are as described above.

[0166] Tensile testing was conducted according to JIS Z 2241:2023, using two test specimens. The yield strength (0.2% yield strength) and tensile strength are the average values ​​of the two specimens. The yield ratio is the ratio of yield strength YS to tensile strength TS, expressed as a percentage, i.e., 100 × (YS / TS). The unit of yield ratio is %.

[0167] Hardness testing involves collecting microscopic samples from a surface parallel to the rolling direction of the steel, and then using a micro Vickers hardness tester. The testing area is a 0.5 mm × 0.5 mm region centered at any t / 4 position within the microscopic observation surface. The measurement interval is 0.05 mm, the testing load is 25 gf, and a total of 121 points are measured (11 points vertically × 11 points horizontally). The average value and standard deviation are calculated from the obtained measurement values.

[0168] In addition, when measuring the segregation degree in the center of the plate thickness, optical microscopy observations are randomly performed within a specific three-dimensional region, for example, taking five fields of view. The hardness of the center of each field of view is measured by a Vickers hardness test with a load of 100g, and the maximum hardness is taken as the maximum hardness HVmax of the central segregated part.

[0169] Regarding the microstructure fractions of martensite and lower bainite, the martensite and lower bainite microstructures were identified by SEM observation in the same cross section as the hardness distribution measurement, and the total area fraction (microstructure fraction) was calculated. The method for determining the microstructure fraction is as described above.

[0170] In addition, the welded joint was fabricated and evaluated. A K-groove was fabricated using argon gas containing 20% ​​CO2 as the shielding gas and YM-69F welding wire manufactured by NIPPON STEEL WELDING & ENGINEERING CO.,LTD. as the welding wire. The heat input was set to 2.0 kJ / mm, and the preheating temperature was set to 100°C. Multi-layer gas-shielded arc welding was performed to fabricate the welded joint. Then, stress-relief annealing (SR) was performed on the base metal and the weld. For stress-relief annealing, the holding temperature was set to 600–620°C, the holding time was set to 2.0–2.8 hours (2 hours 48 minutes), and the cooling rate in the temperature range above 425°C was set to 55–100°C / h.

[0171] Tables 6A and 6B show the yield strength and tensile strength of the base material after SR.

[0172] In addition, 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.

[0173] Furthermore, Tables 7A and 7B show the Charpy absorbed energy at -70°C and the δ value of the CTOD test at -35°C for the surface layer (I side FL) of the base material on the surface of the weld adjacent to the weld before SR and at position t / 2.

[0174] Furthermore, Tables 7A and 7B show the Charpy absorbed energy at -45°C and the δ value of the CTOD test at -35°C for the surface layer (I side FL) of the base material on the side of the weld adjacent to the weld after SR and at position t / 2.

[0175] Charpy energy absorption of the base material and weld: Three V-notch test pieces were collected from both the base material and the weld, and Charpy impact tests were conducted at a specified temperature to determine the absorbed energy (vE). -65 The V-notch test piece was prepared according to the V-notch test piece described in JIS Z 2242:2023. Additionally, the Charpy impact test was performed according to JIS Z 2242:2023. The Charpy absorbed energy at -70°C was recorded as the minimum value measured at three different measurement locations at t / 4.

[0176] Regarding the σ value of the CTOD test, (δc at-10℃ The measurements were performed according to BS7448 Part 1 (1991) and BS7448 Part 2 (1997). Specifically, gas-shielded arc welding was performed on the butt joint of steel plates with a K-groove, with a heat input of 35 kJ / mm. The plate was processed such that the tip of the fatigue crack of the CTOD test piece of the welded part became the central part of the fusion line on the I side of the welded part. The CTOD test was performed at a specified temperature. For the base material, the C direction (plate width direction) perpendicular to the rolling direction of the test piece was evaluated. For the welded joint, only the L direction (rolling direction) was evaluated. In the evaluation of the CTOD of the welded joint, the test piece was collected with the tip of the fatigue crack corresponding to the welded joint. Three tests were performed at each test temperature, and the lowest value of the measured data was taken as the δ value of the CTOD test. The CTOD units shown in Tables 6A to 7B are in mm.

[0177] The following conditions are considered acceptable: the minimum Charpy absorption energy of the base material at -70℃ is 50J or more; the CTOD of the base material at -35℃ is 0.05mm or more; and the CTOD of the joints before and after SR at -35℃ is 0.05mm or more.

[0178] As shown in Tables 1A to 7B, examples No. 1 to 14 of this invention exhibit excellent low-temperature toughness in the base material both before and after the SR treatment. In particular, the δ value of the CTOD test at -35°C at the t / 4 position after SR is 0.05 mm or higher, demonstrating excellent resistance to brittle fracture at low temperatures even after SR treatment. Furthermore, the yield strength after SR treatment is 670–870 MPa, and the tensile strength is 780–940 MPa, showing favorable values. Moreover, for examples No. 1 to 14, the δ value of the weld joint at -35°C before and after SR is 0.05 mm or higher, demonstrating excellent resistance to brittle fracture at low temperatures.

[0179] On the other hand, as shown in Tables 1A to 7B, the chemical composition of steels No. 15 to 44 and 58, which are comparative examples, deviates from the range specified in this invention, resulting in poor low-temperature toughness. That is, the minimum Charpy absorption energy at -70°C of the base material is less than 50 J, or the CTOD of the base material at -35°C is less than 0.05 mm, or the CTOD at -35°C before or after the SR of the weld joint is less than 0.05 mm.

[0180] Furthermore, while the steel compositions of Nos. 45 to 57 meet the composition range of this invention, their low-temperature toughness deteriorates because the manufacturing conditions do not meet the preferred manufacturing conditions. Specifically, the minimum Charpy absorption energy at -70°C of the base material is less than 50 J, or the CTOD of the base material at -35°C is less than 0.05 mm, or the CTOD at -35°C before or after the SR of the weld joint is less than 0.05 mm.

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[0195] Industrial availability According to the present invention, a steel plate for a liquefied CO2 transport tank and a container for liquefied CO2 are provided, exhibiting excellent strength, low-temperature toughness, and low-temperature toughness after stress-relief annealing when the steel plate is manufactured into a welded joint. Therefore, it has high potential for industrial application.

[0196] Explanation of reference numerals in the attached figures 10 steel plates 11. Specimens for observing metallic structures

Claims

1. A steel plate, which is used for liquefied CO2 transport tanks, wherein, Chemical composition is expressed as a percentage by mass. C：0.070~0.110%、 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：0.030~0.100%、 B：0.0005~0.0030%、 N：0.0015~0.0050%、 P: below 0.006% S: Below 0.0030% 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: Below 0.0040% Balance: Fe and impurities, The value of α, as defined by equation (1) below, is 1.00~1.50% by mass. The β value defined by equation (2) below is 10.00~15.

00. The value of γ, as defined by equation (3) below, is 0.70~1.50% by mass. Ceq, as defined by equation (4) below, is 0.550~0.620% by mass. The yield strength is 670~870MPa. The tensile strength is 780~940MPa. When the plate thickness is defined as t, t is 25~60mm. In the hardness distribution measurement at the t / 4 position with spacings of 0.5mm × 0.5mm and 0.05mm, the average hardness at the measurement location of point 121 was 265Hv~290Hv, with a standard deviation of less than 20. The maximum hardness HVmax of the central segregation region is below 400 HV. The segregation at the center of the plate thickness fully satisfies the following equations (5) to (8). Within a rectangular region centered at positions t / 4 and t / 2 along the thickness direction of the plate, with one side measuring 4 mm, the inclusion particles having an equivalent circle diameter of 0.5 μm or more and containing 20% ​​by mass of Ti or more, and 99% or more of the inclusion particles having an equivalent circle diameter of 4.0 μm or less, the inclusion particles in this region have an equivalent circle diameter of 4.0 μm or less. The region surrounded by grain boundaries with a crystal orientation difference of 15° or more, as determined by crystal orientation analysis using electron beam backscatter diffraction pattern analysis, is defined as a grain. The equivalent spherical diameter of the grain is defined as the grain size. When the average grain size is defined as the area-weighted average calculated by weighting the area of ​​each grain, the average grain size at the center of the thickness of the steel plate 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+3.33×[Cr])×(1+3.44×[2] Mo 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 equations (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. This also includes the amount of elements mixed in as impurities. Elements not present are substituted with 0. In addition, [Si]max, [P]max, [Cu]max, and [Ni]max in equations (5) to (8) are respectively the average concentration values ​​of each element in the three regions of the steel plate in the thickness direction. The first region is a rectangular region with a thickness of ±5 mm and a rolling direction of 10 mm, centered at position t / 2. The second region is a rectangular region with a side of 1 mm where the concentration of each element is the largest, selected based on the EPMA line analysis results of each element. The third region is a rectangular region with a side of 20 μm where the concentration of each element is the largest, selected based on the EPMA surface analysis results of each element. The third region is set as the central segregation part.

2. The steel sheet according to claim 1, wherein, The [fB] obtained by equations (A) to (E) below is greater than 0.0003%: [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, in equations (A) to (E), [B], [N], [Ti], [Al], [O], [Ca], [Mg], and [REM] represent the contents (mass%) of B, N, Ti, Al, O, Ca, Mg, and REM, respectively. They also include the amount of elements mixed in as impurities. Elements that are not present are substituted with 0. In addition, if the calculated values ​​of [fN], [fTi], [fAl], and [fO] are less than 0%, they are substituted with 0.

3. The steel sheet according to claim 1 or 2, wherein, When the steel plate is subjected to stress-relief annealing with a holding temperature of 600~620℃, a holding time of 2.0~2.8 hours, and a cooling rate of 55~100℃ / h above 425℃, the yield strength of the stress-relief annealed portion is 670~870MPa, the tensile strength is 780~940MPa, and the Charpy absorption energy at -45℃ is above 40J.

4. A container for liquefying CO2, comprising the steel plate as described in claim 1 or 2.

5. A container for liquefying CO2, comprising the steel plate of claim 3.

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