Method for manufacturing welded joint, welded joint, method for designing low-temperature liquefied gas tank, and low-temperature liquefied gas tank

Abstract

A method for manufacturing a welded joint includes: a thermal stress calculation step for calculating thermal stress generated in the welded joint due to the difference between the linear expansion coefficient of a weld metal and the linear expansion coefficient of a base material at the temperature at which a low-temperature liquid gas tank is used; a necessary CTOD value calculation step for obtaining a CTOD value necessary to prevent brittle fracture of the low-temperature liquid gas tank during use of the low-temperature liquid gas tank by taking into account the influence of thermal stress while assuming that the welded joint has defects; a CTOD value measurement step for measuring the CTOD value of the heat-affected zone of the welded joint at the use temperature of the low-temperature liquid gas tank; and a welding step in which the base material is welded using, as the base material, a thick steel plate in which the CTOD value of the heat-affected zone has been confirmed to be equal to or greater than the necessary CTOD value.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a welded joint, a welded joint, a design method for a cryogenic liquefied gas tank, and a cryogenic liquefied gas tank. This application claims priority based on patent application No. 2022-099498 filed in Japan on June 21, 2022, the contents of which are incorporated herein by reference. Background Art

[0002] Since cryogenic liquefied gas is processed at ultra-low temperatures, when the inner tank of the tank storing the cryogenic liquefied gas and the pipe for transferring the gas are cooled, stress (thermal stress) caused by the difference in linear expansion coefficient between the base material and the weld metal constituting the gas may occur. This thermal stress may cause brittle fracture.

[0003] Patent Document 1 discloses a piping structure for cryogenic liquefied gas as a technique for reducing thermal stress, characterized in that dissimilar materials of a piping made of austenitic stainless steel and a piping made of Invar alloy are joined by welding using a welding material whose linear expansion coefficients satisfy a predetermined relationship. Patent Document 1 exemplifies a welding material containing 55% or more of Ni.

[0004] Prior Art Literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent No. 4578084

[0007] Non-patent literature

[0008] Non-patent document 1: Okawa et al.: Characteristics of 7% Ni-TMCP steel and its applicability to large LNG tanks, Pressure Technology, Vol. 57, No. 4, pp. 221-230, 2019 Summary of the invention

[0009] The inner tank of an LNG (Liquefied Natural Gas) tank, which is a type of cryogenic liquefied gas tank, is made of, for example, Ni steel that has excellent toughness at ultra-low temperatures, such as 9% Ni steel, 7% Ni steel, etc., and Ni-based welding materials are used for welding of Ni steel. However, Ni-based welding materials are expensive because they contain a large amount of Ni. Therefore, it is required to reduce the cost of welding materials and thus LNG tanks. As a method of reducing costs, it is possible to consider reducing the content of Ni contained in the welding material, but such welding materials tend to have a higher linear expansion coefficient than the base material of the LNG tank and the previous Ni-based welding materials. In an LNG tank, if the difference in linear expansion coefficient between the base material and the weld metal becomes larger, thermal stress can be expected to be generated. Such thermal stress was not envisioned in the design of previous welded structures, and acts additionally as a driving force for brittle fracture. Therefore, there is a demand for welded joints and welded structures that can prevent brittle fracture even when using welding materials with a large linear expansion coefficient.

[0010] The present invention has been completed in view of the above situation, and an object of the present invention is to provide a method for manufacturing a welded joint, a welded joint, a method for designing a cryogenic liquefied gas tank, and a cryogenic liquefied gas tank that can suppress brittle fracture from the weld portion even when a low-cost welding material with a large linear expansion coefficient is used in the manufacture of an inner tank of a tank in contact with cryogenic liquefied gas.

[0011] The gist of the present invention is as follows.

[0012] [1] A method for manufacturing a welded joint according to one embodiment of the present invention is a method for manufacturing a welded joint for use in an inner tank of a cryogenic liquefied gas tank, wherein the linear expansion coefficient of the weld metal is larger than the linear expansion coefficient of the base material, comprising:

[0013] a thermal stress calculation step in which a thermal stress generated in the weld joint due to a difference between a linear expansion coefficient of the weld metal and a linear expansion coefficient of the base material at a use temperature of the cryogenic liquefied gas tank is calculated;

[0014] a necessary CTOD value calculation step, in which the CTOD value necessary for preventing the cryogenic liquefied gas tank from brittle fracture during operation of the cryogenic liquefied gas tank is calculated by assuming that the weld joint has defects and taking into account the influence of the thermal stress;

[0015] a CTOD value measuring step, in which the CTOD value of the heat-affected zone of the weld joint at the use temperature of the cryogenic liquefied gas tank is measured; and

[0016] A welding step in which a thick steel plate in which the CTOD value of the heat-affected zone is confirmed to be equal to or greater than the required CTOD value is used as the base material, and the base material is welded.

[0017] [2] Another embodiment of the present invention relates to a welded joint, in which the linear expansion coefficient of the weld metal is larger than the linear expansion coefficient of the base material and the welded joint is used for the inner tank of a cryogenic liquefied gas tank, and the CTOD value at the operating temperature of the cryogenic liquefied gas tank is greater than the required CTOD value after taking into account the influence of thermal stress caused by the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material.

[0018] [3] According to the weld joint described in [2], the CTOD value at the operating temperature of the cryogenic liquefied gas tank may be the required CTOD value δ calculated by the following formula (1): 0 above.

[0019] [Number 1]

[0020]

[0021] In the above formula (1), c 0 is the characteristic crack size, ε Y is the yield strain of the parent material, and ε is the evaluation strain.

[0022] [4] Another aspect of the present invention relates to a method for designing a cryogenic liquefied gas tank, which is a method for designing a cryogenic liquefied gas tank having a butt-welded portion formed of a base material and a weld metal in an inner tank, comprising:

[0023] a thermal stress calculation step of calculating a thermal stress generated in the butt weld portion due to a difference between a linear expansion coefficient of the weld metal and a linear expansion coefficient of the base material at a use temperature of the cryogenic liquefied gas tank;

[0024] a necessary CTOD value calculation step, in which the CTOD value necessary for preventing the cryogenic liquefied gas tank from brittle fracture during operation of the cryogenic liquefied gas tank is calculated by assuming that the butt weld portion has defects and taking the influence of the thermal stress into consideration; and

[0025] A base material selection step in which the base material is selected so that the CTOD value of the heat-affected zone becomes equal to or higher than the required CTOD value.

[0026] [5] Another embodiment of the present invention relates to a cryogenic liquefied gas tank, which is a cryogenic liquefied gas tank having a butt-welded portion in an inner tank composed of a base material and a weld metal, wherein the CTOD value of the butt-welded portion at the operating temperature of the cryogenic liquefied gas tank is greater than the required CTOD value after taking into account the influence of thermal stress generated at the butt-welded portion due to the difference in linear expansion coefficient between the weld metal and the base material.

[0027] [6] According to the cryogenic liquefied gas tank described in [5], the CTOD value of the butt weld at the operating temperature of the cryogenic liquefied gas tank may be the required CTOD value δ calculated by the following formula (2): 0 above.

[0028] [Number 2]

[0029]

[0030] In the above formula (2), c 0 is the characteristic crack size, ε Y is the yield strain of the parent material, and ε is the evaluation strain.

[0031] [7] The cryogenic liquefied gas tank according to [4] or [5] above may be an LNG tank.

[0032] [8] According to the cryogenic liquefied gas tank described in [7] above, it can be an LNG tank for ships, and the CTOD value δ of the above-mentioned butt welded portion of the inner groove material with a plate thickness of less than 25 mm at -165°C can also satisfy the following equations (3) and (4).

[0033] δ≥0.001×(Tw / Tb) 2 +0.057×(Tw / Tb)+0.0035 …(3)

[0034] 1<Tw / Tb<2.2 …(4)

[0035] In the above formula (3) and the above formula (4), Tw is the linear expansion coefficient [1 / °C] of the above weld metal, and Tb is the linear expansion coefficient [1 / °C] of the above base material.

[0036] [9] According to the cryogenic liquefied gas tank described in [7] above, it can be an LNG tank for ships, and the CTOD value δ of the above-mentioned butt welded portion of the inner groove material with a plate thickness of more than 25 mm and less than 40 mm at -165°C can also satisfy the following formula (5) and the following formula (6).

[0037] δ≥0.0038×(Tw / Tb) 2 +0.074×(Tw / Tb)+0.003 …(5)

[0038] 1<Tw / Tb<2.2 …(6)

[0039] In the above formula (5) and the above formula (6), Tw is the linear expansion coefficient [1 / °C] of the above weld metal, and Tb is the linear expansion coefficient [1 / °C] of the above base material.

[0040]

[10] According to the cryogenic liquefied gas tank described in [7] above, it can be an above-ground LNG tank, and the CTOD value δ of the above-mentioned butt welded portion of the inner groove material with a plate thickness of less than 25 mm at -165°C can also satisfy the following equations (7) and (8).

[0041] δ≥0.0153×(Tw / Tb) 2 +0.0274×(Tw / Tb)+0.0135 …(7)

[0042] 1<Tw / Tb<2.2 …(8)

[0043] In the above formula (7) and the above formula (8), Tw is the linear expansion coefficient [1 / °C] of the above weld metal, and Tb is the linear expansion coefficient [1 / °C] of the above base material.

[0044]

[11] According to the cryogenic liquefied gas tank described in [7] above, it can be an above-ground LNG tank, and the CTOD value δ of the above-mentioned butt welded portion of the inner groove material with a plate thickness of more than 25 mm and less than 50 mm at -165°C can also satisfy the following equations (9) and (10).

[0045] δ≥0.03×(Tw / Tb) 2 -0.024×(Tw / Tb)+0.0694 …(9)

[0046] 1<Tw / Tb<2.2 …(10)

[0047] In the above formula (9) and the above formula (10), Tw is the linear expansion coefficient [1 / °C] of the above weld metal, and Tb is the linear expansion coefficient [1 / °C] of the above base material.

[0048]

[12] According to any one of [5] to

[11] above, the base material may contain, in terms of mass%, 0.1 to 5.0% Mn and 6.0 to 16.0% Ni.

[0049]

[13] According to any one of [5] to

[12] above, the weld metal may contain, in terms of mass%, 5.0 to 20.0% Mn and 5.0 to 30.0% Ni.

[0050] According to the above aspect of the present invention, even when a low-cost welding material having a large linear expansion coefficient is used in the manufacture of the inner tank of a tank in contact with cryogenic liquefied gas, brittle fracture at the welded portion can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 FIG. 1 is a schematic diagram showing an analysis model of a welded joint to be subjected to FEM analysis.

[0052] Figure 2 is the stress σ in the weld line direction at -196°C under various conditions of FEM analysis x Contour map of .

[0053] Figure 3 is the stress σ in the direction perpendicular to the weld line at -196°C under each condition of FEM analysis y Contour map of .

[0054] Figure 4 This is a graph showing the relationship between the linear expansion coefficient ratio Tw / Tb and the thermal stress at -165°C based on FEM analysis.

[0055] Figure 5 This is a schematic diagram showing assumed defect locations of welded joints in a C-type free-standing ship LNG tank.

[0056] Figure 6 This is a schematic diagram for explaining the assumed defect size of the welded joint in a C-type free-standing LNG tank for ships.

[0057] Figure 7 Schematic diagram showing an example of a butt weld joint having misalignment and angular deformation.

[0058] Figure 8 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in a C-type free-standing ship LNG tank with a plate thickness of 25 mm are shown. 0 A curve graph of the relationship.

[0059] Fig. 9 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ at the defect type position in a C-type free-standing ship LNG tank with a plate thickness of 40 mm are shown. 0 A curve graph of the relationship.

[0060] Fig.10 This is a schematic diagram showing an example of the structure of an above-ground LNG tank.

[0061] Fig.11 This is a schematic diagram showing the assumed defect locations of welded joints in an above-ground LNG tank.

[0062] Fig.12 This is a schematic diagram for explaining the assumed defect size of the weld joint in the above-ground LNG tank.

[0063] Fig.13 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in an above-ground LNG tank with a plate thickness of 25 mm are shown. 0 A curve graph of the relationship.

[0064] Fig.14 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in an above-ground LNG tank with a plate thickness of 50 mm are shown. 0 A curve graph of the relationship. DETAILED DESCRIPTION

[0065] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0066] <Method for manufacturing welded joint>

[0067] A method for manufacturing a welded joint according to an embodiment of the present invention is described. The method for manufacturing a welded joint according to the embodiment is a method for manufacturing a welded joint for an inner tank of a cryogenic liquefied gas tank, wherein the linear expansion coefficient of the weld metal is larger than the linear expansion coefficient of the base material, and comprises:

[0068] a thermal stress calculation step in which the thermal stress generated in the weld joint due to the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material at the use temperature of the cryogenic liquefied gas tank is calculated;

[0069] A necessary CTOD value calculation process, in which the CTOD value necessary to prevent brittle fracture of the cryogenic liquefied gas tank during operation is calculated by assuming that the weld joint has defects and taking into account the influence of thermal stress;

[0070] a CTOD value measuring step, in which the CTOD value of the heat-affected zone of the welded joint at the use temperature of the cryogenic liquefied gas tank is measured; and

[0071] A welding step in which a thick steel plate having a heat-affected zone having a CTOD value greater than or equal to a required CTOD value is used as the above-mentioned base material and the base material is welded.

[0072] [Thermal stress calculation process]

[0073] In this process, the thermal stress generated in the weld joint due to the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material at the operating temperature of the cryogenic liquefied gas tank is calculated. The thermal stress is calculated, for example, by FEM (Finite Element Method) analysis. In the FEM analysis, the operating temperature of the cryogenic liquefied gas tank can be used as the design temperature. For example, in the case of an LNG tank, the temperature of the cryogenic liquefied gas is -162°C, but the FEM analysis can also be performed assuming a design temperature of -164°C or -165°C. Referring to Figures 1 to 3 , taking the thermal stress analysis of welding joints based on FEM as an example, the calculation method of the above thermal stress is explained. Figure 1 FIG. 1 is a schematic diagram showing an analysis model of a welded joint to be subjected to FEM analysis.

[0074] exist Figure 1 The FEM analysis model of the welded joint as the analysis object is shown in , and the linear expansion coefficients of the base material and weld metal used for FEM analysis are shown in Table 1. The FEM analysis model is set as: welded joint 1 with length I = 1000mm, width w = 1000mm, and thickness t = 30mm. Welded joint 1 is composed of base materials 2A, 2B and weld metal 3. The width h of weld metal 3 is set to 30mm, and weld metal 3 extends along the length direction (X direction) at the center position (w / 2 position) in the width direction of welded joint 1. The analysis range of thermal stress is Figure 1 The range indicated by the double-dotted line is from one end of the weld joint 1 to the 1 / 2 position in the length direction (X direction) and the width direction (Y direction), and is from the surface of the weld joint 1 to the 1 / 2 position in the thickness direction (Z direction).

[0075] Table 1

[0076]

[0077] As the linear expansion coefficient Tb of the base materials 2A and 2B, the value of 9% Ni steel recorded in the "LNG Aboveground Storage Tank Guide" (Japan Gas Association, issued in September 2019) (average value from 20°C to -162°C) is used. As shown in Table 1, three different values ​​are set for the linear expansion coefficient Tw of the weld metal 3. The linear expansion coefficient Tw of the weld metal 3 in Example 1 is equal to the linear expansion coefficient Tb of the base materials 2A and 2B. The linear expansion coefficient is 9.2×10 -6[1 / °C] is 9% Ni steel (JIS G 3127SL9N590) and 7% Ni steel (JIS G 3127SL7N590). In addition, the linear expansion coefficient (average value from 20°C to -162°C) of austenitic stainless steel (such as SUS304, SUS304L, SUS316, SUS316L described in JIS G 4304:2021) in the pointer of LNG above-ground storage tank is 15×10 -6 [1 / °C]. Therefore, the linear expansion coefficient Tw of the weld metal 3 in Example 2 is set to 15×10 -6 [1 / °C]. In Example 3, assuming that the linear expansion coefficient Tw of the weld metal 3 is significantly large, the linear expansion coefficient Tw of the weld metal 3 is set to 20×10 -6 [1 / ℃]. The FEM analysis model was set to a 1 / 8 model using symmetry conditions, and the mesh size was set to 5 mm. The ends were set to free ends, and the thermal stress generated when the temperature of the entire model was cooled from 25℃ to -196℃ was analyzed.

[0078] Figure 2 The weld line stress σ at -196°C under various conditions is shown. x The contour map of Figure 3 The stress σ in the direction perpendicular to the weld line is shown y The welding line direction is parallel to the length direction, and the direction perpendicular to the welding line is parallel to the width direction. Figure 2 , Figure 3 As shown, the thermal stress of Example 1, in which the linear expansion coefficient Tw of the weld metal is the smallest, is the smallest, and the thermal stress of Example 3, in which the linear expansion coefficient Tw of the weld metal is the largest, is the largest. x The maximum value is shown at the center of the width direction of the weld metal 3, and the stress σ in the direction perpendicular to the weld line y The maximum value is shown in the base material portion near the weld metal 3. It is believed that the same tendency exists even at -165°C. Therefore, the relationship between the ratio of the linear expansion coefficient Tw of the weld metal 3 to the linear expansion coefficient Tb of the base materials 2A and 2B, i.e., the linear expansion coefficient ratio Tw / Tb, and the maximum value of the thermal stress at -165°C was sorted out. Figure 4 It is the ratio of linear expansion coefficient Tw / Tb to the stress in the welding line direction σ at -165℃ x And the stress σ in the direction perpendicular to the welding line y The relationship between Figure 4 As shown in the figure, the thermal stress (stress in the welding line direction σ x And the stress σ in the direction perpendicular to the welding line y) is roughly proportional to the linear expansion coefficient ratio Tw / Tb. Therefore, the thermal stress generated in the weld joint due to the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material at the operating temperature of the cryogenic liquefied gas tank can be calculated by FEM analysis.

[0079] The linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material are measured by the following method. First, samples with a diameter of 4 mm and a length of 15 mm are prepared from the weld metal and the base material, respectively. Using a vertical thermal dilatometer (TM-7000 manufactured by Vacuum Riko Co., Ltd.), the reference temperature is set to 20°C, and the temperature is lowered from 20°C to -190°C at a rate of 5°C / min to measure the thermal expansion coefficient of the sample during cooling. He gas is used as the atmosphere at this time. The average value of the linear expansion coefficient from 20°C to the design temperature (for example, -165°C) can be calculated based on the relationship between the temperature and the thermal expansion coefficient measured by the above method. The average value of the linear expansion coefficient of the weld metal and the average value of the linear expansion coefficient of the base material are respectively used as the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material. The measured values ​​of the linear expansion coefficients of the weld metal and the base material obtained in this way can also be used to perform FEM analysis to calculate thermal stress.

[0080] [Required CTOD value calculation process]

[0081] In this process, the CTOD value δ required to prevent brittle fracture of the cryogenic liquefied gas tank during operation is calculated by assuming that the weld joint has defects and taking into account the influence of thermal stress. 0 The following uses the case where the cryogenic liquefied gas tank is a C-type independent ship LNG tank and the case where it is an above-ground LNG tank as examples to explain the required CTOD value δ 0 The calculation method of .

[0082] (Required CTOD value of welded joints of C-type independent LNG tanks for ships 0 )

[0083] First, refer to Figures 5 to 9 , to illustrate the necessary CTOD value δ required for the butt weld (welded joint) in a C-type tank that complies with the IGC code established by the International Maritime Organization, taking into account the influence of thermal stress generated in the butt weld. 0 The above-mentioned C-type tank is a LNG tank for ships and is a stand-alone tank. Figure 5 It is a schematic diagram showing the assumed defect position of the butt weld portion in the C-type tank.

[0084] like Figure 5As shown, in the welded joint 1A, the base material 2C and the base material 2D, the base material 2D and the base material 2E, and the base material 2E and the base material 2C are butt welded. The base materials 2C and the base materials 2D, and the base materials 2C and the base materials 2E are girth welded by flux-cored arc welding or submerged arc welding to form weld metal 3A. The base materials 2D and the base materials 2E are seam welded by flux-cored arc welding to form weld metal 3B.

[0085] Based on the required CTOD value δ of WES2805-2011 0 In the calculation of Figure 5 As shown in FIG. 1 , a defect 4A or a defect 4B is assumed. Defect 4A is a defect located at the fusion line between the base material 2D and the weld metal 3B near the junction of the weld metal 3A and the weld metal 3B. Defect 4B is a defect located at the fusion line between the base material 2D and the weld metal 3B at a position away from the weld metal 3A (the influence of the circumferential weld can be ignored). The shapes of defects 4A and 4B are as follows: Figure 6 The semi-elliptical surface crack with a length (long axis) of 2c and a depth (minor axis radius) of a steel plate surface as shown in the figure is calculated based on the following premises: The locations of defects 4A and 4B are where relatively large thermal stress is likely to occur during the use of C-type LNG tanks.

[0086] Plate thickness t: 25mm or 40mm

[0087] Defect: major axis 2c=1.5t, minor axis radius a=0.5t, t is the plate thickness.

[0088] Yield stress σy of the base material at evaluation temperature (-165°C): 853MPa

[0089] Young's modulus E: 206GPa

[0090] Poisson's ratio ν: 0.3

[0091] Welding construction error (angular deformation W + misalignment H);

[0092] Defect A (25mm and 40mm thickness): W+H=3mm

[0093] Defect B (board thickness 25 mm): W + H = 10 mm

[0094] Defect B (board thickness 40 mm): W + H = 12 mm

[0095] The value of the yield stress σy of the base material at the evaluation temperature, 853 MPa, is a value set assuming 7% Ni steel.

[0096] The angular deformation W is the bending amount of the welded joint 1A, such as Figure 7 As shown in FIG. 1 , it is the length of the crack from the highest position in the center of the plate thickness direction of the base materials 2D and 2E to the welded joint 1A per horizontal unit length (1 m). Figure 7 As shown, the misalignment amount H is the magnitude of the misalignment in the height direction between the center position in the plate thickness direction of the base material 2D and the center position in the plate thickness direction of the base material 2E in the welded joint 1A.

[0097] Based on the above premise, the characteristic crack size c of the surface crack is calculated by the following formula: 0 . Characteristic crack size c 0 Corresponds to the characteristic crack size in WES 2805:2011.

[0098] [Number 3]

[0099] c 0 =aF t 2 …(1)

[0100] The so-called characteristic crack size c 0 It is the half length of the crack when various cracks are replaced by a through-thickness crack with the same K value (stress amplification coefficient). t is the shape correction factor, the shape correction factor F t It is obtained by the following formula (2).

[0101] [Number 4]

[0102]

[0103] In the above formula (2), F 0 It is calculated using the following equations (3) to (6).

[0104] [Number 5]

[0105]

[0106] [Number 6]

[0107]

[0108] [Number 7]

[0109]

[0110] [Number 8]

[0111]

[0112] In addition, in the above formula (2), Φ is calculated by the following formula (7).

[0113] [Number 9]

[0114]

[0115] Next, the evaluation strain ε is calculated. The evaluation strain ε is the strain ε caused by the boundary force. 1 , strain ε caused by welding residual stress 2 , and the strain ε caused by stress concentration 3 The sum is obtained by the following formula (8).

[0116] [Number 10]

[0117] ε=ε 1 +ε 2 +ε 3 …(8)

[0118] The strain ε caused by the boundary force 1 Use the following formula to calculate.

[0119] [Number 11]

[0120]

[0121] In the above formula (9), E is Young's modulus, ν is Poisson's ratio, σ 1 is the stress caused by the boundary force. The stress caused by the boundary force σ 1 The value is obtained by adding the thermal stress obtained by the above FEM analysis to the allowable stress of 230 MPa of 7% Ni steel. For defect 4A, the thermal stress in the weld line direction caused by the circumferential weld is applied. Figure 4 The stress σ in the weld line direction is shown x In the case of defect 4B, the thermal stress caused by the seam welding in the direction perpendicular to the welding line acts, so the Figure 4 The stress σ in the direction perpendicular to the welding line is shown y .

[0122] Strain ε caused by welding residual stress 2 Yield strain ε of used steel plate (base material) Y (=σ Y ×(1-v 2 ) / E) and coefficient α R It is obtained by the following formula (10).

[0123] [Number 12]

[0124] ε 2 =α R ε Y …(10)

[0125] The coefficient α in the above formula (10) is R It is a value given by the type of crack and the relationship between the weld line and the direction of the crack. For defect 4A, the coefficient α R =0.6, for defect 4B, coefficient α R = 0.36. These values ​​are the values ​​described in Table 9.1 of WES2805:2011.

[0126] The strain ε caused by strain concentration in the above formula (8) is 3 , through the average strain concentration factor K ε and the strain ε caused by the boundary force 1 The average strain concentration factor K is expressed by the following formula (11): ε It is calculated using the following equations (12) to (15).

[0127] [Number 13]

[0128] ε 3 =(K ε -1)ε 1 …(11)

[0129] [Number 14]

[0130]

[0131] [Number 15]

[0132]

[0133] [Number 16]

[0134]

[0135] [Number 17]

[0136]

[0137] K in the above formula (12) t is the average elastic stress concentration factor, σ net It is the stress of the actual cross section excluding the crack size. In this embodiment, since it can be considered that the defect is sufficiently small compared with the component size, it is set to σ net =σ 1 Therefore, in the above formula (15), λ=1.

[0138] Average elastic stress concentration factor K of butt welded joints with misalignment and angular deformation t It can be obtained by the following formula (16).

[0139] [Number 18]

[0140]

[0141] Required CTOD value δ 0 , using the characteristic crack size c obtained from the above 0 and the evaluation strain ε are expressed by the following formula (17).

[0142] [Number 19]

[0143]

[0144] In this way, assuming that the weld joint has defects and taking into account the influence of thermal stress, the CTOD value δ required to prevent brittle fracture of the cryogenic liquefied gas tank in the operation of the C-type LNG tank is calculated. 0 Therefore, the required CTOD value δ 0 It can be calculated using the above formula (17).

[0145] Figure 8 , Fig. 9 The required CTOD value δ calculated by the above formula (17) is shown as 0 . Figure 8 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in a C-type free-standing ship LNG tank with a plate thickness of 25 mm are shown. 0 A curve graph of the relationship. Fig. 9 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in a C-type free-standing ship LNG tank with a plate thickness of 40 mm are shown. 0 The relationship between Figure 8 , Fig. 9 As shown, the required CTOD value δ of defect 4A is higher than that of defect 4B. 0 Therefore, by selecting the necessary CTOD value δ to satisfy defect 4A 0 The combination of the base metal and the weld metal can prevent brittle fracture from the weld. Therefore, when the plate thickness is 25 mm, Figure 8 The approximate formula for defect 4A in the figure can prevent brittle fracture from the weld by satisfying the following formula (18) with the CTOD value δ. In addition, when the plate thickness is 40 mm, Fig. 9 The approximate formula for defect 4A in can prevent brittle fracture from the weld by satisfying the following formula (19) with the CTOD value δ.

[0146] [Number 20]

[0147] δ≥0.001×(Tw / Tb) 2 +0.057×(Tw / Tb)+0.0035 …(18)

[0148] [Number 21]

[0149] δ≥0.0038×(Tw / Tb) 2 +0.074×(Tw / Tb)0.003 …(29)

[0150] (Required CTOD value of welded joints of above-ground LNG tanks 0 )

[0151] Next, refer to Figures 10 to 14 , to illustrate the necessary CTOD value δ required for the butt weld (welded joint) in the aboveground LNG tank 0 Calculation example. Fig.10 Schematic diagram showing an example of an above-ground LNG tank. Fig.10 As shown, the aboveground LNG tank is a cylindrical tank arranged on the ground, and is a tank that is covered from above. The inner tank of the aboveground LNG tank has a weld joint 1B. In the weld joint 1B, the base material 2F and the base material 2G, the base material 2G and the base material 2H, and the base material 2H and the base material 2G are butt-welded. The base materials 2F and the base materials 2G, and the base materials 2F and the base materials 2H are respectively circumferentially welded by submerged arc welding to form weld metal 3C. The base materials 2G and the base materials 2H are welded in the vertical direction by metal arc welding or gas tungsten arc welding to form weld metal 3D.

[0152] Based on the required CTOD value δ of WES2805-2011 0 In the calculation of Fig.11 As shown in FIG. 1 , a defect 4C or a defect 4D is assumed. Defect 4C is a defect located near the junction of weld metal 3C and weld metal 3D and located at the fusion line between base metal 2G and weld metal 3D. Defect 4D is a defect located at a position far from weld metal 3C (the influence of circumferential welding can be ignored) in the fusion line between base metal 2G and weld metal 3D. The shapes of defect 4C and defect 4D are as follows: Fig.12 The semi-elliptical surface crack shown in FIG. 1 has a length (major axis) of 2c (=1.5t) and a depth (minor axis radius) of a (=0.25t) in the steel plate surface. The calculation premise is as follows.

[0153] Plate thickness t: 25mm or 50mm

[0154] Defect: major axis 2c=1.5t, minor axis radius a=0.25t, t is the plate thickness.

[0155] Yield stress σ of the base material at the evaluation temperature (-165℃) y :853MPa

[0156] Young's modulus E: 206GPa

[0157] Poisson's ratio ν: 0.3

[0158] Welding construction error (angular deformation W + misalignment H);

[0159] Defect 4C (25 mm and 50 mm thick): W = 3 mm, H = 0 mm

[0160] Defect 4D (for plate thickness 25 mm): W = 10 mm, H = 0 mm

[0161] Defect 4D (for plate thickness 50 mm): W = 12 mm, H = 0 mm

[0162] Yield stress σ of the base material at the evaluation temperature y The value of 853 MPa is a value set assuming 7% Ni steel.

[0163] Characteristic crack size c 0 The strain ε is calculated by the above equations (1) to (7). The evaluation strain ε is calculated by the above equation (8). In the above equation (8), the strain ε caused by the boundary force is 1 The stress σ caused by the boundary force is calculated by the above formula (9). 1 , referring to Non-Patent Document 1, the tensile stress σ of the evaluation part relative to level 2 earthquake motion is used. t =352.5MPa, bending stress σ b =51MPa, calculated by the following equations (20) to (23). In the following equation (20), H t It is set to 1. In the following formulas (21) to (23), a is the depth of the defect, t is the plate thickness, and c is the half length of the defect on the plate surface.

[0164] [Number 22]

[0165] σ 1 =H t σ t +H b σ b …(20)

[0166] [Number 23]

[0167]

[0168] [Number 24]

[0169]

[0170] [Number 25]

[0171]

[0172] The strain ε caused by welding residual stress in formula (8) 2 It is obtained by the following formula (24).

[0173] [Number 26]

[0174] ε 2 =σ R ×(1-v 2 ) / E …(24)

[0175] In the above formula (24), σ R is the residual stress. For defect 4C, let σ R =280MPa, for defect 4D, let σ R =168MPa.

[0176] The strain ε caused by stress concentration in the above formula (8) is 3 The required CTOD value δ of the welded joint 1B in the LNG tank is then obtained by using the above equations (11) to (16). 0 .

[0177] Assuming that the weld joint has defects and taking into account the influence of thermal stress, the CTOD value δ required to prevent brittle fracture of the cryogenic liquefied gas tank in the operation of the above-ground LNG tank is calculated. 0 .

[0178] Fig.13 , Fig.14 The required CTOD value δ calculated by the above formula (17) is shown as 0 . Fig.13 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in an above-ground LNG tank with a plate thickness of 25 mm are shown. 0 A curve graph of the relationship. Fig.14 The linear expansion coefficient ratio Tw / Tb and the required CTOD value δ for each defect position in an above-ground LNG tank with a plate thickness of 50 mm are shown. 0 The relationship between Fig.13 , Fig.14 As shown, the required CTOD value δ of defect 4C is higher than that of defect 4D. 0 Therefore, by selecting the required CTOD value δ to satisfy defect 4D 0 The combination of the base metal and the weld metal can prevent brittle fracture from the weld. Therefore, when the plate thickness is 25 mm, Fig.13The approximate formula for defect 4D in the formula (25) can prevent brittle fracture from the weld by satisfying the CTOD value δ. In addition, when the plate thickness is 50 mm, Fig.14 The approximate formula for defect 4D in can prevent brittle fracture from the weld by satisfying the following formula (26) with the CTOD value δ.

[0179] [Number 27]

[0180] δ≥0.0153×(Tw / Tb) 2 +0.0274×(Tw / Tb)0.00135 …(25)

[0181] [Number 28]

[0182] δ≥0.003×(Tw / Tb) 2 -0.024×(Tw / Tb)0.0694 …(26)

[0183] [CTOD value measurement process]

[0184] In this process, the CTOD value of the heat affected zone of the welded joint at the operating temperature of the cryogenic liquefied gas tank is measured. The CTOD test is performed in accordance with ISO15653-2018. Specifically, a three-point bending test piece with a plate thickness of the tank plate thickness t and a test piece width of t to 2t can be used. Prepare three test pieces with cuts introduced at the fusion line and at positions 1, 3, and 5 mm from the fusion line to the parent material side. Cool the test piece to the operating temperature of the cryogenic liquefied gas tank and perform the CTOD test. The minimum value among the 12 measured CTOD values ​​is taken as the CTOD value of the heat affected zone of the welded joint.

[0185] [Welding process]

[0186] In this process, the CTOD value δ in which the heat affected zone is confirmed is used as the required CTOD value δ 0 The thick steel plate is used as a base material and the base material is welded. For example, a thick steel plate whose CTOD value δ satisfies the above formula (18), formula (19), formula (25), or formula (26) is used as a base material.

[0187] The welding material is selected in consideration of the above-mentioned linear expansion coefficient. For example, the welding material is selected in such a way that the linear expansion coefficient ratio Tw / Tb exceeds 1 and is less than 2.2. The welding material is, for example, a Ni-based alloy, steel, etc. The welding material preferably contains, by mass%, Mn: 5.0-20.0%, Ni: 5.0-30.0%, and the balance: Fe, any elements and impurities. If it is a welding material containing Mn: 5.0-15.0%, Ni: 5.0-20.0%, and the balance: Fe, any elements and impurities, the manufacturing cost can be further reduced.

[0188] The welding method is not particularly limited, as long as the welding method used for the welded joint of the inner tank of the cryogenic liquefied gas tank is appropriately adopted. For example, gas shielded arc welding using flux-cored wire and submerged arc welding using solid wire and flux can be adopted. In addition, arc welding with a stick, simple gas-electric welding, electroslag welding, TIG welding, and gas shielded welding using solid wire are sometimes adopted.

[0189] Flux-cored wire is produced by supplying flux through the opening of an open tube formed by forming a steel strip into a U-shape, butt-welding the opening, and drawing the wire. Heat treatment may be performed during the drawing process. Solid wire is produced by drawing steel that is melted, forged, and rolled into a rod.

[0190] The manufacturing method of the welded joint according to the present embodiment has been described above. According to the present embodiment, the CTOD value δ of the heat-affected zone is used to prevent the cryogenic liquefied gas tank from brittle fracture during operation. 0 In addition, the weld metal having a linear expansion coefficient greater than that of the base material has a lower content of expensive elements than the weld metal having a linear expansion coefficient less than that of the base material. Therefore, the weld joint can be manufactured at a lower cost.

[0191] <Welded joints>

[0192] A weld joint involved in one embodiment of the present invention is a weld joint that includes a base material and a weld metal, the linear expansion coefficient of the weld metal being larger than the linear expansion coefficient of the base material, and is used for an inner tank of a cryogenic liquefied gas tank, and the CTOD value at the operating temperature of the cryogenic liquefied gas tank is greater than the required CTOD value after taking into account the influence of thermal stress caused by the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material.

[0193] [Base material]

[0194] As the base material, a material whose CTOD value at the operating temperature of the cryogenic liquefied gas tank reaches the required CTOD value or more after taking into account the influence of thermal stress caused by the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material. The base material contains 0.1 to 5.0 mass% of Mn and 6.0 to 16.0 mass% of Ni, which can further improve the strength at ultra-low temperatures. Therefore, the base material preferably contains 0.1 to 5.0 mass% of Mn and 6.0 to 16.0 mass% of Ni. The Mn content of the base material is more preferably 0.3 mass% or more, 0.4 mass% or more, or 0.5 mass% or more. In addition, the Mn content of the base material is more preferably 3.0 mass% or less, 2.0 mass% or less, or 1.5 mass% or less. The Ni content of the base material is more preferably 7 mass% or more. In addition, the Ni content of the base material is more preferably 10 mass% or less. The remainder (remainder) of the chemical composition of the base material is Fe, any element and impurities.

[0195] As the arbitrary element, C, Si, Cu, Cr, Mo, W, Co, B, Nb, V, Ti, Zr, N, Al, Ca, REM, Mg, Sn, Te, Se, etc. can be listed. C, Si, Cu, Cr, Mo, W, Co, B, Nb, V, Ti, Zr, and N are elements that improve strength. Al, Ca, REM, and Mg are elements for deoxidation and controlling the form of inclusions. Sn, Te, and Se are elements that improve corrosion resistance. The respective contents may be, for example, C: 0.08% or less, Si: 0.5% or less, Cu: 1% or less, Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.1% or less, Co: 1% or less, Sn: 0.1% or less, W: 0.1% or less, Mg: 0.1% or less, Al: 0.1% or less, Ca: 0.01% or less, Ti: 0.1% or less, B: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less, N: 0.01% or less, Te: 0.1% or less, and Se: 0.1% or less. Since any element may not be contained, the lower limit of the content of each of the arbitrary elements is 0%.

[0196] Impurities include P, S, O, and Sb, As, Zn, etc. that may be mixed from scraps, etc. The content of P may be 0.05 mass % or less, and the content of each of the other impurities may be 0.01 mass % or less.

[0197] In the production of the base material, for example, a steel billet produced by melting steel and continuous casting is used. The base material is produced by, for example, hot rolling and heat treating the steel billet. In some cases, accelerated cooling such as water cooling is performed after hot rolling.

[0198] The thickness of the parent material is, for example, 6 to 50 mm. In the case of a ship tank, it can be less than 25 mm, or more than 25 mm and less than 40 mm. In the case of an above-ground tank, it can be less than 25 mm, or more than 25 mm and less than 50 mm. The plate thickness varies depending on the size and location of the tank. Generally, in the case of a large tank, thick materials exceeding 25 mm and less than 50 mm are mainly used, and in the case of a small tank, thin materials less than 25 mm are mainly used. Therefore, the thickness of the parent material can be less than 50 mm, less than 40 mm, or less than 25 mm. In addition, the thickness of the parent material can be more than 6 mm, more than 10 mm, more than 15 mm, or more than 25 mm.

[0199] [Weld Metal]

[0200] The weld metal preferably contains, by mass%, 5.0 to 20.0% Mn, 5.0 to 30.0% Ni, and the balance: Fe, any elements, and impurities. If the welding material contains 5.0 to 20.0% Mn and 5.0 to 30.0% Ni, the manufacturing cost can be further reduced. The weld metal more preferably contains 5.0 to 15.0% Mn and 5.0 to 20.0% Ni.

[0201] As the arbitrary element, C, Mo, W, Nb, V, Co, Cu, Cr, B, N, Pb, Sn, Si, Al, Ti, Mg, Ca, REM, Zr, etc. can be listed. C, Mo, W, Nb, V, and Co are elements that improve strength. Cu, Cr, B, and N are elements that improve the stability of austenite. Pb is an element that improves machinability. Sn is an element that improves corrosion resistance. Si, Al, Ti, Mg, and Ca are elements for deoxidation and controlling the form of inclusions. REM and Zr are elements that improve welding workability. The respective contents may be, for example, C: 1% or less, Mo: 22% or less, W: 20% or less, Nb: 3% or less, V: 1% or less, Co: 1% or less, Cu: 5% or less, Cr: 20% or less, B: 0.5% or less, N: 0.5% or less, Pb: 1% or less, Sn: 1% or less, Si: 0.75% or less, Al: 0.1% or less, Ti: 0.1% or less, Mg: 5% or less, Ca: 5% or less, REM: 0.5% or less, and Zr: 0.5% or less in terms of mass %. Since any element may not be contained, the lower limit of the content of each of the arbitrary elements is 0%.

[0202] Examples of impurities include P, S, and O. The respective contents may be 0.05 mass % or less for P, 0.05 mass % or less for S, and 0.15 mass % or less for O.

[0203] In the weld joint involved in this embodiment, the linear expansion coefficient ratio Tw / Tb is preferably greater than 1 and less than 2.2. The closer the linear expansion coefficient ratio Tw / Tb is to 1, the smaller the thermal stress generated, but the weld joint in this case contains a large amount of Ni. In order to reduce the Ni content and reduce the manufacturing cost of the weld joint, the linear expansion coefficient ratio Tw / Tb is more preferably 1.2 or more, and further preferably 1.3 or more. On the other hand, if the linear expansion coefficient ratio Tw / Tb is too large, the thermal stress increases, and the required CTOD value δ 0 Increased, so in order to ensure the toughness of the heat affected zone, it is necessary to add a large amount of Ni to the base material, and as a result, the manufacturing cost of the welded joint increases. In addition, if the linear expansion coefficient ratio Tw / Tb is less than 2.2, the brittle fracture from the weld can be fully suppressed. Therefore, the linear expansion coefficient ratio Tw / Tb is preferably less than 2.2, more preferably less than 2.0, and further preferably less than 1.6. In the case of a base material of 9% Ni steel (linear expansion coefficient of 9.2×10 -6 ), welding material is 13Mn%-13Ni% (linear expansion coefficient is 12.5×10 -6 ), the linear expansion coefficient ratio Tw / Tb becomes 1.4.

[0204] The weld joint according to the present embodiment can be manufactured by the above-mentioned method for manufacturing a weld joint.

[0205] The welded joint according to the present embodiment has been described above. According to the present embodiment, in the base material of the welded joint, the CTOD value δ of the heat-affected zone is the CTOD value δ required to prevent the cryogenic liquefied gas tank from brittle fracture during operation. 0 For example, the CTOD value at the operating temperature of the cryogenic liquefied gas tank is the required CTOD value δ calculated by the above formula (17): 0 In addition, the weld metal having a linear expansion coefficient larger than that of the base material has a smaller content of expensive elements than the weld metal having a linear expansion coefficient smaller than that of the base material. Therefore, the weld joint involved in this embodiment is cheaper than the conventional weld joint using Ni-based welding materials.

[0206] <Design Method for Cryogenic Liquefied Gas Tanks>

[0207] The above-mentioned design method of the welded joint can be applied to the design of the cryogenic liquefied gas tank. Therefore, as one embodiment of the present invention, there is a design method of a cryogenic liquefied gas tank, which is a design method of a cryogenic liquefied gas tank having a butt-welded portion composed of a base material and a weld metal in an inner tank, and includes: a thermal stress calculation process, in which the thermal stress generated in the butt-welded portion due to the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material at the use temperature of the cryogenic liquefied gas tank is calculated; a required CTOD value calculation process, in which the butt-welded portion is assumed to have defects and the influence of the thermal stress is taken into account to calculate the CTOD value required for the cryogenic liquefied gas tank not to suffer brittle fracture during operation of the cryogenic liquefied gas tank; and a base material selection process, in which the base material is selected in such a way that the CTOD value of the heat-affected zone reaches or exceeds the required CTOD value.

[0208] <Low-temperature liquefied gas tank>

[0209] The above-mentioned welded joint can be applied to the inner tank of a cryogenic liquefied gas tank. Therefore, as one embodiment of the present invention, there is a cryogenic liquefied gas tank, which is a cryogenic liquefied gas tank having a butt weld portion composed of a base metal and a weld metal in the inner tank, and the CTOD value of the butt weld portion at the operating temperature of the cryogenic liquefied gas tank is greater than the required CTOD value after taking into account the influence of the thermal stress generated in the butt weld portion due to the difference in the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base metal. The cryogenic liquefied gas tank involved in this embodiment may also be an LNG tank.

[0210] When the cryogenic liquefied gas tank is an LNG tank for ships and the plate thickness of the inner tank is 25 mm or less, the CTOD value δ of the butt weld of the inner tank at -165°C preferably satisfies the above formula (18) and the following formula (27).

[0211] [Number 29]

[0212] 1<Tw / Tb<2.2 …(27)

[0213] When the cryogenic liquefied gas tank is an LNG tank for ships and the plate thickness of the inner tank material exceeds 25 mm and is less than 40 mm, it is preferred that the CTOD value δ of the butt weld of the inner tank material at -165°C satisfies the above formula (19) and the above formula (27).

[0214] When the cryogenic liquefied gas tank is an above-ground LNG tank and the plate thickness of the inner tank is 25 mm or less, it is preferred that the CTOD value δ of the butt weld of the inner tank at -165°C satisfies the above equations (25) and (27).

[0215] When the cryogenic liquefied gas tank is an above-ground LNG tank and the plate thickness of the inner tank material exceeds 25 mm and is less than 50 mm, it is preferred that the CTOD value δ of the butt weld of the inner tank material at -165°C satisfies the above-mentioned formula (26) and the above-mentioned formula (27).

[0216] The present invention has been described above based on one embodiment of the present invention, but the present invention is not limited thereto. The above is merely an example, and any technical solution having substantially the same structure as the technical idea described in the claims of the present invention and having the same effect is included in the technical scope of the present invention.

[0217] For example, the cryogenic liquefied gas tank is not limited to an LNG tank, but may be a gas tank storing cryogenic liquefied gas other than LNG. The cryogenic liquefied gas tank may also be a liquefied hydrogen storage tank storing liquefied hydrogen, for example. Even if the cryogenic liquefied gas tank is a liquefied hydrogen storage tank, as described above, as long as the CTOD value δ in units of mm of a part or all of the butt weld at the use temperature is the required CTOD value δ after taking into account the influence of the thermal stress generated in the butt weld due to the difference in the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material 0 For example, the CTOD value at the operating temperature of the cryogenic liquefied gas tank is the required CTOD value δ calculated by the above formula (17): 0 Thus, in the liquefied hydrogen storage tank, even when a welding material having a low cost and a large linear expansion coefficient is used, brittle fracture from the weld portion can be suppressed.

[0218] In addition, for example, in the thermal stress calculation step in the above-mentioned method for manufacturing a welded joint, the thermal stress generated in the welded joint due to the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material at the use temperature of the cryogenic liquefied gas tank is calculated by FEM analysis. However, the calculation of the thermal stress may be performed by a method other than FEM analysis.

[0219] The linear expansion coefficient of the base material and the linear expansion coefficient of the weld metal are values ​​determined by their respective chemical compositions, and the values ​​of the linear expansion coefficients described in known documents may be used, or may be measured by experiments.

[0220] The position of the defect assumed in the necessary CTOD value calculation step in the above-mentioned method for manufacturing a welded joint may be determined based on the result of analyzing in advance the position where relatively large thermal stress occurs by thermal stress analysis based on FEM or the like.

[0221] Example

[0222] The base material of the welded joint in the C-type independent ship LNG tank is assumed to be 7% Ni steel (linear expansion coefficient Tb is 9.2×10 -6 [1 / ℃]), welding material is 13Mn%-13Ni% (linear expansion coefficient Tw is 12.5×10 -6 [1 / ℃]). In this case, the linear expansion coefficient ratio Tw / Tb becomes 1.4. Figure 5 As shown, defect 4A or defect 4B is assumed. The shapes of defect 4A and defect 4B are set as follows Figure 6 As shown in FIG. 1 , a semi-elliptical surface crack with a length (major axis) of 2c and a depth (minor axis radius) of a steel plate surface is formed. Assuming the following conditions, the required CTOD value is obtained when the linear expansion coefficient is taken into account (the example of the present invention) and when the linear expansion coefficient is not taken into account (the prior art).

[0223] Plate thickness: 40mm

[0224] Defect: major axis 2c=1.5t, minor axis radius a=0.5t, t is the plate thickness.

[0225] Yield stress σ of the base material at the evaluation temperature (-165℃) y :853MPa

[0226] Young's modulus E: 206GPa

[0227] Poisson's ratio ν: 0.3

[0228] Welding construction error (angular deformation W + misalignment H);

[0229] Defect A: W+H=3mm

[0230] Defect B: W+H=12mm

[0231] Applied stress: 230MPa

[0232] The calculation results of the required CTOD value are shown in Table 2. The required CTOD value of the example of the present invention taking into account the linear expansion coefficient is 33 to 41% higher than the required CTOD value of the prior art not taking into account the linear expansion coefficient. That is, when a welding material with a low Ni content and a large linear expansion coefficient is used, thermal stress is generated due to the influence of the difference in linear expansion coefficient between the welding material and the base material, but the prior art does not take this thermal stress into account, so the required CTOD value is estimated too low.

[0233] Table 2

[0234]

[0235] Description of Reference Numerals

[0236] 1.1A, 1B: Welded joints

[0237] 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H: Base material

[0238] 3.3A, 3B, 3C, 3D: Weld metal

[0239] 4A, 4B, 4C, 4D: Defects

Claims

1. A method for manufacturing a welded joint, which is used for an inner tank of a cryogenic liquefied gas tank and in which the linear expansion coefficient of the weld metal is larger than the linear expansion coefficient of the base material, comprising: a thermal stress calculation step in which a thermal stress generated in the weld joint due to a difference between a linear expansion coefficient of the weld metal and a linear expansion coefficient of the base material at a use temperature of the cryogenic liquefied gas tank is calculated; a required CTOD value calculation step, in which the CTOD value required for preventing the cryogenic liquefied gas tank from brittle fracture during operation of the cryogenic liquefied gas tank is calculated by assuming that the weld joint has defects and taking the influence of the thermal stress into consideration; a CTOD value measuring step, in which the CTOD value of the heat-affected zone of the weld joint at the use temperature of the cryogenic liquefied gas tank is measured; and A welding step in which a thick steel plate in which the CTOD value of the heat-affected zone is confirmed to be equal to or greater than the required CTOD value is used as the base material, and the base material is welded.

2. A welded joint comprising a base material and a weld metal, wherein the linear expansion coefficient of the weld metal is larger than the linear expansion coefficient of the base material, and is used for an inner tank of a cryogenic liquefied gas tank, The CTOD value at the use temperature of the cryogenic liquefied gas tank is equal to or larger than the required CTOD value after taking into account the influence of thermal stress caused by the difference between the linear expansion coefficient of the weld metal and the linear expansion coefficient of the base material.

3. The welded joint according to claim 2, The CTOD value at the operating temperature of the cryogenic liquefied gas tank is equal to or greater than the required CTOD value δ0 calculated by the following formula (1). In the above formula (1), c0 is the characteristic crack size, ε Y is the yield strain of the parent material, and ε is the evaluation strain.

4. A design method for a cryogenic liquefied gas tank having a butt-welded portion composed of a base metal and a weld metal in an inner tank, comprising: a thermal stress calculation step in which a thermal stress generated in the butt weld portion due to a difference between a linear expansion coefficient of the weld metal and a linear expansion coefficient of the base material at a use temperature of the cryogenic liquefied gas tank is calculated; a required CTOD value calculation step, in which the CTOD value required for preventing the cryogenic liquefied gas tank from brittle fracture during operation of the cryogenic liquefied gas tank is calculated by assuming that the butt weld portion has defects and taking the influence of the thermal stress into consideration; and A base material selection process, in which the CTOD value of the heat affected zone is made to be above the required CTOD value. The parent material is selected in a manner.

5. A cryogenic liquefied gas tank having a butt-welded portion composed of a base metal and a weld metal in an inner tank, The CTOD value of the butt weld at the operating temperature of the cryogenic liquefied gas tank is greater than or equal to the required CTOD value after taking into account the influence of thermal stress generated in the butt weld due to the difference in linear expansion coefficient between the weld metal and the base material.

6. The cryogenic liquefied gas tank according to claim 5, The CTOD value of the butt weld at the operating temperature of the cryogenic liquefied gas tank is equal to or greater than the required CTOD value δ0 calculated by the following formula (2). In the above formula (2), c0 is the characteristic crack size, ε Y is the yield strain of the parent material, and ε is the evaluation strain.

7. The cryogenic liquefied gas tank according to claim 5 or 6, The cryogenic liquefied gas tank is an LNG tank.

8. The cryogenic liquefied gas tank according to claim 7, It is an LNG tank for ships. The CTOD value δ of the butt welded portion of the inner channel material having a plate thickness of 25 mm or less at -165°C satisfies the following equations (3) and (4): δ≥0.001×(Tw / Tb) 2 +0.057×(Tw / Tb)+0.0035… Equation (3) 1<Tw / Tb<2.2…(4) In the above formula (3) and the above formula (4), Tw is the linear expansion coefficient of the weld metal expressed in units of 1 / °C, and Tb is the linear expansion coefficient of the base material expressed in units of 1 / °C.

9. The cryogenic liquefied gas tank according to claim 7, It is an LNG tank for ships. The CTOD value δ of the butt welded portion of the inner channel material having a plate thickness of more than 25 mm and less than 40 mm at -165°C satisfies the following formula (5) and the following formula (6): δ≥0.0038×(Tw / Tb) 2 +0.074×(Tw / Tb)+0.003…Equation (5) 1<Tw / Tb<2.2…(6) In the above formula (5) and the above formula (6), Tw is the linear expansion coefficient of the weld metal expressed in units of 1 / °C, and Tb is the linear expansion coefficient of the base material expressed in units of 1 / °C.

10. The cryogenic liquefied gas tank according to claim 7, It is an above-ground LNG tank. The CTOD value δ of the butt welded portion of the inner channel material having a plate thickness of 25 mm or less at -165°C satisfies the following equations (7) and (8): δ≥0.0153×(Tw / Tb) 2 +0.0274×(Tw / Tb)+0.0135… Equation (7) 1<Tw / Tb<2.2…(8) In the above formula (7) and the above formula (8), Tw is the linear expansion coefficient of the weld metal expressed in units of 1 / °C, and Tb is the linear expansion coefficient of the base material expressed in units of 1 / °C.

11. The cryogenic liquefied gas tank according to claim 7, It is an above-ground LNG tank. The CTOD value δ of the butt welded portion of the inner channel material having a plate thickness of more than 25 mm and less than 50 mm at -165°C satisfies the following equations (9) and (10): δ≥0.03×(Tw / Tb) 2 -0.024×(Tw / Tb)+0.0694… Equation (9) 1<Tw / Tb<2.2…(10) In the above formula (9) and the above formula (10), Tw is the linear expansion coefficient of the weld metal expressed in units of 1 / °C, and Tb is the linear expansion coefficient of the base material expressed in units of 1 / °C.

12. The cryogenic liquefied gas tank according to claim 5 or 6, The base material contains, by mass%, 0.1 to 5.0% Mn and 6.0 to 16.0% Ni.

13. The cryogenic liquefied gas tank according to claim 5 or 6, The weld metal contains, by mass%, 5.0 to 20.0% Mn and 5.0 to 30.0% Ni.

14. The cryogenic liquefied gas tank according to claim 12, The weld metal contains, by mass%, 5.0 to 20.0% Mn and 5.0 to 30.0% Ni.

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