Welding method and welded joint for low-temperature steel materials, and container for storing low-temperature fluids.

Abstract

The present invention provides a welding method for low-temperature steel materials that yields welded joints with excellent low-temperature toughness and suppressed fracture of the weld metal when tensile load is applied. [Solution] A welding method for joining low-temperature steel materials (10, 20) together by multi-layer welding is provided, characterized in that, in the base layer 100, a welding material with a Ni content of 2.5 mass% or more and less than 7.0 mass% is used, and welding is performed such that the tensile strength of the base layer 100 is undermatched to the tensile strength of the low-temperature steel material (10, 20); and in the surface layer 200, a welding material with a Ni content of less than 2.5 mass% is used, and welding is performed such that the tensile strength of the surface layer 200 is overmatched to the tensile strength of the low-temperature steel material (10, 20); and the tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer satisfy a predetermined relational expression.

Description

[Technical Field]

[0001] This disclosure relates to a welding method and welded joint for low-temperature steel materials, as well as a container for storing low-temperature fluids. [Background technology]

[0002] Low-temperature steel materials include, for example, "carbon steel sheets for low-temperature pressure vessels" as described in JIS G 3126 and "nickel steel sheets for low-temperature pressure vessels" as described in JIS G 3127, and are used as materials for the pressure-resistant parts of tanks (storage vessels) that store low-temperature liquids and gases. When increasing the storage capacity or storage pressure of such tanks, higher-strength, high-tensile steel is used for the pressure-resistant parts. High-tensile steel generally has a tensile strength of 490 N / mm². 2 This refers to steel materials with a pressure of 490 MPa or higher.

[0003] When manufacturing tanks like the ones described above, low-temperature steel materials are joined together by welding. Normally, the welded joint requires a higher joint strength than the base material. However, when welding low-temperature steel materials, if a welding material with a high nickel content that has excellent low-temperature toughness is used, the joint strength may be lower than that of the base material, resulting in what is known as an undermatched joint.

[0004] Therefore, in welding high-tensile steel for low-temperature applications, there is a need for a welding method that ensures joint strength while maintaining low-temperature toughness.

[0005] Patent Document 1 discloses a technology relating to the structure of a welded joint that has higher joint strength than the base material while applying a partially soft joint that is highly effective in suppressing initial cracking and preventing brittle fracture.

[0006] Furthermore, Patent Document 2 discloses a technology that ensures the strength of the welded joint and prevents brittle fracture even when the tensile strength of the weld metal is lower than the tensile strength of the steel material (undermatch). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-22599 [Patent Document 2] International Publication No. 2017 / 130830 [Overview of the project] [Problems that the invention aims to solve]

[0008] Generally, welded joints are constructed by selecting a welding material with a tensile strength equal to or greater than that of the base material (the material being welded), resulting in what is known as an overmatched joint. However, selecting a high-strength welding material can lead to problems such as increased hardness and decreased low-temperature toughness as the strength of the welding material increases. Therefore, welding materials with a strength as close as possible to that of the base material are selected to create what is known as an even-matched joint.

[0009] However, in such even-matched joints, tensile tests of welded joints can result in undermatched joints where fracture occurs at the weld metal. The reason for this is that although both the base material and the welding material are manufactured to have a higher actual strength than the specified strength, there are differences in the actual strength of the base material and the welding material. For example, the actual strength of the base material differs depending on the rolling direction during manufacturing, and there is a difference in strength between the leading edge (top) and trailing edge (bottom) during rolling. The actual strength of the welding material also varies slightly depending on the manufacturing lot. On the other hand, the actual strength of the weld metal depends on the welding conditions. Generally, as the heat input increases, the actual strength of the weld metal decreases (one reason for this is that as the heat input increases, the cooling rate decreases and the amount of ferrite increases). Conversely, as the heat input decreases, the actual strength of the weld metal tends to increase. In particular, when welding is performed using welding materials with a Ni content of approximately 2.5% to 6.7% and with a high heat input, the actual strength of the weld metal tends to decrease significantly.

[0010] Thus, even when welding materials are selected to match the strength of the base material and welding is performed to create an even-match joint, it is not always the case that an even-match joint is achieved, and an under-match joint may occur. In the tensile test results of a welded joint, even if an under-match joint fractures at the weld metal, it will pass if the tensile test result is equal to or greater than the standard strength (tensile strength) of the base material. However, it is desirable for an over-match joint to fracture at the base material.

[0011] This disclosure is made in view of the above circumstances and aims to provide a welding method for low-temperature steel materials that can obtain a welded joint that has excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied, or a welded joint for low-temperature steel materials that has excellent low-temperature toughness and can suppress fracture of the weld metal when a tensile load is applied, or a low-temperature fluid storage container equipped with a welded joint that has excellent low-temperature toughness and suppresses fracture of the weld metal. [Means for solving the problem]

[0012] The welding method for low-temperature steel materials relating to this disclosure is: A welding method for joining low-temperature steel materials by multi-layer welding, The layers from the first layer to a predetermined layer of the weld metal are considered the base layer. When the layer outside the base layer of the weld metal is defined as the surface layer, In the base layer, welding is performed using a welding material with a Ni content of 2.5% or more and less than 7.0%, such that the tensile strength of the base layer is undermatched to the tensile strength of the low-temperature steel material. In the aforementioned surface layer, welding is performed using a welding material with a Ni content of less than 2.5% such that the tensile strength of the surface layer is an overmatch to the tensile strength of the low-temperature steel material. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. It is characterized by the following: Furthermore, the welded joints of low-temperature steel materials relating to this disclosure are A welded joint in which low-temperature steel materials with a thickness of 32 mm or more are joined together by multi-layer welding with weld metal, The aforementioned weld metal, A base layer having a tensile strength that is undermatched to the tensile strength of the aforementioned low-temperature steel material, A surface layer located outside the base layer and having a tensile strength that is an overmatch for the tensile strength of the low-temperature steel material, Equipped with, The base layer consists of a welding material with a Ni content of 2.5% by mass or more and less than 7.0% by mass. The aforementioned surface layer is made of a welding material with a Ni content of less than 2.5% by mass. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. It is characterized by the following: Furthermore, the cryogenic fluid storage container relating to this disclosure is The outer surface is made of low-temperature steel welded using the low-temperature steel welding method described above. -Storing fluids between 10°C and 60°C, It is characterized by the following: [Effects of the Invention]

[0013] According to this disclosure, it is possible to provide a welding method for low-temperature steel materials that can obtain a welded joint that has excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied. Alternatively, it is possible to provide a welded joint for low-temperature steel materials that has excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied. Alternatively, it is possible to provide a low-temperature fluid storage container equipped with a welded joint that has excellent low-temperature toughness and suppresses fracture of the weld metal. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic cross-sectional view showing low-temperature steel materials with X-shaped grooves arranged with their end faces butted together. [Figure 2] Figure 1 is a schematic cross-sectional view showing the state in which a base layer has been formed on the end face of the low-temperature steel material. [Figure 3]Figure 2 is a schematic cross-sectional view showing the state in which a surface layer has been formed on top of the base layer formed in Figure 2. [Figure 4] This is a schematic cross-sectional view showing the state after a portion of the surface layer has been cut away from the state shown in Figure 3. [Figure 5] This is a schematic cross-sectional view showing low-temperature steel materials with V-shaped grooves arranged in a butt-jointed configuration. [Figure 6] Figure 5 is a schematic cross-sectional view showing the state in which a base layer has been formed on the end face of the low-temperature steel material. [Figure 7] Figure 6 is a schematic cross-sectional view showing the state in which a surface layer has been formed on top of the base layer formed in Figure 6. [Figure 8] This is a schematic cross-sectional view showing the state after a portion of the surface layer has been removed from the state shown in Figure 7. [Modes for carrying out the invention]

[0015] The embodiments of this disclosure will be described below with examples, but it is obvious that this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of the invention relating to this disclosure are obtained. In addition, each component of the embodiments described below can be combined with one another. Furthermore, in this specification, numerical ranges represented by "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0016] [First Embodiment] The welding method for low-temperature steel materials according to this embodiment is a welding method that joins low-temperature steel materials together by multi-layer welding.

[0017] Low-temperature steel has a tensile strength of 490 N / mm². 2 (490 MPa) or higher is preferable. While there are no particular limitations on low-temperature steel materials from the perspective of their use as pressure-resistant materials for tanks (storage vessels) that store low-temperature fluids (liquids or gases), the following are examples of low-temperature steel materials: SLA365 (minimum usable temperature: -60℃, tensile strength: 490~610N / mm²) as described in JIS G 3126 "Carbon steel sheets for low-temperature pressure vessels". 2 ) or SLA410 (minimum usable temperature: -60℃, tensile strength: 520~640N / mm) 2 ) is preferable. While there are no particular limitations on low-temperature steel materials from the perspective of their use as pressure-resistant materials for tanks (storage vessels) that store low-temperature fluids (liquids or gases), SL3N275 (minimum usable temperature: -101℃, tensile strength: 480~620N / mm²) as described in JIS G 3127 "Nickel steel sheets for low-temperature pressure vessels" is a good example. 2 ), SL3N440 (Minimum usable temperature: -110℃, Tensile strength: 540~690N / mm) 2 ) or SL3N590 (Minimum usable temperature: -130℃, Tensile strength: 690~830N / mm) 2 ) is preferable.

[0018] In the welding method for low-temperature steel materials according to this embodiment, the plate thickness (thickness) of the low-temperature steel material is preferably 32 mm or more. This configuration has the advantage of allowing the application of a welding method that joins materials by multi-layer welding. The plate thickness of low-temperature steel is determined by measuring the plate thickness at any point on the steel plate using calipers, a micrometer, or an ultrasonic thickness gauge, and adopting the minimum value. Alternatively, the design plate thickness of the tank (storage tank) can be used.

[0019] While there are no particular limitations on low-temperature steel materials from the perspective of their use as pressure-resistant materials for tanks (storage vessels) that store low-temperature fluids (liquids or gases), LT365-VI-70G-40AQ (plate thickness 40-50 mm, minimum operating temperature: -60 to -70°C), which has been certified under WES 3003 (criteria for low-temperature rolled steel sheets), is more preferable. While there are no particular limitations on low-temperature steel materials from the perspective of their use as pressure-resistant material for tanks (storage vessels) that store low-temperature fluids (liquids or gases), LT490-VI-60G-30AQ (plate thickness 40-50 mm, minimum operating temperature: -50 to -60°C), which has been certified under WES 3003 (criteria for low-temperature rolled steel sheets), is more preferable.

[0020] It is preferable that the low-temperature steel materials to be joined are of the same type and have the same plate thickness. The low-temperature steel materials described above may be used, for example, as materials for the pressure-resistant parts of tanks (storage vessels) that store low-temperature fluids (liquids or gases).

[0021] Multilayer welding is a welding method in which multiple weld beads are layered during welding to form the weld metal. In multilayer welding, the weld metal is formed in multiple layers. Here, one layer may be formed in two or more passes. By polishing and etching the cross-section of the weld metal formed by multi-layer welding, it can be observed and confirmed that the weld metal is composed of multiple layers, thus verifying the formation method of the weld metal.

[0022] In the welding method for low-temperature steel materials according to this embodiment, the low-temperature steel material has a Ni content of less than 7% by mass and a tensile strength of 490 N / mm². 2 The above configuration is preferable. This configuration has the effect of reducing the weight of the material while ensuring the low-temperature toughness of the material in the pressure-resistant part of a tank (storage vessel) for storing low-temperature fluids (liquids or gases). The Ni content can be measured according to JIS G 1253 (Iron and Steel - Spark Discharge Emission Spectroscopy Method). The tensile strength of a welded joint can be measured by processing low-temperature steel into a test specimen conforming to JIS Z 3121 (Tensile Test Method for Butt Welded Joints) and using the method specified in JIS Z 2241 (Tensile Test Method for Metallic Materials).

[0023] In the welding method for low-temperature steel materials according to this embodiment, the layers from the first layer to a predetermined layer of the weld metal are defined as the base layer. The first layer of weld metal is the first layer formed in multi-layer welding. The first layer of weld metal is formed in the first layer of multi-layer welding. A "predetermined layer" refers to a layer formed from the second layer onwards in multi-layer welding. Here, each layer may consist of multiple paths.

[0024] The base layer consists of layers from the first layer to a predetermined layer. For example, if a predetermined layer is formed as the nth layer, the base layer is composed of n layers of weld metal. The predetermined layer becomes the nth layer and is the outermost layer in the base layer.

[0025] In the welding method for low-temperature steel materials according to this embodiment, the layer outside the base layer of the weld metal is defined as the surface layer. The surface layer includes the final layer. The final layer is the layer that is exposed to the outside among the layers that make up the weld metal. The layers constituting the surface layer are formed from the following layers, which form the predetermined layer (the outermost layer in the base layer) included in the base layer as described above.

[0026] The surface layer preferably consists of at least two layers, including the final layer located on the outermost side of the weld metal and a layer adjacent to the final layer. This configuration has the effect of ensuring the necessary strength in the surface layer. Here, strength refers to tensile strength.

[0027] If the surface layer consists of two or more layers, the surface layer consists of layers from the layer adjacent to the predetermined layer and located outside the predetermined layer to the final layer. For example, if a predetermined layer is formed in n layers, the base layer consists of n layers, and the surface layer includes the (n+1)th layer and layers located outside the (n+1)th layer (including the final layer).

[0028] Here, "outside" refers to the side on which the final layer is located relative to the first layer in the direction in which the weld beads are overlapped.

[0029] In the welding method for low-temperature steel materials according to the present embodiment, a welding material having a Ni content of 2.5% or more and less than 7.0% is used for the base layer portion, and welding is performed such that the tensile strength of the base layer portion is under-matched with respect to the tensile strength of the low-temperature steel material. Note that the base layer portion does not need to be intentionally under-matched, and may consequently be under-matched. That is, even if over-matching is intended, as described above, there are differences in the actual strengths of the base material and the welding material, and due to differences in actual strength between production lots and differences in the actual strength of the weld metal depending on the welding heat input, unintended under-matching may occur.

[0030] The Ni content of the welding material can be measured in accordance with JIS G 1253.

[0031] Under-matching means that when a joint is formed by welding, the actual strength of the base material (ATS B ) > the actual strength of the weld metal (ATS W ) is satisfied. Here, the actual strength refers to the actual tensile strength. In such a joint (under-matched joint), the weld metal becomes the fracture position in a tensile test.

[0032] In the welding method for low-temperature steel materials according to the present embodiment, a welding material having a Ni content of less than 2.5 mass% is used for the surface layer portion, and welding is performed such that the tensile strength of the surface layer portion is over-matched with respect to the tensile strength of the low-temperature steel material. Note that as long as the low-temperature toughness specified for the low-temperature steel material is satisfied, the welding material having a Ni content of less than 2.5 mass% may have lower low-temperature toughness than the welding material having a Ni content of 2.5 mass% or more and less than 7.0 mass%. That is, the welding material having a Ni content of 2.5 mass% or more and less than 7.0 mass% preferably has superior low-temperature toughness to the welding material having a Ni content of less than 2.5 mass%.

[0033] Over-matching means that when a joint is formed by welding, the actual strength of the base material (ATS B ) < the actual strength of the weld metal (ATSW ) This means that the following relationship is satisfied. In such a joint (overmatch joint), the base material will be at the fracture point during a tensile test.

[0034] Even match refers to the condition when a joint is formed by welding. Actual strength of the base material (ATS) B ) = Actual strength of weld metal (ATS) W ) This means that the following relationship is satisfied. In an even-match joint, the fracture location is either the base metal or the weld metal. Even if fracture occurs in the weld metal in an even-match joint, the joint performance is acceptable if the tensile test result is equal to or greater than the specified strength of the base metal.

[0035] In the welding method for low-temperature steel materials according to this embodiment, the tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer satisfy a predetermined relational expression. For example, in the welding method for low-temperature steel materials according to this embodiment, it is preferable that the following equations 1 and 2 are satisfied.

[0036] ATS B ×t B ≦ ATS W1 × (t B -(t 2w +t 2w '))+ATS W2 × (t 2w +t 2w ') ...expression 1

[0037] ATS W1 < ATS B < ATS W2 ...Formula 2

[0038] In equations 1 and 2, The actual tensile strength (actual strength) of low-temperature steel materials is measured using ATS. B , The actual tensile strength of the base layer is measured by ATS. W1 , The actual tensile strength of the surface layer is measured by ATS W2 , The thickness of the low-temperature steel material is t B , The size of the remaining groove in the surface layer on one side of the low-temperature steel material is t 2w , The size of the remaining groove in the surface layer on the other side of the low-temperature steel material is t 2w ', Let's assume that.

[0039] By satisfying the above equations 1 and 2, a welded joint is obtained in which fracture of the weld metal is suppressed when a tensile load is applied.

[0040] Actual Strength (ATS) of the Base Layer of Weld Metal W1 and the actual strength of the surface layer ATS W2 The measured values ​​from the tensile test are used. Specifically, the actual strength ATS is determined in advance using the tensile testing method specified in JIS Z 3111. W1 and actual strength ATS W2 It can be measured.

[0041] In order to make a correct determination using Equations 1 and 2, it is preferable to confirm the actual tensile strength of the low-temperature steel material, the welding material for the base layer, and the welding material for the surface layer by fabricating the welded joint in advance and conducting tensile tests in accordance with JIS Z 2241 or JIS Z 3111.

[0042] Using Figures 1 to 4, the size of the remaining groove (t 2w or t 2w Let's explain the '). Figures 1 to 4 show the process from the state in which the low-temperature steel material to be welded is placed to the state in which the final finishing treatment (removal of excess weld material) is performed to create the welded joint 1. In the examples in these figures, the groove shape is X-shaped. The cross-sections in Figures 1 to 4 are surfaces perpendicular to the welding direction (the direction in which the weld bead is formed) of the base material, which is low-temperature steel.

[0043] Figure 1 shows the low-temperature steel materials arranged with their end faces butted together. In Figure 1, the surfaces (faces 11a and 11b) of the low-temperature steel material 10 and the surfaces (faces 21a and 21b) of the low-temperature steel material 20 are the same height in the thickness direction. In the example in Figure 1, the thickness of the low-temperature steel material 10 and the thickness of the low-temperature steel material 20 are the same, and their thicknesses are t B Let's assume that. B This thickness is defined as the thickness perpendicular to the surface of the low-temperature steel material. A beveled surface 12 is provided on the end face of the low-temperature steel material 10. A beveled surface 22 is provided on the end face of the low-temperature steel material 20.

[0044] Figure 2 shows a state in which multi-layer welding is performed on the butted end faces to form a base layer 100 of the weld metal. Although not shown in detail in Figure 2, the base layer 100 may consist of multiple layers. As shown in Figure 2, t 2w This is the thickness direction (t) of the low-temperature steel materials (10 and 20). B This is the distance from the surface (11a and 21a) of the low-temperature steel material (10 and 20) to the base layer 100 in the same direction as the direction that defines the temperature. Since the surface of the base layer 100 is not flat, the distance from the surface (11a and 21a) of the low-temperature steel material (10 and 20) to the point closest to the surface (11a and 21a) of the low-temperature steel material (10 and 20) in the thickness direction is t 2w Let's assume that.

[0045] t 2w Regarding ', t 2w Similarly, the thickness direction (t) of the low-temperature steel materials (10 and 20) B This is defined as the distance from the opposite surface (11b and 21b) of the low-temperature steel material (10 and 20) to the base layer 100 in the same direction as the direction that defines the function.

[0046] Figure 3 shows a state in which a surface layer 200 is formed on top of the base layer 100 by multi-layer welding. Figure 3 shows an example in which two layers, layer 201 and layer 202, are provided as the surface layer 200. In this example, layer 202 is the final layer.

[0047] As described above, in the welding method for low-temperature steel materials according to this embodiment, the welding material forming the base layer 100 has a Ni content of 2.5% by mass or more and less than 7.0% by mass, and welding is performed so that the tensile strength of the base layer 100 is undermatched to the tensile strength of the low-temperature steel materials (10 and 20). Furthermore, the welding material forming the surface layer 200 has a Ni content of less than 2.5% by mass, and welding is performed so that the tensile strength of the surface layer 200 is overmatched to the tensile strength of the low-temperature steel materials (10 and 20).

[0048] Figure 4 shows a welded joint 1 with weld metal 300. Figure 4 shows a state in which a portion of the surface layer 200 has been cut to match the height of the surface (faces 11a and 11b) of the low-temperature steel material 10 and the surface (faces 21a and 21b) of the low-temperature steel material 20.

[0049] As shown in Figure 4, the excess weld material on the surface layer 200 is removed, so the strength of the welded joint 1 does not reflect the excess material. When used as a material for the pressure-resistant part of a tank (storage vessel) for storing low-temperature fluids (liquids or gases), there is a regulation that the surface of the weld must be smooth, not lower than the surface of the base metal, and without any steps between it and the surface of the base metal. However, the welding method for low-temperature steel materials specified in the above formulas 1 and 2 is preferable because it has the effect of producing a welded joint in which fracture of the weld metal is suppressed when a tensile load is applied, even in a welded joint without excess weld metal.

[0050] A weld refers to the area formed by welding, including the heat-affected zone and weld metal. Weld metal is defined as the area within the weld that melted and solidified during welding. When filler materials such as covered arc welding rods or welding wires are used, it refers to the area where the base metal and filler material have molten composition due to welding. For example, when a cross-sectional sample is taken perpendicular to the weld line containing the weld, etched with Nital, and observed with an optical microscope, the area located in the center of the weld surrounded by the bond is determined to be the weld metal.

[0051] During welding, a portion of the weld metal and base metal from the lower layer melts into the molten weld material, resulting in a mixture with a portion of the weld metal. However, the volume of the mixed portion is generally about 10% of the total weld metal, and the reduction in actual strength due to this mixing is less than 10%. Similarly, the increase in layer thickness due to penetration into the lower layer is also about 10%, so the reduction in actual strength is less than the increase in layer thickness. Therefore, the impact of the weld metal of the base layer 100 that has melted into the surface layer 200 on the actual strength can be ignored.

[0052] In the welding method for low-temperature steel materials according to this embodiment, it is preferable to perform multi-layer welding from both sides of the low-temperature steel materials (10 and 20) to form a base layer 100 and a surface layer 200 on both sides. This configuration has the effect of obtaining a welded joint that does not break at the weld metal when a tensile load is applied more reliably. Figures 1 to 4 show examples of welding performed on both sides of low-temperature steel materials (10 and 20).

[0053] Thickness t of low-temperature steel material B The thickness of the steel plate is measured at any point using a caliper, micrometer, or ultrasonic thickness gauge, and the minimum value is adopted. Alternatively, the design plate thickness of the tank (storage tank) can be used. Size of remaining bevel t 2w , and the size of the remaining groove t 2w ' is measured using calipers or calipers and a metal ruler (square ruler).

[0054] When welding is performed from only one side of the surface of the low-temperature steel materials (10 and 20), the remaining groove size on the other side will be 0.

[0055] In the welding method for low-temperature steel materials according to this embodiment, it is preferable that the thickness of the surface layer is smaller than the thickness of the base layer. The thickness of the surface layer and the thickness of the base layer are the thicknesses in the direction perpendicular to the surface of the low-temperature steel material. The thickness of the surface layer is the thickness after the excess weld material has been removed. This configuration has the effect of balancing the strength and toughness of the base layer and the surface layer.

[0056] In the welding method for low-temperature steel materials according to this embodiment, the groove shape of the weld is preferably X-shaped or K-shaped. This configuration has the effect of maintaining a balance in the strength of the surface layers on both sides of the steel material.

[0057] In the welding method for low-temperature steel materials according to this embodiment, the groove shape of the weld is preferably V-shaped or L-shaped. When the groove shape of the weld is V-shaped or L-shaped, it is preferable to perform back gouging on the side with the smaller groove. This configuration has the effect of maintaining a balance in the strength of the surface layer on both sides of the steel material.

[0058] Figures 5 to 8 illustrate the process from the initial positioning of the low-temperature steel material to be welded to the final finishing treatment (removal of excess weld material) to the completed welded joint 2, similar to Figures 1 to 4. In these examples, the groove shape is V-shaped. The cross-sections in Figures 5 to 8 are surfaces perpendicular to the surface of the base material, the low-temperature steel material, and to the welding direction (the direction in which the weld bead is formed). Figure 5 shows the low-temperature steel material with its end faces butted together. Figure 6 shows the base layer 400 of the weld metal formed by multi-layer welding at the butted end faces. Figure 7 shows the surface layer 500 (including two layers, 501 and 502) formed on top of the base layer 400 by multi-layer welding. Figure 8 shows the welded joint 2 with the weld metal 600.

[0059] The welding method for low-temperature steel materials according to this embodiment provides a welding method that can produce a joint with excellent low-temperature toughness and suppressed fracture of the weld metal when a tensile load is applied.

[0060] [Second Embodiment] The welded joint for low-temperature steel materials according to this embodiment is a welded joint in which low-temperature steel materials with a plate thickness of 32 mm or more are joined together by multi-layer welding with weld metal. The plate thickness of the low-temperature steel material is determined, as in the first embodiment, by measuring the plate thickness at any point on the steel plate using a caliper, micrometer, or ultrasonic thickness gauge, and adopting the minimum value. Alternatively, the design plate thickness of the tank (storage tank) can be used.

[0061] The welded joint for low-temperature steel materials according to this embodiment is, for example, the welded joint 1 shown in Figure 4.

[0062] The weld metal 300 comprises a base layer 100 having a tensile strength that is undermatched to the tensile strength of the low-temperature steel materials (10 and 20), and a surface layer 200 located outside the base layer 100 and having a tensile strength that is overmatched to the tensile strength of the low-temperature steel materials (10 and 20). Here, the relationship between undermatch and overmatch is as described in the first embodiment.

[0063] The base layer 100 and surface layer 200 of the weld metal 300 may consist of multiple layers. For weld metal 300 formed by multi-layer welding, a sample containing the weld metal 300 and a cross-section perpendicular to the welding direction is taken, and the cross-section is polished and etched for observation to confirm the weld metal formation method, thereby confirming that it is composed of multiple layers.

[0064] The base layer 100 is made of a welding material with a Ni content of 2.5% by mass or more and less than 7.0% by mass, and the surface layer 200 is made of a welding material with a Ni content of less than 2.5% by mass. The boundary between the base layer 100 and the surface layer 200 can be determined by examining the Ni concentration using EPMA (electron probe microanalyzer) or XRF (X-ray fluorescence analyzer).

[0065] The welded joint of the low-temperature steel material according to the second embodiment can preferably be manufactured by the welding method for the low-temperature steel material according to the first embodiment.

[0066] The welded joint of the low-temperature steel material according to this embodiment exhibits excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied.

[0067] [Third Embodiment] The cryogenic fluid storage container according to this embodiment has an outer surface made of cryogenic steel welded using the cryogenic steel welding method according to the first embodiment. The low-temperature fluid storage container according to this embodiment is used for storing fluids at temperatures below -10°C or above -60°C.

[0068] The fluids stored in the cryogenic fluid storage containers are not particularly limited, but examples include liquefied ammonia (liquefaction point at atmospheric pressure -33.4°C) and carbon dioxide (melting point at 5.2 atmospheres -56.6°C). Atmospheric pressure is defined as 1 atmosphere.

[0069] The low-temperature fluid storage container according to this embodiment is preferable because it is welded using a low-temperature steel welding method that exhibits excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied.

[0070] (Note) The welding method for low-temperature steel materials, the welded joint for low-temperature steel materials, and the container for storing low-temperature fluids according to the above embodiment can be understood, for example, as follows. (1) The welding method for low-temperature steel materials relating to this disclosure is A welding method for joining low-temperature steel materials by multi-layer welding, The layers from the first layer to a predetermined layer of the weld metal are considered the base layer. When the layer outside the base layer of the weld metal is defined as the surface layer, In the base layer, welding is performed using a welding material with a Ni content of 2.5% by mass or more and less than 7.0% by mass, such that the tensile strength of the base layer is undermatched to the tensile strength of the low-temperature steel material. In the aforementioned surface layer, welding is performed using a welding material with a Ni content of less than 2.5% by mass, such that the tensile strength of the surface layer overmatches the tensile strength of the low-temperature steel material. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. It is characterized by the following: The welding method for low-temperature steel materials, as described above, can produce a welded joint that exhibits excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied.

[0071] (2) In the welding method for low-temperature steel materials described in (1) above, Multi-layer welding is performed on both sides of the aforementioned low-temperature steel material. The base layer and the surface layer may be formed on any of the surfaces. This has the effect of maintaining a balance in the strength of the surface layers on both sides of the steel material. (3) In the welding method for low-temperature steel materials described in (1) or (2) above, The groove shape of the weld is X-shaped, K-shaped, V-shaped, or L-shaped, and back gouging may be performed. This has the effect of maintaining a balance in the strength of the surface layers on both sides of the steel material. (4) In the welding method for low-temperature steel materials described in any one of the above items (1) to (3), The thickness of the surface layer may be less than the thickness of the base layer. This has the effect of ensuring the toughness of the base layer. (5) In the welding method for low-temperature steel materials described in any one of the above items (1) to (4), The actual tensile strength of the aforementioned low-temperature steel material is determined by ATS B , The actual tensile strength of the base layer is determined by ATS W1 , The actual tensile strength of the surface layer is determined by ATS W2 , The thickness of the aforementioned low-temperature steel material is t B , The size of the remaining groove in the surface layer on one side of the low-temperature steel material is t 2w , The size of the remaining groove in the surface layer on the other side of the low-temperature steel material is t 2w ', In this case, the following equations 1 and 2 may also be satisfied. ATS B ×t B ≦ ATS W1 × (t B -(t 2w +t 2w '))+ATS W2 × (t 2w +t 2w ') ...expression 1 ATS W1 < ATS B < ATS W2 ...Formula 2 This has the effect of providing a welded joint in which fracture of the weld metal is suppressed when a tensile load is applied. (6) In the welding method for low-temperature steel materials described in any one of the above items (1) to (5), The surface layer may consist of at least two layers, including the outermost final layer of the weld metal and a layer adjacent to the final layer. This has the effect of adjusting the balance of strength between the base layer and the surface layer. (7) In the welding method for low-temperature steel materials described in any one of the above items (1) to (6), The aforementioned low-temperature steel material may have a plate thickness of 32 mm or more. This makes it easier to perform multi-layer welding and also has the effect of increasing the storage capacity of the tank or raising the storage pressure. (8) In the welding method for low-temperature steel materials described in any one of the above items (1) to (7), The aforementioned low-temperature steel material may have a Ni content of less than 7% by mass. This allows for the production of lower-cost tanks by reducing the amount of expensive nickel, while also achieving excellent low-temperature toughness and suppressing fracture of the weld metal when tensile loads are applied. (9) In the welding method for low-temperature steel materials described in any one of the above items (1) to (8), The aforementioned low-temperature steel material has a tensile strength of 490 N / mm². 2 That's fine too. This has the effect of increasing the storage capacity of the tank or raising the storage pressure.

[0072] (10) The welded joints of low-temperature steel materials relating to this disclosure are A welded joint in which low-temperature steel materials with a thickness of 32 mm or more are joined together by multi-layer welding with weld metal, The aforementioned weld metal, A base layer having a tensile strength that is undermatched to the tensile strength of the aforementioned low-temperature steel material, A surface layer located outside the base layer and having a tensile strength that is an overmatch for the tensile strength of the low-temperature steel material, Equipped with, The base layer consists of a welding material with a Ni content of 2.5% or more and less than 7.0%. The aforementioned surface layer is made of a welding material with a Ni content of less than 2.5%. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. This is a welded joint characterized by the following: In a welded joint with the above configuration, low-temperature toughness is excellent, and fracture of the weld metal is suppressed when a tensile load is applied.

[0073] (11) The cryogenic fluid storage container relating to this disclosure is The outer surface is made of low-temperature steel welded using the welding method for low-temperature steel described in any one of the above items (1) to (9), -Storing fluids between 10°C and 60°C, This is a container for storing low-temperature fluids, characterized by the following features. The low-temperature fluid storage container with the above configuration has the advantage of being applicable to the storage of liquefied ammonia at room temperature and carbon dioxide at low temperature and medium pressure. [Examples]

[0074] The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

[0075] [Example 1] In this example, welded joints were fabricated by joining steel materials having the tensile strength and Ni content shown in Table 1 using welding material 1 and welding material 2 shown in Table 2. Specifically, two identical steel materials A were welded together. The plate thickness of steel material A was 50 mm. Welding material 1 was used as the base layer, and welding material 2 was used as the surface layer. The welding conditions were a welding current of 170 A, a welding voltage of 24 V, and a heat input of 14 to 17 kJ / cm as standard.

[0076] [Table 1]

[0077] The tensile strength and Ni content shown in Table 1 were referenced from the values ​​listed on the steel mill sheet (inspection certificate). The tensile strengths in Table 1 were measured using test specimens conforming to JIS Z 2241 and methods conforming to JIS Z 2241. Chemical components such as Ni were analyzed in accordance with JIS G 0321 (Product analysis methods and permissible variations thereof for steel materials). Actual tensile strength (ATS) of steel material A. B The average value is 669 N / mm², the same as the value listed on the mill sheet. 2 That was the case.

[0078] [Table 2]

[0079] The tensile strength and Ni content shown in Table 2 were obtained by referring to the values ​​listed on the mill sheet (inspection certificate) of the welding material.

[0080] The tensile strengths in Table 2 were measured according to the method specified in JIS Z 3211 (Covered arc welding rods for mild steel, high-tensile steel, and low-temperature steel). Specifically, the tensile strength was measured in accordance with JIS Z 3111 (Method for Tension and Impact Tests of Deposited Metal). Chemical components such as Ni were analyzed in accordance with JIS G 0321. Note that the welding conditions specified in JIS Z 3111 are in the flat position. The standard welding conditions are a welding current of 170 A, a welding voltage of 24 V, and a heat input of 14 to 17 kJ / cm.

[0081] A welded joint fabricated at a heat input of 14 to 17 kJ / cm under standard welding conditions in the flat position satisfied the conditions of the following Formula 1 and Formula 2. As a result of measurement, the actual tensile strength (ATS W1 ) of the base layer portion welded with welding material 1 is, for example, 661 N / mm 2 , and the actual tensile strength (ATS W2 ) of the surface layer portion welded with welding material 2 is, for example, 680 N / mm 2 . The actual measured value of the steel material thickness (t B ) is 50.5 mm, the size of the remaining groove of the surface layer portion on one side (t 2w ) is, in one example, 15 mm, and the size of the remaining groove of the surface layer portion on the other side (t 2w ') is, in one example, 8 mm. t B , t 2w and t 2w ' were measured with a vernier caliper.

[0082] ATS B ×t B ≦ ATS W1 ×(t B -(t 2w +t 2w '))+ATS W2 ×(t 2w +t 2w ') ···Formula 1

[0083] ATS W1 < ATS B < ATS W2 ···Formula 2

[0084] This Formula 2 is the establishment condition of the present disclosure. Specifically, the term ATS on the left side of Formula 2W1 < ATS B indicates that the actual strength of the base layer portion is lower than the actual strength of the low-temperature steel material, and the term ATS on the right side of Formula 2 B < ATS W2 indicates that the actual strength of the surface layer portion is higher than the actual strength of the low-temperature steel material, which is the establishment condition.

[0085] If the term on the left side of Formula 2 is ATS W1 > ATS B and does not satisfy the above establishment condition, fracture occurs in the base material unconditionally. Further, if the term on the right side of Formula 2 is ATS B > ATS W2 and does not satisfy the aforementioned establishment condition, fracture occurs in the welded portion unconditionally. When the term on the left side of Formula 2 is ATS W1 = ATS B or the term on the right side is ATS B = ATS W2 , the joint is an even match joint, and it becomes impossible to determine whether fracture occurs in the base material or in the welded portion.

[0086] A welded joint produced by the method for welding a low-temperature steel material according to the present disclosure will result in fracture in the base material instead of the weld metal if Formula 1 is satisfied.

[0087] [Example 2] The following example is a joint strength verification test for the purpose of proving the validity of Formula 1, and for confirming that the estimated tensile strength of the weld metal in the welded joint matches the actual tensile strength of the weld metal. This example (joint strength verification test) is intended to prove the validity of Formula 1. Therefore, experimental examples that do not satisfy the welding material in which the Ni content in the base layer portion is 2.5 mass% or more and less than 7.0 mass%, and experimental examples that do not satisfy Formula 2 are also included.

[0088] An overmatched joint that satisfies Equation 1 will result in base metal fracture in a tensile test, making it impossible to obtain the actual tensile strength (actual strength) of the weld metal. Therefore, an undermatched joint that does not satisfy Equation 1 was fabricated and intentionally fractured at the weld metal to determine the actual strength of the weld metal. Specifically, in order to prove the validity of Equation 1, t B t 2w t 2w A welded joint (undermatched joint) was fabricated with the weld metal set to be undermatched, and a tensile test was conducted to verify that the joint fractured at the weld metal and that the tensile strength of the joint could be correctly evaluated.

[0089] Equation 3 below is derived from Equation 1', which is a variation of Equation 1, and is an equation for estimating the tensile strength of the weld metal, and can be directly compared with the actual strength of the weld metal obtained from confirmation tests. Specifically, Equation 1' is obtained by multiplying both sides of Equation 1 by t B This is the result of dividing by . Equation 3 is the right-hand side of Equation 1', and it is an estimated value of the tensile strength of the weld metal.

[0090] ATS B ≤ (ATS W1 × (t B -(t 2w +t 2w '))+ATS W2 × (t 2w +t 2w '))÷t B ...Formula 1' (ATS W1 × (t B -(t 2w +t 2w '))+ATS W2 × (t 2w +t 2w '))÷t B ...Formula 3

[0091] The joint strength of an undermatched joint that fractures at the weld metal is the actual strength of the weld metal. Therefore, by comparing the estimated tensile strength of the weld metal in Equation 3 with the actual strength of the weld metal, if they match, then Equation 3 can be said to be valid. If Equation 3 is valid, then Equation 1', a variation of Equation 1, is valid as a conditional expression for determining whether or not the base metal fractures. Therefore, Equation 1 is also valid.

[0092] In the joint strength verification test, specifically, welding material 1 was used as the base layer and welding material 2 as the surface layer to join two steel plates A with a thickness of 50 mm (actual measurement 50.6 mm). Welded joints A and B were then fabricated according to Figures 1 to 4 and the test was conducted. Welded joints A and B were each welded joints having an X-shaped groove.

[0093] Furthermore, a welded joint C was fabricated by joining two steel plates A with a thickness of 38 mm (actual measurement 38.6 mm) using welding material 3 as the base layer and welding material 4 as the surface layer. Furthermore, as shown in Figures 5 to 8, the welded joint C was formed by first positioning the low-temperature steel material to be welded, then performing the final finishing treatment (removal of excess weld material) to create a welded joint with a V-shaped groove.

[0094] Welded joints A, B, and C were fabricated by welding in a horizontal position, which is commonly used in tanks (storage vessels). The welding conditions were appropriate for the horizontal position. Specifically, the welding conditions were a combination of welding current, welding voltage, and welding speed that resulted in a welding heat input of 20 to 35 kJ / cm. The welding heat input [kJ / cm] was calculated as (welding current [A] × welding voltage [V] × 60) ÷ welding speed [cm / min].

[0095] (Tensile strength test of welding materials) To confirm the actual strength of the weld metals formed using welding materials 1 through 4, test specimens were taken from welded joint B, where welding material 1 was used as the base layer and welding material 2 as the surface layer, and from welded joint C, where welding material 3 was used as the base layer and welding material 4 as the surface layer, including either the base layer or the surface layer, in accordance with JIS Z 3111. The tensile strength of the weld metal was measured for these test specimens using a method in accordance with JIS Z 2241. Tensile strength measurements were performed twice for each test specimen. Test specimens were taken from the base layer and surface layer of welded joints B and C, respectively. Table 3 shows the tensile strength measurement results for each weld metal. The average tensile strength in Table 3 is the arithmetic mean of the measured tensile strengths.

[0096] [Table 3]

[0097] Table 3 shows that the tensile strength of the weld metal for welding materials 1 to 4 is lower than the tensile strength in Table 2. This demonstrates that, as described in

[0009] , the actual strength of the weld metal decreases as the heat input increases. As a result, the actual strength (ATS) of welding material 2 in the surface layer of Table 3 is lower. W2 ) is 663.5 N / mm 2 Therefore, the actual strength of the base material (ATS B ) 669N / mm 2 The result was lower than the ATS term on the right side of Equation 2. B < ATS W2 The result was that it did not meet the requirements. However, in Equation 3, the actual strength of the base material (ATS) B () is not included, therefore the estimated tensile strength of the weld metal is not calculated using the actual strength of the base metal (ATS). B ) does not have any effect. Therefore, it does not affect the comparison between the estimated tensile strength of the weld metal and the actual strength of the weld metal.

[0098] As described in

[0083] , the right-hand term of Equation 2 is ATS B > ATS W2In that case, the weld would unconditionally fracture. However, the purpose of this verification test is to cause fracture at the weld metal, and this does not contradict that.

[0099] Welding material 3 and welding material 4 were selected for use in joint strength verification tests, with welding material 3 used in the base layer and welding material 4 in the surface layer. Welding material 3, used in the base layer, has a lower Ni content and lower tensile strength than welding material 1. Welding material 4, used in the surface layer, has a higher Ni content and higher tensile strength than welding material 2. Furthermore, the welding material 3 used in the base layer has a Ni content of 2.28 mass%, which does not satisfy the requirement of a Ni content of 2.5 mass% or more and less than 7.0 mass%. The actual tensile strength of the weld metal shown in Table 3 is a measured value, and the Ni content does not affect the comparison between the estimated tensile strength of the weld metal and the actual tensile strength of the weld metal.

[0100] (Comparison of measured values ​​and actual values ​​in welded joints) Tensile tests were performed on welded joints A, B, and C in accordance with JIS Z 3121. Table 4 shows the estimated tensile strength values ​​using Equation 3 and the tensile test results (measured values) of the welded joint. For the estimated values, use the ATS formula in Equation 3. W1 and ATS W2 The average tensile strength of the welding material in Table 3 was used to calculate it. B t 2w t 2w ' represents the actual measured value using calipers.

[0101] [Table 4]

[0102] The estimated tensile strength of the weld metal in Equation 3 and the tensile test results (measured values) of the welded joint (undermatched joint) showed good agreement, thus proving the validity of Equation 1. Therefore, it can be understood that if Equation 1 is satisfied, fracture will occur in the base metal and not in the weld metal. [Industrial applicability]

[0103] The welding method for low-temperature steel materials according to this disclosure provides a welded joint that exhibits excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied. Furthermore, the welded joint for low-temperature steel materials according to this disclosure exhibits excellent low-temperature toughness and suppresses fracture of the weld metal when a tensile load is applied. In addition, the low-temperature fluid storage container according to this disclosure is equipped with a welded joint that exhibits excellent low-temperature toughness and suppresses fracture of the weld metal. Therefore, the invention according to this disclosure is extremely useful in industry. [Explanation of Symbols]

[0104] 1, 2 Welded joints 10, 20, 30, 40 Low temperature steel 11a, 11b, 21a, 21b, 31a, 31b, 41a, 41b side 12, 22, 32, 42 bevel surfaces 100, 400 base layer 200, 500 surface layer 300, 600 weld metal

Claims

1. A welding method for joining low-temperature steel materials by multi-layer welding, The layers from the first layer to a predetermined layer of the weld metal are considered the base layer. When the layer outside the base layer of the weld metal is defined as the surface layer, In the base layer, welding is performed using a welding material with a Ni content of 2.5% by mass or more and less than 7.0% by mass, such that the tensile strength of the base layer is undermatched to the tensile strength of the low-temperature steel material. In the aforementioned surface layer, welding is performed using a welding material with a Ni content of less than 2.5% by mass, such that the tensile strength of the surface layer overmatches the tensile strength of the low-temperature steel material. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. A welding method for low-temperature steel materials, characterized by the following features.

2. Multi-layer welding is performed on both sides of the aforementioned low-temperature steel material. In each of the surfaces, the base layer and the surface layer are formed. The welding method for low-temperature steel materials according to claim 1, characterized by its features.

3. The groove shape of the weld is X-shaped, K-shaped, V-shaped, or L-shaped, and back gouging is performed. The welding method for low-temperature steel materials according to claim 2, characterized in that it is a welding method for low-temperature steel materials as described in claim 2.

4. The thickness of the surface layer is less than the thickness of the base layer. A welding method for low-temperature steel materials according to any one of claims 1 to 3.

5. The actual tensile strength of the aforementioned low-temperature steel material is determined by ATS Ｂ , The actual tensile strength of the base layer is determined by ATS Ｗ１ , The actual tensile strength of the surface layer is determined by ATS Ｗ２ , The thickness of the aforementioned low-temperature steel material is t Ｂ , The size of the remaining groove in the surface layer on one side of the low-temperature steel material is t ２ｗ , The size of the remaining groove in the surface layer on the other side of the low-temperature steel material is t ２ｗ ', When this is the case, the following equations 1 and 2 are satisfied: A welding method for low-temperature steel materials according to claim 2 or 3, characterized by the features described above. ATS Ｂ ×t Ｂ ≤ ATS Ｗ１ ×(t Ｂ -(t ２ｗ +t ２ｗ ')) + ATS Ｗ２ ×(t ２ｗ +t ２ｗ ') ・・・Equation 1 ATS Ｗ１ < ATS Ｂ < ATS Ｗ２ ... Equation 2

6. The surface layer consists of at least two layers, including the outermost final layer of the weld metal and a layer adjacent to the final layer. A welding method for low-temperature steel materials according to any one of claims 1 to 3.

7. The aforementioned low-temperature steel material has a plate thickness of 32 mm or more. A welding method for low-temperature steel materials according to any one of claims 1 to 3.

8. The aforementioned low-temperature steel material has a Ni content of less than 7% by mass and a tensile strength of 490 N / mm². ２ That's all. A welding method for low-temperature steel materials according to any one of claims 1 to 3.

9. A welded joint in which low-temperature steel materials with a thickness of 32 mm or more are joined together by multi-layer welding with weld metal, The aforementioned weld metal, A base layer having a tensile strength that is undermatched to the tensile strength of the aforementioned low-temperature steel material, A surface layer located outside the base layer and having a tensile strength that is an overmatch for the tensile strength of the low-temperature steel material, Equipped with, The base layer consists of a welding material with a Ni content of 2.5% by mass or more and less than 7.0% by mass. The aforementioned surface layer is made of a welding material with a Ni content of less than 2.5% by mass. The tensile strength and thickness of the low-temperature steel material, the base layer, and the surface layer, respectively, satisfy a predetermined relational expression. A welded joint for low-temperature steel materials, characterized by the following features.

10. The outer surface is made of low-temperature steel material welded by the low-temperature steel material welding method described in any one of claims 1 to 3, -Storing fluids between 10°C and 60°C, A container for storing low-temperature fluids, characterized by the following features.

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