Weld metal, weld joint, and liquid hydrogen pressure vessel

Abstract

This weld metal includes, in mass% relative to the total mass, 0.005%-0.20% of C, 0.01%-0.50% of Si, 0%-3.0% of Mn, 0.010% or less of P, 0.010% or less of S, 50.0%-80.0% of Ni, 0.1%-25.0% of Mo, 0.003%-1.00% of Al, and 0.0001%-0.1000% of B, with the balance being Fe and impurities. The value F represented by F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] in the weld metal is 8.600 or more. [Cr], [Mo], [Nb], [W] and [B] represent the content of each element relative to the total mass of the weld metal.

Description

Weld metal, weld joints, and liquid hydrogen pressure vessels

[0001] The present disclosure relates to weld metals, weld joints, and liquid hydrogen pressure vessels.

[0002] In recent years, due to the tightening of regulations on carbon dioxide emissions in response to the issue of global warming, there has been an increasing demand for hydrogen fuel, which emits less carbon dioxide than petroleum, coal, and the like. Accordingly, there has also been an increasing demand for liquid hydrogen pressure vessels, such as liquid hydrogen tanks for use on ships and on land. Ni-based low-temperature steels containing 6% to 9% Ni are used for the steel materials used in liquid hydrogen pressure vessels, in order to ensure toughness at extremely low temperatures (e.g., −196°C). Furthermore, when welding these Ni-based low-temperature steels, austenitic welding materials are used to form weld metals that have excellent low-temperature toughness. These welding materials are typically designed with a Ni content of 70%.

[0003] For example, Japanese Patent Laid-Open Publication No. 6-277875 discloses a Hastelloy-based TIG welding wire for cryogenic steel, which contains specific amounts of Ni, Mo, W, Cu, and Ti, and in which the active elements Ca, Mg, Al, and Si and the gas components O and N are controlled to extremely low levels, and the OX equivalent calculated from the amounts of Ca, Mg, Al, Si, Ti, O, and N is regulated to less than a specific value.

[0004]

[0003] Meanwhile, steel materials used in high-pressure hydrogen gas, such as in liquid hydrogen pressure vessels, are required to have excellent hydrogen embrittlement resistance. Ni-based welding materials are generally used for welding Ni-based low-temperature steels, but it is known that Ni-based alloys are sensitive to hydrogen embrittlement, which causes grain boundary fracture and reduces strength and toughness. The aforementioned Patent Document 1 neither describes nor suggests the use of weld metal in high-pressure hydrogen gas, such as in liquid hydrogen pressure vessels, nor specifically mentions improving hydrogen embrittlement resistance.

[0005] In view of the above, the weld joint according to the present disclosure has an object to provide a weld metal in which hydrogen embrittlement in high-pressure hydrogen gas is suppressed, a weld joint having the weld metal, and a liquid hydrogen pressure vessel having the weld metal or weld joint.

[0006] The means for solving the problem include the following aspects. <1> The chemical composition at the center position in a cross section perpendicular to the weld line direction and at a t / 4 position in the plate thickness direction (t represents the plate thickness) is, in mass % with respect to the total mass, C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, Co: 0% to 1.0%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, A weld metal comprising Al: 0.003% to 1.00%, Ti: 0% to 1.000%, B: 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance consisting of Fe and impurities, wherein a value F represented by the following formula 1 is 8.600 or more: F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.) <2> A weld metal used in a liquid hydrogen pressure vessel, wherein the chemical composition at the center position in a cross section perpendicular to the weld line direction and at the t / 4 position in the plate thickness direction (t represents the plate thickness) is, in mass % with respect to the total mass, C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, Co: 0% to 1.0%, Pb: 0% to 1.00%, A weld metal comprising Sn: 0% to 1.00%, Al: 0% to 1.00%, Ti: 0% to 1.000%, B: 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance consisting of Fe and impurities, wherein a value F represented by the following formula 1 is 8.600 or more. F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Formula 1 (In Formula 1, [Cr] represents the Cr content with respect to the total mass of the weld metal, [Mo] represents the Mo content with respect to the total mass of the weld metal, [Nb] represents the Nb content with respect to the total mass of the weld metal, [W] represents the W content with respect to the total mass of the weld metal, and [B] represents the B content with respect to the total mass of the weld metal.) <3> The weld metal according to <1> or <2>, wherein the total content of the Nb and the W with respect to the total mass of the weld metal is 1.0% or more. <4> The weld metal according to any one of <1> to <3>, wherein in the chemical composition, the content of at least one of the Co and the Ca is in the range shown below. <5> The weld metal according to any one of <1> to <4>, wherein the ratio of the content of Mo to the content of Cr is more than 1 / 1. <6> The weld metal according to any one of <1> to <5>, wherein the occupancy rate Lc of precipitates on γ grain boundaries is 30.0% or less.<7> The chemical composition, in mass% relative to the total mass of the base material, is: C: 0.01% to 0.07%, Si: 0.03% to 0.3%, Mn: 0.10% to 0.80%, P: 0.008% or less, S: 0.004% or less, Ni: 6.10% to 10.40%, Cr: 0% to 1.00%, Mo: 0% to 0.60%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Co: 0% to 0.050%, Al: 0.01% to 0.06%, N: 0.0015% to 0.006%, O: 0.003% or less, V: 0% to 0.080%, Ti: 0% to 0.020%, <8> A welded joint comprising: a base metal containing B: 0% to 0.0020%, Ca: 0% to 0.0040%, and REM: 0% to 0.0050%, with the balance being Fe and impurities; and a weld metal, wherein the weld metal is the weld metal described in any one of <1> to <6>. <8> A liquid hydrogen pressure vessel having the weld metal described in any one of <1> to <7>. <9> A liquid hydrogen pressure vessel having the weld joint described in <8>.

[0007] According to the present disclosure, there are provided a weld metal in which hydrogen embrittlement in high-pressure hydrogen gas is suppressed, a weld joint having the weld metal, and a liquid hydrogen pressure vessel having the weld metal or weld joint.

[0008] 1 is a top view showing the shape of an SSRT test piece in an SSRT test; 2 is a schematic cross-sectional view showing the extraction position of an SSRT test piece in an SSRT test; 3 is a schematic cross-sectional view showing the extraction position of an SSRT test piece in an SSRT test when the base material is thin;

[0009] An embodiment that is an example of the present disclosure will be described. In this specification, when a numerical range expressed using "to" is used, unless the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that includes these numerical values ​​as the lower and upper limits. Furthermore, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, with regard to the content, "%" means "mass %." When the content (%) is "0 to," it means that the component is an optional component and may not be included.

[0010] <Weld Metal> A weld metal according to an embodiment of the present disclosure has a chemical composition at a central position in a cross section perpendicular to the weld line direction and at a t / 4 position in the plate thickness direction (t represents the plate thickness of the steel plate that becomes the base metal. That is, the t / 4 position refers to a position that is ¼ of the way from the surface of the base metal in the plate thickness direction), which is expressed in mass % with respect to the total mass: C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, The alloy contains Co: 0% to 1.0%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, Al: 0.003% to 1.00%, Ti: 0% to 1.000%, B: 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance being Fe and impurities. Furthermore, the value F represented by the following formula 1 is 8.600 or more. F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.) Note that the weld metal according to the embodiment of the present disclosure is used in high-pressure hydrogen gas, such as in a liquid hydrogen pressure vessel (e.g., a liquid hydrogen tank).

[0011] The weld metal according to the embodiment of the present disclosure, due to the above-described configuration, is suppressed from hydrogen embrittlement (e.g., reduction in strength and toughness due to hydrogen embrittlement) even when used in high-pressure hydrogen gas such as in a liquid hydrogen pressure vessel. The weld metal according to the present disclosure was discovered based on the following findings.

[0012]

[0003] Steel materials used in high-pressure hydrogen gas, such as in liquid hydrogen pressure vessels, require excellent resistance to hydrogen embrittlement. Ni-based welding materials are generally used to weld Ni-based low-temperature steels. However, Ni-based alloys are known to be sensitive to hydrogen embrittlement, which can cause grain boundary fracture and reduce strength and toughness. Therefore, the present inventors investigated the relationship between the chemical composition of the weld metal and hydrogen embrittlement. As a result, the following findings were obtained, leading to the discovery of a welded joint according to an embodiment of the present disclosure.

[0013] When hydrogen penetrates into a weld metal, it accumulates mainly near the γ grain boundaries. Therefore, it is believed that the accumulated hydrogen embrittles the γ grain boundaries. The inventors therefore considered that by increasing the content of grain boundary strengthening elements in the weld metal to increase the strength of the γ grain boundaries, it would be possible to offset the effects of hydrogen embrittlement of the γ grain boundaries, and as a result, it would be possible to suppress hydrogen embrittlement in the weld metal (e.g., reductions in strength and toughness due to hydrogen embrittlement).

[0014] In light of this, in the weld metal according to the embodiment of the present disclosure, the content of the grain boundary strengthening elements Cr, Mo, Nb, W, and B is set to have a value F expressed by Formula 1 of 8.600 or more: F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 In Formula 1, the values ​​of the coefficients related to the content of each of Cr, Mo, Nb, W, and B represent the degree to which each element affects grain boundary strengthening.

[0015] From the above findings, the weld metal according to the present disclosure is suppressed from hydrogen embrittlement (e.g., reduction in strength and toughness due to hydrogen embrittlement) even when used in high-pressure hydrogen gas such as in a liquid hydrogen pressure vessel.

[0016] Hereinafter, the weld metal according to the embodiment of the present disclosure will be specifically described.

[0017] (Chemical Composition of Weld Metal) The chemical composition of the weld metal will be described in detail below. The chemical composition of the weld metal is measured on a sample taken from the center of a cross section of the weld metal perpendicular to the weld line direction and at a t / 4 position in the plate thickness direction (t represents the plate thickness). In the description of the chemical composition of the weld metal, "%" means "mass % with respect to the total mass of the weld metal" unless otherwise specified.

[0018] The chemical composition of the weld metal is: C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, Co: 0% to 1.0%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, Al: 0.003% to 1.00%, Ti: 0% to 1.000%, B : 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance being Fe and impurities, and the value F represented by the following formula 1 being 8.600 or more. F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.)

[0019] (C: 0.005% to 0.20%) C is an element that improves the strength of the weld metal and ensures the strength of the weld metal. On the other hand, if the C content of the weld metal is excessive, carbides precipitate in the weld metal, which has a significant effect of deteriorating toughness and reduces the low-temperature toughness of the weld metal. It also promotes hot cracking during welding. Therefore, the C content of the weld metal is set to 0.005% to 0.20%. The lower limit of the C content of the weld metal is preferably 0.01%, 0.02%, or 0.03%. The upper limit of the C content of the weld metal is preferably 0.18%, 0.16%, 0.14%, 0.12%, 0.10%, or 0.07%.

[0020] (Si: 0.01% to 0.50%) Si is a deoxidizing element. If the Si content of the weld metal is too low, the O content of the weld metal increases. On the other hand, Si has low solid solubility in the austenite phase, and the greater the Si content, the more likely solidification segregation occurs, resulting in hot cracking. Therefore, the Si content of the weld metal is set to 0.01% to 0.50%. The lower limit of the Si content of the weld metal is preferably 0.02%, 0.03%, or 0.05%. The upper limit of the Si content of the weld metal is preferably 0.45%, 0.40%, 0.35%, or 0.30%.

[0021] (Mn: 0% to 3.0%) Mn is an austenite-stabilizing element and may be contained in the weld metal to improve low-temperature toughness. On the other hand, if the Mn content in the weld metal is excessive, solidification segregation occurs in the weld metal, resulting in hot cracking. Therefore, the Mn content in the weld metal is set to 0% to 3.0%. The lower limit of the Mn content in the weld metal is preferably 0.01%, 0.05%, 0.08%, or 0.1%. The upper limit of the Mn content in the weld metal is preferably 2.8%, 2.5%, 2.3%, or 2.0%.

[0022] (P: 0.010% or less) P is an impurity element that promotes hot cracking or reduces toughness, so it is preferable to reduce the P content of the weld metal as much as possible. Therefore, the lower limit of the P content of the weld metal may be 0%. However, from the viewpoint of reducing deP costs, the P content of the weld metal may be 0.001% or more. On the other hand, if the P content of the weld metal is 0.010% or less, the adverse effects of P are within an acceptable range. Therefore, the P content of the weld metal is set to 0.010% or less. To effectively suppress hot cracking or a decrease in toughness, the P content of the weld metal is preferably 0.008% or less, 0.005% or less, 0.004% or less, or 0.003% or less.

[0023] (S: 0.010% or less) S is an impurity element that promotes hot cracking or reduces toughness, so it is preferable to reduce the S content of the weld metal as much as possible. Therefore, the lower limit of the S content of the weld metal may be 0%. However, from the viewpoint of reducing desulfurization costs, the S content of the weld metal should be 0.001% or more. On the other hand, if the S content of the weld metal is 0.010% or less, the adverse effects of S are within an acceptable range. Therefore, the S content of the weld metal is set to 0.010% or less. In order to effectively suppress hot cracking or a decrease in toughness, the S content of the weld metal is preferably 0.008% or less, 0.005% or less, 0.004% or less, or 0.003% or less.

[0024] (Ni: 50.0% to 80.0%) Ni is an austenite-stabilizing element and an element that enhances toughness. If the Ni content of the weld metal is too low, the austenitization of the weld metal becomes difficult to proceed, and low-temperature toughness deteriorates. On the other hand, increasing the Ni content of the weld metal increases the cost of the weld metal. Therefore, the Ni content of the weld metal is set to 50.0% to 80.0%. The lower limit of the Ni content of the weld metal is preferably 52.0%, 55.0%, 58.0%, or 60.0%. The upper limit of the Ni content of the weld metal is preferably 78.0%, 76.0%, or 75.0%.

[0025] (Cr: 0% to 0.44%) Cr is an austenite-stabilizing element and may be contained in the weld metal to improve the low-temperature toughness of the weld metal. Cr, a grain boundary strengthening element, is preferably contained in the weld metal from the viewpoint of suppressing hydrogen embrittlement (e.g., reduction in the strength and toughness of the weld metal due to hydrogen embrittlement). On the other hand, if the Cr content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal becomes wider, making hot cracking more likely to occur. Therefore, the Cr content in the weld metal is set to 0% to 0.44%. The lower limit of the Cr content in the weld metal is preferably 0.02%, 0.05%, or 0.10%. The upper limit of the Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%.

[0026] (Mo: 0.1% to 25.0%) Mo is a solid solution strengthening element or a precipitation strengthening element, and is contained in the weld metal to improve the strength of the weld metal. Furthermore, Mo, a grain boundary strengthening element, is contained in the weld metal from the perspective of suppressing hydrogen embrittlement (e.g., reduction in the strength and toughness of the weld metal due to hydrogen embrittlement). On the other hand, if the Mo content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal widens, making hot cracking more likely to occur. Therefore, the Mo content in the weld metal is set to 0.1% to 25.0%. The lower limit of the Mo content in the weld metal is preferably 1.0%, 5.0%, 10.0%, or 12.0%. The upper limit of the Mo content in the weld metal is preferably 24.0%, 23.0%, 22.0%, 20.0%, 18.0%, or 15.0%.

[0027] (Nb: 0% to 5.0%) Nb may be contained in the weld metal because it forms carbides in the weld metal and increases the strength of the weld metal. Furthermore, Nb, a grain boundary strengthening element, is preferably contained in the weld metal from the viewpoint of suppressing hydrogen embrittlement (e.g., reduction in the strength and toughness of the weld metal due to hydrogen embrittlement). On the other hand, if the Nb content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal widens, making hot cracking more likely to occur. Therefore, the Nb content in the weld metal is set to 0% to 5.0%. The lower limit of the Nb content in the weld metal is preferably 0.1%, 0.2%, 0.3%, or 0.5%. The upper limit of the Nb content in the weld metal is preferably 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, 2.0%, 1.0%, or 0.5%.

[0028] (W: 0% to 5.0%) W is a solid solution strengthening element and may be contained in the weld metal to improve strength. Furthermore, W, a grain boundary strengthening element, is preferably contained in the weld metal from the viewpoint of suppressing hydrogen embrittlement (e.g., reduction in the strength and toughness of the weld metal due to hydrogen embrittlement). On the other hand, if the W content in the weld metal is excessive, the strength of the weld metal may become excessive, potentially resulting in a reduction in toughness. Therefore, the W content in the weld metal is set to 0% to 5.0%. The lower limit of the W content in the weld metal is preferably 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. The upper limit of the W content in the weld metal is preferably 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, or 2.0%.

[0029] (Ta: 0% to 3.0%) Ta is a solid solution strengthening element and may be contained in the weld metal to improve strength. On the other hand, if the Ta content of the weld metal is excessive, the strength of the weld metal becomes excessive, and there is a possibility that a decrease in toughness may occur. Therefore, the Ta content of the weld metal is set to 0% to 3.0%. The lower limit of the Ta content of the weld metal is preferably 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. The upper limit of the Ta content of the weld metal is preferably 2.5%, 2.0%, or 1.5%.

[0030] (Cu: 0% to 1.00%) Cu is a precipitation strengthening element and may be contained in the weld metal to improve the strength of the weld metal. Cu is also an austenite stabilizing element and may be contained in the weld metal to improve the low-temperature toughness of the weld metal. On the other hand, if the Cu content in the weld metal is excessive, the above effects will saturate. Therefore, the Cu content in the weld metal is set to 0% to 1.00%. The lower limit of the Cu content in the weld metal is preferably 0.02%, 0.05%, or 0.08%. The upper limit of the Cu content in the weld metal is preferably 0.90%, 0.80%, or 0.50%.

[0031] (Co: 0% to 1.0%) Co is an element that increases the strength of the weld metal through solid solution strengthening, and therefore may be contained in the weld metal. On the other hand, if the Co content of the weld metal is excessive, the ductility of the weld metal decreases and toughness cannot be ensured. Therefore, the Co content of the weld metal is set to 0% to 1.0%. The lower limit of the Co content of the weld metal is preferably 0.0001%, 0.0003%, 0.0005%, 0.0010%, 0.0020%, 0.01%, 0.05%, or 0.1%. The upper limit of the Co content of the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0032] (Pb: 0% to 1.00%) Pb may be contained in the weld metal because it has the effect of improving the toe formability between the base steel material and the weld metal and improving the machinability of the weld metal. On the other hand, if the Pb content of the weld metal is excessive, hot cracking occurs. Therefore, the Pb content of the weld metal is set to 0% to 1.00%. The lower limit of the Pb content of the weld metal is preferably 0.005%, 0.01%, or 0.1%. The upper limit of the Pb content of the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0033] (Sn: 0% to 1.00%) Sn is an element that improves the corrosion resistance of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Sn content of the weld metal is excessive, there is a concern that hot cracking may occur in the weld metal. Therefore, the Sn content of the weld metal is set to 0% to 1.00%. The lower limit of the Sn content of the weld metal is preferably 0.005%, 0.01%, or 0.1%. The upper limit of the Sn content of the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0034] (Al: 0.003% to 1.00%) Al is a deoxidizing element and is contained in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, if the Al content of the weld metal is excessive, Al may form nitrides or oxides in the weld metal, which may reduce the low-temperature toughness of the weld metal. Therefore, the Al content of the weld metal is set to 0.003% to 1.00%. The lower limit of the Al content of the weld metal is preferably 0.01%, 0.02%, 0.05%, or 0.10%. The upper limit of the Al content of the weld metal is preferably 0.90%, 0.80%, or 0.50%.

[0035] (Ti: 0% to 1.000%) Ti is a deoxidizing element and may be contained in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, if the Ti content of the weld metal is excessive, carbides and intermetallic compounds may be formed in the weld metal, which may deteriorate the toughness of the weld metal. Therefore, the Ti content of the weld metal is set to 0% to 1.000%. The lower limit of the Ti content of the weld metal is preferably 0.003%, 0.010%, or 0.020%. The upper limit of the Ti content of the weld metal is preferably 0.900%, 0.700%, 0.500%, 0.300%, 0.250%, 0.200%, 0.150%, or 0.100%.

[0036] (B: 0.0001% to 0.1000%) B is an austenite stabilizing element and an interstitial solid solution strengthening element, and is contained in the weld metal to improve the low-temperature toughness and strength of the weld metal. In addition, B, which is a grain boundary strengthening element, is contained in the weld metal from the viewpoint of suppressing hydrogen embrittlement (for example, reduction in the strength and toughness of the weld metal due to hydrogen embrittlement). On the other hand, if the B content in the weld metal is excessive, M ２３ (C, B) ６ precipitates, causing a deterioration in toughness. It also expands the solid-liquid coexistence temperature range, promoting hot cracking. Therefore, the B content of the weld metal is set to 0.0001% to 0.1000%. The lower limit of the B content of the weld metal is preferably 0.0003%, 0.0005%, 0.0010%, or 0.0020%. The upper limit of the B content of the weld metal is preferably 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0037] (V: 0% to 3.0000%) V may be contained in the weld metal because it forms carbonitrides in the weld metal and increases the strength of the weld metal. On the other hand, if the V content in the weld metal is excessive, the carbonitrides become coarse and the toughness deteriorates. In addition, the V content widens the solid-liquid coexistence temperature range, which may cause hot cracking in the weld metal. Therefore, the V content in the weld metal is set to 0% to 3.0000%. The lower limit of the V content in the weld metal is preferably 0.0100%, 0.0500%, 0.1000%, 0.1500%, or 0.2000%. The upper limit of the V content in the weld metal is preferably 2.5000%, 2.0000%, or 1.5000%.

[0038] (Ca: 0% to 1.0000%) Ca changes the structure of sulfides in the weld metal and also functions to refine the size of sulfides and oxides in the weld metal, and is therefore effective in improving the ductility and toughness of the weld metal. Therefore, Ca may be contained in the weld metal. On the other hand, an excessive Ca content in the weld metal may cause coarsening of sulfides and oxides, which may lead to a deterioration in the low-temperature toughness of the weld metal. Therefore, the Ca content of the weld metal is set to 0% to 1.0000%. The lower limit of the Ca content in the weld metal is preferably 0.0001%, 0.0003%, 0.0005%, 0.0010%, or 0.0020%. The upper limit of the Ca content in the weld metal is preferably 0.8000%, 0.5000%, or 0.3000%.

[0039] (Mg: 0% to 1.0000%) Mg is a deoxidizing element that reduces the amount of oxygen in the weld metal and has the effect of improving the cleanliness of the weld metal. Therefore, Mg may be contained in the weld metal. On the other hand, if the Mg content of the weld metal is excessive, Mg and oxygen may react violently in the arc, which may increase the amount of spatter and fumes generated. Therefore, the Mg content of the weld metal is set to 0% to 1.0000%. The lower limit of the Mg content of the weld metal is preferably 0.0100%, 0.0200%, or 0.0300%. The upper limit of the Mg content of the weld metal is preferably 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0040] (Zr: 0% to 1.0000%) Zr is a deoxidizing element that reduces the amount of oxygen in the weld metal and has the effect of improving the cleanliness of the weld metal. Therefore, Zr may be contained in the weld metal. On the other hand, if the Zr content of the weld metal is excessive, Zr and oxygen may react violently in the arc, which may increase the amount of spatter and fumes generated. Therefore, the Zr content of the weld metal is set to 0% to 1.0000%. The lower limit of the Zr content of the weld metal is preferably 0.0100%, 0.0200%, or 0.0300%. The upper limit of the Zr content of the weld metal is preferably 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0041] (REM: 0% to 0.5000%) REM is an element that stabilizes the arc during welding to obtain the weld metal, and therefore may be contained in the weld metal. On the other hand, if the REM content in the weld metal is excessive, spattering may become severe during welding to obtain the weld metal, potentially resulting in poor welding workability. Therefore, the REM content in the weld metal is set to 0% to 0.5000%. The lower limit of the REM content in the weld metal is preferably 0.0010%, 0.0020%, or 0.0050%. The upper limit of the REM content in the weld metal is preferably 0.4000%, 0.2000%, or 0.1000%.

[0042] Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanides such as La and Nd, and the REM content refers to the total content of these elements. REM is added to molten steel using, for example, an Fe—Si—REM alloy, and this alloy contains, for example, La, Nd, Ce, and Pr.

[0043] (N: 0% to 0.300%) N is an austenite stabilizing element and an interstitial solid solution strengthening element, and may be contained in the weld metal to improve the low-temperature toughness and strength of the weld metal. On the other hand, if the N content of the weld metal is excessive, the occurrence of blowout increases, causing welding defects. Therefore, the N content of the weld metal is set to 0% to 0.300%. The lower limit of the N content of the weld metal is preferably 0.001%, 0.005%, 0.010%, or 0.020%. The upper limit of the N content of the weld metal is preferably 0.250%, 0.200%, 0.150%, or 0.100%.

[0044] (O: 0.050% or less) O ​​is an element that may be contained in the weld metal as an impurity. An excessive O content leads to deterioration of toughness and ductility, so the upper limit of the O content in the weld metal is set to 0.150% or less. On the other hand, an extreme reduction in the O content leads to an increase in manufacturing costs, so the lower limit of the O content in the weld metal may be set to 0.001% or less. The lower limit of the O content in the weld metal is preferably 0.002% or 0.003%. The upper limit of the O content in the weld metal is preferably 0.040% or 0.030%.

[0045] (Balance: Fe and Impurities) The balance of the chemical composition of the weld metal is Fe and impurities. The impurities refer to components that are mixed in during industrial production of the weld metal due to raw materials such as ore or scrap, or various factors in the production process, and are acceptable within a range that does not adversely affect the properties of the weld metal.

[0046] The upper and lower limits of the contents of each element in the above chemical composition are set for different reasons, and therefore the preferred upper and lower limits can be selected independently.

[0047] (Value F) The weld metal has a value F, expressed by Formula 1, of 8.600 or more for the contents of the grain boundary strengthening elements Cr, Mo, Nb, W, and B. F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.)

[0048] In Formula 1, the coefficient values ​​for the respective contents of Cr, Mo, Nb, W, and B represent the degree to which each element affects grain boundary strengthening. When the value F is 8.600 or more, hydrogen embrittlement (e.g., reduction in strength and toughness due to hydrogen embrittlement) is suppressed even when the weld metal is used in high-pressure hydrogen gas, such as in a liquid hydrogen pressure vessel. From the viewpoint of suppressing hydrogen embrittlement (e.g., reduction in hydrogen embrittlement strength and toughness), the value F is preferably 9.000 or more, 9.500 or more, 10.000 or more, or 10.500 or more.

[0049] (Total Content of Nb and W) In the weld metal, the total content of Nb and W relative to the total mass of the weld metal is preferably 1.0% or more. Nb and W contribute to strengthening the strength of the weld metal, and when the total content of Nb and W is within the above range, the tensile strength TS of the weld metal can be increased. From the viewpoint of increasing the tensile strength TS of the weld metal, the total content of Nb and W is more preferably 1.5% or more, and even more preferably 2.0% or more.

[0050] (Ratio of Mo to Cr Content) It is preferable that the ratio of Mo to Cr content in the weld metal is greater than 1 / 1 (i.e., the Mo content is greater than the Cr content). Mo has a greater effect of suppressing hydrogen embrittlement than Cr, so by having the ratio of Mo to Cr content in the above range, hydrogen embrittlement of the weld metal is suppressed. The ratio of Mo to Cr content is more preferably 5 / 1 or more, 10 / 1 or more, 20 / 1 or more, 30 / 1 or more, or 40 / 1 or more.

[0051] (Tensile Strength) The tensile strength of the weld metal is preferably, for example, 590 to 1200 MPa. The tensile strength can be measured by conducting a tensile test on the weld metal in accordance with JIS Z3111:2005.

[0052] (Occupancy ratio Lc of grain boundary strengthening element precipitates on γ grain boundaries) Element depletion regions occur around precipitates present on γ grain boundaries. For example, when Cr carbides exist as precipitates on γ grain boundaries, depletion regions with low Cr and C occur locally around the carbides. The formation of regions depleted in Cr, a grain boundary strengthening element, reduces grain boundary strength around the precipitates. Therefore, in the weld metal according to the embodiment of the present disclosure, the occupancy ratio Lc of the precipitates on γ grain boundaries is preferably Lc≦30.0%. The precipitates referred to here are considered to contain at least one element selected from the group consisting of Cr, Mo, Nb, W, and B, which are grain boundary strengthening elements (hereinafter, these precipitates are referred to as "grain boundary strengthening element precipitates"). By setting the occupancy rate Lc of the grain boundary strengthening element precipitates on the γ grain boundaries to 30.0% or less, the strength of the γ grain boundaries can be increased, and hydrogen embrittlement in the weld metal (e.g., reduction in strength and toughness due to hydrogen embrittlement) can be suppressed.

[0053] The occupancy rate of the grain boundary strengthening element precipitates on the γ grain boundaries is preferably 25.0% or less, more preferably 20.0% or less, and even more preferably 16.0% or less, from the viewpoint of suppressing hydrogen embrittlement in the weld metal (e.g., reduction in strength and toughness due to hydrogen embrittlement). The lower limit of the occupancy rate is not particularly limited, but may be 5.0% or more, from the viewpoint that it is not easy to completely suppress the precipitation of precipitates.

[0054] The occupancy rate of grain boundary strengthening element precipitates on γ grain boundaries is measured by the following method. A backscattered electron image is observed with a scanning electron microscope (SEM) at the center of a cross section of the weld metal perpendicular to the weld line direction and at a t / 4 position in the plate thickness direction (t represents the plate thickness). Note that by observing the backscattered electron image, the precipitates are observed with a contrast different from that of the matrix. Five γ grains randomly selected in the observation field are designated as measurement targets, the extent of the presence of precipitates on the γ grain boundaries of these measurement targets is measured, and the occupied length of the precipitates per grain boundary length of the five measured γ grains is measured. For example, if the total grain boundary length is 1000 μm and a total of 50 μm of precipitates are present, the occupancy rate Lc is "50 μm / 1000 μm × 100 = 5%." This measurement is performed at five cross sections (the center position in a cross section perpendicular to the weld line direction of the weld metal and at a position t / 4 in the plate thickness direction (t represents the plate thickness)), and the arithmetic average value is taken as the occupation ratio Lc.

[0055] Whether or not the precipitates contain a grain boundary strengthening element, i.e., at least one element selected from the group consisting of Cr, Mo, Nb, W, and B, can be confirmed by the following method. The weld metal is observed using backscattered electron images from a scanning electron microscope (SEM), and precipitates on grain boundaries that have a contrast different from that of the matrix are subjected to point analysis by energy dispersive X-ray spectroscopy (EDS). It can be confirmed whether or not the grain boundary strengthening element is contained in the obtained analytical values. An electron probe microanalyzer (EPMA) may also be used for the analysis.

[0056] The following four methods can be cited as examples of methods for reducing the occupancy rate Lc of grain boundary strengthening element precipitates on γ grain boundaries and controlling it to the above range. (1) Method of reducing occupancy rate Lc by reducing the C content By reducing the C content in the weld metal, the amount of carbides of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) that precipitate can be reduced, and the occupancy rate Lc of grain boundary strengthening element precipitates on γ grain boundaries can be lowered. From this perspective, the upper limit of the C content in the weld metal is preferably 0.12%, 0.10%, or 0.07%.

[0057] (2) Method of Reducing the Occupancy Rate Lc by Reducing the Content of Grain Boundary Strengthening Elements By reducing the content of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) in the weld metal, the amount of grain boundary strengthening element precipitates can be reduced, and the occupancy rate Lc of the grain boundary strengthening element precipitates on the γ grain boundaries can be lowered. From this perspective, the contents of the grain boundary strengthening elements are preferably set as follows. The upper limit of the Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%. The upper limit of the Mo content in the weld metal is preferably 20.0%, 18.0%, or 15.0%. The upper limit of the Nb content in the weld metal is preferably 2.5%, 2.0%, 1.0%, or 0.5%. The upper limit of the W content in the weld metal is preferably 3.0%, 2.5%, or 2.0%. The upper limit of the B content in the weld metal is preferably 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0058] (3) Method of Reducing the Occupancy Rate Lc by Reducing the C Content and the Content of Grain Boundary Strengthening Elements By reducing the C content in the weld metal to a not too high range and also reducing the contents of the grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) to a not too high range, the amount of carbides of the grain boundary strengthening elements can be reduced, and the occupancy rate Lc of the grain boundary strengthening element precipitates on the γ grain boundaries can be lowered. From this perspective, the contents of C and grain boundary strengthening elements are preferably set as follows. The upper limit of the C content in the weld metal is preferably 0.18%, 0.16%, or 0.14%. The upper limit of the Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%. The upper limit of the Mo content in the weld metal is preferably 24.0%, 23.0%, 22.0%, or 20.0%. The upper limit of the Nb content in the weld metal is preferably 4.5%, 4.0%, 3.5%, 3.0%, or 2.5%. The upper limit of the W content in the weld metal is preferably 4.5%, 4.0%, 3.5%, or 3.0%. The upper limit of the B content in the weld metal is preferably 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0059] (4) Method of Reducing the Occupancy Rate Lc by Reducing the Interpass Temperature During Welding The weld metal is formed by multi-pass welding. By reducing the interpass temperature (the temperature between passes in a multi-pass weld in which welding is performed through multiple passes), the amount of grain boundary strengthening element precipitates can be reduced, and the occupancy rate Lc of the grain boundary strengthening element precipitates on the γ grain boundaries can be lowered. The interpass temperature is preferably 150°C or less, and more preferably 125°C or less.

[0060] <Welded joint> Next, a welded joint according to an embodiment of the present disclosure will be described. The welded joint according to the present disclosure has a base material and a weld metal according to the present disclosure. For example, the welded joint according to the present disclosure includes a steel material serving as the base material, and a welded part constituted by a weld metal and a weld heat-affected zone.

[0061] (Chemical Composition of Base Metal) In the weld joint according to the embodiment of the present disclosure, the composition of the base metal is not particularly limited. However, from the viewpoint of use in high-pressure hydrogen gas such as in a liquid hydrogen pressure vessel, it is preferable that the base metal have the chemical composition shown below. In mass % relative to the total mass of the base metal, C: 0.01% to 0.07%, Si: 0.03% to 0.3%, Mn: 0.10% to 0.80%, P: 0.008% or less, S: 0.004% or less, Ni: 6.10% to 10.40%, Cr: 0% to 1.00%, Mo: 0% to 0.60%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Co: 0% to 0.050%, Al: 0.01% to 0.06%, N: 0.0015% to 0.006%, O: 0.003% or less, V: 0% to 0.080%, Ti: 0% to 0.020%, B : 0% to 0.0020%, Ca: 0% to 0.0040%, and REM: 0% to 0.0050%, with the balance being Fe and impurities.

[0062] C: 0.01% to 0.07% C is an element that improves the hardenability of steel and contributes to improving strength. On the other hand, an excessive C content reduces toughness. Therefore, from the viewpoint of improving the strength and toughness of the base metal and heat-affected zone (HAZ), the C content of the base metal is set to 0.010% to 0.070%. The lower limit of the C content of the base metal is preferably 0.015%, 0.02%, or 0.025%. The upper limit of the C content of the base metal is preferably 0.065%, 0.06%, 0.055%, or 0.05%.

[0063] Si: 0.03% to 0.3% Si is an element that increases the strength of steel through solid solution strengthening. On the other hand, excessive Si content reduces workability and the strength of the steel. Therefore, the Si content of the base material is set to 0.03% to 0.3%. The lower limit of the Si content of the base material is preferably 0.05%, 0.08%, or 0.1%. The upper limit of the Si content of the base material is preferably 0.28%, 0.25%, or 0.2%.

[0064] Mn: 0.10% to 0.80% Mn is an element that increases the strength of steel. On the other hand, if the Mn content is excessive, workability deteriorates and toughness becomes unstable. Therefore, the Mn content of the base material is set to 0.10% to 0.80%. The lower limit of the Mn content of the base material is preferably 0.20% or 0.30%. The upper limit of the Mn content of the base material is preferably 0.70% or 0.60%.

[0065] P: 0.008% or less Since P is an impurity and an element that causes embrittlement, it is preferable to reduce the P content of the base material as much as possible. Therefore, the lower limit of the P content of the base material may be 0%. However, from the viewpoint of reducing the dephosphorization cost, the P content of the base material may be 0.002% or more. On the other hand, if the P content of the base material is 0.008% or less, the adverse effect of P on embrittlement falls within an acceptable range. Therefore, the P content of the base material is set to 0.008% or less. In order to effectively suppress embrittlement, the P content of the base material is preferably 0.007% or less, 0.006% or less, or 0.005% or less.

[0066] S: 0.004% or less S is an impurity and an element that causes embrittlement. Furthermore, S forms coarse MnS in steel, which reduces the workability and strength of the steel. Therefore, it is preferable to reduce the S content of the base material as much as possible, and the lower limit of the S content of the base material may be 0%. However, from the viewpoint of reducing desulfurization costs, the S content of the base material may be 0.0002% or more. On the other hand, if the S content of the base material is 0.004% or less, the adverse effects of S, such as embrittlement, are within an acceptable range. Therefore, the S content of the base material is set to 0.004% or less. In order to effectively suppress embrittlement, the S content of the base material is preferably 0.003% or less, or 0.002% or less.

[0067] Ni: 6.10% to 10.40% Ni is an element that contributes to improving the strength and low-temperature toughness of steel. On the other hand, if the Ni content is excessive, it is difficult to improve toughness commensurate with the increased cost, and problems may occur during pickling and hot working. Therefore, the Ni content of the base metal is set to 6.10% to 10.40%. The lower limit of the Ni content of the base metal may preferably be 6.50%, 7.00%, or 8.00%. The upper limit of the Ni content of the base metal is preferably 10.00%, 9.800%, or 9.500%.

[0068] Cr: 0% to 1.00% Cr is an element that contributes to improving the strength of steel and may be contained in the base material. On the other hand, an excessive Cr content may result in a decrease in the strength of the steel and may cause problems during pickling or hot working. Therefore, the Cr content of the base material is set to 0% to 1.00%. The lower limit of the Cr content of the base material is preferably 0.05%, 0.10%, or 0.20%. The upper limit of the Cr content of the base material is preferably 0.90%, 0.80%, or 0.70%.

[0069] Mo: 0% to 0.60% Mo is an element that contributes to improving the strength of steel and may be contained in the base material. On the other hand, an excessive Mo content may lead to a decrease in the strength of the steel and may cause problems during pickling or hot working. Therefore, the Mo content of the base material is set to 0% to 0.60%. The lower limit of the Mo content of the base material may preferably be 0.01%, 0.02%, or 0.03%. The upper limit of the Mo content of the base material is preferably 0.55%, 0.50%, or 0.45%.

[0070] Nb: 0% to 0.050% Nb is an element that forms fine carbonitrides and suppresses grain coarsening, and may be contained in the base material. On the other hand, excessive Nb content may reduce toughness and further reduce the strength of the steel. Therefore, the Nb content of the base material is set to 0% to 0.050%. The lower limit of the Nb content of the base material may preferably be 0.001%, 0.002%, or 0.003%. The upper limit of the Nb content of the base material is preferably 0.045%, 0.040%, or 0.035%.

[0071] Cu: 0% to 1.00% Cu is an element that contributes to improving the strength of steel and may be contained in the base material. On the other hand, an excessive Cu content may lead to a decrease in the strength of the steel and may cause problems during pickling or hot working. Therefore, the Cu content of the base material is set to 0% to 1.00%. The lower limit of the Cu content of the base material is preferably 0.05%, 0.10%, or 0.20%. The upper limit of the Cu content of the base material is preferably 0.90%, 0.80%, or 0.70%.

[0072] Co: 0% to 0.050% Co is an element that improves corrosion resistance in a corrosive environment. Therefore, Co may be contained in the base material. To obtain the above effects, the Co content is preferably 0.006% or more. On the other hand, if the Co content is excessive, the above effects saturate and economic efficiency decreases. Therefore, the Co content of the base material is set to 0% to 0.050%. The lower limit of the Co content of the base material is preferably 0.001%, 0.003%, or 0.006%. The upper limit of the Co content of the base material is preferably 0.040%, 0.030%, or 0.020%.

[0073] Al: 0.01% to 0.06% Al is an element that has a deoxidizing effect. On the other hand, if the Al content is excessive, the number of inclusions increases, reducing workability and toughness of the steel. Therefore, the Al content of the base material is set to 0.01% to 0.06%. The lower limit of the Al content of the base material is preferably 0.015% or 0.02%. The upper limit of the Al content of the base material is preferably 0.055% or 0.05%.

[0074] N: 0.0015% to 0.006% N is an element that increases the strength of steel. On the other hand, N is an element that forms coarse nitrides in steel and acts to deteriorate the formability of steel. Therefore, the N content of the base metal is set to 0.0015% to 0.006%. The lower limit of the N content of the base metal is preferably 0.002% or 0.0025%. The upper limit of the N content of the base metal is preferably 0.0055% or 0.005%.

[0075] O: 0.003% or less O is an element that may be contained in steel as an impurity. Excessive O content leads to deterioration of toughness and ductility. The O content may be 0%, but an extreme reduction leads to an increase in manufacturing costs, so the lower limit of the O content of the base material may be 0.0001%. On the other hand, if the O content of the base material is 0.003% or less, the adverse effects of O on toughness, etc., fall within an acceptable range. In order to effectively suppress deterioration of toughness and ductility, the O content of the base material is preferably 0.0025% or less, or 0.002% or less.

[0076] V: 0% to 0.080% V is an element that forms fine carbonitrides and suppresses grain coarsening, and may be contained in the base material. On the other hand, excessive V content may reduce toughness and further reduce the strength of the steel. Therefore, the V content of the base material is set to 0% to 0.080%. The lower limit of the V content of the base material may preferably be 0.001%, 0.002%, or 0.003%. The upper limit of the V content of the base material is preferably 0.075%, 0.070%, or 0.065%.

[0077] Ti: 0% to 0.020% Ti is an element that forms precipitates and refines the steel structure, and may be contained in the base material. On the other hand, if the Ti content is excessive, there is a concern that the manufacturability of the steel will decrease, cracks will occur during processing, and the strength of the steel will decrease. Therefore, the Ti content of the base material is set to 0% to 0.020%. The lower limit of the Ti content of the base material may preferably be 0.001%, 0.002%, or 0.003%. The upper limit of the Ti content of the base material is preferably 0.018%, 0.016%, or 0.014%.

[0078] B: 0% to 0.0020% B is an element that segregates at grain boundaries to increase grain boundary strength, and may be contained in the base material. On the other hand, excessive B content may reduce toughness and further reduce the strength of the steel. Therefore, the B content of the base material is set to 0% to 0.0020%. The lower limit of the B content of the base material may preferably be 0.0001%, 0.0002%, or 0.0003%. The upper limit of the B content of the base material is preferably 0.0018%, 0.0016%, or 0.0014%.

[0079] Ca: 0% to 0.0040% Ca has the function of reducing the size of sulfides and oxides, and is therefore effective in improving ductility and toughness. Therefore, Ca may be contained in the base material. On the other hand, an excessive Ca content may cause coarsening of sulfides and oxides, leading to a deterioration in the low-temperature toughness of the base material. Therefore, the Ca content of the base material is set to 0% to 0.0040%. The lower limit of the Ca content of the base material is preferably 0.0001%, 0.0002%, or 0.0005%. The upper limit of the Ca content of the base material is preferably 0.0038%, 0.0035%, or 0.0030%.

[0080] REM: 0% to 0.0050% Like Ca, REM is an element that has the effect of refining inclusions in steel and improving the toughness of steel members, and therefore may be contained in the base material. On the other hand, if the REM content of the base material is excessive, the effect saturates and costs increase. Therefore, the REM content of the base material is set to 0% to 0.0050%. The lower limit of the REM content of the base material is preferably 0.0001%, 0.0002%, or 0.0005%. The upper limit of the REM content of the base material is preferably 0.0048%, 0.0045%, or 0.0040%.

[0081] Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanides such as La and Nd, and the REM content refers to the total content of these elements. REM is added to molten steel using, for example, an Fe—Si—REM alloy, and this alloy contains, for example, La, Nd, Ce, and Pr.

[0082] The balance in the chemical composition of the base material is Fe and impurities. The impurities are components that are mixed in from raw materials such as ore and scrap during industrial production of the base material, or are mixed in due to various factors in the production process, and are acceptable as long as they do not adversely affect the properties of the base material.

[0083] The upper and lower limits of the contents of each element in the above chemical composition are set for different reasons, and therefore the preferred upper and lower limits can be selected independently.

[0084] Next, a method for manufacturing a welded joint according to an embodiment of the present disclosure will be described. Note that the manufacturing method described below is an example, and the method for manufacturing a welded joint according to the present disclosure is not limited to the following method.

[0085] The welded joint according to the present disclosure can be produced by welding a steel base material using a welding material. For example, a welded joint can be produced by submerged arc welding using a solid wire and flux. For example, in submerged arc welding, granular flux is dispersed on the weld line in advance, a solid wire is fed into the flux, and welding is performed using arc heat generated between the solid wire and the steel in the flux. A typical submerged arc welding device can be used. In this case, the chemical composition of the weld metal includes components derived from the solid wire and flux, which are the welding materials, and the steel base material.

[0086] From the viewpoint of controlling the occupancy rate Lc of grain boundary strengthening element precipitates on γ grain boundaries within the aforementioned range, it is preferable to produce a welded joint in submerged arc welding using a solid wire and flux under the following conditions, for example: Current: 330 A to 390 A (e.g., 360 A) Voltage: 20 V to 40 V (e.g., 30 V) Wire travel speed: 25 cm / min to 40 cm / min (e.g., 35 cm / min) Heat input: 17.0 kJ / cm to 20.0 kJ / cm (e.g., 18.5 kJ / cm) Welding method: multi-layer welding Interpass temperature: 150°C or less (more preferably 125°C or less)

[0087] Welded joints can also be obtained by other welding methods, such as TIG welding, etc. In this case, the chemical composition of the weld metal contains components derived from the welding material and the base steel material.

[0088] <Applications> The weld metal according to the embodiment of the present disclosure and the weld joint according to the embodiment of the present disclosure having the weld metal are used in high-pressure hydrogen gas, for example, in a liquid hydrogen pressure vessel (e.g., a liquid hydrogen tank). That is, the liquid hydrogen pressure vessel according to the embodiment of the present disclosure has the weld metal according to the embodiment of the present disclosure or the weld joint according to the embodiment of the present disclosure.

[0089] Next, the feasibility and effects of the present disclosure will be explained in more detail using examples of the present invention and comparative examples. However, the following examples of the present invention do not limit the present disclosure, and any design changes that are made in accordance with the above and below-described intent are all included in the technical scope of the present disclosure.

[0090] (Manufacturing of Welded Joint) Base metals were welded together by submerged arc welding (SAW) using a solid wire and flux, according to the method described below, to obtain a welded joint having a weld metal. First, submerged arc welding was performed using a solid wire in combination with NITTETSU FLUX 10H, a submerged arc welding flux manufactured by Nippon Steel Welding Co., Ltd. Steel plates (20 mm (t) × 120 mm (w) × 300 mm (l)) having the compositions shown in Table 1 were used as the steel plates (base metals) to be welded. As shown in FIG. 2 , two steel plates (base metals 22) were butted together, and a groove was formed and welded so that the groove shape was an X groove with an angle θ = 60°. Specifically, after two steel plates (base metal 22) were butted together, a Y-shaped groove was first formed on one side, and buildup welding was performed within this Y-groove. Next, the back side of the Y-groove was subjected to a chipping process to form an X-shaped groove, and buildup welding was performed within this back side groove to form weld metal 42 having an X-groove shape. All welding currents during welding were DC, all wire polarities were positive, and the welding conditions were as shown in Table 2. The chemical composition of the weld metal was controlled to be the chemical composition shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4 by selecting the base metal and adjusting the compositions of the solid wire and flux. The weld metals shown in Tables 3-1 to 3-4 (Invention Examples 1 to 36, Comparative Examples 1 to 5) used base metal No. 1 shown in Table 1 as the base metal, and the weld metals shown in Tables 4-1 to 4-4 (Invention Examples 41 to 76, Comparative Examples 41 to 45) used base metal No. 2 shown in Table 1 as the base metal.

[0091] The chemical compositions of each weld metal are shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4. Furthermore, the occupancy rate Lc of precipitates on the γ grain boundaries in the weld metal was measured using the method described above. The results are shown in Tables 3-2, 3-4, 4-2, and 4-4. An investigation of the compositions of several precipitates revealed that they contained at least one element selected from the group consisting of Cr, Mo, Nb, W, and B.

[0092] <Evaluation Test> (Relative Reduction of Area (RRA) of Weld Metal) As an index of hydrogen embrittlement resistance, the relative reduction of area (RRA) of the weld metal was measured by the following method. The relative reduction of area (RRA) of the weld metal was measured by a slow strain rate tensile test (SSRT) in a hydrogen environment. First, SSRT test specimens shown in FIG. 1 were taken from the weld metal. The locations from which the test specimens were taken are shown in FIGS. 2 and 3. FIG. 2 shows an example of a weld joint having an X-shaped groove (X-groove) with an angle θ = 60°. This weld joint has a base metal 22 (steel plate) and a weld metal 42 formed in the X-groove. FIG. 3 shows the locations from which test specimens were taken when the base metal was thin. This weld joint has an X-groove with an angle θ = 60° and has a base metal 24 (steel plate) and a weld metal 44 formed in the X-groove. As shown in Fig. 2, a test piece of weld metal 42 is sampled so that the entire parallel portion of the test piece is weld metal 42. The test piece of weld metal 42 is sampled from sampling position 62B so that the central axis of the parallel portion of the test piece is parallel to the weld line direction along a position that is t / 4 from the surface of base metal 22 in the plate thickness direction. However, if the base metal is too thin to sample a test piece centered on the t / 4 position, a test piece of weld metal 44 is sampled from sampling position 64B so that the edge of the gripping portion is aligned with a position 1 mm from the surface of base metal 24 in the plate thickness direction and parallel to the weld line direction, as shown in Fig. 3.

[0093] The collected SSRT test specimens were subjected to slow strain rate tensile tests (SSRT tests) in a 0.1 MPa nitrogen gas environment (simulating an atmospheric environment) and a 10 MPa hydrogen gas environment. The test temperature was room temperature (25°C) and the strain rate was 4.2 × 10 －５The strain rate was calculated from the crosshead movement speed during the tensile test.

[0094] The results of the relative squeezing RRA were evaluated according to the following evaluation criteria. The results are shown in Tables 3-2, 3-4, 4-2, and 4-4. A (◎): 75% or more B (○): 50% or more but less than 75% C (×): Less than 50%

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101]

[0102]

[0103]

[0104]

[0105] In Comparative Example 1, the Cr content of the weld metal is too high, and the value F is below 8.600. In Comparative Example 2, the Cr content and Ni content of the weld metal are too high, and the value F is below 8.600. In Comparative Example 3, the C content, Cr content, and Mo content of the weld metal are too high. In Comparative Example 4, the value F is below 8.600. In Comparative Example 5, the Cr content of the weld metal is too high. In Comparative Example 41, the Cr content of the weld metal is too high, and the value F is below 8.600. In Comparative Example 42, the Cr content and Ni content of the weld metal are too high, and the value F is below 8.600. In Comparative Example 43, the C content and Cr content of the weld metal are too high. In Comparative Example 44, the value F is below 8.600. In Comparative Example 45, the Cr content of the weld metal is too high.

[0106] As shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4, the weld metals of the invention examples, in which the chemical composition of the weld metal is within the specified range and the value F is 8.600 or more, are superior in hydrogen embrittlement resistance compared to the weld metals of the comparative examples, in which the chemical composition of the weld metal is outside the specified range or the value F is less than 8.600.

[0107] The disclosure of Japanese Patent Application No. 2024-060439 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

[0108] 22, 24 Base material 42, 44 Weld metal 62B, 64B Sampling position

Claims

1. The chemical composition at the center position in a cross section perpendicular to the weld line direction and at the t / 4 position in the plate thickness direction (t represents plate thickness) is, in mass % relative to the total mass, C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, Co: 0% to 1.0%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, A weld metal comprising Al: 0.003% to 1.00%, Ti: 0% to 1.000%, B: 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance consisting of Fe and impurities, wherein a value F represented by the following formula 1 is 8.600 or more: F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.) 2. Weld metal used in liquid hydrogen pressure vessels, the chemical composition at the center position in a cross section perpendicular to the weld line direction and at the t / 4 position in the plate thickness direction (t represents plate thickness) is, in mass % relative to the total mass, C: 0.005% to 0.20%, Si: 0.01% to 0.50%, Mn: 0% to 3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0% to 80.0%, Cr: 0% to 0.44%, Mo: 0.1% to 25.0%, Nb: 0% to 5.0%, W: 0% to 5.0%, Ta: 0% to 3.0%, Cu: 0% to 1.00%, Co: 0% to 1.0%, Pb: 0% to 1.00%, A weld metal comprising Sn: 0% to 1.00%, Al: 0% to 1.00%, Ti: 0% to 1.000%, B: 0.0001% to 0.1000%, V: 0% to 3.0000%, Ca: 0% to 1.0000%, Mg: 0% to 1.0000%, Zr: 0% to 1.0000%, REM: 0% to 0.5000%, N: 0% to 0.300%, and O: 0.050% or less, with the balance consisting of Fe and impurities, wherein a value F represented by the following formula 1 is 8.600 or more. F = 0.23 [Cr] + 0.68 [Mo] + 0.51 [Nb] + 0.42 [W] + 102.8 [B] Formula 1 (In Formula 1, [Cr] represents the Cr content relative to the total mass of the weld metal, [Mo] represents the Mo content relative to the total mass of the weld metal, [Nb] represents the Nb content relative to the total mass of the weld metal, [W] represents the W content relative to the total mass of the weld metal, and [B] represents the B content relative to the total mass of the weld metal.) 3. The weld metal according to claim 1 or 2, wherein the total content of the Nb and the W relative to the total mass of the weld metal is 1.0% or more.

4. The weld metal according to any one of claims 1 to 3, wherein in the chemical composition, the content of at least one of the Co and the Ca is within the following range: Co: 0.0001% to 1.0% Ca: 0.0001% to 1.0000% 5. A weld metal according to any one of claims 1 to 4, wherein the ratio of the content of Mo to the content of Cr is greater than 1 / 1.

6. A weld metal according to any one of claims 1 to 5, wherein the occupancy rate Lc of the precipitates on the γ grain boundaries is 30.0% or less.

7. The chemical composition, expressed in mass% relative to the total mass of the base material, is as follows: C: 0.01% to 0.07%, Si: 0.03% to 0.3%, Mn: 0.10% to 0.80%, P: 0.008% or less, S: 0.004% or less, Ni: 6.10% to 10.40%, Cr: 0% to 1.00%, Mo: 0% to 0.60%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Co: 0% to 0.050%, Al: 0.01% to 0.06%, N: 0.0015% to 0.006%, O: 0.003% or less, V: 0% to 0.080%, Ti: 0% to 0.020%, A welded joint comprising: a base metal containing B: 0% to 0.0020%, Ca: 0% to 0.0040%, and REM: 0% to 0.0050%, with the balance being Fe and impurities; and a weld metal, wherein the weld metal is the weld metal according to any one of claims 1 to 6.

8. A liquid hydrogen pressure vessel having the weld metal according to any one of claims 1 to 7.

9. A liquid hydrogen pressure vessel having the weld joint of claim 8.

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