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

TITLE OF INVENTION: WELD METAL, WELD JOINT, AND LIQUID HYDROGEN PRESSURE VESSEL Technical Field

[0001] The present disclosure relates to a weld metal, a weld joint, and a liquid hydrogen pressure vessel. Background Art

[0002] In recent years, due to tightening of regulations on carbon dioxide emissions in response to the issue of global warming, there has been an increasing demand for hydrogen fuel that emits less carbon dioxide as compared to petroleum, coal, and the like. Along with this, there has also been an increasing demand for liquid hydrogen pressure vessels such as liquid hydrogen tanks that are used on ships and land. In steel materials used for liquid hydrogen pressure vessels, nickel-alloyed cryogenic steels containing from 6% to 9% of Ni are used because of the requirement to ensure toughness at extremely low temperatures (e.g., -196°C). In welding of such nickel-alloyed cryogenic steels, an austenite-based welding material that can give a weld metal having excellent low-temperature toughness is used to form a weld metal. This welding material is mainly designed to have a Ni content of 70%.

[0003] For example, Japanese Patent Application Laid-Open (JP-A) No. H6-277875 discloses a HASTELLOY-based TIG welding wire for cryogenic steels, which contains specific amounts of Ni, Mo, W, Cu, and Ti, and in which not only the amounts of active elements Ca, Mg, Al, and Si, and gas components O and N are controlled at extremely low levels, but also the OX equivalent calculated from the amounts of Ca, Mg, Al, Si, Ti, O, and N is regulated to be less than a specific value. SUMMARY OF THE INVENTION Technical Problem

[0004] Incidentally, excellent hydrogen embrittlement resistance is required for steel materials used in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like. Ni-based welding materials are generally used for welding nickel-alloyed cryogenic steels; however, Ni-based alloys are known to be sensitive to hydrogen embrittlement, which causes grain boundary fracture and reduces the strength and the toughness. It is noted here that the above-described Patent Document 1 offers neither description nor suggestion about the use of a weld metal in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like, and does not particularly mention an improvement of hydrogen embrittlement resistance.

[0005] In light of the above, an object of the disclosure is to provide: a weld metal that is inhibited from hydrogen embrittlement in high-pressure hydrogen gas; a weld joint including the weld metal; and a liquid hydrogen pressure vessel including the weld metal or the weld joint. Solution to Problem

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

[0007] According to the disclosure, the following are provided: a weld metal that is inhibited from hydrogen embrittlement in high-pressure hydrogen gas; a weld joint including the weld metal; and a liquid hydrogen pressure vessel including the weld metal or the weld joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] [FIG. 1] FIG. 1 is a top view illustrating the shape of an SSRT test piece used in an SSRT test. [FIG. 2] FIG. 2 is a schematic cross-sectional view illustrating a position of collecting the SSRT test piece in the SSRT test. [FIG. 3] FIG. 3 is a schematic cross-sectional view illustrating a position of collecting the SSRT test piece in the SSRT test when a base metal is thin. DESCRIPTION OF EMBODIMENTS

[0009] An exemplary embodiment of the disclosure will now be described. In the present specification, those numerical ranges that are expressed with "to" each denote a range that includes the numerical values stated before and after "to" as the lower limit value and the upper limit value, respectively, unless "more than" or "less than" is attached to the numerical values. A numerical range in which "more than" or "less than" is attached to a numerical value stated before or after "to" means a range that does not include such numerical value as the lower limit value or the upper limit value. In a set of numerical ranges that are stated in a stepwise manner in the present specification, the upper limit value of one numerical range described in a stepwise manner may be replaced with the upper limit value of other numerical range described in a stepwise manner, or may be replaced with a relevant value indicated in any of Examples. Further, the lower limit value of one numerical range described in a stepwise manner may be replaced with the lower limit value of other numerical range described in a stepwise manner, or may be replaced with a relevant value indicated in any of Examples. With regard to the content, "%" means "% by mass". The expression "from 0 to" used for the content (%) of a component means that the component is an optional component, and does not have to be contained.

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

[0011] In the weld metal according to one embodiment of the disclosure, by the abovedescribed configuration, hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement) is inhibited even when the weld metal is used in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like. This weld metal according to the disclosure was discovered based on the following findings.

[0012] Incidentally, excellent hydrogen embrittlement resistance is required for steel materials used in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like. Ni-based welding materials are generally used for welding nickel-alloyed cryogenic steels; however, Ni-based alloys are known to be sensitive to hydrogen embrittlement, which causes grain boundary fracture and reduces the strength and the toughness. Thus, the present inventors investigated the relationship between the chemical composition of a weld metal and hydrogen embrittlement. As a result, the present inventors obtained the following findings, which lead to the discovery of the weld joint according to one embodiment of the disclosure.

[0013] When hydrogen penetrates into a weld metal, it accumulates mainly in the vicinity of 7 Y-grain boundaries. Therefore, it is believed that the y-grain boundaries are embrittled by the accumulated hydrogen. The present inventors thought that, by increasing the content of grain boundary strengthening elements in the weld metal and thereby increasing the strength of the Y-grain boundaries, the effects of the embrittlement of the Y-grain boundaries by hydrogen can be offset and, as a result, hydrogen embrittlement in the weld metal (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement) can be inhibited.

[0014] In light of this, in the weld metal according to one embodiment of the disclosure, the contents of Cr, Mo, Nb, W, and B, which are grain boundary strengthening elements, are set such that a value F represented by Equation 1 is 8.600 or more: F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 In Equation 1, the coefficient values relating to the contents of Cr, Mo, Nb, W, and B each represent the degree of the effect of each element on grain boundary strengthening.

[0015] Based on the above findings, in the weld metal according to the disclosure, hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement) is inhibited even when the weld metal is used in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like.

[0016] The weld metal according to one embodiment of the disclosure is concretely described below.

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

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

[0019] (C: from 0.005% to 0.20%) C is an element that improves the strength of the weld metal, and is used for ensuring the strength of the weld metal. However, when C content in the weld metal is excessive, carbides precipitate in the weld metal, which has a profound effect of deteriorating the toughness, and the low-temperature toughness of the weld metal is reduced. In addition, hot cracking during welding is accelerated. Therefore, C content in the weld metal is set from 0.005% to 0.20%. A lower limit of C content in the weld metal is preferably 0.01%, 0.02%, or 0.03%. An upper limit of C content in the weld metal is preferably 0.18%, 0.16%, 0.14%, 0.12%, 0.10%, or 0.07%.

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

[0021] (Mn: from 0% to 3.0%) Mn is an austenite stabilizing element, and may be incorporated into the weld metal for the purpose of improving the low-temperature toughness. However, when Mn content in the weld metal is excessive, solidification segregation occurs in the weld metal, causing hot cracking. Therefore, Mn content in the weld metal is set from 0% to 3.0%. A lower limit of Mn content in the weld metal is preferably 0.01%, 0.05%, 0.08%, or 0.1%. An upper limit of 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, and accelerates hot cracking or reduces the toughness; therefore, it is preferred to reduce P content in the weld metal as much as possible. Accordingly, a lower limit of P content in the weld metal may be set at 0%. However, from the standpoint of reducing P removal cost, P content in the weld metal may be 0.001% or more. On the other hand, when P content in the weld metal is 0.010% or less, the adverse effects of P are within an acceptable range. Therefore, P content in the weld metal is set at 0.010% or less. In order to effectively inhibit hot cracking or reduction in toughness, the P content in 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, and accelerates hot cracking or reduces the toughness; therefore, it is preferred to reduce S content in the weld metal as much as possible. Accordingly, a lower limit of S content in the weld metal may be set at 0%. However, from the standpoint of reducing S removal cost, S content in the weld metal is preferably 0.001% or more. On the other hand, when S content in the weld metal is 0.010% or less, the adverse effects of S are within an acceptable range. Therefore, S content in the weld metal is set at 0.010% or less. In order to effectively inhibit hot cracking or reduction in toughness, S content in the weld metal is preferably 0.008% or less, 0.005% or less, 0.004% or less, or 0.003% or less.

[0024] (Ni: from 50.0% to 80.0%) Ni is an austenite stabilizing element that improves the toughness. When Ni content in the weld metal is excessively low, austenitization of the weld metal hardly proceeds, and the low-temperature toughness is deteriorated. On the other hand, an increase in Ni content in the weld metal leads to an increase in the cost of the weld metal. Therefore, Ni content in the weld metal is set from 50.0% to 80.0%. A lower limit of Ni content in the weld metal is preferably 52.0%, 55.0%, 58.0%, or 60.0%. An upper limit of Ni content in the weld metal is preferably 78.0%, 76.0%, or 75.0%.

[0025] (Cr: from 0% to 0.44%) Cr is an austenite stabilizing element, and may be incorporated into the weld metal for the purpose of improving the low-temperature toughness of the weld metal. Further, Cr, which is a grain boundary strengthening element, is preferably incorporated into the weld metal from the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness of the weld metal due to hydrogen embrittlement). On the other hand, when Cr content in the weld metal is excessive, the solid-liquid coexistence temperature range of a molten metal is widened, making hot cracking more likely to occur. Therefore, Cr content in the weld metal is set from 0% to 0.44%. A lower limit of Cr content in the weld metal is preferably 0.02%, 0.05%, or 0.10%. An upper limit of Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%.

[0026] (Mo: from 0.1% to 25.0%) Mo, which is a solid-solution strengthening element or a precipitation strengthening element, is incorporated into the weld metal for the purpose of improving the strength of the weld metal. Further, Mo, which is a grain boundary strengthening element, is incorporated into the weld metal from the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness of the weld metal due to hydrogen embrittlement). On the other hand, when Mo content in the weld metal is excessive, the solid-liquid coexistence temperature range of a molten metal is widened, making hot cracking more likely to occur. Therefore, Mo content in the weld metal is set from 0.1% to 25.0%. A lower limit of Mo content in the weld metal is preferably 1.0%, 5.0%, 10.0%, or 12.0%. An upper limit of Mo content in the weld metal is preferably 24.0%, 23.0%, 22.0%, 20.0%, 18.0%, or 15.0%.

[0027] (Nb: from 0% to 5.0%) Nb is an element that forms a carbide in the weld metal and increases the strength of the weld metal, and thus may be incorporated into the weld metal. Further, Nb, which is a grain boundary strengthening element, is preferably incorporated into the weld metal from the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness of the weld metal due to hydrogen embrittlement). On the other hand, when Nb content in the weld metal is excessive, the solid-liquid coexistence temperature range of a molten metal is widened, making hot cracking more likely to occur. Therefore, Nb content in the weld metal is set from 0% to 5.0%. A lower limit of Nb content in the weld metal is preferably 0.1%, 0.2%, 0.3%, or 0.5%. An upper limit of 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: from 0% to 5.0%) W is a solid-solution strengthening element, and may be incorporated into the weld metal for the purpose of improving the strength. Further, W, which is a grain boundary strengthening element, is preferably incorporated into the weld metal from the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness of the weld metal due to hydrogen embrittlement). On the other hand, when W content in the weld metal is excessive, the weld metal is excessively strong, which may lead to a reduction in toughness. Therefore, W content in the weld metal is set from 0% to 5.0%. A lower limit of W content in the weld metal is preferably 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. An upper limit of W content in the weld metal is preferably 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, or 2.0%.

[0029] (Ta: from 0% to 3.0%) Ta is a solid-solution strengthening element, and may be incorporated into the weld metal for the purpose of improving the strength. On the other hand, when Ta content in the weld metal is excessive, the weld metal is excessively strong, which may lead to a reduction in toughness. Therefore, Ta content in the weld metal is set from 0% to 3.0%. A lower limit of Ta content in the weld metal is preferably 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. An upper limit of Ta content in the weld metal is preferably 2.5%, 2.0%, or 1.5%.

[0030] (Cu: from 0% to 1.00%) Cu is a precipitation strengthening element, and may be incorporated into the weld metal for the purpose of improving the strength of the weld metal. Further, Cu is an austenite stabilizing element, and may be incorporated into the weld metal for the purpose of improving the low-temperature toughness of the weld metal. However, when Cu content in the weld metal is excessive, the above-described effects are saturated. Therefore, Cu content in the weld metal is set from 0% to 1.00%. A lower limit of Cu content in the weld metal is preferably 0.02%, 0.05%, or 0.08%. An upper limit of Cu content in the weld metal is preferably 0.90%, 0.80%, or 0.50%.

[0031] (Co: from 0% to 1.0%) Co may be incorporated into the weld metal since it is an element that increases the strength of the weld metal through solid-solution strengthening. However, when Co content in the weld metal is excessive, toughness cannot be ensured due to a reduction in the ductility of the weld metal. Therefore, Co content in the weld metal is set from 0% to 1.0%. A lower limit of Co content in the weld metal is preferably 0.0001%, 0.0003%, 0.0005%, 0.0010%, 0.0020%, 0.01%, 0.05%, or 0.1%. An upper limit of Co content in the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0032] (Pb: from 0% to 1.00%) Pb may be incorporated into the weld metal since it has an effect of improving the toe formability between a steel material that is a base metal and the weld metal, and thereby improving the machinability of the weld metal. However, when Pb content in the weld metal is excessive, hot cracking occurs. Therefore, Pb content in the weld metal is set from 0% to 1.00%. A lower limit of Pb content in the weld metal is preferably 0.005%, 0.01%, or 0.1%. An upper limit of Pb content in the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0033] (Sn: from 0% to 1.00%) Sn may be incorporated into the weld metal since it is an element that improves the corrosion resistance of the weld metal. However, when Sn content in the weld metal is excessive, there is a concern that hot cracking may occur in the weld metal. Therefore, Sn content in the weld metal is set from 0% to 1.00%. A lower limit of Sn content in the weld metal is preferably 0.005%, 0.01%, or 0.1%. An upper limit of Sn content in the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0034] (Al: from 0.003% to 1.00%) Al is a deoxidizing element, and is incorporated into the weld metal for the purposes of reducing welding defects and improving the cleanliness of the weld metal. However, when Al content in the weld metal is excessive, Al may form a nitride or an oxide in the weld metal, and this may cause a reduction in the low-temperature toughness of the weld metal. Therefore, Al content in the weld metal is set from 0.003% to 1.00%. A lower limit of Al content in the weld metal is preferably 0.01%, 0.02%, 0.05%, or 0.10%. An upper limit of Al content in the weld metal is preferably 0.90%, 0.80%, or 0.50%.

[0035] (Ti: from 0% to 1.000%) Ti is a deoxidizing element, and may be incorporated into the weld metal for the purposes of reducing welding defects and improving the cleanliness of the weld metal. However, when Ti content in the weld metal is excessive, carbides and intermetallic compounds may be generated in the weld metal and deteriorate the toughness of the weld metal. Therefore, Ti content in the weld metal is set from 0% to 1.000%. A lower limit of Ti content in the weld metal is preferably 0.003%, 0.010%, or 0.020%. An upper limit of Ti content in the weld metal is preferably 0.900%, 0.700%, 0.500%, 0.300%, 0.250%, 0.200%, 0.150%, or 0.100%.

[0036] (B: from 0.0001% to 0.1000%) B is an austenite stabilizing element and also an interstitial solid-solution strengthening element, and is incorporated into the weld metal for the purpose of improving the low-temperature toughness and the strength of the weld metal. Further, B, which is a grain boundary strengthening element, is incorporated into the weld metal from the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness of the weld metal due to hydrogen embrittlement). However, when B content in the weld metal is excessive, M23(C,B)6 precipitates, causing deterioration of the toughness. In addition, since the solid-liquid coexistence temperature range is widened, hot cracking is accelerated. Therefore, B content in the weld metal is set from 0.0001% to 0.1000%. A lower limit of B content in the weld metal is preferably 0.0003%, 0.0005%, 0.0010%, or 0.0020%. An upper limit of B content in the weld metal is preferably 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0037] (V: from 0% to 3.0000%) V may be incorporated into the weld metal since it is an element that forms a carbonitride in the weld metal and increases the strength of the weld metal. However, when V content in the weld metal is excessive, the toughness is deteriorated due to coarsening of the carbonitride. In addition, since the solid-liquid coexistence temperature range is widened, hot cracking of the weld metal may occur. Therefore, V content in the weld metal is set from 0% to 3.0000%. A lower limit of V content in the weld metal is preferably 0.0100%, 0.0500%, 0.1000%, 0.1500%, or 0.2000%. An upper limit of V content in the weld metal is preferably 2.5000%, 2.0000%, or 1.5000%.

[0038] (Ca: from 0% to 1.0000%) Ca is effective for improving the ductility and the toughness of the weld metal since it has a function of changing the structures of sulfides and reducing the size of sulfides and oxides in the weld metal. Thus, Ca may be incorporated into the weld metal. However, when Ca content in the weld metal is excessive, sulfides and oxides are coarsened, which may lead to deterioration of the low-temperature toughness of the weld metal. Therefore, Ca content in the weld metal is set from 0% to 1.0000%. A lower limit of Ca content in the weld metal is preferably 0.0001%, 0.0003%, 0.0005%, 0.0010%, or 0.0020%. An upper limit of Ca content in the weld metal is preferably 0.8000%, 0.5000%, or 0.3000%.

[0039] (Mg: from 0% to 1.0000%) Mg is a deoxidizing element, and has an effect of reducing the amount of oxygen in the weld metal and improving the cleanliness of the weld metal. Thus, Mg may be incorporated into the weld metal. However, when Mg content in the weld metal is excessive, Mg violently reacts with oxygen in an arc, which may increase the amount of spatter and fumes generated. Therefore, Mg content in the weld metal is set from 0% to 1.0000%. A lower limit of Mg content in the weld metal is preferably 0.0100%, 0.0200%, or 0.0300%. An upper limit of Mg content in the weld metal is preferably 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0040] (Zr: from 0% to 1.0000%) Zr is a deoxidizing element, and has an effect of reducing the amount of oxygen in the weld metal and improving the cleanliness of the weld metal. Thus, Zr may be incorporated into the weld metal. However, when Zr content in the weld metal is excessive, Zr violently reacts with oxygen in an arc, which may increase the amount of spatter and fumes generated. Therefore, Zr content in the weld metal is set from 0% to 1.0000%. A lower limit of Zr content in the weld metal is preferably 0.0100%, 0.0200%, or 0.0300%. An upper limit of Zr content in the weld metal is preferably 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0041] (REM: from 0% to 0.5000%) REM may be incorporated into the weld metal since it is an element that stabilizes an arc during the welding operation for obtaining the weld metal. However, when the REM content in the weld metal is excessive, severe spattering may occur during the welding operation for obtaining the weld metal, which can lead to poor welding workability. Therefore, the REM content in the weld metal is set from 0% to 0.5000%. A lower limit of the REM content in the weld metal is preferably 0.0010%, 0.0020%, or 0.0050%. An upper limit of the REM content in the weld metal is preferably 0.4000%, 0.2000%, or 0.1000%.

[0042] It is noted here that "REM" refers to a total of 17 elements including Sc, Y, and lanthanoids such as La and Nd, and the "REM content" means a total content of these elements. The REM is added to molten steel using, for example, an Fe-Si-REM alloy that contains, for example, La, Nd, Ce, and Pr.

[0043] (N: from 0% to 0.300%) N is an austenite stabilizing element and also an interstitial solid-solution strengthening element, and may be incorporated into the weld metal for the purpose of improving the low-temperature toughness and the strength of the weld metal. However, when N content in the weld metal is excessive, the occurrence of blowing increases, causing welding defects. Therefore, N content in the weld metal is set from 0% to 0.300%. A lower limit of N content in the weld metal is preferably 0.001%, 0.005%, 0.010%, or 0.020%. An upper limit of N content in 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 can be contained in the weld metal as an impurity. An excessive O content leads to deterioration of the toughness and the ductility; therefore, an upper limit of O content in the weld metal is set at 0.150% or less. On the other hand, an excessive reduction in O content leads to an increase in the production cost; therefore, a lower limit of O content in the weld metal may be set at 0.001% or less. The lower limit of O content in the weld metal is preferably 0.002% or 0.003%. The upper limit of O content in the weld metal is preferably 0.040% or 0.030%.

[0045] (Remainder: Fe and Impurities) The other remainder components in the chemical composition of the weld metal are Fe and impurities. The "impurities" refer to components that are, during the industrial production of the weld metal, included from raw materials such as ores and scrap or included due to various factors of the production process, and are acceptable within a range that does not adversely affect the properties of the weld metal.

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

[0047] (Value F) With regard to the contents of Cr, Mo, Nb, W, and B that are grain boundary strengthening elements, the weld metal has a value F, which is represented by Equation 1, of 8.600 or more. Equation 1: F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] wherein, in Equation 1, [Cr] represents a Cr content with respect to a total mass of the weld metal, [Mo] represents an Mo content with respect to the total mass of the weld metal, [Nb] represents an Nb content with respect to the total mass of the weld metal, [W] represents a W content with respect to the total mass of the weld metal, and [B] represents a B content with respect to the total mass of the weld metal.

[0048] In Equation 1, the coefficient values relating to the contents of Cr, Mo, Nb, W, and B each represent the degree of the effect of each element on grain boundary strengthening. By controlling the value F to be 8.600 or more, hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement) is inhibited even when the weld metal is used in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like. From the standpoint of inhibiting hydrogen embrittlement (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement), 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, a total content of Nb and W is preferably 1.0% or more with respect to the total mass of the weld metal. Nb and W contribute to enhancing the strength of the weld metal, and when the total content of Nb and W is in the above-described range, the tensile strength TS of the weld metal can be increased. From the standpoint of increasing the tensile strength TS of the weld metal, the total content of Nb and W is more preferably 1.5% or more, still more preferably 2.0% or more.

[0050] (Content Ratio of Mo to Cr) The weld metal preferably has a content ratio of Mo to Cr of higher than 1 / 1 (i.e., Mo content is greater than Cr content). Mo has a higher effect of inhibiting hydrogen embrittlement than Cr; therefore, when the content ratio of Mo to Cr is in the above-described range, hydrogen embrittlement of the weld metal is inhibited. The content ratio of Mo to Cr is more preferably 5 / 1 or higher, 10 / 1 or higher, 20 / 1 or higher, 30 / 1 or higher, or 40 / 1 or higher.

[0051] (Tensile Strength) The weld metal preferably has a tensile strength of, for example, from 590 to 1,200 MPa. The tensile strength can be measured by conducting a tensile test of the weld metal in accordance with JIS Z3111:2005.

[0052] (Occupancy Lc of Grain Boundary Strengthening Element Precipitates on y-Grain Boundaries) Element-depleted regions are formed around precipitates existing on y-grain boundaries. For example, when a Cr carbide exists as a precipitate on a y-grain boundary, a depleted region with a low amount of Cr and C is locally formed around the carbide. As a result of the formation of a region depleted in Cr that is a grain boundary strengthening element, the grain boundary strength is reduced around the precipitate. Therefore, in the weld metal according to one embodiment of the disclosure, the occupancy Lc of precipitates on y-grain boundaries is preferably Lc < 30.0%. The "precipitates" are believed to contain at least one element selected from the group consisting of Cr, Mo, Nb, W, and B that are grain boundary strengthening elements (such precipitates are hereinafter referred to as "grain boundary strengthening element precipitates"). By controlling the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries to be 30.0% or less, not only the strength of the y-grain boundaries can be increased, but also hydrogen embrittlement of the weld metal (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement) can be inhibited.

[0053] From the standpoint of inhibiting hydrogen embrittlement of the weld metal (e.g., a reduction in strength and a reduction in toughness due to hydrogen embrittlement), the occupancy of grain boundary strengthening element precipitates on y-grain boundaries is preferably 25.0% or less, more preferably 20.0% or less, still more preferably 16.0% or less. Further, a lower limit value of the occupancy is not particularly limited; however, it may be 5.0% or more from the standpoint of the difficulty of completely inhibiting the formation of precipitates.

[0054] The occupancy of grain boundary strengthening element precipitates on y-grain boundaries is determined by the following method. On a backscattered electron image taken under a scanning electron microscope (SEM), the weld metal is observed at a center position of a cross section perpendicular to the weld line direction, and at a t / 4 position in the plate thickness direction (wherein, t represents a plate thickness). By the observation of the backscattered electron image, precipitates are observed with a contrast different from that of the matrix phase. Five y grains randomly selected in an observation field are designated as measurement targets, and the ranges in which precipitates exist on the y-grain boundaries of the measurement targets are measured, after which the length occupied by the precipitates per grain boundary length of the measured five y grains is determined. For example, when the total grain boundary length is 1,000 pm and there are precipitates of 50 pm in total, the occupancy Lc is "50 pm / 1,000 pm x 100 = 5%". This measurement is performed for five cross sections (at the center position of each cross section perpendicular to the weld line direction of the weld metal, and at a t / 4 position in the plate thickness direction (wherein, t represents the plate thickness)), and an arithmetic mean of the measured values is defined as the occupancy 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 verified by the following method. The weld metal is observed on its backscattered electron image taken under a scanning electron microscope (SEM), and a point analysis based on energy dispersive X-ray spectroscopy (EDS) is performed for precipitates on grain boundaries having a contrast different from that of the matrix phase. Verification can be made based on whether or not the grain boundary strengthening element is included in the thus obtained values. For the analysis, an electron-probe microanalyzer (EPMA) may be used.

[0056] Examples of a method of reducing and controlling the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries to be in the abovedescribed range include the following four methods. (1) Method of Reducing Occupancy Lc by Reducing C Content By reducing the C content in the weld metal, the amount of precipitation of carbides of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) can be reduced, as a result of which the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries can be lowered. From this standpoint, an upper limit of the C content in the weld metal is preferably 0.12%, 0.10%, or 0.07%.

[0057] (2) Method of Reducing Occupancy Lc by Reducing Contents of Grain Boundary Strengthening Elements By reducing the contents 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, as a result of which the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries can be lowered. From this standpoint, the content of each grain boundary strengthening element is preferably as follows. An upper limit of the Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%. An upper limit of the Mo content in the weld metal is preferably 20.0%, 18.0%, or 15.0%. An upper limit of Nb content in the weld metal is preferably 2.5%, 2.0%, 1.0%, or 0.5%. An upper limit of the W content in the weld metal is preferably 3.0%, 2.5%, or 2.0%. An 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 Occupancy Lc by Reducing C Content and Contents of Grain Boundary Strengthening Elements By reducing the contents of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) in the weld metal to ranges that are not excessively high while also reducing the C content in the weld metal to a range that is not excessively high, the amount of carbides of the grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) can be reduced, as a result of which the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries can be lowered. From this standpoint, the content of C and that of each grain boundary strengthening element are preferably as follows. An upper limit of the C content in the weld metal is preferably 0.18%, 0.16%, or 0.14%. An upper limit of the Cr content in the weld metal is preferably 0.40%, 0.35%, or 0.30%. An upper limit of the Mo content in the weld metal is preferably 24.0%, 23.0%, 22.0%, or 20.0%. An upper limit of Nb content in the weld metal is preferably 4.5%, 4.0%, 3.5%, 3.0%, or 2.5%. An upper limit of the W content in the weld metal is preferably 4.5%, 4.0%, 3.5%, or 3.0%. An 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 Occupancy Lc by Lowering Interpass Temperature During Welding The weld metal is formed by multi-pass welding. By reducing the interpass temperature (the temperature between passes in the multi-pass welding in which welding is performed through multiple passes), the amount of grain boundary strengthening element precipitates can be reduced, as a result of which the occupancy Lc of the grain boundary strengthening element precipitates on y-grain boundaries can be lowered. The interpass temperature is preferably 150°C or lower, more preferably 125°C or lower.

[0060] <Weld Joint> Next, the weld joint according to one embodiment of the disclosure will be described. The weld joint according to the disclosure includes a base metal and the weld metal according to the disclosure. For example, the weld joint according to the disclosure includes a steel material serving as the base metal, and a welded portion formed of the weld metal and a heat-affected zone.

[0061] (Chemical Composition of Base metal) In the weld joint according to one embodiment of the disclosure, the composition of the base metal is not particularly limited. However, from the standpoint of using the weld joint in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel or the like, the base metal preferably has the following chemical composition. The chemical composition includes, in terms of % by mass with respect to a total mass of the base metal: C: from 0.01% to 0.07%, Si: from 0.03% to 0.3%, Mn: from 0.10% to 0.80%, P: 0.008% or less, S: 0.004% or less, Ni: from 6.10% to 10.40%, Cr: from 0% to 1.00%, Mo: from 0% to 0.60%, Nb: from 0% to 0.050%, Cu: from 0% to 1.00%, Co: from 0% to 0.050%, Al: from 0.01% to 0.06%, N: from 0.0015% to 0.006%, O: 0.003% or less, V: from 0% to 0.080%, Ti: from 0% to 0.020%, B: from 0% to 0.0020%, Ca: from 0% to 0.0040%, and REM: from 0% to 0.0050%, and the remainder is composed of Fe and impurities.

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

[0063] Si: from 0.03% to 0.3% Si is an element that increases the strength of steel through solid-solution strengthening. However, when the Si content is excessive, not only the workability is deteriorated but also the strength of steel is reduced. Therefore, the Si content in the base metal is set from 0.03% to 0.3%. A lower limit of the Si content in the base metal is preferably 0.05%, 0.08%, or 0.1%. An upper limit of the Si content in the base metal is preferably 0.28%, 0.25%, or 0.2%.

[0064] Mn: from 0.10% to 0.80% Mn is an element that increases the strength of steel. However, when the Mn content is excessive, not only the workability is deteriorated but also the toughness is made unstable. Therefore, the Mn content in the base metal is set from 0.10% to 0.80%. A lower limit of the Mn content in the base metal is preferably 0.20% or 0.30%. An upper limit of the Mn content in the base metal is preferably 0.70% or 0.60%.

[0065] P: 0.008% or less P, which is an impurity, is an element that causes embrittlement; therefore, it is preferred to reduce the P content in the base metal as much as possible. Accordingly, a lower limit of the P content in the base metal may be set at 0%. However, from the standpoint of reducing the P removal cost, the P content in the base metal may be set at 0.002% or more. On the other hand, when the P content in the base metal is 0.008% or less, the adverse effect of P on embrittlement is within an acceptable range. Therefore, the P content in the base metal is set at 0.008% or less. In order to effectively inhibit embrittlement, the P content in the base metal is preferably 0.007% or less, 0.006% or less, or 0.005% or less.

[0066] S: 0.004% or less S, which is an impurity, is an element that causes embrittlement. In addition, S is an element that forms coarse MnS in steel, and not only deteriorates the workability of steel but also reduces the strength of steel. Therefore, it is preferred to reduce the S content in the base metal as much as possible, and a lower limit of the S content in the base metal may be set at 0%. However, from the standpoint of reducing the S removal cost, the S content in the base metal may be set at 0.0002% or more. On the other hand, when the S content in the base metal is 0.004% or less, the adverse effects of S, such as embrittlement, are within an acceptable range. Therefore, the S content in the base metal is set at 0.004% or less. In order to effectively inhibit embrittlement, the S content in the base metal is preferably 0.003% or less, or 0.002% or less.

[0067] Ni: from 6.10% to 10.40% Ni is an element that contributes to improving the strength and the low-temperature toughness of steel. However, when the Ni content is excessive, it is difficult to improve the toughness in a way that justifies the increased cost, and problems may arise during acid pickling and hot working. Therefore, the Ni content in the base metal is set from 6.10% to 10.40%. A lower limit of the Ni content in the base metal may be preferably 6.50%, 7.00%, or 8.00%. An upper limit of the Ni content in the base metal is preferably 10.00%, 9.800%, or 9.500%.

[0068] Cr: from 0% to 1.00% Cr is an element that contributes to improving the strength of steel, and may be incorporated into the base metal. However, when the Cr content is excessive, the strength of steel may be reduced, and problems may arise during acid pickling and hot working. Therefore, the Cr content in the base metal is set from 0% to 1.00%. A lower limit of the Cr content in the base metal is preferably 0.05%, 0.10%, or 0.20%. An upper limit of the Cr content in the base metal is preferably 0.90%, 0.80%, or 0.70%.

[0069] Mo: from 0% to 0.60% Mo is an element that contributes to improving the strength of steel, and may be incorporated into the base metal. However, when the Mo content is excessive, the strength of steel may be reduced, and problems may arise during acid pickling and hot working. Therefore, the Mo content in the base metal is set from 0% to 0.60%. A lower limit of the Mo content in the base metal may be preferably 0.01%, 0.02%, or 0.03%. An upper limit of the Mo content in the base metal is preferably 0.55%, 0.50%, or 0.45%.

[0070] Nb: from 0% to 0.050% Nb is an element that forms a fine carbonitride and inhibits the coarsening of crystal grains, and may be incorporated into the base metal. However, when Nb content is excessive, there is a concern that it may cause deterioration of the toughness and a reduction in the strength of steel. Therefore, Nb content in the base metal is set from 0% to 0.050%. A lower limit of Nb content in the base metal may be preferably 0.001%, 0.002%, or 0.003%. An upper limit of Nb content in the base metal is preferably 0.045%, 0.040%, or 0.035%.

[0071] Cu: from 0% to 1.00% Cu is an element that contributes to improving the strength of steel, and may be incorporated into the base metal. However, when the Cu content is excessive, the strength of steel may be reduced, and problems may arise during acid pickling and hot working. Therefore, the Cu content in the base metal is set from 0% to 1.00%. A lower limit of the Cu content in the base metal is preferably 0.05%, 0.10%, or 0.20%. An upper limit of the Cu content in the base metal is preferably 0.90%, 0.80%, or 0.70%.

[0072] Co: from 0% to 0.050% Co is an element that improves the corrosion resistance in a corrosive environment. Thus, Co may be incorporated into the base metal. In order to obtain the above-described effect, Co content is preferably 0.006% or more. However, when Co content is excessive, the above-described effect is saturated, and the economic efficiency is deteriorated. Therefore, Co content in the base metal is set from 0% to 0.050%. A lower limit of Co content in the base metal is preferably 0.001%, 0.003%, or 0.006%. An upper limit of Co content in the base metal is preferably 0.040%, 0.030%, or 0.020%.

[0073] Al: from 0.01% to 0.06% Al is an element that exerts a deoxidizing effect. However, when the Al content is excessive, an increase in the amount of inclusions leads to deterioration of the workability and a reduction in the toughness of steel. Therefore, Al content in the base metal is set from 0.01% to 0.06%. A lower limit of Al content in the base metal is preferably 0.015% or 0.02%. An upper limit of Al content in the base metal is preferably 0.055% or 0.05%.

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

[0075] O: 0.003% or less O is an element that can be contained in steel as an impurity. An excessive O content leads to deterioration of the toughness and the ductility. Although O content may be 0%, an excessive reduction in O content leads to an increase in the production cost; therefore, a lower limit of O content in the base metal may be set at 0.0001%. On the other hand, when O content in the base metal is 0.003% or less, the adverse effects of O on the toughness and the like are within an acceptable range. In order to effectively inhibit the deterioration of the toughness and the ductility, the O content in the base metal is preferably 0.0025% or less, or 0.002% or less.

[0076] V: from 0% to 0.080% V is an element that forms a fine carbonitride and inhibits the coarsening of crystal grains, and may be incorporated into the base metal. However, when V content is excessive, there is a concern that it may cause deterioration of the toughness and a reduction in the strength of steel. Therefore, V content in the base metal is set from 0% to 0.080%. A lower limit of V content in the base metal may be preferably 0.001%, 0.002%, or 0.003%. An upper limit of V content in the base metal is preferably 0.075%, 0.070%, or 0.065%.

[0077] Ti: from 0% to 0.020% Ti is an element that forms precipitates and makes the structure of the steel material into fine grains, and may be incorporated into the base metal. However, when Ti content is excessive, there is a concern that not only the productivity of the steel material may be deteriorated and cracking may occur during processing, but also the strength of the steel may be reduced. Therefore, Ti content in the base metal is set from 0% to 0.020%. A lower limit of Ti content in the base metal may be preferably 0.001%, 0.002%, or 0.003%. An upper limit of Ti content in the base metal is preferably 0.018%, 0.016%, or 0.014%.

[0078] B: from 0% to 0.0020% B is an element that is segregated at grain boundaries and increases the strength of the grain boundaries, and may be incorporated into the base metal. However, when B content is excessive, there is a concern that it may cause deterioration of the toughness and a reduction in the strength of steel. Therefore, B content in the base metal is set from 0% to 0.0020%. A lower limit of B content in the base metal may be preferably 0.0001%, 0.0002%, or 0.0003%. An upper limit of B content in the base metal is preferably 0.0018%, 0.0016%, or 0.0014%.

[0079] Ca: from 0% to 0.0040% Ca is effective for improving the ductility and the toughness since it has a function of reducing the size of sulfides and oxides. Thus, Ca may be incorporated into the base metal. However, when Ca content is excessive, sulfides and oxides are coarsened, which may lead to deterioration of the low-temperature toughness of the base metal. Therefore, Ca content in the base metal is set from 0% to 0.0040%. A lower limit of Ca content in the base metal is preferably 0.0001%, 0.0002%, or 0.0005%. An upper limit of Ca content in the base metal is preferably 0.0038%, 0.0035%, or 0.0030%.

[0080] REM: from 0% to 0.0050% Similarly to Ca, REM may be incorporated into the base metal since it is an element that has an effect of reducing the size of inclusions in steel and improving the toughness of a steel member. However, when the REM content in the base metal is excessive, not only the above-described effect is saturated, but also the cost is increased. Therefore, the REM content in the base metal is set from 0% to 0.0050%. A lower limit of the REM content in the base metal is preferably 0.0001%, 0.0002%, or 0.0005%. An upper limit of the REM content in the base metal is preferably 0.0048%, 0.0045%, or 0.0040%.

[0081] It is noted here that "REM" refers to a total of 17 elements including Sc, Y, and lanthanoids such as La and Nd, and the "REM content" means a total content of these elements. The REM is added to molten steel using, for example, an Fe-Si-REM alloy that contains, for example, La, Nd, Ce, and Pr.

[0082] Remainder: Fe and Impurities The other remainder components in the chemical composition of the base metal are Fe and impurities. The "impurities" refer to components that are, during the industrial production of the base metal, included from raw materials such as ores and scrap or included due to various factors of the production process, and are acceptable within a range that does not adversely affect the properties of the base metal.

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

[0084] Next, a method of producing the weld joint according to one embodiment of the disclosure will be described. It is noted that the production method described below is merely one example, and the method of producing the weld joint according to the disclosure is not limited to the method described below.

[0085] The weld joint according to the disclosure can be produced by welding a steel material as a base metal using a welding material. For example, the weld joint can be obtained by submerged arc welding using a solid wire and a flux. In the submerged arc welding, for example, it is possible to apply a general submerged arc welding apparatus in which a granular flux is sprayed on a weld line in advance, the solid wire is fed into the flux, and then welding is performed with arc heat generated from an arc between the solid wire and the steel material in the flux. In this case, the chemical composition of the weld metal includes components derived from the solid wire and the flux that are welding materials, as well as the steel material that is a base metal.

[0086] From the standpoint of controlling the occupancy Lc of grain boundary strengthening element precipitates on y-grain boundaries to be in the above-described range, in submerged arc welding using a solid wire and a flux, it is preferred to produce the weld joint under, for example, the following conditions: Current: from 330 A to 390 A (e.g., 360 A) Voltage: from 20 V to 40 V (e.g., 30 V) Wire moving speed: from 25 cm / min to 40 cm / min (e.g., 35 cm / min) Heat input: from 17.0 kJ / cm to 20.0 kJ / cm (e.g., 18.5 kJ / cm) Welding method: multi-pass welding Interpass temperature: 150°C or lower (more preferably 125°C or lower)

[0087] Alternatively, the weld joint can also be obtained by other welding method such as TIG welding. In this case, the chemical composition of the weld metal includes components derived from a welding material and a steel material that is a base metal.

[0088] <Use> The weld metal according to one embodiment of the disclosure and the weld joint according to one embodiment of the disclosure that includes the weld metal are used, for example, in a high-pressure hydrogen gas of a liquid hydrogen pressure vessel (e.g., a liquid hydrogen tank) or the like. In other words, the liquid hydrogen pressure vessel according to one embodiment of the disclosure includes the weld metal according to one embodiment of the disclosure, or the weld joint according to one embodiment of the disclosure. EXAMPLES

[0089] The feasibility and the effects of the disclosure will now be described in more detail by way of Inventive Examples and Comparative Examples; however, the below-described Inventive Examples do not limit the disclosure, and any design modification conforming to the gist described above and below is included in the technical scope of the disclosure.

[0090] (Production of Weld Joint) In accordance with the following method, base metals were welded together by submerged arc welding (SAW) using a solid wire and a flux to obtain a weld joint including a weld metal. First, submerged arc welding was performed using a solid wire in combination with NITTETSU FLUX 10H manufactured by Nippon Steel Welding & Engineering Co., Ltd., which is a flux for submerged arc welding. As steel sheets (base metals) to be welded, steel sheets (20 mm(t) x 120 mm(w) x 300 mm(l)) having the respective compositions shown in Table 1 were used. As illustrated in FIG. 2, the two steel sheets (base metals 22) were butted together, and groove formation and welding were performed such that the resulting groove was an X-shaped groove with an angle 0 = 60°. Specifically, after the two steel sheets (base metals 22) were butted together, a Y-shaped groove was first formed on one side, and build-up welding was performed in this Y-shaped groove, after which a chipping process was performed on the back side of the Y-shaped groove to form an X-shaped groove, and build-up welding was then performed in this back-side groove, whereby a weld metal 42 having an X-shaped groove was formed. The welding currents during welding were all direct currents, all wires had a positive polarity, and the welding conditions were as shown in Table 2. The chemical composition of the weld metal was controlled to be the respective 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 the flux. The base metal No. 1 shown in Table 1 was used as the base metal in the weld metals shown in Tables 3-1 to 3-4 (Inventive Examples 1 to 36 and Comparative Examples 1 to 5), and the base metal No. 2 shown in Table 1 was used as the base metal in the weld metals shown in Tables 4-1 to 4-4 (Inventive Examples 41 to 76 and Comparative Examples 41 to 45).

[0091] The chemical compositions of the weld metals are shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4. Further, for each weld metal, the occupancy Lc of precipitates on y-grain boundaries was measured by the above-described method. The results thereof are shown in Tables 3-2, 3-4, 4-2, and 4-4. When the compositions of several precipitates were examined, the precipitates were found to contain 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 each weld metal was measured by the following method. The relative reduction of area (RRA) of each weld metal was measured by a slow strain rate tensile test (SSRT) in a hydrogen environment. First, an SSRT test piece illustrated in FIG. 1 was collected from the weld metal. FIGs. 2 and 3 each illustrate a test piece collection position. FIG. 2 illustrates an example of a weld joint having an X-shaped groove (X groove) with an angle 0 = 60°, and this weld joint includes base metals 22 (steel sheets) and a weld metal 42 formed in the X groove. FIG. 3 illustrates a test piece collection position of a case where the base metals are thin, and the weld joint having an X groove with an angle 0 = 60° includes base metals 24 (steel sheets) and a weld metal 44 formed in the X groove. As illustrated in FIG. 2, a test piece of the weld metal 42 is collected such that the parallel portion of the test piece is entirely the weld metal 42. In addition, the test piece of the weld metal 42 is collected from a collection position 62B such 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 the base metals 22 in the plate thickness direction. However, when the base metals are thin, and a test piece centered on the t / 4 position cannot be collected, as illustrated in FIG. 3, a test piece of the weld metal 44 is collected from a collection position 64B such that the edge of a gripping portion not only is aligned with a position that is 1 mm from the surface of the base metals 24 in the plate thickness direction but also is parallel to the weld line direction.

[0093] The thus collected SSRT test piece was subjected to a slow strain rate tensile test (SSRT test) in a 0.1-MPa nitrogen gas environment (simulating the atmospheric environment) and a 10-MPa hydrogen gas environment. The test temperature and the strain rate were set at normal temperature (25°C) and 4.2 x 10-5 / s, respectively, and the ratio (%) of the reduction of area at break in the hydrogen gas environment with respect to the reduction of area at break in the nitrogen gas environment was calculated as the relative reduction of area (RRA). The strain rate was determined from the crosshead moving speed during the tensile test.

[0094] The results of the relative reduction of area (RRA) were evaluated based on the following evaluation criteria. The results thereof are shown in Tables 3-2, 3-4, 4-2, and 4-4. A (◎): 75% or more B (o): 50% or more but less than 75% C (x): less than 50% Base metal No. C Si Mn P S Ni Mo Al N O 1 0.05 0.2 0.65 0.005 0.001 9.01 0.03 0.003 0.001 2 0.04 0.2 0.57 0.005 0.001 7.61 0.05 0.03 0.003 0.001 Welding method Current A Voltage V Speed mm / min Heat input kJ / cm Interpass temperature °C SAW 360 30 35 18.5 150 or lower Weld metal C Si Mn P S Ni Cr Mo Nb W Ta Cu Co Pb Sn Al Ti Inventive Example 1 0.07 0.12 1.5 0.004 0.003 55.7 14.8 0.21 0.089 Inventive Example 2 0.11 0.28 0.8 0.003 0.001 64.0 17.9 0.1 0.6 0.48 Inventive Example 3 0.04 0.14 0.3 0.003 0.001 61.9 24.6 0.06 0.18 0.017 Inventive Example 4 0.03 0.38 0.4 0.003 0.002 75.0 14.7 0.1 0.13 Inventive Example 5 0.13 0.06 0.4 0.003 0.004 60.4 0.10 14.9 0.5 0.10 0.1 0.01 0.43 0.024 Inventive Example 6 0.12 0.18 1.6 0.003 0.004 69.4 0.20 25.0 1.6 0.10 0.050 Inventive Example 7 0.16 0.24 0.4 0.005 0.003 57.6 24.6 1.6 1.5 0.15 0.087 Inventive Example 8 0.06 0.25 1.1 0.002 0.001 63.4 0.40 23.1 2.4 0.2 0.2 0.10 0.07 Inventive Example 9 0.15 0.28 1.3 0.005 0.004 71.0 19.9 1.9 0.9 0.4 0.06 0.030 Inventive Example 10 0.03 0.35 1.2 0.004 0.003 66.5 12.6 2.1 0.10 0.075 Inventive Example 11 0.04 0.36 0.6 0.002 0.004 71.4 19.1 2.8 1.1 0.29 0.055 Inventive Example 12 0.07 0.16 0.9 0.002 0.004 77.1 13.4 0.1 1.3 0.1 0.06 0.01 0.02 Inventive Example 13 0.03 0.16 0.4 0.004 0.001 55.0 0.40 18.6 0.3 1.9 0.25 0.017 Inventive Example 14 0.05 0.24 0.4 0.005 0.003 78.2 14.6 1.5 0.15 0.087 Inventive Example 15 0.05 0.30 0.004 0.004 75.6 20.1 0.01 Inventive Example 16 0.19 0.22 0.5 0.005 0.003 64.5 15.1 0.03 Inventive Example 17 0.06 0.50 1.1 0.006 0.006 56.9 16.5 0.04 Inventive Example 18 0.03 0.31 3.0 0.004 0.001 74.6 14.6 0.03 Inventive Example 19 0.03 0.25 1.5 0.001 0.004 64.9 25.0 0.10 Inventive Example 20 0.03 0.26 0.8 0.004 0.002 77.1 8.9 4.9 0.09 Inventive Example 21 0.09 0.35 0.9 0.002 0.002 59.8 10.3 5.0 0.08 Weld metal B V Ca Mg Zr REM N 0 Value F Nb+W Lc (%) RRA (%) Evaluation Inventive Example 1 0.0023 0.1000 0.015 0.020 10.300 0 24.1 74 B(o) Inventive Example 2 0.0005 0.1000 0.1000 0.006 0.010 12.526 0.7 16.3 86 A(©) Inventive Example 3 0.0013 0.5000 0.002 0.002 16.862 0 34.6 71 B(o) Inventive Example 4 0.0038 0.1000 0.001 0.004 10.387 0 9.6 72 B(o) Inventive Example 5 0.0009 0.003 0.005 10.503 0.5 12.8 89 A(©) Inventive Example 6 0.0010 0.1000 0.003 0.003 17.821 1.6 32.1 87 A(©) Inventive Example 7 0.0006 1.2000 0.001 0.004 18.236 3.1 33.5 86 A(©) Inventive Example 8 0.0010 0.010 0.010 17.211 2.6 31.4 86 A(©) Inventive Example 9 0.0006 0.020 0.020 14.941 2.8 30.1 80 A(©) Inventive Example 10 0.0005 0.1000 0.020 0.030 9.690 2.1 14.9 81 A(©) Inventive Example 11 0.0080 0.001 0.004 14.986 2.8 19.6 86 A(©) Inventive Example 12 0.0009 0.2000 0.1000 0.007 0.010 9.802 1.4 14.4 89 A(©) Inventive Example 13 0.0010 0.003 0.004 13.794 2.2 12.4 94 A(©) Inventive Example 14 0.0013 0.1000 0.1000 0.001 0.004 10.692 1.5 10.9 92 A(©) Inventive Example 15 0.0005 0.001 0.002 13.719 0 24.0 75 A(©) Inventive Example 16 0.0006 0.050 0.005 10.330 0 21.3 74 B(o) Inventive Example 17 0.0009 0.030 0.020 11.313 0 28.6 70 B(o) Inventive Example 18 0.0008 0.040 0.010 10.010 0 16.0 95 A(©) Inventive Example 19 0.0012 0.005 0.030 17.123 0 23.8 90 A(©) Inventive Example 20 0.0011 0.080 0.040 8.664 4.9 18.0 90 A(©) Inventive Example 21 0.0005 0.003 0.005 9.155 5.0 10.9 98 A(©) Weld metal C Si Mn P S Ni Cr Mo Nb W Ta Cu Co Pb Sn Al Ti Inventive Example 22 0.03 0.15 1.5 0.007 0.007 68.0 10.9 2.8 0.12 Inventive Example 23 0.11 0.26 1.3 0.003 0.005 64.3 15.6 1.00 0.05 Inventive Example 24 0.16 0.31 1.3 0.006 0.006 58.1 0.15 17.8 0.8 0.06 Inventive Example 25 0.05 0.33 0.9 0.007 0.002 70.1 20.1 0.80 0.04 Inventive Example 26 0.03 0.24 0.8 0.002 0.003 70.9 19.1 0.90 0.03 Inventive Example 27 0.03 0.21 1.5 0.001 0.002 55.8 19.8 0.90 Inventive Example 28 0.04 0.15 1.6 0.001 0.001 59.4 15.6 0.01 0.950 Inventive Example 29 0.02 0.26 1.8 0.009 0.002 68.1 15.7 0.01 Inventive Example 30 0.08 0.19 1.9 0.007 0.003 63.1 18.9 0.06 Inventive Example 31 0.06 0.21 2.6 0.002 0.008 61.8 0.26 13.4 0.08 Inventive Example 32 0.07 0.34 0.5 0.007 0.002 79.1 0.40 15.3 0.06 Inventive Example 33 0.16 0.39 0.3 0.001 0.003 58.1 15.6 0.03 Inventive Example 34 0.03 0.15 0.7 0.002 0.007 68.1 18.9 0.11 Inventive Example 35 0.05 0.26 0.9 0.003 0.009 69.4 17.7 0.15 Inventive Example 36 0.03 0.27 1.1 0.001 0.005 71.1 16.4 0.16 Comparative Example 1 0.16 0.23 2.0 0.003 0.002 60.1 15.4 3.4 2.5 0.10 0.080 Comparative Example 2 0.14 0.23 1.6 0.003 0.003 85.4 9.6 1.2 1.1 0.1 0.07 0.031 Comparative Example 3 0.21 0.16 0.3 0.004 0.003 64.3 2.3 25.1 0.5 2.6 0.13 0.053 Comparative Example 4 0.04 0.16 0.3 0.004 0.003 64.3 0.30 5.4 0.5 2.6 0.13 0.053 Comparative Example 5 0.15 0.18 0.12 0.003 0.004 69.4 0.61 12.6 0.05 Weld metal B V Ca Mg Zr REM N O Value F Nb+W Lc (%) RRA (%) Evaluation Inventive Example 22 0.0003 0.070 0.003 7.443 0 18.4 88 A(©) Inventive Example 23 0.0004 0.060 0.007 10.649 0 17.4 94 A(©) Inventive Example 24 0.0009 0.030 0.009 12.231 0 24.1 78 A(©) Inventive Example 25 0.0008 0.040 0.003 13.750 0 24.0 81 A(©) Inventive Example 26 0.0016 0.090 0.004 13.152 0 28.5 69 B(o) Inventive Example 27 0.0060 0.020 0.020 14.081 0 31.5 70 B(o) Inventive Example 28 0.0005 0.080 0.030 10.659 0 26.0 70 B(o) Inventive Example 29 0.0900 0.090 0.005 19.928 0 28.1 90 A(©) Inventive Example 30 0.0030 3.0000 0.030 0.020 13.160 0 8.5 98 A(©) Inventive Example 31 0.0040 0.8000 0.004 0.007 9.583 0 17.6 95 A(©) Inventive Example 32 0.0080 0.9000 0.060 0.009 11.318 0 23.4 70 B(o) Inventive Example 33 0.0003 0.9500 0.003 0.006 10.639 0 34.1 65 B(o) Inventive Example 34 0.0005 0.4500 0.003 0.020 12.903 0 13.3 98 A(©) Inventive Example 35 0.0060 0.300 0.001 12.653 0 23.1 88 A(©) Inventive Example 36 0.0080 0.007 0.050 11.974 0 27.6 75 A(©) Comparative Example 1 0.0010 0.1000 0.019 0.030 7.232 2.5 34.2 47 C(x) Comparative Example 2 0.0010 0.016 0.024 3.688 1.1 8.4 36 C(x) Comparative Example 3 0.0021 0.001 0.008 19.160 3.1 46.9 42 C(x) Comparative Example 4 0.0021 0.001 0.008 5.304 3.1 3.1 42 C(x) Comparative Example 5 0.0003 0.012 0.015 8.739 0 25.6 40 C(x) 00 Weld metal C Si Mn P S Ni Cr Mo Nb W Ta Cu Co Pb Sn Al Ti Inventive Example 41 0.07 0.12 1.5 0.004 0.003 53.5 14.5 0.21 0.089 Inventive Example 42 0.11 0.27 0.8 0.003 0.001 61.4 17.5 0.1 0.6 0.48 Inventive Example 43 0.04 0.14 0.3 0.003 0.001 59.4 24.1 0.06 0.18 0.017 Inventive Example 44 0.03 0.37 0.4 0.003 0.002 72.0 14.4 0.1 0.13 Inventive Example 45 0.13 0.06 0.4 0.003 0.004 58.5 0.10 14.6 0.5 0.10 0.1 0.01 0.43 0.024 Inventive Example 46 0.12 0.18 1.6 0.003 0.004 66.4 0.20 24.5 1.6 0.10 0.050 Inventive Example 47 0.16 0.24 0.4 0.005 0.003 55.3 24.1 1.6 1.5 0.15 0.087 Inventive Example 48 0.06 0.25 1.1 0.002 0.001 60.7 0.40 22.6 2.3 0.2 0.2 0.10 0.07 Inventive Example 49 0.15 0.27 1.3 0.005 0.004 68.5 19.5 1.9 0.9 0.4 0.06 0.030 Inventive Example 50 0.03 0.34 1.2 0.004 0.003 63.9 12.3 2.1 0.10 0.075 Inventive Example 51 0.04 0.36 0.6 0.002 0.004 67.1 18.7 2.7 1.1 0.29 0.055 Inventive Example 52 0.07 0.16 0.9 0.002 0.004 72.2 13.1 0.1 1.3 0.1 0.06 0.01 0.02 Inventive Example 53 0.03 0.16 0.4 0.004 0.001 52.8 0.40 18.2 0.3 1.9 0.25 0.017 Inventive Example 54 0.05 0.24 0.4 0.005 0.003 75.6 14.3 1.5 0.15 0.087 Inventive Example 55 0.06 0.15 0.005 0.004 70.1 16.8 0.11 Inventive Example 56 0.20 0.11 1.6 0.001 0.006 76.1 21.7 0.03 Inventive Example 57 0.07 0.49 2.8 0.009 0.007 55.6 20.9 0.03 Inventive Example 58 0.11 0.11 2.9 0.009 0.001 71.6 18.1 0.51 Inventive Example 59 0.03 0.44 0.9 0.004 0.006 72.4 24.6 0.07 Inventive Example 60 0.10 0.44 0.9 0.008 0.007 76.1 15.4 5.0 0.31 Inventive Example 61 0.04 0.39 2.0 0.006 0.007 60.8 0.04 9.6 4.9 0.17 Weld metal B V Ca Mg Zr REM N 0 Value F Nb+W Lc (%) RRA (%) Evaluation Inventive Example 41 0.0023 0.1000 0.015 0.020 10.096 0 24.1 74 B(o) Inventive Example 42 0.0005 0.1000 0.1000 0.006 0.010 12.254 0.7 16.3 86 A(©) Inventive Example 43 0.0013 0.5000 0.002 0.002 16.522 0 34.6 71 B(o) Inventive Example 44 0.0038 0.1000 0.001 0.004 10.183 0 9.6 72 B(o) Inventive Example 45 0.0009 0.003 0.005 10.299 0.5 12.8 89 A(©) Inventive Example 46 0.0010 0.1000 0.003 0.003 17.481 1.6 32.1 87 A(©) Inventive Example 47 0.0006 1.2000 0.001 0.004 17.896 3.1 33.5 86 A(©) Inventive Example 48 0.0010 0.010 0.010 16.820 2.5 31.4 86 A(©) Inventive Example 49 0.0006 0.020 0.020 14.669 2.8 30.1 80 A(©) Inventive Example 50 0.0005 0.1000 0.020 0.030 9.486 2.1 14.9 81 A(©) Inventive Example 51 0.0080 0.001 0.004 14.672 2.7 19.6 86 A(©) Inventive Example 52 0.0009 0.2000 0.1000 0.007 0.010 9.598 1.4 14.4 89 A(©) Inventive Example 53 0.0010 0.003 0.004 13.522 2.2 12.4 94 A(©) Inventive Example 54 0.0013 0.1000 0.1000 0.001 0.004 10.488 1.5 10.9 92 A(©) Inventive Example 55 0.0009 0.006 0.015 11.517 0 13.3 95 A(©) Inventive Example 56 0.0009 0.100 0.026 14.849 0 5.4 96 A(©) Inventive Example 57 0.0015 0.020 0.024 14.366 0 9.3 96 A(©) Inventive Example 58 0.0017 0.050 0.044 12.483 0 17.3 90 A(©) Inventive Example 59 0.0031 0.020 0.021 17.047 0 27.8 84 A(©) Inventive Example 60 0.0024 0.006 0.044 13.269 5 20.5 88 A(©) Inventive Example 61 0.0016 0.060 0.038 8.760 4.9 15.6 81 A(©) Weld metal C Si Mn P S Ni Cr Mo Nb W Ta Cu Co Pb Sn Al Ti Inventive Example 62 0.16 0.09 0.4 0.006 0.006 65.1 18.9 3.0 0.50 Inventive Example 63 0.10 0.21 2.6 0.006 0.006 69.1 20.6 0.90 0.26 Inventive Example 64 0.05 0.25 2.6 0.009 0.004 63.3 18.6 0.88 0.38 Inventive Example 65 0.09 0.19 2.7 0.006 0.009 59.9 20.7 0.94 0.40 Inventive Example 66 0.18 0.38 1.2 0.001 0.006 76.1 16.0 0.90 0.44 Inventive Example 67 0.07 0.50 0.3 0.002 0.004 71.3 0.36 16.7 0.99 Inventive Example 68 0.03 0.34 2.5 0.002 0.005 70.0 13.1 0.41 0.950 Inventive Example 69 0.11 0.19 2.1 0.002 0.004 68.4 14.1 0.27 Inventive Example 70 0.19 0.43 1.7 0.004 0.003 66.6 15.1 0.41 Inventive Example 71 0.14 0.33 1.2 0.008 0.008 73.5 12.9 0.57 Inventive Example 72 0.15 0.44 1.0 0.004 0.007 76.4 12.6 0.41 Inventive Example 73 0.03 0.34 0.5 0.007 0.001 70.3 14.9 0.16 Inventive Example 74 0.13 0.37 0.6 0.002 0.008 59.9 20.7 0.50 Inventive Example 75 0.02 0.43 1.5 0.002 0.005 65.9 0.40 19.0 0.56 Inventive Example 76 0.06 0.37 2.4 0.007 0.008 69.7 16.2 0.29 Comparative Example 41 0.16 0.23 2.0 0.003 0.002 61.7 15.4 3.3 2.5 0.10 0.080 Comparative Example 42 0.14 0.23 1.6 0.003 0.003 82.0 9.6 1.2 1.1 0.1 0.07 0.031 Comparative Example 43 0.21 0.16 0.3 0.004 0.003 61.7 2.3 24.6 0.5 2.5 0.13 0.053 Comparative Example 44 0.04 0.16 0.3 0.004 0.003 72.4 0.30 5.3 0.5 2.6 0.13 0.053 Comparative Example 45 0.20 0.34 2.6 0.004 0.006 70.6 0.59 10.3 0.5 2.6 0.05 Weld metal B V Ca Mg Zr REM N O Value F Nb+W Lc (%) RRA (%) Evaluation Inventive Example 62 0.0003 0.080 0.023 12.883 0 5.7 99 A(©) Inventive Example 63 0.0034 0.020 0.026 14.358 0 24.8 74 B(o) Inventive Example 64 0.0016 0.180 0.043 12.812 0 5.2 99 A(©) Inventive Example 65 0.0024 0.170 0.035 14.323 0 28.6 70 B(o) Inventive Example 66 0.0031 0.090 0.007 11.199 0 13.7 90 A(©) Inventive Example 67 0.0016 0.060 0.033 11.603 0 26.8 69 A(©) Inventive Example 68 0.0049 0.005 0.039 9.412 0 23.9 68 A(©) Inventive Example 69 0.0940 0.080 0.022 19.251 0 9.0 95 A(©) Inventive Example 70 0.0039 2.9000 0.180 0.031 10.669 0 6.4 95 A(©) Inventive Example 71 0.0049 0.9400 0.210 0.021 9.276 0 7.1 94 A(©) Inventive Example 72 0.0016 0.9000 0.040 0.028 8.732 0 21.1 74 B(o) Inventive Example 73 0.0036 0.9300 0.060 0.014 10.502 0 23.8 78 A(©) Inventive Example 74 0.0058 0.4800 0.060 0.038 14.672 0 11.1 94 A(©) Inventive Example 75 0.0016 0.300 0.025 13.176 0 24.4 76 A(©) Inventive Example 76 0.0011 0.040 0.490 11.129 0 26.8 70 B(o) Comparative Example 41 0.0010 0.1000 0.019 0.030 7.164 2.5 34.2 47 C(x) Comparative Example 42 0.0010 0.016 0.024 3.688 1.1 8.4 36 C(x) Comparative Example 43 0.0021 0.001 0.008 18.778 3.0 46.9 42 C(x) Comparative Example 44 0.0021 0.001 0.008 5.236 3.1 3.1 42 C(x) Comparative Example 45 0.0016 0.010 0.024 8.651 3.1 34.6 40 C(x)

[0105] In Comparative Example 1, the Cr amount in the weld metal was excessively large, and the value F was below 8.600. In Comparative Example 2, the Cr amount and the Ni amount in the weld metal were excessively large, and the value F was below 8.600. In Comparative Example 3, the C amount, the Cr amount, and the Mo amount in the weld metal were excessively large. In Comparative Example 4, the value F was below 8.600. In Comparative Example 5, the Cr amount in the weld metal was excessively large. In Comparative Example 41, the Cr amount in the weld metal was excessively large, and the value F was below 8.600. In Comparative Example 42, the Cr amount and the Ni amount in the weld metal were excessively large, and the value F was below 8.600. In Comparative Example 43, the C amount and the Cr amount in the weld metal were excessively large. In Comparative Example 44, the value F was below 8.600. In Comparative Example 45, the Cr amount in the weld metal was excessively large.

[0106] As shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4, it is seen that the weld metals of Inventive Examples, in which the chemical composition was in a prescribed range and the value F was 8.600 or more, exhibited superior hydrogen embrittlement resistance as compared to the weld metals of Comparative Examples in which the chemical composition was outside a prescribed range or the value F was less than 8.600.

[0107] The disclosure of Japanese Patent Application No. 2024-060439 is hereby incorporated by reference in its entirety. All the documents, patent applications and technical standards that are described in the present specification are hereby incorporated by reference to the same extent as if each individual document, patent application or technical standard is concretely and individually described to be incorporated by reference. DESCRIPTION OF SYMBOLS

[0108] 22, 24: Base metal 42, 44: Weld metal 62B, 64B: Collection position

Claims

Claim 1. A weld metal, having a chemical composition comprising, in terms of % by mass with respect to a total mass, at a center position of a cross section perpendicular to a weld line direction and at a t / 4 position in a plate thickness direction, wherein t represents a plate thickness:C: from 0.005% to 0.20%,Si: from 0.01% to 0.50%,Mn: from 0% to 3.0%,P: 0.010% or less,S: 0.010% or less,Ni: from 50.0% to 80.0%,Cr: from 0% to 0.44%,Mo: from 0.1% to 25.0%,Nb: from 0% to 5.0%,W: from 0% to 5.0%,Ta: from 0% to 3.0%,Cu: from 0% to 1.00%,Co: from 0% to 1.0%,Pb: from 0% to 1.00%,Sn: from 0% to 1.00%,Al: from 0.003% to 1.00%,Ti: from 0% to 1.000%,B: from 0.0001% to 0.1000%,V: from 0% to 3.0000%,Ca: from 0% to 1.0000%,Mg: from 0% to 1.0000%,Zr: from 0% to 1.0000%,REM: from 0% to 0.5000%,N: from 0% to 0.300%, andO: 0.050% or less,with a balance being Fe and impurities,wherein a value F represented by the following Equation 1 is 8.600 or more: Equation 1: F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B]wherein, in Equation 1, [Cr] represents a Cr content with respect to a total mass of the weld metal, [Mo] represents an Mo content with respect to the total mass of the weldmetal, [Nb] represents an Nb content with respect to the total mass of the weld metal, [W] represents a W content with respect to the total mass of the weld metal, and [B] represents a B content with respect to the total mass of the weld metal.Claim 2. A weld metal used in a liquid hydrogen pressure vessel, the weld metal having a chemical composition comprising, in terms of % by mass with respect to a total mass, at a center position of a cross section perpendicular to a weld line direction and at a t / 4 position in a plate thickness direction, wherein t represents a plate thickness:C: from 0.005% to 0.20%,Si: from 0.01% to 0.50%,Mn: from 0% to 3.0%,P: 0.010% or less,S: 0.010% or less,Ni: from 50.0% to 80.0%,Cr: from 0% to 0.44%,Mo: from 0.1% to 25.0%,Nb: from 0% to 5.0%,W: from 0% to 5.0%,Ta: from 0% to 3.0%,Cu: from 0% to 1.00%,Co: from 0% to 1.0%,Pb: from 0% to 1.00%,Sn: from 0% to 1.00%,Al: from 0% to 1.00%,Ti: from 0% to 1.000%,B: from 0.0001% to 0.1000%,V: from 0% to 3.0000%,Ca: from 0% to 1.0000%,Mg: from 0% to 1.0000%,Zr: from 0% to 1.0000%,REM: from 0% to 0.5000%,N: from 0% to 0.300%, andO: 0.050% or less,with a balance being Fe and impurities,wherein a value F represented by the following Equation 1 is 8.600 or more:Equation 1: F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B]wherein, in Equation 1, [Cr] represents a Cr content with respect to a total mass of the weld metal, [Mo] represents an Mo content with respect to the total mass of the weld metal, [Nb] represents an Nb content with respect to the total mass of the weld metal, [W] represents a W content with respect to the total mass of the weld metal, and [B] represents a B content with respect to the total mass of the weld metal.Claim 3. The weld metal according to claim 1 or 2, wherein a total content of Nb and W is 1.0% or more with respect to the total mass of the weld metal.Claim 4. The weld metal according to any one of claims 1 to 3, wherein, in the chemical composition, a content of at least one of Co or Ca is in the following range:Co: from 0.0001% to 1.0%,Ca: from 0.0001% to 1.0000%.Claim 5. The weld metal according to any one of claims 1 to 4, wherein a content ratio of Mo to Cr is higher than 1 / 1.Claim 6. The weld metal according to any one of claims 1 to 5, wherein an occupancy Lc of precipitates on y-grain boundaries is 30.0% or less.Claim 7. A weld joint, comprising:a base metal having a chemical composition that comprises, in terms of % by mass with respect to a total mass of the base metal:C: from 0.01% to 0.07%,Si: from 0.03% to 0.3%,Mn: from 0.10% to 0.80%,P: 0.008% or less,S: 0.004% or less,Ni: from 6.10% to 10.40%,Cr: from 0% to 1.00%,Mo: from 0% to 0.60%,Nb: from 0% to 0.050%,Cu: from 0% to 1.00%,Co: from 0% to 0.050%,Al: from 0.01% to 0.06%,N: from 0.0015% to 0.006%,O: 0.003% or less,V: from 0% to 0.080%,Ti: from 0% to 0.020%,B: from 0% to 0.0020%,Ca: from 0% to 0.0040%, andREM: from 0% to 0.0050%,with a 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.Claim 8. A liquid hydrogen pressure vessel, comprising the weld metal according to any one of claims 1 to 7.Claim 9. A liquid hydrogen pressure vessel, comprising the weld joint according to claim 8.

Images