Weld metal, welded joint and liquid hydrogen pressure vessel

Abstract

A weld metal, the chemical composition of which, by mass% relative to the total mass, comprises: C: 0.005~0.20%, Si: 0.01~0.50%, Mn: 0~3.0%, P: less than 0.010%, S: less than 0.010%, Ni: 50.0~80.0%, Mo: 0.1~25.0%, Al: 0.003%~1.00%, B: 0.0001%~0.1000%, with the remainder consisting of Fe and impurities; the value of F shown in F=0.23[Cr]+0.68[Mo]+0.51[Nb]+0.42[W]+102.8[B] 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

[Technical Field]

[0001] Field of Invention This disclosure relates to welded metal, welded joints, and liquid hydrogen pressure vessels. [Previous Technology]

[0002] Background of the Invention In recent years, due to the problem of global warming, restrictions on carbon dioxide emissions have been strengthened. Therefore, the demand for hydrogen fuel, which does not emit carbon dioxide compared to oil and coal, has continued to increase. Along with this, the demand for liquid hydrogen pressure vessels, such as liquid hydrogen tanks used on ships and land, has also continued to increase. As for the steel used in liquid hydrogen pressure vessels, since it is required to ensure toughness at extremely low temperatures (e.g., -196°C), Ni-based cryogenic steel containing 6% to 9% Ni has been used. Then, when these Ni-based cryogenic steels are welded, a Wostian iron-based welding material that can obtain welding metal with excellent low-temperature toughness is used to form a welding metal. This welding material is mainly designed with a Ni content of 70%.

[0003] For example, Japanese Patent Application Publication No. 6-277875 discloses a Hastelloy-based TIG welding wire for ultra-low temperature steel, which contains Ni, Mo, W, Cu, and Ti in specific amounts, and controls the active elements Ca, Mg, Al, and Si, as well as the gaseous components O and N, to be extremely low, and the OX equivalent calculated from the amounts of Ca, Mg, Al, Si, Ti, O, and N is limited to less than a specific value. [Summary of the Invention]

[0004] Summary of the Invention

[0005] Problem to be Solved by the Invention However, steels used in high-pressure hydrogen environments such as liquid hydrogen pressure vessels require excellent resistance to hydrogen embrittlement. It is known that Ni-based low-temperature steels generally use Ni-based welding materials for welding, but Ni-based alloys are sensitive to hydrogen embrittlement, which can cause grain boundary damage, resulting in a decrease in strength and toughness. Furthermore, the aforementioned Patent Document 1 does not disclose or teach the use of welded metals in high-pressure hydrogen environments such as liquid hydrogen pressure vessels, nor does it specifically mention improving resistance to hydrogen embrittlement.

[0006] From the above viewpoints, the subject matter of the fusion joint disclosed herein is: to provide a fusion metal that suppresses hydrogen embrittlement in high-pressure hydrogen, a fusion joint having the fusion metal, and a liquid hydrogen pressure vessel having the fusion metal or the fusion joint.

[0007] Means for solving problems The means for solving problems include the following forms. <1> A welded metal, the chemical composition of which, in the central position of a cross-section perpendicular to the weld line direction and at position t / 4 of the plate thickness direction (t represents the plate thickness), comprises, by mass% relative to the total mass: C: 0.005%~0.20%, Si: 0.01%~0.50%, Mn: 0%~3.0%, P: less than 0.010%, S: less than 0.010%, Ni: 50.0%~80.0%, Cr: 0%~0.44%, Mo: 0.1%~25.0%, Nb: 0%~5.0%, W: 0%~5.0%, Ta: 0%~3.0%, Cu: 0%~1.00%, Co: 0%~1.0%, Pb: 0%~1.00%, Sn: 0%~1.00%, Al: 0.003%~1.00%, Ti: 0%~1.000%. B: 0.0001%~0.1000%, V: 0%~3.0000%, Ca: 0%~1.0000%, Mg: 0%~1.0000%, Zr: 0%~1.0000%, REM: 0%~0.5000%, N: 0%~0.300%, and O: less than 0.050%, with the remainder consisting of Fe and impurities. The value F shown in Equation 1 below is 8.600 or higher; F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 (In Equation 1, [Cr] represents the percentage of Cr relative to the total mass of the weld metal, [Mo] represents the percentage of Mo relative to the total mass of the weld metal, [Nb] represents the percentage of Nb relative to the total mass of the weld metal, [W] represents the percentage of W relative to the total mass of the weld metal, and [B] represents the percentage of B relative to the total mass of the weld metal).<2> A weld metal used in liquid hydrogen pressure vessels, wherein the chemical composition of the weld metal at the central position in a cross-section perpendicular to the weld line direction and at position t / 4 relative to the plate thickness direction (t represents the plate thickness), in mass% relative to the total mass, comprises: C: 0.005%~0.20%, Si: 0.01%~0.50%, Mn: 0%~3.0%, P: less than 0.010%, S: less than 0.010%, Ni: 50.0%~80.0%, Cr: 0%~0.44%, Mo: 0.1%~25.0%, Nb: 0%~5.0%, W: 0%~5.0%, Ta: 0%~3.0%, Cu: 0%~1.00%, Co: 0%~1.0%, Pb: 0%~1.00%, Sn: 0%~1.00%, Al: 0%~1.00%. Ti: 0%~1.000%, B: 0.0001%~0.1000%, V: 0%~3.0000%, Ca: 0%~1.0000%, Mg: 0%~1.0000%, Zr: 0%~1.0000%, REM: 0%~0.5000%, N: 0%~0.300%, and O: less than 0.050%, with the remainder consisting of Fe and impurities. The value F shown in Equation 1 below is 8.600 or higher; F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 (In Formula 1, [Cr] represents the content of Cr relative to the total mass of the weld metal, [Mo] represents the content of Mo relative to the total mass of the weld metal, [Nb] represents the content of Nb relative to the total mass of the weld metal, [W] represents the content of W relative to the total mass of the weld metal, and [B] represents the content of B relative to the total mass of the weld metal.) <3> The weld metal as described in <1> or <2>, wherein the total content of the aforementioned Nb and the aforementioned W relative to the total mass of the weld metal is 1.0% or more. <4> The weld metal as described in any one of <1> to <3>, wherein in the aforementioned chemical composition, the content of at least one of the aforementioned Co and the aforementioned Ca is within the following range: Co: 0.0001%~1.0%, Ca: 0.0001%~1.0000%. <5> The weld metal as described in any one of <1> to <4>, wherein the content ratio of the aforementioned Mo relative to the aforementioned Cr is greater than 1 / 1. <6> The welded metal described in any of <1> to <5>, wherein the occency of the precipitate on the γ grain boundary is less than 30.0%.<7> A fusion joint comprising: a base material and a fusion metal; the chemical composition of the base material, in terms of mass% relative to the total mass of the base material, includes: C: 0.01%~0.07%, Si: 0.03%~0.3%, Mn: 0.10%~0.80%, P: less than 0.008%, S: less than 0.004%, Ni: 6.10%~10.40%, Cr: 0%~1.00%, Mo: 0%~0.60%, Nb: 0%~0.050%, Cu: 0%~1.00%, Co: 0%~0.050%, Al: 0.01%~0.06%, N: 0.0015%~0.006%, O: less than 0.003%, V: 0%~0.080%, Ti: 0%~0.020%, B: 0%~0.0020%. Ca: 0%~0.0040%, and REM: 0%~0.0050%, with the remainder consisting of Fe and impurities; the aforementioned 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>.

[0008] Effects of the invention According to the present disclosure, a weld metal that suppresses hydrogen embrittlement in high-pressure hydrogen gas, a weld joint having the weld metal, and a liquid hydrogen pressure vessel having the weld metal or the weld joint can be provided.

Implementation Method

[0010] The embodiments of the present invention will be described with reference to an embodiment of the present disclosure. Furthermore, in this specification, regarding the numerical range indicated by "~", if the values ​​before and after "~" are not accompanied by "greater than" or "less than", the numerical range means a range that includes such values ​​as a lower limit and an upper limit. Conversely, if the values ​​before and after "~" are accompanied by "greater than" or "less than", the numerical range means a range that does not include such values ​​as a lower limit or an upper limit. In this specification, in the numerical ranges described in stages, the upper limit of a certain stage's numerical range can be replaced by the upper limit of other stage's numerical ranges, or it can be replaced by the value shown in the embodiment. Additionally, the lower limit of a certain stage's numerical range can be replaced by the lower limit of other stage's numerical ranges, or it can be replaced by the value shown in the embodiment. Furthermore, regarding content, "%" means "mass %". As a percentage, "0~" means that the ingredient is any ingredient and may not be present.

[0011] <Welded Metal> The welded metal of this embodiment, in the central position of a cross-section perpendicular to the weld line direction and at a position t / 4 of the plate thickness direction (t represents the thickness of the steel plate used as the base material; that is, the t / 4 position refers to the position at 1 / 4 of the plate thickness direction from the surface of the base material), comprises, in mass% relative to the total mass: C: 0.005%~0.20%, Si: 0.01%~0.50%, Mn: 0%~3.0%, P: 0.010% or less, S: 0.010% or less, Ni: 50.0%~80.0%, Cr: 0%~0.44%, Mo: 0.1%~25.0%, Nb: 0%~5.0%, W: 0%~5.0%, Ta: 0%~3.0%, Cu: 0%~1.00%, Co: 0%~1.0%. Pb: 0%~1.00%, Sn: 0%~1.00%, Al: 0.003%~1.00%, Ti: 0%~1.000%, B: 0.0001%~0.1000%, V: 0%~3.0000%, Ca: 0%~1.0000%, Mg: 0%~1.0000%, Zr: 0%~1.0000%, REM: 0%~0.5000%, N: 0%~0.300%, and O: less than 0.050%, with the remainder consisting of Fe and impurities. Furthermore, the value F shown in Equation 1 below is 8.600 or higher. F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 (In Equation 1, [Cr] represents the percentage of Cr relative to the total mass of the weld metal, [Mo] represents the percentage of Mo relative to the total mass of the weld metal, [Nb] represents the percentage of Nb relative to the total mass of the weld metal, [W] represents the percentage of W relative to the total mass of the weld metal, and [B] represents the percentage of B relative to the total mass of the weld metal). Furthermore, the weld metal of this embodiment can be used, for example, in high-pressure hydrogen in a liquid hydrogen pressure vessel (e.g., a liquid hydrogen tank).

[0012] The weld metal of the present disclosure, through the above-described structure, can suppress hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in strength and toughness) even when used in high-pressure hydrogen gas such as in liquid hydrogen pressure vessels. The weld metal disclosed herein was discovered based on the following insights.

[0013] It is known that steels used in high-pressure hydrogen environments, such as liquid hydrogen pressure vessels, require excellent resistance to hydrogen embrittlement. It is known that Ni-based welding materials are generally used for welding Ni-based low-temperature steels; however, Ni-based alloys are sensitive to hydrogen embrittlement, which can lead to grain boundary damage, reducing strength and toughness. Therefore, the inventors investigated the relationship between the chemical composition of the weld metal and hydrogen embrittlement. The results yielded the following insights, and the weld joint of the embodiment disclosed herein was discovered.

[0014] When hydrogen intrudes into the weld metal, it mainly accumulates near the γ grain boundaries. It is believed that the accumulated hydrogen leads to γ ​​grain boundary embrittlement. Therefore, the inventors of this invention believe that increasing the content of grain boundary strengthening elements in the weld metal to enhance the γ grain boundary strength can counteract the γ grain boundary embrittlement caused by hydrogen, thereby suppressing hydrogen embrittlement of the weld metal (e.g., hydrogen embrittlement leads to reduced strength and toughness).

[0015] Based on this point, in the weld metal of the disclosed embodiment, the content of Cr, Mo, Nb, W, and B, which are grain boundary strengthening elements, is set to F, as shown in Formula 1, to be 8.600 or higher. F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Formula 1 Furthermore, in Formula 1, the correlation coefficient values ​​of the contents of Cr, Mo, Nb, W, and B represent the degree of influence of each element on grain boundary strengthening.

[0016] Based on the above insights, the welded metal disclosed herein can suppress hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in strength and toughness) even when used in high-pressure hydrogen gas such as liquid hydrogen pressure vessels.

[0017] The following describes in detail the welded metal of the embodiments disclosed herein.

[0018] (Chemical Composition of Welded Metal) The chemical composition of the welded metal will be described in detail below. Furthermore, the chemical composition of the welded metal is determined by taking a sample from the welded metal at the central position of a cross-section perpendicular to the weld line direction and at position t / 4 of the thickness direction (t represents the thickness). Unless otherwise specified, "%" in the description of the chemical composition of the welded metal refers to "mass percentage relative to the total mass of the welded metal".

[0019] The chemical composition of the welded metal contains: C: 0.005%~0.20%, Si: 0.01%~0.50%, Mn: 0%~3.0%, P: less than 0.010%, S: less than 0.010%, Ni: 50.0%~80.0%, Cr: 0%~0.44%, Mo: 0.1%~25.0%, Nb: 0%~5.0%, W: 0%~5.0%, Ta: 0%~3.0%, Cu: 0%~1.00%, Co: 0%~1.0%, Pb: 0%~1.00%, Sn: 0%~1.00%, Al: 0.003%~1.00%, Ti: 0%~1.000%, B: 0.0001%~0.1000%, V: 0%~3.0000%, Ca: 0%~1.0000%. Mg: 0%~1.0000%, Zr: 0%~1.0000%, REM: 0%~0.5000%, N: 0%~0.300%, and O: less than 0.050%, with the remainder consisting of Fe and impurities. The value F shown in Equation 1 below is 8.600 or higher. F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 (In Equation 1, [Cr] represents the content of Cr relative to the total mass of the weld metal, [Mo] represents the content of Mo relative to the total mass of the weld metal, [Nb] represents the content of Nb relative to the total mass of the weld metal, [W] represents the content of W relative to the total mass of the weld metal, and [B] represents the content of B relative to the total mass of the weld metal).

[0020] (C: 0.005%~0.20%) C is an element that enhances the strength of welded metal and is also used to ensure the strength of the welded metal. On the other hand, when the C content of the welded metal is excessive, carbides will precipitate in the welded metal, which has a significant impact on toughness deterioration, and the low-temperature toughness of the welded metal will decrease. It will also promote high-temperature cracking during welding. Therefore, the C content of the welded metal is set at 0.005%~0.20%. The lower limit of the C content of the welded metal should preferably be 0.01%, 0.02%, or 0.03%. The upper limit of the C content of the welded metal should preferably be 0.18%, 0.16%, 0.14%, 0.12%, 0.10%, or 0.07%.

[0021] (Si: 0.01%~0.50%) Si is a deoxidizing element. If the Si content in the weld metal is too low, the O content in the weld metal will increase. On the other hand, Si has low solid solubility in the ferroic phase; the higher the Si content, the more likely solidification segregation will occur, leading to high-temperature cracking. Therefore, the Si content in the weld metal is set at 0.01%~0.50%. The lower limit of the Si content in the weld metal should preferably be 0.02%, 0.03%, or 0.05%. The upper limit of the Si content in the weld metal should preferably be 0.45%, 0.40%, 0.35%, or 0.30%.

[0022] (Mn: 0%~3.0%) Mn is a stabilizing element for ferroalloys and can also be included in the weld metal to improve low-temperature toughness. On the other hand, when the Mn content in the weld metal is excessive, solidification segregation will occur in the weld metal, leading to high-temperature cracks. Therefore, the Mn content of the weld metal is set at 0%~3.0%. The lower limit of the Mn content in the weld metal should preferably be 0.01%, 0.05%, 0.08%, or 0.1%. The upper limit of the Mn content in the weld metal should preferably be 2.8%, 2.5%, 2.3%, or 2.0%.

[0023] (P: 0.010% or less) P is an impurity element. Since it promotes high-temperature cracking or reduces toughness, the P content in the weld metal should be minimized as much as possible. Therefore, the lower limit of the P content in the weld metal can also be set to 0%. However, from the viewpoint of reducing the cost of P removal, the P content in the weld metal can also be set to 0.001% or more. On the other hand, if the P content in the weld metal is 0.010% or less, the adverse effects of P will fall within an acceptable range. Therefore, the P content in the weld metal is set to 0.010% or less. To effectively suppress high-temperature cracking or reduction in toughness, the P content in the weld metal should preferably be 0.008% or less, 0.005% or less, 0.004% or less, or 0.003% or less.

[0024] (S: 0.010% or less) S is an impure element. Since it promotes high-temperature cracking or reduces toughness, the S content in the weld metal should be minimized as much as possible. Therefore, the lower limit of the S content in the weld metal can also be set to 0%. However, from the viewpoint of reducing desulfurization costs, the S content in the weld metal should be 0.001% or more. On the other hand, if the S content in the weld metal is 0.010% or less, the adverse effects of S will fall within an acceptable range. Therefore, the S content in the weld metal is set to 0.010% or less. To effectively suppress high-temperature cracking or reduction in toughness, the S content in the weld metal should preferably be 0.008% or less, 0.005% or less, 0.004% or less, or 0.003% or less.

[0025] (Ni: 50.0%~80.0%) Ni is a ferroplatin stabilizing element and also an element that improves toughness. If the Ni content in the weld metal is too low, ferroplatinization becomes difficult, and low-temperature toughness deteriorates. On the other hand, increasing the Ni content in the weld metal increases its cost. Therefore, the Ni content in the weld metal is set at 50.0%~80.0%. The lower limit of the Ni content in the weld metal should preferably be 52.0%, 55.0%, 58.0%, or 60.0%. The upper limit of the Ni content in the weld metal should preferably be 78.0%, 76.0%, or 75.0%.

[0026] (Cr: 0%~0.44%) Cr is a stabilizer element for ferroalloys and can also be included in the weld metal to improve its low-temperature toughness. Furthermore, based on the viewpoint of suppressing hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in the strength and toughness of the weld metal), Cr, as a grain boundary strengthening element, should preferably be included in the weld metal. On the other hand, when the Cr content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal will widen, thus making it easier to cause high-temperature cracks. Therefore, the Cr content of the weld metal is set at 0%~0.44%. The lower limit of the Cr content in the weld metal should preferably be 0.02%, 0.05%, or 0.10%. The upper limit of the Cr content in the weld metal should preferably be 0.40%, 0.35%, or 0.30%.

[0027] (Mo: 0.1%~25.0%) Mo is a solid solution strengthening element or precipitation strengthening element, which is included in the weld metal to improve its strength. Furthermore, based on the viewpoint of suppressing hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in the strength and toughness of the weld metal), Mo, as a grain boundary strengthening element, is included in the weld metal. On the other hand, when the Mo content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal becomes wider, thus making it easier to cause high-temperature cracks. Therefore, the Mo content in the weld metal is set to 0.1%~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%.

[0028] (Nb: 0%~5.0%) Nb is an element that forms carbides in weld metal, thereby increasing the strength of the weld metal, and therefore can also be contained in the weld metal. Furthermore, based on the viewpoint of suppressing hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in the strength and toughness of the weld metal), Nb, as a grain boundary strengthening element, should preferably be contained in the weld metal. On the other hand, when the Nb content in the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal will widen, thus making it easier to cause high-temperature cracks. Therefore, the Nb content of the weld metal is set to 0%~5.0%. The lower limit of the Nb content in the weld metal should preferably be 0.1%, 0.2%, 0.3%, or 0.5%. The upper limit of the Nb content in the weld metal should preferably be 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, 2.0%, 1.0%, or 0.5%.

[0029] (W: 0%~5.0%) W is a solid solution strengthening element and can also be included in the weld metal to improve strength. Furthermore, based on the viewpoint of suppressing hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in the strength and toughness of the weld metal), W, as a grain boundary strengthening element, should preferably be included in the weld metal. On the other hand, if the W content in the weld metal is excessive, the strength of the weld metal will become excessive, which may lead to a decrease in toughness. Therefore, the W content of the weld metal is set at 0%~5.0%. The lower limit of the W content of the weld metal should preferably be 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. The upper limit of the W content of the weld metal should preferably be 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, or 2.0%.

[0030] (Ta: 0%~3.0%) Ta is a solid solution strengthening element and can also be included in weld metal to improve strength. On the other hand, if the Ta content in the weld metal is excessive, the strength of the weld metal will become excessive, which may lead to a decrease in toughness. Therefore, the Ta content of the weld metal is set at 0%~3.0%. The lower limit of the Ta content in the weld metal should preferably be 0.1%, 0.2%, 0.5%, 0.8%, or 1.0%. The upper limit of the Ta content in the weld metal should preferably be 2.5%, 2.0%, or 1.5%.

[0031] (Cu: 0%~1.00%) Cu is a precipitation strengthening element and can also be included in the weld metal to improve its strength. Furthermore, Cu is a ferrophosphate stabilizing element and can also be included in the weld metal to improve its low-temperature toughness. On the other hand, the above effects will saturate if the Cu content of the weld metal is excessive. Therefore, the Cu content of the weld metal is set to 0%~1.00%. The lower limit of the Cu content of the weld metal should preferably be 0.02%, 0.05%, or 0.08%. The upper limit of the Cu content of the weld metal should preferably be 0.90%, 0.80%, or 0.50%.

[0032] (Co: 0%~1.0%) Co is an element that can improve the strength of welded metal through solid solution strengthening, and therefore can also be contained in welded metal. On the other hand, when the Co content of the welded metal is excessive, the ductility of the welded metal will decrease, and toughness cannot be guaranteed. Therefore, the Co content of the welded metal is set at 0%~1.0%. The lower limit of the Co content of the welded metal should preferably be 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 welded metal should preferably be 0.9%, 0.8%, or 0.5%.

[0033] (Pb: 0%~1.00%) Pb has the following effects: it improves the weld toe formability between the base material steel and the weld metal, and improves the machinability of the weld metal; therefore, it can also be contained in the weld metal. On the other hand, excessive Pb content in the weld metal can cause high-temperature cracking. Therefore, the Pb content of the weld metal is set to 0%~1.00%. The lower limit of the Pb content in the weld metal is preferably 0.005%, 0.01%, or 0.1%. The upper limit of the Pb content in the weld metal is preferably 0.9%, 0.8%, or 0.5%.

[0034] (Sn: 0%~1.00%) Sn is an element that improves the corrosion resistance of welded metal, and therefore can be included in welded metal. On the other hand, if the Sn content in the welded metal is excessive, high-temperature cracking may occur in the welded metal. Therefore, the Sn content of the welded metal is set at 0%~1.00%. The lower limit of the Sn content of the welded metal should preferably be 0.005%, 0.01%, or 0.1%. The upper limit of the Sn content of the welded metal should preferably be 0.9%, 0.8%, or 0.5%.

[0035] (Al: 0.003%~1.00%) Al is a deoxidizing element, included in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, when the Al content of the weld metal is excessive, Al will 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 at 0.003%~1.00%. The lower limit of the Al content of the weld metal should preferably be 0.01%, 0.02%, 0.05%, or 0.10%. The upper limit of the Al content of the weld metal should preferably be 0.90%, 0.80%, or 0.50%.

[0036] (Ti: 0%~1.000%) Ti is a deoxidizing element and can also be included in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, when the Ti content of the weld metal is excessive, carbides and intermetallic compounds will be generated in the weld metal, which may deteriorate the toughness of the weld metal. Therefore, the Ti content of the weld metal is set at 0%~1.000%. The lower limit of the Ti content of the weld metal should preferably be 0.003%, 0.010%, or 0.020%. The upper limit of the Ti content of the weld metal should preferably be 0.900%, 0.700%, 0.500%, 0.300%, 0.250%, 0.200%, 0.150%, or 0.100%.

[0037] (B: 0.0001%~0.1000%) B is a stabilizer for ferrosilicon and an interstitial solid solution strengthening element. It is included in the weld metal to improve its low-temperature toughness and strength. Furthermore, based on the viewpoint of suppressing hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in the strength and toughness of the weld metal), B, as a grain boundary strengthening element, is included in the weld metal. On the other hand, when the B content in the weld metal is excessive, M23(C,B)6 will precipitate, which will be a cause of toughness deterioration. It will also expand the solid-liquid coexistence temperature range, thus promoting high-temperature cracking. Therefore, the B content of the weld metal is set to 0.0001%~0.1000%. The lower limit of the B content in the weld metal should preferably be 0.0003%, 0.0005%, 0.0010%, or 0.0020%. The upper limit of the B content in the welded metal should be 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0038] (V: 0%~3.0000%) V is an element that forms carbonitrides in welded metal, thereby increasing the strength of the welded metal, and therefore can also be contained in the welded metal. On the other hand, when the V content of the welded metal is excessive, the carbonitrides will coarsen and the toughness will deteriorate. It will also expand the solid-liquid coexistence temperature range, which may lead to high-temperature cracking of the welded metal. Therefore, the V content of the welded metal is set to 0%~3.0000%. The lower limit of the V content of the welded metal should preferably be 0.0100%, 0.0500%, 0.1000%, 0.1500%, or 0.2000%. The upper limit of the V content of the welded metal should preferably be 2.5000%, 2.0000%, or 1.5000%.

[0039] (Ca: 0%~1.0000%) Ca has the following functions: it alters the structure of sulfides in the weld metal, or makes the size of sulfides and oxides in the weld metal finer, thus effectively improving the ductility and toughness of the weld metal. Therefore, Ca can also be contained in the weld metal. On the other hand, when the Ca content of the weld metal is excessive, sulfides and oxides will coarsen, 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 at 0%~1.0000%. The lower limit of the Ca content of the weld metal should preferably be 0.0001%, 0.0003%, 0.0005%, 0.0010%, or 0.0020%. The upper limit of the Ca content of the weld metal should preferably be 0.8000%, 0.5000%, or 0.3000%.

[0040] (Mg: 0%~1.0000%) Mg is a deoxidizing element that reduces the oxygen content in the weld metal, thereby improving its cleanliness. Therefore, Mg can also be contained in the weld metal. On the other hand, when the Mg content in the weld metal is excessive, Mg reacts violently with oxygen in the electric arc, sometimes increasing the amount of spatter and fume. Therefore, the Mg content of the weld metal is set at 0%~1.0000%. The lower limit of the Mg content in the weld metal should preferably be 0.0100%, 0.0200%, or 0.0300%. The upper limit of the Mg content in the weld metal should preferably be 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0041] (Zr: 0%~1.0000%) Zr is a deoxidizing element that reduces the oxygen content in the welded metal, thereby improving its cleanliness. Therefore, Zr can also be included in the welded metal. On the other hand, when the Zr content in the welded metal is excessive, Zr reacts violently with oxygen in the electric arc, sometimes increasing the amount of slag splashing and fumes. Therefore, the Zr content in the welded metal is set at 0%~1.0000%. The lower limit of the Zr content in the welded metal should preferably be 0.0100%, 0.0200%, or 0.0300%. The upper limit of the Zr content in the welded metal should preferably be 0.8000%, 0.7000%, 0.6000%, or 0.5000%.

[0042] (REM: 0%~0.5000%) REM is an element that stabilizes the electric arc during welding operations to obtain weld metal, and therefore can also be contained in the weld metal. On the other hand, if the REM content of the weld metal is excessive, slag spatter will become more intense during welding operations to obtain the weld metal, and the weldability may deteriorate. Therefore, the REM content of the weld metal is set to 0%~0.5000%. The lower limit of the REM content of the weld metal should preferably be 0.0010%, 0.0020%, or 0.0050%. The upper limit of the REM content of the weld metal should preferably be 0.4000%, 0.2000%, or 0.1000%.

[0043] Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanide elements such as La and Nd, and the REM content refers to the total content of these elements. REM can be added to molten steel, for example, using an Fe-Si-REM alloy, which contains, for example, La, Nd, Ce, and Pr.

[0044] (N: 0%~0.300%) N is a stabilizer for ferrosilicon and an interstitial solid solution strengthening element. It can also be included in the weld metal to improve its low-temperature toughness and strength. On the other hand, excessive N content in the weld metal will increase the formation of blow, which can be a cause of weld defects. Therefore, the N content of the weld metal is set at 0%~0.300%. The lower limit of the N content of the weld metal should preferably be 0.001%, 0.005%, 0.010%, or 0.020%. The upper limit of the N content of the weld metal should preferably be 0.250%, 0.200%, 0.150%, or 0.100%.

[0045] (O: 0.050% or less) O ​​is an element that may be contained in the weld metal as impurities. Excessive O content leads to a deterioration in toughness and ductility; therefore, the upper limit of O content in the weld metal is set at 0.150% or less. On the other hand, drastically reducing the O content increases manufacturing costs; therefore, the lower limit of O content in the weld metal can also be set at 0.001% or less. The lower limit of O content in the weld metal should preferably be 0.002% or 0.003%. The upper limit of O content in the weld metal should preferably be 0.040% or 0.030%.

[0046] (Remaining components: Fe and impurities) The remaining components in the chemical composition of the welded metal are Fe and impurities. Impurities refer to components introduced during the industrial manufacturing of the welded metal due to raw materials such as ores or waste, or due to various factors in the manufacturing process, and are permissible within a range that does not adversely affect the properties of the welded metal. Regarding the content of each element in the above chemical composition, since the reasons for setting the upper and lower limits differ, suitable values ​​for the upper and lower limits can be selected independently.

[0047] (Value F) Regarding the content of Cr, Mo, Nb, W, and B, which are grain boundary strengthening elements, in the weld metal, the value F shown in Formula 1 is 8.600 or higher. F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Formula 1 (In Formula 1, [Cr] represents the content of Cr relative to the total mass of the weld metal, [Mo] represents the content of Mo relative to the total mass of the weld metal, [Nb] represents the content of Nb relative to the total mass of the weld metal, [W] represents the content of W relative to the total mass of the weld metal, and [B] represents the content of B relative to the total mass of the weld metal).

[0048] Furthermore, in Formula 1, the correlation coefficients of the contents of Cr, Mo, Nb, W, and B represent the degree of influence of each element on grain boundary strengthening. By having a value of F of 8.600 or higher, the welded metal can suppress hydrogen embrittlement (e.g., hydrogen embrittlement leads to a decrease in strength and toughness) even when used in high-pressure hydrogen gas such as in liquid hydrogen pressure vessels. From the viewpoint of suppressing hydrogen embrittlement (e.g., the decrease in strength and toughness caused by hydrogen embrittlement), the value of F should preferably be 9.000 or higher, 9.500 or higher, 10.000 or higher, or 10.500 or higher.

[0049] (Total content of Nb and W) The total content of Nb and W in the weld metal relative to the total mass of the weld metal should preferably be 1.0% or more. Nb and W contribute to the strengthening of the weld metal, and by having the total content of Nb and W within the above-mentioned range, the tensile strength TS of the weld metal can be increased. Furthermore, from the viewpoint of increasing the tensile strength TS of the weld metal, the total content of Nb and W should preferably be 1.5% or more, and more preferably 2.0% or more.

[0050] (Mo to Cr content ratio) The Mo to Cr content ratio of the weld metal should preferably be greater than 1 / 1 (that is, the Mo content is more than the Cr content). Mo has a higher effect on inhibiting hydrogen embrittlement than Cr. Therefore, by having the Mo to Cr content ratio within the above range, hydrogen embrittlement of the weld metal can be inhibited. The Mo to Cr content ratio should preferably be 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 welded metal should preferably be set to, for example, 590~1200 MPa. Alternatively, the tensile strength can be determined by tensile testing of the welded metal according to JIS Z3111:2005.

[0052] (Occupancy rate Lc of grain boundary strengthening element precipitates on γ grain boundaries) Precipitates present on γ grain boundaries will have element-deficient regions around them. For example, when Cr carbide exists as a precipitate on a γ grain boundary, a local deficiency region with less Cr and C will appear around the carbide. Then, the appearance of the deficiency region of Cr, as a grain boundary strengthening element, will reduce the grain boundary strength around the presence of the precipitate. Therefore, in the weld metal of the disclosed embodiment, the occency rate Lc of the precipitate on the γ grain boundary should preferably be: Lc ≤ 30.0%. It can also be considered that the precipitate here includes at least one element selected from the group consisting of Cr, Mo, Nb, W and B, which are grain boundary strengthening elements (the precipitate is hereinafter referred to as "grain boundary strengthening element precipitate"). The proportion of grain boundary strengthening element precipitates on the γ grain boundary (Lc) is less than 30.0%, thereby improving the strength of the γ grain boundary and suppressing hydrogen embrittlement in the welded metal (e.g., hydrogen embrittlement leads to a decrease in strength and toughness).

[0053] Furthermore, from the viewpoint of suppressing hydrogen embrittlement in welded metal (e.g., hydrogen embrittlement leads to a decrease in strength and toughness), the proportion of grain boundary strengthening element precipitates on γ grain boundaries should preferably be 25.0% or less, more preferably 20.0% or less, and even more preferably 16.0% or less. Also, the lower limit of the above proportion is not particularly limited; however, based on the viewpoint that it is not easy to completely suppress precipitate precipitation, it can also be set to 5.0% or more.

[0054] The occupancy of grain boundary strengthening element precipitates on γ grain boundaries is determined by the following method. The weld metal is observed using a scanning electron microscope (SEM) with backscattered electron imaging at its central position in a cross-section perpendicular to the weld line direction and at position t / 4 of the plate thickness (t represents the plate thickness). Furthermore, by observing the backscattered electron imaging, the precipitates can be observed through contrast with the parent phase. Five γ grains are randomly selected from the field of view as the measurement targets. The extent of the precipitate's presence on the γ grain boundaries of these targets is measured, and the precipitate occupancy length within the grain boundary length of the five measured γ grains is determined. For example, if the total grain boundary length is 1000 μm, and a total of 50 μm of precipitate exists, then the occupancy rate Lc will be "50 μm / 1000 μm × 100 = 5%". The measurement was performed at five cross-sections (the center of the cross-section in which the weld metal is perpendicular to the direction of the weld line and at position t / 4 relative to the thickness direction (t represents the thickness)), and the arithmetic mean of these cross-sections was defined as the occupancy rate Lc.

[0055] Furthermore, whether the precipitates contain grain boundary strengthening elements, i.e., whether they contain at least one element selected from the group consisting of Cr, Mo, Nb, W, and B, can be confirmed by the following method: Observation of the welded metal using backscattered electron microscopy (SEM), and point analysis of precipitates on grain boundaries that are different from the parent phase using energy-dispersive X-ray spectroscopy (EDS). Confirmation can be made by checking whether the obtained analytical values ​​contain grain boundary strengthening elements. Electron probe microanalysis (EPMA) can also be used during analysis.

[0056] Regarding methods for reducing the occency (Lc) of grain boundary strengthening element precipitates on γ grain boundaries and controlling it within the aforementioned range, the following four methods can be cited as examples. (1) Method of reducing occency (Lc) by reducing C content By reducing the C content in the weld metal, the amount of carbide precipitation of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) can be reduced, and the occency (Lc) of grain boundary strengthening element precipitates on γ grain boundaries can be reduced. According to this view, the upper limit of C content in the weld metal should be 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 grain boundary strengthening element precipitates on the γ grain boundary can be reduced. According to this view, the content of grain boundary strengthening elements should be set as follows: The upper limit of Cr content in the weld metal should be 0.40%, 0.35%, or 0.30%. The upper limit of Mo content in the weld metal should be 20.0%, 18.0%, or 15.0%. The upper limit of Nb content in the weld metal should be 2.5%, 2.0%, 1.0%, or 0.5%. The upper limit of W content in the weld metal should be 3.0%, 2.5%, or 2.0%. The upper limit of the B content in the welded metal should be 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0058] (3) By reducing the C content and the content of grain boundary strengthening elements to reduce the occupancy rate Lc, the C content in the weld metal is reduced to a range that is not excessive, and the content of grain boundary strengthening elements (i.e., Cr, Mo, Nb, W, and B) is also reduced to a range that is not excessive. This reduces the amount of carbides of grain boundary strengthening elements and reduces the occupancy rate Lc of grain boundary strengthening element precipitates on the γ grain boundary. According to this view, the C and grain boundary strengthening element contents should be set as follows: The upper limit of the C content in the weld metal should be 0.18%, 0.16%, or 0.14%. The upper limit of the Cr content in the weld metal should be 0.40%, 0.35%, or 0.30%. The upper limit of the Mo content in the weld metal should be 24.0%, 23.0%, 22.0%, or 20.0%. The upper limit of the Nb content in the weld metal should be 4.5%, 4.0%, 3.5%, 3.0%, or 2.5%. The upper limit of W content in the welded metal should be 4.5%, 4.0%, 3.5%, or 3.0%. The upper limit of B content in the welded metal should be 0.0800%, 0.0500%, 0.0200%, or 0.0100%.

[0059] (4) Method of reducing the occupancy rate Lc by lowering the temperature between passes during welding: When forming the weld metal, it is carried out by multi-layer welding. Therefore, by lowering the temperature between passes (the temperature between passes in multi-layer welding where multiple passes are welded), the amount of grain boundary strengthening element precipitates can be reduced, and the occupancy rate Lc of grain boundary strengthening element precipitates on the γ grain boundary can be reduced. The temperature between passes should preferably be below 150°C, and more preferably below 125°C.

[0060] <Welded Joint> Next, a welded joint according to an embodiment of the present disclosure will be described. The welded joint disclosed herein includes: a base material and the weld metal disclosed herein. For example, the welded joint disclosed herein includes: steel as the base material and a weld portion composed of the weld metal and a heat-affected zone.

[0061] (Chemical Composition of Base Material) For the fusion joint of the embodiments disclosed herein, the composition of the base material is not particularly limited. However, from the viewpoint of use in high-pressure hydrogen gas such as liquid hydrogen pressure vessels, the chemical composition of the base material should preferably be as shown below. The following components are included by mass percentage relative to the total mass of the parent material: C: 0.01%~0.07%, Si: 0.03%~0.3%, Mn: 0.10%~0.80%, P: less than 0.008%, S: less than 0.004%, Ni: 6.10%~10.40%, Cr: 0%~1.00%, Mo: 0%~0.60%, Nb: 0%~0.050%, Cu: 0%~1.00%, Co: 0%~0.050%, Al: 0.01%~0.06%, N: 0.0015%~0.006%, O: less than 0.003%, V: 0%~0.080%, Ti: 0%~0.020%, B: 0%~0.0020%, Ca: 0%~0.0040%, and REM: 0%~0.0050%. The remaining part consists of Fe and impurities.

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

[0063] Si: 0.03%~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 steel strength. Therefore, the Si content of the base material is set at 0.03%~0.3%. The lower limit of the Si content of the base material should preferably be 0.05%, 0.08%, or 0.1%. The upper limit of the Si content of the base material should preferably be 0.28%, 0.25%, or 0.2%.

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

[0065] P: P content below 0.008% is impurity and an element that can cause embrittlement; therefore, the P content in the base material should be minimized as much as possible. Thus, the lower limit of the P content in the base material can also be set to 0%. However, from the viewpoint of reducing P removal costs, the P content in the base material can also be set to 0.002% or higher. On the other hand, if the P content in the base material is below 0.008%, the adverse effects of P on embrittlement will fall within an acceptable range. Therefore, the P content in the base material is set to below 0.008%. To effectively suppress embrittlement, the P content in the base material should preferably be below 0.007%, 0.006%, or 0.005%.

[0066] S: S content below 0.004% is impurity and an element that causes embrittlement. S also forms coarse MnS in steel, reducing both its workability and strength. Therefore, the S content in the base material should be minimized, and the lower limit of S content in the base material can be set to 0%. However, from the viewpoint of reducing desaturation costs, the S content in the base material can also be set to 0.0002% or higher. On the other hand, if the S content in the base material is below 0.004%, the adverse effects of S-induced embrittlement will fall within an acceptable range. Therefore, the S content in the base material is set to below 0.004%. To effectively suppress embrittlement, the S content in the base material should preferably be below 0.003% or 0.002%.

[0067] Ni: 6.10%~10.40% Ni is an element that helps improve the strength and low-temperature toughness of steel. On the other hand, when the Ni content is excessive, the improvement in toughness is difficult to match the increase in cost, and it may sometimes hinder pickling and hot working. Therefore, the Ni content of the base material is set at 6.10%~10.40%. The lower limit of the Ni content of the base material can also be set at 6.50%, 7.00%, or 8.00%. The upper limit of the Ni content of the base material should be 10.00%, 9.800%, or 9.500%.

[0068] Cr: 0%~1.00% Cr is an element that helps improve the strength of steel and can also be contained in the base material. On the other hand, excessive Cr content can sometimes lead to a decrease in steel strength and may also cause obstacles during pickling and hot working. Therefore, the Cr content of the base material is set at 0%~1.00%. The lower limit of the Cr content of the base material should preferably be 0.05%, 0.10%, or 0.20%. The upper limit of the Cr content of the base material should preferably be 0.90%, 0.80%, or 0.70%.

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

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

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

[0072] Co: 0%~0.050% Co is an element that enhances corrosion resistance in corrosive environments. Therefore, Co can also be contained in the base material. To achieve the above effect, the Co content should preferably be 0.006% or higher. On the other hand, when the Co content is excessive, the above effect reaches saturation and the economy will decrease. Therefore, the Co content of the base material should be set at 0%~0.050%. The lower limit of the Co content of the base material should preferably be 0.001%, 0.003%, or 0.006%. The upper limit of the Co content of the base material should preferably be 0.040%, 0.030%, or 0.020%.

[0073] Al: 0.01%~0.06% Al is an element that plays a deoxidizing role. On the other hand, when the Al content is excessive, inclusions will increase, workability will decrease, and the toughness of the steel will also decrease. Therefore, the Al content of the base material is set at 0.01%~0.06%. The lower limit of the Al content of the base material should preferably be 0.015% or 0.02%. The upper limit of the Al content of the base material should preferably be 0.055% or 0.05%.

[0074] N: 0.0015%~0.006% N is an element that increases the strength of steel. On the other hand, N forms coarse nitrides in steel, which deteriorates the formability of the steel. Therefore, the N content of the base material is set at 0.0015%~0.006%. The lower limit of the N content of the base material should preferably be 0.002% or 0.0025%. The upper limit of the N content of the base material should preferably be 0.0055% or 0.005%.

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

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

[0077] Ti: 0%~0.020% Ti is an element that forms precipitates, resulting in a fine-grained steel microstructure, and can also be contained in the base metal. On the other hand, excessive Ti content may lead to: reduced steel manufacturability, cracking during processing, and reduced steel strength. Therefore, the Ti content of the base metal is set at 0%~0.020%. The lower limit of the Ti content in the base metal can also be set at 0.001%, 0.002%, or 0.003%. The upper limit of the Ti content in the base metal should be 0.018%, 0.016%, or 0.014%.

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

[0079] Ca: 0%~0.0040% Ca has the function of refining the size of sulfides and oxides, thus effectively improving ductility and toughness. Therefore, Ca can also be contained in the base material. On the other hand, excessive Ca content can lead to the coarsening of sulfides and oxides, potentially causing a deterioration in the low-temperature toughness of the base material. Therefore, the Ca content of the base material is set at 0%~0.0040%. The lower limit of the Ca content of the base material should preferably be 0.0001%, 0.0002%, or 0.0005%. The upper limit of the Ca content of the base material should preferably be 0.0038%, 0.0035%, or 0.0030%.

[0080] REM: 0%~0.0050% Like Ca, REM elements can refine inclusions in steel, thus improving the toughness of steel components, and therefore can also be included in the base material. On the other hand, if the REM content in the base material is excessive, this effect will reach saturation, and costs will increase. Therefore, the REM content of the base material is set at 0%~0.0050%. The lower limit of the REM content in the base material should preferably be 0.0001%, 0.0002%, or 0.0005%. The upper limit of the REM content in the base material should preferably be 0.0048%, 0.0045%, or 0.0040%.

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

[0082] Remaining components: Fe and impurities. The remaining components in the chemical composition of the parent material are Fe and impurities. Impurities refer to components mixed in from raw materials such as ores and waste during the industrial manufacturing of the parent material, components mixed in due to various factors in the manufacturing process, and components that are permissible within the range that do not adversely affect the properties of the parent material. Regarding the content of each element in the above chemical composition, since the reasons for setting the upper and lower limits are different, the appropriate values ​​for the upper and lower limits can be selected independently.

[0083] Next, a method for manufacturing a fusion joint according to an embodiment of the present disclosure will be described. Furthermore, the manufacturing method described below is an example, and the method for manufacturing a fusion joint disclosed herein is not limited to the following methods.

[0084] The fusion joint disclosed herein can be manufactured by using a fusion material to fuse steel as the base material. For example, in the method of manufacturing the fusion joint, a solid welding wire and a flux can be used and obtained through submerged arc welding. For example, in submerged arc welding, a general submerged arc welding machine can be used, a pre-mixed granular flux can be distributed on the fusion line, the solid welding wire can be fed into it, and fusion can be performed by the arc heat generated by the electric arc between the solid welding wire and the steel in the flux. At this time, the chemical composition of the fusion metal includes: the fusion material, i.e., the solid welding wire and the flux, as well as components from the base material, i.e., the steel.

[0085] Based on the viewpoint of controlling the occence Lc of grain boundary strengthening element precipitates on γ grain boundaries within the aforementioned range, for submerged arc welding using solid bonding wire and flux, the weld joint should preferably be manufactured under the following conditions: Current: 330A~390A (e.g., 360A) Voltage: 20V~40V (e.g., 30V) Bonding wire speed: 25cm / min~40cm / min (e.g., 35cm / min) Heat input: 17.0kJ / cm~20.0kJ / cm (e.g., 18.5kJ / cm) Welding method: Multi-layer overlay welding Temperature between weld passes: 150℃ or less (preferably 125℃ or less)

[0086] Furthermore, welded joints can also be obtained through other welding methods such as TIG welding. In this case, the chemical composition of the welded metal includes: the welding material and components from the base material, i.e., the steel.

[0087] <Application> The weld metal of this disclosure embodiment and the weld joint of this disclosure embodiment having the weld metal can be used, for example, in high-pressure hydrogen in a liquid hydrogen pressure vessel (e.g., a liquid hydrogen tank). That is, the liquid hydrogen pressure vessel of this disclosure embodiment has: the weld metal of this disclosure embodiment or the weld joint of this disclosure embodiment. [Example]

[0088] Next, the feasibility and effects of this disclosure will be described in more detail through examples of the present invention and comparative examples, but this disclosure is not limited to the following examples of the invention; any changes made to the design in accordance with the above and below are included in the technical scope of this disclosure.

[0089] (Manufacturing of a Welded Joint) A welded joint with weld metal is obtained by fusing base materials together using submerged arc welding (SAW) with solid welding wire and flux, as shown below. First, solid welding wire and submerged arc welding flux, namely NITTETSU FLUX 10H manufactured by NIPPON STEEL WELDING & ENGINEERING CO.,LTD., are combined and submerged arc welding is performed. The steel plate (base material) used for welding is a steel plate with the composition shown in Table 1 (20mm(t) × 120mm(w) × 300mm(l)). As shown in Figure 2, the two steel plates (base material 22) are butt-jointed, and a bevel is formed and welded so that the bevel shape is an X bevel with an angle θ = 60°. Specifically, after joining two steel plates (base material 22), a Y-shaped bevel is first formed on one side, and welding is performed within this Y-shaped bevel. Next, an X-shaped bevel is formed on the back side of the Y-shaped bevel, and welding is performed within this back side bevel, thereby forming a weld metal 42 with an X-shaped bevel. The welding current used during welding is direct current, the polarity of the weld wire is positive, and the welding conditions are as described in Table 2. Furthermore, the chemical composition of the weld metal is controlled to the chemical compositions described in Tables 3-1 to 3-4 and Tables 4-1 to 4-4 by selecting the base material and adjusting the composition of the solid weld wire and flux. The welded metals shown in Tables 3-1 to 3-4 (Invention Examples 1 to 36, Comparative Examples 1 to 5) use base material number 1 shown in Table 1 as the base material, while the welded metals shown in Tables 4-1 to 4-4 (Invention Examples 41 to 76, Comparative Examples 41 to 45) use base material number 2 shown in Table 1 as the base material.

[0090] The chemical compositions of each weld metal are listed in Tables 3-1 to 3-4 and Tables 4-1 to 4-4. Furthermore, the occumulus Lc of precipitates on the γ-grain boundaries in the weld metal was determined using the aforementioned method. The results are listed in Tables 3-2, 3-4, 4-2, and 4-4. In addition, investigations into the composition of several precipitates revealed that they contain at least one element selected from the group consisting of Cr, Mo, Nb, W, and B.

[0091] <Evaluation Test> (Relative Reduction of Area (RRA) of Welded Metal) The relative reduction of area (RRA) of the welded metal is used as an indicator of its resistance to hydrogen embrittlement and is determined by the following method. The relative reduction of area of ​​the welded metal is determined by a low strain rate tensile test (SSRT) in a hydrogen environment. First, an SSRT test piece as shown in Figure 1 is taken from the welded metal. The locations of the test pieces are shown in Figures 2 and 3. Figure 2 shows an example of a welded joint with an X-shaped bevel (X-groove) at an angle θ = 60°, which has: a base material 22 (steel plate) and welded metal 42 formed in the X-groove. Figure 3 shows the locations of the test pieces when the base material is thinner; the welded joint with an X-groove at an angle θ = 60° has: a base material 24 (steel plate) and welded metal 44 formed in the X-groove. As shown in Figure 2, the test piece of welded metal 42 is taken in such a way that the entire parallel portion of the test piece is welded metal 42. Furthermore, the test piece of welded metal 42 is taken from position 62B at a position t / 4 of the thickness direction, measured from the center axis of the parallel portion of the test piece along the surface of the base material 22, and parallel to the weld line direction. However, if the base material is too thin to be centered at t / 4, then as shown in Figure 3, the test piece of welded metal 44 is taken from position 64B at a position 1 mm in the thickness direction, measured from the end of the clamping part along the surface of the base material 24, and parallel to the weld line direction.

[0092] The SSRT test specimens were subjected to low strain rate tensile tests (SSRT tests) under 0.1 MPa nitrogen (assumed to be an atmospheric environment) and 10 MPa hydrogen. The test temperature was set to room temperature (25°C), and the strain rate was set to 4.2 × 10⁻⁵ / s. The ratio (%) of the reduction of area at break under hydrogen to that under nitrogen was calculated as the relative reduction of area (RRA). The strain rate was calculated from the crosshead movement speed during the tensile test.

[0093] The results of the relative reduction of area (RRA) were used to determine the results based on 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 and less than 75%; C(×): less than 50%.

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3-1]

[0097] [Table 3-2][Table 3-3][Table 3-4]

[0098] [Table 4-1]

[0099] [Table 4-2][Table 4-3][Table 4-4]

[0100] Comparative Example 1: The Cr content in the weld metal is excessive, and the value F is below 8.600. Comparative Example 2: The Cr and Ni content in the weld metal are excessive, and the value F is below 8.600. Comparative Example 3: The C, Cr, and Mo content in the weld metal are excessive. Comparative Example 4: The value F is below 8.600. Comparative Example 5: The Cr content in the weld metal is excessive. Comparative Example 41: The Cr content in the weld metal is excessive, and the value F is below 8.600. Comparative Example 42: The Cr and Ni content in the weld metal are excessive, and the value F is below 8.600. Comparative Example 43: The C and Cr content in the weld metal are excessive. Comparative Example 44: The value F is below 8.600. Comparative Example 45: The Cr content in the weld metal is excessive.

[0101] As shown in Tables 3-1 to 3-4 and Tables 4-1 to 4-4, the chemical composition of the weld metal in the invention example is within a predetermined range and the value F is 8.600 or higher. The chemical composition of the weld metal in the comparative example is outside the predetermined range or the value F is less than 8.600. Compared with the weld metal in the comparative example, the weld metal in the invention example has superior resistance to hydrogen embrittlement.

[0102] Furthermore, this specification is incorporated herein by reference in its entirety with respect to the disclosure of Japanese Patent Application No. 2024-060439. All documents, patent applications and technical specifications described in this specification are incorporated herein by reference to the same extent as when each document, patent application and technical specification is specifically and individually described by reference. [Simplified Explanation of the Diagram]

[0009] Figure 1 is a top view showing the shape of the SSRT test piece in the SSRT test. Figure 2 is a schematic cross-sectional view showing the location of the SSRT test piece in the SSRT test. Figure 3 is a schematic cross-sectional view showing the location of the SSRT test piece when the base material is relatively thin in the SSRT test.

Claims

1. A weld metal, wherein the chemical composition, in the central position of a cross-section perpendicular to the weld line direction and at position t / 4 relative to the thickness direction (t representing the plate thickness), comprises, by mass% relative to the total mass: C: 0.005%~0.20%, Si: 0.01%~0.50%, Mn: 0%~3.0%, P: less than 0.010%, S: less than 0.010%, Ni: 50.0%~80.0%, Cr: 0%~0.44%, Mo: 0.1%~25.0%, Nb: 0%~5.0%, W: 0%~5.0%, Ta: 0%~3.0%, Cu: 0%~1.00%, Co: 0%~1.0%, Pb: 0%~1.00%, Sn: 0%~1.00%, Al: 0.003%~1.00%, Ti: 0%~1.000%. B: 0.0001%~0.1000%, V: 0%~3.0000%, Ca: 0%~1.0000%, Mg: 0%~1.0000%, Zr: 0%~1.0000%, REM: 0%~0.5000%, N: 0%~0.300%, and O: less than 0.050%, with the remainder consisting of Fe and impurities. The value F shown in Equation 1 below is 8.600 or higher; F = 0.23[Cr] + 0.68[Mo] + 0.51[Nb] + 0.42[W] + 102.8[B] Equation 1 (In Equation 1, [Cr] represents the percentage of Cr relative to the total mass of the weld metal, [Mo] represents the percentage of Mo relative to the total mass of the weld metal, [Nb] represents the percentage of Nb relative to the total mass of the weld metal, [W] represents the percentage of W relative to the total mass of the weld metal, and [B] represents the percentage of B relative to the total mass of the weld metal).

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

3. The welded metal as in Request 1 or Request 2, wherein, The total content of Nb and W relative to the total mass of the welded metal is 1.0% or more.

4. The welded metal as in Request 1 or Request 2, wherein, In the aforementioned chemical composition, the content of at least one of the aforementioned Co and Ca is within the range shown below: Co: 0.0001%~1.0%, Ca: 0.0001%~1.0000%.

5. The welded metal as in Request 1 or Request 2, wherein, The content ratio of Mo to Cr is greater than 1 / 1.

6. The welded metal as in claim 1 or claim 2, wherein, The occupancy of the precipitates on the γ grain boundaries is less than 30.0% (Lc).

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

8. A liquid hydrogen pressure vessel having: a welded metal as described in any one of claims 1 to 6.

9. A liquid hydrogen pressure vessel having: a welded joint as claimed in claim 7.

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