TIG welded joint

Abstract

This invention provides a TIG welding head for high-Mn steel that suppresses high-temperature cracking during welding, exhibits high strength, and possesses excellent extremely low-temperature impact toughness. A TIG welding head is provided, wherein the high-Mn steel contains, by mass%, C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 18.0–30.0%, P: less than 0.030%, S: less than 0.0070%, Al: 0.010–0.070%, Cr: 2.5–7.0%, N: 0.0050–0.0500%, and O: less than 0.0050%. The balance consists of Fe and unavoidable impurities in the chemical composition of the weld metal, which contains C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 15.0–30.0%, P: less than 0.030%, S: less than 0.030%, Al: less than 0.100%, Cr: 6.0–14.0%, and N: less than 0.100%, with the balance consisting of Fe and unavoidable impurities.

Description

Technical Field

[0001] This invention relates to TIG (Tungsten Inert Gas) welding joints, and particularly to welded steel structures made of high-Mn steel used in extremely low temperature environments, namely, welding joints that suppress high-temperature cracking during welding, have high strength, and possess excellent extremely low temperature impact toughness. Background Technology

[0002] TIG welding, or Tungsten Inert Gas welding, is a welding method in which tungsten, a non-consumable material, is used as the electrode rod, while argon or helium, an inert gas, is blown in to isolate the air, and other filler materials (welding rods) are melted in an electric arc. TIG welding can be applied to various alloy steels, non-ferrous metals, and even complex shapes, producing excellent weld quality, and is therefore used for welding all metals.

[0003] In recent years, environmental restrictions have become increasingly stringent. Liquefied natural gas (LNG), being sulfur-free, is considered a clean fuel that does not produce sulfur oxides or other air pollutants, leading to increased demand. Furthermore, for the transportation and storage of LNG, containers (tanks) are required to maintain excellent cryogenic impact toughness at temperatures below the liquefaction temperature of LNG, which is -162°C.

[0004] However, based on the necessity of maintaining excellent low-temperature impact toughness, aluminum alloys, 9% Ni steel, and austenitic stainless steel have traditionally been used as materials for containers (cans).

[0005] However, aluminum alloys have low tensile strength, requiring larger plate thicknesses for structures, and they also suffer from poor weldability. Furthermore, 9% Ni steel requires expensive Ni-based materials for welding, making it economically disadvantageous. Additionally, austenitic stainless steels are expensive and have low base metal strength.

[0006] Due to this issue, recently, the application of high-Mn steel (hereinafter also referred to as high-Mn steel) containing approximately 10% to 35% Mn by mass as a material for containers (tanks) used for transporting or storing LNG is being investigated. High-Mn steel has the following characteristics: it remains in the austenitic phase even at extremely low temperatures, does not undergo brittle fracture, and has high strength compared to austenitic stainless steels. Therefore, it is desirable to develop welding materials capable of stably welding such high-Mn steel.

[0007] For example, Patent Document 1 discloses a "high-Mn steel for ultra-low temperatures". The "high-Mn steel for ultra-low temperatures" disclosed in Patent Document 1 is as follows: it contains, by mass %: C: 0.001–0.80%, Mn: 15.0–35.0%, S: 0.001–0.01%, Cr: 0.01–10.0%, Ti: 0.001–0.05%, N: 0.0001–0.10%, O: 0.001–0.010%, P limited to 0.02% or less, and also contains Si: 0.001–5.00% and Al: 0.001–2.0%. One or both of them, and also contain one or more of the following: Mg: less than 0.01%, Ca: less than 0.01%, REM: less than 0.01%, totaling more than 0.0002%, satisfying 30C+0.5Mn+Ni+0.8Cr+1.2Si+0.8Mo≥25…(Equation 1), O / S≥1…(Equation 2), with the balance consisting of Fe and unavoidable impurities, the volume fraction of austenite being more than 95%, the grain size of the aforementioned austenite being 20 to 200 μm, and the carbide coverage at the grain boundaries of the aforementioned austenite being less than 50%. Regarding the high-Mn steel disclosed in Patent Document 1, it is described that in order to prevent carbides generated at grain boundaries from becoming fracture initiation points and crack propagation paths, the austenite grain size is controlled to an appropriate size. This is achieved by appropriately adjusting the amount and balance of alloying elements, as well as the amount of S and O, and by adding Mg, Ca, and REM. This appropriate adjustment of the austenite grain size can also suppress the coarsening of the crystal grain size in the weld heat-affected zone.

[0008] Furthermore, Patent Document 2 discloses a "thick steel plate for low-temperature applications". The "thick steel plate for low-temperature applications" disclosed in Patent Document 2 is a steel material containing, by mass%, C: 0.30–0.65%, Si: 0.05–0.30%, Mn: greater than 20.00% and less than 30.00%, Ni: 0.10% or more and less than 3.00%, Cr: 3.00% or more and less than 8.00%, Al: 0.005–0.100%, N: 0.0050% or more and less than 0.0500%, and P limited. The content of Mn is limited to 0.0040% or less, S to 0.020% or less, O to 0.0050% or less, with the balance consisting of Fe and impurities. The Mn segregation ratio XMn (XMn=Mn1 / Mn0) calculated from the Mn concentration Mn1 in the Mn-rich region and the Mn concentration Mn0 in the Mn-sparse region is 1.6 or less. The yield stress at room temperature (25℃) is 400 MPa or more, the tensile stress is 800 MPa or more, and the Charpy impact absorption energy (vE) of the weld heat-affected zone is [not specified]. -196 The J value is 70J or higher. According to the technology described in Patent Document 2, a material for transporting or storing LNG can be provided while maintaining a hot-rolled state.

[0009] In addition, Patent Document 3 discloses a "high-strength welded joint with excellent impact toughness at extremely low temperatures and a flux-cored arc welding wire for the welded joint". The "flux-cored arc welding wire" disclosed in Patent Document 3 is a welding wire as follows: it contains, by weight %: C: 0.15-0.8%, Si: 0.2-1.2%, Mn: 15-34%, Cr: less than 6%, Mo: 1.5-4%, S: less than 0.02%, P: less than 0.02%, B: less than 0.01%, Ti: 0.09-0.5%, N: 0.001-0.3%, TiO2: 4-15%, total of one or more selected from SiO2, ZrO2 and Al2O3: 0.01-9%, total of one or more selected from K, Na and Li: 0.5-1.7%, one or more selected from F and Ca: 0.2-1.5%, and the balance includes Fe and other unavoidable impurities. It is recorded that if the flux-cored arc welding wire disclosed in Patent Document 3 is used for welding, a weld joint with excellent low-temperature toughness of more than 27J in Charpy impact test at a test temperature of -196℃ and high strength of more than 400MPa at room temperature can be effectively obtained. In addition, by adjusting the composition of the welding wire to Mo: 1.5% or more, a weld joint with excellent resistance to high-temperature cracking can be ensured.

[0010] Furthermore, Patent Document 4 discloses a "solid welding wire for gas metal arc welding". The "solid welding wire for gas metal arc welding" disclosed in Patent Document 4 is as follows: it contains, by mass%, C: 0.2–0.8%, Si: 0.15–0.90%, Mn: 17.0–28.0%, P: less than 0.03%, S: less than 0.03%, Ni: 0.01–10.00%, Cr: 0.4–4.0%, Mo: 0.01–3.50%, B: less than 0.0010%, N: less than 0.12%, with the balance being Fe and unavoidable impurities. It should be noted that, depending on the need, it may contain one or more selected from V, Ti, and Nb, and one or more selected from Cu, Al, Ca, and REM. The document states that if the solid welding wire for gas metal arc welding disclosed in Patent Document 4 is used, it is possible to produce high-strength Charpy impact test absorption energy vE at a test temperature of -196°C, with low fume generation and a room temperature yield strength (0.2% yield strength) of over 400 MPa. -196 It is a welded joint with high strength above 28J and excellent impact toughness at extremely low temperatures.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2016-196703

[0014] Patent Document 2: Japanese Patent Application Publication No. 2017-071817

[0015] Patent Document 3: Japanese Patent Publication No. 2017-502842

[0016] Patent Document 4: International Publication No. WO2020 / 039643 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] However, according to the inventor's research, even when using the welding wire described in Patent Documents 3 and 4 to perform TIG welding on the high-Mn steels described in Patent Documents 1 and 2, there is still a problem of high-temperature cracking during welding.

[0019] The purpose of this invention is to solve the problems of the prior art and provide a TIG weld joint that can suppress the occurrence of high-temperature cracking during welding and is suitable as a weld joint for high-Mn steel used in extremely low-temperature environments, and has both high strength and excellent extremely low-temperature impact toughness.

[0020] It should be noted that "high strength" as used here refers to the yield strength (0.2% yield strength) of weld metal at room temperature (25°C) of 400 MPa or more and its tensile strength of 660 MPa or more, which is manufactured according to JIS Z 3111. The tensile strength of weld joints at room temperature (25°C) of 660 MPa or more is also specified in JIS Z 3121.

[0021] In addition, "excellent low-temperature impact toughness" refers to the Charpy impact absorption energy vE of the weld metal and heat-affected zone of the weld joint manufactured according to JIS Z 3128 at a test temperature of -196°C. -196 It is above 28J.

[0022] Methods for solving problems

[0023] To achieve the above objectives, the inventors first conducted an in-depth study on the factors affecting high-temperature cracking during TIG welding. The results showed that segregation of the final solidification portion of the P-axis in the weld metal is a contributing factor to high-temperature cracking. Furthermore, it was found that when the weld metal contains 6.0% or more Cr by mass, Cr phosphides form in the liquid phase of the weld metal, thereby suppressing segregation of the final solidification portion of the P-axis in the weld metal and inhibiting the occurrence of high-temperature cracking.

[0024] Next, the steel composition and weld metal composition required for TIG welding joints manufactured according to JIS Z 3121 to achieve both the desired high strength and the desired excellent cryogenic impact toughness were investigated. The results showed that the following TIG weld joints were required: for the steel, the chemical composition of the high-Mn steel was adjusted by mass% to the range of C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 18.0–30.0%, P reduced to below 0.030%, S reduced to below 0.0070%, and adjusted to a specific range such as Al: 0.010–0.070%, Cr: 2.5–7.0%, N: 0.0050%–0.0500%, and O(… The chemical composition of the TIG weld metal is reduced to less than 0.0050% by mass. Furthermore, for the weld metal, the chemical composition of the weld metal is adjusted to a specific range by mass% as follows: C: 0.10-0.80%, Si: 0.05-1.00%, Mn: 15.0-30.0%, P reduced to less than 0.030%, S reduced to less than 0.030%, Al reduced to less than 0.100%, and Cr adjusted to a specific range as follows: 6.0-14.0%, and N reduced to less than 0.100%.

[0025] This invention was completed based on further research into the above-mentioned insights.

[0026] The main points of this invention are as follows.

[0027] [1] A TIG welding head, which is a TIG welding head made of high-Mn steel, wherein,

[0028] The aforementioned high-Mn steel has the following chemical composition (by mass%): C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 18.0–30.0%, P: less than 0.030%, S: less than 0.0070%, Al: 0.010–0.070%, Cr: 2.5–7.0%, N: 0.0050–0.0500%, and O: less than 0.0050%, with the balance being Fe and unavoidable impurities.

[0029] The welding metal has a chemical composition, by mass%, of C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 15.0–30.0%, P: less than 0.030%, S: less than 0.030%, Al: less than 0.100%, Cr: 6.0–14.0%, and N: less than 0.100%, with the balance being Fe and unavoidable impurities.

[0030] [2] According to the TIG welding head described in [1], the chemical composition of the above-mentioned high-Mn steel also contains, by mass %, one or more of the following: Mo: less than 2.00%, V: less than 2.0% and W: less than 2.00%.

[0031] [3] According to the TIG welding head described in [1] or [2], wherein the chemical composition of the above-mentioned high-Mn steel also contains, by mass %, one or two of REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%.

[0032] [4] The TIG welded joint according to any one of [1] to [3], wherein the high-Mn steel has a yield strength (0.2% yield strength) of 400 MPa or more at room temperature in a tensile test, and a Charpy impact absorption energy vE at a test temperature of -196°C. -196 It is above 28J.

[0033] [5] The TIG welding head according to any one of [1] to [4], wherein the chemical composition of the welding metal further contains, by mass%, one or two of the following: Mo: less than 3.50% and Ni: less than 10.00%.

[0034] [6] The TIG welding head according to any one of [1] to [5], wherein the chemical composition of the welding metal further contains, by mass %, one or more of the following: V: less than 1.60%, Ti: less than 1.00%, Nb: less than 1.00%, and W: less than 1.00%.

[0035] [7] The TIG welding head according to any one of [1] to [6], wherein the chemical composition of the welding metal further contains, by mass %, one or more of the following: Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020%.

[0036] [8] The TIG welded joint according to any one of [1] to [7], wherein the yield strength (0.2% yield strength) of the weld metal at room temperature in the tensile test is 400 MPa or more and the tensile strength is 660 MPa or more, the tensile strength of the welded joint at room temperature is 660 MPa or more, and the Charpy impact absorption energy vE of the heat-affected zone of the weld metal and the welded joint at a test temperature of -196°C. -196 It is above 28J.

[0037] Invention Effects

[0038] According to the present invention, it is possible to easily manufacture TIG welding heads that suppress high-temperature cracking during welding of TIG welding heads made of high-Mn steel, and that have high strength and excellent low-temperature impact toughness, which has had a significant effect on industry. Detailed Implementation

[0039] This invention relates to a TIG welded joint suitable for use as a cryogenic steel joint obtained by TIG welding high-Mn steels together. The welded joint of this invention is a TIG welded joint manufactured according to JIS Z 3121, and is characterized by: suppressing high-temperature cracking during welding; and, according to JIS Z 3111, having a room-temperature (25°C) yield strength (0.2% yield strength) of 400 MPa or more and a tensile strength of 660 MPa or more. In other words, it possesses both high strength (room-temperature (25°C) tensile strength of 660 MPa or more) and excellent cryogenic impact toughness (with weld metal and weld heat-affected zone and steel exhibiting Charpy impact test absorption energy of 28 J or more at a test temperature of -196°C) according to JIS Z 3128.

[0040] [TIG welding]

[0041] As described above, TIG welding is a welding method in which an electrode rod uses tungsten as a non-consumable material, and other filler materials are melted in an electric arc while argon or helium gas is blown in to isolate the air. TIG welding can be applied to various alloy steels, non-ferrous metals, etc., and can weld even complex shapes, achieving excellent weld quality. Therefore, it is used for welding all metals.

[0042] As an example of the TIG welding method, steel plates or steel materials (thickness: 3-100mm) are butt-welded according to JIS Z3111 to form a 45° V-groove. Pure tungsten rods (3.2mmφ) and filler material (diameter 2.0mmφ) are used as electrodes. The welding is carried out in a downward orientation without preheating, under the following conditions: current of 180-250A (DCEN), voltage of 10-15V, welding speed of 5-15cm / min, welding heat input of 0.7-4.0kJ / mm, interpass temperature of 100-150°C, shielding gas of Ar, and gas flow rate of 10-25L / min.

[0043] [Steel with high Mn content]

[0044] First, let's describe the steel used. It should be noted that in the following, "%" in "chemical composition" refers to "mass %".

[0045] The steel used in this invention is a high-Mn steel as described below, having a chemical composition of C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 18.0–30.0%, P: less than 0.030%, S: less than 0.0070%, Al: 0.010–0.070%, Cr: 2.5–7.0%, N: 0.0050–0.0500%, O (oxygen): less than 0.0050%, with the balance being Fe and unavoidable impurities. The rationale for this limited chemical composition is as follows.

[0046] [C: 0.10~0.80%]

[0047] Carbon (C) is an inexpensive and important element that stabilizes the austenitic phase and improves low-temperature impact toughness. To achieve this effect, a content of 0.10% or more is required. Therefore, the C content is set to 0.10% or more. Preferably, the C content is 0.20% or more, more preferably 0.25% or more, further preferably 0.30% or more, and most preferably 0.35% or more. On the other hand, when the C content exceeds 0.80%, excessive Cr carbides are formed, reducing the low-temperature impact toughness. Therefore, the C content is set to 0.80% or less. It should be noted that the C content is preferably 0.75% or less, more preferably 0.70% or less, further preferably 0.65% or less, and most preferably 0.63% or less.

[0048] [Si: 0.05~1.00%]

[0049] Si is an element that acts as a deoxidizer and contributes to the high strength of steel through solid solution strengthening when dissolved in steel. To achieve this effect, a content of 0.05% or more is required. Therefore, the Si content is set to 0.05% or more. Preferably, the Si content is 0.07% or more, more preferably 0.10% or more, further preferably 0.15% or more, and most preferably 0.20% or more. On the other hand, when the content exceeds 1.00%, weldability decreases. Therefore, the Si content is set to 1.00% or less. It should be noted that the Si content is preferably 0.80% or less, more preferably 0.70% or less, further preferably 0.65% or less, and most preferably 0.50% or less.

[0050] [Mn: 18.0~30.0%]

[0051] Mn is a relatively inexpensive element that stabilizes the austenitic phase, and in this invention, it is an important element for achieving both high strength and excellent cryogenic impact toughness. To obtain this effect, a content of 18.0% or more is required. Therefore, the Mn content is set to 18.0% or more. Preferably, the Mn content is 20.0% or more, more preferably 22.0% or more, and even more preferably 24.0% or more. On the other hand, even when the content exceeds 30.0%, the effect of improving cryogenic impact toughness becomes saturated, and an effect matching the content cannot be expected, which becomes economically disadvantageous. Furthermore, a high content exceeding 30.0% leads to a decrease in machinability and promotes Mn segregation, thus promoting stress corrosion cracking. Therefore, the Mn content is set to 30.0% or less. It should be noted that preferably, it is 29.0% or less, more preferably 28.0% or less, and even more preferably 27.0% or less.

[0052] [P: below 0.030%]

[0053] Phosphorus (P) is an element that segregates at grain boundaries as an impurity and becomes the starting point for stress corrosion cracking. In this invention, it is preferable to minimize its content as much as possible, but it is permissible if it is below 0.030%. Therefore, the P content is set to 0.030% or less. It should be noted that it is preferably 0.028% or less. More preferably, it is 0.024% or less, further preferably 0.020% or less, and most preferably 0.015% or less. It should be noted that reducing P to less than 0.002% requires lengthy refining processes, resulting in high refining costs. Therefore, from an economic point of view, P is preferably set to 0.002% or more.

[0054] [S: below 0.0070%]

[0055] Sulfur (S) exists in steel as sulfide inclusions, reducing the ductility and low-temperature impact toughness of steel and weld metal. Therefore, S is preferably minimized, but levels below 0.0070% are acceptable. Thus, the S content is set to 0.0070% or less. It should be noted that the S content is preferably 0.0050% or less, more preferably 0.0040% or less. Extremely reducing S to less than 0.0005% requires lengthy refining processes, leading to high refining costs. Therefore, from an economic point of view, S is preferably set to 0.0005% or more.

[0056] [Al: 0.010~0.070%]

[0057] Al is the most commonly used element in the deoxidation process of molten steel, acting as a deoxidizer. To achieve this effect, it needs to contain 0.010% or more. Therefore, the Al content is set to 0.010% or more. Preferably, the Al content is 0.020% or more, more preferably 0.030% or more. On the other hand, when the content exceeds 0.070%, Al mixes into the weld metal during welding, reducing the toughness of the weld metal. Therefore, the Al content is set to 0.070% or less. It should be noted that preferably, it is 0.060% or less, more preferably 0.050% or less.

[0058] [Cr: 2.5–7.0%]

[0059] Cr is an element that stabilizes the austenite phase and effectively contributes to improving low-temperature impact toughness and steel strength. It is also an effective element for forming microcrystalline regions. To achieve this effect, a Cr content of 2.5% or more is required. Therefore, the Cr content is set to 2.5% or more. The Cr content is preferably 3.0% or more, more preferably 3.3% or more, further preferably 3.5% or more, and most preferably 4.0% or more. On the other hand, when the Cr content exceeds 7.0%, Cr carbides are formed, reducing low-temperature impact toughness and resistance to stress corrosion cracking. Therefore, the Cr content is set to 7.0% or less. It should be noted that the Cr content is preferably 6.8% or less, more preferably 6.5% or less, and further preferably 6.0% or less.

[0060] [N: 0.0050~0.0500%]

[0061] Nitrogen (N) is an element that stabilizes the austenitic phase, effectively contributing to improved low-temperature impact toughness. To achieve this effect, the N content needs to be 0.0050% or more. Therefore, the N content is set to 0.0050% or more. Preferably, the N content is 0.0060% or more, more preferably 0.0070% or more, and even more preferably 0.0080% or more. On the other hand, when the content exceeds 0.0500%, nitrides or carbonitrides coarsen, and the low-temperature impact toughness decreases. Therefore, the N content is set to 0.0500% or less. It should be noted that the N content is preferably 0.0400% or less, more preferably 0.0300% or less, and even more preferably 0.0200% or less.

[0062] [O (Oxygen): less than 0.0050%]

[0063] Oxygen (O) exists in steel as oxide inclusions, reducing the steel's low-temperature impact toughness. Therefore, O content is preferably minimized, but 0.0050% or less is acceptable. Thus, the O content is set to 0.0050% or less. It should be noted that 0.0045% or less is preferred, and 0.0040% or less is more preferred. Furthermore, reducing O content to less than 0.0005% requires lengthy refining processes, leading to high refining costs. Therefore, from an economic point of view, O content is preferably set to 0.0005% or more. An O content of 0.0006% or more is more preferred.

[0064] [Optional selection components]

[0065] The above composition is the basic chemical composition. Alternatively, one or more chemical compositions selected from Mo: less than 2.00%, V: less than 2.0%, and W: less than 2.00%, and one or two chemical compositions selected from REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%, may be added to the basic chemical composition as needed.

[0066] [Mo: less than 2.00%, V: less than 2.0%, and W: less than 2.00%]

[0067] Mo, V, and W are all elements that contribute to the stabilization of the austenitic phase and also help improve the strength and cryogenic impact toughness of steel. One or more of these elements can be selected as needed. To achieve this effect, it is preferable that each of Mo, V, and W contains 0.001% or more. On the other hand, when Mo and W each exceed 2.00% and V exceeds 2.0%, coarse carbonitrides increase, becoming the initiation point for fracture and reducing cryogenic impact toughness. Therefore, when containing Mo, V, and W, the content is set to Mo: 2.00% or less, V: 2.0% or less, and W: 2.00% or less. It should be noted that Mo: 1.70% or less, V: 1.7% or less, and W: 1.70% or less are preferred, and Mo: 1.50% or less, V: 1.5% or less, and W: 1.50% or less are even more preferred.

[0068] [REM: 0.0010–0.0200% and B: 0.0005–0.0020%]

[0069] REM elements, such as Sc, Y, La, and Ce, are rare earth elements that improve the toughness of steel by controlling the morphology of inclusions, thereby enhancing its ductility and resistance to sulfide stress corrosion cracking. Additionally, B is an element that contributes to improving the toughness of steel through grain boundary segregation. One or two of these elements can be selected as needed.

[0070] To achieve the aforementioned effects, REM needs to be present at a concentration of 0.0010% or more. Therefore, when REM is present, the REM content is set to 0.0010% or more. Preferably, the REM content is 0.0015% or more. On the other hand, when the content exceeds 0.0200%, the amount of non-metallic inclusions increases, and toughness, ductility, and resistance to sulfide stress cracking decrease. Therefore, when REM is present, the REM content is set to 0.0200% or less. Preferably, it is 0.0180% or less.

[0071] Furthermore, to achieve the aforementioned effects, B needs to be present at a concentration of 0.0005% or higher. Therefore, when B is present, the B content is set to 0.0005% or higher. Preferably, the B content is 0.0008% or higher. On the other hand, when the content exceeds 0.0020%, coarse nitrides and carbides increase, and toughness decreases. Therefore, when B is present, the B content is set to 0.0020% or lower. It should be noted that the B content is preferably 0.0018% or lower.

[0072] [Composition of Balance]

[0073] The balance other than the above chemical composition consists of Fe and unavoidable impurities. It should be noted that unavoidable impurities include Ca, Mg, Ti, Nb, and Cu, which are permissible if their combined content is less than 0.05%. Furthermore, elements other than those described above may be included, provided the basic and selected compositions are satisfied; such embodiments are also within the scope of this invention.

[0074] [Manufacturing method for high-Mn steel]

[0075] Hereinafter, a preferred manufacturing method for the high-Mn steel used in this invention will be described.

[0076] Molten steel with the above-mentioned composition is smelted using common smelting methods such as converters and electric furnaces, and then produced into steel billets and other raw materials of specified dimensions using common casting methods such as continuous casting or ingot-rolling. It should be noted that secondary refining can also be performed during smelting using a vacuum degassing furnace, which is self-evident. The resulting steel raw material is further heated, hot-rolled, and then cooled to produce steel products of specified dimensions. It should be noted that heating at a temperature ranging from 1100 to 1300°C, ending hot rolling at a finishing temperature of 790 to 980°C, and immediately cooling, etc., can produce steel with excellent low-temperature impact toughness. Furthermore, to adjust the steel properties, further heat treatments such as annealing can be performed, which is self-evident.

[0077] [Properties of Steel]

[0078] Here, preferred characteristics of the high-Mn steel used in this invention will be described.

[0079] For high-strength steel for ultra-low temperatures with the above-mentioned steel composition, the plate thickness is, for example, 6 to 100 mm, and the yield strength (0.2% yield strength) at room temperature (25°C) in the tensile test is preferably 400 MPa or higher, and the Charpy impact absorption energy vE at a test temperature of -196°C is... -196 Preferably, the tensile strength is 28 J or higher. Furthermore, a tensile strength of 660 MPa or higher is preferred. More preferably, a tensile strength of 800 MPa or higher is preferred.

[0080] [Welding metal]

[0081] In this invention, the above-mentioned high-Mn steels are welded together by TIG welding to form a welded joint consisting of one or more layers of weld metal.

[0082] The welding metal of the present invention is a welding metal as described below: As a basic chemical composition, it has the following components: C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 15.0–30.0%, P: 0.030% or less, S: 0.030% or less, Al: 0.100% or less, Cr: 6.0–14.0%, N: 0.100% or less, with the balance being Fe and unavoidable impurities. The reasons for limiting the chemical composition are as follows.

[0083] [C: 0.10~0.80%]

[0084] Carbon (C) is an element that increases the strength of weld metal through solid solution strengthening. Furthermore, C is an inexpensive and important element that stabilizes the austenite phase and improves the low-temperature impact toughness of weld metal. To achieve these effects, a content of 0.10% or more is required. Therefore, the C content is set to 0.10% or more. The C content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, when the C content exceeds 0.80%, excessive carbide formation occurs in the weld metal, reducing the low-temperature impact toughness and making high-temperature cracking during welding more likely. Therefore, the C content is set to 0.80% or less. Preferably 0.75% or less, more preferably 0.70% or less, and most preferably 0.63% or less.

[0085] [Si: 0.05~1.00%]

[0086] Si acts as a deoxidizer, improving the yield of Mn, increasing the viscosity of the molten metal, and stably maintaining the weld bead shape. To achieve this effect, a content of 0.05% or more is required. Therefore, the Si content is set to 0.05% or more. Preferably, the Si content is 0.10% or more, more preferably 0.15% or more, further preferably 0.20% or more, and most preferably 0.25% or more. However, when the content exceeds 1.00%, the extremely low temperature impact toughness of the weld metal decreases. Furthermore, Si segregates during solidification, forming a liquid phase at the solidification unit interface, reducing high-temperature crack resistance. Therefore, the Si content is set to 1.00% or less. It should be noted that the Si content is preferably 0.80% or less, more preferably 0.75% or less, and further preferably 0.70% or less.

[0087] [Mn: 15.0~30.0%]

[0088] Mn is an element that stabilizes the austenitic phase at a low cost, and is required to contain 15.0% or more in this invention. When Mn is less than 15.0%, a ferrite phase is formed in the weld metal, significantly reducing the low-temperature impact toughness. Therefore, the Mn content is set to 15.0% or more. The Mn content is preferably 17.0% or more, more preferably 18.0% or more. On the other hand, when Mn exceeds 30.0%, excessive Mn segregation occurs during solidification, inducing high-temperature cracking. Therefore, the Mn content is set to 30.0% or less. It should be noted that the Mn content is preferably 28.0% or less, more preferably 27.0% or less.

[0089] [P: below 0.030%]

[0090] Phosphorus (P) is an element that induces high-temperature cracking due to grain boundary segregation. In this invention, it is preferable to reduce it as much as possible, but it is permissible if it is below 0.030%. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.020% or less, more preferably 0.018% or less, further preferably 0.016% or less, and most preferably 0.014% or less. It should be noted that reducing P to less than 0.002% requires a long refining process, resulting in high refining costs. Therefore, from an economic point of view, P is preferably set to 0.002% or more.

[0091] [S: below 0.030%]

[0092] Sulfide (S) exists in weld metal as sulfide inclusions, specifically MnS. MnS acts as a fracture initiation point, thus reducing cryogenic impact toughness. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.017% or less. It should be noted that excessive reduction leads to increased refining costs. Therefore, the S content is preferably adjusted to 0.001% or more.

[0093] [Al: below 0.100%]

[0094] Al acts as a deoxidizer, playing a crucial role in increasing the viscosity of the molten metal and stabilizing the weld bead shape. Furthermore, Al narrows the temperature range of the solid-liquid coexistence region of the molten metal, helping to suppress high-temperature cracking in the weld metal. This effect becomes significant when the content is 0.001% or more; therefore, an Al content of 0.001% or more is preferred. However, when the content exceeds 0.100%, the viscosity of the molten metal becomes excessively high, leading to an increase in defects such as poor weld bead extension and fusion. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.060% or less, more preferably 0.050% or less, further preferably 0.040% or less, and most preferably 0.030% or less.

[0095] [Cr: 6.0~14.0%]

[0096] Cr is an element that stabilizes the austenite phase and effectively contributes to improving low-temperature impact toughness and weld metal strength. Furthermore, Cr narrows the temperature range of the solid-liquid coexistence region in molten metal, effectively suppressing high-temperature cracking. In addition, Cr also suppresses high-temperature cracking caused by phosphorus (P) by forming Cr phosphides in the liquid phase. To achieve this effect, a content of 6.0% or more is required. When the Cr content is less than 6.0%, the above-mentioned effects cannot be ensured. Therefore, the Cr content is set to 6.0% or more. The Cr content is preferably 6.5% or more, more preferably 7.0% or more, further preferably 7.5% or more, and most preferably 8.0% or more. On the other hand, when the Cr content exceeds 14.0%, Cr carbides are formed, leading to a decrease in low-temperature impact toughness. Therefore, the Cr content is set to 14.0% or less. Preferably 13.0% or less, more preferably 12.0% or less, further preferably 11.5% or less, and most preferably 11.0% or less.

[0097] [N: below 0.100%]

[0098] Nitrogen (N) is an unavoidable element, but like carbon (C), it effectively contributes to increasing the strength of the weld metal and stabilizing the austenite phase, thus contributing to a stable increase in low-temperature impact toughness. This effect becomes significant when the content is 0.003% or more. Therefore, the N content is preferably set to 0.003% or more, more preferably 0.004% or more, and even more preferably 0.006% or more. On the other hand, when the content exceeds 0.100%, nitrides are formed, and low-temperature toughness decreases. Therefore, the N content is set to 0.100% or less. It should be noted that the N content is preferably 0.080% or less, more preferably 0.050% or less.

[0099] [Optional selection components]

[0100] For the welding metal of the present invention, the above composition is the basic chemical composition, but it may, as needed, have a composition of Mo: 3.50% or less and / or Ni: 10.00% or less, and further selectively contain one or more of V: ​​1.60% or less, Ti: 1.00% or less, Nb: 1.00% or less and W: 1.00% or less, and selectively contain one or more of Cu: 1.00% or less, Ca: 0.010% or less, B: 0.010% or less and REM: 0.020% or less as optional compositions.

[0101] [Mo: less than 3.50% and Ni: less than 10.00%]

[0102] Both Mo and Ni are elements that strengthen austenite grain boundaries, and one or both can be selected as needed.

[0103] Mo is an element that strengthens austenite grain boundaries, segregating at these boundaries and increasing the strength of the weld metal. It also enhances the strength of the weld metal through solid solution strengthening. However, when the content exceeds 3.50%, it precipitates as carbides, becoming a fracture initiation point and leading to a decrease in low-temperature impact toughness. Therefore, in cases where Mo is present, the Mo content is set to 3.50% or less. It should be noted that the Mo content is preferably 3.20% or less, more preferably 3.00% or less, and even more preferably 2.50% or less.

[0104] Ni is an element that strengthens austenite grain boundaries, segregating at these boundaries and improving low-temperature impact toughness. Furthermore, Ni also has a stabilizing effect on the austenite phase; therefore, further increasing its content stabilizes the austenite phase and improves the low-temperature impact toughness of the weld metal. To achieve this effect, the Ni content is preferably set to 0.01% or more, more preferably 1.00% or more. However, Ni is an expensive element, and a content exceeding 10.00% becomes economically unfavorable. Therefore, when Ni is present, the Ni content is set to 10.00% or less. It should be noted that it is preferably 8.00% or less, more preferably 7.00% or less, further preferably 6.50% or less, and most preferably 6.00% or less.

[0105] [V: ≤1.60%, Ti: ≤1.00%, Nb: ≤1.00%, and W: ≤1.00%]

[0106] V, Ti, Nb, and W are all elements that promote the formation of carbides and help improve the strength of weld metal. One or more of these elements can be selected as needed.

[0107] V is a carbide-forming element that causes the precipitation of fine carbides, which helps to improve the strength of the weld metal. To achieve this effect, when V is present, it is preferable to contain 0.001% or more. The V content is preferably 0.002% or more, more preferably 0.005% or more. On the other hand, when the content exceeds 1.60%, the carbides become coarse, becoming the starting point for fracture and leading to a decrease in cryogenic impact toughness. Therefore, when V is present, the V content is set to 1.60% or less. Preferably 1.00% or less, more preferably 0.80% or less, further preferably 0.60% or less, and most preferably 0.40% or less.

[0108] Ti is a carbide-forming element that causes the precipitation of fine carbides, which helps improve the strength of the weld metal. Furthermore, Ti causes carbides to precipitate at the solidification unit interfaces of the weld metal, helping to suppress high-temperature cracking. To achieve this effect, when Ti is present, it is preferable that the Ti content is 0.001% or more. More preferably, the Ti content is set to 0.002% or more, and even more preferably, 0.005% or more. On the other hand, when Ti content exceeds 1.00%, the carbides become coarse, becoming the initiation point for fracture and leading to a decrease in extremely low-temperature impact toughness. Therefore, when Ti is present, the Ti content is set to 1.00% or less. It should be noted that the Ti content is preferably 0.80% or less, more preferably 0.60% or less, and even more preferably 0.50% or less.

[0109] Nitrogen (Nb) is a carbide-forming element that helps increase the strength of weld metal by causing carbide precipitation. Furthermore, Nb causes carbides to precipitate at the solidification unit interfaces of the weld metal, helping to suppress high-temperature cracking. To achieve this effect, it is preferable to contain 0.001% or more of Nb. More preferably, the Nb content is set to 0.002% or more, and even more preferably, 0.005% or more. On the other hand, when Nb exceeds 1.00%, the carbides coarsen, becoming the initiation point for fracture and leading to a decrease in cryogenic impact toughness. Therefore, when Nb is present, the Nb content is set to 1.00% or less. It should be noted that the Nb content is preferably 0.80% or less, more preferably 0.60% or less, and most preferably 0.50% or less.

[0110] W is a carbide-forming element that helps increase the strength of weld metal by causing carbide precipitation. Furthermore, it contributes to the stabilization of the austenite phase, thus improving low-temperature impact toughness. Additionally, W causes carbides to precipitate at the solidification unit interfaces of the weld metal, helping to suppress high-temperature cracking. To achieve this effect, it is preferable to contain 0.001% or more of W. More preferably, the W content is 0.002% or more, and even more preferably 0.005% or more. On the other hand, when W exceeds 1.00%, the carbides coarsen, becoming the initiation point for fracture and leading to a decrease in low-temperature impact toughness. Therefore, when W is present, the W content is set to 1.00% or less. It should be noted that the W content is preferably 0.80% or less, more preferably 0.60% or less, and even more preferably 0.40% or less.

[0111] [Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020%]

[0112] Cu is an element that helps stabilize austenite. In addition, Ca, B and REM are elements that help improve processability. One or more of these elements can be selected as needed.

[0113] Cu is an element that stabilizes the austenite phase, even at extremely low temperatures, thereby improving the low-temperature impact toughness of weld metals. To achieve this effect, when Cu is present, it is preferable to contain 0.01% or more. More preferably, the Cu content is 0.04% or more. However, when Cu exceeds 1.00% and is present in large quantities, segregation occurs during solidification, inducing high-temperature cracking. Therefore, when Cu is present, the Cu content is set to 1.00% or less. It should be noted that the Cu content is preferably 0.80% or less.

[0114] In molten metal, Ca combines with S to form a high-melting-point sulfide, CaS. CaS has a higher melting point than MnS, thus helping to suppress high-temperature cracking in the weld metal. This effect becomes significant when the content is 0.001% or more. Therefore, when Ca is present, the Ca content is preferably set to 0.001% or more. On the other hand, when the content exceeds 0.010%, the arc becomes disordered during welding, making stable welding difficult. Therefore, when Ca is present, the Ca content is set to 0.010% or less. It should be noted that 0.008% or less is preferred, and 0.006% or less is more preferred.

[0115] Boron (B) is an unavoidable element that segregates at austenite grain boundaries. When the B content exceeds 0.010%, boron nitride forms at the austenite grain boundaries, reducing strength. Furthermore, boron nitride becomes a fracture initiation point, reducing low-temperature impact toughness. Therefore, when B is present, the B content is set to 0.010% or less. It should be noted that the B content is preferably 0.008% or less, more preferably 0.005% or less. It should also be noted that while there is no specific lower limit, excessive reduction leads to increased refining costs; therefore, when B is present, the B content is preferably set to 0.0001% or more.

[0116] REM is a powerful deoxidizer, existing in the weld metal as REM oxide. REM oxide contributes to grain refinement by acting as nucleation sites during solidification, thus improving the strength of the weld metal. This effect becomes significant when the content is 0.001% or more. Therefore, when REM is present, the REM content is preferably set to 0.001% or more. More preferably, it is 0.002% or more, and even more preferably 0.003% or more. However, when the content exceeds 0.020%, the stability of the arc decreases. Therefore, when REM is present, the REM content is set to 0.020% or less. It should be noted that it is preferably 0.018% or less, and more preferably 0.015% or less.

[0117] [Composition of Balance]

[0118] The balance other than the above chemical composition consists of Fe and unavoidable impurities. It should be noted that unavoidable impurities include H, O, Mg, Zn, Re, etc., and are permissible if their total content is less than 0.0100%. Furthermore, as long as the above basic and selected compositions are satisfied, elements other than these may be included, and such embodiments are also within the scope of this invention.

[0119] It should be noted that the chemical composition of the weld metal is mainly determined by the inflow ratio of the base metal and welding materials such as TIG welding filler material (welding electrode).

[0120] [Manufacturing method of welded joint]

[0121] Next, the manufacturing method of the welded joint of the present invention will be described.

[0122] First, prepare steel with a high Mn content and the chemical composition described above. Then, beveling is performed by forming the prepared steel pieces together in a prescribed bevel shape. The bevel shape is not particularly limited; for welded steel structures, examples include the common V-groove, X-groove, and K-groove according to JIS Z 3001-1.

[0123] Next, the beveled steel pieces are butt-jointed, and while inactive gas is being sprayed, a specified TIG welding filler material (electrode) is continuously supplied. An arc is generated using a tungsten electrode to perform welding, forming weld metal and creating a welded joint. The filler material used (diameter: 1.2–3.0 mm φ) can be a filler material capable of forming weld metal with desired properties. The chemical composition of the filler material is not particularly limited, but preferably consists of, by mass%, C: 0.20–0.80%, Si: 0.20–1.00%, Mn: 16.0–30.0%, P: 0.030% or less, S: 0.030% or less, Cr: 6.0–15.0%, Mo: 0.01–4.0%, Ni: 0.01–9.0%, and N: 0.100% or less.

[0124] It should be noted that the heat-affected zone of the aforementioned weld joint refers to the region whose properties have changed compared to the original part of the base material or the original part of the weld metal due to the influence of the heat applied during welding, such as changes in crystal structure, formation of new phases, changes in crystal grain size, element diffusion, dislocation recovery, etc. It should also be noted that the reasons for these property changes are not limited to those described above.

[0125] Example

[0126] The present invention will be further described below based on embodiments. However, the following embodiments are merely illustrative of the invention in more detail and are not intended to limit the scope of the invention.

[0127] Molten steel was melted in a vacuum melting furnace, cast in a mold, and then rolled to produce steel billets (wall thickness: 150 mm) with the chemical composition shown in Table 1, thus obtaining steel raw material. Next, the obtained steel raw material was placed in a heating furnace and heated to 1250°C. Hot rolling was then performed with the finishing rolling temperature set at 850°C, followed immediately by water cooling to obtain steel plates with a thickness of 12 mm (high-Mn steel).

[0128] Next, the molten steel with the chemical composition shown in Table 2 was melted in a vacuum melting furnace and cast to produce 1000 kg of steel ingots. The obtained steel ingots were heated to 1200°C and then hot-rolled, followed by cold rolling and annealing (900-1200°C) as needed to obtain TIG welding filler material (electrode) with a diameter of 2.0 mm and a length of 1000 mm.

[0129] Next, using the obtained steel (12mm thick), a 45° V-groove was formed by butt welding according to JIS Z 3121. The obtained TIG welding filler material (electrode) was used as the welding material, and TIG welding was performed using the combinations shown in Table 3. Weld metal was formed within the groove to obtain a welded joint. The above TIG welding was performed without preheating using the filler materials (2.0mm φ diameter) shown in Table 2, in a downward orientation, under the following conditions: current 200A (DCEN), voltage 12V, welding speed 8cm / min, inter-pass temperature 100–150°C, and shielding gas Ar. A pure tungsten rod (3.2mm φ) was used as the electrode.

[0130] [Property Evaluation of High-Mn Steel]

[0131] Tensile test specimens were cut from the high-Mn steel according to JIS Z 2241, and Charpy impact test specimens (V-notch) were cut according to JIS Z 2242. Tensile and impact tests were then performed. The tensile tests were conducted as follows: three specimens were tested at room temperature, and the average value (0.2% yield strength) was taken as the tensile property of the high-Mn steel. The Charpy impact tests were conducted as follows: three specimens were tested, and the absorbed energy vE at a test temperature of -196℃ was calculated. -196 The average value of this value is taken as the extremely low temperature impact toughness of the high Mn steel.

[0132] [Compositional Analysis of Weld Metal]

[0133] Regarding the chemical composition of the weld metal, samples were taken from the center of the weld metal and analyzed using combustion-infrared absorption method, inactive gas melting-thermal conductivity method, inactive gas melting-infrared absorption method, spectrophotometry, ICP-AES method, precipitation analysis method, volumetric method, and wet chemical analysis method.

[0134] [Evaluation of high-temperature crack resistance]

[0135] After welding, a 10mm thick macroscopic test piece was cut from the center of the weld line using a micro-cutting tool, with the observation surface perpendicular to the weld line. The cross-section of the weld metal was observed using an optical microscope (30x magnification) to determine the presence or absence of high-temperature cracking. It should be noted that high-temperature cracking is identified in the microstructure photographs obtained by the optical microscope as a thin, elongated black area with a width of 25μm × a length of 80μm or more. If high-temperature cracking is observed, the high-temperature crack resistance is considered reduced and rated as "×". If no cracks are observed, the high-temperature crack resistance is considered excellent and rated as "○".

[0136] [Evaluation of Weld Metal Properties]

[0137] Tensile test specimens (parallel portion diameter 6 mm φ) and Charpy impact test specimens (V-notch) were cut from the obtained weld joint according to JIS Z 3111, and tensile and impact tests were performed. For the tensile test, three specimens of each type were performed at room temperature, and the average value (0.2% yield strength and tensile strength) was taken as the tensile properties of the weld metal of the weld joint. Similarly, three specimens of each type were performed for the Charpy impact test, and the absorbed energy vE at a test temperature of -196℃ was determined. -196 The average value of these values ​​is taken as the cryogenic impact toughness of the weld metal of the weld joint. Regarding the target values ​​of this invention, as described above, the 0.2% yield strength at room temperature is set to 400 MPa or higher, and the tensile strength is set to 660 MPa or higher. Regarding the target values ​​of this invention, as described above, the absorbed energy vE... -196 Set to 28J or higher.

[0138] [Evaluation of Welded Joint Characteristics]

[0139] The obtained weld joints were subjected to tensile tests at room temperature according to JIS Z 3121. For the test pieces, test pieces No. 1A were cut in a direction perpendicular to the welding axis, with the welding axis as the center of the parallel length of the test piece, and their thickness was equal to the total thickness of the weld joint. In the tests, three pieces were tested, and the average value obtained was taken as the tensile characteristic of the weld joint. Regarding the target value of the present invention, as described above, the tensile strength at room temperature was set to 660 MPa or higher.

[0140] In addition, according to JIS Z 3128, Charpy impact test pieces (V-notch) were cut from the weld heat-affected zone and impact tests were conducted. It should be noted that the V-notch direction of the test piece is perpendicular to the base metal surface, and the test piece was cut from the center of the plate thickness, at the center of the weld metal, and 1 mm from the melt line. Three pieces were tested, and the absorbed energy vE at a test temperature of -196℃ was calculated. -196The average value of this value is taken as the extremely low temperature impact toughness of the weld heat-affected zone. Regarding the target value of this invention, as described above, the absorbed energy vE... -196 Set to 28J or higher.

[0141] The results are shown in Tables 1-4.

[0142]

[0143]

[0144]

[0145] [Table 4]

[0146]

[0147] Underlined: indicates outside the scope of this invention.

[0148] All examples of this invention are welded joints that do not produce high-temperature cracking during welding and have excellent resistance to high-temperature cracking.

[0149] Furthermore, in all examples of the present invention, the yield strength (0.2% yield strength) of the weld metal at room temperature is 400 MPa or more, and its tensile strength is 660 MPa or more; the tensile strength of the heat-affected zone of the weld joint at room temperature is 660 MPa or more; and the absorbed energy vE of the Charpy impact test of the weld metal and the weld heat-affected zone at a test temperature of -196°C is... -196 Welded joints with a strength of 28J or higher, possessing both high strength and excellent low-temperature impact toughness.

[0150] On the other hand, comparative examples outside the scope of this invention exhibited high-temperature cracking and reduced high-temperature cracking resistance, or the room-temperature yield strength (0.2% yield strength) of the weld metal was less than 400 MPa, or the tensile strength was less than 660 MPa, or the absorbed energy vE of the Charpy impact test of the weld metal or the weld heat-affected zone at a test temperature of -196°C. -196 Less than 28J, no welded joint with both high strength and excellent low-temperature impact toughness was obtained.

[0151] As for the welded joints No. 13 and 22, which are used as comparative examples, the Mn content of the weld metal is lower than that of the present invention. Therefore, the austenitic stability of the weld metal is low. Consequently, the Charpy impact test energy absorbed at a test temperature of -196°C is less than 28J, and the expected excellent low-temperature impact toughness is not ensured.

[0152] Furthermore, regarding the welded joints No. 14, 23, 26, 29, and 32 used as comparative examples, the Cr content in the weld metal was lower than the range of the present invention. Therefore, the 0.2% yield strength of the weld metal was less than 400 MPa, the tensile strength was less than 660 MPa, and the tensile strength of the joint was less than 660 MPa, failing to ensure the desired high strength. Moreover, segregation of the final solidification portion during P-axis welding could not be suppressed, resulting in high-temperature cracking. Additionally, the absorbed energy vE of the weld metal at the test temperature of -196°C was... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0153] Furthermore, in the case of welded joint No. 15, which serves as a comparative example, the P content in the weld metal is higher than that of the present invention. As a result, P segregates in the final solidification part of the weld metal, leading to high-temperature cracking. In addition, the S content in the weld metal is higher than that of the present invention, resulting in the precipitation of MnS, which would become the fracture initiation point. The Charpy impact test absorbed less than 28 J at a test temperature of -196°C, failing to ensure the expected excellent low-temperature impact toughness.

[0154] Furthermore, in the case of welded joint No. 16, which serves as a comparative example, the C content in the weld metal is higher than that of the present invention. As a result, carbides are generated in the weld metal, leading to high-temperature cracking. Moreover, these carbides become the fracture initiation point, and the Charpy impact test energy absorbed at a test temperature of -196°C is less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0155] Furthermore, in the case of welded joint No. 17, which serves as a comparative example, the Mn content in the weld metal is higher than that of the present invention. As a result, Mn segregates into the final solidification portion during welding, leading to high-temperature cracking.

[0156] Furthermore, in the case of welded joint No. 18, which serves as a comparative example, the Cr content in the weld metal is higher than that of the present invention. As a result, Cr carbides that would become the fracture initiation point are precipitated in the weld metal. The Charpy impact test absorbed less than 28 J at a test temperature of -196°C, failing to ensure the expected excellent low-temperature impact toughness.

[0157] Furthermore, for the welded joints No. 33, 34, and 35 used as comparative examples, the Mn content of the steel is lower than that of the present invention, and the austenitic phase stability of the steel is low. Therefore, the Charpy impact test absorption energy of the weld heat-affected zone at a test temperature of -196°C is less than 28J, which fails to ensure the expected excellent low-temperature impact toughness.

Claims

1. A TIG welding head, which is a TIG welding head made of high-Mn steel, wherein, The high-Mn steel has the following chemical composition by mass percent: C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 18.0–30.0%, P: less than 0.030%, S: less than 0.0070%, Al: 0.010–0.070%, Cr: 2.5–7.0%, N: 0.0050–0.0500%, and O: less than 0.0050%, with the balance being Fe and unavoidable impurities. The welding metal has a chemical composition, by mass%, of C: 0.10–0.80%, Si: 0.05–1.00%, Mn: 15.0–30.0%, P: less than 0.030%, S: less than 0.030%, Al: less than 0.100%, Cr: 7.0–14.0%, and N: less than 0.100%, with the balance being Fe and unavoidable impurities.

2. The TIG welding head according to claim 1, wherein, The chemical composition of the high-Mn steel also contains, by mass%, at least one of the following groups A and B. Group A: Selected from one or more of the following: Mo: less than 2.00%, V: less than 2.0%, and W: less than 2.00%; Group B: Selected from one or both of REM: 0.0010–0.0200% and B: 0.0005–0.0020%.

3. The TIG welding head according to claim 1, wherein, The high-Mn steel exhibits a yield strength of over 400 MPa at room temperature in a tensile test, and a Charpy impact absorption energy vE at a test temperature of -196℃. -196 It is above 28J.

4. The TIG welding head according to claim 2, wherein, The high-Mn steel exhibits a yield strength of over 400 MPa at room temperature in a tensile test, and a Charpy impact absorption energy vE at a test temperature of -196℃. -196 It is above 28J.

5. The TIG welding joint according to any one of claims 1 to 4, wherein, The chemical composition of the weld metal, by mass%, also contains at least one of the following groups C to E. Group C: Selected from one or both of Mo: less than 3.50% and Ni: less than 10.00%; Group D: Selected from one or more of the following: V: less than 1.60%, Ti: less than 1.00%, Nb: less than 1.00%, and W: less than 1.00%; Group E: Selected from one or more of the following: Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020%.

6. The TIG welding joint according to any one of claims 1 to 4, wherein, The weld metal exhibits a yield strength of ≥400 MPa and a tensile strength of ≥660 MPa at room temperature during tensile testing. The weld joint also exhibits a tensile strength of ≥660 MPa at room temperature. Furthermore, the heat-affected zone of both the weld metal and the weld joint has a Charpy impact absorption energy vE at a test temperature of -196°C. -196 It is above 28J.

7. The TIG welding joint according to claim 5, wherein, The weld metal exhibits a yield strength of ≥400 MPa and a tensile strength of ≥660 MPa at room temperature during tensile testing. The weld joint also exhibits a tensile strength of ≥660 MPa at room temperature. Furthermore, the heat-affected zone of both the weld metal and the weld joint has a Charpy impact absorption energy vE at a test temperature of -196°C. -196 It is above 28J.

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