Submerged arc welding joint

Abstract

The present invention provides a high-Mn-containing steel material submerged arc weld joint that is inhibited from generating high-temperature cracking at the time of welding, has high strength, and has excellent extremely low-temperature impact toughness. A welded joint in which a high-Mn-containing steel material has a chemical composition containing, in mass%, C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 18.0 to 30.0%, P: 0.030% or less, S: 0.0070% or less, Al: 0.010 to 0.070%, Cr: 2.5 to 7.0%, N: 0.0050 to 0.0500%, and O: 0.0050% or less, with the balance consisting of Fe and inevitable impurities, and a weld metal has a chemical composition containing C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 15.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Al: 0.100% or less, Cr: 6.0 to 14.0%, and N: 0.100% or less, with the balance consisting of Fe and inevitable impurities.

Description

TECHNICAL FIELD

[0001] The present application relates to a submerged arc welded joint, and more particularly to a high Mn-containing steel material welded steel structure for use in an extremely low temperature environment, i.e., a welded joint in which generation of high-temperature cracking during welding is suppressed, which has high strength and excellent extremely low temperature impact toughness. BACKGROUND

[0002] In recent years, environmental restrictions are becoming more and more strict. Liquefied natural gas (hereinafter also referred to as LNG) does not contain sulfur, and is therefore called a clean fuel that does not generate atmospheric pollutants such as sulfur oxides, and its demand is increasing. For the transport or storage of LNG, a container (tank) for transporting or storing LNG is required to maintain excellent extremely low temperature impact toughness at a temperature of -162°C or lower, which is the liquefaction temperature of LNG.

[0003] However, from the necessity of maintaining excellent extremely low temperature impact toughness, as a material for a container (tank) or the like, in the past, an aluminum alloy, 9% Ni steel, an austenitic stainless steel, or the like has been used.

[0004] However, an aluminum alloy has a low tensile strength, and therefore requires a large plate thickness of the structure, and has a problem of poor weldability. In addition, 9% Ni steel requires the use of an expensive Ni-based material as a welding material, and therefore becomes economically disadvantageous. In addition, an austenitic stainless steel has a problem of being expensive, and also has a low base material strength.

[0005] In view of such problems, as a material for a container (tank) for transporting or storing LNG, a high Mn-containing steel (hereinafter also referred to as high Mn steel) containing about 10% to about 35% of Mn in mass% is being recently studied for application. A high Mn steel has the following characteristics: it is an austenite phase even at an extremely low temperature, does not undergo brittle fracture, and has a high strength compared to an austenitic stainless steel. Therefore, it is desired to develop a welding material that can stably weld such a high Mn-containing steel material.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] Furthermore, Patent Document 5 discloses a "manufacturing method for a high-strength welded joint for extremely low temperatures". The "manufacturing method for a high-strength welded joint for extremely low temperatures" disclosed in Patent Document 5 is as follows: It is characterized in that, for steels having a mass percentage content of C: 0.10–0.70%, Si: 0.05–1.00%, Mn: 18–30%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01–0.07%, Cr: 2.5–7.0%, N: 0.0050–0.0500%, and O (oxygen): 0.0050% or less, with the balance being Fe and unavoidable impurities, a mixture of steels having a mass percentage content of C: 0.2–0.8%, Si: 0.15% or less, and a mass percentage of O: 0.0050% or less, with the balance being Fe and unavoidable impurities, is used. The welding conditions for gas metal arc welding are adjusted such that the content of Mn and Cr in the solid core welding wire is less than that in the steel, and the dilution rate of the steel to the first layer of the multilayer weld metal is 35 to 60%, as defined by the following formula (1). The solid core welding wire is composed of Fe and unavoidable impurities.

[0011] Dilution rate (%) = 100 × {(content of constituent elements in the first layer of welding metal: mass%) - (content of constituent elements in the solid welding wire: mass%)} / {(content of constituent elements in the steel: mass%) - (content of constituent elements in the solid welding wire: mass%)}…(1)

[0012] It should be noted that, as needed, the steel may also contain, by mass percent, one or more of the following: Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, and / or one or two of the following: REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%. Furthermore, the solid welding wire may also contain, by mass percent, one or more of the following: V: 1.0% or less, Ti: 1.0% or less, and Nb: 1.0% or less. It is described that, according to the manufacturing method described in Patent Document 5, it is possible to easily manufacture high-strength welded steel structures suitable for use in extremely low temperature environments, with a room temperature yield strength (0.2% yield strength) of 400 MPa or more, and absorbable energy vE of Charpy impact test at a test temperature of -196°C. -196 Welded joints for multi-layered welded metal parts with high strength and excellent low-temperature impact toughness (28J or higher).

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Application Publication No. 2016-196703

[0016] Patent Document 2: Japanese Patent Application Publication No. 2017-071817

[0017] Patent Document 3: Japanese Patent Publication No. 2017-502842

[0018] Patent Document 4: International Publication No. WO2020 / 039643

[0019] Patent Document 5: International Publication No. WO2020 / 203335 Summary of the Invention

[0020] The problem that the invention aims to solve

[0021] However, according to the inventor's research, even when using the welding wires described in Patent Documents 3 and 4 and the dilution rate described in Patent Document 5 to perform submerged arc welding on the high-Mn steels described in Patent Documents 1 and 2, there is still a problem of high-temperature cracking during welding.

[0022] The purpose of this invention is to solve the problems of the prior art mentioned above and to provide a submerged arc welded joint that can suppress the generation of high-temperature cracking during welding and is suitable as a welded joint for high-Mn steel used in extremely low temperature environments, and which has both high strength and excellent extremely low temperature impact toughness.

[0023] It should be noted that "high strength" here refers to a yield strength (0.2% yield strength) of 400 MPa or higher at room temperature (25°C) for weld metal manufactured according to JIS Z 3111, and a tensile strength of 660 MPa or higher; and a tensile strength of 660 MPa or higher for weld joints manufactured according to JIS Z 3121. Furthermore, "excellent extremely low temperature impact toughness" refers to the absorbed energy vE of the Charpy impact test of the weld metal and the weld 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.

[0024] Methods for solving problems

[0025] To achieve the aforementioned objectives, the inventors first conducted an in-depth study on the factors influencing high-temperature cracking during submerged arc welding. The results revealed that segregation of phosphorus (P) into the final solidification region of the weld metal is a contributing factor to high-temperature cracking. Specifically, it was found that submerged arc welding, being a highly efficient welding method with a higher heat input (J) than gas metal arc welding (GMAW), has a slower cooling rate and a longer residence time in the high-temperature zone. Therefore, elemental diffusion is promoted, and P tends to segregate into the final solidification region. However, it was discovered that by including Cr of 6.0% or more by mass in the weld metal to form Cr phosphides in the liquid phase of the weld metal, the segregation of P into the final solidification region of the weld metal can be suppressed, thereby inhibiting the occurrence of high-temperature cracking.

[0026] Next, the steel composition and weld metal composition required for submerged arc welding (SAW) produced according to JIS Z 3121 to achieve both the desired high strength and the desired excellent low-temperature impact toughness were investigated. The results showed that the following SAW joints need to be produced: for the steel, the chemical composition of the high-Mn steel should be 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%. The chemical composition of O (oxygen) is reduced to less than 0.0050%. Furthermore, for the welding metal, the chemical composition of the welding metal for submerged arc welding is adjusted by mass% to a specific range of C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 15.0 to 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 of 6.0 to 14.0%, and N reduced to less than 0.100%.

[0027] This invention was completed based on further research into the above-mentioned insights.

[0028] The main points of this invention are as follows.

[0029] [1] A submerged arc welding joint, which is a submerged arc welding joint made of high-Mn steel, wherein,

[0030] 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%, O: less than 0.0050%, with the balance being Fe and unavoidable impurities.

[0031] 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%, N: less than 0.100%, with the balance being Fe and unavoidable impurities.

[0032] [2] According to the submerged arc welding joint 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%.

[0033] [3] According to the submerged arc welding joint 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%.

[0034] [4] Submerged arc welded joint according to any one of [1] to [3], wherein the above-mentioned high-Mn steel has a 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.

[0035] [5] The submerged arc welding joint 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: 3.50% or less and Ni: 10.00% or less.

[0036] [6] The submerged arc welding joint 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%.

[0037] [7] The submerged arc welding joint 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 0.0100%, REM: less than 0.020%.

[0038] [8] The submerged arc welded joint according to any one of [1] to [7], wherein the weld metal has a yield strength of 400 MPa or more and a tensile strength of 660 MPa or more at room temperature in a tensile test, the weld joint has a tensile strength of 660 MPa or more at room temperature, and the weld heat-affected zone of 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.

[0039] Invention Effects

[0040] According to the present invention, it is possible to easily manufacture submerged arc welded joints that suppress high-temperature cracking during welding of high-Mn steel, and that possess high strength and excellent low-temperature impact toughness, resulting in significant industrial benefits. Detailed Implementation

[0041] This invention relates to a submerged arc welded joint suitable for use as a cryogenic steel joint obtained by submerged arc welding high-Mn steels together. The welded joint of this invention is a submerged arc welded joint manufactured according to JIS Z 3121 and other standards, and is characterized by: suppressing high-temperature cracking during welding; having a yield strength (0.2% yield strength) of 400 MPa or more at room temperature (25°C) and a tensile strength of 660 MPa or more; that is, possessing both the high strength of a welded joint with a tensile strength of 660 MPa or more at room temperature (25°C) and the Charpy impact test absorption energy vE of the weld metal and the weld heat-affected zone of the welded joint manufactured according to JIS Z 3128 at a test temperature of -196°C. -196 It exhibits excellent ultra-low temperature impact toughness of over 28J.

[0042] Submerged arc welding

[0043] Submerged arc welding (hereinafter also referred to as "SAW") is a welding method in which an electrode wire is continuously supplied to a base material (in this invention, a high-Mn steel) through a pre-dispersed powdered flux, thereby generating an electric arc between the tip of the electrode wire and the base material, thus performing continuous welding. This submerged arc welding method has the advantage of increasing the deposition rate of the welding wire by applying a large current, thus enabling efficient welding.

[0044] [Steel with high Mn content]

[0045] First, let's describe the steel used. It should be noted that in the following, "%" in "chemical composition" refers to "mass %".

[0046] 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.

[0047] [C: 0.10~0.80%]

[0048] C is an inexpensive and important element that stabilizes the austenitic phase. 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 impact toughness at extremely low temperatures. 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.

[0049] [Si: 0.05~1.00%]

[0050] 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.

[0051] [Mn: 18.0~30.0%]

[0052] 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 low-temperature 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 low-temperature 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 the Mn content is preferably 29.0% or less, more preferably 28.0% or less, and even more preferably 27.0% or less.

[0053] [P: below 0.030%]

[0054] 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 the P content 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.

[0055] [S: below 0.0070%]

[0056] 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.

[0057] [Al: 0.010~0.070%]

[0058] 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.

[0059] [Cr: 2.5–7.0%]

[0060] 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 limited to a range of 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.

[0061] [N: 0.0050~0.0500%]

[0062] 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, reducing low-temperature impact toughness. Therefore, the N content is set to 0.0500% or less. It should be noted that preferably, it is 0.0400% or less, more preferably 0.0300% or less, and even more preferably 0.0200% or less.

[0063] [O (Oxygen): less than 0.0050%]

[0064] 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 the O content is preferably 0.0045% or less, more preferably 0.0040% or less. Furthermore, drastically reducing the O content to less than 0.0005% requires lengthy refining processes, leading to high refining costs. Therefore, from an economic point of view, the O content is preferably set to 0.0005% or more, more preferably 0.0006% or more.

[0065] [Optional selection components]

[0066] The above composition is the basic chemical composition, but it is also possible to add, as needed, one or more of the following chemical compositions selected from Mo: less than 2.00%, V: less than 2.0% and W: less than 2.00%, and containing one or two of the following chemical compositions selected from REM: 0.0010 to 0.0200% and B: 0.0005 to 0.0020%.

[0067] [Mo: less than 2.00%, V: less than 2.0%, and W: less than 2.00%]

[0068] 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, when Mo, V, and W are present, it is preferable that each of Mo, V, and W contains 0.001% or more. More preferably, it is preferable that each of Mo, V, and W contains 0.003% 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 of fracture and reducing cryogenic impact toughness. Therefore, when Mo, V, and W are present, the amounts are set as follows: Mo: 2.00% or less, V: 2.0% or less, and W: 2.00% or less. It should be noted that it is preferable that Mo: 1.70% or less, V: 1.7% or less, and W: 1.70% or less, and more preferably Mo: 1.50% or less, V: 1.5% or less, and W: 1.50% or less.

[0069] [REM: 0.0010–0.0200% and B: 0.0005–0.0020%]

[0070] 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 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.

[0071] 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, the REM content is 0.0180% or less.

[0072] 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. Preferably, it is 0.0018% or lower.

[0073] [Composition of Balance]

[0074] 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.

[0075] [Manufacturing method for high-Mn steel]

[0076] Hereinafter, a preferred manufacturing method for the high-Mn steel used in this invention will be described.

[0077] 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.

[0078] [Properties of Steel]

[0079] Here, preferred characteristics of the high-Mn steel used in this invention will be described.

[0080] 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, the tensile strength is more preferably 800 MPa or higher.

[0081] [Welding metal]

[0082] In this invention, the above-mentioned high-Mn steels are welded together by submerged arc welding to form a welded joint consisting of one or more layers of weld metal.

[0083] 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.

[0084] [C: 0.10~0.80%]

[0085] Carbon (C) is an element that increases the strength of weld metal through solid solution strengthening. Furthermore, C stabilizes the austenite phase, improving the extremely low temperature impact toughness of the weld metal. To achieve this effect, 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. However, when the content exceeds 0.80%, carbide precipitation occurs, the extremely low temperature impact toughness decreases, and high-temperature cracking during welding is more likely to occur. Therefore, the C content is set to 0.80% or less. 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.

[0086] [Si: 0.05~1.00%]

[0087] 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.

[0088] [Mn: 15.0~30.0%]

[0089] Mn is an inexpensive element that stabilizes the austenitic phase, and in this invention, it is required to contain 15.0% or more. When Mn is less than 15.0%, a ferrite phase is formed in the weld metal, significantly reducing the extremely 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.

[0090] [P: below 0.030%]

[0091] Phosphorus (P) is an element that induces high-temperature cracking due to grain boundary segregation. In this invention, it is preferable to reduce 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. 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 excessive reduction leads to a significant increase in refining costs. Therefore, the P content is preferably adjusted to 0.002% or more.

[0092] [S: below 0.030%]

[0093] Sulfur (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. Preferably, the S content is 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.

[0094] [Al: below 0.100%]

[0095] 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.

[0096] [Cr: 6.0~14.0%]

[0097] Cr acts as an element that stabilizes the austenite phase at extremely low temperatures, thereby improving the low-temperature impact toughness of the weld metal. In addition, Cr also increases the strength of the weld metal. Furthermore, Cr narrows the temperature range of the solid-liquid coexistence region of the molten metal, effectively suppressing high-temperature cracking. Moreover, 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 effects cannot be guaranteed. 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. The Cr content is preferably 13.0% or less, more preferably 12.0% or less, further preferably 11.5% or less, and most preferably 11.0% or less.

[0098] [N: below 0.100%]

[0099] Nitrogen (N) is an unavoidable element, but like carbon (C), it effectively contributes to increasing the strength of the weld metal and stabilizes 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 0.090% or less, more preferably 0.080% or less, even more preferably 0.070% or less, and most preferably 0.050% or less.

[0100] [Optional selection components]

[0101] 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 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: 0.0100% or less and REM: 0.020% or less as optional compositions.

[0102] [Mo: less than 3.50% and Ni: less than 10.00%]

[0103] Both Mo and Ni are elements that strengthen austenite grain boundaries, and one or both can be selected as needed.

[0104] 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, when 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. Furthermore, to exhibit the above effects, when Mo is present, the Mo content is preferably 0.005% or more, more preferably 0.30% or more, even more preferably 0.50% or more, and most preferably 1.00% or more.

[0105] 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. Thus, when Ni is present, it is preferable to set the Ni content to 0.02% or more, more preferably 0.05% or more, and even 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 the Ni content is preferably 8.00% or less, more preferably 7.00% or less, even more preferably 6.50% or less, and most preferably 6.00% or less.

[0106] [V: ≤1.60%, Ti: ≤1.00%, Nb: ≤1.00%, and W: ≤1.00%]

[0107] 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.

[0108] 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. However, when the content exceeds 1.60%, the carbides coarsen, becoming the initiation 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. It should be noted that the V content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.

[0109] 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, it is preferable to contain 0.001% or more Ti. The Ti content is more preferably set to 0.005% or more. When the Ti content exceeds 1.00%, the carbides coarsen, 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 it is preferably 0.80% or less, more preferably 0.60% or less, and even more preferably 0.50% or less.

[0110] 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, Nb content is preferably 0.001% or more. More preferably, the Nb content is 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 extremely low-temperature 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 0.70% or less, further preferably 0.60% or less, and most preferably 0.50% or less.

[0111] 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 cryogenic 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, when W is present, 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 cryogenic impact toughness. Therefore, when W is present, the W content is set to 1.00% or less. It should be noted that 0.80% or less is preferred. More preferably, 0.60% or less, and even more preferably 0.40% or less.

[0112] [Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.0100%, and REM: less than 0.020%]

[0113] Cu is an element that contributes to the stabilization of austenite. Furthermore, Ca, B, and REM are elements that contribute to improved machinability; one or more of these can be selected as needed. Cu stabilizes the austenite phase, even at extremely low temperatures, thus improving the low-temperature impact toughness of the weld metal. To achieve this effect, a content of 0.01% or more is preferred. A Cu content of 0.04% or more is more preferable. However, when Cu exceeds 1.00% and is present in large quantities, segregation occurs during solidification, inducing high-temperature cracking during welding. Therefore, when Cu is present, the Cu content is set to 1.00% or less. A Cu content of 0.80% or less is preferred.

[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 the Ca content is preferably 0.008% or less, more preferably 0.006% or less.

[0115] Boron (B) is an unavoidable element that segregates at austenite grain boundaries. When B content exceeds 0.0100%, boron nitride forms at the austenite grain boundaries, reducing strength. Furthermore, boron nitride becomes a fracture initiation point, decreasing low-temperature impact toughness. Therefore, in cases containing B, the B content is set to 0.0100% or less. It should be noted that 0.0080% or less is preferred. More preferably, 0.0050% or less. On the other hand, excessive reduction leads to increased refining costs; therefore, in cases containing B, 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 the REM content 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, and Re, which are permissible if their combined content is less than 0.0100%. 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.

[0119] It should be noted that the chemical composition of the weld metal is mainly determined by the ratio of the base metal to the welding materials such as SAW welding wire.

[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 steel pieces after beveling are submerged arc welded together to form one or more layers of weld metal, thus creating a welded joint. Any welding material used can form a weld metal portion with the desired properties; welding wire with the aforementioned chemical composition can be used initially, and the type of welding wire or flux is not particularly limited.

[0124] It should be noted that, as welding wire, there are solid welding wires or flux-cored welding wires with flux encased inside; in this invention, either type of welding wire can be used. When using flux-cored welding wire, it is manufactured in a manner where the total composition of the steel sheath, metal powder, and flux powder used is taken as the composition of the target welding material.

[0125] [Manufacturing method of welding wire]

[0126] Furthermore, the manufacturing method of SAW welding wire (solid wire and flux-cored wire) will be explained.

[0127] Solid core welding wire is preferably produced by: casting process, in which molten steel with the target composition is melted in a commonly used smelting furnace such as an electric furnace or a vacuum melting furnace and cast in a mold of a specified shape; heating process, in which the obtained steel ingot is heated to a specified temperature; hot rolling process, in which the heated steel ingot is hot rolled to form a steel raw material (bar) of a specified shape; and cold rolling process, in which the obtained steel raw material (bar) is cold rolled (cold drawn wire processing) more than twice and annealed at a temperature of 900-1200°C as needed to produce welding wire of the desired size. It should be noted that the composition of the solid welding wire is not particularly limited, but preferably by mass percent: C: 0.20-0.80%, Si: 0.20-1.00%, Mn: 16.0-30.0%, P: less than 0.030%, S: less than 0.030%, Cr: 6.0-15.0%, Mo: 0.01-4.0%, Ni: 0.01-10.0%, O: less than 0.050%, and N: less than 0.150%.

[0128] Furthermore, regarding flux-cored welding wire, it is preferable to use, for example, a thin steel sheet (0.5 mm thick) having a composition of 0.05–0.20% C, 0.15–0.30% Si, 0.2–1.2% Mn, and the balance Fe as the steel outer sheath material, and to perform cold bending in the width direction to form a U-shape. Then, in a manner that achieves the desired welding wire composition, metal powder with adjusted composition and welding flux powder are sealed into the obtained steel outer sheath, and the wire is drawn by cold working to produce a flux-cored welding wire for SAW.

[0129] The composition of the aforementioned metal powder is not particularly limited, and it is made into metal powder or alloy powder having a metal composition that supplements the composition of the steel outer sheath material to form the overall composition of the welding wire. The composition of the flux-cored wire is not particularly limited, but preferably, by mass percent, it is 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%, O: 0.200% or less, and N: 0.100% or less. Furthermore, the composition of the flux powder for the welding wire is not particularly limited, and it can be flux powder having a composition equal to or similar to the welding flux described later.

[0130] [Welding Flux]

[0131] When using the above-mentioned SAW welding wires (solid wire and flux-cored wire), the welding flux used is not particularly limited, and any commonly known sintered flux or molten flux can be used. It should be noted that, as a specific chemical composition, a powder material containing SiO2: 20-40%, MnO: 8-15%, TiO2: 5-10%, Al2O3: 10-20%, MgO: 20-30%, etc., can be used. As an example, there is a powder material with a composition of 38% SiO2-11% MnO-8% TiO2-16% Al2O3-27% MgO. However, in this invention, the welding flux is not limited to this.

[0132] 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.

[0133] Example

[0134] 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.

[0135] 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 materials. Next, the obtained steel raw materials were 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 (containing high Mn) with a thickness of 20 mm.

[0136] Next, the molten steel with the chemical composition (welding wire composition) shown in Table 2 is melted in a vacuum melting furnace and cast to obtain steel ingots. The obtained steel ingots are heated to 1200°C and then hot-rolled and subsequently cold-rolled to produce solid welding wire for submerged arc welding with a diameter of 4.0 mm.

[0137] In addition, a steel outer sheath and a flux-cored welding wire containing metal powder and flux powder are separately manufactured. A thin steel sheet (0.5 mm thick) with a composition of 0.1% C, 0.2% Si, 0.5% Mn, and the balance Fe is used as the raw material for the steel outer sheath. This material is cold-bent in the width direction to form a U-shape. Then, with the chemical composition shown in Table 3, the metal powder and flux powder, whose composition has been adjusted, are sealed into the obtained steel outer sheath. The wire is then cold-drawn to produce a flux-cored welding wire (diameter: 3.2 mm). That is, the chemical composition shown in Table 3 is the total value of the steel outer sheath, metal powder, and flux powder.

[0138] Next, using the obtained steel (20mm thick), a butt joint constraint was formed according to JIS Z 3121 to create a 45° V-groove. The obtained solid or flux-cored welding wire was then used as the welding material for submerged arc welding, forming weld metal within the groove to obtain a welded joint. It should be noted that a flux composed of 38% SiO2, 11% MnO, 8% TiO2, 16% Al2O3, and 27% MgO was used during welding.

[0139] Submerged arc welding is performed as follows: For the steels shown in Table 1, use the solid wires (4.0 mm φ diameter) or flux-cored wires (3.2 mm diameter) as shown in Tables 2 and 3, in the combination shown in Table 4, without preheating, in a downward orientation, set the current to 450–650 A (DCEP), the voltage to 28–36 V, the welding speed to 20 cm / min, and the interpass temperature to 100–150 °C.

[0140] [Property Evaluation of High-Mn Steel]

[0141] 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.

[0142] [Compositional Analysis of Weld Metal]

[0143] Regarding the chemical composition of the weld metal, shavings were cut 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.

[0144] [Evaluation of high-temperature crack resistance]

[0145] 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 microscopic images 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 high-temperature cracking is observed, the high-temperature crack resistance is considered excellent and rated as "○".

[0146] [Evaluation of Weld Metal Properties]

[0147] 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 this value is taken as the cryogenic impact toughness of the weld metal of the weld joint.

[0148] [Evaluation of Welded Joint Characteristics]

[0149] In addition, tensile tests on the welded joints at room temperature were conducted in accordance with JIS Z 3121. For the test pieces, test pieces No. 1A were cut with the weld axis as the center of the parallel length of the test piece, perpendicular to the weld axis, and their thickness was equal to the total thickness of the welded joint. Three tests were performed, and the average value obtained was taken as the tensile characteristic of the welded joint.

[0150] In addition, Charpy impact tests were conducted on the weld heat-affected zone of the weld joint in accordance with JIS Z 3128. The V-notch of the test piece was perpendicular to the surface of the base metal, 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 tests were conducted, and the average value obtained was taken as the cryogenic impact toughness of the weld heat-affected zone of the weld joint.

[0151] Regarding the target values ​​of this invention, as described above, the yield strength (0.2% yield strength) of high-Mn steel at room temperature (25°C) is above 400 MPa, and the absorbed energy vE in the Charpy impact test at a test temperature of -196°C is... -196 The energy required is 28J or higher. Furthermore, the yield strength (0.2% yield strength) of the weld metal at room temperature (25℃) is 400MPa or higher, and its tensile strength is 660MPa or higher. The tensile strength of the weld joint at room temperature (25℃) is also 660MPa or higher. Additionally, the absorbed energy vE of the Charpy impact test on the weld metal and the weld heat-affected zone of the weld joint at a test temperature of -196℃ is also considered. -196 The value is above 28J. The results are shown in Tables 1-5.

[0152]

[0153]

[0154]

[0155]

[0156] [Table 5]

[0157]

[0158] Underlined: indicates outside the scope of this invention.

[0159] The examples of this invention are all welded joints of weld metals that do not produce high-temperature cracking during welding, have excellent high-temperature cracking resistance, and have good weld appearance.

[0160] Furthermore, the examples of this invention all involve Charpy impact tests on high-Mn steel with a yield strength (0.2% yield strength) of 400 MPa or higher at room temperature (25°C) and a test temperature of -196°C, measuring the absorbed energy vE. -196 The Charpy impact test absorption energy vE of the weld metal and the weld heat-affected zone at a test temperature of -196℃ is ≥28J, the yield strength (0.2% yield strength) of the weld metal at room temperature (25℃) is ≥400MPa, and its tensile strength is ≥660MPa, the tensile strength of the weld joint at room temperature (25℃) is ≥660MPa, and the absorbed energy vE of the weld heat-affected zone of the weld metal and the weld joint is ≥660MPa.-196 Welded joints of weld metal with a strength of 28J or higher, possessing both high strength and excellent low-temperature impact toughness.

[0161] On the other hand, in comparative examples outside the scope of this invention, high-temperature cracking and reduced high-temperature cracking resistance occurred, or the yield strength (0.2% yield strength) of the weld metal at room temperature (25°C) was less than 400 MPa, or the tensile strength was less than 660 MPa, or the tensile strength of the weld joint at room temperature (25°C) 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 was... -196 Less than 28J, no weld metal with both high strength and excellent low-temperature impact toughness was obtained.

[0162] As for welded joint No. 13, which serves as a comparative example, the Mn content of the weld metal is lower than that of the present invention. Therefore, the stability of the austenitic phase in the weld metal is low, and consequently, the absorbed energy vE of the weld metal at the test temperature of -196°C is low. -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0163] Furthermore, regarding welded joint No. 14, which serves as a comparative example, the Si and Mn content of the weld metal is higher than the range of the present invention, while the Cr content of the weld metal is lower than the range of the present invention. Therefore, Si, Mn, and P segregate towards the final solidification portion during welding, resulting in high-temperature cracking. Additionally, the absorbed energy vE of the weld metal at a test temperature of -196°C... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0164] Furthermore, regarding weld joint No. 15, which serves as a comparative example, the S and Mo content of the weld metal is higher than that of the present invention, thus generating MnS and Mo carbides that become fracture initiation points. The absorbed energy vE of the weld metal at a test temperature of -196°C... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0165] Furthermore, regarding weld joint No. 16 as a comparative example, the Cr content of 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, and the tensile strength was less than 660 MPa, failing to ensure the desired high strength. In addition, segregation of the final solidification portion during P-axis welding could not be suppressed, resulting in high-temperature cracking. Moreover, 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.

[0166] Furthermore, regarding welded joint No. 17, which serves as a comparative example, the C content of the weld metal is higher than the range of this invention, resulting in the formation of carbides in the weld metal and causing high-temperature cracking. Additionally, the absorbed energy vE at the test temperature of -196°C... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0167] Furthermore, regarding joint No. 18, which serves as a comparative example, the Cr content of the weld metal is higher than that of the present invention, thus generating Cr carbides. The absorbed energy vE of the weld metal at a test temperature of -196°C... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0168] Furthermore, regarding weld joint No. 22, which serves as a comparative example, the P content of the weld metal is higher than the range of the present invention. P segregates in the final solidification portion of the weld metal, resulting in high-temperature cracking. In addition, the S content of the weld metal is higher than the range of the present invention, leading to the precipitation of MnS, which would become the fracture initiation point. Therefore, the absorbed energy vE of the weld metal at the test temperature of -196°C is... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0169] Furthermore, regarding weld joint No. 23 as a comparative example, the Cr content of 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, and the tensile strength was less than 660 MPa, failing to ensure the desired high strength. In addition, segregation of the final solidification portion during P-axis welding could not be suppressed, resulting in high-temperature cracking. Moreover, the absorbed energy vE of the weld metal at the test temperature of -196°C was... -196 The J value was less than 28, which failed to ensure the expected excellent cryogenic impact toughness. In addition, the tensile strength of the welded joint was less than 660 MPa, which failed to ensure the expected high strength.

[0170] Furthermore, regarding weld joint No. 26 as a comparative example, the Si and Mn content of the weld metal is higher than the range of the present invention, while the Cr content of the weld metal is lower than the range of the present invention. Therefore, Si, Mn, and P segregate towards the final solidification portion during welding, resulting in high-temperature cracking. The 0.2% yield strength of the weld metal is less than 400 MPa, failing to ensure the desired high strength. Additionally, the absorbed energy vE of the weld metal at the test temperature of -196°C is... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

[0171] Furthermore, regarding 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 absorbed energy vE of the weld heat-affected zone at the test temperature of -196°C is... -196 Less than 28J, failing to ensure the expected excellent low-temperature impact toughness.

Claims

1. A submerged arc welding joint, which is a submerged arc welding joint made of high-Mn steel, wherein, The 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%, 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%, N: less than 0.100%, with the balance being Fe and unavoidable impurities.

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

3. The submerged arc welding joint 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 submerged arc welding joint 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 submerged arc welding joint according to claim 1, wherein, The chemical composition of the weld metal, by mass%, also contains components selected from at least one group from groups C to E below. Group C: Selected from one or both of the following: Mo: less than 3.50% and Ni: less than 10.00%. Group D: Selected from one or more of the following: V: 1.60% or less, Ti: 1.00% or less, Nb: 1.00% or less, W: 1.00% or less. Group E: Selected from one or more of the following: Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.0100%, REM: less than 0.020%.

6. The submerged arc welded joint according to any one of claims 2 to 4, wherein, The chemical composition of the weld metal, by mass%, also contains components selected from at least one group from groups C to E below. Group C: Selected from one or both of the following: Mo: less than 3.50% and Ni: less than 10.00%. Group D: Selected from one or more of the following: V: 1.60% or less, Ti: 1.00% or less, Nb: 1.00% or less, W: 1.00% or less. Group E: Selected from one or more of the following: Cu: less than 1.00%, Ca: less than 0.010%, B: less than 0.0100%, REM: less than 0.020%.

7. The submerged arc welded joint according to any one of claims 1 to 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 Charpy impact absorption energy vE of the weld metal and the weld joint's heat-affected zone at a test temperature of -196°C is [missing value]. -196 It is above 28J.

8. The submerged arc welding joint according to claim 6, 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 Charpy impact absorption energy vE of the weld metal and the weld joint's heat-affected zone at a test temperature of -196°C is [missing value]. -196 It is above 28J.

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