Submerged arc welding wire and method for manufacturing welded joint portion using the same

Abstract

The present invention provides a submerged arc welding wire which is suitable as a welding material for high Mn steel used in an extremely low temperature environment and which has excellent high-temperature crack resistance with the generation of high-temperature cracks being suppressed at the time of welding. A submerged arc welding wire having the following composition: C: 0.20 to 0.80%, Si: 0.15 to 0.90%, Mn: 15.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Cr: 6.0 to 15.0%, N: 0.120% or less, the balance consisting of Fe and inevitable impurities. Note that, as necessary, one or both of Ni and Mo can be contained, one or two or more of V, Ti and Nb can be contained, and one or two or more of Cu, Al, Ca and REM can be contained in addition thereto.

Description

Technical Field

[0001] This invention relates to welding wire for submerged arc welding, and particularly to a welding wire for welding high-Mn steel used in extremely low temperature environments that exhibits excellent resistance to high-temperature cracking and suppresses the formation of high-temperature cracks during welding, and a method for manufacturing a welded joint using the welding wire. Background Technology

[0002] Submerged arc welding (SAW) is a welding method in which an electrode wire is continuously supplied to a powdered flux pre-distributed on the base material, thereby generating an electric arc between the tip of the electrode wire and the base material, and thus continuously welding is performed. SAW can efficiently produce weld metal with stable weldability and excellent mechanical properties, and is therefore used in various large structures such as shipbuilding, construction, and bridges.

[0003] In recent years, environmental regulations have become more stringent. Furthermore, 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. In addition, for the transportation or storage of LNG, the containers (tanks) used for transporting or storing LNG need to maintain excellent cryogenic impact toughness at temperatures below the liquefaction temperature of LNG, which is -162°C.

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

[0005] However, aluminum alloys have low tensile strength, necessitating thicker structural plates and exhibiting poor weldability. Furthermore, 9% Ni steel requires expensive Ni-based welding materials, making it economically disadvantageous. Additionally, austenitic stainless steels are expensive and have low base metal strength.

[0006] Starting from this problem, 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 has recently been studied. High-Mn steel has the following characteristics: it remains an 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 methods and welding materials that can stably weld such high-Mn steel.

[0007] To address such expectations, Patent Document 1, for example, proposes "a high-strength welded joint with excellent low-temperature impact toughness and a flux-cored arc welding wire for use therein". The flux-cored arc welding wire described in Patent Document 1 contains, by weight percent: C: 0.15–0.8%, Si: 0.2–1.2%, Mn: 15–34%, Cr: ≤6%, Mo: 1.5–4%, S: ≤0.02%, P: ≤0.02%, B: ≤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%, with the balance being Fe and other unavoidable impurities. When welding with the flux-cored arc welding wire described in Patent Document 1, it is possible to effectively obtain a weld joint with excellent low-temperature toughness, Charpy impact absorption energy of 28J or more at a test temperature of -196°C, and high strength of 400MPa or more at room temperature. In addition, by adjusting the welding wire composition to Mo: 1.5% or more, it is possible to ensure a weld joint with excellent resistance to high-temperature cracking.

[0008] Furthermore, Patent Document 2 discloses a "solid welding wire for gas metal arc welding". The solid welding wire for gas metal arc welding described in Patent Document 2 is a welding wire containing, by mass percent: 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 requirements, it may contain one or more of V, Ti, and Nb, and one or more of Cu, Al, Ca, and REM. If the solid welding wire for gas metal arc welding described in Patent Document 2 is used for welding, it is possible to produce high-strength materials with low fume generation and a room temperature yield strength (0.2% yield strength) of over 400 MPa, and a Charpy impact test absorption energy vE at a test temperature of -196°C. -196 It is a welded joint with high strength above 28J and excellent impact toughness at extremely low temperatures.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Publication No. 2017-502842

[0012] Patent Document 2: International Publication No. WO2020 / 039643 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, according to the inventor's research, the technologies described in Patent Documents 1 and 2 have the problem of generating high-temperature cracks during welding.

[0015] The purpose of this invention is to solve the problems of the prior art mentioned above, and to provide a welding wire that can suppress the generation of high-temperature cracks during welding, is suitable as a welding material for high-Mn steel used in extremely low-temperature environments, and is suitable for submerged arc welding that can stably manufacture weld joints with both high strength and excellent extremely low-temperature toughness.

[0016] It should be noted that "high strength" here refers to the room temperature yield strength (0.2% yield strength) of the weld metal prepared according to JIS Z 3111 being 400 MPa or higher. Furthermore, "excellent low-temperature toughness" refers to the Charpy impact test absorption energy vE of the weld metal prepared according to JIS Z 3111 at a test temperature of -196°C. -196 For cases with a value of 28J or higher.

[0017] Methods for solving problems

[0018] To achieve the above objectives, the inventors first conducted an in-depth study on the factors influencing high-temperature cracking during submerged arc welding of high-Mn steel. The results showed that segregation of the final solidification portion of the P-axis weld metal is a contributing factor to high-temperature cracking. Furthermore, it was found that if the welding wire contains 6.0% by mass or more Cr, Cr phosphides are formed in the liquid phase of the weld metal, which suppresses segregation of the final solidification portion of the P-axis weld metal and also inhibits the formation of high-temperature cracks.

[0019] Furthermore, the composition of submerged arc welding wire required to produce a weld metal with both desired high strength and desired excellent low-temperature toughness according to JIS Z 3111 was studied. It was found that a welding wire with a composition of C adjusted to 0.20–0.80% and Si adjusted to 0.15–0.90% by mass, further adjusted to Mn to 15.0–30.0% and Cr adjusted to 6.0–15.0% by mass, and reduced P to 0.030% or less, S to 0.030% or less, and N to 0.120% or less, and a welded joint using this welding wire, were required.

[0020] This invention was completed based on the above insights and further research, and the main points of this invention are as follows.

[0021] [1] A submerged arc welding wire has the following composition: by mass % it contains C: 0.20-0.80%, Si: 0.15-0.90%, Mn: 15.0-30.0%, P: less than 0.030%, S: less than 0.030%, Cr: 6.0-15.0%, N: less than 0.120%, and the balance is composed of Fe and unavoidable impurities.

[0022] [2] The submerged arc welding wire according to [1], wherein, based on the composition, it further contains, by mass%, one or both selected from Ni: less than 10.00% and Mo: less than 3.50%.

[0023] [3] The submerged arc welding wire according to [1] or [2], wherein, based on the composition, it further contains, by mass%, one or more of V: ​​less than 1.0%, Ti: less than 1.0% and Nb: less than 1.00%.

[0024] [4] The submerged arc welding wire according to any one of [1] to [3] is characterized in that, based on the composition, it further contains, by mass%, one or more of the following: Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less and REM: 0.020% or less.

[0025] [5] The submerged arc welding wire according to any one of [1] to [4] is characterized in that the welding wire is a solid welding wire or a flux-cored welding wire.

[0026] [6] A method for manufacturing a welded joint, wherein a high-Mn steel is submerged arc welded using any one of [1] to [5].

[0027] [7] In the manufacturing method of the welded joint according to [6], the Mn content of the above-mentioned high Mn steel is 15.0 to 30.0% by mass.

[0028] [8] According to the manufacturing method of the welded joint described in [7], the high Mn steel has the following composition: by mass % it contains C: 0.10 to 0.80%, Si: 0.05 to 1.00%, Mn: 15.0 to 30.0%, P: less than 0.030%, S: less than 0.030%, Cr: 2.5 to 15.0%, N: less than 0.120%, with the balance consisting of Fe and unavoidable impurities.

[0029] [9] According to the manufacturing method of the welded joint described in [8], the high Mn steel, based on the composition, further contains, by mass %, one or two of Ni: less than 10.00% and Mo: less than 3.50%.

[0030]

[10] The method for manufacturing a welded joint according to any one of [8] to [9], wherein the high Mn steel, based on the composition, further contains, by mass %, one or more of V: ​​2.0% or less, Ti: 1.0% or less and Nb: 1.00% or less.

[0031]

[11] The method for manufacturing a welded joint according to any one of [8] to

[10] , wherein the high Mn steel, based on the composition, contains, by mass %, one or more of the following: Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less.

[0032] Invention Effects

[0033] The submerged arc welding wire of the present invention, as a welding material for high-Mn steel, can suppress high-temperature cracking during SAW and can easily manufacture welded joints with high strength and excellent low-temperature toughness, which has significant industrial benefits. Detailed Implementation

[0034] This invention relates to a welding wire suitable for submerged arc welding of high-Mn steel. Using the welding wire of this invention, high-temperature cracking can be suppressed during submerged arc welding of high-Mn steels. Furthermore, the welding wire of this invention, produced by submerged arc welding according to JIS Z3111, produces a weld metal with both high strength (at 0.2% yield strength at room temperature of 400 MPa or more) and excellent low-temperature toughness (with a Charpy impact test absorption energy of 28 J or more at a test temperature of -196°C). This allows for the manufacture of welding materials that produce weld joints with both high strength and excellent low-temperature toughness.

[0035] Submerged arc welding

[0036] As described above, submerged arc welding (SAW) is a welding method in which an electrode wire is continuously supplied to a powdered flux pre-dispersed on the base material, thereby generating an electric arc between the tip of the electrode wire and the base material, and thus performing continuous welding. This submerged arc welding has the advantage of increasing the deposition rate of the welding wire by applying a large current, thus enabling efficient welding.

[0037] As welding wire, there are solid welding wires or flux-cored welding wires with flux encased inside the wire. In this invention, any type of welding wire can be used. When using flux-cored welding wire, it is manufactured in such a way that the total composition of the steel sheath, metal powder, and flux powder used is taken as the composition of the target welding material.

[0038] As an example of submerged arc welding, for steel plates or steel materials (thickness: 6–100 mm) as the base material, according to JIS Z 3111, two steel plates or steel materials are butt-jointed to form a 45° V-groove. Using prepared solid welding wire (approximately 4.0 mm φ in diameter) or flux-cored welding wire (approximately 3.2 mm φ in diameter), after distributing flux, without preheating, the welding is performed in a flat welding position under the following conditions: current 350–650 A (DCEP), voltage 28–36 V, welding speed 20–80 cm / min, welding heat input 0.7–8.0 kJ / mm, and inter-pass temperature 100–150 °C. More preferably, the thickness of the base material steel plate or steel material is 9–80 mm. Even more preferably, the thickness is 9–60 mm.

[0039] [Basic Components of Welding Wire]

[0040] The submerged arc welding wire of the present invention has the following basic composition: by mass%, it contains C: 0.20-0.80%, Si: 0.15-0.90%, Mn: 15.0-30.0%, P: less than 0.030%, S: less than 0.030%, Cr: 6.0-15.0%, N: less than 0.120%, with the balance consisting of Fe and unavoidable impurities. First, the rationale for limiting the basic composition will be explained. It should be noted that, in the following, "%" in the composition refers to "mass%".

[0041] [C: 0.20~0.80%]

[0042] 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 0.20% or more is required. However, when the content exceeds 0.80%, carbide precipitation occurs, reducing the extremely low temperature toughness and making welding cracks (high temperature cracks) more likely to occur during welding. Therefore, the C content is limited to the range of 0.20% to 0.80%. It should be noted that C is preferably 0.40% or more. Furthermore, C is preferably 0.60% or less. More preferably, C is 0.45% or more. Preferably, C is 0.55% or less.

[0043] [Si: 0.15~0.90%]

[0044] Si acts as a deoxidizer, improving the yield of Mn, increasing the viscosity of the molten metal, and stabilizing the weld bead shape. To achieve these effects, a content of 0.15% or more is required. However, a content exceeding 0.90% reduces the extremely low temperature toughness of the weld metal. Furthermore, Si segregates during solidification, forming a liquid phase at the solidification unit interface, which reduces resistance to high-temperature cracking. Therefore, the Si content is limited to the range of 0.15% to 0.90%. Preferably, Si is 0.20% or more. More preferably, Si is 0.70% or less. More preferably, Si is 0.30% or more. Even more preferably, Si is 0.60% or less.

[0045] [Mn: 15.0~30.0%]

[0046] Mn is an inexpensive element that stabilizes the austenitic phase, and it 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 toughness at extremely low temperatures. On the other hand, when Mn exceeds 30.0%, excessive Mn segregation occurs during solidification, inducing weld cracks (high-temperature cracks). Therefore, the Mn content is limited to the range of 15.0% to 30.0%. Preferably, Mn is 18.0% or more. Preferably, Mn is 27.0% or less. More preferably, Mn is 20.0% or more. More preferably, Mn is 26.0% or less.

[0047] [P: below 0.030%]

[0048] Phosphorus (P) is an element that causes segregation at grain boundaries and induces high-temperature cracks. It is preferable to minimize its content, but a content below 0.030% is permissible. Therefore, P is limited to 0.030% or less. It should be noted that excessive reduction leads to a significant increase in refining costs. Therefore, P is preferably adjusted to 0.003% or more. More preferably, P is 0.003% or more. Even more preferably, P is 0.020% or less.

[0049] [S: below 0.030%]

[0050] Sulfur (S) exists in the weld metal as MnS, a sulfide inclusion. MnS becomes the initiation point for fracture, thus reducing cryogenic toughness. Therefore, S is limited to 0.030% or less. It should be noted that excessive reduction leads to increased refining costs. Therefore, S is preferably adjusted to 0.001% or more. More preferably, S is 0.001% or more. More preferably, S is 0.020% or less.

[0051] [Cr: 6.0~15.0%]

[0052] Cr acts as an element that stabilizes the austenite phase at extremely low temperatures, thereby improving the cryogenic toughness of the weld metal. Additionally, it increases the strength of the weld metal. Furthermore, it effectively narrows the temperature range of the solid-liquid coexistence region of the molten metal, suppresses the formation of high-temperature cracks, and also inhibits high-temperature cracks caused by phosphorus (P) by forming Cr phosphides in the liquid phase. To achieve these effects, a Cr content of 6.0% or more is required. When Cr is less than 6.0%, the above effects cannot be guaranteed. On the other hand, when the Cr content exceeds 15.0%, Cr carbides are formed, leading to a decrease in cryogenic toughness. Therefore, the Cr content is limited to the range of 6.0% to 15.0%. It should be noted that a Cr content greater than 7.0% is preferred. A Cr content of 15.0% or less is preferred. A Cr content of 8.0% or more is more preferred. A Cr content of 13.0% or less is even more preferred.

[0053] [N: below 0.120%]

[0054] Nitrogen (N) is an unavoidable element that, like carbon (C), effectively contributes to increasing the strength of the weld metal and stabilizes the austenite phase, thus contributing to the stable improvement of low-temperature toughness. This effect becomes significant when the content is 0.003% or more. On the other hand, when the content exceeds 0.120%, nitrides are formed, and low-temperature toughness decreases. Therefore, the content of N is limited to 0.120% or less. It should be noted that N is preferably 0.004% or more. Preferably, N is 0.080% or less. More preferably, N is 0.004% or more. Even more preferably, N is 0.060% or less.

[0055] [Optional Ingredients]

[0056] In the welding wire of the present invention, the above-mentioned components are the basic components. However, in the present invention, based on the above basic components, one or two components selected from Ni: 10.00% or less and Mo: 3.50% or less may be selected as optional components. In addition, one or more components selected from V: 1.0% or less, Ti: 1.0% or less, and Nb: 1.00% or less may be selected. Furthermore, one or more components selected from Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less may be selected.

[0057] [Ni: less than 10.00% and Mo: less than 3.50%]

[0058] Both Ni and Mo are elements that strengthen austenite grain boundaries, and one or both can be selected as needed.

[0059] [Ni: below 10.00%]

[0060] Ni is an element that strengthens austenite grain boundaries, causing segregation at these boundaries and improving low-temperature toughness. Additionally, Ni has a stabilizing effect on the austenite phase; therefore, further increasing its content stabilizes the austenite phase and improves the low-temperature toughness of the weld metal. However, Ni is an expensive element, and its content exceeding 10.00% becomes economically unfavorable. Therefore, Ni is preferably limited to 10.00% or less. It should be noted that a Ni content of 8.00% or less is more preferred. A Ni content of 6.00% or less is even more preferred. A Ni content of 1.00% or more is preferred.

[0061] [Mo: 3.50% or less]

[0062] 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. On the other hand, when its content exceeds 3.50%, it can sometimes precipitate as carbides, becoming the initiation point for fracture and leading to a decrease in extremely low temperature toughness. Therefore, Mo is preferably limited to a range of 3.50% or less. It should be noted that a range of 3.00% or less is more preferred. Further preferred is 1.00% or more. Preferably, Mo is 3.00% or less.

[0063] [V: below 1.0%, Ti: below 1.0%, and Nb: below 1.00%]

[0064] V, Ti, and Nb are all elements that promote the formation of carbides and help improve the strength of weld metal. One or more of them can be selected as needed.

[0065] [V: below 1.0%]

[0066] V is a carbide-forming element that causes the precipitation of fine carbides, contributing to increased strength of the weld metal. To achieve this effect, a content of 0.001% or more is preferred. However, when the content exceeds 1.0%, the carbides coarsen, becoming the initiation point for fracture and leading to a decrease in cryogenic toughness. Therefore, when V is present, its content is preferably limited to 1.0% or less. More preferably, V is 0.002% or more. Preferably, V is 0.8% or less. Further preferably, V is 0.005% or more. Preferably, V is 0.6% or less.

[0067] [Ti: below 1.0%]

[0068] Ti is a carbide-forming element that promotes the precipitation of fine carbides, contributing to increased strength in the weld metal. Furthermore, the precipitation of carbides at the solidification unit interfaces of the weld metal helps suppress the initiation of high-temperature cracks. To achieve this effect, a content of 0.001% or more is preferred. However, when the content exceeds 1.0%, the carbides coarsen, becoming the initiation point for fracture and leading to a decrease in extremely low-temperature toughness. Therefore, when Ti is present, the Ti content is preferably limited to 1.0% or less. It should be noted that 0.002% or more Ti is more preferred. 0.8% or less Ti is more preferred. 0.005% or more Ti is even more preferred. 0.6% or less Ti is more preferred.

[0069] [Nb: below 1.00%]

[0070] Nitrogen (Nb) is a carbide-forming element that contributes to the strength of weld metal by causing carbide precipitation. Furthermore, the precipitation of carbides at the solidification unit interfaces of the weld metal helps suppress the formation of high-temperature cracks. To achieve this effect, a content of 0.001% or more is preferred. However, when it exceeds 1.00%, the carbides coarsen, becoming the initiation point for fracture and leading to a decrease in extremely low-temperature toughness. Therefore, when Nb is present, it is preferable to limit Nb content to 1.00% or less. More preferably, Nb content is 0.002% or more. Preferably, Nb content is 0.80% or less. Further preferably, Nb content is 0.005% or more. Nb content is preferably 0.60% or less.

[0071] [Cu: less than 1.00%, Al: less than 0.100%, Ca: less than 0.010%, and REM: less than 0.020%]

[0072] Cu is an element that helps stabilize austenite, Al is an element that helps stabilize weld bead shape, and Ca and REM are elements that help improve machinability. You can choose to include one or more of these elements as needed.

[0073] [Cu: less than 1.00%]

[0074] Cu is an element that stabilizes the austenite phase, even at extremely low temperatures, thereby improving the low-temperature toughness of the weld metal. To achieve this effect, a content of 0.01% or more is preferred. However, when the content 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 preferably limited to 1.00% or less. It should be noted that 0.02% or more Cu is more preferred. 0.90% or less Cu is more preferred. 0.05% or more Cu is even more preferred. 0.60% or more Cu is preferred.

[0075] [Al: below 0.100%]

[0076] Al acts as a deoxidizer, playing a crucial role in increasing the viscosity of the molten metal and stably maintaining the weld bead shape. Furthermore, Al narrows the temperature range of the solid-liquid coexistence region of the molten metal, helping to suppress the formation of high-temperature cracks in the weld metal. This effect becomes significant when the content is 0.002% or more, therefore, a content of 0.002% 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 propagation and fusion. Therefore, when Al is present, the Al content is preferably limited to 0.100% or less. It should be noted that Al is more preferably 0.002% or more. Preferably, Al is 0.060% or less. Further preferably, Al is 0.005% or more. Preferably, Al is 0.040% or more.

[0077] [Ca: below 0.010%]

[0078] In molten metal, Ca combines with S to form high-melting-point sulfides, 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. On the other hand, when the content exceeds 0.010%, arc disturbance occurs during SAW welding, making stable welding difficult. Therefore, when Ca is present, the Ca content is preferably limited to 0.010% or less. It should be noted that 0.001% or more is more preferred. 0.008% or less is preferred. 0.006% or less is even more preferred.

[0079] [REM: below 0.020%]

[0080] REM refers to rare earth elements such as Sc, Y, La, and Ce. REM is a powerful deoxidizer, existing in the weld metal as REM oxides. REM oxides serve as nucleation sites during solidification, thereby refining the grain size and contributing to increased weld metal strength. This effect becomes significant when the content is 0.001% or more. However, when the content exceeds 0.020%, the stability of the arc decreases. Therefore, when REM is present, it is preferable to limit the REM content to 0.020% or less. It should be noted that 0.002% or more is more preferred. 0.018% or less is more preferred. 0.005% or more is even more preferred. 0.015% or less is more preferred.

[0081] [Balance of Ingredients]

[0082] The balance other than the above-mentioned components consists of Fe and unavoidable impurities. Examples of unavoidable impurities include O, Sn, Sb, As, Pb, and Bi. The amount of O in the welding wire is preferably set to 0.15% or less. The amounts of Sn, Sb, and As are preferably set to 0.005% or less each. The amounts of Pb and Bi are preferably set to 0.0001% or less each. Furthermore, as long as the above basic composition and optional components are satisfied, elements other than these may be included, and such embodiments are also within the scope of the present invention.

[0083] [Manufacturing method of welding wire]

[0084] Next, the manufacturing method of the SAW welding wire (solid welding wire and flux-cored welding wire) of the present invention will be described.

[0085] In the manufacture of the welding wire of the present invention, apart from using molten steel having the above-mentioned components, there is no need to limit the manufacturing method, and commonly used manufacturing methods for welding wires can be applied.

[0086] The solid welding wire of the present invention preferably comprises: a casting process in which molten steel having the above-mentioned components 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; a heating process in which the obtained steel ingot is heated to a specified temperature; a hot rolling process in which the heated steel ingot is hot rolled to form a steel raw material (bar) of a specified shape; and a cold rolling process in which the obtained steel raw material (bar) is cold rolled (cold drawn wire processing) twice or more and annealed at an annealing temperature of 900 to 1200°C as needed to form a welding wire of the desired size.

[0087] Furthermore, the flux-cored wire of the present invention preferably comprises, 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 sheath material, which is then cold-bent in the width direction to form a U-shape. Then, in a manner that forms the target welding wire composition, metal powder with adjusted composition and flux powder for welding wire are sealed into the obtained steel sheath, and the wire is drawn by cold working to produce a flux-cored wire for SAW.

[0088] The composition of the aforementioned metal powder is not particularly limited; it is a metal powder or alloy powder that supplements the composition of the steel outer sheath material to form the overall composition of the welding wire. Furthermore, the composition of the flux powder for the welding wire is not particularly limited; it can be flux powder having the same or similar composition as the welding flux described below.

[0089] [Welding Flux]

[0090] When using the above-mentioned SAW welding wires (solid wires and flux-cored wires), 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% can be used. As an example, a powder material with a composition of 38% SiO2-11% MnO-8% TiO2-16% Al2O3-27% MgO can be used. However, in this invention, the welding flux is not limited to this.

[0091] [Manufacturing method for welded joints]

[0092] The manufacturing method of the welded joint of steel as the base material by welding with the above-mentioned submerged arc welding wire is described.

[0093] By using a submerged arc welding wire with the above-mentioned components, steel materials serving as the base material are butted together, and the above-mentioned welding flux is distributed, the welding wire is continuously supplied to generate an electric arc for welding, thereby enabling the production of a welded joint.

[0094] [Steel]

[0095] The preferred base material is high-Mn steel. High-Mn steel is a high-strength steel for extremely low temperatures, and its content is preferably 15.0% to 30.0% by mass. Specifically, it is a steel with a basic composition containing, by mass, 0.10% to 0.80% C, 0.05% to 1.00% Si, 15.0% to 30.0% Mn, less than 0.030% P, less than 0.030% S, less than 0.030% Cr, 2.5% to 15.0% N, less than 0.120% N, with the balance being Fe and unavoidable impurities. In high-Mn steel, based on the basic composition, as optional components, one or two of the following can be selected as needed: Ni: less than 10.00% and Mo: less than 3.50%. In addition, one or more of the following can be selected as: V: less than 2.0%, Ti: less than 1.0% and Nb: less than 1.00%. In addition, one or more of the following can be selected as: Cu: less than 1.00%, Al: less than 0.100%, Ca: less than 0.010% and REM: less than 0.020%.

[0096] As a method for manufacturing high-Mn steel, there are methods such as hot rolling of steel raw materials obtained through conventional steelmaking and casting processes by adjusting heating conditions and reduction rates, followed by cooling to obtain steel (steel plate). The thickness of the rolled steel plate is, for example, 6 to 100 mm. Preferably, it is 9 to 80 mm. More preferably, it is 9 to 60 mm.

[0097] Example

[0098] The present invention will be further described below based on embodiments. However, the following embodiments are merely examples for illustrating the present invention in a more detailed manner and do not limit the scope of the present invention.

[0099] The molten steel with the composition shown in Table 1 was melted in a vacuum melting furnace and cast to produce 1000 kg steel ingots. The resulting steel ingots were heated to 1200℃ and then hot-rolled and subsequently cold-rolled to produce solid welding wire for submerged arc welding with a diameter of 4.0 mmφ.

[0100] In addition, a steel outer sheath and a flux-cored welding wire containing metal powder and flux powder encapsulated within the steel outer sheath are manufactured. A thin steel sheet (0.5 mm thick) having 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 then cold-bent in the width direction to form a U-shape. Next, in a manner consistent with the welding wire composition shown in Table 2, the compositionally adjusted metal powder and flux powder for welding are encapsulated within the obtained steel outer sheath. The wire is then cold-drawn to produce a flux-cored welding wire (3.2 mm φ in diameter). It should be noted that the composition shown in Table 2 represents the total values ​​of the steel outer sheath, metal powder, and flux powder for welding.

[0101] Next, a high-Mn steel plate (thickness: 20mm) for cryogenic use was prepared as a test plate. Following JIS Z 3111, two test plates were butt-jointed to form a 45° V-groove. The resulting solid or flux-cored welding wire was used as the welding material for submerged arc welding, and deposited metal was obtained within the aforementioned groove. It should be noted that the high-Mn steel plate used as the test plate is a steel plate with a composition of 0.5% C, 0.4% Si, 25% Mn, 3% Cr, and the balance Fe. During welding, a sintered flux powder with a composition of 38% SiO2, 11% MnO, 8% TiO2, 16% Al2O3, and 27% MgO was used.

[0102] In submerged arc welding, solid welding wires (4.0 mm φ in diameter) or flux-cored welding wires (3.2 mm φ in diameter) with the compositions shown in Tables 1 and 2 are used without preheating. The welding is carried out in a flat position under the following conditions: current of 450–650 A (DCEP), voltage of 28–36 V, welding speed of 20 cm / min, heat input of 3.5–7.0 kJ / mm, and interpass temperature of 100–150 °C.

[0103] [High-Temperature Crack Resistance]

[0104] After welding, the cross-section of the weld metal is observed using an optical microscope (30x magnification) to determine the presence or absence of high-temperature cracks. If high-temperature cracks are confirmed, it is evaluated as a reduction in high-temperature crack resistance and marked "×". If no high-temperature cracks are confirmed, it is evaluated as excellent high-temperature crack resistance and marked "○".

[0105] [Weld Appearance]

[0106] In addition, the appearance of the weld bead is visually inspected to determine its quality. If undercut, weld beads, or pits are found, the weld bead is considered defective and rated as "×". If these defects are not found, the weld bead is considered to have good appearance and rated as "○".

[0107] [Characteristics of deposited metal]

[0108] According to JIS Z 3111, tensile test pieces (parallel diameter 6mmφ) of the deposited metal and Charpy impact test pieces (V-notch) of the deposited metal are cut from the obtained deposited metal, and tensile tests and impact tests are carried out.

[0109] [Tensile test: 0.2% yield strength (MPa)]

[0110] Tensile tests were performed on three wires at room temperature, and the average value (0.2% yield strength) was taken as the tensile property of the weld metal using the welding wire. The target value of the present invention is set as described above, with a 0.2% yield strength of 400 MPa or higher at room temperature.

[0111] Impact test: Absorbed energy vE -196 (J)]

[0112] In addition, three Charpy impact tests were conducted on each specimen, and the absorption energy vE at a test temperature of -196℃ was calculated. -196 The average value of this value is taken as the cryogenic toughness of the weld metal using this welding wire.

[0113] The target value of this invention is set as the absorption energy vE as described above. -196 It is above 28J.

[0114] The results are shown in Table 3.

[0115] [Table 1]

[0116]

[0117]

[0118] [Table 3]

[0119]

[0120] *) Based on JIS Z 3111

[0121] The examples of this invention are all welding materials that do not produce high-temperature cracks during welding, and can produce weld metal with excellent high-temperature crack resistance and good weld appearance.

[0122] Furthermore, the examples of this invention all involve Charpy impact tests with a yield strength (0.2% yield strength) of 400 MPa or higher at room temperature and a test temperature of -196°C, yielding absorbed energy vE. -196 A value of 28J or higher, meeting the above target value, indicates that it is a welding material (welding wire) capable of producing welding metals with both high strength and excellent low-temperature toughness.

[0123] On the other hand, in comparative examples outside the scope of this invention, high-temperature cracks occur, high-temperature crack resistance is reduced, or there are defects in the weld bead, poor weld bead appearance, or the 0.2% yield strength at room temperature is less than 400 MPa, and the absorbed energy vE is... -196 Less than 28J, no weld metal with both target strength and extremely low temperature toughness was obtained.

[0124] The following is an explanation of each comparative example.

[0125] In welding wire No. 19, the C content is below the range of this invention, therefore the 0.2% yield strength of the deposited metal is less than 400 MPa, which cannot ensure the desired high strength. Furthermore, the Mn content is below the range of this invention, therefore the absorbed energy vE at a test temperature of -196°C is... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0126] In welding wire No. 20, the Cr content is lower than the range of this invention, therefore the 0.2% yield strength of the deposited metal is less than 400 MPa, which cannot ensure the desired high strength. Furthermore, it cannot suppress the segregation of P towards the final solidification zone during welding, thus causing high-temperature cracking. Additionally, the absorbed energy vE at the test temperature of -196°C is... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0127] In welding wire No. 21, the Si and Mn content is higher than the range of this invention, while the Cr content is lower than the range of this invention. Therefore, during welding, Si, Mn, and P segregate towards the final solidification region, resulting in high-temperature cracks. Furthermore, the 0.2% yield strength of the deposited metal is less than 400 MPa, which cannot ensure the desired high strength. In addition, the absorbed energy vE at the test temperature of -196°C is... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0128] In welding wire No. 22, the S and Mo content is higher than the range of this invention, resulting in the formation of MnS and Mo carbides that become the initiation point of fracture. Therefore, the absorbed energy vE at the test temperature of -196°C is higher. -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0129] In welding wire No. 23, the C and Cr content is lower than the range of this invention, therefore the 0.2% yield strength of the deposited metal is less than 400 MPa, which cannot ensure the desired high strength. Furthermore, it cannot suppress the segregation of P towards the final solidification zone during welding, thus causing high-temperature cracking. Additionally, the absorbed energy vE at the test temperature of -196°C is... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0130] Welding wire No. 24 has a higher P content than the range of this invention, and welding wire No. 25 has a higher C content than the range of this invention. Therefore, during welding, P segregates or carbides segregate in the final solidification part, resulting in high-temperature cracks.

[0131] In welding wire No. 26, the Si content is lower than the range of this invention, so a good weld bead shape is not obtained, resulting in pits.

[0132] In welding wire No. 34, the Mn content is lower than the range specified in this invention, resulting in low stability of the austenite phase. Consequently, the absorbed energy vE at the test temperature of -196°C is low. -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0133] In welding wire No. 35, the P and S content is higher than that of this invention. Therefore, during welding, P and S segregate in the final solidification region, resulting in high-temperature cracks. In addition, MnS is generated, which becomes the initiation point of fracture. Therefore, the absorbed energy vE at the test temperature of -196°C is higher. -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0134] In welding wire No. 36, the Cr content is lower than the range of this invention, therefore the 0.2% yield strength of the deposited metal is less than 400 MPa, which cannot ensure the desired high strength. Furthermore, it cannot suppress the segregation of P towards the final solidification zone during welding, thus causing high-temperature cracking. Additionally, the absorbed energy vE at the test temperature of -196°C is... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

[0135] In welding wire No. 37, the Si content is higher than the range of this invention. Therefore, during welding, Si segregates towards the final solidification part, resulting in high-temperature cracks. In addition, the absorbed energy vE at the test temperature of -196°C... -196 At a temperature below 28J, the desired excellent low-temperature toughness cannot be guaranteed.

Claims

1. A method for manufacturing a welded joint, wherein, Submerged arc welding of high-Mn steel was performed using submerged arc welding wire. The high-Mn steel has the following composition by mass%, comprising: 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%, Cr: 2.5–15.0%, N: less than 0.120%, with the balance consisting of Fe and unavoidable impurities. The submerged arc welding wire has the following composition by mass: C: 0.20–0.80%, Si: 0.15–0.90%, Mn: 15.0–30.0%, P: ≥0.009% and ≤0.030%, S: ≤0.030%, Cr: 6.3–15.0%, N: ≤0.120%, with the balance consisting of Fe and unavoidable impurities. The absorbed energy vE of Charpy impact test when the 0.2% yield strength of the weld metal prepared according to JIS Z 3111 is 400 MPa or higher at room temperature and the test temperature is -196°C. -196 It is above 28J.

2. The method for manufacturing a welded joint according to claim 1, wherein, In addition to the aforementioned composition, the submerged arc welding wire also contains, by mass percent, at least one group of elements from groups A to C. Group A: Selected from one or both of Ni: less than 10.00% and Mo: less than 3.50%. Group B: Selected from one or more of the following: V: less than 1.0%, Ti: less than 1.0%, and Nb: less than 1.00%. Group C: Selected from one or more of the following: Cu: less than 1.00%, Al: less than 0.100%, Ca: less than 0.010%, and REM: less than 0.020%.

3. The method for manufacturing a welded joint according to claim 1, wherein, The welding wire is either a solid core welding wire or a flux-cored welding wire.

4. The method for manufacturing a welded joint according to claim 2, wherein, The welding wire is either a solid core welding wire or a flux-cored welding wire.

5. The method for manufacturing a welded joint according to any one of claims 1 to 4, wherein, In addition to the aforementioned composition, the high-Mn steel also contains, by mass%, at least one group of elements selected from groups D to F below. Group D: Selected from one or both of Ni: less than 10.00% and Mo: less than 3.50%. Group E: Selected from one or more of the following: V: less than 2.0%, Ti: less than 1.0%, and Nb: less than 1.00%. Group F: Selected from one or more of the following: Cu: less than 1.00%, Al: less than 0.100%, Ca: less than 0.010%, and REM: less than 0.020%.

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