Filler metal for TIG welding

Abstract

To provide a filler material for TIG welding which is suitable as a welding material for a high Mn-containing steel material used under an ultralow-temperature environment, suppresses occurrence of high temperature cracks in welding, and is excellent in high temperature crack resistance.SOLUTION: A filler material for TIG welding has a composition comprising, by mass%, 0.20-0.80% C, 0.15-0.90% Si, 15.0-30.0% Mn, 0.030% or less P, 0.030% or less S, 6.0-15.0% Cr, and 0.120% or less N, with the balance being Fe and inevitable impurities. The filler material for TIG welding may contain, if necessary, one or two kinds selected from Ni and Mo, and one or two or more kinds selected from V, Ti and Nb, and one or two or more kinds selected from Cu, Al, Ca and REM. Thereby, a weld joint part which suppresses occurrence of weld cracks in TIG welding, is excellent in high temperature crack resistance, and has high strength and excellent ultralow-temperature impact toughness can be easily manufactured.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a filler metal for TIG welding, and particularly to a filler metal for TIG welding of high-Mn-containing steel materials used in an extremely low temperature environment, which is excellent in crack resistance at high temperature and suppresses the occurrence of hot cracks during welding, and a method for manufacturing a welded joint using the same.

Background Art

[0002] TIG welding is Tungsten Inert Gas welding. Using tungsten, a material that is not consumed by the electrode rod, while blowing an inert gas such as argon gas or helium gas to cut off the air, another filler metal (welding rod) is melted in the arc for welding. According to this TIG welding, it can be applied to various alloy steels and non-ferrous metals, can weld even complex shapes, and excellent welding quality can be obtained, so it is applied to the welding of all metals.

[0003] In recent years, environmental regulations have become stricter. And liquefied natural gas (hereinafter also referred to as LNG) is said to be a clean fuel that does not generate air pollutants such as sulfur oxides because it does not contain sulfur, and its demand is increasing. Furthermore, for the transportation or storage of LNG, containers (tanks) for transporting or storing LNG are required to maintain excellent extremely low temperature impact toughness at temperatures of -162°C or lower, which is the liquefaction temperature of LNG.

[0004] Conventionally, due to the necessity of maintaining excellent extremely low temperature impact toughness, aluminum alloys, 9% Ni steel, austenitic stainless steel, etc. have been used as materials for containers (tanks) and the like.

[0005] However, aluminum alloys have problems such as low tensile strength, so it is necessary to design a large plate thickness for the structure, and poor weldability. For 9% Ni steel, it is necessary to use an expensive Ni-based material as a welding material, which is economically disadvantageous. In addition, austenitic stainless steel is expensive and has a problem of low base metal strength.

[0006] Due to such problems, recently, the application of high-Mn steels (hereinafter also referred to as "high-Mn steels") containing about 10 to 35 mass% of Mn has been considered as a material for containers (tanks) for transporting or storing LNG. High-Mn steels are in the austenite phase even at extremely low temperatures, do not undergo brittle fracture, and have the characteristic of having high strength compared to austenitic stainless steels. Also, the development of welding methods and welding materials that can stably weld such high-Mn steel materials is desired.

[0007] In response to such demands, for example, Patent Document 1 proposes "a high-strength welded joint excellent in cryogenic impact toughness and a wire for flux-cored arc welding therefor". The wire for flux-cored arc welding described in Patent Document 1 has a composition containing, by weight%, C: 0.15 to 0.8%, Si: 0.2 to 1.2%, Mn: 15 to 34%, Cr: 6% or less, Mo: 1.5 to 4%, S: 0.02% or less, P: 0.02% or less, B: 0.01% or less, Ti: 0.09 to 0.5%, N: 0.001 to 0.3%, TiO2: 4 to 15%, one or more selected from SiO2, ZrO2 and Al2O3 in total: 0.01 to 9%, one or more selected from K, Na and Li in total: 0.5 to 1.7%, one or more of F and Ca: 0.2 to 1.5%, and the balance being Fe and other inevitable impurities. When welding using the wire for flux-cored arc welding described in Patent Document 1, a welded joint having excellent low-temperature toughness with a Charpy impact test absorbed energy of 28 J or more at a test temperature of -196°C and high strength with a normal-temperature tensile strength of 400 MPa or more can be effectively obtained. Also, it is stated that by adjusting the wire composition to Mo: 1.5% or more, a welded joint having excellent resistance to hot cracking can be ensured.

[0008] In addition, Patent Document 2 proposes a "solid wire for gas metal arc welding". The solid wire for gas metal arc welding described in Patent Document 2 has, by mass%, C: 0.2 to 0.8%, Si: 0.15 to 0.90%, Mn: 17.0 to 28.0%, P: 0.03% or less, S: 0.03% or less, Ni: 0.01 to 10.00%, Cr: 0.4 to 4.0%, Mo: 0.01 to 3.50%, B: less than 0.0010%, N: 0.12% or less, and the balance is Fe and inevitable impurities. Optionally, it may contain one or more selected from V, Ti, and Nb, and one or more selected from Cu, Al, Ca, and REM. Welding using the solid wire for gas metal arc welding described in Patent Document 2 results in less fume generation, and moreover, it has a high strength with a normal temperature yield strength (0.2% proof stress) of 400 MPa or more, and the absorbed energy vE-196 of the Charpy impact test at a test temperature of -196°C is 28 J or more, enabling the production of a welded joint with high strength and excellent extremely low temperature impact toughness.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, according to the study by the present inventors, the techniques described in Patent Document 1 and Patent Document 2 have a problem that hot cracking occurs during welding.

[0011] The present invention solves the above-mentioned problems of the prior art, can suppress the occurrence of hot cracking during welding, and is suitable as a welding material for high-Mn-containing steel materials used in extremely low-temperature environments. The object is to provide a filler metal for TIG welding that can stably manufacture a welded joint having both high strength and excellent extremely low-temperature toughness.

[0012] Here, the "high strength" refers to the case where the room-temperature yield strength (0.2% proof stress) of the weld metal manufactured in accordance with the provisions of JIS Z 3111 is 400 MPa or more. The "excellent extremely low-temperature toughness" refers to the case where the absorbed energy vE-196 of the Charpy impact test at a test temperature of -196°C of the weld metal manufactured in accordance with the provisions of JIS Z 3111 is 28 J or more.

Means for Solving the Problems

[0013] In order to achieve the above object, the inventors of the present invention first intensively studied the factors affecting hot cracking during TIG welding of high-Mn steel. As a result, it was found that segregation of P to the final solidification part of the weld metal is a factor in the occurrence of hot cracking. Furthermore, it was found that when the composition of the filler metal contains 6.0 mass% or more of Cr, Cr phosphide is formed in the liquid phase of the weld metal, thereby suppressing the segregation of P to the final solidification part of the weld metal and further suppressing the occurrence of hot cracking.

[0014] In addition, the composition of the TIG welding filler metal required for the weld metal manufactured in accordance with the provisions of JIS Z 3111 to be a weld metal having both the desired high strength and the desired excellent extremely low-temperature toughness was studied. As a result, the composition of the filler metal was adjusted in the ranges of C: 0.20 to 0.80% and Si: 0.15 to 0.90% by mass, and further, after adjusting Mn: 15.0 to 30.0% and Cr: 6.0 to 15.0% to specific ranges, it was found that it is necessary to use a TIG welding filler metal having a composition reduced to P: 0.030% or less, S: 0.030% or less, and N: 0.120% or less and a welded joint using the same.

[0015] Based on such findings, the present invention has been further studied and completed, and the gist of the present invention is as follows. [1] A filler metal for TIG welding having a composition containing, by mass%, 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, and the balance being Fe and unavoidable impurities. [2] A filler metal for TIG welding containing, in addition to the composition in [1], one or two selected from Ni: 10.00% or less and Mo: 3.50% or less by mass%. [3] A filler metal for TIG welding containing, in addition to the composition in [1] or [2], one or more selected from V: 1.00% or less, Ti: 1.00% or less, and Nb: 1.00% or less by mass%. [4] A filler metal for TIG welding containing, in addition to the composition in any one of [1] to [3], one or more selected from Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less by mass%. [5] A method for manufacturing a welded joint of a high-Mn steel using the filler metal for TIG welding according to any one of [1] to [4] and a non-consumable electrode by TIG welding. [6] A method for manufacturing a welded joint in [5], wherein the Mn content of the high-Mn steel is 15.0 to 30.0% by mass. [7] In [5] or [6], the high-Mn steel contains, by mass%, 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, Cr: 2.5 to 15.0%, N: 0.120% or less, and the balance being Fe and unavoidable impurities, and a method for manufacturing a welded joint having such a composition. [8] In any one of [5] to [7], a method for manufacturing a welded joint containing, in addition to the composition, one or two selected from Ni: 10.00% or less and Mo: 3.50% or less by mass%. In any one of [9][5] to [8], a method for manufacturing a welded joint further containing, in addition to the above composition, one or more selected from V: 2.00% or less, Ti: 1.00% or less, and Nb: 1.00% or less by mass. In any one of

[10] [5] to [9], a method for manufacturing a welded joint further containing, in addition to the above composition, one or more selected from Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less by mass.

Advantages of the Invention

[0016] According to the filler metal for TIG welding according to the present invention, as a welding material for high-Mn-containing steel materials, it is possible to suppress hot cracking during TIG welding, and furthermore, it is possible to easily manufacture a welded joint having high strength and excellent toughness at extremely low temperatures, which has an extremely high industrial effect.

Embodiments for Carrying Out the Invention

[0017] The present invention is a filler metal suitable for TIG welding of high-Mn-containing steel materials. By using the filler metal of the present invention, hot cracking can be suppressed during TIG welding of high-Mn-containing steel materials. The test plate must be pre-constrained or given reverse strain so that the angular deformation after welding does not exceed 5° so that test pieces can be taken. The groove shape is a V-groove and is composed of a backing bar. Welding is performed by downward welding except when specifically specified, with 1 or 2 passes for the first and second layers and 1 or 2 passes or more for the third layer and subsequent layers. The test pieces after welding must not be heat-treated.

[0018] More preferably, the filler metal of the present invention is a weld metal that complies with JIS Z 3111 and is manufactured by TIG welding, having high strength with a 0.2% proof stress at room temperature of 400 MPa or more and excellent toughness at extremely low temperatures with an absorbed energy of the Charpy impact test at a test temperature of -196°C of 28 J or more, and is a welding material capable of manufacturing a welded joint having high strength and excellent toughness at extremely low temperatures.

[0019] [TIG Welding] As described above, TIG welding is a method of welding by using tungsten, a material that does not consume the electrode rod, blowing argon gas or helium gas to cut off the air, and melting another filler metal in the arc. This TIG welding can be applied to various alloy steels and non-ferrous metals, can weld even complex shapes, and excellent welding quality can be obtained, so it is applied to the welding of all metals.

[0020] As an example of the TIG welding method, on a steel plate or steel material (plate thickness: 3 to 100 mm) as the base material, in accordance with JIS Z 3111, butting is performed to form a 45° V-groove. Using a pure tungsten rod (3.2 mm φ) as the electrode and a filler metal (2.0 mm φ in diameter), without preheating, in a downward position, current: 180 to 250 A (DCEN), voltage: 10 to 15 V, welding speed: 5 to 15 cm / min, heat input per weld: 0.7 to 4.0 kJ / mm, interpass temperature: 100 to 150 °C, shielding gas: Ar, gas flow rate: 10 to 25 L / min.

[0021] [Basic Composition of Filler Metal] The filler metal for TIG welding of the present invention has, as its basic composition, by mass%, 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, and the balance consists of Fe and unavoidable impurities. First, the reasons for limiting the composition of the basic composition will be explained. Hereinafter, "%" in the composition means "mass%".

[0022] [C: 0.20 to 0.80%] C is an element that has the effect of increasing the strength of the weld metal by solid solution strengthening, and also stabilizes the austenite phase and improves the extremely low temperature impact toughness of the weld metal. To obtain such an effect, a content of 0.20% or more is required. However, if the content exceeds 0.80%, carbides precipitate, the extremely low temperature toughness decreases, and furthermore, welding cracks (hot cracks) are likely to occur during welding. Therefore, C is limited to the range of 0.20 to 0.80%. Preferably, C is 0.40% or more. Preferably, C is 0.60% or less. More preferably, C is 0.45% or more. More preferably, C is 0.55% or less.

[0023] [Si: 0.15~0.90%] Si acts as a deoxidizer, increases the yield of Mn, and has the effect of increasing the viscosity of the molten metal and stably maintaining the bead shape. To obtain such an effect, a content of 0.15% or more is required. However, if the content exceeds 0.90%, the extremely low temperature toughness of the weld metal decreases, and also, Si segregates during solidification, generates a liquid phase at the solidification cell interface, and reduces the high temperature crack resistance. Therefore, Si is limited to the range of 0.15 to 0.90%. Preferably, Si is 0.20% or more. Preferably, Si is 0.70% or less. More preferably, Si is 0.30% or more. More preferably, Si is 0.60% or less.

[0024] [Mn: 15.0~30.0%] Mn is an element that stabilizes the austenite phase inexpensively, and in the present invention, a content of 15.0% or more is required. If Mn is less than 15.0%, a ferrite phase is generated in the weld metal, and the toughness at extremely low temperatures is significantly reduced. On the other hand, if Mn exceeds 30.0%, excessive Mn segregation occurs during solidification, inducing welding cracks (hot cracks). Therefore, Mn 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.

[0025] [P: Below 0.030%] P is an element that segregates at grain boundaries and induces hot cracking, and it is preferably reduced as much as possible. However, if it is 0.030% or less, it is acceptable. Therefore, P is limited to 0.030% or less. Note that excessive reduction will lead to an increase in refining costs. Therefore, it is preferable to adjust P to 0.003% or more. More preferably, P is 0.005% or more. More preferably, P is 0.020% or less.

[0026] [S: Below 0.030%] In the weld metal, S exists as MnS, a sulfide-based inclusion. Since MnS serves as a starting point for fracture, it reduces the toughness at extremely low temperatures. Therefore, S is limited to 0.030% or less. Note that excessive reduction will lead to an increase in refining costs. Therefore, it is preferable to adjust S to 0.001% or more. More preferably, S is 0.003% or more. More preferably, P is 0.020% or less.

[0027] [Cr: 6.0 - 15.0%] At extremely low temperatures, Cr acts as an element that stabilizes the austenite phase, improving the toughness of the weld metal at extremely low temperatures. It also has the effect of improving the strength of the weld metal. Furthermore, it effectively acts to narrow the temperature range of the solid-liquid coexistence region of the molten metal and suppress the occurrence of hot cracking, and also has the effect of suppressing hot cracking caused by P by forming Cr phosphides in the liquid phase. To obtain such effects, a content of 6.0% or more is required. If Cr is less than 6.0%, the above-described effects cannot be ensured. On the other hand, if the content exceeds 15.0%, Cr carbides are generated, leading to a decrease in toughness at extremely low temperatures. Therefore, Cr is limited to the range of 6.0 - 15.0%. Note that preferably, Cr is more than 7.0%. Preferably, Cr is 15.0% or less. More preferably, Cr is 8.0% or more. More preferably, Cr is 13.0% or less.

[0028] [N: Below 0.120%] N is an element that inevitably mixes in. Similar to C, it effectively contributes to improving the strength of the weld metal, stabilizes the austenite phase, and contributes to a stable improvement in toughness at extremely low temperatures. Such effects become prominent when the content is 0.003% or more. On the other hand, when the content exceeds 0.120%, nitrides are formed and the toughness at low temperatures decreases. Therefore, N is limited to 0.120% or less. Preferably, N is 0.004% or more. Preferably, N is 0.080% or less. More preferably, N is 0.010% or more. More preferably, N is 0.060% or less.

[0029] [Optional component] The filler metal of the present invention has the above-described components as basic components. In the present invention, in addition to the above-described basic composition, one or two selected from Ni: 10.00% or less and Mo: 3.50% or less can be optionally selected and contained as optional components as needed. In addition to these, one or more selected from V: 1.00% or less, Ti: 1.00% or less, and Nb: 1.00% or less can be selected and contained. Further, in addition to these, one or more selected from Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less can be selected and contained.

[0030] [Ni: 10.00% or less and Mo: 3.50% or less] Both Ni and Mo are elements that strengthen the austenite grain boundaries and can be optionally selected and contained either one or both as needed.

[0031] [Ni: 10.00% or less] Ni is an element that strengthens the austenite grain boundaries, segregates at the grain boundaries, and improves the toughness at extremely low temperatures. Also, since Ni has the effect of stabilizing the austenite phase, further increasing its content will stabilize the austenite phase and improve the toughness at extremely low temperatures of the weld metal. However, Ni is an expensive element, and a content exceeding 10.00% is economically disadvantageous. Therefore, it is preferable to limit Ni to 10.00% or less. More preferably, it is in the range of 8.00% or less. Even more preferably, it is in the range of 6.00% or less.

[0032] [Mo: 3.50% or less] Mo is an element that strengthens the austenite grain boundaries, segregates at the grain boundaries, and improves the strength of the weld metal. It also has the effect of improving the strength of the weld metal by solid solution strengthening. On the other hand, when the content exceeds 3.50%, it precipitates as carbides, which may become the starting point of fracture and lead to a decrease in toughness at extremely low temperatures. Therefore, it is preferable to limit Mo to the range of 3.50% or less. More preferably, it is in the range of 3.00% or less. Further, preferably, Mo is 1.00% or more. Preferably, Mo is 3.00% or less.

[0033] [V: 1.00% or less, Ti: 1.00% or less, and Nb: 1.00% or less] V, Ti, and Nb are all elements that promote the formation of carbides and contribute to the improvement of the strength of the weld metal, and can be selectively contained one or more kinds as needed.

[0034] [V: 1.00% or less] V is a carbide-forming element that precipitates fine carbides and contributes to improving the strength of the weld metal. To obtain such an effect, it is preferably contained in an amount of 0.001% or more. However, if it is contained in an amount exceeding 1.00%, the carbides will coarsen, becoming the starting point of fracture and leading to a decrease in the very low temperature toughness. Therefore, when contained, V is preferably limited to 1.00% or less. Preferably, V is 0.002% or more. Preferably, V is 0.60% or less. More preferably, V is 0.005% or more. More preferably, V is 0.20% or less.

[0035] [Ti: 1.00% or less] Ti is a carbide-forming element that precipitates fine carbides and contributes to improving the strength of the weld metal. Also, it precipitates carbides at the solidification cell interface of the weld metal, contributing to suppressing the occurrence of hot cracking. To obtain such an effect, it is preferably contained in an amount of 0.001% or more. However, if it is contained in an amount exceeding 1.00%, the carbides will coarsen, becoming the starting point of fracture and leading to a decrease in the very low temperature toughness. Therefore, when contained, Ti is preferably limited to 1.00% or less. Preferably, Ti is 0.002% or more. Preferably, Ti is 0.60% or less. More preferably, Ti is 0.005% or more. More preferably, Ti is 0.20% or less.

[0036] [Nb: 1.00% or less] Nb is a carbide-forming element that precipitates carbides and contributes to improving the strength of the weld metal. Also, it precipitates carbides at the solidification cell interface of the weld metal, contributing to suppressing the occurrence of hot cracking. To obtain such an effect, it is preferably contained in an amount of 0.001% or more. However, if it exceeds 1.00%, the carbides will coarsen, becoming the starting point of fracture and leading to a decrease in the very low temperature toughness. Therefore, when contained, Nb is preferably limited to 1.00% or less. More preferably, Nb is 0.002% or more. Preferably, Nb is 0.60% or less. More preferably, Nb is 0.005% or more. More preferably, Nb is 0.30% or less.

[0037] [Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less] Cu is an element that contributes to austenite stabilization, Al contributes to bead shape stabilization, and Ca and REM are elements that contribute to improved workability. They can be selectively contained, one or more of them, as needed.

[0038] [Cu: 1.00% or less] Cu is an element that stabilizes the austenite phase. It stabilizes the austenite phase even at extremely low temperatures, improving the toughness of the weld metal at extremely low temperatures. To obtain such an effect, it is preferably contained at 0.01% or more. However, if it is contained in a large amount exceeding 1.00%, it segregates during solidification and induces hot cracking. Therefore, when contained, Cu is preferably limited to 1.00% or less. Preferably, Cu is 0.01% or more. Preferably, Cu is 0.60% or less. More preferably, Cu is 0.10% or less.

[0039] [Al: 0.100% or less] Al acts as a deoxidizer, increases the viscosity of the molten metal, and has an important effect of stably maintaining the bead shape. It also narrows the temperature range of the solid-liquid coexistence region of the molten metal, contributing to the suppression of hot cracking in the weld metal. Such an effect becomes remarkable when contained at 0.005% or more, so it is preferably contained at 0.005% or more. However, if it is contained in an amount exceeding 0.100%, the viscosity of the molten metal becomes too high, and conversely, defects such as bead spreading and poor fusion increase. Therefore, when contained, Al is preferably limited to 0.100% or less. More preferably, Al is 0.005% or more. More preferably, Al is 0.060% or less. More preferably, Al is 0.020% or less.

[0040] [Ca: 0.010% or less] Ca combines with S in the molten metal to form high-melting sulfide CaS. Since CaS has a higher melting point than MnS, it contributes to suppressing the occurrence of high-temperature cracking in the weld metal. Such an effect becomes remarkable when the content is 0.001% or more. On the other hand, when the content exceeds 0.010%, arc disturbance occurs during welding, making stable welding difficult. Therefore, when containing Ca, it is preferably limited to 0.010% or less. More preferably, Ca is 0.001% or more. More preferably, Ca is 0.008% or less.

[0041] [REM: 0.020% or less] REM refers to rare earth elements such as Sc, Y, La, Ce, etc. It is a strong deoxidizer and exists in the form of REM oxides in the weld metal. REM oxides serve as nucleation sites during solidification, refining the crystal grains and contributing to the improvement of the strength of the weld metal. Such an effect becomes remarkable when the content is 0.001% or more. However, when the content exceeds 0.020%, the arc stability decreases. Therefore, when containing REM, it is preferably limited to 0.020% or less. More preferably, REM is 0.002% or more. Preferably, REM is 0.018% or less. More preferably, REM is 0.010% or less.

[0042] [Remaining components] The remainder other than the above-mentioned components consists of Fe and inevitable impurities. Examples of inevitable impurities include O, Sn, Sb, As, Pb, Bi, etc. The amount of O in the wire is preferably 0.15% or less, the amounts of Sn, Sb, and As are each preferably 0.005% or less, and the amounts of Pb and Bi are each preferably 0.0001% or less. Also, as long as the above-mentioned basic composition and selected components are satisfied, other elements may be contained, and such embodiments are also included in the technical scope of the present invention.

[0043] [Manufacturing method of filler metal] Next, the manufacturing method of the filler metal for TIG welding of the present invention will be described.

[0044] The production of the filler metal of the present invention does not need to limit the production method other than using molten steel having the above-described composition components, and any of the common production methods of filler metals can be applied.

[0045] The filler metal of the present invention, for example, melts molten steel having the above-described composition components in a common melting furnace such as an electric furnace or a vacuum melting furnace, and then a casting process of casting it into a mold of a predetermined shape, etc., and then, a heating process of heating the obtained steel ingot to a predetermined temperature, and a hot rolling process of performing hot rolling on the heated steel ingot to obtain a steel material (rod shape) of a predetermined shape are sequentially performed. Then, it is preferable to perform a cold rolling process of obtaining a wire of a desired size by subjecting the obtained steel material (rod shape) to multiple cold rolling (cold wire drawing) and, if necessary, an annealing process with an annealing temperature of 900 to 1200°C.

[0046] [Manufacturing method of welded joint] A manufacturing method of a welded joint in which a steel material as a base material is welded by the TIG welding method using the above-described filler metal for TIG welding will be described.

[0047] Using the filler metal for TIG welding having the above-described composition components, the steel materials as the base materials are butted together, and while spraying an inert gas, the filler metal is continuously supplied, and an arc is generated with an electrode of a tungsten rod to perform welding, thereby manufacturing a welded joint.

[0048] [Steel material] The steel material used as the base material is preferably a high-Mn-containing steel material. The high-Mn-containing steel material is a high-strength steel material for ultra-low temperatures, and preferably contains 15.0 to 30.0% of Mn by mass%. Specifically, by mass%, 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, Cr: 2.5 to 15.0%, N: 0.120% or less, and the balance consists of Fe and inevitable impurities. The steel material having this basic composition may further contain, as optional components, one or two selected from Ni: 10.00% or less and Mo: 3.50% or less, as required. In addition to these, one or more selected from V: 2.00% or less, Ti: 1.00% or less, and Nb: 1.00% or less can be selected and contained. Further, in addition to these, one or more selected from Cu: 1.00% or less, Al: 0.100% or less, Ca: 0.010% or less, and REM: 0.020% or less can be selected and contained.

[0049] As a method for manufacturing the high-Mn-containing steel material, there is a method in which a steel material obtained through a normal steelmaking process and casting process is hot-rolled by adjusting heating conditions, reduction ratio, etc., and then cooled to obtain a steel material (steel plate). The plate thickness of the steel plate after rolling is, for example, 3 to 100 mm.

Examples

[0050] Hereinafter, the present invention will be further described based on examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of rights of the present invention.

[0051] Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace and cast into a steel ingot of 1000 kg. The obtained steel ingot was heated to 1200°C, then hot-rolled, then cold-rolled, and annealed (900 to 1200°C) as required to obtain a filler metal for TIG welding having a diameter of 2.0 mmφ and a length of 1000 mm.

[0052] Next, as a test plate, a high-Mn-containing steel plate for cryogenic use (plate thickness: 12 mm) was prepared. In accordance with JIS Z 3111, they were butted to form a 45° V-groove. Using the obtained filler metal, TIG welding was performed to obtain a weld metal in the above-mentioned groove. The high-Mn-containing steel plate for cryogenic use used as the test plate was a steel plate having a composition of 0.5% C - 0.4% Si - 25% Mn - 3% Cr - the balance Fe.

[0053] For TIG welding, each filler metal (diameter 2.0 mmφ) having the composition shown in Table 1 was used. The electrode was a pure tungsten rod (3.2 mmφ). Without preheating, in the downward position, the current was 200 A (DCEN), the voltage was 12 V, the welding speed was 8 cm / min, the interpass temperature was 100 - 150°C, and the shielding gas was Ar.

[0054] [High temperature cracking resistance] After welding, the weld metal was observed with an optical microscope (30 times) to determine the presence or absence of welding cracks. The welding cracks were high-temperature cracks. When crack generation was observed, it was evaluated as "×" assuming that the high temperature cracking resistance was reduced. When no crack generation was observed, it was evaluated as "○" assuming excellent high temperature cracking resistance.

[0055] [Weld metal properties] From the obtained weld metal, in accordance with the provisions of JIS Z 3111, a tensile test piece (parallel part diameter 6 mmφ) of the weld metal and a Charpy impact test piece (V-notch) of the weld metal were taken, and a tensile test and an impact test were carried out.

[0056] [Tensile test: 0.2% proof stress (MPa)] The tensile test was carried out three times each at room temperature, and the average value of the obtained values (0.2% proof stress) was taken as the tensile property of the weld metal using the said wire. As described above, the target value of the present invention was that the 0.2% proof stress at normal temperature was 400 MPa or more.

[0057] [Impact test: Absorbed energy vE-196 (J)] In addition, the Charpy impact test was carried out three times each, and the absorbed energy vE-196 at the test temperature of -196°C was determined, and the average value was taken as the extremely low temperature toughness of the weld metal using the wire. As described above, the target value of the present invention was set such that the absorbed energy vE-196 was 28 J or more.

[0058] The results obtained are shown in Table 2.

[0059]

Table 1

[0060]

Table 2

[0061] All the examples of the present invention are welding materials that do not generate high-temperature cracks during welding and can obtain a weld metal with excellent high-temperature crack resistance.

[0062] Furthermore, all the examples of the present invention have a yield strength (0.2% proof stress) at room temperature of 400 MPa or more, and the absorbed energy vE-196 of the Charpy impact test at the test temperature of -196°C is 28 J or more, clearing the above target value, and it has been found that they are filler metals capable of obtaining a weld metal having both high strength and excellent extremely low temperature toughness.

[0063] On the other hand, in the comparative examples outside the scope of the present invention, high-temperature cracks occur, and the high-temperature crack resistance is reduced, or the 0.2% proof stress at room temperature is less than 400 MPa, and the absorbed energy vE-196 is less than 28 J, and a weld metal having both the target strength and extremely low temperature toughness has not been obtained.

[0064] Below, each comparative example will be described.

[0065] In the case of filler metal No. 19, since the P content is far outside the scope of the present invention, P segregates at the final solidification part during welding, and high-temperature cracks occur.

[0066] Since the S content of the filler metal No. 20 is far outside the scope of the present invention, MnS that can be a starting point of fracture is generated, and the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0067] In the case of the filler metal No. 21, since the Cr content is far outside the scope of the present invention, the 0.2% proof stress of the weld metal is less than 400 MPa, failing to ensure the desired high strength. Furthermore, since segregation of P to the final solidified part during welding cannot be suppressed, welding cracks (hot cracks) occur. Moreover, the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0068] Since the C content and the Mn content of the filler metal No. 22 are far outside the scope of the present invention, carbide and Mn segregate to the final solidified part during welding, resulting in the occurrence of welding cracks (hot cracks). Also, the carbide becomes a starting point of fracture, and the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0069] Since the C content of the filler metal No. 23 is far outside the scope of the present invention, the 0.2% proof stress of the weld metal is less than 400 MPa, failing to ensure the desired high strength. Furthermore, the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0070] Since the Mn content of the filler metal No. 24 is far outside the scope of the present invention, the stability of the austenite phase is low. Therefore, the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0071] Since the Mo content of the filler metal No. 25 is far outside the scope of the present invention and Mo carbide that can be a starting point of fracture is generated, the absorbed energy vE-196 at the test temperature of -196 °C is less than 28 J, failing to ensure the desired excellent cryogenic toughness.

[0072] Since the Si content of the additive No. 26 is far higher than the scope of the present invention, Si segregates to the final solidification part during welding, resulting in welding cracks (hot cracks). Furthermore, the absorbed energy vE-196 at the test temperature of -196°C is less than 28 J, and thus the desired excellent cryogenic toughness cannot be ensured.

Claims

1. A filler metal for TIG welding used when TIG welding a high-Mn-containing steel material with Mn being 15.0 to 30.0% by mass, having a composition containing, by mass%, C: 0.20 to 0.80%, Si: 0.15 to 0.90%, Mn: 15.0 to 27.1%, P: 0.030% or less, S: 0.030% or less, Cr: 6.0 to 15.0%, N: 0.120% or less, further containing one or two selected from Ni: 0.25% or more and 10.00% or less and Mo: 0.02% or more and 3.50% or less, and the balance being Fe and inevitable impurities.

2. The filler metal for TIG welding used when TIG welding the high-Mn-containing steel material according to Claim 1, further containing, in addition to the above composition, one or more selected from, by mass%, V: 0.001% or more and 0.20% or less, Ti: 0.001% or more and 0.20% or less, and Nb: 0.001% or more and 0.30% or less.

3. The filler metal for TIG welding used when TIG welding the high-Mn-containing steel material according to Claim 1 or 2, further containing, in addition to the above composition, one or more selected from, by mass%, Cu: 0.01% or more and 0.10% or less, Al: 0.002% or more and 0.100% or less, Ca: 0.001% or more and 0.010% or less, and REM: 0.001% or more and 0.020% or less.

Images