Solid wire for arc welding

Abstract

The purpose of the present invention is to provide a solid wire for arc welding. This solid wire for arc welding according to the present invention has a chemical composition containing, in mass%, C: 0.05-1.00%, Si: 0.05-1.00%, Mn: 30.0-40.0%, P: 0.100% or less, S: 0.100% or less, Al: 0.005-5.000%, Cr: 0.400-10.000% and, if necessary, at least one selected from the group consisting of Ni: 15.0% or less, Mo: 5.00% or less, Cu: 1.00% or less, V: 1.000% or less, Nb: 1.000% or less, Ti: 1.000% or less, N: 0.400% or less, W: 1.000% or less, Ca: 0.020% or less, REM: 0.020% or less, Sn: 0.016% or less, Zn: 0.005% or less, with the balance being Fe and unavoidable impurities.

Description

Solid wire for arc welding

[0001] The present invention relates to a welding material suitable for arc welding of structural steel used in extremely low-temperature environments, such as tanks for storing liquid hydrogen, liquid helium, and liquefied gases. In particular, it relates to a solid wire for arc welding (hereinafter also simply referred to as "welding wire") that exhibits excellent toughness at cryogenic temperatures.

[0002] Welded joints in structures for liquid hydrogen, liquid helium, and liquefied gas storage tanks are required to have excellent toughness at cryogenic temperatures because they are used in extremely low-temperature environments. For example, welded structures for liquid helium storage tanks must have excellent toughness at temperatures below -269°C, the boiling point of helium. Note that the boiling point of liquid hydrogen is -253°C, and that of liquid nitrogen is -196°C, but in the description of this invention, "cryogenic temperature" refers to temperatures below -269°C. Poor cryogenic toughness of welded structures may compromise their safety as cryogenic welded structures; therefore, there is a high demand for improved cryogenic toughness in welded joints used in this application.

[0003] In response to this requirement, austenitic stainless steels have been proposed that have an austenitic phase in their steel sheet structure that does not exhibit brittleness at extremely low temperatures (for example, Patent Documents 1 and 2).

[0004] However, the weld metal contains several percent of ferrite to suppress high-temperature cracking during welding. The ferrite phase may become brittle at extremely low temperatures, posing a challenge in terms of cryogenic toughness. Furthermore, due to the high cost of the alloy and manufacturing, there is a demand for a welding material that is inexpensive and has excellent cryogenic toughness.

[0005] Furthermore, the use of high-Ni steel, which has a large amount of Ni added as an austenite-stabilizing element, as structural steel in a -253°C environment has been proposed, for example, in Patent Documents 3 to 5. However, since a large amount of Ni is added to the steel sheet, and Ni groups are often used in the welding material, the problem of high alloy costs remains.

[0006] Therefore, as a new alternative to these, high-Mn austenitic steel, which ensures austenitic stability with relatively inexpensive Mn, and its welding materials have been proposed (for example, Patent Documents 6-8).

[0007] Japanese Patent Publication No. 2001-287077, Japanese Patent Publication No. 2015-171729, Japanese Patent No. 7188648, Japanese Patent No. 6620659, International Publication No. 2019 / 082325, Japanese Patent No. 6693217, Japanese Patent No. 6978615, Japanese Patent No. 7029034

[0008] However, when a δ-ferrite phase is formed in the weld metal to prevent weld solidification cracking, the welds obtained by the techniques described in Patent Documents 1 and 2 do not achieve the desired cryogenic toughness. On the other hand, when the content of impurity elements such as P and S is drastically reduced to prevent weld solidification cracking, the cost of refining increases, and the amount of Ni and Cr added also increases, leading to the problem of increased costs for the material itself.

[0009] Furthermore, welds obtained by the technologies described in Patent Documents 3 to 5 can sometimes ensure cryogenic toughness. However, they have a high Ni content, and there was a need to reduce material costs.

[0010] Furthermore, the low-temperature toughness of welds obtained by the technologies described in Patent Documents 6 to 8 is guaranteed at -196°C. However, these technologies have the problem that they cannot guarantee the toughness of welds at extremely low temperatures such as liquid hydrogen and liquid helium, and are therefore unsuitable for welds in structures for storage tanks of liquefied gases such as liquid hydrogen and liquid helium.

[0011] Therefore, the present invention aims to provide a solid wire for arc welding that can produce welded joints made of weld metal with excellent weldability, high strength, and excellent cryogenic toughness, while reducing material costs.

[0012] To achieve the above objectives, the inventors investigated the weld metal composition necessary for welded joints fabricated by MAG welding to possess the desired high strength. As a result, they found that using a welding wire containing 30% by mass or more of Mn, an austenite (γ) stabilizing element, is effective in obtaining inexpensive, high-strength welded joints with excellent cryogenic toughness. Since Mn is a strong austenite (γ) stabilizing element yet is inexpensive compared to Ni, they considered it effective to utilize Mn.

[0013] Next, we conducted intensive research on various factors determining the chemical composition of welding materials, focusing on steel types with relatively high Mn content. As a result, we identified segregation of impurity elements such as P and S into the final solidified portion of the weld metal as a factor in the occurrence of high-temperature cracking. Furthermore, we found that segregation of impurity elements becomes more pronounced when the temperature range from the start to the completion of solidification is wide during the solidification of the weld metal. Based on this, we found that by appropriately controlling the chemical composition of the welding material, the impurity elements P and S can be crystallized from the liquid phase during solidification. This suppresses segregation into the final solidified portion, thereby preventing solidification cracking.

[0014] In particular, we found that when the weld metal reaches a temperature above the solidus due to the thermal effects of welding in subsequent weld passes and liquefies, segregation becomes significant when the temperature range from the start of resolidification to the completion of resolidification (hereinafter also referred to as the "solid-liquid coexistence temperature range") is large.

[0015] However, it was found that even if P and S crystallize to prevent solidification cracking, and these crystallized parts liquefy due to subsequent thermal history, segregation can be suppressed by preferentially crystallizing again during cooling. This revealed that liquefaction cracking can be prevented.

[0016] Furthermore, it was discovered that blowholes and pits during MAG welding occur when the nitrogen (N) content exceeds a certain value. Note that "MAG welding" may be referred to simply as "arc welding" below.

[0017] The present invention was obtained by conducting intensive research on the factors of various additive elements in welding wires, in addition to the above findings, and its gist is as follows: [1] A solid wire for arc welding used in arc welding, wherein the chemical composition of the solid wire is, in mass%, C: 0.05% to 1.00%, Si: 0.05% to 1.00%, Mn: 30.0% to 40.0%, P: 0.100% or less, S: 0.100% or less, Al: 0.005% to 5.000%, Cr: 0.400% to 10.000%, with the remainder being Fe and unavoidable impurities. [2] The solid wire for arc welding, wherein the chemical composition of the solid wire in the above [1] further contains one or more selected from Ni: 15.0% or less and Mo: 5.00% or less by mass%. [3] The solid wire for arc welding, wherein the chemical composition of the solid wire in the above [1] or [2] further contains one or more selected from Cu: 1.00% or less, V: 1.000% or less, Nb: 1.000% or less, Ti: 1.000% or less, N: 0.400% or less, W: 1.000% or less, Ca: 0.020% or less, REM: 0.020% or less, Sn: 0.016% or less, and Zn: 0.005% or less by mass%. [4] A solid wire for arc welding whose chemical composition in any one of [1] to [3] satisfies the following formulas (1) and (2): 30.0% ≤ [%Mn] - [%S] ≤ 40.0% ... (1) 0.40% ≤ [%Cr] - 3 × [%P] ≤ 10.00% ... (2) Here, in each formula, [%Mn], [%S], [%Cr], and [%P] are the mass percentages of Mn, S, Cr, and P contained in the solid wire, respectively.

[0018] According to the present invention, it is possible to provide a solid wire for arc welding that yields welded joints with excellent weldability, high strength, and excellent cryogenic toughness. Therefore, welded structures using the welding wire according to the present invention can be used in cryogenic environments such as storage tanks for liquefied gases such as liquid hydrogen and liquid helium, and further contribute greatly to improving the safety and lifespan of such structures, resulting in remarkable industrial benefits. In addition, the solid wire for arc welding according to the present invention is an economically superior material because it does not cause an increase in material costs.

[0019] Figure 1 is a schematic diagram showing an example of the macroscopic cross-sectional shape of a multilayer weld fabricated using the welding wire according to the present invention. Figure 2 is a schematic diagram showing an example of the groove shape for arc welding. Figure 3 is an overhead view showing an example of the preparation process for arc welding using the welding wire according to the present invention. Figure 4 is a schematic cross-sectional view showing an example of the specimen sampling position for a Charpy impact test.

[0020] The welding wire according to the present invention provides a welded joint with excellent weldability, high strength, and excellent cryogenic toughness. The welding wire mentioned above refers to a solid wire for arc welding, and in the following description, it may also be simply referred to as "solid wire."

[0021] Here, "excellent weldability" means that the welded area is free from blowholes, pits, hot cracks, etc., and is judged as Class 1 in non-destructive testing in accordance with JIS Z 3104 (1995). Furthermore, "high strength" means having a 0.2% proof stress or yield stress of 325 MPa or higher and a tensile strength of 490 MPa or higher at room temperature. Note that "room temperature" above refers to 25°C. In addition, "excellent cryogenic toughness" means that the absorbed energy (vE) in Charpy impact tests at -196°C and -269°C is excellent. －１９６ and veE －２６９ This refers to a value of 27J or higher.

[0022] [Solid Wire] First, the solid wire for arc welding according to the present invention will be described. Hereafter, "%" in the chemical composition means "mass%".

[0023] [Basic Chemical Composition of the Wire] The welding wire according to the present invention is a solid wire having the following basic chemical composition: C: 0.05% to 1.00%, Si: 0.05% to 1.00%, Mn: 30.0% to 40.0%, P: 0.100% or less, S: 0.100% or less, Al: 0.005% to 5.000%, Cr: 0.400% to 10.000%, with the remainder being Fe and unavoidable impurities. The reasons for these limitations on chemical composition will be explained below.

[0024] [C: 0.05% to 1.00%] Carbon (C) is an element that increases the strength of weld metal through solid solution strengthening. Furthermore, C stabilizes the austenite phase and improves the cryogenic impact toughness of the weld metal. To obtain these effects, a carbon content of 0.05% or more is required.

[0025] On the other hand, if the carbon content exceeds 1.00%, high-temperature cracking during welding becomes more likely. Also, if the carbon content exceeds 1.00%, carbides precipitate, reducing cryogenic impact toughness. For this reason, the carbon content was limited to a range of 0.05% to 1.00%. Preferably, the carbon content is 0.15% or more, and more preferably 0.80% or less. More preferably, the carbon content is 0.60% or less. Most preferably, the carbon content is 0.25% to 0.45%.

[0026] [Si: 0.05% to 1.00%] Si acts as a deoxidizing agent, increasing the yield of Mn, and also increases the viscosity of the weld metal, stably maintaining the bead shape. Furthermore, Si suppresses the precipitation of carbides, causing C to solid-solve in the austenite phase, stabilizing the austenite phase and improving cryogenic toughness. In addition, Si reduces the stacking fault energy of the weld metal, increasing work hardening ability and improving the tensile strength of the weld metal. To obtain such effects, a Si content of 0.05% or more is required.

[0027] On the other hand, if the Si content exceeds 1.00%, the Si segregates during solidification, forming a liquid phase at the solidification cell interface and reducing the high-temperature crack resistance. Si also reduces the cryogenic toughness of the weld metal. For this reason, the Si content was limited to a range of 0.05% to 1.00%. Preferably, the Si content is 0.15% or more, and more preferably 0.80% or less. More preferably, the Si content is 0.60% or less. Most preferably, the Si content is 0.25% to 0.45%.

[0028] [Mn: 30.0% to 40.0%] Mn is an element that stabilizes the austenite phase inexpensively, and in this invention, it is an essential element for ensuring cryogenic toughness. In addition, during solidification immediately after welding, Mn combines with the impurity element S (sulfur) to crystallize MnS, suppressing solidification segregation of S and improving resistance to high-temperature cracking. Furthermore, by lowering the melting point of the weld metal, Cr phosphides (e.g., CrP, Cr) consisting of the impurity element P (phosphorus) and the alloying element Cr are formed. ２ P, Cr ３ It promotes the crystallization of P (such as phosphorus), suppresses solidification segregation of P, and improves resistance to high-temperature cracking. To obtain these effects, Mn must be present in a content of 30.0% or more.

[0029] On the other hand, if the Mn content exceeds 40.0%, excessive Mn segregation occurs during solidification, inducing high-temperature cracking and degrading cryogenic toughness. Therefore, the Mn content was limited to a range of 30.0% to 40.0%. Preferably, the Mn content is 38.0% or less. More preferably, the Mn content is 36.5% or less.

[0030] [P: 0.100% or less] P is an element that segregates at grain boundaries and induces high-temperature cracking, and also deteriorates cryogenic toughness, so it is desirable to reduce P as much as possible. However, in this invention, the Mn content is 30% or more, the melting point of the molten pool is low, and furthermore, Cr is added, so segregation is suppressed by crystallization as the aforementioned Cr phosphide. Therefore, a P content of 0.100% or less is acceptable. For this reason, the P content is limited to 0.100% or less. The P content is preferably 0.030% or less, and more preferably less than 0.015%. Note that excessive reduction of P leads to a surge in refining costs, so it is preferable to adjust the P content to 0.002% or more.

[0031] [S: 0.100% or less] S is an element that segregates at grain boundaries and induces high-temperature cracking, and also deteriorates cryogenic toughness, so it is preferable to reduce it as much as possible. However, in this invention, the Mn content is 30% or more, and segregation is suppressed by crystallization as MnS during solidification, so S can be tolerated if it is 0.100% or less. For this reason, the S content is limited to 0.100% or less. The S content is preferably 0.030% or less, and more preferably less than 0.015%. Note that excessive reduction of S will lead to a surge in refining costs, so it is preferable to adjust the S content to 0.002% or more.

[0032] [Al: 0.005% to 5.000%] Al acts as a deoxidizer, increasing the viscosity of the molten metal, stably maintaining the bead shape, and playing an important role in reducing spatter generation. Al also contributes to suppressing the occurrence of hot cracks in the weld metal by narrowing the solid-liquid coexistence temperature range. Furthermore, Al contributes to improving the yield strength and local elongation during tensile testing. To obtain these effects, an Al content of 0.005% or more is required.

[0033] On the other hand, if the Al content exceeds 5,000%, a large amount of inclusions will be present, reducing the cryogenic toughness. Therefore, the Al content was limited to a range of 0.005% to 5.000%. Preferably, the Al content is 0.010% or more, and more preferably 4.000% or less. More preferably, the Al content is 0.015% to 3.500%.

[0034] [Cr: 0.400% to 10.000%] Cr acts as an element that stabilizes the austenite phase at extremely low temperatures, improving the cryogenic toughness of the weld metal. Cr also improves the strength of the weld metal, effectively suppressing the occurrence of hot cracking by reducing the temperature range in which solid-liquid material coexists. Furthermore, Cr combines with P (phosphorus) during the solidification process to crystallize Cr phosphide, thereby suppressing hot cracking caused by P and effectively improving the corrosion resistance of the weld metal. To obtain these effects, a Cr content of 0.400% or more is required. The above effects cannot be ensured if the Cr content is less than 0.400%.

[0035] On the other hand, if the Cr content exceeds 10,000%, Cr carbides precipitate at the grain boundaries, causing them to become brittle. These boundaries then open up due to thermal strain introduced during welding, resulting in hot cracking. Furthermore, fracture tends to propagate more easily from these hot cracks, worsening the cryogenic toughness. For this reason, the Cr content was limited to a range of 0.400% to 10.000%. Preferably, the Cr content is 0.600% or more, and more preferably 8.000% or less. More preferably, the Cr content is 5.500% or less.

[0036] [Optional Composition] In addition to the basic chemical composition described above, the welding wire according to the present invention may optionally contain the first group and / or the second group, which are optional compositions described below.

[0037] [First Group of Optional Components] In addition to the basic chemical composition described above, the welding wire according to the present invention can contain, as needed, one or two selected from the first group of optional components, Ni: 15.0% or less and Mo: 5.00% or less. Both Ni and Mo in this first group are elements that strengthen the austenite grain boundaries and have the effect of suppressing the occurrence of hot cracking by suppressing the embrittlement of the grain boundaries. Note that each component described in this first group can be contained as needed, so the content of each of these components may be 0%. These will be described below.

[0038] [Ni: 15.0% or less] As described above, Ni is an element that strengthens the austenite grain boundaries and suppresses the occurrence of hot cracking by suppressing the embrittlement of the grain boundaries. In order to obtain such an effect, it is preferable to contain Ni at 0.1% or more. Also, since Ni has the effect of stabilizing the austenite phase, further increasing the Ni content stabilizes the austenite phase and improves the extremely low temperature toughness of the weld metal.

[0039] On the other hand, Ni is an expensive element, and containing more than 15.0% is economically disadvantageous. Therefore, when containing Ni, the Ni content is preferably in the range of 0.1% or more and 15.0% or less. Note that the Ni content is more preferably 0.5% or more and more preferably 11.0% or less. The Ni content is even more preferably 1.0% or more and even more preferably 9.5% or less.

[0040] [Mo: 5.00% or less] Mo is also an element that strengthens the austenite grain boundaries as described above and suppresses the occurrence of hot cracking by suppressing the embrittlement of the grain boundaries. Note that Mo also has the effect of improving the strength of the weld metal. In order to obtain such an effect, it is preferable to contain Mo at 0.01% or more.

[0041] On the other hand, if the content of Mo exceeds 5.00%, the interior of the grains will be overly hardened, making the grain boundaries relatively weak and leading to high-temperature cracking. Moreover, starting from the generated high-temperature cracking, fracture is likely to progress, deteriorating the toughness at extremely low temperatures. Therefore, when Mo is contained, the Mo content is preferably in the range of 0.01% or more and 5.00% or less. Note that the Mo content is more preferably 0.50% or more and more preferably 4.00% or less. The Mo content is even more preferably 3.00% or less.

[0042] [Second group of optional components]In addition to the aforementioned basic chemical composition, in the present invention, as a second group of optional components, as necessary, Cu: 1.00% or less, V: 1.000% or less, Nb: 1.000% or less, Ti: 1.000% or less, N: 0.400% or less, W: 1.000% or less, Ca: 0.020% or less, REM: 0.020% or less, Sn: 0.016% or less, Zn: 0.005% or less can be contained, selected from one or more of these. Among these components in the second group, Cu, V, Nb, Ti, N, W, Ca, and REM are all elements that stabilize the austenite phase and contribute to improving the strength of the weld metal.

[0043] Note that each component described in this second group can be contained as necessary, so the content of each of these components may be 0%. These will be described below.

[0044] [Cu: 1.00% or less]Cu is an element that stabilizes the austenite phase, and in order to obtain such an effect, it is preferably contained at 0.01% or more. The Cu content is more preferably 0.02% or more.

[0045] On the other hand, if a large amount of Cu is contained in excess of 1.00%, a low-melting liquid phase will form at the austenite grain boundaries, making high-temperature cracking likely to occur. Moreover, starting from the generated high-temperature cracking, fracture is likely to progress, deteriorating the toughness at extremely low temperatures. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less and even more preferably 0.60% or less.

[0046] [V: 1.000% or less] V is a carbide-forming element that precipitates fine carbides within austenite grains, contributing to improved strength of the weld metal. To obtain this effect, it is preferable that V be present in an amount of 0.001% or more. More preferably, the V content is 0.002% or more.

[0047] On the other hand, if the V content exceeds 1.000%, the excess carbides become the starting point for fracture, resulting in a decrease in low-temperature toughness. Therefore, when V is included, it is preferable that the V content be 1.000% or less. More preferably, the V content is 0.600% or less.

[0048] [Nb: 1.000% or less] Nb is a carbide-forming element that precipitates fine carbides within austenite grains, contributing to improved strength of the weld metal. To obtain this effect, it is preferable that the Nb content be 0.001% or more. More preferably, the Nb content is 0.002% or more.

[0049] On the other hand, if the Nb content exceeds 1.000%, the excess carbides become the starting point for fracture, resulting in a decrease in low-temperature toughness. Therefore, when Nb is included, it is preferable that the Nb content be 1.000% or less. More preferably, the Nb content is 0.600% or less.

[0050] [Ti: 1.000% or less] Ti is a carbide-forming element that precipitates fine carbides within austenite grains, contributing to improved strength of the weld metal. To obtain this effect, it is preferable that the Ti content be 0.001% or more. More preferably, the Ti content is 0.002% or more.

[0051] On the other hand, if the Ti content exceeds 1.000%, the excess carbides become the starting point for fracture, resulting in a decrease in low-temperature toughness. Therefore, when Ti is included, it is preferable that the Ti content be 1.000% or less. More preferably, the Ti content is 0.600% or less, even more preferably 0.500% or less, even more preferably 0.450% or less, and most preferably 0.400% or less.

[0052] [N: 0.400% or less] Although nitrogen is an unavoidable element, like carbon, it effectively contributes to improving the strength of the weld metal, stabilizing the austenite phase, and stably improving cryogenic toughness. Furthermore, increasing the content of nitrogen has the effect of narrowing the temperature range in which solid and liquid coexist, and is an additive element that suppresses the occurrence of high-temperature cracking in high-Mn steel. Since this effect becomes significant with a nitrogen content of 0.003% or more, it is preferable to have a nitrogen content of 0.003% or more.

[0053] On the other hand, if the N content exceeds 0.400%, nitrides are formed, reducing low-temperature toughness, and the amount of solid-solution N becomes excessive, leading to welding defects such as blowholes and pits. Therefore, when N is included, it is preferable that the N content be 0.400% or less. More preferably, the N content is 0.300% or less, even more preferably 0.200% or less, even more preferably 0.100% or less, and most preferably 0.050% or less.

[0054] [W: 1.000% or less] W is a carbide-forming element that precipitates fine carbides within austenite grains, contributing to improved strength of the weld metal. To obtain this effect, it is preferable that W be present in an amount of 0.001% or more. More preferably, the W content is 0.002% or more.

[0055] On the other hand, if the W content exceeds 1.000%, the excess carbides become the starting point for fracture, reducing low-temperature toughness. Also, high-temperature cracking occurs due to segregation of W in the final solidification area. Therefore, when W is included, it is preferable that the W content be 1.000% or less. More preferably, the W content is 0.600% or less, even more preferably 0.550% or less, and most preferably 0.500% or less.

[0056] [Ca: 0.020% or less] Ca is an element that contributes to suppressing hot cracking. Ca suppresses hot cracking by bonding with S in the molten metal to form a high-melting-point sulfide called CaS. This effect becomes significant with a Ca content of 0.001% or more. A more preferable Ca content is 0.002% or more.

[0057] On the other hand, if the Ca content exceeds 0.020%, arc disturbance occurs during welding, making stable welding difficult. Furthermore, cryogenic toughness deteriorates. In addition, hot cracking occurs due to segregation of Ca in the final solidification area. For this reason, when Ca is included, the Ca content is preferably 0.020% or less. More preferably, the Ca content is 0.016% or less. Even more preferably, the Ca content is 0.010% or less.

[0058] [REM: 0.020% or less] REM is a powerful deoxidizing agent and exists in the weld metal in the form of REM oxide. REM oxide acts as a nucleation site during solidification, altering the solidification morphology of the weld metal and contributing to the suppression of hot cracking. This effect becomes significant with a REM content of 0.001% or more. A more preferable REM content is 0.002% or more.

[0059] On the other hand, if the REM content exceeds 0.020%, the arc stability decreases. Also, the cryogenic toughness deteriorates. Therefore, when REM is included, it is preferable that the REM content be 0.020% or less. More preferably, the REM content is 0.016% or less, even more preferably 0.080% or less, and even more preferably 0.010% or less.

[0060] Here, REM is a collective term for 17 elements, consisting of the 15 lanthanide elements plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content refers to the total content of these elements.

[0061] [Sn: 0.016% or less] Sn is an element that improves weldability when present in appropriate amounts, and additionally improves corrosion resistance. These effects become significant when the Sn content is 0.001% or higher.

[0062] On the other hand, if the Sn content exceeds 0.016%, grain boundary segregation occurs during solidification, causing solidification cracks. Therefore, when Sn is included, it is preferable that the Sn content be 0.016% or less. More preferably, the Sn content is 0.012% or less, and even more preferably 0.008% or less.

[0063] [Zn: 0.005% or less] Zn is an element that improves weldability when present in appropriate amounts, and additionally improves corrosion resistance. These effects appear when the Zn content is 0.001% or higher.

[0064] On the other hand, if the Zn content exceeds 0.005%, grain boundary segregation occurs during solidification, forming a low-melting-point alloy and causing solidification cracking. Therefore, when Zn is included, it is preferable that the Zn content be 0.005% or less. More preferably, the Zn content is 0.004% or less, and even more preferably 0.003% or less.

[0065] [Remaining Composition] The remainder of the composition other than that described above consists of Fe and unavoidable impurities. Unavoidable impurities include, for example, O (oxygen), B (boron), Mg, Pb, As, Sb, and Bi, which are inevitably mixed in during the melting of wire material, wire drawing, or steel material. The total amount of these is permitted to be 0.1% or less. However, as long as the basic chemical composition and optional composition described above are satisfied, there is no prejudice that other unavoidable impurity elements may be included, and such embodiments are also included within the technical scope of the present invention.

[0066] [Relationships between specific elements] In addition to the content requirements for each individual element of the composition described above, it has been found that for the following compositions, it is preferable that the relationships shown in formulas (1) and (2) below be satisfied. 30.0% ≤ [%Mn] - [%S] ≤ 40.0% ... (1) 0.40% ≤ [%Cr] - 3 × [%P] ≤ 10.00% ... (2) Here, in each formula, [%Mn], [%S], [%Cr], and [%P] are the mass percentages of the content of Mn, S, Cr, and P in the solid wire, respectively.

[0067] First, regarding equation (1), since Mn and S crystallize as MnS during solidification, the yield of Mn changes depending on the amount of S. For the solid-solution Mn to exhibit the aforementioned effects, it is preferable that the value of "[%Mn] - [%S]" is 30.0% or more. On the other hand, if the value of "[%Mn] - [%S]" exceeds 40.0%, there will be an excess of solid-solution Mn, which may induce hot cracking and may also degrade the cryogenic toughness. Therefore, it is preferable that the value of "[%Mn] - [%S]" be in the range of 30.0% to 40.0%. The value of "[%Mn] - [%S]" is more preferably 31.0% or more, and more preferably 39.0% or less. This value is even more preferably 32.0% or more, and even more preferably 38.0% or less. This value is most preferably 33.0% or more.

[0068] Furthermore, regarding equation (2), Cr and P are Cr phosphides (CrP, Cr) produced during coagulation. ２ P, Cr ３ Because it crystallizes as P (such as P), the yield of Cr changes depending on the amount of P. For solid-solution Cr to exhibit the aforementioned effects, it is preferable that the value of "[%Cr] - 3 × [%P]" is 0.40% or more. On the other hand, if the value of "[%Cr] - 3 × [%P]" exceeds 10.00%, Cr carbides precipitate at the grain boundaries, causing the grain boundaries to become brittle. These boundaries may open up due to thermal strain introduced during welding, potentially leading to hot cracking. Therefore, it is preferable that the value of "[%Cr] - 3 × [%P]" be in the range of 0.40% to 10.00%. The value of "[%Cr] - 3 × [%P]" is more preferably 0.60% or more, and more preferably 8.00% or less. This value is even more preferably 0.80% or more, and even more preferably 7.00% or less. This value is most preferably 1.00% or more.

[0069] [Method for Manufacturing Welding Wire] Next, a method for manufacturing welding wire according to the present invention will be described.

[0070] The method for manufacturing welding wire according to the present invention does not need to be particularly limited to the manufacturing method, other than using molten steel having the chemical composition described above, and any commonly used method for manufacturing welding wire can be applied.

[0071] For example, molten steel having the chemical composition described above is melted in a conventional melting furnace such as an electric furnace or a vacuum melting furnace, and cast into a mold of a predetermined shape to obtain a steel ingot. This process is called the "casting process". Next, the obtained steel ingot is heated to a predetermined temperature (for example, heating temperature: 1100°C to 1300°C). This process is called the "heating process". Next, the heated steel ingot is subjected to hot rolling to obtain a steel material of a predetermined shape. This process is called the "hot rolling process". Next, the obtained steel material (in rod shape) is subjected to multiple cold rolling (i.e., cold wire drawing) processes to obtain a steel wire with a diameter of, for example, 1.0 mm to 2.8 mm. If necessary, annealing (for example, annealing temperature: 900°C to 1200°C) is performed to obtain a welding wire of the desired dimensions (for example, a diameter of 0.8 mm to 1.6 mm). This process is called the "cold rolling process". By sequentially performing the above processes, the welding wire according to the present invention can be manufactured.

[0072] [Steel Material] Examples of steel materials applicable to this invention include high-Mn austenitic steel materials that have excellent austenitic stability and cryogenic toughness, achieved with relatively inexpensive Mn. The plate thickness of the steel material is not particularly limited, but examples include thick steel plates of 5 mm to 102 mm.

[0073] As an example of the chemical composition of the steel material used, it is preferable that it contains, by mass%, C: 0.30 to 0.60%, Si: 0.05 to 1.00%, Mn: 25.0 to 40.0%, P: 0.030% or less, S: 0.010% or less, Al: 0.100% or less, Cr: 6.0% or less, and N: 0.020% or less, with the remainder being Fe and unavoidable impurities.

[0074] Furthermore, the chemical composition of the steel material preferably contains one or more elements selected from the following as an optional composition: Ni: 2.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.050% or less, W: 0.050% or less, Ca: 0.005% or less, and REM: 0.050% or less. Each of these optional components can be included as needed, and the content of each component may be 0%. The optional composition can be determined considering strength, toughness, etc., according to the intended use of the steel material.

[0075] [Welding Method] As an example of an arc welding method using the welding wire of the present invention, the MAG welding method will be described. The MAG welding method is one of the gas shielded arc welding methods and is widely used. The MAG welding method uses a gas in which gases such as Ar, CO ２ , O ２ are appropriately mixed to protect (shield) the arc and the molten metal from the atmosphere (air). As an example of the mixing ratio of the gases in the shielding gas of MAG welding, there is one in which the CO ２ gas is 5 vol% to 30 vol% and the balance is Ar gas.

[0076] Next, specific examples of welding conditions and the like will be illustrated and described with respect to the arc welding method. An example of the macro shape of the welding cross section in the case of multi-layer welding is shown in FIG. 1. The plate thickness t of the steel material serving as the base material 1 is preferably in the range of 5 mm to 102 mm. In the example of FIG. 1, the plate thickness t is 12 mm.

[0077] When producing the test piece described later, in accordance with JIS Z 3111 (2005), the base materials 1 are butted together to form a V-groove with a groove angle θ of 35° to 80°. The diameter of the welding wire used as the electrode for arc welding is preferably in the range of 1.0 mmφ to 1.6 mmφ. Preheating may not be performed.

[0078] Other welding conditions are carried out at a current: 180 A to 300 A (DCEP), a voltage: 20 V to 40 V, a welding speed: 8 cm / min to 50 cm / min, and a heat input per unit length of weld: 1.0 kJ / cm to 52.0 kJ / cm.

[0079] Here, when the current is less than 180 A, the welding wire cannot be sufficiently melted and the arc becomes unstable. On the other hand, when the current exceeds 300 A, the transfer of the molten droplets becomes unstable and a large amount of spatter occurs.

[0080] When the voltage is less than 20 V, the arc length becomes short, the arc becomes unstable and the spatter increases. Also, the spread of the arc is insufficient and the weld bead becomes convex. On the other hand, when the voltage exceeds 40 V, the arc length extends excessively, the arc spreads too much, and as a result, the penetration becomes shallow and fusion defects are likely to occur.

[0081] If the welding speed is less than 8 cm / min, the penetration will be too deep, leading to see-through, increased deformation during welding, and a wider heat-affected zone, which reduces the toughness of the weld. On the other hand, if the welding speed exceeds 50 cm / min, the bead will be thin and convex, resulting in poor fusion and a decrease in the quality of the appearance.

[0082] Furthermore, if the welding heat input is less than 1.0 kJ / cm, the heat input is low, and the base metal and weld metal do not fuse properly, resulting in a humping bead. Also, if the welding heat input exceeds 52.0 kJ / cm, the bead shape becomes irregular, the heat-affected zone becomes large, the toughness of the welded joint decreases, and the amount of deformation during welding increases. A more preferable welding heat input is 1.0 kJ / cm to 48.0 kJ / cm. An even more preferable welding heat input is 5.0 kJ / cm to 45.0 kJ / cm. An even more preferable welding heat input is 8.0 kJ / cm to 40.0 kJ / cm.

[0083] Furthermore, the interpass temperature should preferably be 200°C or lower. Examples of welding positions include downward, sideways, upward, and vertical upward.

[0084] Furthermore, while the V-groove shown in Figure 2 is used as the groove shape, other types include the L-groove, K-groove, and X-groove. As mentioned above, the groove angle θ in the V-groove shown in Figure 2 is preferably 35° to 80°, and more preferably 40° to 60°.

[0085] Under the aforementioned welding conditions, weld metal 2 is formed in the V-groove by single-layer or multi-layer welding, and this weld metal 2 is in the form of multiple layers of weld metal stacked on top of each other. Furthermore, by performing multi-layer welding under the above welding conditions, welding defects such as hot cracking do not occur in any part of the weld metal layer, and the strength of the weld metal can be ensured.

[0086] The chemical composition of this weld metal 2 is not necessarily within the numerical ranges of the chemical composition of the welding wire described above. This is because the wire and base metal dilute each other during welding to form the weld metal. The chemical composition of this weld metal 2 is the average value at the center of the weld metal at the 1 / 2 position in the plate thickness direction, as described in the examples below.

[0087] The present invention will be further described below based on the following examples.

[0088] Molten steel having a chemical composition corresponding to the welding wire chemical composition shown in Table 1 was melted in a vacuum melting furnace and cast to obtain a 1000 kg steel ingot. The obtained steel ingot was heated to 1200°C, then hot-rolled, followed by cold-rolling, and annealed as necessary (annealing temperature: 900-1200°C) to obtain a 1.2 mmφ solid wire for MAG welding with the chemical composition shown in Table 1.

[0089] Note that the numerical value shown in "Equation (1)" in the "Relationship Formula" column of Table 1 is the calculated value of "[%Mn] - [%S]" in Equation (1), and the value shown in "Equation (2)" is the calculated value of "[%Cr] - 3 × [%P]" in Equation (2).

[0090] Furthermore, the "-" in Table 1 indicates that elements were intentionally omitted, and this includes not only cases where the elements are not present (i.e., 0%), but also cases where the elements are inevitably present.

[0091]

[0092] Next, using high-Mn steel (12 mm thick, 500 mm wide) with the chemical composition shown in Table 2 as the base material, a V-groove with a groove angle θ: 60° was formed by butting the pieces together with a groove gap G: 2 mm, as shown in Figure 1. A ceramic material was selected for the backing material 4.

[0093]

[0094] Furthermore, as shown in Figure 3, in order to prevent angular deformation during welding, a U-shaped restraint plate 5, measuring 25 mm thick x 150 mm wide x 350 mm long and made of the same steel type as the base material 1, was welded to the back of the welded joint. The dimensions of the restraint plate 5 are shown from left to right as plate thickness, plate width, and plate height. The welding length was 500 mm, and the restraint plate 5 was welded in a total of three places at 100 mm, 250 mm, and 400 mm from the welding start side. Using the welding wire obtained above, MAG welding was performed to form weld metal in the V-groove described above, and a welded joint was obtained.

[0095] This MAG welding was performed using welding wires (diameter 1.2 mmφ) with the compositions shown in Table 1, without preheating, and in a downward position. The welding conditions were: current: 180A to 300A (DCEP), voltage: 20V to 40V, welding speed: 8 cm / min to 50 cm / min, and interpass temperature: 100°C to 150°C. The welding heat input range was 1.0 kJ / cm to 52.0 kJ / cm. The shielding gas was Ar + 20% CO2. ２ The gas flow rate was set to 20 L / min. Furthermore, multi-layer welding was performed, with 7 to 10 passes.

[0096] [Weldability Evaluation] After welding, the welded joint was visually inspected to determine whether or not pits were present in the weld metal. Subsequently, a radiographic test was performed in accordance with JIS Z 3104 (1995) to determine whether or not blowholes and hot cracks were present. If there were any defects that could not be determined, a 10 mm thick macro specimen was taken from the center of the weld line using a microcutter, with the observation surface perpendicular to the weld line. The cross-section of the weld metal was observed using an optical microscope (30x magnification) on this macro specimen to determine whether or not hot cracks were present. In specimens where hot cracks were observed, an elongated black area measuring 25 μm in width and 80 μm in length or more was observed. If defects such as hot cracks, blowholes, or pits were observed, they were judged as "×", and if they were not observed, they were judged as "○". The symbol "×" used here means failure, and the symbol "○" means passing. The same applies hereafter.

[0097] [Evaluation of Weld Metal Properties (Strength, Cryogenic Toughness)] Tensile test specimens (parallel section diameter 6 mmφ) of the weld metal were taken from the obtained welded joints in accordance with the provisions of JIS Z 3111 (2005), and tensile tests were performed. Three tensile tests were performed on each specimen at room temperature, and the average value of the obtained values ​​(0.2% proof stress, tensile strength) was used as the tensile properties of the weld metal of the welded joint. A "○" was given if the 0.2% proof stress or yield stress was 325 MPa or higher and the tensile strength was 490 MPa or higher at room temperature, and a "×" was given if either property fell below the specified value.

[0098] In addition, a Charpy impact test of the weld metal was also performed in accordance with the provisions of JIS Z 3111 (2005). The sampling position of the test specimen (V-notch) is shown in Figure 4. The direction of the V-notch of the test specimen 6 is perpendicular to the surface of the base metal 1, and the position of the V-notch is the midpoint of the melting line of the weld metal 2 on the center line of the test specimen. The test specimen 6 was taken from a position 1 mm below the surface of the base metal 1. In the case of the Charpy impact test, three specimens were also performed, and the absorbed energy (vE) at test temperatures of -196°C and -269°C was measured. －１９６ and veE －２６９ The following was determined: The average of the absorbed energy from three Charpy impact tests at each test temperature was used as the value of the cryogenic impact toughness of the weld metal of the welded joint. If this value exceeded 27 J, it was evaluated as "○", and if it fell below 27 J, it was evaluated as "×".

[0099] Furthermore, the chemical composition of the weld metal was measured by wet analysis using a chip-like test specimen taken from the center of the weld metal at the 1 / 2 position in the plate thickness direction.

[0100] The results obtained are shown in Table 3.

[0101]

[0102] All welded joints fabricated using the welding wire of the present invention were welded joints that did not exhibit hot cracking, blowholes, or pitting during welding, and were made of weld metal with high strength and excellent cryogenic toughness. On the other hand, welded joints fabricated using the welding wire of the comparative example exhibited hot cracking, blowholes, or pitting during welding, or were made of weld metal with low strength, inferior cryogenic toughness, or a combination thereof.

[0103] Furthermore, for welded joint No. 34 (welding wire No. 34) in Table 3, a sound test specimen could not be obtained due to the large number of pits and blowholes within the weld metal. Therefore, the above evaluation of weld metal properties (strength, cryogenic toughness) was not performed.

[0104] 1. Base material (steel) 2. Weld metal 3. Groove shape 4. Backing material 5. Retaining plate 6. Test specimen t: Plate thickness θ: Groove angle G: Groove gap

Claims

1. A solid wire for arc welding used in arc welding, wherein the chemical composition of the solid wire is, by mass%, C: 0.05% to 1.00%, Si: 0.05% to 1.00%, Mn: 30.0% to 40.0%, P: 0.100% or less, S: 0.100% or less, Al: 0.005% to 5.000%, Cr: 0.400% to 10.000%, with the remainder being Fe and unavoidable impurities.

2. The arc welding solid wire according to claim 1, wherein the chemical composition of the solid wire further contains one or two selected from Ni: 15.0% or less and Mo: 5.00% or less by mass.

3. The arc welding solid wire according to claim 1 or 2, wherein the chemical composition of the solid wire further contains one or more selected by mass% from: Cu: 1.00% or less, V: 1.000% or less, Nb: 1.000% or less, Ti: 1.000% or less, N: 0.400% or less, W: 1.000% or less, Ca: 0.020% or less, REM: 0.020% or less, Sn: 0.016% or less, Zn: 0.005% or less.

4. The arc welding solid wire according to claim 1 or 2, wherein the chemical composition of the solid wire satisfies the following formulas (1) and (2): 30.0% ≤ [%Mn] - [%S] ≤ 40.0% ... (1) 0.40% ≤ [%Cr] - 3 × [%P] ≤ 10.00% ... (2) Here, [%Mn], [%S], [%Cr], and [%P] in each formula are the mass percentages of Mn, S, Cr, and P contained in the solid wire, respectively.

5. The arc welding solid wire according to claim 3, wherein the chemical composition of the solid wire satisfies the following formulas (1) and (2): 30.0% ≤ [%Mn] - [%S] ≤ 40.0% ... (1) 0.40% ≤ [%Cr] - 3 × [%P] ≤ 10.00% ... (2) Here, [%Mn], [%S], [%Cr], and [%P] in each formula are the mass percentages of Mn, S, Cr, and P contained in the solid wire, respectively.

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