Flux-cored wire for gas-shielded arc welding and gas-shielded arc welding method

Abstract

The present invention provides a flux-cored wire for gas-shielded arc welding in which the Ni content is 1% or less by mass and weld metal of favorable strength and low-temperature toughness can be obtained both as-welded and after PWHT, regardless of the location of the weld metal. The flux-cored wire for gas-shielded arc welding contains the following with respect to the total wire mass. C: between 0.025% and 0.080% inclusive by mass; Si: between 0.15% and 0.50% inclusive by mass; Mn: between 1.5% and 3.0% inclusive by mass; Ni: at least 0.50% and less than 1.0% by mass; Mo: between 0.05% and 0.40% inclusive by mass; B: between 0.004% and 0.010% inclusive by mass; Fe: at least 85% by mass. The Cu content is not more than 0.25% by mass.

Description

Flux-cored wire for gas-shielded arc welding and gas-shielded arc welding method

[0001] The present invention relates to a flux-cored wire for gas-shielded arc welding used for welding steel materials having a tensile strength of 490 to 670 MPa, and a gas-shielded arc welding method.

[0002] Offshore structures used in the drilling and production of oil, gas, etc., and pipelines used in the transportation of oil, gas, etc., are becoming larger in size and are increasingly operating in cold regions, and the steel plates and welding materials used in the construction of these welded structures are required to have high strength as well as excellent toughness at low temperatures. When constructing such welded structures, post-weld heat treatment (PWHT) is sometimes performed after welding to relieve stress.

[0003] However, the PWHT may reduce the strength and toughness of the weld, making it impossible to obtain the required properties. Therefore, in order to improve the strength and toughness after the PWHT, a flux-cored wire containing Ni may be used.

[0004] However, in the above-mentioned offshore structures and pipelines, problems such as sulfide stress corrosion cracking (SSCC) and hydrogen embrittlement occur due to a hydrogen sulfide-containing (sour) environment. To address these problems, the National Association of Corrosion Engineers (NACE) standard (NACE MR0175) restricts the Ni content in weld metal to 1 mass% or less.

[0005] Therefore, Patent Document 1 discloses a flux-cored wire for gas-shielded arc welding that contains 0.03 to 0.09% C, 0.1 to 0.6% Si, 1.3 to 2.6% Mn, 0.01 to 0.5% Cu, 0.05 to 0.5% Ti, 0.002 to 0.015% B, and 0.05% or less Al, and the contents of oxides, compounds, etc. in the flux are limited, thereby producing a weld metal with good low-temperature toughness (hereinafter also simply referred to as "toughness"). Patent Document 1 also describes that the flux-cored wire further containing 0.1 to 0.5% Ni can have the effect of making the low-temperature toughness of the weld metal more stable. The flux-cored wire described in Patent Document 2 contains, relative to the total mass of the wire, 0.01 to 0.12 mass% of C, 0.05 mass% or more and less than 0.30 mass% of Si, 1.0 to 3.5 mass% of Mn, 0.1 mass% or more and less than 1.0 mass% of Ni, 0.10 to 0.30 mass% of Mo, 0.1 to 0.9 mass% of Cr, and TiO ２ 4.5 to 8.5 mass% of SiO ２ 0.10 to 0.40 mass% of Al ２ O ３ It contains 0.03 to 0.23 mass % of sodium and potassium compounds, and predetermined amounts of sodium compounds and potassium compounds.

[0006] Japanese Patent Publication No. 2017-94360 Japanese Patent No. 6322093

[0007] The flux-cored wire described in Patent Document 1 contains 0.1 to 0.5 mass % of Ni to ensure excellent low-temperature toughness, but does not take into consideration the strength and low-temperature toughness after PWHT.

[0008] Furthermore, in the flux-cored wires described in both Patent Documents 1 and 2, the toughness at temperatures lower than −40°C has not been fully studied, and there is a possibility that sufficient toughness cannot be obtained in lower temperature ranges. Furthermore, in weld metal, the central portion of the weld metal generally has the highest toughness, and the toughness decreases as the distance from the central portion (pass meeting point) of the weld metal approaches the heat-affected zone due to a decrease in cooling rate, etc. Thus, the toughness of the weld metal may rapidly deteriorate depending on the location of the test specimen. However, both Patent Documents 1 and 2 only consider the toughness of the central portion of the weld metal, and there is a possibility that the predetermined toughness cannot be obtained at locations outside the central portion of the weld metal. Therefore, there is a growing demand for flux-cored wires that satisfy the Ni content in the weld metal specified in NACE MR0175 and can obtain weld metal with excellent strength and toughness at low temperatures regardless of the location in the weld metal, both as-welded (as-welded) and after PWHT.

[0009] The present invention has been made in view of the above-mentioned problems, and has an object to provide a flux-cored wire for gas-shielded arc welding and a gas-shielded arc welding method that can provide a weld metal that has good strength and low-temperature toughness regardless of the position in the weld metal, both as-welded and after PWHT, even when the Ni content is 1 mass % or less.

[0010] The above object of the present invention is achieved by the following configuration [1] relating to a flux-cored wire for gas-shielded arc welding.

[0011] [1] A flux-cored wire for gas-shielded arc welding, in which a steel sheath is filled with flux, characterized in that the wire contains, relative to the total mass of the wire: C: 0.025% by mass or more and 0.080% by mass or less, Si: 0.15% by mass or more and 0.50% by mass or less, Mn: 1.5% by mass or more and 3.0% by mass or less, Ni: 0.50% by mass or more and less than 1.00% by mass, Mo: 0.05% by mass or more and 0.40% by mass or less, B: 0.004% by mass or more and 0.010% by mass or less, and Fe: 85% by mass or more, and Cu: 0.25% by mass or less.

[0012] Preferred embodiments of the present invention relating to a flux-cored wire for gas-shielded arc welding relate to the following [2] to [5].

[0013] [2] The flux-cored wire for gas shielded arc welding according to [1], characterized in that, when the Ni content in the wire is expressed as [Ni] in mass % relative to the total mass of the wire, the Mo content in the wire is expressed as [Mo] in mass % relative to the total mass of the wire, the B content in the wire is expressed as [B] in mass % relative to the total mass of the wire, the Si content in the wire is expressed as [Si] in mass % relative to the total mass of the wire, the Mn content in the wire is expressed as [Mn] in mass % relative to the total mass of the wire, the Cu content in the wire is expressed as [Cu] in mass % relative to the total mass of the wire, and the C content in the wire is expressed as [C] in mass % relative to the total mass of the wire, the following relationship is satisfied: ([Ni] + 10 × [Mo] + 10 × [B]) / (5 × [Si] + [Mn] + 10 × [Cu] + 2 × [C]): 0.49 or more and 1.10 or less.

[0014] [3] The flux-cored wire for gas-shielded arc welding according to [1] or [2], further comprising: Mg: 0.2 mass% or more and 1.0 mass% or less relative to the total mass of the wire.

[0015] [4] The flux-cored wire for gas-shielded arc welding according to any one of [1] to [3], characterized in that, based on the total mass of the wire, Cr: 0.09 mass% or less, and Al: 0.15 mass% or less.

[0016] [5] The flux-cored wire for gas-shielded arc welding according to any one of [1] to [4], further comprising at least one selected from Ti, F, Na, and K in the following ranges relative to the total mass of the wire: Ti: 2.5 mass% to 7.0 mass%; F: 0.05 mass% to 0.30 mass%; and the sum of Na and K: 0.05 mass% to 0.30 mass%.

[0017] The above object of the present invention is achieved by the gas-shielded arc welding method according to the following item [6].

[0018] [6] A gas-shielded arc welding method, characterized by performing gas-shielded arc welding using the flux-cored wire for gas-shielded arc welding according to any one of [1] to [5].

[0019] The above object of the present invention is achieved by the following configuration [7] relating to the weld metal.

[0020] [7] A weld metal produced by gas-shielded arc welding using the flux-cored wire for gas-shielded arc welding according to any one of [1] to [5].

[0021] According to the present invention, it is possible to provide a flux-cored wire for gas-shielded arc welding and a gas-shielded arc welding method that can provide a weld metal having a Ni content of 1 mass % or less and that has good strength and low-temperature toughness regardless of the position in the weld metal both in the as-welded state and after PWHT.

[0022] The present inventors have conducted extensive research to obtain a weld metal that has excellent toughness even after PWHT. It has been found that increasing the content of alloying elements in the weld metal can suppress the precipitation of grain boundary ferrite, but the disappearance of grain boundary ferrite leaves prior γ grain boundaries, which are embrittled areas after SR, and reduces the toughness of the weld metal. Therefore, the present inventors have discovered that by adding Mo, an element effective in suppressing grain boundary embrittlement, to the weld metal to suppress the precipitation of grain boundary ferrite, it is possible to improve the low-temperature toughness after SR.

[0023] Furthermore, the present inventors have conducted extensive research to suppress variations in toughness depending on the position in the weld metal. As described above, the toughness of the weld metal decreases at positions shifted from the center compared to the center. This is because the effects of grain boundary ferrite and grain boundary embrittlement become more pronounced depending on the location in the weld metal. Therefore, the present inventors have discovered that the precipitation of grain boundary ferrite can be suppressed by using Mo, and the effects of grain boundary embrittlement can be suppressed by controlling the contents of C, Si, Mn, Ni, Mo, B, and Cu, thereby solving the above-mentioned problem.

[0024] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present invention is not limited to the embodiment described below, and can be carried out with any modifications within the scope of the gist of the present invention.

[0025] Unless otherwise specified, the content in this embodiment means mass % relative to the total mass of the flux-cored wire for gas-shielded arc welding (hereinafter also simply referred to as "flux-cored wire" or "wire.") Furthermore, each element contained in the flux-cored wire for gas-shielded arc welding according to this embodiment may be contained in either the steel sheath (hereinafter also simply referred to as "sheath") or the flux, or may be contained in both the steel sheath and the flux.

[0026] Furthermore, unless otherwise specified, each of the above elements may be contained in the flux-cored wire in the form of a metal, in the form of a compound, or in the form of both a metal and a compound. Therefore, regardless of the form in which each of the above elements is contained in the flux-cored wire, the content is defined as the converted value of the element. For example, in the case of Si, the Si content refers to the sum of the converted values ​​of metal Si and Si compounds. Note that metal Si includes both elemental Si and Si alloys.

[0027] [Flux-cored wire for gas-shielded arc welding] The flux-cored wire for gas-shielded arc welding according to the present embodiment has a steel sheath filled with flux. Hereinafter, the chemical components contained in the flux-cored wire according to the present embodiment and the reasons for limiting the numerical values ​​of the contents thereof will be described in more detail.

[0028] <C: 0.025% by mass or more and 0.080% by mass or less> C is a component that segregates at grain boundaries and has the effect of suppressing the precipitation of grain boundary ferrite, thereby ensuring the strength of the weld metal both as welded and after PWHT. ２ By finely precipitating C within the grains of the weld metal, the growth of carbides precipitated at grain boundaries can be suppressed, and a decrease in low-temperature toughness after PWHT can be suppressed. If the C content is less than 0.025 mass%, the desired strength and low-temperature toughness of the weld metal cannot be obtained. Therefore, the C content relative to the total mass of the wire is set to 0.025 mass% or more, preferably 0.035 mass% or more, and even more preferably 0.040 mass% or more. On the other hand, if the C content exceeds 0.080 mass%, the strength of the weld metal increases excessively and the low-temperature toughness decreases. Therefore, the C content relative to the total mass of the wire is set to 0.080 mass% or less, and preferably 0.070 mass% or less.

[0029] <Si: 0.15% by mass or more and 0.50% by mass or less> Si is a component that ensures the strength of the weld metal both as welded and after PWHT and maintains a good bead shape. If the Si content is less than 0.15% by mass, the desired strength of the weld metal cannot be obtained. Furthermore, in vertical upward welding, the bead sags, resulting in poor bead shape and poor welding workability. Therefore, the Si content relative to the total mass of the wire is set to 0.15% by mass or more, preferably 0.18% by mass or more, more preferably 0.21% by mass or more, and even more preferably 0.25% by mass or more. On the other hand, if the Si content exceeds 0.50% by mass, intergranular fracture occurs and low-temperature toughness deteriorates. Therefore, the Si content relative to the total mass of the wire is set to 0.50% by mass or less, more preferably 0.40% by mass or less, and even more preferably 0.35% by mass or less.

[0030] <Mn: 1.5% by mass or more and 3.0% by mass or less> Mn is a component that ensures hardenability and has the effect of improving the strength and toughness of the weld metal both as-welded and after PWHT. If the Mn content is less than 1.5% by mass, the hardenability is insufficient, and the desired strength and toughness of the weld metal cannot be obtained. Therefore, the Mn content relative to the total mass of the wire is set to 1.5% by mass or more, preferably 1.7% by mass or more, more preferably 1.8% by mass or more, and even more preferably 1.9% by mass or more. On the other hand, if the Mn content exceeds 3.0% by mass, the strength of the grain boundary weld becomes excessive, and the toughness of the weld metal becomes insufficient. Therefore, the Mn content relative to the total mass of the wire is set to 3.0% by mass or less, preferably 2.8% by mass or less, and more preferably 2.6% by mass or less.

[0031] <Ni: 0.50 mass% or more and less than 1.00 mass%> Ni is a component that has the effect of improving the low-temperature toughness of the weld metal by strengthening the matrix. If the Ni content is less than 0.50 mass%, the desired low-temperature toughness of the weld metal cannot be obtained. Therefore, the Ni content relative to the total mass of the wire is set to 0.50 mass% or more, preferably 0.60 mass% or more, and more preferably 0.70 mass% or more. On the other hand, if the Ni content is 1.00 mass% or more, the Ni content in the weld metal falls outside the range specified in NACE MR0175, and the susceptibility to sulfide stress corrosion cracking in a hydrogen sulfide environment increases. Therefore, the Ni content relative to the total mass of the wire is set to less than 1.00 mass%, preferably 0.95 mass% or less, more preferably 0.90 mass% or less, and even more preferably 0.80 mass% or less.

[0032] <Mo: 0.05% by mass or more and 0.40% by mass or less> Mo is a component that has the effect of improving strength and suppressing temper embrittlement. ２The fine precipitation of C within the grains of the weld metal suppresses the growth of carbides precipitated at grain boundaries, thereby suppressing a decrease in toughness at low temperatures after PWHT. Mo is also an element that has the effect of suppressing annealing softening. In this embodiment, as described above, Mo, an element effective in suppressing grain boundary embrittlement, is utilized to suppress grain boundary ferrite, thereby achieving excellent strength and toughness. If the Mo content is less than 0.05 mass%, not only will the desired strength not be obtained, but the suppression of grain boundary ferrite precipitation will be insufficient, and tempering may cause embrittlement, resulting in a deterioration in low-temperature toughness after PWHT. Therefore, the desired strength and toughness of the weld metal both as welded and after PWHT cannot be simultaneously ensured. Therefore, the Mo content relative to the total mass of the wire is set to 0.05 mass% or more, preferably 0.10 mass% or more, more preferably 0.13 mass% or more, and even more preferably 0.15 mass% or more. On the other hand, if the Mo content exceeds 0.40 mass%, fine carbides are excessively precipitated by PWHT, which excessively increases the strength of the weld metal and deteriorates the low-temperature toughness. Therefore, the Mo content relative to the total mass of the wire is set to 0.40 mass% or less, preferably 0.35 mass% or less, and more preferably 0.30 mass% or less.

[0033] <B: 0.004% by Mass or More and 0.010% by Mass or Less> B is a component that segregates at austenite grain boundaries and suppresses the formation of pro-eutectoid ferrite, thereby improving the toughness of the weld metal. If the B content relative to the total mass of the wire is less than 0.004% by mass, most of the B is fixed as nitrides as BN, making it impossible to suppress the formation of pro-eutectoid ferrite and preventing the effect of improving the toughness of the weld metal from being fully achieved. Therefore, the B content relative to the total mass of the wire is set to 0.004% by mass or more, preferably 0.005% by mass or more, and more preferably 0.006% by mass or more. On the other hand, if the B content exceeds 0.010% by mass, coarse bainite structures are generated. Furthermore, hot cracking of the weld metal is more likely to occur. Therefore, the B content relative to the total mass of the wire is set to 0.010% by mass or less, and preferably 0.009% by mass or less.

[0034] <Fe: 85% by mass or more> Fe is a main component of the wire according to this embodiment. In order to obtain a desired deposition amount and in relation to the contents of other components contained in the wire, the Fe content relative to the total mass of the wire is 85% by mass or more, preferably 86% by mass or more, and more preferably 87% by mass or more. Note that, in practice, the Fe content relative to the total mass of the wire is 96% by mass or less.

[0035] <Cu: 0.25% by mass or less> Cu is a component that has the effect of improving the matrix toughness of the weld metal at low temperatures. However, in this embodiment, Cu does not necessarily need to be contained in the wire, and it may be 0% by mass. Furthermore, Cu has a small effect of suppressing grain boundary ferrite. If the Cu content exceeds 0.25% by mass, the strength of the weld metal increases excessively without fully suppressing grain boundary ferrite, resulting in deterioration of low-temperature toughness. Therefore, the Cu content relative to the total mass of the wire is set to 0.25% by mass or less, preferably 0.15% by mass or less, and more preferably 0.10% by mass or less. Note that, when a Cu plating is applied to the wire surface, the Cu contained in the plating also falls within the Cu content range specified in this embodiment, i.e., 0.25% by mass or less.

[0036] In addition, the wire according to this embodiment preferably contains Mg within the following content ranges for the purpose of further improving the desired low-temperature toughness and welding workability of the weld metal. It is also preferable to restrict the contents of Cr and Al, which are components that reduce low-temperature toughness, to their respective predetermined contents or less. The reasons for limiting the contents of Mg, Cr, and Al will be explained below.

[0037] <Mg: 0.2% by mass or more and 1.0% by mass or less> Although Mg is not necessarily an essential element, it has a deoxidizing effect and is an ingredient that has the effect of improving the low-temperature toughness of the weld metal. A Mg content of 0.2% by mass or more can achieve the effect of further improving the desired low-temperature toughness. Therefore, the Mg content relative to the total mass of the wire is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more. On the other hand, if the Mg content exceeds 1.0% by mass, spatter increases, and depending on the working environment, spatter removal work may be required after the welding operation. Therefore, the Mg content relative to the total mass of the wire is preferably 1.0% by mass or less, more preferably 0.9% by mass or less, even more preferably 0.8% by mass or less, and even more preferably 0.7% by mass or less.

[0038] <Cr: 0.09% by mass or less> Cr is a component that promotes the precipitation and growth of coarse grain boundary carbides during PWHT and reduces toughness at low temperatures, and may be 0% by mass. If the Cr content relative to the total mass of the wire exceeds 0.09% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Cr content relative to the total mass of the wire is preferably 0.09% by mass or less, more preferably 0.07% by mass or less, and even more preferably 0.05% by mass or less.

[0039] <Al: 0.15 mass% or less> Al is a component that, when contained in excess in the wire, inhibits nucleation of acicular ferrite and reduces toughness regardless of whether PWHT is performed or not, and may be 0 mass%. If the Al content exceeds 0.15 mass%, the toughness of the weld metal decreases. Therefore, the Al content relative to the total mass of the wire is preferably 0.15 mass% or less, more preferably 0.10 mass% or less, and even more preferably 0.08 mass% or less.

[0040] Furthermore, for the purpose of improving welding workability, etc., the wire according to this embodiment preferably further contains at least one element selected from the group consisting of Ti, F, Na, and K, each within the content range shown below. The reasons for limiting the content of each element when these elements are contained in the wire will be explained below.

[0041] <Ti: 2.5% by Mass or More and 7.0% by Mass or Less> Ti is a slag forming agent and a component that has the effect of improving welding workability in the vertical up welding position and the overhead welding position. When Ti is contained in the wire, if the Ti content is 2.5% by mass or more, the above effect can be obtained, and excellent welding workability can be obtained. Therefore, when Ti is contained in the wire, the Ti content relative to the total mass of the wire is preferably 2.5% by mass or more, more preferably 3.0% by mass or more, and even more preferably 3.5% by mass or more. Furthermore, when Ti is contained in the wire, if the Ti content is 7.0% by mass or less, excessive slag generation can be suppressed, and excellent welding workability can be obtained in the vertical up welding position and the overhead welding position. Therefore, when Ti is contained in the wire, the Ti content relative to the total mass of the wire is preferably 7.0% by mass or less, more preferably 6.0% by mass or less, and even more preferably 5.5% by mass or less.

[0042] <F: 0.05% by mass or more and 0.30% by mass or less> F is a component that has the effect of stabilizing the arc. Furthermore, F is also a component that has the effect of reducing the hydrogen partial pressure in the welding atmosphere and lowering the amount of diffusible hydrogen in the weld metal. When F is contained in the wire, if the F content is 0.05% by mass or more, good arc stability can be obtained and an increase in the amount of spatter can be suppressed. Furthermore, an increase in the amount of diffusible hydrogen can be suppressed and the occurrence of low-temperature cracking in the weld can be suppressed. Therefore, when F is contained in the wire, the F content relative to the total mass of the wire is preferably 0.05% by mass or more, more preferably 0.08% by mass or more, and even more preferably 0.12% by mass or more. F may be contained in the wire in the form of a fluoride, for example, NaF, K ２SiF ６ , CaF ２ The wire may contain F. When the F content is 0.30% by mass or less, an increase in the amount of fumes generated can be suppressed, and good welding workability can be obtained. Therefore, when the wire contains F, the F content relative to the total mass of the wire is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, and even more preferably 0.20% by mass or less.

[0043] <Total of Na and K: 0.05% by Mass or More and 0.30% by Mass or Less> Na and K are components that have the effect of stabilizing the droplet transfer of the arc during welding. When at least one of Na and K is contained in the wire, if the total Na content and K content is 0.05% by mass or more, the droplet transfer of the arc during welding is stabilized and an increase in the amount of spatter generation can be suppressed. Therefore, when at least one of Na and K is contained in the wire, the total Na content and K content relative to the total mass of the wire is preferably 0.05% by mass or more, more preferably 0.08% by mass or more, and even more preferably 0.12% by mass or more. Furthermore, when at least one of Na and K is contained in the wire, if the total Na content and K content is 0.30% by mass or less, the moisture absorption resistance of the wire can be maintained well. Therefore, when at least one of Na and K is contained in the wire, the total content of Na and K relative to the total mass of the wire is preferably 0.30 mass% or less, more preferably 0.25 mass% or less, and even more preferably 0.20 mass% or less.

[0044] Furthermore, the flux-cored wire according to the present embodiment may contain Zr. The content of Zr in the wire will be described below.

[0045] <Zr: 0.20% by mass or less> Zr is a component that has the effect of improving bead conformability and obtaining a flat bead shape, so Zr may be contained in the wire. When Zr is contained in the wire, if the Zr content is 0.20% by mass or less, good slag removability can be maintained. Therefore, when Zr is contained in the wire, the Zr content relative to the total mass of the wire is preferably 0.20% by mass or less, more preferably 0.15% by mass or less, and even more preferably 0.10% by mass or less. Furthermore, the Zr content in the wire may be 0% by mass, but when Zr is contained in the wire, if the Zr content is 0.01% by mass or more, bead conformability can be improved and a flat bead shape can be obtained. Therefore, when Zr is contained in the wire, the Zr content relative to the total mass of the wire is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.08% by mass or more.

[0046] <Total Content of the Above Elements: 90% by Mass or More> In the flux-cored wire according to the present embodiment, the total content of the above C, Si, Mn, Ni, Mo, B, Fe, Cu, Mg, Cr, and Al is preferably 90% by mass or more, more preferably 92% by mass or more, and even more preferably 93% by mass or more. Furthermore, the total content of the above C, Si, Mn, Ni, Mo, B, Fe, Cu, Mg, Cr, Al, Ti, F, Na, K, and Zr is preferably 94% by mass or more, more preferably 95% by mass or more, even more preferably 96% by mass or more, and particularly preferably 97% by mass or more.

[0047] <Other Elements> In addition to the above components, the flux-cored wire according to this embodiment may contain one or more of Ca, Ba, and Li. Furthermore, the flux-cored wire may contain O (oxygen) as an oxide in the flux or in the sheath. When O (oxygen) is contained in the wire for the purpose of improving welding workability, the O content relative to the total mass of the wire is preferably 5.0 mass% or less, and more preferably 4.0 mass% or less. Examples of oxides in the flux include TiO ２ , SiO ２ , K. ２ O, Na ２ Examples include O.

[0048] <Balance> The balance other than the above elements is unavoidable impurities. Examples of the unavoidable impurities include N, As, Sb, Sn, Co, and Zn in addition to Nb, V, P, and S. The upper limits of the contents of Nb, V, P, and S are described below.

[0049] <Total of Nb and V: 0.03% by mass or less> Nb and V are components that segregate at grain boundaries and thereby deteriorate the toughness of the weld metal, and either one or both may be 0% by mass. If the Nb content and V content in the wire exceed 0.03% by mass relative to the total mass of the wire, the toughness of the weld metal decreases. Therefore, the total of the Nb content and V content in the wire is preferably 0.03% by mass or less, and more preferably 0.02% by mass or less.

[0050] <P: 0.015% by mass or less> P is an impurity element that reduces the toughness of the weld metal, and the lower the P content in the wire, the better. If the P content in the wire exceeds 0.015% by mass, the toughness and hot cracking resistance of the weld metal deteriorate. Therefore, the P content relative to the total mass of the wire is set to 0.015% by mass or less, preferably 0.012% by mass or less, and more preferably 0.010% by mass or less.

[0051] <S: 0.015% by mass or less> S is an impurity element that reduces the toughness of the weld metal, and the lower the S content in the wire, the better. If the S content in the wire exceeds 0.015% by mass, the toughness and hot cracking resistance of the weld metal deteriorate. Therefore, the S content relative to the total mass of the wire is set to 0.015% by mass or less, preferably 0.012% by mass or less, and more preferably 0.010% by mass or less.

[0052] The total amount of the unavoidable impurities other than Nb, V, P, and S is preferably restricted to 0.15 mass % or less with respect to the total mass of the wire.

[0053] <Value A Calculated by Formula (1): 0.49 to 1.10> Generally, PWHT conditions are composed of holding temperature and holding time, and can be organized using the Larson-Miller parameter (LMP), which uses these factors as parameters. The higher the LMP, the lower the PWHT conditions. However, not only the strength but also the toughness may decrease. Therefore, in the flux-cored wire of this embodiment, it is preferable not only to specify the contents of the above-mentioned components within a predetermined range, but also to appropriately adjust the relationships between Ni, Mo, B, Si, Mn, Cu, and C in the wire. Specifically, by controlling the value A calculated by the following formula (1) using the Ni content, Mo content, B content, Si content, Mn content, Cu content, and C content, grain boundary carbides can be refined and grain boundary ferrite can be suppressed. As a result, it is possible to obtain a weld metal that has good low-temperature toughness while maintaining the desired strength, not only in the as-welded state but also after PWHT under a wide range of conditions.

[0054] When the value A calculated by the following formula (1) is 0.49 or more, the low-temperature toughness after a wide range of PWHT can be further improved. Therefore, the value A calculated by the following formula (1) is preferably 0.49 or more, more preferably 0.60 or more, even more preferably 0.65 or more, and even more preferably 0.70 or more. Furthermore, when the value A calculated by the following formula (1) is 1.10 or less, the desired strength can be further ensured for the weld metal after a wide range of PWHT. Therefore, the value A calculated by the following formula (1) is preferably 1.10 or less, more preferably 0.90 or less, even more preferably 0.85 or less, and even more preferably 0.80 or less.

[0055] A = ([Ni] + 10 × [Mo] + 10 × [B]) / (5 × [Si] + [Mn] + 10 × [Cu] + 2 × [C]) Equation (1) In the above equation (1), [Ni] is a value representing the Ni content in the wire in mass % with respect to the total mass of the wire. [Mo] is a value representing the Mo content in the wire in mass % with respect to the total mass of the wire. [B] is a value representing the B content in the wire in mass % with respect to the total mass of the wire. [Si] is a value representing the Si content in the wire in mass % with respect to the total mass of the wire. [Mn] is a value representing the Mn content in the wire in mass % with respect to the total mass of the wire. [Cu] is a value representing the Cu content in the wire in mass % with respect to the total mass of the wire. [C] is a value representing the C content in the wire in mass % with respect to the total mass of the wire.

[0056] <Flux Filling Rate: 10% by Mass or More and 20% by Mass or Less> In the flux-cored wire according to the present embodiment, the flux filling rate, which is the mass of flux relative to the total mass of the wire, is not particularly limited as long as the contents of the components in the wire are within the above-mentioned ranges, and can be set to any range. In order to further improve the drawability and wire feedability of the wire, the flux filling rate is preferably, for example, 10% by mass or more and 20% by mass or less relative to the total mass of the wire.

[0057] [Method for Manufacturing Flux-Cored Wire] The flux-cored wire according to this embodiment can be manufactured, for example, by the following method. First, a steel strip constituting the outer sheath is formed by a forming roll while being fed in the longitudinal direction to form a U-shaped open tube. Next, a flux containing a predetermined amount of metal, alloy, compound, etc. so as to have a predetermined chemical composition is filled into the open tube, and the tube is then processed to have a circular cross section. Thereafter, the wire is drawn by cold working, and a flux-cored wire having a desired wire diameter can be manufactured. Annealing may be performed during the cold working.

[0058] In this embodiment, the outer diameter of the wire is not particularly limited, but is preferably, for example, 0.9 mm or more and 1.6 mm or less. Furthermore, the outer sheath can be made seamless by welding or the like, and there are no limitations on the presence or absence of a seam in the outer sheath, the shape of the seam, or the cross-sectional shape.

[0059] [Gas-shielded arc welding method] The gas-shielded arc welding method according to this embodiment is a method of gas-shielded arc welding using the flux-cored wire according to the embodiment. Note that in the gas-shielded arc welding method according to this embodiment, various welding conditions other than the use of the flux-cored wire according to this embodiment are not particularly limited, and general conditions for welding methods using flux-cored wires can be used for the type of base material, welding voltage, welding current, welding position, etc. The shielding gas is also not limited, but from the viewpoint of further improving welding workability, MAG welding (Metal Active Gas Welding) is preferable, and 80% by volume Ar-20% by volume CO ２ It is more preferable that:

[0060] Hereinafter, examples of the flux-cored wire according to the present embodiment and comparative examples will be described.

[0061] [Evaluation of Mechanical Properties of Deposited Metal] <Wire Preparation> First, flux-cored wires having various components and a diameter of 1.2 mm were prepared by filling a strip-shaped steel sheath containing the components shown in Table 1 below with flux. The flux filling rate was set to be in the range of 10 mass % to 20 mass %.

[0062] <Gas-shielded arc welding> Next, gas-shielded arc welding was performed using the obtained flux-cored wire on a base metal having the plate thickness and chemical composition shown in the following Table 2. In this example, a V-groove was formed in the base metal, and gas-shielded arc welding was performed under the welding conditions shown in the following Table 3 to form a weld metal.

[0063] <Evaluation of Mechanical Properties> The mechanical properties of the weld metal were evaluated in accordance with "Tensile and impact test methods for weld metal" specified in JIS Z 3111:2005, by taking a tensile test piece (A0 type) and an impact test piece (V-notch test piece) from the center of the weld metal in the plate thickness direction, and evaluating the tensile performance and impact performance.

[0064] (Collection of test specimens) Tensile test specimens were prepared from as-welded weld metal, PWHT (LMP = 17.3 × 10) at 580°C for 2 hours, and ３ ) and PWHT (LMP = 18.6 × 10) at 620°C for 6 hours. ３ The impact test specimens were taken from the weld metal that had been subjected to PWHT at 580°C for 2 hours (LMP = 17.3 × 10 ３ ), and the more severe PWHT condition, PWHT at 620°C for 6 hours (LMP = 18.6 × 10 ３ For each of the weld metals subjected to the reheat treatment, a test piece (center notch) in which a notch was formed in the center of the structure consisting mainly of the reheated portion, and a test piece (center + 4 mm notch) in which a notch was formed in a position shifted by 4 mm from the center consisting mainly of the original portion were prepared.

[0065] (Tensile Test) The tensile test was carried out on each test piece at room temperature (approximately 20±2°C), and the yield stress and tensile strength were measured to evaluate the tensile performance. In the present invention, the as-welded weld metal and the PWHT (LMP = 17.3 × 10) were used. ３ For test pieces taken from the weld metal subjected to PWHT (LMP = 18.6 × 10) for 6 hours at a temperature of 620°C, a yield stress of 480 MPa or more and a tensile strength of 560 MPa or more were judged to have good strength and the desired toughness. ３ For test pieces taken from weld metals subjected to the above-mentioned process, if the yield stress was 480 MPa or more and the tensile strength was 560 MPa or more, the strength was judged to be excellent.

[0066] (Impact Test) In the impact test, the toughness was evaluated by measuring the Charpy absorbed energy (vE-60°C) of each test specimen at a test temperature of -60°C. In the present invention, when the average value of the Charpy absorbed energy (vE-60°C) of each of three test specimens at -60°C was 50J or more, the toughness was judged to be good. Then, as-welded test specimens were taken from a test specimen in which a notch was formed at a central position, which is a structure mainly consisting of the reheated portion (center notch), and a test specimen in which a notch was formed at a position 4mm shifted from the center, which is a structure mainly consisting of the original portion (center + 4mm notch), and PWHT (LMP = 17.3 × 10) was performed for 2 hours at a temperature of 580°C. ３ All test pieces that were subjected to PWHT (LMP = 18.6 × 10) for 6 hours were judged to have the desired toughness. Furthermore, test pieces that were good in both strength and toughness were judged to have passed, and those that were not were judged to have failed. ３ For test specimens taken from weld metals subjected to the Charpy thermal expansion test (vE-60°C), the toughness was determined to be excellent when the average value of the Charpy absorbed energy (vE-60°C) of each of three test specimens at -60°C was 50 J or more.

[0067] The chemical components of the wires used are shown in Tables 4 and 5 below, and the evaluation results of the mechanical properties are also shown in Table 5 below. The remainder of the chemical components of the wires shown in Tables 4 and 5 were unavoidable impurities. In addition, in the column for the content of each component in Tables 4 and 5, "-" indicates that the component was not added during the production of the wire or was below the detection limit.

[0068] Furthermore, in Tables 4 and 5, the value A in formula (1) represents the following value: A = ([Ni] + 10 × [Mo] + 10 × [B]) / (5 × [Si] + [Mn] + 10 × [Cu] + 2 × [C]) formula (1)

[0069] However, in the above formula (1), [Ni] is the value of the Ni content in the wire expressed as mass% with respect to the total mass of the wire. [Mo] is the value of the Mo content in the wire expressed as mass% with respect to the total mass of the wire. [B] is the value of the B content in the wire expressed as mass% with respect to the total mass of the wire. [Si] is the value of the Si content in the wire expressed as mass% with respect to the total mass of the wire. [Mn] is the value of the Mn content in the wire expressed as mass% with respect to the total mass of the wire. [Cu] is the value of the Cu content in the wire expressed as mass% with respect to the total mass of the wire. [C] is the value of the C content in the wire expressed as mass% with respect to the total mass of the wire.

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] As shown in Tables 4 to 8 above, in Invention Examples 1 to 16, the contents of specific components in the wire were within the ranges specified in the present invention, and therefore, even when the Ni content was 1 mass% or less, weld metals with good tensile properties and low-temperature toughness were obtained both as-welded and after PWHT. Furthermore, in Invention Examples 1 to 11 and 14 to 16, the value A calculated by formula (1) was within the more preferable range specified in the present invention, and therefore, the weld metals after heat treatment under strict PWHT conditions had excellent tensile properties and low-temperature toughness.

[0079] On the other hand, in Comparative Example No. 1, the Mo content in the wire was below the lower limit of the range specified by the present invention, and therefore the tensile performance of the deposited metal after PWHT at a temperature of 580°C for 2 hours was reduced. In Comparative Example No. 2, the B content in the wire was below the lower limit of the range specified by the present invention, and therefore the toughness of the deposited metal was reduced. In Comparative Examples Nos. 3 to 5 and 8 to 11, the Mo content in the wire was below the lower limit of the range specified by the present invention, and in Comparative Examples Nos. 8 and 11, the Cu content in the wire exceeded the upper limit of the range specified by the present invention, and therefore the tensile performance or toughness was poor. In Comparative Examples Nos. 6 and 7, the Cu content in the wire exceeded the upper limit of the range specified by the present invention, and therefore the toughness of the deposited metal was reduced.

[0080] Although various embodiments have been described above, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any manner as long as they do not deviate from the spirit of the invention.

[0081] This application is based on a Japanese patent application (Patent Application No. 2024-068481) filed on April 19, 2024, the contents of which are incorporated herein by reference.

Claims

1. A flux-cored wire for gas-shielded arc welding, in which a steel sheath is filled with flux, characterized in that it contains, relative to the total mass of the wire: C: 0.025% by mass or more and 0.080% by mass or less; Si: 0.15% by mass or more and 0.50% by mass or less; Mn: 1.5% by mass or more and 3.0% by mass or less; Ni: 0.50% by mass or more and less than 1.00% by mass; Mo: 0.05% by mass or more and 0.40% by mass or less; B: 0.004% by mass or more and 0.010% by mass or less; and Fe: 85% by mass or more; and Cu: 0.25% by mass or less.

2. The flux-cored wire for gas shielded arc welding according to claim 1, characterized in that, when the Ni content in the wire is expressed as [Ni] in mass % relative to the total mass of the wire, the Mo content in the wire is expressed as [Mo] in mass % relative to the total mass of the wire, the B content in the wire is expressed as [B] in mass % relative to the total mass of the wire, the Si content in the wire is expressed as [Si] in mass % relative to the total mass of the wire, the Mn content in the wire is expressed as [Mn] in mass % relative to the total mass of the wire, the Si content in the wire is expressed as [Cu] in mass % relative to the total mass of the wire, and the Si content in the wire is expressed as [C] in mass % relative to the total mass of the wire, the following relationship is satisfied: ([Ni] + 10 x [Mo] + 10 x [B]) / (5 x [Si] + [Mn] + 10 x [Cu] + 2 x [C]): 0.49 or more and 1.10 or less.

3. A flux-cored wire for gas-shielded arc welding according to claim 1 or 2, further comprising: Mg: 0.2 mass % or more and 1.0 mass % or less relative to the total mass of the wire.

4. A flux-cored wire for gas-shielded arc welding according to claim 1 or 2, characterized in that the wire contains 0.09 mass% or less of Cr and 0.15 mass% or less of Al, based on the total mass of the wire.

5. A flux-cored wire for gas-shielded arc welding according to claim 1 or 2, further comprising at least one element selected from Ti, F, Na and K in the following ranges relative to the total mass of the wire: Ti: 2.5 mass% to 7.0 mass%, F: 0.05 mass% to 0.30 mass%, and the sum of Na and K: 0.05 mass% to 0.30 mass%, both inclusive.

6. A gas-shielded arc welding method, comprising carrying out gas-shielded arc welding using the flux-cored wire for gas-shielded arc welding according to claim 1 or 2.

7. A weld metal produced by gas-shielded arc welding using the flux-cored wire for gas-shielded arc welding according to claim 1 or 2.

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