Flux cored wire for gas shield arc welding, and gas shield arc welding method

Abstract

To provide a flux cored wire for gas shield arc welding which can obtain weld metal having a Ni content of 1 mass% or less, and having good strength and low temperature toughness, regardless of the position of the weld metal even in both cases of as it is or after PWHT.SOLUTION: A flux cored wire for gas shield arc welding contains, with respect to the total mass of the wire, 0.025 mass% or more and 0.080 mass% or less C, 0.15 mass% or more and 0.50 mass% or less Si, 1.5 mass% or more and 3.0 mass% or less Mn, 0.50 mass% or more and less than 1.00 mass% Ni, 0.05 mass% or more and 0.40 mass% or less Mo, 0.004 mass% or more and 0.010 mass% or less B, and 85 mass% or more Fe, and 0.25 mass% or less Cu.SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[0002] Offshore structures used in the drilling and production of oil, gas, etc., and pipelines used to transport oil, gas, etc., are becoming larger 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, PWHT can reduce the strength and toughness of the weld, making it difficult to obtain the required properties. Therefore, in order to improve the strength and toughness after PWHT, flux-cored wires containing Ni are sometimes used.

[0004] However, in the above-mentioned offshore structures and pipelines, problems such as sulfide stress corrosion cracking (SSCC) and hydrogen embrittlement occur in a sour environment containing hydrogen sulfide. To address these problems, the standard (NACE MR0175) of the National Association of Corrosion Engineers (NACE) 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 0.03-0.09% C, 0.1-0.6% Si, 1.3-2.6% Mn, 0.01-0.5% Cu, 0.05-0.5% Ti, 0.002-0.015% 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 simply referred to as "toughness"). Patent Document 1 also describes that the flux-cored wire further containing 0.1-0.5% Ni has the effect of further stabilizing the low-temperature toughness of the weld metal. Furthermore, 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, 4.5 to 8.5 mass% of TiO2, 0.10 to 0.40 mass% of SiO2, and 0.03 to 0.23 mass% of Al2O3, as well as predetermined amounts of Na compounds and K compounds. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-94360 [Patent Document 2] Patent No. 6322093 Summary of the Invention [Problem to be solved by the invention]

[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 Patent Documents 1 and 2, the toughness at temperatures below −40°C has not been fully investigated, and there is a possibility that sufficient toughness cannot be obtained at lower temperatures. Furthermore, in weld metal, the center of the weld metal generally has the highest toughness, and the toughness decreases as the weld metal approaches the heat-affected zone from the center (pass meeting point) due to factors such as a decrease in cooling rate. Thus, the toughness of the weld metal can rapidly deteriorate depending on the location of the test specimen. However, both Patent Documents 1 and 2 only consider the toughness of the center of the weld metal, and there is a possibility that the required toughness cannot be obtained at locations outside the center 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 low-temperature toughness regardless of the location in the weld metal, both 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. [Means for solving the problem]

[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, For 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 mass% or more and less than 1.00 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, Cu: 0.25% by mass or less, 1. A flux-cored wire for gas shielded arc welding, comprising:

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

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

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

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

[0016] [5] Furthermore, at least one selected from Ti, F, Na and K is added to the total mass of the wire, Ti: 2.5% by mass or more and 7.0% by mass or less, F: 0.05% by mass or more and 0.30% by mass or less, The flux-cored wire for gas-shielded arc welding according to any one of [1] to [4], characterized in that the total content of Na and K is in the range of 0.05% by mass or more and 0.30% by mass or less.

[0017] The above object of the present invention is achieved by the following gas-shielded arc welding method [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]. [Effects of the Invention]

[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 as-welded and after PWHT. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present inventors conducted extensive research to obtain weld metal with excellent toughness even after PWHT. It has been reported 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. The present inventors have therefore discovered that the low-temperature toughness after SR can be improved by adding Mo, an element effective in suppressing grain boundary embrittlement, to the weld metal to suppress the precipitation of grain boundary ferrite.

[0023] The present inventors have also 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. The present inventors have therefore discovered that the effects of grain boundary embrittlement can be suppressed by using Mo to suppress the precipitation of grain boundary ferrite and 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 noted, each of the above elements may be contained in the flux-containing wire in the form of a metal, in the form of a compound, or in both the form of a metal and a compound. Therefore, regardless of the form in which each of the above elements is contained in the flux-containing wire, it is defined by the conversion value converted to the elemental form. For example, when taking Si as an example, the Si content refers to the total of the Si conversion values of metallic Si and Si compounds. Note that metallic Si includes Si单质 and Si alloys.

[0027] [Flux-containing wire for gas shielded arc welding] The flux-containing wire for gas shielded arc welding according to the present embodiment is one in which a flux is filled in a steel outer skin. Hereinafter, the chemical components contained in the flux-containing wire according to the present embodiment and the reasons for numerically limiting their contents will be described in more detail.

[0028] <C: 0.025 mass% or more and 0.080 mass% or less> C is a component that segregates at grain boundaries, suppresses the precipitation of grain boundary ferrite, and has the effect of ensuring the strength of the weld metal during welding and after PWHT. In addition, C can suppress the growth of carbides precipitating at grain boundaries and the decrease in low-temperature toughness after PWHT by finely precipitating as Mo2C, which is a carbide with Mo, into the grains of the weld metal. 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 with respect to the total mass of the wire is set to 0.025 mass% or more, preferably 0.035 mass% or more, and 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 rises excessively and the low-temperature toughness decreases. Therefore, the C content with respect to the total mass of the wire is set to 0.080 mass% or less, preferably 0.070 mass% or less.

[0029] <Si: 0.15 mass% or more and 0.50 mass% or less> Si is a component that ensures the strength of the weld metal as-welded and after PWHT, and has the effect of maintaining a good bead shape. When the Si content is less than 0.15% by mass, the desired strength of the weld metal cannot be obtained, and in vertical up welding, the bead sags, the bead shape deteriorates, and the welding workability deteriorates. Therefore, the Si content based on the total mass of the wire is 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, when the Si content exceeds 0.50% by mass, intergranular fracture occurs and the low-temperature toughness deteriorates. Therefore, the Si content based on the total mass of the wire is 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 as-welded and after PWHT. When 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 based on the total mass of the wire is 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, when the Mn content exceeds 3.0% by mass, the strength of the grain boundary weld part becomes excessive and the toughness of the weld metal is insufficient. Therefore, the Mn content based on the total mass of the wire is 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% by mass or more and less than 1.00% by mass> Ni is a component that has the effect of improving the toughness of the weld metal at low temperatures through matrix strengthening. When the Ni content is less than 0.50% by mass, the desired low-temperature toughness of the weld metal cannot be obtained. Therefore, the Ni content based on the total mass of the wire is 0.50% by mass or more, preferably 0.60% by mass or more, and more preferably 0.70% by mass or more. On the other hand, when the Ni content is 1.00% by mass or more, the Ni content in the weld metal is outside the range defined by NACE MR0175, and in a hydrogen sulfide environment, the susceptibility to sulfide stress corrosion cracking increases. Therefore, the Ni content based on the total mass of the wire is less than 1.00% by mass, preferably 0.95% by mass or less, more preferably 0.90% by mass or less, and even more preferably 0.80% by 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. By the fine precipitation of Mo2C into the grains of the weld metal, the growth of carbides precipitating at the grain boundaries can be suppressed, and the decrease in toughness at low temperatures after PWHT can be suppressed. Also, Mo is a component that has the effect of suppressing annealing softening. In the present embodiment, as described above, by utilizing Mo, which is an element effective for suppressing grain boundary embrittlement, to suppress grain boundary ferrite, excellent strength and toughness can be obtained. When the Mo content is less than 0.05% by mass, not only the desired strength cannot be obtained, but also the suppression of the precipitation of grain boundary ferrite becomes insufficient, and embrittlement is caused by tempering, and the low-temperature toughness after PWHT may deteriorate. For this reason, the desired strength and toughness of the weld metal as-welded and after PWHT cannot be ensured simultaneously. Therefore, the Mo content based on the total mass of the wire is 0.05% by mass or more, preferably 0.10% by mass or more, more preferably 0.13% by mass or more, and even more preferably 0.15% by mass or more. On the one hand, when the Mo content exceeds 0.40% by mass, fine carbides are excessively precipitated by PWHT, so the strength of the weld metal rises excessively and the low-temperature toughness deteriorates. Therefore, the Mo content with respect to the total mass of the wire is 0.40% by mass or less, preferably 0.35% by mass or less, and more preferably 0.30% by mass or less.

[0033] <B: 0.004% by mass or more and 0.010% by mass or less> B is a component that has the effect of improving the toughness of the weld metal by segregating at the austenite grain boundaries and suppressing the formation of primary ferrite. When the B content with respect to the total mass of the wire is less than 0.004% by mass, most of the B is immobilized as a nitride in the form of BN, the formation of primary ferrite cannot be suppressed, and the effect of improving the toughness of the weld metal cannot be sufficiently obtained. Therefore, the B content with respect to the total mass of the wire is 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, when the B content exceeds 0.010% by mass, a coarse bainite structure occurs. Also, hot cracking of the weld metal is likely to occur. Therefore, the B content with respect to the total mass of the wire is 0.010% by mass or less, preferably 0.009% by mass or less.

[0034] <Fe: 85% by mass or more> Fe is the main component of the wire according to this embodiment. In order to obtain a desired welding amount and in relation to the content of other components contained in the wire, the Fe content with respect 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. In addition, it is practical that the Fe content with respect 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 toughness of the matrix at low temperatures of the weld metal. However, in this embodiment, it is not necessarily required to contain Cu in the wire, and it may be 0% by mass. Also, Cu has a small effect of suppressing grain boundary ferrite. When the Cu content exceeds 0.25% by mass, the strength of the weld metal rises excessively while the grain boundary ferrite cannot be completely suppressed, and the low-temperature toughness deteriorates. Therefore, the Cu content with respect to the total mass of the wire is 0.25% by mass or less, preferably 0.15% by mass or less, and more preferably 0.10% by mass or less. When Cu plating is applied to the wire surface, the Cu contained in the plating is also included in the range of the Cu content defined in this embodiment, that is, 0.25% by mass or less.

[0036] In addition, for the wire according to this embodiment, for the purpose of further improving the desired low-temperature toughness and welding workability of the weld metal, it is preferable to contain Mg within the range of the content shown below. It is also preferable to regulate the contents of Cr and Al, which are components that lower the low-temperature toughness, to be below predetermined contents respectively. The reasons for limiting the respective 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> Mg is not an essential element, but it has a deoxidizing effect and is a component that has the effect of improving the low-temperature toughness of the weld metal. When the Mg content is 0.2% by mass or more, the effect of further improving the desired low-temperature toughness can be obtained. Therefore, the Mg content with respect 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, when the Mg content exceeds 1.0% by mass, spatter increases, and spatter removal work after welding may occur depending on the working environment. Therefore, the Mg content with respect 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 mass% or less> Cr promotes the precipitation and growth of coarse grain boundary carbides by PWHT and is a component that reduces toughness at low temperatures. It may be 0 mass%. When the Cr content with respect to the total mass of the wire exceeds 0.09 mass%, the toughness at low temperatures after PWHT decreases. Therefore, the Cr content with respect to the total mass of the wire is preferably 0.09 mass% or less, more preferably 0.07 mass% or less, and even more preferably 0.05 mass% or less.

[0039] <Al: 0.15 mass% or less> Al is a component that hinders the nucleation of acicular ferrite and reduces toughness regardless of the presence or absence of PWHT when contained in excess in the wire. It may be 0 mass%. When the Al content exceeds 0.15 mass%, the toughness of the weld metal decreases. Therefore, the Al content with respect 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] Further, for the purpose of improving welding workability and the like, the wire according to the present embodiment preferably contains at least one selected from the group consisting of Ti, F, Na, and K within the respective content ranges shown below. The reasons for limiting the respective contents when these elements are contained in the wire will be explained below.

[0041] <Ti: 2.5 mass% or more and 7.0 mass% or less> Ti is a slag former and is a component having the effect of improving welding workability in the vertical upward posture and upward posture. When Ti is contained in the wire, when the Ti content is 2.5 mass% or more, the above effects can be obtained and excellent welding workability can be obtained. Therefore, when Ti is contained in the wire, the Ti content with respect to the total mass of the wire is preferably 2.5 mass% or more, more preferably 3.0 mass% or more, and even more preferably 3.5 mass% or more. Also, when Ti is contained in the wire, if the Ti content is 7.0 mass% or less, it is possible to suppress an excessive amount of slag generation and obtain excellent welding workability in the vertical-up position or the overhead position. Therefore, when Ti is contained in the wire, the Ti content with respect to the total mass of the wire is preferably 7.0 mass% or less, more preferably 6.0 mass% or less, and even more preferably 5.5 mass% or less.

[0042] <F: 0.05 mass% or more and 0.30 mass% or less> F is a component having an effect of stabilizing the arc. Also, F is a component having an effect of reducing the hydrogen partial pressure in the welding atmosphere and decreasing the amount of diffusible hydrogen in the weld metal. When F is contained in the wire, if the F content is 0.05 mass% or more, good arc stability can be obtained and an increase in the amount of spatter generation can be suppressed. Also, an increase in the amount of diffusible hydrogen can be suppressed, and the occurrence of cold cracking in the welded part can be suppressed. Therefore, when F is contained in the wire, the F content with respect to the total mass of the wire is preferably 0.05 mass% or more, more preferably 0.08 mass% or more, and even more preferably 0.12 mass% or more. F may be contained in the wire in the form of a fluoride, for example, it may contain NaF, K2SiF6, CaF2, etc. Also, when F is contained in the wire, if the F content is 0.30 mass% or less, an increase in the amount of fume generation can be suppressed and good welding workability can be obtained. Therefore, when F is contained in the wire, the F content with respect 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.

[0043] <Total of Na and K: 0.05 mass% or more and 0.30 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 of the Na content and the 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 of the Na content and the K content with respect 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. Also, when at least one of Na and K is contained in the wire, if the total of the Na content and the 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 of the Na content and the K content with respect 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.

[0044] [[ID=*]] 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 the bead conformity and obtaining a flat bead shape, so the wire may contain Zr. When Zr is contained in the wire, if the Zr content is 0.20% by mass or less, good slag detachability can be maintained. Therefore, when Zr is contained in the wire, the Zr content with respect 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, the bead conformability is 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 this embodiment, the total content of the above C, Si, Mn, Ni, Mo, B, Fe, Cu, Mg, Cr, and Al is preferably 90 mass% or more, more preferably 92 mass% or more, and even more preferably 93 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 mass% or more, more preferably 95 mass% or more, even more preferably 96 mass% or more, and particularly preferably 97 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 TiO2, SiO2, KO, and Na2O.

[0048] <Remainder> The remainder other than the above elements is inevitable impurities. Examples of inevitable impurities include N, As, Sb, Sn, Co, Zn, etc. in addition to Nb, V, P, and S. The upper limit values of the contents of Nb, V, P, and S will be described below.

[0049] <Total of Nb and V: 0.03% by mass or less> Nb and V are components that deteriorate the toughness of the weld metal by segregating at the grain boundaries, and either one or both of them may be 0% by mass. If the Nb content and V content in the wire exceed 0.03% by mass based on the total mass of the wire, the toughness of the weld metal will decrease. 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 deteriorates 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 high-temperature crack resistance of the weld metal will deteriorate. Therefore, the P content based on the total mass of the wire is 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 deteriorates 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 high-temperature crack resistance of the weld metal will deteriorate. Therefore, the S content based on the total mass of the wire is 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 inevitable impurities excluding the above Nb, V, P, and S is preferably regulated to be 0.15% by mass or less based on the total mass of the wire.

[0053] <Value A calculated by formula (1): 0.49 or more and 1.10 or less> Generally, PWHT conditions consist of holding temperature and holding time, and can be summarized using the Larson-Miller parameter (LMP), which uses these factors as parameters. The higher the LMP, the lower the strength of the PWHT conditions. However, not only strength but also toughness may be reduced. 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, 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 can refine grain boundary carbides and suppress grain boundary ferrite. As a result, weld metal with good low-temperature toughness can be obtained 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 PWHTs 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 PWHTs. 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) However, in the above formula (1), [Ni] is a value representing the Ni content in the wire in mass % relative to the total mass of the wire. [Mo] is a value representing the Mo content in the wire in mass % relative to the total mass of the wire. [B] is a value representing the B content in the wire in mass % relative to the total mass of the wire. [Si] is a value representing the Si content in the wire in mass % relative to the total mass of the wire. [Mn] is a value representing the Mn content in the wire in mass % relative to the total mass of the wire. [Cu] is a value representing the Cu content in the wire in mass % relative to the total mass of the wire. [C] is a value representing the C content in the wire in mass % relative 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 the flux relative to the total mass of the wire, is not particularly limited and can be set to any range as long as the contents of the components in the wire are within the above-mentioned ranges. In order to further improve the drawability and wire feedability of the wire, the flux filling rate is preferably set to, for example, 10% by mass or more and 20% by mass or less relative to the total mass of the wire.

[0057] [Method of 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 into a U-shaped open tube by a forming roll while being fed in the longitudinal direction. 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 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 described above. 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 CO2 is more preferable. [Example]

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

[0061] [Evaluation of the mechanical properties of the deposited metal] <Wire production> 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% by mass or more and 20% by mass or less.

[0062] <Gas shielded arc welding> Next, gas-shielded arc welding was carried out on base metals having thicknesses and chemical compositions shown in Table 2 below, using the obtained flux-cored wires. In this example, a V-groove was formed in the base metal, and gas-shielded arc welding was carried out under the welding conditions shown in Table 3 below to form a weld metal.

[0063] <Evaluation of mechanical properties> The mechanical properties of the weld metal were evaluated in accordance with the "Tensile and impact test method for weld metal" specified in JIS Z 3111:2005. Tensile test pieces (A0 type) and impact test pieces (V-notch test pieces) were taken from the center of the weld metal in the thickness direction, and the tensile performance and impact performance were evaluated.

[0064] (Collection of test specimens) The tensile test specimens were as-welded weld metal, PWHT at 580°C for 2 hours (LMP = 17.3 × 10 3 ) and PWHT at 620°C for 6 hours (LMP = 18.6 × 10 3 The impact test specimens were taken from the as-welded weld metal, which had been subjected to PWHT at 580°C for 2 hours (LMP = 17.3 × 10 3 ), and the more severe PWHT condition of 620℃ for 6 hours (LMP=18.6×10 3 For each of the deposited metals subjected to the above treatment, two test pieces were prepared: one with a notch formed in the center of the structure consisting mainly of the reheated portion (center notch), and the other with a notch formed 4 mm away from the center of the structure consisting mainly of the original portion (center + 4 mm notch).

[0065] (Tensile test) The tensile test was carried out on each test piece at room temperature (about 20±2°C), and the tensile performance was evaluated by measuring the yield stress and tensile strength. In the present invention, the as-welded weld metal and the PWHT at 580°C for 2 hours (LMP = 17.3 × 10 3For test pieces taken from the weld metal subjected to PWHT (LMP = 18.6 × 10) for 6 hours at 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 metal subjected to the annealing treatment, those with a yield stress of 480 MPa or more and a tensile strength of 560 MPa or more were judged to have excellent strength.

[0066] (Impact test) In the impact test, the test temperature was set to -60°C and the Charpy absorbed energy (vE-60°C) was measured for each test piece to evaluate the toughness. In the examples of the present invention, when the average value of the Charpy absorbed energy (vE-60°C) of each of three test pieces at -60°C was 50 J or more, the toughness was judged to be good. Then, as-welded specimens were taken from a test piece with a notch formed in the center (center notch), which is the structure of the reheated portion, and a test piece with a notch formed 4 mm from the center (center + 4 mm notch), which is the structure of the original portion, and were subjected to PWHT at 580°C for 2 hours (LMP = 17. 3×10 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 the test, and all other test pieces were judged to have failed the test. Furthermore, test pieces were subjected to PWHT (LMP = 18.6 × 10) for 6 hours at 620°C. 3 For test pieces taken from weld metals subjected to the Charpy test, if the average value of the Charpy absorbed energy (vE-60℃) of three test pieces at -60℃ was 50J or more, the toughness was judged to be excellent.

[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 was unavoidable impurities. In Tables 4 and 5, in the column for the content of each component, "-" indicates that the component was not added during the production of the wire or that the content 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])...Equation (1)

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

[0070] [Table 1]

[0071] [Table 2]

[0072] [Table 3]

[0073] [Table 4]

[0074] [Table 5]

[0075] [Table 6]

[0076] [Table 7]

[0077] [Table 8]

[0078] As shown in Tables 4 to 8 above, in Examples 1 to 16, the contents of specific components in the wire were within the ranges specified in the present invention, so even though 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 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, so 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 in the present invention, and therefore the tensile properties of the deposited metal after PWHT at a temperature of 580°C for 2 hours were reduced. In Comparative Example No. 2, the B content in the wire was below the lower limit of the range specified in the present invention, and therefore the toughness of the deposited metal was reduced. In Comparative Examples 3 to 5 and 8 to 11, the Mo content in the wire was below the lower limit of the range specified in the present invention, and in Comparative Examples 8 and 11, the Cu content in the wire exceeded the upper limit of the range specified in the present invention, resulting in poor tensile performance or toughness. In Comparative Examples 6 and 7, the Cu content in the wire exceeded the upper limit of the range specified in the present invention, and therefore the toughness of the deposited metal was reduced.

Claims

1. A flux-cored wire for gas shielded arc welding, in which a steel sheath is filled with flux, For 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 mass% or more and less than 1.00 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; Cu: 0.25% by mass or less, 1. A flux-cored wire for gas shielded arc welding, comprising:

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

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

4. For the total mass of the wire, Cr: 0.09% by mass or less, and 3. The flux-cored wire for gas-shielded arc welding according to claim 1, wherein Al is 0.15 mass % or less.

5. Furthermore, at least one selected from Ti, F, Na and K is added to the total mass of the wire. Ti: 2.5% by mass or more and 7.0% by mass or less, F: 0.05% by mass or more and 0.30% by mass or less, 3. The flux-cored wire for gas-shielded arc welding according to claim 1, further comprising a total content of Na and K in the range of 0.05 mass % to 0.30 mass %.

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

7. 3. 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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