Flux cored wire and method for forming welded joint

Abstract

This solid wire comprises an outer skin and a flux, and has a Vickers hardness Hv of 180-500, an fcc ratio of at least 70% as obtained by a magnetic induction method, and a chemical composition of C: 0-0.650%, Si: 0.03-0.50%, Mn: 4.1-30.0%, P: 0.050% or less, S: 0.050% or less, Cu: 0-5.0%, Ni: 1.0-30.0%, Cr: 0-10.0%, Mo: 0-10.0%, Nb: 0-1.00%, V: 0-1.00%, Co: 0-1.00%, Pb: 0-1.00%, Sn: 0-1.000%, Al: 0-0.10%, Ti: 0-0.10%, Ta: 0-1.00%, Hf: 0-1.00%, W: 0-30.00%, Mg: 0-0.50%, REM: 0-0.50%, Zr: 0-5.00%, B: 0-0.1000%, N: 0-0.500%, and O: 0.0500% or less, with the balance being Fe and impurities.

Description

Flux-cored wire and method for manufacturing welded joint

[0001] The present disclosure relates to a method for manufacturing a flux-cored wire and a weld joint.

[0002] In recent years, due to the tightening of carbon dioxide emission regulations in response to the issue of global warming, there has been an increasing demand for hydrogen fuel, which emits no carbon dioxide compared to petroleum and coal, as well as natural gas, which also emits less carbon dioxide. Accordingly, there has been a global increase in demand for the construction of liquid hydrogen tanks, liquid carbon dioxide tanks, LNG tanks, and the like, which are used on ships and on land. Ni-based low-temperature steels containing 6 to 9% Ni are used for the steel materials used in liquid hydrogen tanks, liquid carbon dioxide tanks, LNG tanks, and the like, because they are required to ensure toughness at extremely low temperatures of -196°C. Furthermore, austenitic wires are used to weld these Ni-based low-temperature steels, which can produce weld metal with excellent low-temperature toughness.

[0003] For example, as an austenitic wire, Japanese Patent Application Laid-Open No. 2008-246507 describes a wire having a Ni content of 35 to 70% and a TiO content of 100% relative to the total mass of the wire in the flux. ２ , SiO ２ and ZrO ２ in a total amount of 4.0 mass % or more, and further, Mn oxide is MnO ２ The content is 0.6 to 1.2 mass % in terms of TiO. ２ , SiO ２ , ZrO ２ and MnO ２ The contents of (equivalent amounts) are expressed in mass % as [TiO ２ ], [SiO ２ ], [ZrO ２ ] and [MnO ２ ], [TiO ２ ] / [ZrO ２ ] is 2.3 to 3.3, [SiO ２ ] / [ZrO ２ ] is 0.9 to 1.5, and ([TiO ２ ]+[SiO ２ ]+[ZrO ２ ]) / [MnO ２] is 5 to 13." Furthermore, JP 2017-502842 A discloses a wire having an outer sheath made of a Ni-based alloy, "wherein, by weight, C: 0.15 to 0.8%, Si: 0.2 to 1.2%, Mn: 15 to 34%, Cr: 6% or less, Mo: 1.5 to 4%, S: 0.02% or less, P: 0.02% or less, B: 0.01% or less, Ti: 0.09 to 0.5%, N: 0.001 to 0.3%, TiO ２ :4~15%, SiO ２ , ZrO ２ and Al ２ O ３ The document discloses a "flux cored arc welding wire containing: 0.01 to 9% of one or more elements selected from the group consisting of: a total of 0.01 to 9% of one or more elements selected from the group consisting of: a total of 0.5 to 1.7% of one or more elements selected from K, Na and Li, 0.2 to 1.5% of one or more elements selected from F and Ca, and the balance being Fe and other unavoidable impurities."

[0004] Furthermore, Japanese Patent Laid-Open Publication No. 2002-18587 discloses "a stainless steel wire for gas-shielded arc welding, in which the surface of a stainless steel core wire is covered with nickel plating having a surface hardness of 300 to 500 Hv and a thickness of 0.2 to 4 μm, and the free expansion diameter of the coated wire is set in the range of 350 to 800 mm."

[0005] One of the important indicators in welding using flux-cored wire (FCW) is wire feedability. That is, when a coiled wire is straightened while being fed, it is required that the wire be straightened and fed to the desired feeding position. However, if the steel sheath of the flux-cored wire is too hard, straightening is not performed properly, the wire feeding position is shifted, the arc generation point becomes unstable, and the bead becomes meandering. Recently, so-called γ-based steels with an FCC ratio of 70% or more have been used for the steel sheath. γ-based steels are high-alloy steels, and wire hardness can lead to unstable feedability. Therefore, reducing the alloy content in the sheath has been considered to soften flux-cored wires with a steel sheath using a steel sheath with an FCC ratio of 70% or more. However, if the alloy content in the sheath is reduced too much, the alloy content in the weld metal becomes insufficient, and the desired performance cannot be achieved. Therefore, for flux-cored wires using a steel outer sheath with an FCC ratio of 70% or more, there is a demand for wires that have excellent feedability during welding (excellent straightening properties during feeding) by using a soft outer sheath.

[0006] Therefore, an object of the present disclosure is to provide a flux-cored wire that is excellent in feedability during welding, and a method for manufacturing a welded joint using the flux-cored wire.

[0007] The means for solving the problem include the following aspects. <1> A flux-cored wire for welding, comprising a steel sheath and flux filled inside the steel sheath, wherein the steel sheath has a Vickers hardness Hv of 180 to 500, and an fcc proportion determined by a magnetic induction method of 70% or more, and a chemical composition of the steel sheath, in mass% with respect to the total mass of the steel sheath, is: C: 0% to 0.650%, Si: 0.03% to 0.50%, Mn: 3.1% to 30.0%, P: 0.050% or less, S: 0.050% or less, Cu: 0% to 5.0%, Ni: 1.0% to 30.0%, Cr: 0% to 10.0%, Mo: 0% to 10.0%, Nb: 0% to 1.00%, V: 0% to 1.00%, <2> A flux-cored wire according to <1>, wherein the thickness of the steel sheath is 150 μm or more and 450 μm or less, the balance consisting of Fe and impurities. <3> The flux-cored wire according to <1> or <2>, wherein the area ratio of inclusions having an equivalent circle diameter of 5 μm or more in the steel sheath is 3.00% or less. <4> The flux-cored wire according to any one of <1> to <3>, wherein, in the chemical composition of the steel sheath, the sum of the Mn content and the Ni content (Mn + Ni) is 5.0% or more, and the sum of the Mn content, the Ni content, and the Cr content (Mn + Ni + Cr) is 15.0% or more. <5> The flux-cored wire according to any one of <1> to <4>, wherein, in the chemical composition of the steel sheath, the mass ratio of the Mn content to the Ni content (Ni / Mn) is 0.10 or more. <6> The flux-cored wire according to <5>, wherein the mass ratio (Ni / Mn) is 1.00 or more.<7> The flux-cored wire according to any one of <1> to <6>, wherein the total content of Nb, V, Ti, Ta, Hf, and Zr (Nb + V + Ti + Ta + Hf + Zr) in the chemical composition of the steel sheath is 0.0005% or more and 5.0% or less. <8> The flux-cored wire according to any one of <1> to <7>, wherein the steel sheath does not have a welded joint at the joint. <9> The flux-cored wire according to any one of <1> to <7>, wherein the steel sheath has a welded joint at the joint. <10> The flux-cored wire according to any one of <1> to <9>, wherein one or both of polytetrafluoroethylene oil and perfluoropolyether oil are applied to the surface. <11> A method for manufacturing a welded joint, comprising the step of welding steel materials using the flux-cored wire according to any one of <1> to <10>.

[0008] According to the present disclosure, it is possible to provide a flux-cored wire having excellent feedability during welding, and a method for manufacturing a welded joint using the flux-cored wire.

[0009] FIG. 2 is a schematic diagram illustrating a test device used in the test for stability of the feeding speed performed in the examples.

[0010] An embodiment that is an example of the present disclosure will be described. In this specification, when a numerical range expressed using "to" is used, unless the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that includes these numerical values ​​as the lower and upper limits. Furthermore, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, with regard to the content, "%" means "mass %." When the content (%) is "0 to," it means that the component is an optional component and may not be included.

[0011] <Flux-cored wire> A flux-cored wire (hereinafter also simply referred to as "wire") according to the present disclosure includes a steel sheath (hereinafter also simply referred to as "sheath") and flux filled inside the sheath. The sheath has a Vickers hardness Hv of 180 to 500, an fcc fraction determined by a magnetic induction method of 70% or more, and a Cr content of 10.0% or less by mass relative to the total mass of the sheath. The chemical composition of the sheath, expressed in mass% relative to the total mass of the sheath, is within the range described below.

[0012] The flux-cored wire according to the present disclosure has the above-described configuration, and is excellent in wire feedability during welding. The reason for this effect is presumed to be as follows.

[0013] One of the important indicators in welding using flux-cored wire (FCW) is wire feedability. During welding, the flux-cored wire is continuously fed toward the welding point. During this process, the coiled flux-cored wire is straightened (i.e., its curl is corrected). Therefore, if the wire is too hard, the wire cannot be straightened properly, resulting in a misalignment of the wire feed position, an unstable arc generation point, and a meandering bead. Conventionally, low-alloy steel has been widely used for the outer sheath of flux-cored wire. In this case, the outer sheath is less likely to harden due to its low alloy content, and feedability has rarely been an issue in terms of wire hardness. On the other hand, in recent years, so-called gamma-based steels with an fcc fraction of 70% or more have been used for the outer sheath of flux-cored wire. Since high-alloy steels are used for gamma-based steels, the outer sheath can harden due to its high alloy content. Therefore, if the outer sheath is too hard, feedability can become unstable. Therefore, in order to obtain a soft outer sheath, it is conceivable to reduce the amount of alloy in the outer sheath. However, if the amount of alloy in the outer sheath is reduced too much, the amount of alloy in the weld metal becomes insufficient, and the desired performance cannot be obtained. Therefore, for a flux-cored wire using a steel outer sheath with an FCC ratio of 70% or more, a wire with excellent feedability during welding (excellent straightening during feeding) is required by using a soft outer sheath.

[0014] In contrast, the flux-cored wire according to the present disclosure has a steel sheath with an fcc ratio of 70% or more, a Cr content controlled to 10.0% or less, and an optimized heating temperature and heating time in annealing performed before wiredrawing in manufacturing the flux-cored wire, thereby achieving a Vickers hardness Hv of 180 or more and 500 or less. This makes it possible to provide a flux-cored wire having a soft sheath while maintaining an fcc ratio of 70% or more, and thus provides a wire that is excellent in feedability during welding (more specifically, straightening ability during feeding).

[0015] The requirements for the flux-cored wire according to the present disclosure will be described below.

[0016] The flux-cored wire according to the present disclosure melts together with a part of the steel material to be welded during welding, and becomes a weld metal after solidification.

[0017] (Vickers hardness Hv of outer sheath) The Vickers hardness Hv of the outer sheath is 180 or more and 500 or less. If the Vickers hardness Hv exceeds 500, the wire becomes too hard and lacks toughness, causing breakage during straightening and poor feedability during welding (straightening ability during feeding). On the other hand, if the alloy amount is too small, the Vickers hardness Hv softens to less than 180 and the wire buckles. From the viewpoints of feedability during welding (straightening ability during feeding) and buckling, the Vickers hardness Hv of the outer sheath is more preferably 250 or more and 470 or less, and even more preferably 300 or more and 450 or less.

[0018] Here, a method for controlling the Vickers hardness Hv of the outer sheath will be described. The hardness of the outer sheath is controlled by optimizing the amount of solid solution hardening and stacking fault energy by adjusting the amount of alloy and the type of alloy contained in the outer sheath. From the viewpoint of softening the outer sheath (i.e., Hv 500 or less), it is preferable to reduce the amount of solid solution hardening of the outer sheath and increase the stacking fault energy. On the other hand, from the viewpoint of preventing the outer sheath from becoming too soft (i.e., Hv 180 or more), it is preferable to increase the amount of solid solution hardening of the outer sheath and decrease the stacking fault energy. Furthermore, in order to control the Vickers hardness Hv of the outer sheath of the wire within the above range, when manufacturing a flux-cored wire, it is important to control the heating temperature and heating time in annealing performed before wiredrawing.

[0019] The Vickers hardness Hv of the outer sheath is measured by the following method. The direction parallel to the drawing direction of the flux-cored wire (L cross section) is mirror-polished, and the Vickers hardness is measured at five arbitrary points in the center of the thickness of the outer sheath according to JIS Z 2244 (2009). The test force is 9.8 N. The average of the five Vickers hardnesses obtained is taken as the Vickers hardness of the outer sheath.

[0020] (Fcc percentage of outer sheath determined by magnetic induction method) The fcc percentage of the outer sheath is 70% or more. By setting the fcc percentage to 70% or more, the percentage of austenite in the structure of the outer sheath is increased, and the low-temperature toughness of the obtained weld metal can be improved. The fcc percentage is preferably 80% or more, more preferably 90% or more, and may be 100%. The remainder of the structure is bcc.

[0021] In order to achieve an fcc ratio of 70% or more in the sheath, the lower limit of the total amount of elements excluding Fe and impurities in the chemical composition of the sheath is preferably 10%, 15%, 20%, 25%, or 30%. On the other hand, the upper limit of the total amount of elements excluding Fe and impurities in the chemical composition of the sheath is not particularly limited, but is preferably, for example, 60% or 55%. The term "impurities" as used herein refers to components that are mixed in due to raw materials such as ore or scrap, or various factors in the manufacturing process, during industrial production of the sheath, and are acceptable within a range that does not adversely affect the properties of the sheath. These impurities do not include P, S, and O.

[0022] The fcc percentage in the tissue of the exine shell can be determined by the following method: A sample is taken from the exine shell, the bcc percentage (%) is measured on the surface of the sample by magnetic induction, and the arithmetic mean value of the measured bcc percentages is determined. Using the obtained mean value of the bcc percentages, the fcc percentage (%) in the tissue of the exine shell is determined by the following formula: fcc percentage = 100 - bcc percentage

[0023] (Sheath Thickness) The thickness of the outer sheath is preferably 150 μm or more and 450 μm or less. A thickness of 150 μm or more allows the wire to maintain an appropriate hardness, thereby further improving feedability during welding. A thickness of 450 μm or less prevents the wire from becoming too hard, thereby further improving feedability during welding. The thickness of the outer sheath is preferably 200 μm or more and 400 μm or less, and more preferably 250 μm or more and 350 μm or less.

[0024] The thickness of the outer jacket is the arithmetic mean value of thicknesses measured at any five points on the outer jacket, except that the measurement points (the above-mentioned five points) do not include the crimped portion (i.e., the portion where the ends of the outer jacket are overlapped and welded together when forming the jacket into a cylindrical shape).

[0025] (Chemical Composition of Sheath) The chemical composition of the sheath of the flux-cored wire according to the present disclosure will be described below. In the description of the chemical composition of the sheath, "%" means "mass % with respect to the total mass of the sheath" unless otherwise specified.

[0026] The chemical composition of the outer covering according to the present disclosure is: C: 0% to 0.650%, Si: 0.03% to 0.50%, Mn: 3.1% to 30.0%, P: 0.050% or less, S: 0.050% or less, Cu: 0% to 5.0%, Ni: 1.0% to 30.0%, Cr: 0% to 10.0%, Mo: 0% to 10.0%, Nb: 0% to 1.00%, V: 0% to 1.00%, Co: 0% to 1.00%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, Al: 0% to 0.10%, Ti: 0% to 0.10%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, W: 0 to 30.00%, Mg: 0 to 0.50%, REM: 0 to 0.50%, Zr: 0 to 5.00%, B: 0% to 0.1000%, N: 0% to 0.500%, and O: 0.0500% or less, with the balance being Fe and impurities.

[0027] (C: 0 to 0.650%) C is an element that generates spatter. In order to reduce spatter, the lower the C content of the sheath, the more advantageous it is. Furthermore, C is an interstitial solid solution strengthening element, and reducing the C content of the sheath softens the sheath. Therefore, the C content of the sheath is set to 0 to 0.650%. However, reducing the C content of the sheath to 0% increases the cost of decarbonization. Furthermore, there is a concern that the C content of the sheath may be insufficient, resulting in insufficient strength of the weld metal. Therefore, if the C content of the sheath is low, it may be necessary to increase the C content of the flux. Therefore, the lower limit of the C content of the sheath may be 0.003%, 0.005%, or 0.008%. The upper limit of the C content of the outer skin is preferably 0.600%, 0.500%, 0.400%, 0.300%, 0.200%, less than 0.200%, 0.190%, 0.180%, 0.150%, or 0.120%.

[0028] (Si: 0.03 to 0.50%) Si is a deoxidizing element. By increasing the Si content of the outer skin, the P content of the outer skin can be reduced. On the other hand, Si has low solid solubility in the austenite phase, and by reducing the Si content, it is possible to suppress the formation of embrittlement phases such as intermetallic compounds and δ ferrite at high temperatures and improve high-temperature ductility. Therefore, the Si content of the outer skin is set to 0.03 to 0.50%. The lower limit of the Si content of the outer skin is preferably 0.04%, 0.05%, or 0.08%. The upper limit of the Si content of the outer skin is preferably less than 0.50%, 0.48%, 0.45%, 0.40%, 0.35%, 0.30%, or 0.20%.

[0029] (Mn: 3.1 to 30.0%) Mn is an element that causes an increase in the amount of fume generation. In order to reduce the amount of fume generation, the lower the Mn content of the sheath, the more advantageous it is. Furthermore, reducing the Mn content increases stacking fault energy, thereby improving toughness. On the other hand, Mn is an austenite-stabilizing element. Increasing the Mn content of the sheath also increases the Mn content of the entire wire, which promotes austenitization of the weld metal and improves low-temperature toughness. Therefore, the Mn content of the sheath is set to 3.1 to 30.0%. The lower limit of the Mn content of the sheath is preferably 3.7%, 5.0%, more than 5.0%, 5.2%, more than 6.0%, 6.2%, 7.0%, more than 7.0%, 7.2%, more than 10.0%, or 10.2%. The upper limit of the Mn content in the outer skin is preferably 28.0%, 26.0%, 25.0%, 23.0%, 21.0%, 20.0%, 19.0%, 18.0%, 15.0%, or 12.0%.

[0030] (P: 0.050% or less) P is an impurity element that reduces the toughness of the weld metal, so it is preferable to reduce the P content of the outer sheath as much as possible. For example, the lower limit of the P content of the outer sheath may be 0%. However, from the viewpoint of reducing the deP cost, the P content of the outer sheath is preferably 0.003% or more. On the other hand, if the P content of the outer sheath is 0.050% or less, the adverse effect of P on the toughness can be reduced. Therefore, the P content of the outer sheath is set to 0.050% or less. In order to effectively suppress the reduction in the toughness of the weld metal, the P content of the outer sheath is preferably 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, 0.010% or less, or 0.005% or less.

[0031] (S: 0.050% or less) S is an impurity element that reduces the toughness of the weld metal, so it is preferable to reduce the S content of the outer sheath as much as possible. For example, the lower limit of the S content of the outer sheath may be 0%. However, from the viewpoint of reducing desulfurization costs, the S content of the outer sheath is preferably 0.003% or more. On the other hand, if the S content of the outer sheath is 0.050% or less, the adverse effect of S on toughness can be reduced. Therefore, the S content of the outer sheath is set to 0.050% or less. In order to effectively suppress a decrease in the toughness of the weld metal, the S content of the outer sheath is preferably 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, 0.010% or less, or 0.005% or less.

[0032] (Cu: 0 to 5.0%) Cu is a precipitation strengthening element and may be contained in the sheath to improve the strength of the weld metal. Cu is also an austenite stabilizing element and may be contained in the sheath to improve the low-temperature toughness of the weld metal. On the other hand, if the Cu content in the sheath is excessive, the above effects will saturate. Furthermore, reducing the Cu content in the sheath will soften the sheath. Therefore, the Cu content in the sheath is set to 0 to 5.0%. The lower limit of the Cu content in the sheath is preferably 0.3%, 0.5%, or 0.7%. The upper limit of the Cu content in the sheath is preferably 4.5%, 4.0%, or 3.5%.

[0033] (Ni: 1.0 to 30.0%) Ni is an austenite-stabilizing element. Increasing the Ni content of the sheath increases the Ni content of the entire wire, which promotes austenitization of the weld metal and improves low-temperature toughness. On the other hand, reducing the Ni content of the sheath can reduce the cost of the sheath. Therefore, the Ni content of the sheath is set to 1.0 to 30.0%. The lower limit of the Ni content of the sheath is preferably 2.0%, 3.0%, 3.2%, 3.7%, 5.0%, more than 6.0%, 6.2%, 7.0%, more than 8.0%, or 8.2%. The upper limit of the Ni content of the sheath is preferably 28.0%, 26.0%, 24.0%, 22.0%, 20.0%, 19.0%, 18.0%, 15.0%, or 12.0%.

[0034] (Cr: 0 to 10.0%) Cr is a ferrite-stabilizing element and may be contained in the sheath to improve the strength of the weld metal. On the other hand, if the Cr content of the sheath is excessive, the sheath becomes too hard, resulting in poor feedability during welding (straightening during feed). Furthermore, if the Cr content of the sheath is excessive, the amount of low-melting-point compounds in the molten metal increases, and the solid-liquid coexistence temperature range of the molten metal widens, making hot cracking more likely to occur. Therefore, the Cr content of the sheath is set to 0 to 10.0%. The lower limit of the Cr content of the sheath is preferably 0.01%, 0.02%, 1.0%, 2.0%, or 3.0%. The upper limit of the Cr content of the sheath is preferably 9.8%, 9.0%, 8.0%, less than 8.0%, 7.8%, 7.0%, less than 6.0%, or 5.8%.

[0035] (Mo: 0 to 10.0%) Mo is a precipitation strengthening element and may be contained in the sheath to improve the strength of the weld metal. On the other hand, by reducing the Mo content in the sheath, the strength of the weld metal can be suppressed and low-temperature toughness can be increased. Furthermore, by reducing the Mo content in the sheath, the sheath becomes softer. Therefore, the Mo content in the sheath is set to 0 to 10.0%. The lower limit of the Mo content in the sheath is preferably 1.0%, 2.0%, or 3.0%. The upper limit of the Mo content in the sheath is preferably 9.0%, 8.0%, or 7.0%.

[0036] (Nb: 0 to 1.00%) Nb is an element that forms carbides in the weld metal and increases the strength of the weld metal, so it may be contained in the sheath. On the other hand, by reducing the Nb content of the sheath, it is possible to suppress the occurrence of hot cracking in the weld metal. Furthermore, by reducing the Nb content of the sheath, the sheath becomes softer. Therefore, the Nb content of the sheath is set to 0 to 1.00%. The lower limit of the Nb content of the sheath is preferably 0.001%, 0.002%, 0.005%, 0.010%, or 0.015%. The upper limit of the Nb content of the sheath is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0037] (V: 0 to 1.00%) V is an element that forms carbonitrides in the weld metal and increases the strength of the weld metal, so it may be contained in the sheath. On the other hand, by reducing the V content in the sheath, it is possible to suppress the occurrence of hot cracking in the weld metal. Furthermore, by reducing the V content in the sheath, the sheath becomes softer. Therefore, the V content in the sheath is set to 0 to 1.00%. The lower limit of the V content in the sheath is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the V content in the sheath is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0038] (Co: 0 to 1.00%) Co is an element that increases the strength of the weld metal through solid solution strengthening, so it may be contained in the sheath. On the other hand, by reducing the Co content of the sheath, the ductility of the weld metal can be increased and toughness can be ensured. Furthermore, by reducing the Co content of the sheath, the sheath becomes softer. Therefore, the Co content of the sheath is set to 0 to 1.00%. The lower limit of the Co content of the sheath is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Co content of the sheath is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0039] (Pb: 0 to 1.00%) Pb may be contained in the sheath because it has the effect of improving the shape formability of the toe between the base steel material and the weld metal and improving the machinability of the weld metal. On the other hand, by reducing the Pb content in the sheath, the arc state can be stabilized and spatter can be reduced. Therefore, the Pb content in the sheath is set to 0 to 1.00%. The lower limit of the Pb content in the sheath is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Pb content in the sheath is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0040] (Sn: 0 to 1.00%) Sn is an element that improves the corrosion resistance of the weld metal, so it may be contained in the sheath. On the other hand, by reducing the Sn content of the sheath, it is possible to suppress the occurrence of cracks in the weld metal. Therefore, the Sn content of the sheath is set to 0 to 1.00%. The lower limit of the Sn content of the sheath is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Sn content of the sheath is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0041] The outer skin preferably contains at least one selected from the group consisting of Co, Pd, and Sn in the above-mentioned content, i.e., the content of at least one selected from the group consisting of Co, Pd, and Sn is preferably more than 0%.

[0042] (Al: 0 to 0.10%) Al is a deoxidizing element and may be contained in the sheath to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, by reducing the Al content of the sheath, the formation of coarse inclusions (e.g., inclusions with an equivalent circle diameter of 5 μm or more) in the sheath is suppressed, and the sheath becomes softer. Furthermore, by reducing the Al content of the sheath, the formation of nitrides or oxides due to Al in the weld metal is suppressed, and a decrease in the low-temperature toughness of the weld metal can be suppressed. Therefore, the Al content of the sheath is set to 0 to 0.10%. The lower limit of the Al content of the sheath is preferably 0.01%, 0.02%, or 0.03%. The upper limit of the Al content of the sheath is preferably 0.09%, 0.08%, or 0.07%.

[0043] (Ti: 0 to 0.10%) Ti is a deoxidizing element and may be contained in the sheath to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, by reducing the Ti content of the sheath, the formation of coarse inclusions (e.g., inclusions with a circular equivalent diameter of 5 μm or more) in the sheath is suppressed, and the sheath becomes softer. Furthermore, by reducing the Ti content of the sheath, the formation of carbides in the weld metal is suppressed, and deterioration of the toughness of the weld metal can be suppressed. Therefore, the Ti content of the sheath is set to 0 to 0.10%. The lower limit of the Ti content of the sheath is preferably 0.003%, 0.01%, 0.02%, or 0.03%. The upper limit of the Ti content of the sheath is preferably 0.09%, 0.08%, or 0.07%.

[0044] (Ta: 0 to 1.00%) Ta may be contained in the sheath because it contributes to suppressing hot cracking. On the other hand, by reducing the Ta content in the sheath, it is possible to suppress the coarsening of inclusions. Therefore, the Ta content in the sheath is set to 0 to 1.00%. The lower limit of the Ta content in the sheath is preferably 0.0001%, 0.0003%, 0.0005%, or 0.0007%. The upper limit of the Ta content in the sheath is preferably 0.90%, 0.80%, 0.70%, or 0.60%.

[0045] (Hf: 0 to 1.00%) Hf may be contained in the outer sheath because it contributes to suppressing hot cracking. On the other hand, by reducing the Hf content in the outer sheath, it is possible to suppress the coarsening of inclusions. Therefore, the Hf content in the outer sheath is set to 0 to 1.00%. The lower limit of the Hf content in the outer sheath is preferably 0.0001%, 0.0003%, 0.0005%, or 0.0007%. The upper limit of the Hf content in the outer sheath is preferably 0.90%, 0.80%, 0.70%, or 0.60%.

[0046] (W: 0 to 30.00%) W may be contained in the sheath because it is an element that increases the strength of the weld metal through solid solution strengthening. On the other hand, by reducing the W content of the sheath, the ductility of the weld metal can be increased and toughness can be ensured. Furthermore, by reducing the W content of the sheath, the sheath becomes softer. Therefore, the W content of the sheath is set to 0 to 30.00%. The lower limit of the W content of the sheath is preferably 0.50%, 1.00%, 3.00%, or 5.00%. The upper limit of the W content of the sheath is preferably 28.00%, 25.00%, 23.00%, or 20.00%.

[0047] (Mg: 0 to 0.50%) Mg may be contained in the sheath because it has a deoxidizing effect, reducing the oxygen content in the weld metal and improving the toughness of the weld metal. On the other hand, reducing the Mg content in the sheath reduces the amount of slag produced and suppresses welding defects such as slag inclusion. Therefore, the Mg content in the sheath is set to 0 to 0.50%. The lower limit of the Mg content in the sheath is preferably 0.0001%, 0.0005%, 0.0010%, 0.0020%, or 0.0030%. The upper limit of the Mg content in the sheath is preferably 0.40% or 0.30%.

[0048] (REM: 0 to 0.50%) REM has the effect of improving hot workability during the production of the outer sheath, and therefore may be contained in the outer sheath. On the other hand, if the REM content of the outer sheath is excessive, the REM combines with oxygen, significantly reducing cleanliness and thereby deteriorating hot workability. Therefore, the REM content of the outer sheath is set to 0 to 0.50%. The lower limit of the REM content of the outer sheath is preferably 0.001%, 0.003%, 0.005%, or 0.010%. The upper limit of the REM content of the outer sheath is preferably 0.40% or 0.30%.

[0049] Note that "REM" is a collective term for 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of one or more REM elements. REM is generally contained in misch metal. Therefore, for example, REM may be added in the form of misch metal so that the amount of REM falls within the above range.

[0050] (Zr: 0 to 5.00%) Zr may be contained in the sheath because it has a deoxidizing effect, reducing the oxygen content in the weld metal and improving the toughness of the weld metal. On the other hand, reducing the Zr content in the sheath suppresses the generation of coarse inclusions (e.g., inclusions with a circular equivalent diameter of 5 μm or more), softens the sheath, and prevents wire breakage. Therefore, the Zr content in the sheath is set to 0 to 5.00%. The lower limit of the Zr content in the sheath is preferably 0.0005%, 0.0010%, 0.0020%, or 0.0050%. The upper limit of the Zr content in the sheath is preferably 4.50%, 4.00%, 3.50%, or 3.00%.

[0051] (B: 0 to 0.1000%) B is an interstitial solid solution strengthening element, and may be contained in the outer sheath to improve the low temperature toughness and strength of the weld metal. On the other hand, by reducing the B content of the outer sheath, M ２３ (C, B) ６ The precipitation of B is suppressed, and deterioration of toughness can be suppressed. Furthermore, reducing the B content of the outer skin makes the outer skin softer. Therefore, the B content of the outer skin is set to 0 to 0.1000%. The lower limit of the B content of the outer skin is preferably 0.0005%, 0.0010%, or 0.0020%. The upper limit of the B content of the outer skin is preferably 0.0800%, 0.0500%, or 0.0100%.

[0052] (N: 0 to 0.500%) N is an austenite stabilizing element and an interstitial solid solution strengthening element, and may be contained in the sheath to improve the low-temperature toughness and strength of the weld metal. On the other hand, by reducing the N content of the sheath, the occurrence of blowouts can be suppressed and welding defects can be reduced. Furthermore, by reducing the N content of the sheath, the sheath becomes softer. Therefore, the N content of the sheath is set to 0 to 0.500%. The lower limit of the N content of the sheath is preferably 0.001%, 0.010%, or 0.050%. The upper limit of the N content of the sheath is preferably 0.450%, 0.400%, or 0.350%.

[0053] (O: 0.0500% or less) O ​​may be contained in the sheath as an impurity. However, an excessive O content leads to deterioration of the toughness and ductility of the weld metal, so the O content of the sheath is preferably reduced, with the upper limit being 0.0050% or less. The upper limit of the O content of the sheath is preferably 0.0400%, 0.0300%, 0.0200%, 0.0100%, or 0.0050%. On the other hand, the lower limit of the O content of the wire may be 0%, but from the viewpoint of suppressing an increase in manufacturing costs due to the reduction of the O content, it is preferably 0.0003% or 0.0005%.

[0054] (Balance: Fe and impurities) The remaining components in the chemical composition of the outer shell are Fe and impurities. The impurities refer to components that are mixed in due to raw materials such as ore or scrap, or various factors in the manufacturing process, when the outer shell is industrially manufactured, and are acceptable within a range that does not adversely affect the properties of the outer shell.

[0055] (Mass Ratio (Cr / Mn) of Cr Content to Mn Content of Outer Sheath) The mass ratio (Cr / Mn) of the Cr content to the Mn content of the outer sheath is preferably 10.00 or less. In other words, by preventing the Cr content from becoming too large relative to the Mn content, excessive solid solution strengthening is not achieved, and the stacking fault energy is not reduced too much, preventing the outer sheath from becoming hard. As a result, feedability during welding (straightening ability during feed) is improved. The lower limit of the mass ratio (Cr / Mn) is 0. The upper limit of the mass ratio (Cr / Mn) in the outer sheath is more preferably 9.00, 8.00, 7.00, 6.00, or 5.00. The lower limit of the mass ratio (Cr / Mn) in the outer sheath is more preferably 0.50, 1.00, 1.50, or 2.00.

[0056] (Total of Mn Content and Ni Content (Mn+Ni)) Mn and Ni are each an austenite stabilizing element and improve the low-temperature toughness of the weld metal. On the other hand, since Ni is an expensive metal, in order to improve the low-temperature toughness of the weld metal while suppressing the cost of the wire, it is preferable that the Mn content and Ni content in the outer sheath each satisfy the above-mentioned ranges, and that the total of the Mn content and Ni content (Mn+Ni) is 5.0% or more. The total of the Mn content and Ni content (Mn+Ni) in the outer sheath is more preferably 7.0% or more, 7.2% or more, 10.0% or more, or 15.0% or more.

[0057] Mn is an element that causes an increase in the amount of fume generation. Furthermore, by reducing the Mn content, a decrease in stacking fault energy is suppressed, and a decrease in toughness can be suppressed. Therefore, from the viewpoint of reducing the amount of fume generation while suppressing wire costs and improving the low-temperature toughness of the weld metal, it is preferable that the Mn content and Ni content in the sheath each satisfy the above-mentioned ranges, and that the total of the Mn content and Ni content (Mn + Ni) in the sheath is 37.0% or less. The total of the Mn content and Ni content (Mn + Ni) in the sheath is more preferably 35.0% or less, 32.0% or less, or 30.0% or less.

[0058] (Total of Mn Content, Ni Content, and Cr Content (Mn+Ni+Cr)) Mn, Ni, and Cr are each an austenite-stabilizing element and improve the low-temperature toughness of the weld metal. On the other hand, because Ni is an expensive metal, in order to improve the low-temperature toughness of the weld metal while suppressing the cost of the wire, it is preferable that the Mn content, Ni content, and Cr content in the outer sheath each satisfy the above-mentioned ranges, and that the total of the Mn content, Ni content, and Cr content (Mn+Ni+Cr) in the outer sheath be 15.0% or more. The total of the Mn content, Ni content, and Cr content (Mn+Ni+Cr) in the outer sheath is more preferably 17.0% or more, 19.0% or more, 20.0% or more, 22.0% or more, 24.0% or more, 26.0% or more, 28.0% or more, or 30.0% or more.

[0059] Mn is an element that causes an increase in the amount of fume generation. Furthermore, reducing the Mn content suppresses a decrease in stacking fault energy, thereby suppressing a decrease in toughness. Reducing the Cr content suppresses the formation of martensite structures, softening the sheath. Furthermore, Cr causes an increase in the amount of low-melting-point compounds in the molten metal. Therefore, from the viewpoint of reducing wire costs, improving the low-temperature toughness of the weld metal, reducing the amount of fume generation, improving wire workability, and reducing the amount of low-melting-point compounds in the molten metal, it is preferable that the Mn content, Ni content, and Cr content in the sheath each satisfy the above-mentioned ranges, and that the total of the Mn content, Ni content, and Cr content (Mn + Ni + Cr) in the sheath is 47.0% or less. The total of the Mn content, Ni content, and Cr content (Mn + Ni + Cr) in the sheath is more preferably 45.0% or less, 42.0% or less, or 40.0% or less.

[0060] (Mass Ratio of Mn Content to Ni Content (Ni / Mn)) Mn and Ni are each an austenite stabilizing element and improve the low-temperature toughness of the weld metal. On the other hand, Ni is an expensive metal and an element that causes an increase in the amount of fumes generated. Furthermore, Mn is an element that, when added in excess, reduces stacking fault energy, thereby causing a decrease in toughness. Note that Ni improves toughness by increasing stacking fault energy. Therefore, from the viewpoint of improving the low-temperature toughness of the weld metal and reducing the amount of fumes generated while suppressing wire costs, it is preferable that the mass ratio of Mn content to Ni content (Ni / Mn) in the sheath be 0.10 or more. The lower limit of the mass ratio of Mn content to Ni content (Ni / Mn) in the sheath is more preferably 0.15, 0.20, 0.30, 0.50, 0.70, 1.00, 1.02, 1.10, 1.20, or 1.22. The upper limit of the mass ratio (Ni / Mn) of the Mn content to the Ni content in the outer skin is preferably 10.00, 8.00, or 5.00.

[0061] (Total Content of Nb, V, Ti, Ta, Hf, and Zr (Nb + V + Ti + Ta + Hf + Zr)) Nb, V, Ti, Ta, Hf, and Zr each contribute to suppressing hot cracking. On the other hand, by reducing the contents of Nb, V, Ti, Ta, Hf, and Zr, coarsening and formation of precipitates can be suppressed, thereby improving toughness. Therefore, the total content of Nb, V, Ti, Ta, Hf, and Zr in the outer sheath (Nb + V + Ti + Ta + Hf + Zr) is preferably 0.0005% or more and 5.0% or less. The lower limit of the total content of these elements in the outer sheath is preferably 0.0007%, 0.0010%, 0.0015%, 0.0020%, 0.0025%, or 0.0030%. The upper limit of the Zr content of the outer skin is preferably 4.00%, 3.50%, 3.00%, or 2.50%.

[0062] (Area Ratio of Coarse Inclusions) In the flux-cored wire according to the present disclosure, the area ratio of inclusions in the sheath having an equivalent circle diameter of 5 μm or more (referred to as "coarse inclusions" in the present disclosure) is preferably 3.00% or less. By making the area ratio of coarse inclusions in the sheath 3.00% or less, that is, by reducing the amount of precipitated coarse inclusions, it is possible to reduce the number of fracture initiation points while maintaining an appropriate alloy content and making the sheath soft, and it is therefore easy to obtain a flux-cored wire that has excellent feedability during welding (more specifically, straightening ability during feeding).

[0063] From the viewpoint of manufacturability of the flux-cored wire, the area ratio of inclusions having an equivalent circle diameter of 5 μm or more is preferably 2.50% or less, more preferably 2.00% or less, even more preferably 1.50% or less, and even more preferably 1.00% or less. The lower limit of the area ratio of inclusions having an equivalent circle diameter of 5 μm or more may be 0%.

[0064] From the viewpoint of manufacturability of the flux-cored wire, the area ratio of inclusions having an equivalent circle diameter of 1 μm or more is preferably 3.00% or less, more preferably 2.00% or less, and even more preferably 1.00% or less. The lower limit of the area ratio of inclusions having an equivalent circle diameter of 1 μm or more may be 0%.

[0065] In the present disclosure, inclusions refer to oxides, carbides, nitrides, carbonitrides, and sulfides, and among these inclusions, those with a circle equivalent diameter of 5 μm or more and those with a circle equivalent diameter of 1 μm or more are subject to measurement.

[0066] The area ratio of inclusions in the sheath with an equivalent circle diameter of 5 μm or more and inclusions with an equivalent circle diameter of 1 μm or more is measured by the following method. A cross section (C section) of the sheath perpendicular to the wire drawing direction is stained with Murakami's reagent (an alkaline solution of prussiate; see page 250 of the Metal Data Book, 2nd edition, by Nippon Kinzoku). The stained cross section is observed under an optical microscope at 500x magnification, and inclusions (i.e., oxides, carbides, nitrides, carbonitrides, and sulfides) with an equivalent circle diameter of 5 μm or more and 1 μm or more are targeted based on the area of ​​the inclusions. The area ratio can be determined by calculating the total area of ​​the target inclusions within the observation field. The conditions for staining with Murakami's reagent include immersion etching followed by cleaning.

[0067] The area ratio of inclusions having an equivalent circle diameter of 5 μm or more and inclusions having an equivalent circle diameter of 1 μm or more in the outer skin can be controlled by adjusting the contents of elements that contribute to the precipitation of inclusions. Specifically, there are methods such as (1) reducing the contents of carbon (C), nitrogen (N), and sulfur (S) in the outer skin, (2) reducing the contents of the elements shown in (1) above and elements that form inclusions (specifically, Al, Ti, Cr, Mo, Nb, V, Ti, Ta, Hf, and Zr) in the outer skin, and (3) reducing both the elements shown in (1) above and the elements shown in (2) above.

[0068] The flux-cored wire according to the present disclosure may further include a lubricant applied to the wire surface. The lubricant applied to the wire surface has the effect of improving wire feedability during welding. Various types of lubricants (e.g., vegetable oils such as palm oil) can be used for the wire, but in order to suppress welding defects, it is preferable to use one or both of H-free polytetrafluoroethylene oil (PTFE oil) and perfluoropolyether oil (PFPE oil). The flux-cored wire according to the present disclosure may further include a plating formed on the wire surface. In this case, the lubricant is applied to the surface of the plating.

[0069] (Wire Shape) Next, the shape (wire structure) of the flux-cored wire according to the present disclosure will be described. Flux-cored wires are generally classified into wires having a shape (seamless shape) in which the seam of the outer sheath is welded and there is no slit-like gap (wires having a weld at the seam of the outer sheath), and wires having a shape (seamed shape) in which the seam of the outer sheath is not welded and there is a slit-like gap (wires having no weld at the seam of the outer sheath).

[0070] The flux-cored wire according to the present disclosure can adopt any of these shapes. However, in order to suppress the occurrence of cold cracking in the weld metal, it is preferable that the outer sheath does not have slit-shaped gaps. H (hydrogen) that penetrates the weld during welding diffuses into the weld metal and the material to be welded, and accumulates in the stress concentration area, causing cold cracking. There are various sources of H, but when welding is performed under strict control of the cleanliness of the weld and the gas shielding conditions, the moisture (H) contained in the wire is the most likely to cause cold cracking. ２ O) is the main source of H, and the amount of this moisture has a strong effect on the amount of diffusible hydrogen in the welded joint.

[0071] When the sheath has a seam, moisture in the atmosphere is likely to penetrate into the flux through the seam. Therefore, it is desirable to prevent moisture in the atmosphere from penetrating into the flux through the sheath during the period from wire production to use by removing the seam of the sheath. When the sheath has a seam and the period from wire production to use is long, it is desirable to vacuum-pack the entire flux-cored wire or store the flux-cored wire in a container that can keep it dry in order to prevent the penetration of sources of H, such as moisture.

[0072] (Wire Diameter) The diameter of the flux-cored wire according to the present disclosure is not particularly limited, but is, for example, φ1.0 to φ2.0 mm. Note that the diameter of a typical flux-cored wire is φ1.2 to φ1.6 mm.

[0073] (Filling rate) The filling rate of the flux in the flux-cored wire according to the present disclosure is not particularly limited as long as the above-described conditions are satisfied. In consideration of the filling rate of a typical flux-cored wire, the lower limit of the filling rate of the flux-cored wire according to the present disclosure may be, for example, 8%, 10%, or 12%. The upper limit of the filling rate of the flux-cored wire according to the present disclosure may be, for example, 28%, 25%, 22%, 20%, or 17%. Note that when calculating the filling rate, the masses of the sheath and the flux are measured separately.

[0074] <Method for Manufacturing Flux-Cored Wire> Next, a method for manufacturing a flux-cored wire according to the present disclosure will be described.

[0075] (In the case of seamless flux-cored wire) A method for producing a seamless flux-cored wire includes the steps of preparing a flux, forming a steel strip using forming rolls while feeding it in the longitudinal direction to obtain a U-shaped open tube, supplying flux into the open tube through an opening of the open tube, butt-welding opposing edges (both circumferential ends) of the opening of the open tube to obtain a seamless tube, annealing the obtained seamless tube before starting a wiredrawing step, and wiredrawing the seamless tube to obtain a flux-cored wire having a predetermined wire diameter. Note that in order to improve feedability, i.e., to prevent the outer sheath from hardening, it is important to control the heating temperature and heating time in the annealing step.

[0076] Specifically, the flux-cored wire according to the present disclosure can be obtained by the following manufacturing process: (1) A steel strip is continuously fed into a forming machine, and both widthwise ends of the steel strip are processed to form a U-shaped cross section perpendicular to the longitudinal direction, thereby obtaining an open tube. (2) Flux is supplied and filled through the U-shaped opening of the open tube. (3) The opposing edges (both circumferential ends) of the open tube opening are butt-welded to form a seamless tube having an O-shaped cross section perpendicular to the longitudinal direction. The outer diameter of the seamless tube is 2.0 mm to 6.0 mm (preferably 4.0 mm to 5.0 mm). (4) The seamless tube is annealed in a bright annealing furnace at a heating temperature of 900°C to 1150°C (preferably 950°C to 1050°C) for 1 minute to 120 minutes (preferably 5 minutes to 20 minutes) for solution treatment. (5) The annealed seamless tube is drawn until the outer diameter reaches 1.0 mm to 1.6 mm. Note that, in order to improve feedability, i.e., to prevent the outer sheath from hardening, it is preferable to repeat the (4) annealing step and the (5) drawing step multiple times (preferably two or more times) to process until the outer diameter reaches 1.0 mm to 1.6 mm. By the above manufacturing method, a flux-cored wire according to the present disclosure can be obtained, in which the outer sheath has a specific Vickers hardness Hv.

[0077] The butt welding is performed by electric resistance welding, laser welding, TIG welding, or the like. The flux-cored wire may be further annealed (post-annealed) during or after the wiredrawing process in order to remove moisture from the flux-cored wire and to soften the outer sheath. To set the H content of the flux-cored wire to 12 ppm or less, the post-annealing temperature is preferably 650°C or higher, and the post-annealing time is preferably 4 hours or longer. To prevent deterioration of the flux, the post-annealing temperature is preferably 900°C or lower.

[0078] If the cross section of a butt seam welded flux-cored wire without slit-shaped gaps is polished and etched, the weld marks can be seen, but if it is not etched, the weld marks cannot be seen. For this reason, it is sometimes called seamless, as mentioned above. For example, in "New Edition: Introduction to Welding and Joining Technology" (2008) edited by the Japan Welding Society, Sanpo Publishing, p. 111, it is described that a butt seam welded flux-cored wire without slit-shaped gaps is a seamless type wire. Even if the gaps in the outer sheath of a flux-cored wire are brazed, a flux-cored wire without slit-shaped gaps can be obtained.

[0079] (In the case of flux-cored wire having slit-shaped gaps) The method for producing a flux-cored wire having slit-shaped gaps is the same as the method for producing a seamless flux-cored wire, except that instead of butt-welding both circumferential ends of an open pipe to obtain a seamless pipe, the method includes a step of forming an open pipe and butt-welding the ends of the open pipe to obtain a pipe with slit-shaped gaps. The method for producing a flux-cored wire having slit-shaped gaps may further include a step of crimping the butted ends of the open pipe. In the method for producing a flux-cored wire having slit-shaped gaps, a pipe with slit-shaped gaps is drawn.

[0080] <Method for manufacturing a welded joint> Next, a method for manufacturing a welded joint (welding method) according to the present disclosure will be described. The method for manufacturing a welded joint according to the present disclosure includes a step of welding steel materials using the flux-cored wire according to the present disclosure described above.

[0081] In the method for manufacturing a welded joint according to the present disclosure, the welding method is preferably gas-shielded arc welding. In the method for manufacturing a welded joint according to the present disclosure, the type of steel material (workpiece) that serves as the base material of the welded joint is not particularly limited. For example, ＣＭ Examples of such steel materials include steel materials with high cold cracking susceptibility having a (weld crack susceptibility composition) of 0.24% or more (particularly high-strength steel plates having a tensile strength of 590 MPa or more and 1700 MPa or less and a plate thickness of 20 mm or more), and Ni-based steel plates for low temperature use having a plate thickness of 20 mm or more and containing 6% to 9% Ni. Of these, Ni-based steel plates for low temperature use having a plate thickness of 20 mm or more and containing 6% to 9% Ni can be preferably used.

[0082] The method for manufacturing a welded joint according to the present disclosure may include a step of welding steel materials using a flux-cored wire according to the present disclosure in one or more of the first through final passes. When the welding is performed in only one pass, the flux-cored wire according to the present disclosure is used in that one pass. The polarity of the flux-cored wire may be either positive or negative, but positive polarity is preferred, since the effect on the amount of diffusible hydrogen in the weld metal and the amount of spatter generation is negligibly small.

[0083] The type of shielding gas used in the method for producing a welded joint according to the present disclosure is not particularly limited. Examples of the shielding gas used in the method for producing a welded joint according to the present disclosure include 100% carbon dioxide by volume, which is commonly used, and a mixture of Ar and 3 to 30% CO by volume. ２ In addition, the shielding gas used in welding using the flux-cored wire according to the present disclosure is preferably a mixed gas containing 5% by volume or less of O. ２ These gases are inexpensive, so welding using these gases is advantageous for industrial use.

[0084] The welding position in the method for manufacturing a welded joint according to the present disclosure is not particularly limited. The method for manufacturing a welded joint according to the present disclosure can exhibit good welding workability (particularly vertical welding ability) regardless of the welding position, which may be a flat position, a horizontal position, a vertical position, or an overhead position.

[0085] The welded joint obtained by the method for manufacturing a welded joint according to the present disclosure includes a base steel material and a welded portion composed of a weld metal and a weld heat-affected zone. The tensile strength of the welded metal obtained is preferably high, for example, 590 to 1200 MPa.

[0086] Next, the feasibility and effects of the present disclosure will be explained in more detail using examples and comparative examples of the present disclosure. However, the following examples do not limit the present disclosure, and any design changes that are made in accordance with the above and below spirit are all included in the technical scope of the present disclosure.

[0087] (Manufacturing of Flux-Cored Wire) The flux-cored wires of the present disclosure and comparative examples were manufactured by the method described below. First, a steel strip having the chemical composition of the outer sheath shown in Tables 1-1 to 1-3 and Tables 2-1 to 2-3 was fed in the longitudinal direction and formed using forming rolls to obtain a U-shaped open tube. Flux was supplied into the open tube through the opening, and opposing edges of the opening of the open tube were butt-welded to obtain a seamless tube. This seamless tube was annealed, and the annealed seamless tube was wiredrawn to obtain a flux-cored wire without slit-like gaps.

[0088] Specifically, the seamless tube was manufactured as follows: (1) A steel strip was continuously fed into a forming machine, and both widthwise ends of the steel strip were processed to form a U-shaped cross section perpendicular to the longitudinal direction, thereby obtaining an open tube. (2) Flux was supplied and filled through the U-shaped opening of the open tube. (3) The opposing edges (both circumferential ends) of the open tube opening were butt-welded to form a seamless tube having an O-shaped cross section perpendicular to the longitudinal direction. The seamless tube had an outer diameter of 4.5 mm. (4) The seamless tube was annealed in a bright annealing furnace at 1000°C for 10 minutes for solution treatment. (5) The annealed seamless tube was drawn to an outer diameter of 1.2 mm. In this manner, a flux-cored wire with a final wire diameter of 1.2 mm was produced. After the production, a lubricant was applied to the wire surface.

[0089] However, in Wire No. 32 (Comparative Example), the annealing conditions in step (4) were changed to 600°C and 5 minutes.

[0090] The units of the contents of the chemical compositions of the exine shells shown in Tables 1-1 to 1-3 and Tables 2-1 to 2-3 are mass % relative to the total mass of the exine shells.

[0091] The remainder of the chemical compositions of the outer skin shown in Tables 1-1 to 1-3 and Tables 2-1 to 2-3 (i.e., components other than the components shown in the tables) is iron and impurities. Note that in Tables 1-1 to 1-3 and Tables 2-1 to 2-3, values ​​outside the ranges specified in this disclosure are underlined. Also, in Tables 1-1 to 1-3 and Tables 2-1 to 2-3, blank spaces in the tables relating to the content of a chemical composition mean that the content of that chemical component is less than the significant digits. These chemical components may be unavoidably mixed in or produced in amounts less than the significant digits.

[0092] The fcc percentage in the tissue of the exine was determined by the following method. A sample was taken from the exine, and the bcc percentage (%) was measured on the surface of the sample by magnetic induction using a FERITSCOPE (registered trademark) FMP30 (manufactured by Fischer Instruments Inc.) and a Fischer Instruments Inc. probe (FGAB 1.3-Fe) as the probe of the measuring instrument, and the arithmetic mean value of the measured bcc percentages was calculated. Using the obtained mean value of the bcc percentages, the fcc percentage (%) in the tissue of the exine was calculated by the following formula: fcc percentage = 100 - bcc percentage

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099] [Evaluation / Stability of Feed Speed] First, a test device 50 for evaluating the stability of the feed speed will be described. As shown in FIG.

[0100] The feeder 52 has a spool 522, a feed roller 524, and a pressure roller 526. The feeder 52 is a CM-2301 manufactured by Daihen Corporation. The flux-cored wire 56 of the present disclosure or the comparative example is wound around the spool 522. The feed roller 524 is driven by a motor (not shown). The feed roller 524, together with the pressure roller 526, can pinch the flux-cored wire 56, pull it out from the spool 522, and feed it into a conduit liner housed inside the torch cable 60. The torch cable 60 is a WT3510-SD (length: 3.0 m) manufactured by Daihen Corporation. The conduit liner is a U4170H02 (length: 3.0 m) manufactured by Daihen Corporation. A welding torch 62 is connected to the tip of the torch cable 60. The flux-cored wire 56 fed into the conduit liner is fed from the tip of a contact tip (not shown) of the welding torch 62 to a welding point on the test material 70 to be welded.

[0101] The cylindrical member 54 is a cylindrical member. The torch cable 60 is wound around the outer circumferential surface of the cylindrical member 54. The diameter of the cylindrical member 54 is, for example, 150 mm. When the torch cable 60 is wound around the cylindrical member 54, the conduit liner housed inside the torch cable 60 is bent and deformed, and resistance (feeding resistance) is applied to the flux-cored wire 56 when it is fed.

[0102] The stability of the feed rate of the flux-cored wires of the present disclosure and the comparative examples was evaluated by the following method using this test device 50. Polytetrafluoroethylene oil (PTFE oil) was applied to the surface of the wire as a lubricant.

[0103] - Test method for feed rate stability - When a flux-cored wire (brand FC-9NI) manufactured by Nippon Steel Welding Industries Co., Ltd. was used as the reference wire, the welding conditions were adjusted so that the wire feed rate was 7±0.5 m / min. Under these conditions, the flux-cored wire of the present disclosure example or the comparative example was used to perform bead-on welding on SM490 (plate thickness 20 mm), and the feed rate was measured. The feed rate stability was then evaluated according to the following evaluation criteria. [Evaluation criteria] A (◎): Within 7±0.5 m / min B (◯): Within 7±1.0 m / min C (×): 7±a (a>1.0) m / min

[0104]

[0105] From the results shown in the table, it can be seen that the flux-cored wires of the present disclosure have excellent feedability, with the Vickers hardness Hv of the outer sheath being in the range of 180 to 500. On the other hand, the flux-cored wires of the comparative examples did not satisfy any of the requirements defined in the present disclosure, with the Vickers hardness Hv of the outer sheath being outside the range of 180 to 500, and were therefore evaluated as poor in feedability.

[0106] 50: Testing device, 52: Feeder, 54: Cylindrical member, 56: Welding wire, 60: Torch cable, 62: Welding torch, 70: Test material, 522: Spool, 524: Feeding roller, 526: Pressure roller

Claims

1. A flux-cored welding wire comprising a steel sheath and flux filled inside the steel sheath, wherein the steel sheath has a Vickers hardness Hv of 180 to 500, and an fcc ratio determined by a magnetic induction method of 70% or more, and the chemical composition of the steel sheath, in mass% relative to the total mass of the steel sheath, is: C: 0% to 0.650%, Si: 0.03% to 0.50%, Mn: 3.1% to 30.0%, P: 0.050% or less, S: 0.050% or less, Cu: 0% to 5.0%, Ni: 1.0% to 30.0%, Cr: 0% to 10.0%, Mo: 0% to 10.0%, Nb: 0% to 1.00%, V: 0% to 1.00%, A flux-cored wire comprising Co: 0% to 1.00%, Pb: 0% to 1.00%, Sn: 0% to 1.00%, Al: 0% to 0.10%, Ti: 0% to 0.10%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, W: 0 to 30.00%, Mg: 0 to 0.50%, REM: 0 to 0.50%, Zr: 0 to 5.00%, B: 0% to 0.1000%, N: 0% to 0.500%, and O: 0.0500% or less, with the balance being Fe and impurities.

2. The flux-cored wire according to claim 1, wherein the thickness of the steel sheath is 150 μm or more and 450 μm or less.

3. The flux-cored wire according to claim 1, wherein the area ratio of inclusions having an equivalent circle diameter of 5 μm or more in the steel sheath is 3.00% or less.

4. The flux-cored wire according to claim 1, wherein in the chemical composition of the steel sheath, the sum of the Mn content and the Ni content (Mn + Ni) is 5.0% or more, and the sum of the Mn content, the Ni content and the Cr content (Mn + Ni + Cr) is 15.0% or more.

5. The flux-cored wire according to claim 1, wherein the mass ratio (Ni / Mn) of the Mn content to the Ni content in the chemical composition of the steel sheath is 0.10 or more.

6. The flux-cored wire according to claim 5, wherein the mass ratio (Ni / Mn) is 1.00 or more.

7. The flux-cored wire according to claim 1, wherein the total content of Nb, V, Ti, Ta, Hf, and Zr (Nb + V + Ti + Ta + Hf + Zr) in the chemical composition of the steel sheath is 0.0005% or more and 5.0% or less.

8. The flux-cored wire according to claim 1, wherein the steel sheath has no welds at the joints.

9. The flux-cored wire according to claim 1, wherein the steel sheath has a weld at the joint.

10. The flux-cored wire according to claim 1, the surface of which is coated with one or both of polytetrafluoroethylene oil and perfluoropolyether oil.

11. A method for manufacturing a welded joint, comprising the step of welding steel materials using the flux-cored wire according to any one of claims 1 to 10.

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