Flux-cored wire, weld metal, gas-shielded arc welding method, and method for manufacturing welded joint

Abstract

To provide a flux-cored wire for high tensile strength steel, enabling acquisition of a weld zone which maintains target strength, furthermore has excellent low temperature toughness, not only under an as-welded condition but also under a wide PWHT(post weld heat treatment) condition.SOLUTION: A flux-cored wire contains, in terms of mass% with respect to whole mass of the wire, C: 0.01% or more and 0.10% or less, Si: over 0% and 0.45% or less, Mn: 1.0% or more and 3.0% or less, Ni: 3.1% or more and 5.5% or less, Mg: 0.10% or more and 1.00% or less, and Mo: 0.05% or more and 0.65% or less, and further contains Cr: 0.65% or less, Cu: 0.85% or less, and V: 0.045% or less, wherein when Ni content is denoted as [Ni], Mn content is denoted as [Mn] and Si content is denoted as [Si], the flux-cored wire satisfies the equation: [Ni] / ([Mn]+[Si])=1.10 or more and 5.40 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a flux-cored wire used for welding high tensile steel, a weld metal, a gas-shielded arc welding method, and a method for manufacturing a welded joint. [Background technology]

[0002] Welded products such as marine structures and tanks used in the drilling and production of oil, gas, etc., are required to be large-sized and to operate in cold regions, and the steel plates and welding materials used in their manufacture are required to have high strength and excellent low-temperature toughness. In addition to these characteristics, post-weld heat treatment (PWHT) may be performed to further improve the quality of welded products. By performing PWHT, for example, residual stress in the weld can be removed, which has the effect of suppressing cracks in the weld.

[0003] However, when this PWHT is performed, there is a risk of a decrease in toughness due to embrittlement of the welded part caused by precipitation hardening, temper embrittlement, etc. In particular, in the welded parts of high-tensile steel, embrittlement during PWHT is significant due to the influence of various elements added to improve strength, and the higher the strength, the more often it becomes impossible to apply PWHT. Therefore, there is a need to develop welding materials for high-tensile steels that have excellent mechanical properties even after PWHT.

[0004] For example, Patent Document 1 proposes a flux-cored wire that can improve the high-temperature strength and low-temperature toughness of a welded part after a long-term PWHT in welding 690 MPa-class high-tensile steel. The above Patent Document 1 describes that the low-temperature toughness was good as a result of carrying out PWHT at 620°C for 12 hours. In addition, Patent Document 2 states that the resistance is 60 kgf / mm 2 The wire disclosed in the above Patent Document 2 is used for gas shielded arc welding of high tensile steels with a strength of 80 Kgf / mm (approximately 588 MPa) or more, and is free from toughness degradation due to PWHT.2 It is also reported that when wire for high tensile steel (approximately 785 MPa) was subjected to PWHT at 600°C for three hours, there was almost no decrease in toughness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-253886 [Patent Document 2] Japanese Patent Application Publication No. 157795 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned Patent Documents 1 and 2 consider only one type of PWHT condition, and it is not necessarily true that excellent toughness can be obtained under other PWHT conditions. In addition, a larger tolerance in the PWHT conditions makes it possible to apply the PWHT to a variety of welded products, simplifies construction management, and further improves the quality stability of the welds.

[0007] On the other hand, it has been known that PWHT may reduce toughness depending on the conditions, and that the PWHT conditions are strictly determined by the type of material, thickness, welded joints, construction conditions, etc. Therefore, it is very difficult to provide a tolerance for the PWHT conditions. In particular, the higher the strength, the more additive elements are contained, making it more likely that embrittlement phenomena such as temper embrittlement will occur, and the more stringent the PWHT conditions become.

[0008] The present invention has been made in view of the above problems, and aims to provide a flux-cored wire for high tensile steel, capable of obtaining a welded joint having excellent low-temperature toughness while maintaining a desired strength not only in an as-welded state (hereinafter also referred to as "as-welded") but also under a wide range of PWHT conditions, a gas-shielded arc welding method, and a method for manufacturing a welded joint. Another aim is to provide a weld metal having excellent low-temperature toughness while maintaining a desired strength under a wide range of PWHT conditions. [Means for solving the problem]

[0009] The above object of the present invention is achieved by the following configuration [1] relating to a flux-cored wire. [1] A flux-cored wire in which a steel sheath is filled with flux, For the total mass of wire, C: 0.01% by mass or more and 0.10% by mass or less, Si: more than 0 mass% and 0.45 mass% or less, Mn: 1.0 mass% or more and 3.0 mass% or less, Ni: 3.1% by mass or more and 5.5% by mass or less, Mg: 0.10 mass% or more and 1.00 mass% or less, Mo: 0.05% by mass or more and 0.65% by mass or less; Cr: 0.65% by mass or less, Cu: 0.85% by mass or less, and V: 0.045% by mass or less, The Ni content in the wire is expressed as [Ni] 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. When the Si content in the wire is expressed as [Si] in mass% relative to the total mass of the wire, A flux-cored wire, characterized in that [Ni] / ([Mn]+[Si]): 1.10 or more and 5.40 or less.

[0010] Preferred embodiments of the present invention relating to the flux-cored wire relate to the following [2] and [3]. [2] Furthermore, for the total mass of the wire, The flux-cored wire according to [1], characterized in containing B: 0.10 mass % or less.

[0011] [3] Furthermore, at least one selected from the group consisting of Ti, Al, Zr, F, Na and K, For the total mass of wire, Ti: 2.5% by mass or more and 6.0% by mass or less, Al: 0.50% by mass or less, Zr: 0.50% by mass or less, F: 0.10% by mass or more and 0.50% by mass or less, and Total amount of Na and K: 0.01% by mass or more and 0.30% by mass or less The flux-cored wire according to [1] or [2],

[0012] The above object of the present invention is achieved by the following configuration [4] relating to the weld metal.

[0013] [4] A weld metal obtained by gas-shielded arc welding using the flux-cored wire according to any one of [1] to [3].

[0014] The above object of the present invention is achieved by the gas shielded arc welding method having the following configuration [5].

[0015] [5] A gas-shielded arc welding method, comprising: performing gas-shielded arc welding using the flux-cored wire according to any one of [1] to [3].

[0016] The above object of the present invention is achieved by the following configuration [6] relating to a method for manufacturing a welded joint. [6] A method for producing a welded joint, comprising gas-shielded arc welding using a high-tensile steel as a base material and the flux-cored wire according to any one of [1] to [3]. Effect of the Invention

[0017] According to the present invention, it is possible to provide a flux-cored wire for high tensile steel, which is capable of obtaining a weld having excellent low-temperature toughness while maintaining a desired strength not only in as-welded condition but also under a wide range of PWHT conditions. Moreover, according to the present invention, it is possible to provide a weld metal that has excellent low-temperature toughness while maintaining a desired strength even under a wide range of PWHT conditions. Furthermore, according to the present invention, it is possible to provide a gas-shielded arc welding method and a method for manufacturing a welded joint, which are capable of obtaining a welded joint having excellent low-temperature toughness while maintaining the desired strength even under a wide range of PWHT conditions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment 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 modification within the scope of the gist of the present invention.

[0019] In the present embodiment, the content means mass% relative to the total mass of the flux-cored wire unless otherwise specified. In addition, each element and each compound contained in the flux-cored wire according to the present embodiment may be contained in either the steel sheath or the flux, or may be contained in both the steel sheath and the flux. Furthermore, unless otherwise specified, each of the above elements may be contained in the flux-cored wire in the form of a metal, in the form of a compound, or in the form of both a metal and a compound. Therefore, regardless of the form in which each of the above elements is contained in the flux-cored wire, it is specified by the converted value of the element. For example, in the case of Si, the Si content refers to the total of the Si converted values ​​of metal Si and Si compounds. Metal Si includes both simple Si and Si alloys.

[0020] [1. Flux-cored wire] The flux-cored wire (hereinafter simply referred to as "wire") according to this embodiment has a steel sheath (hereinafter simply referred to as "sheath") filled with flux. 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. The flux filling rate can be set to any value as long as the content of each element in the wire is within the range of the present invention, but in order to further improve the wire drawability and wire feedability, it is preferable to set the flux filling rate to, for example, 10 mass % or more and 20 mass % or less with respect to the total mass of the wire. Furthermore, the wire may have a seam in the outer sheath, or may not have a seam, and there is no limitation on the shape of the seam or the cross-sectional shape.

[0021] The present inventors have studied the mechanism of toughness reduction caused by PWHT in high-strength welds, and have also conducted extensive research to obtain welds that have excellent low-temperature toughness even when PWHT is performed under a wide range of conditions. As a result, they have found the following and completed the present invention. Note that the weld refers to the weld metal and heat-affected zone (HAZ), but in the following description of this application, the weld metal will be described.

[0022] First, a conventional mechanism of toughness degradation in high strength steel will be described. Conventionally, the following two factors have been considered to be the main causes of the decrease in weld metal toughness after PWHT.

[0023] (Cause 1) When the weld metal contains a large amount of Cr, Mo, Nb, V, etc., these elements combine with C to form carbides which precipitate, causing the weld metal to harden. (Cause 2) Embrittlement phenomena such as temper embrittlement occur due to slow cooling from the PWHT temperature.

[0024] It was previously believed that the above-mentioned (Cause 1) and (Cause 2) mainly caused a decrease in grain boundary strength of the weld metal, resulting in a decrease in toughness. Therefore, in the past, the following measures were taken to ensure excellent toughness while maintaining the desired strength after PWHT.

[0025] (Countermeasure 1) Suppress the precipitation and growth of carbides at prior austenite grain boundaries. (Countermeasure 2) Suppress the segregation of impurity elements at prior austenite grain boundaries.

[0026] However, depending on the PWHT conditions, simply implementing the above measures (Measures 1) and (Measures 2) may not be sufficient. Generally, PWHT conditions consist of the elements of holding temperature and holding time, and can be organized using the Larson-Miller parameter (hereinafter referred to as "LMP"), which uses these elements as parameters.

[0027] Therefore, in this specification, a flux-cored wire that can obtain a weld metal excellent in low-temperature toughness while maintaining a target strength under the following three conditions a to c is determined to have a wide range of PWHT conditions.

[0028] (Condition a) As-welded. (Condition b) A PWHT condition with a high LMP, in which the temperature is 620°C for 8 hours (hereinafter referred to as "high LMP condition"). (Condition c) A PWHT condition with low LMP, in which the temperature is 580°C for 2 hours (hereinafter referred to as "low LMP condition").

[0029] It is known that in the as-welded condition (Condition a) and in the high LMP condition (Condition b), good low-temperature toughness can be obtained by adding an appropriate amount of Ni to the wire in addition to the above (Measures 1) and (Measures 2). On the other hand, under the low LMP condition of (Condition c), good low-temperature toughness cannot be obtained simply by adding an appropriate amount of Ni. The present inventors have found a mechanism by which low-temperature toughness decreases when PWHT is performed under the low LMP condition of (Condition c), and have also found a measure by which a weld metal with excellent low-temperature toughness can be obtained while maintaining the desired strength even under a wide range of PWHT conditions. The mechanism by which low-temperature toughness decreases under the low LMP condition of (Condition c) and the measure will be described below.

[0030] When the weld metal contains a large amount of Ni, a segregation zone containing Ni (hereinafter referred to as "Ni segregation zone"), i.e., a region where Ni is concentrated, occurs on the weld metal structure. This Ni segregation zone does not affect the low temperature toughness when welded as is under (Condition a) or under high LMP conditions under (Condition b). However, under the low LMP condition (condition c), the C content in the Ni segregation zone becomes high, and the formation of island martensite and the formation and coarsening of carbides in the Ni segregation zone are promoted. In particular, the formation of island martensite is remarkable, and due to the influence of these products, brittle fracture is likely to occur in the Ni segregation zone, resulting in a decrease in low-temperature toughness.

[0031] Based on these findings, the inventors have discovered that, in order to ensure a weld metal with excellent low-temperature toughness while maintaining the desired strength after PWHT, it is effective to implement (Measure 3) described below, in addition to (Measure 1) and (Measure 2).

[0032] (Countermeasure 1) Suppress the precipitation and growth of carbides at prior austenite grain boundaries. (Countermeasure 2) Suppress the segregation of impurity elements at prior austenite grain boundaries. (Countermeasure 3) Suppress the formation of island martensite in the Ni segregation zone.

[0033] Furthermore, the inventors of the present invention have found that by appropriately controlling the chemical composition of the flux-cored wire and the parameters calculated from the contents of Ni, Si, and Mn, the above (Measures 1) to (Measures 3) can be achieved. That is, by using the wire in which (Measures 1) to (Measures 3) are realized, it is possible to obtain a weld metal having excellent low-temperature toughness while maintaining the target strength not only in the as-welded state but also after PWHT under a wide range of conditions.

[0034] In addition to the above (Measures 1) to (Measures 3), it is preferable to add the following (Measure 4) because it can prevent excessive strength increase and suppress the decrease in low-temperature toughness. (Measure 4) Suppress the formation of carbides within the grains.

[0035] The suppression of carbide formation can be achieved by setting the contents of at least C, Cr, Mo, and V within the specified ranges described later.

[0036] Hereinafter, the chemical composition of the specific wire and the reasons for the numerical limitations of its content will be described in more detail.

[0037] <C: 0.01 mass% or more and 0.10 mass% or less> C is a component having the effect of improving the strength of the weld metal. If the C content is less than 0.01 mass%, the target strength cannot be obtained. Therefore, the C content with respect to the total mass of the wire is 0.01 mass% or more, preferably 0.02 mass% or more, and more preferably 0.03 mass% or more. On the other hand, if the C content exceeds 0.10 mass%, it promotes the coarsening of carbides and the low-temperature toughness after PWHT decreases. Therefore, the C content with respect to the total mass of the wire is 0.10 mass% or less, preferably 0.09 mass% or less, and more preferably 0.08 mass% or less.

[0038] <Si: More than 0 mass% and 0.45 mass% or less> Si is a component that is particularly likely to generate island martensite in the Ni segregation band. With an increase in the Si content in the weld metal, temper embrittlement is promoted and the low-temperature toughness decreases. When the Si content exceeds 0.45% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Si content with respect to the total mass of the wire should be 0.45% by mass or less, preferably 0.40% by mass or less, and more preferably 0.35% by mass or less. On the other hand, Si is a component that cannot be completely 0% by mass and is also a component that has the effect of suppressing pore defects as a deoxidizing element. And even if Si is contained in the wire in a trace amount, the effect of suppressing pore defects can be obtained. Therefore, the Si content with respect to the total mass of the wire should be more than 0% by mass, preferably 0.10% by mass or more, and more preferably 0.15% by mass or more.

[0039] <Mn: 1.0% by mass or more and 3.0% by mass or less> Mn is a component that has the effect of improving the strength of the weld metal. If the Mn content is less than 1.0% by mass, the target strength cannot be obtained. Therefore, the Mn content with respect to the total mass of the wire should be 1.0% by mass or more, preferably 1.2% by mass or more, and more preferably 1.4% by mass or more. Also, since Mn is a component that is particularly likely to generate island martensite in the Ni segregation band, with an increase in the Mn content in the weld metal, temper embrittlement is promoted and the low-temperature toughness decreases. When the Mn content exceeds 3.0% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Mn content with respect to the total mass of the wire should be 3.0% by mass or less, preferably 2.9% by mass or less, and more preferably 2.8% by mass or less.

[0040] <Ni: 3.1% by mass or more and 5.5% by mass or less> Ni is a component that has the effect of improving strength and improving the low-temperature toughness of the weld metal by matrix strengthening. When the Ni content is less than 3.1% by mass, the target strength and low-temperature toughness cannot be obtained. Therefore, the Ni content based on the total mass of the wire is 3.1% by mass or more, preferably 3.2% by mass or more, and more preferably 3.4% by mass or more. On the other hand, when the Ni content exceeds 5.5% by mass, there is a risk of hot cracking. Therefore, the Ni content based on the total mass of the wire is 5.5% by mass or less, preferably 5.3% by mass or less, and more preferably 5.0% by mass or less.

[0041] <Mg: 0.10% by mass or more and 1.00% by mass or less> Mg is a component that has a deoxidizing effect and improves strength. When the Mg content is less than 0.10% by mass, the target strength cannot be obtained. Therefore, the Mg content based on the total mass of the wire is 0.10% by mass or more, preferably 0.30% by mass or more, and more preferably 0.35% by mass or more. On the other hand, when the Mg content exceeds 1.00% by mass, the strength increases excessively and the low-temperature toughness decreases. Therefore, the Mg content based on the total mass of the wire is 1.00% by mass or less, preferably 0.90% by mass or less, and more preferably 0.80% by mass or less.

[0042] <Mo: 0.05% by mass or more and 0.65% by mass or less> Mo is a component that improves the strength of the weld metal and is effective in suppressing temper embrittlement. By precipitating Mo carbides inside the grains of the weld metal, the growth of carbides precipitating at the grain boundaries can be suppressed, and the decrease in low-temperature toughness after PWHT can be suppressed. When the Mo content is less than 0.05% by mass, the target low-temperature toughness after PWHT cannot be obtained while ensuring the target strength. 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, and more preferably 0.13% by mass or more. On the one hand, when the Mo content exceeds 0.65% by mass, Mo carbides precipitate and grow within the grains, resulting in an excessive increase in strength and a decrease in low-temperature toughness due to PWHT. Therefore, the Mo content relative to the total mass of the wire should be 0.65% by mass or less, preferably 0.55% by mass or less, and more preferably 0.44% by mass or less.

[0043] <Cr: 0.65% by mass or less (including 0% by mass)> Cr is a component effective for achieving both strength and toughness in the weld metal. In this embodiment, the lower limit of the Cr content is not particularly specified. However, for the purpose of achieving both strength and toughness, when Cr is contained in the wire, the Cr 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 0.10% by mass or more. On the other hand, Cr is also a component that promotes the precipitation and growth of mainly coarse grain boundary carbides and reduces low-temperature toughness by PWHT. When the Cr content relative to the total mass of the wire exceeds 0.65% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Cr content relative to the total mass of the wire should be 0.65% by mass or less, preferably 0.55% by mass or less, and more preferably 0.45% by mass or less.

[0044] <Cu: 0.85% by mass or less (including 0% by mass)> Cu is a component that has the effect of refining the structure of the weld metal and improving low-temperature toughness while maintaining strength. In this embodiment, the lower limit of the Cu content is not particularly specified. However, for the purpose of improving low-temperature toughness, when Cu is contained in the wire, the Cu content relative to the total mass of the wire is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.05% by mass or more. On the one hand, when the Cu content exceeds 0.85% by mass, precipitation is promoted by PWHT, and the low-temperature toughness decreases. Therefore, the Cu content based on the total mass of the wire should be 0.85% by mass or less, preferably 0.65% by mass or less, more preferably 0.45% by mass or less, even more preferably 0.35% by mass or less, and even more preferably 0.20% by mass or less. When Cu plating is applied to the wire surface, the Cu contained during plating is also included in the range of the Cu content defined in this embodiment.

[0045] <V: 0.045% by mass or less (including 0% by mass)> While V has the effect of improving the strength of the weld metal, it is a component that promotes the precipitation and growth of grain boundary carbides and reduces the low-temperature toughness. In this embodiment, the lower limit of the V content is not particularly defined. However, when V is contained in the wire for the purpose of improving the strength of the weld metal, the V content based on the total mass of the wire is preferably 0.010% by mass or more, and more preferably 0.015% by mass or more. On the other hand, when the V content exceeds 0.045% by mass, the low-temperature toughness after PWHT decreases. Therefore, the V content based on the total mass of the wire should be 0.045% by mass or less, preferably 0.035% by mass or less.

[0046] <[Ni] / ([Mn] + [Si]): 1.10 or more and 5.40 or less> As described above, by appropriately controlling the value of this parameter calculated from the contents of Ni, Si, and Mn, while maintaining the target strength, the generation of island-shaped martensite in the Ni segregation zone, which is the above (countermeasure 3), can be suppressed, and a weld metal with excellent low-temperature toughness can be obtained. When the value obtained by [Ni] / ([Mn] + [Si]) is less than 1.10, island-shaped martensite is likely to be generated in the Ni segregation zone, and the low-temperature toughness after PWHT decreases. Therefore, [Ni] / ([Mn] + [Si]) should be 1.10 or more, preferably 1.15 or more, and more preferably 1.20 or more. On the one hand, when the value obtained by [Ni] / ([Mn]+[Si]) exceeds 5.40, since the [Mn]+[Si] value is low, the as-welded strength decreases, and further the strength after PWHT also decreases. Therefore, [Ni] / ([Mn]+[Si]) should be 5.40 or less, preferably 5.00 or less, more preferably 4.50 or less, and even more preferably 4.00 or less.

[0047] In the above formula, [Ni] is the value representing the Ni content in the wire as a mass% with respect to the total mass of the wire. [Mn] is the value representing the Mn content in the wire as a mass% with respect to the total mass of the wire. [Si] is the value representing the Si content in the wire as a mass% with respect to the total mass of the wire.

[0048] <Total content of C, Si, Mn, Ni, Mg, Mo, Cr, Cu and V: 5.0 mass% or more and 10.0 mass% or less> In the present embodiment, from the viewpoint of improving welding workability, the total content of C, Si, Mn, Ni, Mg, Mo, Cr, Cu and V is preferably 5.0 mass% or more, more preferably 6.0 mass% or more. Also, from the viewpoint of improving the wire drawing property of the wire, the total content of the above components is preferably 10.0 mass% or less, more preferably 9.0 mass% or less.

[0049] In addition, the wire according to the present embodiment preferably further contains B within the content range shown below. The reason for limiting the content when B is contained in the wire will be described below.

[0050] <B: 0.10 mass% or less (including 0 mass%)> B is a component that segregates at the prior austenite grain boundaries and has the effect of improving the toughness of the weld metal by suppressing the proeutectoid ferrite. However, in the case of weld metal mainly composed of bainite with a strength exceeding 590 MPa in particular, B exists as free B, and it is considered that free B reduces the interfacial energy between the Ti-based oxide and austenite according to its amount of existence, thereby suppressing the transformation of intragranular bainite. That is, when B is added in excess, the formation of bainite around the Ti-based oxide is suppressed, and lath-like bainite is formed from the austenite grain boundaries, and a fine structure effective for improving toughness cannot be obtained. In the present embodiment, the lower limit of the B content is not particularly defined, but when B is contained in the wire, the B content with respect to the total mass of the wire is preferably 0.10 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.05 mass% or less.

[0051] Further, the wire according to the present embodiment preferably contains at least one selected from the group consisting of Ti, Al, Zr, F, Na, and K within the range of the content shown below respectively. The reasons for limiting the respective contents when these elements are contained in the wire will be described below.

[0052] <Ti: 2.5 mass% or more and 6.0 mass% or less> Ti functions as a slag former and has the effect of facilitating welding in welding positions other than the downward position, for example, vertical-up progression or upward position, and obtaining good welding workability in all positions. When Ti is contained in the wire, in order to maintain an appropriate amount of slag formation and obtain good welding workability, the Ti content with respect to the total mass of the wire is generally preferably 2.5 mass% or more, and more preferably 3.0 mass% or more. Also, the Ti content with respect to the total mass of the wire is generally preferably 6.0 mass% or less, and more preferably 5.0 mass% or less.

[0053] <Al: 0.50 mass% or less (including 0 mass%)> Al is a component that has the effect of improving bead wettability and obtaining a flat bead shape. When Al is contained in the wire, in order to maintain good slag detachability while improving bead wettability and obtaining a flat bead shape, the Al content based on the total mass of the wire is preferably 0.50% by mass or less, and more preferably 0.25% by mass or less.

[0054] <Zr: 0.50% by mass or less (including 0% by mass)> Zr is a component that has the effect of improving bead wettability and obtaining a flat bead shape. When Zr is contained in the wire, in order to maintain good slag detachability while improving bead wettability and obtaining a flat bead shape, the Zr content based on the total mass of the wire is preferably 0.50% by mass or less, and more preferably 0.25% by mass or less.

[0055] <F: 0.10% by mass or more and 0.50% by mass or less> F is a component that has the effect of stabilizing the arc. When F is contained in the wire, the amount of sputtering can be suppressed and sufficient arc stability can be obtained. Therefore, the F content based on the total mass of the wire is preferably 0.10% by mass or more, and more preferably 0.15% by mass or more. Also, the F content based on the total mass of the wire is preferably 0.50% by mass or less, and more preferably 0.45% by mass or less.

[0056] <Total amount of Na and K: 0.01% by mass or more and 0.30% by mass or less> Na and K are components that have the effect of stabilizing the arc. When at least one of Na and K is contained in the wire, the amount of sputtering can be suppressed and sufficient arc stability can be obtained. Therefore, the total amount of Na and K based on the total mass of the wire is preferably 0.01% by mass or more, and more preferably 0.03% by mass or more. The total amount of Na and K relative to the total mass of the wire is preferably 0.30 mass % or less, and more preferably 0.25 mass % or less.

[0057] <Other ingredients and impurities> Fe is a main component of the wire according to this embodiment. The Fe content relative to the total mass of the wire is preferably 82 mass % or more, and more preferably 84 mass % or more. In addition to the above components, the wire may contain oxygen (O), Ca, Ba, Li, Nb, etc., and the content of oxygen (O) in the total wire mass is preferably 1% by mass or more and 5% by mass or less. The total content of Ca, Ba, Li, Nb, etc. in the total wire mass is preferably 1% by mass or less. The remainder is unavoidable impurities, and the total amount of the unavoidable impurities is preferably regulated to 0.15% by mass or less based on the total mass of the wire. Examples of the unavoidable impurities include P and S, and from the viewpoint of preventing hot cracking, the contents of P and S are preferably 0.015% by mass or less based on the total mass of the wire.

[0058] [2. Manufacturing method of flux-cored wire] The flux-cored wire according to the present embodiment can be manufactured, for example, by the following method. First, the steel strip constituting the sheath is formed by a forming roll while being fed in the longitudinal direction to form a U-shaped open tube. Next, flux containing a predetermined amount of metal or alloy and compound, etc., is filled into the sheath to obtain a predetermined chemical composition, and then processed to have a circular cross section. At this time, the sheath can be seamless by welding, etc. at the seam. Then, the wire is drawn by cold working to obtain a wire diameter of, for example, 0.9 mm or more and 2.0 mm or less, thereby manufacturing the flux-cored wire. Annealing may be performed during the cold working.

[0059] [3. Welding metal] The weld metal according to this embodiment is obtained by gas-shielded arc welding using the flux-cored wire according to this embodiment described above in [1. Flux-cored wire]. In addition, in the weld metal according to this embodiment, there are no particular limitations on the conditions other than the use of the flux-cored wire according to this embodiment, and the type of base metal can be appropriately selected depending on the required characteristics.

[0060] [4. Gas shielded arc welding method] The gas-shielded arc welding method according to this embodiment is a method of welding using the flux-cored wire according to this embodiment described above in [1. Flux-cored wire]. In the gas-shielded arc welding method according to the present embodiment, various welding conditions other than the use of the flux-cored wire according to the present embodiment are not particularly limited, and general conditions in a welding method using a flux-cored wire can be used for the type of base material, welding voltage, welding current, welding position, etc. The shielding gas is also not limited, but is preferably MAG from the viewpoint of further improving welding workability, and more preferably 80 vol. % Ar-20 vol. % CO2.

[0061] [5. Manufacturing method of welded joints] The method for manufacturing a welded joint according to this embodiment is a method for gas-shielded arc welding using high-tensile steel as a welding base material and the flux-cored wire according to this embodiment described above in [1. Flux-cored wire]. In the method for manufacturing a welded joint, the welding conditions are not particularly limited except that high tensile steel is used as the weld base material and that gas-shielded arc welding is performed using the flux-cored wire according to this embodiment, and general conditions for welding methods using flux-cored wires can be used for the welding voltage, welding current, welding position, etc. There is also no limitation on the shielding gas, but it is preferably MAG from the viewpoint of improving the welding workability, and more preferably 80 vol. % Ar-20 vol. % CO2. In addition, any high-tensile steel can be used as the base material, but it is preferable that it is of 720 MPa class or higher, such as P690Q, P690QH, P690QL1, and P690QL2 of EN 10028-6:2017, KD620, KD690, KE620, and KE690 specified by Nippon Kaiji Kyokai (NK), and VL690 specified by DNV GL. EXAMPLES

[0062] Hereinafter, examples of the flux-cored wire according to the present embodiment and comparative examples will be described.

[0063] [Evaluation of mechanical properties of weld metal] (Wire preparation) First, flux-cored wires having various components and a diameter of 1.2 mm were prepared by filling a strip-shaped steel sheath having the chemical composition shown in Table 1 below with flux. The flux filling rate was set to be in the range of 13.0 mass % to 15.5 mass % with respect to the total mass of the wire.

[0064] (Gas shielded arc welding) Next, the obtained flux-cored wire was used to perform gas-shielded arc welding on base metals having the plate thicknesses and chemical compositions shown in Table 2 below. In this example, a V-groove was formed in the base metal, and gas-shielded arc welding was performed under the welding conditions shown in Table 3 below to produce a welded joint.

[0065] (Mechanical property evaluation) The mechanical properties of the weld metal were evaluated with reference to "Tensile and impact test methods for weld metal" specified in JIS Z 3111:2005, and a tensile test piece (A2 type) and an impact test piece (V-notch test piece) were taken from the center of the weld metal in the plate thickness direction to evaluate the tensile performance and impact performance. In examining a wide range of PWHT conditions, the changes in tensile strength and toughness when PWHT was performed at various holding temperatures and holding times were organized using the Larson-Miller parameter (LMP) as the PWHT condition. In this example, the low LMP condition was a PWHT condition at a temperature of 580°C for 2 hours (LMP = 17.3 x 10 3 ) was adopted, and as the high LMP condition, a PWHT condition of 8 hours at a temperature of 620°C (LMP = 18.7 × 10 3 ) was adopted.

[0066] The tensile tests were carried out at room temperature (approximately 20±2°C) on the as-welded test pieces, the test pieces under the low LMP condition, and the test pieces under the high LMP condition to measure the tensile strength. In this example, for each of the as-welded test piece, the test piece under the low LMP condition, and the high LMP test piece, it was determined that the target strength was satisfied when the tensile strength (TS) was 720 MPa or more.

[0067] Impact tests were performed on as-welded test pieces, test pieces under low LMP conditions, and test pieces under high LMP conditions at a test temperature of -40°C, and the toughness was evaluated by measuring the Charpy absorbed energy (vE-40°C). In this example, for each of the as-welded test piece, the test piece under the low LMP condition, and the test piece under the high LMP condition, the low temperature toughness was determined to be excellent when the absorbed energy at -40°C was 50 J or more. For the as-welded, low LMP condition and high LMP condition test pieces, those that satisfied the target strength and had excellent low temperature toughness were deemed to have passed, and those that did not were deemed to have failed.

[0068] The chemical components of the wire used are shown in Tables 4 and 5 below, and the evaluation results of the mechanical properties are shown in Table 6 below. In Tables 4 and 5 below, [Ni] is the Ni 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, and [Si] is the Si content in the wire expressed as mass% relative to the total mass of the wire.

[0069] The balance of the chemical components of the wires shown in Tables 4 and 5 were Fe, O (oxygen), Ca, Ba, Li, and unavoidable impurities. In addition, in Table 4, the Ni content marked with "-" indicates that it was less than 0.01 mass% below the quantification limit, the Cr content marked with "-" indicates that it was less than 0.01 mass% below the quantification limit, the Cu content marked with "-" indicates that it was less than 0.01 mass% below the quantification limit, the B content marked with "-" indicates that it was less than 0.0002 mass% below the quantification limit, and the V content marked with "-" indicates that it was less than 0.002 mass% below the quantification limit. Furthermore, in the column "[Ni] / ([Mn]+[Si])" in Table 5, the Ni content (numerator) was below the quantification limit and was therefore indicated as "-" since calculation was not possible.

[0070] In Table 6, test pieces for which the mechanical properties could not be evaluated due to the occurrence of cracks or breakage, etc., are indicated with "-" in the evaluation result column.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] [Table 4]

[0075] [Table 5]

[0076] [Table 6]

[0077] As shown in Tables 4 and 5 above, inventive examples Nos. 1 to 24, in which the content of each component in the wire was within the numerical range of the present invention, the tensile strength (TS) after as-welded and two different types of PWHT was the desired 720 MPa or more, and excellent low-temperature toughness was obtained with an absorbed energy of 50 J or more at -40°C. This means that weld metal with excellent low-temperature toughness could be obtained while maintaining the desired strength not only in as-welded but also in a wide range of PWHT conditions. Furthermore, since invention examples Nos. 1 to 24 were obtained by gas-shielded arc welding using wires whose content of each component was within the numerical range of the present invention, it was possible to produce welded joints having welds with excellent low-temperature toughness while maintaining the desired strength under a wide range of PWHT conditions, not just as-welded.

[0078] On the other hand, in Comparative Example No. 25, the Ni content in the wire was less than the lower limit of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) was less than the lower limit of the numerical range of the present invention, so that the low-temperature toughness after PWHT under low LMP conditions and the tensile strength after high LMP were reduced. In Comparative Example No. 26, the Mo content in the wire exceeded the upper limit of the numerical range of the present invention, and therefore the low-temperature toughness after PWHT under low LMP conditions was reduced.

[0079] In Comparative Example No. 27, the Ni content and Mo content in the wire were below the lower limit values ​​of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) was below the lower limit value of the numerical range of the present invention. Therefore, the tensile strength and low-temperature toughness after PWHT under low LMP conditions were decreased, and the tensile strength after PWHT under high LMP conditions was decreased. In Comparative Example No. 28, the Ni content in the wire was below the lower limit of the numerical range of the present invention, and therefore the tensile strength after PWHT under high LMP conditions was reduced.

[0080] In Comparative Example No. 29, the Mn content in the wire was less than the lower limit of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) exceeded the upper limit of the numerical range of the present invention, so the tensile strength decreased as-welded, after PWHT under low LMP conditions, and after PWHT under high LMP conditions. In Comparative Example No. 30, the Mn content in the wire exceeded the upper limit of the numerical range of the present invention, so the test piece became excessively embrittled and fractured after PWHT under the low LMP condition and the high LMP condition.

[0081] In Comparative Example No. 31, the Ni content in the wire was less than the lower limit of the numerical range of the present invention, and the Cu content in the wire exceeded the upper limit of the numerical range of the present invention. Furthermore, the value obtained by [Ni] / ([Mn]+[Si]) was less than the lower limit of the numerical range of the present invention, so that the low-temperature toughness after PWHT under low LMP conditions was reduced. In Comparative Example No. 32, the Ni and Mo contents in the wire were below the lower limit of the numerical range of the present invention, the C and Cr contents in the wire exceeded the upper limit of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) was below the lower limit of the numerical range of the present invention, so that the test piece was excessively embrittled and fractured after PWHT under low LMP conditions. In addition, the low temperature toughness after PWHT under high LMP conditions was reduced.

[0082] In Comparative Example No. 33, the V content in the wire exceeded the upper limit of the numerical range of the present invention, and therefore the low-temperature toughness after PWHT under low LMP conditions was reduced. In Comparative Example No. 34, the Ni content and Mo content in the wire were below the lower limit values ​​of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) was below the lower limit value of the numerical range of the present invention. Therefore, the tensile strength and low-temperature toughness after PWHT under low LMP conditions were decreased, and the tensile strength after PWHT under high LMP conditions was decreased.

[0083] In Comparative Example No. 35, the Mn content in the wire exceeded the upper limit of the numerical range of the present invention, and the value obtained by [Ni] / ([Mn]+[Si]) was less than the lower limit of the numerical range of the present invention, so that the low-temperature toughness after PWHT under low LMP conditions was reduced. In Comparative Example No. 36, the value obtained by [Ni] / ([Mn]+[Si]) was below the lower limit of the numerical range of the present invention, and therefore the low-temperature toughness after PWHT under low LMP conditions was reduced.

Claims

1. A flux-cored wire in which a steel sheath is filled with flux, For the total mass of wire, C: 0.01% by mass or more and 0.10% by mass or less, Si: more than 0% by mass and not more than 0.45% by mass, Mn: 1.0% by mass or more and 3.0% by mass or less, Ni: 3.1% by mass or more and 5.5% by mass or less, Mg: 0.10% by mass or more and 1.00% by mass or less, Mo: 0.05% by mass or more and 0.65% by mass or less, Cr: 0.01% by mass or more and 0.65% by mass or less, Ti: 2.5% by mass or more and 6.0% by mass or less, Al: 0.50% by mass or less, Zr: 0.50% by mass or less, F: 0.10% by mass or more and 0.50% by mass or less, Total amount of Na and K: 0.01% by mass or more and 0.30% by mass or less, Contains Cu: 0.85% by mass or less, and V: 0.045% by mass or less; The Ni content in the wire is expressed as [Ni] 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, When the Si content in the wire is expressed as [Si] in mass% relative to the total mass of the wire, A flux-cored wire characterized in that [Ni] / ([Mn]+[Si]): 1.10 or more and 5.40 or less.

2. Furthermore, for the total mass of the wire, The flux-cored wire according to claim 1, further comprising: B: 0.10 mass % or less.

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

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

5. 3. A method for producing a welded joint, comprising gas-shielded arc welding using the flux-cored wire according to claim 1 or 2, using a high-tensile steel as a base material.

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