Method for manufacturing welded joint, and welded joint

Abstract

The present invention provides a method for manufacturing a welded joint having excellent toughness, and a welded joint.　The manufacturing method according to the present invention is a method for manufacturing a welded joint (1) comprising a steel material (4) and a plurality of layers of weld metal (31), wherein the weld metal (31) has a specific chemical composition, and the manufacturing method includes: a multi-layer overlay welding step in which layers, other than a first layer, are formed by weaving and are welded with a specific welding heat input amount such that the average layer height of each layer of the weld metal (31) in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material is more than 0 mm and is 4.0 mm or less; and a heat treatment step in which heat treatment is performed at a temperature of 580°C to 620°C for 2 hours or more. A welded joint (1) according to the present invention comprises a steel material (4) and a plurality of layers of weld metal (31) having the specific chemical composition described above, wherein: the average layer height of each layer in the weld metal (31) is more than 0 mm and 4.0 mm or less; the average width of each layer is 15 mm or more; and the absorbed energy in a Charpy impact test at -40°C is 30 J or more.

Description

Welded joint manufacturing method and welded joint

[0001] The present invention relates to a method for manufacturing a welded joint and a welded joint obtained thereby.

[0002] Welding techniques using weld metal, such as gas-shielded arc welding, are widely used in various fields, such as various industrial facilities and structures. Welded joints formed using such welding techniques require the weld or the weld metal contained in the weld to have excellent strength and toughness.

[0003] For example, Patent Document 1 discloses a solid wire for gas-shielded arc welding containing, by mass%, 0.02 to 0.14% C, 0.4 to 1.5% Si, 1.0 to 2.5% Mn, 0.05 to 0.4% Ti, 0.0003 to 0.010% Mg, and 0.0005 to 0.010% B, a total of 0.005 to 0.050% of one or more elements selected from the group consisting of Al and Zr, N limited to 0.005% or less, the balance consisting of Fe and unavoidable impurities, the total content of Si and Mn being 1.5 to 3.5%, and the ratio of the Mn content (%) to the Si content (%) (Mn / Si) being 0.85 or more. The solid wire for gas-shielded arc welding disclosed in Patent Document 1 has a tensile strength of 490 to 780 N / mm ２ It is said that when gas-shielded arc welding of high-tensile steel of the above grade is performed, it is possible to stably obtain weld metal with excellent toughness and strength even under highly efficient welding conditions.

[0004] In addition, Patent Document 2 discloses that either or both of the metal sheath and the flux are added with C: 0.01 to 0.3%, Si: 0.01 to 1.5%, Mn: 0.8 to 8.5%, Mo: 0.2 to 1.5%, Nb: 0.005 to 0.05%, and V: 0.005 to 0.05%, while the flux contains TiO ２Patent Document 2 discloses a flux-cored wire for gas-shielded arc welding, which is formed by filling a metal sheath with a flux containing 1.8 to 7.5% but less than 15% iron powder, with the remainder being a slag-forming agent including an arc stabilizer, at a filling rate of 10 to 25%. It is said that the flux-cored wire for gas-shielded arc welding disclosed in Patent Document 2 can produce a weld metal with excellent mechanical properties, with little coarsening of ferrite grains in the weld metal and little generation of ferrite bands, even when subjected to a long-term post-weld heat treatment (PWHT).

[0005] Furthermore, Patent Document 3 discloses a method for manufacturing a joint by multi-layer welding using a thick steel plate with a plate thickness of 25 mm or more, in which the heat input of each welding pass and the lamination interval of the weld metal satisfy the following formula (1): 0<0.077 H -１.１８ ・d ３．２６ ≦1.1 (1) (where H is the heat input (kJ / mm), and d is the lamination spacing (mm)) is disclosed. According to the manufacturing method of a joint by multi-pass welding disclosed in Patent Document 3, by controlling the welding heat input during welding and the lamination spacing of each pass, it is possible to completely eliminate areas where deterioration of toughness becomes significant, and it is said that this can significantly improve the HAZ toughness of steel materials that undergo multi-pass welding when welding thick marine structures and the like, and increase the safety of structures without reducing welding productivity.

[0006] JP 2007-253163 A JP 5-77086 A JP 2019-5770 A

[0007] Welded joints formed using the above-described welding techniques are sometimes subjected to post-weld heat treatment (PWHT) for the purpose of stress relief. However, in the weld metal after PWHT, P segregates at prior-γ grain boundaries during the heat treatment, which can cause fractures at the prior-γ grain boundaries and lead to deterioration. If the weld metal deteriorates in this way, the toughness of the weld may decrease, and the required properties may not be obtained.

[0008] Therefore, an object of the present invention is to provide a method for manufacturing a welded joint having excellent toughness, and a welded joint.

[0009] The present invention includes the following aspects.

[0010] (Aspect 1) A method for manufacturing a welded joint including a steel material and a plurality of layers of weld metal, wherein the chemical composition of the weld metal is, in mass %, C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Ti: 0.005 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V: 0 to 0.010%, Al: 0 to 0.010%, W: 0 to 0.70%, a multi-layer welding process in which the weld metal of the plurality of layers contains Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, and REM: 0 to 0.0050%, with the balance being Fe and impurities, and the weld metal of the plurality of layers is formed by weaving so that the average layer height of each layer in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding a first layer, is greater than 0 mm and not greater than 4.0 mm; a heat treatment step of performing heat treatment at a temperature of 580°C to 620°C for 2 hours or more after the multi-layer welding step, wherein the multi-layer welding step has a welding heat input calculated by the following formula (1) of 13.0 kJ / cm to 25.0 kJ / cm.

[0011] (Aspect 2) The manufacturing method according to aspect 1, wherein the multi-layer welding step is performed at a weaving speed of 20 mm / sec or more and a weaving width of 15 mm or more.

[0012] (Aspect 3) A welded joint including a steel material and a plurality of layers of weld metal, wherein the chemical composition of the weld metal is, in mass%, C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Ti: 0.005 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V: 0 to 0.010%, Al: 0 to 0.010%, W: 0 to 0.70%, Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%, with the balance consisting of Fe and impurities, wherein in the multiple-layer weld metal, excluding a first layer, each layer in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material has an average layer height of more than 0 mm and 4.0 mm or less, and each layer has an average width of 15 mm or more, A welded joint characterized in that the absorbed energy in the Charpy impact test at -40°C is 30 J or more.

[0013] According to the manufacturing method of the present invention, a welded joint having excellent toughness can be obtained.

[0014] Fig. 1 is a schematic diagram showing a cross section of a welded joint 1 obtained by a manufacturing method according to one embodiment of the present invention, centered on a welded portion 2. Fig. 2 is a diagram showing a cross section of a steel material used in the examples. Fig. 3 is a diagram showing notch positions in a Charpy impact test at -40°C for the welded joints produced in the examples. Fig. 4 is a graph showing the measurement results of absorbed energy in a Charpy impact test at -40°C for welded joints No. 1 to 28 produced in the examples. Fig. 5 is a schematic diagram for explaining a general weaving method.

[0015] Hereinafter, preferred embodiments of the method for manufacturing a welded joint of the present invention and the welded joint obtained thereby will be described in detail with reference to the drawings. In this specification, various numerical ranges mean ranges that include the upper and lower limits unless otherwise specified.

[0016] In order to achieve the above object, the present inventors have conducted extensive research, focusing on reducing the amount of P that segregates at prior-γ grain boundaries during PWHT. As a result, the present inventors have found that, when performing multi-layer welding in which multiple layers of weld metal are stacked, applying heat to the weld metal of each layer in a subsequent pass causes a ferrite structure to form on the grain boundaries of the weld metal, and further increasing the number of layers of weld metal so that the ferrite structure region is at least a certain amount, thereby ensuring excellent toughness after heat treatment under specific conditions. Specifically, the inventors have discovered that when multi-layer welding is performed, a weld metal having a specific chemical composition is used, at least one welding pass is performed by weaving so that the average layer height of each layer in the multiple layers of weld metal is greater than 0 mm and less than or equal to 4.0 mm, and welding is performed under conditions of a specific welding heat input, thereby ensuring a certain amount or more of ferrite structure regions formed on the grain boundaries of the weld metal by the heat of the above-mentioned subsequent passes, and further, by performing heat treatment under specific conditions of a temperature of 580°C to 620°C for 2 hours or more after welding, a weld joint with excellent toughness can be obtained.

[0017] The weaving method is a welding method in which a welding line is formed by moving a welding torch left and right in the welding direction, as shown in FIG.

[0018] The present invention was completed based on these findings, and includes the following aspects of each embodiment of the manufacturing method and welded joint.

[0019] First, a preferred embodiment of the method for manufacturing a welded joint of the present invention will be described in detail.

[0020] <Method for manufacturing a welded joint> A method for manufacturing a welded joint according to one embodiment of the present invention is a method for manufacturing a welded joint including a steel material and multiple layers of weld metal having a specific chemical composition. The manufacturing method of this embodiment includes a multi-layer welding process in which each layer of the multiple layers of weld metal is formed by weaving so that the average layer height of each layer, excluding a first layer, in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material is greater than 0 mm and less than 4.0 mm, and a heat treatment process in which, after the multi-layer welding process, heat treatment is performed at a temperature of 580°C to 620°C for two hours or more, wherein the welding heat input calculated by the following formula (1) in the multi-layer welding process is 13.0 kJ / cm to 25.0 kJ / cm.

[0021] In this specification, "a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material" means a range including the thickness center between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material in the thickness direction of the steel material. In other words, it means a range centered at the thickness center and having a length in the thickness direction of the steel material that is thickness t-10 mm.

[0022] 1 is a diagram schematically showing a cross section of a welded joint 1 obtained by the manufacturing method of this embodiment, centered on a welded portion 2. As shown in Fig. 1, the welded joint 1 obtained by the manufacturing method of this embodiment is composed of a welded portion 2 including multiple layers of weld metal 31 and a steel material 4, which is a material to be welded. In this embodiment, a steel plate is used as the steel material 4, although this is not particularly limited.

[0023] 1, the up-down direction in Fig. 1 corresponds to the plate thickness direction of the welded joint 1, and the left-right direction in Fig. 1 corresponds to the direction perpendicular to the direction in which the welded portion 2 extends, i.e., corresponds to the width direction of the welded portion 2. Furthermore, the direction perpendicular to the cross section of the welded joint 1 shown in Fig. 1, i.e., the direction perpendicular to both the up-down direction and the left-right direction in Fig. 1, corresponds to the direction in which the welded portion 2 extends.

[0024] 1, the welded portion 2 of the welded joint 1 is formed by multi-layer welding such as gas-shielded arc welding, so that multiple layers of weld metal 31 are stacked one on top of the other. Furthermore, the welded portion 2 is formed so that the average layer height h of each layer, excluding the first layer of the welded metal 31, in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material is 4.0 mm or less.

[0025] Furthermore, the fusion lines (thick lines in FIG. 1 ) located at the boundaries between the layers of the multiple layers of weld metal 31 are reheated to the Ac1 transformation point or higher by subsequent passes during multi-layer welding, forming reheated regions 32. These reheated regions 32 are regions where the structure of the weld metal 31 is transformed from above the grain boundaries to austenite (γ) due to the reheating of the weld metal 31, and then transformed again to ferrite during the subsequent cooling process.

[0026] The welded joint 1 is formed so that the average layer height h of each of the above-mentioned layers of the weld metal 31 is 4.0 mm or less, and as a result, the above-mentioned reheated region 32 is densely formed, and a certain amount or more of ferrite structure region is formed at the grain boundary where P originally segregated. Therefore, the welded joint 1 has few regions where P segregation can occur during heat treatment after welding, and P segregation at the grain boundary is less likely to occur, and the welded joint 1 can exhibit excellent toughness even after heat treatment described below.

[0027] In this specification, having excellent toughness means that the absorbed energy in a Charpy impact test at -40°C is 30 J or more.

[0028] Such a welded joint 1 can be obtained by the manufacturing method of this embodiment, as described above. Specifically, in the manufacturing method of this embodiment, in the multi-layer welding process, weld metal having a specific chemical composition is used, and each layer of the weld metal is formed by weaving so that the average layer height h of each layer, excluding the first layer, in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material is 4.0 mm or less. Furthermore, by welding with a specific welding heat input, it is possible to ensure a certain or higher proportion of ferrite structure regions (reheated portion regions 32) formed on the grain boundaries of the weld metal by the heat of subsequent passes. This reduces the areas where P segregation can occur during heat treatment after welding, making it difficult for P to segregate on grain boundaries. Furthermore, in the manufacturing method of this embodiment, by performing heat treatment under specific conditions of a temperature of 580°C to 620°C for 2 hours or more after welding, stress can be removed while suppressing P segregation on grain boundaries in the welded portion 2. As a result, a welded joint 1 with excellent toughness can be obtained.

[0029] Depending on the welding conditions, the reheated region 32 may have structures such as martensite or retained austenite. In any case, however, by forming a certain amount or more of another structure at the prior γ grain boundaries, the region where P segregation can occur during heat treatment is reduced, and as a result, P segregation on the grain boundaries is less likely to occur.

[0030] Each step in the method for manufacturing a welded joint according to this embodiment will now be described in more detail.

[0031] [Multi-layer welding process] In the manufacturing method of the welded joint 1 of the present embodiment, the multi-layer welding process is a process of performing multi-layer welding such as gas-shielded arc welding, and is a process of performing welding passes by weaving to form at least each of the layers, excluding the first layer in the multiple layers of weld metal 31, in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, so that the average layer height of each layer is greater than 0 mm and not greater than 4.0 mm.

[0032] In this embodiment, the multi-layer welding process is such that the average layer height of each layer in the above range in the plurality of layers of weld metal is greater than 0 mm and less than 4.0 mm. Other specific procedures and conditions are not particularly limited as long as they are formed by weaving.

[0033] For example, the multi-layer welding process may be performed as follows. That is, as an example of the multi-layer welding process, first, a step (a) is performed in which a plurality of welding passes are welded by weaving using a weld metal having the above-mentioned specific chemical composition at a location on the steel material 4 where the weld 2 is to be formed, thereby forming n layers of weld metal 31, each with an average layer height of more than 0 mm and not more than 4.0 mm. Here, n is a natural number. Further, a step (b) is performed in which a subsequent pass is similarly welded on the nth layer of weld metal 31 to form an (n+1)th layer of weld metal 31, and a reheated portion region 32 is formed on the upper surface of the nth layer of weld metal 31 and in its vicinity. By performing these steps (a) and (b), the above-mentioned weld 2 is formed at a location on the steel material 4 where the weld 2 is to be formed, in which layers of weld metal 31 are stacked in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding the first layer, and each layer has an average layer height of more than 0 mm and not more than 4.0 mm.

[0034] In the multi-pass welding process, another weld metal 31 of a subsequent pass is stacked on top of the weld metal 31, and a reheated area 32 is formed at the boundary between the stacked layers of weld metal 31, where the heat from the subsequent pass reheats the area to above the Ac1 transformation point.

[0035] In this embodiment, in the multi-pass welding process, the layer height (layer thickness) of the weld metal 31 in each pass is set to 4.0 mm or less in the above range, thereby relatively increasing the proportion of the reheated portion region 32. Specific means for setting the layer height (layer thickness) of the weld metal 31 in each pass in the above range to 4.0 mm or less are not particularly limited, but for example, when welding multiple welding passes by weaving, the layer height of the weld metal 31 in each pass can be easily controlled to a predetermined range by appropriately adjusting the conditions for swinging the welding torch in a direction perpendicular to the weld line.

[0036] (Weaving speed and weaving width) In this multi-layer welding process, weaving is preferably performed at a weaving speed of 20 mm / s or more and a weaving width of 15 mm or more. When weaving is performed under these conditions, the thickness of the reheated portion increases and the proportion of the reheated portion also increases, thereby further improving the toughness of the resulting welded joint. Note that the heat input is calculated based on the movement distance in the welding direction without taking into account the weaving swing width. Therefore, even if the heat input is high, a wide bead can be formed, i.e., the layer thickness can be thinned.

[0037] The weaving speed may be 22 mm / sec or more, 24 mm / sec or more, or 25 mm / sec or more. The upper limit of the weaving speed is not particularly limited, but is, for example, 40 mm / sec for reasons of the performance of the welding equipment.

[0038] The weaving width may be 16 mm or more, 18 mm or more, or 20 mm or more. The upper limit of the weaving width is not particularly limited, but is set to, for example, 25 mm for reasons of the performance of welding equipment.

[0039] Here, the weaving speed refers to the moving speed of the welding torch in a direction perpendicular to the welding direction when the welding torch is moved in the welding direction while being swung, i.e., the swing speed of the welding torch. The moving speed of the welding torch in the welding direction is the welding speed.

[0040] The weaving width refers to the maximum movement distance of the welding torch in a direction perpendicular to the welding direction when the welding torch is moved in the welding direction while swinging it, i.e., the maximum swing width of the welding torch. By setting the above weaving conditions in automatic welding, welding can be performed with good reproducibility.

[0041] Note that, as a means for suppressing the average layer height h of the weld metal 31 in each pass, for example, a means for reducing the welding heat input can be considered. However, with the former means, the reduction in the welding heat input reduces the area to which the heat of the subsequent pass is applied, and there is a risk that the reheated portion region 32 may not be sufficiently formed. On the other hand, with the means using weaving as in this embodiment, as described above, the layer height of the weld metal 31 in each pass can be easily controlled within a predetermined range simply by appropriately adjusting the conditions for oscillating the welding torch in a direction perpendicular to the weld line.

[0042] In this specification, the average layer height of multiple layers of weld metal can be determined from a cross section (similar to the cross section shown in FIG. 1 ) obtained by cutting the weld joint along the width direction of the weld, which is perpendicular to the direction in which the weld extends. Specifically, the cut surface of the weld joint is observed using a scanning electron microscope, and the thicknesses of the thickest portions of 10 consecutive layers of weld metal that have been weaved in the weld are determined, that is, the average length in the thickness direction of the portions with the longest length in the thickness direction. This average value is the average layer height of the multiple layers of weld metal. A specific method for measuring the average layer height of multiple layers of weld metal will be described later.

[0043] In this embodiment, the average layer height of the weld metal of the multiple layers is preferably 3.9 mm or less, and may be 3.8 mm or less, 3.7 mm or less, or 3.6 mm or less. On the other hand, if the average layer height of the weld metal of the multiple layers is too small, the number of welding passes increases, which requires time and cost, so the average layer height of the weld metal of the multiple layers is preferably 0.5 mm or more, and may be 1.0 mm or more, 1.5 mm or more, or 2.0 mm or more.

[0044] In this embodiment, the welding method that can be used in the multi-layer welding process is not particularly limited as long as the effects of the present invention can be obtained, and examples thereof include gas-shielded arc welding using a welding wire that becomes the weld metal and a shielding gas. Note that the shielding gas used in this gas-shielded arc welding is not particularly limited, and examples thereof include 100% by volume Ar gas (pure Ar gas), 100% by volume carbon dioxide gas (pure carbon dioxide gas), Ar and 3 to 30% by volume CO ２A mixed gas of a gas containing a base metal and a mixture of a base metal and a welding current, a welding voltage, a welding current, a welding speed, and the like can be used.

[0045] However, in the multi-layer welding process, welding must be performed under conditions where the welding heat input calculated by the following formula (1) is 13.0 kJ / cm to 25.0 kJ / cm.

[0046] By performing the multi-layer welding process with such a specific welding heat input, the above-mentioned reheated portion region 32 can be secured more reliably, and as a result, a welded joint with excellent toughness can be obtained more reliably.

[0047] To more reliably obtain such effects, the welding heat input in the multi-layer welding process is preferably 13.2 kJ / cm or more. The welding heat input may be 13.3 kJ / cm or more, 13.4 kJ / cm or more, or 13.5 kJ / cm or more. Furthermore, to facilitate control of the average layer height of the multiple layers of weld metal, the welding heat input in the multi-layer welding process is preferably 24.5 kJ / cm or less. The welding heat input may be 24.0 kJ / cm or less, 23.5 kJ / cm or less, or 23.0 kJ / cm or less.

[0048] The individual welding conditions for the welding current (A), welding voltage (V), and welding speed (cm / min) in the multi-layer welding process are not particularly limited as described above, but are conditions that ensure that the welding heat input falls within the above-mentioned specific range. Examples of such welding conditions include a current of 150 to 500 A, a voltage of 10 to 60 V, and a welding speed of 10 to 60 cm / min.

[0049] [Heat Treatment Step] In the method for manufacturing the welded joint 1 of this embodiment, the heat treatment step is a step in which the welded joint is subjected to heat treatment for the purpose of stress relief, and is a step in which the heat treatment is performed under specific conditions of a temperature of 580°C to 620°C for 2 hours or more.

[0050] In this embodiment, the above-described multi-layer welding process ensures that a certain amount or more of the ferrite structure region (reheated region 32) formed on the grain boundaries of the weld metal in the weld 2 is secured, making it difficult for P to segregate on the grain boundaries during heat treatment after welding. In this embodiment, by subjecting a weld joint in this state to heat treatment under specific conditions of a temperature of 580°C to 620°C for two hours or more, stress can be removed while suppressing P segregation on the grain boundaries. As a result, it is possible to obtain a weld joint 1 with the above-described excellent toughness.

[0051] In order to more reliably obtain such effects, the temperature in the heat treatment step is preferably 585°C or higher. The temperature in the heat treatment step may be 590°C or higher or 595°C or higher. The temperature in the heat treatment step may be 615°C or lower or 610°C or lower. Furthermore, the time period in the heat treatment step may be 3 hours or longer or 4 hours or longer, but is preferably 6 hours or shorter from the viewpoint of productivity.

[0052] In the heat treatment step, the heating rate when heating to the above temperature and the cooling rate when cooling from the above temperature are not particularly limited, and any heating rate and cooling rate can be adopted depending on the desired productivity, etc. Examples of such heating rate and cooling rate include a heating rate of 20 to 60°C / h and a cooling rate of 30 to 70°C / h.

[0053] In the manufacturing method of this embodiment, any process performed in a conventional manufacturing method of a welded joint may be performed before or after each of the multi-layer welding process and the heat treatment process described above, as long as the effect of the present invention is not impaired. Examples of such any process include a pretreatment process or cooling process for the steel material, and various surface treatment processes.

[0054] Next, a steel material that can be used in the manufacturing method of this embodiment will be described.

[0055] [Steel Material] In the manufacturing method of this embodiment, the steel material 4 to be welded is not particularly limited, and any steel material can be used depending on the type of component to which the weld joint 1 is applied, etc. Examples of such steel materials include steel plates and steel rods having a predetermined tensile strength.

[0056] When a steel plate is used as the steel material, the tensile strength (TS) of the steel plate is not particularly limited, but examples thereof include tensile strengths of 780 MPa or more, 980 MPa or more, 1200 MPa or more, 1500 MPa or more, or 1800 MPa or more. The upper limit of the tensile strength of the steel plate is not particularly limited, but is, for example, 3000 MPa from the viewpoint of workability, etc. The tensile strength (TS) of the steel plate can be measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.

[0057] Furthermore, when a steel plate is used as the steel material, the thickness of the steel plate is not particularly limited, and a thickness can be adopted according to the type of component to which the welded joint is applied. Examples of such thicknesses include thicknesses of 15 mm or more, 20 mm or more, 25 mm or more, or 30 mm or more. Furthermore, the thickness of the steel plate may be, for example, 150 mm or less, 100 mm or less, or 80 mm or less.

[0058] In the manufacturing method of this embodiment, the chemical composition of the steel material that can be used as the material to be welded is not particularly limited, but it is preferable that the steel material have the following chemical composition. That is, the steel material contains, in mass%, C: 0.07 to 0.11%, Si: 0.10 to 0.15%, Mn: 0.70 to 1.20%, Ni: 1.00 to 2.50%, Mo: 0.20 to 0.80%, V: 0.005 to 0.070%, Al: 0.010 to 0.100%, B: 0.0005 to 0.0030%, N: 0.0015 to 0.0050%, P: 0.006% or less, S: 0.0030% or less, O: 0.0040% or less, Cu: 0 to 1.00%, Cr: 0 to 0.80%, Nb: 0 to 0.030%, Ti: 0 to 0.010%, A steel having a chemical composition of Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, and the balance: Fe and impurities is preferred. When the steel has such a specific chemical composition, it is possible to more reliably obtain a welded joint with excellent toughness.

[0059] Each component constituting such a steel material will be described in detail below. In the following description, "%" means "mass %" unless otherwise specified, and the content of each component means the mass ratio of each component to the total mass of the steel material.

[0060] (C: 0.07 to 0.11%) C is an element necessary for ensuring the strength of steel. To fully obtain this effect, the C content is preferably 0.07% or more. The C content may be 0.08% or more. On the other hand, excessive C content may lead to coarsening of grain boundary carbides depending on the conditions of heat treatment after welding. Therefore, the C content is preferably 0.11% or less. The C content may be 0.10% or less.

[0061] (Si: 0.10 to 0.15%) Si is a deoxidizing element for steel and is an element effective in increasing the strength of steel without impairing its ductility. To fully obtain these effects, the Si content is preferably 0.10% or more. The Si content may be 0.11% or more. On the other hand, if Si is contained in an excessive amount, island martensite may be more likely to form near the weld. Therefore, the Si content is preferably 0.15% or less. The Si content may be 0.14% or less.

[0062] (Mn: 0.70 to 1.20%) Mn is an element necessary for ensuring the strength of steel materials. To fully obtain this effect, the Mn content is preferably 0.70% or more. The Mn content may be 0.75% or more or 0.80% or more. On the other hand, excessive Mn content may easily cause microsegregation and macrosegregation, which may reduce the workability of the steel material. Therefore, the Mn content is preferably 1.20% or less. The Mn content may be 1.10% or less or 1.00% or less.

[0063] (Ni: 1.00 to 2.50%) Ni is an element that suppresses phase transformation at high temperatures, contributes to improving the strength of the steel material, and also improves toughness. The Ni content may be 0%, but in order to fully obtain such effects, the Ni content is preferably 1.00% or more. The Ni content may be 1.20% or more or 1.40% or more. On the other hand, if Ni is contained excessively, the weldability of the steel material may be reduced. Therefore, the Ni content is preferably 2.50% or less. The Ni content may be 2.40% or less, 2.20% or less, or 2.00% or less.

[0064] (Mo: 0.20 to 0.80%) Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel. The Mo content may be 0%, but in order to fully obtain this effect, the Mo content is preferably 0.20% or more. The Mo content may be 0.25% or more or 0.30% or more. On the other hand, excessive Mo content may decrease workability and reduce productivity. Therefore, the Mo content is preferably 0.80% or less. The Mo content may be 0.75% or less or 0.70% or less.

[0065] (V: 0.005 to 0.070%) V is an element that contributes to improving the strength of steel materials due to strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and / or dislocation strengthening by inhibiting recrystallization. The V content may be 0%, but in order to fully obtain these effects, the V content is preferably 0.005% or more. The V content may be 0.010% or more or 0.015% or more. On the other hand, excessive V content may decrease the formability of the steel material. Therefore, the V content is preferably 0.070% or less. The V content may be 0.065% or less or 0.060% or less.

[0066] (Al: 0.010 to 0.100%) Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. To fully obtain this effect, the Al content is preferably 0.010% or more. The Al content may be 0.020% or more or 0.030% or more. On the other hand, excessive Al may combine with N to form nitrides, which may reduce the fatigue strength of the welded joint. Therefore, the Al content is preferably 0.100% or less. The Al content may be 0.090% or less or 0.080% or less.

[0067] (B: 0.0005 to 0.0030%) B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel material. The B content may be 0%, but in order to fully obtain this effect, the B content is preferably 0.0005% or more. The B content may be 0.0008% or more or 0.0010% or more. On the other hand, if B is contained excessively, B precipitates may be formed, which may reduce the strength of the steel material. Therefore, the B content is preferably 0.0030% or less. The B content may be 0.0025% or less or 0.0020% or less.

[0068] (N: 0.0015 to 0.0050%) N is an element that is inevitably mixed in during steel refining. The N content may be 0%, but reducing the N content to less than 0.0015% increases production costs, so the lower limit of the N content is preferably 0.0015%. The N content may be 0.0020% or more, or 0.0025% or more. On the other hand, N may form nitrides and deteriorate workability, so the N content is preferably 0.0050% or less. The N content may be 0.0045% or less, or 0.0040% or less.

[0069] (P: 0.006% or less) P is an element that is inevitably mixed in during steel refining. The P content may be 0%, but reducing the P content to less than 0.0001% increases the dephosphorization cost, so the lower limit of the P content is preferably 0.0001%. The P content may be 0.0005% or more or 0.001% or more. On the other hand, if the P content exceeds 0.006%, solidification segregation during casting may be significant, causing internal cracking and reduced workability, as well as embrittlement of welds. Therefore, the P content is preferably 0.006% or less. The P content may be 0.005% or less or 0.003% or less.

[0070] (S: 0.0030% or less) S is an element that is inevitably mixed in during steel refining. The S content may be 0%, but reducing the S content to less than 0.0001% increases production costs, so the lower limit of the S content is preferably 0.0001%. The S content may be 0.0005% or more or 0.0010% or more. On the other hand, S may combine with Mn and Ti to form precipitates, which may reduce workability. Therefore, the S content is preferably 0.0030% or less. The S content may be 0.0025% or less or 0.0020% or less.

[0071] (O: 0.0040% or less) O ​​is an element that is inevitably mixed in during steel refining. The O content may be 0%, but reducing the O content to less than 0.0001% increases production costs, so the lower limit of the O content is preferably 0.0001%. The O content may be 0.0005% or more, or 0.0010% or more. On the other hand, O may form coarse oxides and deteriorate workability. Therefore, the O content is preferably 0.0040% or less. The O content may be 0.0035% or less, or 0.0030% or less.

[0072] The preferred basic chemical composition of the steel material that can be used in the manufacturing method of this embodiment is as described above. Furthermore, in this embodiment, the steel material may contain one or more of the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0073] (Cu: 0 to 1.00%) Cu is an element effective in improving hardenability and ensuring the desired high strength. The Cu content may be 0%, but to fully obtain this effect, the Cu content may be 0.001% or more. The Cu content may be 0.005% or more, or 0.01% or more. On the other hand, even if Cu is contained excessively, the above effect saturates. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.95% or less, 0.90% or less, 0.85% or less, or 0.80% or less.

[0074] (Cr: 0 to 0.80%) Cr is an element that contributes to improving the strength of steel and the fatigue strength of welded joints. The Cr content may be 0%, but to fully obtain this effect, the Cr content may be 0.10% or more. The Cr content may be 0.15% or more, 0.20% or more, 0.25% or more, or 0.30% or more. On the other hand, if Cr is contained excessively, there is a risk of forming coarse Cr carbides that become the starting points of fracture. Therefore, the Cr content is preferably 0.80% or less. The Cr content may be 0.75% or less, or 0.70% or less.

[0075] (Nb: 0 to 0.030%) Nb is an element that contributes to improving the strength of steel due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but to fully obtain these effects, the Nb content may be 0.001% or more. The Nb content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, excessive Nb content may increase unrecrystallized ferrite, thereby reducing the formability of the steel. Therefore, the Nb content is preferably 0.030% or less. The Nb content may be 0.025% or less, or 0.020% or less.

[0076] (Ti: 0 to 0.010%) Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation points. Ti is also an element that is effective in refining the structure and improving the balance between the strength and formability of the steel material. The Ti content may be 0%, but to fully obtain these effects, the Ti content may be 0.0001% or more. The Ti content may be 0.0005% or more or 0.001% or more. On the other hand, excessive Ti content may form coarse Ti sulfides, Ti nitrides, and / or Ti oxides, which may reduce the formability of the steel material. Therefore, the Ti content is preferably 0.010% or less. The Ti content may be 0.008% or less, 0.006% or less, or 0.005% or less.

[0077] (Ca: 0 to 0.0030%) Ca is an element that functions as a deoxidizer and a desulfurizer, and reduces the amount of dissolved oxygen and S in molten steel. The Ca content may be 0%, but to fully obtain this effect, the Ca content may be 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive Ca content may lead to an increase in oxide-based or sulfide-based inclusions. Therefore, the Ca content is preferably 0.0030% or less. The Ca content may be 0.0025% or less, or 0.0020% or less.

[0078] (Mg: 0 to 0.0030%) (REM: 0 to 0.0030%) Mg and REM are elements that disperse as oxides and contribute to improving the toughness of steel and welds. Furthermore, Mg and REM are elements that contribute to improving toughness via sulfide-based inclusions. The Mg and REM contents may each be 0%, but to fully obtain these effects, the Mg and REM contents may each be 0.0001% or more. The Mg and REM contents may each be 0.0005% or more or 0.0010% or more. On the other hand, even if Mg and REM are contained excessively, the above effects will saturate. Therefore, the Mg and REM contents are preferably 0.0030% or less. The Mg and REM contents may each be 0.0025% or less or 0.0020% or less.

[0079] (Balance: Fe and Impurities) The balance of the steel material other than the above-mentioned components is Fe and impurities.

[0080] Furthermore, it is preferable that the steel material that can be used as the material to be welded has a carbon equivalent Ceq, expressed by the following formula (2), of 0.550 to 0.620: Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (2) In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the mass % content of each element, and are 0 when no element is contained.

[0081] When the steel material has such a specific carbon equivalent Ceq, it is possible to more reliably obtain a welded joint having excellent toughness.

[0082] The chemical composition of the steel material may be measured by a general analytical method. For example, the chemical composition of the steel material may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, when the steel material is a steel plate, a 35 mm square test piece is taken from a depth position approximately half the plate thickness of the steel plate, and the chemical composition of the steel plate can be identified by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0083] Next, a welding wire, i.e., a welding metal, that can be used in the manufacturing method of this embodiment will be described.

[0084] [Weld Metal] The weld metal used in the manufacturing method of this embodiment has the following specific chemical composition. That is, the weld metal contains, in mass %, C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Ti: 0.005 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V: 0 to 0.010%, Al: 0 to 0.010%, W: 0 to 0.70%, Ta: 0 to 0.0050%, Cu: 0 to 1.00%, The weld metal has a chemical composition containing Co: 0-0.50%, Pb: 0-0.100%, Sn: 0-0.100%, B: 0-0.0030%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.0100%, Hf: 0-0.0050%, REM: 0-0.0050%, and the balance being Fe and impurities. When the weld metal has this specific chemical composition, it is possible to more reliably ensure excellent toughness after heat treatment.

[0085] Each component constituting such a weld metal will be described in detail below. In the following description, "%" means "mass %" unless otherwise specified, and the content of each component means the mass ratio of each component to the total mass of the weld metal.

[0086] (C: 0.020 to 0.080%) C is an element that improves the strength of the weld metal. In order to improve the strength of the weld metal, the C content is set to 0.020% or more. In order to further improve the strength of the weld metal, the lower limit of the C content may be 0.040%, 0.050%, or 0.060%. Furthermore, in order to improve the toughness of the weld metal and suppress susceptibility to both hot cracking and cold cracking, the C content is set to 0.080% or less. In order to ensure stable low-temperature toughness, the upper limit of the C content may be 0.075% or 0.070%.

[0087] (Si: 0.10 to 0.70%) Si is a deoxidizing element that reduces the amount of O in the weld metal and increases cleanliness. To achieve this effect, the Si content is set to 0.10% or more. To sufficiently reduce the amount of O in the weld metal, the lower limit of the Si content may be 0.15%, 0.20%, or 0.25%. To further improve the toughness of the weld metal, the Si content is set to 0.70% or less. To stably ensure the toughness of the weld metal, the upper limit of the Si content may be 0.60%, 0.50%, or 0.40%.

[0088] (Cr: 0.10 to 0.60%) Cr improves the hardenability of the weld metal and is therefore an effective element for increasing the strength of the weld metal. In order to increase the strength of the weld metal, the Cr content is set to 0.10% or more. In order to further increase the strength of the weld metal, the lower limit of the Cr content may be 0.15%, 0.20%, 0.25%, or 0.30%. In addition, in order to uniformly harden the bainite structure of the weld metal and further improve the toughness of the weld metal, the Cr content is set to 0.60% or less. In order to further improve the toughness of the weld metal, the upper limit of the Cr content may be 0.58%, 0.57%, 0.56%, or 0.55%.

[0089] (Mo: 0.10 to 0.70%) Mo is an element that improves the hardenability of the weld metal and forms fine carbides, which is effective in ensuring tensile strength through precipitation strengthening. Mo also has the effect of suppressing a decrease in strength when reheated by a subsequent pass during multi-pass welding, making it easier to ensure the toughness of the weld metal. To fully achieve this effect, the Mo content is set to 0.10% or more. To more reliably ensure the strength and toughness of the weld metal, the lower limit of the Mo content may be 0.15%, 0.20%, or 0.25%. To ensure stable toughness, the Mo content is set to 0.70% or less. To ensure more stable toughness, the upper limit of the Mo content may be 0.60%, 0.55%, or 0.50%.

[0090] (Ti: 0.005 to 0.050%) Ti is an effective deoxidizing element and has the effect of reducing the amount of O in the weld metal. It is also an effective element for fixing solute N and mitigating the adverse effect of N on toughness. In order to fully exert these effects, the Ti content is set to 0.005% or more. In order to more fully exert the above-mentioned effects, the lower limit of the Ti content may be 0.008%, 0.010%, 0.011%, or 0.012%. In addition, in order to further improve the toughness of the weld metal, the Ti content is set to 0.050% or less. In order to more reliably improve the toughness of the weld metal, the upper limit of the Ti content may be 0.045%, 0.040%, 0.035%, or 0.030%.

[0091] (P: 0.020% or less) P is an impurity element that reduces both the toughness and ductility of the weld metal, so it is preferable to reduce the P content as much as possible. The P content is set to 0.020% or less. To more reliably prevent a decrease in the toughness and ductility of the weld metal, the upper limit of the P content may be 0.018%, 0.016%, or 0.015%. The lower limit of the P content may be 0%, 0.0001%, 0.0005%, or 0.0010%.

[0092] (S: 0.0200% or less) S is an impurity element that reduces both the toughness and ductility of the weld metal, so it is preferable to reduce the S content as much as possible. The S content is set to 0.0200% or less. To more reliably prevent a decrease in the toughness and ductility of the weld metal, the upper limit of the S content may be 0.0180%, 0.0160%, 0.0140%, or 0.0120%. The lower limit of the S content may be 0%, 0.0001%, 0.0005%, or 0.0010%.

[0093] (N: 0.0200% or less) N is a component that can excessively increase the strength of the weld metal and cause a decrease in low-temperature toughness. Furthermore, N is a component that is inevitably contained in the weld metal. In order to prevent an excessive increase in the strength of the weld metal and to ensure low-temperature toughness, the N content is set to 0.0200% or less. To more reliably obtain these effects, the upper limit of the N content may be 0.0180%, 0.0170%, or 0.0160%. Note that the lower the N content, the more preferable it is. The lower limit of the N content may be 0%, but since it is industrially difficult to achieve 0%, it is practically 0.0010% or 0.0020%.

[0094] (O: 0.0400% or less) O ​​is a component that forms oxides that serve as starting points for the generation of a fine structure during welding, thereby facilitating the improvement of the toughness of the weld metal. Furthermore, O is a component that is inevitably contained in the weld metal. From the viewpoint of preventing the coarsening of oxides and further improving the toughness of the weld metal, the O content is set to 0.0100% or less. To more reliably obtain these effects, the upper limit of the O content may be 0.0080% or 0.0060%. Furthermore, the O content may be 0%, but from the viewpoint of improving the toughness of the weld metal, it is preferably 0.0050% or more. To more reliably improve the toughness of the weld metal, the lower limit of the O content may be 0.0060%, 0.0070%, 0.0080%, or 0.0090%.

[0095] (Mn: 0 to 1.50%) Mn is an element that ensures the hardenability of the weld metal and increases its strength. The lower limit of the Mn content is 0%. To more reliably obtain the effect of increasing the strength of the weld metal, the Mn content is preferably 0.01% or more. To more reliably increase the strength of the weld metal, the lower limit of the Mn content may be 0.05%, 0.10%, or 0.50%. Furthermore, to suppress the susceptibility to intergranular embrittlement and further improve the toughness of the weld metal, the Mn content is set to 1.50% or less. To further improve the toughness of the weld metal, the upper limit of the Mn content may be 1.48%, 1.46%, or 1.44%.

[0096] (Ni: 0 to 5.00%) Ni is an element that can further improve the toughness of the weld metal regardless of the structure and components due to the effect of increasing toughness through solid solution. Ni is particularly effective in further increasing the toughness of high-strength weld metals having a tensile strength of 780 MPa or more. The lower limit of the Ni content is 0%. To more reliably obtain the above-mentioned effect, the Ni content is preferably 0.001% or more. To further improve the toughness of the weld metal, the lower limit of the Ni content may be 0.10%, 0.50%, or 1.00%. Furthermore, to ensure weld crack resistance, the Ni content is set to 5.00% or less. To more reliably ensure weld crack resistance, the upper limit of the Ni content may be 4.00%, 3.50%, or 3.00%.

[0097] (Nb: 0 to 0.010%) Nb forms fine carbides in the weld metal, and is therefore an element effective in ensuring the tensile strength of the weld metal through precipitation strengthening. The lower limit of the Nb content is 0%. In order to improve the tensile strength of the weld metal, the Nb content may be 0.0001% or more. The lower limit of the Nb content may be 0.0005% or 0.0010%. In addition, in order to further improve the toughness of the weld metal, the Nb content is set to 0.010% or less. In order to more reliably improve the toughness of the weld metal, the upper limit of the Nb content may be 0.008%, 0.007%, 0.006%, or 0.005%.

[0098] (V: 0 to 0.010%) V increases the hardenability of the weld metal and is therefore an element effective in increasing the strength of the weld metal. The lower limit of the V content is 0%. To further increase the strength of the weld metal, the V content may be 0.0001% or more. The lower limit of the V content may be 0.0005% or 0.0010%. Furthermore, to suppress hardening of the weld metal and further improve the toughness of the weld metal, the V content is set to 0.010% or less. To ensure these effects, the upper limit of the V content may be 0.008%, 0.007%, 0.006%, or 0.005%.

[0099] (Al: 0 to 0.010%) Al is a deoxidizing element and is effective in reducing the amount of O in the weld metal and improving the cleanliness of the weld metal. The lower limit of the Al content is 0%. In order to fully exert these effects, the Al content may be 0.0001% or more. The lower limit of the Al content may be 0.0005% or 0.0010%. Furthermore, in order to suppress the formation of nitrides and oxides and further improve the toughness of the weld metal, the Al content is set to 0.010% or less. In order to more reliably improve the toughness of the weld metal, the upper limit of the Al content may be 0.008%, 0.007%, 0.006%, or 0.005%.

[0100] [W: 0 to 0.70%, Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%] In the present embodiment, the weld metal may optionally contain W, Ta, Cu, Co, Pb, Sn, B, Ca, Mg, Zr, Hf, and REM, each in a predetermined content. For example, W may be contained in an amount of 0 to 0.70%, Ta, Ca, and Mg in an amount of 0 to 0.0050%, Cu in an amount of 0 to 1.00%, Co in an amount of 0 to 0.50%, Pb and Sn in an amount of 0 to 0.100%, B in an amount of 0 to 0.0030%, Zr in an amount of 0 to 0.0100%, and Hf and REM in an amount of 0 to 0.0050%. These elements may be mixed in due to various factors in the raw materials or manufacturing process, or may be intentionally included to achieve effects such as high strength. These elements may be contained in a total amount of 0.100% or less.

[0101] In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0102] In this embodiment, the chemical composition of the weld metal is, in mass %, Mn: 0.01 to 1.50%, Ni: 0.001 to 5.00%, Nb: 0.0001 to 0.010%, V: 0.0001 to 0.010%, Al: 0.0001 to 0.010%, W: 0.01 to 0.70%, Ta: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Co: 0.001 to 0.50%, Pb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0050%, It may contain one or more selected from the group consisting of Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0050%, and REM: 0.0001 to 0.0050%.

[0103] (Balance) The balance of the weld metal excluding the above components is Fe and impurities, such as Sb, As, and Bi.

[0104] The above-mentioned C, Si, Mn, Ni, Cr, Mo, and V do not necessarily need to be pure substances, and may be contained in the weld metal in the form of an alloy such as Cu—Ni.

[0105] The chemical composition of the weld metal may be measured by a common analytical method, similar to the chemical composition of the steel material described above. For example, the chemical composition of the weld metal may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a 20 mm square test piece is taken from a central portion of the weld at a depth of approximately 7 mm from the surface, and the test piece is measured using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-prepared calibration curve, thereby identifying the chemical composition of the weld metal. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0106] <Welded joint> As described above, the welded joint 1 obtained by the manufacturing method of this embodiment is composed of the welded portion 2 including multiple layers of weld metal 31 and the steel material 4 that is the material to be welded. Note that this welded joint 1 is also one embodiment of the present invention.

[0107] As shown in FIG. 1 , the weld 2 of the welded joint 1 is formed by stacking multiple layers of weld metal 31, and further formed so that the average layer height h of each layer in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding the initial layer, in the weld metal 31, is 4.0 mm or less.

[0108] Furthermore, at the boundary between each layer of the multiple layers of weld metal 31, a reheated region 32 is formed that has been reheated to the Ac1 transformation point or higher by a subsequent pass during multi-pass welding. The welded joint 1 is formed so that the average layer height h of each layer in the weld metal 31 within the above range is 4.0 mm or less, thereby forming a dense reheated region 32 and forming a certain amount or more of a ferrite structure region at the grain boundary where P originally segregated. Therefore, the welded joint 1 has few regions where P segregation can occur during heat treatment after welding, making it difficult for P to segregate at the grain boundary. Even after heat treatment, the welded joint has excellent toughness, with an absorbed energy of 30 J or more in a Charpy impact test at −40°C.

[0109] That is, the welded joint 1 according to one embodiment of the present invention is a welded joint including a steel material 4 and a plurality of layers of weld metal 31, in which the weld metal has the specific chemical composition described above, and the welded joint has the following characteristic configuration: the average layer height of each layer in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding the initial layer, is greater than 0 mm and is 4.0 mm or less, the average width of each layer is 15 mm or more, and the absorbed energy in a Charpy impact test at -40°C is 30 J or more.

[0110] Each configuration of the welded joint 1 of this embodiment will be described below.

[0111] [Average layer height of each layer in multiple layers of weld metal: more than 0 mm and 4.0 mm or less] As described above, in the weld joint 1 of the present embodiment, the average layer height h of each layer, excluding the initial layer, in the weld metal 31 of the weld 2 in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material is 4.0 mm or less. By providing such a configuration, the weld joint 1 of the present embodiment has a ferrite structure region formed in a certain amount or more at the grain boundaries where P originally segregated, and there are fewer regions where P segregation can occur during heat treatment after welding, making it difficult for P to segregate at the grain boundaries, and therefore has excellent toughness even after heat treatment under specific conditions.

[0112] Here, the average layer height h of each layer in the above range of the weld metal 31 can be measured by the following measuring method.

[0113] <Method for Measuring the Average Layer Height of Each Layer in Multiple Layers of Weld Metal> (1) The weld of a welded joint is cut along the width direction of the weld, which is perpendicular to the direction in which the weld extends. (2) The cut surface is corroded with a 3% nital etchant. (3) The corroded cut surface is observed using a scanning electron microscope. The observation is centered around the center of the weld, and the multiple layers of weld metal in the weld and the reheated area, which is visually defined as a relatively dark area (i.e., a fusion line), are identified. (4) Near the center of the weld, 10 consecutive layers of weld metal are selected as measurement targets from among the layers that have weaved between a depth of 5 mm from one surface of the steel material and a depth of 5 mm from the other surface of the steel material, excluding the first layer. If there are fewer than 10 layers that have weaved within the above range, all layers of weld metal are selected as measurement targets. The thickness of the thickest portion of each selected layer of weld metal, i.e., the length in the thickness direction of the portion with the longest length in the thickness direction, is measured. The thickness measured in this way is defined as the layer height (mm) of each layer. (5) The average value is calculated from the measured layer heights of the 10 layers, and this is defined as the average layer height (mm) of each layer in the multiple-layer weld metal.

[0114] The means for controlling the average layer height of the weld metal of the plurality of layers to a range of more than 0 mm and not more than 4.0 mm, the preferable range of the average layer height of the weld metal of the plurality of layers, etc. are as described in the above-mentioned multi-layer welding process.

[0115] (Average Width of Each Layer in Multiple-Layer Weld Metal) In the weld joint 1 of this embodiment, the average width of each layer in the multiple-layer weld metal 31 of the weld 2, excluding the first layer, in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, is 15 mm or more. Such a weld joint can be obtained by using a weld metal having the specific chemical composition in the above-mentioned multi-layer welding process, weaving at a weaving speed of 20 mm / sec or more and a weaving width of 15 mm or more, and welding under conditions where the welding heat input calculated by the above formula (1) is 13.0 kJ / cm to 25.0 kJ / cm. The weld joint obtained in this manner has an increased thickness of the reheated portion region and an increased proportion of the reheated portion region, and therefore can exhibit even superior toughness, with an absorbed energy of 30 J or more, or even 35 J or more, in a Charpy impact test at -40 °C.

[0116] The average width of each layer in the multiple layers of weld metal 31 may be 16 mm or more, 18 mm or more, or 20 mm or more. The upper limit of the average width of each layer is not particularly limited, but is, for example, 25 mm.

[0117] Here, in this specification, the average width of each layer in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding the first layer, in a multiple-layer weld metal means the width of the widest part in five consecutive layers of weld metal among the layers that have been weaved in the range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding the first layer, near the center of the weld, that is, the average value of the length of the part that has the greatest length in the direction perpendicular to the plate thickness direction.

[0118] The average width of each layer in the multiple layers of weld metal 31 can be measured by the following measurement method.

[0119] <Method for Measuring the Average Width of Each Layer in Multiple Layers of Weld Metal> (1) The weld of a welded joint is cut along the width direction of the weld, which is perpendicular to the direction in which the weld extends. (2) The cut surface is corroded with a 3% nital etchant. (3) The corroded cut surface is observed using a scanning electron microscope. The observation is centered around the center of the weld, and the multiple layers of weld metal in the weld and the reheated area, which is visually defined as a relatively dark area (i.e., a fusion line), are identified. (4) Near the center of the weld, five consecutive layers of weld metal are selected as measurement targets from among the weaved layers between a depth of 5 mm from one surface of the steel material and a depth of 5 mm from the other surface of the steel material, excluding the first layer. If there are fewer than five weaved layers, all layers of weld metal are selected as measurement targets. The width of the widest portion of each selected layer of weld metal, i.e., the length of the portion with the longest length in the direction perpendicular to the plate thickness direction, is measured. The thickness measured in this manner is the width (mm) of each layer. (5) The average value is calculated from the measured widths of the five layers, and this is set as the average width (mm) of each layer in the multi-layer weld metal.

[0120] [Absorbed energy in Charpy impact test at −40° C.: 30 J or more] As described above, the welded joint 1 of this embodiment has an absorbed energy of 30 J or more in a Charpy impact test at −40° C. That is, the welded joint 1 of this embodiment has excellent toughness, specifically excellent impact resistance at low temperatures, even after heat treatment.

[0121] The absorbed energy in a Charpy impact test at −40° C. is preferably 31 J or more, and may be 33 J or more, 35 J or more, 36 J or more, 37 J or more, 38 J or more, 39 J or more, or 40 J or more. A method for measuring the absorbed energy in a Charpy impact test at −40° C. will be described later.

[0122] As described above, the welded joint 1 having such excellent toughness can be manufactured by the manufacturing method including the specific multi-layer welding process and heat treatment process.

[0123] Furthermore, in this embodiment, the shape of the welded joint 1 is not particularly limited, and the welded joint can be in a shape that corresponds to the desired joint strength, application, etc. Examples of such welded joint shapes include butt joints and T-joints formed by groove welding, T-joints and cross joints formed by fillet welding, and corner joints.

[0124] (Application Examples) The welded joint obtained by the manufacturing method of the present invention has excellent toughness, and therefore can be applied to structural members in a variety of fields, such as various structures in buildings; various structural parts (e.g., tanks) for transportation machinery such as automobiles, aircraft, and ships; and various structural parts for industrial machinery.

[0125] The manufacturing method of the present invention and the welded joint obtained thereby are not limited to the above-mentioned embodiments or the examples described below, and can be appropriately combined, substituted, modified, etc. within the scope that does not deviate from the purpose and intent of the present invention.

[0126] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0127] (Fabrication of Welded Joints) To verify the effects of the present invention, various welded joints were fabricated as follows. First, a 50 mm thick steel plate with an X-shaped groove as shown in FIG. 2 was prepared. The chemical composition of the steel plate is as shown in Table 1 below. This steel plate and welding wires of weld metals No. 1 to 8 shown in Table 2 below were used to perform gas-shielded arc welding on the front side under the various welding conditions shown in Table 3 below. The groove shape on the back side was then machined into the back-chamfered shape shown in FIG. 2. Next, gas-shielded arc welding on the back side was performed under the same welding conditions as for the front side. In this manner, welded joints Nos. 1 and 3 of the present invention and welded joints Nos. 2 and 4 to 8 of comparative examples were fabricated.

[0128] In addition, Comparative Example No. 6 is an example in which a sufficient weld bead could not be formed during multi-layer welding, that is, the average layer height of the weld metal could not be measured, and a welded joint could not be produced.

[0129] The fabricated welded joints Nos. 1 to 5, 7, and 8 were subjected to heat treatment at 600°C for 2 hours as shown in Table 3. In this heat treatment, the joints were heated to 600°C at a temperature increase rate of 40°C / h, and after 2 hours at 600°C, they were cooled at a cooling rate of 50°C / h.

[0130]

[0131]

[0132]

[0133] In Table 3, the average layer height of each layer of the welded joints of No. 7 and No. 8 is 4.2 mm, but specifically, the average layer height of the welded joint of No. 7 is 4.24 mm, and the average layer height of the welded joint of No. 8 is 4.16 mm.

[0134] For the heat-treated welded joints Nos. 1 to 5, 7, and 8 obtained as described above, the average layer height of each layer in the multiple weld metal layers of the weld was measured, and the absorbed energy value in a Charpy impact test at −40°C was measured according to the <−40°C Charpy Impact Test Method> described below. The measurement results are shown in Table 4 below and FIG. 4.

[0135] The underlines next to various numerical values ​​in Tables 2 and 3 indicate values ​​that are outside the range of the present invention or that do not result in the welded joints of the present invention. The weld metal numbers in Table 2 correspond to the welded joint numbers in Table 3 and the following Tables 5 and 7. The two dashed lines in Fig. 4 indicate the absorbed energy values ​​of 30 J and 35 J in a Charpy impact test at -40°C.

[0136] <Method for Charpy impact test at -40°C> (1) Charpy test specimens (V-notch test specimens) conforming to JIS Z3111-2005 (Method for tensile and impact test of weld metal) are taken from the weld metal of a welded joint. At this time, 15 Charpy test specimens in total are taken, three from each of five positions on one welded joint, so as to correspond to the five notch positions I to V shown in Figure 3. The specific positions of the five notch positions I to V shown in Figure 3 are as follows: I: At the widthwise center of the weld, at a depth of 2 to 12 mm from the surface (upper surface) of the steel material as the starting point. II: At a position 1 / 4 of the widthwise length of the weld from the widthwise center of the weld, at a depth of 2 to 12 mm from the surface (upper surface) of the steel material as the starting point. III: At the widthwise center of the weld, at a depth of 1 / 2 of the plate thickness of the steel material (i.e., a depth of 20 to 30 mm from the surface (upper surface) of the steel material as the starting point. IV: At the widthwise center of the weld, at a depth of 2 to 12 mm from the back surface (lower surface) of the steel material as the starting point. V: At a position 1 / 4 of the widthwise length of the weld from the widthwise center of the weld, at a depth of 2 to 12 mm from the back surface (lower surface) of the steel material as the starting point. (2) Using the Charpy test specimens thus obtained, a Charpy impact test is carried out at -40°C to measure the absorbed energy. The absorbed energy is measured for 15 Charpy test specimens per welded joint as described above. (3) Welded joints having an absorbed energy of 30 J or more for all 15 Charpy test specimens are evaluated as having excellent toughness.

[0137]

[0138] As shown in Tables 3, 4, and Fig. 4, the welded joints of the present invention examples Nos. 1 and 3, in which weaving was performed in the multi-layer welding process so that the average layer height of each layer in the weld metal of the multiple layers was greater than 0 mm and less than 4.0 mm, all had an absorbed energy of 30 J or more in a Charpy impact test at -40 ° C., and were found to have excellent toughness.

[0139] On the other hand, in the multi-layer welding process, the average layer height of each layer in the multiple layers of weld metal was greater than 0 mm and less than 4.0 mm, but weaving was not performed on the weld joints of Comparative Examples Nos. 2, 4, and 5. None of them achieved an absorbed energy of 30 J or more in a Charpy impact test at -40 ° C., indicating poor toughness. Furthermore, in the multi-layer welding process, the average layer height of each layer in the multiple layers of weld metal was greater than 4.0 mm. All of the weld joints of Comparative Examples Nos. 7 and 8 demonstrated absorbed energy significantly below 30 J in a Charpy impact test at -40 ° C, indicating significantly poor toughness.

[0140] Furthermore, in the same manner as in the case of the weld joint of the invention example No. 1, gas-shielded arc welding was performed using welding wires of weld metals Nos. 9 to 14 shown in Table 2 under various welding conditions of a weaving speed of 20 mm / sec or more and a weaving width of 15 mm or more shown in Table 5 below, to produce weld joints of the invention examples Nos. 9 to 14.

[0141] The fabricated welded joints Nos. 9 to 14 were subjected to heat treatment for 2 hours at heat treatment temperatures of 580°C to 620°C as shown in Table 5. In this heat treatment, the joints were heated to the various heat treatment temperatures at a temperature increase rate of 40°C / h, and after 2 hours at the various heat treatment temperatures, they were cooled at a cooling rate of 50°C / h.

[0142] For the welded joints No. 1 and Nos. 9 to 14 obtained as described above, the average width of each layer in the multiple layers of weld metal in the weld was measured, and it was confirmed that the average width was the same as the weaving width under the welding conditions.

[0143] Furthermore, according to the above-mentioned <Method for Charpy Impact Test at -40°C>, the absorbed energy values ​​of the Charpy impact test at -40°C were measured for the welded joints Nos. 9 to 14. The results of these measurements, along with the measurement result for the welded joint No. 1, are shown in Table 6 and Figure 4 below.

[0144]

[0145]

[0146] As shown in Tables 5, 6 and Fig. 4, the welded joints of Examples Nos. 1 and 9 to 13 of the present invention, which were subjected to gas-shielded arc welding weaving under various welding conditions with a weaving speed of 20 mm / sec or more and a weaving width of 15 mm or more in the multi-pass welding process, all had an absorbed energy of 35 J or more in a Charpy impact test at -40°C compared to the welded joint No. 14, which had a weaving width of 5 mm, and were found to have even better toughness.

[0147] Furthermore, in order to verify the influence of the chemical composition of the weld metal, gas-shielded arc welding was carried out under the various welding conditions shown in Table 7 below using welding wires of weld metal Nos. 15 to 28 shown in Table 2. In this manner, welded joints Nos. 15 to 22 and 24 to 26, which are invention examples, and welded joints Nos. 23, 27, and 28, which are comparative examples, were produced.

[0148] According to the above-mentioned <-40°C Charpy impact test method>, the absorbed energy values ​​of the Charpy impact test at -40°C were measured for the welded joints Nos. 15 to 28. The measurement results are shown in Table 8 below and FIG.

[0149]

[0150]

[0151] As shown in Tables 2, 7, 8 and Fig. 4, the welded joints of Examples Nos. 15 to 22 and 24 to 26 of the present invention, which used weld metals having the above-mentioned specific chemical compositions, all had absorbed energies of 30 J or more in Charpy impact tests at -40°C, demonstrating excellent toughness. On the other hand, the welded joints of Comparative Examples Nos. 23, 27 and 28, which used weld metals not having the above-mentioned specific chemical compositions, all failed to achieve absorbed energies of 30 J or more in Charpy impact tests at -40°C, demonstrating poor toughness.

[0152] 1 welded joint 2 welded portion 31 weld metal 32 reheated portion region 4 steel material

Claims

1. A method for manufacturing a welded joint comprising a steel material and a plurality of layers of weld metal, wherein the chemical composition of the weld metal is, in mass%, C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Ti: 0.005 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V: 0 to 0.010%, Al: 0 to 0.010%, W: 0 to 0.70%, Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%, with the balance consisting of Fe and impurities; and a multi-layer welding process in which each layer in the multiple layers of weld metal is formed by weaving so that the average layer height of each layer in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material, excluding a first layer, is greater than 0 mm and less than 4.0 mm. a heat treatment step of performing heat treatment at a temperature of 580°C to 620°C for 2 hours or more after the multi-layer welding step, wherein the multi-layer welding step has a welding heat input calculated by the following formula (1) of 13.0 kJ / cm to 25.0 kJ / cm.

2. The manufacturing method according to claim 1, wherein the multi-layer welding process is performed at a weaving speed of 20 mm / sec or more and a weaving width of 15 mm or more.

3. A welded joint comprising a steel material and a plurality of layers of weld metal, wherein the chemical composition of the weld metal is, in mass%, C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Ti: 0.005 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V: 0 to 0.010%, Al: 0 to 0.010%, W: 0 to 0.70%, Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%, with the balance consisting of Fe and impurities, wherein in the multiple-layer weld metal, excluding an initial layer, each layer in a range between a depth position of 5 mm from one surface of the steel material and a depth position of 5 mm from the other surface of the steel material has an average layer height of more than 0 mm and 4.0 mm or less, and each layer has an average width of 15 mm or more, A welded joint, characterized in that the welded joint has an absorbed energy of 30 J or more in a Charpy impact test at -40 ° C.

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