Method of manufacturing welded joints

Abstract

The present invention provides a method for manufacturing welded joints with improved fatigue properties. [Solution] A method for manufacturing a welded joint made by welding a plurality of steel plates, wherein at least one of the plurality of steel plates is a steel plate containing 0.10 mass% or more of Cu and a total of 0.30 mass% or more of Si and Cu, and the plurality of steel plates are welded together to obtain an as-welded joint with a weld bead formed thereon, and the welding step is to remelt the weld bead toe and the steel plate portion in contact with the weld bead toe in the as-welded joint to form a remelted portion containing 0.09 mass% or more of Cu, the method for manufacturing a welded joint.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a welded joint.

Background Art

[0002] From the viewpoint of increasing the fatigue strength of a welded steel structure, it is conceivable to enhance the fatigue crack growth resistance characteristics of the steel plates constituting the welded steel structure. For example, Patent Document 1 discloses a steel plate having a predetermined chemical composition, whose structure consists of a hard portion and a soft portion, and the average hardness in terms of the ratio occupied in the structures of these two portions and the Vickers hardness satisfies a predetermined formula (1). Further, Patent Document 2 discloses a steel plate having a predetermined chemical composition, wherein the structure at a position of depth t / 4 (t = plate thickness) substantially consists of a mixed structure of ferrite and a hard phase, the ferrite grain size is 30 μm or less, the hard phase fraction is 15 to 80%, the hard phase hardness (HV) is 200 to 800, the hard phase grain size is 200 μm or less, and the H value represented by a predetermined formula is 3.6 or less.

[0003] In manufacturing a welded steel structure, since welding work is generally performed on the steel materials for welded structures, fatigue cracks are likely to occur starting from the welded portion. Therefore, as techniques for improving the fatigue strength of welded joints, techniques such as improving the shape of the end of the welding bead and reducing the welding residual stress have been proposed.

[0004] As a technique for improving the shape of the end of the welding bead, for example, Patent Document 3 discloses a method of grinding the welding end portion of a fillet weld joint, and Patent Document 4 discloses a method of melting and smoothing the welding end portion by TIG arc heat. As a technique for reducing the welding residual stress, for example, Patent Document 5 discloses a method of introducing compressive residual stress by hammer peening or ultrasonic impact treatment along the welding bead to improve the fatigue strength.

Prior Art Documents

Patent Documents

[0005] <00神仙道025>

Patent Document 1

[0006] As described in Patent Documents 1 and 2, suppressing crack propagation in steel plates improves the fatigue strength of the steel plate itself. However, even if the fatigue strength of the steel plate itself improves, the fatigue strength of the welded joint does not improve, as is known, for example, from Watanabe et al., "Fatigue Strength of High-Strength Steel Welded Joints and Its Governing Factors - Effects of Stress Concentration Coefficient and Welding Residual Stress -", ​​Transactions of the Japan Welding Society, 1995, Vol. 13, No. 3, pp. 438-443. For this reason, high-strength steel has not progressed in structures where fatigue characteristics are required. The reason for this is thought to be that stress concentration due to the shape of the weld bead toe and the presence of tensile residual stress generated during welding create conditions in which fatigue cracks are likely to occur within the weld bead, and the fatigue strength of the steel plate does not affect these phenomena.

[0007] While techniques for improving the toe shape, as described in Patent Documents 3 and 4, and techniques for reducing welding residual stress, as described in Patent Document 5, contribute to improving the fatigue life of joints, it is difficult to sufficiently enhance the fatigue characteristics of joints in a simple manner, and further improvements are considered necessary.

[0008] In other words, there are various methods for improving the fatigue strength of welded joints, but these methods do not easily and sufficiently increase the fatigue strength of welded joints, and further improvements are considered necessary. This disclosure has been made in view of these circumstances, and its purpose is to provide a simple method for manufacturing welded joints with improved fatigue properties. [Means for solving the problem]

[0009] One aspect of the present invention is: A method for manufacturing a welded joint formed by welding multiple steel plates, A welding step is to use a steel plate containing 0.10 mass% or more of Cu and a total of 0.30 mass% or more of Si and Cu as at least one of the plurality of steel plates, and to weld the plurality of steel plates to obtain an as-welded joint with a weld bead formed, The remelting step involves remelting the weld bead toe and the steel plate portion in contact with the weld bead toe in the aforementioned as-welded joint to form a remelted portion containing 0.09% by mass or more of Cu. This is a method for manufacturing welded joints, including [the specified element].

[0010] Aspect 2 of the present invention is, The remelted portion contains a total of 0.5% by mass or more of Si and Cu, according to the method for manufacturing a welded joint as described in Embodiment 1.

[0011] A third aspect of the present invention is: The method for manufacturing a welded joint according to embodiment 1 or 2, wherein in the remelting step, the target position of the heating source for forming the remelted portion is set to a position 0.5 mm to 4.0 mm from the weld bead toe of the as-welded joint, perpendicular to the longitudinal direction of the weld bead and away from the weld bead.

[0012] Aspect 4 of the present invention is The method for manufacturing a welded joint according to embodiment 1 or 2, wherein the remelting step is performed such that the steel plate dilution ratio in the remelted area, determined from the following formula (1), is 0.35 or more. Steel plate dilution ratio = (Area where steel plate melted) / (Area where steel plate melted + Area where weld bead melted) ... (1)

[0013] Aspect 5 of the present invention is The method for manufacturing a welded joint according to Embodiment 3, wherein the remelting step is performed such that the steel plate dilution ratio in the remelted area, determined from the following formula (1), is 0.35 or more. Steel plate dilution rate = (area where the steel plate melted) / (area where the steel plate melted + area where the weld bead melted) ··· (1)

Advantages of the Invention

[0014] According to the present disclosure, it is possible to provide a method that can simply manufacture a welded joint with improved fatigue characteristics, which is different from the prior art.

Brief Description of the Drawings

[0015] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an intermediate process of cruciform fillet welding. [Figure 2] FIG. 2 is a schematic cross-sectional view of a welded joint obtained by performing the remelting shown in FIG. 1. [Figure 3] FIG. 3 is a graph showing the relationship between the distance L from the end of the weld bead and the steel plate dilution rate. [Figure 4] FIG. 4 is a view showing the shape of a cruciform fillet welded joint fabricated in an example. [Figure 5] FIG. 5 is a view showing the results of a fatigue test in an example. [Figure 6] FIG. 6 is a photograph of an optical microscope observation showing the crack generation position of the welded joint after the fatigue test in an example. [Figure 7A] FIG. 7A is a photograph of an optical microscope observation of a cross-section of a welded joint obtained in an example. [Figure 7B] FIG. 7B is a view explaining a method of dividing the remelted portion of the welded joint of FIG. 7A into a region where the steel plate melted and a region where the weld bead melted. [Figure 8] FIG. 8 is a graph showing the relationship between the steel plate dilution rate and the amount of Cu in the remelted portion calculated from Equation (2). [Figure 9] FIG. 9 is a graph showing the relationship between the steel plate dilution rate and the amount of Si + Cu in the remelted portion calculated from Equation (2).

Embodiments for Carrying Out the Invention

[0016] The inventors diligently conducted research to realize a simple method for manufacturing welded joints with improved fatigue properties. As a result, they found that it is sufficient to manufacture welded joints that satisfy the following conditions (I) and (II). (I) A steel plate containing 0.10 mass% or more of Cu and a total of 0.30 mass% or more of Si and Cu is used as the base material for welding, and multiple steel plates are welded together to obtain an as-welded joint (hereinafter referred to as an as-welded joint). (II) In the as-welded joint, the weld bead toe and the steel plate portion in contact with the weld bead toe are remelted to form a remelted portion containing 0.09% by mass or more of Cu.

[0017] The following sections will detail (I) and (II).

[0018] First, the inventors focused on the fact that Cu and Si are elements (hereinafter sometimes simply referred to as "reinforcing elements") that are effective in extending crack initiation life and improving the fatigue limit ratio by suppressing dislocation proliferation, and that if these reinforcing elements are present in large quantities in the weld bead, the fatigue strength of the welded joint can be increased. In this specification, "weld bead" refers to the weld metal, and if slag is formed by welding, it refers to the weld metal excluding this slag. In this specification, a portion formed by remelting is referred to as the "remelted portion" and is distinguished from the weld bead formed by welding.

[0019] One way to incorporate a high concentration of reinforcing elements into the weld bead is to supply these elements from the welding material and / or the steel plate (base metal) to the weld bead. However, as shown in the literature (Toshiyuki Kajitani, Masamitsu Wakao, Naoki Tokumitsu, Shigeaki Ogibayashi, Shozo Mizoguchi, "Effects of Temperature and Strain on Brittlement of Carbon Steel by Cu", Iron and Steel, Vol. 81, No. 3 (1995), pp. 185-190), when a steel plate containing a large amount of Cu is heated at high temperatures, the selective oxidation of Fe at high temperatures causes the Cu concentration on the steel surface to exceed the solid solubility limit of solid-solution Cu in the austenite, and Cu precipitates on the steel surface as liquid Cu. This Cu can penetrate the austenite grain boundaries, potentially causing cracks. Whether or not cracks occur when using welding materials with a high Cu content depends on the amount of Cu in the welding material and the stress applied to the welded joint, so it cannot be said definitively, but there is a possibility that cracks may occur during welding when using welding materials with a high Cu content. In other words, welded joints with a high copper content in the weld bead, obtained by welding using welding materials with a high copper content, are more likely to crack.

[0020] Furthermore, as mentioned above, in as-welded joints, stress concentration is likely to occur at the weld bead toe due to the shape of the weld bead toe, making it difficult to achieve improved fatigue properties. Therefore, we conducted research based on the idea that it is important to eliminate such sources of stress concentration and to form a region containing Cu. As described above, we found that by satisfying (I) and (II), it is possible to easily realize a welded joint with improved fatigue properties by incorporating at least Cu, an element that enhances the fatigue strength of the welded joint, in the area of ​​the welded joint where crack formation is a concern, without cracking during welding.

[0021] In the following, the welding process and the remelting process in the manufacturing method of the welded joint of this embodiment will be described, including the composition of the steel plate mentioned above.

[0022] [Welding Process] A steel plate containing 0.10 mass% or more of Cu and a total of 0.30 mass% or more of Si and Cu is used as at least one of a plurality of steel plates, and the plurality of steel plates are welded together to obtain an as-welded joint. First, the amounts of Cu and Si+Cu in the steel plates will be explained.

[0023] (Cu content of steel plate (base material): 0.10% by mass or more) Cu is an effective element for extending crack life and improving the fatigue limit ratio by suppressing dislocation proliferation. To effectively exert this effect, the Cu content should be 0.10 mass% or more. Preferably, the Cu content is 0.15 mass% or more, more preferably 0.20 mass% or more, and even more preferably 0.25 mass% or more. However, if the Cu content is excessive, not only will the hardenability become excessive, but cracks and other problems are more likely to occur during hot working, so it is preferable that the Cu content be 0.60 mass% or less. More preferably, the Cu content is 0.55 mass% or less, even more preferably 0.50 mass% or less, even more preferably 0.40 mass% or less, and particularly preferably 0.30 mass% or less.

[0024] (Total amount of Si and Cu in the steel plate (base material): 0.30% by mass or more) Like Cu, Si is an effective element for extending crack initiation life and improving the fatigue limit ratio by suppressing dislocation proliferation. A combined amount of Si and Cu of 0.30 mass% or more can reliably increase fatigue strength. The combined amount of Si and Cu is preferably 0.40 mass% or more, more preferably 0.50 mass% or more, and even more preferably 0.60 mass% or more. A preferred upper limit for the combined amount of Si and Cu may be, for example, 1.2 mass%.

[0025] The above Cu and components other than Si+Cu are not limited. For example, if the chemical composition of the steel sheet (base material) is: C: 0.02~0.10% by mass, Si: 0.10~0.60% by mass, Mn: 1.00~2.00% by mass, P: more than 0% by mass and 0.035% by mass or less, S: More than 0% by mass and 0.035% by mass or less, Cu: 0.10~0.60% by mass, Al: 0.010~0.060% by mass, Nb: more than 0% by mass and 0.050% by mass or less, Ti: more than 0% by mass and not more than 0.050% by mass, N:0.0010~0.0100% by mass, Ni: more than 0% by mass and not more than 1.00% by mass, Ca: greater than 0% by mass and less than or equal to 0.0050% by mass, The remainder includes fatigue-resistant steel sheets made of Fe and unavoidable impurities.

[0026] The fatigue-resistant steel sheet may further contain one or more of the following elements as optional elements: Cr: more than 0.01% by mass but not more than 1.00% by mass, Mo: more than 0.001% by mass and not more than 0.500% by mass, V: greater than 0.001% by mass and less than or equal to 0.500% by mass, and B: More than 0.0003 mass% and 0.0050 mass% or less It may contain one or more elements selected from the group.

[0027] Furthermore, examples of fatigue-resistant steel include duplex steel in which ferrite (soft phase) and bainite (hard phase) are optimally arranged (for example, Japanese Patent Publication No. 7-242992). However, since the remelted portion melts and solidifies once, it is thought that the microstructure of fatigue-resistant steel with duplex control is reset and the fatigue improvement effect is not achieved. On the other hand, as mentioned above, in the case of fatigue-resistant steel strengthened by solid solution that satisfies predetermined Cu and Si+Cu amounts, remelting does not affect the solid solution of elements and the fatigue improvement effect can be achieved. Therefore, in this disclosure, a fatigue-resistant steel sheet that satisfies at least predetermined Cu and Si+Cu amounts is used as the steel sheet (base material). Preferably, the steel sheet (base material) is a fatigue-resistant steel sheet that satisfies predetermined Cu and Si+Cu amounts as well as the chemical composition described above.

[0028] The strength and thickness of the steel plate (base material) are not limited. One example of a steel plate (base material) is one with a tensile strength of 440 MPa or more and 620 MPa or less. Another example of a steel plate (base material) is one with a thickness of 10 mm or more. There is no specific upper limit on the plate thickness, but it may be set appropriately depending on the required characteristics such as the strength of the steel plate.

[0029] (Welding materials) While there are no particular restrictions on the welding material, as mentioned above, if the amount of Cu in the welding material is too high, cracks may occur during welding. Therefore, it is preferable that the welding material used for welding has a Cu content of less than 0.5 mass%. As an example of a welding material, its chemical composition is such that the Cu content is less than 0.5 mass% (including 0 mass%), C: 0.03~0.10% by mass, Si: 0.10~2.0% by mass, Mn: 0.50~2.00% by mass, P: 0.020% by mass or less, S: 0.020% by mass or less Examples include materials that satisfy the above conditions, with the remainder consisting of Fe and unavoidable impurities. The chemical composition of the welding material may further contain, as an optional element, one or more of Ni, Cr, Mo, V, Ti, Mo, Zr, and Al in a total amount of 5.0% by mass or less.

[0030] (Welding method) In the manufacturing method of the welded joint of this embodiment, the welding method is not limited. Various welding methods such as shielded metal arc welding, gas metal arc welding, submerged arc welding, and welding using FCW (flux-cored wire) can be suitably applied as welding methods for manufacturing the welded joint of this disclosure.

[0031] If the weld bead of an as-welded joint obtained by welding contains 0.5% by mass or more of Cu, the risk of cracking in the as-welded joint and in welded joints obtained using said as-welded joint increases. Therefore, it is preferable that the weld bead of an as-welded joint obtained by welding contains less than 0.5% by mass of Cu.

[0032] [Remelting process] In as-welded joints, the weld bead toe and the steel plate portion adjacent to the weld bead toe (collectively referred to as the "remeltable region") are remelted to form a remelted region containing 0.09% by mass or more of Cu. The range of the "remeltable region" may vary depending on the required fatigue characteristics, but the relationship between the remeltable region and fatigue characteristics should be confirmed in advance through experiments, etc., and the remeltable region necessary to effectively utilize the effect of introducing Cu, etc., into the remeltable region by diluting the steel plate should be determined based on that relationship. The "remeltable region" refers to the remeltable region determined in this way and may differ from the size of the remelted region actually obtained after remelting.

[0033] (Remelting method) In as-welded joints, the goal is to remelt the weld bead toe and the steel plate portion in contact with the weld bead toe to form a remelted portion containing 0.09% by mass or more of Cu. The method of remelting is not limited. Remelting can be carried out by methods such as arc heating as performed in the examples, or by heating by laser irradiation.

[0034] One method of remelting that more reliably forms a remelted area containing 0.09 mass% or more of Cu will be explained with reference to Figure 1. Figure 1 is a schematic cross-sectional view showing an intermediate step in a cross fillet weld as an example, where steel plates 1a and 1b have been fillet welded, but steel plates 1a and 1c have not yet been fillet welded. Figure 2 is a schematic cross-sectional view of a welded joint in which a remelted area 13 has been formed by performing the remelting shown in Figure 1. Note that in Figure 1, for the sake of clarity, the scale differs from actual operation, such as by increasing the distance L from the weld bead toe 5a to the target position 9 of the torch 11 as a heat source.

[0035] In Figure 1, the target position 9 of the torch 11 as a heating source on the steel plate (base material) 1a is preferably 0.5 mm to 4.0 mm away from the weld bead toe 5a, perpendicular to the longitudinal direction of the weld bead 3a (the direction perpendicular to the plane of the paper in Figure 1) and away from the weld bead 3a (especially the weld bead toe 5a), as indicated by the white arrow (i.e., the distance L in Figure 1 is in the range of 0.5 mm to 4.0 mm). This allows for more efficient melting of the remelted region 8 (shown as a circle in Figure 1 for ease of understanding), which includes the weld bead toe of the as-welded joint and the steel plate portion in contact with the weld bead toe. This promotes the melting of the steel plate and facilitates the supply of steel plate components to the remelted portion 13 formed by remelting. As a result, it is preferable to easily achieve a Cu content of 0.09 mass% or more in the remelted portion 13, which can suppress the occurrence of fatigue cracks. On the other hand, in order to keep the steel plate dilution ratio described later within a certain range and to effectively exert the effect of introducing Cu and other elements into the remelted portion due to the dilution of the steel plate, it is preferable to set the upper limit of the distance L to 4.0 mm. The above distance L is more preferably 0.6 mm or more, and more preferably 3.7 mm or less.

[0036] As an example of ensuring reliable heating of the target position 9, when performing arc heating, as shown in Figure 1, a torch 11 is used, and the angle θ is set to, for example, within the range of 60° to 80°. The angle θ refers to the angle between the surface of the steel plate 1a and the torch 11, as shown in Figure 1. In addition, to suppress oxidation during heating, for example, a shielding gas of 100% Ar is supplied during heating. The welding conditions vary depending on the welding method and construction, but for example, as shown in the embodiment described later, in the case of fillet welding in one pass by gas shielded arc welding, the current is set to 180 to 230 A, the voltage to 10 to 12 V, and the speed to 10 to 15 cm / min.

[0037] The remelting process only requires one remelting step. However, this does not preclude the possibility of melting the material in two or more stages to further dilute the copper, etc., from the steel sheet.

[0038] In the remelting process, it is preferable to perform remelting so that the steel sheet dilution ratio in the remelted area, calculated from the following formula (1), is 0.35 or higher. This is preferable because a higher steel sheet dilution ratio makes it easier for the components of the steel sheet to be supplied to the remelted area, making it easier to achieve a Cu content of 0.09% by mass or higher in the remelted area, which can suppress the occurrence of fatigue cracks. The steel sheet dilution ratio is more preferably 0.40 or higher, and even more preferably 0.45 or higher. The upper limit of the steel sheet dilution ratio is approximately 0.99, as melting of the weld bead toe is also necessary. Steel plate dilution ratio = (Area where steel plate melted) / (Area where steel plate melted + Area where weld bead melted) ... (1)

[0039] In the above formula (1), the area of ​​the steel plate that melted and the area of ​​the weld bead that melted can be determined by the method described in the examples below.

[0040] Figure 3 is a graph summarizing the relationship between the distance L from the weld bead toe to the target position of the heating source and the steel plate dilution ratio, using the values ​​in Table 3 from the examples described later. As shown in Figure 3, it can be seen that the steel plate dilution ratio increases when the target position of the heating source is moved away from the weld bead toe. As mentioned above, the steel plate dilution ratio is preferably 0.35 or higher, more preferably 0.40 or higher, and even more preferably 0.45 or higher. On the other hand, when the target position is directly above the weld bead toe (i.e., L=0mm), remelting of the steel plate is unlikely to occur, and a steel plate dilution ratio of 0.35 or higher cannot be achieved. From Figure 3, it can be seen that in order to achieve a steel plate dilution ratio of 0.35 or higher, the distance L from the weld bead toe should be 0.5mm or more and 4.0mm or less, and preferably 1.0mm or more.

[0041] (Remelted area in welded joints) The remelted portion obtained by the above remelting process contains 0.09 mass% or more of Cu, and preferably 0.5 mass% or more of Si and Cu combined. As described above, this makes it possible to achieve a welded joint in which fatigue crack initiation is sufficiently suppressed. The remelted portion can be confirmed by optical microscope observation photographs after etching, as shown in Figure 7A of the examples described later. Furthermore, the amount of Cu and Si+Cu in the remelted portion can be measured by taking an analytical sample from the remelted portion and determining it by ICP emission spectrometry, as shown in the examples. Alternatively, although less accurate than ICP emission spectrometry, it can also be measured by energy-dispersive X-ray spectroscopy (EDS) equipped with a scanning electron microscope (SEM), which allows for simple and rapid analysis.

[0042] Furthermore, ensuring that the remelted portion contains a total of 0.5% by mass or more of Si and Cu can be achieved, for example, by controlling the chemical composition of the steel sheet (base material) used and the dilution ratio of the steel sheet, as shown in Figure 9 later.

[0043] The manufacturing method for a welded joint according to this disclosure only needs to include the welding step and the remelting step, and other steps are not particularly limited. For example, it may include finishing steps such as post-heat treatment or surface finishing with a grinder. The post-heat treatment refers to a treatment performed at 500°C to 600°C for about 30 minutes to 2 hours for the purpose of reducing residual stress and removing hydrogen. This post-heat treatment does not affect the composition of the already formed remelted section.

[0044] Preferably, the amount of Cu in the weld bead of the welded joint obtained by remelting is less than 0.5% by mass, similar to the as-welded joint.

[0045] In addition to the cross fillet welded joints fabricated in the embodiments described below, other examples of welded joints of this disclosure include cross butt welded joints, T-joints, butt joints, corner joints, and lap joints obtained by fillet welding or butt welding. These welded joints may have multiple weld beads, but it is sufficient that a remelted area is formed in at least one of the multiple weld beads, preferably that remelted areas are formed in multiple weld beads, and most preferably that remelted areas are formed in all of the weld beads. [Examples]

[0046] The present invention will be described in more detail below with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope consistent with the spirit described above and below, and all such modifications are included within the technical scope of this disclosure.

[0047] 1. Fabrication of welded joints 1-1. Welding As the steel plates (base material) used for welding, we used general-purpose steel (JIS G 3106) and fatigue-resistant steel (steel based on the chemical composition disclosed in Japanese Patent Publication No. 2022-013657, with an expanded lower limit for the optional additive element Cr) with the chemical compositions shown in Table 1. Two plates measuring 12t × 50W × 500L (unit: mm) were welded to the general-purpose steel or fatigue-resistant steel plate (size: 12t × 400W × 500L, unit: mm) as rib material using a cross fillet weld. A cross fillet welded joint with the shape shown in Figure 4 was obtained as the as-welded joint. In Table 1, blank spaces indicate that even if trace amounts are present, they are at the level of impurities and were not detected in the steel plates of this embodiment. The tensile strength of both the general-purpose steel and fatigue-resistant steel (base material) was 490 MPa class, but the fatigue strength was greater for the fatigue-resistant steel than for the general-purpose steel. In this embodiment, a fillet welded joint was fabricated as an example of a welded joint, but the joint in this embodiment may be other welded joints such as a butt joint.

[0048] [Table 1]

[0049] Other welding conditions are as follows. The leg lengths (R1 and R2 in Figures 1 and 2) were aimed at 6 mm and 4 mm, respectively, as shown below. The chemical composition of the weld bead of the obtained as-welded joint is shown in Table 2. (Other welding conditions) Welding material: FAMILIARC (registered trademark) MX-Z200 Welding method: Gas shielded arc welding, fillet welding, 1 pass. Shielding gas: 100% CO2 Posture: Horizontal corner meat Leg length: 6mm, 4mm

[0050] [Table 2]

[0051] 1-2. Remelting Next, arc heating was used to remelt the weld bead toe of the as-welded joint and the steel plate portion in contact with the weld bead toe to obtain a welded joint. Specifically, arc heating (TIG treatment) of the toe of a cross fillet weld was performed so that the target position (the "distance from the toe to the base material side" in Table 3 refers to the distance L in Figure 1) was as shown in Table 3 to obtain a welded joint. The remelting conditions by arc heating are as shown below and in Table 3. (Remelting conditions) Welding method: TIG melt-run Welding position: Horizontal fillet weld Torch angle: 60°~80° Shielding gas: 100% Ar

[0052] [Table 3]

[0053] In this embodiment, welding and remelting were performed on all four corners of the cross fillet welded joint, and the following fatigue tests were conducted.

[0054] 2. Fatigue testing using welded joints Using the fillet welded joints No. 9, No. 18, and No. 19 shown in Table 3, fatigue tests were conducted on load-nontransmission type fillet welded joints, and the number of cycles in the stress range Δσ = 340 MPa was obtained. The fatigue test conditions are shown below. Figure 5 shows the fatigue life results obtained from the fatigue tests in this stress range Δσ = 340 MPa. (Conditions for fatigue testing) • Test environment: Room temperature, in the atmosphere Stress ratio = 0.0 • Stress range: Δσ = 340 MPa • Control method: Load control • Control waveform: sine wave • Test frequency: 1~15Hz • Stopping condition: Breaking

[0055] From the results in Figure 5 above, in the case of No. 9 (base material is general-purpose steel), the fatigue life (number of cycles) is 1.57 × 10⁻⁶. 5 In contrast, for No. 18 (base material is fatigue-resistant steel) and No. 19 (base material is fatigue-resistant steel), the values ​​were 7.40 × 10 5 , 5.96×10 5 As a result, the fatigue life was extended to 4.7 times or 3.8 times that of No. 9. Figure 5 shows the number of cycles corresponding to Δσ = 340 MPa for each fatigue strength grade, based on the fatigue strength grades in the fatigue design guidelines and commentary for steel structures of the Japan Society of Steel Construction (JSSC), indicated by dotted lines. In the case of this fatigue test result, No. 9 (base material is general-purpose steel) met grade C, while No. 18 (base material is fatigue-resistant steel) and No. 19 (base material is fatigue-resistant steel) fully met grade B, showing an improvement of at least one grade. In actual operation, the conditions are more severe than those of this fatigue test, so the fatigue life in fatigue tests under actual operation may be about one grade lower overall than the results of this fatigue test. However, even in that case, this embodiment can be said to have the effect of relatively improving the fatigue life by one grade compared to the comparative example (when general-purpose steel is used).

[0056] 3. Verification of effectiveness (Verification by photographs) Using the welded joint No. 11 in Table 3, a fatigue test was conducted as described above (after fracture, which is the stopping condition). After polishing the cross section perpendicular to the longitudinal direction of the weld bead of the welded joint to a mirror finish, it was etched by Nital corrosion. The location of the crack was then observed with an optical microscope. The optical microscope image is shown in Figure 6. As shown in Figure 6, it was found that the fatigue crack originated from the remelted area. From this, it can be seen that if the location of the crack in the welded joint is not in the remelted area, it is necessary to examine the parts other than the remelted area. However, according to this disclosure, it was confirmed that the control of the composition and structure of the remelted area affects the fatigue characteristics of the welded joint.

[0057] The dilution ratio of the steel plate in the remelted area was calculated as follows. First, a section perpendicular to the longitudinal direction of the weld was cut at a point at least 50 mm away from the start and end of the longitudinal direction of the weld (the center if the total length of the weld is 100 mm or less). Then, the cross section was polished to a mirror finish and etched by Nital corrosion. Photographs were obtained using an optical microscope, and image analysis (image analysis software: Image-Pro v.10 from Media Cybernetics) was used to distinguish between the area where the steel plate melted and the area where the weld bead melted, and the respective areas were determined. An example is explained with reference to Figures 7A and 7B. Figure 7A is an optical microscope photograph obtained as described above for the welded joint of this disclosure, in which the remelted area 13 is formed. Figure 7B is a diagram that divides the remelted area 13 of Figure 7A into the area where the steel plate melted and the area where the weld bead melted. In Figure 7B, first, a white dashed line S1 representing the steel plate surface and a black line S2 representing the remelted area based on the shape of the weld bead (white solid line in Figure 7B) were drawn. Then, the vertical line region 131 above the white dashed line S1 in the remelted area 13 was defined as the melted area of ​​the weld bead. Furthermore, the shaded area 132 below the white dashed line S1 in the remelted area 13, closer to the weld bead than the black line S2, was defined as the melted area of ​​the weld bead, and the grid line region 133 closer to the base metal than the black line S2 was defined as the melted area of ​​the steel plate. The area of ​​each region was then calculated, and the steel plate dilution ratio was determined from the following formula (1). In the case of Figure 7B, the steel plate dilution ratio is calculated from (area of ​​grid line region 133) / {(area of ​​grid line region 133)+(area of ​​vertical line region 131)+(area of ​​shaded area 132)}. This method for determining the steel plate dilution ratio can be applied not only to fillet welds shown in Figures 7A and 7B, but also to other welded joints such as butt welds. Steel plate dilution ratio = (Area where steel plate melted) / (Area where steel plate melted + Area where weld bead melted) ... (1)

[0058] Using the above steel plate dilution ratio, the calculated chemical composition of the remelted portion can be obtained as follows. That is, the amount of component A in the remelted portion (Si, Cu in this embodiment) (unit: mass%) can be calculated from the following formula (2). Amount of component A in the remelted area (mass%) = (dilution rate of steel plate) × (amount of component A in the steel plate (mass%)) + {1 - (dilution rate of steel plate)} × (amount of component A in the weld bead (mass%)) ... (2)

[0059] Table 3, mentioned above, shows the calculated values ​​for the steel sheet dilution ratio and the chemical components (Si, Cu, Si+Cu) of the remelted portion for each example. Figure 8 is a graph showing the relationship between the steel sheet dilution ratio and the amount of Cu in the remelted portion calculated by formula (2) above, and Figure 9 is a graph showing the relationship between the steel sheet dilution ratio and the amount of Si+Cu in the remelted portion calculated by formula (2) above.

[0060] Figure 8 shows that when fatigue-resistant steel is used as the steel plate (base material), increasing the dilution ratio of the steel plate increases the amount of Cu in the remelted area. In this embodiment, as shown in Table 2, the amount of Cu in the weld bead is small, at 0.01 mass% or 0.07 mass%, so the amount of Cu in the remelted area depends on the amount of Cu in the steel plate. Next, Figure 9 will be explained. As shown in Table 2, the weld bead has a very high Si content and a low Cu content. Therefore, as shown in Figure 9, when the dilution ratio of the steel plate is low (= the dilution ratio of the weld bead is high), it can be seen that it converges to the total amount of Si + Cu in the weld bead (0.50 mass%).

[0061] In Figures 8 and 9, comparing the cases where general-purpose steel and fatigue-resistant steel are used as the steel plate (base material), even when the steel plate dilution ratio of general-purpose steel is as high as 0.80, the amount of Cu in the remelted portion obtained is low at 0.01 mass%, and the total amount of Si+Cu is low at 0.22 mass% (No. 9 in Table 3 above). In contrast, when fatigue-resistant steel is used, it can be seen that the amount of Cu in the remelted portion can be obtained at a steel plate dilution ratio similar to or lower than that of general-purpose steel, and the total amount of Si+Cu can be obtained at 0.58 to 0.63 mass%.

[0062] Next, the validity of the calculated values ​​of each component obtained by equations (1) and (2) above was verified by the following method. First, 5g of analytical blocks were taken from the remelted portions of No. 9, No. 18, and No. 19 (the same samples as in the fatigue test) in Table 3, and the amount of Cu was analyzed by ICP emission spectrometry. In addition, the amount of Cu in the remelted portions of each example was analyzed using energy-dispersive X-ray spectroscopy (EDS) mounted on a scanning electron microscope (SEM). The results are shown in Table 4.

[0063] [Table 4]

[0064] Table 4 shows that the analytical values ​​obtained by ICP emission spectroscopy closely match the calculated values ​​for the amount of Cu. On the other hand, the analytical values ​​obtained by SEM-EDS, although having lower analytical accuracy and being skewed upwards due to background effects, showed the same trend as the calculated values. Compared to the highly accurate ICP emission spectroscopy, EDS has lower analytical accuracy, but it has the advantage of being simple and quick to analyze. [Explanation of Symbols]

[0065] 1a, 1b, 1c steel plate (base metal) 3a, 3b Weld bead (weld metal) 5a, 5b, 5c, 5d Weld bead toe 7 HAZ (heat affected zone) 8 Area to be remelted 9. Target heating location during remelting 11 Torches 13 Remelted section 131, 132 Molten region of the weld bead in the remelted area 133 Region of the steel plate that has melted in the remelted area R1, R2 Leg Length θ Torch angle

Claims

1. A method for manufacturing a welded joint formed by welding multiple steel plates, A welding step is to use a steel plate containing 0.10% by mass or more of Cu and a total of 0.30% by mass or more of Si and Cu as at least one of the plurality of steel plates, and to weld the plurality of steel plates to obtain an as-welded joint with a weld bead formed, The remelting step involves remelting the weld bead toe and the steel plate portion in contact with the weld bead toe in the aforementioned as-welded joint to form a remelted portion containing 0.09% by mass or more of Cu. A method for manufacturing welded joints, including [the specified part of the method].

2. The method for manufacturing a welded joint according to claim 1, wherein the remelted portion contains a total of 0.5% by mass or more of Si and Cu.

3. The method for manufacturing a welded joint according to claim 1 or 2, wherein in the remelting step, the target position of the heating source for forming the remelted portion is set to a position 0.5 mm to 4.0 mm from the weld bead toe of the as-welded joint, perpendicular to the longitudinal direction of the weld bead and away from the weld bead.

4. The method for manufacturing a welded joint according to claim 1 or 2, wherein the remelting step is performed such that the steel plate dilution ratio in the remelted section, determined from the following formula (1), is 0.35 or more. Steel plate dilution ratio = (Area where the steel plate melted) / (Area where the steel plate melted + Area where the weld bead melted) ... (1)

5. The method for manufacturing a welded joint according to claim 3, wherein in the remelting step, remelting is performed such that the steel plate dilution ratio in the remelted section, determined from the following formula (1), is 0.35 or more. Steel plate dilution ratio = (Area where the steel plate melted) / (Area where the steel plate melted + Area where the weld bead melted) ... (1)

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