Overlap fillet welds and T-fillet welds

Abstract

This lap fillet welded joint comprises a first Zn-plated steel sheet, a second Zn-plated steel sheet positioned above the first Zn-plated steel sheet, a weld metal that joins the second Zn-plated steel sheet and the first Zn-plated steel sheet. This T-shaped fillet welded joint comprises a third-generation Zn-plated steel sheet, a fourth-generation Zn-plated steel sheet provided perpendicular to the surface of the third-generation Zn-plated steel sheet, the fourth-generation Zn-plated steel sheet, and a weld metal that joins the third-generation Zn-plated steel sheet. In both lap fillet welds and T-fillet welds, the chemical composition of the weld metal includes Si: greater than 0% and 0.70% or less, and Ti: 0.04-0.15%, and the upper limit WBU and lower limit WBL of the bead width on the lower side of the weld metal, and the upper limit WSU and lower limit WSL of the bead width on the upper side, are equal to the plate thickness t H Each satisfies the desired range.

Description

[Technical Field]

[0001] This disclosure relates to lap fillet welds and T-fillet welds. This application claims priority based on Japanese Patent Application No. 2024-209806 and Japanese Patent Application No. 2024-209767, filed in Japan on December 2, 2024, and the contents thereof are incorporated herein by reference. [Background technology]

[0002] In recent years, the application of high-strength galvanized steel sheets has been considered from the perspective of improving the corrosion resistance of mechanical structural components, such as automobile parts and building material components.

[0003] Overlap fillet arc welding of steel plates involves overlapping two steel plates, overlapping the edge (end face and its vicinity) of one plate onto the surface of the other plate, and then performing fillet arc welding. The steel plate whose edge is welded is often called the upper plate, and the steel plate with the surface to be welded to the upper plate's edge is often called the lower plate. However, in actual overlap fillet arc welding, the upper and lower plates are not limited to the vertical up and down direction, but are defined by the relative positional relationship of the steel plates when they are overlapped as described above.

[0004] Furthermore, T-shaped fillet arc welding of steel plates involves positioning one steel plate perpendicular to the other, and then performing fillet arc welding on the surface of one steel plate and the edge (end face and its vicinity) of the other steel plate. It should be noted that "positioning the other steel plate perpendicular to the other steel plate" here means that the angle between the two steel plates is not limited to 90°, but can be within the range of 90° ± 45°.

[0005] When overlapping steel plates are galvanized steel plates, a large amount of zinc plating evaporates due to the heat input during welding. This leads to unstable droplet transfer during welding, increasing spatter, and the inclusion of zinc vapor in the molten metal causes blowholes. Such increased spatter and blowholes in the weld could lead to a decrease in joint strength.

[0006] Furthermore, electrodeposition coating is known as a means of improving the corrosion resistance of automobile parts. Electrodeposition coating is a painting method in which the object to be coated and electrodes are placed in a tank containing electrodeposition paint, a potential difference is created between the two, and the paint film components are electrophoresed, thereby depositing a paint film on the surface of the object to be coated.

[0007] However, when automotive parts have welded joints, the slag formed in the weld can degrade the corrosion resistance of the part. "Slag" refers to non-metallic substances that partially or completely cover the weld bead. Slag consists of oxides and other substances discharged from the weld bead when the base steel plate and welding material melt and solidify to form the weld bead.

[0008] Generally, slag tends to have low electrical conductivity. Therefore, even if electrodeposition coating is applied to a weld where slag is present, a coating will not form in the areas where slag is attached. As a result, the weld will have areas where a coating has formed and areas where the slag and underlying metal are exposed without a coating. Thus, slag formed in the weld can cause defects in electrodeposition coating.

[0009] To address the above-mentioned problems, for example, Patent Document 1 discloses an overlap fillet weld joint in which the end face of the upper plate and the upper plate side surface of the lower plate are connected via weld metal, and the end face of the lower plate and the lower plate side surface of the upper plate are also connected via weld metal.

[0010] Patent Document 2 discloses a fillet arc welding method for galvanized steel sheets, wherein the Si content in the weld metal is 0.5% or less by mass, and the total Si and Al content in the base steel sheet of the upper plate of the galvanized steel sheet is 0.35% or more by mass. Patent Document 2 also discloses a welding method in which a gap is provided between the upper and lower plates, having a predetermined ratio to the thickness of the upper plate. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2012-183542 [Patent Document 2] Japanese Patent Publication No. 2012-101232 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, conventional techniques sometimes failed to sufficiently reduce the occurrence of pore defects in blowholes.

[0013] For example, in the technology disclosed in Patent Document 1, the melting area on the back side of the weld bead was narrow, and zinc vapor was not sufficiently discharged from the molten metal. Furthermore, while the technology disclosed in Patent Document 2 attempts to suppress the occurrence of blowholes by optimizing the Si and Al content of the base material and creating gaps between steel sheets, it does not take electrodeposition coating properties into consideration.

[0014] The present invention has been made in view of the above circumstances, and aims to provide lap fillet welded joints and T-fillet welded joints that have excellent electrodeposition coating properties and can reduce the occurrence of porosity defects in blowholes. [Means for solving the problem]

[0015] This invention is based on the above findings, and its gist is as follows.

[0016] (1) A lap fillet welded joint according to one aspect of the present invention comprises a first Zn-plated steel sheet, a second Zn-plated steel sheet disposed above the first surface of the first Zn-plated steel sheet, and a weld metal that joins the end surface of the second Zn-plated steel sheet and the first surface of the first Zn-plated steel sheet, The chemical composition of the weld metal is, in mass%, Si: greater than 0%, 0.70% or less, Ti: 0.04~0.15% Includes, At least a part of the end face of the first Zn-based plated steel sheet is covered by the weld metal. The plate thickness t of the first Zn-based plated steel sheet H is 1.0 to 3.4 mm, The upper limit value WBU (mm) of the bead width on the lower surface side of the weld metal is plate thickness t H When it is 1.0 mm or more and less than 2.0 mm, it satisfies the following formula (1), plate thickness t H When it is 2.0 mm or more and 3.4 mm or less, it satisfies the following formula (2), The lower limit value WBL (mm) of the bead width on the lower surface side of the weld metal is plate thickness t H When it is 1.0 mm or more and less than 2.0 mm, it satisfies the following formula (3), plate thickness t H When it is 2.0 mm or more and 3.4 mm or less, it satisfies the following formula (4), The upper limit value WSU (mm) of the bead width on the upper surface side of the weld metal is plate thickness t H When it is 1.0 mm or more and less than 1.6 mm, it satisfies the following formula (5), plate thickness t H When it is 1.6 mm or more and less than 2.9 mm, it satisfies the following formula (6), plate thickness t H When it is 2.9 mm or more and 3.4 mm or less, it satisfies the following formula (7), The lower limit value WSL (mm) of the bead width on the upper surface side of the weld metal is plate thickness t H When it is 1.0 mm or more and less than 1.6 mm, it satisfies the following formula (8), plate thickness t H When it is 1.6 mm or more and less than 2.9 mm, it satisfies the following formula (9), plate thickness t H When it is 2.9 mm or more and 3.4 mm or less, it satisfies the following formula (10). The lap fillet weld joint is characterized by this. WBU ≦ t H +0.8 ···(1) WBU ≦ 0.86t H +1.09 ···(2) WBL ≧ 0.5tH +0.5 ···(3) WBL≧0.21t H +1.07 ···(4) WSU ≤ 5.0 ···(5) WSU ≤ 3.07t H +0.08 ···(6) WSU ≤ 9.0 ···(7) WSL ≥ 3.0 ···(8) WSL ≥ 1.54t H +0.54 ···(9) WSL ≥ 5.0 ···(10) In one aspect of the present invention, the "bead width on the lower side" in the lap fillet weld joint refers to the distance along the thickness direction of the first Zn-plated steel sheet from the lower surface of the second Zn-plated steel sheet to the lower end of the weld metal on the back side, in a cross section perpendicular to the extending direction of the weld metal (longitudinal direction of the weld bead). The "bead width on the upper side" refers to the width of the weld metal on the front side in a direction parallel to the first surface, in a cross section perpendicular to the extending direction of the weld metal (longitudinal direction of the weld bead). (2) In the lap fillet welded joint described in (1) above, the chemical composition of the weld metal is, in mass%, C: 0.06~0.25%, Si: more than 0%, less than 0.70%, Mn: 1.4~2.3%, Ti: 0.04~0.15%, Al: 0.001~0.20%, N: 0.003~0.015%, O: 0.01~0.06%, Cr: 0-2.0%, Ni: 0-2.5%, B: 0~0.0100%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.013%, Sb: 0~0.10%, Sn: 0~0.4%, Cu: 0~0.50%, Nb: 0~0.3%, V: 0~0.3%, Mo: 0~1.0%, The remainder consists of iron and impurities, and further, The following equation (11) may also be satisfied. 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0···(11) However, the element symbols in equation (11) represent the mass percentage content of each element in the weld metal. (3) In the lap fillet welded joint described in (1) or (2) above, the Si content of each base steel sheet of the first Zn-plated steel sheet and the second Zn-plated steel sheet is 0.2 to 1.4% by mass, the ratio of the Si content of the weld metal to the Si content of the base steel sheet of the first Zn-plated steel sheet is less than 1, and the ratio of the Si content of the weld metal to the Si content of the base steel sheet of the second Zn-plated steel sheet is also less than 1. (4) In the lap fillet welded joint described in any of (1) to (3) above, the tensile strength of the first Zn-plated steel sheet and the second Zn-plated steel sheet may be 780 MPa or more. (5) In the lap fillet weld joint described in any of (1) to (4) above, the weld metal may be formed over the entire surface of the end face of the first Zn-plated steel sheet.

[0017] (6) A T-fillet welded joint according to one aspect of the present invention comprises a third Zn-plated steel sheet, a fourth Zn-plated steel sheet provided perpendicular to the surface of the third Zn-plated steel sheet, and a weld metal that joins the end face of the fourth Zn-plated steel sheet on the third Zn-plated steel sheet side and the first face of the third Zn-plated steel sheet on the fourth Zn-plated steel sheet side, The chemical composition of the weld metal is, in mass%, Si: greater than 0%, 0.70% or less, Ti: 0.04~0.15% Includes, Parts of both surfaces of the fourth Zn-plated steel sheet are connected to the first surface of the third Zn-plated steel sheet via the weld metal. The thickness t of the above-mentioned 4 Zn-plated steel sheetH The size is 1.0 to 3.4 mm. The upper limit WBU of the bead width on the lower side of the weld metal is, Plate thickness t H If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (12) is satisfied. Plate thickness t H If it is 2.0 mm or more and less than 2.9 mm, then the following formula (13) is satisfied. Plate thickness t H If the length is 2.9 mm or more and 3.4 mm or less, then the following formula (14) is satisfied. The lower limit value WBL of the bead width on the lower side of the weld metal satisfies the following formula (15): The upper limit value WSU of the bead width on the upper surface side of the weld metal is, Plate thickness t H If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (16) is satisfied. Plate thickness t H If the length is 2.0 mm or more and less than 2.9 mm, then the following formula (17) is satisfied. Plate thickness t H If the length is 2.9 mm or more and 3.4 mm or less, then the following formula (18) is satisfied. The lower limit value WSL of the bead width on the upper surface of the weld metal is, Plate thickness t H If the size is 1.0 mm or more and less than 2.9 mm, then the following formula (19) is satisfied. Plate thickness t H A T-shaped fillet welded joint characterized in that it satisfies the following formula (20) when the diameter is 2.9 mm or more and 3.4 mm or less. WBU ≤ 2.8mm ···(12) WBU ≤ 1.33t H +0.13 ···(13) WBU ≤ 4.0mm ···(14) WBL≧0.7mm ···(15) WSU ≤ 5.0 mm ···(16) WSU ≤ 2.44t H +0.11 ···(17) WSU ≤ 7.2 mm ···(18) WSL ≥ 3.5mm ···(19) WSL≧1.16t H+0.14 ···(20) In a T-fillet welded joint according to one aspect of the present invention, the "bead width on the lower side" refers to the width of the weld metal on the back side along the plate surface direction of the fourth Zn-plated steel sheet in a cross section perpendicular to the extending direction of the weld metal (longitudinal direction of the weld bead). The "bead width on the upper side" refers to the width of the weld metal on the surface of the weld metal in the plate surface direction of the fourth Zn-plated steel sheet in a cross section perpendicular to the extending direction of the weld metal (longitudinal direction of the weld bead). In the case of a T-fillet weld joint where it is difficult to determine the front and back surfaces of the weld metal, the side being welded is considered the front surface. If it is still difficult to determine, the side with the wider bead is considered the front surface, i.e., the "top surface," and the side with the narrower bead is considered the back surface, i.e., the "bottom surface." Alternatively, the distance in the direction of the third Zn-plated steel sheet can be compared between the intersection point of the plate surface of the fourth Zn-plated steel sheet and the weld metal in a cross section perpendicular to the extension direction of the weld metal (longitudinal direction of the weld bead), and the side with the larger distance is considered the front surface (i.e., the top surface). (7) In the T-fillet welded joint described in (6) above, the chemical composition of the weld metal is, in mass%, C: 0.06~0.25%, Si: more than 0%, less than 0.70%, Mn: 1.4~2.3%, Ti: 0.04~0.15%, Al: 0.001~0.20%, N: 0.003~0.015%, O: 0.01~0.06%, Cr: 0-2.0%, Ni: 0-2.5%, B: 0~0.0100%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.013%, Sb: 0~0.10%, Sn: 0~0.4%, Cu: 0~0.50%, Nb: 0~0.3%, V: 0~0.3%, Mo: 0~1.0%, The remainder consists of iron and impurities, and further, The following equation (21) may also be satisfied. 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0···(21) However, the element symbols in formula (21) represent the content of each element in the weld metal. (8) In the T-fillet welded joint described in (6) or (7) above, the Si content of each base steel sheet of the third Zn-plated steel sheet and the fourth Zn-plated steel sheet may be 0.2 to 1.4% by mass, the ratio of the Si content of the weld metal to the Si content of the base steel sheet of the third Zn-plated steel sheet may be less than 1, and the ratio of the Si content of the weld metal to the Si content of the base steel sheet of the fourth Zn-plated steel sheet may also be less than 1. (9) In the T-shaped fillet welded joint described in any of (6) to (8) above, the tensile strength of the third Zn-plated steel sheet and the fourth Zn-plated steel sheet may be 780 MPa or more. (10) In the T-fillet weld joint described in any of (6) to (9) above, the intersection point PB between the bead on the lower surface of the weld metal and the third Zn-plated steel sheet may be at the same position as the intersection point PS between the surface of the fourth Zn-plated steel sheet and the surface of the third Zn-plated steel sheet, or it may be outside the intersection point PS. [Effects of the Invention]

[0018] According to the above embodiment of the present invention, it is possible to provide lap fillet welded joints and T-fillet welded joints that have excellent electrodeposition coating properties and can reduce the occurrence of porosity defects in blowholes. [Brief explanation of the drawing]

[0019] [Figure 1] This is a schematic cross-sectional diagram illustrating a lap fillet welded joint for investigating the relationship between bead shape and porosity defects. [Figure 2]This is a schematic cross-sectional diagram illustrating a lap fillet welded joint for investigating the relationship between bead shape and porosity defects. [Figure 3] This figure shows the results of an investigation into the relationship between the bead shape and porosity defects in lap fillet weld joints, and is a graph showing the relationship between the width of the bead on the front side and the plate thickness. [Figure 4] This figure shows the results of an investigation into the relationship between the bead shape and porosity defects in lap fillet weld joints, and is a graph showing the relationship between the width of the back side of the bead and the plate thickness. [Figure 5] This is a schematic cross-sectional diagram illustrating perforation that occurs in the weld of an overlap fillet weld joint when the melting area is excessively expanded. [Figure 6] This is a schematic cross-sectional diagram of a lap fillet welded joint to illustrate the convection phenomenon of molten metal. [Figure 7] This is a schematic cross-sectional view of a lap fillet welded joint according to one embodiment of the present invention. [Figure 8] This is a schematic cross-sectional diagram illustrating a T-fillet weld joint for investigating the relationship between bead shape and porosity defects. [Figure 9] This is a schematic cross-sectional diagram illustrating a T-fillet weld joint for investigating the relationship between bead shape and porosity defects. [Figure 10] This figure shows the results of an investigation into the relationship between the bead shape and porosity defects in a T-fillet weld joint, and is a graph showing the relationship between the width of the bead on the front side and the plate thickness. [Figure 11] This figure shows the results of an investigation into the relationship between the bead shape and porosity defects in T-fillet weld joints, and is a graph showing the relationship between the width of the back side of the bead and the plate thickness. [Figure 12] This is a schematic cross-sectional diagram illustrating the perforation that occurs in the weld of a T-fillet weld joint when the melting area is excessively expanded. [Figure 13] This is a schematic cross-sectional diagram of a T-fillet welded joint to illustrate the convection phenomenon of molten metal. [Figure 14] This is a schematic cross-sectional view of a T-fillet welded joint according to one embodiment of the present invention. [Figure 15]This is a schematic cross-sectional view showing a modified example of a T-fillet welded joint according to one embodiment of the present invention. [Modes for carrying out the invention]

[0020] (1. Overlap fillet weld joint) The following describes a lap fillet welded joint according to one embodiment of the present invention and a preferred method for manufacturing the same. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range.

[0021] <1.1 Results of the inventors' studies> In typical conventional lap fillet welding, as shown in Figure 1, one steel plate is first overlapped so as to cover a portion of the surface of the other steel plate, and then welded so that the edge of one steel plate and the surface of the other steel plate are joined by a weld. In other words, in typical lap fillet welding, the weld is formed without penetrating the other steel plate, which is the so-called bottom plate, in the thickness direction. Now, if at least one of the two steel plates is a zinc-plated steel plate, as shown in Figure 1, zinc vapor generated from the contact area of ​​the two steel plates during welding cannot be discharged from the weld, and blowholes occur as zinc vapor mixes into the molten metal. In other words, in order to avoid blowholes caused by zinc vapor, it is important to secure a way for the zinc vapor to escape. Therefore, the inventors investigated the effect of melting and welding the edge of the other steel plate, which is the bottom plate. Note that "edge of the steel plate" refers to the peripheral region including the edge face of the steel plate.

[0022] <1.1.1 Regarding the bead shape> First, to investigate the relationship between the bead shape of the weld and the porosity defects of the blowhole, the following experiment was conducted.

[0023] First, two Zn-plated steel sheets (test sheets) with a thickness of 1.0 to 3.4 mm and Zn-plated on both sides were prepared. As shown in Figure 1, the other test sheet 102 was overlapped so as to cover a portion of the surface of the other test sheet 101, and fillet welding was performed to create an overlap fillet welded joint 100. When welding, as shown in Figure 2, the two test sheets were positioned so that the gap G between them was 0 to 0.7 mm and the overlap of the two steel sheets was 2.5 mm, and welding was performed. The plate combination of the overlap fillet welded joint was of the same steel type and thickness. In this experiment, the gap G was kept within the range of 0 to 0.7 mm, but the gap G may be 0 to 0.5 mm or 0 to 0.4 mm.

[0024] Table 1 shows the chemical composition of the two Zn-plated steel sheets (test sheets) and the welding wire used in the investigation. Note that the chemical composition of the test sheets in Table 1 is the chemical composition of the base steel sheet of the Zn-plated steel sheet.

[0025] [Table 1]

[0026] The base material of the tested steel plate was mild steel with a tensile strength of 440 MPa, and the amount of zinc plating adhering to one side of the tested steel plate was 45 g / m². 2 The welding wire used had a relatively low Si content to reduce the amount of slag adhering to the weld bead. A solid wire with a diameter of 1.2 mm was used as the welding wire.

[0027] The welding conditions were set as follows: shielding gas: Ar + 5-20% CO2, welding current: 150-250 A, welding voltage: 20-25 V, welding speed: 60-80 cm / min, with appropriate welding conditions set according to the plate thickness. A Fronius welding power supply was used. The plate thickness of the lower plate, test steel plate 101, was t. H Regarding the plate thickness t, H When using steel plates thicker than 1.6 mm, apply the pulsed MAG welding mode, and the plate thickness t HWhen using steel plates with a thickness of 1.6 mm or less, welding was performed using a wire feed control type low heat input welding mode.

[0028] Figures 3 and 4 show the welding results. Figure 3 is a graph showing the appropriate range for the bead width on the top side (front side bead width), with respect to the thickness t of the test steel plate 101. H Figure 4 shows the relationship between (mm) and the upper and lower limits of the bead width on the upper surface (mm). Figure 4 is a graph showing the appropriate range of the bead width on the lower surface (back side bead width), with the plate thickness t of the test steel plate 101. H This shows the relationship between the upper and lower limits of the bead width on the underside.

[0029] As shown in Figure 3, the thickness t of the test steel plate 101 H It was found that for each case, there is an optimal range for the upper limit (WSU) and lower limit (WSL) of the bead width (mm) on the upper side. Specifically, the upper limit of the bead width (mm) on the upper side is shown by the following equations (5) to (7), and exceeding these upper limits results in excessive heat input during welding, causing welding defects such as holes and burn-through in the weld. On the other hand, the lower limit of the bead width (mm) on the upper side is shown by the following equations (8) to (10), and the plate thickness t H Regardless of the other factors, when the bead width was less than 3.0 mm, insufficient heat input resulted in insufficient penetration, leading to a discontinuous bead shape. Furthermore, when the bead width did not satisfy equations (8) to (10), porosity defects such as blowholes occurred frequently.

[0030] [Upper bead width WSU (mm)] <Plate thickness t H If it is 1.0 mm or more and less than 1.6 mm > WSU ≤ 5.0 ···(5) <Plate thickness t H If it is 1.6 mm or more and less than 2.9 mm > WSU ≤ 3.07t H +0.08 ···(6) <Plate thickness t H If it is between 2.9mm and 3.4mm > WSU ≤ 9.0 ···(7)

[0031] [Lower limit WSL (mm) of the bead width on the top side] <Plate thickness t H If it is 1.0 mm or more and less than 1.6 mm > WSL≧3.0mm ···(8) <Plate thickness t H If it is 1.6 mm or more and less than 2.9 mm > WSL ≥ 1.54t H +0.54 ···(9) <Plate thickness t H If it is between 2.9mm and 3.4mm > WSL ≥ 5.0 ···(10)

[0032] Furthermore, the upper and lower limits of the bead width on the lower side are determined by the thickness t of the test steel plate 101, as shown in Figure 4. H It was found that there is an optimal range for each. Specifically, the upper limit of the bead width (mm) on the lower side is shown by equations (1) and (2) below. If these upper limits are exceeded, similar to the upper limit of the bead width on the upper side, excessive heat input occurs during welding, resulting in welding defects such as holes and burn-through in the weld. On the other hand, the lower limit of the bead width (mm) on the lower side is shown by equations (3) and (4) below. If the bead width on the lower side does not satisfy equations (3) and (4), porosity defects such as blowholes occur frequently.

[0033] [Upper limit WBU (mm) of the bead width on the bottom side] <Plate thickness t H If it is 1.0 mm or more and less than 2.0 mm > WBU≦t H +0.8mm ···(1) <Plate thickness t H If it is between 2.0 mm and 3.4 mm > WBU ≤ 0.86t H +1.09 ···(2)

[0034] [Lower limit of bead width on the bottom side WBL (mm)] <Plate thickness t H If it is 1.0 mm or more and less than 2.0 mm > WBL≧0.5t H +0.5 ···(3) <Plate thickness tH If it is between 2.0 mm and 3.4 mm > WBL≧0.21t H +1.07 ···(4)

[0035] <1.1.2 Composition of Weld Metal> Next, we investigated the relationship between the suppression of porosity defects and welding defects and the composition of the weld metal.

[0036] Conventionally, methods have been known to ensure a passage for zinc vapor to escape from the weld metal in order to suppress porosity defects such as blowholes caused by zinc vapor. In addition, creating a gap between steel plates is effective in promoting the escape of zinc vapor by melting the back surface of the weld bead and exposing that back surface.

[0037] However, when applying lap fillet welds to curved automotive parts, for example, it is difficult to maintain a consistent gap between the overlapping steel plates. Furthermore, excessively large gaps can exacerbate hole formation due to burn-through. This is especially true for parts with complex curved shapes, where maintaining a consistent gap becomes more difficult, leading to a more pronounced occurrence of hole formation due to burn-through. In addition, if there is a gap between the steel plates, spatter will scatter from this gap towards the back of the weld bead, resulting in spatter adhering to a wide area of ​​the part and causing a poor appearance. Furthermore, if a large amount of sputter is scattered and adheres to the surface of a component, it requires a significant amount of labor to remove the sputter, which is undesirable from an economic standpoint.

[0038] Therefore, it is desirable to ensure stable melting of the back surface of the weld bead by keeping the steel plates in close contact with each other without leaving any gaps between them, thereby allowing zinc vapor to be released from the weld metal.

[0039] Therefore, we investigated a method for stably melting the back side of the weld bead while keeping the overlapping steel plates in close contact in lap fillet weld joints, thereby sufficiently exposing the weld metal. In this embodiment, "a state in which the steel plates are in close contact with each other" is not limited to a state in which the gap between the steel plates is 0 mm (i.e., a state in which the steel plates are in contact with each other), but also includes a state in which two steel plates are placed without leaving an excessive gap between them.

[0040] First, in typical lap fillet welds, insufficient penetration can occur, resulting in inadequate formation of the back surface of the weld bead. Therefore, to ensure stable melting of the back surface of the weld bead while the steel plates of a lap fillet weld are in close contact, it is effective to shorten the overlap of the overlapping steel plates and expand the melting area with sufficient welding heat input. However, if the melting area is expanded excessively, as shown in Figure 5, the surface tension on the back surface of the weld bead supporting the molten metal cannot withstand gravity, causing the molten metal to sag downwards and resulting in holes due to burn-through. To prevent such holes due to burn-through, it has been found that it is desirable to minimize the melting area, especially the bead width on the upper surface of the weld bead, while increasing the penetration depth.

[0041] Further investigation revealed that reducing the Si content in the weld metal and welding wire composition is effective in obtaining a weld with a narrow bead width and a large penetration depth. The following describes the effects and actions of Si on the shape of welded joints, particularly the weld bead.

[0042] (Effect 1: Enhanced penetration due to arc pressure) In arc welding, arc pressure is one of the factors influencing the penetration depth. Arc pressure has the effect of pushing down the molten metal and is proportional to the welding current value. In other words, increasing the welding current value allows the downward pushing effect of arc pressure on the molten metal to be maximized. However, if the Si content of the welding wire increases, the electrical resistance of the wire itself increases, which causes the welding current value to decrease.

[0043] Therefore, in order to obtain a bead with a large penetration depth due to the downward pushing effect of arc pressure, it is desirable to use a welding wire with a Si content of 0.30% or less. There are no special restrictions on the lower limit of the Si content of the welding wire, but from the perspective of cost as an industrial product, it is preferable to have more than 0%. It is more preferable that the lower limit of the Si content of the welding wire be 0.01% or more, even more preferably 0.02% or more, and even more preferably 0.10% or more.

[0044] Furthermore, for the same reasons as above, it is preferable to reduce the Si content of the resulting weld metal. To obtain a bead with a large penetration depth, the Si content of the weld metal should be 0.70% or less. Preferably, it should be 0.60% or less, and more preferably 0.50% or less. A lower Si content in the weld metal is preferable, and there is no lower limit. The Si content of the weld metal may be greater than 0%.

[0045] (Effect 2: Flow of molten metal due to surface tension) Another factor affecting the penetration depth in arc welding is the convection phenomenon of the molten metal, which is based on the distribution of surface tension in the molten metal. In particular, the difference in surface tension between the steel plate side (near the steel plate) and the center of the molten metal significantly influences the convection phenomenon in the surface tension distribution on the back side of the molten metal.

[0046] Figure 6 is a schematic cross-sectional view of a lap fillet weld joint to illustrate the convection phenomenon of molten metal. When the surface tension near the steel plate is higher than that in the center of the molten metal, a flow of weld metal occurs from the inside to the outside in the width direction of the molten metal, as shown by the arrows in Figure 6. As a result, the high-temperature molten metal heated by the arc plasma is transported in large quantities to the back side of the weld metal. In other words, by making the surface tension near the steel plate (see area E in Figure 6) relatively higher than that in the center of the molten metal (see area C in Figure 6), a digging effect due to surface tension flow is obtained, resulting in a bead shape with a large penetration depth.

[0047] Here, the surface tension of the molten metal depends on the Si content of the molten metal. Specifically, if the Si content of the molten metal is low, the deoxidation effect of the weld metal decreases, resulting in lower surface tension. Generally, the composition of molten metal during welding varies depending on the region. The majority of the weld metal is a well-mixed mixture of molten steel plate and welding wire, resulting in a generally uniform composition. However, in the region of the weld metal closest to the steel plate (see region E in Figure 6), the molten steel plate and welding wire are not sufficiently mixed, resulting in a higher proportion of steel plate components. Therefore, by making the average Si content of the molten metal, i.e., the Si content of the weld metal, lower than the Si content of the steel plate, the surface tension near the steel plate can be made relatively higher than in the center of the molten metal. As a result, the molten metal, heated to a high temperature, can flow toward the back side, and a bead shape with greater penetration depth can be obtained.

[0048] The method for measuring the Si content of weld metal will be explained below. In this embodiment, the "Si content of the weld metal" refers to the average Si content near the center of the weld metal. Therefore, in this embodiment, first, the lap fillet weld joint is cut out by machining so that it includes the weld metal portion and the surrounding base metal portion, and the weld metal region is identified in advance by visually observing the cross section perpendicular to the longitudinal direction of the weld metal portion. Then, the weld metal chips are collected by cutting that region with a drill or the like, and these chips are used as a sample for measurement by emission spectroscopy using inductively coupled plasma (ICP). In this way, the average Si content near the center of the weld metal, i.e., the "Si content of the weld metal," is measured.

[0049] <1.2. Overlap fillet weld joints> Based on the inventors' findings described above, the lap fillet welded joint according to this embodiment is as follows.

[0050] Figure 7 is a schematic cross-sectional view of the lap fillet welded joint 10 according to this embodiment. The overlap fillet weld joint 10 according to this embodiment comprises a first Zn-plated steel sheet 11, a second Zn-plated steel sheet 12 positioned above the first surface 11a of the first Zn-plated steel sheet 11, and a weld metal 13 that joins the end of the second Zn-plated steel sheet 12 to the first surface 11a of the first Zn-plated steel sheet 11. The second Zn-plated steel sheet 12 is overlapped on the first surface 11a of the first Zn-plated steel sheet 11 in the "arrangement step" described later, so as to cover a part of the first surface 11a.

[0051] <1.2.1 Shape of overlapping fillet welded joints> The lap fillet welded joint according to this embodiment may be a lap fillet welded joint 10 as illustrated in Figure 7. In the case of the lap fillet welded joint 10, the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 are approximately parallel to each other. That is, the second Zn-plated steel sheet 12 is positioned so as to be approximately parallel to the surface of the first Zn-plated steel sheet 11.

[0052] Of the two surfaces of the first Zn-plated steel sheet 11, the surface that is welded to the edge of the second Zn-plated steel sheet 12 is referred to as the first surface 11a, and the surface that is not welded is referred to as the second surface.

[0053] The lap fillet weld joint 10 has a weld metal (weld bead) 13 that joins the end of the second Zn-plated steel sheet 12 and the first surface 11a of the first Zn-plated steel sheet 11. The weld metal (weld bead) is the metal that melts and solidifies during welding. Furthermore, the lap fillet weld joint 10 has a HAZ formed around the weld metal 13. The HAZ (Heat Affected Zone) is the part that does not melt during welding, but whose structure, metallurgical properties, and mechanical properties are changed by the welding heat. Hereinafter, the weld metal and HAZ may be collectively referred to as the "welded area".

[0054] <1.2.2 First-generation Zn-plated steel sheet and second-generation Zn-plated steel sheet> In the lap fillet welded joint 10 of this embodiment, both the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 are Zn-plated steel sheets. Examples of Zn-plated steel sheets include Zn-Ni plated steel sheets, Zn-Al plated steel sheets, Zn-Mg plated steel sheets, and Zn-Mg-Al plated steel sheets. In this embodiment, the lap fillet welded joint 10 is shown as an example in which the first Zn-plated steel sheet 11 is used as the lower plate and the second Zn-plated steel sheet 12 is used as the upper plate, but the welded joint of the present invention is not limited to this combination. In other words, only one of the upper or lower plates may be a "Zn-plated steel sheet". In this case, the other steel sheet may remain as the base steel sheet described later, without plating. Furthermore, the plating layer on the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 only needs to be provided on the overlapping surfaces of the steel sheets, and does not necessarily need to be formed on both sides of each steel sheet. Furthermore, the plating layer only needs to be provided on at least one side of the overlapping surface, either the first Zn-plated steel sheet 11 or the second Zn-plated steel sheet 12.

[0055] The type of base steel sheet used for the plated steel sheet is not particularly limited, but it is preferable to use a high-strength steel sheet with a tensile strength of 780 MPa or higher. This makes it possible to improve the strength of the machine parts to which the lap fillet weld joint according to this embodiment is applied.

[0056] The Si content of the base steel sheets of the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 is preferably 0.2 to 1.4% by mass. If the Si content of the base steel sheet is less than 0.2%, the difference in Si content between the weld metal and the base steel sheet is small, and the desirable convection effect of surface tension described above cannot be sufficiently obtained. By setting the Si content of the base steel sheet to 0.2% or more, the convection effect on the back surface of the weld is promoted and a deep penetration depth can be obtained. On the other hand, if the Si content exceeds 1.4%, the increase in slag in the weld becomes significant, which may adversely affect the electrodeposition coating properties of the weld. The Si content of the base steel sheet can be measured using emission spectroscopy with inductively coupled plasma (ICP) as a sample of cuttings from the base steel sheet.

[0057] As explained above (Function 2), in order to obtain a bead shape with a large penetration depth, it is effective to make the Si content of the molten metal relatively lower than the Si content of the steel sheet, thereby obtaining a flow of the molten metal, heated to a high temperature, toward the back side. From this viewpoint, it is preferable that the ratio of the Si content of the weld metal to the Si content of the first Zn-plated steel sheet 11 is less than 1, and the ratio of the Si content of the weld metal to the Si content of the second Zn-plated steel sheet 12 is less than 1.

[0058] There are no particular limitations on the type of high-strength steel sheet. Examples of high-strength steel sheets include DP steel sheets, TRIP steel sheets, composite structure steel sheets, martensitic steel sheets, and hot-stamped steel sheets. The greater the tensile strength of the steel plate, the lower the joint strength in a conventional welded joint. Therefore, the greater the tensile strength of the steel plate, the more superior the effect of the lap fillet welded joint according to this embodiment becomes compared to a conventional welded joint. The tensile strength of the high-strength steel plate is preferably 780 MPa or higher, more preferably 980 MPa or higher, and even more preferably 1300 MPa or higher, or 1700 MPa or higher. The high-strength steel plate may be cold-rolled or hot-rolled. The tensile strength of steel plates can be measured by a tensile test in accordance with the JIS Z 2241:2022 standard. If it is not possible to obtain a standard test specimen as defined in the standard due to the dimensions of the steel plate, the value measured using a proportional test specimen or a test specimen of appropriate dimensions as defined in the standard may be used. In this embodiment, the tensile strength of the high-strength steel sheet, which is the base steel sheet, and the tensile strength of the Zn-plated steel sheet may be considered to be the same.

[0059] Thickness t of the first Zn-plated steel sheet 11 HThe thickness is within the range of 1.0 to 3.4 mm. Similarly, the thickness of the second Zn-plated steel sheet may also be within the range of 1.0 to 3.4 mm. Overlap fillet welded joints using Zn-plated steel sheets having such thicknesses can be suitably applied to automotive parts and machine parts. Furthermore, the thicknesses of the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 constituting the overlap fillet welded joint 10 may be different. For example, the thickness of the second Zn-plated steel sheet 12 may be twice or less the thickness of the first Zn-plated steel sheet 11, and the thickness of the first Zn-plated steel sheet 11 may be twice or less the thickness of the second Zn-plated steel sheet 12.

[0060] <1.2.3 Weld Metal> The overlap fillet weld joint 10 has a weld metal (weld bead) 13 that joins the end face of the upper plate, the second Zn-plated steel sheet 12, and the first surface 11a of the lower plate, the first Zn-plated steel sheet 11. Furthermore, the surface of the weld metal 13 is exposed at the end face of the first Zn-plated steel sheet 11. In other words, the bead on the lower side of the weld metal 13 is exposed on the lower side of the second Zn-plated steel sheet 12. In Figure 7, the bead on the lower side of the weld metal 13 is formed to cover a portion of the end face of the first Zn-plated steel sheet 11, but it may also be formed over the entire end face of the first Zn-plated steel sheet 11. The chemical composition of the weld metal 13 will be described later.

[0061] <1.2.4 Weld Bead Width> In this embodiment, the upper limit value WBU (mm) of the bead width on the lower side of the weld metal 13 is: Plate thickness t H If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (1) is satisfied. Plate thickness t H If the length is between 2.0 mm and 3.4 mm, then the following equation (2) is satisfied. Furthermore, the lower limit value WBL (mm) of the bead width on the lower side of the weld metal 13 is: Plate thickness t H If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (3) is satisfied. Plate thickness t HIf the length is between 2.0 mm and 3.4 mm, then the following formula (4) is satisfied.

[0062] WBU≦t H +0.8mm ···(1) WBU ≤ 0.86t H +1.09 ···(2)

[0063] WBL≧0.5t H +0.5 ···(3) WBL≧0.21t H +1.07 ···(4)

[0064] As described above, the bead width is optimized for each plate thickness to be within a predetermined range, in order to suppress welding defects such as holes and burn-through that occur in the weld, and porosity defects such as blowholes. Specifically, the upper limit of the bead width on the underside (back side) of the weld metal 13 satisfies the following equations (1) and (2). If the bead width on the underside (back side) exceeds these upper limits, excessive heat input during welding may occur, potentially causing welding defects such as holes and burn-through in the weld. On the other hand, the lower limit of the back side bead width satisfies the following equations (3) and (4). If the back side bead width does not satisfy equations (3) and (4), porosity defects such as blowholes may occur.

[0065] Furthermore, the upper limit WSU of the bead width on the upper surface of the weld metal 13 is, Plate thickness t H If the size is 1.0 mm or more and less than 1.6 mm, then the following formula (5) is satisfied. Plate thickness t H If the length is 1.6 mm or more and less than 2.9 mm, then the following formula (6) is satisfied. Plate thickness t H If the length is between 2.9 mm and 3.4 mm, then the following formula (7) is satisfied.

[0066] Furthermore, the lower limit of the bead width (WSL) on the upper surface of the weld metal is: Plate thickness t H If it is 1.0 mm or more and less than 1.6 mm, then the following formula (8) is satisfied. Plate thickness t HIf the length is 1.6 mm or more and less than 2.9 mm, then the following formula (9) is satisfied. Plate thickness t H If the length is between 2.9 mm and 3.4 mm, then the following formula (10) is satisfied.

[0067] WSU ≤ 5.0 ···(5) WSU ≤ 3.07t H +0.08 ···(6) WSU ≤ 9.0 ···(7)

[0068] WSL≧3.0mm ···(8) WSL ≥ 1.54t H +0.54 ···(9) WSL ≥ 5.0 ···(10)

[0069] As described above, in order to suppress welding defects such as holes and burn-through that occur in the welded area, as well as porosity defects such as blowholes, and to ensure the continuity of the bead shape, it is preferable to optimize the bead width for each plate thickness so that it falls within a predetermined range, similar to the specification of the bead width on the lower surface side.

[0070] Specifically, the upper limit of the bead width on the top (front) side is shown by equations (5) to (7) below. Exceeding these upper limits results in excessive heat input during welding, which can cause welding defects such as holes or burn-through in the weld. On the other hand, the lower limit of the bead width on the top (front) side is shown by equations (8) to (10) below, depending on the plate thickness t. H Regardless of the above, if the bead width is less than 3.5 mm, insufficient heat input during welding may result in insufficient penetration, causing the bead to have a discontinuous shape. Furthermore, if the bead width does not satisfy equations (8) to (10), porosity defects such as blowholes may occur.

[0071] Here, the "bead width on the top side (front side)" and the "bead width on the bottom side (back side)" in this embodiment will be explained.

[0072] "The bead width on the top (front) side" refers to the width of the weld metal (bead) 13 in the direction parallel to the surface of the base steel plate when the weld is viewed in cross-section along the width direction of the weld metal (bead) 13. In other words, as shown in Figure 7, the "bead width on the top (front) side" is the distance between the two ends of the weld metal 13 in the width direction. To put it another way, as shown in Figure 7, the "bead width on the top (front) side" is the distance between the two ends in the direction parallel to the surface of the base steel plate when the weld metal 13 is viewed in cross-section along the width direction of the base steel plate.

[0073] "Bead width on the underside (back side)" refers to the width of the weld metal perpendicular to the surface of the base steel plate when the weld is viewed in cross-section along the width direction of the weld metal (bead) 13. In other words, as shown in Figure 7, "bead width on the underside (back side)" is the distance in the thickness direction of the exposed weld metal at the end face of the lower plate, the first Zn-plated steel plate 11. To put it another way, as shown in Figure 7, "bead width on the underside (back side)" is the distance along the thickness direction from the underside of the upper plate, the second Zn-plated steel plate 12, to the lower end of the back side bead in the thickness direction.

[0074] The "top (front) bead width" is determined by measuring the width of the weld metal on the front surface of the bead from a cross-sectional photograph of the weld, and the "bottom (back) bead width" is determined by measuring the width of the weld metal on the back surface of the bead. When observing the cross-section of the weld, the overlap fillet weld joint is cut perpendicular to the direction of extension of the weld metal (weld bead). Then, the cut surface is polished and etched to reveal the weld metal on the cut surface. Since the bead width may vary depending on the welding position, in this embodiment, the average value of three cross-sections is used for confirmation.

[0075] <1.2.5 Chemical composition of weld metal> Next, the chemical composition of the weld metal will be described. In this embodiment, "weld metal" refers to the metal formed when the base material (first Zn-plated steel sheet and second Zn-plated steel sheet) and the welding wire melt and mix together. Furthermore, the chemical composition of the weld metal will be expressed as a mass % relative to the total mass of the weld metal, and the description of this mass % will simply be indicated as %.

[0076] Furthermore, the composition of the weld metal in lap fillet welded joints can be adjusted by the steel plate components and welding wire components.

[0077] The chemical composition of weld metal can be measured by emission spectroscopy using inductively coupled plasma (ICP). Specifically, (1) the weld metal region is identified in advance by visually observing a cross section perpendicular to the longitudinal direction in the longitudinal center of the weld, (2) weld metal chips are collected by drilling that region, and (3) these chips are used as a sample and measured by emission spectroscopy using inductively coupled plasma (ICP).

[0078] [C: 0.06~0.25%] Carbon (C) has the effect of stabilizing the arc and atomizing molten droplets. If the C content is less than 0.06%, the molten droplets become larger, the arc becomes unstable, and the amount of spatter increases. As a result, the bead shape becomes uneven and defective, leading to the formation of red rust. The reason why red rust occurs due to a defective bead shape is that the depressions caused by the defect are prone to the generation of welding slag, and water or mud containing moisture, which causes red rust, tends to accumulate there. Furthermore, if the C content is less than 0.06%, the tensile strength of the weld metal cannot be obtained, and the desired tensile strength cannot be achieved. Therefore, the lower limit of the C content is 0.06% or more, and preferably 0.08% or more. On the other hand, if the carbon content exceeds 0.25%, the weld metal hardens, reducing its crack resistance and making it more prone to fracture. Therefore, the upper limit of the carbon content is 0.25% or less, preferably 0.20% or less, and more preferably 0.15% or less.

[0079] [Si: more than 0%, 0.70% or less] Si is contained in the welding wire or base metal as a deoxidizing element. In particular, Si in the welding wire improves the tensile strength of the weld metal by promoting deoxidation of the molten pool. However, as explained above in (Effect 1) and (Effect 2), in order to obtain a bead with a large penetration depth due to the downward pushing effect of the arc pressure, it is effective to reduce the amount of Si in the welding wire used and to suppress the Si content of the resulting weld metal. Furthermore, if the weld metal contains an excess of Si, the amount of non-conductive Si-based slag increases, and red rust may occur between the slag and the weld metal. Therefore, the upper limit of the Si content is 0.70% or less, preferably 0.60% or less, and more preferably 0.50% or less. The lower limit of the Si content is not particularly limited, but it may be greater than 0%, and may be 0.02% or more.

[0080] [Mn: 1.4~2.3%] Like Si, Mn is a deoxidizing element that promotes deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal. If the Mn content is too low, the tensile strength of the weld metal cannot be sufficiently ensured, and the weld metal becomes prone to fracture. Therefore, the lower limit of Mn is 1.4% or more, and preferably 1.8% or more. On the other hand, if Mn is present in excess, the viscosity of the molten metal increases, and at high welding speeds, the molten metal cannot flow properly into the weld area, resulting in a humping bead and a tendency for poor bead shape to occur. As a result, the bead shape becomes uneven and defective, leading to the formation of red rust. Therefore, the upper limit of the Mn content is 2.3% or less, preferably 2.1% or less.

[0081] [Ti: 0.04~0.15%] Since Ti is a deoxidizing element, it is effective in suppressing the occurrence of blowholes. Furthermore, Ti is an effective element for ensuring the conductivity of welding slag and is also an effective element for improving electrodeposition coating properties. Therefore, the lower limit of the Ti content is 0.04% or more, preferably 0.05% or more, and more preferably 0.06% or more. Even more preferably, the Ti content is 0.07% or more. On the other hand, if the weld contains an excessive amount of Ti, the amount of Ti-based slag increases, reducing the adhesion between the Ti-based slag and the weld metal, making it more prone to delamination. As a result, red rust is more likely to occur in the delaminate areas. Therefore, the upper limit of the Ti content is 0.15% or less, preferably 0.14% or less, and more preferably 0.13% or less.

[0082] [Al: 0.001~0.20%] Al is a powerful deoxidizing element that promotes the deoxidation of molten metal during arc welding, thereby suppressing the occurrence of blowholes. Furthermore, the presence of a small amount of Al reduces Si-based slag, which can negatively affect electrodeposition coating of the welded area. Therefore, the lower limit of the Al content is 0.001% or higher, preferably 0.005% or higher, and more preferably 0.010% or higher. On the other hand, if the Al content is excessive, the amount of non-conductive Al-based slag increases, making it easier for red rust to form between the slag and the weld metal. Therefore, the upper limit of the Al content of the weld metal is 0.20% or less, preferably 0.18% or less, and more preferably 0.15% or less.

[0083] [Si, Mn, Ti, Al] Furthermore, it is preferable that the content of Si, Mn, Ti, and Al satisfies the following formula (11). As mentioned above, increasing the Si content reduces electrodeposition coating properties, but increasing the Ti and Al content leads to improved electrodeposition coating properties. Regarding Mn, a single oxide of Mn does not affect electrodeposition coating properties, but a composite oxide of Si and Mn has the effect of reducing electrodeposition coating properties. Therefore, it is not desirable to include an excessive amount of Mn.

[0084] The inventors investigated the occurrence of red rust between slag and weld metal in weld metals with various composition systems. As a result, it was found that when the value of 7×[Si]+7×[Mn]-112×[Ti]-30×[Al], an index for the occurrence of red rust, exceeds 12.0, red rust occurs prematurely, and corrosion resistance deteriorates. Therefore, it is preferable that the chemical composition of the weld metal in this embodiment satisfies formula (11).

[0085] 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0...Formula (11)

[0086] [N: 0.003~0.015%] In addition to the amount of nitrogen (N) present in steel plates and welding wires, its content increases due to contamination from the atmosphere during welding. Since N is an element that reduces the toughness of the weld metal, it is desirable to keep the upper limit of the N content below 0.015%. There is no particular limit to the lower limit of the N content, and it may be greater than 0%, but it may be set to 0.003% or higher, which is the content of standard steel plates and welding wires. The N content may also be 0.005% or higher.

[0087] [O: 0.01~0.06%] Like nitrogen, the amount of oxygen (O) present in steel plates and welding wires increases due to contamination from the atmosphere during welding. Excessive oxygen reduces the toughness of the weld metal. Therefore, it is desirable to keep the upper limit of the oxygen content below 0.06%. There is no particular lower limit for the oxygen content. In standard arc welding, oxygen often exceeds 0.01%, so a lower limit of 0.01% is acceptable. The oxygen content may also be 0.02% or higher.

[0088] [P: more than 0%, less than 0.015%] P is an element that is generally present as an impurity in steel, and is also typically found as an impurity in steel plates and welding wires, and therefore is also present in the weld metal. Here, since P is one of the main elements that cause hot cracking in weld metal, it is desirable to suppress it as much as possible. If the P content exceeds 0.015%, hot cracking of the weld metal becomes significant, so the upper limit for the P content in the weld metal is 0.015% or less. Furthermore, there is no particular limit to the phosphorus content, so it is greater than 0%, but from the viewpoint of cost and productivity of phosphorus removal, it may be as low as 0.001%.

[0089] [S: more than 0%, less than 0.013%] S, like P, is an element that is generally present as an impurity in steel, and is also usually found as an impurity in welding wire, and therefore is also present in the weld metal. Here, S is an element that inhibits the crack resistance of the weld metal, and it is preferable to suppress it as much as possible. If the S content exceeds 0.013%, the crack resistance of the weld metal deteriorates, so the S content of the weld metal is 0.013% or less. Furthermore, there is no particular limit to the sulfur content, so it is greater than 0%, but from the viewpoint of the cost and productivity of sulfur removal, it may be 0.001%.

[0090] Cu, Cr, Nb, V, Mo, Ni, Sb, Sn, and B are not essential elements, but one or more of them may be included simultaneously as needed. The effects obtained by including each element and the upper limit will be explained. The lower limit when these elements are not included is 0%.

[0091] [Cu: 0~0.50%] Although not essential, copper (Cu) may be present in the weld metal due to the copper plating on the welding wire, and therefore may be included at a concentration of 0.005% or more. On the other hand, excessive Cu content can increase the likelihood of weld cracking, so the upper limit for Cu content is 0.50% or less.

[0092] [Cr: 0-2.0%] While chromium (Cr) is not essential for improving the hardenability and tensile strength of the weld, it may be included in amounts of 0.05% or more. On the other hand, excessive Cr content reduces the elongation of the weld. Therefore, the upper limit for Cr content is 2.0% or less.

[0093] [Nb: 0~0.3%] While not essential, Nb may be included in amounts of 0.005% or more to improve the hardenability and tensile strength of the weld. On the other hand, excessive Nb content reduces the elongation of the weld. Therefore, the upper limit for Nb content is 0.3% or less.

[0094] [V: 0~0.3%] V is not essential for improving the hardenability of the weld and increasing its tensile strength, but it may be included in amounts of 0.005% or more. On the other hand, if V is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the V content is 0.3% or less.

[0095] [Mo: 0~1.0%] Mo (Mo) is not essential for improving the hardenability and tensile strength of the weld, but it may be included in amounts of 0.005% or more. On the other hand, if Mo is included in excess, the elongation of the weld will decrease. Therefore, the upper limit for Mo content is 1.0% or less.

[0096] [Ni: 0-2.5%] Ni may be included in amounts of 0.05% or more, although it is not essential, to improve the tensile strength and elongation of the weld. On the other hand, excessive Ni content makes welding cracks more likely. Therefore, the upper limit of Ni content is 2.5% or less, preferably 2.0% or less.

[0097] [B: 0~0.0100%] Although not essential, B may be included in amounts of 0.0005% or more to improve the hardenability of the weld and enhance its tensile strength. On the other hand, if B is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the B content is 0.0100% or less. Preferably, it is 0.0030% or less.

[0098] [Sb: 0~0.10%] Sb has the effect of generating convection in the molten metal, causing the slag to gather in the center of the weld bead. This can further improve the electrodeposition coating properties. To obtain this effect, although not essential, it is preferable to have an Sb content of 0.01% or more, 0.02% or more, or 0.04% or more. On the other hand, if the Sb content exceeds 0.10%, solidification cracking may occur in the weld metal. Therefore, the upper limit of the Sb content should be 0.10% or less. Preferably, the upper limit of the Sb content is 0.05% or less.

[0099] [Sn: 0~0.4%] Sn is an element that improves the corrosion resistance of the weld metal itself. While Sn content is not essential in this embodiment because it does not significantly affect electrodeposition coating properties, improving the corrosion resistance of the weld metal itself is advantageous for machine parts. To obtain the corrosion-reducing effect of Sn, the Sn content may be 0.02% or higher. On the other hand, if the Sn content exceeds 0.4%, the crack susceptibility of the weld metal increases, making it more prone to hot cracking. Furthermore, excessive Sn can lead to segregation at the grain boundaries of the weld metal, resulting in a decrease in toughness. Therefore, the upper limit of the Sn content should be 0.4% or less.

[0100] The remainder of the components described above consists of Fe and impurities. Impurities refer to components contained in the raw materials or components introduced during the manufacturing process, which are not components intentionally included in the weld metal, or components that are acceptable as long as they do not hinder the effects of this embodiment.

[0101] <1.3. Method for manufacturing lap fillet welded joints> Next, a preferred manufacturing method for the lap fillet welded joint according to this embodiment will be described. The method for manufacturing an overlap fillet welded joint according to this embodiment is a method for manufacturing an overlap fillet welded joint having two Zn-plated steel sheets and a weld metal for joining the Zn-plated steel sheets together, comprising the steps of: arranging the Zn-plated steel sheets so that the welding surfaces of the Zn-plated steel sheets are not separated by excessive gaps (arrangement step); and welding the Zn-plated steel sheets together using a welding wire (welding step).

[0102] (Placement process) In the arrangement process, the welded surfaces of the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 are placed facing each other. At this time, it is preferable to place the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 without leaving an excessive gap between them. Specifically, for example, if the gap between the steel sheets exceeds 0.5 mm, it may promote hole formation due to burn-through. However, due to the dimensional accuracy of the steel sheets, it is difficult to minimize the variation in the gap. In this embodiment, a gap of 0.5 mm or less is acceptable. More preferably, the gap is 0.2 mm or less. Note that the gap between the welded surfaces of the steel sheets may be 0 mm. Furthermore, the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 are arranged overlapping each other, but it is preferable not to make the overlap excessively long. Specifically, if the overlap is 4 mm or more, there is a risk that the melting on the back surface of the weld will be insufficient, leading to porosity defects. Furthermore, if the overlap is less than 1 mm, it becomes difficult to maintain a stable overlap state due to the dimensional accuracy of the steel plate, which may exacerbate hole formation due to melt-through.

[0103] (Welding process) The two Zn-plated steel sheets described above are preferably joined by gas-shielded arc welding. Gas-shielded arc welding is an arc welding method that uses a shielding gas, such as in consumable electrode gas-shielded arc welding. In gas-shielded arc welding, the shielding gas shields the molten metal from the atmosphere.

[0104] The shielding gas used is preferably a gas mainly composed of Ar. More preferably, it is a gas mainly composed of Ar and containing 5% to 20% CO2 by volume.

[0105] While there are no particular limitations on welding conditions, for example, welding current: 150-250A, welding voltage: 20-25V, welding speed: 60-80cm / min may be used, and appropriate welding conditions may be set according to the plate thickness. Also, for example, the lower plate may have a plate thickness t H When using steel plates thicker than 1.6 mm, apply the pulsed MAG welding mode, and the plate thickness t HWhen using steel plates with a thickness of 1.6 mm or less, it is recommended to apply a wire feed control type low heat input welding mode.

[0106] <Welding wire> The welding wire used is, for example, a solid wire for gas shielded arc welding. From the viewpoint of ensuring sufficient penetration depth of the weld metal and improving the electroplatable properties of the weld, it is desirable to set the amount of Si added to a low level. In addition, it is preferable to add Ti to impart conductivity to the welding slag in order to improve the electroplatable properties. Furthermore, upper and lower limits for each component are set as appropriate from the viewpoint of ensuring the strength of the weld metal and preventing cracking.

[0107] The composition of the welding wire is expressed as a mass percentage of the total mass. C: 0.04~0.12%, Si: more than 0%, less than 0.30%, Mn: 1.40~2.30%, Ti: 0.04~0.25%, Al: 0.001~0.050%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.015%, N: more than 0%, less than 0.01%, O: more than 0%, less than 0.01%, Cr: 0-3%, Ni: 0-3%, Mo: 0~0.5%, B: 0~0.0100%, Cu: 0~0.50%, Nb: 0~0.3%, V: 0~0.5%, Sb: 0~0.10% and Sn: 0~0.4% The remainder consists of iron and impurities.

[0108] Although a preferred manufacturing method for producing the lap fillet welded joint of this embodiment has been described above, other methods are not particularly limited and may be set and adjusted as appropriate within a range that does not hinder the operation and effect of the lap fillet welded joint of the present invention.

[0109] (2. T-shaped fillet weld joint) The following describes a T-fillet welded joint and a method for manufacturing the same according to one embodiment of the present invention. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range.

[0110] <2.1. Results of the inventors' studies> In a typical conventional T-fillet weld, as shown in Figure 8, first, the edge of one steel plate is abutted perpendicular to the surface of the other steel plate, which is positioned vertically, and the weld is performed so that the edge of the first steel plate is not penetrated in the thickness direction by the weld. Here, if at least one of the two steel plates, the vertically positioned steel plate, is a zinc-plated steel plate, zinc vapor generated from the contact area of ​​the two steel plates during welding cannot be discharged from the weld, and blowholes occur as zinc vapor mixes into the molten metal. In other words, in order to avoid blowholes caused by zinc vapor, it is important to secure a way for the zinc vapor to escape. Therefore, the inventors investigated the effect of melting and welding the edge of the horizontally positioned steel plate. Note that "edge of the steel plate" refers to the peripheral region including the edge face of the steel plate.

[0111] <2.1.1 Regarding the bead shape> First, to investigate the relationship between the bead shape of the weld and the porosity defects of the blowhole, the following experiment was conducted.

[0112] First, two Zn-plated steel sheets (test sheets) with a thickness of 1.0 to 3.4 mm and Zn-plated on both sides were prepared. As shown in Figure 8, the other test sheet (butt plate) 202 was placed perpendicularly to the surface of one test sheet 201, and a T-fillet weld was performed to create a T-fillet welded joint 200. When welding, as shown in Figure 9, the two test sheets were positioned so that the gap G between them was 0 to 0.7 mm. The T-fillet joint was assembled using a combination of the same steel type and thickness. In this experiment, the gap G was kept within the range of 0 to 0.7 mm, but the gap G may be 0 to 0.5 mm or 0 to 0.4 mm.

[0113] The chemical compositions of the two Zn-plated steel sheets (test sheets) and the welding wire used in the investigation are shown in Table 1 above.

[0114] The base material of the tested steel plate was mild steel with a tensile strength of 440 MPa, and the amount of zinc plating adhering to one side of the tested steel plate was 45 g / m². 2 The welding wire used had a relatively low Si content to reduce the amount of slag adhering to the weld bead. A solid wire with a diameter of 1.2 mm was used as the welding wire.

[0115] The welding conditions were set as follows: shielding gas: Ar + 5-20% CO2, welding current: 150-250 A, welding voltage: 20-25 V, welding speed: 60-80 cm / min, with appropriate welding conditions set according to the plate thickness. A Fronius welding power supply was used. The plate thickness of the test steel plate 202, which is the butt plate, was t H Regarding the plate thickness t, H When using test steel plate 202 with a thickness exceeding 1.6 mm, apply the pulse MAG welding mode, and the plate thickness t H When using test steel plates 202 with a thickness of 1.6 mm or less, welding was performed using a wire feed control type low heat input welding mode.

[0116] Figures 10 and 11 show the welding results. Figure 10 is a graph showing the appropriate range for the bead width on the top side (front side bead width), with respect to the thickness t of the test steel plate 202. HFigure 11 shows the relationship between the thickness (mm) and the upper and lower limits of the bead width (mm) on the top surface. Figure 11 is a graph showing the appropriate range of the bead width on the bottom surface (back side bead width), with the plate thickness t of the test steel plate 202. H This shows the relationship between the upper and lower limits of the bead width on the underside.

[0117] As shown in Figure 10, the thickness t of the test steel plate 202 H It was found that there is an optimal range for the upper limit (WSU) and lower limit (WSL) of the bead width on the upper side for each case. Specifically, the upper limit of the bead width on the upper side is shown by equations (16) to (18) below. Exceeding these upper limits resulted in excessive heat input during welding, causing welding defects such as holes and burn-through in the weld. On the other hand, the lower limit of the bead width on the upper side is shown by equations (19) and (20) below. Regardless of the steel plate thickness, if the bead width was less than 3.5 mm, insufficient heat input resulted in insufficient penetration, and the bead became discontinuous in shape. Furthermore, when the bead width did not satisfy equations (19) and (20), porosity defects such as blowholes occurred frequently.

[0118] [Upper limit of bead width on the top surface] <Plate thickness t H If it is 1.0 mm or more and less than 2.0 mm > WSU ≤ 5.0 mm ···(16) <Plate thickness t H If it is 2.0 mm or more and less than 2.9 mm > WSU ≤ 2.44t H +0.11 ···(17) <Plate thickness t H If it is between 2.9mm and 3.4mm > WSU ≤ 7.2 mm ···(18)

[0119] [Lower limit of bead width on the top side] <Plate thickness t H If it is 1.0 mm or more and less than 2.9 mm > WSL ≥ 3.5mm ···(19) <Plate thickness t H If it is between 2.9mm and 3.4mm > WSL≧1.16t H +0.14 ···(20)

[0120] Also, regarding the upper and lower limits of the bead width on the lower surface side, as shown in Fig. 11, for each plate thickness t of the test steel plate 202 H it was found that there is an optimal range. Specifically, the upper limit of the bead width on the lower surface side is represented by the following formulas (12) to (14). When exceeding these upper limit values, similar to the upper limit of the bead width on the upper surface side, excessive welding heat is generated, and welding defects such as holes and melting away occur in the welded part. In addition, when the bead width on the lower surface side is larger than that on the upper surface side, welding defects such as holes and melting away may occur. Therefore, it is important that the upper limit of the bead width on the lower surface side satisfies the following formulas (12) to (14) and is smaller than the bead width on the upper surface side. On the other hand, the lower limit of the bead width on the lower surface side is represented by the following formula (15), and for all plate thicknesses t H even when the bead width is less than 0.7 mm, pore defects such as blowholes frequently occurred.

[0121] [Upper limit of bead width on lower surface side] <When plate thickness t H is less than 1.0 to 2.0 mm> WBU ≤ 2.8 mm ···(12) [[ID=2l]]<When plate thickness t H is less than 2.0 to 2.9 mm> WBU ≤ 1.33t + 0.13 ···(13) <When plate thickness t H is 2.9 to 3.4 mm> WBU ≤ 4.0 mm ···(14)

[0122] [Lower limit of bead width on lower surface side] <When plate thickness t H is 1.0 to 3.4 mm> WBL ≥ 0.7 mm ···(15)

[0123] <2.1.2 Regarding the composition of the weld metal> Next, the relationship between the suppression of pore defects and welding defects and the composition of the weld metal was examined.

[0124] Conventionally, methods have been known to ensure a passage for zinc vapor to escape from the weld metal in order to suppress porosity defects such as blowholes caused by zinc vapor. In addition, creating a gap between steel plates is effective in promoting the escape of zinc vapor by melting the back surface of the weld bead and exposing that back surface.

[0125] However, when applying a T-shaped weld joint to a curved automotive part, for example, it is difficult to maintain a consistent gap between the joined steel plates, and excessively large gaps can exacerbate hole formation due to burn-through. This is especially true for parts with complex curved shapes, where maintaining a consistent gap becomes more difficult, leading to more pronounced burn-through and hole formation. In addition, if there is a gap between the T-shaped joined steel plates, spatter will scatter from this gap towards the back of the weld bead, resulting in spatter adhering to a wide area of ​​the part and causing a poor appearance. Furthermore, if a large amount of spatter scatters and adheres to the surface of the part, it requires a great deal of labor to remove it, which is also undesirable from an economic standpoint.

[0126] Therefore, it is desirable to achieve stable melting on the back surface of the weld bead while keeping the steel plates in close contact with each other, without leaving any gaps between them, thereby allowing zinc vapor to be released from the weld metal.

[0127] Therefore, we investigated a method for stably melting the back side of the weld bead while the steel plates are in close contact with each other in a T-shaped welded joint, thereby sufficiently exposing the weld metal. In this embodiment, "a state in which the steel plates are in close contact with each other" is not limited to a state in which the gap between the steel plates is 0 mm (i.e., a state in which the steel plates are in contact with each other), but also includes a state in which two steel plates are placed without leaving an excessive gap between them.

[0128] Specifically, to adequately release zinc vapor from the weld metal, it is necessary to increase the welding heat input to widen the melting area and secure a path for the zinc vapor. However, if the melting area is excessively widened, as shown in Figure 12, the surface tension on the back of the weld bead supporting the molten metal will not be able to withstand gravity, causing the molten metal to sag downwards and resulting in holes due to burn-through. To prevent such holes due to burn-through, it has been found that it is desirable to keep the melting area, especially the bead width on the upper side of the weld bead, to the minimum necessary while increasing the penetration depth.

[0129] Further investigation revealed that reducing the Si content in the weld metal and welding wire composition is effective in obtaining a weld with a narrow bead width and a large penetration depth. The following describes the effects and actions of Si on the shape of welded joints, particularly the weld bead.

[0130] (Effect 3: Enhanced penetration due to arc pressure) In arc welding, arc pressure is one of the factors influencing the penetration depth. Arc pressure has the effect of pushing down the molten metal and is proportional to the welding current value. In other words, increasing the welding current value allows the downward pushing effect of arc pressure on the molten metal to be maximized. However, if the Si content of the welding wire increases, the electrical resistance of the wire itself increases, which causes the welding current value to decrease.

[0131] Therefore, in order to obtain a bead with a large penetration depth due to the downward pushing effect of arc pressure, it is desirable to use a welding wire with a Si content of 0.30% or less. There are no special restrictions on the lower limit of the Si content of the welding wire, but from the perspective of cost as an industrial product, it is preferable to have more than 0%. It is more preferable that the lower limit of the Si content of the welding wire be 0.01% or more, even more preferably 0.02% or more, and even more preferably 0.10% or more.

[0132] Furthermore, for the same reasons as above, it is preferable to reduce the Si content of the resulting weld metal. To obtain a bead with a large penetration depth, the Si content of the weld metal should be 0.70% or less. Preferably, it should be 0.60% or less, and more preferably 0.50% or less. A lower Si content in the weld metal is preferable, and there is no lower limit. The Si content of the weld metal may be greater than 0%.

[0133] (Function 4: Flow of molten metal due to surface tension) Another factor affecting the penetration depth in arc welding is the convection phenomenon of the molten metal, which is based on the distribution of surface tension in the molten metal. In particular, the difference in surface tension between the steel plate side (near the steel plate) and the center of the molten metal significantly influences the convection phenomenon in the surface tension distribution on the back side of the molten metal.

[0134] Figure 13 is a schematic cross-sectional view of a T-fillet weld joint to illustrate the convection phenomenon of molten metal. When the surface tension near the steel plate is higher than that in the center of the molten metal, a flow of weld metal occurs from the inside to the outside in the width direction of the molten metal, as shown by the arrows in Figure 13. As a result, the high-temperature molten metal heated by the arc plasma is transported in large quantities to the back side of the weld metal. In other words, by making the surface tension near the steel plate (see area E in Figure 13) relatively higher than that in the center of the molten metal (see area C in Figure 13), a sculpting effect due to surface tension flow is obtained, resulting in a bead shape with a large penetration depth.

[0135] Here, the surface tension of the molten metal depends on the Si content of the molten metal. Specifically, if the Si content of the molten metal is low, the deoxidation effect of the weld metal decreases, resulting in lower surface tension. Generally, the composition of molten metal during welding varies depending on the region. The majority of the weld metal is a fairly uniform composition because the molten steel plate components and welding wire components are sufficiently mixed. However, in the region of the weld metal closest to the steel plate (see region E in Figure 13), the molten steel plate and welding wire are not sufficiently mixed, resulting in a higher proportion of steel plate components. Therefore, by making the average Si content of the molten metal, i.e., the Si content of the weld metal, lower than the Si content of the steel plate, the surface tension near the steel plate can be made relatively higher than in the center of the molten metal. As a result, the molten metal, heated to a high temperature, can flow toward the back side, and a bead shape with a large penetration depth can be obtained.

[0136] The method for measuring the Si content of weld metal will be explained below. In this embodiment, the "Si content of the weld metal" refers to the average Si content near the center of the weld metal. Therefore, in this embodiment, first, a T-fillet weld joint is cut out by machining so that it includes the weld metal portion and the surrounding base metal portion, and the weld metal region is identified in advance by visually observing the cross section perpendicular to the longitudinal direction of the weld metal portion. Then, weld metal chips are collected by cutting that region with a drill or the like, and these chips are used as a sample for measurement by emission spectroscopy using inductively coupled plasma (ICP). In this way, the average Si content near the center of the weld metal, i.e., the "Si content of the weld metal," is measured.

[0137] <2.2. T-shaped fillet weld joint> Based on the inventors' findings described above, the T-shaped fillet welded joint according to this embodiment is as follows.

[0138] Figure 14 is a schematic cross-sectional view of the T-shaped fillet welded joint 20 according to this embodiment. The T-shaped fillet weld joint 20 according to this embodiment comprises a third Zn-plated steel sheet 21, a fourth Zn-plated steel sheet 22 provided perpendicular to the surface of the third Zn-plated steel sheet 21, and a weld metal 23 that joins the end face of the fourth Zn-plated steel sheet 22 on the third Zn-plated steel sheet 21 side with the first surface 21a of the third Zn-plated steel sheet 21 on the fourth Zn-plated steel sheet 22 side.

[0139] <2.2.1 Shape of T-shaped fillet weld joint> In the T-shaped fillet weld joint 20 according to this embodiment shown in Figure 14, the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22, which is a butt plate, are provided perpendicular to each other. In other words, the fourth Zn-plated steel sheet 22 is provided perpendicular to the surface of the third Zn-plated steel sheet 21. Note that the angle between the surface direction of the third Zn-plated steel sheet 21 and the surface direction of the fourth Zn-plated steel sheet 22 does not have to be 90°. That is, the fourth Zn-plated steel sheet 22 may be provided at an inclination relative to the third Zn-plated steel sheet 21. For example, the angle between the surface direction of the third Zn-plated steel sheet 21 and the surface direction of the fourth Zn-plated steel sheet 22 may be in the range of 45° to 135°. However, the angle between the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 is not particularly limited, and various values ​​between 0° and 180° can be applied.

[0140] Of the two surfaces of the third Zn-plated steel sheet 21, the surface that is welded to the end of the fourth Zn-plated steel sheet 22 is referred to as the first surface 21a, and the surface that is not welded is referred to as the second surface 21b.

[0141] The T-fillet welded joint 20 has a weld metal (weld bead) 23 that joins the end of the fourth Zn-plated steel sheet 22 and the first surface 21a of the third Zn-plated steel sheet 21. Weld metal (weld bead) is the metal that melts and solidifies during welding. Furthermore, the T-fillet welded joint 20 has a HAZ formed around the weld metal 23. HAZ (Heat Affected Zone) is the part that does not melt during welding, but whose structure, metallurgical properties, and mechanical properties are changed by the welding heat. Hereinafter, the weld metal and HAZ may be collectively referred to as the "welded area".

[0142] <2.2.2 Third-Zinc Plated Steel Sheets and Fourth-Zinc Plated Steel Sheets> In the T-fillet welded joint 20 of this embodiment, both the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 are Zn-plated steel sheets. Examples of Zn-plated steel sheets include Zn-Ni plated steel sheets, Zn-Al plated steel sheets, Zn-Mg plated steel sheets, and Zn-Mg-Al plated steel sheets. Although the T-fillet welded joint 20 of this embodiment shows an example using the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22, the T-fillet welded joint of the present invention is not limited to this combination. In other words, only one of them may be a "Zn-plated steel sheet". In this case, the other steel sheet may remain as the base steel sheet described later, without plating. Furthermore, the plating layer on the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 only needs to be provided on at least the weld surface of the steel sheet, and does not necessarily need to be formed on both sides of each steel sheet. Furthermore, the plating layer only needs to be provided on at least one side of the weld surface, either the third Zn-plated steel sheet 21 or the fourth Zn-plated steel sheet 22. More specifically, the plating layer only needs to be provided on at least one of the first surface 21a of the third Zn-plated steel sheet 21 and the upper surface (the surface on the upper side of the weld bead) of the fourth Zn-plated steel sheet 22.

[0143] The type of base steel sheet used for the plated steel sheet is not particularly limited, but it is preferable, for example, to use a high-strength steel sheet with a tensile strength of 780 MPa or higher. This makes it possible to improve the strength of the machine parts to which the welded joint according to this embodiment is applied.

[0144] The Si content of the base steel sheets for the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 is preferably 0.2 to 1.4% by mass. A Si content of 0.2% or more promotes the convection effect on the back surface of the weld, resulting in a deeper penetration depth. On the other hand, if the Si content exceeds 1.4%, the increase in slag in the weld becomes significant, which may adversely affect the paintability, i.e., the corrosion resistance, of the weld. The Si content of the base steel sheet can be measured using emission spectroscopy with inductively coupled plasma (ICP) as a sample of cuttings from the base steel sheet.

[0145] As explained above (Function 4), in order to obtain a bead shape with a large penetration depth, it is effective to make the Si content of the molten metal relatively lower than the Si content of the steel sheet, and to obtain a flow of the molten metal heated to a high temperature toward the back side. From this viewpoint, it is preferable that the ratio of the Si content of the weld metal to the Si content of the third Zn-plated steel sheet 21 is less than 1, and the ratio of the Si content of the weld metal to the Si content of the fourth Zn-plated steel sheet 22 is less than 1.

[0146] There are no particular limitations on the type of high-strength steel sheet. Examples of high-strength steel sheets include DP steel sheets, TRIP steel sheets, composite structure steel sheets, martensitic steel sheets, and hot-stamped steel sheets. The greater the tensile strength of the steel plate, the lower the joint strength in a conventional welded joint. Therefore, the greater the tensile strength of the steel plate, the more superior the effect of the welded joint according to this embodiment becomes compared to a conventional welded joint. The tensile strength of the high-strength steel plate is preferably 780 MPa or higher, preferably 980 MPa or higher, and more preferably 1300 MPa or higher, or 1700 MPa or higher. The high-strength steel plate may be cold-rolled or hot-rolled. The tensile strength of steel plates can be measured by a tensile test in accordance with the JIS Z 2241:2022 standard. However, if it is not possible to obtain a standard test specimen as defined in the same standard due to the dimensions of the steel plate, the value measured using a proportional test specimen or a test specimen of appropriate dimensions as defined in the same standard may be used. In this embodiment, the tensile strength of the high-strength steel sheet, which is the base steel sheet, and the tensile strength of the Zn-plated steel sheet may be considered to be the same.

[0147] Thickness t of the 4th Zn-plated steel sheet 22 H The thickness is within the range of 1.0 to 3.4 mm. Similarly, the thickness of the third Zn-plated steel sheet 22 may also be within the range of 1.0 to 3.4 mm. T-fillet welded joints using Zn-plated steel sheets having such thicknesses can be suitably applied to automotive parts and machine parts. Furthermore, the thicknesses of the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 constituting the T-fillet welded joint 20 may be different.

[0148] <2.2.3 Weld Metal> As shown in Figure 14, the T-shaped fillet weld joint 20 has a weld metal (weld bead) 23 that joins the end of the fourth Zn-plated steel sheet 22 and the first surface 21a of the third Zn-plated steel sheet 21. In other words, both surfaces of the fourth Zn-plated steel sheet 22 are connected to the first surface 21a of the third Zn-plated steel sheet 21 via the weld metal 23. The bead shape of the T-fillet weld joint according to this embodiment is not limited to the shape shown in Figure 14. For example, as shown in the modified example of this embodiment in Figure 15, the intersection point PB between the bead on the lower surface of the weld metal 23A and the third Zn-plated steel sheet 21 may be at the same position as the intersection point PS between the plate surface of the fourth Zn-plated steel sheet 22 and the first surface 21a of the third Zn-plated steel sheet 21, or it may be outside the intersection point PS. By providing a bead shape in which the intersection point PB is outside the intersection point PS, the strength of the weld can be increased. The chemical composition of the weld metal 23 will be described later.

[0149] <2.2.4 Weld Bead Width> In this embodiment, the upper limit value WBU (mm) of the bead width on the lower surface side of the weld metal 23 is plate thickness t H When it is 1.0 mm or more and less than 2.0 mm, it satisfies the following formula (12), plate thickness t H When it is 2.0 mm or more and less than 2.9 mm, it satisfies the following formula (13), plate thickness t H When it is 2.9 to 3.4 mm, it satisfies the following formula (14). Also, the lower limit value WBL (mm) of the bead width on the lower surface side of the weld metal 13 is 0.7 mm or more.

[0150] WBU ≦ 2.8 ···(12) WBU ≦ 1.33t H +0.13 ···(13) WBU ≦ 4.0 ···(14)

[0151] As described above, from the viewpoint of suppressing welding defects such as holes and melting drops generated in the welded part and pore defects such as blowholes, for each plate thickness, the bead width is optimized so as to be within a predetermined range. Specifically, the upper limit of the bead width on the lower surface side (back side) of the weld metal satisfies the following formulas (12) to (14). If the bead width on the lower surface side (back side) exceeds these upper limit values, excessive welding heat will occur, and welding defects such as holes and melting drops may occur in the welded part. On the other hand, the lower limit of the back side bead width is shown by the following formula (15). In all cases of plate thickness, when the bead width is less than 0.7 mm, pore defects such as blowholes may occur.

[0152] Also, the upper limit value WSU (mm) of the bead width on the upper surface side of the weld metal 23 is plate thickness t H When it is 1.0 mm or more and less than 2.0 mm, it satisfies the following formula (16), plate thickness t H When it is 2.0 mm or more and less than 2.9 mm, it satisfies the following formula (17), plate thickness t H When it is 2.9 to 3.4 mm, it satisfies the following formula (18).

[0153] Also, the lower limit value WSL (mm) of the bead width on the upper surface side of the weld metal 23 is plate thickness t H When it is 1.0 mm or more and less than 2.9 mm, it satisfies the following formula (19), plate thickness t H When it is 2.9 mm or more and less than 3.4 mm, it satisfies the following formula (20).

[0154] WSU ≦ 5.0 ···(16) WSU ≦ 2.44t + 0.11 ···(17) WSU ≦ 7.2 ···(18) WSL ≧ 3.5 ···(19) WSL ≧ 1.16t H + 0.14 ···(20)

[0155] As described above, from the viewpoints of suppressing welding defects such as holes and melting drops generated in the welded part, pore defects such as blowholes, and ensuring the continuity of the bead shape, similar to the regulation of the bead width on the lower surface side, it is preferable to optimize the bead width for each plate thickness so as to be within a predetermined range.

[0156] Specifically, the upper limit of the bead width on the upper surface side (front side) is shown by the following formulas (16) to (18). If it exceeds these upper limit values, excessive heat input during welding may occur, and welding defects such as holes and melting drops may occur in the welded part. On the other hand, the lower limit of the bead width on the upper surface side (front side) is shown by the following formulas (19) and (20). Regardless of the plate thickness t H When the bead width is less than 3.5 mm, insufficient penetration may occur due to insufficient heat input during welding, and the bead may have a discontinuous shape. Also, in the case of a bead width that does not satisfy formulas (19) and (20), pore defects such as blowholes may occur.

[0157] Here, the "bead width" in this embodiment refers to the width of the weld metal 23 in the direction parallel to the plate surface of the T-shaped abutting fourth Zn-plated steel sheet 22 when the weld is viewed in cross-section along the width direction of the weld metal (bead) 23. In other words, the "bead width" is the width of the bead along the plate surface direction of the fourth Zn-plated steel sheet 22, which is the one of the two Zn-plated steel sheets constituting the welded joint 20 whose end face is welded. The "bead width" on the upper side (front side) is determined by measuring the width of the weld metal on the front surface of the bead from a cross-sectional photograph of the weld, and the "bead width" on the lower side (back side) is determined by measuring the width of the weld metal on the back surface of the bead. When observing the cross-section of the weld, the T-fillet weld joint is cut perpendicular to the direction of extension of the weld metal (weld bead). Then, the cut surface is polished and etched to reveal the weld metal on the cut surface. Since the weld bead width may vary depending on the welding position, in this embodiment, the average value of three cross-sections is used for confirmation.

[0158] <2.2.5 Chemical composition of weld metal> Next, the chemical composition of the weld metal will be described. In this embodiment, "weld metal" refers to the metal formed when the base material (third-Zn plated steel sheet and fourth-Zn plated steel sheet) and the welding wire melt and mix together. Furthermore, the chemical composition of the weld metal will be expressed as a mass % relative to the total mass of the weld metal, and the description of this mass % will simply be indicated as %.

[0159] Furthermore, the composition of the weld metal in a welded joint can be adjusted by the steel plate component and the welding wire component.

[0160] The chemical composition of weld metal can be measured by emission spectroscopy using inductively coupled plasma (ICP). Specifically, (1) the weld metal region is identified in advance by visually observing a cross section perpendicular to the longitudinal direction in the longitudinal center of the weld, (2) weld metal chips are collected by drilling that region, and (3) these chips are used as a sample and measured by emission spectroscopy using inductively coupled plasma (ICP).

[0161] [C: 0.06~0.25%] Carbon (C) has the effect of stabilizing the arc and atomizing molten droplets. If the C content is less than 0.06%, the molten droplets become larger, the arc becomes unstable, and the amount of spatter increases. As a result, the bead shape becomes uneven and defective, leading to the formation of red rust. The reason why red rust occurs due to a defective bead shape is that the depressions caused by the defect are prone to the generation of welding slag, and water or mud containing moisture, which causes red rust, tends to accumulate there. Furthermore, if the C content is less than 0.06%, the tensile strength of the weld metal cannot be obtained, and the desired tensile strength cannot be achieved. Therefore, the lower limit of the C content is 0.06% or more, and preferably 0.08% or more. On the other hand, if the carbon content exceeds 0.25%, the weld metal hardens, reducing its crack resistance and making it more prone to fracture. Therefore, the upper limit of the carbon content is 0.25% or less, preferably 0.20% or less, and more preferably 0.15% or less.

[0162] [Si: more than 0%, 0.70% or less] Si is contained in the welding wire or base metal as a deoxidizing element. In particular, Si in the welding wire improves the tensile strength of the weld metal by promoting deoxidation of the molten pool. However, as explained above in (Effect 1) and (Effect 2), in order to obtain a bead with a large penetration depth due to the downward pushing effect of the arc pressure, it is effective to reduce the amount of Si in the welding wire used and to suppress the Si content of the resulting weld metal. Furthermore, if the weld metal contains an excess of Si, the amount of non-conductive Si-based slag increases, and red rust may occur between the slag and the weld metal. Therefore, the upper limit of the Si content is 0.70% or less, preferably 0.60% or less, and more preferably 0.50% or less. The lower limit of the Si content is not particularly limited, but it may be greater than 0%, and may be 0.02% or more.

[0163] [Mn: 1.4~2.3%] Like Si, Mn is a deoxidizing element that promotes deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal. If the Mn content is too low, the tensile strength of the weld metal cannot be sufficiently ensured, and the weld metal becomes prone to fracture. Therefore, the lower limit of Mn is 1.4% or more, and preferably 1.8% or more. On the other hand, if Mn is present in excess, the viscosity of the molten metal increases, and at high welding speeds, the molten metal cannot flow properly into the weld area, resulting in a humping bead and a tendency for poor bead shape to occur. As a result, the bead shape becomes uneven and defective, leading to the formation of red rust. Therefore, the upper limit of the Mn content is 2.3% or less, preferably 2.1% or less.

[0164] [Ti: 0.04~0.15%] Since Ti is a deoxidizing element, it is effective in suppressing the occurrence of blowholes. Furthermore, Ti is an effective element for ensuring the conductivity of welding slag and is also an effective element for improving electrodeposition coating properties. Therefore, the lower limit of the Ti content is 0.04% or more, preferably 0.05% or more, and more preferably 0.06% or more. Even more preferably, the Ti content is 0.07% or more. On the other hand, if the weld contains an excessive amount of Ti, the amount of Ti-based slag increases, reducing the adhesion between the Ti-based slag and the weld metal, making it more prone to delamination. As a result, red rust is more likely to occur in the delaminate areas. Therefore, the upper limit of the Ti content is 0.15% or less, preferably 0.14% or less, and more preferably 0.13% or less.

[0165] [Al: 0.001~0.20%] Al is a powerful deoxidizing element that promotes the deoxidation of molten metal during arc welding, thereby suppressing the occurrence of blowholes. Furthermore, the presence of a small amount of Al reduces Si-based slag, which can negatively affect electrodeposition coating of the welded area. Therefore, the lower limit of the Al content is 0.001% or higher, preferably 0.005% or higher, and more preferably 0.010% or higher. On the one hand, if the Al content is excessive, non-conductive Al-based slag increases, and red rust is likely to occur between the slag and the weld metal. Therefore, the upper limit of the Al content in the weld metal is 0.20% or less, preferably 0.18% or less, and more preferably 0.15% or less.

[0166] [Si, Mn, Ti, Al] Also, the contents of Si, Mn, Ti, and Al preferably satisfy the following formula (21). As described above, an increase in the Si content reduces the electrodeposition paintability, but an increase in the contents of Ti and Al leads to an improvement in the electrodeposition paintability. Regarding Mn, if it is a single oxide of Mn, it does not affect the electrodeposition paintability, but a composite oxide of Si and Mn has an effect of reducing the electrodeposition paintability. Therefore, it is not preferable to contain Mn excessively.

[0167] The inventors investigated the presence or absence of red rust generation between the slag and the weld metal for weld metals having various component systems. As a result, it became clear that when the value of 7×[Si]+7×[Mn]-112×[Ti]-30×[Al], which is an index related to the generation of red rust, exceeds 12.0, red rust occurs early and the corrosion resistance deteriorates. Therefore, in the chemical composition of the weld metal of the present embodiment, it is preferable to satisfy the formula (21).

[0168] 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0 ···· Formula (21)

[0169] [N: 0.003 to 0.015%] In addition to the amount contained in the steel sheet and the welding wire, N increases its content by being mixed from the atmosphere during welding. N is an element that lowers the toughness of the weld metal. Therefore, it is desirable that the upper limit of the N content is 0.015% or less. The lower limit value of the N content is not particularly limited and may exceed 0%, but it may also be 0.003% or more, which is the content of standard steel sheets and welding wires. The N content may also be 0.005% or more.

[0170] [O: 0.01~0.06%] Like nitrogen, the amount of oxygen (O) present in steel plates and welding wires increases due to contamination from the atmosphere during welding. Excessive oxygen reduces the toughness of the weld metal. Therefore, it is desirable to keep the upper limit of the oxygen content below 0.06%. There is no particular lower limit for the oxygen content. In standard arc welding, oxygen often exceeds 0.01%, so a lower limit of 0.01% is acceptable. The oxygen content may also be 0.02% or higher.

[0171] [P: more than 0%, less than 0.015%] P is an element that is generally present as an impurity in steel, and is also typically found as an impurity in steel plates and welding wires, and therefore is also present in the weld metal. Here, since P is one of the main elements that cause hot cracking in weld metal, it is desirable to suppress it as much as possible. If the P content exceeds 0.015%, hot cracking of the weld metal becomes significant, so the upper limit for the P content in the weld metal is 0.015% or less. Furthermore, there is no particular limit to the phosphorus content, so it is greater than 0%, but from the viewpoint of cost and productivity of phosphorus removal, it may be as low as 0.001%.

[0172] [S: more than 0%, less than 0.013%] S, like P, is an element that is generally present as an impurity in steel, and is also usually found as an impurity in welding wire, and therefore is also present in the weld metal. Here, S is an element that inhibits the crack resistance of the weld metal, and it is preferable to suppress it as much as possible. If the S content exceeds 0.013%, the crack resistance of the weld metal deteriorates, so the S content of the weld metal is 0.013% or less. Furthermore, there is no particular limit to the sulfur content, so it is greater than 0%, but from the viewpoint of the cost and productivity of sulfur removal, it may be 0.001%.

[0173] Cu, Cr, Nb, V, Mo, Ni, Sb, Sn, and B are not essential elements, but one or more of them may be included simultaneously as needed. The effects obtained by including each element and the upper limit will be explained. The lower limit when these elements are not included is 0%.

[0174] [Cu: 0~0.50%] Although not essential, copper (Cu) may be present in the weld metal due to the copper plating on the welding wire, and therefore may be included at a concentration of 0.005% or more. On the other hand, excessive Cu content can increase the likelihood of weld cracking, so the upper limit for Cu content is 0.50% or less.

[0175] [Cr: 0-2.0%] While not essential, chromium (Cr) may be included in amounts of 0.05% or more to improve the hardenability and tensile strength of the weld. On the other hand, excessive Cr content reduces the elongation of the weld. Therefore, the upper limit for Cr content is 2.0% or less.

[0176] [Nb: 0~0.3%] While not essential, Nb may be included in amounts of 0.005% or more to improve the hardenability and tensile strength of the weld. On the other hand, excessive Nb content reduces the elongation of the weld. Therefore, the upper limit for Nb content is 0.3% or less.

[0177] [V: 0~0.3%] While not essential, V may be included in amounts of 0.005% or more to improve the hardenability and tensile strength of the weld. On the other hand, excessive V content reduces the elongation of the weld. Therefore, the upper limit for V content is 0.3% or less.

[0178] [Mo: 0~1.0%] Mo (Mo) is not essential for improving the hardenability and tensile strength of the weld, but it may be included in amounts of 0.005% or more. On the other hand, if Mo is included in excess, the elongation of the weld will decrease. Therefore, the upper limit for Mo content is 1.0% or less.

[0179] [Ni: 0-2.5%] While not essential, nickel (Ni) may be included in amounts of 0.05% or more to improve the tensile strength and elongation of the weld. On the other hand, excessive nickel content increases the likelihood of weld cracking. Therefore, the upper limit for nickel content is 2.5% or less, preferably 2.0% or less.

[0180] [B: 0~0.0100%] Although not essential, B may be included in amounts of 0.0005% or more to improve the hardenability and tensile strength of the weld. On the other hand, if B is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the B content is 0.0100% or less. Preferably, it is 0.0030% or less.

[0181] [Sb: 0~0.10%] Sb has the effect of generating convection in the molten metal, causing the slag to gather in the center of the weld bead. This can further improve the electrodeposition coating properties. To obtain this effect, although not essential, it is preferable to have an Sb content of 0.01% or more, 0.02% or more, or 0.04% or more. On the other hand, if the Sb content exceeds 0.10%, solidification cracking may occur in the weld metal. Therefore, the upper limit of the Sb content should be 0.10% or less. Preferably, the upper limit of the Sb content is 0.05% or less.

[0182] [Sn: 0~0.4%] Sn is an element that improves the corrosion resistance of the weld metal itself. While Sn content is not essential in this embodiment because it does not significantly affect electrodeposition coating properties, improving the corrosion resistance of the weld metal itself is advantageous for machine parts. To obtain the corrosion resistance improvement effect of Sn, the Sn content may be 0.02% or higher. On the other hand, if the Sn content exceeds 0.4%, the crack susceptibility of the weld metal increases, making it more prone to hot cracking. Furthermore, excessive Sn can lead to segregation at the grain boundaries of the weld metal, resulting in a decrease in toughness. Therefore, the upper limit of the Sn content should be 0.4% or less.

[0183] The remainder of the components described above consists of Fe and impurities. Impurities refer to components contained in the raw materials or components introduced during the manufacturing process, which are not components intentionally included in the weld metal, or components that are acceptable as long as they do not hinder the effects of this embodiment.

[0184] <2.3. Manufacturing method> Next, a preferred manufacturing method for the T-shaped fillet welded joint according to this embodiment will be described. The method for manufacturing a T-shaped fillet welded joint according to this embodiment is a method for manufacturing a T-shaped fillet welded joint having two Zn-plated steel sheets and a weld metal for joining the Zn-plated steel sheets together, comprising the steps of: arranging the Zn-plated steel sheets so that the welding surfaces of the Zn-plated steel sheets are not separated by excessive gaps (arrangement step); and welding the Zn-plated steel sheets together using a welding wire (welding step).

[0185] (Placement process) In the arrangement process, the welded surfaces of the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 are placed facing each other. That is, the fourth Zn-plated steel sheet 22 is placed perpendicular to the surface of the third Zn-plated steel sheet 21. At this time, it is preferable to place the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 without leaving an excessive gap. Specifically, for example, if the gap between the steel sheets exceeds 0.5 mm, it may promote hole formation due to burn-through. However, due to the dimensional accuracy of the steel sheets, it is difficult to minimize the variation in the gap. In this embodiment, a gap in the range of 0 to 0.7 mm is acceptable. More preferably, the gap is 0.2 mm or less. Note that the gap between the welded surfaces of the steel sheets may be 0 mm.

[0186] In the arrangement process, it is preferable that the gap between the welded surfaces of the Zn-plated steel sheets be 0 mm. That is, when butting the Zn-plated steel sheets together, it is preferable that the welded surfaces of the Zn-plated steel sheets be in contact with each other.

[0187] (Welding process) The two Zn-plated steel sheets described above are preferably joined by gas-shielded arc welding. Gas-shielded arc welding is an arc welding method that uses a shielding gas, such as in consumable electrode gas-shielded arc welding. In gas-shielded arc welding, the shielding gas shields the molten metal from the atmosphere.

[0188] The shielding gas used is preferably a gas mainly composed of Ar. More preferably, it is a gas mainly composed of Ar and containing 5% to 20% CO2 by volume.

[0189] The welding conditions are not particularly limited, but for example, welding current: 150~250A, welding voltage: 20~25V, welding speed: 60~80cm / min may be used, and appropriate welding conditions may be set according to the plate thickness. Also, the plate thickness t of the butt plate. H Regarding the plate thickness t, H When using steel plates thicker than 1.6 mm, apply the pulse MAG welding mode, and the plate thickness t H When using steel plates 1.6 mm or thinner, it is recommended to apply a wire feed control type low heat input welding mode.

[0190] <Welding wire> The welding wire used is, for example, a solid wire for gas shielded arc welding. From the viewpoint of ensuring sufficient penetration depth of the weld metal and improving the electroplatable properties of the weld, it is desirable to set the amount of Si added to a low level. In addition, it is preferable to add Ti to impart conductivity to the welding slag in order to improve the electroplatable properties. Furthermore, upper and lower limits for each component are set as appropriate from the viewpoint of ensuring the strength of the weld metal and preventing cracking.

[0191] The composition of the welding wire is expressed as a mass percentage of the total mass. C: 0.04~0.12%, Si: more than 0%, less than 0.30%, Mn: 1.40~2.30%, Ti: 0.04~0.25%, Al: 0.001~0.050%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.015%, N: more than 0%, less than 0.01%, O: more than 0%, less than 0.01%, Cr: 0-3%, Ni: 0-3%, Mo: 0~0.5%, B: 0~0.0100%, Cu: 0~0.50%, Nb: 0~0.3%, V: 0~0.5%, Sb: 0~0.10% and Sn: 0~0.4% The remainder consists of iron and impurities.

[0192] Although preferred manufacturing methods for producing the T-fillet welded joint of this embodiment have been described above, other methods are not particularly limited and may be set and adjusted as appropriate within a range that does not impede the operation and effect of the T-fillet welded joint of the present invention. [Examples]

[0193] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0194] (Example 1) Various lap fillet welded joints with coatings were manufactured by arc welding and electrodeposition coating on a 150mm x 100mm first Zn-plated steel sheet (steel sheet 11; lower steel sheet) and a 150mm x 50m second Zn-plated steel sheet (steel sheet 12; upper steel sheet). The chemical composition of the base materials of the first Zn-plated steel sheet (steel sheet 11) and the second Zn-plated steel sheet (steel sheet 12) is shown in Table 2, and in both chemical compositions, the remainder was iron and impurities. In addition, a Zn-Fe alloy plating layer was formed on the surface of both base materials by alloying a molten Zn plating layer with a trace amount of Al (less than 0.15%) added.

[0195] Next, steel plates 11 and 12 were arranged in the configuration and orientation shown in Table 4, and fillet welding was performed to produce a lap fillet welded joint. The chemical composition of the welding wire used is shown in Table 3, and in all cases, the remainder was iron and impurities, with a diameter of 1.2 mm. The welding conditions were as follows.

[0196] • Welding power supply: CMT power supply (manufactured by Fronius) If the thickness of steel plate 11 (lower steel plate) exceeds 1.6 mm, apply a welding current of 150-250 A, an arc voltage of 20-26 V, and pulse MAG welding mode. • If the thickness of steel plate 11 (lower steel plate) is 1.6m or less, apply a low heat input welding mode with a welding current of 150-200A, an arc voltage in the range of 20-23V, and wire feed control. Welding speed: 80cm / min

[0197] During welding, specifically, steel plates 11 and 12 were overlapped to the extent shown in Table 4, and both ends of the overlapping portion were fixed with tack welding. The gap between the steel plates near the tack welds at each end was measured with a gauge, and the average value was defined as the gap between the steel plates (Table 4). Subsequently, a 120mm length main weld was performed, and the porosity ratio, the appearance of the weld bead, and any paint defects after electrodeposition coating were evaluated.

[0198] <Measurement of porosity ratio> The porosity ratio was determined by measuring the weld length and porosity length from X-ray radiographs taken after welding, and expressing it as the ratio of the sum of porosity lengths to the weld length. Specifically, the porosity ratio was calculated for a 90mm length of weld bead, excluding the 15mm at the start and end of the weld, from a 120mm bead length of the weld test piece. A calculated porosity ratio of 10% or less was evaluated as "◎", a ratio between 10% and 15% was evaluated as "〇", and a ratio greater than 15% was evaluated as "× (fail)".

[0199] <Electrodeposition coating properties> The prepared weld test specimens were degreased and chemically treated, and then electrodeposited to a film thickness of 20 μm. The electrodeposited areas of the weld beads were then photographed, and the ratio of the area of ​​electrodeposited defects to the weld bead area was measured from the images. The defect rate of the electrodeposited coating was calculated for a 90 mm length of the weld bead, excluding the 15 mm at the start and end of the weld, from a total bead length of 120 mm. Electrodeposition coating was performed using gray paint. This made it easier to distinguish between areas with electrodeposition defects, such as exposed reddish-brown or black slag, and the coated areas. Welded joints with a defective area of ​​10% or less were rated "○", those with a defective area of ​​5% or less were rated "◎", and welded joints with a defective area exceeding 10% were rated "× (fail)". In this example, welded joints with a defective area ratio of 10% or less ("○") and 5% or less ("◎") were judged to be welded joints with suppressed electrodeposition defects.

[0200] <Appearance of weld bead> The shape of the weld bead was evaluated by visual inspection. Specifically, a weld bead was judged to be good if it was a "continuous, uniform bead" as observed visually. On the other hand, weld beads that showed "holes" or "serpentine" patterns as observed visually were judged to have poor appearance.

[0201] [Table 2]

[0202] [Table 3]

[0203] The results are shown in Tables 4 and 5. Examples No. A1 to A11 are examples of the invention, satisfying the requirements for upper bead width (front bead width) (mm), lower bead width (back bead width) (mm), and Si content (mass%) and Ti content (mass%) of the weld metal. As a result, they exhibited a good bead shape with a porosity ratio of 15% or less, free from holes and meandering, and also showed good results in the post-weld electrodeposition coating evaluation with a coating defect rate of 15% or less.

[0204] On the other hand, No. A12 had a large overlap, a narrow back bead width, and a porosity defect rate of over 15%. In No. A13, the gap between the steel plates was large at 0.7 mm, which resulted in an excessive widening of the back bead width and caused hole-punching defects. In No. A14, insufficient heat input resulted in narrow weld beads on both the front and back sides, leading to porosity defects and a meandering weld bead. In case No. A15, the weld metal had an excessive Si content, and in case No. A16, the weld metal had an insufficient Ti content. In both cases, the coating defect rate during electrodeposition coating exceeded the 15% standard. In No. A17, the gap between the steel plates was wide at 0.6 mm, which resulted in a larger spread of the back bead width and caused hole-punching defects. In No. A18, insufficient heat input resulted in a narrow surface bead width and a meandering weld bead. In No. A19, the overlap was small, making it difficult to maintain a stable overlap state, which resulted in an increased width of the back bead and a defect in hole punching. In No. A20, the gap between the steel plates was large at 0.6 mm, resulting in excessive expansion of both the front and back bead widths, leading to defective holes. In both cases, No. A21 and A22, the welding heat input was excessively high, resulting in defective holes in the welded area. In welding No. A23, insufficient heat input resulted in a narrow surface bead width and a meandering weld bead. In welding case No. A24, insufficient heat input resulted in a narrow surface bead width, porosity defects, and a meandering weld bead.

[0205] [Table 4]

[0206] [Table 5]

[0207] (Example 2) Next, we investigated the chemical composition of the weld metal. The steel plates 11, 12, and welding wire used were the same as in Example 1, as shown in Tables 2 and 3, and the welding conditions were also the same as in Example 1.

[0208] In the obtained welded joints, the porosity ratio, the external shape of the weld bead, and any defects in the coating after electrodeposition were evaluated, as in Example 1. Furthermore, the chemical composition (unit: mass%) of the welded joints was also measured. The results are shown in Tables 6 to 8. In all examples of the chemical composition shown in Table 7, the remainder consisted of iron and impurities.

[0209] As shown in Tables 7 and 8, in the case of a weld bead having the preferred chemical composition of this embodiment, the desired bead shape was obtained, and good results were obtained in terms of porosity ratio, bead shape, and electrodeposition coating properties. In particular, the invention examples excluding No. A25 and No. A30 showed extremely good results, with a porosity defect rate of 10% or less and an electrodeposition coating defect rate of 10% or less on the back surface bead.

[0210] For welds No. A32 and No. A34, the Si ratio of the weld metal to the base steel plate (steel plate 11) exceeded 1.0, resulting in a slightly narrower back bead width, and thus the porosity defect rate was over 10% and 15% or less, respectively. Nos. A25 and A30 did not satisfy equation (11), resulting in electrodeposition coating defects on the back surface bead being greater than 10% and less than or equal to 15%.

[0211] [Table 6]

[0212] [Table 7]

[0213] [Table 8]

[0214] (Example 3) Various T-shaped fillet welded joints with a coating were manufactured by arc welding and electrodeposition coating on 150mm x 100mm third-generation Zn-plated steel sheets (steel sheet 21) and 150mm x 50m fourth-generation Zn-plated steel sheets (steel sheet 22; butt steel sheet). The chemical composition of the base materials of the third-generation Zn-plated steel sheet (steel sheet 21) and the second-generation Zn-plated steel sheet (steel sheet 22) is shown in Table 2, and in both chemical compositions, the remainder was iron and impurities. In addition, in both base materials, a Zn-Fe alloy plating layer was formed on the surface by alloying a molten Zn plating layer with a trace amount of Al (less than 0.15%) added.

[0215] Next, the butt plates, steel plate 22 and steel plate 21, were arranged in the plate configuration and orientation shown in Table 9, and fillet welding was performed to manufacture a T-shaped fillet welded joint. The chemical composition of the welding wire used is shown in Table 3, and in all chemical compositions, the remainder was iron and impurities, and the size was 1.2 mm in diameter. The welding conditions were as follows. Note that "vertical" or "horizontal" in "Position of steel plate 22" in Table 9 refers to the orientation of steel plate 22 when focusing only on steel plate 22. In other words, in this embodiment, the angle between steel plate 21 and steel plate 22 is a right angle, and in the case of a joint as shown in Figure 14, it is "horizontal", and the orientation when the arrangement in Figure 14 is rotated 90° is "vertical".

[0216] • Welding power supply: CMT power supply (manufactured by Fronius) If the thickness of steel plate 12 (butt plate) exceeds 1.6 mm, apply a welding current of 150-250 A, an arc voltage of 20-25 V, and pulse MAG welding mode. • If the thickness of the steel plate 12 (butt plate) is 1.6m or less, apply a low heat input welding mode with a welding current of 150-200A, an arc voltage in the range of 20-23V, and wire feed control. Welding speed: 80 m / min

[0217] During welding, a 150mm x 50m steel plate 22 (butt plate) was butted against a 150mm x 100mm steel plate 21, and both ends of the butt section were fixed with tack welding. The gap at the butt section near the tack welds at each end was measured with a gauge, and the average value was taken as the gap between the steel plates (Table 9). Subsequently, a 120mm length main weld was performed, and the porosity ratio, the appearance of the weld bead, and any paint defects after electrodeposition coating were evaluated.

[0218] <Measurement of porosity ratio> The porosity ratio was determined by measuring the weld length and porosity length from X-ray radiographs taken after welding, and expressing it as the ratio of the sum of porosity lengths to the weld length. Specifically, the porosity ratio was calculated for a 90mm length of weld bead, excluding the 15mm at the start and end of the weld, from a 120mm bead length of the weld test piece. A calculated porosity ratio of 10% or less was evaluated as "◎", a ratio between 10% and 15% was evaluated as "〇", and a ratio greater than 15% was evaluated as "× (fail)".

[0219] <Electrodeposition coating properties> The prepared weld test specimens were degreased and chemically treated, and then electrodeposited to a film thickness of 20 μm. The electrodeposited areas of the weld beads were then photographed, and the ratio of the area of ​​electrodeposited defects to the weld bead area was measured from the images. The defect rate of the electrodeposited coating was calculated for a 90 mm length of the weld bead, excluding the 15 mm at the start and end of the weld, from a total bead length of 120 mm. Electrodeposition coating was performed using gray paint. This made it easier to distinguish between areas with electrodeposition defects, such as exposed reddish-brown or black slag, and the coated areas. Welded joints with a defective area of ​​10% or less were rated "○", those with a defective area of ​​5% or less were rated "◎", and welded joints with a defective area exceeding 10% were rated "× (fail)". In this example, welded joints with a defective area ratio of 10% or less ("○") and 5% or less ("◎") were judged to be welded joints with suppressed electrodeposition defects.

[0220] <Appearance of weld bead> The shape of the weld bead was evaluated by visual inspection. Specifically, a weld bead was judged to be good if it was a "continuous, uniform bead" as observed visually. On the other hand, weld beads that showed "holes" or "serpentine" patterns as observed visually were judged to have poor appearance.

[0221] The results are shown in Tables 9 and 10. Examples B1 to B10 are inventive examples that satisfy the standard upper bead width (front bead width) (mm), lower bead width (back bead width) (mm), and Si content (mass%) and Ti content (mass%) of the weld metal. As a result, they exhibit a good bead shape with a porosity ratio of 15% or less, free from holes and meandering, and also showed good results in the post-weld electrodeposition coating evaluation with a coating defect rate of 15% or less.

[0222] On the other hand, while No. B11 had a narrow gap between the steel plates, the back bead width was narrow and the porosity defect rate exceeded 15%. In No. B12, the gap between the steel plates was wide at 0.6 mm, which resulted in a larger spread of the back bead width and caused hole-punching defects. In No. B13, insufficient heat input resulted in narrow weld bead widths on both the front and back sides, leading to porosity defects and a meandering weld bead. In case No. B14, the weld metal had insufficient Ti content, and in case No. B15, the weld metal had excessive Si content. In both cases, the coating defect rate during electrodeposition coating exceeded the 15% standard. In No. B16, excessive heat input caused the surface bead width to widen, resulting in a hole-punching defect. In all cases, numbers B17-B19 and B21 were affected by excessive heat input, resulting in either a wide front or back bead width and poor hole drilling. No. B20 had a narrow gap between the steel plates, but the back bead width was narrow and the porosity defect rate exceeded 15%.

[0223] [Table 9]

[0224] [Table 10]

[0225] (Example 4) Next, we investigated the chemical composition of the weld metal. The steel plates 21 and 22, and welding wire used were the same as in Example 3, as shown in Tables 2 and 3, and the welding conditions were also the same as in Example 3.

[0226] In the obtained welded joints, the porosity ratio, the external shape of the weld bead, and any defects in the coating after electrodeposition were evaluated, as in Example 3. Furthermore, the chemical composition (unit: mass%) of the welded joints was also measured. The results are shown in Tables 11 to 13. In all examples of the chemical composition shown in Table 12, the remainder consisted of iron and impurities.

[0227] As shown in Tables 12 and 13, in the case of a weld bead having the preferred chemical composition of this embodiment, the desired bead shape was obtained, and good results were obtained in terms of porosity ratio, bead shape, and electrodeposition coating properties. In particular, the invention examples excluding No. B23, No. B25, and No. B31 showed extremely good results, with a porosity defect rate of 10% or less and an electrodeposition coating defect rate of 10% or less. For welds No. B23 and No. B25, the Si ratio of the weld metal to the base steel plate exceeded 1.0, resulting in a slightly narrower back bead width, and thus the porosity defect rates were over 10% and 15% or less, respectively. Since No. B31 did not satisfy equation (21), the electrodeposition coating defect rate was between 10% and 15%.

[0228] [Table 11]

[0229] [Table 12]

[0230] [Table 13] [Explanation of Symbols]

[0231] 10,100 lap fillet welded joints 11. First Zn-plated steel sheet 12. Second-type Zn-plated steel sheet 13. Weld metal (bead) 20,200 T-shaped fillet welded joint 21. Third-Zinc plated steel sheet 22. 4th Zn-plated steel sheet 23, 23A Weld metal (bead) 101,102,201,202 Test steel plate [Industrial applicability]

[0232] According to the above embodiment of the present invention, lap fillet welded joints and T-fillet welded joints are obtained that have excellent electrodeposition coating properties and can reduce the occurrence of porosity defects such as blowholes. Therefore, the obtained lap fillet welded joints and T-fillet welded joints can be suitably applied to mechanical structural parts such as automobile parts and building material parts, and thus have high industrial applicability.

Claims

1. First Zn-based plated steel sheet, A second Zn-plated steel sheet is positioned above the first surface of the first Zn-plated steel sheet, A lap fillet welded joint comprising a weld metal that joins the end face of the second Zn-plated steel sheet and the first face of the first Zn-plated steel sheet, The chemical composition of the weld metal is, in mass%, Si: greater than 0%, 0.70% or less, Ti: 0.04~0.15% Includes, At least a portion of the end face of the first Zn-plated steel sheet is covered by the weld metal, The thickness t of the first Zn-plated steel sheet Ｈ The size is 1.0 to 3.4 mm. The upper limit WBU (mm) of the bead width on the lower side of the weld metal is, Plate thickness t Ｈ If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (1) is satisfied. Plate thickness t Ｈ If the size is 2.0 mm or more and 3.4 mm or less, then the following formula (2) is satisfied. The lower limit value WBL (mm) of the bead width on the lower side of the weld metal is, Plate thickness t Ｈ If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (3) is satisfied. Plate thickness t Ｈ If the size is 2.0 mm or more and 3.4 mm or less, then the following formula (4) is satisfied. The upper limit value WSU (mm) of the bead width on the upper surface side of the weld metal is, Plate thickness t Ｈ If it is 1.0 mm or more and less than 1.6 mm, then the following formula (5) is satisfied. Plate thickness t Ｈ If the size is 1.6 mm or more and less than 2.9 mm, then the following formula (6) is satisfied. Plate thickness t Ｈ If the size is 2.9 mm or more and 3.4 mm or less, then the following formula (7) is satisfied. The lower limit value WSL (mm) of the bead width on the upper surface of the weld metal is, The plate thickness t Ｈ When it is 1.0 mm or more and less than 1.6 mm, the following formula (8) is satisfied, Plate thickness t Ｈ If the size is 1.6 mm or more and less than 2.9 mm, then the following formula (9) is satisfied. Plate thickness t Ｈ A lap fillet welded joint characterized in that when the thickness is 2.9 mm or more and 3.4 mm or less, it satisfies the following formula (10). WBU≦t Ｈ +0.8 ・・・(1) WBU≦0.86t Ｈ +1.09 ・・・(2) WBL≧0.5t Ｈ +0.5 ・・・(3) WBL≧0.21t Ｈ +1.07 ・・・(4) WSU ≤ 5.0 ... (5) WSU≦3.07t Ｈ +0.08 ・・・(6) WSU ≤ 9.0 ... (7) WSL ≥ 3.0 ... (8) WSL≧1.54t Ｈ +0.54 ・・・(9) WSL ≥ 5.0 ... (10)

2. The chemical composition of the weld metal is, in mass%, C: 0.06-0.25%, Si: more than 0%, 0.70% or less, Mn: 1.4-2.3%, Ti: 0.04-0.15%, Al: 0.001-0.20%, N: 0.003 to 0.015%, O: 0.01-0.06%, Cr: 0-2.0%, Ni: 0 to 2.5%, B: 0 to 0.0100%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.013%, Sb: 0 to 0.10%, Sn: 0-0.4%, Cu: 0 to 0.50%, Nb: 0 to 0.3%, V: 0-0.3%, Mo: 0 to 1.0%, The remainder consists of iron and impurities, and further, The following equation (11) is satisfied The lap fillet welded joint according to claim 1, characterized in that... 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0...(11) However, the element symbols in formula (11) represent the mass percentage content of each element in the weld metal.

3. The Si content of each base steel sheet of the first Zn-plated steel sheet and the second Zn-plated steel sheet is 0.2 to 1.4% by mass. The ratio of the Si content of the weld metal to the Si content of the base steel sheet in the first Zn-plated steel sheet is less than 1. The ratio of the Si content of the weld metal to the Si content of the base steel sheet in the second Zn-plated steel sheet is less than 1. A lap fillet welded joint according to claim 1 or 2, characterized in that it is a lap fillet welded joint.

4. The tensile strength of the first Zn-plated steel sheet and the second Zn-plated steel sheet is 780 MPa or more. A lap fillet welded joint according to claim 1 or 2, characterized in that it is a lap fillet welded joint.

5. The lap fillet welded joint according to claim 1 or 2, characterized in that the weld metal is formed over the entire surface of the end face of the first Zn-plated steel sheet.

6. Third-Zn plated steel sheet, A fourth Zn-plated steel sheet is provided perpendicular to the surface of the third Zn-plated steel sheet, A T-shaped fillet welded joint comprising a weld metal that joins the end face of the 4Zn-plated steel sheet on the side of the 3Zn-plated steel sheet and the first surface of the 3Zn-plated steel sheet on the side of the 4Zn-plated steel sheet, The chemical composition of the weld metal is, in mass%, Si: greater than 0%, 0.70% or less, Ti: 0.04~0.15% Includes, Both surfaces of the fourth Zn-plated steel sheet are connected to the first surface of the third Zn-plated steel sheet via the weld metal. The thickness t of the 4 Zn-plated steel sheet Ｈ The size is 1.0 to 3.4 mm. The upper limit WBU of the bead width on the lower side of the weld metal is, Plate thickness t Ｈ If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (12) is satisfied. Plate thickness t Ｈ If the size is 2.0 mm or more and less than 2.9 mm, then the following formula (13) is satisfied. Plate thickness t Ｈ If the size is 2.9 mm or more and 3.4 mm or less, then the following formula (14) is satisfied. The lower limit WBL of the bead width on the lower side of the weld metal satisfies the following formula (15): The upper limit value WSU of the bead width on the upper surface side of the weld metal is Plate thickness t Ｈ If the size is 1.0 mm or more and less than 2.0 mm, then the following formula (16) is satisfied. Plate thickness t Ｈ If the size is 2.0 mm or more and less than 2.9 mm, then the following formula (17) is satisfied. Plate thickness t Ｈ If the size is 2.9 mm or more and 3.4 mm or less, then the following formula (18) is satisfied. The lower limit value WSL of the bead width on the upper surface side of the weld metal is, Plate thickness t Ｈ If the size is 1.0 mm or more and less than 2.9 mm, then the following formula (19) is satisfied. Plate thickness t Ｈ A T-shaped fillet welded joint characterized in that when the diameter is 2.9 mm or more and 3.4 mm or less, it satisfies the following formula (20). WBU≦2.8mm...(12) WBU≦1.33t Ｈ +0.13 ・・・(13) WBU≦4.0mm...(14) WBL≧0.7mm (15) WSU≦5.0mm...(16) WSU≦2.44t Ｈ +0.11 ... (17) WSU≦7.2mm...(18) WSL≧3.5mm...(19) WSL≧1.16t Ｈ +0.14 ・・・(20)

7. The chemical composition of the weld metal is, in mass%, C: 0.06-0.25%, Si: more than 0%, 0.70% or less, Mn: 1.4-2.3%, Ti: 0.04-0.15%, Al: 0.001-0.20%, N: 0.003 to 0.015%, O: 0.01-0.06%, Cr: 0-2.0%, Ni: 0 to 2.5%, B: 0 to 0.0100%, P: more than 0%, less than 0.015%, S: more than 0%, less than 0.013%, Sb: 0 to 0.10%, Sn: 0-0.4%, Cu: 0 to 0.50%, Nb: 0 to 0.3%, V: 0-0.3%, Mo: 0 to 1.0%, The remainder consists of iron and impurities, and further, The following equation (21) is satisfied A T-shaped fillet welded joint according to claim 6, characterized in that it is a T-shaped fillet welded joint. 7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≦12.0...(21) However, the element symbols in formula (21) represent the content of each element in the weld metal.

8. The Si content of each base steel sheet of the third Zn-plated steel sheet and the fourth Zn-plated steel sheet is 0.2 to 1.4% by mass. The ratio of the Si content of the weld metal to the Si content of the base steel sheet in the 3Zn-plated steel sheet is less than 1. The ratio of the Si content of the weld metal to the Si content of the base steel sheet in the 4 Zn-plated steel sheet is less than 1. A T-shaped fillet welded joint according to claim 6 or 7, characterized in that it is a T-shaped fillet welded joint.

9. The tensile strength of the 3Zn-plated steel sheet and the 4Zn-plated steel sheet is 780 MPa or more. A T-shaped fillet welded joint according to claim 6 or 7, characterized in that it is a T-shaped fillet welded joint.

10. The intersection point PB between the bead on the lower surface of the weld metal and the third Zn-plated steel sheet is at the same position as the intersection point PS between the surface of the fourth Zn-plated steel sheet and the surface of the third Zn-plated steel sheet, or is located outside of the intersection point PS. A T-shaped fillet welded joint according to claim 6 or 7, characterized in that it is a T-shaped fillet welded joint.

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