Lap filet welded joint and t-shaped filet welded joint

Abstract

This lap filet welded joint comprises a first Zn-based plated steel sheet, a second Zn-based plated steel sheet that is disposed above the first Zn-based plated steel sheet, and a weld metal that joins the second Zn-based plated steel sheet and the first Zn-based plated steel sheet.　This T-shaped filet welded joint comprises a third Zn-based plated steel sheet, a fourth Zn-based plated steel sheet that is provided perpendicular to the sheet surface of the third Zn-based plated steel sheet, and a weld metal that joins the fourth Zn-based plated steel sheet and the third Zn-based plated steel sheet.　In both of the lap filet welded joint and the T-shaped filet welded joint: the chemical components of the weld metal include greater than 0% and no greater than 0.70% of Si, and 0.04-0.15% of Ti; and the upper limit value WBU and the lower limit value WBL of the bead width on the lower-surface side of the weld metal and the upper limit value WSU and the lower limit value WSL of the bead width on the upper-surface side satisfy desired ranges for every thickness tH.

Description

Overlap fillet welds and T-fillet welds

[0001] This disclosure relates to lap fillet weld joints and T-fillet weld joints. This application claims priority under Japanese Patent Application No. 2024-209806 and Japanese Patent Application No. 2024-209767, filed in Japan on 2 December 2024, the contents of which are incorporated herein by reference.

[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 at which the two steel plates are positioned relative to each other is not limited to 90°, but may 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 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.

[0011] Japanese Patent Publication No. 2012-183542 Japanese Patent Publication No. 2012-101232

[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. Also, in the technology disclosed in Patent Document 2, blowhole formation is suppressed by optimizing the Si and Al content of the base material and creating gaps between steel plates, but electrodeposition coating properties are not taken 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.

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

[0016] (1) An overlap 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 a first surface of the first Zn-plated steel sheet, and a weld metal joining the end surface of the second Zn-plated steel sheet and the first surface of the first Zn-plated steel sheet, wherein the chemical composition of the weld metal is, by mass%, Si: greater than 0% and 0.70% or less, and Ti: 0.04 to 0.15%, at least a portion of the end surface of the first Zn-plated steel sheet is covered by the weld metal, and the thickness of the first Zn-plated steel sheet is t Ｈ The thickness is 1.0 to 3.4 mm, and the upper limit WBU (mm) of the bead width on the lower side of the weld metal is the thickness of the plate t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, the following formula (1) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and 3.4 mm or less, the following formula (2) is satisfied, and the lower limit value WBL (mm) of the bead width on the lower surface side of the weld metal is the plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (3) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and 3.4 mm or less, the following formula (4) is satisfied, and the upper limit value WSU (mm) of the bead width on the upper surface side of the weld metal is the plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 1.6 mm, then the following formula (5) is satisfied, and the plate thickness tＨ When it is 1.6 mm or more and less than 2.9 mm, the following formula (6) is satisfied, and the plate thickness t Ｈ When it is 2.9 mm or more and 3.4 mm or less, the following formula (7) is satisfied, and the lower limit value WSL (mm) of the bead width on the upper surface side 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, and the plate thickness t Ｈ When it is 1.6 mm or more and less than 2.9 mm, the following formula (9) is satisfied, and the plate thickness t Ｈ When it is 2.9 mm or more and 3.4 mm or less, the following formula (10) is satisfied. The lap fillet weld joint is characterized by: 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) In one aspect of the present invention, the "bead width on the lower side" in the overlap fillet weld joint is 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" is 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 to 0.25%, Si: greater than 0% and 0.70% or less, Mn: 1.4 to 2.3%, Ti: 0.04 to 0.15%, Al: 0.001 to 0.20%, N: 0.003 to 0.015%, O: 0.01 to 0.06%, Cr: 0 to 2.0%, Ni: 0 to 2.5%, B: 0 to 0.0100%, P: greater than 0% and 0.015% or less, S: greater than 0% and 0.013% or less, Sb: 0 to 0.10%, Sn: 0 to 0.4%, Cu: 0 to 0.50%, Nb: 0-0.3%, V: 0-0.3%, Mo: 0-1.0%, with the remainder being iron and impurities, and further satisfying the following equation (11): 7 × [Si] + 7 × [Mn] - 112 × [Ti] - 30 × [Al] ≤ 12.0 ... (11) where the element symbols in equation (11) are the mass percent content of each element contained in the weld metal. (3) In the lap fillet weld 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 may be 0.2-1.4% 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 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 second Zn-plated steel sheet may be 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 joining the end face of the fourth Zn-plated steel sheet on the third Zn-plated steel sheet side and the first surface of the third Zn-plated steel sheet on the fourth Zn-plated steel sheet side, wherein the chemical composition of the weld metal is, by mass%, Si: greater than 0% and 0.70% or less, and Ti: 0.04 to 0.15%, a portion of both surfaces of the fourth Zn-plated steel sheet is connected to the first surface of the third Zn-plated steel sheet via the weld metal, and the thickness of the fourth Zn-plated steel sheet is t Ｈ The thickness is 1.0 to 3.4 mm, and the upper limit WBU of the bead width on the lower surface side of the weld metal is the thickness of the plate t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (12) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (13) is satisfied, and the plate thickness t Ｈ If the thickness is 2.9 mm or more and 3.4 mm or less, the following formula (14) is satisfied, the lower limit WBL of the bead width on the lower surface of the weld metal satisfies the following formula (15), and the upper limit WSU of the bead width on the upper surface of the weld metal is the thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (16) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (17) is satisfied, and the plate thickness t Ｈ If the thickness is 2.9 mm or more and 3.4 mm or less, the following formula (18) is satisfied, and the lower limit value WSL of the bead width on the upper surface side of the weld metal is the plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.9 mm, then the following formula (19) is satisfied, and the plate thickness t ＨA T-shaped 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 (20). WBU ≤ 2.8 mm ... (12) WBU ≤ 1.33 t 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) In a T-fillet welded joint according to one aspect of the present invention, the "bead width on the lower side" is the width of the weld metal on the back side along the plate surface direction of the 4 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" is the width of the weld metal on the surface of the weld metal in the plate surface direction of the 4 Zn plated steel sheet in a cross section perpendicular to the extending direction of the weld metal (longitudinal direction of the weld bead). If it is difficult to determine the front and back sides of the weld metal in a T-fillet welded joint, the side being welded is determined to be the front side. If it is still difficult to determine, the side with the larger bead width is determined to be the front side, i.e., the "upper side," and the side with the smaller bead width is determined to be the back side, i.e., the "lower side." Furthermore, by comparing the distance in the direction of the third Zn-plated steel sheet's surface between the intersection point of the fourth Zn-plated steel sheet and the weld metal in a cross section perpendicular to the extending direction of the weld metal (the longitudinal direction of the weld bead), the larger distance can be determined to be the 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 to 0.25%, Si: greater than 0% and 0.70% or less, Mn: 1.4 to 2.3%, Ti: 0.04 to 0.15%, Al: 0.001 to 0.20%, N: 0.003 to 0.015%, O: 0.01 to 0.06%, Cr: 0 to 2.0%, Ni: 0 to 2.5%, B: 0 to 0.0100%, P: greater than 0% and 0.015% or less, S: greater than 0% and 0.013% or less, Sb: 0 to 0.10%, Sn: 0 to 0.4%, Cu: 0 to 0.50%, Nb: 0-0.3%, V: 0-0.3%, Mo: 0-1.0%, with the remainder being iron and impurities, and furthermore, the following equation (21) may be satisfied: 7 × [Si] + 7 × [Mn] - 112 × [Ti] - 30 × [Al] ≤ 12.0 ... (21) where the element symbols in equation (21) are the content of each element contained 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-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.

[0018] According to the above-described 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.

[0019] This is a schematic cross-sectional diagram illustrating a lap fillet welded joint for investigating the relationship between bead shape and porosity defects. This is a schematic cross-sectional diagram illustrating a lap fillet welded joint for investigating the relationship between bead shape and porosity defects. This is a diagram showing the results of an investigation into the relationship between bead shape and porosity defects in a lap fillet welded joint, and is a graph showing the relationship between the width of the front side of the bead and the plate thickness. This is a diagram showing the results of an investigation into the relationship between bead shape and porosity defects in a lap fillet welded joint, and is a graph showing the relationship between the width of the back side of the bead and the plate thickness. This is a schematic cross-sectional diagram illustrating perforation that occurs when the melting range is excessively expanded in the weld of a lap fillet welded joint. This is a schematic cross-sectional diagram of a lap fillet welded joint for illustrating the convection phenomenon of molten metal. This is a schematic cross-sectional diagram of a lap fillet welded joint according to one embodiment of the present invention. This is a schematic cross-sectional diagram illustrating a T-fillet welded joint for investigating the relationship between bead shape and porosity defects. This is a schematic cross-sectional diagram illustrating a T-fillet welded joint for investigating the relationship between bead shape and porosity defects. This is a diagram showing the results of an investigation into the relationship between bead shape and porosity defects in a T-fillet welded joint, and is a graph showing the relationship between the width of the front side of the bead and the plate thickness. This is a diagram showing the results of an investigation into the relationship between bead shape and porosity defects in a T-fillet welded joint, and is a graph showing the relationship between the width of the back side of the bead and the plate thickness. This is a schematic cross-sectional diagram illustrating perforation that occurs when the melting range is excessively expanded in the weld of a T-fillet welded joint. This is a schematic cross-sectional diagram illustrating the convection phenomenon of molten metal in a T-fillet welded joint. This is a schematic cross-sectional diagram illustrating a T-fillet welded joint according to one embodiment of the present invention. This is a schematic cross-sectional diagram illustrating a modified example of a T-fillet welded joint according to one embodiment of the present invention.

[0020] (1. Overlap Fillet Weld Joint) An overlap fillet weld joint according to one embodiment of the present invention and a preferred method for manufacturing the same will be described below. 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. In addition, 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 Invention> In general conventional lap fillet welding, as shown in Figure 1, first, one steel plate is overlapped so as to cover a part of the surface of the other steel plate, and the end of one steel plate and the surface of the other steel plate are joined by a weld. In other words, in general lap fillet welding, the weld is formed without penetrating the other steel plate, which is the so-called bottom plate, in the thickness direction. Here, if at least one of the two steel plates is a Zn-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 when zinc vapor is mixed 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 end of the other steel plate, which is the bottom plate. Note that "end of the steel plate" refers to the peripheral region including the end face of the steel plate.

[0022] <1.1.1 Regarding the bead shape> First, in order 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 layers 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]

[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] Welding conditions: Shielding gas: Ar + 5-20% CO ２ The welding conditions were set appropriately according to the plate thickness, with a welding current of 150-250A, a welding voltage of 20-25V, and a welding speed of 60-80 cm / min. A Fronius welding power supply was used. The plate thickness of the lower 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 Ｈ When using test steel plates of 1.6 mm or less in thickness, 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 of the bead width on the upper 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 ＨRegardless 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 the thickness is 1.0 mm or more and less than 1.6 mm, then WSU ≤ 5.0 ... (5) < plate thickness t Ｈ If the value is 1.6 mm or more and less than 2.9 mm, then WSU ≤ 3.07t Ｈ +0.08...(6) <Plate thickness t Ｈ If the diameter is between 2.9 mm and 3.4 mm, then WSU ≤ 9.0 ... (7)

[0031] [Lower limit of bead width on the top side WSL (mm)] < Plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 1.6 mm, then WSL ≥ 3.0 mm ... (8) < plate thickness t Ｈ If the thickness is 1.6 mm or more and less than 2.9 mm, then WSL ≥ 1.54t Ｈ +0.54...(9) <Plate thickness t Ｈ If the diameter is between 2.9 mm and 3.4 mm, then 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 the value is 1.0 mm or more and less than 2.0 mm, then WBU ≤ t Ｈ +0.8mm...(1) <Plate thickness t ＨIf the thickness is between 2.0 mm and 3.4 mm, then WBU ≤ 0.86t Ｈ +1.09 ... (2)

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

[0035] <1.1.2 Regarding the composition of the weld metal> Next, we examined 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. Moreover, 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.

[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, in lap fillet weld joints, we investigated a method for stably melting the back side of the weld bead while the overlapping steel plates are in close contact, thereby sufficiently exposing the weld metal. In this embodiment, "a state in which the steel plates are in close contact" is not limited to a state where the gap between the steel plates is 0 mm (i.e., the steel plates are in contact with each other), but also includes a state in which two steel plates are arranged 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 amount of Si 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 influence and effects of Si on the shape of the weld, particularly the weld bead.

[0042] (Effect 1: Enhanced Penetration by Arc Pressure) In arc welding, arc pressure is one of the factors influencing the depth of penetration. Arc pressure has the effect of pushing down the molten metal and is proportional to the welding current value. In other words, by increasing the welding current value, the effect of pushing down the molten metal by the arc pressure can be exerted. 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 viewpoint 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. In order 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. The lower the Si content in the weld metal, the better, 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 molten metal based on the distribution of surface tension of the molten metal. In particular, in the surface tension distribution on the back side of the molten metal, the difference between the surface tension on the steel plate side (near the steel plate) and the surface tension near the center of the molten metal greatly affects the convection phenomenon.

[0046] Figure 6 is a schematic cross-sectional view of a 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 the molten metal during welding varies depending on the region. Most 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 well-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.

[0048] The method for measuring the Si content of the weld metal is described below. In this embodiment, "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., "Si content of the weld metal," is measured.

[0049] <1.2. Overlap Fillet Welded Joint> Based on the results of the inventors' studies described above, the overlap 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 lap fillet welded 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 placed on the first surface 11a of the first Zn-plated steel sheet 11 in the "arrangement process" described later, so as to cover a part of the first surface 11a.

[0051] <1.2.1 Shape of the lap fillet welded joint> 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 substantially parallel to each other. That is, the second Zn-plated steel sheet 12 is provided so as to be substantially parallel to the plate 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 end 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 Zn-plated steel sheet and second 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. That is, either the upper plate or the lower plate may be a "Zn-plated steel sheet". In this case, the other steel sheet may be the base steel sheet described later, which does not have plating formed on it. 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. Also, the plating layer only needs to be provided on at least one side of the overlapping surface of 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 for 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 by emission spectroscopy using inductively coupled plasma (ICP) analysis with metal chips of the base steel sheet as a sample.

[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, 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 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] The type of high-strength steel sheet is not particularly limited. 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 sheet, the lower the joint strength in a normal welded joint. Therefore, the greater the tensile strength of the steel sheet, the more superior the effect of the lap fillet welded joint according to this embodiment becomes compared to a normal welded joint. The tensile strength of the high-strength steel sheet 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 sheet may be cold-rolled or hot-rolled. The tensile strength of the steel sheet can be measured by a tensile test in accordance with the JIS Z 2241:2022 standard. If a standard test piece specified in the same standard cannot be obtained due to the dimensions of the steel sheet, the value measured using a proportional test piece specified in the same standard or a test piece of appropriate dimensions 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 automobile parts and machine parts. Furthermore, the thicknesses of the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 that constitute 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 lap 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. 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 at 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 part 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 WBU (mm) of the bead width on the lower surface of the weld metal 13 is, Plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, the following formula (1) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and 3.4 mm or less, the following formula (2) is satisfied. Also, the lower limit value WBL (mm) of the bead width on the lower side of the weld metal 13 is the plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (3) is satisfied, and the plate thickness t Ｈ If the size is 2.0 mm or more and 3.4 mm or less, 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 lower side (back side) of the weld metal 13 satisfies the following equations (1) and (2). If the bead width on the lower side (back side) exceeds these upper limits, excessive heat input during welding may occur, resulting in 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 determined by the plate thickness t. Ｈ If the thickness is 1.0 mm or more and less than 1.6 mm, then the following formula (5) is satisfied, and the plate thickness t Ｈ If the thickness is 1.6 mm or more and less than 2.9 mm, then the following formula (6) is satisfied, and the plate thickness t Ｈ If the size is between 2.9 mm and 3.4 mm, the following formula (7) is satisfied.

[0066] Furthermore, the lower limit value WSL for the bead width on the upper surface of the weld metal is the plate thickness t. Ｈ If the thickness is 1.0 mm or more and less than 1.6 mm, then the following formula (8) is satisfied, and the plate thickness t Ｈ If the thickness is 1.6 mm or more and less than 2.9 mm, then the following formula (9) is satisfied, and the plate thickness t Ｈ If the size is 2.9 mm or more and 3.4 mm or less, 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 the following equations (5) to (7). 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 the following equations (8) to (10), 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 plate 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 weld metal exposed 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 lap 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 it is confirmed by the average value of three cross-sections.

[0075] <1.2.5 Chemical Composition of Weld Metal> Next, the chemical composition of the weld metal will be explained. 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 written 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. Also, 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, preferably 0.08% or more. On the other hand, if the C content exceeds 0.25%, the weld metal hardens, reducing its crack resistance and making the weld metal more prone to fracture. Therefore, the upper limit of the C content is 0.25% or less, preferably 0.20% or less, and more preferably 0.15% or less.

[0079] [Si: greater than 0%, 0.70% or less] Si is contained in the welding wire or base material 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 by 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. Also, if the weld metal contains an excess of Si, the amount of non-conductive Si-based slag will increase, 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 secured, and the weld metal becomes prone to fracture. Therefore, the lower limit of Mn is 1.4% or more, preferably 1.8% or more. On the other hand, if there is an excess of Mn, the viscosity of the molten metal increases, and when the welding speed is high, the molten metal cannot flow properly into the weld area, resulting in a humping bead and making it easy for defects in the bead shape to occur. As a result, the bead shape becomes uneven and defective, and red rust occurs. 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. In addition, Ti is an effective element for ensuring the conductivity of the 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 there is an excess of Ti, the amount of Ti-based slag increases, which reduces the adhesion between the Ti-based slag and the weld metal, making it easier to peel off. Therefore, red rust is more likely to occur in the peeled 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 strong deoxidizing element and has the effect of promoting the deoxidation of molten metal during arc welding, thereby suppressing the occurrence of blowholes. In addition, when a small amount of Al is included, it has the effect of reducing Si-based slag that adversely affects the electrodeposition coating of the weld. Therefore, the lower limit of the Al content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Al content is excessive, the amount of non-conductive Al-based slag increases, and red rust is more likely to occur 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, if it is an oxide of Mn alone, it does not affect electrodeposition coating properties, but a composite oxide of Si and Mn has the effect of reducing electrodeposition coating properties. For this reason, 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 for weld metals with various composition systems. As a result, it became clear 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 contained in the steel plate and welding wire, the amount of nitrogen (N) increases due to contamination from the atmosphere during welding. Since nitrogen is an element that reduces the toughness of the weld metal, it is desirable that the upper limit of the nitrogen content be 0.015% or less. The lower limit of the nitrogen content is not particularly limited and may be greater than 0%, but it may be set to 0.003% or more, which is the content of standard steel plates and welding wires. The nitrogen content may be 0.005% or more.

[0087] [O: 0.01-0.06%] Similar to N, the amount of O increases not only in the steel plate and welding wire, but also due to contamination from the atmosphere during welding. Excess O reduces the toughness of the weld metal. Therefore, it is desirable to keep the upper limit of the O content below 0.06%. There is no particular lower limit for the O content. In standard arc welding, more than 0.01% O is often present, so it may be 0.01% or higher. The O content may also be 0.02% or higher.

[0088] [P: greater than 0%, 0.015% or less] P is an element that is generally present as an impurity in steel, and is also normally present 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 noticeable, so the upper limit of the P content in the weld metal is 0.015% or less. The lower limit of the P content is not particularly limited, so it is greater than 0%, but from the viewpoint of cost and productivity of P removal, it may be 0.001%.

[0089] [S: greater than 0%, 0.013% or less] Like P, S is an element that is generally mixed in steel as an impurity, and is also usually contained as an impurity in welding wire, and therefore is also contained 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. There is no particular lower limit to the S content, so it is greater than 0%, but from the viewpoint of the cost and productivity of S 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. Note that the lower limit when these elements are not included is 0%.

[0091] [Cu: 0-0.50%] Although not essential, 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 welding cracks, so the upper limit for Cu content is 0.50% or less.

[0092] [Cr: 0-2.0%] Cr is not essential for improving the hardenability and tensile strength of the weld, but it may be included in amounts of 0.05% or more. On the other hand, if Cr is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Cr content is 2.0% or less.

[0093] [Nb: 0-0.3%] Nb 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 Nb is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Nb content is 0.3% or less.

[0094] [V: 0-0.3%] Although 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, 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 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 Mo is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Mo content is 1.0% or less.

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

[0097] [B: 0 to 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 and collecting the slag 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 cracks 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. Since it does not significantly affect the electrodeposition coating properties, the inclusion of Sn is not essential in this embodiment, but improving the corrosion resistance of the weld metal itself is also advantageous for machine parts. To obtain the corrosion resistance improvement effect of Sn, the Sn content may be 0.02% or more. On the other hand, if the Sn content exceeds 0.4%, the crack susceptibility of the weld metal increases, and hot cracking becomes more likely. In addition, excessive Sn may cause Sn segregation at the grain boundaries of the weld metal, leading to 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, components introduced during the manufacturing process, and 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 Overlap Fillet Welded Joints> Next, a preferred method for manufacturing an overlap 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, and comprises the steps of: arranging the Zn-plated steel sheets so as not to leave an excessive gap between the weld surfaces of the Zn-plated steel sheets (arrangement step); and welding the Zn-plated steel sheets together using a welding wire (welding step).

[0102] (Placement Process) In the placement 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 melt-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. The gap between the welded surfaces of the steel sheets may be 0 mm. In addition, the first Zn-plated steel sheet 11 and the second Zn-plated steel sheet 12 are placed 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, causing 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, the main component is Ar, and CO2 is present in a volume ratio of 5% to 20%. ２ It is a gas that contains [something].

[0105] The welding conditions are not particularly limited, but for example, the welding current may be 150 to 250 A, the welding voltage 20 to 25 V, and the welding speed 60 to 80 cm / min, 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 the penetration depth of the weld metal and improving the electroplating 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 electroplating 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, expressed as mass % of the total mass, is as follows: C: 0.04-0.12%, Si: greater than 0% and 0.30% or less, Mn: 1.40-2.30%, Ti: 0.04-0.25%, Al: 0.001-0.050%, P: greater than 0% and 0.015% or less, S: greater than 0% and 0.015% or less, N: greater than 0% and 0.01% or less, O: greater than 0% and 0.01% or less, 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-fillet welded joint) A T-fillet welded joint and a method for manufacturing the same according to one embodiment of the present invention will be described below. 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. In addition, 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 Invention Inventors' Study> In a typical conventional T-fillet weld, as shown in Figure 8, first, the end of one steel plate is abutted against the surface of one steel plate arranged in the vertical direction so as to be perpendicular to it, and the end of one steel plate is welded so that it does not penetrate in the thickness direction by the weld. Here, if at least the vertical steel plate of the two steel plates is a Zn-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 when zinc vapor is mixed 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 end of a steel plate arranged in the horizontal direction. Note that "end of the steel plate" refers to the peripheral region including the end face of the steel plate.

[0111] <2.1.1 Regarding the bead shape> First, in order 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 positioned 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. ２The 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] Welding conditions: Shielding gas: Ar + 5-20% CO ２ The welding conditions were set appropriately according to the plate thickness, with a welding current of 150-250A, a welding voltage of 20-25V, and a welding speed of 60-80 cm / min. 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 a test steel plate 202 thicker than 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 upper side (front side bead width), with respect to the thickness t of the test steel plate 202. Ｈ This shows the relationship between (mm) and the upper and lower limits of the bead width on the upper surface (mm). Figure 11 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 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 surface for each case. Specifically, the upper limit of the bead width on the upper surface 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 surface 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, causing the bead to have a discontinuous 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 tＨ When the plate thickness t is 1.0 mm or more and less than 2.0 mm > WSU ≤ 5.0 mm... (16) < Plate thickness t Ｈ When the plate thickness t is 2.0 mm or more and less than 2.9 mm > WSU ≤ 2.44t Ｈ + 0.11... (17) < Plate thickness t Ｈ When the plate thickness t is 2.9 mm or more and 3.4 mm or less > WSU ≤ 7.2 mm... (18)

[0119] [Lower limit of bead width on the upper surface side] < Plate thickness t Ｈ When the plate thickness t is 1.0 mm or more and less than 2.9 mm > WSL ≥ 3.5 mm... (19) < Plate thickness t Ｈ When the plate thickness t is 2.9 mm or more and 3.4 mm or less > 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, it was found that there is an optimal range for each plate thickness t of the test steel plate 202. Specifically, the upper limit of the bead width on the lower surface side is shown by the following formulas (12) to (14). If it exceeds these upper limit values, similar to the upper limit of the bead width on the upper surface side, the welding heat input becomes excessive, and welding defects such as holes and melting drops occur in the welded part. Note that if the bead width on the lower surface side is larger than the bead width on the upper surface side, welding defects such as holes and melting drops 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 shown by the following formula (15). In all cases of the plate thickness t Ｈ When the bead width is less than 0.7 mm, porosity defects such as blowholes occurred frequently. Ｈ

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

[0122] [Lower limit of bead width on the lower side] < plate thickness t Ｈ When it 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 porosity defects and welding defects and the composition of the weld metal was examined.

[0124] Conventionally, in order to suppress porosity defects such as blowholes caused by zinc vapor, a method of securing a path for discharging zinc vapor from the weld metal is known. Also, in order to promote the discharge of zinc vapor by melting the back surface of the weld bead to expose the back surface, it is effective to provide a gap between the steel plates.

[0125] However, for example, when applying a T-shaped welded joint to a curved automotive part, it is difficult to maintain a constant gap between the abutting steel plates, and an excessively large gap promotes perforation due to melting. Especially for parts with a more complex curved shape, it becomes more difficult to maintain a constant gap, so the occurrence of perforation due to melting becomes more prominent. In addition, if there is a gap between the steel plates abutted in a T-shape, spatter scatters from this gap toward the back side of the weld bead, so spatter adheres to a wide range of the part, resulting in poor appearance. Furthermore, when a large amount of spatter scatters and adheres to the part surface, a large amount of labor is required to remove the spatter, which is not economically preferable.

[0126] Therefore, it is desirable to discharge zinc vapor from the weld metal by realizing stable melting of the back surface of the weld bead in a state where the steel plates are in close contact with each other without providing a gap between the steel plates.

[0127] Therefore, in a T-shaped welded joint, a method of stably melting the back side of the weld bead in a state where the steel plates are in close contact with each other to sufficiently expose the weld metal was examined. Note that the "state where the steel plates are in close contact with each other" in this embodiment is not limited to a state where the gap between the steel plates is 0 mm (that is, a state where the steel plates are in contact with each other), but also includes a state where two steel plates are arranged without providing an excessive gap between the steel plates.

[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 amount of Si 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 influence and effects of Si on the shape of the weld, particularly the weld bead.

[0130] (Function 3: Enhanced Penetration by Arc Pressure) In arc welding, arc pressure is one of the factors influencing the depth of penetration. Arc pressure has the effect of pushing down the molten metal and is proportional to the welding current value. In other words, by increasing the welding current value, the effect of pushing down the molten metal by the arc pressure can be exerted. 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 viewpoint 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. In order 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. The lower the Si content in the weld metal, the better, and there is no lower limit. The Si content of the weld metal may be greater than 0%.

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

[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 the molten metal during welding varies depending on the region. Most 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 13), the molten steel plate and welding wire are not well-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 the weld metal is described below. In this embodiment, "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 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, the weld metal chips are collected by cutting that region with a drill or the like, and these chips are used as a sample to measure by emission spectroscopy using inductively coupled plasma (ICP). In this way, the average Si content near the center of the weld metal, i.e., "Si content of the weld metal," is measured.

[0137] <2.2. T-shaped fillet welded joint> Based on the results of the inventors' studies 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 a T-fillet welded joint 20 according to this embodiment. The T-fillet welded 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-fillet welded joint> In the T-fillet welded 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 steel sheet, are provided perpendicular to each other. That is, 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 with respect 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 to it.

[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. The 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. 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 part".

[0142] <2.2.2 Third Zn-plated steel sheet and fourth Zn-plated steel sheet> In the T-fillet welded joint 20 of this embodiment, the third Zn-plated steel 21 and the fourth Zn-plated steel sheet 22 are both 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 T-fillet welded joint 20 is shown as an example using the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22, but 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 the weld surface of the steel sheet, and does not necessarily need to be formed on both sides of each steel sheet. Also, 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 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 by emission spectrometry using inductively coupled plasma (ICP) analysis with metal chips from the base steel sheet as a sample.

[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] The type of high-strength steel sheet is not particularly limited. 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 sheet, the lower the joint strength in a normal welded joint. Therefore, the greater the tensile strength of the steel sheet, the more superior the effect of the welded joint according to this embodiment will be compared to a normal welded joint. The tensile strength of the high-strength steel sheet 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 sheet may be cold-rolled or hot-rolled. The tensile strength of the steel sheet can be measured by a tensile test in accordance with the JIS Z 2241:2022 standard. If a standard test piece specified in the same standard cannot be obtained due to the dimensions of the steel sheet, the value measured using a proportional test piece specified in the same standard or a test piece of appropriate dimensions 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-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. Note that 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. In this way, by creating a bead shape in which the intersection point PB is located 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 WBU (mm) of the bead width on the lower side of the weld metal 23 is, Plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (12) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (13) is satisfied, and the plate thickness t Ｈ If the value is between 2.9 and 3.4 mm, the following formula (14) is satisfied. Also, the lower limit value WBL (mm) of the bead width on the lower 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, in order to suppress welding defects such as holes and burn-through that occur in the weld, and porosity defects such as blowholes, the bead width is optimized to be within a predetermined range for each plate thickness. Specifically, the upper limit of the bead width on the underside (back side) of the weld metal satisfies the following equations (12) to (14). 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 is shown by the following equation (15), and in all plate thicknesses, if the bead width is less than 0.7 mm, porosity defects such as blowholes may occur.

[0152] Furthermore, the upper limit WSU (mm) of the bead width on the upper surface of the weld metal 23 is determined by the plate thickness t. Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (16) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (17) is satisfied, and the plate thickness t Ｈ If the value is between 2.9 and 3.4 mm, the following equation (18) is satisfied.

[0153] Furthermore, the lower limit value WSL (mm) of the bead width on the upper surface of the weld metal 23 is determined by the plate thickness t. Ｈ If the thickness is 1.0 mm or more and less than 2.9 mm, then the following formula (19) is satisfied, and the plate thickness t Ｈ If the size is 2.9 mm or more and less than 3.4 mm, then the following formula (20) is satisfied.

[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, 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.

[0156] Specifically, the upper limit of the bead width on the upper side (front side) is shown by the following equations (16) to (18). Exceeding these upper limits results in excessive heat input during welding, which may 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 upper side (front side) is shown by the following equations (19) and (20), 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 (19) and (20), porosity defects such as blowholes may occur.

[0157] Here, the "bead width" in this embodiment refers to the width of the weld metal (bead) 23 in the direction parallel to the surface of the T-shaped Zn-plated steel sheet 22 when the weld is viewed in cross-section along the width direction of the weld. In other words, the "bead width" is the width of the bead along the surface direction of the T-shaped 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 surface of the front side 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 surface of the back side of the bead. When observing the cross-section of the weld, the T-shaped fillet weld joint is cut perpendicular to the direction of extension of the weld metal (weld bead). Then, the weld metal on the cut surface is exposed by polishing and etching 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 explained. 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. The chemical composition of the weld metal will be expressed as a mass % relative to the total mass of the weld metal, and the mass % will be simply indicated as % in the explanation.

[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. Also, 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, preferably 0.08% or more. On the other hand, if the C content exceeds 0.25%, the weld metal hardens, reducing its crack resistance and making the weld metal more prone to fracture. Therefore, the upper limit of the C content is 0.25% or less, preferably 0.20% or less, and more preferably 0.15% or less.

[0162] [Si: greater than 0%, 0.70% or less] Si is contained in the welding wire or base material 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 by 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. Also, if the weld metal contains an excess of Si, the amount of non-conductive Si-based slag will increase, 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 secured, and the weld metal becomes prone to fracture. Therefore, the lower limit of Mn is 1.4% or more, preferably 1.8% or more. On the other hand, if there is an excess of Mn, the viscosity of the molten metal increases, and when the welding speed is high, the molten metal cannot flow properly into the weld area, resulting in a humping bead and making it easy for defects in the bead shape to occur. As a result, the bead shape becomes uneven and defective, and red rust occurs. 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. In addition, Ti is an effective element for ensuring the conductivity of the 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 there is an excess of Ti, the amount of Ti-based slag increases, which reduces the adhesion between the Ti-based slag and the weld metal, making it easier to peel off. Therefore, red rust is more likely to occur in the peeled 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 strong deoxidizing element and has the effect of promoting the deoxidation of molten metal during arc welding, thereby suppressing the occurrence of blowholes. In addition, when a small amount of Al is included, it has the effect of reducing Si-based slag that adversely affects the electrodeposition coating of the weld. Therefore, the lower limit of the Al content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Al content is excessive, the amount of non-conductive Al-based slag increases, and red rust is more likely to occur 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.

[0166] [Si, Mn, Ti, Al] Furthermore, it is preferable that the content of Si, Mn, Ti, and Al satisfies the following formula (21). As mentioned above, increasing the Si content reduces the electrodeposition coating properties, but increasing the Ti and Al content leads to an improvement in electrodeposition coating properties. Regarding Mn, if it is an oxide of Mn alone, it does not affect the electrodeposition coating properties, but a composite oxide of Si and Mn has the effect of reducing the electrodeposition coating properties. For this reason, it is not desirable to include an excessive amount of Mn.

[0167] The inventors investigated the occurrence of red rust between slag and weld metal for weld metals with various composition systems. As a result, it became clear 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 (21).

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

[0169] [N: 0.003-0.015%] In addition to the amount contained in the steel plate and welding wire, the amount of nitrogen (N) increases due to contamination from the atmosphere during welding. Since nitrogen is an element that reduces the toughness of the weld metal, it is desirable that the upper limit of the nitrogen content be 0.015% or less. The lower limit of the nitrogen content is not particularly limited and may be greater than 0%, but it may be set to 0.003% or more, which is the content of standard steel plates and welding wires. The nitrogen content may be 0.005% or more.

[0170] [O: 0.01-0.06%] Similar to N, the amount of O increases not only in the steel plate and welding wire, but also due to contamination from the atmosphere during welding. Excess O reduces the toughness of the weld metal. Therefore, it is desirable to keep the upper limit of the O content below 0.06%. There is no particular lower limit for the O content. In standard arc welding, more than 0.01% O is often present, so it may be 0.01% or higher. The O content may also be 0.02% or higher.

[0171] [P: greater than 0%, 0.015% or less] P is an element that is generally present as an impurity in steel, and is also normally present 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 noticeable, so the upper limit of the P content in the weld metal is 0.015% or less. The lower limit of the P content is not particularly limited, so it is greater than 0%, but from the viewpoint of cost and productivity of P removal, it may be 0.001%.

[0172] [S: greater than 0%, 0.013% or less] Like P, S is an element that is generally mixed in steel as an impurity, and is also usually contained as an impurity in welding wire, and therefore is also contained 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. There is no particular lower limit to the S content, so it is greater than 0%, but from the viewpoint of the cost and productivity of S 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. Note that the lower limit when these elements are not included is 0%.

[0174] [Cu: 0-0.50%] Although not essential, 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 welding cracks, so the upper limit for Cu content is 0.50% or less.

[0175] [Cr: 0-2.0%] Cr is not essential, but it may be included in amounts of 0.05% or more to improve the hardenability and tensile strength of the weld. On the other hand, if Cr is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Cr content is 2.0% or less.

[0176] [Nb: 0-0.3%] Nb 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 Nb is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Nb content is 0.3% or less.

[0177] [V: 0-0.3%] Although 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, 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.

[0178] [Mo: 0-1.0%] Mo 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 Mo is included in excess, the elongation of the weld will decrease. Therefore, the upper limit of the Mo content is 1.0% or less.

[0179] [Ni: 0-2.5%] Ni is not essential for improving the tensile strength and elongation of the weld, but it may be included in amounts of 0.05% or more. On the other hand, if Ni is included in excess, welding cracks are more likely to occur. Therefore, the upper limit of the Ni content is 2.5% or less. Preferably, it is 2.0% or less.

[0180] [B: 0 to 0.0100%] B is not essential for improving the hardenability of the weld and increasing its tensile strength, but it may be included in amounts of 0.0005% or more. 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 and collecting the slag 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 cracks 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. Since it does not significantly affect electrodeposition coating properties, the inclusion of Sn is not essential in this embodiment. However, 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 cause segregation at the grain boundaries of the weld metal, leading to 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, components introduced during the manufacturing process, and 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-fillet welded joint according to this embodiment will be described. The manufacturing method for the T-fillet welded joint according to this embodiment is a manufacturing method for a T-fillet welded joint having two Zn-plated steel sheets and a weld metal for joining the Zn-plated steel sheets together, and comprises a step of arranging the Zn-plated steel sheets so as not to leave an excessive gap between the weld surfaces of the Zn-plated steel sheets (arrangement step), and a step of welding the Zn-plated steel sheets together using a welding wire (welding step).

[0185] (Placement Process) In the placement process, the welded surfaces of the third Zn-plated steel sheet 21 and the fourth Zn-plated steel sheet 22 described above 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 melt-through. However, due to the dimensional accuracy of the steel sheets, it is difficult to reduce 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. 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 placed 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, the main component is Ar, and CO2 is present in a volume ratio of 5% to 20%. ２ It is a gas that contains [something].

[0189] The welding conditions are not particularly limited, but for example, welding current: 150-250A, welding voltage: 20-25V, welding speed: 60-80cm / min, 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 the penetration depth of the weld metal and improving the electroplating 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 electroplating 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, expressed as mass % of the total mass, is as follows: C: 0.04-0.12%, Si: greater than 0% and 0.30% or less, Mn: 1.40-2.30%, Ti: 0.04-0.25%, Al: 0.001-0.050%, P: greater than 0% and 0.015% or less, S: greater than 0% and 0.015% or less, N: greater than 0% and 0.01% or less, O: greater than 0% and 0.01% or less, 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 a preferred manufacturing method for producing the T-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 impede the operation and effect of the T-fillet welded joint of the present invention.

[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 overlap fillet welded joints with a coating were manufactured by arc welding and electrodeposition coating on a 150 mm x 100 mm first Zn-plated steel sheet (steel sheet 11; lower steel sheet) and a 150 mm x 50 m 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, in both base materials, a Zn-Fe alloy plating layer was formed on the surface by alloying a Zn hot-dip plating layer to which a trace amount of Al (less than 0.15%) was 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, and the size was 1.2 mm in diameter. The welding conditions were as follows.

[0196] • Welding power supply: CMT power supply (Fronius) • If the thickness of the steel plate 11 (lower steel plate) is greater than 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 the steel plate 11 (lower steel plate) is 1.6 m or less, apply a welding current of 150-200 A, an arc voltage of 20-23 V, and a wire feed control type low heat input welding mode. • Welding speed: 80 cm / 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 welds. 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 120 mm length main weld was performed, and the porosity ratio, the appearance of the weld bead, and the paint defects after electrodeposition coating were evaluated.

[0198] <Measurement of Pore Ratio> The porosity ratio was determined by measuring the weld length and pore length from X-ray radiographs taken after welding, and expressed as the ratio of the sum of pore lengths to the weld length. The porosity ratio was calculated for a 90 mm length of weld bead, excluding the 15 mm at the start and end of the weld, from a 120 mm 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 above 15% was evaluated as "× (fail)".

[0199] <Electrodeposition Coating Performance> After degreasing and chemical treatment of the prepared weld test specimens, electrodeposition coating was applied to a film thickness of 20 μm. The electrodeposited areas of the weld beads were then photographed, and the ratio of the area of ​​electrodeposition defects to the area of ​​the weld bead was measured from the images. The defect rate of electrodeposition coating was determined for a 90 mm length of weld bead, excluding the 15 mm at the start and end of the weld, from a bead length of 120 mm of the weld test specimen. Electrodeposition coating was performed using gray paint. This made it easier to distinguish between electrodeposition defects where reddish-brown or black slag was exposed and the painted areas. A "○" was given if the area of ​​defective coating was 10% or less by area ratio, and a "◎" was given if it was 5% or less. Welded joints with a defective coating area exceeding 10% by area ratio were evaluated as "× (fail)". In this example, welded joints with a paint defect area ratio of 10% or less ("〇") and 5% or less ("◎") were judged to be welded joints in which electrodeposition coating defects were suppressed.

[0200] <Appearance of Weld Beads> The shape of the weld beads was evaluated by visual inspection. Specifically, weld beads that appeared as "continuous and uniform beads" were judged to be of good quality. On the other hand, weld beads that showed "perforations" or "serpentine patterns" upon visual inspection were judged to have poor appearance.

[0201]

[0202]

[0203] The results are shown in Tables 4 and 5. Examples A1 to A11 are examples of the invention and satisfy 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 showed a good bead shape with a porosity ratio of 15% or less, no holes or 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 and a narrow back bead width, resulting in a porosity defect rate of over 15%. No. A13 had a large gap of 0.7 mm between the steel plates, which resulted in a wide back bead and a poorly drilled hole. No. A14 had insufficient heat input during welding, resulting in narrow front and back bead widths, porosity defects, and a meandering weld bead. No. A15 had an excessive Si content in the weld metal, and No. A16 had an insufficient Ti content in the weld metal, both exceeding the 15% standard for paint defects during electrodeposition coating. No. A17 had a wide gap of 0.6 mm between the steel plates, which resulted in a wide back bead and a poorly drilled hole. No. A18 had insufficient heat input during welding, resulting in a narrow front bead and a meandering weld bead. In No. A19, the overlap was small, making it difficult to maintain a stable overlap state, resulting in a widening of the back bead width and a hole-punching defect. In No. A20, the gap between the steel plates was large at 0.6 mm, resulting in a widening of both the front and back bead widths and a hole-punching defect. In both No. A21 and A22, the welding heat input was excessively high, resulting in hole-punching defects in the weld area. In No. A23, the welding heat input was insufficient, resulting in a narrow front bead width and a meandering weld bead. In No. A24, the welding heat input was insufficient, resulting in a narrow front bead width, resulting in porosity defects and a meandering weld bead.

[0205]

[0206]

[0207] (Example 2) Next, the chemical composition of the weld metal was investigated. The steel plates 11 and 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, weld bead appearance, and coating defects after electrodeposition coating were evaluated in the same manner 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 the chemical composition shown in Table 7, in all examples, the remainder consisted of iron and impurities.

[0209] As shown in Tables 7 and 8, in the case of weld beads 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 inventive 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 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 between 10% and 15%. For No. A25 and A30, equation (11) was not satisfied, and therefore the electrodeposition coating defect rate on the back bead was between 10% and 15%.

[0211]

[0212]

[0213]

[0214] (Example 3) Various T-shaped fillet welded joints with a coating were manufactured by arc welding and electrodeposition coating on a 150 mm x 100 mm third-Zn plated steel sheet (steel sheet 21) and a 150 mm x 50 m fourth-Zn plated steel sheet (steel sheet 22; butt steel sheet). The chemical composition of the base materials of the third-Zn plated steel sheet (steel sheet 21) and the second-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 Zn hot-dip plating layer to which a trace amount of Al of 0.15% or less was 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 (Fronius) • If the thickness of the steel plate 12 (butt plate) is greater than 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 1.6 m or less, apply a welding current of 150-200 A, an arc voltage of 20-23 V, and a wire feed control type low heat input welding mode. • Welding speed: 80 m / min

[0217] During welding, a 150 mm x 50 m steel plate 22 (butt plate) was butted against a 150 mm x 100 mm 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 120 mm length of main welding was performed, and the porosity ratio, the appearance shape of the weld bead, and the paint defects after electrodeposition coating were evaluated.

[0218] <Measurement of Pore Ratio> The porosity ratio was determined by measuring the weld length and pore length from X-ray radiographs taken after welding, and expressed as the ratio of the sum of pore lengths to the weld length. The porosity ratio was calculated for a 90 mm length of weld bead, excluding the 15 mm at the start and end of the weld, from a 120 mm 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 above 15% was evaluated as "× (fail)".

[0219] <Electrodeposition Coating Performance> After degreasing and chemical treatment of the prepared weld test specimens, electrodeposition coating was applied to a film thickness of 20 μm. The electrodeposited areas of the weld beads were then photographed, and the ratio of the area of ​​electrodeposition defects to the area of ​​the weld bead was measured from the images. The defect rate of electrodeposition coating was determined for a 90 mm length of weld bead, excluding the 15 mm at the start and end of the weld, from a bead length of 120 mm of the weld test specimen. Electrodeposition coating was performed using gray paint. This made it easier to distinguish between electrodeposition defects where reddish-brown or black slag was exposed and the painted areas. A "○" was given if the area of ​​defective coating was 10% or less by area ratio, and a "◎" was given if it was 5% or less. Welded joints with a defective coating area exceeding 10% by area ratio were evaluated as "× (fail)". In this example, welded joints with a paint defect area ratio of 10% or less ("〇") and 5% or less ("◎") were judged to be welded joints in which electrodeposition coating defects were suppressed.

[0220] <Appearance of Weld Beads> The shape of the weld beads was evaluated by visual inspection. Specifically, weld beads that appeared as "continuous and uniform beads" were judged to be of good quality. On the other hand, weld beads that showed "perforations" or "serpentine patterns" upon visual inspection were judged to have poor appearance.

[0221] The results are shown in Tables 9 and 10. Nos. B1 to B10 are examples of inventions and 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 showed a good bead shape with a porosity ratio of 15% or less, no holes or 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, in No. B11, the gap between the steel plates was set narrowly, but 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, the back bead width was wide, and a hole-punching defect occurred. In No. B13, the welding heat input was insufficient, resulting in narrow front and back bead widths, porosity defects occurred, and the weld bead was meandering. In No. B14, the Ti content of the weld metal was insufficient, and in No. B15, the Si content of the weld metal was excessive, and in both cases, the coating defect during electrodeposition coating exceeded the 15% standard. In No. B16, the front bead width was wide due to excessive heat input and a hole-punching defect occurred. In Nos. B17-B19 and B21, excessive heat input resulted in wide front or back bead widths and hole-punching defects. Although B20 had a narrow gap between the steel plates, the back bead width was narrow and the porosity defect rate exceeded 15%.

[0223]

[0224]

[0225] (Example 4) Next, the chemical composition of the weld metal was investigated. 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 the paint defects after electrodeposition coating 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 the chemical composition shown in Table 12, in all examples, the remainder consisted of iron and impurities.

[0227] As shown in Tables 12 and 13, in the case of weld beads 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. In No. B23 and No. B25, the Si ratio of the weld metal to the base steel plate exceeded 1.0, and the width of the back bead was slightly narrower, resulting in a porosity defect rate of more than 10% and 15% or less. In No. B31, formula (21) was not satisfied, so the electrodeposition coating defect rate was more than 10% and 15% or less.

[0228]

[0229]

[0230]

[0231] 10,100 Overlap fillet weld joint 11 First Zn-plated steel sheet 12 Second Zn-plated steel sheet 13 Weld metal (bead) 20,200 T-fillet weld joint 21 Third Zn-plated steel sheet 22 Fourth Zn-plated steel sheet 23, 23A Weld metal (bead) 101, 102, 201, 202 Test steel sheet

[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. A lap fillet weld joint comprising a first Zn-based plated steel sheet, a second Zn-based plated steel sheet disposed above a first surface of the first Zn-based plated steel sheet, a welding metal joining an end face of the second Zn-based plated steel sheet and the first surface of the first Zn-based plated steel sheet, wherein the chemical composition of the welding metal contains, in mass%, Si: more than 0% and 0.70% or less, and Ti: 0.04 to 0.15%, at least a part of the end face of the first Zn-based plated steel sheet is covered by the welding metal, and the plate thickness t of the first Zn-based plated steel sheet Ｈ is 1.0 to 3.4 mm, and the upper limit value WBU (mm) of the bead width on the lower surface side of the welding metal satisfies the following formula (1) when the plate thickness t Ｈ is 1.0 mm or more and less than 2.0 mm, and satisfies the following formula (2) when the plate thickness t Ｈ is 2.0 mm or more and 3.4 mm or less. The lower limit value WBL (mm) of the bead width on the lower surface side of the welding metal satisfies the following formula (3) when the plate thickness t Ｈ is 1.0 mm or more and less than 2.0 mm, and satisfies the following formula (4) when the plate thickness t Ｈ is 2.0 mm or more and 3.4 mm or less. The upper limit value WSU (mm) of the bead width on the upper surface side of the welding metal satisfies the following formula (5) when the plate thickness t Ｈ is 1.0 mm or more and less than 1.6 mm, and satisfies the following formula (6) when the plate thickness t Ｈ is 1.6 mm or more and less than 2.9 mm, and satisfies the following formula (7) when the plate thickness t Ｈ is 2.9 mm or more and 3.4 mm or less. The lower limit value WSL (mm) of the bead width on the upper surface side of the welding metal satisfies the following formula (8) when the plate thickness t Ｈ is 1.0 mm or more and less than 1.6 mm, and satisfies the following formula (9) when the plate thickness t Ｈ is 1.6 mm or more and less than 2.9 mm, and satisfies the following formula (10) when the plate thickness t Ｈ is 2.9 mm or more and 3.4 mm or less, characterized in that the lap fillet weld joint satisfies the following. 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 as follows, in mass%, C: 0.06-0.25%, Si: greater than 0% and 0.70% or less, 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: greater than 0% and 0.015% or less, S: greater than 0% and 0.013% or less, Sb: 0-0.10%, Sn: 0-0.4%, Cu: 0-0.50%, Nb: 0-0.3%, V: 0-0.3% The lap fillet welded joint according to claim 1, characterized in that Mo: 0 to 1.0%, the remainder consisting of iron and impurities, and further satisfying the following formula (11): 7 × [Si] + 7 × [Mn] - 112 × [Ti] - 30 × [Al] ≤ 12.0 ... (11) where the element symbols in formula (11) represent the mass percentage content of each element contained in the weld metal.

3. The lap fillet welded joint according to claim 1 or 2, characterized in that 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 less than 1.

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

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. A T-shaped fillet welded joint comprising: 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 joining the end face of the fourth-Zn plated steel sheet on the third-Zn plated steel sheet side and the first surface of the third-Zn plated steel sheet on the fourth-Zn plated steel sheet side, wherein the chemical composition of the weld metal is, by mass%, Si: greater than 0% and 0.70% or less, and Ti: 0.04 to 0.15%, 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, and the thickness of the fourth-Zn plated steel sheet is t Ｈ The thickness is 1.0 to 3.4 mm, and the upper limit WBU of the bead width on the lower surface side of the weld metal is the thickness of the plate t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (12) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (13) is satisfied, and the plate thickness t Ｈ If the thickness is 2.9 mm or more and 3.4 mm or less, the following formula (14) is satisfied, the lower limit WBL of the bead width on the lower surface of the weld metal satisfies the following formula (15), and the upper limit WSU of the bead width on the upper surface of the weld metal is the thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.0 mm, then the following formula (16) is satisfied, and the plate thickness t Ｈ If the thickness is 2.0 mm or more and less than 2.9 mm, then the following formula (17) is satisfied, and the plate thickness t Ｈ If the thickness is 2.9 mm or more and 3.4 mm or less, the following formula (18) is satisfied, and the lower limit value WSL of the bead width on the upper surface side of the weld metal is the plate thickness t Ｈ If the thickness is 1.0 mm or more and less than 2.9 mm, then the following formula (19) is satisfied, and the plate thickness t Ｈ A T-shaped 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 (20). WBU ≤ 2.8 mm ... (12) WBU ≤ 1.33 t 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 as follows, in mass%, C: 0.06-0.25%, Si: greater than 0% and 0.70% or less, 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: greater than 0% and 0.015% or less, S: greater than 0% and 0.013% or less, Sb: 0-0.10%, Sn: 0-0.4%, Cu: 0-0.50%, Nb: 0-0.3%, V: 0-0.3% A T-fillet welded joint according to claim 6, characterized in that Mo: 0 to 1.0%, the remainder consisting of iron and impurities, and further satisfying the following formula (21): 7 × [Si] + 7 × [Mn] - 112 × [Ti] - 30 × [Al] ≤ 12.0 ... (21) where the element symbols in formula (21) are the content of each element contained in the weld metal.

8. The T-fillet welded joint according to claim 6 or 7, characterized in that 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 of the third 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 fourth Zn-plated steel sheet is less than 1.

9. The T-fillet welded joint according to claim 6 or 7, characterized in that the tensile strength of the third Zn-plated steel sheet and the fourth Zn-plated steel sheet is 780 MPa or more.

10. The T-fillet welded joint according to claim 6 or 7, characterized in that 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.

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