Welded joints, welded joint manufacturing methods, automobile parts, and building material parts

Abstract

The welded joint according to an embodiment of the present invention comprises: a first steel plate; a second steel plate; a weld bead with which an end of the first steel plate is bonded to a first surface of the second steel plate; an HAZ; a coating film disposed on the surfaces of the welded joint; and a scale lying between the HAZ and the coating film so as to be in contact with the HAZ on the second-surface side of the second steel plate that is opposite from the first surface. The second steel plate has a chemical composition satisfying Ti+1.2×Al-1.2×C-7×P>0. The weld bead has a penetration depth of 50-90% with respect to the thickness of the second steel plate. The scale contains, in terms of atm.%, 2.00-60.00% Ti and 2.00-30.00% Al. In the region overlying the HAZ on the second-surface side of the second steel plate, the areal proportion of the coating film which peels off in a coating-film peel test is 15% or less.

Description

[Technical Field]

[0001] The present invention relates to a welded joint, a method for manufacturing a welded joint, an automobile part, and a building material part. This application claims priority based on Japanese Patent Application No. 2022-107799, filed on July 4, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] For automobile chassis components, plate thickness is designed with corrosion in mind to ensure long-term strength reliability in salt-damaged areas. This places a limit on the minimum plate thickness that can be applied. In order to use thin, high-strength steel plates for chassis components and reduce the weight of automobiles, it is necessary to improve the corrosion resistance of chassis components after painting. The corrosion resistance of welds after painting refers to the corrosion resistance of welds after painting and forming a paint film.

[0003] In order to improve the corrosion resistance of welds after painting, it is important to improve the paintability of the welds while also improving the paint adhesion. Paintability or paint adhesion refers to the ease of forming a paint film. A paint film with fewer defects can be formed on a weld with high paintability. Furthermore, paint adhesion refers to the adhesion of the formed paint film. A paint film formed on a weld with high paint adhesion maintains its corrosion resistance effect for a long period of time.

[0004] Furthermore, the back side of a weld is often located inside a component. Here, the front side of a weld refers to the side where the weld bead is located, and the back side refers to the side opposite the front side. It is difficult to improve the corrosion resistance of the back side of a weld located inside a component after painting using post-welding processes. Therefore, it is necessary to improve the corrosion resistance of the weld after painting while maintaining the welded state.

[0005] Patent Document 1 discloses a cleaning solution for cleaning steel welded structures, which is an acidic solution containing EDTA or a salt thereof, and a cleaning method for steel welded structures in which the steel welded structure is immersed in the acidic solution containing EDTA or a salt thereof. The technology in Patent Document 1 is said to effectively remove oxide films and fumes from the steel welded structure, improving the paintability of the welded parts.

[0006] Patent Document 2 discloses a solid wire for gas-shielded arc welding for joining multiple thin steel plates by gas-shielded arc welding, containing, by mass percent relative to the total mass of the wire, 0.05-0.20% C, 0.01-0.18% Si, 1.0-3.0% Mn, 0.06-0.25% Ti, 0.003-0.10% Al, 0-0.0100% B, more than 0-0.015% P, more than 0-0.015% S, and optional elements, with the balance being iron and impurities, satisfying Si×Mn≦0.30 and (Si+Mn / 5) / (Ti+Al)≦3.0, and further having a Ceq of 0.40-0.90%. The technology described in Patent Document 2 is said to be capable of forming welds with excellent electrodeposition coatability and mechanical properties.

[0007] Patent Document 3 discloses a gas-shielded arc welding method for welding steel sheet W having a tensile strength of 780 MPa or more using a shielding gas containing 92 to 99.5% by volume of Ar, in which the value calculated by equation (1) is 0.20 or more, where CAr is the Ar content (volume %) in the shielding gas, D is the inner diameter (mm) of the nozzle supplying the shielding gas, v is the welding speed (cm / min), and I is the welding current (A). The technology in Patent Document 3 is said to be able to suppress defects in electrodeposition coating after welding and improve the corrosion resistance of structural members. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-21227 [Patent Document 2] Patent Publication No. 2021-3732 [Patent Document 3] Japanese Patent Publication No. 2020-66036 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the technique of Patent Document 1 improves paintability by removing the oxide film. For components with structures that make post-processing after welding difficult, the cleaning effect of immersion is insufficient, and the technique of Patent Document 1 cannot be applied.

[0010] The techniques of Patent Documents 2 and 3 can improve the paintability of welds without requiring post-treatment such as slag removal. However, the techniques of Patent Documents 2 and 3 do not take into consideration the adhesion of the coating. Furthermore, while the techniques of Patent Documents 2 and 3 modify the oxides adhering to the front side of the weld, i.e., the side where the weld bead is formed, they do not consider at all the modification of the oxides formed on the back side of the weld.

[0011] In view of the above circumstances, an object of the present invention is to provide a welded joint that can be produced by painting in an as-welded state and that has high corrosion resistance after painting on both the front and back sides of the weld, a method for producing the same, an automobile part, and a building material part. [Means for solving the problem]

[0012] The gist of the present invention is as follows.

[0013] (1) A welded joint according to one aspect of the present invention comprises a first steel plate, a second steel plate, a weld bead joining an end of the first steel plate and a first surface of the second steel plate, a HAZ formed around the weld bead, coating films provided on the surfaces of the first steel plate, the second steel plate, and the weld bead, respectively, and scale in contact with the HAZ and between the HAZ and the coating film on a second surface side of the second steel plate, which is the reverse side of the first surface, wherein the chemical composition of the second steel plate includes, in unit mass%, C: 0.03 to 0.20%, Si: 0.02 to 0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%, and the chemical composition of the second steel plate satisfies Formula 1, Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) The element symbols included in Formula 1 are the contents in unit mass% of the elements corresponding to the element symbols in the second steel plate, the penetration depth of the weld bead is 50% or more and 90% or less of the plate thickness of the second steel plate, the scale contains, in unit atomic%, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%, and in the region on the HAZ on the second surface side of the second steel plate, the area proportion of the coating that peels off in a coating peeling test in accordance with JIS H 8504:1999 is 15% or less. (2) The welded joint described in (1) above may be a lap joint or a T-joint.

[0014] (3) A method for manufacturing a welded joint according to another aspect of the present invention includes a step of arc welding an end portion of a first steel plate and a first surface of a second steel plate to form a weld bead joining the end portion of the first steel plate and the first surface of the second steel plate, and a step of applying a coating to the first steel plate, the second steel plate, and the weld bead to provide a coating film on each surface. In the step of forming the weld bead, a HAZ is formed around the weld bead, the HAZ is exposed from the second surface side of the second steel plate, a scale is formed on the surface of the exposed HAZ, and in the step of providing the coating film, the coating film is provided on the HAZ and the scale on the second surface side of the second steel plate. The second steel plate has a chemical composition containing, in mass%, C: 0.03 to 0.20%, Si: 0.02 to 0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%. The second steel plate has a chemical composition satisfying Formula 1. Ti + 1.2×Al - 1.2×C - 7×P > 0 Formula (1) The element symbols included in Formula 1 are the contents in mass% of the elements corresponding to the element symbols in the second steel plate. The penetration depth of the weld bead is 50% or more and 90% or less with respect to the plate thickness of the second steel plate. The scale contains, in atomic%, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%. Before the arc welding, the arithmetic mean height Sa and the maximum height Sz defined in ISO 25178:2021 of the region on the second surface of the second steel plate where the HAZ is formed by the arc welding are within the range satisfying Formula 2. Thereby, the area ratio of the coating film peeled by the coating film peeling test conforming to JIS H 8504:1999 in the region on the HAZ on the second surface side of the second steel plate is 15% or less. 20μm < Sz - Sa < 100μm (Formula 2). (4) In the method for manufacturing a welded joint described in (3) above, the welded joint may be an overlap joint or a T-joint.

[0015] (5) An automobile component according to another aspect of the present invention includes the welded joint described in (1) or (2) above.

[0016] (6) A building material component according to another aspect of the present invention includes the welded joint described in (1) or (2) above. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a welded joint that can be produced by painting in an as-welded state and that has high corrosion resistance after painting on both the front and back surfaces, a method for producing the same, an automobile part, and a building material part. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a cross-sectional view perpendicular to the weld line of the lap fillet welded joint according to the present embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a T-joint according to the present embodiment, taken perpendicular to the weld line. [Figure 3] FIG. 1 is a perspective view of a lap fillet weld joint before painting. [Figure 4] FIG. 4 is an enlarged cross-sectional view of a portion of the second surface of the second steel plate where a HAZ is formed. [Figure 5A] This is a cross-sectional photograph of the scale of a welded joint with a shallow penetration depth D. [Figure 5B] This is a cross-sectional photograph of the scale of a welded joint with a deep penetration depth D. [Figure 6] FIG. 1 is a schematic diagram of a coating peeling test. DETAILED DESCRIPTION OF THE INVENTION

[0019] As illustrated in FIGS. 1 to 4 , a welded joint 1 according to one embodiment of the present invention comprises a first steel plate 11, a second steel plate 12, a weld bead 13 joining an end 111 of the first steel plate 11 and a first surface 121 of the second steel plate 12, a HAZ 14 formed around the weld bead 13, coating films 15 provided on the surfaces of the first steel plate 11, the second steel plate 12, and the weld bead 13, and a coating film 15 on the first surface 121 of the second steel plate 12. and a scale 16 on a second surface 122 side, which is the opposite surface to the HAZ 14, in contact with the HAZ 14 and between the HAZ 14 and the coating film 15, the chemical composition of the second steel plate 12 containing, in unit mass %, C: 0.03 to 0.20%, Si: 0.02 to 0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%, and the chemical composition of the second steel plate 12 satisfies formula 1, Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) The element symbols included in formula 1 are the contents in unit mass % of the elements corresponding to the element symbols in the second steel plate 12, the penetration depth D of the weld bead 13 is 50% or more and 90% or less of the plate thickness t2 of the second steel plate 12, the scale 16 contains, in unit atomic %, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%, and in the region on the HAZ 14 on the second surface 122 side of the second steel plate 12, the area proportion of the coating 15 that peels off in a coating peeling test in accordance with JIS H 8504:1999 is 15% or less.

[0020] The first surface 121 of the second steel plate 12 refers to the surface of the two surfaces of the second steel plate 12 that is joined to the first steel plate 11 by the weld bead 13. The second surface 122 of the second steel plate 12 refers to the surface opposite to the first surface 121. The surface corresponding to the front side of the welded portion described above is the first surface 121 of the second steel plate 12, and the surface corresponding to the back side of the welded portion is the second surface 122 of the second steel plate 12. The welded portion is a general term for the portion including the weld bead 13 and the HAZ 14.

[0021] The present inventors have found that by setting the chemical composition of the second steel plate 12, the surface roughness of the second surface 122 of the second steel plate 12 before welding, and the heat input during welding all within predetermined ranges, the corrosion resistance of the welded portion after painting can be dramatically improved without post-treatment such as pickling. The inventors' findings are described in detail below.

[0022] When considering ways to improve the corrosion resistance of welds after painting, it is necessary to consider the characteristics of both the front and back sides of the weld. On the front side of the weld, slag affects the corrosion resistance after painting, while on the back side, scale 16 affects the corrosion resistance after painting. Slag is an oxide that is expelled from the weld metal during welding. Scale 16 is an oxide that is formed when the surface of the base steel plate is oxidized by the welding heat. Slag and scale 16 are distinguished by their composition. Slag is an oxide primarily composed of easily oxidizable elements such as Si, Mn, Ti, Al, Mg, Zn, Cr, Zr, and Ca, while scale 16 is an oxide primarily composed of iron.

[0023] The slag formed on the surface of the weld bead 13 on the front side of the weld is usually non-conductive silicon-based slag, which causes coating defects during painting. Coating defects impair corrosion resistance after painting, causing a reduction in plate thickness due to corrosion and resulting in a decline in component performance. Scale 16 formed on the surface of the HAZ 14 on the back side of the weld reduces paint film adhesion. When paint film adhesion decreases, the paint film 15 peels off, resulting in a decline in corrosion resistance after painting. To solve these two problems, (1) Countermeasures against non-conductive slag that reduces paintability of the weld bead 13 on the front side of the weld (2) Measures to prevent scale 16, which reduces paint adhesion in the HAZ 14 on the back side of the weld However, this is necessary to ensure corrosion resistance after painting on both the front and back sides of the weld. To solve these problems, the inventors investigated various factors that affect paintability and paint adhesion. As a result, the following findings were obtained.

[0024] (1. Chemical Composition of Second Steel Plate 12) The chemical composition of the second steel plate 12 includes Ti. The Ti contained in the second steel plate 12 becomes an oxide in the molten metal during welding. When the molten metal solidifies, the Ti is expelled from the weld bead 13 as slag. Ti also improves the electrical conductivity of the slag formed on the surface of the weld bead 13. The slag that adheres to the surface of the weld bead 13 is typically an insulating material primarily composed of Si, which can cause coating defects during painting. However, by using the Ti contained in the second steel plate 12 to increase the electrical conductivity of the slag, coating defects on the front side of the weld can be suppressed.

[0025] Furthermore, the chemical composition of the second steel plate 12 contains Al. The present inventors have found that by applying the welding method described below to a steel plate containing Ti and Al, it is possible to dramatically improve the adhesion of the coating film 15 on the back side of the weld, particularly on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12.

[0026] It is presumed that the adhesion of the coating film 15 is improved by the Al-Ti-Fe oxides contained in the scale 16 formed on the surface of the second steel sheet 12. According to the results of investigations by the present inventors, when the scale 16 between the surface of the HAZ 14 and the coating film 15 contains, in atomic %, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%, the coating adhesion of the HAZ 14 is improved. It is presumed that the Ti and Al in the scale 16 form oxides together with Fe. The scale formed on the surface of a steel plate with a high Si content is composed of fayalite (Fe2SiO4). Scale composed of fayalite does not contain 2.00 mass% or more of Ti or 2.00 mass% or more of Al. In the welded joint according to this embodiment, the second steel plate 12 has a low Si content, so fayalite is not formed on the surface of the second steel plate 12.

[0027] In addition, the chemical composition of the second steel plate 12 satisfies Equation 1. Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) Here, the element symbols included in Formula 1 represent the contents in unit mass % of the elements corresponding to the element symbols in the second steel plate 12. Formula 1 determines the upper limits of C and P depending on the contents of Ti and Al.

[0028] C and P are elements that cause blistering. Some of the C contained in the steel sheet becomes CO gas or CO2 gas due to welding heat and is released outside the steel sheet. These gases peel off the scale 16, impairing paint adhesion. P also embrittles the grain boundaries of the steel sheet. In this case, the adhesion at the interface between the base steel and the scale falls below the pressure of the CO gas or CO2 gas, causing peeling. In other words, P further promotes the blistering caused by C. On the other hand, when the chemical composition of the second steel sheet 12 satisfies Formula 1, blistering is suppressed and paint adhesion in the HAZ 14 is further improved.

[0029] (2. Welding conditions) In order to control the composition of scale 16 as described above, it is necessary to optimize the welding conditions as well as the chemical composition of the second steel plate 12. Specifically, it is necessary to control the welding heat input so that the ratio of the penetration depth D to the plate thickness t2 of the second steel plate is 50% or more and 90% or less.

[0030] This penetration depth D is greater than that under normal welding conditions. For example, in lap joints used in automobile parts, the penetration depth D is usually set to about 40%. This is to reliably prevent a phenomenon known as strike-through, in which the weld metal penetrates all the way to the second surface 122 of the second steel sheet 12. If strike-through occurs, irregularities will form on the second surface 122 of the second steel sheet 12, impairing corrosion resistance after painting.

[0031] However, according to the results of investigations by the present inventors, it was considered that the Ti content and Al content of the scale 16 could not be increased unless welding was performed under heat input conditions that set the penetration depth D to 50% or more. It is presumed that in order to form Al-Ti-Fe oxides on the second surface 122 of the second steel plate 12, it is necessary to heat the second surface 122 of the second steel plate 12 sufficiently.

[0032] The inventors also discovered that the crystalline structure of the scale 16 differs between when the penetration depth D is 50% or more and when it is less than 50%. The scale 16 is primarily composed of FeO (wüstite), Fe2O3 (hematite), and Fe3O4. The higher the heat input, the thicker the layer composed of Fe2O3 and Fe3O4 becomes. FIGS. 5A and 5B show backscattered electron images of a cross section of the scale 16. FIG. 5A is a backscattered electron image of a cross section of the scale 16 formed under heat input conditions that result in a shallow penetration depth D, and FIG. 5B is a backscattered electron image of a cross section of the scale 16 formed under heat input conditions that result in a deep penetration depth D. The layer composed of FeO is shown in a light-colored region at the top of the backscattered electron image, and the layer composed of Fe2O3 and Fe3O4 is shown in a dark-colored region at the bottom of the backscattered electron image. These backscattered electron images show that the greater the heat input, the thicker the layer composed of Fe2O3 and Fe3O4.

[0033] The thermal expansion coefficients of Fe2O3 and Fe3O4 are lower than that of FeO and are closer to that of the second steel sheet 12. Therefore, it is estimated that the more Fe2O3 and Fe3O4 there are, the more the peeling of the scale 16 is suppressed, and the more the paint film adhesion in the HAZ 14 exposed on the second surface 122 of the second steel sheet 12 is improved.

[0034] According to the results of investigations by the present inventors, even when the penetration depth D was set to 50% or more of the plate thickness t2 of the second steel plate 12, strike-through did not occur over the entire weld bead 13. However, when the penetration depth D was set to more than 90% of the plate thickness t2 of the second steel plate 12, strike-through occurred in part of the weld bead 13. For this reason, the penetration depth D is set to 90% or less of the plate thickness t2 of the second steel plate 12.

[0035] (3. Surface roughness of the second surface 122 of the second steel plate 12 before welding) The unevenness on the surface of the steel plate is a factor contributing to the adhesion of Scale 16. The greater the surface roughness, the greater the anchor effect, and the higher the adhesion of Scale 16. Therefore, it is necessary to increase the surface roughness before welding in at least the region where the HAZ 14 is formed on the second surface 122 of the second steel plate 12. Specifically, the surface roughness of this region is adjusted before welding so that the arithmetic mean height Sa and the maximum height Sz of this region, as defined in ISO 25178:2021, satisfy the following formula 2. 20μm < Sz - Sa < 100μm (Formula 2) By setting Sz - Sa to be 20μm or more, the effect of improving adhesion due to the anchor effect can be obtained. However, if Sz - Sa is excessively large, the paintability is impaired. Therefore, Sz - Sa is less than 100μm. The above values are the values measured before welding. The surface roughness of the steel plate changes slightly before and after welding. Therefore, the surface roughness of the second steel plate 12 after welding is not particularly limited.

[0036] By combining the three elements described above, namely the chemical composition of the second steel plate 12, the heat input, and the surface roughness of the second surface 122 of the second steel plate 12 before welding, it is possible to improve the coating adhesion of the HAZ 14 on the back side of the welded joint, that is, on the second surface 122 of the second steel plate 12. Specifically, when a coating peel test is performed on the region above the HAZ on the second surface 122 side of the second steel plate 12, the area ratio of the peeling coating 15 can be made 15% or less. Furthermore, by combining the three elements described above, it is also possible to suppress coating defects on the front side of the welded joint. As a result, it becomes possible to provide a welded joint 1 with excellent surface quality and corrosion resistance after coating, which has high corrosion resistance after coating and does not cause a decrease in production efficiency or an increase in production cost. By improving the corrosion resistance after coating on the front and back sides of the welded joint, an improvement in the added value and reliability of the product to which the welded joint 1 according to the present embodiment is applied is achieved.

[0037] Next, each component of the welded joint 1 according to this embodiment will be specifically described. Hereinafter, unless otherwise specified, the unit "%" relating to the chemical composition of the steel plate means "mass %." Furthermore, the unit "%" relating to the chemical composition of the scale 16 means "atomic %."

[0038] The welded joint 1 according to this embodiment comprises a first steel plate 11, a second steel plate 12, and a weld bead 13 joining an end 111 of the first steel plate 11 and a first surface 121 of the second steel plate 12. The welded joint 1 according to this embodiment may be a lap fillet welded joint as exemplified in Fig. 1. When the welded joint 1 is a lap welded joint 1, a first steel plate 11 and a second steel plate 12 are overlapped. In other words, the magnitude of the included angle between the first steel plate 11 and the second steel plate 12 is within the range of 0° to 10°, for example. Furthermore, the welded joint 1 according to this embodiment may be a T-joint as shown in Fig. 2. When the welded joint 1 is a T-joint, the first steel plate 11 and the second steel plate 12 are arranged at a substantially right angle. For example, when the welded joint 1 is a T-joint, the angle formed by the first steel plate 11 and the second steel plate 12 may be within a range of 80° to 100°. However, the angle formed by the first steel plate 11 and the second steel plate 12 is not particularly limited, and various values ​​between 0° and 180° can be applied to this angle.

[0039] In either case, the welded joint 1 according to this embodiment has a shape in which the end of one steel plate is welded to the surface of the other steel plate. For convenience, in the welded joint 1 according to this embodiment, the steel plate whose end is welded will be referred to as the first steel plate 11, and the steel plate whose surface is welded will be referred to as the second steel plate 12. Furthermore, of the two surfaces of the second steel plate 12, the surface that is welded to the first steel plate 11 will be referred to as the first surface 121, and the surface that is not welded to the first steel plate 11 will be referred to as the second surface 122.

[0040] The weld bead 13 of the weld joint 1 is the metal that melts and solidifies during welding, i.e., the weld metal. Furthermore, the weld joint 1 has a HAZ 14 formed around the weld bead 13. The HAZ 14 (heat affected zone) is a portion that is not melted during welding, but whose structure, metallurgical properties, mechanical properties, etc. have changed due to the welding heat. Hereinafter, the weld bead 13 and HAZ 14 may be collectively referred to as the "weld."

[0041] In the welded joint 1 according to this embodiment, scale 16 is attached to the surface of the HAZ 14 on the second surface 122 of the second steel plate 12. Therefore, the HAZ 14 to which the scale 16 is attached is exposed to the outside of the second steel plate 12 on the second surface 122 of the second steel plate 12. The scale 16 is an oxide film that forms on the surface of the metal due to welding heat. When the second steel plate 12 is viewed from the back side of the weld bead 13 before painting, areas that have been discolored by the welding heat can be seen, as shown in FIG. 3 . These discolored areas are the areas where the scale 16 is attached.

[0042] The welded joint 1 further has a coating film 15 that covers the first steel plate 11, the second steel plate 12, the weld bead 13, the HAZ 14, and the scale 16. This dramatically improves the corrosion resistance of the welded joint 1. The coating to form the coating film 15 is performed without removing the slag and the scale 16. Therefore, the scale 16 that has adhered to the HAZ 14 is located between the coating film 15 and the HAZ 14.

[0043] (First steel plate 11) There are no particular limitations on the first steel plate 11. The first steel plate 11 may have any thickness, chemical composition, and other configurations that allow fillet welding to the second steel plate 12.

[0044] For example, the Si content of the first steel plate 11 may be 0 to 0.20% by mass. The Si contained in the steel plate combines with oxygen during welding to form oxides, which are discharged as slag to the outside of the weld bead 13. Si oxides are amorphous and therefore have low electrical conductivity. Slag containing a large amount of Si oxide leads to coating defects. In the welded joint 1 according to this embodiment, coating defects due to slag are suppressed by setting the chemical composition of the second steel plate 12 within a predetermined range. However, by setting the Si content of the first steel plate 11 to 0.20% by mass or less, the amount of slag adhering to the weld bead 13 can be further reduced. The Si content of the first steel plate 11 may be 0.18% by mass or less, 0.15% by mass or less, or 0.12% by mass or less.

[0045] The lower the Si content of the first steel plate 11, the more effectively it is possible to suppress the Si content in the welding slag and reduce the amount of the slag. Therefore, it is desirable that the Si content of the first steel plate 11 be 0 mass%. However, in consideration of refining costs, the Si content of the first steel plate 11 may be 0.02 mass% or more, 0.05 mass% or less, or 0.08 mass% or more.

[0046] As long as the Si content is within the above range, the thickness of the first steel plate 11 is not particularly limited. The thickness of the first steel plate 11 may be, for example, 1 mm or more, 2 mm or more, or 4 mm or more. The thickness of the first steel plate 11 may be, for example, 8 mm or less, 6 mm or less, or 4 mm or less.

[0047] There is no particular limitation on the amount of alloying elements other than Si contained in the first steel plate 11. For example, the chemical composition of the first steel plate 11 may satisfy the requirements for the chemical composition of the second steel plate 12 described below.

[0048] There are no particular limitations on the mechanical properties of the first steel plate 11. For example, the higher the tensile strength of the first steel plate 11, the more preferable, and it may be 440 MPa or more, 780 MPa or more, or 980 MPa or more.

[0049] The first steel sheet 11 may be surface-treated. For example, the surface of the first steel sheet 11 may be plated with Zn-based plating, Al-based plating, Mg-based plating, Sn-based plating, or the like. This can further improve the corrosion resistance of the first steel sheet 11.

[0050] (Second steel plate 12) The second steel plate 12 has the chemical composition described below.

[0051] (C: 0.03 to 0.20%) C increases the tensile strength of the second steel plate 12. Therefore, the C content of the second steel plate 12 is set to 0.03% or more. Preferably, the C content of the second steel plate 12 is 0.05% or more, 0.08% or more, or 0.10% or more.

[0052] On the other hand, if the C content of the second steel plate 12 is excessive, the ductility of the second steel plate 12 may be impaired. Furthermore, if the C content of the second steel plate 12 is excessive, some of the C contained in the second steel plate 12 will become CO gas or CO gas due to the welding heat and be released to the outside of the second steel plate 12. These gases will peel off the scale 16 and impair the paint adhesion on the back side of the weld. Therefore, the C content of the second steel plate 12 is set to 0.20% or less. Preferably, the C content of the second steel plate 12 is 0.18% or less, 0.15% or less, or 0.12% or less.

[0053] (Si: 0.02 to 0.30%) Si increases the ductility and tensile strength of the second steel plate 12. Furthermore, Si also has a deoxidizing effect. Therefore, the Si content of the second steel plate 12 is set to 0.03% or more. Preferably, the Si content of the second steel plate 12 is 0.05% or more, 0.08% or more, or 0.010% or more.

[0054] On the other hand, if the second steel sheet 12 has an excessive Si content, the ductility and weldability of the second steel sheet 12 may be impaired. Furthermore, if the second steel sheet 12 has an excessive Si content, an excessive amount of insulating slag may adhere to the surface of the weld, which may impair the paintability of the surface of the weld. Furthermore, if the second steel sheet 12 has an excessive Si content, the appearance of the second steel sheet 12 after painting may be impaired. This is because uneven scale 16, for example, in a striped pattern, is formed on the second steel sheet 12, and this scale 16 makes the surface of the paint film 15 uneven. Therefore, the Si content of the second steel sheet 12 is set to 0.30% or less. Preferably, the Si content of the second steel sheet 12 is 0.28% or less, 0.25% or less, 0.20% or less, less than 0.20%, 0.19% or less, or 0.18% or less.

[0055] (P:0.080% or less) P segregates at the grain boundaries of the second steel sheet 12, degrading the ductility of the second steel sheet 12. P also embrittles the grain boundaries of the second steel sheet 12. In this case, the adhesion between the base steel and the scale interface decreases below the pressure of the CO gas or CO2 gas, causing peeling. That is, P further accelerates the blister phenomenon caused by C, thereby impairing the paint adhesion on the back side of the weld. For these reasons, the P content of the second steel sheet 12 is set to 0.080% or less. The P content of the second steel sheet 12 may also be set to 0.060% or less, 0.040% or less, or 0.020% or less.

[0056] In the welded joint 1 according to this embodiment, the P contained in the second steel plate 12 is not required to solve the problem. Therefore, the lower the P content, the better, and the lower limit is not particularly limited. The P content may be 0%. On the other hand, excessively reducing the P content of the second steel plate 12 significantly increases the manufacturing cost of the second steel plate 12. Furthermore, P has the effect of increasing the tensile strength of the steel plate by solid solution strengthening. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.010% or more.

[0057] (Ti: 0.05 to 0.20%) The Ti contained in the second steel plate 12 becomes an oxide in the molten metal during welding. When the molten metal solidifies, the Ti is expelled from the weld bead 13 as slag. The Ti improves the electrical conductivity of the slag formed on the surface of the weld bead 13. The slag that adheres to the surface of the weld bead 13 is typically an insulating material primarily composed of Si, which can cause coating defects. However, by using the Ti contained in the second steel plate 12 to increase the electrical conductivity of the slag, coating defects on the front side of the weld can be suppressed. Furthermore, the Ti contained in the second steel plate 12 modifies the scale 16 formed on the second surface 122 of the second steel plate 12, improving the adhesion of the scale 16. This improves the paint adhesion on the back side of the weld. Therefore, the Ti content of the second steel plate 12 is set to 0.05% or more. Preferably, the Ti content of the second steel plate 12 is 0.08% or more, 0.10% or more, or 0.12% or more.

[0058] On the other hand, if the Ti content of the second steel plate 12 is excessive, the workability of the second steel plate 12 may be impaired. Therefore, the Ti content of the second steel plate 12 is set to 0.20% or less. Preferably, the Ti content of the second steel plate 12 is 0.18% or less, 0.16% or less, or 0.15% or less.

[0059] (Al: 0.05 to 0.40%) The Al contained in the second steel plate 12 modifies the scale 16 formed on the second surface 122 of the second steel plate 12, improving the adhesion of the scale 16. This improves the paint adhesion on the back side of the weld. Therefore, the Al content of the second steel plate 12 is set to 0.05% or more. Preferably, the Al content of the second steel plate 12 is 0.10% or more, 0.15% or more, or 0.20% or more.

[0060] On the other hand, if the Al content of the second steel plate 12 is excessive, the adhesion of the scale 16 is impaired. This is presumably because a large amount of alumina is formed in the scale 16, which causes the scale to peel off. Therefore, the Al content of the second steel plate 12 is set to 0.40% or less. Preferably, the Al content of the second steel plate 12 is 0.35% or less, 0.30% or less, or 0.25% or less.

[0061] The second steel plate 12 may contain various alloy elements other than C, Si, P, Ti, and Al. Elements other than C, Si, P, Ti, and Al are thought not to affect the corrosion resistance of the weld after painting. This is because elements other than these elements have almost no effect on the composition, etc., of the slag formed on the surface of the weld bead 13 and the scale 16 formed on the surface of the HAZ 14 of the second steel plate 12. Therefore, the types and contents of elements other than C, Si, P, Ti, and Al in the second steel plate 12 are not particularly limited. Suitable examples of elements that the second steel plate 12 may contain and their contents are described below.

[0062] (Mn: e.g. 0.50 to 3.00%) Mn is not essential in the second steel plate 12 of the welded joint 1 according to this embodiment. Therefore, the Mn content of the second steel plate 12 may be 0%. However, Mn increases the tensile strength of the second steel plate 12. Therefore, the Mn content of the second steel plate 12 may be 0.50% or more. Preferably, the Mn content of the second steel plate 12 is 0.80% or more, 1.00% or more, or 1.50% or more.

[0063] On the other hand, by setting the Mn content of the second steel plate 12 to 3.00% or less, it is possible to ensure the workability of the second steel plate 12. Therefore, the Mn content of the second steel plate 12 may be set to 3.00% or less. Preferably, the Mn content of the second steel plate 12 is 2.80% or less, 2.50% or less, or 2.00% or less.

[0064] (S: for example, 0.1000% or less) S is thought not to affect the corrosion resistance of the welded portion after painting. On the other hand, by reducing the S content, it is possible to suppress the formation of sulfide-based inclusions and increase the ductility of the second steel plate 12. For the above reasons, the S content of the second steel plate 12 may be set to 0.1000% or less. The S content of the second steel plate 12 may also be set to 0.0800% or less, 0.0500% or less, or 0.0200% or less.

[0065] In the welded joint 1 according to this embodiment, the S contained in the second steel plate 12 is not required to solve the problem. Therefore, the lower the S content, the better, and the lower limit is not particularly limited. The S content may be 0%. On the other hand, excessively reducing the S content of the second steel plate 12 significantly increases the manufacturing cost of the second steel plate 12. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0100% or more.

[0066] (B: For example, 0 to 0.0100%) In the second steel plate 12 of the welded joint 1 according to this embodiment, B is not essential. Therefore, the B content of the second steel plate 12 may be 0%. However, B improves the hardenability of the second steel plate 12. Therefore, the B content of the second steel plate 12 may be 0.0010% or more. More preferably, the B content of the second steel plate 12 is 0.0020% or more, 0.0030% or more, or 0.0050% or more.

[0067] On the other hand, by reducing the B content of the second steel plate 12, it is possible to prevent deterioration of the ductility, etc. of the second steel plate 12. Therefore, the B content of the second steel plate 12 may be set to 0.2000% or less. More preferably, the B content of the second steel plate 12 is 0.1800% or less, 0.1600% or less, or 0.1500% or less.

[0068] (Cr: e.g. 0 to 1.00%) Cr is not essential for the second steel plate 12 of the welded joint 1 according to this embodiment. Therefore, the Cr content of the second steel plate 12 may be 0%. However, Cr improves the hardenability of the second steel plate 12. Therefore, the Cr content of the second steel plate 12 may be 0.10% or more. More preferably, the Cr content of the second steel plate 12 is 0.20% or more, 0.30% or more, or 0.50% or more.

[0069] On the other hand, by reducing the Cr content of the second steel plate 12, it is possible to suppress deterioration of the chemical conversion treatability of the second steel plate 12. Therefore, the Cr content of the second steel plate 12 may be set to 1.00% or less. More preferably, the Cr content of the second steel plate 12 is 0.90% or less, 0.80% or less, or 0.60% or less.

[0070] (Mo: e.g. 0-0.50%) Mo is not essential in the second steel plate 12 of the welded joint 1 according to this embodiment. Therefore, the Mo content of the second steel plate 12 may be 0%. However, Mo improves the hardenability of the second steel plate 12. Therefore, the Mo content of the second steel plate 12 may be 0.01% or more. More preferably, the Mo content of the second steel plate 12 is 0.05% or more, 0.10% or more, or 0.20% or more.

[0071] On the other hand, by reducing the Mo content of the second steel plate 12, it is possible to reduce the raw material cost of the second steel plate 12. Therefore, the Mo content of the second steel plate 12 may be set to 0.50% or less. More preferably, the Mo content of the second steel plate 12 is 0.40% or less, 0.35% or less, or 0.30% or less.

[0072] (Remainder: e.g., Fe and impurities) The remainder of the chemical components of the second steel plate 12 is, for example, iron and impurities. The impurities refer to components that are mixed in due to raw materials such as ore or scrap, or various factors in the manufacturing process, during the industrial production of steel, and are acceptable as long as they do not adversely affect the welded joint 1 according to this embodiment.

[0073] (Relationship between Ti content, Al content, C content, and P content of second steel plate 12) The chemical composition of the second steel plate 12 satisfies the following formula 1. Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) Here, the element symbols included in formula 1 represent the content of the element corresponding to the element symbol in the second steel plate 12.

[0074] Formula 1 defines the upper limits of C and P depending on the Ti and Al contents. Ti and Al modify the scale 16 and improve its adhesion. On the other hand, C and P impair the adhesion of the scale 16 by causing blistering. By satisfying the above formula 1, blistering is further suppressed and the paint adhesion of the HAZ 14 is further improved.

[0075] As long as the chemical composition is within the above-mentioned range, the thickness of the second steel plate 12 is not particularly limited. The thickness of the second steel plate 12 may be, for example, 1 mm or more, 2 mm or more, or 4 mm or more. The thickness of the second steel plate 12 may be, for example, 8 mm or less, 6 mm or less, or 4 mm or less.

[0076] There are also no particular limitations on the mechanical properties of the second steel plate 12. For example, the higher the tensile strength of the second steel plate 12, the more preferable, and it may be 440 MPa or more, 780 MPa or more, or 980 MPa or more. The second steel sheet 12 may be surface-treated. For example, the surface of the second steel sheet 12 may be plated with Zn-based plating, Al-based plating, Mg-based plating, Sn-based plating, or the like. This can further improve the corrosion resistance of the second steel sheet 12. On the other hand, from the viewpoint of more suitably forming the scale 16 on the second steel sheet, it is preferable that the second steel sheet be an unplated steel sheet, i.e., a steel sheet without a plating layer.

[0077] (Penetration depth D of weld bead 13) The penetration depth D of the weld bead 13 is closely related to the amount of heat input during welding. The greater the amount of heat input during welding, the greater the penetration depth D. When the penetration depth D of the weld bead 13 is 50% or more of the thickness t2 of the second steel plate, the second surface 122 of the second steel plate 12 is sufficiently heated. As a result, Ti and Al contained in the second steel plate 12 migrate to the scale 16, and the crystal structure of the scale 16 is improved, thereby improving the adhesion of the scale 16. As a result, the adhesion of the coating film 15 formed to cover the scale 16 is improved. The penetration depth D may be 55% or more, 60% or more, or 70% or more of the thickness t2 of the second steel plate.

[0078] On the other hand, if the penetration depth D of the weld bead 13 is excessive, there is a risk of strike-through occurring in part of the weld bead 13. The strike-through causes unevenness on the second surface 122 of the second steel plate 12, impairing paint adhesion. Therefore, the penetration depth D of the weld bead 13 is set to 90% or less of the plate thickness t2 of the second steel plate. The penetration depth D of the weld bead 13 may also be 85% or less, 80% or less, or 75% or less of the plate thickness t2 of the second steel plate.

[0079] The penetration depth D of the weld bead 13 is measured in a cross section perpendicular to the weld line direction of the weld bead 13. As shown in Figures 1 and 2, the penetration depth D is the depth of the weld bead 13 relative to the first surface 121 of the second steel plate 12. The boundary between the weld bead 13 and the second steel plate 12 can be easily visualized by etching the cross section. The cross section used to measure the penetration depth D is formed at a location 30 mm or more away from both ends of the weld bead in the longitudinal direction. Cross sections used to measure the penetration depth D are formed at three locations, and the penetration depth D is measured at the three cross sections. The average value of the penetration depth D in the three cross sections is considered to be the penetration depth D of the weld bead 13.

[0080] (Ti and Al contents of scale 16) The scale 16 between the surface of the HAZ 14 and the coating film 15 contains 2.00 to 60.00% Ti and 2.00 to 30.00% Al. Experimental results by the present inventors have shown that when the Ti and Al contents of the scale 16 are within the above-mentioned ranges, the coating adhesion to the HAZ 14 is improved. This is presumably because Ti-Al-Fe oxides are formed within the scale 16, and these Ti-Al-Fe oxides improve the adhesion between the scale 16 and the HAZ 14. The improved adhesion of the scale 16 also improves the adhesion of the coating film 15 that is provided to cover the scale 16.

[0081] However, if the Al content of the scale 16 is excessive, the adhesion of the scale 16 is impaired. It is presumed that a large amount of alumina is formed in the scale 16, which causes the scale 16 to peel off. Furthermore, if the Ti content of the scale 16 is excessive, the adhesion of the scale 16 is also impaired. Therefore, in the scale 16, Ti is set to 60.00% or less, and Al is set to 30.00% or less.

[0082] The Ti content and Al content of the scale 16 can be preferably controlled, for example, by setting the chemical composition of the second steel plate 12 and the penetration depth D of the weld bead 13 within the above-mentioned ranges. The Ti content of the scale 16 may be 5.00% or more, 10.00% or more, or 20.00% or more. The Ti content of the scale 16 may be 55.00% or less, 50.00% or less, or 40.00% or less. The Al content of the scale 16 may be 5.00% or more, 10.00% or more, or 15.00% or more. The Al content of the scale 16 may be 25.00% or less, 20.00% or less, or 18.00% or less.

[0083] The Ti and Al contents of the scale 16 are measured in a cross section perpendicular to the weld line direction of the weld bead 13. As shown in Figure 4, when the cross section is observed with an electron microscope, the scale 16 present between the surface of the HAZ 14 and the coating film 15 can be confirmed. The scale 16 confirmed in the cross section is then subjected to local analysis using an EPMA (Electron Probe Micro Analyzer). The analysis target for the local analysis is an area within 10 μm from the interface between the scale 16 and the HAZ 14 toward the scale 16. This allows the Ti and Al contents of the scale 16 to be measured. Measurements are performed at multiple locations (e.g., five locations) selected from the analysis target, and the average values ​​obtained are considered to be the Ti and Al contents of the scale 16. Note that the FeO, Fe2O3, and Fe3O4 components of the scale 16 may not be uniformly distributed, but this need not be taken into consideration when analyzing the scale 16 composition. The analysis conditions for the scale 16 are as follows: ·Analysis equipment: Light element X-ray microanalyzer Acceleration voltage: 15kV ·Irradiation current: 0.05μA ·Irradiation time: 100msec

[0084] As long as the contents of Ti and Al are within the above-mentioned ranges, the contents of other elements contained in the scale 16 are not particularly limited. Elements other than Ti and Al contained in the scale 16 are mainly Fe and O. The Fe content in the scale 16 may be, for example, 2.00 to 40.00%. The O content in the scale 16 may be, for example, 20.00 to 65.00%. Furthermore, alloy elements such as Si and Mn contained in the second steel sheet 12 may also be detected in the scale 16. The Si content in the scale 16 may be, for example, 0 to 10.00%. The Mn content in the scale 16 may be, for example, 0 to 10.00%. These elements constitute the scale 16 so that their total value is 100%. Naturally, elements not listed above may also be contained in the scale 16.

[0085] (Area of ​​the coating film peeled off when a coating film peeling test is performed on the second surface 122 of the second steel plate 12 in the region above the HAZ 14) The back surface of the welded portion of the welded joint 1 according to this embodiment has high paint adhesion. Therefore, when a paint peeling test conforming to JIS H 8504:1999 is performed on the location on the second surface 122 of the second steel plate 12 where the HAZ 14 is formed, almost no paint peeling occurs. The specific details of the paint film peeling test are as follows: For the paint film peeling test, adhesive tape (with adhesive strength of approximately 8 N per 25 mm width) specified in JIS Z 1522:2009 is used. As shown in Figure 6, adhesive tape T is applied to the location on the second surface 122 of the second steel plate 12 where the HAZ 14 is formed, and then peeled off. Note that a coating film 15 is provided on the second surface 122 of the second steel plate 12 of the welded joint 1 shown in Figure 6. The HAZ 14 cannot be directly seen on the second surface 122 on which the coating film 15 is provided. However, in the location where the HAZ 14 is formed, a raised portion 151 is formed in the coating film 15. The position and shape of the raised portion 151 of the coating film 15 approximately match the position and shape of the HAZ 14 present underneath the coating film 15. Therefore, the raised portion 151 identified by observing the coating film 15 with the naked eye can be considered to be the location where the HAZ 14 is formed. The area of ​​the coating film peeled off by the coating film peeling test is the ratio of the area of ​​the coating film peeled off by the coating film peeling test, calculated by dividing the area of ​​the raised portion 151 on the adhesive tape T. The area of ​​the raised portion 151 on the adhesive tape T refers to the area where the adhesive tape T and the raised portion 151 overlap in Figure 6, i.e., the area of ​​the area surrounded by the dashed line marked with symbol A. The area of ​​the raised portion 151 is approximately the same as the area of ​​the HAZ 14. In the welded joint 1 according to this embodiment, the area ratio of the coating that peels off in a coating peeling test is 15% or less, so the back surface of the welded portion of the welded joint 1 according to this embodiment has high corrosion resistance after painting. The paint peeling area ratio measured by the above procedure may be 12% or less, 10% or less, 9% or less, 8% or less, or 5% or less. Such a paint peeling area ratio can be achieved, for example, by setting the penetration depth D of the weld bead 13 and the components of the scale 16 within the above-mentioned ranges and further setting the surface roughness of the relevant area before welding within the ranges described below. Note that JIS H 8504:1999 is a coating adhesion test method, but it is also suitable for evaluating paint adhesion in the HAZ 14 of the welded joint 1 according to this embodiment.

[0086] (Method for manufacturing welded joint 1) Next, a method for manufacturing a welded joint 1 according to another aspect of the present invention will be described. According to the method for manufacturing a welded joint 1 according to this embodiment, the welded joint 1 according to this embodiment described above can be suitably manufactured.

[0087] The method for manufacturing a welded joint according to this embodiment includes a step S1 of arc-welding an end portion 111 of a first steel plate 11 and a first surface 121 of a second steel plate 12 to form a weld bead 13 that joins the end portion 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12, and a step S2 of painting the first steel plate 11, the second steel plate 12, and the weld bead 13 to provide a coating film 15 on each surface. In the step of forming the weld bead 13, a HAZ 14 is formed around the weld bead 13, and the HAZ 14 is exposed from the second surface 122 side of the second steel plate 12, and a scale 16 is formed on the surface of the exposed HAZ 14; in the step of providing a coating film 15, the coating film 15 is provided on the HAZ 14 and the scale 16 on the second surface 122 side of the second steel plate 12; the chemical composition of the second steel plate 12 includes, in unit mass %, C: 0.03 to 0.20%, Si: 0.02 to 0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%, and the chemical composition of the second steel plate 12 satisfies formula (1); Ti+1.2×Al-1.2×C-7×P>0 Formula (1) The elemental symbol included in Formula 1 is the content in mass % per unit of the element corresponding to the elemental symbol in the second steel plate 12. The penetration depth D of the weld bead 13 is set to 50% or more and 90% or less with respect to the plate thickness t2 of the second steel plate. The scale 16 contains Ti: 2.00 to 60.00% and Al: 2.00 to 30.00% in atomic %. Before arc welding, the arithmetic mean height Sa and the maximum height Sz defined in ISO 25178:2021 of the region where the HAZ 14 is formed by arc welding on the second surface 122 of the second steel plate 12 are within the range that satisfies Formula 2. Thereby, in the region on the HAZ 14 on the second surface 122 side of the second steel plate 12, the area ratio of the coating film 15 peeled off by the coating film peeling test conforming to JIS H 8504:1999 is 15% or less. 20μm < Sz - Sa < 100μm (Formula 2)

[0088] (Arc welding S1) In the arc welding S1, the end portion 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12 are arc welded. Thereby, a weld bead 13 that joins the end portion 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12 is formed. In the process of forming the weld bead 13, a HAZ is formed around the weld bead 13. The HAZ 14 is exposed from the second surface 122 side of the second steel plate 12. Further, a scale is formed on the surface of the exposed HAZ.

[0089] The chemical composition of the second steel plate 12 subjected to the arc welding S1 needs to be within the above-described predetermined range. Also, the chemical composition of the second steel plate 12 needs to satisfy the above-described Formula 1.

[0090] Also, in arc welding S1, it is necessary to adopt a heat input that can make the penetration depth D of the weld bead 13 be 50% or more and 90% or less with respect to the plate thickness t2 of the second steel plate. The heat input can be controlled through the arc voltage value, arc current value, welding speed, etc. By welding the first steel plate 11 and the second steel plate 12 having the above chemical composition with such a heat input, the scale 16 formed on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12 can contain Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%.

[0091] Furthermore, it is necessary to keep the surface roughness of the second surface 122 of the second steel plate 12 subjected to arc welding S1 within a predetermined range. Specifically, the arithmetic mean height Sa and the maximum height Sz of the region where the HAZ 14 is formed by arc welding on the second surface 122 of the second steel plate 12 need to be within the range that satisfies Equation 2. 20μm < Sz - Sa < 100μm (Equation 2) Note that the arithmetic mean height Sa and the maximum height Sz are values defined in ISO 25178:2021. The arithmetic mean height Sa is the average value of the absolute values of the height differences of each point with respect to the average plane of the surface. The maximum height Sz is the distance from the highest point to the lowest point on the surface. The arithmetic mean height Sa and the maximum height Sz of the second surface 122 of the second steel plate 12 before welding can be measured using an ordinary surface roughness meter.

[0092] In the manufacturing method of the welded joint 1 according to the present embodiment, by making Sz - Sa exceed 20μm on the second surface 122 of the second steel plate 12 and utilizing the anchor effect, the adhesion of the scale 16 can be further enhanced. As a result, the adhesion of the coating film 15 formed so as to cover the scale 16 can be enhanced, and the area ratio of the coating film 15 peeled off by the coating film peeling test can be made 15% or less. However, when Sz - Sa is 100μm or more, the paintability of the second surface 122 of the second steel plate 12 may be impaired. Therefore, Sz - Sa is made less than 100μm.

[0093] It is not necessary to control the surface roughness over the entire second surface 122 of the second steel plate 12. It is sufficient that Sz-Sa is more than 20 μm and less than 100 μm only in the area where the HAZ 14 will be formed. In other words, the welding position should be determined so that the HAZ 14 will be formed in the area where Sz-Sa is more than 20 μm and less than 100 μm.

[0094] Furthermore, the surface roughness of the welded joint 1 obtained by the manufacturing method according to this embodiment is not limited. This is because the surface roughness of the second surface 122 of the second steel plate 12 may change slightly due to welding. In addition, if a coating film 15 is provided on the second steel plate 12, it becomes difficult to accurately measure the arithmetic mean height Sa and maximum height Sz of the second surface 122 of the second steel plate 12. This is because the work of removing the coating film 15 affects the surface roughness of the second surface 122 of the second steel plate 12.

[0095] (Painting S2) In the coating S2, the first steel plate 11, the second steel plate 12, and the weld bead 13 of the welded joint 1 obtained by arc welding S1 are coated to form a coating film 15 on each surface. The coating is, for example, electrodeposition coating. In this case, the coating film 15 is formed on the HAZ 14 and the scale 16 on the second surface 122 side of the second steel plate 12. This leaves the scale 16 between the surface of the HAZ 14 on the second surface 122 of the second steel plate 12 and the coating film 15. Therefore, the method for manufacturing the welded joint 1 according to this embodiment does not require a scale 16 removal step such as pickling. The coating is performed on the welded joint 1 with the scale 16 attached, and the coating film 15 is formed to cover the scale 16.

[0096] Other conditions for performing arc welding S1 and painting S2 are not particularly limited. For example, the components of the shielding gas in arc welding S1 are not particularly limited, but the proportion of oxidizing gas in the shielding gas may be 10% or less. An oxidizing gas is a gas that has the effect of oxidizing slag-forming elements, such as Si, contained in the first steel plate 11 and the second steel plate 12. Examples of oxidizing gases include CO2 and O2. By setting the volume fraction of the oxidizing gas in the shielding gas to 10% or less, the effect of reducing the area fraction of slag adhering to the weld bead 13 can be obtained. The volume fraction of the oxidizing gas in the shielding gas may be 9% or less, 8% or less, 5% or less, or 3% or less.

[0097] Furthermore, for example, the preferred configurations of the first steel plate 11 and the second steel plate 12 in the welded joint 1 according to the present embodiment described above can be applied to the first steel plate 11 and the second steel plate 12. The preferred configurations of the welded joint 1 according to the present embodiment described above can also be applied to the positional relationship between the first steel plate 11 and the second steel plate 12. That is, the angle between the first steel plate 11 and the second steel plate 12 can be any value between 0° and 180°. The first steel plate 11 and the second steel plate 12 may be overlapped to form the welded joint 1 as a lap fillet welded joint. The angle between the first steel plate 11 and the second steel plate 12 may be set to 80° to 100° to form the welded joint 1 as a T-joint.

[0098] (Automotive parts and building materials) Next, automobile parts and building materials according to another embodiment of the present invention will be described. The automobile parts and building materials according to this embodiment have the welded joint 1 according to this embodiment. Both automobile parts and building materials are primarily used outdoors, and therefore require high corrosion resistance. The automobile parts and building materials according to this embodiment have the welded joint 1 with high corrosion resistance after painting, and therefore can maintain performance over a long period of time. For example, in the automobile parts according to this embodiment, thinning due to corrosion is suppressed. Therefore, the thickness of the steel plates constituting the automobile parts can be reduced, thereby achieving weight reduction of the automobile parts.

[0099] However, the use of the welded joint 1 according to this embodiment is not particularly limited. The welded joint 1 according to this embodiment can be applied to the joints of machine parts in a wide range of fields other than automobile parts and building material parts. [Example]

[0100] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0101] Various lap fillet welded joints or T-fillet welded joints with coatings were produced by arc welding and electroplating the first and second steel plates. The chemical composition of the first steel plate was 0.7 mass% C, 0.15 mass% Si, 0.18 mass% Mn, 0.009 mass% P, 0.0033 mass% S, 0.11 mass% Ti, and 0.15 mass% Al. The chemical composition of the second steel plate was as shown in Table 1. The balance of both the chemical compositions of the first and second steel plates was iron and impurities. Electroplating was performed to a thickness of 20 μm in the base metal. The penetration depth and scale components of these welded joints were measured and are listed in Table 2.

[0102] The coating defect area rate on the first surface of these welded joints was also measured. The coating defect area rate was measured by photographing the electrodeposition-coated weld bead, analyzing the photograph, and determining the projected area of ​​the weld bead and the projected area of ​​the electrodeposition-coated defect. Welded joints with a coating defect area rate of 7% or less were determined to be welded joints in which electrodeposition-coating defects were suppressed.

[0103] Furthermore, a paint peeling test was carried out on the region on the HAZ on the second surface side of the second steel plate of these welded joints, and the area ratio of the peeled paint film is shown in Table 2. The "coating peeling area ratio A" in Table 2 is the value obtained by dividing the area of ​​the coating peeled portion by the area of ​​the adhesive tape used in the coating peeling test. The adhesive tape was rectangular, 150 mm long and 24 mm wide. Therefore, the area of ​​the adhesive tape was 3600 mm. 2 The "coating peeling area ratio B" is the value obtained by dividing the area of ​​the coating peeled part by the area of ​​the raised part on the adhesive tape. The raised part on the adhesive tape was a quadrilateral with a length of 120 mm and a width of 20 mm. The area of ​​the raised part on the adhesive tape was 2400 mm 2 The paint peeling area ratio specified for the welded joint according to this embodiment is the paint peeling area ratio B, i.e., the value obtained by dividing the area of ​​the paint peeled portion by the area of ​​the raised portion of the pressure-sensitive adhesive tape. The paint peeling area ratio A listed in Table 2 is a reference value. Furthermore, poor painting is usually not a problem on the second surface of the second steel plate, which is the back side of the weld, so evaluation of the rate of poor painting on the second surface was omitted.

[0104] In Table 1, values ​​outside the range of the invention are underlined. The units of the chemical composition of the second steel plate are mass %, with the remainder being iron and impurities. For elements not added to the second steel plate, the content column is left blank. The "Formula 1" column in Table 1 lists the value of the left side of Formula 1. The "Surface roughness" column in Table 1 lists Sz-Sa, measured before welding, at the location on the second surface of the second steel plate where the HAZ will be formed.

[0105] In Table 2, values ​​outside the scope of the invention and values ​​that do not meet the above-mentioned pass / fail criteria are underlined. The chemical composition of the scale is expressed in atomic %. The "D / t2" column in Table 2 lists the ratio of the penetration depth D of the weld bead to the plate thickness t2 of the second steel plate in %.

[0106] During welding, a jig with a hollow structure was used where it would come into contact with the backside of the weld. This prevented heat from being dissipated from the backside of the weld to the jig. The welding wire used was a low-Si welding wire prototyped based on YGW11. The wire diameter was 1.2 mm. Other welding conditions were as shown in Table 3.

[0107] [Table 1]

[0108] [Table 2]

[0109] [Table 3]

[0110] In Comparative Example B1, the P content of the second steel plate was excessive, and the requirement of Formula 1 was not satisfied. As a result, significant blistering occurred in the welded joint of Comparative Example B1, and the paint adhesion on the second surface was insufficient.

[0111] In Comparative Example B2, the Al content of the second steel plate was insufficient, and furthermore, the requirement of Formula 1 was not satisfied. As a result, the Al content of the scale in the welded joint of Comparative Example B2 was insufficient, and furthermore, the paint adhesion on the second surface was insufficient.

[0112] In Comparative Example B3, the Ti content of the second steel plate was insufficient, and the requirement of Formula 1 was not satisfied. As a result, the Ti content of the scale in the welded joint of Comparative Example B3 was insufficient, and the paintability of the first surface and the paint adhesion of the second surface were insufficient.

[0113] Comparative Example B4 did not satisfy the requirement of Formula 1. As a result, the welded joint of Comparative Example B4 suffered from significant blistering, and the paint film adhesion on the second surface was insufficient.

[0114] In Comparative Example B5, the second steel plate had an excessive Al content, which resulted in an excessive Al content in the scale of the welded joint of Comparative Example B5, and insufficient paint adhesion on the second surface.

[0115] In Comparative Example B6, the Ti content of the second steel plate was excessive, and the requirement of Formula 1 was not satisfied. As a result, the Ti content of the scale in the welded joint of Comparative Example B6 was excessive, and the paint adhesion on the second surface was insufficient.

[0116] In Comparative Example B7, the C content of the second steel plate was excessive, and the requirement of Formula 1 was not satisfied. As a result, significant blistering occurred in the welded joint of Comparative Example B7, and the paint adhesion on the second surface was insufficient.

[0117] In Comparative Example B8, the chemical composition of the second steel plate was appropriate and Formula 1 was satisfied, but the heat input was insufficient, resulting in insufficient penetration depth. As a result, the paint adhesion on the second surface of the welded joint in Comparative Example B8 was insufficient.

[0118] In Comparative Example B9, the heat input was excessive, resulting in an excessive penetration depth, which resulted in strike-through in the welded joint of Comparative Example B9 and also impaired the paintability of the first surface.

[0119] In Comparative Example B10, the chemical composition of the second steel plate was appropriate and Formula 1 was satisfied, but the surface roughness of the second surface of the second steel plate before welding was excessive, resulting in insufficient paint adhesion on the second surface of the welded joint in Comparative Example B10. In Comparative Example B11, the chemical composition of the second steel plate was appropriate and Formula 1 was satisfied, but the surface roughness of the second surface of the second steel plate before welding was too small, resulting in insufficient paint adhesion on the second surface of the welded joint in Comparative Example B11.

[0120] On the other hand, in the inventive examples in which the surface roughness of the second steel plate before welding, and the chemical composition of the second steel plate, the chemical composition of the scale, and the penetration depth after welding were appropriate, the paint peeling area ratio was 15% or less, and therefore the inventive examples have high corrosion resistance after painting. [Explanation of symbols]

[0121] 1 Welded joints 11 First Steel Plate 111 End 12 Second Steel Plate 121 Page 1 122 2nd page 13 Weld bead 14 HAZ 15 Paint film 151 Exciting part 16 scale D Penetration depth t2 Thickness of the second steel plate T adhesive tape

Claims

1. a first steel plate; a second steel plate; a weld bead joining the end of the first steel plate and the first surface of the second steel plate; a heat-affected zone (HAZ) formed around the weld bead; a coating film provided on a surface of each of the first steel plate, the second steel plate, and the weld bead; a scale in contact with the HAZ and between the HAZ and the coating film on a second surface side of the second steel plate, which is the back surface of the second steel plate opposite to the first surface; Equipped with The chemical composition of the second steel plate is, in mass%, C: 0.03-0.20%, Si: 0.02-0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%; The chemical composition of the second steel plate satisfies Formula 1, Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) The element symbols included in Formula 1 represent the contents, in unit mass%, of the elements corresponding to the element symbols in the second steel plate, The penetration depth of the weld bead is 50% or more and 90% or less of the plate thickness of the second steel plate, The scale contains, in atomic %, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%, In the region on the HAZ on the second surface side of the second steel plate, the area ratio of the coating film peeling off in a coating film peeling test in accordance with JIS H 8504:1999 is 15% or less. Welded joints.

2. 2. The welded joint of claim 1, wherein the welded joint is a lap joint or a T-joint.

3. arc welding an end portion of a first steel plate and a first surface of a second steel plate to form a weld bead joining the end portion of the first steel plate and the first surface of the second steel plate; a step of painting the first steel plate, the second steel plate, and the weld bead to form a coating film on each surface; A method for manufacturing a welded joint comprising: In the step of forming the weld bead, A HAZ is formed around the weld bead, The HAZ is exposed from the second surface side of the second steel plate, A scale is formed on the exposed surface of the HAZ, In the step of providing the coating film, The coating film is provided on the HAZ and the scale on the second surface side of the second steel plate, The second steel plate has a chemical composition, in mass%, of: C: 0.03-0.20%, Si: 0.02-0.30%, P: 0.080% or less, Ti: 0.05 to 0.20%, and Al: 0.05 to 0.40%; The second steel plate has a chemical composition that satisfies Formula 1, Ti+1.2×Al-1.2×C-7×P>0 (Formula 1) The element symbols included in Formula 1 represent the contents, in unit mass%, of the elements corresponding to the element symbols in the second steel plate, The penetration depth of the weld bead is 50% or more and 90% or less of the plate thickness of the second steel plate, The scale contains, in atomic %, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%; Before the arc welding, the arithmetic mean height Sa and maximum height Sz defined in ISO 25178:2021 of the region on the second surface of the second steel plate where the HAZ is formed by the arc welding are set within a range that satisfies Formula 2, thereby making the area ratio of the coating that peels off in a coating peeling test in accordance with JIS H 8504:1999 in the region on the HAZ on the second surface side of the second steel plate 15% or less. 20μm<Sz-Sa<100μm (Formula 2) Methods for manufacturing welded joints.

4. The method for manufacturing a welded joint according to claim 3, wherein the welded joint is a lap joint or a T-joint.

5. An automobile part comprising the welded joint according to claim 1 or 2.

6. A building component comprising the welded joint according to claim 1 or 2.

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