JP7795136B2 - Welded joints, welded joint manufacturing methods, automobile parts, and building material parts - Google Patents

Description

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.

In order to ensure long-term strength and reliability in salt-damaged areas, the thickness of automobile chassis components is designed with corrosion in mind. This places a limit on the minimum thickness that can be applied. In order to use thin, high-strength steel plates in 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 they have been painted and a paint film has been formed.

To improve the corrosion resistance of welds after painting, it is important to improve the paintability of the welds while also improving paint adhesion. Paintability, or paint adhesion, refers to the ease with which a paint film can be formed. Welds with high paintability can be formed with paint films with fewer defects. Paint adhesion also refers to the adhesion of the paint film that is formed. Paint films formed on welds with high paint adhesion maintain their corrosion resistance effects for a long period of time.

In addition, the back side of a weld is often located inside the 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 after painting on the back side of a weld, which is located inside the component, using post-weld processes. Therefore, it is necessary to improve the corrosion resistance of welds after painting while they are still in the welded state.

Patent Document 1 discloses a cleaning solution for cleaning steel welded structures, which is an acidic solution containing EDTA or its salts, and a cleaning method for steel welded structures in which the steel welded structure is immersed in the acidic solution containing EDTA or its salts. 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 welds.

Patent Document 2 discloses a solid wire for gas-shielded arc welding used to join 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, greater than 0-0.015% P, greater than 0-0.015% S, and optional elements, with the balance consisting of iron and impurities, satisfying the conditions 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 enable the formation of welds with excellent electrodeposition paintability and mechanical properties.

Patent Document 3 discloses a gas-shielded arc welding method for welding steel sheets (W) with a tensile strength of 780 MPa or greater using a shielding gas containing 92 to 99.5% by volume of Ar, in which the value calculated using equation (1) is 0.20 or greater, 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.

Japanese Patent Application Laid-Open No. 2018-21227 Japanese Patent Application Laid-Open No. 2021-3732 Japanese Patent Application Laid-Open No. 2020-66036

However, the technology in 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 technology in Patent Document 1 cannot be applied.

The techniques of Patent Documents 2 and 3 can improve the paintability of welds without requiring post-processing 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 take into consideration the modification of the oxides that form on the back side of the weld.

In light of the above circumstances, the present invention aims to provide a welded joint that can be manufactured 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 manufacturing method for the same, an automobile part, and a building material part.

The gist of the present invention is as follows.

(1) A weld 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 back surface of the second steel plate opposite to 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% to 90% 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%, and in a region on the HAZ on the second surface side of the second steel plate, the area ratio 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.

(3) A method for manufacturing a welded joint according to another aspect of the present invention includes the steps of arc-welding an end of a first steel plate and a first surface of a second steel plate to form a weld bead that joins the end of the first steel plate and the first surface of the second steel plate, and painting the first steel plate, the second steel plate, and the weld bead to provide a coating film on each surface, wherein in the step of forming the weld bead, a HAZ is formed around the weld bead, and the HAZ is formed on the second steel plate. a scale is formed on the surface of the exposed HAZ exposed from the second surface side of the steel plate, 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, and 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% to 90% 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%, and before the arc welding, the arithmetic mean height Sa and maximum height Sz defined in ISO 25178:2021 in the region on the second surface of the second steel plate where the HAZ is formed by the arc welding are set within ranges that satisfy 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 20 μm<Sz-Sa<100 μm (Formula 2).
(4) In the method for manufacturing a welded joint described in (3) above, the welded joint may be a lap joint or a T-joint.

(5) Another aspect of the present invention relates to an automotive part having a welded joint described in (1) or (2) above.

(6) Another aspect of the present invention relates to a building material component having a welded joint described in (1) or (2) above.

The present invention provides welded joints that can be manufactured by painting them in the as-welded state and have high corrosion resistance after painting on both the front and back surfaces, as well as methods for manufacturing such joints, automotive parts, and building material parts.

FIG. 2 is a cross-sectional view perpendicular to the weld line of the lap fillet welded joint according to the present embodiment. FIG. 2 is a cross-sectional view of a T-joint according to the present embodiment, taken perpendicular to the weld line. FIG. 1 is a perspective view of a lap fillet weld joint before painting. 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. 1 is a cross-sectional photograph of the scale of a welded joint with a shallow penetration depth D. 1 is a cross-sectional photograph of the scale of a welded joint with a deep penetration depth D. FIG. 1 is a schematic diagram of a coating peeling test.

As illustrated in FIGS. 1 to 4 , a weld 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 second steel plate 12, 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 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 ratio of the coating 15 that peels off in a coating peeling test in accordance with JIS H 8504:1999 is 15% or less.

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 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.

The inventors have discovered that by keeping 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 specified 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.

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.

The slag formed on the surface of the weld bead 13 on the front side of the weld is typically non-conductive silicon-based slag, which can cause coating defects during painting. Coating defects impair post-painting corrosion resistance, resulting in a reduction in plate thickness due to corrosion and a decline in component performance. The scale 16 formed on the surface of the HAZ 14 on the back side of the weld reduces paint adhesion. This reduced paint adhesion leads to peeling of the coating 15, resulting in a decline in post-painting corrosion resistance. To ensure post-painting corrosion resistance on both the front and back sides of the weld, it is necessary to address these two problems: (1) measures to address the non-conductive slag that reduces paintability on the weld bead 13 on the front side of the weld, and (2) measures to address the scale 16 that reduces paint adhesion in the HAZ 14 on the back side of the weld. To address these problems, the inventors investigated various factors that affect paintability and paint adhesion. As a result, the following findings were obtained.

(1. Chemical Composition of the Second Steel Plate 12)
The chemical components of the second steel plate 12 include 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.

Furthermore, the chemical composition of the second steel plate 12 includes Al. The inventors have discovered that by applying the welding method described below to a steel plate containing Ti and Al, 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, can be dramatically improved.

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 plate 12. According to the results of studies by the present inventors, the coating adhesion of the HAZ 14 was improved when the scale 16 between the surface of the HAZ 14 and the coating film 15 contained, in atomic %, 2.00 to 60.00% Ti and 2.00 to 30.00% Al. 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 (Fe ₂ SiO ₄ ). 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, since the Si content of the second steel plate 12 is low, fayalite is not formed on the surface of the second steel plate 12.

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.

C and P are elements that cause blistering. Some of the C contained in the steel plate is converted into CO gas or _CO2 gas by welding heat and released outside the steel plate. These gases peel off the scale 16, impairing paint adhesion. P also embrittles the grain boundaries of the steel plate. In this case, the adhesion at the base steel-scale interface 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 plate 12 satisfies Formula 1, blistering is suppressed and paint adhesion in the HAZ 14 is further improved.

(2. Welding conditions)
In order to control the composition of the scale 16 as described above, it is necessary to optimize the welding conditions in addition to 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.

This penetration depth D is greater than that found under normal welding conditions. For example, in lap joints used in automotive parts, the penetration depth D is typically set to approximately 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 plate 12. If strike-through occurs, irregularities will form on the second surface 122 of the second steel plate 12, impairing corrosion resistance after painting.

However, according to the results of the inventors' investigations, it was determined that the Ti and Al contents of the scale 16 could not be increased unless welding was performed under heat input conditions that ensured a penetration depth D of 50% or more. It is estimated 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.

The inventors also discovered that the crystal 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 mainly composed of FeO (wustite), _Fe2O3 (hematite), and _Fe3O4 . The higher the heat input, _the thicker the layer composed of _Fe2O3 and _Fe3O4 becomes. Figures 5A and _5B show backscattered electron images of _a cross section _of the scale 16. Figure 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 Figure 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 the region shown in light color at the top of the backscattered electron image, and the layer composed of Fe ₂ O ₃ and Fe ₃ O ₄ is the region shown in dark color at the bottom of the backscattered electron image. These backscattered electron images show that the layer composed of Fe ₂ O ₃ and Fe ₃ O ₄ becomes thicker as the heat input increases.

The thermal expansion coefficients of _Fe2O3 and _Fe3O4 are lower than that of FeO and are closer to the thermal expansion coefficient of the second steel plate 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 adhesion in the HAZ 14 exposed on the second surface 122 of the second steel plate 12 is improved.

According to the inventors' investigations, 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 throughout 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. Therefore, the penetration depth D is set to 90% or less of the plate thickness t2 of the second steel plate 12.

(3. Surface roughness of the second surface 122 of the second steel plate 12 before welding)
The unevenness of the surface of the steel plate is a factor that contributes to the adhesion of the scale 16. The greater the surface roughness, the greater the anchor effect, and the better the adhesion of the scale 16. Therefore, it is necessary to increase the surface roughness of at least the region of the second surface 122 of the second steel plate 12 where the HAZ 14 is to be formed before welding. Specifically, the surface roughness of the region is adjusted before welding so that the arithmetic mean height Sa and maximum height Sz of the region specified in ISO 25178:2021 satisfy the following formula 2:
20μm<Sz-Sa<100μm (Formula 2)
By making Sz-Sa 20 μm or more, an anchor effect can be achieved to improve adhesion. However, if Sz-Sa is excessively large, paintability is impaired. Therefore, Sz-Sa is set to less than 100 μm. Note that the above values are values measured before welding. The surface roughness of the steel plate changes slightly before and after welding. Therefore, there are no particular restrictions on the surface roughness of the second steel plate 12 after welding.

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 paint adhesion of the HAZ 14 on the back side of the weld, i.e., the second surface 122 of the second steel plate 12. Specifically, when a paint peeling test is performed on the area on the HAZ on the second surface 122 side of the second steel plate 12, the area ratio of the peeled paint 15 can be reduced to 15% or less. Furthermore, by combining the three elements described above, it is possible to suppress painting defects on the front side of the weld. This makes it possible to provide a welded joint 1 with excellent surface quality and corrosion resistance after painting, without reducing production efficiency or increasing production costs. By improving the corrosion resistance after painting on both the front and back sides of the weld, it is possible to achieve added value and improved reliability for products to which the welded joint 1 of this embodiment is applied.

Next, each component of the welded joint 1 according to this embodiment will be described in detail. Hereinafter, unless otherwise specified, the unit "%" used to refer to the chemical composition of the steel plate means "mass %." Furthermore, the unit "%" used to refer to the chemical composition of the scale 16 means "atomic %."

The weld joint 1 of 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, the first steel plate 11 and the 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 a range of 0° to 10°, for example.
Furthermore, the welded joint 1 according to this embodiment may be a T-joint as exemplified in Fig. 2. When the welded joint 1 is a T-joint, the first steel plate 11 and the second steel plate 12 are disposed 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.

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.

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."

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 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. 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 discolored by the welding heat can be seen, as shown in Figure 3. These discolored areas are the areas where scale 16 is attached.

The welded joint 1 further has a coating 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 15 is applied without removing the slag and scale 16. Therefore, the scale 16 adhered to the HAZ 14 is located between the coating 15 and the HAZ 14.

(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 properties that allow it to be fillet welded to the second steel plate 12.

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

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 also to reduce the amount of slag. Therefore, it is desirable to set the Si content of the first steel plate 11 to 0% by mass. However, taking refining costs into consideration, the Si content of the first steel plate 11 may be set to 0.02% by mass or more, 0.05% by mass or less, or 0.08% by mass or more.

As long as the Si content is within the above-mentioned 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.

The amount of alloying elements other than Si contained in the first steel plate 11 is not particularly limited. For example, the chemical composition of the first steel plate 11 may satisfy the requirements of the chemical composition of the second steel plate 12 described below.

The mechanical properties of the first steel plate 11 are also not particularly limited. For example, the higher the tensile strength of the first steel plate 11, the better, and it may be 440 MPa or more, 780 MPa or more, or 980 MPa or more.

The first steel plate 11 may have a surface treatment. For example, the first steel plate 11 may have a Zn-based plating, Al-based plating, Mg-based plating, Sn-based plating, etc. on its surface. This can further improve the corrosion resistance of the first steel plate 11.

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

(C: 0.03-0.20%)
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.

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 _CO2 gas due to 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.

(Si: 0.02-0.30%)
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.

On the other hand, if the Si content of the second steel plate 12 is excessive, the ductility and weldability of the second steel plate 12 may be impaired. Furthermore, if the Si content of the second steel plate 12 is excessive, the amount of insulating slag adhering to the surface of the weld may be excessive, which may impair the paintability of the surface of the weld. In addition, if the Si content of the second steel plate 12 is excessive, the aesthetic appearance of the second steel plate 12 after painting may be impaired. This is because uneven scale 16, for example, in a striped pattern, is formed on the second steel plate 12, and this scale 16 makes the surface of the paint film 15 uneven. Therefore, the Si content of the second steel plate 12 is set to 0.30% or less. Preferably, the Si content of the second steel plate 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.

(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 at the base steel-scale interface falls below the pressure of the CO gas or _CO2 gas, causing peeling. That is, P further promotes the blister phenomenon caused by C, thereby impairing the paint film 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.

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 there is no particular limit on the lower limit. 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 through solid solution strengthening. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.010% or more.

(Ti: 0.05-0.20%)
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 substance 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.

On the other hand, if the Ti content of the second steel plate 12 is excessive, the workability, etc. 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.

(Al: 0.05-0.40%)
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.

On the other hand, if the Al content of the second steel plate 12 is excessive, the adhesion of the scale 16 will be impaired. This is presumably because a large amount of alumina is formed in the scale 16, causing 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.

The second steel plate 12 may contain various alloy elements other than C, Si, P, Ti, and Al. It is believed that elements other than C, Si, P, Ti, and Al do not affect the corrosion resistance of the weld after painting. This is because elements other than these elements have little 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.

(Mn: for example, 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.

On the other hand, by setting the Mn content of the second steel plate 12 to 3.00% or less, the workability of the second steel plate 12 can be ensured. 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.

(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.

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 there is no particular limit on the lower limit. 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.

(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.

On the other hand, by reducing the B content of the second steel plate 12, deterioration of the ductility, etc. of the second steel plate 12 can be suppressed. 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.

(Cr: for example, 0 to 1.00%)
Cr is not essential in 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.

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.

(Mo: for example, 0 to 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.

On the other hand, by reducing the Mo content of the second steel plate 12, the raw material costs of the second steel plate 12 can be reduced. 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.

(balance: 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 within a range that does not adversely affect the welded joint 1 according to this embodiment.

(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 .

Formula 1 defines the upper limits of C and P depending on the Ti and Al content. Ti and Al modify the scale 16, improving 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, further improving the paint film adhesion in the HAZ 14.

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.

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.

(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 crystalline 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.

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. 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.

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 longitudinal ends of the weld bead. 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 of the penetration depths D at the three cross sections is considered to be the penetration depth D of the weld bead 13.

(Ti content and Al content of scale 16)
The scale 16 present 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 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 provided to cover the scale 16.

However, if the Al content of scale 16 is excessive, the adhesion of scale 16 will be impaired. It is believed that a large amount of alumina will form in scale 16, causing it to peel off. Furthermore, if the Ti content of scale 16 is excessive, the adhesion of scale 16 will also be impaired. Therefore, in scale 16, Ti should be 60.00% or less and Al should be 30.00% or less.

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.

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 FIG. 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 subjected to local analysis using an electron probe micro analyzer (EPMA). In the local analysis, the analysis target 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 in 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 regarded as the Ti and Al contents of the scale 16. Note that FeO, _Fe2O3 , and _{Fe3O4 ,} which constitute the scale ₁₆ , may not be uniformly distributed, but this need not be taken into consideration in the component analysis of the scale 16. The analysis conditions for scale 16 are as follows:
・Analysis equipment: Light element X-ray microanalyzer ・Accelerating voltage: 15 kV
・Irradiation current: 0.05μA
・Irradiation time: 100msec

As long as the Ti and Al contents 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 plate 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.

(Area of coating peeled off by coating peeling test on the region above the HAZ 14 on the second surface 122 of the second steel plate 12)
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 portion of 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 attached 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 substantially coincide with the position and shape of the HAZ 14 present below 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 the symbol A. The area of the raised portion 151 is approximately equal to 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 that 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 location 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.

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

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, and in the process 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, and 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 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 set to 50% or more and 90% or less of the plate thickness t2 of the second steel plate, the scale 16 contains, in unit atomic %, Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%, and before arc welding, the arithmetic mean height Sa and maximum height Sz specified in ISO 25178:2021 of the region on the second surface 122 of the second steel plate 12 where the HAZ 14 is formed by arc welding are set to fall within ranges that satisfy Formula 2, and thereby the area proportion of the coating 15 that peels in the region on the HAZ 14 on the second surface 122 side of the second steel plate 12 in a coating peeling test in accordance with JIS H 8504:1999 is set to 15% or less.
20μm<Sz-Sa<100μm (Formula 2)

(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. This forms 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. In the process of forming the weld bead 13, a HAZ 14 is formed around the weld bead 13. The HAZ 14 is exposed from the second surface 122 side of the second steel plate 12. Furthermore, scale is formed on the surface of the exposed HAZ.

The chemical composition of the second steel plate 12 used in arc welding S1 must be within the specified range described above. Furthermore, the chemical composition of the second steel plate 12 must satisfy the above-mentioned formula 1.

In addition, in arc welding S1, it is necessary to adopt a heat input that can set the penetration depth D of the weld bead 13 to 50% or more and 90% or less of the plate thickness t2 of the second steel plate. The heat input can be controlled via 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-mentioned chemical composition with such a heat input, the scale 16 formed on the surface of the HAZ 14 on the second side 122 of the second steel plate 12 can be made to contain Ti: 2.00 to 60.00% and Al: 2.00 to 30.00%.

Furthermore, the surface roughness of the second surface 122 of the second steel plate 12 to be subjected to arc welding S1 needs to be set within a predetermined range. Specifically, the arithmetic mean height Sa and maximum height Sz of the region on the second surface 122 of the second steel plate 12 where the HAZ 14 is formed by arc welding need to be set within a range that satisfies Equation 2.
20μm<Sz-Sa<100μm (Formula 2)
The arithmetic mean height Sa and maximum height Sz are values specified in ISO 25178:2021. The arithmetic mean height Sa is the average of the absolute values of the height differences at each point relative 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 maximum height Sz of the second surface 122 of the second steel plate 12 before welding can be measured using a conventional surface roughness meter.

In the manufacturing method for the welded joint 1 according to this embodiment, the Sz-Sa is set to more than 20 μm on the second surface 122 of the second steel plate 12, and the anchor effect is utilized to further enhance the adhesion of the scale 16. This enhances the adhesion of the coating film 15 formed to cover the scale 16, and the area percentage of the coating film 15 that peels off in a coating peeling test can be reduced to 15% or less. However, if Sz-Sa is 100 μm or more, the paintability of the second surface 122 of the second steel plate 12 may be impaired. Therefore, Sz-Sa is set to less than 100 μm.

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

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 15 is applied to 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 removal of the coating 15 affects the surface roughness of the second surface 122 of the second steel plate 12.

(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 manufacturing method of the welded joint 1 according to this embodiment does not require a scale 16 removal process 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.

Other conditions for performing the arc welding S1 and the painting S2 are not particularly limited. For example, the components of the shielding gas in the arc welding S1 are not particularly limited, but the proportion of oxidizing gas in the shielding gas may be 10% or less. The 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 CO ₂ and O _2. 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.

Furthermore, for example, the preferred configuration of the first steel plate 11 and the second steel plate 12 in the welded joint 1 according to this embodiment described above can be applied to the first steel plate 11 and the second steel plate 12. Furthermore, the preferred configuration of the welded joint 1 according to this 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 weld joint. Furthermore, 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.

(Automotive parts and building materials)
Next, an automobile part and a building material part according to another aspect of the present invention will be described. The automobile part and the building material part according to this embodiment have the welded joint 1 according to this embodiment. Both the automobile part and the building material part are mainly used outdoors, and therefore require high corrosion resistance. The automobile part and the building material part 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 part according to this embodiment, thinning due to corrosion is suppressed. Therefore, the thickness of the steel plate constituting the automobile part can be reduced, thereby achieving weight reduction of the automobile part.

However, the applications of the welded joint 1 according to this embodiment are not particularly limited. The welded joint 1 according to this embodiment can be applied to the joints of mechanical parts in a wide range of fields other than automotive parts and building material parts.

The effects of one aspect of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely one example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example of conditions. 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 objectives of the present invention.

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% by mass C, 0.15% by mass Si, 0.18% by mass Mn, 0.009% by mass P, 0.0033% by mass S, 0.11% by mass Ti, and 0.15% by 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 achieve a coating thickness of 20 μm in the base material. The penetration depth and scale components of these welded joints were measured and are listed in Table 2.

The paint defect area rate on the first surface of these welded joints was also measured. The paint 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 area. Welded joints with a paint defect area rate of 7% or less were determined to be welded joints in which electrodeposition-coating defects were suppressed.

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 is shown in Table 2.
The "coat peeling area ratio A" shown in Table 2 is the value obtained by dividing the area of the paint peeled portion by the area of the adhesive tape used in the paint peeling test. The adhesive tape was rectangular, 150 mm long and 24 mm wide. Therefore, the area of the adhesive tape was 3,600 ^mm2 . The "coat peeling area ratio B" is the value obtained by dividing the area of the paint peeled portion by the area of the raised portion on the adhesive tape. The raised portion on the adhesive tape was quadrilateral, 120 mm long and 20 mm wide. The area of the raised portion on the adhesive tape was 2,400 ^mm2 . 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 on the adhesive tape. The paint peeling area ratio A shown in Table 2 is a reference value.
Furthermore, on the second surface of the second steel plate, which is the back surface of the welded portion, poor painting is usually not a problem, and therefore evaluation of the rate of poor painting on the second surface was omitted.

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 "Equation 1" column in Table 1 lists the value of the left side of Equation 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 form.

In Table 2, values outside the scope of the invention and values that do not meet the pass/fail criteria described above are underlined. The chemical composition of the scale was expressed in atomic percent. 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 percent.

When 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 transferred 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.

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

In Comparative Example B2, the Al content of the second steel plate was insufficient, and the requirements of Formula 1 were not met. As a result, the Al content of the scale in the welded joint of Comparative Example B2 was insufficient, and the paint adhesion on the second surface was insufficient.

In Comparative Example B3, the Ti content of the second steel plate was insufficient, and the requirements of Formula 1 were not met. 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 film adhesion of the second surface were insufficient.

In Comparative Example B4, the requirements of Formula 1 were not met. As a result, significant blistering occurred in the welded joint of Comparative Example B4, and the paint film adhesion on the second surface was insufficient.

In Comparative Example B5, the second steel plate had an excessive Al content. As a result, the scale in the welded joint of Comparative Example B5 had an excessive Al content, and the paint film adhesion on the second surface was insufficient.

In Comparative Example B6, the Ti content of the second steel plate was excessive, and the requirements of Formula 1 were not met. 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.

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

In Comparative Example B8, the chemical composition of the second steel plate was appropriate and Equation 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.

In Comparative Example B9, the heat input was excessive, resulting in excessive penetration depth. As a result, strike-through occurred in the welded joint of Comparative Example B9, and the paintability of the first surface was also impaired.

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, which resulted 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.

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 rate was 15% or less. Therefore, the inventive examples have high corrosion resistance after painting.

1 weld joint 11 first steel plate 111 end 12 second steel plate 121 first surface 122 second surface 13 weld bead 14 HAZ
15 Coating film 151 Protuberance 16 Scale D Penetration depth t2 Plate thickness T of second steel plate Adhesive tape

Claims (6)

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%,
A welded joint in which 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 is 15% or less. The welded joint described in claim 1, characterized in that it is a lap joint or a T-joint. 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: 20 μm<Sz-Sa<100 μm (Formula 2)
Methods for manufacturing welded joints. A method for manufacturing a welded joint as described in claim 3, characterized in that the welded joint is a lap joint or a T-joint. An automotive part having the welded joint described in claim 1 or 2. A building component having the welded joint described in claim 1 or 2.