JP7741377B2 - Welded joints, welded joint manufacturing methods, automobile parts, and building material parts - Google Patents

Description

The present invention relates to welded joints, methods for manufacturing welded joints, automotive parts, and building material parts.

Electrodeposition coating is a method for improving the corrosion resistance of mechanical structural parts, such as automobile parts and building materials. Electrodeposition coating is a coating method in which the object to be coated and an electrode are placed in a tank containing electrodeposition paint, creating a potential difference between the two and electrophoresing the coating components, resulting in the deposition of a coating film on the surface of the object.

However, when mechanical structural components have welds, slag and scale formed at the welds can degrade the corrosion resistance of the components. Slag is a non-metallic substance that partially or completely covers the weld bead. Slag is composed of oxides and other materials that are expelled from the weld bead when the base steel plate and welding material melt and solidify to form the weld bead. Scale is an oxide film that forms on the surface of metal due to the heat of welding. Scale forms at the weld bead and in the heat-affected zone (HAZ) around it.

Generally, slag tends to have low conductivity. Therefore, when a weld with slag attached is electrocoated, the coating does not deposit in the areas where the slag has adhered. As a result, there are areas in the weld where no coating is formed, exposing the slag and underlying metal. In other words, slag causes poor electrocoating results.

Scale is unlikely to cause electrodeposition coating defects. However, when a weld with scale attached is electrodeposition coated, the scale and chemical conversion coating are positioned between the base metal and the paint film. Scale generally peels easily from the base metal. Therefore, there is a risk that the scale will peel off from the base metal along with the paint film.

In areas where coating defects have occurred or where scale adhesion has been impaired, the corrosion resistance improvement effect of the electrodeposition coating cannot be achieved. In such areas, red rust easily forms and grows, causing thickness loss due to corrosion progression and impairing the mechanical properties of the part. Therefore, there is a need to reduce the rate of coating defects in welds and improve scale adhesion.

Patent Document 1 discloses a solid wire for gas-shielded arc welding used to join multiple thin steel plates by gas-shielded arc welding, which contains, in mass percent relative to the total mass of the wire, C: 0.05-0.20%, Si: 0.01-0.18%, Mn: 1.0-3.0%, Ti: 0.06-0.25%, Al: 0.003-0.10%, B: 0-0.0100%, P: more than 0-0.015%, S: more than 0-0.015%, and optional elements, with the balance consisting of 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%.

Patent Document 2 discloses a gas-shielded arc welding method for welding steel plates having a tensile strength of 780 MPa or more using a shielding gas containing 92 to 99.5% by volume of Ar, characterized in that, when the Ar content (volume %) in the shielding gas is C _Ar , the inner diameter (mm) of the nozzle supplying the shielding gas is D, the welding speed (cm/min) is v, and the welding current (A) is I, the value calculated by formula (1) is 0.20 or more.
{√v/(D/2) ² }×10-{(100-C _Ar )×I/v}×0.1...(1)

Patent Document 3 discloses a method for manufacturing a steel welded structure, which includes the steps of welding to a steel material to obtain a steel welded structure, and cleaning the steel welded structure using a cleaning method for steel welded structures, in which the steel welded structure is immersed in an acidic solution containing EDTA or a salt thereof.

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

The technology disclosed in Patent Document 1 prevents poor electrodeposition coating of welded parts by making the slag generated on the surface of the weld bead conductive. However, Patent Document 1 does not disclose a means to resolve the problem of reduced scale adhesion caused by scale formed on the surface of the heat-affected zone.

The technology disclosed in Patent Document 2 embeds slag in the oxide film that forms uniformly on the weld bead, further reducing the amount of slag produced, thereby preventing electrodeposition coating defects at the weld. However, Patent Document 2 does not disclose a means to resolve the problem of reduced scale adhesion caused by scale formed on the surface of the heat-affected zone.

The technology disclosed in Patent Document 3 improves corrosion resistance by cleaning steel welded structures before chemical conversion treatment to remove scale. However, the cleaning process used to remove scale reduces the productivity of mechanical structural parts and increases manufacturing costs.

In light of the above, the present invention aims to provide welded joints, automotive parts, and building material parts that can suppress electrodeposition coating defects and improve scale adhesion, as well as a method for manufacturing welded joints.

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 and exposed to the outside of the second steel plate on the second surface of the second steel plate, and scale adhered to the HAZ on the second surface of the second steel plate, wherein the Si content of the first steel plate is 0 to 0.20 mass%, the Si content of the second steel plate is 0.20 to 1.20 mass%, a ratio of an area of slag adhered to the weld bead to an area of the weld bead is 9% or less, a penetration depth of the weld bead is 15% or more and 85% or less of a thickness of the second steel plate, and the weld joint further comprises fayalite detected in the HAZ by X-ray crystal structure analysis on the second surface of the second steel plate.
(2) The welded joint described in (1) above may be a lap fillet welded joint in which the first steel plate and the second steel plate are overlapped.
(3) The welded joint described in (1) above may be a T-joint in which the first steel plate and the second steel plate are arranged at a right angle.
(4) The welded joint described in any one of (1) to (3) above may further include an electrodeposition coating that covers the first steel plate, the second steel plate, the weld bead, the HAZ, the slag, and the scale.
(5) In the welded joint described in (4) above, the coating defect area rate may be 7% or less.

(6) A method for manufacturing a welded joint according to another aspect of the present invention is a method for manufacturing a welded joint comprising a step of gas-shielded arc welding an end of a first steel plate and a first surface of a second steel plate to form a weld bead, wherein the first steel plate has a Si content of 0 to 0.20 mass%, the second steel plate has a Si content of 0.20 to 1.20 mass%, a HAZ formed around the weld bead is exposed to the outside of the second steel plate on the second surface of the second steel plate, the penetration depth of the weld bead is 15% to 85% of the plate thickness of the second steel plate, and a proportion of oxidizing gas in a shielding gas used for the gas-shielded arc welding is 10% or less.
(7) In the method for manufacturing a welded joint described in (6) above, the proportion of oxidizing gas in the shielding gas may be 5% or less.
(8) The method for manufacturing a welded joint described in (6) or (7) above may further include a step of electrocoating the first steel plate, the second steel plate, the weld bead, the HAZ, the slag, and the scale after the gas-shielded arc welding.
(9) In the method for manufacturing a welded joint described in any one of (6) to (8) above, the first steel plate and the second steel plate may be overlapped during the gas-shielded arc welding.
(10) In the method for manufacturing a welded joint described in any one of (6) to (8) above, the angle formed between the first steel plate and the second steel plate during the gas-shielded arc welding may be set to 80° to 100°.

(11) An automobile component according to another aspect of the present invention includes the welded joint described in any one of (1) to (5) above.
(12) A building material component according to another aspect of the present invention includes the welded joint described in any one of (1) to (5) above.

The present invention provides welded joints, automotive parts, and building material parts, as well as a method for manufacturing welded joints, that can suppress electrodeposition coating defects and improve scale adhesion.

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 welded joint according to the present embodiment. Photographs of the second surface of the second steel plate of welded joints a to c immediately after welding and after chemical conversion treatment are shown. Photographs of the second surface of the second steel plate of welded joints a to c immediately after electrodeposition coating and after a peel test.

A welded joint according to one aspect of the present invention will now be described with reference to the drawings. As shown in FIG. 1 or 2, the welded joint 1 according to this embodiment comprises a first steel plate 11, a second steel plate 12, a weld bead 13 joining the end portion 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12, a HAZ 14 formed around the weld bead 13 and exposed on the second surface 122 of the second steel plate 12, slag 15 adhering to the surface of the weld bead 13, and scale 16 adhering to the HAZ 14 on the second surface 122 of the second steel plate 12.

(Regarding the shape of the welded joint 1)
The welded joint 1 according to this embodiment may be a lap fillet welded joint as illustrated 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 included angle between the first steel plate 11 and the second steel plate 12 is, for example, in the range of 0° to 10°. The welded joint 1 according to this embodiment may also be a T-joint as illustrated 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 between the first steel plate 11 and the second steel plate 12 may be in the range of 80° to 100°. However, the angle between 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.

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. Hereinafter, 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 joint 1 has a weld bead 13 joining the end 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12. The weld bead 13 is metal that melts and solidifies during welding. The weld joint 1 also 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, the HAZ 14 is exposed to the outside of the second steel plate 12 at the second surface 122 of the second steel plate 12. Furthermore, scale 16 is attached to the surface of the HAZ 14 at the second surface 122 of the second steel plate 12. Scale 16 is an oxide film that forms on the surface of metal due to welding heat. As shown in Figure 3, when the second steel plate 12 is viewed from the back side of the weld bead 13, areas discolored by the welding heat can be seen. These discolored areas are the areas where scale 16 is attached.

In addition, by reducing the heat input during welding, it is possible to reduce the size of the HAZ 14 and prevent it from being exposed to the outside of the second steel plate 12. This reduces the amount of scale 16 that forms on the second surface 122 of the second steel plate 12, improves the adhesion of the scale, and maintains the electrocoating film. However, in this case, there is a risk that the penetration depth of the weld metal will be insufficient, and the joining strength of the weld joint 1 will not be ensured. Therefore, the weld joint 1 according to this embodiment is manufactured by welding using a heat input that is sufficient to extend the HAZ 14 to the second surface 122 of the second steel plate 12.

(Regarding the first steel plate 11)
The first steel plate 11 has a Si content of 0 to 0.20 mass%. The Si contained in the steel plate combines with oxygen during welding to form oxides, which are discharged to the outside of the weld bead 13 as slag 15. Si oxides are amorphous and have low electrical conductivity, and slag 15 containing a large amount of Si oxide leads to poor electrodeposition. By setting the Si content of the first steel plate 11 to 0.20 mass% or less, the amount of slag 15 adhering to the weld bead 13 can be 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.

On the other hand, 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 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. Examples of the chemical composition of the first steel plate 11 include C: 0.07%, Si: 0.02%, Mn: 1.2%, Ti: 0.08%, Al: 0.04%, B: 0.005%, P: 0.007%, S: 0.001%, O: 0.0017%, and N: 0.0022%.

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 be surface-treated. For example, the first steel plate 11 may have a Zn-based plating, an Al-based plating, an Sn-based plating, or the like on its surface. This can further improve the corrosion resistance of the first steel plate 11.

(Regarding the second steel plate 12)
In contrast to the first steel plate 11, the second steel plate 12 has a Si content of 0.20 to 1.20 mass %, which allows fayalite to be formed on the surface of the HAZ 14 exposed on the second surface 122 of the second steel plate 12, thereby improving scale adhesion.

The inventors conducted extensive research to improve the corrosion resistance of the second surface 122 of the second steel plate 12, i.e., the surface opposite the weld bead 13. The cause of the deterioration of corrosion resistance on the second surface 122 of the second steel plate 12 was the peeling of the coating film along with the scale. Furthermore, when the scale adhesion on the second surface 122 of the second steel plate 12 was evaluated, it was found that the scale adhesion in the area where scale 16 was attached was inferior to the area where scale 16 was not attached. However, when various steel types were applied to the second steel plate 12 and tests were conducted under various welding conditions, some welded joints 1 exhibited excellent scale adhesion in the areas where scale 16 was attached. When the scale 16 of such welded joints 1 was analyzed, fayalite was detected on the surface of the HAZ 14 exposed on the second surface 122 of the second steel plate 12 in all welded joints 1.

Fayalite is a composite oxide of iron and silicon, represented _{by the} chemical formula _Fe2SiO4 . Scale 16 on ordinary steel is composed of iron oxides such as magnetite ( _Fe3O4 ), hematite ( _Fe2O3 ), and wustite ( _FeO ), and does not contain fayalite. On the other hand, when steel with a high Si content is oxidized at high temperatures, the scale 16 formed on the surface will contain fayalite.

Experimental results by the inventors have shown that in order to generate fayalite on the surface of the HAZ 14 of the second steel plate 12, the Si content of the second steel plate 12 must be 0.20 mass% or more. Therefore, in the welded joint 1 according to this embodiment, the Si content of the second steel plate 12 is set to 0.20 mass% or more. The Si content of the second steel plate 12 may also be set to 0.25 mass% or more, 0.30 mass% or more, or 0.50 mass% or more.

However, as the Si content of the second steel plate 12 increases, the amount of slag 15 formed on the surface of the weld metal increases. In the weld joint 1 according to this embodiment, the Si content of the first steel plate 11 is reduced to dilute the Si in the molten metal and suppress the amount of slag 15 produced. However, if the Si content of the second steel plate 12 exceeds 1.20 mass%, the amount of slag 15 becomes excessive, and the coating defect rate cannot be sufficiently reduced. Therefore, in the weld joint 1 according to this embodiment, the Si content of the second steel plate 12 is set to 1.20 mass% or less. The Si content of the second steel plate 12 may also be set to 1.00 mass% or less, 0.80 mass% or less, or 0.60 mass% or less.

As explained in Patent Document 1, the Si contained in steel sheets forms insulating slag 15, which leads to poor electrodeposition coating. Therefore, when attempting to improve the corrosion resistance of welded joints through electrodeposition coating, there have been no examples that simultaneously address both electrodeposition coating defects caused by welding slag and Fayalite, which is effective in improving scale adhesion.

As long as the Si content 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.

The amount of alloying elements other than Si contained in the second steel plate 12 is not particularly limited. Examples of the chemical composition of the second steel plate 12 include C: 0.04%, Si: 0.85%, Mn: 1.5%, Ti: 0.10%, Al: 0.06%, B: 0.002%, P: 0.009%, S: 0.003%, O: 0.0011%, and N: 0.0030%.

The mechanical properties of the second steel plate 12 are also not particularly limited. For example, the higher the tensile strength of the second steel plate 12, the better, and it may be 440 MPa or more, 780 MPa or more, or 980 MPa or more.

(Regarding slag area ratio)
In the welded joint 1 according to this embodiment, slag 15 may adhere to the surface of the weld bead 13. The slag 15 prevents the deposition of a coating film during electrodeposition coating, resulting in poor coating. However, if the ratio of the projected area of the slag 15 to the projected area of the weld bead 13, measured by observing the surface of the weld bead 13, i.e., the slag area ratio, is 9% or less, the corrosion resistance required for the welded joint 1 can be ensured. Therefore, in the welded joint 1 according to this embodiment, the slag area ratio is set to 9% or less. The smaller the slag area ratio, the more preferable it is, and it may be 8% or less, 7% or less, 6% or less, or 5% or less. Furthermore, there is no need to particularly limit the lower limit of the slag area ratio, and the slag area ratio may be 0%. On the other hand, the slag area ratio may be 1% or more, 2% or more, or 4% or more.

The slag area ratio can be measured by photographing the weld bead 13 in a direction perpendicular to the surface on which the weld bead 13 is provided, and then performing image analysis on the photograph to determine the projected area of the weld bead 13 and the projected area of the slag 15. If an electrodeposition coating is provided on the surface of the weld bead 13, the electrodeposition coating can be removed using a remover before photographing the weld bead 13. Note that the slag area ratio and the electrodeposition coating defect rate are generally the same. Therefore, if an electrodeposition coating is provided on the surface of the weld bead 13, the electrodeposition coating defect rate can be determined using the method described below, and this defect rate can be considered to be the slag area ratio.

(Regarding penetration depth of weld bead 13)
As described above, in the welded joint 1 according to this embodiment, it is necessary to form fayalite in the HAZ 14 on the second surface 122 of the second steel plate 12. To achieve this, it is necessary to set the Si content of the second steel plate 12 to 0.20 mass% or more, and also to sufficiently heat the second surface 122 of the second steel plate 12 during welding. If the second surface 122 is not heated sufficiently, fayalite will not be detected on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12.

It is not easy to measure the temperature of the second surface 122 of the second steel plate 12 during welding, and therefore the heating temperature required to form fayalite is unclear. However, as a result of experiments conducted by the present inventors, it was confirmed that Fe 2 SiO 4 is detected from the surface of the HAZ 14 on the second surface 122 of the second steel plate 12 when the Si content of the second steel plate 12 is 0.20 mass% or more and the penetration depth of the weld bead ₁₃ is 15% or more of the plate thickness of the second steel plate 12 (a steel plate having a _HAZ on the back surface). Therefore, in the welded joint 1 according to this embodiment, the penetration depth of the weld bead 13 is 15% or more of the plate thickness of the second steel plate 12 (a steel plate having a HAZ on the back surface). The penetration depth of the weld bead 13 may be 20% or more, 30% or more, or 40% or more of the plate thickness of the second steel plate 12. The penetration depth of the weld bead 13 is measured using the first surface 121 of the second steel plate 12 as a reference.

However, if the penetration depth is excessive, the molten metal will penetrate the second steel plate 12 during welding. In this case, the weld metal will be exposed on the second surface 122 of the second steel plate 12. In this case, the fayalite will melt, promoting the growth of scale and reducing the adhesion of the scale. Therefore, in the weld joint 1 according to this embodiment, the penetration depth of the weld bead 13 is set to 85% or less of the thickness of the second steel plate 12 (the steel plate with a HAZ on the back surface). The penetration depth of the weld bead 13 may also be set to 80% or less, 75% or less, or 70% of the thickness of the second steel plate 12.

(About Firelite)
As described above, the welded joint 1 according to this embodiment further has fayalite detected on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12. Although the reason is not clear, by generating fayalite on the surface of the HAZ 14, it is possible to improve the scale adhesion in the HAZ 14 even if scale 16 is attached to the HAZ 14.

Fayalite is detected on the surface of the HAZ 14 by performing X-ray crystal structure analysis on the second surface 122 of the second steel sheet 12. The procedure for X-ray crystal structure analysis is as follows: X-rays are irradiated at the relevant location at 2θ (5 to 100 deg), and the reflected X-ray diffraction is captured to obtain a diffraction pattern. The obtained diffraction pattern is then compared with diffraction patterns in a database to identify the substance. Note that if an electrocoat coating is disposed on the second surface 122 of the second steel sheet 12, the electrocoat coating can be removed using a remover appropriate for the type of electrocoat coating before performing the analysis using the above procedure. However, if a large amount of oxides other than fayalite are attached to the second surface 122 of the second steel sheet 12, these oxides may interfere and make it unclear whether or not fayalite is present. If the presence or absence of fayalite cannot be determined through X-ray crystal structure analysis, fayalite may be identified by further considering the composition ratio of materials obtained by evaluating the scale cross section using SEM/EDS/EPMA.

(electrodeposition coating)
The welded joint 1 may further include an electrodeposition coating covering the first steel plate 11, the second steel plate 12, the weld bead 13, the HAZ 14, the slag 15, and the scale 16. This dramatically improves the corrosion resistance of the welded joint 1. In the welded joint 1 according to this embodiment, the area ratio of the slag 15 adhering to the bead is limited to 9% or less, and fayalite is provided on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12. Therefore, the welded joint 1 according to this embodiment reduces the coating defect rate of the electrodeposition coating and further improves the scale adhesion of the electrodeposition coating. For example, the welded joint 1 according to this embodiment can achieve a coating defect area ratio of 7% or less, 6% or less, 5% or less, or 3% or less. The type of electrodeposition coating is not particularly limited and can be selected appropriately depending on the application of the welded joint 1.

The defective coating area rate of the electrodeposition coating film can be measured by photographing the electrodeposition-coated weld bead 13, analyzing the photograph, and determining the projected area of the weld bead 13 and the projected area of the defective electrodeposition coating.

(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 manufacturing method for the welded joint 1 according to this embodiment includes a step of gas-shielded arc welding the end 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12 to form a weld bead 13. In gas-shielded arc welding, the Si content of the first steel plate 11 and the second steel plate 12, the heat input, and the components of the shielding gas are important factors.

(Si content of first steel plate 11 and second steel plate 12)
In the manufacturing method of the welded joint 1 according to this embodiment, the Si content of the first steel plate 11 is set to 0 to 0.20 mass %, and the Si content of the second steel plate 12 is set to 0.20 to 1.20 mass %. By combining two steel plates having such Si contents, it is possible to obtain both the effect of reducing the area ratio of slag 15 adhering to the weld bead 13 and the effect of forming fayalite on the surface of the HAZ 14 on the second surface 122 of the second steel plate 12, i.e., the surface opposite the weld bead 13.

The first steel plate 11 and the second steel plate 12 can be, 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. The positional relationship between the first steel plate 11 and the second steel plate 12 can also be the preferred configuration of the welded joint 1 according to this embodiment described above. 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. The angle between the first steel plate 11 and the second steel plate 12 may be 80° to 100° to form the welded joint 1 as a T-joint.

(Shielding gas components)
In the manufacturing method for the welded joint 1 according to this embodiment, the proportion of oxidizing gas in the shielding gas used for gas-shielded arc welding is set to 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 15 adhering to the weld bead 13 can be obtained. The volume fraction of the oxidizing gas in the shielding gas may also be set to 9% or less, 8% or less, 5% or less, or 3% or less.

(Size of HAZ14 and penetration depth)
In the manufacturing method of the welded joint 1 according to this embodiment, it is necessary to control welding conditions such as the heat input and the torch angle so that the HAZ 14 formed around the weld bead 13 is exposed to the outside of the second steel plate 12 at the second surface 122 of the second steel plate 12 and the penetration depth of the weld bead 13 is 15% to 85% of the plate thickness of the second steel plate 12. This makes it possible to form the weld bead 13 joining the first steel plate 11 and the second steel plate 12 and to ensure the strength normally required for the welded joint 1. Furthermore, according to the above-mentioned welding conditions, the second surface 122 of the second steel plate 12 can be sufficiently heated to form fayalite on the surface of the HAZ 14.

In gas-shielded arc welding, the welding conditions other than those described above are not particularly limited and can be selected appropriately depending on the thickness of the first steel plate 11 and the second steel plate 12, etc. The composition of the filler metal in gas-shielded arc welding is also not particularly limited and can be selected appropriately depending on the composition of the first steel plate 11 and the second steel plate 12, etc. A suitable example of a filler metal is solid wire, and a suitable example of the chemical composition of the solid wire is, in mass%, C: 0.05 to 0.20%, Si: 0.15% or less, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.04% or less, Ti: 0.02 to 0.20%, B: 0.012% or less, Al: 0.22% or less, Cr: 0.5% or less, Nb: 0.3% or less, V: 0.3% or less, Mo: 1.0% or less, Ni: 3.0% or less, Zr: 0.200% or less, Cu: 0.5% or less, and the balance: iron and impurities.

(electrodeposition coating)
In the method for manufacturing the welded joint 1 according to this embodiment, after gas-shielded arc welding, the first steel plate 11, the second steel plate 12, the weld bead 13, the HAZ 14, the slag 15, and the scale 16 may be subjected to electrodeposition coating. In a typical method for manufacturing a welded joint, the slag 15 and the scale 16 may be removed before electrodeposition coating. However, such a step is not required in the method for manufacturing the welded joint 1 according to this embodiment. The slag 15 and the scale 16 may be partially removed, or the step of removing the slag 15 and the scale 16 may be omitted. By gas-shielded arc welding according to the above conditions, the slag 15 and the scale 16 are rendered harmless, so a good electrodeposition coating can be obtained without removing them.

(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 a low paint defect rate and a welded joint 1 with high scale adhesion, and therefore have high corrosion resistance.

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 an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this 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 the objectives of the present invention are achieved.

Example 1
Various weld joints were produced by gas-shielded arc welding the ends of various first steel plates shown in Table 1 to the first surface of a second steel plate to form weld beads. The shielding gas used for gas-shielded arc welding contained _CO2 as an oxidizing gas, and the proportions thereof were as shown in Table 1.

The welding conditions other than the shielding gas were the same for all examples and comparative examples. These conditions are shown in Table 2. The shape of the jig used during welding was such that it did not come into contact with the back side of the weld bead. This minimized heat transfer from the workpiece to the jig. The welding wire used was a 1.2 mm diameter low-Si welding wire prototyped based on YGW11.

The penetration depths of the various welded joints obtained in this manner are shown in Table 1. Table 1 also shows whether fayalite was detected in the HAZ when X-ray crystal structure analysis was performed on the second surface of the second steel plate of these welded joints. In all examples and comparative examples, the HAZ was exposed to the outside of the second steel plate.

Furthermore, these welded joints were subjected to electrodeposition coating, and then the coating defect area rate and coating peeling area rate were measured. Electrodeposition coating was performed so that the thickness of the base material was 20 μm. 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 coating defect. The coating peeling area rate was determined by performing a peeling test on the back side of the weld bead on the second surface of the second steel plate. The peeling test was performed by applying tape to the test area and peeling it off, and measuring the area of the peeled coating. The tape was Nichiban cellophane tape, 24 mm wide and 150 mm long. The coating peeling area rate was calculated by dividing the peeled area by the area of the cellophane tape (24 × 150 = 3600 mm ² ). 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. Furthermore, welded joints with a coating peeling area rate of 10% or less were judged to have excellent scale adhesion. In Table 1, values outside the range of the invention are underlined.

Examples 7 to 14, and 18 are examples in which the Si content of the first steel plate and the second steel plate, the penetration depth, and the proportion of oxidizing gas in the shielding gas were appropriate. In the welded joints of these examples, the slag area ratio was suppressed to 9% or less, and fayalite was detected on the surface of the HAZ on the second surface of the second steel plate. In the welded joints of these examples, electrodeposition coating defects were suppressed and scale adhesion was improved.

On the other hand, the welded joint of Comparative Example 1 had an excessive slag area ratio, and no fayalite was detected. This is presumably because the first steel plate had an excessive Si content, the second steel plate had an insufficient Si content, and the proportion of oxidizing gas in the shielding gas was excessive. As a result, the welded joint of Comparative Example 1 exceeded the pass/fail criteria for both the paint defect area ratio and the paint peeling area ratio.

The welded joint of Comparative Example 2 had an excessive slag area ratio. This is presumably due to an excessive Si content in the first steel plate and an excessive proportion of oxidizing gas in the shielding gas. As a result, the coating defect area ratio of the welded joint of Comparative Example 2 exceeded the pass/fail criteria.

The welded joint of Comparative Example 3 had an excessive slag area ratio. This is presumably due to the excessive Si content of the first steel plate. As a result, the coating defect area ratio of the welded joint of Comparative Example 3 exceeded the pass/fail criteria.

No fayalite was detected in the weld joint of Comparative Example 4. This is presumably due to an insufficient Si content in the second steel plate. As a result, the paint peeling area rate of the weld joint of Comparative Example 4 exceeded the pass/fail criteria.

In the welded joint of Comparative Example 5, the slag area ratio was excessive, and no fayalite was detected. This is presumably because the Si content of the first steel plate was excessive and the Si content of the second steel plate was insufficient. As a result, in the welded joint of Comparative Example 5, both the paint defect area ratio and the paint peeling area ratio exceeded the pass/fail criteria.

In the welded joint of Comparative Example 6, the first steel plate had an excessive Si content. As a result, the coating defect area rate in the welded joint of Comparative Example 6 exceeded the pass/fail criteria.

No fayalite was detected in the weld joint of Comparative Example 15. This is presumably because the penetration depth was insufficient and the second surface of the second steel plate was not sufficiently heated. As a result, the paint peeling area rate of the weld joint of Comparative Example 15 exceeded the pass/fail criteria.

No fayalite was detected in the weld joint of Comparative Example 16. This is presumably due to an insufficient Si content in the second steel plate. As a result, the paint peeling area rate of the weld joint of Comparative Example 16 exceeded the pass/fail criteria.

The welded joint of Comparative Example 17 had an excessive slag area ratio. This is presumably due to the excessive Si content of the first steel plate. As a result, the coating defect area ratio of the welded joint of Comparative Example 17 exceeded the pass/fail criteria.

Example 2
The following three types of welded joints were fabricated and their scale adhesion was evaluated. The welding conditions and the method for evaluating scale adhesion were the same as those in the examples and comparative examples in Table 1 above.
(a) The Si content of the second steel plate is 0.05%
(b) The Si content of the second steel plate is 0.15%
(c) The Si content of the second steel plate is 0.30%

Figure 4 shows photographs of the second surface of the second steel plate in welded joints a-c immediately after welding and after chemical conversion treatment. Figure 5 shows photographs of the second surface of the second steel plate in welded joints a-c immediately after electrocoating and after a peel test. As shown in Figures 4 and 5, the second surface of the second steel plate in welded joints a-c appeared almost identical before and immediately after electrocoating. However, clear differences were observed after the peel test. Welded joints a and b, which had an Si content of less than 0.20%, experienced significant loss of the electrocoating, but welded joint c, which had an Si content of 0.20% or more, showed no loss of the electrocoating.

1 Weld joint 11 First steel plate 111 End of first steel plate 12 Second steel plate 121 First surface of second steel plate 122 Second surface of second steel plate 13 Weld bead 14 HAZ
15 Slag 16 Scale

Claims (12)

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 HAZ formed around the weld bead and exposed to the outside of the second steel plate on the second surface of the second steel plate;
Scale adhering to the HAZ on the second surface of the second steel plate;
Equipped with
The Si content of the first steel plate is 0 to 0.20 mass%,
The Si content of the second steel plate is 0.20 to 1.20 mass%,
The ratio of the area of the slag adhering to the weld bead to the area of the weld bead is 9% or less,
The penetration depth of the weld bead is 15% or more and 85% or less of the plate thickness of the second steel plate,
Further comprising fayalite detected from the HAZ by performing X-ray crystal structure analysis on the second surface of the second steel plate ;
The chemical formula of _the fayalite is _Fe2SiO4
Welded joints. The welded joint described in claim 1, characterized in that the first steel plate and the second steel plate are overlapped to form a lap fillet welded joint. The welded joint described in claim 1, characterized in that it is a T-joint in which the angle between the first steel plate and the second steel plate is 80° to 100°. The welded joint described in any one of claims 1 to 3, further comprising an electrocoating film covering the first steel plate, the second steel plate, the weld bead, the HAZ, the slag, and the scale. The welded joint described in claim 4, characterized in that the paint defect area rate is 7% or less. A method for manufacturing a welded joint, comprising: a step of gas-shielded arc welding an end portion of a first steel plate and a first surface of a second steel plate to form a weld bead,
The Si content of the first steel plate is 0 to 0.20 mass%,
The Si content of the second steel plate is 0.20 to 1.20 mass%,
a HAZ formed around the weld bead is exposed to the outside of the second steel plate on a second surface of the second steel plate;
The penetration depth of the weld bead is 15% or more and 85% or less of the plate thickness of the second steel plate,
A method for producing a welded joint, wherein the proportion of oxidizing gas in the shielding gas used for the gas-shielded arc welding is 10% or less. A method for manufacturing a welded joint as described in claim 6, characterized in that the proportion of oxidizing gas in the shielding gas is 5% or less. The method for manufacturing a welded joint described in claim 6 or 7 further comprises a step of electrocoating the first steel plate, the second steel plate, the weld bead, the HAZ, slag, and scale after the gas-shielded arc welding. A method for manufacturing a welded joint as described in any one of claims 6 to 8, characterized in that the first steel plate and the second steel plate are overlapped during the gas-shielded arc welding. A method for manufacturing a welded joint as described in any one of claims 6 to 8, characterized in that the angle between the first steel plate and the second steel plate during gas-shielded arc welding is 80° to 100°. An automotive part having the welded joint described in any one of claims 1 to 5. A building component having the welded joint described in any one of claims 1 to 5.