WELDED JOINT

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to a welded joint.

Description of the Background

Zn—Al—Mg-based hot-dip plated steel sheets having a hot-dip Zn plating layer containing Al and Mg have excellent red rust resistance and coating adhesion. Therefore, Zn—Al—Mg-based hot-dip plated steel sheets are widely used as materials for structural members such as building materials that require corrosion resistance.

In the production of structural members, fusion welding such as arc welding may be performed in joining materials together. In fusion welding of a Zn—Al—Mg-based hot-dip plated steel sheet, a region where a plating layer remains and a region where no plating layer remains are formed in a heat-affected zone of a back bead surface. In the region where the plating layer remains in the heat-affected zone, the plating layer has a different composition from that of the plating layer of a non-heat-affected zone, and thus expected red rust resistance and coating adhesion may not be obtained.

For example, Patent Document 1 discloses a vehicle chassis member having a joining portion obtained by joining together hot-dip Zn—Al—Mg-based alloy-plated steel sheet members having a sheet thickness of 1.0 to 3.0 mm by arc welding, in which a steel sheet surface having a plating layer before welding is continuously covered with a Zn—Al—Mg-based alloy layer up to a weld bead toe portion, an Fe—Al-based alloy layer is present between the Zn—Al—Mg-based alloy layer and a steel base, and in a steel sheet surface layer portion within 2 mm away from the weld bead toe portion, the Zn—Al—Mg-based alloy layer has an average Al concentration of 0.2 to 22.0 mass % and an average Mg concentration of 1.0 to 10.0 mass %, and the Fe—Al-based alloy layer has an average Fe concentration of 70.0 mass % or less. Patent Document 1 discloses that, with the above-described configuration, a decrease in corrosion resistance is avoided in a portion near the bead toe portion of the arc welding portion, and thus a vehicle chassis having high strength and high corrosion resistance can be constructed.

Background Art Document

Patent Document

• Patent Document 1: Japanese Patent No. 5700394

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, Patent Document 1 does not consider red rust resistance and coating adhesion of the back bead surface.

The present disclosure has been made in view of the above circumstances. An object of the present disclosure is to provide a welded joint having excellent coating adhesion and red rust resistance of a back bead surface.

Means for Solving the Problem

The gist of the present disclosure is as follows.

[1] A welded joint in which a first steel sheet and a second steel sheet are welded, the welded joint including:

• • the first steel sheet and the second steel sheet; and
  • a weld bead portion formed by welding,
  • in which each of the first steel sheet and the second steel sheet has a heat-affected zone positioned around the weld bead portion, and a non-heat-affected zone not affected by heat due to the welding,
  • the first steel sheet has a plating layer on a surface in the heat-affected zone and the non-heat-affected zone,
  • the plating layer of the non-heat-affected zone includes, as a chemical composition, by mass %,
  • Al: 5.0% to 40.0%,
  • Mg: 3.0% to 15.0%,
  • Fe: 0.01% to 15.00%,
  • Si: 0% to 10.00%,
  • Ca: 0% to 1.5000%,
  • Sb: 0% to 0.5000%,
  • Pb: 0% to 0.5000%,
  • Sr: 0% to 0.5000%,
  • Cu: 0% to 1.0000%,
  • Ti: 0% to 1.0000%,
  • V: 0% to 1.0000%,
  • Cr: 0% to 1.0000%,
  • Nb: 0% to 1.0000%,
  • Ni: 0% to 1.0000%,
  • Mn: 0% to 1.0000%,
  • Mo: 0% to 1.0000%,
  • Sn: 0% to 1.0000%,
  • Zr: 0% to 1.0000%,
  • Co: 0% to 1.0000%,
  • W: 0% to 1.0000%,
  • Ag: 0% to 1.0000%,
  • Li: 0% to 1.0000%,
  • La: 0% to 0.5000%,
  • Ce: 0% to 0.5000%,
  • Y: 0% to 0.5000%,
  • Bi: 0% to 0.5000%,
  • In: 0% to 0.5000%,
  • B: 0% to 0.5000%, and
  • a remainder comprising Zn of 20.000% or more and impurities,
  • when observing a cross section orthogonal to an extending direction of the weld bead portion, in a back surface of a surface having the weld bead portion,
  • toward a direction orthogonal to the extending direction of the weld bead portion and away from a toe of the weld bead portion, in a region from a position set as a starting point where coating with the plating layer is started to a position 1,000 μm away from the starting point,
  • Expression (1) is satisfied, where L₀ is a horizontal length of an observation field and L is a surface unevenness length of the plating layer in the observation field, and
  • an area ratio of an Mg—Zn phase in the plating layer of the region from the starting point to the position 1,000 μm away from the starting point is 5% or more.

(L−L₀)/L₀×100≥3  (1)

[2] The welded joint according to [1], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %, one or more of

• • Si: 0.01% to 10.00%,
  • Ca: 0.0001% to 1.5000%,
  • Sb: 0.0001% to 0.5000%,
  • Pb: 0.0001% to 0.5000%,
  • Sr: 0.0001% to 0.5000%,
  • Cu: 0.0001% to 1.0000%,
  • Ti: 0.0001% to 1.0000%,
  • V: 0.0001% to 1.0000%,
  • Cr: 0.0001% to 1.0000%,
  • Nb: 0.0001% to 1.0000%,
  • Ni: 0.0001% to 1.0000%,
  • Mn: 0.0001% to 1.0000%,
  • Mo: 0.0001% to 1.0000%,
  • Sn: 0.0001% to 1.0000%,
  • Zr: 0.0001% to 1.0000%,
  • Co: 0.0001% to 1.0000%,
  • W: 0.0001% to 1.0000%,
  • Ag: 0.0001% to 1.0000%,
  • Li: 0.0001% to 1.0000%,
  • La: 0.0001% to 0.5000%,
  • Ce: 0.0001% to 0.5000%,
  • Y: 0.0001% to 0.5000%,
  • Bi: 0.0001% to 0.5000%,
  • In: 0.0001% to 0.5000%, and
  • B: 0.0001% to 0.5000%.

[3] The welded joint according to [1] or [2], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %,

• • Mg: 4.5% to 15.0%, and
  • the area ratio of the Mg—Zn phase in the plating layer of the region is 20% or more.

[4] The welded joint according to [1] or [2], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %,

• • Mg: 5.5% to 15.0%, and
  • the area ratio of the Mg—Zn phase in the plating layer of the region is 30% or more.

[5] The welded joint according to any one of [1] to [4], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %,

• • Al: 10.0% to 40.0%, and
  • a value on a left side of Expression (1) is 6 or more.

[6] The welded joint according to any one of [1] to [4], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %,

• • Al: 15.0% to 40.0%, and
  • a value on a left side of Expression (1) is 8 or more.

[7] The welded joint according to any one of [1] to [6], in which the plating layer of the non-heat-affected zone includes, as the chemical composition, by mass %,

• • Sn: 0.0200% to 1.0000%, and
  • the plating layer of the non-heat-affected zone has an Mg₂Sn phase.

Effects of the Invention

According to the aspect of the present disclosure, it is possible to provide a welded joint having excellent coating adhesion and red rust resistance of a back bead surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a cross section near a weld bead portion of a welded joint.

FIG. 2 is an enlarged view of a heat-affected zone of a back bead surface of the welded joint.

FIG. 3 is an enlarged view of a part of a region from a starting point S to a position 1,000 μm away from the starting point S.

FIG. 4 is a diagram showing a cross section near a weld bead portion of a lap joint.

FIG. 5 is a diagram showing a cross section near a weld bead portion of a T-joint.

DETAILED DESCRIPTION OF THE INVENTION

A welded joint according to an embodiment of the present disclosure (hereinafter, may be referred to as the welded joint according to the present embodiment) will be described. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present disclosure.

Hereinafter, individual configuration requirements of the present disclosure will be described in detail.

The numerical limit range described below with “to” in between includes the lower limit and the upper limit. Numerical values indicated as “less than” or “more than” do not fall within the numerical range. In the following description, % regarding the chemical composition is mass % unless otherwise specified.

In addition, terms related to welding conform to JIS Z 3001:2018-1 to 7.

A welded joint according to the present embodiment is a welded joint 10 in which a first steel sheet 1 and a second steel sheet 2 are welded as shown in FIG. 1, including: the first steel sheet 1 and the second steel sheet 2; and a weld bead portion 3 formed by welding, in which each of the first steel sheet 1 and the second steel sheet 2 has a heat-affected zone a positioned around the weld bead portion 3 and a non-heat-affected zone b not affected by heat due to the welding, and the first steel sheet 1 or the second steel sheet 2 or combination thereof has a plating layer 4 (not shown in FIG. 1) positioned on at least a part of a surface in the heat-affected zone a and the non-heat-affected zone b.

Hereinafter, each configuration will be described in detail.

The materials of the first steel sheet 1 and the second steel sheet 2 are not particularly limited. For example, various kinds of steel sheets such as general steels, Al-killed steels, ultra-low-carbon steels, high carbon steels, various high tensile strength steels, and some high alloy steels (steels containing strengthening elements such as Ni and Cr).

The method of producing the first steel sheet 1 and the second steel sheet 2 (hot rolling method, pickling method, cold rolling method, and the like) is not particularly limited.

The weld bead portion 3 is formed of a weld bead formed by welding. The shape and composition of the weld bead portion 3 are not particularly limited.

The heat-affected zone a affected by heat due to welding and the non-heat-affected zone b not affected by heat due to welding are present around the weld bead portion 3. Zn in the plating layer 4 may be evaporated by heat during welding. Therefore, the composition of the plating layer 4 of the heat-affected zone a may be different from that of the plating layer 4 of the non-heat-affected zone b. In addition, the heat-affected zone a has a part where no plating layer 4 is present, since the plating layer 4 melts and infiltrates into a surface layer region of the first steel sheet 1 and the second steel sheet 2, or melts away.

First, the chemical composition of the plating layer 4 of the non-heat-affected zone b will be described. In the welded joint 10 according to the present embodiment, in a case where the chemical composition of the plating layer 4 of the non-heat-affected zone b is within ranges described below, the chemical composition of the plating layer 4 remaining in the heat-affected zone a can also be preferably controlled, and thus the coating adhesion and the red rust resistance of the back bead surface A can be improved. Note that the chemical composition of the plating layer 4 of the non-heat-affected zone b is the same on both the surface having the weld bead and the back bead surface.

The chemical composition of the plating layer 4 of the non-heat-affected zone b includes, by mass %, Al: 5.0% to 40.0%, Mg: 3.0% to 15.0%, Fe: 0.01% to 15.00%, and a remainder comprising Zn of 20.000% or more and impurities.

Each element will be described below.

• • Al: 5.0% to 40.0%

Al contributes to an improvement in the coating adhesion and the red rust resistance. In a case where the Al content is less than 5.0%, the coating adhesion and the red rust resistance of the back bead surface A deteriorate. Therefore, the Al content is set to 5.0% or more. The Al content is preferably 10.0% or more, and more preferably 15.0% or more.

On the other hand, in a case where the Al content is more than 40.0%, the coating adhesion of the back bead surface A deteriorates. Therefore, the Al content is set to 40.0% or less. The Al content is preferably 35.0% or less or 30.0% or less, and more preferably 25.0% or less.

• • Mg: 3.0% to 15.0%

Mg is a necessary element for forming an Mg—Zn phase. In a case where the Mg content is less than 3.0%, the Mg—Zn phase cannot be formed in a sufficient amount in the back bead surface A, and the coating adhesion and the red rust resistance of the back bead surface A deteriorate. Therefore, the Mg content is set to 3.0% or more. The Mg content is preferably 4.0% or more or 4.5% or more, and more preferably 5.0% or more or 5.5% or more.

On the other hand, in a case where the Mg content is more than 15.0%, a large amount of dross mainly containing Mg is generated in a plating bath, and the dross is likely to adhere to the plating original sheet. Therefore, the plating layer 4 cannot be formed in some cases. Therefore, the Mg content is set to 15.0% or less. The Mg content is preferably 12.0% or less or 10.0% or less, and more preferably 7.0% or less.

• • Fe: 0.01% to 15.00%

Since Fe may be mixed in the plating layer 4 from the first steel sheet 1 during the formation of the plating layer 4, it is difficult to reduce the Fe content in the plating layer 4 to 0%. Therefore, the Fe content is set to 0.01% or more. The Fe content may be 0.05% or more or 0.10% or more.

In addition, in a case where the Fe content is 15.00% or less, the properties of the plating layer 4 are not adversely affected. Therefore, the Fe content is set to 15.00% or less. The Fe content may be 10.00% or less, 5.00% or less, or 3.00% or less.

The plating layer 4 of the non-heat-affected zone b may have the above-described chemical composition and a remainder comprising Zn of 20.000% or more and impurities. In a case where the Zn content is less than 20.000%, desired red rust resistance and coating adhesion of the back bead surface A cannot be obtained. The Zn content is preferably 40.000% or more or 50.000% or more, and more preferably 55.000% or more, 60.000% or more, 65.000% or more, or 70.000% or more.

In the present embodiment, impurities mean those mixed from the production environment or the like and/or those allowed within a range that does not adversely affect the properties of the welded joint 10 according to the present embodiment.

Although not essential for providing desired properties, the following optional elements may be contained in the plating layer 4 according to the present embodiment. However, since it is not essential that these elements be contained, the lower limits of the amounts of these elements are 0%.

• • Si: 0.01% to 10.00%

Si contributes to an improvement in the red rust resistance. In order to reliably obtain this effect, the Si content is preferably set to 0.01% or more. The Si content is more preferably 0.05% or more or 0.10% or more.

On the other hand, in a case where the Si content is more than 10.00%, the red rust resistance deteriorates instead. Therefore, the Si content is set to 10.00% or less. The Si content is more preferably 5.00% or less, 3.00% or less, or 1.00% or less.

• • Ca: 0.0001% to 1.5000%

Ca is an element that can adjust the optimum amount of Mg eluted for imparting red rust resistance. In order to reliably obtain this effect, the Ca content is preferably set to 0.0001% or more. The Ca content is more preferably 0.1000% or more or 0.3000% or more.

On the other hand, in a case where the Ca content is excessive, the red rust resistance and workability deteriorate. Therefore, the Ca content is set to 1.5000% or less. The Ca content is more preferably 1.0000% or less or 0.8000% or less.

• • Sb: 0.0001% to 0.5000%
  • Pb: 0.0001% to 0.5000%
  • Sr: 0.0001% to 0.5000%

Sb, Pb, and Sr contribute to an improvement in the red rust resistance. In order to reliably obtain this effect, the amount of any one of Sb, Pb, and Sr is preferably set to 0.0001% or more. Each of the Sb content, the Pb content, and the Sr content is more preferably 0.0005% or more or 0.0050% or more.

On the other hand, in a case where the amount of any one of Sb, Pb, and Sr is more than 0.5000%, the red rust resistance deteriorates instead. Therefore, each of the Sb content, the Pb content, and the Sr content is set to 0.5000% or less. Each of the Sb content, the Pb content, and the Sr content is more preferably 0.3000% or less or 0.2000% or less.

• • Cu: 0.0001% to 1.0000%
  • Ti: 0.0001% to 1.0000%
  • V: 0.0001% to 1.0000%
  • Cr: 0.0001% to 1.0000%
  • Nb: 0.0001% to 1.0000%
  • Ni: 0.0001% to 1.0000%
  • Mn: 0.0001% to 1.0000%
  • Mo: 0.0001% to 1.0000%

Cu, Ti, V, Cr, Nb, Ni, Mn, and Mo contribute to an improvement in the red rust resistance. In order to reliably obtain this effect, the amount of any one of the above elements is preferably set to 0.0001% or more. Each of the amounts of the above elements is more preferably 0.0005% or more or 0.0050% or more.

On the other hand, in a case where the amount of any one of the above elements is more than 1.0000%, the red rust resistance deteriorates instead. Therefore, each of the amounts of the above elements is set to 1.0000% or less. Each of the amounts of the above elements is more preferably 0.3000% or less or 0.2000% or less.

• • Sn: 0.0001% to 1.0000%

Sn is an element that forms an Mg₂Sn phase with Mg and improves red rust resistance. In order to reliably obtain this effect, the Sn content is preferably set to 0.0001% or more. In order to form an Mg₂Sn phase in the plating layer 4 of the heat-affected zone a and further improve the red rust resistance of the back bead surface, the Sn content is more preferably set to 0.0200% or more.

On the other hand, in a case where the Sn content is more than 1.0000%, the red rust resistance deteriorates instead. Therefore, the Sn content is set to 1.0000% or less. The Sn content is preferably 0.5000% or less or 0.3000% or less.

• • Zr: 0.0001% to 1.0000%
  • Co: 0.0001% to 1.0000%
  • W: 0.0001% to 1.0000%
  • Ag: 0.0001% to 1.0000%
  • Li: 0.0001% to 1.0000%

Zr, Co, W, Ag, and Li are elements that improve red rust resistance. In order to reliably obtain this effect, the amount of any one of Zr, Co, W, Ag, and Li is preferably set to 0.0001% or more. Each of the Zr content, the Co content, the W content, the Ag content, and the Li content is more preferably 0.0005% or more or 0.0020% or more.

On the other hand, in a case where the Zr content, the Co content, the W content, the Ag content, and the Li content are excessive, the red rust resistance deteriorates. In a case where the amount of any one of Zr, Co, W, Ag, and Li is more than 1.0000%, the red rust resistance significantly deteriorates. Therefore, each of the Zr content, the Co content, the W content, the Ag content, and the Li content is set to 1.0000% or less. Each of the Zr content, the Co content, the W content, the Ag content, and the Li content is preferably 0.5000% or less or 0.1000% or less.

• • La: 0.0001% to 0.5000%
  • Ce: 0.0001% to 0.5000%
  • Y: 0.0001% to 0.5000%

La, Ce, and Y contribute to an improvement in the red rust resistance. In order to reliably obtain this effect, the amount of any one of La, Ce, and Y is preferably set to 0.0001% or more. Each of the La content, the Ce content, and the Y content is more preferably 0.0005% or more or 0.0050% or more.

On the other hand, in a case where the amount of any one of La, Ce, and Y is more than 0.5000%, the red rust resistance deteriorates instead. Therefore, each of the La content, the Ce content, and the Y content is set to 0.5000% or less. Each of the La content, the Ce content, and the Y content is more preferably 0.2000% or less or 0.1000% or less.

• • Bi: 0.0001% to 0.5000%
  • In: 0.0001% to 0.5000%
  • B: 0.0001% to 0.5000%

Bi, In, and B contribute to an improvement in the red rust resistance. In order to reliably obtain this effect, the amount of any one of Bi, In, and B is preferably set to 0.0001% or more. Each of the Bi content, the In content, and the B content is more preferably 0.0005% or more or 0.0050% or more.

On the other hand, in a case where the amount of any one of Bi, In, and B is more than 0.5000%, the red rust resistance deteriorates instead. Therefore, each of the Bi content, the In content, and the B content is set to 0.5000% or less. Each of the Bi content, the In content, and the B content is more preferably 0.2000% or less or 0.1000% or less.

The chemical composition of the plating layer 4 is measured by the following method.

A test piece having a size of 20 mm×20 mm×sheet thickness is collected from the non-heat-affected zone b (a position 100 mm or more away from the weld bead portion 3) of the first steel sheet 1. An acid solution is obtained by exfoliating and dissolving the plating layer 4 using 10 vol % of HCl containing an inhibitor that suppresses the corrosion of the first steel sheet 1. Next, the obtained acid solution is subjected to ICP analysis. As a result, the chemical composition of the plating layer 4 is obtained.

In a case where the welded joint 10 is provided with a coating on its surface, the above-described measurement is performed after the coating is removed with DESCOAT 110B manufactured by NEOS COMPANY LIMITED.

Next, a back bead surface (surface A in FIG. 1) of the welded joint 10 according to the present embodiment will be described with reference to FIGS. 1 to 3.

In the welded joint 10 according to the present embodiment, when observing a cross section orthogonal to an extending direction of the weld bead portion 3, in a back surface (back bead surface A) of a surface having the weld bead portion 3, toward a direction orthogonal to the extending direction of the weld bead portion 3 and away from a toe of the weld bead portion 3, in a region from a position set as a starting point S where the coating with the plating layer 4 is started to a position 1,000 μm away from the starting point S, Expression (1) is satisfied, where L₀ is a horizontal length of an observation field and L is a surface unevenness length of the plating layer in the observation field, and an area ratio of an Mg—Zn phase in the plating layer 4 of the region from the starting point S to the position 1,000 μm away from the starting point S is 5% or more.

(L−L₀)/L₀×100≥3  (1)

FIG. 1 is a diagram showing a cross section orthogonal to the extending direction of the weld bead portion 3 of the welded joint 10. The back surface (the back bead surface A) of the surface having the weld bead portion 3 has the heat-affected zone a affected by heat due to welding and the non-heat-affected zone b not affected by heat. FIG. 2 is an enlarged view of the heat-affected zone a of the back bead surface A in FIG. 1. As shown in FIG. 2, in the heat-affected zone a of the back bead surface A, a part where the plating layer 4 is not present and the first steel sheet 1 is exposed, and a part coated with the plating layer 4 are present.

In the present embodiment, toward a direction orthogonal to the extending direction of the weld bead portion 3 and away from the toe of the weld bead portion 3, in a region from a position set as a starting point where the coating with the plating layer 4 is started to a position 1,000 μm away from the starting point, Expression (1) is satisfied, where L₀ is a horizontal length of an observation field and L is a surface unevenness length of the plating layer 4 in the observation field.

Note that the toe of the weld bead portion 3 is a boundary between the weld bead portion 3 and the first steel sheet 1, and is a point E shown in FIG. 1. In addition, the direction away from the toe (the point E in FIG. 1) of the weld bead portion 3 is a direction opposite to the weld bead portion 3 when viewed from the toe E, and is a direction D shown in FIG. 2. In addition, the position (starting point) where the coating with the plating layer 4 is started is a boundary between a part where the first steel sheet 1 is exposed and a part coated with the plating layer 4 on the surface of the first steel sheet 1 of the back bead surface A, and is a point S shown in FIGS. 1 and 2.

In the region from the starting point S to the position 1,000 μm away from the starting point S of the back bead surface A, the heat-affected zone a affected by heat due to welding is present. The size of the heat-affected zone a changes depending on the heat input amount during welding, the sheet thickness of the first steel sheet 1 and the second steel sheet 2, and the like. However, in the region from the starting point S to the position 1,000 μm away from the starting point S, the heat-affected zone a is present in at least a part (particularly, the starting point S side) of the region. In the present embodiment, the coating adhesion of the back bead surface A is improved by preferably controlling the surface unevenness of the plating layer 4 in the heat-affected zone a of the back bead surface A.

The chemical composition of the plating layer 4 in the heat-affected zone a may include, for example, by mass %, Zn+Mg: 50% or more, Fe: 5% or less, and a remainder comprising Al and impurities.

FIG. 3 is an enlarged view of a part of the region from the starting point S to the position 1,000 μm away from the starting point S in FIG. 2. Note that FIG. 3 is upside down compared to FIG. 2. Zn in the plating layer 4 evaporates due to the heat during welding. However, when the plating layer 4 contains Mg and Al in addition to Zn, and is manufactured under the manufacturing conditions described below, Zn bonds with Mg. Because of this, an Mg—Zn phase with a higher melting point is formed. Although Zn evaporates due to the heat during welding, the Mg—Zn phase does not evaporate and remains in the plating layer 4, unevenness on the surface of the plating layer 4 is caused. The unevenness on the surface of the plating layer 4 can improve the coating adhesion due to an anchor effect.

In a case where the value on the left side of Expression (1) is less than 3, the surface unevenness of the plating layer 4 cannot be formed favorably, resulting in poor coating adhesion. Therefore, the value on the left side of Expression (1) is set to 3 or more. In order to further increase the surface unevenness of the plating layer 4 and thereby further improving the coating adhesion, it is preferable to increase the Al content in the plating layer 4 and set the value on the left side of Expression (1) to 6 or more, and more preferably 8 or more.

The value on the left side of Expression (1) may be, for example, 30 or less, or 25 or less.

The the surface unevenness length L of the plating layer 4 is measured by the following method.

A cross section of the first steel sheet along the sheet thickness direction is observed as shown in FIG. 3 using a scanning electron microscope, and the surface unevenness length of the plating layer 4 is measured by μm. For the region from the starting point S to the position 1,000 μm away from the starting point S, and for an arbitrarily selected observation field of 200 μm in the horizontal direction ×120 μm in the sheet thickness direction, the surface unevenness length of the plating layer 4 is measured by μm. Note that L₀ is 200 μm. For the region from the starting point S to the position 1,000 μm away from the starting point S, the above operation is performed for five arbitrarily selected fields from the region where the plating layer remains, and the average value is calculated to obtain L. The obtained L is used to calculate the left side of Expression (1).

In a case where multiple starting points S exist on the back bead surface A, the above measurement is performed for a region from the starting point S closest to the center of the weld bead portion 3 to the position 1,000 μm away from the starting point S when viewing the sheet thickness cross section of the first steel sheet 1 as shown in FIG. 1. In a case of a T-joint of FIG. 5, the above measurement is performed for a region from the starting point S closest to the center of any one of the weld bead portions 3 to a position 1,000 μm away from the starting point S.

Regarding a sample to be collected, a sample having a size of 20 mm×15 mm×sheet thickness is collected from the welded joint 10 so that the above cross section can be observed. The sample is embedded in a resin, mirror-polished to finish the cross section, and then subjected to the measurement. In addition, a reflected electron image of the cross section is taken, and from a difference in brightness, a region positioned closest to the center in the sheet thickness is determined as the first steel sheet 1, and a layer excluding the region is determined as the plating layer 4. In a case where the welded joint 10 is provided with a coating on its surface, the layer present in the middle in the reflected electron image is subjected to SEM-EPMA analysis described later. In a case where the obtained chemical composition satisfies the chemical composition (by mass %, Zn+Mg: 50% or more, Fe: 5% or less, and a remainder comprising Al and impurities) of the plating layer 4 in the heat-affected zone a described above, the layer is determined as the plating layer 4 in the heat-affected zone a.

• • Area Ratio of Mg—Zn Phase: 5% or more

The Mg—Zn phase has an effect of increasing the red rust resistance. In a case where the area ratio of the Mg—Zn phase in the plating layer 4 in the region from the starting point S to the position 1,000 μm away from the starting point S is less than 5%, the red rust resistance of the back bead surface A deteriorates. Therefore, the area ratio of the Mg—Zn phase is set to 5% or more. In order to further improve the red rust resistance of the back bead surface A, the area ratio of the Mg—Zn phase is preferably set to 20% or more, and more preferably 30% or more after the increase of the Mg content in the plating layer 4.

The area ratio of the Mg—Zn phase may be set to 95% or less.

The area ratio of the Mg—Zn phase in the plating layer 4 is measured by the following method.

A sample is collected by the same method as when measuring the surface unevenness length L of the plating layer 4, and the cross section is finished by mirror-polishing. Next, point analysis is performed on the cross section using SEM-EPMA (manufactured by JEOL Ltd., JXA-8500F) to quantitatively analyze elements (Mg, Zn, and Fe). A phase with an Mg content of 20% to 60%, a Zn content of 40% to 80%, and a remainder of 5% or less is determined as the Mg—Zn phase. The above-described analysis is performed on the plating layer 4 in the region from the starting point S to the position 1,000 μm away from the starting point S, thereby obtaining the area ratio of the Mg—Zn phase.

A distribution image of a range of plating layer thickness×1,000 μm is measured with an acceleration voltage of 15 kV, a magnification of 5,000 times, and a measurement interval of 1.0 μm. The area ratio is calculated using the “Analyze” function of image analysis software “ImageJ”.

Mg₂Sn Phase

In the welded joint 10 according to the present embodiment, the plating layer 4 of the non-heat-affected zone b preferably has an Mg₂Sn phase. In a case where the plating layer 4 of the non-heat-affected zone b has an Mg₂Sn phase, the red rust resistance in the non-heat-affected zone can be increased.

Whether or not the plating layer 4 of the non-heat-affected zone b has an Mg₂Sn phase is determined by the following method.

Since the amount of the Mg₂Sn phase is small, the presence of the Mg₂Sn phase is detected and confirmed by X-ray diffraction measurement using a 0-20 method. The X-ray diffraction measurement in the detection of the Mg₂Sn phase is performed by a θ-2θ measurement method. In addition, in the X-ray diffraction measurement, in a case where a peak is detected at 23.4±0.3° using Kα rays of a Cu bulb for the plating layer 4 in a 20 mm square sample collected from the non-heat-affected zone b (the position 100 mm or more away from the weld bead portion 3), it is determined that the Mg₂Sn phase is present.

The amount of the plating layer 4 attached per surface may be, for example, within a range of 20 to 250 g/m². By setting the amount of the plating layer 4 attached per surface to 20 g/m² or more, the red rust resistance can be further increased. On the other hand, by setting the amount of the plating layer 4 attached per surface to 250 g/m² or less, the workability can be further increased.

In addition, the above-described welded joint 10 is a lap joint, but the welded joint 10 according to the present embodiment is not limited thereto. The welded joint 10 according to the present embodiment may be, for example, a butt joint having a cross section as shown in FIG. 4, a T-joint having a cross section as shown in FIG. 5, or any other joint. In a case where the welded joint 10 is a butt joint, a surface B shown in FIG. 4 is regarded as a back bead surface. In a case where the welded joint 10 is a T-joint, a surface C (a surface of the first steel sheet 1 opposite to the surface welded to the second steel sheet 2) shown in FIG. 5 is regarded as a back bead surface.

In the butt joint shown in FIG. 4, the weld bead portion 3 does not reach a back bead surface B, but the weld bead portion 3 may have a shape that it reaches the back bead surface B. In addition, the weld bead portion 3 may be formed on both surfaces. In a case where the weld bead portion 3 reaches the back bead surface B, the surface where the weld bead portion 3 is smaller is regarded as the back bead surface B. In a case where the weld bead portion 3 is formed on both surfaces, the surface where the weld bead portion 3 is smaller is regarded as the back bead surface B, and in a case where the weld bead portions 3 on both surfaces have the same size, any one of the surfaces is regarded as the back bead surface B.

The first steel sheet has been described for convenience of description, but the same applies even in a case where the first steel sheet and the second steel sheet are interchanged.

Next, a preferable method of producing the welded joint 10 according to the present embodiment will be described.

A preferable method of producing the welded joint 10 according to the present embodiment includes:

• • a step of performing shot blasting on the first steel sheet 1;
  • a step of performing annealing;
  • a step of applying a plating;
  • a step of cooling; and
  • a step of welding the first steel sheet 1 and the second steel sheet 2.

The producing method for the second steel sheet is not particularly limited, but the second steel sheet may be produced by the same method as that for the first steel sheet.

Hereinafter, each step will be described.

Shot Blasting

By performing shot blasting on the first steel sheet 1, strain is applied before application of a plating. In a case where shot blasting is performed under preferable conditions, strain is preferably applied to the first steel sheet 1, and thus it is possible to promote alloying in the plating layer 4 in combination with the below-described cooling after application of a plating. As a result, it is possible to preferably control the morphologies of the plating layer 4.

In the shot blasting, steel balls having a central grain size of 40 to 450 μm can be used. Examples of the device include TSH30 manufactured by WINOA IKK JAPAN CO., LTD. The projection amount of the shot blasting is preferably set to 10 to 500 kg/m². In a case where the projection amount of the shot blasting is less than 10 kg/m², strain cannot be preferably applied to the first steel sheet 1, and as a result, the surface unevenness of the plating layer 4 cannot be preferably controlled in some cases.

Strain can also be applied by surface grinding. However, since the amount of strain applied by surface grinding is smaller than that applied by shot blasting, a desired amount of strain cannot be applied.

Annealing

After the shot blasting is performed, the first steel sheet 1 is annealed. The annealing temperature during annealing is set within a temperature range of 400° C. to 600° C., and the holding time (annealing time) in the temperature range is set to be longer than 0 second and 100 seconds or shorter. In a case where the annealing temperature is higher than 600° C. or the annealing time is longer than 100 seconds, strains in the first steel sheet 1 and the second steel sheet 2 are released, and as a result, the surface unevenness of the plating layer 4 cannot be preferably controlled in some cases. In a case where the annealing temperature is lower than 400° C. or the annealing is not performed, plating cannot be preferably applied in some cases.

The temperature mentioned here is the surface temperature at a center portion of the sheet surface of the first steel sheet 1, and can be measured using a thermocouple joined by spot welding or the like.

Plating

After the annealing, the first steel sheet 1 is immersed in a plating bath. The composition of the plating bath is controlled so that the plating layer 4 has the chemical composition of the plating layer 4 described above. The bath temperature of the plating bath is preferably set to 600° C. or lower. In a case where the bath temperature of the plating bath is higher than 600° C., strain in the first steel sheet 1 is released, and as a result, the surface unevenness of the plating layer 4 cannot be preferably controlled in some cases.

After the first steel sheet 1 is lifted from the plating bath, the plating adhesion amount may be adjusted by gas wiping or the like.

Cooling

After the plating is applied, cooling is preferably performed using a cooling gas having a dew point of −20° C. or lower so that the average cooling rate in a temperature range of the bath temperature to 250° C. is 15° C./s or faster. Then, cooling is preferably performed using a cooling gas having a dew point of 0° C. or higher so that the average cooling rate in a temperature range of 250° C. to 50° C. is 5° C./s or slower.

In the cooling in the temperature range of the bath temperature to 250° C., in a case where the dew point of the cooling gas is higher than −20° C., coarse oxides are formed on the surface of the plating layer, as a result, the morphologies of the plating layer 4 cannot be preferably controlled in some cases. In the cooling in the temperature range of the bath temperature to 250° C., in a case where the average cooling rate is slower than 15° C./s, coarse-surface oxides are formed during the cooling, as a result, the surface unevenness of the plating layer 4 cannot be preferably controlled in some cases.

In the cooling in the temperature range of 250° C. to 50° C., in a case where the dew point of the cooling gas is lower than 0° C., the oxide of Al or Mg cannot be sufficiently formed in the plating layer 4, and as a result, Zn in the plating layer 4 may evaporate too much, and the morphologies of the plating layer 4 cannot be preferably controlled in some cases. In addition, in a case where the average cooling rate in the temperature range of 250° C. to 50° C. is faster than 5° C./s, and as a result, the surface unevenness of the plating layer 4 cannot be preferably controlled in some cases.

Welding

After the cooling, the first steel sheet 1 and the second steel sheet 2 are welded, and thus the welded joint 10 is obtained. The welding method is not particularly limited as long as the weld bead portion 3 is formed. For example, in a case where arc welding and laser welding are performed, the following conditions can be set for each welding.

• • Arc Welding
  • Welding Current: 250 A
  • Welding Voltage: 26.4 V
  • Welding Speed: 100 cm/min
  • Welding Gas: 20% CO₂+Ar
  • Gas Flow Rate: 20 L/min
  • Welding Wire: YGW16 manufactured by NIPPON STEEL WELDING & ENGINEERING CO., LTD. (§ 1.2 mm (C: 0.1 mass %, Si: 0.80 mass %, Mn: 1.5 mass %, P: 0.015 mass %, S: 0.008 mass %, Cu: 0.36 mass %)
  • Inclination Angle of Welding Torch: 45°
  • Laser Welding
  • Output: 7 kW
  • Welding Speed: 400 cm/min
  • Advance/Retreat angle: 0°

The welded joint 10 according to the present embodiment can be stably produced using the above-described method. Since the welded joint 10 according to the present embodiment has excellent coating adhesion, coating may be performed on the surface for the purpose of improving the corrosion resistance or the like of the welded joint 10.

Examples

First steel sheets and second steel sheets having the mechanical properties of SS400 of JIS G 3101:2020 were subjected to shot blasting, annealing, plating, and cooling under the conditions shown in Tables 2A and 2B, and then welded by the welding methods shown in Tables 3A and 3B to obtain lap joints (welded joints).

The first steel sheets and the second steels sheet had a size of 200 mm×100 mm×3.2 mm. The lifting speed from a plating bath was set to 20 to 200 mm/sec. In the lifting, the plating adhesion amount was adjusted by gas wiping using N₂ gas.

For conditions not shown in the tables, the same conditions as those described above were employed.

In a case where arc welding was employed as a welding method, regarding steel sheet sizes, 150×50 mm was set for the upper sheet side (first steel sheet) and 150×30 mm was set for the lower sheet side (second steel sheet). In addition, the overlapping margin was set to 10 mm, and the sheet gap was set to 0 mm.

In a case where laser welding was employed as a welding method, regarding steel sheet sizes, 150×50 mm was set for the upper sheet side (first steel sheet) and 150 ×30 mm was set for the lower sheet side (second steel sheet). In addition, the overlapping margin was set to 50 mm, and the sheet gap was set to 0 mm.

The obtained welded joints were subjected to measurements of the chemical composition of a plating layer of a non-heat-affected zone, evaluation of the surface unevenness length of the plating layer in a region from a starting point to a position 1,000 μm away from the starting point of back bead surface, and measurement of the area ratio of an Mg—Zn phase in the region in accordance with the above-described methods, and further subjected to determination of the presence or absence of an Mg₂Sn phase. The measurement results of the chemical compositions of the plating layers are shown in Tables 1A and 1B, and other measurement results are shown in Tables 3A and 3B.

Next, the welded joints were subjected to a phosphoric acid chemical conversion treatment P-01 for building materials (Nippon Paint Industrial Coatings Co., Ltd. standard) and baking at a maximum achieving temperature of 210° C. for 40 seconds using a polyester-based coating material NSC300HQ (Nippon Paint Industrial Coatings Co., Ltd. standard), and thus coatings were applied thereon so that their coating thicknesses after drying were 15 μm.

Evaluation of Coating Adhesion

For the welded joints after coating, a combined cycle corrosion test was performed in accordance with JASO (M609-91). After the test, the coating adhesion was evaluated according to the timing of blister occurrence in the region from the starting point to the position 1,000 μm away from the starting point of the back bead surface of the welded joint. The above region was observed with an optical microscope, and in a case where a protrusion of the coating film was confirmed, it was determined that a blister had occurred. The evaluation criteria were as follows. In a case where the evaluation result was at Level A or higher, the coating adhesion of the back bead surface was determined to be excellent and successful. On the other hand, in a case where the evaluation result was at Level B, the coating adhesion of the back bead surface was determined to be inferior and not successful.

• • AAA: No blister occurred at 180 cycles.
  • AA: A blister occurred in 120 or more cycles and less than 180 cycles.
  • A: A blister occurred in 60 or more cycles and less than 120 cycles.
  • B: A blister occurred in less than 60 cycles.

Evaluation of Red Rust Resistance

While evaluating the coating adhesion by the above combined cycle corrosion test, a region from a starting point to a position 1,000 μm away from the starting point of the back bead surface of the welded joint was subjected to red rust resistance evaluation according to the timing of the occurrence of a red rust. The region was observed with an optical microscope. In a case where a red rust was confirmed, it was determined that a red rust had occurred. The evaluation criteria were as follows. In a case where the evaluation result was at Level A or higher, the red rust resistance of the back bead surface was determined to be excellent and successful. On the other hand, in a case where the evaluation result was at Level B, the red rust resistance of the back bead surface was determined to be inferior and not successful.

• • AAA: No red rust was generated in 360 cycles.
  • AA: Red rust was generated in 150 or more cycles and less than 360 cycles.
  • A: Red rust was generated in 90 or more cycles and less than 150 cycles.
  • B: Red rust was generated in less than 90 cycles.

In addition, the red rust resistance in the non-heat-affected zone of the welded joint was evaluated by the same method. As a region for evaluation, a region of 50 mm×50 mm was set at a position 100 mm or more away from a weld bead portion in a surface having a weld bead. The evaluation criteria were as follows, and in a case where the evaluation result was at Level AA or higher, the red rust resistance in the non-heat-affected zone was determined to be excellent.

• • AAA: No red rust was generated in 360 cycles.
  • AA: Red rust was generated in 150 or more cycles and less than 360 cycles.
  • A: Red rust was generated in 90 or more cycles and less than 150 cycles.
  • B: Red rust was generated in less than 90 cycles.

	TABLE 1A
	Chemical Composition of Plating Layer
	of Non-Heat-Affected Zone (mass %)

No	Classification	Zn	Al	Mg	Fe	Si	Ca	Sn	Others

1	Example	91.900	5.0	3.0	0.10	0	0	0	—
2	Example	75.800	7.6	15.0	0.40	0	1.1000	0	Ni	0.1000
3	Example	85.500	10.1	4.0	0.40	0	0	0	—
4	Example	85.160	10.0	4.5	0.10	0.10	0.1000	0	Ti	0.0400
5	Example	82.650	12.0	4.9	0.10	0.20	0.1000	0	Zr	0.0500
6	Example	82.450	12.0	5.2	0.20	0	0.1000	0	Co	0.0500
7	Example	77.256	16.0	6.0	0.20	0.20	0.1000	0.2400	V	0.0040
8	Example	77.650	16.0	5.6	0.20	0.20	0.1000	0.2400	Sb	0.0100
9	Example	73.845	19.0	6.5	0.10	0.20	0.1000	0.2400	Li	0.0150
10	Example	74.200	19.0	6.0	0.20	0.20	0.1000	0.2400	W	0.0600
11	Example	73.120	20.0	6.0	0.40	0.10	0.1000	0.2400	La	0.0400
12	Example	73.010	20.3	6.0	0.20	0.10	0.1000	0.2400	Ce	0.0500
13	Example	75.970	20.3	3.5	0.10	0.10	0	0.0200	Pb	0.0100
14	Example	75.595	20.1	4.0	0.20	0.10	0	0	B	0.0050
15	Example	71.599	20.3	7.3	0.40	0.10	0.1000	0.2000	In	0.0010
16	Example	72.170	20.4	7.0	0.20	0.10	0.1000	0.0200	Nb	0.0100
17	Example	60.560	22.2	15.0	0.30	0.10	1.5000	0.2400	Sr	0.1000
18	Example	68.260	23.4	7.5	0.40	0.10	0.1000	0.2400	—
19	Example	64.330	24.5	10.0	0.50	0.10	0.3000	0.2400	Bi	0.0300
20	Example	58.490	29.4	11.0	0.60	0.10	0.3000	0.1000	Ag	0.0100
21	Example	56.940	30.5	10.9	0.90	0.20	0.3000	0.2400	Cu	0.0200

	TABLE 1B
	Chemical Composition of Plating Layer
	of Non-Heat-Affected Zone (mass %)

No	Classification	Zn	Al	Mg	Fe	Si	Ca	Sn	Others

22	Example	53.140	34.1	11.0	0.90	0.40	0.3000	0.1000	Y	0.0600
23	Example	46.530	38.0	12.0	1.20	0.50	1.5000	0.2400	Mo	0.0300
24	Example	46.080	38.9	11.9	1.50	0.70	0.8000	0.1000	Cr	0.0200
25	Example	43.810	40.0	12.0	2.20	0.80	0.9000	0.2400	Mn	0.0500
26	Comparative	91.300	4.8	3.0	0.90	0	0	0	—
	Example
27	Comparative	54.000	40.5	4.0	1.50	0	0	0	—
	Example
28	Comparative	83.600	11.0	2.7	2.70	0	0	0	—
	Example
29	Comparative	69.900	12.0	15.2	2.40	0	0.5000	0	—
	Example
30	Comparative	81.700	12.0	3.0	3.30	0	0	0	—
	Example
31	Comparative	81.900	12.0	3.0	3.10	0	0	0	—
	Example
32	Comparative	84.800	10.0	3.0	2.20	0	0	0	—
	Example
33	Comparative	82.700	12.0	3.0	2.30	0	0	0	—
	Example
34	Comparative	83.500	10.0	4.0	2.50	0	0	0	—
	Example
35	Comparative	81.600	12.0	4.0	2.40	0	0	0	—
	Example
36	Comparative	82.400	12.0	3.0	2.60	0	0	0	—
	Example
37	Example	75.400	20.4	3.0	0.10	0.10	0	1.0000	—
38	Example	35.300	38.9	10.0	15.00	0	0.7000	0.1000	—
39	Example	41.560	38.0	7.5	1.20	10.00	1.5000	0.2400	—
40	Comparative	83.100	11.0	3.5	2.40	0	0	0	—
	Example
41	Comparative	84.700	10.0	3.0	2.30	0	0	0	—
	Example
42	Comparative	83.500	11.0	3.0	2.50	0	0	0	—
	Example
The underline indicates that the value was outside the range of the present disclosure.

	TABLE 2A
	Cooling

	Bath
	Temperature to

Projection

250° C.

250° C. to 50° C.

		Amount				Average		Average
		of Shot	Annealing	Annealing	Bath	Cooling	Dew	Cooling	Dew
		Blasting	Temperature	Time	Temperature	Rate	Point	Rate	Point
No	Classification	(kg/m2)	(° C.)	(s)	(° C.)	(° C./s)	(° C.)	(° C./s)	(° C.)

1	Example	10	600	100	440	15	−40	5	0
2	Example	50	550	50	460	15	−40	5	10
3	Example	10	550	50	460	15	−40	5	10
4	Example	10	550	10	460	15	−40	5	0
5	Example	10	550	20	430	15	−40	5	0
6	Example	50	550	50	430	15	−40	5	0
7	Example	10	550	20	470	15	−40	5	0
8	Example	10	550	20	470	15	−20	5	0
9	Example	10	550	20	480	15	−40	5	0
10	Example	50	550	20	480	15	−40	5	0
11	Example	50	550	5	480	15	−40	5	0
12	Example	50	550	5	480	15	−40	5	0
13	Example	50	550	5	480	15	−40	5	0
14	Example	200	550	5	480	15	−40	5	0
15	Example	500	550	5	480	15	−40	5	0
16	Example	50	550	5	480	15	−40	5	0
17	Example	50	550	5	600	15	−40	5	0
18	Example	50	550	5	580	15	−40	5	0
19	Example	50	550	5	580	15	−40	5	0
20	Example	50	550	5	600	15	−40	5	0
21	Example	50	550	5	600	15	−40	5	0

	TABLE 2B
	Cooling

	Bath
	Temperature to

Projection

250° C.

250° C. to 50° C.

		Amount				Average		Average
		of Shot	Annealing	Annealing	Bath	Cooling	Dew	Cooling	Dew
		Blasting	Temperature	Time	Temperature	Rate	Point	Rate	Point
No	Classification	(kg/m2)	(° C.)	(s)	(° C.)	(° C./s)	(° C.)	(° C./s)	(° C.)

22	Example	50	550	5	600	15	−40	5	0
23	Example	50	550	5	600	15	−40	5	0
24	Example	50	550	5	600	15	−40	5	0
25	Example	50	550	5	600	15	−40	5	0
26	Comparative	10	600	100	570	15	−40	5	0
	Example
27	Comparative	10	600	100	620	15	−40	5	0
	Example
28	Comparative	10	600	100	570	15	−40	5	0
	Example
29	Comparative	10	600	100	570	15	−40	5	0
	Example
30	Comparative	10	600	100	570	11	−40	5	0
	Example
31	Comparative	10	600	100	570	15	0	5	0
	Example
32	Comparative	10	600	100	570	15	−40	9	0
	Example
33	Comparative	10	600	100	570	15	−40	5	−40
	Example
34	Comparative	6	600	100	570	15	−40	5	0
	Example
35	Comparative	10	650	100	570	15	−40	5	0
	Example
36	Comparative	10	600	110	570	15	−40	5	0
	Example
37	Example	50	550	5	480	15	−40	5	0
38	Example	50	550	5	600	15	−40	5	0
39	Example	50	550	5	600	15	−40	5	0
40	Comparative	0	600	100	570	15	−40	5	0
	Example
41	Comparative	Surface	600	100	570	15	−40	5	0
	Example	Grinding
42	Comparative	9	600	100	570	15	−40	5	0
	Example
The underline indicates that the value was outside the range of the present disclosure, or the production condition was not preferable.

	TABLE 3A
	Cross Section
	Structure of Back			Evaluation

Bead Surface

Plating Layer

Red Rust

		Left Side		of Non-Heat-		Coating	Red Rust	Resistance
		of		Affected Zone		Adhesion	Resistance	of Non-
		Expression	Mg—Zn	Presence or		of Back	of Back	Heat-
		(1)	Phase	Absence of	Welding	Bead	Bead	Affected
No	Classification	(—)	(area %)	Mg2Sn Phase	Method	Surface	Surface	Zone

1	Example	3	5	Absent	Arc	A	A	A
2	Example	6	92	Absent	Arc	AA	AAA	AAA
3	Example	3	18	Absent	Arc	A	A	A
4	Example	9	20	Absent	Arc	AAA	AA	AA
5	Example	5	26	Absent	Arc	AA	AA	AA
6	Example	10	20	Absent	Arc	AAA	AA	AA
7	Example	5	39	Present	Arc	AA	AAA	AAA
8	Example	4	30	Present	Arc	AA	AAA	AAA
9	Example	5	34	Present	Arc	AA	AAA	AAA
10	Example	12	34	Present	Arc	AAA	AAA	AAA
11	Example	11	32	Present	Arc	AAA	AAA	AAA
12	Example	14	34	Present	Arc	AAA	AAA	AAA
13	Example	13	17	Present	Arc	AAA	A	AA
14	Example	15	18	Absent	Arc	AAA	A	A
15	Example	20	39	Present	Laser	AAA	AAA	AAA
16	Example	12	33	Present	Arc	AAA	AAA	AAA
17	Example	11	89	Present	Arc	AAA	AAA	AAA
18	Example	12	44	Present	Arc	AAA	AAA	AAA
19	Example	9	54	Present	Arc	AAA	AAA	AAA
20	Example	9	44	Present	Arc	AAA	AAA	AAA
21	Example	10	43	Present	Laser	AAA	AAA	AAA

	TABLE 3B
	Cross Section
	Structure of Back			Evaluation

Bead Surface

Plating Layer

Red Rust

		Left Side		of Non-Heat-		Coating	Red Rust	Resistance
		of		Affected Zone		Adhesion	Resistance	of Non-
		Expression	Mg—Zn	Presence or		of Back	of Back	Heat-
		(1)	Phase	Absence of	Welding	Bead	Bead	Affected
No	Classification	(—)	(area %)	Mg2Sn Phase	Method	Surface	Surface	Zone

22	Example	10	45	Present	Arc	AAA	AAA	AAA
23	Example	9	33	Present	Arc	AAA	AAA	AAA
24	Example	8	30	Present	Arc	AAA	AAA	AAA
25	Example	8	32	Present	Arc	AAA	AAA	AAA
26	Comparative	0	2	Absent	Arc	B	B	A
	Example
27	Comparative	0	5	Absent	Arc	B	A	A
	Example
28	Comparative	4	4	Absent	Arc	B	B	B
	Example

29

Comparative

Plating was not possible

	Example
30	Comparative	0	0	Absent	Arc	B	B	B
	Example
31	Comparative	0	0	Absent	Arc	B	B	B
	Example
32	Comparative	0	0	Absent	Arc	B	B	B
	Example
33	Comparative	0	0	Absent	Arc	B	B	B
	Example
34	Comparative	0	5	Absent	Arc	B	A	A
	Example
35	Comparative	0	5	Absent	Arc	B	A	A
	Example
36	Comparative	0	6	Absent	Arc	B	A	A
	Example
37	Example	13	16	Present	Arc	AAA	A	AA
38	Example	8	30	Present	Arc	AAA	AAA	AAA
39	Example	8	31	Present	Arc	AAA	AAA	AAA
40	Comparative	0	6	Absent	Arc	B	A	A
	Example
41	Comparative	0	5	Absent	Arc	B	A	A
	Example
42	Comparative	2	5	Absent	Arc	B	A	A
	Example
The underline indicates that the value was outside the range of the present disclosure, or the property was not preferable.

As shown in Tables 3A and 3B, it was found that welded joints having excellent coating adhesion and red rust resistance of the back bead surface were obtained in the examples according to the present disclosure. On the other hand, it was found that one or more properties deteriorated in the comparative examples.

In No. 40 in which strain was applied by performing surface grinding instead of shot blasting, it was not possible to apply a desired amount of strain, and it was not possible to preferably control the length L of the surface unevenness of the plating layer of the back bead surface.

INDUSTRIAL APPLICABILITY

According to the aspect of the present disclosure, it is possible to provide a welded joint having excellent coating adhesion and red rust resistance of the back bead surface.

EXPLANATION OF REFERENCES

• • 1: first steel sheet
  • 2: second steel sheet
  • 3: weld bead portion
  • 4: plating layer
  • 10: welded joint
  • a: heat-affected zone
  • b: non-heat-affected zone
  • A, B, C: back bead surface
  • S: starting point (position where coating with plating layer is started)
  • E: toe of weld bead portion
  • D: direction away from toe of weld bead portion