Welded joint

Abstract

A welded joint 10 has a first steel plate 1 and a second steel plate 2 having a plating layer 4 at least partially welded thereto. When a cross-section orthogonal to the extension direction of the weld bead 3 is observed, it is found that: when, on a rear surface A of the surface having a weld bead 3, in a direction orthogonal to the extension direction of the weld bead 3 and moving away from an end E of the weld bead 3, the covering start position of the plating layer 4 is designated as a start point S, the horizontal length of an observation field of view for a region extending from the start point S to a position of 1000 μm away is designated as L0, and the surface concavo-convex length of the plating layer 4 in the observation field of view is designated as L, formula (1) is satisfied. The area ratio of the Mg-Zn phase in the plating layer 4 in the region extending from the start point S to the position of 1000 μm away is at least 5%.　Formula (1): (L-L0) / L0×100≥3

Description

Welded joints

[0001] The present disclosure relates to weld joints.

[0002] Zn-Al-Mg hot-dip plated steel sheets having a hot-dip Zn coating layer containing Al and Mg have excellent red rust resistance and paint adhesion, and are therefore widely used as materials for structural members that require corrosion resistance, such as building materials.

[0003] In the manufacture of structural components, fusion welding such as arc welding is sometimes performed to join materials together. When Zn-Al-Mg hot-dip galvanized steel sheets are fusion welded, the heat-affected zone on the backside of the bead is divided into areas where the plating layer remains and areas where the plating layer does not remain. In the areas of the heat-affected zone where the plating layer remains, the composition of the plating layer differs from the composition of the plating layer in the non-heat-affected zone, so the expected red rust resistance and paint adhesion may not be achieved.

[0004] For example, Patent Document 1 discloses an automobile chassis member having a joint formed by arc welding two hot-dip Zn-Al-Mg alloy-plated steel sheet members having a thickness of 1.0 to 3.0 mm, in which the steel sheet surfaces that had a plating layer before welding are continuously covered with a Zn-Al-Mg alloy layer up to the toe of the weld bead, an Fe-Al alloy layer exists between the Zn-Al-Mg alloy layer and the steel base, and in a steel sheet surface layer within 2 mm from the toe of the weld bead, the Zn-Al-Mg alloy layer has an average Al concentration of 0.2 to 22.0 mass %, an average Mg concentration of 1.0 to 10.0 mass %, and the Fe-Al alloy layer has an average Fe concentration of 70.0 mass % or less. Patent Document 1 discloses that this configuration prevents a decrease in corrosion resistance in the vicinity of the toe of the bead of the arc weld and enables the construction of an automobile chassis with high strength and excellent corrosion resistance.

[0005] Japanese Patent No. 5700394

[0006] However, Patent Document 1 does not take into consideration paint adhesion and red rust resistance on the back surface of the bead.

[0007] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a welded joint that has excellent paint adhesion and red rust resistance on the back surface of the bead.

[0008] The gist of the present disclosure is as follows. [1] A welded joint formed by welding a first steel plate and a second steel plate, comprising: the first steel plate and the second steel plate; and a weld bead portion formed by the welding; wherein the first steel plate and the second steel plate each have a heat-affected zone located around the weld bead portion and a non-heat-affected zone that is not thermally affected by the welding; and the first steel plate has a plating layer on the surface of the heat-affected zone and the non-heat-affected zone, and the chemical composition of the plating layer in the non-heat-affected zone is, in 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: 0-1.0000%, Ti: 0-1.0000%, V: 0-1.0000%, Cr: 0-1.0000%, Nb: 0-1.0000%, Ni: 0-1.0000%, Mn: 0-1.0000%, Mo: 0-1.0000%, Sn: 0-1.0000%, Zr: 0-1.0000%, Co: 0-1.0000%, W: 0-1.0000%, Ag: 0-1.0000%, Li: 0-1.0000%, La: 0-0.5000%, Ce: 0-0.5000%, Y :0~0.5000%, The steel sheet contains Bi: 0 to 0.5000%, In: 0 to 0.5000%, and B: 0 to 0.5000%, with the balance consisting of 20.000% or more of Zn and impurities, and when a cross section perpendicular to the extension direction of the weld bead portion is observed, on the back side of the surface having the weld bead portion, the horizontal length of an observation field is L in a region perpendicular to the extension direction of the weld bead portion and extending in a direction away from the toe of the weld bead portion, with the coating start position of the plating layer as the starting point, for a region from the starting point to a position 1000 μm away. ０and the length of the surface irregularities of the plating layer within the observation field is L, the following formula (1) is satisfied, and the area ratio of the Mg—Zn phase in the plating layer in the region from the starting point to a position of 1000 μm is 5% or more. (L-L ０ ) / L ０× 100≧3 ... (1) [2] The chemical composition of the plating layer in the non-heat-affected zone is, in mass %, 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: 0.0001-1.0000%, Mo: 0.0001-1.0000%, Sn: 0.0001-1.0000%, Zr: 0.0001-1.0000%, Co: 0.0001-1.0000%, W: 0.0001-1.0000%, 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%, The welded joint according to [1], characterized in that it contains one or more elements selected from the group consisting of In: 0.0001 to 0.5000%, and B: 0.0001 to 0.5000%. [3] The welded joint according to [1] or [2], characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass%, 4.5 to 15.0% Mg, and the area ratio of the Mg—Zn phase in the plating layer in the region is 20% or more. [4] The welded joint according to [1] or [2], characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass%, 5.5 to 15.0% Mg, and the area ratio of the Mg—Zn phase in the plating layer in the region is 30% or more. [5] The welded joint according to any one of [1] to [4], characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass%, Al: 10.0 to 40.0%, and the left side of the formula (1) is 6 or more.[6] The welded joint according to any one of [1] to [4], characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass%, Al: 15.0 to 40.0%, and the left side of the formula (1) is 8 or more. [7] The chemical composition of the plating layer in the non-heat-affected zone contains, in mass%, Sn: 0.0200 to 1.0000%, and Mg is contained in the plating layer in the non-heat-affected zone. ２ The welded joint according to any one of [1] to [6], characterized in that it has a Sn phase.

[0009] According to the above-described aspects of the present disclosure, it is possible to provide a welded joint that has excellent paint adhesion and red rust resistance on the back surface of the bead.

[0010] 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 on the back surface of the bead of a welded joint. Fig. 3 is an enlarged view of a portion of a region from a starting point S to 1000 μm. 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.

[0011] A welded joint according to an embodiment of the present disclosure (hereinafter, sometimes referred to as a 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 are possible within the scope of the present disclosure.

[0012] Each constituent element of the present disclosure will be described in detail below. Below, the numerical ranges indicated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical composition are mass percent unless otherwise specified. Furthermore, welding-related terms conform to JIS Z 3001:2018-1 to 7.

[0013] 1 , the welded joint according to this embodiment is a welded joint 10 formed by welding a first steel plate 1 and a second steel plate 2, and includes the first steel plate 1, the second steel plate 2, and a weld bead 3 formed by the welding, the first steel plate 1 and the second steel plate 2 each having a heat-affected zone a located around the weld bead 3 and a non-heat-affected zone b that is not thermally affected by the welding, and at least one of the first steel plate 1 or the second steel plate 2 has a plating layer 4 (not shown in FIG. 1 ) located on at least a portion of the surface of the heat-affected zone a and the non-heat-affected zone b. Each component will be described in detail below.

[0014] There are no particular limitations on the materials of the first steel plate 1 and the second steel plate 2. For example, various steel plates can be used, such as general steel, Al-killed steel, extra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). There are also no particular limitations on the manufacturing methods (hot rolling, pickling, cold rolling, etc.) for the first steel plate 1 and the second steel plate 2.

[0015] The weld bead portion 3 is a weld bead formed by welding. The shape and composition of the weld bead portion 3 are not particularly limited.

[0016] Around the weld bead 3, there are a heat-affected zone a that has been thermally affected by welding, and a non-heat-affected zone b that has not been thermally affected. Because Zn in the coating layer 4 evaporates due to the heat during welding, the composition of the coating layer 4 in the heat-affected zone a may differ from the composition of the coating layer 4 in the non-heat-affected zone b. Furthermore, in the heat-affected zone a, the coating layer 4 melts and penetrates into the surface regions of the first steel sheet 1 and the second steel sheet 2, or melts off, resulting in areas where the coating layer 4 is not present.

[0017] First, the chemical composition of the plating layer 4 in the non-heat-affected zone b will be described below. In the welded joint 10 according to this embodiment, as long as the chemical composition of the plating layer 4 in the non-heat-affected zone b is within the range described below, the chemical composition of the plating layer 4 remaining in the heat-affected zone a can also be favorably controlled, thereby improving paint adhesion and red rust resistance on the bead back surface A. Note that the chemical composition of the plating layer 4 in the non-heat-affected zone b is the same on both the surface having the weld bead and the bead back surface.

[0018] The chemical composition of the plating layer 4 of the non-heat-affected zone (b) is, in mass %, 5.0 to 40.0% Al, 3.0 to 15.0% Mg, and 0.01 to 15.00% Fe, with the balance being 20.000% or more Zn and impurities. Each element will be described below.

[0019] Al: 5.0 to 40.0% Al contributes to improving paint adhesion and red rust resistance. If the Al content is less than 5.0%, the paint adhesion and red rust resistance on the bead back surface A will 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, if the Al content exceeds 40.0%, the paint adhesion on the bead back surface A will deteriorate. Therefore, the Al content is set to 40.0% or less. The Al content is preferably 35.0% or less, 30.0% or less, and more preferably 25.0% or less.

[0020] Mg: 3.0 to 15.0% Mg is an element necessary for forming the Mg—Zn phase. If the Mg content is less than 3.0%, a sufficient amount of the Mg—Zn phase cannot be formed on the bead back surface A, resulting in deterioration of paint adhesion and red rust resistance on the bead back surface A. 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, if the Mg content exceeds 15.0%, a large amount of dross mainly composed of Mg is generated in the coating bath, which tends to adhere to the base sheet, making it difficult to form the coating layer 4. 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.

[0021] Fe: 0.01 to 15.00% Since Fe may be mixed into the plating layer 4 from the first steel sheet 1 during formation of the plating layer 4, it is difficult to set 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. Furthermore, an Fe content of 15.00% or less does not adversely affect the properties of the plating layer 4, so 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.

[0022] The plating layer 4 in the non-heat-affected zone (b) may have the above chemical composition, with the balance being 20.000% or more of Zn and impurities. If the Zn content is less than 20.000%, the desired red rust resistance and paint adhesion cannot be obtained on the bead back surface (A). 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.

[0023] In this embodiment, impurities refer to substances that are mixed in from the manufacturing environment, etc., and / or substances that are tolerated within a range that does not adversely affect the characteristics of the weld joint 10 according to this embodiment.

[0024] Although not essential for providing the desired properties, the plating layer 4 according to this embodiment may contain the following optional elements. However, since the inclusion of these elements is not essential, the lower limit of the content of these elements is 0%.

[0025] Si: 0.01 to 10.00% Si contributes to improving red rust resistance. To reliably obtain this effect, the Si content is preferably 0.01% or more. The Si content is more preferably 0.05% or more, or 0.10% or more. On the other hand, if the Si content exceeds 10.00%, the red rust resistance will deteriorate. 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.

[0026] Ca: 0.0001 to 1.5000% Ca is an element that can adjust the optimal amount of Mg elution to impart red rust resistance. To reliably obtain this effect, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.1000% or more, or 0.3000% or more. On the other hand, if the Ca content is excessive, 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.

[0027] Sb: 0.0001 to 0.5000% Pb: 0.0001 to 0.5000% Sr: 0.0001 to 0.5000% Sb, Pb, and Sr contribute to improving red rust resistance. To reliably obtain this effect, it is preferable that the content of any one of Sb, Pb, and Sr is 0.0001% or more. The contents of Sb, Pb, and Sr are more preferably 0.0005% or more and 0.0050% or more, respectively. On the other hand, if the content of any one of Sb, Pb, and Sr exceeds 0.5000%, red rust resistance will actually deteriorate. Therefore, the contents of Sb, Pb, and Sr are each 0.5000% or less. The contents of Sb, Pb, and Sr are more preferably 0.3000% or less and 0.2000% or less, respectively.

[0028] 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 improving red rust resistance. To ensure this effect, it is preferable that the content of any one of the above elements be 0.0001% or more. The contents of the above elements are more preferably 0.0005% or more and 0.0050% or more, respectively. On the other hand, if the content of any one of the above elements exceeds 1.0000%, red rust resistance will be deteriorated. Therefore, the content of each of the above elements is set to 1.0000% or less. The content of each of the above elements is more preferably 0.3000% or less and 0.2000% or less, respectively.

[0029] Sn: 0.0001 to 1.0000% Sn is a mixture of Mg and Mg ２ It is an element that forms a Sn phase and improves red rust resistance. To reliably obtain this effect, the Sn content is preferably 0.0001% or more. ２ In order to form a Sn phase and further improve the red rust resistance on the bead back surface, the Sn content is more preferably 0.0200% or more. On the other hand, if the Sn content exceeds 1.0000%, the red rust resistance will deteriorate. 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.

[0030] 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. To reliably obtain this effect, it is preferable that the content of at least one of Zr, Co, W, Ag, and Li is 0.0001% or more. The contents of Zr, Co, W, Ag, and Li are more preferably 0.0005% or more and 0.0020% or more, respectively. On the other hand, excessive contents of Zr, Co, W, Ag, and Li deteriorate red rust resistance. If the content of any one of Zr, Co, W, Ag, and Li exceeds 1.0000%, red rust resistance is significantly deteriorated. Therefore, the contents of Zr, Co, W, Ag, and Li are each set to 1.0000% or less. The contents of Zr, Co, W, Ag, and Li are preferably set to 0.5000% or less and 0.1000% or less, respectively.

[0031] La: 0.0001 to 0.5000% Ce: 0.0001 to 0.5000% Y: 0.0001 to 0.5000% La, Ce, and Y contribute to improving red rust resistance. To reliably obtain this effect, it is preferable that the content of at least one of La, Ce, and Y be 0.0001% or more. The contents of La, Ce, and Y are more preferably 0.0005% or more and 0.0050% or more, respectively. On the other hand, if the content of at least one of La, Ce, and Y exceeds 0.5000%, red rust resistance will actually deteriorate. Therefore, the contents of La, Ce, and Y are each set to 0.5000% or less. The contents of La, Ce, and Y are more preferably 0.2000% or less and 0.1000% or less, respectively.

[0032] Bi: 0.0001 to 0.5000% In: 0.0001 to 0.5000% B: 0.0001 to 0.5000% Bi, In, and B contribute to improving red rust resistance. To reliably obtain this effect, it is preferable that the content of at least one of Bi, In, and B be 0.0001% or more. The contents of Bi, In, and B are more preferably 0.0005% or more and 0.0050% or more, respectively. On the other hand, if the content of at least one of Bi, In, and B exceeds 0.5000%, red rust resistance will actually deteriorate. Therefore, the contents of Bi, In, and B are each set to 0.5000% or less. The contents of Bi, In, and B are more preferably 0.2000% or less and 0.1000% or less, respectively.

[0033] The chemical composition of the plating layer 4 is measured by the following method. A test piece measuring 20 mm × 20 mm × plate thickness is taken from the non-heat-affected zone b of the first steel sheet 1 (at a position at least 100 mm away from the weld bead 3). An acid solution is obtained by peeling and dissolving the plating layer 4 using 10% by volume of HCl containing an inhibitor that suppresses corrosion of the first steel sheet 1. The obtained acid solution is then subjected to ICP analysis. This determines the chemical composition of the plating layer 4. If the weld joint 10 has a painted surface, the paint is removed using Descoat 110B manufactured by Neos Corporation before the above-mentioned measurements are performed.

[0034] Next, the bead back surface (surface A in FIG. 1) of the welded joint 10 according to this embodiment will be described with reference to FIGS. 1 to 3. When observing the welded joint 10 according to this embodiment in a cross section perpendicular to the extension direction of the weld bead 3, the back surface of the surface having the weld bead 3 (bead back surface A) is observed in a direction perpendicular to the extension direction of the weld bead 3 and away from the toe of the weld bead 3, with the coating start position of the plating layer 4 set as a start point S, and the horizontal length of the observation field is L for a region from the start point S to a position 1000 μm away. ０ When the length of the surface irregularities of the plating layer 4 within the observation field is L, the following formula (1) is satisfied, and the area ratio of the Mg—Zn phase in the plating layer 4 in the region from the starting point S to a position of 1000 μm is 5% or more. (L-L ０ ) / L０ × 100 ≧ 3 ... (1)

[0035] Fig. 1 is a diagram showing a cross section of a welded joint 10, perpendicular to the extension direction of a weld bead 3. On the back side of the surface having the weld bead 3 (bead back side A), there is a heat-affected zone a that has been thermally affected by welding, and a non-heat-affected zone b that has not been thermally affected. Fig. 2 is an enlarged view of the heat-affected zone a on the bead back side A in Fig. 1. As shown in Fig. 2, in the heat-affected zone a on the bead back side A, there is a portion where the plating layer 4 is absent and the first steel sheet 1 is exposed, and a portion where it is covered with the plating layer 4.

[0036] In this embodiment, the horizontal length of the observation field is set to L, which is a region from the coating start position of the plating layer 4 to a position 1000 μm from the starting point perpendicular to the extension direction of the weld bead 3 and in a direction away from the toe of the weld bead 3. ０ and the length of the surface irregularities of the plating layer 4 within the observation field is L, the above formula (1) is satisfied.

[0037] The toe of the weld bead 3 refers to the boundary between the weld bead 3 and the first steel sheet 1, and is point E shown in Figure 1. The direction away from the toe of the weld bead 3 (point E in Figure 1) refers to the direction opposite to the weld bead 3 when viewed from the toe E, and is direction D shown in Figure 2. The coverage start position (start point) of the coating of the coating layer 4 refers to the boundary between the portion of the first steel sheet 1 exposed on the surface of the first steel sheet 1 at the bead back surface A and the portion covered by the coating layer 4, and is point S shown in Figures 1 and 2.

[0038] In the region of the bead back surface A from the start point S to a position 1000 μm away, there is a heat-affected zone a that has been thermally affected by welding. The size of the heat-affected zone a varies depending on the amount of heat input during welding and the plate thicknesses of the first steel sheet 1 and the second steel sheet 2, but as long as it is in the region from the start point S to a position 1000 μm away, the heat-affected zone a is present in at least a portion (particularly on the side of the start point S). In this embodiment, by preferably controlling the surface unevenness of the plating layer 4 in the heat-affected zone a on the bead back surface A, the paint adhesion on the bead back surface A is improved. Note that the chemical composition of the plating layer 4 in the heat-affected zone a may be, for example, in mass %, Zn + Mg: 50% or more, Fe: 5% or less, with the balance being Al and impurities.

[0039] Figure 3 is an enlarged view of a portion of the region extending from the starting point S to 1000 μm in Figure 2. Note that Figure 3 is upside down compared to Figure 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 undergoes the manufacturing conditions described below, Zn bonds with Mg. This forms an Mg-Zn phase with a higher melting point. Zn evaporates due to the heat during welding, but the Mg-Zn phase remains in the plating layer 4 without evaporating, resulting in unevenness on the surface of the plating layer 4. The unevenness on the surface of the plating layer 4 can improve paint adhesion through an anchor effect.

[0040] If the value of the left side of the above formula (1) is less than 3, the surface unevenness of the plating layer 4 cannot be formed favorably, resulting in poor paint adhesion. Therefore, the value of the left side of the above formula (1) is set to 3 or more. In order to increase the surface unevenness of the plating layer 4 and thereby improve paint adhesion, it is preferable to increase the Al content in the plating layer 4 and set the value of the left side of the above formula (1) to 6 or more, and more preferably 8 or more. The value of the left side of the above formula (1) may be, for example, 30 or less, or 25 or less.

[0041] The length L of the surface irregularities of the plating layer 4 is measured by the following method. Using a scanning electron microscope, a cross section of the first steel sheet along the sheet thickness direction is observed as shown in Figure 3, and the length of the surface irregularities of the plating layer 4 is measured in μm. In the region from the starting point S to a position 1000 μm, the length of the surface irregularities of the plating layer 4 is measured in μm for an arbitrarily selected observation field of 200 μm in the horizontal direction × 120 μm in the sheet thickness direction. Note that L ０ is 200 μm. The above operation is performed for five fields arbitrarily selected from the region where the plating layer remains in the region from the start point S to the position 1000 μm, and the average value is calculated to obtain L. The obtained L is used to calculate the left side of equation (1). Note that if there are multiple start points S on the back surface A of the bead, the above measurement is performed for the region from the start point S closest to the center of the weld bead portion 3 to the position 1000 μm when viewing the plate thickness cross section of the first steel plate 1 as shown in Figure 1. In the case of the T-joint of Figure 5, the above measurement is performed for the region from the start point S closest to the center of any of the weld bead portions 3 to the position 1000 μm.

[0042] In addition, a sample measuring 20 mm × 15 mm × thickness is taken from the welded joint 10 so that the cross section can be observed. After embedding in resin, the cross section is mirror-polished to finish it before measurement. A backscattered electron image of the cross section is also taken, and based on differences in brightness, the region located closest to the center of the plate thickness is identified as the first steel sheet 1, and the remaining layers are identified as the plating layer 4. If the welded joint 10 has a painted surface, the SEM-EPMA analysis described below is performed on the intermediate layer in the backscattered electron image. If the resulting chemical composition satisfies the chemical composition of the plating layer 4 in the heat-affected zone a described above (in mass %, Zn + Mg: 50% or more, Fe: 5% or less, the remainder being Al and impurities), that layer is identified as the plating layer 4 in the heat-affected zone a.

[0043] Area ratio of Mg—Zn phase: 5% or more The Mg—Zn phase has the effect of enhancing red rust resistance. If the area ratio of the Mg—Zn phase in the coating layer 4 in the region from the starting point S to 1000 μm is less than 5%, the red rust resistance at the bead back surface A will deteriorate. Therefore, the area ratio of the Mg—Zn phase is set to 5% or more. To further improve the red rust resistance at the bead back surface A, the Mg content in the coating layer 4 is increased, and the area ratio of the Mg—Zn phase is preferably set to 20% or more, and more preferably 30% or more. The area ratio of the Mg—Zn phase may be set to 95% or less.

[0044] The area ratio of the Mg—Zn phase in the plating layer 4 is measured using the following method. A sample is collected using the same method as used to measure the surface irregularity length L of the plating layer 4, and the cross section is mirror-polished to a finished surface. Next, quantitative analysis of elements (Mg, Zn, Fe) is performed on the cross section by point analysis using a SEM-EPMA (JEOL, JXA-8500F). A phase with a Mg content of 20 to 60%, a Zn content of 40 to 80%, and the remainder of 5% or less is identified as the Mg—Zn phase. The above analysis is performed on the plating layer 4 in the region extending from the starting point S to a position 1000 μm away, thereby obtaining the area ratio of the Mg—Zn phase. The distribution image is measured over a range of the plating layer thickness × 1000 μm using an acceleration voltage of 15 kV, a magnification of 5000x, and a measurement interval of 1.0 μm. The area ratio is calculated using the "Analyze" function of the image analysis software "ImageJ."

[0045] Mg ２ In the welded joint 10 according to this embodiment, Mg is present in the plating layer 4 of the non-heat-affected zone b. ２ It is preferable that the plating layer 4 of the non-heat-affected zone b contains an Sn phase. ２ The presence of the Sn phase can improve the red rust resistance in the non-heat-affected zone.

[0046] The plating layer 4 of the non-heat-affected zone b is Mg ２ Whether or not a material has an Sn phase is determined by the following method. ２ Since the Sn phase is present in small amounts, its presence can be detected and confirmed by X-ray diffraction measurement using the θ-2θ method. ２X-ray diffraction measurement for detecting the Sn phase is performed by the θ-2θ measurement method. In addition, the X-ray diffraction measurement is performed on the plating layer 4 of a 20 mm square sample taken from the non-heat-affected zone b (at a position 100 mm or more away from the weld bead portion 3) using Kα rays from a Cu tube. If a peak is detected at 23.4±0.3°, it is determined that the Sn phase is Mg. ２ It is determined that a Sn phase is present.

[0047] The coating weight of the plating layer 4 per side is, for example, 20 to 250 g / m ２ The amount of adhesion per side should be within the range of 20 g / m ２ On the other hand, if the coating weight per side is 250 g / m or more, the red rust resistance can be further improved. ２ By setting the following, the processability can be further improved.

[0048] Furthermore, although the welded joint 10 described above is a lap joint, the welded joint 10 according to this embodiment is not limited to this. The welded joint 10 according to this 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 some other joint. When the welded joint 10 is a butt joint, surface B shown in Fig. 4 is considered to be the bead back surface. When the welded joint 10 is a T-joint, surface C shown in Fig. 5 (the surface of the first steel plate 1 opposite to the surface welded to the second steel plate 2) is considered to be the bead back surface.

[0049] In the butt joint shown in Figure 4, the weld bead 3 does not reach the bead back surface B, but the weld bead 3 may reach the bead back surface B. Also, the weld bead 3 may be formed on both sides. When the weld bead 3 reaches the bead back surface B, the surface on which the weld bead 3 is smaller is considered to be the bead back surface B. When the weld bead 3 is formed on both sides, the surface on which the weld bead 3 is smaller is considered to be the bead back surface B, and when the weld bead 3 on both surfaces is the same size, either one of the surfaces is considered to be the bead back surface B. Note that for convenience of explanation, the first steel plate has been described, but the same applies if the first steel plate and the second steel plate are interchanged.

[0050] Next, a preferred method for manufacturing the welded joint 10 according to this embodiment will be described. The preferred method for manufacturing the welded joint 10 according to this embodiment comprises the steps of shot blasting the first steel plate 1, annealing, plating, cooling, and welding the first steel plate 1 and the second steel plate 2. There are no particular restrictions on the manufacturing method for the second steel plate, but it may be manufactured by the same method as for the first steel plate. Each step will be described below.

[0051] Shot blasting The first steel sheet 1 is subjected to shot blasting to impart strain before plating. By performing shot blasting under preferred conditions, preferred strain is imparted to the first steel sheet 1, which, combined with cooling after plating (described later), can promote alloying in the plating layer 4. As a result, the morphology of the plating layer 4 can be controlled in a preferred manner.

[0052] For shot blasting, steel balls with a median particle size of 40 to 450 μm can be used. An example of the equipment used is the TSH30 manufactured by IKK Shot Co., Ltd. The shot blasting rate is 10 to 500 kg / m ２ The shot blasting amount is preferably 10 kg / m ２ If the thickness is less than 1 / 2 mm, it may be impossible to impart strain to the first steel sheet 1 in a desirable manner, and as a result, it may be impossible to suitably control the surface unevenness of the plating layer 4. Although strain can also be imparted by surface grinding, the amount of strain imparted by surface grinding is smaller than that imparted by shot blasting, and therefore the desired amount of strain cannot be imparted.

[0053] Annealing After shot blasting, the first steel sheet 1 is annealed. The annealing temperature during annealing is in the range of 400 to 600°C, and the holding time in this temperature range (annealing time) is more than 0 seconds and not more than 100 seconds. If the annealing temperature exceeds 600°C or the annealing time exceeds 100 seconds, strain in the first steel sheet 1 and the second steel sheet 2 is released, and as a result, the surface unevenness in the coating layer 4 may not be controlled favorably. If the annealing temperature is less than 400°C, or if annealing is not performed, the coating may not be applied favorably. Note that the temperature referred to here refers to the surface temperature at the center of the sheet surface of the first steel sheet 1, and can be measured using a thermocouple joined by spot welding, for example.

[0054] Plating After 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 600°C or lower. If the bath temperature of the plating bath exceeds 600°C, strain in the first steel sheet 1 is released, and as a result, the surface unevenness in the plating layer 4 may not be controlled in a desirable manner. After the first steel sheet 1 is pulled out of the plating bath, the coating weight may be adjusted by gas wiping or the like.

[0055] Cooling After plating, it is preferable to cool the plating using a cooling gas with a dew point of -20°C or lower, so that the average cooling rate in the temperature range from the bath temperature to 250°C is 15°C / s or higher. Next, it is preferable to cool the plating using a cooling gas with a dew point of 0°C or higher, so that the average cooling rate in the temperature range from 250°C to 50°C is 5°C / s or lower. When cooling in the temperature range from the bath temperature to 250°C, if the dew point of the cooling gas is higher than -20°C, coarse oxides may form on the plating layer surface, resulting in poor control of the morphology of the plating layer 4. When cooling in the temperature range from the bath temperature to 250°C, if the average cooling rate is less than 15°C / s, coarse surface oxides may form during cooling, resulting in poor control of the surface irregularities of the plating layer 4. When cooling in the temperature range from 250°C to 50°C, if the dew point of the cooling gas is lower than 0°C, it is difficult to form sufficient oxides of Al and Mg in the plating layer 4, resulting in excessive evaporation of Zn in the plating layer 4, resulting in poor control of the morphology of the plating layer 4. Furthermore, if the average cooling rate in the temperature range of 250 to 50°C exceeds 5°C / s, the surface irregularities of the plating layer 4 may not be controlled in a favorable manner.

[0056] After cooling, the first steel plate 1 and the second steel plate 2 are welded together to obtain a welded joint 10. The welding method is not particularly limited as long as it can form a weld bead 3. For example, when arc welding or laser welding is performed, the following conditions can be used, respectively.

[0057] Arc welding Welding current: 250 A Welding voltage: 26.4 V Welding speed: 100 cm / min Welding gas: 20% CO ２ +Ar Gas flow rate: 20 L / min Welding wire: YGW16 manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 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%) Welding torch tilt angle: 45°

[0058] Laser welding Output: 7 kW Welding speed: 400 cm / min Forward / reverse angle: 0°

[0059] The method described above allows for stable production of the welded joint 10 according to this embodiment. Because the welded joint 10 according to this embodiment has excellent paint adhesion, the surface of the welded joint 10 may be painted in order to improve the corrosion resistance of the welded joint 10, for example.

[0060] First and second steel plates having mechanical properties of SS400 according to JIS G 3101:2020 were shot blasted, annealed, plated, and cooled under the conditions shown in Tables 2A and 2B, and then welded using the welding method shown in Tables 3A and 3B to obtain lap joints (welded joints). The first and second steel plates were 200 mm x 100 mm x 3.2 mm steel plates. The pulling speed from the plating bath was 20 to 200 mm / sec. During pulling, N ２ The plating weight was adjusted by gas wiping using a gas. Conditions not listed in the table were the same as those described above.

[0061] When the welding method was arc welding, the steel plate sizes were 150 x 50 mm on the upper plate side (first steel plate) and 150 x 30 mm on the lower plate side (second steel plate), with an overlap of 10 mm and a gap of 0 mm.When the welding method was laser welding, the steel plate sizes were 150 x 50 mm on the upper plate side (first steel plate) and 150 x 30 mm on the lower plate side (second steel plate), with an overlap of 50 mm and a gap of 0 mm.

[0062] For the obtained welded joint, the chemical composition of the plating layer in the non-heat-affected zone, the length of the surface irregularities of the plating layer in the region from the starting point to the position 1000 μm on the back surface of the bead, and the area ratio of the Mg—Zn phase in the region were measured by the above-mentioned method. ２ The presence or absence of an Sn phase was determined. The measurement results of the chemical composition of the plating layer are shown in Tables 1A and 1B, and other measurement results are shown in Tables 3A and 3B.

[0063] Next, the welded joint was coated with P-01 phosphate conversion coating for building materials (Nippon Paint Industrial Coatings Co., Ltd. standard) and NSC300HQ polyester paint (Nippon Paint Industrial Coatings Co., Ltd. standard) at a maximum temperature of 210°C and baked for 40 seconds to give a dry coating thickness of 15 μm.

[0064] Paint Adhesion Evaluation The painted welded joints were subjected to a combined cyclic corrosion test in accordance with JASO (M609-91). After the test, paint adhesion was evaluated according to the timing of blister occurrence in a region from the starting point to a position 1000 μm from the bead backside of the welded joint. The region was observed with an optical microscope, and if a bulge in the paint film was confirmed, it was determined that a blister had occurred. The evaluation criteria were as follows. A rating of A or higher was determined to have excellent paint adhesion on the bead backside and was judged to have passed. On the other hand, a rating of B was determined to have poor paint adhesion on the bead backside and was judged to have failed. AAA: No blister occurrence at 180 cycles AA: Blisters occurred at 120 cycles or more but less than 180 cycles A: Blisters occurred at 60 cycles or more but less than 120 cycles B: Blisters occurred at less than 60 cycles

[0065] Red Rust Resistance Evaluation While evaluating the paint film adhesion in the above-mentioned combined cyclic corrosion test, red rust resistance was evaluated in a region from the starting point to a position of 1000 μm on the back surface of the bead of the welded joint according to the timing of red rust occurrence. The region was observed with an optical microscope, and if red rust was confirmed, it was determined that red rust had occurred. The evaluation criteria were as follows. A rating of A or higher was determined to be excellent in red rust resistance on the back surface of the bead, and the test was judged to have passed. On the other hand, a rating of B was determined to be poor in red rust resistance on the back surface of the bead, and the test was judged to have failed. AAA: No red rust occurred at 360 cycles AA: Red rust occurred at 150 cycles or more but less than 360 cycles A: Red rust occurred at 90 cycles or more but less than 150 cycles B: Red rust occurred at less than 90 cycles

[0066] The red rust resistance in the non-heat-affected zone of the welded joint was also evaluated using the same method. The evaluation area was a 50 mm x 50 mm area on the surface having the weld bead, at a position 100 mm or more away from the weld bead. The evaluation criteria were as follows, and a rating of AA or higher was determined to be excellent in red rust resistance in the non-heat-affected zone. AAA: No red rust occurred at 360 cycles AA: Red rust occurred at 150 cycles or more but less than 360 cycles A: Red rust occurred at 90 cycles or more but less than 150 cycles B: Red rust occurred at less than 90 cycles

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073] As shown in Tables 3A and 3B, the examples according to the present disclosure produced welded joints with excellent paint adhesion and red rust resistance on the backside of the bead. On the other hand, the comparative examples showed deterioration in one or more properties. Note that in No. 41, in which strain was imparted by surface grinding instead of shot blasting, the desired amount of strain could not be imparted, and the length L of the surface irregularities of the plating layer on the backside of the bead could not be controlled appropriately.

[0074] According to the above-described aspects of the present disclosure, it is possible to provide a welded joint that has excellent paint adhesion and red rust resistance on the back surface of the bead.

[0075] REFERENCE SIGNS LIST 1 ... First steel plate 2 ... Second steel plate 3 ... Weld bead portion 4 ... Plating layer 10 ... Weld joint a ... Heat-affected zone b ... Non-heat-affected zone A, B, C ... Back surface of bead S ... Start point (position where coating of plating layer starts) E ... Toe of weld bead portion D ... Direction away from the toe of weld bead portion

Claims

1. A welded joint formed by welding a first steel plate and a second steel plate, comprising: the first steel plate, the second steel plate, and a weld bead portion formed by the welding; the first steel plate and the second steel plate each have a heat-affected zone located around the weld bead portion and a non-heat-affected zone that is not affected by the heat of the welding; the first steel plate has a plating layer on the surface of the heat-affected zone and the non-heat-affected zone; and the chemical composition of the plating layer in the non-heat-affected zone is, in 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: 0-1.0000%, Ti: 0-1.0000%, V: 0-1.0000%, Cr: 0-1.0000%, Nb: 0-1.0000%, Ni: 0-1.0000%, Mn: 0-1.0000%, Mo: 0-1.0000%, Sn: 0-1.0000%, Zr: 0-1.0000%, Co: 0-1.0000%, W: 0-1.0000%, Ag: 0-1.0000%, Li: 0-1.0000%, La: 0-0.5000%, Ce: 0-0.5000%, Y :0~0.5000%, The steel sheet contains Bi: 0 to 0.5000%, In: 0 to 0.5000%, and B: 0 to 0.5000%, with the balance consisting of 20.000% or more of Zn and impurities, and when a cross section perpendicular to the extension direction of the weld bead portion is observed, on the back side of the surface having the weld bead portion, the horizontal length of an observation field is L in a region perpendicular to the extension direction of the weld bead portion and extending in a direction away from the toe of the weld bead portion, with the coating start position of the plating layer as the starting point, for a region from the starting point to a position 1000 μm away. ０ and the length of the surface irregularities of the plating layer within the observation field is L, the following formula (1) is satisfied, and the area ratio of the Mg—Zn phase in the plating layer in the region from the starting point to a position of 1000 μm is 5% or more. (L-L ０ ) / L ０ × 100 ≧ 3 ... (1) 2. The chemical composition of the plating layer in the non-heat-affected zone is, in mass %, 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: 0.0001-1.0000%, La: 0.0001-0.5000%, Ce: 0.0001-0.5000%, Y: 0.0001-0.5000%, Bi: 0.0001-0.5000%, In: 0.0001-0.5000%, and B 2. The welded joint according to claim 1, characterized in that it contains one or more selected from the group consisting of: 0.0001 to 0.5000% of Si.

3. A welded joint as set forth in claim 1 or 2, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Mg: 4.5 to 15.0%, and the area ratio of the Mg-Zn phase in the plating layer in said region is 20% or more.

4. A welded joint as set forth in claim 1 or 2, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Mg: 5.5 to 15.0%, and the area ratio of the Mg-Zn phase in the plating layer in said region is 30% or more.

5. A welded joint according to claim 1 or 2, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 10.0 to 40.0%, and the left side of formula (1) is 6 or more.

6. The welded joint according to claim 3, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 10.0 to 40.0%, and the left side of formula (1) is 6 or more.

7. The welded joint according to claim 4, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 10.0 to 40.0%, and the left side of formula (1) is 6 or more.

8. A welded joint according to claim 1 or 2, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 15.0 to 40.0%, and the left side of formula (1) is 8 or more.

9. The welded joint according to claim 3, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 15.0 to 40.0%, and the left side of formula (1) is 8 or more.

10. The welded joint according to claim 4, characterized in that the chemical composition of the plating layer in the non-heat-affected zone contains, in mass %, Al: 15.0 to 40.0%, and the left side of formula (1) is 8 or more.

11. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ 3. The welded joint according to claim 1, further comprising an Sn phase.

12. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 3, characterized in that it contains an Sn phase.

13. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 4, characterized in that it contains an Sn phase.

14. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ 6. The welded joint according to claim 5, characterized in that it contains an Sn phase.

15. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 6, characterized in that it has an Sn phase.

16. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 7, characterized in that it has a Sn phase.

17. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 8, characterized in that it has a Sn phase.

18. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 9, characterized in that it has a Sn phase.

19. The chemical composition of the plating layer in the non-heat-affected portion contains, in mass%, Sn: 0.0200 to 1.0000%, and the plating layer in the non-heat-affected portion contains Mg ２ The welded joint according to claim 10, characterized in that it has a Sn phase.

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