Connection point

Abstract

Invention details;

Description

Welded joints

[0001] The present invention relates to a welded joint.

[0002] Automotive components, such as chassis components, and various building materials are often manufactured using welded joints formed by welding multiple steel materials. Because these automotive and building materials are used while exposed to a variety of environments, the manufactured welded joints are desired to have excellent corrosion resistance. Therefore, various zinc-based plated steel sheets, including galvannealed steel sheets, are used as materials for such welded joints.

[0003] Here, a problem specific to manufacturing a welded joint by welding galvanized steel sheets is a decrease in corrosion resistance in the vicinity of the "toe" defined in JIS Z3001 (2018) due to evaporation of Zn from the plating during welding.

[0004] In order to solve the problem of the formation of blowholes as described above, various proposals have been made up to now. For example, Patent Document 1 below discloses a steel sheet having a plating layer including a Zn—Al—Mg alloy layer disposed on the surface of the steel sheet, and wherein the cross section of the Zn—Al—Mg alloy layer contains MnZn. ２ Phase area fraction is 45-75%, MgZn ２ The total area fraction of the Zn-Al-MgZn phase and the Al phase is 70% or more, and ２ Plated steel materials have been proposed in which the area fraction of the ternary eutectic structure is 0 to 5% and the plating layer has a predetermined chemical composition.

[0005] WO 2018 / 139620

[0006] Here, it is possible to solve the problem of blowhole formation by using the plated steel material proposed in Patent Document 1. However, as a result of intensive research, the present inventors have found that there is still room for improvement in the technology proposed in Patent Document 1, and that further improvement can be expected with respect to the electrodeposition paintability of weld beads in welded joints made of zinc-plated steel sheet.

[0007] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a welded joint that can further improve the electrodeposition paintability at the weld bead portion.

[0008]

[0005] As a result of extensive research conducted by the present inventors to solve the above problems, they have concluded that the deterioration of electrodeposition paintability at the weld bead is caused by the formation of non-conductive Si-based slag on the surface of the weld metal at the weld bead, which inhibits current flow from the surface of the weld metal to the base steel in the area where the Si-based slag is formed. Therefore, they have discovered that if the formation of Si-based slag on the surface of the weld metal can be suppressed, a current path can be formed from the surface of the weld metal to the base steel, and electrodeposition paintability can be further improved.

[0009] Based on this finding, the inventors of the present invention conducted further studies and, by improving the plated steel sheet used as the raw material and by examining appropriate welding conditions, have discovered a technology that can suppress the formation of Si-based slag, which inhibits the electrodeposition of paint, on the surface of the weld metal in the weld bead. The gist of the present invention, which was completed based on this finding, is as follows.

[0010] (1) A weld joint in which a first steel plate and a second steel plate are welded by arc welding or laser welding, the weld joint comprising the first steel plate and the second steel plate, and a weld bead portion formed by the arc welding or laser welding, and when a portion of the first steel plate and the second steel plate that is not affected by heat due to the welding is defined as a non-heat-affected zone, at least one of the first steel plate and the second steel plate has a surface of a base steel in the non-heat-affected zone. and an oxide layer located on the plating layer, wherein the plating layer contains, in mass %, Al: 1.00 to 80.00%, Mg: 1.00 to 20.00%, Fe: 0.01 to 15.00%, Si: 0 to 10.00%, Ca: 0 to 4.00%, and further, selectively, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, In: : 0 to 1.000%, Bi: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Ni: 0 to 1.000%, Mn: 0 to 1.000%, V: 0-1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, S (2) A welded joint according to claim 1, wherein the slag layer contains, in mass %, at least 1.0 to 45.0% Al and 1.0 to 30.0% Mg, with a total of 0 to 5.000% of Al and 0 to 0.500% Mg, with the balance being 5.00 mass % or more of Zn and impurities, and the weld bead portion has a weld metal and a slag layer formed on a part of the surface of the weld metal, the slag layer containing, in mass % excluding oxygen, 1.0 to 45.0% Al and 1.0 to 30.0% Mg, with the balance being Fe, easily oxidized metal elements, and impurities. (3) The welded joint according to (1) or (2), wherein, when a position at a depth of 5 nm from the outermost surface of the oxide layer is observed by X-ray photoelectron spectroscopy (XPS), the intensity ratio ([Al—O]+[Mg—O]) / [Zn—O] calculated from the intensities of peaks respectively assigned to Al—O bonds, Mg—O bonds, and Zn—O bonds is 5.0 or more.(4) The welded joint according to (3), wherein the strength ratio ([Al—O] + [Mg—O]) / [Zn—O] is 10.0 or more. (5) The welded joint according to any one of (1) to (4), wherein the plating layer in the non-heat-affected zone contains at least 18.00 to 60.00 mass% of Al and 5.00 to 15.00 mass% of Mg. (6) The plating layer in the non-heat-affected zone contains at least 35.00 to 60.00 mass% of Al and 7.00 to 15.00 mass% of Mg, and the plating layer does not contain Mg. ３２ (Al, Zn) ４９ phase is present, and the Mg ３２ (Al, Zn) ４９ The welded joint according to any one of (1) to (5), wherein the Mg content [Mg], the Zn content [Zn], and the Al content [Al] (each unit: atomic %) in the Al phase satisfy the relationship 0.50≦[Mg] / ([Zn]+[Al])≦0.83.

[0011] As described above, according to the present invention, it is possible to further improve the electrodeposition paintability of the weld bead portion in a welded joint made of plated steel sheet.

[0012] FIG. 1B is an explanatory diagram schematically showing an example of the structure of a welded joint according to an embodiment of the present invention. FIG. 1C is an explanatory diagram schematically showing an example of the structure of a welded joint according to another embodiment. FIG. 1D is an explanatory diagram schematically showing an example of the structure of a welded joint according to another embodiment. FIG. 1F is an explanatory diagram for explaining the welded joint according to the embodiment shown in FIG. 1A. FIG. 1G is an explanatory diagram for explaining the welded joint according to the same embodiment. FIG. 1H is an explanatory diagram for explaining the peak intensity in the XPS measurement results. FIG. 1I is an explanatory diagram for explaining a method for evaluating electrodeposition coatability.

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0014] (Regarding the welded joint) First, the overall configuration of a welded joint according to an embodiment of the present invention will be described with reference to Fig. 1A. Fig. 1A is an explanatory diagram that schematically shows an example of the structure of a welded joint according to this embodiment.

[0015] For convenience, the following description will be made using the coordinate system shown in Fig. 1A where appropriate. Fig. 1A illustrates a welded joint formed by arc welding two steel plates. A welded joint formed by laser welding two steel plates also has a similar configuration to that shown in Fig. 1A, although the detailed shape of the weld bead differs.

[0016] 1A is a schematic diagram showing the overall configuration of a welded joint obtained by lap fillet welding a first steel plate and a second steel plate by arc welding, and shows a cross section of the welded joint perpendicular to the extension direction of the weld bead. As shown in FIG. 1A, the welded joint 1 according to this embodiment has a first steel plate 10, a second steel plate 20, and a weld bead 30.

[0017] Here, it is preferable to use various types of plated steel sheets as the material for at least one of the first steel sheet 10 and the second steel sheet 20 that constitute the welded joint 1, and it is more preferable to use plated steel sheets having a plating layer as described in detail below as the material for both the first steel sheet 10 and the second steel sheet 20 that constitute the welded joint 1.

[0018] The weld bead 30 is a portion formed by arc welding and includes a weld metal 31 and a slag layer 32 formed on a portion of the surface of the weld metal 31. During welding, interdiffusion of constituent elements occurs between the weld metal 31 and the first and second steel plates 10 and 20. Therefore, in FIG. 1A , the joint interface between the weld metal 31 of the weld bead 30 and the first or second steel plate 10 or 20 is shown as a flat surface (using straight lines) for convenience of illustration, but the actual joint interface is a complex curved surface. Also, in FIG. 1A , the slag layer 32 is shown as having a uniform thickness for convenience of illustration, but the actual thickness of the slag layer 32 may not be uniform. Also, in FIG. 1A , the slag layer 32 is shown to cover the entire surface of the weld metal 31, but as described below, the slag layer 32 is formed to cover only a portion of the surface of the weld metal 31. The weld bead portion 30 extends along the Y-axis direction in the figure, and the first steel plate 10 and the second steel plate 20 are joined by the weld bead portion 30 .

[0019] The components of the weld metal 31 vary depending on the type of welding wire used and the chemical compositions of the first steel sheet 10 and the second steel sheet 20 used as raw materials, making it difficult to uniquely define a composition that covers all possibilities. However, the weld metal 31 generally contains, as its main component, an oxide of an element that is easily oxidized among the various elements that make up the plated steel sheet used as raw material. Examples of such elements that are easily oxidized include Al and Mg.

[0020] Furthermore, when identifying a portion of the weld joint 1 of interest corresponding to the weld metal 31, measurement may be performed, for example, as follows. That is, a sample having the weld metal 31 is prepared, the sample is cut along a plane (the X-Z plane in FIG. 1A ) perpendicular to the welding direction (the Y-axis direction in FIG. 1A ), and the sample is embedded in resin and polished so that a cross section of the weld bead portion 30 (the X-Z cross section in FIG. 1A ) can be observed. After polishing, the cross section of the weld metal 31 is observed using a scanning electron microscope (SEM), and element distribution images of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.) are obtained using SEM-EDS (Energy Dispersive X-ray Spectroscopy), which makes it possible to identify the slag layer 32 on the weld metal 31. The portion located closer to the steel plate than the portion of the slag layer 32 thus identified becomes the weld metal 31 .

[0021] Oxides produced during welding are broadly divided into two types: scale and slag. Scale contains, by mass % excluding oxygen, 50% or more of Fe, with the remainder consisting of easily oxidized elements and impurities. Slag contains, by mass % excluding oxygen, 50% or more of easily oxidized elements, with the remainder consisting of less than 50% Fe and impurities. Here, "easily oxidized elements" are metal elements that are more easily oxidized than Fe in the Ellingham diagram and that can be added to a plating layer. Specific examples of such easily oxidized metal elements include Ca, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, and Ti.

[0022] In JIS Z3001 (2018), the point where the surface of the base metal and the surface of the weld bead intersect is defined as the "toe." In the weld joint 1 shown in FIG. 1A, the point where the surface of the weld bead 30 intersects with the surface of the first steel plate 10 or the second steel plate 20 corresponds to this "toe." The weld joint 1 according to this embodiment focuses on the components of the slag layer 32 on the surface of the weld metal 31 in the vicinity of this toe T.

[0023] Furthermore, this "toe T" is defined not only in a lap fillet weld joint as shown in FIG. 1A, but also in a butt weld joint as shown in FIG. 1B, a T-shaped weld joint as shown in FIGS. 1C and 1D, and the like.

[0024] <Regarding Non-Heat Affected Zone> Next, the configuration of a portion of the welded joint 1 according to this embodiment that is not affected by heat due to welding will be described in detail with reference to Fig. 2. Fig. 2 is a diagram schematically showing a cross section of the welded joint 1 perpendicular to the extension direction of the weld bead portion 30.

[0025] In the following description, a portion of the welded joint 1 that is not affected by heat due to welding will be referred to as the "non-heat-affected zone." In the welded joint 1 shown in Figure 2, for example, the region R1 surrounded by the dashed line in the figure, which is sufficiently separated from the vicinity of the toe T, corresponds to this non-heat-affected zone. The position of this non-heat-affected zone can be considered to be, for example, a region separated from the toe T shown in Figure 2 by 3 mm or more in a direction perpendicular to the extension direction of the weld bead 30 (the Y-axis direction in Figure 2) and away from the toe T (the X-axis direction in Figure 2).

[0026] Fig. 3 is a diagram schematically showing a portion of a cross section parallel to the sheet thickness direction in the non-heat-affected zone R1. As schematically shown in Fig. 3, the non-heat-affected zone R1 in at least either the first steel sheet 10 or the second steel sheet 20 has a base steel 101, a plating layer 103 located on at least a portion of the surface of the base steel 101, and an oxide layer 105 located on the plating layer 103. In the welded joint 1 according to this embodiment, the plating layer 103 and the oxide layer 105 may be present on one surface of the base steel 101, but it is more preferable that they be present on both surfaces of the base steel 101. Below, the base steel 101, the plating layer 103, and the oxide layer 105 will each be described in detail.

[0027] <Regarding the Base Steel 101> In the welded joint 1 according to this embodiment, the base steel 101 corresponding to the base metal of the plated steel sheet that is the raw material is not particularly limited. Various steel sheets can be used as the base steel 101 depending on the mechanical strength (e.g., tensile strength) required of the welded joint 1. Examples of such steel sheets include various steel sheets such as various types of Al-killed steel, extra-low carbon steel containing Ti, Nb, etc., and high-strength steel in which extra-low carbon steel further contains strengthening elements such as P, Si, Mn, etc.

[0028] Furthermore, the thickness of the base steel 101 is not particularly limited, and is set appropriately depending on the mechanical properties required for the welded joint 1.

[0029] <Regarding the plating layer 103> As schematically shown in Fig. 3 , the plating layer 103 is provided on at least a portion of the surface of the base steel 101, and is preferably provided over the entire surface of the base steel 101. This plating layer 103 is derived from the plating layer of the plated steel sheet that is the material of the welded joint 1. First, the chemical composition of this plating layer 103 will be described in detail below.

[0030] Regarding the Chemical Composition of Plating Layer 103 The chemical composition of plating layer 103 according to this embodiment contains, in mass%, 1.00 to 80.00% Al, 1.00 to 20.00% Mg, 0.01 to 15.00% Fe, 0 to 10.00% Si, and 0 to 4.00% Ca, with the balance consisting of 5.00 mass% or more Zn and impurities. In other words, in the chemical composition of plating layer 103 according to this embodiment, the contents of Al, Mg, Fe, Si, and Ca are within the above ranges and the total of these contents is less than 100 mass%, with the balance consisting of 5.00 mass% or more Zn and impurities.

[0031] These components and their contents will be described in detail below.

[0032] [Al: 1.00 to 80.00 mass%] Al is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment. Al is contained in a content equal to or greater than a predetermined amount in order to ensure corrosion resistance in the non-heat-affected zone. If the Al content in the plating layer 103 is less than 1.00 mass%, the corrosion resistance of the non-heat-affected zone described above cannot be ensured. Therefore, in the plating layer 103 according to this embodiment, the Al content is set to 1.00 mass% or more. The Al content is preferably 18.00 mass% or more, and more preferably 35.00 mass% or more. By ensuring the Al content within the above range, it is possible to ensure corrosion resistance in the non-heat-affected zone.

[0033] On the other hand, if the Al content in the plating layer 103 exceeds 80.00 mass%, the Al phase that functions as a cathode when placed in a corrosive environment increases excessively, facilitating corrosion of the base steel, making it impossible to ensure corrosion resistance of the non-heat-affected zone. Therefore, in the plating layer 103 according to this embodiment, the Al content is set to 80.00 mass% or less. The Al content is preferably 60.00 mass% or less, and more preferably 50.00 mass% or less.

[0034] [Mg: 1.00 to 20.00 mass%] Mg is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment. Mg is contained in a content equal to or greater than a predetermined amount in order to ensure the corrosion resistance of the non-heat-affected zone. Therefore, in the plating layer 103 according to this embodiment, the Mg content is 1.00 mass% or more. The Mg content is preferably 5.00 mass% or more, and more preferably 7.00 mass% or more. By ensuring the Mg content in the above range, it is possible to ensure the corrosion resistance of the non-heat-affected zone.

[0035] On the other hand, if the Mg content in the plating layer 103 exceeds 20.00% by mass, anodic dissolution of the plating layer is likely to proceed when placed in a corrosive environment, making it impossible to ensure the corrosion resistance of the non-heat-affected zone. Therefore, in the plating layer 103 according to this embodiment, the Mg content is set to 20.00% by mass or less. The Mg content is preferably 15.00% by mass or less, and more preferably 13.00% by mass or less. By ensuring the Mg content within the above range, it is possible to ensure the corrosion resistance of the non-heat-affected zone.

[0036] [Fe: 0.01 to 15.00 mass%] Elements constituting the base steel 101, which is the base material, may be mixed into the coating layer 103. In particular, when the coating layer 103 is formed by hot-dip galvanizing, elements constituting the base steel 101 are more likely to be mixed into the coating layer 103 due to interdiffusion of elements caused by solid-liquid reactions between the base steel 101 and the coating layer 103. Due to this mixing of elements, a predetermined amount of Fe is contained in the coating layer 103, and the content is generally 0.01 mass% or more. Promoting this interdiffusion improves the adhesion between the base steel 101 and the coating layer 103. From the viewpoint of improving the adhesion between the base steel 101 and the coating layer 103, the Fe content in the coating layer 103 is preferably 0.20 mass% or more.

[0037] Furthermore, Fe may be intentionally added to the plating bath used to produce the plating layer 103, as long as the effects of the present invention are not impaired. However, if the Fe content in the plating layer 103 is 15.00 mass% or more, high-melting-point intermetallic compounds of Fe and Al are formed in the plating bath, and these high-melting-point intermetallic compounds adhere to the plating layer as dross, significantly degrading the appearance quality, which is undesirable. From this perspective, the Fe content in the plating bath is adjusted so that the Fe content in the plating layer 103 is 15.00 mass% or less. The Fe content in the plating layer 103 is more preferably 10.00 mass% or less.

[0038] [Si: 0 to 10.00 mass%] Si is an element that can suppress the excessive growth of Fe-Al intermetallic compounds that form at the interface between the coating layer and the base steel, thereby improving the adhesion between the coating layer and the base steel. To suppress the excessive growth of Fe-Al intermetallic compounds, the Si content is preferably 0.05 mass% or more, and more preferably 0.20 mass% or more. On the other hand, if the Si content exceeds 10.00 mass%, high-melting-point intermetallic compounds are formed in excess with Mg, inhibiting the formation of an Al-Mg oxide film that has the effect of suppressing Zn evaporation, making it difficult to suppress Zn evaporation during welding.

[0039] On the other hand, if the Si content in the plating bath for producing the plating layer 103 is too high, the viscosity of the plating bath may increase more than necessary, which may reduce plating operability. Therefore, from the viewpoint of plating operability, the Si content in the plating bath is adjusted to 10.00 mass% or less. The Si content in the plating layer 103 is preferably 5.00 mass% or less, and more preferably 2.00 mass% or less.

[0040] [Ca: 0 to 4.00% by mass] When contained in the plating layer 103, Ca forms an intermetallic compound with Al and Zn. Furthermore, when Si is contained together with Ca in the plating layer 103, Ca forms an intermetallic compound with Si. These intermetallic compounds have a high melting point and a stable structure, making it possible to suppress the formation of blowholes caused by Zn evaporation during welding of plated steel sheets and LME. The effect of suppressing the formation of blowholes and LME during welding is achieved by setting the Ca content to 0.01% by mass or more. The Ca content in the plating layer 103 is more preferably 0.10% by mass or more.

[0041] On the other hand, if the Ca content in the plating layer 103 exceeds 4.00 mass%, the corrosion resistance of the non-heat-affected zone decreases. From this viewpoint, the Ca content in the plating layer 103 is 4.00 mass% or less. The Ca content in the plating layer 103 is preferably 2.50 mass% or less, and more preferably 1.50 mass% or less.

[0042] The balance of the plating layer 103, other than the above-mentioned Al, Mg, Fe, Si, and Ca, is 5.00% by mass or more of Zn and impurities. Zn is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment, and is an important element for improving the corrosion resistance of the non-heat-affected zone. The effect of improving the corrosion resistance of the non-heat-affected zone is realized when the Zn content is 5.00% by mass or more, so the Zn content is set to 5.00% by mass or more.

[0043] Furthermore, the plating layer 103 according to this embodiment further selectively contains, in place of a portion of the remaining Zn, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, In: 0 to 1.000%, Bi: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1. 000%, Ni: 0 to 1.000%, Mn: 0 to 1.000%, V: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, in total 0 to 5.000%. In other words, the plating layer 103 according to this embodiment may contain at least one of Sb, Pb, Cu, Sn, In, Bi, Ti, Cr, Nb, Zr, Ni, Mn, V, Mo, Ag, Li, La, Ce, B, Y, and Sr as an optional added element, within the above content range and with a total content of 5.000 mass% or less.

[0044] By setting the total content of the above optional additional elements to 5.000% by mass or less, it becomes possible to enjoy the effects exerted by the addition of each optional additional element as described in detail below without mutually impairing each other. The total content of the above optional additional elements is preferably 1.000% by mass or less, and more preferably 0.200% by mass or less. The content of each optional additional element will be described in detail below.

[0045] [Sb: 0 to 0.500 mass%] [Pb: 0 to 0.500 mass%] [Sr: 0 to 0.500 mass%] When at least one of Sb, Pb, and Sr is contained in the plating layer 103, spangles are formed on the surface of the plating layer 103, making it possible to improve metallic luster. Therefore, from the viewpoint of improving the design of the plated steel sheet, it is preferable that at least one of Sb, Pb, and Sr is contained in the plating layer 103. Such an effect of improving design is realized when the content of at least one of Sb, Pb, and Sr is 0.050 mass% or more. Therefore, when at least one of Sb, Pb, and Sr is contained in the plating layer 103, it is preferable that the content of each of these elements is independently 0.050 mass% or more.

[0046] On the other hand, if the coating layer 103 is formed such that any one of the Sb, Pb, and Sr contents exceeds 0.500 mass%, the amount of dross generated in the coating bath used to form the coating layer 103 increases, making it impossible to produce a coated steel sheet with good coating properties. Therefore, the contents of Sb, Pb, and Sr in the coating layer 103 are each independently 0.500 mass% or less. The contents of Sb, Pb, and Sr are each independently preferably 0.200 mass% or less.

[0047] [Cu: 0 to 1.000 mass%] [Ti: 0 to 1.000 mass%] [Cr: 0 to 1.000 mass%] [Nb: 0 to 1.000 mass%] [Ni: 0 to 1.000 mass%] [Mn: 0 to 1.000 mass%] [V: 0 to 1.000 mass%] When at least one of Cu, Ti, Cr, Nb, Ni, Mn, and V is contained in the plating layer 103, these elements are incorporated into the Al—Fe alloy phase generated by welding when such plated steel sheets are welded, thereby improving the corrosion resistance of the formed weld bead 30. This effect of improving the corrosion resistance of the weld is achieved when the content of any of Cu, Ti, Cr, Nb, Ni, Mn, and V in the plating layer 103 is 0.005 mass% or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, and V is contained in the plating layer 103, it is preferable that the content of these elements is each independently 0.005 mass% or more.

[0048] On the other hand, if the plating layer 103 is formed such that the content of any of Cu, Ti, Cr, Nb, Ni, Mn, and V exceeds 1.000 mass%, these elements tend to form various intermetallic compounds in the plating bath used to form the plating layer 103. This increases the viscosity of the plating bath, making it impossible to produce a plated steel sheet with good plating properties. Therefore, the contents of Cu, Ti, Cr, Nb, Ni, Mn, and V in the plating layer 103 are each independently set to 1.000 mass% or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, and V are each independently set to preferably 0.200 mass% or less.

[0049] [Sn: 0 to 1.000 mass%] [In: 0 to 1.000 mass%] [Bi: 0 to 1.000 mass%] Sn, In, and Bi are elements that increase the Mg dissolution rate when the plating layer 103 containing Zn, Al, and Mg is placed in a corrosive environment. When the Mg dissolution rate increases, Mg ions are supplied to the exposed areas of the base steel, improving corrosion resistance. From this perspective, when Sn, In, and Bi are contained, the contents of Sn, In, and Bi are each independently set to 0.0050 mass% or more. On the other hand, adding excessive Sn, In, or Bi may excessively accelerate the Mg dissolution rate, potentially reducing the corrosion resistance of the non-heat-affected zone. Such an increase in the Mg dissolution rate becomes significant when the content of any of Sn, In, and Bi exceeds 1.000 mass%, so the contents of Sn, In, and Bi are each independently 1.000 mass% or less, and preferably 0.200 mass% or less.

[0050] [Zr: 0 to 1.000 mass %] When Zr is contained in the plating layer 103, it is possible to improve plating operability. Such an effect of improving plating operability can be achieved when the Zr content is 0.010 mass % or more. Therefore, when Zr is contained, the content is preferably 0.010 mass % or more.

[0051] On the other hand, when forming the plating layer 103 with a Zr content exceeding 1.000 mass%, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Zr content is set to 1.000 mass% or less. The Zr content is preferably 0.010 mass% or less.

[0052] [Mo: 0 to 1.000 mass%] When Mo is contained in the plating layer 103, it is possible to improve corrosion resistance. This effect of improving corrosion resistance can be achieved when the Mo content is 0.010 mass% or more. Therefore, when Mo is contained, the content is preferably 0.010 mass% or more.

[0053] On the other hand, when forming the plating layer 103 with a Mo content exceeding 1.000 mass%, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Mo content is set to 1.000 mass% or less, and preferably 0.010 mass% or less.

[0054] [Ag: 0 to 1.000 mass %] When Ag is contained in the plating layer 103, it is possible to improve plating operability. Such an effect of improving plating operability can be achieved when the Ag content is 0.010 mass % or more. Therefore, when Ag is contained, the content is preferably 0.010 mass % or more.

[0055] On the other hand, when forming the plating layer 103 having an Ag content exceeding 1.000% by mass, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Ag content is 1.000% by mass or less. The Ag content is preferably 0.050% by mass or less.

[0056] [Li: 0 to 1.000 mass %] When Li is contained in the plating layer 103, it is possible to improve plating operability. Such an effect of improving plating operability can be achieved when the Li content is 0.010 mass % or more. Therefore, when Li is contained, the content is preferably 0.010 mass % or more.

[0057] On the other hand, when forming the plating layer 103 with a Li content exceeding 1.000 mass%, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Li content is set to 1.000 mass% or less. The Li content is preferably 0.050 mass% or less.

[0058] [La: 0 to 0.500 mass%] [Ce: 0 to 0.500 mass%] [Y: 0 to 0.500 mass%] La, Ce, and Y are elements that exhibit almost the same effect as Ca, suppressing the formation of blowholes during welding. This is because the atomic radius of each element is close to that of Ca. When these elements are contained in the plating layer 103, they substitute for Ca. Therefore, these elements are detected at the same position as Ca in EDS. Furthermore, when these elements become oxides, the oxides of these elements are detected at the same position as CaO.

[0059] The effect of suppressing the formation of blowholes during welding can be achieved by independently setting the contents of these elements to 0.010 mass% or more. The contents of La, Ce, and Y in the plating layer 103 are more preferably independently 0.050 mass% or more.

[0060] On the other hand, if the La, Ce, and Y contents in the plating bath for producing the plating layer 103 are too high, the viscosity of the plating bath may increase more than necessary, potentially reducing plating operability. Therefore, from the viewpoint of plating operability, the La, Ce, and Y contents in the plating bath are adjusted so that the La, Ce, and Y contents are each independently 0.500 mass% or less. Preferably, the La, Ce, and Y contents are each independently 0.100 mass% or less.

[0061] [B: 0 to 0.500 mass%] When B is contained in the coating layer 103, it has the effect of further suppressing LME. This is presumably because B, when contained in the coating layer 103, combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. Furthermore, the presence of B in the coating layer 103 is thought to have the effect of diffusing from the coating layer 103 to the steel substrate 101, strengthening the grain boundaries and further suppressing LME in the steel substrate 101. Furthermore, because the various intermetallic compounds formed with B have extremely high melting points, they are presumed to also act to suppress Zn evaporation during welding. These improving effects can be achieved by including 0.050 mass% or more of B. Therefore, the B content in the coating layer 103 is more preferably 0.050 mass% or more.

[0062] On the other hand, if an excessive amount of B is added to the plating bath in order to incorporate B into the plating layer 103, a rapid rise in the plating melting point occurs, deteriorating plating operability and making it impossible to produce a plated steel sheet with excellent plating properties. Such a deterioration in operability becomes significant when the B content exceeds 0.500 mass%, so the B content is set to 0.500 mass% or less. The B content is preferably 0.100 mass% or less.

[0063] [Method for Measuring Chemical Components] The chemical components of the plating layer 103 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). ICP-AES is used when analyzing chemical components down to 0.1% by mass, and ICP-MS is used when analyzing trace amounts of chemical components less than 0.1% by mass. A target portion of the non-heat-affected zone is immersed in a 10% HCl aqueous solution containing an inhibitor for about 1 minute to peel off the plating layer, and a solution in which the plating layer is dissolved is prepared. The resulting solution can be analyzed by ICP-AES or ICP-MS to obtain the overall average chemical composition of the plating layer.

[0064] More Preferred Chemical Composition of Plating Layer 103 While the plating layer 103 according to this embodiment has the chemical composition described above, a more preferred chemical composition is as follows: That is, the plating layer 103 according to this embodiment preferably contains at least 18.00 to 60.00 mass% Al and 5.00 to 15.00 mass% Mg as a chemical composition, and more preferably further contains the above-described optional additional elements as needed.

[0065] The plating layer 103 according to this embodiment contains, as a chemical composition, at least 35.00 to 60.00 mass % of Al and 7.00 to 15.00 mass % of Mg, and may further contain the above-mentioned optional additional elements as needed. ３２ (Al, Zn) ４９ It is even more preferred that a phase is present.

[0066] Here, Mg ３２ (Al, Zn) ４９ The phase is Mg ３２ (Al, Zn) ４９ It is defined as a phase in which the Mg content [Mg], Zn content [Zn], and Al content [Al] contained within the grains of the phase satisfy the following atomic percentages: 0.5≦[Mg] / ([Zn]+[Al])≦0.83. In other words, it is defined as a crystalline phase or quasicrystalline phase in which the ratio of Mg atoms to the total of Zn atoms and Al atoms, Mg:(Zn+Al), is 3:6 to 5:6. Mg ３２ (Al, Zn) ４９ The chemical composition of the phase is preferably measured using TEM-EDX (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). ３２ (Al, Zn) ４９ The phase may be detected as both a crystalline phase and a quasi-crystalline phase. In the case of a crystalline phase, the electron diffraction pattern in TEM observation shows that the crystal structure is Mg ３２ (Al, Zn) ４９ It is possible to identify the Mg phase. ３２ (Al, Zn) ４９When the phase is a quasicrystalline phase, it can be confirmed by taking an electron beam diffraction image using a TEM and checking whether a crystalline structure with five-fold symmetry is observed in the electron beam diffraction image. The crystalline structure with five-fold symmetry can be identified by obtaining an electron beam diffraction image called a Penrose pattern.

[0067] Mg ３２ (Al, Zn) ４９ The Mg phase exerts sacrificial corrosion protection on plated steel sheets, thereby suppressing corrosion of the base steel from cuts and welds where the base steel is exposed, and improving red rust resistance. ３２ (Al, Zn) ４９ The corrosion resistance of the phase itself is also excellent, ３２ (Al, Zn) ４９ Since the corrosion rate of this phase is slow even in a corrosive environment, it suppresses under-film corrosion and also has the effect of improving corrosion resistance after painting in terms of the width of blistering of the paint film.

[0068] Regarding the coating weight of the plating layer 103, the coating weight of the plating layer 103 as described above is not particularly specified. For example, it is 15 to 250 g / m per one side of the base steel 101. ２ When the coating weight of the plating layer 103 falls within the above range, the non-heat-affected zone of the welded joint 1 according to this embodiment can exhibit sufficient corrosion resistance.

[0069] The deposition weight of the plating layer 103 is measured as follows. First, a sample measuring 30 mm x 30 mm is cut out from the plated steel sheet, and the mass of the sample is measured in advance. A tape seal is attached to one side of the sample to prevent the plating layer on this side from dissolving in the next process. The sample is then immersed in a 10% HCl aqueous solution containing an inhibitor to peel off the plating layer by pickling, and the mass of the sample after pickling is measured. The deposition weight of the plating layer 103 per side can be determined from the change in mass of the sample before and after pickling.

[0070] <<Regarding the Oxide Layer 105>> Next, the oxide layer 105 present in the non-heat-affected zone of the welded joint 1 according to this embodiment will be described in detail. As shown schematically in Fig. 3 , the oxide layer 105 is located on the surface of the plating layer 103 described above. This oxide layer 105 originates from the oxide layer present in the plated steel sheet that is the material of the welded joint 1.

[0071] Such oxide layer 105 is formed when easily oxidized elements constituting the plating layer 103 react with oxygen in the heat treatment atmosphere during the cooling process for solidifying the plating layer, which is carried out during the production of the plated steel sheet.

[0072] As described above, the oxide layer 105 is mainly composed of oxides of elements that constitute the plating layer 103, and therefore its chemical composition varies depending on the elements contained in the plating layer 103. The oxide layer 105 is presumed to contain Zn oxide, Mg oxide, and Al oxide in a total amount of 50 mass% or more, and may further contain hydroxides of these Zn, Mg, and Al, at least one of oxides or hydroxides of other constituent elements in the plating layer 103, impurities, etc.

[0073] Here, the oxide layer 105 according to this embodiment exists in the following specific state by undergoing a specific heat treatment step, as described in detail below, during the production of the plated steel sheet that serves as the base material. Hereinafter, this state will be described in detail with reference to Figs. 4 and 5. Fig. 4 is a diagram schematically showing a part of a cross section of the oxide layer parallel to the sheet thickness direction. Fig. 5 is an explanatory diagram for explaining the peak intensity in the XPS measurement results.

[0074] The oxide layer 105 according to this embodiment is produced by a specific heat treatment process, as described in detail below, during the production of the plated steel sheet that serves as the base material, and is therefore a dense coating in which the total amount of at least one of Al oxide or hydroxide and at least one of Mg oxide or hydroxide is greater than the amount of at least one of Zn oxide or hydroxide. This will be explained in more detail below.

[0075] 4, attention is now focused on a position 5 nm deep from the outermost surface of the oxide layer 105 (position B in FIG. 4). In the oxide layer 105 according to this embodiment, when this position is observed by X-ray photoelectron spectroscopy (XPS), it is preferable that the value of the intensity ratio ([Al—O]+[Mg—O]) / [Zn—O] calculated from the intensities of the peaks respectively attributed to Al—O bonds, Mg—O bonds, and Zn—O bonds is 5.0 or more.

[0076] Here, there is a possibility that dirt such as oil and fat is attached to the outermost surface of the oxide layer 105. Therefore, it is desirable to carry out the above-mentioned XPS measurement in a state where such dirt and the like are not present. From this viewpoint, the surface of the oxide layer 105 is subjected to a treatment such as ultrasonic cleaning in ethanol to remove the dirt and the like, and the surface obtained by such treatment is used as the "outermost surface of the oxide layer 105" when carrying out the above-mentioned XPS measurement.

[0077] Then, the oxide layer 105 obtained as described above is removed from the outermost surface to a depth of 5 nm by Ar ion etching, and the surface of the obtained oxide layer 105 is measured by XPS. Here, the XPS measurement conditions may be, for example, as follows: X-ray source: mono-Al Kα (1486.6 eV) X-ray diameter: 50 to 200 μm Measurement area: 100 to 700 μm × 100 to 700 μm Vacuum degree: 1×10 －１０ ~1 x 10 －１１ torr (1 torr is 133.32 Pa) Acceleration voltage: 1 to 10 kV

[0078] In this embodiment, attention is focused on the peaks attributable to Al—O bonds, Mg—O bonds, and Zn—O bonds in the obtained XPS measurement results. The above-mentioned bonds are characteristic of at least one of the oxides and hydroxides of Al, Mg, and Zn. It can be considered that the intensity of the peaks attributable to these bonds is positively correlated with the amount of at least one of the oxides and hydroxides of Al, Mg, and Zn present.

[0079] Here, the peaks attributed to Al—O bonds are those observed in the range of 72 to 76 eV in the XPS spectrum focusing on Al 2p3 / 2, the peaks attributed to Mg—O bonds are those observed in the range of 48 to 52 eV in the XPS spectrum focusing on Mg 2p3 / 2, and the peaks attributed to Zn—O bonds are those observed in the range of 1018 to 1024 eV in the XPS spectrum focusing on Zn 2p3 / 2.

[0080] The intensity of the peaks attributable to each bond is calculated by taking into consideration the baseline of the peak in the XPS spectrum shown in FIG. 5 and calculating the intensity I ｐ to baseline intensity I ｂ (i.e., "I ｐ -I ｂ ").

[0081] A more detailed method for calculating the intensity ratio is as follows. That is, XPS is measured as described above at any location on the surface corresponding to a position 5 nm deep from the outermost surface (the surface at "Position B" in FIG. 4 ), obtained as described above, and the intensity ratio ([Al—O]+[Mg—O]) / [Zn—O] is calculated. This measurement and calculation process is performed at any five locations on the surface corresponding to "Position B," and the average of the five obtained intensity ratios is used as the intensity ratio ([Al—O]+[Mg—O]) / [Zn—O] in the oxide layer 105 according to this embodiment.

[0082] In the oxide layer 105 according to this embodiment, a dense coating is formed such that the intensity ratio is 5.0 or more, thereby suppressing Zn evaporation during welding and preventing the formation of blowholes due to Zn evaporation. If the intensity ratio is less than 5.0, the density required for the oxide layer 105 may be insufficient. The intensity ratio is more preferably 10.0 or more. On the other hand, the upper limit of the intensity ratio ([Al—O] + [Mg—O]) / [Zn—O] is not particularly specified, but a practical upper limit is about 100.0.

[0083] The thickness (more specifically, the average thickness) of the oxide layer 105 described above is not particularly limited, but is preferably, for example, approximately 0.05 to 2.00 μm per side of the base steel 101. When the thickness of the oxide layer 105 falls within the above range, the non-heat-affected zone according to this embodiment can sufficiently suppress the formation of blowholes due to Zn evaporation during welding. Furthermore, the oxide layer 105 having the above thickness can be achieved by controlling the steel sheet threading speed within an appropriate range during the production of the plated steel sheet, which serves as the base material, and performing a heat treatment process, as described in detail below.

[0084] The thickness of the oxide layer 105 can be measured using XPS. XPS measurement is performed in the depth direction from the surface of the plated steel sheet at intervals of 1 to 3 nm, and the depth at which the maximum oxygen intensity reaches 1 / 20 of the maximum intensity at the outermost surface is defined as the thickness of the oxide layer. The XPS measurement conditions may be the same as those described above.

[0085] The non-heat-affected zone in the welded joint 1 according to this embodiment has been described in detail above with reference to Figures 2 to 5. The non-heat-affected zone of the welded joint 1 according to this embodiment may further have one or more of various coating layers on the oxide layer 105. Examples of such coatings include a chromate coating, a phosphate coating, a chromate-free coating, and an organic resin coating.

[0086] <Regarding the Weld Bead Portion> Next, features of the weld bead portion in the weld joint 1 according to this embodiment will be described with reference to Figure 2. In the weld joint 1 according to this embodiment, a plated steel sheet having the above-described plating layer 103 and oxide layer 105 is used as the material for at least one of the first steel sheet 10 or the second steel sheet 20. As a result, a slag layer 32 exhibiting the following composition is formed, which suppresses the formation of Si-based slag on the surface of the weld metal 31.

[0087] More specifically, the formation of Si-based slag is suppressed by forming slag containing Al and Mg (Al-Mg-containing slag) as the slag layer 32 formed on the surface of the weld metal 31. Like Si-based slag, Al-Mg-containing slag is not electrically conductive. However, when Al-Mg-containing slag is formed on the surface of the weld metal 31, the area covering the surface of the weld metal 31 is smaller than when Si-based slag is formed. In other words, the slag can be made finer. As a result, the weld joint 1 according to this embodiment can improve the electrodeposition paintability of the weld bead.

[0088] To achieve the above-described effects, attention is focused on the chemical composition of the slag layer 32. In the weld bead portion 30 according to this embodiment, the chemical composition of the slag layer 32 at position A, expressed in mass % excluding oxygen, is 1.0 to 45.0% Al, 1.0 to 30.0% Mg, and the remainder is Fe, easily oxidized metal elements, and impurities. Here, "easily oxidized metal elements" refer to metal elements that are more easily oxidized than Fe in the Ellingham diagram and that can be added to the plating layer 103. Specific examples of such easily oxidized metal elements include Ca, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, and Ti.

[0089] These components and their contents will be described in detail below.

[0090] [Al: 1.0 to 45.0 mass%] An Al content of less than 1.0 mass% indicates excessive evaporation of Al during welding. In such cases, the Al-Mg-containing slag formed on the surface of the weld metal 31 is insufficient, making it difficult to suppress the formation of Si-based slag. Therefore, the Al content of the slag layer 32 at position A is required to be 1.0 mass% or more. The Al content is preferably 15.0 mass% or more, and more preferably 30.0 mass% or more. With an Al content within this range, the Al-Mg-containing slag suppresses the formation of Si-based slag, improving the electrodeposition paintability of the weld bead 30.

[0091] On the other hand, if the Al content exceeds 45.0 mass%, the amount of Mg consumed to form the Al-Mg-containing slag may be insufficient. As a result, the Al-Mg-containing slag is less likely to form on the surface of the weld metal 31, making it difficult to suppress the formation of Si-based slag. Therefore, the Al content of the slag layer 32 at position A is required to be 45.0 mass% or less. The Al content is preferably 43.0 mass% or less, and more preferably 41.0 mass% or less. With the Al content within this range, the Al-Mg-containing slag suppresses the formation of Si-based slag, improving the electrodeposition paintability of the weld bead 30.

[0092] [Mg: 1.0 to 30.0% by mass] A Mg content of less than 1.0% by mass indicates that excessive Mg evaporated during welding. In such cases, the Al-Mg-containing slag formed on the surface of the weld metal 31 is insufficient, making it difficult to suppress the formation of Si-based slag. Therefore, the Mg content of the slag layer 32 at position A is required to be 1.0% by mass or more. The Mg content is preferably 7.0% by mass or more, and more preferably 8.0% by mass or more. When the Mg content is within this range, the Al-Mg-containing slag suppresses the formation of Si-based slag, improving the electrodeposition paintability of the weld bead 30.

[0093] On the other hand, if the Mg content exceeds 30.0 mass%, the amount of Al consumed to form the Al-Mg-containing slag may be insufficient. As a result, the Al-Mg-containing slag is less likely to form on the surface of the weld metal 31, making it difficult to suppress the formation of Si-based slag. Therefore, the Mg content of the slag layer 32 at position A is required to be 30.0 mass% or less.

[0094] To identify the components of the slag layer 32, cross-sectional SEM-EPMA (Electron Probe Micro Analyzer) measurement is performed at a position (position A in FIG. 2) 1 mm from the toe T of the weld bead 30 in a direction perpendicular to the extension direction of the weld bead 30 (Y-axis direction in FIG. 2) and toward the inside of the weld bead 30 (X-axis direction in FIG. 2), as shown in FIG. 2. This EPMA point composition analysis allows the concentration of each element constituting the slag layer 32 to be measured.

[0095] As described above, the chemical composition of slag layer 32 is limited to the chemical composition at position A. However, if Al-Mg-containing slag is formed at position A, it is estimated that Al-Mg-containing slag is formed not only at position A but also in the vicinity of position A. Therefore, if the chemical composition of slag layer 32 at position A satisfies the above chemical composition, the formation of Si-based slag is suppressed in the vicinity of position A as well, and the electrodeposition paintability of weld bead 30 can be improved.

[0096] Furthermore, the Al and Mg contained in the slag layer 32 at position A are mainly diffused from the plating layer 103 having a Zn-Al-Mg alloy phase. Therefore, the welded joint 1 according to this embodiment, which has excellent electrodeposition paintability, can be produced by using a plated steel sheet having such a plating layer 103 as the raw material for at least one of the first steel sheet 10 and the second steel sheet 20.

[0097] Furthermore, when a filler metal is used to weld the first steel plate 10 and the second steel plate 20, a suitable example of the filler metal is solid wire. An example of a suitable chemical composition of the solid wire is, in mass %, C: 0.05 to 0.20%, Si: 0 to 0.15%, Mn: 0.3 to 2.5%, P: 0 to 0.02%, S: 0 to 0.04%, Ti: 0.02 to 0.20%, B: 0 to 0.012%, Al: 0 to 0.22%, Cr: 0 to 0.5%, Nb: 0 to 0.3%, V: 0 to 0.3%, Mo: 0 to 1.0%, Ni: 0 to 3.0%, Zr: 0 to 0.200%, Cu: 0 to 0.5%, with the balance being Fe and impurities.

[0098] The features of the weld bead portion in the weld joint 1 according to this embodiment have been described in detail above with reference to FIG. 2 .

[0099] The welded joint 1 according to this embodiment has been described in detail above. The welded joint 1 according to this embodiment as described above can be suitably used, for example, as an automobile suspension part.

[0100] (Method for manufacturing plated steel sheet as base material) Next, a description will be given of an example of a method for manufacturing the plated steel sheet as base material for the welded joint 1 as described above. The plated steel sheet as base material for the welded joint 1 according to this embodiment is manufactured by using a steel sheet made of the above-described base steel 101 as a base material and forming a plating layer 103 and an oxide layer 105 on the surface of the base steel 101.

[0101] Here, in addition to hot-dip plating, methods such as thermal spraying, cold spraying, sputtering, vapor deposition, and electroplating can be used to form the plating layer 103. However, hot-dip plating is most preferable in terms of cost for forming a plating layer of a thickness generally used in automobiles and the like.

[0102] Thereafter, the obtained plated steel sheet (a plated steel sheet consisting of the base steel 101 and the plated layer 103) is subjected to a specific heat treatment process as described below, thereby forming an oxide layer 105 on the surface of the plated layer 103. In this way, a plated steel sheet to be used as a material for the welded joint 1 according to this embodiment can be manufactured.

[0103] An example of a manufacturing method for obtaining a plated steel sheet according to the present embodiment using a hot-dip galvanizing method will be described in detail below. In the manufacturing process of such a plated steel sheet, first, a steel sheet made of a base steel 101 used as a base material is rolled by the Sendzimir method to a desired thickness, and then the steel sheet is wound into a coil and placed in a hot-dip galvanizing line.

[0104] In the hot dip plating line, the steel sheet is continuously fed from the coil and passed through the line. At this time, the steel sheet is annealed by an annealing facility installed on the line in an environment where oxidation is unlikely to occur, for example, an oxygen concentration of 20 ppm or less, and N ２ -5% H ２The material is heated and reduced at 800°C in a gas atmosphere. After that, the temperature is reduced to about 20°C above the bath temperature of the subsequent plating bath. ２ The plate is cooled with gas and then immersed in a plating bath.

[0105] Here, a molten plating alloy having the above-described chemical composition is prepared in the plating bath. The temperature of the plating bath is set to be equal to or higher than the melting point of the plating alloy (e.g., approximately 460 to 600°C). When preparing the plating alloy material, it is preferable to use pure metals (purity of 99% or higher) as the alloy material. First, predetermined amounts of alloy metals are mixed to obtain the above-described plating layer composition, and then completely melted to obtain an alloy using a high-frequency induction furnace or an arc furnace under vacuum or inert gas replacement conditions. Next, the alloy mixed with the predetermined components (the above-described plating layer composition) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

[0106] In addition, there is no particular restriction on using pure metals for the production of the plating alloys described above, and existing Zn alloys, Mg alloys, and Al alloys may be melted and used. In this case, there is no problem as long as an alloy of a predetermined composition with few impurities is used.

[0107] After immersing the steel sheet in the plating bath, it is pulled up at a predetermined speed. At this time, the plating layer 103 formed has a desired thickness. ２ The plating coating weight is controlled by wiping gas. Here, with the exception of the bath temperature, general plating operating conditions may be applied, and no special equipment or conditions are required.

[0108] Subsequently, the molten plating alloy located on the steel sheet is subjected to the following first and second cooling steps to convert the molten plating alloy into a plating layer 103 and to form an oxide layer 105 on the surface of the plating layer 103. The first and second cooling steps will be described in detail below.

[0109] The first cooling step is a cooling step carried out when the temperature of the plating alloy is within a range of 250°C or more below the bath temperature, and the plated steel sheet within the above temperature range is quenched at an average cooling rate of 10°C / second or more in an atmosphere with a dew point of -20°C or less. When a hot-dip plating method is adopted in the plating step, the first cooling step is carried out immediately after the steel sheet leaves the plating bath. As a result, the plating alloy located on the surface of the steel sheet solidifies, forming a plating layer.

[0110] Thereafter, when the temperature of the plated alloy (plated layer) is within a range of less than 250° C. and not less than 50° C., a second cooling step is carried out. This second cooling step is a step in which the plated steel sheet, which is within a temperature range of less than 250° C. and not less than 50° C., is slowly cooled at an average cooling rate of less than 10° C. / second in an atmosphere with a dew point of not less than 0° C. This controls the state of oxides formed on the surface of the plated layer, and a desired oxide layer is formed.

[0111] As described above, by undergoing a two-stage cooling process in which rapid cooling is performed in the temperature range of 250°C or higher below the bath temperature, and slow cooling is performed in the temperature range of 50°C or higher but lower than 250°C, a dense oxide layer 105 is formed on the surface of the plating layer 103, which satisfies specific conditions in the XPS measurement results.

[0112] Here, the interval between the end of the first cooling step and the start of the second cooling step is preferably 3 seconds or less, and the second cooling step is preferably started immediately after the end of the first cooling step. If the interval between the end of the first cooling step and the start of the second cooling step exceeds 3 seconds, an unintended cooling process occurs, making it impossible to achieve the desired oxide layer 105.

[0113] Here, in the first cooling step, the lower limit of the dew point is not particularly specified, but for example, about −90° C. is a practical lower limit. The average cooling rate is more preferably 40° C. / sec or more. The upper limit of the average cooling rate is not particularly specified, but for example, about 90° C. / sec is a practical upper limit.

[0114] In the second cooling step, the upper limit of the dew point is not particularly specified, but a practical upper limit is, for example, about 20° C. In addition, the average cooling rate is more preferably 4° C. / second or less.

[0115] Note that if either the first cooling step or the second cooling step is not performed, it is not possible to realize the desired oxide layer 105. By performing both the first cooling step and the second cooling step, the oxide layer 105 according to this embodiment can be realized.

[0116] Furthermore, if an alloying heat treatment step (for example, a heat treatment step involving heating to a sheet temperature of about 480 to 550°C), which is generally performed in the manufacture of galvannealed steel sheets, is performed after the second cooling step, the oxide generation state controlled by the first cooling step and the second cooling step is disrupted, and the oxide grows excessively, making it impossible to obtain the Zn evaporation suppression effect that is the focus of this embodiment. From this perspective, it is important not to perform a heat treatment step after the second cooling step.

[0117] In the above cooling process, N ２ Commonly known methods such as gas cooling, mist cooling, submersion in water, etc. can be applied. ２ In addition to gas, gases with a high heat removal effect, such as He gas and hydrogen gas, may also be used.

[0118] The temperature of the coating layer can be measured using, for example, a contact thermocouple (K-type). By attaching a contact thermocouple to the base steel sheet, the average temperature of the entire coating layer can be constantly monitored. Furthermore, by mechanically controlling various speeds and thicknesses and standardizing various operational conditions such as the preheating temperature of the steel sheet and the temperature of the hot-dip coating bath, it becomes possible to monitor the temperature of the entire coating layer at that time under those manufacturing conditions almost accurately. This makes it possible to precisely control the cooling treatments in the first cooling step and the second cooling step. The surface temperature of the coating layer may also be measured using a non-contact radiation thermometer, although this is not as accurate as a contact thermometer.

[0119] Alternatively, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be determined by a simulation using heat conduction analysis. Specifically, the surface temperature of the plating layer and the average temperature of the entire plating layer are determined based on various manufacturing conditions, such as the preheating temperature of the steel sheet, the temperature of the hot-dip plating bath, the pulling speed of the steel sheet from the plating bath, the thickness of the steel sheet, the thickness of the plating layer, the amount of heat exchanged between the plating layer and the manufacturing equipment, and the amount of heat dissipated from the plating layer. The relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can then be determined using the obtained results. This makes it possible to estimate the average temperature of the entire plating layer at that time under those manufacturing conditions by actually measuring the surface temperature of the plating layer during the manufacturing of the plated steel sheet. As a result, it becomes possible to precisely control the cooling treatments in the first cooling step and the second cooling step.

[0120] An example of the method for manufacturing a plated steel sheet according to this embodiment has been specifically described above.

[0121] In the method for producing a plated steel sheet according to this embodiment, after the second cooling step, a treatment may be further carried out to form one or more various coating layers, such as a chromate treatment, a phosphate treatment, a chromate-free treatment, or an organic resin coating formation treatment.

[0122] Chromate treatments include electrolytic chromate treatments in which a chromate film is formed by electrolysis, reactive chromate treatments in which a film is formed by utilizing a reaction with the material and then excess treatment liquid is washed away, and application-type chromate treatments in which a treatment liquid is applied and then dried without rinsing with water to form a film, and any of these chromate treatments may be used.

[0123] Examples of electrolytic chromate treatments include electrolytic chromate treatments using chromic acid, silica sol, resins (phosphate resins, acrylic resins, vinyl ester resins, vinyl acetate acrylic emulsions, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resins, etc.), and hard silica.

[0124] Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

[0125] Chromate-free treatments are particularly suitable because they do not impose a burden on the environment. Examples of such chromate-free treatments include electrolytic chromate-free treatments that form a chromate-free film by electrolysis, reactive chromate-free treatments that form a film by utilizing a reaction with the material and then wash away excess treatment liquid, and application-type chromate-free treatments that apply a treatment liquid and dry it without rinsing with water to form a film. Any of these chromate-free treatments may be used.

[0126] Furthermore, the organic resin used in the organic resin film formation process is not limited to a specific resin, and various resins can be used, such as polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, modified versions of these resins, etc. Here, the modified version refers to a resin obtained by reacting a reactive functional group contained in the structure of these resins with another compound (e.g., a monomer, a crosslinking agent, etc.) containing a functional group in its structure that can react with the reactive functional group.

[0127] As the organic resin, one of the above-mentioned organic resins may be used alone, or two or more organic resins (unmodified) may be used in combination. Furthermore, one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be used in combination. Furthermore, an aqueous organic resin may be used by dissolving or dispersing it in water. Furthermore, various color pigments and rust-preventive pigments may be contained in such an organic resin film.

[0128] (Regarding the manufacturing method of the welded joint) The welded joint according to this embodiment is manufactured by using the plated steel sheet manufactured as described above as the material for at least one of the first steel sheet and the second steel sheet when manufacturing a welded joint, arranging the first steel sheet and the second steel sheet to form a desired shape for the welded joint, and welding the first steel sheet and the second steel sheet together.

[0129] Here, the first steel plate and the second steel plate can be welded by arc welding or laser welding. In this case, by performing the welding under the welding conditions described below in each welding method, it is possible to achieve the above-mentioned state of the weld bead portion.

[0130] More specifically, when producing a welded joint by arc welding, the first steel plate and the second steel plate may be welded under the following welding conditions, for example: 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°

[0131] When manufacturing a welded joint by laser welding, the first steel plate and the second steel plate may be welded under the following welding conditions, for example: power: 7 kW, welding speed: 400 cm / min, advance / reverse angle: 0°.

[0132] The above describes an example of a method for manufacturing a welded joint according to this embodiment. Although it may be conceivable to use various resistance welding methods, such as high-frequency resistance welding or butt resistance welding, using such welding methods would not achieve the state of the weld bead that is the focus of this embodiment.

[0133] The welded joint according to the present invention will be specifically described below with reference to examples and comparative examples. Note that the examples shown below are merely examples of the welded joint according to the present invention, and the welded joint according to the present invention is not limited to the examples shown below.

[0134] In the examples and comparative examples shown below, a 2.6 mm thick hot-rolled steel plate (0.05 mass% C-0.2 mass% Si-1.3 mass% Mn-0.12 mass% Ti-0.07 mass% Al, manufactured by Nippon Steel Corporation) was used as the steel plate serving as the base material. This hot-rolled steel plate was cut into a size of 100 mm x 200 mm to prepare test specimens.

[0135] The plating baths for realizing the plating layers with the compositions shown in Table 1 below were prepared and installed in a batch-type hot-dip plating test apparatus manufactured by our company, and the above test pieces were plated. The temperature of the test pieces was measured using a thermocouple spot-welded to the center of the test pieces. In addition, before immersion in the plating bath, the test pieces were heated in a furnace with an oxygen concentration of 20 ppm or less, and then heated in a furnace with an N ２ -5% H ２ The surface of the plated original sheet was subjected to a heat reduction treatment at 800°C in a gas atmosphere. After the heat reduction treatment, the test piece was ２ The test piece was cooled by air with gas, and after the temperature of the test piece reached the bath temperature plus 20°C, the test piece was immersed in the plating bath of the hot dip plating tester for about 3 seconds.

[0136] After immersion in the plating bath, the test piece was pulled up at a pulling speed of 20 to 200 mm / sec. ２ The wiping gas was used to control the plating deposition weight to a desired level. In the following examples and comparative examples, the plating deposition weight per side of the test piece after drying was 15 to 250 g / m ２ After the test piece was removed from the coating bath, the test piece was cooled from the coating bath temperature to room temperature under the conditions shown in Table 1 below. In the examples and comparative examples shown below, the second cooling step was started immediately after the first cooling step was completed (i.e., the interval between the completion of the first cooling step and the start of the second cooling step was set to 0.2 seconds or less).

[0137] Here, a steel sheet measuring 30 mm × 30 mm was cut out from the test piece plated as described above, and the plated steel sheet was immersed in a 10% HCl aqueous solution containing an inhibitor to peel off the plating layer by pickling. The elements dissolved in the aqueous solution were then subjected to ICP analysis to measure the composition of the plating layer.

[0138] Further, an electron diffraction image of the obtained plating layer was taken by TEM, and based on whether or not a 5-fold symmetric crystal structure was observed in the electron diffraction image, Mg ３２ (Al, Zn) ４９ The presence or absence of phases was confirmed.

[0139] Furthermore, the XPS spectrum of the obtained oxide layer was measured according to the above-mentioned method, and the intensity ratio ([Al-O] + [Mg-O]) / [Zn-O] was calculated. The obtained intensity ratio was evaluated based on the following criteria. <Evaluation criteria> Rating "A": Intensity ratio value is 10.0 or more "B": Intensity ratio value is 5.0 or more and less than 10.0 "C": Intensity ratio value is less than 5.0

[0140] Furthermore, a steel plate cut out from the obtained test piece to a size of 150 mm × 50 mm was used as the first steel plate, and a steel plate cut out to a size of 150 mm × 30 mm was used as the second steel plate. The long sides of these steel plates were overlapped and welded by arc welding or laser welding (lap fillet welding) to form a welded joint.

[0141] The welding conditions for the arc welding were as follows: Welding mode: pulse MAG welding, Welding current: 230 A, Welding voltage: 23.4 V, Welding speed: 100 cm / min, Welding gas: 20% CO ２ +Ar, gas flow rate: 20 L / min Welding wire: YM-TX manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm (C: 0.11 mass%, Si: 0.01 mass%, Mn: 1.21 mass%, P: 0.006 mass%, S: 0.007 mass%, Cu: 0.22 mass%) Welding torch tilt angle: 60° Overlap: 10 mm Steel plate size: upper plate side (first steel plate) 150 x 50 mm, lower plate side (second steel plate) 150 x 30 mm Plate gap: 0 mm Wire protrusion length: 15 mm

[0142] The welding conditions for the laser welding were as follows: power: 7 kW, welding speed: 400 cm / min, forward / backward angle: 0°, steel plate size: upper plate (first steel plate) 150 x 50 mm, lower plate (second steel plate) 150 x 30 mm, overlap: 50 mm, plate gap: 0 mm

[0143] <Electrodeposition Coatability of Weld Bead Portion> The welded joint obtained as described above was subjected to an automotive phosphate conversion treatment (Zn phosphate treatment, SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard) and electrodeposition coating (PN110 Powernics Gray: Nippon Paint Industrial Coating Co., Ltd. standard). In this case, the electrodeposition film thickness was 20 μm. As shown in FIG. 6, the maximum length L of the weld bead portion in the extension direction of the electrodeposition coating sample was measured. ０ and the length L of the electrodeposition defect that occurred on the sample. ｎ (n: integer of 1 or more) was measured. The defective electrodeposition areas have a reddish brown to black appearance, which is a different color from the color of the electrodeposition coated area, so it is possible to identify the defective electrodeposition areas from the appearance. That is, the length of each defective electrodeposition area identified from the appearance in the direction parallel to the extension direction (welding direction) of the weld bead area is the length L of the defective electrodeposition area. ｎ is.

[0144] The total length of the electrodeposition defects ΣLn and the maximum length L of the weld bead ０ The ratio (ΣL ｎ / L ０ The electrodeposition coatability was evaluated by calculating the ΣL. The evaluation criteria were as follows. The results are summarized in Table 1 below. <Evaluation criteria> Rating "AAA": ΣL ｎ / L ０ The value of ΣL is 0.3 or less "AA": ｎ / L ０ The value is greater than 0.3 and less than 0.5 "A": ΣL ｎ / L ０ The value is greater than 0.5 and less than 0.7 "B": ΣL ｎ / L ０ The value is greater than 0.7

[0145]

[0146] As is clear from Table 1 above, the examples corresponding to the examples of the present invention show excellent electrodeposition paintability at the weld bead portion, whereas the examples corresponding to the comparative examples of the present invention do not show sufficient performance in terms of electrodeposition paintability at the weld bead portion.

[0147] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0148] REFERENCE SIGNS LIST 1 welded joint 10 first steel plate 20 second steel plate 30 weld bead portion 31 weld metal 32 slag layer 101 base steel 103 plating layer 105 oxide layer

Claims

DEPCT671. A joint in which a first and second steel plate are welded by arc welding or laser welding, where the joint comprises the first and second steel plates and the weld area created by arc welding or laser welding, where the area unaffected by heat due to welding is defined as the unaffected area in the first and second steel plates, at least one of the first or second steel plates shall have at least a quenching layer on the subsurface of the base steel and an oxide layer on the quenching layer in the unaffected area. The quenching layer contains the following elements: % by mass Al: 1.00 to 80.00% Mg: 1.00 to 20.00% Fe: 0.01 to 15.00% Si: 0 to 10.00% Ca: 0 to 4.00% It is also possible to choose to contain the following elements in all proportions: Sb: 0 to 5.000%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, In: 0 to 1.000%, Bi: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Ni: 0 to 1.000%, Mn: 0 to 1.000%, V: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0 to 1.000%, Li: 0 to 1.000%.000%La:0 to 0.500%Ce:0 to 0.500%B:0 to 0.500%Y:0 to 0.500%Sr:0 to 0.500%, with the remainder consisting of Zn 5.00% by mass or more and impurities, where the weld area has weld metal and slag layer formed on the weld surface and the slag layer has the following elements in % by mass when oxygen is not included: Al:1.0 to 45.0% and Mg:1.0 to 30.0%, with the remainder consisting of Fe, easily oxidizing metals and impurities.

2. Weld joint according to claim 1, where the slag layer has the following elements in % by mass... At least Al:15.0 to 45.0% and Mg:7.0 to 30.0%.

3. Boundary point according to claim 1 or 2, where when the position at a depth of 5 nm from the top surface of the oxide layer is observed by an X-ray photoelectron spectrometer (XPS) analysis instrument, the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O]) calculated from the peak intensity due to Al-O bonds, Mg-O bonds, and Zn-O bonds, respectively, is 5.0 or more.

4. Boundary point according to claim 3, where the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] is 10.0 or more. 5.

6. Any one of the claims 1 to 4 where the unaffected plating layer contains at least: Al: 18.00 to 60.00% by mass and Mg: 5.00 to 15.00% by mass; and the Mg32(Al,Zn)49 phase is present in the plating layer with the amount of Mg[Mg], amount of Zn[Zn], and amount of Al[Al] (each unit: % by atom) in the Mg32(Al,Zn)49 phase satisfying the relationship 0.50 less than or equal to [Mg] / ([Zn]+[Al]) less than or equal to 0.83;