welded joint

Abstract

[Problem] To further improve the electroplating properties in the weld bead portion. [Solution] A weld joint according to the present invention has a first steel plate and a second steel plate and a weld bead portion formed by arc welding or laser welding, and at least one of the first steel plate or the second steel plate has, in the non-heat-affected portion, a plating layer composed of a predetermined component and an oxide layer located on the plating layer, and the weld bead portion has a weld metal and a slag layer formed on a part of the surface of said weld metal, and the slag layer contains Al: 1.0 to 45.0% and Mg: 1.0 to 30.0% in mass% excluding oxygen, and the remainder consists of Fe, metal elements that are prone to oxidation, and impurities.

Description

Technology Field

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

[0002] Automotive components, including suspension parts, and various construction materials are often manufactured using welded joints formed by welding multiple steel materials. Since these automotive and construction materials are used while exposed to various environments, it is required that the manufactured welded joints possess excellent corrosion resistance. Therefore, various zinc-plated steel sheets, including alloyed hot-dip galvanized steel sheets, are used as materials for these welded joints.

[0003] Here, as a specific problem when manufacturing welded joints by welding galvanized steel sheets, there is a decrease in corrosion resistance due to the evaporation of Zn during plating in the vicinity of the "edge" specified in JIS Z3001 (2018).

[0004] In order to solve the problem of blowhole formation as described above, various proposals have been made in the past. For example, in the following patent document 1, a plated steel material is proposed having a steel plate and a plating layer disposed on the surface of the steel plate and comprising a Zn-Al-Mg alloy layer, wherein in the cross-section of the Zn-Al-Mg alloy layer, the area fraction of the MnZn2 phase is 45 to 75%, the total area fraction of the MgZn2 phase and the Al phase is 70% or more, and the area fraction of the Zn-Al-MgZn2 ternary eutectic structure is 0 to 5%, and the plating layer has a predetermined chemical composition. Prior art literature

[0005] International Publication No. 2018 / 139620 The problem to be solved

[0006] Here, it is possible to solve the problem of blowhole formation by using the plated steel material proposed in the above Patent Document 1. However, as a result of careful examination by the inventors, it was found that there is still room for improvement in the technology proposed in the above Patent Document 1, and further improvement can be expected regarding the electrodeposition coating properties of the weld bead portion in a welded joint made of galvanized steel sheet.

[0007] Therefore, the present invention has been made in consideration of the above problem, and the objective of the present invention is to provide a welded joint capable of further improving the electrodeposition coating properties in the weld bead portion. means of solving the problem

[0008] In order to solve the above problem, the inventors carefully examined the issue and found that the deterioration of electrodeposition paintability in the weld bead area is caused by the formation of non-conductive Si-based slag on the surface of the weld metal in the weld bead area, and the inhibition of current flow from the surface of the weld metal to the base metal in the region where the Si-based slag is formed. Therefore, they discovered that if the formation of Si-based slag on the surface of the weld metal can be suppressed, a current path from the surface of the weld metal to the base metal can be formed, thereby making it possible to further improve electrodeposition paintability.

[0009] Based on these findings, the inventors conducted further investigations and, by improving the plated steel sheet as a material and examining appropriate welding conditions, were able to discover a technology capable of suppressing the formation of Si-based slag that inhibits the electrodeposition of paint on the surface of the weld metal in the weld bead.

[0010] The gist of the present invention, completed based on these findings, is as follows.

[0011] (1) A first steel plate and a second steel plate are welded joints by arc welding or laser welding, and the first steel plate and the second steel plate have a weld bead portion formed by the arc welding or laser welding, and in the first steel plate and the second steel plate, when the portion that is not affected by heat from the welding is called a non-heat-affected portion, at least one of the first steel plate or the second steel plate has a plating layer located on at least a portion of the surface of the base steel in the non-heat-affected portion and an oxide layer located on said plating layer, and 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 optionally, 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 A welded joint comprising 0.500%, 0 to 5.000% Sr, and the remainder being 5.00% by mass of Zn and impurities, wherein the weld bead portion comprises a weld metal and a slag layer formed on a portion of the surface of the weld metal, wherein the slag layer contains 1.0 to 45.0% Al and 1.0 to 30.0% Mg by mass excluding oxygen, and the remainder being Fe, easily oxidizable metal elements, and impurities.

[0012] (2) The above slag layer contains at least 15.0 to 45.0% Al and 7.0 to 30.0% Mg in mass%, the welded joint described in (1).

[0013] (3) When observing a position at a depth of 5 nm from the outermost surface of the oxide layer by X-ray photoelectron spectroscopy (XPS), the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] calculated from the intensity of the peaks attributed to Al-O bonds, Mg-O bonds, and Zn-O bonds, respectively, is 5.0 or greater, the weld joint described in (1) or (2).

[0014] (4) A welded joint described in (3) in which the value of the strength ratio ([Al-O]+[Mg-O]) / [Zn-O] is 10.0 or greater.

[0015] (5) The plating layer in the non-heat-affected portion contains at least 18.00 to 60.00 mass% Al and 5.00 to 15.00 mass% Mg, and is a welded joint described in any one of (1) to (4).

[0016] (6) The plating layer in the above-mentioned non-heat-affected zone contains at least 35.00 to 60.00 mass% Al and 7.00 to 15.00 mass% Mg, and furthermore, in the plating layer, Mg 32 (Al, Zn) 49 Since a phase exists, the above Mg 32 (Al, Zn) 49 A welded joint described in any one of (1) to (5), wherein the Mg content [Mg], Zn content [Zn], and Al content [Al] (each unit: atomic %) in the upper portion satisfy the relationship 0.50 ≤ [Mg] / ([Zn] + [Al]) ≤ 0.83. Effects of the invention

[0017] As explained above, according to the present invention, in a welded joint made of plated steel sheet, it is possible to further improve the electrodeposition coating properties in the weld bead portion. Brief explanation of the drawing

[0018] FIG. 1a is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to an embodiment of the present invention. FIG. 1b is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. FIG. 1c is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. FIG. 1d is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. FIG. 2 is an explanatory diagram for describing a welded joint according to the embodiment shown in FIG. 1a. FIG. 3 is an explanatory diagram for explaining a welded joint according to the same embodiment. FIG. 4 is an explanatory diagram for explaining a welded joint according to the same embodiment. Figure 5 is an explanatory diagram for explaining the peak intensity in XPS measurement results. Figure 6 is an explanatory diagram for explaining the method of evaluating electrodeposition coating properties. Specific details for implementing the invention

[0019] Suitable embodiments of the present invention will be described in detail below with reference to the attached drawings. Furthermore, in this specification and drawings, components having substantially the same functional configuration are assigned the same number to avoid redundant descriptions.

[0020] (Regarding welded joints)

[0021] First, with reference to FIG. 1a, the overall configuration of a welded joint according to an embodiment of the present invention will be described. FIG. 1a is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to the present embodiment.

[0022] In addition, for convenience, the following description will be made using the coordinate system shown in FIG. 1a. Also, FIG. 1a illustrates a welded joint formed by welding two steel plates by arc welding as an example. A welded joint formed by welding two steel plates by laser welding also has the same configuration as FIG. 1a, although the detailed shape of the weld bead portion differs.

[0023] FIG. 1a schematically illustrates the overall configuration of a welded joint obtained by overlapping 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 elongation direction of the weld bead portion. As schematically illustrated in FIG. 1a, the welded joint (1) according to the present embodiment has a first steel plate (10), a second steel plate (20), and a weld bead portion (30).

[0024] Here, it is preferable to use various plated steel plates as the material for at least one of the first steel plate (10) and the second steel plate (20) constituting the weld joint (1), and it is more preferable to use plated steel plates having a plating layer as described in detail below as the material for both the first steel plate (10) and the second steel plate (20) constituting the weld joint (1).

[0025] Additionally, the weld bead portion (30) is a portion formed by arc welding and has a weld metal (31) and a slag layer (32) formed on a portion of the surface of the weld metal (31). During welding, mutual diffusion of constituent elements occurs between the weld metal (31) and the first steel plate (10) and the second steel plate (20). Therefore, in FIG. 1a, the bonding interface between the weld metal (31) of the weld bead portion (30) and the first steel plate (10) or the second steel plate (20) is shown as a flat plane (using a straight line) for convenience of illustration, but the actual bonding 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. In addition, in FIG. 1a, the slag layer (32) is shown to cover the entire surface of the weld metal (31), but as described later, the slag layer (32) is formed to cover a part of the surface of the weld metal (31). Also, this weld bead portion (30) is extended along the Y-axis direction in the drawing, and the first steel plate (10) and the second steel plate (20) are joined by this weld bead portion (30).

[0026] In addition, since the components constituting this welding metal (31) vary depending on the type of welding wire used and the chemical composition of the first steel plate (10) and second steel plate (20) as materials, it is difficult to unilaterally determine a component that encompasses all possibilities. However, this welding metal (31) generally consists mainly of oxides of elements that are prone to oxidation among the various elements constituting the plated steel plate as materials. Examples of such elements that are prone to oxidation include Al and Mg.

[0027] In addition, when specifying the area corresponding to the weld metal (31) in the weld joint (1) of interest, measurements can be performed as follows, for example. That is, a sample having weld metal (31) is prepared, the sample is cut in a plane perpendicular to the welding direction (the Y-axis direction in FIG. 1a) (the XZ plane in FIG. 1a), and the sample is polished in resin so that the cross-section of the weld bead portion (30) (the XZ 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 the slag layer (32) on the weld metal (31) can be specified by obtaining the elemental distribution of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.) using SEM-EDS (Energy Dispersive X-ray Spectroscopy). The portion located on the steel plate side, rather than the portion of the slag layer (32) specified in this way, becomes the weld metal (31).

[0028] In addition, oxides generated during welding are broadly classified into two types: scale and slag. Scale consists of 50% or more of Fe in mass% excluding oxygen, with the remainder consisting of easily oxidizable elements and impurities. Slag consists of 50% or more of easily oxidizable elements in mass% excluding oxygen, with the remainder consisting of less than 50% of Fe and impurities. Here, "easily oxidizable elements" refers to metal elements considered to be more oxidizable than Fe in the Ellingham diagram, and are also metal elements that can be added to the plating layer. Specific examples of such easily oxidizable metal elements include Ca, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, and Ti.

[0029] Here, in JIS Z3001 (2018), the point where the surface of the base material and the surface of the weld bead intersect is defined as the “edge.” In the weld joint (1) as 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 “edge.” The weld joint (1) according to the present embodiment is designed based on the composition of the slag layer (32) on the surface of the weld metal (31) near this edge T.

[0030] In addition, this “T-shaped” is not defined only for overlapping fillet weld joints as shown in FIG. 1a, but is likewise defined for butt weld joints as shown in FIG. 1b, and T-shaped weld joints as shown in FIG. 1c and FIG. 1d.

[0031] <Regarding the non-heat-affected zone>

[0032] Next, referring to FIG. 2, the configuration of the portion of the weld joint (1) according to the present embodiment that is not affected by heat from welding will be described in detail. FIG. 2 is a schematic drawing illustrating a cross-section of the weld joint (1) perpendicular to the elongation direction of the weld bead portion (30).

[0033] In the following description, the part of the weld joint (1) that is not affected by heat from welding is referred to as the "non-heat-affected zone." In the weld joint (1) as shown in FIG. 2, for example, the region R1 that is sufficiently spaced away from the vicinity of the end T, enclosed by the dashed line in the drawing, corresponds to this non-heat-affected zone. The location of this non-heat-affected zone can be considered, for example, as a region spaced at least 3 mm away from the end T as shown in FIG. 2, in a direction that is orthogonal to the elongation direction of the weld bead (30) (the Y-axis direction in FIG. 2) and also spaced away from the end T (the X-axis direction in FIG. 2).

[0034] FIG. 3 is a schematic drawing illustrating a portion of a cross-section parallel to the plate thickness direction in the non-heat-affected zone R1. The non-heat-affected zone R1 in at least either the first steel plate (10) or the second steel plate (20) comprises, as schematically shown in FIG. 3, 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). Furthermore, in the weld joint (1) according to the present embodiment, the plating layer (103) and the oxide layer (105) may exist on one surface of the base steel (101), but it is more preferable for them to exist on both surfaces of the base steel (101).

[0035] Below, each of these, the iron (101), the plating layer (103), and the oxide layer (105), will be described in detail.

[0036] ≪About Jicheol (101)≫

[0037] In the welding joint (1) according to the present embodiment, the base steel (101) corresponding to the base material of the plated steel sheet is not particularly limited. Depending on the mechanical strength (e.g., tensile strength) required for the welding joint (1), various steel sheets may be used as the base steel (101). Examples of such steel sheets include various types of Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., and high-strength steel containing reinforcing elements such as P, Si, Mn in addition to ultra-low carbon steel.

[0038] In addition, the thickness of the steel (101) is not specifically limited and is appropriately set according to the mechanical properties required for the welded joint (1).

[0039] ≪Regarding the plating layer (103)≫

[0040] The plating layer (103) is provided on at least a portion of the surface of the base iron (101), as schematically shown in FIG. 3, and is preferably provided over the entire surface of the base iron (101). This plating layer (103) is derived from the plating layer of the plated steel sheet that is the material of the weld joint (1).

[0041] Below, the chemical composition of the plating layer (103) will be explained in detail first.

[0042] ◇ Regarding the chemical composition of the plating layer (103)

[0043] The chemical composition of the plating layer (103) according to the present embodiment 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 the remainder consists of Zn and impurities of 5.00 mass% or more. That is, in the chemical composition of the plating layer (103) according to the present embodiment, the content of Al, Mg, Fe, Si, and Ca is within the above ranges, and the sum of these contents is less than 100 mass%, and the remainder consists of Zn and impurities of 5.00 mass% or more.

[0044] The following describes these components and their contents in detail.

[0045] [Al: 1.00 to 80.00 mass%]

[0046] Al is an element necessary to form the main phase (Zn-Al-Mg alloy phase) of the plating layer (103) according to the present embodiment. Al is contained in a predetermined amount to ensure corrosion resistance of 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 as described above cannot be guaranteed. Therefore, in the plating layer (103) according to the present 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 having the Al content within the above range, it becomes possible to ensure corrosion resistance of the non-heat-affected zone.

[0047] Meanwhile, 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, making it easier for the corrosion of the base iron to proceed, and thus the corrosion resistance of the non-heat-affected zone cannot be guaranteed. Therefore, in the plating layer (103) according to the present 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.

[0048] [Mg: 1.00 to 20.00 mass%]

[0049] Mg is an element necessary to form the main phase (Zn-Al-Mg alloy phase) of the plating layer (103) according to the present embodiment. Mg is contained in an amount greater than a predetermined amount to ensure corrosion resistance of the non-heat-affected zone. Therefore, in the plating layer (103) according to the present 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 having the Mg content within the above range, it becomes possible to ensure corrosion resistance of the non-heat-affected zone.

[0050] Meanwhile, if the Mg content in the plating layer (103) exceeds 20.00 mass%, the anode dissolution of the plating layer becomes more likely to proceed when placed in a corrosive environment, so the corrosion resistance of the non-heat-affected zone cannot be guaranteed. Therefore, in the plating layer (103) according to the present embodiment, the Mg content is set to 20.00 mass% or less. The Mg content is preferably 15.00 mass% or less, and more preferably 13.00 mass% or less. By having the Mg content within the above range, it becomes possible to guarantee the corrosion resistance of the non-heat-affected zone.

[0051] [Fe: 0.01 to 15.00 mass%]

[0052] In the plating layer (103), elements constituting the base iron (101) may be incorporated from the base iron (101). In particular, when the plating layer (103) is formed by a molten plating method, elements constituting the base iron (101) are easily incorporated into the plating layer (103) due to mutual diffusion of elements caused by a solid-liquid reaction between the base iron (101) and the plating layer (103). Due to this incorporation of elements, a predetermined amount of Fe is contained in the plating layer (103), and the content is generally 0.01 mass% or more. When the above mutual diffusion is promoted, the adhesion between the base iron (101) and the plating layer (103) is improved. From the perspective of improving the adhesion between the base iron (101) and the plating layer (103), it is preferable that the Fe content in the plating layer (103) be 0.20 mass% or more.

[0053] In addition, Fe may be intentionally added to the plating bath used when manufacturing the plating layer (103) within a range that does not impair the effects of the present invention. 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, so this is undesirable. In this regard, the Fe content in the plating bath is adjusted so that the Fe content in the plating layer (103) becomes 15.00 mass% or less. More preferably, the Fe content in the plating layer (103) is 10.00 mass% or less.

[0054] [Si: 0 to 10.00 mass%]

[0055] Si is an element capable of suppressing the excessive growth of Fe-Al intermetallic compounds formed at the interface between the plating layer and the base metal, and improving the adhesion between the plating layer and the base metal. In order 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%, it forms an excessive amount of high-melting-point intermetallic compounds with Mg and inhibits the formation of an Al-Mg oxide film that has a Zn evaporation suppression effect, making it difficult to suppress Zn evaporation during welding.

[0056] Meanwhile, if the Si content in the plating bath for manufacturing the plating layer (103) is excessively high, the viscosity of the plating bath may increase more than necessary, potentially leading to a decrease in plating operability. Therefore, the Si content in the plating bath is adjusted from the perspective of plating operability, so that the Si content in the plating layer (103) becomes 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.

[0057] [Ca: 0 to 4.00 mass%]

[0058] When Ca is contained in the plating layer (103), it forms intermetallic compounds with Al and Zn. Additionally, when Si is contained along with Ca in the plating layer (103), Ca forms intermetallic compounds with Si. Since these intermetallic compounds have a high melting point and a stable structure, it becomes possible to suppress the formation of blowholes and LME caused by Zn evaporation during welding of the plated steel sheet. This effect of suppressing blowhole formation and LME during welding is achieved by making the Ca content 0.01 mass% or more. More preferably, the Ca content in the plating layer (103) is 0.10 mass% or more.

[0059] Meanwhile, if the Ca content in the plating layer (103) exceeds 4.00 mass%, the corrosion resistance of the non-heat-affected zone is reduced. In this regard, 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.

[0060] In the plating layer (103), the remainder of the above Al, Mg, Fe, Si, and Ca is 5.00 mass% or more of Zn and impurities. Zn is an element necessary to form the main phase (Zn-Al-Mg alloy phase) of the plating layer (103) according to the present embodiment, and is an important element for improving the corrosion resistance of the non-heat-affected zone. Since the effect of improving the corrosion resistance of the non-heat-affected zone is manifested by the Zn content being 5.00 mass% or more, the Zn content is set to 5.00 mass% or more.

[0061] Additionally, the plating layer (103) according to the present embodiment comprises, instead of a portion of the remainder of Zn, optionally 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 It may contain 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, and Sr: 0 to 0.500% in total of 0 to 5.000%. That is, the plating layer (103) according to the present embodiment may further contain, as optional additive elements, at least one of the elements Sb, Pb, Cu, Sn, In, Bi, Ti, Cr, Nb, Zr, Ni, Mn, V, Mo, Ag, Li, La, Ce, B, Y, and Sr within the above range of content and in a total content of 5.000 mass% or less.

[0062] By making the total content of the above optional additive elements 5.000 mass% or less, it becomes possible to enjoy the effects expressed by the addition of each optional additive element, as described in detail below, without damaging each other. The total content of the above optional additive elements is preferably 1.000 mass% or less, and more preferably 0.200 mass% or less.

[0063] The content of each optional added element is described in detail below.

[0064] [Sb: 0 to 0.500 mass%]

[0065] [Pb: 0 to 0.500 mass%]

[0066] [Sr: 0 to 0.500 mass%]

[0067] If 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 the metallic luster. Therefore, from the perspective of improving the aesthetic appearance of the plated steel sheet, it is preferable that at least one of Sb, Pb, and Sr be contained in the plating layer (103). This effect of improving aesthetic appearance is manifested 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 these elements be 0.050 mass% or more, independently.

[0068] Meanwhile, if a plating layer (103) is formed in which any of the contents of Sb, Pb, and Sr exceeds 0.500 mass%, the amount of dross generated in the plating bath used to form the plating layer (103) increases, and thus a plated steel sheet with good plating properties cannot be manufactured. Therefore, the contents of Sb, Pb, and Sr in the plating 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.

[0069] [Cu: 0 to 1.000 mass%]

[0070] [Ti: 0 to 1.000 mass%]

[0071] [Cr: 0 to 1.000 mass%]

[0072] [Nb: 0 to 1.000 mass%]

[0073] [Ni: 0 to 1.000 mass%]

[0074] [Mn: 0 to 1.000 mass%]

[0075] [V: 0 to 1.000 mass%]

[0076] If at least one of Cu, Ti, Cr, Nb, Ni, Mn, and V is contained in the plating layer (103), when welding the plated steel sheet, these elements are introduced into the Al-Fe alloy produced by welding, making it possible to improve the corrosion resistance of the weld bead portion (30) formed. This effect of improving the corrosion resistance of the weld portion is manifested when the content of any one 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 each of these elements be 0.005 mass% or more independently.

[0077] Meanwhile, if a plating layer (103) is formed in which any of the contents of Cu, Ti, Cr, Nb, Ni, Mn, and V exceed 1.000 mass%, these elements are prone to forming various intermetallic compounds in the plating bath for forming the plating layer (103). Consequently, this causes an increase in the viscosity of the plating bath, making it impossible to manufacture 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 1.000 mass% or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, and V are each independently preferably 0.200 mass% or less.

[0078] [Sn: 0 to 1.000 mass%]

[0079] [In: 0 to 1.000 mass%]

[0080] [Bi: 0 to 1.000 mass%]

[0081] Sn, In, and Bi are elements that increase the Mg leaching rate when a plating layer (103) containing Zn, Al, and Mg is placed in a corrosive environment. When the Mg leaching rate increases, Mg ions are supplied to the exposed parts of the base iron, thereby improving corrosion resistance. In this regard, when Sn, In, and Bi are included, the content of Sn, In, and Bi is each independently 0.0050 mass% or more. On the other hand, excessive addition of Sn, In, and Bi may excessively accelerate the Mg leaching rate, potentially lowering the corrosion resistance of the non-heat-affected zone. Since this increase in the Mg leaching rate becomes significant when any of the contents of Sn, In, and Bi exceeds 1.000 mass%, the content of Sn, In, and Bi is each independently 1.000 mass% or less. The content of Sn, In, and Bi is each independently preferably 0.200 mass% or less.

[0082] [Zr: 0 to 1.000 mass%]

[0083] When Zr is included in the plating layer (103), it becomes possible to improve plating workability. This effect of improving plating workability can be achieved when the Zr content is 0.010 mass% or more. Therefore, when Zr is included, it is preferable that the content be 0.010 mass% or more.

[0084] Meanwhile, when forming a plating layer (103) in which the Zr content exceeds 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.

[0085] [Mo: 0 to 1.000 mass%]

[0086] When Mo is included in the plating layer (103), it becomes possible to improve corrosion resistance. This improvement in corrosion resistance can be achieved when the Mo content is 0.010 mass% or more. Therefore, when Mo is included, it is preferable that the content be 0.010 mass% or more.

[0087] Meanwhile, when forming a plating layer (103) in which the Mo content exceeds 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. The Mo content is preferably 0.010 mass% or less.

[0088] [Ag: 0 to 1.000 mass%]

[0089] When Ag is contained in the plating layer (103), it becomes possible to improve plating workability. This effect of improving plating workability can be achieved when the Ag content is 0.010 mass% or more. Therefore, when Ag is included, it is preferable that the content be 0.010 mass% or more.

[0090] Meanwhile, when forming a plating layer (103) in which the Ag content exceeds 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 Ag content is 1.000 mass% or less. The Ag content is preferably 0.050 mass% or less.

[0091] [Li: 0 to 1.000 mass%]

[0092] When Li is included in the plating layer (103), it becomes possible to improve plating workability. This effect of improving plating workability can be achieved when the Li content is 0.010 mass% or more. Therefore, when Li is included, it is preferable that the content be 0.010 mass% or more.

[0093] Meanwhile, when forming a plating layer (103) in which the Li content exceeds 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.

[0094] [La: 0 to 0.500 mass%]

[0095] [Ce: 0 to 0.500 mass%]

[0096] [Y: 0 to 0.500 mass%]

[0097] La, Ce, and Y are elements that exhibit an effect nearly equivalent to Ca and suppress the formation of blowholes during welding. This is due to the fact that the atomic radius of each element is close to the atomic radius of Ca. When these elements are contained in the plating layer (103), they substitute for Ca. Therefore, these elements are detected at the same location as Ca in EDS. Furthermore, when these elements become oxides, the oxides of these elements are detected at the same location as CaO.

[0098] The effect of suppressing blowhole formation during welding can be achieved by making the content of these elements independently 0.010 mass% or more. More preferably, the content of La, Ce, and Y in the plating layer (103) is independently 0.050 mass% or more.

[0099] Meanwhile, if the content of La, Ce, and Y in the plating bath for manufacturing the plating layer (103) is excessively high, the viscosity of the plating bath may increase more than necessary, potentially leading to a decrease in plating operability. Therefore, the content of La, Ce, and Y in the plating bath is adjusted from the perspective of plating operability, so that the content of La, Ce, and Y is each independently 0.500 mass% or less. The content of La, Ce, and Y is preferably each independently 0.100 mass% or less.

[0100] [B: 0 to 0.500 mass%]

[0101] When B is contained in the plating layer (103), it has the effect of further suppressing LME. This is presumed to be because when B is contained in the plating layer (103), it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. Furthermore, it is thought that the presence of B in the plating layer (103) causes B to diffuse from the plating layer (103) to the base iron (101), and through grain boundary strengthening, it has the effect of further suppressing LME in the base iron (101). In addition, since the various intermetallic compounds formed with respect to B have extremely high melting points, it is presumed that they also act to suppress Zn evaporation during welding. These improved effects can be achieved by containing B at a level of 0.050 mass% or more. Therefore, the content of B in the plating layer (103) is more preferably 0.050 mass% or more.

[0102] Meanwhile, in order to include B in the plating layer (103), if B is excessively included in the plating bath, it causes a rapid increase in the plating melting point, which lowers plating operability and makes it impossible to manufacture a plated steel sheet with excellent plating properties. Since this lowering of operability becomes significant when the B content exceeds 0.500 mass%, the B content is set to 0.500 mass% or less. The B content is preferably 0.100 mass% or less.

[0103] [Method for Measuring Chemical Components]

[0104] The chemical composition of the plating layer (103) described above can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). Additionally, when analyzing chemical composition down to 0.1 mass%, ICP-AES is used, and when analyzing trace amounts of chemical composition less than 0.1 mass%, ICP-MS is used. A portion of the plating layer is peeled off by immersing the non-heat-affected zone in a 10% HCl aqueous solution containing an inhibitor for about 1 minute, and a solution is prepared in which the plating layer is dissolved. The obtained solution is analyzed by ICP-AES or ICP-MS to obtain the chemical composition as the overall average of the plating layer.

[0105] ◇ Regarding a more desirable chemical composition of the plating layer (103)

[0106] The plating layer (103) according to the present embodiment has the chemical composition as described above, but a more preferred chemical composition is as follows.

[0107] That is, the plating layer (103) according to the present embodiment contains at least 18.00 to 60.00 mass% of Al and 5.00 to 15.00 mass% of Mg as a chemical composition, and if necessary, additionally contains optional additive elements as described above.

[0108] In addition, the plating layer (103) according to the present 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 if necessary, additionally contains optional additive elements as described above, and further, in the plating layer (103), Mg32 (Al, Zn) 49 It is also more desirable that an image exists.

[0109] Here, Mg 32 (Al, Zn) 49 Plaque, 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 particles of the phase satisfy, in atomic %, 0.5 ≤ [Mg] / ([Zn] + [Al]) ≤ 0.83. That is, it is defined as a crystalline or quasicrystalline phase in which the ratio of the sum of Mg atoms, Zn atoms, and Al atoms, Mg:(Zn+Al), is 3:6 to 5:6. Mg 32 (Al, Zn) 49 It is preferable to measure the chemical composition of the phase using TEM-EDX (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). Mg 32 (Al, Zn) 49 The phase may be detected as both a crystalline phase and a quasicrystalline phase. In the case of the crystalline phase, its crystal structure is identified as Mg from the electron diffraction pattern during TEM observation. 32 (Al, Zn) 49 It is possible to sympathize with the merchant. Also, Mg 32 (Al, Zn) 49 In the case where the phase is a quasicrystal phase, it can be confirmed by taking an electron beam diffraction image using TEM and checking whether a crystal structure with five-fold symmetry is observed in the electron beam diffraction image. A crystal structure with five-fold symmetry can be identified by obtaining an electron beam diffraction image called a Penrose pattern.

[0110] Mg 32 (Al, Zn) 49The phase exhibits sacrificial corrosion resistance to plated steel sheets, thereby suppressing corrosion of the base steel from exposed cut or weld sections and improving corrosion resistance. In addition to this, Mg 32 (Al, Zn) 49 The corrosion resistance of the phase itself is excellent, and Mg 32 (Al, Zn) 49 Since the phase exhibits a slow corrosion rate even in corrosive environments, it suppresses corrosion beneath the coating and also has the effect of improving post-coating corrosion resistance in terms of coating expansion width.

[0111] ◇ Regarding the amount of plating layer (103) attached

[0112] Regarding the amount of plating layer (103) applied as described above, it is not specifically defined, but for example, 15 to 250 g / m² per side of the base iron (101). 2 It is desirable that the amount of plating layer (103) applied is within the range described above, thereby enabling the non-heat-affected zone of the welded joint (1) according to the present embodiment to exhibit sufficient corrosion resistance.

[0113] In addition, the amount of the plating layer (103) attached is measured as follows. First, a sample of 30 mm × 30 mm is cut from a plated steel plate, and the mass of the sample is measured in advance. Also, a tape seal is attached to one side of the sample so that the plating layer on this side does not dissolve in the next process. Then, the sample is immersed in a 10% HCl aqueous solution to which an inhibitor has been added to pickle the plating layer, and the mass of the sample after pickling is measured. It is possible to determine the amount of the plating layer (103) attached per side from the change in mass of the sample before and after pickling.

[0114] ≪Regarding the oxide layer (105)≫

[0115] Next, the oxide layer (105) having a non-heat-affected zone of the weld joint (1) according to the present embodiment will be described in detail.

[0116] As schematically illustrated in FIG. 3, an oxide layer (105) is located on the surface of the plating layer (103) as described above. This oxide layer (105) originates from the oxide layer of the plated steel sheet, which is the material of the weld joint (1).

[0117] This oxide layer (105) is formed when an element that is prone to oxidation among the elements constituting the plating layer (103) reacts with oxygen in the heat treatment atmosphere during the cooling process for solidifying the plating layer during the manufacture of the plated steel sheet.

[0118] As such, the oxide layer (105) is composed mainly of oxides of elements constituting the plating layer (103) as described above, and its chemical composition varies depending on the elements contained in the plating layer (103). The oxide layer (105) is presumed to contain at least 50 mass% of Zn oxide, Mg oxide, and Al oxide in total, and also to be a layer that may contain impurities, etc., such as at least any of the hydroxides of Zn, Mg, and Al, or oxides or hydroxides of other constituent elements in the plating layer (103).

[0119] Here, the oxide layer (105) according to the present embodiment exists in a specific state as described below by undergoing a specific heat treatment process as described in detail below during the manufacture of the plated steel sheet that serves as the material. Below, this state will be described in detail with reference to FIGS. 4 and FIGS. 5. FIGS. 4 is a schematic diagram illustrating a part of a cross-section parallel to the plate thickness direction of the oxide layer. FIGS. 5 is an explanatory diagram for explaining the intensity of the peak in the XPS measurement results.

[0120] By manufacturing the plated steel sheet that serves as the material through a specific heat treatment process as described in detail below, the oxide layer (105) according to this embodiment is formed into a dense film in which the sum of the amount of at least one of the oxide or hydroxide of Al and the amount of at least one of the oxide or hydroxide of Mg is greater than the amount of at least one of the oxide or hydroxide of Zn. This will be explained in more detail below.

[0121] Now, as schematically illustrated in FIG. 4, focus 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 the present 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 intensity of the peaks attributed to the Al-O bond, Mg-O bond, and Zn-O bond, respectively, is 5.0 or higher.

[0122] Here, there is a possibility that contaminants such as oil may be attached to the outermost surface of the oxide layer (105). Therefore, it is desired that the XPS measurement as described above be performed in a state where such contaminants are not present. In this regard, contaminants are removed by performing a treatment such as ultrasonic cleaning in ethanol on the surface of the oxide layer (105), and the surface obtained by such treatment is designated as the "outermost surface of the oxide layer (105)" when performing the XPS measurement as described above.

[0123] Then, the oxide layer (105) is removed by Ar ion etching to a depth of 5 nm from the outermost surface obtained as described above, and the surface of the obtained oxide layer (105) is measured by XPS. Here, the measurement conditions for XPS can be, for example, as follows.

[0124] X-ray source: mono-Al Kα(1486.6 eV)

[0125] X-ray diameter: 50 to 200 µm

[0126] Measurement area: 100 to 700㎛ × 100 to 700㎛

[0127] Vacuum level: 1×10⁻⁶ -10 Up to 1×10 -11 torr(1 torr is 133.32 Pa.)

[0128] Acceleration voltage: 1 to 10 kV

[0129] In the present embodiment, attention is focused on the peaks attributed to Al-O bonds, Mg-O bonds, and Zn-O bonds, respectively, in the obtained XPS measurement results. Such bonds are characteristic of at least one of the oxides or hydroxides of Al, Mg, and Zn. It can be assumed that the intensity of the peaks attributed to these bonds has a positive correlation with the amount of at least one of the oxides or hydroxides of Al, Mg, and Zn.

[0130] Here, the peak attributed to the Al-O bond is the peak observed within the range of 72 to 76 eV in the XPS spectrum focused on Al 2p3 / 2. The peak attributed to the Mg-O bond is the peak observed within the range of 48 to 52 eV in the XPS spectrum focused on Mg 2p3 / 2. The peak attributed to the Zn-O bond is the peak observed within the range of 1018 to 1024 eV in the XPS spectrum focused on Zn 2p3 / 2.

[0131] In addition, the intensity of the peak attributed to each combination is, in the XPS spectrum schematically shown in FIG. 5, the intensity I of the peak after considering the baseline of the peak being focused. p Baseline intensity I from b That which is excluding (i.e., "I p -I b 」)

[0132] A more detailed method for calculating the strength ratio is as follows.

[0133] That is, XPS is measured at any location on the surface corresponding to a depth of 5 nm from the outermost surface obtained as described above (the surface of “Position B” in FIG. 4), and the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] is calculated. This measurement and calculation process is performed at any 5 locations on the surface corresponding to “Position B”, and the average value of the 5 obtained intensity ratios is taken as the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] in the oxide layer (105) according to the present embodiment.

[0134] In the oxide layer (105) according to the present embodiment, a dense film is formed such that the strength ratio value is 5.0 or higher, thereby suppressing the evaporation of Zn during welding and suppressing the formation of blowholes caused by Zn evaporation. If the strength ratio value is less than 5.0, there is a possibility that the density required for the oxide layer (105) will be insufficient. The strength ratio value is more preferably 10.0 or higher. Meanwhile, the upper limit of the strength ratio ([Al-O]+[Mg-O]) / [Zn-O] is not specifically defined, but approximately 100.0 is the practical upper limit.

[0135] The thickness (more specifically, average thickness) of the oxide layer (105) described above is not specifically defined, but, for example, it is preferably about 0.05 to 2.00 μm per side of the steel (101). By having the thickness of the oxide layer (105) within the above range, the non-heat-affected zone according to this embodiment can sufficiently suppress the formation of blowholes caused by Zn evaporation during welding. In addition, the oxide layer (105) with the above thickness is realized by undergoing a heat treatment process described in detail below while controlling the passing speed of the steel plate to an appropriate range during the manufacture of the plated steel plate that serves as the material.

[0136] In addition, the thickness of this oxide layer (105) can be measured using XPS. XPS measurements are performed in the depth direction at a pitch of 1 to 3 nm from the surface of the plated steel sheet, and the depth until the maximum oxygen intensity becomes 1 / 20 of the maximum intensity of the outermost surface is defined as the thickness of the oxide layer. In addition, regarding the XPS measurement conditions, the same conditions as above may be used.

[0137] Hereinafter, with reference to FIGS. 2 to 5, the non-heat-affected zone in the welded joint (1) according to the present embodiment has been described in detail.

[0138] In addition, the non-heat-affected portion of the welded joint (1) according to the present embodiment may have one or more layers of various coatings on the oxide layer (105) above. Examples of such coatings include a chromate coating, a phosphate coating, a chromate-free coating, an organic resin coating, etc.

[0139] <Regarding the weld bead>

[0140] Next, referring to FIG. 2, the characteristics of the weld bead portion in the weld joint (1) according to the present embodiment will be explained.

[0141] In the weld joint (1) according to the present embodiment, a plated steel plate having the plating layer (103) and oxide layer (105) as described above is used as the material for at least one of the first steel plate (10) or the second steel plate (20). As a result, a slag layer (32) having the following components is formed, and thereby, the formation of Si-based slag on the surface of the weld metal (31) is suppressed.

[0142] More specifically, the formation of Si-based slag is suppressed by forming a slag layer (32) containing Al and Mg (Al-Mg containing slag) as the slag layer (32) formed on the surface of the weld metal (31). Although the Al-Mg containing slag does not have conductivity like the Si-based slag, when the 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 reduced compared to the case where the Si-based slag is formed. In other words, the slag can be refined. By doing so, the electrodeposition coating properties of the weld bead portion of the weld joint (1) according to the present embodiment can be improved.

[0143] In order to produce the above effect, attention is paid to the chemical composition of the slag layer (32). In the welding bead portion (30) according to the present embodiment, the chemical composition of the slag layer (32) at position A contains Al: 1.0 to 45.0% and Mg: 1.0 to 30.0% when expressed in mass% excluding oxygen, and the remainder consists of Fe, easily oxidizable metal elements and impurities. Here, "easily oxidizable metal elements" refers to metal elements considered to be more oxidizable than Fe in the Ellingham diagram, and are also metal elements that can be added to the plating layer (103). Specific examples of such easily oxidizable metal elements include Ca, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, and Ti.

[0144] The following describes these components and their contents in detail.

[0145] [Al: 1.0 to 45.0 mass%]

[0146] If the Al content is less than 1.0 mass%, it means that Al has evaporated excessively during welding. In this case, the Al-Mg containing slag formed on the surface of the weld metal (31) is insufficient, and it is 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. By having the Al content within this range, the formation of Si-based slag is suppressed by the Al-Mg containing slag, thereby improving the electrodeposition coating properties of the weld bead (30).

[0147] Meanwhile, if the Al content exceeds 45.0 mass%, there may be a shortage of Mg content consumed to form Al-Mg containing slag. As a result, it becomes difficult to form Al-Mg containing slag on the surface of the weld metal (31), and it becomes difficult to suppress the formation of Si-based slag. Therefore, it is required that the Al content of the slag layer (32) at position A be 45.0 mass% or less. The Al content is preferably 43.0 mass% or less, and more preferably 41.0 mass% or less. By having the Al content within this range, the formation of Si-based slag is suppressed by the Al-Mg containing slag, thereby improving the electrodeposition coating properties of the weld bead (30).

[0148] [Mg: 1.0 to 30.0 mass%]

[0149] If the Mg content is less than 1.0 mass%, it means that Mg has evaporated excessively during welding. In this case, the Al-Mg containing slag formed on the surface of the weld metal (31) is insufficient, and it is difficult to suppress the formation of Si-based slag. Therefore, it is required that the Mg content of the slag layer (32) at position A be 1.0 mass% or more. The Mg content is preferably 7.0 mass% or more, and more preferably 8.0 mass% or more. By having the Mg content within this range, the formation of Si-based slag is suppressed by the Al-Mg containing slag, thereby improving the electrodeposition coating properties of the weld bead (30).

[0150] Meanwhile, if the Mg content exceeds 30.0 mass%, there may be a shortage of Al consumed to form the Al-Mg containing slag. As a result, it becomes difficult to form the Al-Mg containing slag on the surface of the weld metal (31), and it becomes difficult to suppress the formation of the Si-based slag. Therefore, the Mg content of the slag layer (32) at position A is required to be 30.0 mass% or less.

[0151] When determining the composition of the slag layer (32), cross-sectional SEM-EPMA (Electron Probe Micro Analyzer) measurement is performed at a position (position A in Fig. 2) that is orthogonal to the elongation direction (Y-axis direction in Fig. 2) of the weld bead portion (30) and also directed inward (X-axis direction in Fig. 2) from the end T as shown in Fig. 2. At this time, the concentration of each element constituting the slag layer (32) can be measured by the EPMA point composition analysis.

[0152] In addition, although the chemical composition of the slag layer (32) is defined only for the chemical composition at position A as described above, if Al-Mg containing slag is formed at position A, it is presumed that Al-Mg containing slag is formed not only at position A but also in the vicinity of position A. Therefore, by satisfying the chemical composition of the slag layer (32) at position A, the formation of Si-based slag in the vicinity of position A is suppressed, making it possible to improve the electrodeposition coating properties of the weld bead portion (30).

[0153] Additionally, 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. Accordingly, the weld joint (1) according to the present embodiment, which has excellent electrodeposition coating properties, can be manufactured by using a plated steel plate having such a plating layer (103) as the material for at least one of the first steel plate (10) or the second steel plate (20).

[0154] In addition, when using a filler material when welding the first steel plate (10) and the second steel plate (20), a suitable example of the filler material is solid wire. An example of a suitable chemical composition of a 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%, and the remainder is Fe and impurities.

[0155] Hereinafter, with reference to FIG. 2, the characteristics of the weld bead portion in the weld joint (1) according to the present embodiment have been described in detail.

[0156] The welded joint (1) according to the present embodiment has been described in detail above. The welded joint (1) according to the present embodiment as described above can be suitably used, for example, as a suspension component of an automobile.

[0157] (Regarding the manufacturing method of the galvanized steel sheet that serves as the material)

[0158] Next, an example of a method for manufacturing a plated steel sheet that serves as the material for a welded joint (1), as described above, will be explained.

[0159] The plated steel plate that serves as the material for the welded joint (1) according to the present embodiment is manufactured by using a steel plate made of the above-mentioned base iron (101) as a base material and forming a plating layer (103) and an oxide layer (105) on the surface of the base iron (101).

[0160] Here, in addition to the molten plating method, the thermal spraying method, cold spraying method, sputtering method, deposition method, electroplating method, etc., can be applied to form the plating layer (103). However, in order to form a plating layer of a thickness generally used in automobiles, the molten plating method is the most preferable in terms of cost.

[0161] After that, an oxide layer (105) is formed on the surface of the plating layer (103) by performing a specific heat treatment process as described below on the obtained plated steel sheet (plated steel sheet consisting of a base iron (101) and a plating layer (103). By doing so, a plated steel sheet used as a material for a welded joint (1) according to the present embodiment can be manufactured.

[0162] Hereinafter, an example of a manufacturing method for obtaining a plated steel sheet according to the present embodiment using a molten plating method will be described in detail.

[0163] In the manufacturing process of such plated steel sheets, first, a steel sheet made of a base iron (101) used as a base material is rolled by the Sendzimir method to a desired thickness, then wound into a coil, and installed in a molten plating line.

[0164] In the hot-dip galvanizing line, steel sheets are continuously passed through while being unwound from the coil. At that time, the steel sheets are heated and reduced to 800°C in an N2-5% H2 gas atmosphere under an environment where oxidation is unlikely to occur, for example, with an oxygen concentration of 20 ppm or less, by an annealing facility provided above the line. Afterwards, they are air-cooled with N2 gas to approximately the bath temperature of the subsequent plating bath + 20°C and immersed in the plating bath.

[0165] Here, a plating alloy in a molten state having the chemical composition as described above is prepared in the plating bath. The temperature of the plating bath is set above the melting point of the plating alloy (e.g., about 460 to 600°C). When preparing the material for the plating alloy, it is preferable to use a pure metal (purity of 99% or more) as the alloy material. First, a predetermined amount of alloy metal is mixed to obtain the composition of the plating layer as described above, and then completely melted into an alloy using a high-frequency induction furnace or an arc furnace under vacuum or inert gas exchange conditions. Furthermore, the alloy mixed with the predetermined components (composition of the plating layer above) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

[0166] Furthermore, in the production of the plating alloys described above, there are no specific restrictions on the use of pure metals; existing Zn, Mg, and Al alloys may be melted and used. In this case, there is no problem as long as only alloys of a specified composition with low impurity levels are used.

[0167] After immersing the steel plate in the plating bath as described above, it is lifted at a predetermined speed. At this time, the amount of plating applied is controlled, for example, by N2 wiping gas, so that the plating layer (103) formed becomes a desired thickness. Here, regarding conditions other than the bath temperature, general plating operation conditions can be applied, and no special equipment or conditions are required.

[0168] Next, the following first cooling process and second cooling process are performed on the molten plating alloy located on the steel plate to form 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 cooling process and the second cooling process will be described in detail below.

[0169] The first cooling process is a cooling process performed when the temperature of the plating alloy is within a range of 250°C or higher below the bath temperature, and the plated steel sheet within such a temperature range is rapidly cooled at an average cooling rate of 10°C / second or higher under an atmosphere of dew point -20°C or lower. In addition, when a molten plating method is adopted in the plating process, this first cooling process is performed immediately after the steel sheet leaves the plating bath. As a result, the plating alloy located on the surface of the steel sheet solidifies, and a plating layer is formed.

[0170] After that, when the temperature of the plating alloy (plating layer) is within the range of less than 250°C and greater than 50°C, a second cooling process is performed. This second cooling process is a process of slowly cooling a plated steel sheet within the temperature range of less than 250°C and greater than 50°C at an average cooling rate of less than 10°C / second under an atmosphere with a dew point of 0°C or higher. By doing so, the state of the oxide formed on the surface of the plating layer is controlled to form a desired oxide layer.

[0171] As described above, by undergoing a two-stage cooling process in which rapid cooling is performed in a temperature range of 250°C or higher below the bath temperature, and slow cooling is performed in a temperature range of 50°C or lower below 250°C, a dense oxide layer (105) that satisfies specific conditions in the XPS measurement results is formed on the surface of the plating layer (103).

[0172] Here, it is preferable that the interval between ending the first cooling process and starting the second cooling process be within 3 seconds, and it is preferable to start the second cooling process immediately after ending the first cooling process. If the interval between ending the first cooling process and starting the second cooling process exceeds 3 seconds, an unintended cooling process occurs, and the desired oxide layer (105) cannot be realized.

[0173] Here, in the first cooling process, the lower limit of the dew point is not specifically defined, but for example, about -90°C is a practical lower limit. Also, the average cooling rate is more preferably 40°C / second or higher. Also, the upper limit of the average cooling rate is not specifically defined, but for example, about 90°C / second is a practical upper limit.

[0174] In addition, in the second cooling process above, the upper limit of the dew point is not specifically defined, but for example, about 20°C is a practical upper limit. In addition, the average cooling rate is more preferably 4°C / second or less.

[0175] In addition, if either the first cooling process or the second cooling process described above is not performed, the desired oxide layer (105) cannot be realized. By performing both the first cooling process and the second cooling process described above, the oxide layer (105) according to the present embodiment can be realized.

[0176] In addition, if an alloying heat treatment process (e.g., a heat treatment process involving heating to a target temperature of about 480 to 550°C), which is commonly performed in the manufacture of alloyed hot-dip galvanized steel sheets, is carried out after the second cooling process described above, the oxide formation state controlled by the first and second cooling processes is disrupted, and as a result, the oxide grows excessively, making it impossible to obtain the Zn evaporation suppression effect conceived in this embodiment. From this perspective, it is important not to carry out the heat treatment process after the second cooling process.

[0177] Here, for the cooling treatment described above, generally known methods such as N2 gas cooling, mist cooling, and submersion can be applied. In addition, as the cooling gas, gases with high heat dissipation effects, such as He gas and hydrogen gas, may be used in addition to N2 gas.

[0178] Furthermore, as a method for measuring the temperature of the plating layer, a contact-type thermocouple (K-type) can be used, for example. By installing a contact-type thermocouple on the base steel plate, the average temperature of the entire plating layer can be monitored at all times. Additionally, by mechanically controlling various speeds and thicknesses and unifying various operating conditions, such as the steel plate preheating temperature and the molten plating bath temperature, it becomes possible to monitor the temperature of the entire plating layer at that specific point in time under these manufacturing conditions with near accuracy. This enables precise control of the cooling treatment in the first and second cooling processes. Furthermore, although not as accurate as the contact type, the surface temperature of the plating layer may be measured using a non-contact radiation thermometer.

[0179] In addition, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be determined through a simulation that performs thermal 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 molten 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 by the plating layer. Subsequently, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can be determined using the obtained results. By doing so, it becomes possible to estimate the average temperature of the entire plating layer at that point in 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 treatment in the first and second cooling processes.

[0180] Above, an example of a method for manufacturing a plated steel sheet according to the present embodiment has been described in detail.

[0181] In addition, in the method for manufacturing a plated steel sheet according to the present embodiment, after the second cooling process above, a treatment to form one or more layers of various films may be further performed. Examples of such treatments include chromate treatment, phosphate treatment, chromate-free treatment, and organic resin film formation treatment.

[0182] Chromate treatments include electrolytic chromate treatment, which forms a chromate film by electrolysis; reactive chromate treatment, which forms a film by utilizing a reaction with the material and then washes off the excess treatment solution; and coating chromate treatment, which forms a film by applying a treatment solution and drying it without rinsing; any of these chromate treatments may be adopted.

[0183] Examples of electrolytic chromate treatment include, for instance, electrolytic chromate treatment using chromic acid, silica sol, resin (phosphate resin, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine modified epoxy resin, etc.), and hard silica.

[0184] Examples of phosphate treatments include zinc phosphate treatment, zinc calcium phosphate treatment, manganese phosphate treatment, etc.

[0185] Chromate-free treatment is particularly suitable because it does not impose a burden on the environment. Such chromate-free treatments include electrolytic chromate-free treatment, which forms a chromate-free film by electrolysis; reactive chromate-free treatment, which forms a film by reacting with the material and then washes off the excess treatment solution; and coating chromate-free treatment, which forms a film by applying a treatment solution and drying it without rinsing; any of these chromate-free treatments may be adopted.

[0186] Furthermore, the organic resin used for the organic resin film formation treatment is not limited to a specific resin; for example, various resins such as polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified versions of these resins may be used. Here, a modified version refers to a resin in which a reactive functional group contained in the structure of these resins is reacted with another compound (e.g., a monomer or a crosslinking agent) that contains a functional group capable of reacting with such functional group.

[0187] As for the organic resin, one of the above-mentioned types may be used alone, or two or more types of organic resins (that are not modified) may be mixed and used. In addition, one or two or more types of organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be mixed and used. In addition, an organic resin that has been made water-based by dissolving or dispersing in water may be used. Furthermore, various coloring pigments or anti-corrosion pigments may be contained in such organic resin films.

[0188] (Regarding the manufacturing method of welded joints)

[0189] The welded joint according to the present embodiment is manufactured by using a plated steel plate manufactured as described above as the material for at least one of the first steel plate or the second steel plate when manufacturing the welded joint, arranging the first steel plate and the second steel plate to form a shape required for the welded joint, and welding the first steel plate and the second steel plate.

[0190] Here, for welding the first steel plate and the second steel plate, it is possible to use an arc welding method or a laser welding method. At this time, for each welding method, by performing welding under welding conditions as described below, it becomes possible to achieve the state of the weld bead portion as described above.

[0191] More specifically, when manufacturing a welded joint by arc welding, the first steel plate and the second steel plate may be welded under welding conditions such as those below, for example.

[0192] Welding current: 250A, Welding voltage: 26.4V, Welding speed: 100cm / min

[0193] Welding gas: 20% CO2+Ar, Gas flow rate: 20L / min

[0194] Welding wire: YGW16 Manufactured by Nittetsu Yosetsu Kogyo Co., Ltd. φ1.2mm

[0195] (C: 0.1 mass%, Si: 0.80 mass%, Mn: 1.5 mass%, P: 0.015 mass%, S: 0.008 mass%, Cu: 0.36 mass%)

[0196] Welding torch bevel angle: 45°

[0197] In addition, when manufacturing a welded joint by laser welding, the first steel plate and the second steel plate may be welded under welding conditions such as those below, for example.

[0198] Output: 7kW, Welding Speed: 400cm / min, Forward / Reverse Angle: 0°

[0199] Above, an example of a method for manufacturing a welded joint according to the present embodiment has been described.

[0200] Furthermore, as a welding method, it is conceivable to use various resistance welding methods such as high-frequency resistance welding or butt resistance welding; however, if such welding methods are used, it is not possible to realize the state of the weld bead portion as envisioned in this embodiment.

[0201] Examples

[0202] Hereinafter, the welded joint according to the present invention will be described in detail while presenting examples and comparative examples. Furthermore, the examples presented 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.

[0203] In the examples and comparative examples shown below, a hot-rolled steel sheet with a thickness of 2.6 mm (0.05 mass% C-0.2 mass% Si-1.3 mass% Mn-0.12 mass% Ti-0.07 mass% Al, manufactured by Nippon Seitetsu Co., Ltd.) was used as the base material. This hot-rolled steel sheet was cut into pieces measuring 100 mm × 200 mm to be used as test specimens.

[0204] A plating bath for realizing a plating layer with the composition shown in Table 1 below was prepared, installed in a batch-type molten plating test apparatus of our company, and plating was performed on the test specimen. Here, the temperature of the test specimen was measured using a thermocouple spot-welded to the center of the test specimen. In addition, regarding the test specimen to be immersed in the plating bath, prior to immersion in the plating bath, the surface of the plating plate was subjected to a heat reduction treatment at 800°C in a furnace with an oxygen concentration of 20 ppm or less and in an N2-5% H2 gas atmosphere. After the heat reduction treatment, the test specimen was air-cooled with N2 gas, and after the temperature of the test specimen reached the bath temperature + 20°C, the test specimen was immersed in the plating bath of the molten plating test apparatus for about 3 seconds.

[0205] After immersion in the plating bath, the test specimen was lifted at a lifting speed of 20 to 200 mm / sec. During lifting, N2 wiping gas was used to control the amount of plating deposited to the desired level. In the following examples and comparative examples, the amount of the plating layer deposited after drying per side of the test specimen was 15 to 250 g / m² 2 The amount of plating deposited was controlled to such an extent. After lifting the test specimen from the plating bath, the test specimen was cooled from the plating bath temperature to room temperature under the conditions shown in Table 1 below. In the examples and comparative examples shown below, the second cooling process was started immediately after the end of the first cooling process (i.e., the interval from the end of the first cooling process to the start of the second cooling process was set to 0.2 seconds or less).

[0206] Here, a steel plate was cut to a size of 30 mm × 30 mm from the test specimen plated as described above, and the plated steel plate was immersed in a 10% HCl aqueous solution containing an inhibitor to pickle and peel off the plating layer, and then the composition of the plating layer was measured by analyzing the elements eluted into the aqueous solution using ICP.

[0207] In addition, regarding the obtained plating layer, electron diffraction patterns were captured by TEM, and based on whether a crystal structure with 5-fold symmetry was observed in the electron diffraction patterns, Mg 32 (Al, Zn) 49 I checked whether the top was present or not.

[0208] In addition, for the obtained oxide layer, the XPS spectrum was measured based on the above method, and the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] was calculated. The obtained intensity ratio was evaluated based on the following criteria.

[0209] ≪Evaluation Criteria≫

[0210] Rating "A": Intensity ratio value is 10.0 or higher

[0211] 「B」: Intensity ratio value is 5.0 or higher and less than 10.0

[0212] 「C」: Intensity ratio value less than 5.0

[0213] In addition, a steel plate cut to a size of 150 mm × 50 mm from the obtained test specimen was designated as the first steel plate, and a steel plate cut to a size of 150 mm × 30 mm was designated as the second steel plate. The long sides of these steel plates were overlapped and welded by arc welding or laser welding (overlap fillet welding) to form a welded joint.

[0214] Here, the welding conditions for arc welding are as follows.

[0215] Welding Mode: Pulse Mug Welding

[0216] Welding current: 230A, Welding voltage: 23.4V, Welding speed: 100cm / min

[0217] Welding gas: 20% CO2+Ar, Gas flow rate: 20L / min

[0218] Welding wire: YM-TX, manufactured by Nittetsu Yosetsu Kogyo Co., Ltd., φ1.2mm

[0219] (C: 0.11 mass%, Si: 0.01 mass%, Mn: 1.21 mass%, P: 0.006 mass%, S: 0.007 mass%, Cu: 0.22 mass%)

[0220] Welding torch inclination: 60°

[0221] Overlap value: 10mm

[0222] Steel plate size: Top side (1st steel plate) 150×50mm, Bottom side (2nd steel plate) 150×30mm

[0223] Plate gap: 0mm

[0224] Wire protrusion length: 15mm

[0225] In addition, the welding conditions for laser welding are as follows.

[0226] Output: 7kW, Welding Speed: 400cm / min, Forward / Reverse Angle: 0°

[0227] Steel plate size: Top side (1st steel plate) 150×50mm, Bottom side (2nd steel plate) 150×30mm

[0228] Overlap value: 50mm

[0229] Plate gap: 0mm

[0230] <Electroplatability of the weld bead>

[0231] The welded joint obtained as described above was subjected to automotive phosphoric acid treatment (Zn phosphoric acid treatment, SD5350 system: Nippon Paint & Industrial Coding Co., Ltd. standard) and electrodeposition coating (PN110 Powernics Gray: Nippon Paint & Industrial Coding Co., Ltd. standard). At this time, the electrodeposition film thickness was set to 20 μm. For the sample after electrodeposition coating, as shown in Fig. 6, the maximum length L0 in the elongation direction of the weld bead and the length L of the electrodeposition defect area that occurred on the sample n(n: an integer greater than or equal to 1) was measured. In addition, the electrodeposition defect area is outwardly reddish-brown to black and exhibits a coloration different from that of the electrodeposited coating area, making it possible to identify the electrodeposition defect area from its appearance. That is, the length in the direction parallel to the elongation direction (welding direction) of the weld bead area at each electrodeposition defect area identified from its appearance is the length L of the electrodeposition defect area. n am.

[0232] And, the total length ΣL of the electrodeposition defect area n The ratio of the maximum length L0 of the weld bead (ΣL n Electroplating performance was evaluated by calculating / L0). The evaluation criteria are as follows. The obtained results are integrated and presented in Table 1 below.

[0233] ≪Evaluation Criteria≫

[0234] Rating 「AAA」: ΣL n / L0 value is 0.3 or less

[0235] 「AA」: ΣL n / L0 value greater than 0.3 and less than or equal to 0.5

[0236] 「A」: ΣL n / L0 value greater than 0.5 and less than or equal to 0.7

[0237] 「B」: ΣL n / L0 value greater than 0.7

[0238]

[0239] As is evident from Table 1 above, in the example corresponding to the embodiment of the present invention, excellent electrodeposition coating performance is exhibited in the weld bead portion, whereas in the example corresponding to the comparative example of the present invention, sufficient performance is not exhibited regarding the electrodeposition coating performance of the weld bead portion.

[0240] Although suitable 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 to those skilled in the art that various modifications or variations can be made within the scope of the technical concept described in the claims, and these are also naturally understood to fall within the technical scope of the present invention. Explanation of the symbols

[0241] 1: Welded joint 10: First steel plate 20: Second steel plate 30: Weld bead 31: Welding metal 32: Slag layer 101: Jicheol 103: Plating layer 105: Oxide layer

Claims

Claim 1 A first steel plate and a second steel plate are welded together by arc welding or laser welding, and the first steel plate and the second steel plate have a weld bead portion formed by the arc welding or laser welding, wherein in the first steel plate and the second steel plate, when a portion not affected by heat from the welding is called a non-heat-affected zone, at least one of the first steel plate or the second steel plate has a plating layer located on at least a portion of the surface of the base steel in the non-heat-affected zone and an oxide layer located on said plating layer, and 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 optionally, 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 A welded joint comprising 0 to 5.000% total of 0 to 0.500%, 0 to 0.500% Sr, and the remainder being 5.00 mass% or more of Zn and impurities, wherein the weld bead portion comprises a weld metal and a slag layer formed on a portion of the surface of the weld metal, wherein the slag layer contains 1.0 to 45.0% Al and 1.0 to 30.0% Mg in mass% when oxygen is excluded from the chemical composition of the slag layer, and the remainder being Fe, easily oxidizable metal elements, and impurities. Claim 2 A welded joint according to claim 1, wherein the slag layer contains, in mass%, at least Al: 15.0 to 45.0% and Mg: 7.0 to 30.0%. Claim 3 A welded joint according to claim 1 or 2, wherein the value of the intensity ratio ([Al-O]+[Mg-O]) / [Zn-O] calculated from the intensity of the peaks attributed to Al-O bonds, Mg-O bonds, and Zn-O bonds, respectively, when the position at a depth of 5 nm from the outermost surface of the oxide layer is observed by X-ray photoelectron spectroscopy (XPS), is 5.0 or higher. Claim 4 A welded joint according to paragraph 3, wherein the value of the strength ratio ([Al-O]+[Mg-O]) / [Zn-O] is 10.0 or greater. Claim 5 A welded joint according to claim 1 or 2, wherein the plating layer in the non-heat-affected portion contains at least Al: 18.00 to 60.00 mass% and Mg: 5.00 to 15.00 mass%. Claim 6 In claim 1 or 2, the plating layer in the non-heat-affected portion contains at least Al: 35.00 to 60.00 mass% and Mg: 7.00 to 15.00 mass%, and furthermore, in the plating layer, Mg 32 (Al, Zn) 49 A phase exists, and the above Mg 32 (Al, Zn) 49 A welded joint in which the Mg content [Mg], Zn content [Zn], and Al content [Al] (each unit: atomic %) satisfy the relationship 0.50 ≤ [Mg] / ([Zn] + [Al]) ≤ 0.83.

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