Welded joints

Abstract

To further improve the corrosion resistance of a heat-affected zone on a back surface of a weld joint.SOLUTION: A weld joint according to the present invention is a weld joint in which a first steel plate and a second steel plate are welded by arc welding, the weld joint including the first steel plate, the second steel plate, a weld bead portion formed by the arc welding, and a heat-affected zone located around the weld bead portion, wherein when an area where there is no heat influence by welding in the first steel plate and the second steel plate is a non-heat-affected zone, at least one of the first steel plate and the second steel plate has, in the non-heat affected zone, a plating layer located at least partially on a surface of the base iron and an oxide layer located above the plating layer; the plating layer has a predetermined chemical composition; and when the side of the weld joint where the weld bead portion is not present is a back surface, the thickness of the Fe scale located above the heat-affected zone on the back side of the weld joint is 0 to 5.0 μm.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[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] A particular problem that arises when manufacturing welded joints by welding galvanized steel sheets is a decrease in corrosion resistance due to blowholes formed near the "toe" defined in JIS Z3001 (2018) as a result of Zn evaporation from the plating during welding.

[0004] Various proposals have been made to solve the problem of blowhole formation as described above. For example, Patent Document 1 listed below proposes a plated steel material that includes a steel sheet and a coating layer that is disposed on the surface of the steel sheet and includes a Zn-Al-Mg alloy layer, in which, in a 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 coating layer has a predetermined chemical composition. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 139620 Summary of the Invention [Problem to be solved by the invention]

[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 by the present inventors, they have found that there is still room for improvement in the technology proposed in Patent Document 1, and that when a portion other than the vicinity of the toe, for example, the surface opposite to the side on which the weld bead is present in a welded joint, is designated as the back surface, further improvement can be expected in the corrosion resistance of the heat-affected zone present on the back surface.

[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 corrosion resistance of the heat-affected zone present on the back surface of the welded joint. [Means for solving the problem]

[0008] As a result of intensive research conducted by the present inventors to solve the above problems, they have concluded that the decrease in corrosion resistance of the heat-affected zone on the back side of a welded joint due to welding is caused by the Zn in the plating evaporating or oxidizing during welding, resulting in no plating components remaining, and as a result, Fe scale forms on the surface of the heat-affected zone on the back side. Therefore, they have found that if the formation of Fe scale on the surface of the heat-affected zone on the back side can be suppressed, the corrosion resistance of the heat-affected zone on the back side can be further improved.

[0009] Based on this finding, the present inventors conducted further studies and found that by studying improvements to the plated steel sheet used as a raw material, it is possible to allow the plating components to remain even after arc welding, thereby further improving the corrosion resistance of the heat-affected zone on the back surface. The gist of the present invention, which was completed based on these findings, is as follows.

[0010] (1) A welded joint formed by arc welding between a first steel plate and a second steel plate, the weld joint comprising the first steel plate and the second steel plate, a weld bead formed by the arc welding, and a heat-affected zone located around the weld bead, wherein when a portion of the first steel plate and the second steel plate that is not thermally affected by the welding is defined as a non-heat-affected zone, at least one of the first steel plate or the second steel plate has a coating layer located on at least a part of a surface of a base steel, and an oxide layer located on the coating layer, and the coating layer contains, 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%, and optionally Sb: 0 to 0.50%. %, Pb:0~0.50%, Cu:0~1.00%, Sn:0~1.00%, In:0~1.00%, Bi:0~1.00%, Ti:0~1.00%, Cr:0~1.00%, Nb:0~1 .00%, Zr:0~1.00%, Ni:0~1.00%, Mn:0~1.00%, V:0~1.00%, Mo:0~1.00%, Ag:0~1.00%, Li:0~1.00%, La:0 0.50%, Ce: 0-0.50%, B: 0-0.50%, Y: 0-0.50%, Sr: 0-0.50%, in a total content of 0-5.00%, with the balance being 5.00 mass% or more of Zn and impurities, and when the surface of the weld joint on which the weld bead portion does not exist is defined as the back surface, the thickness of Fe scale located on the heat-affected zone on the back surface is 0-5.0 μm. (2) The welded joint according to (1), wherein the thickness of the Fe scale is 0 to 1.0 μm. (3) The welded joint according to (1) or (2), wherein, when the oxide layer is observed at a depth of 5 nm from the outermost surface by X-ray photoelectron spectroscopy (XPS), 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. (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 contains at least 18.00 to 60.00 mass % of Al and 5.00 to 15.00 mass % of Mg. (6) The plating layer contains at least 35.00 to 60.00 mass% of Al and 7.00 to 15.00 mass% of Mg, and the plating layer contains 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], Zn content [Zn], and Al content [Al] (each unit: atomic %) in the phase satisfy the relationship 0.50≦[Mg] / ([Zn]+[Al])≦0.83. [Effects of the Invention]

[0011] As described above, according to the present invention, it is possible to further improve the corrosion resistance of the heat-affected zone present on the back surface of the welded joint. [Brief explanation of the drawings]

[0012] [Figure 1A] 1 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to an embodiment of the present invention. FIG. [Figure 1B] FIG. 10 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. [Figure 1C] FIG. 10 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. [Figure 2] 1B is an explanatory view for explaining the welded joint according to the embodiment shown in FIG. 1A. FIG. [Figure 3] 3 is an explanatory diagram for explaining a welded joint according to the same embodiment. FIG. [Figure 4] 3 is an explanatory diagram for explaining a welded joint according to the same embodiment. FIG. [Figure 5] FIG. 2 is an explanatory diagram for explaining peak intensities in the results of XPS measurement. DETAILED DESCRIPTION OF THE INVENTION

[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] (About welded joints) 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 given using the coordinate system shown in Fig. 1A, which illustrates an example of a welded joint formed by arc welding two steel plates.

[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, a weld bead 30, and a heat-affected zone 40.

[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 region formed by arc welding, and interdiffusion of constituent elements occurs between the welding wire, which is used as needed during welding, and the first steel plate 10 and second steel plate 20. The weld bead 30 is formed by the oxidation of these diffusible elements. Therefore, in FIG. 1A, the bonding interface between the weld bead 30 and the first steel plate 10 or the second steel plate 20 is shown as a smooth curve or line for convenience of illustration, but the actual bonding interface has a complex curved surface. The weld bead 30 extends along the Y-axis direction in the figure, and the first steel plate 10 and the second steel plate 20 are bonded by the weld bead 30.

[0019] The components constituting the weld bead portion 30 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 components that cover all possibilities. However, the weld bead portion 30 generally contains, as its main component, an oxide of an element that is easily oxidized among the various elements constituting the plated steel sheet used as raw material. Examples of such elements that are easily oxidized include Al and Mg.

[0020] Furthermore, to identify the region corresponding to the weld bead 30 in the welded joint 1 of interest, measurements can be performed, for example, as follows. Specifically, a sample having the weld bead 30 is prepared, cut along a plane (the XZ plane in FIG. 1A) perpendicular to the welding direction (the Y-axis direction in FIG. 1A), and then embedded in resin and polished so that the cross section of the weld bead 30 (the XZ cross section in FIG. 1A) can be observed. After polishing, the cross section of the weld bead 30 is observed using a scanning electron microscope (SEM). An element distribution image of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.) is obtained using SEM-EDS, which allows identification of the slag layer present on the weld bead. The region located closer to the steel plate than the slag layer identified in this way is the weld bead.

[0021] Oxides produced during welding are broadly divided into two types: scale and slag. Scale, excluding oxygen, contains 50% or more Fe by mass, with the remainder consisting of easily oxidized elements and impurities. Slag, excluding oxygen by mass, contains 50% or more 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 on the Ellingham diagram and that can be added to the 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] Here, JIS Z3001 (2018) defines the point where the surface of the base metal and the surface of the weld bead intersect as the "toe." In the weld joint 1 shown in FIG. 1A, the point where the surface of the weld bead portion 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 corrosion resistance in the vicinity of this toe T.

[0023] Such a "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 and a T-shaped weld joint as shown in FIG. 1C.

[0024] Furthermore, heat input during arc welding is generally performed from one side of the welded joint 1, and weld bead 30 is exposed on the surface of the base steel plate on the heat input side of the arc welding, and the size of weld bead 30 decreases in the direction in which the heat input from the arc welding propagates. Therefore, the heat input direction during arc welding can be identified by focusing on the presence or absence of weld bead 30 exposed on the surface and the extent of weld bead 30 (for example, the extent when focusing on a cross section such as that shown in FIG. 1B).

[0025] In this embodiment, in a joint in which two steel plates at least partially overlap, such as the lap fillet welded joint shown in FIG. 1A or the T-welded joint shown in FIG. 1C, the steel plate located on the heat input side of the arc welding as described above is referred to as the first steel plate 10, and the steel plate located on the side in which the heat input from the arc welding propagates is referred to as the second steel plate 20.

[0026] As shown schematically in Figures 1A to 1C, a heat-affected zone 40 is formed around the weld bead portion 30. This heat-affected zone 40 is generated when the metallurgical structure of the first steel plate 10 and the second steel plate 20 is altered as a result of heat input to the first steel plate 10 and the second steel plate 20 by arc welding. The size of the heat-affected zone 40 depends on the amount of heat input during arc welding, and generally, the larger the amount of heat input, the larger the size of the heat-affected zone 40. Furthermore, due to the alteration of the metallurgical structure of the first steel plate 10 and the second steel plate 20, the appearance (appearance) of the altered metallurgical structure differs visually from the unaltered portions of the first steel plate 10 and the second steel plate 20, making it easy to identify the portion corresponding to the heat-affected zone 40.

[0027] <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 from 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.

[0028] In the following description, a portion of the welded joint 1 that is not affected by heat due to welding (in other words, a portion that is not the weld bead 30 or the heat-affected zone 40) will be referred to as the "non-heat-affected zone." In the welded joint 1 shown in FIG. 2, for example, the region R1, which is sufficiently separated from the vicinity of the toe T as enclosed by the dashed line in the figure, corresponds to such a non-heat-affected zone. The position of such a non-heat-affected zone can be considered to be, for example, a region that is separated from the toe T shown in FIG. 2 by 10 mm or more in a direction perpendicular to the extension direction of the weld bead 30 (the Y-axis direction in FIG. 2) and away from the toe T (the X-axis direction in FIG. 2).

[0029] Fig. 3 is a diagram schematically showing a portion of a cross section parallel to the plate thickness direction in the non-heat-affected zone R1. As shown schematically 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 coating layer 103 located on at least a portion of the surface of the base steel 101, and an oxide layer 105 located on the coating layer 103. In the welded joint 1 according to this embodiment, the coating 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. The base steel 101, the plating layer 103, and the oxide layer 105 will be described in detail below.

[0030] <About Chitetsudo 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, ultra-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.

[0031] Furthermore, the thickness of the base steel 101 is not particularly limited, and is set appropriately depending on the mechanical strength required of the welded joint 1, etc.

[0032] <About plating layer 103> 3, the plating layer 103 is provided on at least a portion of the surface of the steel substrate 101, and more preferably over the entire surface of the steel substrate 101. Such plating layer 103 originates from the plating layer that is present in the plated steel sheet that is the material of the welded joint 1.

[0033] Here, such a plating layer 103 is preferably provided on the surface where the weld bead portion 30 of the steel plate (first steel plate 10) located on the heat input side of the arc welding in the welded joint 1 is exposed (for example, the surface on the positive Z-axis side in FIG. 1A etc.), and on the surface where the heat affected zone 40 of the steel plate (second steel plate 20) on the heat propagation side of the arc welding is exposed (for example, the surface on the negative Z-axis side in FIG. 1A etc.). On surfaces other than those mentioned above of the first steel plate 10 and the second steel plate 20, it is preferable that a plating layer 103 as described below is provided, but the plating layer 103 does not have to be provided.

[0034] First, the chemical composition of the plating layer 103 will be described in detail below.

[0035] ◇Chemical composition of plating layer 103 The chemical composition of the plating layer 103 according to this embodiment contains, in mass%, 1.00-80.00% Al, 1.00-20.00% Mg, 0.01-15.00% Fe, 0-10.00% Si, and 0-4.00% Ca, with the balance being 5.00 mass% or more of Zn and impurities. That is, in the chemical composition of the 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 being 5.00 mass% or more of Zn and impurities.

[0036] These components and their contents will be described in detail below.

[0037] [Al:1.00~80.00% by 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, and is contained in a certain amount or more 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 ensured. Therefore, in the plating layer 103 according to this embodiment, the Al content is 1.00 mass% or more. The Al content is preferably 18.00 mass% or more, and more preferably 35.00 mass% or more. When the Al content is within the above range, it becomes possible to ensure corrosion resistance of the non-heat-affected zone.

[0038] 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 in the non-heat-affected zone. Therefore, in the plating layer 103 according to this embodiment, the Al content is 80.00 mass% or less. The Al content is preferably 60.00 mass% or less, and more preferably 50.00 mass% or less.

[0039] [Mg:1.00~20.00% by 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, and a certain amount of Mg is contained to ensure the corrosion resistance of the non-heat-affected zone. Therefore, the plating layer 103 according to this embodiment has an Mg content of 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 that the Mg content falls within the above range, it is possible to ensure the corrosion resistance of the non-heat-affected zone.

[0040] 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 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 that the Mg content falls within the above range, it is possible to reliably ensure the corrosion resistance of the non-heat-affected zone.

[0041] [Fe:0.01~15.00% by mass] Elements constituting the base steel 101 may be mixed into the coating layer 103 from the base steel 101, which is the base material. In particular, when the coating layer 103 is formed by hot-dip galvanizing, the 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 certain amount of Fe is contained in the coating layer 103, and this 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 perspective 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.

[0042] Furthermore, Fe may be intentionally added to the plating bath used to produce the plating layer 103, provided that the effects of the present invention are not impaired. However, if the Fe content in the plating layer 103 is 15.00% by 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% by mass or less. The Fe content in the plating layer 103 is more preferably 10.00% by mass or less.

[0043] [Si:0~10.00% by mass] Si is an element that can suppress the excessive growth of the Fe-Al intermetallic compound phase that forms 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 the Fe-Al intermetallic compound phase, 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%, an excessive amount of a high-melting-point intermetallic compound phase is formed 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.

[0044] 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 operability. Therefore, the Si content in the plating bath is adjusted from the perspective of operability, so that the Si content in the plating layer 103 is 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.

[0045] [Ca:0~4.00% by mass] When contained in the coating layer 103, Ca forms an intermetallic compound phase with Al and Zn. Furthermore, when Si is contained together with Ca in the coating layer 103, Ca forms an intermetallic compound phase with Si. These intermetallic compound phases have a high melting point and a stable structure, making it possible to suppress the formation of blowholes and LME caused by Zn evaporation during welding of coated steel sheets. 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 coating layer 103 is more preferably 0.10% by mass or more.

[0046] 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 perspective, 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.

[0047] In the plating layer 103, the balance of the above-mentioned Al, Mg, Fe, Si, and Ca is 5.00 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 manifested when the Zn content is 5.00% by mass or more, so the Zn content is set to 5.00% by mass or more.

[0048] Furthermore, the plating layer 103 according to this embodiment further selectively contains, in place of a part of the remaining Zn, Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, In: 0 to 1.00%, Bi: 0 to 1.00%, Ti: 0 to 1.00%, Cr: 0 to 1.00%, Nb: 0 to 1.00%, Zr: 0 to The plating layer 103 according to this embodiment may contain, as optional additive elements, 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 in a total content of 0 to 5.00 mass% or less within the above content ranges.

[0049] By setting the total content of the above optional additional elements to 5.00% 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 impairing each other. The total content of the above optional additional elements is preferably 1.00% by mass or less, and more preferably 0.20% by mass or less. The content of each optional added element will be described in detail below.

[0050] [Sb:0~0.50% by mass] [Pb:0~0.50% by mass] [Sr:0~0.50% by 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 the design is realized when the content of at least one of Sb, Pb, and Sr is 0.05 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 0.05 mass% or more.

[0051] On the other hand, if the coating layer 103 is formed such that any one of the Sb, Pb, and Sr contents exceeds 0.50 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 set to 0.50 mass% or less. The contents of Sb, Pb, and Sr are each independently set to preferably 0.20 mass% or less.

[0052] [Cu:0~1.00% by mass] [Ti:0~1.00% by mass] [Cr:0~1.00% by mass] [Nb:0~1.00% by mass] [Ni:0~1.00% by mass] [Mn:0~1.00% by mass] [V:0~1.00% by 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 layer formed by welding when the plated steel sheet is welded, thereby improving the corrosion resistance of the weld bead 30. This effect of improving the corrosion resistance of the weld is realized 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 each of these elements is independently 0.005 mass% or more.

[0053] 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 1.00 mass%, these elements tend to form various intermetallic compound phases in the plating bath used to form the plating layer 103. This increases the viscosity of the plating bath, making it difficult 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 1.00 mass% or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, and V are each independently set to preferably 0.20 mass% or less.

[0054] [Sn:0~1.00% by mass] [In:0~1.00% by mass] [Bi:0~1.00% by 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 steel substrate, improving sacrificial corrosion protection. On the other hand, excessive addition of Sn, In, or Bi can excessively accelerate the Mg dissolution rate, potentially reducing the corrosion resistance of the non-heat-affected zone. This increase in the Mg dissolution rate becomes significant when the Sn, In, or Bi content exceeds 1.00 mass%, so the Sn, In, and Bi contents are each independently 1.00 mass% or less. The Sn, In, and Bi contents are each independently preferably 0.20 mass% or less. The lower limits of the Sn, In, and Bi contents are not particularly specified and may each independently be 0 mass%. However, when Sn, In, and Bi are contained, their respective contents are preferably independently 0.005 mass% or more. This makes it possible to improve the sacrificial corrosion resistance of the plating layer 103.

[0055] [Zr:0~1.00% by mass] When Zr is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties is realized when the Zr content is 0.01 mass% or more. Therefore, when Zr is contained, the content is preferably 0.01 mass% or more.

[0056] On the other hand, when forming a plating layer 103 having a Zr content exceeding 1.00% 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 Zr content is set to 1.00% by mass or less. The Zr content is preferably set to 0.10% by mass or less.

[0057] [Mo:0~1.00% by mass] When Mo is contained in the plating layer 103, it is possible to improve corrosion resistance. Such an effect of improving corrosion resistance is realized when the Mo content is 0.01 mass% or more. Therefore, when Mo is contained, the content is preferably 0.01 mass% or more.

[0058] On the other hand, when forming a coating layer 103 having a Mo content exceeding 1.00% by mass, a large amount of dross is likely to be generated in the coating bath used to form the coating layer 103. Therefore, the Mo content is set to 1.00% by mass or less. The Mo content is preferably set to 0.05% by mass or less.

[0059] [Ag:0~1.00% by mass] When Ag is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties is realized when the Ag content is 0.01% by mass or more. Therefore, when Ag is contained, the content is preferably 0.01% by mass or more.

[0060] On the other hand, when forming a plating layer 103 having an Ag content exceeding 1.00% 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 1.00% by mass or less. The Ag content is preferably 0.05% by mass or less.

[0061] [Li:0~1.00% by mass] When Li is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties can be achieved when the Li content is 0.01 mass % or more. Therefore, when Li is contained, the content is preferably 0.01 mass % or more.

[0062] On the other hand, when forming a plating layer 103 having a Li content exceeding 1.00% 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 Li content is 1.00% by mass or less. The Li content is preferably 0.05% by mass or less.

[0063] [La:0~0.50% by mass] [Ce:0~0.50% by mass] [Y:0~0.50% by mass] La, Ce, and Y are elements that exhibit almost the same effect as Ca, suppressing blowhole formation and LME during welding. This is because the atomic radius of each element is close to that of Ca. When these elements are contained in the coating layer 103, they substitute for Ca. Therefore, these elements are detected at the same position as Ca by EDS. Furthermore, even when these elements become oxides, the oxides of these elements are detected at the same position as CaO.

[0064] The effect of suppressing the formation of blowholes and LME during welding is achieved by making the contents of these elements independently 0.01% by mass or more. Therefore, the contents of La, Ce, and Y in the plating layer 103 are more preferably independently 0.05% by mass or more.

[0065] 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, which may reduce operability. Therefore, from the viewpoint of operability, the La, Ce, and Y contents in the plating bath are adjusted so that the La, Ce, and Y contents are each independently 0.50 mass% or less. The La, Ce, and Y contents are each preferably independently 0.10 mass% or less.

[0066] [B:0~0.50% by mass] When B is contained in the coating layer 103, it has the effect of suppressing LME. This is thought to be because when B is contained in the coating layer 103, it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compound phases. 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 suppressing LME in the steel substrate 101. Furthermore, because the various intermetallic compounds formed with B have extremely high melting points, they are thought to also act to suppress Zn evaporation during welding. These improving effects are achieved by including 0.05% or more by mass of B. Therefore, the B content in the coating layer 103 is more preferably 0.05% or more by mass.

[0067] On the other hand, if excessive 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, reducing plating operability and making it impossible to produce a plated steel sheet with excellent plating properties. Such a reduction in operability becomes significant when the B content exceeds 0.50 mass%, so the B content is 0.50 mass% or less. The B content is preferably 0.10 mass% or less.

[0068] [Method of measuring chemical components] The chemical composition 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 to analyze chemical compositions down to 0.1% by mass, while ICP-MS is used to analyze trace amounts of chemical compositions less than 0.1% by mass. The target area of ​​the non-heat-affected zone is immersed in 10% hydrochloric acid containing an inhibitor for approximately 1 minute to remove the plating layer, and a solution containing the dissolved plating layer is prepared. The resulting solution is analyzed using ICP-AES or ICP-MS to obtain the overall average chemical composition of the plating layer.

[0069] ◇More preferable chemical composition of plating layer 103 The plating layer 103 according to this embodiment has the above-mentioned chemical composition, but a more preferable chemical composition is as follows. That is, the plating layer 103 according to this embodiment preferably contains, as a chemical composition, at least 18.00 to 60.00 mass% Al and 5.00 to 15.00 mass% Mg, and further contains the above-mentioned optional additional elements as required.

[0070] 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.

[0071] where 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 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 quantitative analysis by 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 quasicrystalline phase. In the case of the 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) 49 If the phase is a quasicrystalline phase, it can be confirmed by taking an electron diffraction image using a TEM and checking whether a five-fold symmetric crystal structure is observed in the electron diffraction image. A five-fold symmetric crystal structure can be identified by obtaining an electron diffraction image called a Penrose pattern.

[0072] Mg 32 (Al, Zn) 49The Mg phase exerts sacrificial corrosion protection on plated steel sheets, 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 phase itself has excellent corrosion resistance, 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.

[0073] ◇About the amount of plating layer 103 The coating weight of the plating layer 103 as described above is not particularly limited, but 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 is within the above range, the non-heat-affected zone of the welded joint 1 according to this embodiment can exhibit sufficient corrosion resistance.

[0074] The deposition weight of the plating layer 103 is determined by cutting a 30 mm x 30 mm sample (the back side is sealed with tape to prevent dissolution) from the plated steel sheet and measuring its mass in advance. The sample is then immersed in a 10% HCl aqueous solution containing an inhibitor to remove the plating layer by pickling, and the mass of the sample after pickling is measured. The deposition weight of the plating layer 103 can be determined from the change in mass of the sample before and after pickling.

[0075] <About oxide layer 105> Next, the oxide layer 105 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, an oxide layer 105 is located on the surface of the above-described plating layer 103. This oxide layer 105 originates from the oxide layer of the plated steel sheet that is the material of the welded joint 1.

[0076] 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 treatment for solidifying the plating layer, which is carried out during the production of the plated steel sheet.

[0077] As described above, the oxide layer 105 is mainly composed of oxides of the elements that make up 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 50 mass% or more of Zn oxide, Mg oxide, and Al oxide in total, and may further contain hydroxides of these Zn, Mg, and Al, at least one of oxides or hydroxides of other constituent elements, impurities, etc.

[0078] 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.

[0079] 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.

[0080] 4, attention is now focused on a position 5 nm deep from the outermost surface of the oxide layer 105 ("position A" 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.

[0081] Here, there is a possibility that dirt such as oil and fat may be 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 perspective, the surface of the oxide layer 105 is subjected to a process such as ultrasonic cleaning in ethanol to remove the dirt and the like, and the surface obtained by such a process is used as the "outermost surface of the oxide layer 105" when carrying out the above-mentioned XPS measurement.

[0082] 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 (1486.6eV) X-ray diameter: 50 to 200 μm Measurement area: 100~700μm×100~700μm Vacuum degree: 1×10 -10 ~1×10 -11 torr (1 torr is 133.32 Pa) Acceleration voltage: 1 to 10 kV

[0083] In this embodiment, we focus on the peaks attributable to Al-O bonds, Mg-O bonds, and Zn-O bonds in the obtained XPS measurement results. These bonds are characteristic of oxides and hydroxides of Al, Mg, and Zn, and 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.

[0084] 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. 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.

[0085] The intensity of the peaks assigned to each bond can be calculated by taking into account the baseline of the peak in the XPS spectrum shown in Figure 5. p to baseline intensity I b (i.e., "I p -I b ").

[0086] 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 A" in FIG. 4 ), 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 A," 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.

[0087] 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.

[0088] 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 raw material, and performing a heat treatment process, as described in detail below.

[0089] 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.

[0090] The non-heat affected zone in the welded joint 1 according to this embodiment has been described in detail above with reference to FIGS. 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.

[0091] <About the heat-affected zone on the back side> As mentioned previously, in the welded joint 1 according to this embodiment, the surface on which the weld bead 30 is present (e.g., the surface on the positive Z-axis side in FIG. 1A ) will be referred to as the "front surface side of the welded joint 1," and the surface opposite the surface on which the weld bead 30 is present will be referred to as the "rear surface side of the welded joint 1." Here, the "front surface side of the welded joint 1" can be understood as the front surface of the component that is exposed to the public when the welded joint 1 according to this embodiment is used as a component such as an automobile suspension part, and the "rear surface side of the welded joint 1" can also be understood as the rear surface of the component that is not exposed to the public. For example, in the welded joint 1 shown in FIGS. 1A to 1C , the surface on the negative Z-axis side of the second steel plate 20 corresponds to the above-mentioned "rear surface side of the welded joint 1."

[0092] In the welded joint 1 according to this embodiment, it is important that a plated steel sheet having the base steel 101, the plating layer 103, and the oxide layer 105 described above is used as the material for at least the second steel sheet 20, and that the plated layer 103 and the oxide layer 105 described above are provided at least on the surface that will become the back side of the second steel sheet 20 after welding. By using such a plated steel sheet in this state, the plating components can be sufficiently retained in the heat-affected zone 40 on the back side of the welded joint 1 obtained after arc welding, even after arc welding. As a result, the formation of Fe scale (not shown) associated with arc welding can be sufficiently suppressed on the surface of the heat-affected zone 40 on the back side (region R2 in FIG. 2 ). This allows the welded joint 1 according to this embodiment to have improved corrosion resistance in the heat-affected zone on the back side.

[0093] Specifically, in the welded joint 1 according to this embodiment, the thickness (average thickness) of the Fe scale (not shown) located on the heat-affected zone 40 on the back surface side is 0 to 5.0 μm. If the thickness of the Fe scale exceeds 5.0 μm, it means that the plating components were not sufficiently retained during arc welding, and the corrosion resistance of the heat-affected zone on the back surface side of the welded joint 1 cannot be ensured. The thickness of the Fe scale in the heat-affected zone 40 on the back surface side is preferably 0 to 1.0 μm. Furthermore, the thinner the thickness of the Fe scale in the heat-affected zone 40 on the back surface side, the better, with the lower limit being 0 μm (in other words, a state in which no Fe scale is present), as described above.

[0094] The thickness of the Fe scale described above can be measured as follows. Specifically, a cross-sectional SEM image is obtained, and point composition analysis is performed using an EPMA to identify the Fe scale portion in an arbitrary field of view (size: 64 μm × 48 μm, equivalent to 2000x magnification). The thickness of the Fe scale is measured at five random locations in the field of view, and the average value of the Fe scale thickness in that field of view is calculated. Similar measurements are performed on four other arbitrary fields of view, and the average value for the total of five fields of view is used as the Fe scale thickness of the sample of interest.

[0095] In the welded joint 1 according to this embodiment, even in the heat-affected zone on the back side where Fe scale is likely to occur, the thickness of the Fe scale is within the range of 0 to 5.0 μm because sufficient plating components remain in the heat-affected zone 40. Therefore, in other areas where sufficient plating components are present (for example, non-heat-affected zones), the thickness of the Fe scale is naturally kept within the range of 0 to 5.0 μm.

[0096] The welded joint 1 according to this embodiment has been described in detail above with reference to Figures 1A to 6. The welded joint 1 according to this embodiment as described above can be suitably used, for example, as an automobile suspension part.

[0097] (About the manufacturing method of the plated steel sheet used as the raw material) Next, an example of a method for manufacturing the plated steel sheet that serves as the material for the welded joint 1 as described above will be described. The plated steel sheet that is the material for the welded joint 1 in this embodiment is manufactured by using a steel sheet made of the above-mentioned 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.

[0098] 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.

[0099] Thereafter, the obtained plated steel sheet (a plated steel sheet consisting of a base steel 101 and a 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.

[0100] An example of a manufacturing method for obtaining a plated steel sheet according to this 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 base steel 101 used as a base material is rolled by the Sendzimir method to a desired thickness, and then wound into a coil and placed on a hot dip plating line.

[0101] In a hot-dip galvanizing line, steel sheets are continuously fed from a coil and threaded through the line. During this process, the steel sheets are subjected to a heating reduction treatment at 800°C in an N2-5% H2 gas atmosphere in a non-oxidizing environment with an oxygen concentration of 20 ppm or less, for example, and then air-cooled with N2 gas to a temperature about 20°C higher than the bath temperature of the subsequent galvanizing bath, and then immersed in the galvanizing bath.

[0102] Here, a molten plating alloy having the above-described chemical components 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 (for example, 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.

[0103] In addition, there is no particular restriction on using pure metals when preparing 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 the alloy has a predetermined composition with few impurities.

[0104] After immersing the steel sheet in the above-described plating bath, it is pulled up at a predetermined pulling speed. At this time, the coating weight is controlled, for example, by using N2 wiping gas, so that the formed plating layer 103 has a desired thickness. Here, general plating operating conditions may be applied except for the bath temperature, and no special equipment or conditions are required.

[0105] Next, 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.

[0106] The first cooling step is a cooling step carried out when the temperature of the plating alloy is within the range of the bath temperature to 250°C, and involves quenching the plated steel sheet in the above temperature range 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 used in the plating step, the first cooling step is carried out immediately after the steel sheet leaves the plating bath. This solidifies the plating alloy present on the surface of the steel sheet, forming a plating layer.

[0107] Thereafter, when the temperature of the plated alloy (plated layer) is within the range of 250 to 50°C, a second cooling step is carried out. This second cooling step involves slowly cooling the plated steel sheet, which is in the temperature range of 250 to 50°C, at an average cooling rate of 10°C / sec or less in an atmosphere with a dew point of 0°C or higher. This controls the state of the oxide formed on the surface of the plated layer, and forms a desired oxide layer.

[0108] As described above, by undergoing a two-stage cooling process of rapid cooling in the temperature range of the bath temperature to 250°C and slow cooling in the temperature range of 250 to 50°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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] Furthermore, if an alloying heat treatment step (e.g., 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 formation state controlled by the first cooling step and the second cooling step is disrupted, and the oxides grow 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.

[0114] Here, the cooling process described above can be performed using commonly known methods such as N2 gas cooling, mist cooling, submersion in water, etc. In addition to N2 gas, gases with high heat removal effects, such as He gas and hydrogen gas, may also be used as the cooling gas.

[0115] The temperature of the coating layer can be measured, for example, using 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 operating conditions such as the steel sheet preheating temperature and the hot-dip coating bath temperature, 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 and second cooling steps. The surface temperature of the coating layer can also be measured using a non-contact radiation thermometer, although this is not as accurate as a contact thermometer.

[0116] Alternatively, the relationship between the surface temperature of the coating layer and the average temperature of the entire coating layer may be determined by a simulation using heat conduction analysis. Specifically, the surface temperature of the coating layer and the average temperature of the entire coating layer are determined based on various manufacturing conditions, such as the preheating temperature of the steel sheet, the temperature of the hot-dip coating bath, the pulling speed of the steel sheet from the coating bath, the thickness of the steel sheet, the thickness of the coating layer, the amount of heat exchanged between the coating layer and the manufacturing equipment, and the amount of heat dissipated from the coating layer. The relationship between the surface temperature of the coating layer and the average temperature of the entire coating layer can then be determined using the obtained results. This makes it possible to estimate the average temperature of the entire coating layer at that time under those manufacturing conditions by actually measuring the surface temperature of the coating layer during the manufacturing of the coated steel sheet. As a result, it becomes possible to precisely control the cooling treatments in the first and second cooling processes.

[0117] An example of the method for manufacturing a plated steel sheet according to this embodiment has been specifically described above.

[0118] 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 layers of various coatings, such as a chromate treatment, a phosphate treatment, a chromate-free treatment, and an organic resin coating formation treatment.

[0119] 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.

[0120] Examples of electrolytic chromate treatments include those 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.

[0121] Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

[0122] 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.

[0123] 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 functional group.

[0124] 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. Alternatively, 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. Alternatively, an aqueous organic resin may be used by dissolving or dispersing it in water. Furthermore, various color pigments and rust-preventive pigments may be incorporated into the organic resin film.

[0125] (Regarding the manufacturing method of welded joints) 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 the desired shape of the welded joint, and welding the first steel sheet and the second steel sheet together.

[0126] Here, the first steel plate and the second steel plate can be welded by arc welding, and by performing the welding under the welding conditions described below, it is possible to achieve the above-mentioned state of the back surface heat affected zone.

[0127] More specifically, the first steel plate and the second steel plate may be arc-welded under the following welding conditions, for example. Welding current: 220A, welding voltage: 25.2V, welding speed: 80cm / min Welding gas: 20% CO2 + Ar, gas flow rate: 20 L / min Welding wire: YGW16, manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm (C: 0.1% by mass, Si: 0.80% by mass, Mn: 1.5% by mass, P: 0.015% by mass, S: 0.008% by mass, Cu: 0.36% by mass) Welding torch tilt angle: 45°

[0128] An example of the method for manufacturing a welded joint according to this embodiment has been described above. [Example]

[0129] 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.

[0130] In the following examples and comparative examples, a 3.2 mm thick hot-rolled steel sheet (0.05 mass % C-0.007 mass % Si-0.25 mass % Mn, manufactured by Nippon Steel Corporation) was used as the steel sheet serving as the base material. The hot-rolled steel sheet was cut into a size of 100 mm × 200 mm to prepare a test piece.

[0131] The coating baths for achieving the coating layer compositions shown in Table 1 below were prepared and installed in a batch-type hot-dip galvanizing test apparatus manufactured by our company, and the test specimens were coated. The test specimen temperatures were measured using a thermocouple spot-welded to the center of the test specimens. Furthermore, before immersion in the coating baths, the surfaces of the test specimens were subjected to a heat reduction treatment at 800°C in an N2-5% H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less. After the heat reduction treatment, the test specimens were air-cooled with N2 gas. After the test specimens reached a temperature 20°C above the bath temperature, they were immersed in the coating bath of the hot-dip galvanizing test apparatus for approximately 3 seconds.

[0132] After immersion in the plating bath, the test piece was pulled up at a pulling speed of 20 to 200 mm / sec. During pulling, N2 wiping gas was used to control the desired plating adhesion weight. In the following examples and comparative examples, the plating layer adhesion 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).

[0133] Here, a steel sheet measuring 30 mm × 30 mm was cut out from the test piece plated as described above, and the steel sheet was immersed in a 10% HCl aqueous solution containing an inhibitor to remove 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.

[0134] In addition, the obtained plating layer was photographed using a TEM to take an electron diffraction image. Based on whether or not a five-fold symmetric crystal structure was observed in the electron diffraction image, the Mg 32 (Al, Zn) 49 The presence or absence of phases was confirmed.

[0135] 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> Grade "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

[0136] 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 (lap fillet welding) to form a welded joint.

[0137] Here, the welding conditions for the arc welding are as follows. Welding current: 220A, welding voltage: 25.2V, welding speed: 80cm / min Welding gas: 20% CO2 + Ar, gas flow rate: 20 L / min Welding wire: YGW16, manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm (C: 0.1% by mass, Si: 0.80% by mass, Mn: 1.5% by mass, P: 0.015% by mass, S: 0.008% by mass, Cu: 0.36% by mass) Welding torch tilt angle: 45° Overlap: 10mm Steel plate size: Upper plate (first steel plate) 150 x 50 mm, lower plate (second steel plate) 150 x 30 mm Gap: 0mm

[0138] <Fe scale thickness in the heat-affected zone on the back side> For the welded joints obtained as described above, the thickness of the Fe scale in the heat-affected zone on the back side was measured by the method previously described. The results obtained are summarized in Table 1 below.

[0139] <Corrosion resistance of weld beads> The welded joints obtained as described above were subjected to an automotive phosphate conversion treatment (Zn phosphate treatment, SD5350 system: Nippon Paint Industrial Coating standard) and electrocoating (PN110 Powernics Gray: Nippon Paint Industrial Coating standard). The electrocoating thickness was 20 μm. After electrocoating, the samples were subjected to a cyclic corrosion test in accordance with JASO (M609-91) to evaluate the timing of red rust formation at the toe. The evaluation criteria were as follows. The results are summarized in Table 1 below. <Evaluation Criteria> Rating "AAA": Red rust occurs over 60 cycles "AA": Red rust occurs between 30 and 60 cycles "A": Red rust occurs between 15 and 30 cycles "B": Red rust occurs within 15 cycles

[0140] [Table 1]

[0141] 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. [Explanation of symbols]

[0142] 1 Welded joints 10 First steel plate 20 Second steel plate 30 Weld bead 40 Heat-affected zone 101 Subway 103 plating layer 105 oxide layer T Toe

Claims

1. A welded joint in which a first steel plate and a second steel plate are welded by arc welding, The first steel plate and the second steel plate; a weld bead portion formed by the arc welding; a heat-affected zone located around the weld bead; It has When a portion of the first steel plate and the second steel plate that is not thermally affected by the welding is defined as a non-heat-affected zone, at least one of the first steel plate or the second steel plate has, in the non-heat-affected zone, a plating layer located on at least a part of a surface of a base steel, and an oxide layer located on the plating layer, The plating layer comprises, in mass %, Al: 1.00-80.00%, Mg: 1.00-20.00%, Fe: 0.01-15.00%, Si: 0 to 10.00%, Ca: 0-4.00%, and optionally further comprising Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0-1.00%, In: 0 to 1.00%, Bi: 0-1.00%, Ti: 0 to 1.00%, Cr: 0-1.00%, Nb: 0 to 1.00%, Zr: 0 to 1.00%, Ni: 0 to 1.00%, Mn: 0 to 1.00%, V: 0-1.00%, Mo: 0-1.00%, Ag: 0-1.00%, Li: 0 to 1.00%, La: 0 to 0.50%, Ce: 0 to 0.50%, B: 0 to 0.50%, Y: 0 to 0.50%, Sr: 0 to 0.50%, The balance is 5.00 mass% or more of Zn and impurities, 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 value of the intensity ratio ([Al—O]+[Mg—O]) / [Zn—O] calculated from the intensities of peaks respectively assigned to an Al—O bond, an Mg—O bond, and a Zn—O bond is 5.0 or more, A welded joint, wherein when the surface of the welded joint on which the weld bead portion does not exist is defined as the back surface, the thickness of Fe scale located on the heat-affected zone on the back surface is 0 to 5.0 μm.

2. 2. The welded joint according to claim 1, wherein the thickness of the Fe scale is 0 to 1.0 μm.

3. The welded joint according to claim 1 or 2, wherein the strength ratio ([Al—O] + [Mg—O]) / [Zn—O] is 10.0 or more.

4. The plating layer is Al: 18.00 to 60.00% by mass, Mg: 5.00 to 15.00% by mass, The welded joint according to any one of claims 1 to 3, containing at least

5. The plating layer is Al: 35.00 to 60.00% by mass, Mg: 7.00 to 15.00% by mass, and The plating layer contains Mg ３２ (Al, Zn) ４９ There are phases, The Mg ３２ (Al, Zn) ４９ 5. The welded joint according to claim 1, wherein the Mg content [Mg], Zn content [Zn], and Al content [Al] (each unit: atomic %) in the Al phase satisfy the relationship 0.50≦[Mg] / ([Zn]+[Al])≦0.83.

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