Welded joint

Abstract

[Problem] To further improve the corrosion resistance of the back surface of a welded joint. [Solution] This welded joint has a first steel plate and a second steel plate that are connected to a bead section having a long side in the extension direction in plan view. An X-axis direction is defined as a direction orthogonal to the extending direction of the bead portion and the direction normal to the surface of the non-heat-affected portion of the second steel plate. A second plating layer containing a Zn-Mg structure is present on the bead portion or the heat-affected portion in a range up to 1 mm on each of both sides in the X-axis direction from the center of the bead portion and the heat-affected portion, and the ratio of sigma Li to the length of 100 [mu] m of the field of view obtained by electron microscope observation of the cross section is 0.05 or more. Furthermore, the average thickness of the ZnO layers present in the range is 1.0 [mu] m or less in a cross-section in the range of 5 mm on both sides in the X-axis direction from the center as a starting point.

Description

Technical Field

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

[0002] Automobile components typified by chassis components of automobiles and various building material components are mostly manufactured using welded joints formed by welding a plurality of steel materials. Since these automobile components and building material components are used while being exposed to various environments, it is desired that the manufactured welded joints have excellent corrosion resistance. Therefore, as the blank material for the welded joint, various galvanized steel sheets typified by alloyed hot-dip galvanized steel sheets are used.

[0003] Here, when manufacturing a welded joint by welding a galvanized steel sheet, there are the following specific problems. There is a concern that pores formed due to evaporation of Zn in the coating near the "weld toe" defined by JIS Z3001 (2018) during welding may reduce the mechanical properties of the welded joint. In addition, the evaporation of Zn in the coating damages the sacrificial anti-corrosion layer, resulting in a reduction in corrosion resistance as well.

[0004] To solve the above-mentioned pore formation problem, various solutions have been proposed in the past. For example, in Patent Document 1 below, a plated steel material is proposed, which has a steel sheet and a coating disposed on the surface of the steel sheet and containing a Zn-Al-Mg alloy layer. 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 coating has a specified chemical composition.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: WO 2018 / 139620 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] Here, by using the plated steel material proposed in Patent Document 1 above, the pore formation problem can be solved. However, the present inventors have conducted in-depth research and found that there is still room for improvement in the technology proposed in Patent Document 1 above. When a part other than the vicinity of the weld toe, for example, the surface on the side opposite to the side where the weld bead portion exists in the welded joint, is set as the back surface, it is desired to further improve the corrosion resistance of the back surface.

[0010] Therefore, the present invention has been completed in view of the above problems, and an object of the present invention is to provide a welded joint capable of further improving the corrosion resistance of the back surface of the welded joint.

[0011] Solution for solving problems

[0012] In order to solve the above technical problems, the inventors of the present invention conducted in-depth research and found that the corrosion resistance of the back surface of the welded joint decreases during welding because Zn in the coating evaporates or oxidizes during welding, resulting in the formation of ZnO or Fe oxide scale on the surface of the back side. Therefore, it was recognized that if the phenomenon peculiar to the zinc-based coating, that is, the evaporation of Zn in the coating during welding, could be suppressed, the corrosion resistance of the back surface could be further improved.

[0013] Based on this insight, the inventors of the present invention conducted further research and found that by improving the coating structure on the heat-affected zone of the back surface of the welded joint, the corrosion resistance of the back surface could be further improved.

[0014] The gist of the present invention completed based on this insight is as follows.

[0015] (1) A welded joint having a first steel plate and a second steel plate connected to a weld bead portion, the weld bead portion having a long side in the extending direction in a plan view, the first steel plate and the second steel plate each having a heat-affected zone around the weld bead portion and a non-heat-affected zone where there is no heat influence caused by welding, the second steel plate having a steel base and a first coating on the steel base in the non-heat-affected zone, the first coating being a coating having the following chemical composition, the chemical composition containing, by mass%, Al: 10.00 to 70.00%, Mg: 3.00 to 20.00%, Fe: 0.01 to 15.00%, selectively containing one or more elements selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, element group F, and element group G, and the balance being Zn and impurities of 5.0000 mass% or more,

[0016] [Element group A]: One or two selected from the group consisting of Si: greater than 0% and 10.00% or less, and Ca: greater than 0% and 4.00% or less;

[0017] [Element group B]: One or more selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less;

[0018] [Element Group C]: selected from one or more of the group consisting of Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less;

[0019] [Element Group D]: selected from one or more of the group consisting of Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less;

[0020] [Element Group E]: selected from one or more of the group consisting of Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less;

[0021] [Element Group F]: selected from one or more of the group consisting of La: greater than 0% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less;

[0022] [Element Group G]: B: greater than 0% and 0.5000% or less,

[0023] Define the normal direction of the surface of the non-heat-affected part relative to the second steel plate as the Z-axis direction, define the extending direction as the Y-axis direction, and define the direction orthogonal to the Z-axis direction and the Y-axis direction as the X-axis direction. In the cross-section obtained by cutting the first steel plate and the second steel plate along the Z-axis direction, set the surface on the positive direction side of the Z-axis as the first surface, set the surface on the negative direction side of the Z-axis as the second surface. When setting the surface with the smaller weld toe pitch in the X-axis direction of the weld bead part on the first surface side and the weld toe pitch in the X-axis direction of the weld bead part on the second surface side as the back surface of the welded joint, or when there is no weld toe on either the first surface side or the second surface side, and set the surface without the weld toe as the back surface of the welded joint, on the back surface of the cross-section, for the X-axis direction, define the center of the heat-affected part on the back surface as the midpoint of the line segment connecting the closest end and the farthest end of the ends of the heat-affected part from one end of the first coating; on the back surface of the cross-section, there is a second coating containing a Zn-Mg structure on the weld bead part or the heat-affected part within the range of 1 mm on each side along the X-axis direction starting from the center; and on the back surface of the cross-section, set the length of the field of view in the X-axis direction when observing the range starting from the center with an electron microscope as 100 μm. When the sum of the lengths ΣLi obtained by vertically projecting the Zn-Mg structures with an equivalent circle diameter of 0.5 μm or more onto a plane parallel to the X-axis direction within this field of view, the ratio of ΣLi to the length 100 μm of the field of view is 0.05 or more; furthermore, on the back surface of the cross-section, the average thickness of the ZnO layer within the range of 5 mm on each side along the X-axis direction starting from the center is 1.0 μm or less.

[0024] (2) The welded joint according to (1) has a chemical composition containing the element group A.

[0025] (3) The welded joint according to (1) has a chemical composition containing the element group B.

[0026] (4) The welded joint according to (1) has a chemical composition containing the element group C.

[0027] (5) The welded joint according to (1) has a chemical composition containing the element group D.

[0028] (6) The welded joint according to (1) has a chemical composition containing the element group E.

[0029] (7) The welded joint according to (1) has a chemical composition containing the element group F.

[0030] (8) The welded joint according to (1) has a chemical composition containing the element group G.

[0031] (9) The welded joint according to any one of (1) to (8), wherein the ratio is 0.15 or more.

[0032] (10) The welded joint according to any one of (1) to (8), wherein the average thickness of the ZnO layer is 0.2 μm or less.

[0033] (11) The welded joint according to (9), wherein the average thickness of the ZnO layer is 0.2 μm or less.

[0034] (12) The welded joint according to any one of (1) to (8), wherein for the C concentration in the steel base of the non-heat-affected zone calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel base and the coating.

[0035] (13) The welded joint according to (9), wherein for the C concentration in the steel base of the non-heat-affected zone calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel base and the coating.

[0036] (14) The welded joint according to (10), wherein for the C concentration in the steel base of the non-heat-affected zone calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel base and the coating.

[0037] (15) The welded joint according to (11), wherein for the C concentration in the steel base of the non-heat-affected zone calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel base and the coating.

[0038] (16) The welded joint according to (1), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0039] Effects of the Invention

[0040] As described above, according to the present invention, the corrosion resistance of the back surface of the welded joint can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 FIG. is an explanatory diagram schematically showing an example of the structure of a welded joint according to an embodiment of the present invention.

[0042] Figure 2 FIG. is an explanatory diagram for explaining the welded joint according to the same embodiment.

[0043] Figure 3 FIG. is an explanatory diagram for explaining the non - heat - affected part of the welded joint according to the same embodiment.

[0044] Figure 4 FIG. is an explanatory diagram for explaining the back side of the welded part of the welded joint according to the same embodiment.

[0045] Figure 5 FIG. is an explanatory diagram for explaining the back side of the welded part of the welded joint according to the same embodiment.

[0046] Figure 6 FIG. is an explanatory diagram schematically showing an example of the structure of a welded joint according to another embodiment of the present invention.

[0047] Figure 7 FIG. is an explanatory diagram schematically showing an example of the structure of a welded joint according to another embodiment of the present invention.

[0048] Figure 8 FIG. is an explanatory diagram schematically showing an example of the structure of a welded joint according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0049] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that in this specification and the drawings, for components having substantially the same functional structure, repeated descriptions are omitted by assigning the same reference numerals.

[0050] (Regarding the welded joint)

[0051] First, with reference to Figure 1 the overall structure of the welded joint according to the embodiment of the present invention will be described. Figure 1 FIG. is an explanatory diagram schematically showing an example of the structure of the welded joint according to this embodiment.

[0052] It should be noted that hereinafter, for convenience, the coordinate system shown as Figure 1 will be appropriately used for description. In Figure 1 a welded joint formed by lap - fillet welding of two steel plates by arc welding is illustrated.

[0053] Figure 1 Schematically shows the overall structure of a welded joint obtained by fillet welding of a first steel plate and a second steel plate by arc welding, and shows a cross-section of the welded joint perpendicular to the extending direction of the weld bead portion. As Figure 1 Schematically shown, the welded joint 1 according to this embodiment has a first steel plate 10, a second steel plate 20, and a weld bead portion 30. In addition, a heat-affected zone 40 is formed near the weld bead portion 30 in the first steel plate 10 and the second steel plate 20. As Figure 1 Shown below, the normal direction with respect to the surface of the second steel plate 20 is set as the Z-axis direction, the extending direction of the weld bead portion 30 is set as the Y-axis direction, and the direction orthogonal to the Z-axis direction and the Y-axis direction is set as the X-axis direction.

[0054] As Figure 1 Shown, a part of the first steel plate 10 overlaps with the second steel plate 20. The included angle between the first steel plate 10 and the second steel plate 20 (the angle formed by the respective surfaces of the first steel plate 10 and the second steel plate 20 in the Figure 1 shown cross-sectional view) is, for example, in the range of 0° to 10°. However, the angle formed by the first steel plate 10 and the second steel plate 20 is not particularly limited.

[0055] In the welded joint 1 formed by fillet welding as Figure 1 shown, it has a shape in which the end of one steel plate is welded to the surface of the other steel plate. For convenience, in the welded joint 1 according to this embodiment, the steel plate whose end is welded is referred to as the "first steel plate 10", and the steel plate whose surface is welded is referred to as the "second steel plate 20". In the Figure 1 shown welded joint 1, the end 11 of the first steel plate 10 is welded to the surface of the second steel plate 20, and the weld bead portion 30 and the heat-affected zone 40 are formed by welding.

[0056] Hereinafter, taking the case where galvanized steel plates provided with the first coating layer described in detail below on both surfaces of a steel base are used as the blanks of the first steel plate 10 and the second steel plate 20 constituting the welded joint 1 as an example for explanation.

[0057] In addition, the weld bead portion 30 is a portion formed by arc welding or laser welding, and mutual diffusion of constituent elements occurs between the welding wire used as required during welding and the steel plate used as the blank (hereinafter referred to as the "blank steel plate"). It should be noted that in Figure 1 , for the convenience of illustration, the joint interface between the weld bead portion 30 and the first steel plate 10 or the second steel plate 20 is shown as a smooth curve or straight line, but the actual joint interface becomes a complex curved surface due to the fluctuation of the molten metal during welding due to arc plasma or the like. In addition, the weld bead portion 30 extends along the Y-axis direction in the drawing, and the first steel plate 10 and the second steel plate 20 are joined by the weld bead portion 30.

[0058] It should be noted that the composition of the weld bead portion 30 varies depending on the type of welding wire used, the chemical composition of the base steel plate, etc., so it is difficult to determine without doubt the composition that encompasses all possibilities. However, the weld bead portion 30 generally has as its main component the oxide of an "element that is easily oxidized" among the various elements constituting the plating present at the lap portion of the base steel plate (the content of the oxide of the "element that is easily oxidized" is 50% by mass or more). Examples of such elements that are easily oxidized include Al, Mg, Si, etc.

[0059] In addition, when determining the weld bead portion 30 of the welded joint 1, it can be easily visualized by etching the cross-section of the welded joint 1 including the weld bead portion 30, the heat-affected zone 40, and the non-heat-affected zone using an etching solution. For example, as the etching solution, nitric acid ethanol (mixing ratio: ethanol 95%, sulfuric acid 5%) can be used; an etching solution prepared by mixing 2400 cc of water, 60 g of sodium dodecylbenzenesulfonate, 36 g of picric acid, 60 cc of ethanol, and 60 cc of a household detergent solution (such as a common type like dishwashing detergent), etc.

[0060] It should be noted that the oxides generated during welding are roughly classified into two types: scale and slag. The scale, which is the oxide generated on the surface of the weld bead portion 30 during welding, contains 50% or more of Fe by mass when oxygen is removed, and the balance is elements that are easily oxidized and impurities. In addition, the slag contains 50% or more of the elements that are easily oxidized by mass when oxygen is removed, and the balance is less than 50% by mass of Fe and impurities. Here, specific examples of the "elements that are easily oxidized" include Ca, Mg, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, Ti.

[0061] It should be noted that the above-mentioned scale and slag can be easily distinguished by performing a composition analysis using a scanning electron microscope (Scanning Electron Microscope: SEM) equipped with an electron probe micro analyzer (Electron Probe Micro Analyzer: EPMA). More specifically, the cross-section of the portion considered to be scale or slag is subjected to point analysis by EPMA, and it is judged based on which of the above-mentioned "elements that are easily oxidized" and Fe has a content of 50% by mass or more. If the content of Fe is 50% by mass or more, it can be determined that the part of interest is slag, and if the content of the "element that is easily oxidized" is 50% by mass or more, it can be determined that the part of interest is scale.

[0062] Here, in JIS Z3001 (2018), the "point where the surface of the steel base intersects the surface of the weld bead" is defined as the "weld toe". InFigure 1 In the welded joint 1 shown, the point where the surface of the weld bead portion 30 intersects the surface of the steel base or the heat affected zone in the first steel plate 10 or the second steel plate 20 corresponds to this "weld toe".

[0063] In addition, the heat input of arc welding or laser welding is usually carried out from one side of the welded joint 1. On the heat input side of arc welding or laser welding, the weld bead portion 30 is exposed on the surface of the steel plate as the base material, and the width of the weld bead becomes narrower as it goes in the direction of heat input propagation generated by welding. Therefore, by paying attention to the presence or absence of the exposed weld bead portion 30 and the shape of the weld bead shown, the heat input direction during arc welding or laser welding can be determined. It should be noted that the above "exposure of the weld bead portion 30" includes the exposure of the scale formed on the weld bead portion 30.

[0064] In addition, in the present embodiment, the steel plate located on the heat input side of the above-mentioned arc welding or laser welding can also be regarded as the first steel plate 10, and the steel plate located on the opposite side of the heat input side of arc welding or laser welding can be regarded as the second steel plate 20.

[0065] As Figure 1 Schematically shown, a heat affected zone 40 is formed around the weld bead portion 30. This heat affected zone 40 is generated due to the change in the metallographic structure of the blank steel plate caused by the heat input of arc welding or laser welding to the blank steel plate. The size of this heat affected zone 40 depends on the heat input amount during arc welding or laser welding. Generally, the higher the heat input amount, the larger the range of the heat affected zone 40. In addition, due to the change in the metallographic structure of the blank steel plate, the morphology (appearance) during visual inspection is different from the non-transformed part of the blank steel plate. Therefore, the heat affected zone 40 can be easily distinguished.

[0066] Hereinafter, the weld bead portion 30 and the heat affected zone 40 are sometimes collectively referred to as the "welded portion".

[0067] As Figure 1 Schematically shown, in the cross-section obtained by cutting the welded joint 1 according to the present embodiment along the Z-axis direction, the surface on the positive direction side of the Z-axis direction is set as the "first surface", and the surface on the negative direction side of the Z-axis direction is set as the "second surface". In Figure 1 In the example shown, the weld bead portion 30 is exposed on the "first surface" side and not exposed on the "second surface" side. From this manner of the weld bead portion 30, it can be known that in Figure 1 In the example shown, the heat input direction during welding is the direction from the first surface side towards the second surface side. In addition, even if the weld bead portion 30 is exposed on the "second surface" side, the width of the weld bead portion 30 in the X-axis direction on the second surface side (for example, the weld toe spacing in the X-axis direction on the second surface side) is smaller than the width of the weld bead portion 30 in the X-axis direction on the first surface side (for example, the weld toe spacing in the X-axis direction on the first surface side).

[0068] In view of the above, in the welded joint 1 according to the present embodiment, in the cross section obtained by cutting the welded joint 1 in the Z-axis direction, the surface on the side where the toe pitch in the X-axis direction of the weld bead portion 30 on the first surface side and the toe pitch in the X-axis direction of the weld bead portion 30 on the second surface side is smaller is defined as the "back surface" of the welded joint 1. Alternatively, when there are no toe portions on either the first surface side or the second surface side, the surface on the side without toe portions is defined as the "back surface" of the welded joint 1.

[0069] In the Figure 1 illustrated example, the first surface side corresponds to the heat input side during welding, and the second surface side corresponds to the "back surface of the welded joint 1" on the side opposite to the heat input side.

[0070] In the case of Figure 1 the back surface of the welded joint 1 as shown, on the surface of the heat affected zone 40 (for example, Figure 1 the region surrounded by the dashed line in Figure 1 ), there is a second plating layer described later. The second plating layer is a plating layer that remains on the surface of the welded joint 1 even after welding of the blank steel plate. In the welded joint 1 according to the present embodiment, by making the second plating layer present on the surface of the welded portion on the back surface of the welded joint 1 (

[0071] the region surrounded by the dashed line in

[0072] ), the corrosion resistance of the back surface of the welded joint 1 can be further improved by the corrosion resistance exhibited by the second plating layer. It should be noted that the first plating layer may also be referred to as a plating layer, and the second plating layer may be referred to as a remaining plating layer. Figure 2 Next, with reference to Figure 2 which schematically shows an example of the lap corner joint of the welded joint 1 according to the present embodiment and shows the X-Z cross section when the welded joint 1 is cut in the Z-axis direction,

[0073] in the following description, the portion of the welded joint 1 where there is no heat affected zone caused by welding (in other words, the portion that is not the weld bead portion 30 and the heat affected zone 40) is referred to as the "non-heat affected zone". In the Figure 2 shown welded joint 1, it can be considered that the non-heat affected zone exists in the direction orthogonal to the extension direction of the weld bead portion 30 ( Figure 2 the Y-axis direction in Figure 2 ) starting from the toe T and in the direction away from the weld bead portion 30 ( Figure 2The region R1 surrounded by the dashed line exists at a certain distance from the weld bead portion 30 (for example, in the X-axis direction, at a position more than 5 mm away from the weld toe T toward the opposite side of the weld bead portion 30), so it can be clearly considered as a non-heat-affected zone.

[0074] Figure 3 It is a diagram schematically showing a partial cross-section parallel to the plate thickness direction of the non-heat-affected zone R1. As Figure 3 Schematically shown, the non-heat-affected zone R1 of the first steel plate 10 and the second steel plate 20 has a steel base 101 and a first coating layer 103 located on the surface of the steel base 101.

[0075] Hereinafter, the steel base 101 and the first coating layer 103 of the non-heat-affected zone will be described in detail.

[0076] 《Regarding the steel base 101》

[0077] In the welded joint 1 according to the present embodiment, the size, composition, structure, and mechanical properties of the steel base 101 corresponding to the base material of the blank steel plate are not particularly limited. For example, according to the mechanical strength (such as tensile strength) required for the welded joint 1, various steel plates can be used as the steel base 101. As such steel plates, for example, various Al-killed steels can be cited; ultra-low carbon steels containing Ti, Nb, etc.; high-strength steels formed by further containing strengthening elements such as P, Si, Mn, etc. in the ultra-low carbon steel; various steel plates containing other various components (Cr, N, Cu, B, Ni, Mg, Ca, V, Co, Zn, As, Y, Zr, Mo, Sn, Sb, Ta, W, Pb, Bi, REM, etc.).

[0078] In the above high-strength steel, for example, using a high-strength steel with a tensile strength of 780 MPa or more (so-called high-strength steel of 780 MPa class or more) can further improve the robustness of the welded joint 1, so it is particularly preferred. Here, the tensile strength of the steel base 101 can be measured by a known method. As an example, from the part corresponding to the steel base 101 of the non-heat-affected zone of the welded joint 1 whose tensile strength is to be measured, a test piece with a size that can be collected from the welded joint 1 and specified in JIS Z 2241 (2011) is manufactured, and the tensile strength of the obtained test piece is measured by the method specified in JIS Z 2241 (2011).

[0079] In addition, the thickness of the steel base 101 is not particularly limited and can be appropriately set according to the mechanical strength required for the welded joint 1, etc.

[0080] 《Regarding the first coating layer 103》

[0081] As Figure 3Schematically shown, the first coating layer 103 is provided on the steel base 101 in the non-heat-affected portion R1. The first coating layer 103 is derived from the coating layer of the coated steel sheet which is the blank of the welded joint 1.

[0082] Hereinafter, first, the chemical composition of the first coating layer 103 will be described in detail.

[0083] ◇Regarding the chemical composition of the first coating layer 103

[0084] According to one aspect, the chemical composition of the first coating layer 103 according to the present embodiment contains, by mass%, Al: 10.00 to 70.00%, Mg: 3.00 to 20.00%, Fe: 0.01 to 15.00, and the balance is Zn and impurities of 5.0000 mass% or more. That is, in the chemical composition of the first coating layer 103 according to the present embodiment, the contents of Al, Mg, and Fe are within the above ranges, and the sum of these contents is 95.0000 mass% or less, and the balance is Zn and impurities of 5.0000 mass% or more.

[0085] In addition, according to another aspect, the chemical composition of the first coating layer 103 according to the present embodiment contains, by mass%, Al: 10.00 to 70.00%, Mg: 3.00 to 20.00%, Fe: 0.01 to 15.00%, and also contains one or more selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, element group F, and element group G, and the balance is Zn and impurities of 5.00 mass% or more. That is, in the chemical composition of the first coating layer 103 according to the present embodiment, the contents of Al, Mg, and Fe are within the above ranges, and the sum of the contents of these Al, Mg, Fe, and element groups A to G is 95.0000 mass% or less, and the balance is Zn and impurities of 5.0000 mass% or more.

[0086] [Element group A]: One or two selected from the group consisting of Si: greater than 0% and 10.00% or less, and Ca: greater than 0% and 4.00% or less

[0087] [Element group B]: One or more selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less

[0088] [Element Group C]: selected from one or more of the group consisting of Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less

[0089] [Element Group D]: selected from one or more of the group consisting of Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less

[0090] [Element Group E]: selected from one or more of the group consisting of Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less

[0091] [Element Group F]: selected from one or more of the group consisting of La: greater than 0% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less

[0092] [Element Group G]: B: greater than 0% and 0.5000% or less

[0093] Thus, the first coating 103 according to the present embodiment is a coating having the following chemical composition, the chemical composition containing, by mass%, Al: 10.00 to 70.00%, Mg: 3.00 to 20.00%, Fe: 0.01 to 15.00%, selectively containing one or more elements selected from the group consisting of Element Group A, Element Group B, Element Group C, Element Group D, Element Group E, Element Group F, and Element Group G, and the balance being Zn and impurities of 5.0000 mass% or more

[0094] [Al: 10.00 to 70.00 mass%]

[0095] Al is an element necessary for the main metallographic structure (Zn-Al-Mg-based metallographic structure) of the first coating layer 103 involved in this embodiment. As a coated steel sheet, it contains a certain amount or more of this element to ensure the corrosion resistance of the part that becomes the heat-affected zone and the part that becomes the non-heat-affected zone. If the Al content in the first coating layer 103 is less than 10.00% by mass, the corrosion resistance of the above-mentioned parts that become the heat-affected zone and the non-heat-affected zone cannot be ensured. This is because when the Al content is insufficient, the alloying reaction between the coating layer and the steel base during welding cannot be controlled, resulting in an increase in the formation amount of the η-Zn phase that is vulnerable to corrosion. Therefore, in the first coating layer 103 involved in this embodiment, the Al content is 10.00% by mass or more. The Al content is preferably 18.00% by mass or more, and more preferably 25.00% by mass or more. By making the Al content within the above range, the corrosion resistance of the coated steel sheet can be ensured.

[0096] On the other hand, if the Al content in the first coating layer 103 is greater than 70.00% by mass, the amount of the Al phase that acts as a cathode when placed in a corrosive environment will increase excessively, resulting in a relatively decrease in the formation amount of the Mg-Zn phase with excellent corrosion resistance, thereby sacrificing the corrosion protection performance and making the corrosion of the steel base easy to proceed. Therefore, the corrosion resistance of the coated steel sheet cannot be ensured. Therefore, in the first coating layer 103 involved in this embodiment, the Al content is 70.00% by mass or less. The Al content is preferably 50.00% by mass or less, and more preferably 48.00% by mass or less.

[0097] [Mg: 3.00 - 20.00% by mass]

[0098] Mg is an element that improves corrosion resistance and is also an element necessary for the main metallographic structure (Zn-Al-Mg-based metallographic structure) of the first coating layer 103 involved in this embodiment. As a coated steel sheet, it contains a certain amount or more of this element to ensure the corrosion resistance of the part that becomes the heat-affected zone and the part that becomes the non-heat-affected zone. Therefore, in order to obtain sufficient corrosion resistance in the welded part, in the first coating layer 103 involved in this embodiment, the Mg content is 3.00% by mass or more. The Mg content is preferably 6.00% by mass or more, and more preferably 9.00% by mass or more. By making the Mg content within the above range, the corrosion resistance of the coated steel sheet can be ensured.

[0099] On the other hand, if the Mg content in the first coating layer 103 is greater than 20.00% by mass, the anodic dissolution of the coating layer is likely to occur when placed in a corrosive environment, and thus the corrosion resistance of the coated steel sheet cannot be ensured. Therefore, in the first coating layer 103 according to the present 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 setting the Mg content within the above range, the corrosion resistance of the coated steel sheet can be ensured.

[0100] [Fe: 0.01 to 15.00% by mass]

[0101] Elements constituting the steel base 101 may mix into the first coating layer 103 from the steel base 101. In particular, in the hot-dip plating method, the mutual diffusion of elements caused by the solid-liquid reaction between the steel base 101 and the first coating layer 103 makes it easy for the elements constituting the steel base 101 to mix into the first coating layer 103. This mixing of elements causes the first coating layer 103 to contain a certain amount of Fe, and its content is usually 0.01% by mass or more. If the above mutual diffusion is promoted, the adhesion between the steel base 101 and the first coating layer 103 is improved. From the perspective of improving the adhesion between the steel base 101 and the first coating layer 103, the Fe content in the first coating layer 103 is preferably 0.20% by mass or more.

[0102] In addition, within the range that does not impair the effects of the present invention, Fe may be intentionally added to the plating bath used in manufacturing the first coating layer 103. However, if the Fe content in the plating bath increases, high-melting-point intermetallic compounds of Fe and Al will be formed in the plating bath, and there is a tendency for the appearance quality to be significantly reduced due to the adhesion of the high-melting-point intermetallic compounds to the first coating layer 103 as dross, so it is not preferred. From this perspective, by adjusting the Fe content in the plating bath, the Fe content in the first coating layer 103 is made 15.00% by mass or less. The Fe content in the first coating layer 103 is more preferably 10.00% by mass or less.

[0103] In the first coating layer 103, the balance of the above-mentioned Al, Mg, and Fe is Zn and impurities of 5.0000% by mass or more.

[0104] Zn is an essential element for the main metallographic structure (Zn-Al-Mg-based metallographic structure) constituting the coating layer 103 according to the present embodiment, and is an important element for improving the corrosion resistance of the coated steel sheet. In addition, by making the coating layer 103 contain the above-mentioned Al, Mg, and Fe within the above range and also contain 5.00% by mass or more of Zn, the corrosion resistance required for the coated steel sheet can be ensured.

[0105] Next, the element groups A to E that the chemical composition of the first coating layer 103 involved in another aspect of the present embodiment may selectively have will be described in detail.

[0106] It should be noted that when the first coating layer 103 involved in the present embodiment contains at least any one of the elements belonging to the following element groups B to E, it is preferably contained at least any one of the elements belonging to the following element groups B to E within the following content range and with a total content of 5.0000 mass% or less.

[0107] By setting the total content of the elements belonging to element groups B to E to 5.0000 mass% or less, the effects exhibited by the addition of each element described in detail below can be enjoyed without mutual impairment. The total content of the elements belonging to element groups B to E is preferably 1.0000 mass% or less, and more preferably 0.2000 mass% or less.

[0108] In addition, in another aspect of the first coating layer 103 involved in the present embodiment, as the chemical composition, the first coating layer 103 more preferably contains 9.00 mass% or more and 15.00 mass% or less of Mg, and contains 0.05 mass% or more and 4.00 mass% or less of Ca as element group A. By making the coating layer 103 have such a chemical composition, more excellent corrosion resistance can be exhibited.

[0109] ◇ Element group A

[0110] In another aspect of the first coating layer 103 involved in the present embodiment, the element group A that the first coating layer 103 may contain will be described. At least any one of the following element group A is an element that can be contained in the first coating layer 103 instead of a part of the balance of Zn.

[0111] [Element group A]: 1 or 2 selected from the group consisting of Si: greater than 0% and 10.00% or less, and Ca: greater than 0% and 4.00% or less

[0112] [Si: 0 to 10.00 mass%]

[0113] Since the case where the first coating layer 103 involved in the present embodiment does not contain Si can also be considered, the lower limit of the content of Si is 0 mass%. On the other hand, Si is an element that can inhibit the excessive growth of the Fe-Al-based metallographic structure formed at the interface between the first coating layer 103 and the steel base 101 and can improve the adhesion between the first coating layer 103 and the steel base 101. When the first coating layer 103 contains Si, in order to inhibit the excessive growth of the Fe-Al-based metallographic structure, the content of Si is preferably 0.05 mass% or more, and more preferably 0.20 mass% or more.

[0114] On the other hand, if the Si content is greater than 10.00 mass%, Si may form an excessive amount of high-melting-point intermetallic compounds with Mg and inhibit the formation of Al-Mg oxides that have the effect of suppressing Zn evaporation. Therefore, it is difficult to suppress Zn evaporation during welding of the plated steel sheet. Accordingly, the Si content in the first plating layer 103 is preferably 10.00 mass% or less. In addition, if the Si content in the plating bath used to manufacture the first plating layer 103 is excessive, the viscosity of the plating bath may increase to more than necessary, resulting in a decrease in the workability (hereinafter referred to as "plating workability") during manufacturing of the plated steel sheet. Therefore, from the perspective of plating workability, by adjusting the Si content in the plating bath, the Si content in the first plating layer 103 is preferably 5.00 mass% or less, more preferably 2.00 mass% or less.

[0115] [Ca: 0 to 4.00 mass%]

[0116] Since the case where the first plating layer 103 according to the present embodiment does not contain Ca can also be considered, the lower limit of the Ca content is 0 mass%. On the other hand, when Ca is contained in the first plating layer 103, it forms intermetallic compounds with Al and Zn. In addition, when Si and Ca are contained together in the first plating layer 103, Ca forms intermetallic compounds with Si. These intermetallic compounds have a high melting point and a stable structure, and thus can suppress liquid metal embrittlement cracks (Liquid Metal Embrittlement: LME) during welding of the plated steel sheet. When Ca is contained in the first plating layer 103, the effect of suppressing LME during welding becomes apparent by setting the Ca content to 0.01 mass% or more. The Ca content in the first plating layer 103 is more preferably 0.05 mass% or more.

[0117] On the other hand, if the Ca content in the first plating layer 103 is greater than 4.00 mass%, the corrosion resistance of the plated steel sheet may decrease. From this perspective, the Ca content in the first plating layer 103 is 4.00 mass% or less. The Ca content in the first plating layer 103 is preferably 2.50 mass% or less, more preferably 1.50 mass% or less.

[0118] ◇Element group B

[0119] Next, in another aspect of the first plating layer 103 according to the present embodiment, an element group B that the first plating layer 103 may contain is described. At least any one of the following element group B is an element that can be contained in the first plating layer 103 in place of a part of the balance of Zn.

[0120] [Element group B]: Select one or more from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less

[0121] [Sb: 0 to 0.5000 mass%]

[0122] [Pb: 0 to 0.5000 mass%]

[0123] [Sr: 0 to 0.5000 mass%]

[0124] Since it is also possible to consider the case where the first coating layer 103 according to the present embodiment does not contain Sb, Pb, and Sr, the lower limit of the content of these elements is 0 mass%. On the other hand, when the first coating layer 103 contains at least any one of Sb, Pb, and Sr, spangles are formed on the surface of the first coating layer 103, and an improvement in metallic luster can be achieved. Therefore, from the viewpoint of improving the appearance of the coated steel sheet, it is preferable that the first coating layer 103 contains at least any one of Sb, Pb, and Sr. This appearance improvement effect appears when the content of at least any one of Sb, Pb, and Sr is 0.0500 mass% or more. Therefore, when the first coating layer 103 contains at least any one of Sb, Pb, and Sr, the content of these elements is preferably set to 0.0500 mass% or more independently of each other.

[0125] On the other hand, if a first coating layer 103 is formed in which the content of any one of Sb, Pb, and Sr is greater than 0.5000 mass%, the amount of scum generated in the plating bath for forming the first coating layer 103 increases, and a coated steel sheet with good coating properties cannot be manufactured. Therefore, the content of Sb, Pb, and Sr in the first coating layer 103 is 0.5000 mass% or less independently of each other. The content of Sb, Pb, and Sr is preferably 0.2000 mass% or less independently of each other.

[0126] ◇Element group C

[0127] Next, in another aspect of the first coating layer 103 according to the present embodiment, an element group C that the first coating layer 103 may contain is described. At least any one of the following element groups C is an element that can be contained in the first coating layer 103 in place of a part of the balance of Zn.

[0128] [Element group C]: selected from one or more of the group consisting of Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less

[0129] [Cu: 0 to 1.0000 mass%]

[0130] [Ti: 0 to 1.0000 mass%]

[0131] [Cr: 0 to 1.0000 mass%]

[0132] [Nb: 0 to 1.0000 mass%]

[0133] [Ni: 0 to 1.0000 mass%]

[0134] [Mn: 0 to 1.0000 mass%]

[0135] [Co: 0 to 1.0000 mass%]

[0136] [V: 0 to 1.0000 mass%]

[0137] Since it is also possible to consider the case where the first coating layer 103 according to the present embodiment does not contain Cu, Ti, Cr, Nb, Ni, Mn, Co, and V, the lower limit of the content of these elements is 0 mass%. On the other hand, if the first coating layer 103 contains at least any one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V, when welding the coated steel sheet, these elements will enter the Fe-Al-based metallographic structure formed by welding, and the corrosion resistance of the formed welded portion can be improved. The effect of improving the corrosion resistance of the welded portion appears when the content of at least any one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the first coating layer 103 is 0.0050 mass% or more. Therefore, when the first coating layer 103 contains at least any one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V, the content of these elements is preferably independently set to 0.0050 mass% or more respectively.

[0138] On the other hand, if a first coating layer 103 is formed in which the content of any one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is greater than 1.0000% by mass, these elements form various intermetallic compounds in the plating bath used to form the first coating layer 103, resulting in an increase in the viscosity of the plating bath and making it impossible to manufacture a plated steel sheet with good plating properties. Therefore, the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the first coating layer 103 are each independently set to 1.0000% by mass or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V are preferably each independently 0.2000% by mass or less.

[0139] [Mo: 0 to 1.0000% by mass]

[0140] Since the case where the first coating layer 103 according to the present embodiment does not contain Mo is also considered, the lower limit of the content of Mo is 0% by mass. On the other hand, when the first coating layer 103 contains Mo, the corrosion resistance can be improved. The effect of improving the corrosion resistance appears when the content of Mo is 0.0100% by mass or more. Therefore, when Mo is contained, its content is preferably set to 0.0100% by mass or more.

[0141] On the other hand, if a first coating layer 103 is formed in which the content of Mo is greater than 1.0000% by mass, a large amount of scum will be generated in the plating bath used, which is not preferable. Therefore, the content of Mo is 1.0000% by mass or less. The content of Mo is preferably 0.0500% by mass or less.

[0142] ◇Element group D

[0143] Next, in another aspect of the first coating layer 103 according to the present embodiment, the element group D that the first coating layer 103 may contain will be described. The elements of the element group D shown below are elements that can be contained in the first coating layer 103 in place of a part of the balance of Zn.

[0144] [Element group D]: One or more selected from the group consisting of Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less

[0145] [Sn: 0 to 1.0000% by mass]

[0146] [In: 0 to 1.0000% by mass]

[0147] [Bi: 0 to 1.0000% by mass]

[0148] Since it is also conceivable that the first coating layer 103 according to this embodiment does not contain Sn, In, or Bi, the lower limit of their content is 0% by mass. Sn, In, and Bi are elements that increase the Mg dissolution rate when the first coating layer 103 is placed in a corrosive environment. When the Mg dissolution rate increases, Mg ions are supplied to the exposed part of the steel base 101, and the corrosion resistance is improved. From this perspective, when Sn, In, and Bi are contained, the contents of Sn, In, and Bi are preferably independently set to 0.0050% by mass or more.

[0149] On the other hand, adding too much Sn, In, or Bi may excessively promote the Mg dissolution rate, resulting in a decrease in the corrosion resistance of the plated steel sheet. The increase in the Mg dissolution rate becomes significant when the contents of Sn, In, and Bi are greater than 1.0000% by mass. Therefore, the contents of Sn, In, and Bi are independently 1.0000% by mass or less. The contents of Sn, In, and Bi are preferably independently 0.2000% by mass or less.

[0150] ◇Element group E

[0151] Next, in another aspect of the first coating layer 103 according to this embodiment, an element group E that the first coating layer 103 may contain is described. At least one of the elements shown below is an element that can be contained in the first coating layer 103 in place of a part of the balance of Zn.

[0152] [Element group E]: One or more selected from the group consisting of Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less

[0153] [Zr: 0 - 1.0000% by mass]

[0154] [Ag: 0 - 1.0000% by mass]

[0155] [Li: 0 - 1.0000% by mass]

[0156] Since it is also conceivable that the first coating layer 103 according to this embodiment does not contain Zr, Ag, or Li, the lower limit of the content of these elements is 0% by mass. On the other hand, when the first coating layer 103 contains at least any one of Zr, Ag, and Li, the plating operability can be improved. The effect of improving the plating operability appears when the content of at least any one of Zr, Ag, and Li in the first coating layer 103 is 0.0100% by mass or more. Therefore, when containing at least any one of Zr, Ag, and Li, the contents of these elements are preferably independently set to 0.0100% by mass or more.

[0157] On the other hand, if a first coating layer 103 is formed in which the content of any one of Zr, Ag, and Li is greater than 1.0000% by mass, a large amount of scum is likely to be generated in the plating bath used to form the first coating layer 103. Therefore, the content of at least any one of Zr, Ag, and Li is independently 1.0000% by mass or less. The content of at least any one of Zr, Ag, and Li is preferably independently 0.1000% by mass or less.

[0158] ◇Element group F

[0159] Next, in another aspect of the first coating layer 103 according to the present embodiment, an element group F that can be contained in the first coating layer 103 will be described. At least any one of the following element groups F is an element that can be contained in the first coating layer 103 in place of a part of the balance of Zn.

[0160] [Element group F]: One or more selected from the group consisting of La: greater than 0% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less

[0161] [La: 0 to 0.5000% by mass]

[0162] [Ce: 0 to 0.5000% by mass]

[0163] [Y: 0 to 0.5000% by mass]

[0164] Since it is also conceivable that the first coating layer 103 according to the present embodiment does not contain La, Ce, and Y, the lower limit of the content of these elements is 0% by mass. On the other hand, La, Ce, and Y are elements that exhibit almost the same effect as Ca and suppress the formation of pores during welding. This is because the atomic radius of each element is close to the atomic radius of Ca. When these elements are contained in the first coating layer 103, they replace the position of Ca. Therefore, these elements are detected at the same position as Ca in EDS (Energy Dispersive X-ray Spectroscopy).

[0165] The effect of suppressing the formation of pores during welding is manifested by setting the content of these elements independently to 0.0100% by mass or more. Therefore, when at least any one of Zr, Ag, and Li is contained, the content of these elements is preferably independently set to 0.0100% by mass or more. The content of La, Ce, and Y in the first coating layer 103 is more preferably independently 0.0500% by mass or more.

[0166] On the other hand, in the plating bath for manufacturing the first plating layer 103, if the contents of La, Ce, and Y are excessive, the viscosity of the plating bath may increase beyond the necessary level, resulting in a reduction in plating operability. Therefore, from the perspective of plating operability, by adjusting the contents of La, Ce, and Y in the plating bath, the contents of La, Ce, and Y are each independently 0.5000 mass% or less. The contents of La, Ce, and Y are preferably each independently 0.1000 mass% or less.

[0167] ◇Element group G

[0168] Next, in another aspect of the first plating layer 103 according to the present embodiment, the element group G that the first plating layer 103 may contain will be described. The elements of the element group G shown below are elements that can be contained in the first plating layer 103 in place of a part of the balance of Zn.

[0169] [Element group G]: B: greater than 0% and 0.5000% or less

[0170] [B: 0 - 0.5000 mass%]

[0171] Since the case where the first plating layer 103 according to the present embodiment does not contain B is also considered, the lower limit of its content is 0 mass%. When B is contained in the first plating layer 103, it has the effect of suppressing LME. It is speculated that this is because when B is contained in the first plating layer 103, it reacts with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. In addition, it is considered that by having B present in the first plating layer 103, B diffuses from the first plating layer 103 to the steel base 101, and has the effect of suppressing LME of the steel base 101 through grain boundary strengthening. Furthermore, it is speculated that the melting points of the various intermetallic compounds formed with B are extremely high, so they also play a role in suppressing Zn evaporation during welding. These improvement effects are manifested by containing 0.0500 mass% or more of B. Therefore, when B is contained, the content of B is preferably 0.0500 mass% or more.

[0172] On the other hand, when the plating bath contains an excessive amount of B in order to contain B in the first plating layer 103, the melting point of the plating will rise sharply, resulting in a reduction in plating operability and making it impossible to manufacture a plated steel sheet with excellent plating properties. This reduction in plating operability becomes significant when the content of B is greater than 0.5000 mass%, so the content of B is 0.5000 mass% or less. The content of B is preferably 0.1000 mass% or less.

[0173] [Method for measuring chemical composition]

[0174] The chemical composition of the above-described first coating 103 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). It should be noted that when analyzing the chemical composition in units of up to 0.1 mass%, ICP-AES is used, and when analyzing trace chemical components less than 0.1 mass%, ICP-MS is used. A sample cut from the non-heat-affected zone of the welded joint 1 is immersed in a 10% HCl aqueous solution containing an inhibitor for about 1 minute, the coating part is peeled off, and a solution in which the coating is dissolved is prepared. By analyzing the obtained solution using ICP-AES or ICP-MS, the overall average chemical composition of the coating can be obtained.

[0175] ◇Regarding the coating amount of the first coating 103

[0176] There is no particular regulation regarding the coating amount of the first coating 103 as described above. For example, on each side of the steel base 101, it is preferably 15 to 250 g / m 2 or so. By making the coating amount of the first coating 103 within the above range, the first coating 103 according to the present embodiment can exhibit sufficient corrosion resistance. The thickness of the first coating 103 having such a coating amount is approximately 5 to 40 μm or so.

[0177] It should be noted that the coating amount of the first coating 103 is measured as follows. First, a sample having a size of 30 mm × 30 mm in plan view is cut from the non-heat-affected zone of the welded joint 1, and the mass of the sample is measured in advance. It should be noted that when cutting the sample, the entire thickness direction is cut out. A tape is pasted on one surface of the sample so that the coating on the one surface side is not dissolved in the next process. On this basis, the sample is immersed in a 10% HCl aqueous solution containing an inhibitor, and the first coating 103 is pickled and peeled off, and the mass of the sample after pickling is measured. The coating amount of the first coating 103 on each surface can be determined based on the mass change of the sample before and after pickling.

[0178] ◇Regarding the metallographic structure of the first coating 103

[0179] Next, the metallographic structure of the first coating 103 having the chemical composition described above will be described.

[0180] The first coating layer 103 involved in this embodiment has the above chemical composition. In addition, the first coating layer 103 is formed by the manufacturing method described in detail below, and thus contains metallographic structures such as Fe2Al5 phase, Fe4Al3 phase, FeAl phase, ηZn phase, α phase, MgZn2 phase, Mg2Zn3 phase, MgZn phase, and Mg phase. In addition, according to the elements that the first coating layer 103 may also contain, in addition to the above metallographic structures, the first coating layer 103 may also contain metallographic structures such as Al-Si-Ca phase, Al-Si-Ca-Fe phase, Mg2Si phase, and Mg2Sn phase. The first coating layer 103 involved in this embodiment shows the properties of suppressing the occurrence of LME and excellent corrosion resistance by having the above metallographic structures.

[0181] Here, regarding what kind of metallographic structure the first coating layer 103 involved in this embodiment has, it can be determined by observing the cross-section of the first coating layer 103 existing in the welded joint 1 using SEM. That is, by observing the solidification structure of the coating layer 103 with SEM and based on the point analysis results of SEM-EPMA in the observation field of view, it is possible to determine what kind of metallographic structure it has.

[0182] More specifically, the observation position is set to the first coating layer 103 within the non-heat-affected zone more than 80 mm from the weld toe on the back side of the weld bead portion 30, and the size of the observation area is set to 300 μm × 300 μm. Set the acceleration voltage: 15.0 kV, irradiation current: 4.999×10 -8 A, irradiation time: 50 milliseconds, and observe this range at a magnification of 2000 times. Under this condition, obtain the backscattered electron image of the area of interest, and then use the contrast of the backscattered electron image to perform three point analyses for each metallographic structure.

[0183] In particular, when determining what kind of metallographic structure the coating layer on the back of the welded joint 1 has, focus on at least the cross-section including part of the welded portion and observe it using SEM.

[0184] More specifically, the observation position is set to within the heat-affected zone on the back side, and the size of the observation area is set to 150 μm × 150 μm. Set the acceleration voltage: 15.0 kV, irradiation current: 4.999×10 -8 A, irradiation time: 50 milliseconds, and observe this range at a magnification of 2000 times. Under this condition, obtain the backscattered electron image of the area of interest, and then use the contrast of the backscattered electron image to perform three point analyses for each metallographic structure.

[0185] ◇ Regarding the carbon (C) concentration of the surface layer part of the steel base in the non-heat-affected zone

[0186] In addition, by glow discharge optical emission spectrometry (GDS), the non-heat-affected part of the welded joint 1 according to the present embodiment is measured in the depth direction (the thickness direction of the steel base 101) starting from the surface of the first coating layer 103, and depth curves related to the distributions of Zn, Fe, and C are measured.

[0187] It should be noted that the measurement based on this GDS can be carried out using a commercially available glow discharge optical emission spectrometer under the following conditions.

[0188] ◇ Commercially available glow discharge optical emission spectrometer (for example, GDS850A manufactured by LECO Japan Co., Ltd., etc.)

[0189] · Argon pressure: 0.27 MPa

[0190] · Anode diameter: 4 mm φ

[0191] · RF (high frequency) output power: 30 W

[0192] In the obtained measurement results, first, the position of the interface between the first coating layer 103 and the steel base 101 is specified. More specifically, in the depth curve related to the distributions of Zn and Fe obtained, the depth at which the curve representing the Zn intensity intersects the curve representing the Fe intensity is taken as the interface between the first coating layer 103 and the steel base 101. Then, in the depth curve related to the distribution of C, starting from the position of the interface between the first coating layer 103 and the steel base 101, the change in the C concentration in the depth direction is confirmed. The C concentration focused on below can be said to be the C concentration of the part of the steel base 101 adjacent to the interface of the first coating layer 103.

[0193] In the non-heat-affected part of the welded joint 1 according to the present embodiment, the depth at which the C concentration becomes 0.05 mass% or less is preferably 10 μm or more from the interface between the first coating layer 103 and the steel base 101. C in steel is an element that promotes LME. Therefore, by reducing the C concentration at the interface part between the steel base 101 and the first coating layer 103, which is the starting point for the generation of LME cracks, LME during welding can be further suppressed, and the LME resistance can be further improved. The depth at which the C concentration becomes 0.05 mass% or less is more preferably 15 μm or more. On the other hand, the upper limit of the depth at which the C concentration becomes 0.05 mass% or less is not particularly limited.

[0194] Here, for the C concentration at the interface part between the steel base 101 and the first coating layer 103, it is set to the desired value by controlling the conditions of the decarburizing annealing applied to the steel base 101. This decarburizing annealing is carried out as needed when manufacturing the plated steel sheet that is the blank of the first steel sheet 10 or the second steel sheet 20.

[0195] <Regarding the back side of the welded part>

[0196] Next, with reference to Figure 2 , Figure 4 and Figure 5 , the state of the back side of the welded part (the part composed of the weld bead part 30 and the heat affected part 40) in the welded joint 1 according to this embodiment will be described in detail.

[0197] In the welded joint 1 according to this embodiment, a plated steel sheet having the first plating layer 103 is used as the blanks of the first steel sheet 10 and the second steel sheet 20. The first plating layer 103 has the chemical composition described above and is formed by a specific manufacturing method described below. At this time, the above-mentioned first plating layer 103 is provided on the steel base 101. Thus, when manufacturing the welded joint 1 according to this embodiment, it is possible to suppress the combustion (oxidation) of Zn in the first plating layer 103 during the above-mentioned welding.

[0198] Here, the part that becomes the weld bead part 30 after arc welding or laser welding reaches about 2000°C during welding. On the other hand, since Zn in the first plating layer 103 starts to evaporate from about 600°C as described above, almost all of the Zn in the first plating layer 103 has evaporated in the part where the weld bead part 30 is formed.

[0199] However, in the back side of the welded part as shown in Figure 2 (R2 in Figure 2 ), the reaching temperature of the part where the heat affected part 40 is formed during welding is about 550 to 1400°C. In such a reaching temperature situation, even after welding, Zn in the first plating layer 103 having the chemical composition described above and formed by a specific manufacturing method described below will not completely burn (oxidize).

[0200] It should be noted that the specific mechanism for suppressing the combustion (oxidation) of Zn is not yet clear, but the following factors can be considered. Generally, when heating a plating layer containing Zn, the plating layer is heated to above the boiling point of Zn, 906°C, and Zn evaporates from the surface layer of the plating layer. The evaporated Zn burns through an oxidation reaction, causing further evaporation of Zn. As the plating layer completely evaporates, ZnO, the combustion product, will adhere. However, for the first plating layer 103 according to this embodiment, it is speculated that Zn is alloyed to transform into a substance with a boiling point greater than 906°C, thereby suppressing the evaporation / combustion of Zn. In this case, with the heat input during welding, the first plating layer 103 derived from the plated steel sheet as the blank remains as the second plating layer having a specific metallographic structure.

[0201] When describing the state of such a second plating layer, in this embodiment, for Figure 2Define the "center of the welded portion at the back surface" C0 shown schematically. That is, in the present embodiment, in the direction orthogonal to the extending direction of the weld bead portion 30 ( Figure 2 the Y-axis direction in it), the direction (which can also be said to be the direction parallel to the width direction of the weld bead portion 30). Figure 2 the X-axis direction in it) cut Figure 2 the cross-section obtained by cutting the welded joint 1 shown (corresponding to Figure 2 the cross-section of the X-Z plane in it) at the back surface ( Figure 2 R2 in it), the midpoint of the line segment connecting the end closest to one end of the first plating layer 103 and the end farthest from it among the ends of the heat-affected zone 40 (a line segment parallel to Figure 2 the X-axis direction in it) is defined as the "center of the welded portion at the back surface" C0.

[0202] Next, with reference to Figure 4 , the structure of the welded joint 1 at the back surface of the welded portion shown Figure 2 will be described. Figure 4 is a diagram schematically showing a partial cross-section parallel to the plate thickness direction of the welded joint 1 according to the present embodiment, including the back surface R2 of the welded portion. As Figure 4 schematically shown, the back surface R2 of the welded portion of the welded joint 1 according to the present embodiment has a second plating layer 105 on the weld bead portion 30 and the heat-affected zone 40, and on at least a part of the weld bead portion 30 or the heat-affected zone 40.

[0203] The second plating layer 105 on the weld bead portion 30 or the heat-affected zone 40 is a plating layer remaining after partial alloying of the plating layer of the blank of the welded joint 1, i.e., the plated steel sheet, by welding. This second plating layer 105 is a zinc-based plating layer containing a Zn-Mg structure 107 in the layer.

[0204] Here, the Zn-Mg structure 107 is a metallographic structure containing at least any one or more of, for example, MgZn2 phase, Mg2Zn3 phase, MgZn phase, and Mg phase. In addition, this Zn-Mg structure 107 also includes a layered structure composed of η-Zn phase and MgZn2 phase, etc.

[0205] In addition, in the present embodiment, the Zn-Mg structure 107 means that when SEM observation of the cross-section is performed according to the method described below and element mapping is obtained using SEM-EPMA, the contents of Mg and Zn are each 10 atomic% or more, and the total content of Mg and Zn is 85 atomic% or more.

[0206] The Zn-Mg structure 107 composed of the above metal phases is a structure that plays a favorable role in the corrosion resistance of the welded part. Therefore, due to the presence of the second coating layer 105 containing such a Zn-Mg structure 107, the back surface of the welded part of the welded joint 1 according to the present embodiment exhibits excellent corrosion resistance.

[0207] Here, in the welded joint 1 according to the present embodiment, as Figure 4 shown, attention is paid to the range up to 1 mm on each side in the cutting direction ( Figure 2 , Figure 4 shown) for obtaining the cross-section along the X-axis direction in Figure 4 ), that is, the positive direction side and the negative direction side of the X-axis direction in Figure 4 ), starting from the center C0 of the welded part on the back surface.

[0208] When the above range up to 1 mm on each side along the X-axis direction starting from the center C0 of the welded part on the back surface is subjected to SEM observation, attention is paid to the Zn-Mg structure 107 with an equivalent circle diameter of 0.5 μm or more. Figure 5 It is an explanatory diagram for explaining the observation method when observing the back surface of the welded part of the welded joint 1 according to the present embodiment by SEM.

[0209] As Figure 5 schematically shown, when observing an arbitrary position in the above range by SEM, the length of the field of view in the cutting direction ( Figure 4 the X-axis direction in Figure 5 ) is denoted as the length L, and L = 100 μm is set. In addition, within such a field of view, the sum of the lengths obtained by vertically projecting the width of the Zn-Mg structure 107 with an equivalent circle diameter of 0.5 μm or more ( Figure 5 the length in the X-axis direction in

[0210] ) onto a plane parallel to the X-axis direction (X-Y plane) is denoted as ΣLi. Here, the subscript i is a parameter representing the number of Zn-Mg structures 107 with an equivalent circle diameter of 0.5 μm or more in the field of view, and is an integer of 1 or more. Figure 5 In the example shown in Figure 5 , there are 6 Zn-Mg structures 107 with an equivalent circle diameter of 0.5 μm or more in the second coating layer 105. At this time, as in the Zn-Mg structures 107A and 107B in Figure 5 , if the Zn-Mg structures overlap when observed along the projection direction, instead of adding the projection lengths of each Zn-Mg structure (not adding the overlapping parts repeatedly), the entire Zn-Mg structure with overlap is regarded as one structure. In the case of the example shown in Figure 5 , ΣLi is the value shown as L1 + L2 + L3 + L4 + L5.

[0211] On the back surface of the welded portion of the welded joint 1 according to this embodiment, the center C0 of the welded portion at the back surface is defined. When the range on both sides in the X-axis direction up to 1 mm each is used for SEM observation, the ratio to ΣLi of L = 100 μm is 0.05 or more. Thereby, the Zn-Mg structure 107 that is beneficial to the corrosion resistance of the welded portion is sufficiently contained in the second plating layer 105, and thus the corrosion resistance of the back surface of the welded portion is improved. If the ratio regarding ΣLi is less than 0.05, the content of the Zn-Mg structure 107 contained in the second plating layer 105 is too small to improve the corrosion resistance of the back surface of the welded portion. The ratio regarding ΣLi is preferably 0.15 or more, and more preferably 0.30 or more. On the other hand, the higher the upper limit of the ratio regarding ΣLi, the better, and this value can be 1.00.

[0212] Here, the ratio regarding ΣLi can be obtained as follows.

[0213] That is, the solidification structure at an arbitrary position in the range on both sides in the X-axis direction up to 1 mm each starting from the center C0 of the welded portion at the back surface in the second plating layer 105 is observed by SEM. At this time, the magnification of the SEM is set to about 1000 to 10000 times that can observe a range of 600 μm × 600 μm in size. Let the length of the observation field of view at this time be Figure 5 the length L = 100 μm shown. In this observation field of view, point analysis using SEM-EPMA is performed, and based on the point analysis results, the regions of the phases belonging to the Zn-Mg structure 107 are determined. For each determined region, the width of the region is vertically projected onto a plane parallel to the X-axis direction, and the sum of the lengths ΣLi is calculated. By dividing ΣLi by L = 100 μm, the value of the ratio regarding ΣLi in the concerned observation field of view can be obtained. Such measurement / calculation processing is performed at a plurality of positions (for example, 10 positions) in the range on both sides in the X-axis direction up to 1 mm each starting from the center C0 of the welded portion at the back surface, and the average value of the obtained plurality of values is used as the value of the ratio regarding ΣLi of the welded joint 1 according to this embodiment.

[0214] In addition, in the welded joint 1 according to this embodiment, as the plated steel sheet that becomes the blank, a plated steel sheet that can suppress the combustion of Zn in the plating layer during welding is used. Therefore, it is difficult to form a ZnO layer that may be formed due to the combustion of Zn in the plating layer. However, depending on the heat input state during welding, it is possible that high temperature acts on a part of the plating layer of the plated steel sheet that is the blank, causing the combustion of Zn, and thus a ZnO layer is locally formed on the back surface of the welded portion. Even in such a case where the heat input amount is large, in the welded joint 1 according to this embodiment, the formation degree of the ZnO layer is extremely suppressed, and the average thickness of the ZnO layer is an extremely small value.

[0215] When explaining the average thickness of this ZnO layer, in the present embodiment, attention is paid to starting from the center C0 of the welding part at the back surface and along the cutting direction of the cross-section shown in Figure 2 ( Figure 2 the X-axis direction in Figure 4 ) on both sides (

[0216] the positive and negative sides of the X-axis direction in

[0217] ) up to a range of 5 mm each. In the welded joint 1 according to the present embodiment, in the range of 5 mm each on both sides along the X-axis direction starting from the center C0 of the welding part at the back surface, the average thickness of the ZnO layer is 1.0 μm or less.

[0218] The average thickness of the ZnO layer being 1.0 μm or less implies that the combustion of Zn in the first plating layer 103 during welding is suppressed. Therefore, the back surface of the welding part of the welded joint 1 with an average thickness of the ZnO layer of 1.0 μm or less exhibits excellent corrosion resistance. If the average thickness of the ZnO layer is greater than 1.0 μm, the back surface of the welding part of the welded joint 1 cannot exhibit excellent corrosion resistance. In the welded joint 1 according to the present embodiment, the average thickness of the ZnO layer on the back surface of the welding part is preferably 0.2 μm or less. On the other hand, the thinner the average thickness of the ZnO layer, the better, and its lower limit value is 0 μm.

[0219] Hereinbefore, with reference to Figure 2 , Figure 4 and Figure 5 , the back surface of the welding part of the welded joint 1 according to the present embodiment has been described in detail.

[0220] Note that the non - heat - affected part of the welded joint 1 involved in this embodiment may further have one or more layers of various films on the above - mentioned first coating 103. Examples of such films include chromate film, phosphate film, non - chromate film, organic resin film, etc.

[0221] (Regarding the manufacturing method of the plated steel sheet as the blank)

[0222] Next, an example of the manufacturing method of the plated steel sheet as the blank of the welded joint 1 as described above will be described.

[0223] The plated steel sheet as the blank of the welded joint 1 involved in this embodiment is manufactured by using the above - mentioned steel base 101 as the base material, applying strain to the surface of the steel base 101 by heavy grinding, and then forming a coating on the strained surface. Then, by performing specific heat treatment on the coating formed on the surface of the steel base 101, a plated steel sheet as the blank of the welded joint 1 having the first coating 103 on the steel base 101 is manufactured.

[0224] Here, by grinding the surface of the steel base 101 with a heavy - duty grinding brush to apply strain to the surface, when the plated steel sheet as the blank is supplied for welding, the alloying of Fe and Al can be promoted. Since the alloying of Fe and Al during welding is promoted, Mg (and Ca depending on the chemical composition of the coating) is enriched in the liquid phase present during welding. As a result, the evaporation of the Zn - Mg structure, which is beneficial to the corrosion resistance of the steel base 101, can be suppressed, and the value of the ratio regarding ΣLi and the average thickness of the ZnO layer can be made to fall within the range of this embodiment.

[0225] In addition to the hot - dip plating method, spraying method, cold - spraying method, sputtering method, evaporation plating method, electroplating method, etc. can also be applied to form the coating. Among them, in terms of cost, the hot - dip plating method is most preferable for forming a coating with a thickness commonly used in automobiles, etc.

[0226] Then, by performing the specific heat treatment process described below on the obtained plated steel sheet, a plated steel sheet having the first coating 103 on the steel base 101 and being the blank of the welded joint 1 can be manufactured.

[0227] Hereinafter, an example of the manufacturing method of the plated steel sheet as the blank using the hot - dip plating method will be described in detail.

[0228] In the manufacturing process of this plated steel sheet, first, the steel base 101 is rolled by the Sendzimir method to make the desired plate thickness, and then wound into a coil shape and set on the hot - dip plating production line.

[0229] In a hot-dip plating production line, the steel sheet 101 is continuously passed while being uncoiled from the coil. During the passing process, strain is applied to the surface of the steel base 101 by a heavy grinding brush provided at a specified position. Then, through an annealing device provided on the production line, in an environment where oxidation hardly occurs, such as an oxygen concentration of 20 ppm or less, in an atmosphere of N2-(1 to 10)% H2 gas with a dew point of -60 to 10°C, the steel base 101 is heated and reduced at 700 to 900°C for more than 0 seconds and 300 seconds or less. Then, it is air-cooled with N2 gas to about the bath temperature of the subsequent plating bath + 20°C and immersed in the plating bath. It should be noted that in the above process, strain is applied to the steel base 101 before annealing. However, even if a part of the applied strain is released by annealing, the first coating layer 103 within the scope of the present embodiment can be achieved.

[0230] Here, in the plating bath, a plating alloy in a molten state having the above-described chemical composition is prepared. The bath temperature of the plating bath is set above the melting point of the plating alloy (for example, about 460 to 660°C). When manufacturing the material of the plating alloy, it is preferable to use pure metal (purity of 99% or more) as the alloy material for formulation. First, a specified amount of alloy metals are mixed in such a way as to form the above-described coating layer, and in a vacuum or inert gas replacement state, a high-frequency induction furnace, an electric arc furnace, etc. are used to completely dissolve it to form an alloy. Further, the alloy mixed with the specified composition (the composition of the first coating layer 103) is melted in the atmosphere, and the obtained melt is used as the plating bath.

[0231] It should be noted that in the production of the above-described plating alloy, it is not particularly limited to using pure metal, and existing Zn alloys, Mg alloys, Al alloys can also be melted and used. At this time, as long as a specified composition alloy with fewer impurities is used, there is no problem.

[0232] The steel base 101 is immersed in the above plating bath and then lifted at a specified speed. At this time, for example, the N2 purge gas is used to control the plating adhesion amount so that the formed coating layer becomes the desired thickness. Here, for conditions other than the bath temperature, conventional plating operation conditions can be applied, and no special equipment or conditions are required.

[0233] Next, the following first cooling process and second cooling process are performed on the molten plating alloy located on the steel base 101 to form the first coating layer 103 from the molten plating alloy. Hereinafter, the first cooling process and the second cooling process will be described in detail.

[0234] The first cooling process is a cooling process carried out when the temperature of the plated alloy is in the range of the bath temperature to 250 °C. The plated steel sheet within the above temperature range is rapidly cooled in an atmosphere with a dew point of -20 °C or lower at an average cooling rate of 10 °C / second or more. This is because the temperature range of the bath temperature to 250 °C is likely to cause the formation of coarse oxides on the surface of the coating. Therefore, by setting the dew point to -20 °C or lower and the average cooling rate to 10 °C / second or more, oxidation at high temperature regions can be prevented. It should be noted that in the case of using the hot-dip plating method in the plating process, this first cooling process is carried out immediately after the steel base 101 comes out of the plating bath. Thus, the plated alloy on the surface of the steel base 101 solidifies to form a coating layer.

[0235] Then, when the temperature of the plated alloy (coating layer) is in the range of 250 to 50 °C, the second cooling process is carried out. This second cooling process is a process of slowly cooling the plated steel sheet within the temperature range of 250 to 50 °C in an atmosphere with a dew point of 0 °C or higher at an average cooling rate of less than 10 °C / second. In this second cooling process, by setting the dew point to 0 °C or higher and the average cooling rate to less than 10 °C / second, dense oxides can be formed in the low-temperature region.

[0236] It should be noted that regarding the switching of the average cooling rate and the dew point from the first cooling process to the second cooling process, by providing two or more pipeline systems for controlling the atmosphere gas blown for the dew point, the switching can be carried out smoothly, so it is preferred. In addition, if it is difficult to switch both the average cooling rate and the dew point simultaneously at the boundary of 250 °C, the average cooling rate can also be switched at the boundary of 250 °C, and the atmosphere gas for controlling the dew point can be switched within the temperature range of 260 to 240 °C.

[0237] As described above, by forming a coating layer on the basis of applying strain to the surface of the steel base 101 using a heavy abrasive brush, and further subjecting this coating layer to a two-stage cooling process, that is, 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, the first coating layer 103 in which Zn is difficult to burn even during welding can be achieved. Thus, in the welded joint 1 using this plated steel sheet as the blank for the first steel sheet 10 and the second steel sheet 20, the second coating layer 105 can be generated on the welded portion on the back side, and the ratio value of ΣLi and the average thickness of the ZnO layer can be made to fall within the range of the present embodiment.

[0238] Here, the interval from the end of the first cooling process to the start of the second cooling process is preferably set within 3 seconds, and it is preferably to start the second cooling process immediately after the end of the first cooling process. If the interval from the end of the first cooling process to the start of the second cooling process is greater than 3 seconds, an undesirable cooling process will occur, and the first coating layer 103 within the scope of the above-described present embodiment cannot be achieved.

[0239] Here, in the above-described first cooling process, the lower limit value of the dew point is not particularly specified. For example, around -90°C is the substantial lower limit. Additionally, the average cooling rate is more preferably 40°C / second or more. It should be noted that the upper limit value of the average cooling rate is not particularly specified. For example, around 90°C / second is the substantial upper limit.

[0240] In addition, in the above-described second cooling process, the upper limit value of the dew point is not particularly specified. For example, around 20°C is the substantial upper limit. Additionally, the average cooling rate is more preferably 4°C / second or less.

[0241] It should be noted that even if strain is appropriately applied to the surface of the steel base 101, if either the first cooling process or the second cooling process is not performed, the first coating layer 103 within the scope of the above-described present embodiment cannot be achieved. By appropriately applying strain to the surface of the steel base 101 and simultaneously performing the above-described first cooling process and second cooling process, the first coating layer 103 according to the present embodiment can be achieved. Thus, in the welded joint 1 using this plated steel sheet as the blank for the first steel sheet 10 and the second steel sheet 20, the second coating layer 105 can be generated on the welded portion on the back side, and the ratio value of ΣLi and the average thickness of the ZnO layer can be made to fall within the scope of the above-described present embodiment.

[0242] In addition, after the above-described second cooling process, if an alloying heat treatment process (for example, a heat treatment process accompanied by heating to a plate temperature of around 480 - 550°C) that is usually performed in the manufacture of alloyed hot-dip galvanized steel sheets is performed, the generation state of the coating layer controlled by the first cooling process and the second cooling process is destroyed, and the Zn evaporation suppression effect focused on in the present embodiment cannot be obtained. From this perspective, it is important not to perform the heat treatment process after the second cooling process.

[0243] Here, in the above-described cooling treatment, generally known methods such as N2 gas cooling can be applied. In addition, for the cooling gas, in addition to N2 gas, gases with higher heat dissipation effects such as He gas and hydrogen gas can also be used.

[0244] Note that, as a method for measuring the actual temperature of the coating layer, for example, a contact thermocouple (type K) can be used. By installing the contact thermocouple on the steel substrate 101, the average temperature of the entire coating layer can be continuously monitored. Additionally, if various speed and thickness controls are mechanically performed to unify various operating conditions such as the preheating temperature of the steel substrate 101 and the plating bath temperature, the temperature of the entire coating layer at that moment under these manufacturing conditions can be approximately accurately monitored. Thus, the cooling processes in the first cooling step and the second cooling step can be precisely controlled. Note that although not as accurate as the contact type, the surface temperature of the coating layer can also be measured by a non-contact radiation thermometer.

[0245] In addition, the relationship between the surface temperature of the coating layer and the average temperature of the entire coating layer can also be obtained through simulation of heat conduction analysis. Specifically, based on various manufacturing conditions such as the preheating temperature of the steel substrate 101, the plating bath temperature, the speed at which the steel substrate 101 is lifted from the plating bath, the plate thickness of the steel substrate 101, the layer thickness of the coating layer, the heat exchange amount between the coating layer and the manufacturing equipment, and the heat dissipation amount of the coating layer, the surface temperature and the average temperature of the entire coating layer of the coating layer are obtained. Then, using the obtained results, the relationship between the surface temperature of the coating layer and the average temperature of the entire coating layer can be obtained. Thus, by actually measuring the surface temperature of the coating layer during the manufacture of the plated steel sheet, the average temperature of the entire coating layer at that moment under these manufacturing conditions can be inferred. As a result, the cooling processes in the first cooling step and the second cooling step can be precisely controlled.

[0246] Above, a specific example of the manufacturing method of the plated steel sheet as the blank in the present embodiment has been specifically described.

[0247] Note that, in the manufacturing method of the plated steel sheet as the blank in the present embodiment, after the above-mentioned second cooling step, a treatment for forming one or two or more layers of various films can be further implemented. As such treatments, for example, chromate treatment, phosphate treatment, non-chromate treatment, organic resin film formation treatment, etc. can be cited.

[0248] In chromate treatment, there are: electrolytic chromate treatment for forming a chromate film by electrolysis; reaction type chromate treatment for forming a film by reacting with the blank and then rinsing the excess treatment liquid; coating type chromate treatment for coating the treatment liquid and drying it without water washing to form a film, etc. Any one of the chromate treatments can be adopted.

[0249] As an example of electrolytic chromate treatment, for example, an electrolytic chromate treatment using chromic acid, silica sol, resin (phosphoric acid resin, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine modified epoxy resin, etc.) and hard silica can be cited.

[0250] As phosphate treatment, for example, zinc phosphate treatment, zinc calcium phosphate treatment, manganese phosphate treatment, etc. can be exemplified.

[0251] In order not to impose a burden on the environment, chromate-free treatment is particularly preferred. In this chromate-free treatment, there are: electrolytic chromate-free treatment in which a chromate-free film is formed by electrolysis; reactive chromate-free treatment in which a film is formed by reaction with the blank and then the excess treatment liquid is rinsed; coating-type chromate-free treatment in which a treatment liquid is coated and dried without water washing to form a film, etc. Any of these chromate-free treatments can be adopted.

[0252] In addition, the organic resin used in the organic resin film forming 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 products of these resins can be used. Here, the modified product refers to a resin formed by reacting other compounds (such as monomers, crosslinking agents, etc.) containing functional groups capable of reacting with the reactive functional groups contained in the structure of these resins with respect to the reactive functional groups contained in the structure of these resins.

[0253] As the organic resin, one of the above-mentioned types can be used alone, or two or more organic resins (unmodified organic resins) can be used in combination. In addition, an organic resin obtained by modifying at least one other organic resin in the presence of at least one organic resin can be used alone or two or more can be used in combination. In addition, an organic resin made water-based by dissolving or dispersing it in water can also be used. In addition, various coloring pigments and rust preventive pigments can also be contained in the organic resin film.

[0254] (Regarding the manufacturing method of the welded joint)

[0255] The welded joint according to this embodiment is manufactured as follows: On the basis of the plated steel sheet manufactured as described above as the blank steel sheets of the first steel sheet and the second steel sheet when manufacturing a welded joint, for example, the blank steel sheets are arranged in the shape required for the welded joint, and the blank steel sheets are welded.

[0256] Here, in the welding of the blank steel sheets, an arc welding method or a laser welding method can be used. At this time, in each welding method, by welding under the welding conditions described below, the state of the above-mentioned welded portion can be achieved.

[0257] More specifically, in the case of manufacturing a welded joint by arc welding, the blank steel sheets can be welded under the following welding conditions, for example.

[0258] Welding current: 250A, welding voltage: 26.4V, welding speed: 100 cm / min

[0259] Welding gas: 20% CO2 + Ar, gas flow rate: 20 L / minute

[0260] Welding wire: φ1.2 mm made by Nittetsu Welding Industries Co., Ltd. YGW16

[0261] (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)

[0262] Torch inclination angle: 45°

[0263] In addition, in the case of manufacturing a welded joint by laser welding, the blank steel plate can be welded under the following welding conditions, for example.

[0264] Output power: 7 kW, welding speed: 400 cm / minute, forward / backward angle: 0°

[0265] Above, an example of the manufacturing method of the welded joint according to this embodiment has been described.

[0266] (Modified example)

[0267] In the above embodiment, attention was paid to the case where the welded joint 1 according to this embodiment is a lap fillet weld, but the form of the welded joint 1 according to this embodiment is not limited to Figure 1 the example shown.

[0268] For example, the welded joint 1A according to the modified example of this embodiment may be Figure 6 the so-called T-joint shown. Figure 6 Schematically shows the overall structure of the welded joint 1A obtained by T-welding the first steel plate and the second steel plate by arc welding, and shows the cross-section of the welded joint 1A perpendicular to the extending direction of the weld bead portion.

[0269] It can be seen that in this T-joint, the end of one steel plate is also welded to the surface of the other steel plate. In Figure 6 the T-joint shown, similar to the above embodiment, the steel plate on the side where the end is welded is referred to as the "first steel plate 10", and the steel plate on the side where the surface is welded is referred to as the "second steel plate 20". In Figure 6 the T-joint of, the end 11 of the first steel plate 10 is also welded to the surface of the second steel plate 20.

[0270] In this T-joint, the first steel plate 10 and the second steel plate 20 are substantially arranged at a right angle. For example, when the welded joint 1A is a T-joint, the angle formed by the first steel plate 10 and the second steel plate 20 can be, for example, in the range of 80° to 100°. However, the angle formed by the first steel plate 10 and the second steel plate 20 is not particularly limited.

[0271] In Figure 6 the T-joint shown, the weld bead portion 30 also protrudes outside the second steel plate 20 on the surface of the second steel plate 20 where the first steel plate 10 is welded. Even if the welded joint 1A of this modified example is a T-joint as Figure 6 shown, it is sufficient to use the surface on the side where the weld bead portion 30 does not protrude as the back surface of the welded joint 1A. In addition, in the welded joint 1 of this modified example, depending on the distribution state of the weld bead portion 30 and the heat affected zone 40, the surface on the side determined to be opposite to the heat input side during welding can be set as the back surface of the welded joint 1A. In Figure 6 the T-joint shown, two portions are assumed as this back surface as surrounded by the dashed line in the figure.

[0272] In addition, the welded joint 1B of this modified example can be Figure 7 the so-called butt joint shown. Figure 7 It schematically shows the overall structure of the welded joint 1B obtained by butt-welding the first steel plate and the second steel plate by arc welding, and shows a cross-section of the welded joint 1B perpendicular to the extending direction of the weld bead portion.

[0273] In Figure 7 the case of the butt joint shown, the ends of the respective steel plates are butted against each other and welded. In Figure 7 the case of the butt joint shown, one steel plate is referred to as the "first steel plate 10" and the other steel plate is referred to as the "second steel plate 20".

[0274] In Figure 7 the butt joint shown, the weld bead portion 30 also protrudes outside the first steel plate 10 and the second steel plate 20. Even if the welded joint 1B of this modified example is a butt joint as Figure 7 shown, it is sufficient to use the surface on the side where the weld bead portion 30 does not protrude as the back surface of the welded joint 1. In addition, in the welded joint 1B of this modified example, depending on the distribution state of the weld bead portion 30 and the heat affected zone 40, the surface determined to be opposite to the heat input side during welding can be used as the back surface of the welded joint 1B.

[0275] It should be noted that in the case of laser welding, the identification of the back surface of the welded portion is usually judged according to the shape shown by the welded portion, and the shape of the back surface of the welded portion tends to be convex. On the other hand, the situation of the back surface of the welded portion varies depending on the penetration degree. In Figure 8 the case of the welded joint 1C formed by lap welding shown, there are cases where the back surface protrudes and cases where it does not protrude due to complete penetration shown in (a) and partial penetration shown in (b). This situation is not limited to the case of lap joints using laser welding. Even in the case of T-joints or butt joints using laser welding, it shows the same as Figure 8The example of the lap joint shown has the same tendency.

[0276] In addition, in the above-described embodiment, as the blanks of the first steel plate 10 and the second steel plate 20, the case where a galvanized steel plate having the above-described first plating layer 103 provided on both surfaces of the steel base 101 is used was taken as an example and described. However, the above-described first plating layer 103 may be provided at least on one side of the surface of the second steel plate 20 that becomes the back surface of the welded joint 1, and the first plating layer 103 may not be provided on the surface of the second steel plate 20 that becomes the front side of the welded joint 1 or a part of the first steel plate 10.

[0277] Examples

[0278] Hereinafter, examples and comparative examples will be shown to specifically describe the welded joint according to the present invention. It should be noted that the examples shown below are merely examples of the welded joint of the present invention, and the welded joint of the present invention is not limited to the examples shown below.

[0279] In the examples and comparative examples shown below, as the steel plates that become the steel base 101, hot-rolled steel plates of 440 MPa grade, 590 MPa grade, 780 MPa grade, 980 MPa grade, and 1180 MPa grade (all manufactured by Nippon Steel & Sumitomo Metal Corporation) were used. The plate thickness of the hot-rolled steel plates was all set to 3.2 mm. Using these hot-rolled steel plates, a plurality of test pieces were prepared for each hot-rolled steel plate.

[0280] For the prepared test pieces, the following two types of heavy-duty abrasive brushes were used to apply strain to the surfaces of the test pieces. It should be noted that during grinding, a 1.0 to 5.0% NaOH aqueous solution can be coated on the steel plate surface. By appropriately adjusting the brush pressing amount within the range of 0.5 to 10.0 mm and the brush rotation speed within the range of 100 to 1000 rpm, the amount of strain applied to the surface was controlled. It should be noted that among the following two types of heavy-duty abrasive brushes, the A-type brush is a brush with stronger grinding force. It should be noted that for comparison, test pieces without applying such heavy-duty grinding were also prepared.

[0281] Brush type A: D-100 manufactured by Hodani Co., Ltd.

[0282] Brush type B: M-33 manufactured by Hodani Co., Ltd.

[0283] Prepare plating baths for the coatings having the compositions shown in Table 1 below for the following purposes respectively, and set them in a batch-type hot dip plating test apparatus made by our company to perform plating on the above test pieces. Here, use a thermocouple spot-welded to the center of the test piece to measure the temperature of the test piece. In addition, for the test piece immersed in the plating bath, before immersing it in the plating bath, in a furnace with an oxygen concentration of 20 ppm or less, in an N2-5% H2 gas atmosphere, heat and reduce the surface of the plating base plate at 800 °C. After the heat reduction treatment, air-cool the test piece with N2 gas. After the temperature of the test piece reaches the bath temperature +20 °C, immerse the test piece in the plating bath of the hot dip plating test apparatus for about 3 seconds.

[0284] After immersing in the plating bath, lift the test piece at a speed of 20 to 200 mm / second. When lifting, use N2 purge gas for control to achieve the desired plating adhesion amount. In the following examples and comparative examples, control the plating adhesion amount so that the adhesion amount of the dried coating on each side of the test piece is 40 to 120 g / m 2 . After lifting the test piece from the plating bath, cool the test piece from the plating bath temperature to room temperature under the conditions shown in Table 1 below. In the examples and comparative examples shown below, immediately start the second cooling process after the end of the first cooling process (that is, set the interval from the end of the first cooling process to the start of the second cooling process to 0.2 seconds or less).

[0285] Here, cut out a steel plate with a size of 30 mm × 30 mm from the test piece plated as described above, immerse the plated steel plate in a 10% HCl aqueous solution added with an inhibitor, perform pickling and peeling on the coating, and perform ICP analysis on the elements dissolved in the aqueous solution to measure the composition of the coating.

[0286] In addition, cut a steel plate with a size of 150 mm × 50 mm from the obtained test piece as the first steel plate, and cut a steel plate with a size of 150 mm × 30 mm as the second steel plate. Overlap the long sides of these steel plates and weld them by arc welding (lap fillet welding) to make a welded joint.

[0287] Here, the welding conditions for arc welding are as follows.

[0288] Welding current: 250 A, welding voltage: 26.4 V, welding speed: 100 cm / minute

[0289] Welding gas: 20% CO2 + Ar, gas flow rate: 20 L / minute

[0290] Welding wire: φ1.2 mm made by Nippon Welding Industry Co., Ltd. YGW16

[0291] (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)

[0292] Torch tilt angle: 45°

[0293] Overlap amount: 10 mm

[0294] Steel plate dimensions: upper plate side (first steel plate) 150×50 mm, lower plate side (second steel plate) 150×30 mm

[0295] Plate gap: 0 mm

[0296] In addition, in laser welding, lap welding is also carried out on the basis of overlapping steel plates as follows. The welding conditions are as follows.

[0297] Output power: 7 kW, welding speed: 400 cm / minute, forward / backward angle: 0°

[0298] Steel plate dimensions: upper plate side (first steel plate) 150×50 mm, lower plate side (second steel plate) 150×50 mm

[0299] Overlap amount: 50 mm

[0300] Plate gap: 0 mm

[0301] <GDS measurement of C concentration in the steel base>

[0302] For the welded joint obtained as described above, the C concentration in the steel base was measured by GDS according to the method described previously, and the depth at which the C concentration became 0.05% by mass or less was measured.

[0303] <Measurement of the ratio ΣLi / L and the average thickness of the ZnO layer on the back side of the welded part>

[0304] For the welded joint obtained as described above, cross-sectional observation of the back side of the welded part was carried out by SEM according to the method described previously, and the ratio ΣLi / L and the average thickness of the ZnO layer were measured.

[0305] <Evaluation of the corrosion resistance of the back side of the welded part>

[0306] For the above-mentioned welded joints, phosphoric acid chemical conversion treatment for automotive use (phosphoric acid Zn treatment, SD5350 system: standard prepared by Nipponpaint Industrial Coatings Co., LTD) and electrodeposition coating (PN110 Power Nix Gray: standard prepared by Nipponpaint Industrial Coatings Co., LTD) are carried out. At this time, the electrodeposited film thickness is set to 20 μm. The samples after electrodeposition coating are subjected to a composite cyclic corrosion test based on JASO (M609-91) to evaluate the timing of red rust generation on the back side of the welded part. The evaluation criteria are as follows.

[0307] "Evaluation Criteria"

[0308] Score "AAA": The timing of red rust generation is 240 cycles or more

[0309] "AA": The timing of red rust generation is more than 180 cycles and less than 240 cycles

[0310] "A": The timing of red rust generation is more than 90 cycles and less than 180 cycles

[0311] "B": The timing of red rust generation is less than 90 cycles

[0312] In addition, if the timing of the above-mentioned red rust generation is more than 90 cycles, the back side of the welded part of the test piece of interest can be evaluated as having good corrosion resistance.

[0313] The results obtained are summarized and shown in Table 1 below.

[0314] [Table 1]

[0315]

[0316] As can be seen from Table 1 above, in the examples belonging to the embodiments of the present invention, excellent corrosion resistance can be achieved on the back side of the welded part. In contrast, in the comparative examples belonging to the present invention, sufficient performance was not exhibited with respect to the corrosion resistance of the back side of the welded part.

[0317] For example, in No. 24 where the Al content of the first coating exceeds the scope of the present invention, excessive burning of Zn in the coating occurs during welding, resulting in the fact that the ratio value of ΣLi and the thickness of the ZnO layer on the back side of the welded part do not fall within the scope of the present invention, thus sufficient corrosion resistance cannot be achieved. In No. 24 where the Al content of the first coating exceeds the scope of the present invention, excessive Al exists, resulting in a shortage of the Zn-Mg phase, and the ratio value of ΣLi and the thickness of the ZnO layer on the back side of the welded part do not fall within the scope of the present invention, thus sufficient corrosion resistance cannot be achieved.

[0318] In No. 26 where the Mg content in the first coating exceeds the scope of the present invention, during welding, Zn in the coating burns excessively, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved. In No. 27 where the Mg content in the first coating exceeds the scope of the present invention, the excessive presence of Mg causes the effect of suppressing Zn burning during welding not to be obtained, and the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part do not fall within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved.

[0319] In No. 28 where the Si content in the first coating exceeds the scope of the present invention, not only does Zn in the coating burn excessively during welding, but excessive Si forms Si-containing compounds, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved. In No. 29 where the Ca content in the first coating exceeds the scope of the present invention, not only does Zn in the coating burn excessively during welding, but excessive Ca forms Ca-containing compounds, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved. In No. 30 where the Ti content in the first coating exceeds the scope of the present invention, Zn in the coating burns excessively during welding, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved.

[0320] In No. 31 where the average cooling rate in the first cooling process exceeds the scope of the present invention, during welding, Zn in the coating burns excessively, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved. In No. 32 where the dew point in the first cooling process exceeds the scope of the present invention, during welding, Zn in the coating burns excessively, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved.

[0321] In No. 33 where the average cooling rate in the second cooling process exceeds the scope of the present invention, during welding, Zn in the coating burns excessively, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved. In No. 34 where the dew point in the second cooling process exceeds the scope of the present invention, during welding, Zn in the coating burns excessively, resulting in the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part not falling within the scope of the present invention, and thus sufficient corrosion resistance cannot be achieved.

[0322] In No. 35 where no strain is applied to the base steel plate, since the alloying behavior in the coating cannot be controlled, the Zn-Mg phase is insufficient, and the ratio value of ΣLi and the thickness of the ZnO layer on the back of the welded part do not fall within the scope of the present invention, resulting in insufficient corrosion resistance.

[0323] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings. However, the present invention is not limited to this example. It is obvious that those with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims. Of course, these are also understood to fall within the protection scope of the present invention.

[0324] The embodiments disclosed this time are exemplary in all aspects and not restrictive. Without departing from the appended claims, the constitution and the gist within the technical scope of the present invention described hereinafter, the above embodiments can be omitted, replaced, and changed in various forms. For example, within the scope that does not damage its effect, the constituent elements of the above embodiments can be arbitrarily combined. In addition, based on this arbitrary combination, the functions and effects of the respective constituent elements involved in the combination can of course be obtained, and other functions and other effects obvious to those skilled in the art based on the description of this specification can also be obtained.

[0325] In addition, the effects described in this specification are merely illustrative or exemplary and not restrictive. That is, the technology related to the present invention can achieve the above effects, or can achieve other effects obvious to those skilled in the art based on the description of this specification to replace the above effects.

[0326] It should be noted that the following constitution also belongs to the technical scope of the present invention.

[0327] (1) A welded joint having a first steel plate and a second steel plate connected to a weld bead portion, the weld bead portion having a long side in the extending direction in a plan view,

[0328] The first steel plate and the second steel plate respectively have a heat-affected zone around the weld bead portion and a non-heat-affected zone where there is no heat influence caused by welding.

[0329] The second steel plate has a steel base and a first coating on the steel base in the non-heat-affected zone.

[0330] The first coating is a coating having the following chemical composition, which contains, by mass%,

[0331] Al: 10.00 - 70.00%,

[0332] Mg: 3.00 to 20.00%,

[0333] Fe: 0.01 to 15.00%,

[0334] Optionally containing one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the balance being Zn and impurities of 5.0000 mass% or more,

[0335] [Element group A]: One or two selected from the group consisting of Si: greater than 0% and 10.00% or less, and Ca: greater than 0% and 4.00% or less;

[0336] [Element group B]: One or more selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less;

[0337] [Element group C]: One or more selected from the group consisting of Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less;

[0338] [Element group D]: One or more selected from the group consisting of Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less;

[0339] [Element group E]: One or more selected from the group consisting of Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less;

[0340] [Element group F]: One or more selected from the group consisting of La: greater than 0% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less;

[0341] [Element group G]: B: greater than 0% and 0.5000% or less,

[0342] Define the normal direction of the surface of the non-heat-affected part with respect to the second steel plate as the Z-axis direction, the extension direction as the Y-axis direction, and the direction orthogonal to the Z-axis direction and the Y-axis direction as the X-axis direction.

[0343] In the cross-section obtained by cutting the first steel plate and the second steel plate along the Z-axis direction,

[0344] When the surface on the positive direction side of the Z-axis direction is defined as the first surface, the surface on the negative direction side of the Z-axis direction is defined as the second surface, and the surface on the side with the smaller weld toe pitch in the X-axis direction of the weld bead part on the first surface side and the weld toe pitch in the X-axis direction of the weld bead part on the second surface side is defined as the back surface of the welded joint; or, when there is no weld toe on either the first surface side or the second surface side, the surface on the side without the weld toe is defined as the back surface of the welded joint,

[0345] On the back surface of the cross-section, for the X-axis direction, define the center of the heat-affected part on the back surface as the midpoint of the line segment connecting the end closest to one end of the first coating and the farthest end among the ends of the heat-affected part.

[0346] On the back surface of the cross-section, there is a second coating containing a Zn-Mg structure on the weld bead part or the heat-affected part within the range of 1 mm on each side along the X-axis direction starting from the center; and,

[0347] On the back surface of the cross-section, when the length of the field of view in the X-axis direction during electron microscope observation of the range starting from the center is set to 100 μm, and the sum of the lengths obtained by vertically projecting the Zn-Mg structures with an equivalent circle diameter of 0.5 μm or more onto a plane parallel to the X-axis direction within this field of view is denoted as ΣLi, the ratio of ΣLi to the length of the field of view of 100 μm is 0.05 or more;

[0348] In addition, on the back surface of the cross-section, the average thickness of the ZnO layer present within the range of 5 mm on each side along the X-axis direction starting from the center is 1.0 μm or less.

[0349] (2) The welded joint according to (1) has a chemical composition containing the element group A.

[0350] (3) The welded joint according to (1) has a chemical composition containing the element group B.

[0351] (4) The welded joint according to (1) has a chemical composition containing the element group C.

[0352] (5) The welded joint according to (1) has a chemical composition containing the element group D.

[0353] (6) The welded joint according to (1) has a chemical composition containing the element group E.

[0354] (7) The welded joint according to (1) has a chemical composition containing the element group F.

[0355] (8) The welded joint according to (1) has a chemical composition containing the element group G.

[0356] (9) The welded joint according to any one of (1) to (8), wherein the ratio is 0.15 or more.

[0357] (10) The welded joint according to any one of (1) to (9), wherein the average thickness of the ZnO layer is 0.2 μm or less.

[0358] (11) The welded joint according to any one of (1) to (10), wherein for the C concentration in the steel base of the non-heat-affected part calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel base and the coating.

[0359] (12) The welded joint according to any one of (1) to (11), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0360] Explanation of reference numerals

[0361] 1, 1A, 1B, 1C Welded joint

[0362] 10 First steel plate

[0363] 11 End of the first steel plate

[0364] 20 Second steel plate

[0365] 30 Weld bead part

[0366] 40 Heat-affected part

[0367] 101 Steel base

[0368] 103 First coating

[0369] 105 Second coating

[0370] 107 Zn-Mg structure

[0371] T Weld toe

Claims

1. A welded joint having a first steel plate and a second steel plate connected to a weld bead portion, the weld bead portion having a long side in the extending direction in a plan view, wherein the first steel plate and the second steel plate each have a heat-affected portion located around the weld bead portion and a non-heat-affected portion not affected by heat caused by welding, the second steel plate has a steel base and a first coating on the steel base in the non-heat-affected portion, the first coating is a coating having the following chemical composition, and the chemical composition contains, by mass%, Al：10.00～70.00％、 Mg: 3.00 to 20.00%, Fe: 0.01 to 15.00%, optionally contains one or more elements selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, element group F, and element group G, and the balance is Zn and impurities of 5.0000 mass% or more, [Element group A]: one or two selected from the group consisting of Si: more than 0% and 10.00% or less, and Ca: more than 0% and 4.00% or less; [Element group B]: one or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less; [Element group C]: one or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less; [Element group D]: one or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less; [Element group E]: one or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less; [Element group F]: one or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less; [Element group G]: B: more than 0% and 0.5000% or less, defining the normal direction of the surface of the non-heat-affected portion of the second steel plate as the Z-axis direction, the extending direction as the Y-axis direction, and the direction orthogonal to the Z-axis direction and the Y-axis direction as the X-axis direction, in a cross section obtained by cutting the first steel plate and the second steel plate along the Z-axis direction, Let the surface on the positive direction side of the Z-axis be the first surface, and the surface on the negative direction side of the Z-axis be the second surface. When the face with the smaller toe spacing in the X-axis direction of the weld bead portion on the first surface side and the toe spacing in the X-axis direction of the weld bead portion on the second surface side is set as the back surface of the welded joint, or when there is no toe on either the first surface side or the second surface side, and the surface on the side without the toe is set as the back surface of the welded joint, On the back surface of the cross-section, for the X-axis direction, the center of the heat-affected zone on the back surface is defined as the midpoint of the line segment connecting the end closest to one end of the first coating and the end farthest from it among the ends of the heat-affected zone. On the back surface of the cross-section, there is a second coating containing a Zn-Mg structure on the weld bead portion or the heat-affected zone within a range of 1 mm on each side along the X-axis direction starting from the center. And, On the back surface of the cross-section, when the length of the field of view in the X-axis direction during electron microscope observation of the range starting from the center is set to 100 μm, and when the sum of the lengths obtained by vertically projecting the Zn-Mg structures with an equivalent circle diameter of 0.5 μm or more onto a plane parallel to the X-axis direction within this field of view is denoted as ΣLi, the ratio of ΣLi to the length of the field of view of 100 μm is 0.05 or more; Furthermore, on the back surface of the cross-section, the average thickness of the ZnO layer present within a range of 5 mm on each side along the X-axis direction starting from the center is 1.0 μm or less.

2. The welded joint according to claim 1, which has a chemical composition containing the element group A.

3. The welded joint according to claim 1, which has a chemical composition containing the element group B.

4. The welded joint according to claim 1, which has a chemical composition containing the element group C.

5. The welded joint according to claim 1, which has a chemical composition containing the element group D.

6. The welded joint according to claim 1, which has a chemical composition containing the element group E.

7. The welded joint according to claim 1, which has a chemical composition containing the element group F.

8. The welded joint according to claim 1, which has a chemical composition containing the element group G.

9. The welded joint according to any one of claims 1 to 8, wherein, The ratio is 0.15 or more.

10. The welded joint according to any one of claims 1 to 8, wherein, The average thickness of the ZnO layer is 0.2 μm or less.

11. The welded joint according to claim 9, wherein, The average thickness of the ZnO layer is 0.2 μm or less.

12. The welded joint according to any one of claims 1 to 8, wherein, For the C concentration in the steel base of the non-heat-affected zone calculated based on the depth curve related to the carbon distribution using glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05 mass% or less is 10 μm or more from the interface between the steel base and the coating.

13. The welded joint according to claim 9, wherein, Regarding the C concentration calculated based on the depth curve related to the carbon distribution in the steel substrate of the non-thermally affected part by glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel substrate and the coating layer.

14. The welded joint according to claim 10, wherein, Regarding the C concentration calculated based on the depth curve related to the carbon distribution in the steel substrate of the non-thermally affected part by glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel substrate and the coating layer.

15. The welded joint according to claim 11, wherein, Regarding the C concentration calculated based on the depth curve related to the carbon distribution in the steel substrate of the non-thermally affected part by glow discharge emission spectrometry, the depth at which the C concentration becomes 0.05% by mass or less is 10 μm or more from the interface between the steel substrate and the coating layer.

16. The welded joint according to claim 1, wherein, The tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

Images