Welded joint

Abstract

[Problem] To further improve the corrosion resistance of a weld lap in a weld joint obtained by lap fillet welding using a galvanized steel sheet as a blank. [Solution] The present invention relates to a welded joint having a first steel plate and a second steel plate which are connected to a bead section having the extension direction as the long side in plan view, the welded joint having a joint gap in a weld overlap section, and having a steel base and a plating layer on the steel base on at least the surfaces of the first steel plate and the second steel plate on the opposite side, in a cross-sectional view cut in a direction orthogonal to the extension direction, the distance between an end portion on the side away from the bead portion of a region where the thickness of the plating layer closest to the bead portion in a direction orthogonal to the extension direction and a normal direction to the surface of the second steel plate is less than 1 [mu] m and a tip portion of the weld overlap portion is 1000 [mu] m or less.

Description

Technical Field

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

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

[0003] Here, when welding a galvanized steel sheet to manufacture a welded joint, 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 anticorrosive 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-Mg-Zn2 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: International Publication No. 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. Here, the present inventors conducted in-depth research and found that in a welded joint formed by welding a galvanized steel sheet, as a part where corrosion resistance is desired, in addition to the vicinity of the weld toe studied in Patent Document 1 above, there is also a part where the steel sheets overlap each other in a lap fillet welded joint (hereinafter also referred to as a welded overlap part).

[0010] The corrosion resistance of the welded overlap part in a lap fillet welded joint based on a galvanized steel sheet as a blank is not mentioned in Patent Document 1 above, and there is room for improvement in the corrosion resistance of the welded overlap part.

[0011] Accordingly, the present invention has been accomplished in view of the above problems, and an object of the present invention is to provide a welded joint that can further improve the corrosion resistance of a welded overlap portion in a lap fillet weld joint using a galvanized steel sheet as a blank.

[0012] Means for Solving the Problem

[0013] In order to solve the above technical problems, the inventors of the present invention conducted in-depth research and found that by controlling the existence state of the plating layer at the welded overlap portion, it is possible to further improve the corrosion resistance at the welded overlap portion.

[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 having a heat-affected portion around the weld bead portion and a non-heat-affected portion where there is no heat affect caused by welding, a joint gap existing in a portion where the surface of the first steel plate faces the surface of the second steel plate, i.e., the welded overlap portion, the joint gap being a gap existing between the first steel plate and the second steel plate, and at least the surfaces of the first steel plate and the second steel plate facing each other having a steel base and a plating layer on the steel base, the plating layer being a plating layer having the following chemical composition, the chemical composition containing, in 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 5.00 mass % or more of Zn and impurities,

[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] When observing the welded overlap portion in the extending direction of the weld bead portion, assuming that the central axis of the joint gap is extended toward the weld bead portion along the direction orthogonal to the extending direction, the intersection point on the central axis with the end portion of the weld bead portion is defined as the front end portion of the welded overlap portion. In a cross-sectional view cut along the direction orthogonal to the extending direction, the distance between the end portion on the side away from the weld bead portion of the region where the thickness of the plating closest to the weld bead portion in the direction orthogonal to both the extending direction and the normal direction to the surface of the second steel plate is less than 1 μm and the front end portion of the welded overlap portion is 1000 μm or less.

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

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

[0026] (4) The welded joint according to (1), which 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 coating contains Al: 18.00 to 50.00% by mass, Mg: 6.00 to 15.00% by mass, and Ca: 0.05 to 4.00% by mass as the element group A.

[0032] (10) The welded joint according to any one of (1) to (8), wherein for the C concentration calculated based on the depth curve of the carbon distribution by glow discharge emission spectrometry in the steel base of the non-heat-affected zone, 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.

[0033] (11) The welded joint according to (9), wherein for the C concentration calculated based on the depth curve of the carbon distribution by glow discharge emission spectrometry in the steel base of the non-heat-affected zone, 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.

[0034] (12) The welded joint according to any one of (1) to (8), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0035] (13) The welded joint according to (9), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0036] (14) The welded joint according to (10), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0037] (15) The welded joint according 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.

[0038] (16) The welded joint according to (1), wherein the distance is 500 μm or less.

[0039] (17) The welded joint according to (1), wherein the distance is 100 μm or less.

[0040] Effects of the Invention

[0041] As described above, according to the present invention, it is possible to further improve the corrosion resistance of the welded overlap portion in a welded joint based on fillet welding with a galvanized steel sheet as a blank. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is an explanatory view schematically showing an example of the structure of a welded joint according to an embodiment of the present invention.

[0043] Figure 2 It is an explanatory view for explaining the welded joint according to the same embodiment.

[0044] Figure 3 It is an explanatory view for explaining the welded overlap portion in the welded joint according to the same embodiment.

[0045] Figure 4 It is an explanatory view for explaining the welded overlap portion in the welded joint according to the same embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0046] 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.

[0047] (Regarding the welded joint)

[0048] 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 It is an explanatory view schematically showing an example of the structure of the welded joint according to the present embodiment.

[0049] It should be noted that hereinafter, for convenience, the following description will be appropriately made using the coordinate system shown in Figure 1 . In Figure 1 a welded joint formed by arc welding two steel plates is illustrated.

[0050] 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 or laser welding, and shows a cross section of the welded joint perpendicular to the extending direction of the weld bead portion. AsFigure 1 Schematically shown, the welded joint 1 according to this embodiment includes 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 to the surface of the second steel plate 20 is defined as the Z-axis direction, the extending direction of the weld bead portion 30 is defined as the Y-axis direction, and the direction orthogonal to the Z-axis direction and the Y-axis direction is defined as the X-axis direction.

[0051] Here, as the steel plates (hereinafter also referred to as "blank steel plates") of the blanks of the first steel plate 10 and the second steel plate 20 constituting the welded joint 1, galvanized steel plates having the coatings described in detail below are preferably used. At this time, in the blank steel plate, at least in the region where the surface of the blank steel plate on the first steel plate 10 side faces the surface of the blank steel plate on the second steel plate 20 side in the Z-axis direction, the coatings described in detail below are provided.

[0052] In addition, the weld bead portion 30 is a portion formed by arc welding or laser welding. During welding, mutual diffusion of constituent elements occurs between the welding wire used as required and the blank steel plate. In Figure 1 the figure, for the sake of easy illustration, the bonding 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. However, the actual bonding interface becomes a complex curved surface because the molten metal fluctuates due to arc plasma or the like during welding. In addition, the weld bead portion 30 extends along the Y-axis direction in the figure, and the first steel plate 10 and the second steel plate 20 are joined by the weld bead portion 30.

[0053] 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 blank steel plate, etc., so it is difficult to definitely determine the composition covering all possibilities without doubt. However, at the overlapping portion (lap) where the two blank steel plates overlap, when there is a coating on the blank steel plate, the weld bead portion 30 usually has the oxide of the "element that is easily oxidized" among the various elements constituting at least the coating existing at the overlapping portion as the main component (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.

[0054] In addition, when determining the portion of the weld bead portion 30 belonging to the weld joint 1 of interest, it can be easily visualized by etching the cross-section of the weld joint 1 including the weld bead portion 30, the heat-affected portion 40, and the non-heat-affected portion 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.

[0055] It should be noted that the oxides generated during welding are roughly classified into two types: mill scale and slag. The oxide generated on the surface of the weld bead portion 30 during welding, i.e., mill scale, contains 50% or more of Fe by mass when removing oxygen, and the balance is elements and impurities that are easily oxidized. In addition, the slag contains 50% or more of the elements that are easily oxidized by mass when removing oxygen, and the balance is Fe and impurities less than 50% by mass. Here, as specific examples of the "elements that are easily oxidized", Ca, Mg, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, Ti can be listed.

[0056] It should be noted that the above-mentioned mill scale and slag can be easily distinguished by performing compositional 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 mill 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% or more by mass. If the content of Fe is 50% or more by mass, it can be determined that the portion of interest is slag, and if the content of the "elements that are easily oxidized" is 50% or more by mass, it can be determined that the portion of interest is mill scale.

[0057] 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". In Figure 1 In the shown weld joint 1, the point where the surface of the weld bead portion 30 intersects the surface of the steel base or the heat-affected portion in the first steel plate 10 or the second steel plate 20 corresponds to this "weld toe". For the weld joint 1 related to this embodiment, the corrosion resistance of the vicinity of the weld toe T is of concern.

[0058] In addition, the heat input of arc welding or laser welding is usually applied 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 serving as the base material, and the width of the weld bead becomes narrower as it goes in the direction in which the heat input generated by welding propagates. Therefore, by paying attention to whether the weld bead portion 30 is exposed on the surface and the shape shown by the weld bead, the heat input direction during arc welding or laser welding can be determined. It should be noted that the way of "exposure of the weld bead portion 30" includes the way of exposure of the scale formed on the weld bead portion 30.

[0059] In addition, in the present embodiment, the steel plate located on the heat input side of the above-mentioned arc welding or laser welding is defined as the first steel plate 10, and the steel plate located on the side opposite to the heat input side of the arc welding or laser welding is defined as the second steel plate 20.

[0060] 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, so the heat affected zone 40 can be easily distinguished.

[0061] In addition, in the welded joint 1 according to the present embodiment, the region where the surface of the first steel plate 10 faces the surface of the second steel plate 20 in the Z-axis direction (more specifically, the part other than the weld bead portion 30 and the heat affected zone 40 in the region where the surface of the first steel plate 10 faces the surface of the second steel plate 20) is referred to as the "weld overlap portion 50". This weld overlap portion 50 will be described again later.

[0062] <Regarding the non-heat affected zone>

[0063] Next, the structure of the part in the welded joint 1 according to the present embodiment where there is no heat affect caused by welding will be described in detail.

[0064] In the following description, the part in the welded joint 1 where there is no heat affect caused by welding (in other words, the part 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 welded joint 1 as Figure 1 shown, 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 1 the Y-axis direction in Figure 1 this) starting from the weld toe T and in the direction away from the weld bead portion 30 ( Figure 1Regions R1 and R1' surrounded by the dashed lines exist 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.

[0065] Figure 2 It is a diagram schematically showing a part of a cross-section parallel to the plate thickness direction in the non-heat-affected zones R1 and R1'. As Figure 2 Schematically shown, the non-heat-affected zones R1 and R1' in at least any one of the first steel plate 10 or the second steel plate 20 have a steel base 101 and a coating layer 103 located on at least a part of the surface of the steel base 101. It should be noted that in the non-heat-affected zone R1 of the welded joint 1 according to this embodiment, the coating layer 103 exists on both surfaces of the steel base 101.

[0066] Hereinafter, the steel base 101 and the coating layer 103 of the non-heat-affected zone will be described in detail.

[0067] 《Regarding the steel base 101》

[0068] In the welded joint 1 according to this 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, and Mn in the ultra-low carbon steel; various steel plates containing various other components (Cr, N, Cu, B, Ni, Mg, Ca, V, Co, Zn, As, Y, Zr, Mo, Sn, Sb, Ta, W, Pb, Bi, REM, etc.).

[0069] 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 grade or more) can further improve the firmness of the welded joint 1, so it is 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 as specified in JIS Z 2241 (2011) is made, and the tensile strength of the obtained test piece is measured by the method specified in JIS Z 2241 (2011).

[0070] 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.

[0071] "Regarding Coating 103"

[0072] As Figure 2 Schematically shown, the coating 103 is provided, for example, on the steel base 101 of the non - heat - affected part. The coating 103 comes from the coating of the plated steel sheet which is the blank of the welded joint 1.

[0073] Hereinafter, first, the chemical composition of the coating 103 will be described in detail.

[0074] ◇Regarding the Chemical Composition of Coating 103

[0075] According to one aspect, the chemical composition of the coating 103 according to the present embodiment contains, by mass%, Al: 10.00 - 70.00%, Mg: 3.00 - 20.00%, Fe: 0.01 - 15.00, and the balance is Zn and impurities of 5.0000 mass% or more. That is, in the chemical composition of the coating 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.

[0076] In addition, according to another aspect, the chemical composition of the coating 103 according to the present embodiment contains, by mass%, Al: 10.00 - 70.00%, Mg: 3.00 - 20.00%, Fe: 0.01 - 15.0000%, and further 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 5.0000 mass% or more. That is, in the chemical composition of the coating 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 - G is 95.0000 mass% or less, and the balance is Zn and impurities of 5.00 mass% or more.

[0077] [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

[0078] [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

[0079] [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

[0080] [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

[0081] [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

[0082] [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

[0083] [Element group G]: B: greater than 0% and 0.5000% or less

[0084] Thus, the first coating 103 according to this embodiment is a coating having the following chemical composition, which contains, by mass%, Al: 10.00 - 70.00%, Mg: 3.00 - 20.00%, Fe: 0.01 - 15.00%, and optionally contains 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.

[0085] [Al: 10.00 - 70.00 mass%]

[0086] Al is an element necessary for the main metallographic structure (Zn-Al-Mg-based metallographic structure) of the 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 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 substrate 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 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 30.00% by mass or more. By making the Al content within the above range, the corrosion resistance of the coated steel sheet can be ensured.

[0087] On the other hand, if the Al content in the coating layer 103 is greater than 70.00% by mass, the Al phase that acts as a cathode when placed in a corrosive environment will increase excessively, resulting in a relatively reduced formation amount of the Mg-Zn phase with excellent corrosion resistance, thereby sacrificing the corrosion protection performance and making the corrosion of the steel substrate easy to proceed. Therefore, the corrosion resistance of the coated steel sheet cannot be ensured. Therefore, in the 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.

[0088] [Mg: 3.00 - 20.00% by mass]

[0089] 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 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 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.

[0090] On the other hand, if the Mg content in the 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 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.

[0091] [Fe: 0.01 - 15.00% by mass]

[0092] Elements constituting the steel sheet may sometimes mix from the steel base 101 as a base material into the coating layer 103. 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 coating layer 103 makes it easy for the elements constituting the steel base 101 to mix into the coating layer 103. This mixing of elements causes the 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 coating layer 103 is improved. From the perspective of improving the adhesion between the steel base 101 and the coating layer 103, the Fe content in the coating layer 103 is preferably 0.20% by mass or more.

[0093] In addition, within the range that does not impair the effects of the present invention, Fe can also be intentionally added to the plating bath used in manufacturing the 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 attachment of the high-melting-point intermetallic compounds to the coating layer as dross, so it is not preferred. From this perspective, by adjusting the Fe content in the plating bath, the Fe content in the coating layer 103 is made 15.00% by mass or less. The Fe content in the coating layer 103 is more preferably 10.00% by mass or less.

[0094] In the coating layer 103, the balance of the above-mentioned Al, Mg, and Fe is Zn and impurities of 5.0000% by mass or more.

[0095] Zn is an essential element for forming the main metallographic structure (Zn-Al-Mg-based metallographic structure) of 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.

[0096] Next, the element groups A to E that the chemical composition of the coating layer 103 according to another aspect of the present embodiment may selectively have will be described in detail.

[0097] It should be noted that when the coating layer 103 involved in this embodiment contains at least any one of the elements belonging to the following element groups B to E, it is preferably contained within the following content ranges and with a total content of 5.0000 mass% or less for at least any one of the elements belonging to the following element groups B to E.

[0098] 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 detailed below can be enjoyed without mutual impairment. The total content of the elements belonging to element groups B to E is preferably 1.00 mass% or less, more preferably 0.2000 mass% or less.

[0099] In addition, in another aspect of the coating layer 103 involved in this embodiment, in terms of chemical composition, the 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.

[0100] ◇Element group A

[0101] In another aspect of the coating layer 103 involved in this embodiment, element group A that the coating layer 103 may contain is described. At least any one of the elements shown below is an element that can be contained in the coating layer 103 in place of a part of the balance of Zn.

[0102] [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

[0103] [Si: 0 to 10.00 mass%]

[0104] Since the case where the coating layer 103 involved in this embodiment does not contain Si is also considered, the lower limit of its content 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 coating layer 103 and the steel substrate 101 and can further improve the adhesion between the coating layer and the steel substrate. When the 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, more preferably 0.20 mass% or more.

[0105] 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, inhibiting 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. Thus, the Si content in the coating layer 103 is preferably 10.00 mass% or less. Additionally, if the Si content in the plating bath used to manufacture the coating layer 103 is excessive, the viscosity of the plating bath may increase beyond necessity, resulting in a reduction in the workability (hereinafter referred to as "plating workability") during the manufacture 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 coating layer 103 is preferably 5.00 mass% or less, more preferably 2.00 mass% or less.

[0106] [Ca: 0 to 4.00 mass%]

[0107] Since the case where the coating layer 103 according to the present embodiment does not contain Ca is also considered, the lower limit of its content is 0 mass%. On the other hand, when Ca is contained in the coating layer 103, it forms intermetallic compounds with Al and Zn. In addition, when Si and Ca are contained together in the coating layer 103, Ca forms intermetallic compounds with Si. These intermetallic compounds have a relatively high melting point and are stable structures, so they can further inhibit liquid metal embrittlement cracks (Liquid Metal Embrittlement: LME) during welding of the plated steel sheet. When Ca is contained in the coating layer 103, the effect of suppressing LME during welding becomes apparent when the Ca content is set to 0.01 mass% or more. The Ca content in the coating layer 103 is more preferably 0.05 mass% or more.

[0108] On the other hand, if the Ca content in the coating 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 coating layer 103 is 4.00 mass% or less. The Ca content in the coating layer 103 is preferably 2.50 mass% or less, more preferably 1.50 mass% or less.

[0109] ◇ Element group B

[0110] Next, in another aspect of the coating layer 103 according to the present embodiment, the element group B that the coating 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 coating layer 103 in place of a part of the balance of Zn.

[0111] [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

[0112] [Sb: 0 to 0.5000 mass%]

[0113] [Pb: 0 to 0.5000 mass%]

[0114] [Sr: 0 to 0.5000 mass%]

[0115] Since it is also conceivable that the 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 coating layer 103 contains at least any one of Sb, Pb, and Sr, spangles are formed on the surface of the coating layer 103, and an improvement in metallic luster can be achieved. Therefore, from the viewpoint of further improving the appearance of the coated steel sheet, it is preferable that the coating layer 103 contains at least any one of Sb, Pb, and Sr. The 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 coating layer 103 contains at least any one of Sb, Pb, and Sr, the content of these elements is preferably independently set to 0.0500 mass% or more respectively.

[0116] On the other hand, if a 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 coating layer 103 increases, and a coated steel sheet with good plating properties cannot be manufactured. Therefore, the content of Sb, Pb, and Sr in the coating layer 103 is independently 0.5000 mass% or less respectively. The content of Sb, Pb, and Sr is preferably independently 0.2000 mass% or less respectively.

[0117] ◇Element group C

[0118] Next, in another aspect of the coating layer 103 according to the present embodiment, the element group C that the coating layer 103 may contain will be described. At least any one of the following element group C is an element that can be contained in the coating layer 103 in place of a part of the balance Zn.

[0119] [Element group C]: 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, one or more

[0120] [Cu: 0 to 1.0000 mass%]

[0121] [Ti: 0 to 1.0000 mass%]

[0122] [Cr: 0 to 1.0000 mass%]

[0123] [Nb: 0 to 1.0000 mass%]

[0124] [Ni: 0 to 1.0000 mass%]

[0125] [Mn: 0 to 1.0000 mass%]

[0126] [Co: 0 to 1.0000 mass%]

[0127] [V: 0 to 1.0000 mass%]

[0128] Since it is also conceivable that the 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 coating layer 103 contains at least any one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V, when welding the plated 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 further 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 coating layer 103 is 0.0050 mass% or more. Therefore, when the 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 set to 0.0050 mass% or more independently.

[0129] On the other hand, if a 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 mass%, these elements form various intermetallic compounds in the plating bath used to form the 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 content of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the coating layer 103 is independently set to 1.0000 mass% or less. The content of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is preferably 0.20 mass% or less independently.

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

[0131] Since it is also possible to consider the case where the coating layer 103 according to the present embodiment does not contain Mo, the lower limit of its content is 0% by mass. On the other hand, when Mo is contained in the coating layer 103, 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.

[0132] On the other hand, if a coating layer 103 with a Mo content greater than 1.0000% by mass is formed, a large amount of scum will be generated in the plating bath used, which is not preferred. Therefore, the content of Mo is 1.0000% by mass or less. The content of Mo is preferably 0.0500% by mass or less.

[0133] ◇Element group D

[0134] Next, in another aspect of the coating layer 103 according to the present embodiment, the element group D that the 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 coating layer 103 in place of a part of the balance of Zn.

[0135] [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

[0136] [Sn: 0 to 1.0000% by mass]

[0137] [In: 0 to 1.0000% by mass]

[0138] [Bi: 0 to 1.0000% by mass]

[0139] Since it is also possible to consider the case where the coating layer 103 according to the present embodiment does not contain Sn, In, and 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 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.

[0140] On the other hand, adding too much Sn, In, and Bi may excessively promote the Mg dissolution rate, and the corrosion resistance of the plated steel sheet may be reduced. 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.

[0141] ◇Element group E

[0142] Next, in another aspect of the coating layer 103 according to the present embodiment, the element group E that the coating layer 103 may contain will be described. At least any one of the following element groups E is an element that can be contained in the coating layer 103 in place of a part of the balance of Zn.

[0143] [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

[0144] [Zr: 0 to 1.0000% by mass]

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

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

[0147] Since the case where the coating layer 103 according to the present embodiment does not contain Zr, Ag, or Li can also be considered, the lower limit of the content of these elements is 0% by mass. On the other hand, when the coating layer 103 contains at least any one of Zr, Ag, and Li, the plating workability can be further improved. The effect of improving the plating property appears when the content of at least any one of Zr, Ag, and Li in the coating layer 103 is 0.0100% by mass or more. Therefore, when containing at least any one of Zr, Ag, and Li, the content of these elements is preferably set to 0.0100% by mass or more independently.

[0148] On the other hand, if a 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 coating layer 103. Therefore, the content of at least any one of Zr, Ag, and Li is 1.0000% by mass or less independently. The content of at least any one of Zr, Ag, and Li is preferably 0.1000% by mass or less independently.

[0149] ◇Element group F

[0150] Next, in another aspect of the coating layer 103 according to the present embodiment, the element group F that the coating layer 103 may contain will be described. At least any one of the following element groups F is an element that can be contained in the coating layer 103 in place of a part of the balance of Zn.

[0151] [Element group F]: Select one or more 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

[0152] [La: 0 - 0.5000 mass%]

[0153] [Ce: 0 - 0.5000 mass%]

[0154] [Y: 0 - 0.5000 mass%]

[0155] Since it is also conceivable that the coating layer 103 according to this embodiment does not contain La, Ce, and Y, the lower limit of the content of these elements is 0 mass%. On the other hand, La, Ce, and Y are elements that exhibit almost the same effect as Ca, and further suppress the formation of pores during welding. This is because the atomic radii of these elements are close to the atomic radius of Ca. When these elements are contained in the 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).

[0156] The effect of suppressing the formation of pores during welding appears by setting the content of these elements to 0.0100 mass% or more independently. Therefore, when at least any one of Zr, Ag, and Li is contained, the content of these elements is preferably set to 0.0100 mass% or more independently. The content of La, Ce, and Y in the coating layer 103 is more preferably 0.0500 mass% or more independently.

[0157] On the other hand, in the plating bath for manufacturing the coating layer 103, if the content of La, Ce, and Y is too high, the viscosity of the plating bath may increase more than necessary, resulting in a decrease in plating operability. Therefore, from the perspective of plating operability, by adjusting the content of La, Ce, and Y in the plating bath, the content of La, Ce, and Y is 0.5000 mass% or less independently. The content of La, Ce, and Y is preferably 0.1000 mass% or less independently.

[0158] ◇Element group G

[0159] Next, in another aspect of the coating layer 103 according to this embodiment, the element group G that the coating layer 103 may contain is described. The elements of the element group G shown below are elements that can be contained in the coating layer 103 by replacing a part of the balance of Zn.

[0160] [Element group G]: B: greater than 0% and 0.5000% or less

[0161] [B: 0 to 0.5000 mass%]

[0162] Since it is also possible to consider the case where the coating layer 103 according to the present embodiment does not contain B, the lower limit of its content is 0 mass%. On the other hand, when B is contained in the coating layer 103, it has the effect of further suppressing LME. It is speculated that this is because when B is contained in the coating layer 103, it combines with at least any one of Zn, Al, Mg, and Ca to form various intermetallic compounds. In addition, it can be considered that by making B present in the coating layer 103, B diffuses from the coating layer 103 to the steel base 101, and has the effect of suppressing LME of the steel base 101 by grain boundary strengthening. Furthermore, it is speculated that the melting points of 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 B of 0.0500 mass% or more. Therefore, when B is contained, the content of B is preferably 0.0500 mass% or more.

[0163] On the other hand, when the plating bath contains too much B in order to make the coating layer 103 contain B, the melting point of the plating rises sharply, resulting in a decrease in plating operability, and it is impossible to manufacture a plated steel sheet with excellent plating properties. This decrease 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.

[0164] [Measurement method of chemical components]

[0165] The chemical components of the above-mentioned coating layer 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 chemical components in units of 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 part of the welded joint 1 is immersed in a 10% HCl aqueous solution added with an inhibitor for about 1 minute, the coating layer part is peeled off, and a solution in which the coating layer is dissolved is prepared. By analyzing the obtained solution with ICP-AES or ICP-MS, the overall average chemical components of the coating layer can be obtained.

[0166] ◇Regarding the coating amount of the coating layer 103

[0167] Regarding the coating amount of the coating layer 103 as described above, there is no particular limitation. For example, on each side of the steel base 101, it is preferably 15 to 250 g / m 2Around. By making the coating amount of the coating layer 103 within the above range, the coating layer 103 involved in the present embodiment can exhibit sufficient corrosion resistance. The thickness of the coating layer 103 having such a coating amount is approximately 5 to 40 μm.

[0168] It should be noted that the coating amount of the coating layer 103 is measured as follows. First, a sample with a size of 30 mm × 30 mm in plan view is cut out from the non-heat-affected part of the welded joint 1, and the mass of this sample is measured in advance. It should be noted that when cutting out the sample, the entire thickness direction is cut out. A tape is pasted on one surface of this sample so that the coating layer on this one surface side does not dissolve in the next process. On this basis, this sample is immersed in a 10% HCl aqueous solution added with an inhibitor, and the coating layer 103 is pickled and peeled off, and the mass of the sample after pickling is measured. The coating amount of the coating layer 103 on each surface can be determined based on the mass change of the sample before and after pickling.

[0169] ◇Regarding the metallographic structure of the coating layer 103

[0170] Next, the metallographic structure of the coating layer 103 having the chemical composition as described above will be described.

[0171] The coating layer 103 involved in the present embodiment has the chemical composition as described above and is formed by the manufacturing method described in detail below, so as to contain 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 coating layer 103 may also contain, in addition to the above metallographic structures, it may also contain metallographic structures such as Al-Si-Ca phase, Al-Si-Ca-Fe phase, Mg2Si phase, and Mg2Sn phase. The coating layer 103 involved in the present embodiment shows the properties of suppressing the occurrence of LME and excellent corrosion resistance by having the above metallographic structures.

[0172] Here, regarding what kind of metallographic structure the coating layer 103 involved in the present embodiment has, it can be determined by observing the cross-section of the coating layer 103 using SEM. That is, by observing the solidification structure of the coating layer 103 using SEM, based on the point analysis results of SEM-EPMA in the observation field of view, it can be determined what kind of metallographic structure it has.

[0173] More specifically, the observation position is set within the non-heat-affected part on the back side, and the size of the observation area is set to 40 μm × 40 μm. Set the acceleration voltage: 15.0 kV, irradiation current: 5.0×10 -7 A, irradiation time: 50 milliseconds, and observe this range at a magnification of 2000 times. Under this condition, a backscattered electron image of the concerned range is obtained, and then using the contrast of the backscattered electron image, three point analyses of each metallographic structure are performed respectively. It is only necessary to perform this measurement on any 5 fields of view.

[0174] ◇Regarding the carbon (C) concentration in the surface layer portion of the steel base of the non-heat-affected portion

[0175] In addition, by glow discharge optical emission spectrometry (GDS: Glow Discharge Optical Emission Spectrometry), the non-heat-affected portion of the welded joint 1 according to the present embodiment is measured along the depth direction (the thickness direction of the steel base 101) starting from the surface of the plating 103, and a depth curve related to the distribution of Zn, Fe, and C is measured.

[0176] 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.

[0177] ◇Commercially available glow discharge optical emission spectrometer (for example, GDS850A manufactured by LECO Japan Co., Ltd., etc.)

[0178] · Argon pressure: 0.27 MPa

[0179] · Anode diameter: 4 mm φ

[0180] · RF (high frequency) output power: 30 W

[0181] In the obtained measurement results, first, the position of the interface between the plating 103 and the steel base 101 is specified. More specifically, in the depth curve related to the distribution 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 plating 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 plating 103 and the steel base 101, the change in the C concentration in the depth direction is confirmed. The C concentration to be focused on below can be said to be the C concentration of the portion of the steel base 101 adjacent to the interface of the plating 103.

[0182] In the non-heat-affected portion 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 plating 103 and the steel base 101. C in steel is an element that promotes LME. Therefore, by reducing the C concentration at the interface portion between the steel base 101 and the plating 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.

[0183] Here, regarding the C concentration in the interface portion between the steel base 101 and the plating layer 103, it is set to a 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 for the welded joint 1 according to this embodiment.

[0184] <Regarding the welding overlap portion 50>

[0185] Next, with reference to Figure 3 and Figure 4 the welding overlap portion 50 of the welded joint 1 according to this embodiment will be described in detail. Figure 3 and Figure 4 are explanatory diagrams for explaining the welding overlap portion 50 of the welded joint 1 according to this embodiment.

[0186] Figure 3 is a diagram schematically showing an enlarged cross-sectional shape near the welding overlap portion 50 shown in Figure 1 as observed from the Y-axis direction.

[0187] In the welded joint 1 formed by arc welding or laser welding of the first steel plate 10 and the second steel plate 20, when the vicinity of the welding overlap portion 50 is observed in an enlarged manner, as Figure 3 schematically shown, there is a gap 60 between the first steel plate 10 and the second steel plate 20. This gap is called the "joint gap" (sometimes also called the lap joint gap). In Figure 3 this, the joint gap 60 is highlighted, so at first glance it seems there is a relatively large gap, but the size of the joint gap 60 ( Figure 3 d in this) is usually about 10 to 500 μm. It can be said that from this size, it is clear that even in the welded joint 1 where the two steel plates seem to be in complete contact to the naked eye, there is a joint gap 60 when observed in an enlarged manner.

[0188] In this welding overlap portion 50, it is considered that assuming the center axis of the joint gap 60 is extended toward the weld bead portion 30 along a direction orthogonal to the extending direction of the weld bead portion 30 ( Figure 3 the Y-axis direction in this) ( Figure 3 the X-axis direction in this). In this embodiment, the virtual straight line representing the center axis of the joint gap 60 is denoted as the straight line L C . In the welded joint 1 according to this embodiment, molten metal sometimes flows in from the joint gap 60, exposing the weld bead portion 30. In this case, the heat-affected zone 40 no longer exists on the virtual straight line L C representing the center axis of the joint gap 60. Considering this situation, in this embodiment, the intersection point of the virtual straight line L C representing the center axis of the joint gap 60 and the end of the weld bead portion 30 is defined as the "front end portion of the welding overlap portion".

[0189] It should be noted that during arc welding or laser welding, so-called pores may be formed depending on the type of plated steel sheet, welding conditions, etc. However, since the pores are formed inside the weld bead portion 30, according to the above definition, the front end portion of the welded overlapping portion can be determined regardless of whether pores are generated or not.

[0190] Here, the boundary position between the heat-affected zone 40 and the weld bead portion 30 in the cross-section of the actual welded joint 1 can be easily visualized by etching the cross-section of the welded joint 1 including the weld bead portion 30 and the heat-affected zone 40 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 household detergent solution, etc.

[0191] It can also be clearly seen from the chemical composition of the plating layer 103 in the non-heat-affected zone that the welded joint 1 according to the present embodiment uses a galvanized steel sheet as the blank steel sheet. During arc welding or laser welding, the heat input during welding is greater the closer to the welding site. For the welded joint 1 according to the present embodiment, since the plating layer 103 is a zinc-based plating layer with a lower melting point, the plating layer 103 will evaporate and disappear during welding. In addition, the less heat reaches the farther away from the welding site, so when at a certain distance from the welding site, the plating layer 103 will still exist even after welding.

[0192] The inventors of the present invention studied the corrosion resistance of the welded overlapping portion 50 and found that if the evaporation of this plating layer 103 can be suppressed, thereby reducing the disappearance amount of the plating layer 103, the area of the remaining plating layer 103 after welding will increase, and thus the corrosion resistance of the welded overlapping portion 50 can be improved.

[0193] In the welded joint 1 according to the present embodiment, by using a plated steel sheet including the plating layer 103 having the above-described chemical composition as the blank steel sheet, the evaporation of the plating layer 103 during welding can be suppressed. As a result, in the welded joint 1 according to the present embodiment, the corrosion resistance of the welded overlapping portion can be improved.

[0194] Next, an explanation will be given on how to specify the degree of disappearance of the above plating layer 103.

[0195] In the gap 60 of the welded joint 1 according to the present embodiment, the region where the thickness of the plating layer 103 closest to the weld bead portion 30 (the front end portion 51 of the welded overlapping portion) in the X-axis direction is less than 1 μm is referred to as the "plating disappearance portion". For example, Figure 4 In the schematic diagram shown, the region surrounded by the dashed line corresponds to the above plating disappearance portion.

[0196] In the welded joint 1 according to the present embodiment, starting from the front end portion 51 of the welding overlap portion defined as above, the size of the plating disappearance portion 53 is defined. That is, it is determined how far the plating disappearance portion 53 extends from the vicinity of the weld bead portion 30 and the heat-affected portion 40 in a direction orthogonal to the extending direction of the weld bead portion 30 ( Figure 4 the Y-axis direction in Figure 4 and in a direction separated from the weld bead portion 30 ( Figure 4 the positive X-axis direction in Figure 4 ). At this time, the position (

[0197] For example, in Figure 4 the example of Figure 4 ), the end of the plating 103 is located at position A. Therefore, Figure 4 position A in Figure 4 is the end of the plating disappearance portion 53 on the side away from the weld bead portion 30 (which can also be understood as the terminal portion of the plating disappearance portion 53). Therefore, for

[0198] Here, in Figure 4 as an example, the case where the position of the terminal portion of the plating disappearance portion 53 on the first steel plate 10 side coincides with the position of the terminal portion of the plating disappearance portion 53 on the second steel plate 20 side is illustrated. When the position of the terminal portion of the plating disappearance portion 53 on the first steel plate 10 side is different from the position of the terminal portion of the plating disappearance portion 53 on the second steel plate 20 side, the position of the terminal portion on the side farther from the front end portion 51 of the welding overlap portion is regarded as the terminal portion of the plating disappearance portion 53 in the welded joint 1 of concern.

[0199] In the welded joint 1 according to the present embodiment, Figure 4The length L of the plating disappearance portion 53 shown in the figure is 1000 μm or less. The length L of the plating disappearance portion 53 is short, being 1000 μm or less, which means that the plating 103 sufficiently exists in the welding overlap portion 50 of the welded joint 1 according to the present embodiment. Due to the existence of such plating 103, the corrosion resistance of the welding overlap portion 50 in the welded joint 1 according to the present embodiment is improved. Figure 4 The shorter the length L of the exemplified plating disappearance portion 53 is, the better. It is preferably 500 μm or less, and more preferably 100 μm or less.

[0200] Here, the length L of the above-mentioned plating disappearance portion 53 can be measured as follows.

[0201] That is, an etching treatment is performed on a cross-section obtained by cutting the welded joint 1 in a direction orthogonal to the Y-axis direction at an arbitrary position in the extending direction of the weld bead portion 30 using the above-mentioned etching solution, and the observation is carried out by using SEM. At this time, the acceleration voltage is set to 15 kV, and a field of view of 200 μm × 180 μm is observed using a backscattered electron image. Through this observation, the position of the front end portion 51 of the welding overlap portion 50 is determined. On this basis, starting from the front end portion 51, the length of the plating disappearance portion is measured. Such observation / measurement operations are performed on the cross-sections of any three parts, and the average value of the obtained measurement values is taken as the length L of the plating disappearance portion 53.

[0202] Above, with reference to Figures 3 - 4 The welding overlap portion 50 of the welded joint 1 according to the present embodiment has been described in detail.

[0203] It should be noted that the non-heat-affected portion of the welded joint 1 according to the present embodiment may further have one or two or more layers of various films on the above-mentioned plating 103. Examples of such films include chromate films, phosphate films, chromium-free films, organic resin films, and the like.

[0204] (Regarding the manufacturing method of the plated steel sheet as the blank)

[0205] 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.

[0206] The plated steel sheet as the blank of the welded joint 1 according to the present embodiment is based on the above-mentioned steel base 101 as the substrate, and strain is applied to the surface of the steel base 101 by heavy grinding. Then, a plating layer is formed on the surface to which the strain is applied, and thus the plated steel sheet as the blank of the welded joint 1 is manufactured. Then, by performing a specific heat treatment on the plating layer formed on the surface of the steel base 101, the plated steel sheet as the blank of the welded joint 1 is manufactured.

[0207] Here, the surface of the steel base 101 is ground using a heavy grinding brush to apply strain to the surface, so that when the plated steel sheet as a 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 plating) is enriched in the liquid phase present during welding, and a dense Mg-Ca oxide is formed on the surface of the liquid phase Mg-Zn metal microstructure. As a result, the evaporation of the Mg-Zn metal microstructure that plays a favorable role in the corrosion resistance of the steel base 101 can be suppressed, and the length of the above-mentioned plating disappearance portion 53 can be within the range of the present embodiment.

[0208] In addition to the hot-dip plating method, spraying methods, cold spraying methods, sputtering methods, evaporation plating methods, electroplating methods, etc. can also be applied to form the plating. Among them, in terms of cost, the hot-dip plating method is most preferably used to form a plating with a thickness of the degree commonly used in automobiles and the like.

[0209] Then, the obtained plated steel sheet is subjected to a specific heat treatment process described below, whereby a plated steel sheet as a blank can be manufactured.

[0210] Hereinafter, an example of a manufacturing method of a plated steel sheet as a blank obtained by the hot-dip plating method will be described in detail.

[0211] In the manufacturing process of this plated steel sheet, first, the steel sheet used as the base material 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.

[0212] In the hot-dip plating production line, the steel sheet is continuously passed while being uncoiled from the coil. During the passing process, strain is applied to the surface of the steel sheet 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 and a dew point of -60 to 10 °C, the steel sheet is heated and reduced at 700 to 900 °C for more than 0 seconds and 300 seconds or less, and then air-cooled to about the bath temperature of the subsequent plating bath + 20 °C with N2 gas and immersed in the plating bath. It should be noted that in the above process, strain is applied to the steel sheet before annealing, but even if at least a part of the applied strain is released by annealing, the alloying of Fe and Al can be promoted.

[0213] 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 to be above the melting point of the plating alloy (for example, around 460 to 660 °C). When producing the material of the plating alloy, it is preferable to use pure metals (purity of 99% or more) as alloy materials for blending. First, a specified amount of alloy metals is mixed in such a way as to form the above-described coating layer, and in a vacuum or an inert gas replacement state, it is completely dissolved using a high-frequency induction furnace, an arc furnace, etc. to produce an alloy. Further, the alloy mixed with the specified composition (the composition of the above-described coating layer) is melted in the atmosphere, and the obtained melt is used as the plating bath.

[0214] It should be noted that in the production of the above-described plating alloy, there is no particular limitation to the use of pure metals, and existing Zn alloys, Mg alloys, and 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.

[0215] The steel plate is immersed in the above-described plating bath and then lifted at a specified speed. At this time, for example, the plating adhesion amount is controlled using N2 purge gas so that the formed coating layer becomes a 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.

[0216] Next, the following first cooling process and second cooling process are performed on the plating alloy in a molten state located on the steel plate to form the plating layer 103 from the plating alloy in a molten state. Hereinafter, the first cooling process and the second cooling process will be described in detail.

[0217] The first cooling process is a cooling process performed when the temperature of the plating alloy is in the range of the bath temperature to 250 °C. The plated steel plate 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 from the bath temperature to 250 °C is likely to cause the formation of coarse oxides on the surface of the coating layer. Therefore, by setting the dew point to -20 °C or lower and the average cooling rate to 10 °C / second or more, oxidation in the high-temperature region is prevented. It should be noted that in the case where the hot-dip plating method is used in the plating process, this first cooling process is performed immediately after the steel plate comes out of the plating bath. Thereby, the plating alloy on the surface of the steel plate solidifies to form a coating layer.

[0218] Then, when the temperature of the plating alloy (coating layer) is in the range of 250 to 50 °C, the second cooling process is performed. This second cooling process is a process of slowly cooling the plated steel plate 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, it is possible to form dense oxides in the low-temperature region.

[0219] 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 the atmosphere gas blown for controlling 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 with 250°C as the boundary, the average cooling rate can also be switched with 250°C as the boundary, and the atmosphere gas for controlling the dew point can be switched within the temperature range of 260 - 240°C.

[0220] As described above, by forming a coating on the basis of applying strain to the surface of the steel base 101 using a heavy grinding brush, and further subjecting this coating to a two-stage cooling process, that is, rapid cooling within the temperature range of the bath temperature to 250°C and slow cooling within the temperature range of 250 - 50°C, a coating 103 in which Zn is difficult to evaporate even during welding can be achieved.

[0221] Here, the interval from the end of the first cooling process to the start of the second cooling process is preferably set to within 3 seconds, and it is preferred 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 coating 103 within the scope of the above-described present embodiment cannot be achieved.

[0222] 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. In addition, 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.

[0223] 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. In addition, the average cooling rate is more preferably 4°C / second or less.

[0224] It should be noted that even if strain is appropriately applied to the surface of the steel base 101, if either the above-described first cooling process or the second cooling process is not carried out, the coating 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 also simultaneously carrying out the above-described first cooling process and the second cooling process, the coating 103 according to the present embodiment can be achieved.

[0225] In addition, after the above-described second cooling step, if an alloying heat treatment step (for example, a heat treatment step accompanied by heating to a plate temperature of about 480 to 550°C) that is usually carried out in the manufacture of alloyed hot-dip galvanized steel sheets is carried out, the formation state of the plating layer controlled by the first cooling step and the second cooling step 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 carry out the heat treatment step after the second cooling step.

[0226] Here, in the above 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.

[0227] It should be noted that as a method for actually measuring the temperature of the plating layer, for example, a contact thermocouple (K type) can be used. By installing the contact thermocouple on the steel base 101 as the base material, the average temperature of the entire plating layer can be monitored at all times. In addition, if various speeds and thicknesses are mechanically controlled to unify various operating conditions such as the preheating temperature of the steel base 101 and the plating bath temperature, the temperature of the entire plating layer at that moment under these manufacturing conditions can be monitored approximately accurately. Thus, the cooling treatment in the first cooling step and the second cooling step can be precisely controlled. It should be noted that although not as accurate as the contact type, the surface temperature of the plating layer can also be measured by a non-contact radiation thermometer.

[0228] In addition, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can also be obtained by performing a simulation of heat conduction analysis. Specifically, based on various manufacturing conditions such as the preheating temperature of the steel base 101, the plating bath temperature, the speed at which the steel plate is lifted from the plating bath, the plate thickness of the steel base 101, the layer thickness of the plating layer, the heat exchange heat quantity between the plating layer and the manufacturing equipment, and the heat dissipation quantity of the plating layer, the surface temperature and the average temperature of the entire plating layer of the plating layer are obtained. Then, using the obtained results, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can be obtained. Thus, by actually measuring the surface temperature of the plating layer when manufacturing the plated steel sheet, the average temperature of the entire plating layer at that moment under these manufacturing conditions can be inferred. As a result, the cooling treatment in the first cooling step and the second cooling step can be precisely controlled.

[0229] Above, an example of the manufacturing method of the plated steel sheet according to the present embodiment has been specifically described.

[0230] It should be noted that in the manufacturing method of the plated steel sheet according to the present embodiment, after the above-described second cooling step, a treatment for forming one or two or more layers of various films can also be further carried out. As such a treatment, for example, chromate treatment, phosphate treatment, chromium-free treatment, organic resin film formation treatment, etc. can be cited.

[0231] In chromate treatment, there are: electrolytic chromate treatment in which a chromate film is formed by electrolysis; reactive chromate treatment in which a film is formed by reacting with the blank and then the excess treatment liquid is rinsed; coating-type chromate treatment in which a treatment liquid is coated and dried without water washing to form a film, etc. Any one of the chromate treatments can be adopted.

[0232] As the electrolytic chromate treatment, for example, an electrolytic chromate treatment using chromic acid, silica sol, resins (phosphate resin, acrylic resin, vinyl ester resin, vinyl acetate-acrylic emulsion, carboxylated styrene-butadiene latex, diisopropanolamine-modified epoxy resin, etc.) and hard silica can be exemplified.

[0233] As the phosphate treatment, for example, zinc phosphate treatment, zinc calcium phosphate treatment, manganese phosphate treatment, etc. can be exemplified.

[0234] In order not to impose a burden on the environment, a chromium-free treatment is particularly preferred. In this chromium-free treatment, there are: electrolytic chromium-free treatment in which a chromium-free film is formed by electrolysis; reactive chromium-free treatment in which a film is formed by reacting with the blank and then the excess treatment liquid is rinsed; coating-type chromium-free treatment in which a treatment liquid is coated and dried without water washing to form a film, etc. Any one of the chromium-free treatments can be adopted.

[0235] 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 means 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.

[0236] As the organic resin, one kind of the above-mentioned resins can be used alone, or two or more kinds of organic resins (unmodified organic resins) can be mixed and used. 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 kinds can be mixed and used. In addition, an organic resin made water-based by dissolving or dispersing in water can also be used. In addition, various coloring pigments and anti-rust pigments can also be contained in the organic resin film.

[0237] (Regarding the manufacturing method of the welded joint)

[0238] The welded joint according to this embodiment is manufactured as follows: The plated steel sheet manufactured as described above is used as the blank steel sheet for the first steel sheet and the second steel sheet when manufacturing the welded joint. On this basis, the blank steel sheet is arranged in the shape required for the welded joint, and the blank steel sheet is welded.

[0239] Here, in the welding of the blank steel sheet, the arc welding method or the 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 portion near the weld toe can be achieved.

[0240] More specifically, in the case of manufacturing a welded joint by arc welding, the blank steel sheet can be welded under the following welding conditions, for example.

[0241] Welding current: 250 A, welding voltage: 26.4 V, welding speed: 100 cm / min

[0242] Welding gas: 20% CO2 + Ar, gas flow rate: 20 L / min

[0243] Welding wire: φ1.2 mm made by Nippon Welding Industry Co., Ltd. YGW16

[0244] (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)

[0245] Torch tilt angle: 45°

[0246] In addition, in the case of manufacturing a welded joint by laser welding, the blank steel sheet can be welded under the following welding conditions, for example.

[0247] Output power: 7 kW, welding speed: 400 cm / min, forward / backward angle: 0°

[0248] Above, an example of the manufacturing method of the welded joint according to this embodiment has been described.

[0249] It should be noted that in the above embodiment, as the blank steel sheet for the first steel sheet 10 and the second steel sheet 20, the case of using the galvanized steel sheet provided with the plating layer 103 on the steel base 101 as described above has been described as an example. However, the above plating layer 103 only needs to exist on at least the surface of the side where the first steel sheet 10 and the second steel sheet 20 overlap each other.

[0250] Examples

[0251] 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.

[0252] In the examples and comparative examples shown below, as the steel plate that becomes the steel base 101, hot-rolled steel plates with a tensile strength of 590 MPa grade, 780 MPa grade, and 980 MPa grade (all manufactured by Nippon Steel Corporation) are used. The plate thickness of the hot-rolled steel plates is set to 3.2 mm uniformly. Using these hot-rolled steel plates, a plurality of test pieces are prepared for each hot-rolled steel plate.

[0253] For the prepared test pieces, the following two types of heavy-duty abrasive brushes are used to apply strain to the surface of the test pieces. It should be noted that during grinding, a 1.0 - 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 - 10.0 mm and the brush rotation speed within the range of 100 - 1000 rpm, the amount of strain applied to the surface is 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 are also prepared.

[0254] Brush type A: D-100 manufactured by Hodani Co., Ltd.

[0255] Brush type B: M-33 manufactured by Hodani Co., Ltd.

[0256] Plating baths for achieving the compositions shown in Table 1 below are prepared respectively, and are respectively set in a batch-type hot-dip plating test apparatus manufactured by our company to perform plating on the above test pieces. Here, a thermocouple spot-welded to the center of the test piece is used to measure the temperature of the test piece. In addition, for the test pieces immersed in the plating bath, before immersion in the plating bath, in a furnace with an oxygen concentration of 20 ppm or less, in an N2 - 5% H2 gas atmosphere, the surface of the plating base plate is heated and reduced at 800°C. After the heat reduction treatment, the test pieces are air-cooled with N2 gas. After the temperature of the test pieces reaches the bath temperature + 20°C, the test pieces are immersed in the plating bath of the hot-dip plating test apparatus for about 3 seconds.

[0257] After immersion in the plating bath, the test pieces are lifted at a lifting speed of 20 - 200 mm / second. During lifting, N2 purge gas is used for control to achieve the desired plating adhesion amount. In the following examples and comparative examples, the plating adhesion amount is controlled so that the adhesion amount of the dried plating on each side of the test piece is 40 - 120 g / m 2After lifting the test piece from the plating bath, the test piece was cooled from the plating bath temperature to room temperature under the conditions shown in Table 1 below. In the examples and comparative examples shown below, immediately after the end of the first cooling process, the second cooling process was started (that is, the interval from the end of the first cooling process to the start of the second cooling process was set to 0.2 seconds or less).

[0258] Here, a steel plate with a size of 30 mm × 30 mm was cut out from the test piece plated as described above, and the plated steel plate was immersed in a 10% HCl aqueous solution containing an inhibitor. After pickling and peeling the plating layer, ICP analysis was performed on the elements dissolved in the aqueous solution to measure the composition of the plating layer.

[0259] In addition, a steel plate cut into a size of 150 mm × 50 mm from the obtained test piece was used as the first steel plate, and a steel plate cut into a size of 150 mm × 30 mm was used as the second steel plate. The long sides of these steel plates were overlapped and welded by arc welding (lap fillet welding) to form a welded joint.

[0260] Here, the welding conditions for arc welding are as follows.

[0261] Welding current: 250 A, Welding voltage: 26.4 V, Welding speed: 100 cm / min

[0262] Welding gas: 20% CO2 + Ar, Gas flow rate: 20 L / min

[0263] Welding wire: YGW16 manufactured by Nippon Welding Industry Co., Ltd., φ1.2 mm

[0264] (C: 0.1 mass%, Si: 0.80 mass%, Mn: 1.5 mass%, P: 0.015 mass%, S: 0.008 mass%, Cu: 0.36 mass%)

[0265] Torch inclination angle: 45°

[0266] Overlap amount: 10 mm

[0267] Steel plate size: Upper plate side (first steel plate) 150 × 50 mm, Lower plate side (second steel plate) 150 × 30 mm

[0268] Plate gap: 0 mm

[0269] In addition, the welding conditions for laser welding are as follows.

[0270] Output power: 7 kW, Welding speed: 400 cm / min, Forward / backward angle: 0°

[0271] Steel plate size: Upper plate side (first steel plate) 150 × 50 mm, Lower plate side (second steel plate) 150 × 30 mm

[0272] Overlap amount: 10 mm

[0273] Gap between plates: 0 mm

[0274] <GDS measurement of C concentration in the steel substrate>

[0275] For the non-heat-affected part of the welded joint obtained as described above, using the method described previously, measure the Zn, Fe, and C concentrations by GDS, and measure the depth at which the C concentration becomes 0.05 mass% or less.

[0276] <Measurement of the length L of the plating disappearance part in the welded overlap part>

[0277] For the welded joint obtained as described above, using the method described previously, observe the cross-section of the welded overlap part by SEM and measure the length L of the plating disappearance part.

[0278] <Evaluation of the corrosion resistance of the welded overlap part>

[0279] For the above-mentioned welded joint, perform automotive phosphating chemical conversion treatment (Zn phosphating treatment, SD5350 system: standard prepared by Nipponpaint Industrial Coatings Co., LTD) and electrodeposition coating (PN110 Power NixGray: standard prepared by Nipponpaint Industrial Coatings Co., LTD). At this time, set the electrodeposition film thickness to 20 μm. Subject the sample after electrodeposition coating to a composite cyclic corrosion test based on JASO (M609-91), and evaluate the timing of red rust generation in the cross-section of the welded overlap part. The evaluation criteria are as follows.

[0280] <<Evaluation Criteria>>

[0281] Score "AAA": Timing of red rust generation is more than 120 cycles

[0282] "AA": Timing of red rust generation is more than 60 cycles and 120 cycles or less

[0283] "A": Timing of red rust generation is more than 30 cycles and 60 cycles or less

[0284] "B": Timing of red rust generation is 30 cycles or less

[0285] In addition, if the timing of red rust generation is more than 30 cycles, the welded overlap part of the test piece of interest can be evaluated as having good corrosion resistance.

[0286] Summarize the results obtained and show them in Table 1 below.

[0287] [Table 1]

[0288]

[0289] 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 in the welded lap joint. In contrast, in the comparative examples belonging to the present invention, sufficient performance was not exhibited in terms of the corrosion resistance of the welded lap joint.

[0290] For example, in No. 26 where the Al content of the coating exceeds the scope of the present invention, the Fe-Al alloying ends prematurely, and the alloying of the liquid-phase Mg-Zn metallographic structure and the steel substrate proceeds excessively, resulting in the inability to form a dense Mg oxide on the surface of the liquid-phase Mg-Zn metallographic structure. Therefore, Zn evaporates during welding, and the corrosion resistance of the welded lap joint is insufficient. In No. 27 where the Al content of the coating exceeds the scope of the present invention, since the Fe-Al alloying takes time, the alloying of the coating and the steel substrate is insufficient, and as a result, a dense Mg-based oxide cannot be formed on the surface of the liquid-phase Mg-Zn metallographic structure. Thus, Zn evaporates during welding, and the corrosion resistance of the welded lap joint is insufficient.

[0291] In No. 28 where the Mg content of the coating exceeds the scope of the present invention, since a dense Mg-based oxide cannot be formed on the surface of the liquid-phase Mg-Zn metallographic structure, Zn evaporates during welding, and the corrosion resistance of the welded lap joint is insufficient. In No. 29 where the Mg content of the coating exceeds the scope of the present invention, due to excessive Mg, the excessive Mg inhibits Fe-Al alloying, and the alloying of the coating and the steel substrate is insufficient. As a result, a dense Mg-based oxide cannot be formed on the surface of the liquid-phase Mg-Zn metallographic structure. Thus, Zn evaporates during welding, and the corrosion resistance of the welded lap joint is insufficient.

[0292] In No. 30 where the average cooling rate of the first cooling process exceeds the scope of the present invention and No. 31 where the flow rate of the cooling medium in the first cooling process exceeds the scope of the present invention, Zn that constitutes the coating burns excessively during welding, resulting in insufficient corrosion resistance.

[0293] In No. 32 where the average cooling rate of the second cooling process exceeds the scope of the present invention and No. 33 where the flow rate of the cooling medium in the second cooling process exceeds the scope of the present invention, Zn that constitutes the coating burns excessively during welding, resulting in insufficient corrosion resistance.

[0294] In No. 36 where no strain is applied to the steel substrate, the formation of the η phase cannot be controlled due to alloying, resulting in insufficient corrosion resistance.

[0295] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to this example. It is obvious that various variations or modifications can be conceived by those with ordinary knowledge in the technical field to which the present invention pertains 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.

[0296] The embodiments disclosed this time are exemplary rather than restrictive in all aspects. Without departing from the appended claims, the constitution and 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 not impairing its effects, 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.

[0297] In addition, the effects described in this specification are merely illustrative or exemplary, rather than 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.

[0298] It should be noted that the following constitution also belongs to the technical scope of the present invention.

[0299] (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,

[0300] The first steel plate and the second steel plate 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.

[0301] A joint gap exists in the welding overlap portion where the surfaces of the first steel plate and the second steel plate face each other, and the joint gap is a gap existing between the first steel plate and the second steel plate.

[0302] At least on the side where the surfaces of the first steel plate and the second steel plate face each other, each has a steel base and a coating on the steel base.

[0303] The coating is a coating having the following chemical composition, and the chemical composition contains, by mass%,

[0304] Al: 10.00 - 70.00%,

[0305] Mg: 3.00 - 20.00%,

[0306] Fe: 0.01 - 15.00%,

[0307] 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, with the balance being Zn and impurities of 5.00 mass% or more,

[0308] [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;

[0309] [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;

[0310] [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;

[0311] [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;

[0312] [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;

[0313] [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;

[0314] [Element group G]: B: greater than 0% and 0.5000% or less,

[0315] When observing the welded overlapping portion in the extending direction of the weld bead portion, assuming that the central axis of the joint gap is extended toward the weld bead portion along a direction orthogonal to the extending direction, the intersection point on the central axis with the end portion of the weld bead portion is defined as the front end portion of the welded overlapping portion.

[0316] In a cross-sectional view cut in a direction orthogonal to the extending direction, the distance between the end portion on the side away from the weld bead portion of the region where the thickness of the coating layer closest to the weld bead portion in the direction orthogonal to both the extending direction and the normal direction to the surface of the second steel plate is less than 1 μm and the front end portion of the welded overlapping portion is 1000 μm or less.

[0317] (2) The welded joint according to (1), which has a chemical composition containing the element group A.

[0318] (3) The welded joint according to (1), which has a chemical composition containing the element group B.

[0319] (4) The welded joint according to (1), which has a chemical composition containing the element group C.

[0320] (5) The welded joint according to (1), which has a chemical composition containing the element group D.

[0321] (6) The welded joint according to (1), which has a chemical composition containing the element group E.

[0322] (7) The welded joint according to (1), which has a chemical composition containing the element group F.

[0323] (8) The welded joint according to (1), which has a chemical composition containing the element group G.

[0324] (9) The welded joint according to any one of (1) to (8), wherein the coating layer contains Al: 18.00 to 50.00% by mass, Mg: 6.00 to 15.00% by mass, and contains Ca: 0.05 to 4.00% by mass as the element group A.

[0325] (10) The welded joint according to any one of (1) to (9), wherein for the C concentration in the steel base of the non-heat-affected portion calculated based on the depth curve of the carbon distribution 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 base and the coating layer.

[0326] (11) The welded joint according to any one of (1) to (10), wherein the tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

[0327] (12) The welded joint according to any one of (1) to (11), wherein the distance is 500 μm or less.

[0328] (13) The welded joint according to any one of (1) to (12), wherein the distance is 100 μm or less.

[0329] Explanation of reference numerals

[0330] 1 Welded joint

[0331] 10 First steel plate

[0332] 20 Second steel plate

[0333] 30 Weld bead portion

[0334] 40 Heat affected zone

[0335] 50 Welding overlap portion

[0336] 51 Front end portion of the welding overlap portion

[0337] 60 Joint gap

[0338] 101 Steel base

[0339] 103 Coating

[0340] 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, The first steel plate and the second steel plate have a heat-affected portion around the weld bead portion and a non-heat-affected portion where there is no heat influence caused by welding. A joint gap exists in a welding overlap portion where the surfaces of the first steel plate and the second steel plate face each other, and the joint gap is a gap existing between the first steel plate and the second steel plate. At least the surfaces of the first steel plate and the second steel plate facing each other have a steel base and a coating on the steel base. The coating is a coating having the following chemical composition, and the chemical composition contains, by mass%, Al：10.00～70.00％、 Mg: 3.00 - 20.00%, Fe: 0.01 - 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 5.00 mass% or more. [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. [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. [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. [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. [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. [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. [Element group G]: B: greater than 0% and 0.5000% or less. When observing the welded overlap portion in the extending direction of the weld bead portion, assuming that the central axis of the joint gap is extended toward the weld bead portion along a direction orthogonal to the extending direction, the intersection point on the central axis with the end portion of the weld bead portion is defined as the front end portion of the welded overlap portion. In a cross-sectional view cut in a direction orthogonal to the extending direction, the distance between the end portion on the side away from the weld bead portion of the region where the thickness of the plating closest to the weld bead portion in the direction orthogonal to both the extending direction and the normal direction with respect to the surface of the second steel plate is less than 1 μm and the front end portion of the welded overlap portion is 1000 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 plating contains 18.00 to 50.00 mass% of Al, 6.00 to 15.00 mass% of Mg, and contains 0.05 to 4.00 mass% of Ca as the element group A.

10. The welded joint according to any one of claims 1 to 8, wherein, Regarding the C concentration in the steel base of the non-heat-affected portion calculated based on the depth curve of 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 plating.

11. The welded joint according to claim 9, wherein, Regarding the C concentration in the steel base of the non-heat-affected portion calculated based on the depth curve of 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 plating.

12. The welded joint according to any one of claims 1 to 8, wherein, The tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

13. The welded joint according to claim 9, wherein, The tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

14. The welded joint according to claim 10, wherein, The tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

15. The welded joint according to claim 11, wherein, The tensile strength of at least one of the first steel plate and the second steel plate is 780 MPa or more.

16. The welded joint according to claim 1, wherein, The distance is 500 μm or less.

17. The welded joint according to claim 1, wherein The distance is 100 μm or less.

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