Welded joints

Abstract

Provided is a welded joint characterized by comprising a plurality of stacked steel sheets, nuggets joining the plurality of steel sheets, and a spot welded section having pressure welded sections and heat affected sections formed around the nuggets, wherein: at least one of the plurality of steel sheets is a plated steel sheet provided with a base material steel sheet and a plating layer formed at least on a surface of the base material steel sheet corresponding to the outermost surface of the plurality of steel sheets; the plating layer in the outermost region of the heat affected sections has a prescribed chemical composition; the ratio "Al concentration in the center of the plating layer" / "Al concentration in a plating layer position where the Fe concentration is 50% of the base material steel sheet" is 0.10-1.50 when the plating layer is measured using GDS; and the surface ratio of the β-CuZn phase is 50% or less in the plating layer of a welded shoulder section of the outermost surface of the plurality of steel sheets.

Description

[Technical Field]

[0001] The present invention relates to a welded joint, and more particularly to a welded joint obtained by spot welding plated steel sheets. [Background technology]

[0002] Steel sheets used in automobiles and other applications require good weldability. Spot welding is primarily used in processes such as assembling automobile bodies and installing parts. Liquid metal embrittlement (LME) cracking must be prevented, particularly when spot welding between galvanized steel sheets or between galvanized and non-galvanized steel sheets. This phenomenon occurs when metals such as zinc, which have been liquefied by welding heat input, penetrate into the steel sheet along the grain boundaries, embrittling the steel and then the tensile stress generated by welding acts on these embrittled areas. If LME cracking occurs during spot welding, the strength of the welded joint cannot be ensured, which can hinder the use of galvanized steel sheets.

[0003] In relation to this, Patent Document 1 teaches that such LME cracking can be addressed by improving the spot welding method. More specifically, Patent Document 1 teaches that by continuing to hold the welding electrodes under pressure after the passage of current between the welding electrodes is terminated (extending the post-weld hold time Ht) and adjusting the post-weld hold time Ht as a function of the total plate thickness t of the welded members, the molten zinc-based coating is solidified before the electrodes are released, and as a result, the molten zinc-based coating does not penetrate into the grain boundaries of the steel sheet in areas where welding residual stress is high, making it possible to suppress cracking just outside the corona bond or at the nugget edge of the corona bond.

[0004] Patent Document 2 teaches that cracks can be prevented by controlling the plating structure near the spot weld after spot welding. More specifically, Patent Document 2 describes a spot-welded component having a spot weld formed by sandwiching a sheet assembly of multiple overlapping steel sheets between a pair of electrodes and spot welding, wherein at least one of the multiple steel sheets is a high-strength galvanized steel sheet having a tensile strength of 780 MPa or more, and the high-strength galvanized steel sheet has a plating layer containing an FeAl alloy layer with an average thickness of 0.3 μm or more at the interface between the base steel sheet of the high-strength galvanized steel sheet and the plating, and a zinc-based plating layer with an average thickness of 2.0 μm or more formed on the FeAl alloy layer. Furthermore, Patent Document 2 teaches that in order to suppress the penetration of Zn into the base steel sheet, it is important to set the Al content in the coating to 0.5 mass % or more, so that an FeAl alloy layer with a high melting point is formed at the interface between the steel sheet and the coating on the steel sheet by heat input during welding. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-047475 [Patent Document 2] International Publication No. 2020 / 130079 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when lapped galvanized steel sheets are spot-welded to form a joint, LME cracking may occur not only inside and immediately outside the pressure-welded joint (corona bond) on the overlapping surfaces of the steel sheets, but also on the surface on the electrode side. Patent Documents 1 and 2 discuss the prevention of LME cracking around the pressure-welded joint on the overlapping surfaces of the steel sheets, from the perspective of improving the spot welding method and controlling the plating structure near the spot weld after spot welding. However, they do not necessarily fully discuss the prevention of LME cracking on the surface of the steel sheet on the electrode side. Therefore, the inventions described in these patent documents still leave room for improvement in terms of improving LME resistance.

[0007] Therefore, an object of the present invention is to provide a welded joint that has a novel configuration and that can suppress or reduce the occurrence of LME cracks on the electrode side surface during spot welding. [Means for solving the problem]

[0008] The present inventors conducted research, focusing particularly on the structure of the coating layer of coated steel sheets used for welded joints, in order to suppress or reduce the occurrence of LME cracking on the electrode-side surface when manufacturing welded joints by spot welding. As a result, the present inventors discovered that by using coated steel sheets having a coating layer containing a relatively small amount of Al and in which the Al concentration distribution is appropriately controlled, it is possible to suppress or reduce the dissolution of Cu from the electrode and its incorporation into the coating layer at high temperatures during spot welding, and in this regard, it is possible to significantly improve the LME resistance on the electrode-side surface of the welded joint, and thus completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) A plurality of overlapping steel plates; a nugget that joins the plurality of steel plates, and a spot weld that has a pressure welding portion and a heat-affected zone formed around the nugget; A welded joint comprising: At least one of the plurality of steel plates is a plated steel plate comprising a base steel plate and a plating layer formed on at least a surface of the base steel plate corresponding to an outermost surface of the plurality of steel plates, The plating layer in the region outside the heat-affected zone comprises, in mass%, Al: 0.10 to 1.50%, and Fe: 0.01 to 2.00% and further comprising Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, When the coating layer in the region outside the heat-affected zone is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" / "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50, A welded joint, characterized in that the area ratio of the β-CuZn phase in the plating layer of the weld shoulder on the outermost surfaces of the plurality of steel plates is 50% or less. (2) The welded joint according to (1) above, characterized in that the area ratio of the β-CuZn phase in the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates is 30% or less. (3) The welded joint according to (1) above, characterized in that the area ratio of the β-CuZn phase in the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates is 10% or less. (4) The welded joint according to any one of the above (1) to (3), wherein the plating layer is a hot-dip galvanized (GI) layer. (5) The welded joint according to any one of (1) to (4) above, wherein the plated steel sheet has a tensile strength of 780 MPa or more. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a welded joint that can suppress or reduce the occurrence of LME cracking during spot welding. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are diagrams showing a cross section of a welded joint according to an embodiment of the present invention, in which FIG. 1A is an overall view of the welded joint, and FIG. 1B is an enlarged view of a weld shoulder on the electrode side surface. [Figure 2] 1A and 1B are diagrams showing the results of GDS analysis of plated steel sheets, where FIG. 1A shows the results of GDS analysis of an Al-containing plated steel sheet produced by a conventional method, and FIG. 1B shows the results of GDS analysis of a plated steel sheet useful for use in a welded joint according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Welded joints> A welded joint according to an embodiment of the present invention includes a plurality of overlapping steel plates, a nugget that joins the plurality of steel plates, and a spot weld that has a pressure welding portion and a heat-affected zone formed around the nugget; A welded joint comprising: At least one of the plurality of steel plates is a plated steel plate comprising a base steel plate and a plating layer formed on at least a surface of the base steel plate corresponding to an outermost surface of the plurality of steel plates, The plating layer in the region outside the heat-affected zone comprises, in mass%, Al: 0.10 to 1.50%, and Fe: 0.01 to 2.00% and further comprising Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, When the coating layer in the region outside the heat-affected zone is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" / "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50, The method is characterized in that the area ratio of the β-CuZn phase in the plating layer of the weld shoulders on the outermost surfaces of the plurality of steel plates is 50% or less.

[0013] As mentioned above, LME cracking must be suppressed during spot welding between galvanized steel sheets or between galvanized and non-galvanized steel sheets. For example, when spot welding two or more steel sheets, including at least one galvanized steel sheet, to form a joint, LME cracking may occur inside or immediately outside the corona bond formed on the outside of the weld metal (nugget), or on the electrode-side surface (e.g., the weld shoulder or its vicinity corresponding to the outer edge of the electrode indentation depressed by the electrode). LME cracking occurs when metals such as zinc that have been liquefied by welding heat input during spot welding penetrate into the steel sheet along grain boundaries, embrittlement occurs, and then tensile stresses generated by welding, such as the electrode pressure, expansion and contraction of the weld, and springback upon electrode release, act on the steel sheet. Therefore, in order to suppress or reduce the penetration of metals such as zinc into the steel sheet, the inventors focused on the microstructure of the coating layer of galvanized steel sheets and conducted research with the aim of optimizing the microstructure of the coating layer.

[0014] First, the inventors discovered that adding a relatively small amount of aluminum (Al), i.e., 0.10 to 1.50 mass%, to a zinc (Zn)-based coating layer is effective from the perspective of suppressing or reducing the penetration of Zn into the steel sheet. As the amount of Al added increases, the composition of the coating layer approaches the Zn-Al eutectic composition, thereby lowering the melting point of the coating layer. Therefore, excessive addition of Al is likely to have a detrimental effect from the perspective of suppressing or reducing the penetration of molten Zn into the steel sheet and improving LME resistance. In particular, when Al is added in an amount significantly exceeding 1.50 mass%, the negative effects of excessive Al addition become significant, and it is believed that the LME cracking suppression effect of Al addition cannot be fully exerted. In addition, the inventors discovered that by using a plated steel sheet in which the Al concentration distribution in the coating layer is controlled so that the ratio of the Al concentration at the center of the coating layer to the Al concentration near the interface between the base steel sheet and the coating layer, more specifically the Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet, is 0.10 to 1.50, the coating structure at the weld shoulder on the electrode side changes due to the thermal influence during spot welding, and the occurrence of LME cracking at or around the weld shoulder is significantly suppressed or reduced. This will be explained in more detail below with reference to the drawings.

[0015] Fig. 1 is a schematic diagram showing a cross section of a welded joint according to an embodiment of the present invention, where (a) is an overall view of the welded joint and (b) is an enlarged view of a weld shoulder on the electrode-side surface. Referring to Fig. 1(a), the welded joint 1 according to an embodiment of the present invention includes two overlapping steel sheets 11, a nugget 12 joining the steel sheets 11, and a spot weld 15 having a pressure weld 13 and a heat-affected zone 14 formed around the nugget 12. The outermost surface of the welded joint 1, i.e., the electrode-side surface, has a weld shoulder 16 formed corresponding to the outer edge of the electrode indentation, i.e., the boundary between the area depressed by the electrode and the flat portion of the steel sheet. In Fig. 1, the two steel sheets 11 are plated steel sheets, each of which has a Zn-based plating layer with an Al concentration distribution controlled within a predetermined range on both sides of the base steel sheet. In this regard, referring to FIG. 1(b), a coating layer 17 derived from the coating layer on the steel sheet 11 before spot welding is formed at the weld shoulder 16 by being pushed from the electrode (not shown) toward the weld shoulder 16. According to an embodiment of the present invention, the structure of the initial coating layer changes at the weld shoulder 16 and its periphery within the heat-affected zone 14 due to factors such as the heat effect of spot welding. More specifically, a coating layer 17 containing a β-CuZn phase with an area ratio of 50% or less is formed. In the present invention, the β-CuZn phase refers to a phase having a Zn concentration of 40 to 60 atomic %, a Cu concentration of 40 to 60 atomic %, an Fe concentration of 0 to 20 atomic %, and 3 atomic % or less of other elements, as measured using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA). Unlike the pressure-welded portion 13 on the opposite overlap surface, the electrode-side surface of the welded joint 1 comes into contact with the electrode. Therefore, Cu may dissolve from the electrode and be mixed into the coating layer at high temperatures during spot welding. Although structural changes in the coating layer due to the heat effect during spot welding and the like occur in the coating layer present on both the electrode side and overlap side surfaces of the steel sheet 11, the structural changes in the coating layer on the electrode side, particularly in the weld shoulder 16, differ from those in the coating layer on the overlap side due to the inclusion of Cu. Such structural changes in the coating layer are significantly influenced by the chemical composition and structure of the initial coating layer before spot welding.In particular, in the case of Zn-based coatings, on the electrode side surface, the heat input during welding can cause alloying between the Zn in the coating layer and the Cu in the electrode in contact with it. This alloying lowers the melting point of Cu, accelerating the incorporation of Cu from the electrode into the coating layer. The present inventors have confirmed that the incorporation of Cu leads to the formation of a relatively large amount of β-CuZn phase in the coating layer, and that this incorporation of Cu accelerates LME cracking. To address this issue, the present inventors conducted extensive research and found that by using a coated steel sheet with a coating layer containing a relatively small amount of Al and with an appropriately controlled Al concentration distribution, the alloying reaction between the Zn in the coating layer and the Cu in the electrode during spot welding can be suppressed, thereby reliably reducing the area fraction of the β-CuZn phase in the coating layer 17 of the weld shoulder 16 to 50% or less. Therefore, according to the embodiment of the present invention, it is possible to significantly suppress or reduce the occurrence of LME cracking in or around the weld shoulder during spot welding, compared to the case of conventional spot welding of Zn-based plated steel sheets.

[0016] For ease of understanding, FIG. 1 illustrates a welded joint in which only two steel sheets 11 are overlapped and both steel sheets 11 have a coating layer formed thereon. However, the welded joint according to the embodiment of the present invention is not necessarily limited to this type of welded joint and may include various welded joints in which a coating layer 17 is formed in a weld shoulder 16 in a heat-affected zone 14, with the area ratio of the β-CuZn phase controlled to 50% or less. For example, in a welded joint in which two steel sheets 11 are spot-welded as shown in FIG. 1 , only one of the steel sheets 11 may be a coated steel sheet. In this case, it is sufficient that a coating layer 17 is present in the weld shoulder 16 of at least the electrode-side surface (the outermost surface of the welded joint 1) of the two surfaces of the coated steel sheet; of course, a coating layer may also be present on the opposite overlapping surface. A coating layer containing a relatively small amount of Al and having an appropriately controlled Al concentration distribution (i.e., a coating layer containing 0.10 to 1.50 mass% Al and having an Al concentration distribution controlled to a ratio of 0.10 to 1.50 relative to the Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet, as described in detail later with reference to FIG. 2 ) is useful for suppressing or reducing the penetration of molten Zn into the steel sheet regardless of contact with the electrode. For example, when such a coating layer is present on the overlapping surfaces of steel sheets, the Al in the coating layer can significantly suppress or reduce the penetration of molten Zn into the steel sheet during spot welding, even in the pressure welded portion 13 and the area immediately surrounding it in the heat-affected zone. This enables further improvement in LME resistance compared to a coating layer 17 simply present on the electrode-side surface of the steel sheet. Similarly, a welded joint made of three or more steel sheets can include various welded joints in which a coating layer 17 is formed in which the area ratio of the β-CuZn phase is controlled to 50% or less in the weld shoulder 16 in the heat-affected zone 14. For example, in a welded joint formed by spot welding three steel plates 11, it is sufficient that a plating layer 17 is present on the weld shoulder 16 of at least the electrode side surface (the outermost surface of the welded joint 1) of one or more of the three steel plates 11.For example, if only one of the three overlapping steel sheets 11 is a plated steel sheet, the plated layer 17 may be present only on the weld shoulder 16 of the electrode-side surface of the plated steel sheet, or may also be present on the overlapping surface on the opposite side. For example, the plated layer 17 may be present only on the weld shoulder 16 of one of the outer steel sheets 11, while another Zn-based plated layer may be present on the weld shoulder 16 of the other outer steel sheet 11. This embodiment is also encompassed by the present invention. In this case, compared to a welded joint in which the plated layer 17 is present on the weld shoulders 16 of both outer steel sheets 11, the risk of LME cracking is increased on the surface where the other Zn-based plated layer is present, which may result in a slight decrease in joint strength. However, the specific number and arrangement of steel sheets 11 on which the plated layer 17, with the area ratio of the β-CuZn phase controlled to 50% or less, is formed in the welded joint may be determined appropriately taking into account the desired joint strength, etc.

[0017] Next, plated steel sheets useful for use in welded joints according to embodiments of the present invention will be described in more detail. Figure 2 shows the results of GDS analysis of plated steel sheets, with Figure 2(a) showing the results of GDS analysis of an Al-containing plated steel sheet manufactured by a conventional method, and Figure 2(b) showing the results of GDS analysis of a plated steel sheet useful for use in welded joints according to embodiments of the present invention. First, referring to Figure 2(a), in an Al-containing plated steel sheet in which a Zn-based coating layer containing 0.20% Al is formed by a conventional method, the Al concentration gradually increases with increasing depth from a depth of 0 μm, which corresponds to the coating surface. The Al concentration peaks near the interface between the base steel sheet and the coating layer, i.e., near the coating layer position where the Fe concentration is 50% of that of the base steel sheet. This Al concentration peak suggests the formation of an Fe-Al barrier layer containing an alloy of Fe and Al at the interface between the base steel sheet and the coating layer. As is clear from the relatively high peak of Al concentration, in aluminum-containing coated steel sheets manufactured by conventional methods, a larger amount of Al in the coating layer is consumed in the formation of the Fe-Al barrier layer, resulting in the formation of a relatively thick Fe-Al barrier layer. Therefore, Figure 2(a) shows that the Al concentration decreases significantly from near the interface between the base steel sheet and the coating layer toward the coating surface, and then becomes almost constant at a very low value. At the center of the coating layer, which corresponds to the midpoint between the coating surface and the position where the Fe concentration is 50% of that of the base steel sheet, the Al concentration is low at around 0.1%.

[0018] In contrast, referring to Figure 2(b), the Al concentration near the interface between the base steel sheet and the coating layer in the coated steel sheet in Figure 2(b) is significantly lower than that in Figure 2(a), even though the coating layer has a similar Al content to that in Figure 2(a). Therefore, a thinner Fe-Al barrier layer is formed in the coated steel sheet in Figure 2(b) than in Figure 2(a). In relation to this, Figure 2(b) shows that the Al concentration in the coating layer does not decrease significantly from near the interface between the base steel sheet and the coating layer toward the coating surface, but decreases relatively gradually and then becomes almost constant. At the center of the coating layer, the Al concentration exceeds approximately 0.2%, a very high value that is more than twice as high as that in Figure 2(a). The GDS analysis results in Figures 2(a) and (b) suggest that in the coated steel sheet in Figure 2(b), much of the Al in the coating layer is not consumed in the formation of the Fe-Al barrier layer, but rather exists as an Al phase in the coating layer outside the Fe-Al barrier layer, e.g., in a solid solution state. Based on the GDS analysis results of these coated steel sheets and the experimental evidence that weld joints obtained by spot welding the coated steel sheets shown in Figure 2(b) exhibit higher LME resistance than weld joints obtained by spot welding the coated steel sheets shown in Figure 2(a), particularly at the pressure welds of the lap surfaces and at the weld shoulder on the electrode-side surface, the inventors believed that the Al phase present in the coated layer other than the Fe-Al barrier layer plays a very important role in suppressing or reducing LME cracking during spot welding. Further investigation was conducted. As a result, the inventors discovered that the effect of adding Al to the coated layer can be fully achieved by controlling the total Al content to a relatively low level of 1.50 mass% or less to suppress deterioration of LME resistance due to a decrease in the melting point of the coated layer, while controlling the ratio of the "Al concentration at the center of the coated layer" to the "Al concentration at the position in the coated layer where the Fe concentration is 50% of that of the base steel sheet" within the range of 0.10 to 1.50, as measured by GDS.Furthermore, the inventors have found that by using such a plated steel sheet, the structure of the initial plating layer changes at the weld shoulder within the heat-affected zone on the electrode-side surface during spot welding, forming a plating layer with an area ratio of β-CuZn phase of 50% or less, and in relation to this, it is possible to significantly suppress or reduce the occurrence of LME cracking at the weld shoulder and its surrounding area during spot welding.

[0019] Without intending to be bound by any particular theory, it is believed that in welded joints according to embodiments of the present invention, Al in the initial coating layer acts as follows to suppress or reduce the occurrence of LME cracking during spot welding, particularly in and around the weld shoulder. More specifically, the Fe-Al barrier layer formed at the interface between the base steel sheet and the coating layer in coated steel sheets is relatively brittle and therefore likely fractures relatively easily due to stress applied to the steel sheet due to the electrode pressure or other factors during spot welding. The fracture of the Fe-Al barrier layer brings molten Zn into direct contact with the base steel sheet during spot welding, increasing the risk of the molten Zn penetrating into the steel sheet along grain boundaries. However, when a coated steel sheet is used that has a coating layer with an Al concentration distribution as shown in FIG. 2 , Al, which is present in relatively large amounts in portions of the coating layer other than the Fe-Al barrier layer, comes into direct contact with the base steel sheet together with Zn due to the fracture of the Fe-Al barrier layer. In this case, the heat input during spot welding likely causes Al in the coating layer to react with Fe in the base steel sheet, forming a new Fe-Al barrier layer, thereby repairing the fractured Fe-Al barrier layer. In other words, the presence of a large amount of Al in the coating layer other than the Fe-Al barrier layer allows a new Fe-Al barrier layer to be immediately formed at the fractured area, even if Zn comes into direct contact with the base steel sheet due to fracture of the Fe-Al barrier layer during spot welding. This significantly reduces or prevents the penetration of molten Zn into the steel sheet during spot welding, thereby potentially reducing or preventing LME cracking. Additionally, the relatively large amount of Al present in the coating layer likely reacts with Cu in the electrode at the high temperatures during spot welding, forming high-melting-point Cu-Al-based metal compounds on the electrode surface.The high-melting-point Cu-Al-based metal compound acts as a barrier to suppress or reduce alloying between Cu in the electrode and Zn in the coating layer, as well as the associated incorporation of Cu into the coating layer. As a result, the formation of the β-CuZn phase in the coating layer at the weld shoulder can be controlled within a predetermined range, and it is believed that this reliably suppresses or reduces the occurrence of LME cracking due to Cu incorporation. Coated steel sheets with Al added to a Zn-based coating layer have been known. However, the fact that the penetration of molten Zn into the steel sheet during spot welding can be suppressed or reduced by keeping the Al content of the entire coating layer relatively low in consideration of a lower melting point of the coating layer, while increasing the Al content in the coating layer other than the Fe-Al barrier layer, can suppress or reduce the occurrence of LME cracking due to Cu incorporation from the electrode, particularly in the weld shoulder and its surroundings on the electrode-side surface, was not previously known and has now been discovered for the first time by the present inventors.

[0020] Hereinafter, each component of the welded joint according to an embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0021] [Plated steel sheet] In a welded joint according to an embodiment of the present invention, at least one of the overlapping steel sheets is a plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface of the base steel sheet that corresponds to the outermost surface of the steel sheets. The plating layer of this plated steel sheet has the following chemical composition in a region outside the heat-affected zone, which is the same as the initial chemical composition before spot welding.

[0022] [Chemical composition of plating layer] [Al: 0.10-1.50%] Al is an element that effectively prevents molten Zn from penetrating into the steel sheet along grain boundaries and / or reacts with Cu in the electrode on the electrode surface to prevent Cu from being mixed into the coating layer. To fully achieve these effects, the Al content is set to 0.10% or more. The Al content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, 0.25% or more, 0.30% or more, more than 0.30%, 0.31% or more, 0.32% or more, 0.35% or more, 0.40% or more, more than 0.60%, 0.62% or more, 0.65% or more, or 0.70% or more. On the other hand, excessive Al content causes the composition of the coating layer to approach the Zn-Al eutectic composition, lowering the melting point of the coating layer. This may result in the Zn in the coating layer melting more easily during spot welding, which may promote LME cracking. Therefore, the Al content is set to 1.50% or less. The Al content may be 1.45% or less, 1.40% or less, 1.30% or less, 1.20% or less, 1.10% or less, 1.00% or less, 0.90% or less, or 0.80% or less.

[0023] [Fe: 0.01 to 2.00%] Fe is an element inevitably contained in the coating layer, for example, by dissolving from the base steel sheet into the coating bath or by reacting with Al during the coating process to form an Fe-Al barrier layer at the interface between the base steel sheet and the coating layer. Therefore, in an embodiment of the present invention, the Fe content in the coating layer is 0.01% or more. The Fe content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.40% or more, or 0.50% or more. On the other hand, if the Fe content in the coating layer is too high, Al in the coating layer may combine with Fe, or a large amount of Al may be consumed to form the Fe-Al barrier layer. As a result, the effect of adding Al, i.e., the effect of Al in the coating layer to suppress penetration of molten Zn into the steel sheet during spot welding and / or to suppress Cu incorporation into the coating layer by reacting with Cu in the electrode, thereby suppressing or reducing the occurrence of LME cracking, may not be fully exhibited. Therefore, the Fe content is set to 2.00% or less. The Fe content may be 1.80% or less, 1.60% or less, 1.50% or less, 1.30% or less, 1.20% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[0024] The basic chemical composition of the plating layer is as described above. The plating layer may further optionally contain: Mg: 0-1.500%, Si: 0-1.000%, Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1.000%, Nb: 0-1.000% The coating layer may contain at least one of the following: Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, P: 0-0.500%, and Sr: 0-0.500%. The total content of these optional elements is limited to 5.000% or less in order to fully exhibit the effects and functions of the basic components constituting the coating layer, particularly Al. The total content of the optional elements may be 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less. These optional elements are described in detail below.

[0025] [Mg: 0-1.500%] Mg is an element effective in improving the corrosion resistance of the coating layer. The Mg content may be 0%, but to achieve this effect, the Mg content is preferably 0.001% or more. The Mg content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Mg content may result in the formation of large amounts of MgZn-based compounds, which are brittle compounds, in the coating layer, which may cause a decrease in workability. Therefore, the Mg content is preferably 1.500% or less. The Mg content may be 1.200% or less, 1.000% or less, 0.800% or less, 0.500% or less, 0.240% or less, 0.220% or less, or 0.200% or less.

[0026] [Si: 0-1.000%] Si is an element effective in improving the corrosion resistance of the plating layer. The Si content may be 0%, but if necessary, Si may be contained in the plating layer in an amount of 0.0001% or more, or 0.001% or more. On the other hand, excessive Si content may reduce the plating adhesion of the plating layer. Therefore, the Si content is preferably 1.000% or less. The Si content may be 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0027] [Ni: 0-1.000%] Ni is an element effective in improving the corrosion resistance of the coating layer. The Ni content may be 0%, but to achieve this effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive Ni content may result in the formation of many intermetallic compounds, which may reduce corrosion resistance. Therefore, the Ni content is preferably 1.000% or less. The Ni content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0028] [Ca: 0-4.000%] Ca is an element effective in ensuring wettability of the coating bath. The Ca content may be 0%, but to obtain this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.010% or more, 0.100% or more, or 1.000% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the coating layer, making the coating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 4.000% or less. The Ca content may be 3.000% or less, 2.000% or less, or 1.500% or less.

[0029] [Sb:0~0.500%, Pb:0~0.500%, Cu:0~1.000%, Sn:0~1.000%, Ti:0~1.000%, Cr:0~1.000%, Nb:0~1.000%, Zr:0~1.000%, Mn:0~1.000 %, Mo:0~1.000%, Ag:0~1.000%, Li:0~1.000%, La:0~0.500%, Ce:0~0.500%, B:0~0.500%, Y:0~0.500%, P:0~0.500% and Sr:0~0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr may not be present in the coating layer, but may be present in the coating layer in amounts of 0.0001% or more, or 0.001% or more. These elements do not adversely affect the performance of the coated steel sheet as long as they are within the specified content range. However, excessive content of each element may reduce corrosion resistance. Therefore, the contents of Sb, Pb, La, Ce, B, Y, P, and Sr are preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, or 0.050% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, or 0.100% or less.

[0030] The remainder of the plating layer other than the above elements is composed of Zn and impurities. The impurities in the plating layer are components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is manufactured.

[0031] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, and measuring the resulting solution using ICP (inductively coupled plasma) atomic emission spectroscopy.

[0032] The coating layer may be any coating layer having the above-mentioned chemical composition, and is not particularly limited, but is preferably, for example, a hot-dip galvanized (GI) layer. For example, when alloying heat treatment is performed, the Fe content in the coating layer increases, and it may not be possible to obtain the desired chemical composition and ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" in the final coating layer. The thickness of the coating layer may be, for example, 3 to 50 μm. The coating weight of the coating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side. 2 The coating weight of the plating layer may be 45 g / m per side. 2 or more than 50g / m 2 Similarly, the coating weight of the plating layer may be 75 g / m per side. 2 or less than 70g / m 2 The coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, and then determining the change in weight before and after pickling.

[0033] [Ratio of "Al concentration at the center of the coating layer" / "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet": 0.10 to 1.50] In an embodiment of the present invention, when the coating layer outside the heat-affected zone is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50. By controlling the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" measured by GDS within this range, the effect of adding Al to the coating layer can be fully exerted, and it is possible to reliably control the area ratio of the β-CuZn phase in the coating layer at the weld shoulder to a range of 50% or less during spot welding. From the viewpoint of reducing the area ratio of the β-CuZn phase in the coating layer at the weld shoulder, the higher the ratio of the "Al concentration at the coating layer center" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet," the better. It is preferably 0.15 or more, more preferably 0.20 or more, and most preferably 0.30 or more, for example, 0.40 or more, 0.42 or more, 0.45 or more, 0.50 or more, 0.55 or more, or 0.60 or more. On the other hand, if the ratio of the "Al concentration at the coating layer center" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" is too high, the melting point of the coating layer may decrease due to the associated increase in the Al content of the entire coating layer. This may result in the Zn in the coating layer being more likely to melt during spot welding, which may promote LME cracking. Therefore, in an embodiment of the present invention, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 1.50 or less, and may be, for example, 1.40 or less, 1.30 or less, 1.20 or less, 1.10 or less, or 1.00 or less.

[0034] [Method for measuring the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet"] The ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is determined as follows: First, a coated steel sheet sample is obtained by cutting a 50 mm × 50 mm sample from the outer region of the heat-affected zone of the welded joint. The coated steel sheet sample is then subjected to glow discharge optical emission spectroscopy (GDS) to obtain the Al concentration distribution from the surface of the coating layer to a depth of 100 μm. Next, the Al concentration at the depth where the Fe intensity is 50% of that of the base steel sheet (Fe intensity at a depth of 100 μm from the surface of the coating layer of the sample) in the GDS measurement is determined as the "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet," and the distance from this depth to the surface is defined as the thickness of the coating layer. The Al concentration measured by GDS at a position halfway through the thickness of the coating layer is determined as the "Al concentration at the center of the coating layer," and the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" is finally determined.

[0035] [Area ratio of β-CuZn phase in the plating layer of the weld shoulder: 50% or less] In an embodiment of the present invention, the area ratio of the β-CuZn phase in the coating layer of the weld shoulder on the outermost surface of a plurality of steel sheets is 50% or less. Because the weld shoulder contacts the electrode, Cu dissolved from the electrode may be mixed into the coating layer at the weld shoulder under high temperatures during spot welding. In such cases, the proportion of the β-CuZn phase in the coating layer increases, and the Cu mixed into the coating layer promotes LME cracking. However, according to an embodiment of the present invention, Al, which is present in a relatively large amount in the coating layer, reacts with Cu in the electrode under high temperatures during spot welding to form high-melting-point Cu-Al-based metal compounds on the electrode surface, thereby suppressing or reducing Cu mixing into the coating layer. In this regard, since the area ratio of the β-CuZn phase in the coating layer of the weld shoulder is controlled to a range of 50% or less, it is possible to reliably suppress or reduce LME cracking due to Cu mixing. Therefore, from the perspective of suppressing LME cracking due to Cu mixing, the lower the area ratio of the β-CuZn phase in the coating layer of the weld shoulder, the better. For example, the area ratio of the β-CuZn phase in the plating layer of the weld shoulder is preferably 40% or less, more preferably 30% or less or 20% or less, and most preferably 10% or less. There is no particular lower limit, and the area ratio of the β-CuZn phase in the plating layer of the weld shoulder may be 0%, or may be, for example, 1% or more or 3% or more.

[0036] [Method for measuring the area ratio of β-CuZn phase in the plating layer of weld shoulder] The area fraction of the β-CuZn phase in the coating layer of the weld shoulder on the outermost surface of multiple steel sheets is determined as follows. First, a cross-sectional sample of the spot weld is prepared. Next, a backscattered electron image (BSE image) including the weld shoulder is obtained using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA), and elemental analysis is performed to measure the area fraction of the β-CuZn phase in the weld shoulder. Specifically, the β-CuZn phase has a Zn concentration of 40 to 60 atomic %, a Cu concentration of 40 to 60 atomic %, an Fe concentration of 0 to 20 atomic %, and other elements of 3 atomic % or less. The field of view of the SEM image is 100 μm × 100 μm, and similar elemental analysis is performed on five different locations on the weld shoulder. The average of the area fractions of the β-CuZn phase obtained in each field of view is determined as the area fraction of the β-CuZn phase in the coating layer of the weld shoulder.

[0037] [Base material steel plate] In an embodiment of the present invention, the base steel sheet for forming the above-mentioned coating layer is not particularly limited and may be any appropriate material, particularly a cold-rolled steel sheet. For example, the base steel sheet may be a material having a chemical composition that provides a tensile strength of 780 MPa or more for the coated steel sheet. It is generally known that LME cracking occurs more frequently when spot welding steel sheets with relatively high strength, and that the higher the strength of the steel sheet, the greater the susceptibility to LME cracking. According to an embodiment of the present invention, even when using a coated steel sheet with a high tensile strength of 780 MPa or more, at which LME cracking is generally more likely to occur in welded joints, it is possible to reliably suppress or reduce the occurrence of LME cracking. On the other hand, LME cracking due to Cu inclusion on the electrode-side surface can occur even when using a coated steel sheet with a tensile strength lower than 780 MPa or sufficiently lower. However, according to an embodiment of the present invention, even when a plated steel sheet having such a relatively low tensile strength is used, Al present in a relatively large amount in the plated layer reacts with Cu in the electrode at high temperatures during spot welding to form a high-melting-point Cu-Al-based metal compound on the electrode surface, thereby suppressing or reducing the incorporation of Cu into the plated layer. Therefore, it is possible to reliably suppress or reduce the occurrence of LME cracking due to the incorporation of Cu, regardless of the tensile strength of the plated steel sheet.

[0038] [Preferred chemical composition of base steel plate] As described above, the present invention aims to provide a welded joint capable of suppressing or reducing the occurrence of LME cracking on the electrode-side surface during spot welding. This objective is achieved by using a plated steel sheet having a plating layer on at least the outermost surface of a plurality of overlapping steel sheets, controlling the plating layer to have a predetermined chemical composition in a region outside the heat-affected zone, and controlling the area ratio of the β-CuZn phase in the plating layer at the weld shoulder to 50% or less. Therefore, it is clear that the chemical composition of the base steel sheet having the above-described plating layer is not an essential technical feature for achieving the objective of the present invention. Preferred chemical compositions of the base steel sheet for plated steel sheets useful for welded joints according to embodiments of the present invention will be described in detail below. However, these descriptions are intended merely as examples of preferred chemical compositions of the base steel sheet for plated steel sheets having a tensile strength of 780 MPa or more, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0039] In an embodiment of the present invention, for example, the base steel plate contains, in mass%, C: 0.01 to 0.50%, Si: 0.01 to 3.50% Mn: 0.10~5.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.0100% or less, O: 0 to 0.020%, Al: 0 to 1.000%, B: 0~0.010%, Nb: 0 to 0.150%, Ti: 0 to 0.20% Mo: 0-3.00%, Cr: 0~2.00%, V: 0~1.00%, Ni: 0-2.00% W: 0~1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0 to 1.00% Sb: 0 to 0.50% Cu: 0-2.00% As: 0~0.050%, Mg: 0 to 0.100%, Ca: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, REM: 0~0.10, and Remainder: Fe and impurities It is preferable that the metal has a chemical composition consisting of the following: Each element will be described in more detail below.

[0040] [C: 0.01 to 0.50%] C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.01% or more. The C content may be 0.05% or more, 0.08% or more, 0.09% or more, 0.10% or more, 0.11% or more, 0.12% or more, or 0.15% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.40% or less, 0.35% or less, or 0.30% or less.

[0041] [Si: 0.01 to 3.50%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during annealing of cold-rolled sheets. To fully obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.08% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.30% or more, or 0.80% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.50% or less. The Si content may be 2.50% or less, 2.00% or less, or 1.50% or less.

[0042] [Mn: 0.10~5.00%] Mn is an element that affects the ferrite transformation of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 0.60% or more, 0.80% or more, 1.00% or more, or 1.50% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 5.00% or less. The Mn content may be 4.00% or less, 3.00% or less, or 2.50% or less.

[0043] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0044] [S:0.0300% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. The lower the S content, the better, ideally 0%. However, excessive reduction in the S content can significantly increase costs. For this reason, the S content may be 0.0001% or more, or may be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.0300% or less. The S content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0045] [N:0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, or may be 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may also be 0.0080% or less, or 0.0050% or less.

[0046] The base steel sheet preferably has the basic chemical composition as described above. Furthermore, the base steel sheet may contain, as necessary, O: 0-0.020%, Al: 0-1.000%, B: 0-0.010%, Nb: 0-0.150%, Ti: 0-0.20%, Mo: 0-3.00%, Cr: 0-2.00%, V: 0-1.00%, Ni: 0-2.00%, W: 0-1.00%, Ta: 0-0.10%, and / or Fe: 0-0.150% instead of the remaining Fe. %, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.50%, Cu: 0-2.00%, As: 0-0.050%, Mg: 0-0.100%, Ca: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and REM: 0-0.100%. The content of each element may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0047] The remainder of the base steel plate other than the above elements is composed of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0048] The chemical composition of the base steel sheet may be measured by a common analytical method. For example, the chemical composition of the base steel sheet may be measured by first removing the coating layer by mechanical grinding and then using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0049] [Base steel plate thickness] The thickness of the base steel plate is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 6.0 mm or less, 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0050] [Mechanical properties of plated steel sheets] Plated steel sheets useful in welded joints according to embodiments of the present invention may have any appropriate tensile strength, and are not particularly limited, but may have a tensile strength of 780 MPa or more. For example, in embodiments of the present invention, the tensile strength of the plated steel sheet may be 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The upper limit is not particularly limited, and the tensile strength of the plated steel sheet may be 2300 MPa or less, 2000 MPa or less, 1800 MPa or less, or 1500 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece so that the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the plated steel sheet, and conducting a tensile test in accordance with JIS Z 2241:2011.

[0051] <Method of manufacturing plated steel sheets> Next, a preferred method for producing a plated steel sheet useful for use in a welded joint according to an embodiment of the present invention, more specifically, a plated steel sheet having a coating layer in which the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that in the base steel sheet" is 0.10 to 1.50 as measured by glow discharge optical emission spectroscopy (GDS), will be described. The following description is intended to exemplify a characteristic method for producing the plated steel sheet, but is not intended to limit the plated steel sheet to one produced by the production method described below.

[0052] The plated steel sheet can be produced by, for example, carrying out a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, a pretreatment process, an annealing process in which the pretreated cold-rolled steel sheet is annealed, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the base steel sheet may be pickled after the hot rolling process without being coiled, and then directly subjected to the cold rolling process. Each process will be described in detail below.

[0053] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0054] [Hot rolling process] A hot-rolled steel sheet can be obtained by hot-rolling the cast steel slab. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0055] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then subjected to the cold rolling step described below without performing the coiling step.

[0056] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be air-cooled to room temperature, for example.

[0057] [Pretreatment process] Next, it is effective to carry out a predetermined pretreatment process before annealing the cold-rolled steel sheet. Such pretreatment processes can include a degreasing process and an optional grinding process. The degreasing process may include, for example, passing an electric current through the cold-rolled steel sheet in a solution having a pH of 8.0 or higher (electrolytic treatment). The current density during the current passing is 1.0 to 8.0 A / dm 2 The current application time may be 5 to 10 seconds. Meanwhile, the optional grinding process is preferably performed using a heavy-duty abrasive brush. By introducing strain into the surface of the cold-rolled steel sheet through grinding using a heavy-duty abrasive brush, the nucleation of the Fe-Al barrier layer during the plating process after the annealing process is promoted, densifying the Fe-Al barrier layer. This slows the growth rate of the Fe-Al barrier layer, allowing for a thinner thickness. As a result, the amount of Al consumed in the formation of the Fe-Al barrier layer can be reduced. Therefore, the amount of Al in the plating layer other than the Fe-Al barrier layer can be increased, thereby increasing the ratio of the "Al concentration at the center of the plating layer" to the "Al concentration at the position of the plating layer where the Fe concentration is 50% of that of the base steel sheet" in the final plated steel sheet. The grinding process is not particularly limited, but for example, a grinding amount of 10 to 200 g / m can be used with a heavy-duty abrasive brush. 2The grinding amount by the heavy-duty abrasive brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited, but can be adjusted, for example, by appropriately selecting the number of heavy-duty abrasive brushes, the rotation speed, the brush pressure, the coating liquid to be used, etc.

[0058] [Annealing process] The cold-rolled steel sheet that has undergone the pretreatment step is then annealed. The holding temperature in the annealing step is preferably 700 to 900°C. If the holding temperature in the annealing step exceeds 900°C, an outer oxide layer may form on the surface of the steel sheet, resulting in a risk of reduced platability. The rate of temperature rise to the holding temperature is not particularly limited, but may be 1 to 10°C / second. The holding time at the holding temperature is preferably 10 to 300 seconds, more preferably 80 to 120 seconds. If the holding time exceeds 300 seconds, the outer oxide may grow excessively, resulting in a risk of reduced platability. The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C. If the dew point is too low, an outer oxide layer may form on the surface of the steel sheet, resulting in a risk of reduced platability. On the other hand, if the dew point is too high, Fe oxide may similarly form as an outer oxide on the surface of the steel sheet, resulting in a risk of reduced platability. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (for example, 4% hydrogen and the balance nitrogen).

[0059] [Plating process] Next, in the plating process, a coating layer having the above-described chemical composition and structure is formed on at least one, preferably both, surfaces of a cold-rolled steel sheet (base steel sheet). More specifically, the plating process is performed, for example, by hot-dip plating using a coating bath whose components are adjusted so that the chemical composition of the coating layer falls within the above-described range. In the plating process, it is extremely important to first control the time from immersion of the steel sheet in the coating bath to the start of cooling to 6 seconds or less, and then control the average cooling rate from the bath temperature (e.g., 420 to 480°C) to 370°C to 20°C / second or more. By satisfying these requirements, the Fe-Al barrier layer can be thinned, reducing the amount of Al consumed in forming the Fe-Al barrier layer and ensuring sufficient Al content in the coating layer other than the Fe-Al barrier layer. As a result, the ratio of the "Al concentration at the center of the coating layer" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" can be 0.10 or more in the finally obtained coated steel sheet. On the other hand, if any one of these requirements is not satisfied, i.e., if the time from the start of immersion of the steel sheet in the coating bath to the start of cooling exceeds 6 seconds and / or the average cooling rate from the bath temperature to 370°C is less than 20°C / s, a large amount of Al is consumed in forming the Fe-Al barrier layer, resulting in a decrease in the Al content in the coating layer other than the Fe-Al barrier layer. As a result, the desired ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" cannot be obtained in the finally obtained coated steel sheet.

[0060] From the viewpoint of further improving the LME resistance of welded joints, particularly at the weld shoulder, it is preferable to shorten the time from the start of immersion of the steel sheet in the coating bath to the start of cooling and to increase the average cooling rate from the bath temperature to 370°C. For example, by setting the time from the start of immersion of the steel sheet in the coating bath to the start of cooling to 4 seconds or less and the average cooling rate from the bath temperature to 370°C to 40°C / second or more, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" can be increased to a high value of, for example, 0.20 or more, and in connection with this, the area fraction of the β-CuZn phase in the coating layer at the weld shoulder can be reduced to, for example, 30% or less, thereby further improving the LME resistance of the weld shoulder of a welded joint. When grinding with a heavy-duty abrasive brush is performed as the pretreatment step described above, even under conditions where the time from the start of immersion of the steel sheet in the coating bath to the start of cooling is 6 seconds or less and the average cooling rate from the bath temperature to 370°C is 20°C / s or more, higher LME resistance can be achieved by reducing the area fraction of the β-CuZn phase in the coating layer at the weld shoulder. Alternatively, by performing grinding with a heavy-duty abrasive brush as the pretreatment step described above and further setting the time from the start of immersion of the steel sheet in the coating bath to the start of cooling to 4 seconds or less and the average cooling rate from the bath temperature to 370°C to 40°C / s or more, the ratio of the "Al concentration at the coating layer center" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" can be set to a high value of, for example, 0.30 or more, and in connection with this, the area fraction of the β-CuZn phase in the coating layer at the weld shoulder can be reduced to, for example, 10% or less, thereby more significantly improving LME resistance at the weld shoulder of a welded joint. The lower limit of the time from the start of immersion of the steel sheet in the coating bath to the start of cooling is not particularly limited, but for example, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling may be 2 seconds or more. Similarly, the upper limit of the average cooling rate from the bath temperature to 370°C is not particularly limited, but for example, the average cooling rate from the bath temperature to 370°C may be 80°C / second or less. Other conditions of the coating process may be set appropriately taking into account the thickness and coating weight of the coating layer, etc.For example, a cold-rolled steel sheet is immersed in a coating bath, then pulled out, and immediately sprayed with N2 gas or air using a gas wiping method, followed by cooling, thereby achieving a coating weight within a predetermined range, for example, 10 to 170 g / m per side. 2 can be adjusted within the range.

[0061] In plated steel sheets manufactured using this manufacturing method, the ratio of "Al concentration at the center of the plated layer" to "Al concentration at the plated layer position where the Fe concentration is 50% of that of the base steel sheet" is controlled within the range of 0.10 to 1.50. Therefore, when used in spot welding, the structure of the initial plated layer changes in the weld shoulder on the electrode side, forming a plated layer with an area ratio of β-CuZn phase of 50% or less. As a result, it is possible to significantly suppress or reduce the occurrence of LME cracking in and around the weld shoulder during spot welding.

[0062] [Other steel plates] Of the multiple steel sheets used in the welded joint according to the embodiment of the present invention, any appropriate steel sheet or plated steel sheet can be used as the steel sheet other than the plated steel sheet described above. Such a steel sheet may have, for example, a tensile strength of 780 MPa or more, as in the preferred embodiment of the plated steel sheet, or may have a tensile strength of less than 780 MPa. Therefore, for steel sheets other than the plated steel sheets described above, an appropriate steel sheet or plated steel sheet may be selected as appropriate depending on the application and desired properties of the welded joint, such as the desired joint strength.

[0063] <Method for manufacturing welded joints> Welded joints according to embodiments of the present invention can be produced by applying any appropriate spot welding method known to those skilled in the art to a plurality of overlapping steel sheets, or to a plurality of overlapping steel sheets, in which all steel sheets are plated steel sheets, or to a plurality of overlapping steel sheets, in which one or more steel sheets are plated steel sheets, and the plated steel sheet is overlapped with another steel sheet or with another plated steel sheet. For example, a welded joint according to embodiments of the present invention can be produced by applying pressure to the overlapping steel sheets as described above using a pair of opposing electrodes while passing current between the electrodes under normal conditions to form a nugget and a pressure weld around the nugget. Spot welding conditions may be any appropriate conditions known to those skilled in the art. For example, the welding electrode may be a dome radius electrode with a tip diameter of 6 to 8 mm, and the welding pressure may be 1.5 to 6.0 kN, the welding time may be 0.1 to 1.0 s (5 to 50 cycles, power supply frequency 50 Hz), the current may be 4 to 15 kA, and the impact angle (the angle between the axial direction of the electrode and the direction perpendicular to the surface of the steel sheet) may be 0 to 10°.

[0064] The welded joint manufactured as described above fully utilizes the effects of adding Al to the coating layer, allowing the area ratio of the β-CuZn phase in the coating layer at the weld shoulder to be controlled within a desired range, thereby making it possible to suppress or reduce the occurrence of LME cracking on the electrode-side surface during spot welding. Therefore, such a welded joint can achieve superior LME resistance compared to a conventional coated steel sheet having a coating layer with a similar chemical composition, more specifically, a Zn-based coating layer with a similar Al content, and can contribute to industrial development by improving crash safety and extending the service life, particularly in the automotive field.

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0066] In the following examples, plated steel sheets were produced under various conditions, and the LME resistance of welded joints produced by spot welding the produced plated steel sheets was investigated.

[0067] [Manufacturing of plated steel sheets] First, molten steel having a chemical composition consisting of, by mass%, 0.15% C, 1.00% Si, 2.60% Mn, 0.010% P, 0.0020% S, 0.0100% N, 0.020% Al, and the balance Fe and impurities was cast by continuous casting to form a steel billet. The steel billet was once cooled, reheated to 1200°C, hot rolled, and then coiled at 600°C. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at 950°C and a reduction of 30%. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheet was subjected to a 5.0 A / dm 2 Then, if necessary, a 2.0% NaOH aqueous solution is applied to the cold-rolled steel sheet, and the sheet is polished with a heavy-duty abrasive brush (Hotani D-100) at a density of 10 to 200 g / m. 2 The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm, a brush reduction of 2.0 mm, and a rotation speed of 600 rpm to introduce strain into the surface of the cold-rolled steel sheet. Table 1 shows whether each cold-rolled steel sheet was ground with a heavy abrasive brush.

[0068] Next, each cold-rolled steel sheet was cut into a size of 100 mm x 200 mm and then annealed under conditions of a dew point of 0°C, a holding temperature of 870°C, and a holding time of 100 seconds (annealing atmosphere: 4% hydrogen and balance nitrogen). For all steel sheet samples, the heating rate during annealing was 5°C / s. Next, the cut steel sheet samples were plated using a hot-dip galvanizing bath having a predetermined bath composition under the conditions of the bath temperature, time from immersion in the galvanizing bath to the start of cooling, and average cooling rate from the bath temperature to 370°C shown in Table 1, thereby obtaining plated steel sheet samples with a coating layer formed on both surfaces of the steel sheet samples. After immersion in the galvanizing bath, the steel sheet samples were removed, and before the start of cooling, the coating weight was adjusted to 50 g / m per side by N2 gas wiping.2 In Comparative Example 28, after the hot dip galvanizing treatment, an alloying heat treatment was carried out at 520°C for 10 seconds.

[0069] [Tensile strength of plated steel sheet] The tensile strength was measured by taking JIS No. 5 test pieces so that the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the plated steel sheet sample, and conducting a tensile test in accordance with JIS Z 2241:2011. As a result, the tensile strength of all plated steel sheet samples was 780 MPa or more.

[0070] [Measurement of Al concentration distribution in plating layer] First, the coated steel sheet samples were cut into 50 mm x 50 mm pieces. The cut samples were then subjected to GDS measurement to obtain the Al concentration distribution from the surface of the coating layer to a depth of 100 μm. Next, the Al concentration at the depth where the Fe intensity was 50% of the Fe intensity of the base steel sheet (the Fe intensity at a depth of 100 μm from the surface of the coating layer of the sample) was determined as the "Al concentration at the coating layer where the Fe concentration is 50% of that of the base steel sheet." The distance from this depth to the surface was defined as the thickness of the coating layer. The Al concentration measured by GDS at a position halfway through the thickness of the coating layer was determined as the "Al concentration at the center of the coating layer." Finally, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer where the Fe concentration is 50% of that of the base steel sheet" was determined.

[0071] [Welded joint manufacturing] Of the obtained plated steel sheet samples, one measuring 100 × 100 mm was subjected to spot welding. Two pieces were cut to a size of 50 mm × 100 mm, and these two plated steel sheet samples were spot welded using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 5°, a pressure of 4.0 kN, a welding time of 0.5 seconds, and a current of 11 kA to produce welded joints.

[0072] [Chemical composition analysis of plating layer] The chemical composition of the coating layer was determined by immersing a 30mm x 30mm sample taken from the outer region of the heat-affected zone of the welded joint in a 10% HCl solution containing an inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.), removing the coating layer by pickling, and then measuring the coating components dissolved in the solution using ICP emission spectroscopy. The results are shown in Table 1.

[0073] [Measurement of the area ratio of β-CuZn phase in the plating layer of the weld shoulder] The area fraction of the β-CuZn phase in the plating layer at the weld shoulder was determined as follows. First, a cross-sectional sample of the spot weld was prepared. Next, a BSE image including the weld shoulder was obtained using an SEM-EPMA, and elemental analysis was performed to measure the area fraction of the β-CuZn phase in the weld shoulder. Specifically, the β-CuZn phase had a Zn concentration of 40 to 60 atomic %, a Cu concentration of 40 to 60 atomic %, an Fe concentration of 0 to 20 atomic %, and other elements of 3 atomic % or less. The field of view of the SEM image was 100 μm × 100 μm, and similar elemental analysis was performed on five different locations on the weld shoulder. The average of the area fractions of the β-CuZn phase obtained in each field of view was determined as the area fraction of the β-CuZn phase in the plating layer at the weld shoulder.

[0074] [Evaluation of LME resistance] The cross section of the welded joint produced was polished and then observed with an optical microscope. The length of the LME crack that had occurred in the cross section of the weld shoulder was measured, and the LME resistance was evaluated as follows. AAA: No LME cracks, AA: LME crack length over 0 μm to 100 μm, A: LME crack length over 100 μm to 500 μm, B: LME crack length over 500 μm

[0075] Welded joints with LME resistance ratings of AAA, AA, and A were evaluated as being able to suppress or reduce the occurrence of LME cracking on the electrode side surface during spot welding. The results are shown in Table 1 below.

[0076] [Table 1]

[0077] Referring to Table 1, in Comparative Example 26, the low Al content of the entire coating layer prevented the Al addition from fully suppressing LME cracking. Consequently, the large amount of Cu dissolved from the electrode resulted in an extremely high area ratio of the β-CuZn phase in the coating layer at the weld shoulder. As a result, LME resistance was reduced. In Comparative Example 27, the high Al content of the entire coating layer is thought to have lowered the melting point of the coating layer. As a result, Zn in the coating layer was more likely to melt during spot welding, resulting in reduced LME resistance. In Comparative Example 28, the alloying heat treatment increased the Fe content in the coating layer, preventing the desired coating chemical composition from being obtained. Furthermore, the ratio of the "Al concentration at the center of the coating layer" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" was less than 0.10. This resulted in an increased area ratio of the β-CuZn phase in the coating layer at the weld shoulder. In Comparative Example 28, the desired ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" was not obtained. This is thought to have prevented the Al in the coating layer from reacting with the Cu in the electrode to form a sufficient amount of high-melting-point Cu-Al-based metal compound on the electrode surface, thereby promoting the incorporation of Cu into the coating layer. This is also thought to have resulted in a decrease in LME resistance. In Comparative Example 29, the long time from immersion in the coating bath to the start of cooling presumably consumed a large amount of Al in the formation of the Fe-Al barrier layer, resulting in a decrease in the amount of Al in the coating layer other than the Fe-Al barrier layer. As a result, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" was less than 0.10. For the same reason as in Comparative Example 28, the area fraction of the β-CuZn phase in the coating layer at the weld shoulder increased, resulting in a decrease in LME resistance. In Comparative Example 30, the average cooling rate from a bath temperature of 370°C was slow, so a large amount of Al was consumed in forming the Fe-Al barrier layer, which is thought to have reduced the amount of Al in the plating layer other than the Fe-Al barrier layer.As a result, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" was less than 0.10, and for the same reasons as in Comparative Examples 28 and 29, the area ratio of the β-CuZn phase in the coating layer at the weld shoulder increased, resulting in reduced LME resistance.

[0078] In contrast, the welded joints of all examples had a specified coating chemical composition, and when the coating layer outside the heat-affected zone was measured using GDS, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" was controlled within the range of 0.10 to 1.50.Furthermore, the area ratio of the β-CuZn phase in the coating layer at the weld shoulder was controlled within the range of 50% or less, thereby fully utilizing the effect of adding Al to the coating layer and reliably suppressing or reducing LME cracking. In particular, in Examples 2, 3, and 5 (without grinding with a heavy abrasive brush), in which the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was 4 seconds and the average cooling rate from the bath temperature to 370°C was 40°C / second, and in Example 14, in which grinding with a heavy abrasive brush was performed as a pretreatment for the annealing process (however, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was 6 seconds and the average cooling rate from the bath temperature to 370°C was 20°C / second), the area ratio of the β-CuZn phase in the coating layer at the weld shoulder was 30% or less, and as a result, the LME resistance was evaluated as AA, further improving LME resistance. Additionally, in Examples 6 to 13, 15 to 17, and 19 to 25, in which grinding with a heavy abrasive brush was performed as pretreatment for the annealing process, and the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was set to 4 seconds, and the average cooling rate from the bath temperature to 370°C was set to 40°C / second, the area ratio of the β-CuZn phase in the coating layer at the weld shoulder was 10% or more, and as a result, the LME resistance was evaluated as AAA, further improving LME resistance. [Explanation of symbols]

[0079] 1 Welded joints 11 Steel plate 12 Nuggets 13 Pressure welding part 14 Heat-affected zone 15 Spot welds 16 Weld shoulder 17 Plating layer

Claims

1. A plurality of overlapping steel plates; a nugget that joins the plurality of steel plates together, and a spot weld that has a pressure welding portion and a heat-affected zone formed around the nugget; A welded joint comprising: At least one of the plurality of steel plates is a plated steel plate comprising a base steel plate and a plating layer formed on at least a surface of the base steel plate corresponding to an outermost surface of the plurality of steel plates, The plating layer in the region outside the heat-affected zone comprises, in mass%, Al: 0.10 to 1.50%, and Fe:0.01~2.00% and further comprising Mg: 0-1.500%, Si: 0 to 1.000%, Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0-1.000%, Ti: 0 to 1.000%, Cr: 0-1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, P: 0 to 0.500%, and Sr: 0-0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, When the coating layer in the region outside the heat-affected zone is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" / "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50, an area ratio of a β-CuZn phase in the plating layer of the weld shoulder portion on the outermost surfaces of the plurality of steel plates is 50% or less; The β-CuZn phase refers to a phase having a Zn concentration of 40 to 60 atomic %, a Cu concentration of 40 to 60 atomic %, an Fe concentration of 0 to 20 atomic %, and other elements of 3 atomic % or less, as measured by a scanning electron microscope equipped with an electron probe microanalyzer (SEM-EPMA), A welded joint, characterized in that the plated steel sheet has a tensile strength of 780 MPa or more.

2. 2. The welded joint according to claim 1, wherein the plating layer in the weld shoulder portion on the outermost surfaces of the plurality of steel plates has an area ratio of β-CuZn phase of 30% or less.

3. 2. The welded joint according to claim 1, wherein the plating layer in the weld shoulder portion on the outermost surfaces of the plurality of steel plates has an area ratio of β-CuZn phase of 10% or less.

4. The welded joint according to any one of claims 1 to 3, characterized in that the plating layer is a hot-dip galvanized (GI) layer.

5. A welded joint described in any one of claims 1 to 3, characterized in that when the plating layer in the area outside the heat-affected zone is measured by GDS, the ratio of "Al concentration at the center of the plating layer" to "Al concentration at the plating layer position where the Fe concentration is 50% of that of the base steel plate" is 0.45 to 1.50.

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