welded joint

Abstract

A welded joint is provided having a plurality of overlapping steel plates, a nugget joining the plurality of steel plates, and a spot welded portion having a pressure welded portion and a heat-affected portion formed around the nugget, wherein at least one of the plurality of steel plates is a plated steel plate having a base steel plate and a plating layer formed on a surface corresponding to the outermost surface of at least the plurality of steel plates among the surfaces of the base steel plates, wherein the plating layer in the region outside the heat-affected portion has a predetermined chemical composition, and when the plating layer is measured by GDS, the ratio of "Al concentration at the center of the plating layer" / "Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate" is 0.10 to 1.50, and 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 50% or less.

Description

Technology Field

[0001] The present invention relates to a welded joint, and more specifically, to a welded joint obtained by spot welding plated steel plates. Background Technology

[0002] Steel sheets used in automobiles and other applications require good weldability. Spot welding is primarily used in processes such as the assembly of vehicle bodies and the installation of parts; in particular, when spot welding galvanized steel sheets together or between galvanized and non-galvanized steel sheets, it is necessary to suppress Liquid Metal Embrittle (LME) cracking. This phenomenon is a cracking that occurs when metals such as zinc, liquefied by welding heat input, penetrate into the interior of the steel sheet along grain boundaries and are subjected to tensile stress generated by welding. If such LME cracking occurs during spot welding, the strength of the welded joint cannot be secured, which may 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 maintain pressure on the welding electrodes after the current flow between the welding electrodes ends (extending the post-weld holding time Ht) and adjusting the said post-weld holding time Ht as a function of the total plate thickness t of the workpiece, the molten zinc-based plating is solidified before the electrodes are opened, and as a result, the molten zinc-based plating does not penetrate into the grain boundaries of the steel plate at locations with high welding residual stress, thereby suppressing cracking just outside the corona bond or near the nugget of the corona bond.

[0004] Patent Document 2 describes a method for suppressing cracking by controlling the plating structure near the spot weld after spot welding. More specifically, Patent Document 2 describes a spot weld member having a spot weld formed by spot welding a plate set of multiple steel plates stacked together and supported by a pair of electrodes, wherein at least one of the multiple steel plates is a high-strength zinc-plated steel plate with a tensile strength of 780 MPa or more, and the Al content in the plating of the high-strength zinc-plated steel plate is 0.5 mass% or more, and the thermal shock region outside the corona bond end of the spot weld is a plating layer having an average thickness of 0.3 μm or more and a zinc-plated layer having an average thickness of 2.0 μm or more formed on the interface between the base steel plate of the high-strength zinc-plated steel plate and the plating. In addition, Patent Document 2 teaches that in order to suppress Zn penetration into the base steel plate, it is important to form a high-melting-point FeAl alloy layer at the interface between the steel plate and the plating of the steel plate by the heat input during welding by setting the Al content in the plating to 0.5 mass% or more. Prior art literature

[0005] Japanese Patent Publication No. 2017-047475, International Publication No. 2020 / 130079 The problem to be solved

[0006] For example, when a joint is fabricated by spot welding overlapping galvanized steel sheets, LME cracking may occur not only within or immediately outside the pressure weld (corona bond) on the overlapping surface of the steel sheets, but also on the surface of the electrode side. Although Patent Documents 1 and 2 have examined the suppression of LME cracking around the pressure weld on the overlapping surface of the steel sheets from the perspective of improving the spot welding method or controlling the plating structure near the spot weld after spot welding, they have not necessarily examined the suppression of LME cracking on the surface of the steel sheet on the electrode side sufficiently. Therefore, in the inventions described in these patent documents, there was still room for improvement regarding the enhancement of LME resistance.

[0007] Therefore, the present invention aims to provide a weld joint capable of suppressing or reducing the occurrence of LME cracking on the electrode-side surface during spot welding through a novel configuration. means of solving the problem

[0008] The inventors conducted an investigation by focusing particularly on the microstructure of the plating layer in the plated steel sheet used for the weld joint in order to suppress or reduce the occurrence of LME cracking on the electrode-side surface when manufacturing a weld joint by spot welding. As a result, the inventors discovered that by using a plated steel sheet equipped with a plating layer containing a relatively small amount of Al and appropriately controlling the Al concentration distribution, it is possible to suppress or reduce the incorporation of Cu from the electrode into the plating layer under high temperatures during spot welding, and in this regard, significantly improve the LME resistance on the electrode-side surface of the weld joint, thereby completing the present invention.

[0009] The present invention obtained by achieving the above objective is as follows.

[0010] (1) Multiple overlapping steel plates, and

[0011] A nugget joining the plurality of steel plates, and a spot weld having a pressure weld and a heat-affected zone formed around the nugget

[0012] It is a welded joint equipped with,

[0013] One or more of the plurality of steel plates is a plated steel plate having a base steel plate and a plating layer formed on a surface of the base steel plate corresponding to at least the outermost surface of the plurality of steel plates, and

[0014] The plating layer in the outer region of the heat-affected zone is, in mass %,

[0015] Al: 0.10 to 1.50%, and

[0016] Fe: 0.01 to 2.00%

[0017] Contains, and also,

[0018] Mg: 0 to 1.500%,

[0019] Si: 0 to 1.000%,

[0020] Ni: 0 to 1.000%,

[0021] Ca: 0 to 4.000%,

[0022] Sb: 0 to 0.500%,

[0023] Pb: 0 to 0.500%,

[0024] Cu: 0 to 1.000%,

[0025] Sn: 0 to 1.000%,

[0026] Ti: 0 to 1.000%,

[0027] Cr: 0 to 1.000%,

[0028] Nb: 0 to 1.000%,

[0029] Zr: 0 to 1.000%,

[0030] Mn: 0 to 1.000%,

[0031] Mo: 0 to 1.000%,

[0032] Ag: 0 to 1.000%,

[0033] Li: 0 to 1.000%,

[0034] La: 0 to 0.500%,

[0035] Ce: 0 to 0.500%,

[0036] B: 0 to 0.500%,

[0037] Y: 0 to 0.500%,

[0038] P: 0 to 0.500%, and

[0039] Sr: 0 to 0.500%

[0040] Containing at least one of the following in total of 5.000% or less,

[0041] It has a chemical composition in which the remainder consists of Zn and impurities,

[0042] When the plating layer in the outer region of the heat-affected zone is measured by glow discharge emission analysis (GDS), the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” is 0.10 to 1.50, and

[0043] A welded joint characterized in that the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates has an area ratio of β-CuZn phase of 50% or less.

[0044] (2) The welded joint described in (1), characterized in that the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates has an area ratio of β-CuZn of 30% or less.

[0045] (3) The welded joint described in (1), characterized in that the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates has an area ratio of β-CuZn of 10% or less.

[0046] (4) A welded joint described in any one of claims (1) to (3), characterized in that the plating layer is a molten zinc plating (GI) layer.

[0047] (5) A welded joint described in any one of claims (1) to (4), characterized in that the plated steel plate has a tensile strength of 780 MPa or more. Effects of the invention

[0048] According to the present invention, a weld joint capable of suppressing or reducing the occurrence of LME cracking during spot welding can be provided. Brief explanation of the drawing

[0049] FIG. 1 is a schematic diagram illustrating a cross-section of a welded joint according to an embodiment of the present invention, wherein (a) is an overall view of the welded joint and (b) is an enlarged view of the welded shoulder on the electrode side surface. FIG. 2 is a diagram showing the results of GDS analysis of a plated steel sheet, (a) showing the results of GDS analysis of an Al-containing plated steel sheet manufactured by a conventional method, and (b) showing 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. Specific details for implementing the invention

[0050] Welded Joint

[0051] A welded joint according to an embodiment of the present invention comprises a plurality of overlapping steel plates, and

[0052] A nugget joining the plurality of steel plates, and a spot weld having a pressure weld and a heat-affected zone formed around the nugget

[0053] It is a welded joint equipped with,

[0054] One or more of the plurality of steel plates is a plated steel plate having a base steel plate and a plating layer formed on a surface of the base steel plate corresponding to at least the outermost surface of the plurality of steel plates, and

[0055] The plating layer in the outer region of the heat-affected zone is, in mass %,

[0056] Al: 0.10 to 1.50%, and

[0057] Fe: 0.01 to 2.00%

[0058] Contains, and also,

[0059] Mg: 0 to 1.500%,

[0060] Si: 0 to 1.000%,

[0061] Ni: 0 to 1.000%,

[0062] Ca: 0 to 4.000%,

[0063] Sb: 0 to 0.500%,

[0064] Pb: 0 to 0.500%,

[0065] Cu: 0 to 1.000%,

[0066] Sn: 0 to 1.000%,

[0067] Ti: 0 to 1.000%,

[0068] Cr: 0 to 1.000%,

[0069] Nb: 0 to 1.000%,

[0070] Zr: 0 to 1.000%,

[0071] Mn: 0 to 1.000%,

[0072] Mo: 0 to 1.000%,

[0073] Ag: 0 to 1.000%,

[0074] Li: 0 to 1.000%,

[0075] La: 0 to 0.500%,

[0076] Ce: 0 to 0.500%,

[0077] B: 0 to 0.500%,

[0078] Y: 0 to 0.500%,

[0079] P: 0 to 0.500%, and

[0080] Sr: 0 to 0.500%

[0081] Containing at least one of the following in total of 5.000% or less,

[0082] It has a chemical composition in which the remainder consists of Zn and impurities,

[0083] When the plating layer in the outer region of the heat-affected zone is measured by glow discharge emission analysis (GDS), the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” is 0.10 to 1.50, and

[0084] The plating layer of the weld shoulder on the outermost surface of the plurality of steel plates is characterized by having an area ratio of β-CuZn phase of 50% or less.

[0085] As previously explained, in spot welding between galvanized steel sheets or between galvanized steel sheets and non-galvanized steel sheets, it is necessary to suppress LME cracking. For example, when a joint is fabricated by spot welding two or more steel sheets, each containing at least one galvanized steel sheet, LME cracking may occur within the pressure weld (corona bond) formed on the outer side of the weld metal (nugget), or immediately outside of it, or on the surface of the electrode side (for example, the weld shoulder or its periphery corresponding to the outer edge of the electrode indentation that is concave compared to the steel sheet surface due to pressure from the electrode). LME cracking occurs when metals such as zinc, which are liquefied by the welding heat input during spot welding, penetrate into the interior of the steel sheet along the grain boundaries and act on the steel sheet in areas where tensile stress generated by welding—such as pressure applied by the electrode, expansion and contraction of the weld area, and springback upon electrode release—acts on the steel sheet. Therefore, in order to suppress or reduce the intrusion of metals such as zinc into the interior of such steel sheets, the inventors focused on the structure of the plating layer in the plated steel sheets and conducted an examination from the perspective of making the structure of the plating layer more suitable.

[0086] First, the inventors have discovered that adding aluminum (Al) in a relatively small amount, namely 0.10 to 1.50 mass%, to a plating layer based on zinc (Zn) is effective in suppressing or reducing the penetration of Zn into the steel sheet. As the amount of Al added increases, the composition of the plating layer approaches the Zn-Al eutectic composition, and thus the melting point of the plating layer decreases. For this reason, the excessive addition of Al is likely to have an adverse effect in terms of improving LME resistance by suppressing or reducing the penetration of molten Zn into the steel sheet. In particular, when Al is added in an amount significantly exceeding 1.50 mass%, the negative effects based on the excessive addition of Al become pronounced, and it is thought that the effect of suppressing LME cracking caused by the addition of Al cannot be fully exerted. Furthermore, the inventors have discovered that by using a plated steel plate in which the Al concentration distribution in the plating layer is controlled such that the Al concentration at the center of the plating layer is in a ratio of 0.10 to 1.50 with respect to the Al concentration near the interface between the base steel plate and the plating layer—more specifically, the Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel plate—the plating structure at the weld shoulder on the electrode side changes due to thermal effects during spot welding, thereby significantly suppressing or reducing the occurrence of LME cracking at the weld shoulder or its surroundings. This will be explained in more detail below with reference to the drawings.

[0087] FIG. 1 is a schematic diagram illustrating a cross-section of a welded joint according to an embodiment of the present invention, wherein (a) is an overall view of the welded joint and (b) is an enlarged view of the weld shoulder on the electrode side surface. First, referring to FIG. 1 (a), the welded joint (1) according to an embodiment of the present invention comprises two overlapping steel plates (11), a nugget (12) joining the steel plates (11), and a spot weld (15) having a pressure weld (13) and a heat-affected zone (14) formed around the nugget (12). On the outermost side of the welded joint (1), that is, on the electrode side surface, a weld shoulder (16) is formed corresponding to the outer edge of the electrode indentation, that is, the boundary between the portion that is pressed by the electrode and is concave compared to the steel plate surface and the flat portion of the steel plate. In FIG. 1, as two steel plates (11), a plated steel plate is used, having a plating layer on both sides of the base steel plate that is mainly composed of Zn and has an Al concentration distribution controlled within a predetermined range. In relation to this, referring to FIG. 1 (b), at the weld shoulder (16), a plating layer (17) originating from the plating layer on the steel plate (11) prior to spot welding is formed by pushing it out from the electrode (not shown) side toward the weld shoulder (16). According to an embodiment of the present invention, at the weld shoulder (16) and its surroundings within the heat-affected zone (14), the structure of the initial plating layer is changed due to the heat effect during spot welding, and more specifically, a plating 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 in which the Zn concentration is 40 to 60 atomic%, the Cu concentration is 40 to 60 atomic%, the Fe concentration is 0 to 20 atomic%, and other elements are 3 atomic% or less, as measured by a scanning electron microscope (SEM-EPMA) equipped with an electron probe microanalyzer.Unlike the pressure weld (13) on the opposite side of the overlapping surface, the electrode-side surface of the weld joint (1) comes into contact with the electrode, so under the high temperature during spot welding, Cu may melt from the electrode and be incorporated into the plating layer. Although changes in the structure of the plating layer due to heat effects during spot welding occur in the plating layer present on both the electrode-side and overlapping-side surfaces of the steel plate (11), in the plating layer on the electrode-side, particularly the weld shoulder (16), the changes in the structure of the plating layer are different from those on the overlapping-side surface due to the incorporation of Cu. Such changes in the structure of the plating layer are greatly influenced by the chemical composition and structure of the initial plating layer before spot welding. In particular, in the case of Zn-based plating, on the electrode-side surface, alloying may proceed between the Zn in the plating layer and the Cu in the electrode in contact with it due to the heat input of welding. Since this alloying acts to lower the melting point of Cu, it promotes the incorporation of Cu from the electrode into the plating layer. This time, the inventors confirmed that when Cu is incorporated, a relatively large amount of β-CuZn phase is formed in the plating layer, and LME cracking is promoted due to this incorporation of Cu. In order to address this problem, the inventors, after repeated investigations, found that by using a plated steel sheet having a plating layer that contains a relatively small amount of Al and appropriately controls the Al concentration distribution, the alloying reaction between Zn in the plating layer and Cu in the electrode during spot welding is suppressed, and the β-CuZn phase in the plating layer (17) of the weld shoulder (16) can be reliably reduced to an area ratio of 50% or less. Accordingly, according to an embodiment of the present invention, compared to the case of spot welding of conventional Zn-plated steel sheets, it is possible to significantly suppress or reduce the occurrence of LME cracking at the weld shoulder or around it during spot welding.

[0088] In FIG. 1, for ease of understanding, only two steel plates (11) are overlapped and a plating layer is formed on both steel plates (11) to form a welded joint. However, the welded joint according to the embodiment of the present invention is not necessarily limited to such a welded joint, and may include various welded joints in which a plating layer (17) is formed in which the area ratio of the β-CuZn phase in the weld shoulder (16) within the heat-affected zone (14) is controlled to be 50% or less. For example, in the welded joint formed by spot welding two sets of steel plates (11) as described in FIG. 1, only one steel plate (11) may be a plated steel plate. In this case, the plating layer (17) may exist on the weld shoulder (16) of at least the electrode-side surface (the outermost surface of the welded joint (1)) among the two surfaces of the plated steel plate, and, of course, the plating layer may also exist on the overlapping surface on the opposite side. A plating layer containing a relatively small amount of Al and having an appropriately controlled Al concentration distribution (i.e., a plating layer containing 0.10 to 1.50 mass% of Al, as described in detail later in relation to FIG. 2, and having an Al concentration distribution controlled such that the ratio of Al concentration at the plating layer location where Fe concentration is 50% of the base steel plate is 0.10 to 1.50) is useful for suppressing or reducing the penetration of molten Zn into the steel plate regardless of contact with the electrode. For example, when the plating layer is present on the overlapping surface of the steel plate, the penetration of Al in the plating layer into the molten Zn into the steel plate can be significantly suppressed or reduced during spot welding, even in the pressure welded portion (13) within the heat-affected zone and the area immediately outside of it. Because of this, it is possible to further improve LME resistance compared to the case where the plating layer (17) is present only on the surface of the steel plate on the electrode side.In the case of a welded joint made of three or more steel plates, various welded joints may be included in which a plating layer (17) is formed in which the area ratio of the β-CuZn phase in the weld shoulder (16) within the heat-affected zone (14) is controlled to be 50% or less. For example, in the case of a welded joint formed by spot welding three sets of steel plates (11), it is sufficient for the plating layer (17) to exist on the weld shoulder (16) of at least one of the three steel plates (11) on the electrode side surface (the outermost surface of the welded joint (1)). For example, if only one outer steel plate (11) among the three overlapping steel plates (11) is a plated steel plate, the plating layer (17) may exist only on the weld shoulder (16) of the electrode side surface among the two surfaces of the plated steel plate, and additionally, the plating layer may also exist on the overlapping surface on the opposite side. For example, a plating layer (17) may exist only on the weld shoulder (16) of one outer steel plate (11), and another Zn-based plating layer may exist on the weld shoulder (16) of the other outer steel plate (11). Such an embodiment is also included in the present invention. In this case, compared to the case of a weld joint where a plating layer (17) exists on the weld shoulder (16) of both outer steel plates (11), there is a possibility that the joint strength may be slightly reduced in that the risk of LME cracking is higher on the surface where the other Zn-based plating layer exists. However, the specific number and arrangement of steel plates (11) on which the plating layer (17) is formed with a β-CuZn phase area ratio controlled to 50% or less in the weld joint can be appropriately determined by considering the desired joint strength, etc.

[0089] Next, a plated steel sheet useful for use in a welded joint according to an embodiment of the present invention will be described in more detail. FIG. 2 is a diagram showing the results of GDS analysis of a plated steel sheet, FIG. 2(a) shows the results of GDS analysis of an Al-containing plated steel sheet manufactured by a conventional method, and FIG. 2(b) 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. First, referring to FIG. 2(a), in an Al-containing plated steel sheet in which 0.20% Al is contained in a Zn-based plating layer by a conventional method, as the depth increases from 0 μm, which corresponds to the plating surface, it can be seen that the Al concentration has a relatively high peak near the interface between the base steel sheet and the plating layer, that is, near the plating layer position where the Fe concentration is 50% of that of the base steel sheet. This peak in Al concentration suggests that an Fe-Al barrier layer containing an alloy of Fe and Al is formed at the interface between the base steel sheet and the plating layer. As is evident from the relatively high peak in Al concentration, in Al-containing plated steel sheets manufactured by conventional methods, more Al in the plating 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, in Figure 2(a), as the Al concentration progresses from near the interface between the base steel sheet and the plating layer toward the plating surface, it decreases significantly and then becomes a very low value that is almost constant. It can be seen that the Al concentration at the center of the plating layer, which corresponds to a position midway between the plating layer where the Fe concentration is 50% of the base steel sheet and the plating surface, shows a low value of about 0.1%.

[0090] In contrast to this, referring to FIG. 2(b), it can be seen that in the plated steel sheet of FIG. 2(b), although the plating layer has the same Al content as in FIG. 2(a), the Al concentration near the interface between the base steel sheet and the plating layer is very low compared to FIG. 2(a). Therefore, in the plated steel sheet of FIG. 2(b), a thin Fe-Al barrier layer is formed compared to FIG. 2(a). In this regard, in FIG. 2(b), even as the process proceeds from near the interface between the base steel sheet and the plating layer toward the plating surface, the Al concentration in the plating layer decreases relatively gradually without significantly decreasing, and then becomes almost constant, and the Al concentration at the center of the plating layer exceeds about 0.2%, showing a very high value of about twice as high as in FIG. 2(a). From the GDS analysis results of FIG. 2(a) and (b), it is believed that in the plated steel sheet of FIG. 2(b), most of the Al in the plating layer is not consumed in the formation of the Fe-Al barrier layer, but exists as an Al phase, for example, in a solid solution state, within the plating layer other than the Fe-Al barrier layer. Based on the GDS analysis results of these plated steel sheets and the fact that the weld joint obtained by spot welding the plated steel sheet of FIG. 2(b) experimentally exhibited higher LME resistance compared to the weld joint obtained by spot welding the plated steel sheet of FIG. 2(a), particularly high LME resistance in the weld shoulder of the electrode-side surface in addition to the pressure weld on the overlapping surface, the inventors believed that the Al phase existing in the plating layer other than the Fe-Al barrier layer plays a very important role in suppressing or reducing LME cracking during spot welding and conducted further investigations. As a result, the inventors determined that the total amount of Al added is 1.It was discovered that by controlling the amount to a relatively low level of 50 mass% or less to suppress the deterioration of LME resistance based on the lowering of the melting point in the plating layer, and by controlling the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” when the plating layer is measured by GDS to within the range of 0.10 to 1.50, the effect of adding Al to the plating layer can be fully exerted. Furthermore, the inventors discovered that by using such a plated steel sheet, the structure of the initial plating layer in the weld shoulder within the heat-affected zone of the electrode-side surface changes during spot welding, forming a plating layer with an area ratio of 50% or less of the β-CuZn phase, and in relation to this, the occurrence of LME cracking in the weld shoulder and its surroundings during spot welding can be significantly suppressed or reduced.

[0091] Although it is not intended to be bound by any specific theory, in the weld joint according to the embodiment of the present invention, it is believed that Al in the initial plating layer acts as follows to suppress or reduce the occurrence of LME cracking during spot welding, particularly LME cracking in the weld shoulder and its surroundings. To explain in more detail, first, the Fe-Al barrier layer formed at the interface between the base steel plate and the plating layer in the plated steel plate is relatively brittle, so it is thought to fracture relatively easily due to the stress applied to the steel plate caused by the pressure exerted by the electrode during spot welding. Since the molten Zn during spot welding comes into direct contact with the base steel plate due to the fracture of this Fe-Al barrier layer, the risk of the molten Zn penetrating into the steel plate along the grain boundaries increases. However, when using a plated steel sheet having a plating layer with an Al concentration distribution as shown in Fig. 2, the Al present in relatively large quantities in the portion of the plating layer other than the Fe-Al barrier layer comes into direct contact with the base steel sheet along with Zn due to the rupture of the Fe-Al barrier layer. In this case, it is believed that the Al in the plating layer reacts with Fe in the base steel sheet due to the heat input during spot welding to form a new Fe-Al barrier layer, and consequently, the ruptured Fe-Al barrier layer is repaired. That is, because there is a large amount of Al in the plating layer other than the Fe-Al barrier layer, even if Zn comes into direct contact with the base steel sheet due to the rupture of the Fe-Al barrier layer during spot welding, a new Fe-Al barrier layer is immediately formed at the rupture site by the Al present nearby. For this reason, it is believed that the penetration of molten Zn into the steel sheet during spot welding can be significantly suppressed or reduced, thereby enabling the suppression or reduction of LME cracking.In addition, particularly on the electrode-side surface of the weld joint, it is thought that Al, which is present in a relatively large amount in the plating layer, reacts with Cu in the electrode under high temperatures during spot welding to form a high-melting-point Cu-Al metal compound on the electrode surface. It is thought that this high-melting-point Cu-Al metal compound acts as a barrier, thereby suppressing or reducing the alloying of Cu in the electrode with Zn in the plating layer, and further suppressing or reducing the incorporation of Cu into the plating layer associated with it. As a result, the formation of the β-CuZn phase in the plating layer of the weld shoulder can be controlled within a predetermined range, making it possible to reliably suppress or reduce the occurrence of LME cracking caused by the incorporation of Cu. Conventionally, plated steel sheets with Al added to a Zn-based plating layer are known. However, the fact that the amount of Al in the entire plating layer can be kept relatively low in consideration of the lowering of the melting point of the plating layer, while the amount of Al in the plating layer other than the Fe-Al barrier layer is increased, thereby suppressing or reducing the penetration of molten Zn into the steel sheet during spot welding, and in particular, suppressing or reducing the occurrence of LME cracking caused by the incorporation of Cu from the electrode in the weld shoulder of the electrode side surface and the surrounding area, has not been known in the prior art and has been revealed for the first time by the inventors of the present invention.

[0092] Hereinafter, each component of the welded joint according to the embodiments of the present invention will be described in more detail. In the following description, "%", which is the unit of the content of each element, means "mass%" unless otherwise specified. Furthermore, in this specification, the term "to" indicating a numerical range is used to mean that the values ​​described before and after it are included as lower and upper limits, unless otherwise specified.

[0093] [Plated Steel Sheet]

[0094] In a welded joint according to an embodiment of the present invention, one or more of a plurality of overlapping steel plates are plated steel plates having a base steel plate and a plating layer formed on a surface of the base steel plate corresponding to at least the outermost surface of the plurality of steel plates. The plating layer of the plated steel plate has the following chemical composition, which is identical to the initial chemical composition prior to spot welding, in the region outside the heat-affected zone.

[0095] [Chemical composition of the plating layer]

[0096] [Al: 0.10 to 1.50%]

[0097] Al is an element effective in suppressing molten Zn from penetrating into the steel sheet along grain boundaries and / or in suppressing the incorporation of Cu into the plating layer by reacting with Cu in the electrode at the electrode surface. To sufficiently obtain 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, exceeding 0.30%, 0.31% or more, 0.32% or more, 0.35% or more, 0.40% or more, exceeding 0.60%, 0.62% or more, 0.65% or more, or 0.70% or more. On the other hand, if Al is contained excessively, the composition of the plating layer approaches the Zn-Al eutectic composition, and thus the melting point of the plating layer is lowered. For this reason, Zn in the plating layer is prone to melting during spot welding, which may accelerate LME cracking. Therefore, the Al content should be 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.

[0098] [Fe: 0.01 to 2.00%]

[0099] Fe is an element that is inevitably included in the plating layer, for example, by dissolving from the base steel sheet into the plating bath or by reacting with Al during the plating process to form an Fe-Al barrier layer at the interface between the base steel sheet and the plating layer. For this reason, in the embodiments of the present invention, the Fe content in the plating 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 plating layer is too high, there may be cases where Al in the plating layer combines with Fe, or where a large amount of Al is consumed in the formation of the Fe-Al barrier layer. As a result, there are cases where the effect of adding Al, that is, by suppressing the penetration of molten Zn into the steel sheet during spot welding by Al in the plating layer and / or by reacting with Cu in the electrode to suppress the incorporation of Cu into the plating layer, can not be fully exerted, thereby suppressing or reducing the occurrence of LME cracking. Therefore, the Fe content is 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.

[0100] The basic chemical composition of the plating layer is as described above. In addition, the plating layer is optionally composed of Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0 to 4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 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 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to It may contain at least one of 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500%. These optional elements shall be present in a total amount of 5.000% or less, from the viewpoint of fully utilizing the action and function of the basic components constituting the plating layer, particularly Al. The optional elements may be present in a total amount of 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 will be described in detail below.

[0101] [Mg: 0 to 1.500%]

[0102] Mg is an element effective for improving the corrosion resistance of the plating layer. The Mg content may be 0%, but to obtain this effect, it is preferable that the Mg content be 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, if Mg is contained excessively, a large amount of brittle MgZn-based compounds may be generated in the plating layer, which can cause a decrease in processability. Therefore, it is preferable that the Mg content be 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.

[0103] [Si: 0 to 1.000%]

[0104] Si is an element effective for 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, if Si is contained excessively, the adhesion of the plating layer may decrease. Therefore, it is preferable that the Si content be 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.

[0105] [Ni: 0 to 1.000%]

[0106] Ni is an element effective for improving the corrosion resistance of the plating layer. The Ni content may be 0%, but to obtain this effect, it is preferable that the Ni content be 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, if Ni is contained excessively, a large amount of intermetallic compounds may be formed, which may reduce corrosion resistance. Therefore, it is preferable that the Ni content be 1.000% or less. The Ni content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0107] [Ca: 0 to 4.000%]

[0108] Ca is an element effective for ensuring the wettability of the plating bath. The Ca content may be 0%, but to obtain this effect, it is preferable that the Ca content be 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, if Ca is contained excessively, a large amount of hard intermetallic compounds are formed within the plating layer, making the plating layer prone to cracking and potentially reducing adhesion to the steel sheet. Therefore, it is preferable that the Ca content be 4.000% or less. The Ca content may be 3.000% or less, 2.000% or less, or 1.500% or less.

[0109] [Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 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 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, P: 0 to 0.500% and Sr: 0 to 0.500%]

[0110] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr do not have to be included in the plating layer, but may be present in the plating layer in an amount of 0.0001% or more or 0.001% or more. These elements do not adversely affect the performance of the plated steel sheet as long as they are within a predetermined content range. However, if the content of each element is excessive, it may reduce corrosion resistance. Therefore, it is preferable that the content of Sb, Pb, La, Ce, B, Y, P, and Sr be 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, or 0.050% or less. Likewise, the content of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li is preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, or 0.100% or less.

[0111] In the plating layer, the remainder other than the above-mentioned elements consists of Zn and impurities. Impurities in the plating layer refer to components that are incorporated due to various factors of the manufacturing process, including raw materials, when the plating layer is manufactured.

[0112] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that inhibits corrosion of the base steel sheet has been added, and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy.

[0113] As for the plating layer, any plating layer having the chemical composition described above may be used and is not particularly limited, but, for example, it is preferably a hot-dip galvanized (GI) layer. For example, if alloying heat treatment is performed, the Fe content in the plating layer increases, and in the final plating layer, it may not be possible to obtain the desired chemical composition and the ratio of "Al concentration at the center of the plating layer" to "Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel sheet." In addition, the thickness of the plating layer may be, for example, 3 to 50 μm. The amount of plating layer applied is not particularly limited, but, for example, 10 to 170 g / m² per side. 2 It is acceptable. The deposition amount of the plating layer is 45 g / m² per side. 2 ≥ 50g / m² 2 It may be more than that. Likewise, the deposition amount of the plating layer is 75 g / m² per side. 2 70g / m² or less 2 The following may be used. The amount of plating layer deposited is determined by dissolving the plating layer in an acid solution to which an inhibitor that inhibits corrosion of the base steel sheet has been added, and by the weight change before and after pickling.

[0114] [Ratio of "Al concentration at the center of the plating layer" / "Al concentration at the location of the plating layer where Fe concentration is 50% of the base steel sheet": 0.10 to 1.50]

[0115] In an embodiment of the present invention, when the plating layer on the outer side of the heat-affected zone is measured by glow discharge emission analysis (GDS), the ratio of “Al concentration at the center of the plating layer” to “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” is 0.10 to 1.50. By controlling the ratio of “Al concentration at the center of the plating layer” and “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” measured by GDS to be within this range, the effect of adding Al to the plating layer is fully exerted, and it is possible to ensure that the area ratio of the β-CuZn phase in the plating layer of the weld shoulder is controlled to be within a range of 50% or less during spot welding. From the perspective of reducing the area ratio of the β-CuZn phase in the plating layer of the weld shoulder, the ratio of “Al concentration at the center of the plating layer” to “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” is better when higher, preferably 0.15 or higher, more preferably 0.20 or higher, most preferably 0.30 or higher, and may be, for example, 0.40 or higher, 0.42 or higher, 0.45 or higher, 0.50 or higher, 0.55 or higher, or 0.60 or higher. On the other hand, if the ratio of “Al concentration at the center of the plating layer” to “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” becomes too high, the melting point of the plating layer may decrease along with an increase in the total Al content of the plating layer associated with it. Because of this, there is a risk that Zn in the plating layer will melt easily during spot welding, thereby promoting LME cracking. Accordingly, in an embodiment of the present invention, the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% 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.

[0116] [Method for measuring the ratio of "Al concentration at the center of the plating layer" / "Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet"]

[0117] The ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel plate” is determined as follows. First, a plated steel plate sample is obtained by cutting a 50mm × 50mm area from the outer region of the heat-affected zone of the weld joint, and then the Al concentration distribution from the surface of the plating layer to a depth of 100㎛ is obtained by measuring the said plated steel plate sample using Glow Discharge Spectrometry (GDS). Next, the Al concentration at the depth location where the Fe strength is 50% of the Fe strength of the base steel plate (Fe strength at a depth of 100㎛ from the surface of the plating layer of the sample) is determined by GDS measurement as the “Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel plate,” and the distance from this depth location to the surface is defined as the thickness of the plating layer. The Al concentration by GDS at a position halfway the thickness of the plating layer is determined as the “Al concentration at the center of the plating layer,” and finally, the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the plating layer position where the Fe concentration is 50% of the base steel sheet” is determined.

[0118] [Area percentage of β-CuZn phase in the plating layer of the weld shoulder: 50% or less]

[0119] In an embodiment of the present invention, the area ratio of the β-CuZn phase in the plating layer of the weld shoulder on the outermost surface of a plurality of steel plates is 50% or less. Since the weld shoulder is in contact with the electrode, Cu that melts from the electrode under high temperatures during spot welding may be incorporated into the plating layer of the weld shoulder. In such cases, the proportion of the β-CuZn phase in the plating layer increases, and LME cracking is promoted due to the Cu incorporated into the plating layer in this manner. However, according to an embodiment of the present invention, Al, which is present in a relatively large amount in the plating layer, reacts with Cu in the electrode under high temperatures during spot welding to form a high-melting-point Cu-Al metal compound on the electrode surface, thereby suppressing or reducing the incorporation of Cu into the plating layer. In this regard, since the area ratio of the β-CuZn phase in the plating layer of the weld shoulder is controlled to within a range of 50% or less, it becomes possible to reliably suppress or reduce the occurrence of LME cracking caused by the incorporation of Cu. Therefore, from the perspective of suppressing the occurrence of LME cracking caused by the incorporation of Cu, it is better for the area ratio of the β-CuZn phase in the plating layer of the weld shoulder to be as low as possible. 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. The lower limit is not particularly limited, and the area ratio of the β-CuZn phase in the plating layer of the weld shoulder may be 0%, or, for example, 1% or more or 3% or more.

[0120] [Method for Measuring the Area Ratio of β-CuZn Phase in the Plating Layer of a Welded Shoulder]

[0121] The area ratio of the β-CuZn phase in the plating layer of the weld shoulder on the outermost surface of a plurality of steel plates is determined as follows. First, a cross-sectional sample of the spot weld is prepared, and then a reflected electron image (BSE image) including the weld shoulder is obtained by a scanning electron microscope (SEM-EPMA) equipped with an electron probe microanalyzer, and elemental analysis is performed, and the area ratio of the β-CuZn phase in the weld shoulder is measured. Specifically, the β-CuZn phase is defined as a phase in which the Zn concentration is 40 to 60 atomic%, the Cu concentration is 40 to 60 atomic%, the Fe concentration is 0 to 20 atomic%, and other elements are 3 atomic% or less. The field of view of the SEM image is 100㎛×100㎛, and the same elemental analysis is performed on five other locations of the weld shoulder, and the average value of the area ratio of the β-CuZn phase obtained in each field of view is determined as the area ratio of the β-CuZn phase in the plating layer of the weld shoulder.

[0122] [Base material steel plate]

[0123] In an embodiment of the present invention, the base steel sheet for forming the plating layer is not particularly limited and may be any suitable material, particularly a cold-rolled steel sheet. For example, the base steel sheet may be a material having a chemical composition such that the tensile strength of the plated steel sheet is 780 MPa or higher. Generally, it is known that LME cracking becomes more pronounced when a steel sheet with relatively high strength is spot-welded, and that the susceptibility to LME cracking tends to increase as the strength of the steel sheet increases. According to an embodiment of the present invention, even when using a plated steel sheet having a high tensile strength of 780 MPa or higher, where LME cracking generally becomes pronounced in welded joints, it is possible to reliably suppress or reduce the occurrence of LME cracking. On the other hand, LME cracking caused by the incorporation of Cu on the electrode-side surface may occur even when using a plated steel sheet having a tensile strength lower than 780 MPa or sufficiently low. However, according to an embodiment of the present invention, even when using a plated steel sheet having such relatively low tensile strength, Al present in a relatively large amount in the plating layer reacts with Cu in the electrode at high temperatures during spot welding to form a high-melting-point Cu-Al metal compound on the electrode surface, thereby suppressing or reducing the incorporation of Cu into the plating layer. Consequently, it is possible to reliably suppress or reduce the occurrence of LME cracking caused by the incorporation of Cu, regardless of the tensile strength of the plated steel sheet.

[0124] [Desirable Chemical Composition of Base Steel Sheet]

[0125] As described above, the present invention aims to provide a weld joint capable of suppressing or reducing the occurrence of LME cracking on the electrode-side surface during spot welding. This objective is achieved by making at least one of a plurality of overlapping steel plates into a plated steel plate having a plating layer on a surface corresponding to the outermost surface of at least the plurality of steel plates, controlling the plating layer to have a predetermined chemical composition in the region outside the heat-affected zone, and controlling the area ratio of the β-CuZn phase in the plating layer of the weld shoulder to within a range of 50% or less. Accordingly, it is clear that the chemical composition of the base steel plate having the above plating layer itself is not an essential technical feature for achieving the objective of the present invention. Hereinafter, a preferred chemical composition of a base steel plate useful for use in welded joints according to embodiments of the present invention will be described in detail, but these descriptions are intended to be simple examples of preferred chemical compositions of a base steel plate having a tensile strength of 780 MPa or more, and are not intended to limit the present invention to the use of a base steel plate having such a specific chemical composition.

[0126] In an embodiment of the present invention, for example, the base steel sheet is, in mass %,

[0127] C: 0.01 to 0.50%,

[0128] Si: 0.01 to 3.50%,

[0129] Mn: 0.10 to 5.00%,

[0130] P: 0.100% or less,

[0131] S: 0.0300% or less,

[0132] N: 0.0100% or less,

[0133] O: 0 to 0.020%,

[0134] Al: 0 to 1.000%,

[0135] B: 0 to 0.010%,

[0136] Nb: 0 to 0.150%,

[0137] Ti: 0 to 0.20%,

[0138] Mo: 0 to 3.00%,

[0139] Cr: 0 to 2.00%,

[0140] V: 0 to 1.00%,

[0141] Ni: 0 to 2.00%,

[0142] W: 0 to 1.00%,

[0143] Ta: 0 to 0.10%,

[0144] Co: 0 to 3.00%,

[0145] Sn: 0 to 1.00%,

[0146] Sb: 0 to 0.50%,

[0147] Cu: 0 to 2.00%,

[0148] As: 0 to 0.050%,

[0149] Mg: 0 to 0.100%,

[0150] Ca: 0 to 0.100%,

[0151] Zr: 0 to 0.100%,

[0152] Hf: 0 to 0.100%,

[0153] REM: 0 to 0.10, and

[0154] Remainder: Fe and impurities

[0155] It is desirable to have a chemical composition consisting of the following. Each element will be explained in more detail below.

[0156] [C: 0.01 to 0.50%]

[0157] C is an element that increases tensile strength at a low cost and is an important element for controlling the strength of steel. To obtain this effect sufficiently, it is desirable to have a C content of 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, it is desirable to have a C content of 0.50% or less. The C content may be 0.40% or less, 0.35% or less, or 0.30% or less.

[0158] [Si: 0.01 to 3.50%]

[0159] Si is an element that acts as a deoxidizer to suppress the precipitation of carbides during the cooling process in the annealing of cold-rolled sheets. To sufficiently obtain this effect, it is desirable to have a Si content of 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 result in a decrease in elongation along with an increase in steel strength. For this reason, it is desirable to have a Si content of 3.50% or less. The Si content may be 2.50% or less, 2.00% or less, or 1.50% or less.

[0160] [Mn: 0.10 to 5.00%]

[0161] Mn is an element that influences the ferrite transformation of steel and is effective for increasing strength. To obtain this effect sufficiently, it is desirable to have a Mn content of 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, if Mn is contained excessively, it may result in a decrease in elongation along with an increase in steel strength. For this reason, it is desirable to have a Mn content of 5.00% or less. The Mn content may be 4.00% or less, 3.00% or less, or 2.50% or less.

[0162] [P: 0.100% or less]

[0163] P is an element that segregates at grain boundaries and promotes the embrittlement of steel. Since a lower P content is desirable, it is ideally 0%. However, an excessive reduction in P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, if P is excessively contained, it may cause embrittlement of steel due to grain boundary segregation as mentioned above. Therefore, it is desirable to keep the P content 0.100% or less. The P content may be 0.050% or less, 0.030% or less, or 0.010% or less.

[0164] [S: 0.0300% or less]

[0165] S is an element that forms non-metallic inclusions, such as MnS, in steel, causing a decrease in the ductility of steel parts. Since a lower S content is desirable, it is ideally 0%. However, excessive reduction of the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content may cause cracking originating from non-metallic inclusions during cold forming. Therefore, it is desirable to keep the S content 0.0300% or less. The S content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0166] [N: 0.0100% or less]

[0167] N is an element that forms coarse nitrides in steel sheets, thereby reducing the workability of the steel sheets. Since a lower N content is desirable, it is ideally 0%. However, an excessive reduction in N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if N is excessively contained, as mentioned above, coarse nitrides may be formed, thereby reducing the workability of the steel sheets. Therefore, it is desirable to keep the N content at 0.0100% or less. The N content may be 0.0080% or less, or 0.0050% or less.

[0168] The preferred basic chemical composition of the base steel sheet is as described above. In addition, the base steel sheet, if necessary, instead of a portion of the remainder of Fe, comprises: O: 0 to 0.020%, Al: 0 to 1.000%, B: 0 to 0.010%, Nb: 0 to 0.150%, Ti: 0 to 0.20%, Mo: 0 to 3.00%, Cr: 0 to 2.00%, V: 0 to 1.00%, Ni: 0 to 2.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Cu: 0 to 2.00%, As: 0 to 0.050%, Mg: 0 to 0.100%, Ca: 0 to 0.100%, It may contain one or more selected from the group consisting of Zr: 0 to 0.100%, Hf: 0 to 0.100%, and REM: 0 to 0.100%. Each element may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0169] In the base steel sheet, the remainder other than the elements mentioned above consists of Fe and impurities. Impurities in the base steel sheet refer to components that are incorporated due to various factors of the manufacturing process, including raw materials such as ore and scrap, when the base steel sheet is manufactured industrially.

[0170] The chemical composition of the base steel sheet can be measured by general analytical methods. For example, the chemical composition of the base steel sheet can be measured by first removing the plating layer by mechanical grinding, and then using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melt-thermal conductivity method, and O using the inert gas melt-non-dispersive infrared absorption method.

[0171] [Thickness of base steel plate]

[0172] The thickness of the base steel sheet is not particularly limited, but 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. Likewise, the thickness of the base steel sheet 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.

[0173] [Mechanical properties of galvanized steel sheets]

[0174] A plated steel sheet useful for use in a welded joint according to an embodiment of the present invention may have any suitable tensile strength and is not particularly limited, but may, for example, have a tensile strength of 780 MPa or more. For example, in an embodiment 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, but for example, 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 specimen from a direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the plated steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.

[0175] Method for manufacturing galvanized steel sheets

[0176] Next, a preferred method for manufacturing a plated steel sheet useful for use in welded joints according to an embodiment of the present invention will be described, more specifically, a plated steel sheet having a plating layer in which the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” is 0.10 to 1.50 when measured by glow discharge emission analysis (GDS). The following description is intended to be an example of a characteristic method for manufacturing the said plated steel sheet, and is not intended to limit the said plated steel sheet to being manufactured by the manufacturing method described below.

[0177] Galvanized steel sheets can be manufactured by performing, for example, a casting process in which molten steel with 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 cold rolling process may be performed directly after pickling without coiling after the hot rolling process. Each process is described in detail below.

[0178] [Casting Process]

[0179] The conditions of the casting process are not particularly limited. For example, various secondary smelting processes can be performed continuously in a flux using a blast furnace or electric furnace, and then casting can be done using methods such as conventional continuous casting or ingot casting.

[0180] [Hot Rolling Process]

[0181] Hot-rolled steel sheets can be obtained by hot-rolling a cast steel billet. The hot-rolling process is carried out by reheating the cast steel billet either directly or after cooling and then hot-rolling it. When reheating, the heating temperature of the steel billet may be, for example, 1100 to 1250°C. In the hot-rolling process, rough rolling and finish rolling are typically performed. The temperature and reduction ratio of each rolling step can be appropriately determined according to the desired metal structure or sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0182] [Winding Process]

[0183] Hot-rolled steel sheets can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined according to the desired metal structure, etc., and, for example, may be 500 to 800°C. Before or after coiling, the hot-rolled steel sheet may be rewound to apply a predetermined heat treatment. Generally, the coiling process may not be performed, and the cold rolling process described later may be carried out by pickling after the hot rolling process.

[0184] [Cold Rolling Process]

[0185] After performing pickling or the like on a hot-rolled steel sheet, a cold-rolled steel sheet can be obtained by cold-rolling the hot-rolled steel sheet. The reduction ratio of cold rolling can be appropriately determined according to the desired metal structure or sheet thickness, and, for example, may be 20 to 80%. After the cold rolling process, the sheet may be cooled to room temperature, for example by air cooling.

[0186] [Pretreatment Process]

[0187] Next, it is effective to perform a predetermined pretreatment process before annealing the cold-rolled steel sheet. Such pretreatment processes may include degreasing treatment and an optional grinding treatment. The degreasing treatment may include, for example, passing an electric current through the cold-rolled steel sheet in a solution with a pH of 8.0 or higher (electrolytic treatment). The current density during application is 1.0 to 8.0 A / dm² 2 It may be continuous, and the current application time may be 5 to 10 seconds. Meanwhile, for optional grinding treatment, it is preferable to perform it using a heavy grinding brush. By introducing deformation to the surface of the cold-rolled steel sheet through grinding using a heavy grinding brush, the nucleation of the Fe-Al barrier layer is promoted during the plating process after the annealing process, thereby densifying the Fe-Al barrier layer. Consequently, since the growth rate of the Fe-Al barrier layer slows down, the thickness can be reduced. As a result, the amount of Al consumed in the formation of the Fe-Al barrier layer can be reduced. Therefore, since the amount of Al in the plating layer other than the Fe-Al barrier layer can be increased, the ratio of "Al concentration at the center of the plating layer" to "Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet" can be increased in the finally obtained plated steel sheet. The grinding treatment is not particularly limited, but for example, a heavy grinding brush can be used with a grinding amount of 10 to 200 g / m² 2 This can be carried out by grinding the surface of a cold-rolled steel sheet under the conditions. The amount of grinding by the heavy grinding brush can be adjusted by any suitable 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 grinding brushes, the rotational speed, the amount of brush reduction, and the coating liquid used.

[0188] [Annealing Process]

[0189] Annealing is performed on a cold-rolled steel sheet that has undergone a pretreatment process. The holding temperature during the annealing process is preferably 700 to 900°C. If the holding temperature during the annealing process exceeds 900°C, an external oxide layer may form on the surface of the steel sheet, potentially leading to a decrease in plating performance. The rate of heating to the above holding temperature is not particularly limited, but may be 1 to 10°C / second. The holding time at the above holding temperature is preferably 10 to 300 seconds, and more preferably 80 to 120 seconds. If the holding time exceeds 300 seconds, the external oxide may grow excessively, potentially leading to a decrease in plating performance. The dew point of the atmosphere during the annealing process is preferably -20 to 10°C, and more preferably -10 to 5°C. If the dew point is too low, an external oxide layer may form on the surface of the steel sheet, potentially leading to a decrease in plating performance. On the other hand, if the dew point is too high, Fe oxide may similarly form on the surface of the steel sheet as an external oxide, potentially leading to a decrease in plating performance. In addition, the atmosphere in the annealing process may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (for example, 4% hydrogen and nitrogen balance).

[0190] [Plating Process]

[0191] Next, in the plating process, a plating layer having the chemical composition and structure described above is formed on at least one, preferably both, surface of a cold-rolled steel sheet (base steel sheet). More specifically, the plating process is carried out by molten plating using a plating bath whose composition is adjusted so that, for example, the chemical composition of the plating layer is within the range described above. In the plating process, it is extremely important to first control the time from immersing the steel sheet in the plating bath until the start of cooling to 6 seconds or less, and then to 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, thereby reducing the amount of Al consumed in forming the Fe-Al barrier layer, and thus it becomes possible to secure a sufficient amount of Al present in the plating layer other than the Fe-Al barrier layer. As a result, in the finally obtained plated steel sheet, the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel sheet” can be 0.10 or higher. On the other hand, if either of these requirements is not satisfied, that is, if the time from the start of immersion of the steel sheet into the plating 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 / second, a large amount of Al is consumed in the formation of the Fe-Al barrier layer, and the amount of Al in the plating layer other than the Fe-Al barrier layer decreases. As a result, in the finally obtained plated steel sheet, the desired ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of that of the base steel sheet” cannot be obtained.

[0192] From the perspective of further improving the resistance to LME in weld joints, particularly in the weld shoulder, it is desirable that the time from the start of immersion of the steel plate in the plating bath to the start of cooling be shorter, and the average cooling rate from the bath temperature to 370°C be faster. For example, by setting the time from the start of immersion of the steel plate in the plating 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 in the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” to a high value of, for example, 0.20 or more, and in relation to this, by reducing the area ratio of the β-CuZn phase in the plating layer of the weld shoulder to, for example, 30% or less, it is possible to further improve the resistance to LME in the weld shoulder of the weld joint. In the case where grinding treatment is performed using a heavy grinding brush as the pretreatment process described above, even if the time from the start of immersion of the steel plate in the plating 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 / second or more, higher LME resistance can be achieved by reducing the area ratio of the β-CuZn phase in the plating layer of the weld shoulder. Alternatively, by performing a grinding treatment with a heavy grinding brush as a pretreatment process as described above, and by setting the time from the start of immersion of the steel plate in the plating 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 in the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” to a high value of, for example, 0.30 or more, and in relation to this, by reducing the area ratio of the β-CuZn phase in the plating layer of the weld shoulder to, for example, 10% or less, it is possible to more significantly improve the LME resistance in the weld shoulder of the weld joint.The lower limit of the time from the start of immersion of the steel sheet in the plating bath to the start of cooling is not specifically limited, but, for example, the time from the start of immersion of the steel sheet in the plating 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 specifically 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 plating process may be appropriately set considering the thickness of the plating layer and the amount of coating, etc. For example, by immersing a cold-rolled steel sheet in the plating bath, lifting it, immediately spraying N2 gas or air by the gas wiping method, and then cooling it, the amount of coating is set within a predetermined range, for example, 10 to 170 g / m per side. 2 It can be adjusted within the range of.

[0193] Since the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” in the plating steel sheet produced by the present manufacturing method is controlled within the range of 0.10 to 1.50, when used for spot welding, the initial plating layer structure at the weld shoulder on the electrode side is changed, and a plating layer with an area ratio of 50% or less of the β-CuZn phase is formed, and in relation to this, it is possible to significantly suppress or reduce the occurrence of LME cracking at the weld shoulder and its surroundings during spot welding.

[0194] [Other steel plates]

[0195] Among the plurality of steel plates used in the welded joint according to the embodiment of the present invention, any suitable steel plate or plated steel plate may be used as a steel plate other than the plated steel plate described above. Such a steel plate may have a tensile strength of 780 MPa or more, for example, similar to the preferred embodiment of the plated steel plate, or may have a tensile strength of less than 780 MPa. Accordingly, regarding the steel plate other than the plated steel plate described above, a suitable steel plate or plated steel plate may be appropriately selected depending on the application of the welded joint or desired characteristics, for example, the desired joint strength.

[0196] Method for manufacturing welded joints

[0197] A welded joint according to an embodiment of the present invention can be manufactured by applying any suitable spot welding method known to those skilled in the art to a plurality of steel plates in which the above-mentioned plated steel plate is used for all steel plates and the steel plates are overlapped, or to a plurality of steel plates in which the above-mentioned plated steel plate is used for one or more steel plates and the plated steel plate is overlapped with another steel plate or another plated steel plate. For example, a welded joint according to an embodiment of the present invention can be manufactured by forming a nugget and a pressure welded portion around it by applying pressure using a pair of opposing electrodes and passing current between the electrodes under normal conditions while applying pressure to the plurality of steel plates overlapped as above. The conditions for spot welding may be any suitable conditions known to those skilled in the art. For example, the welding electrode may be a dome-shaped welding electrode with a tip diameter of 6 to 8 mm, the applying force may be 1.5 to 6.0 kN, the current application time may be 0.1 to 1.0 s (5 to 50 cycles, power frequency 50 Hz), the current application current may be 4 to 15 kA, and the angle of impact (the angle formed by the axial direction of the electrode and the direction perpendicular to the surface of the steel plate) may be 0 to 10°.

[0198] According to the weld joint manufactured as described above, the effect of adding Al to the plating layer is fully exerted, allowing the area ratio of the β-CuZn phase in the plating layer at the weld shoulder to be controlled within a desired range, and in this regard, it becomes possible to suppress or reduce the occurrence of LME cracking on the electrode-side surface during spot welding. Accordingly, according to this weld joint, it is possible to achieve superior LME resistance compared to the case where a conventional plated steel sheet with a plating layer having the same chemical composition, more specifically a Zn-based plating layer having the same Al content, and particularly in use in the automotive field, it can contribute to industrial development by improving collision safety and extending lifespan.

[0199] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples at all.

[0200] Examples

[0201] In the following examples, plated steel sheets were manufactured under various conditions, and the LME resistance of welded joints manufactured by spot welding the manufactured plated steel sheets was investigated.

[0202] [Manufacture of galvanized steel sheets]

[0203] First, molten steel having a chemical composition in mass % consisting of C: 0.15%, Si: 1.00%, Mn: 2.60%, P: 0.010%, S: 0.0020%, N: 0.0100%, Al: 0.020%, and the remainder: Fe and impurities was cast by a continuous casting method to form a steel billet. The steel billet was cooled once, then reheated to 1200°C for hot rolling, and subsequently coiled at 600°C. Hot rolling was performed by carrying out rough rolling and finish rolling, with the finish rolling end temperature being 950°C and the reduction rate of the finish rolling being 30%. Next, the obtained hot-rolled steel sheet was pickled and subsequently cold-rolled at a reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Next, for the obtained cold-rolled steel sheet, 5.0 A / dm in a solution of pH 9.2 2 Pretreatment is performed by passing current at a current density for 8 seconds, and then, if necessary, a 2.0% aqueous NaOH solution is applied to the cold-rolled steel sheet, and then a heavy grinding brush (Hotani D-100) is used to grind at a rate of 10 to 200 g / m² 2 The surface of a cold-rolled steel sheet was ground with a grinding amount, a brush reduction of 2.0 mm, and a rotational speed of 600 rpm, and deformation was introduced to the surface of the cold-rolled steel sheet. The presence or absence of grinding by a heavy grinding brush for each cold-rolled steel sheet is shown in Table 1.

[0204] Next, each cold-rolled steel sheet was cut to a size of 100 mm × 200 mm, and subsequently subjected to annealing treatment (annealing atmosphere: 4% hydrogen and nitrogen balance) under conditions of a dew point of 0 °C, a holding temperature of 870 °C, and a holding time of 100 seconds. For all steel sheet samples, the heating rate during annealing was set to 5 °C / second. Next, the cut steel sheet samples were subjected to plating treatment using a molten zinc plating bath having a predetermined bath composition under the conditions of the bath temperature, the time from immersion in the plating bath to the start of cooling, and an average cooling rate of 370 °C from the bath temperature as shown in Table 1, thereby obtaining plated steel sheet samples with plating layers formed on both surfaces of the steel sheet samples. The plating thickness was 50 g / m² per side by N2 gas wiping before the start of cooling, after lifting the steel sheet samples following immersion in the plating bath. 2 It was adjusted. In Comparative Example 28, an alloying heat treatment of 520°C × 10 seconds was performed after the hot-dip galvanizing treatment.

[0205] [Tensile strength of galvanized steel sheets]

[0206] Tensile strength was measured by taking a JIS No. 5 specimen from a direction in which the longitudinal direction of the specimen was parallel to the rolling direction of the galvanized steel sheet specimen, and performing a tensile test in accordance with JIS Z 2241:2011. As a result, the tensile strength of all galvanized steel sheet specimens was 780 MPa or higher.

[0207] [Measurement of Al concentration distribution in the plating layer]

[0208] First, a plated steel sheet sample was cut to a size of 50 mm × 50 mm, and then the Al concentration distribution from the surface of the plating layer to a depth of 100 μm was obtained by measuring the cut plated steel sheet sample using GDS. Next, the Al concentration at a depth position where the Fe strength is 50% of the Fe strength of the base steel sheet (Fe strength at a depth of 100 μm from the surface of the plating layer of the sample) was determined by GDS measurement as the “Al concentration at the plating layer position where the Fe concentration is 50% of that of the base steel sheet,” and the distance from this depth position to the surface was defined as the thickness of the plating layer. The Al concentration by GDS at a position halfway the thickness of the plating layer was determined as the “Al concentration at the center of the plating layer,” and finally, the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the plating layer position where the Fe concentration is 50% of that of the base steel sheet” was determined.

[0209] [Manufacturing of welded joints]

[0210] Among the obtained plated steel sheet samples, a plated steel sheet sample with a size of 100×100mm was provided for spot welding. Two sheets were prepared by cutting them to a size of 50mm×100mm, and a welded joint was manufactured by performing spot welding on these two plated steel sheet samples using a dome-shaped welding electrode with a tip diameter of 8mm, with a welding angle of 5°, a pressure of 4.0kN, a current application time of 0.5 seconds, and a current of 11kA.

[0211] [Analysis of Chemical Composition of Plating Layer]

[0212] The chemical composition of the plating layer was determined by immersing a 30 mm × 30 mm sample taken from the outer region of the heat-affected zone of the welded joint in a 10% HCl aqueous solution containing an inhibitor (Evit, manufactured by Asahi Kagaku Kogyo), acid-peeling the plating layer, and then measuring the plating components dissolved in the aqueous solution by ICP emission spectroscopy. The results are shown in Table 1.

[0213] [Measurement of Area Ratio of β-CuZn Phase in the Plating Layer of the Welded Shoulder]

[0214] The area ratio of the β-CuZn phase in the plating layer of the weld shoulder was determined as follows. First, a cross-sectional sample of the spot weld was prepared, and then a BSE phase including the weld shoulder was obtained by SEM-EPMA, and elemental analysis was performed, and the area ratio of the β-CuZn phase in the weld shoulder was measured. Specifically, the β-CuZn phase was defined as a phase in which the Zn concentration was 40 to 60 atomic%, the Cu concentration was 40 to 60 atomic%, the Fe concentration was 0 to 20 atomic%, and other elements were 3 atomic% or less. The field of view of the SEM image was 100㎛ × 100㎛, and the same elemental analysis was performed on five other locations of the weld shoulder, and the average value of the area ratio of the β-CuZn phase obtained in each field of view was determined as the area ratio of the β-CuZn phase in the plating layer of the weld shoulder.

[0215] [Assessment of LME Resistance]

[0216] After cross-sectionally grinding the weld of the manufactured welded joint, it was observed under an optical microscope, the length of the LME cracks that occurred on the cross-section of the weld shoulder was measured, and the LME resistance was evaluated as follows.

[0217] AAA: No LME split,

[0218] AA: LME split length greater than 0㎛ to 100㎛,

[0219] A: LME split length greater than 100㎛ to 500㎛,

[0220] B: LME crack length exceeding 500㎛

[0221] Cases where the LME resistance was evaluated as AAA, AA, and A were evaluated as weld joints capable of suppressing or reducing the occurrence of LME cracking on the electrode-side surface during spot welding. The results are shown in Table 1 below.

[0222]

[0223] Referring to Table 1, in Comparative Example 26, since the total Al content of the plating layer was low, the effect of suppressing LME cracking by adding Al could not be fully exerted. Consequently, due to a large amount of Cu melted from the electrode, the area ratio of the β-CuZn phase in the plating layer at the weld shoulder became extremely high. As a result, LME resistance decreased. In Comparative Example 27, since the total Al content of the plating layer was high, it is thought that the melting point of the plating layer decreased. As a result, Zn in the plating layer became more prone to melting during spot welding, leading to a decrease in LME resistance. In Comparative Example 28, the Fe content in the plating layer increased due to alloying heat treatment, so the desired plating chemical composition was not obtained. Furthermore, the ratio of "Al concentration at the center of the plating layer" to "Al concentration at the plating layer location where the Fe concentration is 50% of the base steel sheet" became less than 0.10. In relation to this, the area ratio of the β-CuZn phase in the plating layer at the weld shoulder increased. In Comparative Example 28, since the desired ratio of “Al concentration at the center of the plating layer” / “Al concentration at the plating layer location where the Fe concentration is 50% of the base steel sheet” was not obtained, it is thought that a high-melting-point Cu-Al metal compound could not be sufficiently formed on the electrode surface by the reaction between Al in the plating layer and Cu in the electrode, and thus the incorporation of Cu into the plating layer was promoted. Furthermore, as a result, the LME resistance was reduced. In Comparative Example 29, since the time from immersion in the plating bath to the start of cooling was long, a large amount of Al was consumed in the formation of the Fe-Al barrier layer, and it is thought that the amount of Al in the plating layer other than the Fe-Al barrier layer was reduced. As a result, the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” is less than 0.10, and for the same reason as in Comparative Example 28, the area ratio of the β-CuZn phase in the plating layer of the weld shoulder is increased, and the LME resistance is reduced.In Comparative Example 30, because the average cooling rate from the bath temperature to 370°C was slow, a large amount of Al was consumed in the formation of the Fe-Al barrier layer, and it is thought that the amount of Al in the plating layer other than the Fe-Al barrier layer decreased. As a result, the ratio of “Al concentration in the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel sheet” became less than 0.10, and for the same reason as in Comparative Examples 28 and 29, the area ratio of the β-CuZn phase in the plating layer of the weld shoulder increased, and the LME resistance decreased.

[0224] In contrast to this, in the weld joints according to all embodiments, by having a predetermined plating chemical composition and controlling the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” when the plating layer on the outer side of the heat-affected zone is measured by GDS to be within the range of 0.10 to 1.50, and also controlling the area ratio of the β-CuZn phase in the plating layer of the weld shoulder to be within the range of 50% or less, the effect of adding Al to the plating layer can be fully exerted and LME cracking can be reliably suppressed or reduced. In particular, in Examples 2, 3, and 5 (no grinding by a heavy grinding brush) in which the time from the start of immersion of the steel plate in the plating 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 (whereby grinding by a heavy grinding brush was performed as a pretreatment for the annealing process, but the time from the start of immersion of the steel plate in the plating 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 plating layer of the weld shoulder was 30% or less, and as a result, the LME resistance evaluation was AA, and the LME resistance was further improved. In addition, in Examples 6 to 13, 15 to 17 and 19 to 25, in which grinding with a heavy grinding brush was performed as a pretreatment for the annealing process, and the time from the start of immersion of the steel plate in the plating 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, the area ratio of the β-CuZn phase in the plating layer of the weld shoulder was 10% or more, and as a result, the LME resistance evaluation was AAA, and the LME resistance was further improved. Explanation of the symbols

[0225] 1: Welded joint 11: Steel plate 12: Nugget 13: Pressure weld 14: Heat-affected zone 15: Spot weld 16: Welding shoulder 17: Plating layer

Claims

Claim 1 A welded joint comprising a plurality of overlapping steel plates, a nugget joining the plurality of steel plates, and a spot weld portion having a pressure weld portion and a heat-affected zone formed around the nugget, wherein one or more of the plurality of steel plates are plated steel plates having a base steel plate and a plating layer formed on a surface of the base steel plate corresponding to at least the outermost surface of the plurality of steel plates, and wherein the plating layer in the region outside the heat-affected zone contains, in mass%, Al: 0.10 to 1.50% and Fe: 0.01 to 2.00%, and also Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0 to 4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: The plating layer in the region outside the heat-affected zone has a chemical composition comprising a total of 5.000% or less of at least one of 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 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500%, and the remainder being Zn and impurities. A welded joint characterized in that, when measured by glow discharge emission analysis (GDS), the ratio of “Al concentration at the center of the plating layer” / “Al concentration at the location of the plating layer where the Fe concentration is 50% of the base steel plate” is 0.10 to 1.50, and in the plating layer of the weld shoulder on the outermost surface of the plurality of steel plates, the area ratio of the β-CuZn phase is 50% or less, and the β-CuZn phase is a phase in which the Zn concentration is 40 to 60 atomic%, the Cu concentration is 40 to 60 atomic%, the Fe concentration is 0 to 20 atomic%, and other elements are 3 atomic% or less. Claim 2 A welded joint according to claim 1, 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. Claim 3 A welded joint according to claim 1, 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. Claim 4 A welded joint characterized in that, in any one of claims 1 to 3, the plating layer is a molten zinc plating (GI) layer. Claim 5 A welded joint characterized in that, in any one of claims 1 to 3, the plated steel sheet has a tensile strength of 780 MPa or more.

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