Welded joint

Abstract

Provided is a welded joint characterized by comprising: a plurality of overlapped steel sheets; a spot welding part having a nugget, a pressure contact part formed around the nugget, and a heat-affected part; and a separation part, at least one of the steel sheets being an Al-containing plated steel sheet provided with a base material steel sheet and a plating layer formed on at least the surface, from among the surfaces of the base material steel sheet, that corresponds to an overlap surface where a plurality of steel sheets overlap, the plating layer of the Al-containing plated steel sheet containing Zn and / or the steel sheet adjacent to the Al-containing plated steel sheet having Zn-containing plating on the surface thereof that corresponds to the overlap surface, the plating layer in the separation part on the outside of the heat-affected part having a prescribed chemical composition, the thickness of an Fe-Al phase being 10-200 μm in the plating layer of the separation part in a region 1 mm from the end section of the pressure contact part, and the area ratio of a Zn-rich phase being 0-20%.

Description

Welded joints

[0001] The present invention relates to a welded joint, and more particularly to a welded joint obtained by spot welding an Al-containing plated steel sheet.

[0002] In recent years, plated steel sheets with improved spot weldability have been proposed.

[0003] For example, Patent Document 1 discloses a steel sheet comprising: a substrate steel sheet; a first alloy plating layer having a thickness of 3 to 30 μm and having a composition containing, in mass %, 40 to 70% Fe, 0.3 to 10% Mn, and the balance being Al and unavoidable impurities, on at least one surface of the substrate steel sheet; a second alloy plating layer having a thickness of 0.10 to 10 μm and having a composition containing, in mass %, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the first alloy plating layer; and a second alloy plating layer having a thickness of 0.10 to 10 μm and having a composition containing, in mass %, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the surface of the second alloy plating layer, in a coating amount of 0 to 1000 mg / m 2 Patent Document 1 also describes a method for forming an Al-Fe-Mn alloy plating layer having two layers, each with a different Mn content, on the surface of a substrate steel sheet by hot-dip plating an Al-Mn alloy on the substrate steel sheet, and a method for forming an Al-Fe-Mn alloy plating layer having two layers, each with a different Mn content, on the surface of the substrate steel sheet, and a method for forming an Al-Fe-Mn alloy plating layer having a coating weight of 1000 mg / m 2 It is taught that by limiting the content to the following ranges, it is possible to achieve both corrosion resistance after painting and resistance spot weldability in an environment similar to the corrosive environment of an automobile exterior panel.

[0004] Japanese Patent Application Laid-Open No. 2020-122205

[0005] Spot welding is primarily used in processes such as the assembly of automobile bodies and the installation of parts. Liquid metal embrittlement (LME) cracking must be suppressed, particularly in spot welding between zinc-containing coated steel sheets or between zinc-containing coated steel sheets and uncoated steel sheets. This phenomenon occurs when zinc, which has been converted into a liquid phase by welding heat input, penetrates into the steel sheet along grain boundaries, embrittling the steel sheet, and then tensile stress generated by welding acts on the embrittled area. If such LME cracking occurs during spot welding, the strength of the welded joint cannot be ensured, which may hinder the use of zinc-containing coated steel sheets.

[0006] Therefore, an object of the present invention is to provide a welded joint that can suppress or reduce the occurrence of LME cracking during spot welding using a novel configuration.

[0007] The present inventors conducted research, focusing particularly on the plating layer of a plated steel sheet used for a welded joint, in order to suppress or reduce the occurrence of LME cracking when manufacturing a welded joint by spot welding. As a result, the present inventors discovered that by optimizing the chemical composition of an Al-containing plating layer and using an Al-containing plated steel sheet in which the morphology of the interface between the Al-containing plating layer and a base steel sheet is appropriately controlled, it is possible to improve the plating layer structure in a portion where LME cracking is likely to occur during spot welding, and in connection with this, it is possible to significantly improve the LME resistance of a welded joint, and thus completed the present invention.

[0008] The present invention, which has achieved the above object, is as follows. (1) A welded joint comprising: a nugget joining the plurality of steel sheets; a spot weld having a pressure-welded portion and a heat-affected portion formed around the nugget; and a separation portion located around the pressure-welded portion, wherein at least one of the plurality of steel sheets is an Al-containing plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface of the base steel sheet corresponding to an overlapping surface of the plurality of steel sheets, the plating layer of the Al-containing plated steel sheet contains Zn, and / or a steel sheet adjacent to the Al-containing plated steel sheet has a Zn-containing plating on a surface corresponding to the overlapping surface, and the plating layer in the separation portion outside the heat-affected zone contains, in mass %, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, Zn: 0 to 30.0%, and further contains Ni: 0 to 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%, 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%, Sr:0~0.500%, In:0~0.500%, (2) A welded joint according to (1), characterized in that the plating layer in the separation portion in a region 1 mm from the end of the pressure-welded portion has an Fe-Al phase thickness of 10 to 200 μm and an area ratio of the Zn-rich phase of 0 to 20%.(3) The welded joint according to (2) above, characterized in that the area fraction of the Zn-rich phase is 0 to 5%. (4) The welded joint according to any one of (1) to (3) above, characterized in that the thickness of the Fe—Al phase is 16 to 200 μm. (5) The welded joint according to any one of (1) to (4) above, characterized in that the chemical composition contains, in mass%, Mg: 0.1 to 10.0%. (6) The welded joint according to any one of (1) to (5) above, characterized in that the chemical composition contains, in mass%, Mg: 0.3 to 10.0% and Si: 0 to 1.0%. (7) The welded joint according to any one of (1) to (6) above, characterized in that the chemical composition contains, in mass %, 0.3 to 10.0% Mg, the plating layer in the separation part outside the heat-affected zone further contains an Mg-containing phase, and in a cross section of the plating layer in the separation part outside the heat-affected zone, the surface coverage of the Mg-containing phase is 20 to 100%. (8) The welded joint according to (7) above, characterized in that the surface coverage of the Mg-containing phase is 60 to 100%. (9) The welded joint according to any one of (1) to (8) above, characterized in that in a cross section of the plating layer in the separation part outside the heat-affected zone, the interface length L between the plating layer and the base steel sheet and the surface length L0 of the base steel sheet satisfy (L - L0) / L0 × 100 ≧ 3.

[0009] 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.

[0010] 1 is a schematic cross-sectional view of a welded joint according to an embodiment of the present invention, in which (a) is an overall view of the welded joint, and (b) is an enlarged view of the end of the pressure-welded portion and the separation portion immediately outside thereof. 2 is a schematic cross-sectional view of an Al-containing plated steel sheet useful for use in a welded joint according to an embodiment of the present invention, showing the interface length L between the plated layer and the base steel sheet and the surface length L0 of the base steel sheet. 3 is a schematic cross-sectional view of an Al-containing plated steel sheet according to a preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase.

[0011] <Welded Joint> A welded joint according to an embodiment of the present invention is a welded joint comprising: a plurality of overlapping steel sheets; a nugget joining the plurality of steel sheets; and a spot weld having a pressure-welded portion and a heat-affected zone formed around the nugget; and a separation portion located around the pressure-welded portion, wherein at least one of the plurality of steel sheets is an Al-containing plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface of the base steel sheet corresponding to an overlapping surface of the plurality of steel sheets, and wherein the plating layer of the Al-containing plated steel sheet contains Zn and / or a steel sheet adjacent to the Al-containing plated steel sheet has a Zn-containing plating on a surface corresponding to the overlapping surface, i.e., the plating layer of the Al-containing plated steel sheet contains Zn, or the steel sheet adjacent to the Al-containing plated steel sheet has a Zn-containing plating on a surface corresponding to the overlapping surface, or the plating layer of the Al-containing plated steel sheet contains Zn and the steel sheet adjacent to the Al-containing plated steel sheet has a Zn-containing plating on a surface corresponding to the overlapping surface, The plating layer in the separation portion outside the heat-affected zone contains, in mass %, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, Zn: 0 to 30.0%, and further contains 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%, It has a chemical composition containing at least one of the following in a total amount of 5.000% or less: 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%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0 to 0.500%, with the balance being 20.0% or more of Al and impurities;In the plating layer of the separation portion in a region 1 mm from the end of the pressure-welded portion, the thickness of the Fe-Al phase is 10 to 200 μm, and the area ratio of the Zn-rich phase is 0 to 20%.

[0012] As mentioned above, LME cracking must be suppressed when spot welding zinc-containing coated steel sheets together or when spot welding zinc-containing coated steel sheets to uncoated steel sheets. For example, when spot welding two or more steel sheets, including at least one zinc-containing coated steel sheet, to form a joint, LME cracking may occur inside a pressure-welded joint (corona bond) formed on the outside of the weld metal (nugget) or just outside the pressure-welded joint, e.g., in a separation zone (a region where the steel sheets are not joined) located around the pressure-welded joint. LME cracking occurs when zinc, which has been converted into a liquid phase by welding heat input during spot welding, penetrates into the steel sheet along grain boundaries, embrittling the steel sheet. This embrittlement is then triggered by tensile stresses generated by welding, such as the pressure applied by the electrode, expansion and contraction of the weld, and springback upon electrode release. Therefore, the present inventors focused on the coating layer of coated steel sheets, and conducted research, particularly from the perspective of optimizing the chemical composition and morphology of the coating layer, in order to suppress or reduce zinc penetration into the steel sheet.

[0013] First, the present inventors discovered that optimizing the chemical composition of a plating layer made of an Al-containing plating and, in connection with this, controlling the interface shape between the plating layer and a base steel sheet to have a shape with greater irregularities are effective in suppressing or reducing the penetration of Zn into the steel sheet during spot welding. Generally, when a Zn-containing plated steel sheet is spot welded, a Zn-rich phase, more specifically, a liquid phase containing 60 mass% or more of Zn, may be formed due to the thermal effects of spot welding or the like. The penetration of Zn in this liquid phase into the steel sheet along grain boundaries causes LME cracking. In response to this, the present inventors discovered that by spot welding an Al-containing plated steel sheet having a plating layer made of an Al-containing plating with an optimized chemical composition and in which the interface shape between the plating layer and the base steel sheet is controlled to have a shape with greater irregularities, the occurrence of LME cracking around the pressure weld can be significantly suppressed or reduced, even when the Al-containing plated steel sheet is spot welded to another Zn-containing plated steel sheet. More specifically, the inventors have found that by using such an Al-containing plated steel sheet in spot welding, the plating structure in the separation area immediately outside the pressure weld can be changed due to the thermal effect during the spot welding, and the formation of a Zn-rich phase can be suppressed or reduced to an area ratio of 0 to 20%, thereby significantly suppressing or reducing the occurrence of LME cracking around the pressure weld. This will be explained in more detail below with reference to the drawings.

[0014] Fig. 1 is a diagram schematically illustrating 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 the end of a pressure-welded portion and a separation portion immediately outside the end. Referring to Fig. 1(a), a welded joint 10 according to an embodiment of the present invention includes two overlapping steel sheets 11, a nugget 12 joining the steel sheets 11, a spot weld 16 having a pressure-welded portion 13 and a heat-affected zone 15 formed around the nugget 12, and a separation portion 17 located around the pressure-welded portion 13. In Fig. 1, two different plated steel sheets are used as the two steel sheets 11. Specifically, one of the plated steel sheets is an Al-containing plated steel sheet having a plated layer made of an Al-containing plating on both sides of the base steel sheet, with the interface between the plated layer and the base steel sheet controlled to have a shape with greater irregularities, and the other plated steel sheet is a conventional galvannealed (GA) steel sheet. In this regard, referring to FIG. 1( b), a plating layer 18 derived from the plating layer on the steel sheet 11 before spot welding is formed in a separation portion 17 immediately outside the pressure weld 13 (i.e., in the separation portion 17 in the region 1 mm from the pressure weld end 14). According to an embodiment of the present invention, in the separation portion 17 in the heat-affected zone 15 located immediately outside the pressure weld 13, the structure of the initial plating layer changes due to the thermal influence during spot welding, and a plating layer 18 having a thickness of 10 to 200 μm is formed, which contains an Fe—Al phase but has a relatively low proportion of Zn-rich phase, more specifically, a plating layer 18 having an area ratio of 0 to 20% Zn-rich phase. It is believed that such structural changes in the plating layer are significantly influenced by the chemical composition and morphology of the initial plating layer before spot welding. In this regard, according to an embodiment of the present invention, an Al-containing plated steel sheet is used which includes a plating layer made of an Al-containing plating having an optimized chemical composition, and in which the interface shape between the plating layer and the base steel sheet is controlled to have a shape with larger irregularities. As a result of this interface shape in particular, a plating layer 18 is formed in which the proportion of Zn-rich phase is relatively low in the separation portion 17 immediately outside the pressure-welded portion 13, i.e., the area ratio of the Zn-rich phase is 0 to 20%, compared to the case of conventional spot welding of Zn-based plated steel sheets.As a result, penetration of molten Zn into the steel plate during spot welding is suppressed or reduced, and the LME resistance of the finally obtained welded joint 10 can be significantly improved.

[0015] 1 illustrates a welded joint 10 in which only two steel sheets 11 are overlapped and both steel sheets 11 are plated steel sheets (one of which is an Al-containing plated steel sheet and the other is a conventional GA steel sheet) for ease of understanding. 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 10 having a plated layer 18 in which a separation portion 17 in a heat-affected zone 15 contains an Fe—Al phase of a predetermined thickness and the area ratio of the Zn-rich phase is limited within a range of 0 to 20%. For example, in a welded joint 10 in which two steel sheets 11 are spot-welded as shown in FIG. 1 , only one of the steel sheets 11 may be a plated steel sheet, more specifically, an Al—Zn-containing plated steel sheet in which the plated layer contains Zn in addition to Al and the interface between the plated layer and the base steel sheet has an uneven shape. In this case, it is sufficient that the plating layer 18 is present in the separation portion 17 immediately outside the pressure-welded portion 13 on at least one of the two surfaces of the Al-Zn-containing plated steel sheet corresponding to the overlapping surface. Naturally, the plating layer 18 and / or an initial plating layer related to the plating layer 18 may be present on both surfaces. For example, if an initial plating layer related to the plating layer 18 is present on the surface opposite the overlapping surface of the steel sheet, i.e., the surface on the electrode side, the initial plating layer on the electrode side also undergoes a structural change during spot welding in the region within the heat-affected zone (e.g., the contact area between the electrode and the steel sheet and its periphery), resulting in the formation of a plating layer 18 with a relatively low area ratio of Zn-rich phase. This further improves LME resistance compared to when the plating layer 18 is simply present on the surface corresponding to the overlapping surface of the steel sheet. Naturally, both of the two steel sheets 11 may be Al-Zn-containing plated steel sheets whose plating layers contain Zn in addition to Al and whose interfaces between the plating layers and the base steel sheet have an uneven shape.

[0016] Similarly, in the case of a welded joint 10 made of three or more steel sheets, various welded joints 10 can be included, each having a plating layer 18 containing an Fe—Al phase of a predetermined thickness in a separation portion 17 within a heat-affected zone 15, and in which the area ratio of the Zn-rich phase is limited within a range of 0 to 20%. For example, in a welded joint 10 formed by spot welding three steel sheets 11, the plating layer 18 may be present in the separation portion 17 immediately outside the pressure-welded portion 13 on at least the surface corresponding to the overlapping surface of one or more of the three steel sheets 11. For example, if the middle steel sheet 11 of the three overlapping steel sheets 11 is an Al-containing plated steel sheet in which the interface between the plated layer and the base steel sheet has an uneven shape, the plating layer 18 may be present only in the separation portion 17 immediately outside the pressure-welded portion 13 on one of the two surfaces of the Al-containing plated steel sheet, or the plating layer 18 may be present in the separation portion 17 immediately outside the pressure-welded portion 13 on both surfaces. For example, the plating layer 18 may be present only in the separation portions 17 immediately outside the pressure-welded portions 13 of some of the steel sheets 11, while another Zn-based plating layer may be present in the separation portions 17 immediately outside the pressure-welded portions 13 of the other steel sheets 11. Such an embodiment is also encompassed by the present invention. In this case, compared to a welded joint 10 in which the plating layer 18 is present in the separation portions 17 immediately outside the pressure-welded portions 13 of all of the steel sheets 11, the risk of LME cracking is increased on the surface where the other Zn-based plating layer is present, and therefore the joint strength may be somewhat reduced. However, the specific number and arrangement of the steel sheets 11 in the welded joint 10, each having a plating layer 18 that contains an Fe—Al phase of a predetermined thickness and in which the area fraction of the Zn-rich phase is limited to a range of 0 to 20%, may be determined appropriately in consideration of the desired joint strength, etc.

[0017] Each component of the welded joint according to an embodiment of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, the term "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0018] [Al-containing plated steel sheet] [Plating layer in separation area outside heat-affected zone: (L - L0) / L0 × 100 ≧ 3] In a welded joint according to an embodiment of the present invention, at least one of a plurality of overlapping steel sheets is an Al-containing plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface of the base steel sheet corresponding to the overlapping surfaces of the plurality of steel sheets. Fig. 2 is a cross-sectional schematic diagram of an Al-containing plated steel sheet useful for a welded joint according to an embodiment of the present invention, showing the interface length L between the plating layer and the base steel sheet and the surface length L0 of the base steel sheet. Referring to Fig. 2, the Al-containing plated steel sheet 1 comprises a base steel sheet 2 and a plating layer 3 formed on the surface of the base steel sheet 2, and the plating layer 3 contains an Fe-Al phase 4. 2, the interface length L between the coating layer 3 and the base steel sheet 2 and the corresponding length L0 of the surface of the base steel sheet 2 satisfy the relationship (L-L0) / L0×100≧3, that is, the interface length L is 3% or more longer than the length L0 of the surface of the base steel sheet 2. It can therefore be seen that the interface between the coating layer 3 and the base steel sheet 2 is controlled to have a shape with greater irregularities.

[0019] While not intending to be bound by any particular theory, it is believed that when the interface between the coating layer 3 and the base steel sheet 2 has a shape with greater irregularities as shown in Fig. 2, i.e., when the interface length L between the coating layer 3 and the base steel sheet 2 and the surface length L0 of the base steel sheet 2 satisfy the relationship (L - L0) / L0 × 100 ≥ 3, the alloying rate of the coating layer 3 or the alloying rate between the coating layer 3 and a coating layer on another coated steel sheet during spot welding is increased. More specifically, firstly, as the irregularities at the interface between the coating layer 3 and the base steel sheet 2 become greater, the surface area of ​​the interface where alloying progresses during spot welding increases. It is therefore believed that the alloying rate can be increased for the coating layer as a whole.

[0020] Second, the state in which the interface between the coating layer 3 and the base steel sheet 2 has greater irregularities is thought to mean that, during the production of the Al-containing plated steel sheet 1, the coating layer 3 was solidified at a stage before alloying of the coating layer 3 had progressed sufficiently. To explain in more detail, in the production of the Al-containing plated steel sheet 1, Fe diffuses from the base steel sheet 2 to the coating layer 3, and alloying of the coating layer 3 progresses from the interface. However, by solidifying the coating layer 3 in a state before the coating layer 3 is sufficiently or completely alloyed, i.e., in a state in which parts of the coating layer 3 where alloying has progressed and parts of the coating layer 3 where alloying has not progressed sufficiently are mixed, it is possible to create an interface shape with greater irregularities between the coating layer 3 and the base steel sheet 2. For this reason, it is thought that the Al-containing plated steel sheet 1 having the interface irregularities shown in Fig. 2 potentially has many parts that can be further alloyed. Therefore, based on this high potential alloying ability and the large surface area resulting from the uneven shape described above, it is thought that it will be possible to significantly increase the alloying rate of the plating layer 3 at high temperatures during spot welding, or the alloying rate between the plating layer 3 and the plating layer on the other plated steel sheet.

[0021] In this case, even if the other plated steel sheet to be spot-welded is a plated steel sheet containing a relatively large amount of Zn, such as a GA steel sheet, it is believed that the Zn component in the plated layer can be sufficiently incorporated into the Fe-Al phase 4 and formed into a solid solution by alloying with the plated layer 3 during spot welding. As a result, it is possible to reliably control the area ratio of the Zn-rich phase in the plated layer 18 in the separation region 17 immediately outside the pressure welded portion 13 of the final welded joint 10 (i.e., the separation region 17 in the region 1 mm from the pressure welded portion end 14) to within the range of 0 to 20%. This also makes it possible to significantly suppress or reduce the occurrence of LME cracking around the pressure welded portion 13. In particular, the inventors have now discovered for the first time that by spot welding an Al-containing plated steel sheet 1 in which the interface length L between the plated layer 3 and the base steel sheet 2 and the surface length L of the base steel sheet 2 satisfy the relationship (L - L) / L × 100 ≥ 3, the area fraction of the Zn-rich phase in the plated layer 18 in the separation zone 17 immediately outside the pressure-welded portion 13 in the heat-affected zone 15 can be controlled within a range of 0 to 20%, thereby significantly improving the LME resistance of the welded joint 10. Therefore, the welded joint 10 according to the embodiment of the present invention is particularly useful in the automotive field, where spot welding is relatively common. Additionally, the objective of suppressing or reducing the occurrence of LME cracking during spot welding in the welded joint 10 according to the embodiment of the present invention can be achieved primarily by controlling the area fraction of the Zn-rich phase in the plated layer 18 in the separation zone 17 immediately outside the pressure-welded portion 13 within a range of 0 to 20%. Therefore, the feature of (L - L) / L × 100 ≥ 3 in the Al-containing plated steel sheet 1 used in the welded joint 10 is not an essential technical feature for achieving the object of the present invention, but is merely one of preferred means for reliably controlling the area ratio of the Zn-rich phase in the plating layer 18 in the separation portion 17 immediately outside the pressure-welded portion 13 to within a desired range.

[0022] From the viewpoint of further reducing the area ratio of the Zn-rich phase in the coating layer 18 in the separation portion 17 immediately outside the pressure-welded portion 13 and further improving the LME resistance of the welded joint 10, it is preferable to control the shape of the interface between the coating layer 3 and the base steel sheet 2 to have greater irregularities, that is, to set a larger value for (L - L0) / L0 x 100. More specifically, the value of (L - L0) / L0 x 100 is preferably 4 or greater, and may be, for example, 5 or greater, 6 or greater, 7 or greater, or 8 or greater. There is no particular upper limit, but the value of (L - L0) / L0 x 100 may be, for example, 30 or less, 20 or less, 15 or less, 12 or less, or 10 or less.

[0023] The present inventors have also investigated the manufacture of an Al-containing plated steel sheet 1 useful for use in a welded joint 10 according to an embodiment of the present invention, more specifically, an Al-containing plated steel sheet 1 having an interface shape with greater irregularities between the plated layer 3 and the base steel sheet 2, i.e., an interface shape in which the interface length L between the plated layer 3 and the base steel sheet 2 and the surface length L0 of the base steel sheet 2 satisfy the relationship (L - L0) / L0 × 100 ≥ 3. As a result, the present inventors have found that increasing the alloying rate during the alloying treatment of the plated layer 3 is effective in creating such an interface shape itself. More specifically, since excessive Si and Mg content in the plated layer 3 can adversely affect the alloying of the plated layer 3, the Si and Mg contents in the plated layer 3 must each be controlled to 10.0 mass% or less. Additionally, in order to increase the alloying rate of the plated layer 3, the metallographic structure of the base steel sheet 2 must be appropriately controlled during the alloying treatment. More specifically, by making the base steel sheet 2 moderately decarburized and have a metallographic structure containing a larger amount of austenite phase during the alloying treatment, the reaction between the coating layer 3 and the austenite phase in the base steel sheet 2 is promoted during the alloying treatment, that is, the alloying rate can be significantly increased. As will be described in detail later in relation to the manufacturing method of the Al-containing coated steel sheet 1, the inventors have found that it is possible to create a metallographic structure of the base steel sheet 2 that is moderately decarburized and contains a larger amount of austenite phase by appropriately controlling the annealing step, cooling step, and coating step of the base steel sheet 2. As a result, it is possible to realize an interface shape with greater irregularities, in which the interface length L between the coating layer 3 and the base steel sheet 2 and the corresponding surface length L0 of the base steel sheet 2 satisfy the relationship (L - L0) / L0 × 100 ≧ 3.

[0024] The plating layer of the above-mentioned Al-containing plated steel sheet may or may not contain Zn. However, when the plating layer of the Al-containing plated steel sheet does not contain Zn, the steel sheet adjacent to the Al-containing plated steel sheet must have at least a Zn-containing plating on the surface corresponding to the overlapping surface with the plated steel sheet. Here, the plating layer of the Al-containing plated steel sheet has the following chemical composition in the separation zone outside the heat-affected zone (i.e., non-heat-affected zone), which is the same as the initial chemical composition before spot welding.

[0025] [Chemical Composition of the Plated Layer in the Separation Zone Outside the Heat-Affected Zone (Non-Heat-Affected Zone)] [Fe: 20.0 to 55.0%] When an Al-containing plated steel sheet is alloyed, Fe from the base steel sheet diffuses into the plated layer and alloys with Al, etc., so the plated layer inevitably contains Fe. To ensure corrosion resistance after painting, the Al-containing plated steel sheet must be appropriately alloyed, and therefore the Fe content is set to 20.0% or more. The Fe content may be 25.0% or more, 30.0% or more, 35.0% or more, or 40.0% or more. On the other hand, if the Fe content is too high, excessive alloying of the plated layer may result in reduced cold workability. Therefore, the Fe content is set to 55.0% or less. The Fe content may be 52.0% or less, 50.0% or less, 48.0% or less, or 45.0% or less.

[0026] [Mg: 0 to 10.0%] Mg is an element effective in improving the corrosion resistance of the coating layer, particularly its chemical conversion treatability. While the Mg content may be 0%, to achieve this effect, the Mg content is preferably 0.1% or more. The Mg content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, excessive Mg content may slow the alloying rate during alloying of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Mg content is set to 10.0% or less. The Mg content may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, less than 2.5%, 2.4% or less, or 2.2% or less.

[0027] [Si: 0 to 10.0%] Si is an element effective in improving the adhesion of the coating layer. The Si content may be 0%, but to fully obtain this effect, the Si content is preferably 0.1% or more. The Si content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.6% or more, or 0.8% or more. On the other hand, excessive Si content may slow the alloying rate during alloying treatment of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Si content is set to 10.0% or less. The Si content may be 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less. Further reducing the Si content can significantly suppress or reduce the formation of Fe-Al-based intermetallic compounds containing relatively large amounts of Si, more specifically, Fe-Al-Si phases containing 3% or more by mass of Si. If a relatively large amount of the Fe—Al—Si phase is present, bimetallic corrosion (galvanic corrosion) may occur between the Fe—Al phase (content of elements other than Fe, Al, and Zn is less than 3%). Therefore, from the viewpoint of further improving corrosion resistance, the Si content is preferably 1.0% or less.

[0028] [Zn: 0 to 30.0%] Zn has a sacrificial corrosion protection effect and is effective in improving the corrosion resistance of the coating layer. The Zn content may be 0%, but to fully obtain this effect, the Zn content is preferably 1.0% or more. The Zn content may be 3.0% or more, 5.0% or more, 10.0% or more, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, excessive Zn content may cause significant Zn melting during welding of Al-containing coated steel sheets, and the molten Zn may penetrate into the steel, causing liquid metal embrittlement (LME) cracking. Therefore, the Zn content is preferably 30.0% or less. The Zn content may be 28.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

[0029] Furthermore, the plating layer may optionally contain 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 At least one of the following may be contained: Cr: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, Sr: 0-0.500%, In: 0-0.500%, Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, W: 0-0.500%, and V: 0-0.500%. These optional elements are not particularly limited, but preferably their total content is 5.000% or less. The optional elements may have a total content 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, 1.000% or less, 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less. These optional elements are described in more detail below.

[0030] [Ni: 0 to 1.000%] Ni is an element effective in improving the corrosion resistance of the plating 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 0.003% or more, 0.005% or more, 0.008% or more, 0.010% or more, or 0.020% or more. While there are no particular upper limits, from the viewpoint of production costs, etc., the Ni content may be 1.000% or less, and may be, for example, 0.500% or less, 0.400% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0031] [Ca: 0 to 4.000%] Ca is an element effective in ensuring the wettability of the plating bath. The Ca content may be 0%, but to achieve this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the plating layer, making the plating 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, 1.000% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0032] [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%, Sr: 0-0.500%, In: 0-0.500%, Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, W: 0-0.500% and V: 0-0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, W and V may not be contained in the plating layer, but may be present in the plating layer in an amount of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements do not adversely affect the performance of the Al-containing plated steel sheet as long as their content is within a predetermined range. However, excessive content of each element may reduce corrosion resistance. Therefore, the contents of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V are preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% 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, 0.100% or less, 0.050% or less, or 0.020% or less.

[0033] The remainder of the plating layer other than the above elements consists of 20.0% or more of Al and impurities. The Al content may be, for example, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. Similarly, the Al content may be, for example, 80.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, or 60.0% or less. Impurities in the plating layer refer to components that are mixed in due to various factors in the manufacturing process, including raw materials, when producing the plating layer.

[0034] [Measurement of Chemical Composition of Plated Layer] The chemical composition of the plated layer is determined as follows. First, an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet is used to peel and dissolve the plated layer from the Al-containing plated steel sheet in the separation zone outside the heat-affected zone (i.e., non-heat-affected zone) of the spot weld. Next, the obtained acid solution is measured by ICP (inductively coupled plasma) emission spectroscopy to determine the chemical composition (average composition) of the plated layer. The acid species is not particularly limited, and may be any acid that can dissolve the plated layer. Note that the chemical composition of the plated layer in this embodiment is the average of measurements taken on three samples.

[0035] [Surface Coverage of Mg-Containing Phase in Separation Zone of Non-Heat-Affected Zone: 20-100%] According to a preferred embodiment of the present invention, the Mg content of the plating layer in the separation zone outside the heat-affected zone (non-heat-affected zone) is 0.3-10.0%. In this regard, the plating layer in the separation zone of the non-heat-affected zone further contains a Mg-containing phase, and the surface coverage of the Mg-containing phase in the cross section of the plating layer is controlled to 20-100%. In this invention, the Mg-containing phase refers to a phase having a chemical composition, by mass, consisting of 0.5-90% Mg, 10-99.5% Al, 0-70% O, 0-3% Fe, and less than 3% other elements. As is clear from this chemical composition, the Mg-containing phase does not include an MgZn phase.

[0036] Fig. 3 is a cross-sectional schematic view of an Al-containing plated steel sheet according to a preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase. Referring to Fig. 3, the Al-containing plated steel sheet 1, like the case of Fig. 2, comprises a base steel sheet 2 and a plating layer 3 formed on the surface of the base steel sheet 2, and the plating layer 3 contains an Fe-Al phase 4. In Fig. 3, the plating layer 3 further contains an Mg-containing phase 5 in its surface portion. Here, the length M of each Mg-containing phase 5 is i The sum of ΣM i (In Figure 3, ΣM i = M1 + M2 + M3), and the surface length L0 of the base steel plate 2 is ΣM iIt can be seen that the relationship / L0 × 100≧20 is satisfied, i.e., the surface coverage of the Mg-containing phase is 20% or more. By controlling the surface coverage of the Mg-containing phase to 20% or more and allowing a relatively large amount of Mg to be present on the surface of the coating layer, the reaction can be promoted by the action of Mg during chemical conversion treatment, thereby improving the adhesion of the chemical conversion coating to the Al-containing plated steel sheet. From the viewpoint of further enhancing the effect of improving chemical conversion treatability, a higher surface coverage of the Mg-containing phase is preferable, and may be, for example, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. There is no particular upper limit, and the surface coverage of the Mg-containing phase may be 100%. For example, the surface coverage of the Mg-containing phase may be 95% or less or 90% or less. To increase the surface coverage of the Mg-containing phase, it is preferable to increase the Mg content in the coating layer. More specifically, the Mg content in the coating layer is preferably 0.3% or more, and more preferably 0.6% or more. However, since the surface coverage of the Mg-containing phase does not depend only on the Mg content, the Mg content in the plating layer may be appropriately determined according to the desired surface coverage, taking into consideration the manufacturing conditions, etc.

[0037] [Fe—Al Phase Thickness in the Separation Zone of the Heat-Affected Zone: 10 to 200 μm] In the welded joint according to an embodiment of the present invention, the plating layer contains an Fe—Al phase in the separation zone in the region (heat-affected zone) 1 mm from the end of the pressure-welded joint, and the thickness of the Fe—Al phase is 10 to 200 μm. In the present invention, the Fe—Al phase refers to a phase having a chemical composition, by mass, of 40 to 70% Fe, 30 to 60% Al, 0 to 20% Zn, and less than 3% other elements (i.e., Fe, Al, and Zn: total greater than 97%). The Fe—Al phase contained in the plating layer 18 shown in FIG. 1 originates from the Fe—Al phase 4 in the aluminum-containing plated steel sheet 1 before spot welding shown in FIG. 2. By controlling the Fe content in the plating layer before spot welding, i.e., the Fe content in the plating layer in the non-heat-affected zone, to 20.0 mass% or more as described above, and controlling the thickness of the Fe—Al phase contained in the plating layer in the separation zone of the heat-affected zone after spot welding to 10 μm or more, the plating layer can be sufficiently alloyed, thereby improving corrosion resistance after painting. From the viewpoint of further improving corrosion resistance after painting, the thickness of the Fe—Al phase is preferably as thick as possible, for example, 12 μm or more, 14 μm or more, 16 μm or more, or 18 μm or more. On the other hand, even if the Fe—Al phase is excessively thick, the effect of improving corrosion resistance after painting saturates. Therefore, the thickness of the Fe—Al phase is set to 200 μm or less, and may be, for example, 150 μm or less, 100 μm or less, 80 μm or less, 60 μm or less, 40 μm or less, or 20 μm or less.

[0038] [Area Fraction of Zn-Rich Phase in Separation Zone of Heat-Affected Zone: 0 to 20%] In the welded joint according to an embodiment of the present invention, the area fraction of the Zn-rich phase in the plating layer in the separation zone in a region 1 mm from the end of the pressure-welded joint is controlled to 0 to 20%. In the present invention, the Zn-rich phase refers to a phase containing 60 mass% or more of Zn, and this Zn-rich phase becomes liquid at high temperatures during welding. As described above with reference to Figure 1, by suppressing or reducing the formation of the Zn-rich phase to an area fraction of 0 to 20% in the separation zone in a region 1 mm from the end of the pressure-welded joint (i.e., the separation zone in the heat-affected zone located immediately outside the pressure-welded joint), it is possible to significantly suppress or reduce the occurrence of LME cracking around the pressure-welded joint. From the viewpoint of further enhancing the effect of suppressing the occurrence of LME cracking, the smaller the area ratio of the Zn-rich phase in the plating layer of the separation portion, the more preferable, and may be, for example, 18% or less, 15% or less, 12% or less, 10% or less, 8% or less, 5% or less, or 2% or less. Similarly, the area ratio of the Zn-rich phase may be 0.5% or more, or 1% or more.

[0039] [Analysis of the Plating Layer in the Separation Zone of the Non-Heat-Affected Zone] Analysis of the plating layer in the separation zone outside the heat-affected zone (non-heat-affected zone) is performed as follows. First, a 15 mm x 20 mm sample is taken from the surface of the Al-containing plated steel sheet in the non-heat-affected zone so that the cross section of the plating layer can be observed. The sample is embedded in resin and then polished. Next, a backscattered electron image (BSE image) is obtained from the obtained mirror-polished sample using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA) in a field of view of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction. The plating layer in the separation zone of the non-heat-affected zone is identified from the BSE image. Next, the composition of each phase in the identified plating layer is analyzed by point analysis. From the obtained composition, the Mg-containing phase (Mg: 0.5 to 90%, Al: 10 to 99.5%, O: 0 to 70%, Fe: 0 to 3%, and other elements: less than 3%) is identified. The specific measurement conditions for the EPMA in the above field of view were as follows: Apparatus: JXA-8500 manufactured by JEOL Ltd. Acceleration voltage: 15 kV Probe current: 5×10 -7A Irradiation time: 50ms

[0040] (L - L0) / L0 x 100 is determined as follows. First, the mirror-polished sample obtained above is observed with an SEM in a field of view of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction to obtain a BSE image. The BSE image is then measured using the "Analyze" function of the image analysis software "ImageJ" to measure the interface length between the coating layer and the base steel sheet (interface length L between the coating layer and the base steel sheet shown in Figure 2). The above operation is performed for five fields of view, and the average value is calculated to determine the interface length L. Next, (L - L0) / L0 x 100 is determined from the obtained interface length L and the length L0 of the corresponding base steel sheet surface, i.e., the length of the long side of the observation field: 100 μm. The resolution of the SEM image is 2560 x 1920. To measure L0, use the "Find edge" function in the "Process" section of the image analysis software "ImageJ," binarize the image using the "Binary" function, and then read "Perim." using the "Measure" function in "Analyze."

[0041] The surface coverage of the Mg-containing phase is determined as follows: First, the length M of the Mg-containing phase present in the surface portion of the coating layer among the Mg-containing phases identified above is determined. i The sum of ΣM i (In Figure 3, ΣM i M = M + M + M is calculated using the "Analyze" function of the image analysis software "ImageJ." Specifically, when a horizontal line is drawn at both ends of each Mg-containing phase using the toolbar "Straight" in ImageJ, M is calculated by reading the value displayed in "Length" on the toolbar. ｉ Then, the calculated ΣM i and the corresponding surface length L0 of the base steel plate (length of the long side of the observation field: 100 μm), ΣM i / L0 x 100 (surface coverage of the Mg-containing phase) is determined.

[0042] [Analysis of the plating layer in the separation zone of the heat-affected zone] Analysis of the plating layer in the separation zone 1 mm from the edge of the pressure-welded zone (heat-affected zone) is performed as follows. First, a 15 mm × 20 mm sample is taken from the surface of the Al-containing plated steel sheet so that the cross section of the plating layer in the separation zone of the heat-affected zone can be observed. The sample is embedded in resin and then mirror-polished to prepare a cross-sectional specimen of the spot weld. Next, a BSE image including the edge of the pressure-welded zone is obtained using SEM-EPMA. From the BSE image, the edge of the pressure-welded zone of the spot weld and a region 1 mm from the edge of the pressure-welded zone toward the separation zone (the boundary between the pressure-welded zone and the separation zone) are identified. Next, elemental analysis is performed on the identified boundary zone to determine the area ratio of the Zn-rich phase in the boundary zone. Specifically, the Zn-rich phase is defined as a phase with a Zn concentration of 60% by mass or more. The field of view of the SEM image is 200 μm × 160 μm, and the same elemental analysis is performed on 10 different locations randomly selected from the boundary portion, the area ratio of the Zn-rich phase is calculated in each field of view, and the maximum value among them is determined as the area ratio of the Zn-rich phase. The specific measurement conditions of the EPMA in each field of view are as follows: Apparatus: JXA-8500 manufactured by JEOL Ltd. Acceleration voltage: 15 kV Probe current: 5 × 10 -7 A Irradiation time: 50 ms Analysis interval: 300 μm or more Area ratio: average value of 5 fields of view First, an elemental distribution image of Zn, Fe, Al, and Mg is obtained using EPMA, and the lower limit of the color bar of the Zn distribution image is displayed as 60% or more and the upper limit as 100%, and the area where a Zn-rich phase with a Zn concentration of 60% or more exists is displayed. Next, using the image analysis software "ImageJ", the Zn elemental distribution image is read into ImageJ, and then in "Make Binary" under "Binary" in "Process", it is binarized so that areas with a Zn concentration of 60% or more are displayed as black and areas with a Zn concentration of less than 60% are displayed as white. After binarization, "Measure" in "Analyze" is used to read the value of "Area fraction" in "Results," and this value is determined as the area fraction of the Zn-rich phase.

[0043] The thickness of the Fe—Al phase is determined as follows. First, elemental analysis is similarly performed at the boundary portion identified above to identify the Fe—Al phase (Fe: 40-70%, Al: 30-60%, Zn: 0-20%, and other elements: less than 3%). Next, the thickness of the identified Fe—Al phase is measured at five different points in the field of view using the "Analyze" function of the image analysis software "ImageJ," and the thickness of the Fe—Al phase is determined by averaging the thicknesses measured at the five points.

[0044] The plating layer may be any plating layer having the above chemical composition, Fe—Al phase and / or Mg-containing phase, and is not particularly limited, but may be, for example, an alloyed hot-dip plating layer.

[0045] [Preferable Chemical Composition of Base Steel Sheet] As described above, the present invention aims to provide a welded joint capable of suppressing or reducing the occurrence of LME cracking during spot welding. This objective is achieved by controlling one or more of a plurality of overlapping steel sheets to be an Al-containing plated steel sheet having a plating layer on at least the surface corresponding to the overlapping surface, controlling the plating layer to have a predetermined chemical composition in a separation region outside the heat-affected zone, and controlling the thickness of the Fe-Al phase in the plating layer in the separation region 1 mm from the edge of the pressure-welded joint to be within a range of 10 to 200 μm and the area ratio of the Zn-rich phase to be within a range of 0 to 20%. 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. Below, preferred chemical compositions of the base steel sheet for Al-containing plated steel sheets useful for use in welded joints according to embodiments of the present invention are described in detail. However, these descriptions are intended merely as examples of preferred chemical compositions of the base steel sheet and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0046] In an embodiment of the present invention, for example, the base steel plate has, in mass %, C: 0.01 to 0.50%, Si: 0.001 to 3.000%, Mn: 0.10 to 3.00%, Al: 0.0002 to 2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Nb: 0 to 0.15%, Ti: 0 to 0.15%, V: 0 to 0.15%, Mo: 0 to 1.0%, Cr: 0 to 1.0%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, It is preferable that the alloy has a chemical composition consisting of Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.30%, Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0047] [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, 0.10% or more, 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% 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 0.45% or less or 0.40% or less.

[0048] [Si: 0.001 to 3.000%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during cold-rolled sheet annealing. To fully obtain this effect, the Si content is preferably 0.001% or more. The Si content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content may increase the steel strength while decreasing elongation. For this reason, the Si content is preferably 3.000% or less. The Si content may be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0049] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability 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 0.30% or more, 0.50% or more, 1.00% or more, or 1.30% 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 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0050] [Al: 0.0002 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.0002% or more. The Al content may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0051] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, the ideal P content is 0%. However, excessive reduction in the P content may result in a significant increase in costs. Therefore, the P content may be 0.0001% or more, 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 be 0.050% or less, 0.030% or less, or 0.010% or less.

[0052] [S: 0.1000% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in 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 cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0053] [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, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 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 be 0.0080% or less or 0.0050% or less.

[0054] The base steel sheet preferably has the basic chemical composition described above. Furthermore, the base steel sheet may contain, as necessary, one or more elements selected from the group consisting of Nb: 0-0.15%, Ti: 0-0.15%, V: 0-0.15%, Mo: 0-1.0%, Cr: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, B: 0-0.0100%, W: 0-1.000%, Hf: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.010%, REM: 0-0.30%, and Ir: 0-1.000%, in place of a portion of the remaining Fe. Each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

[0055] The remainder of the base steel plate other than the above elements consists 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.

[0056] 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 plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece may be obtained from the base steel sheet at approximately half the thickness, and the composition may be determined by measuring the test piece using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0057] [Thickness of Base Steel Plate] 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, or 4.0 mm or less.

[0058] [Other Steel Sheets] 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 other steel sheet other than the above-described Al-containing plated steel sheet. When the other steel sheet is a plated steel sheet, the plated steel sheet may or may not contain Zn. However, when the plating layer of the above-described Al-containing plated steel sheet does not contain Zn, the steel sheet adjacent to the Al-containing plated steel sheet must have at least a Zn-containing plating on the surface corresponding to the overlapping surface with the Al-containing plated steel sheet. In any case, for the steel sheet other than the Al-containing plated steel sheet, 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.

[0059] <Method for manufacturing plated steel sheet> Next, a description will be given of a preferred method for manufacturing an Al-containing plated steel sheet useful for use in a welded joint according to an embodiment of the present invention, more specifically, an Al-containing plated steel sheet in which the interface length L between the plating layer and the base steel sheet and the corresponding length L0 of the surface of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3. The following description is intended to exemplify a characteristic method for manufacturing the Al-containing plated steel sheet, but is not intended to limit the Al-containing plated steel sheet to one manufactured by the manufacturing method described below.

[0060] The Al-containing plated steel sheet can be produced by, for example, 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, a cooling process in which the annealed cold-rolled steel sheet is cooled, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the 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.

[0061] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an 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.

[0062] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, 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%.

[0063] [Coiling 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 process and then subjected to the cold-rolling process described below, without performing the coiling process.

[0064] [Cold Rolling Step] 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 of the 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 cooled to room temperature, for example, by air cooling.

[0065] [Pretreatment Step] Next, a predetermined pretreatment step may be performed before annealing the cold-rolled steel sheet. Such a pretreatment step may include a degreasing treatment. The degreasing treatment 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 energization time may be 5 to 10 seconds.

[0066] [Annealing Step] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 780 to 900°C in an atmosphere with a dew point of -10 to 10°C and holding the temperature for 10 to 300 seconds. By performing the annealing step under these conditions, the surface layer of the cold-rolled steel sheet can be adequately decarburized. In this case, the reaction between the coating layer and the base steel sheet during the alloying treatment in the subsequent coating step is promoted, i.e., the alloying rate can be increased. As a result, an interface shape with greater irregularities can be achieved, in which the interface length L between the coating layer and the base steel sheet and the corresponding surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3. By using an Al-containing plated steel sheet having such an interface shape in spot welding, even when the Al-containing plated steel sheet is spot welded to another Zn-containing plated steel sheet, the Zn component in the plating layer of the other Zn-containing plated steel sheet can be sufficiently incorporated into the plating layer of the Al-containing plated steel sheet at the high temperature during spot welding to form a solid solution. As a result, the formation of a Zn-rich phase can be suppressed or reduced to an area ratio of 0 to 20% in the plating layer of the separation portion in a region 1 mm from the end of the pressure weld portion of the finally obtained welded joint (the separation portion immediately outside the pressure weld portion), thereby making it possible to significantly suppress or reduce the occurrence of LME cracking around the pressure weld portion.

[0067] If the dew point is lower than −10°C, the annealing temperature is lower than 780°C, and / or the annealing time is shorter than 10 seconds, the decarburization of the surface layer of the cold-rolled steel sheet is insufficient, making it impossible to obtain a sufficient alloying rate during the alloying treatment of the coating layer. As a result, it becomes impossible to achieve an interface shape that satisfies the relationship (L − L) / L × 100 ≧ 3 between the coating layer and the base steel sheet. On the other hand, if the dew point is higher than 10°C, the heating temperature is higher than 900°C, and / or the annealing time is longer than 300 seconds, an outer oxide layer may form on the surface of the base steel sheet, resulting in reduced galvanic properties, or excessive decarburization may result in reduced strength of the finally obtained Al-containing coated steel sheet. 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 of 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen).

[0068] [Cooling Step] The cold-rolled steel sheet whose surface layer has been decarburized in the annealing step needs to be appropriately cooled in the subsequent cooling step to obtain a desired surface layer structure. Specifically, the cooling step includes cooling from the heating temperature (annealing temperature) in the annealing step to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more. This will be described in detail below.

[0069] Typically, the annealed cold-rolled steel sheet is then cooled to a temperature below 500°C, for example, to a temperature of about 200°C, and then reheated and plated. However, this temperature history results in the transformation of the austenitized metal structure into a structure such as bainite or martensite in the annealing process. Therefore, in the subsequent plating process, the metal structure such as bainite or martensite is alloyed with the plating layer. However, because the alloying rate between these metal structures and the plating layer is relatively slow, the finally obtained Al-containing plated steel sheet cannot achieve an interface shape between the plating layer and the base steel sheet that satisfies the relationship (L - L) / L × 100 ≥ 3. Therefore, in the cooling process of the present manufacturing method, it is extremely important to immerse the metal structure of the cold-rolled steel sheet, the surface of which has been decarburized in the annealing process, in a plating bath while still containing a large amount of austenite phase, thereby directly alloying the austenite phase with the plating layer. In this regard, in this cooling step, by cooling from the annealing temperature to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more, the metal structure of the cold-rolled steel sheet can be maintained in a state containing a larger amount of austenite phase. As a result, in the subsequent plating step, it becomes possible to achieve an alloying rate sufficient to directly alloy the austenite phase with the plating layer and realize a desired interface shape.

[0070] While not intending to be bound by any particular theory, it is believed that increasing the alloying rate by combining decarburization and the austenite phase causes unevenness in the alloying rate between areas where austenite grain boundaries exist and areas where they do not, and that this unevenness in the alloying rate results in the formation of an uneven surface at the interface between the coating layer and the base steel sheet. If the controlled temperature is less than 500°C, the austenite phase transforms into bainite or martensite, making it impossible to achieve a sufficient alloying rate in the subsequent plating process. Furthermore, if the average cooling rate from the annealing temperature to the controlled temperature of 500 to 750°C is less than 5°C / s, the transformation to ferrite becomes significant, similarly making it impossible to achieve a sufficient alloying rate in the subsequent plating process. As a result, in either case, it becomes impossible to achieve an interface shape that satisfies the relationship (L - L) / L × 100 ≥ 3 between the coating layer and the base steel sheet. On the other hand, if the controlled temperature exceeds 750°C, the temperature becomes higher than that suitable for the subsequent plating process, and the desired coating layer may not be obtained. From the viewpoint of realizing an interface shape with greater irregularities, the higher the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C, the more preferable, and for example, 15°C / s or more is preferable. Although the upper limit is not necessarily limited, the average cooling rate is preferably, for example, 30°C / s or less.

[0071] [Plating Step] Next, in the plating step, a plating layer is formed on at least one, preferably both, surfaces of a cold-rolled steel sheet (base steel sheet). More specifically, the plating step is carried out by immersing the cold-rolled steel sheet cooled to the above-mentioned controlled temperature in a plating bath having a predetermined chemical composition (plating bath temperature: e.g., 680 to 750°C) while maintaining a state in which the austenite phase is contained in a large amount, and then heat-treating the cold-rolled steel sheet at an alloying temperature of 680 to 750°C for 0.5 to 20 seconds. By performing the alloying treatment under these conditions, the plating layer is appropriately alloyed so that the Fe—Al phase has a desired thickness, and a sufficient alloying rate can be achieved based on the combination of decarburization and austenite phase. As a result, an interface shape with greater irregularities can be achieved, in which the interface length L between the plating layer and the base steel sheet and the corresponding surface length L of the base steel sheet satisfy the relationship (L - L) / L × 100 ≥ 3. By using an Al-containing plated steel sheet having such an interface shape in spot welding, the Zn component in the plating layer of another Zn-containing plated steel sheet can be sufficiently incorporated into the plating layer of the Al-containing plated steel sheet at high temperatures during spot welding to form a solid solution, and as a result, the formation of a Zn-rich phase can be suppressed or reduced to an area ratio of 0 to 20% in the plating layer of the separation portion in a region 1 mm from the end of the pressure-welded portion of the finally obtained welded joint.

[0072] If the alloying temperature is lower than 680°C, the coating layer solidifies without sufficient alloying, resulting in a reduced Fe content in the coating layer and / or making it impossible to obtain the desired Fe-Al phase thickness in the separation zone immediately outside the pressure welded joint during spot welding. As a result, the corrosion resistance of the Al-containing coated steel sheet after painting is reduced. Furthermore, if the alloying treatment time is shorter than 0.5 seconds, the coating layer is insufficiently alloyed, making it impossible to create an uneven interface between the coating layer and the base steel sheet and / or making it impossible to obtain the desired Fe-Al phase thickness in the separation zone immediately outside the pressure welded joint during spot welding. On the other hand, if the alloying temperature is higher than 750°C or the alloying treatment time is longer than 20 seconds, excessive alloying of the coating layer progresses, resulting in a flatter interface with fewer unevenness, and the resulting Al-containing coated steel sheet may not satisfy the relationship (L - L) / L × 100 ≥ 3. In this case, it becomes impossible to suppress or reduce the formation of the Zn-rich phase to within a range of 0 to 20% in terms of area ratio in the plating layer of the separation portion in the region 1 mm from the end of the pressure-welded portion of the finally obtained welded joint. From the viewpoint of ensuring the desired alloying, the alloying treatment time is preferably set to 5 to 20 seconds.

[0073] The plating step is carried out by, for example, hot-dip plating. The plating step is not limited to hot-dip plating, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions of the plating step may be appropriately set taking into consideration the thickness and coating weight of the plating layer. For example, a cold-rolled steel sheet is immersed in a plating bath, then pulled out, and immediately sprayed with N2 gas or air by a gas wiping method, followed by cooling. This allows the coating weight of the plating layer to be adjusted within a predetermined range, for example, within a range such that the thickness of the Fe—Al phase is 4 to 50 μm.

[0074] [Cooling after plating] Finally, the base steel sheet to which the plating layer is attached is cooled to obtain an Al-containing plated steel sheet. The cooling after plating is not particularly limited and can be performed under any appropriate conditions known to those skilled in the art. For example, the cooling after plating can be performed at an average cooling rate of 10°C / s or more. The cooling stop temperature is also not particularly limited and may be set appropriately in the range of, for example, 100 to 350°C.

[0075] According to this production method, it is possible to produce an aluminum-containing plated steel sheet provided with a plating layer in which the chemical composition of the plating layer is optimized within a predetermined range, i.e., by mass %, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, and Al: 20.0% or more, and the shape of the interface between the plating layer and the base steel sheet is controlled to satisfy the relationship (L - L0) / L0 × 100 ≥ 3. Therefore, by controlling the shape of the interface between the plating layer and the base steel sheet to have greater irregularities, even when the aluminum-containing plated steel sheet is spot welded to another zinc-containing plated steel sheet, the Zn component in the plating layer of the other zinc-containing plated steel sheet can be sufficiently incorporated into the plating layer of the aluminum-containing plated steel sheet and formed into a solid solution at the high temperature during spot welding. As a result, in the plating layer of the separation portion in the region 1 mm from the end of the pressure weld portion of the finally obtained welded joint, the formation of a Zn-rich phase can be suppressed or reduced to an area ratio of 0 to 20%, thereby making it possible to significantly suppress or reduce the occurrence of LME cracking around the pressure weld portion.

[0076] <Method for Manufacturing Welded Joint> Welded joints according to embodiments of the present invention can be manufactured 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 steel sheets, in which all steel sheets are the above-described Al-containing plated steel sheets (provided that in this case, any one of the Al-containing plated steel sheets is an Al-Zn-containing plated steel sheet), or to a plurality of steel sheets, in which one or more steel sheets are the above-described Al-containing plated steel sheet and the Al-containing plated steel sheet is overlapped with another steel sheet or another Zn-containing plated steel sheet. For example, a welded joint according to embodiments of the present invention can be manufactured 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 type welding electrode having a tip diameter of 6 to 8 mm, the pressure may be 1.5 to 6.0 kN, the current flow 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°.

[0077] With the welded joint manufactured as described above, the area ratio of the Zn-rich phase in the coating layer in the separation area immediately outside the pressure welded joint can be controlled within a desired range, which in turn makes it possible to suppress or reduce the penetration of molten Zn into the steel sheet during spot welding. Therefore, with such a welded joint, it is possible to achieve superior LME resistance compared to when conventional coated steel sheets are used. This can contribute to industrial development by extending the service life of products in the automotive and construction industries.

[0078] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0079] In the following examples, Al-containing plated steel sheets were produced under various conditions, and the LME resistance of welded joints obtained by spot welding the produced Al-containing plated steel sheets was examined.

[0080] First, molten steel was cast by a continuous casting method to form a steel slab having a chemical composition consisting of, by mass%, C: 0.20%, Si: 0.012%, Mn: 1.30%, Al: 0.030%, P: 0.005%, S: 0.0020%, and N: 0.0030%, with the balance consisting of Fe and impurities. The steel slab was once cooled, reheated to 1200 °C, hot rolled, and then coiled at a temperature of 600 °C or less. Hot rolling was performed by performing rough rolling and finish rolling, with the finish rolling ending temperature being 900 to 1050 °C and the finish rolling reduction being 30%. Next, the obtained hot-rolled steel sheet was subjected to pickling and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheet was subjected to a 5.0 A / dm 2 A pretreatment (degreasing treatment) was carried out by passing a current at a current density of 1000 kJ / min for 8 seconds.

[0081] Next, each cold-rolled steel sheet was cut into a size of 100 mm x 200 mm and then subjected to annealing treatment (annealing atmosphere: 3% hydrogen and balance nitrogen) under the conditions shown in Table 1. Next, the cut steel sheet samples were cooled from the annealing temperature to the control temperature at the average cooling rate shown in Table 1, and then immersed in a hot-dip galvanizing bath having a predetermined bath composition (galvanizing bath temperature: 680 to 750°C) and subjected to alloying treatment under the conditions shown in Table 1. After immersion in the galvanizing bath, the steel sheet samples were pulled out and subjected to N2 gas wiping to adjust the coating weight. Finally, the base steel sheet with the coating layer attached was cooled at an average cooling rate of 10°C / s or more to obtain an Al-containing coated steel sheet (first steel sheet in Table 1) in which a coating layer was formed on both sides of the base steel sheet.

[0082] [Production of Welded Joints] Of the obtained Al-containing plated steel sheet samples, a plated steel sheet sample measuring 100 × 100 mm was subjected to spot welding. Two samples 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 a 2° impact angle, a pressure of 4.0 kN, a welding time of 0.8 seconds, and a current of 12 kA to produce welded joints. At least one of the two plated steel sheet samples was a Zn-containing plated steel sheet. More specifically, as shown in Table 1, a conventional galvannealed (GA) steel sheet was used as the second steel sheet except for Examples 2 and 3, and in Examples 2 and 3, the same type of Al-Zn-containing plated steel sheet was used as the first and second steel sheets.

[0083] The physical properties and characteristics of the obtained welded joints were measured and evaluated by the following methods.

[0084] [Analysis of Chemical Composition of Plating Layer] The chemical composition of the plating layer was determined by immersing a 30 mm × 30 mm sample taken from the separation zone outside the heat-affected zone of the welded joint (i.e., non-heat-affected zone) in a 10% HCl aqueous solution containing 0.04% IBIT 710K (manufactured by Asahi Chemical Industry Co., Ltd.) as an inhibitor, pickling 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.

[0085] [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 occurred in the cross section of the separation part around the pressure welded part was measured, and the LME resistance was evaluated as follows: AAA: No LME crack AA: LME crack length more than 0 μm to 60 μm A: LME crack length more than 60 μm to 120 μm B: LME crack length more than 120 μm

[0086] [Evaluation of Chemical Conversion Treatability] Chemical conversion treatability was evaluated as follows. First, a 50 mm x 100 mm sample (corresponding to the non-heat-affected zone) of the produced Al-containing plated steel sheet was subjected to zinc phosphate treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) to form a chemical conversion coating. Next, the sample surface was observed using a secondary electron image of an SEM, and the area ratio of the portion where no chemical conversion coating was formed, commonly called "clear," was measured. The chemical conversion treatability of the Al-containing plated steel sheet was evaluated according to the area ratio of the clear-cut area using the following evaluation criteria: AA: Clear-cut area ratio 0 to 5% A: Clear-cut area ratio more than 5 to 15% -: Clear-cut area ratio more than 15%

[0087] Welded joints with LME resistance ratings of AAA, AA, and A were evaluated as being capable of suppressing or reducing the occurrence of LME cracking during spot welding. The results are shown in Table 1.

[0088]

[0089]

[0090] Referring to Table 1, in Comparative Examples 29 and 30, the high Mg and Si contents in the coating layers presumably prevented a sufficient alloying rate during the alloying treatment of the coating layers. In relation to this, the value of (L - L) / L × 100 at the interface between the coating layer and the base steel sheet of the Al-containing coated steel sheet was less than 3, meaning the interface had a flatter shape with fewer irregularities. As a result, the area fraction of the Zn-rich phase in the coating layer in the separation zone 1 mm from the edge of the pressure-welded joint (the separation zone immediately outside the pressure-welded joint) exceeded 20%, resulting in reduced LME resistance. In Comparative Example 31, the low annealing temperature presumably prevented insufficient decarburization of the surface layer of the cold-rolled steel sheet, preventing a sufficient alloying rate during the alloying treatment of the coating layers. As a result, the value of (L - L) / L × 100 was less than 3, and the area fraction of the Zn-rich phase in the coating layer in the separation zone immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 32, the annealing time was too short, which likely resulted in insufficient decarburization of the surface layer of the cold-rolled steel sheet, and thus a sufficient alloying rate could not be achieved during the alloying treatment of the coating layer. As a result, the value of (L - L) / L × 100 was less than 3, and the area fraction of the Zn-rich phase in the coating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 33, the dew point in the annealing process was too low, which likely resulted in insufficient decarburization of the surface layer of the cold-rolled steel sheet, and thus a sufficient alloying rate could not be achieved during the alloying treatment of the coating layer. As a result, the value of (L - L) / L × 100 was less than 3, and the area fraction of the Zn-rich phase in the coating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 34, the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C was slow, which is thought to have resulted in a significant transformation from the austenite phase to ferrite in the metal structure of the cold-rolled steel sheet, making it impossible to obtain a sufficient alloying rate in the subsequent plating process. As a result, the value of (L - L0) / L0 x 100 was less than 3, and the area ratio of the Zn-rich phase in the plating layer in the separation area immediately outside the pressure-welded area exceeded 20%, resulting in a decrease in LME resistance.

[0091] In Comparative Examples 35 and 36, the controlled temperature in the annealing process was low, which resulted in significant transformation from austenite to bainite or martensite in the metal structure of the cold-rolled steel sheet, and it is believed that this prevented a sufficient alloying rate from being achieved in the subsequent plating process. As a result, the value of (L - L0) / L0 x 100 was less than 3, and the area fraction of the Zn-rich phase in the plating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 37, the alloying temperature of the plating layer was high, which is believed to have caused excessive alloying of the plating layer. As a result, the value of (L - L0) / L0 x 100 was less than 3, and the area fraction of the Zn-rich phase in the plating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 38, the alloying treatment time of the coating layer was short, resulting in insufficient alloying of the coating layer, and the interface between the coating layer and the base steel sheet could not be formed into an uneven shape. This meant that the value of (L - L) / L × 100 was less than 3, and the desired Fe-Al phase thickness could not be obtained. As a result, the area ratio of the Zn-rich phase in the coating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance. In Comparative Example 39, the alloying treatment time of the coating layer was long, which is thought to have caused excessive alloying of the coating layer. As a result, the value of (L - L) / L × 100 was less than 3, resulting in the area ratio of the Zn-rich phase in the coating layer in the separation area immediately outside the pressure-welded joint exceeded 20%, resulting in reduced LME resistance.

[0092] In contrast, in all welded joints according to the examples, the chemical composition of the plating layer in the separation area of ​​the non-heat-affected zone was optimized within the specified ranges, i.e., Fe: 20.0-55.0%, Mg: 0-10.0%, Si: 0-10.0%, and Al: 20.0% or more, by mass %, and the thickness of the Fe-Al phase in the separation area immediately outside the pressure welded joint was controlled to 10-200 μm and the area fraction of the Zn-rich phase was controlled to 0-20%, thereby reliably suppressing or reducing LME cracking. In particular, in Examples 6-9, in which the area fraction of the Zn-rich phase in the separation area immediately outside the pressure welded joint was controlled to 10% or less, the LME resistance was evaluated as AA, further improving LME resistance. Similarly, in Examples 10-28, in which the area fraction of the Zn-rich phase in the separation area immediately outside the pressure welded joint was controlled to 5% or less, the LME resistance was evaluated as AAA, further improving LME resistance. Additionally, in Examples 6 to 9, in which the surface coverage of the Mg-containing phase was controlled to 20% or more, the evaluation of chemical conversion treatability was A. Similarly, in Examples 10 to 23 and 25 to 28, in which the surface coverage of the Mg-containing phase was controlled to 60% or more, the evaluation of chemical conversion treatability was AA, indicating that very high chemical conversion treatability was achieved.

[0093] REFERENCE SIGNS LIST 1 Al-containing plated steel sheet 2 Base steel sheet 3 Plated layer 4 Fe-Al phase 5 Mg-containing phase L Interface length between plated layer and base steel sheet L0 Surface length of base steel sheet 10 Welded joint 11 Steel sheet 12 Nugget 13 Pressure welded portion 14 End of pressure welded portion 15 Heat-affected zone 16 Spot welded portion 17 Separation portion 18 Plated layer

Claims

1. A welded joint comprising: a plurality of overlapping steel sheets; a nugget joining the plurality of steel sheets; a spot weld having a pressure weld portion and a heat-affected zone formed around the nugget; and a separation portion located around the pressure weld portion, wherein at least one of the plurality of steel sheets is an Al-containing plated steel sheet comprising a base steel sheet and a plating layer formed on at least a surface of the base steel sheet corresponding to an overlapping surface of the plurality of steel sheets, wherein the plating layer of the Al-containing plated steel sheet contains Zn and / or a steel sheet adjacent to the Al-containing plated steel sheet has a Zn-containing plating on a surface corresponding to the overlapping surface, and the plating layer in the separation portion outside the heat-affected zone contains, by mass%, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, Zn: 0 to 30.0%, and further Ni: 0 to 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%, 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%, Sr:0~0.500%, In:0~0.500%, A welded joint having a chemical composition containing a total of 5.000% or less of at least one of Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, W: 0-0.500%, and V: 0-0.500%, with the balance being 20.0% or more of Al and impurities, wherein in the plating layer in the separation portion in a region 1 mm from an end of the pressure welded portion, the thickness of the Fe-Al phase is 10-200 μm and the area ratio of a Zn-rich phase is 0-20%.

2. The welded joint according to claim 1, characterized in that the area ratio of the Zn-rich phase is 0 to 10%.

3. The welded joint according to claim 2, characterized in that the area ratio of the Zn-rich phase is 0 to 5%.

4. The welded joint according to any one of claims 1 to 3, characterized in that the thickness of the Fe-Al phase is 16 to 200 µm.

5. A welded joint according to any one of claims 1 to 4, characterized in that the chemical composition contains, in mass percent, Mg: 0.1 to 10.0%.

6. The welded joint according to any one of claims 1 to 5, characterized in that the chemical composition contains, in mass%, Mg: 0.3 to 10.0%, and Si: 0 to 1.0%.

7. A welded joint as claimed in any one of claims 1 to 6, characterized in that the chemical composition contains, in mass %, Mg: 0.3 to 10.0%, the plating layer in the separation area outside the heat-affected zone further contains an Mg-containing phase, and in a cross section of the plating layer in the separation area outside the heat-affected zone, the surface coverage of the Mg-containing phase is 20 to 100%.

8. The welded joint according to claim 7, characterized in that the surface coverage of the Mg-containing phase is 60 to 100%.

9. A welded joint as claimed in any one of claims 1 to 8, characterized in that, in a cross section of the plating layer in the separation area outside the heat-affected zone, the interface length L between the plating layer and the base steel sheet and the length L0 of the surface of the base steel sheet satisfy (L - L0) / L0 × 100 ≧ 3.

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