Plated steel sheet

Abstract

The present invention provides a plated steel sheet which is provided with a base steel sheet and a plating layer that is formed on the surface of the base steel sheet, and which is characterized in that: the plating layer has a specific chemical composition; and if the plating layer is measured by glow discharge optical emission spectrometry (GDS), the ratio of (Al concentration in center of plating layer) / (Al concentration at position of plating layer, at which Fe concentration is 50% of that of base steel sheet) is 0.10 to 1.50.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet. [Background technology]

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

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

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

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

[0006] Patent Documents 1 and 2 discuss the prevention of LME cracking from the viewpoint of improving the spot welding method and controlling the coating structure in the vicinity of the spot weld after spot welding. However, Patent Documents 1 and 2 do not necessarily provide sufficient discussion from the viewpoint of improving the coating structure of the plated steel sheet before spot welding. Therefore, the inventions described in these patent documents still have room for improvement in terms of improving LME resistance.

[0007] Therefore, an object of the present invention is to provide a plated steel sheet having improved LME resistance, which is capable of suppressing or reducing the occurrence of LME cracks during spot welding, through a novel configuration. [Means for solving the problem]

[0008] The present inventors conducted research, focusing particularly on the structure of the coating layer of a coated steel sheet, in order to suppress or reduce the occurrence of LME cracking. As a result, the present inventors discovered that in a coating layer containing a relatively small amount of Al, the LME resistance of the coated steel sheet can be significantly improved by controlling the Al concentration at the center of the coating layer to be within a predetermined range relative to the Al concentration near the interface between the base steel sheet and the coating layer, and completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) A steel plate having a base steel sheet and a plating layer formed on the surface of the base steel sheet, The plating layer is composed of, in mass %, Al: 0.10 to 1.50%, and Fe: 0.01 to 2.00% and further comprising Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0 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: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, A plated steel sheet characterized in that, when the plated layer is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the plated layer" to "Al concentration at the position of the plated layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50. (2) The plated steel sheet according to (1) above, characterized in that the chemical composition contains, in mass %, Al: 0.30 to 1.50%, and when the plated layer is measured by GDS, the ratio of "Al concentration at the center of the plated layer" to "Al concentration at the position in the plated layer where the Fe concentration is 50% of that of the base steel sheet" is 0.20 to 1.50. (3) The plated steel sheet according to (1) above, characterized in that the chemical composition contains, in mass %, Al: 0.30 to 1.50%, and when the plated layer is measured by GDS, the ratio of "Al concentration at the center of the plated layer" to "Al concentration at the position in the plated layer where the Fe concentration is 50% of that of the base steel sheet" is 0.30 to 1.50. (4) The plated steel sheet according to any one of the above (1) to (3), wherein the plated layer is a hot-dip galvanized (GI) layer. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that it has a tensile strength of 780 MPa or more. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a plated steel sheet having improved LME resistance that can suppress or reduce the occurrence of LME cracks during spot welding. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are diagrams showing the results of analysis of plated steel sheets by GDS, in which FIG. 1A shows the results of analysis of an Al-containing plated steel sheet produced by a conventional method by GDS, and FIG. 1B shows the results of analysis of an Al-containing plated steel sheet according to an embodiment of the present invention by GDS. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Plated steel sheet> A plated steel sheet according to an embodiment of the present invention comprises a base steel sheet and a plating layer formed on a surface of the base steel sheet, The plating layer is composed of, in mass %, Al: 0.10 to 1.50%, and Fe: 0.01 to 2.00% and further comprising Mg: 0 to 1.500%, Si: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0 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: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, P: 0 to 0.500%, and Sr: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, When the coating layer is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50.

[0013] As mentioned above, LME cracking must be suppressed during spot welding between galvanized steel sheets or between galvanized and non-galvanized steel sheets. For example, when spot welding two or more steel sheets, including at least one galvanized steel sheet, to form a joint, LME cracking may occur inside or immediately outside the pressure weld formed on the outside of the weld metal (nugget), or on the surface of the electrode side. 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, embrittles it, and then tensile stresses generated by welding, such as the applied pressure of the electrode, expansion and contraction of the weld, and springback when the electrode is released, act on the steel sheet. Therefore, in order to suppress or reduce the penetration of zinc into the steel sheet, the inventors focused on the structure of the coating layer of galvanized steel sheets and conducted research from the perspective of optimizing the structure of the coating layer.

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

[0015] FIG. 1 shows the results of GDS analysis of plated steel sheets. FIG. 1(a) shows the results of GDS analysis of an Al-containing plated steel sheet manufactured by a conventional method, and FIG. 1(b) shows the results of GDS analysis of a plated steel sheet according to an embodiment of the present invention. First, referring to FIG. 1(a), in an Al-containing plated steel sheet in which 0.20% Al is added to a Zn-based coating layer manufactured by a conventional method, the Al concentration gradually increases with increasing depth from a depth of 0 μm, which corresponds to the coating surface, and the Al concentration has a relatively high peak near the interface between the base steel sheet and the coating layer, i.e., near the coating layer position where the Fe concentration is 50% of that of the base steel sheet. This Al concentration peak suggests the formation of an Fe-Al barrier layer containing an alloy of Fe and Al at the interface between the base steel sheet and the coating layer. As is clear from the relatively high Al concentration peak, in the Al-containing plated steel sheet manufactured by a conventional method, a larger amount of Al in the coating layer is consumed in the formation of the Fe-Al barrier layer, resulting in the formation of a relatively thick Fe-Al barrier layer. Therefore, Figure 1(a) shows that the Al concentration decreases significantly from near the interface between the base steel sheet and the coating layer toward the coating surface, and then becomes a fairly constant, very low value. At the center of the coating layer, which corresponds to the midpoint between the coating surface and the position where the Fe concentration is 50% of that of the base steel sheet, the Al concentration is a low value of approximately 0.1%.

[0016] In contrast, referring to FIG. 1(b), it can be seen that the Al concentration near the interface between the base steel sheet and the coating layer in the coated steel sheet according to the embodiment of the present invention, which has a coating layer with a similar Al content, is significantly lower than that in FIG. 1(a). Therefore, the coated steel sheet according to the embodiment of the present invention forms a thinner Fe-Al barrier layer than that in FIG. 1(a). In relation to this, FIG. 1(b) shows that the Al concentration in the coating layer does not decrease significantly from near the interface between the base steel sheet and the coating layer toward the coating surface, but decreases relatively gradually and then becomes approximately constant. At the center of the coating layer, the Al concentration exceeds approximately 0.2%, which is a very high value, more than twice that in FIG. 1(a). The GDS analysis results in FIGS. 1(a) and 1(b) suggest that in the coated steel sheet according to the embodiment of the present invention, much of the Al in the coating layer is not consumed in the formation of the Fe-Al barrier layer, but is present in the coating layer outside the Fe-Al barrier layer as an Al phase, e.g., in a solid solution state. Based on the GDS analysis results of these coated steel sheets and the fact that the coated steel sheet of Figure 1(b) experimentally demonstrated higher LME resistance than the coated steel sheet of Figure 1(a), the inventors conducted further studies, believing that the Al phase present in the coating layer other than the Fe-Al barrier layer plays a very important role in suppressing or reducing the penetration of molten Zn into the steel sheet. As a result, the inventors discovered that by controlling the total Al content to a relatively low amount of 1.50 mass% or less to suppress deterioration of LME resistance due to a decrease in the melting point of the coating layer, and by controlling the ratio of the "Al concentration at the center of the coating layer" to the "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" as measured by GDS within the range of 0.10 to 1.50, the effect of adding Al to the coating layer can be fully exerted to suppress or reduce the penetration of molten Zn into the steel sheet, and relatedly, the LME resistance of the coated steel sheet can be significantly improved.

[0017] Without intending to be bound by any particular theory, it is believed that in the plated steel sheet according to the embodiment of the present invention, Al in the plated layer acts as follows to inhibit or reduce the penetration of Zn, which has been liquefied by welding heat input during spot welding, into the steel sheet along grain boundaries. More specifically, it is believed that the Fe-Al barrier layer formed at the interface between the base steel sheet and the plated layer in the plated steel sheet is relatively brittle and therefore fractures relatively easily due to stress applied to the steel sheet due to the electrode pressure or the like during spot welding. The fracture of the Fe-Al barrier layer brings molten Zn into direct contact with the base steel sheet during spot welding, increasing the risk of the molten Zn penetrating into the steel sheet along grain boundaries. However, in the plated steel sheet according to the embodiment of the present invention, Al, which is present in a relatively large amount in the portions of the plated steel layer other than the Fe-Al barrier layer, comes into direct contact with the base steel sheet together with Zn due to the fracture of the Fe-Al barrier layer. In this case, it is believed that the heat input during spot welding causes Al in the coating layer to react with Fe in the base steel sheet to form a new Fe-Al barrier layer, thereby repairing the fractured Fe-Al barrier layer. In other words, the presence of a large amount of Al in the coating layer other than the Fe-Al barrier layer allows a new Fe-Al barrier layer to be immediately formed at the fractured portion, even if Zn comes into direct contact with the base steel sheet due to fracture of the Fe-Al barrier layer during spot welding, thanks to the presence of Al in the immediate vicinity. Therefore, according to the coated steel sheet according to the embodiment of the present invention, the penetration of molten Zn into the steel sheet during spot welding can be significantly suppressed or reduced, and it is believed that the occurrence of LME cracking in the coated steel sheet can be reliably suppressed or reduced. Although coated steel sheets with Al added to a Zn-based coating layer have been known, the fact that the penetration of molten Zn into the steel sheet during spot welding can be suppressed or reduced by keeping the Al content of the entire coating layer relatively low in consideration of a lowering of the melting point of the coating layer, while increasing the Al content in the coating layer other than the Fe-Al barrier layer, was not previously known and has now been discovered for the first time by the present inventors.

[0018] Hereinafter, a plated steel sheet according to an embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0019] [Plating layer] According to an embodiment of the present invention, a plating layer is formed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces of the base steel sheet. The plating layer has the following chemical composition:

[0020] [Al: 0.10-1.50%] Al is an element effective in preventing molten Zn from penetrating into the steel sheet along grain boundaries. To fully achieve this effect, the Al content is set to 0.10% or more. The Al content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.35% or more, 0.40% or more, more than 0.60%, 0.62% or more, 0.65% or more, or 0.70% or more. On the other hand, excessive Al content causes the coating layer to approach the Zn-Al eutectic composition, lowering the melting point of the coating layer. This may result in the Zn in the coating layer melting more easily during spot welding, which may promote LME cracking. Therefore, the Al content is set to 1.50% or less. The Al content may be 1.45% or less, 1.40% or less, 1.30% or less, 1.20% or less, 1.10% or less, 1.00% or less, 0.90% or less, or 0.80% or less.

[0021] [Fe: 0.01 to 2.00%] Fe is an element that is inevitably present in the coating layer, for example, by dissolving from the base steel sheet into the coating bath or by reacting with Al during the coating process to form an Fe-Al barrier layer at the interface between the base steel sheet and the coating layer. Therefore, in the coated steel sheet according to the embodiment of the present invention, the Fe content in the coating layer is 0.01% or more. The Fe content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.40% or more, or 0.50% or more. On the other hand, if the Fe content in the coating layer is too high, Al in the coating layer may combine with Fe, or a large amount of Al may be consumed to form the Fe-Al barrier layer. As a result, the effect of Al addition in suppressing LME cracking may not be fully exerted. Therefore, the Fe content is set to 2.00% or less. The Fe content may be 1.80% or less, 1.60% or less, 1.50% or less, 1.30% or less, 1.20% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[0022] The basic chemical composition of the plating layer is as described above. The plating layer may further optionally contain: Mg: 0-1.500%, Si: 0-1.000%, Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1.000%, Nb: 0-1.000% The coating layer may contain at least one of the following: Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, P: 0-0.500%, and Sr: 0-0.500%. The total content of these optional elements is limited to 5.000% or less in order to fully exhibit the effects and functions of the basic components constituting the coating layer, particularly Al. The total content of the optional elements may be 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less. These optional elements are described in detail below.

[0023] [Mg: 0-1.500%] Mg is an element effective in improving the corrosion resistance of the coating layer. The Mg content may be 0%, but to achieve this effect, the Mg content is preferably 0.001% or more. The Mg content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Mg content may result in the formation of large amounts of MgZn-based compounds, which are brittle compounds, in the coating layer, which may cause a decrease in workability. Therefore, the Mg content is preferably 1.500% or less. The Mg content may be 1.200% or less, 1.000% or less, 0.800% or less, 0.500% or less, 0.240% or less, 0.220% or less, or 0.200% or less.

[0024] [Si: 0-1.000%] Si is an element effective in improving the corrosion resistance of the plating layer. The Si content may be 0%, but if necessary, Si may be contained in the plating layer in an amount of 0.0001% or more, or 0.001% or more. On the other hand, excessive Si content may reduce the plating adhesion of the plating layer. Therefore, the Si content is preferably 1.000% or less. The Si content may be 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0025] [Ni: 0-1.000%] Ni is an element effective in improving the corrosion resistance of the coating layer. The Ni content may be 0%, but to achieve this effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive Ni content may result in the formation of many intermetallic compounds, which may reduce corrosion resistance. Therefore, the Ni content is preferably 1.000% or less. The Ni content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0026] [Ca: 0-4.000%] Ca is an element effective in ensuring wettability of the coating bath. The Ca content may be 0%, but to obtain this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.010% or more, 0.100% or more, or 1.000% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the coating layer, making the coating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 4.000% or less. The Ca content may be 3.000% or less, 2.000% or less, or 1.500% or less.

[0027] [Sb:0~0.500%, Pb:0~0.500%, Cu:0~1.000%, Sn:0~1.000%, Ti:0~1.000%, Cr:0~1.000%, Nb:0~1.000%, Zr:0~1.000%, Mn:0~1.000 %, Mo:0~1.000%, Ag:0~1.000%, Li:0~1.000%, La:0~0.500%, Ce:0~0.500%, B:0~0.500%, Y:0~0.500%, P:0~0.500% and Sr:0~0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr may not be present in the coating layer, but may be present in the coating layer in amounts of 0.0001% or more, or 0.001% or more. These elements do not adversely affect the performance of the coated steel sheet as long as they are within the specified content range. However, excessive content of each element may reduce corrosion resistance. Therefore, the contents of Sb, Pb, La, Ce, B, Y, P, and Sr are preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, or 0.050% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, or 0.100% or less.

[0028] The remainder of the plating layer other than the above elements is composed of Zn and impurities. The impurities in the plating layer are components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is manufactured.

[0029] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, and measuring the resulting solution using ICP (inductively coupled plasma) atomic emission spectroscopy.

[0030] The coating layer may be any coating layer having the above-mentioned chemical composition, and is not particularly limited, but is preferably, for example, a hot-dip galvanized (GI) layer. For example, when alloying heat treatment is performed, the Fe content in the coating layer increases, and it may not be possible to obtain the desired chemical composition and ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" in the final coating layer. The thickness of the coating layer may be, for example, 3 to 50 μm. The coating weight of the coating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side. 2 The coating weight of the plating layer may be 45 g / m per side. 2 or more than 50g / m 2 Similarly, the coating weight of the plating layer may be 75 g / m per side. 2 Less than or equal to 70g / m 2 The coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, and measuring the change in weight before and after pickling.

[0031] [Ratio of "Al concentration at the center of the coating layer" / "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet": 0.10 to 1.50] In an embodiment of the present invention, when the coating layer is measured by glow discharge optical emission spectroscopy (GDS), the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is 0.10 to 1.50. By controlling the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" measured by GDS within this range, the effect of adding Al to the coating layer can be fully exerted, making it possible to suppress or reduce the penetration of molten Zn into the steel sheet, and as a result, it becomes possible to significantly improve the LME resistance of the coated steel sheet. From the viewpoint of improving LME resistance, the higher the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet," the better. The ratio is preferably 0.15 or more, more preferably 0.20 or more, and most preferably 0.30 or more, for example, 0.40 or more, 0.42 or more, 0.45 or more, 0.50 or more, 0.55 or more, or 0.60 or more. This ratio can be increased by increasing the Al content of the entire coating layer and applying a method for producing a coated steel sheet, which will be described in detail later. For example, to achieve this ratio of 0.20 or more or 0.30 or more, the Al content of the entire coating layer is preferably 0.30% or more. On the other hand, if the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" becomes too high, the melting point of the coating layer may decrease due to the associated increase in the Al content of the entire coating layer. This may cause Zn in the coating layer to melt more easily during spot welding, which may accelerate LME cracking. Therefore, in an embodiment of the present invention, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that in the base steel sheet" is set to 1.50 or less, and may be, for example, 1.40 or less, 1.30 or less, 1.20 or less, 1.10 or less, or 1.00 or less.

[0032] [Method for measuring the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet"] The ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is determined as follows. First, a coated steel sheet sample is obtained by cutting a 50 mm x 50 mm piece from the coated steel sheet. The coated steel sheet sample is then subjected to glow discharge optical emission spectroscopy (GDS) to obtain the Al concentration distribution from the surface of the coating layer to a depth of 100 μm. Next, the Al concentration at a depth where the Fe intensity is 50% of that of the base steel sheet (Fe intensity at a depth of 100 μm from the surface of the coating layer of the sample) in the GDS measurement is determined as "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet." The distance from this depth to the surface is defined as the thickness of the coating layer. The Al concentration measured by GDS at a position halfway through the thickness of the coating layer is determined as "Al concentration at the center of the coating layer." Finally, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is determined.

[0033] [Base material steel plate] In an embodiment of the present invention, the base steel sheet for forming the above-described coating layer is not particularly limited and may be any appropriate material. For example, the base steel sheet may be a material having a chemical composition that provides a tensile strength of 780 MPa or more for the coated steel sheet. It is generally known that LME cracking occurs more frequently when steel sheets having relatively high strength are spot-welded, and that the higher the strength of the steel sheet, the greater the susceptibility to LME cracking. Therefore, when the coated steel sheet according to an embodiment of the present invention has a high tensile strength of 780 MPa or more, the effect of suppressing LME cracking is particularly significant compared to conventional coated steel sheets having the same tensile strength.

[0034] [Preferred chemical composition of base steel plate] As described above, the present invention aims to provide a plated steel sheet with improved LME resistance that can suppress or reduce the occurrence of LME cracking during spot welding. This objective is achieved by forming a plated layer on the surface of a base steel sheet, which has a predetermined chemical composition and, when measured by GDS, has a ratio of "Al concentration at the center of the plated layer" to "Al concentration at the position in the plated layer where the Fe concentration is 50% of that of the base steel sheet" of 0.10 to 1.50. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the objective of the present invention. Below, we will explain in detail the preferred chemical composition of the base steel sheet used in the plated steel sheets according to embodiments of the present invention. However, these explanations are intended merely as examples of preferred chemical compositions of base steel sheets for plated steel sheets having a tensile strength of 780 MPa or more, at which LME occurrence is significant when spot welded, i.e., plated steel sheets that exhibit a particularly significant effect of suppressing LME cracking according to the present invention. However, it is not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0035] In an embodiment of the present invention, for example, the base steel plate contains, in mass%, C: 0.01 to 0.50%, Si: 0.01 to 3.50% Mn: 0.10~5.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.0100% or less, O: 0 to 0.020%, Al: 0 to 1.000%, B: 0~0.010%, Nb: 0 to 0.150%, Ti: 0 to 0.20% Mo: 0-3.00%, Cr: 0~2.00%, V: 0~1.00%, Ni: 0-2.00% W: 0~1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0 to 1.00% Sb: 0 to 0.50% Cu: 0-2.00% As: 0~0.050%, Mg: 0 to 0.100%, Ca: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, REM: 0~0.10, and Remainder: Fe and impurities It is preferable that the metal has a chemical composition consisting of the following: Each element will be described in more detail below.

[0036] [C: 0.01 to 0.50%] C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.01% or more. The C content may be 0.05% or more, 0.08% or more, 0.09% or more, 0.10% or more, or 0.15% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.40% or less, 0.35% or less, or 0.30% or less.

[0037] [Si: 0.01 to 3.50%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during annealing of cold-rolled sheets. To fully obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.08% or more, 0.10% or more, 0.30% or more, or 0.80% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.50% or less. The Si content may be 2.50% or less, 2.00% or less, or 1.50% or less.

[0038] [Mn: 0.10~5.00%] Mn is an element that affects the ferrite transformation of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 0.60% or more, 0.80% or more, 1.00% or more, or 1.50% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 5.00% or less. The Mn content may be 4.00% or less, 3.00% or less, or 2.50% or less.

[0039] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0040] [S:0.0300% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. The lower the S content, the better, ideally 0%. However, excessive reduction in the S content can significantly increase costs. For this reason, the S content may be 0.0001% or more, or may be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.0300% or less. The S content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0041] [N:0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. The lower the N content, the better, and ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, or may be 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, or 0.0050% or less.

[0042] The base steel sheet preferably has the basic chemical composition as described above. Furthermore, the base steel sheet may contain, as necessary, O: 0-0.020%, Al: 0-1.000%, B: 0-0.010%, Nb: 0-0.150%, Ti: 0-0.20%, Mo: 0-3.00%, Cr: 0-2.00%, V: 0-1.00%, Ni: 0-2.00%, W: 0-1.00%, Ta: 0-0.10%, and / or Fe: 0-0.150% instead of the remaining Fe. %, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.50%, Cu: 0-2.00%, As: 0-0.050%, Mg: 0-0.100%, Ca: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and REM: 0-0.100%. The content of each element may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0043] The remainder of the base steel plate other than the above elements is composed of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0044] The chemical composition of the base steel sheet may be measured by a common analytical method. For example, the chemical composition of the base steel sheet may be measured by first removing the coating layer by mechanical grinding and then using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0045] [Base steel plate thickness] The thickness of the base steel plate is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 6.0 mm or less, 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0046] [Mechanical properties of plated steel sheets] The plated steel sheet according to the embodiment of the present invention may have any appropriate tensile strength, and is not particularly limited. For example, it preferably has a tensile strength of 780 MPa or more. As described above, LME cracking occurs significantly when steel sheets having relatively high strength are spot-welded. Therefore, when the plated steel sheet according to the embodiment of the present invention has a high tensile strength of 780 MPa or more, the LME cracking suppression effect is particularly significant compared to conventional plated steel sheets having the same tensile strength. For example, in the embodiment of the present invention, the tensile strength of the plated steel sheet may be 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The upper limit is not particularly limited, and the tensile strength of the plated steel sheet may be 2300 MPa or less, 2000 MPa or less, 1800 MPa or less, or 1500 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from a specimen with the longitudinal direction parallel to the rolling direction of the plated steel sheet and conducting a tensile test in accordance with JIS Z 2241:2011.

[0047] <Method of manufacturing plated steel sheets> Next, a preferred method for producing a plated steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for producing a plated steel sheet according to an embodiment of the present invention, but is not intended to limit the plated steel sheet to one produced by the production method described below.

[0048] The plated steel sheet according to the present invention can be produced by, for example, carrying out a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, a pretreatment process, an annealing process in which the pretreated cold-rolled steel sheet is annealed, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the base steel sheet may be pickled after the hot rolling process without being coiled, and then directly subjected to the cold rolling process. Each process will be described in detail below.

[0049] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional continuous casting method, an ingot casting method, or the like.

[0050] [Hot rolling process] A hot-rolled steel sheet can be obtained by hot-rolling the cast steel slab. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0051] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then subjected to the cold rolling step described below without performing the coiling step.

[0052] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be air-cooled to room temperature, for example.

[0053] [Pretreatment process] Next, it is effective to carry out a predetermined pretreatment process before annealing the cold-rolled steel sheet. Such pretreatment processes can include a degreasing process and an optional grinding process. The degreasing process may include, for example, passing an electric current through the cold-rolled steel sheet in a solution having a pH of 8.0 or higher (electrolytic treatment). The current density during the current passing is 1.0 to 8.0 A / dm 2 The current application time may be 5 to 10 seconds. Meanwhile, the optional grinding process is preferably performed using a heavy-duty abrasive brush. By introducing strain into the surface of the cold-rolled steel sheet through grinding using a heavy-duty abrasive brush, the nucleation of the Fe-Al barrier layer during the plating process after the annealing process is promoted, densifying the Fe-Al barrier layer. This slows the growth rate of the Fe-Al barrier layer, allowing for a thinner layer. As a result, the amount of Al consumed in the formation of the Fe-Al barrier layer can be reduced. Therefore, the amount of Al in the plating layer other than the Fe-Al barrier layer can be increased, thereby increasing the ratio of the "Al concentration at the center of the plating layer" to the "Al concentration at the position of the plating layer where the Fe concentration is 50% of that of the base steel sheet" in the final plated steel sheet, thereby achieving higher LME resistance. The grinding process is not particularly limited, but for example, a grinding amount of 10 to 200 g / m can be used with a heavy-duty abrasive brush. 2The grinding amount by the heavy-duty abrasive brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited, but can be adjusted, for example, by appropriately selecting the number of heavy-duty abrasive brushes, the rotation speed, the brush pressure, the coating liquid to be used, etc.

[0054] [Annealing process] The cold-rolled steel sheet that has undergone the pretreatment step is then annealed. The holding temperature in the annealing step is preferably 700 to 900°C. If the holding temperature in the annealing step exceeds 900°C, an outer oxide layer may form on the surface of the steel sheet, resulting in a risk of reduced platability. The rate of temperature rise to the holding temperature is not particularly limited, but may be 1 to 10°C / second. The holding time at the holding temperature is preferably 10 to 300 seconds, more preferably 80 to 120 seconds. If the holding time exceeds 300 seconds, the outer oxide may grow excessively, resulting in a risk of reduced platability. The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C. If the dew point is too low, an outer oxide layer may form on the surface of the steel sheet, resulting in a risk of reduced platability. On the other hand, if the dew point is too high, Fe oxide may similarly form as an outer oxide on the surface of the steel sheet, resulting in a risk of reduced platability. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (for example, 4% hydrogen and the balance nitrogen).

[0055] [Plating process] Next, in the plating process, a coating layer having the above-described chemical composition and structure is formed on at least one, preferably both, surfaces of a cold-rolled steel sheet (base steel sheet). More specifically, the plating process is performed, for example, by hot-dip galvanization using a coating bath whose components are adjusted so that the chemical composition of the coating layer falls within the above-described range. In the plating process, it is extremely important to first control the time from immersion of the steel sheet in the coating bath to the start of cooling to 6 seconds or less, and then control the average cooling rate from the bath temperature (e.g., 420 to 480°C) to 370°C to 20°C / second or more. By satisfying these requirements, the Fe-Al barrier layer can be thinned, reducing the amount of Al consumed in forming the Fe-Al barrier layer and ensuring sufficient Al content in the coating layer other than the Fe-Al barrier layer. As a result, the ratio of the "Al concentration at the center of the coating layer" to the "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" in the finally obtained coated steel sheet can be set to 0.10 or more, thereby reliably improving the LME resistance of the coated steel sheet. On the other hand, if any one of these requirements is not satisfied, i.e., if the time from the start of immersion of the steel sheet in the coating bath to the start of cooling exceeds 6 seconds and / or the average cooling rate from the bath temperature to 370°C is less than 20°C / s, a large amount of Al is consumed in forming the Fe-Al barrier layer, resulting in a decrease in the Al content in the coating layer other than the Fe-Al barrier layer.As a result, the desired ratio of "Al concentration in the coating layer center" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet" cannot be obtained in the finally obtained coated steel sheet, and the LME resistance of the coated steel sheet is reduced.

[0056] From the viewpoint of further improving the LME resistance of coated steel sheets, it is preferable that the time from the start of immersion of the steel sheet in the coating bath to the start of cooling is shorter and that the average cooling rate from the bath temperature to 370°C is faster. For example, the LME resistance of coated steel sheets can be further improved by setting the Al content of the entire coating layer to 0.30% or more, setting the time from the start of immersion of the steel sheet in the coating bath to the start of cooling to 4 seconds or less, and setting the average cooling rate from the bath temperature to 370°C to 40°C / second or more. When grinding treatment using a heavy abrasive brush is performed as the pretreatment step described above, similarly higher LME resistance can be achieved even under conditions where the time from the start of immersion of the steel sheet in the coating bath to the start of cooling is 6 seconds or less and the average cooling rate from the bath temperature to 370°C is 20°C / second or more, by combining this with the condition that the Al content of the entire coating layer is 0.30% or more. Alternatively, the LME resistance of the plated steel sheet can be more significantly improved by setting the Al content of the entire coating layer to 0.30% or more, performing grinding using a heavy-duty abrasive brush as the pretreatment step described above, and setting the time from the start of immersion of the steel sheet in the coating bath to the start of cooling to 4 seconds or less and the average cooling rate from the bath temperature to 370°C to 40°C / second or more. The lower limit of the time from the start of immersion of the steel sheet in the coating bath to the start of cooling is not particularly limited, but for example, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling may be 2 seconds or more. Similarly, the upper limit of the average cooling rate from the bath temperature to 370°C is not particularly limited, but for example, the average cooling rate from the bath temperature to 370°C may be 80°C / second or less. Other conditions of the coating process may be appropriately set taking into account the thickness and coating weight of the coating layer, etc. For example, after immersing a cold-rolled steel sheet in a coating bath, it is pulled out, and immediately sprayed with N2 gas or air by a gas wiping method, and then cooled, thereby making it possible to keep the coating weight of the coating layer within a predetermined range, for example, 10 to 170 g / m per side. 2 can be adjusted within the range.

[0057] In coated steel sheets manufactured by this manufacturing method, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" is controlled within the range of 0.10 to 1.50, thereby fully utilizing the effect of adding Al to the coating layer to inhibit or reduce the penetration of molten Zn into the steel sheet during spot welding, thereby significantly improving the LME resistance of the coated steel sheet. Therefore, such coated steel sheets can achieve superior LME resistance compared to conventional coated steel sheets having a coating layer with a similar chemical composition, more specifically, a Zn-based coating layer with a similar Al content, and can contribute to industrial development by improving crash safety and extending the life of coated steel sheets, particularly when used as coated steel sheets for automobiles.

[0058] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0059] In the following examples, plated steel sheets according to the embodiments of the present invention were produced under various conditions, and the LME resistance of the produced plated steel sheets was examined.

[0060] First, molten steel having a chemical composition consisting of, by mass%, 0.15% C, 1.00% Si, 2.60% Mn, 0.010% P, 0.0020% S, 0.0100% N, 0.020% Al, and the balance Fe and impurities was cast by continuous casting to form a steel billet. The steel billet was once cooled, reheated to 1200°C, hot rolled, and then coiled at 600°C. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at 950°C and a reduction of 30%. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheet was subjected to a 5.0 A / dm 2Then, if necessary, a 2.0% NaOH aqueous solution is applied to the cold-rolled steel sheet, and the sheet is polished with a heavy-duty abrasive brush (Hotani D-100) at a density of 10 to 200 g / m. 2 The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm, a brush reduction of 2.0 mm, and a rotation speed of 600 rpm to introduce strain into the surface of the cold-rolled steel sheet. Table 1 shows whether each cold-rolled steel sheet was ground with a heavy abrasive brush.

[0061] Next, each cold-rolled steel sheet was cut into a size of 100 mm x 200 mm and then annealed under conditions of a dew point of 0°C, a holding temperature of 870°C, and a holding time of 100 seconds (annealing atmosphere: 4% hydrogen and balance nitrogen). For all steel sheet samples, the heating rate during annealing was 5°C / s. Next, the cut steel sheet samples were plated using a hot-dip galvanizing bath having a predetermined bath composition under the conditions of the bath temperature, time from immersion in the galvanizing bath to the start of cooling, and average cooling rate from the bath temperature to 370°C shown in Table 1, thereby obtaining plated steel sheet samples with a coating layer formed on both surfaces of the steel sheet samples. After immersion in the galvanizing bath, the steel sheet samples were removed, and before the start of cooling, the coating weight was adjusted to 50 g / m per side by N2 gas wiping. 2 In Comparative Examples 28 and 29, after the hot dip galvanizing treatment, alloying heat treatment was carried out at 520°C for 10 seconds and at 570°C for 30 seconds, respectively.

[0062] [Chemical composition analysis of plating layer] The chemical composition of the plating layer was determined by immersing a sample cut to 30 mm x 30 mm in a 10% HCl solution containing an inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.), pickling the plating layer, and then measuring the plating components dissolved in the solution using ICP emission spectroscopy. The results are shown in Table 1.

[0063] [Tensile strength of plated steel sheet] The tensile strength was measured by taking JIS No. 5 test pieces so that the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the plated steel sheet sample, and conducting a tensile test in accordance with JIS Z 2241:2011. As a result, the tensile strength of all plated steel sheet samples was 780 MPa or more.

[0064] [Measurement of Al concentration distribution in plating layer] First, the coated steel sheet samples were cut into 50 mm x 50 mm pieces. The cut samples were then subjected to GDS measurement to obtain the Al concentration distribution from the surface of the coating layer to a depth of 100 μm. Next, the Al concentration at the depth where the Fe intensity was 50% of the Fe intensity of the base steel sheet (the Fe intensity at a depth of 100 μm from the surface of the coating layer of the sample) was determined as the "Al concentration at the coating layer where the Fe concentration is 50% of that of the base steel sheet." The distance from this depth to the surface was defined as the thickness of the coating layer. The Al concentration measured by GDS at a position halfway through the thickness of the coating layer was determined as the "Al concentration at the center of the coating layer." Finally, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer where the Fe concentration is 50% of that of the base steel sheet" was determined.

[0065] [Evaluation of LME resistance by spot welding] A 100 x 100 mm plated steel sheet sample was subjected to spot welding. Two pieces were cut to a size of 50 x 100 mm, and welded joints were fabricated by spot welding these two plated steel sheet samples using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 5°, a pressure of 4.0 kN, a welding time of 0.5 seconds, and a current of 9 kA. Next, the cross section of the welded joint was polished and observed with an optical microscope. The length of the LME crack that occurred in the cross section of the weld was measured, and LME resistance was evaluated as follows. AAA: No LME cracks, AA: LME crack length over 0 μm to 100 μm, A: LME crack length over 100 μm to 500 μm, B: LME crack length over 500 μm

[0066] Coated steel sheets with LME resistance ratings of AAA, AA, and A were evaluated as having improved LME resistance. The results are shown in Table 1 below.

[0067] [Table 1]

[0068] Referring to Table 1, in Comparative Example 26, the Al content of the entire coating layer was low, so the effect of adding Al to suppress LME cracking could not be fully exerted, and LME resistance was reduced. In Comparative Example 27, the Al content of the entire coating layer was high, which is thought to have lowered the melting point of the coating layer. As a result, Zn in the coating layer was more likely to melt during spot welding, resulting in reduced LME resistance. In Comparative Examples 28 and 29, the Fe content in the coating layer was increased by the alloying heat treatment, making it impossible to obtain the desired coating chemical composition and the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the coating layer position where the Fe concentration is 50% of that of the base steel sheet," resulting in reduced LME resistance. In Comparative Example 30, the time from immersion in the coating bath to the start of cooling was long, so a large amount of Al was consumed in forming the Fe-Al barrier layer, which is thought to have reduced the Al content in the coating layer other than the Fe-Al barrier layer. As a result, the desired ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" could not be obtained, and LME resistance was reduced. In Comparative Example 31, the average cooling rate from a bath temperature of up to 370°C was slow, which is thought to have resulted in a large amount of Al being consumed in forming the Fe-Al barrier layer, and the amount of Al in the coating layer other than the Fe-Al barrier layer being reduced. As a result, the desired ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position of the coating layer where the Fe concentration is 50% of that of the base steel sheet" could not be obtained, and LME resistance was reduced.

[0069] In contrast, the plated steel sheets according to all of the examples had a predetermined plating chemical composition, and the ratio of "Al concentration at the center of the plating layer" to "Al concentration at the position in the plating layer where the Fe concentration is 50% of that of the base steel sheet" was controlled within the range of 0.10 to 1.50, thereby fully demonstrating the effect of adding Al to the plating layer and reliably suppressing or reducing LME cracking. In particular, in Examples 4 and 5 (without grinding with a heavy abrasive brush) in which the Al content of the entire coating layer was 0.30% or more, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was 4 seconds, and the average cooling rate from the bath temperature to 370°C was 40°C / second, and in Examples 12 and 19 in which the Al content of the entire coating layer was 0.30% or more and grinding with a heavy abrasive brush was performed as pretreatment for the annealing step (however, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was 6 seconds, and the average cooling rate from the bath temperature to 370°C was 20°C / second), the ratio of "Al concentration in the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" was 0.20 or more, and as a result, the LME resistance was evaluated as AA, further improving the LME resistance of the coated steel sheet. Additionally, in Examples 6 to 11, 13 to 18, and 20 to 25, in which the Al content of the entire coating layer was set to 0.30% or more, grinding with a heavy abrasive brush was performed as pretreatment for the annealing process, the time from the start of immersion of the steel sheet in the coating bath to the start of cooling was set to 4 seconds, and the average cooling rate from the bath temperature to 370°C was set to 40°C / second, the ratio of "Al concentration at the center of the coating layer" to "Al concentration at the position in the coating layer where the Fe concentration is 50% of that of the base steel sheet" was 0.30 or more, and as a result, the LME resistance was evaluated as AAA, further improving the LME resistance of the coated steel sheet.

Claims

1. A steel plate having a base steel sheet and a plating layer formed on a surface of the base steel sheet, The plating layer comprises, in mass %, Al: 0.10 to 1.50%, and Fe:0.01~2.00% and further comprising Mg: 0-1.500%, Si: 0 to 1.000%, Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0-1.000%, Ti: 0 to 1.000%, Cr: 0-1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, P: 0 to 0.500%, and Sr: 0~0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, When the coating layer is measured by glow discharge optical emission spectrometry (GDS), the ratio of "Al concentration at the center of the coating layer" to "Al concentration at a position in the coating layer where the Fe concentration is 50% of that in the base steel sheet" is 0.10 to 1.50, A plated steel sheet having a tensile strength of 780 MPa or more.

2. 2. The plated steel sheet according to claim 1, characterized in that the chemical composition contains, in mass%, Al: 0.30 to 1.50%, and when the plated layer is measured by GDS, the ratio of "Al concentration at a center of the plated layer" to "Al concentration at a position in the plated layer where the Fe concentration is 50% of that of the base steel sheet" is 0.20 to 1.

50.

3. 2. The plated steel sheet according to claim 1, characterized in that the chemical composition contains, in mass%, Al: 0.30 to 1.50%, and when the plated layer is measured by GDS, the ratio of "Al concentration at the center of the plated layer" to "Al concentration at a position in the plated layer where the Fe concentration is 50% of that in the base steel sheet" is 0.30 to 1.

50.

4. The plated steel sheet according to any one of claims 1 to 3, wherein the plated layer is a hot-dip galvanized (GI) layer.

5. A plated steel sheet as described in any one of claims 1 to 3, characterized in that when the plated layer is measured by GDS, the ratio of "Al concentration at the center of the plated layer" to "Al concentration at the plated layer position where the Fe concentration is 50% of that of the base steel sheet" is 0.45 to 1.50.

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