Plated steel sheet

Abstract

Provided is a plated steel sheet characterized by comprising a base steel sheet and a plating layer that is formed on the surface of the base steel sheet, wherein the plating layer has a prescribed chemical composition, the depth, in the sheet thickness direction from the interface between the base steel sheet and the plating layer, where the area fraction of pearlite is 0-20% is 3-100 μm, the area fraction of pearlite having a circle-equivalent diameter of 5 μm or more is 0-30% at the depth where the area fraction of pearlite is 0-20%, and the adhesion amount of the plating layer is 40 g / m2 or more per surface.

Description

[Technical Field]

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

[0002] Hot stamping is known as a technique for press-forming difficult-to-form materials such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated before being formed. With this technique, the steel is soft and has good formability at the time of forming because the material is heated before being formed. Therefore, even high-strength steel can be formed with high precision into complex shapes. In addition, since the steel is quenched at the same time as forming using a press die, the steel after forming is known to have sufficient strength.

[0003] In this regard, various studies have been conducted on plated steel sheets for hot stamping.

[0004] For example, Patent Document 1 describes a galvanized steel sheet for hot stamping, comprising a base steel sheet and a coating layer formed on the surface of the base steel sheet, the base steel sheet containing, by mass%, 0.10-0.5% C, 0.7-2.5% Si, 1.0-3% Mn, and 0.01-0.5% Al, with the remainder being iron and unavoidable impurities, the base steel sheet having an internal oxidation layer containing oxides of at least one of Si and Mn and having a thickness of 1 μm or more, and a decarburized layer extending from the interface with the coating layer toward the interior of the base steel sheet and having a thickness of 20 μm or less. Patent Document 1 also teaches that by making the thickness of the internal oxidation layer of the base steel sheet 1 μm or more, it is possible to sufficiently suppress the occurrence of bare spots in the galvanized steel sheet and to achieve sufficiently high adhesion between the formed coating layer and the base steel sheet. In addition, Patent Document 1 teaches that an internal oxidation layer is formed near the surface of a base steel sheet by high dew-point annealing, while a decarburized layer is formed on the surface and near the surface of the base steel sheet by the high dew-point annealing, and that since the carbon content in the decarburized layer is low, the tensile strength is lower than that of a non-decarburized portion. However, if the thickness of the decarburized layer is 20 μm or less, the effect of the decarburized layer on the strength of a galvanized steel sheet and a hot-stamped product produced using the same can be suppressed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-151883 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when a galvanized steel sheet such as that described in Patent Document 1 is used in hot stamping, the galvanized steel sheet is processed at high temperatures (e.g., approximately 900°C), which can cause the Zn contained in the coating layer to be in a molten state. As a result, the molten Zn may penetrate the steel and cause cracks inside the steel sheet. This phenomenon is called liquid metal embrittlement (LME), and it is known that LME can cause a decrease in the fatigue properties of steel sheets. Furthermore, after hot stamping, the coating layer may alloy with the base steel (base steel sheet), resulting in a decrease in corrosion resistance. On the other hand, even if a high-Zn concentration phase consisting of a Zn-based intermetallic compound, which has relatively high corrosion resistance, is included in the coating layer to address this issue, this high-Zn concentration phase exists as a liquid phase at the high temperatures during hot stamping, thereby increasing the risk of LME cracking.

[0007] Therefore, an object of the present invention is to provide a plated steel sheet that can suppress LME cracking while maintaining high corrosion resistance even when applied to hot stamping. [Means for solving the problem]

[0008] As a result of investigations conducted by the present inventors to achieve the above-mentioned object, they found that forming a coating layer containing Zn in a coating weight equal to or greater than a predetermined amount maintains sufficient corrosion resistance even when applied to hot stamping, and that by appropriately modifying the surface layer structure of the base steel sheet, even with a coating layer formed in such a coating weight, it is possible to significantly suppress or reduce the occurrence of LME cracking during high-temperature heating in hot stamping, and thus 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 to 0.50%, and Fe: 0 to 17.00% and further comprising Mg: 0 to less than 0.500% Si: 0 to 0.200%, Ni: 0 to less than 0.500% Ca: 0~3.000%, 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%, Sr: 0 to 0.500% In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0 to 0.500%, and W: 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, the depth of the interface between the base steel sheet and the coating layer in the sheet thickness direction is 3 to 100 μm, and the area ratio of pearlite is 0 to 20%, The area ratio of pearlite having a circle equivalent diameter of 5 μm or more at a depth of 0 to 20% is 0 to 30%, The coating weight of the plating layer is 40 g / m per side 2 A plated steel sheet characterized by the above. (2) The plated steel sheet according to (1) above, wherein the depth at which the pearlite area ratio is 0 to 20% is 10 to 100 μm. (3) The plated steel sheet according to (2) above, characterized in that the depth at which the pearlite area ratio is 0 to 20% is 30 to 100 μm. (4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the area ratio of pearlite having a circle equivalent diameter of 5 μm or more at a depth where the area ratio of the pearlite is 0 to 20% is 0 to 15%. (5) The surface Al content of the plating layer measured by fluorescent X-rays is 5 mg / m 2 The plated steel sheet according to any one of the above items (1) to (4), wherein the thickness of the plated steel sheet is larger than that of the plated steel sheet. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a plated steel sheet that can suppress LME cracking while maintaining high corrosion resistance even when applied to hot stamping. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram showing the shape of a hot-stamped steel produced in the examples. 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 to 0.50%, and Fe: 0 to 17.00% and further comprising Mg: 0 to less than 0.500% Si: 0 to 0.200%, Ni: 0 to less than 0.500% Ca: 0~3.000%, 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%, Sr: 0 to 0.500% In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0 to 0.500%, and W: 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, the depth of the interface between the base steel sheet and the coating layer in the sheet thickness direction is 3 to 100 μm, and the area ratio of pearlite is 0 to 20%, The area ratio of pearlite having a circle equivalent diameter of 5 μm or more at a depth of 0 to 20% is 0 to 30%, The coating weight of the plating layer is 40 g / m per side 2 The present invention is characterized by the above.

[0013] As mentioned above, when a galvanized steel sheet is used in hot stamping, the galvanized steel sheet is processed at high temperatures (e.g., about 900°C), which means that the Zn contained in the coating layer is processed in a molten state. This molten Zn may penetrate the steel and cause LME cracking inside the steel sheet. Although the reason for this is not entirely clear, research by the present inventors and others has revealed that carbon contained in the steel sheet is an element that promotes such LME cracking. Therefore, it is believed that the occurrence of LME cracking can be suppressed or reduced by reducing the carbon concentration in the surface layer of the steel sheet, where LME cracking occurs, for example, by decarburization. However, in reality, the LME suppression effect based on such a reduction in the carbon concentration in the surface layer of the steel sheet during hot stamping is limited and may not necessarily be satisfactory.

[0014] As a result of various investigations, the inventors have found that even if the carbon concentration in the surface layer of a base steel sheet is reduced by decarburization or the like in order to improve LME resistance, carbon contained in the base steel sheet diffuses into the steel surface layer during high-temperature heating in hot stamping, and this recarburization in the steel surface layer eliminates or reduces the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer of the base steel sheet. Therefore, the inventors have conducted further investigations and found that by creating a structure in the surface layer of the base steel sheet that can inhibit such recarburization, the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer of the base steel sheet can be fully exerted, even when the coating weight of the Zn-containing coating layer is relatively large to maintain sufficient corrosion resistance, and the occurrence of LME cracking during high-temperature heating in hot stamping can be reliably inhibited or reduced. More specifically, the inventors have found that by creating a structure in the surface layer of the base steel sheet that can inhibit such recarburization, the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer of the base steel sheet can be fully exerted, and the occurrence of LME cracking during high-temperature heating in hot stamping can be reliably inhibited or reduced. 2It was discovered that by satisfying the above conditions, sufficient corrosion resistance is maintained even when applied to hot stamping, and by forming a structure in the surface layer of the base steel sheet in which the depth at which pearlite has an area ratio of 0 to 20% is 3 to 100 μm from the interface between the base steel sheet and the coating layer in the sheet thickness direction, and the area ratio of pearlite with a circle equivalent diameter of 5 μm or more at the depth at which the pearlite area ratio is 0 to 20% is controlled to 0 to 30%, LME cracking during high-temperature heating in hot stamping can be reliably suppressed or reduced.

[0015] 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, the structure in the surface layer of the base steel sheet acts as follows to suppress or reduce the diffusion of carbon contained in the base steel sheet into the steel surface layer and recarburization during high-temperature heating in hot stamping. More specifically, when the carbon concentration in the surface layer of the base steel sheet is reduced by decarburization or the like, the amount of pearlite formed in the microstructure in the surface layer of the base steel sheet is relatively small in association with this low carbon concentration. Here, in the plated steel sheet according to the embodiment of the present invention, it is important to first reduce the carbon concentration in the surface layer of the base steel sheet by decarburization or the like so that the depth in the thickness direction from the interface between the base steel sheet and the coating layer where the pearlite area ratio is 0 to 20%, i.e., the depth of the region where the pearlite area ratio is relatively low, is 3 to 100 μm. This makes it possible to fully exert the LME suppression effect based on the reduced carbon concentration. However, simply reducing the area fraction of pearlite to within a predetermined range is not enough. In cases where pearlite precipitates along grain boundaries, the pearlite transforms into austenite during high-temperature heating during hot stamping, forming a carbon diffusion path (i.e., a carbon recarburization path) through the austenite along the grain boundaries. During high-temperature heating during hot stamping, carbon in the bulk of the base steel sheet attempts to diffuse toward the surface due to the concentration gradient between the high carbon concentration in the bulk and the low carbon concentration at the surface. If austenite-induced carbon recarburization paths exist along the grain boundaries, carbon in the bulk diffuses toward the surface through the recarburization path, accelerating recarburization in the steel surface layer. As a result, the LME suppression effect achieved by initially reducing the carbon concentration in the surface layer of the base steel sheet cannot be fully achieved.In contrast, according to the plated steel sheet according to the embodiment of the present invention, in the above-mentioned depth region where the area ratio of pearlite is relatively low, the area ratio of pearlite having a circle equivalent diameter of 5 μm or more is controlled to be within a range of 0 to 30% to reduce the amount of relatively large pearlite. This makes it possible to cause austenite transformed from pearlite to be dispersed and present on grain boundaries even during high-temperature heating in hot stamping, thereby reliably interrupting the recarburization pathway of carbon by austenite.

[0016] More specifically, when pearlite transforms to austenite during high-temperature heating in hot stamping, a two-phase structure of ferrite and austenite is formed. In such a case, the austenite present at the interface between ferrite and austenite extends to the surface side of the base steel sheet, forming a carbon recarburization path, which in turn promotes carbon diffusion from the bulk of the base steel sheet to the surface side. In relation to this, in the plated steel sheet according to an embodiment of the present invention, it is important to reduce the area fraction of pearlite in the surface layer portion of the base steel sheet, i.e., the region 3 to 100 μm deep in the sheet thickness direction from the interface between the base steel sheet and the coating layer, to 0 to 20% and to limit the area fraction of relatively coarse pearlite, i.e., pearlite with a circle equivalent diameter of 5 μm or more, in this depth region to the range of 0 to 30%. Such a surface layer structure can reduce the amount of austenite transformed from pearlite even during high-temperature heating in hot stamping. Furthermore, the austenite can be dispersed and present on grain boundaries, thereby reliably disrupting the carbon recarburization path by austenite. Therefore, according to the plated steel sheet according to the embodiment of the present invention, even though the coating weight of the Zn-containing coating layer is relatively large to maintain sufficient corrosion resistance, creating conditions that make LME more likely to occur, recarburization during high-temperature heating in hot stamping is significantly suppressed, thereby fully demonstrating the LME-inhibiting effect due to the initial low carbon concentration in the surface layer of the base steel sheet, thereby reliably suppressing or reducing the occurrence of LME cracking during hot stamping. The fact that the occurrence of LME cracking can be suppressed or reduced as described above by appropriately modifying the surface layer structure of the base steel sheet in a plated steel sheet provided with a Zn-containing coating layer was first discovered by the present inventors. Furthermore, because this LME-inhibiting effect can be exerted not only during high-temperature heating in hot stamping but also during spot welding after hot stamping, the plated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where spot welding is relatively frequently used.

[0017] 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.

[0018] [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:

[0019] [Al: 0-0.50%] Al is an element effective in improving the corrosion resistance of the coating layer. The Al content may be 0%, but to achieve this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.01% or more, 0.03% or more, 0.05% or more, 0.08% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Al content is excessive, the composition of the coating layer approaches the Zn-Al eutectic composition, which may lower the melting point of the coating layer. Therefore, the Al content is set to 0.50% or less. The Al content may be 0.45% or less, 0.40% or less, 0.30% or less, or 0.25% or less.

[0020] [Fe: 0-17.00%] Fe is an element that can be included 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. The Fe content may be 0%, but when Fe is contained, the Fe content may be 0.01% or more, 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, Fe may be contained in the coating layer up to approximately 17.00%, but this range does not adversely affect the coated steel sheet according to the embodiment of the present invention. Therefore, the Fe content is set to 17.00% or less, and may be, for example, 15.00% or less, 12.00% or less, 10.00% or less, 8.00% or less, 5.00% or less, 3.00% or less, 1.00% or less, or 0.80% or less.

[0021] Furthermore, the plating layer may optionally contain Mg: 0 to less than 0.500%, Si: 0 to 0.200%, Ni: 0 to less than 0.500%, Ca: 0 to 3.000%, 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 1.00 The alloy may contain at least one of the following optional elements: 0%, 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%, and W: 0-0.500%. Although there are no particular limitations on the amount of these optional elements, it is preferable that the total amount be 5.000% or less. The optional elements may total up to 4.500%, 4.000%, 3.500%, 3.000%, 2.500%, 2.000%, 1.500%, or 1.000%. These optional elements are described in more detail below.

[0022] [Mg: 0 to less than 0.500%] 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, from the viewpoint of improving workability, the Mg content may be less than 0.500%. The Mg content may be 0.490% or less, 0.480% or less, 0.470% or less, 0.450% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0023] [Si: 0 to 0.200%] 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, 0.001% or more, 0.010% or more, or 0.050% or more. On the other hand, from the viewpoint of improving the plating adhesion of the plating layer, the Si content may be 0.200% or less. The Si content may be 0.180% or less, 0.150% or less, 0.120% or less, or 0.100% or less.

[0024] [Ni: 0 to less than 0.500%] 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 0.0001% or more. The Ni content may be 0.0004% or more, 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more. While there is no particular upper limit, from the viewpoint of production costs, etc., the Ni content may be less than 0.500%, for example, 0.490% or less, 0.480% or less, 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0025] [Ca: 0-3.000%] 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 0.005% or more, 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 3.000% or less. The Ca content may be 2.500% or less, 2.000% or less, or 1.500% or less.

[0026] [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% and W: 0~0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W may not be present in the coating layer, but may be present in the coating layer in amounts of 0.0001% or more, 0.001% or more, or 0.01% 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 content of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, and W is 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.

[0027] 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.

[0028] [Measurement of the chemical composition of the plating layer] The chemical composition of the plating layer is determined as follows. First, the plating layer is stripped and dissolved from the plated steel sheet using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the resulting acid solution is measured by ICP (inductively coupled plasma) atomic emission spectroscopy to determine the chemical composition (average composition) of the plating layer. The type of acid is not particularly limited, and any acid that can dissolve the plating layer may be used.

[0029] The plating layer may be any plating layer having the above chemical composition, and is not particularly limited, but may be, for example, a hot-dip galvanized (GI) layer, a galvannealed (GA) layer, an electrogalvanized (EG) layer, etc. Preferably, the plating layer is a hot-dip galvanized (GI) layer or a galvannealed (GA) layer.

[0030] [Plating layer adhesion: 40g / m per side 2 End] In an embodiment of the present invention, the coating weight of the plating layer is 40 g / m per side. 2 That's all. Generally, a coating layer may be alloyed with the base steel sheet during high-temperature heating in hot stamping, resulting in a decrease in corrosion resistance. However, according to an embodiment of the present invention, although the reason is not necessarily clear, it is believed that the alloying of the coating layer and the base steel sheet can be delayed due to the surface layer structure of the base steel sheet, i.e., the surface layer structure in which the area fraction of pearlite is 0 to 20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction is 3 to 100 μm deep, and the area fraction of pearlite with a circle equivalent diameter of 5 μm or more at that depth is 0 to 30%. Therefore, the coating weight of the coating layer is made relatively large, specifically 40 g / m per side. 2By controlling the coating weight as described above, when the steel sheet is applied to hot stamping, a coating layer in which alloying has not progressed sufficiently will be present, and it is believed that the presence of such a coating layer will make it possible to maintain sufficient corrosion resistance. On the other hand, if the coating weight of the coating layer is small, the effects related to the delay in alloying as described above will not be fully obtained, and the corrosion resistance after hot stamping may decrease. From the viewpoint of improving corrosion resistance, the coating weight of the coating layer is preferably 45 g / m per side. 2 or more than 50g / m 2 More preferably, 60 g / m 2 or more, and even more preferably 70 g / m 2 More than 80 g / m 2 Although the upper limit is not particularly limited, the coating weight of the plating layer is, for example, 200 g / m 2 Below 190g / m 2 Below 180g / m 2 or less than 170g / m 2 It may be the following:

[0031] [Measurement of plating adhesion weight] The coating weight of the plating layer is determined as follows. First, a 30 mm × 30 mm sample is taken from the plated steel sheet. Then, the plating layer is stripped and dissolved from this sample using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet. The coating weight of the plating layer is determined from the change in weight of the sample before and after stripping and dissolution. The type of acid is not particularly limited, and any acid that can dissolve the plating layer may be used.

[0032] [Amount of Al on the surface of the plating layer measured by fluorescent X-ray: 5 mg / m 2 super] According to an embodiment of the present invention, the surface Al amount of the plating layer measured by fluorescent X-rays is 5 mg / m 2It is preferable that the surface Al content of the plating layer measured by X-ray fluorescence measurement is greater than 5 mg / m. The surface Al content of the plating layer measured by X-ray fluorescence measurement correlates with the formation of an Al oxide film on the surface of the plating layer. Therefore, a larger surface Al content of the plating layer measured by X-ray fluorescence measurement indicates a thicker Al oxide film formed on the surface of the plating layer. For example, when a conventional Zn-based plated steel sheet or an Al-Zn-based plated steel sheet is used in hot stamping, the plated steel sheet is generally heated to a temperature of about 900°C or higher during hot stamping. Since Zn has a relatively low boiling point of about 907°C, the Zn in the plating layer may partially evaporate and / or oxidize at such high temperatures, which may result in a decrease in corrosion resistance after hot stamping. According to a preferred embodiment of the present invention, the surface Al content of the plating layer measured by X-ray fluorescence measurement is greater than 5 mg / m. 2 By forming an Al oxide film on the surface of the coating layer in such a range that the Al oxide film is larger than the Al oxide film on the surface of the coating layer, evaporation and / or oxidation of Zn in the coating layer can be suppressed or reduced even when the coating layer is applied to hot stamping. In addition, in relation to this, compared to when an Al oxide film is not formed or is only formed insufficiently on the surface of the coating layer, the Zn concentration in the coating layer of the obtained hot stamped article can be maintained relatively high and the Fe concentration can be relatively reduced, making it possible to further improve corrosion resistance after hot stamping. From the viewpoint of further improving corrosion resistance, the surface Al content of the coating layer as determined by fluorescent X-ray measurement is preferably as high as possible, and is preferably, for example, 10 mg / m 2 More than 12mg / m 2 More than 15mg / m 2 or more than 20 mg / m 2 The upper limit is not particularly limited, but for example, the surface Al amount of the plating layer measured by fluorescent X-rays is 70 mg / m 2 or less than 60 mg / m 2 It may be the following:

[0033] The surface Al content of the plating layer measured by X-ray fluorescence measurement is calculated from the surface Al intensity obtained by X-ray fluorescence measurement of a plated steel sheet based on a calibration curve prepared in advance using plated steel sheets with known surface Al content.

[0034] [Depth of pearlite with an area ratio of 0 to 20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction: 3 to 100 μm] In an embodiment of the present invention, the depth at which the area ratio of pearlite is 0 to 20% in the sheet thickness direction from the interface between the base steel sheet and the coating layer is 3 to 100 μm. This feature is related to the low carbon concentration in the surface layer portion of the base steel sheet. Therefore, by having this feature, the LME suppression effect due to the low carbon concentration in the surface layer portion of the base steel sheet can be exerted, thereby suppressing or reducing the occurrence of LME cracking during high-temperature heating in hot stamping. In addition, by reducing the amount of pearlite in the surface layer portion of the steel sheet to within the above range, the amount of austenite transformed from pearlite during high-temperature heating in hot stamping can be reduced. Therefore, this feature can be said to be very important in preventing the formation of carbon recarburization paths by austenite along grain boundaries during hot stamping. From the perspective of further improving these effects, it is preferable to increase the surface layer region with little pearlite. More specifically, the depth at which the area ratio of pearlite is 0 to 20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction is preferably 5 μm or more or 10 μm or more, more preferably 20 μm or more or 30 μm or more, and most preferably 40 μm or more or 50 μm or more. The upper limit of the depth may be, for example, 90 μm or 80 μm.

[0035] [Area ratio of pearlite with a circle equivalent diameter of 5 μm or more at a depth of 0 to 20% of the area ratio of pearlite from the interface between the base steel sheet and the coating layer in the sheet thickness direction: 0 to 30%] In an embodiment of the present invention, the area fraction of pearlite having a circle equivalent diameter of 5 μm or more is 0 to 30% at a depth where the area fraction of pearlite is 0 to 20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction. By controlling the area fraction of pearlite having a circle equivalent diameter of 5 μm or more to be within the range of 0 to 30% in the depth region where the area fraction of pearlite is relatively low as described above and reducing the amount of relatively large pearlite, austenite transformed from pearlite can be dispersed and present on grain boundaries even during high-temperature heating in hot stamping, thereby reliably interrupting the recarburization pathway of carbon by austenite. Therefore, by significantly suppressing recarburization during high-temperature heating in hot stamping, the LME suppression effect due to the initial low carbon concentration in the surface layer of the base steel sheet can be fully exerted, making it possible to reliably suppress or reduce the occurrence of LME cracking during hot stamping. From the viewpoint of further improving such effects, the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at a depth where the pearlite area ratio is 0 to 20% is preferably 25% or less or 20% or less, more preferably 15% or less or 12% or less, and most preferably 10% or less or 8% or less. The lower limit of the area ratio of pearlite having an equivalent circle diameter of 5 μm or more may be, for example, 1% or 3%.

[0036] [Measurement of pearlite area ratio at depths of 0-20% and pearlite area ratio with a circle equivalent diameter of 5 μm or more] The area fraction of pearlite in the microstructure of the surface layer of the base steel sheet, at a depth of 0-20% and with a circle equivalent diameter of 5 μm or more, is determined as follows. First, five samples are taken from the surface of the plated steel sheet so that cross sections parallel to the rolling direction and the thickness direction can be observed. Next, these observation surfaces are mirror-polished and etched with a picral etching solution, after which the structure is observed using a scanning electron microscope (SEM). The measurement range for each sample is a rectangular area extending 100 μm in the thickness direction from the interface between the base steel sheet and the plated layer and 500 μm in the direction perpendicular to the thickness direction, with one field of view being a total of five fields of view for the five samples. The interface between the base steel sheet and the plated layer can be identified by the difference in color tone between the base steel sheet and the plated layer in the SEM backscattered electron image (BSE image). The area fraction of pearlite is calculated using the point counting method from a microstructure photograph, for example, at a magnification of approximately 5000x. Here, a region surrounded by grain boundaries where the ferrite crystal orientation misorientation is 15° or more, where ferrite and cementite phases are mixed, and where the cementite is lamellar and / or spherical, is recognized as pearlite, and its area fraction is calculated. For each sample, the depth position from the interface between the base steel sheet and the coating layer where the area fraction of pearlite gradually increases to 20% is identified, and the distance from the identified depth position to the interface is calculated, and the arithmetic average of these distances is determined as the "depth from the interface between the base steel sheet and the coating layer in the thickness direction where the area fraction of pearlite is 0 to 20%." Similarly, for each sample, the area fraction of pearlite with a circle equivalent diameter of 5 μm or more is calculated by image processing in the depth region from the interface where the area fraction of pearlite is 20%, and the arithmetic average of these distances is determined as the "area fraction of pearlite with a circle equivalent diameter of 5 μm or more at a depth from the interface between the base steel sheet and the coating layer in the thickness direction where the area fraction of pearlite is 0 to 20%."

[0037] [Area ratio of martensite: less than 1%] As described above, the microstructure of the base steel sheet may be formed in the surface layer portion of the base steel sheet such that the depth from the interface between the base steel sheet and the coating layer in the sheet thickness direction, where the area fraction of pearlite is 0 to 20%, is 3 to 100 μm, and the area fraction of pearlite with a circle-equivalent diameter of 5 μm or more at the depth where the area fraction of pearlite is 0 to 20%, is controlled to 0 to 30%. Therefore, other microstructures are not particularly limited. For example, in a preferred embodiment of the present invention, the area fraction of martensite contained in the base steel sheet is less than 1%. As will be described in detail later with respect to the method for producing a coated steel sheet, in order to obtain the above-mentioned pearlite structure, it is preferable to cool the base steel sheet at a relatively slow average cooling rate of 10°C / s or less from a controlled temperature of 620 to 670°C to the coating bath temperature in the cooling step after the annealing step. With such a relatively slow average cooling rate, martensite rarely precipitates, and even if it does precipitate, its area fraction will be less than 1%. The area fraction of martensite may be 0.5% or less, or even 0%.

[0038] [Identification of martensite and calculation of area fraction] The identification and calculation of the area fraction of martensite are performed as follows. First, a sample is taken so that the cross section parallel to the rolling direction and thickness direction of the plated steel sheet serves as the observation surface. The observation surface is then mirror-polished and etched with a nital etchant, after which the microstructure is observed using a scanning electron microscope (SEM). A 300 μm × 300 μm area is photographed at 1000x magnification at a depth of 1 / 2 the sheet thickness of the observation surface. The obtained microstructure photograph is then binarized to black and white and then analyzed to identify pearlite, bainite, and ferrite. The total area fraction of these components is then calculated using a method based on the "Method for Microscopic Testing of Grain Size in Steels" specified in JIS G 0551:2020. Because retained austenite is difficult to distinguish from martensite using an SEM, the area fraction of retained austenite is measured using X-ray diffraction. Finally, the area fraction of martensite is determined by subtracting the total area fraction of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0039] [Preferred chemical composition of base steel plate] As described above, the present invention aims to provide a plated steel sheet that can suppress LME cracking while maintaining high corrosion resistance even when applied to hot stamping, and the coating weight of the coating layer is set to 40 g / m per side. 2 The object is achieved by forming a structure in the surface layer portion of the base steel sheet, in which the depth at which pearlite has an area ratio of 0 to 20% is 3 to 100 μm from the interface between the base steel sheet and the coating layer in the sheet thickness direction, and the area ratio of pearlite having a circle equivalent diameter of 5 μm or more at the depth at which the pearlite area ratio is 0 to 20% is controlled to 0 to 30%. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the object of the present invention. Preferred chemical compositions of the base steel sheet used in the coated steel sheets according to embodiments of the present invention will be described in detail below. However, these descriptions are intended merely as examples of preferred chemical compositions of the base steel sheet suitable for achieving a Vickers hardness of 400 HV or more in the formed product after hot stamping, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0040] In an embodiment of the present invention, for example, the base steel plate contains, in mass%, C: 0.13~0.50%, Si: 0.001 to 3.000%, Mn: 0.30-3.00%, 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: 0~0.15%, Mo: 0-1.0% Cr: 0 to 1.0%, Cu: 0-1.0% Ni: 0 to 1.0% B: 0~0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.010% REM: 0~0.30%, Ir: 0 to 1.000%, 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.

[0041] [C:0.13~0.50%] 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 0.13% or more. The C content may be 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.

[0042] [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 annealing of cold-rolled sheets. 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 but decrease the 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.

[0043] [Mn: 0.30~3.00%] 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 0.30% or more. The Mn content may be 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.

[0044] [Al: 0.0002~2.000%] 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.

[0045] [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.

[0046] [S:0.1000% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content can result in a significant increase in 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 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0047] [N:0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, or may be 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may also be 0.0080% or less, or 0.0050% or less.

[0048] 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. The content of each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

[0049] 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.

[0050] The chemical composition of the base steel sheet can be measured using a common analytical method. For example, the chemical composition of the base steel sheet can be measured by first removing the coating 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 can be obtained from the base steel sheet at approximately half the thickness position, and the components can be determined by measuring them using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.

[0051] [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, or 4.0 mm or less.

[0052] [Mechanical properties] The plated steel sheet according to an embodiment of the present invention, more specifically, the plated steel sheet before hot stamping, has a tensile strength of, for example, less than 980 MPa, although this is not particularly limited. The tensile strength may be 950 MPa or less, 900 MPa or less, 850 MPa or less, or 800 MPa or less. The lower limit is not particularly limited, but the tensile strength may be, for example, 500 MPa or more, 550 MPa or more, or 590 MPa or more. According to an embodiment of the present invention, even if the tensile strength of the plated steel sheet before hot stamping is less than 980 MPa, the microstructure of the formed product after hot stamping becomes a martensite-based structure, and therefore it is possible to sufficiently achieve a Vickers hardness of 400 HV or more. The tensile strength is measured by conducting a tensile test in accordance with JIS Z 2241:2011 using a JIS No. 5 test piece taken from a position where the longitudinal direction of the test piece is parallel to the rolling direction of the plated steel sheet.

[0053] <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.

[0054] The plated steel sheet according to the embodiment of the present invention can be manufactured 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, an annealing process in which the 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 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.

[0055] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0056] [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%.

[0057] [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.

[0058] [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.

[0059] [Annealing process] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 730 to 900°C in an atmosphere with a dew point of -20 to 10°C and holding the temperature for 10 to 300 seconds. By carrying out the annealing step under such relatively high dew point conditions, it is possible to appropriately decarburize the surface layer of the cold-rolled steel sheet. Therefore, in the finally obtained plated steel sheet, it is possible to control the depth at which the area fraction of pearlite is 0 to 20% in the sheet thickness direction from the interface between the base steel sheet and the coating layer to within a range of 3 to 100 μm. If the dew point is lower than -20°C, the heating temperature is lower than 730°C, and / or the holding time is shorter than 10 seconds, decarburization in the surface layer of the cold-rolled steel sheet becomes insufficient. As a result, in the finally obtained plated steel sheet, it becomes impossible to achieve the depth at which the area fraction of pearlite is 0 to 20% in the sheet thickness direction from the interface between the base steel sheet and the coating layer to be 3 μm or more. On the other hand, if the dew point exceeds 10°C, the heating temperature exceeds 900°C, and / or the holding time exceeds 300 seconds, an outer oxide layer may form on the surface of the steel sheet, reducing galvanizability, or excessive decarburization may reduce the strength of the finally obtained plated steel sheet. The dew point is preferably -10 to 5°C, and more preferably -5 to 5°C. 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., 4% hydrogen and the balance nitrogen).

[0060] [Cooling process] The cold-rolled steel sheet whose surface layer has been decarburized in the annealing process must be appropriately cooled in the subsequent cooling process to obtain the desired surface layer structure. Specifically, the cooling process includes cooling from the heating temperature of the annealing process to a controlled temperature of 620 to 670°C at an average cooling rate of 20°C / s or more (primary cooling), and cooling from the controlled temperature to a coating bath temperature (e.g., the melting point of the coating bath + 20°C) at an average cooling rate of 10°C / s or less (secondary cooling). The primary cooling and secondary cooling will be described in more detail below.

[0061] [Primary cooling] In the primary cooling, it is important to suppress pearlite precipitation at high temperatures. More specifically, pearlite precipitates at high temperatures from the heating temperature of 730 to 900°C in the annealing process to the control temperature of 620 to 670°C. Because of its fast diffusion, it easily diffuses to grain boundaries and forms pearlite along the grain boundaries. This pearlite formed along the grain boundaries transforms to austenite during high-temperature heating in hot stamping, forming austenite-mediated carbon recarburization paths along the grain boundaries and promoting the recarburization of carbon in the bulk to the steel surface layer. Therefore, in the temperature range from the heating temperature in the annealing process to the control temperature, it is extremely important to suppress pearlite precipitation at high temperatures in the steel surface layer by cooling the cold-rolled steel sheet at a relatively fast average cooling rate of 20°C / s or more. If the average cooling rate is less than 20°C / s and / or the control temperature is higher than 670°C, pearlite precipitates at high temperatures where diffusion is fast, promoting the formation of pearlite along the grain boundaries. As a result, in the finally obtained plated steel sheet, the area ratio of pearlite with a circle equivalent diameter of 5 μm or more at a depth where the area ratio of pearlite is 0 to 20% from the interface between the base steel sheet and the plated layer in the sheet thickness direction exceeds 30%, and sufficient LME resistance cannot be achieved when applied to hot stamping.

[0062] [Secondary cooling] On the other hand, in the secondary cooling after the primary cooling, it is important to precipitate pearlite at a low temperature where diffusion is relatively slow. More specifically, pearlite can be precipitated by cooling from a controlled temperature of 620 to 670°C to the plating bath temperature (for example, the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less. Since the diffusion of pearlite precipitated in such a low temperature range below the controlled temperature is relatively slow, it is not formed in a form connected along the grain boundaries, but rather the pearlite can be dispersed and present on the grain boundaries. In such a structure, even during high-temperature heating in hot stamping, A c1Since austenite transformed from pearlite at or above this temperature can be dispersed and present on the grain boundaries in the same way, it is possible to reliably cut off the recarburization pathway of carbon by austenite. On the other hand, if the average cooling rate is more than 10°C / s and / or the controlled temperature is less than 620°C, martensite and bainite will mainly precipitate instead of pearlite, and in the finally obtained plated steel sheet, it will be impossible to make the depth where the area ratio of pearlite is 0 to 20% 100 μm or less in the sheet thickness direction from the interface between the base steel sheet and the plated layer. Martensite and bainite transform into austenite faster than pearlite, and c1 The ferrite-austenite two-phase structure is exposed to high temperatures for a longer period of time during hot stamping than pearlite. In such cases, recarburization paths are likely to form at the grain boundaries, making it difficult to achieve sufficient LME resistance.

[0063] [Plating process] Next, in the plating process, a plating layer having the above-described chemical composition is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). More specifically, the plating process is performed by hot-dip galvanizing using a plating bath (bath temperature: e.g., 420 to 480°C) whose components have been adjusted so that the chemical composition of the plating layer falls within the above-described range. After the hot-dip galvanizing process, an alloying treatment may be performed. The plating process is not limited to hot-dip galvanizing, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions for the plating process may be appropriately set taking into account 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 gas wiping, followed by cooling, to achieve a coating weight of the plating layer within a predetermined range, e.g., 40 to 200 g / m per side. 2 can be adjusted within the range.

[0064] [Cooling after plating] During cooling after plating, it is preferable to control the dew point of the cooling gas (e.g., nitrogen gas) to within a range of −10 to 10°C. By cooling the plated steel sheet in such an atmosphere with a relatively high dew point, a relatively thick Al oxide film can be formed on the surface of the plating layer. Due to the formation of this Al oxide film, evaporation and / or oxidation of Zn in the plating layer can be significantly suppressed or reduced, even during high-temperature heating in hot stamping. Furthermore, the Fe concentration in the plating layer of the obtained hot-stamped product can be relatively reduced. Therefore, it is possible to further improve corrosion resistance after hot stamping. As mentioned above, the formation of an Al oxide film can be confirmed based on the surface Al content of the plating layer by fluorescent X-ray measurement.

[0065] The coated steel sheet manufactured by this manufacturing method has a coating weight of 40 g / m per side. 2 In addition, a structure can be formed in the surface layer of the base steel sheet, in which the depth where the pearlite area ratio is 0-20% is 3-100 μm from the interface between the base steel sheet and the coating layer in the sheet thickness direction, and the area ratio of pearlite with a circle equivalent diameter of 5 μm or more at the depth where the pearlite area ratio is 0-20% is controlled to 0-30%. Therefore, even when exposed to high temperatures such as those during hot stamping, recarburization of carbon in the bulk to the steel surface layer can be significantly suppressed while maintaining high corrosion resistance. This allows the LME suppression effect achieved by the initial low carbon concentration in the surface layer of the base steel sheet to be fully exerted, reliably suppressing or reducing the occurrence of LME cracking during hot stamping. Therefore, when used as a coated steel sheet for hot stamping, such a coated steel sheet can maintain sufficient corrosion resistance and achieve better LME resistance than conventional coated steel sheets. This can contribute to industrial development by extending the life of coated steel sheets for automobiles and building materials.

[0066] 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]

[0067] In the following examples, plated steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced plated steel sheets were investigated.

[0068] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. The steel billet 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 rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°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 an annealing process under the conditions shown in Table 2 in a mixed gas atmosphere of 4% hydrogen and the balance nitrogen in a furnace with an oxygen concentration of 20 ppm or less, and then a cooling process was similarly performed under the conditions shown in Table 2 to produce a base steel sheet.

[0069] Next, the produced base steel sheet was cut into 100 mm x 200 mm pieces and plated using a batch-type hot-dip galvanizing test device manufactured by our company. More specifically, the produced base steel sheet was first immersed in a coating bath having a predetermined chemical composition for approximately 3 seconds, then pulled up at a pulling speed of 20 to 200 mm / s, and the coating layer thickness was adjusted to the value shown in Table 2 by N2 gas wiping. Next, the base steel sheet with the coating layer attached was cooled from the coating bath temperature (approximately 420 to 480°C) to room temperature using nitrogen gas controlled to the dew point shown in Table 2 as a cooling gas, thereby obtaining a plated steel sheet with a coating layer formed on both sides of the base steel sheet. The sheet temperature was measured using a thermocouple spot-welded to the center of the base steel sheet.

[0070] The physical properties and characteristics of the resulting plated steel sheets were measured and evaluated by the following methods.

[0071] [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, removing the plating layer by pickling, and then measuring the plating components dissolved in the solution using ICP emission spectroscopy. The results are shown in Table 2.

[0072] [Evaluation of LME resistance during hot stamping (HS) forming] First, a 180 mm x 50 mm blank was cut from the plated steel sheet and placed in a furnace at 900 °C. After the blank reached a temperature of -10 °C below the furnace temperature, it was held in the furnace for 100 seconds. Next, the blank was removed from the furnace and, after reaching 800 °C, hat-forming and die-quenching were performed using a hat-forming die set at approximately room temperature. The forming speed was varied between 200 mm / s, 100 mm / s, and 50 mm / s. The shape of the hat-formed body (hot-stamped body) is shown in Figure 1. The cross-section of the bent portion of the hat-formed body was cut out and observed under an SEM to determine whether LME cracking occurred at each forming speed. LME resistance was evaluated as follows. AAA: No LME cracks at forming speed of 200 mm / s AA: No LME cracks at a forming speed of 100 mm / s A: No LME cracks at a forming speed of 50 mm / s B: LME cracking occurs at forming speed of 50 mm / s

[0073] [Corrosion resistance evaluation] The corrosion resistance of plated steel sheets was evaluated as follows. First, plated steel sheets were placed in an atmospheric heating furnace at 900°C. After the plated steel sheet reached a furnace temperature of -10°C, they were held there for 100 seconds. Next, the plated steel sheets were removed from the furnace and clamped between flat molds at room temperature for quenching. After heating and quenching, 50mm x 100mm samples of the plated steel sheets were treated with zinc phosphate (SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard) and then electrocoated (PN110 Powernics Gray: Nippon Paint Industrial Coating Co., Ltd. standard) to a thickness of 20µm and baked at 150°C for 20 minutes. Next, a cut was made in the center of the sample, reaching the steel substrate (base steel sheet). Then, a 360-cycle cyclic corrosion test was performed in accordance with JASO (M609-91). Next, the electrodeposition coating was removed using a descoat, and the plating layer was removed using a 10% aqueous HCl solution containing an inhibitor. After that, the depth of corrosion of the steel substrate was measured using a laser meter, and the corrosion resistance was evaluated as follows. AAA: Substrate corrosion depth 0.1 mm or less AA: Substrate corrosion depth: over 0.1 to 0.3 mm A: Substrate corrosion depth: over 0.3 to 0.4 mm B: Substrate corrosion depth over 0.4 mm

[0074] [Hardness evaluation] First, as in the corrosion resistance evaluation, test specimens were cut from any position, excluding the edge, of the heated and quenched plated steel sheet so that a cross section perpendicular to the surface (thickness cross section) could be observed. The thickness cross section of the test specimen was polished using #600 to #1500 silicon carbide paper and then polished to a mirror finish using a diluted solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed. This thickness cross section served as the measurement surface. Next, Vickers hardness was measured using a micro-Vickers hardness tester at a load of 1 kgf at intervals of at least three times the indentation. A total of 20 measurements were taken at random points near the half-thickness position of the base steel sheet, excluding the low-carbon surface layer, and the arithmetic mean of these measurements was determined as the hardness after hot stamping (HS), and evaluated as follows. AAA: Hardness after HS is over 550HV AA: Hardness after HS is over 500 to 550HV A: Hardness after HS is 400-500HV B: Hardness after HS is less than 400HV

[0075] Plated steel sheets with LME resistance ratings of AAA, AA, and A and corrosion resistance ratings of AAA, AA, and A were evaluated as being capable of suppressing LME cracking while maintaining high corrosion resistance even when applied to hot stamping. The results are shown in Table 2.

[0076] [Table 1]

[0077] [Table 2-1]

[0078] [Table 2-2]

[0079] Referring to Table 2, in Comparative Example 38, the heating temperature in the annealing step was low, which is thought to have resulted in insufficient decarburization in the surface layer portion of the cold-rolled steel sheet. As a result, the depth at which the pearlite area ratio was 0 to 20% in the sheet thickness direction from the interface between the base steel sheet and the coating layer could not be made 3 μm or more, resulting in reduced LME resistance. In Comparative Example 39, the holding time in the annealing step was short, which is thought to have resulted in insufficient decarburization in the surface layer portion of the cold-rolled steel sheet. Similarly, the depth at which the pearlite area ratio was 0 to 20% could not be made 3 μm or more, resulting in reduced LME resistance. In Comparative Example 40, the dew point in the annealing step was low, which is thought to have resulted in insufficient decarburization in the surface layer portion of the cold-rolled steel sheet. Similarly, the depth at which the pearlite area ratio was 0 to 20% could not be made 3 μm or more, resulting in reduced LME resistance. In Comparative Example 41, the average cooling rate of the primary cooling in the cooling step was low, which is thought to have resulted in pearlite precipitation at high temperatures and the formation of the pearlite along grain boundaries. As a result, the area ratio of pearlite with a circle-equivalent diameter of 5 μm or more at a depth where the pearlite area ratio was 0 to 20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction exceeded 30%, resulting in reduced LME resistance. In Comparative Example 42, the controlled temperature for the primary cooling in the cooling process was high, which is thought to have caused pearlite to precipitate at high temperatures and form along grain boundaries. Similarly, the area ratio of pearlite with a circle-equivalent diameter of 5 μm or more at a depth where the pearlite area ratio was 0 to 20% exceeded 30%, resulting in reduced LME resistance. In Comparative Example 43, the controlled temperature for the secondary cooling in the cooling process was low, which resulted in the precipitation of bainite rather than pearlite. As a result, the desired depth where the pearlite area ratio was 0 to 20% in the sheet thickness direction from the interface between the base steel sheet and the coating layer could not be achieved, resulting in reduced LME resistance. In Comparative Example 44, the average cooling rate of the secondary cooling in the cooling step was too fast, so that bainite was mainly precipitated instead of pearlite, and similarly, the desired depth at which the pearlite area ratio was 0 to 20% could not be achieved, resulting in a decrease in LME resistance.In Comparative Example 45, the coating weight of the coating layer was insufficient, resulting in a decrease in corrosion resistance after HS.In Comparative Example 46, the average cooling rate of the secondary cooling in the cooling step was too fast, so that bainite was mainly precipitated instead of pearlite, and similarly, the depth at which the pearlite area ratio was 0 to 20% could not be achieved as desired, resulting in reduced LME resistance.

[0080] In contrast, the plated steel sheets according to all of the examples had a predetermined plating chemical composition and a plating layer coating weight of 40 g / m per side. 2 By controlling the depth where the pearlite area ratio is 0-20% from the interface between the base steel sheet and the coating layer in the sheet thickness direction to 3-100 μm and the area ratio of pearlite with a circle equivalent diameter of 5 μm or more at the depth where the pearlite area ratio is 0-20% to 0-30%, high corrosion resistance can be maintained even when exposed to high temperatures of 900°C, and the LME suppression effect of the initial low carbon concentration in the surface layer of the base steel sheet can be fully exerted, thereby reliably suppressing or reducing the occurrence of LME cracking during hot stamping. In particular, in Examples 13 to 36, in which the depth where the pearlite area ratio is 0-20% is 30-100 μm and the area ratio of pearlite with a circle equivalent diameter of 5 μm or more at that depth is controlled to 0-15%, the LME resistance was evaluated as AAA, further improving LME resistance. In addition, when the surface Al content of the coating layer was 5 mg / m or more as measured by fluorescent X-ray measurement, the LME suppression effect of the initial low carbon concentration in the surface layer of the base steel sheet was fully exerted, and the LME cracking during hot stamping was reliably suppressed or reduced. 2 In Examples 2 to 18, 20 to 31, and 33 to 36, in which the value was controlled to be larger than , the corrosion resistance was evaluated as AA, which was a further improvement in corrosion resistance compared to Examples 1, 19, 32, and 37, which were evaluated as A.

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 to 0.50%, and Fe: 0-17.00% and further comprising Mg: 0 to less than 0.500% Si: 0-0.200%, Ni: 0 to less than 0.500% Ca: 0-3.000%, 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%, Sr: 0-0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0-0.500%, P: 0 to 0.500%, and W: 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, a depth of 3 to 100 μm from the interface between the base steel sheet and the plating layer in the sheet thickness direction, where the area ratio of pearlite is 0 to 20%, The area ratio of pearlite having a circle equivalent diameter of 5 μm or more at a depth of 0 to 20% is 0 to 30%, The area ratio of martensite contained in the base steel plate is less than 1%, The coating weight of the plating layer is 40 g / m per side 2 A plated steel sheet characterized by the above.

2. 2. The plated steel sheet according to claim 1, wherein the depth at which the area ratio of pearlite is 0 to 20% is 10 to 100 μm.

3. The plated steel sheet according to claim 2, wherein the depth at which the area ratio of pearlite is 0 to 20% is 30 to 100 μm.

4. The plated steel sheet according to any one of claims 1 to 3, characterized in that the area ratio of pearlite having an equivalent circle diameter of 5 µm or more at a depth where the area ratio of pearlite is 0 to 20% is 0 to 15%.

5. The surface Al content of the plating layer measured by fluorescent X-rays was 5 mg / m 2 The plated steel sheet according to any one of claims 1 to 3, wherein the thickness of the plated steel sheet is larger than that of the plated steel sheet.

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