Plated steel sheet

Abstract

The present invention provides a plated steel sheet which is characterized by comprising a base steel sheet and a plating layer that is formed on the surface of the base steel sheet, and which is also characterized in that: the plating layer has a specific 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% to 20% is 3 μm to 100 μm; the area fraction of pearlite having a circle-equivalent diameter of 5 μm or more is 0% to 30% at the depth where the area fraction of pearlite is 0% to 20%; the adhesion amount of the plating layer is 20 g / m2 or more per surface; the plating layer has an oxide layer in the surface; and if the oxide layer is measured by XPS, the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 3 or more.

Description

[Technical Field]

[0001] This invention relates to plated steel sheets. [Background technology]

[0002] Hot stamping (hot pressing) is a well-known technique for press-forming materials that are difficult to form, such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated before forming. In this technique, because the material is heated before forming, the steel is soft and has good formability during the forming process. Therefore, even high-strength steel can be formed into complex shapes with high precision, and because the steel is hardened simultaneously with forming using a press die, the formed steel is known to have sufficient strength.

[0003] In connection with this, 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 plating layer provided on the surface of the base steel sheet, wherein the base steel sheet contains, by mass%, C: 0.10-0.5%, Si: 0.7-2.5%, Mn: 1.0-3%, and Al: 0.01-0.5%, with the remainder being iron and unavoidable impurities, and the base steel sheet has an internal oxide layer containing at least one oxide of Si and Mn with a thickness of 1 μm or more, and a decarburization layer with a thickness of 20 μm or less extending inward from the interface with the plating layer to the base steel sheet. Furthermore, Patent Document 1 teaches that by making the thickness of the internal oxide layer of the base steel sheet 1 μm or more, the occurrence of unplated areas in the galvanized steel sheet can be sufficiently suppressed, and the adhesion between the formed plating layer and the base steel sheet can be sufficiently high. In addition, Patent Document 1 teaches that an internal oxide layer is formed near the surface of the base steel sheet by high dew point annealing, while a decarburized layer is formed on and near the surface of the base steel sheet by the same high dew point annealing. Although the tensile strength of the decarburized layer is lower than that of the non-decarburized portion because it has a low carbon content, if the thickness of the decarburized layer is 20 μm or less, the influence of the decarburized layer on the strength of galvanized steel sheets and hot-stamped molded products manufactured therefrom can be suppressed.

[0005] Patent Document 2 describes a steel sheet coated with a metal coating comprising 2.0 to 24.0 wt% zinc, 7.1 to 12.0 wt% silicon, any 1.1 to 8.0 wt% magnesium, and optionally additional elements selected from Pb, Ni, Zr, or Hf, wherein the weight content of each additional element is less than 0.3 wt%, and the remainder is aluminum and any unavoidable impurities and residual elements, wherein the Al / Zn ratio is greater than 2.9, and teaches that parts obtained by hot stamping the steel sheet exhibit high sacrificial corrosion protection.

[0006] Patent Document 3 provides a steel sheet pre-coated with a metal coating comprising: A) 2.0 to 24.0 wt% zinc, 1.1 to 7.0 wt% silicon, optionally 1.1 to 8.0 wt% magnesium if the silicon content is between 1.1 to 4.0 wt%, and optionally additional elements selected from Pb, Ni, Zr, or Hf, wherein the weight content of each additional element is less than 0.3 wt%, and the remainder is aluminum and unavoidable impurities and residual elements, wherein the Al / Zn ratio is greater than 2.9; B) cutting the coated steel sheet to make a blank A method for manufacturing a hardened part is described, comprising the steps of: C) heat-treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel; D) transferring the blank into a press tool; E) hot-forming the blank to obtain a part; and F) cooling the part obtained in step E) to obtain a microstructure in the steel that is martensite or martensite-bainite, or composed of at least 75% equiaxed ferrite, 5 to 20% martensite and 10% or less bainite. Furthermore, Patent Document 3 teaches that a hardened part without LME can be obtained according to the above manufacturing method. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2019-151883 [Patent Document 2] Special Publication No. 2018-528324 [Patent Document 3] Special Publication No. 2018-527462 [Overview of the project] [Problems that the invention aims to solve]

[0008] For example, when galvanized steel sheets, such as those described in Patent Documents 1 to 3, are used in hot stamping, the plating layer after hot stamping may alloy with the base metal (base steel sheet), resulting in reduced corrosion resistance. Furthermore, hot-stamped bodies obtained by hot stamping galvanized steel sheets are then joined using spot welding or the like, but it is necessary to suppress liquid metal embrittlement (LME) cracking at this time. This phenomenon occurs when Zn, which has liquefied due to the heat input of welding, penetrates into the steel material along the grain boundaries, causing embrittlement, and then cracks occur when tensile stress generated by welding acts on it. In this regard, Patent Document 3 teaches how to suppress LME that occurs during hot stamping, but Patent Document 3 does not necessarily provide sufficient consideration from the perspective of suppressing LME cracking during spot welding after hot stamping, or from the perspective of achieving both suppression of LME cracking and improvement of corrosion resistance.

[0009] Therefore, the present invention aims to provide a plated steel sheet that can suppress LME cracking during spot welding after hot stamping, while maintaining high corrosion resistance even when applied to hot stamping. [Means for solving the problem]

[0010] As a result of studies conducted to achieve the above objective, the present inventors have found that by forming a Zn-containing plating layer with an adhesion amount greater than a predetermined amount, and further by making the surface structure of the plating layer appropriate, sufficient corrosion resistance can be maintained even when applied to hot stamping molding. In addition, by appropriately modifying the surface structure of the base steel sheet, even with a plating layer formed with such an adhesion amount, the occurrence of LME cracks during spot welding after hot stamping molding can be significantly suppressed or reduced, thus completing the present invention.

[0011] The present invention, which has achieved the above objectives, is as follows. (1) comprising a base steel sheet and a plating layer formed on the surface of the base steel sheet, The aforementioned plating layer is, by mass%, Al: 0.5 to 50.0%, Mg: 0.50 to 15.00%, Si: 0 to 4.0%, and Fe: 0 to 15.0% contains, and further Ni: 0 to 1.000%, Ca: 0 to 3.0%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, and W: 0 to 0.500% contains at least one of them in a total amount of 5.000% or less, the balance: having a chemical composition consisting of Zn and impurities, from the interface between the base steel plate and the plating layer, in the thickness direction, the depth where the area ratio of pearlite is 0 to 20% is 3 to 100 μm, at the depth where the area ratio of pearlite is 0 to 20%, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is 0 to 30%, the adhesion amount of the plating layer is 20 g / m per side 2 or more, A plated steel sheet characterized in that the plated layer has an oxide layer on its surface, and when the oxide layer is measured by XPS, the peak intensity ratio of (Al-O+Mg-O) / Zn-O is 3 or more. Here, Al-O, Mg-O, and Zn-O represent the intensities of the peaks attributed to the Al-O, Mg-O, and Zn-O bonds, respectively. (2) The plated steel sheet according to (1) above, characterized in that the area ratio of the perlite is 0 to 20% and the depth is 10 to 100 μm. (3) The plated steel sheet according to (2) above, characterized in that the area ratio of the perlite is 0 to 20% and the depth 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 perlite is 0 to 15% of the perlite with an equivalent circle diameter of 5 μm or more at a depth where the area ratio of perlite is 0 to 20%. (5) The plated steel sheet according to any one of the above items (1) to (4), characterized in that the chemical composition contains, by mass%, Al: 10.0 to 50.0% and Mg: 4.00 to 15.00%, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 5 or more. (6) The plated steel sheet according to (5) above, characterized in that the chemical composition contains, by mass%, Al: 30.0 to 50.0% and Mg: 7.00 to 15.00%, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 10 or more. [Effects of the Invention]

[0012] According to the present invention, even when applied to hot stamping, it is possible to provide a plated steel sheet that can suppress LME cracking during spot welding after hot stamping while maintaining high corrosion resistance. [Modes for carrying out the invention]

[0013] <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 the surface of the base steel sheet. The aforementioned plating layer is, by mass%, Al: 0.5-50.0% Mg: 0.50~15.00%, Si: 0~4.0%, and Fe: 0-15.0% It contains, and further, Ni: 0~1,000%, Ca: 0-3.0%, 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% It contains at least one of the following in total amount of 5.000% or less: The remainder has a chemical composition consisting of Zn and impurities. From the interface between the base steel sheet and the plating layer, the area ratio of pearlite in the thickness direction is 0-20% and the depth is 3-100 μm. The area ratio of the perlite is 0-30% at a depth where the area ratio of the perlite is 0-20%, and the area ratio of perlite with an equivalent circle diameter of 5 μm or more is 0-30%. The amount of the aforementioned plating layer is 20 g / m² per side. 2 That's all. The aforementioned plating layer has an oxide layer on its surface, and when the oxide layer is measured by XPS, the peak intensity ratio of (Al-O+Mg-O) / Zn-O is 3 or more. Here, Al-O, Mg-O, and Zn-O represent the intensities of the peaks attributed to the Al-O, Mg-O, and Zn-O bonds, respectively.

[0014] As mentioned earlier, when hot-stamped bodies obtained by hot-stamping galvanized steel sheets are joined by spot welding, it is necessary to suppress liquid metal embrittlement (LME) cracking. Although the reason is not entirely clear, research by the inventors has shown that carbon contained in the steel material is an element that promotes such LME cracking. Therefore, it is thought that the occurrence of LME cracking can be suppressed or reduced by reducing the carbon concentration in the surface layer of the steel material, for example, by decarburization. However, in practice, when applied to hot-stamping, the LME suppression effect based on such a reduction in the carbon concentration in the surface layer of the steel material is limited and may not always be satisfactory.

[0015] As a result of various studies, the inventors have found that even if the carbon concentration in the surface layer of the base steel sheet is reduced by decarburization or the like from the viewpoint of improving LME resistance, carbon contained in the base steel sheet diffuses to the steel surface layer during high-temperature heating in hot stamping, and this recarburization to the steel surface layer causes the initial LME suppression effect due to the low carbon concentration in the surface layer of the base steel sheet to disappear or be reduced. Therefore, the inventors conducted further studies and found that by creating a structure in the surface layer of the base steel sheet that can suppress such recarburization, even when a Zn-containing plating layer is included in a predetermined amount to maintain sufficient corrosion resistance, the LME suppression effect due to the initial low carbon concentration in the surface layer of the base steel sheet can be fully exerted, and the occurrence of LME cracks during spot welding after hot stamping can be reliably suppressed or reduced. More specifically, as will be explained in detail later in relation to the manufacturing method of the hot stamped molded body, the inventors formed a predetermined oxide layer on the surface of the plating layer and the amount of said plating layer attached was 20 g / m² per side.2 By doing so, we found that it is possible to suppress or reduce the evaporation of Zn and / or Mg even during high-temperature heating in hot stamping molding, thereby maintaining sufficient corrosion resistance. Furthermore, by forming a structure on the surface layer of the base steel sheet in which the area ratio of pearlite is controlled to be 0-30% at a depth of 3-100 μm in the thickness direction from the interface between the base steel sheet and the plating layer, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth of 0-20% pearlite is controlled to be 0-30%, it is possible to reliably suppress or reduce LME cracking during spot welding after hot stamping molding.

[0016] While not intended to be bound by any particular theory, in the plated steel sheet according to the embodiment of the present invention, it is believed that the structure of 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 subsequent recarburization during high-temperature heating in hot stamping formation. 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 generated in the microstructure of the surface layer of the base steel sheet becomes relatively small in relation to this reduction in 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 of the surface layer of the base steel sheet by decarburization or the like so that the depth of the region with a relatively low pearlite area ratio is 3 to 100 μm, i.e., the depth of the region with a relatively low pearlite area ratio. This makes it possible to fully exhibit the LME suppression effect based on the reduction in carbon concentration. However, simply reducing the area ratio of pearlite within a predetermined range is insufficient. In cases where such pearlite precipitates along grain boundaries, it is thought that during high-temperature heating in hot stamping, the pearlite transforms into austenite, forming a carbon diffusion pathway (i.e., a carbon recarburization pathway) along the grain boundaries. During high-temperature heating in hot stamping, carbon in the bulk of the base steel sheet attempts to diffuse to the surface based on the concentration gradient between the high carbon concentration in the bulk and the low carbon concentration on the surface. If a carbon recarburization pathway by austenite along the grain boundaries exists, the carbon in the bulk will diffuse to the surface through this pathway, promoting recarburization in the steel surface layer. As a result, the initial LME suppression effect achieved by reducing the carbon concentration in the surface layer of the base steel sheet cannot be fully realized.In contrast, according to the plated steel sheet according to the embodiment of the present invention, in the depth region where the area ratio of pearlite is relatively low, the area ratio of pearlite with a circular equivalent diameter of 5 μm or more is controlled to be within the range of 0 to 30%, thereby reducing the amount of relatively large pearlite. This makes it possible to disperse austenite, which has been transformed from pearlite, on the grain boundaries even during high-temperature heating in hot stamping molding, thereby reliably interrupting the carbon recarburization pathway by austenite.

[0017] More specifically, when pearlite transforms into austenite during high-temperature heating in hot stamping, a two-phase structure of ferrite and austenite is formed. In such cases, the austenite present at the interface between the different phases of ferrite and austenite connects to the surface side of the base steel sheet, forming a carbon recarburization pathway, which in turn promotes the diffusion of carbon from the bulk to the surface side of the base steel sheet. In this regard, in the plated steel sheet according to the embodiment of the present invention, it is important to reduce the area ratio of pearlite in the surface layer of the base steel sheet, i.e., in the depth region of 3 to 100 μm in the thickness direction from the interface between the base steel sheet and the plating layer, to 0 to 20%, and to limit the area ratio of relatively coarse pearlite in the said depth region, i.e., pearlite with an equivalent circle diameter of 5 μm or more, to within the range of 0 to 30%. With such a surface layer structure, even during high-temperature heating in hot stamping, the amount of austenite that transforms from pearlite can be reduced, and furthermore, the austenite can be dispersed on the grain boundaries, making it possible to reliably interrupt the carbon recarburization pathway by austenite. Therefore, according to the plated steel sheet according to the embodiment of the present invention, even though the amount of Zn-containing plating layer is relatively large to maintain sufficient corrosion resistance, creating conditions where LME is more likely to occur, recarburization during high-temperature heating in hot stamping is significantly suppressed. This allows the LME suppression effect due to the initial low carbon concentration of the base steel sheet surface to be fully realized, and reliably suppresses or reduces the occurrence of LME cracks during subsequent spot welding. The fact that the occurrence of LME cracks can be suppressed or reduced as described above by appropriately modifying the surface structure of the base steel sheet in a plated steel sheet equipped with a Zn-containing plating layer is something that the inventors have revealed for the first time. Therefore, the plated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where spot welding is relatively common.

[0018] The plated steel sheet according to the embodiment of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

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

[0020] [Al: 0.5~50.0%] Al is an effective element for improving the corrosion resistance of the plating layer. To obtain this effect sufficiently, the Al content should be 0.5% or more. The Al content may be 1.0% or more, 3.0% or more, 5.0% or more, 8.0% or more, 10.0% or more, 15.0% or more, or 20.0% or more. On the other hand, if the Al content is excessive, the amount of Zn required to impart sacrificial corrosion protection will decrease. Therefore, the Al content should be 50.0% or less. The Al content may be 45.0% or less, 40.0% or less, 35.0% or less, or 30.0% or less.

[0021] [Mg: 0.50~15.00%] Mg is an effective element for improving the corrosion resistance of the plating layer. To obtain this effect fully, the Mg content should be 0.50% or more. The Mg content may also be 0.51% or more, 0.52% or more, 0.53% or more, 0.55% or more, 0.60% or more, 0.80% or more, 1.00% or more, 1.50% or more, 2.00% or more, or 3.00% or more. On the other hand, if the Mg content is excessive, excessive sacrificial corrosion protection may cause blistering of the coating and rust runoff. Therefore, the Mg content should be 15.00% or less. The Mg content may also be 12.00% or less, 10.00% or less, 9.00% or less, 8.00% or less, 7.00% or less, 6.00% or less, or 5.00% or less.

[0022] [Si: 0~4.0%] Si is an effective element for improving the corrosion resistance of the plating layer. The Si content may be 0%, but if necessary, Si may be included in the plating layer in amounts of 0.01% or more, 0.05% or more, 0.1% or more, 0.5% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, from the viewpoint of improving the plating adhesion of the plating layer, the Si content may be 4.0% or less. The Si content may also be 3.5% or less, 3.0% or less, 2.7% or less, or 2.5% or less.

[0023] [Fe: 0~15.0%] Fe is an element that can be included in the plating layer, for example, by dissolving from the base steel sheet into the plating bath or by reacting with Al during the plating process to form an Fe-Al barrier layer at the interface between the base steel sheet and the plating layer. The Fe content may be 0%, but if Fe is included, the Fe content may be 0.01% or more, 0.05% or more, 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. On the other hand, Fe may be included in the plating layer up to about 15.0%, but within this range, it will not adversely affect the plated steel sheet according to the embodiment of the present invention. Therefore, the Fe content may be 15.0% or less, for example, 12.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, 3.0% or less, 1.0% or less, or 0.8% or less.

[0024] Furthermore, the plating layer can be optionally composed of Ni: 0-1,000%, Ca: 0-3.0%, 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%. The mixture may contain at least one of the following elements: 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%. These optional elements are not particularly limited, but it is preferable that their total is 5.000% or less. The optional elements may also total 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. The following provides a detailed explanation of these optional elements.

[0025] [Ni: 0~1.000%] Ni is an effective element for improving the corrosion resistance of the plating layer. The Ni content may be 0%, but to obtain such an effect, it is preferable that the Ni content be 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. There is no particular upper limit, but from the viewpoint of manufacturing costs, etc., the Ni content should be 1.000% or less, for example, 0.980% or less, 0.950% or less, 0.900% or less, 0.700% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0026] [Ca: 0~3.0%] Ca is an effective element for ensuring the wettability of the plating bath. While the Ca content may be 0%, it is preferable that the Ca content be 0.01% or more to obtain this effect. The Ca content may also be 0.05% or more, 0.1% or more, 0.5% or more, or 1.0% or more. On the other hand, if the Ca content is excessive, a large amount of hard intermetallic compounds may be formed in the plating layer, making the plating layer brittle and reducing its adhesion to the steel sheet. Therefore, it is preferable that the Ca content be 3.0% or less. The Ca content may also be 2.5% or less, 2.0% or less, or 1.5% 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%, 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 do not necessarily have to be included in the plating layer, but they may be present in the plating 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 plated steel sheet as long as they are within the predetermined content range. However, if the content of each element is excessive, it 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 content of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li is preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, or 0.100% or less.

[0028] In the plating layer, the remainder of the elements other than those mentioned above consists of Zn and impurities. Impurities in the plating layer refer to components that are mixed in during the manufacturing process, including raw materials and various other factors in the manufacturing process.

[0029] [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 peeled and dissolved from the plated steel sheet using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet. The chemical composition (average composition) of the plating layer is then determined by measuring the obtained acid solution using ICP (inductively coupled plasma) emission spectroscopy. The type of acid is not particularly limited and can be any acid that can dissolve the plating layer.

[0030] The plating layer may be any plating layer having the above chemical composition and the following oxide layer on its surface, and is not particularly limited, but may be, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, etc.

[0031] [Plating layer adhesion: 20g / m² per side] 2 [End] In an embodiment of the present invention, the amount of plating layer deposited is 20 g / m² per side. 2 This concludes the explanation. Generally, the plating layer may alloy with the base steel sheet during high-temperature heating in hot stamping, resulting in reduced corrosion resistance. However, according to the embodiments of the present invention, although the reason is not entirely clear, it is believed that the alloying of the plating layer and the base steel sheet can be delayed due to the surface structure of the base steel sheet, specifically, the surface structure in which the area ratio of pearlite is 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer to a depth of 3-100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at that depth is 0-30%. Therefore, by increasing the amount of plating layer, specifically to 20 g / m² per side, the alloying of the plating layer and the base steel sheet can be delayed. 2By controlling as described above, when applied to hot stamping forming, there will be a plating layer where alloying has not sufficiently progressed, and it is considered that sufficient corrosion resistance can be maintained due to the presence of such a plating layer. On the other hand, if the deposition amount of the plating layer is small, the effects related to the delay of alloying as described above cannot be fully obtained, and the corrosion resistance after hot stamping forming may decrease. From the perspective of improving corrosion resistance, the deposition amount of the plating layer is preferably 25 g / m 2 or more, or 30 g / m 2 or more, more preferably 40 g / m 2 or more, even more preferably 50 g / m 2 or more, most preferably 60 g / m 2 or more. The upper limit is not particularly limited, but the deposition amount of the plating layer is, for example, 200 g / m 2 or less, 180 g / m 2 or less, 170 g / m 2 or less, 160 g / m 2 or less, or 150 g / m 2 or less may also be acceptable.

[0032] [Measurement of plating deposition amount] The deposition amount of the plating layer is determined as follows. First, a 30 mm × 30 mm sample is taken from the plated steel sheet, and then the plating layer is stripped and dissolved from this sample using an acid solution containing an inhibitor that suppresses the corrosion of the base steel sheet. The deposition amount of the plating layer is determined from the weight change of the sample before and after the stripping and dissolution. The type of acid is not particularly limited and may be any acid that can dissolve the plating layer.

[0033] [Peak intensity ratio by XPS measurement: (Al - O + Mg - O) / Zn - O ≥ 3] In embodiments of the present invention, the plating layer has an oxide layer on its surface, and when the oxide layer is measured by XPS (X-ray photoelectron spectroscopy), the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 3 or more, where Al-O, Mg-O, and Zn-O represent the peak intensities attributed to the Al-O, Mg-O, and Zn-O bonds, respectively. For example, when conventional Zn-based plated steel sheets or Al-Zn-based plated steel sheets are used in hot stamping, the plated steel sheets are generally heated to a temperature of approximately 900°C or higher during hot stamping. Since Zn has a relatively low boiling point of approximately 907°C, the Zn in the plating layer may partially evaporate at such high temperatures, which can reduce the corrosion resistance after hot stamping. Similarly, some of the Mg added to Zn-based plated steel sheets or Al-Zn-based plated steel sheets may evaporate during heating in hot stamping at high temperatures, which can reduce the corrosion resistance after hot stamping, just as with Zn. In contrast, since Al oxide and Mg oxide are stronger and / or denser oxides than Zn oxide, an oxide layer with a peak intensity ratio of (Al-O+Mg-O) / Zn-O of 3 or more, as measured by XPS, is formed on the surface of the plating layer. This means that the surface of the plating layer is covered with a relatively large amount of Al oxide and / or Mg oxide, which can suppress or reduce the evaporation of Zn and / or Mg in the plating layer even when applied to hot stamping molding. In connection with this, it is possible to maintain a relatively high concentration of Zn and Mg in the plating layer of the resulting hot stamped molded body, while relatively reducing the Fe concentration, thereby improving the corrosion resistance after hot stamping molding. From the viewpoint of further improving corrosion resistance, a higher peak intensity ratio of (Al-O+Mg-O) / Zn-O as measured by XPS is preferable, for example, it may be 5 or more, 8 or more, or 10 or more. The peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS can be controlled within a desired range by appropriately adjusting the chemical composition of the plating layer, in addition to controlling the dew point during cooling after plating, which will be explained in more detail later in relation to the manufacturing method of hot-stamped molded articles.For example, the peak intensity ratio can be controlled to 5 or higher by ensuring that the chemical composition of the plating layer contains, by mass%, Al: 10.0-50.0% and Mg: 4.00-15.00%. Similarly, the peak intensity ratio can be controlled to 10 or higher by ensuring that the chemical composition of the plating layer contains, by mass%, Al: 30.0-50.0% and Mg: 7.00-15.00%. The upper limit of the peak intensity is not particularly limited, but for example, the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS may be 20 or less, 18 or less, or 15 or less.

[0034] The measurement of the oxide layer on the surface of the plated layer by XPS is performed at a position 5 nm from the surface of the plated layer in the thickness direction, under the following conditions. X-ray source: mono-Al Kα (1486.6eV) X-ray diameter: 50-200 μm Measurement area: 100~700μm×100~700μm Vacuum degree: 1×10 -10 ~1 × 10 -11 torr (1 torr: 133.32 Pa) Acceleration voltage: 1~10kV Here, the peaks attributed to the Al-O bond are those observed in the range of 72-76 eV in the XPS spectrum focusing on Al 2p3 / 2. The peaks attributed to the Mg-O bond are those observed in the range of 48-52 eV in the XPS spectrum focusing on Mg 2p3 / 2. The peaks attributed to the Zn-O bond are those observed in the range of 10¹⁸-10²⁴ eV in the XPS spectrum focusing on Zn 2p3 / 2. The intensity of each peak is calculated by subtracting the baseline intensity Ib from the peak intensity Ip (i.e., "Ip-Ib"), taking the peak baseline into consideration. Based on the peak intensities obtained in this way, the value of (Al-O+Mg-O) / Zn-O is calculated.

[0035] [From the interface between the base steel sheet and the plating layer, the pearlite area ratio is 0-20% in the thickness direction, with a depth of 3-100 μm.] In embodiments of the present invention, the depth at which the area ratio of pearlite is 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer is 3-100 μm. This characteristic is related to the reduction of carbon concentration in the surface layer of the base steel sheet, and therefore, by having this characteristic, the LME suppression effect due to the reduction of carbon concentration in the surface layer of the base steel sheet can be exerted, thereby suppressing or reducing the occurrence of LME cracks during spot welding after hot stamping. In addition, by reducing the amount of pearlite in the surface layer of the steel sheet to the above range, the amount of austenite that transforms from pearlite during high-temperature heating in hot stamping can be reduced. Therefore, this characteristic is also very important in preventing the formation of carbon recarburization paths by austenite along grain boundaries during hot stamping. From the viewpoint of further improving these effects, it is preferable to increase the area of ​​the surface layer with less pearlite. More specifically, the depth in the thickness direction from the interface between the base steel sheet and the plating layer where the area ratio of pearlite is 0-20% 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.

[0036] [Perlite area ratio of 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer, with an equivalent circle diameter of 5 μm or more: 0-30%] In this embodiment of the present invention, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is 0-30% at a depth in the thickness direction from the interface between the base steel sheet and the plating layer where the area ratio of pearlite is 0-20%. By controlling the area ratio of pearlite with an equivalent circle diameter of 5 μm or more to within the range of 0-30% in the depth region where the area ratio of pearlite is relatively low as described above, the amount of relatively large pearlite can be reduced. This allows austenite, which has been transformed from pearlite, to be dispersed and present on the grain boundaries even during high-temperature heating in hot stamping, thereby reliably interrupting the carbon recarburization pathway 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, and the occurrence of LME cracks during subsequent spot welding can be reliably suppressed or reduced. From the viewpoint of further improving these effects, the area ratio of perlite with an equivalent circle diameter of 5 μm or more at a depth where the area ratio of perlite 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 perlite with an equivalent circle diameter of 5 μm or more may be, for example, 1% or 3%.

[0037] [Measurement of the area ratio of perlite at depths of 0-20% and perlite with an equivalent circle diameter of 5 μm or more] The area ratio of pearlite in the microstructure of the surface layer of the base steel sheet, specifically the depth at which it accounts for 0-20% and the area ratio of pearlite with a circular diameter of 5 μm or more, are 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 thickness direction can be observed. Next, these observation surfaces are mirror-polished, etched with Picral etchant, and then microstructure observation is performed using a scanning electron microscope (SEM). For each sample, the measurement range is defined as a rectangular area of ​​100 μm in the thickness direction and 500 μm perpendicular to the thickness direction from the interface between the base steel sheet and the plating layer, with a total of five fields of view measured for the five samples. The interface between the base steel sheet and the plating layer can be identified by the difference in color tone between the base steel sheet and the plating layer in the backscattered electron image (BSE image) of the SEM. The area ratio of pearlite is calculated, for example, from microstructure photographs at a magnification of about 5000x using the dot method. Here, regions surrounded by grain boundaries where the crystal orientation difference of ferrite is 15° or more, where ferrite and cementite phases are mixed, and the cementite morphology is layered and / or spherical, are identified as pearlite, and their area ratio is calculated. For each sample, the depth position from the interface between the base steel sheet and the plating layer where the area ratio of pearlite gradually increases to 20% is identified, and then the distance from the identified depth position to the interface is calculated. The arithmetic mean of these distances is determined as "the depth from the interface between the base steel sheet and the plating layer in the thickness direction where the area ratio of pearlite is 0-20%". Similarly, for each sample, in the depth region from the interface where the area ratio of pearlite is 20%, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is calculated by image processing, and the arithmetic mean of these distances is determined as "the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth from the interface between the base steel sheet and the plating layer in the thickness direction where the area ratio of pearlite is 0-20%".

[0038] [Preferred chemical composition of the base steel sheet] 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 amount of plating layer adhesion is 20 g / m² per side. 2As described above, the objective is achieved by forming an oxide layer on the surface of the plating layer in which the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS is 3 or more, and by forming a structure on the surface layer of the base steel sheet in which the area ratio of pearlite is controlled to 0-30% at a depth of 3-100 μm in the thickness direction from the interface between the base steel sheet and the plating layer, with a pearlite area ratio of 0-20% at that depth, and a pearlite area ratio of 0-30% with an equivalent circle diameter of 5 μm or more at that depth. 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. The following describes in detail preferred chemical compositions of base steel sheets used in plated steel sheets according to embodiments of the present invention, but these descriptions are intended to be merely examples of preferred chemical compositions of base steel sheets suitable for achieving a Vickers hardness of 400 HV or more in a molded article after hot stamping, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0039] In embodiments of the present invention, for example, the base steel sheet is, by mass%, C: 0.13~0.50%, Si: 0.001~3.000%, Mn: 0.30~3.00%, Al: 0.0002~2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, 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%, Ir: 0~1.000%, and Remainder: Fe and impurities It is preferable to have a chemical composition consisting of the following. Each element will be described in more detail below.

[0040] [C:0.13~0.50%] Carbon (C) is an element that increases tensile strength inexpensively and is an important element for controlling the strength of steel. To obtain this effect sufficiently, it is preferable that the C content be 0.13% or more. The C content may also be 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, an excessive amount of C may lead to a decrease in elongation. For this reason, it is preferable that the C content be 0.50% or less. The C content may also be 0.45% or less, or 0.40% or less.

[0041] [Si: 0.001~3.000%] Si acts as a deoxidizing agent and suppresses the precipitation of carbides during the cooling process in cold-rolled sheet annealing. To obtain this effect sufficiently, 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 lead to an increase in steel strength and a decrease in 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.

[0042] [Mn: 0.30~3.00%] Mn is an element that enhances the hardenability of steel and is effective in increasing its strength. To fully obtain these effects, it is preferable that the Mn content be 0.30% or more. The Mn content may also be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, if the Mn content is excessive, it may lead to a decrease in elongation along with an increase in steel strength. For this reason, it is preferable that the Mn content be 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less.

[0043] [Al:0.0002~2.000%] Al acts as a deoxidizing agent for steel and is an element that has the effect of sounding down steel. To obtain this effect sufficiently, it is preferable that the Al content be 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, if the Al content is excessive, coarse Al oxide may be generated, which may reduce the elongation of the steel sheet. For this reason, it is preferable that the Al content be 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.

[0044] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes steel embrittlement. A lower P content is preferable, and ideally it should be 0%. However, excessive reduction of the P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content can lead to steel embrittlement due to grain boundary segregation, as described above. Therefore, it is preferable to have a P content of 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0045] [S:0.1000% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel parts. A lower S content is preferable, and ideally it should be 0%. However, excessive reduction of the S content can lead to a significant increase in cost. For this reason, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can lead to cracking during cold forming, originating from nonmetallic inclusions. Therefore, it is preferable to have an S content of 0.1000% or less. The S content may also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0046] [N:0.0100% or less] N is an element that forms coarse nitrides in steel sheets, reducing their workability. A lower N content is preferable, ideally 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or higher, or 0.0005% or higher, or 0.0010% or higher. On the other hand, excessive N content can lead to the formation of coarse nitrides, as described above, reducing the workability of the steel sheet. Therefore, it is preferable to have an N content of 0.0100% or less. The N content may also be 0.0080% or lower, or 0.0050% or lower.

[0047] The preferred basic chemical composition of the base steel sheet is as described above. Furthermore, the base steel sheet may, if necessary, contain 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. These elements may be present in amounts of 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more, respectively.

[0048] In the base steel sheet, the remainder other than the elements mentioned above consists of Fe and impurities. Impurities in the base steel sheet refer to components that are mixed in during the industrial production of the base steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0049] The chemical composition of the base steel sheet can be measured using general analytical methods. For example, the chemical composition of the base steel sheet can be determined by first removing the plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from around the 1 / 2 thickness point of the base steel sheet, and the composition can be determined by measuring it 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 can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0050] [Thickness of the base steel plate] The thickness of the base steel sheet is not particularly limited, but may be, for example, 0.2 mm or more, and may be 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 sheet may be, for example, 6.0 mm or less, and may be 5.0 mm or less, or 4.0 mm or less.

[0051] <Method for manufacturing plated steel sheets> Next, preferred manufacturing methods for plated steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing plated steel sheets according to embodiments of the present invention, and is not intended to limit the plated steel sheets to those manufactured by the manufacturing methods described below.

[0052] A plated steel sheet according to an embodiment of the present invention can be manufactured by, for example, a casting step of casting molten steel with an adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a winding step of winding the hot-rolled steel sheet, a cold rolling step of cold rolling the winded hot-rolled steel sheet to obtain a cold-rolled steel sheet, an annealing step of annealing the cold-rolled steel sheet, a cooling step of cooling the annealed cold-rolled steel sheet, and a plating step of forming a plating layer on the obtained base steel sheet. Alternatively, the hot-rolled steel sheet may be pickled and then immediately subjected to the cold-rolling step without winding. Each step will be described in detail below.

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

[0054] [Hot rolling process] Hot-rolled steel sheets can be obtained by hot-rolling cast steel billets. The hot-rolling process is carried out by hot-rolling the cast steel billet either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the steel billet may be, for example, 1100 to 1250°C. In the hot-rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling step can be appropriately determined according to the desired metal structure and sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0055] [Winding process] The hot-rolled steel sheet can be wound at a predetermined temperature. The winding temperature can be appropriately determined according to the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be unwound before or after winding to be subjected to a predetermined heat treatment. Alternatively, the winding process can be omitted, and the hot-rolled process can be followed by pickling and the cold-rolling process described later.

[0056] [Cold rolling process] After pickling or performing other processes on hot-rolled steel sheets, cold-rolled steel sheets can be obtained by cold-rolling them. The reduction ratio during cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20-80%. After the cold-rolling process, the sheet may be cooled to room temperature by air cooling, for example.

[0057] [Annealing process] Next, the obtained cold-rolled steel sheet is annealed. The annealing process 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 it for 10 to 300 seconds. By performing the annealing process under such relatively high dew point conditions, the surface layer of the cold-rolled steel sheet can be properly decarburized. Therefore, in the final plated steel sheet, it is possible to control the depth of the pearlite area ratio of 0 to 20% in the thickness direction from the interface between the base steel sheet and the plating layer to within the range of 3 to 100 μm. If the dew point is below -20°C, the heating temperature is below 730°C, and / or the holding time is less than 10 seconds, decarburization of the surface layer of the cold-rolled steel sheet will be insufficient. As a result, in the final plated steel sheet, it will not be possible to achieve a depth of 3 μm or more of pearlite area ratio of 0 to 20% in the thickness direction from the interface between the base steel sheet and the plating layer. On the other hand, if the dew point is above 10°C, the heating temperature is above 900°C, and / or the holding time is above 300 seconds, an external oxide layer may form on the surface of the steel sheet, which may reduce the plating properties or reduce the strength of the final plated steel sheet due to excessive decarburization. The dew point is preferably -10 to 5°C, and more preferably -5 to 5°C. Furthermore, the atmosphere in the annealing process may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere with 1 to 10% hydrogen (for example, a balance of 4% hydrogen and nitrogen).

[0058] [Cooling process] Cold-rolled steel sheets, whose surface layer has been decarburized in the annealing process, need to be properly cooled in the subsequent cooling process in order to obtain the desired surface structure. Specifically, the cooling process includes cooling from the heating temperature of the annealing process to a control temperature of 620-670°C at an average cooling rate of 20°C / s or more (primary cooling), and cooling from the control temperature to the plating bath temperature (e.g., the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less (secondary cooling). Primary and secondary cooling will be explained in more detail below.

[0059] [Primary cooling] In primary cooling, it is important to suppress the precipitation of pearlite at high temperatures. More specifically, pearlite that precipitates at high temperatures from the heating temperature of 730-900°C in the annealing process to the control temperature of 620-670°C diffuses rapidly, making it easy for it to diffuse into grain boundaries after precipitation and form pearlite along those boundaries. This pearlite formed along grain boundaries undergoes austenite transformation during the high-temperature heating of hot stamping, creating a carbon recarburization pathway along the grain boundaries by the austenite, thereby promoting the recarburization of carbon in the bulk to the surface layer of the steel. Therefore, in the temperature range from the heating temperature of the annealing process to the control temperature mentioned above, it is extremely important to suppress the precipitation of pearlite at high temperatures on the surface layer of the steel sheet 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 above 670°C, pearlite will precipitate at high temperatures due to rapid diffusion, thus promoting the formation of pearlite along the grain boundaries. As a result, in the final plated steel sheet, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more exceeds 30% at a depth of 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer. When applied to hot stamping, this makes it impossible to achieve sufficient LME resistance during subsequent spot welding.

[0060] [Secondary cooling] On the other hand, in the secondary cooling after the primary cooling, it is important to precipitate pearlite at a relatively low temperature where diffusion is slow. More specifically, by cooling from a control temperature of 620-670°C to the plating bath temperature (e.g., the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less, pearlite can be precipitated. Because the pearlite precipitated in such a low temperature range below the control temperature diffuses relatively slowly, it does not form in a connected form along the grain boundaries, but rather the pearlite can be dispersed on the grain boundaries. In the case of such a structure, even during high-temperature heating in hot stamping molding, A c1 Since austenite, which has transformed from pearlite at a certain point or higher, can be similarly dispersed on the grain boundaries, it is possible to reliably interrupt the carbon recarburization pathway by austenite. On the other hand, if the average cooling rate is greater than 10°C / s and / or the control temperature is less than 620°C, martensite or bainite will precipitate mainly instead of pearlite, and in the final plated steel sheet, it will not be possible to have a pearlite area ratio of 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer to a depth of 100 μm or less. Martensite and bainite have a faster transformation rate to austenite compared to pearlite, and A c1 It instantly transforms into austenite directly above the point. Therefore, compared to pearlite, the ferrite-austenite two-phase structure is exposed to high temperatures for a longer period during hot stamping. In such cases as well, recarburization pathways are more likely to form at the grain boundaries, making it impossible to achieve sufficient LME resistance.

[0061] [Plating process] Next, in the plating process, a plating layer having the chemical composition described above is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). More specifically, the plating process is carried out by hot-dip galvanizing using a plating bath (plating bath temperature: e.g., 420-480°C) whose components have been adjusted so that the chemical composition of the plating layer falls within the range described above, and an alloying treatment may be performed after the hot-dip galvanizing treatment. Furthermore, the plating treatment is not limited to hot-dip galvanizing, but may also be electroplating, vapor deposition, thermal spraying, or cold spraying. Other conditions of the plating process can be appropriately set considering the thickness and amount of plating layer, etc. For example, after immersing the cold-rolled steel sheet in the plating bath, it is removed, and N2 gas or air is immediately blown onto it using the gas wiping method, and then it is cooled to ensure that the amount of plating layer adheres within a predetermined range, for example, 20-200 g / m² per side. 2 It can be adjusted within the range.

[0062] [Cooling after plating] During cooling after plating, it is necessary to control the dew point of the cooling gas (e.g., nitrogen gas) within the range of -10 to 10°C. By cooling the plated steel sheet in such a relatively high dew point atmosphere, an oxide layer containing a relatively large amount of Al oxide and Mg oxide can be formed on the surface of the plating layer. More specifically, an oxide layer can be formed in which the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS is 3 or higher. Due to the formation of such an oxide layer, evaporation of Zn and / or Mg in the plating layer can be significantly suppressed or reduced even during high-temperature heating in hot stamping molding, and furthermore, the Fe concentration in the plating layer of the resulting hot stamped molded body can be relatively reduced. Therefore, it is possible to further improve the corrosion resistance after hot stamping molding. In addition to controlling the dew point of the cooling gas during cooling after plating within the range of -10 to 10°C, it is possible to control the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS within a desired range by appropriately adjusting the chemical composition of the plating layer. For example, in addition to the dew point control described above, the peak intensity ratio can be controlled to 5 or higher by ensuring that the chemical composition of the plating layer contains, by mass%, Al: 10.0-50.0% and Mg: 4.00-15.00%. Similarly, the peak intensity ratio can be controlled to 10 or higher by ensuring that the chemical composition of the plating layer contains, by mass%, Al: 30.0-50.0% and Mg: 7.00-15.00%.

[0063] The plated steel sheets produced by this manufacturing method have a plating layer adhesion amount of 20 g / m² per side. 2As described above, an oxide layer is formed on the surface of the plating layer, with a peak intensity ratio of (Al-O+Mg-O) / Zn-O of 3 or more as measured by XPS. Furthermore, a structure is formed on the surface of the base steel sheet where the area ratio of pearlite is 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer to a depth of 3-100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth of 0-20% is controlled to 0-30%. Therefore, even when exposed to high temperatures such as during hot stamping, high corrosion resistance can be maintained while significantly suppressing the recarburization of carbon in the bulk to the steel surface layer. This allows the LME suppression effect due to the initial low carbon concentration on the surface layer of the base steel sheet to be fully realized, and the occurrence of LME cracks during subsequent spot welding can be reliably suppressed or reduced. Consequently, when such a plated steel sheet is applied as a plated steel sheet for hot stamping, it is possible to maintain sufficient corrosion resistance and achieve better LME resistance compared to conventional plated steel sheets. Therefore, by extending the lifespan of plated steel sheets used in automobiles and building materials, it can contribute to the development of industry.

[0064] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples. [Examples]

[0065] In the following embodiments, plated steel sheets according to the present invention were manufactured under various conditions, and the properties of the manufactured plated steel sheets were investigated.

[0066] First, molten steel was cast using a continuous casting method to form steel billets having the chemical composition shown in Table 1. After the steel billets were cooled, they were reheated to 1200°C and hot-rolled, and then coiled at a temperature of 600°C or lower. Hot rolling was carried out by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900-1050°C and a reduction ratio of 30%. Next, the obtained hot-rolled steel sheets were pickled and then cold-rolled at a reduction ratio of 50% to obtain cold-rolled steel sheets with a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheets were subjected to an annealing process in a furnace with an oxygen concentration of 20 ppm or less in a mixed gas atmosphere of 4% hydrogen and nitrogen under the conditions shown in Table 2, and then a cooling process was carried out under the same conditions shown in Table 2 to produce base steel sheets.

[0067] Next, the manufactured base steel sheet was cut into 100 mm x 200 mm pieces, and plated using a batch-type hot-dip galvanizing test apparatus manufactured in-house. More specifically, the manufactured base steel sheet was first immersed in a plating bath having a predetermined chemical composition for approximately 3 seconds, then pulled up at a lifting speed of 20 to 200 mm / s, and the amount of plating layer attached was adjusted to the values ​​shown in Table 2 by N2 gas wiping. Next, the base steel sheet with the plated layer attached was cooled from the plating bath temperature (approximately 420 to 480°C) to room temperature using nitrogen gas controlled to the dew point shown in Table 2 as the cooling gas, thereby obtaining a plated steel sheet with a plating layer formed on both sides of the base steel sheet. The plate temperature was measured using a thermocouple spot-welded to the center of the base steel sheet.

[0068] The physical properties and characteristics of the obtained plated steel sheets were measured and evaluated by the following methods.

[0069] [Chemical composition analysis of the plating layer] The chemical composition of the plating layer was determined by immersing a 30mm x 30mm sample in a 10% HCl aqueous solution containing an inhibitor, pickling and stripping the plating layer, and then measuring the plating components dissolved in the aqueous solution by ICP emission spectroscopy. The results are shown in Table 2.

[0070] [Evaluation of LME resistance during spot welding] First, the plated steel sheet was placed in an atmospheric heating furnace at 900°C, and after the temperature of the plated steel sheet reached the furnace temperature -10°C, it was held at that temperature for 100 seconds. Next, the plated steel sheet was removed from the furnace and rapidly cooled by sandwiching it between flat plates at room temperature. Two 50mm x 100mm samples of the plated steel sheet were prepared after heating and rapid cooling. Welded joints were fabricated on these two plated steel sheet samples by spot welding using a dome radius type welding electrode with a tip diameter of 8mm, at a striking angle of 2°, a pressing force of 4.0kN, an energizing time of 0.5 seconds, and an energizing current of 12kA. Next, the length of the LME crack that formed on the outside of the shoulder of the weld was measured, and the LME resistance was evaluated as follows. AAA: 0μm AA: More than 0~20μm A: Over 20 μm to less than 80 μm B:80μm or more

[0071] [Evaluation of corrosion resistance] The corrosion resistance of the plated steel sheet was evaluated as follows. First, the plated steel sheet was placed in an air-heated furnace at 900°C, and after the temperature of the plated steel sheet reached the furnace temperature -10°C, it was held for 100 seconds. Next, the plated steel sheet was removed from the furnace and rapidly cooled by sandwiching it between flat plates at room temperature. A 50mm x 100mm sample of the plated steel sheet after heating and rapid cooling was treated with zinc phosphate (SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard), followed by electrodeposition coating (PN110 Powernix Gray: Nippon Paint Industrial Coating Co., Ltd. standard) at a thickness of 20 μm, followed by baking at a temperature of 150°C for 20 minutes. Next, a cut was introduced in the center of the sample, leading to the base metal (base steel sheet). Then, a combined cycle corrosion test was conducted according to JASO (M609-91), and the number of cycles required for red rust to occur from the cut area was measured, and the corrosion resistance was evaluated as follows. AAA: Over 240 cycles AA: 180-240 cycles A: Less than 90-180 cycles B: Less than 90 cycles

[0072] [Hardness evaluation] First, in the same manner as in the evaluation of corrosion resistance, test specimens were cut from any position other than the edges of the plated steel sheet, which had been heated and rapidly cooled, 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 sandpaper, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluent such as alcohol or pure water, and this thickness cross section was used as the measurement surface. Next, the Vickers hardness was measured using a micro Vickers hardness tester with a load of 1 kgf at intervals of more than 3 times the indentation. A total of 20 points were randomly measured around the 1 / 2 position of the thickness of the base steel sheet, without including the low-carbon concentration 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 exceeds 550HV AA: After HS, hardness exceeds 500~550HV A: Hardness after HS is 400-500HV B: Hardness after HS is less than 400HV

[0073] When LME resistance was rated AAA, AA, and A, and corrosion resistance was rated AAA, AA, and A, the plated steel sheets were evaluated as being able to suppress LME cracking during spot welding after hot stamping while maintaining high corrosion resistance when applied to hot stamping. The results are shown in Table 2.

[0074] [Table 1]

[0075] [Table 2-1]

[0076] [Table 2-2]

[0077] Referring to Table 2, in Comparative Example 35, it is thought that the decarburization of the surface layer of the cold-rolled steel sheet was insufficient due to the low heating temperature in the annealing process. As a result, it was not possible to achieve a pearlite area ratio of 0-20% and a depth of 3 μm or more in the thickness direction from the interface between the base steel sheet and the plating layer, resulting in reduced LME resistance. In Comparative Example 36, it is thought that the decarburization of the surface layer of the cold-rolled steel sheet was insufficient due to the short holding time in the annealing process, and similarly, it was not possible to achieve a pearlite area ratio of 0-20% and a depth of 3 μm or more, resulting in reduced LME resistance. In Comparative Example 37, it is thought that the dew point in the annealing process was low, resulting in insufficient decarburization of the surface layer of the cold-rolled steel sheet, and similarly, it was not possible to achieve a pearlite area ratio of 0-20% and a depth of 3 μm or more, resulting in reduced LME resistance. In Comparative Example 38, it is thought that the average cooling rate of the primary cooling in the cooling process was low, causing pearlite to precipitate at high temperatures and form along the grain boundaries. As a result, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more exceeded 30% at a depth in the thickness direction from the interface between the base steel sheet and the plating layer where the pearlite area ratio is 0-20%, resulting in reduced LME resistance. In Comparative Example 39, it is thought that the control temperature of the primary cooling in the cooling process was too high, causing pearlite to precipitate at high temperatures and form along the grain boundaries. Similarly, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at a depth where the pearlite area ratio is 0-20% exceeded 30%, resulting in reduced LME resistance. In Comparative Example 40, the control temperature of the secondary cooling in the cooling process was too low, causing bainite, not pearlite, to precipitate. As a result, the desired depth could not be achieved for the pearlite area ratio of 0-20% in the thickness direction from the interface between the base steel sheet and the plating layer, resulting in reduced LME resistance. In Comparative Example 41, the average cooling rate of the secondary cooling in the cooling process was too fast, resulting in the deposition of bainite rather than pearlite. Similarly, the desired depth of 0-20% pearlite area could not be achieved, leading to a decrease in LME resistance. Furthermore, in Comparative Examples 35 and 42, the low dew point of the cooling gas after plating prevented the formation of the desired oxide layer on the surface of the plated layer.As a result, the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS became less than 3, indicating a decrease in corrosion resistance.

[0078] In contrast, all plated steel sheets in the examples had a predetermined plating chemical composition, and the amount of plating layer adhesion was 20 g / m² per side. 2 As described above, by forming an oxide layer on the surface of the plating layer with a peak intensity ratio of (Al-O+Mg-O) / Zn-O of 3 or more as measured by XPS, and by controlling the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth of 3 to 100 μm in the thickness direction from the interface between the base steel sheet and the plating layer to 0 to 30%, it was possible to maintain high corrosion resistance even when applied to hot stamping, fully exert the LME suppression effect due to the initial low carbon concentration of the surface layer of the base steel sheet, and reliably suppress or reduce the occurrence of LME cracks during subsequent spot welding. In particular, in Examples 13 to 34, where the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth of 30 to 100 μm was controlled to 0 to 15%, the LME resistance evaluation was AAA, indicating a further improvement in LME resistance. In addition, in Examples 6-8, 16, 17, 24-27, 33, and 34, where the Al and Mg content in the plating layer was 10.0% by mass or more and 4.00% by mass or more, respectively, and the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS was 5 or more, the corrosion resistance was evaluated as AA, demonstrating further improvement in corrosion resistance compared to Example 1, which received a corrosion resistance evaluation of A. Similarly, in Examples 9-12, 18-20, and 28-32, where the Al and Mg content in the plating layer was 30.0% by mass or more and 7.00% by mass or more, respectively, and the peak intensity ratio of (Al-O+Mg-O) / Zn-O measured by XPS was 10 or more, the corrosion resistance was evaluated as AAA, demonstrating further improvement in corrosion resistance.

Claims

1. The device comprises a base steel sheet and a plating layer formed on the surface of the base steel sheet. The aforementioned plating layer is, by mass%, Al: 0.5-50.0%, Mg: 0.50-15.00%, Si: 0-4.0%, and Fe: 0-15.0% It contains, and further, Ni: 0-1.000%, Ca: 0-3.0%, 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: 0-0.500%, and W: 0~0.500% It contains at least one of the following in total amount of 5,000% or less: The remainder has a chemical composition consisting of Zn and impurities. From the interface between the base steel sheet and the plating layer, the area ratio of pearlite in the thickness direction is 0 to 20% and the depth is 3 to 100 μm. The area ratio of the perlite is 0-30% at a depth where the area ratio of the perlite is 0-20%, and the area ratio of perlite with an equivalent circle diameter of 5 μm or more is 0-30%. The amount of the aforementioned plating layer is 20 g / m² per side. 2 That's all. A plated steel sheet characterized in that the plated layer has an oxide layer on its surface, and when the oxide layer is measured by XPS, the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 3 or more. Here, Al-O, Mg-O, and Zn-O represent the intensities of the peaks attributed to the Al-O, Mg-O, and Zn-O bonds, respectively.

2. The plated steel sheet according to claim 1, characterized in that the area ratio of the perlite is 0 to 20% and the depth is 10 to 100 μm.

3. The plated steel sheet according to claim 2, characterized in that the area ratio of the perlite is 0 to 20% and the depth 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 perlite is 0 to 15% of the perlite with an equivalent circle diameter of 5 μm or more at a depth where the area ratio of perlite is 0 to 20%.

5. The plated steel sheet according to any one of claims 1 to 3, characterized in that the chemical composition contains, by mass%, Al: 10.0 to 50.0% and Mg: 4.00 to 15.00%, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 5 or more.

6. The plated steel sheet according to claim 5, characterized in that the chemical composition contains, by mass%, Al: 30.0 to 50.0% and Mg: 7.00 to 15.00%, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 10 or more.

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