Hot stamped molded parts

Abstract

Provided is a hot-stamp molded body characterized by comprising a steel base material and a plating layer disposed on a surface of the steel base material, wherein: the plating layer has a prescribed chemical composition; the average C concentration from the surface of the steel base material to 1 μm in the depth direction is 0.25 mass% or less; the steel base material contains 90% or more of martensite in terms of area ratio; the adhesion amount of the plating layer is 60 g / m2 or more per one surface; and the hot-stamp molded body has a Vickers hardness is 400 HV or more.

Description

[Technical Field]

[0001] This invention relates to a hot-stamped molded body. [Background technology]

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel plates used in structural components is one effective method, and for this reason, the development of high-strength steel plates is progressing.

[0003] 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.

[0004] In connection with this, various studies have been conducted on hot-stamped molded products that have been plated with zinc.

[0005] For example, Patent Document 1 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 2 provides a step of providing 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 amount of silicon 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) obtaining the blank, D) heat-treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel, E) transferring the blank into a press tool, F) 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 2 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] Special Publication No. 2018-528324 [Patent Document 2] 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 and 2, are used in hot stamping, the plating layer after hot stamping may alloy with the base metal (steel matrix), 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 2 teaches how to suppress LME that occurs during hot stamping, but Patent Document 2 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. In addition, it is known that LME cracking is generally more pronounced in steel materials with relatively high strength, and the susceptibility to LME cracking tends to increase as the strength of the steel material increases. On the other hand, the automotive industry and other sectors are also demanding further weight reduction of steel materials, and achieving such weight reduction requires increasing the strength of the steel materials more than ever before. Therefore, even when the strength is increased to the same level as or higher than before, there is a high demand for steel materials that can solve the problem of LME cracking, and more specifically for hot-stamped molded products.

[0009] Therefore, the present invention aims to provide a hot-stamped molded article that is high-strength, maintains high corrosion resistance even after hot-stamping, and can suppress LME cracking during spot welding. [Means for solving the problem]

[0010] The inventors of the present invention conducted studies to achieve the above object, and found that, first, forming a Zn-containing plating layer with an adhesion amount of a predetermined value or more makes it possible to maintain sufficient corrosion resistance even when applied to hot stamping. In addition, the inventors of the present invention found that despite having a high Vickers hardness of 400 HV or higher, by appropriately modifying the surface layer structure of the steel base material before hot stamping so that the C concentration in the surface layer of the steel base material after hot stamping is relatively low, the occurrence of LME cracking during spot welding after hot stamping can be remarkably suppressed or reduced even with the plating layer formed with the relatively large plating adhesion amount as described above, thereby completing the present invention.

[0011] The present invention that has achieved the above object is as follows. (1) Comprising a steel base material and a plating layer disposed on a surface of the steel base material, The plating layer contains, in mass%, Ni: 0.5 to 25.0%, and Fe: 20.0 to 70.0%, and further contains, Al: 0 to 1.000%, Mg: 0 to less than 0.500%, Si: 0 to 0.200%, 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 1.000%, 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 the above elements in a total content of 5.000% or less, has a chemical composition consisting of the balance Zn and impurities, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is 0.25 mass% or less, the steel base material contains 90% or more of martensite in terms of area fraction, the adhesion amount of the plating layer per one side is 60 g / m 2 or more, has a Vickers hardness of 400 HV or more, which is a hot-stamped molded product. (2) The hot-stamped molded product according to the above (1), wherein the average C concentration is 0.18 mass% or less, and the Fe content in the plating layer is 20.0 to 60.0 mass%. (3) The hot-stamped molded product according to the above (1) or (2), wherein the average C concentration is 0.10 mass% or less. Effects of the Invention

[0012] According to the present invention, there can be provided a hot-stamped molded product that has high strength, maintains high corrosion resistance even after hot stamping, and can suppress LME cracking during spot welding. Mode for Carrying Out the Invention

[0013] <Hot-stamped molded product> The hot-stamped molded product according to an embodiment of the present invention includes a steel base material and a plating layer disposed on a surface of the steel base material, the plating layer comprises, in mass%: Ni: 0.5 to 25.0%, and Fe: 20.0~70.0% It contains, and further, Al: 0-1,000%, Mg: 0-0.500% Si: 0~0.200%, Ca: 0-3,000%, Sb: 0~0.500%, Pb: 0~0.500%, Cu: 0~1.000%, Sn: 0~1.000%, Ti: 0~1,000%, Cr: 0~1.000%, Nb: 0~1.000%, Zr: 0~1.000%, Mn: 0~1.000%, Mo: 0~1.000%, Ag: 0~1,000%, Li: 0~1.000%, La: 0~0.500%, Ce: 0~0.500%, B: 0~0.500%, Y: 0~0.500%, Sr: 0~0.500%, In: 0~0.500%, 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. The average C concentration from the surface of the steel base material to a depth of 1 μm is 0.25% by mass or less. The aforementioned steel base material contains 90% or more martensite by area ratio, The amount of the aforementioned plating layer is 60 g / m² per side. 2 That's all. It is characterized by having a Vickers hardness of 400 HV or higher.

[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 where LME cracking occurs, 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 sheet is limited and may not always be satisfactory.

[0015] As a result of various studies, the inventors have found that, from the viewpoint of improving LME resistance, even if the carbon concentration in the surface layer of the steel base material before hot stamping is reduced by decarburization, etc., carbon contained in the bulk of the steel base material diffuses to the surface layer during high-temperature heating in hot stamping, and this recarburization to the surface layer causes the initial LME suppression effect due to the low carbon concentration of the surface layer to disappear or be reduced. Therefore, the inventors conducted further studies and found that by creating a structure in the surface layer of the steel base material before hot stamping that can suppress such recarburization, even when the amount of Zn-containing plating layer attached is relatively large in order to maintain sufficient corrosion resistance, the LME suppression effect due to the initial low carbon concentration of the surface layer 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 set the amount of plating layer attached to 60 g / m² per side. 2By doing so, sufficient corrosion resistance is maintained even when applied to hot stamping, and a structure is formed in the surface layer of the steel base material with a relatively small amount of pearlite and a reduced amount of coarse pearlite. As a result, when applied to hot stamping, the average C concentration from the surface of the steel base material to a depth of 1 μm can be suppressed to 0.25 mass% or less, and in connection with this, it has been found that LME cracking during spot welding after hot stamping can be reliably suppressed or reduced.

[0016] While not intended to be bound by any particular theory, when obtaining a hot-stamped molded body according to an embodiment of the present invention by hot stamping, it is believed that the microstructure in the surface layer of the steel base material acts as follows to suppress or reduce the diffusion of carbon contained in the bulk of the steel base material to the surface layer and subsequent recarburization during high-temperature heating in hot stamping. More specifically, when the carbon concentration in the surface layer of the steel base material is reduced by decarburization or the like, the amount of pearlite generated in the microstructure of the surface layer of the steel base material becomes relatively small in relation to this reduction in carbon concentration. By forming a region with a relatively small amount of pearlite in the surface layer of the steel base material, it is possible to fully exert the LME suppression effect based on the reduction in carbon concentration. However, simply reducing the amount of pearlite is insufficient, as in cases where pearlite precipitates along grain boundaries, it is believed 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 material attempts to diffuse towards 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 exists along the grain boundaries as described above, the carbon in the bulk will diffuse towards the surface through this pathway, promoting recarburization in the surface layer. As a result, the initial LME suppression effect due to the low carbon concentration in the surface layer cannot be fully realized. In contrast, according to the embodiment of the present invention, by further reducing the amount of coarse pearlite in the surface layer region where the amount of pearlite is relatively small, austenite transformed from pearlite can be dispersed and present on the grain boundaries even during high-temperature heating in hot stamping, thereby reliably disrupting 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 steel matrix, forming a carbon recarburization pathway, which in turn promotes the diffusion of carbon from the bulk to the surface side of the steel matrix. In this regard, when obtaining a hot-stamped molded article according to the embodiment of the present invention by hot stamping, it is important to reduce the amount of pearlite in the surface layer of the steel matrix, as well as the amount of coarse pearlite in the surface layer. 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, thus reliably disrupting the carbon recarburization pathway by the austenite. As a result, in the final hot-stamped molded article, the average C concentration from the surface of the steel matrix to a depth of 1 μm can be suppressed to 0.25 mass% or less. In particular, when hot-stamping a plated steel sheet with Zn-containing plating, the plating layer and Fe alloy together due to interdiffusion between the Fe in the steel base material and the plating layer during high-temperature heating in the hot-stamping process. On the other hand, since the C in the steel base material is not contained in the plating layer, the C concentration near the surface of the steel base material becomes relatively concentrated relative to Fe. Therefore, even if the surface layer of the steel base material has been decarburized or otherwise reduced in carbon concentration before hot-stamping, the C concentration in the surface layer of the steel base material may become much higher than the C concentration in the bulk after hot-stamping. In contrast, according to the embodiment of the present invention, the diffusion of C from the bulk to the surface layer is suppressed by interrupting the recarburization pathway as described above, so that the concentration of C near the surface of the steel base material can be significantly suppressed or reduced compared to conventional plated steel sheets, and more specifically, the average C concentration from the surface of the steel base material to a depth of 1 μm can be suppressed to 0.25 mass% or less.Therefore, according to the embodiments 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 molding is significantly suppressed. This allows the LME suppression effect due to the initial reduction of carbon concentration in the surface layer of the steel base material to be fully realized, thereby reliably suppressing or reducing 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 steel base material before hot stamping molding in a hot stamped body equipped with a Zn-containing plating layer is a fact that has been revealed for the first time by the present inventors. Therefore, the hot stamped body according to the embodiments of the present invention is particularly useful in the automotive field, where spot welding is relatively common.

[0018] The following describes in more detail the hot-stamped molded articles according to embodiments of the present invention. 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 and upper limits, respectively, unless otherwise specified.

[0019] [Plating layer] According to embodiments of the present invention, the plating layer is disposed on the surface of the steel base material, for example, on at least one, preferably both, surfaces of the steel base material. Here, the expression "disposed on the surface of the steel base material" includes not only cases where the plating layer is directly disposed on the surface of the steel base material, but also cases where the plating layer is indirectly disposed on the surface of the steel base material, for example, when a solid solution layer, which will be described later, is included between the steel base material and the plating layer. The plating layer has the following chemical composition.

[0020] [Ni: 0.5~25.0%] Ni is an effective element for improving the corrosion resistance of the plating layer. To obtain this effect fully, the Ni content should be 0.5% or more. The Ni content may also be 0.51% or more, 0.52% or more, 0.53% or more, 0.55% or more, 0.6% or more, 1.0% or more, 5.0% or more, 7.0% or more, 10.0% or more, or 12.0% or more. There is no particular upper limit, but from the viewpoint of manufacturing costs, etc., the Ni content should be 25.0% or less, for example, 23.0% or less, 20.0% or less, 18.0% or less, or 16.0% or less.

[0021] [Fe: 20.0~70.0%] When a plated steel sheet is heated during hot stamping, Fe from the steel base material diffuses into the plating layer and alloys with Zn, etc., so the plating layer inevitably contains Fe. Therefore, the Fe content is 20.0% or more, and may be, for example, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. On the other hand, according to embodiments of the present invention, as will be explained in detail later in relation to the manufacturing method of the hot stamped molded body, it is thought that by appropriately modifying the surface layer structure of the steel base material before hot stamping, the alloying of Zn, etc. in the plating layer with Fe in the steel base material can be delayed, although the reason is not entirely clear. Therefore, according to embodiments of the present invention, the Fe content in the plating layer can be suppressed to a maximum of 70.0% or less. In this regard, the amount of plating layer attached is made relatively large, specifically 60 g / m² per side. 2 By controlling the process as described above, it is possible to maintain a relatively large number of plating layers in which alloying has not progressed sufficiently, and the presence of such plating layers makes it possible to maintain sufficient corrosion resistance. From the viewpoint of improving corrosion resistance, a lower Fe content is preferable, and may be, for example, 67.0% or less, 65.0% or less, 62.0% or less, 60.0% or less, 57.0% or less, or 55.0% or less.

[0022] Furthermore, the plating layer can be optionally composed of: Al: 0-1.000%, Mg: 0-0.500%, Si: 0-0.200%, Ca: 0-3.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1.000%, Nb: 0-1.000%, Zr: 0-1.000%, Mn: 0-1.000% It may contain at least one of the following elements: 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%. These optional elements are not particularly limited, but it is preferable that their total content be 5.000% or less. The optional elements may be present in amounts of 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less in total. These optional elements will be described in detail below.

[0023] [Al: 0~1.000%] Al is an effective element for improving the corrosion resistance of the plating layer. The Al content may be 0%, but to obtain this effect, it is preferable that the Al content be 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.080% or more, 0.100% or more, or 0.150% or more. On the other hand, if the Al content is excessive, the composition of the plating layer will approach the Zn-Al eutectic composition, which may lower the melting point of the plating layer. Therefore, it is preferable that the Al content be 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0024] [Mg: 0-0.500%] Mg is an effective element for improving the corrosion resistance of the plating layer. The Mg content may be 0%, but to obtain this effect, it is preferable that the Mg content be 0.001% or more. The Mg content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, from the viewpoint of improving processability, 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.

[0025] [Si: 0~0.200%] 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 0.0001% 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.

[0026] [Ca: 0~3.000%] 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 0.001% or more to obtain this effect. The Ca content may also be 0.005% or more, 0.010% or more, 0.100% or more, or 1.000% 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 3.000% or less. The Ca content may also be 2.500% or less, 2.000% or less, or 1.500% or less.

[0027] [Sb:0~0.500%, Pb:0~0.500%, Cu:0~1.000%, Sn:0~1.000%, Ti:0~1.000%, Cr:0 ~1.000%, Nb:0~1.000%, Zr:0~1.000%, Mn:0~1.000%, Mo:0~1.000%, Ag:0~1.000 %, Li:0~1.000%, La:0~0.500%, Ce:0~0.500%, B:0~0.500%, Y:0~0.500%, 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 hot-stamped molded product as long as they are within the predetermined content range. However, excessive amounts 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 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. The Zn content of the remainder is not particularly limited, but may be, for example, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more.

[0029] [Measurement of the chemical composition of the plating layer] The chemical composition of the plating layer is determined by dissolving only the plating layer in an acidic aqueous solution and performing chemical analysis. Specifically, the plating layer is dissolved in a room-temperature acidic aqueous solution containing an inhibitor that suppresses the dissolution of steel, more specifically, an acidic aqueous solution obtained by adding 1% Hibiron (A-6) (manufactured by Sugimura Chemical Industrial Co., Ltd.) to 10% hydrochloric acid, and the obtained acidic aqueous solution is analyzed by ICP (inductively coupled plasma optical emission spectrometry), whereby the chemical composition of the plating layer is obtained.

[0030] The plating layer may be any plating layer having the above chemical composition and is not particularly limited, and may be, for example, a hot-dip plating layer, an alloyed hot-dip plating layer, or an electroplating layer. Preferably, the plating layer is an electroplating layer.

[0031] [Coating weight of plating layer: 60 g / m per side 2 or more] In the hot-stamped molded article according to the embodiment of the present invention, the coating weight of the plating layer is 60 g / m per side 2 or more. In general, during high-temperature heating in hot stamp molding, the plating layer may alloy with the steel base material, leading to a decrease in corrosion resistance. However, according to the embodiment of the present invention, although the reason is not necessarily clear, it is considered that alloying of the plating layer and the steel base material can be delayed due to the appropriately modified surface layer structure of the steel base material before hot stamp molding, that is, the surface layer structure in which the amount of pearlite is relatively small and the amount of coarse pearlite is reduced. Therefore, when the coating weight of the plating layer is set to a relatively large value, specifically 60 g / m per side 2 or more, there will be plating layers that have not been sufficiently alloyed after hot stamp molding, and it is considered that sufficient corrosion resistance can be maintained due to the presence of such plating layers. On the other hand, if the coating weight of the plating layer is small, the effect related to the above-described delay of alloying cannot be sufficiently obtained, and the corrosion resistance after hot stamp molding may decrease. From the viewpoint of improving corrosion resistance, the coating weight of the plating layer is preferably 65 g / m per side 2 or more, or 70 g / m 2 or more, more preferably 80 g / m 2In addition, it is even more comfortable at 90g / m². 2 In summary, the most preferred value is 100 g / m². 2 That concludes the explanation. There is no particular upper limit, but the amount of plating layer attached is, for example, 200 g / m². 2 Below 190g / m 2 Below 180g / m 2 The following or 170g / m 2 The following is also acceptable.

[0032] [Measurement of plating adhesion] The amount of plating layer attached is determined by dissolving only the plating layer in an acidic aqueous solution. Specifically, a 30mm x 30mm sample is taken from the hot-stamped molded body, and then the plating layer is dissolved in an acidic aqueous solution containing an inhibitor that suppresses the dissolution of steel, specifically a room-temperature acidic aqueous solution of 10% hydrochloric acid with 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) added. The amount of plating layer attached is determined from the change in weight of the sample before and after the dissolution of the plating layer.

[0033] [Average carbon concentration from the surface of the steel base material to a depth of 1 μm: 0.25% by mass or less] In the hot-stamped molded article according to the embodiment of the present invention, the average carbon concentration from the surface of the steel base material to a depth of 1 μm is 0.25% by mass or less. As will be explained in detail later in relation to the manufacturing method of the hot-stamped molded article, the microstructure related to the low carbon concentration of the surface layer of the steel base material before hot-stamping, namely the surface layer microstructure in which the amount of pearlite is relatively small and the amount of coarse pearlite is reduced, significantly suppresses recarburization during high-temperature heating of hot-stamping. In connection with this, in the final hot-stamped molded article, the average carbon concentration from the surface of the steel base material to a depth of 1 μm is reduced to 0.25% by mass or less. As a result, the LME suppression effect due to the initial low carbon concentration of the surface layer of the steel base material is fully exerted, making it possible to reliably suppress or reduce the occurrence of LME cracks during spot welding after hot-stamping. Therefore, from the viewpoint of improving LME resistance during spot welding after hot stamping, it is preferable that the average C concentration from the surface of the steel base material to a depth of 1 μm be as low as possible, for example, 0.22 mass% or less, 0.20 mass% or less, 0.18 mass% or less, 0.15 mass% or less, 0.12 mass% or less, 0.10 mass% or less, 0.08 mass% or less, or 0.06 mass% or less. In particular, when the average C concentration is set to 0.18 mass% or less, the alloying delay effect between Zn etc. in the plating layer and Fe in the steel base material, as mentioned above, becomes more pronounced. As a result, the Fe content in the plating layer can be relatively reduced, and more specifically, the Fe content in the plating layer can be reduced to 60.0 mass% or less. Therefore, it is possible to further improve corrosion resistance after hot stamping. The lower limit of the average C concentration from the surface of the steel base material to a depth of 1 μm is not particularly limited, but for example, the average C concentration may be 0.001% by mass or more, 0.005% by mass or more, or 0.01% by mass or more.

[0034] [Measurement of average carbon concentration from the surface of the steel base material down to a depth of 1 μm] The average carbon concentration from the surface of the steel matrix down to a depth of 1 μm is determined using a radiofrequency glow discharge emission spectrometer (GDS) as follows. Specifically, the surface of the hot-stamped molded body is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the hot-stamped molded body is then sputtered and analyzed in the depth direction. The elements contained in the material are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass percent by creating a calibration curve. When a hot-stamped molded body is measured using GDS in this manner, the position where the Zn concentration in the depth direction becomes 0.1% or less is determined as the surface of the steel base material, and the average C concentration in the region from that surface to a depth of 1 μm is determined as the "average C concentration from the surface of the steel base material to a depth of 1 μm".

[0035] As mentioned earlier, when hot-stamping a plated steel sheet with conventional Zn-containing plating, Fe in the steel base material diffuses into the plating layer during high-temperature heating in hot-stamping, while C in the steel base material does not diffuse into the plating layer. As a result, C becomes relatively concentrated near the surface of the steel base material relative to Fe. Therefore, even if the surface layer of the steel base material has been decarburized or otherwise reduced in carbon concentration before hot-stamping, the C concentration in the surface layer of the steel base material may become much higher than the C concentration in the bulk after hot-stamping. However, according to the embodiment of the present invention, since the diffusion of C from the bulk to the surface layer is suppressed due to the suppression of recarburization, the concentration of C near the surface of the steel base material can be significantly suppressed or reduced compared to the case of conventional plated steel sheets. From the viewpoint of improving LME resistance, it is preferable that the average C concentration from the surface of the steel base material to a depth of 1 μm is equal to or lower than the C content of the steel base material. More specifically, the average C concentration from the surface of the steel base material to a depth of 1 μm is less than 1.10 times the C content of the steel base material, and may be, for example, 1.05 times or less, 1.00 times or less, 0.90 times or less, 0.80 times or less, 0.70 times or less, 0.60 times or less, or 0.50 times or less. The lower limit is not particularly limited, but for example, the average C concentration from the surface of the steel base material to a depth of 1 μm may be 0.05 times or more, 0.10 times or more, or 0.15 times or more the C content of the steel base material. In the present invention, "C content of the steel base material" refers to the value measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014, using a test piece obtained from around the 1 / 2 position of the thickness of the steel base material.

[0036] [Solid solution layer] In embodiments of the present invention, the hot-stamped molded body may further include a solid solution layer between the steel base material and the plating layer. Depending on the plating composition and hot-stamping conditions before hot-stamping, solid solution formation may occur between Fe diffusing from the steel base material to the plating layer and Al and / or Zn in the plating layer, and a solid solution layer containing solid solutions of these elements may be formed between the steel base material and the plating layer after hot-stamping. Therefore, if the hot-stamped molded body further includes a solid solution layer, the solid solution layer includes Fe and one or both of Al and Zn. More specifically, the solid solution layer includes an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an Fe-Al-Zn solid solution layer.

[0037] The presence of a solid solution layer can be confirmed as follows. First, the plating layer is dissolved only in an acidic aqueous solution containing an inhibitor that suppresses the dissolution of steel, specifically a room-temperature acidic aqueous solution of 10% hydrochloric acid with 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) added. Next, using a high-frequency glow discharge emission spectrometer (GDS), the surface of the hot-stamped molded body is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the hot-stamped molded body is then sputtered and analyzed in the depth direction. The elements contained in the material are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. The data in the depth direction can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass % by creating a calibration curve. When a hot-stamped molded body is measured using GDS in this manner, the region where the Zn concentration exceeds 0.1% in the depth direction is determined to be the solid solution layer.

[0038] [Martensite area ratio of steel base material: 90% or more] The steel base material of the hot-stamped molded article according to the embodiment of the present invention contains 90% or more martensite by area. The remaining structure is not particularly limited, but may consist of at least one of bainite, ferrite, retained austenite, and pearlite in amounts of 10% or less. Martensite is a very hard structure, and therefore, by including 90% or more martensite by area in the hot-stamped molded article, it is possible to achieve high strength, specifically a Vickers hardness of 400 HV. On the other hand, if the area ratio of martensite is low and the proportion of soft structures such as ferrite is high, it may not be possible to achieve a Vickers hardness of 400 HV. Therefore, a larger area ratio of martensite is preferable, and may be, for example, 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the area ratio of martensite is not particularly limited and may be 100%.

[0039] [Identification of martensite and calculation of area ratio] The identification and calculation of the area fraction of martensite are performed as follows. First, a sample is taken from the hot-stamped molded body so that the cross section parallel to the rolling direction and thickness direction becomes the observation surface. Next, the observation surface is mirror-polished, etched with Nital etching solution, and then the microstructure is observed using a scanning electron microscope (SEM). At a depth of 1 / 4 of the thickness of the observation surface, a 300 μm × 300 μm area is photographed at 1000x magnification. The obtained microstructural images are binarized to black and white and then image analysis is performed to identify pearlite, bainite, and ferrite, and the total area fraction of these is determined using the method based on "Microscopic Test Method for Grain Size of Steel" as defined in JIS G 0551:2020. Since retained austenite is difficult to distinguish from martensite using SEM, the area fraction of retained austenite is measured by 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%.

[0040] [Mechanical properties] According to the hot-stamped molded article of the present invention, excellent mechanical properties, such as a Vickers hardness of 400 HV or more, can be achieved, more specifically, a Vickers hardness of 400 HV or more at the 1 / 2 position of the thickness of the steel base material. A Vickers hardness of 500 HV or more is preferred, and 550 HV or more is more preferred. There is no particular upper limit, but for example, the Vickers hardness may be 650 HV or less or 600 HV or less.

[0041] [Hardness measurement] Vickers hardness is determined as follows: First, a test specimen is cut from any position except the edges of the hot-stamped molded body so that a cross-section perpendicular to the surface (thickness cross-section) can be observed. The thickness cross-section of the test specimen is polished using #600 to #1500 silicon carbide sandpaper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and this thickness cross-section is used as the measurement surface. Next, the Vickers hardness is measured using a micro-Vickers hardness tester with a load of 1 kgf at intervals of at least 3 times the indentation length. A total of 20 measurements are taken randomly around the 1 / 2 position of the thickness of the steel base material, without including the low-carbon concentration surface layer, and the arithmetic mean of these measurements is determined as the hardness of the hot-stamped molded body.

[0042] [Preferred chemical composition of steel base material] The chemical composition of the steel base material according to the embodiments of the present invention may be any chemical composition that is commonly applied in hot-stamped molded articles and capable of achieving a Vickers hardness of 400 HV or higher. Preferred chemical compositions of the steel base material will be described in detail below, but these descriptions are intended merely as examples of preferred chemical compositions of steel base materials suitable for achieving a Vickers hardness of 400 HV or higher, and are not intended to limit the present invention to those using steel base materials having such specific chemical compositions.

[0043] In embodiments of the present invention, for example, the steel base material 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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] [Al:0.0002~2.000%] Al acts as a deoxidizing agent for steel and is an element that has the effect of making steel sound. To obtain such an effect to the fullest extent, it is preferable that the Al content be 0.0002% or more. The Al content may also 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 material. For this reason, it is preferable that the Al content be 2.000% or less. The Al content may also be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0048] [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.

[0049] [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.

[0050] [N:0.0100% or less] N is an element that forms coarse nitrides in steel, reducing its 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. 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.

[0051] The preferred basic chemical composition of the steel matrix is ​​as described above. Furthermore, the steel matrix 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.

[0052] In the steel base material, the remainder other than the elements mentioned above consists of Fe and impurities. Impurities in the steel base material refer to components that are mixed in during the industrial production of the steel base material due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0053] The chemical composition of the steel base material can be measured using general analytical methods. For example, the chemical composition of the steel base material can be determined by first removing the plating layer by mechanical grinding, and then measuring it using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on the chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from approximately half the thickness of the steel base material, and measured 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.

[0054] <Method for manufacturing hot-stamped molded products> Next, preferred methods for manufacturing hot-stamped molded articles according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing hot-stamped molded articles according to embodiments of the present invention, and is not intended to limit the hot-stamped molded articles to those manufactured by the manufacturing methods described below.

[0055] A hot-stamped molded article 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 wound 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, a plating step of forming a plating layer on the obtained steel base material, and a hot-stamping step of hot-stamping the obtained plated steel sheet. Alternatively, the hot-rolling step may be performed without winding, after which the steel may be pickled and then immediately subjected to the cold-rolling step. Each step will be described in detail below.

[0056] [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.

[0057] [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%.

[0058] [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.

[0059] [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.

[0060] [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 carrying out the annealing process under such relatively high dew point conditions, the surface layer of the cold-rolled steel sheet can be properly decarburized. This decarburization treatment makes it possible to form a region with a relatively small amount of pearlite on the surface of the steel base material in the plated steel sheet before hot stamping, and more specifically, it is possible to form a depth region of 3 μm or more in the thickness direction from the surface of the steel base material where the area ratio of pearlite is 20% or less. By forming such a structure with a relatively small amount of pearlite on the surface of the steel base material, it is possible to reliably suppress the average C concentration from the surface of the steel base material to a depth of 1 μm in the final hot-stamped molded product to 0.25 mass% or less. If the dew point is below -20°C, the heating temperature is below 730°C, and / or the holding time is below 10 seconds, decarburization of the surface layer of the cold-rolled steel sheet will be insufficient. As a result, it will not be possible to form a region with a relatively small amount of pearlite on the surface layer of the steel base material. 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 will form on the surface of the steel sheet, which may reduce the plating properties or reduce the strength of the hot-stamped molded article obtained 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).

[0061] [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.

[0062] [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 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 material to the surface layer of the steel base material. 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 base material 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 grain boundaries. In connection with this, in plated steel sheets before hot stamping, it becomes impossible to reduce the amount of coarse pearlite in the surface layer of the steel base material. More specifically, a large amount of coarse pearlite with an equivalent circular diameter of 5 μm or more is formed along the grain boundaries in the surface layer of the steel base material. When the amount of such coarse pearlite formed along the grain boundaries increases, the formation of carbon recarburization pathways by austenite along the grain boundaries is promoted during high-temperature heating in hot stamping. As a result, in the final hot-stamped molded body, it becomes impossible to suppress the average C concentration from the surface of the steel base material to a depth of 1 μm to 0.25 mass% or less, and sufficient LME resistance cannot be achieved during spot welding.

[0063] [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, can be dispersed on the grain boundaries at a certain point or higher, 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. Martensite and bainite have a faster transformation rate to austenite than 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.

[0064] [Plating process] Next, in the plating process, a plating layer is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (steel base material). More specifically, the plating process is carried out by hot-dip galvanizing using a plating bath (plating bath temperature: e.g., 420-480°C) having a predetermined chemical composition, and an alloying treatment may be performed after the hot-dip galvanizing process. Furthermore, the plating process 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, 60-200 g / m² per side. 2 It can be adjusted within the range.

[0065] [Hot stamping molding process] Finally, the obtained plated steel sheet is hot-stamped in a hot-stamping process to produce a hot-stamped molded body having the desired surface layer composition and hard structure. From the viewpoint of obtaining the desired hard structure, it is preferable to charge the plated steel sheet into a furnace at 800 to 1000°C and hold it in the furnace for 60 to 600 seconds after the temperature of the plated steel sheet reaches a predetermined temperature, for example, the furnace temperature -10°C. If the heating temperature is less than 800°C and / or the holding time is less than 60 seconds, austenitization will be insufficient, and it may not be possible to obtain the desired area ratio of the hard structure (i.e., an area ratio of 90% or more of martensite), and the final hot-stamped molded body may not achieve a Vickers hardness of 400 HV or more. The heating atmosphere is not particularly limited and can be under normal conditions, for example, in the atmosphere, in a gas combustion atmosphere with a controlled ratio of air to fuel, or in a nitrogen atmosphere, and the dew point may be controlled in these gases. After heating and holding in the furnace, the plated steel sheet is removed from the furnace, and then, after the plated steel sheet reaches a predetermined temperature, for example, 850°C or lower, hot stamping can be performed under normal conditions. After hot stamping, although not particularly limited, it is sufficient to cool the sheet to a temperature range of, for example, 250°C or lower at an average cooling rate of 20°C / second or more.

[0066] 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]

[0067] In the following examples, hot-stamped molded articles according to the present invention were manufactured under various conditions, and the properties of the manufactured hot-stamped molded articles were investigated.

[0068] First, molten steel was cast using a continuous casting method to form steel billets having the chemical composition shown in Table 1. After the 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 annealed 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 cooled under the same conditions shown in Table 2 to produce steel base materials.

[0069] Next, the manufactured steel base material 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 steel base material 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 steel base material with the plated layer attached was cooled from the plating bath temperature (approximately 420 to 480°C) to room temperature using nitrogen gas as a cooling gas, thereby obtaining a plated steel sheet with a plating layer formed on both sides of the steel base material. The sheet temperature was measured using a thermocouple spot-welded to the center of the steel base material.

[0070] [Chemical composition analysis of the plating layer] The chemical composition of the plating layer was determined 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 there 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. After heating and rapid cooling, samples cut to 30 mm x 30 mm were immersed in a room-temperature acidic aqueous solution of 10% hydrochloric acid with 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) added. After pickling and stripping the plating layer, the plating components dissolved in the aqueous solution were determined by measurement using ICP emission spectroscopy. The results are shown in Table 2.

[0071] [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 directly below the electrode in 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

[0072] [Evaluation of corrosion resistance] The corrosion resistance after hot stamping 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 die plates at room temperature. A 50mm x 100mm sample of the hot-stamped molded body 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 reaching the base metal (steel matrix) was introduced in the center of the sample. Then, a combined cycle corrosion test in accordance with JASO (M609-91) was performed for 120 cycles, and the width of the coating blister was measured, and the corrosion resistance was evaluated as follows. AAA: 2mm or less AA: over 2~3mm A: More than 3~4mm B: More than 4mm

[0073] [Evaluation of hardness after hot stamping (HS)] First, similar to the evaluation of corrosion resistance, 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 a hot-stamped molded body was obtained by sandwiching the plated steel sheet between flat molds at room temperature and rapidly cooling it (HS condition A). On the other hand, a hot-stamped molded body of Comparative Example 42 was obtained in the same manner as HS condition A, except that the plated steel sheet was placed in an air-heated furnace at 700°C (HS condition B). A test piece was cut out from any position other than the edges of the obtained hot-stamped molded body so that a cross section perpendicular to the surface (thickness cross section) could be observed. The thickness cross section of the test piece 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, Vickers hardness was measured using a micro-Vickers hardness tester at a load of 1 kgf, at intervals of more than three times the indentation length. A total of 20 measurements were taken randomly at approximately half the thickness of the steel base material, avoiding the low-carbon concentration surface layer. 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

[0074] Hot-stamped molded articles that exhibited high strength, maintained high corrosion resistance even after hot-stamping, and suppressed LME cracking during spot welding were evaluated as having hardness ratings of AAA, AA, and A, LME resistance ratings of AAA, AA, and A, and corrosion resistance ratings of AA and A. The results are shown in Table 2. In the hot-stamped molded articles shown in Table 2, the remaining microstructure other than martensite consisted of bainite, ferrite, retained austenite, and / or pearlite.

[0075] [Table 1]

[0076] Table 2-1

[0077] Table 2-2

[0078] Referring to Table 2, in Comparative Example 35, although the LME resistance and corrosion resistance were good, the hardness after HS was low, and therefore the desired high strength could not be achieved. 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 low heating temperature in the annealing process. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm became high, and the LME resistance decreased. In Comparative Example 37, it is thought that the decarburization of the surface layer of the cold-rolled steel sheet was similarly insufficient due to the short holding time in the annealing process. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm became high, and the LME resistance decreased. In Comparative Example 38, it is thought that the dew point in the annealing process was similarly insufficient due to the low decarburization of the surface layer of the cold-rolled steel sheet. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm became high, and the LME resistance decreased. Cross-sectional observation of plated steel sheets before hot stamping using SEM for Comparative Examples 36-38 revealed that the depth region where the area ratio of pearlite was 20% or less in the thickness direction from the surface of the steel base material was less than 3 μm in all cases. In Comparative Example 39, it is thought that the low average cooling rate of the primary cooling in the cooling process caused pearlite to precipitate at high temperatures, forming along the grain boundaries, and promoting recarburization during high-temperature heating in hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm increased, reducing LME resistance. In Comparative Example 40, it is thought that the high control temperature of the primary cooling in the cooling process similarly caused pearlite to precipitate at high temperatures and form along the grain boundaries, promoting recarburization during high-temperature heating in hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm increased, reducing LME resistance. When the plated steel sheets of Comparative Examples 39 and 40 were observed in cross-section by SEM before hot stamping, a relatively large amount of coarse pearlite with an equivalent circular diameter of 5 μm or more was found to have formed on the surface layer of the steel base material. In Comparative Examples 41 and 42, the control temperature of the secondary cooling in the cooling process was low, so bainite, rather than pearlite, precipitated mainly, which is thought to have promoted recarburization during the high-temperature heating of hot stamping.As a result, the average carbon concentration from the surface of the steel base material to a depth of 1 μm became high, reducing LME resistance. In Comparative Example 43, the average cooling rate of the secondary cooling in the cooling process was fast, so bainite, rather than pearlite, precipitated mainly, which is thought to have promoted recarburization during high-temperature heating in hot stamping. As a result, the average carbon concentration from the surface of the steel base material to a depth of 1 μm became high, reducing LME resistance. In Comparative Example 44, the amount of plating layer adhesion was insufficient, resulting in reduced corrosion resistance after HS. In Comparative Example 45, the hot stamping conditions were not appropriate, resulting in insufficient austenitization, failure to achieve the desired martensite area ratio, and reduced hardness after HS.

[0079] In contrast, all examples of hot-stamped molded articles have a predetermined plating chemical composition and a plating layer adhesion amount of 60 g / m² per side. 2 In addition to the above, by controlling the average C concentration from the surface of the steel base material to a depth of 1 μm to 0.25 mass% or less, even when applied to hot stamping, it was possible to maintain high strength and high corrosion resistance while fully exhibiting the LME suppression effect due to the initial low carbon concentration in the surface layer of the steel base material, thereby reliably suppressing or reducing the occurrence of LME cracks during subsequent spot welding. When the plated steel sheets of all examples were observed in cross-section by SEM before hot stamping, the depth region in the thickness direction from the surface of the steel base material where the area ratio of pearlite was 20% or less was 3 μm or more in all cases, and the proportion of coarse pearlite with an equivalent circle diameter of 5 μm or more in the surface layer of the steel base material was sufficiently lower compared to Comparative Examples 39 and 40. In particular, in Examples 4 to 34, where the average C concentration from the surface of the steel base material to a depth of 1 μm was controlled to 0.18 mass% or less, the Fe content in the plating layer could be reduced to 60.0 mass% or less, resulting in a corrosion resistance evaluation of AA, and further improvement in corrosion resistance was achieved. Furthermore, GDS measurements were performed on each hot-stamped molded body, and the presence of a solid solution layer containing an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an Fe-Al-Zn solid solution layer was confirmed in all examples of the hot-stamped molded bodies.

Claims

1. The system comprises a steel base material and a plating layer disposed on the surface of the steel base material. The aforementioned plating layer is, by mass%, Ni: 0.5–25.0%, and Fe:20.0~70.0% It contains, and further, Al: 0-1.000%, Mg: 0-0.500% Si: 0-0.200%, 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: 0-0.500%, 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. The average C concentration from the surface of the steel base material to a depth of 1 μm is 0.25% by mass or less. The aforementioned steel base material contains 90% or more martensite by area ratio. The amount of the aforementioned plating layer is 60 g / m² per side. 2 That's all. A hot-stamped molded article characterized by having a Vickers hardness of 400 HV or higher.

2. The hot-stamped molded article according to claim 1, characterized in that the average C concentration is 0.18% by mass or less, and the Fe content of the plating layer is 20.0 to 60.0% by mass.

3. The hot-stamped molded article according to claim 1 or 2, characterized in that the average C concentration is 0.10% by mass or less.

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