Coated steel sheets for hot stamping.

Abstract

DEPCT6525 / 02 / 2565 This invention relates to coated steel sheets for hot stamping, which... It consists of steel plates and at least one Zn-Ni coating layer formed on the surface. The surface of a steel plate with a Zn-Ni coating layer containing 8% Ni by mass. Furthermore, the deposition amount during plating is equal to 10 grams / square meter or more and 90 grams / square meter. Or even less, with a surface area and average grain size of 50 nanometers or less. ----------------------------------------------------------- DEPCT65 This invention relates to a coated steel sheet for hot stamping, which is assembled. This includes steel plates and a Zn-Ni coating layer created on at least one surface of the plate. Steel with a Zn-Ni coating containing 8% Ni by mass or more. The deposition rate during plating is 10 grams / square meter or more, and 90 grams / square meter or less. For surface area and average grain size of 50 nanometers or less. -----------------------------------------------------------

Description

Coated steel sheets for hot stamping

[0001] The present invention relates to a plated steel sheet for hot stamping, and more specifically to a plated steel sheet for hot stamping having a Zn—Ni plating layer.

[0002] In recent years, hot stamping (hot pressing) has become widely used for forming steel sheets for use in automotive components. Hot stamping is a method in which a steel sheet is press-formed while heated to a temperature in the austenite region and quenched (cooled) in a press die simultaneously with forming. This method is one of the methods for forming steel sheets that are excellent in strength and dimensional accuracy.

[0003] In some cases, a Zn-Ni plating layer is provided on the surface of a steel sheet used for hot stamping. In Patent Document 1, a Zn-Ni plating layer having a melting point of 800°C or higher and a coating weight of 10 to 90 g / m per side is provided on the surface of the steel sheet. 2 Furthermore, Patent Document 2 discloses a steel sheet for hot stamping having a plating layer such as a Zn-Ni plating layer of 10 to 25 mass % on the surface of the steel sheet, with the remainder consisting of Zn and unavoidable impurities, with a coating weight of 10 to 90 g / m 2 and the η phase content of the coating layer is 5 mass % or less. Furthermore, Patent Document 3 discloses a steel sheet for hot stamping having a coating layer of 60 mass % or more of Ni, the balance of which is Zn and unavoidable impurities, in that order, on the surface of the steel sheet, and a coating weight of 0.01 to 5 g / m 2 and a plating layer I containing 10 to 25 mass% of Ni, the balance being Zn and unavoidable impurities, with a coating weight of 10 to 90 g / m 2 Patent Document 4 also discloses a hot stamping steel sheet comprising a base steel sheet and a plating layer formed on the base steel sheet, the plating layer containing 10 to 25 mass % of Ni with the balance being Zn and unavoidable impurities, and the coating weight of the plating layer per side is 10 to 90 g / m 2 The present invention describes a method for producing a hot-pressed member, which comprises heating a plated steel sheet of the formula (I) to 850 to 950°C, and starting hot press forming when the temperature of the plated steel sheet after heating is 650 to 800°C.

[0004] In relation to the Zn—Ni plating layer, Patent Documents 5 to 7 each teach that by adjusting the average crystal grain size of the plating layer, the appearance after chromate treatment can be kept stable, press workability can be improved, and chemical conversion treatability can be improved.

[0005] JP 2012-197505 A JP 2016-29214 A JP 2012-233247 A International Publication No. 2015 / 001705 JP 2009-127126 A JP 6-116781 A JP 3-68793 A

[0006] When hot stamping is applied to a plated steel sheet having a Zn-plated layer or a Zn alloy-plated layer such as a Zn—Ni-plated layer formed on the surface of the steel sheet, liquid metal embrittlement (hereinafter referred to as "LME") may occur. LME is a phenomenon in which, when stress (e.g., tensile stress) is applied to a solid metal surface while the liquid metal is in contact with the solid metal, the liquid metal penetrates into the solid metal, causing the solid metal to become embrittled. When hot stamping is applied to a steel sheet having a Zn-plated layer or a Zn alloy-plated layer, if the melting point of the plated layer is lower than the heating temperature for hot stamping, the plated layer may melt and become liquid on the surface of the steel sheet. Therefore, when hot stamping is performed at such a heating temperature, Zn in the Zn—Ni-plated layer may penetrate into the grain boundaries of the steel sheet, potentially causing LME. This may result in cracks (also referred to as "LME cracking") in the resulting hot-stamped product.

[0007] In the hot stamping steel sheet described in Patent Document 1, a coating layer having a melting point of 800°C or higher is provided on the steel sheet to achieve LME resistance. Patent Document 1 also describes that when the melting point of the coating layer is lower than the heating temperature of hot stamping, coating with a metal having as high a melting point as possible shortens the contact time between the molten metal and the base steel sheet, thereby improving liquid metal embrittlement resistance. However, even if the melting point of the coating layer is 800°C or higher, if the melting point of the coating layer is lower than the heating temperature during hot stamping, melting of the coating layer during hot stamping heating cannot be completely prevented. Therefore, there is a risk that the coating layer melted during hot stamping may penetrate into the grain boundaries of the steel sheet, causing LME.

[0008] Patent Documents 2 and 3 disclose a steel sheet for hot stamping having a Zn—Ni plated layer thereon, but do not consider suppressing LME. On the other hand, Patent Document 4 teaches that LME cracking can be prevented by starting hot press forming at a predetermined temperature, but does not sufficiently consider the configuration of the Zn—Ni plated steel sheet to prevent LME cracking.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a plated steel sheet for hot stamping that can prevent melting of a Zn—Ni plating layer during hot stamping and sufficiently suppress LME.

[0010] The present inventors have found that minimizing the average grain size of the Zn—Ni plating layer formed on a steel sheet is effective in sufficiently suppressing LME during hot stamping. By achieving a finer grain size, the Zn in the Zn—Ni plating layer becomes more mobile during heating in hot stamping, allowing more Zn in the plating layer to diffuse into the underlying steel sheet. This reduces the Zn concentration in the plating layer, i.e., relatively increases the Ni concentration in the plating layer, and raises the melting point of the plating layer. Furthermore, the amount of Fe diffusing from the base steel sheet into the plating layer also increases, and the increased Fe concentration in the plating layer also raises the melting point of the plating layer. Therefore, during hot stamping, the plating layer does not exist as a liquid phase, and the Zn diffused into the steel sheet during heating alloys with the Fe in the steel sheet to form a complete solid solution, thereby suppressing LME. Furthermore, the present inventors have found that, in addition to controlling the average crystal grain size, it is effective to set the Ni concentration in the plating layer before hot stamping to a predetermined value or more in order for the plating layer to have a sufficient melting point after Zn has diffused into the steel sheet, and further, it is effective to avoid making the plating layer excessively thick in order to facilitate the diffusion of Zn into the steel sheet.

[0011] The present invention was made based on the above findings, and its gist is as follows: (1) A steel sheet having a Zn-Ni plating layer formed on at least one side of the steel sheet, wherein the Zn-Ni plating layer has a Ni concentration of 8 mass % or more and a coating weight of 10 g / m per side. 2 90g / m or more 2and an average crystal grain size of 50 nm or less. (2) The steel plate contains, in mass%, C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.00% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, O: 0.010% or less, B: 0% or more and 0.0040% or less, Cr: 0% or more and 2.00% or less, Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 0.300% or less, Zr: 0% or more and 0.300% or less, Mo: 0% or more and 2.000% or less, Cu: 0% or more and 2.000% or less, The plated steel sheet for hot stamping according to (1), comprising: Ni: 0% or more and 2.000% or less; Sb: 0% or more and 0.100% or less; Ca: 0% or more and 0.0100% or less; Mg: 0% or more and 0.0100% or less; and REM: 0% or more and 0.1000% or less, with the balance being iron and impurities. (3) The steel plate contains, in mass%, B: 0.0005% or more and 0.0040% or less, Cr: 0.01% or more and 2.00% or less, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more and 0.300% or less, V: 0.001% or more and 0.300% or less, Zr: 0.001% or more and 0.300% or less, Mo: 0.001% or more and 2.000% or less, Cu: 0.001% or more and 2.000% or less, Ni: 0.001% or more and 2.000% or less, Sb: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, and (4) The plated steel sheet for hot stamping according to any one of (1) to (3), wherein the average grain size is 40 nm or less. (5) The plated steel sheet for hot stamping according to any one of (1) to (4), wherein the C concentration in the Zn—Ni plating layer is less than 1 mass %.

[0012] According to the present invention, by diffusing a large amount of Zn in the coating layer into the base steel sheet during heating in hot stamping and also increasing the amount of Fe diffused from the base steel sheet into the coating layer, the Fe concentration in the coating layer is increased, thereby increasing the melting point of the coating layer and preventing the coating layer from becoming liquid during hot stamping, and it is possible to provide a coated steel sheet for hot stamping that does not cause LME.As a result, it is possible to obtain a hot-stamped product that is free of LME cracking.

[0013] <Plated steel sheet for hot stamping> The plated steel sheet for hot stamping according to the present invention comprises a steel sheet and a Zn—Ni plating layer formed on at least one side of the steel sheet. Preferably, the Zn—Ni plating layer is formed on both sides of the steel sheet. In addition, in the present invention, it is sufficient that the Zn—Ni plating layer is formed on the steel sheet, and another plating layer may be provided between the steel sheet and the Zn—Ni plating layer.

[0014] [Steel Sheet] The chemical composition of the steel sheet in the present invention is not particularly limited as long as the steel sheet can be used for hot stamping. Elements that may be contained in the steel sheet in the present invention will be described below. Note that "%" representing the content of each element in the chemical composition means mass % unless otherwise specified.

[0015] Preferably, the steel sheet in the present invention can contain, in mass%, C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.00% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, and O: 0.010% or less.

[0016] (C: 0.05% or more and 0.70% or less) C (carbon) is an element effective in improving the strength of steel sheet. Automotive components may require high strength, for example, of 980 MPa or more. To ensure sufficient strength, the C content is preferably 0.05% or more. On the other hand, excessive C content may reduce the workability of the steel sheet, so the C content is preferably 0.70% or less. The lower limit of the C content is preferably 0.10%, more preferably 0.12%, even more preferably 0.15%, and most preferably 0.20%. The upper limit of the C content is preferably 0.65%, more preferably 0.60%, even more preferably 0.55%, and most preferably 0.50%.

[0017] (Mn: 0.5% or more and 11.0% or less) Mn (manganese) is an element effective in improving hardenability during hot stamping. To ensure this effect, the Mn content is preferably 0.5% or more. On the other hand, if Mn is contained excessively, Mn may segregate, causing the strength of the formed body after hot stamping to become non-uniform. Therefore, the Mn content is preferably 11.0% or less. The lower limit of the Mn content is preferably 1.0%, more preferably 2.0%, even more preferably 2.5%, still more preferably 3.0%, and most preferably 3.5%. The upper limit of the Mn content is preferably 10.0%, more preferably 9.5%, even more preferably 9.0%, still more preferably 8.5%, and most preferably 8.0%.

[0018] (Si: 0.05% or more and 2.00% or less) Si (silicon) is an element effective in improving the strength of steel sheet. In order to ensure sufficient strength, the Si content is preferably 0.05% or more. On the other hand, if Si is contained excessively, workability may decrease, so the Si content is preferably 2.00% or less. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%, even more preferably 0.20%, and most preferably 0.30%. The upper limit of the Si content is preferably 1.80%, more preferably 1.50%, even more preferably 1.20%, and most preferably 1.00%.

[0019] (Al: 0.001% or more and 1.500% or less) Al (aluminum) is an element that acts as a deoxidizing element. In order to obtain the deoxidizing effect, the Al content is preferably 0.001% or more. On the other hand, since an excessive amount of Al may reduce workability, the Al content is preferably 1.500% or less. The lower limit of the Al content is preferably 0.010%, more preferably 0.020%, even more preferably 0.050%, and most preferably 0.100%. The upper limit of the Al content is preferably 1.000%, more preferably 0.800%, even more preferably 0.700%, and most preferably 0.500%.

[0020] (P: 0.100% or less) (S: 0.100% or less) (N: 0.010% or less) (O: 0.010% or less) P (phosphorus), S (sulfur), N (nitrogen), and oxygen (O) are impurities, and the lower the content, the better. Therefore, the lower limits of these elements are not particularly limited. However, the contents of these elements may be more than 0% or 0.001% or more. On the other hand, excessive inclusion of these elements may deteriorate toughness, ductility, and / or workability. Therefore, it is preferable to set the upper limits of P and S to 0.100%, and the upper limits of N and O to 0.010%. The upper limits of P and S are preferably 0.080%, more preferably 0.050%. The upper limits of N and O are preferably 0.008%, more preferably 0.005%.

[0021] The basic chemical composition of the steel sheet in the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements, as needed, in place of a portion of the remaining Fe. For example, the steel sheet may contain B: 0% to 0.0040%. The steel sheet may also contain Cr: 0% to 2.00%. The steel sheet may also contain at least one element selected from the group consisting of Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, and Zr: 0% to 0.300%. The steel sheet may also contain at least one element selected from the group consisting of Mo: 0% to 2.000%, Cu: 0% to 2.000%, and Ni: 0% to 2.000%. The steel sheet may also contain Sb: 0% to 0.100%. The steel sheet may also contain at least one element selected from the group consisting of Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, and REM: 0% to 0.1000%. These optional elements will be described in detail below.

[0022] (B: 0% or more and 0.0040% or less) B (boron) is an element effective in improving hardenability during hot stamping. The B content may be 0%, but to ensure this effect, the B content is preferably 0.0005% or more. On the other hand, if B is contained excessively, the workability of the steel sheet may be reduced, so the B content is preferably 0.0040% or less. The lower limit of the B content is preferably 0.0008%, more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the B content is preferably 0.0035%, more preferably 0.0030%.

[0023] (Cr: 0% or more and 2.00% or less) Cr (chromium) is an element effective in improving hardenability during hot stamping. The Cr content may be 0%, but to ensure this effect, the Cr content is preferably 0.01% or more. The Cr content may be 0.10% or more, 0.50% or more, or 0.70% or more. On the other hand, excessive Cr content may reduce the thermal stability of the steel material. Therefore, the Cr content is preferably 2.00% or less. The Cr content may be 1.50% or less, 1.20% or less, or 1.00% or less.

[0024] (Ti: 0% or more and 0.300% or less) (Nb: 0% or more and 0.300% or less) (V: 0% or more and 0.300% or less) (Zr: 0% or more and 0.300% or less) Ti (titanium), Nb (niobium), V (vanadium), and Zr (zirconium) are elements that improve tensile strength by refining the metal structure. The contents of these elements may be 0%, but to ensure this effect, the contents of Ti, Nb, V, and Zr are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if Ti, Nb, V, and Zr are excessively contained, the effect saturates and the production cost increases. Therefore, the Ti, Nb, V and Zr contents are preferably set to 0.300% or less, and may be set to 0.150% or less, 0.100% or less, or 0.060% or less.

[0025] (Mo: 0% or more and 2.000% or less) (Cu: 0% or more and 2.000% or less) (Ni: 0% or more and 2.000% or less) Mo (molybdenum), Cu (copper), and Ni (nickel) have the effect of increasing tensile strength. The contents of these elements may be 0%, but to ensure this effect, the Mo, Cu, and Ni contents are preferably 0.001% or more, and may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive inclusion of Mo, Cu, and Ni may reduce the thermal stability of the steel material. Therefore, the Mo, Cu, and Ni contents are preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.800% or less.

[0026] (Sb: 0% or more and 0.100% or less) Sb (antimony) is an element effective in improving the wettability and adhesion of plating. The Sb content may be 0%, but to ensure this effect, the Sb content is preferably 0.001% or more. The Sb content may be 0.005% or more, 0.010% or more, or 0.020% or less. On the other hand, excessive Sb content may cause a decrease in toughness. Therefore, the Sb content is preferably 0.100% or less. The Sb content may be 0.080% or less, 0.060% or less, or 0.050% or less.

[0027] (Ca: 0% or more and 0.0100% or less) (Mg: 0% or more and 0.0100% or less) (REM: 0% or more and 0.1000% or less) Ca (calcium), Mg (magnesium), and REM (rare earth metals) are elements that improve toughness after hot stamping by adjusting the shape of inclusions. The contents of these elements may be 0%, but to ensure this effect, the Ca, Mg, and REM contents are preferably 0.0001% or more, and may be 0.0010% or more, 0.0020% or more, or 0.0040% or more. On the other hand, if Ca, Mg, and REM are excessively contained, the effects will saturate and the manufacturing cost will increase. For this reason, the Ca and Mg contents are preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0050% or less. Similarly, the REM content is preferably 0.1000% or less, and may be 0.0800% or less, or 0.0500% or less and 0.0100% or less.

[0028] The balance other than the above elements consists of iron and impurities. Here, "impurities" include components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scrap, when industrially manufacturing the base steel sheet, and are not components that are intentionally added to the base steel sheet according to the embodiment of the present invention. Furthermore, impurities also include elements other than those described above that are contained in the base steel sheet at a level that does not affect the properties of the hot-dip galvanized steel sheet according to the embodiment of the present invention in terms of the specific action and effect of the element.

[0029] The steel sheet in the present invention is not particularly limited, and may be a common steel sheet such as a hot-rolled steel sheet, a cold-rolled steel sheet, etc. The steel sheet in the present invention may have any thickness, for example, 0.1 to 3.2 mm, as long as a Zn—Ni plating layer described below can be formed on the steel sheet and hot stamping can be performed.

[0030] [Zn—Ni Plating Layer] The Zn—Ni plating layer in the present invention is a plating layer containing at least Zn and Ni, with no particular limitations on the other components. For example, the Zn—Ni plating layer may be a plating layer containing Zn as the main component (i.e., a Zn concentration of 50% by mass or more) and a Ni concentration of 8% by mass or more, with no particular limitations on the other components. In the plating layer, Zn and Ni form a solid solution of Ni in Zn or an intermetallic compound of Zn and Ni. The plating layer may be formed by any plating method, but is preferably formed by, for example, electroplating. The Zn—Ni plating layer is formed on at least one side of the steel sheet, and preferably on both sides of the steel sheet. Naturally, hot stamping causes diffusion of Fe and other elements from the base steel sheet to the plating layer and diffusion of Zn and other elements from the plating layer to the base steel sheet. Therefore, the chemical composition of the plating layer after hot stamping varies depending on the heating conditions (heating temperature, holding time, etc.) during hot stamping.

[0031] (Ni Concentration) In the Zn—Ni plating layer of the present invention, the lower limit of the Ni concentration is 8 mass%. By setting the Ni concentration to 8 mass% or more, the Ni concentration in the Zn—Ni plating layer can be made sufficiently higher than the Zn concentration after Zn diffuses into the steel sheet due to heating during hot stamping, resulting in a plating layer with a sufficiently high melting point. This prevents the plating layer from becoming liquid during hot stamping, thereby suppressing LME. If the Ni concentration is less than 8 mass%, even if Zn diffuses into the steel sheet, the Zn concentration in the plating layer cannot be sufficiently reduced compared to the Ni concentration. Therefore, the melting point of the plating layer cannot be sufficiently increased, and LME may occur during hot stamping. For example, the lower limit of the Ni concentration is preferably 10 mass%, more preferably 12 mass%.

[0032] The upper limit of the Ni concentration is not particularly limited, but from the viewpoint of economic efficiency, it is preferably 30% by mass or less, for example, 28%, 25%, or 20% by mass.

[0033] (C Concentration) In the present invention, the C concentration in the Zn—Ni plating layer is preferably less than 1 mass%. Reducing the C concentration in the Zn—Ni plating layer to less than 1 mass% can improve the adhesion of the plating layer to the steel sheet. On the other hand, if the C concentration exceeds 1 mass%, the plating layer becomes embrittled during heating in hot stamping, making the plating more likely to peel off from the steel sheet after hot stamping. Therefore, from the viewpoint of improving the adhesion of the plating layer, a lower C concentration is preferable, and it may be 0.8 mass% or less, 0.5 mass% or less, 0.1 mass% or less, 0.01 mass% or less, or 0%. For example, by forming the Zn—Ni plating layer using a plating bath containing no organic additives, it is possible to reliably reduce the C concentration in the Zn—Ni plating layer to 0.1 mass% or less or 0.01 mass% or less.

[0034] The Zn—Ni plating layer of the present invention may further contain one or more of Fe, Cr, and Co. These elements may be intentionally added or may be unavoidably mixed in during production. The balance of the Zn—Ni plating layer's composition is Zn and impurities. In a specific embodiment of the present invention, the Zn—Ni plating layer contains, by mass%, 8% to 30% Ni, 0% to 5% one or more of Fe, Cr, and Co, and less than 1% C, with the balance being iron and impurities. Preferably, the Zn—Ni plating layer contains, by mass%, 8% to 30% Ni, with the balance being iron and impurities. The "impurities" in the Zn—Ni plating layer refer to components and the like that are mixed in due to various factors in the production process, including raw materials, when producing the Zn—Ni plating layer.

[0035] (Plating Weight) In the Zn-Ni plating layer of the present invention, the lower limit of the plating weight per one side of the steel sheet is 10 g / m 2 The plating weight per side is 10 g / m 2 By setting the coating weight per side to 10 g / m or more, it is possible to sufficiently prevent the formation of surface scale during heating in hot stamping. 2 If the thickness is less than 16 g / m, the thickness of the Zn-Ni plating layer will be insufficient, and surface scale will form during heating in hot stamping, making it necessary to remove the scale by shot blasting before painting. 2 , more preferably 20 g / m 2 , more preferably 24 g / m 2 , most preferably 30 g / m 2 is.

[0036] The upper limit of the coating weight per side of the steel sheet is 90 g / m 2 The plating weight per side is 90 g / m 2By setting the coating weight per side to 90 g / m or less, the coating layer does not become excessively thick, and Zn in the coating layer is easily diffused into the steel sheet during heating in hot stamping, and as a result, the melting point of the coating layer is sufficiently improved during hot stamping, making it possible to effectively prevent LME. 2 If the coating weight exceeds 80 g / m, the thickness of the coating layer becomes too thick, and the diffusion of Zn into the steel sheet does not proceed sufficiently. As a result, the coating layer may become liquid during hot stamping, which may make it impossible to suppress LME. In addition, an excessive coating weight is not preferable from an economical point of view. The upper limit of the coating weight per side of the steel sheet is preferably 80 g / m. 2 , more preferably 76 g / m 2 , more preferably 70 g / m 2 , most preferably 60 g / m 2 is.

[0037] In the present invention, the Ni concentration and coating weight of the Zn—Ni plating layer are measured by inductively coupled plasma (ICP) atomic emission spectroscopy. Specifically, the coating weight is determined by dissolving the Zn—Ni plating layer from a plated steel sheet with the layer in 10% HCl and performing ICP analysis on the resulting solution. Note that the coating weight in the present invention is the amount per side, and therefore, when a Zn—Ni plating layer is formed on both sides of a steel sheet, the coating weight is calculated assuming that both sides have the same coating weight. In addition, the C concentration in the Zn—Ni plating layer in the present invention is measured by high-frequency combustion-infrared absorption spectroscopy.

[0038] (Average Crystal Grain Size) In the Zn—Ni plating layer of the present invention, the upper limit of the average crystal grain size of the Zn—Ni plating is 50 nm. By setting the average crystal grain size to 50 nm or less, Zn in the Zn—Ni plating layer becomes more mobile during heating in hot stamping, facilitating Zn diffusion into the steel sheet. Note that this diffusion can occur not only when Zn is in a liquid phase but also when Zn is in a solid phase. When Zn in the Zn—Ni plating layer diffuses into the steel sheet, the Ni concentration in the plating layer becomes relatively higher than the Zn concentration, thereby improving the melting point of the plating layer. Furthermore, setting the average grain size of the plating layer to 50 nm or less also promotes diffusion of Fe from the base steel sheet into the plating layer, and the increase in the Fe concentration in the plating layer also increases the melting point of the plating layer. Therefore, the plating layer is less likely to exist in a liquid phase during hot stamping, making it possible to suppress the occurrence of LME. In particular, LME is more likely to occur when hot stamping is performed at a high temperature, rapidly heated, and / or without a holding time. However, by reducing the average grain size of the coating as in the present invention, Zn diffuses into the steel sheet during heating in hot stamping, increasing the melting point of the coating layer. This prevents the coating layer from melting during hot stamping. Furthermore, the Zn diffused during heating forms a solid solution with Fe in the steel sheet. As a result, LME can be suppressed even under conditions that are prone to LME, such as those described above. If the average grain size exceeds 50 nm, Zn may not sufficiently diffuse into the steel sheet, making it difficult to sufficiently increase the melting point of the coating layer, and LME may not be suppressed. The upper limit of the average grain size of the coating is preferably 45 nm, more preferably 40 nm, even more preferably 35 nm, and most preferably 30 nm.

[0039] The lower limit of the average crystal grain size of the plating is not particularly limited, but since the lower limit of the average crystal grain size that can be practically produced is 10 nm, the lower limit may be 10 nm. The lower limit of the average crystal grain size of the plating is preferably 12 nm, more preferably 15 nm, even more preferably 18 nm, and most preferably 20 nm.

[0040] The average crystal grain size of the Zn—Ni plating is measured by X-ray diffraction (XRD). Specifically, the half-width B of the diffraction peak measured by XRD using Co-Kα radiation (tube voltage: 40 kV and tube current: 200 mA) is used to calculate the average crystal grain size according to the following Scherrer equation: Average crystal grain size (nm) = Kλ / B cos θ (1) (where K is the Scherrer constant, λ is the wavelength of the Co-Kα radiation (nm), and θ is the Bragg angle (radian)). Note that K is a value that varies depending on the shape of the crystallite, but in the present invention, it is sufficient to set K = 0.9.

[0041] As described above, in the present invention, the average crystal grain size of the Zn—Ni plating is 50 nm or less. Such fine Zn—Ni plating can be obtained by, for example, applying a high current density (typically 300 A / dm 2 The resulting alloy can be obtained by electroplating at a temperature of 1000 K or more.

[0042] The plated steel sheet having a Zn—Ni plating layer thereon as described above can be used for hot stamping under any conditions known to those skilled in the art. The heating method for hot stamping is not limited, but examples include furnace heating, electrical heating, and induction heating. The heating temperature during hot stamping may be any temperature that heats the steel sheet to the austenite region depending on the chemical composition of the steel sheet, such as 800°C or higher, 850°C or higher, 900°C or higher, or 950°C or higher. After heating the plated steel sheet to the austenite region using the heating method described above, it can be formed in a press die and quenched. After heating, the steel sheet may be held at the temperature for 1 to 10 minutes and then cooled, or it may not be held at that temperature at all. Quenching (cooling) can be performed at a cooling rate of 1 to 100°C / second.

[0043] When the plated steel sheet for hot stamping according to the present invention is used, Zn in the Zn—Ni plating layer can be diffused into the steel sheet during heating in hot stamping, thereby increasing the melting point of the plating layer during hot stamping. As a result, the plating layer does not become liquid during hot stamping. Furthermore, the Zn diffused into the steel sheet during heating dissolves in Fe in the steel sheet, and LME does not occur under any of the conditions described above, particularly conditions under which LME is likely to occur (high temperature, rapid heating, and / or no holding time). Therefore, a hot-stamped product free from LME cracking can be obtained.

[0044] [Method for producing plated steel sheet for hot stamping] An example of a method for producing a plated steel sheet for hot stamping according to the present invention is described below. The plated steel sheet for hot stamping according to the present invention can be obtained by forming a Zn—Ni plating layer on at least one side, preferably both sides, of a steel sheet by, for example, electroplating.

[0045] (Production of Steel Sheet) The method for producing the steel sheet used to produce the plated steel sheet for hot stamping according to the present invention is not particularly limited. For example, the steel sheet can be obtained by adjusting the chemical composition of molten steel to a desired range, hot rolling, coiling, and further cold rolling. The thickness of the steel sheet according to the present invention may be, for example, 0.1 mm to 3.2 mm.

[0046] The chemical composition of the steel plate to be used is not particularly limited, but as described above, the steel plate preferably contains, by mass%, C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.00% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, and O: 0.010% or less, with the balance being iron and impurities. The steel sheet contains, in mass%, B: 0.0005% or more and 0.0040% or less, Cr: 0.01% or more and 2.00% or less, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more and 0.300% or less, V: 0.001% or more and 0.300% or less, Zr: 0.001% or more and 0.300% or less, Mo: 0.001% or more and 2.000% or less, Cu: 0. It may further contain at least one selected from the group consisting of: Sb: 0.001% or more and 0.100% or less, Ni: 0.001% or more and 2.000% or less, Sb: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.1000% or less.

[0047] (Formation of Zn—Ni Plated Layer) The method for forming the Zn—Ni plated layer in the present invention is not particularly limited as long as the Ni concentration, plating coverage, and average crystal grain size according to the present invention can be obtained, but it can be formed by electroplating. In particular, in order to obtain a Zn—Ni plated layer with a fine grain size, it is preferable to perform electroplating at a high current density, for example, 280 A / dm 2 Above, 300A / dm 2 or more, or 350 A / dm 2 Electroplating can be performed at 350 A / dm or more. 2 When electroplating is performed at the above current density, it is possible to obtain an average crystal grain size of 40 nm or less. The reason why the average crystal grain size of plating becomes finer when plating is performed at a high current density is thought to be because the rate of precipitation nuclei generation increases rapidly in proportion to the exponential function of the overvoltage, which increases with increasing current density. Note that, at a relatively low current density (for example, about 100 A / dm 2) cannot obtain an average plating crystal grain size of 50 nm or less as in the present invention. The bath composition used to form the Zn—Ni plating layer may be, for example, nickel sulfate hexahydrate: 150 to 350 g / L, zinc sulfate heptahydrate: 10 to 150 g / L, and sodium sulfate: 25 to 75 g / L. By using such a bath composition, electroplating at a high current density as described above becomes possible, and a fine average crystal grain size can be obtained.

[0048] On the other hand, generally, electroplating at a high current density is likely to cause plating burn, which can lead to poor appearance and / or poor adhesion. Therefore, in order to prevent plating burn while using a high current density, the pH of the plating bath is set to 2.0 or less, preferably 1.5 or less, more preferably 1.0 or less, using, for example, sulfuric acid, and the temperature of the plating bath is set to 60°C or higher, preferably 65°C or higher, more preferably 70°C or higher. With the bath composition, bath pH, ​​and bath temperature as described above, it is possible to form a Zn—Ni plating layer without causing plating burn even at a high current density. Furthermore, from the viewpoint of preventing plating burn, the upper limit of the current density is set to 400 A / dm 2 , or 450 A / dm 2 It is preferable that:

[0049] When the plated steel sheet for hot stamping according to the present invention is produced by electroplating, the Ni concentration, coating weight, and average grain size of the Zn—Ni plating layer can be adjusted by appropriately changing the current density, bath composition, and current application time during electroplating. More specifically, the Ni concentration can be adjusted by changing the current density and bath composition, the coating weight can be adjusted by changing the current density and current application time, and the grain size can be adjusted by changing the current density.

[0050] For example, to improve various properties of the plating layer, organic additives such as dextrin, diallylamine polymer, and diallyldialkylammonium salt polymer may be added to the plating bath. Examples of diallylamine polymers include diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, and diallylamine hydrochloride / sulfur dioxide copolymer. Examples of diallyldialkylammonium salt polymers include diallyldimethylammonium chloride polymer and diallylmethylethylammonium ethyl sulfate polymer. However, when electroplating using a plating bath containing such organic additives, the carbon components contained in the additives can embrittle the plating layer during heating in hot stamping, potentially resulting in plating peeling on the steel sheet after hot stamping. Therefore, from the perspective of improving the adhesion of the plating layer, it is necessary to reduce the C concentration in the Zn—Ni plating layer to, for example, less than 1 mass%, preferably 0.01 mass% or less. For this reason, it is preferable to perform the above-mentioned electroplating using an additive-free plating bath.

[0051] The plated steel sheet for hot stamping according to the present invention will be described in more detail below using several examples, but the scope of the present invention as defined in the claims is not intended to be limited by the specific examples described below.

[0052] [Preparation of Samples of Plated Steel Sheet for Hot Stamping] A cold-rolled steel sheet having a thickness of 1.4 mm was immersed in a plating bath having the following plating bath composition, and a Zn—Ni plating layer was formed on both surfaces of the cold-rolled steel sheet by electroplating, thereby obtaining plated steel sheet samples No. 1 to 10 for hot stamping. All of the steel sheets used contained, in mass%, C: 0.50%, Mn: 3.0%, Si: 0.50%, Al: 0.100%, P: 0.010%, S: 0.020%, N: 0.003%, O: 0.003%, and B: 0.0010%, with the balance being iron and impurities. Plating bath composition: Nickel sulfate hexahydrate: 250 g / L (fixed); Zinc sulfate heptahydrate: 10 to 150 g / L (variable); Sodium sulfate: 50 g / L (fixed); Organic additives: none, dextrin (3 g / L), or diallylamine hydrochloride-sulfur dioxide copolymer (molecular weight 5000, 3 g / L)

[0053] To prevent plating burn, the pH of the plating bath was adjusted to 1.0 using sulfuric acid, and the bath temperature was maintained at 70°C. The current density and current application time were adjusted to obtain the desired Zn-Ni plating weight and average crystal grain size. Furthermore, to obtain the desired Ni concentration, the concentration of zinc sulfate heptahydrate was appropriately adjusted based on the set current density. The current densities set for producing each sample are shown in Table 1.

[0054] The Ni concentration and plating weight per side of each of Samples No. 1 to 10 obtained by electroplating under the above conditions were determined by ICP analysis. Specifically, the plating layer alone was dissolved from each sample in 10% HCl, and the resulting solution was subjected to ICP analysis to determine the Ni concentration and plating weight per side. The Ni concentration and plating weight per side of each sample are shown in Table 1.

[0055] The average crystal grain size of the Zn—Ni plating was determined by XRD. First, the half-width B of the diffraction peak was determined for each sample by XRD using Co-Kα radiation (tube voltage: 40 kV and tube current: 200 mA). Then, using the determined half-width B, the average crystal grain size was calculated using the following Scherrer equation: Average crystal grain size (nm) = Kλ / B cos θ (1) (where K is the Scherrer constant = 0.9, λ is the wavelength of the Co-Kα radiation (nm), and θ is the Bragg angle (radian)). Here, the Co-Kα radiation wavelength λ = 0.179 nm, and the Bragg angle θ was the angle of the diffraction line observed in the range of 50.1 to 50.3°. The average crystal grain size of each sample is shown in Table 1.

[0056] The carbon concentration in the Zn--Ni plating layer was measured using a high-frequency combustion-infrared absorption apparatus CS-6000 manufactured by LECO Corporation.

[0057] [Evaluation of Plated Steel Sheets for Hot Stamping] [Evaluation of LME Resistance] Hot stamping was performed on Samples 1 to 10 of the plated steel sheets for hot stamping obtained as described above. For hot stamping, each sample was heated to 950°C by electrical heating, and immediately formed and quenched (cooling rate: 50°C / sec) using a V-bend die with a tip radius of 3 mm without being held. The V-bend portion of each hot-stamped product obtained was embedded in epoxy resin and polished, and the cross section was observed with an optical microscope. Observation was performed at 250x magnification at five locations randomly selected from around the top of the V-bend. In the five observation locations, samples in which no cracks were observed were rated "Crack rating: ◯," and samples in which cracks were observed at even one location were rated "Crack rating: ×."

[0058] [Evaluation of Plating Adhesion] Plating adhesion was evaluated by applying tape to the above-mentioned V-bend portion, then peeling it off, and measuring the area ratio of the plating layer adhering to the peeled tape. Cellotape (registered trademark) CT-18 manufactured by Nichiban Co., Ltd. was used as the tape, and it was applied perpendicular to the V-bend direction. The tape was strongly rubbed with a rubber stopper to ensure sufficient adhesion, and then peeled off perpendicularly. The evaluation range was 10 mm from the center of the sample in the longitudinal direction of the tape, and 5 mm across from the top of the V-bend in the width direction of the tape. The evaluation results for each sample are shown in Table 1.

[0059]

[0060] It was found that no cracks occurred in the hot-stamped steel sheet according to the present invention, Samples 1 to 4, 7 to 9, 11, and 12, and that LME was sufficiently suppressed. In particular, Samples 1 to 4 and 7 to 9 had a C concentration in the Zn—Ni coating layer below the analytical lower limit (less than 0.01 mass%), and therefore achieved high coating adhesion.

[0061] LME cracking occurred in the hot stamped steel sheet of Sample No. 5. This is thought to be because the coating weight was excessive and the Zn—Ni coating layer was thick, so Zn was unable to sufficiently diffuse into the steel sheet during heating in hot stamping.

[0062] LME cracking occurred in the hot stamped steel sheet of Sample No. 6. This is thought to be because the current density during electroplating was low, which increased the average grain size of the plated steel, preventing Zn from sufficiently diffusing into the steel sheet during heating in hot stamping.

[0063] LME cracking occurred after hot stamping in Sample No. 10. This is thought to be because, although Zn diffused into the steel sheet during heating for hot stamping, the initial Ni concentration in the sample was low, and the melting point of the Zn-Ni plating layer after diffusion could not be sufficiently increased.

[0064] In Samples No. 11 and 12, LME was sufficiently suppressed, but the plating adhesion was poor, and a large amount of plating adhered to the peeled tape, with the area ratio exceeding 50%. This is thought to be because the use of organic additives in the plating bath increased the C concentration in the Zn-Ni plating layer, causing the plating layer to become embrittled during heating in hot stamping.

[0065] According to the present invention, it is possible to provide a plated steel sheet for hot stamping that can suppress LME and prevent LME cracking in a hot-stamped product, thereby making it possible to provide a high-strength automotive part. Therefore, the present invention can be said to be an invention of extremely high industrial value.

Claims

Revised 15 / 05 / 681. Coated steel plates for hot stamping comprising a steel plate and a Zn-Ni plating layer formed on at least one surface of the steel plate, where the Zn-Ni plating layer has a Ni concentration of 8% by mass or more, a Zn concentration of 50% by mass or more, a plating deposition of 30 g / m² or more and 90 g / m² or less per surface, and an average grain size of the Zn-Ni plating portion of 50 nm or less. DEPCT6525 / 02 / 25651. Coated steel plates for hot stamping comprising a steel plate and a Zn-Ni plating layer formed on at least one surface of the steel plate, where the Zn-Ni plating layer has a Ni concentration of 8% by mass or more, a plating deposition of 10 g / m² or more and 90 g / m² or less per surface, and an average grain size of 50 nm or less. 2.Coated steel plates for hot stamping according to claim1, where the steel plate contains, in percentage by mass, the following elements: C: 0.05% or more and 0.70% or less; Mn: 0.5% or more and 11.0% or less; Si: 0.05% or more and 2.00% or less; Al: 0.001% or more and 1.500% or less; P: 0.100% or less; S: 0.100% or less; N: 0.010% or less; O: 0.010% or less; B: 0% or more and 0.0040% or less; Cr: 0% or more and 2.00% or less. That is, Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 0.300% or less, Zr: 0% or more and 0.300% or less, Mo: 0% or more and 2.000% or less, Cu: 0% or more and 2.000% or less, Ni: 0% or more and 2.000% or less, Sb: 0% or more and 0.100% or less, Ca: 0% or more and 0.0100% or less, Mg: 0% or more and 0.0100% or less, REM: 0% or more and 0.300% or less.1000% or less and the remainder being iron and impurities.

3. Coated steel plates for hot stamping according to claim 2, where the steel plate is composed in % by mass of at least one element selected from the group consisting of: B: 0.0005% or more and 0.0040% or less; Cr: 0.01% or more and 2.00% or less; Ti: 0.001% or more and 0.300% or less; Nb: 0.001% or more and 0.300% or less; V: 0.001% or more. That and 0.300% or less. Zr: 0.001% or more and 0.300% or less. Mo: 0.001% or more and 2.000% or less. Cu: 0.001% or more and 2.000% or less. Ni: 0.001% or more and 2.000% or less. Sb: 0.001% or more and 0.100% or less. Ca: 0.0001% or more and 0.0100% or less. Mg: 0.0001% or more and 0.0100% or less. And REM: 0.0001% or more and 0.1000% or less. 4.Coated steel plates for hot stamping according to any one of Patents 1 to 3 where the average grain size is 40 nm or less.

5. Coated steel plates for hot stamping according to any one of Patents 1 to 4 where the concentration of C in the Zn-Ni coating layer is less than 1% by mass. -----------------------------------------------------------DEPCT651. Coated steel plates for hot stamping which consist of a steel plate and a Zn-Ni coating layer formed on at least one surface of the steel plate.

1. Zn-Ni coated steel plates with a Ni concentration of 8% by mass or more, a plating deposition of 10 g / m² or more and 90 g / m² or less per surface area, and an average grain size of 50 nm or less.

2. Coated steel plates for hot stamping according to claim 1, where the steel plate contains the following elements as a percentage by mass: C: 0.05% or more and 0.70% or less; Mn: 0.5% or more and 11.0% or less; Si: 0.05% or more and 2.00% or less; Al: 0.0.001% or more and 1.500% or less; P: 0.100% or less; S: 0.100% or less; N: 0.010% or less; O: 0.010% or less; B: 0% or more and 0.0040% or less; Cr: 0% or more and 2.00% or less; Ti: 0% or more and 0.300% or less; Nb: 0% or more and 0.300% or less; V: 0% or more and 0.300% or less; Zr: 0% or more and 0.300% or less; Mo: 0% or more and 2.000% or less; Cu: 0% or more and 2.000% or less. Ni: 0% or more and 2.000% or less; Sb: 0% or more and 0.100% or less; Ca: 0% or more and 0.0100% or less; Mg: 0% or more and 0.0100% or less; REM: 0% or more and 0.1000% or less; and the remainder being iron and impurities.

3. Coated steel plates for hot stamping according to claim 2, where the steel plate is composed as a percentage by mass of at least one element selected from the group consisting of B: 0.0005% or more and 0.0040% or less; Cr: 0.0.001% or more and 2.00% or less; Ti: 0.001% or more and 0.300% or less; Nb: 0.001% or more and 0.300% or less; V: 0.001% or more and 0.300% or less; Zr: 0.001% or more and 0.300% or less; Mo: 0.001% or more and 2.000% or less; Cu: 0.001% or more and 2.000% or less; Ni: 0.001% or more and 2.000% or less; Sb: 0.001% or more and 0.100% or less; Ca: 0.0001% or more and... 0.0100% or less; Mg: 0.0001% or more and 0.0100% or less; and REM: 0.0001% or more and 0.1000% or less.

4. Coated steel plates for hot stamping according to any one of claims 1 to 3 where the average grain size is 40 nm or less.

5. Coated steel plates for hot stamping according to any one of claims 1 to 4 where the concentration of C in the Zn-Ni coating layer is less than 1% by mass.