Plated steel sheet

Abstract

Provided is a plated steel sheet provided with a base steel sheet and a plating layer formed on the surface of the base steel sheet, the plating layer having a prescribed chemical composition, and the depth of 3-100 [mu] m at which the area ratio of pearlite in the sheet thickness direction from the interface between the base steel sheet and the plating layer is 0-20%. The area ratio of pearlite having an equivalent circle diameter of 5 [mu] m or more at a depth at which the area ratio of the pearlite is 0-20% is 0-30%, and the amount of deposited plating layer is 40 g / m2 or more per surface.

Description

Technical Field

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

[0002] As a technique for stamping a material that is difficult to form, such as high-strength steel, hot stamping (hot pressing) is known. Hot stamping is a hot forming technique in which a material to be formed is heated and then formed. In this technique, since the material is heated and then formed, the steel is soft and has good formability during forming. Therefore, it is known that even high-strength steel can be formed into a complex shape with high precision. In addition, since quenching is performed simultaneously with forming by a stamping die, the formed steel has sufficient strength.

[0003] In connection with this, various studies have been made on plated steel sheets for hot stamping in the past.

[0004] For example, Patent Document 1 describes a galvanized steel sheet for hot stamping, which is characterized by including a base steel sheet and a plating layer provided on the surface of the base steel sheet. The base steel sheet contains, by mass%, C: 0.10 to 0.5%, Si: 0.7 to 2.5%, Mn: 1.0 to 3%, and Al: 0.01 to 0.5%, and the balance is iron and unavoidable impurities. The base steel sheet has inside: an internal oxide layer containing at least one of Si and Mn with a thickness of 1 μm or more; and a decarburized layer with a thickness of 20 μm or less from the interface with the plating layer toward the inside of the base steel sheet. In addition, in Patent Document 1, it is taught that by making the thickness of the internal oxide layer of the base steel sheet 1 μm or more, the occurrence of non-plating in the galvanized steel sheet can be sufficiently suppressed, and the adhesion between the formed plating layer and the base steel sheet is sufficiently high. Further, in Patent Document 1, it is taught that the internal oxide layer is formed near the surface of the base steel sheet by high dew point annealing. On the other hand, a decarburized layer is formed on the surface and near the surface of the base steel sheet by this high dew point annealing. In this decarburized layer, the carbon content is low, so the tensile strength is lower than that of the non-decarburized part. However, if the thickness of the decarburized layer is 20 μm or less, the influence of the decarburized layer on the strength of the galvanized steel sheet and the hot stamping formed product manufactured using the same can be suppressed.

[0005] Patent Document 2 describes a steel sheet for hot pressing, which has on at least one surface of the steel sheet a composition containing 10 to 25 mass% of Ni and the balance being composed of Zn and unavoidable impurities, and an adhesion amount of 10 to 90 g / m 2The Zn-Ni alloy plating layer has cracks inside the Zn-Ni alloy plating layer that cut the Zn-Ni alloy plating layer, and the crack density per unit cross section in at least one cross section of the Zn-Ni alloy plating layer is 10 cut-off parts / mm or more. In addition, Patent Document 2 states that according to the above-mentioned structure, a hot-pressing steel plate suitable for hot-pressing parts with excellent corrosion resistance and hydrogen release characteristics after coating can be obtained.

[0006] Patent Document 3 describes that a coating weight of 120 g / m2 is applied to a steel sheet surface. 2 The following Zn alloy coating is provided, and a hot-pressed steel sheet having a metal oxide layer composed of a metal oxide having a melting point exceeding 900° C. on the surface of the above Zn alloy coating is provided, and the use of a Zn-Ni alloy coating or the like is taught as the Zn alloy coating. In addition, Patent Document 3 states that: According to the above-mentioned structure, a hot-pressed steel sheet having improved sliding properties during hot press forming and excellent scratch resistance can be obtained.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2019-151883

[0010] Patent Document 2: International Publication No. 2021 / 019829

[0011] Patent Document 3: International Publication No. 2020 / 049832 Summary of the invention

[0012] Problems to be Solved by the Invention

[0013] For example, when the galvanized steel sheets described in Patent Documents 1 to 3 are used in hot stamping, the coating after hot stamping is sometimes alloyed with the iron base (parent steel sheet) and the corrosion resistance is reduced. In addition, the hot stamped body obtained by hot stamping the galvanized steel sheet is then joined by spot welding or the like, but at this time it is necessary to suppress liquid metal embrittlement (LME) cracks. This phenomenon is that Zn that has been liquidified due to the heat input of welding intrudes along the grain boundaries into the embrittled parts of the steel, and cracks are generated due to the tensile stress generated by welding. In connection with this, in Patent Document 3, a Ni2Zn alloy with a high melting point is formed by controlling the amount of Ni in the Zn alloy coating to 10 to 25% by mass. 11 、NiZn3、Ni5Zn 21 The γ phase of any crystal structure in the Zn alloy coating is advantageous in terms of resistance to liquid metal embrittlement compared to other Zn alloy coatings. However, in Patent Document 3, sufficient research has not necessarily been conducted from the perspective of both suppressing LME cracks and improving corrosion resistance.

[0014] Accordingly, an object of the present invention is to provide a plated steel sheet that can maintain high corrosion resistance and suppress LME cracks during spot welding after hot stamping even when applied to hot stamping forming.

[0015] Means for Solving the Problems

[0016] The inventors of the present invention conducted research to achieve the above object, and as a result, it was found that by forming a Zn-containing coating layer in an amount equal to or more than a specified amount, sufficient corrosion resistance can be maintained even when applied to hot stamping forming, and by appropriately modifying the structure of the surface layer portion of the base steel sheet, generation of LME cracks during spot welding after hot stamping can be significantly suppressed or reduced even for a coating layer formed in such an amount, thereby completing the present invention.

[0017] The present invention for achieving the above object is as follows.

[0018] (1) A plated steel sheet, characterized by comprising a base steel sheet and a coating layer formed on the surface of the base steel sheet,

[0019] The coating layer has the following chemical composition,

[0020] Containing by mass%:

[0021] Ni: 1.0 to 25.0% and

[0022] Fe: 0 to 3.0%,

[0023] Further containing in a total amount of 5.000% or less:

[0024] Al: 0 to 1.000%,

[0025] Mg: 0 to less than 0.500%,

[0026] Si: 0 to 0.200%,

[0027] Ca: 0 to 3.000%,

[0028] Sb: 0 to 0.500%,

[0029] Pb: 0 to 0.500%,

[0030] Cu: 0 to 1.000%,

[0031] Sn: 0 to 1.000%,

[0032] Ti: 0 to 1.000%,

[0033] Cr: 0 to 1.000%,

[0034] Nb: 0 to 1.000%,

[0035] Zr: 0 to 1.000%,

[0036] Mn: 0 to 1.000%,

[0037] Mo: 0 to 1.000%,

[0038] Ag: 0 to 1.000%,

[0039] Li: 0 to 1.000%,

[0040] La: 0 to 0.500%,

[0041] Ce: 0 to 0.500%,

[0042] B: 0 to 0.500%,

[0043] Y: 0 to 0.500%,

[0044] Sr: 0 to 0.500%,

[0045] In: 0 to 0.500%,

[0046] Co: 0 to 0.500%,

[0047] Bi: 0 to 0.500%,

[0048] P: 0 to 0.500%, and

[0049] W: at least one of 0 to 0.500%,

[0050] The balance: composed of Zn and impurities,

[0051] The depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the above base metal steel plate and the above plating layer is 3 to 100 μm,

[0052] The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the above pearlite is 0 to 20% is 0 to 30%,

[0053] The coating amount of the above plating layer is 40 g / m per single side 2 or more.

[0054] (2) The plated steel sheet according to the above (1), characterized in that the depth at which the area ratio of the above pearlite is 0 to 20% is 10 to 100 μm.

[0055] (3) The plated steel sheet according to (2) above is characterized in that the depth at which the area ratio of the pearlite is 0 to 20% is 30 to 100 μm.

[0056] (4) The plated steel sheet according to any one of (1) to (3) above is characterized in that the area ratio of the pearlite having an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the pearlite is 0 to 20% is 0 to 15%.

[0057] Effects of the Invention

[0058] According to the present invention, it is possible to provide a plated steel sheet that can maintain high corrosion resistance even when applied to hot stamping forming and suppress LME cracks during spot welding after hot stamping forming. Detailed Description of the Invention

[0059] <Plated Steel Sheet>

[0060] The plated steel sheet according to an embodiment of the present invention is characterized by including a base steel sheet and a plating layer formed on the surface of the base steel sheet.

[0061] The plating layer has the following chemical composition.

[0062] Containing by mass%:

[0063] Ni: 1.0 to 25.0% and

[0064] Fe: 0 to 3.0%,

[0065] Further containing in a total amount of 5.000% or less:

[0066] Al: 0 to 1.000%,

[0067] Mg: 0 to less than 0.500%,

[0068] Si: 0 to 0.200%,

[0069] Ca: 0 to 3.000%,

[0070] Sb: 0 to 0.500%,

[0071] Pb: 0 to 0.500%,

[0072] Cu: 0 to 1.000%,

[0073] Sn: 0 to 1.000%,

[0074] Ti: 0 to 1.000%,

[0075] Cr: 0 to 1.000%,

[0076] Nb: 0 to 1.000%,

[0077] Zr: 0 to 1.000%,

[0078] Mn: 0 to 1.000%,

[0079] Mo: 0 to 1.000%,

[0080] Ag: 0 to 1.000%,

[0081] Li: 0 to 1.000%,

[0082] La: 0 to 0.500%,

[0083] Ce: 0 to 0.500%,

[0084] B: 0 to 0.500%,

[0085] Y: 0 to 0.500%,

[0086] Sr: 0 to 0.500%,

[0087] In: 0 to 0.500%,

[0088] Co: 0 to 0.500%,

[0089] Bi: 0 to 0.500%,

[0090] P: 0 to 0.500%, and

[0091] W: at least one of 0 to 0.500%,

[0092] Balance: consisting of Zn and impurities,

[0093] The depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating is 3 to 100 μm,

[0094] The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is 0 to 30%,

[0095] The coating adhesion amount is 40 g / m 2 or more.

[0096] As described above, when a hot stamping formed body obtained by hot stamping a galvanized steel sheet is joined by spot welding, it is necessary to suppress liquid metal embrittlement (LME) cracks. The reason is not necessarily clear, but in the research of the present inventors and the like, it is known that carbon contained in the steel is an element that promotes such LME cracks. Therefore, it is considered that by reducing the carbon concentration in the surface layer portion of the steel that generates LME cracks by, for example, decarburization, the generation of LME cracks can be suppressed or reduced. However, in practice, when applied to hot stamping forming, the LME suppression effect based on such low carbon concentration in the surface layer portion of the steel is limited and sometimes not satisfactory.

[0097] The present inventors have conducted various studies and found that: even from the viewpoint of improving LME resistance, by reducing the carbon concentration in the surface layer portion of the base steel sheet by decarburization or the like, during high-temperature heating in hot stamping forming, the carbon contained in the base steel sheet diffuses to the steel surface layer portion. Through such re-carbonization to the steel surface layer portion, the LME suppression effect based on the low carbon concentration in the original surface layer portion of the base steel sheet disappears or decreases. Therefore, the present inventors further conducted research and found that by forming a structure capable of suppressing such re-carbonization in the surface layer portion of the base steel sheet, even when the adhesion amount of the Zn-containing coating is relatively large in order to maintain sufficient corrosion resistance, the LME suppression effect brought about by the low carbon concentration in the original surface layer portion of the base steel sheet can be fully exerted, and the generation of LME cracks during spot welding after hot stamping forming can be reliably suppressed or reduced. More specifically, the present inventors found that by setting the adhesion amount of the coating to 40 g / m per single side 2 As described above, even when applied to hot stamping forming, sufficient corrosion resistance can be maintained, and the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the interface between the base steel sheet and the coating is set to 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is controlled to 0 to 30%, and a structure is formed in the surface layer portion of the base steel sheet, whereby the generation of LME cracks during spot welding after hot stamping forming can be reliably suppressed or reduced.

[0098] It is not intended to be bound by any specific theory, but it is considered that in the coated steel sheet of the embodiment of the present invention, the structure of the surface layer portion of the base steel sheet acts as follows to suppress or reduce the re-carbonization of carbon contained in the base steel sheet during high-temperature heating in hot stamping forming. More specifically, when the carbon concentration in the surface layer portion of the base steel sheet is reduced by decarburization or the like, the amount of pearlite generated in the microstructure of the surface layer portion of the base steel sheet becomes relatively small in association with such a low-carbon concentration. Here, in the coated steel sheet of the embodiment of the present invention, first, importantly, the surface layer portion of the base steel sheet is made to have a low carbon concentration by decarburization or the like such that the area ratio of pearlite in the thickness direction from the interface between the base steel sheet and the coating layer is 0 to 20%, that is, the depth of the region with a relatively low area ratio of pearlite is 3 to 100 μm. Thereby, the LME suppression effect based on low-carbon concentration can be fully exerted. However, when only the area ratio of pearlite is reduced to a specified range, it is considered that in the case where such pearlite precipitates along the grain boundaries, during high-temperature heating in hot stamping forming, this pearlite transforms into austenite, thereby forming a diffusion path of carbon caused by austenite along the grain boundaries (i.e., a carbon re-carbonization path). During high-temperature heating in hot stamping forming, based on the concentration gradient between the high carbon concentration in the bulk of the base steel sheet and the low carbon concentration on the surface side, the carbon in the bulk diffuses toward the surface side. At this time, when there is a carbon re-carbonization path caused by austenite along the grain boundaries as described above, the carbon in the bulk diffuses toward the surface side through this re-carbonization path, thereby promoting the re-carbonization of the steel surface layer portion. As a result, the LME suppression effect brought about by the low-carbon concentration of the original surface layer portion of the base steel sheet cannot be fully exerted. In contrast, according to the coated steel sheet of the embodiment of the present invention, in the above-mentioned depth region where the area ratio of pearlite is relatively low, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is controlled within the range of 0 to 30%, reducing the amount of relatively large pearlite. Thereby, even during high-temperature heating in hot stamping forming, the austenite transformed from pearlite can be made to exist dispersedly at the grain boundaries, and thus, the carbon re-carbonization path caused by austenite can be reliably cut off.

[0099] More specifically, when pearlite transforms into austenite during high-temperature heating in hot stamping forming, a two-phase structure of ferrite and austenite is formed. In such a case, the austenite present at the heterogeneous interface between ferrite and austenite connects to the surface side of the base steel sheet, thereby forming a carbon re-carbonization path. As a result, the diffusion of carbon from the bulk of the base steel sheet to the surface side is promoted. In connection with this, in the coated steel sheet according to an embodiment of the present invention, it is important that the pearlite in the surface layer portion of the base steel sheet, that is, in the depth range of 3 to 100 μm in the thickness direction from the interface between the base steel sheet and the coating layer, is reduced to 0 to 20% by area ratio, and the relatively coarse pearlite in this depth range, that is, the pearlite with an equivalent circle diameter of 5 μm or more, is limited within the range of 0 to 30% by area ratio. Through such a surface layer structure, even during high-temperature heating in hot stamping forming, the amount of austenite transformed from pearlite can be reduced, and further, the austenite can be dispersed and present at grain boundaries. Therefore, the carbon re-carbonization path caused by austenite can be reliably blocked. Thus, in the coated steel sheet according to an embodiment of the present invention, although the coating amount of the Zn-containing coating layer is relatively large to maintain sufficient corrosion resistance and thus becomes a condition more likely to generate LME, by significantly suppressing re-carbonization during high-temperature heating in hot stamping forming, the LME suppression effect brought about by the low carbon concentration in the surface layer portion of the original base steel sheet can be fully exerted, and the generation of LME cracks during subsequent spot welding can be reliably suppressed or reduced. In the coated steel sheet having a Zn-containing coating layer, the fact that the generation of LME cracks can be suppressed or reduced as described above by appropriately modifying the surface layer structure of the base steel sheet was first clarified by the inventors in this case. Therefore, the coated steel sheet according to an embodiment of the present invention is particularly useful in the automotive field where a large number of spot welds are used.

[0100] Hereinafter, the coated steel sheet according to an embodiment of the present invention will be described in more detail. In the following description, unless otherwise specified, the unit of the content of each element, that is, "%" means "% by mass". In addition, in this specification, the "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value without special specification.

[0101] [Coating layer]

[0102] According to an embodiment of the present invention, a coating layer is formed on the surface of the base steel sheet, for example, on at least one side of the base steel sheet, and preferably on both surfaces. The coating layer has the following chemical composition.

[0103] [Ni: 1.0 to 25.0%]

[0104] Ni is an element effective in improving the corrosion resistance of the coating. In particular, the inventors have found that by combining a Zn-Ni based plating with the base steel plate of the embodiment of the present invention in which the surface layer portion has a low carbon concentration due to decarburization or the like, alloying of the coating and the iron base (base steel plate) can be suppressed during high-temperature heating in hot stamping forming. As a result, the corrosion resistance can be significantly improved. Such a fact was unknown in the past and was first clarified by the inventors this time. In the embodiment of the present invention, in order to fully obtain such an effect brought about by the addition of Ni, the Ni content is 1.0% or more. The Ni content can be 1.1% or more, 1.2% or more, 1.3% or more, 1.5% or more, 2.0% or more, 3.0% or more, 5.0% or more, 7.0% or more, or 10.0% or more. The upper limit is not particularly limited, and from the viewpoint of manufacturing cost and the like, the Ni content is set to 25.0% or less. For example, it can be 23.0% or less, 20.0% or less, 18.0% or less, 16.0% or less, 14.0% or less, or 12.0% or less.

[0105] [Fe: 0 to 3.0%]

[0106] Fe is an element that can be contained in the coating by dissolving from the base steel plate into the plating bath, for example. The Fe content can be 0%, but when Fe is contained, the Fe content can be 0.01% or more, 0.05% or more, 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. On the other hand, Fe is sometimes contained in the coating up to about 3.0%, but if it is in this range, it will not have an adverse effect on the plated steel plate of the embodiment of the present invention. Therefore, the Fe content is set to 3.0% or less. For example, it can be 2.5% or less, 2.2% or less, 2.0% or less, 1.8% or less, 1.5% or less, 1.2% or less, or 1.0% or less.

[0107] Furthermore, the coating layer may optionally contain at least one of 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%. These optional elements are not particularly limited, and preferably the total is 5.000% or less. The total of the optional elements may be 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less or 1.000% or less. Hereinafter, these optional elements will be described in detail.

[0108] [Al: 0 to 1.000%]

[0109] Al is an element effective in improving the corrosion resistance of the coating layer. The Al content may be 0%, but in order to obtain such an effect, the Al content is preferably 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, when Al is excessively contained, the composition of the coating layer approaches the Zn-Al eutectic composition, so that the melting point of the coating layer sometimes decreases. Therefore, the Al content is preferably 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.

[0110] [Mg: 0 to less than 0.500%]

[0111] Mg is an element effective in improving the corrosion resistance of the coating. The Mg content can be 0%, but in order to obtain such an effect, the Mg content is preferably 0.001% or more. The Mg content can be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, from the viewpoint of improving workability, the Mg content can be less than 0.500%. The Mg content can 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.

[0112] [Si: 0 to 0.200%]

[0113] Si is an element effective in improving the corrosion resistance of the coating. The Si content can be 0%, but if necessary, Si can also be contained in the coating in an amount of 0.0001% or more, 0.001% or more, 0.010% or more, or 0.050% or more. On the other hand, from the viewpoint of improving the coating adhesion of the coating, the Si content can be 0.200% or less. The Si content can be 0.180% or less, 0.150% or less, 0.120% or less, or 0.100% or less.

[0114] [Ca: 0 to 3.000%]

[0115] Ca is an element effective in ensuring the wettability of the plating bath. The Ca content can be 0%, but in order to obtain such an effect, the Ca content is preferably 0.001% or more. The Ca content can be 0.005% or more, 0.010% or more, 0.100% or more, or 1.000% or more. On the other hand, when Ca is excessively contained, sometimes a large amount of hard intermetallic compounds are formed in the coating, the coating becomes brittle, and the adhesion to the steel sheet is reduced. Therefore, the Ca content is preferably 3.000% or less. The Ca content can be 2.500% or less, 2.000% or less, or 1.500% or less.

[0116] [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%]

[0117] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W may not be included in the coating, but may be present in the coating in an amount of 0.0001% or more, 0.001% or more, or 0.01% or more. As long as these elements are within the specified content range, they will not have an adverse effect on the properties of the coated steel sheet. However, when the content of each element is excessive, the corrosion resistance may sometimes be reduced. 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.

[0118] In the coating, the balance other than the above elements consists of Zn and impurities. Impurities in the coating refer to components that are mixed in due to various reasons in the manufacturing process, typically represented by raw materials, during the production of the coating.

[0119] [Determination of the chemical composition of the coating]

[0120] The chemical composition of the coating is determined as follows. First, the coating is stripped and dissolved from the coated steel sheet using an acid solution containing an inhibitor that suppresses the corrosion of the base steel sheet, and the resulting acid solution is measured by ICP (inductively coupled plasma) optical emission spectrometry to determine the chemical composition (average composition) of the coating. The type of acid is not particularly limited and can be any acid that can dissolve the coating.

[0121] As the coating, any coating having the above chemical composition can be used, and there is no particular limitation. For example, it can be a hot-dip coating, an alloyed hot-dip coating, or an electroplated coating, etc. The preferred coating is an electroplated coating.

[0122] [Coating adhesion amount: 40 g / m per single side 2 or more]

[0123] In an embodiment of the present invention, the coating adhesion amount is 40 g / m per single side 2As described above. Generally, the plating sometimes alloyizes with the base steel plate during high-temperature heating in hot stamping forming, resulting in a reduction in corrosion resistance. However, according to the embodiments of the present invention, although the reason is not necessarily clear, it is considered that the surface layer structure of the base steel plate, that is, the depth from the interface between the base steel plate and the plating where the area ratio of pearlite is 0 to 20% in the plate thickness direction is 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at this depth is 0 to 30%, can delay the alloying of the plating and the base steel plate. Therefore, it is considered that by making the coating amount of the plating relatively large, specifically controlled to be 40 g / m per single side 2 As described above, in the case of being applied to hot stamping forming, there is a plating layer where alloying is not fully carried out. Due to the existence of such a plating layer, sufficient corrosion resistance can be maintained. On the other hand, when the coating amount of the plating is small, the effect related to the delay of alloying as described above cannot be obtained sufficiently, and sometimes the corrosion resistance after hot stamping forming decreases. From the viewpoint of improving corrosion resistance, the coating amount of the plating is preferably 45 g / m per single side 2 or more or 50 g / m 2 or more, more preferably 60 g / m 2 or more, further more preferably 70 g / m 2 or more, most preferably 80 g / m 2 or more. There is no particular limitation on the upper limit. For example, the coating amount of the plating can be 200 g / m 2 or less, 190 g / m 2 or less, 180 g / m 2 or less or 170 g / m 2 or less.

[0124] [Measurement of plating adhesion amount]

[0125] The coating amount of the plating is determined as follows. First, a 30 mm × 30 mm sample is collected from the plated steel sheet, and then the plating layer is stripped and dissolved from this sample using an acid solution containing an inhibitor that inhibits the corrosion of the base steel plate. The coating amount of the plating is determined based on the weight change of the sample before and after stripping and dissolution. There is no particular limitation on the type of acid, and any acid that can dissolve the plating can be used.

[0126] [Depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the interface between the base steel plate and the plating: 3 to 100 μm]

[0127] In an embodiment of the present invention, the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating is 3 to 100 μm. This feature is associated with the low carbon concentration in the surface layer portion of the base metal steel plate. Therefore, by having this feature, the LME suppression effect brought about by the low carbon concentration in the surface layer portion of the base metal steel plate can be exerted, and the generation of LME cracks during spot welding after hot stamping forming can be suppressed or reduced. In addition, by reducing the amount of pearlite in the surface layer portion of the steel plate to the above range, the amount of austenite transformed from pearlite during high-temperature heating in hot stamping forming can be reduced. Therefore, this feature can also be said to be a very important feature in preventing the re-carbonization path of carbon caused by austenite along the grain boundaries during hot stamping forming. From the viewpoint of further improving these effects, it is preferable to increase the area of the surface layer portion with less pearlite. More specifically, the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating is preferably 5 μm or more, or 10 μm or more, more preferably 20 μm or more, or 30 μm or more, and most preferably 40 μm or more, or 50 μm or more. By making the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating 10 μm or more, the alloying delay effect between the coating and the base metal steel plate described above can be further improved. Therefore, by making the depth at which the area ratio of this pearlite is 0 to 20% 10 μm or more, the corrosion resistance after hot stamping forming can be further improved. The upper limit of the depth can be, for example, 90 μm or 80 μm.

[0128] [The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating: 0 to 30%]

[0129] In an embodiment of the present invention, the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at a depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the interface between the base metal steel sheet and the plating layer is 0 to 30%. By controlling the area ratio of pearlite having an equivalent circle diameter of 5 μm or more within the range of 0 to 30% in a relatively low depth region as described above, the amount of relatively large pearlite is reduced. Thus, even during high-temperature heating in hot stamping forming, austenite transformed from pearlite can be dispersed and present at grain boundaries, and thus the carbon re-carbonization path caused by austenite can be reliably interrupted. Therefore, by significantly suppressing re-carbonization during high-temperature heating in hot stamping forming, the LME suppression effect brought about by low carbon concentration in the surface layer portion of the original base metal steel sheet can be fully exerted, and the generation of LME cracks during subsequent spot welding can be reliably suppressed or reduced. From the viewpoint of further improving such an effect, the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at a depth where the area ratio of pearlite is 0 to 20% is preferably 25% or less or 20% or less, more preferably 15% or less or 12% or less, and most preferably 10% or less or 8% or less. The lower limit of the area ratio of pearlite having an equivalent circle diameter of 5 μm or more can be, for example, 1% or 3%.

[0130] [Measurement of the depth of the pearlite area ratio of 0 to 20% and the area ratio of pearlite having an equivalent circle diameter of 5 μm or more]

[0131] The depth of the area ratio of pearlite in the microstructure in the surface layer part of the base metal steel plate being 0 to 20% and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more are determined as follows. First, five samples are collected in such a way that a cross-section parallel to the rolling direction and the plate thickness direction can be observed from the surface of the plated steel plate. Next, these observation surfaces are mirror-polished, etched with picric alcohol etching solution, and then the microstructure is observed using a scanning electron microscope (SEM). Regarding the measurement range, for each sample, a rectangular range with a thickness of 100 μm in the plate thickness direction and 500 μm in the direction perpendicular to the plate thickness direction starting from the interface between the base metal steel plate and the coating layer is set as one field of view, and five fields of view are measured for the total of five samples. The interface between the base metal steel plate and the coating layer can be discriminated based on the difference in color tones between the base metal steel plate and the coating layer in the backscattered electron image (BSE image) of the SEM. The area ratio of pearlite is calculated using the point algorithm from a microstructure photograph with a magnification of about 5000 times, for example. Here, a region surrounded by grain boundaries with a crystal orientation difference of 15° or more in ferrite, a region where a ferrite phase and a cementite phase coexist, and a region where the morphology of cementite is lamellar and / or spherical are recognized as pearlite, and its area ratio is calculated. For each sample, the depth position where the area ratio of pearlite gradually increases to 20% is determined from the interface between the base metal steel plate and the coating layer. Then, the distance from the determined depth position to the interface is calculated, and their arithmetic mean is determined as the "depth of the area ratio of pearlite being 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating layer". Similarly, for each sample, in the depth region starting from the interface where the area ratio of pearlite is 20%, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is calculated by image processing, and their arithmetic mean is determined as the "area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth of the area ratio of pearlite being 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the coating layer".

[0132] [Area ratio of martensite: less than 1%]

[0133] Regarding the microstructure of the base metal steel sheet, as described above, it is only necessary to form a structure in the surface layer portion of the base metal steel sheet such that the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the interface between the base metal steel sheet and the plating layer is 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is controlled to 0 to 30%. Therefore, there is no particular limitation on other structures. For example, in a preferred embodiment of the present invention, the area ratio of martensite contained in the base metal steel sheet is less than 1%. Regarding the manufacturing method of the plated steel sheet, as will be described in detail later, in order to obtain the above-mentioned pearlite structure, in the cooling process after the annealing process, it is particularly preferable to cool from the controlled temperature of 620 to 670 °C to the plating bath temperature at a relatively slow average cooling rate of 10 °C / s or less. In such a case of a relatively slow average cooling rate, martensite hardly precipitates, and even if it precipitates, its area ratio is less than 1%. The area ratio of martensite can be 0.5% or less or 0%.

[0134] [Identification and calculation of area ratio of martensite]

[0135] The identification and calculation of the area ratio of martensite are carried out as follows. First, a specimen is collected such that the cross-section parallel to the rolling direction and the plate thickness direction of the plated steel sheet becomes the observation surface. Next, the observation surface is mirror-polished, etched with a nitric acid ethanol etching solution, and then the microstructure is observed using a scanning electron microscope (SEM). At the position of 1 / 2 the plate thickness depth of the above observation surface, a range of 300 μm × 300 μm is photographed at a magnification of 1000 times. After performing black-and-white binary processing on the obtained microstructure photograph, image analysis is performed to identify pearlite, bainite, and ferrite, and the total area ratio of them is obtained using the method based on the "Microscopic test method for crystal grain size of steel" specified in JIS G 0551:2020. Retained austenite is difficult to distinguish from martensite by SEM, so the area ratio of retained austenite is measured by X-ray diffraction method. Finally, the area ratio of martensite is determined by subtracting the total area ratio of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0136] [Preferred chemical composition of base metal steel sheet]

[0137] As described above, the object of the present invention is to provide a plated steel sheet that can maintain high corrosion resistance and suppress LME cracks even when applied to hot stamping forming. By setting the coating amount of the plating layer to 40 g / m per single side 2Above, and the depth where the area ratio of pearlite in the plate thickness direction from the interface between the base metal steel plate and the plating layer is 0 to 20% is set to 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of this pearlite is 0 to 20% is controlled to 0 to 30% to form a structure in the surface layer portion of the base metal steel plate, thereby achieving this object. Therefore, it is clear that the chemical composition of the base metal steel plate itself is not an essential technical feature in achieving the object of the present invention. Hereinafter, the preferred chemical composition of the base metal steel plate used in the plated steel plate of the embodiment of the present invention will be described in detail, but these descriptions are only intended to exemplify the preferred chemical composition of the base metal steel plate suitable for achieving a Vickers hardness of 400 HV or more in the formed body after hot stamping forming, and are not intended to limit the present invention to the use of a base metal steel plate having such a specific chemical composition.

[0138] In an embodiment of the present invention, for example, the base metal steel plate preferably has a chemical composition composed of the following components by mass%:

[0139] C: 0.13 to 0.50%,

[0140] Si: 0.001 to 3.000%,

[0141] Mn: 0.30 to 3.00%,

[0142] Al: 0.0002 to 2.000%,

[0143] P: 0.100% or less,

[0144] S: 0.1000% or less,

[0145] N: 0.0100% or less,

[0146] Nb: 0 to 0.15%,

[0147] Ti: 0 to 0.15%,

[0148] V: 0 to 0.15%,

[0149] Mo: 0 to 1.0%,

[0150] Cr: 0 to 1.0%,

[0151] Cu: 0 to 1.0%,

[0152] Ni: 0 to 1.0%,

[0153] B: 0 to 0.0100%,

[0154] W: 0 to 1.000%,

[0155] Hf: 0 to 0.050%,

[0156] Mg: 0 to 0.050%,

[0157] Zr: 0 to 0.050%,

[0158] Ca: 0 to 0.010%,

[0159] REM: 0 to 0.30%,

[0160] Ir: 0 to 1.000%, and

[0161] Balance: Fe and impurities. Hereinafter, each element will be described in more detail.

[0162] [C: 0.13 to 0.50%]

[0163] C is an element that is inexpensive and increases the tensile strength, and is an important element for controlling the strength of steel. In order to fully obtain such an effect, the C content is preferably 0.13% or more. The C content can be 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, when C is contained excessively, sometimes the elongation rate decreases. Therefore, the C content is preferably 0.50% or less. The C content can be 0.45% or less or 0.40% or less.

[0164] [Si: 0.001 to 3.000%]

[0165] Si acts as a deoxidizer and is an element that inhibits the precipitation of carbides during the cooling process in the annealing of cold-rolled sheets. In order to fully obtain such an effect, the Si content is preferably 0.001% or more. The Si content can be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, when Si is contained excessively, sometimes the strength of the steel increases and the elongation rate decreases. Therefore, the Si content is preferably 3.000% or less. The Si content can be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0166] [Mn: 0.30 to 3.00%]

[0167] Mn is an element that improves the hardenability of steel and is an element effective for increasing the strength. In order to fully obtain such an effect, the Mn content is preferably 0.30% or more. The Mn content can be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, when Mn is contained excessively, sometimes the strength of the steel increases and the elongation rate decreases. Therefore, the Mn content is preferably 3.00% or less. The Mn content can be 2.80% or less, 2.50% or less, or 2.00% or less.

[0168] [Al: 0.0002 to 2.000%]

[0169] Al acts as a deoxidizer for steel and is an element that has the effect of making the steel sound. In order to fully obtain such an effect, the Al content is preferably 0.0002% or more. The Al content can be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, when Al is excessively contained, sometimes coarse Al oxides are generated and the elongation of the steel plate decreases. Therefore, the Al content is preferably 2.000% or less. The Al content can be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0170] [P: 0.100% or less]

[0171] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the more preferable, and thus 0% is ideal. However, an excessive reduction in the P content sometimes leads to a large increase in cost. Therefore, the P content can be 0.0001% or more, and can also be 0.001% or more or 0.005% or more. On the other hand, when P is excessively contained, as described above, sometimes embrittlement of steel occurs due to grain boundary segregation. Therefore, the P content is preferably 0.100% or less. The P content can be 0.050% or less, 0.030% or less, or 0.010% or less.

[0172] [S: 0.1000% or less]

[0173] S is an element that forms non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel components. The lower the S content, the more preferable, and thus 0% is ideal. However, an excessive reduction in the S content sometimes leads to a large increase in cost. Therefore, the S content can be 0.0001% or more, and can also be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when S is excessively contained, sometimes cracks starting from non-metallic inclusions occur during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content can be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0174] [N: 0.0100% or less]

[0175] N is an element that forms coarse nitrides in the steel plate and reduces the workability of the steel plate. The lower the N content, the more preferable it is, and thus 0% is ideal. However, an excessive reduction in the N content sometimes leads to a significant increase in manufacturing costs. Therefore, the N content can be 0.0001% or more, and can also be 0.0005% or more or 0.0010% or more. On the other hand, when N is excessively contained, as described above, coarse nitrides are sometimes formed, reducing the workability of the steel plate. Therefore, the N content is preferably set to 0.0100% or less. The N content can be 0.0080% or less or 0.0050% or less.

[0176] The preferred basic chemical composition of the base steel plate is as described above. Further, the base steel plate may also contain, as needed, one or more selected from Nb: 0 to 0.15%, Ti: 0 to 0.15%, V: 0 to 0.15%, Mo: 0 to 1.0%, Cr: 0 to 1.0%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.30% and Ir: 0 to 1.000% to replace a part of the balance of Fe. These elements can be 0.0001% or more, 0.0005% or more, 0.001% or more or 0.01% or more, respectively.

[0177] In the base steel plate, the balance other than the above elements consists of Fe and impurities. Impurities in the base steel plate refer to components such as those mixed in due to various reasons in the manufacturing process, typically represented by raw materials such as ores and scrap.

[0178] The chemical composition of the base steel plate can be determined by general analytical methods. For example, for the chemical composition of the base steel plate, first, the coating is removed by mechanical grinding, and then it can be determined using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry) for the cut powder in accordance with JIS G 1201:2014. Specifically, for example, a test piece of 35 mm square can be taken from near the 1 / 2 position of the plate thickness of the base steel plate, and measured using an ICPS - 8100 etc. (measurement device) manufactured by Shimadzu Corporation under the conditions based on a pre - made standard curve. C and S that cannot be measured by ICP - AES can be measured using combustion - infrared absorption method, N can be measured using inert gas fusion - thermal conductivity method, and O can be measured using inert gas fusion - non - dispersive infrared absorption method.

[0179] [Thickness of the base steel plate]

[0180] The plate thickness of the base material steel plate is not particularly limited. For example, it is 0.2 mm or more, and can be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the plate thickness of the base material steel plate is, for example, 6.0 mm or less, and can also be 5.0 mm or less or 4.0 mm or less.

[0181] [Mechanical properties]

[0182] The coated steel plate according to the embodiment of the present invention, more specifically, the coated steel plate before hot stamping forming, is not particularly limited. For example, it has a tensile strength of less than 980 MPa. The tensile strength can be 950 MPa or less, 900 MPa or less, 850 MPa or less, or 800 MPa or less. The lower limit is not particularly limited. For example, the tensile strength can be 500 MPa or more, 550 MPa or more, or 590 MPa or more. According to the embodiment of the present invention, even if the tensile strength of the coated steel plate before hot stamping forming is less than 980 MPa, in the formed body after hot stamping forming, by making the microstructure a martensite-based structure, a Vickers hardness of 400 HV or more can be sufficiently achieved. The tensile strength is measured by a tensile test in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece collected from a direction parallel to the rolling right angle direction of the length direction of the test piece of the coated steel plate.

[0183] [Manufacturing method of coated steel plate]

[0184] Next, a preferred manufacturing method of the coated steel plate according to the embodiment of the present invention will be described. The following description is an exemplification of the characteristic method for manufacturing the coated steel plate according to the embodiment of the present invention, and it is not intended to limit the coated steel plate to the coated steel plate manufactured by the manufacturing method described below.

[0185] The coated steel plate according to the embodiment of the present invention can be manufactured, for example, by performing the following processes: a casting process of casting molten steel with an adjusted chemical composition to form a steel sheet; a hot rolling process of hot rolling the steel sheet to obtain a hot rolled steel plate; a coiling process of coiling the hot rolled steel plate; a cold rolling process of cold rolling the coiled hot rolled steel plate to obtain a cold rolled steel plate; an annealing process of annealing the cold rolled steel plate; a cooling process of cooling the annealed cold rolled steel plate; and a plating process of forming a plating layer on the obtained base material steel plate. As an alternative, it is also possible not to coil after the hot rolling process, perform pickling, and directly perform the cold rolling process. Hereinafter, each process will be described in detail.

[0186] [Casting process]

[0187] The conditions of the casting process are not particularly limited. For example, after smelting using a blast furnace, an electric furnace, etc., various secondary smelting processes are carried out, and then casting can be carried out by methods such as ordinary continuous casting, ingot casting method, or thin slab casting.

[0188] [Hot rolling process]

[0189] The cast steel sheet is hot rolled to obtain a hot rolled steel sheet. The hot rolling process is carried out as follows: the cast steel sheet is directly or temporarily cooled and then reheated and hot rolled. In the case of reheating, the heating temperature of the steel sheet can be, for example, 1100 to 1250 °C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling can be appropriately changed according to the desired metal structure and plate thickness. For example, the finish rolling end temperature can be 900 to 1050 °C, and the finish rolling reduction ratio can be 10 to 50%.

[0190] [Coiling process]

[0191] The hot rolled steel sheet can be coiled at a specified temperature. The coiling temperature can be appropriately determined according to the desired metal structure, etc., and can be, for example, 500 to 800 °C. It is also possible to uncoil before or after coiling and apply a specified heat treatment to the hot rolled steel sheet. As an alternative, the coiling process can be omitted and pickling can be carried out after the hot rolling process and the cold rolling process described below can be carried out.

[0192] [Cold rolling process]

[0193] After pickling etc. of the hot rolled steel sheet, the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet. The reduction ratio of cold rolling can be appropriately determined according to the desired metal structure and plate thickness, and can be, for example, 20 to 80%. After the cold rolling process, it can be air cooled to room temperature, for example.

[0194] [Annealing process]

[0195] Next, the obtained cold-rolled steel sheet is annealed. The annealing process includes 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 for 10 to 300 seconds. By performing the annealing process under such conditions of a relatively high dew point, decarburization of the surface layer of the cold-rolled steel sheet can be appropriately carried out. Therefore, in the finally obtained plated steel sheet, the depth at which the area ratio of pearlite is 0 to 20% in the thickness direction starting from the interface between the base metal steel sheet and the plating layer can be controlled within the range of 3 to 100 μm. When the dew point is lower than -20°C, or the heating temperature is lower than 730°C and / or the holding time is shorter than 10 seconds, decarburization of the surface layer of the cold-rolled steel sheet is insufficient. As a result, in the finally obtained plated steel sheet, it becomes impossible to make the depth at which the area ratio of pearlite is 0 to 20% in the thickness direction starting from the interface between the base metal steel sheet and the plating layer be 3 μm or more. On the other hand, when the dew point exceeds 10°C, or the heating temperature exceeds 900°C and / or the holding time exceeds 300 seconds, an external oxide layer may sometimes be formed on the steel sheet surface, the plating property may be reduced, or the strength of the finally obtained plated steel sheet may be reduced due to excessive decarburization. The dew point is preferably -10 to 5°C, more preferably -5 to 5°C. In addition, the atmosphere in the annealing process can be a reducing atmosphere, and more specifically, it can be a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with 1 to 10% hydrogen (for example, 4% hydrogen and nitrogen balance).

[0196] [Cooling process]

[0197] In order to obtain a desired surface layer structure, the cold-rolled steel sheet decarburized in the surface layer during the annealing process needs to be appropriately cooled in the subsequent cooling process. Specifically, the cooling process includes: cooling from the heating temperature of the annealing process to a control temperature of 620 to 670°C at an average cooling rate of 20°C / s or more (primary cooling); and cooling from this control temperature to the plating bath temperature (for example, the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less (secondary cooling). Hereinafter, the primary cooling and the secondary cooling will be described in more detail.

[0198] [Primary cooling]

[0199] In the primary cooling, it is important to suppress the precipitation of pearlite at high temperatures. More specifically, in the annealing process, the diffusion of pearlite precipitated at high temperatures from the heating temperature of 730 to 900 °C to the control temperature of 620 to 670 °C is fast. Therefore, after precipitation, it is easy to diffuse along the grain boundaries and form pearlite along the grain boundaries. The pearlite formed along these grain boundaries undergoes austenite phase transformation during the high-temperature heating of hot stamping forming, thereby forming a carbon re-carbonization path caused by austenite along the grain boundaries and promoting the re-carbonization of carbon in the bulk to the steel surface layer. Therefore, in the temperature range from the heating temperature of the annealing process to the above control temperature, it is extremely important to cool the cold-rolled steel sheet at an average cooling rate of 20 °C / s or more to suppress the precipitation of pearlite at high temperatures in the steel sheet surface layer. When the above average cooling rate is less than 20 °C / s and / or the control temperature exceeds 670 °C, pearlite precipitates at high temperatures with fast diffusion, thus promoting the formation of pearlite along the grain boundaries. As a result, in the finally obtained coated steel sheet, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at a depth where the area ratio of pearlite in the plate thickness direction from the interface between the base steel sheet and the coating layer is 0 to 20% exceeds 30%. In the case of applying it to hot stamping forming, sufficient LME resistance cannot be achieved during subsequent spot welding.

[0200] [Secondary cooling]

[0201] On the other hand, in the secondary cooling after the primary cooling, it is important to precipitate pearlite at low temperatures where the diffusion is relatively slow. More specifically, by cooling from the control temperature of 620 to 670 °C to the plating bath temperature (for example, the melting point of the plating bath + 20 °C) at an average cooling rate of 10 °C / s or less, pearlite can be precipitated. In the low-temperature region below such a control temperature, the diffusion of pearlite precipitated is relatively slow. Therefore, it will not form in a form connected along the grain boundaries, and pearlite can be dispersed and exist on the grain boundaries. In such a tissue case, even during the high-temperature heating of hot stamping forming, austenite transformed from pearlite can also be dispersed and exist on the grain boundaries above the A c1 point, so the carbon re-carbonization path caused by austenite can be reliably truncated. On the other hand, when the above average cooling rate exceeds 10 °C / s and / or the control temperature is lower than 620 °C, martensite and bainite are mainly precipitated instead of pearlite. In the finally obtained coated steel sheet, it is impossible to make the depth where the area ratio of pearlite in the plate thickness direction from the interface between the base steel sheet and the coating layer is 0 to 20% less than 100 μm. Compared with pearlite, the phase transformation rate of martensite and bainite to austenite is fast, and it immediately transforms into austenite slightly higher than the A c1 point. Therefore, compared with the case of pearlite, the exposure time to high temperatures in the two-phase structure of ferrite and austenite during hot stamping forming becomes longer. In such a case as well, it is easy to form a re-carbonization path along the grain boundaries, so sufficient LME resistance cannot be achieved.

[0202] [Plating Process]

[0203] Next, in the plating process, a plating layer having the chemical composition described above is formed on at least one surface, preferably both surfaces, of the cold-rolled steel sheet (base steel sheet). More specifically, the plating process can be carried out, for example, by hot-dip plating using a plating bath whose composition has been adjusted so that the chemical composition of the plating layer falls within the range described above (plating bath temperature: for example, 420 to 480 °C), and an alloying treatment is carried out after this hot-dip plating treatment. In addition, the plating treatment is not limited to the molten plating method, and can also be an electroplating method, a vapor deposition method, a spraying or cold spraying method, etc. Other conditions of the plating process can be appropriately set by considering the thickness and adhesion amount of the plating layer, etc. For example, after immersing the cold-rolled steel sheet in the plating bath, it is lifted up, and N2 gas or air is immediately blown by the gas wiping method, and then cooled, whereby the adhesion amount of the plating layer can be adjusted to within a specified range, for example, 40 to 200 g / m per single side 2 within the range of.

[0204] The plated steel sheet manufactured by this manufacturing method can set the adhesion amount of the plating layer to 40 g / m per single side 2 or more, and set the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the interface between the base steel sheet and the plating layer to 3 to 100 μm, and control the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% to 0 to 30% to form a structure in the surface layer portion of the base steel sheet. Therefore, even when exposed to high temperatures such as during hot stamping forming, high corrosion resistance can be maintained, and the re-carbonization of carbon in the block to the steel surface layer portion can be significantly suppressed. Therefore, the LME suppression effect brought about by the low carbon concentration in the surface layer portion of the original base steel sheet can be fully exerted, and the generation of LME cracks during subsequent spot welding can be reliably suppressed or reduced. Therefore, according to such a plated steel sheet, when applied as a plated steel sheet for hot stamping, compared with conventional plated steel sheets, sufficient corrosion resistance can be maintained, and more excellent LME resistance can be achieved. Therefore, in the use of plated steel sheets for automobiles and building materials, through extended service life, it can contribute to the development of the industry.

[0205] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0206] Examples

[0207] In the following examples, the plated steel sheets of the embodiments of the present invention were manufactured under various conditions, and the properties of the manufactured plated steel sheets were investigated.

[0208] First, molten steel is cast by the continuous casting method to form a steel sheet having the chemical composition shown in Table 1. After the steel sheet is temporarily cooled, it is reheated to 1200 °C and hot-rolled, and then coiled at a temperature below 600 °C. The hot rolling is carried out by rough rolling and finish rolling, the finishing temperature of the finish rolling is 900 - 1050 °C, and the reduction ratio of the finish rolling is 30%. Next, pickling is performed on the obtained hot-rolled steel sheet, and then cold rolling is carried out at a reduction ratio of 50% to obtain a cold-rolled steel sheet with a plate thickness of 1.6 mm. Then, for the obtained cold-rolled steel sheet, an annealing process is carried out under the conditions shown in Table 2 in a furnace with an oxygen concentration of 20 ppm or less in a mixed gas atmosphere of 4% hydrogen and nitrogen balance, and then a cooling process is carried out under the conditions shown in Table 2 in the same way to manufacture a base metal steel sheet.

[0209] Next, the manufactured base metal steel sheet is cut into 100 mm × 200 mm, and the base metal steel sheet is plated using the batch melting plating test device manufactured by our company. More specifically, first, the manufactured base metal steel sheet is immersed in a plating bath having a prescribed chemical composition for about 3 seconds, and then it is lifted at a lifting speed of 20 - 200 mm / s, and the amount of the deposited layer is adjusted to the value shown in Table 2 by wiping with N2 gas. Next, nitrogen gas is used as the cooling gas, and the base metal steel sheet with the deposited layer is cooled from the plating bath temperature (about 420 - 480 °C) to room temperature, thereby obtaining a plated steel sheet having deposited layers formed on both surfaces of the base metal steel sheet. The plate temperature is measured using a thermocouple spot-welded to the center of the base metal steel sheet.

[0210] The physical properties and characteristics of the obtained plated steel sheet are measured and evaluated by the following methods.

[0211] [Analysis of Chemical Composition of Deposited Layer]

[0212] The chemical composition of the deposited layer is determined as follows: A sample cut into 30 mm × 30 mm is immersed in a 10% HCl aqueous solution added with an inhibitor. After pickling and peeling the deposited layer, the plating components dissolved in the aqueous solution are measured by ICP emission spectrometry. The results are shown in Table 2.

[0213] [Evaluation of LME Resistance during Spot Welding]

[0214] First, load the coated steel sheet into an atmospheric heating furnace at 900 °C. After the temperature of the coated steel sheet reaches the furnace temperature - 10 °C, hold for 100 seconds. Then, take out the coated steel sheet from the furnace, clamp the coated steel sheet with a flat die at a temperature around room temperature and quench it rapidly. Prepare 2 samples of the coated steel sheet after heating and quenching, each with a size of 50 mm × 100 mm. For these 2 coated steel sheet specimens, use a welding electrode with a dome radius type and a tip diameter of 8 mm, and perform spot welding with a welding angle of 2°, a pressing force of 4.0 kN, a welding current time of 0.5 seconds, and a welding current of 12 kA to fabricate a welded joint. Then, measure the length of the LME crack generated directly under the electrode at the welded part, and evaluate the LME resistance as follows.

[0215] AAA: 0 μm

[0216] AA: Exceeding 0 - 20 μm

[0217] A: Exceeding 20 μm and less than 80 μm

[0218] B: 80 μm or more

[0219] [Evaluation of corrosion resistance]

[0220] The corrosion resistance of the coated steel sheet is evaluated as follows. First, load the coated steel sheet into an atmospheric heating furnace at 900 °C. After the temperature of the coated steel sheet reaches the furnace temperature - 10 °C, hold for 100 seconds. Then, take out the coated steel sheet from the furnace, clamp the coated steel sheet with a flat die at a temperature around room temperature and quench it rapidly. Apply the coated steel sheet sample with a size of 50 mm × 100 mm according to the phosphoric acid Zn treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). Then, perform electrodeposition coating at 20 μm (PN110 POWERNICS GRAY: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.), and conduct sintering treatment at a temperature of 150 °C for 20 minutes. Then, introduce a cut reaching the iron base (base metal steel sheet) into the center of the sample. Then, perform 120 cycles according to the composite cyclic corrosion test of JASO (M609 - 91), measure the film swelling width, and evaluate the corrosion resistance as follows.

[0221] AAA: 2 mm or less

[0222] AA: Exceeding 2 - 3 mm

[0223] A: Exceeding 3 - 4 mm

[0224] B: Exceeding 4 mm

[0225] [Evaluation of hardness]

[0226] First, in the same manner as in the case of evaluating the corrosion resistance, a test piece is cut out from an arbitrary position other than the end portion of the plated steel sheet except for heating and quenching in such a way that a cross-section perpendicular to the surface (plate thickness cross-section) can be observed. The plate thickness cross-section of the test piece is polished using silicon carbide sandpaper with #600 to #1500 grit, and then it is finish-machined into a mirror surface 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 plate thickness cross-section is used as the measurement surface. Next, a micro-Vickers hardness tester is used to measure the Vickers hardness at a load of 1 kgf at intervals of more than three times the indentation. A total of 20 points are randomly measured near the 1 / 2 position of the plate thickness of the base metal steel sheet in such a way as not to include the surface layer portion that has undergone low-carbon concentration, and their arithmetic mean is determined as the hardness after hot stamping (HS), and the evaluation is carried out as follows.

[0227] AAA: The hardness after HS exceeds 550 HV

[0228] AA: The hardness after HS exceeds 500 - 550 HV

[0229] A: The hardness after HS is 400 - 500 HV

[0230] B: The hardness after HS is less than 400 HV

[0231] The cases where the evaluation of the LME resistance is AAA, AA, and A, and the evaluation of the corrosion resistance is AAA, AA, and A are evaluated as plated steel sheets that can maintain high corrosion resistance even when applied to hot stamping forming and can suppress LME cracks during spot welding after hot stamping forming. The results are shown in Table 2.

[0232] Table 1

[0233]

[0234] Table 2-1

[0235]

[0236] Table 2-2

[0237]

[0238] Referring to Table 2, it is considered that in Comparative Example 35, the heating temperature in the annealing process is low, so the decarburization of the surface layer of the cold-rolled steel sheet is insufficient. As a result, it is impossible to make the depth of the area ratio of pearlite in the thickness direction from the interface between the base steel sheet and the coating layer be 3 μm or more with an area ratio of 0 to 20%, and the LME resistance is reduced. In Comparative Example 36, since the holding time in the annealing process is short, it is considered that the decarburization of the surface layer of the cold-rolled steel sheet is insufficient. Similarly, it is impossible to make the depth of the area ratio of pearlite be 3 μm or more with an area ratio of 0 to 20%, and the LME resistance is reduced. In Comparative Example 37, since the dew point in the annealing process is low, it is considered that the decarburization of the surface layer of the cold-rolled steel sheet is insufficient. Similarly, it is impossible to make the depth of the area ratio of pearlite be 3 μm or more with an area ratio of 0 to 20%, and the LME resistance is reduced. In Comparative Example 38, the average cooling rate of the primary cooling in the cooling process is low, so it is considered that pearlite precipitates at a high temperature and forms along the grain boundaries. As a result, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite in the thickness direction from the interface between the base steel sheet and the coating layer is 0 to 20% exceeds 30%, and the LME resistance is reduced. In Comparative Example 39, the control temperature of the primary cooling in the cooling process is high, so it is considered that pearlite precipitates at a high temperature and forms along the grain boundaries. Similarly, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% exceeds 30%, and the LME resistance is reduced. In Comparative Example 40, the control temperature of the secondary cooling in the cooling process is low, so mainly bainite precipitates instead of pearlite. As a result, it is impossible to make the depth of the area ratio of pearlite in the thickness direction from the interface between the base steel sheet and the coating layer be the desired depth with an area ratio of 0 to 20%, and the LME resistance is reduced. In Comparative Example 41, since the average cooling rate of the secondary cooling in the cooling process is fast, mainly bainite precipitates instead of pearlite. Similarly, it is impossible to make the depth of the area ratio of pearlite be the desired depth with an area ratio of 0 to 20%, and the LME resistance is reduced. In Comparative Example 42, since the coating adhesion amount is insufficient, the corrosion resistance after HS is reduced.

[0239] In contrast, the plated steel sheets in all the examples have a specified plating chemical composition, and the coating adhesion amount is set to 40 g / m per single side 2Furthermore, the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base material steel plate and the coating is set to 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is controlled to 0 to 30%. Thus, even when applied to hot stamping forming, high corrosion resistance can be maintained, and the LME suppression effect brought about by low carbon concentration in the surface layer part of the original base material steel plate can be fully exerted, and the generation of LME cracks during subsequent spot welding can be reliably suppressed or reduced. In particular, in Examples 3 to 34 where the depth where the area ratio of pearlite is 0 to 20% is controlled to 10 to 100 μm, the corrosion resistance is evaluated as AA, achieving further improvement in corrosion resistance. Furthermore, in Examples 13 to 34 where the depth where the area ratio of pearlite is 0 to 20% is set to 30 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at this depth is controlled to 0 to 15%, the LME resistance is evaluated as AAA, and the LME resistance is further improved.

Claims

1. A coated steel sheet, characterized in that, It has a base metal steel plate and a plating layer formed on the surface of the base metal steel plate. The plating layer has the following chemical composition. By mass%, it contains: Ni: 1.0 to 25.0% and Fe: 0 to 3.0%, Further contains in a total amount of 5.000% or less: Al：0～1.000％、 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：0～1.000％、 Nb: 0 to 1.000%, Zr：0～1.000％、 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：0～0.500％、 Y：0～0.500％、 Sr：0～0.500％、 In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, and at least one of W: 0 to 0.500%, The balance: consists of Zn and impurities. The depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction starting from the interface between the base metal steel plate and the plating layer is 3 to 100 μm. The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is 0 to 30%. The coating amount is 40 g / m per single side 2 or more.

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

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

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

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