Plated steel sheet

Abstract

Provided is a plated steel sheet which is provided with a base steel sheet and a plating layer formed on the surface of the base steel sheet, and which is characterized in that the plating layer has a prescribed chemical composition, and the depth 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% is 3-100 [mu] m. The surface area ratio of pearlite having an equivalent circle diameter of 5 [mu] m or more at a depth at which the surface area ratio of pearlite is 0-20% is 0-30%, the deposition amount of the plating layer is 20 g / m2 or more per surface, the plating layer has an oxide layer on the surface, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 3 or more when the oxide layer is measured by XPS.

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

[0004] For example, Patent Document 1 describes a galvanized steel sheet for hot stamping, 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%, with the balance being iron and inevitable 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 it can be suppressed.

[0005] Patent Document 2 describes a steel sheet coated with a metal coating. The metal coating contains 2.0 to 24.0% by weight of zinc, 7.1 to 12.0% by weight of silicon, optional 1.1 to 8.0% by weight of magnesium, and an additional element optionally selected from Pb, Ni, Zr, or Hf. The weight content ratio of each additional element is less than 0.3% by weight, and the balance is aluminum and any inevitable impurities and residual elements. Among them, the Al / Zn ratio exceeds 2.9, and it is taught that a component obtained by hot stamping the steel sheet exhibits high sacrificial corrosion protection.

[0006] Patent Document 3 describes a method for manufacturing a hardened component, which includes the following steps: A) a step of providing a steel sheet pre-coated with a metal coating, the metal coating containing 2.0 to 24.0% by weight of zinc, 1.1 to 7.0% by weight of silicon, when the amount of silicon is between 1.1 and 4.0% by weight, optionally containing 1.1 to 8.0% by weight of magnesium, and an additional element optionally selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less than 0.3% by weight, with the remainder being aluminum and inevitable impurities and residual elements, wherein the Al / Zn ratio exceeds 2.9; B) a step of cutting the coated steel sheet to obtain a blank; C) a step of heat-treating the blank at a temperature between 840 and 950 °C to obtain a fully austenitic fine structure in the steel; D) a step of transferring the blank into a stamping tool; E) a step of hot-forming the blank to obtain a component; F) a step of cooling the component obtained in step E) in order to obtain a fine structure in the steel composed of martensite or martensite-bainite, or at least 75% equiaxed ferrite, 5 to 20% martensite, and 10% or less of bainite. Additionally, Patent Document 3 teaches that a hardened component without LME can be obtained according to the above manufacturing method.

[0007] Prior art documents

[0008] Patent documents

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

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2018-528324

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2018-527462 Summary of the invention

[0012] Problems to be Solved by the Invention

[0013] For example, when using the galvanized steel sheets described in Patent Documents 1 to 3 in hot stamping, the coating after hot stamping sometimes alloyizes with the iron-based (base metal steel sheet) and the corrosion resistance decreases. In addition, the hot-stamped formed body obtained by hot stamping the galvanized steel sheet is then joined by spot welding or the like. At this time, it is necessary to suppress liquid metal embrittlement (LME) cracks. This phenomenon is a crack generated when tensile stress generated by welding acts on a part where Zn liquefied due to welding heat input penetrates into the steel inside along the grain boundaries and embrittles. In relation to this, Patent Document 3 teaches suppression of LME generated during hot stamping. However, in Patent Document 3, from the viewpoints of suppressing LME cracks during spot welding after hot stamping and achieving both suppression of such LME cracks and improvement of corrosion resistance, sufficient research has not necessarily been conducted.

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

[0015] Means for Solving the Problems

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

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

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

[0019] The coating has the following chemical composition,

[0020] Containing by mass%:

[0021] Al: 0.5 to 50.0%,

[0022] Mg: 0.50 to 15.00%,

[0023] Si: 0 to 4.0%, and

[0024] Fe: 0 to 15.0%,

[0025] Further containing in a total amount of 5.000% or less:

[0026] Ni: 0 to 1.000%,

[0027] Ca: 0 to 3.0%,

[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] Balance: Consisting of Zn and impurities,

[0051] The depth from the interface between the base metal steel sheet and the above plating layer in the plate thickness direction where the area ratio of pearlite is 0 to 20% 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 20 g / m per single side. 2 or more.

[0054] The above plating layer has an oxide layer on the surface. When measuring the above oxide layer by XPS, the peak intensity ratio of (Al - O + Mg - O) / Zn - O is 3 or more.

[0055] Here, Al - O, Mg - O, and Zn - O respectively represent the intensities of the peaks attributed to the Al - O, Mg - O, and Zn - O bonds.

[0056] (2) The plated steel sheet according to the above (1), characterized in that the depth where the area ratio of the above pearlite is 0 to 20% is 10 to 100 μm.

[0057] (3) The plated steel sheet according to the above (2), characterized in that the depth where the area ratio of the above pearlite is 0 to 20% is 30 to 100 μm.

[0058] (4) The plated steel sheet according to any one of the above (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 the above pearlite is 0 to 20% is 0 to 15%.

[0059] (5) The plated steel sheet according to any one of the above (1) to (4), characterized in that the chemical composition contains Al: 10.0 to 50.0% and Mg: 4.00 to 15.00% by mass%, and the peak intensity ratio of (Al - O + Mg - O) / Zn - O is 5 or more.

[0060] (6) The plated steel sheet according to the above (5), characterized in that the chemical composition contains Al: 30.0 to 50.0% and Mg: 7.00 to 15.00% by mass%, and the peak intensity ratio of (Al - O + Mg - O) / Zn - O is 10 or more.

[0061] Effects of the Invention

[0062] 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 can suppress LME cracks during spot welding after hot stamping forming. Detailed embodiments

[0063] <Plated steel sheet>

[0064] The plated steel sheet according to an embodiment of the present invention is characterized by comprising a base steel sheet and a plating layer formed on the surface of the base steel sheet.

[0065] The plating layer has the following chemical composition.

[0066] Containing by mass%:

[0067] Al: 0.5 to 50.0%,

[0068] Mg: 0.50 to 15.00%,

[0069] Si: 0 to 4.0%, and

[0070] Fe: 0 to 15.0%,

[0071] Further containing in a total amount of 5.000% or less:

[0072] Ni: 0 to 1.000%,

[0073] Ca: 0 to 3.0%,

[0074] Sb: 0 to 0.500%,

[0075] Pb: 0 to 0.500%,

[0076] Cu: 0 to 1.000%,

[0077] Sn: 0 to 1.000%,

[0078] Ti: 0 to 1.000%,

[0079] Cr: 0 to 1.000%,

[0080] Nb: 0 to 1.000%,

[0081] Zr: 0 to 1.000%,

[0082] Mn: 0 to 1.000%,

[0083] Mo: 0 to 1.000%,

[0084] Ag: 0 to 1.000%,

[0085] Li: 0 to 1.000%,

[0086] La: 0 to 0.500%,

[0087] Ce: 0 to 0.500%,

[0088] B: 0 to 0.500%,

[0089] Y: 0 to 0.500%,

[0090] Sr: 0 to 0.500%,

[0091] In: 0 to 0.500%,

[0092] Co: 0 to 0.500%,

[0093] Bi: 0 to 0.500%,

[0094] P: 0 to 0.500%, and

[0095] W: at least one of 0 to 0.500%,

[0096] Balance: composed of Zn and impurities,

[0097] The depth at which 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 coating is 3 to 100 μm,

[0098] 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%,

[0099] The coating amount of the coating is 20 g / m 2 or more,

[0100] The coating has an oxide layer on the surface. When the oxide layer is measured by XPS, the peak intensity ratio of (Al - O + Mg - O) / Zn - O is 3 or more.

[0101] Here, Al - O, Mg - O, and Zn - O represent the intensities of the peaks attributed to the Al - O, Mg - O, and Zn - O bonds, respectively.

[0102] As described above, when a hot - stamped 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, etc., it is known that carbon contained in steel is an element that promotes such LME cracks. Therefore, it is considered that by reducing the carbon concentration in the surface layer of the steel that generates LME cracks, such as by 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 of the steel is limited and sometimes not satisfactory.

[0103] The inventors have conducted various studies and found that: even from the perspective of improving LME resistance, by reducing the carbon concentration in the surface layer of the base steel plate through decarburization or the like, during high-temperature heating in hot stamping forming, the carbon contained in the base steel plate diffuses to the steel surface layer, and through such re-carbonization of the steel surface layer, the LME suppression effect based on low carbon concentration in the surface layer of the original base steel plate disappears or decreases. Therefore, the inventors have further conducted research and found that by forming a structure capable of suppressing such re-carbonization in the surface layer of the base steel plate, even when the Zn-containing coating contains a specified adhesion amount in order to maintain sufficient corrosion resistance, the LME suppression effect brought about by low carbon concentration in the surface layer of the original base steel plate 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, as will be described in detail later in connection with the manufacturing method of the hot stamping formed body, the inventors have found that by forming a specified oxide layer on the surface of the coating and making the adhesion amount of the coating 20 g / m per single side 2 As described above, even during high-temperature heating in hot stamping forming, the evaporation of Zn and / or Mg is suppressed or reduced to maintain sufficient corrosion resistance, and in the surface layer of the base steel plate, a structure is formed with a depth of 3 to 100 μm and a pearlite area ratio of 0 to 20% in the plate thickness direction starting from the interface between the base steel plate and the coating, and the pearlite area ratio with an equivalent circle diameter of 5 μm or more at the depth where the pearlite area ratio is 0 to 20% is controlled to 0 to 30%. Thus, the generation of LME cracks during spot welding after hot stamping forming can be reliably suppressed or reduced.

[0104] It is not intended to be bound by any specific theory, but it is considered that in the plated 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-carburization 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 low-carbon concentration. Here, in the plated 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 in such a manner that the area ratio of pearlite in the plate thickness direction from the interface between the base steel sheet and the plating layer becomes 0 to 20%, that is, the depth of the region with a relatively low area ratio of pearlite becomes 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 re-carburization path of carbon). 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 tends to diffuse toward the surface side. At this time, when there is a re-carburization path of carbon caused by austenite along the grain boundaries as described above, the carbon in the bulk diffuses toward the surface side through this re-carburization path, thereby promoting the re-carburization 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 plated 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%, and the amount of relatively large pearlite is reduced. 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 re-carburization path of carbon caused by austenite can be reliably truncated.

[0105] 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 plate, thereby forming a carbon re-carbonization path. As a result, the diffusion of carbon from the bulk of the base steel plate to the surface side is promoted. In connection with this, in the coated steel plate according to an embodiment of the present invention, it is important that the pearlite in the surface layer portion of the base steel plate, that is, in the depth region of 3 to 100 μm in the plate thickness direction from the interface between the base steel plate and the coating layer, is reduced to 0 to 20% by area ratio, and the relatively coarse pearlite in this depth region, 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. With such a surface layer portion structure, even during high-temperature heating in hot stamping forming, the amount of austenite transformed from pearlite can be reduced, and furthermore, the austenite can be dispersed and present at the grain boundaries. Therefore, the carbon re-carbonization path caused by austenite can be reliably cut off. Thus, in the coated steel plate according to an embodiment of the present invention, although the amount of 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 low carbon concentration in the surface layer portion of the original base steel plate can be fully exerted, and the generation of LME cracks during subsequent spot welding can be reliably suppressed or reduced. In the coated steel plate 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 portion structure of the base steel plate was first clearly demonstrated by the inventors of the present invention this time. Therefore, the coated steel plate according to an embodiment of the present invention is particularly useful in the automotive field where a large number of spot welds are used.

[0106] Hereinafter, the coated steel plate 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 "mass%". In addition, in this specification, the "~" indicating a numerical range is used in the meaning of including the values described before and after it as the lower limit value and the upper limit value without special description.

[0107] [Coating layer]

[0108] According to an embodiment of the present invention, the coating layer is formed on the surface of the base steel plate, for example, formed on at least one side of the base steel plate, and preferably formed on both surfaces. The coating layer has the following chemical composition.

[0109] [Al: 0.5 to 50.0%]

[0110] Al is an element effective in improving the corrosion resistance of the coating. To fully obtain such an effect, the Al content is 0.5% or more. The Al content can be 1.0% or more, 3.0% or more, 5.0% or more, 8.0% or more, 10.0% or more, 15.0% or more, or 20.0% or more. On the other hand, when Al is excessively contained, the amount of Zn required to impart sacrificial corrosion protection decreases. Therefore, the Al content is 50.0% or less. The Al content can be 45.0% or less, 40.0% or less, 35.0% or less, or 30.0% or less.

[0111] [Mg: 0.50 - 15.00%]

[0112] Mg is an element effective in improving the corrosion resistance of the coating. To fully obtain such an effect, the Mg content is 0.50% or more. The Mg content can be 0.51% or more, 0.52% or more, 0.53% or more, 0.55% or more, 0.60% or more, 0.80% or more, 1.00% or more, 1.50% or more, 2.00% or more, or 3.00% or more. On the other hand, when Mg is excessively contained, film swelling and flowing rust sometimes occur due to excessive sacrificial corrosion protection. Therefore, the Mg content is 15.00% or less. The Mg content can be 12.00% or less, 10.00% or less, 9.00% or less, 8.00% or less, 7.00% or less, 6.00% or less, or 5.00% or less.

[0113] [Si: 0 - 4.0%]

[0114] 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 0.01% or more, 0.05% or more, 0.1% or more, 0.5% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, from the viewpoint of improving the coating adhesion of the coating, the Si content can be 4.0% or less. The Si content can be 3.5% or less, 3.0% or less, 2.7% or less, or 2.5% or less.

[0115] [Fe: 0 - 15.0%]

[0116] Fe is an element that can be contained in the coating layer, for example, by being dissolved from the base metal steel plate into the plating bath or reacting with Al during the plating process to form an Fe-Al barrier layer at the interface between the base metal steel plate and the coating layer. 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 layer up to about 15.0%, but if it is in this range, it will not have an adverse effect on the plated steel sheet according to the embodiment of the present invention. Therefore, the Fe content is set to 15.0% or less, and for example, it can be 12.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, 3.0% or less, 1.0% or less, or 0.8% or less.

[0117] Furthermore, the coating layer may optionally contain at least one of Ni: 0 to 1.000%, Ca: 0 to 3.0%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, and W: 0 to 0.500%. These optional elements are not particularly limited, and preferably, the total is 5.000% or less. The total of the optional elements can 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.

[0118] [Ni: 0 to 1.000%]

[0119] Ni is an element effective in improving the corrosion resistance of the coating. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.0001% or more. The Ni content may be 0.0004% or more, 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more. The upper limit is not particularly limited, and from the viewpoints of manufacturing cost and the like, the Ni content is set to 1.000% or less, and for example, it may be 0.980% or less, 0.950% or less, 0.900% or less, 0.700% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0120] [Ca: 0 to 3.0%]

[0121] Ca is an element effective in ensuring the wettability of the plating bath. The Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.01% or more. The Ca content may be 0.05% or more, 0.1% or more, 0.5% or more, or 1.0% or more. On the other hand, when Ca is contained excessively, 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 3.0% or less. The Ca content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0122] [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%]

[0123] 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.

[0124] In the coating, the balance other than the above elements consists of Zn and impurities. Impurities in the coating refer to components such as those mixed in due to various reasons in the manufacturing process, typically represented by raw materials, during the manufacture of the coating.

[0125] [Determination of the Chemical Composition of the Coating]

[0126] 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.

[0127] As the coating, any coating having the above chemical composition and having the following oxide layer on the surface may be used, and there is no particular limitation. For example, it may be a hot-dip coating, an alloyed hot-dip coating, etc.

[0128] [Coating Adhesion Amount: 20 g / m² per Single Side 2 or more]

[0129] In an embodiment of the present invention, the coating adhesion amount is 20 g / m² per single side 2As described above. Generally, during high-temperature heating in hot stamping forming, the coating sometimes alloyizes with the base steel plate, resulting in a decrease in corrosion resistance. However, according to the embodiments of the present invention, although the reason is not 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 coating where the area ratio of pearlite in the plate thickness direction is 0 to 20% 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 coating and the base steel plate. Therefore, it is considered that by making the coating attachment amount relatively large, specifically controlled to be 20 g / m per single side 2 As described above, when applied to hot stamping forming, there is a coating where alloying is not fully carried out. Due to the existence of such a coating, sufficient corrosion resistance can be maintained. On the other hand, when the coating attachment amount 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 attachment amount is preferably 25 g / m per single side 2 or more, or 30 g / m 2 or more, more preferably 40 g / m 2 or more, still more preferably 50 g / m 2 or more, and most preferably 60 g / m 2 or more. There is no particular limitation on the upper limit. For example, the coating attachment amount can be 200 g / m 2 or less, 180 g / m 2 or less, 170 g / m 2 or less, 160 g / m 2 or less, or 150 g / m 2 or less.

[0130] [Measurement of Coating Attachment Amount]

[0131] The coating attachment amount is determined as follows. First, a 30 mm × 30 mm sample is collected from the coated steel plate. Then, the coating is stripped and dissolved from this sample using an acid solution containing an inhibitor that inhibits the corrosion of the base steel plate, and the coating attachment amount is determined based on the weight change of the sample before and after stripping and dissolution. The type of acid is not particularly limited, and any acid that can dissolve the coating can be used.

[0132] [Peak Intensity Ratio Based on XPS Measurement: (Al - O + Mg - O) / Zn - O ≥ 3]

[0133] In an embodiment of the present invention, the coating has an oxide layer on its surface. When the oxide layer is measured by XPS (X-ray photoelectron spectroscopy), the peak intensity ratio of (Al-O + Mg-O) / Zn-O is 3 or more. Here, Al-O, Mg-O, and Zn-O respectively represent the intensities of the peaks attributed to the Al-O, Mg-O, and Zn-O bonds. For example, when a conventional Zn-based coated steel sheet or Al-Zn-based coated steel sheet is used in hot stamping forming, generally, the coated steel sheet is heated to about 900 °C or a higher temperature in hot stamping forming. The boiling point of Zn is about 907 °C, which is relatively low. Therefore, sometimes part of the Zn in the coating evaporates at such a high temperature, resulting in a decrease in the corrosion resistance after hot stamping forming. In addition, for Mg added to the Zn-based coated steel sheet or Al-Zn-based coated steel sheet, during heating in hot stamping forming at a high temperature, part of it also evaporates, and like the case of Zn, sometimes it causes a decrease in the corrosion resistance after hot stamping forming. In contrast, Al oxide and Mg oxide are oxides that are more firm and / or dense than Zn oxide. Therefore, by forming an oxide layer on the surface of the coating with a peak intensity ratio of (Al-O + Mg-O) / Zn-O of 3 or more based on XPS measurement, that is, the surface of the coating is covered with a relatively large amount of Al oxide and / or Mg oxide, it is possible to suppress or reduce the evaporation of Zn and / or Mg in the coating even when applied to hot stamping forming. In addition, in connection with this, it is possible to maintain a relatively high state of the Zn and Mg concentrations in the coating of the obtained hot stamping formed body, and relatively reduce the Fe concentration, and improve the corrosion resistance after hot stamping forming. From the viewpoint of further improving the corrosion resistance, the higher the peak intensity ratio of (Al-O + Mg-O) / Zn-O based on XPS measurement, the more preferable it is. For example, it can be 5 or more, 8 or more, or 10 or more. The peak intensity ratio of (Al-O + Mg-O) / Zn-O based on XPS measurement can be controlled within a desired range by appropriately adjusting the chemical composition of the coating, in addition to the dew point control during cooling after plating, which will be described in detail later in connection with the manufacturing method of the hot stamping formed body. For example, by making the chemical composition of the coating contain Al: 10.0 to 50.0% and Mg: 4.00 to 15.00% by mass, the above peak intensity ratio can be controlled to be 5 or more. Similarly, by making the chemical composition of the coating contain Al: 30.0 to 50.0% and Mg: 7.00 to 15.00% by mass, the above peak intensity ratio can be controlled to be 10 or more. The upper limit of the above peak intensity is not particularly limited. For example, the peak intensity ratio of (Al-O + Mg-O) / Zn-O based on XPS measurement can be 20 or less, 18 or less, or 15 or less.

[0134] The measurement of the oxide layer on the surface of the coating based on XPS is carried out at a position 5 nm in the thickness direction of the coating surface under the following conditions.

[0135] X-ray source: mono-Al Kα (1486.6 eV)

[0136] X-ray diameter: 50 - 200 μm

[0137] Measurement area: 100 - 700 μm × 100 - 700 μm

[0138] Vacuum degree: 1×10 -10 ~1×10 -11 torr (1 torr: 133.32 Pa)

[0139] Accelerating voltage: 1 - 10 kV

[0140] Here, the peak attributed to the Al-O bond is the peak observed in the XPS spectrum focusing on Al 2p3 / 2 within the range of 72 - 76 eV. The peak attributed to the Mg-O bond is the peak observed in the XPS spectrum focusing on Mg 2p3 / 2 within the range of 48 - 52 eV. The peak attributed to the Zn-O bond is the peak observed in the XPS spectrum focusing on Zn 2p3 / 2 within the range of 1018 - 1024 eV. The intensity of each peak, on the basis of considering the baseline of the peak, is set as the intensity obtained by subtracting the intensity Ib of the baseline from the intensity Ip of the peak (i.e., "Ip - Ib"). Based on the intensities of each peak thus obtained, the value of (Al-O + Mg-O) / Zn-O is calculated.

[0141] [Depth at which the area ratio of pearlite in the thickness direction from the interface between the base metal steel plate and the coating is 0 - 20%: 3 - 100 μm]

[0142] 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 material steel plate and the coating is 3 to 100 μm. This feature is associated with the low-carbon concentration in the surface layer of the base material steel plate. Therefore, by having this feature, the LME suppression effect brought about by the low-carbon concentration in the surface layer of the base material 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 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 carbon re-carbonization path 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 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 material 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. The upper limit of the depth can be, for example, 90 μm or 80 μm.

[0143] [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 material steel plate and the coating: 0 to 30%]

[0144] In an embodiment of the present invention, 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 material steel plate and the coating is 0 to 30%. As described above, by controlling the area ratio of pearlite with an equivalent circle diameter of 5 μm or more within the range of 0 to 30% in the relatively low-depth region, 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 the grain boundaries, thereby reliably truncating the carbon re-carbonization path caused by austenite. Therefore, 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 initial surface layer of the base material steel plate 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 effects, 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 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 with an equivalent circle diameter of 5 μm or more can be, for example, 1% or 3%.

[0145] [Measurement of the depth of the area ratio of pearlite of 0 to 20% and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more]

[0146] The depth of the pearlite in the microstructure in the surface layer portion of the base metal steel sheet where the area ratio of pearlite is 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 sheet. 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 a width of 500 μm in the direction perpendicular to the plate thickness direction starting from the interface between the base metal steel sheet and the coating is set as one field of view, and five fields of view are measured for the total of the five samples. The interface between the base metal steel sheet and the coating can be discriminated based on the difference in color tone between the base metal steel sheet and the coating in the backscattered electron image (BSE image) of the SEM. The area ratio of pearlite is calculated, for example, using a dot algorithm from a microstructure photograph at a magnification of about 5000 times. 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 is defined 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 sheet and the coating. Then, the distance from the determined depth position to the interface is calculated, and their arithmetic mean is determined as the "depth of the pearlite in the plate thickness direction from the interface between the base metal steel sheet and the coating where the area ratio of pearlite is 0 to 20%". 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 pearlite in the plate thickness direction from the interface between the base metal steel sheet and the coating where the area ratio of pearlite is 0 to 20%".

[0147] [Preferred Chemical Composition of Base Metal Steel Sheet]

[0148] As described above, an 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 coating to 20 g / m per single side 2As described above, an oxide layer having a peak intensity ratio of (Al-O + Mg-O) / Zn-O of 3 or more as measured by XPS is formed on the surface of the plating layer, and the depth at which the area ratio of pearlite in the plate thickness direction from the interface between the base steel plate and the plating layer is 0 to 20% is set to 3 to 100 μm, and the area ratio of pearlite having 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%, thereby forming the structure in the surface layer portion of the base steel plate, and thus achieving this object. Therefore, it is clear that the chemical composition of the base steel plate itself is not an essential technical feature for achieving the object of the present invention. Hereinafter, the preferred chemical composition of the base steel plate used in the plated steel sheet of the embodiment of the present invention will be described in detail, but these descriptions are intended only to illustrate the preferred chemical composition of the base 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 base steel plate having such a specific chemical composition.

[0149] In an embodiment of the present invention, for example, the base steel plate preferably has a chemical composition consisting of the following by mass%:

[0150] C: 0.13 to 0.50%,

[0151] Si: 0.001 to 3.000%,

[0152] Mn: 0.30 to 3.00%,

[0153] Al: 0.0002 to 2.000%,

[0154] P: 0.100% or less,

[0155] S: 0.1000% or less,

[0156] N: 0.0100% or less,

[0157] Nb: 0 to 0.15%,

[0158] Ti: 0 to 0.15%,

[0159] V: 0 to 0.15%,

[0160] Mo: 0 to 1.0%,

[0161] Cr: 0 to 1.0%,

[0162] Cu: 0 to 1.0%,

[0163] Ni: 0 to 1.0%,

[0164] B: 0 to 0.0100%,

[0165] W: 0 to 1.000%,

[0166] Hf: 0 to 0.050%,

[0167] Mg: 0 to 0.050%,

[0168] Zr: 0 to 0.050%,

[0169] Ca: 0 to 0.010%,

[0170] REM: 0 to 0.30%,

[0171] Ir: 0 to 1.000%, and

[0172] Balance: Fe and impurities. Hereinafter, each element will be described in more detail.

[0173] [C: 0.13 to 0.50%]

[0174] 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.

[0175] [Si: 0.001 to 3.000%]

[0176] 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.

[0177] [Mn: 0.30 to 3.00%]

[0178] 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.

[0179] [Al: 0.0002 to 2.000%]

[0180] Al acts as a deoxidizer for steel and is an element that has the effect of making the steel sound. 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.

[0181] [P: 0.100% or less]

[0182] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The less the P content, the more preferable it is, and thus 0% is ideal. However, an excessive reduction in the P content sometimes leads to a significant 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 the steel becomes brittle 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.

[0183] [S: 0.1000% or less]

[0184] S is an element that forms non-metallic inclusions such as MnS in steel and causes a decrease in the ductility of steel components. The less the S content, the more preferable it is, and thus 0% is ideal. However, an excessive reduction in the S content sometimes leads to a significant 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.

[0185] [N: 0.0100% or less]

[0186] 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 contained excessively, 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.

[0187] The preferred basic chemical composition of the base steel plate is as described above. Furthermore, the base steel plate may also contain, as required, 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.

[0188] In the base steel plate, the balance other than the above elements consists of Fe and impurities. The 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.

[0189] 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 or the like (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.

[0190] [Thickness of the base steel plate]

[0191] The plate thickness of the base material steel plate is not particularly limited. For example, it is 0.2 mm or more, and it 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 it can also be 5.0 mm or less or 4.0 mm or less.

[0192] <Manufacturing method of coated steel plate>

[0193] Next, a preferred manufacturing method of the coated steel plate according to an embodiment of the present invention will be described. The following description is an example of a characteristic method for manufacturing the coated steel plate according to an 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.

[0194] The coated steel plate according to an embodiment of the present invention can be manufactured, for example, by performing the following steps: a casting step of casting molten steel with an adjusted chemical composition to form a steel sheet; a hot rolling step of hot rolling the steel sheet to obtain a hot rolled steel plate; a coiling step of coiling the hot rolled steel plate; a cold rolling step of cold rolling the coiled hot rolled steel plate to obtain a cold rolled steel plate; an annealing step of annealing the cold rolled steel plate; a cooling step of cooling the annealed cold rolled steel plate; and a plating step 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 step, perform pickling, and directly perform the cold rolling step. Hereinafter, each step will be described in detail.

[0195] [Casting step]

[0196] The conditions of the casting step are not particularly limited. For example, after melting using a blast furnace, an electric furnace, etc., various secondary smelting is performed, and then casting can be performed using a normal continuous casting, a casting method using an ingot method, or a thin slab casting method.

[0197] [Hot rolling step]

[0198] The cast steel sheet is hot rolled to obtain a hot rolled steel plate. The hot rolling step is performed by directly or temporarily cooling the cast steel sheet and then reheating and performing hot rolling. In the case of reheating, the heating temperature of the steel sheet can be, for example, 1100 to 1250 °C. In the hot rolling step, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling can be appropriately changed according to the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling can be 900 to 1050 °C, and the reduction ratio of finish rolling can be 10 to 50%.

[0199] [Coiling step]

[0200] 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 to impart a specified heat treatment to the hot-rolled steel sheet. As an alternative, it is also possible to perform pickling after the hot-rolling process and then perform the subsequent cold-rolling process without performing the coiling process.

[0201] [Cold-rolling process]

[0202] After pickling the hot-rolled steel sheet, etc., 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 sheet thickness, and can be, for example, 20 to 80%. After the cold-rolling process, it can be air-cooled to room temperature, for example.

[0203] [Annealing process]

[0204] 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 with 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 in the thickness direction where the area ratio of pearlite is 0 to 20% starting from the interface between the base 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 in the thickness direction where the area ratio of pearlite is 0 to 20% starting from the interface between the base steel sheet and the plating layer more than 3 μm. 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 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).

[0205] [Cooling process]

[0206] In order to obtain a desired surface layer structure in a cold-rolled steel sheet decarburized in the annealing process in the surface layer portion, it is necessary to cool it appropriately in the subsequent cooling process. Specifically, the cooling process includes: cooling from the heating temperature in the annealing process to a control temperature of 620 to 670 °C (primary cooling) at an average cooling rate of 20 °C / s or more; 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.

[0207] [Primary cooling]

[0208] In the primary cooling, it is important to suppress the precipitation of pearlite at high temperatures. More specifically, the diffusion of pearlite precipitated at high temperatures from the heating temperature of 730 to 900 °C in the annealing process to the control temperature of 620 to 670 °C is fast, so it is easy to diffuse along the grain boundaries and form pearlite along the grain boundaries after precipitation. The pearlite formed along the grain boundaries undergoes austenite phase transformation during the high-temperature heating of hot stamping forming, thereby forming a carbon re-carburization path caused by austenite along the grain boundaries, which promotes the re-carburization of carbon in the bulk to the steel surface layer portion. Therefore, in the temperature range from the heating temperature in the annealing process to the above control temperature, it is extremely important to cool the cold-rolled steel sheet at a relatively fast average cooling rate of 20 °C / s or more to suppress the precipitation of pearlite at high temperatures in the steel sheet surface layer portion. 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, so the formation of pearlite along the grain boundaries is promoted. As a result, in the finally obtained plated 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 is 0 to 20% in the plate thickness direction from the interface between the base steel sheet and the plating layer exceeds 30%. In the case of being applied to hot stamping forming, sufficient LME resistance cannot be achieved during subsequent spot welding.

[0209] [Secondary cooling]

[0210] On the other hand, in the secondary cooling after the primary cooling, it is important to precipitate pearlite at a low temperature 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. The diffusion of pearlite precipitated in the low-temperature region below such a control temperature is relatively slow, so it will not form in a form connected along the grain boundaries, and pearlite can be dispersed and present on the grain boundaries. In the case of such a structure, even during the high-temperature heating of hot stamping forming, it is possible to c1Above a certain point, austenite transformed from pearlite is also dispersed at grain boundaries, so that the re-carbonization path of carbon 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 rather than pearlite mainly precipitate. In the finally obtained plated steel sheet, it is impossible to make the depth with a pearlite area ratio of 0 to 20% in the plate thickness direction starting from the interface between the base metal steel sheet and the coating layer be 100 μm or less. Compared with pearlite, martensite and bainite have a faster phase transformation rate into austenite, slightly higher than the A c1 point and immediately transform into austenite. Therefore, compared with the case of pearlite, the exposure time to high temperature in the two-phase structure of ferrite and austenite during hot stamping forming becomes longer. Also in such a case, it is easy to form a re-carbonization path at grain boundaries, so sufficient LME resistance cannot be achieved.

[0211] [Plating process]

[0212] Next, in the plating process, a coating layer having the chemical composition described above is formed on at least one surface, preferably both surfaces, of the cold-rolled steel sheet (base metal steel sheet). More specifically, for example, the plating process can be carried out by hot-dip plating using a plating bath whose composition has been adjusted so that the chemical composition of the coating layer falls within the range described above (plating bath temperature: for example, 420 to 480 °C), and an alloying treatment is carried out after the 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 as long as the thickness and adhesion amount of the coating layer are considered. 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 coating layer can be adjusted to a specified range, for example, 20 to 200 g / m per single side 2 within the range.

[0213] [Cooling after plating]

[0214] When cooling after plating, it is necessary to control the dew point of the cooling gas (such as nitrogen) within the range of -10 to 10°C. By cooling the plated steel sheet in an atmosphere with such a relatively high dew point, an oxide layer containing a relatively large amount of Al oxide and Mg oxide can be formed on the surface of the plating layer. More specifically, an oxide layer with a peak intensity ratio of (Al-O + Mg-O) / Zn-O determined by XPS measurement of 3 or more can be formed. Due to the formation of such an oxide layer, even during high-temperature heating in hot stamping forming, the evaporation of Zn and / or Mg in the plating layer can be significantly suppressed or reduced. Furthermore, the Fe concentration in the plating layer of the obtained hot stamping formed body can be relatively reduced. Therefore, the corrosion resistance after hot stamping forming can be further improved. In addition to controlling the dew point of the cooling gas during cooling after plating within the range of -10 to 10°C, by appropriately adjusting the chemical composition of the plating layer, the peak intensity ratio of (Al-O + Mg-O) / Zn-O determined by XPS measurement can be controlled within a desired range. For example, in addition to the above dew point control, by making the chemical composition of the plating layer contain Al: 10.0 to 50.0% and Mg: 4.00 to 15.00% by mass, the above peak intensity ratio can be controlled to be 5 or more. Similarly, by making the chemical composition of the plating layer contain Al: 30.0 to 50.0% and Mg: 7.00 to 15.00% by mass, the above peak intensity ratio can be controlled to be 10 or more.

[0215] The plated steel sheet manufactured by this manufacturing method can set the coating amount of the plating layer to 20 g / m per single side 2 or more, form an oxide layer with a peak intensity ratio of (Al-O + Mg-O) / Zn-O determined by XPS measurement of 3 or more on the surface of the plating layer, set the depth with a pearlite area ratio of 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 with a pearlite area ratio of 0 to 20% to 0 to 30% in the surface layer part of the base steel sheet. Therefore, even when exposed to high temperatures such as in hot stamping forming, high corrosion resistance can be maintained, and the re-carbonization of carbon in the bulk to the steel surface layer part can be significantly suppressed. Therefore, the LME suppression effect brought about by the low carbon concentration in the surface layer part 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.

[0216] 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.

[0217] Example

[0218] 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.

[0219] First, molten steel was cast by a continuous casting method to form a steel sheet having the chemical composition shown in Table 1. After the steel sheet was temporarily cooled, it was reheated to 1200 °C and hot-rolled, and then coiled at a temperature of 600 °C or lower. The hot rolling was carried out by rough rolling and finish rolling, the finishing temperature of the finish rolling was 900 - 1050 °C, and the reduction ratio of the finish rolling was 30%. Then, pickling was performed on the obtained hot-rolled steel sheet, and then cold rolling was carried out at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm. Then, for the obtained cold-rolled steel sheet, an annealing process was carried out in a furnace with an oxygen concentration of 20 ppm or less in a mixed gas atmosphere of 4% hydrogen and nitrogen balance under the conditions shown in Table 2, and then a cooling process was carried out under the conditions shown in Table 2 in the same manner to manufacture a base material steel sheet.

[0220] Next, the manufactured base material steel sheet was cut into 100 mm × 200 mm, and the base material steel sheet was plated using an intermittent melting plating test apparatus manufactured by our company. More specifically, first, the manufactured base material steel sheet was immersed in a plating bath having a prescribed chemical composition for about 3 seconds, and then it was lifted at a lifting speed of 20 - 200 mm / s, and the amount of deposit of the coating layer was adjusted to the value shown in Table 2 by wiping with N2 gas. Then, the base material steel sheet with the coating layer attached was cooled from the plating bath temperature (about 420 - 480 °C) to room temperature using nitrogen gas controlled at the dew point shown in Table 2 as a cooling gas, thereby obtaining a plated steel sheet having coating layers formed on both sides of the base material steel sheet. The sheet temperature was measured using a thermocouple spot-welded to the center of the base material steel sheet.

[0221] The physical properties and characteristics of the obtained plated steel sheet were measured and evaluated by the following methods.

[0222] [Analysis of Chemical Composition of Coating Layer]

[0223] The chemical composition of the coating layer was determined as follows: A sample cut into 30 mm × 30 mm was immersed in a 10% HCl aqueous solution containing an inhibitor. After pickling and peeling the coating layer, the plating components dissolved in the aqueous solution were measured by ICP emission spectrometry. The results are shown in Table 2.

[0224] [Evaluation of LME Resistance during Spot Welding]

[0225] 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 two samples of the coated steel sheet after heating and quenching, each with dimensions 50 mm × 100 mm. For these two coated steel sheet specimens, use a welding electrode with a dome radius type and a tip diameter of 8 mm, and perform spot welding at a welding angle of 2°, a pressing force of 4.0 kN, a welding 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 outside the shoulder of the welded part and evaluate the LME resistance as follows.

[0226] AAA: 0 μm

[0227] AA: Exceeding 0 - 20 μm

[0228] A: Exceeding 20 μm and less than 80 μm

[0229] B: 80 μm or more

[0230] [Evaluation of Corrosion Resistance]

[0231] 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 dimensions 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 perform 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 a composite cyclic corrosion test according to JASO (M609 - 91), measure the number of cycles until red rust is generated from the cut part, and evaluate the corrosion resistance as follows.

[0232] AAA: Exceeding 240 cycles

[0233] AA: 180 - 240 cycles

[0234] A: Less than 90 - 180 cycles

[0235] B: Less than 90 cycles

[0236] [Evaluation of Hardness]

[0237] First, in the same manner as in the evaluation of corrosion resistance, a test piece is cut out from an arbitrary position other than the end of the plated steel sheet except for heating and quenching so as to be able to observe a cross-section perpendicular to the surface (plate thickness cross-section). The plate thickness cross-section of the test piece is polished with #600 to #1500 silicon carbide sandpaper, and then finished 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. This plate thickness cross-section is used as the measurement surface. Then, 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 steel sheet in a manner that does not 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.

[0238] AAA: The hardness after HS exceeds 550 HV

[0239] AA: The hardness after HS exceeds 500 to 550 HV

[0240] A: The hardness after HS is 400 to 500 HV

[0241] B: The hardness after HS is less than 400 HV

[0242] The cases where the evaluation of LME resistance is AAA, AA, and A, and the evaluation of 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 suppress LME cracks during spot welding after hot stamping forming. The results are shown in

[0243] Table 2.

[0244] Table 1

[0245]

[0246] Table 2-1

[0247]

[0248] Table 2-2

[0249]

[0250] 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 from the interface between the base steel sheet and the coating layer in the thickness direction where the area ratio of pearlite is 0 to 20% be 3 μm or more, 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 where the area ratio of pearlite is 0 to 20% be 3 μm or more, 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 where the area ratio of pearlite is 0 to 20% be 3 μm or more, 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 is 0 to 20% from the interface between the base steel sheet and the coating layer in the thickness direction 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 from the interface between the base steel sheet and the coating layer in the thickness direction where the area ratio of pearlite is 0 to 20% be the desired depth, 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 where the area ratio of pearlite is 0 to 20% be the desired depth, and the LME resistance is reduced. In addition, in Comparative Examples 35 and 42, since the dew point of the cooling gas after plating is low, it is impossible to form the desired oxide layer on the surface of the coating layer. As a result, the peak intensity ratio of (Al-O + Mg-O) / Zn-O measured by XPS is less than 3, and the corrosion resistance is reduced.

[0251] In contrast, the plated steel sheets in all the examples have a specified plating chemical composition, and the coating amount is set to 20 g / m per single side 2As described above, an oxide layer having a peak intensity ratio of (Al-O + Mg-O) / Zn-O of 3 or more based on XPS measurement is formed on the surface of the plating layer. The depth at which the area ratio of pearlite in the plate thickness direction from the interface between the base steel plate and the plating layer is 0 to 20% is set to 3 to 100 μm, and the area ratio of pearlite having 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 portion of the original base 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 13 to 34 where the depth at which the area ratio of pearlite is 0 to 20% is set to 30 to 100 μm and the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at this depth is controlled to 0 to 15%, the evaluation of LME resistance is AAA, and the LME resistance is further improved. In addition, in Examples 6 to 8, 16, 17, 24 to 27, 33, and 34 where the Al and Mg contents in the plating layer are 10.0 mass% or more and 4.00 mass% or more, respectively, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O based on XPS measurement is 5 or more, the evaluation of corrosion resistance is AA, and the corrosion resistance can be further improved compared to Example 1 where the evaluation of corrosion resistance is A. Similarly, in Examples 9 to 12, 18 to 20, and 28 to 32 where the Al and Mg contents in the plating layer are 30.0 mass% or more and 7.00 mass% or more, respectively, and the peak intensity ratio of (Al-O + Mg-O) / Zn-O based on XPS measurement is 10 or more, the evaluation of corrosion resistance is AAA, and the corrosion resistance can be 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: Al：0.5～50.0％、 Mg: 0.50 - 15.00%, Si: 0 - 4.0%, and Fe: 0 - 15.0%, Furthermore, it contains in a total amount of 5.000% or less: Ni: 0 - 1.000%, Ca: 0 - 3.0%, Sb: 0 - 0.500%, Pb: 0 - 0.500%, Cu: 0 - 1.000%, Sn: 0 - 1.000%, Ti: 0 - 1.000%, Cr：0～1.000％、 Nb: 0 - 1.000%, Zr：0～1.000％、 Mn: 0 - 1.000%, Mo: 0 - 1.000%, Ag: 0 - 1.000%, Li: 0 - 1.000%, La: 0 - 0.500%, Ce: 0 - 0.500%, B：0～0.500％、 Y：0～0.500％、 Sr：0～0.500％、 In: 0 - 0.500%, Co: 0 - 0.500%, Bi: 0 - 0.500%, P: 0 - 0.500%, and at least one of W: 0 - 0.500%, The balance: consists of Zn and impurities. The depth at which the area ratio of pearlite is 0 - 20% in the plate thickness direction starting from the interface between the base metal steel plate and the plating layer is 3 - 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 - 20% is 0 - 30%. The coating amount is 20 g / m per single side 2 or more. The plating layer has an oxide layer on the surface. When measuring the oxide layer by XPS, the peak intensity ratio of (Al - O + Mg - O) / Zn - O is 3 or more. Here, Al - O, Mg - O, and Zn - O respectively represent the intensities of the peaks attributed to the Al - O, Mg - O, and Zn - O bonds.

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

3. The coated steel sheet according to claim 2, wherein The depth at which the area ratio of pearlite is 0 - 20% is 30 - 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 - 20% is 0 - 15%.

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

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

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