Hot press molded body

Abstract

Provided is a hot-press molded body which is provided with a steel base material and a plating layer disposed on the surface of the steel base material, and which is characterized in that the plating layer has a prescribed chemical composition, the average C concentration from the surface of the steel base material up to 1 [mu] m in the depth direction is 0.25 mass% or less, and the steel base material contains 90% or more of martensite in terms of area ratio, the plating layer contains an [eta]-Zn phase, the amount of the plating layer deposited per surface is 50 g / m2 or more, and the hot-press molded article has a Vickers hardness of 400 HV or more.

Description

Technical Field

[0001] The present invention relates to a hot stamping formed body. Background Art

[0002] In recent years, in the automotive industry, from the viewpoint of improving fuel efficiency, weight reduction of vehicle bodies has been required. In order to achieve both weight reduction of vehicle bodies and collision safety, high-strengthening of steel sheets used in frame members and the like is one of the effective methods, and development of high-strength steel sheets has been promoted based on such a background.

[0003] As a technique for press-forming a material that is difficult to form such as a high-strength steel sheet, 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 material is soft and has good formability during forming. Therefore, it is known that even a high-strength steel material can be accurately formed into a complex shape, and since quenching is performed simultaneously with forming by a pressing die, the formed steel material has sufficient strength.

[0004] In connection therewith, various studies have also been conducted on hot stamping formed bodies having Zn plating.

[0005] For example, Patent Document 1 describes a steel sheet covered with a metal coating, the metal coating containing 2.0 to 24.0% by weight of zinc, 7.1 to 12.0% by weight of silicon, optionally 1.1 to 8.0% by weight of magnesium, and additional elements selected from Pb, Ni, Zr, or Hf, the weight content ratio of each additional element being less than 0.3% by weight, the balance being aluminum and any inevitable impurities and residual elements, wherein the Al / Zn ratio exceeds 2.9, and teaches that a component obtained by hot stamping forming of the steel sheet exhibits high substitution corrosion resistance.

[0006] If a plated steel sheet having Zn plating as described in Patent Document 1 is used in hot stamping forming, since the plated steel sheet is processed at a high temperature (for example, about 900° C.), Zn contained in the plating layer may be processed in a molten state. Therefore, the molten Zn may invade the steel and cause cracking inside the steel sheet. It is known that such a phenomenon is called liquid metal embrittlement (LME), and the fatigue characteristics of the steel sheet are reduced due to this LME.

[0007] Associated therewith, Patent Document 2 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, optionally containing 1.1 to 8.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0% by weight, and optionally containing an additional element selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less than 0.3% by weight, with the balance being aluminum, unavoidable 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 pressing 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 bainite. Further, Patent Document 2 teaches that: according to the above manufacturing method, a hardened component without LME can be obtained.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-528324

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2018-527462 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] After hot stamping forming, the coating may alloy with the base metal (steel base material), resulting in a reduction in corrosion resistance. On the other hand, even if, in order to address this situation, the coating contains a high-Zn concentration phase formed from intermetallic compounds mainly composed of Zn with relatively high corrosion resistance, since such a high-Zn concentration phase exists as a liquid phase at the high temperature during hot stamping forming, there is also a problem of increasing the risk of LME cracking. In addition, it is known that LME cracking generally becomes significant in the case of steel materials with relatively high strength, and the higher the strength of the steel material, the more likely the sensitivity of LME cracking is to increase. On the other hand, in the automotive industry and the like, further lightweighting of steel materials is also required. In order to achieve such lightweighting, it is necessary to increase the strength of the steel material more than before. Therefore, even in the case of achieving the same or higher strength as before, there is a high demand for steel materials that can solve the problem of LME cracking, and more specifically, for hot stamping formed bodies.

[0014] Accordingly, an object of the present invention is to provide a hot stamping formed body that is high-strength and has excellent LME resistance during hot stamping forming, and furthermore, can maintain high corrosion resistance even after hot stamping forming.

[0015] Means for Solving the Problem

[0016] The inventors of the present invention conducted research to achieve the above object, and as a result, first found that by forming a Zn-containing coating added with an appropriate amount of Al with an adhesion amount of a specified amount or more, sufficient corrosion resistance can be maintained even when applied to hot stamping forming. In addition, the inventors of the present invention found that although having a high Vickers hardness of 400 HV or more, by appropriately modifying the structure of the surface layer portion of the steel base material before hot stamping forming to make the C concentration in the surface layer portion of the steel base material after hot stamping forming relatively low, even a coating formed with the above relatively large coating adhesion amount can significantly suppress or reduce the generation of LME cracking during high-temperature heating of hot stamping forming, thereby completing the present invention.

[0017] The present invention that has achieved the above object is as follows.

[0018] (1) A hot stamping formed body, characterized by comprising a steel base material and a coating disposed on the surface of the steel base material,

[0019] The coating has the following chemical composition: containing, by mass%:

[0020] Al: 5.0 to 80.0%,

[0021] Si: 0 to 15.0%, and

[0022] Fe: 15.0 to 70.0%,

[0023] Further contains at least one of the following elements in a total amount of 5.000% or less:

[0024] Mg: 0% or more and less than 0.500%,

[0025] Ni: 0% or more and less than 0.500%,

[0026] Ca: 0 to 3.000%,

[0027] Sb: 0 to 0.500%,

[0028] Pb: 0 to 0.500%,

[0029] Cu: 0 to 1.000%,

[0030] Sn: 0 to 1.000%,

[0031] Ti: 0 to 1.000%,

[0032] Cr: 0 to 1.000%,

[0033] Nb: 0 to 1.000%,

[0034] Zr: 0 to 1.000%,

[0035] Mn: 0 to 1.000%,

[0036] Mo: 0 to 1.000%,

[0037] Ag: 0 to 1.000%,

[0038] Li: 0 to 1.000%,

[0039] La: 0 to 0.500%,

[0040] Ce: 0 to 0.500%,

[0041] B: 0 to 0.500%,

[0042] Y: 0 to 0.500%,

[0043] Sr: 0 to 0.500%,

[0044] In: 0 to 0.500%,

[0045] Co: 0 to 0.500%,

[0046] Bi: 0 to 0.500%,

[0047] P: 0 to 0.500%, and

[0048] W: 0 to 0.500%,

[0049] The remaining part: composed of Zn and impurities,

[0050] The average C concentration from the surface of the above steel base material to a depth of 1 μm is 0.25 mass% or less,

[0051] The above steel base material contains 90% or more martensite by area ratio,

[0052] The above coating contains an η-Zn phase,

[0053] The coating amount of the above coating is 50 g / m per single side 2 or more,

[0054] The above hot stamping formed body has a Vickers hardness of 400 HV or more.

[0055] (2) The hot stamping formed body according to (1) above, characterized in that the average C concentration is 0.18 mass% or less.

[0056] (3) The hot stamping formed body according to (2) above, characterized in that the average C concentration is 0.10 mass% or less.

[0057] (4) The hot stamping formed body according to any one of (1) to (3) above, characterized in that when the above coating is measured by X-ray diffraction method, the ratio of the peak height derived from the η-Zn phase to the peak height derived from Fe2Al5 is 0.05 or more.

[0058] Advantages of the Invention

[0059] According to the present invention, it is possible to provide a hot stamping formed body having high strength, excellent liquid metal embrittlement resistance during hot stamping, and further maintaining high corrosion resistance even after hot stamping. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic view showing the shape of the hot stamping formed body manufactured in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0061] <Hot stamping formed body>

[0062] The hot stamping formed body according to an embodiment of the present invention is characterized by including a steel base material and a coating disposed on the surface of the steel base material,

[0063] The above coating has the following chemical composition: containing, by mass%:

[0064] Al: 5.0 to 80.0%,

[0065] Si: 0 to 15.0%, and

[0066] Fe: 15.0 to 70.0%,

[0067] Further containing at least one of the following elements in a total amount of 5.000% or less:

[0068] Mg: 0% or more and less than 0.500%,

[0069] Ni: 0% or more and less than 0.500%,

[0070] Ca: 0 to 3.000%,

[0071] Sb: 0 to 0.500%,

[0072] Pb: 0 to 0.500%,

[0073] Cu: 0 to 1.000%,

[0074] Sn: 0 to 1.000%,

[0075] Ti: 0 to 1.000%,

[0076] Cr: 0 to 1.000%,

[0077] Nb: 0 to 1.000%,

[0078] Zr: 0 to 1.000%,

[0079] Mn: 0 to 1.000%,

[0080] Mo: 0 to 1.000%,

[0081] Ag: 0 to 1.000%,

[0082] Li: 0 to 1.000%,

[0083] La: 0 to 0.500%,

[0084] Ce: 0 to 0.500%,

[0085] B: 0 to 0.500%,

[0086] Y: 0 to 0.500%,

[0087] Sr: 0 to 0.500%,

[0088] In: 0 to 0.500%,

[0089] Co: 0 to 0.500%,

[0090] Bi: 0 to 0.500%,

[0091] P: 0 to 0.500%, and

[0092] W: 0 to 0.500%,

[0093] The balance: composed of Zn and impurities,

[0094] The average C concentration from the surface of the above steel base material to a depth of 1 μm in the depth direction is 0.25 mass% or less,

[0095] The above steel base material contains 90% or more martensite by area ratio,

[0096] The above coating contains an η-Zn phase,

[0097] The coating amount of the above coating is 50 g / m per single side 2 or more,

[0098] The above hot stamping formed body has a Vickers hardness of 400 HV or more.

[0099] As described above, if a plated steel sheet with Zn-containing plating is used in hot stamping forming, since the plated steel sheet is processed at a high temperature (for example, around 900 °C), the Zn contained in the plating is processed in a molten state, and the molten Zn may invade the steel and cause LME cracking inside the steel sheet. The reason is not necessarily clear, but through the research of the inventors of the present invention, etc., it is known that the carbon contained in the steel sheet is an element that promotes such LME cracking. Therefore, it is considered that by reducing the carbon concentration in the surface layer of the steel sheet that generates LME cracking by, for example, decarburization, the generation of LME cracking can be suppressed or reduced. However, in fact, during hot stamping forming, the LME suppression effect due to such low carbon concentration in the surface layer of the steel sheet is limited and may not be satisfactory.

[0100] The inventors of the present invention conducted various studies and found that: even from the viewpoint of improving LME resistance, by decarburization or the like, the carbon concentration in the surface layer portion of the steel base material before hot stamping forming is reduced, and the carbon contained in the bulk of the steel base material during high-temperature heating in hot stamping forming diffuses into the surface layer portion. Due to such re-carbonization of the surface layer portion, the initial LME suppression effect brought about by the low carbon concentration of the surface layer portion disappears or decreases. Then, the inventors of the present invention further conducted research and found that: by forming a structure in the surface layer portion of the steel base material before hot stamping forming that can suppress such re-carbonization, even when the coating amount of the Zn-containing coating is set to be relatively large in order to maintain sufficient corrosion resistance, the initial LME suppression effect brought about by the low carbon concentration of the surface layer portion can be fully exerted, and the generation of LME cracking during high-temperature heating of hot stamping forming can be reliably suppressed or reduced. More specifically, as will be described in detail below in connection with the method for manufacturing a hot stamping formed body, the inventors of the present invention found that: by setting the coating amount to 50 g / m per single side 2 or more, sufficient corrosion resistance can be maintained even when applied to hot stamping forming, and by forming a structure in the surface layer portion of the steel base material with a relatively small amount of pearlite and a reduced amount of coarse pearlite, when applied to hot stamping forming, the average C concentration from the surface of the steel base material to a depth of 1 μm can be suppressed to 0.25 mass% or less, and in connection therewith, the generation of LME cracking during high-temperature heating of hot stamping forming can be reliably suppressed or reduced.

[0101] Although not intending to be bound by any particular theory, it is considered that when obtaining the hot stamping formed body of the embodiment of the present invention by hot stamping forming, the structure in the surface layer portion of the steel base material functions as follows, suppressing or reducing the re-carburization by the diffusion of carbon contained in the body of the steel base material into the surface layer portion during the high-temperature heating of hot stamping forming. To explain in more detail, if the carbon concentration in the surface layer portion of the steel base material is reduced by decarburization or the like, then in association with such a low-carbon concentration, the amount of pearlite generated in the microstructure in the surface layer portion of the steel base material becomes relatively small. By forming a region with a relatively small amount of pearlite in the surface layer portion of the steel base material, the LME suppression effect brought about by low-carbon concentration can be fully exerted. However, it is considered that when only the amount of pearlite is simply reduced, in the case where pearlite precipitates along the grain boundaries, during the high-temperature heating of hot stamping forming, the pearlite transforms into austenite, resulting in the formation of a carbon diffusion path (i.e., a carbon re-carburization path) generated by the austenite along the grain boundaries. During the high-temperature heating of hot stamping forming, due to the concentration gradient between the high-carbon concentration in the body of the steel base material and the low-carbon concentration on the surface side, the carbon in the body wants to diffuse to the surface side. At this time, if there is the above-mentioned carbon re-carburization path generated by the austenite along the grain boundaries, the carbon in the body diffuses to the surface side via this re-carburization path, thereby promoting re-carburization into the surface layer portion. As a result, the initial LME suppression effect brought about by the low-carbon concentration of the surface layer portion cannot be fully exerted. In contrast, according to the embodiment of the present invention, in the region of the surface layer portion with a relatively small amount of pearlite, by further reducing the amount of coarse pearlite, even during the high-temperature heating of hot stamping forming, the austenite transformed from pearlite can be dispersed and present at the grain boundaries, thereby reliably cutting off the carbon re-carburization path generated by the austenite.

[0102] To explain in more detail, when pearlite transforms into austenite during high-temperature heating in hot stamping forming, a duplex structure of ferrite and austenite is formed. In such a case, a carbon re-carbonization path is formed by the austenite existing at the heterogeneous interface between ferrite and austenite connecting to the surface side of the steel base material. As a result, it promotes the diffusion of carbon from the bulk of the steel base material to the surface side. In association with this, when obtaining the hot stamping formed body of the embodiment of the present invention by hot stamping forming, the following matters become important: reducing the amount of pearlite in the surface layer portion of the steel base material and at the same time reducing the amount of coarse pearlite in this surface layer portion. 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, so that the carbon re-carbonization path generated by the austenite can be reliably cut off. As a result, in the finally obtained hot stamping formed body, the average C concentration from the surface of the steel base material to a depth of 1 μm can be suppressed to 0.25 mass% or less. In particular, in the case of hot stamping forming a Zn-coated steel sheet, during high-temperature heating in hot stamping forming, the coating layer and Fe are alloyed by the mutual diffusion of Fe in the steel base material and the coating layer. On the other hand, since C in the steel base material is not contained in the coating layer, C is relatively concentrated with respect to Fe near the surface of the steel base material. Therefore, even if the surface layer portion of the steel base material is decarburized or the like before hot stamping forming to make the carbon concentration low, the C concentration in the surface layer portion of the steel base material after hot stamping forming may become much higher than the C concentration in the bulk. In contrast, according to the embodiment of the present invention, since the diffusion of C from the bulk to the surface layer portion is suppressed by cutting off the re-carbonization path as described above, compared with the case of the conventional coated steel sheet, the concentration of C near the surface of the steel base material can be significantly suppressed or reduced. More specifically, the average C concentration from the surface of the steel base material to a depth of 1 μm can be suppressed to 0.25 mass% or less. Therefore, according to the embodiment of the present invention, although the amount of Zn coating is set relatively large to maintain sufficient corrosion resistance and thus becomes a condition more likely to cause LME, by significantly suppressing re-carbonization during high-temperature heating in hot stamping forming, the initial LME suppression effect brought about by the low carbon concentration of the surface layer portion of the steel base material can be fully exerted, and the generation of LME cracking during hot stamping forming can be reliably suppressed or reduced. The fact that the generation of LME cracking can be suppressed or reduced as described above by appropriately modifying the surface layer portion structure of the steel base material before hot stamping forming in a hot stamping formed body having a Zn coating layer was first discovered by the inventors of the present invention this time. In addition, such an LME suppression effect can be exerted not only during high-temperature heating in hot stamping forming but also during spot welding after hot stamping forming. Therefore, the hot stamping formed body of the embodiment of the present invention is particularly useful in the automotive field where spot welding is used relatively frequently.

[0103] Hereinafter, the hot stamping formed body according to the embodiment of the present invention will be described in more detail. In the following description, the unit of the content of each element, i.e., “%”, means “mass %” unless otherwise specified. In addition, in this specification, “~” indicating a numerical range is used to mean including the values described before and after it as the lower limit value and the upper limit value without special description.

[0104] [Coating layer]

[0105] According to the embodiment of the present invention, the coating layer is disposed on the surface of the steel base material, for example, disposed on at least one surface of the steel base material, preferably two surfaces. Here, the expression “disposed on the surface of the steel base material” includes not only the case where the coating layer is directly disposed on the surface of the steel base material, but also the case where the coating layer is indirectly disposed on the surface of the steel base material, for example, the case where a solid solution layer described below is included between the steel base material and the coating layer. The coating layer has the following chemical composition.

[0106] [Al: 5.0 to 80.0%]

[0107] Al is an element effective for improving the corrosion resistance of the coating layer. In addition, during high-temperature heating in hot stamping forming, Al alloyizes with Fe diffused from the steel base material into the coating layer to form intermetallic compounds such as Fe2Al5. Due to such consumption of Fe caused by Al, alloying of Zn and Fe is indirectly suppressed, and η-Zn phase mainly composed of Zn can exist relatively more in the coating layer, which is effective from the viewpoint of improving corrosion resistance. In order to fully obtain these effects, the Al content is set to 5.0% or more. The Al content can also be 8.0% or more, 10.0% or more, 15.0% or more, 20.0% or more, 25.0% or more, 30.0% or more, or 40.0% or more. On the other hand, if Al is excessively contained, the amount of Zn required for imparting substitutional corrosion resistance decreases. Therefore, the Al content is set to 80.0% or less. The Al content can also be 75.0% or less, 70.0% or less, 65.0% or less, or 60.0% or less.

[0108] [Si: 0 to 15.0%]

[0109] Si is an element effective for improving the corrosion resistance of the coating layer. The Si content can also be 0%, but in order to obtain such an effect, the Si content is preferably 0.01% or more. The Si content can also be 0.1% or more, 1.0% or more, 3.0% or more, 5.0% or more, or 8.0% or more. On the other hand, from the viewpoint of improving the plating adhesion of the coating layer, the Si content is set to 15.0% or less. The Si content can also be 14.0% or less, 12.0% or less, or 10.0% or less.

[0110] [Fe: 15.0 to 70.0%]

[0111] If the plated steel sheet is heated during hot stamping forming, Fe from the steel base material diffuses into the coating and alloying occurs with Al, Zn, etc. Therefore, Fe is necessarily contained in the coating. Thus, the Fe content is 15.0% or more, and for example, it can also be 20.0% or more, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. On the other hand, according to the embodiments of the present invention, as will be described in detail below in connection with the manufacturing method of the hot stamping formed body, by appropriately modifying the structure of the surface layer portion of the steel base material before hot stamping forming, although the reason is not necessarily clear, it is considered that alloying of Al, Zn, etc. in the coating with Fe in the steel base material can be delayed. Therefore, according to the embodiments of the present invention, the Fe content in the coating can be suppressed to 70.0% or less at most. In association therewith, by setting the coating adhesion amount to be relatively large, specifically controlled to be 50 g / m 2 or more on each single side, there can be relatively many coatings in which alloying has not been fully carried out, and due to the existence of such coatings, sufficient corrosion resistance can be maintained. From the viewpoint of improving corrosion resistance, the lower the Fe content, the more preferable, and for example, it can also be 67.0% or less, 65.0% or less, 62.0% or less, 60.0% or less, 57.0% or less, or 55.0% or less.

[0112] Furthermore, the coating may optionally contain at least one of Mg: 0% or more and less than 0.500%, Ni: 0% or more and less than 0.500%, 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, but are preferably 5.000% or less in total. The optional elements can also 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 in total. Hereinafter, these optional elements will be described in detail.

[0113] [Mg: 0% or more and less than 0.500%]

[0114] Mg is an element effective for improving the corrosion resistance of the coating. The Mg content can also be 0%, but in order to obtain such an effect, the Mg content is preferably 0.001% or more. The Mg content can also 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 also be less than 0.500%. The Mg content can also 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.

[0115] [Ni: 0% or more and less than 0.500%]

[0116] Ni is an element effective for improving the corrosion resistance of the coating. The Ni content can also be 0%, but in order to obtain such an effect, the Ni content is preferably 0.0001% or more. The Ni content can also 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, but from the viewpoints of manufacturing cost, etc., the Ni content is set to be less than 0.500%, and for example, it can also be 0.490% or less, 0.480% or less, 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0117] [Ca: 0 to 3.000%]

[0118] Ca is an element effective for ensuring the wettability of the plating bath. The Ca content can also be 0%, but in order to obtain such an effect, the Ca content is preferably 0.001% or more. The Ca content can also be 0.005% or more, 0.010% or more, 0.100% or more, or 1.000% or more. On the other hand, if Ca is excessively contained, it is possible to form a large amount of hard intermetallic compounds in the coating, making the coating brittle and reducing the adhesion to the steel sheet. Therefore, the Ca content is preferably 3.000% or less. The Ca content can also be 2.500% or less, 2.000% or less, or 1.500% or less.

[0119] [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%]

[0120] 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 performance of the hot stamping formed body. However, in the case where the content of each element is excessive, the corrosion resistance may 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 also 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 also be, for example, 0.800% or less, 0.500% or less or 0.100% or less.

[0121] In the coating, the remainder other than the above elements is composed of Zn and impurities. The impurities in the coating refer to components mixed in through various factors in the manufacturing process represented by raw materials during the manufacture of the coating. The Zn content of the remainder is not particularly limited, but may also be, for example, 1.0% or more, 2.0% or more, 3.0% or more or 4.0% or more.

[0122] [Determination of the chemical composition of the coating]

[0123] The chemical composition of the coating is determined by dissolving only the coating in an acidic aqueous solution and performing chemical analysis. That is, the chemical composition of the coating is obtained by the following method: dissolving the coating in an acidic aqueous solution containing an inhibitor that inhibits the dissolution of steel, specifically, an acidic aqueous solution at room temperature containing 1% of HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.) in 10% hydrochloric acid, and performing ICP (high-frequency inductively coupled plasma optical emission spectrometry) analysis on the obtained acidic aqueous solution.

[0124] As the coating, it can be any coating having the above chemical composition and containing the following η-Zn phase, and there is no particular limitation. For example, it can be a hot-dip coating or an alloyed hot-dip coating, etc.

[0125] [η-Zn phase]

[0126] In an embodiment of the present invention, the coating contains an η-Zn phase. As described above, Al contained in the coating alloyizes with Fe diffused from the steel base material during high-temperature heating in hot stamping forming to form intermetallic compounds such as Fe2Al5. Due to such consumption of Fe caused by Al, the alloying of Zr with Fe is indirectly inhibited, and an η-Zn phase mainly composed of Zn can exist relatively abundantly in the coating. In the present invention, the η-Zn phase refers to a phase mainly composed of Zn and containing other elements such as Fe in a solid solution state. More specifically, it refers to a phase in which the Zn concentration is 97 atomic% or more, the Fe concentration is 3 atomic% or less, and the other element concentration is 3 atomic% or less in the measurement using a scanning electron microscope (SEM-EPMA) equipped with an electron probe microanalyzer. By the coating containing such an η-Zn phase mainly composed of Zn, the corrosion resistance of the hot stamping formed body can be improved. The presence of the η-Zn phase in the coating itself can be confirmed by the measurement using X-ray diffraction method described below.

[0127] [Peak height ratio of η-Zn phase / Fe2Al5 obtained by X-ray diffraction method: 0.05 or more]

[0128] According to a preferred embodiment of the present invention, when the coating layer is measured by X-ray diffraction method (XRD), the ratio of the peak height derived from the η-Zn phase to the peak height derived from Fe2Al5 (peak height ratio of η-Zn phase / Fe2Al5) is 0.05 or more. Since the η-Zn phase is a phase mainly composed of Zn as described above, it is a phase effective for improving corrosion resistance. Therefore, by containing the η-Zn phase relatively more in the coating layer, that is, containing it in an amount such that the peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD becomes 0.05 or more, the corrosion resistance of the hot stamping formed body can be significantly improved. From the viewpoint of further improving corrosion resistance, the higher the peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD, the more preferred. For example, it may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more. The upper limit is not particularly limited. For example, the peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD may also be 1.50 or less, 1.20 or less, or 1.00 or less.

[0129] [Method for measuring the peak height ratio of η-Zn phase / Fe2Al5 obtained by X-ray diffraction method]

[0130] The peak height ratio of η-Zn phase / Fe2Al5 is determined by the following operation. First, a specimen cut into a size of 50 mm × 30 mm is obtained from the hot stamping formed body. Then, by subjecting this specimen to X-ray diffraction measurement (tube target: Cu, acceleration voltage: 15 kV, step: 0.1°, 2θ: 20 to 60°), the peak height of the peak of the (002) plane derived from the η-Zn phase detected in the range of 2θ = 35.5 to 37.0° and the peak height of the peak of the (002) plane derived from Fe2Al5 detected in the range of 2θ = 42.0 to 43.1° are measured, and the peak height ratio of η-Zn phase / Fe2Al5 is determined.

[0131] [Coating amount: 50 g / m per single side 2 above]

[0132] In the hot stamping formed body according to the embodiment of the present invention, the coating amount is 50 g / m per single side 2 above. Generally, the coating layer may be alloyed with the steel base material during high-temperature heating in hot stamping, resulting in a decrease in corrosion resistance. However, according to the embodiment of the present invention, although the reason is not clear, it is considered that: due to the surface layer structure of the steel base material appropriately modified before hot stamping, that is, the surface layer structure with a relatively small amount of pearlite and a reduced amount of coarse pearlite, the alloying of the coating layer with the steel base material can be delayed. Therefore, by setting the coating amount to be relatively large, specifically controlled to be 50 g / m per single side2 As described above, there is a coating layer in which alloying has not been sufficiently carried out after hot stamping forming. Further, as described above, Al contained in the coating layer suppresses the alloying of Zr and Fe by consuming Fe, and can also make the η-Zn phase mainly composed of Zn exist relatively more in the coating layer. It is considered that due to such a delay in alloying and the addition effect of Al in the coating layer, sufficient corrosion resistance can be maintained even after hot stamping forming. On the other hand, if the coating amount is small, the effect associated with the delay in alloying as described above cannot be obtained sufficiently, and the corrosion resistance after hot stamping forming may decrease. From the viewpoint of improving corrosion resistance, the coating amount per single side is preferably 55 g / m 2 or more, or 60 g / m 2 or more, more preferably 70 g / m 2 or more, or 80 g / m 2 or more, further more preferably 90 g / m 2 or more, most preferably 100 g / m 2 or more. There is no particular limitation on the upper limit, but the coating amount of the coating layer can also be, for example, 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.

[0133] [Measurement of Coating Amount]

[0134] The coating amount of the coating layer is determined by dissolving only the coating layer with an acidic aqueous solution. That is, a 30 mm × 30 mm sample is collected from the hot stamping formed body, and then the coating layer is dissolved with an acidic aqueous solution containing an inhibitor that inhibits the dissolution of steel, specifically, an acidic aqueous solution at room temperature containing 1% HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.) in 10% hydrochloric acid, and the coating amount of the coating layer is determined from the weight change of the sample before and after the dissolution of the coating layer.

[0135] [Average C Concentration from the Surface of the Steel Base Metal to 1 μm in the Depth Direction: 0.25 mass% or Less]

[0136] In the hot stamping formed body according to an embodiment of the present invention, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is 0.25 mass% or less. As will be described in detail below in connection with the manufacturing method of the hot stamping formed body, due to the structure related to the low carbon concentration of the surface layer portion of the steel base material before hot stamping, that is, the surface layer structure in which the amount of pearlite is relatively small and the amount of coarse pearlite is reduced, the re-carbonization during the high-temperature heating of hot stamping is significantly suppressed. As a result, the LME suppression effect initially brought about by the low carbon concentration of the surface layer portion of the steel base material can be fully exerted, and the generation of LME cracking during hot stamping can be reliably suppressed or reduced. Moreover, in the finally obtained hot stamping formed body, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is reduced to 0.25 mass% or less. Therefore, from the viewpoint of improving the LME resistance during hot stamping, the lower the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction, the more preferable it is. For example, it may also be 0.22 mass% or less, 0.20 mass% or less, 0.18 mass% or less, 0.15 mass% or less, 0.12 mass% or less, 0.10 mass% or less, 0.08 mass% or less, or 0.06 mass% or less. The lower limit is not particularly limited. For example, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction may also be 0.001 mass% or more, 0.005 mass% or more, or 0.01 mass% or more.

[0137] [Measurement of the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction]

[0138] The average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is determined by operating a high-frequency glow discharge optical emission spectrometry (GDS) apparatus as follows. Specifically, the following method is adopted: the surface of the hot stamping formed body is set in an Ar atmosphere, and in a state where glow plasma is generated by applying a voltage, the surface of the hot stamping formed body is analyzed in the depth direction while sputtering. Then, the elements contained in the material are identified from the emission spectral wavelengths unique to the elements emitted when the atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. The data in the depth direction can be estimated from the sputtering time. Specifically, by previously obtaining the relationship between the sputtering time and the sputtering depth using a standard sample, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted into mass% by creating a calibration curve. In the case of performing GDS measurement on the hot stamping formed body by operating in this way, the position where the Al concentration in the depth direction is below "the Al concentration in the steel base material + 0.1%" and the Zn concentration in the depth direction becomes 0.1% or less is determined as the surface of the steel base material, and the average C concentration in the region from this surface to a depth of 1 μm in the depth direction is determined as "the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction". The Al concentration in the steel base material refers to the Al concentration corresponding to the average value of the Al emission intensity obtained by GDS at a depth of 100 to 150 μm from the surface of the hot stamping formed body.

[0139] As described above, in the case of hot stamping a conventional plated steel sheet with Zn plating, during the high-temperature heating of hot stamping, Fe in the steel base material diffuses into the plating layer. On the other hand, C in the steel base material does not diffuse into the plating layer, resulting in the relative concentration of C with respect to Fe near the surface of the steel base material. Therefore, even if the surface layer of the steel base material is made to have a low carbon concentration by decarburization or the like before hot stamping, the C concentration in the surface layer of the steel base material after hot stamping may become much higher than the C concentration in the body. However, according to the embodiment of the present invention, due to the suppression of recarburization, the diffusion of C from the body to the surface layer is suppressed, so that the concentration of C near the surface of the steel base material can be significantly suppressed or reduced compared with the case of the conventional plated steel sheet. From the viewpoint of improving the LME resistance, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is preferably equal to or lower than the C content of the steel base material. More specifically, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is less than 1.10 times the C content of the steel base material, and for example, it may also be 1.05 times or less, 1.00 times or less, 0.90 times or less, 0.80 times or less, 0.70 times or less, 0.60 times or less, or 0.50 times or less. The lower limit is not particularly limited, but for example, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction may also be 0.05 times or more, 0.10 times or more, or 0.15 times or more of the C content of the steel base material. In the present invention, the "C content of the steel base material" refers to the value measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for chips based on JIS G 1201:2014 using a test piece obtained from near the 1 / 2 position of the thickness of the steel base material.

[0140] [Solid solution layer]

[0141] In an embodiment of the present invention, the hot-stamped formed body may further include a solid solution layer between the steel base material and the plating layer. Depending on the plating composition before hot stamping and the hot stamping conditions, it is possible to form a solid solution of these elements between the Fe diffused from the steel base material into the plating layer and Al and / or Zn in the plating layer, and a solid solution layer containing a solid solution of these elements is formed between the steel base material and the plating layer after hot stamping. Therefore, when the hot-stamped formed body further includes a solid solution layer, the solid solution layer contains Fe and one or both of Al and Zn. More specifically, the solid solution layer includes an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an Fe-Al-Zn solid solution layer.

[0142] The presence of the solid solution layer can be confirmed by the following procedure. First, the coating is dissolved only using an acidic aqueous solution containing an inhibitor that suppresses the dissolution of steel, specifically, a room-temperature acidic aqueous solution of 10% hydrochloric acid with 1% HIBIRON (A-6) (manufactured by SUGIMURAChemicalIndustrial Co., Ltd.). Next, using a high-frequency glow discharge optical emission spectrometry device (GDS), the surface of the hot-stamped formed body is set to an Ar atmosphere, and while applying a voltage to generate a glow plasma, the surface of the hot-stamped formed body is analyzed in the depth direction while sputtering. Then, based on the emission spectral wavelength specific to the element emitted when the atoms are excited in the glow plasma, the elements contained in the material are identified, and the emission intensity of the identified elements is estimated. The data in the depth direction can be estimated from the sputtering time. Specifically, by previously obtaining the relationship between the sputtering time and the sputtering depth using a standard sample, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted into mass% by creating a calibration curve. In the case of performing GDS measurement on the hot-stamped formed body by operating in this way, a region where the Al concentration exceeds "the Al concentration in the steel base material + 0.1%" in the depth direction, or the Zn concentration exceeds 0.1% in the depth direction, or both conditions are met, is determined as the solid solution layer. The Al concentration in the steel base material refers to the Al concentration corresponding to the average value of the Al emission intensity obtained by GDS at a depth of 100 to 150 μm from the surface of the hot-stamped formed body after the coating has been dissolved.

[0143] [Martensite area ratio of the steel base material: 90% or more]

[0144] The steel base material of the hot-stamped formed body according to the embodiment of the present invention contains 90% or more of martensite by area ratio. The remaining structure is not particularly limited, but may also contain at least one of bainite, ferrite, retained austenite, and pearlite in an amount of 10% or less. Since martensite is a very hard structure, by including 90% or more of martensite by area ratio in the hot-stamped formed body, high strength, specifically, a Vickers hardness of 400 HV, can be achieved. On the other hand, if the area ratio of martensite decreases and the proportion of soft structures such as ferrite increases, it may not be possible to achieve a Vickers hardness of 400 HV. Therefore, the larger the area ratio of martensite, the more preferable it is. For example, it may also be 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the area ratio of martensite is not particularly limited and may also be 100%.

[0145] [Identification of martensite and calculation of area ratio]

[0146] The identification of martensite and the calculation of the area ratio 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 hot stamping formed body 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 / 4 depth of the plate thickness on the above observation surface, a range of 300 μm × 300 μm is photographed at 1000 times magnification. After performing black-and-white binarization processing on the obtained microstructure photograph, image analysis is carried out to identify pearlite, bainite, and ferrite, and the total sum of their area ratios is obtained using the method based on the "Microscopic Test Method for Grain Size of Steel" specified in JIS G 0551:2020. Since retained austenite is difficult to distinguish from martensite by SEM, 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%.

[0147] [Mechanical properties]

[0148] The hot stamping formed body according to an embodiment of the present invention can achieve excellent mechanical properties, such as a Vickers hardness of 400 HV or more. More specifically, a Vickers hardness of 400 HV or more can be achieved at the position of 1 / 2 of the thickness of the steel base material. The Vickers hardness is preferably 500 HV or more, more preferably 550 HV or more. The upper limit is not particularly limited, but for example, the Vickers hardness can also be 650 HV or less or 600 HV or less.

[0149] [Measurement of hardness]

[0150] The Vickers hardness is determined as follows. First, a test piece is cut out from an arbitrary position other than the end of the hot stamping formed body in such a way that a cross-section perpendicular to the surface (thickness cross-section) can be observed. The plate thickness cross-section of the test piece is ground using silicon carbide paper of #600 to #1500, and then finished into a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water, and this thickness cross-section is used as the measurement surface. Next, a micro-Vickers hardness tester is used to measure the Vickers hardness with a load of 1 kgf at an interval of more than 3 times the indentation. A total of 20 points are randomly measured near the position of 1 / 2 of the thickness of the steel base material in such a way that the surface layer portion with low carbon concentration is not included, and their arithmetic mean is determined as the hardness of the hot stamping formed body.

[0151] [Preferred chemical composition of the steel base material]

[0152] The chemical composition of the steel base material according to an embodiment of the present invention may be any chemical composition that is generally used in a hot stamping formed body and can achieve a Vickers hardness of 400 HV or more. Hereinafter, the preferred chemical composition of the steel base material will be described in detail, but these descriptions are merely illustrative of the preferred chemical composition of the steel base material suitable for achieving a Vickers hardness of 400 HV or more, and are not intended to limit the present invention to a steel base material having such a specific chemical composition.

[0153] In an embodiment of the present invention, for example, the steel base material preferably has the following chemical composition: by mass%:

[0154] C: 0.13 to 0.50%,

[0155] Si: 0.001 to 3.000%,

[0156] Mn: 0.30 to 3.00%,

[0157] Al: 0.0002 to 2.000%,

[0158] P: 0.100% or less,

[0159] S: 0.1000% or less,

[0160] N: 0.0100% or less,

[0161] Nb: 0 to 0.15%,

[0162] Ti: 0 to 0.15%,

[0163] V: 0 to 0.15%,

[0164] Mo: 0 to 1.0%,

[0165] Cr: 0 to 1.0%,

[0166] Cu: 0 to 1.0%,

[0167] Ni: 0 to 1.0%,

[0168] B: 0 to 0.0100%,

[0169] W: 0 to 1.000%,

[0170] Hf: 0 to 0.050%,

[0171] Mg: 0 to 0.050%,

[0172] Zr: 0 to 0.050%,

[0173] Ca: 0 to 0.010%,

[0174] REM: 0 to 0.30%,

[0175] Ir: 0 to 1.000%, and

[0176] The balance: consists of Fe and impurities.

[0177] Hereinafter, each element will be described in more detail.

[0178] [C: 0.13 to 0.50%]

[0179] C is an element that cheaply increases the tensile strength and is important for controlling the strength of steel. To fully obtain such an effect, the C content is preferably set at 0.13% or more. The C content can also be 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, if C is contained excessively, it may lead to a decrease in the elongation rate. Therefore, the C content is preferably set at 0.50% or less. The C content can also be 0.45% or less or 0.40% or less.

[0180] [Si: 0.001 to 3.000%]

[0181] Si is an element that acts as a deoxidizer and suppresses the precipitation of carbides during the cooling process in the annealing of cold-rolled sheets. To fully obtain such an effect, the Si content is preferably set at 0.001% or more. The Si content can also be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, if Si is contained excessively, it may lead to an increase in the steel strength and a decrease in the elongation rate. Therefore, the Si content is preferably set at 3.000% or less. The Si content can also be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0182] [Mn: 0.30 to 3.00%]

[0183] Mn is an element that improves the hardenability of steel and is effective for increasing the strength. To fully obtain such an effect, the Mn content is preferably set at 0.30% or more. The Mn content can also be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, if Mn is contained excessively, it may lead to an increase in the steel strength and a decrease in the elongation rate. Therefore, the Mn content is preferably set at 3.00% or less. The Mn content can also be 2.80% or less, 2.50% or less, or 2.00% or less.

[0184] [Al: 0.0002 to 2.000%]

[0185] Al acts as a deoxidizer for steel and has the effect of soundifying steel. In order to fully obtain such an effect, the Al content is preferably set to 0.0002% or more. The Al content can also be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if Al is contained excessively, it is possible to form coarse Al oxides, resulting in a decrease in the elongation of the steel. Therefore, the Al content is preferably set to 2.000% or less. The Al content can also be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0186] [P: 0.100% or less]

[0187] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the more preferable it is, and thus ideally it is 0%. However, an excessive reduction in the P content may lead to a significant increase in cost. Therefore, the P content can also be set to 0.0001% or more, and can also be 0.001% or more or 0.005% or more. On the other hand, if P is contained excessively, as described above, it is possible to cause embrittlement of steel due to grain boundary segregation. Therefore, the P content is preferably set to 0.100% or less. The P content can also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0188] [S: 0.1000% or less]

[0189] 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 it is, and thus ideally it is 0%. However, an excessive reduction in the S content may lead to a significant increase in cost. Therefore, the S content can also be set to 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, if S is contained excessively, it is possible to cause cracking starting from non-metallic inclusions during cold forming. Therefore, the S content is preferably set to 0.1000% or less. The S content can also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0190] [N: 0.0100% or less]

[0191] N is an element that forms coarse nitrides in steel and reduces the workability of the steel. The lower the N content, the more preferable, and thus ideally it is 0%. However, an excessive reduction in the N content may lead to a significant increase in manufacturing costs. Therefore, the N content can also be set to 0.0001% or more, and can also be 0.0005% or more or 0.0010% or more. On the other hand, if N is contained excessively, as described above, coarse nitrides may be formed and the workability of the steel may be reduced. Therefore, the N content is preferably set to 0.0100% or less. The N content can also be 0.0080% or less or 0.0050% or less.

[0192] The preferred basic chemical composition of the steel base material is as described above. Furthermore, the steel base material 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 remaining Fe. These elements can also be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more, respectively.

[0193] In the steel base material, the remaining part other than the above elements is composed of Fe and impurities. The impurities in the steel base material refer to components mixed in due to various factors in the manufacturing process represented by raw materials such as ores and scraps when manufacturing the steel base material industrially.

[0194] The chemical composition of the steel base material can be determined by general analysis methods. For example, the chemical composition of the steel base material is first ground mechanically to remove the coating, and then measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for chips according to JIS G 1201:2014. Specifically, for example, by taking a test piece of 35 mm square from near the 1 / 2 position of the thickness of the steel base material and using an ICPS-8100 or the like (measurement device) manufactured by Shimadzu Corporation, measurement can be performed under the conditions based on the standard curve prepared in advance, and thus determination can be made. C and S that cannot be measured by ICP-AES can be measured by combustion-infrared absorption method, N can be measured by inert gas fusion-thermal conductivity method, and O can be measured by inert gas fusion-non-dispersive infrared absorption method.

[0195] <Method for manufacturing hot stamping formed body>

[0196] Next, a preferred method for manufacturing a hot stamping formed body according to an embodiment of the present invention will be described. The following description aims to illustrate the characteristic method for manufacturing the hot stamping formed body according to the embodiment of the present invention, and does not intend to limit the hot stamping formed body to be manufactured by the manufacturing method as described below.

[0197] The hot stamping formed body according to the 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 billet; a hot rolling step of hot rolling the steel billet to obtain a hot rolled steel sheet; a coiling step of coiling the hot rolled steel sheet; a cold rolling step of cold rolling the coiled hot rolled steel sheet to obtain a cold rolled steel sheet; an annealing step of annealing the cold rolled steel sheet; a cooling step of cooling the annealed cold rolled steel sheet; a plating step of forming a plating layer on the obtained steel base material; and a hot stamping step of hot stamping the obtained plated steel sheet. As an alternative, after the hot rolling step, pickling may be performed instead of coiling, and then the cold rolling step may be directly performed. Hereinafter, each step will be described in detail.

[0198] [Casting step]

[0199] The conditions of the casting step are not particularly limited. For example, after melting using a blast furnace, an electric furnace, etc., various secondary refinings are then carried out, and then casting can be performed by methods such as ordinary continuous casting and casting using the ingot method.

[0200] [Hot rolling step]

[0201] The cast steel billet can be hot rolled to obtain a hot rolled steel sheet. The hot rolling step is carried out by directly hot rolling the cast steel billet or reheating it after temporary cooling and then hot rolling. In the case of reheating, the heating temperature of the steel billet can be, for example, 1100 - 1250 °C. In the hot rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling can be appropriately determined according to the desired metal structure and plate thickness. For example, the finishing temperature can be 900 - 1050 °C, and the reduction ratio of finish rolling can be 10 - 50%.

[0202] [Coiling step]

[0203] 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 - 800 °C. It is also possible to uncoil before or after coiling and give the hot rolled steel sheet a specified heat treatment. As an alternative, the coiling step may not be performed, and pickling may be carried out after the hot rolling step and then the subsequent cold rolling step may be performed.

[0204] [Cold rolling process]

[0205] After pickling the hot-rolled steel sheet, etc., the hot-rolled steel sheet can be 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 also be air-cooled to room temperature, for example.

[0206] [Annealing process]

[0207] 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 and holding for 10 to 300 seconds in an atmosphere with a dew point of -20 to 10°C. By performing the annealing process under such relatively high dew point conditions, the surface layer of the cold-rolled steel sheet can be appropriately decarburized. Through such decarburization treatment, in the plated steel sheet before hot stamping forming, a region with a relatively small amount of pearlite can be formed in the surface layer of the steel base material. More specifically, a depth region with an area ratio of pearlite of 20% or less and a thickness of 3 μm or more can be formed in the plate thickness direction starting from the surface of the steel base material. By forming such a structure with a relatively small amount of pearlite in the surface layer of the steel base material, the average C concentration from the surface of the steel base material to a depth of 1 μm in the finally obtained hot stamping formed body can be reliably suppressed to 0.25 mass% or less. If 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, the decarburization in the surface layer of the cold-rolled steel sheet becomes insufficient. As a result, it becomes impossible to form a region with a relatively small amount of pearlite as described above in the surface layer of the steel base material. On the other hand, if 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 generated on the steel sheet surface, the plating property may be reduced, or the strength of the finally obtained hot stamping formed body 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, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere with hydrogen of 1 to 10% (for example, hydrogen of 4% and the balance of nitrogen).

[0208] [Cooling process]

[0209] The cold-rolled steel sheet with the surface layer decarburized in the annealing process needs to be appropriately cooled in the subsequent cooling process to obtain the desired surface layer structure. Specifically, the cooling process includes the following cooling: 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 secondary cooling will be described in more detail.

[0210] [Primary cooling]

[0211] In primary cooling, it is important to suppress the precipitation of pearlite at high temperatures. More specifically, the pearlite precipitated at high temperatures from the heating temperature of 730 - 900°C in the annealing process to the controlled temperature of 620 - 670°C diffuses quickly, and thus is likely to diffuse to the grain boundaries after precipitation to form pearlite along the grain boundaries. 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-carbonization path generated by the austenite along the grain boundaries, which promotes the re-carbonization of carbon in the body to the surface layer of the steel base material. Therefore, the following matters become extremely important: in the temperature range from the heating temperature in the annealing process to the above-mentioned controlled temperature, by cooling the cold-rolled steel sheet at a relatively fast average cooling rate of 20°C / s or more, the precipitation of pearlite at high temperatures in the surface layer of the steel base material is suppressed. If the above average cooling rate is lower than 20°C / s and / or the controlled temperature exceeds 670°C, pearlite precipitates at high temperatures where diffusion is fast, thus promoting the formation of pearlite along the grain boundaries. Associated therewith, in the plated steel sheet before hot stamping forming, it becomes impossible to reduce the amount of coarse pearlite in the surface layer of the steel base material. More specifically, a large amount of coarse pearlite with an equivalent circle diameter of 5 μm or more is formed along the grain boundaries in the surface layer of the steel base material. If the amount of such coarse pearlite formed along the grain boundaries increases, it leads to the promotion of the formation of a carbon re-carbonization path generated by the austenite along the grain boundaries during the high-temperature heating of hot stamping forming. As a result, in the finally obtained hot stamping formed body, it becomes impossible to suppress the average C concentration from the surface of the steel base material to a depth of 1 μm to below 0.25 mass%, and it becomes impossible to achieve sufficient LME resistance.

[0212] [Secondary cooling]

[0213] On the other hand, in the secondary cooling after primary cooling, it is important to precipitate pearlite at low temperatures where diffusion is relatively slow. More specifically, by cooling from the controlled temperature of 620 - 670°C to the plating bath temperature (e.g., the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less, pearlite can be precipitated. The pearlite precipitated in the low-temperature region below such a controlled temperature diffuses relatively slowly, and thus does not form in a form connected along the grain boundaries, and the pearlite can be dispersed at the grain boundaries. In such a microstructure, even during the high-temperature heating of hot stamping forming, it is possible to make the pearlite in the A c1Above a certain point, austenite formed by pearlite transformation also exists dispersedly at grain boundaries, so it can reliably cut off the carbon re-carbonization path generated by austenite. On the other hand, if the above average cooling rate exceeds 10 °C / s and / or the control temperature is lower than 620 °C, martensite and bainite mainly precipitate instead of pearlite. The phase transformation rate of martensite and bainite into austenite is faster than that of pearlite, and they immediately transform into austenite just above the A c1 point. Therefore, compared with the case of pearlite, the time during which the dual-phase structure of ferrite and austenite is exposed to high temperature during hot stamping forming becomes longer. In such a case, it also becomes easy to form a re-carbonization path at grain boundaries, so sufficient LME resistance cannot be achieved.

[0214] [Plating process]

[0215] Next, in the plating process, a plating layer is formed on at least one surface, preferably two surfaces, of the cold-rolled steel sheet (steel base material). More specifically, the plating process can also be carried out by hot-dip plating treatment using a plating bath with a specified chemical composition (plating bath temperature: for example, 420 - 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 hot-dip plating method, and can also be an electroplating method, an evaporation plating method, a spraying or cold spraying method, etc. Other conditions of the plating process can be appropriately determined 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 and then fishing it out, N2 gas or air is immediately blown by the gas wiping method, and then it is cooled, so that the adhesion amount of the plating layer can be adjusted within a specified range, for example, 50 - 200 g / m per single side 2 in the range.

[0216] [Cooling after plating]

[0217] During the cooling after plating, it is preferable to control the dew point of the cooling gas (for example, nitrogen) within the range of -10 to 10 °C. By cooling the plated steel sheet in such an atmosphere with a relatively high dew point, an Al oxide film can be formed relatively thickly on the surface of the plating layer. Due to the formation of this Al oxide film, even during the high-temperature heating of hot stamping forming, the evaporation of Zn in the plating layer can be significantly suppressed or reduced. As a result, the proportion of η-Zn phase in the plating layer can be further increased, and the peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD can be reliably set to 0.05 or more. Furthermore, due to the suppression of Zn evaporation in the plating layer, the Fe content in the plating layer of the obtained hot stamping formed body can be relatively reduced, so the corrosion resistance after hot stamping forming can be further improved.

[0218] [Hot stamping forming process]

[0219] Finally, the obtained plated steel sheet is hot stamping formed in a hot stamping forming process to manufacture a hot stamping formed body having a desired surface layer portion composition and a hard structure. From the viewpoint of obtaining a desired hard structure, it is preferable to load the plated steel sheet into a furnace at 800 to 1000 °C. After the temperature of the plated steel sheet reaches a specified temperature, for example, the furnace temperature - 10 °C, it is held in the furnace for 60 to 600 seconds. If the heating temperature is lower than 800 °C and / or the holding time is shorter than 60 seconds, austenitization becomes insufficient, and the area ratio of the desired hard structure (i.e., the area ratio of martensite is 90% or more) cannot be obtained, and it may not be possible to achieve a Vickers hardness of 400 HV or more in the finally obtained hot stamping formed body. The heating atmosphere is not particularly limited and can be normal conditions. For example, as long as it is in the atmosphere, a gas combustion atmosphere with the ratio of air to fuel controlled, a nitrogen atmosphere is acceptable, and the dew point can also be controlled in these gases. After the heating and holding in the furnace, the plated steel sheet is taken out of the furnace. Then, after the plated steel sheet reaches a specified temperature, for example, a specified temperature of 850 °C or lower, hot stamping forming can be carried out under normal conditions. After hot stamping forming, although not particularly limited, for example, it is sufficient to cool to a temperature range of 250 °C or lower at an average cooling rate of 20 °C / second or more.

[0220] 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.

[0221] Examples

[0222] In the following examples, hot stamping formed bodies of the embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured hot stamping formed bodies were investigated.

[0223] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. After the steel billet was temporarily cooled, it was reheated to 1200 °C for hot rolling, and then coiled at a temperature of 600 °C or lower. The hot rolling was carried out by rough rolling and finish rolling. The finish rolling end temperature was 900 to 1050 °C, and the reduction ratio of the finish rolling was 30%. Then, the obtained hot rolled steel sheet was pickled, and then cold rolled with a reduction ratio of 50% to obtain a cold rolled steel sheet with a 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 the balance nitrogen 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 steel base material.

[0224] Next, the manufactured steel base material was cut into 100 mm × 200 mm, and this steel base material was plated using an intermittent hot-dip plating test apparatus made by our company. More specifically, first, the manufactured steel base material 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 to 200 mm / s. The amount of coating adhered was adjusted to the value shown in Table 2 by wiping with N2 gas. Next, the steel base material with the coating adhered was cooled from the plating bath temperature (about 420 to 480°C) to room temperature using nitrogen gas controlled to the dew point shown in Table 2 as the cooling gas, whereby a plated steel sheet having coatings formed on both surfaces of the steel base material was obtained. The plate temperature was measured using a thermocouple spot-welded to the center of the steel base material.

[0225] [Analysis of Chemical Composition of Coating]

[0226] The chemical composition of the coating was determined by the following procedure. First, the plated steel sheet was loaded into an atmospheric heating furnace at 900°C, and after the temperature of the plated steel sheet reached the furnace temperature - 10°C, it was held for 100 seconds. Next, the plated steel sheet was taken out of the furnace and clamped with a flat die at a temperature around room temperature and rapidly cooled. A sample cut into 30 mm × 30 mm after heating and rapid cooling was immersed in an acidic aqueous solution at room temperature containing 1% HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.) in 10% hydrochloric acid. After pickling and peeling the coating, the coating components dissolved in the aqueous solution were determined by ICP emission spectrometry, whereby it was determined. The results are shown in Table 2.

[0227] [Evaluation of Liquid Metal Embrittlement Resistance during Hot Stamping (HS) Forming]

[0228] First, a blank of 180 mm × 50 mm was collected from the plated steel sheet, and this blank was loaded into a furnace at 900°C. After the temperature of the blank reached the furnace temperature - 10°C, it was held in the furnace for 100 seconds. Next, the blank was taken out of the furnace, and after reaching 750°C, hat-shaped forming and die quenching were carried out using a hat-shaped forming die at a temperature around room temperature. At this time, the forming speed was changed to 200 mm / s, 100 mm / s, and 50 mm / s. The shape of the formed body (hot stamping formed body) after hat-shaped forming was as shown in Figure 1 . A cross-section of the bent portion in the formed body after hat-shaped forming was cut out and observed by SEM, and the presence or absence of liquid metal embrittlement cracking at each forming speed was investigated, and the liquid metal embrittlement resistance was evaluated as follows.

[0229] AAA: No liquid metal embrittlement cracking occurred when the forming speed was 200 mm / s

[0230] AA: There is no LME cracking when the forming speed is 100 mm / s

[0231] A: There is no LME cracking when the forming speed is 50 mm / s

[0232] B: There is LME cracking when the forming speed is 50 mm / s

[0233] [Evaluation of corrosion resistance]

[0234] The corrosion resistance after hot stamping forming 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 and clamp the coated steel sheet with a flat die at a room - temperature level and perform quenching. Samples of the hot - stamped formed body after heating and quenching, 50 mm × 100 mm, are carried out according to phosphating Zn treatment (SD5350 system: standard of Nipponpaint Industrial Coatings Co., Ltd.). Then, electrodeposition coating is carried out at 20 μm (PN110POWERNICS GRAY: standard of Nipponpaint Industrial Coatings Co., Ltd.), and baking treatment is carried out at a temperature of 150 °C for 20 minutes. Then, introduce a cut mark reaching the base metal (steel base material) in 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 mark part, and evaluate the corrosion resistance as follows.

[0235] AAA: More than 120 cycles

[0236] AA: More than 60 and less than or equal to 120 cycles

[0237] A: 30 - 60 cycles

[0238] B: Less than 30 cycles

[0239] [Evaluation of hardness after hot stamping (HS)]

[0240] First, in the same manner as in the evaluation of corrosion resistance, the plated steel sheet is placed in an atmospheric heating furnace at 900°C. After the temperature of the plated steel sheet reaches the furnace temperature - 10°C, it is maintained for 100 seconds. Then, the plated steel sheet is taken out of the furnace and quenched by clamping it with a flat die at a temperature around room temperature to obtain a hot stamping formed body (HS condition A). On the other hand, except that the plated steel sheet is placed in an atmospheric heating furnace at 700°C, the same operation as in HS condition A is performed to obtain a hot stamping formed body of Comparative Example 42 (HS condition B). A test piece is cut out from an arbitrary position except the end of the obtained hot stamping formed body 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 paper with #600 to #1500, and then finished into a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water, and this plate thickness cross-section is used as the measurement surface. Then, a micro-Vickers hardness tester is used to measure the Vickers hardness with a load of 1 kgf at intervals of more than 3 times the indentation. A total of 20 points are randomly measured near the 1 / 2 position of the thickness of the steel base material in a manner that does not include the surface layer portion with low carbon concentration, and their arithmetic mean is determined as the hardness after hot stamping (HS), and the evaluation is carried out as follows.

[0241] AAA: Hardness after HS exceeds 550 HV

[0242] AA: Hardness after HS exceeds 500 HV and is 550 HV or less

[0243] A: Hardness after HS is 400 - 500 HV

[0244] B: Hardness after HS is less than 400 HV

[0245] [Peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD]

[0246] First, a specimen cut into a size of 50 mm × 30 mm is obtained from the hot stamping formed body. Then, by subjecting this specimen to X-ray diffraction measurement (tube target: Cu, accelerating voltage: 15 kV, step: 0.1°, 2θ: 20 - 60°), the peak height of the peak of the (002) plane of the η-Zn phase detected in the range of 2θ = 35.5 - 37.0° and the peak height of the peak of the (002) plane of Fe2Al5 detected in the range of 2θ = 42.0 - 43.1° are measured, and the peak height ratio of η-Zn phase / Fe2Al5 is determined. In all the examples and comparative examples, as indicated by the fact that the peak height ratio of η-Zn phase / Fe2Al5 exceeds 0, it is confirmed that the η-Zn phase is contained in all the coatings.

[0247] The case where the hardness after HS is evaluated as AAA, AA, and A, the LME resistance is evaluated as AAA, AA, and A, and the corrosion resistance is evaluated as AA and A is evaluated as a hot-stamped formed body with high strength, excellent LME resistance during hot stamping forming, and furthermore, high corrosion resistance can be maintained even after hot stamping forming. The results are shown in Table 2. In the hot-stamped formed bodies shown in Table 2, the remaining structure other than martensite is bainite, ferrite, retained austenite, and / or pearlite.

[0248] Table 1

[0249]

[0250] Table 2-1

[0251]

[0252] Table 2-2

[0253]

[0254] Referring to Table 2, for Comparative Example 38, although the LME resistance and corrosion resistance are good, the hardness after HS is low, so the desired high strength cannot be achieved. For Comparative Example 39, it is considered that since the heating temperature in the annealing process is low, decarburization in the surface layer of the cold-rolled steel sheet is insufficient. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. For Comparative Example 40, it is considered that since the holding time in the annealing process is short, decarburization in the surface layer of the cold-rolled steel sheet is similarly insufficient. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. For Comparative Example 41, it is considered that since the dew point in the annealing process is low, decarburization in the surface layer of the cold-rolled steel sheet is similarly insufficient. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. The cross-section of the plated steel sheet before hot stamping of Comparative Examples 39 to 41 was observed by SEM, and as a result, the depth region where the area ratio of pearlite is 20% or less in the plate thickness direction from the surface of the steel base material is less than 3 μm. For Comparative Example 42, it is considered that since the average cooling rate of the first cooling in the cooling process is low, pearlite precipitates at a high temperature, which forms along the grain boundaries and promotes re-carbonization during the high-temperature heating of hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. For Comparative Example 43, it is considered that since the control temperature of the first cooling in the cooling process is high, pearlite similarly precipitates at a high temperature and forms along the grain boundaries, promoting re-carbonization during the high-temperature heating of hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. The cross-section of the plated steel sheet before hot stamping of Comparative Examples 42 and 43 was observed by SEM, and as a result, coarse pearlite with an equivalent circle diameter of 5 μm or more was formed relatively more in the surface layer of the steel base material. For Comparative Examples 44 and 45, it is considered that since the control temperature of the second cooling in the cooling process is low, bainite mainly precipitates instead of pearlite, promoting re-carbonization during the high-temperature heating of hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. For Comparative Example 46, it is considered that since the average cooling rate of the second cooling in the cooling process is fast, bainite mainly precipitates instead of pearlite similarly, promoting re-carbonization during the high-temperature heating of hot stamping. As a result, the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction becomes high, and the LME resistance decreases. For Comparative Example 47, since the coating adhesion amount is insufficient, the corrosion resistance after HS decreases. For Comparative Example 48, since the hot stamping conditions are inappropriate, austenitization becomes insufficient, and the desired martensite area ratio cannot be achieved, and the hardness after HS decreases.

[0255] In contrast, in the hot-stamped formed bodies of all the examples, by having a specified plating chemical composition, the plating layer contains an η-Zn phase and the adhesion amount of the plating layer is set to 50 g / m per single side 2 or more, and the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is controlled to be 0.25 mass% or less. Thus, even when exposed to a high temperature of 900 °C, it has high strength and maintains high corrosion resistance, and can fully exhibit the initial LME suppression effect brought about by the low carbon concentration of the surface layer of the steel base material, and reliably suppress or reduce the generation of LME cracking during hot stamping. The plated steel sheets before hot stamping in all the examples were observed by SEM in cross section. As a result, in the depth region where the area ratio of pearlite becomes 20% or less in the plate thickness direction from the surface of the steel base material, it is 3 μm or more. In addition, the proportion of coarse pearlite with an equivalent circle diameter of 5 μm or more in the surface layer of the steel base material is sufficiently lower than that of Comparative Examples 42 and 43. In particular, for Examples 2 to 13, 15 to 18, 20, 22 to 31, and 33 to 36 in which the dew point of the cooling gas after plating is controlled within the range of -10 to 10 °C, the peak height ratio of η-Zn phase / Fe2Al5 obtained by XRD can be set to 0.05 or more. As a result, the evaluation of corrosion resistance becomes AA, and the corrosion resistance can be further improved compared with Examples 1, 14, 19, 21, 32, and 37 in which the dew point of the cooling gas after plating is set to -40 °C or -15 °C. In addition, GDS measurement was performed on each hot-stamped formed body. As a result, the presence of a solid solution layer containing a Fe-Al solid solution layer, a Fe-Zn solid solution layer, and / or a Fe-Al-Zn solid solution layer was confirmed in the hot-stamped formed bodies of all the examples.

Claims

1. A hot stamping formed body, characterized in that, It has a steel base material and a coating disposed on the surface of the steel base material. The coating has the following chemical composition: containing, by mass%: Al：5.0～80.0％、 Si: 0 to 15.0%, and Fe: 15.0 to 70.0%, Further containing at least one of the following elements in a total of 5.000% or less: Mg: 0% or more and less than 0.500%, Ni: 0% or more and less than 0.500%, 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 W：0～0.500％， The balance: composed of Zn and impurities, The average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction is 0.25 mass% or less. The steel base material contains 90% or more of martensite by area ratio. The coating contains an η-Zn phase. The coating amount is 50 g / m per single side 2 or more. The hot stamping formed body has a Vickers hardness of 400 HV or more.

2. The hot stamping formed body according to claim 1, characterized in that, The average C concentration is 0.18 mass% or less.

3. The hot stamping formed body according to claim 2, wherein The average C concentration is 0.10 mass% or less.

4. The hot stamping formed body according to any one of claims 1 to 3, characterized in that, When the coating is measured by X-ray diffraction, the ratio of the peak height from the η-Zn phase to the peak height from Fe2Al5 is 0.05 or more.

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