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 deposition amount of the plating layer is 30 g / m2 or more per surface, the plating layer contains a Zn-containing phase containing at least one of an [eta]-Zn phase, an Fe-Zn intermetallic compound phase, and an Mg-Zn intermetallic compound phase, 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, lightweighting of vehicle bodies has been demanded. In order to balance the lightweighting 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 based on such a background, the development of high-strength steel sheets has been promoted.

[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 high-strength steel materials can be accurately formed into complex shapes, and since quenching is performed simultaneously with forming by a pressing die, the formed steel material has sufficient strength.

[0004] In association 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 it is taught that a component obtained by hot stamping this steel sheet exhibits high sacrificial corrosion protection.

[0006] A method for manufacturing a hardened component is described in Patent Document 2, 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 additional elements 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, 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 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. In addition, Patent Document 2 teaches that: according to the above manufacturing method, a hardened component without LME can be obtained.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-528324

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2018-527462 Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] For example, if galvanized steel sheets as described in Patent Documents 1 and 2 are used in hot stamping forming, the coating after hot stamping forming may alloy with the base metal (steel base material), resulting in a reduction in corrosion resistance. In addition, the hot stamping formed body obtained by hot stamping a galvanized steel sheet is then joined by spot welding or the like. At this time, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon is caused by cracking that occurs when the tensile stress generated by welding acts when Zn liquefied by the input of welding heat penetrates into the interior of the steel along the crystal grain boundaries and causes embrittlement. In connection with this, in Patent Document 2, although instructions for suppressing LME generated during hot stamping forming are provided, in Patent Document 2, from the viewpoint of suppressing LME cracking during spot welding after hot stamping forming and further taking into account both the suppression of this LME cracking and the improvement of corrosion resistance, sufficient research has not necessarily been conducted. 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 the sensitivity to LME cracking tends to increase. On the other hand, in the automotive industry and the like, further weight reduction of steel materials is also required. To achieve such weight reduction, it is necessary to strengthen the steel material more than before. Therefore, even in the case of achieving the same or higher strength than 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.

[0013] Accordingly, an object of the present invention is to provide a hot stamping formed body having high strength, capable of maintaining high corrosion resistance even after hot stamping forming, and suppressing LME cracking during spot welding.

[0014] Means for Solving the Problem

[0015] The inventors of the present invention conducted research to achieve the above object and first found that by forming a coating containing a specific Zn-containing phase with an adhesion amount equal to or more than a specified amount, 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, the C concentration in the surface layer portion of the steel base material after hot stamping forming becomes relatively low. Thus, even for a coating formed with such an adhesion amount, the generation of LME cracking during spot welding after hot stamping forming can be significantly suppressed or reduced, thereby completing the present invention.

[0016] The present invention that has achieved the above object is as follows.

[0017] (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,

[0018] The above-mentioned coating has the following chemical composition: by mass %, it contains:

[0019] Al: 0.5 to 30.0%,

[0020] Mg: 0.50 to 15.00%,

[0021] Si: 0 to 2.0%, and

[0022] Fe: 15.0 to 70.0%

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

[0024] Ni: 0 to 1.000%,

[0025] Ca: 0 to 3.0%,

[0026] Sb: 0 to 0.500%,

[0027] Pb: 0 to 0.500%,

[0028] Cu: 0 to 1.000%,

[0029] Sn: 0 to 1.000%,

[0030] Ti: 0 to 1.000%,

[0031] Cr: 0 to 1.000%,

[0032] Nb: 0 to 1.000%,

[0033] Zr: 0 to 1.000%,

[0034] Mn: 0 to 1.000%,

[0035] Mo: 0 to 1.000%,

[0036] Ag: 0 to 1.000%,

[0037] Li: 0 to 1.000%,

[0038] La: 0 to 0.500%,

[0039] Ce: 0 to 0.500%,

[0040] B: 0 to 0.500%,

[0041] Y: 0 to 0.500%,

[0042] Sr: 0 to 0.500%,

[0043] In: 0 to 0.500%,

[0044] Co: 0 to 0.500%,

[0045] Bi: 0 to 0.500%,

[0046] P: 0 to 0.500%, and

[0047] W: 0 to 0.500%,

[0048] The balance: composed of Zn and impurities,

[0049] 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,

[0050] The above steel base material contains 90% or more martensite by area ratio,

[0051] The coating amount of the above coating is 30 g / m per single side 2 or more,

[0052] The above coating contains a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn intermetallic compound phase, and an Mg-Zn intermetallic compound phase,

[0053] The above hot stamping formed body has a Vickers hardness of 400 HV or more.

[0054] (2) The hot stamping formed body according to (1) above, characterized in that the average C concentration is 0.18 mass% or less.

[0055] (3) The hot stamping formed body according to (2) above, characterized in that the average C concentration is 0.10 mass% or less.

[0056] (4) The hot stamping formed body according to any one of (1) to (3) above, characterized in that the chemical composition contains Al: 6.0 to 30.0% and Mg: 3.00 to 15.00% by mass%, and the Zn-containing phase contains at least one of an η-Zn phase and an Fe-Zn intermetallic compound phase.

[0057] (5) The hot stamping formed body according to any one of (1) to (4) above, characterized in that the chemical composition contains Al: 20.0 to 30.0% and Mg: 5.00 to 15.00% by mass%, and the Zn-containing phase contains an Mg-Zn intermetallic compound phase.

[0058] Advantages of the Invention

[0059] According to the present invention, it is possible to provide a hot stamping formed body having high strength, maintaining high corrosion resistance even after hot stamping forming, and suppressing LME cracking during spot welding. Detailed Implementation Modes

[0060] <Hot Stamping Formed Body>

[0061] The hot stamping formed body according to an embodiment of the present invention is characterized by comprising a steel base material and a coating layer disposed on the surface of the above-mentioned steel base material.

[0062] The above-mentioned coating layer has the following chemical composition: containing, by mass%:

[0063] Al: 0.5 to 30.0%,

[0064] Mg: 0.50 to 15.00%,

[0065] Si: 0 to 2.0%, 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] Ni: 0 to 1.000%,

[0069] Ca: 0 to 3.0%,

[0070] Sb: 0 to 0.500%,

[0071] Pb: 0 to 0.500%,

[0072] Cu: 0 to 1.000%,

[0073] Sn: 0 to 1.000%,

[0074] Ti: 0 to 1.000%,

[0075] Cr: 0 to 1.000%,

[0076] Nb: 0 to 1.000%,

[0077] Zr: 0 to 1.000%,

[0078] Mn: 0 to 1.000%,

[0079] Mo: 0 to 1.000%,

[0080] Ag: 0 to 1.000%,

[0081] Li: 0 to 1.000%,

[0082] La: 0 to 0.500%,

[0083] Ce: 0 to 0.500%,

[0084] B: 0 to 0.500%,

[0085] Y: 0 to 0.500%,

[0086] Sr: 0 to 0.500%,

[0087] In: 0 to 0.500%,

[0088] Co: 0 to 0.500%,

[0089] Bi: 0 to 0.500%,

[0090] P: 0 to 0.500%, and

[0091] W: 0 to 0.500%,

[0092] The balance: composed of Zn and impurities,

[0093] 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,

[0094] The above steel base material contains 90% or more martensite by area ratio,

[0095] The coating amount of the above coating is 30 g / m per single side 2 or more,

[0096] The above coating contains a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase,

[0097] The above hot stamping formed body has a Vickers hardness of 400 HV or more.

[0098] As described above, when joining a hot stamping formed body obtained by hot stamping a galvanized steel sheet by spot welding, it is necessary to suppress liquid metal embrittlement (LME) cracking. The reason is not necessarily clear, but through the research of the inventors of the present invention, etc., it is known that carbon contained in steel 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 that causes LME cracking by, for example, decarburization, etc., the generation of LME cracking can be suppressed or reduced. However, in fact, in the case of application to hot stamping, the LME suppression effect based on such low carbon concentration in the surface layer of the steel sheet is limited and may not be satisfactory.

[0099] The inventors of the present invention have conducted various studies and found that: even from the perspective 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 is reduced, and the carbon contained in the bulk of the steel base material during high-temperature heating in hot stamping 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 that can suppress such re-carbonization, even when a Zn-containing coating is included in a specified coating amount 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 spot welding after hot stamping can be reliably suppressed or reduced. More specifically, as will be described in detail below in connection with the manufacturing method of the hot stamping formed body, the inventors of the present invention found that: by setting the coating amount of a coating containing a specific Zn-containing phase, more specifically a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn intermetallic compound phase, and an Mg-Zn intermetallic compound phase to 30 g / m per single side 2 as described above, even when applied to hot stamping, sufficient corrosion resistance is maintained, 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, 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 spot welding after hot stamping can be reliably suppressed or reduced.

[0100] 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 of carbon contained in the body of the steel base material from diffusing 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, when this pearlite transforms into austenite, a carbon diffusion path (i.e., a carbon re-carburization path) is formed 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 formed 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 in 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 where the amount of pearlite is relatively small, 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 truncating the carbon re-carburization path generated by the austenite.

[0101] More specifically, if 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 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 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 a Zn-containing coating layer is provided in an amount sufficient to maintain corrosion resistance, and thus it becomes a condition where LME is more likely to occur, 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 subsequent spot welding 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-containing coating layer was first discovered by the inventors of the present invention this time. Therefore, the hot stamping formed body of the embodiment of the present invention is particularly useful in the automotive field where spot welding is frequently used.

[0102] Hereinafter, the hot stamping formed body of 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, the "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value without special description.

[0103] [Coating layer]

[0104] According to an 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, including a solid solution layer described below between the steel base material and the coating layer. The coating layer has the following chemical composition.

[0105] [Al: 0.5 to 30.0%]

[0106] Al is an element effective for improving the corrosion resistance of the coating layer. In order to fully obtain such an effect, the Al content is set to 0.5% or more. The Al content may also be 0.7% or more, 1.0% or more, 3.0% or more, 5.0% or more, 7.0% or more, 10.0% or more, or 15.0% or more. On the other hand, if Al is contained excessively, the amount of Zn required for substitutional corrosion prevention decreases. Therefore, the Al content is set to 30.0% or less. The Al content may also be 27.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

[0107] [Mg: 0.50 to 15.00%]

[0108] Mg is an element effective for improving the corrosion resistance of the coating layer. In order to fully obtain such an effect, the Mg content is set to 0.50% or more. The Mg content may also 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, if Mg is contained excessively, film swelling and running rust may occur due to excessive substitutional corrosion prevention. Therefore, the Mg content is set to 15.00% or less. The Mg content may also 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.

[0109] [Si: 0 to 2.0%]

[0110] Si is an element effective in improving the corrosion resistance of the coating. The Si content may also be 0%, but as needed, Si may also be contained in the coating in an amount of 0.01% or more, 0.05% or more, 0.1% or more, 0.2% or more, 0.4% or more, or 0.6% or more. On the other hand, from the viewpoint of improving the coating adhesion of the coating, the Si content may also be 2.0% or less. The Si content may also be 1.8% or less, 1.5% or less, 1.2% or less, 1.0% or less, or 0.8% or less.

[0111] [Fe: 15.0 to 70.0%]

[0112] If the coated steel sheet is heated during hot stamping forming, Fe from the steel base material diffuses into the coating and alloying occurs with Zn, etc., so Fe is necessarily contained in the coating. Therefore, the Fe content is 15.0% or more, and for example, it may 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 an embodiment 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 surface layer structure of the steel base material before hot stamping forming, although the reason is not necessarily clear, it is considered that the alloying of Zn, etc. in the coating with Fe in the steel base material can be delayed. Therefore, according to an embodiment of the present invention, the Fe content in the coating can be suppressed to a maximum of 70.0% or less. In connection therewith, by controlling the coating adhesion amount to 30 g / m 2 or more on each side, there can be a relatively large amount of the coating in which alloying has not been fully carried out, and due to the existence of such a coating, and further due to the existence of a specific Zn-containing phase in the coating, more specifically, the existence of at least one of the η-Zn phase, the Fe-Zn-based intermetallic compound phase, and the Mg-Zn-based intermetallic compound phase, 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 may 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.

[0113] 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, but preferably the total is 5.000% or less. The total of the optional elements may 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. Hereinafter, these optional elements will be described in detail.

[0114] [Ni: 0% or more and less than 1.000%]

[0115] Ni is an element effective for improving the corrosion resistance of the coating layer. 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 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 and the like, the Ni content is set to 1.000% or less. For example, it may also 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.

[0116] [Ca: 0 to 3.0%]

[0117] Ca is an element effective for 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 also be 0.05% or more, 0.1% or more, 0.5% or more or 1.0% or more. On the other hand, if Ca is contained excessively, there is a possibility that hard intermetallic compounds are formed in a large amount in the coating layer, the coating layer 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 also be 2.5% or less, 2.0% or less or 1.5% or less.

[0118] [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%]

[0119] 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 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.

[0120] In the coating, the remaining portion 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 content of Zn in the remaining portion is not particularly limited, but may be, for example, 5.0% or more, 10.0% or more, 15.0% or more, or 20.0% or more.

[0121] [Determination of the chemical composition of the coating]

[0122] 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 (inductively coupled plasma optical emission spectrometry) analysis on the obtained acidic aqueous solution.

[0123] As the coating, any coating having the above chemical composition and containing the following Zn-containing phase can be used, and there is no particular limitation. For example, it can be a hot-dip coating, an alloyed hot-dip coating, etc.

[0124] [Zn-containing phase]

[0125] In an embodiment of the present invention, the coating contains a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn intermetallic compound phase, and an Mg-Zn intermetallic compound phase. It is considered that, according to the embodiment 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 surface layer structure of the steel base material before hot stamping, the alloying of Zn, etc. in the coating with Fe in the steel base material can be delayed. In addition, due to the Al oxide film formed on the surface of the coating, even during the high-temperature heating of hot stamping, the evaporation of Zn and / or Mg in the coating can be significantly suppressed or reduced. As a result, at least one of the η-Zn phase, the Fe-Zn intermetallic compound phase, and the Mg-Zn intermetallic compound phase having excellent corrosion resistance can be contained as the Zn-containing phase in the coating after hot stamping. 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 other elements are 3 atomic% or less in the measurement using a scanning electron microscope (SEM-EPMA) with an electron probe microanalyzer. The Fe-Zn intermetallic compound phase can be, for example, at least one of the δ-FeZn 10 phase and the Γ-Fe3Zn 10 phase. For example, by the Zn-containing phase in the coating containing at least one of the η-Zn phase and the Fe-Zn intermetallic compound phase, the corrosion resistance of the hot stamping formed body can be improved. On the other hand, the Mg-Zn intermetallic compound phase can be, for example, the MgZn2 phase, the Mg2Zn3 phase, the MgZn phase, the Mg2Zn 11 phase, and Mg 21 Zn 25At least one of the phases. The Mg-Zn based intermetallic compound phase has higher corrosion resistance compared to the η-Zn phase and the Fe-Zn based intermetallic compound phase. Therefore, in a preferred embodiment of the present invention, by having the Zn-containing phase in the coating contain at least one of the η-Zn phase and the Fe-Zn based intermetallic compound phase or contain the Mg-Zn based intermetallic compound phase in place of them, the corrosion resistance can be further improved. In a more preferred embodiment of the present invention, the Zn-containing phase in the coating is composed only of the Mg-Zn based intermetallic compound phase. By forming the Zn-containing phase only of the Mg-Zn based intermetallic compound phase having higher corrosion resistance, the corrosion resistance of the coating can be more significantly improved. Regarding the η-Zn phase, the Fe-Zn based intermetallic compound phase, and the Mg-Zn based intermetallic compound phase, as will be described in detail below in connection with the manufacturing method, by not only forming an Al oxide film on the surface of the coating to suppress or reduce the evaporation of Zn and / or Mg during high-temperature heating of hot stamping forming, but also appropriately controlling the chemical composition of the coating, it is possible to manufacture at a desired ratio. For example, by making the chemical composition of the coating contain Al: 6.0 to 30.0% and Mg: 3.00 to 15.00% by mass, at least one of the η-Zn phase and the Fe-Zn based intermetallic compound phase can be formed as the Zn-containing phase in the coating. Similarly, by making the chemical composition of the coating contain Al: 20.0 to 30.0% and Mg: 5.00 to 15.00% by mass, the Mg-Zn based intermetallic compound phase, particularly only the Mg-Zn based intermetallic compound phase, can be formed as the Zn-containing phase in the coating.

[0126] [Identification of Zn-containing phase]

[0127] The identification of the Zn-containing phase is performed by X-ray diffraction method (XRD). Specifically, a Cu tube target is used as the X-ray source, and XRD measurement is performed on the surface of the coating. When there is a peak in the range of 2θ = 42.9 to 43.6°, it is determined that the Zn-containing phase in the coating contains the η-Zn phase. Regarding the Fe-Zn based intermetallic compound phase and the Mg-Zn based intermetallic compound phase, when there is a peak in the range of ±0.3° of the diffraction peak with the highest intensity in the ICDD card, it is determined that the Zn-containing phase in the coating contains the Fe-Zn based intermetallic compound phase and / or the Mg-Zn based intermetallic compound phase.

[0128] [Coating adhesion amount: 30 g / m per single side 2 above]

[0129] In the hot stamping formed body of the embodiment of the present invention, the coating adhesion amount is 30 g / m per single side 2The above. Generally speaking, during the high-temperature heating in hot stamping forming, the coating may alloy with the steel base material, resulting in a reduction in corrosion resistance. However, according to the embodiments 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 forming, that is, the amount of pearlite is relatively small and the amount of coarse pearlite is reduced in the surface layer structure, the alloying of the coating with the steel base material can be delayed. Therefore, it is considered that by controlling the coating adhesion amount to be 30 g / m 2 or more per single side, after hot stamping forming, there can be relatively more coatings where alloying has not been fully carried out. Due to the existence of such coatings, and further due to the existence of specific Zn-containing phases in the coatings, more specifically, the existence of at least one of η-Zn phase, Fe-Zn-based intermetallic compound phase, and Mg-Zn-based intermetallic compound phase, sufficient corrosion resistance can be maintained. On the other hand, if the coating adhesion amount is small, the effect associated with the delay of 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 adhesion amount per single side is preferably 30 g / m 2 or more or 40 g / m 2 or more, more preferably 50 g / m 2 or more, even more preferably 60 g / m 2 or more, and most preferably 80 g / m 2 or more. There is no particular limitation on the upper limit, but the coating adhesion amount can 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.

[0130] [Measurement of Coating Adhesion Amount]

[0131] The coating adhesion amount is determined by dissolving only the coating 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 is dissolved with an acidic aqueous solution containing an inhibitor that inhibits the dissolution of steel, specifically, an acidic aqueous solution at room temperature with 1% HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.) added to 10% hydrochloric acid. The coating adhesion amount is determined from the weight change of the sample before and after the dissolution of the coating.

[0132] [Average C Concentration from the Surface of the Steel Base Material to a Depth of 1 μm: 0.25 mass% or Less]

[0133] In the hot-stamped formed body according to the 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-stamped formed body, due to the structure associated with 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, re-carbonization during high-temperature heating of hot stamping is significantly suppressed. In connection therewith, in the finally obtained hot-stamped 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. 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 spot welding after hot stamping can be reliably suppressed or reduced. Therefore, from the viewpoint of improving the LME resistance during spot welding after 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.

[0134] [Measurement of the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction]

[0135] 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 spectrometer (GDS) as follows. Specifically, the following method is adopted: the surface of the hot stamping formed body is set in an Ar atmosphere, and analysis is performed in the depth direction while sputtering the surface of the hot stamping formed body in a state where glow plasma is generated by applying a voltage. Then, the elements contained in the material are identified from the emission spectral wavelengths characteristic of 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 Zn concentration becomes 0.1% or less in the depth direction 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".

[0136] As described above, in the case of hot stamping a conventional plated steel sheet with Zn plating, during the high-temperature heating in 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, so that 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 made to have a low carbon concentration by decarburization or the like before hot stamping, the C concentration in the surface layer portion of the steel base material after hot stamping may become much higher than the C concentration in the bulk. However, according to the embodiment of the present invention, since the diffusion of C from the bulk to the surface layer portion is suppressed due to the suppression of re-carbonization, the concentration of C near the surface of the steel base material can be significantly suppressed or reduced as 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 the 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.

[0137] [Solid solution layer]

[0138] In the embodiment of the present invention, the hot stamping 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 Fe diffused from the steel base material into the plating layer with 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 stamping formed body further includes a solid solution layer, the solid solution layer contains one or both of Fe and 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.

[0139] The presence of the solid solution layer can be confirmed as follows. First, only the plating layer is dissolved 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 added with 1% HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.). Next, using a high-frequency glow discharge optical emission spectrometry device (GDS), the surface of the hot-stamped 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-stamped formed body is analyzed in the depth direction while sputtering. Then, the elements contained in the material are identified from the characteristic emission spectral wavelengths of 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 using a standard sample to obtain the relationship between the sputtering time and the sputtering depth, 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 material surface. The obtained emission intensity is converted into mass% by creating a standard curve. In the case of performing GDS measurement on the hot-stamped formed body by operating in this way, a region where the Zn concentration becomes more than 0.1% in the depth direction is determined as the solid solution layer.

[0140] [Martensite area ratio of steel base material: 90% or more]

[0141] 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. Martensite is a very hard structure, and therefore, by containing 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 becomes low and the proportion of soft structures such as ferrite becomes high, 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%.

[0142] [Identification of martensite and calculation of area ratio]

[0143] The identification of martensite and the calculation of the area ratio are carried out by the following operations. First, a specimen is collected in such a way 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 a magnification of 1000 times. After performing black-and-white binarization processing on the obtained microstructural 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 ratios of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0144] [Mechanical properties]

[0145] The hot stamping formed body according to the 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.

[0146] [Measurement of hardness]

[0147] The Vickers hardness is determined by the following operations. 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 intervals 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 part with low carbon concentration is not included, and their arithmetic mean is determined as the hardness of the hot stamping formed body.

[0148] [Preferred chemical composition of the steel base material]

[0149] The chemical composition of the steel base material according to the embodiments of the present invention can be any chemical composition that is generally used in hot stamping formed bodies 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 the steel base material having such a specific chemical composition.

[0150] In the embodiments of the present invention, for example, the steel base material preferably has the following chemical composition: by mass%:

[0151] C: 0.13 to 0.50%,

[0152] Si: 0.001 to 3.000%,

[0153] Mn: 0.30 to 3.00%,

[0154] Al: 0.0002 to 2.000%,

[0155] P: 0.100% or less,

[0156] S: 0.1000% or less,

[0157] N: 0.0100% or less,

[0158] Nb: 0 to 0.15%,

[0159] Ti: 0 to 0.15%,

[0160] V: 0 to 0.15%,

[0161] Mo: 0 to 1.0%,

[0162] Cr: 0 to 1.0%,

[0163] Cu: 0 to 1.0%,

[0164] Ni: 0 to 1.0%,

[0165] B: 0 to 0.0100%,

[0166] W: 0 to 1.000%,

[0167] Hf: 0 to 0.050%,

[0168] Mg: 0 to 0.050%,

[0169] Zr: 0 to 0.050%,

[0170] Ca: 0 to 0.010%,

[0171] REM: 0 to 0.30%,

[0172] Ir: 0 to 1.000%, and

[0173] The balance: consisting of Fe and impurities.

[0174] Hereinafter, each element will be described in more detail.

[0175] [C: 0.13 to 0.50%]

[0176] C is an element that cheaply increases the tensile strength and is important for controlling the strength of steel. In order to fully obtain such an effect, the C content is preferably set to 0.13% or more. The C content may also be 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, if C is contained excessively, it may lead to a decrease in elongation. Therefore, the C content is preferably set to 0.50% or less. The C content may also be 0.45% or less or 0.40% or less.

[0177] [Si: 0.001 to 3.000%]

[0178] 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. In order to fully obtain such an effect, the Si content is preferably set to 0.001% or more. The Si content may 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 steel strength and a decrease in elongation. Therefore, the Si content is preferably set to 3.000% or less. The Si content may also be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0179] [Mn: 0.30 to 3.00%]

[0180] Mn is an element that improves the hardenability of steel and is effective for increasing strength. In order to fully obtain such an effect, the Mn content is preferably set to 0.30% or more. The Mn content may also be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, if Mn is contained excessively, it may lead to an increase in steel strength and a decrease in elongation. Therefore, the Mn content is preferably set to 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less.

[0181] [Al: 0.0002 to 2.000%]

[0182] Al acts as a deoxidizer for steel and has the effect of making the steel sound. To fully obtain such an effect, the Al content is preferably set at 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 excessively contained, there is a possibility of generating coarse Al oxides, resulting in a decrease in the elongation of the steel. Therefore, the Al content is preferably set at 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.

[0183] [P: 0.100% or less]

[0184] 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 at 0.0001% or more, and can also be 0.001% or more or 0.005% or more. On the other hand, if P is excessively contained, as described above, there is a possibility of embrittlement of steel due to grain boundary segregation. Therefore, the P content is preferably set at 0.100% or less. The P content can also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0185] [S: 0.1000% or less]

[0186] S is an element that generates non-metallic inclusions such as MnS in steel and causes 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 at 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 excessively contained, there is a possibility of cracking starting from non-metallic inclusions during cold forming. Therefore, the S content is preferably set at 0.1000% or less. The S content can also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0187] [N: 0.0100% or less]

[0188] N is an element that forms coarse nitrides in steel and reduces the workability of steel. The lower the N content, the more preferable it is, 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, or can 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 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.

[0189] 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.

[0190] In the steel base material, the remaining part other than the above elements is composed of Fe and impurities. 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 scrap during the industrial manufacturing of the steel base material.

[0191] The chemical composition of the steel base material can be determined by general analytical methods. For example, the chemical composition of the steel base material is first ground mechanically to remove the coating, and then it can be determined by using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry) for the 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 etc. (measurement device) manufactured by Shimadzu Corporation, and measuring under the conditions based on the pre - made standard line, it can be determined. 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.

[0192] <Method for manufacturing hot stamping formed body>

[0193] 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 is intended to illustrate a characteristic method for manufacturing the hot stamping formed body according to the embodiment of the present invention, and is not intended to limit the hot stamping formed body to be manufactured by the manufacturing method as described below.

[0194] 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 forming 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.

[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 refinings are then carried out, and then casting can be carried out by methods such as ordinary continuous casting and casting using the ingot method.

[0197] [Hot rolling step]

[0198] 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 to 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 finish rolling end temperature can be 900 to 1050 °C, and the finish rolling reduction ratio 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 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 cold rolling step described below may be performed.

[0201] [Cold rolling process]

[0202] 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 sheet thickness, for example, it can be 20 to 80%. After the cold rolling process, for example, it can also be air-cooled to room temperature.

[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 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 is 4% and the balance is nitrogen).

[0205] [Cooling process]

[0206] In order to obtain the desired surface layer structure, the cold-rolled steel sheet with the surface layer decarburized in the annealing process needs to be appropriately cooled in the subsequent cooling process. 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 the secondary cooling will be described in more detail.

[0207] [Primary cooling]

[0208] In primary cooling, it is important to suppress the precipitation of pearlite at high temperatures. To explain in more detail, pearlite that precipitates 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 and 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 becomes 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 less than 0.25 mass%, and it becomes impossible to achieve sufficient LME resistance during spot welding.

[0209] [Secondary cooling]

[0210] 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 (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. Pearlite that precipitates in the low-temperature region below such a controlled temperature diffuses relatively slowly, and thus does not form in a shape that connects 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... c1Above the point, austenite from pearlite phase transformation also exists dispersedly at grain boundaries, so that the carbon re-carbonization path generated by austenite can be reliably truncated. On the other hand, if the above average cooling rate exceeds 10 °C / s and / or the controlled temperature is lower than 620 °C, instead of mainly precipitating pearlite, martensite and bainite mainly precipitate. Compared with pearlite, the phase transformation rate of martensite and bainite into austenite is faster, 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 the grain boundary, so that sufficient LME resistance cannot be achieved.

[0211] [Plating process]

[0212] 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 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 hot-dip plating method, and can also be an electroplating method, a vapor deposition 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 cooled, so that the adhesion amount of the plating layer can be adjusted within a specified range, for example, 30 to 200 g / m per single side 2 in the range.

[0213] [Cooling after plating]

[0214] When cooling after plating, the dew point of the cooling gas (e.g., nitrogen) needs to be controlled 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 coating. Due to the formation of this Al oxide film, even during high-temperature heating in hot stamping forming, the evaporation of Zn and / or Mg in the coating can be significantly suppressed or reduced. As a result, at least one of the η-Zn phase, Fe-Zn-based intermetallic compound phase, and Mg-Zn-based intermetallic compound phase with excellent corrosion resistance can be reliably included as the Zn-containing phase in the coating after hot stamping forming. Furthermore, due to the suppression of the evaporation of Zn and / or Mg in the coating, the Fe content in the coating of the obtained hot stamping formed body can be relatively reduced, so that the corrosion resistance after hot stamping forming can be further improved. By not only controlling the dew point of the cooling gas during cooling after plating within the range of -10 to 10°C, but also appropriately adjusting the chemical composition of the coating, the desired Zn-containing phase can be formed in the coating. For example, by not only performing the above dew point control, but also making the chemical composition of the coating contain Al: 6.0 to 30.0% and Mg: 3.00 to 15.00% by mass, at least one of the η-Zn phase and Fe-Zn-based intermetallic compound phase can be formed as the Zn-containing phase in the coating. Similarly, by making the chemical composition of the coating contain Al: 20.0 to 30.0% and Mg: 5.00 to 15.00% by mass, a Mg-Zn-based intermetallic compound phase, particularly only a Mg-Zn-based intermetallic compound phase, can be formed as the Zn-containing phase in the coating.

[0215] [Hot stamping forming process]

[0216] Finally, the obtained plated steel sheet is hot-stamped in a hot stamping process to manufacture a hot-stamped body having a desired surface layer 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 and, after the temperature of the plated steel sheet reaches a specified temperature, for example, the furnace temperature - 10°C, hold it 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-stamped body. The heating atmosphere is not particularly limited and can be under normal conditions. For example, as long as it is in the atmosphere, a gas combustion atmosphere in which the ratio of air to fuel is controlled, or a nitrogen atmosphere, and the dew point can also be controlled in these gases. After heating and holding in the furnace, the plated steel sheet is taken out of the furnace, and then, after the plated steel sheet reaches a specified temperature, for example, a specified temperature of 850°C or lower, hot stamping can be performed under normal conditions. After hot stamping, 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.

[0217] 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.

[0218] Examples

[0219] In the following examples, hot-stamped bodies of the embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured hot-stamped bodies were investigated.

[0220] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. After temporarily cooling the steel billet, 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 performing rough rolling and finish rolling, and the finishing temperature of the finish rolling was 900 to 1050°C, and the reduction ratio of the finish rolling was 30%. Next, pickling was performed on the obtained hot-rolled steel sheet, and then cold rolling was performed at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a plate thickness of 1.6 mm. Next, for the obtained cold-rolled steel sheet, an annealing process was carried out under the conditions shown in Table 2 in a mixed gas atmosphere of 4% hydrogen and the balance nitrogen in a furnace with an oxygen concentration of 20 ppm or less, 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.

[0221] Next, the manufactured steel base material was cut into 100 mm × 200 mm, and the steel base material was plated using an intermittent hot-dip plating test apparatus manufactured by our company. More specifically, first, the manufactured steel base material was immersed in a plating bath having a specified 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 attachment was adjusted to the value shown in Table 2 by wiping with N2 gas. Next, the steel base material with the coating attached 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, thereby obtaining a plated steel sheet having coatings formed on both sides of the steel base material. The plate temperature was measured using a thermocouple spot-welded to the center of the steel base material.

[0222] [Analysis of Chemical Composition of Coating]

[0223] 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 between flat dies at a temperature around room temperature and quenched. Samples cut into 30 mm × 30 mm after heating and quenching were immersed in an acidic aqueous solution at room temperature containing 1% HIBIRON (A-6) (manufactured by SUGIMURA Chemical Industrial Co., Ltd.) added to 10% hydrochloric acid. After pickling and peeling the coating, the coating components dissolved in the aqueous solution were determined by ICP emission spectrometry, thereby determining. The results are shown in Table 2.

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

[0225] 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 between flat dies at a temperature around room temperature and quenched. Two samples of the plated steel sheet after heating and quenching, 50 mm × 100 mm, were prepared, and spot welding was performed on these two plated steel sheet specimens using a welding electrode with a dome radius type and a tip diameter of 8 mm at an inclination angle of 2°, a pressing force of 4.0 kN, a welding time of 0.5 seconds, and a welding current of 12 kA, thereby fabricating a welded joint. Next, the length of the LME crack generated directly below the electrode at the welded part was measured, and the LME resistance was evaluated as follows.

[0226] AAA: 0 μm

[0227] AA: More than 0 μm and 20 μm or less

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

[0229] B: 80 μm or more

[0230] [Evaluation of corrosion resistance]

[0231] The corrosion resistance after hot stamping forming is evaluated as follows. First, load the plated steel sheet into an atmospheric heating furnace at 900 °C. After the temperature of the plated steel sheet reaches the furnace temperature - 10 °C, hold for 100 seconds. Then, take out the plated steel sheet from the furnace and clamp it with a flat die at a temperature close to room temperature for rapid cooling. Treat a 50 mm × 100 mm sample of the hot stamping formed body after heating and rapid cooling according to phosphating Zn treatment (SD5350 system: standard of Nipponpaint Industrial Coatings Co., Ltd.), and then apply electrodeposition coating at 20 μm (PN110 POWERNICS GRAY: standard of Nipponpaint Industrial Coatings Co., Ltd.), and perform baking treatment 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 appears from the cut mark part, and evaluate the corrosion resistance as follows.

[0232] AAA: Over 240 cycles

[0233] AA: 180 - 240 cycles

[0234] A: 90 or more and less than 180 cycles

[0235] B: Less than 90 cycles

[0236] [Evaluation of hardness after hot stamping (HS)]

[0237] First, in the same manner as in the evaluation of corrosion resistance, the plated steel sheet is loaded into an atmospheric heating furnace at 900 °C. After the temperature of the plated steel sheet reaches the furnace temperature - 10 °C, it is held 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 loaded into 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 41 (HS condition B). A test piece is cut out from an arbitrary position except the end of the obtained hot stamping formed body in a manner that allows observation of a cross-section perpendicular to the surface (plate thickness cross-section). The plate thickness cross-section of the test piece is polished using silicon carbide paper with #600 - #1500, and then finished to a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 - 6 μm 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 with a load of 1 kgf and at intervals more than three 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 part 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.

[0238] AAA: Hardness after HS exceeds 550 HV

[0239] AA: Hardness after HS exceeds 500 HV and is 550 HV or less

[0240] A: Hardness after HS is 400 - 500 HV

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

[0242] [Identification of Zn-containing phases]

[0243] The identification of Zn-containing phases is carried out by X-ray diffraction method (XRD). Specifically, a Cu tube target is used as the X-ray source, and XRD measurement is performed on the surface of the coating. When a peak exists in the range of 2θ = 42.9 - 43.6 °, it is judged that the Zn-containing phase in the coating contains η-Zn phase. Regarding the Fe-Zn-based intermetallic compound phase and the Mg-Zn-based intermetallic compound phase, when a peak exists in the range of ±0.3 ° of the diffraction peak with the highest intensity in the ICDD card, it is judged that the Zn-containing phase in the coating contains the Fe-Zn-based intermetallic compound phase and / or the Mg-Zn-based intermetallic compound phase. In Table 2, only the presence or absence of at least one of the η-Zn phase and the Fe-Zn-based intermetallic compound phase, and the Mg-Zn-based intermetallic compound phase is shown. However, for example, in Examples 1 and 13, the η-Zn phase and δ-FeZn are confirmed. 10Phase and Γ-Fe3Zn 10 The presence of the Fe-Zn intermetallic compound phases of the phase was confirmed for the η-Zn phase and Γ-Fe3Zn in Examples 2 and 14. 10 The presence of the phase was detected for the η-Zn phase and MgZn2 phase in Examples 5 and 17. The MgZn2 phase was detected in Examples 9 and 20, and the MgZn2 phase, Mg2Zn3 phase, and MgZn phase were detected in Examples 12 and 19.

[0244] A hot-stamped body in which 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 AAA, AA, and A is evaluated as high strength, and which can maintain high corrosion resistance even after hot stamping forming and suppress LME cracking during spot welding. The results are shown in Table 2. In the hot-stamped bodies shown in Table 2, the remaining structure other than martensite is bainite, ferrite, retained austenite, and / or pearlite.

[0245] Table 1

[0246]

[0247] Table 2-1

[0248]

[0249] Table 2-2

[0250]

[0251] Referring to Table 2, for Comparative Example 37, 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 38, 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 39, it is considered that since the holding time in the annealing process is short, similarly, 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 dew point in the annealing process is low, similarly, 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. The cross-section of the coated steel sheet before hot stamping of Comparative Examples 38 to 40 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 41, it is considered that since the average cooling rate of the first cooling in the cooling process is low, pearlite precipitates at 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 42, it is considered that since the control temperature of the first cooling in the cooling process is high, similarly, pearlite precipitates at 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 coated steel sheet before hot stamping of Comparative Examples 41 and 42 was observed by SEM, and as a result, thick 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 Example 43, 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 44, it is considered that since the average cooling rate of the second cooling in the cooling process is fast, similarly, 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 45, due to inappropriate hot stamping conditions, austenitization becomes insufficient, and the desired martensite area ratio cannot be achieved, and the hardness after HS decreases. For Comparative Example 46, it is considered that although the coating adhesion amount is 30 g / m per single side2 However, since the dew point of the cooling gas after plating is low, an Al oxide film cannot be sufficiently formed on the surface of the plating layer. As a result, the evaporation of Zn and / or Mg in the plating layer cannot be sufficiently suppressed or reduced, and accordingly, a specific Zn-containing phase cannot be formed in the plating layer, and the corrosion resistance after HS is reduced.

[0252] In contrast, in the hot-stamped formed bodies of all the examples, by having a specified plating chemical composition and setting the adhesion amount of the plating layer containing a specific Zn-containing phase to 30 g / m per single side 2 or more, and controlling the average C concentration from the surface of the steel base material to a depth of 1 μm in the depth direction to 0.25 mass% or less, even when applied to hot stamping forming, it is high-strength and maintains high corrosion resistance, and sufficiently exhibits the initial LME suppression effect brought about by the low carbon concentration of the steel base material surface layer portion, and can reliably suppress or reduce the generation of LME cracking during subsequent spot welding. The cross-sectional observation of the plated steel sheets before hot stamping of all the examples was carried out using SEM. As a result, 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 is 3 μm or more. In addition, the proportion of coarse pearlite with an equivalent circle diameter of 5 μm or more in the steel base material surface layer portion is sufficiently low compared with Comparative Examples 41 and 42. In particular, for Examples 5 to 8, 16 to 18, and 24 to 28 in which the Al and Mg contents in the plating layer are 6.0 mass% or more and 3.00 mass% or more, respectively, the Zn-containing phase in the plating layer includes at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase. As a result, the evaluation of corrosion resistance becomes AA, and the corrosion resistance can be further improved compared with Example 1 and the like in which the evaluation of corrosion resistance is A. Similarly, for Examples 9 to 12, 19, 20, and 29 to 32 in which the Al and Mg contents in the plating layer are 20.0 mass% or more and 5.00 mass% or more, respectively, the Zn-containing phase in the plating layer includes a Mg-Zn-based intermetallic compound phase, and in particular, only includes a Mg-Zn-based intermetallic compound phase. As a result, the evaluation of corrosion resistance becomes AAA, and the corrosion resistance is further improved. In addition, GDS measurement was carried out on each hot-stamped formed body, and as a result, the presence of a solid solution layer including an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an 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 plating layer disposed on the surface of the steel base material. The plating layer has the following chemical composition: containing, by mass%: Al：0.5～30.0％、 Mg: 0.50 to 15.00%, Si: 0 to 2.0%, and Fe: 15.0 to 70.0%, Further containing at least one of the following elements in a total of 5.000% or less: 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：0～1.000％、 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 martensite by area ratio. The coating amount is 30 g / m per single side 2 or more The plating layer contains a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase. The hot stamping formed body has a Vickers hardness of 400 HV or more.

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

3. The hot stamping formed body according to claim 2, characterized in that, 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, The chemical composition contains, by mass%, Al: 6.0 to 30.0% and Mg: 3.00 to 15.00%, and the Zn-containing phase contains at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase.

5. The hot stamping formed body according to any one of claims 1 to 3, characterized in that The chemical composition contains, by mass%, Al: 20.0 to 30.0% and Mg: 5.00 to 15.00%, and the Zn-containing phase contains an Mg-Zn-based intermetallic compound phase.

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