Overlapped blank for hot stamping and overlapped hot-stamped molded body

Abstract

When an Al-based plated steel sheet is used as a blank, both the slow rate of temperature increase in an overlapping portion and the difference in the rate of temperature increase between the overlapping portion and a single-layer portion are improved. This laminated blank for hot stamping is provided with: a first Al-based plated steel sheet having a sheet thickness of t1; and a second Al-based plated steel sheet having a sheet thickness t2 welded to the first steel sheet in an overlapping manner, having a smaller surface area than the first steel sheet, and having a carbon-based black coating as an upper layer of the Al-based plating layer on the side thereof not in contact with the first Al-based plated steel sheet, the overlapping blank for hot stamping satisfying formulae (1) to (5).

Description

Technical Field

[0001] The present invention relates to overlapping blanks for hot stamping and overlapping hot stamping formed bodies. Background Art

[0002] In recent years, there has been a demand for steel sheets that achieve both high strength and high formability in automotive applications. One type of steel suitable for achieving both high strength and formability is TRIP (Transformation Induced Plasticity) steel, which utilizes the martensitic transformation of retained austenite. TRIP steel enables the production of high-strength steel sheets with excellent formability and strengths of approximately 1000 MPa. However, the use of TRIP steel has difficulty ensuring formability in ultra-high-strength steels with even higher strengths (e.g., 1500 MPa and above). Furthermore, there are issues with poor shape retention after forming and poor dimensional accuracy of the formed product.

[0003] Compared to the above-mentioned forming method near room temperature (so-called cold pressing method), the method that has recently attracted attention is hot stamping (also known as hot pressing, hot pressing, die quenching, press quenching, etc.). Hot stamping is a method for manufacturing the following parts: after heating the steel plate to above the Ac3 point (for example, above 800°C) and austenitizing, the steel plate is immediately pressed under heating conditions to ensure formability, and the die is rapidly cooled to below the Ms point (for example, below 400°C) while holding the bottom dead point, thereby martensited and quenched, thereby obtaining the desired high-strength material after pressing. According to this method, automotive parts with excellent shape retention after forming can be obtained.

[0004] On the other hand, various press-formed articles used in components that form automobile bodies are being required to have improved performance and characteristics from various perspectives, including static strength, dynamic strength, collision safety, and lightweighting. For example, in automotive parts such as A-pillar reinforcements, B-pillar reinforcements, bumper reinforcements, tunnel reinforcements, side sill reinforcements, roof reinforcements, and floor cross members, only specific portions of each part are required to have collision resistance characteristics comparable to those of the general parts.

[0005] Therefore, the following method has been actually adopted since around 2007: after overlapping and welding a plurality of steel plates only at the portion of the automotive part corresponding to the specific portion that needs to be strengthened, the obtained steel plates are hot stamped to produce overlapping hot stamped parts (see Patent Documents 1 and 2). According to this method, it is possible to partially strengthen only specific portions of the overlapping hot stamped parts while reducing the number of stamping dies, and since the thickness of the parts is not unnecessarily increased, it is also possible to contribute to the lightweighting of the parts. In addition, the blanks produced by such overlapping and welding are called overlapping blanks (also called spliced ​​blanks).

[0006] In the case where the overlapping steel plates are non-plated steel plates, oxide scale is generated on the surface of the manufactured overlapping hot stamping components due to the high temperature heating accompanying hot press forming. Therefore, there is the following problem: after hot press forming, the generated oxide scale needs to be removed, for example, by shot peening, or the corrosion resistance of the manufactured overlapping hot stamping components is easily reduced. In addition, as a unique problem in the case of using non-plated steel plates as blanks for overlapping blanks, there is the following problem: although the non-overlapping parts (hereinafter also referred to as "single-layer parts") can be shot peened, the oxide scale formed between the steel plates in the overlapping parts (hereinafter also referred to as "overlapping parts") is difficult to remove by shot peening, and the corrosion resistance is particularly easily reduced.

[0007] If the stacked steel sheets are plated, the need for shot peening of the stacked hot-pressed components after hot-pressing is eliminated. Typical examples of plated steel sheets used for hot-pressing include Zn-plated steel sheets and Al-plated steel sheets. With either Zn-plating or Al-plating, the alloying reaction caused by the diffusion of Fe in the plating causes Zn-plating to become Zn-Fe plating, while Al-plating to become Al-Fe plating, after heating during hot stamping.

[0008] As shown in Patent Documents 2 and 3, Zn-plated steel sheets (i.e., plated steel sheets containing 50% by mass or more of Zn (Zn plating or Zn-Fe alloy, Zn-Ni alloy, Zn-Fe-Al alloy, etc. Zn alloy plating)) suppress the formation of oxide scale and eliminate the need for shot peening. However, when Zn-plated steel sheets are used as overlapping blanks and the overlapping portions are bent during hot stamping, problems sometimes arise in the collision resistance due to cracks in the base metal. This is caused by the Zn becoming liquid metal and penetrating the base metal from the plated surface when zinc with a relatively low melting point remains, which is caused by a problem called liquid metal embrittlement. In addition, bending is a means of ensuring collision resistance from the aspect of shape, and bending the overlapping portions is an extremely important method of utilizing overlapping formed bodies.

[0009] As shown in Patent Documents 2 and 3, as a countermeasure for liquid metal embrittlement when using Zn-plated steel sheets for hot stamping, there are generally listed countermeasures such as performing a Zn-Fe alloying reaction during hot stamping heating to increase the melting point of the plating, and lowering the forming temperature during hot stamping bending forming to wait for the zinc to solidify. However, as a unique problem when using zinc-plated steel sheets as blanks for overlapping blanks, there is the following problem: since the thickness of the overlapping part is thicker than that of the single-layer part, the heating rate is slow, making it difficult to perform a Zn-Fe alloying reaction during hot stamping heating. In addition, regarding the forming temperature during hot stamping forming, there is also the following problem: since the thickness of the overlapping part is thicker than that of the single-layer part, the cooling rate is also slow. If the overlapping part is left to cool, the single-layer part will cool faster and the martensitic structure cannot be ensured. In addition, in the single-layer portion, Zn forms a zinc oxide film, which suppresses Zn evaporation. However, due to oxygen deficiency in the atmosphere between the steel plates in the overlapping portion, Zn evaporates, further increasing the problem of reduced corrosion resistance in the overlapping portion and embrittlement of the liquid metal.

[0010] In the Al-plated steel sheet disclosed in Patent Document 4 (i.e., a plated steel sheet containing 50% by mass or more of Al (Al plating, or Al-based alloy plating such as Al-Si alloy, Al-Fe-Si alloy, etc.)), the formation of oxide scale is suppressed similarly to Zn, eliminating the need for shot peening. Furthermore, in the case of Al, which has a higher melting point than Zn, the problem of liquid metal embrittlement does not occur. Therefore, the Al-plated steel sheet is suitable for use as a blank for stacking blanks.

[0011] Therefore, the present inventors have conducted intensive research in recent years on Al-plated steel sheets for laminated blanks and laminated blanks using the Al-plated steel sheets, and have proposed various proposals (for example, see Patent Documents 5 to 8 below).

[0012] Prior art literature

[0013] Patent Literature

[0014] Patent Document 1: Japanese Patent Application Laid-Open No. 2011-88484

[0015] Patent Document 2: Japanese Patent No. 6178301

[0016] Patent Document 3: Japanese Patent Application Laid-Open No. 2016-124029

[0017] Patent Document 4: International Publication No. 2002 / 103073

[0018] Patent Document 5: International Publication No. 2019 / 194308

[0019] Patent Document 6: Japanese Patent Application Laid-Open No. 2011-149084

[0020] Patent Document 7: International Publication No. 2010 / 005121

[0021] Patent Document 8: International Publication No. 2021 / 172379 Summary of the Invention

[0022] Problems to be solved by the invention

[0023] However, as a unique problem when using Al-based plated steel sheets as disclosed in Patent Document 4 as the blanks for overlapping blanks, there are problems such as reduced productivity of hot stamped parts due to the slow heating rate of the overlapping portion during hot stamping and the difference in heating rate between the overlapping portion and the non-overlapping portion (single-layer portion), as well as problems with spot weldability.

[0024] That is, in order to make the overlapping portion reach the austenitizing temperature or above, it is necessary to make the heating time longer, thereby causing the problem of reduced component productivity. In particular, in the case of a steel plate without Al-plating (i.e., when the steel plate surface is Fe), Fe will oxidize and become black. In the case of Zn-plating, compared with Al-plating, the Zn-Fe alloying reaction is more likely to proceed, causing the metallic luster to disappear and become black. Compared with these steel plates, in the case of Al-plating steel plates, not only does it have a silvery-white metallic luster, but the oxidation resistance of Al-plating is excellent, so the metallic luster can be maintained for a longer time during heating, thereby slowing the heating rate. As a result, the problem of reduced component productivity in hot stamping is easily generated.

[0025] Furthermore, as described in Patent Document 3, the Al-Fe alloying reaction that occurs during hot stamping in Al-plated steel sheets significantly affects the spot weldability of the coating. When the hot stamping heating time is prolonged, the Al-Fe alloying reaction excessively progresses in the single-layer portion of the steel sheet, forming an Al-Fe alloy coating with a high Fe concentration. This results in a further problem of reduced spot weldability in the single-layer portion of the steel sheet.

[0026] Therefore, in the context of the above description, in order to suppress the oxide scale of the base metal and avoid the problem of embrittlement of the liquid metal, it is desired to solve the problems of hot stamping component productivity and the problem of spot weldability of the single-layer portion by an invention that can improve both the slow heating rate of the overlapping portion and the difference in heating rate between the overlapping portion and the single-layer portion as an Al-coated steel sheet suitable for use as a blank for hot stamping overlapped blanks. Regarding these problems of hot stamping component productivity and the problem of spot weldability of the single-layer portion, there is room for improvement in the technologies described in Patent Documents 5 to 8.

[0027] Therefore, the present invention is made in view of the above-mentioned problems, and the purpose of the present invention is to provide overlapping blanks and overlapping hot stamping bodies for hot stamping that can improve both the slow heating rate of the overlapping part and the difference in heating rate between the overlapping part and the single-layer part when Al-coated steel plates are used as blanks.

[0028] Solutions for solving problems

[0029] To solve the above-mentioned problem, the present inventors have repeatedly conducted in-depth research and paid attention to the brightness L* specified in JIS Z 8781-4 of the surface of the Al-coated steel sheet. As a result, it was confirmed that the lower the brightness, the higher the temperature rise rate during hot stamping of the Al-coated steel sheet. It is believed that this is because the lower the brightness, the more black the surface of the Al-coated steel sheet, thus obtaining the characteristic of easily absorbing heat. In particular, it was found that in order to improve the situation where the temperature rise rate of the spliced ​​overlapping part is slow, it is important to reduce the brightness of the overlapping part relative to the thickness of the overlapping part (i.e., the sum of the thicknesses of the two steel sheets). Furthermore, it was found that in order to improve the difference in temperature rise rate between the overlapping part and the single-layer part, it is important to set a difference between the brightness of the overlapping part and the brightness of the single-layer part (i.e., in the overlapping part with a thicker plate thickness, the surface brightness is reduced, and conversely, in the single-layer part, the surface brightness is increased).

[0030] In addition, it was discovered that by using Al-plated steel sheets, a higher brightness can be achieved by having a surface with a silvery-white metallic luster, and by increasing the thickness of the Al-plated layer, the blackening of the plated surface caused by alloying of the plating reaching the surface can be suppressed, thereby maintaining a high brightness even during heating in hot stamping.

[0031] Furthermore, it was discovered that the brightness can be reduced by using a carbon-based black coating on the upper layer of the Al-plated steel sheet. In particular, it was discovered that the carbon-based black coating burns away due to an oxidation reaction during hot stamping, thus suppressing the degradation of the spot weldability of overlapping hot stamped parts caused by the remaining carbon-based black coating.

[0032] The gist of the present invention, which has been accomplished based on the above findings, is as follows.

[0033] [1] A hot stamping overlapped blank comprising: a first Al-plated steel sheet having a thickness of t1; and a second Al-plated steel sheet having a thickness of t2, which is overlapped and welded to the first steel sheet, has a smaller area than the first steel sheet, and has a carbon-based black coating as an upper layer of the Al-plated layer on a side thereof not in contact with the first Al-plated steel sheet.

[0034] The hot stamping superimposed blank satisfies the following equations (1) to (5):

[0035] 14.0≤(L*1-L*2)×(W1 / W2) 2 ≤32.0 ... Formula (1)

[0036] 0.86≤{t1 / (t1+t2)}×(L*1 / L*2) 2 ... Formula (2)

[0037] 20≤W1a≤120 ... Formula (3)

[0038] 20≤W1b≤120 ... Formula (4)

[0039] 20≤W2b≤120 ... Formula (5)

[0040] Here,

[0041] W1=0.5×(W1a+W1b)

[0042] W2=0.5×(W1b+W2b)

[0043] L*1=0.5×(L*1a+L*1b)

[0044] L*2=0.5×(L*1b+L*2b)

[0045] W1a: The amount of Al-based plating deposited on the surface of the first Al-based plated steel sheet that contacts the second Al-based plated steel sheet,

[0046] W1b: The amount of Al-based plating layer deposited on the surface of the first Al-based plated steel sheet that is not in contact with the second Al-based plated steel sheet,

[0047] W2b: The amount of Al-based plating layer deposited on the surface of the second Al-based plated steel sheet that is not in contact with the first Al-based plated steel sheet,

[0048] L*1a: Brightness L* of the surface of the first Al-plated steel sheet on the side in contact with the second Al-plated steel sheet,

[0049] L*1b: Brightness L* of the surface of the first Al-based plated steel sheet that is not in contact with the second Al-based plated steel sheet,

[0050] L*2b: Brightness L* of the surface of the second Al-plated steel sheet that is not in contact with the first Al-plated steel sheet,

[0051] The units of t1 and t2 are mm, and the units of W1a, W1b, W2a, and W2b are g / m 2 .

[0052] [2] The hot stamping overlapping blank according to [1], wherein the hot stamping overlapping blank satisfies the following formula (6):

[0053] 14.0≤(L*1-L*2)×(W1 / W2) 2 ≤30.0...Formula (6).

[0054] [3] The hot stamping overlapping blank according to [1] or [2], wherein the first Al-plated steel sheet has a carbon-based black coating as an upper layer of the Al-plated layer on the side thereof not in contact with the second Al-plated steel sheet.

[0055] [4] The hot stamping stacked blank according to any one of [1] to [3], wherein the carbon-based black coating has a thickness of 0.3 to 10.0 μm.

[0056] [5] The hot stamping overlapping blank according to any one of [1] to [4], wherein the total of the Zn content, Ti content, Cu content and V content of the carbon-based black coating is 0.2 to 3.0 g / m 2 .

[0057] [6] The hot stamping overlapping blank according to any one of [1] to [4], wherein the upper layer of the Al plating layer further has a total Zn content, Ti content, Cu content and V content of 0.2 to 3.0 g / m 2 layer.

[0058] [7] The hot stamping stacked blank according to any one of [1] to [6], wherein the carbon-based black coating has a nitrogen content of 2 to 18% by mass.

[0059] [8] A superposed hot stamped steel body comprising: a first Al-Fe alloy plated steel sheet having a thickness of T1; and a second Al-Fe alloy plated steel sheet having a thickness of T2, the second Al-Fe alloy plated steel sheet being superposed and welded to the first Al-Fe alloy plated steel sheet and having a smaller area than the first Al-Fe alloy plated steel sheet.

[0060] The stacked hot stamped body satisfies the following relationships (7) to (9):

[0061] 25≤K1≤60 ... Formula (7)

[0062] 25≤K2≤60 ... Formula (8)

[0063] 0≤(D1-D2)×(K1 / K2) 2 ≤5.0 ... Formula (9)

[0064] Here,

[0065] K1: Average value of the plating thickness of the Al-Fe alloy plating layer on the side of the first Al-Fe alloy plated steel sheet that is in contact with the second Al-Fe alloy plated steel sheet and the plating thickness of the Al-Fe alloy plating layer on the side that is not in contact with the second Al-Fe alloy plated steel sheet at the non-overlapping portion of the first Al-Fe alloy plated steel sheet

[0066] K2: the plating thickness of the Al—Fe alloy plating layer at the overlapping portion of the second Al—Fe alloy plated steel sheet, which is not in contact with the first Al—Fe alloy plated steel sheet;

[0067] D1: the average value of the thickness of the diffusion layer in the Al-Fe alloy plated steel sheet located on the side in contact with the second Al-Fe alloy plated steel sheet so as to be in contact with the steel sheet substrate, and the thickness of the diffusion layer in the Al-Fe alloy plated steel sheet located on the side not in contact with the second Al-Fe alloy plated steel sheet so as to be in contact with the steel sheet substrate,

[0068] D2: thickness of the diffusion layer in the Al-Fe alloy plated layer on the side of the second Al-Fe alloy plated steel sheet that is not in contact with the first Al-Fe alloy plated steel sheet, so as to be in contact with the steel sheet substrate;

[0069] The units of the plate thickness T1 and the plate thickness T2 are mm, and the units of K1, K2, D1, and D2 are μm.

[0070] [9] The overlapped hot stamped body according to [8], wherein the sum of the Zn content, Ti content, Cu content and V content in the second Al-Fe alloy plated steel sheet is 0.2 to 3.0 g / m 2 The layer is located on the Al-Fe alloy plating layer on the side not in contact with the first Al-Fe alloy plated steel sheet,

[0071] Furthermore, in the first Al-Fe alloy plated steel sheet, the total of the Zn content, the Ti content, the Cu content, and the V content is 0 to 3.0 g / m 2 The layer is located on the Al-Fe alloy plating layer on the side not in contact with the second Al-Fe alloy plated steel sheet.

[0072] Effects of the Invention

[0073] As described above, according to the present invention, when Al-based plated steel sheets are used as blanks for overlapping blanks for hot stamping, both the slow heating rate of the overlapping portion and the difference in heating rate between the overlapping portion and the single-layer portion can be improved, thereby solving the problems of component productivity of hot stamped parts and spot weldability of the single-layer portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 It is an explanatory diagram schematically showing an example of a stacked blank for hot stamping, a method for manufacturing a stacked hot stamped product, and a stacked hot stamped product according to an embodiment of the present invention.

[0075] Figure 2 This is an explanatory diagram schematically showing the structure of an Al-based plated steel sheet having an Al-based plated layer on the surface of a base steel sheet of a hot stamping stacked blank according to this embodiment.

[0076] Figure 3 This is an explanatory diagram schematically showing the structure of an Al-plated steel sheet having an Al-plated layer on the surface of a base steel sheet, and a carbon-based black coating or a carbon-based black coating selectively containing Zn, Ti, Cu, and V on the upper layer of the Al-plated layer, which is a superimposed blank for hot stamping according to this embodiment.

[0077] Figure 4 The figure shows the results of optical microscope observation after etching a cross section of an Al-plated steel sheet having an Al-plated layer on the surface of a base steel sheet of a hot stamping stacked blank according to the embodiment with Nital.

[0078] Figure 5 This is an explanatory diagram schematically showing the structure of an Al-Fe alloy-plated steel sheet of a stacked hot stamped body of this embodiment, which has an Al-Fe alloy plating layer on the surface of a base steel sheet and a diffusion layer contained in the Al-Fe alloy plating and in contact with the steel sheet substrate.

[0079] Figure 6 The figure shows the results of optical microscope observation after etching a cross section of an Al—Fe alloy plated steel sheet having an Al—Fe alloy plated layer on the surface of a base steel sheet of the hot stamped product according to the embodiment with nital.

[0080] Figure 7 This is an explanatory diagram schematically showing the structure of the overlapping hot stamping body of this embodiment, which has an Al-Fe alloy coating (including a diffusion layer) on the surface of the base steel plate and Zn, Ti, Cu, and V on the upper layer of the Al-Fe alloy coating. DETAILED DESCRIPTION

[0081] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, in this specification and the drawings, components having substantially the same functional structure are denoted by the same reference numerals to omit repeated description.

[0082] Figure 1 This is an explanatory diagram schematically showing an example of a stacked blank for hot stamping and a stacked hot stamped product according to an embodiment of the present invention.

[0083] The hot stamping overlapping blank of the present embodiment is a type of tailor-welded blank, also called a spliced ​​blank. The hot stamping overlapping blank of the present embodiment is used as a blank for overlapping hot stamped bodies.

[0084] like Figure 1 As schematically shown, the hot stamping overlapping blank 4 of this embodiment is formed by welding a first Al-plated steel sheet 1 and a second Al-plated steel sheet 2 having a smaller area than the first Al-plated steel sheet 1 together at a weld portion 3. At this time, in the hot stamping overlapping blank 4, the portion where the second Al-plated steel sheet 2 overlaps is referred to as an overlapping portion 4a, and the portion where the second Al-plated steel sheet 2 does not overlap is referred to as a single layer portion 4b. In addition, in the hot stamping overlapping blank 4 of this embodiment, the second Al-plated steel sheet 2 is also preferably as shown in FIG. Figure 1 As schematically shown in , it is arranged inside the outer edge of the first Al-based plated steel sheet 1 so that there is no portion protruding from the first Al-based plated steel sheet 1 .

[0085] Furthermore, on the surface of the first Al-plated steel sheet 1, Al-plating is applied to both the surface 1a on the side in contact with the second Al-plated steel sheet 2 and the surface 1b on the side not in contact with the second Al-plated steel sheet 2. Similarly, Al-plating is applied to both the surface 2a on the side in contact with the first Al-plated steel sheet 1 and the surface 2b on the side not in contact with the first Al-plated steel sheet 1 of the second Al-plated steel sheet 2. Furthermore, a carbon-based black coating (not shown) is provided on the upper layer of the Al-plating on the surface 2b of the second Al-plated steel sheet 2 not in contact with the first Al-plated steel sheet 1.

[0086] In the method for manufacturing a stacked hot stamped body according to this embodiment, a hot stamping stacked blank 4 is heated to a point above Ac3 in a heating furnace 5, thereby austenitizing the steel sheet. Immediately after removal from the furnace, the hot stamping stacked blank 4 is press-formed and rapidly cooled in a die 6, causing the steel sheet to undergo a martensitic transformation. As a result, the hot stamping stacked blank 4 becomes the stacked hot stamped body 12 of this embodiment, which exhibits excellent crash resistance. At this point, at least a portion of the overlapping portion 4a includes a portion that will become the bent portion 8 when the stacked hot stamped body 12 is formed.

[0087] exist Figure 1, a molded product using a hat-shaped mold is shown as an example of an overlapping hot stamped body 12. Here, the parts of the hot stamped body 12 are referred to as the top portion 7, the curved portion 8 of the top portion, the vertical wall portion 10, the flange portion 11, and the curved portion 9 of the flange portion.

[0088] In addition, Figure 1 In the present embodiment, the second Al-plated steel sheet 2 is arranged outside the top portion 7 , but the object of the present invention can be achieved even if the second Al-plated steel sheet 2 is arranged inside the top portion 7 .

[0089] (1. Overlapping blanks for hot stamping)

[0090] Hereinafter, the hot stamping stacked blank 4 according to the present embodiment will be described in detail.

[0091] As described above, the hot stamping superimposed blank 4 of this embodiment includes a first Al-plated steel sheet 1 and a second Al-plated steel sheet 2 welded to the first Al-plated steel sheet 1 and having a smaller area than the first Al-plated steel sheet 1. Al-plating is applied to both surfaces of each of the first Al-plated steel sheet 1 and the second Al-plated steel sheet 2. In other words, the first Al-plated steel sheet 1 and the second Al-plated steel sheet 2 of this embodiment are aluminum-plated steel sheets having Al-plating layers on the surfaces of both base steel sheets.

[0092] <Base Steel Plate>

[0093] In the hot stamping overlapped blank 4 of the present embodiment, the chemical composition of the base steel sheet of each of the first Al-plated steel sheet 1 and the second Al-plated steel sheet 2 is not particularly limited. However, for example, in order to obtain a tensile strength of 1500 MPa or more (a Vickers hardness of 400 or more when the test force is set to 9.8107 N, i.e., HV1 in JIS Z2244-1:2020), it is preferable to use a base steel sheet having the following chemical composition, in terms of mass %, C: 0.19-0.50%, Si: 0.01-1.50%, Mn: 0.4-2.0%, Cr: 0.01-1.00%, Ti: 0.001-0.100%, B: 0.0005 The following chemical compositions are present: 1) 10% to 0.0100%; 2) 0% to 0.0100%; 3) 0% to 0.0100%; 4) 0% to 0.0100%; 5) 0% to 0.0100%; 6) 0% to 0.0100%; 7) 0% to 0.0100%; 8) 0% to 0.0100%; 9) 0% to 0.0100%; 10) 0% to 0.0100%; 11) 0% to 0.0100%; 12) 0% to 0.0100%; 13) 0% to 0.0100%; 14) 0% to 0.0100%; 15) 0% to 0.0100%; 16) 0% to 0.0100%; 17) 0% to 0.0100%; 18) 0% to 0.0100%; 19) 0% to 0.0100%; 20) 0% to 0.0100%; 21) 0% to 0.0100%; 22) 0% to 0.0100%; 23) 0% to 0.0100%; 24) 0% to 0.0100%; 25) 0% to 0.0100%; 26) 0% to 0.0100%; 27) 0% to 0.0100%; 28) 0% to 0.0100%; 29) 0% to 0.0100%; 30) 0% to 0.0100%; 3

[0094] The method for producing the Al-plated steel sheet using the above-mentioned chemical composition as the base steel sheet is not particularly limited. For example, the aluminum-plated steel sheet can be produced by a conventional ironmaking process and steelmaking process, followed by hot rolling, pickling, cold rolling, and Sendzimir hot-dip aluminum coating.

[0095] In this embodiment, the thickness t1 (mm) of the first Al-plated steel sheet 1 and the thickness t2 (mm) of the second Al-plated steel sheet 2 are preferably selected to be 0.5 mm or greater and 3.2 mm or less. By setting the thickness to 0.5 mm or greater, the productivity of the hot rolling and cold rolling processes can be maintained at a desired level. In addition, by setting the thickness to 3.2 mm or less, the following phenomenon can be prevented: during press quenching in hot stamping, the cooling rate is reduced, resulting in insufficient hardenability, which results in failure to achieve the desired tensile strength.

[0096] The thickness t1 of the first Al-plated steel plate 1 and the thickness t2 of the second Al-plated steel plate 2 can be measured using a micrometer conforming to JIS B 7502: 2016, for example. The thicknesses t1 and t2 are defined as thicknesses that include not only the thickness of the base steel plate but also the thickness of the Al-plated layers provided on both surfaces.

[0097] <Al-based plating>

[0098] The adhesion amount of the Al-based plating layer applied to both surfaces of the first Al-based plated steel sheet 1 is W1a (g / m 2 ), W1b (g / m 2 ). In addition, the adhesion amount of the Al-based plating layer applied to both surfaces of the second Al-based plated steel sheet 2 is W2b (g / m 2 ). Here, regardless of which of the above W1a, W1b, and W2b, the value thereof is independently 20 g / m per single side. 2 Above and 120g / m 2 That is, W1a, W1b, and W2b satisfy the following formulas (3), (4), and (5), respectively.

[0099] 20≤W1a≤120 ... Formula (3)

[0100] 20≤W1b≤120 ... Formula (4)

[0101] 20≤W2b≤120 ... Formula (5)

[0102] Here, the average amount of Al-based plating deposited on the non-overlapped portion (single-layer portion 4b) of the hot stamping overlapped blank 4 is W1 (g / m 2 In the first Al-plated steel sheet 1, the amount of Al-plated layer deposited on one side of the surface in contact with the second Al-plated steel sheet 2 is W1a (g / m 2 ) and the amount of Al-based plating on one side of the surface not in contact with the second Al-based plated steel sheet 2 is W1b (g / m 2 ), W1 = 0.5 × (W1a + W1b). In addition, the average amount of Al-based plating applied to the overlapping portion 4a of the hot stamping overlapping blank 4 is set to W2 (g / m 2 The amount of Al-based coating on one side of the surface 1b of the first Al-based plated steel sheet 1 that is not in contact with the second Al-based plated steel sheet 2 is W1b (g / m 2 ) and the amount of Al-based coating on one side of the second Al-based plated steel sheet 2 that is not in contact with the first Al-based plated steel sheet 1 is W2b (g / m 2 ), W2=0.5×(W1b+W2b).

[0103] In addition, the surface 1a of the first Al-plated steel plate 1 on the side in contact with the second Al-plated steel plate 2, the surface 1b on the side not in contact with the second Al-plated steel plate 2, and the surface 2b of the second Al-plated steel plate 2 on the side not in contact with the first Al-plated steel plate 1 are the surfaces of the manufactured overlapping blanks exposed to the heat source when subjected to heating in hot stamping, and are important surfaces in controlling the temperature rise rate during heating in hot stamping.

[0104] The properties required of the Al-based plating layer of this embodiment include: (a) suppressing the generation of Fe oxide scale during hot stamping heating; and (b) suppressing plating defects and bruises caused by plating slippage (also called powdering) during hot stamping.

[0105] Powdering is caused by compressive stress applied to the coating on the inner surface of the bent portion during forming, or shear stress applied to the coating due to the sliding of the mold during forming. When the adhesion amount W1 and W2 of the Al-based coating in each steel plate is less than 20 g / m 2 Therefore, the adhesion amount W1 and W2 of the Al-based coating in each Al-based plated steel sheet is independently set to 20 g / m 2 The coating weights W1 and W2 of the Al-based plating layers in each Al-based plated steel sheet are preferably 30 g / m², respectively. 2 More preferably, in the order of preference, 35 g / m2 Above, 40g / m 2 Above, 45g / m 2 Above or 50g / m 2 above.

[0106] On the other hand, the coating weights W1 and W2 per single surface of each Al-based plated steel sheet exceeded 120 g / m 2 Therefore, in this embodiment, the coating weights W1 and W2 per single side of each Al-based plated steel sheet are independently set to 120 g / m 2 The coating weights W1 and W2 per single surface of each Al-based plated steel sheet are preferably 110 g / m², respectively. 2 Hereinafter, it is more preferred that the preferred order is 100 g / m 2 Below, 95g / m 2 Below, 90g / m 2 Hereinafter, in the second Al-plated steel sheet 2 , the amount of the Al-plated layer deposited on the surface in contact with the first Al-plated steel sheet 1 is not particularly limited.

[0107] Furthermore, the thickness (μm) of the Al-based plating layer in each Al-based plated steel sheet can be determined from the coating weight (g / m 2 The thickness (μm) of the Al-based plating layer in each Al-based plated steel sheet also depends on the chemical composition of the Al-based plating layer, but can be roughly calculated by the following formula (10).

[0108] (Thickness of the coating) = (Coating adhesion) / 3... Formula (10)

[0109] Figure 2 The layer structure of one side of a plated steel sheet 13 having an Al-based plating layer according to this embodiment provided on the surface of a base steel sheet is schematically shown. When the Al-based plating layer according to this embodiment is produced by hot-dip plating, an aluminum-iron (Al-Fe) alloy layer (not shown) is formed near the boundary between the Al-based plating layer 14 and the base steel sheet 15.

[0110] In addition, as a method of applying Al-based plating to a base steel sheet, according to a conventional hot-dip plating method, the base steel sheet can be immersed in a hot-dip aluminum bath and gas-wiped with nitrogen, air, etc. to produce an Al-based plated steel sheet with an adjusted adhesion amount. Figure 2 An aluminum-iron alloy layer is formed at the interface between the Al-based coating 14 and the base steel plate 15 due to the dissolution of Fe during hot-dip coating. Figure 2 The Al-based plating layer 14 also includes an aluminum-iron alloy layer.

[0111] The chemical composition of the hot-dip aluminizing bath used to form the above-mentioned Al-based coating (that is, except for Fe, the chemical composition is substantially the same as the chemical composition of the Al-based coating 14) is not particularly limited. However, from the perspective of excellent heat resistance required during hot stamping heating, the Al content of the hot-dip aluminizing bath is preferably 80% by mass or more. In addition, from the perspective of easy control of the thickness of the aluminum-iron alloy layer, the Si content of the hot-dip aluminizing bath is preferably 2% by mass or more. By setting the Si content to 2% by mass or more, it is possible to prevent the aluminum-iron alloy layer from becoming too thick and the formability from decreasing. On the other hand, by setting the Si content of the hot-dip aluminizing bath to 15% by mass or less, it is possible to prevent the alloying reaction during hot stamping heating from slowing down and reducing the productivity of hot stamping.

[0112] When the hot-dip aluminum coating bath contains 2% to 15% Si by mass, a eutectic structure of Al and Si is formed in the Al-based coating layer 14 formed using this coating bath, based on the phase diagram. When hot-dip coating is used, Fe may inevitably be contained in an amount of 1% to 5% by mass as a component eluted from the base steel sheet. Other unavoidable impurities include Cr, Mn, Zn, V, Ti, Sn, Ni, Cu, W, Bi, Mg, and Ca, which are eluted from hot-dip coating equipment and impurities in the ingot cast in the hot-dip aluminum coating bath. These elements may be contained in an amount of less than 1% by mass.

[0113] That is, the chemical composition (average chemical composition) of the Al-based plating layer 14 of this embodiment may include, in mass%, Al: 80-97%, Si: 2-15%, Fe: 1-15%, Cr: 0% or more and less than 1%, Mo: 0% or more and less than 1%, Zn: 0% or more and less than 1%, V: 0% or more and less than 1%, Ti: 0% or more and less than 1%, Sn: 0% or more and less than 1%, Ni: 0% or more and less than 1%, Cu: 0% or more and less than 1%, W: 0% or more and less than 1%, Bi: 0% or more and less than 1%, Mg: 0% or more and less than 1%, Ca: 0% or more and less than 1%, with the remainder being impurities. As described above, the hot-dip aluminum plating bath inevitably contains 1% or more and 5% or less of Fe. However, since the Al-based plating layer forms an aluminum-iron alloy layer, the Fe ratio increases. Therefore, the chemical composition of the Al-based plating layer 14 may be Fe: 1 to 15%.

[0114] Examples of the metallographic structure of the aluminum-iron alloy layer include binary alloys of Al and Fe, namely, θ phase (FeAl3), η phase (Fe2Al5), ζ phase (FeAl2), Fe3Al, FeAl, and BCC phases (α2, α) of Al solid solution. The aluminum-iron alloy layer is formed by a combination of these plating phases. Examples of the metallographic structure of the aluminum-iron alloy layer containing Si include τ1 phase (Al2Fe3Si3), τ2 phase (Al3FeSi), τ3 phase (Al2FeSi), τ4 phase (Al3FeSi2), τ5 phase (Al8Fe2Si), τ6 phase (Al9Fe2Si2), τ7 phase (Al3Fe2Si3), τ8 phase (Al2Fe3Si4), τ10 phase (Al4Fe 1.7 Si), τ11 phase (Al5Fe2Si), etc. (wherein each phase is sometimes not a stoichiometric composition.), in most cases, the metallographic structure of the aluminum-iron alloy layer is mainly composed of τ5 phase or θ phase.

[0115] The amount of Al-based plating deposited on one side was measured using the JIS G 3314:2019 JB.3 method (sodium hydroxide-hexamethylenetetramine / hydrochloric acid fractionation weight method) after protecting the Al-based plating on one side with a sealant.

[0116] Brightness

[0117] Below, refer to Figure 1 Brightness, which is important in the present invention, is described below. This brightness is the CIE 1976 lightness index L* (CIE 1976 lightness) specified in 3.3 of JIS Z 8781-4:2013, and is hereinafter referred to as "brightness" or "brightness L*."

[0118] Regarding the brightness L* of the surface of the first Al-plated steel sheet 1, the surface 1a on the side in contact with the second Al-plated steel sheet 2 is marked as L*1a, and the surface 1b on the side not in contact with the second Al-plated steel sheet 2 is marked as L*1b. Furthermore, regarding the brightness L* of the surface of the second Al-plated steel sheet 2, the surface 2b on the side not in contact with the first Al-plated steel sheet 1 is marked as L*2b. On this basis, it is defined as L*1=0.5×(L*1a+L*1b), L*2=0.5×(L*1b+L*2b). In this case, the overlapping blank 4 for hot stamping satisfies the following formulas (1) and (2). In addition, in the second Al-plated steel sheet 2, there is no particular limitation on the brightness of the surface of the Al-plated layer on the surface 2a on the side in contact with the first Al-plated steel sheet 1.

[0119] 14.0≤(L*1-L*2)×(W1 / W2) 2≤32.0 ... Formula (1)

[0120] 0.86≤{t1 / (t1+t2)}×(L*1 / L*2) 2 ... Formula (2)

[0121] Here, L*1 refers to the average brightness of the surface of the Al-based plating layer in the non-overlapped portion (single-layer portion 4b) of the hot stamping stacked blanks 4. Furthermore, L*2 refers to the average brightness of the surface of the Al-based plating layer in the overlapped portion 4a of the hot stamping stacked blanks 4.

[0122] [(L*1-L*2)×(W1 / W2) 2 : 14.0 or more and 32.0 or less]

[0123] The lower the brightness value, the higher the temperature rise rate during the hot stamping of the aluminum-plated steel sheet. It is believed that the reason for this is that the lower the brightness value, the more black the surface of the aluminum-plated steel sheet is, and thus the characteristic of easily absorbing heat is obtained. That is, in this embodiment, brightness is used as an indicator for evaluating the temperature rise rate during the hot stamping. In particular, it was found that in order to improve the difference in temperature rise rate between the overlapping portion and the single-layer portion, it is important to set the difference between the brightness of the overlapping portion and the brightness of the single-layer portion and increase (L*1-L*2) (that is, in the overlapping portion with a thicker plate thickness, reduce the brightness of the surface, and conversely, in the single-layer portion, increase the brightness of the surface).

[0124] Furthermore, the following situations were found: by using an Al-plated steel sheet, a higher brightness can be obtained by using a surface with a silvery-white metallic luster, and by increasing the thickness of the Al-plated layer (which can also be considered as the amount of Al-plated layer attached), the blackening of the plated surface caused by the alloying of the plating reaching the surface can be suppressed, thereby maintaining a higher brightness even during the heating of hot stamping. That is, it is important to increase the ratio of the amount of Al-plated layer attached W1 of the single-layer portion 4b to the amount of Al-plated layer attached W2 of the overlapping portion 4a, that is, (W1 / W2). In addition, by using a carbon-based black coating on the upper layer of the Al-plated steel sheet, the brightness can be reduced. In this way, by appropriately controlling the Al-plated layer and the carbon-based black coating used for the Al-plated steel sheet, the brightness can be adjusted to the desired value.

[0125] As mentioned above, in order to improve the difference in temperature rise rate between the overlapped portion and the single layer portion, since the brightness and the adhesion amount are related to each other, (L*1-L*2) and (W1 / W2) are used to calculate the difference. 2 It is important to control the product within a certain range.

[0126] (L*1-L*2)×(W1 / W2) 2When the value of is less than 14.0, the improvement of the heating rate during hot stamping is insufficient, and the difference in the heating rate between the overlapping portion and the single layer portion is not sufficiently improved, and the spot weldability of the overlapping hot stamped body is reduced. Therefore, (L*1-L*2)×(W1 / W2) 2 The value should be 14.0 or more. (L*1-L*2)×(W1 / W2) 2 The value of is preferably 16.0 or more, more preferably 18.0 or more.

[0127] In addition, (L*1-L*2)×(W1 / W2) 2 If the value exceeds 32.0, the improvement of the heating rate during hot stamping will be saturated, and the amount of Al-based coating W1 of the first Al-based plated steel sheet 1, which is larger in area, will increase, thereby reducing the powdering property of the coating during hot stamping. Therefore, (L*1-L*2)×(W1 / W2) 2 The value should be 32.0 or less. (L*1-L*2)×(W1 / W2) 2 The value of is preferably 31.0 or less, more preferably 30.0 or less, 29.0 or less, 28.0 or less, 27.0 or less, or 26.0 or less.

[0128] [{t1 / (t1+t2)}×(L*1 / L*2) 2 :0.86 or above]

[0129] The lower the brightness value, the higher the heating rate during the hot stamping of the aluminum-plated steel sheet. It is believed that the reason for this is that the lower the brightness value, the more black the surface of the aluminum-plated steel sheet is, and thus the characteristic of easily absorbing heat is obtained. In particular, in order to improve the situation where the heating rate of the overlapping portion of the splicing is slow, it is important to reduce the brightness of the overlapping portion relative to the thickness of the overlapping portion (i.e., the sum t1+t2 of the thickness t1 of the first Al-plated steel sheet 1 and the thickness t2 of the second Al-plated steel sheet 2). In the above formula (2), the power exponent of the thickness ratio t1 / (t1+t2) is 1, while the power exponent of the brightness ratio (L*1 / L*2) is 2. It can be seen from this that in the present invention, the brightness ratio (L*1 / L*2) is more important than the thickness ratio t1 / (t1+t2).

[0130] Furthermore, in order to improve the difference in temperature rise rate between the overlapped portion and the single-layer portion, it is important to reduce the surface brightness of the thick overlapped portion and conversely increase the surface brightness of the single-layer portion.

[0131] That is, in order to improve the difference in temperature rise rate between the overlapping portion and the single-layer portion of the spliced ​​part, it is important to increase the square of the ratio of the brightness L*2 of the overlapping portion to the brightness L*1 of the single-layer portion (L*1 / L*2) to 0.86 or more relative to the ratio t1 / (t1+t2) of the thickness (t1+t2) of the overlapping portion to the thickness t1 of the single-layer portion, which is obtained from the thickness t1 of the first Al-plated steel sheet 1 and the thickness t2 of the second Al-plated steel sheet 2. 2 If the value is less than 0.86, a difference in the heating rate of the overlapping portion will occur, and the spot weldability will be reduced. {t1 / (t1+t2)}×(L*1 / L*2) 2 The higher the value, the better. Therefore, as needed, the lower limit can be set to 0.90 or more, 0.94 or more, 0.98 or more, or 1.02 or more. However, {t1 / (t1+t2)}×(L*1 / L*2) 2 There is no particular upper limit on the value of , but if it exceeds 3.00, it is economically difficult to create a brightness difference. Therefore, {t1 / (t1+t2)}×(L*1 / L*2) 2 The upper limit of the value is substantially 3.00. {t1 / (t1+t2)}×(L*1 / L*2) 2 The value of is preferably 2.50 or less, more preferably 2.00 or less, and further preferably 1.50 or less.

[0132] As a method for measuring brightness, for example, a test piece of 50×50 mm is cut out and measured using a spectrocolorimeter (SC-T-GV5 manufactured by SugaTest Instruments Co., Ltd., including specular reflected light) with a measurement beam diameter of φ15 mm.

[0133] Furthermore, the above equations (1) and (2) were obtained through a series of verifications in which overlapping blanks were produced while varying the thickness of the Al-plated steel sheet and the amount of Al-plated coating, hot stamping was actually performed, and the resulting overlapping hot-stamped parts were evaluated. In this verification, each of the resulting overlapping hot-stamped parts was evaluated from the perspectives of the heating rate of the overlapping portion and spot weldability, and the relationships satisfied by the overlapping hot-stamped parts that exhibited good evaluation results were experimentally formulated.

[0134] Carbon Black Coating

[0135] Figure 3It schematically shows the layer structure of one side of an Al-plated steel plate 16 in which an Al-plated layer 14 is provided on the surface of a base steel plate 15 in this embodiment and a carbon-based black coating 17 is further provided on the upper layer of the Al-plated layer 14, or the layer structure of one side of an Al-plated steel plate 16 in which a carbon-based black coating 17 containing Zn, Ti, Cu, and V is selectively provided on the upper layer of the Al-plated layer 14.

[0136] In the second Al-plated steel sheet 2, a carbon-based black coating 17 is provided on the upper layer of the Al-plated layer on the surface 2b on the side not in contact with the first Al-plated steel sheet 1. In the Al-plated steel sheet, the surface has a silvery-white metallic luster, so the brightness is high. Here, by providing a carbon-based black coating on the upper layer of the Al-plated steel sheet, the brightness can be reduced. In particular, the carbon-based black coating 17 is burned and burned away due to an oxidation reaction during heating during hot stamping, and is discharged in the form of CO2 or the like. As a result, the reduction in the spot weldability of the overlapping hot stamped products caused by the remaining carbon-based black coating 17 can be suppressed.

[0137] Another method for further increasing the brightness of an Al-plated steel sheet having a silvery-white metallic luster is, for example, heating the Al-plated steel sheet to approximately 600°C to oxidize the surface and thereby whiten the surface. This further increases the brightness of the Al-plated steel sheet's surface.

[0138] [Thickness of Carbon-Based Black Film 17]

[0139] The film thickness of the carbon-based black coating 17 is preferably not less than 0.3 μm and not more than 10 μm. By setting the film thickness of the carbon-based black coating 17 to be not less than 0.3 μm, the increase in brightness can be suppressed and the temperature rise rate of the overlapping portion can be increased. The film thickness of the carbon-based black coating 17 is more preferably not less than 0.3 μm, further preferably not less than 0.4 μm, and further preferably, in the order of preference, not less than 0.5 μm, not less than 0.6 μm, and not less than 0.7 μm. On the other hand, by setting the film thickness of the carbon-based black coating 17 to be not more than 10 μm, the brightness can be reduced while ensuring economy, and no film will remain after heating during hot stamping, ensuring spot weldability. The film thickness of the carbon-based black coating 17 is more preferably not more than 8 μm, and further preferably, in the order of preference, not more than 6 μm, not more than 5 μm, and not more than 4 μm.

[0140] The thickness of the carbon-based black coating 17 is determined by observing the plated cross section (without etching) using an optical microscope (e.g., area: 100 μm×100 μm). Figure 3As shown, the thickness of the coating layer above the aluminum plating layer was measured in three viewing fields, and the average of the thicknesses measured in the three viewing fields was calculated as the film thickness of the carbon-based black coating 17. At this time, the coating was analyzed from a cross-section using an electron probe microanalyzer (EPMA). If the carbon content was 30% by mass or greater, it was determined to be a carbon-based black coating. If the lightness L* value from the surface was 60 or less, the coating was determined to be black.

[0141] [Binder]

[0142] The carbon-based black coating 17 may optionally contain a resin as a binder for improving the adhesion between the carbon-based black coating 17 and the Al-based plating layer. The type of resin is not particularly limited, and examples thereof include polyethylene resin, polyolefin resin, polyacrylic resin, polymethacrylic resin, polyepoxy resin, polyurethane resin, and polycarbonate resin.

[0143] [Nitrogen content of carbon black coating]

[0144] The nitrogen content of the carbon-based black coating 17 is preferably 2% to 18% by mass. Generally, when a carbon-based coating is applied over an Al-plated layer, the adhesion between the coating and the Al-plated layer decreases. However, for example, by using a nitrogen-containing polyurethane resin as a binder component or by incorporating ammonia to increase the nitrogen content, adhesion to the Al-plated layer can be further improved. This can further increase the heating rate of the overlapping portion.

[0145] Since the effect of improving adhesion with the Al-based plating layer can be achieved by setting the nitrogen content to 2% by mass or more, the nitrogen content of the carbon-based black coating 17 is preferably 2% by mass or more. Furthermore, by setting the nitrogen content of the carbon-based black coating 17 to 18% by mass or less, the decomposition reaction of the resin can be suppressed and peeling of the coating can be prevented. Furthermore, the nitrogen content can be determined by analyzing the coating cross-section using an electron probe microanalyzer (EPMA).

[0146] [The carbon-based black film contains at least one of Zn, Ti, Cu, and V]

[0147] It is preferable that the carbon-based black coating 17 located above the Al-based plating layer applied to the surface of the second steel sheet 2 selectively contains 0.2 g / m 2 Above and 3.0g / m 2At least one of the following: Zn, Ti, Cu, and V. The amount of adhesion shown here refers to the amount of Zn, Ti, Cu, or V adhered per unit area. Zn, Ti, Cu, and V are elements that form oxides that increase emissivity and improve infrared absorption. Therefore, by providing a carbon-based black coating 17 containing these elements, it is possible to suppress a technical problem when using as overlapping blanks, namely, the difference in heating rate between the overlapping portion with a slower heating rate and the single-layer portion with a faster heating rate. In particular, compared to the carbon-based black coating 17 being burned away during hot stamping heating, Zn, Ti, Cu, or V will also remain during hot stamping heating. Therefore, the carbon-based black coating 17 containing these elements can further contribute to an increase in the heating rate at high temperatures. In addition, Zn, Ti, Cu, and V can be contained in either a metallic state or an oxide state. This is because they all become oxides during the heating process, which helps to increase emissivity.

[0148] The total amount of the elements deposited can be set to 0.2 g / m 2 Therefore, the total amount of the elements deposited in the carbon-based black film 17 is preferably 0.2 g / m 2 The total amount of the elements deposited in the carbon-based black film 17 is more preferably 0.4 g / m 2 More preferably, 0.6 g / m 2 above.

[0149] On the other hand, by setting the total adhesion amount of the above elements in the carbon-based black film 17 to 3.0 g / m 2 In addition, Zn, Ti, Cu or V will remain after hot stamping heating, but by setting the total amount of the elements in the carbon black film 17 to 3.0 g / m 2 The total amount of the elements deposited in the carbon-based black film 17 is more preferably 2.8 g / m 2 Below, more preferably 2.6g / m 2 the following.

[0150] The amount (content) of Zn, Ti, Cu, and V can be determined by, for example, performing elemental analysis on the surface using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation) to quantitatively determine the amount of Zn, Ti, Cu, and V deposited.

[0151] The method for forming the carbon-based black coating 17 described above is not particularly limited. For example, a water-based coating liquid containing water-dispersible carbon black (e.g., RCF#52 manufactured by Mitsubishi Chemical Corporation), zinc oxide (e.g., NanoTek manufactured by CIKasei Co., Ltd.), titanium oxide (e.g., NanoTek manufactured by CIKasei Co., Ltd.), copper oxide (e.g., NanoTek manufactured by CIKasei Co., Ltd.), or vanadium oxide (manufactured by Hongwu International Group Ltd.) dispersed in water can be prepared. After the above-mentioned hot-dip aluminum coating process, the coating can be performed using a roll coater and then dried and baked. Alternatively, the coating can be formed by vacuum-depositing Zn, Ti, Cu, or V onto an aluminum-coated steel sheet.

[0152] Furthermore, as another aspect of this embodiment, both the carbon-based black coating 17 and a coating layer 17' comprising at least one of Zn, Ti, Cu, and V (more specifically, these elements in a metallic state or oxides of these elements) may be provided on the Al-based plating layer. In this case, the order in which the carbon-based black coating 17 and the coating layer 17' comprising at least one of Zn, Ti, Cu, and V are disposed is not particularly limited; the carbon-based black coating 17 may be disposed on top of the coating layer 17' comprising at least one of Zn, Ti, Cu, and V, or the coating layer 17' comprising at least one of Zn, Ti, Cu, and V may be disposed on top of the carbon-based black coating 17.

[0153] Furthermore, even when the coating layer 17' containing at least one of Zn, Ti, Cu, and V is located below the carbon-based black coating 17, if the film thickness of the carbon-based black coating 17 is the same as that of the present embodiment, fluorescent X-rays can easily penetrate the carbon-based black coating 17. Therefore, even when the coating layer 17' containing at least one of Zn, Ti, Cu, and V is located below the carbon-based black coating 17, the adhesion amounts of Zn, Ti, Cu, and V can be measured by fluorescent X-ray analysis.

[0154] The carbon black coating 17 and the coating layer 17' containing at least one of Zn, Ti, Cu and V may be provided on both surfaces of the base steel plate, but are preferably provided only on the surface of the base steel plate that is exposed to the heat source during hot stamping.

[0155] Furthermore, the first Al-based plated steel sheet 1 may have the carbon-based black coating as described above as an upper layer of the Al-based plating layer on the side not in contact with the second Al-based plated steel sheet 2 .

[0156] <Welding>

[0157] In the hot stamping overlapped blank 4 formed by overlapping and welding the first Al-plated steel sheet 1 and the second Al-plated steel sheet 2, the welding type can be selected from spot welding, seam welding, brazing, laser welding, plasma welding, arc welding, etc. Here, from the perspective of achieving good contact between the overlapping portions, spot welding is more preferred, which allows contact at multiple points extending into the overlapping portion and directly joins the steel sheets by applying pressure between the steel sheets.

[0158] (2. Overlapping hot stamping parts)

[0159] In the method for manufacturing a hot stamped body of this embodiment, as Figure 1 As shown, the stacked hot stamped product 12 of this embodiment is manufactured by heating the stacked blank 4 for hot stamping and providing a bent portion by bending at least partially in the stacked portion when forming immediately after the heating.

[0160] The heating temperature is not particularly limited, but is generally set to a temperature range of not less than the Ac3 point (e.g., 800°C) and not more than 1000°C. When forming immediately after heating, cooling using a mold or a refrigerant such as water can produce a laminated hot stamped body 12 with excellent collision resistance. The time for which the laminated hot stamping blank 4 is maintained at the heated temperature is not particularly limited, but can be, for example, not less than 4 minutes and not more than 20 minutes.

[0161] For example, preliminary tests can be conducted to measure the heating time or heating rate when the hot stamping stacked blank 4 is heated to a temperature between the Ac3 point and 1000°C. The holding time can then be determined using the test results. This can lead to a condition such as "holding a temperature of 910-920°C for 250-1200 seconds," for example.

[0162] The above-mentioned heating temperature refers to the maximum temperature reached by the steel plates at the overlapped portion. In addition, heating methods include heating using an electric furnace, a gas furnace, a far infrared furnace, a near infrared furnace, etc., electric heating, high-frequency heating, induction heating, etc.

[0163] The overlapping hot stamping formed body 12 of this embodiment manufactured as described above comprises: a first Al-Fe alloy-plated steel plate having a plate thickness of T1 (mm); and at least one second Al-Fe alloy-plated steel plate, which is overlapped and welded on the first Al-Fe alloy-plated steel plate, has an area smaller than that of the first Al-Fe alloy-plated steel plate and a plate thickness of T2 (mm).

[0164] Furthermore, as described above, the hot stamped body 12 is produced by heating the hot stamping overlapped blank 4 formed by overlapping and welding the first Al-plated steel sheet 1 and the second Al-plated steel sheet 2, and then further performing bending processing. Therefore, the first Al-Fe alloy plated steel sheet and the second Al-Fe alloy plated steel sheet constituting the hot stamped body 12 are not necessarily flat. For example, Figure 1 In this way, the first Al-Fe alloy-plated steel plate has a bent portion 8 at the top of the head, and the second Al-Fe alloy-plated steel plate has a bent portion 8 at the top of the head and a bent portion 9 at the flange. Although the name "steel plate" is used, it is not necessarily a flat shape. In this embodiment, in order to distinguish between the steel plate constituting the hot stamping body 12 and the steel plate constituting the overlapping blank 4, the former is referred to as an alloy-plated steel plate (for example, an Al-Fe alloy-plated steel plate) for convenience, and the latter is referred to as a plated steel plate (for example, an Al-plated steel plate) (without the addition of "alloy").

[0165] The first Al-Fe alloy-plated steel sheet in the overlapped hot stamped formed body 12 is a plated steel sheet having an Al-Fe alloy plating layer having an average plating thickness of K1 (μm) on both sides of the first Al-Fe alloy-plated steel sheet. Here, K1 is the average of the plating thickness of the Al-Fe alloy plating layer on the side of the non-overlapping portion of the first Al-Fe alloy-plated steel sheet that contacts the second Al-Fe alloy-plated steel sheet and the plating thickness of the Al-Fe alloy plating layer on the side that does not contact the second Al-Fe alloy-plated steel sheet.

[0166] The second Al-Fe alloy-plated steel sheet in the overlapped hot stamped body 12 is a plated steel sheet having an Al-Fe alloy plating layer having a thickness of K2 (μm) on the surface of the side not in contact with the first Al-Fe alloy-plated steel sheet. Here, K2 is the plating thickness of the Al-Fe alloy plating layer on the side of the overlapped portion of the second Al-Fe alloy-plated steel sheet not in contact with the first Al-Fe alloy-plated steel sheet.

[0167] In addition, in the second Al—Fe alloy plated steel sheet, the plating thickness of the Al—Fe alloy plated layer on the surface in contact with the first Al—Fe alloy plated steel sheet is not particularly limited.

[0168] Here, the average plating thicknesses K1 and K2 of the Al-Fe alloy plating layers in each Al-Fe alloy-plated steel sheet of the stacked hot stamped body 12 are each independently 25 μm or greater. Furthermore, the average plating thicknesses K1 and K2 of the Al-Fe alloy plating layers in each Al-Fe alloy-plated steel sheet are each independently 60 μm or less. In other words, K1 and K2 satisfy the following equations (7) and (8).

[0169] 25≤K1≤60 ... Formula (7)

[0170] 25≤K2≤60 ... Formula (8)

[0171] Furthermore, the average plating thicknesses K1 and K2 of the Al—Fe alloy plating layers in the Al—Fe alloy plated steel sheets are each independently preferably 30 μm or more, and more preferably 35 μm or more.

[0172] The average plating thicknesses K1 and K2 of the Al—Fe alloy plating layers in each Al—Fe alloy plated steel sheet are each independently preferably 58 μm or less, more preferably 56 μm or less, 52 μm or less, or 48 μm or less.

[0173] By setting the plating thickness of the Al—Fe alloy plating layer of the first Al—Fe alloy plated steel sheet and the second Al—Fe alloy plated steel sheet within the above-mentioned range, the spot weldability of the stacked hot stamped body 12 can be maintained in a good state.

[0174] The above-mentioned coating thickness was determined by etching the coating cross-section with nital and observing it with an optical microscope (area: 100 μm × 100 μm). The coating thickness was measured in three viewing fields, and the average of the coating thicknesses measured in each of the three viewing fields was calculated as the above-mentioned coating thickness. Furthermore, the coating thickness of the first Al-Fe alloy-plated steel sheet is measured at a location 4b of the single-layer portion and at a location of the overlapping portion 4a in contact with the second Al-Fe alloy-plated steel sheet. However, since the heating rate is fast, the heating time during hot stamping is maximized, and spot weldability is easily degraded, the coating thickness of the first Al-Fe alloy-plated steel sheet is measured at the single-layer portion.

[0175] The Al-Fe alloy plating is a layer formed as a result of Fe diffusing from the Al-based plating to the surface due to heating during hot stamping (in other words, an alloy plating containing at least Al and Fe). The Al-Fe alloy plating is composed of a combination of the θ phase (FeAl3), η phase (Fe2Al5), ζ phase (FeAl2), Fe3Al, FeAl, Al solid solution Fe and other phases, which are compound layers of Al and Fe. In addition, the Al-Fe alloy plating containing Si in the plating also includes τ1 phase (Al2Fe3Si3), τ2 phase (Al3FeSi), τ3 phase (Al2FeSi), τ4 phase (Al3FeSi2), τ5 phase (Al8Fe2Si), τ6 phase (Al9Fe2Si2), τ7 phase (Al3Fe2Si3), τ8 phase (Al2Fe3Si4), τ10 phase (Al4Fe 1.7 Si), τ11 phase (Al5Fe2Si) (however, each phase is sometimes not a stoichiometric composition.), in most cases, the Al-Fe alloy plating is mainly composed of any one of the τ1 phase (Al2Fe3Si3), η phase (Fe2Al5), FeAl phase, and BCC phase of Fe with Al solid solution, or multiple phases thereof.

[0176] In particular, due to the heating during hot stamping, Al in the Al-based plating and Fe in the base steel plate diffuse into each other. Therefore, as phases formed by the diffusion of Al into the base steel plate, layers of BCC phase and FeAl phase containing Fe in which Al is solid-dissolved are formed in sequence from the base steel plate side. The layer containing these phases is also called a diffusion layer (Diffusion Layer). Here, as described below, the diffusion layer can be identified by observing the cross section after nitric acid etching treatment using an optical microscope. In addition, when the diffusion layer cannot be identified by observation with an optical microscope, the diffusion layer can be identified by analyzing the cross section using an electron probe microanalyzer (EPMA). In this case, in the EPMA analysis results, the layer with Al: 30% by mass or less and Fe: 70% by mass or more can be regarded as the diffusion layer 20.

[0177] Specifically, if Figure 5 、 Figure 6 As shown, in the overlapped hot stamped body 12, the layer 19 is an Al-Fe alloy plating layer, including a diffusion layer 20. The thickness of the layer 19 is measured as the thickness of the Al-Fe alloy layer, and the thickness of the layer 20 is measured as the thickness of the diffusion layer. Figure 6 ] shows an example of observing a cross section using an optical microscope after nital etching.

[0178] The thickness of the diffusion layer included in the Al-Fe alloy plating layer of the first Al-Fe alloy plated steel sheet that does not overlap with the second Al-Fe alloy plated steel sheet is denoted as D1 (μm), and the thickness of the diffusion layer included in the Al-Fe alloy plating layer of the second Al-Fe alloy plated steel sheet is denoted as D2 (μm). In this case, the difference (D1-D2) between D1 (μm) and D2 (μm) is calculated as the ratio (K1 / K2) of the plating thickness K1 of the non-overlapping portion of the first Al-Fe alloy plated steel sheet and the plating thickness K2 of the overlapping portion of the second Al-Fe alloy plated steel sheet. 2 The product of (D1-D2)×(K1 / K2) 2 5.0 μm or less.

[0179] Here, D1 is the average of the thickness of the diffusion layer in the Al-Fe alloy plating layer located on the side of the first Al-Fe alloy-plated steel sheet that is in contact with the steel sheet substrate and the thickness of the diffusion layer in the Al-Fe alloy plating layer located on the side that is not in contact with the second Al-Fe alloy-plated steel sheet and is in contact with the steel sheet substrate. D2 is the thickness of the diffusion layer in the Al-Fe alloy plating layer located on the side of the second Al-Fe alloy-plated steel sheet that is not in contact with the first Al-Fe alloy-plated steel sheet and is in contact with the steel sheet substrate.

[0180] In the Al-Fe coating, the Al-Fe binary alloy (FeAl3, Fe2Al5, FeAl2) contains a phase with a relatively low melting point, in which Al exceeds 30% by mass and Fe is less than 70% by mass, and a phase with a relatively high melting point, in which Al is less than 30% by mass and Fe is greater than 70% by mass. When the heating time of hot stamping is long or the heating temperature is high, there is a relationship in which the diffusion layer increases and the binary alloy (FeAl3, Fe2Al5, FeAl2) decreases. Regarding spot weldability, it is known that the presence of a coating with a low melting point promotes sufficient welding between the materials. Therefore, when the diffusion layer is thick, the binary alloy (FeAl3, Fe2Al5, FeAl2) decreases, and the spot weldability is reduced.

[0181] At (D1-D2)×(K1 / K2) 2 When the value of exceeds 5.0 μm, the diffusion layer of the first Al-Fe alloy plated steel sheet increases, and conversely, the Al-Fe binary alloy becomes thinner, and the spot weldability decreases. Therefore, in terms of the spot weldability of the overlapping portion, it is important to adjust (D1-D2)×(K1 / K2) 2 Suppressed to below 5.0μm. (D1-D2)×(K1 / K2) 2The value of is preferably 4.5 μm or less, more preferably 4.0 μm or less.

[0182] On the other hand, (D1-D2)×(K1 / K2) 2 The lower limit is 0μm. However, in (D1-D2)×(K1 / K2) 2 When the value is less than 0.5 μm, the effect is saturated.

[0183] As a method for determining the coating thickness K1, K2 of the Al-Fe alloy coating and the thickness D1, D2 of the diffusion layer, a cross section of the coating is subjected to nitric acid etching in a viewing field of 100 μm × 100 μm and the cross section is observed using an optical microscope. Figure 6 As shown, by optical microscope, the thickness of coating thickness and diffusion layer is measured. In more detail, at least 3 positions, the coating cross section is observed by the above method, and the coating thickness and the thickness of diffusion layer of each observation position are determined. Afterwards, the average value of the thickness obtained is calculated, and the average value obtained is used as the coating thickness and the thickness of diffusion layer. However, when the diffusion layer cannot be determined by optical microscope observation, an electron probe microanalyzer (EPMA) is used to analyze, and the thickness of the layer in which Al is less than 30 mass % and Fe is more than 70 mass % is measured at least 3 positions, and its average value is used as the thickness of diffusion layer 20.

[0184] Furthermore, the diffusion layer described above is formed by inward diffusion within the Al-based plating layer. Therefore, the thicknesses D1 and D2 of the diffusion layers are largely influenced only by the hot stamping heating conditions and are not affected by the initial plating thickness. Furthermore, even if the coating weight of the second Al-based plated steel sheet is increased, the thickness D2 of the diffusion layer of the second Al-Fe alloy plated steel sheet, which has a slower heating rate, cannot be made substantially the same as the thickness D1 of the diffusion layer of the first Al-Fe alloy plated steel sheet. When a conventional hot stamping process is performed on a stacked blank for hot stamping that satisfies equations (1) to (5), a stacked hot stamped body that satisfies equations (7) to (9) can be readily obtained.

[0185] Furthermore, the plating thicknesses K1 and K2 of the Al-Fe alloy plating are affected not only by the plating adhesion before hot stamping but also by the heating conditions (heating temperature, holding time) during hot stamping. Furthermore, the plating thicknesses K1 and K2 of the Al-Fe alloy plating are not affected solely by the initial plating thickness, as they are not affected by these factors alone, but also by the plate thickness and brightness, which affect the heating time at high temperatures.

[0186] The above relationship was obtained through a series of verifications in which overlapping blanks were produced while varying the thickness of the Al-plated steel sheets and the amount of Al-plating applied, hot stamping was performed, and the resulting overlapping hot-stamped parts were evaluated. In this verification, each resulting overlapping hot-stamped part was evaluated from the perspectives of the temperature rise rate of the overlapping portion and spot weldability. The relationship satisfied by the overlapping hot-stamped parts exhibiting favorable evaluation results was experimentally formulated.

[0187] Alternatively, a layer containing at least one of Zn, Ti, Cu, and V may be selectively provided on the upper layer of the Al-Fe alloy plating layer applied to the surface of the second Al-Fe alloy plated steel sheet. In this case, the adhesion amount (content) of at least one of Zn, Ti, Cu, and V is preferably set to be greater than 0 g / m 2 The adhesion amount (content) of at least one of Zn, Ti, Cu or V is more preferably 0.2 g / m 2 Above and 3.0g / m 2 The following. A layer containing Zn, Ti, Cu, and V can be obtained by forming a layer containing Zn, Ti, Cu, and V in a metallic or oxide state before heating for hot stamping. Most of these become oxides during the heating process, but some may remain in a metallic state.

[0188] The adhesion amount (content) shown here means the amount adhered as Zn, Ti, Cu or V per unit area.

[0189] exist Figure 7 Schematically shows the structure of an Al-Fe alloy-plated steel sheet in which a layer containing at least one of Zn, Ti, Cu, and V is provided on the upper layer of the Al-Fe alloy plating applied to the surface of the second Al-Fe alloy-plated steel sheet of the overlapping hot stamped formed body. The Al-Fe alloy-plated steel sheet has an Al-Fe alloy plating layer 19 (including a diffusion layer 20) and a coating layer 23 containing at least one of Zn, Ti, Cu, and V on the surface of a base steel sheet 21. The coating layer 23 is preferably provided on the Al-Fe alloy plating layer 19 on the side of the second Al-Fe alloy-plated steel sheet that is not in contact with the first Al-Fe alloy-plated steel sheet.

[0190] In addition to increasing emissivity, Zn, Ti, Cu, and V also improve infrared absorption when in the form of oxides. Therefore, by providing this layer, it is possible to suppress the difference in heating rate between the overlapping portion (which heats up more slowly) and the single-layer portion (which heats up more quickly), a problem encountered when using the blank as a stacked part. As a result, overheating of the single-layer portion can be suppressed, improving the spot weldability of the single-layer portion of the hot-stamped part.

[0191] In particular, oxides of Zn, Ti, Cu, or V also remain during the heating process during hot stamping. Therefore, the coating layer 23 can further contribute to increasing the heating rate at high temperatures. When the coating layer 23 is provided on the surface of the Al-Fe alloy plating layer 19 applied to the surface of the second Al-Fe alloy plated steel sheet, the coating layer 23 can be applied to the steel sheet with a coating weight of 0.2 g / m 2 The above can fully demonstrate the effect of suppressing the difference in temperature rise rate. The adhesion amount of the coating layer 23 is more preferably 0.4 g / m 2 More preferably, 0.6 g / m 2 On the other hand, by setting the coating layer 23 adhesion amount to 3.0 g / m 2 In addition, even after hot stamping heating, Zn, Ti, Cu or V oxides will remain, but by setting the adhesion amount to 3.0 g / m 2 The following can prevent the deterioration of the spot weldability of the hot stamped product. The coating weight of the coating layer 23 is more preferably 2.8 g / m 2 Below, more preferably 2.6g / m 2 the following.

[0192] In addition, when the coating layer 23 is not provided on the surface of the Al-Fe alloy plating layer 19 applied on the surface of the second Al-Fe alloy plated steel sheet, the adhesion amount (content) of Zn, Ti, Cu or V is 0 g / m 2 .

[0193] Alternatively, in the first Al-Fe alloy plated steel sheet, a layer containing at least one of Zn, Ti, Cu, and V may be selectively provided on the Al-Fe alloy plated layer on the side not in contact with the second Al-Fe alloy plated steel sheet. In this case, the adhesion amount (content) of at least one of Zn, Ti, Cu, and V is preferably set to be greater than 0 g / m 2 The adhesion amount (content) of at least one of Zn, Ti, Cu or V is more preferably 0.2 g / m 2 Above and 3.0g / m 2In addition, when the layer containing at least one of Zn, Ti, Cu and V is not provided, the adhesion amount (content) of Zn, Ti, Cu or V is 0 g / m 2 .

[0194] The adhesion amount of the layer containing Zn, Ti, Cu, or V can be determined by quantitatively analyzing Zn, Ti, Cu, and V from the surface using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation), for example.

[0195] As described above, in the laminated hot stamped body of the present embodiment, the total of the Zn content, the Ti content, the Cu content, and the V content may be 0.2 to 3.0 g / m 2 The layer is located on the Al-Fe alloy coating layer on the side of the second Al-Fe alloy coating steel sheet that is not in contact with the first Al-Fe alloy coating steel sheet, and the total Zn content, Ti content, Cu content and V content is 0 to 3.0 g / m 2 The layer is located on the Al—Fe alloy plating layer on the surface of the first Al—Fe alloy plated steel sheet that is not in contact with the second Al—Fe alloy plated steel sheet.

[0196] When the stacked hot stamped parts 12 of this embodiment are used as automotive parts, they are typically subjected to welding, phosphoric acid chemical conversion treatment, electrodeposition coating, and the like. For example, a zinc phosphate coating and a phosphoric acid coating may be formed on the surface of the hot stamped parts 12 by phosphoric acid chemical conversion treatment, and an organic coating having a thickness of 5 μm to 50 μm may be formed on the surface of these coatings by electrodeposition coating. After electrodeposition coating, a mid-coat or topcoat may be further applied to improve appearance quality and corrosion resistance.

[0197] Example

[0198] Hereinafter, the present invention will be described in more detail using examples.

[0199] (Example 1)

[0200] The first Al-plated steel sheet 1 and the second Al-plated steel sheet 2 shown in Table 1 were prepared by the following method. Figure 1Spot welding was then performed at the welded portion 3 to produce a hot stamping overlapped blank 4. A cold-rolled steel sheet (chemical composition, by mass%, of C: 0.23%, Si: 0.30%, Mn: 1.2%, P: 0.010%, S: 0.002%, Cr: 0.25%, Ti: 0.020%, Al: 0.042%, N: 0.0030%, B: 0.0020%, balance: Fe and impurities) that had undergone conventional hot and cold rolling processes was used as a sample material. Aluminum was applied to both surfaces using a Sendzimir hot-dip aluminum coating line. After coating, the coating weight was adjusted using a gas wiping method, and then cooled to produce the first Al-plated steel sheet 1. The coating bath composition at this time was 89 mass% Al, 9 mass% Si, and 2 mass% Fe. The second Al-based plated steel sheet 2 is also produced in the same manner as the first Al-based plated steel sheet 1 and coated with a carbon-based black coating.

[0201] The blank was hot stamped and heated at 920°C to investigate the temperature rise rate of the overlapping portion. After heating the overlapping portion at 910-920°C for 300 seconds, the die was immediately cooled to obtain an overlapping hot stamped body 12. Two flange portions 11 were cut from the non-overlapping portion (single-layer portion) and their spot weldability was investigated in a set of the same steel sheets.

[0202] Regarding each level in Table 1, examples of the present invention (hereinafter referred to as "invention examples") are denoted as A2 to A7, A10, A11, A13, A20, and A21, and comparative examples are denoted as A1, A8, A9, A12, and A15 to A19.

[0203] The thickness t1 and t2 of the steel plate were measured using a micrometer based on JIS B 7502:2016 as described above. In addition, the coating adhesion amount W1a, W1b, and W2b on each single side was measured using the method of JIS G 3314:2019JB.3 after sealing the surface on the back side of the surface to be measured. In addition, regarding the brightness L*1a, L*1b, and L*2b, a test piece was cut out of 50×50 mm and measured using a spectrophotometer (SC-T-GV5 manufactured by Suga Test Instruments Co., Ltd., including specular reflected light) with a measuring beam diameter of φ=15 mm. As described above, the film thickness of the carbon-based black coating was measured by observing the cross section.

[0204] In order to investigate the heating rate of the overlapping part of the blank, Figure 1The K-type thermocouple is welded in the center of the overlapping portion of the overlapping blank 4 for hot stamping shown. On this basis, the heating time is calculated and the heating rate is evaluated. The heating time is calculated based on the time to reach 910°C and evaluated. The evaluation standard is based on the comparative example A15 in which the second Al-based plated steel plate does not have a carbon-based black coating, and the heating time is shortened as follows. The evaluation G3 (Good3) indicates that it is judged to be good, G2 (Good2) indicates that it is judged to be better, G1 (Good1) indicates that it is judged to be particularly good, and the evaluation NG (No Good) indicates that it is judged to be poor. In addition, in order to indicate that it is a benchmark, the record in the column of "Heating rate of overlapping portion" of the comparative example A15, which serves as the evaluation benchmark, is recorded as "-".

[0205] <Evaluation Criteria>

[0206] G1: Heating time shortened by more than 90 seconds

[0207] G2: Heating time is shortened by more than 60 seconds and less than 90 seconds

[0208] G3: Heating time is shortened by more than 30 seconds and less than 60 seconds

[0209] NG: The heating time is shortened to less than 30 seconds or the heating time is prolonged

[0210] In order to investigate the spot weldability of the hot stamping blanks after heating, the hot stamping blanks were welded at 920°C. Figure 1 The hot stamping overlapped blank 4 shown was hot stamped and heated to investigate the heating rate of the overlapping portion. At this time, after heating the overlapping portion at 910-920°C for 300 seconds, the die was immediately cooled, thereby obtaining an overlapping hot stamped body 12. Two flange portions 11 of the non-overlapping portion (single-layer portion) were cut out, and the spot weldability was investigated in a set of the same steel plates under the welding conditions shown below. In its evaluation criteria, the range between the welding current value at which the diameter of the weld nugget during spot welding is 4√t (t is the plate thickness) and the welding current value at which splash (Splash) is generated (called the appropriate current range) is evaluated as G3 (Good3) for good, G2 (Good2) for better, G1 (Good1) for particularly good, and NG (No Good) for poor.

[0211] <Evaluation Criteria>

[0212] G1: Appropriate current range is 2.0kA or above

[0213] G2: Appropriate current range is 1.5kA or more and less than 2.0kA

[0214] G3: Appropriate current range is 1.0kA or more and less than 1.5kA

[0215] NG: Appropriate current range is less than 1.0kA

[0216] Welding power source: DC inverter, electrode pressure: 400kgf

[0217] Electrode shape: DR, tip diameter 6φ (R40) (Material: Chrome copper)

[0218] Initial pressurization time: 60 cycles, welding time: 22 cycles, holding time: 10 cycles

[0219] Welding current value: 4kA to 15kA, welding in 0.2kA intervals

[0220] A comprehensive evaluation was performed based on the evaluation results related to the heating rate of the overlapping portion of the above-mentioned blank and the evaluation results related to the spot weldability after hot stamping (HS). More specifically, based on the score "Gx" (x is an integer from 1 to 3) of each item, the product of the numbers x was used as the evaluation value. If the obtained evaluation value was "9" or the evaluation result of at least one item was "NG", it was set as the comprehensive evaluation "NG". In addition, for Comparative Example A15, since it became an evaluation benchmark related to the heating rate, the above-mentioned comprehensive evaluation study was not carried out, and the comprehensive evaluation column was recorded as "-".

[0221] Table 1 summarizes the evaluation results of the temperature increase rate of the overlapped portion of the blank and the spot weldability after hot stamping (HS).

[0222] [Table 1]

[0223]

[0224] In the invention examples A3 to A7, A10, A11, A13, A20, A21, and A23, the coating adhesion amount is within the scope of the present invention, and there is a carbon-based black coating. The relationship between the plate thickness, the coating adhesion amount, and the brightness satisfies the formula (1) and the formula (2). Therefore, the heating rate of the overlapping portion and the spot weldability after hot stamping are good. In the comparative example A1, although there is a carbon-based black coating, it does not satisfy the formula (1) and the formula (2). Therefore, the heating rate of the overlapping portion and the spot weldability after hot stamping are both poor. In the comparative examples A8, A9, and A12, since the formula (1) is not satisfied, the spot weldability after hot stamping is poor. In addition, in the comparative examples A2 and A22, although there is a carbon-based black coating, the formula (2) is not satisfied. Therefore, the comprehensive evaluation is NG (poor: No Good). In addition, when the coating adhesion amount is less than 20 g / m 2 In Comparative Examples A16 and A18, scale was formed and the coating weight exceeded 120 g / m2 In Comparative Examples A17 and A19, powdering also occurred during press forming, which is considered to have led to a decrease in spot weldability. In addition, in Comparative Example A15, there was no carbon-based black coating and it did not satisfy Formula (1) and Formula (2), so the spot weldability after hot stamping was poor.

[0225] (Example 2)

[0226] Hot stamping blanks were prepared by producing first Al-plated steel sheets and second Al-plated steel sheets with thicknesses of 1.0 mm, 1.6 mm, and 2.0 mm, as shown in Table 2, under the same manufacturing conditions as in Example 1. The first Al-plated steel sheet 1 and the second Al-plated steel sheet 2 were produced by the following method. Figure 1 By performing the spot welding 3 in this manner, a superimposed blank 4 for hot stamping is produced.

[0227] As in Example 1, the blank was hot stamped and heated at 920°C to investigate the heating rate of the overlapping portion. After heating the overlapping portion at 910-920°C for 300 seconds, the die was immediately cooled to obtain an overlapping hot stamped body 12. The two flange portions 11 of the non-overlapping portion (single-layer portion) were cut out, and the spot weldability was investigated in a set of the same steel plates. The grades in Table 2 are B1, B5, and B6 for the invention examples, and B2, B3, B4, and B7 for the comparative examples.

[0228] In the investigation of the heating rate of the overlapping portion of the blank, the heating time was calculated and evaluated in the same manner as in Example 1. In the evaluation criteria, with respect to B1, the heating time was shortened as follows, using as a benchmark the comparative example B2 in which the first Al-plated steel plate and the second Al-plated steel plate had the same plate thickness, but the second Al-plated steel plate did not have a carbon-based black coating. Similarly, with respect to B3, the heating time was shortened as follows, using as a benchmark the comparative example B4 in which the first Al-plated steel plate and the second Al-plated steel plate had the same plate thickness, but the second Al-plated steel plate did not have a carbon-based black coating. In addition, with respect to B5 and B6, the heating time was shortened as follows, using as a benchmark the comparative example B7 in which the first Al-plated steel plate and the second Al-plated steel plate had the same plate thickness, but the second Al-plated steel plate did not have a carbon-based black coating. A rating of G3 (Good3) indicates good, G2 (Good2) indicates better, G1 (Good1) indicates particularly good, and NG (No Good) indicates poor. Furthermore, for each comparative example serving as the evaluation benchmark, the "Overlapping Portion Temperature Rise Rate" column is marked with "-" to indicate that it serves as the benchmark.

[0229] <Evaluation Criteria>

[0230] G1: Heating time shortened by more than 90 seconds

[0231] G2: Heating time is shortened by more than 60 seconds and less than 90 seconds

[0232] G3: Heating time is shortened by more than 30 seconds and less than 60 seconds

[0233] NG: The heating time is shortened to less than 30 seconds or the heating time is prolonged

[0234] The spot weldability of the hot stamping superimposed blanks after heating was evaluated using the same investigation method and evaluation criteria as in Example 1.

[0235] A comprehensive evaluation is performed based on the evaluation results related to the heating rate of the overlapping portion of the above-mentioned blank and the evaluation results related to the spot weldability after hot stamping (HS). More specifically, based on the score "Gx" (x is an integer from 1 to 3) of each item, the product of the numbers x is used as the evaluation value. If the obtained evaluation value is "9" or the evaluation result of at least one item is "NG", it is set as the comprehensive evaluation "NG". In addition, for each comparative example that serves as the evaluation standard related to the heating rate, the above-mentioned comprehensive evaluation study is not carried out, and the comprehensive evaluation column is recorded as "-".

[0236] [Table 2]

[0237]

[0238] Inventive Examples B1, B5, and B6, the coating weights were within the scope of the present invention, and they had a carbon-based black coating. The relationships among plate thickness, coating weight, and brightness satisfied equations (1) and (2), resulting in good heating rates at the overlapped portion and good spot weldability after hot stamping. Comparative Example B3, while having a carbon-based black coating, did not satisfy equation (2), resulting in a poor heating rate at the overlapped portion compared to B4. Comparative Examples B2, B4, and B7, which did not have a carbon-based black coating and did not satisfy equations (1) and (2), were evaluated as poor spot weldability after hot stamping.

[0239] (Example 3)

[0240] A first Al-plated steel sheet and a second Al-plated steel sheet were produced under the same manufacturing conditions as those of Grade A3 in Example 1. Zn, V, Ti, and Cu were formed on the Al-plated steel sheet of the second Al-plated steel sheet (vacuum deposition method), and a carbon-based black coating was further formed thereon, thereby producing Inventive Examples C1 to C4. In addition, Inventive Examples C5 to C10 were produced by making the carbon-based black coating contain a water-dispersed sol of Zn oxide, V oxide, Ti oxide, and Cu oxide. Figure 1These were spot welded 3 in this manner to produce a hot stamping laminated blank 4. Table 3 shows the details of Invention Examples C1 to C10.

[0241] As in Example 1, the blank was hot stamped and heated at 920°C to investigate the heating rate of the overlapping portion (the evaluation criteria were the same as in Example 1, and were evaluated based on Comparative Example A15 in which the second Al-plated steel plate did not have a carbon-based black coating.), and after heating the overlapping portion at 910-920°C for 300 seconds, the mold was immediately cooled to obtain an overlapping hot stamped body 12. The two flange portions 11 of the non-overlapping portion (single-layer portion) were cut out, and the spot weldability was investigated in a group of the same steel plates (the evaluation criteria were the same as in Example 1). The results are shown in Table 3.

[0242] [Table 3]

[0243]

[0244] Inventive Examples C1 to C10 have a preferred coating containing Zn, V, Ti, and Cu in the carbon-based black coating, and therefore exhibit better results in the heating rate of the overlapped portion and spot weldability after HS than A3.

[0245] (Example 4)

[0246] A first Al-plated steel sheet and a second Al-plated steel sheet were produced under the same manufacturing conditions as those for Grade A3 in Example 1. A carbon-based black coating was formed on the Al-plated steel sheet of the second Al-plated steel sheet. The nitrogen content was adjusted to 1% using the above-described method. The resulting blank was designated D1, and blanks with nitrogen contents of 2%, 5%, 8%, 18%, and 20% were designated D2, D3, D4, D5, and D6, respectively. Adhesive tape peeling tests (tape: CT405AP-24 manufactured by Nichiban Co., Ltd.) were performed. The results showed that peeling of the coating was observed in D1, which had a nitrogen content of 1%, and D6, which had a nitrogen content of 20%, while no peeling was observed in D2, D3, D4, and D5.

[0247] (Example 5)

[0248] The overlapping hot stamped bodies shown in Table 4 were produced under the same manufacturing conditions as the grades A3, A7, A10, A12, A13, A15, and A16 of Example 1 and the grades C5, C6, C9, and C10 of Example 3, and were designated as E1, E6, E7, E8, E9, E10, and E11, and E2, E3, E4, and E5, respectively. Furthermore, Al-plated steel sheets E2, E3, E4, and E5, each having a carbon-based black coating containing Zn oxide, V oxide, Ti oxide, and Cu oxide used in the second Al-plated steel sheet, were used for both the second Al-plated steel sheet and the first Al-plated steel sheet, and overlapping hot stamped bodies were produced under the same manufacturing conditions as in Example 3, and were designated as E12, E13, E14, and E15, respectively. In addition, Table 4 includes a section titled "Applied Manufacturing Conditions" that describes the manufacturing conditions used in Examples 1 and 3 to obtain overlapping hot-stamped products of grades E1 to E15. Two non-overlapping portions (single-layer portions) were cut out, and spot weldability was investigated in a set of the same steel sheets. In each grade in Table 4, the invention examples are designated E1 to E5, E7, E9, and E12 to E15, and the comparative examples are designated E6, E8, E10, and E11. The evaluation method and criteria for spot weldability after hot stamping were the same as those in Example 1.

[0249] As described above, the plating thickness K1 of the Al-Fe alloy plating layer and the thickness D1 of the diffusion layer of the first Al-Fe alloy-plated steel sheet, and the plating thickness K2 of the Al-Fe alloy plating layer and the thickness D2 of the diffusion layer of the second Al-Fe alloy-plated steel sheet were measured by observing the cross section using an optical microscope (all measurements were performed at three locations and the average value was calculated).

[0250] [Table 4]

[0251]

[0252] In the inventive examples E1 to E5, E7, E9, and E12 to E15, the plating thicknesses K1 and K2 of the Al-Fe alloy plating layer were within the range of the present invention, and the plating thickness of the Al-Fe alloy plating layer and the thickness of the diffusion layer satisfied equation (9), resulting in good spot weldability after hot stamping. In the comparative examples E6, E8, and E10, since equation (9) was not satisfied, spot weldability after hot stamping was poor. In the comparative example E11, the plating thickness of the Al-Fe alloy plating layer was less than 25 μm, and equation (9) was also not satisfied, resulting in poor spot weldability after hot stamping.

[0253] Furthermore, in the inventive examples E2, E3, E4, and E5, since the oxides of Zn, V, Ti, and Cu are present on the Al-Fe alloy plating layer of the second Al-Fe alloy-plated steel sheet, the spot weldability after HS is better than that of E1.

[0254] In addition, in E12, E13, E14, and E15, which are inventive examples, oxides of Zn, V, Ti, and Cu are present on the Al-Fe alloy plating layer of the first Al-Fe alloy-plated steel sheet and on the Al-Fe alloy plating layer of the second Al-Fe alloy-plated steel sheet. Therefore, compared with E1, better spot weldability after HS is achieved.

[0255] (Example 6)

[0256] Again, focusing on the grades A2, A3, A5, and A15 of Example 1 shown in Table 1, the same manufacturing conditions were used to manufacture the same grades except that a carbon-based black coating was further provided on the first Al-based coating layer. Figure 1 By performing the spot welding 3 in this manner, a superimposed blank 4 for hot stamping is produced.

[0257] As in Example 1, the blank was hot stamped and heated at 920°C to investigate the heating rate of the overlapping portion (the evaluation criteria were the same as in Example 1, and the evaluation was conducted based on Comparative Example A15, in which the second Al-plated steel sheet did not have a carbon-based black coating). After heating the overlapping portion at 910-920°C for 300 seconds, the die was immediately cooled to obtain an overlapping hot stamped body 12. The two flange portions 11 of the non-overlapping portion (single-layer portion) were cut out, and the spot weldability was investigated in a group of the same steel sheets (the evaluation criteria were the same as in Example 1).

[0258] Table 5 shows the grades F1 to F4. In grade F1, a carbon-based black coating is further provided on the first Al-plated steel sheet of grade A2. In grade F2, a carbon-based black coating is further provided on the first Al-plated steel sheet of grade A3. In grade F3, a carbon-based black coating is further provided on the first Al-plated steel sheet of grade A5. Furthermore, in grade F4, a carbon-based black coating is further provided on the first Al-plated steel sheet of grade A15.

[0259] [Table 5]

[0260]

[0261] As can be seen from Table 5, in F1 to F4, the temperature rise rate of the overlapping portion is further increased compared to the corresponding A2, A3, A5, and A15, and more excellent evaluation results are obtained.

[0262] The above, while referring to the attached Figure 1 While preferred embodiments of the present invention have been described in detail, the present invention is not limited to these examples. It is understood that anyone with common sense in the technical field to which the present invention pertains can readily conceive of various variations or modifications within the scope of the technical concept set forth in the claims, and such variations or modifications naturally fall within the technical scope of the present invention.

[0263] Furthermore, the present invention claims priority based on an invention filed on May 19, 2022, having the following subject matter.

[0264] [a] A hot stamping overlapping blank comprising: a first steel plate having a thickness of t1 (mm), which serves as the blank; and at least one second steel plate having a thickness of t2 (mm), which is overlapped and welded to the first steel plate and has a smaller area than the first steel plate, wherein:

[0265] The first steel plate and the second steel plate are both plated steel plates having Al-based plating layers on both sides of the steel plates.

[0266] The second steel sheet has a carbon-based black coating as at least a partial upper layer of the Al-based plating layer on the side thereof not in contact with the first steel sheet.

[0267] In the first steel sheet, the amount of the Al-based coating on the surface in contact with the second steel sheet is W1a (g / m 2 ), the surface on the side not in contact with the second steel plate is W1b (g / m 2 ), and regarding the brightness L* specified in JIS Z 8781-4 of the surface of the first steel sheet, the surface on the side in contact with the second steel sheet is L*1a, and the surface on the side not in contact with the second steel sheet is L*1b,

[0268] In the second steel sheet, the amount of the Al-based coating on the surface not in contact with the first steel sheet is W2b (g / m 2 ), and regarding the brightness L* of the surface of the second steel plate, the surface on the side not in contact with the first steel plate is L*2b,

[0269] At the portion where the first steel plate overlaps with the second steel plate, the average adhesion amount W2 of the Al-based coating is set to 0.5×(W1b+W2b), and the average brightness L*2 is set to 0.5×(L*1b+L*2b). At the portion of the first steel plate that does not overlap with the second steel plate, the average adhesion amount W1 of the Al-based coating is set to 0.5×(W1a+W1b), and the average brightness L*1 is set to 0.5×(L*1a+L*1b). At this time,

[0270] The W1a, W1b and W2b are all 20g / m 2 Above and 120g / m 2 Within the following range,

[0271] The L*1, the L*2, the W1, the W2, the t1, and the t2 satisfy the following relationship between the formula (a) and the formula (b):

[0272] 14≤(L*1-L*2)×(W1 / W2) 2 ≤32...Formula (a)

[0273] 0.80≤{t1 / (t1+t2)}×(L*1 / L*2) 2 ...Formula (b).

[0274] [b] The hot stamping stacked blank according to [a], wherein the carbon-based black coating has a thickness of 0.3 μm or more and 10.0 μm or less.

[0275] [c] The hot stamping laminated blank according to [a] or [b], wherein the carbon-based black coating contains a total of 0.2 g / m 2 Above and 3.0g / m 2 At least any one of the following Zn, Ti, Cu and V.

[0276] [d] The hot stamping stacked blank according to [a] or [b], wherein the carbon-based black coating has a nitrogen content of 1 mass % or more and 20 mass % or less.

[0277] [e] The stacked blank for hot stamping according to [c], wherein the carbon-based black coating has a nitrogen content of 1 mass % or more and 20 mass % or less.

[0278] [f] An overlapping hot stamped body comprising: a first steel plate having a thickness of T1 (mm); and at least one second steel plate having a thickness of T2 (mm) which is overlapped and welded to the first steel plate and has a smaller area than the first steel plate, wherein:

[0279] The first steel plate and the second steel plate are both plated steel plates having Al-Fe alloy plating layers on both surfaces.

[0280] In the first steel sheet, the coating thickness K1 of the Al-Fe alloy coating layer in the non-overlapping portion is 25 μm or more and 60 μm or less.

[0281] In the second steel sheet, the coating thickness K2 of the Al-Fe alloy coating layer in the overlapping portion is 25 μm or more and 60 μm or less.

[0282] In the first steel plate, the thickness of the diffusion layer containing 30% by mass or less of Al and 70% by mass or more of Fe, which is located in the Al-Fe alloy plating layer in contact with the steel plate substrate, is denoted as D1 (μm). In the second steel plate, the thickness of the diffusion layer containing 30% by mass or less of Al and 70% by mass or more of Fe, which is located in the Al-Fe alloy plating layer in contact with the steel plate substrate, is denoted as D2 (μm). In this case,

[0283] The K1, K2, D1 and D2 satisfy the relationship of the following formula (c),

[0284] The first steel plate and the second steel plate have a bent portion,

[0285] 0.0≤(D1-D2)×(K1 / K2) 2 ≤5.0...Formula (c).

[0286] [g] The overlapped hot stamped body according to [f], wherein, in the second steel sheet, on at least a portion of the Al-Fe alloy coating on the side not in contact with the first steel sheet, there is a total of 0.2 g / m 2 Above and 3.0g / m 2 At least any one of the following oxides of Zn, Ti, Cu, and V.

[0287] Description of Reference Numerals

[0288] 1. First Al-plated steel sheet; 1a. Surface of the first Al-plated steel sheet in contact with the second Al-plated steel sheet; 1b. Surface of the first Al-plated steel sheet not in contact with the second Al-plated steel sheet; 2. Second Al-plated steel sheet; 2a. Surface of the second Al-plated steel sheet in contact with the first Al-plated steel sheet; 2b. Surface of the second Al-plated steel sheet not in contact with the first Al-plated steel sheet; 3. Welding portion; 4. Overlapping blanks for hot stamping; 4a. Overlapping portion of overlapping blanks for hot stamping; 4b. Single-layer portion of overlapping blanks for hot stamping; 5. Heating furnace for hot stamping; 6. Pressing die for hot stamping; 7. Top of head; 8. Bent portion on the top of head side; 9. Bent portion on the flange side; 10. Vertical wall Part; 11. Flange part; 12. Overlapping hot stamping formed body; 13. Surface of one side of Al-plated steel plate; 14. Al-plating layer; 15. Base steel plate; 16. Surface of one side of Al-plated steel plate having a carbon-based black coating on the upper layer of Al-plating layer; 17. Carbon-based black coating; 18. Surface of one side of Al-Fe alloy-plated steel plate; 19. Al-Fe alloy coating; 20. Diffusion layer; 21. Base steel plate; 22. Surface of one side of Al-Fe alloy-plated steel plate having a coating containing any one of Zn, Ti, Cu, and V on the upper part of Al-Fe alloy coating; 23. Coating containing any one of Zn, Ti, Cu, and V.

Claims

1. A hot stamping overlapping blank, comprising: A first Al-based plated steel sheet having a sheet thickness of t1; and A second Al-plated steel plate having a plate thickness of t2, which is overlapped and welded on the first steel plate, has a smaller area than the first steel plate, and has a carbon-based black coating as an upper layer of the Al-plated layer on the side not in contact with the first Al-plated steel plate, The hot stamping overlapped blank satisfies the following equations (1) to (5): 14.0≤(L*1-L*2)×(W1 / W2) 2 ≤32.0 ... Formula (1) 0.86≤{t1 / (t1+t2)}×(L*1 / L*2) 2 ... form(2) 20≤W1a≤120 ... Formula (3) 20≤W1b≤120 ... Formula (4) 20≤W2b≤120 ... Formula (5) Here, W1=0.5×(W1a+W1b) W2=0.5×(W1b+W2b) L*1=0.5×(L*1a+L*1b) L*2=0.5×(L*1b+L*2b) W1a: the amount of Al-based plating layer deposited on the surface of the first Al-based plated steel sheet that is in contact with the second Al-based plated steel sheet, W1b: the amount of Al-based plating layer deposited on the surface of the first Al-based plated steel sheet that is not in contact with the second Al-based plated steel sheet, W2b: the amount of Al-based plating layer deposited on the surface of the second Al-based plated steel sheet that is not in contact with the first Al-based plated steel sheet, L*1a: Brightness L* of the surface of the first Al-plated steel sheet on the side in contact with the second Al-plated steel sheet, L*1b: Brightness L* of the surface of the first Al-based plated steel sheet that is not in contact with the second Al-based plated steel sheet, L*2b: Brightness L* of the surface of the second Al-plated steel sheet that is not in contact with the first Al-plated steel sheet, in, The unit of t1 and t2 is mm, the unit of W1a, W1b, W2a and W2b is g / m 2 .

2. The hot stamping overlapping blank according to claim 1, wherein: The hot stamping overlapped blank satisfies the following formula (6): 14.0≤(L*1-L*2)×(W1 / W2) 2 ≤30.0...Formula (6).

3. The hot stamping overlapping blank according to claim 1 or 2, wherein: The first Al-based plated steel sheet has a carbon-based black coating as an upper layer of the Al-based plated layer on the side thereof that is not in contact with the second Al-based plated steel sheet.

4. The hot stamping overlapping blank according to any one of claims 1 to 3, wherein: The carbon-based black coating has a thickness of 0.3 to 10.0 μm.

5. The hot stamping overlapping blank according to any one of claims 1 to 4, wherein: The total content of Zn, Ti, Cu and V in the carbon black coating is 0.2-3.0 g / m 2 .

6. The hot stamping overlapping blank according to any one of claims 1 to 4, wherein: The upper layer of the Al coating also has a total Zn content, Ti content, Cu content and V content of 0.2 to 3.0 g / m 2 layer.

7. The hot stamping overlapping blank according to any one of claims 1 to 6, wherein: The carbon-based black coating has a nitrogen content of 2 to 18% by mass.

8. An overlapping hot stamping formed body, comprising: a first Al-Fe alloy plated steel plate having a plate thickness of T1; and a second Al-Fe alloy plated steel plate having a plate thickness of T2, which is overlapped and welded on the first Al-Fe alloy plated steel plate and has a smaller area than the first Al-Fe alloy plated steel plate, The overlapped hot stamped body satisfies the following relationships (7) to (9): 25≤K1≤60 ... Formula (7) 25≤K2≤60 ... Formula (8) 0≤(D1-D2)×(K1 / K2) 2 ≤5.0 ... Formula (9) Here, K1: the average value of the plating thickness of the Al-Fe alloy plating layer on the side in contact with the second Al-Fe alloy plating steel sheet and the plating thickness of the Al-Fe alloy plating layer on the side not in contact with the second Al-Fe alloy plating steel sheet at the non-overlapping portion of the first Al-Fe alloy plating steel sheet, K2: the plating thickness of the Al—Fe alloy plating layer at the overlapping portion of the second Al—Fe alloy plated steel sheet, which is not in contact with the first Al—Fe alloy plated steel sheet, D1: in the first Al-Fe alloy-plated steel sheet, an average value of the thickness of the diffusion layer in the Al-Fe alloy-plated steel sheet located on the side in contact with the second Al-Fe alloy-plated steel sheet in contact with the steel sheet substrate and the thickness of the diffusion layer in the Al-Fe alloy-plated steel sheet located on the side not in contact with the second Al-Fe alloy-plated steel sheet in contact with the steel sheet substrate, D2: thickness of a diffusion layer in the Al-Fe alloy plated layer in contact with the steel sheet substrate on the side of the second Al-Fe alloy plated steel sheet that is not in contact with the first Al-Fe alloy plated steel sheet, in, The units of the plate thickness T1 and the plate thickness T2 are mm, and the units of K1, K2, D1, and D2 are μm.

9. The overlapping hot stamping formed body according to claim 8, wherein: In the second Al-Fe alloy plated steel sheet, the total of the Zn content, the Ti content, the Cu content and the V content is 0.2 to 3.0 g / m 2 The layer is located on the Al-Fe alloy plated layer on the side not in contact with the first Al-Fe alloy plated steel sheet, Furthermore, in the first Al-Fe alloy plated steel sheet, the total of the Zn content, the Ti content, the Cu content and the V content is 0 to 3.0 g / m 2 The layer is located on the Al—Fe alloy plated layer on the side not in contact with the second Al—Fe alloy plated steel sheet.

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