Method for manufacturing an overlapping hot stamping formed body and overlapping hot stamping formed body

Abstract

Solve the problem related to the warping of the steel plate caused by the difference in the heating rates of the overlapping part and the single-sheet part. This manufacturing method includes a process of heating the overlapping billets, a process of conveying the heated overlapping billets, and a process of pressing the heated overlapping billets using a die. In the heating process, when the thickness of the first steel plate is set as t1, the thickness of the second steel plate is set as t2, the average heating rate of the part with the total thickness (t1 + t2) of the overlapping part between the plate temperatures of 20°C and 800°C is set as V, and the average heating rate of the non-overlapping part between the plate temperatures of 20°C and 800°C is set as v1, the total thickness (t1 + t2) is 2.5 mm to 5.0 mm, the maximum length L of the overlapping part is 100 mm to 1100 mm, and the area S1 of the first steel plate, the area S2 of the part of the second steel plate overlapping with the first steel plate, the average heating rates V and v1 satisfy equations (1) to (3), and heating is performed at the heating temperature and heating time within a specific range in the coordinate plane defined by the heating time and the heating temperature.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an overlapped hot stamping formed body and an overlapped hot stamping formed body. Background Art

[0002] In recent years, in the applications of automotive steel sheets, steel sheets that can achieve both high strength and high formability have been desired. As one of the steel sheets that can achieve both high strength and high formability, there is a TRIP (Transformation Induced Plasticity) steel that utilizes the martensitic transformation of retained austenite. By using this TRIP steel, it is possible to manufacture a high-strength steel sheet with excellent formability and a strength of about 1000 MPa. However, the technology using TRIP steel has difficulty in ensuring formability in ultra-high-strength steels with higher strength (for example, 1500 MPa or more). In addition, there are problems such as poor shape retention after forming and poor dimensional accuracy of formed products.

[0003] Compared with the above-mentioned forming method near room temperature (so-called cold pressing method), a recently concerned method is hot stamping (also called hot die forging, hot pressing, die quenching, press quenching, etc.). This hot stamping is a manufacturing method for the following components: after heating a steel sheet to above the Ac3 point (for example, 800 °C or more) to austenitize it, immediately transporting the heated steel sheet to a press using, for example, a robot, and pressing the heated steel sheet under heating conditions to ensure formability, and rapidly cooling it to below the Ms point (for example, 400 °C or less) using a die during the bottom dead center holding period, thereby martensiticizing the material and quenching it, so as to obtain a material with the desired high strength after pressing. According to this method, it is possible to obtain automotive components with excellent shape retention after forming.

[0004] On the other hand, for various press-formed bodies used in components constituting an automotive body, improvements in various performances and characteristics are required from various viewpoints such as static strength, dynamic strength, collision safety, and weight reduction. For example, in automotive components such as A-pillar reinforcements, B-pillar reinforcements, bumper reinforcements, tunnel reinforcements, side sill reinforcements, roof reinforcements, or floor cross members, it is required that only specific parts in each automotive component have collision resistance characteristics compared to general parts other than the specific parts.

[0005] Therefore, from around 2007, the following method has been actually adopted: only at the part corresponding to the specific part that needs to be strengthened in automotive parts, multiple steel sheets are overlapped and joined (e.g., spot welding), and then the obtained steel sheet is hot stamping formed to manufacture an overlapped hot stamping formed body. This method is also called splicing. According to this method, it is possible to strengthen by overlapping the steel sheets only at the specific part of the hot stamping formed body while reducing the pressure modulus, and since the thickness of the part is not unnecessarily increased, it can also contribute to the weight reduction of the part. It should be noted that the blank produced by overlapping and welding in this way is called an overlapped blank (also called a spliced blank).

[0006] The schematic diagram of the process for manufacturing the overlapped hot stamping formed body is shown in Figure 1 . As will be described in detail later, in Figure 1 , reference numeral 4 represents the overlapped blank, and reference numeral 12 represents the overlapped hot stamping formed body.

[0007] When the overlapped steel sheets ( Figure 1 reference numerals 1 and 2 in the figure) are non-coated steel sheets, scale is generated on the surface of the manufactured overlapped hot stamping member through high-temperature heating accompanied by hot pressing forming. Therefore, there are the following problems: after hot pressing forming, it is necessary to remove the generated scale by, for example, shot peening, or the corrosion resistance of the manufactured overlapped hot stamping member is likely to decrease.

[0008] Furthermore, as a specific problem in the case of using non-coated steel sheets as the raw material of the overlapped blank, there is the following problem. That is, there is such a problem: the non-overlapped part (hereinafter, also called the "single-sheet part") can be shot peened, so the scale can be removed, and the reduction of corrosion resistance can be suppressed. On the other hand, the scale formed between the steel sheets in the overlapped part (hereinafter, also called the "overlapped part") is difficult to remove by shot peening, and the corrosion resistance is particularly likely to decrease.

[0009] If the overlapped steel sheets are coated steel sheets, the necessity of shot peening the overlapped hot pressing member after hot pressing forming is eliminated. As the coated steel sheets used for hot pressing, Zn-based coated steel sheets and Al-based coated steel sheets are generally listed. For either Zn-based coating or Al-based coating, through the alloying reaction of Fe diffusing in the coating, after hot stamping heating, the Zn-based coating becomes a Zn-Fe-based coating, and the Al-based coating becomes an Al-Fe-based coating. The schematic diagram of the coated steel sheet is shown in Figure 2 . Here, reference numeral 13 represents the coated steel sheet, reference numeral 15 represents the base material of the steel sheet, and reference numeral 14 represents the coating. This reference numeral 14 corresponds to the Zn-based coating and the Al-based coating.

[0010] It should be noted that the above Zn-based plating refers to plating with a Zn content of 50% by mass or more, and the above Zn-Fe-based plating refers to plating with a total content of Zn and Fe of 50% by mass or more. In addition, the Al-based plating refers to plating with an Al content of 50% by mass or more, and the above Al-Fe-based plating refers to plating with a total content of Al and Fe of 50% by mass or more.

[0011] As disclosed in Patent Document 1 and Patent Document 2, Zn-based plated steel sheets (i.e., plated steel sheets containing 50% by mass or more of Zn (such as Zn plating, or alloy plating such as Zn-Fe alloy, Zn-Ni alloy, Zn-Fe-Al alloy, etc.)) suppress the formation of scale and eliminate the problem of the need for shot peening. However, when using a Zn-based plated steel sheet as a raw material for an overlapping blank and performing bending forming on the overlapping portion during hot stamping forming, cracks sometimes occur in the base metal due to galvanization, and there are problems with the collision resistance characteristics. This is because, when zinc, which is a metal with a relatively low melting point, remains, Zn becomes a liquid metal and enters the base metal from the plating surface. Such a phenomenon is called liquid metal embrittlement (LME: Liquid Metal Embrittlement). It should be noted that bending forming is a means of ensuring collision resistance characteristics from the aspect of shape. Performing bending forming on the overlapping portion is an extremely important method of using the overlapping formed body.

[0012] As disclosed in Patent Document 1 and Patent Document 2, as countermeasures against liquid metal embrittlement when using Zn-based plated steel sheets for hot stamping, generally, countermeasures such as performing a Zn-Fe alloying reaction during hot stamping heating to increase the melting point of the plating and reducing the forming temperature during bending forming of hot stamping and waiting for the zinc to solidify can be cited. However, as unique problems when using Zn-based plated steel sheets as raw materials for overlapping blanks, the following three problems can be cited. First, since the plate thickness of the overlapping portion is thicker than that of the single sheet portion, both the heating rate and the cooling rate are slow, and there is a problem that it is difficult to perform a Zn-Fe alloying reaction during hot stamping heating. Second, regarding the forming temperature during hot stamping forming, if waiting for the overlapping portion to cool, the single sheet portion cools rapidly, and there is a problem that a martensite structure cannot be ensured in the single sheet portion. Third, in the single sheet portion, Zn forms a zinc oxide film to suppress the evaporation of Zn, but in the atmosphere between the steel sheets in the overlapping portion, due to the lack of oxygen, Zn evaporates. As a result, there is a problem that the corrosion resistance decreases due to the reduction of the plating in the overlapping portion.

[0013] In the Al-based plated steel sheets disclosed in Patent Document 3 and Patent Document 4 (i.e., plated steel sheets containing 50% by mass or more of Al (Al plating, or alloy plating such as Al-Si alloy, Al-Fe alloy, Al-Fe-Si alloy, etc.)), the formation of scale is suppressed in the same manner as Zn, and the problem of the need for shot peening treatment is eliminated. In addition, the Al-based plated steel sheet does not cause the problem of liquid metal embrittlement (LME), and has a boiling point as high as 2470 °C, so it is suitable as a material for overlapping blanks.

[0014] Prior Art Documents

[0015] Patent Documents

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-112569

[0017] Patent Document 2: Japanese Patent Application Laid-Open No. 2016-124029

[0018] Patent Document 3: International Publication No. WO2002 / 103073

[0019] Patent Document 4: International Publication No. WO2008 / 053273 Summary of the Invention

[0020] Problems to be Solved by the Invention

[0021] However, when using the Al-based plated steel sheets disclosed in Patent Document 3 and Patent Document 4 as raw materials for overlapping blanks, when heating during hot stamping, there is a problem that the heating rate of the overlapping portion is slow. That is, the heating rate when heating the overlapping blank is slower in the overlapping part (overlapping portion) and faster in the non-overlapping part (single sheet portion). Therefore, during the heating process, a temperature difference is formed between the overlapping portion and the single sheet portion. Due to the temperature difference, according to the linear expansion coefficient (Fe: 11.7×10 -6 [1 / °C]), the single sheet portion at a high temperature expands more than the overlapping portion. As a result, as Figure 3 shown, there is a problem that warping of the steel sheet occurs during the heating process. It should be noted that if heated for a certain period of time, during the period when the heating is completed and maintained at a high temperature, the temperature inside the blank becomes uniform, and the warping gradually converges and finally flattens.

[0022] Warpage of the steel sheet during the temperature rise causes problems related to the heating productivity as described below. Generally, the heating furnaces used in hot stamping include the following types: a roller hearth furnace (also known as a straight furnace), in which the steel sheet is placed on rollers continuous in the horizontal direction, and the steel sheet is heated while being moved between the rollers by the rotation of the rollers; and a multi-stage furnace (also known as a pizza furnace), in which the steel sheet is placed in a heating furnace having a plurality of heating sites in the horizontal and vertical directions, and the steel sheet is heated without being moved. In either type of furnace, the warpage of the aforementioned steel sheet hinders the heating productivity. More specifically, in the roller hearth furnace, the generation of warpage may change the traveling direction of the conveyance of the steel sheet based on the rotation of the rollers, thereby hindering the movement of the steel sheet in the furnace, or the steel sheet may fall between the rollers. In addition, in the multi-stage furnace, in addition to the possibility that the positions of the steel sheet before and after heating may be displaced, there is also the possibility that, due to the narrow heating space, the steel sheet may come into contact with the furnace wall due to warpage, causing equipment damage.

[0023] It should be noted that, in either the roller hearth furnace or the multi-stage furnace, after the heated blank is sent out from the heating furnace, it needs to be conveyed to the press. Generally, a robot is used to grasp the heated blank and convey it to the press. However, when the blank after heating remains warped, it is difficult to grasp it with a robot. When a large amount of warpage occurs during the heating process, the position of the blank moves. As a result, in the worst case, production stops, causing problems in the conveyance productivity of hot stamping.

[0024] In particular, in the heating with a heating rate of 4°C / s to 12°C / s as disclosed in Patent Document 4, since the heating rate is relatively fast, the difference in the heating rate between the single-piece part and the overlapping part is enlarged. As a result, there is a problem of further generating warpage of the steel sheet. Such a difference in the heating rate between the single-piece part and the overlapping part also hinders the uniformity at a high heating temperature, further significantly generating warpage.

[0025] Therefore, regarding the aluminum-based coated steel sheet that is suitably used as a raw material for a hot-stamped overlapping blank in order to suppress the scale of the base metal and not cause liquid metal embrittlement as described above, the following aspects are required. That is, to solve the problem related to the warpage of the steel sheet caused by the difference in the heating rate between the overlapping part and the single-piece part, and regarding the manufacturing method of the overlapping hot-stamped molded body, it is required to improve the productivity during hot-stamping heating.

[0026] Therefore, the present invention has been completed in view of the above problems, and the object of the present invention is to provide a manufacturing method of an overlapping hot-stamped molded body and an overlapping hot-stamped molded body that can solve the problem related to the warpage of the steel sheet caused by the difference in the heating rate between the overlapping part and the single-piece part and further improve the productivity during hot-stamping heating when using an aluminum-based coated steel sheet as a raw material.

[0027] Solution for solving problems

[0028] The inventors of the present invention repeatedly conducted intensive research to solve the above problems and found that it is important to suppress the difference in linear expansion between the overlapping part (i.e., the overlapping portion) and the non-overlapping part (i.e., the single-sheet part). Specifically, the difference in linear expansion ΔL [mm] affecting warping is represented by the product of the linear expansion rate α [1 / °C] inherent in the material, the maximum length L [mm] of the overlapping part, and the temperature difference ΔT [°C] between the overlapping part and the single-sheet part (ΔL = α × L × ΔT). Therefore, it was found that by suppressing the length of the overlapping part to 100 mm to 1100 mm and suppressing the difference in average heating rate between the overlapping part and the single-sheet part to 3.0 °C / s or less, warping can be improved.

[0029] In addition, heating is gradually performed from the single-sheet part toward the overlapping part, and also gradually from the end in the plane of the blank toward the center in the single-sheet part. Therefore, it was found that by slowly heating the overlapping part within the range of an average heating rate of 1.0 °C / s to 4.0 °C / s, the temperature unevenness of the overlapping part within the blank can be suppressed and warping can be improved.

[0030] Furthermore, the inventors of the present invention also found that for the overlapping part of the first steel plate with a plate thickness t1 (mm) having an area S1 (cm 2 ) and the second steel plate with a plate thickness t2 (mm) having an area smaller than that of the first steel plate, warping can be suppressed by increasing the rigidity of the overlapping part. That is, it was found that when the area of the part of the area of the second steel plate overlapping with the first steel plate is set to S2 (cm 2 ), by setting the total plate thickness (t1 + t2) to 2.5 mm or more and 5.0 mm or less, and the above-mentioned areas S1, S2 and the above-mentioned plate thickness t1 satisfy specific conditions, warping during temperature rise can be improved.

[0031] In addition, when taking out the overlapped steel plate after heating from the heating furnace, from the aspect of the stability of the conveyance of the steel plate, it is also necessary to equalize the plate temperatures of the overlapping part and the single-sheet part to the furnace temperature and for the warping to converge. The inventors of the present invention found that by heating the overlapped steel plate at a heating temperature and a heating time within the figure ABCD determined by points A (4 minutes, 930 °C), B (10 minutes, 930 °C), C (20 minutes, 870 °C) and D (8 minutes, 870 °C) in the coordinate plane defined by (heating time, temperature in the furnace after preheating), warping when sending out from the heating furnace can be improved.

[0032] Furthermore, the inventors of the present invention have found that when investigating the corrosion resistance of the overlapped hot-stamped formed body while suppressing warpage, in the first steel sheet in the overlapped portion of the first steel sheet and the second steel sheet, red rust generated in the plating layer on the surface not in contact with the second steel sheet is suppressed. It is presumed that this is due to the improvement of warpage, resulting in a reduction in the tensile stress formed in the Al-Fe-based plating layer and suppression of cracks in the plating.

[0033] The gist of the present invention completed based on the above insights is as follows.

[0034] [1] A method for manufacturing an overlapped hot-stamped formed body, which uses an overlapped blank obtained by overlapping and joining a first steel sheet having an area S1 (cm 2 ) and at least one second steel sheet having an area smaller than the area of the first steel sheet to manufacture the overlapped hot-stamped formed body. The first steel sheet and the second steel sheet are Al-based plated steel sheets having an Al-based plating layer on the base material. The manufacturing method includes: an overlapped blank heating step of heating the overlapped blank using a heating furnace; a heated blank conveying step of sending out the heated overlapped blank from the heating furnace and conveying it to a pressing device; and a hot stamping step of pressing the heated overlapped blank using a die provided in the pressing device to obtain the overlapped hot-stamped formed body. In the overlapped blank heating step, when the plate thickness of the first steel sheet is set as t1 (mm), the plate thickness of the second steel sheet is set as t2 (mm), the average heating rate of the portion with the total plate thickness (t1 + t2) formed by overlapping the first steel sheet and the second steel sheet is set as V (°C / s) between a plate temperature of 20°C and 800°C, and the average heating rate of the portion of the first steel sheet not overlapping with the second steel sheet is set as v1 (°C / s) between a plate temperature of 20°C and 800°C, the total plate thickness (t1 + t2) of the overlapped portion is 2.5 mm or more and 5.0 mm or less, the maximum length L of the overlapped portion of the second steel sheet is 100 mm or more and 1100 mm or less, the average heating rates V and v1 satisfy the relational expressions of the following formula (1) and formula (2). When the area of the portion of the area of the second steel sheet overlapping with the first steel sheet is set as S2 (cm 2 ), the area S1, S2, and the plate thickness t1 satisfy the relational expression of the following formula (3). In the coordinate plane defined by the heating time and the heating temperature, the overlapped blank is heated with the heating temperature and the heating time located inside the figure ABCD determined by point A (4 minutes, 930°C), point B (10 minutes, 930°C), point C (20 minutes, 870°C), and point D (8 minutes, 870°C).

[0035] [2]The manufacturing method of the overlapping hot stamping formed body according to [1], wherein the maximum length L of the overlapping portion of the second steel plate is 300 mm or more.

[0036] [3]The manufacturing method of the overlapping hot stamping formed body according to [1] or [2], wherein the base materials of the first steel plate and the second steel plate contain, by mass%, C: 0.10% or more and 0.50% or less, Si: 0.01% or more and 2.00% or less, Mn: 0.30% or more and 5.00% or less, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Al: 0.500% or less, B: 0.0002% or more and 0.0100% or less, and the balance is Fe and impurities.

[0037] [4]The manufacturing method of the overlapping hot stamping formed body according to [3], wherein the base materials of the first steel plate and the second steel plate further contain, by mass%, instead of a part of the balance of Fe, W: 0% or more and 3.0% or less, Cr: 0% or more and 2.0% or less, Mo: 0% or more and 3.0% or less, V: 0% or more and 2.0% or less, Ti: 0% or more and 0.5% or less, Nb: 0% or more and 1.0% or less, Ni: 0% or more and 5.0% or less, Cu: 0% or more and 3.0% or less, Co: 0% or more and 3.0% or less, Sn: 0% or more and 0.10% or less, Sb: 0% or more and 0.10% or less, Mg: 0% or more and 0.0050% or less, Ca: 0% or more and 0.0050% or less, O: 0% or more and 0.0070% or less, one or more of them.

[0038] [5]The manufacturing method of the overlapping hot stamping formed body according to [3] or [4], wherein the C content C1 (mass%) of the base material of the first steel plate and the C content C2 (mass%) of the base material of the second steel plate satisfy the relational expression of the following formula (4).

[0039] [6]An overlapping hot stamping formed body, which has an area S1 (cm 2) It is formed by laminating a first steel plate and at least one second steel plate having an area smaller than that of the first steel plate. An Al-Fe-based coating is provided on the surfaces of the first steel plate and the second steel plate. The Al-Fe-based coating is composed of a compound layer of Al and Fe and an Al solid solution Fe layer. When the plate thicknesses of the first steel plate and the second steel plate are set as t1 and t2 (mm) respectively, the total plate thickness (t1 + t2) of the overlapping portion of the first steel plate and the second steel plate is 2.5 mm or more and 5.0 mm or less. The maximum length L of the overlapping portion of the second steel plate is 100 mm or more and 1100 mm or less. When the area of the portion of the area of the second steel plate overlapping with the first steel plate is set as S2 (cm 2 ), the area S1, S2, and plate thickness t1 satisfy the relational expression of the following formula (3). In the Al-Fe-based coating on the surface of the first steel plate where the first steel plate does not contact the second steel plate in the overlapping portion of the first steel plate and the second steel plate, the number of cracks reaching the Al solid solution Fe layer is 5 or less per 100 μm length parallel to the Al-Fe-based coating. The thickness D1 (μm) of the Al solid solution Fe layer of the portion of the first steel plate not overlapping with the second steel plate and the thickness D2 (μm) of the Al solid solution Fe layer of the second steel plate satisfy the relational expression of the following formula (5).

[0040] [7] The overlapping hot stamping formed body according to [6], wherein the maximum length L of the overlapping portion of the second steel plate is 300 mm or more.

[0041] [8] The overlapping hot stamping formed body according to [6] or [7], wherein the base materials of the first steel plate and the second steel plate contain, by mass%, C: 0.10% or more and 0.50% or less, Si: 0.01% or more and 2.00% or less, Mn: 0.30% or more and 5.00% or less, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Al: 0.500% or less, B: 0.0002% or more and 0.0100% or less, and the balance is Fe and impurities.

[0042] [9] The overlapping hot stamping formed body according to [8], wherein the base materials of the first steel sheet and the second steel sheet further contain, in mass %, one or more of W: 0% or more and 3.0% or less, Cr: 0% or more and 2.0% or less, Mo: 0% or more and 3.0% or less, V: 0% or more and 2.0% or less, Ti: 0% or more and 0.5% or less, Nb: 0% or more and 1.0% or less, Ni: 0% or more and 5.0% or less, Cu: 0% or more and 3.0% or less, Co: 0% or more and 3.0% or less, Sn: 0% or more and 0.10% or less, Sb: 0% or more and 0.10% or less, Mg: 0% or more and 0.0050% or less, Ca: 0% or more and 0.0050% or less, O: 0% or more and 0.0070% or less, and REM: 0% or more and 0.0070% or less, in place of a part of the surplus Fe.

[0043]

[10] The overlapping hot stamping formed body according to [8] or [9], wherein the C content C1 (mass %) of the base material of the first steel sheet and the C content C2 (mass %) of the base material of the second steel sheet satisfy the relational expression of the following formula (4).

[0044] 1.0 ≤ V ≤ 4.0... Formula (1)

[0045] (v1 - V) ≤ 3.0... Formula (2)

[0046] 400 ≤ (S1 - S2) × (t1 / 10) ≤ 950... Formula (3)

[0047] 0.03 ≤ (C2 - C1) ≤ 0.30... Formula (4)

[0048] (D1 - D2) ≤ 6.0... Formula (5)

[0049] Effects of the Invention

[0050] As described above, according to the present invention, in the case of using an Al-based plated steel sheet as a raw material, it is possible to improve the problem of warping of the steel sheet during heating in the process of manufacturing an overlapping hot stamping formed body. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic view showing an outline of the process of manufacturing an overlapping hot stamping formed body.

[0052] Figure 2 It is a view showing a cross section of a steel sheet coated with an Al-based coating.

[0053] Figure 3 It is a view schematically showing a case where warping occurs during temperature rise in the heating process of a blank and an example of a side photograph actually taken of the warping during the temperature rise.

[0054] Figure 4 It is a diagram schematically showing the maximum length L of the overlapping portion of the first steel plate and the second steel plate.

[0055] Figure 5 It is a diagram schematically showing an example in which warping is suppressed when the difference in the heating rate between the single portion and the overlapping portion at 20°C to 800°C is 2°C / s during the temperature increase in the process of heating the workpiece.

[0056] Figure 6 It is a diagram schematically showing an example in which warping occurs when the difference in the heating rate between the single portion and the overlapping portion at 20°C to 800°C is 4°C / s during the temperature increase in the process of heating the workpiece.

[0057] Figure 7 It is a diagram showing the heating temperature and heating time within the figure ABCD determined by points A (4 minutes, 930°C), B (10 minutes, 930°C), C (20 minutes, 870°C), and D (8 minutes, 870°C) in the coordinate plane defined by (heating time, temperature in the furnace after preheating) in the process of heating the overlapped hot stamping formed body.

[0058] Figure 8 It shows in the overlapped hot stamping formed body, in the portion corresponding to Figure 1 an example of a crack formed in the plating surface of 1b.

[0059] Figure 9 It is a diagram schematically showing the shape of the cap-shaped overlapped hot stamping formed body used in the embodiment of the present invention. Detailed Description of the Embodiment

[0060] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in this specification and the drawings, components having substantially the same functional structure are denoted by the same reference numerals, and thus redundant descriptions are omitted.

[0061] 《1. Outline of the Manufacturing Method of the Overlapped Hot Stamping Formed Body》

[0062] Figure 1 It is a diagram schematically showing an example of the manufacturing method of the overlapped hot stamping formed body using the overlapped blank for hot stamping and the overlapped hot stamping formed body. Hereinafter, based on Figure 1 and Figure 2 it will be described.

[0063] The manufacturing method of the overlapped hot stamping formed body is used as a method for manufacturing the overlapped hot stamping formed body using the overlapped blank for hot stamping as a raw material.

[0064] The overlapped blank 4 for hot stamping is formed by joining a first steel plate 1 ( Figure 1 with the reference numeral 1) and a second steel plate 2 having an area smaller than that of the first steel plate ( Figure 1 with the reference numeral 2) ( Figure 1 with the reference numeral 3). At this time, in the overlapped blank 4 for hot stamping ( Figure 1 with the reference numeral 4), the portion where the second steel plate 2 is overlapped is referred to as an overlapping portion 4a, and the non-overlapped portion is referred to as a single-sheet portion 4b.

[0065] Regarding the overlapped blank 4 for hot stamping according to the embodiment of the present invention described in detail below, the outline of the manufacturing method is also as Figure 1 shown, and the outline of its configuration is also as Figure 2 shown.

[0066] It should be noted that in the overlapped blank 4 for hot stamping according to the embodiment of the present invention, the second steel plate 2 is also preferably arranged inside the first steel plate 1 in such a manner that there is no portion protruding from the first steel plate 1, as schematically shown in Figure 1 . However, there may be a portion of the second steel plate 2 protruding from the first steel plate 1.

[0067] In addition, on the surface of the first steel plate 1, Al-based coatings ( Figure 2 with the reference numeral 14) are coated on both the surface 1a on the side in contact with the second steel plate 2 and the surface 1b on the side not in contact with the second steel plate 2. Similarly, for the second steel plate 2, Al-based coatings are coated on both the surface 2a on the side in contact with the first steel plate 1 and the surface 2b on the side not in contact with the first steel plate 1.

[0068] The overlapped blank 4 for hot stamping is heated to a temperature above the Ac3 point in a heating furnace 5, so that the base metal portion of the steel plate is austenitized. After the heated steel plate is taken out of the furnace, it is immediately conveyed, press-formed with a die 6 and quenched, whereby martensite transformation occurs in the steel plate. Thus, the overlapped blank 4 for hot stamping becomes an overlapped hot-stamped formed body 12 having excellent collision resistance characteristics.

[0069] In Figure 1 , as an example of the overlapped hot-stamped formed body 12, a formed product using a cap-shaped die is illustrated. In this specification, the parts of the hot-stamped formed body 12 are referred to as a top portion 7, a bent portion 8 of the top portion, a longitudinal wall portion 10, a flange portion 11, and a bent portion 9 of the flange portion.

[0070] It should be noted that in Figure 1 , the second steel plate 2 is arranged outside the side of the top portion 7, but the second steel plate 2 may also be arranged inside the top portion 7.

[0071] "Method for Manufacturing Overlapped Hot-Stamped Formed Body"

[0072] Hereinafter, a characteristic manufacturing method of the overlapped hot-stamped formed body according to the embodiment of the present invention will be described in detail.

[0073] (2-1. Overlapped Blanks)

[0074] The overlapped blank 4 for hot stamping in this embodiment (hereinafter, sometimes simply referred to as "blank".) is the same as the overlapped blank 4 for hot stamping shown in the above Figure 1 and Figure 2 and has: a first steel plate 1 having an area S1 (cm 2 ); and a second steel plate 2 joined to the first steel plate 1 and having an area smaller than the area of the first steel plate 1. In addition, an Al-based plating is coated on both surfaces of the first steel plate 1 and the second steel plate 2 respectively. That is, the first steel plate 1 and the second steel plate 2 in this embodiment are Al-based plated steel plates having Al-based plating layers on both surfaces of the steel plate as the base material. It should be noted that the area S1 of the first steel plate 1 refers to the area of the steel plate plane substantially orthogonal to the plate thickness direction of the first steel plate 1 (the area per single surface).

[0075] <Base Material>

[0076] In the overlapped blank 4 for hot stamping in this embodiment, the chemical composition of the base material in each of the first steel plate 1 and the second steel plate 2 is not particularly limited. However, for example, for the purpose of obtaining a tensile strength of 1000 MPa or more (about 300 HV or more with a Vickers hardness tester when the load is 9.81 N), it is preferable to use a base material having the following chemical composition. In addition, within the following chemical composition range, the chemical composition of the base material of the first steel plate 1 and the chemical composition of the base material of the second steel plate 2 may be the same or different.

[0077] That is, the chemical components of the base materials of the first steel plate 1 and the second steel plate 2 in the present embodiment contain, by mass%, C: 0.10% to 0.50%, Si: 0.01% to 2.00%, Mn: 0.30% to 5.00%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Al: 0.500% or less, B: 0.0002% or more and 0.0100% or less, and the balance is Fe and impurities. In addition, in order to improve the collision resistance characteristics of the steel plate, the chemical components of the base materials of the first steel plate 1 and the second steel plate 2 in the present embodiment preferably further have the following chemical components to replace a part of the balance of Fe, containing Ti: 0% or more and 0.5% or less, Nb: 0% or more and 1.0% or less, Cr: 0% or more and 2.0% or less, W, Mo: 0% or more and 3.0% or less, V: 0% or more and 2.0% or less, Ni: 0% or more and 5.0% or less, Cu, Co: 0% or more and 3.0% or less, Sn, Sb: 0% or more and 0.10% or less, Mg, Ca: 0% or more and 0.0050% or less, O, REM: 0% or more and 0.0070% or less, and one or more of them.

[0078] In addition, in order to improve the collision safety, steel plates used in automotive components use steel plates having a high C content and a high tensile strength. Therefore, in the steel plates used in the overlapping hot stamping formed body, it is common to use steel plates having a high C content for both the first steel plate and the second steel plate. However, when the C content of the base material of the first steel plate 1 is set to C1 (mass%) and the C content of the base material of the second steel plate 2 is set to C2 (mass%), C1 and C2 preferably satisfy the relational expression of 0.03 ≤ (C2 - C1) ≤ 0.30. Due to the increase in the C content, the deformation resistance of the steel plate at high temperature increases. Therefore, in order to suppress the warping of the blank 4 during heating, it is better to increase the C content. From this viewpoint, it is preferable that the second steel plate with uniform temperature increases the C content, and the first steel plate with non-uniform temperature in the single-layer part and the overlapping part reduces the C content. The inventors of the present invention have conducted intensive research, and as a result, it has been clarified that by making the difference between C2 and C1 (C2 - C1) 0.03 mass% or more, the warping of the blank 4 can be more reliably suppressed. The difference between C2 and C1 (C2 - C1) is preferably 0.04 mass% or more, and more preferably 0.05 mass% or more. On the other hand, by setting the difference between C2 and C1 (C2 - C1) to 0.30 mass% or less, the embrittlement of the base material of the second steel plate and the extreme reduction of the tensile strength of the first steel plate can be more reliably suppressed. As a result, the collision characteristics of the components manufactured using this blank can be more reliably guaranteed, and the practicality of the components can be ensured. The difference between C2 and C1 (C2 - C1) is more preferably 0.28 mass% or less, and more preferably 0.25 mass% or less.

[0079] The method for manufacturing an Al-based plated steel sheet using a base material having the above chemical composition is not particularly limited. For example, an Al-based plated steel sheet can be manufactured through ironmaking and steelmaking processes by conventional methods, followed by hot rolling, pickling, cold rolling, and Sendzimir hot-dip aluminizing processes.

[0080] <Regarding the Al-based coating>

[0081] In the present embodiment, the Al-based coating is coated on the front and back surfaces of each of the first steel sheet 1 and the second steel sheet 2.

[0082] As the characteristics required for the Al-based coating, the suppression of the generation of Fe scale during hot stamping heating, and the defects of the coating caused by the peeling of the coating during hot stamping forming (also referred to as chalking), and the indentation caused by the adhered peeled coating to other parts can be cited. Chalking is caused by the compressive stress of the surface load on the coating on the inner side of the bent portion generated during forming, the shear stress on the coating due to the sliding from the die during forming, etc. Therefore, the coating thickness of the Al-based coating is preferably independently 10 μm or more and 50 μm or less in each of the first steel sheet 1 and the second steel sheet 2. When the coating thickness is less than 10 μm, the suppression effect of Fe scale generation may be insufficient. By making the coating thickness of the Al-based coating 10 μm or more, the suppression effect of Fe scale generation can be more reliably exhibited. The coating thickness of the Al-based coating is more preferably 15 μm or more. On the other hand, when the coating thickness exceeds 50 μm, a large amount of chalking may occur. By setting the coating thickness to 50 μm or less, the occurrence of chalking can be more reliably prevented. The coating thickness of the Al-based coating is more preferably 45 μm or less.

[0083] It should be noted that as a method for determining the coating thickness of the Al-based coating, by using an optical microscope, observing the coating cross-section without etching treatment in a field of view of 100 μm × 100 μm, and measuring the coating thickness, the coating thickness can be obtained. More specifically, at any arbitrary multiple sites (for example, 3 sites), observe the coating cross-section by the above method, and determine the coating thickness of each observed site. Then, calculate the average value of the obtained coating thickness, and use the obtained average value as the coating thickness of the Al-based coating.

[0084] As a method for coating the base material with the Al-based coating, according to the general hot-dip plating method, by immersing the steel sheet in a hot-dip aluminum bath and performing gas wiping with nitrogen, air, etc., an Al-based plated steel sheet ( Figure 2 with reference numeral 13) having an adjusted coating amount can be manufactured. At this time, through the alloying reaction between the Al-based coating and Fe of the base material during hot-dip plating, inevitably in the Al-based coating ( Figure 2 with reference numeral 14) and the base material ( Figure 2The interface of the attached drawing reference numeral 15 forms an Al-Fe-based interfacial alloy layer of about several μm. The thickness of the formed interfacial alloy layer can be controlled by adjusting the immersion time in the hot-dip aluminizing bath, and can be increased by extending the immersion time.

[0085] The chemical composition of the hot-dip aluminizing bath for forming the above-mentioned Al-based coating is not particularly limited. However, from the aspect of excellent heat resistance, the content of Al in the hot-dip aluminizing bath is preferably 80% by mass or more. In addition, from the aspect of easily controlling the thickness of the interfacial alloy layer, the content of Si in the hot-dip aluminizing bath is preferably 2% by mass or more. When the content of Si is less than 2% by mass, the interfacial alloy layer becomes too thick and the formability may be reduced. On the other hand, when the content of Si in the hot-dip aluminizing bath exceeds 15% by mass, the alloying rate of the Al-based coating during hot stamping heating becomes slow, and the productivity of hot stamping may be reduced. Therefore, the content of Si in the hot-dip aluminizing bath is preferably 15% by mass or less. When the interfacial alloy layer does not contain Si in the hot-dip aluminizing bath, it is composed of a binary alloy layer of Al-Fe system, and when Si is contained, in addition to the above binary system, it is also composed of a ternary alloy layer of Al-Fe-Si system. In addition, in the hot-dip aluminizing bath as described above, various impurities sometimes exist.

[0086] When the Al-based coating 14 contains 2% by mass or more and 15% by mass or less of Si, in the Al-based coating 14, a eutectic structure of Al and Si is formed based on the phase diagram. In the case of using the hot-dip method, sometimes 1% by mass or more and 5% by mass or less of Fe is inevitably contained in the hot-dip aluminizing bath as a dissolution component from the steel sheet. As other inevitable impurities, elements such as Cr, Mn, V, Ti, Sn, Ni, Cu, W, Bi, Mg, Ca, etc. which are dissolution components from the hot-dip equipment and impurities in the ingot of the hot-dip aluminizing bath can be cited, and sometimes these elements are contained in less than 1% by mass.

[0087] The above-mentioned interfacial alloy layer is composed of a combination of phases such as a binary alloy of Al and Fe, namely, θ phase (FeAl 3 ), η phase (Fe 2 Al 5 ), ζ phase (FeAl 2 ), Fe 3 Al, FeAl, and the BCC phase of Fe solid-solved with Al. As the chemical composition of the interfacial alloy layer in the case of containing Si, for example, τ1 phase (Al 2 Fe 3 Si 3 ), τ2 phase (Al 3 FeSi), τ3 phase (Al 2 FeSi), τ4 phase (Al 3FeSi 2 ) and τ5 phase (Al 8 Fe 2 Si), τ6 phase (Al 9 Fe 2 Si 2 ), τ7 phase (Al 3 Fe 2 Si 3 ), τ8 phase (Al 2 Fe 3 Si 4 ), τ10 phase (Al 4 Fe 1.7 Si), τ11 phase (Al 5 Fe 2 Si), etc., mainly composed of any one of τ5 phase, τ6 phase, θ phase, η phase or multiple phases thereof. It should be noted that the above phases are sometimes not stoichiometric compositions (i.e., the element ratios are not integers).

[0088] <Regarding the plate thickness>

[0089] In this embodiment, the total plate thickness (t1 + t2) formed by overlapping the first steel plate 1 with a plate thickness t1 (mm) and the second steel plate 2 with a plate thickness t2 (mm) is 2.5 mm or more and 5.0 mm or less.

[0090] In this embodiment, as a characteristic required for the Al-based plated steel sheet, it is important to further suppress the problems in the case of using it as an overlapping blank, that is, the warping caused by the difference in the heating rate between the overlapping part with a slow heating rate and the single-sheet part with a fast heating rate. In order to suppress the warping as described above, the total plate thickness (t1 + t2) of the overlapping part (overlapping part) of the plate thickness t1 (mm) of the first steel plate 1 and the plate thickness t2 (mm) of the second steel plate 2 is set to 2.5 mm or more and 5.0 mm or less. When the total plate thickness (t1 + t2) is less than 2.5 mm, significant warping occurs, reducing the productivity during hot stamping heating. The total plate thickness (t1 + t2) is preferably 2.8 mm or more, more preferably 3.0 mm or more. On the other hand, when the total plate thickness (t1 + t2) exceeds 5.0 mm, the heat capacity becomes too large, the heating rate during hot stamping heating becomes slow, and the heating productivity decreases, so it is not preferred. The total plate thickness (t1 + t2) is preferably 4.8 mm or less, more preferably 4.5 mm or less.

[0091] Here, regarding the plate thickness t1 of the first steel plate 1 and the plate thickness t2 of the second steel plate 2, they are each preferably in the range of about 1.0 mm to 4.0 mm, for example.

[0092] Note that the thickness t1 of the first steel plate 1 and the thickness t2 of the second steel plate 2 can be measured using a micrometer, or can also be measured by observing a cross-section using an optical microscope. In addition, the above-mentioned thicknesses t1 and t2 are set to be the thicknesses including the Al-based plating layers provided on both sides in addition to the thickness of the base material.

[0093] <Regarding the maximum length L of the overlapping portion>

[0094] In the present embodiment, the maximum length L of the overlapping portion (overlap portion) of the first steel plate 1 and the second steel plate 2 is 100 mm or more and 1100 mm or less. The reason for setting the maximum length L of the overlapping portion within the above range will be described again below.

[0095] Note that the maximum length L of the overlapping portion (overlap portion) of the first steel plate 1 and the second steel plate 2 can be measured using a known measuring device such as a vernier caliper or a tape measure. In addition, the maximum length L of the overlapping portion (overlap portion) is set to the diameter of the smallest circumscribed circle that encloses the overlapping portion of the first steel plate 1 and the second steel plate 2. According to this definition, for example, Figure 5 in the case where the overlapping portion shown in (a) is a quadrilateral, the length of the diagonal at the four corners is the maximum length L. In addition, Figure 5 in the case shown in (b), the maximum length L becomes the diameter of the smallest circumscribed circle shown in the figure.

[0096] (2-2. Regarding the heating of the overlapping blank during hot stamping)

[0097] According to the following formula (A), warping is caused by the difference in the heating rate between the overlapping portion with a slow heating rate and the single-sheet portion with a fast heating rate due to the temperature difference between the overlapping portion and the single-sheet portion.

[0098] The difference in linear expansion ΔL [mm] in the following formula (A) causes warping, and ΔL is represented by the product of the linear expansion rate α [1 / °C] inherent in the material, the length Ls [mm] of the material, and the temperature difference ΔT [°C] of the material. Therefore, in the blank of the present embodiment, the length Ls in the following formula (A) corresponds to the maximum length L of the overlapping portion.

[0099] ΔL = α × Ls × ΔT... Formula (A)

[0100] Therefore, if the maximum length L of the overlapping portion is short, ΔL becomes small and warping is also suppressed. However, when the maximum length L of the overlapping portion is less than 100 mm, a temperature difference is generated in the blank of the non-overlapping portion from the end with faster temperature rise toward the center with slower temperature rise, thus causing warping. From this viewpoint, the maximum length L of the overlapping portion is set to 100 mm or more. Thereby, the generation of warping during heating of the blank can be prevented. The maximum length L of the overlapping portion is preferably 200 mm or more, more preferably 400 mm or more. On the other hand, when the maximum length L of the overlapping portion exceeds 1100 mm, the warping becomes large and the productivity during hot stamping heating decreases. From this viewpoint, the maximum length L of the overlapping portion is set to 1100 mm or less. Thereby, the productivity can be ensured and the generation of warping during heating can be prevented. The maximum length L of the overlapping portion is preferably 1050 mm or less, more preferably 1000 mm or less.

[0101] <Regarding the relationship between the area S1 of the first steel plate and the area S2 of the second steel plate>

[0102] The warping of the blank during heating is suppressed by the self-weight of the non-overlapping portion (single-sheet portion) of the first steel plate 1 in the first steel plate 1. Therefore, in the present embodiment, the area of the portion of the area of the second steel plate 2 that overlaps with the first steel plate 1 is set to S2 (cm 2 ), and the value obtained by multiplying the difference between the area S1 of the first steel plate and the above area S2 by the plate thickness t1 of the first steel plate 1, {(S1 - S2) × (t1 / 10)} (unit: cm 3 ) is used as an index corresponding to the self-weight of the above single-sheet portion. Here, the reason for dividing the plate thickness t1 (mm) by 10 is to convert the unit of the plate thickness t1 from mm to cm. In addition, regarding the area S2, when there is no portion of the second steel plate 2 that extends from the first steel plate 1, the area of the second steel plate 2 becomes the above area S2.

[0103] The inventors of the present invention and the like conducted dedicated research using the above-mentioned indices, and as a result, it was clarified that warping during heating can be suppressed by setting the value of the index {(S1 - S2) × (t1 / 10)} to 400 or more and 950 or less. Here, in conventional overlapping blanks, weight reduction is important for automotive steel sheets. Therefore, by limiting the area S2 of the second steel sheet that bears the strengthening function to the minimum, the value of the index {(S1 - S2) × (t1 / 10)} sometimes exceeds 950, or by limiting the area S1 or the plate thickness t1 of the first steel sheet to the minimum, the value of the index {(S1 - S2) × (t1 / 10)} sometimes falls below 400. However, in order to meet the increasing requirements for collision safety in recent years, it is necessary to increase the values of S1, S2, and t1 respectively, and a new problem of warping of the blank occurs. Therefore, the inventors of the present invention and the like found that warping during heating can be suppressed by setting the value of the index {(S1 - S2) × (t1 / 10)} to 400 or more and 950 or less. When the value of the index {(S1 - S2) × (t1 / 10)} is less than 400, the warping suppression effect is insufficient. By setting the value of the index {(S1 - S2) × (t1 / 10)} to 400 or more, warping that may occur during heating can be suppressed. The value of the index {(S1 - S2) × (t1 / 10)} is preferably 420, and more preferably 440. On the other hand, when the value of the index {(S1 - S2) × (t1 / 10)} exceeds 950, the size of the entire blank becomes larger and the height of warping becomes larger. By setting the value of the index {(S1 - S2) × (t1 / 10)} to 950 or less, the height of warping that may occur during heating can be reduced. The value of the index {(S1 - S2) × (t1 / 10)} is preferably 930 or less, and more preferably 900 or less.

[0104] <Regarding joining>

[0105] In a hot stamping overlapping blank in which the first steel sheet 1 and the second steel sheet 2 are overlapped and joined, the above joining is preferably spot welding. The reasons therefor are described below.

[0106] In the overlapping portion, heat transfer is improved by making the first steel sheet 1 and the second steel sheet 2 contact well with each other. Thereby, it is possible to suppress the problem in the case of using an overlapping blank, that is, the difference in the heating rate between the overlapping portion (slow heating rate) and the single sheet portion (fast heating rate), and suppress warping.

[0107] As the type of joining, spot welding, seam welding, brazing, laser welding, plasma welding, arc welding, etc. can be selected. From the aspect of making the large-area overlapping portion contact efficiently and well, spot welding that can make multiple points contact up to the inside of the overlapping portion and directly join by applying pressure between the steel sheets is preferred.

[0108] At this time, the dot density of spot welding is preferably 1 dot / 200 cm 2 or more. When the dot density is less than 1 dot / 200 cm 2 , the contact between the steel plates becomes insufficient, and the improvement of the temperature rise in the overlapping portion becomes insufficient. The dot density of spot welding is more preferably 1 dot / 40 cm 2 or more. On the other hand, regarding the dot density of spot welding, there is no particular upper limit specified. However, if the density is too high, the welding current will be shunted and welding will become difficult. Therefore, it is preferably 1 dot / 1 cm 2 or less.

[0109] The above-mentioned dot density of spot welding (dots / cm 2 ) is obtained by dividing the number of spot welding dots in the second steel plate 2 where the blank has been treated by the area of the portion of the second steel plate 2 that overlaps with the first steel plate 1.

[0110] <Regarding the heating rate during heating>

[0111] In the present embodiment, the average heating rate V (℃ / s) from 20 °C to 800 °C of the plate temperature in the portion with the total plate thickness (t1 + t2) (mm) formed by overlapping the first steel plate 1 and the second steel plate 2 and the average heating rate v1 (℃ / s) from 20 °C to 800 °C of the plate temperature in the portion of the first steel plate 1 that does not overlap with the second steel plate 2 satisfy the following relational expressions of formula (1) and formula (2). Hereinafter, the reasons will be explained.

[0112] 1.0 ≤ V ≤ 4.0... Formula (1)

[0113] (v1 - V) ≤ 3.0... Formula (2)

[0114] According to the aforementioned formula (A), the warpage caused by the difference in the heating rate between the overlapping portion with a slow heating rate and the single-layer portion with a fast heating rate is caused by the temperature difference between the overlapping portion and the non-overlapping portion. Therefore, in order to reduce the temperature difference ΔT of the materials between the overlapping portion and the non-overlapping portion, by suppressing the difference in the average heating rate (v1 - V), the warpage becomes smaller. More specifically, by setting the difference in the average heating rate (v1 - V) to 3.0 °C / s or less, as schematically shown in Figure 5 , the warpage is suppressed, and the reduction in productivity during hot stamping heating is improved. On the other hand, when the difference in the average heating rate (v1 - V) exceeds 3.0 °C / s, as schematically shown in Figure 6 , the warpage becomes larger, and the productivity during hot stamping decreases. The difference in the average heating rate (v1 - V) is preferably 2.8 °C / s or less, and more preferably 2.6 °C / s or less. It should be noted that the lower limit of the difference in the average heating rate (v1 - V) is not particularly limited. Industrially, the lower limit of the difference in the average heating rate (v1 - V) is 0.5 °C / s or more.

[0115] Furthermore, the overlapping blank is gradually heated from the end portion within the blank surface with a fast heating rate toward the central portion with a slow heating rate. Therefore, by gradually heating with the average heating rate V of the overlapping portion set within the range of 1.0 °C / s or more and 4.0 °C / s or less, the temperature difference between the single-layer portion and the overlapping portion can be suppressed, and warpage can be improved. When the average heating rate V of the overlapping portion exceeds 4.0 °C / s, there is a problem of excessive warpage formation. The upper limit of the average heating rate V of the overlapping portion is preferably 3.8 °C / s or less, more preferably 3.6 °C / s or less. On the other hand, when the average heating rate V of the overlapping portion is less than 1.0 °C / s, the heating rate during heating is too slow, and the heating productivity decreases. The lower limit of the average heating rate V of the overlapping portion is preferably 1.2 °C / s or more, more preferably 1.4 °C / s or more.

[0116] It should be noted that the average heating rate V [°C / second] of the overlapping portion and the average heating rate v1 [°C / second] of the single-layer portion are obtained as follows: Spot weld K-type thermocouples on the steel plate and connect them. Measure the plate temperature from the start of heating until the heating temperature reaches 800 °C. After the start of heating, divide 780 °C (=800 °C - 20 °C) by the time [seconds] from when the plate temperature reaches 20 °C until it reaches 800 °C. However, due to reasons such as a high room temperature at the start of temperature rise, when the plate temperature exceeds 20 °C from before heating, for example, when it is 25 °C, it is obtained by dividing 775 °C (=800 °C - 25 °C) by the time [seconds] from 25 °C until it reaches 800 °C.

[0117] <Regarding the time and temperature during heating>

[0118] In the present embodiment, as Figure 7 shown, the overlapping blank ( Figure 1 reference numeral 4) is heated at a heating temperature and heating time located within the figure ABCD determined by points A (4 minutes, 930 °C), B (10 minutes, 930 °C), C (20 minutes, 870 °C), and D (8 minutes, 870 °C) in the coordinate plane defined by (heating time, heating temperature). The heating temperature here refers to the temperature inside the preheated heating furnace, and the overlapping blank fed into the furnace is heated to the temperature of the preheated furnace. In addition, the heating time here refers to the time from when the overlapping blank is fed into the heating furnace until it is discharged.

[0119] When discharging the heated overlapping blank from the heating furnace, warpage also needs to be improved in terms of the stability of the conveyance of the overlapping blank. However, the difference in the heating rate between the overlapping portion with a slow temperature rise and the single-layer portion with a fast temperature rise must be such that the temperature inside the blank is made uniform between the overlapping portion and the single-layer portion by heating for a certain time or more in the furnace. Therefore, by being located Figure 7Heating the overlapping blanks at the heating temperature and heating time within the shown figure ABCD can improve the warpage when the heated overlapping blanks are sent out from the heating furnace.

[0120] When the heating time at a heating temperature of 930 °C is less than 4 minutes, the temperature difference between the overlapping part with a slow heating rate and the single-sheet part with a fast heating rate is not sufficiently homogenized, the warpage is not sufficiently restored, and it cannot be stably grasped during the conveyance of the heated overlapping blanks. The heating time is preferably 4.5 minutes or more, more preferably 5 minutes or more. In addition, when the heating time at a heating temperature of 870 °C is less than 8 minutes, similarly, the warpage cannot be sufficiently restored, and it cannot be stably grasped during the conveyance of the heated overlapping blanks. The heating time is preferably 8.5 minutes or more, more preferably 9 minutes or more.

[0121] Furthermore, when the heating time at a heating temperature of 930 °C exceeds 10 minutes, in addition to the reduction in heating productivity, the diffusion of Fe into the plating proceeds excessively, and the corrosion resistance of the hot stamping formed body decreases. In particular, the corrosion resistance of the single-sheet part with a fast heating rate decreases. Therefore, the heating time at a heating temperature of 930 °C is preferably 9.5 minutes or less, more preferably 9 minutes or less. Similarly, when the heating time at a heating temperature of 870 °C exceeds 20 minutes, the corrosion resistance of the single-sheet part with a fast heating rate decreases. Therefore, the heating time at 870 °C is preferably 18 minutes or less, more preferably 16 minutes or less.

[0122] When the heating temperature exceeds 930 °C, the difference in heating rate between the overlapping part and the single-sheet part becomes larger, and the warpage becomes larger. The upper limit of the heating temperature is preferably 920 °C, more preferably 910 °C. On the other hand, when the heating temperature is less than 870 °C, the γ-phase transformation (austenitization) of the base material of the overlapping blanks becomes insufficient, the hardness after die quenching decreases, and in addition, the heating speed becomes slow, resulting in a reduction in productivity. The lower limit of the heating temperature is preferably 875 °C, more preferably 880 °C.

[0123] In the present embodiment, the overlapping blanks are heated at a heating temperature and heating time within the range of the Figure 7 shown figure ABCD. Therefore, for example, point E (6 minutes, 900 °C) located between line segments AD, point F (15 minutes, 900 °C) located between line segments BC, point G (10 minutes, 900 minutes) located between line segments EF, etc. are also within the scope of the present invention.

[0124] As the heating furnace used in the above heating method, a roller hearth furnace, a multi-stage furnace can be used. As the heat source, examples 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.

[0125] (2-3. Regarding the transfer from the heating furnace to the stamping device)

[0126] The heated stacked blanks are sent out from the heating furnace and transferred to the stamping device. If the heated stacked blanks are cooled to below 650°C before the die quenching, the martensitic transformation becomes insufficient. Therefore, the time from when the blanks are sent out from the heating furnace until they are transferred to the stamping device is preferably within 20 seconds.

[0127] (2-4. Regarding the hot pressing process)

[0128] By pressing the heated stacked blanks with a die, a hot-stamped formed body can be obtained. When performing the pressing process using the die, martensitic transformation is carried out by quenching the heated stacked blanks using the die. Thus, a formed body with a Vickers hardness of 300 HV or more when the load is set to 9.81 N can be obtained. Regarding the quenching rate in the die, both the overlapping part and the single-sheet part are preferably 30°C / s or more, and more preferably 50°C / s or more. It should be noted that the quenching rate mentioned here refers to the average cooling rate of the heated stacked blanks from when they leave the heating furnace until they are cooled to below 400°C.

[0129] Above, the manufacturing method of the stacked hot-stamped formed body of the present embodiment has been described in detail.

[0130] (3. Regarding the stacked hot-stamped formed body)

[0131] The stacked hot-stamped formed body 12 of the present embodiment includes a first steel plate with a thickness of t1 (mm), and at least one second steel plate that overlaps and is joined to the first steel plate, has an area smaller than the area of the first steel plate, and a thickness of t2 (mm).

[0132] Both sides of the first steel plate and the second steel plate in the stacked hot-stamped formed body 12 are coated with an Al-Fe-based coating.

[0133] The Al-Fe-based coating is a layer formed as a result of Fe diffusing to the surface in the Al-based coating during hot stamping (in other words, an alloy coating containing at least Al and Fe). The Al-Fe-based coating is composed of a combination of phases such as the θ phase (FeAl 3 ), η phase (Fe 2 Al 5 ), ζ phase (FeAl 2 ), Fe 3 Al, FeAl, etc. In addition, when Si is contained in the plating, the Al-Fe-based coating also includes the τ1 phase (Al 2 Fe 3 Si 3 ), τ2 phase (Al 3FeSi), τ3 phase (Al 2 FeSi), τ4 phase (Al 3 FeSi 2 ), τ5 phase (Al 8 Fe 2 Si), τ6 phase (Al 9 Fe 2 Si 2 ), τ7 phase (Al 3 Fe 2 Si 3 ), τ8 phase (Al 2 Fe 3 Si 4 ), τ10 phase (Al 4 Fe 1.7 Si), τ11 phase (Al 5 Fe 2 Si), as the compound layer of Al and Fe, is mainly composed of any one or more of the τ1 phase, η phase (Fe 2 Al 5 ). In particular, Al in the plating diffuses with Fe in the base material. The layer formed by the diffusion of Al into the base material and containing the BCC phase of Fe with Al in solid solution or the phase of FeAl is called the Al solid solution Fe layer, and this layer is adjacent to the base material as shown in Figure 8 . Under the heating conditions of the present embodiment, as exemplified in Figure 8 , in addition to the above-described compound layer containing at least Al and Fe, an Al solid solution Fe layer is formed in the lowermost layer of the plating on the base material side. As shown in Figure 8 , the Al-Fe-based plating layer of the present embodiment includes the compound layer of Al and Fe and the Al solid solution Fe layer as described above.

[0134] The plating thickness of the Al-Fe-based plating layer is preferably 10 μm to 50 μm for each of the first steel plate and the second steel plate independently. When the plating thickness of the Al-Fe-based plating layer is less than 10 μm, the corrosion resistance of the overlapping hot-stamped formed body decreases. On the other hand, when the plating thickness of the Al-Fe-based plating layer exceeds 50 μm, there is a problem that more chalking occurs during press forming. The plating thickness of the Al-Fe-based plating layer is more preferably 15 μm to 45 μm.

[0135] The difference (D1 - D2) between the thickness D1 (μm) of the Al solid solution Fe layer of the portion of the first steel plate that does not overlap with the second steel plate and the thickness D2 (μm) of the Al solid solution Fe layer of the second steel plate is 6.0 μm or less. It is known that the corrosion resistance of the Al-Fe-based plating layer is affected by the binary alloy of Al-Fe (FeAl 3 , Fe 2 Al 5 , FeAl2 ) Inhibition, where there is a relationship that if the Al solid solution Fe layer becomes thinner, the binary alloy of Al-Fe becomes thicker. Therefore, when the difference (D1 - D2) exceeds 6.0 μm, the Al solid solution Fe layer in the first steel plate becomes more, the binary alloy of Al-Fe becomes thinner, and the corrosion resistance decreases. Furthermore, in the overlapping part of the first steel plate and the second steel plate, when the structure of the Al-Fe based coating is different, dissimilar metal contact corrosion sometimes occurs and the corrosion resistance decreases. Therefore, it is found that suppressing the thickness difference (D1 - D2) of the Al solid solution Fe layer between the first steel plate and the second steel plate to 6 μm or less is important for the corrosion resistance of the overlapping part. The upper limit of the difference (D1 - D2) is preferably 5.5 μm or less, more preferably 5.0 μm or less. The lower limit of the difference (D1 - D2) is not particularly limited, and when it is less than 0.5 μm, the effect saturates.

[0136] As a method for determining the plating thickness of the Al-Fe based coating and the thickness of the Al solid solution Fe layer, by using an optical microscope, nitric acid ethanol etching treatment is performed on the plating cross-section in a field of view of 100 μm × 100 μm, and its cross-section is observed, as Figure 8 shown, to obtain the plating thickness and the thickness of the Al solid solution Fe layer adjacent to the base material. More specifically, at any plurality of sites (for example, 3 sites), the plating cross-section is observed by the above method, and the plating thickness and the thickness of the Al solid solution Fe layer at each observation site are determined. Then, the average value of the obtained thicknesses is calculated, and the obtained average value can be used as the plating thickness and the thickness of the Al solid solution Fe layer.

[0137] In addition, focusing on the cracks formed in the Al-Fe based coating on the surface ( Figure 1 reference numeral 1b) of the first steel plate in the overlapping part of the first steel plate and the second steel plate after hot stamping, which reach the Al solid solution Fe layer. By making the number of such cracks 5 or less per 100 μm length parallel to the Al-Fe based coating (in other words, 1 or less per 20 μm length parallel to the Al-Fe based coating), the corrosion resistance is improved. It is considered that the cracks are the cause of the generation of red rust of the plating, and the cracks are generated due to warping during hot stamping heating. By improving the warping by using the overlapping hot stamping manufacturing method of the foregoing embodiment, the generation of cracks can also be suppressed. When the number of the above cracks exceeds 5 per 100 μm length, the generation of red rust becomes a problem. The number of the above cracks is preferably 3 or less per 100 μm length, and further preferably 2 or less per 100 μm length.

[0138] As Figure 8As exemplified, as a method for measuring cracks in the Al solid solution Fe layer formed in the Al-Fe based coating, it can be obtained by subjecting the plating cross-section to nitric acid ethanol etching treatment with a field of view of 100 μm × 100 μm or more using an optical microscope, observing the cross-section, and measuring the number of cracks. As shown in Figure 8 As also shown in, the Al solid solution Fe layer is a layer formed directly above the base material with a martensite structure. For the example of Figure 8 , there are 2 cracks per 135 μm, so it becomes 1.5 cracks / 100 μm.

[0139] The above has described the overlapping hot stamping formed body of the present embodiment in detail.

[0140] Examples

[0141] Hereinafter, the present invention will be described more specifically using examples.

[0142] <Example 1>

[0143] Slabs having steel compositions with chemical components by mass% of C: 0.21%, Si: 0.20%, Mn: 1.20%, P: 0.010%, S: 0.0020%, N: 0.0030%, Al: 0.04%, B: 0.0020%, the balance being Fe and impurities were made into cold-rolled steel sheets through ordinary hot rolling processes and cold rolling processes, and aluminized on both sides using a Sendzimir type hot dip aluminizing production line to make test specimens A of Al-based coated steel sheets. Similarly, slabs having steel compositions with chemical components by mass% of C: 0.21%, Si: 0.20%, Mn: 1.20%, P: 0.010%, S: 0.0080%, N: 0.0030%, Al: 0.04%, B: 0.0020%, W: 0.1%, Cr: 0.3%, Mo: 0.1%, V: 0.1%, Ti: 0.02%, Nb: 0.02%, Ni: 0.1%, Cu: 0.1%, Co: 0.1%, Sn: 0.01%, Sb: 0.01%, Mg: 0.0010%, Ca: 0.0020%, O: 0.0020%, REM: 0.0030%, the balance being Fe and impurities were made into cold-rolled steel sheets through hot rolling processes and cold rolling processes, and aluminized on both sides to make test specimen B. Furthermore, materials with the C content of test specimen A being 0.35%, 0.27%, and 0.45% were used as test specimens C, D, and E respectively. After plating, test specimens A, B, C, D, and E were all adjusted for the plating adhesion amount by the gas wiping method and then cooled. As the plating bath composition during aluminizing treatment, it was 89% Al - 9% Si - 2% Fe. The plating thickness of the Al-based coating was 25 μm. The plate thickness was adjusted to a thickness of 1.0 mm to 4.0 mm as shown in Table 1 below.

[0144] The first steel plate is set to a size of 1200 mm × 300 mm, and the second steel plate is cut to a size of 40 mm × 30 mm to 1196 mm × 100 mm, and is overlapped to prepare a total plate thickness (t1 + t2) and a maximum length L as shown in Table 1 below. In this embodiment, the second steel plate is overlapped in such a manner that there is no portion protruding from the first steel plate. Therefore, in this embodiment, the area S2 is the same as the size of the second steel plate. By spot-welding these two steel plates as shown by the dots (joint portion 3) of Figure 1 , a blank for hot stamping 4 is fabricated.

[0145] As shown in Table 1, in the process of heating the blank fabricated as above in a preheated furnace for a certain period of time, the average heating rate between 20°C and 800°C of the plate temperature is investigated. After maintaining the target temperature and time, it is sent out from the heating furnace and conveyed for 10 seconds, and immediately pressed with a die at a load of 100 tons, and at the same time cooled in the die, whereby a hat-shaped overlapped hot stamping formed body is obtained. The cooling rate at this time is 50°C / s.

[0146] K-type thermocouples are spot-welded on the non-overlapped portion (single sheet portion with a fast heating rate) of the first steel plate and the overlapped second steel plate (overlapped portion with a slow heating rate) to measure the plate temperature of the blank during heating.

[0147] In addition, in order to confirm the warping of the blank during heating, a gap capable of observing the inside of the furnace is provided, and the maximum value of the warping of the blank during heating is actually measured. As a method of actual measurement, blocks with heights of 40 mm, 50 mm, and 70 mm are placed in the furnace. On this basis, when the warping exceeds 70 mm, problems occur in mass production, so it is judged as non-conforming (NG: No Good). When the warping is 70 mm or less and exceeds 50 mm, it is judged as qualified 3 (G3: Good No3). When the warping is 50 mm or less and exceeds 40 mm, it is judged as qualified 2 (G2: Good No2). When the warping is 40 mm or less, it is judged as qualified 1 (G1: Good No1). In addition, if warping remains when the blank is sent out after heating is completed, problems with productivity will occur during conveyance to the press. Therefore, even when warping remains more than 40 mm after heating is completed, problems will occur during mass production, so it is judged as non-conforming (NG: No Good). The results of the judgment are shown in Table 1.

[0148] Each level shows the inventive examples of the present application (hereinafter, only recorded as "inventive examples") as A1 to A16 and the comparative examples as a1 to a8 in Table 1.

[0149] It should be noted that, as described above, the plate thickness of the steel plate was measured using a micrometer by the method described in JIS G 3314:2011.

[0150] [Table 1]

[0151]

[0152] As is clear from Table 1 above, A1 to A16 as inventive examples suppressed warping during heating and were qualified. However, a1 to a3, a5 to a8 as comparative examples had large warping during heating and were unqualified. a4 of the comparative examples had more than 40 mm of warping remaining after heating was completed and was unqualified.

[0153] <Example 2>

[0154] Similar to Example 1, a slab having a steel composition composed of the chemical components of test materials A, B, C, D, and E was made into a cold-rolled steel sheet through a normal hot-rolling process and a cold-rolling process, and aluminizing treatment was performed on both sides using a Sendzimir-type hot-dip aluminizing treatment production line to make a test material of an Al-based coated steel sheet. After aluminizing, the test materials A, B, C, D, and E were all adjusted for the aluminizing adhesion amount by the gas wiping method and then cooled. As the bath composition at this time, it was 89% Al - 9% Si - 2% Fe. In addition, the coating thickness of the Al-based coating was 25 μm. The plate thickness was adjusted to a thickness of 1.0 mm to 4.0 mm as shown in Table 2 below.

[0155] The first steel plate was set to a size of 1200 mm × 300 mm, and the second steel plate was cut into sizes from 40 mm × 30 mm to 1196 mm × 100 mm and overlapped to achieve the total plate thickness (t1 + t2) and the maximum length L shown in Table 2 below for preparation. In this example, the second steel plate was overlapped in such a way that there was no part protruding from the first steel plate. Therefore, in this example, the area S2 was the same as the size of the second steel plate. By spot-welding these two steel plates as shown by the dots (joint portion 3) Figure 1 a hot stamping overlapping blank 4 was made.

[0156] As shown in Table 2, in the process of heating the overlapping blank made as above in a preheated furnace for a certain period of time, the average heating rate between 20°C and 800°C of the plate temperature was investigated. After maintaining the target temperature and time, it was sent out from the heating furnace and conveyed for 10 seconds, and immediately pressed with a die at a load of 100 tons, and at the same time cooled in the die, thereby obtaining Figure 9 a hat-shaped overlapping hot stamping molded body as shown. The cooling rate at this time was 50°C / s or more.

[0157] Spot weld K-type thermocouples on the non-overlapped part of the first steel plate (the single-piece part with a fast heating rate) and the overlapped second steel plate (the overlapped part with a slow heating rate), and measure the plate temperature of the overlapped billets during heating.

[0158] Cut out the top part ( Figure 1 with the reference numeral 7) from the cap molded product after this test in a size of 100 mm × 50 mm. On the basis of protecting the end face with tape, conduct a salt spray test (JIS Z 2371: 2015) to evaluate the corrosion resistance. The evaluation is carried out on the surface of the first steel plate that does not contact the second steel plate ( Figure 1 with the reference numeral 1b). Judge that the case where the red rust area ratio exceeds 50% after 24 hours is unqualified (NG: No Good), judge that the case where the red rust area ratio exceeds 30% and is 50% or less is qualified 3 (G2: Good No3), judge that the case where the red rust area ratio exceeds 20% and is 30% or less is qualified 2 (G2: Good No2), and judge that the case where the red rust area ratio is 20% or less is qualified 1 (G1: Good No1).

[0159] In addition, similarly, cut out a 20 mm × 20 mm size from the top part, and conduct the nitric acid ethanol etching treatment on the cross-section of the Al-Fe-based coating as described above. For the cross-section of the Al-Fe-based coating, observe a 100 μm × 100 μm field of view with an optical microscope, and measure the plating thickness and the thickness of the Al solid solution Fe layer. At the same time, observe the structure of the coating, and measure the number of cracks per unit length reaching the Al solid solution Fe layer in the Al-Fe-based coating.

[0160] Show the measured results in Table 2.

[0161] If the number of cracks per 100 μm reaching the Al solid solution Fe layer exceeds 5, it is unqualified (NG: No Good); if it exceeds 2 and is 5 or less, it is qualified 3 (G3: Good No3); if it is 2 or less, it is qualified 2 (G2: Good No2); if it is 1 or less, it is qualified 1 (G1: Good No1).

[0162] For each level, take the inventive examples of the present application (hereinafter, only recorded as "inventive examples") as B1 to B16, and take the comparative examples as b1 to b7, as shown in Table 2.

[0163] [Table 2]

[0164]

[0165] In Table 2, B1 to B16, which are the inventive examples of the present application, show good corrosion resistance, while the corrosion resistance of b1 to b7, which are the comparative examples, is unqualified.

[0166] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings. However, the present invention is not limited to these examples. It should be understood that those having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims, and these modification examples or correction examples naturally also fall within the technical scope of the present invention.

[0167] Description of Reference Numerals

[0168] 1 First steel plate;

[0169] 1a Surface of the first steel plate that contacts the second steel plate

[0170] 1b Surface of the first steel plate that does not contact the second steel plate

[0171] 2 Second steel plate

[0172] 2a Surface of the second steel plate that contacts the first steel plate

[0173] 2b Surface of the second steel plate that does not contact the first steel plate

[0174] 3 Joint part

[0175] 4 Overlapped blank for hot stamping

[0176] 4a Overlapped part in the overlapped blank for hot stamping

[0177] 4b Single - sheet part in the overlapped blank for hot stamping

[0178] 5 Heating furnace for hot stamping

[0179] 6 Pressing die for hot stamping

[0180] 7 Top part

[0181] 8 Bending part on the top - part side

[0182] 9 Bending part on the flange side

[0183] 10 Longitudinal wall part

[0184] 11 Flange part

[0185] 12 Overlapped hot - stamped formed body

[0186] 13 One - side surface of the Al - based plated steel sheet

[0187] 14 Al - based plating layer

[0188] 15 Base material

Claims

1. A method for manufacturing an overlapping hot stamping formed body, which uses an overlapping blank formed by overlapping and joining a first steel plate having an area S1 and at least one second steel plate having an area smaller than that of the first steel plate to manufacture the overlapping hot stamping formed body, wherein the unit of the area S1 is cm 2 , The first steel plate and the second steel plate are Al-based coated steel plates having an Al-based coating on a base material. This manufacturing method includes: An overlapping blank heating process of heating the overlapping blank using a heating furnace; A heated blank conveying process of sending out the heated overlapping blank from the heating furnace and conveying it to a pressing device; and A hot stamping process of performing pressing processing on the heated overlapping blank using a die provided in the pressing device to obtain an overlapping hot stamping formed body. In the overlapping blank heating process, when the thickness of the first steel plate is set as t1, the thickness of the second steel plate is set as t2, the average heating rate of the portion with a total thickness of t1 + t2 formed by overlapping the first steel plate and the second steel plate between a plate temperature of 20°C and 800°C is set as V, and the average heating rate of the portion of the first steel plate that does not overlap with the second steel plate between a plate temperature of 20°C and 800°C is set as v1, the total thickness t1 + t2 of the overlapping portion is 2.5 mm or more and 5.0 mm or less, the maximum length L of the overlapping portion of the second steel plate is 100 mm or more and 1100 mm or less, the average heating rates V and v1 satisfy the relational expressions of the following formula (1) and formula (2), where the units of t1 and t2 are mm, and the units of V and v1 are °C / s. When the area of the portion of the second steel plate that overlaps with the first steel plate is set as S2, the areas S1 and S2 and the plate thickness t1 satisfy the relational expression of the following formula (3), where the unit of S2 is cm 2 , In the coordinate plane defined by the heating time and the heating temperature, the overlapping blank is heated with a heating temperature and a heating time located inside the figure ABCD determined by point A, point B, point C, and point D, where the coordinates of point A are 4 minutes and 930°C, the coordinates of point B are 10 minutes and 930°C, the coordinates of point C are 20 minutes and 870°C, and the coordinates of point D are 8 minutes and 870°C. 1.0 ≤ V ≤ 4.0 … formula (1) (v1 - V) ≤ 3.0 … formula (2) 400 ≤ (S1 - S2) × (t1 / 10) ≤ 950 … formula (3).

2. The manufacturing method of the overlapping hot stamping formed body according to claim 1, wherein, the maximum length L of the overlapping portion of the second steel plate is 300 mm or more.

3. The manufacturing method of the overlapping hot stamping formed body according to claim 1 or 2, wherein, the base material of the first steel plate and the second steel plate contains by mass% C: 0.10% or more and 0.50% or less, Si: 0.01% or more and 2.00% or less, Mn: 0.30% or more and 5.00% or less, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Al: 0.500% or less, B: 0.0002% or more and 0.0100% or less, optionally further contains W: 0% or more and 3.0% or less, Cr: 0% or more and 2.0% or less, Mo: 0% or more and 3.0% or less, V: 0% or more and 2.0% or less, Ti: 0% or more and 0.5% or less, Nb: 0% or more and 1.0% or less, Ni: 0% or more and 5.0% or less, Cu: 0% or more and 3.0% or less. Co: not less than 0% and not more than 3.0%, Sn: not less than 0% and not more than 0.10%, Sb: not less than 0% and not more than 0.10%, Mg: not less than 0% and not more than 0.0050%, Ca: not less than 0% and not more than 0.0050%, O: not less than 0% and not more than 0.0070%, REM: one or more of not less than 0% and not more than 0.0070%, with the balance being Fe and impurities.

4. The manufacturing method of the overlapping hot stamping formed body according to claim 3, wherein, the C content C1 of the base material of the first steel plate and the C content C2 of the base material of the second steel plate satisfy the relational expression of the following formula (4), where the units of the C content C1 and the C content C2 are mass%, 0.03 ≤ (C2 - C1) ≤ 0.30... Formula (4).

5. An overlapping hot stamping formed body, which is formed by laminating a first steel plate having an area S1 and at least one second steel plate having an area smaller than that of the first steel plate, wherein the unit of the area S1 is cm 2 , An Al-Fe based coating is provided on the surfaces of the first steel plate and the second steel plate, the Al-Fe based coating is composed of a compound layer of Al and Fe and an Al solid solution Fe layer, when the plate thicknesses of the first steel plate and the second steel plate are set as t1 and t2 respectively, the total plate thickness t1 + t2 of the overlapping part of the first steel plate and the second steel plate is not less than 2.5 mm and not more than 5.0 mm, where the units of t1 and t2 are mm, the maximum length L of the overlapping part of the second steel plate is not less than 100 mm and not more than 1100 mm, When the area of the portion of the second steel plate that overlaps with the first steel plate is set as S2, the areas S1 and S2 and the plate thickness t1 satisfy the relational expression of the following formula (3), where the unit of S2 is cm 2 , in the Al-Fe based coating on the surface of the overlapping part of the first steel plate and the second steel plate where the first steel plate does not contact the second steel plate, the number of cracks reaching the Al solid solution Fe layer is not more than 5 in every 100 μm length parallel to the Al-Fe based coating, the thickness D1 of the Al solid solution Fe layer of the part of the first steel plate that does not overlap with the second steel plate and the thickness D2 of the Al solid solution Fe layer of the second steel plate satisfy the relational expression of the following formula (5), where the units of the thickness D1 and the thickness D2 are μm, 400 ≤ (S1 - S2) × (t1 / 10) ≤ 950... Formula (3) (D1 - D2) ≤ 6.0... Formula (5).

6. The overlapping hot stamping formed body according to claim 5, wherein, the maximum length L of the overlapping part of the second steel plate is not less than 300 mm.

7. The overlapping hot stamping formed body according to claim 5 or 6, wherein, the base materials of the first steel plate and the second steel plate contain, by mass%, C: not less than 0.10% and not more than 0.50%, Si: not less than 0.01% and not more than 2.00%, Mn: not less than 0.30% and not more than 5.00%, P: not more than 0.100%, S: not more than 0.1000%, N: not more than 0.0100%, Al: not more than 0.500%, B: not less than 0.0002% and not more than 0.0100%, optionally further containing W: not less than 0% and not more than 3.0%, Cr: not less than 0% and not more than 2.0%, Mo: not less than 0% and not more than 3.0%, V: not less than 0% and not more than 2.0%, Ti: not less than 0% and not more than 0.5%, Nb: 0% or more and 1.0% or less, Ni: 0% or more and 5.0% or less, Cu: 0% or more and 3.0% or less, Co: 0% or more and 3.0% or less, Sn: 0% or more and 0.10% or less, Sb: 0% or more and 0.10% or less, Mg: 0% or more and 0.0050% or less, Ca: 0% or more and 0.0050% or less, O: 0% or more and 0.0070% or less, REM: one or more of 0% or more and 0.0070% or less, with the balance being Fe and impurities.

8. The overlapping hot stamping formed body according to claim 7, wherein the C content C1 of the base material of the first steel sheet and the C content C2 of the base material of the second steel sheet satisfy the relational expression of the following formula (4), where the units of the C content C1 and the C content C2 are mass%, 0.03 ≤ (C2 - C1) ≤ 0.30... formula (4).

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