Layered blank for hot stamping, layered hot stamp molded body, and automobile component

Abstract

The layered blank for hot stamping according to an embodiment of the present invention comprises a first steel plate and at least one second steel plate that is layered on and welded to the first steel plate and has a smaller area than the first steel plate, the first steel plate and the second steel plate each having an Al-based plating layer on both surfaces of the steel plate and having brazing between the first steel plate and the second steel plate, the thickness of the brazing being 40-180 µm, the liquidus temperature TLL being 950°C or lower, and the spot welding dot density being 0.2 dots / 100 cm2 or greater.

Description

Laminated hot stamping blank, laminated hot stamped compact, and automotive part

[0001] The present invention relates to a hot stamping overlap blank, a hot stamped overlap formed body, and an automotive part. This application claims priority based on Japanese Patent Application No. 2023-31896, filed on March 2, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, the automotive industry has been seeking to reduce the thickness of automotive components in order to reduce the weight of vehicle bodies in order to reduce carbon dioxide emissions. At the same time, there is also a need to ensure collision safety performance, and one way to ensure collision safety performance while reducing the thickness of automotive components is to increase the strength of the steel sheets that make up the automotive components. For example, in the manufacture of center pillars, the use of hot stamping technology, which can produce high-strength components with high dimensional accuracy by simultaneously quenching and press forming, is becoming more common.

[0003] Patent Document 1 discloses a technique in which hard solder is applied to either a base sheet or a reinforcing sheet, each of which is cut out from a coil material, and the two sheets are overlapped and tack-welded, and then placed in a heating furnace in this state, heated to a structural deformation temperature (850°C to 930°C), and then hot stamped.

[0004] Patent Document 2 discloses a technique in which a filler metal having a solidus temperature of 1050°C or less and a liquidus temperature of 700°C or more is sandwiched between the plate surfaces of two steel plates, which are then placed in a heating furnace and heated to 900°C to 1050°C, and then hot stamped.

[0005] Patent Document 3 discloses a method of pressurizing a plurality of metal plates with a brazing filler material sandwiched therebetween in the plate thickness direction during a heating step for hot stamping or during a transition from the heating step to a hot stamp forming step.

[0006] Patent Document 4 discloses a technique in which a sheet assembly formed by sandwiching a filler metal between steel sheets is heated to a temperature equal to or higher than the Ac3 point of the steel sheets, so that the Ar3 point of the steel sheets in the vicinity of the filler metal is made higher than the Ar3 point of the steel sheets, and then a hot stamp forming process is carried out with the quenching start temperature set to a temperature equal to or lower than the Ar3 point of the steel sheets.

[0007] Japanese Patent Publication No. 2002-178069 Japanese Patent Publication No. 2004-141913 Japanese Patent Publication No. 2014-200840 International Publication No. 2016 / 052738

[0008] In the techniques of Patent Documents 1 to 4, in order to achieve high strength, multiple steel sheets sandwiched between them are stacked in the sheet thickness direction and hot stamped. However, with the techniques described in Patent Documents 1 and 2, there are areas between the stacked steel sheets where the filler metal is not filled, reducing the joining strength and making it difficult to obtain the desired strength and rigidity. Furthermore, while the technique described in Patent Document 3 can avoid areas between the stacked steel sheets where the filler metal is not filled, it requires large-scale equipment and significantly increases the number of steps. The technique described in Patent Document 4 changes the temperature during hot stamping, requiring precise temperature control.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a laminated blank for hot stamping and a laminated hot-stamped product that can be produced without making any significant changes to the hot stamping process, and that are lightweight, high in strength, and high in rigidity.

[0010] The present inventors have come up with the idea of ​​supplementing the lack of joining strength due to possible localized peeling of the brazing filler metal by inserting brazing filler metal between overlapping steel sheets and performing spot welding.

[0011] The gist of the present invention, which was completed based on the above findings, is as follows: [1] A lap blank for hot stamping according to one aspect of the present invention is a lap blank for hot stamping comprising a first steel sheet and at least one second steel sheet having an area smaller than that of the first steel sheet, the second steel sheet being overlapped with the first steel sheet and having a weld spot-welded thereto, the first steel sheet and the second steel sheet each having an Al-based plating layer on both sides of the steel sheet, a filler metal is formed between the first steel sheet and the second steel sheet, the thickness of the filler metal is 40 μm to 180 μm, the liquidus temperature TLL is 950° C. or less, and the spot welding point density is 0.2 points / 100 cm ２ [2] In the overlapping blank for hot stamping described in [1] above, the spot welding point density is 0.7 points / 100 cm ２ [3] In the overlapping blank for hot stamping according to the above [1] or [2], the periphery of the nugget may be a non-brazed portion free of the brazing material, and the diameter of the non-brazed portion may be 2.00 times or less the nugget diameter. [4] In the overlapping blank for hot stamping according to the above [3], the diameter of the non-brazed portion may be 1.10 times or more the nugget diameter.

[0012] [5] Also, a hot-stamped lap-stamped product according to another aspect of the present invention comprises a first steel sheet and at least one second steel sheet having an area smaller than that of the first steel sheet, the second steel sheet being overlapped on the first steel sheet and having a weld spot-welded thereto, the first steel sheet and the second steel sheet each having an Al-based plating layer on both surfaces of the steel sheet, the hot-stamped lap-stamped product further comprising a brazing filler layer between the first steel sheet and the second steel sheet, the brazing filler layer having a thickness of 40 to μm and a liquidus temperature TLL of 950° C. or less, a bent portion at least in a part of the overlapping portion between the first steel sheet and the second steel sheet, and a spot-welding density of 0.2 points / 100 cm ２ [6] In the lap hot-stamped compact described in [5] above, the spot welding density is 0.7 points / 100 cm ２[7] In the lap hot-stamped product according to the above item [5] or [6], the periphery of the nugget may be a non-brazed layer portion free of the braze layer, and the diameter of the non-brazed layer portion may be 2.00 times or less the nugget diameter. [8] In the lap hot-stamped product according to the above item [7], the diameter of the non-brazed layer portion may be 1.10 times or more the nugget diameter.

[0013] [9] Furthermore, an automobile part according to yet another aspect of the present invention includes the overlap hot-stamped product according to any one of the above [5] to [8].

[0014] According to the present invention, it is possible to provide a laminated blank for hot stamping, a laminated hot-stamped product, and an automotive part that are lightweight, high in strength, and high in rigidity, and that can be produced without making any significant changes to the hot stamping process.

[0015] FIG. 1 is a partial cross-sectional view of a overlapping blank for hot stamping according to one embodiment of the present invention. FIG. 2 is a view of the overlapping blank for hot stamping according to the same embodiment, viewed from the second steel plate side. FIG. 3 is a cross-sectional view of a overlapping hot-stamped body according to one embodiment of the present invention. FIG. 4 is a schematic view of an overlapping blank for hot stamping produced in an example. FIG. 5 is a schematic view of a hat member (overlapping hot-stamped body) produced in an example. FIG. 6 is a diagram showing an automobile frame as an example to which a hot-stamped body is applied. FIG. 7 is a schematic view for explaining test conditions for a three-point bending test.

[0016] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the following description, unless otherwise specified, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. However, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit.

[0017] <Hot stamping overlapping blank 1> Fig. 1 shows a partial cross-sectional view of a hot stamping overlapping blank 1 according to one embodiment of the present invention. Fig. 2 shows a view of the hot stamping overlapping blank according to this embodiment as viewed from the second steel sheet side. As shown in Fig. 1, the hot stamping overlapping blank 1 according to this embodiment includes a first steel sheet 10 and at least one second steel sheet 20 having an area smaller than that of the first steel sheet 10, and has a brazing filler metal 30 between the first steel sheet 10 and the second steel sheet 20. The second steel sheet 20 is overlapped and welded to the first steel sheet 10. The brazing filler metal 30 is also sometimes called a brazing filler metal.

[0018] [First Steel Sheet 10 ] The first steel sheet 10 includes a base steel sheet 11 and Al-based plating layers 12 disposed on both sides of the base steel sheet 11 .

[0019] (Base steel plate 11) There is no particular limitation on the chemical composition of the base steel plate 11. For example, when a tensile strength of 1000 MPa or more (Vickers hardness (Vickers hardness when the test force is 9.807 N) conforming to JIS Z2244-1:2020 of about 300 HV or more) is to be obtained in the lap hot stamped compact 100, the chemical composition of the base steel plate 11 may be, for example, in mass %, C: 0.10 to 0.60%, 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 to 0.0100%, Ti: 0.10 to 0.60%, 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 to 0.0100%, : 0 to 0.500%, Nb: 0 to 1.00%, Cr: 0 to 2.00%, W: 0 to 3.00%, Mo: 0 to 3.00%, V: 0 to 2.00%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Co: 0 to 3.00%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, Zr: 0 to 0.0050%, O: 0 to 0.0070%, REM: 0 to 0.0070%, and the balance: Fe and impurities. When a Vickers hardness of about 400 HV or more is to be obtained in accordance with Z2244-1:2020 (Vickers hardness when the test force is 9.807 N), the chemical composition of the base steel plate 11 is, for example, in mass %, C: 0.19 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.40 to 2.00%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Al: 0.500% or less, B: 0.0005 to 0.0050%, Ti: 0.0 01 to 0.100%, Nb: 0 to 0.10%, Cr: 0.01 to 1.00%, W: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.50%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Co: 0 to 0.50%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, Zr: 0 to 0.0050%, O: 0 to 0.0050%, REM: 0 to 0.0050%, and the balance: Fe and impurities. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the content of the REM refers to the total content of these elements.The tensile strength (Vickers hardness in accordance with JIS Z2244-1:2020 (Vickers hardness when the test force is 9.807 N) of the base steel sheet 11 of the overlapping blank for hot stamping 1 does not need to be particularly specified, but may be, for example, a tensile strength of 300 to 600 MPa or a Vickers hardness of 100 to 200.

[0020] The thickness of the base steel sheet 11 does not need to be particularly specified, but may be, for example, 0.5 mm or more and 3.0 mm or less. The thickness of the base steel sheet 11 is preferably 0.8 mm or more and 2.3 mm or less.

[0021] (Al-based plating layer 12) The Al-based plating layer 12 is a plating layer containing Al as a primary component. Here, "Al as a primary component" means that the Al content is 80% or more by mass. It is not necessary to specify the content of elements other than Al. However, when the Al-based plating layer 12 contains 2 to 15% by mass of Si, a eutectic structure of Al and Si is formed in the Al-based plating layer 12 based on the phase diagram. When the Al-based plating layer 12 is formed by a hot-dip galvanizing method, the Al-based plating layer 12 may contain 1% to 5% by mass of Fe as a component eluted from the steel sheet. Examples of other impurities include elements such as Cr, Mn, V, Ti, Sn, Ni, Cu, W, Bi, Mg, and Ca, which are derived from components eluted from the hot-dip galvanizing equipment or impurities in the ingots used in the hot-dip Al plating bath. The Al-based plating layer 12 may contain less than 1% by mass of these elements. Therefore, the chemical composition of the Al-based plating layer 12 may be, for example, in mass %, 80 to 97% Al, 2 to 15% Si, 0 to 5% Fe, 0 to 1% Cr, 0 to 1% Mo, 0 to 1% Zn, 0 to 1% V, 0 to 1% Ti, 0 to 1% Sn, 0 to 1% Ni, 0 to 1% Cu, 0 to 1% W, 0 to 1% Bi, 0 to 1% Mg, 0 to 1% Ca, 0 to 1% REM, and the balance: impurities. Alternatively, the Al-based plating layer 12 may have a two-layer structure of an Al layer and an Al—Fe alloy layer.

[0022] The composition of the Al-based plating layer 12 is determined by polishing a cross section of a test piece cut out from the first steel sheet 10, then performing nital etching, and observing the cross section as a composition image at a magnification of, for example, 1000 times using an electron probe microanalyzer (EPMA), followed by elemental analysis. In the elemental analysis, 10 points are analyzed at equal intervals from the plating surface toward the base material. The average value of the measured values ​​at each analysis point is taken as the chemical composition of the Al-based plating layer 12.

[0023] The Al-based plating layer is required to have the following characteristics: suppression of iron scale formation during hot stamping heating; and suppression of plating chipping due to plating peeling (also known as powdering) during hot stamping, and suppression of indentation marks due to the adhesion of peeled plating to other locations. Powdering occurs due to compressive stress imposed on the plating on the inner surface of a bent portion generated during forming, or shear stress imposed on the plating due to sliding from the mold during forming. For this reason, the plating thickness of the Al-based plating layer 12 is preferably 10 to 50 μm independently for each of the first steel sheet 10 and the second steel sheet 20. If the plating thickness is less than 10 μm, the effect of suppressing iron scale formation may be insufficient. By making the plating thickness of the Al-based plating layer 12 10 μm or more, the effect of suppressing iron scale formation can be more reliably achieved. The plating thickness of the Al-based plating layer 12 is more preferably 15 μm or more. On the other hand, if the plating thickness exceeds 50 μm, there is a possibility of significant powdering. By setting the plating thickness to 50 μm or less, it is possible to more reliably prevent the occurrence of powdering. The plating thickness of the Al-based plating layer 12 is more preferably 45 μm or less.

[0024] The plating thickness of the Al-based plating layer 12 can be determined by observing the cross section of the plating without etching using an optical microscope in a field of view of, for example, 100 μm × 100 μm, and measuring the plating thickness. More specifically, the plating cross section is observed at any multiple locations (for example, three locations) using the above method, and the plating thickness at each observation location is determined. The obtained plating thicknesses are then averaged, and the obtained average value can be used as the plating thickness of the Al-based plating layer 12.

[0025] Since the Al-based plating layer 12 does not cause liquid metal embrittlement (LME), LME does not occur when hot stamping the overlapping blank with the brazing filler metal 30 sandwiched therebetween.

[0026] [Second steel sheet 20] The second steel sheet 20 comprises a base steel sheet 21 and Al-based plating layers 22 disposed on both sides of the base steel sheet 21. As shown in Fig. 2 , the area of ​​the second steel sheet 20 is smaller than the area of ​​the first steel sheet 10. By making the area of ​​the second steel sheet 20 smaller than the area of ​​the first steel sheet 10, the weight of the overlapping blank for hot stamping can be reduced.

[0027] The base steel sheet 21 and the Al-based plating layer 22 are similar to the base steel sheet 11 and the Al-based plating layer 12 of the first steel sheet 10, respectively, and therefore detailed description thereof will be omitted here. However, the base steel sheet 11 and the base steel sheet 21 may have the same chemical composition or different chemical compositions. Furthermore, they may have the same sheet thickness or different sheet thicknesses. Furthermore, the Al-based plating layer 12 and the Al-based plating layer 22 may have the same chemical composition or different chemical compositions. Furthermore, they may have the same plating thickness or different plating thicknesses.

[0028] [Brass filler 30] The brazing filler 30 is a brazing filler having a liquidus temperature TLL of 950°C or less. Examples of such brazing filler include copper-based brazing filler, nickel-based brazing filler, silver-based brazing filler, gold-based brazing filler, and palladium-based brazing filler. Examples of copper-based brazing filler include Cu, Cu-Zn, Cu-Sn, Cu-P, or Cu-P-Ag. Examples of nickel-based brazing filler include Ni, Ni-Cr-B-Si, or Ni-B-Si. Examples of silver-based brazing filler include Ag, Ag-Cu, Ag-Cu-Zn, or Ag-Cu-Ni. Examples of gold-based brazing filler include Au, Au-Cu, Au-Cu-Ni, or Au-Pd-Ni. Examples of palladium-based brazing filler include Pd-Ag, Pd-Cu, or Pd-Ag-Cu. Specific types of brazing filler metal 30 include, for example, copper and copper alloy brazing filler metals according to JIS Z3262:1998, nickel brazing filler metals according to JIS Z3265:1998, silver brazing filler metals according to JIS Z3261:1998, gold brazing filler metals according to JIS Z3266:1998, and palladium brazing filler metals according to JIS Z3267:1998. Typically, the upper limit of the heating temperature in the hot stamping process is 950°C. Therefore, if the brazing filler metal 30 has a liquidus temperature TLL of 950°C or less, the brazing filler metal 30 is in a molten state during heating in hot stamping, and therefore can join the joining surfaces of the first steel sheet 10 and the second steel sheet 20. If the brazing filler metal has a liquidus temperature above TLL of 950°C, the brazing filler metal does not melt during heating in hot stamping, and therefore cannot join the joining surfaces of the first steel sheet 10 and the second steel sheet 20. The liquidus temperature TLL of the brazing filler metal 30 is preferably 910° C. or lower, and more preferably 895° C. or lower. The liquidus temperature TLL of the brazing filler metal 30 is preferably 450° C. or higher, and more preferably 585° C. or higher.

[0029] The liquidus temperature TLL can be determined by the following method. For example, the hot stamping overlap blank 1 or the hot stamped body 100 described below is disassembled to collect filler metal and the like, and the components of the collected filler metal and the like are analyzed. The liquidus temperature TLL is determined from the chemical composition of the filler metal, using a catalog value or a known correlation equation between the chemical composition and the liquidus temperature TLL. The liquidus temperature TLL can be easily confirmed in the above manner. The same applies to the solidus temperature TSL. Note that if the liquidus temperature TLL is known by investigating the hot stamping overlap blank 1, there is no need to investigate the hot stamped body 100. For example, if the type of filler metal used is known in advance, or if the TLL of the filler metal is known in advance, there is no need to disassemble and investigate the hot stamping overlap blank 1 or the hot stamped body 100.

[0030] The hot stamped body may be subjected to paint baking. In the paint baking process, the hot stamped body is subjected to a thermal history of 150 to 200° C. Therefore, to prevent melting during the paint baking process, the brazing filler metal 30 preferably has a solidus temperature TSL of 250° C. or higher, and more preferably 400° C. or higher.

[0031] The thickness of the filler metal 30 is 40 to 180 μm. If the thickness of the filler metal 30 is less than 40 μm, the bonding strength of the filler metal 30 in the thickness direction is low, and at least a portion of the filler metal peels off, resulting in a small maximum load. If the thickness of the filler metal 30 is more than 180 μm, the bonding strength of the filler metal 30 in the in-plane direction is low, and at least a portion of the filler metal peels off, resulting in a small maximum load. Therefore, the thickness of the filler metal 30 is set to 40 to 180 μm. The thickness of the filler metal 30 is preferably 50 to 180 μm. The thickness of the filler metal 30 of the hot stamping overlapping blank 1 can be measured using a method similar to the method for measuring the plating thickness of the Al-based plating layer 12 described above (the average value of measured values ​​at three locations by cross-sectional observation).

[0032] The brazing filler 30 may be a foil or a paste.

[0033] The density of spot welding points is 0.2 points / 100 cm２ The spot welding density is 0.2 points / 100 cm ２ If the density is more than 0.7 points / 100cm, the effect of improving rigidity can be obtained. ２ It is preferable that the spot welding density is 0.7 points / 100 cm or more. ２ If the density is equal to or greater than 10.0 points / 100 cm, the peeling of the brazing filler metal is suppressed even when the member (the overlap hot-stamped body 100) is deformed, and a more reliable effect of improving rigidity can be achieved. There is no particular upper limit to the spot welding density, but if the spot welding density is too high, the welding current may be shunted during welding, making welding difficult. Therefore, the spot welding density should be 10.0 points / 100 cm. ２ If necessary, the spot welding density is set to 7.0 points / 100 cm or less. ２ Below, 5.0 points / 100cm ２ Below, 4.0 points / 100cm ２ or less, or 2.0 points / 100cm ２ The density of spot welds can be calculated by dividing the number of spot welds by the area of ​​the overlapping portion of the first steel sheet and the second steel sheet. The number of spot welds, including whether or not they are spot welded, can be visually confirmed.

[0034] If the filler metal 30 is not present between the first steel plate 10 and the second steel plate 20, these steel plates remain in a state where they are discretely joined by welding, and out-of-plane deformation (elastic buckling) is likely to occur in at least one of the first steel plate 10 or the second steel plate 20. However, by having the filler metal 30 present between the first steel plate 10 and the second steel plate 20, the two steel plates are continuously joined via the filler metal 30, improving rigidity and strength. Note that the rigidity has an effect of the second to third power of the plate thickness t. For example, if the rigidity is proportional to the square of the plate thickness t, and assuming that two steel plates with a plate thickness t1 are joined by patchwork, the rigidity will be ２ +t1 ２ On the other hand, when two steel plates with a plate thickness of t1 are surface-joined, the plate thickness becomes 2t1 (t1 + t1), and (2t1) ２Therefore, a member in which two steel plates are surface-bonded has a greater improvement in rigidity than a patchwork member.

[0035] The filler metal 30 has a lower electrical resistance than the Al-based plating layer 22. Therefore, if the filler metal 30 is provided at the welding position, current shunting may occur during welding, reducing the joint strength of the spot weld. In other words, in such cases, the strength and rigidity of the hot-stamped body 100 may not be significantly improved. For this reason, it is preferable that the entire periphery (circumference) of the nugget of the overlapping blank 1 for hot stamping be a non-brazed portion 31 that is free of the filler metal 30, that is, that the periphery of the nugget is not in contact with the filler metal 30. The diameter of the non-brazed portion 31 is preferably more than 1.00 times the nugget diameter. In order to ensure that the entire periphery (circumference) of the nugget of the overlapping blank 1 for hot stamping is a non-brazed portion 31 that is free of the filler metal 30, at least in a steel sheet having an Al-based plating layer 22 before spot welding, the non-brazed portion needs to have a diameter larger than the nugget diameter to be formed. Even if the nugget diameter and spot welding position are predicted using such a preliminary test or the like, considering variations in the nugget diameter and spot welding position, it is preferable to form a non-brazed portion with a diameter larger than the predicted nugget diameter in a steel sheet having an Al-based coating layer 22 before spot welding. Therefore, when the periphery (entire periphery) of the nugget of the overlapping blank 1 for hot stamping is a non-brazed portion 31 without a brazing filler metal 30, the diameter of the non-brazed portion 31 of the overlapping blank 1 for hot stamping is often larger than the nugget diameter. Therefore, the diameter of the non-brazed portion 31 of the overlapping blank 1 for hot stamping may be 1.10 times or more, 1.20 times or more, or 1.30 times or more the nugget diameter. Furthermore, during processing of the hot-stamped body (specifically, during press forming after heating to a liquidus temperature TLL or higher), the area of ​​the brazed portion in the overlapping blank may increase slightly, resulting in the diameter of the non-brazed portion of the hot-stamped body being smaller than the diameter of the non-brazed portion of the blank. However, the expansion margin of the brazed portion is often about 0.03 to 0.05 times the nugget diameter, and does not exceed at least 0.10 times. Therefore, in order to make the periphery of the nugget of the hot-stamped body a non-brazed layer portion, it is preferable to set the diameter of the non-brazed portion of the overlapping blank to 1.05 times or more or 1.10 times or more the nugget diameter.However, increasing the diameter of the non-brazed portion 31 may result in a decrease in the joining strength, which in turn may reduce the strength and rigidity of the hot-stamped body 100. Therefore, it is preferable that the diameter of the non-brazed portion 31 of the overlapping blank for hot stamping 1 be 2.00 times or less the nugget diameter. Since the non-brazed portion 31 has a nugget at least in the center, it is basically annular in shape, and its outer diameter is referred to as the diameter. The diameter of the non-brazed portion 31 and the nugget diameter can be measured by disassembling the first steel sheet 10 and the second steel sheet 20 of the overlapping blank for hot stamping 1 with a chisel and observing and measuring the overlapping surfaces visually or with a magnifying glass, etc.

[0036] The area ratio of the brazing filler metal 30 is preferably 50% or more, and more preferably 70% or more or 80% or more. Here, the area ratio of the brazing filler metal 30 refers to the ratio of the total area of ​​the region sandwiched between the first steel plate 10 and the second steel plate 20 where the brazing filler metal 30 is disposed to the area of ​​the region excluding the non-brazed portion 31. The brazing filler metal 30 may be disposed over the entire portion excluding the non-brazed portion 31 between the first steel plate 10 and the second steel plate 20. In this embodiment, since a nugget is present, the area ratio of the brazing filler metal 30 is not 100%, but may be 99% or less, 98% or less, 95% or less, or 90% or less depending on the area ratio of the nugget.

[0037] <Method for manufacturing overlapping blank 1 for hot stamping> The method for manufacturing the overlapping blank 1 for hot stamping is not particularly limited, and for example, two steel sheets having an Al-based plating layer, which will become the first steel sheet 10 and the second steel sheet 20, may be prepared, a brazing filler metal 30 may be sandwiched between them, and the first steel sheet 10 and the second steel sheet 20 may be welded together. The steel sheet having an Al-based plating layer may be one manufactured by a known method.

[0038] When providing a non-brazed portion in the brazing material, a preliminary test is conducted to measure the nugget diameter, and a non-brazed portion having a diameter greater than 1.00 to 2.00 times the nugget diameter is provided in the brazing material. If necessary, the diameter of the non-brazed portion may be increased to, for example, 1.10 times or more the nugget diameter.

[0039] <Laminated Hot-Stamped Steel 100> Next, with reference to Fig. 3, a laminated hot-stamped steel 100 according to an embodiment of the present invention will be described. Fig. 3 is a cross-sectional view of the laminated hot-stamped steel 100 according to the present embodiment. The laminated hot-stamped steel 100 according to the present embodiment is a laminated hot-stamped steel including a first steel sheet 110 and a second steel sheet 120 having an area smaller than that of the first steel sheet 110 and having a welded portion overlapping and welded onto the first steel sheet 110, wherein the first steel sheet 110 and the second steel sheet 120 each have an Al-based plating layer (not shown) on both sides of the steel sheet. The laminated hot-stamped steel 100 has a brazing layer 130 between the first steel sheet 110 and the second steel sheet 120, the brazing layer 130 having a thickness of 40 µm or more and 180 µm or less, and a liquidus temperature TLL of 950°C or less. The overlap hot-stamped steel 100 has a bent portion 140 in at least a part of the overlapping portion of the first steel plate 110 and the second steel plate 120. The spot welding density in the overlap hot-stamped steel 100 is 0.2 points / 100 cm. ２That's all. The overlap hot-stamped compact 100 according to this embodiment is obtained by hot-stamping the overlap blank for hot stamping described above. In the overlap blank for hot stamping 1, the "filler layer 30" between the first steel sheet 10 and the second steel sheet 20 is the "filler layer" before hot stamping, i.e., before brazing. On the other hand, in the hot-stamped compact 100, the "filler layer" between the first steel sheet 110 and the second steel sheet 120 is the "filler layer" after hot stamping, i.e., after brazing. To distinguish it from the filler layer 30 of the overlap blank for hot stamping 1, in this embodiment, the "filler layer" of the hot-stamped compact 100 after brazing is generally referred to as the "filler layer 130." Furthermore, in this embodiment, even for the hot-stamped compact 100, if the "filler layer" is not brazed during hot stamping because the liquidus temperature TLL of the "filler layer" is above 950°C, it is not referred to as a "filler layer" but as a "filler layer." In other words, it is only referred to as a "filler layer" when it is brazed. The filler layer 130 is also sometimes referred to as a filler metal. The thickness of the braze layer 130 is 40 to 180 μm, similar to the braze 30. The thickness of the braze layer 130 is preferably 50 to 180 μm. The thickness of the braze layer 130 can be measured using the same method as the method for measuring the braze 30 (average of measurements taken at three locations by cross-sectional observation).

[0040] [First steel sheet 110] The first steel sheet 110 comprises a base steel sheet 111 and Al-based plating layers disposed on both sides of the base steel sheet 111. The first steel sheet 110 is obtained by hot stamping the first steel sheet 10 in the overlapping blank 1 for hot stamping. The shape of a hot-stamped body is often not flat, and the first steel sheet 110 often has a non-flat shape. However, in this embodiment, the first steel sheet 110 will be referred to as a "steel sheet" for convenience, like the base steel sheet and the second steel sheet 120 described below.

[0041] (Base Steel Plate 111) The chemical composition of the base steel plate 111 is not particularly limited, as is the base steel plate 11 of the lap hot-stamped body. The chemical composition of the base steel plate 111 may be the chemical composition exemplified for the base steel plate 11 (a chemical composition for obtaining a tensile strength of 1000 MPa or more or a Vickers hardness of approximately 300 HV or more in a hot-stamped body, or a chemical composition for obtaining a tensile strength of 1500 MPa or more or a Vickers hardness of approximately 400 HV or more). The metallographic structure of the base steel plate 111 does not need to be particularly specified, but it is preferable that the total of martensite and bainite is 98 area % or more. The total of martensite, bainite, and retained austenite may be 100 area %.

[0042] The metal structure of the base steel sheet 111 can be observed by the following method. The observation surface is polished and immersed in an acetylacetone-based electrolyte, followed by electrolytic etching. A field-emission scanning electron microscope equipped with a secondary electron detector is used to observe a secondary electron image of the observation surface after electrolytic etching at, for example, 5,000x magnification to distinguish between ferrite and pearlite and structures other than ferrite and pearlite. A structure consisting of massive crystal grains that does not contain a substructure such as lath within the structure is considered to be ferrite. A structure consisting of layered plate-like ferrite and Fe-based carbides is considered to be pearlite. Furthermore, a secondary electron image is observed at, for example, 10,000x magnification at the same observation location as above to distinguish between tempered martensite and bainite. A structure consisting of a collection of lath-shaped crystal grains and containing Fe-based carbides with a major axis of 20 nm or more and elongated in different directions within the structure is considered to be tempered martensite. Among structures that are a collection of lath-shaped crystal grains and do not contain Fe-based carbides with a major axis of 20 nm or more within the structure, structures in which Fe-based carbides precipitate between the laths and structures in which Fe carbides precipitate within the laths and the Fe-based carbides elongate in the same direction are considered to be bainite. Furthermore, at the same observation location as above, the observation surface is re-polished to a mirror finish, strain is removed, and crystal orientation information is obtained at measurement intervals of 0.4 μm using electron backscatter diffraction. Regions with an FCC crystal structure are considered to be retained austenite. Furthermore, regions other than ferrite, pearlite, tempered martensite, bainite, and retained austenite obtained by the above method are considered to be fresh martensite. The area ratios of fresh martensite and tempered martensite are collectively referred to as the area ratio of martensite.

[0043] Furthermore, if the chemical composition of the base steel plate 111 is within the range of the chemical composition exemplified in the base steel plate 11 (a chemical composition when a tensile strength of 1000 MPa or more or a Vickers hardness of approximately 300 HV or more is to be obtained in a hot stamped body, or a chemical composition when a tensile strength of 1500 MPa or more or a Vickers hardness of approximately 400 HV or more is to be obtained), and the Vickers hardness (Vickers hardness when the test force is 9.807 N) in accordance with JIS Z2244-1:2020 is 300 or more or 400, the total of the martensite, bainite, and retained austenite in the base steel plate 111 can be considered to be 100% by area.

[0044] (Al-based plating layer 112) The Al-based plating layer 112 is a layer corresponding to the Al-based plating layer 12 in the hot stamping overlapping blank 1. The Al-based plating layer 112 is a plating layer containing Al and Fe as its main components. Here, containing Al and Fe as its main components means that the Al+Fe content is 80% or more by mass %, and the Al content is 30% or more by mass %. The chemical composition of the Al-based plating layer 112 may be, for example, in mass %, 30 to 97% Al, 2 to 15% Si, 0 to 70% Fe, 0 to 1% Cr, 0 to 1% Mo, 0 to 1% Zn, 0 to 1% V, 0 to 1% Ti, 0 to 1% Sn, 0 to 1% Ni, 0 to 1% Cu, 0 to 1% W, 0 to 1% Bi, 0 to 1% Mg, 0 to 1% Ca, 0 to 1% REM, and the balance: impurities. The chemical composition of the Al-based plating layer 112 is determined by polishing a cross section of a test piece cut out from the overlap hot stamped body, performing nital etching, and observing the cross section as a composition image at a magnification of, for example, 1000 times using an electron beam microanalyzer (EPMA), and performing elemental analysis. When performing elemental analysis, 10 points are analyzed at equal intervals from the plating surface toward the base material. The average value of the measurements at each analysis point is taken as the chemical composition of the Al-based plating layer 112. Because Fe diffuses to the surface of the Al-based plating layer 12 in the overlapping blank 1 for hot stamping due to heating during hot stamping, the Al-based plating layer 112 is an Al-Fe-based plating layer (in other words, an alloy plating layer containing at least Al and Fe). The Al-Fe-based plating layer is a θ-phase (FeAl ３ ), η phase (Fe ２ Al ５ ), ζ phase (FeAl ２ ), Fe ３ In addition, when the Al-based coating layer 12 of the first steel sheet 10 contains Si, the Al-Fe-based coating layer of the lap hot stamped steel 100 is composed of a combination of phases such as τ1 phase (Al ２ Fe ３ Si ３ ), τ2 phase (Al ３ FeSi), τ3 phase (Al ２ FeSi), τ4 phase (Al ３ FeSi２ ), 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 The Al and Fe compound layer mainly consists of τ1 phase and η phase (Fe ２ Al ５ ) or a plurality of such phases. In particular, Al in the coating and Fe in the base material diffuse inter-diffusively. A layer containing a BCC phase of Fe with Al dissolved therein or a FeAl phase formed by Al diffusion into the base material is called an Al-dissolved Fe layer, and this layer is adjacent to the base steel sheet. Under the heating conditions of this embodiment, in addition to the compound layer containing at least Al and Fe, an Al-dissolved Fe layer is formed in the bottom layer of the coating located on the base steel sheet 111 side. The Al-Fe-based coating layer includes the Al-Fe compound layer and the Al-dissolved Fe layer.

[0045] [Second steel sheet 120] The second steel sheet 120 includes a base steel sheet 121 and Al-based plating layers 122 disposed on both sides of the base steel sheet. The second steel sheet 120 is obtained by hot stamping the second steel sheet 20 in the overlapping blank 1 for hot stamping.

[0046] The base steel plate 121 and the Al-based plating layer 122 of the second steel plate 120 are similar to the base steel plate 111 and the Al-based plating layer 112 of the first steel plate 110, respectively, and therefore detailed description thereof will be omitted here. However, the base steel plates of the first steel plate 110 and the second steel plate 120 may have the same chemical composition or different chemical compositions. Furthermore, they may have the same plate thickness or different plate thicknesses. Furthermore, the Al-based plating layers of the first steel plate 110 and the second steel plate 120 may have the same chemical composition or different chemical compositions. Furthermore, they may have the same plating thickness or different plating thicknesses.

[0047] The spot welding density is 0.2 points / 100cm ２ The spot welding density is 0.2 points / 100 cm ２ If the density is more than 0.7 points / 100cm, the effect of improving rigidity can be obtained. ２ It is preferable that the spot welding density is 0.7 points / 100 cm or more. ２ If the density is equal to or greater than this, peeling of the brazing layer is suppressed even when deformation is applied to the member, and a more reliable effect of improving rigidity can be obtained. The upper limit of the spot welding point density is not particularly limited, as with the overlapping blank for hot stamping, but the spot welding point density should be 10.0 points / 100 cm ２ If necessary, the spot welding density is set to 7.0 points / 100 cm or less. ２ Below, 5.0 points / 100cm ２ Below, 4.0 points / 100cm ２ or less, or 2.0 points / 100cm ２ The number of spot welds, including whether or not they are spot welded, can be confirmed visually.

[0048] The overlapping hot-stamped body 100 according to this embodiment is obtained by hot stamping the overlapping blank for hot stamping 1 described above. Therefore, the braze layer 130 of the hot-stamped body 100 corresponds to the braze layer 30 in the overlapping blank for hot stamping 1 described above. Therefore, the overlapping hot-stamped body 100 preferably has a non-braze layer portion 131 corresponding to the non-braze portion 31 in the overlapping blank for hot stamping 1. Specifically, in order to further improve the strength and rigidity of the hot-stamped body 100, it is preferable that the entire periphery (entire periphery) of the nugget of the hot-stamped body 100 be the non-braze layer portion 131 free of the braze layer 130, that is, that the periphery of the nugget is not in contact with the braze layer 130. It is preferable that the diameter of the non-braze layer portion 131 be more than 1.00 times the nugget diameter. As with the overlapping hot stamping blank 1 described above, the diameter of the non-brazed layer portion 131 of the overlapping hot-stamped body 100 may be 1.10 times or more, 1.20 times or more, or 1.30 times or more the nugget diameter. Increasing the diameter of the non-brazed layer portion 131 may result in a decrease in the bonding strength, thereby reducing the strength and rigidity of the hot-stamped body 100. Therefore, the diameter of the non-brazed layer portion 131 is preferably 2.00 times or less the nugget diameter. The non-brazed layer portion 131 has a nugget at least in the center, so it is basically annular in shape, and its outer diameter is referred to as the diameter. The diameter of the non-brazed layer portion 131 and the nugget diameter can be measured by disassembling the first steel sheet 110 and the second steel sheet 120 of the overlapping hot-stamped body 100 with a chisel and observing and measuring the overlapping surfaces visually or with a magnifying glass, etc.

[0049] The area ratio of the brazing layer 130 is preferably 50% or more, and more preferably 70% or more or 80% or more. The brazing layer 130 may be disposed over the entire portion between the first steel plate 110 and the second steel plate 120 excluding non-brazing layer portions. In this embodiment, the area ratio of the brazing layer 130 is not 100% due to the presence of a nugget, but may be 99% or less, 98% or less, 95% or less, or 90% or less depending on the area ratio of the nugget.

[0050] In the overlap hot-stamped steel 100 according to this embodiment, the first steel sheet 10 and the second steel sheet 20 are strongly bonded to each other in the overlap blank for hot stamping 1, which is the material of the overlap hot-stamped steel 100, and therefore the first steel sheet 110 and the second steel sheet 120 are also firmly bonded to each other in the overlap hot-stamped steel 100 after hot stamping. As a result, the overlap hot-stamped steel 100 is lightweight and has high strength and rigidity.

[0051] The overlap hot-stamped body 100 may be painted or the like.

[0052] <Method for manufacturing the overlap hot-stamped product 100> In the method for manufacturing the overlap hot-stamped product 100 according to this embodiment, the above-described overlap blank for hot stamping 1 is heated and formed immediately after the heating, and a bent portion is provided in at least a part of the overlap portion, thereby manufacturing the overlap hot-stamped product 100 according to this embodiment.

[0053] The heating temperature is not particularly limited, but is generally in the range of from the Ac3 point (e.g., 800°C) of the base steel sheet 111 and the base steel sheet 121 to 950°C. By performing cooling using a mold or a coolant such as water during forming immediately after heating, it is possible to obtain a lap hot-stamped product 100 with excellent crash resistance. The heating temperature refers to the maximum temperature reached by the steel sheets at the overlapping portion. Examples of heating methods include heating with an electric furnace, gas furnace, far-infrared furnace, near-infrared furnace, etc., electrical heating, high-frequency heating, and induction heating. As described above, a lap hot-stamped product can be produced without making significant changes to the hot stamping process.

[0054] An application example of a structural member of the present invention will be described with reference to Fig. 6. Fig. 6 is a diagram showing an automobile frame 2 as an example to which a hot-stamped steel 100 is applied. The hot-stamped steel 100 can form the automobile frame 2 as a cabin frame or an impact-absorbing frame.

[0055] Application examples of the hot stamped body 100 include a roof central reinforcement 201, a roof side rail 203, a B-pillar 207, a side sill 209, a tunnel 211, an A-pillar lower 213, an A-pillar upper 215, a kick reinforcement 227, a floor cross member 229, an under reinforcement 231, and a front header 233. Application examples of the hot stamped body include a rear side member 205, an apron upper member 217, a bumper reinforcement 219, a crash box 221, and a front side member 223.

[0056] The present invention has been described above based on an embodiment of the present invention, but the above is merely an example, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention.

[0057] In the above-described embodiment, the case where there is one second steel plate has been described, but there may be two or more second steel plates. In this case, each of the plurality of second steel plates is joined to the first steel plate.

[0058] Furthermore, for example, the overlap hot-stamped product is not limited to the hat-shaped member shown in FIG. 3, but may have various shapes.

[0059] The present invention will be described in more detail below with reference to examples. Note that the examples described below are merely examples of the present invention and are not intended to limit the present invention.

[0060] Example 1 Two overlapping blanks for hot stamping were produced for each of the first steel sheet, the second steel sheet, the thickness and type of filler metal, and the spot welding conditions, as shown in Table 1. Specifically, the first steel sheet and the second steel sheet were overlapped, and a filler metal having the thickness shown in Table 1 and a solidus temperature TSL and a liquidus temperature TLL was sandwiched between the two steel sheets, thereby producing one overlapping blank for hot stamping. Here, the first steel sheet was an Al-based plated steel sheet having a tensile strength of 1500 GPa or more at a thickness of 1.4 mm (however, some of the steel sheets were cold-rolled steel sheets without an Al-based plated layer, and are indicated as "bare" in Table 1), and the second steel sheet was an Al-based plated steel sheet having a tensile strength of 1500 GPa or more at a thickness of 1.4 mm (however, some of the steel sheets were hot-dip galvanized steel sheets, and are indicated as "GA" in Table 1). The solidus temperature TSL and liquidus temperature TLL listed in Table 1 are catalog values ​​for the brazing filler metal used. For reference, Table 1 also lists the type of brazing filler metal used. In Example 1, no non-brazing filler metal was provided. One of the overlapping blanks was disassembled for each condition, and the thickness of the brazing filler metal and the like were measured. The remaining blanks were subjected to hot stamping using the method described below, and subsequent rigidity and maximum load measurement tests, etc., described below.

[0061] Specifically, each of the overlapping blanks was heated at 910°C for 480 seconds, and the hat member was prepared at a forming start temperature of 750°C, which was the overlapping hot-stamped product 100 shown in Fig. 5. Then, as shown in Fig. 5, the hat member and a back plate (closing plate) 200 were joined by spot welding, and the hat member was subjected to a BH treatment at 170°C for 20 minutes and then subjected to a three-point bending test. The closing plate was a 780 MPa-class cold-rolled steel plate measuring 800 mm in length, 130 mm in width, and 1.2 mm in thickness.

[0062] In the three-point bending test, a hat member was placed between an impactor and a pair of support members, as shown in Figure 7. The radius of curvature R1 of the impactor was 150 mm, the distance L between the support points was 600 mm, and the radius of curvature R0 of the support points on the support members was 30 mm. At this time, the convex portion of the hat member was in contact with the impactor, and the closing plate joined to the flange portion was in contact with the pair of support members. The crushing speed was 1 mm / sec, and the load-displacement relationship was evaluated until the impactor crushed to 80 mm.

[0063] The spot welding density was calculated by dividing the number of spot welding points by the area of ​​the overlapping portion of the first steel plate and the second steel plate. The hat member was also disassembled into the first steel plate and the second steel plate using a chisel, and the thickness of the brazing layer and other properties were measured.

[0064] Table 1 shows the rigidity and maximum load of each hat member, as well as the presence or absence (appearance) of peeling of the braze layer after the maximum load was applied. For each pair of test specimens, the thickness of the braze layer of the overlapping hot stamping blank and the thickness of the braze layer of the overlapping hot stamped body were all the same. Therefore, the thickness is listed in the "Braze" column of Table 1.

[0065]

[0066] The evaluation results were judged to be good when the rigidity was 4000 N / mm or more, the maximum load was 61 kN or more, and the appearance was acceptable (Good).

[0067] In Examples A1 to A5, in which the first steel sheet and the second steel sheet each have an Al-based plating layer on both sides of the steel sheet, a brazing filler layer is provided between the first steel sheet and the second steel sheet, the thickness of the brazing filler layer is 40 μm or more and 180 μm or less, and the liquidus temperature TLL is 950°C or less, high rigidity and maximum load were obtained, and no peeling of the brazing filler layer was observed.

[0068] Example a7 is an example in which the first steel plate and the second steel plate were not spot-welded, and the rigidity was low and the sample was rejected.

[0069] In the example a6, the TLL of the brazing filler metal was too high, so the material was not brazed during hot stamping (i.e., the brazing filler layer was not formed), and the rigidity was low, resulting in failure.

[0070] The example a3 is an example in which no brazing filler was used, and was found to have low rigidity and to have failed.

[0071] In the example a4, the thickness of the brazing filler was too small, so the maximum load was small and the example was rejected.

[0072] In the example of a1, the first steel sheet did not have an Al-based plating layer, and scale was formed on the surface of the hat member, resulting in failure.

[0073] In the example a5, the thickness of the brazing filler metal was too large and the density of the spot welding points was small, so the maximum load was small and the example a5 was unacceptable.

[0074] In the example of a2, the plating layer of the second steel sheet was a Zn plating layer, and since the melting point of the Zn plating layer was low, LME occurred, resulting in a small maximum load and failure.

[0075] In the example a5', the maximum load was small because the spot welding point density was small, and the example was unacceptable.

[0076] Example 2 Two overlapping blanks for hot stamping were manufactured under the conditions shown in Table 2. Specifically, a first steel sheet and a second steel sheet were overlapped, a filler metal shown in Table 2 was sandwiched between the two steel sheets, and the spot welding density was changed to manufacture two overlapping blanks for hot stamping. Here, the first steel sheet was an Al-based plated steel sheet having a thickness of 1.4 mm and a tensile strength of 1500 GPa or more, and the second steel sheet was an Al-based plated steel sheet having a thickness of 1.4 mm and a tensile strength of 1500 GPa or more. The spot welding points were spaced equally apart. The spot welding point density was 0.7 points / 100 cm ２ (Longitudinal intervals of 190 mm x width intervals of 75 mm), 1.0 point / 100 cm ２ (Longitudinal intervals of 133 mm x widthwise intervals of 75 mm), and 4.0 points / 100 cm ２The spacing was 33 mm in the longitudinal direction x 75 mm in the width direction. The solidus temperature TSL and liquidus temperature TLL listed in Table 2 are catalog values ​​for the brazing filler metal used. For reference, Table 2 also lists the type of brazing filler metal used. In Example 2, no non-brazing filler metal was provided. Two overlapping blanks were produced for each condition, and one blank was disassembled to measure the thickness of the brazing filler metal, etc. The remaining blank was subjected to hot stamping using the method described below, and subsequent rigidity and maximum load measurement tests, etc., described below.

[0077] Specifically, hat members having a closing plate were manufactured using overlapping blanks for hot stamping under the same conditions as in Example 1, and a three-point bending test was performed on each hat member.

[0078] The spot welding density was calculated by dividing the number of spot welding points by the area of ​​the overlapping portion of the first steel plate and the second steel plate. The hat member was also disassembled into the first steel plate and the second steel plate using a chisel, and the thickness of the brazing layer and other properties were measured.

[0079] Table 2 shows the rigidity and maximum load of each hat member, as well as the presence or absence (appearance) of peeling of the braze layer after the maximum load was applied. For each pair of test specimens, the thickness of the braze layer of the overlapping hot stamping blank and the thickness of the braze layer of the overlapping hot stamped body were all the same. Therefore, the thickness is listed in the "Braze" column of Table 2.

[0080]

[0081] The first steel sheet and the second steel sheet each have an Al-based plating layer on both sides of the steel sheet, and a brazing filler layer is formed between the first steel sheet and the second steel sheet, the brazing filler layer has a thickness of 40 μm or more and 180 μm or less, a liquidus temperature TLL is 950°C or less, and a dot density is 0.7 dots / cm ２ In the examples B1 to B3, high rigidity and maximum load were obtained, and no peeling of the brazing layer was observed.

[0082] Example 3 Two overlapping blanks for hot stamping were produced under the conditions shown in Table 3. Specifically, a first steel sheet and a second steel sheet were overlapped, and the filler metal shown in Table 3 was sandwiched between the two steel sheets. The spot welding density was varied to produce two overlapping blanks for hot stamping. Here, the first steel sheet was an Al-plated steel sheet having a tensile strength of 1500 GPa or more at a thickness of 1.4 mm, and the second steel sheet was an Al-plated steel sheet having a tensile strength of 1500 GPa or more at a thickness of 1.4 mm. The spot welding points were spaced equally apart. The solidus temperature TSL and liquidus temperature TLL listed in Table 3 are catalog values. For reference, Table 3 also lists the type of filler metal used. Two overlapping blanks were produced under each condition, and one blank was disassembled to measure the filler metal thickness, the diameter of the non-filler metal portion, and the nugget diameter. The filler metal thickness, the diameter of the non-filler metal portion, and the nugget diameter were measured. The remaining one was subjected to hot stamping and subsequent tests such as rigidity.

[0083] Using one overlapping blank for each hat member, hat members each having a closing plate were manufactured under the same conditions as in Example 1, and a three-point bending test was performed on each hat member.

[0084] The spot welding point density was calculated by (number of spot welds) / (area of ​​the overlapping portion of the first steel plate and the second steel plate). Furthermore, the overlapping blank and the hat member were disassembled into the first steel plate and the second steel plate using a chisel, and the presence or absence of a non-brazed layer around the nugget, the thickness of the brazing layer, the diameter of the non-brazed portion and non-brazed layer, and the nugget diameter were measured. With the exception of the hat member C5, which had a brazing layer around the nugget, there was no brazing layer around the nugget, and the nuggets of C1 to C4 and C6 all had non-brazed layer areas around them.

[0085] Table 3 shows the rigidity and maximum load of each hat member, as well as the presence or absence (appearance) of peeling of the braze layer after the maximum load was applied. For each pair of test specimens, the thickness of the braze layer of the overlapping hot stamping blank and the thickness of the braze layer of the overlapping hot stamped body were all the same. Therefore, the thickness is listed in the "Braze" column of Table 3.

[0086]

[0087] The first steel sheet and the second steel sheet each have an Al-based plating layer on both sides of the steel sheet, and a brazing filler layer is formed between the first steel sheet and the second steel sheet, the brazing filler layer has a thickness of 40 to 180 μm, a liquidus temperature TLL of 950° C. or less, and a dot density of 0.7 dots / cm ２ In examples C1 to C6, where the minimum length of the non-brazed layer portion was 1.00 to 2.00 times the nugget diameter, high rigidity and maximum load were obtained, and no delamination of the braze layer was observed. In example C5, the overlapping hot stamping blank had a non-brazed portion, the diameter of which was 2.00 times the nugget diameter or less, but the overlapping hot-stamped product had a braze layer around the nugget. This is thought to be because the braze spread during hot stamping, forming a braze layer around the nugget in the hot-stamped product. For this reason, the maximum loads of C1 to C4 and C6 are thought to be higher than the maximum load of C5.

[0088] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0089] REFERENCE SIGNS LIST 1 Hot stamping overlap blank 10 First steel sheet 11 Base steel sheet 12 Al-based plating layer 20 Second steel sheet 21 Base steel sheet 22 Al-based plating layer 30 Brazing filler metal 31 Non-brazing filler metal portion 100 Lap-stamped hot stamped body 110 First steel sheet 111 Base steel sheet 112 Al-based plating layer 120 Second steel sheet 121 Base steel sheet 122 Al-based plating layer 130 Brazing filler metal portion 131 Non-brazing filler metal portion 140 Bent portion 200 Back plate

Claims

1. A hot stamping overlapping blank comprising: a first steel plate; and at least one second steel plate having an area smaller than that of the first steel plate, the second steel plate being overlapped and spot welded to the first steel plate; The first steel sheet and the second steel sheet each have an Al-based plating layer on both sides of the steel sheet, A brazing material is provided between the first steel plate and the second steel plate, The thickness of the brazing filler metal is 40 to 180 μm, and the liquidus temperature TLL is 950° C. or less; Spot welding density is 0.2 points / 100cm ２ That's all. The periphery of the nugget is a non-brazed portion that is free of the brazing material, The diameter of the non-brazed portion is 2.00 times or less than the nugget diameter.

1. A hot stamping overlapping blank comprising:

2. The spot welding density is 0.7 points / 100 cm ２ That's all.

2. The hot stamping laminate blank according to claim 1.

3. The diameter of the non-brazed portion is 1.10 times or more the nugget diameter. The hot stamping laminated blank according to claim 1 or 2.

4. A lap hot stamp formed body comprising: a first steel sheet; and at least one second steel sheet having an area smaller than that of the first steel sheet, the second steel sheet being overlapped on the first steel sheet and spot welded thereto, the first steel sheet and the second steel sheet each having an Al-based plating layer on both sides of the steel sheet, A brazing layer is provided between the first steel plate and the second steel plate, The brazing layer has a thickness of 40 to 180 μm and a liquidus temperature TLL of 950° C. or less; At least a part of the overlapping portion of the first steel plate and the second steel plate has a bent portion, Spot welding density is 0.2 points / 100cm ２ That's all. The periphery of the nugget is a non-brazed portion that is free of the brazing material, The diameter of the non-brazed portion is 2.00 times or less than the nugget diameter. The overlapping hot stamped compact is characterized by the above.

5. The spot welding density is 0.7 points / 100 cm ２ That's all. The overlap hot stamped compact according to claim 4.

6. The diameter of the non-brazed layer portion is 1.10 times or more the nugget diameter. The overlap hot stamped compact according to claim 4 or 5.

7. An automobile part comprising the overlap hot stamped compact according to claim 4 or 5.