Blank, method for manufacturing structural member, and structural member

Abstract

A blank (30, 30A, 30B) is provided with a plurality of steel sheets. The plurality of steel sheets are disposed and joined so as to form two long sections (34L, 34R) and a connection section (35). The plurality of steel sheets include a first steel sheet (31) and a second steel sheet (32). An end of the second steel sheet (32) and an end of the first steel sheet (31) form an overlapping section (361). The overlapping section (361) has the maximum plate thickness (t max) of the blank (30, 30A, 30B). With respect to at least one of the first steel sheet (31) and the second steel sheet (32), the surface (311, 321) positioned on the outer side in the overlapping section (361) has been subjected to a treatment for increasing the emissivity compared to at least one of other surface of the plurality of steel sheets.

Description

Blank, manufacturing method of structural member, and structural member

[0001] The present disclosure relates to a blank, a method for manufacturing a structural member, and a structural member.

[0002] Structures such as automobile bodies are formed from multiple structural members. Structural members are manufactured, for example, by press-forming a blank. To ensure high strength and good dimensional accuracy, structural members are sometimes manufactured by a press-forming method called hot stamping. Hot stamping is a technique in which a blank, which is a steel plate, is heated to a temperature in the austenite range, and then press-formed using a die. The blank is then held in the die and quenched by removing heat (rapid cooling).

[0003] Patent Document 1 discloses a steel sheet (blank) for hot stamping. The steel sheet of Patent Document 1 has a surface treatment film on the entire surface of at least one surface, the surface treatment film having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. According to Patent Document 1, the surface of the steel sheet to which the surface treatment film is applied has an increased emissivity, and the heat transfer effect by radiation is large. Therefore, when the steel sheet is heated during hot stamping, the steel sheet undergoes a transformation into an austenite phase, where the metal structure of the steel sheet is transformed into an austenite phase. ｃ３ Patent Document 1 describes that this makes it possible to shorten the heating time and improve the productivity of hot stamped members.

[0004] Patent Document 2 discloses an overlapping blank for hot stamping. The overlapping blank in Patent Document 2 includes a first steel sheet and a second steel sheet having an area smaller than that of the first steel sheet. The second steel sheet is overlapped on the surface of the first steel sheet and welded to the first steel sheet. Both the first steel sheet and the second steel sheet are aluminum-plated steel sheets, and the coating weight of the plating layer on both surfaces is 20 g / m. ２ 120g / m or more ２When the average coating weight of the aluminum-based plating layer on both surfaces of the first steel sheet is W1, the coating weight of the aluminum-based plating layer on the surface of the second steel sheet not in contact with the first steel sheet is W2, the sheet thickness of the first steel sheet is t1, and the sheet thickness of the second steel sheet is t2, the overlapping blank of Patent Document 2 satisfies the following conditions: 30≦(W1−W2)≦100, and (W1 / W2) ２ × (t1 / t2) ≥ 1.5 is satisfied. According to Patent Document 2, by satisfying these relationships, it is possible to quickly progress the alloying reaction of the plating layer, which increases the emissivity during heating in hot stamping, to the surface in the overlapping portion between the first steel sheet and the second steel sheet.

[0005] International Publication No. 2022 / 215229 Patent No. 6642777

[0006] In recent years, in order to simplify the manufacturing process of structures, it has been considered to integrate two or more components from the blank stage. That is, it has been considered to subject a composite blank including multiple steel plates (sub-blanks) to hot stamping to form structural components that were previously formed separately into a single component. In a composite blank, adjacent steel plates are joined by spot welding or the like, for example, with their edges overlapping. When a composite blank includes such an overlapping portion, it is difficult to ensure a process window in the manufacturing of a structural component. Specifically, during hot stamping, the blank is heated until its microstructure is austenitized. However, the overlapping portion formed by partial overlapping of the steel plates is thicker than the non-overlapping portion, and therefore is difficult to heat. For example, when a blank includes a plated steel sheet, while the overlap portion, which has the largest sheet thickness in the blank, is heated to ensure its strength by hot stamping, excessive alloying of the plating layer may occur in the steel sheet (non-overlapping portion) that has been heated first, resulting in a thick diffusion layer and a decrease or loss of the corrosion resistance (rust prevention) of the plating layer. Therefore, when an overlap portion is present in a blank, it is difficult to ensure a process window for the heating conditions. Furthermore, the heating rate of the overlap portion, which has the largest sheet thickness in the blank, may become a bottleneck, potentially reducing the productivity of the structural component.

[0007] An object of the present disclosure is to provide a blank for hot stamping that can be used to manufacture structural components that combine the strength of the overlap portion, which has the maximum plate thickness, with rust prevention functionality.

[0008] A hot stamping blank according to the present disclosure includes a plurality of steel sheets. The plurality of steel sheets are arranged and joined to form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel sheets include a first steel sheet and a second steel sheet. The second steel sheet has an end portion that is overlapped and joined to an end portion of the first steel sheet to form an overlap portion together with the end portion of the first steel sheet. The overlap portion has the maximum sheet thickness in the blank. At least one of the plurality of steel sheets is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. A surface located outside the overlap portion of at least one of the first steel sheet and the second steel sheet is treated to increase the emissivity compared to at least one other surface of the plurality of steel sheets.

[0009] According to the hot stamping blank of the present disclosure, a structural component can be manufactured that combines the strength of the overlap portion, which has the maximum plate thickness, with rust prevention functionality.

[0010] FIG. 1 is an exploded perspective view of a structural member according to the first embodiment. FIG. 2 is a cross-sectional view of a side frame included in each of the structural members shown in FIG. 1. FIG. 3A is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in FIG. 1. FIG. 3B is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a cross-sectional view of the blank shown in FIG. 3A. FIG. 3C is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing another cross-sectional view of the blank shown in FIG. 3A. FIG. 3D is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other of the structural members shown in FIG. 1. FIG. 3E is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 3F is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 3G is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 4 is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 5A is a cross-sectional view of a blank according to a second embodiment. FIG. 5B is another cross-sectional view of the blank according to the second embodiment. FIG. 6A is a cross-sectional view of a structural member according to the second embodiment. FIG. 6B is another cross-sectional view of the structural member according to the second embodiment. FIG. 7 is a cross-sectional view of a blank according to a modified example of the second embodiment. FIG. 8 is an exploded perspective view of a structural member according to the third embodiment. FIG. 9 is a plan view of a blank according to the third embodiment. FIG. 10 is a plan view of another blank according to the third embodiment. FIG. 11 is a plan view of a blank according to a modified example of the first embodiment. FIG. 12 is a plan view of a blank according to another modified example of the first embodiment. FIG. 13 is a plan view of a blank according to yet another modified example of the first embodiment. FIG. 14 is a plan view of a blank according to a modified example of the third embodiment. FIG. 15 is a plan view of a blank according to another modified example of the third embodiment. FIG. 16 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 17 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 18 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 19 is a plan view of a blank according to yet another modified example of the third embodiment.Fig. 20 is a plan view of a blank according to yet another modified example of the third embodiment. Fig. 21 is a plan view of a blank according to yet another modified example of the third embodiment. Fig. 22 is a plan view of a blank according to yet another modified example of the third embodiment. Fig. 23 is a plan view of a blank according to yet another modified example of the third embodiment. Fig. 24 is a plan view of a blank according to yet another modified example of the third embodiment. Fig. 25 is a cross-sectional view of a side frame included in a structural member according to the modified examples of each embodiment. Fig. 26A is a diagram showing a division pattern of a structural member in an example. Fig. 26B is a diagram showing another division pattern of a structural member in an example.

[0011] A hot stamping blank according to an embodiment includes a plurality of steel sheets. The plurality of steel sheets are arranged and joined to form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel sheets include a first steel sheet and a second steel sheet. The second steel sheet has an end portion that is overlapped and joined to an end portion of the first steel sheet to form an overlap portion together with the end portion of the first steel sheet. The overlap portion has the maximum sheet thickness in the blank. At least one of the plurality of steel sheets is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. A surface located outside the overlap portion of at least one of the first steel sheet and the second steel sheet is treated to increase the emissivity compared to at least one other surface of the plurality of steel sheets (first configuration).

[0012] In a blank according to the first configuration, the end of the first steel sheet and the end of the second steel sheet form an overlap portion having the maximum sheet thickness in the blank. The surface of at least one of the first steel sheet and the second steel sheet located outside the overlap portion is treated to increase the emissivity compared to the surface of at least one other steel sheet included in the blank. This allows the temperature rise rate of the overlap portion to be increased when the blank is heated during hot stamping, thereby shortening the heating time of the overlap portion. Therefore, the heating of the blank required for hot stamping can be completed before excessive alloying of the plating layer of the plated steel sheet included in the blank progresses and the diffusion layer becomes thick. As a result, in a structural component formed from the blank, the strength of the overlap portion can be ensured by hot stamping, and corrosion resistance (rust prevention) can be ensured. The phrase "treated to increase emissivity" not only refers to a case where the surface of at least one of the first steel plate and the second steel plate located outside the overlap portion has a higher emissivity than the other surfaces before the blank is heated, but also includes a case where the surface of at least one of the first steel plate and the second steel plate located outside the overlap portion has a higher emissivity than the other surfaces during heating of the blank.

[0013] As described above, the blank according to the first configuration enables the manufacture of structural components that combine the strength of the overlap portion, which has the greatest sheet thickness, with the rust prevention function of the plated steel sheet portion. In other words, the blank allows the temperature rise rate of the overlap portion, which has the greatest sheet thickness, to be increased, thereby making it easier to ensure the process window of heating conditions in the manufacture of structural components. Furthermore, by increasing the temperature rise rate of the overlap portion, the heating time of the blank for hot stamping can be shortened, thereby improving the productivity of structural components. Furthermore, shortening the heating time of the blank reduces energy consumption in the manufacture of structural components and reduces the amount of greenhouse gases generated when heating the blank for hot stamping.

[0014] In the blank according to the first configuration, the steel sheet that constitutes the portion of the blank having the smallest thickness among the plurality of steel sheets may be a plated steel sheet (second configuration).

[0015] When a blank is heated during hot stamping, the temperature of the steel sheet with the smallest thickness rises relatively quickly. Therefore, if the steel sheet with the smallest thickness in the blank is a plated steel sheet, while the overlap portion with the largest thickness is being heated to ensure its strength by hot stamping, alloying of the coating layer of the plated steel sheet with the smallest thickness, which was heated first, tends to progress, resulting in a decrease or loss of corrosion resistance due to the coating layer. However, in the blank according to the embodiment, the surface outside the overlap portion of at least one of the first steel sheet and the second steel sheet is treated to increase the emissivity. This increases the heating rate of the overlap portion and shortens the heating time. This allows the heating of the blank required for hot stamping to be completed before excessive alloying of the coating layer of the steel sheet with the smallest thickness, which was heated first, progresses. Therefore, even if the steel sheet with the smallest thickness is a plated steel sheet, as in the second configuration, its corrosion resistance can be ensured.

[0016] In a blank relating to the first or second configuration, the surface of each of the first steel plate and the second steel plate located outside the overlap portion may be treated to increase the emissivity compared to at least one other surface of the multiple steel plates (third configuration).

[0017] In the blank according to the third configuration, the surfaces of the first steel sheet and the second steel sheet located outside the overlap portion are each treated to increase the emissivity. That is, the emissivity of both outer surfaces of the overlap portion is higher than that of the other surfaces before heating the blank, or becomes higher than that of the other surfaces during heating of the blank. In this case, the temperature rise rate of the overlap portion during heating of the blank can be increased, and the heating time of the overlap portion can be shortened. Therefore, the process window of heating conditions in manufacturing the structural member can be more easily secured.

[0018] In the blank according to any one of the first to third configurations, each of the plurality of steel sheets may be a plated steel sheet (fourth configuration).

[0019] In a fourth configuration, each steel sheet included in the blank is a plated steel sheet. In this case, when the blank is formed into a structural component by hot stamping, the formation of oxide scale can be suppressed. Therefore, after hot stamping, there is no need to subject the structural component to a process for removing oxide scale, such as shot blasting. This can improve the productivity of the structural component. Furthermore, since each steel sheet is a plated steel sheet, the corrosion resistance of the structural component can be more easily ensured.

[0020] In the blank according to the fourth configuration, the plating layer of each steel plate is an aluminum-based plating layer (fifth configuration).

[0021] When each steel sheet is a plated steel sheet having an aluminum-based plating layer, as in the fifth configuration, when the blank is heated during hot stamping, a difference in temperature rise rate is likely to occur, particularly between the overlap portion, which has the greatest sheet thickness, and the non-overlapping portion. Because the aluminum-based plating layer is nearly white, it easily reflects thermal energy and inhibits temperature rise in the overlap portion. However, even when each steel sheet is a plated steel sheet having an aluminum-based plating layer, by treating at least one of the outer surfaces of the overlap portion to increase the emissivity, temperature rise in the overlap portion can be accelerated when the blank is heated during hot stamping, thereby shortening the heating time of the overlap portion. Therefore, corrosion resistance in the plated steel sheet portion can be ensured, and the productivity of structural components manufactured from the blank can be improved.

[0022] In the blank according to any one of the third to fifth configurations, a coating may be formed on the surface of each of the first steel plate and the second steel plate located outside the overlap portion as a treatment for increasing emissivity. This coating may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. (sixth configuration).

[0023] In the blank according to any one of the third to fifth configurations, a coating may be formed on the surface of each of the first steel plate and the second steel plate located outside the overlap portion as a treatment for increasing emissivity. This coating comprises carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m ２ In this case, the content of carbon black in the coating can be X ＣＢ (g / m ２ ), the oxide content is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ may satisfy the following formula (1) (see Patent Document 1) (seventh configuration): 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )≦332.0 (1)

[0024] In the blank according to any one of the third to seventh configurations, the plurality of steel plates may include two or more steel plates having different plate thicknesses. In this case, the plate thickness of the overlap portion is t ｍａｘ , the thickness of the steel plate constituting the part with the smallest plate thickness in the blank is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ It may be ≦3.2 (eighth configuration).

[0025] When two or more steel sheets with different thicknesses are included in a blank, the difference in thickness between the overlap portion and the steel sheet constituting the portion with the smallest thickness in the blank becomes larger compared to when all the steel sheets included in the blank have the same thickness. If the difference in thickness is excessive, when a structural component is manufactured from the blank by hot stamping, the temperature rise in the overlap portion becomes slower compared to the steel sheet constituting the portion with the smallest thickness in the blank. Therefore, for example, when the thinnest steel sheet is a plated steel sheet, there is a possibility that alloying of the plated layer in the thinnest steel sheet may proceed excessively, making it more difficult to ensure the process window of the heating conditions. However, in the eighth configuration, the thickness t of the overlap portion ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ is set to 3.2 or less. This makes it easier to complete heating of the thickest overlap portion before alloying of the coating layer progresses excessively, even when two or more steel sheets with different thicknesses are included in the blank, and makes it easier to ensure the process window of the heating conditions.

[0026] In the blank according to the first configuration, the plurality of steel sheets may further include a third steel sheet. At least one of the first steel sheet and the second steel sheet, and the third steel sheet may each be a plated steel sheet having an aluminum-based plating layer as a plating layer on both surfaces of the base steel sheet. In this case, as a treatment for increasing the emissivity of the surface located outside the overlap portion compared to the emissivity of the surface of the third steel sheet, for example, the coating amount (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet of at least one of the first steel sheet and the second steel sheet may be ２ ) is 60 or less, and the coating weight (g / m ２ ) (ninth configuration).

[0027] A blank according to a ninth configuration includes a third steel sheet in addition to a first steel sheet and a second steel sheet. At least one of the first steel sheet and the second steel sheet and the third steel sheet are aluminum-based plated steel sheets. The coating weight of the aluminum-based plated layer on at least one of the first steel sheet and the second steel sheet is less than the coating weight of the aluminum-based plated layer on the third steel sheet and is 60 g / m ２ The following is true. By using a relatively thinly plated steel sheet for the first steel sheet and / or the second steel sheet, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron in the first steel sheet and / or the second steel sheet proceeds quickly, and both surfaces of the first steel sheet and / or the second steel sheet quickly change from silvery white to black or a color close to black. Therefore, during heating of the blank, the emissivity of the overlap portion formed by the first steel sheet and the second steel sheet is higher than that of the third steel sheet. This allows the overlap portion to be heated to the austenite temperature relatively quickly, thereby shortening the heating time of the overlap portion. Therefore, the heating of the blank required for hot stamping can be completed before excessive alloying of the aluminum-based plating layer in each steel sheet progresses and the diffusion layer becomes thick. As a result, the strength of the overlap portion can be ensured by hot stamping, and the corrosion resistance (rust resistance) of each steel sheet can be ensured.

[0028] In the blank according to the ninth configuration, the third steel sheet has a larger coating mass of the aluminum-based plating layer than at least one of the first steel sheet and the second steel sheet forming the overlap portion. That is, the third steel sheet has higher rust prevention performance than the first steel sheet and / or the second steel sheet. Therefore, the rust prevention performance of the structural member formed from the blank can be partially enhanced. For example, by disposing the third steel sheet in a portion of the structural member requiring relatively high rust prevention performance, the rust prevention performance required for the structural member as a whole can be ensured.

[0029] In the blank according to the ninth configuration, the plurality of steel plates may include two or more steel plates having different plate thicknesses. In this case, the plate thickness of the overlap portion is t ｍａｘ, the thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ It may be ≦3.0 (tenth configuration).

[0030] When two or more steel sheets with different thicknesses are included in a blank, the difference in thickness between the overlap portion and the non-overlap portion becomes larger compared to when all the steel sheets included in the blank have the same thickness. That is, the difference in thickness between the overlap portion having the largest thickness in the blank and the steel sheet having the smallest thickness becomes larger. If the difference in thickness is excessive, when a structural component is manufactured from the blank by hot stamping, the temperature rise in the overlap portion becomes slower than that of the steel sheet having the smallest thickness. Therefore, for example, when the thinnest steel sheet is a plated steel sheet, there is a possibility that alloying of the plated layer in the thinnest steel sheet may proceed excessively, making it difficult to ensure the process window of the heating conditions. Therefore, in the tenth configuration, when the first steel sheet and / or the second steel sheet forming the overlap portion are relatively thin plated steel sheets, the thickness t of the overlap portion is increased. ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ is set to 3.0 or less. This makes it easier to complete heating of the thickest overlap portion before alloying of the plating layer of each plated steel sheet progresses excessively, and makes it easier to ensure the process window of the heating conditions.

[0031] In the blank according to the ninth configuration, the plurality of steel plates may include two or more steel plates having different thicknesses. In this case, the thickness of the overlapping portion may be t ｍａｘ , the thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ Furthermore, a surface of at least one of the first steel plate and the second steel plate positioned outside the overlap portion may be coated with a black coating (eleventh configuration).

[0032] In the eleventh configuration, the first steel sheet and / or the second steel sheet forming the overlap portion are relatively thinly plated steel sheets, and at least one of the first steel sheet and the second steel sheet has a substantially black coating applied to the surface located outside the overlap portion. This allows the emissivity of the overlap portion to be increased in advance, and the temperature of the overlap portion can be increased more quickly when the blank is heated during hot stamping. Therefore, even if there is a large difference in thickness between the overlap portion having the maximum thickness in the blank and the steel sheet having the minimum thickness, the process window of the heating conditions can be easily ensured. For example, ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ Even if the value is increased to 4.0, the process window is easily secured.

[0033] A method for manufacturing a structural component according to an embodiment includes the steps of preparing a blank according to any one of the first to eleventh configurations, heating a plurality of steel plates included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming and quenching the heated blank using a mold (twelfth configuration).

[0034] A vehicle body structural member according to an embodiment can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate and a second steel plate. The second steel plate has an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. A coating is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. The coating contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of 0.001 g / m ２ or more (thirteenth configuration).

[0035] A vehicle body structural member according to another embodiment can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate and a second steel plate. The second steel plate has an end portion that is overlapped and joined to the end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. A coating is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. The coating contains carbon black at 0.500 g / m ２ The following is contained (fourteenth configuration).

[0036] According to yet another embodiment, a vehicle body structural member can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate, a second steel plate, and a third steel plate. The second steel plate has an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. At least one of the first steel plate and the second steel plate, and the third steel plate, are each plated steel plates having aluminum-based plating layers on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer on the third steel plate (fifteenth configuration).

[0037] In the structural member according to the fifteenth configuration, in the overlap portion, the maximum value of the Vickers hardness of the steel plate located on the surface side of the structural member out of the first steel plate and the second steel plate is HV ｍａｘ , the minimum value of Vickers hardness is HV ｍｉｎ When this is done, HV ｍａｘ -HV ｍｉｎ is HV ｍａｘ It may be 30% or less of the above (sixteenth configuration).

[0038] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0039] <First embodiment> [Structural member] Fig. 1 is an exploded perspective view of structural members 10, 20 according to this embodiment. The structural members 10, 20 are used in the body of an automobile or the like. In the example shown in Fig. 1, the structural members 10, 20 constitute a front under module of the vehicle body.

[0040] The structural member 10 is an upper module. That is, when assembled to the vehicle body, the structural member 10 is disposed above the structural member 20. The structural member 10 includes a pair of side frames 11L, 11R and at least one cross member 12. The side frames 11L, 11R and the cross member 12 each have an elongated shape.

[0041] The side frames 11L, 11R are arranged side by side in the left-right direction of the vehicle body when the structural member 10 is assembled to the vehicle body. The side frames 11L, 11R each extend in the front-to-rear direction of the vehicle body. Each of the side frames 11L, 11R includes a front portion 111 and a rear portion 112. The rear portion 112 is arranged behind the front portion 111 when the structural member 10 is assembled to the vehicle body.

[0042] When the structural member 10 is assembled to the vehicle body, the cross member 12 extends in the left-right direction of the vehicle body. The cross member 12 extends from the side frame 11L to the side frame 11R. The cross member 12 connects the side frames 11L, 11R. In the example shown in FIG. 1 , the cross member 12 connects the side frames 11L, 11R to each other at one end of the side frames 11L, 11R in the longitudinal direction. When the structural member 10 is assembled to the vehicle body, the cross member 12 is disposed, for example, at the rear end of the structural member 10. However, the cross member 12 may also connect the middle portions of the side frames 11L, 11R.

[0043] The structural member 20 is a lower module. That is, when assembled to the vehicle body, the structural member 20 is disposed below the structural member 10. The structural member 20 includes a pair of side frames 21L, 21R and at least one cross member 22. The side frames 21L, 21R and the cross member 22 each have an elongated shape.

[0044] The side frames 21L, 21R are arranged side by side in the left-right direction of the vehicle body when the structural member 20 is assembled to the vehicle body. The side frames 21L, 21R each extend in the front-rear direction of the vehicle body. Each of the side frames 21L, 21R includes a front portion 211 and a rear portion 212. The rear portion 212 is arranged behind the front portion 211 when the structural member 20 is assembled to the vehicle body.

[0045] The lower side frames 21L, 21R are joined to the upper side frames 11L, 11R, respectively. The side frames 21L, 21R form a closed cross section together with the side frames 11L, 11R. Figure 2 shows the closed cross section formed by the side frames 21L, 21R together with the side frames 11L, 11R, respectively. Hereinafter, when there is no need to particularly distinguish between the side frames 11L, 11R, the side frames 11L, 11R will be collectively referred to as side frames 11. Similarly, when there is no need to particularly distinguish between the side frames 21L, 21R, the side frames 21L, 21R will be collectively referred to as side frames 21.

[0046] 2 is a cross-sectional view (horizontal cross-section) of the side frames 11, 21 cut along a plane perpendicular to the longitudinal direction. In the example of FIG. 2, each of the side frames 11, 21 has a substantially hat-shaped transverse cross-section.

[0047] 2, the side frame 11 includes a top plate 113, vertical walls 114 and 115, and flanges 116 and 117. In a cross-sectional view of the side frame 11, one ends of the vertical walls 114 and 115 are connected by the top plate 113. In a cross-sectional view of the side frame 11, the other ends of the vertical walls 114 and 115 are connected to flanges 116 and 117, respectively. The flanges 116 and 117 protrude outward from the vertical walls 114 and 115, respectively.

[0048] The side frame 21 includes a top plate 213, vertical walls 214 and 215, and flanges 216 and 217. In a cross-sectional view of the side frame 21, one ends of the vertical walls 214 and 215 are connected by the top plate 213. In a cross-sectional view of the side frame 21, flanges 216 and 217 are connected to the other ends of the vertical walls 214 and 215, respectively. The flanges 216 and 217 protrude outward from the vertical walls 214 and 215, respectively.

[0049] The top plate 213 of the lower side frame 21 is disposed to face the top plate 113 of the upper side frame 11. In a cross-sectional view of the side frames 11, 21, the vertical walls 214, 215 of the side frame 21 extend from the top plate 213 toward the side frame 11. The flanges 216, 217 of the side frame 21 are joined to the flanges 116, 117 of the side frame 11, respectively. The flanges 216, 217 are joined to the flanges 116, 117 by, for example, spot welding. In the example of FIG. 2 , the flanges 116, 117 of the upper side frame 11 are directly joined to the flanges 216, 217 of the lower side frame 21. However, other members, such as a floor panel, may be provided between the side frame 11 and the side frame 21.

[0050] Returning to FIG. 1 , the cross member 22 extends in the left-right direction of the vehicle body when the structural member 20 is assembled to the vehicle body. The cross member 22 extends from the side frame 21L to the side frame 21R. The cross member 22 connects the side frames 21L and 21R. In the example shown in FIG. 1 , the cross member 22 connects the side frames 21L and 21R to each other at one longitudinal end of the side frames 21L and 21R. Like the upper cross member 12, the cross member 22 is disposed, for example, at the rear end of the structural member 20 when the structural member 20 is assembled to the vehicle body. However, the cross member 22 may also connect the middle portions of the side frames 21L and 21R. The cross member 22 may also be joined to the upper cross member 12 by, for example, spot welding.

[0051] The structural members 10 and 20 are hot-stamped members. That is, the structural member 10 is formed by hot stamping (hot press working) a blank formed from a plurality of steel plates (sub-blanks). Similarly, the structural member 20 is formed by hot stamping a blank formed from a plurality of steel plates.

[0052] In the upper structural member 10, for example, the side frames 11L, 11R may each be formed from a plurality of steel plates 31, 32. In each of the side frames 11L, 11R, for example, the rear portion 112 may be formed from a steel plate 31, and the front portion 111 may be formed from a steel plate 32. The steel plate 31 forming the rear portion 112 may be larger in at least one of plate thickness and tensile strength than the steel plate 32 forming the front portion 111. The cross member 12 may be formed mainly from a steel plate 33 that is different from the steel plates 31, 32 forming the side frames 11L, 11R. Adjacent steel plates 31, 32, 33 are joined by welding.

[0053] Similarly, in the lower structural member 20, for example, the side frames 21L, 21R may each be formed from a plurality of steel plates 41, 42. In each of the side frames 21L, 21R, for example, the rear portion 212 may be formed from the steel plate 41, and the front portion 211 may be formed from the steel plate 42. The steel plate 41 forming the rear portion 212 may be greater in at least one of thickness and tensile strength than the steel plate 42 forming the front portion 211. The cross member 22 may be formed mainly from a steel plate 43 that is different from the steel plates 41, 42 forming the side frames 21L, 21R. Adjacent steel plates 41, 42, 43 are joined together by welding.

[0054] 3A to 3G, a method for manufacturing the structural members 10 and 20 according to this embodiment will be described. The method for manufacturing the structural member 10 includes the steps of preparing a blank 30, heating the blank 30, and forming the heated blank 30 into the structural member 10. Similarly, the method for manufacturing the structural member 20 includes the steps of preparing a blank 40, heating the blank 40, and forming the heated blank 40 into the structural member 20.

[0055] (Preparation Process) As shown in Figure 3A, in the preparation process for manufacturing the upper structural member 10 (Figure 1), a blank 30 is prepared. The blank 30 has the shape of the structural member 10 when unfolded. The blank 30 includes a plurality of steel plates (sub-blanks) 31, 32, and 33. The steel plates 31, 32, and 33 are arranged and joined to form two long portions 34L and 34R and at least one connecting portion 35.

[0056] The long portions 34L, 34R are arranged side by side in the horizontal direction when viewed from above the blank 30. The long portion 34L is a portion of the blank 30 that corresponds to the side frame 11L ( FIG. 1 ). The long portion 34R is a portion of the blank 30 that corresponds to the side frame 11R ( FIG. 1 ). In the example of FIG. 3A , the long portions 34L, 34R are formed from steel plates 31, 32, respectively.

[0057] The connecting portion 35 connects the long portions 34L, 34R to each other. The connecting portion 35 is a portion of the blank 30 that corresponds to the cross member 12 ( FIG. 1 ). In the example of FIG. 3A , the connecting portion 35 includes a steel plate 33. The connecting portion 35 may further include a portion of the steel plate 31.

[0058] 3B and 3C are cross-sectional views of the blank 30 showing the joints between the steel plates 31, 32, and 33. FIGS. 3B and 3C are cross-sectional views taken along lines IIIB-IIIB and IIIC-IIIC in FIG. 3A, respectively. Referring to FIGS. 3B and 3C, in this embodiment, the steel plate 31 is lap-joined to the steel plate 32. The steel plate 31 is also lap-joined to the steel plate 33. That is, the end of the steel plate 31 is lap-joined to the end of the steel plate 32, thereby forming an overlap portion 361 together with the end of the steel plate 32. Similarly, the other end of the steel plate 31 is lap-joined to the end of the steel plate 33, thereby forming an overlap portion 362 together with the end of the steel plate 33. The steel plates 31, 32, and 33 are joined by, for example, spot welding or laser welding.

[0059] 3B and 3C, the steel plate 31 has a plate thickness t １ The steel plate 32 has a plate thickness t ２ The steel plate 33 has a plate thickness t ３ In the example of this embodiment, the steel plate 31 has a plate thickness t １ is the t of the steel plate 32 ２ and the thickness t of the steel plate 33 ３ The overlap portion 361 of the steel plates 31 and 32 has a maximum plate thickness t ｍａｘ The thickness t of the overlap portion 361 ｍａｘ is greater than the thickness of the overlap portion 362 of the steel plates 31 and 33.

[0060] In the example of this embodiment, among the plurality of steel plates 31, 32, and 33 included in the blank 30, the plate thickness t ３ Therefore, the steel plate 33 has a minimum thickness t ｍｉｎ That is, the part of the steel plate 33 that is not overlapped with the other steel plates 31 constitutes a part having the minimum plate thickness t ｍｉｎ and the thickness t ｍｉｎ is the thickness t of the steel plate 33 ３ is.

[0061] As in this embodiment, the plurality of steel plates 31, 32, and 33 have different plate thicknesses t １, t ２ , t ３ When having a maximum thickness t ｍａｘ and minimum plate thickness t ｍｉｎ Ratio of: t ｍａｘ / t ｍｉｎ In this case, the maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ For example, t ｍａｘ / t ｍｉｎ <4.0 Maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ is t ｍａｘ / t ｍｉｎ It is preferable that the maximum plate thickness t satisfies ≦3.2. ｍａｘ and minimum plate thickness t ｍｉｎ is t ｍａｘ / t ｍｉｎ The maximum plate thickness t may be ≧2.5. ｍａｘ is, for example, 4.6 mm or less. ｍａｘ may be 1.6 mm or more.

[0062] The steel plates 31 and 32 have a maximum plate thickness t ｍａｘ The surfaces 311 and 321 of the steel plates 31 and 32 located outside the overlap portion 361 are treated to increase the emissivity compared to at least one other surface of the steel plates 31, 32, and 33 included in the blank 30. In this embodiment, the emissivity of the surface 311 of the steel plate 31 and the surface 321 of the steel plate 32 is higher than the emissivity of at least one other surface even before the heating process. That is, the emissivity of the surface 311 of the steel plate 31 and the surface 321 of the steel plate 32 is higher than the emissivity of the surfaces 312 and 322 of the steel plates 31 and 32 located inside the overlap portion 361 and / or the emissivity of one or both surfaces of the other steel plate 33. The surfaces 311 and 321 of the steel plates 31 and 32 located outside the overlap portion 361 are treated to increase the emissivity compared to at least one other surface of the steel plates 31, 32, and 33 included in the blank 30. ｍａｘ These are the surfaces that constitute the front and back surfaces of the overlap portion 361 having the above structure.

[0063] In this embodiment, the emissivity of the surfaces 311 and 321 located outside the overlap portion 361 of the steel plates 31 and 32 is higher than the emissivity of the surfaces 312 and 322 located inside the overlap portion 361. In addition, the emissivity of the surfaces 311 and 321 located outside the overlap portion 361 of the steel plates 31 and 32 is higher than the emissivity of the surfaces 312 and 322 located inside the overlap portion 361 of the steel plates 31 and 32 when the steel plates 31 and 32 have a minimum plate thickness t ｍｉｎ The emissivity at 25°C of the surfaces 311 and 321 of the steel plates 31 and 32 is greater than the emissivity at both surfaces of the steel plate 33 constituting the portion having the overlapping portion 361. For example, the emissivity at 25°C of the surfaces 311 and 321 of the steel plates 31 and 32 at a wavelength of 8.0 μm is 60% or greater. The emissivity at 25°C of the surfaces 311 and 321 of the steel plates 31 and 32 at a wavelength of 8.0 μm is more preferably 70% or greater, and even more preferably 80% or greater. The emissivity at 25°C of at least one surface other than the surfaces 311 and 321 of the steel plates 31, 32, and 33 included in the blank 30 may be less than 60%. The difference in emissivity at 25°C of the surfaces 311 and 321 on both outer sides of the overlapping portion 361 and at least one surface other than the surfaces 311 and 321 at a wavelength of 8.0 μm is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity can be measured in accordance with JIS R 1801:2002. In this case, a sample taken from the steel sheet to be measured is placed in a Fourier transform infrared spectrophotometer, and the radiation intensity at a wavelength of 8.0 μm is measured at 25° C. to calculate the emissivity. Alternatively, it is also possible to measure the radiation intensity of a target area at 25° C. using a radiation thermometer set to a measurement wavelength of 8.0 μm, and calculate the emissivity from the ratio to the radiation intensity of a black body.

[0064] In this embodiment, a coating 50 is formed on the surfaces 311, 321 of the steel plates 31, 32 located outside the overlap portion 361 as a treatment to increase emissivity. The surface 311 of the steel plate 31 located opposite the steel plate 32 is, for example, entirely covered with the coating 50. Furthermore, the surface 321 of the steel plate 32 located opposite the steel plate 31 is, for example, entirely covered with the coating 50. On the other hand, the surfaces 312, 322 of the steel plates 31, 32 located inside the overlap portion 361 are not provided with the coating 50. Furthermore, the steel plate 33 is not provided with the coating 50. As a result, the emissivity of both outer surfaces of the overlap portion 361 in the blank 30 is higher than the emissivity of the other surfaces.

[0065] The coating 50 is, for example, a substantially black coating. For example, the lightness L ＊ Value (CIE 1976 lightness index L defined in JIS Z8781-4:2013) ＊ If the emissivity at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, and more preferably 80% or more, the coating 50 can be determined to be substantially black. The coating 50 may be a carbon-based surface treatment coating (a coating containing carbon (C)). The emissivity at a wavelength of 8.0 μm at 25°C of the coating 50 is 60% or more, preferably 70% or more, and more preferably 80% or more. That is, the emissivity at a wavelength of 8.0 μm at 25°C of the surfaces 311, 321 of the steel sheets 31, 32 to which the coating 50 is applied is 60% or more, preferably 70% or more, and more preferably 80% or more. The coating 50 may have an emissivity at a wavelength of 8.0 μm at 700°C of 60% or more. For example, the surface treatment coating described in Patent Document 1 can be used as the coating 50. Specifically, the coating 50 can contain carbon black and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating 50 may or may not contain silica. That is, the silica content of the coating 50 is 0 g / m ２ The silica content of the coating 50 is 0.30 g / m ２ The silica content may be more preferably 0.10 g / m or less. ２More preferably, it is 0.05 g / m or less. ２ The following is the result.

[0066] The carbon black and oxides can be dispersed throughout the entire surface of the coating 50 that is perpendicular to the thickness direction of the steel sheet 31. ＣＢ (g / m ２ ), the content of one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ It is preferable that the following formula (1) is satisfied: 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )≦332.0 (1)

[0067] In formula (1), the central formula: 24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )) is preferably 119.0 or more, more preferably 170.0 or more, and even more preferably 220.0 or more. The value calculated by the middle formula is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

[0068] The dispersion state of the carbon black and metal oxide in the coating 50 can be confirmed by performing an area analysis of the coating 50 with an electron probe micro analyzer (EPMA) for elements derived from the carbon black (e.g., C) and elements derived from the oxide (Zr, Zn, and Ti). ＣＢcan be measured by cross-sectional analysis of the coating 50 using a transmission electron microscope (TEM). That is, a cross-sectional analysis of the coating 50 is performed by TEM-EDS analysis on an area of ​​a predetermined size (thickness of the coating 50 × 5 μm), and the thickness of the coating 50 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is defined as ρ (ton / m ３ ), the film thickness is d (μm), and the area ratio is a (%), the value expressed by ρ×d×a is the carbon black content X ＣＢ (g / m ２ ) The oxide content X Ｏｘｉｄｅ can be determined by performing elemental analysis on the surface of the coating 50 using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation) and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0069] Carbon black content X in the coating 50 ＣＢ is 0.030 g / m ２ It is preferable that the content is 0.100 g / m or more. ２ It is more preferable that the content X is equal to or greater than this. ＣＢ is set within a range that satisfies the formula (1), but is preferably 0.800 g / m ２ or less, more preferably 0.600 g / m ２ The following is the result.

[0070] The coating 50 may contain 5.0% or more carbon black by volume, preferably 8.0% or more carbon black by volume, and may contain 40.0% or less carbon black by volume, preferably 30.0% or less carbon black by volume.

[0071] Metal oxide content X in the coating 50 Ｏｘｉｄｅ is 0.030 g / m ２ It is preferable that the content is 0.060 g / m or more. ２ It is more preferable that the content X is equal to or greater than this. Ｏｘｉｄｅ is set within a range that satisfies the formula (1), but is preferably 0.500 g / m２ More preferably, it is 0.300 g / m or less. ２ The following is the result.

[0072] The coating 50 can contain 1.0% or more metal oxide by volume. The coating 50 can also contain 30.0% or less metal oxide by volume, and preferably contains 25.0% or less metal oxide by volume.

[0073] Carbon black content X ＣＢ (g / m ２ ) and the metal oxide content X Ｏｘｉｄｅ (g / m ２ ) ratio: X Ｏｘｉｄｅ / X ＣＢ is preferably 0.20 or more and 200.00 or less. Ｏｘｉｄｅ / X ＣＢ is more preferably 0.40 or more and 10.00 or less, and further preferably 0.60 or more and 5.00 or less.

[0074] The coating 50 may contain various binder components and additives in addition to the carbon black and metal oxides described above.

[0075] The binder component is preferably a water-dispersible or water-soluble resin. The content of the binder component is preferably 40% by volume or more relative to the total volume of the coating 50. As the binder component selected from water-dispersible or water-soluble resins, various known resins exhibiting water dispersibility or water solubility can be used. Examples of such water-dispersible or water-soluble resins include polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluororesins, polyamide resins, polyolefin resins, and polymer compounds obtained by hydrolysis and condensation polymerization of silane coupling agents. It is more preferable that the binder component be one or more selected from the group consisting of polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, fluororesins, and polyamide resins. When a polyurethane resin is used as the binder component, the polyurethane resin is preferably a polyether-based polyurethane resin.

[0076] Examples of additives include leveling agents, water-soluble solvents, metal stabilizers, and etching inhibitors. The leveling agent is, for example, a nonionic or cationic surfactant. Examples of nonionic or cationic surfactants include polyethylene oxide or polypropylene oxide adducts, acetylene glycol compounds, and the like. Examples of water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol, and propylene glycol; cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether; esters such as ethyl acetate and butyl acetate; and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of metal stabilizers include chelate compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). Examples of etching inhibitors include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine.

[0077] The coating 50 can be formed by applying an organic or inorganic treatment liquid containing, for example, carbon black and a metal oxide to the entire surfaces 311, 321 of the steel sheets 31, 32, and then drying the volatile components in the treatment liquid. The treatment liquid can be applied to the surfaces 311, 321 of the steel sheets 31, 32, for example, by a roll coater, a curtain coater, or an inkjet. In the case of inkjet application, the thickness of the coating 50 can also be changed continuously. The thickness of the coating 50 is, for example, 0.5 μm or more and 5.0 μm or less. The thickness of the coating 50 is preferably 1.0 μm or more and 3.0 μm or less. The thickness of the coating 50 is determined by the thickness t of the steel sheets 31, 32. １ , t ２ Therefore, the thicknesses of the steel plates 31 and 32 measured including the coating 50 are respectively defined as the thicknesses t １ , t ２ can be treated as

[0078] At least one of the steel sheets 31, 32, and 33 included in the blank 30 is a plated steel sheet. ｍｉｎThe steel sheet 33 having the above-mentioned characteristic may be a plated steel sheet. In this case, the steel sheet 33 has a base steel sheet 33a and a plated layer 33b. The type of the base steel sheet 33a is not particularly limited. The plated layer 33b is provided on the base steel sheet 33a. The plated layer 33b covers the entire or almost the entire both sides of the base steel sheet 33a. The plated layer 33b is a metal plated layer. The plated layer 33b may be, for example, hot-dip aluminum plating, hot-dip galvanneal plating, or electrogalvanized plating. As the steel sheet 33, a known aluminum-plated steel sheet, zinc-plated steel sheet, etc. may be used. The thickness t of the steel sheet 33 ３ (t ｍｉｎ ) is the combined thickness of the base steel sheet 33a and the plating layer 33b. ３ is the average thickness of the steel plate 33.

[0079] The steel sheets 31 and 32 may be known plated steel sheets, similar to the steel sheet 33. That is, the steel sheet 31 may have a base steel sheet 31a and a plated layer 31b provided on the base steel sheet 31a. The steel sheet 32 ​​may have a base steel sheet 32a and a plated layer 32b provided on the base steel sheet 32a. The type of the base steel sheets 31a and 32a is not particularly limited. The plated layers 31b and 32b cover the entire or almost the entire surfaces of the base steel sheets 31a and 32a, respectively. The plated layers 31b and 32b are metal plated layers. The plated layers 31b and 32b may be the same as those exemplified for the steel sheet 33. That is, the steel sheets 31 and 32 may be aluminum plated steel sheets or zinc plated steel sheets, similar to the steel sheet 33. When the steel sheet 31 is a plated steel sheet, the thickness t of the steel sheet 31 may be 0.05 mm or less. １ Similarly, when the steel sheet 32 ​​is a plated steel sheet, the thickness t ２ is the total thickness of the base steel sheet 32a and the plating layer 32b. １ , t ２ are the average thicknesses of the steel plates 31 and 32, respectively.

[0080] The plating layers 31b, 32b, and 33b are typically plating layers containing aluminum as a main component (aluminum-based plating layers). The configuration of the aluminum-based plating layers is not particularly limited. Known aluminum-based plating layers can be used as the plating layers 31b, 32b, and 33b.

[0081] The steel sheets 31, 32, and 33 may be plated steel sheets of the same type as the other steel sheets, or may be plated steel sheets of a different type from the other steel sheets. At least one of the steel sheets 31, 32, and 33 may be a steel sheet (bare material) that does not have a plated layer on its surface. When two or more of the steel sheets 31, 32, and 33 are plated steel sheets, the coating weight (g / m) of each steel sheet is ２ ) may be the same as or different from the other steel plates.

[0082] As shown in Figure 3D, in manufacturing the lower structural member 20 (Figure 1), a blank 40 is prepared in the preparation step. The blank 40 has the shape of the structural member 20 when unfolded. The blank 40 includes a plurality of steel plates (sub-blanks) 41, 42, and 43. The steel plates 41, 42, and 43 are arranged and joined to form two long portions 44L and 44R and at least one connecting portion 45.

[0083] The long portions 44L, 44R are arranged side by side in the horizontal direction when viewed from above the blank 40. The long portion 44L is a portion of the blank 40 that corresponds to the side frame 21L ( FIG. 1 ). The long portion 44R is a portion of the blank 40 that corresponds to the side frame 21R ( FIG. 1 ). In the example of FIG. 3D , the long portions 44L, 44R are formed from steel plates 41, 42, respectively.

[0084] The connecting portion 45 connects the long portions 44L, 44R to each other. The connecting portion 45 is a portion of the blank 40 that corresponds to the cross member 22 ( FIG. 1 ). In the example of FIG. 3D , the connecting portion 45 includes a steel plate 43. The connecting portion 45 may further include a portion of the steel plate 41.

[0085] The blank 40 has the same configuration as the blank 30 (FIGS. 3B and 3C) with respect to the steel plates 41, 42, and 43. That is, the configurations of the steel plates 31, 32, and 33 of the blank 30 (FIGS. 3B and 3C) can be applied directly to the steel plates 41, 42, and 43. Therefore, detailed description of the configurations of the steel plates 41, 42, and 43 will be omitted.

[0086] (Heating Step) The prepared blanks 30, 40 are formed into the structural members 10, 20 (FIG. 1) by hot stamping. During the hot stamping, the blanks 30, 40 are subjected to a heating step. Referring to FIG. 3E, in the heating step, the blank 30 is heated, for example, by a heating furnace. The plurality of steel plates 31, 32, 33 included in the blank 30 are heated to an austenite transformation completion temperature (A ｃ３ The steel plates 31, 32, and 33 are heated to a temperature above the austenite transformation completion temperature (A ). The steel plates 31, 32, and 33 are heated to, for example, 900°C or higher. This causes the microstructures of the steel plates 31, 32, and 33 to transform into an austenite phase. Although not shown, the plurality of steel plates 41, 42, and 43 ( FIG. 3D ) included in the blank 40 are also heated to a temperature above the austenite transformation completion temperature (A ). ｃ３ It is heated above this temperature (point).

[0087] (Forming Process) Referring to Fig. 3F, in the forming process, the heated blank 30 is formed into the structural member 10 (Fig. 1) using a die 60 and quenched. The blank 30 heated in the heating process is removed from the heating furnace and transported to the die 60. The die 60 may be attached to a known press device. The die 60 includes, for example, a punch 61 and a die 62. The blank 30 is placed between the punch 61 and the die 62.

[0088] 3G , after the blank 30 is placed between the punch 61 and the die 62, the die 62 moves relatively close to the punch 61. The blank 30 is clamped (pressed) between the punch 61 and the die 62 and formed into a shape that conforms to the forming surfaces of the punch 61 and the die 62. The blank 30 remains clamped between the punch 61 and the die 62. The blank 30 is cooled (quenched) by the die 60, and its microstructure is transformed into martensite. In this way, the structural member 10 can be manufactured from the blank 30.

[0089] Although not shown, the blank 40 shown in Fig. 3D is also subjected to the same forming process as the blank 30. That is, using a die, the heated blank 40 is formed into the structural member 20 (Fig. 1) and quenched. The structural member 20 is joined to the structural member 10 (Fig. 1) by, for example, welding.

[0090] FIG. 4 is a cross-sectional view of the structural component 10 after hot stamping. FIG. 4 shows a cross-section of the structural component 10 at the position of the steel plate 32 ( FIG. 3B ) to which the black coating 50 was applied at the blank 30 stage. In the example of FIG. 4 , the structural component 10 includes a coating 13. The coating 13 is provided on the steel plate 32. The black coating 50 ( FIG. 3B ) applied to the steel plate 32 in the blank 30 becomes the coating 13 after hot stamping. Although not shown, the black coating 50 ( FIG. 3B ) applied to the steel plate 31 in the blank 30 also becomes the coating 13 after hot stamping. The coating 13 is provided on surfaces 311 and 321 ( FIG. 3B ) located outside the overlap portion 361 of each of the steel plates 31 and 32. If the coating 50 before hot stamping contains carbon black, this carbon black is almost entirely eliminated by the high-temperature heating during hot stamping, but some carbon black may remain. When the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping may contain no carbon black or may contain 0.500 g / m ２ However, when the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping may contain 0.500 g / m ２It is preferable that the coating 13 contains the following carbon black: When the coating 13 after hot stamping contains carbon black, the content of carbon black in the coating 13 is 0 g / m ２ More preferably, it is greater than 0.001 g / m ２ That's all. If the coating 50 ( FIG. 3B ) is provided on the surfaces 311, 321 of the steel sheets 31, 32 before hot stamping so that the coating 13 contains carbon black after hot stamping, carbon black will be present in the overlap portion 361 even in the later stages of the heating process, and the emissivity of the overlap portion 361 will be ensured, making the overlap portion 361 more likely to be heated even in the later stages of the heating process. Furthermore, if the steel sheets 31, 32 are plated steel sheets, the coating 13 after the heating process containing carbon black will suppress adhesion of the plated layers 31b, 32b ( FIG. 3B ) to the mold 60 in the forming process (hot stamping), thereby reducing the coefficient of friction between the steel sheets 31, 32 and the mold 60 ( FIGS. 3F and 3G ). If the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping will satisfy the central formula: 24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )} is, for example, 120.0 or more and 150.0 or less.

[0091] When the coating 50 (FIG. 3B) before hot stamping satisfies the above formula (1), the coating 13 after hot stamping contains one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of, for example, 0 g / m ２ More preferably, 0.001 g / m ２ The metal oxide content of the coating 13 is, for example, 0.500 g / m ２ In this way, when the metal oxide remains on the structural member 10, that is, when the coating 13 is 0 g / m or less, ２ When the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping has a metal oxide content of 0 to 0.30 g / m or more, which is more desirable because it improves the corrosion resistance of the structural member 10. ２ Contains silica.

[0092] The carbon black content, metal oxide content, and silica content in the coating 13 can be measured in the same manner as for the coating 50 ( FIG. 3B ) at the blank 30 stage. Specifically, a vehicle body part is disassembled to obtain a structural member 10, and an analytical sample is obtained from the structural member 10, for example, by laser cutting. For example, an analytical sample is obtained from each of the multiple steel plates included in the structural member 10. The analytical sample is obtained, for example, from the center or its vicinity of the top plate of each steel plate having an open cross section. The obtained analytical sample is adjusted by polishing the cross section to outside the heat-affected zone during laser cutting, to prepare a coating analysis sample. Surface analysis of the coating 13 for elements derived from carbon black (e.g., C) and elements derived from oxides (Zr, Zn, and Ti) can be performed using EPMA to confirm the dispersion state of carbon black and metal oxide in the coating 13. Because the coating 13 is present on the front and / or back side of the structural member 10 depending on the location, both the front and back sides of the analytical sample are analyzed.

[0093] In many cases, the outermost surface layer of the structural member 10 is, for example, an electrodeposition coating film. In such cases, the coating layer that is below the electrodeposition coating film layer and above the alloyed metal plating layer is analyzed. The carbon black content X in the coating 13 is ＣＢ can be measured by cross-sectional analysis of the coating 50 using a TEM. That is, a cross-sectional analysis of the coating 13 is performed by TEM-EDS analysis on an area of ​​a predetermined size (thickness of the coating 13 × 5 μm), and the thickness of the coating 13 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is defined as ρ (ton / m ３ ), the film thickness is d (μm), and the area ratio is a (%), the value expressed by ρ×d×a is the carbon black content X ＣＢ (g / m ２ ) The oxide content X Ｏｘｉｄｅ can be determined by performing elemental analysis of the coating layer that is present below the electrodeposition coating layer and above the alloyed metal plating layer using the above-mentioned X-ray fluorescence analyzer, and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0094] After the forming process (hot stamping), the steel sheets 31, 32, and 33 (FIG. 1) can have a tensile strength of, for example, 0.5 GPa or more, and preferably 1.0 GPa or more. At least one of the steel sheets 31, 32, and 33 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of the steel sheets 31, 32, and 33 may be the same as or different from the tensile strength of the other steel sheets.

[0095] Although not shown, the lower structural member 20 (FIG. 1) may also include the coating 13 similar to that of the upper structural member 10 after hot stamping. ｍａｘ The overlapping portions are disposed on both outer surfaces of the overlapping portions.

[0096] [Effect] When the blank 30 is heated during hot stamping, the minimum plate thickness t ｍｉｎ In the steel sheet 33 having the above-mentioned thickness, iron diffuses from the base steel sheet 33a to the plating layer 33b, forming a diffusion layer. The diffusion layer becomes thicker as the heating time of the blank 30 becomes longer, which may reduce the corrosion resistance or weldability of the steel sheet 33 due to the plating layer 33b. In order to ensure the corrosion resistance and weldability of the steel sheet 33, it is preferable that the thickness of the diffusion layer be kept below a predetermined value. For example, when the steel sheet 33 has a plating layer 33b with a weight of 50 to 80 g / m ２ In the case of a steel sheet having a thickness of about 15 μm, the thickness of the diffusion layer is preferably 15 μm or less. ２ In the case of a thin steel sheet, the thickness of the diffusion layer is preferably 10 μm or less. ｍａｘThe emissivity of the surfaces 311, 321 of the steel sheets 31, 32 located outside the overlap portion 361 having a thickness of 1000 Å is higher than the emissivity of at least one other surface of the steel sheets 31, 32, 33. That is, the surfaces 311, 321 on both sides of the overlap portion 361 of the steel sheets 31, 32 are treated to increase the emissivity compared to at least one other surface. This allows the heating rate of the overlap portion 361 to be increased when the blank 30 is heated during hot stamping, thereby shortening the heating time of the overlap portion 361. Therefore, for example, in the thinnest steel sheet 33 whose temperature is raised first, excessive alloying of the plating layer 33b progresses, and heating of the blank 30 can be terminated before the diffusion layer grows to a thickness exceeding a predetermined thickness. As a result, the maximum sheet thickness t ｍａｘ The strength of the overlap portion 361 having the above-mentioned structure can be ensured by hot stamping, and the corrosion resistance (rust prevention) of the structural member 10 can be ensured. Furthermore, by reducing the thickness of the diffusion layer, the weldability of the steel plate 33 can be more easily ensured.

[0097] As described above, according to the blank 30 of this embodiment, the maximum plate thickness t ｍａｘ Therefore, it is possible to heat the entire blank 30 to the temperature required for hot stamping before the alloying of the plating layer 33b in the thinnest steel sheet 33 progresses excessively, and it is possible to secure a process window for the heating conditions in the manufacture of the structural member 10. Therefore, it is possible to increase the temperature rise rate of the overlap portion 361 having the maximum sheet thickness t ｍａｘ This allows the manufacture of a structural member 10 that combines the strength of the overlap portion 361 and the rust prevention function of the plating layer 33b. Furthermore, by increasing the temperature rise rate of the overlap portion 361, the heating time of the blank 30 can be shortened, thereby improving the productivity of the structural member 10. Furthermore, by shortening the heating time of the blank 30, energy consumption in the manufacture of the structural member 10 is reduced, and the amount of greenhouse gases generated during heating can be reduced.

[0098] In this embodiment, the steel sheets 31, 32, and 33 included in the blank 30 may all be plated steel sheets. In this case, when the blank 30 is formed into the structural member 10 by hot stamping, the generation of oxide scale can be suppressed. Therefore, after hot stamping, there is no need to subject the structural member 10 to a process for removing oxide scale, such as shot blasting. This allows for increased productivity of the structural member 10.

[0099] For example, when the plating layers 31b, 32b, and 33b of the steel sheets 31, 32, and 33 are aluminum-based plating layers, the maximum plate thickness t ｍａｘ The overlap portion 361 has a minimum plate thickness t ｍｉｎ The aluminum-based plating layer is white, which tends to reflect heat energy and inhibit the temperature rise of the overlap portion 361. However, in the blank 30 according to the present embodiment, the surfaces 311, 321 of the steel sheets 31, 32 arranged on both sides of the overlap portion 361 are treated to increase the emissivity. Therefore, even if the steel sheets 31, 32 forming the overlap portion 361 are plated steel sheets having an aluminum-based plating layer, the temperature rise of the overlap portion 361 in the heating process can be promoted and the heating time of the overlap portion 361 can be shortened. Therefore, the corrosion resistance and weldability of the thin-walled steel sheet 33 can be ensured, and the productivity of the structural member 10 can be improved.

[0100] In this embodiment, in the heating process, first, the blank 30 is heated to the minimum thickness t ｍｉｎ The steel plate 33 having a thickness of t reaches the temperature in the austenite region, and then the temperature in the austenite region is reached in order from the thinner portion of the plate. ｍａｘ In the present embodiment, when the steel plates 31, 32, and 33 included in the blank 30 have different thicknesses, the overlap portion 361 having the minimum thickness t ｍｉｎ and maximum plate thickness t ｍａｘ If there is too much difference between the thickness t ｍｉｎ For a steel plate 33 having a plate thickness t ｍａｘThe temperature rise of the overlap portion 361 having the minimum thickness t ｍｉｎ and maximum plate thickness t ｍａｘ Ratio to: t ｍａｘ / t ｍｉｎ is preferably 3.2 or less. This allows the overlap portion 361 to be sufficiently heated until the phase transformation to austenite is completed before the alloying of the plating layer 33b of the thinnest steel sheet 33 progresses and the diffusion layer becomes thicker, causing a loss of corrosion resistance, even if the steel sheets 31, 32, and 33 have different thicknesses. This makes it easier to ensure a process window in the manufacture of the structural member 10.

[0101] In this embodiment, in order to increase the emissivity of the overlap portion 361, a coating 50 can be applied to the surfaces 311, 321 of the steel sheets 31, 32 opposite the mating steel sheet. The emissivity of the coating 50 (at a temperature of 25°C and a wavelength of 8.0 μm) is, for example, 60% or more. This allows efficient radiative heating of the overlap portion 361, making it easier to increase the temperature rise rate of the overlap portion 361 during the heating process.

[0102] In this embodiment, the coating 50 is made of a material containing carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a carbon black content of 0 to 0.30 g / m ２ The carbon black content X may be the following: ＣＢ (g / m ２ ), and the oxide content X Ｏｘｉｄｅ (g / m ２ ) preferably satisfies the above formula (1). As described in Patent Document 1, formula (1) is a function of the ratio (%) of the increase in the temperature rise rate (°C / s) and the carbon black content X ＣＢ and oxide content X ＯｘｉｄｅFormula (1) defines the relationship between the carbon black and the oxide. Formula (1) indicates that carbon black primarily functions as a heat absorbing material in the temperature range up to 700°C, and that oxide primarily functions as a heat absorbing material in the temperature range of 700°C or higher. When coating 50 satisfies formula (1), the emissivity at a wavelength of 8.0 μm at 25°C tends to be 60% or higher for both outer surfaces 311, 321 of overlap portion 361 to which coating 50 is applied.

[0103] The carbon black and oxides can be dispersed throughout the entire surface of the coating 50 that is perpendicular to the thickness direction of the steel sheet 31. This facilitates uniform emissivity of the surfaces 311, 321 of the steel sheets 31, 32 located on both outer sides of the overlap portion 361. Therefore, the overlap portion 361, which is the thickest, can be heated quickly and uniformly in the heating process.

[0104] However, the configuration of the coating 50 is not limited thereto. The coating 50 may be a substantially black coating in order to increase the emissivity of both outer surfaces 311 and 321 of the overlap portion 361 compared to an untreated coating. For example, the coating 50 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating 50 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 to 50 in order to increase the emissivity of both outer surfaces 311 and 321 of the overlap portion 361. The compound having a hexagonal crystal structure is typically graphite (C), but may also be lanthanum silicate, magnesium diboride, beryllium oxide (beryllia), zinc oxide, β-quartz, goethite (NiS), wurtzite (ZnS), or the like.

[0105] The lower-side structural member 20 and blank 40 have the same configuration as the upper-side structural member 10 and blank 30. Therefore, the lower-side structural member 20 and blank 40 can achieve the same effects as those described above.

[0106] Second Embodiment [Blank] Figures 5A and 5B are cross-sectional views of a blank 30A according to a second embodiment. Figures 5A and 5B are cross-sectional views of the blank 30 according to the first embodiment, corresponding to Figures 3B and 3C. In the first embodiment, the blank 30 for the structural member 10 has a maximum plate thickness t ｍａｘ The surfaces 311 and 321 located outside the overlap portion 361 are treated so that the emissivity of these surfaces 311 and 321 is higher than that of the other surfaces before the heating process. In the first embodiment, for example, a coating 50 for increasing the emissivity is formed on the surfaces 311 and 321 on both outer sides of the overlap portion 361 ( FIG. 3B ). On the other hand, in the present embodiment, the surfaces 311 and 321 of the blank 30A are treated so that the emissivity of the surfaces 311 and 321 located outside the overlap portion 361 is higher than that of the other surfaces during the heating process.

[0107] 5A and 5B , in this embodiment, the steel sheets 31, 32, and 33 are each plated steel sheets. More specifically, the steel sheets 31, 32, and 33 are all aluminum-plated steel sheets. That is, the plating layers 31b, 32b, and 33b of the steel sheets 31, 32, and 33 are aluminum-based plating layers. In the steel sheet 31, the aluminum-based plating layer 31b covers both surfaces of the base steel sheet 31a. The aluminum-based plating layer 31b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 31a. Similarly, in the steel sheet 32, the aluminum-based plating layer 32b covers both surfaces of the base steel sheet 32a. The aluminum-based plating layer 32b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 32a. Furthermore, in the steel sheet 33, the aluminum-based plating layer 33b covers both surfaces of the base steel sheet 33a. The aluminum-based plating layer 33b is provided over the entire or almost the entire surface of both surfaces of the base steel sheet 33a.

[0108] The chemical composition of the aluminum-based plating layers 31b, 32b, 33b is not particularly limited. As in the first embodiment, known aluminum-based plating layers (plating layers containing aluminum as the main component) can be used as the aluminum-based plating layers 31b, 32b, 33b. Although not particularly limited, the aluminum-based plating layers 31b, 32b, 33b are, for example, Al-Si-based plating layers. The aluminum-based plating layers 31b, 32b, 33b may be the same as or different from the aluminum-based plating layers of the other steel sheets.

[0109] The steel plates 31 and 32 have a maximum plate thickness t ｍａｘ The overlap portion 361 has a coating weight W1 (g / m ２ ) is the coating weight W3 (g / m) of the aluminum-based plating layer 33b on the other steel plate 33. ２ ) is smaller than the coating weight W2 (g / m ２ ) is also smaller than the coating weight W3 of the aluminum-based plating layer 33b on the other steel plates 33.

[0110] In the steel sheet 31, the coating weight W1 is the coating weight of the aluminum-based plating layer 31b on both surfaces of the base steel sheet 31a, and is the average coating weight on both surfaces of the base steel sheet 31a. Usually, the coating weight (g / m) of the aluminum-based plating layer 31b on one surface of the base steel sheet 31a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer 31b on the other surface of the base steel sheet 31a. ２ ) is substantially equal to the coating weight (g / m) of the aluminum-based plating layer 31b on one surface of the base steel sheet 31a. However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 31b may vary between the front and back surfaces of the base steel sheet 31a. In other words, the coating weight of the aluminum-based plating layer 31b may be different between one surface and the other surface of the base steel sheet 31a. Similarly, in the steel sheet 32, the coating weight W2 is the coating weight of the aluminum-based plating layer 32b on both surfaces of the base steel sheet 32a, and is the average coating weight on both surfaces of the base steel sheet 32a. Usually, the coating weight (g / m) of the aluminum-based plating layer 32b on one surface of the base steel sheet 32a is ２) is the coating weight (g / m) of the aluminum-based plating layer 32b on the other surface of the base steel sheet 32a. ２ ) is substantially equal to the coating weight (g / m) of the aluminum-based plating layer 32b on one surface of the base steel sheet 32a. However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 32b may vary between the front and back surfaces of the base steel sheet 32a. In other words, the coating weight of the aluminum-based plating layer 32b may be different between one surface and the other surface of the base steel sheet 32a. In addition, in the steel sheet 33, the coating weight W3 is the coating weight of the aluminum-based plating layer 33b on both surfaces of the base steel sheet 33a, and is the average coating weight on both surfaces of the base steel sheet 33a. Usually, the coating weight (g / m) of the aluminum-based plating layer 33b on one surface of the base steel sheet 33a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer 33b on the other surface of the base steel sheet 33a. ２ ) However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 33b may vary between the front and back surfaces of the base steel sheet 33a. In other words, the coating weight of the aluminum-based plating layer 33b may be different between one surface and the other surface of the base steel sheet 33a.

[0111] The coating weights W1 and W2 of the aluminum-based plating layers 31b and 32b on the steel sheets 31 and 32 are 60 g / m ２ and preferably 50 g / m ２ The adhesion amounts W1 and W2 are, for example, 10 g / m ２ or more, preferably 20 g / m ２ The coating weight W3 of the aluminum-based plating layer 33b on the steel plate 33 is, for example, 20 g / m ２ 120g / m or more ２ W3 is preferably 30 g / m ２ More preferably, 35 g / m ２ However, the adhesion amount W3 is greater than the adhesion amounts W1 and W2. The adhesion amount W3 is preferably 115 g / m ２ More preferably, it is 100 g / m or less. ２ The difference between the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 and the coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33, i.e., W3-W1, is, for example, 20 (g / m２ ) or more. W3-W1 is 80 (g / m ２ Similarly, the difference W3-W2 between the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​and the coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 may be, for example, 20 (g / m ２ ) or more. W3-W2 is 80 (g / m ２ ) or less.

[0112] As described above, in the steel sheet 31, the coating weight of the aluminum-based plating layer 31b may vary between the front and back of the base steel sheet 32a. That is, in the steel sheet 31, the coating weight of the aluminum-based plating layer 31b may differ between the surfaces 311 and 312. In this case, in the steel sheet 31, it is preferable that the coating weight of the aluminum-based plating layer 31b on the surface 311 located outside the overlap portion 361 is smaller than the coating weight of the aluminum-based plating layer 31b on the surface 312 located inside the overlap portion 361. The difference between the coating weight of the aluminum-based plating layer 31b on the surface 311 and the coating weight of the aluminum-based plating layer 31b on the surface 312 is, for example, 1 g / m ２ The difference between the coating weight of the aluminum-based plating layer 31b on the surface 311 and the coating weight of the aluminum-based plating layer 31b on the surface 312 is 10 g / m ２ It may be the following:

[0113] Similarly, in the steel sheet 32, the coating weight of the aluminum-based plating layer 32b may vary between the front and back of the base steel sheet 32a. That is, in the steel sheet 32, the coating weight of the aluminum-based plating layer 32b may differ between the surfaces 321 and 322. In this case, in the steel sheet 32, it is preferable that the coating weight of the aluminum-based plating layer 32b on the surface 321 located outside the overlap portion 361 is smaller than the coating weight of the aluminum-based plating layer 32b on the surface 322 located inside the overlap portion 361. The difference between the coating weight of the aluminum-based plating layer 32b on the surface 321 and the coating weight of the aluminum-based plating layer 32b on the surface 322 is, for example, 1 g / m ２ The difference between the coating weight of the aluminum-based plating layer 32b on the surface 321 and the coating weight of the aluminum-based plating layer 32b on the surface 322 is 10 g / m２ It may be the following:

[0114] The method for forming the aluminum-based plating layers 31b, 32b, and 33b on the base steel sheets 31a, 32a, and 33a, respectively, is not particularly limited, but may be, for example, a general hot-dip plating method. That is, by immersing the base steel sheet 31a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 31 having an adjusted coating weight W1 of the aluminum-based plating layer 31b can be obtained. Similarly, by immersing the base steel sheet 32a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 32 ​​having an adjusted coating weight W2 of the aluminum-based plating layer 32b can be obtained. Furthermore, by immersing the base steel sheet 33a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 33 having an adjusted coating weight W3 of the aluminum-based plating layer 33b can be obtained. When an aluminum-based plating layer is formed by hot-dip plating, an Al—Fe-based alloy layer is formed at the interface between the base steel sheet and the aluminum-based plating layer due to the elution of Fe during the hot-dip plating process.

[0115] The coating weights W1, W2, and W3 of the aluminum-based plating layers 31b, 32b, and 33b can be measured, for example, by the sodium hydroxide-hexamethylenetetramine-hydrochloric acid stripping weight method described in JIS G 3314:2019. Specifically, in accordance with JIS G 3314:2019, a predetermined surface area S (mm ２ ) (for example, 50 mm × 50 mm) and measure the weight w1 (g) of each test piece. Then, each test piece is immersed in an aqueous sodium hydroxide solution, and after confirming that foaming caused by dissolution of the plating has subsided, each test piece is removed from the aqueous sodium hydroxide solution and rinsed with water. Next, each test piece that is still wet after rinsing with water is immersed in an aqueous hydrochloric acid solution containing added hexamethylenetetramine until foaming caused by dissolution of the plating has subsided. The test piece is removed from the hexamethylenetetramine-hydrochloric acid solution, immediately rinsed with water, dried, and the weight w2 (g) of the test piece is measured again. The coating weight W (g / m) of the aluminum-based plating layer of each test piece is measured.２ ) is {(w1-w2) / S}×10 ６ The average value of the coating weight W of five or more test pieces taken from each steel sheet is taken as the coating weight of the aluminum-based plating layer on that steel sheet.

[0116] However, when the size of the test specimens taken from each of the steel sheets 31, 32, and 33 is small, the cross section of each of the aluminum-based plating layers 31b, 32b, and 33b can be observed with an optical microscope (area: 100 μm × 100 μm), and the thickness (μm) of the plating layer can be measured in three fields of view in the same manner. The average value of the thicknesses measured in the three fields of view can then be tripled to convert it into a coating weight. In this case, the coating weights of the aluminum-based plating layers 31b, 32b, and 33b can be obtained for each side of each of the steel sheets 31, 32, and 33. The average values ​​of the obtained coating weights (average values ​​for both sides) for the steel sheets 31, 32, and 33 are designated as coating weights W1, W2, and W3 of the aluminum-based plating layer. When an Al-Fe-based alloy layer is present at the interface between the base steel sheet and the aluminum-based plating layer, the thickness of the aluminum-based plating layer includes the thickness of the Al-Fe-based alloy layer. The thicknesses of the aluminum-based plating layers 31b, 32b of the steel plates 31, 32 forming the thickest overlap portion 361 are smaller than the thickness of the aluminum-based plating layer 33b of the other steel plate 33. The thickness of the aluminum-based plating layer 31b of the steel plate 31 may be equal to or different from the thickness of the aluminum-based plating layer 32b of the steel plate 32.

[0117] In the blank 30A according to this embodiment, the plurality of steel plates 31, 32, and 33 have different plate thicknesses t １ , t ２ , t ３ When having, the maximum plate thickness t in the blank 30A ｍａｘ and minimum plate thickness t ｍｉｎ Ratio of: t ｍａｘ / t ｍｉｎ In this case, the maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ For example, t ｍａｘ / t ｍｉｎ ≦4.0, preferably t ｍａｘ / t ｍｉｎ≦3.0. Maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ is t ｍａｘ / t ｍｉｎ The maximum plate thickness t may be ≧2.5. ｍａｘ is, for example, 4.6 mm or less. ｍａｘ may be 1.6 mm or more.

[0118] [Structural Member] The blank 30A is subjected to the heating process and forming process similar to those in the first embodiment. As a result, as shown in Figures 6A and 6B, a structural member 10A similar to that in the first embodiment is manufactured from the blank 30A. Figures 6A and 6B are cross-sectional views of the structural member 10A after the forming process (hot stamping). Figure 6A shows the structural member 10A at the maximum plate thickness t ｍａｘ 6A shows a cross section of structural member 10A at an overlap 361 having a cross section of structural member 10A. FIG. 6B shows a cross section of structural member 10A at another overlap 362.

[0119] Referring to Fig. 6A, in the structural member 10A after hot stamping, the steel sheet 31 is also a plated steel sheet having aluminum-based plating layers 31b on both surfaces of a base steel sheet 31a. Similarly, the steel sheet 32 ​​is also a plated steel sheet having aluminum-based plating layers 32b on both surfaces of a base steel sheet 32a. Referring to Fig. 6B, the steel sheet 33 is a plated steel sheet having aluminum-based plating layers 33b on both surfaces of a base steel sheet 33a. However, compared to the state of the blank 30A (Figs. 5A and 5B), the aluminum-based plating layers 31b, 32b, 33b in the structural member 10A have been alloyed with iron by the heating process.

[0120] 6A and 6B , the thicknesses K1 and K2 of the aluminum-based plating layers 31b and 32b of the steel plates 31 and 32 that form the thickest overlap portion 361 are each smaller than the thickness K3 of the aluminum-based plating layer 33b of the other steel plate 33. The thickness K1 is the average thickness of the aluminum-based plating layer 31b on both surfaces of the base steel plate 31a of the steel plate 31. The thickness K2 is the average thickness of the aluminum-based plating layer 32b on both surfaces of the base steel plate 32a of the steel plate 32. The thickness K3 is the average thickness of the aluminum-based plating layer 33b on both surfaces of the base steel plate 33a of the steel plate 33. The difference K3 - K1 between the thickness K1 of the aluminum-based plating layer 31b of the steel plate 31 and the thickness K3 of the aluminum-based plating layer 33b of the steel plate 33 is, for example, 7 μm or more. K3 - K1 may be 33 μm or less. Similarly, the difference (K3-K2) between the thickness K2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​and the thickness K3 of the aluminum-based plating layer 33b on the steel sheet 33 is, for example, 7 μm or more. K3-K2 may be 33 μm or less. The thickness K1 of the aluminum-based plating layer 31b on the steel sheet 31 may be equal to or different from the thickness K2 of the aluminum-based plating layer 32b on the steel sheet 32.

[0121] In the blank 30 ( FIGS. 3A and 3B ) before hot stamping, if the coating mass of the aluminum-based plating layer 31b on the surface 311 located outside the overlap portion 361 of the steel sheet 31 is smaller than the coating mass of the aluminum-based plating layer 31b on the surface 312 located inside the overlap portion 361, the thickness of the aluminum-based plating layer 31b on the surface 311 will also be smaller than the thickness of the aluminum-based plating layer 31b on the surface 312 in the structural member 10A after hot stamping. The difference in thickness between the surfaces 311 and 312 is, for example, 0.3 μm or more. The difference in thickness between the surfaces 311 and 312 may be, for example, 3.0 μm or less. Similarly, in the blank 30 before hot stamping, if the coating mass of the aluminum-based plating layer 32b on the surface 321 located outside the overlap portion 361 of the steel sheet 32 ​​is smaller than the coating mass of the aluminum-based plating layer 32b on the surface 322 located inside the overlap portion 361, the thickness of the aluminum-based plating layer 32b on the surface 321 will also be smaller than the thickness of the aluminum-based plating layer 32b on the surface 322 in the structural member 10A after hot stamping. The difference in thickness between the surfaces 321 and 322 is, for example, 0.3 μm or more. The difference in thickness between the surfaces 321 and 322 may be, for example, 3.0 μm or less.

[0122] The thicknesses K1, K2, and K3 of the aluminum-based plating layers 31b, 32b, and 33b in the structural member 10A can be measured as follows. Specifically, a vehicle body part is disassembled to obtain the structural member 10A, and an analysis sample is obtained from the structural member 10A, for example, by laser cutting. For example, an analysis sample is obtained from each of the multiple steel plates included in the structural member 10A. The analysis sample is obtained from the center or its vicinity of the top plate of each steel plate having an open cross section. For test pieces obtained from each of the multiple steel plates, the cross section of the aluminum-based plating layer is nital-etched and then observed with an optical microscope (area: 100 μm × 100 μm), and the thickness of the plating layer is measured in three fields of view. The thickness of the plating layer can be measured on each side of the steel plates 31, 32, and 33. The average values ​​of the thicknesses of the plating layer measured on both sides in the three fields of view for the steel plates 31, 32, and 33 can be used as the plating thicknesses K1, K2, and K3. The outermost surface of the structural member 10A often contains, for example, an electrodeposition coating film. In this case, the plating layer that is present below the electrodeposition coating layer and above the base steel sheet is observed.

[0123] Referring to FIG. 6A, in the structural member 10A, the maximum plate thickness t ｍａｘ The overlap portion 361 is a joint between two adjacent steel plates 31 and 32. The steel plate located on the surface side of the structural member 10A among the steel plates 31 and 32 has a maximum Vickers hardness of HV ｍａｘ , the minimum value of Vickers hardness is HV ｍｉｎ When this is done, HV ｍａｘ -HV ｍｉｎ For example, HV ｍａｘ In this embodiment, the steel sheets 31, 32 having relatively thin plating are joined together to form an overlap portion 361, and then the steel sheets 31, 32 are subjected to hot stamping. In the overlap portion 361 after hot stamping, the maximum Vickers hardness HV of at least the steel sheet arranged on the front surface side of the structural member 10A of the steel sheets 31, 32 is 30% or less. ｍａｘ and minimum value HV ｍｉｎ is HV ｍａｘ -HV ｍｉｎ ≦HV ｍａｘ× 30% is satisfied. In this embodiment, the steel plate 32 is disposed on the front surface side (outside) of the structural member 10A with respect to the steel plate 31. However, the steel plate 31 may also be disposed on the front surface side (outside) of the structural member 10A with respect to the steel plate 32.

[0124] Maximum Vickers hardness HV in the overlap portion 361 ｍａｘ and minimum value HV ｍｉｎ can be measured as follows. First, a test piece including a cross section of the steel plate 31, 32 located on the surface side of the structural member 10A in the overlap portion 361 is taken from the structural member 10A by laser cutting. Then, in accordance with JIS Z 2244:2020, the cross section of the steel plate (base steel plate) is prepared as the test surface, and a Vickers hardness test is carried out on 30 arbitrary locations on this test surface that are spaced apart by at least the total plate thickness of the overlap portion 361, with a test force of 10 kgf (98.07 N) and a test force holding time of 10 seconds. Of the 30 Vickers hardness values ​​(HV) obtained in this way, the maximum Vickers hardness is designated as HV. ｍａｘ , the minimum Vickers hardness is HV ｍｉｎ It can be said that:

[0125] 6B, in the structural member 10A, the overlap portion 362 is a joint between two adjacent steel plates 31 and 33. As in the thickest overlap portion 361, the overlap portion 362 has a maximum Vickers hardness of HV ｍａｘ , the minimum value of Vickers hardness is HV ｍｉｎ When this is done, HV ｍａｘ -HV ｍｉｎ For example, HV ｍａｘ In the manufacturing method according to the present embodiment, the steel sheets 31, 33 are subjected to hot stamping after the overlap portion 362 is formed. In the overlap portion 362 after hot stamping, for example, the maximum Vickers hardness HV ｍａｘ and minimum value HV ｍｉｎ is HV ｍａｘ -HV ｍｉｎ ≦HV ｍａｘ x30% is met.

[0126] Maximum Vickers hardness HV of the overlap portion 362ｍａｘ and minimum value HV ｍｉｎ is measured on the steel plate located on the surface side of the structural member 10A, out of the steel plates 31 and 33, in the overlap portion 362. ｍａｘ and minimum value HV ｍｉｎ can be measured in the same manner as the overlap portion 361.

[0127] Although not shown, the blank 40 (FIGS. 1 and 3D) for the structural member 20 can have the same configuration as the blank 30A. That is, the configuration of the steel plates 31, 32, and 33 in the blank 30A can be directly applied to the steel plates 41, 42, and 43 (FIG. 3D) of the blank 40. In this case, in the structural member 20 manufactured from the blank 40 through the heating process and the forming process, the steel plates 41, 42, and 43 have the same configuration as the steel plates 31, 32, and 33 of the structural member 10A shown in FIGS. 6A and 6B.

[0128] [Effect] In the blank 30A according to this embodiment, the maximum plate thickness t ｍａｘ The coating weights W1 and W2 of the aluminum-based plating layers 31b and 32b on the base steel sheets 31a and 32a of the steel sheets 31 and 32 forming the overlap portion 361 are smaller than the coating weight W3 of the aluminum-based plating layer 33b on the base steel sheet 33a of the other steel sheet 33. The coating weights W1 and W2 are 60 g / m ２ As a result, when the blank 30A is heated during hot stamping, the temperature rise rate of the overlap portion 361 can be increased, and the heating time of the overlap portion 361 can be shortened. Specifically, the aluminum-based plating layers 31b, 32b on the surface layers of the steel sheets 31, 32 have a coating weight of 60 g / m ２Because the thickness is less than 1 / 3 mm, when the blank 30A is heated, alloying of the aluminum-based plating layers 31b, 32b with the iron contained in the base steel sheets 31a, 32a progresses rapidly to the surfaces of the steel sheets 31, 32, and both surfaces of the steel sheets 31, 32, including the overlap portion 361, turn black or a color close to black relatively quickly. This increases the emissivity of the outer surfaces 311, 321 of the overlap portion 361, accelerating the temperature rise and shortening the heating time of the overlap portion 361. Therefore, heating of the blank 30A can be terminated before excessive alloying of the aluminum-based plating layers 31b, 32b, 33b in the steel sheets 31, 32, 33 progresses and the diffusion layer grows to a thickness exceeding a predetermined value. As a result, the maximum sheet thickness t ｍａｘ The strength of the overlap portion 361 having the above-mentioned structure can be ensured by hot stamping, and the corrosion resistance (rust prevention) of the structural member 10A can be ensured. Furthermore, by reducing the thickness of the diffusion layer, the weldability of the steel plates 31, 32, and 33 can be more easily ensured.

[0129] As described above, according to the blank 30A of this embodiment, the maximum plate thickness t ｍａｘ Therefore, it is possible to heat the entire blank 30A to a temperature required for hot stamping before the alloying of the aluminum-based plating layers 31b, 32b, and 33b in the steel sheets 31, 32, and 33 progresses excessively, and it is possible to ensure a process window for the heating conditions in the manufacture of the structural member 10A. ｍａｘ This allows the manufacture of a structural member 10A that combines the strength of the overlap portion 361 and the rust prevention properties of each of the steel plates 31, 32, and 33. In addition, by increasing the temperature rise rate of the overlap portion 361, the heating time of the blank 30A can be shortened, thereby improving the productivity of the structural member 10A. Furthermore, by shortening the heating time of the blank 30A, energy consumption in the manufacture of the structural member 10A is suppressed, and the amount of greenhouse gases generated during heating can be reduced.

[0130] In this embodiment, the coating weight W3 of the aluminum-based plating layer 33b on the steel plate 33 is greater than that on the steel plates 31 and 32 that form the thickest overlap portion 361. That is, the steel plate 33 has higher rust prevention performance than the steel plates 31 and 32. However, the arrangement of the steel plates 31, 32, and 33 is not limited to the example of this embodiment. The steel plate 33 with a relatively thick plating can be arranged in a portion of the structural member 10A that requires high rust prevention performance.

[0131] In this embodiment, the coating weights W1 and W2 of the aluminum-based plating layers 31b and 32b on the steel sheets 31 and 32 may be the same or different. However, from the viewpoint of uniformly heating the blank 30A, it is preferable that the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is substantially equal to the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32.

[0132] In the steel sheet 31, the coating mass of the aluminum-based plating layer 31b on the surface 311 located outside the overlap portion 361 is preferably smaller than the coating mass of the aluminum-based plating layer 31b on the surface 312 located inside the overlap portion 361. In addition, in the steel sheet 32, the coating mass of the aluminum-based plating layer 32b on the surface 321 located outside the overlap portion 361 is preferably smaller than the coating mass of the aluminum-based plating layer 32b on the surface 322 located inside the overlap portion 361. The small coating masses of the aluminum-based plating layers 31b, 32b on the surfaces 311, 321 outside the overlap portion 361 facilitate heating of the overlap portion 361, and the heating time of the overlap portion 361 is likely to be shorter.

[0133] In this embodiment, in the heating process, first, the blank 30A is heated to the minimum thickness t ｍｉｎ The steel plate 33 having a thickness of t reaches the temperature in the austenite region, and then the temperature in the austenite region is reached in order from the thinner portion of the plate. ｍａｘIn the present embodiment, when the steel plates 31, 32, and 33 included in the blank 30A have different thicknesses, the overlap portion 361 having the minimum thickness t ｍｉｎ and maximum plate thickness t ｍａｘ If there is too much difference between the thickness t ｍｉｎ For a steel plate 33 having a plate thickness t ｍａｘ The temperature rise of the overlap portion 361 having the minimum thickness t ｍｉｎ and maximum plate thickness t ｍａｘ Ratio to: t ｍａｘ / t ｍｉｎ is preferably 3.0 or less. This allows the overlap portion 361 to be sufficiently heated until the phase transformation to austenite is completed before the alloying of the aluminum-based plating layer 33b of the thinnest steel plate 33 progresses and the corrosion resistance is lost, even if there is a difference in plate thickness among the steel plates 31, 32, and 33. This makes it easier to ensure a process window in the manufacture of the structural member 10A.

[0134] As shown in FIG. 7 , in this embodiment, the thickest overlap portion 361 may be provided with the same coating 50 as in the first embodiment. That is, the surfaces 311, 321 located outside the overlap portion 361 of each of the steel plates 31, 32 may be coated with a substantially black coating 50. In the example shown in FIG. 7 , the surfaces 311, 321 of the steel plates 31, 32 are coated with the coating 50. In the steel plate 31, the entire surface 311 opposite the mating steel plate 32 is coated with the coating 50. In the steel plate 32, the entire surface opposite the mating steel plate 31 is also coated with the coating 50. That is, the surfaces 311, 321 of the steel plates 31, 32 that constitute the front and back surfaces of the overlap portion 361 are each coated with the coating 50.

[0135] The substantially black coating 50 can increase the emissivity of the surfaces 311, 321 located outside the overlap portion 361 of the steel sheets 31, 32 in advance. This allows the temperature of the overlap portion 361 to rise more quickly when the blank 30A is heated during hot stamping. Therefore, for example, the maximum thickness t ｍａｘ The overlap portion 361 has a minimum plate thickness t ｍｉｎ Even if the difference in plate thickness between the steel plate 31 having the same thickness as the steel plate 33 having the same thickness is large, the process window of the heating conditions can be easily ensured.

[0136] In the blank 30A shown in FIG. 7, the maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ is, for example, 2.5 or more. ｍａｘ / t ｍｉｎ In the example of FIG. 7, the emissivity of the overlap portion 361 is increased in advance by the substantially black coating 50, so that when the blank 30A is heated during hot stamping, the temperature rise of the overlap portion 361 is accelerated. Therefore, t ｍａｘ / t ｍｉｎ When is large, for example, t ｍａｘ / t ｍｉｎ Even if the maximum thickness t of the blank 30A exceeds 3.0, it becomes easier to complete heating of the thickest overlap portion 361 before alloying of the aluminum-based plating layer 33b in the thinnest steel sheet 33 progresses excessively, and the process window of the heating conditions can be secured. ｍａｘ and minimum plate thickness t ｍｉｎ is t ｍａｘ / t ｍｉｎ It is preferable that the value satisfies ≦4.0.

[0137] In the example of FIG. 7 , the surfaces located outside the overlap portion 361 of each of the steel plates 31 and 32 are coated with the coating 50. However, the surfaces located outside the overlap portion 361 of only one of the steel plates 31 and 32 may be coated with the coating 50. Also, in the example of FIG. 7 , the surfaces 312 and 322 located inside the overlap portion 361 of each of the steel plates 31 and 32 are not coated with the coating 50. However, the surface located inside the overlap portion 361 of at least one of the steel plates 31 and 32 may also be coated with the coating 50. However, from the viewpoint of uniformly heating the blank 30A during hot stamping, it is preferable that the surfaces 311 and 321 outside the overlap portion 361 of each of the steel plates 31 and 32 are coated with the coating 50, and the surfaces 312 and 322 located inside the overlap portion 361 are not coated with the coating 50.

[0138] The lower structural member 20 (FIG. 1) and blank 40 (FIG. 3D) can have the same configuration as the upper structural member 10A and blank 30A in this embodiment, and therefore the lower structural member 20 and blank 40 can also achieve the same effects as described above.

[0139] 8 is an exploded perspective view of structural members 10B, 20B according to this embodiment. The structural members 10, 10A, 20 according to the first and second embodiments constitute a front under module of the vehicle body. On the other hand, the structural members 10B, 20B according to this embodiment constitute a rear under module of the vehicle body.

[0140] Referring to Figure 8, the structural member 10B, like the above-described embodiment, includes a pair of side frames 11L, 11R and at least one cross member 12. Similarly, the structural member 20B, like the above-described embodiment, includes a pair of side frames 21L, 21R and at least one cross member 22. In the example shown in Figure 8, the cross member 12 connects the middle portions of the side frames 11L, 11R. Similarly, the cross member 22 connects the middle portions of the side frames 21L, 21R. The configurations of the structural members 10, 10A and the structural member 20 described in the first and second embodiments can be applied to the structural members 10B, 20B of this embodiment, respectively.

[0141] The structural member 10B can be manufactured from a blank 30B shown in Figure 9 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank 30B includes steel plates 31, 32, and 33. The steel plates 31, 32, and 33 are arranged and joined to form two long portions 34L and 34R and at least one connecting portion 35. The blank 30B can have a configuration similar to the blank 30 or 30A described in other embodiments (Figures 3B and 3C, or Figures 5A and 5B).

[0142] In this embodiment, the steel plates 32 forming the rear portions 112 (FIG. 8) of the side frames 11L, 11R may be smaller in at least one of thickness and tensile strength than the steel plates 31 forming the front portions 111 (FIG. 8). In the structural member 10B according to this embodiment and the structural members 10, 10A according to the above embodiments (FIGS. 1, 6A, and 6B), it is preferable that the thickness and / or tensile strength of the steel plates located further outward in the fore-and-aft direction of the vehicle body be smaller than that of the steel plates located further inward. As a result, when a longitudinal collision load is input to the vehicle body, in the structural members 10, 10A, 10B, the portions located further outward in the vehicle body deform to absorb the collision energy, while the portions located further inward in the vehicle body are less likely to deform, thereby protecting surrounding components.

[0143] The structural member 20B can be manufactured from a blank 40B shown in Figure 10 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank 40B includes steel plates 41, 42, and 43. The steel plates 41, 42, and 43 are arranged and joined to form two long portions 44L and 44R and at least one connecting portion 45. The blank 40B can have a configuration similar to the blank 30 or 30A described in other embodiments (Figures 3B and 3C, or Figures 5A and 5B).

[0144] In this embodiment, the steel plates 42 forming the rear portions 212 (FIG. 8) of the side frames 21L, 21R may be smaller in at least one of thickness and tensile strength than the steel plates 41 forming the front portions 211 (FIG. 8). In the lower structural members 20, 20B (FIGS. 1 and 8), similar to the upper structural members, it is preferable that the thickness and / or tensile strength of the steel plates located further outward in the fore-and-aft direction of the vehicle body be smaller than those of the steel plates located further inward. As a result, when a longitudinal collision load is applied to the vehicle body, the outer portions of the structural members 20, 20B deform to absorb the collision energy, while the inner portions are less likely to deform, thereby protecting surrounding components.

[0145] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0146] In the above-described embodiments, examples have been described in which the surfaces 311, 321 located outside the thickest overlap portion 361 of each of the steel plates 31, 32 have been treated to increase the emissivity compared to other surfaces of the steel plates 31, 32, 33. However, it is sufficient if at least one of the surfaces 311, 321 outside the thickest overlap portion 361 has been treated to increase the emissivity compared to at least one other surface of the steel plates 31, 32, 33.

[0147] For example, in the blank 30 according to the first embodiment, the maximum plate thickness t ｍａｘThe emissivity of the surfaces 311 and 321 of the steel plates 31 and 32 located outside the overlapping portion 361 is ｍｉｎ The emissivity of the coating 50 is already greater than the emissivity of both surfaces of the steel sheet 33 having a thickness of 1000 Å. That is, the coating 50 is applied to the surfaces 311 and 321 of the steel sheets 31 and 32, while the coating 50 is not applied to both surfaces of the steel sheet 33. However, it is sufficient that the coating 50 is applied to at least one of the surfaces 311 and 321 of the steel sheets 31 and 32, and that the coating 50 is not applied to one or more other surfaces of the steel sheets 31, 32, and 33. For example, one or both surfaces of the steel sheet 33 may be covered with the coating 50. However, when the thinnest steel sheet 33 is a plated steel sheet, it is preferable that the coating 50 is not applied to at least one surface of the steel sheet 33, from the viewpoint of ensuring the process window.

[0148] In the blank 30 according to the first embodiment, the emissivity of the surfaces 311 and 321 located outside the overlap portion 361 of the steel plates 31 and 32 is set higher than the emissivity of the surfaces 312 and 322 located inside the overlap portion 361. That is, in the steel plates 31 and 32, the coating 50 is applied to the outer surfaces 311 and 321, while the coating 50 is not applied to the inner surfaces 312 and 322. However, in the steel plate 31, the surface 311 located inside the overlap portion 361 may be coated with the coating 50. Similarly, in the steel plate 32, the surface 321 located inside the overlap portion 361 may be coated with the coating 50. However, from the viewpoint of uniform heating of the blank 30 during hot stamping, it is preferable that the outer surfaces 311, 321 of the overlap portion 361 of each of the steel sheets 31, 32 are coated with the coating 50, and the inner surfaces 312, 322 of the overlap portion 361 are not coated with the coating 50. Specifically, if both surfaces of each of the steel sheets 31, 32 constituting the overlap portion 361 are coated with the coating 50, the portions of the steel sheets 31, 32 that do not overlap with the other steel sheet are excessively heated in the heating process, which makes it easier for the diffusion layer to grow to a thickness exceeding a predetermined value, and therefore it may become impossible to ensure the corrosion resistance (rust prevention) of the plating layers 31b, 32b. In other words, providing the coating 50 on both surfaces of each of the steel sheets 31, 32 may actually narrow the process window of the heating conditions. Therefore, in the steel plates 31, 32, it is preferable that the outer surfaces 311, 321 of the overlap portion 361 are coated with the coating 50, and the inner surfaces 312, 322 of the overlap portion 361 are not coated with the coating 50. In this case, even in the structural member 10 after hot stamping, in the steel plates 31, 32, the coating 13 is present on the outer surfaces 311, 321 of the overlap portion 361, and the coating 13 is not present on the inner surfaces 312, 322 of the overlap portion 361.

[0149] In the blank 30A according to the second embodiment, the maximum plate thickness t ｍａｘ The emissivity of the surfaces 311 and 321 of the steel plates 31 and 32 located outside the overlapping portion 361 isｍｉｎ That is, in both of the steel plates 31 and 32 forming the overlap portion 361, the coating weights W1 and W2 of the aluminum-based plating layers 31b and 32b are 60 g / m or more. ２ 32. However, in the blank 30A, the coating weight of the aluminum-based plating layer on at least one of the steel sheets 31, 32 is 60 g / m or less, which is smaller than the coating weight W3 of the aluminum-based plating layer 33b on the other steel sheets 33. ２ It is sufficient that the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is 60 g / m or less and is smaller than the coating weight W1 of the aluminum-based plating layer on one or more of the other surfaces of the steel sheets 31, 32, and 33. For example, when the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is 60 g / m ２ or less, the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​is 60 g / m ２ In this case, the coating weight W2 may be equal to or greater than the coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33. Similarly, if the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​is 60 g / m or more, ２ or less, the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is 60 g / m ２ In this case, the coating weight W1 may be equal to or greater than the coating weight W3 of the aluminum-based plating layer 33b on the steel plate 33.

[0150] In the structural member 10A according to the second embodiment, the maximum plate thickness t ｍａｘIn both of the steel plates 31, 32 forming the overlap portion 361, the thicknesses K1, K2 of the aluminum-based plating layers 31b, 32b are smaller than the thickness K3 of the aluminum-based plating layer 33b on the other steel plate 33. However, in the structural member 10A, it is sufficient that the thickness of the aluminum-based plating layer on at least one of the steel plates 31, 32 is smaller than the thickness K3 of the aluminum-based plating layer 33b on the other steel plate 33. For example, when the thickness K1 of the aluminum-based plating layer 31b on the steel plate 31 is smaller than the thickness K3 of the aluminum-based plating layer 33b on the steel plate 33, the thickness K2 of the aluminum-based plating layer 32b on the steel plate 32 may be equal to or greater than the thickness K3 of the aluminum-based plating layer 33b on the steel plate 33. Similarly, when the thickness K2 of the aluminum-based plating layer 32b on the steel plate 32 is smaller than the thickness K3 of the aluminum-based plating layer 33b on the steel plate 33, the thickness K1 of the aluminum-based plating layer 31b on the steel plate 31 may be equal to or greater than the thickness K3 of the aluminum-based plating layer 33b on the steel plate 33.

[0151] In the second embodiment, the maximum plate thickness t ｍａｘ Both of the steel sheets 31, 32 forming the overlap portion 361 are aluminum-plated steel sheets. However, it is sufficient if at least one of the steel sheets 31, 32 is a plated steel sheet having a base steel sheet and an aluminum-based plating layer. When one of the steel sheets 31, 32 is an aluminum-plated steel sheet, the other of the steel sheets 31, 32 may be, for example, a zinc-plated steel sheet.

[0152] In the blank 30 according to the first embodiment, the coating 50 is applied to reduce the maximum plate thickness t ｍａｘThe emissivity of the outer surfaces 311, 321 of the overlap portion 361 having a thickness of 1000 nm is increased. In the blank 30A according to the second embodiment, the amount of aluminum-based plating layer on the outer surfaces 311, 321 of the overlap portion 361 is reduced compared to the other portions, thereby increasing the emissivity of the overlap portion 361 during the heating process. However, the treatment for increasing the emissivity of the overlap portion is not limited to this. For example, the emissivity of the overlap portion 361 can also be increased by increasing the surface roughness of the overlap portion 361 compared to the surface roughness of other portions.

[0153] In the above embodiment, the steel plates 31, 32, and 33 included in the blanks 30 and 30A may each be a single layer or multiple layers. That is, the steel plates 31, 32, and 33 may each be a single steel plate or a plate material formed by overlapping multiple steel plates.

[0154] In the above embodiment, the ends of the steel plates 31 and 32 are lap-joined to form the overlap portion 361. Similarly, the ends of the steel plates 31 and 33 are lap-joined to form the overlap portion 362. However, the blanks 30, 30A, and 30B and the structural members 10 and 10A only need to include at least one overlap portion. In other words, the steel plate that does not form the thickest overlap portion 361 does not necessarily need to be lap-joined to another steel plate, and may be butt-joined to another steel plate.

[0155] In the first embodiment, the blank 30 includes steel plates 31, 32 corresponding to the long portions 34L, 34R (side frames 11L, 11R), and a steel plate 33 corresponding to the connecting portion 35 (cross member 12). However, the number and arrangement of steel plates included in the blank 30 and the structural member 10 manufactured from the blank 30 are not limited to this. As shown in Figures 11 to 13, the number and arrangement of steel plates can be changed as appropriate.

[0156] For example, as shown in Figure 11, in a blank 30 corresponding to the structural member 10 (Figure 1) of a front under module, the long portions 34L, 34R may each be formed from a single steel plate 31. In the blank 30, the connecting portion 35 may connect one longitudinal end of the long portions 34L, 34R to each other, as in the first embodiment. In other words, the long portions 34L, 34R may be connected by the connecting portion 35 at the end side that is disposed forward or rearward when the structural member 10 is assembled to the vehicle body.

[0157] In the first embodiment described above, the structural member 10 is provided with a single cross member 12, and therefore the blank 30 for the structural member 10 also includes a single connecting portion 35. However, the structural member 10 may include multiple cross members 12. In this case, as shown in Figures 12 and 13, the blank 30 also includes multiple connecting portions 35. These connecting portions 35 are formed from separate steel plates 32, 33 or steel plates 33, 36. The long portions 34L, 34R may each be formed from a single steel plate 31 as shown in Figure 12, or may each be formed from multiple steel plates 31, 32 as shown in Figure 13.

[0158] Although not shown, the number and arrangement of steel plates in the structural member 10A and blank 30A according to other embodiments, and in the lower structural member 20 and blank 40, are not particularly limited. The structural members 10, 10A, 20 and blanks 30, 30A, 40 each need only include two or more joined steel plates. Preferably, the structural members 10, 10A, 20 and blanks 30, 30A, 40 each include three or more steel plates. In each of the blanks 30, 30A, 40, the maximum plate thickness t ｍａｘ One or both of the steel sheets forming the overlapping portion having the above structure are treated to increase emissivity. The other steel sheets may or may not be treated to increase emissivity. Each of the blanks 30, 30A, and 40 may include at least one steel sheet that has not been treated to increase emissivity on one or both sides.

[0159] In the third embodiment, the blank 30B corresponds to the structural member 10B of the rear under module. This blank 30B includes steel plates 31 and 32 corresponding to the long portions 34L and 34R (side frames 11L and 11R), respectively, and a steel plate 33 corresponding to the connecting portion 35 (cross member 12). However, the number and arrangement of steel plates included in the blank 30B and the structural member 10B manufactured from the blank 30B are not limited to this. As shown in Figures 14 to 24, the number and arrangement of steel plates can be changed as appropriate.

[0160] 14 and 15 , in a blank 30B, the long portions 34L, 34R may each be formed from a single steel plate 31. In this case, the connecting portion 35 may connect the long portions 34L, 34R at one end in the longitudinal direction. For example, the long portions 34L, 34R may be connected by the connecting portion 35 at the end that is disposed forward when the structural member 10B ( FIG. 8 ) is assembled to the vehicle body. The long portions 34L, 34R may also be connected by the connecting portion 35 at their intermediate portions.

[0161] As shown in Figure 16, even when the long portions 34L, 34R of the blank 30B are formed from a plurality of steel plates 31, 32, the long portions 34L, 34R may be connected by a connecting portion 35 at the end side that is disposed forward when the structural member 10B (Figure 8) is assembled to the vehicle body. For example, when the portion corresponding to the front portion 111 (Figure 8) of the side frames 11L, 11R is formed from a steel plate 31 and the portion corresponding to the rear portion 112 (Figure 8) is formed from a steel plate 32, the connecting portion 35 may be joined to the steel plate 31 as shown in Figures 16 and 17, or may be joined to the steel plate 32 as shown in Figure 18.

[0162] In the third embodiment described above, because the structural member 10B ( FIG. 8 ) is provided with a single cross member 12, the blank 30B for the structural member 10B also includes a single connecting portion 35. However, the structural member 10B may include multiple cross members 12. In this case, as shown in FIGS. 19 to 24 , the blank 30B also includes multiple connecting portions 35. The connecting portion 35 is formed from separate steel plates 32, 33, steel plates 33, 36, or steel plates 32, 33, 36. In this case, the long portions 34L, 34R may each be formed from a single steel plate 31, as shown in FIGS. 19 , 20 , and 24 , or may each be formed from multiple steel plates 31, 32, as shown in FIGS. 21 to 23 .

[0163] Although not shown, in the third embodiment, the number and arrangement of steel plates in the lower structural member 20B and blank 40B are not particularly limited. The structural members 10B, 20B and blanks 30B, 40B each need only include two or more joined steel plates. Preferably, the structural members 10B, 20B and blanks 30B, 40B each include three or more steel plates. In each of the blanks 30B, 40B, the maximum plate thickness t ｍａｘ One or both of the steel sheets forming the overlapping portion having the above structure are treated to increase emissivity. The other steel sheets may or may not be treated to increase emissivity. Each of the blanks 30B and 40B may include at least one steel sheet that has not been treated to increase emissivity on one or both sides.

[0164] Typically, all of the steel sheets included in the blanks 30, 30A, 30B and the structural members 10, 10A, 10B according to the above embodiments are plated steel sheets. However, some of the steel sheets in the blanks 30, 30A, 30B and the structural members 10, 10A, 10B may be steel sheets without a plated layer (bare material). Similarly, all of the steel sheets included in the blanks 40, 40B and the structural members 20, 20B may be plated steel sheets, or some of them may be bare material.

[0165] In the blanks 30, 30A, and 30B according to the above embodiments, the maximum plate thickness t ｍａｘThe steel plate 33 that does not form the overlap portion 361 has the minimum plate thickness t ｍｉｎ However, the maximum plate thickness t ｍａｘ One or both of the steel plates 31, 32 forming the overlap portion 361 of the blank 30, 30A, 30B has a minimum plate thickness t ｍｉｎ Similarly, in the blanks 40 and 40B, the maximum plate thickness t ｍａｘ One or both of the steel plates forming the overlapping portion have a minimum plate thickness t ｍｉｎ The steel plate may have a minimum thickness of t ｍｉｎ The steel plate may be made of a material other than steel.

[0166] In the above embodiment, the maximum plate thickness t ｍａｘ However, in each of the blanks 30, 30A, and 30B, there is only one overlap portion 361 having a maximum plate thickness t ｍａｘ Similarly, in each of the blanks 40 and 40B, there may be a plurality of overlap portions 361 having the maximum plate thickness t ｍａｘ In this case, there may be a plurality of overlapping portions having the maximum plate thickness t ｍａｘ Preferably, all of the overlapping portions have one or both of their outer surfaces treated to increase emissivity.

[0167] In each of the blanks 30, 30A, and 30B according to the above embodiments, all of the steel plates have different thicknesses. However, among the multiple steel plates included in each of the blanks 30, 30A, and 30B, some of the steel plates may have the same thickness, or all of the steel plates may have the same thickness. When the steel plates included in the blanks 30, 30A, and 30B all have the same thickness, t ｍａｘ / t ｍｉｎ is 2.0. Similarly, in the blanks 40, 40B, some of the steel plates may have the same plate thickness, or all of the steel plates may have the same plate thickness.

[0168] In the above embodiment, the mold 60 used in the forming process includes a punch 61 and a die 62. However, the configuration of the mold 60 is not limited to the example described in the above embodiment. The mold 60 may further include, for example, a pad and a blank holder. The mold 60 may be configured according to the desired structural member.

[0169] In the structural members 10, 10A, 10B, 20, and 20B according to the above embodiments, the side frames 11 and 21 have a substantially hat-shaped cross section. However, the cross-sectional shape of the side frames 11 and 21 is not necessarily limited to this. For example, as shown in FIG. 25 , the side frames 11 and 21 may have a shape in which one widthwise side is open in cross section. In this case, another member (not shown) may be joined to the open portion of the side frame 11 and 21, so that the side frame 11 and the other member form a closed cross section. Similarly, the cross members 12 and 22 may each have a substantially hat-shaped cross section or a cross-sectional shape of another shape.

[0170] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to the following examples.

[0171] In order to confirm the effects of the present disclosure, CAE analysis was performed on the press-formed (hot stamped) structural members of the front or rear under module using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) while changing the type (material type) and thickness of the steel plate included in the structural member, as well as the division pattern of the structural member.

[0172] The types of steel plates (material types) are shown in Table 1.

[0173]

[0174] In Table 1, the material type is listed in the order of plating type, tensile strength, and application (hot stamping). Regarding the coating specifications, the black coating is a black coating containing carbon black and metal oxide. "Black coating - one side" means that one entire side of the steel sheet is covered with a black coating. "Black coating - both sides" means that both entire sides of the steel sheet are covered with a black coating. In this analysis, material types were selected from Table 1 to construct the target structural members.

[0175] The division patterns of the structural members are shown in Figures 26A and 26B. The structural member shown in Figure 26A is a structural member on the upper side of the front under module. The structural member shown in Figure 26B is a structural member on the upper side of the rear under module. Figures 26A and 26B show the number of steel plates (materials) included in the structural member and the positions of the joints between the steel plates in the structural member. In Figures 26A and 26B, each steel plate is given a number in parentheses.

[0176] Table 2 shows the analysis conditions and results for the structural member shown in Figures 26A and 26B. In Figures 26A and 26B, the side frames of the structural member are each formed from two pieces of material (1) and (2). In Figure 26A, material (2) forms the front portion of the side frame, and material (1) forms the rear portion of the side frame. In Figure 26B, material (1) forms the front portion of the side frame, and material (2) forms the rear portion of the side frame. The cross member is formed from material (3). Materials (1) to (3) are each overlapped and joined with the adjacent material.

[0177]

[0178] In Table 2, the "time to reach 910°C" is the time required for the blank to reach 910°C. ｍｉｎ The material having the above temperature reaches 910°C (A ｃ３The "heating completion time" is the time required for the part of the blank that has the slowest temperature rise to reach 910°C from the start of heating the blank until it becomes hot stampable. The "process window (PW)" is the time required for the part of the blank that has the slowest temperature rise to reach 910°C from the start of heating the blank until it becomes hot stampable. ｍｉｎ This is the value obtained by adding the allowable heating time (245 seconds) after a material having the above temperature reaches 910°C to the time it takes to reach 910°C, and subtracting the time it takes to complete heating. The larger this value, the wider the process window of heating conditions is for hot stamping structural members.

[0179] Referring to Table 2 and FIG. 26B, in Example 1, the material (1) and the material (2) have the maximum plate thickness t ｍａｘ = 2.8 mm, and the surfaces of the raw materials (1) and (2) located outside the overlapping portions are coated with a substantially black coating (black coating) as described in the first embodiment. On the other hand, the surfaces of the raw materials (1) and (2) located inside the overlapping portions are not coated with a black coating. In Example 1, the raw material (3) has a minimum plate thickness t ｍｉｎ = 1.0 mm, and a black coating was applied to only one side of the blank (3). In Comparative Example 1, which had the same combination of material type and plate thickness as Example 1, a black coating was not applied to any of the blanks (1) to (3). In Example 1, a black coating was applied to both outer sides of the thickest overlap portion, which promoted the temperature rise of this overlap portion and significantly reduced the time to complete heating of the blank compared to Comparative Example 1. In Example 1, although the time to reach 910°C was faster than in Comparative Example 1 because a black coating was applied to the thinnest blank (3), the process window was expanded by more than 60 seconds compared to Comparative Example 1.

[0180] Referring to Table 2 and FIG. 26A, in Example 3, the material (1) and the material (3) have the maximum plate thickness t ｍａｘIn Example 3, the blank (2) has a minimum thickness t = 3.4 mm, and the surfaces of the blanks (1) and (3) located outside the overlapping portions are provided with a black coating. On the other hand, the surfaces of the blanks (1) and (3) located inside the overlapping portions are not provided with a black coating. ｍｉｎ = 1.2 mm, and a black coating was applied to only one side of the raw material (2). In Comparative Example 2, which had the same combination of material type and plate thickness as Example 3, a black coating was not applied to any of the raw materials (1) to (3). In Example 3, a black coating was applied to both outer sides of the thickest overlap portion, which promoted the temperature rise of this overlap portion and significantly reduced the time to complete heating of the blank compared to Comparative Example 2. In Example 3, a black coating was applied to the thinnest raw material (2), which resulted in an earlier time to reach 910°C than in Comparative Example 2, but the process window was expanded by more than 70 seconds compared to Comparative Example 2. In Comparative Example 2, the process window of the heating conditions was eliminated (negative).

[0181] In Example 2, the material (1) and the material (2) have the maximum plate thickness t ｍａｘ An overlap portion having a thickness of t = 3.0 mm is formed, and a black coating is applied to the surface located outside the overlap portion in each of the raw materials (1) and (2). On the other hand, a black coating is not applied to the surface located inside the overlap portion in each of the raw materials (1) and (2). In Example 2, the raw material (3) has a minimum plate thickness t ｍｉｎ = 1.0 mm. It was also confirmed that the process window of the heating conditions was wider in Example 2 compared to the comparative examples.

[0182] This example confirmed that by treating the surface of the thickest overlap portion to increase the emissivity, the temperature rise rate of the overlap portion can be increased, and the process window of heating conditions in the manufacture of structural components can be expanded.

[0183] DESCRIPTION OF SYMBOLS 10, 10A, 10B: Structural member 11, 11L, 11R: Side frame 12: Cross member 13: Coating 20, 20B: Structural member 21, 21L, 21R: Side frame 22: Cross member 30, 30A, 30B: Blank 31, 32, 33, 36: Steel plate 31a, 32a, 33a: Base steel plate 31b, 32b, 33b: Plating layer 311, 312, 321, 322: Surface 361, 362: Overlap portion 34L, 34R: Long portion 35: Connection portion 40, 40B: Blank 41, 42, 43: Steel plate 44L, 44R: Long portion 45: Connection portion 50: Coating 60: Mold

Claims

1. A blank for hot stamping, comprising a plurality of steel plates arranged and joined to form two long portions arranged side by side in a plan view of the blank and a connecting portion connecting the long portions together, the plurality of steel plates including: a first steel plate; and a second steel plate having an end portion which is overlapped and joined to an end portion of the first steel plate to form, together with the end portion of the first steel plate, an overlap portion having the maximum sheet thickness in the blank, at least one of the plurality of steel plates being a plated steel plate having a base steel plate and a plating layer provided on the base steel plate, and a surface of at least one of the first steel plate and the second steel plate located outside the overlap portion has been treated to increase the emissivity compared to at least one other surface of the plurality of steel plates.

2. A blank according to claim 1, wherein, among said plurality of steel sheets, the steel sheet constituting the portion having the smallest sheet thickness in said blank is said plated steel sheet.

3. A blank as described in claim 1, wherein a surface of each of the first steel plate and the second steel plate located outside the overlap portion is subjected to the treatment for increasing the emissivity compared to at least one other surface of the plurality of steel plates.

4. A blank according to claim 1, wherein each of said plurality of steel sheets is a plated steel sheet.

5. A blank according to claim 4, wherein the plating layer is an aluminum-based plating layer.

6. A blank as described in claim 3, wherein a coating having an emissivity of 60% or more at a wavelength of 8.0 μm at 25°C is formed on the surface of each of the first steel plate and the second steel plate located outside the overlap portion as the treatment for increasing emissivity.

7. A blank according to claim 3, wherein a coating is formed on the surface of each of the first steel plate and the second steel plate located outside the overlapping portion as the treatment for increasing emissivity, and the coating is a mixture of carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m ２ and silica, and the content of the carbon black in the coating is X ＣＢ (g / m ２ ), the content of the oxide is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ A blank that satisfies the following formula (1). 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ ))≦332.0 (1) 8. A blank according to claim 3, wherein the plurality of steel plates include two or more steel plates having different plate thicknesses, and the plate thickness of the overlapping portion is t ｍａｘ The thickness of the steel plate constituting the part having the smallest thickness in the blank is t ｍｉｎ Then, t ｍａｘ / t ｍｉｎ ≦3.2, blank.

9. A blank according to claim 1, wherein the plurality of steel sheets further includes a third steel sheet, and at least one of the first steel sheet and the second steel sheet and the third steel sheet are plated steel sheets having an aluminum-based plating layer as the plating layer on both surfaces of the base steel sheet, and the treatment for increasing the emissivity of the surface located outside the overlap portion compared to the emissivity of the surface of the third steel sheet includes a coating amount (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in at least one of the first steel sheet and the second steel sheet. ２ ) is 60 or less, and the adhesion amount (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in the third steel sheet ２ ) are less than blank.

10. A blank according to claim 9, wherein the plurality of steel plates include two or more steel plates having different plate thicknesses, and the plate thickness of the overlapping portion is t ｍａｘ The thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ Then, t ｍａｘ / t ｍｉｎ Blank, which is ≦3.

0.

11. A blank according to claim 9, wherein the plurality of steel plates include two or more steel plates having different plate thicknesses, and the plate thickness of the overlapping portion is t ｍａｘ The thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ Then, t ｍａｘ / t ｍｉｎ ≦4.0, and a surface of at least one of the first steel plate and the second steel plate located outside the overlap portion is coated with a black coating.

12. A method for manufacturing a structural component, comprising the steps of: preparing a blank as described in any one of claims 1 to 11; heating the plurality of steel plates contained in the blank to a temperature equal to or higher than the austenite transformation completion temperature; and forming the heated blank using a die and quenching it.

13. A structural member for a vehicle body, comprising: a pair of side frames; and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate and a second steel plate having an end portion which is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and wherein the first steel plate and the second steel plate each have a surface located outside the overlap portion, and the surface of each of the first steel plate and the second steel plate is coated with at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide in an amount of 0.001 g / m ２ A structural member provided with a coating containing the above.

14. A structural member for a vehicle body, comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate and a second steel plate having an end portion which is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and wherein carbon black is applied at a rate of 0.500 g / m on a surface of each of the first steel plate and the second steel plate located outside the overlap portion. ２ A structural member provided with a coating comprising:

15. A structural member for a vehicle body, comprising: a pair of side frames; and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined to each other, including a first steel plate, a second steel plate having an end portion which is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate, wherein at least one of the first steel plate and the second steel plate, and the third steel plate are each plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate, and the thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer on the third steel plate.

16. A structural member according to claim 15, wherein in the overlap portion, the maximum Vickers hardness of the steel plate located on the surface side of the structural member among the first steel plate and the second steel plate is HV ｍａｘ , the minimum value of Vickers hardness is HV ｍｉｎ Then, HV ｍａｘ -HV ｍｉｎ is HV ｍａｘ 30% or less of the structural member.

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