ชิ้นงานดิบ, วิธีการผลิตสำหรับชิ้นประกอบโครงสร้าง และชิ้นประกอบโครงสร้าง

Abstract

DEPCT69 ชิ้นงานดิบ(20,20A)รวมถึงแผ่นเหล็กกล้าหลายแผ่นแผ่นเหล็กกล้าหลายแผ่นได้รับการ จัดวางไว้เพื่อก่อรูปรูปทรงวงแหวนในรูปมองด้านบนของชิ้นงานดิบ(20,20A)และได้รับการเชื่อม ติดเข้ากับกันและกันอย่างน้อยที่สุดหนึ่งแผ่นของแผ่นเหล็กกล้าที่หนึ่งและที่สอง(21,22)และ แผ่นเหล็กกล้าที่สาม(23)โดยแต่ละแผ่นเหล็กกล้าที่ได้รับการชุบเคลือบจะมีแผ่นเหล็กกล้าพื้นฐาน (21a,22a,23a)และชั้นการชุบเคลือบที่มีพื้นฐานเป็นอะลูมินัม(21b,22b,23b)ส่วนปลายของ แผ่นเหล็กกล้าที่หนึ่ง(21)และส่วนปลายของแผ่นเหล็กกล้าที่สอง(22)ก่อรูปส่วนซ้อนทับ(241)ที่มี ความหนาของแผ่นมากที่สุด(tmax)ปริมาณการตกสะสม(W1,W2)ของชั้นการชุบเคลือบ(21b,22b) ของอย่างน้อยที่สุดหนึ่งแผ่นของเหล็กกล้าที่หนึ่งและที่สอง(21,22)มีค่า60g / ตารางเมตรหรือน้อยกว่าและ มีค่าน้อยกว่าปริมาณการตกสะสม(W3)ของชั้นการชุบเคลือบ(23b)ของแผ่นเหล็กกล้าที่สาม(23);

Description

Blank, manufacturing method of structural member, and structural member

[0001] The present disclosure relates to a blank for hot stamping, a method for manufacturing a structural member using the blank, and the 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 an overlapping blank for hot stamping. The overlapping blank in Patent Document 1 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 ２ When 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 1 satisfies the following conditions: 30≦(W1−W2)≦100, and (W1 / W2) ２ × (t1 / t2) ≥ 1.5 is satisfied. According to Patent Document 1, 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.

[0004] Patent Document 2 discloses a method for manufacturing an automobile body side structural frame from a plurality of blanks. In Patent Document 2, the plurality of blanks are joined to form a composite blank, and the composite blank is press-formed to manufacture the body side structural frame. Patent Document 2 also describes hot forming (hot stamping) the composite blank.

[0005] Patent No. 6642777 Publication Special Publication No. 2021-528248

[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. Patent Document 2, for example, discloses a method of hot stamping a composite blank having an annular shape in a plan view to form an annular body side structural frame integrated with pillars, rockers, etc. Patent Document 2 also describes a method of overlapping adjacent steel sheets in the composite blank and joining them by spot welding. When a composite blank includes an overlap portion, it is difficult to ensure a process window in the manufacturing of structural components. Specifically, during hot stamping, the blank is heated until its microstructure is austenitized. However, the overlap portion formed by partial overlapping of steel sheets 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 the rust prevention function of each steel plate.

[0008] A hot stamping blank according to the present disclosure comprises a plurality of steel sheets. The plurality of steel sheets are arranged and joined to form an annular shape in a plan view of the blank. The plurality of steel sheets include a first steel sheet, a second steel sheet, and a third steel sheet. An end of the first steel sheet is overlapped and joined to an end of the second steel sheet, thereby forming an overlap portion together with the end of the second steel sheet. The overlap portion has the maximum sheet thickness in the blank. At least one of the first steel sheet and the second steel sheet, and the third steel sheet are each plated steel sheets. The plated steel sheet comprises a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. The coating weight (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 is ２ ) is 60 or less, and the coating weight (g / m ２ ) less than

[0009] According to the hot stamping blank of the present disclosure, it is possible to manufacture a structural component that combines the strength of the overlap portion, which has the maximum plate thickness, with the rust prevention function of each steel plate.

[0010] FIG. 1 is a plan view of a structural member according to the first embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. FIG. 3A is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 3B is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 3C is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 3D is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 3E is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment. FIG. 3F is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment. FIG. 3G is a schematic view illustrating a manufacturing method for a structural member according to the first embodiment. FIG. 4A is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 4B is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 4C is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 5 is a cross-sectional view of a blank according to a second embodiment. Fig. 6 is a plan view of a structural member according to a modified example of the above embodiment. Fig. 7A is a diagram showing a division pattern of a structural member in the first example. Fig. 7B is a diagram showing another division pattern of a structural member in the first example. Fig. 7C is a diagram showing yet another division pattern of a structural member in the first example. Fig. 7D is a diagram showing yet another division pattern of a structural member in the first example. Fig. 7E is a diagram showing yet another division pattern of a structural member in the first example. Fig. 7F is a diagram showing yet another division pattern of a structural member in the first example. Fig. 7G is a diagram showing yet another division pattern of a structural member in the first example. Fig. 8A is a diagram showing a division pattern of a structural member in the second example. Fig. 8B is a diagram showing another division pattern of a structural member in the second example. Fig. 8C is a diagram showing yet another division pattern of a structural member in the second example. Fig. 8D is a diagram showing yet another division pattern of a structural member in the second example.

[0011] A blank for hot stamping according to an embodiment includes a plurality of steel sheets. The plurality of steel sheets are arranged and joined to form an annular shape in a plan view of the blank. The plurality of steel sheets include a first steel sheet, a second steel sheet, and a third steel sheet. An end of the first steel sheet is overlapped and joined to an end of the second steel sheet, thereby forming an overlap portion together with the end of the second steel sheet. The overlap portion has the maximum sheet thickness in the blank. At least one of the first steel sheet and the second steel sheet, and the third steel sheet are each plated steel sheets. The plated steel sheet has a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. The coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet on at least one of the first steel sheet and the second steel sheet is ２ ) is 60 or less, and the coating weight (g / m ２ ) (first configuration).

[0012] The blank according to the first configuration includes a first steel plate, a second steel plate, and a third steel plate. An end portion of the first steel plate and an end portion of the second steel plate form an overlap portion having the maximum plate thickness in the blank. The coating weight of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is less than the coating weight of the aluminum-based plating layer on the third steel plate. The coating weight of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is 60 g / m ２The 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 on 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, the emissivity of the overlap portion formed by the first steel sheet and the second steel sheet increases during heating of the blank. This allows the overlap portion to be heated to the austenite temperature more 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 on 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.

[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 having the maximum plate thickness with the rust prevention function of each steel sheet. In other words, the blank allows the temperature rise rate of the overlap portion having the maximum plate thickness to be increased, 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 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.

[0015] In the blank according to the first 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 It is preferable that the ratio is ≦3.0 (second configuration).

[0016] When two or more steel plates 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 plates included in the blank have the same thickness. In other words, the difference in thickness between the overlap portion having the largest thickness in the blank and the steel plate 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 in the steel plate having the smallest thickness, making it difficult to ensure the process window of the heating conditions. Therefore, in the second configuration, the thickness t of the overlap portion is 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 coating layer in the thinnest steel sheet progresses excessively, and makes it easier to ensure the process window of the heating conditions.

[0017] In the blank according to the first configuration, the plurality of steel plates may include two or more steel plates having different plate thicknesses. 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 In this case, it is preferable that the 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 (third configuration).

[0018] In the third configuration, a black coating is applied to the surface located outside the overlap portion of at least one of the first steel sheet and the second steel sheet forming 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.

[0019] 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 third 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 using a mold to form the heated blank into a structural component that is annular in plan view and quenching it (fourth configuration).

[0020] A structural member according to an embodiment includes a member body. The member body is formed from a plurality of steel plates joined together. The member body has an annular shape in plan view. The plurality of steel plates include a first steel plate, a second steel plate, and a third steel plate. An end of the first steel plate is overlapped and joined to an end of the second steel plate, thereby forming an overlap portion in the member body together with the end of the second steel plate. The overlap portion has the greatest plate thickness in the member body. At least one of the first steel plate and the second steel plate, and the third steel plate are 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 (fifth configuration).

[0021] Regarding the structural member according to the fifth configuration, in the overlap portion, the maximum value of the Vickers hardness of the steel plate located on the surface side of the member body 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 The sixth configuration may be 30% or less of the above.

[0022] In the structural member according to the fifth or sixth configuration, the member body may include a curved portion. The curved portion is, for example, curved convexly outward from the member body in a plan view of the structural member. At least one of the first steel plate and the second steel plate may be positioned in the curved portion (seventh configuration).

[0023] The structural member according to any one of the fifth to seventh configurations may be a door ring part for an automobile. In this case, the member body may include a front pillar, a center pillar, and a rocker connecting the front pillar and the center pillar (eighth configuration).

[0024] 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.

[0025] <First embodiment> [Structural member] Fig. 1 is a diagram (plan view) of a structural member 10 according to this embodiment, viewed from above in a state where it is placed on a horizontal surface. The structural member 10 is used, for example, in the body of an automobile. The structural member 10 is typically a door ring component of the automobile. In this embodiment, an example in which the structural member 10 is a door ring component will be described.

[0026] The structural member 10 is a hot-stamped member. That is, the structural member 10 is formed by hot stamping (hot press processing) a blank made of a plurality of steel plates. The structural member 10 includes a member body 11. The member body 11 has an annular shape in a plan view of the structural member 10. The member body 11 includes a front pillar 111, a center pillar 112, and a rocker 113.

[0027] When the structural member 10 is assembled to the vehicle body, the center pillar 112 is disposed behind the front pillar 111. The center pillar 112 extends generally in the vertical direction of the vehicle body. The front pillar 111 extends toward the center pillar 112. The front pillar 111 includes a curved portion 111a. The curved portion 111a is a portion that curves convexly outward from the member body 11 in a plan view of the structural member 10. In the example of FIG. 1 , the curved portion 111a is disposed at the upper portion (upper front pillar portion) of the front pillar 111. When the structural member 10 is assembled to the vehicle body, the rocker 113 is disposed below the front pillar 111 and the center pillar 112. The rocker 113 connects the front pillar 111 and the center pillar 112.

[0028] In this embodiment, the member body 11 is formed from a plurality of steel plates 21, 22, and 23 joined together. In the example of FIG. 1 , the front pillar 111 is mainly formed from the steel plates 21 and 23. The steel plate 21 is positioned at the curved portion 111a of the front pillar 111. In other words, at least a portion of the steel plate 21 forms part or all of the curved portion 111a. The center pillar 112 is mainly formed from the steel plate 22. The rocker 113 is formed from the steel plates 22 and 23.

[0029] FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1 . FIG. 2 shows a cross-section of the structural member 10 cut in the thickness direction at the position of the steel plate 21. As shown in FIG. 2 , the steel plate 21 has an open cross-section. In a cross-sectional view of the structural member 10, the steel plate 21 has, for example, a generally hat-shaped shape. More specifically, the steel plate 21 includes a top plate 211, vertical walls 212 and 213, and flanges 214 and 215. The vertical wall 212 is disposed on the opposite side of the vertical wall 213 with respect to the top plate 211. In a cross-sectional view of the structural member 10, one end of the vertical walls 212 and 213 is connected by the top plate 211. In a cross-sectional view of the structural member 10, the other end of the vertical walls 212 and 213 is connected to flanges 214 and 215, respectively. The flanges 214 and 215 protrude from the vertical walls 212 and 213, respectively, to the outside of the structural member 10.

[0030] In the structural member 10, the width W of the steel plate 21, i.e., the portion located above the front pillar 111 (FIG. 1), may be 15 mm or more and 300 mm or less. The height H of the steel plate 21, i.e., the portion located above the front pillar 111, may be 10 mm or more and 150 mm or less. The width W is the distance from the end of the rounded corner between the top plate 211 and the vertical wall 212 on the vertical wall 212 side to the end of the rounded corner between the top plate 211 and the vertical wall 213 on the vertical wall 213 side in the cross section of the structural member 10. The height H is the distance from the top plate 211 to the flanges 214, 215 in the thickness direction of the top plate 211.

[0031] Although not shown, the other steel plates 22, 23 also have an open cross section similar to the steel plate 21. The steel plates 22, 23 may also have, for example, a generally hat-shaped cross section in the cross section of the structural member 10. The width of the portion of the steel plate 22 corresponding to the center pillar 112 ( FIG. 1 ) may be 15 mm or more and 300 mm or less. The height of the portion of the steel plate 22 corresponding to the center pillar 112 may be 10 mm or more and 150 mm or less. The width of the portion of the steel plate 23 corresponding to the lower part of the front pillar 111 ( FIG. 1 ) may be, for example, 30 mm or more and 750 mm or less. The height of the portion of the steel plate 23 corresponding to the lower part of the front pillar 111 may be 25 mm or more and 150 mm or less. The width of the portion of the steel plate 23 corresponding to the rocker 113 ( FIG. 1 ) may be, for example, 30 mm or more and 300 mm or less. The height of the portion of the steel plate 23 corresponding to the rocker 113 may be 25 mm or more and 150 mm or less.

[0032] The size of the annular structural member 10 in a plan view is, for example, 1.0 m or more. The size of the structural member 10 may be, for example, 4.0 m or less. The size of the structural member 10 is the length of a line segment connecting any two points on the outer periphery of the structural member 10 that are farthest apart when the structural member 10 is placed on a horizontal surface and viewed vertically.

[0033] 3A to 3G, a method for manufacturing the structural member 10 will be described. The method for manufacturing the structural member 10 according to this embodiment includes the steps of preparing a blank 20, heating the blank 20, and forming the heated blank 20 into the structural member 10.

[0034] 3A , in the preparation step, a blank 20 having a shape obtained by unfolding the structural member 10 is prepared. The blank 20 includes a plurality of steel plates 21, 22, and 23. The steel plates 21, 22, and 23 are arranged and joined together so as to have an annular shape when the blank 20 is viewed from above.

[0035] 3B, 3C, and 3D are cross-sectional views of the blank 20 showing the joints between the steel plates 21, 22, and 23. FIGS. 3B, 3C, and 3D are cross-sectional views taken along lines IIIB-IIIB, IIIC-IIIC, and IIID-IIID in FIG. 3A, respectively. Referring to FIGS. 3B and 3C, the steel plate 21 is lap-joined to each of the steel plates 22 and 23. That is, the end of the steel plate 21 is lap-joined to the end of the steel plate 22, thereby forming an overlap portion 241 together with the end of the steel plate 22. Similarly, the other end of the steel plate 21 is lap-joined to the end of the steel plate 23, thereby forming an overlap portion 242 together with the end of the steel plate 23. Referring to FIG. 3D, the steel plate 22 is lap-joined to the steel plate 21 as well as the steel plate 23. The end of the steel plate 22 is overlapped and joined to the end of the steel plate 23, thereby forming an overlap portion 243 together with the end of the steel plate 23. The steel plates 21, 22, and 23 are joined by, for example, spot welding or laser welding.

[0036] 3B to 3D, the steel plate 21 has a plate thickness t １ The steel plate 22 has a plate thickness t ２ The steel plate 23 has a plate thickness t ３ In the example of this embodiment, the steel plate 21 has a plate thickness t １ and t of the steel plate 22 ２ is the thickness t of the steel plate 23 ３ Therefore, the overlap portion 241 of the steel plates 21 and 22 has a maximum plate thickness t ｍａｘ The thickness t of the overlap portion 241 ｍａｘ is greater than the plate thickness of the overlap portion 242 between the steel plates 21 and 23 and the plate thickness of the overlap portion 243 between the steel plates 22 and 23. ３ is the minimum plate thickness t of the plurality of steel plates 21, 22, and 23 included in the blank 20. ｍｉｎ is.

[0037] As in this embodiment, the plurality of steel plates 21, 22, and 23 have different plate thicknesses t １ , t ２ , t ３ When having the maximum plate thickness t ｍａｘand 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 ≦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, 4.2 mm or less. ｍａｘ may be 1.6 mm or more.

[0038] The steel sheets 21, 22, and 23 are each plated steel sheets, more specifically, aluminum-plated steel sheets. The steel sheet 21 has a base steel sheet 21a and an aluminum-based plating layer 21b. The steel sheet 22 has a base steel sheet 22a and an aluminum-based plating layer 22b. The steel sheet 23 has a base steel sheet 23a and an aluminum-based plating layer 23b. The sheet thickness t of the steel sheet 21 １ is the combined thickness of the base steel sheet 21a and the aluminum-based plating layer 21b, and is the average thickness of the steel sheet 21. ２ is the combined thickness of the base steel sheet 22a and the aluminum-based plating layer 22b, and is the average thickness of the steel sheet 22. ３ is the combined thickness of the base steel sheet 23 a and the aluminum-based plating layer 23 b, and is the average thickness of the steel sheet 23 .

[0039] In the steel sheet 21, an aluminum-based plating layer 21b covers both surfaces of the base steel sheet 21a. The aluminum-based plating layer 21b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 21a. Similarly, in the steel sheet 22, an aluminum-based plating layer 22b covers both surfaces of the base steel sheet 22a. The aluminum-based plating layer 22b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 22a. Furthermore, in the steel sheet 23, an aluminum-based plating layer 23b covers both surfaces of the base steel sheet 23a. The aluminum-based plating layer 23b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 23a.

[0040] The chemical composition of the aluminum-based plating layers 21b, 22b, 23b is not particularly limited. Known aluminum-based plating layers (plating layers containing aluminum as a main component) can be used as the aluminum-based plating layers 21b, 22b, 23b. While not particularly limited, the aluminum-based plating layers 21b, 22b, 23b are, for example, Al-Si-based plating layers. The aluminum-based plating layers 21b, 22b, 23b may be the same as or different from the aluminum-based plating layers of the other steel sheets.

[0041] Referring to FIG. 3B, the steel plates 21 and 22 have a maximum plate thickness t ｍａｘ The overlap portion 241 has a coating weight W1 (g / m ２ ) is the coating weight W3 (g / m) of the aluminum-based plating layer 23b on the other steel plate 23 (FIGS. 3C and 3D). 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 23b on the other steel sheets 23.

[0042] In the steel sheet 21, the coating weight W1 is the coating weight of the aluminum-based plating layer 21b on both surfaces of the base steel sheet 21a, and is the average coating weight on both surfaces of the base steel sheet 21a. Usually, the coating weight (g / m) of the aluminum-based plating layer 21b on one surface of the base steel sheet 21a is ２) is the coating weight (g / m) of the aluminum-based plating layer 21b on the other surface of the base steel sheet 21a. ２ ) is substantially equal to the coating weight (g / m) of the aluminum-based plating layer 21b on one surface of the base steel sheet 21a. However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 21b may vary between the front and back surfaces of the base steel sheet 21a. The coating weight of the aluminum-based plating layer 21b may be different between one surface and the other surface of the base steel sheet 21a. Similarly, in the steel sheet 22, the coating weight W2 is the coating weight of the aluminum-based plating layer 22b on both surfaces of the base steel sheet 22a, and is the average coating weight on both surfaces of the base steel sheet 22a. Usually, the coating weight (g / m) of the aluminum-based plating layer 22b on one surface of the base steel sheet 22a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer 22b on the other surface of the base steel sheet 22a. ２ ). However, depending on various conditions during manufacturing, for example, the adhesion weight of the aluminum-based plating layer 22b may vary between the front and back of the base steel sheet 22a. The adhesion weight of the aluminum-based plating layer 22b may be different between one surface and the other surface of the base steel sheet 22a. In addition, in the steel sheet 23, the adhesion weight W3 is the adhesion weight of the aluminum-based plating layer 23b on both surfaces of the base steel sheet 23a, and is the average adhesion weight on both surfaces of the base steel sheet 23a. Usually, the adhesion weight (g / m) of the aluminum-based plating layer 23b on one surface of the base steel sheet 23a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer 23b on the other surface of the base steel sheet 23a. ２ ) However, depending on various conditions during manufacturing, for example, the adhesion weight of the aluminum-based plating layer 23b may vary between the front and back surfaces of the base steel sheet 23a. The adhesion weight of the aluminum-based plating layer 23b may be different between one surface and the other surface of the base steel sheet 23a.

[0043] The coating weights W1 and W2 of the aluminum-based plating layers 21b and 22b on the steel sheets 21 and 22 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 ２The coating weight W3 of the aluminum-based plating layer 23b on the steel plate 23 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 21b on the steel sheet 21 and the coating weight W3 of the aluminum-based plating layer 23b on the steel sheet 23, i.e., W3-W1, is, for example, 20 (g / m ２ ) 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 22b on the steel sheet 22 and the coating weight W3 of the aluminum-based plating layer 23b on the steel sheet 23 may be, for example, 20 (g / m ２ ) or more. W3-W2 is 80 (g / m ２ ) or less.

[0044] The method for forming the aluminum-based plating layers 21b, 22b, and 23b on the base steel sheet 21a, 22a, and 23a, respectively, is not particularly limited, but may be, for example, a general hot-dip plating method. That is, by immersing the base steel sheet 21a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 21 having an adjusted coating weight W1 of the aluminum-based plating layer 21b can be obtained. Similarly, by immersing the base steel sheet 22a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 22 having an adjusted coating weight W2 of the aluminum-based plating layer 22b can be obtained. Furthermore, by immersing the base steel sheet 23a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 23 having an adjusted coating weight W3 of the aluminum-based plating layer 23b 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.

[0045] The coating weights W1, W2, and W3 of the aluminum-based plating layers 21b, 22b, and 23b 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 ６The 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.

[0046] However, when the size of the test specimens taken from each of the steel sheets 21, 22, and 22 is small, the cross section of each of the aluminum-based plating layers 21b, 22b, and 23b is observed with an optical microscope (area: 100 μm × 100 μm), and the thickness (μm) of the plating layer is measured in three fields of view. The average of the thicknesses measured in the three fields of view is tripled to convert it into the coating weight. In this case, the coating weight is calculated for each side of the base steel sheet for each of the steel sheets 21, 22, and 22, and the average of the obtained coating weights (average of both sides) is taken as the coating weight of the aluminum-based plating layer. If 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 thickness of the aluminum-based plating layers 21b and 22b of the steel sheets 21 and 22 forming the thickest overlap portion 241 is smaller than the thickness of the aluminum-based plating layer 23b of the other steel sheet 23. The thickness of the aluminum-based plating layer 21b of the steel plate 21 may be equal to or different from the thickness of the aluminum-based plating layer 22b of the steel plate 22.

[0047] (Heating Step) The prepared blank 20 is formed into the structural member 10 (FIGS. 1 and 2) by hot stamping (hot press working). During the hot stamping, the blank 20 is subjected to a heating step. Referring to FIG. 3E, in the heating step, the blank 20 is heated, for example, by a heating furnace 30. The plurality of steel plates 21, 22, and 23 included in the blank 20 are heated to an austenite transformation completion temperature (A ｃ３ The steel plates 21, 22, and 23 are heated to, for example, 900° C. or higher, so that the microstructures of the steel plates 21, 22, and 23 are transformed, for example, entirely or almost entirely into an austenite phase.

[0048] (Forming Process) Referring to Fig. 3F, in the forming process, a die 40 is used to form the heated blank 20 into a structural member 10 (Figs. 1 and 2) that is annular in plan view and quench it. The blank 20 heated in the heating process is removed from the heating furnace 30 (Fig. 3E) and transported to the die 40. The die 40 is attached to a known press device. The die 40 includes, for example, a punch 41 and a die 42. The blank 20 is placed between the punch 41 and the die 42.

[0049] 3G , after the blank 20 is placed between the punch 41 and the die 42, the die 42 moves relatively close to the punch 41. The blank 20 is clamped (pressed) between the punch 41 and the die 42 and formed into a shape that conforms to the forming surfaces of the punch 41 and the die 42. The blank 20 remains clamped between the punch 41 and the die 42. The blank 20 is cooled (quenched) by the die 40, and its microstructure is transformed into martensite. This allows the structural member 10 to be manufactured from the blank 20.

[0050] 4A to 4C are cross-sectional views of the structural member 10 after hot stamping. ｍａｘ 4B and 4C show cross sections of the structural member 10 at other overlapping portions 242, 243.

[0051] Referring to Fig. 4A, in the structural member 10 after hot stamping, the steel sheet 21 is also a plated steel sheet having aluminum-based plating layers 21b on both surfaces of a base steel sheet 21a. Similarly, the steel sheet 22 is also a plated steel sheet having aluminum-based plating layers 22b on both surfaces of a base steel sheet 22a. Referring to Figs. 4B and 4C, the steel sheet 23 is a plated steel sheet having aluminum-based plating layers 23b on both surfaces of a base steel sheet 23a. However, compared to the state of the blank 20 (Figs. 3B to 3D), the aluminum-based plating layers 21b, 22b, 23b in the structural member 10 have been alloyed with iron by the heating process.

[0052] 4A to 4C , the thicknesses K1 and K2 of the aluminum-based plating layers 21b and 22b of the steel sheets 21 and 22 that form the thickest overlap portion 241 are each smaller than the thickness K3 of the aluminum-based plating layer 23b of the other steel sheet 23. The thickness K1 is the average thickness of the aluminum-based plating layer 21b on both surfaces of the base steel sheet 21a of the steel sheet 21. The thickness K2 is the average thickness of the aluminum-based plating layer 22b on both surfaces of the base steel sheet 22a of the steel sheet 22. The thickness K3 is the average thickness of the aluminum-based plating layer 23b on both surfaces of the base steel sheet 23a of the steel sheet 23. The difference K3 - K1 between the thickness K1 of the aluminum-based plating layer 21b of the steel sheet 21 and the thickness K3 of the aluminum-based plating layer 23b of the steel sheet 23 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 22b on the steel sheet 22 and the thickness K3 of the aluminum-based plating layer 23b on the steel sheet 23 is, for example, 7 μm or more. K3-K2 may be 33 μm or less. The thickness K1 of the aluminum-based plating layer 21b on the steel sheet 21 may be equal to or different from the thickness K2 of the aluminum-based plating layer 22b on the steel sheet 22.

[0053] The thicknesses K1, K2, and K3 of the aluminum-based plating layers 21b, 22b, and 23b in the structural member 10 can be measured as follows. Specifically, a vehicle body part is disassembled to obtain the annular structural member 10, and an analysis sample is obtained from the structural member 10, for example, by laser cutting. For example, an analysis sample is obtained from each of the multiple steel plates included in the structural member 10. 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 subjected to nital etching 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 average of the thicknesses of the plating layer measured in the three fields of view can be used as the plating thickness. In many cases, the outermost layer of the structural member 10 contains, for example, an electrodeposition coating film. In such cases, the plating layer present below the electrodeposition coating film and above the base steel sheet is observed.

[0054] Referring to FIG. 4A, in the structural member 10, the maximum plate thickness t ｍａｘ The overlap portion 241 is a joint between two adjacent steel plates 21, 22. Of the steel plates 21, 22 constituting the overlap portion 241, the steel plate on the surface side of the member body 11 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 21, 22 are joined together with relatively thin plating to form an overlap portion 241, and then the steel sheets 21, 22 are subjected to hot stamping. In the overlap portion 241 after hot stamping, the maximum Vickers hardness HV of at least the steel sheet arranged on the surface side of the member body 11 of the steel sheets 21, 22 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 22 is arranged on the surface side (outside) of the member body 11 with respect to the steel plate 21. However, the steel plate 21 may be arranged on the surface side (outside) of the member body 11 with respect to the steel plate 22.

[0055] Maximum Vickers hardness HV in the overlap portion 241 ｍａｘ and minimum value HV ｍｉｎ can be measured as follows. First, a test piece including a cross section of the steel plate 21, 22 located on the surface side of the member body 11 in the overlap portion 241 is taken from the structural member 10 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 241, 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 HVｍｉｎ It can be said that:

[0056] Referring to Fig. 4B, in the structural member 10, the overlap portion 242 is the joint between two adjacent steel plates 21 and 23. Referring to Fig. 4C, the overlap portion 243 is the joint between two adjacent steel plates 22 and 23. As with the thickest overlap portion 241, the maximum Vickers hardness values ​​in each of the overlap portions 242 and 243 are 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 21, 22, and 23 are subjected to hot stamping after the overlap portions 242 and 243 are formed. In the overlap portions 242 and 243 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.

[0057] Maximum Vickers hardness HV of the overlap portion 242 ｍａｘ and minimum value HV ｍｉｎ is measured on the steel plate located on the surface side of the member body 11, of the steel plates 21 and 23, in the overlap portion 242. ｍａｘ and minimum value HV ｍｉｎ is measured on the steel plate located on the surface side of the member body 11 of the steel plates 22 and 23 in the overlap portion 243. ｍａｘ and minimum value HV ｍｉｎ can be measured in the same manner as for the overlap portion 241.

[0058] After the forming process (hot stamping), the steel sheets 21, 22, and 23 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 21, 22, and 23 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of the steel sheets 21, 22, and 23 may be the same as or different from the tensile strength of the other steel sheets.

[0059] [Effect] When the blank 20 is heated during hot stamping, a diffusion layer is formed in the steel sheets 21, 22, 23 by diffusing iron from the base steel sheets 21a, 22a, 23a into the aluminum-based plating layers 21b, 22b, 23b. The diffusion layer becomes thicker as the heating time of the blank 20 becomes longer, which may reduce the corrosion resistance or weldability of the steel sheets 21, 22, 23 due to the aluminum-based plating layers 21b, 22b, 23b. In order to ensure the corrosion resistance and weldability of the steel sheets 21, 22, 23, it is preferable that the thickness of the diffusion layer be kept below a predetermined value. For example, when the steel sheets 21, 22, 23 have a basis weight of 50 to 80 g / m ２ In the case of aluminum-plated steel sheets with a weight of about 40 to 50 g / m, the thickness of the diffusion layer is preferably 15 μm or less. ２ In the case of a thin aluminum-plated steel sheet, the thickness of the diffusion layer is preferably 10 μm or less. ｍａｘ The coating weights W1 and W2 of the aluminum-based plating layers 21b and 22b on the base steel sheets 21a and 22a of the steel sheets 21 and 22 forming the overlap portion 241 are smaller than the coating weight W3 of the aluminum-based plating layer 23b on the base steel sheet 21a of the other steel sheet 23. The coating weights W1 and W2 are 60 g / m ２ As a result, when the blank 20 is heated during hot stamping, the temperature rise rate of the overlap portion 241 can be increased, and the heating time of the overlap portion 241 can be shortened. Specifically, when the aluminum-based plating layers 21b, 22b are formed on the surface layers of the steel sheets 21, 22 at a coating weight of 60 g / m ２Since the thickness is less than 1 / 2 mm, when the blank 20 is heated, alloying of the aluminum-based plating layers 21b, 22b with the iron contained in the base steel sheets 21a, 22a progresses rapidly to the surfaces of the steel sheets 21, 22, and both surfaces of the steel sheets 21, 22, including the overlap portion 241, turn black or a color close to black relatively quickly. This allows the temperature rise of the overlap portion 241 to be accelerated, and the heating time of the overlap portion 241 to be shortened. Therefore, the heating of the blank 20 can be terminated before the alloying of the aluminum-based plating layers 21b, 22b, 23b in the steel sheets 21, 22, 23 progresses excessively and 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 241 having the above-mentioned thickness can be ensured by hot stamping, and the corrosion resistance (rust prevention) of the steel plates 21, 22, and 23 can be ensured. Furthermore, by reducing the thickness of the diffusion layer, the weldability of the steel plates 21, 22, and 23 can be more easily ensured.

[0060] As described above, according to the blank 20 of this embodiment, the maximum plate thickness t ｍａｘ Therefore, it is possible to heat the entire blank 20 to a temperature required for hot stamping before the alloying of the aluminum-based plating layers 21b, 22b, and 23b in the steel sheets 21, 22, and 23 progresses excessively, and it is possible to ensure a process window for the heating conditions in the manufacture of the structural member 10. Therefore, the maximum sheet thickness t ｍａｘ This allows the manufacture of a structural member 10 that combines the strength of the overlap portion 241 and the rust prevention function of each of the steel plates 21, 22, and 23. In addition, by increasing the temperature rise rate of the overlap portion 241, the heating time of the blank 20 can be shortened, thereby improving the productivity of the structural member 10. Furthermore, by shortening the heating time of the blank 20, energy consumption in the manufacture of the structural member 10 is suppressed, and the amount of greenhouse gases generated during heating can be reduced.

[0061] In this embodiment, the coating weight W3 of the aluminum-based plating layer 23b on the steel plate 23 is greater than that on the steel plates 21, 22 that form the thickest overlap portion 241. That is, the steel plate 23 has higher rust prevention performance than the steel plates 21, 22. The steel plate 23 is usually disposed in a portion of the structural member 10 that requires relatively high rust prevention performance. This ensures the rust prevention performance required for the annular structural member 10.

[0062] In this embodiment, the coating weights W1 and W2 of the aluminum-based plating layers 21b and 22b on the steel sheets 21 and 22 may be the same or different. However, from the viewpoint of uniformly heating the blank 20, it is preferable that the coating weight W1 of the aluminum-based plating layer 21b on the steel sheet 21 is substantially equal to the coating weight W2 of the aluminum-based plating layer 22b on the steel sheet 22.

[0063] In this embodiment, in the heating process, first, the blank 20 is heated to the minimum thickness t ｍｉｎ The steel plate 23 having a maximum thickness 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 case where the steel plates 21, 22, and 23 included in the blank 20 have different thicknesses as in the present embodiment, the overlap portion 241 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 23 having a plate thickness t ｍａｘ The temperature rise of the overlap portion 241 having the minimum thickness t ｍｉｎ and maximum plate thickness t ｍａｘ Ratio to: t ｍａｘ / t ｍｉｎis preferably 3.0 or less. This allows the overlap portion 241 to be sufficiently heated until the phase transformation to austenite is completed before the alloying of the aluminum-based plating layer 23b of the thinnest steel sheet 23 progresses and the corrosion resistance is lost, even if there is a difference in sheet thickness among the steel sheets 21, 22, and 23. This makes it easier to ensure a process window in the manufacture of the structural member 10.

[0064] Second Embodiment Fig. 5 is a cross-sectional view of a blank 20A according to a second embodiment. Fig. 5 shows an overlap portion 241 formed by steel plates 21 and 22. The blank 20A according to this embodiment has a maximum plate thickness t ｍａｘ The blank 20 differs from the blank 20 according to the first embodiment in that a coating 25 is applied to the overlap portion 241 having the overlap portion 241 .

[0065] In each of the steel plates 21 and 22, the surface located outside the overlap portion 241 is coated with a coating 25. In the steel plate 21, the entire surface opposite the mating steel plate 22 is coated with the coating 25. In the steel plate 22, the entire surface opposite the mating steel plate 21 is also coated with the coating 25. In other words, the surfaces of the steel plates 21 and 22 that constitute the front and back surfaces of the overlap portion 241 are each coated with the coating 25.

[0066] The coating 25 is a substantially black coating. For example, the lightness L ＊ Value (CIE 1976 lightness index L defined in JIS Z8781-4:2013) ＊ ) is 60 or less, the coating 25 can be determined to be black. The coating 25 may be a carbon-based surface treatment coating (a coating containing carbon (C)).

[0067] The coating 25 may be, for example, a surface treatment coating described in International Publication No. 2022 / 215229. That is, the coating 25 may contain, for example, carbon black. The coating 25 may further contain a metal oxide. The metal oxide may be, for example, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating 25 may also contain silica.

[0068] The coating 25 may contain graphite, soot, or the like instead of or in addition to carbon black. Alternatively, the coating 25 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 the steel sheet 21. 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.

[0069] The thickness of the coating 25 is, for example, 0.5 μm or more and 5.0 μm or less. The thickness of the coating 25 is preferably 1.0 μm or more and 3.0 μm or less. The thickness of the coating 25 is negligibly small compared to the thicknesses of the steel plates 21, 22, and 23. Therefore, the thickness measured including the coating 25 is subtracted from the thickness t of the overlap portion 241. ｍａｘ can be treated as

[0070] In this embodiment, the substantially black coating 25 is applied to the surface located outside the overlap portion 241 of each of the steel sheets 21 and 22, thereby increasing the emissivity of both outer surfaces of the overlap portion 241. For example, in each of the steel sheets 21 and 22, the surface coated with the coating 25 has an emissivity of 60% or more at a measurement temperature of 25°C and a wavelength of 8.0 μm. By applying the coating 25, the surface of the steel sheets 21 and 22 located outside the overlap portion 241 has an emissivity at a wavelength of 8.0 μm at 25°C that is, for example, more than 5%, preferably more than 10%, and more preferably more than 20%, higher than that of other steel sheets not provided with the coating 25. By increasing the emissivity of the overlap portion 241 in advance in this way, the temperature rise of the overlap portion 241 can be more quickly achieved when the blank 20A is heated during hot stamping. Therefore, the maximum sheet thickness t of the blank 20A is increased. ｍａｘ and an overlap portion 241 having a minimum plate thickness t ｍｉｎ Even when the difference in plate thickness between the steel plate 21 having the same thickness as the steel plate 23 (FIGS. 3C and 3D) is large, the process window of the heating conditions can be more easily ensured.

[0071] In the blank 20 according to this embodiment, 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 this embodiment, the emissivity of the overlap portion 241 is increased in advance by the substantially black coating 25, and therefore, when the blank 20A is heated during hot stamping, the temperature rise of the overlap portion 241 is accelerated. ｍａｘ / t ｍｉｎ When is large, for example, t ｍａｘ / t ｍｉｎEven if the maximum thickness t of the blank 20A exceeds 3.0, it becomes easier to complete heating of the thickest overlap portion 241 before alloying of the aluminum-based plating layer 23b in the thinnest steel sheet 23 (FIGS. 3C and 3D) 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.

[0072] In the example of FIG. 5 , in each of the steel plates 21 and 22, the surface located outside the overlap portion 241 is substantially coated with a black coating 25. However, in one of the steel plates 21 and 22, the surface located outside the overlap portion 241 may be coated with the coating 25, and in the other of the steel plates 21 and 22, the surface located outside the overlap portion 241 may not be coated with the coating 25. Also, in the example of FIG. 5 , in each of the steel plates 21 and 22, the surface located inside the overlap portion 241 is not substantially coated with the black coating 25. However, in each of the steel plates 21 and 22, the surface located inside the overlap portion 241 may also be coated with the coating 25. However, from the viewpoint of uniformly heating the blank 20A during hot stamping, it is preferable that in each of the steel plates 21 and 22, the surface outside the overlap portion 241 is coated with the coating 25 and the surface located inside the overlap portion 241 is not covered with the coating 25.

[0073] 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.

[0074] In the blanks 20 and 20A according to the above embodiments, the maximum plate thickness t ｍａｘ In both of the steel plates 21 and 22 forming the overlap portion 241, the coating weights W1 and W2 of the aluminum-based plating layers 21b and 22b are 60 g / m ２, which is smaller than the coating weight W3 of the aluminum-based plating layer 23b of the other steel sheets 23. However, in the blanks 20 and 20A, the coating weight of the aluminum-based plating layer on at least one of the steel sheets 21 and 22 is 60 g / m or less. ２ It is sufficient that the coating weight W1 of the aluminum-based plating layer 21b on the steel sheet 21 is 60 g / m or less and is smaller than the coating weight W3 of the aluminum-based plating layer 23b on the other steel sheet 23. For example, when the coating weight W1 of the aluminum-based plating layer 21b on the steel sheet 21 is 60 g / m ２ or less, the coating weight W2 of the aluminum-based plating layer 22b on the steel sheet 22 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 23b on the steel sheet 23. Similarly, if the coating weight W2 of the aluminum-based plating layer 22b on the steel sheet 22 is 60 g / m ２ or less, the coating weight W1 of the aluminum-based plating layer 21b on the steel sheet 21 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 23b on the steel sheet 23.

[0075] In the structural member 10 according to the above embodiment, the maximum plate thickness t ｍａｘIn both of the steel plates 21, 22 forming the overlap portion 241, the thicknesses K1, K2 of the aluminum-based plating layers 21b, 22b are smaller than the thickness K3 of the aluminum-based plating layer 23b on the other steel plate 23. However, in the structural member 10, it is sufficient that the thickness of the aluminum-based plating layer on at least one of the steel plates 21, 22 is smaller than the thickness K3 of the aluminum-based plating layer 23b on the other steel plate 23. For example, when the thickness K1 of the aluminum-based plating layer 21b on the steel plate 21 is smaller than the thickness K3 of the aluminum-based plating layer 23b on the steel plate 23, the thickness K2 of the aluminum-based plating layer 22b on the steel plate 22 may be equal to or greater than the thickness K3 of the aluminum-based plating layer 23b on the steel plate 23. Similarly, when the thickness K2 of the aluminum-based plating layer 22b on the steel plate 22 is smaller than the thickness K3 of the aluminum-based plating layer 23b on the steel plate 23, the thickness K1 of the aluminum-based plating layer 21b on the steel plate 21 may be equal to or greater than the thickness K3 of the aluminum-based plating layer 23b on the steel plate 23.

[0076] In the above embodiment, the maximum plate thickness t ｍａｘ Both of the steel sheets 21, 22 forming the overlap portion 241 are aluminum-plated steel sheets. That is, the steel sheets 21, 22 each have a base steel sheet 21a, 22a and an aluminum-based plating layer 21b, 22b. However, it is sufficient if at least one of the steel sheets 21, 22 is a plated steel sheet having a base steel sheet and an aluminum-based plating layer. That is, either one of the steel sheets 21, 22 does not have to be an aluminum-plated steel sheet. When one of the steel sheets 21, 22 is an aluminum-plated steel sheet, the other of the steel sheets 21, 22 may be, for example, a zinc-plated steel sheet.

[0077] In the blanks 20 and 20A according to the above embodiments, the ends of the steel plates 21 and 22 are overlapped and joined to form an overlap portion 241. Similarly, the ends of the steel plates 21 and 23 are overlapped and joined to form an overlap portion 242. Furthermore, the ends of the steel plates 22 and 23 are overlapped and joined to form an overlap portion 243. However, it is sufficient for the blanks 20 and 20A to include at least one overlap portion. In other words, the steel plate 23 does not necessarily have to be overlapped and joined to the steel plates 21 and 22. The steel plate 23 may be butt-joined to one or both of the steel plates 21 and 22.

[0078] In the above embodiment, the steel plates 21, 22, and 23 included in the blank 20 or 20A may each be a single layer or multiple layers. That is, the steel plates 21, 22, and 23 may each be a single steel plate or a plate material formed by overlapping multiple steel plates.

[0079] In the above embodiment, each of the blanks 20 and 20A includes three steel sheets 21, 22, and 23. However, the number of steel sheets included in the blanks 20 and 20A is not limited to this. The blanks 20 and 20A may include four or more steel sheets. When the blanks 20 and 20A include four or more steel sheets, the steel sheets other than the steel sheets 21, 22, and 23 may be aluminum-plated steel sheets, other plated steel sheets, or steel sheets without a plating layer (bare material). When the steel sheets other than the steel sheets 21, 22, and 23 are plated steel sheets, the coating weight of the plating layer on the steel sheet may be equal to or different from any of the coating weights W1, W2, and W3 of the aluminum-based plating layers 21b, 22b, and 23b on the steel sheets 21, 22, and 23.

[0080] In the blanks 20 and 20A according to the above embodiments, the maximum plate thickness t ｍａｘ The steel plate 23 that does not form the overlap portion 241 has the minimum plate thickness t ｍｉｎ However, the maximum plate thickness t ｍａｘThe minimum thickness t of the annular blank 20, 20A is t ｍｉｎ Alternatively, when the blank 20 or 20A includes four or more steel plates, the steel plates other than the steel plates 21, 22, and 23 may have a minimum plate thickness t ｍｉｎ may have

[0081] In the above embodiment, the maximum plate thickness t ｍａｘ However, in the blank 20 or 20A, there is only one overlap portion 241 having a maximum thickness t ｍａｘ In this case, there may be a plurality of overlapping portions having the maximum plate thickness t ｍａｘ For all of the overlapping portions having the above structure, the coating weight of the aluminum-based plating layer of at least one of the two steel sheets forming the overlapping portion is 60 g / m ２ It is preferable that:

[0082] In the blanks 20 and 20A according to the above embodiments, the steel plates 21, 22, and 23 have different thicknesses. However, among the three or more steel plates included in each of the blanks 20 and 20A, two or more may have the same thickness, or all of the steel plates may have the same thickness. When the steel plates included in the blanks 20 and 20A all have the same thickness, t ｍａｘ / t ｍｉｎ is 2.0.

[0083] In the above embodiment, the mold 40 used for hot stamping the blank 20 includes a punch 41 and a die 42. However, the configuration of the mold 40 is not limited to the example described in the above embodiment. The mold 40 may further include, for example, a pad and a blank holder.

[0084] In the above embodiment, the main body 11 of the structural member 10 includes a front pillar 111, a center pillar 112, and a rocker 113. However, the main body 11 may further include other components. For example, as shown in FIG. 6 , the main body 11 may further include a rear pillar 114. The rear pillar 114 may include a curved portion 114a that curves convexly outward from the main body 11 in a plan view of the structural member 10. In the example shown in FIG. 6 , the curved portion 114a is located in the upper portion (upper rear pillar portion) of the rear pillar 114. The structural member 10 according to the above embodiment is a door ring component having a single ring shape (single door ring component). On the other hand, the structural member shown in FIG. 6 is a door ring component having a double ring shape (double door ring component). When manufacturing a double door ring component, the blank used as the material also has a double ring shape.

[0085] In the structural member 10 according to the first embodiment, the steel sheet 21, which has a relatively small amount of aluminum-based plating layer 21b, is positioned in the curved portion 111a of the front pillar 111. However, the steel sheet 22, which forms the overlap portion 241 together with the steel sheet 21, may also be positioned in the curved portion 111a. In the structural member shown in FIG. 6, at least one of the steel sheets 21, 22 may be positioned in the curved portion 114a of the rear pillar 114. In door ring components, the curved portions 111a, 114a are locations requiring relatively low corrosion resistance. Because the thinly plated steel sheets 21 and / or 22 have inferior corrosion resistance compared to thickly plated steel sheets, they are preferably positioned in the curved portion 111a or 114a, where high corrosion resistance is not required. However, the positions of the steel sheets 21, 22 are not limited thereto. The steel plates 21 and 22 may be disposed in the lower part of the front pillar 111 (lower front pillar part), the lower part of the rear pillar 114 (lower front pillar part), or the center pillar 112 .

[0086] 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.

[0087] [First Example] In order to confirm the effects of the present disclosure, a CAE analysis was performed on a press-formed (hot stamped) structural member that is a single door ring part, 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.

[0088] The steel plates used in this analysis are shown in Table 1.

[0089]

[0090] In Table 1, the material type is listed in the order of plating type, tensile strength, and application (hot stamping). However, in cases where a black coating containing carbon (C) is provided on the plating layer, the material type is prefixed with the words "black coating (one side)" or "black coating (both sides)." "Black coating (one side)" means that one entire side of the plated steel sheet is covered with a black coating. "Black coating (both sides)" means that both entire sides of the plated steel sheet are covered with a black coating. In Table 1, the coating weight of the plating layer is shown as a value per side, but in this analysis, the coating weight of the plating layer is the same on both sides of the material.

[0091] The division patterns of the structural members are shown in Figures 7A to 7G. Figures 7A to 7G show the number of steel plates (materials) included in the structural member, which is a single door ring component, and the positions of the joints between the steel plates in the structural member. In Figures 7A to 7G, each steel plate is given a number in parentheses.

[0092] The analysis conditions and results for division patterns 1 and 2 shown in Figures 7A and 7B are shown in Table 2. In Figures 7A and 7B, the structural member is formed from three materials (1) to (3). Each of materials (1) to (3) is overlapped and joined with the adjacent material.

[0093]

[0094] 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 (Aｃ３ The "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.

[0095] Referring to Table 2 and FIG. 7A, in Example 1, the material (1) and the material (2) have the maximum plate thickness t ｍａｘ In Example 1, the coating weight of the aluminum-based plating layer on both the base material (1) and the base material (2) was 40 g / m ２ In Comparative Example 1, which has the same combination of material type and plate thickness as Example 1, the coating weight of the aluminum-based plating layer was 80 g / m for materials (1) to (3). ２ In Example 1, the thickest overlap portion was made the thinly plated portion, which accelerated the temperature rise in this overlap portion, and the time required to complete heating of the blank was significantly reduced compared to Comparative Example 1. As a result, the process window in Example 1 was extended by nearly 50 seconds compared to Comparative Example 1.

[0096] Referring to Table 2 and FIG. 7B, in Example 3, the material (1) and the material (3) have the maximum plate thickness t ｍａｘ In Example 3, the coating weight of the aluminum-based plating layer on both the materials (1) and (3) was 40 g / m. ２ In Comparative Example 2, which has the same combination of material type and plate thickness as Example 3, the coating weight of the aluminum-based plating layer was 80 g / m for materials (1) to (3). ２In Example 3, the thickest overlap portion was made the thinly plated portion, which accelerated the temperature rise in this overlap portion, and the time required to complete heating of the blank was significantly reduced compared to Comparative Example 2. As a result, the process window in Example 3 was extended by more than 50 seconds compared to Comparative Example 2.

[0097] In Example 2, the material (2) and the material (3) have the maximum plate thickness t ｍａｘ In Example 2, the coating weight of the aluminum-based plating layer on both the materials (2) and (3) was 40 g / m ２ , which is smaller than the amount of aluminum-based plating layer deposited on the other material (1). It was also confirmed that the process window was sufficiently secured in Example 2.

[0098] The analysis conditions and results for division patterns 3 and 4 shown in Figures 7C and 7D are shown in Table 3. In Figures 7C and 7D, the structural member is formed from four materials (1) to (4). Each of materials (1) to (4) is overlapped and joined with the adjacent material.

[0099]

[0100] In Examples 4 to 11 in Table 3, similarly to Examples 1 to 3 (Table 2), the maximum plate thickness t ｍａｘ In the material forming the overlap portion having the above structure, the coating amount of the aluminum-based plating layer is 40 g / m ２ , which is less than that of other materials. Therefore, the process window of the heating conditions was able to be secured for Examples 4 to 11. When each Example is compared with the corresponding Comparative Example, it can be seen that the process window is expanded.

[0101] For example, in Example 8, the material (1) and the material (3) have the maximum plate thickness t ｍａｘ In Example 8, the coating weight of the aluminum-based plating layer on each of the materials (1) and (3) was 40 g / m ２, which is less than the coating weight of the aluminum-based plating layer in the material (4) (Table 3 and FIG. 7D). On the other hand, in Comparative Example 4, the combination of material type and plate thickness was the same as in Example 8, but the coating weight of the aluminum-based plating layer was 80 g / m for all materials. ２ In Example 8, the temperature rise in the overlapping portion, which is the thinly plated portion, was accelerated, and the time required to complete heating of the blank was significantly reduced compared to Comparative Example 4. In Example 8, the process window was extended by 60 seconds or more compared to Comparative Example 4. In Comparative Example 4, the process window of the heating conditions was eliminated (negative).

[0102] For example, in Example 9, the material (3) and the material (4) have the maximum plate thickness t ｍａｘ In Example 9, the coating weight of the aluminum-based plating layer on each of the materials (3) and (4) was 40 g / m. ２ , which is less than the coating weight of the aluminum-based plating layer in the material (2) (Table 3 and FIG. 7D). On the other hand, in Comparative Example 7, the combination of material type and plate thickness was the same as in Example 9, but the coating weight of the aluminum-based plating layer was 80 g / m for all materials. ２ In Example 9, the temperature rise in the overlap portion, which is the thinly plated portion, was accelerated, and the blank heating completion time was significantly shortened compared to Comparative Example 7. In Example 9, the process window was expanded by nearly 60 seconds compared to Comparative Example 7. In Comparative Example 7, a black coating was applied to both sides of the thinnest material (4), and the time to reach 910°C was slightly more than 10 seconds earlier than in Example 9. However, in Example 9, the blank heating completion time was 45 seconds shorter than in Comparative Example 7, so it can be said that the process window was expanded compared to Comparative Example 7 even when taking into account the difference in the time to reach 910°C.

[0103] The analysis conditions and results for division patterns 5 to 7 shown in Figures 7E to 7G are shown in Table 4. In Figures 7E to 7G, the structural member is formed from five materials (1) to (5). Materials (1) to (5) are each overlapped and joined with the adjacent material.

[0104]

[0105] In Examples 12 to 21 in Table 4, the maximum plate thickness t ｍａｘ In at least one of the two materials forming the overlap portion, the coating amount of the aluminum-based plating layer is 60 g / m ２ or less, which is less than that of other materials. For these Examples 12 to 21, the process window of the heating conditions was also sufficiently secured. When each Example is compared with the corresponding Comparative Example, it can be seen that the process window is expanded.

[0106] For example, in Example 14, the material (1) and the material (2) have the maximum plate thickness t ｍａｘ In Example 14, the coating weight of the aluminum-based plating layer on each of the materials (1) and (2) was 40 g / m ２ On the other hand, in Comparative Example 9, the combination of material type and plate thickness was the same as in Example 14, but the coating weight of the aluminum-based plating layer was 80 g / m or less for all materials. ２ The same applies below. In Example 14, the temperature rise in the overlapping portion, which is the thinly plated portion, was accelerated, and the time required to complete heating of the blank was significantly shortened compared to Comparative Example 9. In Example 14, the process window was extended by 50 seconds or more compared to Comparative Example 9. In Comparative Example 9, the process window of the heating conditions was eliminated (negative).

[0107] For example, in Example 16, the material (1) and the material (3) have the maximum plate thickness t ｍａｘ In Example 16, the coating weight of the aluminum-based plating layer on each of the materials (1) and (3) was 40 g / m ２ , which is smaller than the deposition amount of the aluminum-based plating layer in each of the materials (2) and (5) (Table 4 and FIG. 7F). In Example 16, the materials (3) and (4) have the maximum plate thickness t ｍａｘIn Example 16, the coating weight of the aluminum-based plating layer on each of the materials (3) and (4) was 40 g / m ２ On the other hand, in Comparative Example 11, the combination of material type and plate thickness was the same as in Example 16, but the coating weight of the aluminum-based plating layer was 80 g / m for all materials. ２ In Example 16, the temperature rise in the overlap portion, which is the thinly plated portion, was accelerated, and the time required to complete heating of the blank was significantly reduced compared to Comparative Example 11. In Example 16, the process window was extended by 30 seconds or more compared to Comparative Example 11.

[0108] In Example 19, the material (1) and the material (3), and the material (3) and the material (4) each have a maximum plate thickness t ｍａｘ In Example 19, the coating weight of the aluminum-based plating layer on the base material (3) was 40 g / m ２ and the coating weight of the aluminum-based plating layer in each of the materials (1) and (4) was 60 g / m ２ In Example 19, the temperature rise in the overlapping portion, which is the thinly plated portion, was also accelerated, and the blank heating completion time was shortened compared to each comparative example. In Example 18, the blank heating completion time was further shortened compared to Example 19. In Example 18, the maximum plate thickness t ｍａｘ In each of the materials (1) and (4) forming the overlapping portion of the aluminum-based plating layer, the coating weight of the aluminum-based plating layer is 50 g / m ２ It is as follows.

[0109] In Example 20, the material (1) and the material (3), and the material (3) and the material (4) each have a maximum plate thickness t ｍａｘ In Example 20, the coating weight of the aluminum-based plating layer in the base material (4) was 80 g / m ２ However, the coating weight of the aluminum-based plating layer in materials (1) and (3) was 60 g / m ２In this Example 20, at least one of the two materials forming the overlapping portion is applied with a deposition amount of 60 g / m ２ By making the following thinly plated areas, the temperature rise in the overlapping area was promoted, and the time required to complete heating of the blank was shortened compared to each comparative example.

[0110] In Example 21, the material (1) and the material (3), and the material (3) and the material (4) each have a maximum plate thickness t ｍａｘ In Example 21, the coating weight of the aluminum-based plating layer in the materials (1) and (4) was 80 g / m. ２ However, the coating weight of the aluminum-based plating layer in the material (3) was 60 g / m ２ In this Example 21, one of the two materials forming the overlapping portion is applied with a deposition amount of 60 g / m ２ By providing the following thinly plated areas, the temperature rise in the overlapping areas was accelerated, and the heating completion time of the blank was shortened compared to each comparative example. In Example 21, by providing a black coating to the materials (1) and (4) to increase the emissivity of the overlapping areas in advance, the heating completion time of the blank was further shortened compared to Example 20.

[0111] Second Example Regarding the press forming (hot stamping) of a structural member that is a double door ring part, an analysis similar to that of the first example was carried out while changing the material type and plate thickness of the structural member, as well as the division pattern of the structural member.

[0112] As with the first embodiment, the steel plates used as raw materials were selected from those shown in Table 1. The division patterns of the structural members are as shown in Figures 8A to 8D. Figures 8A to 8D show the number of steel plates (raw materials) included in the structural members that are double door ring components, and the positions of the joints between the steel plates in the structural members. In Figures 8A to 8D, each steel plate used as raw material is given a number in parentheses.

[0113] The analysis conditions and results for division patterns 8 and 9 shown in Figures 8A and 8B are shown in Table 5. In Figures 8A and 8B, the structural member is formed from six materials (1) to (6). Each of materials (1) to (6) is overlapped and joined with the adjacent material.

[0114]

[0115] Referring to Table 5, in Example 22, the materials (1) and (3), and the materials (3) and (4) each have the maximum plate thickness t ｍａｘ In Example 23, the materials (1) and (3) have an overlap portion with a maximum sheet thickness t = 3.2 mm, and the coating weight of the aluminum-based plating layer on each of the materials (1), (3), and (4) is smaller than the coating weight of the aluminum-based plating layer on the material (2). ｍａｘ In Example 24, the materials (1) and (3), and the materials (3) and (4) each had an overlapping portion with a maximum plate thickness t ｍａｘ = 3.0 mm, and the coating weight of the aluminum-based plating layer on each of the materials (1), (3), and (4) was less than the coating weight of the aluminum-based plating layer on the material (2). In Examples 22 to 24, the coating weight of the aluminum-based plating layer on the material forming the thickest overlap portion was 40 g / m ２ On the other hand, in Comparative Examples 12 and 13, the coating weight of the aluminum-based plating layer was 80 g / m for all materials. ２ are identical.

[0116] From Table 5, in Examples 22 to 24, the maximum plate thickness t ｍａｘ By making the overlapping portion a thinly plated portion, the temperature rise in the overlapping portion was accelerated, and the time to complete heating of the blank was significantly shortened compared to Comparative Examples 12 and 13. As a result, in Examples 22 to 24, the process window of the heating conditions was significantly expanded compared to Comparative Examples 12 and 13.

[0117] The analysis conditions and results for division patterns 10 and 11 shown in Figures 8C and 8D are shown in Table 6. In Figures 8C and 8D, the structural member is formed from seven materials (1) to (7). Each of materials (1) to (7) is overlapped and joined with the adjacent material.

[0118]

[0119] Referring to Table 6, in Examples 25 and 27, the materials (1) and (3) had the maximum plate thickness t ｍａｘ In Example 26, the materials (1) and (3) formed overlap portions with a maximum sheet thickness t = 3.4 mm, and the deposition weight of the aluminum-based plating layer on each of the materials (1) and (3) was smaller than the deposition weight of the aluminum-based plating layer on each of the materials (2) and (5). ｍａｘ In Examples 25 to 27, the coating weight of the aluminum-based plating layer on the material forming the thickest overlap portion was 40 g / m². ２ On the other hand, in Comparative Examples 14 and 15, the coating weight of the aluminum-based plating layer was 80 g / m for all materials. ２ are identical.

[0120] From Table 6, in Examples 25 to 27, the maximum plate thickness t ｍａｘ By making the overlapping portion a thinly plated portion, the temperature rise in the overlapping portion was accelerated, and the time to complete heating of the blank was significantly shortened compared to Comparative Examples 14 and 15. As a result, in Examples 25 to 27, the process window of the heating conditions was significantly expanded compared to Comparative Examples 14 and 15.

[0121] [Third Example] Regarding press forming (hot stamping) of a structural member that is a single door ring part, the maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ In order to check the effect of the difference between ｍａｘ / t ｍｉｎ The analysis conditions and results are shown in Table 7.

[0122]

[0123] The division pattern of Test Examples 1, 3, and 5 and Comparative Example 16 is division pattern 7 shown in FIG. 7G. In Test Examples 1 and 3, the materials (1) and (3) have a maximum plate thickness t ｍａｘ In addition, in Test Examples 1 and 3, the coating weight of the aluminum-based plating layer in each of the materials (1) and (3) was 60 g / m ２ In Test Example 5, the materials (1) and (3), and the materials (3) and (4) each had a maximum plate thickness t ｍａｘ In addition, in Test Example 5, the coating weight of the aluminum-based plating layer in each of the materials (1), (3), and (4) was 60 g / m ２ or less, which is smaller than the deposition weight of the aluminum-based plating layer in the other materials. In Test Example 3, in materials (1) and (3), the surfaces located outside the overlap portion are coated with a black film. In Test Example 5, in materials (1), (3), and (4), the surfaces located outside the overlap portion are coated with a black film. In Comparative Example 16, in all materials, the deposition weight of the aluminum-based plating layer was 80 g / m ２ is equal to.

[0124] The division pattern of Test Examples 2 and 4 and the Reference Example is division pattern 5 shown in FIG. 7E. In Test Examples 2 and 4, the materials (1) and (3), and the materials (3) and (4) each have a maximum plate thickness t ｍａｘ In addition, in Test Examples 2 and 4, the coating weight of the aluminum-based plating layer in each of the materials (1), (3), and (4) was 60 g / m ２ In the reference example, all materials (1) to (5) have the same plate thickness and an aluminum-based plating layer coating weight of 80 g / m or less, which is smaller than the aluminum-based plating layer coating weight of the other materials. ２ is equal to.

[0125] In the reference example, there is no difference in thickness between the materials (1) to (5), so the minimum thickness t ｍｉｎ Maximum thickness t for (thickness of materials (1) to (5)) ｍａｘ (Thickness of overlapping portion) ratio: t ｍａｘ / t ｍｉｎ In the reference example, the maximum plate thickness t ｍａｘ and minimum plate thickness t ｍｉｎ The difference between (t ｍａｘ / t ｍｉｎ ) is not large, so the coating weight of the aluminum-based plating layer in the overlapping portion is 80 g / m ２ On the other hand, if there is a difference in thickness between materials (1) to (5), the maximum thickness t ｍａｘ and minimum plate thickness t ｍｉｎ The difference between (t ｍａｘ / t ｍｉｎ In this case, the effect of forming the overlap portion into a thinly plated portion is likely to be significantly exhibited.

[0126] From Table 7, the maximum plate thickness t ｍａｘ The coating weight of the aluminum-based plating layer on the material forming the overlap portion is 60 g / m ２ If we set the following, t ｍａｘ / t ｍｉｎ It can be seen that even if the maximum plate thickness t is about 3.0, the heating completion time is unlikely to be long and the process window can be secured (Test Examples 1 and 2). ｍａｘ When a black coating is applied to the outside of the overlapping portion of t ｍａｘ / t ｍｉｎ It can be seen that even if the maximum plate thickness t exceeds 3.0, the heating completion time is unlikely to become long, and the process window can be secured (Test Example 3). ｍａｘ Even if a black coating is applied to each of the materials forming the overlapping portion of t ｍａｘ / t ｍｉｎ When the value exceeded 4.0, it became difficult to ensure the process window (Test Example 5).

[0127] Under the conditions of this analysis, if there is a difference in thickness between the materials, ｍａｘ / t ｍｉｎIt is preferable that the maximum plate thickness t ｍａｘ If a black coating is applied to the outside of the overlapping portion of t ｍａｘ / t ｍｉｎ It may be ≦4.0.

[0128] The productivity of the structural members and the greenhouse gas emission ratio were evaluated for Test Examples 1 and 3 and Comparative Example 16. The evaluation results are shown in Table 8.

[0129]

[0130] As shown in Table 8, in Test Examples 1 and 3, the time required to complete heating of the blank was shorter than in Comparative Example 16 (Table 7), and therefore the productivity of the structural components was improved compared to Comparative Example 16. Furthermore, in Test Examples 1 and 3, the time required to complete heating of the blank was shorter than in Comparative Example 16, and the amount of energy consumed in manufacturing the structural components was reduced, so the greenhouse gas generation rate during manufacturing was also reduced. In Table 8, "Productivity" is the number of blanks (structural components) that can be heat-treated per minute in a heating furnace. "Greenhouse gas generation rate" is the ratio of greenhouse gas generation rate to Comparative Example 16. The amount of greenhouse gases generated during the heating process was calculated by setting the hourly power consumption of the multi-stage electric heating furnace for heating hot stamp blanks at 450 kWh, the furnace temperature at 920 °C, the daily operating time at 15 hours, and the heating and waiting time in the heating furnace at 3 hours, assuming component production at the productivity shown in Table 8. The amount was allocated to the power consumption per structural component and multiplied by the greenhouse gas intensity per kWh of the average electricity in Japan in 2018 (from the LCA database, AIST IDEA v3.2). Other greenhouse gas emissions (other than the heating process) during the manufacture of structural components were calculated using the method described in a published literature on LCA calculations for automotive parts (Masahiro Kubo and two others, "Evaluation of Greenhouse Gas Emissions over the Life Cycle of Lightweight Steel Bodies and Parts," Spring Meeting Academic Lecture Proceedings, Society of Automotive Engineers of Japan, 2022).

[0131] DESCRIPTION OF SYMBOLS 10: Structural member 11: Member body 20, 20A: Blank 21: Steel plate (first steel plate) 21a: Base steel plate 21b: Aluminum-based plating layer 22: Steel plate (second steel plate) 22a: Base steel plate 22b: Aluminum-based plating layer 23: Steel plate (third steel plate) 23a: Base steel plate 23b: Aluminum-based plating layer 25: Coating 40: Mold 111: Front pillar 112: Center pillar 113: Rocker 111a, 114a: Curved portion 241, 242, 243: Overlap portion

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