Automobile rear module

Abstract

This automobile rear module is for reinforcing side outers of an automobile and is formed by hot-stamp molding a plurality of integrated steel plates. When the projected area as viewed from the vertical direction of a reference plane is indicated by S (m2), and among components of the automobile rear module, the total weight of the components is indicated by W (kg), and the total weight of steel components each having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is indicated by WA, W / S is 24 or less and WA / W is 0.30 or more.

Description

Automotive rear module

[0001] The present invention relates to an automobile rear module for reinforcing the outer side of an automobile. This application claims priority to Japanese Patent Application No. 2024-045131, filed on March 21, 2024, the contents of which are incorporated herein by reference.

[0002] Recently, from the viewpoint of preventing global warming, carbon dioxide (CO ２ It is becoming increasingly important to reduce emissions of greenhouse gases (Green House Gases, hereinafter referred to as GHGs), including CO₂. Under these circumstances, the emergence of electric vehicles, hybrid vehicles, and other vehicles that emit less GHG than conventional vehicles powered by internal combustion engines is expected to reduce GHG emissions from vehicles while they are in motion. In addition, the use of lightweight materials such as aluminum and carbon as materials for constructing vehicles is expected to reduce GHG emissions from vehicles while they are in motion.

[0003] Regarding automobile bodies, for example, Patent Document 1 listed below discloses a vehicle body structure with excellent productivity. Also, in relation to vehicle body structures, Patent Document 2 listed below discloses a vehicle rear body structure that provides improved crashworthiness in the event of a rear impact to a vehicle, and a manufacturing method that includes a step of forming a tailored welded blank into a desired shape. Furthermore, Patent Document 3 listed below discloses an automobile body that can reduce the total amount of GHG generated throughout the entire life cycle of an automobile, from its manufacture, use, and disposal.

[0004] International Publication No. 2021 / 001813 Special Publication No. 2019-503920 International Publication No. 2022 / 250091

[0005] Considering the life cycle of an automobile, reducing GHGs only during vehicle use (driving) is insufficient to reduce the total amount of GHGs emitted into the global environment. Furthermore, no studies have been conducted to date to focus on reducing LC-GHGs (hereinafter referred to as life cycle GHGs or LC-GHGs) generated throughout the entire life cycle of an automobile rear module, from its manufacture, use, and disposal.

[0006] Therefore, an object of the present invention is to provide an automobile rear module that can reduce LC-GHG per unit area.

[0007] The gist of the present disclosure is as follows.

[0008] (1) A first aspect of the present invention is an automobile rear module having an integrated part formed by hot stamping a plurality of integrated steel plates, and a projected area when viewed from a direction perpendicular to a reference plane is S (m ２ ), the total weight of the components of the automobile rear module is W (kg), and the total weight of the steel components having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W Ａ When W / S is 24 or less, W Ａ / W is 0.30 or more. (2) The automobile rear module described in (1) above may include elemental technology A1 and at least one of elemental technology B1, elemental technology C1, elemental technology C2, elemental technology D1, and elemental technology D2. The elemental technology A1 is a skeleton member formed by hot stamping a steel plate, the skeleton member having a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-section portion having at least two flat portions whose radius of curvature is larger than the maximum outer dimension of the cross-section, and a concave bead portion formed between the two flat portions, the concave bead portion having a pair of wall portions whose radius of curvature is 50 mm or more and which protrude from opposing ends of the two flat portions toward the inside of the closed cross-section portion via a pair of bent portions that bend toward the inside of the closed cross-section, the Vickers hardness of the wall portions at the plate thickness center portion is 520 Hv or more, and the width of the wall portions is an effective width W calculated from Karman's effective width formula ｅ The structural member has a standard deviation ratio of less than 1.0, calculated by dividing the standard deviation of the hardness frequency distribution in the surface layer of the wall by the standard deviation of the hardness frequency distribution in the center of the wall. The elemental technology B1 is a structural member having a first steel plate with a minimum thickness and a second steel plate with a thickness greater than that of the first steel plate, the structural member having a component body with an annular shape in a plan view, the component body being formed by a plurality of steel plates joined together, and the value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate: A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２(1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements. The element technology C1 is a structural member for a vehicle body, comprising: (C1a) a pair of side frames and a cross member connecting the side frames, the side frames and the cross member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the side frames and the cross member being formed by a plurality of steel plates joined together, the first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and the first steel plate having a thickness of 0.001 g / m ２ (C1b) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and a coating containing 0.500 g / m or more of carbon black is provided on the first steel plate. ２(C1c) a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the first steel plate and the second steel plate are plated steel plates each having an aluminum-based plating layer on both surfaces of a base steel plate, and the thickness of the aluminum-based plating layer on the first steel plate is smaller than the thickness of the aluminum-based plating layer on the second steel plate. The elemental technology C2 is a structural member for a vehicle body, comprising: (C2a) a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined to each other, including a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and wherein the first steel plate and the second steel plate each have a surface located outside the overlap portion, and the surface of each of the first steel plate and the second steel plate contains 0.001 g / m of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. ２ (C2b) A structural member comprising a pair of side frames and a cross member connecting the side frames, wherein the structural member includes a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and is formed by a plurality of steel plates joined to each other, and a coating containing 0.500 g / m2 of carbon black is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. ２(C2c) A structural member that satisfies at least one of the above: (C2c) A structural member that is provided with a coating containing the following: (C2c) A structural member that is provided with a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member include a first steel plate, a second steel plate having an end that is overlapped and joined to an end of the first steel plate to form an overlap portion together with the end of the first steel plate, and a third steel plate, and is formed by a plurality of steel plates that are joined to each other, 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, and the thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer on the third steel plate.The elemental technology D1 is a technology in which a plurality of partial blanks made of steel plates are joined together, and (D1a) at least two of the partial blanks are joined by a plurality of joining portions at overlapping portions formed by partially overlapping the partial blanks, and in a cross section perpendicular to the surface of the partial blank including the center of the joining portion of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank in contact with the other partial blanks, when the Vickers hardness at a position 15 mm or more away from the center of the joining portion and not joined is Hvm, a part of the plurality of joining portions has a Vickers hardness of ΔHv, which is the difference between the maximum hardness and the minimum hardness in Vickers hardness within a range of 5 mm from the end of the joining portion toward the base material (or within 12 mm from the center of the joining portion), of less than 0.2 Hvm, preferably 0.1 Hvm or less, and the other joining portions of the plurality of joining portions (joining portions other than the part) have a ΔHv of 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more, (D1b) At least two of the partial blanks are joined by a plurality of spot welds at the overlapping portions where they are partially overlapped, and in a cross section including the center of the spot weld of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank, when the hardness at a position 15 mm or more away from the center of the spot weld and where no spot weld has been applied is Hvm, some of the plurality of spot welds have a ΔHv, which is the difference between the maximum hardness and the minimum hardness within a range of a radius of 12 mm from the center, of less than 0.2 Hvm, and the spot welds other than the portion have a ΔHv of 0.2 Hvm or more.The elemental technology D2 is: (D2a) A press-formed part having a bent portion, in which a patchwork made of steel plates is superimposed on the surface of a base blank made of steel plates and joined at a joint, wherein the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joint exists only in one region of the surface of the base blank across the bent portion when there is one bent portion, and only in a region between two adjacent bent portions on the surface of the base blank when there are two or more bent portions. (D2b) A press-formed part having a bent portion, in which a patchwork of steel plates is overlapped on the surface of a base blank made of steel plate and spot-welded at joint points, wherein the difference between the maximum hardness at a position 5 mm away from the center of the joint point on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joint satisfies at least one of the following: if there is only one bent portion, the joint exists only in one region of the surface of the base blank across the bent portion; if there are two or more bent portions, the joint exists only in a region between two adjacent bent portions on the surface of the base blank. (3) The automobile rear module described in (2) above may include the elemental technology A1 and the elemental technology B1. (4) The automobile rear module described in (2) above may include the elemental technology A1 and at least one of the elemental technology C1 and the elemental technology C2. (5) The automobile rear module described in (2) above may include at least one of the elemental technology A1, the elemental technology D1, and the elemental technology D2. (6) The automobile rear module described in (2) above may include at least one of the elemental technology A1, the elemental technology B1, the elemental technology C1, and the elemental technology C2, and at least one of the elemental technology D1 and the elemental technology D2.

[0009] According to the present disclosure, it is possible to provide an automobile rear module that can reduce LC-GHG per unit area.

[0010] 1 is a characteristic diagram showing the environmental load (GHG emissions) during the manufacture of each material of an automobile rear module. FIG. 2 is an exploded perspective view showing an automobile rear module according to the present embodiment. FIG. 3 is a graph showing the weight of each component in Example 1 of the invention. FIG. 4 is a graph showing the weight of each component in Comparative Example 1. For the examples, the horizontal axis shows the weight of the component. A / W, and W / S (kg / m ２ ) is plotted on the horizontal axis. A / W, and the vertical axis is LC-GHG / S (kg, CO ２ -eq / m ２) is a graph showing a plot of the linearity of the blank according to the first embodiment. FIG. 5 is a plan view of a structural member according to the first embodiment. FIG. 6 is a cross-sectional view taken along line II-II of FIG. 5. FIG. 7A is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 7B is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 7C is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 7D is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment, showing a blank according to the first embodiment. FIG. 7E is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment. FIG. 7F is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment. FIG. 7G is a schematic view illustrating a method for manufacturing a structural member according to the first embodiment. FIG. 8 is a cross-sectional view of a blank according to the second embodiment. FIG. 9 is a cross-sectional view of a blank according to the third embodiment. FIG. 10A is a cross-sectional view of a blank according to a modification of each embodiment. FIG. 10B is another cross-sectional view of a blank according to a modification of each embodiment. FIG. 10C is another cross-sectional view of a blank according to a modified example of each embodiment. FIG. 11 is a plan view of a structural member according to a modified example of each embodiment. FIG. 12A is a diagram showing a division pattern of a structural member in the first example. FIG. 12B is a diagram showing another division pattern of a structural member in the first example. FIG. 12C is a diagram showing yet another division pattern of a structural member in the first example. FIG. 12D is a diagram showing yet another division pattern of a structural member in the first example. FIG. 12E is a diagram showing yet another division pattern of a structural member in the first example. FIG. 12F is a diagram showing yet another division pattern of a structural member in the first example. FIG. 12G is a diagram showing yet another division pattern of a structural member in the first example. FIG. 13A is a diagram showing a division pattern of a structural member in the second example. FIG. 13B is a diagram showing another division pattern of a structural member in the second example. FIG. 13C is a diagram showing yet another division pattern of a structural member in the second example. FIG. 13D is a diagram showing yet another division pattern of a structural member in the second example.FIG. 14 is an exploded perspective view of a structural member according to the first embodiment. FIG. 15 is a cross-sectional view of a side frame included in each of the structural members shown in FIG. 14. FIG. 16A is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in FIG. 14. FIG. 16B is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a cross-sectional view of the blank shown in FIG. 16A. FIG. 16C is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing another cross-sectional view of the blank shown in FIG. 16A. FIG. 16D is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other of the structural members shown in FIG. 14. FIG. 16E is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 16F is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 16G is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 17 is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 18A is a cross-sectional view of a blank according to the second embodiment. FIG. 18B is another cross-sectional view of a blank according to the second embodiment. FIG. 19A is a cross-sectional view of a structural member according to the second embodiment. FIG. 19B is another cross-sectional view of a structural member according to the second embodiment. FIG. 19C is yet another cross-sectional view of a structural member according to the second embodiment. FIG. 20 is a cross-sectional view of a blank according to a modified example of the second embodiment. FIG. 21 is a plan view of a blank according to a third embodiment. FIG. 22 is a cross-sectional view of a blank according to the third embodiment. FIG. 23 is a cross-sectional view of a blank according to a modified example of the third embodiment. FIG. 24 is a cross-sectional view of a blank according to another modified example of the third embodiment. FIG. 25 is an exploded perspective view of a structural member according to the fourth embodiment. FIG. 26 is a plan view of a blank according to the fourth embodiment. FIG. 27 is a plan view of another blank according to the fourth embodiment. FIG. 28 is a plan view of a blank according to a modified example of the first embodiment. FIG. 29 is a plan view of a blank according to another modified example of the first embodiment. FIG. 30 is a plan view of a blank according to a modified example of the fourth embodiment. FIG. 31 is a plan view of a blank according to another modified example of the fourth embodiment.FIG. 32 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 33 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 34 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 35 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 36 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 37 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 38 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 39 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 40 is a plan view of a blank according to yet another modified example of the fourth embodiment. FIG. 41 is a cross-sectional view of a side frame included in a structural member according to the modified examples of each embodiment. FIG. 42A is a diagram showing a division pattern of a structural member in an example. FIG. 42B is a diagram showing another division pattern of a structural member in an example. FIG. 42C is a diagram showing another division pattern of a structural member in an example. FIG. 42D is a diagram showing another division pattern of a structural member in an example.

[0083] Fig. 42E is a diagram showing yet another division pattern of a structural member in an example. Fig. 42F is a diagram showing yet another division pattern of a structural member in an example. Fig. 43 is an exploded perspective view of a structural member according to the first embodiment. Fig. 44 is a cross-sectional view of a side frame included in each of the structural members shown in Fig. 43. Fig. 45A is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in Fig. 43. Fig. 45B is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a cross-sectional view of the blank shown in Fig. 45A. Fig. 45C is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing another cross-sectional view of the blank shown in Fig. 45A. Fig. 45D is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other of the structural member shown in Fig. 45. Fig. 45E is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. Fig. 45F is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment.FIG. 45G is a schematic diagram for explaining the manufacturing method of a structural member according to the first embodiment. FIG. 46 is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. FIG. 47A is a cross-sectional view of a blank according to the second embodiment. FIG. 47B is another cross-sectional view of a blank according to the second embodiment. FIG. 48A is a cross-sectional view of a structural member according to the second embodiment. FIG. 48B is another cross-sectional view of a structural member according to the second embodiment. FIG. 49 is a cross-sectional view of a blank according to a modified example of the second embodiment. FIG. 50 is an exploded perspective view of a structural member according to the third embodiment. FIG. 51 is a plan view of a blank according to the third embodiment. FIG. 52 is a plan view of another blank according to the third embodiment. FIG. 53 is a plan view of a blank according to a modified example of the first embodiment. FIG. 54 is a plan view of a blank according to yet another modified example of the first embodiment. FIG. 55 is a plan view of a blank according to yet another modified example of the first embodiment. FIG. 56 is a plan view of a blank according to a modified example of the third embodiment. FIG. 57 is a plan view of a blank according to another modified example of the third embodiment. 58 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 59 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 60 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 61 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 62 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 63 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 64 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 65 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 66 is a plan view of a blank according to yet another modified example of the third embodiment. FIG. 67 is a cross-sectional view of a side frame included in a structural member according to each modified example. FIG. 68A is a diagram showing a division pattern of a structural member in an example. FIG. 68B is a diagram showing another division pattern of a structural member in an example. FIG. 68A is a schematic diagram showing the appearance of a single door ring for an automobile. FIG. 68B is a diagram explaining a part manufacturing process by press forming a conventional general TWB. FIG. 1 is a schematic diagram for explaining the configuration of a blank for press-molding a door ring.This is a conceptual diagram showing the results of stress analysis of the overlapping portion of the A-pillar lower. Figure 72(a) is a conceptual diagram showing the stress state of the A-pillar lower as viewed from the outside, and Figure 72(b) is a conceptual diagram showing the stress state of the rocker as viewed from the inside, using contour diagrams. This is a conceptual diagram showing an example of spot welding points at the overlapping portion of the A-pillar lower. Figure 73(a) shows an example of spot welding points at the overlapping portion of the A-pillar lower, and Figure 73(b) is an explanatory conceptual diagram showing, among the spot welding points, areas at risk of fracture surrounded by solid lines and areas without risk of fracture surrounded by dotted lines. This is a conceptual diagram showing the spot welding points at the overlapping portion determined by simulation analysis. Figure 74(a) shows an example of spot welding at the overlapping portion (L-shaped) of the A-pillar lower and the overlapping portion (T-shaped) of the B-pillar lower as viewed from the outside, and Figure 74(b) is a conceptual diagram showing the same thing as viewed from the inside. 75(a) is a conceptual diagram showing the stress state after hot pressing the integrated blank of FIG. 75, obtained by FEM simulation. FIG. 75(a) is a conceptual diagram showing the stress state at the overlapping portion (L-shaped) of the A-pillar lower section and the overlapping portion (T-shaped) of the B-pillar lower section, as viewed from the outside, and FIG. 75(b) is a conceptual diagram showing the same as viewed from the inside. This is a diagram explaining the part manufacturing process by press forming the TWB according to the present invention. This is an explanatory diagram for explaining the HAZ softened portion by spot welding. This is a conceptual diagram for explaining an example of application of the present invention to an automobile floor module. This is a conceptual diagram showing the overlap welding position of the overlapping portion determined by simulation analysis, and showing an example in which lap welding is performed instead of the spot welding of FIG. 74. FIG. 79(a) is a conceptual diagram showing an example of lap welding at the overlapping portion (L-shaped) of the A-pillar lower section and the overlapping portion (T-shaped) of the B-pillar lower section, as viewed from the outside, and FIG. 79(b) is a conceptual diagram showing the same as viewed from the inside. 80(a) and 80(b) are diagrams for explaining the present invention, and are schematic diagrams showing an example of a press-formed part (hat-shaped part) with a hat-shaped cross section. Fig. 80(a) shows an external view thereof, and Fig. 80(b) shows a cross-sectional view thereof. Fig. 80(b) is a schematic diagram showing an example of a blank (patchwork blank) used in press-forming the hat-shaped part of Fig. 80. Fig. 80(b) is a schematic diagram showing an example of a cross-section of the hat-shaped part of Fig. 80, which has a recess in the top surface thereof.FIG. 85 is a schematic diagram showing an example of a blank (patchwork blank) used in press-forming the hat-shaped part having a recess in the top surface of FIG. 82. FIG. 86 is a diagram for explaining an example of a hat-shaped part to which a conventional manufacturing method is applied. FIG. 84(a) is an external view thereof, FIG. 84(b) is a cross-sectional view thereof, and FIG. 84(c) is a schematic diagram showing an example of a patchwork blank used in press-forming the hat-shaped part of FIG. 84(a). FIG. 85 is a diagram for explaining an example of a hat-shaped part according to one embodiment of the present invention. FIG. 85(a) to (c) show an example of joining by spot welding. FIG. 85(a) is an external view thereof, FIG. 85(b) is a cross-sectional view thereof, and FIG. 85(c) is a schematic diagram showing an example of a patchwork blank used in press-forming the hat-shaped part of FIG. 85(a). FIG. 86 is a diagram for explaining an example of a hat-shaped part according to one embodiment of the present invention. FIG. 86(d) to (f) are schematic diagrams showing an example of a patchwork blank when a joining method other than spot welding is applied to the same hat-shaped part. FIG. 86(d) shows an example in which lap fillet welding is applied, FIG. 86(e) shows an example in which lap welding is applied, and FIG. 86(f) shows an example in which adhesive is applied before press forming and laser spot welding is applied after press forming.

[0111] FIG. 86(d) is a diagram for explaining an example of a hat-shaped part according to one embodiment of the present invention, and is a schematic diagram showing an example in which the patchwork of the hat-shaped part shown in FIG. 85(a) is extended to the flange.

[0112] FIG. 88(a) is a schematic diagram showing an example of a hat-shaped part according to one embodiment of the present invention, in which there is no patchwork on one side of the standing wall of the hat-shaped part shown in FIG. 85(a).

[0113] FIG. 88(b) is a schematic diagram showing an example of a hat-shaped part according to one embodiment of the present invention, in which there is no patchwork on one side of the standing wall of the hat-shaped part shown in FIG. 87.

[0114] FIG. 89(a) is a diagram for explaining an example of a hat-shaped part according to one embodiment of the present invention, in which there is no patchwork on the top surface, FIG. 89(b) is a diagram showing an example in which there is a convex portion on the top surface, and FIG. 89(c) is a schematic diagram showing another example in which there is a convex portion on the top surface.

[0011] As mentioned above, when considering the life cycle of an automobile, reducing GHG emissions only while the vehicle is in use (while driving) is insufficient to reduce the total amount of GHG emitted into the global environment. Furthermore, because the automobile rear module accounts for approximately 3-10% of the total weight of the vehicle, reducing GHG emissions related to the automobile rear module will contribute significantly to the reduction. Currently, the focus is on making automobiles multi-material by using materials such as aluminum and carbon to reduce the weight of automobiles, but the inventors have focused on the following: 1. GHG generated in the manufacturing of materials for the automobile rear module (hereinafter referred to as "material manufacturing GHG"); 2. GHG generated in the automobile rear module manufacturing process (hereinafter referred to as "process GHG"); 3. The portion of GHG generated while the automobile is driving that contributes to the automobile rear module (hereinafter referred to as "driving GHG"); and 4. The present inventors have focused on four types of GHGs that are generated when an automobile rear module is disposed of (hereinafter referred to as "disposal GHGs"), and have studied ways of reducing the total amount of these GHGs.

[0012] In this specification, the rear module refers to a group of structural members including the rear floor of the vehicle body and the frame parts joined thereto. The rear module is mainly composed of an upper frame and an underframe. Furthermore, the rear module includes reinforcing or stiffening parts joined to these parts or members. In addition, in this specification, the CO generated during the life cycle ２ GHGs including CO are called LC-GHGs. ２ The amount is CO ２ GHGs other than CO are also calculated as equivalent masses and added together. ２ Other GHGs include methane, nitrous oxide, and ozone-depleting substances such as chlorofluorocarbon compounds. ２ The equivalent mass is calculated using the conversion factors listed in Table 1, which are set for each category of material, process, use, and recycling. ２ "equivalent" means "CO ２ Also referred to as "equivalent mass" and in this specification, "CO ２ "equivalent", "CO ２Equivalent mass” “CO ２ "CO equivalent" is defined as the same meaning. ２ The "equivalent mass" is CO ２ (global warming potential: 1) and CO ２ Gases other than methane CH ４ (Greenhouse effect per unit mass is CO ２ 25 times: Global warming potential 25), nitrous oxide N ２ O (greenhouse effect per unit mass is CO ２ 298 times the global warming potential (global warming potential 298), and then weighted by the global warming potential, CO ２ The converted mass is calculated.

[0013]

[0014] (GHG emissions in material manufacturing) Steel materials as raw materials have the smallest GHG emissions per unit of weight compared to other materials. Figure 1 is a characteristic diagram showing the environmental impact (GHG emissions) during the manufacturing of each material in an automobile rear module. The vertical axis shows the materials used in the automobile rear module: ordinary steel plate, high-strength steel plate, aluminum, and carbon fiber reinforced plastic (CFRP). The horizontal axis shows GHG emissions per equivalent function [kg-CO ２ As shown in Figure 1, steel materials (normal steel plate, high-strength steel plate) have overwhelmingly smaller GHG emissions than other materials (aluminum, carbon fiber reinforced plastic), and it can be seen that the primary use of steel materials as materials for constructing automobile rear modules will contribute greatly to reducing LC-GHG.

[0015] (Process GHG) In the manufacture of automotive rear modules, GHG is generated mainly in the welding process, heating process, painting process, etc. Therefore, appropriate process design while ensuring the performance required for automotive rear modules can contribute to reducing LC-GHG.

[0016] (GHG during driving) By reducing the weight of the rear module of the automobile, it is possible to reduce the load on the drive source such as an internal combustion engine, etc. Therefore, by reducing the weight of the rear module of the automobile, it is possible to contribute to the reduction of LC-GHG.

[0017] (GHG emissions at the time of disposal) Steel reduces CO emissions by 1.60 kg per kg through scrap recycling ２ eq. Compared to aluminum, steel has a smaller effect on reducing emissions per kg, but when high-strength steel is used, the weight required to obtain the required strength is small, so it can be said that using high-strength steel can contribute to reducing LC-GHG. In other words, as with material manufacturing GHG, GHG emissions at disposal can be reduced by primarily using steel materials as the material that makes up the automobile rear module, which can contribute to reducing LC-GHG.

[0018] As described above, in order to reduce LC-GHG, total GHG emissions must be reduced from four perspectives. For example, it is believed that there is a trade-off between "material production GHG, disposal GHG, and driving GHG." Furthermore, there is generally a tendency for "process GHG" to increase in the manufacture of lightweight, highly functional parts, and there is a trade-off between "process GHG" and "driving GHG." While methods for reducing GHG at each stage of a product's life cycle have been studied and discussed, no optimal examples of material selection and process design for reducing LC-GHG have been disclosed.

[0019] The inventors focused on reducing LC-GHG by taking into consideration the above four categories of GHG, including the trade-off relationships, and discovered that by controlling the weight of components per specified area and the weight of high-strength components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness value of HV230 or more within an appropriate range, it is possible to reduce the LC-GHG of an automobile rear module while still satisfying the required strength (crash resistance).

[0020] An automobile rear module according to an embodiment of the present invention, which has been developed based on the above findings, will now be described with reference to the drawings. However, these descriptions are intended to merely exemplify preferred embodiments of the present invention, and are not intended to limit the present invention to such specific embodiments.

[0021] The automobile rear module 100 according to this embodiment is applied to an automobile body configured with a monocoque frame equipped with an impact-absorbing framework. Fig. 2 shows an exploded perspective view of the automobile rear module 100 according to this embodiment. As shown in Fig. 2, the automobile rear module 100 includes an integrated upper frame 10 and an integrated underframe 20.

[0022] The integrated upper frame 10 is composed of a rear member front portion 11, a rear member rear portion 12, two cross members 13, a cross member extension equivalent portion 14 joined to the cross members 13, a rear floor portion 15, a rear floor side panel portion 16, and a rear wheel house portion 17.

[0023] The integrated upper frame 10 is manufactured by hot stamping a single tailored blank to form it into an integrated part. A single tailored blank can be obtained by joining multiple hot stamping steel sheets corresponding to the respective portions to be made into the integrated part, and by overlapping and joining (patchwork joining) hot stamping steel sheets to be formed into reinforcement members as necessary.

[0024] The integrated underframe 20 is composed of a rear member front portion 21, a rear member rear portion 22, three cross members 23, and a cross member extension equivalent portion 24 joined to the cross members 23.

[0025] The integrated underframe 20 is manufactured by hot stamping a single tailored blank to form it into an integrated part. A single tailored blank can be obtained by joining multiple hot stamping steel sheets corresponding to the respective portions to be made into the integrated part, and by overlapping and joining (patchwork joining) hot stamping steel sheets formed into reinforcement members as needed.

[0026] As described above, the automobile rear module 100 according to this embodiment has portions (the integrated underframe 110 and the integrated upper frame 120) that are each formed into an integrated part by hot stamping a single tailored blank. By hot stamping the tailored blanks, an integrated part with different properties (weight, hardness, strength, thickness) can be obtained for each portion. Note that additional reinforcing parts and brackets may be attached to the automobile rear module 100 after hot stamping.

[0027] The integrated parts are not limited to the above example. For example, in this embodiment, the rear floor section 15, the rear floor side panel section 16, and the rear wheel housing section 17 are configured to be integrated with the upper frame 10. However, an integrated part consisting of the rear member front section 11, the rear member rear section 12, the three cross members 13, and the cross member extension equivalent section 14 joined to the cross members 13 may be attached after molding. Furthermore, the rear wheel housing section 17 may be integrated with the integrated underframe 20.

[0028] As described above, the automobile rear module 100 according to this embodiment uses an integrated component formed by joining multiple hot stamping steel plates and then hot stamping them, which reduces material production GHG compared to when press-forming each section, trimming unnecessary portions as needed, and then joining them by welding. Furthermore, when hot stamping each section and then welding them, the hot stamping heating process requires multiple steps and time, but the automobile rear module 100 according to this embodiment welds multiple steel plates and then hot stamps them, which reduces the number and time of the heating process. Therefore, the automobile rear module 100 according to this embodiment can reduce process GHG while still achieving the performance required of an automobile rear module.

[0029] Furthermore, in the automobile rear module 100 according to this embodiment, it is possible to reduce the LC-GHG of the automobile rear module by controlling the weight of the components per projected area and the weight of the high-strength components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness value of HV230 or more within an appropriate range.

[0030] Specifically, the projected area when viewed from the perpendicular direction of the reference plane is S (m ２ ), the total weight of the components of the automobile rear module is W (kg), and the total weight of the steel components having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W Ａ When W / S is 24 or less, W Ａ By satisfying a ratio of 0.30 or more, it is possible to reduce LC-GHG emissions from the automobile rear module. No automobile rear module that satisfies the above conditions has existed in the past, and these conditions would not have been easily conceived without the knowledge of the inventors mentioned above. The reference plane is a plane perpendicular to the vehicle height direction in the automobile rear module when attached to the vehicle body.

[0031] W Ａ The larger the vehicle body or side door module, the larger the value of W. Therefore, in this application, W / S(m ２ ) is used as an index. When the W / S value is 24 or less, weight reduction according to the size of the side door module is achieved, making it possible to reduce GHG during driving. From the perspective of reducing GHG during driving, the smaller the W / S value, the better, preferably 23 or less, and more preferably 22 or less. The lower limit of W / S is set based on the required safety performance. To ensure both rigidity as a module and safety performance, W / S may be, for example, 5.0 or more.

[0032] W ＡWhen the value of / W is 0.30 or more, a high proportion of the components used in the side door module are high strength components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more, which makes it possible to reduce GHG emissions from material production. Ａ The higher the value of / W, the better, preferably it is more than 0.40, more preferably more than 0.45. Furthermore, from the viewpoint of reducing GHG in material production, the weight of a component having a plate thickness of 1.3 mm or less and a minimum Vickers hardness of HV230 or more is determined by the ratio of W Ｂ When W Ｂ The value of / W is preferably 0.18 or more, and more preferably 0.20 or more.

[0033] The Vickers hardness measurement method is as follows. A sample having a cross section perpendicular to the plate surface is taken from the flat portion of each portion, and the cross section is prepared as the measurement surface, which is then subjected to a hardness test. The measurement surface is prepared in accordance with JIS Z 2244:2009. The measurement surface is polished using silicon carbide paper of #600 to #1500, and then the measurement surface is mirror-finished using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The hardness test is performed according to the method described in JIS Z 2244:2009. Using a micro Vickers hardness tester, measurements are taken at 30 points at 3 / 8 of the plate thickness of the sample, with a load of 1000 gf, at intervals of at least three times the indentation, and the average value is the hardness at the center of the plate thickness.

[0034] In the present application, by applying (1) elemental technology A1, and (2) at least one of elemental technology B1, elemental technology C1, elemental technology C2, elemental technology D1, and elemental technology D2 to the components of the automobile rear module 100, the weight ratio of steel material in the automobile body is increased to reduce the GHG emitted during material production, and the weight of the automobile body is reduced to reduce GHG emitted during driving, resulting in a significant reduction in LC-GHG compared to conventional automobile bodies.

[0035] It is more preferable to apply the elemental technology A1 and the elemental technology B1. It is more preferable to apply the elemental technology A1 and at least one of the elemental technology C1 and the elemental technology C2. It is more preferable to apply the elemental technology A1 and at least one of the elemental technology D1 and the elemental technology D2. It is more preferable to apply the elemental technology A1, the elemental technology B1, at least one of the elemental technology C1 and the elemental technology C2, and at least one of the elemental technology D1 and the elemental technology D2.

[0036] In this specification, the term "automobile" refers to an automobile that has excellent rear-end collision safety. For example, if an automobile receives the highest rating in the rear-end collision neck injury protection test by the Insurance Institute for Highway Safety (IIHS), it can be said to be fully suitable for driving on public roads.

[0037] Furthermore, the automobile body of a public road vehicle to which the automobile rear module according to this embodiment is applied is not limited to the body of an internal combustion vehicle or an electric vehicle, but may also be a hybrid vehicle powered by an internal combustion engine and an electric motor, a fuel cell vehicle, a hydrogen engine vehicle, or the like. Furthermore, the automobile body to which the automobile rear module is applied is not limited to an automobile body having a monocoque frame, but may also be an automobile body having a ladder frame structure. Furthermore, vehicle types of public road vehicles include passenger cars and commercial vehicles such as sedans, hatchbacks, station wagons, minivans, and pickup trucks. Furthermore, public road vehicles also include loaded vehicles such as trucks.

[0038] (Examples) The present invention will be specifically described below by way of examples. Note that the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the conditions in the examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and the purpose is achieved.

[0039] Table 2 shows various characteristic values ​​of the automobile rear modules according to the invention example and the comparative example, including: Number of components Weight ratio of steel sheet Projected area S (m ２ ), Total weight W (kg) of components, W / S, Total weight W of components having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more Ａ ・W Ａ / W (kg / m ２ ), the total weight W of components with a plate thickness of 1.3 mm or less and a minimum Vickers hardness of HV230 or more Ｂ , ・W Ｂ / W, the calculated values ​​are shown.

[0040]

[0041] The automobile rear module of Example 1 includes 56 components. Of these, four are integrated components. The weight of each component is shown in the graph in Figure 3A. In the graph in Figure 3A, the vertical axis represents the component weight of that portion. The shaded areas represent portions that meet the conditions of a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more. The dotted areas represent portions that have a minimum Vickers hardness of HV230 or more but a plate thickness that is not 1.5 mm or less. The components are arranged from left to right on the horizontal axis in order of decreasing plate thickness. Components to the left of the dashed line have a plate thickness of 1.5 mm or less. Component Nos. 2, 3, 4, and 5 are each integrated parts obtained by hot stamping from a single tailored blank.

[0042] The same weight, thickness and hardness were also measured for invention examples 2 to 8.

[0043] The automobile rear module of Comparative Example 1 includes 129 components. There are no integrated components, only individually molded components. The weight of each component is as shown in the graph in Figure 3B. In the graph in Figure 3B, the vertical axis represents the weight of the component at that location. The shaded areas represent areas that meet the conditions of a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more. The dotted areas represent areas that have a minimum Vickers hardness of HV230 or more but are not 1.5 mm or less in plate thickness. The components are arranged from left to right on the horizontal axis in order of decreasing plate thickness. To the left of the dashed line are components with a plate thickness of 1.5 mm or less.

[0044] Similar weight, thickness and hardness measurements were also carried out for Comparative Examples 2 to 8.

[0045] The weights (kg) of the parts shown in Table 2 are the weights when the automobile rear module is cut along the weld lines of the tailored blank. The weights in the comparative examples were determined by disassembling the body of a commonly available road-legal automobile and measuring and analyzing the shape and weight data. The weights in some comparative examples and inventive examples were determined by measuring and analyzing design and development data using CAD (Computer-Aided Design). The hardness HV was determined as follows. A sample having a cross section perpendicular to the plate surface was taken from the flat portion of each part, and the cross section was prepared as the measurement surface, which was then subjected to the hardness test. The measurement surface was prepared in accordance with JIS Z 2244:2009. The measurement surface was polished using #600 to #1500 silicon carbide paper, and then the measurement surface was mirror-finished using a liquid in which diamond powder with a particle size of 1 μm to 6 μm was dispersed in a diluted solution such as alcohol or pure water. The hardness test was carried out according to the method described in JIS Z 2244: 2009. Using a micro-Vickers hardness tester, measurements were taken at 30 points at 3 / 8 of the thickness of the sample, with a load of 1000 gf, and at intervals of at least three times the indentation, and the average value was taken as the hardness at the center of the thickness.

[0046] Table 3 shows the following for each of the invention examples and comparative examples: Total GHG emissions LC-GHG (kg, CO ２ -eq), and LC-GHG / S (kg, CO２ -eq / m ２ ) are shown as calculated values.

[0047]

[0048] The total GHG emissions are calculated by subtracting the CO from the above-mentioned GHGs from the material manufacturing GHGs, process GHGs, GHGs during driving, and GHGs during disposal. ２ The equivalent mass is calculated and summed up, which corresponds to the amount of LC-GHG emissions. The total GHG emissions is a value calculated using the method described below.

[0049] The characteristic values ​​of Examples 1-8 were obtained by measuring and analyzing automobile bodies constructed by the inventors using the above-mentioned elemental technologies. The characteristic values ​​of Comparative Examples 1-8 were obtained by measuring and analyzing automobile bodies of publicly available road vehicles. For some Comparative Examples, default values ​​listed on the World Auto Steel (WAS) website were used. WAS is the Automotive Subcommittee of the World Steel Association, consisting of 17 steel manufacturers worldwide. The analysis of LC-GHG emissions was based on "Roland Geyer, Parametric Assessment of Climate Change Impacts of Automotive Material Substitution, Environmental Science & Technology 2008 42 (18), 6973-6979, DOI: 10.1021 / es800314w."

[0050] (Calculation of GHG emissions from material production) The default settings of the GHG analysis software were used as the basic conditions. With these default settings, the scrap charging rate to the blast furnace was 11.9%, and the usage rates of recycled materials made from scrap were set based on statistical data as follows: plate material 5%, wire rod 85%, and cast iron 100%. Assuming these as the base conditions, values ​​were entered to obtain the various material compositions shown in Table 1, and calculations were performed.

[0051] (Calculation of process GHG) The default settings of the GHG analysis software were used as the basic conditions. The material yield in automobile part production was assumed to be 55% for steel plate, 52% for aluminum alloy plate, 75% for plate, bar, and wire rod, and 80% for cast iron, aluminum extrusion, and aluminum casting. Calculations were performed by inputting values ​​for the various material compositions shown in Table 1.

[0052] (Calculation of Driving GHG) An electric vehicle was selected as the powertrain type for the target vehicle. A mid-size electric vehicle was selected based on the size and weight of each vehicle to be analyzed. The vehicle's driving pattern was set to the following WLTP (Class 3b) mode. WLTP mode: Average speed: 36.57 km / h; Maximum speed: 97.4 km / h; Driving time: 1,477 seconds; Driving distance: 15.01 km; Idling ratio: 15.4%; Cold start ratio: 100%. The driving distance was assumed to be 110,000 km, and the settings were made to take into account the weight reduction of the vehicle body and the resizing of the powertrain. The electric vehicle's power consumption during driving was set to be generated in Japan, and the calculation was performed by inputting the power consumption contribution of the rear module obtained by the inventors' analysis.

[0053] (Calculation of GHG at disposal) The default settings of the software for GHG analysis at disposal are used as the basic conditions, and the recycling rate of steel is assumed to be 90.3%, and that of aluminum alloy material is 78.6%. Energy recovery from recycling other than automobiles is also calculated as CO2. ２ This setting is taken into consideration as the amount of absorption.

[0054] CO ２ The calculation of the equivalent mass is based on the GHG emissions from the material manufacturing process, vehicle manufacturing process, fuel manufacturing and use process, and material and vehicle recycling process shown in Table 1. ２ Use a coefficient to calculate the equivalent mass, and use the weight or energy amount to calculate CO ２These values ​​are the default settings of the GHG analysis software, and are set based on statistical data on GHG emissions for each substance and each process. By following the above procedure, CO2 can be calculated from the material production GHG, process GHG, GHG during driving, and GHG during disposal. ２ The equivalent masses were calculated and summed to calculate the LC-GHGs listed in Table 3.

[0055] In Example 1-8, the ratio of steel used in the automobile rear module was increased, and the above-mentioned elemental technologies were combined and applied to form an integrated part, resulting in a W / S of 24 or less and a W Ａ This enabled us to reduce LC-GHG emissions from the rear module of the automobile.

[0056] On the other hand, in Comparative Examples 1 to 8, W Ａ The W / S ratio was 0.27 or less, and in order to ensure collision performance, it was necessary to increase the W / S ratio, and it was not possible to obtain a sufficient reduction effect in GHG emissions during material production and GHG emissions during driving.

[0057] Furthermore, in Comparative Examples 1 to 8, which did not employ an integrated structure, each section was press-formed, and unnecessary sections were trimmed and removed before being joined by welding, so it was not possible to obtain a sufficient effect in reducing GHG during material production and GHG during driving.

[0058] FIG. 4A shows the results of the embodiment, with the horizontal axis representing W Ａ / W, and W / S (kg / m ２ 4B is a graph plotting W Ａ / W, and the vertical axis is LC-GHG / S (kg, CO ２ -eq / m ２ ) are plotted. From these graphs, it can be seen that the example of the present invention significantly reduces LC-GHG compared to the conventional structure. Thus, according to the present invention, the projected area when viewed from the perpendicular direction of the reference plane is S (m ２), the total weight of the components of the automobile rear module is W (kg), and the total weight of the steel components having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W Ａ When W / S is 24 or less, W Ａ By satisfying the condition of / W being 0.30 or more, it is possible to reduce LC-GHG emissions from the automobile rear module.

[0059] (Example 2) Table 4 shows the results of collision tests conducted on the invention examples and comparative examples. Table 4 presents the collision test results as evaluation values ​​A and B. In this evaluation, the test results of the IIHS rear-impact test were first disclosed. For the vehicle body of Comparative Example 6, which received the highest evaluation for collision safety performance, a numerical analysis of the rear-impact test was conducted using the IIHS rear-impact analysis model, and the resulting intrusion amount was used as the standard (evaluation B). The vehicle bodies of Comparative Examples 1-8 were also certified (type-approved) under the laws and regulations of each country. The safety performance evaluation results of some vehicle bodies are listed in comparison with Comparative Example 6. For Inventive Examples 1-8, a numerical analysis of the rear-impact test was conducted with only the rear module replaced, and safety performance was evaluated based on the relative intrusion amount into the rear module and the absorbed energy during the collision. A vehicle with test results superior to the vehicle with the highest evaluation in the IIHS rear-impact simulation test (Comparative Example 6) was given an evaluation of A. Furthermore, a vehicle with results equivalent to those of Comparative Example 6, in which no parts were separated, was given an evaluation of B.

[0060]

[0061] As shown in Table 4, inventive examples 1-8, rear collision results (rating A or B) equivalent to or better than the safety test results of vehicles that received a good rating in the IIHS rear collision test were obtained.

[0062] Therefore, according to Invention Examples 1-8, it is possible to reduce LC-GHG while satisfying the vehicle safety test results of a good evaluation in the IIHS rear-end collision test.

[0063] The outline of the elemental technologies applied to the automobile rear module 100 according to this embodiment is as follows. Note that the symbols for components, formulas, embodiments, examples, etc. in the explanation of each elemental technology are assigned to each elemental technology for the sake of simplicity. Therefore, the same symbols may be assigned in the explanation of different elemental technologies. Also, the word "invention" in the description of the elemental technology can be read as "elemental technology."

[0064] Elemental technology A1 is a skeletal member formed by hot stamping a steel plate, the skeletal member having a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, the closed cross-section portion having at least two flat portions which are portions having a radius of curvature larger than the maximum outer dimension of the cross-section, and a concave bead portion formed between the two flat portions, the concave bead portion having a pair of wall portions having a radius of curvature of 50 mm or more, the pair of wall portions protruding from opposing ends of the two flat portions toward the inside of the closed cross-section portion via a pair of bent portions which bend toward the inside of the closed cross-section, the Vickers hardness of the wall portions at the plate thickness center portion being 520 Hv or more, the width of the wall portions being an effective width W calculated from Karman's effective width formula ｅ and the standard deviation ratio calculated by dividing the standard deviation of the hardness frequency distribution in the surface layer portion of the wall portion by the standard deviation of the hardness frequency distribution in the thickness center portion of the wall portion is smaller than 1.0.

[0065] Elemental technology A1 is a technology disclosed in International Publication No. 2022 / 018963. This elemental technology A1 makes it possible to provide a framework member with excellent energy absorption efficiency.

[0066] (Elemental Technology B1) The elemental technology B1 is a structural member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the structural member comprising a component body formed by a plurality of steel plates joined together and having an annular shape in a plan view, wherein the value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate: A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[0067] According to the elemental technology B1, it is possible to provide a structural member with excellent impact absorbing performance.

[0068] For example, as described in "Ueno Masakatsu and Ito Kametaro, 'A new prediction formula for the hardenability of steel to replace the GROSSMANN formula,' Iron and Steel, The Iron and Steel Institute of Japan, 74th year (1988) No. 6, pp. 1073-1080," the critical cooling rate V has conventionally been used as an index of the hardenability of steel. ｃ９０ The critical cooling rate V ｃ９０ is the critical cooling rate (°C / s) at which a martensite structure of 90% or more in volume fraction is obtained, and logV ｃ９０ = 2.94 - 0.75β. β is calculated as 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo. β represents the influence of each element on hardenability based on the amount of Mn. The larger β is, the higher the critical cooling rate V ｃ９０ becomes smaller, and the hardenability of the steel material becomes good.

[0069] There is a correlation between β, which indicates the influence of each element on hardenability, and the time until the diffusion transformation starts after the heating of the steel material is completed (transformation start time). The inventors held hot stamping steel sheets A, B, C, and D (plate thickness: 1.2 mm) similar to those in the examples (Table 5) described below in a heating furnace at 900°C for 1 minute, then removed from the heating furnace and air-cooled, and measured the time until the phase transformation starts (transformation start time). Then, regression analysis was performed using the measured transformation start time data, and the formula for converting β to the transformation start time was: A = 1.48 × β ３．４２ The inventors further utilized coefficient A to study the appropriate arrangement of steel materials in a blank. As a result, the inventors completed a blank according to an embodiment.

[0070] A hot stamping blank according to an embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined to form an annular shape in a plan view of the blank. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the smallest thickness among the plurality of steel plates. The second steel plate has a thickness greater than that of the first steel plate. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (first configuration). A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[0071] For example, if a first steel plate having a minimum thickness and a second steel plate having a greater thickness have the same hardenability, the thin-walled first steel plate will begin its austenite-to-ferrite phase transformation (diffusion transformation) earlier than the thick-walled second steel plate after the blank is heated for hot stamping. However, in a blank according to the first configuration, the coefficient A calculated based on the chemical composition of the first steel plate is greater than the coefficient A calculated based on the chemical composition of the second steel plate. That is, the first steel plate is made of a material with higher hardenability than the second steel plate, i.e., a material that initiates diffusion transformation later during cooling. Therefore, after the blank is heated, the initiation of diffusion transformation in the first steel plate is delayed, thereby reducing the difference in transformation initiation time between the first steel plate and the second steel plate. As a result, when the blank is hot stamped, not only the relatively thick second steel plate but also the minimum-thick first steel plate can be well hardened, which facilitates uniform hardness of the structural component formed from the blank. Furthermore, since the first steel plate is well quenched, stress is offset by transformation plasticity and residual stress is reduced, so that the occurrence of twisting due to the concentration of residual stress in the annular structural member can be suppressed, and as a result, deterioration of dimensional accuracy can be reduced. Here, the application of the invention to an annular door ring part for an automobile will be described, but it can also be used in the rear module of an automobile, etc., and the occurrence of twisting, warping, etc. due to the concentration of residual stress can be suppressed.

[0072] In this way, in the first configuration, even though the annular blank includes the first steel plate that is thinner than the second steel plate, the hardness of the structural member formed by hot stamping from the blank can be made uniform and deterioration of dimensional accuracy can be suppressed. As a result, the impact absorption performance (crash resistance performance) of annular structural members, particularly large annular structural members or structural members such as rear modules, can be improved.

[0073] In the blank according to the first configuration, the thickness of the first steel plate is t ｍｉｎ , the maximum thickness among the thicknesses of multiple steel plates is t ｍａｘ When this is the case, t ｍａｘ -t ｍｉｎ≧0.2 (mm) (second configuration).

[0074] A method for manufacturing a structural component according to an embodiment includes the steps of preparing a blank according to the first or second configuration, heating a plurality of steel plates included in the blank to an austenite transformation completion temperature or higher, and using a mold to form the heated blank into a structural component that is annular in plan view and quenching it (third configuration).

[0075] A structural member according to an embodiment includes a member body. The member body has an annular shape in plan view. The member body is formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (fourth configuration). A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[0076] The structural member according to the fourth configuration may be a door ring part and a rear module of an automobile, etc. In the case of a door ring part of an automobile, the member body may include a front pillar, a center pillar, and a rocker connecting the front pillar and the center pillar (fifth configuration).

[0077] 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.

[0078] <First embodiment> [Structural member] Fig. 5 is a diagram (plan view) of a structural member B1-10 according to this embodiment, viewed from above when placed on a horizontal surface. The structural member B1-10 is used, for example, in the body of an automobile. The structural member B1-10 is typically a door ring component of an automobile. In this embodiment, an example in which the structural member B1-10 is a door ring component will be described.

[0079] The structural member B1-10 is a hot-stamped member. That is, the structural member B1-10 is formed by hot stamping (hot pressing) a blank made of multiple steel plates. The structural member B1-10 includes a member body B1-11. The member body B1-11 has an annular shape in a plan view of the structural member B1-10. The member body B1-11 includes a front pillar B1-111, a B1-center pillar B1-112, and a rocker B1-113. When the structural member B1-10 is assembled to the body of an automobile, the center pillar B1-112 is positioned behind the front pillar B1-111. The center pillar B1-112 extends generally in the vertical direction of the vehicle body. The front pillar B1-111 extends toward the center pillar B1-112. When the structural member B1-10 is assembled to the body of an automobile, the rocker B1-113 is disposed below the front pillar B1-111 and the center pillar B1-112. The rocker B1-113 connects the front pillar B1-111 and the center pillar B1-112.

[0080] In this embodiment, the member body B1-11 is formed of a plurality of steel plates B1-21, B1-22, and B1-23 joined together. In the example of FIG. 5, the front pillar B1-111 is mainly composed of steel plates B1-21 and B1-22. The center pillar B1-112 is mainly composed of steel plate B1-23. The rocker B1-113 is mainly composed of steel plates B1-21 and B1-23.

[0081] FIG. 6 is a cross-sectional view taken along line II-II of FIG. 5. FIG. 6 shows a cross-section of the structural member B1-10 cut along the thickness direction at the position of the steel plate B1-21. As shown in FIG. 6, the steel plate B1-21 has an open cross-section. In a cross-sectional view of the structural member B1-10, the steel plate B1-21 has, for example, a roughly hat-like shape. More specifically, the steel plate B1-21 includes a top plate B1-211, vertical walls B1-212 and B1-213, and flanges B1-214 and B1-215. The vertical wall B1-212 is located on the opposite side of the top plate B1-211 from the vertical wall B1-213. In a cross-sectional view of the structural member B1-10, one end of the vertical walls B1-212 and B1-213 is connected by the top plate B1-211. In a cross-sectional view of the structural member B1-10, flanges B1-214 and B1-215 are connected to the other ends of the vertical walls B1-212 and B1-213, respectively. The flanges B1-214 and B1-215 protrude from the vertical walls B1-212 and B1-213, respectively, to the outside of the structural member B1-10.

[0082] In the structural member B1-10, the width W of the steel plate B1-21 at the position shown in FIG. 6, i.e., the width W of the lower part of the front pillar B1-111 (FIG. 5), may be 30 mm or more and 750 mm or less. The height H of the lower part of the front pillar B1-111 may be 25 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 B1-211 and the vertical wall B1-212 on the vertical wall B1-212 side to the end of the rounded corner between the top plate B1-211 and the vertical wall B1-213 on the vertical wall B1-213 side in the cross section of the structural member B1-10. The height H is the distance from the top plate B1-211 to the flanges B1-214 and B1-215 along the thickness direction of the top plate B1-211. The width W of the portion of the steel plate B1-21 corresponding to the locker B1-113 (FIG. 5) is, for example, 30 mm or more and 300 mm or less. The height of the portion of the steel plate B1-21 corresponding to the locker B1-113 may be 25 mm or more and 150 mm or less.

[0083] Although not shown, the other steel plates B1-22 and B1-23 (FIG. 5) also have an open cross section similar to the steel plate B1-21. The steel plates B1-22 and B1-23 may also have, for example, a generally hat-shaped cross section of the structural member B1-10. The width of the portion of the steel plate B1-22 corresponding to the upper part of the front pillar B1-111 may be 15 mm or more and 300 mm or less. The height of the portion of the steel plate B1-22 corresponding to the upper part of the front pillar B1-111 may be 10 mm or more and 150 mm or less. The width of the portion of the steel plate B1-23 corresponding to the center pillar B1-112 may be 15 mm or more and 300 mm or less. The height of the portion of the steel plate B1-23 corresponding to the center pillar B1-112 may be 10 mm or more and 150 mm or less.

[0084] The size of the structural member B1-10, which is annular in plan view, is, for example, 1.0 m or more. The size of the structural member B1-10 may be, for example, 4.0 m or less. The size of the structural member B1-10 is the length of the line segment connecting the two farthest points on the outer periphery of the structural member B1-10 when the structural member B1-10 is placed on a horizontal surface and viewed vertically.

[0085] [Method for manufacturing a structural member] A method for manufacturing a structural member B1-10 according to this embodiment will be described below with reference to Figures 7A to 7G. The method for manufacturing a structural member B1-10 according to this embodiment includes the steps of preparing a blank B1-20, heating the blank B1-20, and forming the heated blank B1-20 into a structural member B1-10.

[0086] 7A, in the preparation process, a blank B1-20 having a shape obtained by expanding the structural member B1-10 is prepared. The blank B1-20 includes a plurality of steel plates B1-21, B1-22, and B1-23. The steel plates B1-21, B1-22, and B1-23 are arranged and joined to form a ring shape in a plan view of the blank B1-20.

[0087] Figures 7B, 7C, and 7D are cross-sectional views of blank B1-20 showing the joints of steel plates B1-21, B1-22, and B1-23. Figures 7B, 7C, and 7D are cross-sectional views taken along lines IIIB-IIIB, IIIC-IIIC, and IIID-IIID in Figure 7A, respectively. Referring to Figures 7B and 7C, steel plate B1-21 is butt-joined to each of steel plates B1-22 and B1-23. That is, the end faces of steel plate B1-21 are joined with the end face of steel plate B1-22 abutting against each other, and the other end face of steel plate B1-21 is joined with the end face of steel plate B1-23 abutting against each other. Referring to Figure 7D, steel plate B1-22 is butt-joined to steel plate B1-21 as well as steel plate B1-23. The end face of the steel plate B1-22 is joined to the end face of the steel plate B1-23 in a state of abutting against the end face of the steel plate B1-23. The steel plates B1-21, B1-22, and B1-23 are joined by, for example, laser welding. In this embodiment, the blank B1-20 is a so-called tailor-welded blank.

[0088] 7B to 7D, the steel plate B1-21 has a plate thickness t １ The steel plate B1-22 has a plate thickness of t ２ The steel plate B1-23 has a plate thickness of t ３ The thickness of steel plates B1-22 and B1-23 is t ２ , t ３ is the thickness t of steel plate B1-21 １ That is, the thickness t of the steel plate B1-21 is larger than １ is the minimum thickness t of steel plates B1-21, B1-22, and B1-23 ｍｉｎ In the example of this embodiment, the plate thickness t ２ is the thickness t of steel plate B1-23 ３ Therefore, the thickness t of steel plate B1-22 is ２ is the maximum plate thickness t of steel plates B1-21, B1-22, and B1-23 ｍａｘ However, the steel plate B1-22 does not necessarily have the maximum plate thickness t ｍａｘ The thickness t of the steel plate B1-22 may not be ２ is the thickness t of the other steel plate B1-23 ３It may be the following:

[0089] Minimum thickness t for steel plates B1-21, B1-22, and B1-23 ｍｉｎ and maximum plate thickness t ｍａｘ For example, t ｍａｘ -t ｍｉｎ ≧0.2 (mm) Plate thickness t ｍｉｎ , t ｍａｘ is t ｍａｘ -t ｍｉｎ The thickness t of the steel plate B1-21 may be ≦3.2 (mm). ｍｉｎ is, for example, less than 1.4 mm. ｍｉｎ The maximum plate thickness t may be 0.8 mm or more. ｍａｘ is, for example, less than 4.0 mm. ｍａｘ may be 1.4 mm or more.

[0090] Steel plates B1-21, B1-22, and B1-23 may have a chemical composition known as a steel plate for hot stamping. For example, the chemical composition of steel plates B1-21, B1-22, and B1-23 contains, in mass%, C: 0.05 to 0.50%, Si: 0.020 to 1.000%, Mn: 0.20 to 2.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.5%, and B: 0.0005 to 0.0050%, respectively. The chemical compositions of steel plates B1-21, B1-22, and B1-23 may further contain, in mass%, one or more elements selected from the group consisting of Cu: 0.005 to 3.000%, Co: 0.005 to 0.500%, Sn: 0.005 to 0.500%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050%, and Sb: 0.0005 to 0.0200%.

[0091] For steel plates B1-21, B1-22, and B1-23, the minimum plate thickness t ｍｉｎThe chemical composition of steel plate B1-21, which has a thickness of 1.5 mm, differs from the chemical compositions of the thicker steel plates B1-22 and B1-23. The value of coefficient A calculated using the chemical composition of steel plate B1-21 in accordance with formula (1) below differs from the coefficient A calculated using the chemical composition of each of steel plates B1-22 and B1-23 in accordance with formula (1): A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo). ３．４２ (1)

[0092] The element symbols in formula (1) are substituted with the content (mass%) of the corresponding element. That is, the coefficient A of steel plate B1-21 is calculated by substituting the content (mass%) of each element in the chemical composition of steel plate B1-21 for the corresponding element symbol in formula (1). Similarly, the coefficient A of steel plate B1-22 is calculated by substituting the content (mass%) of each element in the chemical composition of steel plate B1-22 for the corresponding element symbol in formula (1). The value of coefficient A calculated by formula (1) using the chemical composition of steel plate B1-21 is larger than the coefficient A calculated by formula (1) using the chemical composition of steel plate B1-22. The coefficient A of steel plate B1-22 is the smallest of the coefficients A calculated by formula (1) for steel plates B1-22 and B1-23 other than steel plate B1-21. The coefficient A of steel plate B1-21 is calculated as A １ , the coefficient A of steel plate B1-22 is A ２ When this is done, A １ -A ２ is preferably 0.10 or more, and more preferably 0.20 or more. １ -A ２ may be, for example, 11.50 or less.

[0093] The coefficient A of steel plate B1-23 is calculated by substituting the content (mass%) of each element in the chemical composition of steel plate B1-23 into the corresponding element symbol in formula (1). The value of coefficient A calculated by formula (1) using the chemical composition of steel plate B1-23 is equal to or greater than the value of coefficient A calculated by formula (1) using the chemical composition of steel plate B1-22. The value of coefficient A of steel plate B1-23 is preferably equal to or greater than the value of coefficient A calculated by formula (1) using the chemical composition of steel plate B1-22. ｍｉｎ That is, among the plurality of steel plates B1-21, B1-22, and B1-23 included in the blank B1-20, the value of the coefficient A of the steel plate B1-21 having the smallest plate thickness t ｍｉｎIt is preferable that the coefficient A of the steel plate B1-21 having the following is the largest. １ , the coefficient A of steel plate B1-23 is A ３ When this is done, A １ -A ３ is preferably 0.10 or more, more preferably 0.20 or more. １ -A ３ However, the coefficient A of the steel plate B1-23 ３ is the coefficient A of steel plate B1-21 １ The above (A １ -A ３ ≦0).

[0094] The chemical compositions of the steel plates B1-21, B1-22, and B1-23 included in the blank B1-20 may be measured by a common analytical method. For example, analytical test specimens may be taken from each of the steel plates B1-21, B1-22, and B1-23, and the test specimens may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) to obtain the chemical compositions of the steel plates B1-21, B1-22, and B1-23. For each analytical test specimen, C may be measured using a combustion-infrared absorption method.

[0095] (Heating Process) The prepared blank B1-20 is formed into a structural member B1-10 (FIGS. 5 and 6) by hot stamping (hot pressing). During hot stamping, the blank B1-20 is subjected to a heating process. Referring to FIG. 7E, in the heating process, the blank B1-20 is heated, for example, by a heating furnace B1-30. The multiple steel plates B1-21, B1-22, and B1-23 included in the blank B1-20 are heated to an austenite transformation completion temperature (Ac3 point) or higher. The steel plates B1-21, B1-22, and B1-23 are heated, for example, to 900°C or higher. This causes the microstructures of the steel plates B1-21, B1-22, and B1-23 to transform, for example, entirely or almost entirely, into an austenite phase.

[0096] (Forming Process) Referring to Figure 7F, in the forming process, a mold B1-40 is used to form the heated blank B1-20 into a structural member B1-10 (Figures 5 and 6) that is annular in plan view and then quenched. The blank B1-20 heated by the heating process is removed from the heating furnace B1-30 (Figure 7E) and transported to the mold B1-40. The mold B1-40 is attached to a known press device. The mold B1-40 includes, for example, a punch B1-41 and a die B1-42. The blank B1-20 is placed between the punch B1-41 and the die B1-42.

[0097] Referring to Figure 7G, after the blank B1-20 is placed between the punch B1-41 and the die B1-42, the die B1-42 moves relatively close to the punch B1-41. The blank B1-20 is clamped (pressed) between the punch B1-41 and the die B1-42 and formed into a shape that conforms to the forming surfaces of the punch B1-41 and the die B1-42. The blank B1-20 remains clamped between the punch B1-41 and the die B1-42. The blank B1-20 is cooled (quenched) by the die B1-40, and its microstructure is transformed to martensite. This allows the structural member B1-10 to be manufactured from the blank B1-20.

[0098] Referring again to Figure 5, the chemical compositions of steel plates B1-21, B1-22, and B1-23 do not change before and after hot stamping. Therefore, in the structural member B1-10, the value of coefficient A calculated using the above-mentioned formula (1) using the chemical composition of steel plate B1-21, which has the minimum plate thickness tmin, is larger than the coefficient A calculated using formula (1) using the chemical composition of steel plate B1-22. The value of coefficient A calculated using formula (1) using the chemical composition of steel plate B1-21 is preferably larger than the coefficient A calculated using formula (1) using the chemical composition of steel plate B1-23. However, the value of coefficient A calculated using formula (1) using the chemical composition of steel plate B1-21 may be equal to or smaller than the coefficient A calculated using formula (1) using the chemical composition of steel plate B1-23.

[0099] The chemical compositions of the steel plates B1-21, B1-22, and B1-23 in the structural member B1-10 after hot stamping can be obtained by the same analytical method as that for the chemical compositions of the steel plates B1-21, B1-22, and B1-23 at the blank B1-20 stage.

[0100] Referring to FIG. 6, the minimum plate thickness t ｍｉｎ In the cross section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum martensite fraction (%), the martensite fraction variation is, for example, 20% or less. The martensite fraction variation is preferably 15% or less, and more preferably 10% or less. The martensite fraction variation can be measured as follows. That is, ｍｉｎ At the cross section of the structural member B1-10 at the position of the steel plate B1-21 having the above structure, 10 or more analysis samples (for example, the long side is about 10 mm in size) are cut out from positions 20 mm or more away from the end and 10 mm or more away from each other, and then each is mirror-polished and etched with LePeller's reagent so that the plate thickness direction becomes the observation surface. Then, an optical microscope is used to observe the area from the steel plate surface to a depth of 1 / 4 of the plate thickness (the area from the steel plate surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness) with a magnification of 1000 times and a field of view of 2,400 μm ２ The above structural photographs are taken from 30 fields of view, and the obtained structural photographs are subjected to image analysis.

[0101] The image analysis method involves obtaining the maximum brightness value Lmax and minimum brightness value Lmin from the image, designating the area with pixels whose brightness ranges from Lmax-0.3 (Lmax-Lmin) to Lmax as a white area, and calculating the ratio of the number of pixels in the white area to the total number of pixels to measure the martensite fraction. This type of image analysis is performed on a total of 30 observation fields of each analysis sample to determine the martensite fraction, and the average value is used as the martensite fraction for each analysis sample. Furthermore, the difference between the maximum and minimum martensite fractions in 10 or more analysis samples is calculated based on the minimum plate thickness t ｍｉｎThe variation in martensite fraction in the cross section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum plate thickness t ｍｉｎ If there are multiple steel plates having the above structure, such analysis is performed on each steel plate to determine the martensite fraction, and the maximum martensite fraction variation among these steel plates is taken as the martensite fraction variation in the structural member B1-10.

[0102] In some steel sheets, the area fraction of martensite obtained by image analysis, i.e., the area fraction of the white region, may contain a few percent of the area fraction of retained austenite. However, since the variation in the martensite fraction is calculated as a difference, the impact is minor.

[0103] After the forming process (hot stamping), steel plate B1-21 may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. Similarly, after the forming process (hot stamping), steel plates B1-22 and B1-23 (FIG. 5) may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. At least one of steel plates B1-21, B1-22, and B1-23 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of steel plates B1-21, B1-22, and B1-23 may be the same as or different from the tensile strength of the other steel plates.

[0104] [Effect] In the blank B1-20 according to this embodiment, the minimum plate thickness t ｍｉｎ Coefficient A of steel plate B1-21 having １ is the coefficient A of the thicker steel plate B1-22 ２ Coefficient A is greater than １ , A ２ is calculated based on the chemical compositions of the steel plates B1-21 and B1-22 using the formula (1): A = 1.48 × (2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo) ３．４２ The coefficient A is the value obtained by calculating １ , A ２corresponds to the time (transformation start time) from the completion of heating of the blank B1-20 for hot stamping from the heating furnace B1-30 until the steel sheets B1-21 and B1-22 start to undergo diffusion transformation, respectively. １ , A ２ is a value corresponding to the transformation start time when only the influence of elements is considered for each of the steel plates B1-21 and B1-22, without considering the influence of the plate thickness. １ is coefficient A ２ If the coefficient A is larger than 0.01, it means that the steel plate B1-21 is made of a material with higher hardenability than the steel plate B1-22, in other words, a material that starts diffusion transformation slowly during cooling. １ > Coefficient A ２ By doing so, the minimum plate thickness t ｍｉｎ The onset of the diffusion transformation in the steel sheet B1-21 having the coefficient A １ is coefficient A ２ Therefore, when the blank B1-20 is hot stamped, not only the relatively thick steel sheet B1-22 but also the steel sheet B1-21 with a minimum thickness of t ｍｉｎ The steel plate B1-21 can also be well quenched, which makes it easier to uniformize the hardness of the structural member formed from the blank B1-20. Furthermore, by well quenching the steel plate B1-21, stress is offset by transformation plasticity, reducing residual stress, which can suppress the occurrence of twisting in the annular structural member B1-10 due to the concentration of residual stress. As a result, the deterioration of the dimensional accuracy of the structural member B1-10 can be reduced.

[0105] In this manner, in this embodiment, even though the annular blank B1-20 includes the steel plate B1-21 that is thinner than the steel plate B1-22, the hardness of the structural member B1-10 formed by hot stamping from the blank B1-20 can be made uniform, and deterioration of dimensional accuracy can be suppressed. As a result, the impact absorption performance (crash resistance performance) of the annular structural member B1-10, especially the large annular structural member B1-10, can be improved.

[0106] In this embodiment, the hardenability of the relatively thin steel plate B1-21 is improved, so that the hardness of the structural member B1-10 formed from the blank B1-20 can be made uniform. More specifically, the minimum plate thickness t ｍｉｎ Since the steel plate B1-21 having the above structure is also well quenched, the variation in the martensite fraction in the steel plate B1-21 can be kept to 20% or less. This makes it less likely for deformation concentration to occur when a collision load is input to the structural member B1-10, making it easier for the structural member B1-10 to exhibit high impact absorption performance. Therefore, even when an annular structural member B1-10 including a thin steel plate B1-21, particularly a large annular structural member B1-10, is formed from the blank B1-20, the strength defect of the structural member B1-10 can be reduced, and the impact absorption performance of the structural member B1-10 can be improved.

[0107] The smaller the variation in the martensite fraction, the less uneven the mechanical properties within the structural member B1-10, which is preferable from the standpoint of the functionality of the structural member B1-10. On the other hand, a large variation in the martensite fraction indicates that there is an uneven distribution of areas with insufficient hardenability, i.e., areas with insufficient hardness, within the structural member B1-10, and when the structural member B1-10 is deformed by impact, deformation tends to concentrate in the areas with insufficient hardness, thereby reducing the functionality of the structural member B1-10.

[0108] Second Embodiment Fig. 8 is a cross-sectional view of a blank B1-20A according to a second embodiment. ｍｉｎ Steel plate B1-21 having a larger plate thickness t ２ The blank B1-20A according to this embodiment has a configuration generally similar to that of the blank B1-20 according to the first embodiment, but differs from the blank B1-20 according to the first embodiment in that a coating B1-24 is provided on the steel plate B1-21. In the example of FIG. 8, ｍｉｎ The steel plate B1-21 having the above structure is coated on one surface with a coating B1-24.

[0109] The coating B1-24 is a substantially black coating. For example, if the lightness L* value (CIE 1976 lightness index L* defined in JIS Z8781-4 (2013)) from the surface of the coating B1-24 is 60 or less, the coating B1-24 can be determined to be black. The coating B1-24 may be a carbon-based surface-treated coating (a coating containing carbon (C)).

[0110] The coating B1-24 may contain, for example, carbon black. The coating B1-24 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 B1-24 may also contain silica.

[0111] The coating B1-24 may be, for example, a surface treatment coating described in International Publication No. 2022 / 215229. That is, the coating B1-24 may contain graphite, soot, or the like instead of or in addition to carbon black. Alternatively, the coating B1-24 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 to 50. 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.

[0112] The blank B1-20A according to this embodiment is formed into an annular structural member B1-10 (FIGS. 5 and 6) by the same manufacturing method as in the first embodiment. In the blank B1-20A, the emissivity of the surface of the steel plate B1-21 is increased by a substantially black coating B1-24 applied to the surface of the steel plate B1-21. The surface of the steel plate B1-21 coated with the coating B1-24 has an emissivity of 60% or more at a measurement temperature of 25°C and a wavelength of 8.0 μm, for example. In this way, the minimum plate thickness t ｍｉｎBy increasing the emissivity of the steel sheet B1-21 having the above-mentioned emissivity, the temperature rise of the steel sheet B1-21 can be accelerated when the blank B1-20A is heated during hot stamping. Therefore, the steel sheet B1-21 can be quickly heated to the austenite temperature range, ensuring a long high-temperature holding time for the steel sheet B1-21. As a result, the austenite grains in the microstructure of the steel sheet B1-21 are coarsened, further delaying the diffusion transformation of the steel sheet B1-21 after the heating process is completed. Therefore, the steel sheet B1-21 can be more effectively hardened. At least a portion of the coating B1-24 can remain on the surface of the steel sheet B1-21 after hot stamping.

[0113] In this embodiment, one surface of the steel plate B1-21 is coated with the coating B1-24. However, both surfaces of the steel plate B1-21 may be coated with the substantially black coating B1-24. The coating B1-24 may or may not be provided on one or both surfaces of the other steel plates B1-22 and B1-23. However, from the viewpoint of ensuring a longer high-temperature holding time for the steel plate B1-21 from the start to the completion of heating of the blank B1-20A, it is preferable that the coating B1-24 is not provided on at least one of the other steel plates B1-22 and B1-23.

[0114] <Third embodiment> Fig. 9 is a cross-sectional view of a blank B1-20B according to a third embodiment. ｍｉｎ Steel plate B1-21 having a larger plate thickness t ２ The blank B1-20B according to this embodiment has a configuration generally similar to that of the blank B1-20 according to the first embodiment, but differs from the blank B1-20 according to the first embodiment in that the steel plates B1-21 and B1-22 are plated steel plates.

[0115] In the example of FIG. 9 , the steel plate B1-21 has a base steel plate B1-21a and an aluminum-based plating layer B1-21b. The aluminum-based plating layer B1-21b covers both surfaces of the base steel plate B1-21a. The aluminum-based plating layer B1-21b is provided over the entire or almost the entire surfaces of both surfaces of the base steel plate B1-21a. Similarly, the steel plate B1-22 has a base steel plate B1-22a and an aluminum-based plating layer B1-22b. The aluminum-based plating layer B1-22b covers both surfaces of the base steel plate B1-22a. The aluminum-based plating layer B1-22b is provided over the entire or almost the entire surfaces of both surfaces of the base steel plate B1-22a. In this embodiment, the plate thickness t ｍｉｎ is the plate thickness including the base steel plate B1-21a and the aluminum-based plating layer B1-21b. ２ is the plate thickness including the base steel plate B1-22a and the aluminum-based plating layer B1-22b.

[0116] The chemical composition of the aluminum-based plating layers B1-21b and B1-22b is not particularly limited. Known aluminum-based plating layers (plating layers containing aluminum as the main component) can be used as the aluminum-based plating layers B1-21b and B1-22b. Although not particularly limited, the aluminum-based plating layers B1-21b and B1-22b are, for example, Al-Si-based plating layers. The chemical compositions of the aluminum-based plating layers B1-21b and B1-22b may be the same or different.

[0117] The coating weight of the aluminum-based plating layer B121b on the steel sheet B1-21 is W1 (g / m ２ ), the coating weight of the aluminum-based plating layer B1-22b on the steel sheet B1-22 is W2 (g / m ２ ), the deposition amounts W1 and W2 are 20 g / m ２ 120g / m or more ２ The coating weight W1 of the aluminum-based plating layer B1-21b is the average coating weight on both surfaces of the base steel sheet B1-21a. The coating weight W2 of the aluminum-based plating layer B1-22b is the average coating weight on both surfaces of the base steel sheet B1-22a.

[0118] The deposition amounts W1 and W2 are preferably 30 g / m ２ More preferably, 35 g / m ２ The deposition amounts W1 and W2 are preferably 115 g / m ２ More preferably, it is 100 g / m or less. ２ However, the minimum plate thickness t ｍｉｎ The coating weight W1 of the aluminum-based plating layer B1-21b in the steel plate B1-21 having a larger plate thickness t ２ The difference between the coating weights W1 and W2 (W2-W1) is, for example, 10 (g / m ２ ) or more. W2-W1 is preferably 20 (g / m ２ ) or more, more preferably 30 (g / m ２ ) or more. W2-W1 is 80 (g / m ２ ) or less. W2-W1 is preferably 70 (g / m ２ ) or less, more preferably 60 (g / m ２ The adhesion amounts W1 and W2 satisfy the relationship W2 / W1>1.0. The adhesion amounts W1 and W2 preferably satisfy the relationship W2 / W1≧1.2, and more preferably W2 / W1≧1.5.

[0119] If steel plates B1-21 and B1-22 are plated steel plates, coefficient A １ , A ２ is calculated using the chemical compositions of the base steel plates B1-21a and B1-22a. １ is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate B1-21a into the above-mentioned formula (1). ２ is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate B1-22a into formula (1). １ is the coefficient A of steel plate B1-22 ２ Coefficient A of steel plate B1-22 is larger than ２is the smallest coefficient A among the coefficients A calculated by equation (1) for each of the multiple steel plates included in blank B1-20B.

[0120] The blank B1-20B according to this embodiment is formed into an annular structural member B1-10 (FIGS. 5 and 6) by the same manufacturing method as in the first embodiment. ｍｉｎ The coating weight W1 of the aluminum-based plating layer B1-21b of the steel plate B1-21 having a larger plate thickness t ２ The coating weight W2 of the aluminum-based plating layer B1-22b of the steel sheet B1-22 having the same thickness is smaller than that of the steel sheet B1-22 having the same thickness. As a result, when the blank B1-20B is heated during hot stamping, the heating rate of the steel sheet B1-21 is significantly higher than that of the steel sheet B1-22. Specifically, because the aluminum-based plating layer B1-21b on the surface of the steel sheet B1-21 is relatively thin, when the blank B1-20B is heated, alloying of the aluminum-based plating layer B1-21b with the iron contained in the base steel sheet B1-21a progresses rapidly to the surface of the steel sheet B1-21, and both surfaces of the steel sheet B1-21 turn black or a color close to black. In other words, the emissivity of both surfaces of the steel sheet B1-21 increases during the heating process. Therefore, the steel sheet B1-21 can be quickly heated to a temperature in the austenite range, ensuring a long high-temperature holding time for the steel sheet B1-21. As a result, the austenite grains in the microstructure of the steel sheet B1-21 become coarser, which can further delay the diffusion transformation of the steel sheet B1-21 after the heating process is completed, and therefore the steel sheet B1-21 can be more effectively hardened.

[0121] In the structural member B1-10 (FIGS. 5 and 6) formed from the blank B1-20B, the average thickness (plating thickness) of the aluminum-based plating layer B1-21b on both surfaces of the steel sheet B1-21 is K1 (μm), and the average thickness (plating thickness) of the aluminum-based plating layer B1-21b on both surfaces of the steel sheet B1-22 is K2 (μm). The plating thickness K1 of the steel sheet B1-21 is smaller than the plating thickness K2 of the steel sheet B1-22. The difference between the plating thicknesses K1 and K2, K2 - K1, is, for example, 7 μm or more. K2 - K1 may be 33 μm or less. Furthermore, the plating thicknesses K1 and K2 may satisfy the relationship K2 / K1 > 1.0. K2 / K1 is preferably 1.2 or more, more preferably 1.5 or more.

[0122] In this embodiment, the steel plate B1-23 (FIGS. 7C and 7D) may be a plated steel plate having a base steel plate and a plating layer, similar to the steel plates B1-21 and B1-22, or may be a steel plate (bare material) without a plating layer on the surface. When the steel plate B1-23 is a plated steel plate, the plating layer may be an aluminum-based plating layer or a metal plating layer other than aluminum. When the steel plate B1-23 is a plated steel plate, the adhesion amount and thickness of the plating layer on the base steel plate are not particularly limited. When the steel plate B1-23 is a plated steel plate, the coefficient A of the steel plate B1-23 ３ is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel sheet into the above-mentioned formula (1). Even when the steel sheets B1-21, B1-22, and B1-23 are plated steel sheets, the chemical compositions of the steel sheets B1-21, B1-22, and B1-23 can be measured by the general analysis method described in the first embodiment. Analysis of the chemical compositions of the steel sheets B1-21, B1-22, and B1-23 can be performed after removing the plated layer on their surfaces by mechanical grinding.

[0123] The configuration of the blank B1-20B according to this embodiment can also be combined with the blanks B1-20 and B1-20A according to the first and second embodiments, respectively. That is, in each of the blanks B1-20 and B1-20A, the steel sheet B1-21 is a plated steel sheet having a base steel sheet B1-21a and an aluminum-based plating layer B1-21b, and the steel sheet B1-22 is a plated steel sheet having a base steel sheet B1-22a and an aluminum-based plating layer B1-22b, and the coating weight W1 of the aluminum-based plating layer B1-21b on the steel sheet B1-21 may be less than the coating weight W2 of the aluminum-based plating layer B1-22b on the steel sheet B1-22.

[0124] However, in the first and second embodiments, the coating weight W1 of the aluminum-based plating layer B1-21b on the steel plate B1-21 may be equal to or greater than the coating weight W2 of the aluminum-based plating layer B1-22b on the steel plate B1-22. Also, in the first and second embodiments, the plating layers of the steel plates B1-21 and B1-22 may be metal plating layers other than aluminum, or the steel plates B1-21 and B1-22 may be steel plates (bare materials) that do not have a plating layer on their surfaces.

[0125] 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.

[0126] In the above embodiment, the blanks B1-20, B1-20A, and B1-20B each include three steel plates B1-21, B1-22, and B1-23. However, the number of steel plates included in the blanks B1-20, B1-20A, and B1-20B is not limited to this. The blanks B1-20, B1-20A, and B1-20B may each be composed of two steel plates B1-21 and B1-22, or may each include four or more steel plates. The blanks B1-20, B1-20A, and B1-20B each have at least a minimum plate thickness t ｍｉｎ Steel plate B1-21 having a plate thickness t ｍｉｎ Plate thickness t is greater than ２ and steel plate B1-22 having the above structure.

[0127] Blanks B1-20, B1-20A, and B1-20B, which are annular in plan view, can typically include three or more steel plates. Steel plate B1-21 is directly or indirectly joined to steel plate B1-22. In blanks B1-20, B1-20A, and B1-20B, the arrangement of multiple steel plates including steel plates B1-21 and B1-22 is not particularly limited. In blanks B1-20, B1-20A, and B1-20B, the coefficient A of steel plate B1-21 is １ is the coefficient A calculated by formula (1) for the other steel plate B1-2i ｉ The coefficient A of steel plate B1-22 is the smallest among (i = 2, 3, ...) ２ The minimum thickness t of blank B1-20, B1-20A, or B1-20B is greater than ｍｉｎ If there are multiple steel plates B1-21 having the coefficient A of all steel plates B1-21 １ is the coefficient A of steel plate B1-22 ２ When blanks B1-20, B1-20A, and B1-20B contain three or more steel plates, the coefficient A of the steel plates other than steel plates B1-21 and B1-22 is preferably greater than the coefficient A of steel plate B1-22. ２ That's all.

[0128] In the above embodiment, the steel plate B1-21 is butt-joined to each of the steel plates B1-22 and B1-23. The steel plate B1-22 is butt-joined to the steel plate B1-23. However, the steel plate B1-21 may be lap-joined to at least one of the steel plates B1-22 and B1-23. That is, as shown in FIG. 10A , the end of the steel plate B1-21 may be overlapped on the end of the steel plate B1-22 and joined to the end of the steel plate B1-22 by, for example, spot welding or laser welding, so that the steel plates B1-21 and B1-22 form an overlap portion B1-25. Similarly, as shown in Figure 10B, the steel plates B1-21 and B1-23 may form an overlap portion B1-25 by joining the end of the steel plate B1-21 to the end of the steel plate B1-23 by, for example, spot welding or laser welding, while being overlapped with the end of the steel plate B1-23. Also, as shown in Figure 10C, the steel plates B1-22 and B1-23 may form an overlap portion B1-25 by joining the end of the steel plate B1-22 to the end of the steel plate B1-23 by, for example, spot welding or laser welding, while being overlapped with the end of the steel plate B1-23. In the blanks B1-20, B1-20A, and B1-20B, the joining method for adjacent steel plates may be butt joining or lap joining.

[0129] In each of the blanks B1-20, B1-20A, and B1-20B according to the above embodiments, the plurality of steel plates arranged to form an annular shape in a plan view may be either a single layer or a multi-layer. That is, each of the plurality of steel plates may be a single steel plate or a plate material formed by stacking a plurality of steel plates.

[0130] In the above embodiment, the mold B1-40 used for hot stamping the blanks B1-20, B1-20A, and B1-20B includes a punch B1-41 and a die B1-42. However, the configuration of the mold B1-40 is not limited to the example described in the above embodiment. The mold B1-40 may further include, for example, a pad or a blank holder.

[0131] In the above embodiment, the main body B1-11 of the structural member B1-10 includes a front pillar B1-111, a center pillar B1-112, and a rocker B1-113. However, the main body B1-11 may include other components. For example, as shown in FIG. 11 , the main body B1-11 may further include a rear pillar B1-114. The structural member B1-10 according to the above embodiment is a door ring component (single door ring component) having a single ring shape. On the other hand, the structural member shown in FIG. 11 is a door ring component (double door ring component) having a double ring shape. When manufacturing a double door ring component, the blank used as the material also has a double ring shape.

[0132] 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.

[0133] [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, which is a single door ring part, using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) while changing the steel plates included in the structural member and the division pattern of the structural member.

[0134]

[0135] The types of steel sheets (materials) used in this analysis are shown in Table 5. For each material, Table 5 shows the content (mass%) of each element in the base material, the type of coating, and the coefficient A calculated by the above-mentioned formula (1).

[0136] The division patterns of the structural members are shown in Figures 12A to 12G. Figures 12A to 12G show the number of steel plates (materials) included in the structural member, which is a single door ring part, and the positions of the joints between the steel plates in the structural member. In Figures 12A to 12G, each steel plate is given a number in parentheses.

[0137]

[0138] Referring to Table 6, in Example 1, the minimum thickness tｍｉｎ In Example 1, the coefficient A of the thinnest material (3) is 1.2 mm. １ is the smallest coefficient A in other materials (1) and (2). ２ In Example 2, the minimum thickness t ｍｉｎ : Coefficient A of material (1) having 1.2 mm １ is the smallest coefficient A in other materials (2) and (3). ２ In Example 3, the minimum thickness t ｍｉｎ The coefficient A value is the largest for the material (2) having a thickness of 1.2 mm. In Example 3, the coefficient A of the thinnest material (2) is １ is the smallest coefficient A in the other materials (1) and (3). ２ In contrast, in Comparative Examples 1 and 2, the coefficient A of the thinnest material is significantly larger than １ is less than the coefficient A of other materials.

[0139] As described above, coefficient A corresponds to the transformation initiation time for each material when only the influence of elements is considered. However, the actual transformation initiation time for each material is also affected by plate thickness, and becomes shorter as the plate thickness decreases. The "phase transformation initiation time" in Table 6 refers to the shortest time until the phase transformation to ferrite begins after the blank is heated at a furnace temperature of 920°C for 5 minutes and 30 seconds and removed from the heating furnace (the time until the thinnest material begins to transform). As shown in Table 6, Examples 1 to 3 had longer phase transformation initiation times than Comparative Examples 1 and 2. For example, comparing Example 3 and Comparative Example 2, which are identical except for the material of the thinnest material (2), it can be seen that Example 3 has a slower phase transformation initiation time than Comparative Example 2. In Example 3, coefficient A for the thinnest material (2) １ The coefficient A is the smallest among the other materials (1) and (3). ２ On the other hand, in Comparative Example 2, the coefficient A of the thinnest material (2) is larger than that of the thinnest material (3), and the hardenability of the thinnest material is higher than that of the other materials. １ The coefficient A is the smallest among the other materials (1) and (3). ２or less, and the hardenability of the thinnest material is equal to or less than that of the other materials. In Example 3, the hardenability of the thinnest material is made higher than that of the other materials, so the phase transformation start time of the thinnest material is extended compared to Comparative Example 2, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials are made uniform. Therefore, in Example 3, after heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite begins in the thinnest material, and it becomes easier to harden the structural member uniformly.

[0140] The analysis conditions and results for division patterns 3 and 4 shown in Figures 12C and 12D are shown in Table 7. In Figures 12C and 12D, the structural member is formed from four pieces of material (1) to (4).

[0141]

[0142] Referring to Table 7, in Examples 4 to 11, the minimum plate thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 3 to 7, the minimum thickness t ｍｉｎ Coefficient A of １ The smallest coefficient A among other relatively thick materials ２ The minimum plate thickness t ｍｉｎ If there are multiple pieces of material with the minimum thickness t ｍｉｎ The smallest value of the coefficient A of the material having the coefficient A １ As the coefficient A of other materials ２ We decided to compare it with the following.

[0143] When each Example is compared with the corresponding Comparative Example, it can be seen that the phase transformation start time is extended. For example, when Example 6 and Comparative Example 5 are compared, which are identical except for the material of the thinnest material (4), the phase transformation start time is delayed in Example 6 compared to Comparative Example 5. In Example 6, the coefficient A of the thinnest material (4) １ The smallest coefficient A among the other materials (1) to (3) ２On the other hand, in Comparative Example 5, the coefficient A of the thinnest material (4) is larger than that of the thinnest material (4), and the hardenability of the thinnest material is higher than that of the other one or more materials. １ The smallest coefficient A among the other materials (1) to (3) ２ or less, and the hardenability of the thinnest material is equal to or less than that of the other materials. In Example 6, the hardenability of the thinnest material is made higher than that of the other materials, so the phase transformation start time of the thinnest material is extended compared to Comparative Example 5, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials are made uniform. Therefore, in Example 6, after heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite begins in the thinnest material, and it becomes easier to harden the structural member uniformly.

[0144] The analysis conditions and results for division patterns 5 to 7 shown in Figures 12E to 12G are shown in Table 8. In Figures 12E to 12G, the structural member is formed from five pieces of material (1) to (5).

[0145]

[0146] Referring to Table 8, in Examples 12 to 17, the minimum plate thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 8 to 11, the minimum thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ The minimum plate thickness t ｍｉｎ If there are multiple pieces of material with the same thickness, the smallest thickness t ｍｉｎ The smallest value of the coefficient A of the material having the coefficient A １ As the coefficient A of other materials ２ We decided to compare it with the following.

[0147] When each Example is compared with the corresponding Comparative Example, it can be seen that the phase transformation start time is extended. For example, when Example 12 and Comparative Example 8 are compared, which are identical except for the materials of the thinnest materials (1) and (4), the phase transformation start time of Example 12 is delayed compared to Comparative Example 8. In Example 12, the coefficient A of the thinnest materials (1) and (4) １ is the smallest coefficient A among the other materials (2), (3), and (5). ２ In Example 12, the coefficient A of the thinnest materials (1) and (4) is greater than that of the thinnest material, and the hardenability of the thinnest material is higher than that of the other one or more materials. １ is larger than the coefficients A of all the other materials (2), (3), and (5). On the other hand, in Comparative Example 8, the coefficients A of the thinnest materials (1) and (4) are １ is the smallest coefficient A among the other materials (2), (3), and (5). ２ or less, and the hardenability of the thinnest material was equal to or less than that of the other materials. In Example 12, the hardenability of the thinnest material was made higher than that of the other materials, so the phase transformation start time of the thinnest material was extended compared to Comparative Example 8, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials were made uniform. Therefore, in Example 12, after heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite began in the thinnest material, and it became easier to harden the structural member uniformly.

[0148] In addition, for example, when Example 13 and Comparative Example 9 are compared, which are identical in condition except for the materials of the thinnest materials (1), (3), and (5), the phase transformation start time is slower in Example 13 than in Comparative Example 9. In Example 13, the smallest coefficient A １ The coefficient A is the smallest among the other materials (2) and (4). ２ In Example 13, all of the coefficients A of the thinnest materials (1), (3), and (5) are larger than the coefficients A of the other materials (2) and (4). On the other hand, in Comparative Example 9, the coefficient A is the smallest among the thinnest materials (1), (3), and (5). １The coefficient A is the smallest among the other materials (2) and (4). ２ or less, and the hardenability of the thinnest material was equal to or less than that of the other materials. In Example 13, the hardenability of the thinnest material was made higher than that of the other materials, so the phase transformation start time of the thinnest material was extended compared to Comparative Example 9, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials were made uniform. Therefore, in Example 13, after heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite began in the thinnest material, and it became easier to harden the structural member uniformly.

[0149] In each of the examples and comparative examples shown in Tables 6 to 8, the materials are butted together and then laser-joined (butt-joined). On the other hand, in each of the examples and comparative examples shown in Table 9 below, some of the materials are joined by, for example, spot welding to form overlapping portions.

[0150]

[0151] Referring to Table 9 and Figure 12F, in Example 18 and Comparative Example 12, material (2) and material (5), material (3) and material (4), and material (4) and material (5) each form an overlapping portion at their joints. Referring to Table 9 and Figure 12E, in Example 19 and Comparative Example 13, material (2) and material (5), and material (3) and material (4) each form an overlapping portion at their joints.

[0152] As shown in Table 9, in Examples 18 and 19, the minimum plate thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 12 and 13, the minimum thickness t ｍｉｎ Coefficient A of １ The coefficient A is the smallest among other relatively thick materials. ２ The details are as follows.

[0153] As can be seen from Table 9, in Examples 18 and 19, the phase transformation start time is delayed compared to Comparative Examples 12 and 13. Therefore, even when an overlap portion exists in the blank, the coefficient A of the thinnest material is １ The smallest coefficient A among other materials ２ It was confirmed that by making the thickness larger than 1 / 2 mm, the phase transformation start time of each material in the blank can be made uniform.

[0154] 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 performed while changing the material and thickness of the material contained in the structural member, as well as the division pattern of the structural member.

[0155] As with the first embodiment, the steel plates used as raw materials were selected from those shown in Table 5. The division pattern of the structural members is as shown in Figures 13A to 13D. Figures 13A to 13D 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 13A to 13D, each steel plate used as raw material is given a number in parentheses.

[0156] The analysis conditions and results for division patterns 8 and 9 shown in Figures 13A and 13B are shown in Table 10. In Figures 13A and 13B, the structural member is formed from six materials (1) to (6).

[0157]

[0158] Referring to Table 10, in Examples 20 and 21, the minimum plate thickness t ｍｉｎ Coefficient A of １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 14 and 15, the minimum thickness t ｍｉｎ Coefficient A of １ The coefficient A is the smallest among other relatively thick materials. ２ The details are as follows.

[0159] From Table 10, A １ -A ２In Examples 20 and 21, A １ -A ２ It can be seen that the phase transformation start time is extended compared to Comparative Examples 14 and 15, where the hardenability is ≦0. Therefore, by making the hardenability of the thinnest material higher than that of one or more other materials, it becomes easier to start forming the blank before the phase transformation to ferrite begins in the thinnest material, and it can be said that it is possible to harden the structural member uniformly.

[0160] The analysis conditions and results for division patterns 10 and 11 shown in Figures 13C and 13D are shown in Table 11. In Figures 13C and 13D, the structural member is formed from seven pieces of material (1) to (7).

[0161]

[0162] Referring to Table 11, in Examples 22 to 24, the minimum plate thickness t ｍｉｎ The smallest coefficient A among materials with １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 16 and 17, the minimum thickness t ｍｉｎ The smallest coefficient A among materials with １ The coefficient A is the smallest among other relatively thick materials. ２ The details are as follows.

[0163] From Table 11, A １ -A ２ In Examples 22 to 24, A １ -A ２ It can be seen that the phase transformation start time is extended compared to Comparative Examples 16 and 17, where the hardenability is ≦0. Therefore, by making the hardenability of the thinnest material higher than that of one or more other materials, it becomes easier to start forming the blank before the phase transformation to ferrite begins in the thinnest material, and it can be said that it is possible to harden the structural member uniformly.

[0164] [Example 3] A blank was heated at a furnace temperature of 920°C until the entire blank reached 910°C, then transferred to a press in 17 seconds. It was then hot stamped at a forming speed of 40 mm / s and held at bottom dead center for 20 seconds while applying a pressure of 3000 kN to obtain a hot-stamped structural component. Analysis samples were taken from the thinnest portions of these structural components using the method described in the above embodiment, and the martensite fraction variation was measured. Furthermore, shape accuracy and impact absorption performance were measured separately for these structural components. The evaluation results are shown in Table 12.

[0165]

[0166] Examples 14 and 16 in Table 12 are examples formed under the same conditions as Examples 14 and 16 shown in Table 8. Example 15A has the same division pattern and material combination as Example 15 shown in Table 8, but some of the materials form overlapping portions at the joints. Comparative Example 10A has the same division pattern and material combination as Comparative Example 10 shown in Table 8, but some of the materials form overlapping portions at the joints. In Example 15A and Comparative Example 10A, material (1) and material (2), material (1) and material (3), material (2) and material (5), and material (4) and material (5) each form overlapping portions at their joints. Comparative Example 11 is a comparative example formed under the same conditions as Comparative Example 11 shown in Table 8.

[0167] In Table 12, the variation in martensite fraction is, as explained in the above embodiment, the variation in the minimum plate thickness t ｍｉｎ The martensite fraction (%) is the maximum martensite fraction (%) minus the minimum martensite fraction (%) in the cross section of the structural member at the location of the material having the martensite fraction (%).

[0168] The shape accuracy was evaluated by measuring the distance between the structural member and the mating member at the overlapping portion when the structural member, which is approximately hat-shaped in cross section, is attached to another member. In Table 12, a value of ○ indicates that the distance from the surface of the mating member is within ±2.0 mm, a value of △ indicates that the distance is between ±2.0 mm and ±3.0 mm, and a value of × indicates that the distance is greater than ±3.0 mm.

[0169] Regarding impact absorption performance, door ring assemblies were prepared by spot welding door ring inner parts to the structural members (door ring outer parts) of each of the examples and comparative examples shown in Table 12, and these were used as test specimens for partial structure evaluation. The periphery of each test specimen was fixed with a restraining jig to reproduce deformation during a vehicle collision, and a barrier was collided with the door ring outer part (the side of the vehicle body). The maximum penetration amount at this time was evaluated as impact absorption performance. Impact absorption performance was evaluated based on the impact absorption performance of a door ring outer part formed by hot stamping each material and then joining them, and compared to the impact absorption performance of the base. In Table 12, impact absorption performance equivalent to the impact absorption performance of the base is represented by "good," impact absorption performance superior to the base impact absorption performance is represented by "better," impact absorption performance slightly lower than the base impact absorption performance is represented by "marginal," and impact absorption performance even lower than the base impact absorption performance is represented by "poor." The overall evaluation of the shape accuracy and impact absorption performance is also shown in four levels: poor, marginal, good, and better.

[0170] Each example shown in Table 12 is A １ -A ２ >0, whereas in each comparative example, A １ -A ２ ≦0. In Comparative Examples 10A and 11, the variation in martensite fraction exceeded 20%, whereas in Examples 14, 15A, and 16, the variation in martensite fraction was 20% or less. In Examples 14, 15A, and 16, the variation in martensite fraction was reduced to 15% or less. In Examples 14, 15A, and 16, the shape accuracy was also better than in Comparative Examples 10A and 11.

[0171] Examples 14, 15A, and 16, which had small variations in martensite fraction, also exhibited improved impact absorption performance compared to Comparative Examples 10A and 11. In particular, Examples 14 and 15A, which had variations in martensite fraction of 10% or less, were able to ensure impact absorption performance equal to or greater than that of the base material. That is, even though the annular structural component was formed by integrating multiple materials at the blank stage, impact absorption performance equal to or greater than that of a structural component formed by joining the materials after press-molding them individually was ensured. While the present invention has been described so far with respect to annular automobile door rings, it can also be applied to rear modules and the like, where dimensional accuracy can be improved and impact absorption performance can be ensured.

[0172] (Elemental Technology C1) The elemental technology C1 is a structural member for a vehicle body, comprising: (C1a) a pair of side frames; and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and wherein 0.001 g / m of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide are deposited on the first steel plate. ２ (C1b) A vehicle comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and a coating containing 0.500 g / m or more of carbon black is provided on the first steel plate. ２(C1c) A structural member comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the first steel plate and the second steel plate are each plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate, and the thickness of the aluminum-based plating layer on the first steel plate is smaller than the thickness of the aluminum-based plating layer on the second steel plate.

[0173] According to the elemental technology C1, it is possible to provide a structural member with excellent shock absorbing performance.

[0174] A hot stamping blank according to an embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined to form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the smallest plate thickness among the plurality of steel plates. The second steel plate has a plate thickness greater than that of the first steel plate. At least one of both surfaces of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate (first configuration).

[0175] The blank according to the first configuration includes a first steel plate having a minimum thickness and a second steel plate having a thickness greater than that of the first steel plate. At least one surface of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate. This allows the heating rate of the thin-walled portion of the first steel plate to be increased when the blank is heated during hot stamping. Therefore, the first steel plate can be heated to the austenite temperature more quickly, ensuring a longer holding time for the first steel plate at that temperature. This allows the austenite grains in the first steel plate to be coarsened. As a result, the ferrite transformation region (ferrite nose) in the CCT diagram is shifted to the long-time side, delaying the onset of ferrite transformation in the first steel plate after heating of the blank. This allows the blank to be formed while maintaining the microstructure of the first steel plate in the austenite phase. In other words, the hardenability of the thin-walled first steel plate can be improved. The phrase "treated to increase emissivity" not only refers to a case where the emissivity of at least one surface of the first steel plate is higher than the emissivity of both surfaces of the second steel plate before the blank is heated, but also refers to a case where the emissivity of at least one surface of the first steel plate becomes higher than the emissivity of both surfaces of the second steel plate during the heating of the blank.

[0176] In the blank according to the first configuration, the hardenability of the first steel plate having a smaller thickness is improved, and therefore the first steel plate can be well hardened when a structural component is formed from the blank by hot stamping. This facilitates uniform hardness in the structural component, thereby preventing partial reductions in the structural component's strength. Furthermore, because uneven stress is less likely to occur in the structural component, twisting, warping, and other problems are less likely to occur even in large structural components, ensuring good dimensional accuracy in the structural component. Therefore, when a structural component, particularly a large structural component, including a first steel plate having a smaller thickness than the second steel plate is formed from the blank, poor strength and dimensional accuracy of the structural component can be reduced, and the impact absorption performance (crash resistance) of the structural component can be improved.

[0177] In the blank according to the first configuration, the first steel sheet having the minimum thickness is treated to have a higher emissivity than the second steel sheet having a relatively large thickness. In this case, when the blank is heated during hot stamping, the first steel sheet heats up faster than the second steel sheet. This increases the high-temperature holding time of the first steel sheet, i.e., the time from when the first steel sheet reaches the austenite temperature range until the second steel sheet and the entire blank reach the austenite temperature range, compared to when the first steel sheet has the same emissivity as the second steel sheet. This reduces non-uniformity in phase transformation due to differences in cooling rates between the steel sheets after the blank is heated. Specifically, the initiation of the austenite-to-ferrite phase transformation in the first steel sheet having the minimum thickness can be delayed, thereby reducing the difference in the phase transformation initiation time between the first steel sheet having the minimum thickness and the other steel sheets. As a result, the hardenability can be made uniform between the first steel sheet having the minimum thickness and the other steel sheets.

[0178] In the blank according to the first configuration, the first steel plate may have a plate thickness of less than 1.4 mm (second configuration).

[0179] When the thickness of the first steel plate is less than 1.4 mm as in the second configuration, the first steel plate is particularly susceptible to heat dissipation after the heating of the blank is completed, making the hardenability of the first steel plate more likely to deteriorate. However, even when the thickness of the first steel plate is less than 1.4 mm, by subjecting at least one surface of the first steel plate to a treatment that increases the emissivity compared to the relatively thick second steel plate, it is possible to accelerate the temperature rise of the first steel plate when the blank is heated during hot stamping, thereby ensuring a long high-temperature holding time for the first steel plate. Therefore, the hardenability of the first steel plate can be improved.

[0180] In the blank according to the first or second configuration, the first steel sheet may be a plated steel sheet. The plated steel sheet may have a base steel sheet and an aluminum-based plating layer provided on the base steel sheet (third configuration).

[0181] When the first steel sheet is a plated steel sheet having an aluminum-based plating layer, as in the third configuration, the temperature rise rate of the first steel sheet tends to be slow when the blank is heated during hot stamping. Because the aluminum-based plating layer is nearly white, it tends to reflect heat energy and inhibit the temperature rise of the first steel sheet. However, even when the first steel sheet is a plated steel sheet having an aluminum-based plating layer, by subjecting at least one surface of the first steel sheet to a treatment that increases the emissivity compared to the relatively thick second steel sheet, the temperature rise of the first steel sheet can be accelerated when the blank is heated during hot stamping. This allows the first steel sheet to maintain a high temperature for a long time, thereby improving the hardenability of the first steel sheet.

[0182] In the blank according to any one of the first to third configurations, a coating may be formed on at least one surface of the first steel plate as a treatment for increasing emissivity, and this coating may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. (fourth configuration).

[0183] In the blank according to any one of the first to third configurations, a coating may be formed on at least one surface of the first steel plate as a treatment for increasing emissivity. The coating may comprise carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a metal oxide having a concentration of 0 to 0.30 g / m ２ In this case, the content of carbon black in the coating can be X ＣＢ (g / m ２ ), the oxide content is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ may satisfy the following formula (1) (see Patent Document 1) (fifth configuration): 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )≦332.0 (1)

[0184] In the blank according to any one of the first to fifth configurations, the first steel sheet and the second steel sheet may each be a plated steel sheet. The plated steel sheet may have a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. As a treatment for increasing the emissivity, the coating amount (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in the first steel sheet may be increased. ２ ) is the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in the second steel sheet. ２ ) (sixth configuration).

[0185] In the sixth configuration, both the first steel sheet and the second steel sheet are plated steel sheets having an aluminum-based plating layer. However, the deposition amount of the aluminum-based plating layer on both surfaces of the base steel sheet is smaller on the first steel sheet than on the second steel sheet. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron progresses to the surface of the thin first steel sheet before that of the relatively thick second steel sheet, causing both surfaces of the first steel sheet to change from silvery white to black or a color close to silvery white. As a result, the emissivity of both surfaces of the first steel sheet is higher than that of both surfaces of the second steel sheet during heating of the blank. Therefore, the first steel sheet can be heated to a temperature in the austenite range more quickly, ensuring a longer holding time for the first steel sheet at that temperature. This allows austenite grains in the first steel sheet to be coarsened. As a result, the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the longer time side, delaying the onset of ferrite transformation in the first steel sheet after heating of the blank. Therefore, the hardenability of the first steel plate can be improved.

[0186] In the blank according to any one of the first to sixth configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. In this case, the thickness of the first steel sheet may be t ｍｉｎ The thickness of the steel plate having the largest thickness among the plurality of steel plates is t ｍａｘ When 1.0<t ｍａｘ / t ｍｉｎ It is preferable that the ratio is ≦3.2 (seventh configuration).

[0187] Among the multiple steel plates included in the blank, the smallest plate thickness t ｍｉｎ The first steel plate having a maximum plate thickness t ｍａｘ When the difference in thickness between the steel plate and other steel plates having the minimum thickness t is large, it becomes difficult to ensure the process window in the manufacture of structural members. ｍｉｎ and maximum plate thickness t ｍａｘ If the difference is large, when the blank is heated during hot stamping, the maximum plate thickness t ｍａｘ While waiting for the steel sheet having the minimum sheet thickness t to reach the austenite temperature range, alloying of the coating layer of the first steel sheet, which has been heated to the austenite temperature range in advance, progresses, and the diffusion layer becomes thick, which may make it impossible to ensure the corrosion resistance or weldability of the first steel sheet due to the coating layer. ｍｉｎ Maximum plate thickness t ｍａｘ The ratio is set to 3.2 or less. ｍａｘ The temperature rise rate and minimum plate thickness t of the steel plate ｍｉｎ Since the heating rate of the first steel sheet does not deviate too much from the heating rate of the other steel sheet, heating of the other steel sheet can be completed before excessive alloying of the coating layer of the first steel sheet progresses. Therefore, a structural component can be manufactured while maintaining the corrosion resistance or weldability of the first steel sheet, and a process window can be secured in the manufacture of the structural component.

[0188] In the blank according to any one of the first to seventh configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. The blank may further include an overlap portion. The overlap portion is formed by overlapping the ends of two adjacent steel sheets among the plurality of steel sheets, the steel sheets being other than the second steel sheet. The overlap portion may have a total thickness of 4.0 mm or less. The surface of each of the two steel sheets located outside the overlap portion may be treated to increase the emissivity compared to both surfaces of the second steel sheet (eighth configuration).

[0189] When a blank has an overlap portion formed by overlapping the ends of two steel sheets, it may be impossible to ensure a process window in the manufacture of structural components. Specifically, when a blank is heated during hot stamping, alloying of the coating layer of the first steel sheet having the smallest sheet thickness progresses while waiting for the overlap portion to reach the austenite temperature range, resulting in a thick diffusion layer, making it impossible to ensure the corrosion resistance or weldability of the first steel sheet. Therefore, in the eighth configuration, the outer surface of each of the two steel sheets forming the overlap portion is treated to increase the emissivity. This accelerates the temperature rise of the overlap portion, allowing heating of the overlap portion to be completed before excessive alloying of the coating layer of the first steel sheet progresses, thereby enabling the manufacture of structural components while maintaining the corrosion resistance or weldability of the first steel sheet. In other words, the process window in the manufacture of structural components is more easily ensured. However, even if the emissivity of the overlap portion is increased, it becomes difficult to ensure a process window if the total sheet thickness of the overlap portion becomes excessive. Therefore, it is preferable that the total sheet thickness of the overlap portion be 4.0 mm or less.

[0190] In the blank according to the eighth configuration, the second steel sheet and the two steel sheets may each be a plated steel sheet having a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. In this case, the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in each of the two steel sheets may be ２ ) is the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in the second steel sheet. ２ ) (ninth configuration).

[0191] In a ninth configuration, the two steel sheets forming the overlap portion and the second steel sheet are aluminum-plated steel sheets. The deposition amount of the aluminum-based plating layer on both surfaces of the base steel sheet of each steel sheet forming the overlap portion is smaller than that of the second steel sheet. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron in the overlap portion progresses relatively quickly to the surface, and both surfaces of the overlap portion change from silver-white to black or a color close to silver-white. That is, the emissivity of both surfaces of the overlap portion increases during heating of the blank. As a result, the temperature rise in the overlap portion can be accelerated, making it easier to ensure a process window in the manufacture of structural components.

[0192] 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 ninth configurations, heating a plurality of steel plates included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming and quenching the heated blank using a mold (tenth configuration).

[0193] A vehicle body structural member according to an embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. A coating is provided on the first steel plate. The coating contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of 0.001 g / m. ２ or more (eleventh configuration).

[0194] A vehicle body structural member according to another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. A coating is provided on the first steel plate. The coating contains 0.500 g / m of carbon black. ２ The following is contained (twelfth configuration).

[0195] A vehicle body structural member according to yet another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. The first steel plate and the second steel plate are each plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer on the first steel plate is smaller than the thickness of the aluminum-based plating layer on the second steel plate (thirteenth configuration).

[0196] 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.

[0197] <First embodiment> [Structural members] Figure 14 is an exploded perspective view of structural members C1-10 and C1-20 according to this embodiment. The structural members C1-10 and C1-20 are used in the body of an automobile or the like. In the example shown in Figure 14, the structural members C1-10 and C1-20 form a front under module of the body.

[0198] The structural member C1-10 is an upper module. That is, when assembled to the vehicle body, the structural member C1-10 is disposed above the structural member C1-20. The structural member C1-10 comprises a pair of side frames C1-11L, C1-11R and at least one cross member C1-12. The side frames C1-11L, C1-11R and the cross member C1-12 each have an elongated shape.

[0199] The side frames C1-11L, C1-11R are arranged side by side in the left-right direction of the vehicle body when the structural member C1-10 is assembled to the vehicle body. The side frames C1-11L, C1-11R each extend in the front-to-rear direction of the vehicle body. Each of the side frames C1-11L, C1-11R includes a front section C1-111 and a rear section C1-112. The rear section C1-112 is arranged behind the front section C1-111 when the structural member C1-10 is assembled to the vehicle body.

[0200] The cross member C1-12 extends in the left-right direction of the vehicle body when the structural member C1-10 is assembled to the vehicle body. The cross member C1-12 extends from the side frame C1-11L to the side frame C1-11R. The cross member C1-12 connects the side frames C1-11L and C1-11R. In the example shown in FIG. 14, the cross member C1-12 connects the side frames C1-11L and C1-11R to each other at one end of the longitudinal direction of the side frames C1-11L and C1-11R. When the structural member C1-10 is assembled to the vehicle body, the cross member C1-12 is positioned, for example, at the rear end of the structural member C1-10. However, the cross member C1-12 may also connect the middle portions of the side frames C1-11L and C1-11R.

[0201] The structural member C1-20 is a lower module. That is, when assembled to the vehicle body, the structural member C1-20 is disposed below the structural member C1-10. The structural member C1-20 comprises a pair of side frames C1-21L, C1-21R and at least one cross member C1-22. The side frames C1-21L, C1-21R and the cross member C1-22 each have an elongated shape.

[0202] The side frames C1-21L, C1-21R are arranged side by side in the left-right direction of the vehicle body when the structural member C1-20 is assembled to the vehicle body. The side frames C1-21L, C1-21R each extend in the front-to-rear direction of the vehicle body. Each of the side frames C1-21L, C1-21R includes a front section C1-211 and a rear section C1-212. The rear section C1-212 is arranged behind the front section C1-211 when the structural member C1-10 is assembled to the vehicle body.

[0203] The lower side frames C1-21L and C1-21R are joined to the upper side frames C1-11L and C1-11R, respectively. The side frames C1-21L and C1-21R form a closed cross section together with the side frames C1-11L and C1-11R. Figure 15 shows the closed cross section formed by the side frames C1-21L and C1-21R together with the side frames C1-11L and C1-11R, respectively. Hereinafter, when there is no need to particularly distinguish between the side frames C1-11L and C1-11R, the side frames C1-11L and C1-11R will be collectively referred to as the side frame C1-11. Similarly, when there is no need to particularly distinguish between the side frames C1-21L and C1-21R, the side frames C1-21L and C1-21R will be collectively referred to as the side frame C1-21.

[0204] Fig. 15 is a cross-sectional view (transverse cross-section) of the side frames C1-11 and C1-21 taken along a plane perpendicular to the longitudinal direction. In the example of Fig. 15, each of the side frames C1-11 and C1-21 has a substantially hat-shaped transverse cross-section.

[0205] Referring to Figure 15, the side frame C1-11 includes a top plate C1-113, vertical walls C1-114 and C1-115, and flanges C1-116 and C1-117. In a cross-sectional view of the side frame C1-11, one end of the vertical walls C1-114 and C1-115 is connected by the top plate C1-113. In a cross-sectional view of the side frame C1-11, the other end of the vertical walls C1-114 and C1-115 is connected to flanges C1-116 and C1-117, respectively. The flanges C1-116 and C1-117 protrude outward from the vertical walls C1-114 and C1-115, respectively.

[0206] The side frame C1-21 includes a top plate C1-213, vertical walls C1-214 and C1-215, and flanges C1-216 and C1-217. In a cross-sectional view of the side frame C1-21, one end of the vertical walls C1-214 and C1-215 is connected by the top plate C1-213. In a cross-sectional view of the side frame C1-21, the other end of the vertical walls C1-214 and C1-215 is connected to flanges C1-216 and C1-217, respectively. The flanges C1-216 and C1-217 protrude outward from the vertical walls C1-214 and C1-215, respectively.

[0207] The top plate C1-213 of the lower side frame C1-21 is positioned opposite the top plate C1-113 of the upper side frame C1-11. In a cross-sectional view of the side frames C1-11 and C1-21, the vertical walls C1-214 and C1-215 of the side frame C1-21 extend from the top plate C1-213 toward the side frame C1-11. The flanges C1-216 and C1-217 of the side frame C1-21 are joined to the flanges C1-116 and C1-117 of the side frame C1-11, respectively. The flanges C1-216 and C1-217 are joined to the flanges C1-116 and C1-117 by, for example, spot welding. In the example of Figure 15, the flanges C1-116 and C1-117 of the upper side frame C1-11 are directly joined to the flanges C1-216 and C1-217 of the lower side frame C1-21. However, other members such as a floor panel may be provided between the side frame C1-11 and the side frame C1-21.

[0208] Returning to FIG. 14 , the cross member C1-22 extends in the left-right direction of the vehicle body when the structural member C1-20 is assembled to the vehicle body. The cross member C1-22 extends from the side frame C1-21L to the side frame C1-21R. The cross member C1-22 connects the side frames C1-21L and C1-21R. In the example shown in FIG. 14 , the cross member C1-22 connects the side frames C1-21L and C1-21R to each other at one end of the longitudinal direction of the side frames C1-21L and C1-21R. Like the upper cross member C1-12, the cross member C1-22 is positioned, for example, at the rear end of the structural member C1-20 when the structural member C1-20 is assembled to the vehicle body. However, the cross member C1-22 may also connect the middle portions of the side frames C1-21L and C1-21R. The cross member C1-22 may be joined to the upper cross member C1-12 by, for example, spot welding.

[0209] The structural members C1-10 and C1-20 are hot-stamped members. That is, the structural member C1-10 is formed by hot stamping (hot pressing) a blank formed from a plurality of steel plates (sub-blanks). Similarly, the structural member C1-20 is formed by hot stamping a blank formed from a plurality of steel plates.

[0210] In the upper structural member C1-10, for example, the side frames C1-11L and C1-11R may each be formed from a plurality of steel plates C1-31 and C1-32. In each of the side frames C1-11L and C1-11R, for example, the front section C1-111 may be formed from the steel plate C1-31, and the rear section C1-112 may be formed from the steel plate C1-32. The thickness of the steel plate C1-32 forming the rear section C1-112 may be greater than the thickness of the steel plate C1-31 forming the front section C1-111. Furthermore, the tensile strength of the steel plate C1-32 may be greater than the tensile strength of the steel plate C1-31. The cross member C1-12 may be primarily formed from a steel plate C1-33 that is different from the steel plates C1-31 and C1-32 forming the side frames C1-11L and C1-11R. Adjacent steel plates C1-31, C1-32, and C1-33 are joined by welding.

[0211] Similarly, in the lower structural member C1-20, for example, the side frames C1-21L, C1-21R may each be formed from a plurality of steel plates C1-41, C1-42. In each of the side frames C1-21L, C1-21R, for example, the front section C1-211 may be formed from the steel plate C1-41, and the rear section C1-212 may be formed from the steel plate C1-42. The thickness of the steel plate C1-42 forming the rear section C1-212 may be greater than the thickness of the steel plate C1-41 forming the front section C1-211. Furthermore, the tensile strength of the steel plate C1-42 may be greater than the tensile strength of the steel plate C1-41. The cross member C1-22 may be primarily formed from a steel plate C1-43 that is different from the steel plates C1-41, C1-42 forming the side frames C1-21L, C1-21R. Adjacent steel plates C1-41, C1-42, and C1-43 are joined by welding.

[0212] [Method for manufacturing structural members] Below, a method for manufacturing structural members C1-10 and C1-20 according to this embodiment will be described with reference to Figures 16A to 16G. The method for manufacturing structural member C1-10 includes the steps of preparing a blank C1-30, heating the blank C1-30, and forming the heated blank C1-30 into the structural member C1-10. Similarly, the method for manufacturing structural member C1-20 includes the steps of preparing a blank C1-40, heating the blank C1-40, and forming the heated blank C1-40 into the structural member C1-20.

[0213] (Preparation Process) As shown in Figure 16A, in the preparation process for manufacturing the upper structural member C1-10 (Figure 14), a blank C1-30 is prepared. The blank C1-30 has a shape obtained by unfolding the structural member C1-10. The blank C1-30 includes multiple steel plates (sub-blanks) C1-31, C1-32, and C1-33. The steel plates C1-31, C1-32, and C1-33 are arranged and joined to form two long portions C1-34L and C1-34R and at least one connecting portion C1-35.

[0214] The long portions C1-34L and C1-34R are arranged side by side in a plan view of the blank C1-30. The long portion C1-34L is a portion of the blank C1-30 that corresponds to the side frame C1-11L (FIG. 14). The long portion C1-34R is a portion of the blank C1-30 that corresponds to the side frame C1-11R (FIG. 14). In the example of FIG. 16A, the long portions C1-34L and C1-34R are formed from steel plates C1-31 and C1-32, respectively.

[0215] The connecting portion C1-35 connects the long portions C1-34L and C1-34R to each other. The connecting portion C1-35 is a portion of the blank C1-30 that corresponds to the cross member C1-12 (FIG. 14). In the example of FIG. 16A, the connecting portion C1-35 includes the steel plate C1-33. The connecting portion C1-35 may further include a portion of the steel plate C1-32.

[0216] 16B and 16C are cross-sectional views of the blank C1-30 showing the joining of the steel plates C1-31, C1-32, and C1-33. FIGS. 16B and 16C are cross-sectional views taken along lines IIIB-IIIB and IIIC-IIIC in FIG. 16A, respectively. Referring to FIG. 16B, the steel plate C1-31 is butt-joined to the steel plate C1-32. That is, the end faces of the steel plate C1-31 are joined together with the end face of the steel plate C1-32 abutting against the end face of the steel plate C1-32. Referring to FIG. 16C, the steel plate C1-32 is butt-joined to the steel plate C1-33. That is, the other end face of the steel plate C1-32 is joined together with the end face of the steel plate C1-33 abutting against the end face of the steel plate C1-33. The steel plates C1-31, C1-32, and C1-33 are joined together by, for example, laser welding. In this embodiment, the blank C1-30 is a so-called tailor-welded blank. However, the steel plates C1-31, C1-32, and C1-33 may be joined with their ends overlapping (overlap joint). In this case, the steel plates C1-31, C1-32, and C1-33 may be joined by spot welding. In particular, the intersections between the cross member C1-12 ( FIG. 14 ) extending in the left-right direction of the vehicle body and the side frames C1-11L and C1-11R ( FIG. 14 ) extending in the front-rear direction of the vehicle body may have an overlap structure as necessary.

[0217] Referring to FIGS. 16B and 16C, the steel plate C1-31 has a thickness t １ Steel plate C1-32 has a plate thickness of t ２ Steel plate C1-33 has a plate thickness of t ３ In this embodiment, the t １ is the minimum thickness t among steel plates C1-31, C1-32, and C1-33 ｍｉｎ The thickness of steel plate C1-32 is t ２ is the thickness t of steel plate C1-31 １ The thickness of steel plate C1-33 is larger than ３ is the thickness t of steel plate C1-31 １ In this embodiment, the plate thickness t ２ is the maximum thickness t among steel plates C1-31, C1-32, and C1-33 ｍａｘHowever, the thickness t of steel plate C1-33 ３ is the maximum thickness t among steel plates C1-31, C1-32, and C1-33 ｍａｘ That is, the thickness t of the steel plate C1-33 ３ is the thickness t of steel plate C1-32 ２ It may be more than that.

[0218] Thickness t of steel plate C1-31 ｍｉｎ is typically less than 1.4 mm. ｍｉｎ The thickness t may be, for example, 0.8 mm or more. ｍｉｎ and plate thickness t ｍａｘ is 1.0<t ｍａｘ / t ｍｉｎ ≦3.2, and 1.3≦t ｍａｘ / t ｍｉｎ It is more preferable that the ratio ≦3.2 is satisfied.

[0219] Minimum plate thickness t ｍｉｎ At least one of the surfaces of steel plate C1-31 having a thickness of 1000 Å is treated to increase the emissivity compared to the surfaces of thicker steel plate C1-32. In this embodiment, the emissivity of at least one surface of steel plate C1-31 is higher than the emissivity of the surfaces of steel plate C1-32 before the heating process of blank C1-30.

[0220] For example, the emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more on one or both surfaces of steel plate C1-31, and less than 60% on both surfaces of steel plate C1-32. The emissivity at a wavelength of 8.0 μm at 25° C. on one or both surfaces of steel plate C1-31 is more preferably 70% or more, and even more preferably 80% or more. ｍｉｎThe difference in emissivity at a wavelength of 8.0 μm at 25°C between steel sheet C1-31 having the above formula and other steel sheet C1-32 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity can be measured in accordance with JIS R 1801:2002. In this case, a sample taken from the steel sheet to be measured is placed in a Fourier transform infrared spectrophotometer, and the radiation intensity at a wavelength of 8.0 μm at 25°C is measured to calculate the emissivity. Alternatively, it is also possible to measure the radiation intensity of a target area at 25°C using a radiation thermometer set to a measurement wavelength of 8.0 μm, and calculate the emissivity from the ratio to the radiation intensity of a blackbody.

[0221] In this embodiment, a coating C1-50 is formed on one surface of the steel plate C1-31 as a treatment to increase the emissivity. For example, the entire one surface of the steel plate C1-31 is covered with the coating C1-50. On the other hand, the coating C1-50 is not provided on both surfaces of the steel plate C1-32. As a result, the emissivity of the one surface of the steel plate C1-31 is higher than the emissivity of both surfaces of the steel plate C1-32. However, the coating C1-50 may be provided on both surfaces of the steel plate C1-31.

[0222] The coating C1-50 is, for example, a substantially black coating. For example, the lightness L ＊ Value (CIE 1976 lightness index L defined in JIS Z8781-4:2013) ＊) is 60 or less, the coating C1-50 can be determined to be substantially black. The coating C1-50 may be a carbon-based surface treatment coating (a coating containing carbon (C)). The emissivity of the coating C1-50 at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, and more preferably 80% or more. That is, the emissivity of the surface of the steel sheet C1-31 to which the coating C1-50 is applied at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, and more preferably 80% or more. The coating C1-50 may have an emissivity of 60% or more at a wavelength of 8.0 μm at 700°C. For example, the surface treatment coating described in Patent Document 1 can be used as the coating C1-50. Specifically, the coating C1-50 can contain carbon black and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating C1-50 may or may not contain silica. That is, the silica content of the coating C1-50 is 0 g / m ２ The silica content of the coating C1-50 is 0.30 g / m ２ The silica content may be more preferably 0.10 g / m or less. ２ More preferably, it is 0.05 g / m or less. ２ The following is the result.

[0223] The carbon black and oxides can be dispersed throughout the entire surface of the coating C1-50 that is perpendicular to the thickness direction of the steel sheet C1-31. ＣＢ (g / m ２ ), the content of one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ It is preferable that the following formula (1) is satisfied: 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )≦332.0 (1)

[0224] In formula (1), the central formula: 24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )) is preferably 119.0 or more, more preferably 170.0 or more, and even more preferably 220.0 or more. The value calculated by the middle formula is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

[0225] The dispersion state of the carbon black and metal oxide in the coating C1-50 can be confirmed by performing an area analysis of the coating C1-50 with an electron probe micro analyzer (EPMA) for elements derived from the carbon black (e.g., C) and elements derived from the oxide (Zr, Zn, and Ti). ＣＢ can be measured by cross-sectional analysis of the coating C1-50 using a transmission electron microscope (TEM). That is, a cross-sectional analysis of the coating C1-50 is performed by TEM-EDS analysis in an area of ​​a predetermined size (thickness of the coating C1-50 × 5 μm), and the thickness of the coating C1-50 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is defined as ρ (ton / m ３ ), the film thickness is d (μm), and the area ratio is a (%), the value expressed by ρ×d×a is the carbon black content X ＣＢ (g / m ２ ) The oxide content X Ｏｘｉｄｅ can be determined by performing elemental analysis on the surface of the coating C1-50 using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation) and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0226] Carbon black content X in film C1-50 ＣＢ is 0.030 g / m ２ It is preferable that the content is 0.100 g / m or more. ２ It is more preferable that the content X is equal to or greater than this.ＣＢ is set within a range that satisfies the formula (1), but is preferably 0.800 g / m ２ or less, more preferably 0.600 g / m ２ The following is the result.

[0227] Coating C1-50 may contain 5.0% or more carbon black by volume, preferably 8.0% or more carbon black by volume, and Coating C1-50 may contain 40.0% or less carbon black by volume, preferably 30.0% or less carbon black by volume.

[0228] Metal oxide content X in coating C1-50 Ｏｘｉｄｅ is 0.030 g / m ２ It is preferable that the content is 0.060 g / m or more. ２ It is more preferable that the content X is equal to or greater than this. Ｏｘｉｄｅ is set within a range that satisfies the formula (1), but is preferably 0.500 g / m ２ More preferably, it is 0.300 g / m or less. ２ The following is the result.

[0229] Coating C1-50 can contain 1.0 or more volume percent metal oxides, and Coating C1-50 can contain 30.0 or less volume percent metal oxides, and preferably 25.0 or less volume percent metal oxides.

[0230] Carbon black content X ＣＢ (g / m ２ ) and the metal oxide content X Ｏｘｉｄｅ (g / m ２ ) ratio: X Ｏｘｉｄｅ / X ＣＢ is preferably 0.20 or more and 200.00 or less. Ｏｘｉｄｅ / X ＣＢ is more preferably 0.40 or more and 10.00 or less, and further preferably 0.60 or more and 5.00 or less.

[0231] In addition to the carbon black and metal oxides described above, the coating C1-50 may contain various binder components and additives.

[0232] The binder component is preferably a water-dispersible or water-soluble resin. The content of the binder component is preferably 40% by volume or more relative to the total volume of the coating C1-50. As the binder component selected from water-dispersible or water-soluble resins, various known resins exhibiting water dispersibility or water solubility can be used. Examples of such water-dispersible or water-soluble resins include polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluororesins, polyamide resins, polyolefin resins, and polymer compounds obtained by hydrolysis and condensation polymerization of silane coupling agents. It is more preferable that the binder component be one or more selected from the group consisting of polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, fluororesins, and polyamide resins. When a polyurethane resin is used as the binder component, the polyurethane resin is preferably a polyether-based polyurethane resin.

[0233] Examples of additives include leveling agents, water-soluble solvents, metal stabilizers, and etching inhibitors. The leveling agent is, for example, a nonionic or cationic surfactant. Examples of nonionic or cationic surfactants include polyethylene oxide or polypropylene oxide adducts, acetylene glycol compounds, and the like. Examples of water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol, and propylene glycol; cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether; esters such as ethyl acetate and butyl acetate; and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of metal stabilizers include chelate compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). Examples of etching inhibitors include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine.

[0234] The coating C1-50 can be formed by applying an organic or inorganic treatment liquid containing, for example, carbon black and a metal oxide to the entire surface of the steel sheet C1-31, and then drying the volatile components in the treatment liquid. The treatment liquid can be applied to the surface of the steel sheet C1-31, for example, by a roll coater, a curtain coater, or an inkjet. In the case of inkjet printing, the film thickness of the coating C1-50 can also be continuously changed. The film thickness of the coating C1-50 is, for example, 0.5 μm or more and 5.0 μm or less. The film thickness of the coating C1-50 is preferably 1.0 μm or more and 3.0 μm or less. The film thickness of the coating C1-50 is determined based on the sheet thickness t of the steel sheet C1-31. ｍｉｎ Therefore, the thickness of the steel plate C1-31 measured with the coating C1-50 included is the thickness t ｍｉｎ can be treated as

[0235] The steel plate C1-31 may be a plated steel plate. In this case, the steel plate C1-31 has a base steel plate C1-31a and a plated layer C1-31b. The type of base steel plate C1-31a is not particularly limited. The plated layer C1-31b is provided on the base steel plate C1-31a. The plated layer C1-31b covers the entire or almost the entire surface of both sides of the base steel plate C1-31a. The plated layer C1-31b is a metal plated layer. The plated layer C1-31b may be, for example, hot-dip aluminum plating, hot-dip galvanneal plating, or electrogalvanized plating. Known aluminum-plated steel plates, zinc-plated steel plates, etc. can be used as the steel plate C1-31.

[0236] The plating layer C1-31b is typically a plating layer containing aluminum as a main component (aluminum-based plating layer). The configuration of the aluminum-based plating layer is not particularly limited. A known aluminum-based plating layer can be adopted as the plating layer C1-31b. When the steel sheet C1-31 is a plated steel sheet, the thickness t ｍｉｎ is the combined thickness of the base steel plate C1-31a and the plating layer C1-31b.

[0237] Like the steel sheet C1-31, the steel sheets C1-32 and C1-33 may be known plated steel sheets. The steel sheets C1-32 and C1-33 may be aluminum plated steel sheets or zinc plated steel sheets. The steel sheets C1-32 and C1-33 may be the same type of plated steel sheet as the steel sheet C1-31, or may be a plated steel sheet of a different type from the steel sheet C1-31. Furthermore, the steel sheet C1-32 may be the same type of plated steel sheet as the steel sheet C1-33, or may be a plated steel sheet of a different type from the steel sheet C1-33. When the steel sheet C1-32 is a plated steel sheet, the thickness t of the steel sheet C1-32 ２ Similarly, when the steel sheet C1-33 is a plated steel sheet, the thickness t ３ is the combined thickness of the base steel sheet and the plating layer. When two or more of the steel sheets C1-31, C1-32, and C1-33 are plated steel sheets, the coating weight of each steel sheet may be the same as or different from that of the other steel sheets. However, in this embodiment, the steel sheets C1-31, C1-32, and C1-33 may be steel sheets (bare materials) that do not have a plating layer on their surfaces.

[0238] As shown in Figure 16D, in the manufacturing of the lower structural member C1-20 (Figure 14), a blank C1-40 is prepared in the preparation process. The blank C1-40 has a shape obtained by expanding the structural member C1-20. The blank C1-40 includes multiple steel plates (sub-blanks) C1-41, C1-42, and C1-43. The steel plates C1-41, C1-42, and C1-43 are arranged and joined to form two long portions C1-44L and C1-44R and at least one connecting portion C1-45.

[0239] The long portions C1-44L and C1-44R are arranged side by side in a plan view of the blank C1-40. The long portion C1-44L is the portion of the blank C1-40 that corresponds to the side frame C1-21L (Figure 14). The long portion C1-44R is the portion of the blank C1-40 that corresponds to the side frame C1-21R (Figure 14). In the example of Figure 16D, the long portions C1-44L and C1-44R are formed from steel plates C1-41 and C1-42, respectively.

[0240] The connecting portion C1-45 connects the long portions C1-44L and C1-44R to each other. The connecting portion C1-45 is a portion of the blank C1-40 that corresponds to the cross member C1-22 (FIG. 14). In the example of FIG. 16D, the connecting portion C1-45 includes the steel plate C1-43. The connecting portion C1-45 may further include a portion of the steel plate C1-42.

[0241] The blank C1-40 is configured in the same manner as the blank C1-30 (FIGS. 16B and 16C) with respect to the steel plates C1-41, C1-42, and C1-43. That is, the configurations of the steel plates C1-31, C1-32, and C1-33 of the blank C1-30 (FIGS. 16B and 16C) can be applied directly to the steel plates C1-41, C1-42, and C1-43. Therefore, detailed descriptions of the configurations of the steel plates C1-41, C1-42, and C1-43 will be omitted.

[0242] (Heating process) The prepared blanks C1-30 and C1-40 are formed into structural members C1-10 and C1-20 (Fig. 14) by hot stamping, respectively. During the hot stamping, the blanks C1-30 and C1-40 are subjected to a heating process. Referring to Fig. 16E, in the heating process, the blank C1-30 is heated, for example, by a heating furnace. The plurality of steel plates C1-31, C1-32, and C1-33 included in the blank C1-30 are heated to an austenite transformation finish temperature (A ｃ３ The steel plates C1-31, C1-32, and C1-33 are heated to a temperature above the austenite transformation finish temperature (A ). The steel plates C1-31, C1-32, and C1-33 are heated, for example, to a temperature above 900°C. This causes the microstructures of the steel plates C1-31, C1-32, and C1-33 to transform into an austenite phase. Although not shown, the multiple steel plates C1-41, C1-42, and C1-43 ( FIG. 16D ) included in the blank C1-40 are also heated to a temperature above the austenite transformation finish temperature (A ). ｃ３ It is heated above this temperature (point).

[0243] (Forming Process) Referring to Figure 16F, in the forming process, the heated blank C1-30 is formed into a structural member C1-10 (Figure 14) using a die C1-60 and quenched. The blank C1-30 heated by the heating process is removed from the heating furnace and transferred to the die C1-60. The die C1-60 may be attached to a known press device. The die C1-60 includes, for example, a punch C1-61 and a die C1-62. The blank C1-30 is placed between the punch C1-61 and the die C1-62.

[0244] Referring to FIG. 16G, after the blank C1-30 is placed between the punch C1-61 and the die C1-62, the die C1-62 approaches the punch C1-61. The blank C1-30 is clamped (pressed) between the punch C1-61 and the die C1-62 and formed into a shape that conforms to the forming surfaces of the punch C1-61 and the die C1-62. The blank C1-30 is held clamped between the punch C1-61 and the die C1-62. The blank C1-30 is cooled (quenched) by the die C1-60, and its microstructure is transformed to martensite. This allows the structural member C1-10 to be manufactured from the blank C1-30.

[0245] Although not shown, blank C1-40 shown in Figure 16D is also subjected to the same forming process as blank C1-30. That is, using a die, heated blank C1-40 is formed into structural member C1-20 (Figure 14) and quenched. Structural member C1-20 is joined to structural member C1-10 (Figure 14) by, for example, welding.

[0246] FIG. 17 is a cross-sectional view of the structural member C1-10 after hot stamping. FIG. 17 shows a cross-section of the structural member C1-10 at the position of the steel plate C1-31 (FIG. 16B) to which the black coating C1-50 was applied at the blank C1-30 stage. In the example of FIG. 17, the structural member C1-10 includes a coating C1-13. The coating C1-13 is provided on the steel plate C1-31. The black coating C1-50 (FIG. 16B) that was applied to the steel plate C1-31 in the blank C1-30 becomes the coating C1-13 after hot stamping. The coating C1-13 is provided on at least one surface of the steel plate C1-31. If the coating C1-50 before hot stamping contains carbon black, this carbon black will almost entirely disappear due to the high temperature heating during hot stamping, but some carbon black may remain. If the coating C1-50 before hot stamping satisfies the above formula (1), the coating C1-13 after hot stamping may contain no carbon black or may contain 0.500 g / m ２ However, if the coating C1-50 before hot stamping satisfies the above formula (1), the coating C1-13 after hot stamping may contain 0.500 g / m ２ When the coating C1-13 after hot stamping contains carbon black, the carbon black content in the coating C1-13 is 0 g / m ２ More preferably, it is greater than 0.001 g / m ２ If the coating C1-50 (FIG. 16B) is provided on the steel sheet C1-31 before hot stamping so that the coating C1-13 contains carbon black after hot stamping, carbon black will be present on the steel sheet C1-31 even in the later stage of the heating process, and the emissivity of the steel sheet C1-31 will be secured, so that the minimum sheet thickness t ｍｉｎThe steel sheet C1-31 having the above formula (1) is easily heated. Furthermore, when the steel sheet C1-31 is a plated steel sheet, the coating C1-13 after the heating process contains carbon black, which suppresses adhesion of the plating layer C1-31b (FIG. 16B) to the mold C1-60 during the forming process (hot stamping), thereby reducing the coefficient of friction between the steel sheet C1-31 and the mold C1-60 (FIGS. 16F and 16G). If the coating C1-50 before hot stamping satisfies the above formula (1), the coating C1-13 after hot stamping satisfies the central formula: 24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )} is, for example, 120.0 or more and 150.0 or less.

[0247] When the coating C1-50 (FIG. 16B) before hot stamping satisfies the above formula (1), the coating C1-13 after hot stamping contains one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of, for example, 0 g / m ２ More preferably, 0.001 g / m ２ The metal oxide content of the coating C1-13 is, for example, 0.500 g / m ２ In this way, when the metal oxide remains on the structural member C1-10, that is, when the coating C1-13 is 0 g / m or less, ２ If the coating C1-50 before hot stamping satisfies the above formula (1), the coating C1-13 after hot stamping will have a metal oxide content of 0 to 0.30 g / m or more, which is more preferable because it improves the corrosion resistance of the structural member C1-10. ２ Contains silica.

[0248] The carbon black content, metal oxide content, and silica content of the coating C1-13 can be measured in the same manner as for the coating C1-50 ( FIG. 16B ) at the blank C1-30 stage. Specifically, a vehicle body part is disassembled to obtain a structural member C1-10, and an analytical sample is obtained from this structural member C1-10, for example, by laser cutting. For example, an analytical sample is obtained from each of the multiple steel plates included in the structural member C1-10. The analytical sample is obtained, for example, from the center or its vicinity of the top plate of each steel plate having an open cross section. The obtained analytical sample is adjusted by polishing the cross section to outside the heat-affected zone during laser cutting, to prepare a coating analysis sample. Surface analysis of the coating C1-13 for elements derived from carbon black (e.g., C) and elements derived from oxides (Zr, Zn, and Ti) can be performed using EPMA to confirm the dispersion state of the carbon black and metal oxide in the coating C1-13. Depending on the location of the structural member C1-10, the coating C1-13 is present on the front and / or back side of the structural member C1-10, so both the front and back sides of the analytical sample are analyzed.

[0249] In many cases, the outermost surface layer of the structural member C1-10 is, for example, an electrodeposition coating film. In such cases, the coating layer that is below the electrodeposition coating film layer and above the alloyed metal plating layer is analyzed. The carbon black content X in the coating C1-13 ＣＢ can be measured by cross-sectional analysis of the coating C1-50 using a TEM. That is, a cross-sectional analysis of the coating C1-13 is performed by TEM-EDS analysis in an area of ​​a predetermined size (film thickness of the coating C1-13 × 5 μm), and the film thickness of the coating C1-13 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is defined as ρ (ton / m ３ ), the film thickness is d (μm), and the area ratio is a (%), the value expressed by ρ×d×a is the carbon black content X ＣＢ (g / m ２ ) The oxide content X Ｏｘｉｄｅcan be determined by performing elemental analysis of the coating layer that is present below the electrodeposition coating layer and above the alloyed metal plating layer using the above-mentioned X-ray fluorescence analyzer, and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0250] Minimum plate thickness t ｍｉｎ In the cross section of the structural member C1-10 at the position of the steel plate C1-31 having the minimum martensite fraction (%), the martensite fraction variation is, for example, 15% or less. The martensite fraction variation is preferably 10% or less. The martensite fraction variation can be measured as follows. That is, ｍｉｎ At the cross section of the structural member C1-10 at the position of the steel plate C1-31 having the above structure, 10 or more analysis samples (for example, the long side is about 10 mm in size) are cut out from positions 20 mm or more away from the end and 10 mm or more away from each other, and then each is mirror-polished and etched with LePeller's reagent so that the plate thickness direction becomes the observation surface. Then, an optical microscope is used to observe the area from the steel plate surface to a depth of 1 / 4 of the plate thickness (the area from the steel plate surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness) with a magnification of 1000 times and a field of view of 2,400 μm ２ The above structural photographs are taken from 30 fields of view, and the obtained structural photographs are subjected to image analysis.

[0251] The image analysis method involves obtaining the maximum brightness value Lmax and minimum brightness value Lmin from the image, designating the area with pixels whose brightness ranges from Lmax-0.3 (Lmax-Lmin) to Lmax as a white area, and calculating the ratio of the number of pixels in the white area to the total number of pixels to measure the martensite fraction. This type of image analysis is performed on a total of 30 observation fields of each analysis sample to determine the martensite fraction, and the average value is used as the martensite fraction for each analysis sample. Furthermore, the difference between the maximum and minimum martensite fractions in 10 or more analysis samples is calculated based on the minimum plate thickness t ｍｉｎThe variation in martensite fraction in the cross section of the structural member C1-10 at the position of the steel plate C1-31 having the minimum plate thickness t ｍｉｎ If there are multiple steel plates having the above structure, such analysis is performed on each steel plate to determine the martensite fraction, and the maximum martensite fraction variation among these steel plates is taken as the martensite fraction variation in the structural member C1-10.

[0252] In some steel sheets, the area fraction of martensite obtained by image analysis, i.e., the area fraction of the white region, may contain a few percent of the area fraction of retained austenite. However, since the variation in the martensite fraction is calculated as a difference, the impact is minor.

[0253] After the forming process (hot stamping), steel plate C1-31 may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. Similarly, after the forming process (hot stamping), steel plates C1-32 and C1-33 (FIG. 14) may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. At least one of steel plates C1-31, C1-32, and C1-33 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of steel plates C1-31, C1-32, and C1-33 may be the same as or different from the tensile strength of the other steel plates.

[0254] Although not shown, the lower structural member C1-20 (FIG. 14) may also include a coating C1-13 similar to that of the upper structural member C1-10 after hot stamping. The coating C1-13 is formed on the structural member C1-20 at a thickness of, for example, the minimum thickness t ｍｉｎ The structural member C1-20 is also arranged on at least one surface of a steel plate having a minimum plate thickness t ｍｉｎ The variation in martensite fraction at the position of the steel plate having the above structure is, for example, 15% or less, more preferably 10% or less, similar to the structural member C1-10.

[0255] [Effect] In the blank C1-30 according to this embodiment, the minimum plate thickness tｍｉｎ The emissivity of one surface of the steel plate C1-31 having a larger plate thickness t ２ The emissivity of the steel sheet C1-31 is higher than that of both surfaces of the steel sheet C1-32. That is, the surface of the steel sheet C1-31 is treated to increase the emissivity compared to both surfaces of the steel sheet C1-32. As a result, when the blank C1-30 is heated during hot stamping, the temperature rise rate of the steel sheet C1-31 is significantly higher than that of the steel sheet C1-32. Therefore, during the heating process, the steel sheet C1-31 can be quickly heated to the austenite temperature range, ensuring a long high-temperature holding time for the steel sheet C1-31. As a result, the austenite grains in the microstructure of the steel sheet C1-31 coarsen, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, the transformation of austenite to ferrite in the steel sheet C1-31 can be prevented between the time when the blank C1-30 is removed from the heating furnace and the time when forming is started using the die C1-60. Therefore, it is possible to start forming the blank C1-30 using the die C1-60 while maintaining the microstructure of the steel plate C1-31 in the austenite phase, and the minimum plate thickness t ｍｉｎ The hardenability of the steel plate C1-31 having the above properties can be improved.

[0256] In this embodiment, the hardenability of the relatively thin steel plate C1-31 is improved, so that the hardness of the structural member C1-10 formed from the blank C1-30 can be made uniform. More specifically, the minimum plate thickness t ｍｉｎ Since the steel plate C1-31 having the above structure is also well quenched, the variation in the martensite fraction in the steel plate C1-31 can be reduced to 15% or less. This reduces the likelihood of concentrated deformation when a collision load is applied to the structural member C1-10, making it easier for the structural member C1-10 to exhibit high impact absorption performance. Therefore, even when a structural member C1-10 including a thin steel plate C1-31, particularly a relatively large structural member C1-10 used in a vehicle body, is formed from the blank C1-30, the strength defect of the structural member C1-10 can be reduced, and the impact absorption performance of the structural member C1-10 can be improved.

[0257] The smaller the variation in the martensite fraction, the less uneven the mechanical properties within the structural component C1-10, which is preferable from the standpoint of the functionality of the structural component C1-10. On the other hand, a large variation in the martensite fraction indicates that there is an uneven distribution of areas with insufficient hardenability, i.e., areas with insufficient hardness, within the structural component C1-10, and when the structural component C1-10 is deformed by impact, deformation tends to concentrate in the areas with insufficient hardness, thereby reducing the functionality of the structural component C1-10.

[0258] In this embodiment, by improving the hardenability of the relatively thin steel plate C1-31, uneven stress is less likely to occur in the structural member C1-10. Therefore, even when a large structural member C1-10 is formed from the blank C1-30, twisting, warping, etc. are less likely to occur in the structural member C1-10. Therefore, even when a large structural member C1-10 including a thin steel plate C1-31 is formed from the blank C1-30, poor dimensional accuracy of the structural member C1-10 can be reduced, and the impact absorption performance of the structural member C1-10 can be improved.

[0259] In the blank C1-30 according to this embodiment, the minimum plate thickness t ｍｉｎ The surface of the steel plate C1-31 is substantially coated with a black coating C1-50, while the thickness t is larger than that of the steel plate C1-31. ２ The steel plate C1-32 does not have the coating C1-50. As a result, the emissivity of the surface of the steel plate C1-31 is already higher than the emissivity of both surfaces of the steel plate C1-32. In this case, when the blank C1-30 is heated during hot stamping, the steel plate C1-31 heats up faster than the steel plate C1-32, so the high-temperature holding time of the steel plate C1-31 is longer than when the steel plate C1-31 has the same emissivity as the steel plate C1-32. Therefore, after the heating of the blank C1-30 is completed, the non-uniformity of the phase transformation caused by the difference in cooling rate between the steel plates C1-31, C1-32, and C1-33 can be reduced. Specifically, the minimum plate thickness t ｍｉｎSince the start of the austenite-to-ferrite phase transformation for the steel plate C1-31 can be delayed, the difference in the phase transformation start time between the steel plate C1-31 and the other steel plates C1-32 and C1-33 can be reduced. As a result, the hardenability of the steel plates C1-31, C1-32, and C1-33 included in the blank C1-30 can be made uniform.

[0260] For example, if the plating layer C1-31b of the steel sheet C1-31 is an aluminum-based plating layer, the temperature rise rate of the steel sheet C1-31 tends to be slow during the heating process. Because the aluminum-based plating layer is white, it tends to reflect heat energy and inhibit the temperature rise of the steel sheet C1-31. However, in the blank C1-30 according to this embodiment, the surface of the steel sheet C1-31 is treated to increase the emissivity. Therefore, even if the steel sheet C1-31 is a plated steel sheet having an aluminum-based plating layer, the temperature rise of the steel sheet C1-31 during the heating process can be accelerated and the high-temperature holding time of the steel sheet C1-31 can be extended. Therefore, the hardenability of the thin-walled steel sheet C1-31 can be ensured.

[0261] In this embodiment, in the heating process, first, the minimum plate thickness t ｍｉｎ The steel plate C1-31 having the smallest thickness t reaches the austenite temperature range, and then the steel plates C1-32 and C1-33 reach the austenite temperature range in the order of thickness. ｍｉｎ and maximum plate thickness t ｍａｘ Ratio to: t ｍａｘ / t ｍｉｎ It is preferable that the thickness t ｍｉｎ The alloying of the plating layer C1-31b of the steel plate C1-31 having the thickness t ｍａｘ The steel plate C1-33 having the above structure can be heated sufficiently to complete the phase transformation to austenite, thereby ensuring a process window for the production of the structural component C1-10.

[0262] In this embodiment, a coating C1-50 can be applied to the steel plate C1-31 to increase the emissivity of the steel plate C1-31. The emissivity of the coating C1-50 (at a temperature of 25°C and a wavelength of 8.0 μm) is, for example, 60% or more. This allows the steel plate C1-31 to be efficiently radiated and makes it easier to increase the temperature rise rate of the steel plate C1-31 during the heating process.

[0263] In this embodiment, the coating C1-50 is a coating containing carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m ２ The carbon black content X may be the following: ＣＢ (g / m ２ ), and the oxide content X Ｏｘｉｄｅ (g / m ２ ) preferably satisfies the above formula (1). As described in Patent Document 1, formula (1) is a function of the ratio (%) of the increase in the temperature rise rate (°C / s) and the carbon black content X ＣＢ and oxide content X Ｏｘｉｄｅ Formula (1) defines the relationship between the temperature and the emissivity of the steel sheet C1-31 at a wavelength of 8.0 μm at 25°C. Formula (1) indicates that carbon black primarily functions as a heat absorbing material in the temperature range up to 700°C, and that oxide primarily functions as a heat absorbing material in the temperature range of 700°C or higher. When the coating C1-50 satisfies formula (1), the surface of the steel sheet C1-31 to which the coating C1-50 is applied tends to have an emissivity of 60% or more at a wavelength of 8.0 μm at 25°C.

[0264] The carbon black and oxides can be dispersed throughout the entire surface of the coating C1-50 that is perpendicular to the thickness direction of the steel sheet C1-31. This makes it easier to make the emissivity of the surface of the steel sheet C1-31 uniform. Therefore, in the heating process, the minimum thickness t ｍｉｎ The steel plate C1-31 having the above structure can be heated quickly and uniformly.

[0265] However, the configuration of the coating C1-50 is not limited thereto. The coating C1-50 may be a substantially black coating to increase the emissivity of the steel sheet C1-31 compared to an untreated steel sheet. For example, the coating C1-50 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating C1-50 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 to 50 in order to increase the emissivity of the steel sheet C1-31. 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.

[0266] The lower structural member C1-20 and blank C1-40 have the same configuration as the upper structural member C1-10 and blank C1-30. Therefore, the lower structural member C1-20 and blank C1-40 can achieve the same effects as those described above.

[0267] 18A and 18B are cross-sectional views of a blank C1-30A according to a second embodiment. 18A and 18B are cross-sectional views of the blank C1-30 according to the first embodiment, corresponding to FIGS. 16B and 16C. In the first embodiment, the blank C1-30 for the structural member C1-10 has a minimum plate thickness t ｍｉｎ At least one surface of the steel plate C1-31 is treated so that the emissivity of the surface of the steel plate C1-31 is higher than that of the surface of the other steel plate C1-32 before the heating process. In the first embodiment, for example, a coating C1-50 for increasing the emissivity is formed on one or both sides of the steel plate C1-31 (FIG. 16B). On the other hand, in the present embodiment, the surface of the steel plate C1-31 in the blank C1-30A is treated so that the emissivity of the surface of the steel plate C1-31 is higher than that of the surface of the steel plate C1-32 during the heating process.

[0268] 18A and 18B, in this embodiment, the steel plates C1-31, C1-32, and C1-33 are each plated steel plates. More specifically, the steel plates C1-31, C1-32, and C1-33 are all aluminum-plated steel plates. The steel plate C1-31 has a base steel plate C1-31a and an aluminum-based plating layer C1-31b. The steel plate C1-32 has a base steel plate C1-32a and an aluminum-based plating layer C1-32b. The steel plate C1-33 has a base steel plate C1-33a and an aluminum-based plating layer C1-33b.

[0269] In steel plate C1-31, an aluminum-based plating layer C1-31b covers both surfaces of the base steel plate C1-31a. The aluminum-based plating layer C1-31b is provided over the entire or almost the entire surface of both surfaces of the base steel plate C1-31a. Similarly, in steel plate C1-32, an aluminum-based plating layer C1-32b covers both surfaces of the base steel plate C1-32a. The aluminum-based plating layer C1-32b is provided over the entire or almost the entire surface of both surfaces of the base steel plate C1-32a. Furthermore, in steel plate C1-33, an aluminum-based plating layer C1-33b covers both surfaces of the base steel plate C1-33a. The aluminum-based plating layer C1-33b is provided over the entire or almost the entire surface of both surfaces of the base steel plate C1-33a.

[0270] The chemical composition of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b is not particularly limited. Known aluminum-based plating layers (plating layers containing aluminum as the main component) can be used as the aluminum-based plating layers C1-31b, C1-32b, and C1-33b. Although not particularly limited, the aluminum-based plating layers C1-31b, C1-32b, and C1-33b are, for example, Al-Si-based plating layers. The aluminum-based plating layers C1-31b, C1-32b, and C1-33b may be the same as or different from the aluminum-based plating layers of the other steel plates C1-31, C1-32, and C1-33, respectively.

[0271] The types of the base steel plates C1-31a, C1-32a, and C1-33a are not particularly limited, and each of the base steel plates C1-31a, C1-32a, and C1-33a may be the same as or different from the other base steel plates.

[0272] 18A and 18B, similarly to the first embodiment, the steel plate C1-31 has the smallest plate thickness t ｍｉｎ The steel plate C1-32 has a thickness t ｍｉｎ Plate thickness t is greater than ２ The steel plate C1-33 has a thickness t ｍｉｎ Plate thickness t ３ The thickness of steel plate C1-31 is t ｍｉｎ is the combined thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average thickness of the steel plate C1-31. ２ is the combined thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average thickness of the steel plate C1-32. ３ is the combined thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average thickness of the steel plate C1-33.

[0273] Referring to FIG. 18A, the minimum plate thickness t ｍｉｎ The coating weight of the aluminum-based plating layer C1-31b on both surfaces of the base steel plate C1-31a in the steel plate C1-31 having the above structure is W1 (g / m ２ ), larger plate thickness t ２ The coating weight of the aluminum-based plating layer C1-32b on both surfaces of the base steel plate C1-32a in the steel plate C1-32 having the above formula W2 (g / m ２ ), the coating weight W1 of the aluminum-based plating layer C1-31b on the steel plate C1-31 is smaller than the coating weight W2 of the aluminum-based plating layer C1-32b on the steel plate C1-32. The coating weight W1 of the aluminum-based plating layer C1-31b on the steel plate C1-31 is the average coating weight on both surfaces of the base steel plate C1-31a. Usually, the coating weight (g / m) of the aluminum-based plating layer C1-31b on one surface of the base steel plate C1-31a is２ ) is the coating weight (g / m) of the aluminum-based plating layer C1-31b on the other surface of the base steel sheet C1-31a. ２ ) is substantially equal to the coating weight (g / m) of the aluminum-based plating layer C1-31b on one surface of the base steel plate C1-31a. However, depending on various manufacturing conditions, for example, the coating weight of the aluminum-based plating layer C1-31b may vary between the front and back surfaces of the base steel plate C1-31a. The coating weight of the aluminum-based plating layer C1-31b on one surface of the base steel plate C1-31a may be different from the coating weight on the other surface of the base steel plate C1-31a. Similarly, the coating weight W2 of the aluminum-based plating layer C1-32b on the steel plate C1-32 is the average coating weight on both surfaces of the base steel plate C1-32a. The coating weight (g / m) of the aluminum-based plating layer C1-32b on one surface of the base steel plate C1-32a is ２ ) is usually the coating weight (g / m) of the aluminum-based plating layer C1-32b on the other surface of the base steel sheet C1-32a. ２ ) However, due to various manufacturing conditions, for example, the coating weight of the aluminum-based plating layer C1-32b may vary between the front and back of the base steel sheet C1-32a. The coating weight of the aluminum-based plating layer C1-32b may be different between one surface and the other surface.

[0274] The coating weight W1 of the aluminum-based plating layer C1-31b on the steel sheet C1-31 and the coating weight W2 of the aluminum-based plating layer C1-32b on the steel sheet C1-32 were each 20 g / m ２ 120g / m or more ２ The deposition amounts W1 and W2 are preferably 30 g / m or less. ２ More preferably, 35 g / m ２ The deposition amounts W1 and W2 are preferably 115 g / m ２ More preferably, it is 100 g / m or less. ２ The difference between the adhesion amounts W1 and W2 (W2-W1) is, for example, 10 (g / m ２ ) or more. W2-W1 is preferably 20 (g / m ２ ) or more, more preferably 30 (g / m ２ ) or more. W2-W1 is 80 (g / m ２ ) or less. W2-W1 is preferably 70 (g / m２ ) or less, more preferably 60 (g / m ２ ) or less. The adhesion amounts W1 and W2 satisfy the relationship W2 / W1>1.0. The adhesion amounts W1 and W2 preferably satisfy the relationship W2 / W1≧1.2, and more preferably W2 / W1≧1.5.

[0275] Referring to FIG. 18B, the plate thickness t ３ The coating weight of the aluminum-based plating layer C1-33b on both surfaces of the base steel sheet C1-33a in the steel sheet C1-33 having the above structure is W3 (g / m ２ ), the coating weight W3 of the aluminum-based plating layer C1-33b on the steel sheet C1-33 may be less than the coating weight W2 of the aluminum-based plating layer C1-32b on the steel sheet C1-32. The coating weight W3 of the aluminum-based plating layer C1-33b is the average coating weight on both surfaces of the base steel sheet C1-33a. Usually, the coating weight (g / m) of the aluminum-based plating layer C1-33b on one surface of the base steel sheet C1-33a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer C1-33b on the other surface of the base steel sheet C1-33a. ２ ) is substantially equal to the thickness of the aluminum-based plating layer C1-33b. However, depending on various manufacturing conditions, for example, the adhesion weight of the aluminum-based plating layer C1-33b may vary between the front and back surfaces of the base steel sheet C1-33a. The adhesion weight of the aluminum-based plating layer C1-33b may be different between one surface and the other surface of the base steel sheet C1-33a.

[0276] The coating weight W3 of the aluminum-based plating layer C1-33b in the steel sheet C1-33 is also, for example, 20 g / m, similar to the steel sheets C1-31 and C1-32. ２ 120g / m or more ２ The deposition amount W3 is preferably 30 g / m ２ More preferably, 35 g / m ２ The adhesion amount W3 is 115 g / m ２ It is preferable that the weight is 100 g / m or less. ２ When W2>W3, the difference between the adhesion amounts W2 and W3 (W2-W3) is, for example, 10 (g / m ２) or more. W2-W3 is preferably 20 (g / m ２ ) or more, more preferably 30 (g / m ２ ) or more. W2-W3 is 80 (g / m ２ ) or less. W2-W3 is preferably 70 (g / m ２ ) or less, more preferably 60 (g / m ２ The coating weight W3 of the aluminum-based plating layer C1-33b on the steel sheet C1-33 is equal to or less than the minimum sheet thickness t ｍｉｎ It may be equal to or greater than the coating weight W1 of the aluminum-based plating layer C1-31b of the steel plate C1-31 having the above-mentioned thickness.

[0277] The method for forming the aluminum-based plating layers C1-31b, C1-32b, and C1-33b on the base steel sheets C1-31a, C1-32a, and C1-33a, respectively, is not particularly limited, but may be, for example, a general hot-dip plating method. That is, by immersing the base steel sheet C1-31a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet C1-31 having an adjusted coating weight W1 of the aluminum-based plating layer C1-31b can be obtained. Similarly, by immersing the base steel sheet C1-32a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet C1-32 having an adjusted coating weight W2 of the aluminum-based plating layer C1-32b can be obtained. Furthermore, an aluminum-plated steel sheet C1-33 having an adjusted coating weight W3 of an aluminum-based plating layer C1-33b can be obtained by immersing a base steel sheet C1-33a in a hot-dip aluminum plating bath and gas wiping with nitrogen, air, or the like. When an aluminum-based plating layer is formed by the hot-dip plating method, 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.

[0278] The method for measuring the coating weights W1, W2, and W3 of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b includes, for example, the sodium hydroxide-hexamethylenetetramine-hydrochloric acid stripping weight method described in JIS G 3314: 2019. Specifically, in accordance with JIS G 3314: 2019, a predetermined surface area S (mm ２ ) (for example, 50 mm × 50 mm) and measure the weight w1 (g) of each test piece. Then, each test piece is immersed in an aqueous sodium hydroxide solution, and after confirming that foaming caused by dissolution of the plating has subsided, each test piece is removed from the aqueous sodium hydroxide solution and rinsed with water. Next, each test piece that is still wet after rinsing with water is immersed in an aqueous hydrochloric acid solution containing added hexamethylenetetramine until foaming caused by dissolution of the plating has subsided. The test piece is removed from the hexamethylenetetramine-hydrochloric acid solution, immediately rinsed with water, dried, and the weight w2 (g) of the test piece is measured again. The coating weight W (g / m) of the aluminum-based plating layer of each test piece is measured. ２ ) is {(w1-w2) / S}×10 ６ The average value of the coating weight W of five or more test pieces taken from each steel sheet is taken as the coating weight of the aluminum-based plating layer on that steel sheet.

[0279] However, if the size of the test specimens taken from each of the steel sheets C1-31, C1-32, and C1-33 is small, the cross section of each of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b can be observed with an optical microscope (area: 100 μm × 100 μm) and the thickness (μm) of the plating layer can be measured in three fields of view in the same way. The average of the thicknesses measured in the three fields of view can be multiplied by three to convert it into the coating weight. In this case, for each of the steel sheets C1-31, C1-32, and C1-33, the coating weight is calculated for each side of the base steel sheet, 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 layer C1-31b of the thinnest steel plate C1-31 is smaller than the thickness of the aluminum-based plating layer C1-32b of the other steel plate C1-32. In this embodiment, the thickness of the aluminum-based plating layer C1-33b of the steel plate C1-33 is also smaller than the thickness of the aluminum-based plating layer C1-32b of the steel plate C1-32. The thicknesses of the aluminum-based plating layers C1-31b and C1-33b may be different from each other or the same.

[0280] [Structural Member] The blank C1-30A is subjected to the heating process and forming process similar to those in the first embodiment. As a result, as shown in Figures 19A to 19C, a structural member C1-10A similar to that in the first embodiment is manufactured from the blank C1-30A. Figures 19A to 19C are cross-sectional views of the structural member C1-10A after the forming process (hot stamping). Figure 19A shows the structural member C1-10A with a minimum plate thickness t ｍｉｎ FIG. 19B shows a cross section of the structural member C1-10A at the location of the steel plate C1-31 having a thickness t ｍｉｎ Plate thickness t is greater than ２ FIG. 19C shows a cross section of the structural member C1-10A at the location of the steel plate C1-32 having a thickness t ｍｉｎ Plate thickness t ３ 1 shows a cross section of structural member C1-10A at the location of steel plate C1-33 having a

[0281] Referring to Fig. 19A, even in the structural member C1-10A after hot stamping, the steel sheet C1-31 is a plated steel sheet having an aluminum-based plating layer C1-31b on both surfaces of the base steel sheet C1-31a. Referring to Fig. 19B, the steel sheet C1-32 is a plated steel sheet having an aluminum-based plating layer C1-32b on both surfaces of the base steel sheet C1-32a. Referring to Fig. 19C, the steel sheet C1-33 is a plated steel sheet having an aluminum-based plating layer C1-33b on both surfaces of the base steel sheet C1-33a. However, compared to the state of the blank C1-30A (Figs. 18A and 18B), the aluminum-based plating layers C1-31b, C1-32b, and C1-33b in the structural member C1-10A have been alloyed with iron by the heating process.

[0282] 19A and 19B, when the average thickness (plating thickness) of the aluminum-based plating layer C1-31b on both surfaces of the steel sheet C1-31 is K1 (μm), and the average thickness (plating thickness) of the aluminum-based plating layer C1-32b on both surfaces of the steel sheet C1-32 is K2 (μm), the plating thickness K1 of the steel sheet C1-31 is smaller than the plating thickness K2 of the steel sheet C1-32. The difference between the plating thicknesses K1 and K2, K2 - K1, is, for example, 7 μm or more. K2 - K1 may be 33 μm or less. Furthermore, the plating thicknesses K1 and K2 may satisfy the relationship K2 / K1 > 1.0. K2 / K1 is preferably 1.2 or more, more preferably 1.5 or more.

[0283] 19B and 19C, when the average thickness (plating thickness) of the aluminum-based plating layer C1-33b on both surfaces of the steel plate C1-33 is K3 (μm), in this embodiment, the plating thickness K3 of the steel plate C1-33 is smaller than the plating thickness K2 of the steel plate C1-32. The difference between the plating thicknesses K2 and K3 (K2 - K3) is, for example, 7 (μm) or more. K2 - K3 may be 33 (μm) or less.

[0284] The thicknesses K1, K2, and K3 of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b in the structural member C1-10A can be measured as follows. Specifically, a vehicle body part is disassembled to obtain the structural member C1-10A, and an analysis sample is obtained from this structural member C1-10A, for example, by laser cutting. For example, an analysis sample is obtained from each of the multiple steel plates included in the structural member C1-10A. The analysis sample is obtained from the center or its vicinity of the top plate of each steel plate having an open cross section. For test pieces obtained from each of the multiple steel plates, the cross section of the aluminum-based plating layer is nital etched and then observed with an optical microscope (area: 100 μm × 100 μm), and the thickness of the plating layer is measured in three fields of view. The average value of the thicknesses of the plating layer measured in the three fields of view can be used as the plating thickness. The outermost layer of the structural member C1-10A often contains, for example, an electrodeposition coating film. In this case, the plating layer that is present below the electrodeposition coating layer and above the base steel sheet is observed.

[0285] In the structural member C1-10A according to this embodiment, as in the first embodiment, the minimum plate thickness t ｍｉｎ The variation in the martensite fraction in the cross section at the position of the steel plate C1-31 having the above structure is, for example, 15% or less, and more preferably 10% or less. The variation in the martensite fraction can be measured by the method described in the first embodiment.

[0286] Although not shown, the blank C1-40 (FIGS. 14 and 16D) for the structural member C1-20 can have the same configuration as the blank C1-30A. That is, the configuration of the steel plates C1-31, C1-32, and C1-33 in the blank C1-30A can be directly applied to the steel plates C1-41, C1-42, and C1-43 (FIG. 16D) of the blank C1-40. In this case, in the structural member C1-20 manufactured from the blank C1-40 through the heating process and forming process, the steel plates C1-41, C1-42, and C1-43 have the same configuration as the steel plates C1-31, C1-32, and C1-33 of the structural member C1-10A shown in FIGS. 19A to 19C.

[0287] [Effect] In the blank C1-30A according to this embodiment, the minimum plate thickness t ｍｉｎ The coating weight W1 of the aluminum-based plating layer C1-31b of the steel plate C1-31 having a larger plate thickness t ２ The coating weight W2 of the aluminum-based plating layer C1-32b of the steel sheet C1-32 having the same thickness is smaller than that of the steel sheet C1-32 having the same thickness. As a result, when the blank C1-30A is heated during hot stamping, the temperature rise rate of the steel sheet C1-31 is significantly higher than that of the steel sheet C1-32. Specifically, because the aluminum-based plating layer C1-31b on the surface of the steel sheet C1-31 is relatively thin, when the blank C1-30A is heated, alloying between the aluminum-based plating layer C1-31b and the iron contained in the base steel sheet C1-31a progresses rapidly to the surface of the steel sheet C1-31, and both surfaces of the steel sheet C1-31 turn black or a color close to black. In other words, the emissivity of both surfaces of the steel sheet C1-31 increases during the heating process. Therefore, the steel sheet C1-31 can be heated to the austenite range temperature more quickly, ensuring a longer high-temperature holding time for the steel sheet C1-31. As a result, the austenite grains in the microstructure of the steel sheet C1-31 become coarse, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, it is possible to prevent the transformation of austenite to ferrite in the steel sheet C1-31 after removal from the heating furnace and before the start of forming with the die C1-60. Therefore, it is possible to start forming the blank C1-30A with the die C1-60 while maintaining the microstructure of the steel sheet C1-31 in the austenite phase, and it is possible to achieve the minimum sheet thickness t ｍｉｎ The hardenability of the steel plate C1-31 having the above properties can be improved.

[0288] In this embodiment, the hardenability of the relatively thin steel plate C1-31 is improved, so that the hardness of the structural member C1-10A formed from the blank C1-30A can be made uniform. More specifically, in this embodiment, the minimum plate thickness t ｍｉｎSince the steel plate C1-31 having the above structure is also well quenched, the variation in martensite fraction in the steel plate C1-31 can be reduced to 15% or less. Therefore, as described in the first embodiment, the strength defect of the structural member C1-10A can be reduced, and the impact absorption performance of the structural member C1-10A can be improved.

[0289] As in the first embodiment, by improving the hardenability of the relatively thin steel plate C1-31, uneven stress is less likely to occur in the structural member C1-10A. As a result, even when forming a large structural member C1-10A from a blank C1-30A including the steel plate C1-31, it is possible to reduce dimensional accuracy defects in the structural member C1-10A and improve the impact absorption performance of the structural member C1-10A.

[0290] In the blank C1-30A according to this embodiment, the minimum plate thickness t ｍｉｎ The coating weight W1 of the aluminum-based plating layer C1-31b on the steel plate C1-31 is smaller than the coating weight W2 of the aluminum-based plating layer C1-32b on the steel plate C1-32, which is thicker than the steel plate C1-31. This allows the steel plate C1-31 to be heated more quickly than the steel plate C1-32, and the high-temperature holding time of the steel plate C1-31 is longer than when the coating weight W1 of the aluminum-based plating layer C1-31b on the steel plate C1-31 is the same as or greater than the coating weight W2 of the aluminum-based plating layer C1-32b on the steel plate C1-32. Therefore, after heating of the blank C1-30A is completed, non-uniformity in phase transformation due to differences in cooling rates among the steel plates C1-31, C1-32, and C1-33 can be reduced. Specifically, the minimum plate thickness t ｍｉｎ Since the start of the austenite-to-ferrite phase transformation for the steel plate C1-31 can be delayed, the difference in the phase transformation start time between the steel plate C1-31 and the other steel plates C1-32 and C1-33 can be reduced. As a result, the hardenability of the steel plates C1-31, C1-32, and C1-33 included in the blank C1-30 can be made uniform.

[0291] As shown in FIG. 20, in the blank C1-30A according to this embodiment, the minimum plate thickness t ｍｉｎThe steel plate C1-31 having the above structure may be provided with a coating C1-50 similar to that of the first embodiment. The coating C1-50 may be provided on at least one surface of the steel plate C1-31. That is, the coating C1-50 may cover only one surface of the steel plate C1-31, or may cover both surfaces of the steel plate C1-31. This allows the emissivity of the surface of the steel plate C1-31 to be increased in advance, so that when the blank C1-30A is heated during hot stamping, the temperature of the steel plate C1-31 rises more quickly. Therefore, the high-temperature holding time of the steel plate C1-31 can be secured longer. Therefore, the hardenability of the thinnest steel plate C1-31 can be further improved.

[0292] When the coating C1-50 is provided on the thinnest steel plate C1-31 in the blank C1-30A, in the formed structural member C1-10A, a coating C1-13 (Figure 17) similar to that in the first embodiment is present on the steel plate C1-31.

[0293] The lower structural member C1-20 (FIG. 14) and blank C1-40 (FIG. 16D) can have the same configuration as the upper structural member C1-10A and blank C1-30A in this embodiment, and therefore the lower structural member C1-20 and blank C1-40 can also achieve the same effects as described above.

[0294] <Third embodiment> Figure 21 is a plan view of a blank C1-30B according to the third embodiment. The blanks C1-30 and C1-30A according to the first and second embodiments are tailored blanks in which steel plates C1-31, C1-32, and C1-33 are butt-joined to each other. The blank C1-30B according to this embodiment differs from the first and second embodiments mainly in the arrangement of the steel plates and the form of the joint.

[0295] 21, the blank C1-30B includes a plurality of steel plates C1-31, C1-32, C1-33, and C1-36. In the example of FIG. 21, the steel plates C1-31, C1-32, C1-33, and C1-36 are arranged and joined to form long portions C1-34L and C1-34R and a plurality of connecting portions C1-35. The minimum plate thickness t ｍｉｎAt least one surface of the steel plate C1-31 having the above structure is treated to increase the emissivity compared to both surfaces of the steel plate C1-32. That is, at least one surface of the steel plate C1-31 is treated in the same manner as in the first or second embodiment so as to increase the emissivity compared to the steel plate C1-32 before or during heating. Therefore, the blank C1-30B according to this embodiment can also achieve the same effect as the first embodiment.

[0296] The blank C1-30B has an overlap portion C1-37. Figure 22 is a cross-sectional view taken along line IX-IX in Figure 21, showing the cross section of the overlap portion C1-37. In this embodiment, the overlap portion C1-37 is formed by overlapping the ends of two adjacent steel plates C1-33 and C1-36. The end of the steel plate C1-33 is joined to the end of the steel plate C1-36 in an overlapping state. The steel plates C1-33 and C1-36 are joined to each other by, for example, spot welding or laser welding.

[0297] The overlap portion C1-37 has a total plate thickness t. The total plate thickness t is the thickness t of the steel plate C1-33. ３ and the thickness t of steel plate C1-36 ６ The total thickness t of the overlap portion C1-37 is the sum of the thicknesses of the overlap portion C1-33 and C1-36. If at least one of the steel plates C1-33 and C1-36 is a plated steel plate, the total thickness t also includes the thickness of the plated layer. The total thickness t of the overlap portion C1-37 is, for example, 4.0 mm or less. The total thickness t of the overlap portion C1-37 may be greater than 2.5 mm.

[0298] The surface of each of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 is treated to increase its emissivity compared to both surfaces of the other steel plate C1-32 (FIG. 16B). The surface of each of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 is treated to increase its emissivity, for example, over its entire surface. For example, the emissivity of the surface of the steel plate C1-33 located outside the overlap portion C1-37, i.e., the surface opposite the mating steel plate C1-36, is pre-set to be higher than the emissivity of both surfaces of the steel plate C1-32. Similarly, the emissivity of the surface of the steel plate C1-36 located outside the overlap portion C1-37, i.e., the surface opposite the mating steel plate C1-33, is pre-set to be higher than the emissivity of both surfaces of the steel plate C1-32. For example, the emissivity at 25°C at a wavelength of 8.0 μm is 60% or more, preferably 70% or more, and more preferably 80% or more, on the surfaces of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37. The difference in emissivity at a wavelength of 8.0 μm at 25°C between the surfaces of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 and both surfaces of the other steel plate C1-32 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity of the surfaces of the steel plates C1-33 and C1-36 located inside the overlap portion C1-37 may be greater than the emissivity of both surfaces of the steel plate C1-32, or may be less than the emissivity of both surfaces of the steel plate C1-32.

[0299] For example, the coating C1-50 described in the first embodiment may cause the surface of each of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 to have a higher emissivity than the steel plate C1-32 (FIG. 16B). The coating C1-50 is provided on at least the surface located outside (the front side) of the overlap portion C1-37 of both surfaces of the steel plate C1-33, covering the entire surface. The coating C1-50 is also provided on at least the surface located outside (the front side) of the overlap portion C1-37 of both surfaces of the steel plate C1-36, covering the entire surface. Because the thickness of the coating C1-50 is very small as described in the first embodiment, the thickness of the overlap portion C1-37 measured including the coating C1-50 can also be treated as the total plate thickness t.

[0300] Alternatively, the surfaces of the steel sheets C1-33 and C1-36 located outside the overlap portion C1-37 may be treated so that the surfaces have a higher emissivity than both surfaces of the steel sheet C1-32 ( FIG. 21 ) when the blank C1-30 is heated. For example, as shown in FIG. 23 , the steel sheets C1-33 and C1-36 may be plated steel sheets with a thinner coating than the steel sheet C1-32 ( FIG. 18A ). In this case, the steel sheet C1-33 is a plated steel sheet having a base steel sheet C1-33a and an aluminum-based plating layer C1-33b, as in the second embodiment. The steel sheet C1-36 is also a plated steel sheet having a base steel sheet C1-36a and an aluminum-based plating layer C1-36b. The base steel sheets C1-33a and C1-36a may be the same type of steel sheet or different types of steel sheets. Similarly, the aluminum-based plating layers C1-33b, C1-36b may be the same type of plating layer or different types of plating layers. The other steel plates C1-31, C1-32 (FIGS. 18A and 18B) are also plated steel plates having base steel plates C1-31a, C1-32a and aluminum-based plating layers C1-33b, respectively, and have the same configuration as the second embodiment.

[0301] When the steel plates C1-33 and C1-36 are plated steel plates, the total plate thickness t of the overlap portion C1-37 is a plate thickness including the thickness of the plated layers C1-33b and C1-36b. In the example of Figure 23, the coating weight of the aluminum-based plated layer C1-33b on both surfaces of the base steel plate C1-33a of the steel plate C1-33 is W3 (g / m ２ ), the coating weight of the aluminum-based plating layer C1-36b on both surfaces of the base steel sheet C1-36a of the steel sheet C1-36 is W6 (g / m ２ ), the deposition weights W3 and W6 are smaller than the deposition weight W2 of the aluminum-based plating layer C1-32b (FIG. 18A) on both surfaces of the base steel plate C1-32a of the steel plate C1-32.

[0302] As explained in the second embodiment, the coating weight W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 is the average coating weight on both surfaces of the base steel plate C1-33a. Similarly, the coating weight W6 of the aluminum-based plating layer C1-36b in the steel plate C1-36 is the average coating weight on both surfaces of the base steel plate C1-36a. Usually, the coating weight (g / m) of the aluminum-based plating layer C1-36b on one surface of the base steel plate C1-36a is ２ ) is the coating weight (g / m) of the aluminum-based plating layer C1-36b on the other surface of the base steel sheet C1-36a. ２ ) However, depending on various manufacturing conditions, for example, the coating weight of the aluminum-based plating layer C1-36b may vary between the front and back of the base steel sheet C1-36a. The coating weight of the aluminum-based plating layer C1-36b may be different between one surface and the other surface of the base steel sheet C1-36a. The coating weights W3 and W6 are each 20 g / m ２ 120g / m or more ２ The deposition amounts W3 and W6 are preferably 30 g / m ２ More preferably, 35 g / m ２ The deposition amounts W3 and W6 are preferably 115 g / m ２ More preferably, it is 100 g / m or less. ２ The difference between the coating weights W2 and W3 between the steel plates C1-32 and C1-33 (W2-W3) is, for example, 20 (g / m ２) or more. W2-W3 is 80 (g / m ２ Similarly, the difference between the coating weights W2 and W6 between the steel plates C1-32 and C1-36 (W2-W6) may be, for example, 20 (g / m ２ ) or more. W2-W6 is 80 (g / m ２ The coating weight W3 of the aluminum-based plating layer C1-33b of the steel plate C1-33 and the coating weight W6 of the aluminum-based plating layer C1-36b of the steel plate C1-36 may be the same or different. ３ and the thickness t of steel plate C1-36 ６ The thickness t of the steel plate C1-33 may be the same or different. ３ and the thickness t of steel plate C1-36 ６ are the average thicknesses of the steel plates C1-33 and C1-36, respectively, and are the thicknesses including the aluminum-based plating layers C1-33b and C1-36b.

[0303] The coating weight W3 of the aluminum-based plating layer C1-33b on the steel plate C1-33 and the coating weight W6 of the aluminum-based plating layer C1-36b on the steel plate C1-36 can be measured by the method described in the second embodiment.

[0304] If the total thickness t of the overlap portion C1-37 exceeds, for example, 2.5 mm, the overlap portion C1-37 is difficult to heat up, so when the blank C1-30B is heated during hot stamping, the overlap portion C1-37 reaches the minimum thickness t before it reaches the austenite temperature. ｍｉｎThe plating layer C1-31b (FIGS. 16B and 18A) of the steel sheet C1-31 having the coating C1-33b (FIG. 16B) may be alloyed, resulting in a thick diffusion layer and impairing the corrosion resistance or weldability of the steel sheet C1-31. However, in the example of FIG. 22, the emissivity of the overlap portion C1-37 is increased in advance, for example, by the coating C1-50, thereby facilitating the heating of the overlap portion C1-37 during the heating process. In the example of FIG. 23, the coating weights W3 and W6 of the plating layers C1-33b and C1-36b in the overlap portion C1-37 are made smaller than the coating weight W2 of the plating layer C1-32b of the other steel sheet C1-32, thereby increasing the emissivity of the overlap portion C1-37 compared to the steel sheet C1-32 during heating and facilitating the heating of the overlap portion C1-37. Therefore, even if the total thickness t of the overlap portion C1-37 is greater than 2.5 mm, the overlap portion C1-37 can be heated sufficiently to complete the austenite phase transformation before the alloying of the plating layer C1-31b of the steel sheet C1-31 progresses and the diffusion layer becomes thick enough to lose corrosion resistance or weldability, thereby ensuring a process window for manufacturing structural components.

[0305] As shown in Figure 24, even when the steel sheets C1-33 and C1-36 are plated steel sheets having aluminum-based plating layers C1-33b and C1-36b, a coating C1-50 may be provided on the surface located outside the overlap portion C1-37 of at least one of the steel sheets C1-33 and C1-36. The coating C1-50 may be provided on the surface located outside the overlap portion C1-37 of both the steel sheets C1-33 and C1-36. The surface located inside the overlap portion C1-37 of each of the steel sheets C1-33 and C1-36 may be coated with the coating C1-50, but it is preferable that it not be coated with the coating C1-50 from the viewpoint of uniform heating of the blank C1-30B.

[0306] This allows the emissivity of the overlap portion C1-37 to be increased in advance, so that when the blank C1-30B is heated during hot stamping, the temperature of the overlap portion C1-37 can be increased more quickly. ｍｉｎThe overlap portion C1-37 of the steel plate C1-31 (FIGS. 16B and 18A) having the above structure is likely to be heated to the austenite temperature range before the alloying of the plating layer C1-31b of the steel plate C1-31 progresses excessively, and a structural component can be manufactured while maintaining the corrosion resistance or weldability of the steel plate C1-31. This makes it easier to ensure a process window in the manufacture of structural components.

[0307] In the blank C1-30B according to this embodiment, the steel plate C1-31 may be butt-joined to the steel plate C1-32, or may form an overlap with the steel plate C1-32, as in the steel plates C1-33 and C1-36. The steel plate C1-32 may be butt-joined to the steel plate C1-36, or may form an overlap with the steel plate C1-36, as in the steel plates C1-33 and C1-36.

[0308] The lower blank C1-40 (FIG. 16D) may have a configuration similar to that of the upper blank C1-30B in this embodiment, in which case the lower blank C1-40 can also achieve the same effects as those described above.

[0309] 25 is an exploded perspective view of structural members C1-10C and C1-20C according to this embodiment. The structural members C1-10, C1-10A, and C1-20 according to the above embodiments constitute a front under module of the vehicle body. Meanwhile, the structural members C1-10C and C1-20C according to this embodiment constitute a rear under module of the vehicle body.

[0310] Referring to FIG. 25 , the structural member C1-10C, like the above embodiment, includes a pair of side frames C1-11L, C1-11R and at least one cross member C1-12. The structural member C1-20C, like the above embodiment, also includes a pair of side frames C1-21L, C1-21R and at least one cross member C1-22. In the example shown in FIG. 25 , the cross member C1-12 connects the middle portions of the side frames C1-11L, C1-11R. Similarly, the cross member C1-22 connects the middle portions of the side frames C1-21L, C1-21R. The configurations of the structural members C1-10, C1-10A, and C1-20 described in other embodiments can be applied to the structural member C1-10C and structural member C1-20C of this embodiment, respectively.

[0311] The structural member C1-10C can be manufactured from the blank C1-30C shown in FIG. 26 by a manufacturing method similar to that described in the first embodiment. The blank C1-30C includes steel plates C1-31, C1-32, and C1-33. The steel plates C1-31, C1-32, and C1-33 are arranged and joined to form two long portions C1-34L and C1-34R and at least one connecting portion C1-35. The blank C1-30C can have a configuration similar to any of the blanks C1-30, C1-30A, and C1-30B described in the other embodiments.

[0312] In this embodiment, the steel plate C1-31 forming the rear portion C1-112 (FIG. 25) of each side frame C1-11L, C1-11R has a thickness smaller than that of the steel plate C1-32 forming the front portion C1-111 (FIG. 25). The tensile strength of the steel plate C1-31 may be smaller than that of the steel plate C1-32. In the structural member C1-10C according to this embodiment and the structural members C1-10, C1-10A according to other embodiments (FIGS. 14 and 19A to 19C), it is preferable that the thickness and / or tensile strength of the steel plates located further outboard in the fore-and-aft direction of the vehicle body be smaller than that of the steel plates located further inboard. As a result, when a longitudinal collision load is input to the vehicle body, in the structural members C1-10, C1-10A, C1-10C, the portions located further outboard in the vehicle body deform to absorb the collision energy, while the portions located further inboard are less likely to deform, thereby protecting surrounding components.

[0313] The structural member C1-20C can be manufactured from the blank C1-40C shown in FIG. 27 by a manufacturing method similar to that described in the first embodiment. The blank C1-40C includes steel plates C1-41, C1-42, and C1-43. The steel plates C1-41, C1-42, and C1-43 are arranged and joined to form two long portions C1-44L and C1-44R and at least one connecting portion C1-45. The blank C1-40C can have a configuration similar to any of the blanks C1-30, C1-30A, and C1-30B described in the other embodiments.

[0314] In this embodiment, the steel plate C1-41 forming the rear portion C1-212 (FIG. 25) of each side frame C1-21L, C1-21R has a thickness smaller than that of the steel plate C1-42 forming the front portion C1-211 (FIG. 25). The tensile strength of the steel plate C1-41 may be smaller than that of the steel plate C1-42. As with the upper side, in the lower structural members C1-20, C1-20C (FIGS. 14 and 25), it is preferable that the thickness and / or tensile strength of the steel plates located further outward in the fore-and-aft direction of the vehicle body be smaller than that of the steel plates located further inward. As a result, when a longitudinal collision load is input to the vehicle body, the outer portions of the structural members C1-20, C1-20C deform to absorb the collision energy, while the inner portions are less likely to deform, thereby protecting surrounding components.

[0315] 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.

[0316] In the blank C1-30 according to the first embodiment, the minimum plate thickness t ｍｉｎAs a treatment for increasing the surface emissivity of the steel sheet C1-31 compared to the other steel sheets C1-32, a coating C1-50 is formed on at least one surface of the steel sheet C1-31. In the blank C1-30A according to the second embodiment, as a treatment for increasing the surface emissivity of the steel sheet C1-31 compared to the other steel sheets C1-32, the coating weight W1 of the aluminum-based plating layer C1-31b on both surfaces of the steel sheet C1-31 is less than the coating weight W2 of the aluminum-based plating layer C1-32b on the other steel sheets C1-32. However, the treatment for increasing the emissivity of the steel sheet C1-31 is not limited to this. For example, the emissivity of at least one surface of the steel sheet C1-31 can also be increased compared to the steel sheet C1-32 by making the surface roughness of at least one surface of the steel sheet C1-31 greater than the surface roughness of both surfaces of the steel sheet C1-32. Similarly, in the blank C1-30B of the third embodiment, when treatment is performed to increase the emissivity on the outer surface of the overlap portion C1-37 of the steel plates C1-33 and C1-36, the method is not limited to the method described in that embodiment.

[0317] In the second embodiment, the minimum plate thickness t ｍｉｎ Not only the steel plate C1-31 having the plate thickness t ３ However, in the above example, the coating weight W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 having the minimum plate thickness t ｍｉｎIt is sufficient that the coating weight W1 of the aluminum-based plating layer C1-31b in the steel plate C1-31 having the above-mentioned thickness is less than the coating weight W2 of the aluminum-based plating layer C1-32b in the steel plate C1-32. The coating weight W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 other than the steel plate C1-32 may be equal to or greater than the coating weight W2 of the aluminum-based plating layer C1-32b in the steel plate C1-32. In this case, in the structural member C1-10A after hot stamping, the plating thickness K3 of the steel plate C1-33 is also equal to or greater than the plating thickness K2 of the steel plate C1-32. In the second embodiment described above, the steel plates other than the steel plates C1-31 and C1-32 do not necessarily have to be aluminum-plated steel plates.

[0318] In the first embodiment, the blank C1-30 includes steel plates C1-31 and C1-32 corresponding to the long sections C1-34L and C1-34R (side frames C1-11L and C1-11R), respectively, and a steel plate C1-33 corresponding to the connecting section C1-35 (cross member C1-12). However, the number and arrangement of steel plates included in the blank C1-30 and the structural member C1-10 manufactured from the blank C1-30 are not limited to this. As shown in Figures 28 and 29, the number and arrangement of steel plates can be changed as appropriate.

[0319] As shown in Figures 28 and 29, in a blank C1-30 corresponding to the structural member C1-10 (Figure 14) of a front under module, the long sections C1-34L and C1-34R may each be constructed from a single steel plate C1-31. In the blank C1-30, the connecting portion C1-35 may connect one longitudinal end of the long sections C1-34L and C1-34R, as in the first embodiment described above. That is, the long sections C1-34L and C1-34R may be connected by the connecting portion C1-35 at the end located forward or rearward when the structural member C1-10 is assembled to the vehicle body. As shown in Figure 29, the blank C1-30 may also include multiple connecting portions C1-35. These connecting portions C1-35 are formed from separate steel plates C1-32 and C1-33.

[0320] Although not shown, the number and arrangement of steel plates in the structural member C1-10A and blanks C1-30A and C1-30B according to other embodiments, as well as the lower structural member C1-20 and blank C1-40, are not particularly limited. Each of the structural members C1-10, C1-10A, C1-20 and blanks C1-30, C1-30A, C1-30B, and C1-40 may include two or more joined steel plates. Each of the structural members C1-10, C1-10A, C1-20 and blanks C1-30, C1-30A, C1-30B, and C1-40 preferably includes three or more steel plates. Each of the blanks C1-30, C1-30A, C1-30B, and C1-40 has at least a minimum plate thickness t ｍｉｎ A first steel plate having a plate thickness t ｍｉｎ The blanks C1-30, C1-30A, C1-30B, and C1-40 may each include a first steel plate having a thickness greater than t and a second steel plate having a thickness greater than t. The first steel plate is joined to the second steel plate directly or indirectly via another steel plate. One or both sides of the first steel plate are treated to increase the emissivity compared to both sides of the second steel plate. The other steel plates may or may not be treated to increase the emissivity. ｍｉｎ When there are multiple steel plates having the above structure, it is preferable that one or both surfaces of all of these steel plates are treated to increase the emissivity more than both surfaces of a steel plate with a greater plate thickness.

[0321] In the fourth embodiment, the blank C1-30C corresponds to the structural member C1-10C of the rear under module. This blank C1-30C includes steel plates C1-31 and C1-32 corresponding to the long sections C1-34L and C1-34R (side frames C1-11L and C1-11R), respectively, and a steel plate C1-33 corresponding to the connecting section C1-35 (cross member C1-12). However, the number and arrangement of steel plates included in the blank C1-30C and the structural member C1-10C manufactured from the blank C1-30C are not limited to this. As shown in Figures 30 to 40, the number and arrangement of steel plates can be changed as appropriate.

[0322] For example, as shown in Figures 30 and 31, in a blank C1-30C, the long sections C1-34L and C1-34R may each be formed from a single steel plate C1-31. In this case, the connecting portion C1-35 may connect the long sections C1-34L and C1-34R at one end in the longitudinal direction. For example, the long sections C1-34L and C1-34R may be connected by the connecting portion C1-35 at the end located forward when the structural member C1-10C (Figure 25) is assembled to the vehicle body. The long sections C1-34L and C1-34R may also be connected by the connecting portion C1-35 at their intermediate portions.

[0323] As shown in Figure 32, in the blank C1-30C, even when the long portions C1-34L, C1-34R are formed of multiple steel plates C1-31, C1-32, the long portions C1-34L, C1-34R may be connected by a connecting portion C1-35 at the end side that is disposed forward when the structural member C1-10C (Figure 25) is assembled to the vehicle body. For example, if the portion corresponding to the front portion C1-111 (Figure 25) of the side frames C1-11L, C1-11R is formed of steel plate C1-31 and the portion corresponding to the rear portion C1-112 (Figure 25) is formed of steel plate C1-32, the connecting portion C1-35 may be joined to the steel plate C1-31 as shown in Figures 32 and 33, or may be joined to the steel plate C1-32 as shown in Figure 34.

[0324] In the fourth embodiment described above, the structural member C1-10C (FIG. 25) is provided with a single cross member C1-12, and therefore the blank C1-30C for the structural member C1-10C also includes a single connecting portion C1-35. However, the structural member C1-10C may include multiple cross members C1-12. In this case, as shown in FIGS. 35 to 40, the blank C1-30C also includes multiple connecting portions C1-35. The connecting portion C1-35 is formed from separate steel plates C1-32 and C1-33, steel plates C1-33 and C1-36, or steel plates C1-32, C1-33 and C1-36. In this case, the long sections C1-34L, C1-34R may each be formed from a single steel plate C1-31, as shown in Figures 35, 36, and 40, or may each be formed from multiple steel plates C1-31, C1-32, as shown in Figures 37 to 39.

[0325] Although not shown, in the fourth embodiment, the number and arrangement of steel plates in the lower structural member C1-20C and blank C1-40C are not particularly limited. The structural members C1-10C, C1-20C and blanks C1-30C, C1-40C each need only include two or more joined steel plates. The structural members C1-10C, C1-20C and blanks C1-30C, C1-40C each preferably include three or more steel plates. Each of the blanks C1-30C, C1-40C has at least a minimum plate thickness t ｍｉｎ A first steel plate having a plate thickness t ｍｉｎ The first steel plate is joined to the second steel plate directly or indirectly via another steel plate. One or both sides of the first steel plate are treated to increase the emissivity compared to both sides of the second steel plate. The other steel plates may or may not be treated to increase the emissivity. ｍｉｎ When there are multiple steel plates having the above structure, it is preferable that one or both surfaces of all of these steel plates are treated to increase the emissivity more than both surfaces of a steel plate with a greater plate thickness.

[0326] In the above embodiment, each of the blanks and the multiple steel plates (sub-blanks) included in each structural member may be single-layered or multi-layered, i.e., each of the sub-blanks may be a single steel plate or a plate material formed by stacking multiple steel plates.

[0327] In the above embodiment, the mold C1-60 used in the forming process includes a punch C1-61 and a die C1-62. However, the configuration of the mold C1-60 is not limited to the example described in the above embodiment. The mold C1-60 may further include, for example, a pad or a blank holder. The mold C1-60 may be configured according to the desired structural member.

[0328] In the structural members C1-10, C1-10A, C1-10C, C1-20, and C1-20C according to the above embodiments, the side frames C1-11 and C1-21 have a substantially hat-shaped cross section. However, the cross-sectional shape of the side frames C1-11 and C1-21 is not necessarily limited to this. For example, as shown in FIG. 41 , the side frames C1-11 and C1-21 may have a shape in which one side in the width direction is open in cross section. In this case, other members (not shown) may be joined to the open portions of the side frames C1-11 and C1-21, so that the side frames C1-11 and C1-21 and the other members form a closed cross section. Similarly, the cross members C1-12 and C1-22 may each have a substantially hat-shaped cross section or a cross-sectional shape of another shape.

[0329] 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.

[0330] In order to confirm the effects of the present disclosure, CAE analysis was performed on the press-formed (hot stamped) structural members of the front or rear under module using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) while changing the type (material type) and thickness of the steel plate included in the structural member, as well as the division pattern of the structural member.

[0331] The steel plates (material types) used in this analysis are shown in Table 13.

[0332]

[0333] In Table 13, the material type is listed in the order of plating type, tensile strength, and application (hot stamping). Regarding the coating specifications, the black coating is a black coating containing carbon black and metal oxide. "Black coating - one side" means that one entire side of the steel sheet is covered with a black coating. "Black coating - both sides" means that both entire sides of the steel sheet are covered with a black coating. In this analysis, material types were selected from Table 13 to construct the target structural members.

[0334] The division patterns of the structural members are shown in Figures 42A to 42F. The structural member shown in Figure 42A is a structural member on the upper or lower side of the front under module. The structural members shown in Figures 42B to 42F are structural members on the upper or lower side of the rear under module. Figures 42A to 42F show the number of steel plates (materials) included in the structural member and the positions of the joints between the steel plates in the structural member. In Figures 42A to 42F, each steel plate is given a number in parentheses.

[0335] Table 14 shows the analysis conditions and results for the structural member shown in Figures 42A and 42B. In Figures 42A and 42B, the side frames of the structural member are each made of two pieces of material (1) and (2). The cross member is made of material (3). Each of materials (1) to (3) is butt-jointed with the adjacent material.

[0336]

[0337] 42A and 42B, Examples 1 and 2 and Comparative Examples 1 and 2 are structural members on the upper side of a rear under module, and Example 3 and Comparative Example 3 are structural members on the upper side of a front under module. In Example 1, the material with the smallest thickness t ｍｉｎ In Example 2, the material (3) having the smallest thickness t of the materials (1) to (3) is provided with a black coating on both sides. ｍｉｎIn Example 3, a black coating is applied to one side of the material (3) having the smallest thickness t ｍｉｎ : A black coating is applied to one side of the material (2) having a thickness of 1.0 mm. On the other hand, in Comparative Examples 1 to 3, a black coating is not applied to any of the materials (1) to (3). In Comparative Examples 1 to 3, ｍｉｎ : No black coating was applied to the material having a thickness of 1.0 mm.

[0338] In Table 14, the "time to reach 910°C" indicates the time to reach 910°C (A ｃ３ The "phase transformation start time" is the time required for a blank heated to 910°C from the start of heating to reach 910°C for a blank heated to a furnace temperature of 920°C for 5 minutes and 30 seconds and then removed from the heating furnace. Table 14 shows that in Examples 1 to 3, in which the emissivity of the thinnest blank was increased by a black coating, the time to reach 910°C was approximately 20 seconds shorter than in Comparative Examples 1 to 3, and the temperature rise rate of the thinnest blank during the heating process was higher. Furthermore, in Examples 1 to 3, the phase transformation start time was slower than in Comparative Examples 1 to 3, making it easier to start forming the blank before the onset of ferrite transformation, and enabling the blank to be uniformly quenched during the forming process.

[0339] Tables 15 and 16 show the analysis conditions and results for the structural members shown in Figures 42C to 42F. Examples 4 to 16 and Comparative Examples 4 to 16 in Tables 15 and 16 are all rear undermodules, but Examples 4 to 8 and 14 to 16 and Comparative Examples 4 to 8 and 14 to 16 are upper-side structural members, while Examples 9 to 13 and Comparative Examples 9 to 13 are lower-side structural members. In Figures 42C to 42E, the side frames of the structural members are each formed from two pieces of material (1) and (2). The structural members in Figures 42C to 42E include multiple cross members formed from material (3) or material (4). In Figure 42F, the side frames of the structural member are formed from a single piece of material (1). The structural member in Figure 42F includes multiple cross members formed from any of materials (2) to (4). Materials (1) to (4) are each butt-jointed to adjacent materials.

[0340]

[0341]

[0342] Referring to Table 15, in Examples 4 to 13, the minimum plate thickness t ｍｉｎ In Examples 4 to 13, a black coating is applied to a material having a minimum plate thickness t ｍｉｎ The material is covered with a black coating on one or both sides. ｍｉｎ In the case where there are multiple blanks having the same thickness, all of these blanks are provided with a black coating. On the other hand, in Comparative Examples 4 to 13, none of the blanks (1) to (4) are provided with a black coating. That is, in Comparative Examples 4 to 13, ｍｉｎ : No black coating was applied to the material having a thickness of 1.0 mm.

[0343] Table 15 shows that in Examples 4 to 13, the time to reach 910°C was about 20 seconds shorter than in Comparative Examples 4 to 13, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 4 to 13, the phase transformation start time was slower than in Comparative Examples 4 to 13, making it easier to start forming the blank before the ferrite transformation begins, and making it possible to uniformly harden the blank in the forming process.

[0344] Referring to Table 16, in Examples 14 to 16, the minimum plate thickness t ｍｉｎ In Examples 14 to 16, a black coating was applied to a material having a minimum plate thickness t ｍｉｎ On the other hand, in Comparative Examples 14 to 16, no black coating was applied to any of the materials (1) to (4). ｍｉｎ : No black coating was applied to the material having a thickness of 1.2 mm.

[0345] Table 16 shows that in Examples 14 to 16, the time to reach 910°C was 45 seconds or more shorter than in Comparative Examples 14 to 16, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 14 to 16, the phase transformation start time exceeded 20 seconds, which was later than in Comparative Examples 14 to 16. This makes it easier to start forming the blank before the onset of ferrite transformation, and enables the blank to be uniformly quenched in the forming process.

[0346] For Examples 6, 11, and 16 and Comparative Examples 6, 11, and 16, analysis samples were taken from the thinnest parts of the structural members using the method described in the above embodiment, and the variation in martensite fraction was measured. Furthermore, shape accuracy and impact absorption performance were separately measured for these structural members. The evaluation results are shown in Table 17.

[0347]

[0348] In Table 17, the variation in martensite fraction is, as explained in the above embodiment, the variation in the minimum plate thickness t ｍｉｎ The martensite fraction (%) is the maximum martensite fraction (%) minus the minimum martensite fraction (%) in the cross section of the structural member at the location of the material having the martensite fraction (%).

[0349] The shape accuracy was evaluated by the distance between the structural member and the mating member at the overlapping portion when the structural member was attached to another member. In Table 17, a distance from the surface of the mating member that was within ±2.0 mm is indicated by ○, a distance between ±2.0 mm and ±3.0 mm is indicated by △, and a distance above ±3.0 mm is indicated by ×.

[0350] Regarding impact absorption performance, rear and side collisions of the structural members mounted on a vehicle were simulated, and an impactor simulating a vehicle was collided with the structural members to evaluate the maximum intrusion amount during a rear collision and the maximum intrusion amount during a side collision. Impact absorption performance was evaluated based on the impact absorption performance of a rear under module formed by hot stamping each material and then joining them, and compared to the impact absorption performance of the base. In Table 17, impact absorption performance equivalent to the impact absorption performance of the base is indicated as "good," impact absorption performance superior to the base is indicated as "better," impact absorption performance slightly lower than the base is indicated as "marginal," and impact absorption performance even lower is indicated as "poor."

[0351] As described above, in Examples 6, 11, and 16, a black coating was applied to the surface of the thinnest material. On the other hand, in Comparative Examples 6, 11, and 16, a black coating was not applied to any material, including the thinnest material. As shown in Table 17, in all of Examples 6, 11, and 16, the variation in martensite fraction was 15% or less, which was significantly reduced compared to Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, the shape accuracy was also better than in Comparative Examples 6, 11, and 16.

[0352] In Examples 6, 11, and 16, where the variation in martensite fraction was small, the impact absorption performance was also improved compared to Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, even though the structural members were formed by integrating multiple materials at the blank stage, it was possible to ensure impact absorption performance equal to or greater than that of structural members formed by joining the materials after press-forming them individually.

[0353] (Elemental Technology C2) The elemental technology C2 is a structural member for a vehicle body, comprising: (C2a) a pair of side frames; and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined to each other, including a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and wherein 0.001 g / m of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide are added on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. ２ (C2b) A vehicle comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and wherein a coating containing 0.500 g / m2 of carbon black is provided on a surface of each of the first steel plate and the second steel plate located outside the overlap portion. ２ (C2c) A structural member comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined to each other, including a first steel plate, a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate, wherein at least one of the first steel plate and the second steel plate and the third steel plate are plated steel plates having aluminum-based plating layers on both surfaces of a base steel plate, and the thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer on the third steel plate.

[0354] According to elemental technology C2, it is possible to provide a structural member that combines the strength of the overlap portion, which has the maximum plate thickness, with rust prevention functionality.

[0355] A hot stamping blank according to an embodiment includes a plurality of steel sheets. The plurality of steel sheets are arranged and joined to form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel sheets include a first steel sheet and a second steel sheet. The second steel sheet has an end portion that is overlapped and joined to an end portion of the first steel sheet to form an overlap portion together with the end portion of the first steel sheet. The overlap portion has the maximum sheet thickness in the blank. At least one of the plurality of steel sheets is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. A surface located outside the overlap portion of at least one of the first steel sheet and the second steel sheet is treated to increase the emissivity compared to at least one other surface of the plurality of steel sheets (first configuration).

[0356] In a blank according to the first configuration, the end of the first steel sheet and the end of the second steel sheet form an overlap portion having the maximum sheet thickness in the blank. The surface of at least one of the first steel sheet and the second steel sheet located outside the overlap portion is treated to increase the emissivity compared to the surface of at least one other steel sheet included in the blank. This allows the temperature rise rate of the overlap portion to be increased when the blank is heated during hot stamping, thereby shortening the heating time of the overlap portion. Therefore, the heating of the blank required for hot stamping can be completed before excessive alloying of the plating layer of the plated steel sheet included in the blank progresses and the diffusion layer becomes thick. As a result, in a structural component formed from the blank, the strength of the overlap portion can be ensured by hot stamping, and corrosion resistance (rust prevention) can be ensured. The phrase "treated to increase emissivity" not only refers to a case where the surface of at least one of the first steel plate and the second steel plate located outside the overlap portion has a higher emissivity than the other surfaces before the blank is heated, but also includes a case where the surface of at least one of the first steel plate and the second steel plate located outside the overlap portion has a higher emissivity than the other surfaces during heating of the blank.

[0357] As described above, the blank according to the first configuration enables the manufacture of structural components that combine the strength of the overlap portion, which has the greatest sheet thickness, with the rust prevention function of the plated steel sheet portion. In other words, the blank allows the temperature rise rate of the overlap portion, which has the greatest sheet thickness, to be increased, thereby making it easier to ensure the process window of heating conditions in the manufacture of structural components. Furthermore, by increasing the temperature rise rate of the overlap portion, the heating time of the blank for hot stamping can be shortened, thereby improving the productivity of structural components. Furthermore, shortening the heating time of the blank reduces energy consumption in the manufacture of structural components and reduces the amount of greenhouse gases generated when heating the blank for hot stamping.

[0358] In the blank according to the first configuration, the steel sheet that constitutes the portion of the blank having the smallest thickness among the plurality of steel sheets may be a plated steel sheet (second configuration).

[0359] When a blank is heated during hot stamping, the temperature of the steel sheet with the smallest thickness rises relatively quickly. Therefore, if the steel sheet with the smallest thickness in the blank is a plated steel sheet, while the overlap portion with the largest thickness is being heated to ensure its strength by hot stamping, alloying of the coating layer of the plated steel sheet with the smallest thickness, which was heated first, tends to progress, resulting in a decrease or loss of corrosion resistance due to the coating layer. However, in the blank according to the embodiment, the surface outside the overlap portion of at least one of the first steel sheet and the second steel sheet is treated to increase the emissivity. This increases the heating rate of the overlap portion and shortens the heating time. This allows the heating of the blank required for hot stamping to be completed before excessive alloying of the coating layer of the steel sheet with the smallest thickness, which was heated first, progresses. Therefore, even if the steel sheet with the smallest thickness is a plated steel sheet, as in the second configuration, its corrosion resistance can be ensured.

[0360] In a blank relating to the first or second configuration, the surface of each of the first steel plate and the second steel plate located outside the overlap portion may be treated to increase the emissivity compared to at least one other surface of the multiple steel plates (third configuration).

[0361] In the blank according to the third configuration, the surfaces of the first steel sheet and the second steel sheet located outside the overlap portion are each treated to increase the emissivity. That is, the emissivity of both outer surfaces of the overlap portion is higher than that of the other surfaces before heating the blank, or becomes higher than that of the other surfaces during heating of the blank. In this case, the temperature rise rate of the overlap portion during heating of the blank can be increased, and the heating time of the overlap portion can be shortened. Therefore, the process window of heating conditions in manufacturing the structural member can be more easily secured.

[0362] In the blank according to any one of the first to third configurations, each of the plurality of steel sheets may be a plated steel sheet (fourth configuration).

[0363] In a fourth configuration, each steel sheet included in the blank is a plated steel sheet. In this case, when the blank is formed into a structural component by hot stamping, the formation of oxide scale can be suppressed. Therefore, after hot stamping, there is no need to subject the structural component to a process for removing oxide scale, such as shot blasting. This can improve the productivity of the structural component. Furthermore, since each steel sheet is a plated steel sheet, the corrosion resistance of the structural component can be more easily ensured.

[0364] In the blank according to the fourth configuration, the plating layer of each steel plate is an aluminum-based plating layer (fifth configuration).

[0365] When each steel sheet is a plated steel sheet having an aluminum-based plating layer, as in the fifth configuration, when the blank is heated during hot stamping, a difference in temperature rise rate is likely to occur, particularly between the overlap portion, which has the greatest sheet thickness, and the non-overlapping portion. Because the aluminum-based plating layer is nearly white, it easily reflects thermal energy and inhibits temperature rise in the overlap portion. However, even when each steel sheet is a plated steel sheet having an aluminum-based plating layer, by treating at least one of the outer surfaces of the overlap portion to increase the emissivity, temperature rise in the overlap portion can be accelerated when the blank is heated during hot stamping, thereby shortening the heating time of the overlap portion. Therefore, corrosion resistance in the plated steel sheet portion can be ensured, and the productivity of structural components manufactured from the blank can be improved.

[0366] In the blank according to any one of the third to fifth configurations, a coating may be formed on the surface of each of the first steel plate and the second steel plate located outside the overlap portion as a treatment for increasing emissivity. This coating may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. (sixth configuration).

[0367] In the blank according to any one of the third to fifth configurations, a coating may be formed on the surface of each of the first steel plate and the second steel plate located outside the overlap portion as a treatment for increasing emissivity. This coating comprises carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a concentration of 0 to 0.30 g / m ２ In this case, the content of carbon black in the coating can be X ＣＢ (g / m ２ ), the oxide content is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ may satisfy the following formula (1) (see Patent Document 1) (seventh configuration): 118.9≦24280 / {6700 / (100+76×X ＣＢ ) + 18000 / (130 + 65 x X Ｏｘｉｄｅ )≦332.0 (1)

[0368] In the blank according to any one of the third to seventh configurations, the plurality of steel plates may include two or more steel plates having different plate thicknesses. In this case, the plate thickness of the overlap portion is t ｍａｘ , the thickness of the steel plate constituting the part with the smallest plate thickness in the blank is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ It may be ≦3.2 (eighth configuration).

[0369] When two or more steel sheets with different thicknesses are included in a blank, the difference in thickness between the overlap portion and the steel sheet constituting the portion with the smallest thickness in the blank becomes larger compared to when all the steel sheets included in the blank have the same thickness. If the difference in thickness is excessive, when a structural component is manufactured from the blank by hot stamping, the temperature rise in the overlap portion becomes slower compared to the steel sheet constituting the portion with the smallest thickness in the blank. Therefore, for example, when the thinnest steel sheet is a plated steel sheet, there is a possibility that alloying of the plated layer in the thinnest steel sheet may proceed excessively, making it more difficult to ensure the process window of the heating conditions. However, in the eighth configuration, the thickness t of the overlap portion ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ is set to 3.2 or less. This makes it easier to complete heating of the thickest overlap portion before alloying of the coating layer progresses excessively, even when two or more steel sheets with different thicknesses are included in the blank, and makes it easier to ensure the process window of the heating conditions.

[0370] In the blank according to the first configuration, the plurality of steel sheets may further include a third steel sheet. At least one of the first steel sheet and the second steel sheet, and the third steel sheet may each be a plated steel sheet having an aluminum-based plating layer as a plating layer on both surfaces of the base steel sheet. In this case, as a treatment for increasing the emissivity of the surface located outside the overlap portion compared to the emissivity of the surface of the third steel sheet, for example, the coating amount (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet of at least one of the first steel sheet and the second steel sheet may be ２ ) is 60 or less, and the coating weight (g / m ２ ) (ninth configuration).

[0371] A blank according to a ninth configuration includes a third steel sheet in addition to a first steel sheet and a second steel sheet. At least one of the first steel sheet and the second steel sheet and the third steel sheet are aluminum-based plated steel sheets. The coating weight of the aluminum-based plated layer on at least one of the first steel sheet and the second steel sheet is less than the coating weight of the aluminum-based plated layer on the third steel sheet and is 60 g / m ２ The following is true. By using a relatively thinly plated steel sheet for the first steel sheet and / or the second steel sheet, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron in the first steel sheet and / or the second steel sheet proceeds quickly, and both surfaces of the first steel sheet and / or the second steel sheet quickly change from silvery white to black or a color close to black. Therefore, during heating of the blank, the emissivity of the overlap portion formed by the first steel sheet and the second steel sheet is higher than that of the third steel sheet. This allows the overlap portion to be heated to the austenite temperature relatively quickly, thereby shortening the heating time of the overlap portion. Therefore, the heating of the blank required for hot stamping can be completed before excessive alloying of the aluminum-based plating layer in each steel sheet progresses and the diffusion layer becomes thick. As a result, the strength of the overlap portion can be ensured by hot stamping, and the corrosion resistance (rust resistance) of each steel sheet can be ensured.

[0372] In the blank according to the ninth configuration, the third steel sheet has a larger coating mass of the aluminum-based plating layer than at least one of the first steel sheet and the second steel sheet forming the overlap portion. That is, the third steel sheet has higher rust prevention performance than the first steel sheet and / or the second steel sheet. Therefore, the rust prevention performance of the structural member formed from the blank can be partially enhanced. For example, by disposing the third steel sheet in a portion of the structural member requiring relatively high rust prevention performance, the rust prevention performance required for the structural member as a whole can be ensured.

[0373] In the blank according to the ninth configuration, the plurality of steel plates may include two or more steel plates having different plate thicknesses. In this case, the plate thickness of the overlap portion is t ｍａｘ, the thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ It may be ≦3.0 (tenth configuration).

[0374] When two or more steel sheets with different thicknesses are included in a blank, the difference in thickness between the overlap portion and the non-overlap portion becomes larger compared to when all the steel sheets included in the blank have the same thickness. That is, the difference in thickness between the overlap portion having the largest thickness in the blank and the steel sheet having the smallest thickness becomes larger. If the difference in thickness is excessive, when a structural component is manufactured from the blank by hot stamping, the temperature rise in the overlap portion becomes slower than that of the steel sheet having the smallest thickness. Therefore, for example, when the thinnest steel sheet is a plated steel sheet, there is a possibility that alloying of the plated layer in the thinnest steel sheet may proceed excessively, making it difficult to ensure the process window of the heating conditions. Therefore, in the tenth configuration, when the first steel sheet and / or the second steel sheet forming the overlap portion are relatively thin plated steel sheets, the thickness t of the overlap portion is increased. ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ is set to 3.0 or less. This makes it easier to complete heating of the thickest overlap portion before alloying of the plating layer of each plated steel sheet progresses excessively, and makes it easier to ensure the process window of the heating conditions.

[0375] In the blank according to the ninth configuration, the plurality of steel plates may include two or more steel plates having different thicknesses. In this case, the thickness of the overlapping portion may be t ｍａｘ , the thickness of the steel plate having the smallest thickness among the plurality of steel plates is t ｍｉｎ When this is the case, t ｍａｘ / t ｍｉｎ Furthermore, a surface of at least one of the first steel plate and the second steel plate positioned outside the overlap portion may be coated with a black coating (eleventh configuration).

[0376] In the eleventh configuration, the first steel sheet and / or the second steel sheet forming the overlap portion are relatively thinly plated steel sheets, and at least one of the first steel sheet and the second steel sheet has a substantially black coating applied to the surface located outside the overlap portion. This allows the emissivity of the overlap portion to be increased in advance, and the temperature of the overlap portion can be increased more quickly when the blank is heated during hot stamping. Therefore, even if there is a large difference in thickness between the overlap portion having the maximum thickness in the blank and the steel sheet having the minimum thickness, the process window of the heating conditions can be easily ensured. For example, ｍａｘ and minimum plate thickness t ｍｉｎ Ratio to: t ｍａｘ / t ｍｉｎ Even if the value is increased to 4.0, the process window is easily secured.

[0377] A method for manufacturing a structural component according to an embodiment includes the steps of preparing a blank according to any one of the first to eleventh configurations, heating a plurality of steel plates included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming and quenching the heated blank using a mold (twelfth configuration).

[0378] A vehicle body structural member according to an embodiment can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate and a second steel plate. The second steel plate has an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. A coating is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. The coating contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of 0.001 g / m ２ or more (thirteenth configuration).

[0379] A vehicle body structural member according to another embodiment can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate and a second steel plate. The second steel plate has an end portion that is overlapped and joined to the end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. A coating is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. The coating contains carbon black at 0.500 g / m ２ The following is contained (fourteenth configuration).

[0380] According to yet another embodiment, a vehicle body structural member can include a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate, a second steel plate, and a third steel plate. The second steel plate has an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. At least one of the first steel plate and the second steel plate, and the third steel plate, are each plated steel plates having aluminum-based plating layers on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer on the third steel plate (fifteenth configuration).

[0381] In the structural member according to the fifteenth configuration, in the overlap portion, the maximum value of the Vickers hardness of the steel plate located on the surface side of the structural member out of the first steel plate and the second steel plate is HV ｍａｘ , the minimum value of Vickers hardness is HV ｍｉｎ When this is done, HV ｍａｘ -HV ｍｉｎ is HV ｍａｘ It may be 30% or less of the above (sixteenth configuration).

[0382] 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.

[0383] <First embodiment> [Structural members] Figure 43 is an exploded perspective view of structural members C2-10 and C2-20 according to this embodiment. The structural members C2-10 and C2-20 are used in the body of an automobile or the like. In the example shown in Figure 43, the structural members C2-10 and C2-20 form a front under module of the body.

[0384] The structural member C2-10 is an upper module. That is, when assembled to the vehicle body, the structural member C2-10 is disposed above the structural member C2-20. The structural member C2-10 comprises a pair of side frames C2-11L, C2-11R and at least one cross member C2-12. The side frames C2-11L, C2-11R and the cross member C2-12 each have an elongated shape.

[0385] The side frames C2-11L, C2-11R are arranged side by side in the left-right direction of the vehicle body when the structural member C2-10 is assembled to the vehicle body. The side frames C2-11L, C2-11R each extend in the front-to-rear direction of the vehicle body. Each of the side frames C2-11L, C2-11R includes a front section C2-111 and a rear section C2-112. The rear section C2-112 is arranged behind the front section C2-111 when the structural member C2-10 is assembled to the vehicle body.

[0386] The cross member C2-12 extends in the left-right direction of the vehicle body when the structural member C2-10 is assembled to the vehicle body. The cross member C2-12 extends from the side frame C2-11L to the side frame C2-11R. The cross member C2-12 connects the side frames C2-11L and C2-11R. In the example shown in FIG. 43 , the cross member C2-12 connects the side frames C2-11L and C2-11R to each other at one end of the longitudinal direction of the side frames C2-11L and C2-11R. When the structural member C2-10 is assembled to the vehicle body, the cross member C2-12 is positioned, for example, at the rear end of the structural member C2-10. However, the cross member C2-12 may also connect the middle portions of the side frames C2-11L and C2-11R.

[0387] The structural member C2-20 is a lower module. That is, when assembled to the vehicle body, the structural member C2-20 is disposed below the structural member C2-10. The structural member C2-20 comprises a pair of side frames C2-21L, C2-21R and at least one cross member C2-22. The side frames C2-21L, C2-21R and the cross member C2-22 each have an elongated shape.

[0388] The side frames C2-21L, C2-21R are arranged side by side in the left-right direction of the vehicle body when the structural member C2-20 is assembled to the vehicle body. The side frames C2-21L, C2-21R each extend in the front-to-rear direction of the vehicle body. Each of the side frames C2-21L, C2-21R includes a front section C2-211 and a rear section C2-212. The rear section C2-212 is arranged behind the front section C2-211 when the structural member C2-10 is assembled to the vehicle body.

[0389] The lower side frames C2-21L and C2-21R are joined to the upper side frames C2-11L and C2-11R, respectively. The side frames C2-21L and C2-21R form a closed cross section together with the side frames C2-11L and C2-11R. Figure 44 shows the closed cross section formed by the side frames C2-21L and C2-21R together with the side frames C2-11L and C2-11R, respectively. Hereinafter, when there is no need to particularly distinguish between the side frames C2-11L and C2-11R, the side frames C2-11L and C2-11R will be collectively referred to as the side frame C2-11. Similarly, when there is no need to particularly distinguish between the side frames C2-21L and C2-21R, the side frames C2-21L and C2-21R will be collectively referred to as the side frame C2-21.

[0390] Figure 44 is a cross-sectional view (horizontal cross-section) of the side frames C2-11 and C2-21 when cut along a plane perpendicular to the longitudinal direction. In the example of Figure 44, the side frames C2-11 and C2-21 each have a substantially hat-shaped transverse cross-section.

[0391] Referring to Figure 44, the side frame C2-11 includes a top plate C2-113, vertical walls C2-114 and C2-115, and flanges C2-116 and C2-117. In a cross-sectional view of the side frame C2-11, one end of the vertical walls C2-114 and C2-115 is connected by the top plate C2-113. In a cross-sectional view of the side frame C2-11, flanges C2-116 and C2-117 are connected to the other end of the vertical walls C2-114 and C2-115, respectively. The flanges C2-116 and C2-117 protrude outward from the vertical walls C2-114 and C2-115, respectively.

[0392] The side frame C2-21 includes a top plate C2-213, vertical walls C2-214 and C2-215, and flanges C2-216 and C2-217. In a cross-sectional view of the side frame C2-21, one end of the vertical walls C2-214 and C2-215 is connected by the top plate C2-213. In a cross-sectional view of the side frame C2-21, flanges C2-216 and C2-217 are connected to the other end of the vertical walls C2-214 and C2-215, respectively. The flanges C2-216 and C2-217 protrude outward from the vertical walls C2-214 and C2-215, respectively.

[0393] The top plate C2-213 of the lower side frame C2-21 is arranged to face the top plate C2-113 of the upper side frame C2-11. In a cross-sectional view of the side frames C2-11 and C2-21, the vertical walls C2-214 and C2-215 of the side frame C2-21 extend from the top plate C2-213 toward the side frame C2-11. The flanges C2-216 and C2-217 of the side frame C2-21 are joined to the flanges C2-116 and C2-117 of the side frame C2-11, respectively. The flanges C2-216 and C2-217 are joined to the flanges C2-116 and C2-117 by, for example, spot welding. In the example of Figure 44, the flanges C2-116 and C2-117 of the upper side frame C2-11 are directly joined to the flanges C2-216 and C2-217 of the lower side frame C2-21. However, other members such as a floor panel may be provided between the side frame C2-11 and the side frame C2-21.

[0394] Returning to FIG. 43 , the cross member C2-22 extends in the left-right direction of the vehicle body when the structural member C2-20 is assembled to the vehicle body. The cross member C2-22 extends from the side frame C2-21L to the side frame C2-21R. The cross member C2-22 connects the side frames C2-21L and C2-21R. In the example shown in FIG. 43 , the cross member C2-22 connects the side frames C2-21L and C2-21R to each other at one end of the longitudinal direction of the side frames C2-21L and C2-21R. Like the upper cross member C2-12, the cross member C2-22 is positioned, for example, at the rear end of the structural member C2-20 when the structural member C2-20 is assembled to the vehicle body. However, the cross member C2-22 may also connect the middle portions of the side frames C2-21L and C2-21R. The cross member C2-22 may be joined to the upper cross member C2-12 by, for example, spot welding.

[0395] The structural members C2-10 and C2-20 are hot-stamped members. That is, the structural member C2-10 is formed by hot stamping (hot pressing) a blank formed from a plurality of steel plates (sub-blanks). Similarly, the structural member C2-20 is formed by hot stamping a blank formed from a plurality of steel plates.

[0396] In the upper structural member C2-10, for example, the side frames C2-11L, C2-11R may be formed from a plurality of steel plates C2-31, C2-32, respectively. In each of the side frames C2-11L, C2-11R, for example, the rear section C2-112 may be formed from the steel plate C2-31, and the front section C2-111 may be formed from the steel plate C2-32. The steel plate C2-31 forming the rear section C2-112 may be larger in at least one of the plate thickness and tensile strength than the steel plate C2-32 forming the front section C2-111. The cross member C2-12 may be formed mainly from a steel plate C2-33 different from the steel plates C2-31, C2-32 forming the side frames C2-11L, C2-11R. Adjacent steel plates C2-31, C2-32, and C2-33 are joined by welding.

[0397] Similarly, in the lower structural member C2-20, for example, the side frames C2-21L, C2-21R may each be formed from a plurality of steel plates C2-41, 42. In each of the side frames C2-21L, C2-21R, for example, the rear section C2-212 may be formed from the steel plate C2-41, and the front section C2-211 may be formed from the steel plate C2-42. The steel plate C2-41 forming the rear section C2-212 may be larger in at least one of the plate thickness and tensile strength than the steel plate C2-42 forming the front section C2-211. The cross member C2-22 may be formed mainly from a steel plate C2-43 different from the steel plates C2-41, 42 forming the side frames C2-21L, C2-21R. Adjacent steel plates C2-41, C2-42, and C2-43 are joined by welding.

[0398] [Method for manufacturing structural members] Below, a method for manufacturing structural members C2-10 and C2-20 according to this embodiment will be described with reference to Figures 45A to 45G. The method for manufacturing structural member C2-10 includes the steps of preparing a blank C2-30, heating the blank C2-30, and forming the heated blank C2-30 into the structural member C2-10. Similarly, the method for manufacturing structural member C2-20 includes the steps of preparing a blank C2-40, heating the blank C2-40, and forming the heated blank C2-40 into the structural member C2-20.

[0399] (Preparation Process) As shown in Figure 45A, in the preparation process for manufacturing the upper structural member C2-10 (Figure 43), a blank C2-30 is prepared. The blank C2-30 has a shape obtained by unfolding the structural member C2-10. The blank C2-30 includes multiple steel plates (sub-blanks) C2-31, C2-32, and C2-33. The steel plates C2-31, C2-32, and C2-33 are arrang...

Claims

1. An automobile rear module having an integrated part formed by hot stamping a plurality of integrated steel plates, the projected area when viewed from the perpendicular direction of the reference plane is S (m ２ ), among the components of the automobile rear module, the total weight of the components is W (kg), and the total weight of the steel components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W Ａ When W / S is 24 or less, W Ａ A rear module for an automobile, wherein / W is 0.30 or more.

2. A skeletal member comprising elemental technology A1 and at least one of elemental technology B1, elemental technology C1, elemental technology C2, elemental technology D1, and elemental technology D2, wherein elemental technology A1 is a skeletal member formed by hot stamping a steel plate, wherein the skeletal member has a closed cross-section portion whose cross section perpendicular to the longitudinal direction is a closed cross-section, wherein the closed cross-section portion has at least two flat portions which are portions having a radius of curvature larger than the maximum outer dimension of the cross-section, and a concave bead portion formed between the two flat portions, wherein the concave bead portion is a pair of wall portions having a radius of curvature of 50 mm or more, and which protrudes from opposing ends of the two flat portions toward the inside of the closed cross-section portion via a pair of bent portions which bend toward the inside of the closed cross-section, wherein the Vickers hardness of the wall portions at the plate thickness center portion is 520 Hv or more, and the width of the wall portions is an effective width W calculated from Karman's effective width formula ｅ and a standard deviation ratio obtained by dividing the standard deviation of the hardness frequency distribution in a surface layer portion of the wall portion by the standard deviation of the hardness frequency distribution in a thickness center portion of the wall portion is less than 1.0; and the elemental technology B1 is a structural member, comprising a member body having an annular shape in a plan view, formed by a plurality of steel plates including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate and joined together, wherein the value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate, A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo). ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements, and the element technology C1 is a structural member for a vehicle body, (C1a) comprising a pair of side frames and a cross member connecting the side frames, the side frames and the cross member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and formed by a plurality of steel plates joined together, and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide are contained on the first steel plate in an amount of 0.001 g / m ２ (C1b) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and wherein a coating containing 0.500 g / m or more of carbon black is provided on the first steel plate. ２ (C1c) A structural member that satisfies at least one of the above, comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates that include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and are joined together, wherein the first steel plate and the second steel plate are plated steel plates that have an aluminum-based plating layer on both surfaces of a base steel plate, and the thickness of the aluminum-based plating layer on the first steel plate is smaller than the thickness of the aluminum-based plating layer on the second steel plate; and the elemental technology C2 is a structural member for a vehicle body, wherein (C2a) A vehicle comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member include a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and are formed by a plurality of steel plates joined to each other, and wherein one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide are contained in an amount of 0.001 g / m on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. ２ (C2b) A structural member comprising a pair of side frames and a cross member connecting the side frames, wherein the structural member includes a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and is formed by a plurality of steel plates joined to each other, and a coating containing 0.500 g / m2 of carbon black is provided on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. ２ (C2c) A structural member comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member include a first steel plate, a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate, and are formed by a plurality of steel plates joined to each other, at least one of the first steel plate and the second steel plate and the third steel plate are plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate, and a thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than a thickness of the aluminum-based plating layer on the third steel plate, and the elemental technology D1 is a structural member comprising a plurality of partial blanks made of steel plates joined together, (D1a) At least two of the partial blanks are joined by a plurality of joints at overlapping portions formed by partial overlapping, and in a cross section perpendicular to the surface of the partial blank including the center of the joint of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank in contact with the other partial blanks, when the Vickers hardness at a position 15 mm or more away from the center of the joint and not joined is Hvm, a part of the plurality of joints has a Vickers hardness of ΔHv, which is the difference between the maximum hardness and the minimum hardness in Vickers hardness within a range of 5 mm from the end of the joint toward the base material (or within 12 mm from the center of the joint), of less than 0.2 Hvm, preferably 0.1 Hvm or less, and the other joints of the plurality of joints (joints other than the part) have a ΔHv of 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more.(D1b) A press-formed part in which at least two of the partial blanks are joined by a plurality of spot welds at overlapping portions where the partial blanks are partially overlapped, and in a cross section including the center of the spot weld of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank, when the hardness at a position 15 mm or more away from the center of the spot weld and at which the spot weld is not formed is Hvm, some of the plurality of spot welds have a difference ΔHv between the maximum hardness and the minimum hardness within a range of a radius of 12 mm from the center of the spot weld that is less than 0.2 Hvm, and the spot welds other than the some have a ΔHv of 0.2 Hvm or more, and the elemental technology D2 is (D2a) A press-molded part having a bent portion, in which a patchwork made of steel plate is superimposed on the surface of a base blank made of steel plate and joined at a joint, wherein the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joint exists only in one region of the surface of the base blank across the bent portion when there is one bent portion, and only in the region between two adjacent bent portions on the surface of the base blank when there are two or more bent portions. (D2b) An automobile rear module as described in claim 1, characterized in that the press-formed part has a bent portion, in which a patchwork of steel plates is superimposed on the surface of a base blank made of steel plate and joined at the joint points by spot welding, and the joint satisfies at least one of the following: if there is one bent portion, the difference between the maximum hardness at a position 5 mm away from the center of the joint point on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joint exists only in one of the regions on the surface of the base blank, separated by the bent portion, or if there are two or more bent portions, only in the region between two adjacent bent portions on the surface of the base blank.

3. The automobile rear module according to claim 2, characterized in that it comprises the elemental technology A1 and the elemental technology B1.

4. The automobile rear module according to claim 2, characterized in that it comprises the elemental technology A1 and at least one of the elemental technology C1 and the elemental technology C2.

5. The automobile rear module according to claim 2, characterized in that it comprises the elemental technology A1 and at least one of the elemental technology D1 and the elemental technology D2.

6. An automobile rear module as described in claim 2, characterized in that it comprises: the element technology A1; the element technology B1; at least one of the element technology C1 and the element technology C2; and at least one of the element technology D1 and the element technology D2.

Images