Rear structure of vehicle body, composite blank, and method for manufacturing rear structure of vehicle body

Abstract

A rear structure (100, 100A) of a vehicle body is provided with a floor panel (30) and a first frame unit (10 or 20). The first frame unit (10 or 20) includes a pair of first side frames (11, 12 or 21, 22) and a first cross member (13 or 23). The first side frames (11, 12 or 21, 22) and the first cross member (13 or 23) are welded to the floor panel (30) to form, together with the floor panel (30), first closed cross-sectional parts (14, 15 or 24, 25), respectively. In the first closed cross-sectional parts (14, 15 or 24, 25), the minimum value of the Vickers hardness of the heat-affected zone of a welded part (44 or 45) of the first frame unit (10 or 20) welded to the floor panel (30) is 70% or more of the Vickers hardness of the non-welded part of the first frame unit (10 or 20).

Description

Vehicle body rear structure, composite blank, and method for manufacturing vehicle body rear structure

[0001] The present disclosure relates to a rear structure for a vehicle body. The present disclosure also relates to a composite blank used in manufacturing the rear structure for a vehicle body. The present disclosure further relates to a method for manufacturing the rear structure for a vehicle body.

[0002] In recent years, there has been a demand for reducing greenhouse gas emissions (life cycle GHG) throughout the entire life cycle of vehicle bodies such as automobiles. In order to reduce the life cycle GHG emissions by reducing the number of vehicle body parts and omitting processes during vehicle body manufacturing, efforts are being made to integrate vehicle body parts.

[0003] For example, Patent Document 1 discloses a rear structure for an automobile body. The rear structure in Patent Document 1 includes left and right rear rails (side frames). Each of the side frames includes a front portion, a rear portion, and a transition region connecting the front and rear portions. Each of the side frames is formed, for example, from a tailor welded blank or a tailor rolled blank. Therefore, in Patent Document 1, the front portion of the side frame and the rear portion of the side frame are integrated from the blank stage.

[0004] Similarly, Patent Document 2 also discloses a technique for integrating the front and rear portions of a side frame from the blank stage.

[0005] The rear structure of the vehicle body in Patent Document 3 includes left and right side members (side frames) and a cross member connecting the side frames. This rear structure is formed from a single tailor-welded blank. In other words, in Patent Document 3, the left and right side frames and the cross member are integrated from the blank stage.

[0006] International Publication No. 2021 / 044234 International Publication No. 2017 / 098470 International Publication No. 2022 / 097058

[0007] The rear structure of a vehicle body, such as an automobile, is an important structural element of the vehicle and is an important component for ensuring safety in the event of a rear collision, for example. Therefore, the rear structure is composed of numerous components, and the manufacturing process of the rear structure requires multiple molding processes for manufacturing each component and an assembly process for these components. Therefore, reducing the number of components in the rear structure is required to improve production efficiency. In response to this, various patent documents disclose techniques for integrating multiple components into a single component by press-forming a tailored blank including multiple sub-blanks with different thicknesses and / or tensile strengths. However, these patent documents only focus on integrating side frame components, and their contribution to reducing the number of components is limited. Therefore, integrating more components is preferable.

[0008] Therefore, an object of the present disclosure is to provide a rear structure for a vehicle body that can further reduce the number of parts.

[0009] The vehicle body rear structure according to the present disclosure includes a floor panel and a first frame unit. The floor panel is formed of a metal plate material. The first frame unit includes a pair of first side frames and a first cross member. The first cross member connects the first side frames. The first frame unit is formed of a metal plate material. The first frame unit is disposed on the floor panel. The first side frames and the first cross member are each welded to the floor panel to form a first closed cross section together with the floor panel. The first closed cross section formed by each of the first side frames and the floor panel is a cross section perpendicular to the fore-and-aft direction of the vehicle body when the rear structure is assembled to the vehicle body. The first closed cross section formed by the first cross member and the floor panel is a cross section perpendicular to the left-and-right direction of the vehicle body when the rear structure is assembled to the vehicle body. In the first closed cross section, the minimum Vickers hardness value of the heat-affected zone of the welded portion of the first frame unit to the floor panel is 70% or more of the Vickers hardness of the non-welded portion of the first frame unit.

[0010] According to the present disclosure, the number of parts in the rear structure of the vehicle body can be further reduced.

[0011] FIG. 1 is a perspective view schematically showing the rear structure of a vehicle body according to the first embodiment. FIG. 2 is another perspective view schematically showing the rear structure of a vehicle body according to the first embodiment. FIG. 3A is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the first embodiment. FIG. 3B is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the first embodiment. FIG. 3C is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the first embodiment. FIG. 3D is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the first embodiment. FIG. 3E is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the first embodiment. FIG. 4 is a horizontal cross-sectional view of the rear structure of a vehicle body according to the first embodiment. FIG. 5 is a vertical cross-sectional view of the rear structure of a vehicle body according to the first embodiment. FIG. 6 is an exploded perspective view showing a composite blank used in manufacturing the rear structure of a vehicle body according to the second embodiment. FIG. 7A is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the second embodiment. FIG. 7B is a schematic view for explaining a manufacturing method of the rear structure of a vehicle body according to the second embodiment. FIG. 8 is a cross-sectional view of a vehicle body rear structure according to a second embodiment. FIG. 9 is a plan view of a composite blank according to a modified example of the first embodiment. FIG. 10 is a schematic diagram for explaining a manufacturing method of a vehicle body rear structure according to a modified example of the first embodiment. FIG. 11 is a perspective view of a composite blank used in the examples. FIG. 12 is another perspective view of a composite blank used in the examples. FIG. 13 is a diagram showing the distribution of Vickers hardness at a spot-welded cross section in a closed cross section formed by a side frame and a floor panel for the examples and comparative examples. FIG. 14 is a graph showing the plate thickness measured at the flange position, the center position in the height direction of the vertical wall, the position of the ridge line, and the center position in the width direction of the top plate in the side frame for the examples. FIG. 15 is a graph showing the plate thickness measured at the flange position, the center position in the height direction of the vertical wall, the position of the ridge line, and the center position in the width direction of the top plate in the side frame for the comparative examples.

[0012] A vehicle body rear structure according to an embodiment includes a floor panel and a first frame unit. The floor panel is formed of a metal plate material. The first frame unit includes a pair of first side frames and a first cross member. The first cross member connects the first side frames. The first frame unit is formed of a metal plate material. The first frame unit is disposed on the floor panel. The first side frames and the first cross member are each welded to the floor panel to form a first closed cross section together with the floor panel. The first closed cross section formed by each of the first side frames and the floor panel is a cross section perpendicular to the fore-and-aft direction of the vehicle body when the rear structure is assembled to the vehicle body. The first closed cross section formed by the first cross member and the floor panel is a cross section perpendicular to the left-and-right direction of the vehicle body when the rear structure is assembled to the vehicle body. In the first closed cross section, the minimum Vickers hardness value of the heat-affected zone of the welded portion of the first frame unit to the floor panel is 70% or more of the Vickers hardness of the non-welded portion of the first frame unit (first configuration).

[0013] In the first configuration, a first frame unit including a pair of first side frames and a first cross member and a floor panel are integrated as a single component. Specifically, the first frame unit and the floor panel are simultaneously formed as a single component by hot blow molding. Therefore, in the first closed cross-section formed by each of the first side frame and the first cross member of the first frame unit together with the floor panel, the minimum Vickers hardness of the heat-affected zone of the welded portion of the first frame unit to the floor panel is 70% or more of the Vickers hardness of the non-welded portion of the first frame unit. In other words, because the first frame unit and the floor panel are integrated by welding at the blank stage and simultaneously formed as a single component, even if softening of the heat-affected zone (HAZ) occurs during welding before forming, the heat treatment during hot blow molding reduces the HAZ softening, thereby reducing the hardness difference between the non-welded portion and the heat-affected zone in the first frame unit.

[0014] In this way, in the vehicle rear structure according to the first configuration, the first frame unit and the floor panel are integrated into a single component, which allows the number of components in the vehicle rear structure to be reduced compared to conventional structures.

[0015] For example, if multiple components are individually molded and then welded, HAZ softening due to welding may occur in each component. Since the HAZ softened portion has relatively low strength, it may become a weak spot during a collision. On the other hand, in the first configuration, by performing hot blow molding after integrating the first frame unit and the floor panel in advance, HAZ softening at the weld between the first frame unit and the floor panel can be reduced during the molding process. Therefore, the vehicle body rear structure according to the first configuration can have good crashworthiness.

[0016] In the vehicle body rear structure according to the first configuration, the first side frame and the first cross member may each include a top plate, a vertical wall, and a flange. The top plate faces a floor panel. The vertical wall is connected to the top plate via a ridge portion. The flange is connected to the vertical wall on an opposite side of the top plate and welded to the floor panel. In the first closed cross-sectional portion, the plate thicknesses measured at a center position in the height direction of the vertical wall, a center position in the width direction of the top plate, and a ridge portion between the top plate and the vertical wall may all be 95% or less of the plate thickness of the flange (second configuration).

[0017] In the second configuration, the thickness of the first closed cross-section at the height center of the vertical wall of the first frame unit, the width center of the top panel, and the ridge line between the top panel and the vertical wall are all 95% or less of the thickness of the flange welded to the floor panel. In other words, the first frame unit experiences a relatively uniform reduction in thickness at the top panel, vertical wall, and ridge line. This eliminates localized thickness reduction in the first frame unit and efficiently distributes the thickness, eliminating the need to increase the thickness of the first frame unit before molding, which facilitates weight reduction of the first frame unit and the rear structure.

[0018] In the vehicle body rear structure according to the first or second configuration, the plate material forming the first frame unit may include a plurality of metal plates joined together (third configuration).

[0019] In a third configuration, the plate material forming the first frame unit includes a plurality of metal plates. In this case, the plate thickness and / or tensile strength can be varied depending on the location in the first frame unit.

[0020] In the vehicle body rear structure according to the third configuration, each of the first side frames may include a front portion and a rear portion. The rear portion is disposed rearward of the front portion when the rear structure is assembled to the vehicle body. The front portion and the rear portion are preferably formed of metal plates having different thicknesses and / or tensile strengths (fourth configuration).

[0021] Generally, the performance required for the front portion of a side frame differs from the performance required for the rear portion. Therefore, in the fourth configuration, the front and rear portions of the first side frame are formed from different metal plates. The metal plate forming the front portion differs from the metal plate forming the rear portion in at least one of the plate thickness and tensile strength. This allows different performance characteristics to be imparted to the front and rear portions. For example, the front portion can be formed from a metal plate that is relatively resistant to deformation, while the rear portion can be formed from a metal plate that allows a certain degree of deformation to absorb collision energy.

[0022] In the vehicle body rear structure according to any one of the first to fourth configurations, each of the first side frames may have a tensile strength of 980 MPa or more (fifth configuration).

[0023] The vehicle body rear structure according to any one of the first to fifth configurations may further include a second frame unit. The second frame unit may include a pair of second side frames. The second frame unit is formed of metal plate material. In this case, the second frame unit is disposed on the floor panel on the opposite side of the first frame unit. The second side frames may each be welded to the floor panel to form a second closed cross-sectional portion together with the floor panel. The second closed cross-sectional portion formed by each of the second side frames and the floor panel is a cross-section perpendicular to the fore-and-aft direction of the vehicle body when the rear structure is assembled to the vehicle body. In the second closed cross-sectional portion, the minimum Vickers hardness value of the heat-affected zone of the welded portion of the second frame unit to the floor panel may be 70% or more of the Vickers hardness of the unwelded portion of the second frame unit (sixth configuration).

[0024] In the sixth configuration, in addition to the first frame unit and the floor panel, the second frame unit is integrated as a single component. This allows for more components to be integrated in the rear structure of the vehicle body, further reducing the number of components.

[0025] In the vehicle body rear structure according to the sixth configuration, the second frame unit may further include a second cross member. The second cross member connects the second side frames. The second side frames and the second cross member may each be welded to a floor panel to form the second closed cross section together with the floor panel. The second closed cross section formed by the second cross member and the floor panel is a cross section perpendicular to the left-right direction of the vehicle body when the rear structure is assembled to the vehicle body (seventh configuration).

[0026] In the seventh configuration, the second frame unit includes a second cross member in addition to the pair of second side frames. Therefore, parts are integrated on a larger scale in the rear structure of the vehicle body, further reducing the number of parts.

[0027] In the vehicle rear structure according to any one of the first to fifth configurations, the floor panel may include a panel main body and a pair of second side frames. The second side frames are provided contiguous with the panel main body. The second side frames correspond to the first side frames. Each of the first side frames can form a first closed cross-sectional portion together with the corresponding second side frame (an eighth configuration).

[0028] In the eighth configuration, the floor panel is provided with a second side frame separate from the first side frame of the first frame unit, which allows for more integration of parts in the rear structure of the vehicle body, further reducing the number of parts.

[0029] In the vehicle rear structure according to any one of the first to eighth configurations, the floor panel may include a panel body and a pair of wheel houses. The wheel houses are disposed on both sides of the panel body (ninth configuration).

[0030] In the ninth configuration, a pair of wheel houses are provided on the floor panel, which allows for a larger scale integration of parts in the rear structure of the vehicle body, further reducing the number of parts.

[0031] A composite blank according to the embodiment is used in the manufacture of a rear structure for a vehicle body. The composite blank comprises a first blank and a second blank. The first blank corresponds to a floor panel. The second blank includes a pair of side frame equivalent portions corresponding to a pair of side frames and a cross member equivalent portion corresponding to a cross member. The cross member equivalent portions connect the side frame equivalent portions. The second blank is overlaid on the first blank. The outer peripheries of the side frame equivalent portions and the cross member equivalent portion are welded to the first blank (tenth configuration).

[0032] In the composite blank according to the tenth configuration, at least one of the first blank and the second blank may include a plurality of metal plates joined together, preferably having different thicknesses and / or tensile strengths (eleventh configuration).

[0033] A manufacturing method for a vehicle rear structure according to an embodiment includes the steps of preparing a composite blank according to the tenth or eleventh configuration, heating the composite blank, and placing the heated composite blank in a mold and using the mold to mold a first blank into a floor panel and a second blank into a frame unit. The frame unit includes a pair of side frames and a cross member connecting the side frames. In the molding step, the side frame equivalent portion and the cross member equivalent portion are molded into the side frame and the cross member, respectively, by hot blow molding (twelfth configuration).

[0034] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In these drawings, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0035] 1 and 2 are perspective views schematically showing a rear structure 100 according to a first embodiment. The rear structure 100 is used in the body of an automobile or the like. As shown in FIGS. 1 and 2, the rear structure 100 includes frame units 10, 20 and a floor panel 30.

[0036] 1 , the frame unit 10 is formed from a metal plate. The frame unit 10 includes a pair of side frames 11, 12 and a cross member 13. In this embodiment, the frame unit 10 includes one cross member 13. However, the frame unit 10 may include multiple cross members 13.

[0037] The side frames 11, 12 and the cross member 13 each have an elongated shape. The side frames 11, 12 extend in the fore-and-aft direction of the vehicle body when the rear structure 100 is attached to the vehicle body. The side frame 11 includes a front section 11F and a rear section 11R. Similarly, the side frame 12 includes a front section 12F and a rear section 12R. When the rear structure 100 is attached to the vehicle body, the rear sections 11R, 12R are positioned rearward of the front sections 11F, 12F, respectively. The rear sections 11R, 12R extend rearward from the front sections 11F, 12F, respectively.

[0038] The cross member 13 extends in the left-right direction of the vehicle body when the rear structure 100 is attached to the vehicle body. The cross member 13 extends from the side frame 11 to the side frame 12. The cross member 13 connects the side frames 11, 12. One longitudinal end of the cross member 13 is connected to one of the side frames 11. The other longitudinal end of the cross member 13 is connected to the other side frame 12.

[0039] Continuing to refer to FIG. 1 , the floor panel 30 is formed from a metal plate material. The floor panel 30 includes a panel main body 31 and a pair of wheel houses 32, 33. The wheel houses 32, 33 are disposed on both sides of the panel main body 31. When the rear structure 100 is assembled to the vehicle body, the wheel houses 32, 33 are positioned on the left and right sides of the panel main body 31. When viewed along the left-right direction of the vehicle body, the wheel houses 32, 33 each have an arch shape that protrudes upward from the vehicle body. In this embodiment, the wheel houses 32, 33 are provided adjacent to the panel main body 31.

[0040] A frame unit 10 is disposed on the floor panel 30. The frame unit 10 is disposed on the upper surface of the floor panel 30 with the rear structure 100 assembled to the vehicle body. The side frames 11, 12 and the cross member 13 are each welded to the floor panel 30. More specifically, the side frames 11, 12 and the cross member 13 are disposed on the upper surface of a panel main body 31 of the floor panel 30 and welded to the panel main body 31.

[0041] 2 , the frame unit 20 is formed from a metal plate. The frame unit 20 includes a pair of side frames 21, 22 and a cross member 23. In this embodiment, the frame unit 20 includes a plurality of cross members 23. However, it is sufficient that the frame unit 20 includes at least one cross member 23.

[0042] The side frames 21, 22 and the cross member 23 each have an elongated shape. The side frames 21, 22 extend in the fore-and-aft direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. The side frame 21 includes a front section 21F and a rear section 21R. Similarly, the side frame 22 includes a front section 22F and a rear section 22R. When the rear structure 100 is assembled to the vehicle body, the rear sections 21R, 22R are positioned rearward of the front sections 21F, 22F, respectively. The rear sections 21R, 22R extend rearward from the front sections 21F, 22F, respectively.

[0043] Each of the cross members 23 extends in the left-right direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. The cross members 23 extend from the side frames 21 to the side frames 22. The cross members 23 connect the side frames 21, 22. One longitudinal end of each of the cross members 23 is connected to one of the side frames 21. The other longitudinal end of each of the cross members 23 is connected to the other side frame 22.

[0044] The frame unit 20 is disposed on the floor panel 30 on the opposite side of the frame unit 10. The frame unit 20 is disposed on the underside of the floor panel 30 when the rear structure 100 is assembled to the vehicle body. That is, the frame unit 20 is the lower frame unit, and the frame unit 10 is the upper frame unit. The side frames 21, 22 and the cross member 23 are each welded to the floor panel 30. More specifically, the side frames 21, 22 and the cross member 23 are disposed on the underside of the panel main body 31 of the floor panel 30 and welded to the panel main body 31.

[0045] [Method for manufacturing rear structure of vehicle body] Hereinafter, an example of a method for manufacturing rear structure 100 will be described with reference to Figures 3A to 3E. The manufacturing method according to this embodiment includes a preparation step, a heating step, and a molding step.

[0046] 3A and 3B, in the preparation step, a composite blank 40 is prepared. The composite blank 40 includes blanks 41, 42, and 43.

[0047] 3A, the blank 41 corresponds to the upper frame unit 10 (FIG. 1). That is, the blank 41 has the shape of the frame unit 10 in an expanded state.

[0048] The blank 41 includes a pair of side frame equivalents 411, 412 and a cross member equivalent 413. The side frame equivalents 411, 412 are portions of the blank 41 that correspond to the side frames 11, 12 (FIG. 1), respectively, of the frame unit 10. The cross member equivalent 413 is a portion that corresponds to the cross member 13 (FIG. 1) of the frame unit 10.

[0049] The side frame portion 411 includes a plurality of metal plates 411F, 411R. The metal plates 411F, 411R are typically joined to each other by welding. The metal plates 411F, 411R may be butt-joined by laser welding. Alternatively, the metal plates 411F, 411R may be joined by spot welding with their ends overlapping.

[0050] The metal plates 411F and 411R differ in at least one of plate thickness and tensile strength. For example, the metal plate 411F has a greater plate thickness and / or tensile strength than the metal plate 411R. The metal plate 411F is a sub-blank corresponding to the front portion 11F (FIG. 1) of the side frame 11. The metal plate 411R is a sub-blank corresponding to the rear portion 11R (FIG. 1) of the side frame 11.

[0051] The side frame equivalent portion 412, like the other side frame equivalent portions 411, includes a plurality of metal plates 412F, 412R. The metal plates 412F, 412R are typically joined to one another by welding. Like the metal plates 411F, 411R, the metal plates 412F, 412R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping.

[0052] The metal plates 412F, 412R differ in at least one of plate thickness and tensile strength. For example, the metal plate 412F has a greater plate thickness and / or tensile strength than the metal plate 412R. The metal plate 412F is a sub-blank corresponding to the front portion 12F (FIG. 1) of the side frame 12. The metal plate 412R is a sub-blank corresponding to the rear portion 12R (FIG. 1) of the side frame 12.

[0053] The metal plate 412F of the side frame equivalent portion 412 usually has the same plate thickness and tensile strength as the metal plate 411F of the side frame equivalent portion 411. Similarly, the metal plate 412R of the side frame equivalent portion 412 usually has the same plate thickness and tensile strength as the metal plate 411R of the side frame equivalent portion 411. However, the metal plates 412F, 412R may differ from the metal plates 411F, 411R in at least one of the plate thickness and tensile strength.

[0054] The cross member equivalent portion 413 connects the side frame equivalent portions 411, 412. The cross member equivalent portion 413 may be formed from a metal plate separate from the side frame equivalent portions 411, 412. In this case, the cross member equivalent portion 413 is typically joined to the side frame equivalent portions 411, 412 by welding. In the example of FIG. 3A , the cross member equivalent portion 413 is joined to the metal plate 411F of the side frame equivalent portion 411 and the metal plate 412F of the side frame equivalent portion 412. The cross member equivalent portion 413 may be butt-joined to the side frame equivalent portions 411, 412 by laser welding, or may be joined by spot welding with the ends overlapping.

[0055] In this embodiment, the cross member equivalent portion 413 is a single metal plate (sub-blank). However, the cross member equivalent portion 413 may include multiple metal plates (sub-blanks) joined together. The cross member equivalent portion 413 may differ in at least one of the plate thickness and tensile strength from the metal plates 411F, 411R, 412F, and 412R included in the side frame equivalent portions 411 and 412.

[0056] At least one air inlet 414 is provided in the blank 41. The air inlet 414 penetrates the blank 41 in the thickness direction. The air inlet 414 is used for blowing air in the molding process described below. In this embodiment, the air inlet 414 is formed in each of the side frame equivalent parts 411, 412. The air inlet 414 is arranged, for example, in a position in each of the side frame equivalent parts 411, 412 where the cross member equivalent part 413 is connected.

[0057] 3A, blank 43 corresponds to floor panel 30 (FIGS. 1 and 2). That is, blank 43 has the shape of floor panel 30 when unfolded. Blank 43 includes a panel main body portion 431 and wheel house portion portions 432, 433.

[0058] The panel main body equivalent portion 431 is a portion of the blank 43 that corresponds to the panel main body 31 ( FIG. 1 ) of the floor panel 30. In the example of FIG. 3A , the panel main body equivalent portion 431 includes multiple metal plates (sub-blanks) 431F, 431C, and 431R. The metal plate 431C is disposed intermediate the metal plates 431F and 431R. The metal plates 431F, 431C, and 431R are typically joined to one another by welding. More specifically, the metal plate 431F is joined to the metal plate 431C, and the metal plate 431R is joined to the metal plate 431C on the opposite side of the metal plate 431F. The metal plates 431F, 431C, and 431R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping. The metal plates 431F, 431C, and 431R may differ from each other in at least one of plate thickness and tensile strength. For example, the plate thickness and / or tensile strength of the metal plates 431F and 431R may be greater than that of the intermediate metal plate 431C.

[0059] The wheel house equivalent portions 432, 433 are portions of the blank 43 that correspond to the wheel houses 32, 33 ( FIG. 1 ) of the floor panel 30, respectively. The wheel house equivalent portions 432, 433 may be formed from metal plates separate from the panel main body equivalent portion 431. In this case, the wheel house equivalent portions 432, 433 are typically joined to the panel main body equivalent portion 431 by welding. In the example of FIG. 3A , the wheel house equivalent portions 432, 433 are joined to the metal plate 431C of the panel main body equivalent portion 431. The wheel house equivalent portions 432, 433 may be butt-joined to the panel main body equivalent portion 431 by laser welding, or may be joined by spot welding with their ends overlapping.

[0060] In this embodiment, the wheel house equivalents 432, 433 are each a single metal plate (sub-blank). However, each of the wheel house equivalents 432, 433 may include multiple metal plates (sub-blanks) joined together. The wheel house equivalents 432, 433 may differ from the metal plates 431F, 431C, 431R of the panel main body equivalent 431 in at least one of the plate thickness and tensile strength. Alternatively, the wheel house equivalents 432, 433 may have the same plate thickness and tensile strength as any of the metal plates 431F, 431C, 431R.

[0061] The blank 41 is placed on the blank 43. The blank 41 is welded to the blank 43. More specifically, the blank 41 is placed on the panel main body equivalent portion 431 of the blank 43. The outer peripheries of the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 are welded to the blank 43. Therefore, the composite blank 40 includes welds 44 that weld the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 to the blank 43. The welds 44 are, for example, spot welds. The welds 44 may also be laser welds. That is, the welds 44 may be formed by spot welding or laser welding.

[0062] 3B, the blank 42 corresponds to the lower frame unit 20 (FIG. 2). That is, the blank 42 has the shape of the frame unit 20 when it is unfolded.

[0063] The blank 42 includes a pair of side frame equivalents 421, 422 and at least one cross member equivalent 423. The side frame equivalents 421, 422 are portions of the blank 42 that correspond to the side frames 21, 22 ( FIG. 2 ) of the frame unit 20, respectively. The cross member equivalent 423 is a portion that corresponds to the cross member 23 ( FIG. 1 ) of the frame unit 20. In this embodiment, the blank 42 includes a plurality of cross member equivalents 423 that correspond to the multiple cross members 23 of the frame unit 20.

[0064] The side frame portion 421 includes a plurality of metal plates 421F, 421R. The metal plates 421F, 421R are typically joined to each other by welding. The metal plates 421F, 421R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping.

[0065] The metal plates 421F and 421R differ in at least one of plate thickness and tensile strength. For example, the metal plate 421F has a greater plate thickness and / or tensile strength than the metal plate 421R. The metal plate 421F is a sub-blank corresponding to the front portion 21F (FIG. 2) of the side frame 21. The metal plate 421R is a sub-blank corresponding to the rear portion 21R (FIG. 1) of the side frame 21.

[0066] The side frame equivalent portion 422 includes a plurality of metal plates 422F, 422R, similar to the side frame equivalent portion 421. The metal plates 422F, 422R are typically joined to each other by welding. The metal plates 422F, 422R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping each other.

[0067] The metal plates 422F, 422R differ in at least one of plate thickness and tensile strength. For example, the metal plate 422F has a greater plate thickness and / or tensile strength than the metal plate 422R. The metal plate 422F is a sub-blank corresponding to the front portion 22F (FIG. 2) of the side frame 22. The metal plate 422R is a sub-blank corresponding to the rear portion 22R (FIG. 2) of the side frame 22.

[0068] The metal plate 422F of the side frame equivalent portion 422 usually has the same plate thickness and tensile strength as the metal plate 421F of the side frame equivalent portion 421. Similarly, the metal plate 422R of the side frame equivalent portion 422 usually has the same plate thickness and tensile strength as the metal plate 421R of the side frame equivalent portion 421. However, the metal plates 422F, 422R may differ from the metal plates 421F, 421R in at least one of the plate thickness and tensile strength.

[0069] In this embodiment, each of the cross member equivalents 423 connects the side frame equivalents 421, 422. The cross member equivalents 423 may be formed from a metal plate separate from the side frame equivalents 421, 422. In this case, the cross member equivalents 423 are typically joined to the side frame equivalents 421, 422 by welding. In the example of FIG. 3B , one cross member equivalent 423 is joined to the metal plate 421F of the side frame equivalent 421 and the metal plate 422F of the side frame equivalent 422, and the other cross member equivalent 423 is joined to the metal plate 421R of the side frame equivalent 421 and the metal plate 422R of the side frame equivalent 422. The cross member equivalents 423 may be butt-joined to the side frame equivalents 421, 422 by laser welding, or may be joined by spot welding with the ends overlapping.

[0070] In this embodiment, each of the cross member equivalents 423 is a single metal plate (sub-blank). However, the cross member equivalents 423 may include multiple metal plates joined together. The cross member equivalents 423 may differ in at least one of the plate thickness and tensile strength from the metal plates 421F, 421R, 422F, and 422R included in the side frame equivalents 421 and 422.

[0071] Similar to the blank 41 ( FIG. 3A ), the blank 42 has at least one air inlet 424. The air inlet 424 penetrates the blank 42 in its thickness direction. The air inlet 424 is used for blowing air in the molding process described below. In this embodiment, the air inlet 424 is formed in each of the side frame equivalents 421, 422. The air inlet 424 is disposed, for example, in each of the side frame equivalents 421, 422 at a position where each cross member equivalent 423 is connected.

[0072] The blank 42 is overlapped on the blank 43 on the opposite side of the blank 41 ( FIG. 3A ). The blank 42 is welded to the blank 43. More specifically, the blank 42 is overlapped on the panel body equivalent portion 431 of the blank 43. The outer peripheries of the side frame equivalent portions 421, 422 and the cross member equivalent portion 423 are welded to the blank 43. Therefore, the composite blank 40 includes welds 45 that weld the side frame equivalent portions 421, 422 and the cross member equivalent portion 423 to the blank 43. The welds 45 are, for example, spot welds. The welds 45 may also be laser welds. That is, the welds 45 may be formed by spot welding or laser welding.

[0073] 3A and 3B , blanks 41, 42, and 43 are each a tailored blank including multiple metal plates (sub-blanks). More specifically, blanks 41, 42, and 43 are tailor welded blanks in which multiple metal plates are joined by welding. The metal plates included in blanks 41, 42, and 43 are, for example, steel plates. However, the metal plates included in blanks 41, 42, and 43 may also be, for example, aluminum alloy plates.

[0074] Of the blank 41, at least the side frame equivalent portions 411, 412 are preferably formed from hot-stamped material. Similarly, of the blank 42, at least the side frame equivalent portions 421, 422 are preferably formed from hot-stamped material. In the blanks 41, 42, the cross member equivalent portions 413, 423 may also be formed from hot-stamped material. A hot-stamped material is generally a steel plate that can achieve a desired strength by hot-forming a blank made of high-strength steel plate (e.g., a steel plate having a high tensile strength of more than 440 MPa) and then pressing it against a die and quenching it, thereby producing a steel plate with a strength ranging from 980 MPa to 2000 MPa. For hot-stamped material, a forming method is used in which a blank (steel plate) is heated to an austenite temperature range of, for example, approximately 900°C, and then hot-forming is performed, followed by pressing it against a die, quenching, and quenching to cause martensitic transformation. Hot forming is characterized by the fact that the forming load can be reduced because the forming is performed at high temperatures, and that the martensitic transformation results in high strength after forming and excellent shape fixability.

[0075] (Heating Step) Referring to FIG. 3C, in the heating step, the prepared composite blank 40 is heated. The composite blank 40 is heated, for example, in a heating furnace. The heating temperature of the composite blank 40 is determined depending on the material of the composite blank 40. When the composite blank 40 is made of steel, the composite blank 40 is heated to a temperature above the austenite transformation finish temperature (A ｃ３ When the composite blank 40 is made of steel, the composite blank 40 is heated to, for example, 900° C. or higher.

[0076] (Forming Process) In the forming process, the composite blank 40 is formed into the rear structure 100 (FIGS. 1 and 2) by hot blow forming. Referring to FIG. 3D, in the forming process, the heated composite blank 40 is placed in a mold 50. Then, using the mold 50, the blank 43 in the composite blank 40 is formed into the floor panel 30 (FIG. 1), and the blank 41 is formed into the upper frame unit 10 (FIG. 1). In the forming process, the mold 50 is further used to form the blank 42 into the lower frame unit 20 (FIG. 2).

[0077] 3D shows a cross section taken along line IIID-IIID in FIG. 3A, i.e., a cross section of the mold 50 taken along the positions of the side frame equivalent portions 411 and 412 of the blank 41. As shown in FIG. 3D, the mold 50 includes an upper mold 51 and a lower mold 52.

[0078] The upper die 51 is capable of moving relatively close to and away from the lower die 52. In the forming process, the composite blank 40 is placed between the upper die 51 and the lower die 52. Of the composite blank 40, the blank 41 faces the forming surface of the upper die 51, and the blank 42 faces the forming surface of the lower die 52. The forming surface of the upper die 51 has a shape corresponding to the upper frame unit 10 and floor panel 30 (FIG. 1). The forming surface of the lower die 52 has a shape corresponding to the lower frame unit 20 and floor panel 30 (FIG. 2).

[0079] In the example of Fig. 3D, the upper mold 51 includes air blow supply passages 511. The supply passages 511 are provided at positions corresponding to the blowing holes 414 (Fig. 3A) of the blank 41. The lower mold 52 also includes air blow supply passages 521. The supply passages 521 are provided at positions corresponding to the blowing holes 424 (Fig. 3B) of the blank 42.

[0080] In the forming process, the upper mold 51 and the lower mold 52 are moved relatively close to each other to grip portions of the composite blank 40. For example, the upper mold 51 and the lower mold 52 can grip flange portions of the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 of the composite blank 40. Air is then supplied to the composite blank 40 via a supply path 511. The air is introduced between the blanks 41 and 43 through the air blowing holes 414 ( FIG. 3A ). As shown in FIG. 3E , the air expands the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 ( FIG. 3A ) of the blank 41 toward the upper mold 51. The temperature of the air blown may be determined appropriately depending on the material of the composite blank 40, and may be, for example, room temperature.

[0081] The side frame equivalent portions 411, 412 and the cross member equivalent portion 413 ( FIG. 3A ) of the blank 41 are inflated by air blowing and then pressed against the upper die 51. As a result, the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 are molded into the side frames 11, 12 and the cross member 13 ( FIG. 1 ), respectively, and are then heat-dissipated (rapidly cooled) and quenched in the die 50. However, quenching using the die 50 is not necessarily required in the molding process. In other words, the side frame equivalent portions 411, 412 and the cross member equivalent portion 413 may be hot-blow molded using a heated die 50.

[0082] Similarly, air is supplied to the composite blank 40 from the supply passage 521 of the lower mold 52. The air is introduced between the blanks 42 and 43 through the air blowing holes 424 (FIG. 3B). The air inflates the side frame-corresponding portions 421 and 422 and the cross member-corresponding portion 423 (FIG. 3B) of the blank 42 and presses them against the lower mold 52. As a result, the side frame-corresponding portions 421 and 422 and the cross member-corresponding portion 423 are molded into the side frames 21 and 22 and the cross member 23 (FIG. 2), respectively, and are then heat-removed (rapidly cooled) and quenched in the mold 50. However, as described above, quenching using the mold 50 is not necessarily required. That is, the side frame-corresponding portions 421 and 422 and the cross member-corresponding portion 423 may be hot-blow molded using a heated mold 50.

[0083] In this way, the blanks 41, 42 corresponding to the frame units 10, 20 (FIGS. 1 and 2) are hot blow-molded. Meanwhile, the blank 43 corresponding to the floor panel 30 (FIGS. 1 and 2) is pressed using a mold 50. The blank 43 is pressed between an upper mold 51 and a lower mold 52 to form the floor panel 30 (FIGS. 1 and 2). The blank 43 may be quenched by removing heat (rapid cooling) in the mold 50.

[0084] In the molding process, both longitudinal ends of the side frames 11, 12, 21, and 22, i.e., the front and rear ends in the longitudinal direction of the vehicle body, are closed by welding. Therefore, the front and rear ends of the side frames 11, 12, 21, and 22 may be cut along a cross section perpendicular to the longitudinal direction after the molding process.

[0085] 4 and 5 are cross-sectional views of the molded rear structure 100. Fig. 4 shows a cross section (transverse cross section) of the rear structure 100 perpendicular to the longitudinal direction of the side frames 11, 12 of the frame unit 10. Fig. 5 shows a cross section (longitudinal cross section) of the rear structure 100 perpendicular to the longitudinal direction of the cross member 13 of the frame unit 10.

[0086] As shown in FIG. 4 , in this embodiment, the side frames 11, 12 of the upper frame unit 10 have a generally hat-shaped cross section. The side frame 11 includes a top plate 111, vertical walls 112, 113, flanges 114, 115, and ridge portions 116, 117. The top plate 111 faces the floor panel 30 with a gap between them. The vertical walls 112, 113 are disposed on both sides of the top plate 111 and are connected by the top plate 111. In a cross-sectional view of the side frame 11, the vertical walls 112, 113 extend from the top plate 111 toward the floor panel 30. The vertical walls 112, 113 are connected to the top plate 111 via ridge portions 116, 117, respectively. In a cross-sectional view of the side frame 11, the ridge portions 116, 117 have, for example, an arc shape. The flange 114 is connected to the vertical wall 112 on the opposite side of the top plate 111. The flange 115 is connected to the vertical wall 113 on the opposite side of the top plate 111. The flanges 114, 115 protrude outward from the vertical walls 112, 113 and are welded to the floor panel 30.

[0087] The side frame 12 also includes a top plate 121, vertical walls 122, 123, flanges 124, 125, and ridge portions 126, 127. The cross-sectional configuration of the side frame 12 is similar to that of the side frame 11, and therefore will not be described here.

[0088] The side frames 11, 12 are each welded to the floor panel 30 to form a closed cross-sectional portion 14 together with the floor panel 30. This closed cross-sectional portion 14 is a cross-section perpendicular to the longitudinal direction of the side frames 11, 12. In other words, the closed cross-sectional portion 14 formed by each of the side frames 11, 12 and the floor panel 30 is a cross-section perpendicular to the longitudinal direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. In the closed cross-sectional portion 14, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 of the side frames 11, 12 to the floor panel 30 is 70% or more of the Vickers hardness of the non-welded portion of the side frames 11, 12. In the closed cross-sectional portion 14, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 of the side frames 11, 12 to the floor panel 30 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion of the side frames 11, 12. In each of the side frames 11, 12, the minimum Vickers hardness of the heat-affected zone is equal to or less than the Vickers hardness of the non-welded zone. That is, in the closed cross-sectional portion 14, the minimum Vickers hardness of the heat-affected zone of the welded zone 44 of the side frames 11, 12 is 100% or less of the Vickers hardness of the non-welded zone of the side frames 11, 12.

[0089] The Vickers hardness of the closed cross-sectional portion 14 can be measured by a Vickers hardness test specified in JIS Z 2244-1:2020. That is, first, a test specimen including one of the side frames 11, 12 and the weld 44 is obtained from the rear structure 100 by laser cutting or the like at a position passing through the weld center of the weld 44. The test specimen is then embedded in resin so that a cross section passing through the weld center of the side frame 11 or 12 and the weld 44 is located on the surface, and the cross section is polished. Next, Vickers hardness is measured in accordance with JIS Z 2244-1:2020 at a test force of 0.49 N and a measurement interval (pitch) of 0.1 to 0.2 mm, for example, at a position 1 / 4 of the plate thickness from the surface on the weld center side of the weld 44 of the side frame 11 or 12 to a position 12.0 mm outward from the weld center. The minimum value of the measured Vickers hardness is defined as the minimum Vickers hardness of the heat-affected zone in the closed cross-section portion 14. In addition, the Vickers hardness is measured in accordance with JIS Z 2244-1:2020 at a position 15.0 mm or more away from the weld center of the welded portion 44 and at a position 1 / 4 of the way in the plate thickness direction from the surface of the welded portion 44 of the side frame 11 or 12 on the weld center side, using a test force of 0.49 N, for example. This Vickers hardness is defined as the Vickers hardness of the non-welded portion of the side frames 11, 12 in the closed cross-section portion 14.

[0090] In the closed cross-sectional portion 14 formed by the side frame 11 and the floor panel 30, for example, the thicknesses measured at the center position in the height direction of the vertical wall 112, the center position in the width direction of the top plate 111, and the position of the ridge line 116 are all 95% or less of the thickness of the flange 114. The thicknesses measured at the center position in the height direction of the vertical wall 112, the center position in the width direction of the top plate 111, and the position of the ridge line 116 may all be 60% or more of the thickness of the flange 114. Similarly, in the closed cross-sectional portion 14 formed by the side frame 12 and the floor panel 30, for example, the thicknesses measured at the center position in the height direction of the vertical wall 122, the center position in the width direction of the top plate 121, and the position of the ridge line 126 are all 95% or less of the thickness of the flange 124. The plate thickness measured at the center position in the height direction of the vertical wall 122, the center position in the width direction of the top plate 121, and the position of the ridge line portion 126 may all be 60% or more of the plate thickness of the flange 124. In the closed cross-sectional portion 14, the width directions of the top plates 111, 121 are directions perpendicular to the plate thickness directions of the top plates 111, 121. In the closed cross-sectional portion 14, the height direction of the vertical walls 112, 122 is the plate thickness direction of the floor panel 30, for example.

[0091] In the lower frame unit 20, the side frames 21, 22 have a generally hat-shaped cross section. The side frame 21 includes a top plate 211, vertical walls 212, 213, flanges 214, 215, and ridge portions 216, 217. The top plate 211 faces the floor panel 30 with a gap between them. The vertical walls 212, 213 are disposed on both sides of the top plate 211 and are connected by the top plate 211. In a cross-sectional view of the side frame 21, the vertical walls 212, 213 extend from the top plate 211 toward the floor panel 30. The vertical walls 212, 213 are connected to the top plate 211 via ridge portions 216, 217, respectively. In a cross-sectional view of the side frame 21, the ridge portions 216, 217 have, for example, an arc shape. The flange 214 is connected to the vertical wall 212 on the opposite side of the top plate 211. The flange 215 is connected to the vertical wall 213 on the opposite side of the top plate 211. The flanges 214, 215 protrude outward from the vertical walls 212, 213 and are welded to the floor panel 30.

[0092] The side frame 22 also includes a top plate 221, vertical walls 222, 223, flanges 224, 225, and ridges 226, 227. The cross-sectional configuration of the side frame 22 is similar to that of the side frame 21, and therefore will not be described here.

[0093] 4, the side frames 21, 22 are disposed at positions corresponding to the side frames 11, 12 in a cross-sectional view of the rear structure 100. In other words, the positions in the left-right direction of the vehicle body of the lower side frames 21, 22 and the upper side frames 11, 12 coincide with each other. However, the positions of the side frames 21, 22 in the left-right direction of the vehicle body do not necessarily have to coincide with the positions of the side frames 11, 12.

[0094] The side frames 21, 22 are each welded to the floor panel 30 to form a closed cross-sectional portion 24 together with the floor panel 30. This closed cross-sectional portion 24 is a cross-section perpendicular to the longitudinal direction of the side frames 21, 22. In other words, the closed cross-sectional portion 24 formed by each of the side frames 21, 22 and the floor panel 30 is a cross-section perpendicular to the longitudinal direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. In the closed cross-sectional portion 24, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 of the side frames 21, 22 to the floor panel 30 is 70% or more of the Vickers hardness of the non-welded portion of the side frames 21, 22. In the closed cross-sectional portion 24, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 of the side frames 21, 22 to the floor panel 30 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion of the side frames 21, 22. In each of the side frames 21, 22, the minimum Vickers hardness of the heat-affected zone is equal to or less than the Vickers hardness of the non-welded zone. That is, in the closed cross-section 24, the minimum Vickers hardness of the heat-affected zone of the welded zone 45 of the side frames 21, 22 is equal to or less than 100% of the Vickers hardness of the non-welded zone of the side frames 21, 22. The Vickers hardness of the closed cross-section 24 can be measured in the same manner as the Vickers hardness of the closed cross-section 14 of the side frames 11, 12.

[0095] In the closed cross-sectional portion 24 formed by the side frame 21 and the floor panel 30, for example, the thicknesses measured at the center position in the height direction of the vertical wall 212, the center position in the width direction of the top plate 211, and the position of the ridge line 216 are all 95% or less of the thickness of the flange 214. The thicknesses measured at the center position in the height direction of the vertical wall 212, the center position in the width direction of the top plate 211, and the position of the ridge line 216 may all be 60% or more of the thickness of the flange 214. Similarly, in the closed cross-sectional portion 24 formed by the side frame 22 and the floor panel 30, for example, the thicknesses measured at the center position in the height direction of the vertical wall 222, the center position in the width direction of the top plate 221, and the position of the ridge line 226 are all 95% or less of the thickness of the flange 224. The plate thicknesses measured at the center position in the height direction of the vertical wall 222, the center position in the width direction of the top plate 221, and the position of the ridge line portion 226 may all be 60% or more of the plate thickness of the flange 224. In the closed cross-sectional portion 24, the width directions of the top plates 211, 221 are directions perpendicular to the plate thickness directions of the top plates 211, 221. In the closed cross-sectional portion 24, the height direction of the vertical walls 212, 222 is the plate thickness direction of the floor panel 30, for example.

[0096] As shown in FIG. 5 , in this embodiment, the cross member 13 of the upper frame unit 10 also has a generally hat-shaped cross section, similar to the side frames 11 and 12 ( FIG. 4 ). The cross member 13 includes a top plate 131, vertical walls 132 and 133, flanges 134 and 135, and ridge portions 136 and 137. The top plate 131 faces the floor panel 30 with a gap between them. The vertical walls 132 and 133 are disposed on both sides of the top plate 131 and are connected by the top plate 131. In a cross-sectional view of the cross member 13, the vertical walls 132 and 133 extend from the top plate 131 toward the floor panel 30. The vertical walls 132 and 133 are connected to the top plate 131 via ridge portions 136 and 137, respectively. In a cross-sectional view of the cross member 13, the ridge portions 136 and 137 have, for example, an arc shape. The flange 134 is connected to the vertical wall 132 on the opposite side of the top plate 131. The flange 135 is connected to the vertical wall 133 on the opposite side of the top plate 131. The flanges 134, 135 protrude outward from the vertical walls 132, 133 and are welded to the floor panel 30.

[0097] The cross member 13 is welded to the floor panel 30 to form a closed cross-section 15 together with the floor panel 30. This closed cross-section 15 is a cross section perpendicular to the longitudinal direction of the cross member 13. In other words, the closed cross-section 15 formed by the cross member 13 and the floor panel 30 is a cross section perpendicular to the left-right direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. In the closed cross-section 15, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 of the cross member 13 to the floor panel 30 may be 70% or more of the Vickers hardness of the non-welded portion of the cross member 13. In the closed cross-section 15, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 of the cross member 13 to the floor panel 30 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion of the cross member 13. In the cross member 13, the minimum Vickers hardness of the heat-affected zone is equal to or less than the Vickers hardness of the non-welded portion. That is, in the closed cross-section portion 15, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 of the cross member 13 is 100% or less of the Vickers hardness of the non-welded portion of the cross member 13. The Vickers hardness in the closed cross-section portion 15 can be measured in the same manner as the Vickers hardness in the closed cross-section portions 14 ( FIG. 4 ) of the side frames 11, 12.

[0098] In the closed cross-sectional portion 15, for example, the thickness measured at the center position in the height direction of the vertical wall 132, the center position in the width direction of the top plate 131, and the position of the ridge line portion 136 may all be 95% or less of the thickness of the flange 134. The thickness measured at the center position in the height direction of the vertical wall 132, the center position in the width direction of the top plate 131, and the position of the ridge line portion 136 may all be 60% or more of the thickness of the flange 134. In the closed cross-sectional portion 15, the width direction of the top plate 131 is a direction perpendicular to the thickness direction of the top plate 131. In the closed cross-sectional portion 15, the height direction of the vertical wall 132 is, for example, the thickness direction of the floor panel 30.

[0099] In the lower frame unit 20, each cross member 23 has a generally hat-shaped cross section. Each cross member 23 includes a top plate 231, vertical walls 232 and 233, flanges 234 and 235, and ridge portions 236 and 237. The top plate 231 faces the floor panel 30 with a gap between them. The vertical walls 232 and 233 are disposed on both sides of the top plate 231 and are connected by the top plate 231. In a cross-sectional view of the cross member 23, the vertical walls 232 and 233 extend from the top plate 231 toward the floor panel 30. The vertical walls 232 and 233 are connected to the top plate 231 via ridge portions 236 and 237, respectively. In a cross-sectional view of the cross member 23, the ridge portions 236 and 237 have, for example, an arc shape. The flange 234 is connected to the vertical wall 232 on the opposite side of the top plate 231. The flange 235 is connected to the vertical wall 233 on the opposite side of the top plate 231. The flanges 234, 235 protrude outward from the vertical walls 232, 233 and are welded to the floor panel 30.

[0100] The cross member 23 is welded to the floor panel 30 to form a closed cross-section 25 together with the floor panel 30. This closed cross-section 25 is a cross section perpendicular to the longitudinal direction of the cross member 23. In other words, the closed cross-section 25 formed by the cross member 23 and the floor panel 30 is a cross section perpendicular to the left-right direction of the vehicle body when the rear structure 100 is assembled to the vehicle body. In the closed cross-section 25, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 of the cross member 23 to the floor panel 30 may be 70% or more of the Vickers hardness of the non-welded portion of the cross member 23. In the closed cross-section 25, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 of the cross member 23 to the floor panel 30 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion of the cross member 23. In the cross member 23, the minimum Vickers hardness of the heat-affected zone is equal to or less than the Vickers hardness of the non-welded portion. That is, in the closed cross-section portion 25, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 of the cross member 23 is 100% or less of the Vickers hardness of the non-welded portion of the cross member 23. The Vickers hardness in the closed cross-section portion 25 can be measured in the same manner as the Vickers hardness in the closed cross-section portion 14 ( FIG. 4 ) of the side frames 11, 12.

[0101] In the closed cross-sectional portion 25, for example, the thickness measured at the center position in the height direction of the vertical wall 232, the center position in the width direction of the top plate 231, and the position of the ridge line portion 236 may all be 95% or less of the thickness of the flange 234. The thickness measured at the center position in the height direction of the vertical wall 232, the center position in the width direction of the top plate 231, and the position of the ridge line portion 236 may all be 60% or more of the thickness of the flange 234. In the closed cross-sectional portion 25, the width direction of the top plate 231 is a direction perpendicular to the thickness direction of the top plate 231. In the closed cross-sectional portion 25, the height direction of the vertical wall 232 is, for example, the thickness direction of the floor panel 30.

[0102] [Effects] In the rear structure 100 according to this embodiment, the upper frame unit 10, the lower frame unit 20, and the floor panel 30 are integrated into a single component. That is, the frame units 10, 20 and the floor panel 30 are simultaneously formed in a single molding process from a composite blank 40 including blanks 41, 42, and 43. This allows the number of components in the rear structure 100 to be reduced compared to conventional methods, and significantly reduces the number of processes required to manufacture the rear structure 100. As a result, lifecycle GHG emissions are likely to be reduced in the manufacture of a vehicle body that uses the rear structure 100.

[0103] When multiple parts are manufactured separately, for example, by press molding, and then joined together by spot welding or the like, softening of the heat-affected zone (HAZ softening) of the weld can occur. In particular, when each part is formed from a high-strength material such as a hot-stamped material, HAZ softening occurs due to welding, and the minimum Vickers hardness of the heat-affected zone is about 60% of the Vickers hardness of the unwelded part. Because the HAZ-softened part has relatively low strength, it may become a weak part during a collision.

[0104] In contrast, in this embodiment, the frame units 10, 20 and the floor panel 30 are molded as a single component by hot blow molding. Therefore, in at least the side frames 11, 12 of the frame unit 10, the minimum Vickers hardness of the heat-affected zone of the welded portion 44 is 70% or more of the Vickers hardness of the non-welded portion. Similarly, in at least the side frames 21, 22 of the frame unit 20, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 is 70% or more of the Vickers hardness of the non-welded portion. In other words, even if HAZ softening due to welding occurs in the composite blank 40, the composite blank 40 is heated to a high temperature of, for example, 900°C or higher for blow molding, and this heat treatment reduces HAZ softening. Therefore, in the side frames 11, 12, 21, 22, the reduction in hardness of the heat-affected zone relative to the non-welded portion is relatively small. This improves the crashworthiness of the rear structure 100.

[0105] In this embodiment, even in the cross member 13 of the frame unit 10, the minimum Vickers hardness value of the heat-affected zone of the welded portion 44 is 70% or more of the Vickers hardness of the non-welded portion. In other words, the reduction in hardness of the heat-affected zone of the welded portion 44 is small in all of the closed cross-sectional portions 14, 15 formed between the frame unit 10 and the floor panel 30. This allows the rear structure 100 to have better collision resistance performance.

[0106] Similarly, in each of the cross members 23 of the frame unit 20, the minimum Vickers hardness of the heat-affected zone of the welded portion 45 is 70% or more of the Vickers hardness of the non-welded portion. In other words, the reduction in hardness of the heat-affected zone of the welded portion 45 is small in all of the closed cross-sectional portions 24, 25 formed between the frame unit 20 and the floor panel 30. This allows the rear structure 100 to have better collision resistance performance.

[0107] When parts are manufactured by press forming, local thickness reduction usually occurs in the part. For example, in the case of a part with a roughly hat-shaped cross section, there is almost no thickness reduction in the top plate, but thickness reduction is likely to occur in the vertical walls and the ridge between the top plate and the vertical walls during press forming.

[0108] In contrast, in the present embodiment, the frame units 10, 20 are formed by hot blow molding. That is, in the forming process, the cross-sectional line lengths of the blanks 41, 42 in the composite blank 40 are entirely elongated by air blowing. As a result, the frame units 10, 20 can be formed with a uniform plate thickness. For example, in at least the side frames 11, 12 of the frame unit 10, the plate thicknesses measured at the center positions in the height direction of the vertical walls 112, 122, the center positions in the width direction of the top plates 111, 121, and the positions of the ridge lines 116, 126 are all 95% or less of the plate thickness of the flanges 114, 124. Similarly, in at least the side frames 21, 22 of the frame unit 20, the plate thicknesses measured at the center positions in the height direction of the vertical walls 212, 222, the center positions in the width direction of the top plates 211, 221, and the positions of the ridge lines 216, 226 are all 95% or less of the plate thickness of the flanges 214, 224. In this way, the plate thickness is efficiently distributed, with a reduction in plate thickness occurring in all locations of each of the side frames 11, 12, 21, and 22. Therefore, even if the blanks 41 and 42, which are the raw materials, are thinned, the minimum plate thickness required for the formed frame units 10 and 20 can be easily ensured, and the weight of the frame units 10 and 20 and the rear structure 100 can be reduced.

[0109] In order to further reduce the weight of the rear structure 100, it is preferable that the thickness of the cross member 13 of the frame unit 10, measured at the center position in the height direction of the vertical wall 132, the center position in the width direction of the top plate 131, and the position of the ridge line 136, is all 95% or less of the thickness of the flange 134. Similarly, it is preferable that the thickness of each cross member 23 of the frame unit 20, measured at the center position in the height direction of the vertical wall 232, the center position in the width direction of the top plate 231, and the position of the ridge line 236, is all 95% or less of the thickness of the flange 234.

[0110] In this embodiment, the plate material forming the upper frame unit 10 includes a plurality of metal plates 411F, 411R, 412F, 412R, and 413. In this case, the plate thickness and / or tensile strength of the frame unit 10 can be changed depending on the location.

[0111] In the frame unit 10, the front portion 11F and the rear portion 11R of the side frame 11 may be formed of metal plates 411F, 411R having different thicknesses and / or tensile strengths. The front portion 12F and the rear portion 12R of the side frame 12 may be formed of metal plates 412F, 412R having different thicknesses and / or tensile strengths. For example, the thickness and / or tensile strength of the metal plates 411F, 412F may be greater than that of the metal plates 411R, 412R so that the rigidity of the front portions 11F, 12F is higher than that of the rear portions 11R, 12R. As a result, for example, when a collision load is input from behind to the vehicle body including the rear structure 100, the rear portions 11R, 12R deform to absorb the collision energy, while deformation of the front portions 11F, 12F is suppressed, thereby protecting components near the front portions 11F, 12F.

[0112] In this embodiment, the plate material forming the lower frame unit 20 includes a plurality of metal plates 421F, 421R, 422F, 422R, and 423. In this case, the plate thickness and / or tensile strength of the frame unit 20 can be changed depending on the location.

[0113] In the frame unit 20, the front portion 21F and the rear portion 21R of the side frame 21 may be formed of metal plates 421F, 421R having different thicknesses and / or tensile strengths. The front portion 22F and the rear portion 22R of the side frame 22 may be formed of metal plates 422F, 422R having different thicknesses and / or tensile strengths. For example, the metal plates 421F, 422F may have a greater thickness and / or tensile strength than the metal plates 421R, 422R so that the front portions 21F, 22F have higher rigidity than the rear portions 21R, 22R. This allows, for example, when a collision load is input from behind to a vehicle body including the rear structure 100, the rear portions 21R, 22R to deform and absorb the collision energy, while suppressing deformation of the front portions 21F, 22F to protect components near the front portions 21F, 22F.

[0114] In this embodiment, at least the side frames 11, 12 of the frame unit 10 may be formed from hot-stamped material. Similarly, at least the side frames 21, 22 of the frame unit 20 may be formed from hot-stamped material. In other words, the side frames 11, 12, 21, 22 are formed from one or more steel plates and can have a tensile strength of 980 MPa or more after the forming process. The tensile strength of the side frames 11, 12, 21, 22 may be 2000 MPa or less. This increases the strength of the side frames 11, 12, 21, 22, so that, for example, when a vehicle body including the rear structure 100 is subjected to a rear collision, the side frames 11, 12, 21, 22 are more likely to support a collision load. The cross members 13, 23 may also be formed from steel plates having a tensile strength of 980 MPa or more after the forming process. In this case, the tensile strength of the cross members 13, 23 may be 2000 MPa or less.

[0115] Second Embodiment FIG. 6 is an exploded perspective view showing a composite blank 40A used in manufacturing a rear structure of a vehicle body in a second embodiment.

[0116] In the first embodiment, the composite blank 40 includes a blank 41 corresponding to the upper frame unit 10, a blank 42 corresponding to the lower frame unit 20, and a blank 43 corresponding to the floor panel 30. The composite blank 40A of this embodiment differs from the composite blank 40 of the first embodiment in that it does not include the blank 41.

[0117] 6, the composite blank 40A includes blanks 42 and 43. The blank 42 is configured similarly to the first embodiment. The blank 43 includes a panel main body equivalent portion 431, wheel house equivalent portions 432 and 433, and a pair of side frame equivalent portions 434 and 435.

[0118] The side frame equivalents 434, 435 are disposed on both sides of the panel main body equivalent 431. One side frame equivalent 434 is disposed between the panel main body equivalent 431 and one wheel house equivalent 432. The side frame equivalent 434 may be formed from a metal plate separate from the panel main body equivalent 431 and the wheel house equivalent 432. In this case, the side frame equivalent 434 is typically joined to the panel main body equivalent 431 and the wheel house equivalent 432 by welding. The side frame equivalent 434 may be butt-joined to the panel main body equivalent 431 and the wheel house equivalent 432 by laser welding, or may be joined by spot welding with the ends overlapping.

[0119] 6, the side frame portion 434 includes a plurality of metal plates 434F, 434R. The metal plates 434F, 434R are typically joined to each other by welding. The metal plates 434F, 434R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping.

[0120] The metal plates 434F and 434R are different in at least one of thickness and tensile strength. For example, the metal plate 434F is greater than the metal plate 434R in at least one of thickness and tensile strength.

[0121] The other side frame equivalent 435 is disposed between the panel main body equivalent 431 and the other wheel house equivalent 433. The side frame equivalent 435 may be formed from a metal plate separate from the panel main body equivalent 431 and the wheel house equivalent 433. In this case, the side frame equivalent 435 is typically joined to the panel main body equivalent 431 and the wheel house equivalent 433 by welding. The side frame equivalent 435 may be butt-joined to the panel main body equivalent 431 and the wheel house equivalent 433 by laser welding, or may be joined by spot welding with the ends overlapping.

[0122] 6 , the side frame equivalent portion 435 includes a plurality of metal plates 435F, 435R, similar to the side frame equivalent portion 434. The metal plates 435F, 435R are typically joined to each other by welding. The metal plates 435F, 435R may be butt-joined by laser welding, or may be joined by spot welding with their ends overlapping.

[0123] The metal plates 435F, 435R differ in at least one of the plate thickness and tensile strength. For example, the metal plate 435F is greater than the metal plate 435R in at least one of the plate thickness and tensile strength. The metal plate 435F of the side frame equivalent portion 435 typically has the same plate thickness and tensile strength as the metal plate 434F of the side frame equivalent portion 434. Similarly, the metal plate 435R of the side frame equivalent portion 435 typically has the same plate thickness and tensile strength as the metal plate 434R of the side frame equivalent portion 434. However, the metal plates 435F, 435R may differ from the metal plates 434F, 434R in at least one of the plate thickness and tensile strength.

[0124] In this embodiment, the panel main body equivalent portion 431 is a single metal plate (sub-blank). However, the panel main body equivalent portion 431 may include a plurality of metal plates (sub-blanks) joined together, as in the first embodiment.

[0125] 6, the blank 42 has a blowing hole 424 for blowing air, while the blank 43 has no blowing hole for blowing air. However, instead of or in addition to the blowing hole 424 of the blank 42, a blowing hole may be provided in the blank 43. When a blowing hole is provided in the blank 43, the blowing hole is disposed in each of the side frame equivalent portions 434, 435.

[0126] In this embodiment, similarly to the first embodiment, the rear structure of the vehicle body is manufactured by hot blow forming the composite blank 40A. That is, similarly to the first embodiment, the heating step of the composite blank 40A is carried out, and then the forming step of the composite blank 40A is carried out.

[0127] 7A and 7B , in the molding process, air is introduced between blanks 42 and 43 from each air blowing hole 424 ( FIG. 6 ) while a portion of composite blank 40A is held by upper mold 51 and lower mold 52. The air blow inflates side frame equivalent portions 434 and 435 of blank 43 toward upper mold 51, and also inflates side frame equivalent portions 421 and 422 and cross member equivalent portion 423 ( FIG. 6 ) of blank 42 toward lower mold 52.

[0128] The side frame equivalent portions 434, 435 of the blank 43 are expanded by air blowing and then pressed against the upper die 51. The panel main body equivalent portion 431 and wheel house equivalent portions 432, 433 of the blank 43 are pressed between the upper die 51 and the lower die 52. Meanwhile, the side frame equivalent portions 421, 422 and the cross member equivalent portion 423 ( FIG. 6 ) of the blank 42 are expanded by air blowing and then pressed against the lower die 52. The blanks 42, 43 are formed by the die 50 and then heat-removed (rapidly cooled) to be quenched. However, as in the first embodiment, the blanks 42, 43 do not necessarily need to be quenched by the die 50.

[0129] FIG. 8 is a cross-sectional view of the rear structure 100A after molding. Referring to FIG. 8, the side frame equivalent portions 434, 435 (FIGS. 7A and 7B) of the blank 43 are molded into the upper side frames 11, 12. In this embodiment, the side frames 11, 12 are included in the floor panel 30. The side frames 11, 12 are disposed on both sides of the panel main body 31. The side frames 11, 12 are disposed between the panel main body 31 and the wheel houses 32, 33. The side frames 11, 12 are provided continuously with the panel main body 31 and the wheel houses 32, 33. The side frames 11, 12 have a convex shape extending from the panel main body 31 on the side opposite the lower frame unit 20.

[0130] The side frames 21, 22 of the frame unit 20, together with the side frames 11, 12 of the floor panel 30, form a closed cross-sectional portion 24 perpendicular to the longitudinal direction of the vehicle body. In the closed cross-sectional portion 24, the relationship between the Vickers hardness of the heat-affected zone and the non-welded zone of the welded portion 45 of the side frames 21, 22 and the method for measuring the same are the same as in the first embodiment. In addition, in the closed cross-sectional portion 24, the distribution of the plate thickness of the side frames 21, 22 is also the same as in the first embodiment.

[0131] As in the first embodiment, the cross member 23 of the frame unit 20, together with the floor panel 30, forms a closed cross-sectional portion 25 that is perpendicular to the left-right direction of the vehicle body (FIG. 5).

[0132] The vehicle body rear structure 100A according to this embodiment and the manufacturing method thereof can also achieve the same effects as those of the first embodiment.

[0133] 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.

[0134] The rear structure 100 according to the first embodiment includes an upper frame unit 10 and a lower frame unit 20. However, the rear structure 100 does not necessarily have to include either of the frame units 10, 20. That is, the rear structure 100 may include one of the frame units 10, 20 as a first frame unit, while not including the other of the frame units 10, 20 as a second frame unit. In this case, the composite blank 40, which is the material for the rear structure 100, does not include the blank 41 or 42.

[0135] When the rear structure 100 includes both frame units 10 and 20 as in the first embodiment, either of the frame units 10 and 20 does not necessarily have to include a cross member. That is, it is sufficient for one of the frame units 10 and 20 to include a pair of first side frames and a cross member as a first frame unit, and the other of the frame units 10 and 20, which is the second frame unit, does not necessarily have to include a cross member. When either the frame unit 10 or 20 does not include a cross member, either of the blanks 41 and 42 in the composite blank 40, which is the material for the rear structure 100, does not include a portion corresponding to a cross member. In other words, the blank 41 or 42 may be a blank divided into one portion corresponding to a side frame and the other portion corresponding to a side frame.

[0136] The rear structure 100A according to the second embodiment includes a lower frame unit 20 and a floor panel 30. However, the rear structure 100A may include an upper frame unit 10 instead of the lower frame unit 20. In this case, the floor panel 30 includes lower side frames 21, 22. The side frames 11, 12 of the frame unit 10 are welded to the floor panel 30 to form a closed cross-sectional portion 14 together with the side frames 21, 22 of the floor panel 30.

[0137] In the first embodiment, the blank 41 of the composite blank 40, i.e., the upper frame unit 10, includes five metal plates 411F, 411R, 412F, 412R, and 413. However, the number of metal plates included in the blank 41 and the frame unit 10 is not limited to this. The blank 41 and the frame unit 10 may include two or more metal plates joined together, or may be formed from a single metal plate. When the blank 41 and the frame unit 10 include two or more metal plates, the arrangement, thickness, and tensile strength of these metal plates can be changed as appropriate.

[0138] Similarly, the blank 42 and the frame unit 20 may include two or more metal plates joined together, or may be formed from a single metal plate. When the blank 42 and the frame unit 20 include two or more metal plates, the arrangement, thickness, and tensile strength of these metal plates can be changed as appropriate. The blank 43 and the floor panel 30 may also include two or more metal plates joined together, or may be formed from a single metal plate. When the blank 43 and the floor panel 30 include two or more metal plates, the arrangement, thickness, and tensile strength of these metal plates can be changed as appropriate.

[0139] As shown in Fig. 9, the composite blank 40 according to the first embodiment may further include at least one patch material 46. The patch material 46 is overlaid on one of the blanks 41, 42, and 43 and joined by spot welding, laser welding, or the like. The composite blank 40 is molded into the rear structure 100 with the patch material 46 included. The patch material 46 may be disposed in a location in the rear structure 100 that requires reinforcement. Although not shown in the drawings, the composite blank 40A according to the second embodiment may also be provided with at least one patch material 46.

[0140] In the composite blank 40 according to the first embodiment, air blowing holes 414 are formed in the portions of the side frame equivalents 411, 412 of the blank 41 that will be molded into the top plates 111, 121. Also, air blowing holes 424 are formed in the portions of the side frame equivalents 421, 422 of the blank 42 that will be molded into the top plates 211, 221. However, the positions of the air blowing holes 414, 424 are not limited to this.

[0141] For example, as shown in FIG. 10 , by extending a portion of the flange 114 of the side frame corresponding portion 411 and a portion of the flange 125 of the side frame corresponding portion 412 outward and providing a convex bead, the bead can be used as the air inlet 414. In this case, air is blown into the side frame corresponding portions 411, 412 from their widthwise end faces through a supply passage 511 communicating with the air inlet 414. Similarly, by extending a portion of the flange 214 of the side frame corresponding portion 421 and a portion of the flange 225 of the side frame corresponding portion 422 outward and providing a convex bead, the bead can be used as the air inlet 424. In this case, air is blown into the side frame corresponding portions 421, 422 from their widthwise end faces through a supply passage 521 communicating with the air inlet 424. Although not shown, a configuration similar to that shown in FIG. 10 can also be applied to the composite blank 40A and mold 50 according to the second embodiment.

[0142] 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.

[0143] In order to confirm the effects of the present disclosure, a rear structure having a configuration similar to that of rear structure 100 according to the first embodiment was fabricated by an integrated molding method using hot blow molding with a composite blank 40 shown in Figures 11 and 12 (Example). On the other hand, a rear structure serving as a comparative example also has a configuration similar to that of rear structure 100 according to the first embodiment, but in the comparative example, each component, such as the side frames and floor panel, is formed individually by press molding, and then these components are combined and joined by spot welding to fabricate the rear structure.

[0144] 11 and 12 , the composite blank 40 includes blanks 41, 42, and 43 overlapped and joined by spot welding. In the composite blank 40, the blank 41 corresponding to the upper frame unit was a hot-stamped material having a thickness of 1.6 mm and a tensile strength (after forming) of 1.5 GPa. In the blank 42 corresponding to the lower frame unit, rear portions 421R and 422R of the side frame equivalent portions 421 and 422 were hot-stamped material having a thickness of 1.0 mm and a tensile strength (after forming) of 1.5 GPa, and the remaining portions were hot-stamped material having a thickness of 1.6 mm and a tensile strength (after forming) of 1.5 GPa. A plurality of patch materials 46 having a thickness of 1.0 mm and a tensile strength of 590 MPa were arranged in the blank 42. In the blank 43 corresponding to the floor panel, the front portion 431F of the panel body equivalent portion 431 was made of hot-stamped material with a thickness of 1.6 mm and a tensile strength (after forming) of 1.5 GPa, the middle portion 431C was made of hot-stamped material with a thickness of 0.6 mm and a tensile strength (after forming) of 1.0 GPa, and the rear portion 431R was made of hot-stamped material with a thickness of 1.0 mm and a tensile strength (after forming) of 1.5 GPa. Furthermore, in the blank 43, the wheel house equivalent portions 432 and 433 were made of hot-stamped material with a thickness of 0.8 mm and a tensile strength (after forming) of 1.5 GPa. The above materials and thickness arrangements were the same in the example and the comparative example.

[0145] The Vickers hardness of the spot-welded cross section at the closed cross section formed by the upper side frame (frame unit) and floor panel and the plate thickness of each part of the side frame were investigated for a rear structure (Example) formed from composite blank 40. For a Comparative Example in which the side frame and floor panel were press-formed separately and then joined by spot welding after forming, the Vickers hardness of the spot-welded cross section at the closed cross section formed by the side frame and floor panel and the plate thickness of each part of the side frame were investigated.

[0146] Figure 13 shows the distribution of Vickers hardness at the spot-welded cross section of the closed cross section formed by the side frame and floor panel for the example and comparative example. In this analysis, Vickers hardness was measured for each cross section of the side frame for the example and comparative example at 0.2 mm intervals along a straight line located 1 / 4 of the way through the plate thickness from the surface on the weld center side in accordance with JIS Z 2244-1:2020 (test force: 0.49 N). As shown in Figure 13 , a heat-affected zone 442 exists around the weld metal portion (nugget portion) 441 of the spot-welded weld 44. At a point approximately 1.0 mm away from the outer edge of the weld metal portion 441, a decrease in Vickers hardness (softening) due to heat influence occurs. However, at a point further away from the outer edge of the weld metal portion 441, the Vickers hardness converges to the base metal hardness of 500 Hv.

[0147] 13 , when the Vickers hardness was measured at 0.2 mm intervals from the weld center toward both outer sides of the weld metal zone 441 to positions spaced apart by, for example, 7.0 mm or more, and the minimum measured Vickers hardness was taken as the minimum Vickers hardness of the heat-affected zone 442, in the comparative example, the minimum Vickers hardness of the heat-affected zone 442 was approximately 300 Hv. In the comparative example, the minimum Vickers hardness of the heat-affected zone 442 was approximately 60% of the Vickers hardness (base material hardness): 500 Hv of the non-welded portion of the side frame.

[0148] On the other hand, in the example, the minimum Vickers hardness of the heat-affected zone 442 was 420 Hv, which was higher than the minimum Vickers hardness of the heat-affected zone 442 in the comparative example. In the example, the minimum Vickers hardness of the heat-affected zone 442 was 84% ​​of the Vickers hardness (base material hardness) of 500 Hv of the non-welded portion of the side frame. For the cross member, the relationship between the minimum Vickers hardness of the heat-affected zone 442 and the Vickers hardness of the non-welded portion was similar to that of the side frame.

[0149] In this way, when the frame unit and floor panel are integrated from the blank stage and then hot blow molded, softening of the heat-affected zone 442 is reduced. It has been confirmed that in a rear structure in which the frame unit and floor panel are integrated, the minimum Vickers hardness of the heat-affected zone 442 in the frame portion is ensured to be 70% or more of the Vickers hardness of the non-welded portion.

[0150] For the example, the plate thickness measured at the flange position, the center position in the height direction of the vertical wall, the position of the ridge line, and the center position in the width direction of the tabletop on the side frame is shown in Figure 14. For the comparative example, the plate thickness measured at the flange position, the center position in the height direction of the vertical wall, the position of the ridge line, and the center position in the width direction of the tabletop on the side frame is shown in Figure 15. In Figures 14 and 15, the plate thickness of each portion is shown as a ratio to the plate thickness of the flange.

[0151] As shown in Figure 14, in the example, the thicknesses measured at the center of the vertical wall height, the ridgeline, and the center of the width of the top plate were all 95% or less of the flange thickness. In other words, in the example, a uniform thickness reduction occurred in the top plate, vertical wall, and ridgeline. For the cross member, the relationship between the thicknesses measured at the center of the vertical wall height, the ridgeline, and the center of the width of the top plate and the thickness measured at the flange was similar to that of the side frame.

[0152] On the other hand, as shown in Figure 15, in the comparative example, the thickness measured at the center position in the height direction of the vertical wall and the center position in the width direction of the top plate exceeded 95% of the thickness of the flange, while the thickness measured at the ridge line position was 90% of the thickness of the flange. In other words, in the comparative example, a local thickness reduction occurred at the ridge line.

[0153] In this example, it was confirmed that when the frame unit is integrated with the floor panel from the blank stage and hot blow-molded, the plate thickness of the frame portion is made uniform.

[0154] 100, 100A: Rear structure 10: Frame unit 11, 12: Side frame 11F, 12F: Front section 11R, 12R: Rear section 13: Cross member 14, 15: Closed cross section section 20: Frame unit 21, 22: Side frame 21F, 22F: Front section 21R, 22R: Rear section 23: Cross member 24, 25: Closed cross section section 30: Floor panel 31: Panel body 32, 33: Wheel house 40, 40A: Composite blank 41: Blank 411, 412: Side frame equivalent section 413: Cross member equivalent section 44, 45: Welded section 42: Blank 421, 422: Side frame equivalent section 423: Cross member equivalent section 43: Blank 50: Mold

Claims

1. A vehicle body rear structure comprising: a floor panel formed from a metal plate; and a first frame unit formed from a metal plate, the first frame unit including a pair of first side frames and a first cross member connecting the first side frames, the first frame unit being disposed on the floor panel, the first side frames and the first cross member being each welded to the floor panel to form a first closed cross section together with the floor panel, the first closed cross section formed by each of the first side frames and the floor panel being a cross section perpendicular to the fore-aft direction of the vehicle body when the rear structure is assembled to the vehicle body, the first closed cross section formed by the first cross member and the floor panel being a cross section perpendicular to the left-right direction of the vehicle body when the rear structure is assembled to the vehicle body, and in the first closed cross section, a minimum Vickers hardness value of a heat-affected zone of a welded portion of the first frame unit to the floor panel is 70% or more of a Vickers hardness value of a non-welded portion of the first frame unit.

2. A rear structure for a vehicle body as described in claim 1, wherein the first side frame and the first cross member each include a top plate facing the floor panel, a vertical wall connected to the top plate via a ridge portion, and a flange connected to the vertical wall on the opposite side of the top plate and welded to the floor panel, and in the first closed cross-sectional portion, the plate thicknesses measured at the center position in the height direction of the vertical wall, the center position in the width direction of the top plate, and the position of the ridge portion are all 95% or less of the plate thickness of the flange.

3. A vehicle body rear structure as claimed in claim 1, wherein the plate material forming the first frame unit includes a plurality of metal plates joined together.

4. A rear structure for a vehicle body as described in claim 3, wherein each of the first side frames includes a front section and a rear section that is positioned rearward of the front section when the rear structure is assembled to the vehicle body, and the front section and the rear section are formed from metal plates that differ from each other in at least one of plate thickness and tensile strength.

5. A rear structure for a vehicle body according to claim 1, wherein each of the first side frames has a tensile strength of 980 MPa or more.

6. A vehicle body rear structure as described in claim 1, further comprising a second frame unit including a pair of second side frames and formed from a metal plate, the second frame unit being arranged on the floor panel on the opposite side of the first frame unit, the second side frames being each welded to the floor panel to form a second closed cross-sectional portion together with the floor panel, the second closed cross-sectional portion formed by each of the second side frames and the floor panel being a cross-section perpendicular to the fore-and-aft direction of the vehicle body when the rear structure is assembled to the vehicle body, and the minimum Vickers hardness value of the heat-affected zone of the welded portion of the second frame unit to the floor panel is 70% or more of the Vickers hardness of the non-welded portion of the second frame unit.

7. A rear structure for a vehicle body as described in claim 6, wherein the second frame unit further includes a second cross member connecting the second side frames, the second side frame and the second cross member are each welded to the floor panel to form the second closed cross section together with the floor panel, and the second closed cross section formed by the second cross member and the floor panel is a cross section perpendicular to the left-right direction of the vehicle body when the rear structure is assembled to the vehicle body.

8. A vehicle body rear structure as described in claim 1, wherein the floor panel includes a panel body and a pair of second side frames that are provided contiguous with the panel body and correspond to the first side frames, and each of the first side frames forms the first closed cross-sectional portion together with the corresponding second side frame.

9. A vehicle body rear structure according to claim 1, wherein the floor panel includes a panel body and a pair of wheel houses disposed on both sides of the panel body.

10. A composite blank used in the manufacture of a rear structure of a vehicle body, comprising: a first blank corresponding to a floor panel; and a second blank including a pair of side frame equivalent portions corresponding to a pair of side frames, and a cross member equivalent portion corresponding to a cross member and connecting the side frame equivalent portions, wherein the second blank is superimposed on the first blank, and the outer peripheries of the side frame equivalent portions and the cross member equivalent portions are welded to the first blank.

11. A composite blank according to claim 10, wherein at least one of the first blank and the second blank comprises a plurality of metal plates joined together, the metal plates having at least one of different thicknesses and tensile strengths.

12. A method for manufacturing a rear structure of a vehicle body, comprising the steps of: preparing a composite blank as described in claim 10 or 11; heating the composite blank; placing the heated composite blank in a mold and using the mold to mold the first blank into a floor panel and mold the second blank into a frame unit including a pair of side frames and a cross member connecting the side frames; wherein in the molding step, the side frame equivalent portion and the cross member equivalent portion are molded into the side frame and the cross member, respectively, by hot blow molding.

Images