Automotive subframe structure and stiffening part for automotive subframe structure

Abstract

To provide a sub-frame structure of a vehicle and a stiffening component of a sub-frame structure of a vehicle, capable of achieving high stiffness.SOLUTION: A sub-frame structure 1 of a vehicle includes: a pair of right and left side members 3 having a hollow member of a substantially rectangular cross section extending in a vehicle longitudinal direction and having a suspension arm connection part 11; and at least one cross member 5 having a hollow member of a substantially rectangular cross section extending in a vehicle longitudinal direction and connecting with the side members 3. The side members 3 and the cross member 5 are connected with each other through a bent part 7. There is also a stiffened part 9 which is stiffened by integrally covering a suspension arm connection part 11a in the side member 3 and a curved ridge part 7a in the bent part 7 from the suspension arm connection part 11a to the cross member 5.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an automobile subframe structure having improved rigidity, and to a stiffening part for an automobile subframe structure. [Background technology]

[0002] Some automobile bodies are provided with a subframe that connects the suspension arm to the body. The subframe generally has a pair of left and right side members extending in the front-rear direction of the vehicle, and the left and right ends of a cross member extending in the left-right direction of the vehicle are connected to the pair of left and right side members. Some subframes have a pair of front and rear cross members that are lattice-shaped when viewed from above, and some have a single cross member on the rear side of the vehicle that is H-shaped when viewed from above. The side members and the cross members are separate parts, and are not limited to those that are connected by separate parts, but there are also subframes in which the side members and the cross members are integrated.

[0003] Subframes are required to have high rigidity because they deform due to the load input from the suspension arms when the vehicle is moving. Increasing the rigidity of subframes leads to improved ride comfort and increases the value of the vehicle. At the same time, from the perspective of the energy efficiency of the vehicle, subframes are also required to be lightweight.

[0004] In order to reduce the weight of a subframe, if the vehicle body parts that make up the subframe are manufactured by press-forming metal plate materials (metal plates), it is effective to reduce the plate thickness. However, reducing the plate thickness generally reduces the rigidity, so there is a trade-off between increasing the rigidity and reducing the weight of the subframe. In addition, vehicle body performance that decreases when the thickness of the metal plate is reduced includes collision performance, deformation strength when a single load is input, fatigue strength, etc., and these vehicle body performances can be ensured by applying high-strength metal plates. However, since the elastic modulus of a general metal plate hardly changes even when the strength is increased, it is not expected that the rigidity of the subframe will be increased by increasing the strength of the metal plate. For this reason, several technologies have been proposed to achieve high rigidity of the subframe without relying on the strength of the metal plate.

[0005] For example, Patent Documents 1 and 2 propose a technique for improving the rigidity of an automobile subframe structure formed of side members and cross members in a grid shape by providing a new member inside the connection between the side members and the cross members. Patent Document 1 states that the rigidity can be improved by providing a tubular member connecting the upper and lower sides inside each of the side members and the cross members at the connection between the side members and the cross members. Patent Document 2 states that the rigidity can be improved by filling or providing a resin stiffening member having a columnar portion connecting the upper and lower sides inside each of the side members and the cross members at the connection between the side members and the cross members.

[0006] Furthermore, Patent Document 3 discloses a subframe for a front or rear body of an automobile having a reinforcing structure for improving torsional rigidity, configured so that a charging module can be incorporated therein. According to Patent Document 3, the charging module incorporated in the reinforcing structure is one component of the reinforcing structure, and the charging module itself contributes to reinforcing the subframe. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-083018 A [Patent Document 2] JP 2020-075656 A [Patent Document 3] Patent No. 5822896 Summary of the Invention [Problem to be solved by the invention]

[0008] The technologies disclosed in Patent Documents 1 and 2 have the problem that new members (stiffening parts) are added and installed or filled in to improve the rigidity of the subframe structure, which increases the number of parts, leading to increased manufacturing costs and reduced productivity. In addition, the technology disclosed in Patent Document 3 can improve the torsional rigidity of the subframe by installing a reinforcing structure with shear strength, but has the problem of increasing the vehicle weight.

[0009] The present invention has been made to solve the above-mentioned problems, and has an object to provide an automobile subframe structure and a stiffening part for an automobile subframe structure that improves rigidity without increasing the number of parts and minimizing weight increase. [Means for solving the problem]

[0010] (1) A subframe structure for an automobile according to the present invention includes a pair of left and right vehicle longitudinal extending portions having a hollow member with a substantially rectangular cross section extending in a vehicle longitudinal direction, and a suspension arm connecting portion provided thereon, and at least one vehicle lateral extending portion having a hollow member with a substantially rectangular cross section extending in the vehicle lateral direction, the vehicle longitudinal extending portions and the vehicle lateral extending portions being connected to each other via a curved portion, The suspension arm is characterized in that a stiffening part is provided which integrally covers and stiffens the suspension arm connection portion and the curved ridge portion of the curved portion from the suspension arm connection portion to the vehicle left-right direction extending portion.

[0011] (2) In the above (1), The stiffening part also covers a ridge portion on the outer side of the vehicle of the vehicle longitudinal direction extending portion of the suspension arm connecting portion.

[0012] (3) In the above (1) or (2), The vehicle longitudinal direction extending portion and the vehicle lateral direction extending portion are characterized in that they are manufactured using steel plates having a tensile strength of 590 MPa or more.

[0013] (4) A stiffening part for a subframe structure of an automobile according to the present invention has a hollow member with a substantially rectangular cross section extending in the fore-and-aft direction of the vehicle, and includes a pair of left and right vehicle fore-and-aft extending portions provided with a suspension arm connecting portion, and at least one vehicle left-and-right extending portion having a hollow member with a substantially rectangular cross section extending in the left-and-right direction of the vehicle, and stiffens a subframe structure of an automobile in which the vehicle fore-and-aft extending portions and the vehicle left-and-right extending portions are connected via a curved portion, The subframe structure is characterized in that a local stiffening portion which covers the suspension arm connecting portion and locally stiffens the suspension arm connecting portion, and a global stiffening portion which extends from the local stiffening portion along the curved ridge portion of the curved portion to the vehicle left-right direction extending portion, covers the curved ridge portion, and globally stiffens the subframe structure, are integrally formed.

[0014] (5) In the above (4), The local stiffening portion also covers a ridge portion on the vehicle outer side of the vehicle longitudinal direction extending portion of the suspension arm connecting portion. Effect of the Invention

[0015] In the present invention, a stiffening part is provided that integrally covers and stiffens the suspension arm connecting part and the curved ridge part at the curved part where the vehicle fore-aft extending part and the vehicle left-right extending part connect from the suspension arm connecting part to the vehicle left-right extending part, thereby suppressing deformation at the curved part and improving the rigidity of the entire subframe structure without increasing the number of parts or reducing productivity. Furthermore, according to the present invention, improvements in collision strength, deformation strength when a single load is input, and fatigue strength can also be expected. [Brief description of the drawings]

[0016] [Figure 1] 1 is a diagram illustrating a subframe structure of an automobile according to an embodiment of the present invention; [Diagram 2] 1 is an enlarged view showing a stiffening part provided at a portion where a side member, which is a portion extending in the vehicle's fore-and-aft direction, is connected to a cross member, which is a portion extending in the vehicle's left-right direction, in a subframe structure for an automobile according to an embodiment of the present invention. FIG. [Diagram 3] FIG. 2 is a diagram showing a stiffening part according to the present invention, the stiffness of which was investigated in Example 1, and a subframe structure to which the stiffening part is applied. [Figure 4] FIG. 11 is a diagram showing a conventional stiffening part that was the subject of a stiffness investigation in Example 2, and a subframe structure to which the stiffening part is applied. [Diagram 5] 5A and 5B are diagrams showing conditions for analysis by a finite element method for calculating the rigidity of a subframe structure in the first embodiment ((a) torsional conditions, (b) bending conditions). [Figure 6] FIG. 4 is a diagram showing conditions in a crash analysis for calculating crash characteristics of a subframe structure in the first embodiment. [Figure 7] FIG. 1 is a diagram illustrating a configuration of a conventional automobile subframe structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] <Background to the invention> Before describing the embodiments of the present invention, the background to the invention will be described based on an automobile subframe structure 41 (hereinafter simply referred to as "subframe structure 41") shown in FIG. 7 as an example.

[0018] A subframe structure 41 shown in FIG. 7 includes a pair of left and right side members 3, and a pair of front and rear cross members 5 (5A, 5B). The side member 3 has a hollow member with a substantially rectangular cross section extending in the vehicle longitudinal direction, and is provided with two suspension arm connecting portions 11a, 11b in the vehicle longitudinal direction to which a suspension arm is connected. The cross member 5 has a hollow member with a substantially rectangular cross section that extends in the left-right direction of the vehicle, and is disposed between a pair of left and right side members 3. In the subframe structure 41, the side member 3 and the cross member 5B on the vehicle rear side are connected via a curved portion 7. The curved portion 7 has a curved surface on the vehicle interior side, and has a curved ridge portion 7a that connects the ridge portion 3a on the vehicle interior side of the side member 3 and the ridge portion 5a on the vehicle front side of the cross member 5. The subframe structure 41 shown in FIG. 7 is manufactured by integrating a pair of left and right side members 3 and a pair of front and rear cross members 5, and the curved portion 7 is formed in the side member 3.

[0019] The inventors have thoroughly studied effective stiffening locations for improving the rigidity of subframe structure 41. As a result, they have found that it is possible to efficiently improve the rigidity of subframe structure 1 by stiffening curved ridge portion 7a on the vehicle interior side of L-shaped curved portion 7 that connects side member 3 and cross member 5B.

[0020] Conventionally, a stiffening part 43 has been used to cover the suspension arm connecting part 11a on the vehicle front side provided on the side member 3 in order to prevent local deformation when connecting the suspension arm. The inventors came up with the idea of ​​extending this stiffening part 43 toward the curved ridge line 7a side to integrally cover the suspension arm connecting part 11a and the curved ridge line 7a from the suspension arm connecting part 11a to the cross member 5B. They then found that by integrally covering and stiffening the suspension arm connecting part 11a and the curved ridge line 7a, it is possible to improve the rigidity of the subframe structure without increasing the number of parts. The present invention has been made based on such studies, and the specific configuration thereof will be described below.

[0021] <Automobile subframe structure> A subframe structure for an automobile according to an embodiment of the present invention will be described below. Fig. 1 shows an overall view of a subframe structure 1 as an example seen from above the vehicle. The subframe structure 1 comprises a pair of left and right side members 3 and a pair of front and rear cross members 5 (vehicle front side: 5A, vehicle rear side: 5B), with the side members 3 and the vehicle rear cross member 5B being connected via a curved portion 7. Here, the subframe structure 1 is formed in a lattice shape when viewed from above, with the pair of left and right side members 3 and the pair of front and rear cross members 5. The subframe structure 1 is provided with stiffening parts 9 as shown in Fig. 1.

[0022] <Extension portion in the vehicle's fore-and-aft direction> The side member 3 is an extending portion in the vehicle longitudinal direction, has a hollow member with a substantially rectangular cross section extending in the vehicle longitudinal direction, and is provided with a suspension arm connecting portion 11 to which a suspension arm is connected. The side member 3 has ridge portions 3a, 3b that form corners of the substantially rectangular cross section on the inside and outside of the vehicle in the left-right direction of the vehicle. Each side member 3 is provided with suspension arm connecting portions 11, namely, suspension arm connecting portions 11a, 11b at two locations, front and rear, on the rear side of the vehicle.

[0023] <Extension portion in the left-right direction of the vehicle> The cross member 5 is an extending portion in the left-right direction of the vehicle, and has a hollow member with a substantially rectangular cross section extending in the left-right direction of the vehicle, and a cross member 5A on the front side of the vehicle and a cross member 5B on the rear side of the vehicle are disposed between a pair of left and right side members 3. The cross member 5B on the rear side of the vehicle has a ridge portion 5a on the front side of the vehicle that is a corner portion of the substantially rectangular cross section.

[0024] <Bend section> The curved portion 7 is a portion curved toward the vehicle interior side from the side member 3 to the cross member 5B near the suspension arm connection portion 11. In the case of a subframe structure 1 having a pair of front and rear cross members 5A, 5B like the subframe structure 1 shown in FIG. 1, the cross member 5B near the suspension arm connection portion 11 refers to the one on the rear side of the vehicle. This is because the suspension arm connection portion 11 is provided on the side member 3 on the rear side of the vehicle. The curved portion 7 has a curved ridgeline portion 7a that connects the ridgeline portion 3a on the vehicle inner side of the side member 3 and the ridgeline portion 5a on the vehicle front side of the cross member 5B. In the subframe structure 1, the curved portion 7 is formed in the side member 3.

[0025] <Stiffening parts> The stiffening part 9 integrally covers and stiffens the suspension arm connecting portion 11a and the curved ridge portion 7a of the curved portion 7 from the suspension arm connecting portion 11a to the cross member 5B.

[0026] In this embodiment, as shown in FIG. 1, the stiffening part 9 has a local stiffening portion 9a and a global stiffening portion 9b. The local stiffening portion 9a covers the suspension arm connecting portion 11a and locally stiffens the suspension arm connecting portion 11a. The global stiffening portion 9b extends from the local stiffening portion 9a along the curved ridge portion 7a in the curved portion 7 to the cross member 5B, covering the curved ridge portion 7a and stiffening the subframe structure 1 globally.

[0027] The reason why the rigidity is improved in the automobile subframe structure 1 according to this embodiment will be explained below.

[0028] In the subframe structure 1, the curved portion 7 where the side member 3 and the cross member 5 are connected is a portion whose shape changes uniformly in the vehicle longitudinal direction or the vehicle lateral direction (hereinafter referred to as a "shape-changing portion"). When a load is input to this type of subframe structure 1 through suspension arm connection part 11, the stress distribution becomes disrupted in the shape-changing part compared to a part of uniform shape, causing localized stress increase and making it more likely for stress concentration to occur. Therefore, deformation is concentrated in the shape-changing part due to stress concentration, causing a decrease in the rigidity of the subframe structure 1.

[0029] In addition, since both the side members 3 and the cross members 5 are hollow members having a substantially rectangular cross section, when a bending or torsional load is applied to the subframe structure 1, stress concentration occurs at the ridges which are the corners of the substantially rectangular cross section. Therefore, as a portion for stiffening the curved portion 7, it is important to stiffen the ridges at the portions where the shape changes, i.e., the curved ridges 7a at the curved portion 7.

[0030] Therefore, in the subframe structure 1 according to the present embodiment, similar to the stiffening part 43 (FIG. 7) provided in the conventional subframe structure 41, the local stiffening part 9a of the stiffening part 9 covers the suspension arm connecting part 11 so as to fit along the upper surface of the side member 3. This makes it possible to increase the apparent plate thickness of the suspension arm connecting part 11 and improve the local rigidity. Furthermore, the large area stiffening portion 9b of the stiffening component 9 covers the curved ridge portion 7a of the curved portion 7 from the suspension arm connecting portion 11a to the cross member 5B. This makes it possible to suppress deformation of the curved portion 7 and improve the rigidity of the entire subframe structure 1.

[0031] In this way, the subframe structure 1 is provided with stiffening parts 9 that integrally cover and stiffen the suspension arm connecting part 11a and the curved ridge part 7a of the curved part 7 from the suspension arm connecting part 11a to the cross member 5. This makes it possible to suppress deformation of the curved part 7 where the side member 3 and the cross member 5 are connected and where stress concentration is likely to occur, suppressing deformation of the curved part without increasing the number of parts or reducing productivity, and improving the rigidity of the entire subframe structure 1. Moreover, with the subframe structure 1 according to the present embodiment, improvements can be expected in terms of collision strength, deformation strength when a single load is input, and fatigue strength.

[0032] Furthermore, in the present invention, it is more preferable that the stiffening part 9 is provided so as to also cover the ridgeline portion 3b on the vehicle exterior side of the side member 3, as in the case of the local stiffening part 9a shown in Fig. 1. This results in a structure in which the ridgeline portion 3b on the vehicle exterior side covered by the local stiffening part 9a and the curved ridgeline portion 7a covered by the global stiffening part 9b are connected to transmit a load, thereby improving the stiffening ability of the stiffening part 9.

[0033] In this embodiment, since the stiffening part 9 is intended to improve the rigidity of the subframe structure 1, it is not necessary to use a high-strength metal plate for the stiffening part 9; it is sufficient if the stiffening part 9 is a metal plate having a tensile strength of 270 MPa or more.

[0034] In contrast to this, by providing the stiffening part 9, the rigidity of the entire subframe structure 1 can be improved, and therefore, for parts other than the stiffening part 9 (side member 3, cross member 5), it is possible to use thinner metal plates in terms of the rigidity of the entire subframe structure 1. Therefore, in order to reduce the weight of the subframe structure 1 while maintaining its strength, it is advisable to use steel plates having a tensile strength of 590 MPa or more for the portions of the subframe structure 1 other than the stiffening parts 9.

[0035] In the above description, the large-area stiffening portion 9b of the stiffening part 9 covers the curved ridgeline portion 7a on the vehicle upper side of the curved portion 7. Here, the curved portion 7 is a portion that connects the side member 3 made of a hollow member having a substantially rectangular cross-sectional shape to the cross member 5 made of a hollow member having a substantially rectangular cross-sectional shape. Therefore, in addition to the curved ridgeline portion 7a on the vehicle upper side, the curved portion 7 may also have a curved ridgeline portion on the vehicle lower side that connects the ridgeline portion on the vehicle inner side of the side member 3 and the ridgeline portion on the vehicle front side of the cross member 5.

[0036] In such a case, the global stiffening portion of the stiffening part may extend from the local stiffening portion so as to cover the curved ridge portion on the vehicle lower side of the curved portion. In other words, the subframe structure according to the present invention is not limited to the curved ridge portion that is stiffened by the global stiffening portion of the stiffening part, as long as the global stiffening portion stiffens the curved ridge portion that is easily covered by the global stiffening portion.

[0037] In addition, the stiffening part 9 shown in FIG. 1 is provided on the outer surface sides of the side members 3, cross members 5 and curved portions 7 that constitute the subframe structure 1, but it may also be provided on the inner surface sides of these.

[0038] The subframe structure 1 shown in Fig. 1 is a structure in which a suspension arm is connected to a suspension arm connection portion 11 provided on the outer surface (the surface on the vehicle outer side and above the vehicle) of a side member 3. However, the subframe structure according to the present invention may also be one in which an opening is provided on the side surface of the side member on the vehicle outer side, and the suspension arm is inserted into the opening and fastened to the side member with a bolt or the like. Even in this case, by providing a stiffening part having a local stiffening portion that covers the suspension arm connection portion and a global stiffening portion that extends from the suspension arm connection portion and covers the curved ridge portion, deformation of the curved portion is suppressed and rigidity is improved.

[0039] It should be noted that the subframe structure 1 in this embodiment is manufactured by integrating the side member 3 and the cross member 5, however, the present invention is not limited to a structure in which the side member and the cross member are integrated. Furthermore, in the subframe structure 1, the curved portion 7 is formed in the side member 3, but the curved portion may be formed in the cross member.

[0040] Furthermore, the above description has been directed to the subframe structure 1 in which a pair of left and right side members 3 and a pair of front and rear cross members 5 are connected via curved portions 7 to form a parallel cross shape, as shown in Fig. 1. However, the present invention may also be directed to a subframe structure 21 in which a pair of left and right side members 23 and one cross member 25 are connected via curved portions 27 to form an H-shape, as shown in Fig. 2.

[0041] In the H-shaped subframe structure 21, the side members 23 are provided with suspension arm connection parts 31a, 31b at two positions, front and rear, in the vehicle longitudinal direction, as suspension arm connection parts 31. In addition, the cross members 25 are disposed in the side members 23 near the suspension arm connection parts 31 to which the suspension arms 33 are connected.

[0042] In the H-shaped subframe structure 21 as well, when a load is input from the suspension arm connecting portion 31, stress concentration occurs at the curved portion 27 connecting the side member 23 and the cross member 25.

[0043] Therefore, in the H-shaped subframe structure 21, a stiffening part 29 is provided that integrally covers the suspension arm connection portion 31a and the curved ridge portion 27a of the curved portion 27 from the suspension arm connection portion 31 to the cross member 25. As a result, when a load is input to the subframe structure 21, deformation at the curved portion 27 can be suppressed, and the rigidity of the subframe structure 21 can be improved.

[0044] <Stiffening parts for automobile subframe structures> Although the above description has been directed to a subframe structure 1 provided with a stiffening part 9, in another aspect of this embodiment, it is possible to configure the subframe structure 1 as a stiffening part 9 that stiffens the subframe structure 1.

[0045] That is, as shown in Fig. 1 described above, the stiffening part 9 of the subframe structure of an automobile is provided with a side member 3 and a cross member 5, and stiffens the subframe structure 1 in which the side member 3 and the cross member 5B are connected via a curved portion 7. The stiffening part 9 is formed integrally with a local stiffening portion 9a and a global stiffening portion 9b. and

[0046] The local stiffening portion 9a covers the suspension arm connecting portion 11a and locally stiffens the suspension arm connecting portion 11a. The global stiffening portion 9b extends from the local stiffening portion 9a along the curved ridge portion 7a in the curved portion 7 to the cross member 5, covering the curved ridge portion 7a and thereby stiffening the subframe structure 1 globally.

[0047] Furthermore, the stiffening component 9 can suppress deformation at the curved portion 7 where stress concentration is likely to occur when a load is input to the subframe structure 1, thereby improving the rigidity of the entire subframe structure 1.

[0048] As described above, it is more preferable that the local stiffening portion 9a of the stiffening part 9 also covers the ridgeline portion 3b on the vehicle exterior side of the side member 3. This makes it possible to achieve a structure in which the ridgeline portion 3b on the vehicle exterior side covered by the local stiffening portion 9a and the curved ridgeline portion 7a covered by the global stiffening portion 9b are connected to transmit a load, thereby further improving the stiffening ability of the stiffening part 9. EXAMPLES

[0049] A study was conducted to verify the effect of the automobile subframe structure according to the present invention, and the results are described below.

[0050] In Example 1, the rigidity of the automobile subframe structure according to the present invention was investigated. The subframe structure 1 surveyed comprises a pair of left and right side members 3 and a pair of front and rear cross members 5, as shown in Fig. 1 of the embodiment described above, with the side members 3 and the cross members 5B being connected via curved portions 7. The subframe structure 1 is provided with stiffening parts 9 as shown in Figs. 1 and 3 (Example 1 of the invention). In Fig. 3, the X-axis direction is to the right of the vehicle, the Y-axis direction is to the front of the vehicle, and the Z-axis direction is to the top of the vehicle (this also applies to Fig. 4 described later).

[0051] 1, the stiffening part 9 integrally covers and stiffens the suspension arm connecting part 11a provided on the side member 3 and the curved ridge part 7a of the curved part 7 from the suspension arm connecting part 11a to the cross member 5B. As shown in FIG. 1 and FIG. 3(a), the stiffening part 9 has a local stiffening part 9a and a global stiffening part 9b. In the first embodiment, as shown in Fig. 1 and Fig. 3(b), the local stiffening portion 9a covers the suspension arm connecting portion 11a and the ridge portion of the side member 3 at the suspension arm connecting portion 11a on the vehicle outer side. On the other hand, as shown in Figs. 1 and 3(b), the global stiffening portion 9b extends from the local stiffening portion 9a along the curved ridge portion 7a of the curved portion 7 to the cross member 5B and covers the curved ridge portion 7a.

[0052] In the subframe structure 1, the side members 3 and the cross members 5 are manufactured as a single unit using steel plates having a thickness of 2.0 mm and a tensile strength of 440 MPa. The stiffening part 9 was manufactured using a steel plate having a thickness of 2.0 mm and a tensile strength of 270 MPa class. The stiffening part 9 was joined to the subframe structure 1 by welding.

[0053] In Example 1, for comparison, the rigidity of a subframe structure 41 provided with a conventional stiffening part 43 that stiffens the suspension arm connecting part 11 as shown in Fig. 4 (conventional example) was also investigated. Here, as shown in Fig. 4(a), the stiffening part 43 does not have the global stiffening part 9b of the stiffening part 9 according to Example 1 of the invention, and has a shape equivalent to the local stiffening part 9a.

[0054] In the subframe structure 41 according to the conventional example, the side members 3 and the cross members 5 were manufactured using steel plates having a plate thickness of 2.0 mm and a tensile strength of 270 MPa, similar to the first invention example. As with Example 1, the stiffening part 43 was manufactured using a steel plate having a thickness of 2.0 mm and a tensile strength of 270 MPa, and joined to the subframe structure 41 by welding.

[0055] In Example 1, a finite element analysis was performed on the subframe structure 1 according to Example 1 of the present invention and the subframe structure 41 according to the conventional example by applying a predetermined load, and the displacement of the load input point for a unit load was calculated, and the reciprocal of the displacement was calculated as the rigidity. Then, the rigidity change rate of Example 1 of the present invention was calculated based on the rigidity of the conventional example.

[0056] The analysis using the finite element method was performed under two conditions: the "torsion condition" shown in FIG. 5(a) and the "bending condition" shown in FIG. 5(b).

[0057] The "torsion condition" shown in Figure 5(a) is a condition in which the rear bushes 15, which are both left and right ends of the cross member 5B, are restrained, and a load (1 kN) is applied upward and downward in the vertical direction of the vehicle to the front bushes 13, which are both left and right ends of the cross member 5A.

[0058] Then, an analysis was performed under the torsional conditions shown in Figure 5(a), and the amount of displacement in the vertical direction of the vehicle per unit load in the front bush 13 of the cross member 5A was calculated as the torsional rigidity for each of the subframe structure 1 of Example 1 of the invention and the subframe structure 41 of the conventional example.

[0059] The torsional rigidity improvement rate of the subframe structure 1 according to the invention example 1 was calculated from the torsional rigidity obtained for each of the invention example 1 and the conventional example. Here, the torsional rigidity improvement rate is the improvement rate of the torsional rigidity of the invention example 1 when the torsional rigidity of the conventional example is used as the reference, and was calculated by the following formula. Torsional rigidity improvement rate [%] = (torsional rigidity of invention example 1 - torsional rigidity of conventional example) / (torsional rigidity of conventional example) x 100

[0060] The torsional rigidity improvement rate of Example 1 of the present invention was 6.2%, which indicates that the torsional rigidity is improved compared to that of subframe structure 41 according to the conventional example.

[0061] The “bending condition” shown in FIG. 5(b) is a condition in which the rear bushes 15 at both left and right ends of the cross member 5B are restrained, and a load (1 kN) is applied to the right in the left-right direction of the vehicle on the front bushes 13 at both left and right ends of the cross member 5A.

[0062] Then, an analysis was performed under the bending conditions shown in Figure 5 (b), and the amount of displacement in the left-right direction of the vehicle per unit load in the front bush 13 of the cross member 5A was calculated as bending rigidity for each of the subframe structure 1 of Example 1 of the invention and the subframe structure 41 of the conventional example.

[0063] The bending rigidity improvement rate of the subframe structure 1 according to the invention example 1 was calculated from the bending rigidity obtained for each of the invention example 1 and the conventional example. Here, the bending rigidity improvement rate is the improvement rate of the bending rigidity of the invention example 1 when the bending rigidity of the conventional example is used as a reference, and was calculated by the following formula. Flexural rigidity improvement rate [%]=(flexural rigidity of invention example 1-flexural rigidity of conventional example) / (flexural rigidity of conventional example)×100

[0064] The bending rigidity improvement rate of Example 1 of the present invention was 4.9%, which indicated that the bending rigidity was improved compared to that of subframe structure 41 according to the conventional example. EXAMPLES

[0065] In Example 2, the relationship between the improvement in rigidity and the increase in weight in the automobile subframe structure according to the present invention was investigated. Furthermore, the collision characteristics of the automobile subframe structure according to the present invention were also investigated.

[0066] In the second embodiment, the same subframe structure 1 (FIG. 3) as in the first embodiment was investigated. In subframe structure 1, the thickness of the steel plates used in side members 3 and cross members 5 was changed from 2.0 mm to 1.8 mm so that the rigidity of the subframe structure 1 would be equivalent to that of subframe structure 41 (FIG. 4) provided with the conventional stiffening parts 43 described above. Furthermore, in subframe structure 1, the tensile strength of the steel plates used in side members 3 and cross members 5 was set to 590 MPa class (invention example 2).

[0067] In Example 2, first, as in the above-described Example 1, analysis was performed by the finite element method under the "torsion conditions" and "bending conditions" shown in FIG. 5 for the subframe structure 1 according to Example 2 to determine each of the torsional rigidity and bending rigidity. Table 1 shows the rigidity (torsional rigidity and bending rigidity) and weight of the subframe structure 1 according to the invention example 2, together with the rigidity and weight results of the invention example 1 according to the above-mentioned embodiment 1 and the conventional example.

[0068] [Table 1]

[0069] As shown in Table 1, the torsional rigidity improvement rate and bending rigidity improvement rate of Example 2 of the invention compared to the conventional example were both ±0%, and the rigidity of the subframe structure 1 of Example 2 of the invention was equivalent to the rigidity of the subframe structure 41 of the conventional example.

[0070] The subframe structure 1 of Example 1 of the invention, in which the plate thickness of the side members 3 and cross members 5 are equivalent to those of the conventional example, has a weight increased by 1.2% compared to the subframe structure 41 of the conventional example by using a stiffening part 9 having a large stiffening portion 9b. In contrast, the subframe structure 1 according to Example 2 of the invention, in which the plate thicknesses of the side members 3 and cross members 5 are reduced, uses the same stiffening parts 9 as in Example 1 of the invention, but has a weight that is 8.5% less than that of the subframe structure 41 according to the conventional example. This shows that the subframe structure 1 according to Example 2 of the invention has a reduced weight while maintaining the same rigidity as the subframe structure 41 according to the conventional example.

[0071] Next, a crash analysis was carried out on the crash characteristics of the subframe structures 1 according to the invention examples 1 and 2 and the subframe structure 41 according to the conventional example.

[0072] In the collision analysis, a rigid wall 51 was collided from the front side of the vehicle as shown in Fig. 6. The following constraint conditions were imposed: all degrees of freedom were restrained for the rear bushes 15 at both left and right ends of the cross member 5B, and the translational degree of freedom in the Z-axis direction (upward direction of the vehicle) was restrained for the front bushes 13 at both left and right ends of the cross member 5A.

[0073] Through a crash analysis, the maximum load generated when the rigid wall 51 crashes and the absorbed energy during the crash process were calculated for each of the subframe structures 1 according to the invention examples 1 and 2 and the subframe structure 41 according to the conventional example. Then, for each of the calculated maximum loads and absorbed energies, the rate of change was calculated when the maximum load or absorbed energy in the conventional example was used as a reference. The above-mentioned Table 1 shows the maximum load and absorbed energy obtained by the collision analysis for each of the invention example 1, the invention example 2, and the conventional example.

[0074] As shown in Table 1, the maximum load generated on the subframe structure during a collision according to the collision analysis was +0.6% for Example 1 and +2.8% for Example 2, both of which exceeded the conventional examples, compared to the conventional examples. In addition, the crash absorbed energy was +1.5% for Example 1 and +4.6% for Example 2, both of which exceeded the conventional examples.

[0075] As described above, it has been demonstrated that in the subframe structure according to the present invention, by reducing the thickness of the steel plates so as to achieve the same rigidity as conventional subframe structures, and by using high-strength steel plates, it is possible to achieve weight reduction and improved collision characteristics while maintaining rigidity. [Explanation of symbols]

[0076] 1 Subframe structure 3 Side members 3a Ridgeline 3b Ridge line 5 Cross member 5A Cross member 5B Cross member 5a Ridgeline 7 Curved section 7a Curved ridge 9. Stiffening parts 9a Local stiffening part 9b Global stiffening part 11 Suspension arm connection 11a Suspension arm connection 11b Suspension arm connection part 13 Front bush 15 Rear Bush 21 Subframe structure 23 Side member 25 Cross member 27 Curved section 27a Curved ridge 29 Stiffening parts 29a Local stiffening part 29b Global stiffening part 31 Suspension arm connection part 31a Suspension arm connection 31b Suspension arm connection part 33 Suspension arm 41 Subframe structure 43 Stiffening parts 51 Rigid Wall

Claims

1. a pair of left and right vehicle longitudinal extending portions each having a hollow member with a substantially rectangular cross section extending in a vehicle longitudinal direction and provided with a suspension arm connecting portion; and at least one vehicle lateral extending portion each having a hollow member with a substantially rectangular cross section extending in the vehicle lateral direction, wherein the vehicle longitudinal extending portions and the vehicle lateral extending portions are connected via a curved portion, a stiffening part is provided that integrally covers and stiffens the suspension arm connection portion and a curved ridge portion that is a corner of a substantially rectangular cross section on the vehicle inner side of the L-shaped curved portion from the suspension arm connection portion to the vehicle left-right direction extending portion, so as to fit along an upper surface of the vehicle front-rear direction extending portion.

2. 2. The subframe structure for an automobile according to claim 1, wherein the stiffening part also covers a ridge portion on an outer side of the vehicle of the vehicle longitudinal direction extending portion of the suspension arm connecting portion.

3. 3. The subframe structure for an automobile according to claim 1, wherein the vehicle longitudinal direction extending portion and the vehicle lateral direction extending portion are manufactured using a steel plate having a tensile strength of 590 MPa or higher.

4. a pair of left and right vehicle longitudinal extending portions each having a hollow member with a substantially rectangular cross section extending in the vehicle longitudinal direction and provided with a suspension arm connecting portion, and at least one vehicle lateral extending portion each having a hollow member with a substantially rectangular cross section extending in the vehicle lateral direction, the vehicle longitudinal extending portions and the vehicle lateral extending portions being connected via a curved portion, a local stiffening part which covers the suspension arm connecting portion and locally stiffens the suspension arm connecting portion, and a global stiffening part which extends from the local stiffening part along a curved ridge portion which is a corner of a substantially rectangular cross section on the vehicle inner side of the L-shaped curved portion to the vehicle left-right extending portion, covering the curved ridge portion, and globally stiffening the subframe structure, which are formed integrally along an upper surface of a side member.

5. 5. The stiffening part of an automobile subframe structure according to claim 4, wherein the local stiffening portion also covers a ridge portion on an outer side of the vehicle of the vehicle longitudinal direction extending portion of the suspension arm connecting portion.

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