Vehicle subframe

Abstract

A vehicle subframe 10 is fixed to a vehicle body, and includes a structural member that extends in a curved manner in a planar view from above the vehicle body, wherein: the structural member is made of a steel material and has an open cross-sectional shape in a direction perpendicular to an extension direction; the cross-sectional shape differs depending on a position in the extension direction of the structural member; and the structural member includes a reinforcing member 14 that is fixed at a predetermined position in the extension direction of the structural member.

Description

Vehicle subframe

[0001] The present invention relates to a subframe for a vehicle.

[0002] Conventionally, a rear subframe has been known that has a U-shaped outer shape in a plan view and includes U-shaped members whose cross sections perpendicular to the extending direction are closed at each position from one end to the other end of the U-shape (Patent Document 1).

[0003] Also, a vehicle subframe is known that is obtained by joining an upper member made of a die-cast member such as aluminum and a lower member obtained by pressing a flat plate member such as a steel plate (Patent Document 2).

[0004] JP 2021-133742 A Patent No. 6611773 A

[0005] The U-shaped member described in Patent Document 1 is made of a simple steel pipe, and has a circular cross section perpendicular to the extension direction that is constant at any position along the extension direction. However, if the cross section is constant at any position along the extension direction, there is a problem that space constraints arise in relation to adjacent members around the subframe.

[0006] Furthermore, the subframe receives loads from the vehicle suspension. If the cross-sectional shape is a constant circle at any position along the extension direction, there may be locations where the suspension strength is excessive for the load, resulting in a problem of being unable to effectively utilize the space around the subframe.

[0007] Furthermore, the vehicle subframe described in Patent Document 2 is configured by joining an upper member made of a die-cast member such as aluminum and a lower member obtained by pressing a flat plate member such as a steel plate, which increases the number of welds or the length of the weld line, making it difficult to ensure rigidity. Furthermore, since the vehicle subframe constitutes a vehicle suspension part in particular, it is used in an environment where cracks, corrosion, and the like are likely to occur. The increased number of welds or the length of the weld line increases the likelihood of cracks, corrosion, and the like due to welding, which reduces fatigue strength.

[0008] In view of the above problems, an object of the present disclosure is to provide a subframe for a vehicle that has sufficient strength to withstand loads and can effectively utilize the surrounding space.

[0009] The gist of the present disclosure is as follows.

[0010] (1) A vehicle subframe having an integral structural member fixed to a vehicle body and extending in a curved manner when viewed in a plan view from above the vehicle body, wherein the structural member is made of a steel material and has an open cross-sectional shape perpendicular to the extension direction, the cross-sectional shape differing depending on the position of the structural member in the extension direction, and the vehicle subframe is provided with a reinforcing member fixed to a predetermined position in the extension direction of the structural member.

[0011] (2) The vehicle subframe according to (1), wherein, in the plan view, both end portions of the curved structural member are arranged on one end side in the vehicle length direction, and the curved convex shape of the structural member is arranged on the other end side in the vehicle length direction, the structural member has a fastening portion for an arm at each of the both end portions, and a distance L (mm) in the vehicle length direction from the fastening portion to the tip of the convex shape satisfies the following formula (1): L≧200 (1)

[0012] (3) A subframe for a vehicle as described in (1) or (2) above, wherein the cross-sectional shape of the structural member in a direction perpendicular to the extension direction is a closed cross-section at the predetermined position due to the reinforcing member, and is an open cross-section at other than the predetermined position.

[0013] (4) The vehicle subframe according to (3), wherein the reinforcing member extends along the extension direction of the structural member and forms a partition wall that separates the inside of the closed cross-section from the outside.

[0014] (5) A vehicle subframe as described in (1) or (2) above, wherein the reinforcing member extends in a direction perpendicular to the extension direction of the structural member and forms a partition wall that divides the interior of the open cross section in the extension direction.

[0015] (6) The vehicle subframe according to (1) or (2) above, wherein the reinforcing member is a block-shaped member filled inside the open cross section.

[0016] (7) The vehicle subframe according to any one of (1) to (6), wherein, in the plan view, the structural member has a straight portion and a curved portion, and the predetermined position is included in the curved portion.

[0017] (8) A subframe for a vehicle as described in (7) above, wherein, in the plan view, the structural member has a shape that is approximately symmetrical with respect to an imaginary line in the vehicle length direction, and the predetermined position is located within a range of 70% of the width of the structural member in the vehicle width direction of the structural member, with the imaginary line as the center.

[0018] (9) The vehicle subframe according to any one of (1) to (8) above, wherein the structural member has a flange at a tip of the open cross section at the predetermined position.

[0019] (10) The vehicle subframe according to any one of (1) to (9) above, wherein the structural member has a tensile strength of 510 MPa or more.

[0020] (11) The vehicle subframe according to any one of (1) to (9), wherein the structural member has a Vickers hardness of 160 Hv or more at a center portion in a plate thickness direction.

[0021] (12) The vehicle subframe according to any one of (1) to (11) above, wherein the structural member does not have a seam that crosses the extension direction.

[0022] According to the present disclosure, a vehicle subframe is provided that has sufficient strength to withstand loads and can effectively utilize the surrounding space.

[0023] 5A is a plan view showing a subframe 10 of a vehicle according to an embodiment of the present disclosure and a configuration of its periphery. FIG. 5B is a diagram showing the state when the subframe 10 is mounted on the vehicle body as viewed from below. FIG. 5C is a perspective view of the subframe 10 of FIG. 1 as viewed from diagonally above. FIG. 5C is a diagram showing a cross section taken along dashed dotted line II-II' in FIG. 1. FIG. 5D is a diagram showing a cross section taken along dashed dotted line II-II' in FIG. 1. FIG. 5E is a diagram showing a cross section taken along dashed dotted line II-II' in FIG. 5A. FIG. 5F is a diagram showing the state when the subframe 10 is mounted on the vehicle body as viewed from below, showing an example in which a reinforcing member 20 is provided instead of the reinforcing member 14. FIG. 5C is a cross section taken along dashed dotted line II-II' in FIG. 5A. FIG. 5C is a diagram showing the state when the subframe 10 is mounted on the vehicle body as viewed from below, showing an example in which a block-shaped reinforcing member 21 is provided instead of the reinforcing member 14. FIG. 5C is a cross section taken along dashed dotted line II-II' in FIG. 5C. 1A is a plan view showing an example in which an arm 52 arranged on the rear side of the vehicle is supported by two support portions 22 adjacent in the vehicle length direction, similar to the arm 50 arranged on the front side of the vehicle. FIG. 1B is a plan view showing a plan view of a subframe similar to FIG. 1A, and shows an example in which a reinforcing member is fixed in the vehicle width direction. FIG. 1C is a plan view showing the configuration of a subframe according to a comparative example. FIG. 1D is a perspective view of the subframe according to the comparative example of FIG. 8, seen obliquely from above. FIG. 1E is a view showing a cross section taken along dashed dotted line III-III' in FIG. 8. FIG. 1F is a view showing a cross section taken along dashed dotted line IV-IV' in FIG. 8.

[0024] Hereinafter, several embodiments of the present invention will be described with reference to the drawings. However, these descriptions are intended to merely exemplify preferred embodiments of the present invention and are not intended to limit the present invention to such specific embodiments. In the following description, similar components will be designated by the same reference numerals.

[0025] 1A is a plan view showing a subframe 10 of a vehicle and its surrounding configuration according to an embodiment of the present disclosure. The subframe 10 is fixed to a lower part of a vehicle body. The subframe 10 may be fixed to the vehicle body directly or via a body mount component (not shown).

[0026] FIG. 1A illustrates the subframe 10 mounted on the vehicle body as viewed from above, i.e., as viewed from above. The X-axis direction in FIG. 1A corresponds to the vehicle width direction, and the Y-axis direction corresponds to the vehicle length direction. The Z-axis direction corresponds to the vehicle height direction, which is perpendicular to the X-axis and Y-axis directions. In the plan view illustrated in FIG. 1A, the subframe 10 has a curved, integral structural member extending in a generally U-shape, and is configured to withstand loads from various directions. In the plan view illustrated in FIG. 1, both ends of the curved, integrally molded structural member are located at one end in the vehicle length direction, and the curved, convex shape of the structural member is located at the other end in the vehicle length direction. The subframe 10 is generally symmetrical with respect to the center line C illustrated in FIG. 1A. FIG. 1B illustrates the subframe 10 mounted on the vehicle body as viewed from below. FIG. 2 is a perspective view of the subframe 10 illustrated in FIG. 1 as viewed obliquely from above.

[0027] The structural members of the subframe 10 are made of steel. The subframe 10 may be made of hot-rolled steel. The tensile strength of the structural members of the subframe 10 is preferably 510 MPa or more. More preferably, the tensile strength of the structural members of the subframe 10 is 780 MPa or more, and even more preferably, 980 MPa or more. Furthermore, when these tensile strengths are converted into Vickers hardness, the Vickers hardness of the structural members of the subframe 10 is preferably 160 Hv or more, more preferably 250 Hv or more, and even more preferably 310 Hv or more.

[0028] The tensile strength of a structural member can be confirmed by taking a test piece of the structural member in the longitudinal direction of the full thickness and conducting a tensile test. The shape of the test piece is preferably a JIS Z 2201 No. 12B test piece, but if a No. 12B test piece cannot be taken due to its dimensions, a test piece of the full thickness but with the gauge length and width of the parallel part reduced by half may be used. The tensile test method follows JIS Z 2241 (2011).

[0029] The method for measuring the hardness of the center of a structural member in the thickness direction is as follows. A cross section perpendicular to the plate surface is taken from the wall of the structural member, and the sample is prepared for the measurement surface and subjected to a hardness test. The measurement surface is prepared in accordance with JIS Z 2244-1:2020. The measurement surface is polished using silicon carbide paper with a grit size of #600 to #1500, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The mirror surface thus polished is subjected to a hardness test using the method described in JIS Z 2244-1:2020. Using a micro-Vickers hardness tester, measurements are taken at 10 points at 3 / 8 of the sample's thickness, with a load of 9.87 N, at intervals of at least three times the indentation width, and the average value is taken as the hardness of the center of the plate in the thickness direction.

[0030] Specifically, the subframe 10 is made of high-strength high-tensile steel plate, for example, a hat-shaped member integrally formed from a thick steel plate having a thickness of approximately 1.8 mm to 4.0 mm. The lower limit of the plate thickness is preferably 1.4 mm, more preferably 1.6 mm. The upper limit of the plate thickness is preferably 4.2 mm, more preferably 3.2 mm. Note that, in the following description, a hat-shaped member is used as an example of a curved, integral structural member. However, the integral structural member is not limited to a hat-shaped member, and various members can be used as long as the cross-sectional shape perpendicular to the extension direction is an open cross-section. In FIG. 1A , the hat-shaped member is positioned so that it opens downward. By being made of the hat-shaped member, the subframe 10 has an open cross-section perpendicular to the extension direction.

[0031] The subframe 10 according to this embodiment is constructed from a curved, one-piece, hat-shaped member. This structure eliminates any seams that cross the direction of extension of the structural member. In other words, "one-piece" means that the structural member of the subframe 10 does not have any seams that cross the direction of extension of the structural member. Therefore, the subframe 10 according to this embodiment is constructed to reduce the number of parts while still satisfying the rigidity and load transmission performance required of the subframe 10.

[0032] When the subframe 10 is configured from a curved, one-piece hat-shaped member, a torsional moment may occur near the center of the subframe 10 in the vehicle width direction, i.e., near the center of the curved shape where the subframe 10 intersects with the center line C. For this reason, a flange 10a is provided on the hat-shaped member near the center of the vehicle width direction of the subframe 10, i.e., in the region where the hat-shaped member is curved, to increase rigidity.

[0033] The structural members constituting the subframe 10 include reinforcing members fixed at predetermined positions in the extension direction. In a plan view, the structural members constituting the subframe 10 have straight and curved portions, and the predetermined positions where the reinforcing members are fixed are included in the curved portions. Note that, in a plan view of the subframe 10, the extension direction may be a direction along a straight line in the straight portions, or a tangent direction to the curve in the curved portions. As shown in FIG. 1A , a reinforcing member 14 is fixed near the center of the subframe 10 in the vehicle width direction, i.e., near the center of the curved shape where the subframe 10 intersects with the center line C, to increase rigidity. As shown in FIG. 1B , the reinforcing member 14 is joined to the flange 10a by arc welding or spot welding.

[0034] In this way, by providing the flanges 10a or reinforcing members 14 in areas with large curvature, i.e., areas with small radius of curvature, when viewed from above, the torsional rigidity of the subframe 10 is particularly increased.

[0035] Furthermore, at the positions where arms 50, 52 (described later) are fastened to the subframe 10, a load is transmitted from the arms 50, 52 to the subframe 10. For this reason, as shown in Figures 1A and 1B, a reinforcing member 16 is fixed to the subframe 10 at each of the two rear ends in the vehicle length direction. As shown in Figure 1B, the reinforcing member 16 is joined to the subframe 10 by arc welding or spot welding. This increases the rigidity of the subframe 10 at the positions where the load is transmitted from the arms 52.

[0036] The reinforcing members 14 and 16 may be made of hot-rolled steel, similar to the subframe 10. Furthermore, the reinforcing members 14 and 16 may be made of high-strength high-tensile steel, similar to the subframe 10.

[0037] The structural members that make up the subframe 10 have an open cross-sectional shape perpendicular to the extension direction, and the cross-sectional shape varies depending on the position of the structural member in the extension direction. Fig. 3 is a diagram showing a cross-section taken along dashed line II' in Fig. 1. Fig. 4A is a diagram showing a cross-section taken along dashed line II-II' (center line C) in Fig. 1.

[0038] As shown in FIGS. 3 and 4A, the cross section of the subframe 10 according to this embodiment taken along dashed dotted line II' has a different cross-sectional shape than the cross section taken along dashed dotted line II-II'. In this embodiment, "different cross-sectional shapes" refers to at least one of different cross-sectional sizes (height or width) and different cross-sectional shapes. The cross sections shown in FIG. 3 and FIG. 4A have different heights and widths of the hat-shaped members. Specifically, the cross section shown in FIG. 3 and the cross section shown in FIG. 4A have larger heights and widths of the hat-shaped members than the cross section shown in FIG. 4A.

[0039] The cross section of the subframe 10 may be a shape other than a rectangle, such as a circle, an ellipse, a polygon, etc. Fig. 4B is a diagram showing another example of a cross section taken along the dashed dotted line II-II' in Fig. 1. In the example shown in Fig. 4B, a portion of the cross section is circular, and the cross sections shown in Fig. 3 and Fig. 4B have different cross-sectional shapes, resulting in "different cross-sectional shapes."

[0040] The subframe 10 receives loads from the left and right arms 50, 52 in the vehicle width direction. Therefore, in order to increase the rigidity of the subframe 10 on the left and right sides in the vehicle width direction, the height H and width W of the hat-shaped member in the cross section taken along the dashed dotted line II' are increased, thereby increasing the second moment of area and the polar second moment of area. This improves the rigidity of the subframe 10, particularly the bending rigidity and torsional rigidity.

[0041] Meanwhile, structures such as an engine, motor, and steering mechanism are often located near the center of the subframe 10 in the vehicle width direction, i.e., near the center of the curved shape where the subframe 10 intersects with the center line C, and this may result in space constraints on the shape of the subframe 10. For this reason, near the center of the curved shape of the subframe 10, the height H and width W of the hat-shaped member in the cross section taken along the dashed dotted line II-II' are reduced to ensure a larger space adjacent to the subframe 10. Furthermore, when the subframe 10 is fixed to the vehicle body at left and right positions in the vehicle width direction, the load from the arms 50, 52 is transmitted to the vehicle body. For this reason, considering the transmission of the load to the vehicle body, even if the height H and width W of the hat-shaped member in the cross section taken along the dashed dotted line II-II' are reduced relative to the height H and width W of the hat-shaped member in the cross section taken along the dashed dotted line I-I', the effect on the rigidity of the subframe 10 is relatively small. Therefore, by making the height H and width W of the hat-shaped member in the cross section taken along the dotted line I-I' larger than the height H and width W of the hat-shaped member in the cross section taken along the dotted line II-II', the rigidity of the subframe 10 is increased, particularly on the left and right sides in the vehicle width direction, and sufficient space is secured in the center in the vehicle width direction for arranging other members.

[0042] As shown in FIGS. 4A and 4B , the reinforcing member 14 is joined to the flange 10a by welding at welds 18. As shown in FIGS. 3 , 4A, and 4B , the cross-sectional shape of the structural members constituting the subframe 10 in a direction perpendicular to the extension direction is closed by the reinforcing member 14 at predetermined positions (positions of dashed dotted lines II-II′) where the reinforcing member 14 is provided, and is open at other positions (positions of dashed dotted lines I-I′). The reinforcing member 14 extends along the extension direction of the structural members and forms a partition wall that separates the interior of the closed cross-section from the outside. Note that the reinforcing member 14 may also be joined above the flange 10a (toward the top plate of the hat-shaped member).

[0043] Two arms 50, 52 are fastened to the subframe 10 on each of the left and right sides in the vehicle width direction. The two arms 50, 52 are arms that constitute the vehicle suspension, and specifically may be lower arms or upper arms. The two arms 50, 52 may also be formed as a single unit.

[0044] 1, the subframe 10 is disposed on the front side of the vehicle, and the arms 50, 52 support the front wheels of the vehicle. However, the subframe 10 may also be disposed on the rear side of the vehicle, and the arms 50, 52 may support the rear wheels of the vehicle.

[0045] The subframe 10 is provided with two support portions 22 on each of the left and right sides in the vehicle width direction, on the front side in the vehicle length direction, extending outward in the vehicle width direction. Each of the two adjacent support portions 22 is provided with a hole 22a, and a shaft 26 is inserted through the hole 22a in the vehicle length direction. Of the two arms 50, 52, the arm 50 located on the front side of the vehicle is rotatably attached to the shaft 26, thereby allowing the outer tip of the arm 50 in the vehicle width direction to move up and down relative to the subframe 10. Preferably, the arm 50 is attached to the shaft 26 via a rubber bushing (not shown) to absorb impacts from the ground.

[0046] The subframe 10 also has two support portions 28 in the vehicle height direction at each of its two rear ends in the vehicle length direction. As shown in FIG. 2 , the side walls of the hat-shaped members constituting the subframe 10 are opened from the vehicle width direction outer sides, and each of the two support portions 28 is formed from a top plate of the hat-shaped member and a reinforcing member 16. Each of the two support portions 28 adjacent in the vehicle height direction has a hole 28a, through which a shaft (not shown) is inserted in the vehicle height direction. Of the two arms 50, 52, the arm 52 located on the vehicle rear side is rotatably attached to the shaft inserted in the hole 28a, thereby allowing the outer tip of the arm 52 in the vehicle width direction to move in the vehicle length direction relative to the subframe 10. Like the arm 50, the arm 52 may be attached to the shaft via a rubber bushing.

[0047] FIG. 5A , like FIG. 1B , is a view showing the subframe 10 mounted on the vehicle body as viewed from below, illustrating an example in which a reinforcing member 20 (bulkhead) is provided instead of the reinforcing member 14. FIG. 5B is a cross-sectional view taken along dashed line II-II′ in FIG. 5A . The reinforcing member 20 extends perpendicular to the extension direction of the subframe 10 and forms a partition wall that divides the interior of the hat-shaped member with an open cross section in the extension direction. The partition wall-like reinforcing member 20 is joined to the hat-shaped member of the subframe 10 by arc welding or spot welding. The partition wall-like reinforcing member 20 may increase the strength of the subframe 10. The reinforcing member 20 may be made of hot-rolled steel, similar to the subframe 10. Furthermore, the reinforcing member 20 may be made of high-strength high-tensile steel, similar to the subframe 10.

[0048] FIG. 5C , like FIG. 1B , shows the subframe 10 mounted on the vehicle body as viewed from below, illustrating an example in which a block-shaped reinforcing member 21 (bulkhead) is filled inside the hat-shaped member with an open cross section instead of the reinforcing member 14. FIG. 5D is a cross-sectional view taken along dashed line II-II′ in FIG. 5C . The reinforcing member 21 is made of, for example, a steel block. The block-shaped reinforcing member 21 is joined to the hat-shaped member of the subframe 10 by arc welding or spot welding. The strength of the subframe 10 may be increased by such a block-shaped reinforcing member 21. The block-shaped reinforcing member 21 may also be made of a foam metal of aluminum or an aluminum alloy.

[0049] It is preferable that the reinforcing member 14, the reinforcing member 20, or the reinforcing member 21 is disposed within a range of 70% of the width of the subframe 10 in the vehicle width direction, centered on the center line C. This makes it possible to effectively increase the torsional rigidity.

[0050] The subframe 10 is a component fixed to the lower part of a vehicle body (not shown). Road loads are transmitted to the subframe 10 via arms 50 and 52. Therefore, the subframe 10 has the rigidity and crashworthiness required to withstand the transmitted loads. More specifically, the subframe 10 has an open cross-section structure formed by hat-shaped members, but rigidity is ensured by the provision of reinforcing members 14, 16, 20, and 21 at key locations. Meanwhile, components such as an engine and a battery located under the hood are fastened to the subframe 10, and the subframe 10 supports and protects these components. For this reason, the subframe 10 has a curved shape, roughly a U-shape or a U-shape, as shown in FIG. 1 in a plan view. This allows the subframe 10 to maintain strength against road loads and, by arranging components such as the engine, motor, and steering mechanism inside the curved shape, improves space utilization efficiency.

[0051] The subframe 10 has a vehicle length direction distance from the support portion 28, which supports the arm 52 disposed on the rear side of the vehicle out of the two arms 50, 52, to the front end of the vehicle, which distance is equal to or greater than a predetermined value. Specifically, as shown in Figure 1, if the distance from the line connecting the centers of the left and right holes 28a to which the arms 52 are attached to the front end of the subframe 10 in the vehicle length direction is L [mm], the relationship of the following equation (1) is established: L≧200 (1)

[0052] If L is less than 200 mm, the rigidity of the subframe 10 will be equivalent to that of a subframe that does not have a curved shape, and sufficient rigidity cannot be ensured. By satisfying the relationship of formula (1), sufficient rigidity can be ensured.

[0053] 6 is a plan view showing an example in which an arm 52 disposed on the rear side of the vehicle is supported by two support portions 22 adjacent in the vehicle length direction, similar to the arm 50 disposed on the front side of the vehicle. Similar to the arm 50, the arm 52 is attached so as to be rotatable about the shaft 26 inserted through the hole 22a. In the configuration shown in FIG. 6, the starting point on the rear side of the vehicle of the distance L is the midpoint of the distance between the two support portions 22 in the vehicle length direction.

[0054] Fig. 7 is a plan view showing the subframe 10 in a plan view, similar to Fig. 1A. In the example shown in Fig. 7, reinforcing members 30 are fixed in the vehicle width direction near the positions of the left and right holes 28a to which the arms 52 are attached. The reinforcing members 30 are fixed to the subframe 10 by arc welding, spot welding, or the like. The provision of the reinforcing members 30 further increases the rigidity of the subframe 10.

[0055] Fig. 8 is a plan view showing the configuration of a subframe according to a comparative example. The subframe according to the comparative example is composed of two cross members 60, 66 and two side members 62, 64, and has a square-shaped configuration in the plan view shown in Fig. 8. Fig. 9 is a perspective view of the subframe according to the comparative example shown in Fig. 8, viewed obliquely from above.

[0056] The subframe according to the comparative example is constructed by welding two cross members 60, 66 and two side members 62, 64 together.

[0057] Each of the cross members 60, 66 and the side members 62, 64 is made up of two members, an upper and a lower. That is, the cross member 60 is made up of members 60a and 60b, the cross member 66 is made up of members 66a and 66b, the side member 62 is made up of members 62a and 62b, and the side member 64 is made up of members 64a and 64b.

[0058] As with the subframe 10 shown in FIG. 1A , in the subframe of the comparative example, an arm 50 is supported by an axle 26 inserted through holes 22 a of two support parts 22, and an arm 52 is supported by an axle inserted through holes 28 a of two support parts 28.

[0059] The subframe of the comparative example is constructed by welding together two cross members 60, 66 and two side members 62, 64, and therefore has a greater number of welds and a longer weld line length than the subframe 10 of the present embodiment, which is constructed from a single hat-shaped member.

[0060] Fig. 10 is a cross-sectional view taken along dashed dotted line III-III' in Fig. 8. Fig. 11 is a cross-sectional view taken along dashed dotted line IV-IV' in Fig. 8. As shown in Fig. 10, side member 62 is formed by line-welding members 62a and 62b together at welds 68. As shown in Fig. 11, cross member 60 is formed by line-welding members 60a and 60b together at welds 68. Side member 64 and cross member 66 are also formed by line-welding the upper and lower members together.

[0061] In the subframe according to the comparative example, the cross members 60, 66 and the side members 62, 64 are each constructed by line welding two upper and lower members together, resulting in long weld lines. According to the present embodiment, the subframe 10 is constructed from hat-shaped members, which significantly reduces the weld line length of the members that make up the subframe 10. This eliminates the reduction in rigidity due to welding, thereby increasing the rigidity of the subframe 10 and increasing the resonance frequency. Furthermore, the reduction in the number of welds and the weld line length reduces fatigue cracking at the welds and reduces corrosion initiation points, thereby suppressing corrosion. Furthermore, the reduction in the number of parts that make up the subframe 10 reduces the cost of the parts, which in turn reduces the manufacturing cost of the subframe 10.

[0062] The present disclosure will be specifically described below using examples. Note that the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the conditions in the examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and the purpose is achieved.

[0063] The inventors compared subframes 10 made from hat-shaped members between examples and comparative examples, which differed in distance L and material. They evaluated the component integration effects, such as the number of components and weld length, as well as component performance, such as static rigidity and vibration mode. Analysis was performed using commercially available structural analysis software (Nastran, manufactured by MSC Software). More specifically, structural analysis was performed assuming a load (lateral force) applied from the outside to the inside of the vehicle in the vehicle width direction, and eigenvalue analysis of the vibration input was performed to evaluate the reaction force and natural frequency for each load. Examples 1 to 4 were the subframes 10 shown in FIG. 1 , while Comparative Example 1 was the subframe 10 shown in FIG. 8 , but without the cross member 66. Examples 5 and 6 were the subframes 10 shown in FIG. 1 , and had the same distance L as Examples 1 and 4 but different weld lengths. Examples 7 and 8 were subframes 10 with reinforcing members 20, as shown in FIG. 5A . Inventive Examples 9 and 10, the subframe 10 had the reinforcing member 21 shown in Fig. 5C. Comparative Example 2 was a subframe 10 made from hat-shaped members similar to Fig. 1A, but the distance L was less than 200 mm.

[0064] In addition, Examples 1 to 10 and Comparative Example 1 were evaluated under conditions in which the weights were approximately the same, and the effects of weight were eliminated so that comparisons based on differences in structure could be made.

[0065] In terms of the number of parts, Comparative Example 1, which excludes cross member 66 from the structure shown in Figure 8, has a total of six parts because cross member 60 and side members 62, 64 each consist of two members, one above the other. In contrast, the subframe 10 of Examples 1 to 6, 9, and 10, which is made from hat-shaped members, has four parts. Therefore, Examples 1 to 6, 8, and 10 were able to reduce the number of parts compared to Comparative Example 1.

[0066] Furthermore, the reduction in the number of parts resulted in a significant reduction in the length of the weld line. The weld line reduction rate in Table 1 indicates the ratio of the reduction in the weld line length in Examples 1 to 10 to the weld line length in Comparative Example 1, and the weld line reduction rates in Examples 1 to 10 were all high, exceeding 50%.

[0067] Regarding component performance, the static rigidity of Examples 1 to 10 was significantly improved compared to that of Comparative Example 1 due to the component integration effect. Static rigidity was calculated by dividing the reaction force generated when a 1 kN load was applied from the outside to the inside of the vehicle by the component displacement. All of Examples 1 to 10 had static rigidity of 33.0 kN / mm or greater, while Comparative Examples 1 and 2 had static rigidity less than 33.0 kN / mm. The performance improvement effect (static rigidity) in Table 1 indicates the ratio of the reduction in static rigidity of Examples 1 to 10 relative to that of Comparative Example 1, resulting in a performance improvement effect of 3% or greater. Furthermore, the natural frequencies (primary mode, secondary mode) of Examples 1 to 10 were significantly improved compared to those of Comparative Example 1. The performance improvement effect (primary mode) in Table 1 indicates the ratio of the increase in the natural frequency (primary mode) of Examples 1 to 10 relative to that of Comparative Example 1. Furthermore, the performance improvement effect (secondary mode) in Table 1 indicates the ratio of the increase in the natural frequency (secondary mode) of invention examples 1 to 10 relative to the natural frequency (secondary mode) of comparative example 1. Performance improvement effects were obtained for all natural frequencies, and the performance improvement effect for the natural frequency (secondary mode) in particular was 4% or more. As the natural frequency increases, resonance with road noise is suppressed. In the subframe 10, as the torsional rigidity increases, the natural frequency (primary mode) increases, and as the bending rigidity increases, the natural frequency (secondary mode) increases. According to invention examples 1 to 10, the natural frequency (primary mode) and the natural frequency (secondary mode) increased, and both the torsional rigidity and bending rigidity increased.

[0068] Furthermore, Comparative Example 2, in which the distance L was less than 200 mm, had lower static rigidity than Invention Examples 1 to 10, and even lower static rigidity than Comparative Example 1. For this reason, it was not possible to obtain a performance improvement effect for the natural frequencies (primary mode, secondary mode).

[0069]

[0070] REFERENCE SIGNS LIST 10 subframe 10a flange 14, 16, 20, 21 reinforcing member 18 welded portion 22, 28 support portion 22a hole 26 shaft 28a hole 30 reinforcing member 50, 52 arm

Claims

1. A vehicle subframe having an integral structural member fixed to a vehicle body and extending in a curved manner when viewed in a plan view from above the vehicle body, wherein the structural member is made of a steel material and has an open cross-sectional shape perpendicular to the extension direction, the cross-sectional shape differing depending on the position of the structural member in the extension direction, and the vehicle subframe is provided with a reinforcing member fixed to a predetermined position of the structural member in the extension direction.

2. The vehicle subframe according to claim 1, wherein, in the plan view, both ends of the curved structural member are disposed on one end side in the vehicle length direction, and the curved convex shape of the structural member is disposed on the other end side in the vehicle length direction, each of the both ends of the structural member has a fastening portion for an arm, and a distance L (mm) in the vehicle length direction from the fastening portion to the tip of the convex shape satisfies the following formula (1): L≧200 ... (1) 3. A vehicle subframe as described in claim 1 or 2, wherein the cross-sectional shape of the structural member in a direction perpendicular to the extension direction is made a closed cross-section at the specified position by the reinforcing member, and is an open cross-section other than at the specified position.

4. A vehicle subframe as set forth in claim 3, wherein the reinforcing member extends along the extending direction of the structural member and constitutes a partition wall that separates the inside of the closed cross section from the outside.

5. A vehicle subframe as claimed in claim 1 or 2, wherein the reinforcing member extends in a direction perpendicular to the extension direction of the structural member and constitutes a partition wall that divides the interior of the open cross section in the extension direction.

6. A vehicle subframe as set forth in claim 1 or 2, wherein the reinforcing member is a block-shaped member filled inside the open cross section.

7. A vehicle subframe as claimed in claim 1 or 2, wherein, in the plan view, the structural member has a straight portion and a curved portion, and the predetermined position is included in the curved portion.

8. A subframe for a vehicle as described in claim 7, wherein, in the plan view, the structural member has a substantially symmetrical shape with respect to an imaginary line in the vehicle length direction, and the predetermined position is located within a range of 70% of the width of the structural member in the vehicle width direction of the structural member, with the imaginary line as the center.

9. A vehicle subframe as set forth in claim 1 or 2, wherein the structural member has a flange at a tip of the open cross section at the predetermined position.

10. A vehicle subframe according to claim 1 or 2, wherein the structural member has a tensile strength of 510 MPa or more.

11. A vehicle subframe as set forth in claim 1 or 2, wherein the structural member has a Vickers hardness of 160 Hv or more at a central portion in a plate thickness direction.

12. A vehicle subframe as claimed in claim 1 or 2, wherein the structural member has no seams transverse to the extension direction.