Sub-frame for vehicle

Abstract

A sub-frame 10 for a vehicle is fixed to a vehicle body, and comprises a structural member which is curved and extends in a plan view from an upper direction of the vehicle body. The structural member is made of steel material, and cross-sectional shapes thereof in a direction perpendicular to the extension direction are closed cross sections and vary according to a position of the structural member in the extension direction.

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 a closed cross-sectional shape perpendicular to the direction of extension, and the cross-sectional shape varies depending on the position of the structural member in the direction of extension.

[0011] (2) The vehicle subframe according to (1), wherein in a cross section of the structural member perpendicular to the extension direction, a ratio of a circumferential length L2 of the thickest cross section to a circumferential length L1 of the thinnest part of the cross section of the structural member satisfies the following formula (1): 1<(L2 / L1)≦3 (1).

[0012] (3) The vehicle subframe according to (1) or (2), 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 (2): L≧200 (2)

[0013] (4) The vehicle subframe according to any one of (1) to (3), further comprising a flange that protrudes from an outer peripheral surface of the structural member and is provided integrally with the structural member along the extension direction of the structural member.

[0014] (5) The vehicle subframe according to any one of (1) to (4), wherein the structural member has a Vickers hardness of 320 Hv or more at a center portion in the plate thickness direction.

[0015] (6) The vehicle subframe according to any one of (1) to (4) above, wherein the structural member has a tensile strength of 440 MPa or more.

[0016] (7) A vehicle subframe according to any one of (1) to (4) above, wherein the structural member has a Vickers hardness of 138 Hv or more at a center portion in the plate thickness direction.

[0017] (8) The vehicle subframe according to any one of (1) to (7), wherein the structural member has no seam that crosses the extension direction.

[0018] According to the present disclosure, a vehicle subframe is provided that has sufficient strength to withstand loads and can effectively utilize the surrounding space.

[0019] 1. FIG. 2 is a plan view showing a subframe of a vehicle according to an embodiment of the present disclosure and a configuration of its periphery. FIG. 3 is a perspective view of the subframe of FIG. 1, viewed obliquely from above. FIG. 4 is a diagram showing a method of forming a closed cross-section member from a steel pipe using a conventional STAF method. FIG. 5 is a diagram showing a method of forming a subframe according to the present embodiment by high-pressure air blow molding. FIG. 6 is a diagram showing a process flow for forming a subframe according to the present embodiment by high-pressure air blow molding in order of processes. FIG. 7 is a diagram showing a process flow for forming a subframe according to the present embodiment by high-pressure air blow molding in order of processes. FIG. 8 is a diagram showing a process flow for forming a subframe according to the present embodiment by high-pressure air blow molding in order of processes. FIG. 9 is a diagram showing a cross section taken along dashed line II-II' in FIG. 1. FIG. 10 is a diagram showing another example of a cross section taken along dashed line II-II' in FIG. 1. FIG. 11 is a diagram showing yet another example of a cross section taken along dashed line II-II' in FIG. 1. FIG. 12 is a diagram showing an example in which a flange is provided on the subframe. FIG. 13 is a diagram showing an example in which a flange is provided on the subframe. FIG. 14 is a plan view showing an example in which an arm arranged on the rear side of the vehicle is supported by two support portions adjacent in the vehicle length direction, similar to the arm arranged on the front side of the vehicle. 10 is a plan view showing a plan view of a subframe similar to FIG. 1, and is a view showing an example in which a reinforcing member is fixed in the vehicle width direction. FIG. 11 is a plan view showing the configuration of a subframe according to a comparative example. FIG. 12 is a perspective view of the subframe according to the comparative example of FIG. 10, seen obliquely from above. FIG. 13 is a view showing a cross section taken along dashed dotted line III-III' in FIG. 10. FIG. 14 is a view showing a cross section taken along dashed dotted line IV-IV' in FIG. 10.

[0020] 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.

[0021] 1 is a plan view showing a subframe 10 for a vehicle according to an embodiment of the present disclosure and the configuration of its surroundings. 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).

[0022] FIG. 1 illustrates a state in which the subframe 10 is mounted on the vehicle body as viewed from above, i.e., a plan view from above. The X-axis direction in FIG. 1 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 direction and the Y-axis direction. In the plan view illustrated in FIG. 1 , the subframe 10 has integral structural members that extend in a curved manner, and is generally U-shaped or C-shaped, and is configured to be able to withstand loads from various directions. That is, in the plan view illustrated in FIG. 1 , the structural members constituting the subframe 10 have linear and curved portions. In the plan view illustrated in FIG. 1 , both ends of the structural members, which are curved integrally molded products and continuous deformed closed cross-section members, are located at one end in the vehicle length direction, and the curved convex shape of the structural members is located at the other end in the vehicle length direction. The subframe 10 is symmetrical with respect to the center line C illustrated in FIG. 1 . FIG. 2 is a perspective view of the subframe 10 illustrated in FIG. 1 as viewed obliquely from above.

[0023] The structural member of the subframe 10 is made of steel material. The subframe 10 is a curved, one-piece structural member manufactured, for example, by hot stamping a steel pipe. The structural member can be formed into a predetermined continuous, irregular closed cross-sectional shape by hot stamping, in which a hot-stamping steel pipe is heated to the austenite region, held at a predetermined temperature, and then quenched using a press die having a rapid cooling mechanism. The subframe 10 is formed in this way as a continuous, irregular closed cross-sectional member. The steel pipe used in the hot-stamping steel pipe method is a welded steel pipe (electric resistance welded pipe) or a seamless steel pipe.

[0024] Steel vehicle suspension components currently available on the market have a strength class of approximately 400 to 600 MPa. In this embodiment, by using hot-stamped steel formed by a steel pipe hot stamping method as the structural members of the subframe 10, it is possible to achieve high strength of 1 to 1.5 GPa. The subframe 10 according to this embodiment is constructed from a curved, integral steel pipe, thereby reducing the number of parts and satisfying the rigidity and load transmission performance required of the subframe 10. The thickness of the steel pipe after forming is, for example, approximately 1 mm to 4.2 mm.

[0025] By hot stamping the steel pipe, the Vickers hardness of the center portion of the plate thickness of the structural member is 320 Hv or more when the test load is 9.87 N in a hardness test performed according to the method described in JIS Z 2244-1:2020. The upper limit of the hardness of the center portion in the plate thickness direction is not particularly specified, but it may be 600 Hv or less in Vickers hardness. The method for measuring the hardness of the center portion in the plate thickness direction of a structural member is as follows. A cross section perpendicular to the plate surface is taken from the wall portion of the structural member, and the sample of the measurement surface is adjusted and subjected to a hardness test. The measurement surface is adjusted according to JIS Z 2244-1:2020. The measurement surface is polished using silicon carbide paper of #600 to #1500, and then finished 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-finished surface is subjected to a hardness test according to 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 thickness of the sample, with a load of 9.87 N, and at intervals of at least three times the indentation, and the average value is taken as the hardness at the center in the thickness direction.

[0026] The structural members of the subframe 10 may be made of, for example, a high-pressure air blow molding die-hardened material. High-pressure air blow molding is a hot air blow molding technology that combines pressing and blow forming. Examples of high-pressure air blow molding die-hardened materials include those formed by Sumitomo Heavy Industries' STAF (registered trademark: No. 5673570, right holder: Sumitomo Heavy Industries, Ltd.) (Steel Tube Air Forming) method.

[0027] Fig. 3 is a diagram showing a method for forming a closed cross-section member from a steel pipe 100, which is an electrically heated material, by ordinary high-pressure air blow molding. When forming a closed cross-section member by high-pressure air blow molding, blow dies 110, 112 are placed at both ends of the steel pipe 100, as shown in Fig. 3. In addition, both ends of the steel pipe 100 are chucked by electrodes 120, 130, and the steel pipe 100 is rapidly heated by electrically heating with the electrodes 120, 130.

[0028] After the steel pipe 100 is heated and both ends of the steel pipe 100 are sealed using two blow dies 110, 112 on either side, the steel pipe 100 is sandwiched between dies 140, 142 and pressurized, and high-pressure air is injected into the interior of the steel pipe 100 from the blow dies 110, 112, thereby forming the steel pipe 100. Furthermore, by bringing the steel pipe 100 into close contact with the dies 140, 142 from the inside, the steel pipe 100 is rapidly cooled by the dies 140, 142, thereby performing quenching. In this way, in high-pressure air blow molding, by rapidly cooling the material (steel pipe 100) using the dies 140, 142 simultaneously with molding, it is possible to form a closed cross-section member using PHS (Press Hardened Steel).

[0029] On the other hand, the subframe 10 of this embodiment extends in a curved shape in a plan view as described above. If such a curved shape is to be formed by high-pressure air blow molding, it is necessary to reliably press the blow mold against the end of the steel pipe 100 to seal the steel pipe 100 when bending the steel pipe 100 during the forming process.

[0030] Therefore, when forming the subframe 10 according to this embodiment by high-pressure air blow molding, as shown in FIG. 4 , the clamping dies 150, 152 are positioned next to the electrode 170, and the clamping dies 150, 152 are operated before the dies 140, 142. The clamping dies 150, 152 then bite into the steel pipe 100, deforming and constraining the steel pipe 100. Then, the blow die 160 is pressed against the end of the steel pipe 100, and high-pressure air is injected into the steel pipe 100. By pressing the blow die 160 against the end of the steel pipe 100 while the steel pipe 100 is constrained by the clamping dies 150, 152, the blow die can be reliably crimped onto the end of the steel pipe 100, even when the steel pipe 100 is formed into a curved shape. This reliably seals the steel pipe 100, preventing high-pressure air from leaking from the inside to the outside of the steel pipe 100. 4 illustrates the clamping dies 150, 152, blow die 160, and electrode 170 only on the right side of the dies 140, 142, but the clamping dies 150, 152, blow die 160, and electrode 170 are also arranged on the left side of the dies 140, 142 in the same manner as on the right side. The same applies to Figures 5A to 5C described below. On the other hand, if the left end of the steel pipe 100 is closed, it is not necessary to provide the clamping dies 150, 152, blow die 160, and electrode 170 on the left side of the dies 140, 142.

[0031] 5A to 5C are diagrams showing the process flow for molding the subframe 10 according to this embodiment using high-pressure air blow molding technology.

[0032] 5A is a diagram showing a state in which the clamping dies 150, 152 are pressed against the steel pipe 100 from the state shown in FIG. 4. As a result, the clamping dies 150, 152 bite into the steel pipe 100, and the steel pipe 100 is restrained by the clamping dies 150, 152. In the state shown in FIG. 5A, the blow die 160 is separated from the end of the steel pipe 100. In addition, the end of the steel pipe 100 is chucked by an electrode 170, and the steel pipe 100 is rapidly heated by applying electrical heat to the electrode 170.

[0033] Next, as shown in Figure 5B, the blow die 160 is pressed against the end of the steel pipe 100, and high-pressure air is injected from the blow die 160 into the inside of the steel pipe 100. At this time, since the steel pipe 100 is restrained by the clamping dies 150, 152, the blow die 160 can be reliably pressed against the end of the steel pipe 100. Therefore, leakage of high-pressure air from the inside of the steel pipe 100 to the outside is suppressed.

[0034] 5C, the steel pipe 100 is sandwiched between the metal molds 140 and 142 and pressurized, and high-pressure air is injected into the steel pipe 100 from the blow mold 160, thereby forming the steel pipe 100. In addition, the steel pipe 100 is brought into close contact with the metal molds 140 and 142 from the inside, and the steel pipe 100 is rapidly cooled by the metal molds 140 and 142, thereby being quenched.

[0035] As described above, when forming the structural members of the subframe 10 according to this embodiment using high-pressure air blow molding technology, by restraining the steel pipe 100 with the clamping dies 150, 152, leakage of high-pressure air from the inside to the outside of the steel pipe 100 is suppressed, and curved structural members can be formed.

[0036] The structural members of the subframe 10 may also be made of cold-hydroformed materials formed by cold hydroforming using, for example, high-strength steel pipes. When forming cold-hydroformed materials, by using a clamping die as in Figures 4 and 5A to 5C, it is possible to form even curved shapes.

[0037] As described above, the structural members of the subframe 10 according to this embodiment may be made of hot-stamped material, high-pressure air blow-molded die-hardened material, or cold-hydroformed material. With this configuration, the structural members of the subframe 10 according to this embodiment can be manufactured using a simplified manufacturing process, reducing manufacturing costs, and achieving lighter weight and higher strength.

[0038] The tensile strength of the structural members of the subframe 10 is preferably 440 MPa or more. More preferably, the tensile strength of the structural members of the subframe 10 is 590 MPa or more, and even more preferably, 1000 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 138 Hv or more, more preferably 180 Hv or more, and even more preferably 320 Hv or more.

[0039] 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).

[0040] The structural members of the subframe 10 according to this embodiment are made of hot-stamped material, high-pressure air blow-molded die-hardened material, or cold-hydroformed material, resulting in integral, curved members. This allows the structural members to have no seams that cross the direction of their extension. Specifically, "integral" means that the structural members of the subframe 10 have no seams that cross the direction of their extension. Therefore, the subframe 10 according to this embodiment is configured to reduce the number of parts while still satisfying the rigidity and load transmission performance required of the subframe 10. Note that if the subframe 10 is made of welded steel pipes, the structural members may have welds (weld marks) as seams along the direction of their extension.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The subframe 10 also has two support portions 28 in the vehicle height direction at each of two rearward ends of the vehicle in the vehicle length direction. As shown in FIG. 2 , the steel pipes constituting the subframe 10 are opened from the outer sides in the vehicle width direction, thereby forming each of the two support portions 28 from a top plate and a bottom plate of the steel pipe. 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 rear side of the vehicle 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.

[0045] The subframe 10 is a component fixed to the lower part of a vehicle body (not shown). Loads from the road surface 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. Meanwhile, components such as the engine and 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 like a U-shape, as shown in FIG. 1 in a plan view. This allows the subframe 10 to maintain strength against loads from the road surface, while also improving space utilization efficiency by arranging components such as the engine, motor, and steering mechanism inside the curved shape.

[0046] Fig. 6 is a diagram showing a cross section taken along dashed dotted line II' in Fig. 1. Fig. 7A is a diagram showing a cross section taken along dashed dotted line II-II' (center line C) in Fig. 1. In the example shown in Figs. 6 and 7A, the cross section of the subframe 10 is rectangular. Because the subframe 10 is formed by the above-described construction method, as shown in Figs. 6 and 7A, welds (welding marks) 12 of the welded steel pipes are present in the cross section.

[0047] The structural members constituting the subframe 10 have a closed 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. That is, as shown in FIGS. 6 and 7A , the cross-sectional shape of the subframe 10 according to this embodiment is different between the cross-section taken along dashed dotted line II' and the cross-section taken along dashed dotted line II-II'. Note that, in a plan view of the subframe 10, the extension direction may be a direction along a straight line in a linear portion, or a tangent direction to the curvature in a curved portion. Here, in this embodiment, "different cross-sectional shapes" includes at least one of different cross-sectional sizes and different cross-sectional shapes. The cross-sections shown in FIGS. 6 and 7A are both rectangular (oblong), but have different cross-sectional sizes. Therefore, the cross-sections shown in FIGS. 6 and 7A have "different cross-sectional shapes" due to their different cross-sectional sizes.

[0048] The cross section of the subframe 10 may be a shape other than a rectangle, such as a circle, an ellipse, or a polygon. FIG. 7B is a diagram showing another example of a cross section taken along the dashed dotted line II-II' in FIG. 1, and FIG. 7C is a diagram showing yet another example of a cross section taken along the dashed dotted line II-II' in FIG. 1. In the example shown in FIG. 7B, the cross section is circular, and in the example shown in FIG. 7C, the cross section is elliptical. The cross sections shown in FIG. 6 and FIG. 7B have different cross-sectional shapes. Therefore, the cross sections shown in FIG. 6 and FIG. 7B have different cross-sectional shapes, and are therefore "different cross-sectional shapes." The same is true for the cross sections shown in FIG. 6 and FIG. 7C.

[0049] The cross section taken along dashed dotted line II' and the cross section taken along dashed dotted line II-II' in Figure 1 have different cross-sectional shapes, and as a result, the perimeter of the outer periphery of the cross section taken along dashed dotted line II-II' is different from the perimeter of the outer periphery of the cross section taken along dashed dotted line II-II'. Specifically, the perimeter of the outer periphery of the cross section taken along dashed dotted line II-II' is longer than the perimeter of the outer periphery of the cross section taken along dashed dotted line II-II'.

[0050] 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 perimeter of the cross section taken along the dashed dotted line II' is increased, thereby increasing the moment of inertia and polar moment of inertia. This improves the rigidity of the subframe 10, particularly its bending rigidity and torsional rigidity.

[0051] 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, the perimeter of the cross section taken along the dashed-dotted line II-II' is shortened near the center of the curved shape of the subframe 10 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, shortening the perimeter of the cross section taken along the dashed-dotted line II-II' relative to the cross section taken along the dashed-dotted line I-I' has a relatively small effect on the rigidity of the subframe 10. Therefore, by making the perimeter of the cross section taken along the dashed dotted line II-II' longer than the perimeter of the cross section taken along the dashed 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 components.

[0052] Preferably, the ratio (=L2 / L1) of the circumferential length L2 of the thickest cross section of the subframe 10 to the circumferential length L1 of the thinnest cross section of the subframe 10 satisfies the following formula (1). Note that the thinnest part is the part with the smallest area enclosed by the outer periphery of the cross section, and the thickest part is the part with the largest area enclosed by the outer periphery of the cross section: 1<(L2 / L1)≦3 (1)

[0053] The subframe 10 further includes flanges that protrude from the outer peripheral surfaces of the structural members that constitute the subframe and are integrally provided with the structural members along the extension direction of the structural members. FIG. 7D is a cross-sectional view of the subframe 10, similar to FIGS. 6 and 7A-7C, showing an example in which the subframe 10 is provided with flanges 10a. As shown in FIG. 7D, the subframe 10 may be formed as a continuous, deformed closed cross-section member with an integral flange 10a by steel pipe hot stamping, high-pressure air blow forming, or cold hydroforming. Providing the flanges 10a further increases the rigidity of the subframe 10. The flanges are preferably provided on the left and right sides of the subframe 10 in the vehicle width direction, for example, near the dashed-dotted line II' in FIG. 1, in order to increase the rigidity of the subframe 10 on both sides in the vehicle width direction. When the subframe 10 is formed as a continuous, deformed closed cross-section member with the flanges 10a, a material seam may be formed in the flanges 10a. FIG. 7E is a cross-sectional view showing in detail how the material seam 10b is formed on the flange 10a by molding in FIG. 7D.

[0054] The subframe 10 has a vehicle length direction distance from a fastening portion where the arm 52, which is located on the rear side of the vehicle, is fastened to its front end of the vehicle that 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 where 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 (2) is established: L≧200 (2)

[0055] 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 (2), sufficient rigidity can be ensured.

[0056] 8 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. 8, 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.

[0057] Fig. 9 is a plan view showing the subframe 10 in a plan view, similar to Fig. 1. In the example shown in Fig. 9, 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.

[0058] Fig. 10 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. 10. Fig. 11 is a perspective view of the subframe according to the comparative example shown in Fig. 10, viewed obliquely from above.

[0059] The subframe according to the comparative example is constructed by welding two cross members 60, 66 and two side members 62, 64 together.

[0060] 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.

[0061] As with the subframe 10 shown in Figure 1, in the subframe of the comparative example, an arm 50 is supported by an axle 26 inserted through holes 22a of two support parts 22, and an arm 52 is supported by an axle inserted through holes 28a of two support parts 28.

[0062] 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 steel pipe with a closed cross section.

[0063] Fig. 12 is a cross-sectional view taken along dashed dotted line III-III' in Fig. 10. Fig. 13 is a cross-sectional view taken along dashed dotted line IV-IV' in Fig. 10. As shown in Fig. 12, side member 62 is formed by line-welding members 62a and 62b together at welds 68. As shown in Fig. 13, 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.

[0064] 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 steel pipes with a closed cross section, which significantly reduces the weld line length of the components that make up the subframe 10. This increases the rigidity of the subframe 10 and the resonant frequency. Furthermore, the reduction in the number of welds and the weld line length reduces fatigue cracking in the welds and reduces corrosion initiation points, thereby suppressing corrosion. Furthermore, the reduction in the number of components that make up the subframe 10 reduces component costs, which in turn reduces the manufacturing cost of the subframe 10.

[0065] 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.

[0066] The inventors compared 14 examples of subframes 10 manufactured by steel pipe hot stamping, high-pressure air blow forming, or cold hydroforming, which differed in distance L and material, with Comparative Examples 1 and 2 shown in Figure 10 . The inventors evaluated component integration effects, such as the number of components and weld length, as well as component performance, such as static rigidity and vibration mode. Examples 1 to 8 were subframes 10 made of hot-stamped steel pipe with a tensile strength of 1500 MPa. Examples 9 to 11 were subframes 10 made of high-pressure air blow-formed and quenched steel with a tensile strength of 1500 MPa. Examples 12 to 14 were subframes 10 made of cold-hydroformed steel with a tensile strength of 980 MPa. Comparative Example 2 was a subframe 10 made of hot-stamped steel pipe with a tensile strength of 1500 MPa, but with a distance L of less than 200 mm. Analysis was performed using commercially available structural analysis software (Nastran, manufactured by MSC Software). More specifically, a structural analysis was performed assuming the input of a load (lateral force) directed from the outside to the inside of the vehicle in the vehicle width direction, and an eigenvalue analysis of vibration input was performed to evaluate the reaction force and natural frequency for each load. Examples 1 to 14 were the subframe 10 shown in Figure 1, and Comparative Example 1 was the same as Examples 1 to 14, except that the cross member 66 was removed from the subframe shown in Figure 10. Comparative Example 1 was a subframe 10 made of hot-rolled material with a tensile strength of 590 MPa.

[0067] In addition, Examples 1 to 14 of the present invention and Comparative Examples 1 and 2 were evaluated under conditions in which the weight of the subframe was approximately the same, and the evaluation was performed so that the influence of weight could be eliminated and comparisons based on differences in structure could be made.

[0068] In terms of the number of parts, Comparative Example 1, which excludes cross member 66 from the structure shown in Figure 10, has a total of six parts because each of cross members 60, 66 and side member 62 is made up of two members, an upper and lower. In contrast, the subframe 10 of Examples 1 to 14, which was manufactured using steel pipe hot stamping, high-pressure air blow forming, or cold hydroforming, has one part. Therefore, Examples 1 to 14 were able to reduce the number of parts by five compared to Comparative Example 1.

[0069] 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 14 to the weld line length in Comparative Example 1, and the weld line reduction rates in Examples 1 to 14 were all high, exceeding 40%.

[0070] Regarding component performance, the static rigidity of Examples 1 to 14 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 the Examples had static rigidity of 33.0 kN / mm or more, 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 14 relative to that of Comparative Example 1, resulting in a performance improvement effect of 3% or more. Furthermore, the natural frequencies (primary mode, secondary mode) of Examples 1 to 14 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 14 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 14 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 3% or more. As the natural frequency increases, resonance with road noise is suppressed. In the subframe 10, as the torsional rigidity increased, the natural frequency (primary mode) increased, and as the bending rigidity increased, the natural frequency (secondary mode) increased. According to invention examples 1 to 14, the natural frequency (primary mode) and the natural frequency (secondary mode) increased, and both the torsional rigidity and bending rigidity increased.

[0071] Furthermore, Comparative Example 2, in which the distance L was less than 200 mm, had lower static rigidity than Invention Examples 1 to 14, 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).

[0072]

[0073] REFERENCE SIGNS LIST 10 subframe 10a flange 12 welded portion (weld trace) 22 support portion 22a hole 26 shaft 28 support portion 28a hole 30 reinforcing member 50, 52 arm

Claims

1. A vehicle subframe having an integral structural member that is fixed to a vehicle body and extends 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 a closed cross-sectional shape perpendicular to the direction of extension, and the cross-sectional shape varies depending on the position of the structural member in the direction of extension.

2. The vehicle subframe according to claim 1, wherein in a cross section perpendicular to the extending direction of the structural member, a ratio of a perimeter L2 of the thickest cross section to a perimeter L1 of the thinnest part of the structural member satisfies the following formula (1): 1<(L2 / L1)≦3 ... (1) 3. The vehicle subframe according to claim 1 or 2, 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 (2): L≧200 ... (2) 4. A vehicle subframe as set forth in claim 1 or 2, further comprising a flange protruding from an outer peripheral surface of said structural member and integrally provided with said structural member along said extending direction of said structural member.

5. A vehicle subframe as set forth in claim 1 or 2, wherein the structural member has a Vickers hardness of 320 Hv or more at a central portion in a plate thickness direction.

6. The vehicle subframe according to claim 1 or 2, wherein the structural member has a tensile strength of 440 MPa or more.

7. A vehicle subframe as set forth in claim 1 or 2, wherein the structural member has a Vickers hardness of 138 Hv or more at a central portion in a plate thickness direction.

8. A subframe for a vehicle according to claim 1 or 2, wherein the structural member has no seams that cross the extension direction.