Vehicular arm member and method for manufacturing vehicular arm member

Abstract

According to the present invention, there is provided a vehicular arm member and a method for manufacturing a vehicular arm member, said arm member having a burred portion capable of ensuring a pull-out load even with higher-strength materials. A vehicular arm member according to the present invention has, in a burred portion, a cylindrical raised portion that mates with and holds a mating member for coupling with another member. The inner circumference of the raised portion has a plurality of coaxial same-diameter inner circumferential portions where the inner diameter increases in a stepwise manner proceeding from a leading-end side to a root side, with an annular step interposed at the boundary therebetween. The annular step serves as pull-out resistance step for the mating member. A method for manufacturing a vehicular arm member according to the present invention comprises molding a burred portion by means of a molding die having an inner diameter corresponding to an outer circumference of a raised portion of said burred portion, and a molding punch having, at a same-diameter inner circumferential portion farthest on a leading-end side, an outer diameter such that a clearance Ct with respect to the molding die is 70% to 100% of a material plate thickness, and having, at a same-diameter inner circumferential portion adjacent on a root side, an outer diameter such that a clearance Cn with respect to the molding die is at least 60% of the material plate thickness and less than the clearance Ct.

Description

Vehicle arm member and method of manufacturing the vehicle arm member

[0001] The present invention relates to a vehicle arm member and a method for manufacturing a vehicle arm member, and more particularly to a vehicle arm member and a method for manufacturing a vehicle arm member that has a burred portion that can ensure pull-out load even when the material is made high-strength.

[0002] A typical example of a vehicle arm component is a suspension arm, a key structural component of a suspension system that supports a vehicle body between the wheels and the vehicle body and absorbs shocks. The suspension system, also known as a suspension, is primarily composed of three components: a spring, a shock absorber, and a suspension arm. On the front wheel side, a suspension arm may consist of a combination of an upper arm and a lower arm, or just a lower arm, depending on the wheel suspension system. In the latter case, the suspension arm and the lower arm refer to the same component, just with different names. On the rear wheel side, a suspension arm may consist of an upper arm, a lower arm, and a trailing arm. The suspension arm illustrated here is sometimes called an automotive undercarriage component, and because they share the same required characteristics, they are collectively referred to as a vehicle arm component.

[0003] In recent years, automobile CO ２ From the viewpoint of reducing emissions and ensuring safety, efforts are being made to use thinner and stronger materials for vehicle arm members. Furthermore, weight reduction is being sought by optimizing the structure and shape of each part of the vehicle arm member. For example, to reduce weight, a known structure is one in which the arm body is composed of upper and lower halves, each of which is formed by press-molding a plate material into a U-shaped cross section, and the openings of the upper and lower halves are aligned to face each other, and the corresponding side walls are welded together.

[0004] Patent Document 1 discloses an invention that further reduces the weight of the upper and lower half structures of a vehicle arm member by forming a second vehicle body side connecting portion 14 with a simple structure in which only burring is performed on an extension portion of a lower half 10L of an arm main body 10, as shown in FIGS. 1 and 2. That is, in the vehicle arm member 1 described in this document, both the upper half 10U and the lower half 10L are press-formed from plate material into a U-shaped cross section, thereby reducing the number of parts and weight of the arm main body 10. Furthermore, in the vehicle arm member 1 described in this document, an outer end extension portion is provided only on the lower half 10L, excluding the upper half 10U, and a cylindrical burring portion 20 is integrally molded thereto to form the second vehicle body side connecting portion 14, thereby achieving weight reduction. To ensure the rigidity of the second vehicle body side connecting portion 14, the rising portion of the burring portion 20 formed on the outer end extension portion of the lower half 10L is connected to the outer end of the upper half 10U by a weld w. This vehicle arm member 1 is connected to a vehicle body subframe 32 on the vehicle body side via a second bushing 30a2 that is fitted into the second vehicle body side connecting portion 14. The vehicle arm member 1 described in Patent Document 1 is also called an L-type suspension arm or an L-type lower arm.

[0005] In the vehicle arm member 1 described in Patent Document 1, as shown in FIGS. 1 to 4, the first vehicle body side coupling section 12 is also formed by welding two members having similar burred portions 20 to the arm main body 10. As a result, the vehicle arm member 1 is coupled to a vehicle body subframe 32 on the vehicle body side via a first bushing 30a1 fitted to the first vehicle body side coupling section 12 (see FIGS. 1 and 3). Also, in the wheel support section 16 of the vehicle arm member 1, a similar burred portion 20 is formed by integrally molding by burring on an outer end extension provided only on the upper half body 10U, excluding the lower half body 10L. As a result, the vehicle arm member 1 is coupled to a wheel support member 34 via a ball joint 30bj fitted to the wheel support section 16 (see FIGS. 1 and 4).

[0006] JP 2010-111226 A

[0007] Recent automobile CO ２In order to reduce emissions, high-strength steel plates have come to be used to reduce the weight of vehicle bodies, and this has led to a problem of a decrease in the pull-out load of fitting members in burred portions. This problem will be explained using Figures 5 and 6.

[0008] 5 is a diagram showing a cross section of one side of a burred portion 20 in which a pilot hole (not shown) drilled in a metal plate is expanded and a rising portion 26 is formed around the pilot hole from a peripheral plate-like portion 22 via a curved portion 24. Generally, the diameter of the pilot hole d ０ The hole expansion ratio d / d is the ratio of the formable burring portion inner diameter d to the hole expansion ratio d (not shown). ０ has an inverse correlation with the material strength. Therefore, when trying to increase the strength of a material, the hole expansion ratio d / d ０ It should be noted that the resulting burring height h becomes smaller as the burring height h decreases. This is because a decrease in the burring height h reduces the contact area between the burring processed portion 20 and the fitting member, which directly leads to a decrease in the pull-out load of the fitting member.

[0009] In addition, while the inventors were studying the problem of a decrease in the pull-out load of a burred portion due to the increase in the strength of a vehicle arm member, they found that the above-mentioned hole expansion ratio d / d ０ We have found that there are other issues to be resolved besides the problem of the reduced contact area due to the decrease in the bearing capacity. For example, even if the strength of the burred portion is increased, it is rare that the specifications of the mating member to be fitted thereto are also changed. In such cases, it is expected that the collar of the mating member will have lower strength or a thinner plate thickness than the strengthened burred portion, resulting in a lower relative rigidity. Figure 6 is a schematic cross-sectional view showing how, when the bushing collar 30a of the mating member 30 under such conditions is fitted to the burred portion 20, the mating member 30, which has relatively low rigidity, bends, leaving only the contact area around the high support pressure portion P. Such contact only around the high support pressure portion P does not ensure a large contact area at the mating portion. Therefore, even if the vehicle arm member is strengthened, it is difficult to ensure the pull-out load of the burred portion.

[0010] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a vehicle arm member and a method for manufacturing a vehicle arm member, which have a burred portion that can ensure a pull-out load even when the material is made to have a high strength.

[0011] [1] A vehicle arm member having a burring processed portion, wherein the burring processed portion has a cylindrical raised portion that fits and holds a fitting member for connecting to another member, and the inner circumference of the cylindrical raised portion has a plurality of coaxial same-diameter inner circumference portions whose inner diameters increase stepwise from the tip side to the base side, with annular steps sandwiched between them at the boundaries, and the annular steps serve as pull-out resistance steps for the fitting member.

[0012] [2] The vehicle arm member described in [1], wherein a step amount Δd of the annular step in a direction perpendicular to the burring direction is 1.8% or more of the plate thickness tL of the same-diameter inner peripheral portion on the root side of the annular step, and a height position of the annular step in the burring direction satisfies hd≧0.49h, where hd is the height of the tip side of the annular step from the surface surrounding the burring processed portion and h is the burring height from the surface surrounding the burring processed portion.

[0013] [3] A method for manufacturing a vehicle arm member having a burring processing portion formed with a cylindrical raised portion, and having a plurality of coaxial same-diameter inner circumferential portions whose inner diameters gradually increase from the tip side to the base side on the inner periphery of the raised portion, with an annular step at the boundary between them, the method comprising: forming the burring processing portion in a single press process using a forming die having an inner diameter corresponding to the outer periphery of the raised portion of the burring processing portion; and a forming punch having an outer circumferential shape corresponding to the inner periphery of the raised portion of the burring processing portion, wherein a portion of the outer circumferential shape of the forming punch corresponding to the same-diameter inner circumferential portion on the tip side has an outer diameter that satisfies the burring processing condition of a clearance Ct between the forming die and the forming die being 70% or more and 100% or less of the thickness of the metal material, and a portion of the outer circumferential shape of the forming punch corresponding to the same-diameter inner circumferential portion on the base side of the same-diameter inner circumferential portion on the tip side has an outer diameter that satisfies the burring processing condition of a clearance Cn between the forming die and the forming die being 60% or more of the thickness of the metal material and less than the clearance Ct.

[0014] [4] A method for manufacturing a vehicle arm member having a burring processing part with a cylindrical rising portion formed thereon, in which a plurality of coaxial same-diameter inner peripheral portions whose inner diameters increase stepwise from the tip side to the base side are formed on the inner periphery of the rising portion with an annular step at the boundary therebetween, the method comprising: a first press forming step in which the same-diameter inner peripheral portion closest to the tip side of the inner periphery of the rising portion of the burring processing part is formed using a forming die having an inner diameter corresponding to the outer periphery of the rising portion of the burring processing part, and a forming punch having an outer diameter that satisfies the burring processing condition such that the clearance Ct between the forming die and the forming die is 70% or more and 100% or less of the thickness of the metal material; a second press forming step of forming the same diameter inner peripheral portion, located from the most distal end side of the inner periphery of the rising portion of the burring processing portion toward the base side, with respect to the burring processing portion formed by the first press forming step, using a forming die having an inner diameter corresponding to the outer periphery of the rising portion of the burring processing portion, and a forming punch having an outer diameter that satisfies burring processing conditions such that a clearance Cn between the forming die and the forming die is 60% or more of the thickness of the metal material and less than the clearance Ct.

[0015] [5] The manufacturing method of a vehicle arm member according to [3] or [4], wherein a step amount Δd of the annular step in a direction perpendicular to the burring direction is 1.8% or more of the plate thickness tL of the same-diameter inner peripheral portion on the root side of the annular step, and a height position of the annular step in the burring direction satisfies hd≧0.49h, where hd is the height of the tip side of the annular step from the surface surrounding the burring processed portion and h is the burring height from the surface surrounding the burring processed portion.

[0016] According to the present invention, the inner periphery of the cylindrical rising portion of the burred portion has a plurality of coaxial, same-diameter inner peripheries, the inner diameter of which increases stepwise from the tip side to the base side, with annular steps sandwiched between them, and this annular step can serve as a pull-out resistance step for the fitting member. Such burred portions can be formed using a forming punch having an outer diameter with different clearances relative to the forming die. As described above, according to the present invention, it is possible to provide a vehicle arm member and a method for manufacturing a vehicle arm member, which have a burred portion that can ensure pull-out load even when the material is made high-strength.

[0017] FIG. 1 is a diagram showing, in a simplified perspective view, a vehicle mounting mode of a vehicle arm member according to a conventional technique. FIG. 2 is a diagram showing, in a simplified cross-sectional view, a main portion of a second vehicle body side connecting portion of a burring processed portion of the vehicle arm member of FIG. 1 , taken along a cross-sectional view including the burring processed axis. FIG. 3 is a diagram showing, in a simplified cross-sectional view, a main portion of a first vehicle body side connecting portion of a burring processed portion of the vehicle arm member of FIG. 1 , taken along a cross-sectional view including the burring processed axis. FIG. 4 is a diagram showing, in a simplified cross-sectional view, a main portion of a wheel support portion of the burring processed portion of the vehicle arm member of FIG. 1 . FIG. 5 is a diagram showing, in a simplified cross-sectional view, a main portion of a wheel support portion of the burring processed portion of the vehicle arm member of FIG. 1 . FIG. 6 is a diagram showing, as a comparative example, a partial cross-sectional view of one side of the burring processed portion in a cross-section including the burring processed axis. FIG. 7 is a diagram showing, as a comparative example, a partial cross-sectional view of one side of the burring processed portion in a cross-section including the burring processed axis. FIG. 8 is a diagram showing, as a comparative example, a state in which a fitting member is fitted into the burring processed portion of a conventional vehicle arm member of a comparative example, taken along a cross-sectional view including the burring processed axis. FIG. 9 is a diagram showing, as a schematic plan view, a vehicle arm member according to an embodiment of the present invention. FIG. 1 is a diagram schematically showing a burred portion of a vehicle arm member according to an embodiment of the present invention, in a partial cross-sectional view of one side of the burred portion in a cross section including the burring axis. FIG. 2 is a diagram schematically showing a state in which a fitting member is fitted into the burred portion of a vehicle arm member according to an embodiment of the present invention, in a partial cross-sectional view of one side of the burred portion in a cross section including the burring axis. FIG. 3 is a diagram showing the influence of the step amount of an annular step provided in the burred portion and the burring height direction position on the effect of improving pull-out load of a vehicle arm member according to an embodiment of the present invention. FIG. 4 is a diagram schematically showing, in a partial cross-sectional view of a cross section including the burring axis, a stage in the burring process in a manufacturing method for a vehicle arm member according to an embodiment of the present invention, where the tip-side same-diameter inner circumferential portion has been completely formed over the entire area of ​​the rising portion. FIG. 5 is a diagram schematically showing, in a partial cross-sectional view of a cross section including the burring axis, a stage in the burring process in a manufacturing method for a vehicle arm member according to an embodiment of the present invention, where the base-side same-diameter inner circumferential portion has been completely formed over the base portion of the rising portion. 1 is a diagram showing a cross-sectional view of a composite forming punch used in a burring process in a manufacturing method of a vehicle arm member according to an embodiment of the present invention, the cross-section including a burring axis. FIG.1 is a partial cross-sectional view showing a stage in which a tip-side same-diameter inner peripheral portion is formed in a rising portion by a first shaping punch during a burring process in a modified example of a manufacturing method for a vehicle arm member according to an embodiment of the present invention, and a partial cross-sectional view showing a stage in which a base-side same-diameter inner peripheral portion is formed in a rising portion by a second shaping punch during a burring process in a modified example of a manufacturing method for a vehicle arm member according to an embodiment of the present invention,

[0018] Embodiments of the present invention will be described in detail below with reference to the drawings. In the following embodiments, a suspension arm or a lower arm will be used as an example of a vehicle arm member, and the description will focus on the burred portion. FIG. 7 is a schematic plan view of an L-shaped lower arm 1a, which is an example of a vehicle arm member 1 according to an embodiment of the present invention. FIG. 8 is a schematic partial cross-sectional view of one side of the burred portion of a vehicle arm member according to an embodiment of the present invention, taken along a cross section including the burred axis. FIG. 9 is a schematic partial cross-sectional view of one side of the burred portion of a vehicle arm member according to an embodiment of the present invention, taken along a cross section including the burred axis, showing how a fitting member is fitted into the burred portion of a vehicle arm member according to an embodiment of the present invention. FIG. 10 is a diagram showing the influence of the step height and burring height position of an annular step provided in the burred portion of a vehicle arm member according to an embodiment of the present invention on the pull-out load improvement effect. In the following embodiments, identical or common parts are designated by the same reference numerals in the drawings, and their description will not be repeated. It should be noted that the present invention is not limited to the following embodiments.

[0019] As shown in Figures 7 to 10, the vehicle arm member 1 (L-shaped lower arm 1a) according to the embodiment of the present invention has a burred portion 20 in the second vehicle body side connecting portion 14, which is capable of ensuring a pull-out load even when the material is made high strength. Note that Figure 8 is a partial cross-sectional view of only the burred portion 20 of the second vehicle body side connecting portion 14, as seen in the direction of the arrow A-A in Figure 7. Furthermore, since the L-shaped lower arm 1a according to the embodiment is equivalent as a vehicle arm member to the L-shaped lower arm 1a according to the prior art shown in Figures 1 to 4, when referring to parts not explicitly shown in Figure 7, the drawings and symbols of the prior art may be used in the description.

[0020] The vehicle arm member or manufacturing method for a vehicle arm member according to this embodiment can be preferably used in particular for steel members having a tensile strength of 590 MPa or more. The vehicle arm member or manufacturing method for a vehicle arm member according to this embodiment can be preferably used for steel members having a tensile strength of 780 MPa or more to 980 MPa or more. The vehicle arm member or manufacturing method for a vehicle arm member according to this embodiment can be more preferably used for steel members having a tensile strength of 1180 MPa or more.

[0021] The tensile strength of the above-mentioned steel member can be evaluated by a tensile test on a tensile test piece obtained from a portion of the vehicle arm member corresponding to the head position of the forming punch of the press molding machine, which is hardly subjected to work hardening due to press molding. On the other hand, even if a member area sufficient for obtaining a tensile test piece cannot be secured in the portion of the vehicle arm member corresponding to the head of the forming punch, the tensile strength of the steel member can be evaluated by cutting a cross-sectional sample from the portion and measuring the Vickers hardness at the 1 / 4 thickness position. In this case, the Vickers hardness test load is preferably 9.807 N, taking into account the size of the indentation that appears in proportion to the strength of the measured sample, for a tensile strength range of 590 MPa to 1480 MPa of steel that can be suitably used in this embodiment. The Vickers hardness of the steel materials with tensile strength levels of 590 MPa, 780 MPa, 980 MPa, and 1180 MPa, which are exemplified as steel materials suitable for use in this embodiment, is estimated to be 184 Hv, 245 Hv, 310 Hv, and 372 Hv or more, respectively.

[0022] The thickness of the steel material used in the vehicle arm member or the method for manufacturing a vehicle arm member according to this embodiment is not particularly limited, but is, for example, 3 mm or more and 6 mm or less.

[0023] 7, the L-shaped lower arm 1a according to this embodiment is formed in a generally L-shape in a plan view, and is provided at one end thereof with a wheel support portion 16 that is swingably connected to a wheel support member 34 (not shown) via a ball joint 30bj (not shown). A first vehicle body-side coupling portion 12 that can be swingably connected to the vehicle body via a first bushing 30a1 (not shown) whose axis is aligned in the longitudinal direction of the vehicle body is provided at a bent intermediate portion of the arm main body 10. Furthermore, a second vehicle body-side coupling portion 14 that can be swingably connected to the vehicle body via a second bushing 30a2 (not shown) whose axis is aligned in the vertical direction is provided at the other end of the arm main body 10, i.e., the rear end in the longitudinal direction of the vehicle body.

[0024] The arm body 10 of the L-shaped lower arm 1a according to this embodiment is preferably lightweight due to its upper and lower half structure, as in the prior art. That is, the arm body 10 according to this embodiment has a hollow, closed cross-section structure in which an upper half 10U and a lower half 10L, each formed by press-forming a steel plate, are integrally joined together. The upper half 10U has an upper wall portion and a pair of side walls (not shown) extending downward from both widthwise sides of the upper wall portion, forming an inverted U-shaped cross section. The lower half 10L is essentially formed as a flat plate and is generally L-shaped in plan view so as to close the open lower end of the upper half 10U. Both widthwise end surfaces (not shown) of the lower half 10L are welded to the inner surfaces of the pair of side walls (not shown) of the upper half 10U.

[0025] Next, the structure of the second vehicle-body-side connecting portion 14 having the burred portion 20 according to this embodiment will be described with reference to Figure 7, as well as Figures 1 and 2. On the second vehicle-body-side connecting portion 14 side of the arm main body 10, the lower half 10L extends outward (toward the rear of the vehicle body) further than the upper half 10U, and the burred portion 20 that becomes the second vehicle-body-side connecting portion 14 is formed on this extending portion. An upward, cylindrical raised portion 26 into which the bushing collar 30a of the second bushing 30a2 can be press-fitted is integrally formed on this burred portion 20. The outer peripheral surface of this cylindrical raised portion 26 near the middle of the arm main body 10 is joined by a weld w to the outer end of the upper half 10U, which is formed in a corresponding arc shape in plan view. The second vehicle body side connecting portion 14 having such a structure ensures strength and rigidity while reducing the amount of extension of the upper half 10U toward the rear of the vehicle body, thereby achieving weight reduction, and as a result, burring of the upper half 10U can be omitted, thereby simplifying the manufacturing process, etc. In the L-shaped lower arm 1a according to this embodiment shown in Figure 7, the burring is employed only in the second vehicle body side connecting portion 14, but it may also be employed in the first vehicle body side connecting portion 12 or the wheel support portion 16, as in the L-shaped lower arm 1a according to the prior art shown in Figures 1 to 4.

[0026] In the burred portion 20 according to this embodiment, an annular step 28 provided on the inner periphery of the rising portion 26 functions as a pull-out resistance step that increases the pull-out load of a fitting member 30 fitted to the inner periphery of the rising portion 26. This will be explained using Figures 5, 6, and 8 to 10. Figure 5 shows the burred portion of a conventional vehicle arm member, and Figure 6 shows a state in which a fitting member is fitted to the burred portion of the conventional vehicle arm member, as a partial cross-sectional view of one side of the burred portion within a cross section including the burred processing axis.

[0027] As described above, when the burring processed portion 20 of the conventional vehicle arm member shown in FIG. 5 is strengthened, the fitting member 30, which has a relatively low rigidity, bends, as shown in FIG. 6, so that the contact portion becomes only around the high support pressure portion P, which causes a problem in that it becomes difficult to ensure the pull-out load.

[0028] In contrast to the conventional technology, the burred portion 20 according to this embodiment has, as shown in FIG. 8 , a plurality of coaxial same-diameter inner circumferential portions 26A (26A1, 26A2) on the inner periphery of a cylindrical raised portion 26, whose inner diameters increase stepwise from the tip side to the base side (d1<d2). An annular step 28 is formed at the boundary between the tip-side same-diameter inner circumferential portion 26A1 and the base-side same-diameter inner circumferential portion 26A2, which have different inner diameters. According to this embodiment, the gap that occurs in the conventional technology shown in FIG. 6 , i.e., the gap distal to the high support pressure portion P between the inner periphery of the raised portion 26 and the outer periphery of the fitting member 30, can be filled by the tip-side same-diameter inner circumferential portion 26A1 sandwiching the annular step 28, as shown in FIG. 9 . As a result, in the burred portion 20 according to this embodiment, contact areas between the inner periphery of the cylindrical rising portion 26 and the fitting member 30 can be generated around at least two high support pressure portions P, namely, around the tip-side high support pressure portion P1 and around the base-side high support pressure portion P2. Therefore, the burred portion 20 according to this embodiment can be a burred portion 20 that can advantageously ensure pull-out load even when the material is made high-strength. Note that, although the description up to this point has been made of a combination of two same-diameter inner peripheries 26A and one boundary annular step 28, this is not limiting, and a combination of three or more same-diameter inner peripheries 26A and two or more boundary annular steps 28 may also be used.

[0029] In the vehicle arm member or the manufacturing method of the vehicle arm member according to this embodiment, the size of the burred portion is not particularly limited, but for example, the inner diameter (see d1 in Figure 8) is 10 mm or more and 100 mm or less, and the burring height (see h in Figure 8) is 3 mm or more and 15 mm or less.

[0030] As a preliminary experiment leading to the present invention, the inventors investigated the influence of the step amount Δd and the burring height position (height hd) of the annular step 28 of the burring-processed portion 20 shown in FIG. 8 on the effect of improving the pull-out load. The material used in the investigation was a high-strength steel plate with a plate thickness of 3.2 mm and a tensile strength of 980 MPa. The burring-processed portion was formed using an experimental device with a mold configuration shown in FIGS. 11A to 11C or 12A and 12B, described below, so that multiple levels of the step amount Δd and the burring height position (height hd) of the annular step were obtained. Note that here, the burring height position is represented by the height hd of the tip side of the annular step from the surrounding surface of the burring-processed portion, as shown in FIG. 8.

[0031] Furthermore, we measured the pull-out load from the burred portion under the various conditions described above using a bushing collar with a plate thickness of 2.3 mm and a tensile strength of 590 MPa as the mating member to be fitted to the burred portion. The results are summarized in Figure 10. The pull-out load improvement rate (Fd-F) / F on the vertical axis represents the maximum pull-out load Fd of the various annular steps prepared, relative to the pull-out load F when there is no annular step. The annular step height Δd / tL on the horizontal axis represents the step height Δd in the direction perpendicular to the burring direction, non-dimensionalized by the plate thickness tL of the same-diameter inner peripheral portion at the base of the annular step. The data in Figure 10 is also stratified by hd / h, which is the height hd of the tip of the annular step from the surrounding surface of the burred portion, non-dimensionalized by the burring height h from the surrounding surface of the burred portion.

[0032] As shown in Figure 10, if the pull-out load improvement rate (Fd-F) / F is preferably 10% or more, when the height hd / h of the annular step in the burring direction is 32%, there is no preferred range for the step amount Δd / tL of the annular step. On the other hand, when the height hd / h of the annular step in the burring direction is 49 to 84%, the step amount Δd / tL of the annular step is 1.8% or more, and the pull-out load improvement rate (Fd-F) / F is 10% or more, so there is a preferred range. However, even in this case, as the step amount Δd / tL of the annular step increases, the pull-out load improvement rate (Fd-F) / F falls below 10%. Thus, the upper limit of the step amount Δd / tL of the annular step at which the pull-out load improvement rate (Fd-F) / F falls below 10% varies depending on the height hd / h of the annular step in the burring direction. That is, when hd / h = 84%, the upper limit of the step amount Δd / tL of the annular step is 23%, when hd / h = 66%, the upper limit of the step amount Δd / tL of the annular step is 10%, and when hd / h = 49%, the upper limit of the step amount Δd / tL of the annular step is 5%.

[0033] In particular, when the height hd / h of the annular step in the burring direction is in the range of 49 to 84% and the step amount Δd / tL of the annular step is in the range of 1.8 to 5%, the maximum value of the pull-out load improvement rate (Fd-F) / F exceeds 30%, which shows that these are particularly preferable conditions for the annular step.

[0034] Based on the results of the preliminary experiments described above, the inventors have determined that the step amount Δd of the annular step in the direction perpendicular to the burring direction should preferably be 1.8% or more of the plate thickness tL of the same-diameter inner peripheral portion on the base side of the annular step. In this regard, the inventors have determined that the height position of the annular step in the burring direction should preferably simultaneously satisfy hd≧0.49h, where hd is the height of the tip side of the annular step from the surface surrounding the burred portion and h is the burring height from the surface surrounding the burred portion.

[0035] If the step amount Δd / tL of the annular step is less than 1.8%, the gap on the tip side of the high support pressure portion P shown in Fig. 6 cannot be filled by the tip-side same-diameter inner circumferential portion 26A1 sandwiched between the annular step 28 shown in Fig. 8, and the tip-side high support pressure portion P1 shown in Fig. 9 does not occur, so the effect of improving the pull-out load cannot be obtained. Conversely, if the step amount Δd / tL of the annular step is large and the pull-out load improvement rate Fd / F is less than 10%, the root-side high support pressure portion P2 of the root-side same-diameter inner circumferential portion 26A2 sandwiched between the annular step 28 as shown in Fig. 9 does not occur, and it is estimated that the effect of improving the pull-out load will be reduced.

[0036] Next, a manufacturing method of a vehicle arm member according to this embodiment will be described with reference to FIGS. 11A to 11C, 12A, and 12B. The burring process occupies a portion of a press-formed product, such as a lower half. While this burring process and the press-forming process for the entire member may be performed separately, the burring process is typically incorporated into the press-forming process for the entire member for production efficiency. Furthermore, the manufacturing method of the vehicle arm member according to this embodiment, excluding the burring process, is similar to conventional manufacturing methods, such as press-forming each component member, such as a lower half, and assembling the components obtained by press-forming by welding, etc. Therefore, the description of the manufacturing method of the vehicle arm member according to this embodiment will be limited to the description of the manufacturing method of the burring process portion, and detailed description of the manufacturing method common to conventional manufacturing methods will be omitted.

[0037] 11A and 11B are diagrams that schematically explain, in partial cross-sectional views at a cross section including the burring processing axis, the final stage of the burring processing step when a burring processing portion is formed in a single press process in a manufacturing method for a vehicle arm member according to an embodiment of the present invention.

[0038] 11C shows the cross-sectional shape of the composite forming punch 40A used herein, including the central axis. A small-diameter cylindrical portion 40Aa is disposed at the tip of the composite forming punch 40A. By inserting this small-diameter cylindrical portion 40Aa into the pilot hole of the blank before burring, the burring position of the blank can be aligned with the central positions of the burring processing portion 20, the forming punch 40, and the forming die 44. The conical portion 40Ab of the punch is a portion that pushes the peripheral plate-like portion 22 around the pilot hole in the out-of-plane direction while the peripheral plate-like portion 22 around the pilot hole is clamped between the holder 42 and the forming die 44, gradually expanding the pilot hole in the blank to the inner diameter of the burring processing portion. The cylindrical front portion 40Ac of the punch is a portion that forms the raised portion 26 having a same-diameter inner periphery 26A of inner diameter d1 in the height direction of the burring processing portion protruding from the conical portion 40Ab, while maintaining a predetermined clearance Ct with the inner diameter of the forming die 44. The cylindrical rear portion 40Ae of the punch is a portion that forms a root-side same-diameter inner periphery 26A2 of inner diameter d2 at a predetermined height on the root side of the same-diameter inner periphery 26A of inner diameter d1 formed by the cylindrical front portion 40Ac, while maintaining a predetermined clearance Cn with the inner diameter of the forming die 44. The step portion 40Ad of the punch is a boundary between the cylindrical front portion 40Ac and the cylindrical rear portion 40Ae, and is a portion that forms an annular step 28 on the inner circumference of the burring processing portion 20.

[0039] In the manufacturing method of a vehicle arm member according to an embodiment of the present invention, a forming die 44 having an inner diameter corresponding to the outer periphery of the rising portion of the burred portion is used. Furthermore, a composite forming punch 40A having an outer periphery shape corresponding to the inner periphery of the rising portion 26 of the burred portion 20, as shown in Fig. 11C, is used as the forming punch 40. By employing such a die configuration, the burred portion 20 of the vehicle arm member according to this embodiment can be formed in a single press process.

[0040] The steps up to Figure 11A in this single pressing process represent the stage where the same-diameter inner peripheral portion 26A, having an inner diameter d1, has been completely formed over the entire inner periphery of the raised portion 26, while maintaining a predetermined clearance Ct between the cylindrical front portion 40Ac of the composite forming punch 40A and the inner diameter portion of the forming die 44. Note that the burring steps up to Figure 11A overlap with the description of the small-diameter cylindrical portion 40Aa and the conical portion 40Ab of the composite forming punch, and therefore will not be described here. Figure 11B shows the stage where the same-diameter inner peripheral portion 26A2, having an inner diameter d2, has been completely formed over the entire inner periphery of the raised portion 26, while maintaining a predetermined clearance Cn between the cylindrical rear portion 40Ae of the composite forming punch 40A and the inner diameter portion of the forming die 44. At this stage, an annular step 28 is formed at a predetermined height position on the inner periphery of the rising portion 26 as the boundary between the tip side same-diameter inner periphery portion 26A1 and the root side same-diameter inner periphery portion 26A2.

[0041] The predetermined clearance Ct will now be described. That is, the clearance Ct between the cylindrical front portion 40Ac of the composite forming punch 40A, which corresponds to the same-diameter inner peripheral portion 26A1 closest to the tip, and the inner diameter portion of the forming die 44 is set to 70% to 100% of the thickness of the metal blank. If this clearance Ct exceeds 100% of the thickness of the metal blank, the cylindrical shape of the rising portion of the burred portion 20 is likely to become unstable, which is undesirable. On the other hand, if this clearance Ct is less than 70% of the thickness of the metal blank, the ironing burring process will be performed under severe working conditions from the tip side to the base side, which is undesirable because it increases the risk of fracture of the burred portion.

[0042] Furthermore, the clearance Cn between the cylindrical rear portion 40Ae of the composite forming punch 40A, which corresponds to the same-diameter inner peripheral portion 26A2 closest to the base from the tip, and the inner diameter portion of the forming die 44 is set to be at least 60% of the thickness of the metal blank but less than the clearance Ct. Setting this clearance Cn to be equal to or greater than the clearance Ct is equivalent to attempting to reduce the inner diameter of the same-diameter inner peripheral portion 26A closer to the base than to the tip, which is undesirable because it would be impossible to form such a shape of the same-diameter inner peripheral portion 26A in a single press process. On the other hand, if this clearance Cn were less than 60% of the thickness of the metal blank, it would result in extremely severe processing conditions that exceed the ironing burring processing conditions, which is undesirable because fracture of the burred portion would be unavoidable.

[0043] As described above, the manufacturing method of the vehicle arm member according to the present embodiment, in which the burred portion of the vehicle arm member is formed in a single press step, has been described using Figures 11A to 11C. However, in this embodiment, the formation of the burred portion is not limited to a single press step. For example, as shown in Figure 12A, the process up to the intermediate stage of the burring process, in which a same-diameter inner peripheral portion 26A having an inner diameter d1 is formed around the entire inner circumference of the raised portion 26 using a first forming punch 40B1, may be divided into burring processes similar to those shown in Figure 11A. Then, as shown in Figure 12B, a burring process similar to that shown in Figure 11B may be added, in which a root-side same-diameter inner peripheral portion 26A2 having an inner diameter d2 is formed at the base of the inner circumference of the raised portion 26 using a second forming punch 40B2, to complete the burring process.

[0044] DESCRIPTION OF SYMBOLS 1 Vehicle arm member 1a L-shaped lower arm 10 Arm body 10L Lower half 10U Upper half 12 First vehicle body side connecting portion 14 Second vehicle body side connecting portion 16 Wheel support portion 20 Burring processed portion 22 Peripheral plate-shaped portion 24 Curved portion 26 Rising portion 26A Same diameter inner peripheral portion 26A1 Tip side same diameter inner peripheral portion 26A2 Root side same diameter inner peripheral portion 28 Annular step 30 Fitting member 30a Bush collar 30a1 First bush 30a2 Second bush 30bj Ball joint 32 Vehicle body subframe 34 Wheel support member 40 Forming punch 40A Composite forming punch 40Aa Thin diameter cylindrical portion 40Ab Conical portion 40Ac Cylindrical front portion 40Ad Step portion 40Ae Rear part of cylinder 40B1 First forming punch 40B2 Second forming punch 42 Holder 44 Forming die d Inner diameter of burring part d1 Inner diameter of same-diameter inner peripheral part on tip side d2 Inner diameter of same-diameter inner peripheral part on root side Δd Step amount of annular step F Pull-out load when there is no annular step Fd Pull-out load when there is annular step h Burring height hd Height of annular step position tL Plate thickness of same-diameter inner peripheral part on root side P High support pressure part P1 Tip-side high support pressure part P2 Root-side high support pressure part w Welded part

Claims

1. A vehicle arm member having a burring processed portion, wherein the burring processed portion has a cylindrical raised portion that fits and holds a fitting member for connecting to another member, and the inner circumference of the cylindrical raised portion has a plurality of coaxial same-diameter inner circumference portions whose inner diameters increase stepwise from the tip side to the base side, with annular steps sandwiched between them at the boundaries, and the annular steps serve as steps that provide resistance to the fitting member being pulled out.

2. A vehicle arm member according to claim 1, wherein a step amount Δd of the annular step in a direction perpendicular to the burring direction is 1.8% or more of the plate thickness tL of the same-diameter inner peripheral portion on the base side of the annular step, and a height position of the annular step in the burring direction satisfies hd≧0.49h, where hd is the height of the tip side of the annular step from the surface surrounding the burring processed portion and h is the burring height from the surface surrounding the burring processed portion.

3. A method for manufacturing a vehicle arm member having a burring processing section formed with a cylindrical raised section, the inner circumference of which is formed with a plurality of coaxial same-diameter inner circumference sections whose inner diameters gradually increase from the tip side to the base side, with annular steps at the boundaries, wherein the burring processing section is formed in a single press process using a forming die having an inner diameter corresponding to the outer circumference of the raised section of the burring processing section, and a forming punch having an outer circumference shape corresponding to the inner circumference of the raised section of the burring processing section, wherein a portion of the outer circumference shape of the forming punch corresponding to the same-diameter inner circumference section on the tip side has an outer diameter that satisfies the burring processing condition of a clearance Ct between the forming die and the punch being 70% or more and 100% or less of the thickness of the metal material, and a portion of the outer circumference shape of the forming punch corresponding to the same-diameter inner circumference section on the base side of the same-diameter inner circumference section on the tip side has an outer diameter that satisfies the burring processing condition of a clearance Cn between the forming die and the punch being 60% or more of the thickness of the metal material and less than the clearance Ct.

4. A method for manufacturing a vehicle arm member having a burring processing part formed with a cylindrical rising portion, wherein a plurality of coaxial same-diameter inner peripheral portions whose inner diameters increase stepwise from the tip side to the base side are formed on the inner periphery of the rising portion with an annular step at the boundary, the method comprising: a first press forming step in which the same-diameter inner peripheral portion closest to the tip side of the inner periphery of the rising portion of the burring processing part is formed using a forming die having an inner diameter corresponding to the outer periphery of the rising portion of the burring processing part, and a forming punch having an outer diameter that satisfies the burring processing condition such that the clearance Ct between the forming die and the forming die is 70% to 100% of the thickness of the metal material; a second press forming step of forming the same diameter inner peripheral portion, located from the most distal end side of the inner periphery of the rising portion of the burring processing portion toward the base side, with respect to the burring processing portion formed by the first press forming step, using a forming die having an inner diameter corresponding to the outer periphery of the rising portion of the burring processing portion, and a forming punch having an outer diameter that satisfies burring processing conditions such that a clearance Cn between the forming die and the forming die is 60% or more of the thickness of the metal material and less than the clearance Ct.

5. A method for manufacturing a vehicle arm member according to claim 3 or 4, wherein a step amount Δd of the annular step in a direction perpendicular to the burring direction is 1.8% or more of the plate thickness tL of the same-diameter inner peripheral portion on the base side of the annular step, and a height position of the annular step in the burring direction satisfies hd≧0.49h, where hd is the height of the tip side of the annular step from the surface surrounding the burring processed portion and h is the burring height from the surface surrounding the burring processed portion.

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