Vehicle arm member and method for manufacturing vehicle arm member

Abstract

The present invention provides a vehicle arm member and a method for manufacturing a vehicle arm member, having a burring section that ensures pull-out load even when the material is made high-strength. The vehicle arm member of the present invention has a cylindrical rising section in the burring section for fitting and holding a fitting member for connecting with other members, and the inner circumference of the rising section has a plurality of coaxial inner circumference sections of the same diameter, with the inner diameter increasing in stages from the tip side to the root side, separated by annular steps, with the annular steps serving as pull-out resistance steps for the fitting member. The method for manufacturing a vehicle arm member of the present invention involves forming the burring section with a forming die having an inner diameter corresponding to the outer circumference of the rising section of the processing section, and a forming punch having an outer diameter with a clearance Ct between the forming die and the inner circumference of the same diameter at the tipmost end that is 70% to 100% of the material plate thickness, and an outer diameter with a clearance Cn between the forming die and the adjacent inner circumference of the same diameter at the root side that is 60% to less than the clearance Ct of the material plate thickness.

Description

Technical Field

[0001] The present invention relates to a vehicle arm member and a method for manufacturing a vehicle arm member. More specifically, the present invention relates to a vehicle arm member and a method for manufacturing a vehicle arm member, which have a burring portion capable of ensuring a drawing load even when the material is strengthened.

Background Art

[0002] A typical vehicle arm member is a suspension arm, which is a main structural member of a suspension device that supports the vehicle body and absorbs shocks between the wheel and the vehicle body. The suspension device is also called a suspension and is mainly composed of three parts: a spring, a shock absorber, and a suspension arm. The suspension arm may be composed of a combination of an upper arm and a lower arm or only a lower arm depending on the wheel suspension method on the front wheel side. In particular, in the latter case, the suspension arm and the lower arm refer to the same member, only with different names. Also, on the rear wheel side, the suspension arm may be composed of adding a trailing arm to the upper arm and the lower arm. The suspension arm exemplified here may also be called an automotive underbody part, and since the required characteristics are common, they are collectively referred to as vehicle arm members here.

[0003] In recent years, from the viewpoints of reducing CO2 emissions and ensuring safety of automobiles, the material of vehicle arm members has been made thinner and stronger. Also, weight reduction has been achieved by optimizing the structure and shape of each part of the vehicle arm member. For example, for weight reduction, the arm body is composed of an upper half body and a lower half body each formed into a U-shaped cross section by press-forming a plate material, and a structure is known in which the corresponding side wall portions are welded to each other with the openings of the upper and lower half bodies facing each other.

[0004] Patent Document 1 discloses an invention that further reduces weight from the upper and lower half structure of a vehicle arm member, and forms a second vehicle body side connecting portion 14 with a simple structure in which only burring is applied to the extension portion of the lower half 10L of the arm body 10, as shown in Figures 1 and 2. In other words, in the vehicle arm member 1 described in the document, both the upper half 10U and the lower half 10L are press-formed from sheet metal into a U-shape in cross-section, thereby reducing the number of parts and weight of the arm body 10. Furthermore, in the vehicle arm member 1 described in the document, weight reduction is also achieved by providing an outer end extension only on the lower half 10L, excluding the upper half 10U, and integrally molding a cylindrical burring portion 20 thereto to form the second vehicle body side connecting portion 14. In order 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 and the outer end of the upper half 10U are connected by a welded portion w. This vehicle arm member 1 is connected to the vehicle body subframe 32 on the vehicle body side via a second bush 30a2 that fits into the second vehicle body side connecting portion 14. The vehicle arm member 1 described in Patent Document 1 is also referred to as an L-shaped suspension arm or an L-shaped lower arm.

[0005] In the vehicle arm member 1 described in Patent Document 1, as shown in Figures 1 to 4, the first vehicle body side connecting portion 12 is also constructed by welding two members having similar burring portions 20 to the arm body 10. As a result, the vehicle arm member 1 is connected to the vehicle body subframe 32 on the vehicle body side via a first bush 30a1 that fits into the first vehicle body side connecting portion 12 (see Figures 1 and 3). In addition, in the wheel support portion 16 of the vehicle arm member 1, a similar burring portion 20 is integrally formed by burring on the outer end extension portion provided only on the upper half 10U, excluding the lower half 10L. As a result, the vehicle arm member 1 is connected to the wheel support member 34 via a ball joint 30bj that fits into the wheel support portion 16 (see Figures 1 and 4). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-111226 [Overview of the project] [Problems that the invention aims to solve]

[0007] In recent years, with the aim of reducing CO2 emissions from automobiles, high-strength steel plates have been used to lighten vehicle bodies, leading to concerns about the reduced pull-out load of fitting members in burred sections. This problem will be explained using Figures 5 and 6.

[0008] Figure 5 schematically shows a cross-section of one side of a burring section 20 in which a pilot hole (not shown) drilled in a metal plate is expanded, and a rising section 26 is formed around the pilot hole via a curved section 24 from a peripheral plate-like section 22. Generally, the hole expansion ratio d / d0, which is the ratio of the formable inner diameter d of the burring section to the pilot hole diameter d0 (not shown), has an inverse correlation with the material strength. Therefore, when trying to increase the strength of the material, it is necessary to note that the hole expansion ratio d / d0 at the processing limit becomes smaller, and the resulting burring height h becomes lower. This is because a decrease in burring height h reduces the contact area with the mating member at the burring section 20, which directly leads to a decrease in the pull-out load of the mating member.

[0009] Furthermore, while investigating the problem of reduced pull-out load in the burred section due to increased strength of the vehicle arm member, the inventors discovered that there were other problems to be solved besides the problem of reduced contact area due to the decrease in the hole expansion ratio d / d0. For example, even if the burred section is made stronger, the specifications of the fitting member that is fitted to it are rarely changed. In such cases, it is expected that the collar of the fitting member will have lower strength or a thinner plate thickness, resulting in lower relative rigidity compared to the strengthened burred section. Figure 6 is a schematic cross-sectional view showing how, when the bush collar 30a of the fitting member 30 under such conditions is fitted to the burred section 20, the relatively less rigid fitting member 30 bends, and the contact area becomes limited to the area around the high-bearing section P. With contact limited to the area around the high-bearing section P, it is not possible to secure a large contact area at the fitting section, and therefore, even if the vehicle arm member is made stronger, it becomes difficult to secure the pull-out load in the burred section.

[0010] The present invention has been made in view 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 burring section that can secure the pull-out load even when the material is made high-strength. [Means for solving the problem]

[0011] [1] A vehicle arm member having a burring section, The burring section has a cylindrical rising portion that fits and holds a fitting member for connecting with other members, The inner circumference of the aforementioned cylindrical rising portion has multiple coaxial inner circumference sections of the same diameter, with the inner diameter gradually increasing from the tip to the base, separated by annular steps at the boundaries. A vehicle arm member wherein the aforementioned annular step serves as a pull-out resistance step for the fitting member.

[0012] [2] The amount of step Δd in the direction perpendicular to the burring direction of the annular step is 1.8% or more of the plate thickness tL of the inner circumference of the same diameter on the root side of the annular step. The height position of the annular step in the burring direction is such that, when hd is the height of the annular step on the tip side from the surrounding surface of the burring portion and h is the burring height from the surrounding surface of the burring portion, hd ≥ 0.49h, as described in [1], for the vehicle arm member.

[0013] [3] A method for manufacturing a vehicle arm member, comprising a burring section having a cylindrical rising portion, wherein a plurality of coaxial inner circumferential portions of the same diameter are formed on the inner circumference of the rising portion, with an annular step at the boundary, the inner diameter of which increases in stages from the tip side to the root side, A forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring process, A forming punch having an outer circumference shape corresponding to the inner circumference of the rising portion of the burring section, wherein the portion of the outer circumference shape of the forming punch corresponding to the inner circumference of the same diameter at the very tip has an outer diameter such that the clearance Ct between it and the forming die is 70% or more and 100% or less of the thickness of the metal material, and the portion of the outer circumference shape of the forming punch corresponding to the inner circumference of the same diameter closer to the root than the inner circumference of the same diameter at the very tip has an outer diameter such that the clearance Cn between it and the forming die is 60% or more of the thickness of the metal material and less than the clearance Ct, A method for manufacturing a vehicle arm member, wherein the burring portion is formed in a single pressing step.

[0014] [4] A method for manufacturing a vehicle arm member, comprising a burring section having a cylindrical rising section, wherein a plurality of coaxial inner circumferential sections of the same diameter are formed on the inner circumference of the rising section, with an annular step at the boundary, the inner diameter of which increases in stages from the tip side to the root side, A first press forming step in which the inner circumference of the outermost tip of the rising portion of the burring section is formed using a forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring section, and a forming punch having an outer diameter such that the clearance Ct between the forming die is 70% or more and 100% or less of the thickness of the metal material, thus creating burring conditions. A method for manufacturing an arm member for a vehicle, comprising: a second press forming step in which the inner circumference portion of the rising portion of the burring portion, closer to the root than the tip, is formed on the burring portion formed by the first press forming step using a forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring portion, and a forming punch having an outer diameter such that the clearance Cn between the forming die and the punch is 60% or more of the thickness of the metal material and less than the clearance Ct.

[0015] [5] The amount of step Δd in the direction perpendicular to the burring direction of the annular step is 1.8% or more of the plate thickness tL of the inner circumference of the same diameter on the root side of the annular step. The method for manufacturing a vehicle arm member according to [3] or [4], wherein the height position of the annular step in the burring direction is such that when hd is the height of the annular step on the tip side from the surrounding surface of the burring process and h is the burring height from the surrounding surface of the burring process, hd ≥ 0.49h.

[0016] According to the present invention, the inner circumference of the cylindrical rising portion of the burring section has multiple coaxial inner circumference sections of the same diameter, where the inner diameter gradually increases from the tip to the root, with an annular step at the boundary. This annular step can be used as a pull-out resistance step for the fitting member. Such a burring section can be formed using a forming punch having outer diameters with different clearances to the forming die. As described above, the present invention provides a vehicle arm member and a method for manufacturing a vehicle arm member, both having a burring section that can secure a pull-out load even when the material is made high-strength. [Brief explanation of the drawing]

[0017] [Figure 1] This diagram shows a simplified perspective view illustrating how a conventional vehicle arm member is mounted on a vehicle. [Figure 2] This figure shows a simplified cross-sectional view of the second vehicle body side connecting portion of the burring-processed section of the vehicle arm member shown in Figure 1, including the burring-processing axis. [Figure 3] It is a figure which shows roughly in sectional view in the cross section containing a burring axis the principal part of the first vehicle body side connection part of the burring part of the arm member for vehicles of FIG. 1. [Figure 4] It is a figure which shows roughly in sectional view in the cross section containing a burring axis the principal part of the wheel support part of the burring part of the arm member for vehicles of FIG. 1. [Figure 5] It is a figure which shows typically in a partial sectional view of one side of the burring part in the cross section containing a burring axis the burring part of the conventional arm member for vehicles as a comparative example. [Figure 6] It is a figure which shows typically in a partial sectional view of one side of the burring part in the cross section containing a burring axis the state where a fitting member is fitted to the burring part of the conventional arm member for vehicles as a comparative example. [Figure 7] It is a figure which shows typically in a plan view the arm member for vehicles according to an embodiment of the present invention. [Figure 8] It is a figure which shows typically in a partial sectional view of one side of the burring part in the cross section containing a burring axis the burring part of the arm member for vehicles according to an embodiment of the present invention. [Figure 9] It is a figure which shows typically in a partial sectional view of one side of the burring part in the cross section containing a burring axis the state where a fitting member is fitted to the burring part of the arm member for vehicles according to an embodiment of the present invention. [Figure 10] It is a figure which shows the influence which the step amount of the annular step provided in the burring part and the position in the burring height direction of the arm member for vehicles according to an embodiment of the present invention have on the effect of improving the drawing-out load. [Figure 11A] Among the burring processes in the manufacturing method of the arm member for vehicles according to an embodiment of the present invention, it is a figure which shows typically in a partial sectional view in the cross section containing a burring axis the stage where the tip side same-diameter inner peripheral part is completed in molding in the entire area of the rising part. [Figure 11B]This figure schematically shows, in a partial cross-sectional view including the burring axis, the stage in the burring process of a method for manufacturing a vehicle arm member according to an embodiment of the present invention, where the inner circumference portion with the same diameter at the base side is formed at the base of the rising portion. [Figure 11C] This figure schematically shows a composite molding punch used in the burring process in a method for manufacturing a vehicle arm member according to an embodiment of the present invention, in a cross-sectional view including the burring axis. [Figure 12A] This figure schematically shows a partial cross-sectional view including the burring axis, in a modified example of the burring process in which the inner circumference portion of the tip side with the same diameter is formed on the rising portion by a first forming punch. [Figure 12B] This figure schematically shows a partial cross-sectional view including the burring axis, in a modified example of the manufacturing method for a vehicle arm member according to an embodiment of the present invention, specifically the step in which the same-diameter inner circumference at the base is formed at the base of the rising portion by a second forming punch. [Modes for carrying out the invention]

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the embodiments described below, 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 burring portion. Figure 7 is a schematic plan view showing an L-shaped lower arm 1a, which is an example of a vehicle arm member 1 according to an embodiment of the present invention. Figure 8 is a schematic diagram showing the burring 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 burring portion within a cross-section including the burring axis. Figure 9 is a schematic diagram showing how a fitting member is fitted into the burring 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 burring portion within a cross-section including the burring axis. Figure 10 is a diagram showing the effect of the amount of the step and the position in the height direction of the burring of the annular step provided in the burring portion of a vehicle arm member according to an embodiment of the present invention on the pull-out load improvement effect. In the embodiments described below, the same or common parts are denoted by the same reference numerals in the figures, and their descriptions will not be repeated. However, 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 an embodiment of the present invention has a burring section 20 on the second vehicle body side connecting section 14 that can secure the pull-out load even when the material is made stronger. Figure 8 is a partial cross-sectional view showing only the burring section 20 of the second vehicle body side connecting section 14, as seen from the direction of arrow AA in Figure 7. Furthermore, since the L-shaped lower arm 1a according to this embodiment is equivalent as a vehicle arm member to the conventional L-shaped lower arm 1a shown in Figures 1 to 4, when referring to parts not explicitly shown in Figure 7, the drawings and reference numerals of the conventional technology may be used for explanation.

[0020] The vehicle arm member or method for manufacturing the vehicle arm member according to this embodiment can be preferably used with steel members having a tensile strength of 590 MPa or more. Furthermore, the vehicle arm member or method for manufacturing the vehicle arm member according to this embodiment can be preferably used with steel members having a tensile strength of 780 MPa or more to 980 MPa or more. Furthermore, the vehicle arm member or method for manufacturing the vehicle arm member according to this embodiment can be more preferably used with 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 tensile testing of a test specimen taken from a part of the vehicle arm member corresponding to the head position of the forming punch of the press forming machine, where the steel member is hardly affected by work hardening due to press forming. On the other hand, even if it is not possible to secure enough material area to take a tensile test specimen from 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 that part and measuring the Vickers hardness at the 1 / 4 thickness position. In this case, the test load for Vickers hardness is preferably 9.807 N, considering the size of the indentation that appears in proportion to the strength of the sample, within the range of tensile strength of steel materials suitable for use in this embodiment, from 590 MPa to 1480 MPa. The Vickers hardness of steel materials with tensile strengths 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 higher, respectively.

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

[0023] As shown in Figure 7, the L-shaped lower arm 1a according to this embodiment is formed in a generally L-shape in plan view, and a wheel support portion 16 is provided at one end thereof, which is pivotably connected to a wheel support member 34 (not shown) via a ball joint 30bj (not shown). Furthermore, a first vehicle body side connecting portion 12 is provided at the bent intermediate portion of the arm body 10, which can be pivotably connected to the vehicle body via a first bush 30a1 (not shown) whose axis is oriented in the longitudinal direction of the vehicle body. In addition, a second vehicle body side connecting portion 14 is provided at the rear end of the arm body 10 in the longitudinal direction of the vehicle body, which can be pivotably connected to the vehicle body via a second bush 30a2 (not shown) whose axis is oriented in the vertical direction.

[0024] In this embodiment, the arm body 10 of the L-shaped lower arm 1a is preferably made lighter by an upper and lower half structure, similar to the prior art. That is, the arm body 10 in 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 of steel plates, are integrally joined together. The upper half 10U has an upper wall portion and a pair of side wall portions (not shown) extending downward from both sides of the upper wall portion in the width direction, and is formed in an inverted U shape in cross-section. The lower half 10L is basically formed in a flat plate shape and is formed in a roughly L shape in plan view so as to close the open lower end of the upper half 10U. The width direction end faces (not shown) of the lower half 10L are welded to the inner surfaces of the pair of side wall portions (not shown) of the upper half 10U.

[0025] Next, with reference to Figure 7, and also to Figures 1 and 2, the structure of the second vehicle body side connecting portion 14 having the burring portion 20 according to this embodiment will be described. On the second vehicle body side connecting portion 14 side of the arm body 10, the lower half 10L extends further outward (towards the rear of the vehicle body) than the upper half 10U, and the burring portion 20, which becomes the second vehicle body side connecting portion 14, is formed in this extended portion. An upward-facing, cylindrical rising portion 26 is integrally molded into this burring portion 20, into which the bush collar 30a of the second bush 30a2 can be press-fitted. The outer circumferential surface of this cylindrical rising portion 26 near the middle of the arm body 10 is joined to the outer end of the upper half 10U, which is formed in a corresponding arc shape in plan view, by a welded portion w. The second body-side connecting portion 14 with this 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. Consequently, burring of the upper half 10U can be omitted, simplifying the manufacturing process. In the L-shaped lower arm 1a according to this embodiment shown in Figure 7, the burring process is used only in the second body-side connecting portion 14, but it may also be used in the first body-side connecting portion 12 or the wheel support portion 16, similar to the conventional L-shaped lower arm 1a shown in Figures 1 to 4.

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

[0027] As described above, when the burring section 20 of the conventional vehicle arm member shown in Figure 5 is made stronger, as shown in Figure 6, the relatively less rigid fitting member 30 bends, and the contact area becomes limited to the area around the high bearing section P, making it difficult to secure the pull-out load.

[0028] In view of such prior art, as shown in FIG. 8, the flanging portion 20 according to the present embodiment has a plurality of coaxial and same-diameter inner peripheral portions 26A (26A1, 26A2) with inner diameters gradually increasing from the tip side to the root side (d1 < d2) on the inner periphery of the cylindrical rising portion 26. An annular step 28 is formed at the boundary between the tip-side same-diameter inner peripheral portion 26A1 and the root-side same-diameter inner peripheral portion 26A2 with different inner diameters. According to the present embodiment, as shown in FIG. 9, the gap generated in the prior art as shown in FIG. 6, that is, the gap on the tip side of the high-support-pressure portion P between the inner periphery of the rising portion 26 and the outer periphery of the mating member 30, can be filled by the tip-side same-diameter inner peripheral portion 26A1 sandwiching the annular step 28. Thereby, in the flanging portion 20 according to the present embodiment, the contact portion between the inner periphery of the cylindrical rising portion 26 and the mating member 30 can be made to occur at least at two high-support-pressure portion P peripheries, namely, around the tip-side high-support-pressure portion P1 and around the root-side high-support-pressure portion P2. Therefore, in the flanging portion 20 according to the present embodiment, even if the material is strengthened to a high strength, the flanging portion 20 can be made such that the drawing-out load can be advantageously ensured. In the description so far, the case where there are two same-diameter inner peripheral portions 26A and one annular step 28 at the boundary has been described, but it is not limited thereto, and combinations where the number of same-diameter inner peripheral portions 26A is three or more and the number of annular steps 28 at the boundary is two or more may also be used.

[0029] The size of the flanging portion in the vehicle arm member or the manufacturing method of the vehicle arm member according to the present embodiment is not particularly limited. For example, the inner diameter (see d1 in FIG. 8) is 10 mm or more and 100 mm or less, and the flanging height (see h in FIG. 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 effect of the step amount Δd and the burring height direction position (height hd) of the annular step 28 of the burring section 20, as shown in Figure 8, on the effect of improving the pull-out load. The material used in the investigation was a high-strength steel plate with a thickness of 3.2 mm and a tensile strength of 980 MPa. The burring section was formed using an experimental apparatus with a mold configuration as shown in Figures 11A to 11C or Figures 12A and 12B, described later, so that multiple levels of the step amount Δd and the burring height direction position (height hd) of the annular step could be obtained. Here, the burring height direction position is represented by the height hd of the tip side of the annular step from the surrounding surface of the burring section, as shown in Figure 8.

[0031] Furthermore, using a bush collar with a plate thickness of 2.3 mm and a tensile strength of 590 MPa as a fitting member to be fitted into the burring section, the pull-out load from the burring section under the various conditions described above was measured. The obtained results are summarized in Figure 10. The vertical axis, the improvement rate of pull-out load (Fd-F) / F, 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 horizontal axis, the step amount Δd / tL of the annular step, is the step amount Δd in the direction perpendicular to the burring direction, non-dimensionalized by the plate thickness tL of the inner circumference of the same diameter at the root side of the annular step. In addition, the data in Figure 10 is shown stratified by hd / h, which is the height hd of the tip side of the annular step from the surrounding surface of the burring section, non-dimensionalized by the burring height h from the surrounding surface of the burring section.

[0032] From Figure 10, assuming that a pull-out load improvement rate (Fd-F) / F of 10% or more is preferable, there is no preferable range for the step amount Δd / tL of the annular step when the height hd / h in the burring direction of the annular step is 32%. On the other hand, when the height hd / h in the burring direction of the annular step is 49-84%, 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 a preferable range exists. However, even in that case, as the step amount Δd / tL of the annular step increases, the pull-out load improvement rate (Fd-F) / F will fall 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% differs for each height hd / h of the annular step in the burring direction. In other words, when hd / h = 84%, the upper limit of the step amount Δd / tL for the annular step is 23%, when hd / h = 66%, the upper limit of the step amount Δd / tL for the annular step is 10%, and when hd / h = 49%, the upper limit of the step amount Δd / tL for the annular step is 5%.

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

[0034] Based on the preliminary experimental results described above, the inventors determined that the step amount Δd in the direction perpendicular to the burring direction of the annular step is preferably 1.8% or more of the plate thickness tL of the inner circumference of the same diameter at the root side of the annular step. In this case, the inventors determined that the height position of the annular step in the burring direction is preferably such that hd ≥ 0.49h, where hd is the height of the tip side of the annular step from the surrounding surface of the burring process, and h is the burring height from the surrounding surface of the burring process.

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

[0036] Next, the method for manufacturing the vehicle arm member according to this embodiment will be described with reference to Figures 11A to 11C, 12A, and 12B. The burring section constitutes a part of the press-formed product, such as the lower half. The burring process and the press-forming process for the entire member may be provided separately, but usually, for production efficiency, the burring process is incorporated into the press-forming process for the entire member. Furthermore, the method for manufacturing the vehicle arm member according to this embodiment, excluding the burring section, is the same as conventional manufacturing methods, such as press-forming each component, such as the lower half, and assembling the components obtained by press-forming by welding, etc. Therefore, this explanation of the method for manufacturing the vehicle arm member according to this embodiment will omit detailed explanations of manufacturing methods common to conventional manufacturing methods and will be limited to the explanation of the method for manufacturing the burring section.

[0037] Figures 11A and 11B schematically illustrate the final stage of the burring process in a method for manufacturing a vehicle arm member according to an embodiment of the present invention, where the burring portion is formed in a single pressing step, using a partial cross-sectional view including the burring axis.

[0038] Figure 11C shows the cross-sectional shape including the central axis of the composite molding punch 40A used here. A small diameter cylindrical portion 40Aa is provided at the tip of the composite molding 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 center position of the burring section 20, the molding punch 40, and the molding die 44. The conical portion 40Ab of the punch is the part that pushes the peripheral plate-like portion 22 around the pilot hole out of plane while gradually expanding the diameter of the pilot hole in the blank to the inner diameter of the burring section, with the peripheral plate-like portion 22 around the pilot hole being held between the holder 42 and the molding die 44. The cylindrical front portion 40Ac of the punch is the part that forms a rising portion 26 having an inner circumference 26A of the same diameter d1 in the height direction, while maintaining a predetermined clearance Ct with the inner diameter of the forming die 44, from the burring portion that protrudes from the conical portion 40Ab. The cylindrical rear portion 40Ae of the punch is the part that forms the root side of the inner circumference 26A of the same diameter d1 formed by the cylindrical front portion 40Ac, from the root side to a predetermined height, while maintaining a predetermined clearance Cn with the inner diameter of the forming die 44, to the root side of the inner circumference 26A2 of the same diameter d2. The stepped portion 40Ad of the punch is the boundary between the cylindrical front portion 40Ac and the cylindrical rear portion 40Ae, and is the part that forms an annular step 28 on the inner circumference of the burring portion 20.

[0039] In the manufacturing method of the vehicle arm member according to the embodiment of the present invention, a molding die 44 having an inner diameter corresponding to the outer circumference of the rising portion of the burring section is used. Furthermore, a composite molding punch 40A is used for the molding punch 40, which has an outer circumference shape corresponding to the inner circumference of the rising portion 26 of the burring section 20, as shown in Figure 11C. By adopting such a mold configuration, the burring section 20 of the vehicle arm member according to this embodiment can be formed in a single pressing step.

[0040] In this single pressing process, the steps up to Figure 11A show the stage in which the inner circumference portion 26A of the same diameter as the inner diameter d1 is formed over the entire inner circumference of the rising portion 26 while maintaining a predetermined clearance Ct between the cylindrical front portion 40Ac of the composite molding punch 40A and the inner diameter portion of the molding die 44. Note that the explanation of each stage of the burring process up to Figure 11A is omitted as it overlaps with the explanation of the small diameter cylindrical portion 40Aa and the conical portion 40Ab of the composite molding punch. Figure 11B shows the stage in this single pressing process in which the inner circumference portion 26A2 of the same diameter as the root side of the inner diameter d2 is formed over the root portion of the inner circumference of the rising portion 26 while maintaining a predetermined clearance Cn between the cylindrical rear portion 40Ae of the composite molding punch 40A and the inner diameter portion of the molding die 44. At this stage, an annular step 28 is formed at a predetermined height position on the inner circumference of the rising portion 26, serving as the boundary between the inner circumference portion 26A1 with the same diameter at the tip and the inner circumference portion 26A2 with the same diameter at the base.

[0041] Here, we will explain the predetermined clearance Ct. Specifically, the clearance Ct between the cylindrical front portion 40Ac of the composite molding punch 40A, which corresponds to the inner circumference portion 26A1 of the same diameter at the very tip, and the inner diameter portion of the molding die 44 shall be 70% or more and 100% or less of the thickness of the metal material. If this clearance Ct exceeds 100% of the thickness of the metal material, the cylindrical shape of the rising portion of the burring section 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 material, it becomes a strong ironing burring process from the tip to the base, increasing the risk of fracture of the burring section, which is undesirable.

[0042] Furthermore, the clearance Cn between the cylindrical rear portion 40Ae of the composite molding punch 40A and the inner diameter portion of the molding die 44, corresponding to the inner circumference portion 26A2 of the same diameter from the tip to the base, shall be 60% or more of the metal material thickness and less than clearance Ct. Setting this clearance Cn to be greater than or equal to clearance Ct would be equivalent to reducing the inner diameter of the inner circumference portion 26A from the tip to the base, making it impossible to form such a shape of the inner circumference portion 26A in a single press process, which is undesirable. On the other hand, if this clearance Cn is less than 60% of the metal material thickness, it would result in extremely harsh processing conditions that exceed the ironing burring processing conditions, making fracture of the burring portion unavoidable, which is undesirable.

[0043] As described above, a method for manufacturing a vehicle arm member in which the burring portion of the vehicle arm member according to this embodiment is formed in a single press step has been explained with reference to Figures 11A to 11C. However, in this embodiment, the formation of the burring portion is not limited to a single press step. For example, as shown in Figure 12A, the process up to the intermediate stage of burring, in which the inner circumference portion 26A of the same diameter as the inner diameter d1 is formed over the entire inner circumference of the rising portion 26 with the first forming punch 40B1, may be divided into burring processes similar to those up to Figure 11A. After that, as shown in Figure 12B, the process of burring, similar to that in Figure 11B, is added to form the inner circumference portion 26A2 of the same diameter at the base side of the inner diameter d2 at the base of the inner circumference of the rising portion 26 with the second forming punch 40B2, thereby completing the burring process. [Explanation of Symbols]

[0044] 1. Arm member for vehicle 1a L-shaped lower arm 10 Arm body 10L lower half 10U upper half 12. First car body side coupling section 14. Second car body side coupling section 16 Wheel support part 20 Burring section 22 Peripheral plate-like portion 24 music section 26. Rising section 26A Same diameter inner circumference 26A1 Same diameter inner circumference on tip side 26A2 Inner circumference of the same diameter at the base 28 Ring-shaped step 30 Fitting member 30a Bush Collar 30a1 First Bush 30a2 Second Bush 30bj ball joint 32. Body Subframe 34 Wheel support member 40 Forming punches 40A Composite Molding Punch 40Aa Thin cylindrical section 40Ab Cone section 40Ac Front of the cylinder 40 AD Step section 40Ae Rear of the cylinder 40B1 First forming punch 40B2 Second forming punch 42 holder 44 molding dies d Inner diameter of the burring section d1 Inner diameter of the inner circumference of the tip side d2 Inner diameter of the inner circumference at the base side Δd: Amount of step difference in the annular step. F Pull-out load when there is no annular step Fd Pull-out load when there is an annular step h burring height HD Ring-shaped step height tL Thickness of the inner circumference plate of the same diameter at the base P High bearing pressure area P1 Tip side high bearing pressure part P2 High bearing section at the base w weld

Claims

1. A vehicle arm member having a burring-processed portion, The burring section has a cylindrical rising portion that fits and holds a fitting member for connecting with other members, The inner circumference of the aforementioned cylindrical rising portion has multiple coaxial inner circumference sections of the same diameter, with the inner diameter gradually increasing from the tip to the base, separated by annular steps at the boundaries. A vehicle arm member wherein the aforementioned annular step serves as a pull-out resistance step for the fitting member.

2. The amount of step Δd in the direction perpendicular to the burring direction of the annular step is 1.8% or more of the plate thickness tL of the inner circumference of the same diameter on the root side of the annular step. The height position of the annular step in the burring direction satisfies hd ≥ 0.49h, where hd is the height of the annular step on the tip side from the surrounding surface of the burring portion, and h is the burring height from the surrounding surface of the burring portion, as described in claim 1.

3. A method for manufacturing a vehicle arm member, comprising a burring section having a cylindrical rising portion, wherein multiple coaxial inner circumferential portions of the same diameter are formed on the inner circumference of the rising portion, with an annular step at the boundary, and the inner diameter gradually increasing from the tip to the base. A forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring process, A forming punch having an outer circumference shape corresponding to the inner circumference of the rising portion of the burring section, wherein the portion of the outer circumference shape of the forming punch corresponding to the inner circumference of the same diameter at the very tip has an outer diameter such that the clearance Ct between it and the forming die is 70% or more and 100% or less of the thickness of the metal material, and the portion of the outer circumference shape of the forming punch corresponding to the inner circumference of the same diameter closer to the root than the inner circumference of the same diameter at the very tip has an outer diameter such that the clearance Cn between it and the forming die is 60% or more of the thickness of the metal material and less than the clearance Ct, A method for manufacturing a vehicle arm member, wherein the burring portion is formed in a single pressing step.

4. A method for manufacturing a vehicle arm member, comprising a burring section having a cylindrical rising portion, wherein multiple coaxial inner circumferential portions of the same diameter are formed on the inner circumference of the rising portion, with an annular step at the boundary, and the inner diameter gradually increasing from the tip to the base. A first press forming step in which the inner circumference of the outermost tip of the rising portion of the burring section is formed using a forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring section, and a forming punch having an outer diameter such that the clearance Ct between the forming die and the punch is 70% or more and 100% or less of the thickness of the metal material for burring processing. A method for manufacturing an arm member for a vehicle, comprising: a second press forming step in which the inner circumference portion of the rising portion of the burring portion, closer to the root than the tip, is formed on the burring portion formed by the first press forming step using a forming die having an inner diameter corresponding to the outer circumference of the rising portion of the burring portion, and a forming punch having an outer diameter such that the 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. The amount of step Δd in the direction perpendicular to the burring direction of the annular step is 1.8% or more of the plate thickness tL of the inner circumference of the same diameter on the root side of the annular step. The method for manufacturing a vehicle arm member according to claim 3 or 4, wherein the height position of the annular step in the burring direction satisfies hd ≥ 0.49h, where hd is the height of the annular step on the tip side from the surrounding surface of the burring process and h is the burring height from the surrounding surface of the burring process.

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