Method for manufacturing metal parts and metal parts
By forming a metal intermediate product with a protruding convex portion and using a tapered punch to fold back the convex portion, the method addresses the issue of stretch flange cracking in high-strength steel sheets, ensuring crack-free extended flange formation through surface smoothing and compressive forces.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NIPPON STEEL CORPORATION
- Filing Date
- 2026-04-15
- Publication Date
- 2026-06-30
Smart Images
Figure 2026108893000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method for manufacturing a metal part and a metal part.
Background Art
[0002] Automobile underbody parts, seat parts, transmission parts, etc. are manufactured by performing press working on metal plates such as high-tensile steel plates and special steel plates. In addition, for parts such as lower arms, seat gears, recliners, bearings, hubs, and various gears, an extended flange (burring flange) may be formed by burring for connection with other parts.
[0003] Burring is a process of expanding a pre-formed initial hole (pilot hole) in a metal plate with a punch and expanding the peripheral portion of the initial hole in the pushing direction of the punch to form an extended flange protruding in a cylindrical shape.
[0004] A defect that can occur due to burring is cracking at the tip of the extended flange (extended flange cracking). As causes of extended flange cracking, it has been pointed out that the cut surface is work-hardened when the initial hole is formed by punching, and the notch effect due to the unevenness of the fracture surface generated on the cut surface. Therefore, there is a method of improving the limit hole expansion ratio by scraping the vicinity of the cut surface after forming the initial hole. However, in this method, the scraped chips are likely to adhere to the punch, and there is a risk of occurrence of indentation defects during press working. In addition, although it is also conceivable to form the initial hole by cutting instead of punching, it is difficult to incorporate such a mechanism into the die of an automatic press device such as a progressive press device.
[0005] Japanese Patent Laid-Open No. 2015-36147 discloses a burring punch. This burring punch is characterized by having a tip portion that expands a pre-formed pilot hole into a hole having a diameter smaller than the final hole diameter, a rear end portion that expands the hole expanded by the tip portion to the final hole diameter, and a concave portion provided at the boundary between the tip portion and the rear end portion and having a diameter smaller than that of the tip portion.
[0006] Japanese Patent Application Laid-Open No. 2014-172089 discloses a punch for flanging. This punch for flanging includes a tip portion that contacts the peripheral edge of a pilot hole at the initial stage of flanging, a large-diameter portion having a columnar shape with the same diameter as the finished inner diameter of the flange extension, and a diameter-expanding portion having a frustum shape that smoothly connects the tip portion and the large-diameter portion and whose diameter expands toward the rear end side. The opening angle of the tip portion is 90° or more, the opening angle of the diameter-expanding portion is 60° or less, and the diameter d1 of the rear end of the tip portion satisfies the condition of d0 < d1 ≦ 1.2 × d0 in relation to the diameter d0 of the pilot hole.
[0007] Japanese Patent Application Laid-Open No. 2007-75869 discloses a flanging method. This flanging method includes a first punching step of punching a plate material to form a preliminary pilot hole, a second punching step of punching the peripheral edge of the preliminary pilot hole with a predetermined scrap width to form a pilot hole having a diameter larger than that of the preliminary pilot hole, and a bending step of bending the peripheral edge of the pilot hole to form a flange.
[0008] Although not related to flanging, Japanese Patent Application Laid-Open No. 2000-288655 discloses a die-less punching method.
[0009] Y. Abe et al., "Improvement of Hole Expansion Property by Smoothing the Fracture Surface of Ultra-High Tensile Steel Sheets," Journal of the JSTP, vol. 52, no. 603 (2011-4), describes that after pushing a taper punch into the fracture surface of a punched steel sheet to smooth the fracture surface, hole expansion is performed by pushing a cone punch with the return side facing the die side.
Prior Art Documents
Patent Documents
[0010]
Patent Document 1
Patent Document 2
Patent Document 3
[0011] [Non-Patent Document 1] Yohei Abe et al., "Improvement of Hole Expansionability by Smoothing of Fracture Surface of Ultra-High-Strength Steel Sheet," Journal of the JSTP, vol. 52, no. 603 (2011-4). [Overview of the project] [Problems that the invention aims to solve]
[0012] In recent years, there has been a growing demand for lighter automotive parts to address environmental concerns. As a result, high-strength steel sheets, such as high-tensile steel sheets that can maintain strength even with thinner sheets, and special steel sheets whose strength can be increased through heat treatment after processing, are increasingly being used as materials for automotive parts. However, high-strength steel sheets have low ductility, making them prone to stretch flange cracking during burring. Among these, special steel sheets made of medium to high carbon steel with a high carbon content are particularly susceptible to stretch flange cracking.
[0013] One objective of the present invention is to provide a method for manufacturing metal parts that can suppress expansion flange cracking. Another objective of the present invention is to provide metal parts in which expansion flange cracking is suppressed. [Means for solving the problem]
[0014] A method for manufacturing a metal part according to an embodiment of the present invention is a method for manufacturing a metal part having an extended flange, comprising a step of preparing a metal intermediate product, and a step of forming an extended flange by pressing a tapered punch into the intermediate product. The intermediate product is a part having a plate-shaped portion, and has an overhanging convex portion protruding on one side in the plate thickness direction and a hole formed in the overhanging convex portion. The hole has a broken cross-section on the inner peripheral surface, and the broken cross-section is on the side where the overhanging convex portion protrudes. In the step of forming the extended flange, the tapered punch is pressed into the hole from the side where the overhanging convex portion protrudes, and the extended flange is formed on the side opposite to the side where the overhanging convex portion protrudes by folding back the overhanging convex portion.
[0015] A metal part according to an embodiment of the present invention is a metal part having an extended flange, and the extended flange has rounded corners on the outer peripheral side of the tip portion of the extended flange.
Advantages of the Invention
[0016] According to the present invention, the occurrence of cracks in the extended flange can be suppressed.
Brief Description of the Drawings
[0017] [Figure 1] FIG. 1 is a flowchart of a method for manufacturing a metal part according to an embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view schematically showing the configuration of an example of an intermediate product. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2. [Figure 4] FIG. 4 is an example of a cross-sectional photograph of an intermediate product. [Figure 5A] FIG. 5A is a schematic diagram for explaining an example of a method for manufacturing an intermediate product. [Figure 5B] FIG. 5B is a schematic diagram for explaining an example of a method for manufacturing an intermediate product. [Figure 5C] FIG. 5C is a schematic diagram for explaining an example of a method for manufacturing an intermediate product. [Figure 5D] FIG. 5D is a schematic diagram for explaining one example of a method for manufacturing an intermediate product. [Figure 6A] FIG. 6A is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 6B] FIG. 6B is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 6C] FIG. 6C is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 6D] FIG. 6D is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 7A] FIG. 7A is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 7B] FIG. 7B is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 7C] FIG. 7C is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 8A] FIG. 8A is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 8B] FIG. 8B is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 8C] FIG. 8C is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 9A] FIG. 9A is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 9B] FIG. 9B is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 9C] FIG. 9C is a schematic diagram for explaining another example of a method for manufacturing an intermediate product. [Figure 10A] FIG. 10A is a schematic diagram for explaining the process of forming an elongation flange. [Figure 10B] FIG. 10B is a schematic diagram for explaining the process of forming an elongation flange. [Figure 10C] FIG. 10C is a schematic diagram for explaining the process of forming an elongation flange. [Figure 11] Figure 11 is a schematic diagram showing the machining process in the process of forming an elongated flange. [Figure 12] Figure 12 is an example of a cross-sectional photograph of the vicinity of the stretched flange portion of a metal part. [Figure 13] Figure 13 is a cross-sectional photograph showing an enlarged view of region A1 in Figure 12. [Figure 14] Figure 14 is a schematic cross-sectional view of a metal part. [Figure 15] Figure 15 schematically shows another example of the cross-sectional shape of the tip of an elongated flange of a metal part. [Figure 16] Figure 16 is a cross-sectional photograph showing yet another example of the cross-sectional shape of the tip of an elongated flange of a metal part. [Figure 17] Figure 17 is a schematic cross-sectional view of the extension flange shown in Figure 16. [Figure 18] Figure 18 is a photograph of an intermediate product, showing the side where the protruding projection is visible. [Figure 19] Figure 19 is a schematic representation of Figure 18. [Figure 20] Figure 20 is a photograph of a metal part manufactured by forming an elongated flange on the intermediate product shown in Figure 18, and the photograph shows the side where the elongated flange protrudes. [Figure 21] Figure 21 is a schematic representation of Figure 20. [Figure 22] Figure 22 is a photograph of a metal part manufactured by forming an elongated flange on the intermediate product shown in Figure 18, and shows the side opposite to the side where the elongated flange protrudes. [Figure 23] Figure 23 is a schematic representation of Figure 22. [Figure 24] Figure 24 is a schematic diagram illustrating the manufacturing method of the comparative example intermediate product. [Figure 25] Figure 25 is a schematic cross-sectional view of the intermediate product of the comparative example. [Figure 26] Figure 26 is a schematic diagram showing the machining process in the process of forming an elongated flange. [Figure 27]Figure 27 is a schematic cross-sectional view of the intermediate product of the comparative example. [Figure 28] Figure 28 is a schematic diagram showing the machining process in the process of forming an elongated flange. [Figure 29] Figure 29 is an example of a cross-sectional photograph of the vicinity of the stretched flange portion of a comparative example metal part. [Figure 30] Figure 30 is a cross-sectional photograph showing an enlarged view of region A2 in Figure 29. [Figure 31] Figure 31 is a schematic cross-sectional view of a comparative example metal part. [Modes for carrying out the invention]
[0018] The embodiments of the present invention will be described in detail below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated. The dimensional ratios between the constituent members shown in each drawing do not necessarily represent the actual dimensional ratios.
[0019] [Manufacturing method for metal parts] Figure 1 is a flowchart of a method for manufacturing a metal part according to one embodiment of the present invention. The method for manufacturing a metal part according to this embodiment is a method for manufacturing a metal part having an expandable flange, and comprises the steps of preparing a metal intermediate product having a predetermined shape (step S1) and pressing a tapered punch into the intermediate product to form an expandable flange (step S2).
[0020] [Intermediate product preparation process] First, a metal intermediate product having a predetermined shape is prepared (Step S1). Figure 2 is a schematic perspective view showing the structure of an intermediate product 10, which is an example of an intermediate product prepared in this step. Figure 3 is a cross-sectional view along line III-III in Figure 2. Figure 4 is an example of a cross-sectional photograph of the intermediate product.
[0021] The material of the intermediate product 10 is not particularly limited as long as it is a metal. Examples of materials for the intermediate product 10 include iron-based alloys (steel), copper, copper alloys, aluminum, aluminum alloys, etc. The manufacturing method of metal parts according to this embodiment is suitable when the intermediate product 10 is made of a metal with high strength and low ductility (for example, high-strength steel such as high-tensile steel). The manufacturing method of metal parts according to this embodiment is particularly suitable when the material of the intermediate product 10 is a medium- to high-carbon steel with a carbon content of 0.20 mass% or more. Examples of medium-carbon steel materials include, but are not limited to, S35C, S40C, S45C, and other carbon steel materials for machine structures. Examples of high-carbon steel materials include, but are not limited to, carbon tool steel materials such as SK85 and high-carbon chromium bearing steel materials such as SUJ2.
[0022] The intermediate part 10 is a component having a plate-like portion, and has a protruding projection 11 that extends outwards on one side in the plate thickness direction (z direction), and a hole 10a formed within the protruding projection 11. The hole 10a penetrates the intermediate part 10 in the plate thickness direction. Preferably, the hole 10a is formed in the center of the protruding projection 11 in the in-plane direction (xy-plane direction).
[0023] The diameter D0 of the hole 10a (Figure 3) is determined by the product shape and is therefore not particularly limited, but is, for example, 2.0 to 50 times the plate thickness t (Figure 3). If the diameter D0 is too small, the strain on the outside of the stretch flange tip increases during the process of forming the stretch flange, which can make stretch flange formation difficult. On the other hand, if the diameter D0 is too large, the pressing effect on the punch during folding decreases, which can also make stretch flange formation difficult. The lower limit of the diameter D0 is more preferably 2.5 times the plate thickness t, and even more preferably 3.0 times. The upper limit of the diameter D0 is more preferably 30 times the plate thickness t, and even more preferably 20 times.
[0024] The hole 10a is formed by punching (shearing), as described later. Therefore, the hole 10a has a shear surface 111 and a fracture surface 112 on its inner circumferential surface. In this embodiment, the shear surface 111 is on the side opposite to the side from which the protruding projection 11 protrudes, and the fracture surface 112 is on the side from which the protruding projection 11 protrudes.
[0025] As shown in Figures 3 and 4, the shear surface 111 is a plane that is roughly parallel to the plate thickness direction (z direction), while the fracture surface 112 has a cross-sectional shape that widens outward in the radial direction of the hole 10a. Furthermore, the shear surface 111 is a smooth surface with a metallic luster, while the fracture surface 112 has minute irregularities and does not have a metallic luster.
[0026] The height h0 of the protruding portion 11 (Figure 3) is not particularly limited, but is, for example, 0.25 to 10 times the plate thickness t (Figure 3). If the height h0 is too small, the effect of smoothing the fracture surface may not be sufficiently obtained in the process of forming the stretched flange (step S2). On the other hand, if the height h0 is too large, a local reduction in plate thickness may occur when forming the protruding portion, which may cause cracking in the hole widening process. The height h0 is the distance from the surface of the flat portion of the intermediate product 10 to the apex of the protruding portion 11.
[0027] Methods for forming the intermediate product 10 are not limited to these, but include: (1) a method of forming a protruding portion 11 by drawing or stretching and then forming a hole 10a by punching; (2) a method of forming a hole 10a by punching and then forming a protruding portion 11 by edge bending; and (3) a method of forming the hole 10a and the protruding portion 11 in a single process. More specifically, the method of forming the hole 10a and the protruding portion 11 in a single process in (3) includes (3-1) a method of forming the protruding portion 11 and the hole 11a by performing punching with a step between the die and the die holder; (3-2) a method of forming the protruding portion 11 and the hole 11a by performing punching using only the die holder without using a die; and (3-3) a method of forming the protruding portion 11 and the hole 11a by performing punching using a stepped die.
[0028] Figures 5A to 5D are schematic diagrams illustrating the method described in (1) above. First, a metal material S is subjected to drawing or stretching using a punch 21 for forming protruding protrusions, a die 22 for forming protruding protrusions, and a plate holder 25 to form a protruding protrusion 11 (see Figures 5A and 5B). Next, the portion of the material S on which the protruding protrusion 11 has been formed is punched using a punching punch 23, a punching die 24, and a plate holder 25 to form a hole 10a within the protruding protrusion 11. At this time, the punching punch 23 is brought into contact with the material S from the side opposite to the side on which the protruding protrusion 11 is protruding. As a result, a shear surface 111 (Figure 3) is formed on the side opposite to the side on which the protruding protrusion 11 is protruding, and a fracture surface 112 (Figure 3) is formed on the side on which the protruding protrusion 11 is protruding.
[0029] Figures 6A to 6D are schematic diagrams illustrating the method described in (2) above. This method is the same as the method described in (1) above, but with the order in which the protruding projection 11 and the hole 10a are formed reversed. Specifically, first, a hole 10a is formed on the metal material S by punching using a punching punch 23, a punching die 241, and a plate presser 25 (see Figures 6A and 6B). Next, the protruding projection 11 is formed on the part of the material S where the hole 10a has been formed by bending the edge using a punch for forming the protruding projection 211, a die for forming the protruding projection 22, and a plate presser 25 (see Figures 6C and 6D). At this time, the punch for forming the protruding projection 211 is brought into contact with the material S from the same side as the punching punch 23. As a result, a shear surface 111 (Figure 3) is formed on the side opposite to the side from which the protruding projection 11 protrudes, and a fracture surface 112 (Figure 3) is formed on the side from which the protruding projection 11 protrudes.
[0030] Figures 7A to 7C are schematic diagrams illustrating the method described in (3-1) above. In this method, punching is performed using a punching die 242 and a die holder 245 instead of the punching die 24 in Figures 5C and 5D. That is, in the method shown in Figures 7A to 7C, punching is performed using a punching punch 23, a punching die 242, a die holder 245 positioned to surround the punching die 242, and a plate holder 25. The punching die 242 is positioned such that, in the thickness direction of the material S, its upper end surface is further from the material S than the upper end surface of the die holder 245.
[0031] First, the metal material S is sandwiched between the plate holder 25 and the die holder 245, and the punching punch 23 is brought into contact with the die holder 245 from the opposite side, moving at least one of the punching punch 23 and the set of punching die 242 and die holder 245 so that they are closer together (see Figures 7A and 7B). Since the upper end surface of the punching die 242 is positioned further from the material S than the upper end surface of the die holder 245, the material S does not come into contact with the punching die 242 in the first half of the stroke. During this time, a protruding projection 11 is formed on the material S (see Figure 7B). By further moving the punching punch 23 and the set of punching die 242 and die holder 245 closer together from the state shown in Figure 7B, a hole 10a is formed in the protruding projection 11 (see Figure 7C). In this case as well, since the punching punch 23 contacts the side opposite to the side from which the protruding projection 11 is protruding, a shear surface 111 (Figure 3) is formed on the side opposite to the side from which the protruding projection 11 is protruding, and a fracture surface 112 (Figure 3) is formed on the side from which the protruding projection 11 is protruding.
[0032] Figures 8A to 8C are schematic diagrams illustrating the method described in (3-2) above. In this method, a die holder 246 is used for punching instead of the punching die 24 shown in Figures 5C and 5D. That is, in the method shown in Figures 8A to 8C, punching is performed using a punching punch 23, a die holder 246, and a plate holder 25. The method shown in Figures 8A to 8C does not use the punching die 24. In other words, the method shown in Figures 8A to 8C is die-less punching.
[0033] Similar to the case in (3-1) above, the metal material S is sandwiched between the plate holder 25 and the die holder 246, and the punching punch 23 is brought into contact with the die holder 246 from the opposite side, and at least one of the punching punch 23 and the die holder 246 is moved so that they are close together (see Figures 8A and 8B). In this case as well, the protruding portion 11 is formed on the material S in the first half of the stroke (see Figure 8B). By bringing the punching punch 23 and the die holder 246 closer together from the state in Figure 8B, a hole 10a is formed in the protruding portion 11 (see Figure 8C). In this case as well, since the punching punch 23 is brought into contact with the side opposite to the side from which the protruding portion 11 is protruding, a shear surface 111 (Figure 3) is formed on the side opposite to the side from which the protruding portion 11 is protruding, and a fracture surface 112 (Figure 3) is formed on the side from which the protruding portion 11 is protruding.
[0034] The clearance (on one side) between the punching punch 23 and the die holder 246 is not limited to this, but is, for example, twice or more the thickness t of the material S. More preferably, the clearance (on one side) is five times or more the thickness t.
[0035] Figures 9A to 9C are schematic diagrams illustrating the method described in (3-3) above. In this method, a stepped die 244 is used for punching instead of the punching die 24 shown in Figures 5C and 5D. That is, in the method shown in Figures 9A to 9C, punching is performed using a punching punch 23, a stepped die 244, and a plate holder 25. The stepped die 244 has a hole 244a through which the punching punch 23 passes. The hole 244a includes, in order from the punching punch 23 side, a large diameter portion 244a1 formed to have a predetermined first clearance relative to the punching punch 23, and a small diameter portion 244a2 formed to have a second clearance smaller than the first clearance relative to the punching punch 23.
[0036] First, the metal material S is sandwiched between the plate holder 25 and the stepped die 244, and the punching punch 23 is brought into contact with the stepped die 244 from the opposite side, and at least one of the punching punch 23 and the stepped die 244 is moved so that they are close together (see Figures 9A and 9B). In this case as well, a protruding projection 11 is formed on the material S in the first half of the stroke (see Figure 9B). By bringing the punching punch 23 and the stepped die 244 closer together from the state in Figure 9B, a hole 10a is formed inside the protruding projection 11 (see Figure 9C). In this case as well, since the punching punch 23 is brought into contact with the material S from the opposite side of the protruding projection 11, a shear surface 111 (Figure 3) is formed on the side opposite to the side where the protruding projection 11 protrudes, and a fracture surface 112 (Figure 3) is formed on the side where the protruding projection 11 protrudes.
[0037] In methods (1) and (2) above, two steps are required to form the protruding portion 11 and the hole 10a, whereas in methods (3-1) to (3-3) above, the protruding portion 11 and the hole 10a can be formed in a single step. Therefore, methods (3-1) to (3-3) above can reduce the number of steps compared to methods (1) and (2) above. In addition, in methods (3-1) to (3-3) above, since the protruding portion 11 and the hole 10a are formed simultaneously, the center of the protruding portion 11 and the center of the hole 10a can be made stable without misalignment.
[0038] According to the method described in (3-2) above, die molds are not required, thus reducing mold costs. In addition, maintenance of the cutting edge is limited to the punching punch 23, making management easier.
[0039] Furthermore, when the intermediate product 10 is formed by the methods (3-1) to (3-3) described above, a groove g (see Figures 7C, 8C, and 9C) may be formed on the protruding side of the protruding projection 11, surrounding the projection 11, due to contact with the corner of the stepped die 244 or the corner of the die holder 245 or 246.
[0040] [Process for forming stretched flanges] Next, a tapered punch is pressed into the intermediate product 10 to form an elongated flange (step S2). More specifically, the tapered punch is pressed into the hole 10a of the intermediate product 10 from the side where the protruding projection 11 is protruding, and the protruding projection 11 is folded back to form an elongated flange on the side opposite to the side where the protruding projection 11 is protruding.
[0041] Figures 10A to 10C are schematic diagrams illustrating the process of forming an elongated flange (step S2). First, the tapered punch 26 is brought into contact with the hole 10a of the intermediate product 10 from the side where the protruding projection 11 is protruding (see Figures 10A and 10B). From this state, the intermediate product 10 is fixed with the elongated flange processing die 27 and the plate holder 28, and the tapered punch 26 is pushed in, folding back the protruding projection 11 to form an elongated flange 31 on the side opposite to the side where the protruding projection 11 is protruding (see Figure 10C). At this time, the hole 10a of the intermediate product 10 is widened by the tapered punch 26, forming a hole 30a with a diameter D1 that is larger than the diameter D0 of the hole 10a. This produces a metal part 30 having an elongated flange 31 (see Figure 10C).
[0042] The tapered punch 26 has a shape in which the diameter decreases towards the tip. The opening angle φ at the tip of the tapered punch 26 (see Figure 10A) is not particularly limited, but is, for example, 5 to 90°, and preferably 15 to 45°.
[0043] The angle θ (see Figure 10B) between the tangent to the outer surface of the tip of the tapered punch 26 and the tangent to the portion of the protruding projection 11 that first contacts the tapered punch 26 is preferably 45° or greater. If the angle θ is too small, the protruding projection 11 cannot be folded back properly, and the tip may buckle or bend. The lower limit of the angle θ is preferably 60°, and more preferably 90°. The upper limit of the angle θ required for folding back varies depending on the opening angle φ of the tip of the tapered punch 26, but the upper limit is the angle θ when the protruding projection 11 becomes flat.
[0044] Furthermore, it is preferable that the tangent line to the outer surface of the tip of the tapered punch 26 is parallel to the fracture surface 112.
[0045] The diameter D1 of hole 30a (Figure 10C) should be larger than the diameter D0 of hole 10a of intermediate product 10 (Figure 10A). For example, diameter D1 is 1.30 to 3.00 times diameter D0. The lower limit of diameter D1 is preferably 1.50 times diameter D0, and more preferably 1.75 times. The upper limit of diameter D1 is preferably 2.70 times diameter D0, and more preferably 2.50 times.
[0046] Figure 11 is a schematic diagram showing the processing in the step of forming the stretch flange (step S2). In Figure 11, the thick line on the inner circumferential surface of the hole 10a represents the fracture surface. As described above, in the step of forming the stretch flange (step S2), the stretch flange 31 is formed on the side opposite to the side from which the protruding projection 11 is protruding by pressing in the tapered punch 26 and folding back the protruding projection 11. According to this embodiment, when the protruding projection 11 is folded back, the inner circumferential surface of the hole 10a and the tapered punch 26 come into contact with high surface pressure. Therefore, compared to a normal burring process, a strong contact pressure is applied to the hole edge in the initial stages of processing, which can suppress the occurrence of cracks at the hole edge.
[0047] Furthermore, at this time, the inner surface of the hole 10a undergoes plastic deformation (coining) due to contact pressure from the tapered punch 26. As a result, minute irregularities on the fracture surface 112 are crushed and smoothed. By crushing and smoothing the minute irregularities on the fracture surface 112, the occurrence of cracks originating from these irregularities is suppressed.
[0048] In this embodiment, the tapered punch 26 is brought into contact with the fracture surface 112 from the side where the protruding projection 11 is located. That is, the tapered punch 26 is brought into contact with the fracture surface 112 from the side of the fracture surface 112. This allows the tapered punch 26 to come into contact with the fracture surface 112 from the beginning of the process of forming the stretched flange (step S2). Furthermore, the tapered punch 26 and the fracture surface 112 can come into contact over a wider area and for a longer period of time. As a result, most of the fracture surface 112 (preferably the entire area) is coined. Note that a portion of the inner circumferential surface of the hole 10a that is farther from the tapered punch 26 does not come into contact with the tapered punch 26, but this portion is a shear surface and is originally a part that has almost no irregularities.
[0049] [Metal parts] Figure 12 is an example of a cross-sectional photograph of the vicinity of the stretch flange 31 of the metal part 30. Figure 13 is a magnified cross-sectional photograph showing the tip portion of the stretch flange 31 (region A1 in Figure 12). Figure 14 is a schematic cross-sectional view of the metal part 30. In Figure 14, the metal structure deformed by plastic flow is schematically illustrated with dashed lines.
[0050] The inner circumferential surface of the tip portion of the stretch flange 31 has a shear surface 311 resulting from the shear surface 111 (Figure 9A) of the hole 10a of the intermediate product 10, and a smooth surface 312 formed by coining the fracture surface 112 (Figure 10A) by the tapered punch 26. In other words, the inner circumferential surface of the tip portion of the stretch flange 31 does not have any irregularities remaining throughout, originating from the fracture surface 112. It is preferable that the inner circumferential surface of the tip portion of the stretch flange 31 has a metallic luster throughout.
[0051] The stretch flange 31 also has rounded corners 31b on the outer circumference of its tip. The rounding of the corners 31b is formed by plastic flow of the material during the process of forming the stretch flange (step S2). Specifically, the rounding of the corners 31b is due to the curve of the inner side surface of the protruding projection 11 of the intermediate product 10 (Figure 10A), and this side surface is formed by compression by the tapered punch 26. The larger the radius of curvature r of the corners 31b, the stronger the compressive force applied during the formation of the stretch flange 31.
[0052] The radius of curvature r of the corner 31b is preferably 0.10 times the plate thickness t (Figure 10C) or more. The lower limit of the radius of curvature r is more preferably 0.15 times the plate thickness t, and even more preferably 0.20 times. The upper limit of the radius of curvature r is, for example, 0.75 times the plate thickness t, and preferably 0.50 times.
[0053] Furthermore, as schematically shown in Figure 14, in the vicinity of the corner 31b, the metal structure also deforms to conform to the outer shape of the corner 31b. The deformed metal structure can sometimes be revealed by etching, although this depends on the material of the metal part 30.
[0054] The stretch flange 31 may also have a protrusion 31a on the inner circumference side of its tip portion that bulges inward in the radial direction of the stretch flange 31. The protrusion 31a is formed by plastic flow of the material during the process of forming the stretch flange (step S2). Specifically, the protrusion 31a is formed when a part of the outer side surface of the protruding protrusion 11 of the intermediate product 10 (Figure 10A) is compressed by the tapered punch 26. If the protrusion 31a is formed, the greater the height h2 of the protrusion 31a, the stronger the compressive force that was applied during the formation of the stretch flange 31 can be evaluated.
[0055] The height h2 of the protrusion 31a is not particularly limited, but is preferably 0.05 times the plate thickness t (Figure 10C) or more. The lower limit of the height h2 is more preferably 0.10 times the plate thickness t. The upper limit of the height h2 is, for example, 0.25 times the plate thickness t, and preferably 0.20 times. The height h2 is the distance from the tangent to the inner circumferential surface of the tip portion of the extension flange 31 to the apex portion of the protrusion 31a.
[0056] Figure 15 schematically shows another example of the cross-sectional shape of the tip of the extension flange 31 of the metal part 30. As shown in Figure 15, the protrusion 31a may be folded and inclined (folded) towards the inner circumferential surface of the tip portion of the extension flange 31. In this case as well, the height h2 is the distance from the tangent to the inner circumferential surface of the tip portion of the extension flange 31 to the apex of the protrusion 31a. The preferred range for the height h2 is the same as in Figures 12 to 14.
[0057] Figure 16 is a cross-sectional photograph showing yet another example of the cross-sectional shape of the stretch flange 31 of the metal part 30. Figure 17 is a schematic cross-sectional view of the stretch flange 31 of Figure 16. As shown in Figures 16 and 17, if the stretch flange 31 is folded back further than in Figures 12 to 14, the protrusion 31a may be crushed and disappear by the tapered punch 26 (Figure 11). Even in this case, as a trace of the protrusion 31a, the stretch flange 31 may have a groove G formed on the inner circumference side of the tip portion, surrounding the hole 30a of the stretch flange 31.
[0058] Apart from the groove G described above, the metal part 30 may have a groove g formed on the side opposite to the side from which the stretch flange 31 protrudes, at the base of the stretch flange 31, surrounding the hole 30a of the stretch flange 31. This groove g is a groove g that remains after the process of forming the stretch flange (step S2) when the intermediate product 10 is manufactured using a specific method (see Figures 7C, 8C, and 9C).
[0059] Figure 18 is a photograph of the intermediate product 10, showing the side opposite to the side where the protruding projection is visible. Figures 20 and 22 are photographs of the metal part 30 manufactured by forming an elongated flange on the intermediate product 10 of Figure 18, showing the side where the elongated flange 31 is visible (Figure 20) and the side opposite to the side where the elongated flange 31 is visible (Figure 22). Figures 19, 21, and 23 are schematic diagrams of Figures 18, 20, and 22, respectively.
[0060] As described above, when the intermediate product 10 is manufactured using the method described in Figures 7A to 7C, Figures 8A to 8C, or Figures 9A to 9C, a groove g (see Figures 7C, 8C, and 9C) may be formed on the protruding side of the protruding projection 11, surrounding the projection 11. This groove g may also remain on the metal part 30. As described above, in the process of forming the stretch flange (step S2), the stretch flange 31 is formed on the side opposite to the side where the projection 11 protrudes by pressing in the tapered punch 26 and folding back the projection 11. Therefore, the groove g is formed on the side opposite to the side where the stretch flange 31 protrudes, surrounding the hole 30a of the stretch flange 31.
[0061] The metal part 30 is not limited to these, but may be an automotive part such as a lower arm, seat gear, recliner, bearing, hub, or various gears. The metal part 30 may also be a part other than an automotive part, such as a pipe or bearing part, tapped hole part, or joint part for industrial machinery or home appliances.
[0062] [Comparative Example] Next, as a hypothetical comparative example, consider the case where, in the process of preparing the intermediate product (step S1), a protruding projection is formed on the material, and then punching is performed from the side where the protruding projection protrudes. Specifically, as shown in Figure 24, consider the case where the punching punch 23 is brought into contact with the side where the protruding projection 11 protrudes, and the die 29 is placed on the opposite side to perform the punching. In this case, as shown in Figure 25, in the intermediate product 40 formed in this way, a fracture surface 411 is formed on the side opposite to the side where the protruding projection 11 protrudes, and a shear surface 412 is formed on the side where the protruding projection 11 protrudes.
[0063] Figure 26 schematically shows the processing when the process of forming an elongated flange (step S2) is performed using the intermediate product 40. In Figure 26, the thick line on the inner circumferential surface of the hole 10a represents the fracture surface. When the intermediate product 40 is used, the fracture surface 411 is located on the outer diameter side where the tensile stress is high, making it easy for cracks to occur at the edge of the hole.
[0064] Furthermore, when the intermediate product 40 is used, the tapered punch 26 contacts the fracture surface 411 from the side furthest from it. As a result, the tapered punch 26 cannot be brought into contact with the fracture surface 411 from the beginning of the process of forming the stretched flange (step S2), resulting in insufficient coining. In addition, uncoined portions remain on the fracture surface 411 (i.e., portions where irregularities originating from the fracture surface 411 remain).
[0065] As another comparative example, consider the case where, in the process of preparing the intermediate product (step S1), only the punching process shown in Figures 6A and 6B is performed, and the process of forming the protruding portion 11 (the process shown in Figures 6C and 6D) is not performed. In this case, as shown in Figure 27, an intermediate product 45 is formed that does not have a protruding portion and only has a hole 10a. In the intermediate product 45, a shear surface 451 is formed on one side in the thickness direction, and a fracture surface 452 is formed on the other side in the thickness direction.
[0066] Figure 28 schematically shows the processing when the process of forming an elongated flange (step S2) is performed using the intermediate part 45. In Figure 28, the thick line on the inner circumferential surface of the hole 10a represents the fracture surface. When using the intermediate part 45, the protruding projection 11 is not folded back as when using the intermediate part 10 (Figure 11), so a strong contact pressure is not applied to the edge of the hole compared to when using the intermediate part 10.
[0067] Furthermore, when using intermediate product 45, compared to when using intermediate product 10 (Figure 11), the contact area between the tapered punch 26 and the fracture surface 452 is smaller, and the contact time between the tapered punch 26 and the fracture surface 452 is shorter, resulting in insufficient coining. In addition, uncoined portions remain on the fracture surface 452 (i.e., portions where irregularities originating from the fracture surface 452 remain).
[0068] Figure 29 is an example of a cross-sectional photograph of the vicinity of the stretched flange portion of a metal part 50 (Figure 28) manufactured by a process of forming a stretched flange on an intermediate product 45. Figure 30 is a magnified cross-sectional photograph showing the tip portion of this stretched flange (region A2 in Figure 29). Figure 31 is a schematic cross-sectional view of the metal part 50. The metal structure is schematically illustrated with dashed lines in Figure 31.
[0069] The inner circumferential surface of the tip portion of the elongated flange 51 of the metal part 50 has a shear surface 511, a fracture surface 512, and a smooth surface 513. The smooth surface 513 is a surface where a part of the fracture surface 452 (Figure 28) of the intermediate product 45 has been smoothed by coining, and the fracture surface 512 is a surface where another part of the fracture surface 452 remains without coining. In other words, the inner circumferential surface of the tip portion of the elongated flange 51 retains irregularities originating from the fracture surface 452. Furthermore, compared to the metal part 30 (Figures 14 and 17), the metal part 50 does not have either a protrusion 31a or a groove G like the metal part 30. Also, unlike the metal part 30, the corner portion 51b of the metal part 50 is not rounded, and retains the shape of a corner where two sides intersect, formed during the punching process, and the metal structure near the corner portion 51b maintains a layered structure laminated in the thickness direction.
[0070] The above describes a method for manufacturing a metal part and a metal part according to one embodiment of the present invention. In this embodiment, a step of forming an elongated flange is performed on an intermediate product 10 having an overhanging projection 11. It is difficult to smooth the entire fracture surface by simply pressing a tapered punch, but according to this embodiment, when the overhanging projection 11 is folded back to the opposite side, the fracture surface contacts the tapered punch with high surface pressure, so the fracture surface can be smoothed over a wide area. In addition, since the initial contact surface pressure on the hole edge can be increased, cracks are less likely to occur at the hole edge when the hole is widened. This makes it possible to suppress elongated flange cracking. [Examples]
[0071] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.
[0072] [Example 1] A 4.0 mm thick steel plate made of S35C (annealed material) was prepared. This steel plate was processed by bulging using a die with a hole diameter of 19.4 mm and a shoulder radius of 0.5 mm, and a punch with a diameter of 13.0 mm and a shoulder radius of 6.5 mm (spherical head), to form a 6.0 mm high protruding portion. From the inside of the protruding portion (opposite the side where the protruding portion is protruding; the same applies hereafter), a punching process was performed using a 10.0 mm diameter punch and a die with a hole diameter of 10.8 mm to form a 10 mm diameter punched hole, which was used as an intermediate product. On the inner circumferential surface of this punched hole, a fracture surface was formed on the outside of the protruding portion (the side where the protruding portion is protruding; the same applies hereafter).
[0073] A metal part with an elongated flange was manufactured by burring this intermediate product. Specifically, the intermediate product was fixed with a plate-holding force of 30kN using a die (plate clamp) with a hole diameter of 41.25mm and a shoulder radius of 1.0mm, and a tapered punch with an opening angle of φ (see Figure 10A) of 30° was pressed against the outside of the protruding part to form an elongated flange while folding the protruding part back. The hole was widened from an initial diameter of 10mm until the hole diameter reached 20mm, at which point the processing was completed.
[0074] Visual inspection confirmed that the entire fracture surface of the inner circumferential surface of the tip portion of the stretched flange of the obtained metal part was coined. Furthermore, no stretched flange cracking was observed (limit hole expansion ratio of 100% or more).
[0075] [Comparative Example 1] An intermediate product was manufactured in the same manner as in Example 1, except that the punching process was performed from the outside of the protruding projection. A fracture surface was formed on the inner circumferential surface of the punched hole, inside the protruding projection.
[0076] Burring was performed on this intermediate product in the same manner as in Example 1. In Comparative Example 1, stretch flange cracking occurred when the hole diameter reached 14 mm (limit hole expansion ratio of 40%). Furthermore, it was visually confirmed that the majority of the fracture surface on the inner circumferential surface of the tip portion of the stretch flange of the obtained metal part was an area that had not been coined.
[0077] [Comparative Example 2] An intermediate product was manufactured in the same manner as in Example 1, except that the protrusion process was not performed.
[0078] Burring was performed on this intermediate product in the same manner as in Example 1. The tapered punch was inserted from the side with the fracture surface of the punched hole. In Comparative Example 2, stretch flange cracking occurred when the hole diameter reached 16 mm (limit hole expansion ratio of 60%). Furthermore, it was visually confirmed that there was an area on the inner circumferential surface of the tip portion of the stretch flange of the obtained metal part that was not coined in part of the fracture surface.
[0079] Although embodiments of the present invention have been described above, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and it is possible to carry out the present invention by appropriately modifying the embodiments described above within the scope of the invention. [Explanation of symbols]
[0080] 10 Intermediate products 10a hole 11 Overhanging convex part 111 Shear surface 112 Fracture surface 21, 211 Punch for forming protruding protrusions 22 Die for forming protruding protrusions 23 Punching 24, 241, 242, 29 Dies for punching 244-step die 245, 246 Die holder 26 Tapered Punch 25, 28 Board holder 27 Die for stretch flange machining 30 metal parts 30a hole 31 Stretch flange
Claims
1. A metal part having an expandable flange, A metal part in which the outer corner of the tip portion of the extension flange is rounded.
2. A metal part according to claim 1, The extension flange is a metal component having a protrusion on the inner circumference side of the tip portion that bulges inward in the radial direction of the extension flange.
3. A metal part according to claim 1, The extension flange is a metal component having a groove formed on the inner circumference side of the tip portion so as to surround the hole in the extension flange.
4. A metal part according to any one of claims 1 to 3, A metal part wherein the inner circumferential surface of the tip portion of the stretch flange is free from irregularities originating from the fracture surface over its entire length.
5. A metal part according to any one of claims 1 to 3, A metal part having a metallic luster throughout the inner circumferential surface of the tip portion of the extension flange.
6. A metal part according to claim 2, A metal part in which the height of the aforementioned protrusion is 0.05 times or more the thickness of the plate.
7. A metal part according to any one of claims 1 to 3, A metal part in which the radius of curvature of the aforementioned corner is 0.10 times or more the thickness of the plate.
8. A metal part according to any one of claims 1 to 3, A metal component having a groove formed on the side opposite to the side from which the extension flange protrudes, at the base portion of the extension flange, so as to surround the hole in the extension flange.