Metal component manufacturing method, and metal component

Abstract

Provided is a metal component manufacturing method with which it is possible to suppress stretch flange cracks. This metal component manufacturing method is a method for manufacturing a metal component having a stretch flange, the method comprising a step for preparing a metal intermediate product, and a step for pressing a tapered punch into the intermediate product to form a stretch flange, wherein: the intermediate product is a component having a plate-shaped part, and includes a projecting portion that projects to one side in a plate thickness direction, and a hole formed within the projecting portion; the hole has a fracture surface on an inner circumferential surface; the fracture surface is on the side from which the projecting portion protrudes; and in the step for forming the stretch flange, the tapered punch is pressed into the hole from the side from which the projecting portion protrudes, thereby folding back the projecting portion to form the stretch flange on the side opposite to the side from which the projecting portion protrudes.

Description

Manufacturing method of metal parts and metal parts

[0001] The present invention relates to a method for manufacturing a metal part and to a metal part.

[0002] Automobile suspension parts, seat parts, transmission parts, etc. are manufactured by pressing metal plates such as high-tensile steel plates, special steel plates, etc. In addition, parts such as lower arms, seat gears, recliners, bearings, hubs, and various gears may be formed with stretch flanges (burring flanges) by burring in order to be joined to other parts.

[0003] Burring is a process in which an initial hole (prepared hole) that has been formed in advance in a metal plate is expanded with a punch, and the peripheral portion of the initial hole is stretched in the direction of the punch's thrust to form a cylindrically protruding stretch flange.

[0004] One of the defects that can occur during burring is cracking at the tip of the stretch flange (stretch flange cracking). The causes of stretch flange cracking have been identified as work hardening of the cut surface when the initial hole is formed by punching, and the notch effect caused by the unevenness of the fracture surface on the cut surface. Therefore, one method to improve the limit hole expansion ratio is to scrape off the area near the cut surface after forming the initial hole. However, with this method, the scraped off shavings tend to adhere to the punch, which may cause dent defects during press processing. It is also possible to form the initial hole by cutting instead of punching, but it is difficult to incorporate such a mechanism into the mold of an automatic press device such as a progressive press.

[0005] Japanese Patent Application Laid-Open Publication No. 2015-36147 discloses a burring punch having a front end portion that enlarges a pre-formed pilot hole to a diameter smaller than the final hole diameter, a rear end portion that enlarges the hole enlarged by the front end portion to the final hole diameter, and a recessed portion that is provided at the boundary between the front end portion and the rear end portion and has a diameter smaller than that of the front end portion.

[0006] Japanese Patent Application Laid-Open No. 2014-172089 discloses a burring punch. This burring punch has a tip portion that contacts the periphery of a prepared hole in the initial stage of burring, a cylindrical large-diameter portion having the same diameter as the finished inner diameter of the stretch flange, and a truncated cone-shaped expanded diameter portion that smoothly connects the tip portion and the large-diameter portion and expands in diameter toward the rear end, with the tip portion having an opening angle of 90° or more and the expanded diameter portion having an opening angle of 60° or less, and the diameter d of the rear end of the tip portion being 90° or more. １ is the diameter of the pilot hole d ０ In relation to d ０ <d １ ≦1.2×d ０ It is characterized by satisfying the following conditions.

[0007] Japanese Patent Application Laid-Open Publication No. 2007-75869 discloses a burring method that includes a first punching step of punching a plate material to form a pilot hole, a second punching step of punching the periphery of the pilot hole with a predetermined scrap width to form a pilot hole with a larger diameter than the pilot hole, and a bending step of bending the periphery of the pilot hole to form a flange.

[0008] Although not related to burring, Japanese Patent Laid-Open Publication No. 2000-288655 discloses a dieless punching method that does not use a die.

[0009] Abe Yohei et al., "Improving hole expandability by smoothing the fracture surface of ultra-high tensile steel sheets," Journal of the JSTP, vol. 52, no. 603 (2011-4), describes how a tapered punch is pressed into the fracture surface of a punched steel sheet to smooth the fracture surface, and then a conical punch is pressed into the sheet with the burr facing the die, thereby performing hole expansion.

[0010] JP 2015-36147 A JP 2014-172089 A JP 2007-75869 A JP 2000-288655 A

[0011] Yohei Abe et al., "Improvement of hole expansion ability by smoothing the fracture surface of ultra-high strength steel sheets," Journal of the JSTP, vol. 52, no. 603 (2011-4)

[0012] In recent years, there has been a demand for lighter automotive parts to address environmental issues. Therefore, high-strength steel sheets, such as high-tensile steel sheets that can maintain strength even when made thin, and special steel sheets that can increase strength through heat treatment after processing, are increasingly being used as materials for automotive parts. High-strength steel sheets have low ductility, making them prone to stretch flange cracking during burring. In particular, special steel sheets made of medium- and high-carbon steels with a high C content are particularly prone to stretch flange cracking.

[0013] An object of the present invention is to provide a method for manufacturing a metal part that can suppress stretch flange cracking. Another object of the present invention is to provide a metal part in which stretch flange cracking is suppressed.

[0014] A method for manufacturing a metal part according to one embodiment of the present invention is a method for manufacturing a metal part having a stretch flange, and includes the steps of preparing a metal intermediate product and forcing a tapered punch into the intermediate product to form a stretch flange, wherein the intermediate product is a part having a plate-shaped portion and has a protruding protrusion protruding to one side in the plate thickness direction and a hole formed within the protruding protrusion, the hole having a fracture surface on its inner surface and the fracture surface being on the side from which the protruding protrusion protrudes, and in the step of forming the stretch flange, the tapered punch is forced into the hole from the side from which the protruding protrusion protrudes and the protruding protrusion is folded back to form the stretch flange on the side opposite to the side from which the protruding protrusion protrudes.

[0015] A metal part according to one embodiment of the present invention is a metal part having a stretch flange, and the corners of the outer periphery of the tip portion of the stretch flange are rounded.

[0016] According to the present invention, the occurrence of stretch flange cracking can be suppressed.

[0017] FIG. 1 is a flow diagram of a method for manufacturing a metal part according to one embodiment of the present invention. FIG. 2 is a perspective view schematically illustrating the configuration of an example of an intermediate product. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. FIG. 4 is an example of a cross-sectional photograph of the intermediate product. FIG. 5A is a schematic diagram illustrating an example of a method for manufacturing the intermediate product. FIG. 5B is a schematic diagram illustrating an example of a method for manufacturing the intermediate product. FIG. 5C is a schematic diagram illustrating an example of a method for manufacturing the intermediate product. FIG. 5D is a schematic diagram illustrating an example of a method for manufacturing the intermediate product. FIG. 6A is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 6B is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 6C is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 6D is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 7A is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 7B is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 7C is a schematic diagram illustrating another example of a method for manufacturing the intermediate product. FIG. 8A is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 8B is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 8C is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 9A is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 9B is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 9C is a schematic diagram illustrating another example of a method for manufacturing an intermediate product. FIG. 10A is a schematic diagram illustrating a step of forming a stretch flange. FIG. 10B is a schematic diagram illustrating a step of forming a stretch flange. FIG. 10C is a schematic diagram illustrating a step of forming a stretch flange. FIG. 11 is a diagram schematically illustrating the processing state in the step of forming a stretch flange. FIG. 12 is an example of a cross-sectional photograph of the vicinity of a stretch flange portion of a metal part. FIG. 13 is a cross-sectional photograph showing an enlarged view of region A1 in FIG. 12. FIG. 14 is a schematic cross-sectional view of a metal part. FIG. 15 is a diagram schematically illustrating another example of the cross-sectional shape of the tip of a stretch flange of a metal part. Fig. 16 is a cross-sectional photograph showing yet another example of the cross-sectional shape of the tip of a stretch flange of a metal part. Fig. 17 is a schematic cross-sectional view of the stretch flange of Fig. 16.18 is a photograph of an intermediate product, showing the side from which the protruding protrusion protrudes. FIG. 19 is a diagram schematically illustrating FIG. 18. FIG. 20 is a photograph of a metal part manufactured by performing a step of forming a stretch flange on the intermediate product of FIG. 18, showing the side from which the stretch flange protrudes. FIG. 21 is a diagram schematically illustrating FIG. 20. FIG. 22 is a photograph of a metal part manufactured by performing a step of forming a stretch flange on the intermediate product of FIG. 18, showing the side opposite to the side from which the stretch flange protrudes. FIG. 23 is a diagram schematically illustrating FIG. 22. FIG. 24 is a schematic diagram for explaining a method for manufacturing an intermediate product of a comparative example. FIG. 25 is a schematic cross-sectional view of an intermediate product of a comparative example. FIG. 26 is a diagram schematically illustrating the processing state in the step of forming a stretch flange. FIG. 27 is a schematic cross-sectional view of an intermediate product of a comparative example. FIG. 28 is a diagram schematically illustrating the processing state in the step of forming a stretch flange. FIG. 29 is an example of a cross-sectional photograph of the vicinity of the stretch flange portion of a metal part of a comparative example. Fig. 30 is an enlarged cross-sectional photograph of region A2 in Fig. 29. Fig. 31 is a schematic cross-sectional view of a metal part of a comparative example.

[0018] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and the description thereof will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[0019] [Method for manufacturing a metal component] Figure 1 is a flow diagram of a method for manufacturing a metal component according to one embodiment of the present invention. The method for manufacturing a metal component according to this embodiment is a method for manufacturing a metal component having a stretch flange, and includes a step of preparing a metal intermediate product having a predetermined shape (step S1), and a step of pressing a tapered punch into the intermediate product to form a stretch flange (step S2).

[0020] [Intermediate Product Preparation Step] First, a metal intermediate product having a predetermined shape is prepared (step S1). Fig. 2 is a perspective view schematically showing the configuration of intermediate product 10, which is an example of the intermediate product prepared in this step. Fig. 3 is a cross-sectional view taken along line III-III in Fig. 2. Fig. 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 the material of the intermediate product 10 include an iron-based alloy (steel), copper, a copper alloy, aluminum, and an aluminum alloy. The method for manufacturing a metal part according to this embodiment is suitable when the intermediate product 10 is made of a metal with high strength and low ductility (e.g., high-strength steel such as high-tensile steel). The method for manufacturing a metal part according to this embodiment is particularly suitable when the intermediate product 10 is made of medium- or high-carbon steel with a C content of 0.20 mass% or more. Examples of medium-carbon steel include, but are not limited to, carbon steels for machine structures such as S35C, S40C, and S45C. Examples of high-carbon steel include, but are not limited to, carbon tool steels such as SK85 and high-carbon chromium bearing steels such as SUJ2.

[0022] The intermediate product 10 is a component having a plate-like portion, and has a protruding protrusion 11 that protrudes to one side in the plate thickness direction (z direction), and a hole 10a formed in the protruding protrusion 11. The hole 10a penetrates the intermediate product 10 in the plate thickness direction. It is preferable that the hole 10a be formed in the center of the protruding protrusion 11 in the in-plane direction (xy in-plane direction).

[0023] Diameter D of hole 10a ０ The diameter D (Fig. 3) is not particularly limited as it is determined by the product shape, but is, for example, 2.0 to 50 times the plate thickness t (Fig. 3). ０ If the diameter D is too small, strain on the outer side of the stretch flange tip increases in the process of forming the stretch flange, which may make stretch flange forming difficult. ０ If the diameter D is too large, the pressing effect against the punch during folding will decrease, making stretch flange forming difficult. ０ The lower limit of the diameter D is more preferably 2.5 times the plate thickness t, and even more preferably 3.0 times. ０ The upper limit of is more preferably 30 times, and even more preferably 20 times the plate thickness t.

[0024] The hole 10a is formed by punching (shearing) as described below. Therefore, the hole 10a has a sheared surface 111 and a fractured surface 112 on its inner circumferential surface. In this embodiment, the sheared surface 111 is located on the side opposite to the side from which the protruding protrusion 11 protrudes, and the fractured surface 112 is located on the side from which the protruding protrusion 11 protrudes.

[0025] 3 and 4, the sheared surface 111 is a surface that is approximately parallel to the plate thickness direction (z direction), whereas the fractured surface 112 has a cross-sectional shape that widens outward in the radial direction of the hole 10a. Furthermore, the sheared surface 111 is a smooth surface that has a metallic luster, whereas the fractured surface 112 is a surface that has minute irregularities and does not have a metallic luster.

[0026] The height h0 (FIG. 3) of the protruding portion 11 is not particularly limited, but is, for example, 0.25 to 10 times the plate thickness t (FIG. 3). If the height h0 is too small, the effect of smoothing the fracture surface in the stretch flange forming process (step S2) may not be sufficiently obtained. On the other hand, if the height h0 is too large, a localized reduction in plate thickness may occur during the formation of the protruding portion, which may cause cracks 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 include, but are not limited to, (1) a method of forming the protruding protrusion 11 by drawing or bulging, and then forming the hole 10a by punching, (2) a method of forming the hole 10a by punching, and then forming the protruding protrusion 11 by edge bending, and (3) a method of forming the hole 10a and the protruding protrusion 11 in a single process. More specifically, the method of forming the hole 10a and the protruding protrusion 11 in a single process (3) includes (3-1) a method of forming the protruding protrusion 11 and the hole 11a by providing a step between a die and a die holder and performing a punching process, (3-2) a method of forming the protruding protrusion 11 and the hole 11a by performing a punching process using only a die holder without using a die, and (3-3) a method of forming the protruding protrusion 11 and the hole 11a by performing a punching process using a stepped die.

[0028] 5A to 5D are schematic diagrams illustrating the method (1) above. First, a metal blank S is subjected to drawing or bulging using a bulging-protrusion-forming punch 21, a bulging-protrusion-forming die 22, and a plate holder 25 to form a bulging protrusion 11 (see FIGS. 5A and 5B ). Next, a punch 23, a punching die 24, and a plate holder 25 are used to punch the portion of the blank S where the bulging protrusion 11 is formed, forming a hole 10a in the bulging protrusion 11. At this time, the punch 23 is abutted from the side opposite the side from which the bulging protrusion 11 protrudes. As a result, a shear surface 111 ( FIG. 3 ) is formed on the side opposite the side from which the bulging protrusion 11 protrudes, and a fracture surface 112 ( FIG. 3 ) is formed on the side from which the bulging protrusion 11 protrudes.

[0029] 6A to 6D are schematic diagrams illustrating the above-described method (2). This method reverses the order in which the overhanging protrusion 11 and the hole 10a are formed in the above-described method (1). Specifically, a metal blank S is first punched using a punch 23, a punching die 241, and a plate holder 25 to form the hole 10a (see FIGS. 6A and 6B). Next, the portion of the blank S where the hole 10a was formed is edge-bent using a punch 211 for forming an overhanging protrusion, a die 22 for forming an overhanging protrusion, and a plate holder 25 to form the overhanging protrusion 11 (see FIGS. 6C and 6D). At this time, the punch 211 for forming an overhanging protrusion is brought into contact with the blank S from the same side as the punch 23. As a result, a shear surface 111 ( FIG. 3 ) is formed on the side opposite the side from which the overhanging protrusion 11 protrudes, and a fracture surface 112 ( FIG. 3 ) is formed on the side from which the overhanging protrusion 11 protrudes.

[0030] 7A to 7C are schematic diagrams illustrating the above-mentioned method (3-1). In this method, punching is performed using a punching die 242 and a die holder 245 instead of the punching die 24 shown in FIGS. 5C and 5D. That is, in the method shown in FIGS. 7A to 7C, punching is performed using a punch 23, a punching die 242, a die holder 245 arranged to surround the punching die 242, and a plate holder 25. The punching die 242 is arranged so that its upper end surface is farther from the material S than the upper end surface of the die holder 245 in the thickness direction of the material S.

[0031] First, the metal blank S is sandwiched between the plate holder 25 and the die holder 245, the punch 23 is brought into contact with the die holder 245 from the opposite side, and at least one of the punching die 242 and die holder 245 is moved toward each other (see FIGS. 7A and 7B ). Because the upper end surface of the punching die 242 is positioned farther from the blank S than the upper end surface of the die holder 245, the blank S does not contact the punching die 242 during the first half of the stroke. During this time, a protruding portion 11 is formed in the blank S (see FIG. 7B ). By further moving the punch 23 and the punching die 242 and die holder 245 closer to each other from the state shown in FIG. 7B , a hole 10a is formed in the protruding portion 11 (see FIG. 7C ). In this case, too, the punching punch 23 contacts the side opposite to the side from which the protruding protrusion 11 protrudes, so that a shear surface 111 (Figure 3) is formed on the side opposite to the side from which the protruding protrusion 11 protrudes, and a fracture surface 112 (Figure 3) is formed on the side from which the protruding protrusion 11 protrudes.

[0032] Figures 8A to 8C are schematic diagrams illustrating the above method (3-2). In this method, punching is performed using a die holder 246 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 punch 23, a die holder 246, and a plate holder 25. In the method shown in Figures 8A to 8C, the punching die 24 is not used. That is, the method shown in Figures 8A to 8C is a punching process that does not use a die (dieless process).

[0033] As in the case of (3-1) above, the metal blank S is sandwiched between the plate holder 25 and the die holder 246, the punch 23 is brought into contact with the die holder 246 from the side opposite the die holder 246, and at least one of the punch 23 and the die holder 246 is moved toward each other (see FIGS. 8A and 8B). In this case, too, a protruding portion 11 is formed in the blank S during the first half of the stroke (see FIG. 8B). By further bringing the punch 23 and the die holder 246 closer to each other from the state shown in FIG. 8B, a hole 10a is formed in the protruding portion 11 (see FIG. 8C). In this case, too, the punch 23 contacts the blank S from the side opposite the protruding portion 11, so that a shear surface 111 ( FIG. 3 ) is formed on the side opposite the protruding portion 11, and a fracture surface 112 ( FIG. 3 ) is formed on the side where the protruding portion 11 protrudes.

[0034] The size of the clearance (one side) between the punch 23 and the die holder 246 is, for example, but not limited to, at least twice the thickness t of the material S. The size of the clearance (one side) is more preferably at least five times the thickness t.

[0035] 9A to 9C are schematic diagrams illustrating the above-mentioned method (3-3). In this method, a stepped die 244 is used to perform the punching process instead of the punching die 24 shown in FIGS. 5C and 5D. That is, in the method shown in FIGS. 9A to 9C, punching is performed using a punch 23, a stepped die 244, and a plate holder 25. The stepped die 244 has a hole 244a for passing the punch 23 therethrough. The hole 244a includes, from the punch 23 side, a large-diameter portion 244a1 formed to have a first clearance of a predetermined size relative to the punch 23, and a small-diameter portion 244a2 formed to have a second clearance relative to the punch 23 that is smaller than the first clearance.

[0036] First, the metal blank S is sandwiched between the sheet holder 25 and the stepped die 244, the punch 23 is brought into contact with the stepped die 244 from the side opposite the stepped die 244, and at least one of the punch 23 and the stepped die 244 is moved toward each other (see FIGS. 9A and 9B ). In this case, too, a protruding portion 11 is formed in the blank S during the first half of the stroke (see FIG. 9B ). By further bringing the punch 23 and the stepped die 244 closer to each other from the state shown in FIG. 9B , a hole 10a is formed in the protruding portion 11 (see FIG. 9C ). In this case, too, the punch 23 contacts the blank S from the side opposite the protruding portion 11, so that a shear surface 111 ( FIG. 3 ) is formed on the side opposite the protruding portion 11, and a fracture surface 112 ( FIG. 3 ) is formed on the side where the protruding portion 11 protrudes.

[0037] While the methods (1) and (2) above require two steps to form the overhanging protrusion 11 and the hole 10a, the methods (3-1) to (3-3) above allow the overhanging protrusion 11 and the hole 10a to be formed in a single step. Therefore, the methods (3-1) to (3-3) above can reduce the number of steps compared to the methods (1) and (2) above. Furthermore, the methods (3-1) to (3-3) above allow the overhanging protrusion 11 and the hole 10a to be formed simultaneously, so that the centers of the overhanging protrusion 11 and the hole 10a can be stabilized without misalignment.

[0038] According to the method (3-2) above, furthermore, since a die is not required, the cost of the die can be reduced. Also, maintenance of the cutting edge is limited to the punch 23, which makes management easier.

[0039] In addition, when the intermediate product 10 is formed by the above methods (3-1) to (3-3), a groove g (see Figures 7C, 8C and 9C) may be formed on the protruding side surface of the protruding protrusion 11, surrounding the protruding protrusion 11, due to contact with the corner of the stepped die 244 or the corner of the die holder 245 or 246.

[0040] [Step of forming a stretch flange] Next, a tapered punch is pressed into the intermediate product 10 to form a stretch 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 protrusion 11 protrudes, and the protruding protrusion 11 is folded back, thereby forming a stretch flange on the side opposite to the side where the protruding protrusion 11 protrudes.

[0041] 10A to 10C are schematic diagrams illustrating the process of forming a stretch flange (step S2). First, a tapered punch 26 is brought into contact with the hole 10a of the intermediate product 10 from the side where the protruding protrusion 11 protrudes (see FIGS. 10A and 10B). From this state, the intermediate product 10 is fixed by a stretch flange forming die 27 and a plate holder 28, and the tapered punch 26 is pressed in, folding back the protruding protrusion 11 to form a stretch flange 31 on the side opposite the side where the protruding protrusion 11 protrudes (see FIG. 10C). At this time, the hole 10a of the intermediate product 10 is expanded by the tapered punch 26, and the diameter D of the hole 10a is increased. ０ Diameter D is larger than １ This forms a hole 30a having a stretch flange 31. This produces a metal part 30 (see FIG. 10C) having a stretch flange 31.

[0042] The tapered punch 26 has a shape in which the diameter decreases toward the tip. The opening angle φ (see FIG. 10A) of the tip of the tapered punch 26 is not particularly limited, but is, for example, 5 to 90°, and preferably 15 to 45°.

[0043] The angle θ (see FIG. 10B ) formed by the tangent to the outer peripheral surface of the tip of the tapered punch 26 and the tangent to the protruding protrusion 11 at the portion where it first comes into contact with the tapered punch 26 is preferably 45° or greater. If the angle θ is too small, the protruding protrusion 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 angle θ at which the protruding protrusion 11 becomes flat is the upper limit.

[0044] It is also preferable that the tangent to the outer peripheral surface of the tip of the tapered punch 26 and the fracture surface 112 are parallel to each other.

[0045] Diameter D of hole 30a １ (FIG. 10C) shows the diameter D of the hole 10a of the intermediate product 10. ０ (Fig. 10A). １ is, for example, the diameter D ０ The diameter D is 1.30 to 3.00 times the diameter D. １ The lower limit of the diameter D ０ The diameter D is preferably 1.50 times, and more preferably 1.75 times. １ The upper limit of the diameter D ０ It is preferably 2.70 times, and more preferably 2.50 times.

[0046] FIG. 11 is a diagram schematically illustrating the processing state in the step of forming a stretch flange (step S2). In FIG. 11, the thick line on the inner peripheral surface of the hole 10a represents the fracture surface. As described above, in the step of forming a stretch flange (step S2), the tapered punch 26 is pressed to fold back the protruding protrusion 11, thereby forming a stretch flange 31 on the side opposite to the side from which the protruding protrusion 11 protrudes. According to this embodiment, when the protruding protrusion 11 is folded back, the inner peripheral surface of the hole 10a and the tapered punch 26 come into contact with each other at a high surface pressure. Therefore, compared to normal burring, a strong contact surface pressure acts on the hole edge at the beginning of processing, which can suppress the occurrence of cracks at the hole edge.

[0047] At this time, the inner peripheral surface of hole 10a is subjected to contact pressure from tapered punch 26 and undergoes plastic deformation (coining), which crushes and smooths minute irregularities on fracture surface 112. By crushing and smoothing the minute irregularities on 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 fractured surface 112 from the side where the overhanging protrusion 11 protrudes. That is, the tapered punch 26 is brought into contact with the fractured surface 112 from the beginning of the stretch flange forming process (step S2). This also allows the tapered punch 26 and the fractured surface 112 to be in contact over a wider area and for a longer period of time. This allows most (preferably the entire area) of the fractured surface 112 to be coined. Note that a portion of the inner peripheral 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 an area that originally had almost no irregularities.

[0049] [Metal part] Figure 12 is an example of a cross-sectional photograph of the vicinity of a stretch flange 31 of a metal part 30. Figure 13 is a cross-sectional photograph showing an enlarged view of the tip portion (area A1 in Figure 12) of the stretch flange 31. 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 shown by dashed lines.

[0050] The inner peripheral surface of the tip portion of the stretch flange 31 has a shear surface 311 resulting from the shear surface 111 (FIG. 9A) of the hole 10a of the intermediate product 10, and a smooth surface 312 formed by coining the fracture surface 112 (FIG. 10A) with the tapered punch 26. In other words, the inner peripheral surface of the tip portion of the stretch flange 31 does not have any unevenness resulting from the fracture surface 112 remaining throughout. It is preferable that the inner peripheral 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 periphery of the tip portion. The rounding of the corners 31b is formed by the plastic flow of the material in the stretch flange forming process (step S2). Specifically, the rounding of the corners 31b is due to the curve of the inner side surface of the protruding protrusion 11 of the intermediate product 10 (FIG. 10A), and is formed by compressing this side surface with the tapered punch 26. It can be evaluated that 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 or more the thickness t (FIG. 10C). The lower limit of the radius of curvature r is more preferably 0.15 times the 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 thickness t, and preferably 0.50 times.

[0053] 14, the metal structure in the vicinity of the corner 31b is also deformed to conform to the outer shape of the corner 31b. The deformed metal structure may be revealed by etching, although this depends on the material of the metal component 30.

[0054] The stretch flange 31 may also have a protrusion 31a on the inner circumferential side of the tip portion that bulges radially inward of the stretch flange 31. The protrusion 31a is formed by plastic flow of the material in the process of forming the stretch flange (step S2). Specifically, the protrusion 31a is formed by compressing a portion of the outer side surface of the protruding protrusion 11 of the intermediate product 10 ( FIG. 10A ) with a tapered punch 26. When the protrusion 31a is formed, it can be evaluated that the greater the height h2 of the protrusion 31a, the stronger the compressive force applied when the stretch flange 31 was formed.

[0055] The height h2 of the protrusion 31a is not particularly limited, but is preferably 0.05 times or more the thickness t (FIG. 10C). The lower limit of the height h2 is more preferably 0.10 times the thickness t. The upper limit of the height h2 is, for example, 0.25 times the thickness t, preferably 0.20 times. The height h2 is the distance from the tangent to the inner peripheral surface of the tip portion of the stretch flange 31 to the apex of the protrusion 31a.

[0056] Figure 15 is a diagram schematically showing another example of the cross-sectional shape of the tip of the stretch flange 31 of the metal part 30. As shown in Figure 15, the protrusion 31a may be collapsed toward the inner circumferential surface of the tip portion of the stretch flange 31 and rolled up (folded). In this case, too, the height h2 is the distance from the tangent to the inner circumferential surface of the tip portion of the stretch flange 31 to the apex of the protrusion 31a. The preferred range of 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, when the stretch flange 31 is folded back further than in the cases of Figures 12 to 14, the convex portion 31a may be crushed and disappeared by the tapered punch 26 (Figure 11). Even in this case, as a trace of the convex portion 31a, the stretch flange 31 may have a groove G formed on the inner peripheral side of the tip portion so as to surround the hole 30a of the stretch flange 31.

[0058] In addition to the groove G, the metal part 30 may have a groove g formed on the surface opposite to the side from which the stretch flange 31 protrudes, at the base of the stretch flange 31, so as to surround the hole 30a of the stretch flange 31. This groove g is a groove g (see Figures 7C, 8C, and 9C) formed when the intermediate product 10 is manufactured by a specific method, which remains even after the step of forming the stretch flange (step S2).

[0059] Figure 18 is a photograph of the intermediate product 10, showing the side opposite to the side from which the protruding protrusion is formed. Figures 20 and 22 are photographs of a metal part 30 manufactured by performing a process of forming a stretch flange on the intermediate product 10 of Figure 18, showing the side from which the stretch flange 31 is protruding (Figure 20) and the side opposite to the side from which the stretch flange 31 is protruding (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, 8A to 8C, or 9A to 9C, a groove g (see Figures 7C, 8C, and 9C) may be formed on the surface of the protruding protrusion 11 so as to surround the protruding protrusion 11. This groove g may also remain in the metal part 30. As described above, in the stretch flange forming step (step S2), the tapered punch 26 is pressed to fold back the protruding protrusion 11, thereby forming a stretch flange 31 on the side opposite the side from which the protruding protrusion 11 protrudes. Therefore, the groove g is formed on the surface opposite the side from which the stretch flange 31 protrudes, so as to surround the hole 30a of the stretch flange 31.

[0061] The metal part 30 may be, for example, an automobile part such as a lower arm, a seat gear, a recliner, a bearing, a hub, various gears, etc. The metal part 30 may also be a part other than an automobile part, such as a pipe or bearing part for industrial machinery or home appliances, a part for a pilot hole for tapping, a joint part, etc.

[0062] [Comparative Example] Next, as a hypothetical comparative example, consider a case where, in the step of preparing an intermediate product (step S1), a protruding protrusion is formed in a material, and then punching is performed from the side where the protruding protrusion protrudes. Specifically, as shown in Fig. 24, consider a case where punching is performed by abutting a punch 23 against the side where the protruding protrusion 11 protrudes, and disposing a die 29 on the opposite side. In this case, as shown in Fig. 25, in the intermediate product 40 thus formed, a fracture surface 411 is formed on the side opposite to the side where the protruding protrusion 11 protrudes, and a shear surface 412 is formed on the side where the protruding protrusion 11 protrudes.

[0063] 26 is a diagram showing a schematic diagram of the processing state when the step (step S2) of forming a stretch flange is performed using an intermediate product 40. In FIG. 26, the thick line on the inner peripheral 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, so cracks are likely to occur at the hole edge.

[0064] Furthermore, when intermediate product 40 is used, tapered punch 26 contacts fracture surface 411 from the side farthest from it. Therefore, tapered punch 26 cannot contact fracture surface 411 from the beginning of the stretch flange forming process (step S2), resulting in insufficient coining. Furthermore, portions of fracture surface 411 that are not coined (i.e., portions where irregularities originating from fracture surface 411 remain) remain.

[0065] As another comparative example, consider a case where only the punching process shown in Figures 6A and 6B is performed in the process of preparing an intermediate product (step S1), and the process of forming the protruding protrusion 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 protrusion and only has a hole 10a. In the intermediate product 45, a shear surface 451 is formed on one side in the plate thickness direction, and a fracture surface 452 is formed on the other side in the plate thickness direction.

[0066] 28 is a diagram showing a schematic diagram of the processing state when the step of forming a stretch flange (step S2) is performed using intermediate product 45. In FIG. 28, the thick line on the inner peripheral surface of hole 10a represents the fracture surface. When intermediate product 45 is used, protruding protrusion 11 is not folded back as in the case of intermediate product 10 (FIG. 11), and therefore, strong contact pressure is not applied to the hole edge compared to the case of intermediate product 10.

[0067] Furthermore, when intermediate product 45 is used, the area of ​​contact between tapered punch 26 and fracture surface 452 is smaller and the time that tapered punch 26 is in contact with fracture surface 452 is shorter than when intermediate product 10 is used ( FIG. 11 ), resulting in insufficient coining. Furthermore, portions of fracture surface 452 that are not coined (i.e., portions where irregularities originating from fracture surface 452 remain) remain.

[0068] Figure 29 is an example of a cross-sectional photograph of the vicinity of the stretch flange portion of a metal part 50 (Figure 28) manufactured by performing a step of forming a stretch flange on an intermediate product 45. Figure 30 is a cross-sectional photograph showing an enlarged view of the tip portion of this stretch flange (area A2 in Figure 29). Figure 31 is a schematic cross-sectional view of the metal part 50. In Figure 31, the metal structure is schematically illustrated by dashed lines.

[0069] The inner peripheral surface of the tip portion of the stretch 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 portion of the fracture surface 452 ( FIG. 28 ) of the intermediate product 45 has been smoothed by coining, and the fracture surface 512 is another portion of the fracture surface 452 that remains without being coined. In other words, the inner peripheral surface of the tip portion of the stretch flange 51 has remaining irregularities derived from the fracture surface 452. Furthermore, compared to the metal part 30 ( FIGS. 14 and 17 ), the metal part 50 does not have any of the convex portions 31 a and grooves G like the metal part 30. Furthermore, unlike the metal part 30, the corners 51 b of the metal part 50 are not rounded, and the shape with the corner where two sides intersect formed during punching remains, and the metal structure near the corners 51 b also maintains a layered structure stacked in the plate 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 process for forming a stretch flange is performed on an intermediate product 10 having a protruding protrusion 11. While it is difficult to smooth the entire fracture surface by simply pressing a tapered punch, according to this embodiment, when the protruding protrusion 11 is folded back to the other side, the fracture surface comes into contact with the tapered punch at a high surface pressure, making it possible to smooth the fracture surface over a wide area. In addition, since the contact surface pressure applied to the initial hole edge can be increased, cracks are less likely to occur at the hole edge during hole expansion. This makes it possible to suppress stretch flange cracking.

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0072] Example 1: A 4.0 mm thick steel plate made of S35C (annealed steel) was prepared. This steel plate was subjected to 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 bulging protrusion. The center of the bulging protrusion was punched from the inside of the bulging protrusion (the side opposite to the protruding protrusion; the same applies below) using a punch with a diameter of 10.0 mm and a die with a hole diameter of 10.8 mm to form a 10 mm diameter punched hole, resulting in an intermediate product. A fracture surface was formed on the inner circumferential surface of this punched hole on the outside of the bulging protrusion (the side from which the bulging protrusion protrudes; the same applies below).

[0073] This intermediate product was subjected to burring to produce a metal part having a stretch flange. Specifically, the intermediate product was fixed with a die (plate holder) having a hole diameter of 41.25 mm and a shoulder radius of 1.0 mm at a plate holding force of 30 kN, and a tapered punch having an opening angle φ (see FIG. 10A) of 30 ° was pressed against the outside of the protruding convex portion to form a stretch flange while folding back the protruding convex portion. The hole was expanded from an initial hole diameter of 10 mm, and the processing was terminated when the hole diameter reached 20 mm.

[0074] It was visually confirmed that the entire fracture surface of the inner peripheral surface of the tip portion of the stretch flange of the obtained metal part was coined. In addition, no stretch flange cracking was observed (limit hole expansion ratio of 100% or more).

[0075] Comparative Example 1 Except for the fact that punching was performed from the outside of the protruding portion, an intermediate product was manufactured in the same manner as in Example 1. A fracture surface was formed on the inner peripheral surface of the punched hole, on the inside of the protruding portion.

[0076] This intermediate product was subjected to burring in the same manner as in Example 1. In Comparative Example 1, stretch flange cracking occurred when the hole diameter reached 14 mm (limiting hole expansion ratio 40%). It was also visually confirmed that the majority of the fracture surface on the inner peripheral 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 produced in the same manner as in Example 1, except that the bulging process was not carried out.

[0078] This intermediate product was subjected to burring in the same manner as in Example 1. The tapered punch was inserted from the side of the punched hole where the fracture surface was located. In Comparative Example 2, stretch flange cracking occurred when the hole diameter reached 16 mm (limiting hole expansion ratio 60%). Furthermore, it was visually confirmed that there was an uncoined area on the inner peripheral surface of the tip portion of the stretch flange of the obtained metal part.

[0079] Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out within the scope of the invention.

[0080] REFERENCE SIGNS LIST 10 Intermediate product 10a Hole 11 Protruding convex portion 111 Shear surface 112 Fracture surface 21, 211 Protruding convex portion forming punch 22 Protruding convex portion forming die 23 Punching punch 24, 241, 242, 29 Punching processing die 244 Stepped die 245, 246 Die holder 26 Tapered punch 25, 28 Sheet holder 27 Stretch flange processing die 30 Metal part 30a Hole 31 Stretch flange

Claims

1. A method for manufacturing a metal part having a stretch flange, comprising: a step of preparing a metal intermediate product; and a step of forcing a tapered punch into the intermediate product to form a stretch flange, wherein the intermediate product is a part having a plate-shaped portion and has a protruding protrusion protruding to one side in the plate thickness direction and a hole formed in the protruding protrusion, wherein the hole has a fracture surface on its inner surface, and the fracture surface is on the side from which the protruding protrusion protrudes, and wherein in the step of forming the stretch flange, the tapered punch is forced into the hole from the side from which the protruding protrusion protrudes, and the protruding protrusion is folded back to form the stretch flange on the side opposite to the side from which the protruding protrusion protrudes.

2. A method for manufacturing a metal part as described in claim 1, wherein the step of preparing the intermediate product includes a step of forming the protruding convex portion by drawing or stretching a metal material, and then forming the hole by punching.

3. A method for manufacturing a metal part as described in claim 1, wherein the step of preparing the intermediate product includes a step of forming the hole in a metal blank by punching and then forming the protruding convex portion by edge bending.

4. A method for manufacturing a metal part as described in claim 1, wherein the step of preparing the intermediate product includes a step of forming the protruding portion and the hole in a single step.

5. A method for manufacturing a metal part as described in claim 4, wherein the step of preparing the intermediate product includes a step of forming the protruding convex portion and the hole by punching the die and providing a step between the die and the die holder.

6. A method for manufacturing a metal part as described in claim 4, wherein the step of preparing the intermediate product includes a step of forming the protruding convex portion and the hole by punching without using a die.

7. A method for manufacturing a metal part as described in claim 4, wherein the step of preparing the intermediate product includes a step of forming the protruding convex portion and the hole by punching using a stepped die.

8. A method for manufacturing a metal part according to any one of claims 1 to 7, wherein in the step of forming the stretch flange, the entire area of ​​the fractured surface is coined by the tapered punch.

9. A metal part having a stretch flange, wherein the outer peripheral corners of a tip portion of the stretch flange are rounded.

10. A metal part according to claim 9, wherein the stretch flange has a protrusion on the inner periphery of the tip portion that bulges inward in the radial direction of the stretch flange.

11. A metal part according to claim 9, wherein the stretch flange has a groove formed on the inner periphery of the tip portion so as to surround a hole in the stretch flange.

12. A metal part according to any one of claims 9 to 11, wherein the inner circumferential surface of the tip portion of the stretch flange is free of irregularities resulting from fracture surfaces.

13. A metal part according to any one of claims 9 to 11, wherein the inner circumferential surface of the tip portion of the stretch flange has a metallic luster over the entirety.

14. A metal part according to claim 10, wherein the height of the protrusion is 0.05 times or more the plate thickness.

15. A metal part according to any one of claims 9 to 11, wherein the radius of curvature of the corner is 0.10 times or more the plate thickness.

16. A metal part according to any one of claims 9 to 11, comprising a groove formed in the base portion of the stretch flange on the surface opposite to the side from which the stretch flange protrudes, the groove surrounding the hole in the stretch flange.

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