TOOL OF PRESS FORMING

Abstract

A press forming tool includes: a punch; and a die, and press forms a press forming product including a punch bottom and a shallow concave portion or a low convex portion in the punch bottom. The punch includes a punch bottom forming surface, a concave shape forming portion or a convex shape forming portion, and a low Young's modulus member installation region formed under the punch bottom forming surface and accommodating a low Young's modulus member having a lower Young's modulus than that of the punch bottom forming surface. The low Young's modulus member installation region includes an area directly under the concave shape forming portion or the convex shape forming portion of the punch and has a projected area in a press forming direction that is 110% or greater as large as a projected area of the concave shape forming portion or the convex shape forming portion.

Description

FIELD

The present invention relates to a tool of press forming, particularly, relates to a tool for press forming a blank into a press forming product by pressing that has a punch bottom and a shallow concave portion or a low convex portion in the punch bottom. In the present application, the blank is a metal sheet made of a variety of metals, such as a hot rolled steel sheet, a cold rolled steel sheet, a surface treated steel sheet made by subjecting a steel sheet to surface treatment, including, for example, electro galvanizing, hot-dip galvanizing, and organic film treatment, a stainless steel sheet, an aluminum sheet, and a magnesium sheet. In the present application, the punch bottom is the bottom of a press forming product formed with a punch and a die. Specifically, the punch bottom is formed with the punch pushed into the die. Furthermore, in the present application, the terms indicative of directions such as “directly under”, “under”, and “top” are used to refer to the vertical direction in which the punch and the die are placed relative to one another in the direction of press forming. For example, if the die is located above the punch, the terms “directly under” and “under” of the punch indicate the direction opposite the die in the press forming direction, whereas “top” of the punch indicates the direction of the die in the press forming direction.

BACKGROUND

Application of a high-tensile steel sheet to the automotive body has been extensively increased in response to the request of fuel efficiency and collision safety along with the weight reduction of the automotive body. However, in manufacturing parts of an automotive body from a high-tensile steel sheet by press forming, formability and accuracy of dimensions are likely to be reduced because of the low ductility of the high-tensile steel sheet and its large material strength, which has been an issue to be solved. In addition, an increase in the forming load applied to form the parts requires changes in the press line and breaking down of parts. These are problems to overcome when using high-tensile steel sheets.

To overcome such issues, various technologies have been developed in order to reduce the forming load applied during press forming on a high-tensile steel sheet. For example, Patent Literature 1 describes a method of forming a metal sheet with a die and a punch. The method reduces the stress of the metal sheet by stopping the punch during the course of operation starting with the punch contacting the metal sheet and ending with the punch reaching the stroke end, and resuming the forming operation of the metal sheet after a predetermined time elapsed, thereby reducing the forming load. Patent Literature 2 describes a method allowing a reduction in the forming load applied during the press forming process on a blank material with a die and a punch. In the method, the blank material is sequentially formed by a predetermined section with several parts of the punch separately operating.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Laid-open No. 2011-245525

Patent Literature 2: Japanese Patent Application Laid-open No. 2010-207907

SUMMARY

Technical Problem

However, the method described in Patent Literature 1 needs a servo press to stop the punch during the forming process, leading to a restriction of the facility. With the method of Patent Literature 2, the complex die structure increases the production cost. Furthermore, the method may cause fracture and wrinkles in the high-tensile steel sheet having low formability, during press forming.

For example, as illustrated in FIG. 2, when forming a press forming product 11 having a concave portion 15 concaved in a punch bottom 13 and having a depth close to the plate thickness in the press forming direction, the forming load is markedly increased around the bottom dead center, as is common with operation of making a plate thinner by pressing. In such a forming process, reducing the forming load has been an important issue. However, the methods of Patent Literatures 1 and 2 are not intended to produce products like the press forming product 11, and therefore it has been difficult to sufficiently reduce the forming load applied in forming a concave or convex shape the depth of which is close to the plate thickness. Therefore, technologies have been demanded to achieve a reduction in the forming load during formation of the press forming product 11 having a shallow concave portion 15 (or a convex portion in a low convex shape in the press forming direction) in the punch bottom 13.

From the above viewpoints, the present invention aims to provide a press forming tool that allows a reduction in the forming load applied during press forming of a press forming product having a concave portion or a convex portion concaved or convexed in the press forming direction in the punch bottom, while eliminating the necessity of using a complex mechanism or structure.

Solution to Problem

A press forming tool according to the present invention includes: a punch; and a die, wherein the press forming tool is configured to press form a press forming product including a punch bottom and a shallow concave portion or a low convex portion in the punch bottom, the punch includes a punch bottom forming surface to form the punch bottom, a concave shape forming portion to form the concave portion or a convex shape forming portion to form the convex portion in the punch bottom forming surface, and a low Young's modulus member installation region formed under the punch bottom forming surface and accommodating a low Young's modulus member having a lower Young's modulus than that of the punch bottom forming surface, and the low Young's modulus member installation region includes an area directly under the concave shape forming portion or the convex shape forming portion of the punch and has a projected area in a press forming direction that is 110% or greater as large as a projected area of the concave shape forming portion or the convex shape forming portion. Advantageous Effects of Invention

The present invention relates to production of a press forming product by press forming that has a punch bottom and a shallow concave portion or a low convex portion in the punch bottom. A punch includes a low Young's modulus member installation region in which a low Young's modulus member having a low Young's modulus is installed, under a punch bottom forming surface of the punch. During the process of press forming, the punch bottom forming surface is flexed to the low Young's modulus member installation region, which facilitates closeness between a concave shape forming portion or a convex shape forming portion formed in the punch bottom forming surface and a blank. This configuration is effective in minimizing an increase in the forming load applied around the bottom dead center.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing illustrating the configuration of a press forming tool according to an embodiment of the present invention.

FIG. 2 is a drawing illustrating an example press forming product having a concave portion concaved in the press forming direction in the punch bottom.

FIG. 3 is a drawing illustrating the configuration of a conventional press forming tool.

FIG. 4 is a schematic drawing illustrating deformation caused in the conventional press forming tool around the bottom dead center.

FIG. 5 is a schematic drawing illustrating deformation and flection caused in a press forming tool according to the embodiment of the present invention, around the bottom dead center.

FIG. 6 is a drawing illustrating the configuration of another aspect of the press forming tool according to the embodiment of the present invention.

FIG. 7 is a drawing indicating results of press forming analyses analyzing the amount of deformation caused in a press forming tool according to a conventional example of the examples, around the bottom dead center.

FIG. 8 is a drawing illustrating results of press forming analyses analyzing the amount of deformation caused in a press forming tool according to the invention example of the examples, around the bottom dead center.

FIG. 9 are graphs indicating the relation between the forming stroke and the forming load in the process of press forming, analyzed by the press forming analyses in the examples (the graph (a) indicates the fluctuations from start of forming until the bottom dead center, (b) is an enlargement view around the bottom dead center).

FIG. 10 is a graph indicating the results of forming load measured at the bottom dead center in the conventional example and the example of the invention, of the examples.

FIG. 11 is a graph indicating the relation between the ratio of the projected area A1 of the low Young's modulus member installation region to the projected area A2 of the concave shape forming portion in the press forming direction and the forming load, in the examples.

DESCRIPTION OF EMBODIMENTS

As illustrated in FIG. 1 as an example, a press forming tool 1 according to an embodiment of the present invention includes a punch 3, a die 5, and a blank holder 7. As exemplarily illustrated in FIG. 2, the press forming tool 1 is used for press forming to press form a press forming product 11 having a punch bottom 13 and a concave portion 15 that is concaved in the punch bottom 13 and is shallow. The shallow of the concave portion 15 means that at least the depth is smaller than the press forming height of the press forming product 11. For example, the depth of the concave portion 15 in the press forming direction is 10 mm or smaller and is close to the plate thickness. Each component of the press forming tool 1 according to the embodiment will now be described. In the following description, components having the same function will be indicated by the same reference numeral, and description thereof will be omitted.

The punch 3 includes a punch bottom forming surface 3a to form the punch bottom 13 of the press forming product 11, a concave shape forming portion 3b to form the concave portion 15 in the punch bottom 13, and a low Young's modulus member installation region 3d formed under the punch bottom forming surface 3a (opposite the die 5 in the press forming direction) inside the punch 3 and accommodating a low Young's modulus member 3c having a lower Young's modulus than that of the punch bottom forming surface 3a.

The low Young's modulus member 3c is made of a material of which Young's modulus is lower than a casting and a steel material usually used for the punch 3 and the die 5. Examples of the material may include resins, such as a rubber material and a urethane material, and metals, such as aluminum and magnesium.

The low Young's modulus member installation region 3d includes the area directly under the concave shape forming portion 3b, and has a projected area A1 in the press forming direction that is 110% or greater as large as a projected area A2 of the concave shape forming portion 3b.

In the press forming tool 1 illustrated in FIG. 1, the low Young's modulus member 3c is installed so as to occupy the entire low Young's modulus member installation region 3d. The low Young's modulus member 3c is installed to the low Young's modulus member installation region 3d by, for example, fittingly inserting the low Young's modulus member 3c into the punch 3, tightening with bolts, and fixing with adhesive.

The die 5 is placed facing the punch 3 with a blank 9 interposed therebetween. The die 5 is designed to relatively move toward the punch 3 in order to form the press forming product 11 in coordination with the punch 3. The die 5 includes a punch bottom forming surface 5a to form the punch bottom 13 of the press forming product 11 and a concave shape forming portion 5b to form the concave portion 15 in the punch bottom 13.

The blank holder 7 is disposed opposite the die 5, on the same side as the punch 3 relative to the blank 9. The blank holder 7 is designed to relatively move toward the punch 3 while holding the ends of the blank 9 with the die 5.

The operation of the press forming tool 1 in the above configuration according to the embodiment to produce the press forming product 11 by press forming, and the effects of the operation will now be described in comparison to the case of using a conventional press forming tool 21 illustrated in FIG. 3.

When the press forming product 11 is produced by press forming using the conventional press forming tool 21 including a punch 23, the die 5, and the blank holder 7 as illustrated in FIG. 3, the blank 9 is placed between the punch 23 and the die 5 with the ends of the blank 9 pinched by the die 5 and the blank holder 7. With the blank 9 pinched by the die 5 and the blank holder 7, the die 5 and the blank holder 7 are relatively moved toward the punch 23, thereby forming the punch bottom 13. When the press forming product 11 is bottomed around the bottom dead center, the concave shape forming portion 5b of the die 5 pushes the blank 9 into a concave shape forming portion 23b of the punch 23, thereby forming the concave portion 15 in the punch bottom 13.

At the same time, around the bottom dead center, the surfaces of the punch 23 and the die 5 are elastically deformed and pressed down in the direction indicated by the arrows of FIG. 4, as illustrated in FIG. 4. The broken lines in FIG. 4 indicate the surface conditions of the punch 23 and the die 5 before deformation. The areas hatched by the diagonal lines are areas where the punch 23 and the die 5 are pressed down around the bottom dead center.

However, since the punch 23 and the die 5 are not easily elastically deformed, the amount of deformation is small in each of them. In order to form the concave portion 15 with the blank 9 sufficiently fitting the concave shape forming portion 23b of the punch 23, the blank 9 needs to be pressed toward the punch 23 by applying large forming load, such that the plate thickness is made thinner by being pressed against the die 5, around the bottom dead center.

On the other hand, when the press forming tool 1 having the punch 3 and the die 5 according to the embodiment produces the press forming product 11 by press forming, as illustrated in FIG. 5, the surfaces of the punch 3 and the die 5 are pressed down around the bottom dead center. The broken lines in FIG. 5 represent the surface conditions of the punch 3 and the die 5 before deformation.

Specifically, as with the conventional press forming tool 21, the surface of the die 5 is elastically deformed and pressed down around the bottom dead center (areas in the die 5 hatched with diagonal lines in FIG. 5). Since the low Young's modulus member installation region 3d is arranged in the region including the area directly under the concave shape forming portion 3b, under the punch bottom forming surface 3a, the punch bottom forming surface 3a around the bottom dead center is flexed to the low Young's modulus member installation region 3d while inducing deformation of the surface of the punch 3 (the hatched areas in the punch 3 in FIG. 5).

Because the amount of deformation of the punch 3 caused by the flection of the punch bottom forming surface 3a is larger than the amount of elastic deformation of the punch 23 of the conventional press forming tool 21, closeness of the concave shape forming portion 3b of the punch 3 and the blank 9 is easily achieved. Consequently, the press forming tool 1 according to the embodiment can produce the press forming product 11 having the concave portion 15 formed by press forming while minimizing an increase in the forming load around the bottom dead center.

The present invention can further effectively minimize an increase in the forming load around the bottom dead center, when the blank 9 to be pressed is a high-tensile steel sheet of 590 MPa or greater in the MPa class.

The press forming tool 1 according to the present invention accommodates the low Young's modulus member 3c inside the punch 3. This structure can reduce the forming load with the punch 3 flexed during the forming process while ensuring the structural strength, even when the structure of the tool does not allow the punch 3 to be hollow (for example, when the press forming tool 1 is too small and thus does not allow the punch 3 to be hollow to secure sufficient strength).

As described earlier, in the present invention, the projected area of the low Young's modulus member installation region 3d of the punch 3 in the press forming direction is 110% or greater as large as the projected area of the concave shape forming portion 3b. This structure allows the punch bottom forming surface 3a to be flexed during the press forming process. The thickness of the punch bottom forming surface 3a in the press forming direction may be appropriately set in the range allowing the flection of the punch bottom forming surface 3a. The projected area of the concave shape forming portion 3b is the area projected from the bottom of the concave shape forming portion 3b of the punch bottom forming surface 3a.

In the above description, the punch 3 has the low Young's modulus member 3c installed in the low Young's modulus member installation region 3d in a fashion that entirely occupies the region. In another configuration of the present invention, as illustrated in FIG. 6, a press forming tool 31 may have low Young's modulus members 33c partially installed in a low Young's modulus member installation region 33d of a punch 33.

With regard to the location and the area where the low Young's modulus members 33c are installed, no restriction is provided if: the low Young's modulus members 33c have the Young's modulus lower than that of a punch bottom forming surface 33a of the punch 33, the low Young's modulus member installation region 33d includes the area directly under a concave shape forming portion 33b, and the projected area A1 in the press forming direction is 110% or greater as large as the projected area A2 of the concave shape forming portion 33b.

The manner of formation of the press forming product 11 having the concave portion 15 concaved in the punch bottom 13 in the press forming direction has been described above. Alternatively, the press forming tools 1 and 31 according to the present invention may be designed to press form a press forming product (not illustrated) that has a convex portion that is convexed in the punch bottom in the press forming direction and is low.

The low convex portion means that at least the height of the convex portion is smaller than the forming height of the press forming product. The present invention is preferably applicable to such a press forming product that has the convex portion whose height is 10 mm or smaller and nearly equal to the plate thickness.

As described above, such a press forming product having a convex portion in the punch bottom can be produced by press forming using the press forming tool according to the present invention. In the process of forming the convex portion around the bottom dead center, the die is pushed toward the punch, causing the punch bottom forming surface of the punch to be flexed to the low Young's modulus member installation region located below. This configuration can easily achieve closeness between the blank and the convex shape forming portion of the punch. In this manner, the convex portion is formed while minimizing an increase in the press forming load applied around the bottom dead center.

The press forming tool 1 according to the embodiment has the die 5 above the punch 3 as illustrated in FIG. 1, with the direction in which the die 5 descends to the punch 3 defined as the press forming direction. In the present invention, the press forming direction may be the direction in which the punch 3 is lifted to the die 5. Furthermore, in the press forming tool according to the present invention, the die may be placed under the punch with the punch configured to descend to the die or the die configured to be lifted to the punch (not illustrated). The present invention is not intended to limit the position of the punch relative to the die or the press forming direction.

In the above description, the press forming tool 1 illustrated in FIG. 1 includes the blank holder 7 and produces the press forming product 11 by draw forming with both ends of the blank 9 pinched with the die 5 and the blank holder 7. The present invention may be applied to a press forming tool having no blank holder and configured to produce the press forming product 11 by crash forming with the punch 3 and the die 5.

EXAMPLES

Several experiments were conducted to validate the effects of the press forming tool according to the present invention, and the detail thereof will now be described.

In the experiments, the press forming process to produce the press forming product 11 of FIG. 2 using the press forming tool 1 according to the present invention illustrated in FIG. 1 was analyzed to evaluate deformation of the punch 3 and the forming load applied during the process of press forming.

In the analyses of press forming, a high-tensile steel sheet having 1180 MPa grade tensile strength and having a plate thickness of 1.2 mm was used as the blank 9. The press forming product 11 is in a hat cross section shape having the punch bottom 13 and the concave portion 15 concaved in the punch bottom 13 in the press forming direction, as illustrated in FIG. 2. The dimension of each part is indicated in FIG. 2. The concave portion 15 to be formed in the punch bottom 13 has a depth of 3 mm, which is smaller than the forming height (20 mm) of the press forming product 11 and close to the plate thickness.

In the experiments, the press forming product 11 was produced using the press forming tool 1 including the punch 3, the die 5, and the blank holder 7 as illustrated in FIG. 1. The analyses of the press forming process are described as an example of the invention. In the example of the invention, the punch 3 of the press forming tool 1 includes the punch bottom forming surface 3a, the concave shape forming portion 3b, and the low Young's modulus member installation region 3d accommodating the low Young's modulus member 3c having Young's modulus lower than that of the punch bottom forming surface 3a, under the punch bottom forming surface.

In the analyses of press forming, the punch 3 made of FC250 (gray cast iron) having a Young's modulus of 115 GPa was used. The Young's modulus of the low Young's modulus member 3c was set at 0.1 GPa that was lower than the Young's modulus of the punch bottom forming surface 3a of the punch 3. The low Young's modulus member installation region 3d includes the area directly under the concave shape forming portion 3b, and has the projected area A1 in the press forming direction 110% as large as the projected area A2 of the concave shape forming portion 3b.

As a comparison, the press forming product 11 was produced using the conventional press forming tool 21 including the punch 23, the die 5, and the blank holder 7 illustrated in FIG. 3. The analyses of the press forming process are described as a conventional example.

FIG. 7 and FIG. 8 illustrate the results of deformation of the punch 3, the punch 23, and the die 5 in the press forming direction measured around the bottom dead center in the conventional example and the example of the invention. In the conventional example illustrated in FIG. 7, the concave shape forming portion 23b of the punch 23 and the concave shape forming portion 5b of the die 5 were deformed in the press forming direction at areas close to the edges. The largest amount of deformation was 0.02 mm. In the example of the invention illustrated in FIG. 8, deformation in the press forming direction was observed in extensive areas under the concave shape forming portion 3b of the punch 3 and above the concave shape forming portion 5b of the die 5. The largest amount of deformation was 0.05 mm, which was a significant increase from the conventional example.

FIG. 9 are graphs indicating the relation between the forming stroke and the forming load in the press forming process. As is obvious from FIG. 9, in the conventional example and the example of the invention, the forming load gradually increases after start of forming (the forming stroke 0 mm), and sharply increases around the bottom dead center (the forming stroke about 21 to 23 mm). As illustrated in FIGS. 9(a) and 9(b), the forming load applied from the start of forming until around the bottom dead center (the forming stroke 22.7 mm) was almost the same between the conventional example and the example of the invention. On the other hand, as illustrated in FIG. 9(b), the forming load around the bottom dead center was small in the example of the invention, in comparison to the conventional example.

FIG. 10 is a graph indicating the forming load of the conventional example and the example of the invention around the bottom dead center (forming stroke 23 mm). As indicated in FIG. 10, the forming load of the example of the invention was smaller than the conventional example by approximately 2.0 tonf.

At the next step, fluctuations in the forming load applied at the bottom dead center were evaluated by changing the dimensions of the concave shape forming portion 3b of the punch 3, thereby changing the ratio of the projected area A1 of the low Young's modulus member installation region 3d including the area directly under the concave shape forming portion 3b to the projected area A2 of the concave shape forming portion 3b. The results are indicated in FIG. 11.

As is obvious from FIG. 11, when the ratio of the projected area A1 of the low Young's modulus member installation region 3d to the projected area A2 of the concave shape forming portion 3b is 110% or greater, which is the scope of the present invention, the forming load is drastically decreased. When the low Young's modulus member installation region 3d was shifted out of the area directly under the concave shape forming portion 3b, the forming load was similar with the results obtained when the ratio of the projected area A1 to the projected area A2 in FIG. 11 was smaller than 100%. The forming load therefore could not effectively get reduced.

Although the examples are adopted to a press forming product having a concave portion concaved in the press forming direction in the punch bottom, the effect of reducing the forming load around the bottom dead center is further exerted to such a press forming product that has a convex portion convexed in the press forming direction in the punch bottom.

As has been proved, the press forming tool 1 according to the present invention allows a reduction in the forming load applied around the bottom dead center, by flexing the punch bottom forming surface 3a of the punch 3 toward the low Young's modulus member installation region 3d formed under the punch bottom forming surface 3a during press forming of the press forming product 11 having the concave portion 15 or convex portion concaved or convexed in the punch bottom 13 in the press forming direction.

INDUSTRIAL APPLICABILITY

According to the present invention, a tool of press forming is provided that allows a reduction in the forming load applied in the press forming process to produce a press forming product having a concave portion or a convex portion concaved or convexed in the press forming direction in the punch bottom, without employing any complex mechanisms or structures.

REFERENCE SIGNS LIST

1 PRESS FORMING TOOL

3 PUNCH

3a PUNCH BOTTOM FORMING SURFACE

3b CONCAVE SHAPE FORMING PORTION

3c LOW YOUNG'S MODULUS MEMBER

3d LOW YOUNG'S MODULUS MEMBER INSTALLATION REGION

5 DIE

5a PUNCH BOTTOM FORMING SURFACE

5b CONCAVE SHAPE FORMING PORTION

7 BLANK HOLDER

9 BLANK

11 PRESS FORMING PRODUCT

13 PUNCH BOTTOM

15 CONCAVE PORTION

21 PRESS FORMING TOOL (CONVENTIONAL STRUCTURE)

23 PUNCH

23a PUNCH BOTTOM FORMING SURFACE

23b CONCAVE SHAPE FORMING PORTION

31 PRESS FORMING TOOL

33 PUNCH

33a PUNCH BOTTOM FORMING SURFACE

33b CONCAVE SHAPE FORMING PORTION

33c LOW YOUNG'S MODULUS MEMBER

33d LOW YOUNG'S MODULUS MEMBER INSTALLATION REGION

Claims

1. A press forming tool comprising:

a punch; and

a die,

wherein the press forming tool is configured to press form a press forming product including a punch bottom and a shallow concave portion or a low convex portion in the punch bottom,

the punch includes a punch bottom forming surface to form the punch bottom, a concave shape forming portion to form the concave portion or a convex shape forming portion to form the convex portion in the punch bottom forming surface, and a low Young's modulus member installation region formed under the punch bottom forming surface and accommodating a low Young's modulus member having a lower Young's modulus than that of the punch bottom forming surface, and the low Young's modulus member installation region includes an area directly under the concave shape forming portion or the convex shape forming portion of the punch and has a projected area in a press forming direction that is 110% or greater as large as a projected area of the concave shape forming portion or the convex shape forming portion.