JPH07178462A - Method for bending metallic sheet - Google Patents

Abstract

PURPOSE:To stably control the camber in the small range without increasing the number of the pressing processes by using a soft sheet in bending a metallic sheet, and setting the clearance between a punch and a die in the prescribed range. CONSTITUTION:When a metallic sheet is formed to a hat-shaped section by using a bending tool of U-shaped section, one or a plurality of resin sheets 5 which is sufficiently soft and highly ductile are overlapped on one side of a punch 1 side of a metallic sheet 4 to be worked, or on both sides of the punch 1 side and a die 2 side. The clearance (c) between the punch 1 and the die 2 is set to value equal to the total thickness of the resin sheets 5 added by 80-95% of the thickness of the metallic sheet 4 to be worked. In the bending operation, the metallic sheet 4 to be worked and the resin sheets 5 are subjected to the compressive deformation in the clearance (c). The distribution of the residual stress generated in the bending and bending-back at the shoulder of the die in the thickness direction can be reduced by the compressive deformation through the similar function and effect as the prevention of the spring-back by providing the tension.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for bending a metal plate, and more particularly to a method for bending a high strength steel plate or an aluminum alloy plate having a low Young's modulus.

[0002]

2. Description of the Related Art Recently, high-strength steel plates and aluminum alloy plates have been frequently used for automobile parts. At that time, however, the problem of shape freeze due to springback after forming has become a problem. That is, when a U-shaped bending tool as shown in FIG. 1 is used to form a part having a hat-shaped cross-sectional shape shown in the perspective view (a) of FIG. 2, a cross-sectional view (b) of FIG. As shown, a shape defect called a warpage occurs on the side wall. This is because the side wall that should be machined perpendicularly to the bottom surface returns to the direction indicated by the arrow due to springback,
This is a phenomenon in which the side wall has a shape with a certain curvature, and the size of the warp is generally evaluated by the radius of curvature ρ.

Springback is caused by the distribution of residual stress generated during bending in the plate thickness direction. In the case of warpage, in addition to springback due to residual stress due to bending / bending back deformation at the die shoulder, punching It is a complicated phenomenon that is affected by the meandering of the work material within the clearance of the die. Meandering means that when the clearance is too large compared to the plate thickness, the bending tendency at the die shoulder remains, resulting in insufficient bending back or excessive bending back, so that the side wall of the work piece is linear in the punch progress direction. A phenomenon that disappears. Bending / unbending at the die shoulder causes residual stress in opposite directions on the front and back sides of the work material, and generally produces a moment that causes outward warpage. However, when the material to be processed meanders within the clearance, reverse bending at the time of bending back may act strongly, and a residual moment that warps inward may occur.

It is generally known that springback can be reduced by applying tension until the plastically deformed portion is subjected to tensile plastic deformation. In that case, measures such as increasing the plate pressing pressure and providing a draw bead are taken, but in a high-strength steel plate or an aluminum alloy plate having poor ductility, if the tension is increased, the side wall may be broken. Therefore, conventionally, a method of correcting the shape by bending the side opposite to the sled in the rest-like process has been adopted. However, in this method, an increase in the number of steps is a major drawback in terms of cost and production efficiency.

In order to solve this problem, Japanese Patent Laid-Open No. 56-1
In Japanese Patent No. 17831, from a detailed study of the mechanism of warpage generation, the bending radius at the die shoulder and the clearance between the punch and the die are set within a certain range, and the bending at the die shoulder portion and the reverse bending at the time of bending back are controlled. We propose a method to prevent warpage by canceling the residual moment. However, in this case, it is easily conceivable that the optimum reverse bending size strongly depends on the plate thickness and mechanical properties of the material. In fact, a high-strength steel sheet having a tensile strength of 490 MPa or more and an aluminum alloy sheet having a Young's modulus of about one third of steel may not provide sufficient effects even within the range of the present invention. Since the range is limited, there is a problem in that the degree of warpage largely fluctuates due to the accuracy of the plate thickness.

[0006]

SUMMARY OF THE INVENTION According to the present invention, when bending a hat-shaped part such as a high-strength steel plate or an aluminum alloy plate, the warpage can be prevented without increasing the number of pressing steps and without being influenced by the error of the plate thickness accuracy. It was made for the purpose of providing a method for stably controlling a small range.

[0007]

In order to solve the above problems, in the present invention, a sheet which is sufficiently soft as compared with a metal plate to be processed is used and the clearance between the punch and the die is set within a specific range. It has a feature. That is, in the method of forming a metal plate into a hat-shaped cross-sectional shape using a bending tool having a U-shaped section, one side of the metal plate to be processed or both the punch side and the die side is sufficiently soft and high. It is possible to stack one or more ductile resin sheets and set the clearance between the punch and the die to a value obtained by adding 80 to 95% of the thickness of the metal plate to be processed to the total thickness of the resin sheets. This is a characteristic metal plate bending method.

[0008]

The present invention will be described in detail below. Figure 3 (a)
Since the clearance c between the punch 1 and the die 2 is smaller than the total thickness of the metal plate 4 and the resin sheet 5 to be processed, the material 4 and the resin sheet 5 are compressed in the thickness direction within the clearance c. Undergo transformation. Compression in the plate thickness direction is equivalent to tension in a certain direction within the plate surface. Therefore, the distribution in the plate thickness direction of the residual stress generated by bending / unbending at the die shoulder can be reduced by compressive deformation with the same action and effect as the prevention of springback by applying tension. The larger the compression deformation is, the more effective it is. However, considering that the thickness as a product should not be impaired and the capacity load of the press processing equipment is not particularly required, the clearance c is set to the thickness of the resin sheet 5. It is necessary to set the range to the size obtained by adding 80 to 95% of the thickness of the metal plate 4 to be processed to the total thickness. If it is less than 80%, the plate thickness of the side wall of the product after processing is too small, and there is a concern that the rigidity and strength necessary for the product will be insufficient. In addition, the load required for compression increases, the load on the press processing equipment increases, and insufficient press capacity may become a problem. If it exceeds 95%, the deformation is concentrated on the soft resin sheet 5, and the work piece 4 cannot be sufficiently plastically deformed by compression.

At this time, when the resin sheet is not overlaid on the work material, a frictional force acts between the work material surface and the tool surface. This frictional force acts on the punch side for compression and on the die side for tension, which causes residual stress distribution in the plate thickness direction. Therefore, it is necessary to stack a soft resin sheet on the work material in order to reduce the frictional force. The soft resin sheets may be laminated only on the punch side surface, but it is more effective to use them on both sides in terms of uniforming the stress distribution in the plate thickness direction.

The resin sheet to be laminated on the material to be processed must be sufficiently soft as compared with the material to be processed. This is due to the following two reasons. One is to alleviate the frictional force acting between the work material and the tool by ironing the resin sheet,
Even if the frictional force between the tool and the resin sheet and between the workpiece and the resin sheet is large, if the resin sheet is easily deformed, the force beyond the deformation resistance of the resin sheet will not be transmitted. The other is that the soft resin sheet is compressed in the plate thickness direction at the time of deformation within the clearance, and is stretched along the pulling direction of the plate surface, so that the metal plate to be processed receives a pulling force. That is, since the tensile force is transmitted to the surface of the material to be processed by compressive deformation, there is an effect of promoting expansion of the material itself due to compression.

This is a phenomenon well known in the upsetting of the cylinder shown in FIG. 3 (b). Of course, if the friction between the resin sheet and the workpiece or the resin sheet and the tool surface is small, it is not necessary to rely on the former effect even if the resin sheet is hard, but in that case, the latter effect will be small. Therefore, the deformation resistance of the resin sheet is determined by the trade-off between the resin sheet and the work material and the friction between the resin sheet and the tool.
Further, as a matter of course, if the resin sheet is damaged during molding, the effect cannot be expected, so the resin sheet needs to have sufficient ductility. Examples of materials for such a resin sheet include fluororesins such as Teflon (registered trademark) and organic resins such as polyethylene. The thickness of the resin sheet is not particularly specified because the effect can be obtained without being much affected by the deformation resistance ratio with the material and the clearance, but from the viewpoint of cost and handling, it is about 0.05 to 0.2 mm. Things are good.

Under the above conditions, even if a material having a large spring back such as a high strength steel plate or an aluminum alloy plate is used, it is stable and good regardless of variations in plate thickness and material characteristics. It is possible to obtain precise shape accuracy.

[0013]

EXAMPLES Next, the present invention will be specifically described with reference to Examples.
Using the tool shown in FIG. 4, a 500 mm square work piece was pressed into a hat shape. At this time, the materials used, the soft resin sheet, and the clearance were tested under the conditions shown in Table 1. The plate pressing pressure is 200 kN for SPCC and aluminum alloy plates, and 600 kN for high strength steel plates.
Set to. As for the evaluation of the processed product, the curvature of the side wall 1 /
ρ was measured and shown in Table 1.

[0014]

[Table 1]

Method A and method B are examples of molding by a conventional method. In the method A using the mild steel plate, the curvature of the warpage is less than 0.001 mm ^-1 , whereas in the method B using the high strength steel plate, the warpage is large. Such a shape freezing defect is greater as the strength of the material is higher. However,
According to the present invention, it can be seen that the sled is set to a small level of less than 0.001 mm ^-1 as shown in methods E, E ', F and G. Method C is an example in which the clearance is large and no compression is applied to the work material. In this case, even if the slidability is increased by using the soft resin sheet, the curvature of the warp is still beyond the allowable range. However, when the clearance is reduced and the compression is applied in the plate thickness direction during processing as in the methods E and G, the difference in residual stress distribution in the plate thickness direction becomes small, and the warpage becomes sufficiently small.

In the methods D and F, the soft resin sheets are laminated only on one side, but the effect is different between the front and back of the plate. When the resin sheets are not overlapped, the frictional force from the wall of the die acts tensilely, whereas the frictional force from the side wall of the punch is compression. However, when the present invention is applied and a soft resin sheet is overlaid on the punch side, a tensile force also acts on the punch side surface of the workpiece, and the stress difference in the plate thickness direction is relaxed. When the resin sheet is used only on the die side, the effect cannot be expected so much, and a relatively large warp remains.

Method E'is a resin sheet having a thickness of 0.0
This is an example of using a 5 mm polyethylene sheet, but also in this case, the warpage can be sufficiently reduced as in the case of the fluororesin sheet. Method H is an example of processing by a conventional method using an aluminum alloy plate whose Young's modulus is about one-third of that of the steel plate. In this case, as in Method B using a high-strength steel plate, warpage large. However, in the method I to which the present invention is applied, the warpage can be sufficiently reduced even if an aluminum alloy plate having a Young's modulus smaller than that of the steel plate is used. As described above, according to the present invention, it is understood that the warpage of the high strength steel plate or the aluminum alloy plate is reduced to the level at the same time as the mild steel plate.

[0018]

According to the present invention, by using a soft resin sheet and setting the clearance to a specific dimension, it is possible to stably reduce the occurrence of warpage without increasing the number of pressing steps. Therefore, the present invention makes it easy to apply a material having a large spring back such as a high-strength steel plate or an aluminum alloy plate to bending-formed parts, and is extremely effective in expanding the applications of these.

[Brief description of drawings]

FIG. 1 is an explanatory view of a conventional method of forming a hat-shaped cross-section-shaped component using a U-shaped bending tool.

FIG. 2 is an explanatory view of a shape defect of a side wall of a hat-shaped cross-section part,

FIG. 3 is an explanatory view of actions and effects at the time of forming the hat-shaped cross-sectional shape of the present invention,

FIG. 4 is a cross-sectional view of a U-shaped bending tool according to an embodiment of the present invention.

[Explanation of symbols] 1 punch 2 die 3 blank holder 4 work material 5 soft resin sheet c clearance ρ sled curvature R tool shoulder radius

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Matsuo Usuda 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technology Development Division

Claims (1)

[Claims] 1. A method for forming a metal plate into a hat-shaped cross-sectional shape by using a bending tool having a U-shaped cross section, wherein the metal plate to be processed is sufficiently punched on one side or both punch side and die side. One or more soft, highly ductile resin sheets are stacked, and the clearance between the punch and the die is set to the total thickness of the resin sheets by 80 to 95 of the thickness of the metal plate to be processed.
A method for bending a metal plate, characterized in that the clearance is set to a value including%.