JP2006061981A - Press forming method excellent in shape fixability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a press forming method which prevents the opening of an angle at the shoulder portion and the warpage of the wall portion of the press-formed component. <P>SOLUTION: The spring-back of a metal plate is lessened by the press forming method, in which a punch is once stopped in process of a stroke during forming and the punch is moved in a direction reverse to the stroke for re-forming after stopping. Re-forming is also performed without moving the punch in the reverse direction after stopping once. Further, the spring-back of the metal plate can be lessened by a press forming process, in which the punch is returned to the bottom dead point of forming in a state of maintaining the added blank holding force after forming, or in another press forming process, in which the punch is moved again to the bottom dead point of forming after the punch is returned. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a press forming apparatus and method for a metal plate, and more particularly to a press forming method for preventing a spring back that occurs in the case of a high-strength steel plate, an aluminum alloy plate, etc. and ensuring a good shape freezing property. Is.

  In recent years, high-strength steel plates and aluminum alloy plates have been diversified in automobile parts, but shape freezing defects that occur when they are formed into automobile parts have become a problem. That is, when a hat-shaped cross-sectional part is formed using a U-shaped bending mold, a shape freezing defect called a spring back occurs, and dimensional accuracy cannot be obtained. Yes. This shape freeze defect not only significantly deteriorates the appearance quality of the final product, but also causes welding defects in assembly operations performed after molding. Therefore, prevention of shape freeze defects is especially important for structural parts such as members and frames. ing.

Springback, which is known as a shape defect frequently observed in bending, is a phenomenon that occurs because the residual stress generated in the mold plate during bending is elastically restored and deformed during unloading. This is because the residual stress is unevenly distributed in the thickness direction. In general, when bent from the mold, it is opened at the shoulder of the punch due to the spring back of the metal plate. In order to solve this problem, Patent Document 1 discloses a press molding method and a molded product for simultaneously molding a concave step portion recessed in a direction opposite to the molding direction on a bending shoulder. This is to counterbalance the spring back and balance it to prevent the angle near the punch shoulder from opening.
However, as shown in FIG. 1, the springback not only changes the angle of the shoulder 1 but also manifests a phenomenon called “wall warp” in which the wall 2 deforms from a flat surface to a curved surface having a curvature as the material strength increases. The invention disclosed in Patent Document 1 cannot prevent the wall from warping.

JP 7-185663 A

  An object of this invention is to provide the press molding method which prevents the opening of the angle in the shoulder part of a press molding component, and the curvature of a wall part.

In order to solve the problem, the gist of the present invention is as follows.
(1) A press molding method excellent in shape freezing property, wherein the punch is stopped in the middle of the stroke during press molding, and the punch is moved again from the stopped position to the bottom dead center.
(2) Freezing of the shape characterized by stopping the punch in the middle of the stroke during press working, returning the punch to the middle in the reverse direction from the stop position, and then moving the punch again from the returned position to the bottom dead center Press molding method with excellent properties.
(3) After the punch reaches the bottom dead center, the punch is returned to the middle in the reverse direction, and then the punch is moved again from the returned position to the bottom dead center. .
(4) A press molding method with excellent shape freezing property, wherein after the punch reaches bottom dead center, the punch is returned in the reverse direction while applying a wrinkle holding force.
(5) The press molding method having excellent shape freezing property according to any one of (1) to (4), wherein molding is performed while applying a wrinkle pressing force.
(6) The press molding method having excellent shape freezing property as described in (2) or (3) above, wherein the punch is returned while a wrinkle pressing force is applied.
(7) Either one of the above (1) and (2), wherein a crease pressing force is applied from the initial stage of molding until the punch stops in the middle of the stroke, and then the crease pressing force is not applied. 2. A press molding method having excellent shape freezing property described in 1.
(8) The molding is performed by applying a crease pressing force from the initial stage of molding until the end of molding without adding a crease pressing force until the punch is stopped in the middle of the stroke. A press molding method having excellent shape freezing property as described in 1) or (2).
(9) The press molding method excellent in shape freezing according to any one of (1) to (8), wherein the press molding is performed by a hydraulic press or a servo press.

  According to the present invention, a good hat-shaped molded part having excellent dimensional accuracy can be obtained.

Hereinafter, the present invention will be described in detail.
The inventors of the present invention have studied a press molding method for preventing the opening of the angle at the shoulder of the hat-shaped molded part having a U-shaped cross-section and warping of the wall, and have found that these problems can be solved.
In the conventional press molding method, as shown in FIG. 2, the punch 3 does not stop until the molding bottom dead center A (FIG. 2 (a) → (b)), and after reaching the molding bottom dead center A (FIG. 2 ( b)), the wrinkle retainer 4 and the punch 3 were simultaneously released (FIG. 2 (c)). The present inventors have found that the spring back of the metal plate can be reduced by temporarily stopping the stroke during forming, moving the punch in the direction opposite to the stroke after the stop, and re-forming. Furthermore, it was found that re-molding without stopping the punch in the reverse direction after stopping was effective. In addition, a press forming method for returning the punch to the bottom dead center after forming with the wrinkle holding force applied was also examined.

The invention according to the above (1) is characterized in that the punch is stopped in the middle of the stroke during the press working, and the punch is moved again from the stopped position to the bottom dead center.
It can be considered that the springback can be reduced by temporarily stopping during the molding because the inertial force of the punch reduces the slack of the punch bottom of the product when the punch is moved after being stopped. About the time to stop, since it affects cycle time, about 1 to 10 seconds may be sufficient.

In the invention according to the above (2), the punch is stopped in the middle of the stroke during the press working, the punch is returned to the middle in the reverse direction from the stop position, and then the punch is moved again from the returned position to the bottom dead center. It is characterized by.
This can reduce the springback because returning the punch increases the distance between the punch and the product, and further increases the inertial force when the product and the punch come into contact with each other, reducing the slack in the punch bottom of the product. It is thought that the effect is improved.

The invention according to (3) is characterized in that after the punch reaches the bottom dead center, the punch is returned halfway in the reverse direction, and then the punch is moved again from the returned position to the bottom dead center.
The spring back can be reduced by moving the punch back to the bottom dead center again after returning the punch to the middle in the reverse direction.Returning the punch generates a spring back of the metal plate in the mold. This is considered to be an effect of re-molding and an effect of increasing the inertial force of the punch when the punch and product re-contact.

The invention according to the above (4) is characterized in that after the punch reaches the bottom dead center, the punch is returned in the reverse direction while applying the crease pressing force to finish the molding.
The spring back can be reduced by returning the punch while applying the wrinkle holding force. By returning the punch, the metal plate springs back in the mold, and the residual stress of the product can be reduced by the binding force of the mold. This is probably because of this.

The invention according to the above (5) is characterized in that in the invention according to any one of the above (1) to (4), molding is performed while applying a wrinkle pressing force.
By adding a wrinkle holding force during molding, the restraining force in the mold is increased and the shape freezing property is improved.

The invention according to (6) above is characterized in that, in the invention of (2) or (3), the punch is returned while a wrinkle pressing force is applied.
By applying a wrinkle holding force when returning the punch, the restraining force in the mold is increased and the shape freezing property is improved.

In the invention according to (7) above, in the invention of (1) or (2) above, a wrinkle pressing force is applied from the initial stage of molding until the punch stops in the middle of the stroke, and then molding is performed without adding the wrinkle pressing force. It is characterized by that.
Since the wrinkle holding force is not applied after the punch is stopped, the distortion and residual stress of the product at the bottom dead center of molding can be reduced, so that the spring back can be reduced.

The invention according to (8) above is the invention according to (1) or (2) above, wherein the punch is re-moved without applying a wrinkle holding force until the punch is stopped in the middle of the stroke from the initial stage of molding. It is characterized in that it is molded with a wrinkle pressing force until the end.
By not adding wrinkle holding force at the initial stage of molding, the slack of the punch bottom is increased, and when the punch is moved again, wrinkle holding force is added to prevent punch bottom slack, thereby reducing spring back. be able to.

The invention according to (9) is characterized in that in any one of the inventions (1) to (8), molding is performed by a hydraulic press machine or a servo press machine.
In order to realize the present invention, since the position control of the punch is indispensable, the present invention can be implemented without any limitation as long as it is an apparatus capable of doing so, but it is formed by a known hydraulic press machine or servo press machine. It is preferable to do.

  As an example of the present invention, a press test of a press mold was performed using a steel plate having a plate thickness of 1.2 mm and a tensile strength of 590 MPa. Specimen width is 100 mm, length is 280 mm, punch width is 73 mm, punch shoulder is R5 mm, die width is 80 mm, die shoulder is R5 mm, and the vertical wall angle on one side is inclined 5 °, and the molding height is 80 mm A hat forming test was conducted. In order to examine the effect of the present invention, the punch was stopped at an initial stroke of 15 mm and 30 mm, and moved from the stopped position to the bottom dead center again. Further, the effect of moving the punch again from the returned position to the bottom dead center after returning the punch by 5 mm and 10 mm in the reverse direction from the stop position was also examined. The wrinkle pressing force during molding and when returning the punch was 0 kN (none) and 300 kN. As a comparative example, forming test by conventional press working method (decision pushing) that moves to bottom dead center without stopping to the bottom dead center and applies compressive force in the product thickness direction at the die bottom and punch bottom. Went.

The test piece subjected to the bending test was measured for the shape of the hat-shaped central cross section with a three-dimensional shape measuring device, and as shown in FIG. 1, the angles θ1 and θ2 of the punch shoulders, the curvature curvature radius ρ1 of the inclined wall The warp curvature ρ2 and the opening width W of the vertical wall were measured. In the drawing, the angles θ1 and θ2 are the angle formed between the ceiling and the left upper vertical wall, and the angle formed between the ceiling and the right upper vertical wall. From each measured value, the angle change Δθ1 [degree], Δθ2 [degree], wall warp 1 / ρ1 [1 / mm], wall warp 1 / ρ2 [1 / mm], and opening width ΔW [mm] The evaluation value was used.
Δθ1 = θ1-95 °
Δθ2 = θ2-90 °
ΔW = W-80

As Example 2, a molding test was performed using the same steel type, test piece size, and press mold as those of the above-described examples of the present invention. In order to examine the effect of the present invention, after the punch reached the bottom dead center, the punch was returned in the reverse direction, and then the molding test was terminated. The wrinkle pressing force during molding and when returning the punch was 300 kN.
The test piece subjected to the bending test was evaluated by the same measurement as described above.

As Example 3, a molding test was performed using the same steel type, test piece size, and press mold as those of the above-described examples of the present invention. In order to examine the effect of the present invention, after the punch reached the bottom dead center, the punch was returned by 30, 50, and 70 mm in the reverse direction, and then the punch was moved again from the returned position to the bottom dead center. The wrinkle pressing force during molding and when returning the punch was 300 kN.
The test piece subjected to the bending test was evaluated by the same measurement as described above.

The measurement results of Examples 1 to 3 are shown in Tables 1 to 3. The wrinkle holding force at the end indicates the wrinkle holding force added when forming the bottom dead center after returning the punch. The effect on the opening width is considered as a reference for each comparative example, and the opening width is 6 mm or more. Those that could be reduced were evaluated as ◎ for 5 mm or more, and Δ for 4 mm or more.
According to the present invention, it is possible to obtain a hat-shaped molded part that has improved dimensional accuracy as compared with the comparative example without applying a pressing force, and that has better dimensional accuracy when a wrinkle holding force is applied.

Sectional drawing of the test piece which measures an angle change, wall curvature, and opening width is shown. It is a conceptual diagram of the conventional press work method, (a) Before shaping | molding, (b) Molding bottom dead center, (c) Punch, wrinkle press release mold, (d) Product shape after springback is shown. It is a conceptual diagram of the press working method of the present invention, (a) before molding, (b) punch stop, (c) punch return, (d) product shape after spring back, (e) molding bottom dead center, (f ) Shows punch, wrinkle holding release, and (g) product shape after springback. It is a conceptual diagram of the press working method of the present invention, (a) before molding, (b) molding bottom dead center, (c) punch return, (d) molding bottom dead center, (f) punch, wrinkle pressing release mold, (G) Shows the product shape after springback.

Explanation of symbols

1 shoulder 2 wall 3 punch 4 wrinkle retainer 5 product 6 die A molding bottom dead center

Claims (9)

  A press molding method excellent in shape freezing property, characterized in that the punch is stopped in the middle of a stroke during press working, and the punch is moved again from the stopped position to the bottom dead center.   The punch is stopped in the middle of the stroke during pressing, and the punch is returned to the middle in the reverse direction from the stop position, and then the punch is moved again from the returned position to the bottom dead center. Press molding method.   A press molding method excellent in shape freezing property, wherein after the punch reaches the bottom dead center, the punch is returned halfway in the reverse direction and then moved again from the returned position to the bottom dead center.   A press molding method with excellent shape freezing property, wherein after the punch reaches bottom dead center, the punch is returned in the reverse direction while applying a wrinkle holding force and the molding is finished.   The press molding method with excellent shape freezing property according to any one of claims 1 to 4, wherein molding is performed while a wrinkle pressing force is applied.   The press molding method with excellent shape freezing property according to claim 2 or 3, wherein the punch is returned while a wrinkle pressing force is applied.   3. The press forming with excellent shape freezing property according to claim 1 or 2, wherein a pressing force is applied from the initial stage until the punch is stopped in the middle of the stroke, and thereafter the pressing is not performed. Method.   3. The crease pressing force is not applied until the punch is stopped in the middle of the stroke from the initial stage of molding, and the crease pressing force is applied after the punch is moved again until the end of molding. 2. A press molding method having excellent shape freezing property described in 1.   The press molding method excellent in shape freezing according to any one of claims 1 to 8, wherein the press molding is performed by a hydraulic press machine or a servo press machine.

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