JPH0237922A - Drawing method - Google Patents

Abstract

PURPOSE:To prevent a forming defect such as a fold, etc., by executing the forming by small blank holder pressing force from the time point when a punch comes into contact with a blank to the vicinity of the time point when a shape failure is generated in a formed part, and thereafter, increasing suddenly the blank holder pressing force within a range in which it is not ruptured. CONSTITUTION:A formed part is formed from a blank 1 of a steel sheet by a forming method by a double action press, etc. When blank holder pressing force is set to a constant prescribed value by about middle pressure and forming is executed, a body fold 16 of height hW is generated. When the blank holder pressing force is fixed and forming is executed, a flange fold is generated in case of a comparatively low pressure value or below, and also, a body hold is generated from the vicinity of a specific forming process. On the other hand, according to a forming method by which forming is executed by small blank holder pressure from the beginning of forming, and thereafter, the blank holder pressure has been increased suddenly within a range in which it is not ruptured, a formed part in which no body fold occurs at all is obtained. In such a way, forming defects such as a rupture and a fold, or a surface strain, etc., are prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a drawing method using a press, and more particularly to a drawing method in which the blank holder pressing force is variably controlled during forming.

Prior art As a basic drawing method of this type, for example, No. 8
There are methods shown in Figures (A) and (B). This is an example of a forming method using a double-acting press, in which blank material 1 is
The blank holder 5 attached to the outer slide 4 is lowered and the blank 1 is placed on the die 3 fixed to the
Apply blank holder pressing force H to the peripheral edge of (see figure (
A)).

Then, the punch 7 attached to the inner slide 6 is lowered to draw the blank material 1 into the die hole 8, thereby obtaining a drawn product 9 of a predetermined shape (FIG. 3(B)).

Here, the purpose of applying the pressing force H by the blank holder 5 is the following three points.

(A) Preventing the occurrence of wrinkles (buckling) in the flange portion 9a during the process in which the flange portion 9a is drawn into the die hole 8.

(a) When the flange portion 9a is drawn in, a frictional force is generated to apply tension to the blank material 1 in the die hole 8, and the shape of the bottom portion 9b of the molded product 9 is made to be faithful to the punch face 7a. Further, this tension prevents formation of defects in shape such as wrinkles in the aperture wall portion 9c.

(1) To increase the amount of elongation deformation by applying tension to the blank material 1 in the die hole 8 within a range that does not cause breakage, so that a molded product of a predetermined shape can be obtained with a smaller area of the blank material 1. .

The tension described in (a) and (b) above is provided not only by the blank holder pressing force H (but also by the uneven drawing beat 10 provided at the peripheral edge of the die hole 8. This drawing beat IO is It has the advantage that the distribution in the circumferential direction of the die holes acting on the molded product 9 can be adjusted by changing the shape depending on the installation position on the die 3 or the location.

Generally, the blank holder pressing force H is as described in (a) above.
It is set to a value that satisfies conditions (a) and (i), and press forming is performed without changing it during molding. That is, if the blank holder pressing force H is too large, breakage will occur, and if it is too small, shape defects such as wrinkles will occur, so the blank holder pressing force is set to a value that will not cause any of these problems.

On the other hand, recently, a method has been proposed in which the blank holder pressing force H is actively changed during the molding process to avoid breakage and wrinkles. As for the change pattern of this blank holder pressing force H, there is a method of controlling the blank holder pressing force H high within a range that does not cause breakage (hereinafter referred to as method A);
There is a method (hereinafter referred to as method B) in which the blank holder pressing force H is controlled to be small within a range where the above-mentioned flange wrinkles do not pose a problem.

To explain these A method and B method using the cylindrical drawing shown in FIG. 9 as an example, FIG. 9 (A) shows the molded product 1 in FIG.
A circular blank 11 for molding 4 is shown, and its diameter is D. The defects that occur in this cylindrical throttle are a break 12 that occurs near the bottom of the throttle wall 31 in FIG. 10(A), and a wrinkle 14 that occurs in the flange 13 in FIG. 10(B). . Figures 11 and 12 show the relationship between the blank holder pressing force H and the diameter of the blank material 11 that causes breakage and wrinkles when drawing forming is performed without changing the blank holder pressing force H during forming. , where LB is the breaking limit line and Lw is the wrinkle limit line. L,
With the blank holder pressing force H6 at the intersection of Lw and Lw, it is possible to form the blank material up to the largest diameter DC without causing breakage or wrinkles. The ratio Dc/d between the blank diameter Dc and the punch diameter d is known as the so-called critical drawing ratio. The relationship in FIG. 12 can be directly replaced with the relationship between the blank holder pressing force H and the drawing depth h at which breakage and wrinkles occur as shown in FIG. 13, and when the blank holder pressing force is Hc, the maximum drawing depth Sa hc is obtained. That is, when drawing is performed without changing the blank holder pressing force H during forming, the depth of the molded product shown in FIG. 9(B) needs to be less than hc.

As the method A mentioned above, for example, there is a method shown in Japanese Patent Application Laid-open No. 61-266130, and when the blank holder pressing force H is changed as shown by the dashed line in FIG. Initially, a blank holder pressing force H1 that is larger than Hc is applied, and when the blank holder pressing force is gradually decreased so as not to intersect the breaking limit line LB, the wrinkle limit line L1 moves to LwI and breaks to point A. Draw forming can be continued without wrinkles. As a result, a molded product having a depth exceeding the maximum drawing depth hc when the blank holder pressing force is constant is obtained.

Next, to explain the basic concept of method B based on Fig. 14, solid line 1a-1b shows the case where the blank holder pressing force is constant and the blank holder pressing force Hw is set to prevent flange wrinkles. This is the lower limit value required for On the other hand, in method B, when the blank holder pressing force is not required, the pressing force is reduced to the extent that flange wrinkles do not become a problem, as shown by the dashed lines 2a and 2b. The frictional force when drawing the blank holder is reduced, and deeper drawing forming is possible than when the blank holder pressing force is constant. There is also the advantage that the energy consumed by the press machine can be saved.

As described above, the press forming method in which the blank holder pressing force is variably controlled during forming is known to be effective in preventing breakage and wrinkles during deep drawing. The reality is that research is limited to experimental research on simple shapes such as cylindrical or square cylindrical apertures.

On the other hand, one of the purposes of applying the blank holder pressing force is to prevent the occurrence of wrinkles in the drawing wall portion 9C of the molded product 9 shown in FIG. As mentioned earlier, it is necessary to make the blank holder faithful to the shape, but it is known that in the conventional method of forming the blank holder with a constant pressing force, there is a phenomenon in which the effect of the pressing force is hardly visible. ing.

These are found in large-sized molded products such as automobile body panels, and include, for example, wrinkles 16 (hereinafter referred to as body wrinkles) on the aperture wall 15 with an extremely large radius of curvature ρ, as shown in FIG. As shown in FIG. 16, slight wrinkles 19 (hereinafter referred to as surface distortion) appear around the dimple pattern 18 on the bottom surface 17 of the molded product. The body wrinkles 16 shown in FIG. 15 are unavoidable in deep drawing processing where the flange portion 9a is largely drawn into the die hole 8 as shown in FIG. 8(B). ), (
As shown in B), a so-called step drawing method in which a stepped portion 20 is formed on the drawn wall portion 15 is common. The idea of the step draw forming method is to allow the occurrence of body wrinkles 16 (FIG. 15) to some extent during the forming process, and to absorb the body wrinkles 16 by forming the stepped portion 20 at the final stage of forming. That is, as shown in FIG. 17, the excess material in the body wrinkle-generated portion is absorbed by the shrinkage deformation ε in the direction orthogonal to the elongation deformation ε1 in the depth direction that occurs when the stepped portion 20 is formed. It is something.

The problem with the step drawing method is that in order to form the step portion 20 redundantly, a blank material with a correspondingly large area is used, resulting in a reduction in material yield.

On the other hand, one of the major causes of the surface strain 19 shown in FIG. 16 is that sufficient elongation deformation is not given to the peripheral portion of the dimple pattern 18Φ. Countermeasures against surface strain 19 include lowering the yield point of the material, or (a) increasing the radius R of a part of the plane corner of the depression pattern 18 shown in FIG. 18, (b) increasing the depression depth. There are methods such as decreasing hc and (C) decreasing the inclination angle θ of the vertical wall portion 21, but there are cases where it is not possible to lower the yield point due to the strength of the molded product, or from a design standpoint. If the shape of the dimple pattern 18 cannot be changed, countermeasures will be difficult.

The present invention solves the conventional problems of body wrinkles 16 and surface distortion 1 by adopting a change pattern of the blank holder pressing force that is completely different from the above-mentioned methods A and B.
This is an attempt to prevent the occurrence of 9.

Means for Solving the Problems and Their Effects In the present invention, the blank holder pressing force is small enough that flange wrinkles do not become a problem from the time when the punch contacts the blank material to the time when shape defects occur in the molded product. After that, the blank holder pressing force is rapidly increased within a range that does not cause the blank holder to break.

The present invention is applicable to both the double-acting press method (double action) and the single-acting press method (single action), and for variable control of the blank holder pressing force,
In the case of the double-acting press method, for example, a hydraulic circuit is added to the overload circuit of the press machine, and the overload cylinder pressure is controlled to change the blank holder pressing force. In the case of the single-acting press method, for example, the cushion cylinder is configured with a combination of air and hydraulic pressure, and the blank holder pressing force is changed by controlling the hydraulic pressure.

FIG. 1 shows a typical example of the change pattern of the blank holder pressing force in the present invention. H is the flange wrinkle limit pressing force when forming with a constant blank holder pressing force. In the present invention, forming with a punch is started with a pressing force H1 that is equal to or slightly larger than H, and the punch is not pressed until a point P immediately before or immediately after the stroke Sw where a defective shape occurs in the die hole. Molding is performed with the pressure H1 maintained. Point P
, or rapidly increase the blank holder pressing force from Pet to point P3 or Pffi3, and then
The molding is completed by holding it until i4. The - dotted chain line HB in the figure indicates a breaking limit line. For example, if the blank holder pressing force is increased from point Pi to point P13 on the line and molding is continued, a break will occur at point P14 in the middle of molding. That is, the blank holder pressing force H2 or HIt at the point P 3 + P 23 is a high pressing force within a range where no breakage occurs until the end of molding. Note that the point H3 on the HB cotton wire is the breaking limit pressing force at the molding start point when molding is performed with the blank holder pressing force constant.

Point P or Pt! Blank holder pressing force H as low as
, in order to give a large elongation deformation to the material in the die hole during the remaining process from point P to the end of forming, especially the material in the die hole up to point P or P. The purpose is to suppress work hardening of the material on the bottom of the punch as much as possible. That is, the yield stress of the punch bottom material immediately before the sudden increase in the blank holder pressing force is kept as low as possible, and the material is stretched all at once by the tensile force caused by the sudden increase in the blank holder pressing force.

Example 1 FIGS. 2(A) and 2(B) show schematic diagrams of molded products used in the experiment, and the plate thickness was 0.81111. Yield point 27 kgf/xx
″.

It was molded from a steel plate with a tensile strength of 42 kg TexN.

Figure 3 shows the change pattern of blank holder pressing force,
The solid line HD indicates the case where the limit blank holder pressing force that does not cause breakage is constant at 130 tons, and the height increases as shown in FIG. 2(B). = Approximately 211 body wrinkles 16 occur. When forming with a constant blank holder pressing force, flange wrinkles occur below 5 Q ton, and body wrinkles occur from around the 7Ozx forming stroke. In contrast, in Fig. 3, the broken line P1 P-Pts Pt
When the change pattern of the blank holder pressing force shown in No. 4 was adopted, a molded product with no visible body wrinkles was obtained.

FIG. 4(A) shows the change in the amount of plastic deformation during molding at the bottom portion 22 in FIG. 5, and FIG. 4(B) shows the change in the amount of plastic deformation at the vertical wall portion 23 in FIG. 5. The solid line and the broken line represent the solid line Ho (130 ton) of the change bar for the link holder pressing force shown in FIG.
constant) and the dashed line (pt-P□-P t3P t4). As for the deformation of the bottom part 22 in FIG. 4(A), when the blank holder pressing force is constant at 130 tons, the elongation deformation ε11 is saturated before the completion of forming, and the elongation deformation ε□ in the direction perpendicular to this is also gradually decreasing. In contrast, in this example, the amount of deformation is small (suppressed) until the blank holder pressing force is increased rapidly, and as the blank holder pressing force increases rapidly, elongation deformation ε0. ε increases rapidly.

Next, looking at the deformation in the vertical wall portion 23 shown in FIG. 4(B), when the blank holder pressing force is constant at 130 tons, the elongation deformation ε1 in the depth direction is saturated in the first half of the forming process; In the deformation ε in the orthogonal direction, the shrinkage deformation progresses monotonically. On the other hand, in this example, the elongation deformation ε increases monotonically until the end of molding, and the negative deformation reverses and begins to decrease as the blank holder pressing force increases rapidly.

That is, by adopting the change pattern of the blank holder pressing force of this embodiment, the bottom 2
2 is rapidly stretched in two axial directions, and the vertical wall portion 23 where body wrinkles occur is also stretched not only in the depth direction but also in a direction perpendicular thereto.

This point is completely different from the step drawing method described above.

It goes without saying that the elongation deformation of the vertical wall portion 23 in the direction orthogonal to the depth direction is effective in preventing or eliminating body wrinkles.

Example 2 Next, an example of the method of the present invention for dealing with surface strain on the bottom surface of a punch will be described.

Figure 6 shows a schematic diagram of the molded product used in the experiment, with a plate thickness of 0.
8xm, yield point 20 kgr/x1. Tensile strength 32 to 9
Figure 7 shows the change pattern of the blank holder pressing force, and the solid line HE is the pattern when the blank holder pressing force is constant at 53 tons. In this molded product, flange wrinkles occur when the blank holder pressing force is less than 5 Q ton, and when molded at a constant pressure of 8 Q ton or more, the ridge line portion 25 shown in FIG. Material movement (line shift) occurs.Furthermore, surface strain around the depression 24 occurs near the forming process 20311.

In contrast, the broken lines P, -P, -P, -P in Fig.
When the change pattern of the blank holder pressing force of this embodiment shown in 4 is adopted, the surface strain around the recess 24 is 20
It was improved to a level that did not pose any problem in terms of appearance. Further, no line shift occurred at all.

Furthermore, looking at the amount of plastic deformation of the bottom surface near the depression 24, when the blank holder pressing force is constant at 53 tons, the elongation deformation ε, not shown in FIG. 6, is 1.8%, but in this example The elongation deformation ε was also 0% when the blank holder pressing force was constant at 53 tons, but increased to 0.3% in this example. That is, in this embodiment, the plastic deformation l of the bottom surface of the punch at the stage where surface strain occurs increases, and as a result, the magnitude of surface strain can be significantly reduced.

Effects of the Invention As described above, according to the method of the present invention, from the time when the punch contacts the blank material to the time when a shape defect occurs in the molded product, molding can be performed with a small blank holder pressing force within a range where flange wrinkles do not become a problem. Then, by rapidly increasing the pressing force on the blank holder without breaking it, molding defects such as breaks, wrinkles, and surface distortion, which have been problems in the past, can be prevented, especially in automobile bodies. It is effective for molded products with large dimensions and complex shapes, such as panels.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a change pattern of the blank holder pressing force of the present invention, FIGS. 2(A) and (B) are explanatory diagrams of a molded product molded by the method of the present invention, and FIG. FIG. 4(A) is a diagram showing a change pattern of the blank holder pressing force during the molding process of the molded product. (B) is a diagram showing the change in the amount of bottom deformation and the amount of wall deformation during the molding process of the molded product shown in FIG. 5, and FIGS. A perspective view, FIG. 7 is a diagram showing the change pattern of the blank holder pressing force during the molding process of the molded product in FIG. 6, and FIGS. 8(A) and (B) are representative examples of the conventional drawing method. An explanatory diagram showing Fig. 9 (A
), (B) are explanatory diagrams of molded products obtained by conventional cylindrical drawing, FIGS. 10 (A) and (B) are explanatory diagrams of molding defects in the molded product of FIG. 9, and FIG. Fig. 12 is a diagram showing changes in the blank holder pressing force during the molding process of Fig. 11, Fig. 13 is JP-A-61-2
66130 publication, FIG. 14 is a diagram showing other patterns of change in blank holder pressing force in the conventional method, and FIGS. 15 and 16 are diagrams showing molding defects in the conventional molding method. Perspective view for explanation, 1st
7(A) and (B) are explanatory views of the conventional step drawing method, and FIGS. 18(A) and (B) are enlarged explanatory views of the main parts of FIG. 16. 1.11...Blank material, 3...Die, 5...Blank holder, 7...Punch, 8...Die hole. Fig. 1 Fig. 2 (A) Fig. 2 (B) E-e+' Shape embedded on Sfr surface (mm) Fig. 5 Fig. 6 Fig. 7 Doll embedding (mrn) Fig. 4 (A) Fig. 4 (B) Hexagonal raft (mm) Fig. 8<A) Fig. 8 (B) 1---Aranque 1゜3--1 type 5-1-F' rank holly 7--Pmanche 8-11 tie hole Fig. 9 (A) Fig. 1o (A) Fig. 10 (B) Fig. 14 Fig. 15 Fig. 16 Fig. 11 Fig. 12 Diagram Figure 17 (A) Figure 17 (B) ρ. Figure 18 (A) Figure 18 (B) e-el cross section

Claims (1)

[Claims] (1) A method in which a blank material that is pressed and held between a die and a blank holder is drawn into the die side and drawn into a predetermined shape by cooperation between the die and a punch, and from the time when the punch contacts the blank material. It is characterized by forming with a small blank holder pressing force within a range where flange wrinkles do not become a problem until near the point where a shape defect occurs in the molded product, and then rapidly increasing the blank holder pressing force within a range that does not cause breakage. A drawing method.