JP7156343B2 - Press molding method and press molding die - Google Patents

Description

TECHNICAL FIELD The present invention relates to a press-forming method and a press-forming die for press-molding a member such as an automobile part from a blank metal plate, and in particular, a top plate portion having a bifurcated shape in plan view, such as a lower arm, and the top plate portion. The present invention relates to a press-molding method and a press-molding mold for a press-molded product having a vertical wall portion continuous to a top plate portion.

Among press-formed products, for example, there is a product that includes a top plate portion that has a bifurcated shape in a plan view and a vertical wall portion that is continuous with the top plate portion, such as a lower arm that is an underbody part of an automobile. . An example of such a press-formed product will be described with reference to FIG.
A press molded product 5 shown in FIG. Hereinafter, such a bifurcated portion of the press-formed product 1 is referred to as a bifurcated portion 23 .

When the vertical wall portion 3 is formed on the top plate portion 1 having a bifurcated shape as shown in FIG. Due to flange deformation, stretch flange cracks are likely to occur at the tip of the vertical wall portion 3 (portion o surrounded by a dashed circle in the figure).

In order to avoid stretch flange cracking as described above, such a press-formed product 5 is generally manufactured in a plurality of steps as described below.
First, a top plate portion 1 and an intermediate vertical wall that is continuous with the top plate portion 1 and serve as the base end side of the vertical wall portion 3 are formed as shown in FIG. The intermediate molded product 7 having the portion 9, the bent portion 11 continuing to the intermediate vertical wall portion 9, and the shelf portion 13 continuing to the bent portion 11 is stretch-molded (stretch molding step).

Next, as shown in FIG. 8, the panel is trimmed along a predetermined trim line, unnecessary portions (hatched portions in the drawing) are removed from the shelf portion 13 of the intermediate molded product 7, and the flange portion 25 is removed. Form (trim process). Then, by bending back the bent portion 11 and erecting the flange portion 25 (restrike), the vertical wall portion 3 composed of the portion corresponding to the intermediate vertical wall portion 9 and the flange portion 25 is formed (restrike step). 6 is produced. The portion shown in gray in FIG. 6 corresponds to the bent portion 11 bent back in the restrike process.

In the above-described method, the amount of deformation during the subsequent restrike process is reduced by interposing the intermediate molded product 7 formed by stretch molding. Furthermore, since the intermediate molded product 7 is provided with a surplus portion (a portion to be removed in the trimming process), the portion corresponding to the portion o shown in FIG. Since the material flow is suppressed at , it is possible to suppress the stretch flange cracking that occurs at the o portion during the restrike process.
As such a press forming method, Patent Document 1 discloses an example in which stretch forming and trimming are performed in the same step.

JP 2017-217698 A

The inventors conducted molding analysis and press molding experiments using the finite element method on press-formed products such as lower arms having various shapes. Not only the tip of part 3 (o part), but also the part bent back in the restrike process in the vicinity of the central part of the bifurcation (parts a and b surrounded by dashed circles in the figure, the central part of the bifurcation in the direction of the bending ridge line (hereinafter referred to simply as "both sides of the bifurcation") also crack during molding. The reason why the cracks are generated in the parts a and b will be described with reference to FIGS. 6 and 9. FIG.

9(a) is an A'-A' sectional view of the intermediate molded product 7 shown in FIG. 8, and FIG. 9(b) is an AA sectional view of the press-formed product 5 shown in FIG. be. The A'-A' cross-sectional view and the AA cross-sectional view show the cross sections of the portion corresponding to the part a in the intermediate molded product 7 and the press-molded product 5. As shown in FIG.
The intermediate vertical wall portion 9, the bent portion 11, and the flange portion 25 (see FIG. 9(a)) formed by the stretch forming process and the trimming process are bent back by the restrike process so that the bent portion 11 is bent back as shown in FIG. As shown in b), it becomes the vertical wall portion 3 . At this time, since work hardening occurs in the portion corresponding to the bent portion 11 that has been bent back, the thickness of the portion indicated by the arrow in the figure is reduced.
Furthermore, since the o-portion, which tends to cause material shortage due to stretch flanging deformation, is in the vicinity (see FIG. 6), a material flow from the a-portion to the o-portion occurs. Cracks occur in the part a due to both the reduction in plate thickness and the material flow. For the same reason, cracks also occur in the b portion.

In order to suppress the cracks that occur in the parts a and b as described above, in order to reduce work hardening, the bending radius of the bent part 11 of the intermediate molded product 7 is increased, etc. Although the amount of deformation may be reduced, if the amount of deformation in the stretch forming process is reduced, the amount of deformation in the subsequent restrike process increases, and cracks are likely to occur at the portion o.
As described above, in the conventional technology, cracking is simultaneously prevented at the tip (o part) of the vertical wall part 3 formed in the central part of the branch part and the bent back parts (a part, b part) on both sides of the branch part. The problem was that it was difficult to

The present invention has been made to solve such problems, and includes a top plate portion having a bifurcated shape in plan view, such as a lower arm, and a vertical wall portion continuing to the top plate portion. A press molding method and a press molding die capable of simultaneously preventing cracks in both the tip of the vertical wall formed in the center of the bifurcation and the bent back portions on both sides of the bifurcation when molding a molded product. intended to provide

(1) A press-molding method according to the present invention press-moldes a press-molded product having a top plate portion having a bifurcated shape in plan view and a vertical wall portion continuing to the top plate portion. Then, in a state in which the metal plate is pressed against the punch by the pad, the top plate portion, the intermediate vertical wall portion that is continuous with the top plate portion and is on the base end side of the vertical wall portion, and the intermediate vertical wall portion that is continuous with the intermediate vertical wall portion. a stretch forming step in which the punch and the die work together to form a bent portion and a shelf portion connected to the bent portion; and a trimming step in which an unnecessary portion is removed from the shelf portion to form a flange portion. and a restrike step of bending back the bent portion to form the vertical wall portion composed of a portion corresponding to the intermediate vertical wall portion and the flange portion, wherein the overhang forming step includes a branch portion at the bent portion It is characterized in that the bending radius of the center portion is formed to be smaller than the bending radius of the portions on both sides of the center portion of the branch portion in the bending ridge line direction.

(2) A press molding die according to the present invention is for realizing the press molding method described in (1) above, and includes the top plate portion and the vertical wall portion that is continuous with the top plate portion. A first mold for molding an intermediate molded product having an intermediate vertical wall portion on the proximal side of the intermediate vertical wall portion, a bent portion continuous to the intermediate vertical wall portion, and a shelf portion continuous to the bent portion, and from the shelf portion A second mold that removes unnecessary portions and forms a flange portion, and a third mold that bends back the bent portion and forms the vertical wall portion composed of a portion corresponding to the intermediate vertical wall portion and the flange portion. a mold, wherein the first mold includes a punch having a top plate forming surface portion for forming the top plate portion; a pad arranged to face the punch and pressing a metal plate against the punch; A die having an intermediate vertical wall molding surface portion for molding the wall portion, a die shoulder portion for molding the bent portion, and a shelf portion molding surface portion for molding the shelf portion, and molding the central portion of the branch portion in the die. It is characterized in that the die shoulder R for molding is set to be smaller than the die shoulder R for molding the portions on both sides in the bending ridgeline direction of the central portion of the branch.

In the present invention, in the stretch forming step of forming the intermediate molded product, the bending radius of the central portion of the branched portion in the bent portion is formed so as to be smaller than the bending radius of the portions on both sides of the central portion of the branched portion in the bending ridge line direction. By doing so, it is possible to suppress the amount of deformation of the central portion of the branch during the restrike process while suppressing the reduction in the plate thickness of the bent back portion on both sides of the branch, so that the vertical wall portion formed in the central portion of the branch. It is possible to simultaneously prevent cracks at both the tip of the branch and the bent back portions on both sides of the branch.

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing of the press molding method which concerns on one embodiment of this invention. It is explanatory drawing of the stretch molding process and the 1st metal mold|die used for this which concerns on one embodiment of this invention. It is a view showing the PP cross section and the OO cross section of the intermediate molded product in FIG. is. FIG. 3 is an explanatory diagram of a trimming process and a second mold used for this according to one embodiment of the present invention; It is explanatory drawing of the restrike process based on one embodiment of this invention, and the 3rd metal mold|die used for this. It is explanatory drawing explaining the subject which arises in the formation process of the target shape which concerns on one embodiment of this invention, and this target shape. It is explanatory drawing of the conventional press molding method (part 1). It is explanatory drawing of the conventional press molding method (the 2). FIG. 7 is an explanatory diagram for explaining the reason why cracks occur in the process of forming the target shape shown in FIG. 6 ;

In the press-molding method according to the present embodiment, a press-formed product 5 (see FIG. 6) having a top plate portion 1 having a bifurcated shape in a plan view and a vertical wall portion 3 continuing to the top plate portion 1 is formed. As shown in FIG. 1, a stretch forming step S1 for forming a metal plate into an intermediate molded product 7 and a trimming step for removing unnecessary portions from the intermediate molded product 7 formed in the stretch forming step S1. S3 and a restriking step S5 for forming the intermediate molded product 7 from which unnecessary portions are removed in the trimming step S3 into a target shape.
Each step will be described below. In FIG. 1, the same reference numerals are given to the same parts and corresponding parts as those in FIGS.

<Overhang forming process>
The overhang molding step S1 is a step of molding an intermediate molded product 7 as shown in FIG. It has an intermediate vertical wall portion 9 on the base end side, a bent portion 11 continuous with the intermediate vertical wall portion 9 , and a shelf portion 13 continuous with the curved portion 11 .

In the stretch molding step S1 for molding such an intermediate molded product 7, as shown in FIG. A pad 17 for pressing the metal plate against the punch 15, an intermediate vertical wall portion forming surface portion 19a for forming the intermediate vertical wall portion 9, a die shoulder portion 19b for forming the bent portion 11, and a shelf portion forming surface portion 19c for forming the shelf portion 13. A first mold 21 having a die 19 with a

FIG. 2(a) is an OO cross section (see the enlarged view of FIG. 1(a)) which is a cross section of the central part of the branch portion of the intermediate molded product 7, and FIG. 2 is a diagram showing the state of the molding bottom dead center of the PP cross section (see the enlarged view of FIG. 1(a)), which is the cross section of both sides of the branch portion at .
As can be seen by comparing FIG. 2(a) and FIG. 2(b), regarding the die shoulder portion 19b that forms the bent portion 11, the die shoulder R ₁ (FIG. 2(a)) at the center of the branch portion is It is smaller than the die shoulder R ₂ (FIG. 2(b)) of the part, that is, R ₁ <R ₂ .
As shown in the enlarged view of FIG. 1(a), between the die shoulder _R1 and the die shoulder _R2 , there is a shape (gradually changing portion) in which R gradually changes in the bending ridgeline direction.

In the stretch forming step S1 using the first mold 21 as described above, the die 19 is moved to the initial position (not shown) while the metal plate is held by the top plate forming surface 15a of the punch 15 and the pad 17. 2(a) and 2(b) to the position of the bottom dead center shown in FIGS. 2(a) and 2(b). An intermediate molded product 7 having a vertical wall portion 9, a bent portion 11 continuing to the intermediate vertical wall portion 9, and a shelf portion 13 continuing to the bent portion 11 is formed.
Then, the bending radius of the bent portion 11 at the central portion of the branch portion of the intermediate molded product 7 is made smaller than the bending radius of the bent portion 11 at both side portions of the branch portion.
In addition, between the bent portion 11 having a small bending radius at the central portion of the bifurcation and the bent portion 11 having a large bending radius at both side portions of the bifurcation, the bending radius gradually increases from the center toward both sides in the direction of the bending ridge line. become a department.

In FIG. 3, the solid line indicates a cross section (OO cross section) of the central part of the branch of the intermediate molded product 7, and the broken line indicates cross sections (PP cross section) of both sides of the branch of the intermediate molded product 7. As shown in FIG. In order to make it easier to understand the difference in the bending radius of the bent portion 11, both cross sections are shown overlapping.

As shown in FIG. 3, by reducing the bending radius of the bending portion 11 at the center of the branch (see OO cross section) and increasing the bending radius of both sides of the branch (see PP cross section), O The line length (distance of the solid line from point X to point Y) on the −O cross section is long, and the line length (distance of the broken line from point X to point Y) on the PP cross section is short. Therefore, in the stretch forming step S1, the amount of deformation at the central portion of the branch portion is larger than that at both side portions of the branch portion.
Note that the X point and the Y point are arbitrary points common to both cross sections when the two cross sections are overlapped.

In this manner, the bending radius of the central portion of the branching portion in the bent portion 11 of the intermediate molded product 7 is made smaller than the bending radius of both side portions of the branching portion, and the amount of deformation of the central portion of the branching portion in the stretch forming step S1 is The reason why it is made larger than the part will be described with reference to FIGS. 1 and 3. FIG.

In the press forming method of the present embodiment, the vertical wall portion 3 is formed at the branch portion 23 by the stretch forming step S1 and the restrike step S5. The amount of deformation in the minute restrike step S5 is reduced.
Therefore, by molding the intermediate molded product 7 with a large amount of deformation (larger area) at the central part of the branch part, the stretch flanging deformation at the time of the restrike step S5 is alleviated, and the part o shown in FIG. cracking is suppressed.

On the other hand, on both sides of the bifurcated portion, since the amount of deformation during molding of the intermediate molded product 7 is kept small, reduction in thickness of the bent portion 11 is suppressed. Furthermore, as described above, the stretch flanging deformation that occurs in the o portion in the restrike step S5 is also reduced, so the material flow toward the o portion is also alleviated, and the cracking of the a portion shown in FIG. 6 is suppressed. be.
For the same reason, cracking at the b portion shown in FIG. 6 is also suppressed.

<Trim process>
The trim step S3 is a step of forming the flange portion 25 by removing unnecessary portions from the shelf portion 13 of the intermediate molded product 7 formed in the stretch forming step S1.

FIG. 4 is a diagram showing a state in the middle of trimming in the BB section of FIG. 1(b).
The trimming step S3 is performed by a second die 33 having an upper blade 27, a lower blade 29, and a plate retainer 31, as shown in FIG. The upper blade 27 and the lower blade 29 work together to remove unnecessary portions from the shelf 13 of the intermediate molded product 7, and each trim blade (trim blade) has a shape corresponding to the cutting line (trim line). not shown).

In the trimming step S3, a part of the top plate portion 1 and the shelf portion 13 of the intermediate molded product 7 formed in the stretch forming step S1 is held between the lower blade 29 and the plate retainer 31, and the upper blade 27 is moved, for example, to the initial position. (not shown) to form the flange portion 25 by relatively moving downward from the shelf portion 13 and cutting off an unnecessary portion (the hatched portion in FIG. 1(b)).

<Restrike process>
The restrike step S5 is a step of bending back the bent portion 11 of the intermediate molded product 7 from which unnecessary portions have been removed in the trimming step S3 to form the vertical wall portion 3, thereby forming the target shape.

FIG. 5 is a diagram showing the state of the restrike bottom dead center in the CC section of FIG. 1(c).
The restrike step S5 is performed by a third die 41 having a punch 35, a pad 37 and a die 39, as shown in FIG.
The punch 35 has a molding surface corresponding to the inner surface of the press-molded product 5 and grips the top plate portion 1 of the intermediate molded product 7 together with the pad 37 . The die 39 has a forming surface corresponding to the outer surface of the vertical wall portion 3 of the press-formed product 5, and bends back the bent portion 11 of the intermediate formed product 7 from which unnecessary portions have been removed in the trimming step S3. .

In the restrike step S5 using the third mold 41 as described above, the die 39 is moved to the initial position (not shown) while the top plate portion 1 of the intermediate molded product 7 is held by the punch 35 and the pad 37. 5 to the position of the bottom dead center shown in FIG. to mold.

As described above, in the present embodiment, in the stretch forming step S1, the bent portion 11 of the intermediate molded product 7 is formed so that the bending radius at the central portion of the branch portion is smaller than the bending radius at both side portions of the branch portion. After forming the flange portion 25 by removing the unnecessary portion in the trimming step S3, in the restrike step S5, the bent portion 11 is bent back and a stretch-flange deformation portion is formed.
As a result, at the central portion of the branch portion, the amount of deformation during the restrike step S5 is reduced and the stretch-flange deformation is suppressed, so cracks occurring at the tip of the vertical wall portion 3 can be prevented.
Furthermore, on both sides of the bifurcation, the thickness reduction of the bent back portion is suppressed, and the stretch flanging deformation of the central part of the bifurcated portion is suppressed, so that the material flow is alleviated, so cracks occurring in the bent back portion can be prevented.

In the above description, an example in which press molding is performed by dividing it into three steps has been described, but the present invention is not limited to this. For example, by molding the intermediate molded product 7 and removing unnecessary portions, the projecting step S1 and the trimming step S3 may be performed in one step, and two steps may be performed as a whole.

Further, the vertical wall portion 3 other than the branch portion 23 may be formed during the above-described three steps, or may be formed in a separate step to implement the present invention. Also, the molding of the top plate portion 1 is the same.

In order to confirm the effects of the present invention, CAE analysis was performed on press forming with a lower arm component having a target shape as shown in FIG.
A hot-rolled steel plate having a thickness of 2.3 mm and a tensile strength of 780 MPa was used as the metal plate.
The press molding process was the three processes of the stretch molding process S1, the trim process S3, and the restrike process S5 described in the first embodiment.
In addition, the bending radius of the bent portion (the portion formed by the punch shoulder) between the top plate portion 1 and the vertical wall portion 3 in the target shape was set to 7 mm.
Here, the bending radius is the radius of the circular arc at the plate thickness center portion.

As a conventional press molding method, two types of press molding methods were analyzed in which the bending radius of the bent portion 11 of the intermediate molded product 7 was the same in the central portion of the bifurcation portion and both side portions of the bifurcation portion. The bending radius of the bent portion 11 in the conventional example was set to 5 mm (conventional example 1) or 7 mm (conventional example 2) for both the central portion of the branch portion and both side portions of the branch portion.

Further, as the press molding method of the example of the present invention, the bend radius of the central portion of the branch portion in the bent portion 11 of the intermediate molded product 7 was made smaller than the bend radius of the both side portions of the branch portion. The bending radius of the bent portion 11 in the invention example was 5 mm at the central portion of the branching portion and 7 mm at both side portions of the branching portion (parts a and b in FIG. 6).
Table 1 shows the maximum plate thickness reduction rates of the parts a, b, and o (see FIG. 6) at the bottom dead center of the restrike step S5, which were obtained by CAE molding analysis.

According to the past knowledge of the inventors who made prototypes of various shapes of lower arms, in the case of a hot-rolled steel sheet with a thickness of 2.3 mm and a tensile strength of 780 MPa, if the thickness reduction rate exceeds 20% in CAE forming analysis, cracking will occur. Occur. Therefore, the forming limit at which cracks occur in this example was set at a plate thickness reduction rate of 20%.

As shown in Table 1, in the case of Conventional Example 1, the plate thickness reduction rate of part o is 20% or less of the forming limit, but the plate thickness reduction rate of parts a and b exceeds 20% of the forming limit. there is Therefore, it was determined that, in the press forming method of Conventional Example 1, cracks did not occur in the o portion, but cracks occurred in the a portion and the b portion.

Further, in the case of Conventional Example 2, the plate thickness reduction rate of parts a and b is 20% or less of the forming limit, but the plate thickness reduction rate of part o exceeds 20% of the forming limit. Therefore, in the press molding method of Conventional Example 2, it was judged that cracks did not occur in the parts a and b, but cracks occurred in the part o.

On the other hand, in the case of the invention example, the plate thickness reduction rate was 20% or less of the forming limit in all of the parts a, b, and o, and it was determined that cracks did not occur in any of the parts.
As described above, from the results of the present example, it was found that cracking of the tip of the vertical wall portion at the center of the branching portion and both sides of the branching portion of the press-formed product having a bifurcated shape in plan view, such as the lower arm, can be prevented at the same time. rice field.

In Conventional Example 1 in Table 1 above, the bending radius of the central part of the branch part (o part) and both sides of the branch part (a part, b part) of the intermediate molded product 7 is the same, and the thickness of the central part of the branch part is reduced. This is an example in which the rate is within the forming limit range, but the plate thickness reduction rate on both sides of the branch exceeds the forming limit range.
Further, in Conventional Example 2 in Table 1, the bending radius of the central portion of the branch portion of the intermediate molded product 7 and the both side portions of the branch portion are the same, and the plate thickness reduction rate of the central portion of the branch portion exceeds the forming limit range. This is an example in which the plate thickness reduction rate of the part is within the forming limit range.
Further, from Conventional Example 1, it is found that the bending radius at the central portion of the branch portion should be equal to or less than the value set in Conventional Example 1, and from Conventional Example 2, the bending radius at both side portions of the branch portion is the same as the conventional example. It can be seen that the value set in 2 or more is sufficient.

From this, it can be seen that the bending radii of the central portion of the branching portion and both side portions of the branching portion should be set as follows.
That is, by setting the bending radius of the central portion of the branch portion of the intermediate molded product 7 and the bending radius of both side portions of the branch portion to be the same, and by variously changing the value and obtaining the plate thickness reduction rate at the restrike bottom dead center, the bending radius of the central portion of the branch portion and the bending radii of both sides of the branch are the same, the thickness reduction rate of the center of the branch is within the forming limit range, and the thickness reduction rate of both sides of the branch exceeds the forming limit (A value); An example (B value) in which the plate thickness reduction rate at the central part of the branch exceeds the forming limit range and the plate thickness reduction rate at both sides of the branch is within the forming limit is obtained in advance. The bending radius is set to be less than or equal to the A value which is within the limit range, and to be equal to or more than the B value which is within the molding limit range at both sides of the branch, and the bending radius at the central portion of the branch is set smaller than the bending radius at both side portions of the branch.

1 top plate portion 3 vertical wall portion 5 press-molded product 7 intermediate molded product 9 intermediate vertical wall portion 11 bent portion 13 shelf portion 15 punch 15a top plate portion molding surface portion 17 pad 19 die 19a intermediate vertical wall portion molding surface portion 19b die shoulder portion 19c shelf molding surface 21 first mold 23 branching portion 25 flange portion 27 upper blade 29 lower blade 31 plate retainer 33 second mold 35 punch 37 pad 39 die 41 third mold

Claims (2)

A press-molding method for press-molding a press-molded product having a top plate portion having a bifurcated shape in plan view and a vertical wall portion continuous to the top plate portion,
In a state in which the metal plate is pressed against the punch by a pad, the top plate portion, the intermediate vertical wall portion continuing from the top plate portion and serving as the base end side of the vertical wall portion, and the bending continuous to the intermediate vertical wall portion. A stretch forming step in which the punch and the die work together to stretch the portion and the shelf portion that is continuous with the bent portion;
a trimming step of removing an unnecessary portion from the shelf portion to form a flange portion;
a restrike step of bending back the bent portion to form the vertical wall portion composed of a portion corresponding to the intermediate vertical wall portion and the flange portion;
In the stretch forming step, the bending radius of the central portion of the branch in the bent portion is formed so as to be smaller than the bending radius of the portions on both sides of the central portion of the branch in the bending ridge line direction. . A press molding die for realizing the press molding method according to claim 1,
The top plate portion, an intermediate vertical wall portion that is continuous with the top plate portion and serves as the base end side of the vertical wall portion, a bent portion that is continuous with the intermediate vertical wall portion, and a shelf portion that is continuous with the bent portion. a first mold for molding an intermediate molded product having
a second mold for removing an unnecessary portion from the shelf to form a flange;
a third mold that bends back the bent portion to form the vertical wall portion composed of a portion corresponding to the intermediate vertical wall portion and the flange portion;
The first mold includes a punch having a top plate forming surface portion for forming the top plate portion, a pad arranged to face the punch and pressing a metal plate against the punch, and the intermediate vertical wall portion. a die having an intermediate vertical wall molding surface, a die shoulder for molding the bent portion, and a shelf molding surface for molding the shelf;
A press molding die, wherein a die shoulder R for forming the central portion of the branched portion of the die is set smaller than a die shoulder R for forming the portions on both sides of the branched central portion in a bending ridge line direction.

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