JP2013027912A - Method for die design and method for press molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for die design and a method for press molding, by which spring back during press molding can be efficiently reduced.SOLUTION: When press-molding a press molding item 11 having a nearly hat-like cross section, a recessed or projected bead 16 is given to a bottom wall 12 to reduce such a tensile stress of a flange 15 before mold releasing that is a factor causing torsion due to spring back. In concrete terms, a bead-molding projecting section or a bead-molding recessed section for forming the projecting bead 16 is provided at a part corresponding to a punch bottom of a molding die. Thus, a material of the flange 15 is put into the side of the punch at a bottom dead center in the middle of press molding, to mold the projecting bead 16, thereby easing tensile stress indirectly by the compressive stress generated at the flange 15 as a result.

Description

  The present invention relates to a mold design method for press molding that suppresses deterioration in shape accuracy of a press molded product caused by springback, and a press molding method using the mold.

  In press forming of a plate material such as a high-tensile steel plate, deterioration of shape accuracy due to springback (elastic deformation) of the plate material is often a problem, and in particular, it is difficult to take measures against the problem of torsion in elongated parts. In particular, when the molding is foam molding (a technique in which molding is performed without constraining the periphery of the blank), it is difficult to take countermeasures because measures cannot be taken by changing molding conditions. Conventionally, it has been dealt with mainly by the prospect of the mold.

However, if the springback is large, the expected amount is large, and thus accurate prediction is difficult, and it is necessary to correct the mold many times. In addition, when the negative angle is obtained with respect to the pressing direction, the prospect cannot be further increased.
Therefore, for example, a technique described in Patent Document 1 has been proposed as a press forming method for reducing spring back generated in press forming of a plate material and preventing shape defects such as warpage and twist. In this technique, an emboss is formed at a deformation occurrence position of a plate material in a previous process, and the emboss is crushed in a subsequent process. However, this method requires at least two steps, a pre-process for forming the emboss and a post-process for crushing the emboss. Furthermore, when the embossing cannot be crushed in the subsequent process, there is a concern of causing a new shape defect.

  Further, in Patent Document 2, as a method of press-forming a member having at least a part of a U-shaped cross-section or a C-shaped cross-section, after forming the horizontal cross section to be a closed cross section, surplus A method of shearing the part has been proposed. However, this method has problems such as that it can be applied only to draw molding, the yield is lowered, and press shearing of the vertical wall portion is difficult, and industrial productivity is poor.

Furthermore, in Patent Document 3, for the defect that the angular opening is generated by the springback after forming in the bent portion of the linear ridgeline, after forming is performed, a concave shape is given to the bottom, and this is performed. A molding method to be corrected has been proposed.
However, in this method, since the dent is provided after the end of the bending, it is only possible to reduce the angular opening of the shape having a linear ridgeline in the case of a box, and the long length having a concave curved ridgeline. It is not possible to reduce the spring back such as the twist in the longitudinal direction caused by causes other than the angular opening, such as the spring back generated in the shape of. Furthermore, since the dent shape is formed after the end of the bending, at least two steps of molding are required, which increases costs and costs.

  Patent Document 4 proposes a press molding method in which spring back is suppressed by relaxing a tensile stress generated in a convex curved vertical wall portion. Here, after a surplus portion (concave portion) in which the peripheral length of the portion increases is provided in a portion corresponding to the convex curved vertical wall portion, press molding is performed. However, this method can relieve the tensile stress generated in the convex curved vertical wall portion, but cannot reduce the tensile stress generated in the flange portion of the concave curved portion. Furthermore, since the dent shape is formed after the end of the bending, at least two steps of molding are required, which increases costs and costs.

  Furthermore, Patent Document 5 proposes a press molding method for preventing an angle opening of a bent portion due to a spring back generated in a bending shoulder. Here, when the bending shoulder is formed at the time of press forming, a concave step portion that is recessed in the direction opposite to the forming bending direction is simultaneously formed on the bending shoulder. However, this method is intended only to prevent angular opening, and a sufficient effect cannot be obtained for longitudinal twisting.

JP 2009-255106 A JP 2008-264857 A JP 58-157527 A JP 2002-1445 A Japanese Patent Laid-Open No. 07-185663

As described above, in the method described in each of the above patent documents, the spring back at the time of press molding of a press-formed product having a long shape in which a flange portion is formed on a concavely curved vertical wall portion is efficiently reduced. I can't.
Then, this invention makes it a subject to provide the metal mold | die design method and press molding method which can reduce the springback at the time of press molding efficiently.

  In order to solve the above-mentioned problems, a mold designing method according to the present invention comprises a main body portion comprising a bottom wall and a side wall having one end connected to at least one width direction end portion of the bottom wall and extending in the longitudinal direction. And at least the other side wall of the side wall, and a flange portion that protrudes outward in the width direction, and the one side wall on which the flange portion is formed has a curved portion that is concavely curved in a horizontal plane. A mold design method for designing a mold for molding a press-formed product having a longitudinal length before release of the flange portion formed on the side wall having the curved portion in the course of molding in only one step. In order to generate a compressive stress against the tensile stress in the direction, the shape and molding position of the concavo-convex portion to be applied to the bottom wall are set, and the body portion is molded based on the shape and molding position of the concavo-convex portion. The punch and die are each recessed. Parts so as to have a surface shaped to mold the is characterized by determining the shape of the mold.

  In this way, unevenness for forming an uneven portion that generates compressive stress against the tensile stress to the outside in the longitudinal direction of the flange portion before releasing at the portion corresponding to the punch bottom (bottom wall) of the molding die Since the shape is arranged, the material of the flange portion can flow into the punch near the bottom dead center in the middle of press forming, and the tensile stress before releasing the flange portion can be relieved. The tensile stress before release of the flange part is known to be the main cause of torsion due to springback by press molding analysis etc., so by relaxing this, the longitudinal direction of the press molded product after release is reduced. Springback such as torsion can be reduced.

Furthermore, in the above, the main body is characterized in that the side wall is substantially U-shaped in cross section formed at both ends in the width direction of the bottom wall.
As a result, due to the effect of deformation of the stretch flange due to the curved portion, a large tensile stress tends to be distributed over a wide range, particularly on the bottom wall. A large spring back reduction effect can be expected.

Moreover, in the above, it is characterized in that the forming position of the concavo-convex portion is in the longitudinal direction region of the curved portion in the bottom wall.
Thereby, the tensile stress before mold release of a flange part can be relieve | moderated effectively, and the spring back of a press-molded product can be reduced significantly.
Furthermore, in the above, the shape of the concavo-convex portion is a shape that is convex toward the side where the side wall having the curved portion exists in a direction orthogonal to the bottom wall.

Thereby, the material of a flange part can be appropriately made to flow into the punch side in the vicinity of the bottom dead center during press molding.
Moreover, in the above, the shape of the said uneven | corrugated | grooved part is made into the shape which becomes convex on the opposite side to the side in which the said side wall which has the said curved part exists in the direction orthogonal to the said bottom wall.

Thereby, the material of a flange part can be directly flowed into the punch side in the vicinity of the bottom dead center in the middle of press molding. Therefore, compared with the case where the shape of the concavo-convex portion is convex to the side where the side wall exists, the effect of relaxing the tensile stress can be obtained with a small increase in line length.
Furthermore, in the above, the length in the longitudinal direction of the concavo-convex portion is set to be ½ times or more of the total of the width of the flange portion and the height of the side wall.

Thereby, the relaxation amount of the tensile stress sufficient to reduce the spring back of the press-formed product can be ensured.
Further, in the above, the amount of increase in the line length in the width direction of the bottom wall due to the provision of the uneven portion is at least 1/50 times the total of the width of the flange portion and the height of the side wall. .

Thereby, the amount of relaxation of the tensile stress sufficient to reduce spring back such as twisting in the longitudinal direction of the press-formed product can be ensured.
Furthermore, the press molding method according to the present invention is characterized in that a material to be molded is press-molded using a mold designed by any one of the above-described mold design methods, and the press-molded product is molded.

  In this way, press molding is performed using a press molding die that allows the flange part material to flow into the punch near the bottom dead center in the middle of press molding. Can be molded. Therefore, it is possible to effectively relieve the tensile stress before releasing the flange portion. In addition, a press-formed product with reduced spring back can be obtained in only one step.

  According to the present invention, it is possible to effectively cope with a shape defect caused by a tensile stress generated in the flange portion before release. At that time, since there is no significant change in the construction method or mold structure or a decrease in yield, an increase in production cost can be suppressed. Moreover, it is applicable regardless of a technique such as draw molding using a wrinkle press, foam molding without using a wrinkle press, or bending.

It is a perspective view which shows the press molded product shape | molded with the press molding method in this embodiment. It is a flowchart which shows a metal mold | die design processing procedure. It is a top view which shows a to-be-molded material typically. It is a figure which shows the area | region which performs a springback analysis. It is a figure which shows a springback analysis result. It is a figure which shows a twist factor site | part and a Z displacement factor site | part. It is a figure which shows a springback factor and its countermeasure technique. It is sectional drawing which shows a concave bead shape. It is a figure which shows a bead shaping area | region. It is a figure which shows the relationship with a twist angle. It is a figure for demonstrating the evaluation method of bead depth. It is sectional drawing which shows a convex bead shape. It is a top view which shows Examples 1-6 typically. It is a figure which shows the effect of Examples 1-6. It is sectional drawing which shows another example of a press-formed product.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Here, a high-tensile material is formed by molding, and a plate-shaped member is processed into a long shape that is twice or more the width of the bottom wall of a substantially hat-shaped cross section seen in a side member of an automobile, etc. The case will be described using an example of manufacturing.
FIG. 1 is a perspective view showing a press-formed product formed by the press-forming method in the present embodiment, wherein (a) is a right perspective view and (b) is a left perspective view.

  In the figure, reference numeral 11 denotes a press-formed product. This press-molded product 11 has a substantially U-shaped body section having a bottom wall 12 and a side wall 13 having one end connected to both ends in the width direction of the bottom wall 12 via shoulder portions (R portions) 13a. 14 has a substantially hat-shaped cross section having outward flange portions 15 on both sides in the width direction. The main body portion 14 extends in the longitudinal direction (normal direction of the cross section), and one side wall 13 has a curved portion 18 that is curved in a concave shape in a horizontal plane. In the present embodiment, the bending portion 18 is formed at a substantially central portion sandwiched between both side portions 17 in the longitudinal direction of the main body portion 14.

Further, in the longitudinal direction region of the curved portion 18 in the bottom wall 12, a concave shape that protrudes inside the U shape (the lower side in FIG. 1) of the main body portion 14, or the U shape outside the main body portion 14 (see FIG. A convex bead (uneven portion) 16 that is convex on the upper side of 1 is applied during one-step press molding.
In addition, although FIG. 1 shows the case where one concave and convex bead 16 are respectively provided, a plurality of bead 16 may be provided, or one of the concave and convex beads 16 may be provided. You may make it provide one or more. Further, the plurality of beads 16 may all have the same shape or different shapes.

  The bead 16 cancels out the residual stress that greatly affects the springback (elastic deformation) after mold release from the residual stress before mold release in each part of the molding material press-molded in the reference mold shape. This is for generating such stress. Here, the reference mold shape is a mold in which the shape of the material to be molded before releasing is a shape in which the bead 16 is not applied to the bottom wall 12 in the press-formed product 11 shown in FIG. Say.

FIG. 2 is a flowchart showing a series of processing (die design process) procedures for determining the shape of a mold for producing the press-formed product 11 described above. This process is executed by a computer such as a personal computer having a CPU and a memory (not shown).
First, in step S1, press molding analysis using a finite element method (FEM) analysis system is performed, and the residual stress before release at each part of the molding material press-molded with the reference mold shape is returned to the spring back. The degree of contribution is calculated.

  Springback is an elastic recovery that uses the residual stress before mold release as the initial stress. Here, the material to be molded is divided into multiple areas, and the residual stress before mold release is erased in each area and before it is erased. Check how the springback amount changes afterwards. Then, as a result of performing the springback analysis after eliminating the residual stress in the specific region, if the springback in the region is effectively reduced, it is determined that the contribution of the residual stress to the springback is high. .

As an index for evaluating the amount of springback, a twist angle (°) and a displacement in the Z direction (mm) are used. First, these evaluation indexes will be described.
FIG. 3 is a plan view schematically showing the molding material 1. In the figure, reference numeral 2 is a bottom wall, 3 is a side wall, 4 is a main body portion, and 5 is a flange portion, which correspond to the bottom wall 12, side wall 13, main body portion 14 and flange portion 15 of the press-formed product 11 shown in FIG. It is a part to do. In the following description, the longitudinal direction of the molding material 1 is the X direction, the width direction is the Y direction, and the height direction is the Z direction. Further, the Y direction plus side is an upward direction on the paper surface, and the Y direction minus side is a downward direction on the paper surface. Further, the plus direction in the Z direction is the direction in front of the page, and the minus direction in the Z direction is the direction in the back of the page. Further, the upper side wall 3 of the molding material 1 is the upper side wall 3a, the lower side wall 3 having a curved portion curved in a concave shape in the horizontal plane is the lower side wall 3b, and the upper flange portion 5 is the upper flange portion 5a. The lower flange portion 5 is defined as a lower flange portion 5b.

  As shown in FIG. 3, the twist angle is a change amount before and after the springback of the angle (tilt) of the twist evaluation point AA ′ when the points P1 to P3 are fixed points. The twist evaluation points A and A ′ are two predetermined points in the evaluation cross section SA that form a cut at one end of the main body portion 4 of the molding material 1. The twist angle becomes a positive value when the twist evaluation point A moves to the plus side in the Z direction from the twist evaluation point A ′. The Z-direction displacement is the amount of displacement in the Z direction of a predetermined displacement evaluation point B in the evaluation section SA.

  And the to-be-molded material 1 is divided | segmented into the some area | region shown in FIG. 4, a springback analysis is performed based on the data which erase | eliminated the residual stress before mold release of each area | region, and the to-be-molded material 1 after mold release is carried out. The shape is calculated, and the torsion angle and the Z direction displacement are calculated. In the springback analysis, for the data from which the residual stress before release in each region is erased, the restraining force by the press die is removed, and the shape of the molding material 1 after release is calculated.

  As shown in FIG. 4, the area division of the molding material 1 is performed in each of the Y direction and the X direction. About the Y direction, as shown to Fig.4 (a), area | region A1-A5 is set for every site | part. That is, in comparison with FIG. 3, the region A1 is a region corresponding to the upper flange portion 5a, the region A2 is a region corresponding to the upper side wall (including the R portion) 3a, the region A3 is a region corresponding to the bottom wall 2, and the region A4. Is a region corresponding to the lower side wall (including the R portion) 3b, and a region A5 is a region corresponding to the lower flange portion 5b. For the X direction, as shown in FIG. 4B, areas B1, B2,..., B5 are set in order from the minus side in the X direction at predetermined intervals.

  FIG. 5 shows the result of the springback analysis performed for each region shown in FIG. As shown in FIG. 5A, when the residual stress in the region A5B2 (region B2 of the lower flange portion 5b) is erased, the reference value (result of performing the spring analysis without erasing the residual stress) Thus, it can be seen that the twist angle is greatly reduced. Further, when the residual stress in the region A5B1 (region B1 of the lower flange portion 5b) and the region A4B4 (region B4 of the lower side wall 3b) is eliminated, the twist angle is relatively reduced. That is, the residual stress in the region A5B2 indicated by the black arrow in FIG. 6A greatly affects the twist angle, and the residual stresses in the regions A5B1 and A4B4 indicated by the white arrows are This affects the twist angle.

  Moreover, as shown in FIG.5 (b), when the residual stress of area | region A3B1 (area | region B1 of the bottom wall 3) is erase | eliminated, it turns out that a Z direction displacement is reduced significantly with respect to a reference value. Further, even when residual stresses in the region A5B2 (region B2 of the lower flange portion 5b), the region A5B3 (region B3 of the lower flange portion 5b), and the region A5B4 (region B4 of the lower flange portion 5b) are erased, The displacement in the Z direction is relatively reduced. That is, the residual stress in the region A3B1 indicated by the black arrow in FIG. 6B greatly affects the displacement in the Z direction, and then the regions A5B2, A5B3, and A5B4 indicated by the white arrows. This means that the residual stress of Z affects the displacement in the Z direction.

In this way, a portion having a large influence on the springback amount is specified.
Next, in step S2 of FIG. 2, the bead 16 applied to the bottom wall 12 in order to generate a stress that cancels out the residual stress that contributes greatly to the springback specified by the press molding analysis in step S1. Set the shape and molding position.
From the result of the springback analysis in step S1, it can be seen that the tensile stress in the region A5B2 (region B2 of the lower flange portion 5b) is the main factor of torsion. Further, the change in the shape of the region A3B1 (region B1 of the bottom wall 3) around the fixed point at the time of spring back and the contraction in the longitudinal direction due to the tensile stress of the region A5 (lower flange portion 5b) are the main causes of the Z-direction displacement. It turns out that it is a cause. That is, if the tensile stress of the lower flange portion 5b common to the twist angle and the Z-direction displacement is relaxed, the amount of springback can be reduced.

  By the way, as a method for eliminating the tensile stress of the lower flange portion, a method of cutting a predetermined region of the lower flange portion can be considered. However, in this case, the effect of reducing the springback can be obtained with certainty, but there is a concern that the rigidity as a press-formed product may be adversely affected. There is also a method to eliminate the tensile stress by giving a new shape to the lower flange part, but since the flange part is generally used as a joint part with other parts, it has a flat shape without giving a new shape. It is desirable to keep it as it is.

  Therefore, in the present embodiment, a method is adopted in which the tensile stress of the lower flange portion 5b is relieved by changing the shape of the bottom wall 2, specifically, adding a bead to the bottom wall 2. This is because the influence on the part performance of the press-formed product due to the shape change of the bottom wall 2 is considered to be limited. Accordingly, the tensile stress outward in the longitudinal direction of the lower flange portion 5b as shown in FIG. 7A is a compressive stress generated by the application of the bead 6 to the bottom wall 2 as shown in FIG. 7B. It is thought that it is indirectly changed and mitigated. This compressive stress is a stress in a direction against the tensile stress, and is generated by the material inflow to the bead 6 side of the lower flange portion 5b.

Next, specific shape setting of the bead 6 will be described.
FIG. 8 is a cross-sectional view showing the shape of the bead 6. In this example, the bead 6 has a concave shape. In FIG. 8, Wp is the width of the lower flange portion 5b, and Hp is the height of the lower side wall 3b. Wb is the width of the bead 6 (linear distance in the width direction), Db is the depth of the bead 6 (depth from the bottom wall 2), and Lb is the line length of the bead 6 (distance in the width direction).

  The shape of the bead 6 is a shape that can effectively reduce the tensile stress before releasing the lower flange portion 5b. Here, the line length increase amount (= Lb−Wb) of the bottom wall 2 by adding the beads 6 is set to 1/50 times or more of the total of the flange width Wp and the side wall height Hp. Further, the depth Db of the bead 6 is set to a depth at which a desired twist angle can be realized as will be described later. Furthermore, the length (bead length) in the longitudinal direction (X direction) of the bead 6 is set to be 1/2 or more of the total of the flange width Wp and the side wall height Hp.

  For example, when the length in the longitudinal direction of the curved portion 18 (curved portion length) is 570 mm, the flange width Wp is 20 mm, and the side wall height Hp is 20 mm, the line length increase is 0.8 mm or more and the bead length is 20 mm. That's it. Also, the forming position of the bead 6 is set to a position where the tensile stress before releasing the lower flange portion 5b can be effectively reduced. Here, in the press-formed product 11 of FIG. 1, the bead 16 is set in the bead forming regions Ea and Eb shown by the oblique lines in FIG. 9 so that the bead 16 is applied in the longitudinal direction region of the curved portion 18 in the bottom wall 12. . In FIG. 9, Ceb indicates the center position of the bead molding region Eb which is the reference position in the X direction.

As the bead length increases, the tensile stress relaxation range of the lower flange portion 5b increases as the length increases, but the amount of relaxation tends to decrease. Therefore, the bead length is about 120 mm (the flange width Wp and the height of the side wall portion). (Approx. 3 times the sum of the height Hp) is appropriate.
FIG. 10 is a diagram showing the relationship between the amount of increase in wire length, the bead length, and the twist angle when a 780 MPa grade cold-rolled steel sheet having a thickness of 1.2 mm is used as the forming target material. Here, the twist evaluation result shown in FIG. 10A is the twist evaluation in the evaluation cross section SA that forms the cut end of one end of the main body 4 when the points P1 to P3 are fixed points as shown in FIG. This is the twist angle of the point A-A ′. Moreover, the twist evaluation result shown in FIG. 10B is the twist angle of the twist evaluation point CC ′ in the evaluation section SC forming the cut end of the other end of the main body 4. As can be seen from FIGS. 10A and 10B, the twist angle can be freely controlled by changing the line length increase amount and the bead length.

  In addition, since the line length increase amount of this embodiment is set to 1/50 times or more of the total of the height Hp of the side wall portion and the flange width Wp, the line length increase amount / (wall height + flange width) ) Is 0.02 or more is the applicable range of the present embodiment. Further, the bead length of the present embodiment is set to be 1/2 or more times the total of the height Hp of the side wall and the flange width Wp, so that the increase in line length / (wall height + flange width) Is within the range of 0.5 or more.

Moreover, when making the bead 6 convex, it is set as the shape shown in FIG. In FIG. 12, Wb is the width of the bead 6 (linear distance in the width direction), and Hb is the height of the bead 6 (height from the bottom wall 2). Also in the case of the convex shape, the setting conditions for the line length increase amount and the bead length are the same as in the case of the concave shape.
Next, in step 3 of FIG. 2, the mold shape for molding the press-formed product 11 is determined, and the mold design process is terminated. Here, on the basis of the shape and molding position of the bead 6 set in step S2, a convex portion and a concave portion corresponding to the bead 6 are arranged in a portion corresponding to the punch bottom portion of the molding die.

  That is, when the bead 6 is a concave bead, a bead molding concave portion corresponding to the shape and molding position of the bead 6 is arranged in the punch, and a bead molding convex portion that fits into the bead molding concave portion is arranged in the die. When the bead 6 is a convex bead, a bead molding concave portion corresponding to the shape and molding position of the bead 6 is disposed on the die, and a bead molding convex portion that is fitted to the bead molding concave portion is disposed on the punch.

  When press molding is performed using the molding die determined by the above-described mold design process, the material of the flange portion 15 flows into the punch side near the bottom dead center in the middle of press molding. That is, after the side wall 13 of the press-formed product 11 starts to be molded, the bead 16 is molded by the bead molding concave portion and the bead molding convex portion. Thereby, the press-formed product 11 in which the spring back is suppressed and good shape freezing property is secured is formed.

(Example)
Hereinafter, the effect of the present invention will be specifically described with reference to examples.
FIG. 13 is a plan view schematically showing the press-formed product 11 of Examples 1-6. Here, as Examples 1 to 6, as shown in FIGS. 13A to 13F, press-formed products 11 having different shapes and molding positions of beads 16 to be applied to the bottom wall 12 are produced, and spring backs are produced. Analysis was performed. Further, as a comparative example, a press-molded product 11 in which the bottom wall 12 was not provided with the beads 16 was produced, and a springback analysis was performed.

  As the press-formed product 11, a product having a flange width Wp of 20 mm, a side wall height Hp of 20 mm, and a bottom wall width of 100 mm was used. When the bead 16 is a concave bead, the bead width Wb is 13.5 mm, the bead depth Db is 4 mm, the bead line length Lb is 16.5 mm, and the line length increase is 3 mm. On the other hand, when the bead 16 is a convex bead, the bead width Wb is 23 mm, the bead height Hb is 1.6 mm, and the line length increase is 1.9 mm. Further, the bead molding region Eb shown in FIG. 9 was a region having a longitudinal direction of 270 mm.

In Example 1, the shape of the bead 16 was a concave bead, and the bead length was 120 mm. Further, as shown in FIG. 13A, the forming position of the bead 16 is a position that is substantially the center in the longitudinal direction of the curved portion 18 in the bead forming region Eb, and in the vicinity of the lower flange portion 15 (lower side). The distance from the side wall 13 to the center of the bead 16 was 15 mm).
In the second embodiment, the shape of the bead 16 is the same as that of the first embodiment, and the forming position of the bead 16 is substantially the center in the longitudinal direction of the curved portion 18 in the bead forming region Eb as shown in FIG. And the vicinity of the upper flange portion 15 (the distance from the upper side wall 13 to the center of the bead 16 is 15 mm).

  In Example 3, the shape of the bead 16 was a convex bead, and the bead length was 60 mm. Further, as shown in FIG. 13C, the forming position of the bead 16 is a position that is substantially the center in the longitudinal direction of the curved portion 18 in the bead forming region Eb, and in the vicinity of the lower flange portion 15 (lower side). The distance from the side wall 13 to the center of the bead 16 was 20 mm), and the convex surface was not directly connected to the side wall 13.

In the fourth embodiment, the shape of the bead 16 is the same as that of the third embodiment, and the molding position of the bead 16 is set in the vicinity of the end in the X direction minus side in the bead molding region Eb (X) as shown in FIG. The distance from the direction reference position Ceb to the center of the bead 16 is 105 mm) and in the vicinity of the lower flange portion 15, and the convex surface is directly connected to the lower side wall 13.
In Example 5, both the concave bead of Example 1 and the convex bead of Example 3 were used. Here, as shown in FIG. 13E, the forming position of the convex bead of Example 3 was set in the vicinity of the X direction minus side end in the bead forming region Eb.

In Example 6, the shape of the bead 16 was a convex bead, and the bead length was 120 mm. Further, as shown in FIG. 13 (f), the bead 16 is molded at a position that is substantially in the center in the longitudinal direction of the curved portion 18 in the bead molding region Eb and in the vicinity of the lower flange portion 15 (lower side). The distance from the side wall 13 to the center of the bead 16 was 20 mm).
A springback analysis was performed on each of the press-formed products 11 press-molded under the above conditions, and the torsion angle and the Z-direction displacement were investigated. The result is shown in FIG. Referring to FIG. 14, it can be seen that Examples 1 to 6 have a springback reduction effect as compared with the comparative example in which no bead 16 is applied. Hereinafter, each example will be specifically verified.

  Example 1 and Example 2 are the same in shape, except that the forming position of the bead 16 is different. And when the analysis results of these two examples are compared, it can be seen that the same effect is obtained. That is, the effect is almost the same when the concave bead is disposed in the vicinity of the lower flange portion 15 and when the concave bead is disposed at a position away from the lower flange portion 15. Thus, the setting of the Y-direction position of the concave bead does not greatly affect the springback reduction effect. However, it is desirable to dispose the bead 16 in the vicinity of the lower flange portion 15 as in the first embodiment because there is no adverse effect on the upper flange portion 15 (less).

  Example 3 is an example in which the bead 16 is a convex bead having a relatively short bead length. The analysis result of Example 3 shows that although there is a springback reduction effect, the effect is low when the bead length is short. On the other hand, even in the case of a convex bead having the same shape as that of the third embodiment, as in the fourth embodiment, the molding position of the bead 16 is set in the vicinity of the end portion on the minus side in the X direction in the bead molding region Eb, and the convex surface is the lower side. If it is directly connected to the side wall 13, the twist angle is greatly improved. This is because the material can flow directly from the lower side wall 13 and the lower flange portion 15 to the bead 16 and the tensile stress of the lower flange portion 15 can be effectively reduced.

  And in Example 5 which gave both the concave bead of Example 1 and the convex bead of Example 3, it turns out that a very high springback reduction effect is acquired. As described above, the bead 16 is a concave bead + convex bead, and the molding position of the convex bead is set in the vicinity of the end in the X direction minus side in the bead molding region Eb, and in particular, the twist angle is improved to a value less than half. can do.

  Example 6 and Example 1 differ only in the shape of the bead 16, and the molding position and bead length are the same. When the analysis result of Example 6 is seen, it turns out that the effect equivalent to Example 1 is acquired. Moreover, even if it compares with the analysis result of Example 3 from which only bead length differs, when the bead 16 is made into a convex bead, it turns out that a high springback reduction effect will be acquired if sufficient bead length is ensured.

  As described above, when the bead 16 is a concave bead, the line length increase is 3 mm, and when the bead 16 is a convex bead, the line length increase is 1.9 mm. That is, it can be seen from the analysis results of Example 6 and Example 1 that in the case of a convex bead, the springback reduction effect can be obtained with a smaller line length increase than that of the concave bead. This is because, in the convex bead, the material inflow during the molding from the side wall 13 on the side where the tensile stress is to be eliminated can be performed more directly than in the case of the concave bead.

  As described above, in this embodiment, when producing a press-molded product having a substantially hat-shaped cross section and having a curved portion in which one side wall is concavely curved in a horizontal plane, a bead is applied to the bottom wall. . As a result, the material of the flange portion formed on the side wall having the curved portion can be caused to flow into the punch side during press molding, so that the tensile stress outside the longitudinal direction of the flange portion is generated by the bead on the bottom wall. It can be changed and relaxed indirectly by the compressive stress. It is known from the press molding analysis that the tensile stress on the outer side in the longitudinal direction of the flange formed on the side wall having the curved portion is the main factor for the occurrence of torsion due to the spring back. The back can be reduced, a good shape freezing property can be secured, and a press-formed product having a desired shape can be obtained.

  At this time, since the bead molding concave portion and the bead molding convex portion fitted to the bead molding concave portion are arranged in the portion corresponding to the punch bottom portion of the molding die, the material of the flange portion is placed on the punch side near the bottom dead center during press molding. The bead can be formed by inflow. That is, the bead can be formed on the bottom wall after the side wall of the press-molded product starts to be formed, and the tensile stress generated in the flange portion can be effectively relieved. Further, since only a bead molding concave portion is added to one of the punch and the die and a bead molding convex portion is added to the other, there is no need to change the mold structure greatly. Furthermore, it is possible to produce a press-molded product in which a springback reduction effect is obtained by press molding in only one step without increasing the number of molding steps.

In addition, this method is applied to a press-molded product having a substantially U-shaped cross section of the main body. In the case of a substantially U-shaped cross section, a large tensile stress tends to be distributed over a wide range, particularly on the bottom wall, due to the influence of stretch flange deformation by the curved portion. Therefore, by applying this method to a shape having such a tendency, a greater springback reduction effect can be obtained.
Further, the bead length is ½ times or more of the total of the width of the flange portion and the height of the side wall portion, and the amount of increase in the line length in the width direction of the bottom wall due to the bead is given as the width of the flange portion and the height of the side wall portion. Since the bead molding position is within a predetermined area in the center of the bottom wall, the tensile stress of the flange portion is ensured by the compressive stress generated by the bead molded under these conditions. Can be relaxed. As a result, the twist in the longitudinal direction can be effectively reduced.
Furthermore, as described above, there is no significant increase in production cost because there is no major change in the construction method or mold structure, and there is no decrease in yield.

(Application examples)
In the above embodiment, the case where the automobile part having the shape of FIG. 1 is applied as the press-formed product 11 is described, but the concave curved portion like the shape of FIG. If it is a part, it will not be limited to a motor vehicle part. Further, the press molding is not limited to foam molding, and can be applied to draw molding and bending molding.

Furthermore, in the said embodiment, although the cross-sectional shape of the press molded product 11 demonstrated the case where a main-body part was made into the substantially U-shaped hat type, one side wall in which the flange part was formed is concave shape in a horizontal surface. As long as it has a curved part that is curved, the present invention can be applied to other parts.
For example, as shown in FIG. 15A, the flange portion may be formed only on the side wall 3b having the curved portion, and the side walls 3a and 3b may be asymmetric in the width direction. Moreover, as shown in FIG.15 (b), it may have a side wall which has a curved part only on one side (only the side wall 3b and the flange part 5b are formed, and there is no side wall 3a and the flange part 5a). Furthermore, as shown in FIG.15 (c), the side wall 3a may extend in a different direction from the side wall 3b which has a curved part in an up-down direction. In these cases, the same springback reduction effect as that of the above embodiment can be obtained.

  Moreover, in the said embodiment, although the case where a torsion angle and a Z direction displacement were used as a springback evaluation parameter | index was demonstrated, it is applicable if it is a value correlated with a springback amount. For example, an angle formed by a predetermined evaluation point in an evaluation cross section that forms a cut at one end of the molding material 1 and a predetermined evaluation point in an evaluation cross section that forms a cut at the other end of the molding material 1 changes before and after the springback. An amount can also be used.

  DESCRIPTION OF SYMBOLS 1 ... Molding material, 2 ... Bottom wall, 3 ... Side wall, 3a ... Upper side wall, 3b ... Lower side wall, 4 ... Main part, 5 ... Flange part, 5a ... Upper flange part, 5b ... Lower flange part, 6 DESCRIPTION OF SYMBOLS ... Bead (uneven part), 11 ... Press-molded product, 12 ... Bottom wall, 13 ... Side wall, 14 ... Main part, 15 ... Flange part, 16 ... Bead (uneven part), 17 ... Both sides, 18 ... Curved part

Claims (8)

A main body that includes a bottom wall and a side wall having one end connected to at least one widthwise end of the bottom wall, and protrudes outward in the width direction at the other end of the at least one side wall. And a mold design method for designing a mold for molding a press-formed product having a curved portion in which the one side wall on which the flange portion is formed is curved in a concave shape in a horizontal plane. Because
Irregularities imparted to the bottom wall in order to generate a compressive stress that resists the tensile stress in the longitudinal direction before the release of the flange portion formed on the side wall having the curved portion in the course of molding in only one step Set the shape and molding position of the part,
Based on the shape and molding position of the uneven portion, the shape of the mold is determined so that the punch and die for forming the main body portion have a surface shape for forming the uneven portion, respectively. Mold design method.   2. The mold design method according to claim 1, wherein the main body has a substantially U-shaped cross section in which the side wall is formed at both ends in the width direction of the bottom wall.   3. The mold design method according to claim 1, wherein a molding position of the concavo-convex portion is set in a longitudinal direction region of the curved portion in the bottom wall.   The shape of the concavo-convex portion is a shape that protrudes toward the side where the side wall having the curved portion exists in a direction orthogonal to the bottom wall. The mold design method described.   The shape of the concavo-convex portion is a shape that is convex on the side opposite to the side on which the side wall having the curved portion exists in a direction orthogonal to the bottom wall. The mold design method according to claim 1.   The length in the longitudinal direction of the concavo-convex part is not less than ½ times the total of the width of the flange part and the height of the side wall. Mold design method.   The amount of increase in the line length in the width direction of the bottom wall due to the provision of the uneven portions is at least 1/50 times the total of the width of the flange portion and the height of the side wall. The mold design method according to any one of the above.   A press molding method comprising press molding a material to be molded using a mold designed by the mold design method according to any one of claims 1 to 7, and molding the press molded product. .

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