JP7617399B2 - Pressing device and method for manufacturing press-molded products - Google Patents

Description

This disclosure relates to a press device and a method for manufacturing press-molded products using the same.

Press-formed products have traditionally been used as components for automobile bodies and the like. Press-formed products are manufactured by pressing metal sheets. More specifically, press-formed products are manufactured by drawing and bending metal sheets as raw materials using a mold consisting of a die and a punch.

In press working of metal sheets, springback occurs when the press-formed product formed by the die is released from the die. That is, when the metal sheet is bent by the die during press working, tensile stress occurs on the outside of the bend and compressive stress occurs on the inside of the bend in the bent part, but when forming is completed and the press-formed product is removed from the die, the material undergoes elastic recovery. As a result, stress occurs in the bent part of the press-formed product in a direction opposite to the direction of the stress during press working. This reverse stress due to springback is added to the stress at the end of forming, so that compressive stress remains on the outside of the bend and tensile stress remains on the inside of the bend in the bent part of the press-formed product.

In press-formed products used as automotive parts, it is preferable for the tensile residual stress on the inside of the bend to be small. For example, in automotive suspension parts such as suspension members, relatively large stresses are repeatedly generated in the bent parts when the automobile starts, is running, or is braking, so the greater the tensile stress remaining on the inside of the bend, the lower the fatigue strength of the bent parts.

In order to resist fatigue failure caused by repeated loads, appropriate materials and plate thicknesses are used when manufacturing press-formed products for automotive parts. Also, to meet the need for lighter vehicles, metal plates, which are the raw material, are becoming stronger and thinner. However, thinner materials result in higher repeated loads on press-formed products manufactured from those materials. Therefore, to achieve both weight reduction (thinner materials) and fatigue strength, it is necessary to reduce the tensile stress remaining on the inside of the bend in the bent parts of press-formed products.

Heat treatment, shot peening, impact treatment with an ultrasonic vibrator, etc. are known as means for removing residual tensile stress from press-formed products. However, these treatments must be performed in a separate process from the press working process, which increases the number of steps and costs required for manufacturing press-formed products. Furthermore, with shot peening, in addition to the difficulty of treating only the bent portions of the press-formed product, there is also the problem of causing surface roughness on the press-formed product. For this reason, it is preferable to reduce the residual tensile stress in press-formed products in the press working process.

For example, Patent Document 1 discloses a technique for performing a so-called ironing process on a metal sheet material in order to reduce tensile residual stress. In Patent Document 1, the clearance between the die and the punch is set to be equal to or greater than the thickness of the metal sheet and equal to or less than 1.02 times the thickness. According to Patent Document 1, by setting the clearance in this manner, the residual stress in the portion of the metal sheet that is in contact with the shoulder of the die before bending begins is smaller than the residual stress in the portion that will finally come into contact with the shoulder, thereby improving the fatigue properties of the press-formed product.

Patent Document 2 discloses a press device that includes a die, a punch, and a pad that faces the top surface of the punch. In this press device, the die descends, and the die and punch shoulder press the metal sheet to form a bent portion in the metal sheet, after which the pad descends toward the top surface of the punch. A recess is formed in the top surface of the punch, and this recess forms a space below the metal sheet, so that when the descending pad presses the metal sheet, material from the metal sheet flows into the recess side of the top surface of the punch. According to Patent Document 2, this inflow of material applies a tensile force to the inside of the bend in the bent portion, reducing the compressive stress and reducing the amount of springback after demolding. This makes it possible to reduce the tensile residual stress on the inside of the bend.

Patent documents 3 to 5 disclose techniques for modifying the shape of the press-formed product itself in order to reduce the amount of springback after demolding. In patent documents 3 to 5, in a press-formed product having a roughly hat-shaped cross section, unevenness or steps are provided on the vertical wall portion.

JP 2002-316215 A International Publication No. 2017/131042 JP 2004-314123 A JP 2006-272378 A JP 2017-196646 A

As described above, when a press-formed product is manufactured by pressing a metal plate, compressive stress remains on the outside of the bend and tensile stress remains on the inside of the bend in the bent portion of the press-formed product. In order to obtain a press-formed product with better fatigue properties, it is necessary to reduce the tensile residual stress on the inside of the bend. Furthermore, from the viewpoint of preventing an increase in the manufacturing steps and manufacturing costs of the press-formed product, it is preferable that the tensile residual stress on the inside of the bend be reduced in the press processing process.

The objective of this disclosure is to provide a new press device that can reduce the tensile residual stress that occurs on the inside of the bend in the bend of a press-molded product during the press working process.

The press apparatus according to the present disclosure is a press apparatus for performing press processing on a material that is a metal plate. The press apparatus includes a punch, a pad, a die, and a holder. The punch has a punch top surface, a punch shoulder, and a punch side surface. The punch shoulder is continuous with the punch top surface. The punch side surface extends downward from the punch shoulder. The pad is arranged above the punch so as to face the punch top surface. The pad is configured to be able to be raised and lowered. The die is arranged above the punch and to the side of the pad. The die is configured to be able to be raised and lowered. The die has a first die shoulder, a die side surface, and a die lower surface. The first die shoulder corresponds to the punch shoulder. The die side surface extends downward from the first die shoulder in correspondence with the punch side surface. The die lower surface is connected to a lower end of the die side surface via a second die shoulder. The holder is arranged to the side of the punch and below the die. The holder is configured to be able to be raised and lowered. The holder has a holder upper surface. The holder upper surface faces the die lower surface. The die lower surface includes an inclined surface. The inclined surface of the die lower surface descends as it moves away from the second die shoulder. The holder upper surface is disposed below the punch top surface at the start of press processing by the press device and above the position of the die lower surface when the die reaches the bottom dead center. The holder upper surface includes an inclined surface. The inclined surface of the holder upper surface descends as it moves away from the punch in correspondence with the inclined surface of the die lower surface.

The press device disclosed herein can reduce the residual tensile stress that occurs on the inside of the bend in the bent portion of a press-molded product during the press processing process.

FIG. 1 is a cross-sectional view showing a schematic configuration of a press device according to a first embodiment. FIG. 2A is a schematic diagram for explaining the method for manufacturing a press-formed product according to the first embodiment. FIG. 2B is a schematic diagram for explaining the method for manufacturing a press-formed product according to the first embodiment. FIG. 2C is a schematic diagram for explaining the method for manufacturing a press-formed product according to the first embodiment. FIG. 2D is a schematic diagram for explaining the method for manufacturing a press-formed product according to the first embodiment. FIG. 2E is a schematic diagram for explaining the manufacturing method of the press-formed product according to the first embodiment. FIG. 2F is a schematic diagram for explaining the manufacturing method of the press-formed product according to the first embodiment. FIG. 2G is a schematic diagram for explaining the manufacturing method of the press-formed product according to the first embodiment. FIG. 2H is a schematic diagram for explaining the manufacturing method of the press-formed product according to the first embodiment. FIG. 2I is a schematic diagram for explaining the manufacturing method of the press-formed product according to the first embodiment. FIG. 2J is a schematic diagram for explaining the method for manufacturing a press-formed product according to the first embodiment. FIG. 3 is a graph showing the relationship between the distance the holder descends while clamping the end of the material and the tensile residual stress on the inside of the bend in the bent portion of the press-molded product for the press apparatus and the manufacturing method of the press-molded product according to the first embodiment. FIG. 4 is a graph showing the relationship between the vertical distance from the R end of the second die shoulder to the end of the material at the completion of forming and the tensile residual stress on the inside of the bend in the bent portion of the press-formed product for the press apparatus and the manufacturing method of the press-formed product according to the first embodiment. FIG. 5 is a graph showing the relationship between the dimensionless number Z of the holder load and the residual stress on the inner side of the bend in the bent portion of the press-formed product for the press apparatus and the method for manufacturing a press-formed product according to the first embodiment. FIG. 6 is a schematic diagram for explaining the method for manufacturing a press-formed product according to the second embodiment. FIG. 7 is a perspective view showing a schematic configuration of the press-formed product according to the first embodiment. FIG. 8 is a graph showing the analysis results of the second embodiment. FIG. 9 is a graph showing the analysis results of the third embodiment. FIG. 10 is a graph showing the stress distribution in the plate thickness direction at the end of forming and the residual stress distribution in the plate thickness direction after springback, with respect to stress generated in the bending circumferential direction in the bent portion of a conventional press-formed product. FIG. 11 is a graph showing the stress distribution in the plate thickness direction at the end of forming and the residual stress distribution in the plate thickness direction after springback, based on the findings of the present inventors, for stress generated in the bending circumferential direction in the bent portion of a press-formed product.

In order to confirm the distribution of stress occurring in the bent portion of the press-formed product, the inventors carried out a CAE analysis of the press working using commercially available software (LS-DYNA ver971 rev7.12, manufactured by ANSYS). The analysis conditions are as follows:
-Material tensile strength: 980MPa
-Material thickness: 3.0 mm
Bend R (inner curvature radius): 4.0 mm
・Shape of press-molded product (after processing): Shape consisting of two vertical walls and a top plate connecting the vertical walls (no flanges)

Figure 10 shows the stress distribution in the plate thickness direction at the end of forming for the stress generated in the circumferential direction in the bent portion of a press-formed product, and the residual stress distribution in the plate thickness direction after springback. Figure 10 shows the stress (stress in the circumferential direction) generated on a line from the R apex (center of R) on the inside of the bend to the R apex on the outside of the bend in the bent portion of the press-formed product. As can be seen from Figure 10, at the end of forming, in the bent portion of the press-formed product, tensile stress occurs on the outside of the bend and compressive stress occurs on the inside of the bend. On the other hand, after springback, in the bent portion of the press-formed product, compressive residual stress occurs on the outside of the bend and tensile residual stress occurs on the inside of the bend.

The inventors noticed that compressive residual stress occurs in the bent part of a press-formed product after springback not only in the region on the outside of the bend but also in the region on the inside of the bend near the neutral axis (center of plate thickness), and came up with the idea of applying tension to the bent part throughout the entire plate thickness during press working. The inventors believed that by applying tension to the bent part, it would be possible to reduce the compressive stress in the region on the inside of the bend at the end of forming, and therefore the tensile residual stress after springback would also be reduced.

When a CAE analysis was performed under the same conditions as above, except that the material was bent under tension, the stress distribution shown in Figure 11 was obtained. As can be seen from Figure 11, when the bent portion of the press-formed product was formed under tension, the compressive stress on the inside of the bend at the end of forming was reduced, and the tensile residual stress on the inside of the bend after springback was reduced and then became a compressive residual stress. Therefore, the inventors investigated the configuration of a new press device that can bend material under tension, and completed the press device according to the embodiment.

The press apparatus according to the embodiment is a press apparatus for performing press processing on a material that is a metal plate. The press apparatus includes a punch, a pad, a die, and a holder. The punch has a punch top surface, a punch shoulder, and a punch side surface. The punch shoulder is continuous with the punch top surface. The punch side surface extends downward from the punch shoulder. The pad is arranged above the punch so as to face the punch top surface. The pad is configured to be able to be raised and lowered. The die is arranged above the punch and to the side of the pad. The die is configured to be able to be raised and lowered. The die has a first die shoulder, a die side surface, and a die lower surface. The first die shoulder corresponds to the punch shoulder. The die side surface extends downward from the first die shoulder in correspondence with the punch side surface. The die lower surface is connected to the lower end of the die side surface via the second die shoulder. The holder is arranged to the side of the punch and below the die. The holder is configured to be able to be raised and lowered. The holder has a holder upper surface. The holder upper surface faces the die lower surface. The die lower surface includes an inclined surface. The inclined surface of the die lower surface descends as it moves away from the second die shoulder. The holder upper surface is disposed below the punch top surface at the start of press processing by the press device and above the position of the die lower surface when the die reaches the bottom dead center. The holder upper surface includes an inclined surface. The inclined surface of the holder upper surface descends as it moves away from the punch in correspondence with the inclined surface of the die lower surface (first configuration).

When performing press processing using a press device with the first configuration, a metal plate material is placed on the top surface of the punch, and the pad and die are lowered to contact the material on the top surface of the punch in that order. That is, first, the pad presses down on the material on the top surface of the punch from above, and then the die is allowed to reach the bottom dead center. When the die reaches the bottom dead center, the material is pressed between the first die shoulder and die side surface and the punch shoulder and punch side surface.

Here, the die underside is connected to the lower end of the die side via the second die shoulder. In other words, the die underside is provided below the first die shoulder and die side corresponding to the punch shoulder and punch side. Meanwhile, the upper surface of the holder provided below the die is located above the position of the die underside when the die reaches the bottom dead center and below the punch top surface. Therefore, when the die descends, the die underside contacts the material before the first die shoulder and die side, and clamps the end of the material together with the holder upper surface. Since the material is held down by the pad in advance, the die underside and the holder upper surface clamp the end of the material, applying tension to the material. After that, when the die descends further, the first die shoulder and die side come into contact with the material in a tensioned state, and a bent portion is formed by the first die shoulder and die side and the punch shoulder and punch side.

In this way, the press device of the first configuration can be used to bend the material while applying tension, thereby producing a press-formed product. This makes it possible to reduce the residual tensile stress that occurs on the inside of the bend in the bent portion of the press-formed product.

When a press-molded product is manufactured using the press device of the first configuration, the end of the material clamped between the lower surface of the die and the upper surface of the holder is pulled out along the inclined surfaces provided on the lower surface of the die and the upper surface of the holder as the die and the holder are lowered. In other words, as the molding of the material by the punch and die progresses, the surface area of the material clamped between the lower surface of the die and the upper surface of the holder decreases. As a result, the load per unit area acting on the material from the lower surface of the die and the upper surface of the holder gradually increases, so the tension applied to the material can be increased in the later stages of molding.

It is preferable that the angle that the inclined surface of the holder top surface makes with the horizontal plane is 10° or more and 55° or less (second configuration).

When the die descends and the lower surface of the die clamps the end of the material together with the upper surface of the holder, a load perpendicular to the inclined surface of the holder is applied to the upper surface of the holder. If the inclined surface of the holder is steep, the horizontal component of the load on the upper surface of the holder increases. Furthermore, the steeper the inclined surface of the holder is, the smaller the width (horizontal dimension) of the holder becomes, and the strength of the holder decreases. Therefore, in the second configuration, the angle that the inclined surface of the holder top surface makes with the horizontal plane is limited to 55° or less. This makes it possible to suppress the horizontal component of the load on the upper surface of the holder and ensure the width of the holder. This makes it possible to make the holder less susceptible to damage, etc.

During press working, the end of the material is clamped between the lower surface of the die and the upper surface of the holder, and the end of the material is bent along the inclined surfaces of the lower surface of the die and the upper surface of the holder, as well as the second die shoulder adjacent to the lower surface of the die. If the inclined surface of the lower surface of the die and the corresponding inclined surface of the upper surface of the holder are too gentle, the material will bend more, affecting the dimensional accuracy of the press-formed product manufactured from the material. In contrast, in the second configuration, the angle between the inclined surface of the upper surface of the holder and the horizontal plane is ensured to be 10° or more. This makes it possible to reduce the bending tendency of the material caused by clamping between the lower surface of the die and the upper surface of the holder, and to manufacture press-formed products with high accuracy.

It is preferable that the lifting stroke of the holder is 5.0 mm or more and 25.0 mm or less (third configuration).

In the above press device, the holder is lowered together with the die with the end of the material being clamped by the lower surface of the die and the upper surface of the holder. The longer the time that the end of the material is clamped by the lower surface of the die and the upper surface of the holder, the greater the tension on the material. However, if the tension on the material is excessive, cracks may occur in the material being pressed or in the press-formed product being manufactured, for example, at the punch shoulder or the punch side. On the other hand, if the tension on the material is too small, the tensile residual stress generated on the inside of the bend in the bent part of the press-formed product to be manufactured may not be effectively reduced. Therefore, in the third configuration, the holder's lifting stroke is set to 5.0 mm and 25.0 mm or less to appropriately ensure the time that the end of the material is clamped by the lower surface of the die and the upper surface of the holder. This makes it possible to suppress the occurrence of cracks due to the application of tension in the material or the press-formed product, and also to effectively exert the effect of reducing tensile residual stress.

It is preferable that the vertical load applied to the upper surface of the holder is set so as to satisfy the following conditional formula, where F [kN] is the tensile strength of the material, TS [MPa] is the sheet thickness of the material, t [mm] is the longitudinal length of the portion of the material that is clamped between the lower surface of the die and the upper surface of the holder, L [mm] (fourth configuration).
0.04≦Z≦0.45
where Z = F / (TS x t x L)

According to the fourth configuration, the vertical load applied to the upper surface of the holder, i.e., the load in the same direction as the lifting stroke of the holder, is set to satisfy a predetermined conditional formula. This allows the load from the lower surface of the die to be applied to the upper surface of the holder appropriately, without excess or deficiency. This prevents the die from succumbing to the reaction force of the material, making it difficult to clamp the end of the material between the lower surface of the die and the upper surface of the holder, or increasing the tension applied, which can cause cracks in the material or press-formed product, for example, at the punch shoulder or side of the punch.

It is preferable that the second die shoulder, when viewed in cross section of the die, has an arc shape with a radius of curvature that is greater than or equal to the thickness of the material and less than or equal to four times the thickness (fifth configuration).

During press working, the material is clamped between the die lower surface and the holder upper surface, whereas the material is not clamped between the second die shoulder adjacent to the die lower surface and the holder upper surface. That is, the portion of the holder upper surface facing the die lower surface is in contact with the material, while the portion facing the second die shoulder is not in contact with the material. In the fifth configuration, the radius of curvature of the second die shoulder is limited to four times the plate thickness of the material, so that the contact area of the holder upper surface with the material can be secured. As a result, sufficient tension can be applied to the material during press working, and the tensile stress remaining on the inside of the bend in the bent portion of the press-formed product can be effectively reduced.

In addition, in the fifth configuration, a radius of curvature equal to or greater than the thickness of the material is ensured at the second die shoulder. Therefore, when the end of the material is clamped between the lower surface of the die and the upper surface of the holder, the material does not deform unnecessarily significantly at the position of the second die shoulder. This makes it possible to prevent the material from cracking at the position of the second die shoulder during press working, or to prevent bending habits at the second die shoulder from remaining in the press-formed product.

The press device may further include a distance block. The distance block is provided on the upper surface of the holder. The distance block can adjust the distance between the lower surface of the die that holds the material and the upper surface of the holder (sixth configuration).

According to the sixth configuration, a distance block is provided on the upper surface of the holder. This distance block prevents the upper surface of the holder from colliding with the lower surface of the die when the die and holder are lowered and the end of the material is pulled out from between the lower surface of the die and the upper surface of the holder.

The manufacturing method according to the embodiment is a method for manufacturing a press-formed product using the above-mentioned press device. This manufacturing method includes the steps of preparing a material, which is a metal plate; placing the material on the top surface of the punch and lowering the pad and die; clamping an end of the material between the lower surface of the die and the upper surface of the holder after the pad presses the material on the top surface of the punch and before the die reaches the bottom dead center; and pressing the material with the first die shoulder and die side surface and the punch shoulder and punch side surface by allowing the die to reach the bottom dead center. The end of the material is pulled out along the inclined surface from between the lower surface of the die and the upper surface of the holder as the die descends, and is positioned between the die side surface and the punch side surface when the die reaches the bottom dead center (seventh configuration).

In the above manufacturing method, when the die reaches the bottom dead center, it is preferable that the upper surface of the holder is located outside the pressed material in the width direction of the press device (8th configuration).

According to the eighth configuration, when the die reaches the bottom dead point, the upper surface of the holder is positioned outside the pressed material in the width direction of the press device. In other words, when the die reaches the bottom dead point, the material falls out from between the lower surface of the die and the upper surface of the holder, and the material is naturally released from the clamping between the upper surface of the die and the upper surface of the holder. Therefore, there is no need to provide the press device with a complex locking mechanism or the like for releasing the clamping of the material. This allows the press device to have a simple configuration.

In the above manufacturing method, it is preferable that the prepared material is concavely bent at a position corresponding to the second die shoulder (ninth configuration).

According to the ninth configuration, the material before press processing is bent concavely in advance at a position corresponding to the second die shoulder. This makes it easier for the material to be clamped between the lower surface of the die, which is continuous with the second die shoulder, and the upper surface of the holder during press processing. This allows tension to be applied to the material more effectively.

In the above manufacturing method, it is preferable that the prepared material is convexly bent at a position corresponding to the punch shoulder (tenth configuration).

In the above manufacturing method, the punch can have a groove or step on the top surface of the punch. In this case, it is preferable that the prepared material has a groove or step formed therein that corresponds to the groove or step on the top surface of the punch (eleventh configuration).

According to the tenth or eleventh configuration, the material is preformed to fit the punch. This allows the material on the punch to be more securely pressed down by the pad during press processing. Therefore, when the end of the material is clamped between the lower surface of the die and the upper surface of the holder, tension can be applied to the material more effectively.

In the above manufacturing method, the work hardening index of the material is preferably 0.04 or less (12th configuration).

According to the twelfth configuration, the work hardening index of the material is sufficiently small, at 0.04 or less, so that the material is less likely to work harden during press working. In this case, the stress generated in the material during press working is reduced, and as a result, the stress due to springback is also reduced. Therefore, the residual stress in the press-formed product, which is the sum of the stress during press working and the stress due to springback, can be further reduced.

Embodiments of the present disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components are given the same reference numerals, and the same description will not be repeated.

First Embodiment
[Configuration of the Pressing Device]
1 is a cross-sectional view showing a schematic configuration of a press machine 100 according to this embodiment. The press machine 100 is used to perform press working on a metal plate material to manufacture press-formed products, such as automobile suspension parts. Examples of automobile suspension parts include arm parts such as upper arms, lower arms, and trail links, as well as suspension members and torsion beams.

With reference to FIG. 1, the press device 100 includes a punch 10, dies 20L and 20R, a pad 30, and holders 40L and 40R. The press device 100 further includes a punch holder 50, a bed 60, a die holder 70, a slide 80, and a cushion 90. The punch 10 and the dies 20L and 20R extend in a direction intersecting the plane of FIG. 1. Hereinafter, for the sake of convenience, with respect to the press device 100 and its components, the direction in which the punch 10 and the dies 20L and 20R extend is referred to as the longitudinal direction, and the direction in which the punch 10 and the dies 20L and 20R approach and move away from each other (the vertical direction on the plane of FIG. 1) is referred to as the vertical direction (vertical direction). The direction perpendicular to the plane consisting of the longitudinal direction and the vertical direction, i.e., the direction crossing the punch 10 and the dies 20L and 20R, is referred to as the left-right direction or width direction. Furthermore, a cross section of the press device 100 and its components cut along a plane perpendicular to the longitudinal direction is called a transverse section.

The punch 10 is supported by a bed 60, which is part of the main body of the press device 100, via a punch holder 50. That is, the punch holder 50 is fixed to the upper surface of the bed 60, and the punch 10 is placed on the punch holder 50. A plate-shaped spacer may be inserted between the bed 60 and the punch holder 50 to adjust the position of the punch 10 in the vertical direction.

The punch 10 has a punch top surface 11, left and right punch shoulders 12L, 12R, and left and right punch side surfaces 13L, 13R. In this embodiment, a groove portion 111 extending in the longitudinal direction is formed in the punch top surface 11. The groove portion 111 may extend in the longitudinal direction over the entire punch top surface 11, or may be provided in a part of the punch top surface 11. A punch shoulder 12L and a punch side surface 13L are provided on the left side of the punch top surface 11. A punch shoulder 12R and a punch side surface 13R are provided on the right side of the punch top surface 11.

The punch shoulders 12L and 12R are provided so as to be continuous with the punch top surface 11. More specifically, the punch shoulder 12L is continuous with the left edge of the punch top surface 11, and the punch shoulder 12R is continuous with the right edge of the punch top surface 11. Each of the punch shoulders 12L and 12R is substantially arc-shaped in the cross-sectional view of the punch 10. The punch shoulders 12L and 12R are ridges (corners) between the punch top surface 11 and the punch side surfaces 13L and 13R, respectively. Hereinafter, the term "substantially arc-shaped" includes not only curves that constitute a part of a perfect circle, but also smooth curves such as elliptical curves and spline curves. In addition, the radius of curvature in the case of a smooth curve other than a curve that constitutes a part of a perfect circle is defined as the radius of a circle that passes through the three points of both ends of the curve and the midpoint of the curve when cut in cross section.

The punch side surface 13L extends downward from the left punch shoulder 12L in a cross-sectional view of the punch 10. The punch side surface 13R extends downward from the right punch shoulder 12R in a cross-sectional view of the punch 10. Each of the punch side surfaces 13L, 13R is inclined with respect to a vertical plane parallel to the longitudinal direction so that the lower end is positioned outward in the width direction relative to the upper end. The angles (acute angles) θ _L , θ _R that the punch side surfaces 13L, 13R make with the vertical plane parallel to the longitudinal direction may be the same or different. The angle θ _L of the punch side surface 13L and the angle θ _R of the punch side surface 13R are preferably 20° or less. The angles (acute angles) θ _L and θ _R that the punch side surfaces 13L and 13R make with a vertical plane parallel to the longitudinal direction refer to the angles (acute angles) that the punch side surfaces 13L and 13R make with the vertical direction in a cross section cut by a plane consisting of the up-down and width directions.

The dies 20L and 20R are disposed above the punch 10 and to the side of the pad 30, which will be described later. The die 20L is disposed above the punch 10, to the left of the pad 30. The die 20R is disposed above the punch 10, to the right of the pad 30. The dies 20L and 20R are configured to be capable of being raised and lowered.

Each of the left and right dies 20L, 20R is attached to the slide 80 via the die holder 70. That is, the die holder 70 is fixed to the underside of the slide 80, and the dies 20L, 20R are fixed to the underside of the die holder 70. A plate-shaped spacer may be inserted between the slide 80 and the die holder 70 to adjust the positions of the dies 20L, 20R in the vertical direction. The slide 80 is configured to be able to rise and fall relative to the punch 10, for example, by a mechanical or hydraulic mechanism (not shown) provided in the press device 100. As the slide 80 rises and falls, the dies 20L, 20R also rise and fall relative to the punch 10.

The left die 20L has a first die shoulder 21L, a die side surface 22L, a second die shoulder 23L, and a die bottom surface 24L. The first die shoulder 21L is provided to correspond to the left punch shoulder 12L. That is, the first die shoulder 21L has a position and shape that allows the material to be pressed between the punch shoulder 12L and the die 20L when the die 20L descends to the bottom dead center. The first die shoulder 21L, like the punch shoulder 12L, is substantially arc-shaped in the cross section of the die 20L. The radii of curvature of the first die shoulder 21L and the punch shoulder 12L can be appropriately determined according to the desired press-molded product.

The die side surface 22L extends downward from the first die shoulder 21L in a cross-sectional view of the die 20L. The die side surface 22L is provided corresponding to the left punch side surface 13L. That is, the die side surface 22L has a position and shape that allows the die side surface 22L to press the material between the die side surface 22L and the punch side surface 13L when the die 20L descends to the bottom dead center. The die side surface 22L, like the punch side surface 13L, is inclined with respect to a vertical plane parallel to the longitudinal direction so that the lower end portion is located outward in the width direction from the upper end portion. The angle (acute angle side) that the die side surface 22L makes with the vertical plane parallel to the longitudinal direction is substantially equal to the angle θ _L that the punch side surface 13L makes with the vertical plane parallel to the longitudinal direction.

The second die shoulder 23L is provided continuous with the lower end of the die side surface 22L. The second die shoulder 23L is substantially arc-shaped in a cross-sectional view of the die 20L. The second die shoulder 23L preferably has a radius of curvature R _L that is equal to or greater than the thickness of the material to be pressed. The radius of curvature R _L of the second die shoulder 23L is preferably four times or less than the thickness of the material.

The second die shoulder 23L is a ridge (corner) between the die side surface 22L and the die bottom surface 24L. The die bottom surface 24L is connected to the lower end of the die side surface 22L via the second die shoulder 23L. The die bottom surface 24L includes an inclined surface 241L that is inclined with respect to the horizontal plane. The inclined surface 241L is connected to the second die shoulder 23L and descends as it moves away from the second die shoulder 23L. In other words, the inclined surface 241L descends from the second die shoulder 23L toward the outside in the width direction. The inclined surface 241L is an inclined surface whose outer end in the width direction is located lower than its inner end in the width direction. In this embodiment, the entire die bottom surface 24L is the inclined surface 241L.

The right die 20R has a first die shoulder 21R, a die side surface 22R, a second die shoulder 23R, and a die bottom surface 24R, similar to the left die 20L. However, the shapes of the die shoulders 21R, 23R, die side surface 22R, and die bottom surface 24R of the right die 20R do not have to be the same as the shapes of the die shoulders 21L, 23R, die side surface 22R, and die bottom surface 24L of the left die 20L. In other words, in the press device 100, the left and right dies 20L, 20R may have symmetrical shapes, or may have asymmetrical shapes.

In the right die 20R, the first die shoulder 21R is provided to correspond to the right punch shoulder 12R. That is, the first die shoulder 21R has a position and shape that allows the material to be pressed between the first die shoulder 21R and the punch shoulder 12R when the die 20R descends to the bottom dead center. The first die shoulder 21R, like the punch shoulder 12R, is substantially arc-shaped in a cross-sectional view of the die 20R. The radii of curvature of the first die shoulder 21R and the punch shoulder 12R can be appropriately determined according to the desired press-molded product.

The die side surface 22R extends downward from the first die shoulder 21R in a cross-sectional view of the die 20R. The die side surface 22R is provided corresponding to the right punch side surface 13R. That is, the die side surface 22R has a position and shape that allows the die side surface 22R to press the material between the die side surface 22R and the punch side surface 13R when the die 20R descends to the bottom dead center. The die side surface 22R is inclined with respect to a vertical plane parallel to the longitudinal direction so that the lower end is located outward in the width direction from the upper end, similar to the punch side surface 13R. The angle (acute angle side) that the die side surface 22R makes with the vertical plane parallel to the longitudinal direction is substantially equal to the angle _θR that the punch side surface 13R makes with the vertical plane parallel to the longitudinal direction.

The second die shoulder 23R is provided continuous with the lower end of the die side surface 22R. The second die shoulder 23R is substantially arc-shaped in a cross-sectional view of the die 20R. The second die shoulder 23R preferably has a radius of curvature R _R that is equal to or greater than the thickness of the material to be pressed. The radius of curvature R _R of the second die shoulder 23R is preferably four times or less than the thickness of the material.

The second die shoulder 23R is a ridge (corner) between the die side surface 22R and the die bottom surface 24R. The die bottom surface 24R is connected to the lower end of the die side surface 22R via the second die shoulder 23R. The die bottom surface 24R includes an inclined surface 241R that is inclined with respect to the horizontal plane. The inclined surface 241R is connected to the second die shoulder 23R and descends as it moves away from the second die shoulder 23R. In other words, the inclined surface 241R descends from the second die shoulder 23R toward the outside in the width direction. The inclined surface 241R is an inclined surface whose outer end in the width direction is located lower than its inner end in the width direction. In this embodiment, the entire die bottom surface 24R is the inclined surface 241R.

The pad 30 is disposed above the punch 10 and between the left and right dies 20L, 20R. The pad 30 is disposed so as to face the punch top surface 11. More specifically, the lower surface of the body 31 of the pad 30 faces the punch top surface 11. The lower surface of the pad body 31 has a shape corresponding to the punch top surface 11. In this embodiment, the lower surface of the pad body 31 is provided with a convex portion 311 extending in the longitudinal direction, which corresponds to the groove portion 111 of the punch top surface 11.

The pad 30 is configured to be movable up and down. The pad 30 is attached to the slide 80 via the die holder 70, so that it can be raised and lowered relative to the punch 10. The pad body 31 is elastically supported on the die holder 70 by an expandable support member 32. The support member 32 is configured, for example, by a spring or a fluid pressure cylinder. When the support member 32 is in an expanded state, the lower surface of the pad body 31 is located at least below the first die shoulders 21L, 21R and the die side surfaces 22L, 22R.

The holders 40L and 40R are disposed to the side of the punch 10 and below the dies 20L and 20R. More specifically, the holder 40L is disposed to the left of the punch 10 and directly below the die lower surface 24L, and the holder 40R is disposed to the right of the punch 10 and directly below the die lower surface 24R.

The holders 40L and 40R are configured to be able to rise and fall. It is preferable that the rise and fall strokes of the holders 40L and 40R are set to 5.0 mm or more and 25.0 mm or less, respectively. The holders 40L and 40R are supported by the cushions 90.

The cushion 90 has left and right cushion pins 91L, 91R and a cushion body 92. The cushion pins 91L, 91R extend vertically through the bed 60 and are attached to the undersides of the holders 40L, 40R. The cushion body 92 is disposed, for example, below the bed 60. The cushion body 92 has a mechanism (for example, a hydraulic mechanism) for raising and lowering the cushion pins 91L, 91R, similar to the die cushion of a typical press device. As the cushion pins 91L, 91R rise and fall, the holders 40L, 40R rise and fall relative to the bed 60. The holders 40L, 40R generate pressure against the corresponding dies 20L, 20R by the cushion body 92 and the cushion pins 91L, 91R.

The left holder 40L has a holder upper surface 41L. The holder upper surface 41L faces the left die lower surface 24L. The holder upper surface 41L includes an inclined surface 411L that is inclined with respect to the horizontal plane, corresponding to the inclined surface 241L of the die lower surface 24L. The inclined surface 411L descends as it moves away from the punch 10. In other words, the inclined surface 411L is an inclined surface whose outer end in the width direction is located lower than its inner end in the width direction. In this embodiment, the entire holder upper surface 41L is the inclined surface 411L.

In this embodiment, the holder upper surface 41L is positioned outboard of the second die shoulder 23L in the width direction. That is, the holder upper surface 41L faces the die lower surface 24L, but does not face the second die shoulder 23L. However, a portion of the holder upper surface 41L may be positioned opposite the second die shoulder 23L. More specifically, a portion of the inclined surface 411L may be positioned opposite the second die shoulder 23L.

In the width direction, it is preferable that the position of the end of the holder upper surface 41L on the punch 10 side substantially coincides with the position of the end of the die lower surface 24L on the second die shoulder 23L side. However, since there may be processing errors or misalignment of the die 20L and holder 40L in the width direction during assembly, even if it is desired to coincide the position of the end of the holder upper surface 41L on the punch 10 side with the position of the end of the die lower surface 24L on the second die shoulder 23L side in the width direction, a part of the holder upper surface 41L can be positioned opposite the second die shoulder 23L.

The angle (acute angle) ψ _L of the inclined surface 411L with respect to the horizontal plane is preferably 10° or more. The angle ψ _L of the inclined surface 411L is preferably 55° or less. The angle ψ _L of the inclined surface 411L is substantially the same as the angle that the inclined surface 241L of the die lower surface 24L makes with respect to the horizontal plane.

The right holder 40R has a holder upper surface 41R. The holder upper surface 41R faces the right die lower surface 24R. The holder upper surface 41R includes an inclined surface 411R that is inclined with respect to the horizontal plane, corresponding to the inclined surface 241R of the die lower surface 24R. The inclined surface 411R descends as it moves away from the punch 10. In other words, the inclined surface 411R is an inclined surface whose outer end in the width direction is located lower than its inner end in the width direction. In this embodiment, the entire holder upper surface 41R is the inclined surface 411R.

In this embodiment, the holder upper surface 41R is positioned outboard of the second die shoulder 23R in the width direction. That is, the holder upper surface 41R faces the die lower surface 24R, but does not face the second die shoulder 23R. However, a portion of the holder upper surface 41R may be positioned opposite the second die shoulder 23R. More specifically, a portion of the inclined surface 411R may be positioned opposite the second die shoulder 23R.

In the width direction, it is preferable that the position of the end of the holder upper surface 41R on the punch 10 side substantially coincides with the position of the end of the die lower surface 24R on the second die shoulder 23R side. However, since there may be processing errors or misalignment of the die 20R and holder 40R in the width direction during assembly, even if it is desired to coincide the position of the end of the holder upper surface 41R on the punch 10 side with the position of the end of the die lower surface 24R on the second die shoulder 23R side in the width direction, a part of the holder upper surface 41R can be positioned opposite the second die shoulder 23R.

The angle (acute angle side) ψ _R of the inclined surface 411R with respect to the horizontal plane is preferably 10° or more. The angle ψ _R of the inclined surface 411R is preferably 55° or less. The angle ψ _R of the inclined surface 411R is substantially the same as the angle that the inclined surface 241R of the die lower surface 24R makes with respect to the horizontal plane. However, the angles ψ _L and ψ _R of the inclined surfaces 411L and 411R of the left and right holder upper surfaces 41L and 41R do not necessarily have to be the same.

In this embodiment, the press device 100 is equipped with distance blocks 42L and 42R. The distance block 42L is provided on the left holder upper surface 41L and adjusts the distance between the die lower surface 24L and the holder upper surface 41L. The distance block 42L is disposed on the holder upper surface 41L toward the outer edge in the width direction. The distance block 42R is provided on the right holder upper surface 41R and adjusts the distance between the die lower surface 24R and the holder upper surface 41R. The distance block 42R is disposed on the holder upper surface 41R toward the outer edge in the width direction. The thickness of each of the distance blocks 42L and 42R is preferably 0.9 times or more and 1.1 times or less than the plate thickness of the material to be subjected to the press processing.

It is preferable that multiple distance blocks 42L are arranged in the longitudinal direction at a well-balanced pitch on the left holder top surface 41L. Similarly, it is preferable that multiple distance blocks 42R are arranged in the longitudinal direction at a well-balanced pitch on the right holder top surface 41R. In this case, the longitudinal length of each distance block 42L, 42R can be, for example, approximately 20.0 mm to 60.0 mm.

The material of the distance blocks 42L, 42R preferably has a lower strength than the material of the dies 20L, 20R and the holders 40L, 40R. Therefore, in this embodiment, the distance blocks 42L, 42R are separate members from the holders 40L, 40R. However, the distance blocks 42L, 42R may be integrally formed with the holders 40L, 40R.

[Method of manufacturing press-molded products]
A method for manufacturing a press-formed product using the press device 100 configured as described above will be described below with reference to Figures 2A to 2J. Figures 2A to 2J are schematic diagrams for explaining the method for manufacturing a press-formed product. The method for manufacturing a press-formed product according to this embodiment includes a step of preparing a material and a step of performing press working on the material.

(Preparation process)
2A , when manufacturing a press-formed product, first, a material M is prepared. The material M is a metal plate, typically a steel plate. The material M has a plate thickness t of, for example, 2.0 mm or more and 8.0 mm or less. The material M has a tensile strength of, for example, 270 MPa or more and 1180 MPa or less. When the press-formed product to be manufactured is an arm part among the suspension parts of an automobile, the tensile strength of the material M is usually 590 MPa or more.

The work hardening index of material M is preferably 0.04 or less. Furthermore, material M is preferably such that the ratio of yield stress YS to tensile strength TS (yield ratio) YR=YS/TS is 0.95 or more. The work hardening index is an index showing how easily material M is work hardened, and the larger the value, the more easily the material is work hardened. The work hardening index (n value) is defined in JIS G 0202:2013, and is measured, for example, by the method specified in JIS Z 2253:2011. The yield ratio YR is a value that is inversely correlated with the n value, so it decreases as the n value increases.

(Press processing process)
Next, the prepared material M is subjected to press working by the press device 100. The press working process includes a step (a) of lowering the dies 20L, 20R and the pad 30, a step (b) of clamping the material M between the dies 20L, 20R and the holders 40L, 40R, and a step (c) of pressing the material M between the dies 20L, 20R, the pad 30, and the punch 10. The press working process will be described in more detail below.

Step (a)
2B , when press working by the press device 100 is started, the dies 20L, 20R and the pad 30 are located at the top dead center. At the start of press working, the holder upper surfaces 41L, 41R are located below the punch top surface 11. Also, at the start of press working, the holder upper surfaces 41L, 41R are located above the positions of the die lower surfaces 24L, 24R when the dies 20L, 20R reach the bottom dead center.

With the dies 20L, 20R and pad 30 positioned at the top dead center, the material M is placed on the punch top surface 11. At this point, the holder upper surfaces 41L, 41R are positioned below the punch top surface 11 and therefore do not come into contact with the material M on the punch top surface 11. After the material M is placed on the punch top surface 11, the slide 80 is lowered, causing the dies 20L, 20R and pad 30 attached to the slide 80 to lower toward the material M. The pad 30 comes into contact with the material M on the punch top surface 11 before the dies 20L, 20R.

Referring to FIG. 2C, the pad 30 presses down the material M on the punch top surface 11. More specifically, the pad body 31, which is supported by the slide 80 via the support member 32, presses down the central portion of the material M on the punch top surface 11 in the width direction from above.

Referring to FIG. 2D, after the material M is pressed down by the pad body 31, the slide 80 further descends. At this time, the support member 32 contracts, causing the dies 20L, 20R to descend relative to the pad body 31 that is pressing down on the material M. As a result, both ends (left and right ends) of the material M in the width direction are pressed down by the die lower surfaces 24L, 24R, and the material M begins to bend along the punch shoulders 12L, 12R.

Step (b)
2E, when the dies 20L and 20R are further lowered, the left and right ends of the material M come into contact with the holder upper surfaces 41L and 41R. The left end of the material M is sandwiched between the holder upper surface 41L and the die lower surface 24L after the pad 30 presses the material M on the punch top surface 11 and before the die 20L reaches the bottom dead point. The right end of the material M is sandwiched between the die lower surface 24R and the holder upper surface 41R after the pad 30 presses the material M on the punch top surface 11 and before the die 20R reaches the bottom dead point. The left and right ends of the material M are sandwiched between the inclined surfaces 241L and 241R of the die lower surfaces 24L and 24R and the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R.

The holders 40L, 40R can generate pressure on the dies 20L, 20R and material M by the cushion 90. The vertical load applied from the dies 20L, 20R and material M to the holder top surfaces 41L, 41R can be set by adjusting the pressure of the holders 40L, 40R by the cushion 90.

The load F _L [kN] applied to the holder upper surface 41L in the vertical direction (the same direction as the lifting and lowering stroke of the holder 40L) is preferably set so as to satisfy the following conditional formula (1). The load F _R [kN] applied to the holder upper surface 41R in the vertical direction (the same direction as the lifting and lowering stroke of the holder 40R) is preferably set so as to satisfy the following conditional formula (2). The loads F _L and F _R may vary within a range that satisfies the conditional formulas (1) and (2) during the press working process.
0.04≦ _ZL ≦0.45...(1)
Here, _ZL = F _L / (TS x t x L _L ).
0.04≦ _ZR ≦0.45...(2)
Here, _ZR = _FR /(TS×t× _LR )

In the conditional formulas (1) and (2), TS is the tensile strength [MPa] of the material M, t is the sheet thickness [mm] of the material M, L _L is the longitudinal length [mm] of the portion of the material M that is sandwiched between the die lower surface 24L and the holder upper surface 41L, and L _R is the longitudinal length [mm] of the portion of the material M that is sandwiched between the die lower surface 24R and the holder upper surface 41R. The length L _L is the length of the left end of the material M in the longitudinal direction at the time when the die lower surface 24L and the holder upper surface 41L start to sandwich the left end of the material M. The length L _R is the length of the right end of the material M in the longitudinal direction at the time when the die lower surface 24R and the holder upper surface 41R start to sandwich the right end of the material M.

Referring to FIG. 2F, after the left and right ends of the material M are clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, the dies 20L, 20R are further lowered. This also causes the holders 40L, 40R and the cushion pins 91L, 91R to lower. As the die 20L and holder 40L lower, the left end of the material M is pulled out from between the die lower surface 24L and the holder upper surface 41L along the inclined surfaces 241L, 411L toward the punch 10. Similarly, as the die 20R and holder 40R move, the right end of the material M is pulled out from between the die lower surface 24R and the holder upper surface 41R along the inclined surfaces 241R, 411R toward the punch 10.

Step (c)
2G, the dies 20L, 20R are finally caused to reach the bottom dead center, and the material M is pressed by the first die shoulders 21L, 21R and die sides 22L, 22R, and the punch shoulders 12L, 12R and punch sides 13L, 13R. The material M is also pressed by the pad body 31 and the punch top surface 11. When a part of the dies 20L, 20R corresponds to a part of the punch top surface 11, the material M is also pressed between the dies 20L, 20R and the punch top surface 11. In this way, a press-formed product is produced from the material M.

When the die 20L reaches the bottom dead point, the left end of the material M is completely pulled out from between the die lower surface 24L and the holder upper surface 41L, and is located between the die side surface 22L and the punch side surface 13L. When the die 20R reaches the bottom dead point, the right end of the material M is completely pulled out from between the die lower surface 24R and the holder upper surface 41R, and is located between the die side surface 22R and the punch side surface 13R. In the example of this embodiment, when the dies 20L and 20R reach the bottom dead point, the holder upper surfaces 41L and 41R are positioned outboard of the pressed material M in the width direction.

Referring to Figures 2H and 2I, the distance D1 by which the holder 40L descends when the die lower surface 24L and the holder upper surface 41L are clamping the left end of the material M is preferably 2.0 mm or more and 10.0 mm or less. After the left end of the material M is completely pulled out from between the die lower surface 24L and the holder upper surface 41L, it is preferable to quickly make the die 20L and the holder 40L reach the bottom dead center. When the die 20L and the holder 40L reach the bottom dead center, the vertical distance D2 between the bottom end of the material M between the die side surface 22L and the punch side surface 13L and the R end (upper side) of the second die shoulder 23L is, for example, 4.5 mm or less. Most preferably, the distance D2 is substantially 0 mm.

Although not shown in the figure, it is preferable that the distance that the holder 40R descends when the right end of the material M is clamped between the die lower surface 24R and the holder upper surface 41R is 2.0 mm or more and 10.0 mm or less. After the right end of the material M is completely pulled out from between the die lower surface 24R and the holder upper surface 41R, it is preferable that the die 20R and the holder 40R quickly reach the bottom dead center. When the die 20L and the holder 40L reach the bottom dead center, the vertical distance between the bottom end of the material M between the die side surface 22R and the punch side surface 13R and the R end (upper side) of the second die shoulder 23R is, for example, 4.5 mm or less. Most preferably, the distance is substantially 0 mm.

Figure 2J is a perspective view partially showing the manufactured press-formed product 200. The press-formed product 200 has a top plate 201, left and right vertical walls 202L, 202R, and left and right bent portions 203L, 203R. A groove portion 204 extending in the longitudinal direction is formed in the top plate 201. The upper end of the left vertical wall 202L is connected to the left edge of the top plate 201 via the left bent portion 203L. The upper end of the right vertical wall 202R is connected to the right edge of the top plate 201 via the right bent portion 203R. The lower ends of the vertical walls 202L, 202R are each free ends. Each of the bent portions 203L, 203R is substantially arc-shaped in a cross-sectional view of the press-formed product 200.

[effect]
In the press device 100 according to this embodiment, at the start of press working, the holder upper surfaces 41L, 41R are disposed below the punch top surface 11 and above the position of the die lower surfaces 24L, 24R when the dies 20L, 20R reach the bottom dead center. Therefore, before the dies 20L, 20R reach the bottom dead center and the first die shoulders 21L, 21R press the material M, the die lower surfaces 24L, 24R come into contact with the material M and, together with the holder upper surfaces 41L, 41R, clamp the left and right ends of the material M pressed by the pads 30. This allows tension to be applied to the portions of the material M that will become the bent portions 203L, 203R of the press-formed product 200. In this way, by performing forming while applying tension to the material M, it is possible to reduce the tensile residual stress that occurs on the inside of the bend in the bent portions 203L, 203R of the press-formed product 200 manufactured from the material M.

In this embodiment, the left and right ends of the material M sandwiched between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R are pulled out along the inclined surfaces 241L, 241R, 411L, 411R as the die 20L, 20R and the holder 40L, 40R descend. Therefore, as the die 20L, 20R approaches the bottom dead center, the surface area of the part of the material M sandwiched between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R decreases, and the load per unit area acting on the material M increases. Therefore, the tension applied to the material M can be gradually increased toward the end of molding. This effectively reduces the tensile residual stress generated on the inside of the bend in the bent parts 203L, 203R of the press-molded product 200.

For example, when a press-formed product is manufactured from a metal plate material by drawing, the widthwise end of the material is clamped between a die having a horizontal lower surface and a holder having a horizontal upper surface from the beginning of the forming process. In this case, tension is applied to the material from the beginning of the forming process. However, in drawing, the end of the material is still clamped between the die and the holder even at the end of the forming process. Therefore, the press-formed product manufactured has a shape having a top plate, vertical walls connected to the left and right side edges of the top plate, and flanges protruding outward in the width direction from the lower ends of each vertical wall, that is, an approximately hat shape in cross section. In order to manufacture a press-formed product without flanges using drawing, it is necessary to first form the material into an approximately hat shape using a punch, a die, and a holder, and then trim the formed material with the left and right vertical walls. However, since the trimming of the material is generally performed parallel to the plate thickness direction, for example, when trimming left and right vertical walls that are inclined relative to a vertical plane, a trimming blade of a pull-out type equipped with a complex cam mechanism or the like is required. In this case, problems arise such as an increase in the number of steps required to design the trim blade and an increase in the manufacturing costs of the trim blade. There is also the problem that trimming the vertical wall reduces the material yield.

On the other hand, in this embodiment, the left and right ends of the material M are clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R not from the beginning of the forming process, but from the middle of the forming process. However, when the dies 20L, 20R reach the bottom dead center, the left and right ends of the material M are completely pulled out from between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, and pressed between the die side surfaces 22L, 22R and the punch side surfaces 13L, 13R. Therefore, the material M can be bent while applying tension, and a press-formed product 200 without a flange can be directly formed. Therefore, there is no need for the design labor and manufacturing costs of trim blades, as in the case of using drawing, and a decrease in material yield can be prevented.

In this embodiment, the holder upper surfaces 41L, 41R are positioned outward in the width direction from the material M after pressing when the dies 20L, 20R reach the bottom dead center. In other words, the left and right ends of the material M are naturally pulled out from between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R when the dies 20L, 20R reach the bottom dead center. Therefore, there is no need to provide the press device 100 with a complex locking mechanism or the like for releasing the clamping of the material M by the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R. Therefore, the press device 100 can be configured simply.

In general press working, misalignment of the material can be a problem. For example, if the heights of the left and right punch sides are different, the die with the larger height of the punch side will come into contact with the material first, which is likely to cause misalignment of the material. When positioning the material using a pilot pin or the like, if the material is misaligned, the peripheral portion of the hole for the pilot pin in the material will be deformed. On the other hand, the press device 100 according to this embodiment is configured so that the pad 30 first presses the center of the material M in the width direction. After the material M on the punch top surface 11 is pressed by the pad 30, the left and right ends of the material M are sandwiched between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, thereby preventing misalignment of the material M during press working. Therefore, the press-molded product 200 can be manufactured with high precision.

In this embodiment, the angles ψ _L and ψ _R of the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R and the horizontal plane are preferably 10° or more. Here, the horizontal plane is a plane perpendicular to the direction in which the holders 40L and 40R rise and fall (up and down direction). By making the angles ψ _L and ψ _R 10° or more, the width of the holder upper surfaces 41L and 41R and the strength of the holders 40L and 40R can be ensured. In addition, the bending tendency of the material M bent along the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R, the inclined surfaces 241L and 241R of the die lower surfaces 24L and 24R, and the second die shoulders 23L and 23R can be reduced. Therefore, the press-molded product 200 can be manufactured with high accuracy. The angles _ψL and _ψR that the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R make with the horizontal plane refer to the angles (acute angle side) that the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R make with the width direction in a cross section cut by a plane consisting of the vertical and width directions.

In this embodiment, the angles ψ _L and ψ _R of the inclined surfaces 411L and 411R of the holder upper surfaces 41L and 41R with the horizontal plane are preferably 55° or less, respectively. This prevents the inclined surfaces 411L and 411R from becoming too steep, and can suppress the horizontal component of the load applied vertically to the inclined surfaces 411L and 411R. This makes it possible to make it difficult for the holders 40L and 40R and the cushion pins 91L and 91R supporting the holders 40L and 40R to be damaged.

In this embodiment, in the later stage of forming the material M, the left and right ends of the material M are clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, and the holders 40L, 40R are lowered. The distance by which each of the holders 40L, 40R descends (lifting stroke) is preferably set to 5.0 mm or more. By setting each of the holders 40L, 40R's lifting stroke to 5.0 mm or more, the clamping time for the left and right ends of the material M can be secured, and the tension applied to the material M can be increased. Therefore, the tensile residual stress generated on the inside of the bend in the bent portions 203L, 203R of the press-formed product 200 can be effectively reduced.

In addition, it is preferable that the distance (lifting stroke) that each of the holders 40L, 40R descends is set to 25.0 mm or less. By setting the lifting stroke of each of the holders 40L, 40R to 25.0 mm or less, it is possible to prevent excessive tension from being applied to the material M. This makes it possible to suppress the occurrence of cracks in the material M or the press-molded product.

In order to apply an appropriate tension to the material M, strictly speaking, it is preferable to adjust the distance D1 by which the holders 40L and 40R are lowered while clamping the left and right ends of the material M. Figure 3 is a graph showing the relationship between the distance D1 by which the holders 40L or 40R are lowered while clamping the end of the material M, and the tensile residual stress on the inner side of the bend in the bent part of the press-formed product. The graphs in Figure 3 and Figures 4 and 5 described later were created from the results obtained when a CAE analysis was performed using commercially available software (LS-DYNA ver971 rev7.12, manufactured by ANSYS) on press working in which a material M with a tensile strength of 980 MPa and a plate thickness of 3.0 mm is bent with a bend R (inner curvature radius of the bend) of 4.0 mm to form a press-formed product without a flange.

As shown in FIG. 3, when the distance D1 is 2.0 mm or more, the tensile stress remaining on the inside of the bend in the bent portion of the press-molded product is reduced. Therefore, it is preferable that the lowering distance D1 of the holders 40L, 40R when clamping the end of the material M is 2.0 mm or more. The larger the distance D1, the longer the clamping time of the end of the material M, so the tensile residual stress on the inside of the bend is reduced. However, when the distance D1 exceeds 10.0 mm, the tension applied to the material M becomes excessive, and there is a possibility that cracks will occur in the bent portion or vertical wall. Therefore, it is preferable that the lowering distance D1 of the holders 40L, 40R when clamping the end of the material M is 10.0 mm or less. By setting the distance D1 to 2.0 mm or more and 10.0 mm or less, an appropriate tension can be applied to the material M.

In this embodiment, when the dies 20L, 20R reach the bottom dead center, the vertical distance D2 between the lower end of the material M located between the die side surfaces 22L, 22R and the punch side surfaces 13L, 13R and the R end (upper side) of the second die shoulders 23L, 23R is set to, for example, 4.5 mm or less. This allows the left and right ends of the material M to be completely pulled out from between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, and after the tension on the material M has decreased, the forming of the material M can be completed quickly.

Figure 4 is a graph showing the relationship between the vertical distance D2 from the R end (upper side) of the second die shoulder 23L or 23R to the end of the material M at the completion of forming, and the tensile residual stress on the inside of the bend in the bent part of the press-formed product. As shown in Figure 4, the greater the distance D2 from the R end of the second die shoulder 23L, 23R to the material M, the greater the residual stress on the inside of the bend in the bent part of the press-formed product. In order to effectively reduce the residual stress on the inside of the bend, it is preferable that the distance D2 is 4.5 mm or less. Most preferably, the distance D2 is 0 mm. By reducing the distance D2, the mold including the punch 10 and the dies 20L, 20R can be made smaller, and the cost of manufacturing the mold can be reduced.

In this embodiment, the vertical load F _L applied to the left holder upper surface 41L is preferably set to satisfy the above conditional formula (1). Moreover, the vertical load F _R applied to the right holder upper surface 41R is preferably set to satisfy the above conditional formula (2). By setting the loads F _L and F _R in this manner, the left and right ends of the material M can be more reliably clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, and an appropriate tension can be applied to the material M.

5 is a graph showing the relationship between the dimensionless number Z=F/(TS×t×L) of the holder load and the residual stress on the inside of the bend in the bent part of the press-formed product. In FIG. 5, the load F _L applied to the holder upper surface 41L and the load F _R applied to the right holder upper surface 41R are not distinguished, and are expressed as the holder load F [kN]. In addition, the longitudinal length L _L of the part of the material M that is sandwiched between the die lower surface 24L and the holder upper surface 41L, and the longitudinal length L _R of the part of the material M that is sandwiched between the die lower surface 24R and the holder upper surface 41R are not distinguished, and are expressed as the length L [mm] of the sandwiched part of the material M. TS is the tensile strength [MPa] of the material M, and t is the plate thickness [mm] of the material M.

Referring to FIG. 5, when the non-dimensional number Z of the holder load F is 0.04 or more, the die 20L, 20R does not succumb to the reaction force of the material M, and the die lower surface 24L, 24R securely clamps the left and right ends of the material M together with the holder upper surface 41L, 41R, so that tension is effectively applied to the material M. Therefore, the tensile stress remaining on the inside of the bend in the bent part of the press-formed product is reduced. The larger the non-dimensional number Z, the greater the tension on the material M, and the more the tensile residual stress on the inside of the bend can be reduced. However, when the non-dimensional number Z exceeds 0.45, the tension on the material M becomes excessive, and there is a possibility that cracks or the like may occur in the bent part or vertical wall. Therefore, as shown in the above conditional expressions (1) and (2), it is preferable that the non-dimensional number Z of the holder load F is 0.04 or more and 0.45 or less.

In this embodiment, the die lower surfaces 24L, 24R, together with the holder upper surfaces 41L, 41R, clamp the left and right ends of the material M, while the second die shoulders 23L, 23R adjacent to the die lower surfaces 24L, 24R do not substantially contribute to clamping the material M. If the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are too large, the clamped portion of the material M is reduced. Therefore, it is preferable that the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are four times or less the plate thickness t of the material M. As a result, the material M is sufficiently clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, so that the tensile residual stress on the inner side of the bend can be effectively reduced in the bent portions 203L, 203R of the press-formed product 200.

In this embodiment, the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are preferably equal to or greater than the thickness t of the material M. This makes it possible to prevent cracks and the like from occurring at the positions of the second die shoulders 23L, 23R in the material M during press working. Also, it is possible to suppress bending tendencies from remaining at the second die shoulders 23L, 23R in the press-formed product 200 manufactured from the material M.

In this embodiment, distance blocks 42L, 42R are provided on the holder upper surfaces 41L, 41R. These distance blocks 42L, 42R can prevent the holder upper surfaces 41L, 41R from colliding with the die lower surfaces 24L, 24R when the left and right ends of the material M are pulled out from between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R. Even after the left and right ends of the material M are completely pulled out from between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, the distance blocks 42L, 42R are pressed by the dies 20L, 20R, so that the holders 40L, 40R can be lowered together with the dies 20L, 20R.

In this embodiment, it is preferable that the thickness of each of the distance blocks 42L, 42R is 1.1 times or less the sheet thickness t of the material M. This allows the left and right ends of the material M to be more securely clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R, and tension can be effectively applied to the material M. To prevent the tension applied to the material M from becoming excessive, it is preferable that the thickness of each of the distance blocks 42L, 42R is 0.9 times or more the sheet thickness t of the material M.

It is preferable that the material of the distance blocks 42L, 42R has a lower strength than the material of the dies 20L, 20R and the holders 40L, 40R. In this case, the distance blocks 42L, 42R function as a buffer between the dies 20L, 20R and the holders 40L, 40R, preventing deformation of the dies 20L, 20R and the holders 40L, 40R. This prevents a decrease in the dimensional accuracy of the press-molded product 200 produced due to deformation of the dies 20L, 20R or the holders 40L, 40R.

The residual stress of the press-formed product 200 is the sum of the stress when the material M is formed into the press-formed product 200 and the reversal stress due to springback when the press-formed product 200 is released from the mold. If the stress generated in the material M during forming is small, the stress due to springback will also be small, and the residual stress of the press-formed product 200 will be reduced. Therefore, in the manufacturing method of the press-formed product 200 according to this embodiment, it is preferable to use a material M having a work hardening index of 0.04 or less. If the work hardening index of the material M is 0.04 or less, work hardening of the material M is unlikely to occur, and the stress generated in the material M during forming will also be small. Therefore, the residual stress of the press-formed product 200 can be reduced.

Second Embodiment
In this embodiment, the same press apparatus 100 as in the first embodiment is used to manufacture a press-formed product 200 in a manner generally similar to that of the first embodiment. However, in this embodiment, the shape of the material M to be prepared is different from that of the first embodiment.

Figure 6 is a cross-sectional view of the press device 100 when the dies 20L, 20R are located at the top dead center, and shows the state in which the material M prepared in this embodiment is placed on the punch top surface 11. As shown in Figure 6, in this embodiment, the material M is pre-formed into a shape that fits along the punch top surface 11 and the left and right punch shoulders 12L, 12R when placed on the punch top surface 11. More specifically, a groove M1 corresponding to the groove 111 of the punch top surface 11 is formed in the material M. In addition, the material M is bent convexly at positions M2 corresponding to each of the punch shoulders 12L, 12R so as to partially fit along the punch shoulders 12L, 12R.

Furthermore, the material M is bent concavely in advance at positions M3 corresponding to the left and right second die shoulders 23L, 23R. In other words, when the material M is placed on the punch top surface 11, the material M is bent so that the left and right ends that are to be clamped between the die lower surfaces 24L, 24R and the holder upper surfaces 41L, 41R will jump up slightly.

6, when the material M is placed on the punch top surface 11, the left end of the material M is substantially parallel to the inclined surface 411L of the holder upper surface 41L. Also, when the material M is placed on the punch top surface 11, the right end of the material M is substantially parallel to the inclined surface 411R of the holder upper surface 41R. However, the left and right ends of the material M do not necessarily have to be parallel to the inclined surfaces 411L, 411R. For example, the angles that the left and right ends of the material M make with the horizontal plane may deviate by about ±10° from the angles ψ _L , ψ _R that the inclined surfaces 411L, 411R make with the horizontal plane.

In this embodiment, the material M is preformed to a shape that conforms to the groove portion 111 of the punch top surface 11 and the punch shoulders 12L, 12R. Therefore, the material M is easily fitted to the punch 10, and when performing press processing, the material M can be more reliably held between the punch 10 and the pad 30. In addition, since the material M is pre-bent at the portion corresponding to the second die shoulders 23L, 23R, the material M can be easily clamped between the die lower surfaces 24L, 24R adjacent to the second die shoulders 23L, 23R and the holder upper surfaces 41L, 41R. Therefore, tension can be applied more effectively to the material M. As a result, the effect of reducing the tensile residual stress generated on the inside of the bend in the bent portions 203L, 203R of the press-formed product 200 (FIG. 2J) can be further improved.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

For example, the press device 100 according to each of the above embodiments includes left and right holders 40L, 40R. However, the press device 100 may include only one of the holders 40L, 40R. That is, one of the left and right bent portions 203L, 203R of the press-formed product 200 can be formed while applying tension by the holder, and the other can be formed without applying tension by the holder. For example, if it is necessary to reduce the tensile residual stress on the inside of the bend and obtain excellent fatigue characteristics in one of the left and right bent portions 203L, 203R of the press-formed product 200, while excellent fatigue characteristics are not required in the other, tension by the holder can be applied only to the side requiring excellent fatigue characteristics. When only one of the holders 40L, 40R is provided in the press device 100, the manufacturing costs of the press device 100 can be reduced.

In the press device 100 according to each of the above embodiments, the entire holder upper surface 41L, 41R is an inclined surface 411L, 411R. However, a part of the holder upper surface 41L, 41R may be an inclined surface 411L, 411R. For example, the outer portion of the holder upper surface 41L, 41R in the width direction may be a horizontal surface, and the portion located inside this horizontal surface in the width direction (the portion on the punch 10 side) may be an inclined surface 411L, 411R. In this case, the distance blocks 42L, 42R may be arranged on a horizontal surface.

In the press device 100 according to each of the above embodiments, the holders 40L, 40R can be raised and lowered by the cushion 90. However, the mechanism for raising and lowering the holders 40L, 40R is not limited to the cushion 90. For example, the holders 40L, 40R can be supported by an expandable support member such as a spring or a fluid pressure cylinder. In this case, when a load is applied from the dies 20L, 20R to the holder upper surfaces 41L, 41R, the support member contracts and the holders 40L, 40R descend. When the load is removed from the holder upper surfaces 41L, 41R, the support member expands and the holders 40L, 40R ascend.

In the press device 100 according to each of the above embodiments, a groove 111 is formed in the punch top surface 11. However, a step (step) may be provided in the punch top surface 11 instead of the groove 111. This step may extend over the entire punch top surface 11 in the longitudinal direction, or may be provided in a part of the punch top surface 11. When a step is provided in the punch top surface 11, a corresponding step is also provided in the underside of the pad 30. When a step is provided in the punch top surface 11, a step corresponding to the step in the punch top surface 11 is formed in the top plate 201 of the press-formed product 200. A step corresponding to the step in the punch top surface 11 can also be formed in advance in the material M before press processing.

However, the punch top surface 11 and the top plate 201 of the press-formed product 200 do not have to have a groove or a step. The punch top surface 11 and the top plate 201 of the press-formed product 200 may have a flat shape overall.

The overall shape of the press-molded product 200 can be appropriately selected depending on the purpose of use, etc. For example, the press-molded product 200 may be straight when viewed from the top plate 201 side (in a plan view), may be curved to the left or right, or may be curved up or down (curved when viewed from the width direction). The press-molded product 200 may have a shape that combines two or more types of straight portions and curved portions when viewed from the width direction. Furthermore, the press-molded product 200 may have a shape in which a part having an approximately U-shaped cross section branches into multiple parts, such as a three-pronged or four-pronged shape when viewed from the width direction.

The press-molded product 200 does not need to have a uniform cross-sectional shape throughout. As long as the press-molded product has at least a portion that is roughly U-shaped, the press device and manufacturing method according to each of the above embodiments can be applied.

The top plate 201 of the press-molded product 200 may have one or more through holes formed therein. The through holes may be simply round holes, or may be holes (burring holes) formed by burring processing with protruding hole edges.

In the second embodiment, preforming is performed on the material M before it is pressed by the press device 100. In the second embodiment, the material M is subjected to the formation of a groove M1 corresponding to the groove 111 of the punch top surface 11, convex bending at a position M2 corresponding to the punch shoulders 12L and 12R, and concave bending at a position M3 corresponding to the second die shoulders 23L and 23R. However, in the second embodiment, it is also possible to perform on the material M one or two of the formation of the groove M1, the convex bending at a position M2 corresponding to the punch shoulders 12L and 12R, and the concave bending at a position M3 corresponding to the second die shoulders 23L and 23R.

The present disclosure will be described in more detail below with reference to examples. However, the present disclosure is not limited to the following examples.

[First embodiment]
In order to confirm the effect of the present disclosure, a CAE analysis was performed on the press processing in which a material (steel plate) having a tensile strength TS of 980 MPa and a plate thickness t of 3.0 mm is formed into a press-formed product 300 shown in FIG. 7 by the press device 100 shown in FIG. 1. The press-formed product 300 has a top plate 301, left and right vertical walls 302, and left and right bending portions 303. The top plate 301 has a groove portion 304. The lower end of each vertical wall 302 is a free end. That is, no flange is provided at the lower end of each vertical wall 302. The width of the top plate 301 is 80.0 mm, and the height of the vertical wall 302 is 25.0 mm. In addition, the bending R (curvature radius on the inner side of the bending) of the bending portion 303 is 4.0 mm. The press-formed product 300 is curved in a plan view. The bending R on the inner side of the bending of the press-formed product 300 is 250.0 mm. The curvature R of the outer curve is 330.0 mm obtained by adding the curvature R of the inner curve to the width of the top plate 301. Note that these dimensions are dimensions on the inside of the press-formed product 300 (the surface on the inner side of the bend of the bent portion 303).

In the CAE analysis, commercially available software (LS-DYNA ver971 rev7.12, manufactured by ANSYS) was used to perform forming analysis and springback analysis to evaluate the maximum value of the residual stress (maximum principal stress) on the inner surface of the bent portion 303 as the residual stress on the inner side of the bend. If the residual stress value is positive, it is tensile, and if it is negative, it is compressive. In the CAE analysis, of the elements divided in the plate thickness direction, the value of the element on the inner surface of the bend was used as the value of the inner surface of the bend. The analysis results are shown in Table 1.

In Table 1, Comparative Example 1 shows the results when the press-formed product 300 was formed by typical pad bending. That is, in Comparative Example 1, the material was pressed by the pad 30 and pressed using the punch 10 and dies 20L, 20R, but the material was not clamped between the dies 20L, 20R and the holders 40L, 40R. In Comparative Example 1, after the press forming was completed and springback occurred, a residual stress of 580 MPa was generated on the inner surface of the bent portion 303 of the press-formed product 300.

Examples 1-1 to 1-18 are results when a press-formed product 300 was formed using the method according to the present disclosure. That is, in Examples 1-1 to 1-18, the widthwise ends of the material were clamped between the dies 20L, 20R and the holders 40L, 40R from the middle of forming, and tension was applied to the material. In Examples 1-1 to 1-11, 1-13, and 1-16 to 1-18, after press forming was completed and springback occurred, the residual stress value on the inner surface of the bent portion 303 of the press-formed product 300 was smaller than that of Comparative Example 1. Therefore, it was confirmed that the effect of reducing tensile residual stress on the inner surface of the bent portion 303 was obtained by applying tension to the material from the middle of forming.

In Examples 1-12, 1-14, and 1-15, the material thickness reduction rate during press forming was 20% or more. If the material thickness reduction rate is 20% or more in the CAE analysis, cracks often occur in the material or press-formed product during actual press forming, so residual stress calculations were not performed in Examples 1-12, 1-14, and 1-15.

(Holder lift stroke)
In Example 1-12, the holder stroke (the lifting and lowering stroke of holders 40L, 40R) is 30.0 mm. In Example 1-12, it is believed that the large holder stroke caused the sheet thickness reduction rate of the material to be 20% or more. On the other hand, in Example 1-2, in which the holder stroke was 25.0 mm, the sheet thickness reduction rate of the material was less than 20%. From this result, in order to make it difficult for cracks to occur in the material or the press-molded product and to effectively exert the effect of reducing tensile residual stress in the present disclosure, it is preferable that the lifting and lowering stroke of holders 40L, 40R is 25.0 mm or less.

In Example 1-11, the holder stroke is 3.0 mm. In Example 1-11, the residual stress on the inside of the bend of the bent portion 303 is 547 MPa, and the effect of reducing the tensile residual stress is small. On the other hand, in Example 1-3, in which the holder stroke is 5.0 mm, the tensile residual stress is significantly reduced compared to Example 1-11. Even in Examples 1-1, 1-2, 1-4 to 1-10, etc., in which the holder stroke is greater than 5.0 mm, the effect of reducing the tensile residual stress is greater compared to Example 1-11. From this result, in order to effectively exert the effect of reducing the tensile residual stress in the present disclosure, it is preferable that the lifting and lowering stroke of the holders 40L and 40R is 5.0 mm or more.

(Holder load)
In Example 1-14, the non-dimensional number Z of the holder load F is 0.50, and the holder load F is large. In Example 1-14, it is believed that the large holder load F caused the sheet thickness reduction rate of the material to be 20% or more. On the other hand, in Example 1-4, in which the non-dimensional number Z of the holder load F is 0.45, the sheet thickness reduction rate of the material was less than 20%. From this result, in order to make it difficult for cracks to occur in the material or the press-molded product and to effectively exert the effect of reducing the tensile residual stress in the present disclosure, it is preferable that the non-dimensional number Z of the holder load F is 0.45 or less.

In Example 1-13, the non-dimensional number Z of the holder load F is 0.01, and the holder load F is insufficient. Therefore, in Example 1-13, the residual stress on the inner side of the bend of the bent portion 303 is 538 MPa, and the effect of reducing the tensile residual stress is small. On the other hand, in Example 1-5, in which the non-dimensional number Z of the holder load F is 0.04, the tensile residual stress is significantly reduced compared to Example 1-13. Even in Examples 1-1 to 1-4, 1-6 to 1-10, etc., in which the non-dimensional number Z is greater than 0.04, the effect of reducing the tensile residual stress is greater compared to Example 1-13. From this result, in order to effectively exert the effect of reducing the tensile residual stress in the present disclosure, it is preferable that the non-dimensional number Z of the holder load F is 0.04 or more.

(Radius of curvature of second die shoulder)
In Example 1-15, the bend R (radius of curvature R _L , R _R ) of the second die shoulders 23L, 23R is 2.0 mm, which is 0.67 times the thickness t of the material. In Example 1-15, it is considered that the thickness reduction rate of the material is 20% or more because the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are small. On the other hand, in Example 1-8, in which the bend R of the second die shoulders 23L, 23R is equal to the thickness t of the material, the thickness reduction rate of the material was less than 20%. Therefore, in order to make it difficult for cracks to occur in the material or the press-formed product and to effectively exert the effect of reducing the tensile residual stress in the present disclosure, it is preferable that the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are equal to or greater than the thickness t of the material.

In Example 1-16, the bending R of the second die shoulders 23L, 23R is 15.0 mm, which is 5 times the plate thickness t of the material. In Example 1-16, the residual stress on the inner side of the bending of the bending portion 303 is 563 MPa, and the effect of reducing the tensile residual stress is small. On the other hand, in Example 1-9, in which the bending R of the second die shoulders 23L, 23R is 4 times the plate thickness t of the material, the tensile residual stress was significantly reduced compared to Example 1-16. Even in Examples 1-1 to 1-8, 1-10, etc., in which the bending R is less than 4 times the plate thickness t, the effect of reducing the tensile residual stress was greater compared to Example 1-16. From this result, in order to effectively exert the effect of reducing the tensile residual stress in the present disclosure, it is preferable that the curvature radii R _L and R _R of the second die shoulders 23L, 23R are less than 5 times the plate thickness t of the material. More preferably, the radii of curvature R _L , R _R of the second die shoulders 23L, 23R are equal to or smaller than four times the plate thickness t of the material.

(Holder face angle)
In Example 1-17, the residual stress on the inner side of the bend of the bent portion 303 is 103 MPa, and the tensile residual stress is significantly reduced compared to Comparative Example 1. However, in Example 1-17, the angles ψ L and ψ R (holder face angle ψ ₎ made by the inclined surfaces 241L and 241R of the die lower surfaces 24L and 24R and the inclined surfaces 411L and _411R of the holder upper surfaces 41L and 41R with the horizontal plane are small at 7°, so the bending tendency when the material passes through the second die shoulders 23L and 23R is large. On the other hand, in Example 1-6, in which the holder face angle ψ is 10°, the bending tendency is smaller than that in Example 1-17. Therefore, in order to suppress the occurrence of bending tendency during press forming, it is preferable that the angles ψ _L and ψ _R of the inclined surfaces 241L, 241R, 411L, and 411R are larger than 7°. More preferably, the angles φ _L and φ _R are equal to or greater than 10°.

In Example 1-18, the residual stress on the inner side of the bend of the bent portion 303 is 414 MPa, and the tensile residual stress is significantly reduced compared to Comparative Example 1. However, in Example 1-18, the face angle ψ of the holder is large at 60°, so the horizontal component of the load on the holder upper surfaces 41L, 41R is large. Therefore, there is concern about deformation or damage of the holders 40L, 40R and the cushion pins 91L, 91R. On the other hand, if the face angle ψ of the holder is 55° as in Example 1-7, the horizontal component of the load on the holder upper surfaces 41L, 41R is reduced to a certain extent, and it is considered that deformation or damage of the holders 40L, 40R and the cushion pins 91L, 91R is unlikely to occur. Therefore, it is preferable that the angles ψ _L and ψ _R of the inclined surfaces 241L, 241R, 411L, and 411R are less than 60°. More preferably, the angles ψ _L and ψ _R are 55° or less.

[Second embodiment]
In order to confirm the effect of preforming the material, a CAE analysis was performed using the same software as in Example 1. In this example, an analysis was performed on the press working of forming a material (steel plate) with a tensile strength TS of 980 MPa and a plate thickness t of 3.0 mm into a press-formed product without a flange. The results of the analysis are shown in FIG.

In Comparative Example 2 in FIG. 8, a press-formed product is formed by general pad bending, as in Comparative Example 1 in the first embodiment. On the other hand, in Examples 2-1 to 2-3, press-formed products are formed by the press device 100 shown in FIG. 1 while applying tension to the material using the holders 40L and 40R. In Example 2-1, the material, which is a metal plate, is not preformed, but is used as is for press forming. In Example 2-2, a groove corresponding to the groove 111 of the punch top surface 11 is formed in the material in advance. In Example 2-3, in addition to the preforming of Example 2-2, the material is preformed by bending convexly at positions corresponding to the punch shoulders 12L and 12R and bending concavely at positions corresponding to the second die shoulders 23L and 23R. Note that the preforming in Example 2-3 is the same as the preforming performed on the material in Example 1-10 (Table 1) of the first embodiment.

As shown in Figure 8, even in Example 2-1, where press forming was performed without preforming the material, the residual stress on the inner side of the bend was reduced by approximately half compared to Comparative Example 2. In Examples 2-2 and 2-3, where press forming was performed after preforming the material, the residual stress on the inner side of the bend was further reduced compared to Example 2-1. Therefore, by preforming the material, the effect of reducing tensile residual stress can be further improved.

[Third Example]
In order to confirm the effect based on the work hardening index (n value) of the material, a CAE analysis was performed using the same software as in Example 1. In this example, as in Example 2, an analysis was performed on the press working of forming a material (steel plate) with a tensile strength TS of 980 MPa and a plate thickness t of 3.0 mm into a press-formed product without a flange. The results of the analysis are shown in Figure 9.

In Comparative Example 3 in FIG. 9, like Comparative Example 1 in the first embodiment, a press-formed product is formed by general pad bending. The n value of the material used in Comparative Example 3 is 0.08. On the other hand, in Examples 3-1 and 3-2, the material is formed into a press-formed product by the press device 100 shown in FIG. 1. The n value of the material used in Example 3-1 is 0.08, like Comparative Example 3, and the n value of the material used in Example 3-2 is 0.04. The yield ratio YR = yield stress YS / tensile strength TS of the material used in Comparative Example 3 and Example 3-1 is 0.88, and the yield ratio YR of the material used in Example 3-2 is 0.95.

As shown in Figure 9, in both Example 3-1 and Example 3-2, the tensile residual stress on the inside of the bend is reduced compared to Comparative Example 3. However, in Example 3-2, in which the n value of the material is small, the effect of reducing the tensile residual stress is better than in Example 3-1. From this result, it can be said that in order to improve the effect of reducing the tensile residual stress, it is preferable that the n value of the material is small. The n value of the material is preferably 0.04 or less. In addition, the yield ratio YR of the material is preferably 0.95 or more.

100: Press device 10: Punch 11: Punch top surface 12L, 12R: Punch shoulder 13L, 13R: Punch side surface 20L, 20R: Die 21L, 21R: First die shoulder 22L, 22R: Die side surface 23L, 23R: Second die shoulder 24L, 24R: Die lower surface 241L, 241R: Inclined surface 30: Pad 40L, 40R: Holder 41L, 41R: Holder upper surface 411L, 411R: Inclined surface 42L, 42R: Distance block

Claims (12)

A press device for performing press working on a metal plate material,
a punch having a punch top surface, a punch shoulder continuous with the punch top surface, and a punch side surface extending downward from the punch shoulder;
a pad disposed above the punch so as to face the top surface of the punch and configured to be liftable and lowerable;
a die arranged above the punch and to the side of the pad, configured to be liftable, and having a first die shoulder corresponding to the punch shoulder, a die side surface extending downward from the first die shoulder corresponding to the punch side surface, and a die lower surface connected to a lower end of the die side surface via a second die shoulder;
a holder arranged to the side of the punch and below the die, configured to be movable up and down, and having a holder upper surface facing a lower surface of the die;
Equipped with
the lower surface of the pad has a shape corresponding to the top surface of the punch;
the die lower surface includes an inclined surface that descends away from the second die shoulder;
a press device, wherein the holder upper surface is positioned below the punch top surface and above the position of the die lower surface when the die reaches bottom dead center at the start of press processing by the press device, and includes an inclined surface that descends as it moves away from the punch in correspondence with the inclined surface of the die lower surface. The press device according to claim 1,
A press apparatus, wherein the angle between the inclined surface of the holder upper surface and a horizontal plane is equal to or greater than 10° and equal to or less than 55°. The press device according to claim 1 or 2,
A press device, wherein the lifting and lowering stroke of the holder is 5.0 mm or more and 25.0 mm or less. The press device according to any one of claims 1 to 3,
A press apparatus in which a vertical load applied to the upper surface of the holder is set to satisfy the following conditional formula, where F [kN] is the load, TS [MPa] is the tensile strength of the material, t [mm] is the plate thickness of the material, and L [mm] is the longitudinal length of the portion of the material that is clamped between the lower surface of the die and the upper surface of the holder.
0.04≦Z≦0.45
where Z = F / (TS x t x L) The press device according to any one of claims 1 to 4,
The second die shoulder has an arc shape having a radius of curvature greater than or equal to a thickness of the material and less than or equal to four times the thickness when viewed in cross section of the die. The press apparatus according to any one of claims 1 to 5, further comprising:
a distance block provided on an upper surface of the holder and configured to adjust a distance between a lower surface of the die that holds the material and the upper surface of the holder;
A press device comprising: A method for producing a press-molded product using the press apparatus according to any one of claims 1 to 6,
Providing a material which is a metal plate;
placing the material on the top surface of the punch and lowering the pad and the die;
a step of clamping an end of the material between a lower surface of the die and an upper surface of the holder after the pad presses the material on the top surface of the punch and before the die reaches a bottom dead center;
a step of pressing the material by the first die shoulder and the die side surface and the punch shoulder and the punch side surface by causing the die to reach the bottom dead center;
Equipped with
the end of the material is pulled out along the inclined surface from between the lower surface of the die and the upper surface of the holder as the die descends, and is positioned between the die side surface and the punch side surface when the die reaches the bottom dead center. The method according to claim 7, further comprising the steps of:
When the die reaches the bottom dead center, an upper surface of the holder is positioned outside the pressed material in the width direction of the press device. The method according to claim 7 or 8,
A manufacturing method in which the prepared blank is concavely bent at a position corresponding to the second die shoulder. The method according to any one of claims 7 to 9,
A manufacturing method in which the prepared blank is convexly bent at a position corresponding to the punch shoulder. The method according to any one of claims 7 to 10,
The punch has a groove or a step on the top surface of the punch,
A manufacturing method in which a groove or a step corresponding to the groove or the step on the top surface of the punch is formed in the prepared material. The method according to any one of claims 7 to 11,
The manufacturing method, wherein the work hardening index of the material is 0.04 or less.

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