JP7705689B2 - Method for press forming metal sheet - Google Patents

Description

The present invention relates to a method for press forming a metal plate.

Press-formed products (hereafter referred to as press-formed products) are used in a variety of fields, including automobiles, home appliances, furniture, and miscellaneous goods. Press-formed products are usually obtained by stretching or elongating a metal sheet held between the periphery of a die and a die cushion pad (also called a blank holder or blank holder) between the forming recess of the die and the forming protrusion of the punch, plastically deforming the metal sheet into the desired shape. By performing this type of press forming, even parts with complex shapes can be mass-produced efficiently.

However, metal sheets with high strength have the characteristic of being difficult to press-form due to their high hardness. In particular, when deep drawing or square tube forming, which involves a large amount of deformation, is performed, there is a risk of cracks occurring in the press-formed product.

Patent Document 1 describes a press forming method in which, in order to perform deep drawing efficiently, a steel sheet, which is the workpiece, is placed between a die having a forming recess and a punch having a forming protrusion, and the steel sheet held by the blank holder and die is drawn by moving the die and the punch relatively in the approaching direction, and the processing speed, which is the time rate of change in the relative movement amount between the die and the punch, is changed from an early processing speed, which is the processing speed before the speed change point, to a late processing speed, which is the processing speed after the speed change point and is higher than the early processing speed, at a speed change point where the relative movement amount becomes a predetermined value. In addition, Figure 2A of Patent Document 1 shows the relationship between the stroke amount of the punch and time when step motion forming is performed so that the average processing speed is approximately the same as linear motion forming, and further Figure 2C suggests that when the step motion forming shown in Figure 2A is applied to deep drawing, the sheet thickness reduction rate is smaller than that of linear motion forming.

JP 2016-221536 A

However, even when step motion forming is performed on a blank material made of a metal plate so that the average processing speed is approximately the same as that of linear motion forming, as described in Patent Document 1, cracks may occur in the press-formed product after forming.

The present invention was made in consideration of the above circumstances, and aims to provide a method for press-forming metal sheets that can prevent the occurrence of cracks.

In order to solve the above problems, the present invention employs the following configuration.
[1] A first step of disposing a metal plate between a die and a punch and sandwiching a portion of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position before contacting the metal sheet, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least twice or more times during the period from the contact position where the punch contacts the metal sheet to the bottom dead center,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force from the final stopping step onward, wherein the second die cushion force is made smaller than the first die cushion force and the third die cushion force.
[2] A first step of disposing a metal plate between a die and a punch and sandwiching a portion of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position before contacting the metal sheet, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal sheet and the bottom dead center,
A method for press forming a metal sheet, comprising: a first stopping step being performed when the punch reaches a range from the maximum load position to a position where the forming load is 80% of the maximum value, where the position of the punch when the forming load of the punch is at its maximum value is defined as a maximum load position.
[3] A first step of disposing a metal plate between a die and a punch and sandwiching a part of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
A stopping step of temporarily stopping the punch and the die is performed at least twice or more between a contact position where the punch contacts the metal plate and a bottom dead center,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force from the final stopping step onward, wherein the second die cushion force is made smaller than the first die cushion force and the third die cushion force.
[4] The press forming method for a metal sheet according to [1] or [3], wherein a first stopping step is performed when the punch reaches a range from the maximum load position to a position where the forming load is 80% of the maximum value, where the position of the punch when the forming load of the punch is at its maximum value is defined as a maximum load position.
[5] The press forming method for a metal sheet according to [1] or [3], wherein a final stopping step is performed when the punch reaches a position 5 to 10 mm away from the bottom dead center.
[6] The press forming method for a metal sheet according to [1], wherein the second speed is 40% or more and 70% or less of the first speed.
[7] The press forming method for a metal sheet according to [1], wherein the intermediate position is a position 10 mm to 20 mm away from the contact position.
[8] The press forming method of a metal sheet according to [1] or [3], wherein a stopping time in the stopping step is in the range of 0.2 to 1.0 seconds.
[9] The press forming method of a metal sheet according to [1] or [3], wherein the second die cushion force is in a range of 0.1 to 0.5 times the first die cushion force or the third die cushion force.
[10] A first step of disposing a metal plate between a die and a punch and sandwiching a portion of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal plate and the bottom dead center,
The press forming method for a metal sheet comprises performing a final stopping step when the punch reaches a position 5 to 10 mm away from the bottom dead center.
[11] A first step of disposing a metal plate between a die and a punch and sandwiching a portion of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal plate and the bottom dead center,
The method for press-forming a metal sheet, wherein the stopping time in the stopping step is in the range of 0.2 to 1.0 seconds.
[12] A first step of disposing a metal plate between a die and a punch and sandwiching a portion of the metal plate between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least twice or more times during the period from the contact position where the punch contacts the metal plate to the bottom dead center,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force after the final stopping step, wherein the second die cushion force is in a range of 0.1 to 0.5 times the first die cushion force or the third die cushion force.

According to the invention described in [1] above, in the second stage, the punch is driven at a first speed until it comes into contact with the metal plate, and then the punch is decelerated to a second speed slower than the first speed until it reaches the bottom dead center, thereby reducing the impact on the metal plate when the punch comes into contact with the metal plate.
In addition, by providing at least one stopping step between the contact position where the punch comes into contact with the metal plate and the bottom dead center, when the stopping step ends and the punch and die are moved again, the strain in the metal plate being formed is dispersed, thereby suppressing breakage of the material due to concentration of strain.
Furthermore, by setting the relative speed of the punch and die before and after the stopping step to a second speed that is slower than the first speed, it becomes easier to control the speed of the punch and die during forming of the metal plate, and concentration of strain in the metal plate during forming can be prevented.
As described above, by suppressing the impact on the metal plate and dispersing the strain in the metal plate during forming, and preventing strain concentration, it is possible to prevent cracks in the press-formed product.
In addition, in the second stage, by setting the relative speed of the punch and the die to a first speed which is faster than the second speed before the punch comes into contact with the metal plate, the increase in forming time caused by providing a stop step can be suppressed, and the forming time as a whole can be shortened.

According to the invention described in [2] above, by making the first die cushion force before the first stopping step higher than the second die cushion force, it is possible to improve the formability of the metal sheet by the punch in the initial stage of forming.
Furthermore, by making the second die cushion force between the first and final stopping steps smaller than the die cushion forces before the first and after the final stopping steps (first and third die cushion forces), the flow of material between the punch and die in the middle of forming is promoted, thereby preventing excessive reduction in plate thickness in the metal plate during forming and dispersing strain better, suppressing breakage of the material due to concentration of strain and preventing cracks in the press-formed product.
Furthermore, by making the third die cushion force after the stopping step higher than the second die cushion force, the formability of the metal sheet by the punch and die in the later stage of forming can be improved, thereby making it possible to obtain a press-formed product having a desired shape and free of cracks.

According to the invention described in [3] above, in the second stage, by providing at least two or more stopping steps between the contact position where the punch comes into contact with the metal plate and the bottom dead center, when the stopping steps are completed and the punch and die are moved again, the strain in the metal plate being formed is dispersed, thereby suppressing breakage of the material due to concentration of strain and preventing cracks in the press-formed product.
Furthermore, by making the first die cushion force before the first stopping step higher than the second die cushion force, the formability of the metal sheet by the punch in the initial stage of forming can be improved.
Furthermore, by making the second die cushion force between the first and final stopping steps smaller than the die cushion forces before the first and after the final stopping steps (first and third die cushion forces), the flow of material between the punch and the die in the middle of forming is promoted, thereby preventing excessive reduction in plate thickness in the metal plate during forming and dispersing strain better, and suppressing breakage of the material due to concentration of strain.
Furthermore, by making the third die cushion force after the stopping step higher than the second die cushion force, the formability of the metal sheet by the punch and the die in the later stage of forming can be improved.
As a result, a press-molded product having the desired shape and free of cracks can be obtained.

According to the invention described in [4] above, by performing the first stopping step when the punch reaches the range from the maximum load position to the position where the forming load is 80% of the maximum value, it is possible to move to the stopping step after the forming of the metal plate by the punch in the initial stage of forming is almost completed. This makes it possible to further enhance the strain distribution effect on the metal plate during forming by the stopping step, and to manufacture a press-formed product of the desired shape while preventing cracks.

According to the invention described in [5] above, when the punch reaches a position 5 to 10 mm away from the bottom dead center, the final stopping step is performed and the die cushion force is changed to the third die cushion force at that time, thereby further improving the formability of the metal sheet by the punch and die in the later stages of forming.

According to the invention described in [6] above, by setting the second speed to be between 40% and 70% of the first speed, it becomes easier to control the speed of the punch and die during forming of the metal plate, and it is possible to prevent concentration of strain in the metal plate during forming.

According to the invention described in [7] above, by setting the intermediate position at a position 10 mm to 20 mm away from the contact position, it is possible to reliably reduce the relative speed between the punch and the die to the second speed before the punch comes into contact with the metal plate.

According to the invention described in [8] above, by setting the stopping time in the stopping step to a range of 0.2 to 1.0 seconds, it is possible to more reliably distribute the strain in the metal plate when the punch and die are moved again after the stopping step.

According to the invention described in [9] above, by setting the second die cushion force in the range of 0.1 to 0.5 times the first and third die cushion forces, it is possible to ensure that the material flows between the punch and the die during forming, suppressing excessive reduction in plate thickness and preventing breakage of the material.

FIG. 1 is a process diagram illustrating a molding method according to an embodiment of the present invention. FIG. 2 is a process diagram illustrating a molding method according to an embodiment of the present invention. FIG. 3 is a process diagram illustrating a molding method according to an embodiment of the present invention. FIG. 4 is a diagram for explaining the press forming method according to the first embodiment of the present invention, and is a graph showing the relationship between the relative position of the punch with respect to the bottom dead center and time. FIG. 5 is a diagram for explaining the press forming method according to the second embodiment of the present invention, and is a graph showing the relationship between the relative position of the punch with respect to the bottom dead center, the die cushion force, and time. FIG. 6 is a diagram for explaining the press forming method according to the third embodiment of the present invention, and is a graph showing the relationship between the relative position of the punch with respect to the bottom dead center, the die cushion force, and time. FIG. 7 is a graph showing the relationship between the forming load of the punch and the forming height of the formed product.

The following describes a method for press-forming a metal sheet according to an embodiment of the present invention. The press-forming method of this embodiment is a press-forming method that uses a punch, a die, and a blank holder, and is particularly suitable for deep drawing or square cylinder drawing.

FIG. 1 shows an example of a molded product that can be molded by the molding method of the present embodiment.
The formed product 2 shown in Fig. 1 is manufactured by subjecting a metal sheet 1 to square cylinder drawing. The formed product 2 has a rectangular bottom surface portion 21, a flange portion 22, and four vertical wall portions 23 connecting the bottom surface portion 21 and the flange portion 22. A first bent portion 1b (punch shoulder portion) is provided between the bottom surface portion 21 and the vertical wall portions 23, and a second bent portion 1c (die shoulder portion) is provided between the vertical wall portions 23 and the flange portions 22. The distance between the bottom surface portion 21 and the flange portions 22, i.e., the height d of the vertical wall portions 23, corresponds to the distance from a contact position to a bottom dead center position D, which will be described later.

The molded product 2 shown in FIG. 1 can be used, for example, in automobile parts. The molded product 2 can also be used as an exterior panel for home appliances and kitchen appliances. When the molded product 2 is used for these purposes, it is not limited to the shape shown in FIG. 1, and the bottom surface portion 21, flange portion 22, and vertical wall portion 23 may be appropriately modified.

The material of the metal plate 1 is not particularly limited, and examples of the material include steel, stainless steel, titanium or titanium alloy, aluminum or aluminum alloy, copper or copper alloy, etc. In this embodiment, the present invention is preferably applied to a metal plate having a relatively high yield strength. One example of such a metal plate is a ferritic-austenitic duplex stainless steel plate.

Next, an example of a press molding machine suitable for use in the molding method of this embodiment will be described. Figures 2 and 3 are process diagrams showing the basic steps of the press molding method of this embodiment. The press molding machine 30 shown in Figure 2(a) is a so-called servo press machine, and is equipped with a die 31, a punch 32, and a die cushion pad 33. The die cushion pad 33 is also generally called a blank holder or blank holder. The press molding machine 30 improves the formability of the molded product 2 by sandwiching a part 1a of the steel sheet 1 between the peripheral portion of the die 31 and the die cushion pad 33 during press molding.

A servo motor (not shown) is connected as a drive source to the die 31 in the press molding machine 30. By using a servo motor as a drive source for the die 31, the slide motion of the die 31 and punch 32 can be freely controlled. In this case, the press molding machine 30 operates with the punch 32 as the fixed side and the die 31 and die cushion pad 33 as the movable side.

The press molding machine 30 applicable to this embodiment is not limited to the one described above, and the punch 32 may be the movable side, and the die 31 and die cushion pad 33 may be the fixed side. In this case, a servo motor may be used as the drive source for each of the punch 32 and the die cushion pad 33. Furthermore, the punch 32, the die 31, and the die cushion pad 33 may each be configured to be movable. In this case, a servo motor may be used as the drive source for each of the punch 32, the die 31, and the die cushion pad 33.

In the press molding machine 30 according to this embodiment, the following are set in the order along the relative movement direction of the punch 32 with respect to the die 31: an initial position of the punch, an intermediate position A closer to the die 31 than the initial position, a contact position where the punch 32 first contacts the metal sheet 1, a first stop position B, a second stop position C, and a bottom dead center position D of the punch 32. The initial position and contact position of the punch 32 are determined by the structure of the press molding machine 30. The intermediate position A, the first stop position B, the second stop position C, and the bottom dead center position D are determined by the relationships described below.

First, the initial position of the punch 32 is the relative position of the punch 32 with respect to the die 31 when the punch 32 is waiting before the start of press forming. Specifically, it is a position separated from the die 31 as shown in FIG. 2(a).

The contact position is the relative position of the punch 32 with respect to the die 31 when the punch 32 first comes into contact with the metal sheet 1. In Figures 2 and 3, the contact position is at the same height as the upper surface 31a of the die 31. The contact position is also the position where the unformed metal sheet 1 is first fixed by the die 31 and the die cushion pad 33.

Intermediate position A is a position where the relative average speed of the punch 32 and die 31 is changed from the first speed to the second speed, and is a position 10 mm to 20 mm away from the contact position toward the initial position.

The first stop position B is a position determined based on the position (maximum load position) where the forming load of the punch 32 during forming is at its maximum value, and is the relative position of the punch 32 with respect to the die 31 when the punch 32 reaches a range from the maximum load position to a position where the forming load is 80%, preferably 90%, of the maximum value. The first stop position B is determined at least by the material, thickness, and mechanical properties (strength, elongation, etc.) of the metal sheet 1, the die cushion force of the die cushion pad 33, and the shapes and sizes of the die 31 and punch 32. The first stop position B may be determined each time press forming is performed by measuring the forming load applied to the punch 32 during press forming, or it may be determined based on the maximum load position of the punch 32 at that time by performing a test press forming on the metal sheet 1 in advance.

The second stop position C is set at a position 5 to 10 mm away from the bottom dead center position D of the punch 32 toward the initial position.

The bottom dead center position D of the punch is arbitrarily determined depending on the target shape of the formed product 2. As shown in FIG. 1, if the forming height of the formed product 2 is d, the bottom dead center position D in FIG. 2 and FIG. 3 is set at a position that is a distance away from the contact position (positioned on the upper surface 31a of the die 31) that corresponds to the forming height d.

(First embodiment)
Next, the press molding method of the first embodiment will be described.
The press forming method of the present embodiment includes a first stage in which the metal sheet 1 is placed in the press forming machine 30, and a second stage in which the metal sheet 1 is formed by relatively approaching the die 31 and the punch 32 while applying a die cushion force to the metal sheet 1 by the die cushion pad 33. In the second stage, a relative average speed of the die 31 and the punch 32 from the initial position to the intermediate position A is set to a first speed, and a relative average speed of the die 31 and the punch 32 from the intermediate position A to the bottom dead center position D is set to a second speed slower than the first speed, and further, a stop step is performed in which the punch 32 and the die 31 are temporarily stopped at a first stop position B and a second stop position C between the contact position and the bottom dead center position D of the punch 31. However, the stop step may be performed only at the first stop position B.

The press forming method of this embodiment will be described in detail below with reference to Figures 2 to 4. Figure 4 is a graph showing the relationship between the relative position of the punch with respect to the bottom dead center and time.

(First Stage)
2(a), the metal sheet 1 is placed between the die 31 and the punch 32, the metal sheet 1 is placed on the upper surface 31a of the die 31, and a part 1a of the metal sheet 1 is restrained by the die cushion pad 33. At this time, the punch 31 is in the initial position. Although it depends on the press molding machine 30, the initial position is, for example, a position about 100 to 200 mm away from the position where the metal sheet 1 is placed.

(Second stage)
Next, in the second stage, the die 31 and the punch 32 are moved relatively closer to each other while applying a die cushion force to the metal sheet 1 by the die cushion pad 33, thereby forming the metal sheet 1. In this embodiment, the die cushion force is constant from the start to the end of forming. Specifically, while the metal sheet 1 is restrained by the die cushion pad 33, the metal sheet 1 is raised together with the die 31 and the die cushion pad 33 and brought closer to the punch 32, and forming is performed while the metal sheet 1 is being deformed by the die 31 and the punch 32. There is no particular limit to the die cushion force, and it may be set in the range of 10 to 100 kN, for example.

2(a) and 2(b), while the steel sheet 1 is restrained by the die cushion pad 33, the die 31 and the punch 31 are moved relatively close to each other at a first speed V1 while the tip 32a of the punch 32 moves from the initial position to the intermediate position A. In FIG. 4, the initial position corresponds to the punch position of 200 mm on the vertical axis of FIG. 4. The intermediate position A corresponds to the punch position of 55 mm in FIG. 4. The movement of the punch 32 during the stroke relative to the die 31 from the initial position to the intermediate position A may be a so-called crank motion. The first speed V1 is the relative average speed of the punch 31 with respect to the die 31 from the initial position to the intermediate position A.

The first speed V1 is preferably set in the range of 100 mm/s to 200 mm/s, for example. By setting the first speed V1 to 100 mm/s or more, the time required for the punch 32 to move from the initial position to the bottom dead center position D and then return to the initial position again (hereinafter referred to as the forming time) can be shortened. Furthermore, by setting the first speed V1 to 200 mm/s or less, the relative speed of the punch 32 with respect to the die 31 can be reliably decelerated from the first speed V1 to the second speed V2 when the punch 32 reaches the intermediate position A.

The first speed V1 corresponds to 15 to 30 spm when expressed in strokes per minute (spm), which is an index of how many times the punch can move back and forth between the initial position and the bottom dead center in one minute.

In the second stage, as shown in Figs. 2(b) to 3(c), the die 31 and punch 32 are moved relatively closer to each other at a second speed V2 while the tip 32a of the punch 32 moves from the intermediate position A to the bottom dead center position D. The bottom dead center position D in Fig. 4 corresponds to the punch position on the vertical axis of Fig. 4 at 0 mm. The movement of the die 31 and punch 32 during the stroke from the intermediate position A to the bottom dead center position D may be a so-called crank motion. The second speed V2 is the relative average speed of the punch 32 with respect to the die 31 from the intermediate position A to the bottom dead center position D. However, the stop time of the intermediate stop step is not included in the calculation of the second speed V2.

The second speed V2 is preferably set in the range of, for example, 65 mm/s to 130 mm/s. By setting the second speed V2 to 65 mm/s or more, an increase in the forming time can be prevented. Furthermore, by setting the second speed V2 to 130 mm/s or less, strain can be prevented from concentrating at a specific location of the metal sheet 1. Furthermore, the second speed V2 is preferably set to a speed between 40% and 70% of the first speed V1. By setting the second speed V2 in this range, it becomes easier to control the speed of the punch 32 and the die 31 during forming of the metal sheet 1. Furthermore, the second speed V2 is a speed equivalent to 10 to 20 spm when expressed in strokes per minute (spm).

The movement of the punch 32 in this embodiment is a crank motion at a first speed V1 from the initial position to the intermediate position A, a crank motion at a second speed V2 from the intermediate position A to the bottom dead center position D, and a crank motion at the first speed V1 from the bottom dead center position D after forming is completed to return to the initial position, so that the punch 32 moves in a so-called link motion overall. For reference, in Figure 4, the crank motion at a constant speed is shown by a dotted line to contrast with the link motion of the punch 32 in this embodiment.

In the second stage, as shown in FIG. 2(c), forming of the metal sheet 1 begins when the tip 32a of the punch 32 comes into contact with the metal sheet 1 at the contact position. Then, as shown in FIG. 3(a), while the tip 32a of the punch 32 moves from the contact position to the first stop position B, the metal sheet 1 is bent and deformed mainly by the punch 32, forming the first bent portion 1b, which is the punch shoulder. In the initial stage of forming from FIG. 2(c) to FIG. 3(a), the bottom surface portion 21 and the first bent portion 1b of the formed product 2 are formed. The forming load on the punch 32 in the initial stage of forming is at its maximum value.

Next, in the second stage, as shown in FIG. 3(a), when the tip 32a of the punch 32 reaches the first stop position B, a stop step is performed to temporarily stop the operation of the die 31 and punch 32. In FIG. 4, the first stop position B corresponds to the punch position of 35 mm on the vertical axis of FIG. 4. The stop time is in the range of 0.2 to 1.0 seconds. After the stop time has elapsed, the operation of the die 31 and punch 32 is resumed. By setting the stop time to 0.2 seconds or more, the distortion dispersion effect is sufficient, and by setting the stop time to 1.0 second or less, the molding time can be shortened. The stop step may be performed only once at the first stop position B.

As described above, the first stop position B is determined based on the position where the forming load of the punch 32 during forming of the metal sheet 1 is at its maximum value. The first stop position B is the relative position of the punch 32 with respect to the die 31 when the punch 32 reaches a position in the range from the maximum load position to a position where the forming load is 80%, preferably 90%, of the maximum value. By performing the stopping step at the first stop position B, the distortion in the metal sheet 1 is dispersed.

Figure 7 shows an example of the change in the forming load of the punch 32 while forming the metal sheet 1. On the horizontal axis (forming height) of Figure 7, the contact position where the punch 32 comes into contact with the metal sheet 1 is set to 0 mm, and the position of the bottom dead center is set to 55 mm. In the example shown in Figure 7, the forming load reaches its maximum value at a forming height of 17 mm, so this position is set as the maximum load position, and the first stop position B can be set in the range from this position to a position where the forming load is 80%, preferably 90%, of the maximum value. In practice, press forming can be actually performed on the metal sheet 1, the change in the forming load at the punch can be measured, and the first stop position B can be set based on the results.

Next, in the second stage, as shown in FIG. 3(b), when the tip 32a of the punch 32 reaches the second stop position C, a stop step is again performed to temporarily halt the operation of the die 31 and punch 32. The stop step may be performed only once at the first stop position B, or may be performed a second time at the second stop position C. In FIG. 4, the second stop position C corresponds to the punch position of 5 mm on the vertical axis of FIG. 4. The stop time is in the range of 0.2 to 1.0 seconds. After the stop time has elapsed, the operation of the die 31 and punch 32 is resumed. By setting the stop time to 0.2 seconds or more, the distortion dispersion effect is sufficient, and by setting the stop time to 1.0 second or less, the molding time can be shortened.

As described above, the second stop position C is set at a position 5 to 10 mm away from the bottom dead center position D of the punch 32 toward the initial position. By performing a stop step again at the second stop position C, it is possible to further distribute the strain in the metal sheet 1. That is, in the intermediate forming period from the first stop step to the last stop step, the metal sheet 1 is mainly tensilely deformed, and the portion corresponding to the vertical wall portion 23 of the formed product 1 is formed. In this intermediate forming period, strain is likely to concentrate in the curved edge portion where the vertical wall portions 23 are joined, especially the curved edge portion near the punch shoulder, but by providing two or more stop steps, the strain is distributed, thereby preventing the material from breaking during forming.

It is preferable to perform the stopping step at least once, and preferably twice or more, since this effectively distributes the strain in the metal plate 1. If the stopping step is not performed, the strain in the metal plate 1 will not be sufficiently distributed, and there is a risk that the material will break during forming or that strength will decrease due to a reduction in the plate thickness.

There is no particular limit to the number of stopping steps, so long as it is one or more, and it may be two, three, or four. If three or more stopping steps are provided, it is effective to perform an additional stopping step between the first stopping position B and the second stopping position C in order to distribute the distortion.

Next, as shown in FIG. 3(c), the die 31 and the die cushion pad 33 are moved relatively closer to the punch 32 until the tip 32a of the punch 32 reaches the bottom dead center position D, thereby completing the forming process.

During the later stage of forming from the last stop step (second stop position C) until the punch 32 reaches the bottom dead center position D, bending deformation is performed at the die shoulder of the metal sheet 1, forming the second bent portion 1c. In addition, when the second bent portion 1c is formed, the part 1a of the metal sheet 1 is sandwiched and restrained between the upper surface 31a of the die 31 and the die cushion pad 33, so the occurrence of wrinkles in the part corresponding to the flange portion 22 of the formed product 2 is suppressed.

To obtain such an effect, it is preferable to set the second stop position C, where the final stopping step is performed, at a position 5 to 10 mm away from the bottom dead center position D of the punch 32 toward the initial position. By setting the second stop position C at a position 5 mm or more away from the bottom dead center position D of the punch 32 toward the initial position, it is possible to reliably form the second bent portion 1c and suppress the occurrence of wrinkles in the flange portion 22. In addition, by setting the second stop position C at a position within 10 mm toward the initial position from the bottom dead center position D of the punch 32, it is possible to widen the distance between the first stop position B and the second stop position C, and it is possible to reliably distribute strain in the metal sheet 1.

As described above, according to this embodiment, in the second stage, the punch 32 is driven at a first speed until it comes into contact with the metal plate 1, and then the punch 32 is decelerated to a second speed slower than the first speed until it reaches the bottom dead center position D, thereby reducing the impact on the metal plate 1 when the punch 32 comes into contact with the metal plate 1.
In addition, by providing at least one stopping step between the contact position where the punch 32 comes into contact with the metal sheet 1 and the bottom dead center position D, when the stopping step is completed and the punch 32 and die 31 are moved again, the strain in the metal sheet 1 being formed is dispersed, thereby suppressing breakage of the material due to concentration of strain.
Furthermore, by setting the relative speed of the punch 32 and the die 31 before and after the stopping step to a second speed that is slower than the first speed, it becomes easier to control the speed of the punch 32 and the die 31 during the forming of the metal sheet 1, and concentration of strain in the metal sheet 1 during forming can be prevented.
As described above, by suppressing the impact on the metal plate 1 and dispersing the strain in the metal plate 1 during forming and preventing strain concentration, it is possible to suppress breakage of the material during forming and prevent cracks in the press-formed product 2.
In addition, in the second stage, by setting the relative speed of the punch 32 and the die 31 to a first speed that is faster than the second speed before the punch 32 comes into contact with the metal plate 1, the increase in forming time due to the provision of a stop step can be suppressed, and the forming time as a whole can be shortened.

In addition, according to this embodiment, the first stopping step is performed when the punch 32 reaches a position in the range from the maximum load position to a position where the forming load is 80% of the maximum value, so that the process can proceed to the stopping step after the forming of the metal sheet 1 by the punch 32 in the early stages of forming is almost completed. This further enhances the strain dispersion effect on the metal sheet 1 during forming by the stopping step, and makes it possible to manufacture a press-formed product 2 of the desired shape while preventing cracks.

In addition, according to this embodiment, by setting the second speed to 40% or more and 70% or less of the first speed, it becomes easier to control the speed of the punch 32 and the die 31 during the forming of the metal sheet 1, and it is possible to prevent concentration of strain in the metal sheet 1 during forming.

In addition, according to this embodiment, by setting the intermediate position A at a position 10 mm to 20 mm away from the contact position, the relative speed between the punch 32 and the die 1 can be reliably reduced to the second speed before the punch 32 comes into contact with the metal plate 1.

In addition, according to this embodiment, by setting the stopping time in the stopping step to a range of 0.2 to 1.0 seconds, it is possible to more reliably distribute the strain in the metal plate when the punch and die are moved again after the stopping step.

Second Embodiment
Next, a second embodiment will be described. In the press molding method of the first embodiment described above, the die cushion force is constant from the start to the end of molding, but in the press molding method of this embodiment, two or more stop steps are provided during press molding in the second stage, and the die cushion force is changed in each stop step. Since the operations other than the control of the die cushion force are the same as in the first embodiment, in this embodiment, the control of the die cushion force will be described and other descriptions will be omitted.

Figure 5 is a diagram explaining the press molding method of this embodiment, and shows in graph form the relationship between the relative position of the punch with respect to the bottom dead center, the die cushion force, and time. In order to make it easier to understand the relationship between the relative position of the punch and the die cushion force, Figure 5 is enlarged in the direction of the vertical axis compared to Figure 4. That is, while the maximum value of the vertical axis is 200 mm in Figure 4, it is 80 mm in Figure 5. Also, the initial position is not shown in Figure 5.

In this embodiment, as shown in FIG. 5, the die cushion force applied to the metal sheet 1 by the die cushion pad 33 is a first die cushion force until the first stop step, a second die cushion force smaller than the first die cushion force from the first stop step to the last stop step, and a third die cushion force larger than the second die cushion force after the last stop step. In this way, the second die cushion force is made smaller than the first die cushion force and the third die cushion force.

More specifically, as shown in Figures 2(a) to 3(a), the die cushion force when the relative position of the punch 32 with respect to the die 31 moves from the initial position to the first stop position B is defined as the first die cushion force, the die cushion force when the punch 32 moves from the first stop position B to the second stop position C is defined as the second die cushion force, and the die cushion force when the punch 32 moves from the second stop position C to the bottom dead center position D is defined as the third die cushion force. After the relative position of the punch 32 with respect to the die 31 reaches the bottom dead center position D, the constraint by the die cushion pad 33 may be released.

The first die cushion force and the third die cushion force may be set, for example, in the range of 10 to 100 kN. The first die cushion force and the third die cushion force may be the same, or one of them may be smaller. Furthermore, it is preferable that the second die cushion force is in the range of 0.1 to 0.5 times the first die cushion force or the third die cushion force. For example, the second die cushion force may be in the range of 5 to 50 kN.

The change from the first die cushion force to the second die cushion force is preferably made during the first stop step, and the change from the second die cushion force to the third die cushion force is preferably made during the last stop step. If the die cushion force is changed at any time other than the stop step, the restraint of the metal sheet 1 during forming becomes unstable, raising concerns about the occurrence of wrinkles, so it is advisable to change the die cushion force during the stop step. Furthermore, by changing the die cushion force in the first stop step and the last stop step, the section in which the second die cushion force is maintained can be widened, further promoting the inflow of material. In this case, it is more preferable that the stop time of the stop step be 0.5 seconds or more.

According to this embodiment, by making the first die cushion force before the first stopping step higher than the second die cushion force, it is possible to improve the formability of the punch shoulder portion (first bent portion 1b) of the metal sheet 1 by the punch 32 in the early stage of forming.
Furthermore, by making the second die cushion force between the first and final stopping steps smaller than the die cushion forces before the first and after the final stopping steps (first and third die cushion forces), the flow of material between the punch 32 and the die 31 in the middle of forming is promoted, thereby preventing excessive reduction in plate thickness in the vertical wall portions of the metal sheet 1 during forming and dispersing strain better, suppressing breakage of the material due to concentration of strain and preventing cracks in the press-formed product.
Furthermore, by making the third die cushion force after the final stopping step higher than the second die cushion force, it is possible to improve the formability of the die shoulder portion (second bent portion 1c) and the flange portion 22 of the metal sheet 1 by the punch 32 and the die 31 in the later stage of forming. This makes it possible to obtain a formed product 2 having a desired shape and free of cracks.

In addition, according to this embodiment, when the punch 32 reaches the second stop position C, which is 5 to 10 mm away from the bottom dead center position D, a final stop step is performed and the die cushion force is changed to the third die cushion force at that time, thereby further improving the formability of the die shoulder portion (second bent portion 1c) and flange portion 22 of the metal sheet 1 by the punch 32 and die 31 in the later stage of forming.

Third Embodiment
Next, a third embodiment will be described. In the press forming method of the second embodiment described above, the die cushion force is changed while changing the relative average speed of the die 31 and the punch 32 from a first speed to a second speed at the intermediate position A, but in the press forming method of this embodiment, the die cushion force is changed without changing the relative average speed of the die 31 and the punch 32 at the intermediate position A. Since the operations other than the control of the relative speed of the die 31 and the punch 32 are the same as those of the second embodiment, in this embodiment, the control of the relative average speed of the die 31 and the punch 32 will be described and other descriptions will be omitted.

FIG. 6 is a diagram for explaining the press forming method of this embodiment, and shows in a graph the relationship between the relative position of the punch with respect to the bottom dead center, the die cushion force, and time.
6, the punch 32 according to this embodiment is operated in a so-called crank motion without changing the relative average speed from the initial position to the intermediate position A and the relative average speed from the intermediate position A to the bottom dead center position D. After defining the first stop position B and the second stop position C, the die cushion force is controlled in the same manner as in the second embodiment.

According to this embodiment, as in the second embodiment, in the second stage, at least two or more stopping steps are provided between contact position A where punch 32 comes into contact with metal sheet 1 and bottom dead center position D. This enables the dispersion of strain in metal sheet 1 being formed when punch 32 and die 31 are moved again after the stopping steps are completed, thereby suppressing breakage of the material due to concentration of strain and preventing cracks in the press-formed product.
Furthermore, by making the first die cushion force before the first stopping step higher than the second die cushion force, the formability of the metal sheet 1 by the punch 32 in the initial stage of forming can be improved.
Furthermore, by making the second die cushion force between the first and final stopping steps smaller than the die cushion forces before the first and after the final stopping steps (first and third die cushion forces), the flow of material between the punch 32 and the die 31 in the middle of forming is promoted, thereby preventing excessive reduction in plate thickness in the metal sheet 1 during forming and dispersing strain more effectively, thereby suppressing breakage of the material due to concentration of strain.
Furthermore, by making the third die cushion force after the stopping step higher than the second die cushion force, the formability of the metal sheet 1 by the punch 32 and the die 31 in the later stage of forming can be improved.
As a result, a press-molded product having the desired shape and free of cracks can be obtained.

The present invention will now be described in more detail with reference to examples.
A metal plate was press-formed under the conditions shown in Table 1A. The metal plate was a duplex stainless steel sheet (NSSC2120 manufactured by Nippon Steel Stainless Steel Corporation) with a size of 170 mm square and a thickness of 1.0 mm. The press-formed product had the shape shown in Fig. 1, with the bottom part measuring 70 mm long x 70 mm wide, the vertical wall part having a height of 50 mm, and the flange part having a width of 10 mm.

In Table 1A, in test examples 1 and 10, the punch motion was changed from a first speed to a second speed at an intermediate position, which is called a link motion, the die cushion force was constant, and the stopping step was performed four times in test example 1 and twice in test example 10. The position of the stopping step is the height position when the height of the bottom dead center is set to 0 mm.

In test examples 2, 3, 6 to 8, and 11, the punch motion was a so-called link motion, and the die cushion force was changed as shown in Table 1A. The die cushion force was changed at the first stop step and the final stop step.

In test example 4, the punch motion was a crank motion, and the die cushion force was constant.

In test example 5, the punch motion was a so-called link motion, and the die cushion force was constant.

In test example 9, the punch motion was a so-called link motion, and the die cushion force was changed as shown in Table 1A. The die cushion force was changed at the first and last stopping steps. The stopping step had a stopping time of 0 seconds. In other words, there was essentially no stopping step, and the die cushion force was changed at positions 35 mm and 5 mm from the bottom dead center.

In addition, in test examples 1 to 3 and 7 to 11, excluding test example 6, the position of the first stopping step was set to a position 35 mm above the bottom dead center. If the position of the punch when the punch forming load reached its maximum value was defined as the maximum load position, this position was within the range from the maximum load position to the position where the forming load was 80% of the maximum value.

The intermediate position where the speed changes from the first to the second was set at a position 15 mm away from the contact position.

The evaluation method was to judge the presence or absence of cracks by visual inspection. Conditions where cracks were observed were marked with ×, and conditions where no cracks were observed were marked with ○ or ◎. Furthermore, for test examples where no cracks occurred, the sheet thickness reduction rate was measured at a location near the vertical wall of the first bend (punch shoulder) between the bottom surface and the vertical wall. A sheet thickness reduction rate of less than 5% of the thickness of the blank metal sheet was marked with ◎, and a sheet thickness reduction rate of 5% or more was marked with ○. The results are shown in Table 1B.

Test example 1 was rated as good because the magnitude of the die cushion force was kept constant.

Test examples 2 and 3 were press molded under the most favorable conditions and were therefore rated as ◎.

Test examples 4 and 5 were rated x because they were pressed under conditions outside the scope of the present invention.

Test example 6 was rated as good because the position of the first stop step was outside the range from the maximum load position to the position where the forming load is 80% of the maximum load, assuming that the maximum load position is the punch position when the punch forming load is at its maximum value.

Test example 7 was rated as good because the final stopping step was performed when the punch reached a position 15 mm away from the bottom dead center.

Test example 8 was rated as good because the second speed was not between 40% and 70% of the first speed.

Test example 9 was rated as x because the die cushion force was changed without actually performing a stopping step.

Test example 10 was rated as good because the magnitude of the die cushion force was kept constant.

In test example 11, the magnitude of the second die cushion force was outside the range of 0.1 to 0.5 times the first and third die cushion forces, and was rated as good.

1...metal plate, 31...die, 32...punch, 33...die cushion pad, A...intermediate position, D...bottom dead center position (bottom dead center).

Claims (12)

A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least twice or more times during the period from the contact position where the punch contacts the metal plate to the bottom dead center,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force from the final stopping step onward, wherein the second die cushion force is made smaller than the first die cushion force and the third die cushion force . A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal plate and the bottom dead center,
A method for press forming a metal sheet, comprising: a first stopping step being performed when the punch reaches a range from the maximum load position to a position where the forming load is 80% of the maximum value, where the position of the punch when the forming load of the punch is at its maximum value is defined as a maximum load position. A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
A stopping step of temporarily stopping the punch and the die is performed at least twice or more between a contact position where the punch contacts the metal plate and a bottom dead center ,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force from the final stopping step onward, wherein the second die cushion force is made smaller than the first die cushion force and the third die cushion force. 4. The press forming method for a metal sheet according to claim 1, wherein a first stopping step is performed when the punch reaches a range from the maximum load position to a position where the forming load is 80% of the maximum value, where the position of the punch when the forming load of the punch is at its maximum value is defined as a maximum load position. The method for press-forming a metal sheet according to claim 1 or 3, further comprising the step of performing a final stopping step when the punch reaches a position 5 to 10 mm away from the bottom dead center. The method for press forming a metal sheet according to claim 1, wherein the second speed is 40% or more and 70% or less of the first speed. The method for press forming a metal sheet according to claim 1, wherein the intermediate position is a position 10 mm to 20 mm away from the contact position. The method for press forming a metal sheet according to claim 1 or 3, wherein the stopping time in the stopping step is in the range of 0.2 to 1.0 seconds. 4. The press forming method for a metal sheet according to claim 1 , wherein the second die cushion force is in a range of 0.1 to 0.5 times the first die cushion force or the third die cushion force. A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal plate and the bottom dead center,
The press forming method for a metal sheet comprises performing a final stopping step when the punch reaches a position 5 to 10 mm away from the bottom dead center . A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least once or more between the contact position where the punch contacts the metal plate and the bottom dead center,
The method for press-forming a metal sheet, wherein the stopping time in the stopping step is in the range of 0.2 to 1.0 seconds . A first step of disposing a metal sheet between a die and a punch and sandwiching a portion of the metal sheet between the die and a die cushion pad;
a second stage in which the die and the punch are moved relatively close to each other while applying a die cushion force to the metal sheet by the die cushion pad, thereby forming the metal sheet;
In the second stage,
a first speed is a relative average speed of the die and the punch from an initial position of the punch to an intermediate position immediately before the punch contacts the metal plate, and a second speed is a relative average speed of the die and the punch from the intermediate position to a bottom dead center, the second speed is set to a speed lower than the first speed, and a stopping step of temporarily stopping the punch and the die is performed at least twice or more times during the period from the contact position where the punch contacts the metal plate to the bottom dead center,
a die cushion force applied to the metal sheet by the die cushion pad up to a first stopping step, a second die cushion force from the first stopping step to a final stopping step, and a third die cushion force after the final stopping step, wherein the second die cushion force is in a range of 0.1 to 0.5 times the first die cushion force or the third die cushion force.

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