JP7476935B2 - Manufacturing method of press-molded products - Google Patents

Description

The present invention relates to a method for manufacturing a press-formed product that is curved when viewed from above and has at least a top plate portion and a vertical wall portion that continues from the top plate portion.

While automobile crash safety standards are becoming stricter and the crash safety of vehicle bodies is improving, there is also a need to reduce the weight of vehicle bodies in order to improve fuel efficiency and promote the shift to electric vehicles in response to carbon dioxide emission regulations.
In order to simultaneously improve the collision safety of these vehicle bodies and reduce their weight, the use of high-strength steel plates (also called high-tensile materials) of 590 MPa class or higher in vehicle body structural parts is increasing.

An automobile part, including a part of its structure, includes a press-formed product 1 that is curved when viewed from above and has at least a top plate portion 3 and a vertical wall portion 5 continuing from the top plate portion 3, as shown in FIG.
When press-molding such a press-molded product, if the die is released after moving it to the bottom dead center of molding, springback occurs, and the press-molded product is likely to become twisted. In particular, in the case of high-tensile steel, the stress generated at the bottom dead center of molding increases due to the high strength, and after release from the die, the large stress is released, making it easy for large twists to occur, which is a problem.

As a method for reducing twisting due to such springback, for example, Patent Document 1 discloses a method for reducing the stress generated by providing a through hole or a groove in a press-formed product.
Furthermore, paragraph [0004] of Patent Document 2 discloses a method of press molding using a die to which a twist angle in the direction opposite to the springback is applied.

JP 2007-253173 A JP 2007-130671 A

The press molding method described in Patent Document 1 creates through holes and grooves in the press molded product, resulting in a press molded product with a shape different from the target, which has problems such as reduced rigidity when assembled to the vehicle body and difficulty in joining parts together.

In addition, the press molding method described in Patent Document 2 above involves applying a twist angle to the die in only one direction, the opposite direction to springback, for press molding, which poses the problem that it is difficult to set the twist angle to be applied to the die. In other words, if the twist angle is too small, the twist that is springback cannot be fully eliminated, and if the twist angle is too large, twist in the opposite direction remains, so it is necessary to apply an appropriate twist angle, which is difficult.

Furthermore, when blanks having different material strengths are press-molded using the same die, or when there is variation in the material, thickness, etc. of the blanks used for press molding, the following problems arise.
In other words, even if press molding is performed using the same die with a conventional twist angle in only one direction, the degree of springback will vary depending on differences in material strength and variations in plate thickness and material quality, resulting in variations in the springback of the press-molded product.

The present invention has been made to solve such problems. In other words, the purpose is to provide a manufacturing method for press-molded products that can reduce shape errors caused by springback after demolding in press-molded products that are curved when viewed from above and have at least a top plate portion and a vertical wall portion continuing from the top plate portion.

(1) The manufacturing method of the press-molded product according to the present invention is a manufacturing method of a press-molded product that is curved in a top view, has at least a top plate portion and a vertical wall portion continuing from the top plate portion, and is manufactured by imparting a projection angle to a die in order to reduce an error from a target shape due to springback after demolding.
a forming process of press-forming using a forming die having a first projection angle such that a twist (positive twist) caused by springback when press-forming is performed in one step without a projection angle being imparted to the die and a twist (negative twist) caused by springback in the opposite direction remains;
and a restriking step of press-molding the molded product produced in the molding step using a restriking die having a second projection angle.

(2) In addition, in the above (1), the first projection angle is larger than the one-step projection angle at which the twist due to springback when the press-formed product is press-formed in one step is equal to or smaller than a predetermined threshold value.

(3) Furthermore, in the above (2), a press forming analysis and a springback analysis of the press-formed product are performed in advance to determine the direction of twist due to springback and the one-step projection angle.

(4) Furthermore, in the above-mentioned (2), the method is characterized in that the actual press forming of the press-formed product is carried out in advance to determine the direction of twist due to springback and the one-step projection angle.

(5) Furthermore, in any of the above (1) to (4), the first projection angle or the second projection angle is an inclination angle of the top plate forming surface portion of the widthwise cross section at or near the longitudinal end of the molding die or the restriking die, based on the top plate forming surface portion of the widthwise cross section at the longitudinal center of the molding die or the restriking die.

(6) Furthermore, in any of the above (1) to (5), the absolute value of the second projection angle in the restriking process is smaller than the absolute value of the first projection angle.

According to the present invention, it is possible to sufficiently reduce the twist, which is the springback after demolding. As a result, it is not necessary to provide through holes or grooves, and it is possible to manufacture press-formed products having a better shape than ever before while maintaining the desired shape of the press-formed product.
Furthermore, even when using blanks with different material strengths or variations in plate thickness or material quality, the same die can be used to manufacture press-formed products with sufficiently reduced springback.

FIG. 1 is an explanatory diagram of the die projection angle in the molding process and the restriking process in an example of the invention. FIG. 1 is an explanatory diagram of the stress state at the bottom dead center of the forming process and the restriking process in an example of the invention. 1 is an explanatory diagram of an example of a press-molded product to which the present invention is directed; 4A to 4C are explanatory diagrams of a forming process and a restriking process according to the embodiment. FIG. 1 is an explanatory diagram of the twist angle of a press-molded product. FIG. 1 is an explanatory diagram of a mold projection angle. FIG. 11 is an explanatory diagram of the torsion angle of a formed product after a forming process and a press product after a restriking process in a conventional example. FIG. 11 is an explanatory diagram of a stress state at the forming bottom dead center in the forming process and the restriking process in a conventional example. FIG. 11 is an explanatory diagram of a die projection angle in a molding process and a restriking process in a comparative example. FIG. 11 is an explanatory diagram of the stress state at the forming bottom dead center in the forming process and the restriking process in the comparative example. FIG. 1 is an explanatory diagram of a press-molded product that is the subject of the examples (part 1). FIG. 2 is an explanatory diagram of the press-molded product used in the examples (part 2).

As shown in Fig. 3 as an example, the press-formed product 1 that is the subject of the present invention has a hat-shaped cross-sectional shape that is curved when viewed from above and has at least a top plate portion 3, a vertical wall portion 5 continuing from the top plate portion 3, and a flange portion 7 at the lower end of the vertical wall portion 5. In the example shown in Fig. 3, the flange portion 7 is included in addition to the top plate portion 3 and the vertical wall portion 5, but the presence of the flange portion 7 is not essential.
The operation of the present invention will be described below with reference to an example of the press-formed product 1 shown in FIG. 3, which is formed using a 1,470 MPa class steel plate having a thickness of 1.0 mm as a blank 9.

As shown in FIG. 4, the press-formed product 1 shown in FIG. 3 is formed in a forming process (FIG. 4(a)) and a restriking process (FIG. 4(b)). In the forming process, a blank 9, which is a metal plate, is pressed by a pad 11 and a forming punch 13, and a forming die 15 is moved relatively to form a formed product 17 having a top plate portion 3, a vertical wall portion 5, and a flange portion 7. Then, in the restriking process, the formed product 17 formed in the forming process is shaped by a restriking punch 19 and a restriking die 21.

The shape of the press-formed product 1 after restriking, particularly the twist angle, will be explained with reference to Figure 5. Figure 5(a) is a plan view of the press-formed product 1, Figure 5(b) is a diagram showing the P-P cross section of the longitudinal center of the press-formed product 1 together with the forming surface of the longitudinal center of the restriking punch 19, and Figure 5(c) is a diagram showing the Q-Q cross section near the longitudinal end of the press-formed product 1 together with the forming surface of the longitudinal center of the restriking punch 19.

As shown in Figure 5, when the cross section of the top plate forming surface at the longitudinal center of the restriking punch 19 is aligned with the top plate portion 3 of the P-P cross section at the longitudinal center of the press-formed product 1, the Q-Q cross section shape near the longitudinal end rotates clockwise (rightward on the page with the outer side of the curve on the left and the inner side of the curve on the right) relative to the cross section of the top plate forming surface at the longitudinal center of the restriking punch 19. This indicates that the press-formed product 1 is twisted due to springback.

In the following explanation, the twist angle of the molded product 17 and the press-molded product 1 is defined as the angle between the cross section of the top plate forming surface at the longitudinal center of the forming punch 13 and the restriking punch 19 and the cross section of the top plate forming surface at the longitudinal center of the forming punch 13 and the restriking punch 19, based on the cross section (see FIG. 5(b)). The cross section of the top plate 3 near the longitudinal end (for example, about 10 mm toward the center from the outermost end) is defined as a + (plus) value when the angle is shifted leftward (counterclockwise) on the paper, with the outer side of the curve on the left and the inner side of the curve on the right, and the cross section of the top plate forming surface at the longitudinal center of the forming punch 13 and the restriking punch 19 is defined as a - (minus) value when the angle is shifted rightward (clockwise) on the paper.

The die projection angle is defined as the angle of the cross section (S-S cross section) near the longitudinal end (for example, about 10 mm inside from the extreme end) (see Figs. 6(a) and (c)) with the cross section (R-R cross section) (see Figs. 6(a) and (b)) of the longitudinal center of the top plate forming surface of the forming punch 13 and the restriking punch 19 as the reference point. The die projection angle is defined as + (plus) when it rotates left (counterclockwise) on the page, with the outside of the curve on the left and the inside of the curve on the right, and - (minus) when it rotates right (clockwise) on the page.

The circumstances leading to the present invention will be described below through explanations of conventional examples and comparative examples.
<Conventional Example>
In the conventional example, after the molding process, the die was restriked with the same shape. Figure 7 shows a Q-Q cross section (see Figure 5) near the longitudinal end of a press-formed product 17A after the molding process (Figure 7(a)) and a press-formed product 1A after the restriking process (Figure 7(b)) in the conventional example.
In the conventional example, as shown in FIG. 7, the twist angle due to springback after the forming process remains negative, and is not corrected even in the restriking process, remaining almost as it is.
The torsion angle due to springback of the press-formed product 17 after the press-forming process shown in FIG. 7(a) is −3.1 degrees, and the torsion angle due to springback of the press-formed product 1A after restriking shown in FIG. 7(b) is −3.4 degrees.

A press forming analysis was performed using the finite element method (FEM) for two steps (forming step and restriking step) in the conventional example. Figure 8 shows contour diagrams of the analysis results, in which Figure 8(a) shows the stress distribution at the bottom dead center of a formed product 17A after the forming step, and Figure 8(b) shows the stress distribution at the bottom dead center of a press formed product 1A after the restriking step.
8, in both the molded product 17A and the press-molded product 1A, in the longitudinal center, large compressive stress due to shrink flange deformation occurs in the outer flange portions 177A and 7A, and large tensile stress due to stretch flange deformation occurs in the inner flange portions 177A and 7A. In addition, large tensile stress occurs on the outer side of the curve of the top plate portions 173A and 3A, and large compressive stress occurs on the inner side of the curve of the top plate portions 173A and 3A.
In addition, in both the molded product 17A and the press-molded product 1A, the compressive stress and tensile stress are small near the longitudinal ends.

Therefore, when these stresses are released by demolding, in conventional examples, these stresses act as a driving force to cause twisting at the longitudinal ends.

Comparative Example
In the comparative example, in order to eliminate the twisting that occurred in the conventional example, in the forming process, a forming die (forming punch 13B) with a one-step lead angle is used near the longitudinal end (10 mm inside from the end) from the longitudinal center to the longitudinal end (see FIG. 9( a)), and then in the restriking process, forming is performed with a restriking die (restriking punch 19B) of the target shape (see FIG. 9( b)).
The one-step projection angle in this example is 6.0 degrees in the direction opposite to the twist of springback with respect to the die having the target shape.

In the comparative example, a one-step angle is provided in the forming process to anticipate springback, so there is almost no twisting due to springback after the forming process, and when the part is formed in the restriking process using the restriking die 19B of the target shape, the resulting shape is close to the target shape.

A press forming analysis was performed using the finite element method (FEM) for the forming process and the restriking process in the comparative example. Figure 10 shows contour diagrams of the analysis results, with Figure 10(a) showing the stress distribution at the bottom dead center of the formed product 17B after the forming process and Figure 10(b) showing the stress distribution at the bottom dead center of the press formed product 1B after the restriking process.
10A, in the comparative example, at the bottom dead point of molding, a large compressive stress due to shrink flange deformation is generated in the outer flange portion 177B, and a large tensile stress due to stretch flange deformation is generated in the inner flange portion 177B. Also, a large tensile stress is generated on the outer side of the curve of the top plate portion 173B, and a large compressive stress is generated on the inner side of the curve of the top plate portion 173B.

After demolding in the forming process, springback occurs, but since the one-step projection angle causes springback in the opposite direction to that of the conventional example, after springback, the part assumes approximately the target shape.
Therefore, at the bottom dead point of the restriking process, as shown in FIG. 10(b), the generated stress is reduced compared to the conventional example, and when the mold is released after the bottom dead point of the restriking process, the target shape is achieved with almost no springback.

However, at the bottom dead point of the forming in the restriking process, some compressive stress remains on the inside of the curve of the outer flange portion 7B and the top plate portion 3B, and tensile stress remains on the outside of the curve of the inner flange portion 7B and the top plate portion 3B, so that the stress is not sufficiently reduced.
Therefore, when using the same die to press-form blanks 9 having different material strengths, or when press-forming blanks 9 having variations in material, plate thickness, etc., twisting due to springback may occur after the restriking process.
In other words, as shown in the comparative example, in the method of imparting a one-step projection angle using only the molding process, the distribution of residual stress in the press-molded product 1B varies depending on differences in material strength and variations in material quality and plate thickness, etc., and as a result, there are cases where springback cannot be sufficiently reduced even when the same mold is used.

Therefore, in the present invention, the forming step and the restriking step are carried out as follows.
<Molding process>
The forming process is performed by press forming using a forming die 13C (see FIG. 1(a)) that is given a first projection angle such that a twist due to springback in the opposite direction (reverse twist) remains compared to the twist (positive twist) that occurs due to springback when press forming is performed in one step without giving the die a projection angle.
As shown in the comparative example, in this example, the torsion after springback is reduced most by setting the one-step angle to 6.0 degrees, so in order to leave the reverse torsion, the first angle is set to be greater than 6.0 degrees. Specifically, in the forming process of Fig. 1(a), the first angle of the S-S cross section in the longitudinal direction of the die is set to 8.0 degrees with respect to the R-R cross section at the center of the longitudinal direction of the die in Fig. 6.

<Restriking process>
In the restriking step, the molded product 17C molded in the molding step is press-molded using a restriking die 19C having a second projection angle for reducing the reverse twist.
After the mold is released from the molding process, reverse twist remains due to springback. In order to reduce this, as shown in FIG. 1(b), a restriking mold 19C is used to impart a second projection angle in the same direction as the springback in order to reduce reverse twist (in this example, the angle of the S-S cross section near the longitudinal end of the mold is −6.0 degrees with respect to the R-R cross section at the longitudinal center of the mold in FIG. 6), forming into a target shape.

The forming process and the restriking process were analyzed using an FEM to determine the stress distribution at the bottom dead center of the forming process, which is shown in FIG.
As shown in FIG. 2A, in the molding process, in the longitudinal center of the molded product 17C, a large compressive stress due to shrinkage flange deformation is generated in the outer flange portion 177C, and a large tensile stress due to stretch flange deformation is generated in the inner flange portion 177C.
In addition, a large tensile stress is generated on the outer side of the curved top plate portion 173C, and a large compressive stress is generated on the inner side of the curved top plate portion 173C.

As shown in Figure 2(b), in the longitudinal center of the press-formed product 1C in the restriking process, the compressive stress of the outer flange portion 7C, the tensile stress on the outer side of the curve of the top plate portion 3C, the tensile stress of the inner flange portion 7C, and the compressive stress on the inner side of the curve of the top plate portion 3C are all significantly reduced.

2(b) of the present invention and Fig. 10(b) of the comparative example, in the case of the present invention shown in Fig. 2(b), the compressive stress of the outer flange portion 7C and the tensile stress of the inner flange portion 7C are reduced compared to the comparative example. Furthermore, the tensile stress on the outer side of the curve of the table top portion 3C and the compressive stress on the inner side of the curve of the table top portion 3C are also reduced compared to the comparative example.
That is, the stress distribution of the press-formed product 1C after the restriking process shown in Fig. 2(b) of the present invention is lower in the entire press-formed product than the stress of the press-formed product 1B after the restriking process shown in Fig. 10(b) of the comparative example, and the torsion due to springback can be sufficiently reduced. At the same time, even when the same die is used to press-form blanks 9 having different material strengths or blanks 9 having variations in material and plate thickness, the stress generated in the press-formed product 1C can be sufficiently reduced, and as a result, the springback of the press-formed product 1C can be reduced.

Although the above has been described with respect to hat-shaped cross-section parts, the present invention is not limited to this. In other words, the present invention can also be applied to U-shaped cross-section parts that are curved when viewed from above and consist of a top plate and vertical wall portions on both sides, Z-shaped cross-section parts that consist of a vertical wall portion and a flange portion on only one side of the top plate, and L-shaped cross-section parts that consist of a top plate and a vertical wall portion on only one side. It can also be applied to cases where a part of a press-molded product is curved.

In order to confirm the effect of the present invention, 1,470 MPa class steel plates and 980 MPa class steel plates having a thickness of 1.0 mm were used as blanks 9 to study the twist angle, which is springback, and the difference in twist angle due to differences in material strength.
As shown in Figure 11, the shape of the press-molded product 1 is a hat-shaped cross-section having a top plate portion 3, a vertical wall portion 5 continuing from the top plate portion 3, and a flange portion 7 continuing from the vertical wall portion 5, and the cross-sectional dimensions are as shown in Figure 12.
As described above, the twist angle due to springback and the die projection angle are based on the cross section of the top plate molding part at the center of the longitudinal direction of the die, with the outside of the curve being the left and the inside of the curve being the right, and the left-handed side on the page is a + (plus) value and the right-handed side on the page is a - (minus) value.

First, the direction of twist (positive twist) due to springback was obtained by molding using a die with no die projection angle. Furthermore, the one-step projection angle that can minimize the twist due to springback in one step was obtained to be 6.0 degrees.
Thereafter, the molding process and restriking process were carried out for each of the conventional example, comparative example, and invention example described above.
The results are shown in Table 1.

For the conventional examples No. 1-1 (1470 MPa class material) and No. 1-2 (980 MPa class material), the die projection angle was set to 0 degrees in both the forming process and the restriking process, and the die of the target shape was used as is. The twist angles of the press-formed products 1A after restriking were -3.4 degrees and -2.3 degrees, respectively, and there was a large amount of twisting due to springback. In addition, the difference in twist angle due to the material strength after the restriking process was large at -1.1 degrees.

In the comparative examples No. 2-1 (1470 MPa class material) and No. 2-2 (980 MPa class material), the one-step projection angle in the forming process was 6.0 degrees, and the die projection angle in the restriking process was 0 degrees. The torsion angle of the press-formed product 1A after restriking was reduced to 0.6 degrees in the case of No. 2-1 (1470 MPa class material), but was 2.0 degrees in the case of No. 2-2 (980 MPa class material), resulting in significant torsion due to springback. As a result, the difference in torsion angle due to material strength after the restriking process was -1.4 degrees, which was greater than the difference between the conventional examples No. 1-1 and No. 1-2.

On the other hand, in the invention examples No. 3-1 (1470 MPa class material) and No. 3-2 (980 MPa class material), the first projection angle in the forming process was 7.0 degrees, and the second projection angle in the restriking process was -2.0 degrees. The twist angle of the press-formed product 1C after restriking was 0.3 degrees in the case of No. 3-1 (1470 MPa class material) and 0.6 degrees in the case of No. 3-2 (980 MPa class material), which reduced the twist caused by springback. As a result, the difference in twist angle due to the material strength after the restriking process was -0.3 degrees, and even though the material strength of the 1470 MPa class material and the 980 MPa class material was significantly different, the difference in twist angle due to the difference in material strength using the same die was small. Therefore, it was found that springback can be sufficiently reduced even when press forming is performed using different materials using the same die.

In the comparative examples No. 4-1 (1470 MPa class material) and No. 4-2 (980 MPa class material), the first projection angle in the forming process was increased to 8.0 degrees, and the die projection angle in the restriking process was 0 degrees. The twist angle of the press-formed product 1B after these restriking processes was 2.2 degrees for No. 4-1 (1470 MPa class material) and 3.4 degrees for No. 4-2 (980 MPa class material). In addition, the difference in twist angle due to material strength after the restriking process was large at -1.2 degrees.

In the invention examples No. 5-1 (1470 MPa class material) and No. 5-2 (980 MPa class material), the first projection angle in the forming process was 8.0 degrees, and the second projection angle in the restriking process was -6.0 degrees. The twist angle of the press-formed product 1C after these restriking processes was 0.1 degrees in the case of No. 5-1 (1470 MPa class material) and 0.2 degrees in the case of No. 5-2 (980 MPa class material), and the twist due to springback was sufficiently reduced in both the 1470 MPa class material and the 980 MPa class material.
In addition, the difference in twist angle due to the material strength after the restriking process was -0.1 degrees. Although the material strength differed greatly between the 1470 MPa class material and the 980 MPa class material, the difference in twist angle due to the difference in material strength was slight even when the same die was used.
Therefore, it was found that according to the present invention, even when press molding is performed using different materials using the same die, springback can be sufficiently reduced.

REFERENCE SIGNS LIST 1 Press-molded product 3 Top plate portion 5 Vertical wall portion 7 Flange portion 9 Blank 11 Pad 13 Forming punch 15 Forming die 17 Molded product 19 Re-striking punch 21 Re-striking die 1A Press-molded product (conventional example)
3A Top plate portion 5A Vertical wall portion 7A Flange portion 1B Press-molded product (Comparative example)
3B Top plate portion 5B Vertical wall portion 7B Flange portion 1C Press-molded product (example of the invention)
3C: Top plate portion 5C: Vertical wall portion 7C: Flange portion 13A: Forming punch (conventional example)
13B Forming punch (comparative example)
13C Forming punch (example of invention)
17A Molded product (conventional example)
173A Top plate portion 175A Vertical wall portion 177A Flange portion 17B Molded product (Comparative example)
173B Top plate portion 175B Vertical wall portion 177B Flange portion 17C Molded product (example of the invention)
173C: Top plate portion 175C: Vertical wall portion 177C: Flange portion 19A: Re-strike punch (conventional example)
19B Re-strike punch (Comparative example)
19C Re-strike punch (example of invention)

Claims (5)

A method for manufacturing a press-molded product that is curved when viewed from above, has at least a top plate portion and a vertical wall portion continuing from the top plate portion, and is manufactured by imparting a projection angle to a die in order to reduce an error from a target shape due to springback after release from the die,
a forming process of press-forming using a forming die having a first projection angle such that a twist (positive twist) caused by springback when press-forming is performed in one step without a projection angle being imparted to the die and a twist (negative twist) caused by springback in the opposite direction remains;
and a restriking process in which the molded product molded in the molding process is press-molded using a restriking die having a second projection angle that reduces the reverse twist .
However, the first projection angle is defined as the inclination angle of the top plate molding surface portion of the widthwise cross section at the longitudinal end or its vicinity of the molding die, based on the top plate molding surface portion of the widthwise cross section at the longitudinal center of the molding die, and the second projection angle is defined as the inclination angle of the top plate molding surface portion of the widthwise cross section at the longitudinal end or its vicinity of the restriking die, based on the top plate molding surface portion of the widthwise cross section at the longitudinal center of the restriking die. The method for manufacturing a press-formed product according to claim 1, characterized in that the first projection angle is greater than a one-step projection angle at which twisting due to springback when the press-formed product is press-formed in one step is equal to or smaller than a predetermined threshold value. The method for manufacturing a press-formed product according to claim 2, characterized in that a press forming analysis and a springback analysis of the press-formed product are performed in advance to determine the direction of twist due to springback and the one-step projection angle. The method for manufacturing a press-formed product according to claim 2, characterized in that the actual press-formed product is first press-formed to determine the direction of twist due to springback and the one-step projection angle. The method for manufacturing a press-formed product according to any one of claims 1 to 4, characterized in that the absolute value of the second projection angle in the restriking process is smaller than the absolute value of the first projection angle.