Methods for manufacturing pressed parts.

Abstract

A method for manufacturing a press-molded article 1 provided with at least a top plate portion 3 having a concave curved portion in a side view and a longitudinal wall portion 7 connecting from the top plate portion 3 through a punch-shoulder-radiused portion 5, the method comprising: a first molding step for press-molding an intermediate molded article 19 having an intermediate top plate portion 21 that curves in the same direction as the top plate portion 3, a step-shaped portion 17 that comprises a step formed continuous with a ridge portion 23 formed in an area corresponding to the punch-shoulder-radiused portion 5, and an outward surface portion 25 that extends outward continuous from the step-shaped portion 17 and curves in the same direction as the intermediate top plate portion 21; and a second molding step for press-molding the intermediate molded article 19 into a press-molded article 1.

Description

Manufacturing method for press-molded products

[0001] The present invention relates to a method for manufacturing a press-formed product having at least a top plate portion having a concavely curved portion that is concavely curved in a side view, and a vertical wall portion that continues from the top plate portion via a punch shoulder R portion. Note that in this specification, for example, when the term "punch shoulder R portion" is used, it means a portion of the formed product, and when the term "punch shoulder" is used, it means a portion of the mold.

[0002] While stricter automobile crash safety standards are improving the crash safety of vehicle bodies, carbon dioxide emission regulations are also urging vehicle weight reduction to improve fuel efficiency and promote the shift to electric vehicles. In order to achieve both improved crash safety and weight reduction, high-strength steel plates (also known as high-tensile steel) with a tensile strength of 590 MPa or higher are increasingly being used in vehicle body structural parts.

[0003] Automotive parts, including some of their structures, include press-formed products that have at least a top plate and a vertical wall portion that continues from the top plate via a punch shoulder R portion, and that have a curved portion on the top plate that is concavely curved in side view. When such press-formed products are press-formed, the bottom of the concavely curved portion in the top plate and the tip of the vertical wall portion that continues to the punch shoulder R portion nearby undergo stretch flange deformation. As a result, the sheet thickness in these areas decreases, making them more susceptible to cracking. Furthermore, the sheet thickness increases at the bottom of the concavely curved portion of the top plate and the punch shoulder R portion nearby, making them more susceptible to buckling (wrinkling, folding).

[0004] In particular, in the case of high tensile strength steel, increasing the strength makes it more likely that cracks will occur in the stretch flange deformation area and that buckling (wrinkles, creases) will occur in the punch shoulder R portion, which is a particular problem.

[0005] In this regard, measures to prevent cracking due to stretch flange deformation in press-formed products having a top plate portion that is flat and has a concave outer peripheral edge when viewed from above, and a vertical wall portion that continues from the top plate portion along the concave outer peripheral edge have been proposed, for example, in Patent Document 1.

[0006] International Publication No. 2014 / 097745

[0007] The press-forming method described in Patent Document 1 targets a press-formed product having a flat top plate portion whose outer peripheral edge is concave in top view. On the other hand, the press-formed product targeted by the present invention is a press-formed product having at least a top plate portion having a concavely curved portion that is concavely curved in side view, and a vertical wall portion that continues from the top plate portion via a punch shoulder R portion. Thus, the press-formed product targeted by Patent Document 1 and the press-formed product targeted by the present invention have different shapes.

[0008] Therefore, even if the press-forming method of Patent Document 1 is applied to the press-formed product targeted by the present invention, it is not possible to prevent cracks in the stretch flange deformation portion of the press-formed product, or wrinkles or folds in the punch shoulder R portion.

[0009] The present invention has been made in consideration of the above-mentioned problems, and its purpose is to provide a method for manufacturing a press-formed product that can suppress cracking and prevent wrinkles and folds in a press-formed product that has at least a top plate portion with a concavely curved portion that is curved concavely in a side view and a vertical wall portion that continues from the top plate portion via a punch shoulder R portion.

[0010] In order to solve the above-mentioned problems and achieve the object, (1) the manufacturing method of the press-molded product of the present invention is a manufacturing method of a press-molded product having at least a top plate portion having a concavely curved portion that is curved concavely in a side view and a vertical wall portion that continues from the top plate portion via a punch shoulder R portion, and is characterized by comprising a first molding process of press-molding an intermediate molded product having an intermediate top plate portion that curves in the same direction as the top plate portion, a step-shaped portion consisting of a step formed continuously with a ridge portion formed in a portion corresponding to the punch shoulder R portion, and an outer surface portion that extends outward continuously with the step-shaped portion and curves in the same direction as the intermediate top plate portion, and a second molding process of press-molding the intermediate molded product into the press-molded product.

[0011] (2) The manufacturing method of a press-formed product according to the present invention is characterized in that, in the invention (1) above, the radius of curvature of the ridge line portion of the intermediate formed product is the same as or greater than the radius of curvature of the punch shoulder R portion of the press-formed product.

[0012] The manufacturing method of the press-molded product according to the present invention has the effect of preventing wrinkles and folds at the bottom of the concave curved portion of the top plate portion and the punch shoulder R portion and its vicinity, and suppressing cracks at the tip of the vertical wall portion continuing to the punch shoulder R portion.

[0013] FIG. 1 is an explanatory diagram of a method for manufacturing a press-formed product according to an embodiment. FIG. 2 is an explanatory diagram of an intermediate product formed in this embodiment. FIG. 3 is an explanatory diagram of the forming process of the second forming step in this embodiment. FIG. 4 is an explanatory diagram of the effects of this embodiment. FIG. 5 is an explanatory diagram of a method for manufacturing a press-formed product according to another aspect of the present invention. FIG. 6 is an explanatory diagram of a press-formed product that was the target of manufacture in the examples. FIG. 7 is an explanatory diagram of a press-formed product that is the target of the present invention. FIG. 8 is an explanatory diagram of a conventional method for manufacturing a press-formed product. FIG. 9 is an explanatory diagram of the forming process in a conventional press-forming method. FIG. 10 is an explanatory diagram of a problem with a press-formed product manufactured by a conventional press-forming method.

[0014] Hereinafter, an embodiment of the method for manufacturing a press-formed product according to the present invention will be described, but the present invention is not limited to the embodiment.

[0015] An example of a press-formed product having a target shape that is a target of the present invention will be described with reference to Fig. 7. The press-formed product 1 that is a target of the present invention has at least a top plate portion 3 having a concavely curved portion that is concavely curved in a side view, and a vertical wall portion 7 that continues from the top plate portion 3 via a punch shoulder R portion 5, and an example of such a product has an L-shaped cross section as shown in Fig. 7.

[0016] Problems that arise when press-forming such a press-formed product 1 will be described below in the case where a 1.4 mm thick steel plate having a tensile strength of 980 MPa is press-formed as a blank. Figure 8 shows a conventional mold used when press-forming the press-formed product 1 of the present invention in one process. The conventional mold includes a punch 9 having a concavely curved portion, a pad 11 having a concavely curved portion similar to that of the punch 9, and a die 13. A blank 15, which is a metal plate, is pressed between the pad 11 and the punch 9 to form a concavely curved portion in the top plate portion 3, while the die 13 is moved relatively to form the vertical wall portion 7.

[0017] A conventional press forming process will be described with reference to Figure 9. Figure 9 shows a cross section (a cross section cut in the direction of arrow A in Figure 7) of the bottom of the curve of the concavely curved portion of the top plate 3 of a press-formed product 1 during press forming. The numerical values ​​such as [28 mm up] in Figure 9 are the gaps in the press forming direction between the punch shoulder 33 of the punch 9 and the die jaws 13a (see Figures 8 and [2 mm up] in Figure 9), taking into account the sheet thickness up to the bottom dead center of forming. For example, when [28 mm up] is selected, the gap between the punch shoulder 33 of the punch 9 and the die jaws 13a is the sheet thickness of the press-formed product 1 + 28 mm.

[0018] 9, the blank 15 is pressed against the punch 9 by the pad 11 at a 47 mm up position, forming a concave curved portion. In this state, the entire blank 15, including the portion to be formed into the vertical wall, is curved concavely in the direction perpendicular to the paper surface.

[0019] At [35 mm up], the die shoulder 13b contacts the blank 15, and the vertical wall portion 7 begins to form. As a result, the end of the portion of the blank 15 corresponding to the vertical wall curves downward. When the vertical wall portion 7 begins to form, the portion corresponding to the vertical wall curves concavely in the direction perpendicular to the paper plane, resulting in high rigidity and little deformation along the punch shoulder R portion 5 (ridge line) that forms the boundary between the top plate portion 3 and the vertical wall portion 7. Therefore, at [28 mm up], the bottom of the concave curved portion of the blank 15 that contacts the die shoulder 13b of the die 13 bends, causing a bending. Further, at [19 mm up], the portion where the bending occurred bends in the forming direction without contacting the punch shoulder 33 of the punch 9. Then, at [8 mm up] and [2 mm up], the portion that broke is formed into the vertical wall portion 7 in the gap between the die 13 and the punch 9, and contacts the punch shoulder 33 of the punch 9 at the bottom dead center of the forming process to form the punch shoulder R portion 5.

[0020] Figure 10 shows the thickness distribution at the bottom dead center of a conventional press forming process analyzed by FEM. The dotted line A-A in Figure 10 indicates the position of the bottom of the concave curved portion, as in Figure 7. As shown in Figure 10, the tip of the vertical wall portion 7, which is continuous with the bottom of the concave curved portion, undergoes stretch flange deformation, resulting in a maximum thickness reduction of 12.3%, making it susceptible to cracking. On the other hand, the maximum thickness increase rate of the punch shoulder R portion 5 is as large as 10.0%, making it susceptible to wrinkles and folds.

[0021] In conventional press-forming processes, the concave curved portion formed in the early stages of press-forming, which appears concave in side view, has high rigidity due to its shape. Therefore, when the vertical wall portion 7 is subsequently formed using the die 13, a large compressive force acts on the blank 15, increasing the plate thickness and causing bending, which is prone to wrinkles and creases. Furthermore, because the top plate portion 3 is concavely curved, the vertical wall portion 7 continuing to the concave curved portion expands to the left and right of the concave curved portion, resulting in stretch flange deformation. This applies large tension to the tip of the vertical wall portion 7 continuing to the bottom of the curve, reducing the plate thickness and making it more susceptible to cracking.

[0022] In studying ways to suppress increases and decreases in thickness during this series of forming processes, the present inventors focused on the deformation of the blank 15 from [28 mm up] to [2 mm up] in Figure 9. That is, if the blank 15 can be brought into contact with the punch shoulder 33 of the punch 9 at the beginning of forming [28 mm up], the blank 15 will not bend in the forming direction without contacting the punch shoulder 33 of the punch 9, and wrinkles and folds as in the conventional method can be prevented. Furthermore, they thought that if the forming height of the vertical wall portion 7 when forming the vertical wall portion 7 into the target shape could be lowered, the amount of stretch flange deformation would be reduced and cracks would be less likely to occur.

[0023] Therefore, in the present invention, as shown in FIG. 1 , the press forming process is divided into two steps, a first forming step and a second forming step. In the first forming step, an intermediate formed product 19 is press-formed. The intermediate formed product 19 is curved in the same direction as the press-formed product 1 and has a step-shaped portion 17. In the second forming step, the intermediate formed product 19 is press-formed into the press-formed product 1, which has the target shape. The intermediate formed product 19 includes an intermediate top plate portion 21 that curves in the same direction as the top plate portion 3 of the press-formed product 1, a ridge portion 23 formed in a region corresponding to the punch shoulder R portion 5, and a step-shaped portion 17 formed continuously with the ridge portion 23. FIG. 2 shows an example in which the intermediate formed product 19 includes an intermediate top plate portion 21 having the same shape as the top plate portion 3 of the press-formed product 1. The intermediate formed product 19 also includes an outer surface portion 25 that extends outward, continuous with the step-shaped portion 17, and curves in the same direction as the intermediate top plate portion 21. The outer surface portion 25 is formed into the vertical wall portion 7 in the second forming step. 2 shows an example in which the outer surface portion 25 is parallel to the intermediate top plate portion 21, but in the present invention, the outer surface portion 25 only needs to curve in the same direction as the intermediate top plate portion 21, and does not need to have the same curvature as the intermediate top plate portion 21. Furthermore, the intermediate top plate portion 21 does not need to completely match the shape of the top plate portion 3 of the press-formed product 1, which is the target shape, as long as it matches the curvature of the top plate portion 3 of the press-formed product 1 in the second forming step. Below, the first forming step and the second forming step will be described in detail with reference to FIG. 1, using as an example the case in which the intermediate formed product 19 has the shape shown in FIG. 2.

[0024] <First Forming Step> In the first forming step, as shown in FIG. 1( a), a step forming punch 27 and a step forming die 29 are used. Using the step forming punch 27 and the step forming die 29, an intermediate formed product 19 having an intermediate top plate portion 21, a step-shaped portion 17, and an outer surface portion 25 is press-formed. The radius of curvature of the ridge line portion 23 of the intermediate formed product 19 is preferably equal to or greater than the radius of curvature of the punch shoulder R portion 5 of the press-formed product 1. While there is no particular upper limit for the radius of curvature of the ridge line portion 23, it is preferable to set the radius of curvature to up to five times the radius of curvature of the punch shoulder R portion 5 of the press-formed product 1, depending on the shape of the press-formed product 1 and the thickness of the metal plate. This makes it easier for the ridge line portion 23 to come into contact with the punch shoulder 33 of the punch 9 in the second forming step.

[0025] <Second forming step> In the second forming step, as shown in Fig. 1(b), a punch 9, a pad 11, and a die 13 are used to form the press-formed product 1 having the target shape. What is shown in Fig. 1(b) is the same as the conventional example shown in Fig. 8. Then, an intermediate formed product 19 is placed on the punch 9 and pressed down with the pad 11, and the die 13 is moved relatively toward the punch 9, thereby forming the outer surface portion 25 into the vertical wall portion 7 and forming the press-formed product 1 having the target shape.

[0026] Figure 3 shows the movement of the punch 9 and die 13 in the second forming step and the deformation process of the intermediate formed product 19, along a cross section cut in the direction of arrow A in Figure 2. Note that, as in the case of Figure 9 described above, values ​​such as [28 mm up] are the gap in the press-forming direction between the punch shoulder 33 and the die jaws 13a, taking into account the plate thickness up to the bottom dead center of forming. Therefore, the gap between the punch shoulder 33 and the die jaws 13a when [28 mm up] is the plate thickness of the press-formed product 1 + 28 mm.

[0027] 3, in the second forming step, the intermediate formed product 19 is placed on the punch 9 and pressed down by the pad 11. At this time, as shown in the diagram [47 mm up], the ridge line 23 of the step-shaped portion 17 comes into contact with the punch shoulder 33 of the punch 9. In this way, the step-shaped portion 17 of the intermediate formed product 19 fits along the punch shoulder 33, thereby suppressing the amount of deformation of the ridge line 23 in the subsequent forming process, suppressing an increase in plate thickness, and preventing bending.

[0028] Thereafter, when the die 13 moves relative to the punch 9, both longitudinal ends of the die 13 come into contact with both ends of the outer surface 25 of the intermediate molded product 19 at [35 mm up], causing the ends of the outer surface 25 of the intermediate molded product 19 to rise slightly.

[0029] Furthermore, when the molding progresses from [28 mm up] to [19 mm up], the die shoulder 13b of the die 13 comes into contact with the bottom of the concave curved portion of the outer surface portion 25, and the entire outer surface portion 25 is bent back, and the molding of the vertical wall portion 7 progresses further at [8 mm up] and [2 mm up], and the molding is completed at the bottom dead center of the molding.

[0030] Since the step-shaped portion 17 is formed in the intermediate formed product 19, in the forming process of the vertical wall portion 7, the forming height of the vertical wall portion in the second forming step is the distance from the bottom of the step-shaped portion 17 to the end of the outer surface portion 25. Therefore, the forming height of the vertical wall portion in the second forming step is shorter than the distance from the R start position of the ridge line portion 23 of the top plate portion 3 to the end of the blank 15, as in the conventional example. This suppresses the reduction in plate thickness due to stretch flange deformation, and prevents cracks from occurring.

[0031] 4A and 4B show the thickness distributions at the bottom dead center in the first and second forming steps when the press forming process of this embodiment is analyzed by FEM. FIG. 4A shows the thickness distribution at the bottom dead center in the first forming step. FIG. 4B shows the thickness distribution at the bottom dead center in the second forming step. As shown in FIG. 4A, in the first forming step, the maximum thickness increase rate was 3.1% at the ridge line portion 23, and the maximum thickness decrease rate was 3.7% at the outer surface portion 25. As shown in FIG. 4B, in the second forming step, the maximum thickness increase rate was 3.1% at the punch shoulder R portion 5, and the maximum thickness decrease rate was 10.8% at the tip of the vertical wall portion 7.

[0032] Comparing FIG. 4(b) of this embodiment with FIG. 10 of the conventional example, the maximum thickness increase rate at the punch shoulder R portion 5 (ridge line portion 23) was 10.0% in the conventional example, while it was significantly reduced to 3.1% in this embodiment. This shows that wrinkles and folds at the ridge line portion 23 can be prevented. Furthermore, the maximum thickness reduction rate at the tip portion of the vertical wall portion 7 was 12.3% in the conventional example, while it was reduced to 10.8% in this embodiment. This shows that cracking due to stretch flange deformation can be suppressed.

[0033] In the above description, as shown in Fig. 1(b), the die 13 used in the second forming step has a straight die shoulder 13b for forming the vertical wall portion 7. However, the die used in the second forming step of the present invention may have a die shoulder 31b curved to fit the outer surface portion 25 of the intermediate formed product 19, as shown in Fig. 5. By using such a shape, when forming the outer surface portion 25 into the vertical wall portion 7, the entire length of the outer surface portion 25 can be simultaneously formed. Note that the reference numeral 31a in Fig. 5 denotes a die jaw.

[0034] A press-forming analysis using FEM was conducted to confirm the effects of the present invention, and is described below. Using a 980 MPa-class steel plate with a thickness of 1.4 mm, a press-formed product 1 having a top plate portion 3 that curves concavely in side view and a vertical wall portion 7 that is continuous with the top plate portion 3, as shown in FIG. 6, was analyzed to determine the thickness distribution. The dimensions of each part of the press-formed product 1 are as shown in FIG. 6. In addition to the conventional example performed in one process shown in FIG. 8, the press-forming analysis was also performed by varying the shape of the outer surface portion 25 of the intermediate formed product 19 (FIG. 2). The maximum thickness increase rate at the ridge line portion 23 and the maximum thickness decrease rate at the tip of the vertical wall of the press-formed product 1, which is the target shape, were determined. It has been found that with the press-formed product 1 shown in FIG. 6, wrinkles and folds occur when the maximum thickness increase rate exceeds 8%, and cracks occur when the maximum thickness decrease rate exceeds 12.0%. The analysis results are shown in Table 1. In Table 1, the step height (mm) is shown as the height of the step-shaped portion 17 at the position of the concave curved bottom of the intermediate top plate portion 21 and the height of the step-shaped portion 17 at the end of the intermediate molded product 19.

[0035]

[0036] In the conventional example shown in No. 1, in which the top plate portion 3 and vertical wall portion 7 were press-formed in one process, the maximum thickness increase rate of the punch shoulder R portion 5 was 10.0%, resulting in wrinkles and folds. Furthermore, the maximum thickness decrease rate at the tip of the vertical wall was 12.3%, resulting in cracks. In the comparative example, No. 2, the outer surface portion 25 of the intermediate formed product 19 was formed in a convex mountain shape toward the top plate portion 3, as in Patent Document 1. In this case, during press-forming of the intermediate formed product 19, cracks occurred at the tip of the convex mountain-shaped outer surface portion 25, forcing the press-forming to be stopped.

[0037] In the invention examples shown in No. 3 to No. 11, an intermediate molded product 19 having a step-shaped portion 17 is molded, and then the intermediate molded product 19 is molded into a target shape. The outer surface portion 25 has a concave curved shape similar to the top plate portion 3, and the height of the step-shaped portion 17 is varied from 1 mm to 20 mm.

[0038] In all of the invention examples No. 3 to No. 11, the maximum thickness increase rate at the punch shoulder R portion 5 of the target shape and the maximum thickness decrease rate at the tip of the vertical wall were reduced compared to the conventional example No. 1. Furthermore, in all of the invention examples No. 3 to No. 11, the maximum thickness increase rate at the punch shoulder R portion 5 of the target shape was less than 8%, which is the rate at which wrinkles occur, thereby preventing wrinkles. Furthermore, in all of the invention examples No. 3 to No. 11, the maximum thickness decrease rate at the tip of the vertical wall of the target shape was 12.0% or less, thereby preventing cracks. In particular, in the invention examples No. 7 to No. 9, by setting the step height of the intermediate formed product 19 to 10 mm to 15 mm, the maximum thickness increase rate at the punch shoulder R portion 5 of the target shape was significantly reduced, reliably preventing wrinkles and folds. In addition, in all of the invention examples No. 7 to No. 11, the maximum thickness increase rate at the punch shoulder R portion 5 of the target shape was significantly reduced, reliably preventing wrinkles and folds. In Example 9 of the present invention, it was found that by setting the step height of the intermediate molded product 19 to 10 mm to 15 mm, the maximum thickness reduction rate at the tip of the vertical wall of the target shape is also reduced, and cracks can be reliably prevented.

[0039] The present invention can provide a method for manufacturing a press-formed product that can suppress cracking and prevent wrinkles and folds in a press-formed product that has at least a top plate portion with a concavely curved portion that is curved concavely when viewed from the side, and a vertical wall portion that continues from the top plate portion via a punch shoulder R portion.

[0040] REFERENCE SIGNS LIST 1 Press-formed product 3 Top plate portion 5 Punch shoulder R portion 7 Vertical wall portion 9 Punch 11 Pad 13 Die 13a Die jaw portion 13b Die shoulder 15 Blank 17 Step-shaped portion 19 Intermediate formed product 21 Intermediate top plate portion 23 Ridge line portion 25 Outer surface portion 27 Step-forming punch 29 Step-forming die 31 Die (other embodiment) 31a Die jaw portion 31b Die shoulder 33 Punch shoulder

Claims

DEPCT6827 / 05 / 25681. Method of fabrication of a pressed part which includes a concave upper section which is concave in the lateral view and a side wall which is continuous from the upper section through the R section at the stamping shoulder, at least where the assembly method includes the first forming step in press forming the intermediate forming part which includes the intermediate upper section which is curved in the same direction as the upper, a stepped section which is formed from the step which is continuous with the ridge section which is formed at the section corresponding to the R section at the stamping shoulder and an outer surface which is extended continuously from the stepped section and is curved in the same direction as the intermediate upper, and the second forming step in press forming the intermediate forming part as the pressed part.

2. Method of fabrication of a pressed part according to claim 1 where the radius of curvature of the ridge section of the intermediate forming part is greater than or equal to the radius of curvature of the R section at the stamping shoulder of the pressed part;

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