Press forming method

Abstract

To achieve both reduction in a press load and excellent dimensional accuracy.SOLUTION: A press molding method includes an intermediate molded article molding step of pressing a metal plate and molding an intermediate molded article, an additional processing step of subjecting the intermediate molded article to additional processing by pressing, and a specification step of specifying a part under a low pressure where a low pressure is acted on the intermediate molded article in the additional processing step, wherein the additional processing step performs the additional processing so that the pressure acted on the part under the low pressure is equal to or less than a predetermined pressure.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a press forming method.

Background Art

[0002] When press forming a metal plate, the press forming load increases as the material strength or plate thickness increases. Also, the press forming load of the metal plate increases due to the enlargement of the press formed product and the complication of the forming shape. On the other hand, there is an upper limit to the load that can be applied to the metal plate by a press machine. Therefore, depending on the existing press machine, it may not be possible to apply the target load to the metal plate, and it may not be possible to manufacture a press formed product with the target shape.

[0003] For the purpose of reducing the press load, Patent Document 1 discloses a method for manufacturing a press formed product in which the press forming shape is arranged to be inclined with respect to the press direction and formed by a cam mechanism, so that the load applied in the press direction is converted to the inclined surface side to reduce the press direction load.

[0004] Also, for example, in a transfer press machine, a press formed product is manufactured by performing a plurality of press processes on a metal plate. In a press process in which a plurality of press processes are performed, a portion formed by a previous press process may be pressurized again. Hereinafter, the case where a portion formed by a previous press process is pressurized again may be referred to as double pressurization.

[0005] As a technique for reducing double pressurization, for example, Patent Document 2 discloses a press forming method for press forming a vehicle body part from a plate-like material in a plurality of steps, in which the clearance between the mold and the material to be formed at a portion of the formed portion of the intermediate formed product that has a large influence on springback in the next step is made larger than the plate thickness.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-75275 [Patent Document 2] Japanese Patent Publication No. 2020-163416 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The technology described in Patent Document 1 reduces the apparent load by changing the direction of the load's application, but it can be difficult to apply to actual part shapes because it involves tilting the molding area. Furthermore, as in the technology described in Patent Document 2, simply increasing the clearance between the mold and the workpiece beyond the plate thickness in areas that have a significant impact on springback can alter the springback after molding, making it impossible to achieve both load reduction and dimensional accuracy.

[0008] This invention has been made in view of the above circumstances, and aims to provide a press forming method that can achieve both a reduction in press load and excellent dimensional accuracy. [Means for solving the problem]

[0009] The inventors have found that areas with small differences in strain before and after press working are areas with small deformation, and that by reducing the pressure applied to such areas, the press load can be reduced, and furthermore, press-worked products with excellent dimensional accuracy can be manufactured.

[0010] Based on the above findings, the gist of this invention is as follows: [1] A press forming method according to one aspect of the present invention comprises: an intermediate molded product forming step of forming an intermediate molded product by press working a metal sheet; an additional processing step of further processing the intermediate molded product by press working; and a specification step of identifying a low-pressure portion in which a low pressure is applied to the intermediate molded product in the additional processing step, wherein the additional processing is performed with the pressure applied to the low-pressure portion being less than or equal to a predetermined pressure. [2] In the press forming method described in [1] above, the additional processing step may be performed with a pressure of 100 MPa or less applied to the low-pressure portion. [3] In the press forming method described in [1] or [2] above, in the specified step, the portion in which the difference in plastic strain before and after press working to further process the intermediate molded product without providing a relief portion is 0.02 or less may be designated as the low-reduction portion. [4] In the press forming method described in any of [1] to [3] above, the metal sheet may be a steel sheet having a tensile strength of 590 MPa or more. [5] In the press forming method described in any of [1] to [4] above, the intermediate molded product forming step and the additional processing step may be steps in which cold press working is performed. [Effects of the Invention]

[0011] According to embodiments of the present invention, it is possible to achieve both a reduction in press load and excellent dimensional accuracy. [Brief explanation of the drawing]

[0012] [Figure 1] This is a flowchart illustrating the flow of a press forming method according to one embodiment of the present invention. [Figure 2] This is a model diagram of the molded product in the example. [Figure 3] This is a contour diagram showing the dimensional difference between the molded product of Example 1 of the present invention and the molded product of a reference example. [Figure 4] This is a contour diagram showing the dimensional difference between the molded product of Example 2 of the present invention and the molded product of the reference example. [Figure 5] This is a contour map showing the dimensional difference between the comparative example molded product and the reference example molded product. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The press forming method according to this embodiment includes an intermediate molded product forming step of press-forming a metal plate to form an intermediate molded product, a secondary processing step of performing secondary processing on the intermediate molded product by press working, and a specifying step of specifying a low-pressure portion where a low pressure is applied to the intermediate molded product in the secondary processing step. In the secondary processing step, the secondary processing is performed with the pressure applied to the low-pressure portion being set to a pressure not exceeding a predetermined pressure.

[0014] <Intermediate Molded Product Forming Step> In the intermediate molded product forming step, a metal plate is press-formed to form an intermediate molded product. The metal plate is not particularly limited, and various metal plates can be used as materials. Examples of the metal plate include a steel plate, a stainless steel plate, an aluminum steel plate, a copper plate, a titanium plate, and the like. However, the higher the strength, the greater the load required for press working, and the more remarkable the effects of this embodiment can be obtained. For example, the metal plate is preferably a steel plate having a tensile strength of 590 MPa or more, more preferably a steel plate having a tensile strength of 980 MPa or more, and even more preferably a steel plate having a tensile strength of 1180 MPa or more or 1470 MPa or more.

[0015] The thickness of the metal plate is not particularly limited. However, as the thickness increases, the load required for press working increases, and thus the effects of this embodiment can be obtained more remarkably. Therefore, the thickness of the metal plate is preferably 1.0 mm or more, more preferably 1.4 mm or more, and even more preferably 1.8 mm or more. Also, the thickness may be, for example, 3.2 mm or less.

[0016] The press working in the intermediate molded product forming step is cold press working, and as the press working, for example, drawing, pad drawing, pad bending, stamping, deep drawing, flanging, or bending can be applied.

[0017] <Secondary Processing Step> In the additional processing step, the intermediate formed product is additionally processed by pressing. In the additional processing step, the additional processing is performed with the pressure applied to the low-pressure portion specified in a specific step described later being set to be equal to or lower than a predetermined pressure. The pressure applied to the low-pressure portion varies depending on the type of metal plate, the plate thickness, the shape of the formed product, etc., and can be determined by the following method according to these. For example, for various forming shapes, the contact pressure between the intermediate formed product and the mold during plastic deformation is observed, and the pressure at which elastic deformation occurs is set as the pressure applied to the low-pressure portion. By setting the pressure applied to the low-pressure portion to be equal to or lower than a predetermined pressure, the load can be reduced. Also, since only a low pressure acts on portions that have little influence on deformation before and after pressing, and only a high pressure acts on the portions to be deformed, a press-formed product with excellent dimensional accuracy can be manufactured. The pressure applied to the low-pressure portion is preferably 100 MPa or less. If the pressure is 100 MPa or less, the load can be sufficiently reduced to the extent that pressing can be performed within the limit load of the press machine. The pressure applied to the low-pressure portion can be 0 MPa, in other words, the low-pressure portion and the mold do not have to be brought into contact with each other.

[0018] The pressing in the additional processing step is cold pressing, and as the pressing, for example, drawing, pad drawing, pad bending, stamping, deep drawing, flanging, or bending, etc. can be applied.

[0019] <Specific process> In a specific process, a low-pressure area is identified in the intermediate molded product during the additional processing step. As mentioned earlier, when a press-formed product is manufactured through multiple press processes, overlapping pressure may occur where a portion formed in a previous press process is pressed again in a subsequent press process. If the overlappingly pressed portion is not deformed in the subsequent press process, the load acting on that portion does not contribute to the deformation. Therefore, by reducing the load on the portion that does not deform, the load that the press machine applies to the intermediate molded product can be reduced. Accordingly, in this process, a portion with small deformation is identified between the previous press process (intermediate molded product forming process) and the subsequent press process (process of processing the intermediate molded product without providing relief portions). In the additional processing step, the pressure applied to that portion is set to a predetermined pressure or less.

[0020] The low-pressure region is identified, for example, by finite element method (FEM) analysis. Specifically, an FEM analysis is performed under the conditions of applying the load required for press working into the desired shape without providing any relief (clearance). The low-pressure region is defined as the area where the difference in plastic strain before and after press working is less than or equal to a predetermined value. Identification of the low-pressure region is not limited to FEM analysis; it may also be identified by conducting preliminary tests, etc.

[0021] The low-reduction section can be identified by considering the reduction in load and the required dimensional accuracy. However, it is preferable to designate the section where the difference in plastic strain before and after press working is 0.02 or less as the low-reduction section, as this allows for further improvement in dimensional accuracy.

[0022] <Press molding process> Referring to Figure 1, the flow of press forming according to the press forming method of this embodiment will be explained. Figure 1 is a flowchart illustrating the flow of press forming.

[0023] First, a specific process is carried out to identify a low-pressure portion in which low pressure is applied to the intermediate molded product during the additional processing process (step S101). Next, an intermediate molded product forming process is carried out in which a metal sheet is press-formed to form the intermediate molded product (step S103), followed by an additional processing process in which the intermediate molded product is further processed by press-forming (step S105).

[0024] The present invention has been described with reference to these embodiments. However, the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention.

[0025] For example, the additional processing step may be performed multiple times. In this case, the intermediate molded product obtained by press processing in the earlier additional processing step will be press processed in the later additional processing step. In this case, the portion to be pressurized at a predetermined pressure or less in the later additional processing step can be specified in the specific process. [Examples]

[0026] Next, embodiments of the present invention will be described. However, the technical scope of the present invention is not limited to the following embodiments.

[0027] A steel plate with a thickness of 1.4 mm and a tensile strength of 1470 MPa was subjected to deep drawing, cam bending, and stamping in that order to produce a molded product with the shape shown in Figure 2. As a reference example, in the areas where stamping was performed with overlapping pressure, stamping was carried out without providing a clearance between the mold and the intermediate molded product. The stamping load was 177 tonf. As Example 1 of the present invention, in the areas where the difference in plastic strain before and after stamping calculated by finite element analysis was 0.02 or less, stamping was carried out with a clearance between the mold and the intermediate molded product (contact pressure of 0 MPa). As Example 2 of the present invention, in the areas where the difference in plastic strain before and after stamping calculated by finite element analysis was 0.02 or less, stamping was carried out with a contact pressure of 100 MPa. As a comparative example, in the overlapping pressurized portion, stamping was performed with a clearance between the mold and the intermediate molded product not only in the portion where the difference in plastic strain before and after stamping, calculated by finite element method analysis, was 0.02 or less, but also in the portion where it was greater than 0.02 and 0.06 or less. Figure 3 shows a contour plot showing the dimensional difference between the molded product of Example 1 of the present invention and the molded product of the reference example. Figure 4 shows a contour plot showing the dimensional difference between the molded product of Example 2 of the present invention and the molded product of the reference example. Figure 5 shows a contour plot showing the dimensional difference between the molded product of the comparative example and the molded product of the reference example. The values ​​shown in Figures 3 to 5 indicate the dimensional difference (mm) with the molded product of the reference example.

[0028] In Example 1 of the present invention, the stamping load was reduced by 42% compared to the reference example to 102 tonf, and the dimensional difference was within ±0.15 mm. In Example 2 of the present invention, the stamping load was reduced by 36.5% compared to the reference example to 112.6 tonf, and the dimensional difference was within ±0.27 mm. On the other hand, in the comparative example, the stamping load was reduced by 54% compared to the reference example, but the dimensional difference was ±1.43 mm. It was found that Examples 1 and 2 of the present invention reduce the stamping load and also exhibit excellent dimensional accuracy.

Claims

1. The intermediate molded product forming process involves pressing a metal sheet to form an intermediate molded product, An additional processing step is performed on the aforementioned intermediate molded product by press working. The process includes a process for identifying a low-pressure portion in which a low pressure is applied to the intermediate molded product during the additional processing step, In the aforementioned additional machining process, the additional machining is performed with the pressure applied to the low-pressure lower portion set to a predetermined pressure or less. A press forming method in which, in the specified step, the portion in which the difference in plastic strain before and after press working on the intermediate molded product without providing a relief portion is 0.02 or less is defined as the low-reduction portion.

2. The press forming method according to claim 1, wherein in the additional processing step, the pressure applied to the low-pressure lower portion is 100 MPa or less when the additional processing is performed.

3. The press forming method according to claim 1 or 2, wherein the metal plate is a steel plate having a tensile strength of 590 MPa or more.

4. The press molding method according to claim 1 or 2, wherein the intermediate molded product molding step and the additional processing step are steps in which cold press working is performed.

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