Press molding method and shape evaluation method for press molded products

Abstract

Provided are a press-forming method and a shape evaluation method for a press-formed product that shorten the time required for the shape change that occurs over time in a press-formed product that has been released from the mold and has spring back, and that avoid problems in the next process due to the shape change. [Solution] The press-molding method according to the present invention shortens the time required for a press-molded product 1 to change shape over time after springback at the moment of release from a mold, and includes a press-molding step S1 in which the press-molded product 1 is press-molded using a mold, a mold-releasing step S3 in which the press-molded product 1 is released from the mold, a mold-fitting step S5 after release in which the released press-molded product 1 is fitted into the mold and fixed to match the shape of the bottom dead center of the mold, a heating and holding step S7 in which the press-molded product 1 fitted into the mold is heated to a predetermined temperature A higher than room temperature and held at the predetermined temperature A for a predetermined time of less than 30 minutes, and a heating and holding step S8 in which the heated and held press-molded product 1 is heated to a predetermined temperature B (B

Description

[Technical field]

[0001] The present invention relates to a press forming method and a shape evaluation method for a press-formed product, and more particularly to a press forming method and a shape evaluation method for a press-formed product whose shape changes over time after springing back at the moment of release from a die. [Background technology]

[0002] Press forming is a method that can manufacture metal parts at low cost and in a short time, and is used to manufacture many automobile parts. In recent years, in order to simultaneously improve the collision safety performance of automobiles and reduce the weight of the vehicle body, higher strength metal sheets are press formed into automobile parts.

[0003] One of the main issues when press-forming high-strength metal sheets is the deterioration of dimensional accuracy due to springback. Springback is a phenomenon in which, when a metal sheet is deformed using a die during press-forming, residual stress generated in the press-formed product acts as a driving force, causing the press-formed product to instantly return to the shape of the metal sheet before press-forming like a spring when it is released from the die.

[0004] The residual stress generated by press forming in a press-formed product is greater in a metal plate with higher strength (e.g., high-tensile steel plate), so the shape change of the press-formed product due to springback is also greater. Therefore, the higher the strength of the metal plate, the more difficult it is to keep the shape of the press-formed product after springback within the specified dimensions. Therefore, technology has been developed to accurately predict the shape change of a press-formed product due to springback.

[0005] To predict the shape change of a press-formed product due to springback, it is common to use press forming simulation using the finite element method. The procedure in this press forming simulation can be divided into a first stage (e.g., Patent Document 1), in which a press forming analysis is performed of the process in which a metal plate is press formed using a die to the bottom dead point, and the residual stress that will occur in the press formed product is predicted; and a second stage (e.g., Patent Document 2), in which a springback analysis is performed in which the shape of the press formed product changes due to springback after being removed from the die, and the shape of the press formed product in which the force moment and residual stress are balanced is predicted. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5795151 [Patent Document 2] Patent No. 5866892 [Patent Document 3] JP 2013-113144 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, when comparing the shape of a press-molded product predicted by press molding simulation with the shape of a press-molded product that was actually press-molded, the inventors noticed that there were some press-molded products for which the shape prediction accuracy by the press molding simulation was low.

[0008] We therefore investigated press-formed products with low shape prediction accuracy in press forming simulations and the reasons for this.We discovered that, for example, in press-formed products with bent parts in which metal plates are bent, the shape differs between immediately after press forming (immediately after release from the die and springback) and several days later.

[0009] Figure 9 shows an example of the change in shape of a press-formed product over time. In a press-formed product 1 having a hat cross-sectional shape as shown as an example in Figure 9(a), springback occurs the moment it is released from the mold, as shown in Figure 9(b), and a shape change occurs in which the wall opening of the vertical wall portion 4 increases compared to the bottom dead center shape of the molded product (dotted line in Figure 9(b)) (dashed line in Figure 9(b)). Even after springback, a shape change occurs in which the wall opening of the vertical wall portion 4 further increases over time (solid line in Figure 9(b)).

[0010] The change in shape of a press-formed product over time after springback appears to be similar to the creep phenomenon in which a structural component that is continuously subjected to a high external load gradually deforms (e.g., Patent Document 3). However, the phenomenon described above that occurs in a press-formed product that is not subjected to an external load has not been known until now.

[0011] Therefore, the method of using a die designed in anticipation of the shape change due to springback, or the method of identifying the part that contributes to springback and taking measures to reduce the springback alone cannot reduce further shape change of the press-formed product after springback. Furthermore, when the press-formed product is assembled with other parts, if the press-formed product changes in shape over time, it will cause problems in the next process. Therefore, it has been found that some kind of measures are necessary to deal with the shape change over time of the press-formed product after springback.

[0012] One way to deal with the change in shape of a press-formed product over time is to leave the press-formed product, which has spring back after being released from the die, in a state in which almost no change in shape occurs before it is assembled with other parts in the next process. However, when investigating the time required to bring the shape change to almost zero, it was found that it takes a long time of at least 30 minutes or more. Therefore, immediately after the press-formed product is demolded, it cannot be sent directly to the assembly process with other parts in the next step, or it takes time to adjust the mold based on the shape measurement results of the press-formed product, resulting in time constraints in the process. For this reason, it has been desired to quickly cause a shape change over time of the press-formed product that has been demolded from the mold and undergone springback.

[0013] The present invention has been made to solve the above problems, and shortens the time required for the shape change of the press-formed product over time after press-forming and springback, and reduces the shape change of the press-formed product over time after springback. An object of the present invention is to provide a press-forming method and a method for evaluating the shape of a press-formed product that shortens the time required for the shape change of the press-formed product before being assembled with other parts in the next step and evaluates the shape of the press-formed product.

Means for Solving the Problems

[0014] (1) The press-forming method according to the present invention shortens the time required for the shape change of the press-formed product over time after springback immediately after the press-formed product is demolded from the mold, a press-forming step of press-forming a metal plate into the press-formed product using the mold, a demolding step of demolding the press-formed product that has been press-formed from the mold, a post-demolding mold fitting step of fitting the demolded press-formed product into the mold and fixing the press-formed product in accordance with the bottom dead center shape of the molding, a heating and holding step of heating the press-formed product fitted into the mold to a predetermined temperature A higher than room temperature and holding it for a predetermined time of less than 30 minutes at the predetermined temperature A, and a cooling step of cooling the heated and held press-formed product to a predetermined temperature B (B < A).

[0015] (2) The press forming method according to the present invention shortens the time required for the shape change of the press-formed product over time after springback at the moment when the press-formed product is released from the mold, and a press forming step of press forming a metal plate into the press-formed product using the mold; a mold release step of releasing the press-formed product from the mold; a post-mold-release jig fitting step of fitting all or part of the released press-formed product into a jig capable of holding it in a predetermined shape including another mold having the same shape as the mold, and fixing all or part of the press-formed product to conform to the predetermined shape; a heating and holding step of heating the press-formed product fitted in the jig to a predetermined temperature A higher than room temperature and holding it for a predetermined time of less than 30 minutes at the predetermined temperature A; a cooling step of cooling the heated and held press-formed product to a predetermined temperature B (B < A), and is characterized by including these steps.

[0016] (3) The press forming method according to the present invention shortens the time required for the shape change of the press-formed product over time after springback at the moment when the press-formed product is released from the mold, and a press forming step of press forming a metal plate into the press-formed product using the mold; a step of releasing the press-formed product from the mold; a post-mold-release jig fitting step of measuring or predicting by calculation the shape after release and springback, fitting all or part of the released press-formed product into a jig capable of holding it in the same shape as the shape after release and springback, and fixing all or part of the press-formed product to conform to the shape after springback; a heating and holding step of heating the press-formed product fitted in the jig to a predetermined temperature A higher than room temperature and holding it for a predetermined time of less than 30 minutes at the predetermined temperature A; a cooling step of cooling the heated and held press-formed product to a predetermined temperature B (B < A), and is characterized by including these steps.

[0017] (4) The press forming method according to the present invention shortens the time required for the shape change of the press formed product over time after springback occurs immediately after the press formed product is released from the mold, and a press forming step of press forming a metal plate into the press formed product using the mold; a mold release step of releasing the press formed product from the mold; a post-mold release heating and holding step of heating the released press formed product to a predetermined temperature A higher than room temperature and holding it for a predetermined time of less than 30 minutes at the predetermined temperature A; and a cooling step of cooling the heated and held press formed product to a predetermined temperature B (B < A).

[0018] (5) In any one of the above (1) to (4), the predetermined temperature A is set to 130°C to 150°C, and the predetermined time is set to 1 minute or more and 3 minutes or less.

[0019] (6) The method for evaluating the shape of a press formed product according to the present invention evaluates the shape of a press formed product that is assembled with other parts after press forming, and a press forming step of pressing a metal plate into the press formed product using a mold; a mold release step of releasing the press formed product from the mold; a post-mold release heating and holding step of heating the released press formed product to a predetermined temperature A higher than room temperature and holding it for a predetermined time of less than 30 minutes at the predetermined temperature A; a cooling step of cooling the heated and held press formed product to a predetermined temperature B (B < A); a shape measurement step of measuring the shape of the press formed product after cooling; and a shape determination step of determining that the press formed product is to be subjected to the assembly process if the shape of the measured press formed product is within a predetermined range set in advance.

[0020] (7) In the above (6), The predetermined temperature A is set to 130°C to 150°C, and the predetermined time is set to 1 minute or more and 3 minutes or less.

Advantages of the Invention

[0021] In the present invention, a metal plate is press-formed into a press-formed product using a mold. Immediately after the press-formed product is released from the mold and undergoes springback, the press-formed product is fitted into the mold and fixed in accordance with the shape at the bottom dead center of the forming process, or all or part of the press-formed product is fitted into a jig capable of holding a predetermined shape and fixed in accordance with the predetermined shape, and then heated to a predetermined temperature A higher than room temperature and held for a predetermined time. Thereafter, by cooling to a predetermined temperature B (B < A), the relaxation of the residual stress in the press-formed product can be achieved in a shorter time compared to performing it only at room temperature. Therefore, the time required for the shape change of the press-formed product over time after being released from the mold again and undergoing springback can be shortened, and furthermore, the shape change after being released again can be reduced.

[0022] Also, in the present invention, before being assembled and processed with other parts in the next process, the press-formed product released from the mold is heated to a predetermined temperature A and held for a predetermined time, and then cooled to a predetermined temperature B (B < A). After that, the shape of the press-formed product is measured. If the shape of the measured press-formed product is within a preset predetermined range, it is determined that the press-formed product is to be supplied to the next process. By doing so, the time required for the shape change of the press-formed product over time can be shortened, and defects in the next process can be efficiently avoided. Thus, according to the present invention, the time constraints in the process can be relaxed, so that the productivity of manufacturing and assembling the press-formed product can be improved.

Brief Description of the Drawings

[0023] [Figure 1] It is a flowchart showing the flow of each step of the press-forming method according to Embodiment 1 of the present invention. [Diagram 2]13 is an example of the results of measuring the change over time in the amount of change Xk in the wall opening of the vertical wall portion of a press-formed product immediately after springback. [Diagram 3] This is an example of the results (●) of the required holding time at a specified temperature A to achieve the same amount of change in wall opening as at room temperature (○) when a press-molded product is heated to that temperature A. [Figure 4] 1 is a graph showing the heating time to each predetermined temperature A, the holding time required to sufficiently change the shape at each predetermined temperature A, and the cooling time from each predetermined temperature A to room temperature (25°C) for a press-molded product which is loaded into a high-temperature furnace and heated to and held at a predetermined temperature A. [Diagram 5] FIG. 4 is a flowchart showing the flow of each step of the press-molding method according to another aspect of the first embodiment of the present invention. [Figure 6] FIG. 11 is a flowchart showing the flow of each step of a press-molding method according to a second embodiment of the present invention. [Figure 7] FIG. 11 is a flowchart showing the flow of each step of a press-molding method according to a third embodiment of the present invention. [Figure 8] FIG. 11 is a flowchart showing the flow of each step of a shape evaluation method for a press-formed product according to a fourth embodiment of the present invention. [Figure 9] 1A to 1C are diagrams showing press-formed products having a hat-shaped cross section that are the subject of embodiments 1 to 4 and examples of the present invention ((a) is a perspective view, (b) is the bottom dead center shape during forming, the shape immediately after springback, and the shape after the passage of time). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In order to solve the above-mentioned problems, the inventors conducted various studies on the causes of shape change over time for a press-formed product 1 with a hat cross-sectional shape shown in Figure 9(a), in order to establish a method for shortening the time required for the shape to change over time of a press-formed product that has been released from a mold and springs back.

[0025] As a result, the inventors focused on the stress relaxation phenomenon, in which stress gradually relaxes and decreases over time, and discovered that even in the press-formed product 1 immediately after springback, the residual stress gradually relaxes over time without being subjected to external force, causing the shape of the press-formed product 1 to change to balance the force moment.

[0026] As mentioned above, the phenomenon in which the shape of a press-formed product changes without external force is different from the creep phenomenon in which a structural component that is continually subjected to a high external load gradually deforms. This phenomenon has not been known until now, and its mechanism has not been elucidated. On the other hand, in order to release the residual stress in machined metal parts, a heat treatment called "stress relief" is carried out, and it is known that the shape of the metal parts changes as a result of this stress relief.

[0027] Therefore, the inventors focused on temperature and investigated the effect of temperature on the phenomenon in which residual stress in a press-formed product after springback gradually relaxes over time without being subjected to external force.

[0028] In this study, a press-formed product 1 with a hat-shaped cross-section as shown in Figure 9(a) was used as an example. After the press-formed product 1, which had been press-formed up to the bottom dead center of the press, was released from the die, the punch shoulder R portion 3 was locally heated to 50°C, 150°C, or 200°C, and the change in shape of the press-formed product 1 over time was measured and compared with the change in shape of the press-formed product 1 that had been left at room temperature (25°C) without being heated over time.

[0029] Here, the target shape change of the press-formed product 1 is wall opening of the vertical wall portion 4. As shown in FIG. 9(b), the press-formed product 1 immediately after springback is used as a reference, and the vertical wall portion 4 of the press-formed product 1 after the passage of time is measured at a distance H s The change in the width direction of the position of the vertical wall is the change in the wall opening of the vertical wall, X k was measured as.

[0030] FIG. 2 shows the change in wall opening X of the vertical wall portion 4 from immediately after the mold release and spring back until two days have passed. k The measurement results are shown below. As shown in Figure 2, the change in the amount of change X immediately after release from the mold and springback (less than 0.1 s) k is 0.05 mm, and the change amount X k increased and reached 0.75 mm after 30 minutes. The amount of change X k It gradually increased and after two days it reached 0.83 mm and remained almost constant. In contrast, when the press-molded product 1 was heated after being released from the die, the shape change occurred in a shorter time than when the press-molded product was left at room temperature (25°C) without being heated. Furthermore, it was found that the higher the heating temperature, the shorter the time it took for the shape change to occur.

[0031] From these results, it was found that the time required for the shape of a press-formed product to change over time can be shortened if the metal part is heated to a temperature range of 50°C to 200°C. The reason for limiting the heating temperature to 200°C is that heating to a temperature above 200°C does not shorten the time required for the shape to change, but rather lengthens the heating and cooling times, resulting in time constraints on the process.

[0032] Furthermore, when heated to 50℃~200℃, the change amount X was the same as the change amount (=0.75mm) when left at room temperature for 30 minutes. k The required holding time at each temperature until the temperature reaches the limit is shown in FIG. As shown in Figure 3, the change in X when left at room temperature (25°C) for 30 minutes k The holding time required to reach this state shortened as the heating temperature increased, falling below 5 minutes at 120°C, beginning to saturate at around 150°C, and being less than 1 minute above 150°C.

[0033] In this way, the higher the predetermined temperature A during heating, the shorter the necessary holding time can be. However, when a press-molded product is loaded into a high-temperature furnace maintained at a constant atmosphere and heated, the heating time for heating the press-molded product to the predetermined temperature A and the cooling time for cooling the press-molded product from the predetermined temperature A to room temperature (25°C) become longer as the predetermined temperature A for heating becomes higher.

[0034] 4 shows, as an example, the heating time to each predetermined temperature A, the holding time required for the shape to change at each predetermined temperature A, and the cooling time from each predetermined temperature A to room temperature (25°C) in the case where a press-molded product 1 using a steel plate with a thickness of 1.5 mm as the metal plate is charged into a high-temperature furnace with an atmospheric temperature of 250°C and heated. Here, the holding time at each predetermined temperature A is given from the results shown in FIG.

[0035] From FIG. 4, when the heating time and cooling time are taken into consideration, it has been found that the preferred range of the predetermined temperature at which the time (heating time + holding time + cooling time) required to load the press-formed product immediately after springback into a high-temperature furnace and to relax the residual stress of the press-formed product to a state where almost no shape change occurs any more is the shortest is 130°C to 150°C.

[0036] Furthermore, the holding time at the specified temperature A in the range of 130°C to 150°C is the longest at 3 minutes at 130°C and the shortest at 1 minute at 150°C, so it was discovered that holding at the specified temperature A of 130°C to 150°C for 1 minute or more and 3 minutes or less will result in a shape change equivalent to that obtained when left at room temperature (25°C) for 30 minutes or more.

[0037] The present invention is based on the above findings and will be described below in the following embodiments 1 to 4. In embodiments 1 to 4, a press-molded product 1 will be described as an example, which has a hat-shaped cross-sectional shape having a top plate portion 2, a vertical wall portion 4, and a flange portion 6, as shown as an example in Figure 9, and has, as bending ridge portions, a punch shoulder R portion 3 connecting the top plate portion 2 and the vertical wall portion 4, and a die shoulder R portion 5 connecting the vertical wall portion 4 and the flange portion 6.

[0038] [Embodiment 1] The press molding method of embodiment 1 of the present invention is intended to shorten the time required for the shape change over time of a press-molded product 1 (Figure 9) that springs back the moment it is released from the mold, and as shown in Figure 1, includes a press molding process S1, a demolding process S3, a post-release mold fitting process S5, a heating and holding process S7, and a cooling process S9.

[0039] <Press molding process> The press-molding step S1 is a step of press-molding a metal plate into a press-molded product 1 using a die. The mold used in the press molding step S1 is not particularly limited as long as it includes, for example, a die and a punch, and the die can be moved relatively to the punch side up to the bottom dead center for press molding.

[0040] <Mold release process> The demolding step S3 is a step of releasing the press-molded product 1 produced in the press molding step S1 from the mold. The press-molded product 1 released from the die in the demolding step S3 springs back at the moment of release, causing an opening in the vertical wall portion 4 as shown by the dashed line in FIG. 9(b).

[0041] <Mold fitting process after mold release> The post-mold release mold fitting process S5 is a process in which the press-molded product 1 released from the mold in the mold releasing process S3 is fitted into the mold used for press molding in the press molding process S1, and the press-molded product 1 is fixed to match the molding bottom dead center shape. Here, the bottom dead point shape refers to the shape of the press-molded product 1 at the bottom dead point of the mold used in the press-molding step S1 (the same applies below).

[0042] <Heating and holding process> The heating and holding step S7 is a step of heating the press-molded product 1 fitted in the die to a predetermined temperature A that is higher than room temperature, and holding it at the predetermined temperature A for a predetermined time of less than 30 minutes.

[0043] As shown in Figs. 3 and 4, in the heating and holding step S7, the predetermined temperature A to which the press-molded product 1 is heated is preferably set to 130°C to 150°C, and the predetermined time is preferably set to 1 minute or more and 3 minutes or less.

[0044] In addition, the means for heating the press-molded product 1 in the heating and holding step S7 is not particularly limited, and may be, for example, a means for heating the press-molded product 1 together with the mold to which it is fixed, by loading it into a high-temperature furnace in which the atmospheric temperature is maintained constant, or a means for heating the press-molded product 1 fixed to the mold using high-frequency induction heating.

[0045] <Cooling process> The cooling step S9 is a step of cooling the press-molded product 1 that has been heated and held at the predetermined temperature A in the heating and holding step S7 to room temperature. Note that the temperature of the press-molded product 1 after cooling does not necessarily have to be room temperature, but may be a predetermined temperature B (B <A)とするとよい。 The cooling method in the cooling step S9 may be either natural air cooling or forced air cooling in which a cooling fan or the like is actively used, but forced air cooling is preferred because it can shorten the cooling time.

[0046] As described above, according to the press molding method of this embodiment 1, the press-molded product 1 that springs back at the moment of release from the mold is fitted into a mold and fixed in the molding bottom dead center shape, and is heated to a predetermined temperature A higher than room temperature and held for a predetermined time of less than 30 minutes, thereby making it possible to relieve the residual stress in the press-molded product 1 in a shorter time than if it were performed at room temperature. Furthermore, according to the press molding method of this embodiment 1, it is possible to reduce the change in shape that occurs over time after the press-molded product 1, which has been fitted into a die and held heated, is released from the die and springs back.

[0047] In addition, in the press molding method of this embodiment 1, in the post-mold release mold fitting process S5, the entire press-molded product 1 is fixed in the molding bottom dead center shape by the mold used for press molding the press-molded product 1. However, as another aspect of the press molding method according to the first embodiment, instead of the post-release mold fitting step S5 and the heating and holding step S7, a post-release jig fitting step S11 and a heating and holding step S13 may be included as shown in FIG. 5.

[0048] <Jig fitting process after demolding> The post-mold release jig fitting process S11 is a process in which all or part of the press-molded product 1 released in the mold release process S3 is fitted into a jig that can hold a predetermined shape, including another mold, having the same shape as the mold used in the press molding process, and all or part of the press-molded product 1 is fixed to match the predetermined shape.

[0049] Furthermore, the shape may not only be the same as that of the die used in the press molding process, but may also be a predetermined shape, such as the target shape of the press-molded product 1 (the shape defined as the product), or an intermediate shape between the bottom dead center shape and the target shape, such as the shape after springing back from the bottom dead center of molding.

[0050] The shape after springback may be predicted by measuring or calculating the shape of the press-molded product 1 after release from the mold and springback. In this case, all or a part of the press-molded product 1 released from the mold is fitted into a jig capable of holding the same shape as the shape after release from the mold and springback, and all or a part of the press-molded product 1 is fixed in accordance with the shape after springback.

[0051] In addition, using a jig to hold the press-formed product 1 in a predetermined shape may mean using a jig that holds the entire press-formed product 1 in a predetermined shape, or may mean using a jig that holds only a part of the press-formed product 1, for example, the punch shoulder R portion 3, in a predetermined shape.

[0052] <Heating and holding process> The heating and holding step S13 is a step of heating the press-molded product 1 fitted in the jig in the post-mold-release jig fitting step S11 to a predetermined temperature A higher than room temperature and holding it at the predetermined temperature A for a predetermined time of less than 30 minutes.

[0053] In this way, in other aspects of the press forming method according to the first embodiment, it is possible to promote the relaxation of residual stress, so that the shape change that occurs over time after springback can be carried out in a short period of time, and further, the subsequent shape change can be suppressed.

[0054] In the heating and holding step S13 of another aspect of the first embodiment, the predetermined temperature A to which the press-molded product 1 is heated is preferably set to 130° C. to 150° C., and the predetermined time is preferably set to 1 minute or more and 3 minutes or less.

[0055] [Embodiment 2] The press molding method of embodiment 2 of the present invention is an example that shortens the time required for the press-molded product 1 to change in shape over time after springing back at the moment the press-molded product 1 is released from a mold, as shown in Figure 9(a), and includes a press molding step S21, a demolding step S23, a post-release heating and holding step S25, and a cooling step S27, as shown in Figure 6. Here, since the press molding process S21 and the demolding process S23 are similar to the press molding process S1 and the demolding process S3 in the above-mentioned embodiment 1, their explanation will be omitted here, and the following will explain the post-demand heating and holding process S25 and the cooling process S27.

[0056] <Post-mold heating process> The post-mold-release heating and holding step S25 is a step of heating the press-molded product 1 released in the mold-releasing step S23 to a predetermined temperature A higher than room temperature and holding it at the predetermined temperature A for a predetermined time of less than 30 minutes.

[0057] In the post-mold-release heating and holding step S25, the press-molded product 1 is held at a predetermined temperature A for a predetermined time, thereby allowing the shape of the press-molded product 1 to be changed.

[0058] It is preferable that the predetermined temperature A in the post-release heating and holding step S25 is in the range of 130° C. to 150° C., and the predetermined time is 1 minute or more and 3 minutes or less.

[0059] <Cooling process> The cooling step S27 is a step of cooling the press-molded product 1, which has been heated and held at the predetermined temperature A for a predetermined time in the post-mold-release heating and holding step S25, to room temperature. The cooling method in the cooling step S27 may be either natural air cooling or forced air cooling in which a cooling fan or the like is actively used, but forced air cooling is preferred because it can shorten the cooling time. The temperature of the press-molded product after cooling does not have to be room temperature, but may be a predetermined temperature B (B <A)とすればよい。

[0060] In this way, according to the press molding method of this embodiment 2, by heating to a predetermined temperature A which is higher than room temperature and holding it for a predetermined time, it is possible to cause the press-molded product 1 to undergo shape change due to stress relaxation over time in a shorter time than would be possible at room temperature before being subjected to assembly processing, thereby easing time constraints on the process, such as the waiting time for shape change before the press-molded product 1 is subjected to assembly processing with other parts in the next process.

[0061] [Embodiment 3] The press molding method according to the third embodiment of the present invention is for shortening the time required for the press-molded product 1 to change in shape over time after springback at the moment of releasing the press-molded product 1 from the die as shown in Fig. 9(a) as an example, and includes a press molding step S31, a pre-release heating and holding step S33, a cooling step S35, and a demolding step S37 as shown in Fig. 7. Note that the press molding step S31 is the same as the press molding step S1 in the first embodiment described above, and therefore the pre-release heating and holding step S33, the cooling step S35, and the demolding step S37 will be described below.

[0062] <Pre-release heating process> The pre-release heating and holding process S33 is a process in which, after the press-molded product 1 is press-molded using a mold in the press molding process S31, the product is heated to a predetermined temperature A higher than room temperature at the bottom dead center position without being released from the mold, and held at the predetermined temperature A for a predetermined time of less than 30 minutes.

[0063] <Cooling process> The cooling step S35 is a step of cooling the press-molded product 1 that has been heated and held in the pre-release heating and holding step S33 to room temperature. In the cooling step S35, the press-molded product 1 is cooled together with the die that fixes the press-molded product 1 in the bottom dead center shape. The temperature of the press-molded product 1 after cooling does not have to be room temperature, but may be a predetermined temperature B (B <A)とすればよい。

[0064] <Mold release process> The demolding step S37 is a step of releasing the press-molded product 1 cooled in the cooling step S35 from the mold.

[0065] According to the press molding method of the third embodiment, the press-molded product that has been press-molded up to the bottom dead point of molding is fixed in the bottom dead point shape without being released from the mold, heated to a predetermined temperature A that is higher than room temperature, and held for a predetermined time of less than 30 minutes. This makes it possible to reduce the residual stress in the press-molded product even if the time it is held in the mold is shortened compared to when it is held in the mold at room temperature, and makes it possible to sufficiently reduce the change in shape that occurs over time after it is released from the mold and springs back.

[0066] [Embodiment 4] The shape evaluation method of a press-molded product according to embodiment 4 of the present invention is an example shown in FIG. 9(a) which evaluates the shape of a press-molded product 1 which is press-molded and then assembled with other parts before the press-molded product is subjected to assembly processing, and includes a press molding process S41, a demolding process S43, a post-demolding heating and holding process S45, a cooling process S47, a shape measurement process S49, and a shape determination process S51, as shown in FIG. 8.

[0067] Here, the press molding process S41 and the demolding process S43 are similar to the press molding process S1 and the demolding process S3 in the above-described embodiment 1, and the post-release heating and holding process S45 and the cooling process S47 are similar to the post-release heating and holding process S25 and the cooling process S27 in the above-described embodiment 2, so below, only the shape measurement process S49 and the shape determination process S51 will be described.

[0068] <Shape measurement process> The shape measurement process S49 is a process of measuring the shape of the press-formed product 1 after cooling in the cooling process S47.

[0069] The reason for heating and holding the press-formed product 1 at a predetermined temperature A for a predetermined time to change its shape and then measuring the shape is that by heating to a predetermined temperature A higher than room temperature, the residual stress in the press-formed product 1 can be relaxed in a shorter time than at room temperature, and a shape change due to stress relaxation can be caused promptly.

[0070] <Shape determination process> The shape determination process S51 is a process of determining that the press-formed product 1 is to be subjected to assembly processing if the shape of the press-formed product 1 measured in the shape measurement process S49 is within a preset predetermined range.

[0071] The preset predetermined range in the shape determination process S51 may be appropriately set within a range such that no problems occur in the assembly processing with other parts.

[0072] Thus, according to the method for evaluating the shape of a press-formed product according to the fourth embodiment, the press-formed product 1 is released from the mold, heated to a predetermined temperature A higher than room temperature, held for a predetermined time, and the shape change due to stress relaxation over time is sufficiently caused in a shorter time than at room temperature, then cooled to a predetermined temperature B (B < A), and its shape is measured. If the measured shape of the press-formed product 1 is within the preset predetermined range, it is determined that it is to be subjected to the next process. Therefore, the time constraints in the process can be relaxed, and furthermore, it is possible to prevent a shape change from occurring in the press-formed product 1 before it is assembled after press forming and a problem from occurring in the next process.

[0073] Note that the press forming method and the method for evaluating the shape of a press-formed product according to the present invention do not particularly limit the metal plate used as a blank, the shape and type of the press-formed product, etc., and are more effective for automotive parts press formed using a metal plate with high residual stress in the press-formed product.

[0074] Specifically, the blank preferably has a tensile strength of 440 to 1470 MPa. Metal sheets with a tensile strength of less than 440 MPa have low residual stress inside the press-formed product, so the effect of changes over time is relatively small, and there are few advantages to using the present invention. However, for parts with low rigidity such as automobile exterior panels, which are susceptible to shape changes due to changes in residual stress, it is desirable to apply the present invention even to metal sheets with a tensile strength of less than 440 MPa.

[0075] On the other hand, although there is no particular upper limit for the tensile strength of a metal sheet, a metal sheet having a tensile strength exceeding 1470 MPa has poor ductility. Therefore, for example, when the target is a press-formed product 1, cracks may occur in the punch shoulder R portion 3 or the die shoulder R portion 5 during the press forming process, making it impossible to perform press forming.

[0076] Furthermore, with regard to the shape of the press-formed product, the present invention is not limited to the press-formed product 1 having a hat cross-sectional shape as shown in FIG. 9 , but can be preferably applied to press-formed products having shapes that have portions with high residual stress, such as press-formed products having a Z-shaped cross-sectional shape, a U-shaped cross-sectional shape, or an L-shaped cross-sectional shape.

[0077] In terms of the types of press-formed products, it is preferable to apply the present invention to automobile parts such as exterior panel parts, such as doors, roofs, and hoods, which have low rigidity, and frame parts, such as A-pillars, B-pillars, roof rails, side rails, front side members, rear side members, and cross members, which use high-strength metal plates. EXAMPLES

[0078] Experiments were conducted to confirm the effects of the press molding method according to the present invention, and the results will be described below.

[0079] In the experiment, a metal plate having the mechanical properties shown in Table 1 below was used, and the press-formed product 1 with a hat-shaped cross section having a top plate portion 2, a vertical wall portion 4, and a flange portion 6, as shown in FIG. 9(a), was used. The bottom dead point shape of the press-formed product 1 was T 50 mm, forming height H is 50 mm, and the inclination angle θ of the vertical wall portion 4 is v The radius of curvature R and the bending angle of the punch shoulder R portion 3 were set to 5 mm and 95°, respectively, and the radius of curvature R and the bending angle of the die shoulder R portion 5 were set to 5 mm and 95°.

[0080] [Table 1]

[0081] As an example of the invention, the change in shape of the press-molded product 1 over time after it was released from the die and spring back was measured in two cases: the press-molded product 1 was released from the die and allowed to spring back, then returned to the die and held in the bottom dead center shape at a predetermined temperature A for a predetermined time, and the press-molded product 1 was press-molded to the bottom dead center and then held in the die at a predetermined temperature A for a predetermined time without being released.

[0082] In addition, the change in shape of the press-molded product 1 over time after release and springback was measured for each of the following cases: a conventional example in which the press-molded product 1 that had been press-molded up to the bottom dead point of molding was released without being held in the mold; and a comparative example in which the press-molded product 1 that had been released and springback was returned to the mold and held in the bottom dead point shape for a predetermined time at room temperature (25°C), and a comparative example in which the press-molded product 1 was not released from the mold and was held in the mold for a predetermined time at room temperature (25°C).

[0083] As shown in FIG. 9(b), the shape of the press-formed product 1 changes over time when the center of the longitudinal direction of the top plate 2 of the press-formed product 1 is aligned with the bottom dead point shape of the press-formed product 1 and the top plate 2 is moved away from the top plate 2 by a distance H s = 40 mm position (widthwise distance (change in wall opening of vertical wall portion 4) X k ) was measured.

[0084] Table 2 shows the change in wall opening X of the vertical wall portion 4 at each elapsed time immediately after press molding of the press-molded product 1 (immediately after release from the mold and spring back) and after being held in the mold for a predetermined time and then released from the mold. k The results of the measurement are shown below. Here, the change amount X shown in Table 2 k is based on the shape of the press-molded product 1 immediately after it is released from the die and springs back.

[0085] [Table 2]

[0086] In the conventional example, the press-molded product 1, which has been press-molded to the bottom dead point, is released from the die and left at room temperature. k The thickness was 1.7 mm immediately after press molding (immediately after release from the mold and spring back) and two days later.

[0087] In Comparative Example 1, the press-molded product 1 that was first released from the mold and then spring-backed was returned to the mold, held at room temperature, and then released again, and the press-molded product 1 was returned to the mold. This process was repeated. k was 0.2 mm 15 minutes after the initial release, and 0.3 mm after 30 minutes. It increased over time and became constant at 0.5 mm after 2 days. These changes were k This is a reduction compared to (=1.7mm).

[0088] In the invention example 1, the press-molded product 1 that was first released from the mold and spring back was returned to the mold, heated to 150°C in a furnace for 6 minutes, held at 150°C for 1 minute, then removed from the furnace and cooled to room temperature by forced air cooling for 8 minutes, and released from the mold again. The press-molded product 1 was then returned to the mold and repeatedly heated, held, and cooled. k was 0.3 mm 15 minutes after the initial demolding and 0.5 mm 30 minutes after, remaining almost constant.

[0089] In this way, the change in wall opening X k The times required from the first demolding until the thickness became 0.3 mm and 0.5 mm were short, 15 minutes and 30 minutes, respectively, in Example 1, whereas they were long, 30 minutes and 2 days, respectively, in Comparative Example 1. This shows that by returning the press-molded product 1 that has been demolded and springs back to the mold, heating it to the specified temperature A, and holding it for a specified time of less than 30 minutes, it is possible to cause a change in shape over time in a short period of time, compared to holding it at room temperature.

[0090] In addition, in Example 1, the change amount X2 after the first release k The difference was 0.5 mm, which was reduced compared to the conventional example.

[0091] Table 3 shows the change in wall opening X after the press-molded product 1, which has been press-molded to the bottom dead center, is held in the mold for a specified time and then released from the mold. k The results of the measurement are shown below. Here, the change amount X shown in Table 3 k is based on the shape of the press-formed product 1 after springing back from the bottom dead center of the press.

[0092] [Table 3]

[0093] In Comparative Example 2, the product was press molded, then held in the mold at room temperature (25°C) and then released from the mold. k The change in thickness is 0.3 mm immediately after it is released from the mold after being held in the mold for 15 minutes, and then immediately after it is released from the mold again after 15 minutes have passed since it was placed back in the mold (the cumulative holding time in the mold is 30 minutes). k = 0.6 mm, and then it is returned to the mold again. After two days, the change immediately after release is X k = 1.0 mm, and the change amount X k (=1.7mm).

[0094] Inventive Example 2, after press molding, the molded product was heated to 150°C by induction heating over 6 minutes, held at 150°C for 1 minute, and then removed from the furnace and cooled in forced air for 8 minutes before being demolded. k After measuring the temperature, the press-molded product 1 was returned to the mold and heated, held, and cooled repeatedly. Amount of change X in Example 2 k was 0.6 mm immediately after being released from the mold after being heated, held, and cooled for 15 minutes, and then returned to the mold, heated, held, and cooled for a further 15 minutes (total holding time in the mold was 30 minutes), and then immediately after being released from the mold, it was 1.0 mm, and then returned to the mold again, and finally held in the mold at room temperature for 2 days, after which the change remained constant at 1.0 mm immediately after being released from the mold.

[0095] From this, in the invention example 2, the change amount X after two days from press molding k was significantly reduced compared to the conventional example (1.7 mm). k The cumulative retention times in the mold required to achieve thicknesses of 0.6 mm and 1.0 mm were 15 minutes and 30 minutes, respectively, in Example 2 of the invention, which showed that the cumulative retention times in the mold could be shortened compared to the 30 minutes and 2 days in Comparative Example 2. [Explanation of symbols]

[0096] 1 Press-molded products 2 Top plate 3 Punch shoulder R 4 Vertical wall section 5 Die shoulder R 6 Flange

Claims

1. A press molding method for shortening the time required for a shape change of a press-molded product over time after the press-molded product springs back at the moment the press-molded product is released from a die, A press forming process in which a metal plate is press-formed into a press-formed product using the die; a demolding step of demolding the press-molded product from the mold; a post-mold release heating and holding step in which the press-molded product after the mold release and springback is heated to a predetermined temperature A higher than room temperature without being subjected to external force and held at the predetermined temperature A for a predetermined time of less than 30 minutes, thereby accelerating the phenomenon in which the residual stress in the press-molded product after the springback gradually relaxes over time, and the springback press-molded product undergoes a shape change over time (excluding the case of a heating step in which the press-molded product, which is an intermediate product, is heated between the press-molding step and a trimming step as a finishing step to attenuate the residual stress in the deformed portion of the metal plate); and a cooling step of cooling the heated and held press-molded product to a predetermined temperature B (B<A).

2. 2. The press molding method according to claim 1, wherein the predetermined temperature A is set to 130° C. to 150° C., and the predetermined time is set to 1 minute or more and 3 minutes or less.

3. A shape evaluation method for a press-molded product that evaluates a shape of a press-molded product that is assembled with other parts after press molding, comprising: a press forming process of press-forming a metal plate into a press-formed product using a die; a demolding step of demolding the press-molded product from the mold; a post-release heating and holding step in which the press-molded product after the release and springback is heated to a predetermined temperature A higher than room temperature without being subjected to external force and held at the predetermined temperature A for a predetermined time of less than 30 minutes, thereby accelerating the phenomenon in which the residual stress in the press-molded product after the springback gradually relaxes over time, and the springback press-molded product undergoes a shape change over time; A cooling process of cooling the heated and held press-molded product to a predetermined temperature B (B<A); a shape measuring step of measuring the shape of the press-molded product after cooling; a shape determination step of determining that the press-molded product is to be subjected to the assembly process if the measured shape of the press-molded product is within a predetermined range.

4. 4. The shape evaluation method for a press-formed product according to claim 3, wherein the predetermined temperature A is set to 130° C. to 150° C., and the predetermined time is set to 1 minute or more and 3 minutes or less.

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