JP2011245525A - Method for press forming of metal plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for press forming of a metal plate, which improves press forming performance without a decrease in production efficiency.SOLUTION: The method for press forming of a metal plate, forms a metal plate into shapes by a die and a punch. The method for press forming of a metal plate, performs at least one cycle of the operation in which the punch is stopped during the time in which the punch comes into contact with the metal plate and reaches the stroke termination of the punch, and after a lapse of a prescribed time, the press forming of the metal plate is resumed.

Description

  The present invention relates to a method of press forming a metal plate, and more particularly to a technique that enables deeper processing by reducing a forming load.

In recent years, global warming has become an international issue, and weight reduction is being promoted in the automobile industry for the purpose of improving fuel efficiency in order to reduce CO ₂ . On the other hand, collision safety standards in recent years have become stricter year by year, and research has been conducted to make safe cars that anticipate such trends. That is, the application of high-tensile steel sheets has been increasing in order to satisfy the conflicting purposes of weight reduction and safety. For example, the increase in weight has been minimized by arranging high-tensile steel plates at the right place in consideration of the impact mode and crash deformation mode.

  However, high-tensile steel sheets have poor formability compared to mild steel materials, and are often limited by the types and shapes of applicable parts. In order to expand the application range to more parts and reduce the weight of automobiles and the like, research on deep drawing of high-tensile steel sheets has been conducted. For example, in Patent Document 1, the punch is retracted from the metal plate after the punch first contacts the metal plate and the molding is started and before the punch reaches the end of the water jet rooke and the molding is completed. A press molding method in which the operation is performed at least once is disclosed. According to this technique, the oil film of the lubricating oil is regenerated on the metal plate by retracting the punch from the metal plate, and deeper press working can be performed by improving the lubricating effect.

JP 2005-199318 A

  However, the technique described in Patent Document 1 has a problem in that the time loss is large for retracting the punch and the production efficiency is lowered. Therefore, an object of the present invention is to provide a press forming method of a metal plate that can improve press formability without reducing production efficiency.

  The present inventors investigated what behavior occurs when the punch is stopped during press forming of the metal plate. As a result, it was found that when the punch was stopped, the load acting on the punch decreased rapidly, and when the punch was moved again, it recovered to almost the same value as the load before the stop. The fact that the limit drawing depth is extended by the operation of stopping the punch has been found out. The reason is considered to be metallographic, but the details are not clear. In this case, since the punch and the metal plate are always in close contact with each other, it is certain that the oil film of the lubricating oil is not regenerated as in Patent Document 1, but the mechanism for improving the formability will be clarified by future research. To be.

  The present invention was made based on the above knowledge, and in the metal plate press forming method of forming a metal plate with a die and a punch, while the punch contacts the metal plate and reaches the stroke end of the punch, The operation of stopping the punch and restarting the forming of the metal plate after a lapse of a predetermined time is performed at least once.

  According to the present invention, the press formability of the metal plate can be improved, and the punch is only stopped for a short time, so the influence on productivity is small. The time for stopping the punch is arbitrary, but the present inventors have examined it by a tensile test, and it has been found that the stress is rapidly relieved within 0.5 seconds after stopping the tension. . It has also been found that such stress relaxation phenomenon is almost completed in 4 seconds. Therefore, the punch stop time is preferably between 0.5 and 4 seconds.

  It is also desirable to stop the punch when the metal plate starts plastic deformation and the amount of strain becomes maximum. According to the study by the present inventors, it has been found that there is not much difference in the rate of stress relaxation when the strain amount is small and large. Therefore, since the stress is larger when the strain amount is large, the amount of stress that is relieved by stopping the punch is large. Further, the number of times the punch is stopped is one or more times. However, when the punch is stopped a plurality of times, it is desirable to stop the punch at least once when the amount of distortion is maximum.

  In the present invention, in order to stop the punch, a press device that can easily stop and restart the press ram, such as a servo press instead of a conventional crank press, is used.

  According to the present invention, it is possible to obtain such an effect that the press formability of the metal plate can be improved without reducing the production efficiency.

It is a sectional side view which shows the metal mold | die used in the Example of this invention. It is a top view which shows the blank shape | molded in the Example. It is a graph which shows the crank motion and step motion of the press ram in an Example. It is a graph which shows the relationship between the stroke of a press ram in an Example, and punch load. It is a graph which shows the relationship between time and punch load in an Example. It is a graph which shows the relationship between the time in a tensile test, and a true stress. It is a graph which shows the relationship between the time in a tensile test, and a true stress. It is the figure which expanded the principal part of FIG. It is a graph which shows the relationship between time and stress relaxation rate.

  A bottomed rectangular tube-shaped product was molded from the blank shown in FIG. 2 by deep drawing using the mold shown in FIG. In FIG. 1, reference numeral 10 denotes, for example, a die fixed to a servo press, and a hole 11 for forming a metal plate W is formed in the die 10. On the die 10, a punch 20 that is removably inserted into the hole 11 is disposed oppositely. The punch 20 is attached to a press ram 21 that can approach and separate from the die 10, and a plate 22 is attached to the press ram 21. A blank presser 23 is supported on the plate 22 so as to be slidable in the moving direction of the punch 20. A coil spring 24 is interposed between the blank presser 23 and the plate 22.

  In the above-described mold, when the press ram 21 is moved to the die 10 side (upward), the blank presser 23 first comes into contact with the metal plate W and presses it. Next, the punch 20 comes into contact with the metal plate W and deforms the metal plate W to the inside of the hole 11. Then, the metal plate W is deep-drawn with the punch 20 while pressing the metal plate W with the blank presser 23. The blank presser 23 can also be configured to be moved by a drive source different from the drive source of the press ram 21.

  A strain gauge was affixed to the lower end of the punch 20, and a molding test was performed to measure the molding load. The servo press used was Aida Engineering's NC1 2500 (D) 2500 kN. In this servo press, the movement of the press ram 21 can be freely programmed by the NC servo mechanism. The blank pressing force was 100 kN, and as the metal plate W, the materials shown in Table 1 were prepared. In the molding test, the molding depth was increased by 5 mm, and the molding limit depth without causing breakage or necking was determined.

  In the drawing process, two types of motion were used for the press ram 21. FIG. 3A shows the motion (crank motion) of a conventional crank press, which is an operation of drawing a locus of a sine wave with respect to time. In this embodiment, the press ram 21 is operated with this crank motion. The other motion is an operation of the press ram 21 that temporarily stops the operation during molding as shown in FIG. 3B (step motion). In the present embodiment, the press ram 21 is also operated by this step motion.

  The material used in this example is JAC440W. In the crank motion, cracks were found in the corner at a molding depth of 65 mm, but when molded with step motion, molding was possible up to 70 mm without cracking. Thus, it was confirmed that the forming limit depth increases by forming with step motion. FIG. 4 shows a change in the forming load in the deep drawing process.

  As shown in FIG. 4, the molding load at each position of the press ram 21 is substantially the same in both crank motion and step motion. However, as shown in FIG. 5, when the time change of the molding load of the step motion is seen, it can be seen that the molding load is reduced while stopping during the molding. When the press ram 21 starts moving again, it recovers to almost the same value as the load before stopping, and thereafter draws a load curve that is almost the same as that in the crank motion. In addition, when the same molding test was performed with JAC590Y, in the crank motion, the molding limit of 50 mm was able to be molded up to 70 mm by the step motion.

  Next, a tensile test was performed on the above materials. The tensile test pieces were strip-shaped, and the load cell speed was two types, low speed and high speed. Moreover, the stop time of the tensile test was 10.0 seconds, and the stop points were two points of low strain and high strain. FIG. 6 shows the result of stopping the tensile test in the low strain region, and FIG. 7 shows the result of stopping the tensile test in the high strain region. As can be seen from these figures, a stress decrease, that is, a stress relaxation phenomenon, was observed at the stop on the low strain side and the stop on the high strain side. By stopping for 10.0 seconds, the load decreased 3.6% from 405 MPa to 390 MPa on the low strain side, and 3.7% from 482 MPa to 464 MPa on the high strain side. From the above results, it was found that the load decreased at substantially the same rate in the low strain region and the high strain region. Therefore, the absolute value of the relaxation amount becomes larger as the stop in the high strain region.

  FIG. 8 shows an enlarged graph of the stress relaxation portion immediately after stopping. As shown in FIG. 8, the amount of stress relaxation changes with the elapsed time after stopping, and it can be seen that the amount of relaxation immediately after stopping is particularly large. FIG. 9 shows the amount of stress relaxation per unit time. As shown in FIG. 9, the relaxation rate decreases to 30% in the initial 0.5 seconds, and decreases to 15% in 1.0 seconds. This means that the effect of 1.0 seconds at the beginning of relaxation is great.

  The above stress relaxation phenomenon was confirmed as the same phenomenon when the strain rate (load cell speed) was changed to be high and low. Moreover, as a result of performing a tensile test with all the materials shown in Table 1, 3-4% stress relaxation was confirmed after stopping for 10.0 seconds.

  Now, in production, it is ideal that the processing time is as short as possible. Therefore, in order to find a limit point at a stop time of 1 second or less where the effect of stress relaxation is large, a molding test was performed with stop times of 0.1 seconds, 0.3 seconds, 0.5 seconds, and 1.0 seconds. As a result, it was possible to mold up to a molding depth of 70 mm without cracking or necking at a stop time of 1.0 seconds and 0.5 seconds. At 0.3 seconds and 0.1 seconds when the stop time was shortened, necking was observed in molding with a depth of 70 mm, and cracking occurred at a molding depth of 65 mm when molding was performed with crank motion without stopping.

  From the above results, it was found that the effect of improving the formability by the step motion depends on the stop time, and it was found that it was necessary to stop for 0.5 seconds or more in order to obtain a sufficient effect. A stop time of 0.5 seconds is considered the optimal time to minimize the impact on productivity. Moreover, since the stress relaxation was confirmed from the result of the tensile test regardless of the steel type and strain rate, the improvement of the formability by the step motion can be expected regardless of the forming speed and the steel type.

  The metal plate press forming method of the present invention can improve the press formability of the metal plate without lowering the production efficiency, so a high-strength steel plate or an ultra-high strength steel plate is applied to more parts in an automobile or the like. This can greatly contribute to the weight reduction of the vehicle body.

Claims (3)

In the metal plate press forming method of forming a metal plate with a die and a punch,
The operation of stopping the punch between the time when the punch contacts the metal plate and the end of the stroke of the punch and restarting the forming of the metal plate after elapse of a predetermined time is performed at least once. A metal plate press forming method.   The metal sheet press forming method according to claim 1, wherein the predetermined time is 0.5 to 4 seconds. The said predetermined time is 0.5-1 second, The press molding method of the metal plate of Claim 1 characterized by the above-mentioned.

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