JP2011218436A - Hot press-forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot press-forming method which can achieve both shortening of cycle time and securing accuracy of a formed article.SOLUTION: A steel sheet is heated at Ac3 point or higher as an average temperature. The steel sheet heated at the average temperature is press formed using a die, and cooled during press forming of the steel sheet. The formed article is released out of the die and cooled further. In the hot press-forming method, the lower the release temperature of the formed article is set, the smaller the shape deviation amount (t) of the formed article becomes. However, when the release temperature of the article is set at lower than 322°C, further improvement of precision of the article cannot be obtained. Furthermore, in such case, occupied time of the die by the article for cooling becomes longer. Meanwhile, when the release temperature is set at 510°C and lower, changing ratio of the shape deviation amount (t) to temperature change becomes remarkably larger so that precision improvement can be obtained. Therefore, the release temperature of the article is set at a temperature in the range between 322°C or higher and 510°C or lower.

Description

  The present invention relates to a hot press forming method in which a steel sheet is formed in a heated state, and more particularly to an improvement in a technique for taking out a steel sheet from a mold after forming.

  The press molding method uses, for example, a mold 100 shown in FIG. In the mold 100, as shown in FIG. 4A, when the work W is placed on the upper surface of the lower mold 120 and then the upper mold 110 is lowered toward the lower mold 120, the mold 100 is shown in FIG. As described above, when the pad 112 of the upper mold 110 abuts against the work W and presses it, the foam molding is performed.

  Next, when the upper die 110 is further lowered, the contact state of the pad 112 with the workpiece W is maintained, the trim blade 115 of the trim portion 111 of the upper die 110 abuts against the workpiece W, and the workpiece W by the trim blade 115 is maintained. The piercing blade 114 of the upper mold 110 comes into contact with the workpiece W, and the workpiece W is cut by the piercing blade 114. Reference numeral 113 denotes an elastic member that supports the pad 112, reference numeral 121 denotes a trim blade provided on the lower mold 120, and reference numeral 122 denotes a hole in which a portion of the workpiece W cut by the piercing blade 114 falls. .

  A hot press forming method in which a steel plate heated at a high temperature of 800 ° C. or higher is used as the work W, the above press forming is performed by the mold 100, and the steel plate after press forming is cooled is a method of obtaining a high strength press formed product. As promising. Specifically, by using a press die 100 as a heat removal material (refrigerant) of the steel sheet, the steel sheet structure is transformed from an austenite structure during heating to a martensite structure at the same time as press forming (so-called baking). The strength is increased (for example, Patent Document 1).

  In the above method, the cooling rate of the steel plate is equal to or higher than the quenching limit rate of the steel plate composition (27 ° C./s), and in order to complete the quenching, the temperature of the molded product immediately after removal from the press die is set to 100. It is generally necessary to reduce to ~ 200 ° C.

JP 2005-288528 A

  However, since it is necessary to occupy the mold until the cooling is completed in order to cool the molded product as described above, the hot press molding method has a longer cycle time and productivity than the cold press molding method. Inferior. Therefore, in order to shorten the cycle time, Patent Document 1 proposes to take out the molded product before cooling and quench it in a separate process. However, in this case, since it is taken out from the press mold before cooling, there is a great possibility that the shape of the molded product is deformed due to cooling, and it is difficult to ensure the accuracy of the molded product.

  Accordingly, an object of the present invention is to provide a hot press molding method capable of simultaneously reducing cycle time and ensuring accuracy of a molded product.

  As a result of intensive studies on a hot press molding method in order to solve the above-mentioned problems, the accuracy of the molded product depends on the temperature of the molded product when it is taken out from the mold after press molding, and the molded product The present inventors have found that a critical point exists in the temperature dependency of the accuracy of the present invention, and have completed the present invention.

  In the hot press forming method of the present invention, the average temperature of the steel sheet is heated to Ac3 point or higher, and the steel sheet set at the average temperature is press formed using a mold, and the steel sheet is cooled during the press forming. A hot press molding method in which the molded product is removed from the mold and further cooled, and the molded product is removed from the mold in a temperature range in which the average temperature of the molded product is 322 ° C. or higher and 510 ° C. or lower. It is characterized by being performed when The average temperature is an average value of temperatures at a plurality of sites set for the entire molded product.

  In the hot press molding method of the present invention, the accuracy of the molded product is improved as the average temperature of the molded product at the time of taking out from the mold (hereinafter referred to as take-out temperature) is lowered, but the take-out temperature of the molded product is 322 ° C. When it is less than the range, the accuracy of the molded product becomes substantially constant, and the accuracy cannot be improved further. In addition, in this case, the occupation time of the mold by the molded product for cooling becomes long. On the other hand, when the temperature is 510 ° C. or lower, the degree of improvement in the accuracy of the molded product with respect to a temperature change increases rapidly.

  From the above, by setting the take-out temperature of the molded product to 322 ° C. or more and 510 ° C. or less, both shortening of the cycle time and ensuring of the accuracy of the molded product can be achieved.

  Various configurations can be used in the hot press molding method of the present invention. The degree of improvement in the accuracy of the molded product with respect to the temperature change is maximized when the removal temperature of the molded product is in the temperature range of 366 ° C. or higher and 451 ° C. or lower. Therefore, it is more preferable to set the lower limit value of the removal temperature of the molded product to 366 ° C., and it is more preferable to set the upper limit value of the removal temperature of the molded product to 451 ° C.

  According to the hot press molding method of the present invention, the cycle time can be shortened and the accuracy of the molded product can be ensured by setting the removal temperature of the molded product to 322 ° C. or more and 510 ° C. or less.

It is a graph showing the relationship between the temperature of the workpiece | work (steel plate) and time in each process of the hot press molding method which concerns on the Example of this invention. It is a graph showing the relationship between the amount of shape changes of the molded article obtained with the hot press molding method which concerns on the Example of this invention, and taking-out temperature. (A), (B) is a figure explaining the dimension of the molded article obtained by the hot press molding method which concerns on the Example of this invention. (A)-(C) are side sectional views showing each process in a press molding method.

  Hereinafter, the present invention will be described in more detail with reference to specific examples. In the example, the heated steel sheet is put into a mold and drawn, and the average temperature (hereinafter referred to as the extraction temperature) of the molded product at the time of taking out from the die is changed for each molded product. The accuracy was evaluated.

  FIG. 1 is a graph showing a specific example of the relationship between the temperature of a work (steel plate) and time in each step of a hot press forming method according to an example. The temperature in the present embodiment is an average value (average temperature) of temperatures at a plurality of points of the steel plate measured with a thermo viewer. Since the temperature of the steel sheet cannot be measured during press forming, in FIG. 1, the temperature of the steel sheet during press forming is predicted to change linearly, and is obtained from the temperature of the steel sheet at the start of forming and at the end of forming. It was. In FIG. 1, a quenching limit line (quenching limit speed 27 ° C./sec) of the steel sheet is also shown.

  In the examples, a steel plate (C (carbon) content of 0.2 wt% or more) was used as a workpiece, and the steel plate was first heated to 950 ° C. in a heating furnace (heating step). Note that the steel plate material suitable for the present invention is, for example, C or a Fe-based material containing C and other elements with the balance being Fe and inevitable impurities. This material preferably contains 0.15 wt% or more of C in order to ensure hardness during quenching. In this case, Cr, Mo, Ti, and B are appropriately added for quenchability. May be. In addition to the above elements, Si, Mn, P, S, or the like may be added as appropriate.

  After the completion of heating, the steel plate was taken out from the heating furnace, and the heated steel plate was put into the mold (input process). Then, while performing press molding by a metal mold | die, the steel plate was cooled (rapid cooling) during press molding (formation process). The forming step includes a step of cooling the steel sheet in a state where the die is held at the bottom dead center (an in-mold restraining cooling step by heat removal from the die). In this case, quenching was started during press molding.

  In such press molding, two types of molded products having cross-sectional shapes shown in FIGS. 3A and 3B were obtained by using two types of molds. 3A shows a cross-sectional shape with a low molding height, and FIG. 3B shows a cross-sectional shape with a high molding height. Subsequently, the molded product was taken out from the mold and carried out (carrying out process), and the molded product was cooled (rapidly cooled) (cooling process).

  In the examples, the cooling rate during press molding with a mold is set within a range of 50 to 100 ° C./sec, and the take-out temperature of the molded product (temperature at the time of molding completion) is set within a range of 250 to 650 ° C. did. In addition, the graph shown in FIG. 1 has shown the result of one specific example of the steel plate set within the said temperature range, In that specific example, the time from the completion | finish of heating of a steel plate to taking out from a heating furnace is 1 second, The time from taking out the steel sheet from the heating furnace to the start of press forming (loading time) is 2.6 seconds, the press forming time is 5.4 seconds, and the time from forming completion to the start of cooling (unloading time) is 1.7 seconds. The cooling time was set to 2 seconds. The temperature of the steel plate at the start of press forming was 850 ° C., and the temperature of the formed product at the time of completion of press forming was 390 ° C.

  For each of the molded products after the cooling step, the shape change amount t of the flange portion of the molded product (the difference t in the height of the flange portion edge between the actually obtained shape and the regular shape (FIGS. 3A and 3) (B))) was measured. 3A and 3B, the shape indicated by the solid line is the normal shape (desired shape), and the shape indicated by the broken line (only the left part is shown) indicates the actually obtained shape. . In addition, the numerical value in FIG. 3 has shown the size (unit: mm) and angle of each site | part of the cross-sectional shape of a molded article. Table 2 and FIG. 2 show the measurement results for those having a high molding height and those having a low molding height.

  FIG. 2 is a graph of the data shown in Table 2. In FIG. 2, a curve l is a data curve for a high molding height, and is an approximate curve for the maximum value of the change amount t at each temperature. A curve m is a data curve for a low molding height, and is an approximate curve for the minimum value of the variation t at each temperature. As described above, the curves l and m are obtained by including all of the data for the high and low forming heights between the curves. The polygonal line N is an image line that emphasizes the critical point for the curve l.

  As can be seen from FIG. 2, the shape change amount t of the molded product decreases as the average temperature (hereinafter referred to as the extraction temperature) of the molded product at the time of taking out from the mold is lowered, but the removal temperature of the molded product is 322 ° C. When it is less than the value, the shape change amount t is substantially constant, and it has been confirmed that the accuracy of the molded product cannot be further improved. In addition, in this case, the occupation time of the mold by the molded product for cooling becomes long. Therefore, it was confirmed that it was necessary to set the temperature for taking out the molded product to 322 ° C. or higher (critical point [1]). On the other hand, when the temperature was 510 ° C. or lower, it was confirmed that the rate of change of the shape change amount t with respect to the temperature change increased rapidly. Therefore, it was confirmed that it was necessary to set the temperature at which the molded product was taken out to 510 ° C. or less (critical point [4]).

  From the above, it was confirmed that the cycle time can be shortened and the accuracy of the molded product can be ensured by setting the removal temperature of the molded product to 322 ° C. or more and 510 ° C. or less. Outside the temperature range, the force that deforms the shape inside the molded product is greater than the resistance against deformation of the material, and it is assumed that the accuracy of the molded product is changed. On the other hand, outside the above temperature range, the relationship between the two forces is reversed, and the resistance to deformation of the material is greater than the force that deforms the shape inside the molded product, ensuring the accuracy of the molded product. It is thought that it was possible to plan.

  Further, as can be seen from FIG. 2, the rate of change of the shape change amount t of the molded product with respect to the temperature change is maximized when the removal temperature of the molded product is in the temperature range of 366 ° C. or higher and 451 ° C. or lower. It was confirmed. Therefore, in order to obtain the above-mentioned effect efficiently, it is more preferable to set the lower limit value of the removal temperature of the molded product to 366 ° C. (critical point [2]). It was confirmed that it was more preferable to set the temperature at 0 ° C. (critical point [3]).

  100 ... Mold

Claims (3)

The average temperature of the steel sheet is heated to Ac3 point or higher,
Using a mold, press forming the steel sheet set to the average temperature, cooling the steel sheet during the press forming,
Remove the molded product from the mold,
In the hot press molding method for further cooling,
The hot press molding method is characterized in that the molded product is taken out from the mold when the average temperature of the molded product is in a temperature range of 322 ° C. or higher and 510 ° C. or lower.   2. The hot press forming method according to claim 1, wherein an average temperature of the steel sheet at the time of taking out from the mold is set to 366 ° C. or more.   The hot press forming method according to claim 1 or 2, wherein an average temperature of the steel sheet at the time of taking out from the mold is set to 451 ° C or lower.

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