1. Field of the Invention
The present invention relates to a hot press forming method for press forming a heated steel plate and more particularly to an improvement in technique for removing a formed steel plate from a mold.
2. Related Art
In a press forming method, for example, a mold 100 shown in
Next, when the upper mold 110 is lowered further following the form shaping, the abutment state of the pad 112 with the workpiece W is maintained. Then, a trimming blade 115 of a trimming portion 111 of the upper mold 110 is brought into abutment with the workpiece W, and a cutting operation of the workpiece W by the trimming blade 15 is effected. A piercing blade 114 of the upper mold 110 is brought into abutment with the workpiece W, and a cutting operation of the workpiece W by the piercing blade 114 is effected. Reference numeral 113 denotes an elastic member which supports the pad 112, reference numeral 121 a trimming blade provided on the lower die 120, and reference numeral 122 a hole portion into which a portion of the workpiece W which is cut by the piercing blade 114 falls.
In the hot press forming method, the steel plate which is heated at a high temperature of 800° C. or higher is used as the workpiece, and the press forming is performed by the mold 100, and the press formed plate steel is cooled. A hot press forming method like the one described above is promising as a method for obtaining a highly strong press formed product. Specifically, making the steel plate highly strong is realized by changing the structure of the steel plate from an austenite structure (resulting when the steel plate is heated) to a martensite structure (executing so-called quenching) at the same time as press forming by making use of the press mold 100 as a heat removal material (a refrigerant) for removing heat that the steel plate possesses.
In the method above, it is generally necessary that the cooling speed of the steel plate is equal to or faster than the quenching limit speed (27° C./s) and that the temperature of the formed product is reduced to 100 to 200° C. just after it is removed from the press mold to complete the quenching.
However, since the mold needs to be occupied exclusively for cooling the formed product in the way described above until the cooling is completed, compared with a cold press forming method, the hot press forming method requires a longer cycle time, and hence, the productivity thereof becomes worse than that of the cold press forming method. Then, to shorten the cycle time, JP-A-2005-288528 proposes that the formed product be removed from the press mold prior to cooling for quenching in a separate step. However, as this occurs, since the formed product is removed from the press mold prior to cooling, there exists a serious risk of occurrence of a deformation in the shape of the formed product in association with cooling, making it difficult to ensure the accuracy of the formed product with respect to shape.
With a view to solving the problem, deep studies have been made on hot press forming methods, and as a result thereof, it has been found that the accuracy of a formed product depends on the temperature thereof when the formed product is removed from the mold after press forming and that there exists a critical point in the dependency on temperature of the accuracy of the formed product. Thus, the invention has been completed based on these findings.
One or more embodiments of the invention provide a hot press forming method which can not only shorten a cycle time but also ensure the accuracy of a formed product with respect to shape.
In accordance with one or more embodiments, a hot press forming method may include the steps of: heating a steel plate so that an average temperature of the steel plate becomes Ac3 point or higher; executing a press forming on the steel plate whose average temperature is so set by use of a mold; cooling the steel plate during press forming; removing a formed product from the mold, while the average temperature of the formed product stays within a temperature range from 322° C. or higher to 510° C. or lower; and cooling the formed product further.
The “average temperature” denotes an average value of temperatures of a plurality of locations which are set over a whole area of the formed product.
In the hot press forming method, although the accuracy of the formed product increases as the average temperature of the formed product when it is removed from the mold (hereinafter, referred to as a removal temperature) decreases, the accuracy of the formed product becomes substantially constant when the removal temperature of the formed product becomes lower than 322° C., and no more improvement in accuracy cannot be realized. Moreover, in this case, the time during which the mold is occupied exclusively to cool the formed product becomes long. On the other hand, when the removal temperature is 510° C. or lower, the degree at which the accuracy of the formed product increases with change in temperature increases drastically.
Based on the understanding above, shortening the cycle time is compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower.
The hot press forming method of the invention can adopt various configurations. The degree at which the accuracy of the formed product increases with change in temperature becomes the largest when the removal temperature of the formed product stays in a temperature range from 366° C. or higher to 451° C. or lower. Consequently, a lower limit value for the removal temperature of the formed product is more preferably set to 366° C., and an upper limit of the removal temperature of the formed product is more preferably set to 451° C. or lower.
According to the hot press forming method of the embodiments, shortening the cycle time can be made compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
Hereinafter, the invention will be described in detail by reference to an exemplary embodiment. In the exemplary embodiment, a draw forming is performed on heated steel plates which are introduced into a mold. Average temperatures of formed products when they are removed from the mold (hereinafter, referred to as removal temperatures) are changed formed product by formed product, and accuracies of the formed products are evaluated.
In the exemplary embodiment, steel plates (whose carbon content is 0.2 wt % or more) are used as workpieces. Steel plates are firstly heated to 950° C. in a heating oven (a heating step). A material of the steel plate suitable for the invention is an Fe-based material containing, for example, C or C and other elements and including Fe and impurities that cannot be avoided as the remaining portion thereof. This material preferably contains 15 wt % or more C to ensure a required hardness in quenching, and as this occurs, Cr, Mo, Ti and B may be added as required for quenching properties. In addition to the elements above, Si, Mn, P, S and the like may also be added as required.
After the completion of heating, the steel plates are removed from the heating oven, and the steel plate which are still being heated are installed into a mold (an installation step). Following this, the steel plates are press formed by the mold, and the steel plates are cooled (quenching) during press forming (a forming step). This forming step includes a step of cooling the steel plates with the mold held at its bottom dead center (an in-mold restrained cooling step by removing heat by the mold). In this case, quenching is designed to start during press forming.
In the press forming described above, two types of formed products having sectional shapes shown in
In the exemplary embodiment, a cooling speed during press forming by the mold is set to fall within a range from 50 to 100° C./s, and a removal temperature of formed products is set to fall within a range from 250° C. to 650° C. The graph shown in
A quantity of change t in shape of a flange portion of the formed product (a different t in height of an edge portion of the flange portion between a shape actually obtained and a normal shape (
As is seen from
Thus, it is confirmed based on the facts above that shortening the cycle time is compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower. Since a force which deforms an internal shape of the formed product becomes larger than a resisting force to deformation of the material outside the above temperature range, it is assumed that a change in accuracy of the formed product is induced. In contrast to this, since the relationship between the two forces is reversed, causing the resisting force to deformation of the material to be larger than the force which deforms the internal shape of the formed product within the temperature range, it is considered that the accuracy of the formed product can be ensured.
As is seen from
While description has been made in connection with the specific exemplary embodiment and the examples thereof, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the present invention. It is aimed, therefore, to cover in the appended claims all such changes and modifications falling within the true spirit and scope of the present invention.
Number | Date | Country | Kind |
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2010-092863 | Apr 2010 | JP | national |