Patent Publication Number: US-2011068519-A1

Title: Press hardening die cooling device

Description:
TECHNICAL FIELD 
     The present invention relates to a press hardening die cooling device, and more particularly, to a press hardening die cooling device which has a structure improved for adjusting cooling rate in order to adjust hardness of the surface structure of a portion that is cut by trimming die of a steel plate blank. 
     BACKGROUND ART 
     In general, a method of manufacturing a high-strength press-hardened product is to heat a steel material of which hardenability is improved by adding B, Mo, and Cr etc. at a high temperature, about 900° C., above an Ac3 transformation point to be completely changed into an austenite state, and then hot-forming the steel plate at one time into a product shape with a press die and rapidly cooling it into martensite structure. 
     As well known in the related art, a steel plate is easy to form, because its ductility is increased when being heated at a high temperature. Thus, the machinability of a steel plate manufactured by press hardening is slightly better than that of typical steel plates for machining and considerably better than that of high-strength steel. 
     Further, a steel plate manufactured by press hardening has very high strength (above 1,400 MPa) such that it is significantly advantageous in terms of specific strength, obtained by dividing yield strength by density, and thus can considerably contribute to reducing weight of vehicles. Further, the steel plate manufactured by press hardening is used to manufacture ultra high-strength parts that are difficult to form, because there is little spring back after machining. 
     It is required to heat a steel plate at about 900° C. or more for several minutes to transform the steel plate into an austenite state in press hardening process, and this should be automated for an efficient process. 
     Press hardening includes heating a blank before being hardened from a wound steel plate coil in a furnace, and carrying the blank into a press with a robot, and pressing it in the related art. 
     The press includes upper and lower dies of a press hardening die, cooling water is supplied to cooling water supplier for the upper and lower dies to harden the surface structure of a steel plate blank and cools the steel plate blank. 
     Steel plate blanks of the related art are cut into a middle part which is substantially used for a part, and scraps which are disposed at both sides and are substantially cut by a trimming die in the post-machining. 
     However, the portion to be trimmed by a trimming die of the related art is hardened by the cooling using a press die described above. Thus, the trimming die is severely worn in trimming, and accordingly machinability of the trimming is reduced, work loss is caused by replacing the trimming die, and productivity decreases. 
     DISCLOSURE 
     Technical Problem 
     In order to solve the above problems, the present invention has been made in an effort to provide a press hardening die cooling device that facilitates trimming a steel plate blank by applying cooling processes to a part differently from the other part of the blank, by using an improved structure of a press hardening die. 
     Technical Solution 
     In order to accomplish the above object, an embodiment of the present invention provides a press hardening die cooling device, which includes: a press hardening die including upper and lower dies and pressing a steel plate blank; a plurality of cooling water suppliers spaced apart from each other in the upper and lower dies to be channels through which cooling water flows; and a cooling water temperature adjuster configured to adjust temperature of the cooling water supplied to the cooling water suppliers. 
     The cooling water temperature adjuster includes a controller that operates such that cooling water is supplied at a higher temperature to the cooling water suppliers close to trimming portions of the steel plate blank than the other cooling water suppliers in the cooling suppliers, and wherein the controller includes a heater configured to heat the cooling water supplied to the cooling water suppliers close to trimming portions. 
     The controller controls the heater to maintain the temperature of the cooling water supplied to the cooling water suppliers close to the trimming portions between 45 and 80° C. 
     Holes are formed between the cooling water suppliers close to the trimming portions of the steel plate blank and the contact surface of the steel plate blank. 
     The press hardening die is provided with insulators. 
     The press hardening die cooling device further includes temperature sensors disposed between the trimming portions of the steel plate blank and the cooling water supplier to sense temperature of the press hardening die, and outputting sensed signals to the controller. 
     The cooling water temperature adjuster controls temperature of the cooling water such that the trimming portions of the steel plate blank changes to a mixed structure of ferrite, pearlite, and bainite. 
     Another embodiment of the present invention provides a press hardening die cooling device, which includes: a press hardening die including upper and lower dies provided with insulators therein and pressing a steel plate blank; a plurality of cooling water suppliers spaced apart from each other in the upper and lower dies to be channels through which cooling water flows; and a cooling water temperature adjuster adjusting temperature of the cooling water supplied to the cooling water suppliers using temperature sensors disposed in the press hardening die. 
     ADVANTAGEOUS EFFECTS 
     The present invention provides an apparatus for a press hardening die which is improved to decrease cooling rate of trimming portions in pressing a steel plate blank for manufacturing a part for press hardening. According to the embodiments of the present invention, it is possible to improve durability and productivity by reducing wear of the trimming die, because it is possible to prevent the trimming portion from changing to a martensite structure having high hardness by decreasing cooling rate by the cooling water suppliers in the upper and lower dies to prevent the trimming portions from being rapidly cooled. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing the configuration of a first embodiment of a press hardening die cooling device according to the present invention. 
         FIG. 2  is a perspective view showing a second embodiment of the present invention. 
         FIG. 3  is a view showing the configuration of a third embodiment of a press hardening die cooling device according to the present invention. 
         FIG. 4  is a view showing the configuration of a fourth embodiment of a press hardening die cooling device according to the present invention. 
         FIG. 5  is a view showing the configuration of a fifth embodiment of a press hardening die cooling device according to the present invention. 
         FIG. 6  is a view showing the configuration of a sixth embodiment of a press hardening die cooling device according to the present invention. 
         FIG. 7  is a graph showing a cooling process to time of a steel plate blank according to the present invention. 
     
    
    
     BEST MODE 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Referring to  FIG. 1 , the first embodiment of a press hardening die cooling device according to the present invention includes: a press hardening die P composed of upper and lower dies  10  and  20  equipped with cooling water suppliers  12  and  22  therein to press a steel plate blank S from the top and the bottom; and a cooling water temperature adjuster disposed between the cooling water suppliers  12  and  22  of the upper and lower dies  10  and  20  and the contact surface of the steel plate blank S to adjust temperature of cooling water supplied to the steel plate blank S. 
     In detail, the cooling water temperature adjuster is provided with a controller  100  for supplying cooling water at different temperatures to the cooling water suppliers  12  and  22 , and the controller  100  includes a heater  150  for heating the cooling water supplied to the cooling water suppliers  12  and  22  close to trimming portions T 1 . 
     The controller  100  controls the turning on/off of the heater  150  such that the temperature of the cooling water, which is supplied to the cooling water suppliers  12  and  22  close to the trimming portions T 1 , is maintained between 45 and 80° C. The other cooling suppliers  12  and  22  are supplied with cooling water at a temperature between 10 and 25° C. which is the same as in the related art. 
     That is, controller  100  operates such that cooing water at 10 to 25° C. is supplied to the cooling water suppliers  12  and  22  close to the middle portion of the steel plate blank S, which will be a part, and cooling water at 45 to 80° C. heated by the heater  150  is supplied to the cooling water suppliers  12  and  22  close to the trimming portions T 1  located at both sides. 
     In this configuration, adjusting the temperature of the cooling water between 45 and 80° C. is for transforming the structure of the trimming portions T 1  in the steel plate blank S into a mixed structure with ferrite, pearlite, and bainite, which has excellent toughness and ductility, as compared with a martensite structure. 
     The operation of the present invention having this configuration is described hereafter. 
     A press hardening die cooling device according to the present invention controls cooling rate of the cooling water suppliers  12  and  22  during the press hardening of the steel plate blank heated at 900° C. or more through a furnace and carried into the press hardening die P using a robot in order to manufacture a press-hardened product such that heat conduction rate of the trimming portions T 1  interfacing each scrap and the middle portion which will be a product is different from the other portions of the steel plate blank S. 
     For this configuration, the some cooling air suppliers  12  and  22  which are close to the trimming portions T 1  among the cooling water suppliers  12  and  22  disposed in the upper and lower dies  10  and  20  of the press hardening die P are supplied with cooling water at a higher temperature than the other portions such that the cooling rate becomes lower at the trimming portions T 1  than the other portions to be trimmed, and accordingly, hardenability can be more decreased than the other portions of the steel plate blank S. 
     Therefore, the trimming portions T 1  have lower cooling rate than the other portions of the steel plate blank S in pressing, and thus, the structures become having lower hardness than the other portions. 
     Thereafter, the other portions of the steel plate blank S except for the portions T 1  to be trimmed in trimming process are change into a martensite structure having high hardness, whereas the trimming portions are changed into a ferrite+pearlite+bainite structure, which has lower hardness and better machinability than the martensite structure. 
     In this case, hardenability of the trimming portions T 1  is variable in accordance with cooling water temperature, and for example, there is a relationship that the cooling rate decreases when the cooling water temperature increases and the cooling rate increases when the cooling water temperature decreases. 
     Hereinafter, repeated description for the components that are the same as those in the above embodiment is not provided and they are given the same reference numeral, in describing the other embodiments of the present invention. 
     On the other hand,  FIG. 2  is a view showing a second embodiment of the present invention, in which holes  210  are formed through the upper and lower dies, between the cooling water suppliers  12  and  22  close to the trimming portions T 1  and the contact surface of the steel plate blank S. 
     The holes are formed to change the trimming portions T 1  of the steel plate blank S into a mixed structure with ferrite+pearlite+bainite by reducing the rate of heat conduction to the cooling water suppliers  12  and  22 , as described above. 
     That is, heat transferred from the cooling water suppliers  12  and  22  is conducted to the trimming portions T 1  through the holes  210  such that cooling performance of the cooling water decreases at the portions than the other portions and the trimming portions T 1  of the steel plate blank S are prevented from being rapidly cooled. 
     In this configuration, although the holes  210  are formed at both of the upper and lower dies  10  and  20 , the hole may be formed only at the upper die or the lower die. 
     Further, it is preferable that the hole  210  has a rectangular or a circular cross section to decrease heat transfer efficiency. 
       FIG. 3  is a view showing the configuration of a third embodiment of the present invention. In addition to the configurations of the embodiments described above insulators  300  are vertically disposed in the upper and lower dies  10  and  20  of the press hardening die P. 
     The insulators  300  are vertically arranged to divide the upper and lower dies of the press hardening die P around the trimming portions T 1 . 
     This is for making the temperature of the cooling water supplied to the trimming portions T 1  of the steel plate blank S different from the temperature of the cooling water supplied to the other portions. 
     That is, since the press hardening die P is divided into the middle portion and both sides by the insulators  300 , it is possible to provide different cooling performance to each portion of the steel plate blank by supplying cooling water of the cooling water suppliers  12  and  22  with different temperatures. 
       FIG. 4  is a view showing the configuration of a fourth embodiment of the present invention, which includes the components of the embodiments described above and further includes temperature sensors  155  that are disposed in the press hardening die P to sense the internal temperature of the die and outputs sensed signals to the controller  100 . 
     The temperature sensors  155  sense the temperature of the die in real time such that the heater  150  heats the cooling water and that temperature of specific portions is increased or maintained when the temperature of the die is decreased under temperature set in advance in the controller  100 . 
       FIG. 5  is a view showing the configuration of a fifth embodiment of the present invention, in which the diameter of the cooling water suppliers  12  and  22  close to the trimming portions T 1  is set smaller than the diameter of the other cooling water suppliers  12  and  22 . 
     Further,  FIG. 6  is a view showing the configuration of a sixth embodiment of the present invention, in which the cooling water suppliers  12  and  22  close to the trimming portions T 1  at both sides in the press hardening die P are dispose further from the contact surface of the steel plate blank than the other cooling water suppliers  12  and  22 . 
     That is, the cooling water suppliers  12  close to the trimming portions T 1  in the upper die are disposed at a level higher than the other water suppliers  12  in the upper die, while the water suppliers  22  close to the trimming portions T 1  in the lower die are disposed at a level lower than the other water suppliers  22 , in the figure. 
     Therefore, the fifth and sixth embodiments of the present invention show that it is possible to make cooling performance of each portion of the steel plate blank different without making temperature of the supplied cooling water different. 
     That is, the embodiments of the present invention can prevent the trimming portions from changing to the martensite structure having high hardness by preventing the trimming portions T 1  of the steel plate blanks S from being rapidly cooled by way of making the cooling rate of portions different using the cooling water suppliers  12  and  22  in the upper and lower dies  10  and  20  when pressing the steel plate blanks S. 
     Therefore, as shown in  FIG. 7 , temperature distribution of the steel plate blank S after the pressing is shown, in which the section A shows cooling rate at the middle portion of the steel plate blanks S and the section B shows cooling rate to time at the trimming portions T 1 . The cooling rates of the portions are different in the steel plate blank S. Accordingly, the structure corresponding to the section A is cooled at a rate of 17 to 24° C. per second from 950° C. to 800° C. and then hardened by rapid cooling into the martensite structure, and the structure corresponding to the section B is slowly cooled from 800° C. into the mixed structure.)