Patent Publication Number: US-11383288-B2

Title: Method of processing steel plate and punching machine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2019-11011, filed on Jan. 25, 2019, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     The present disclosure relates to a method for processing a steel plate and a punching machine. 
     One of the issues in press-forming of a high tensile strength steel is cracking in a stretch flange. This cracking in a stretch flange occurs due to residual strain on a shear end face. As a method for reducing this residual strain, a heating method is known. As a method for heating a residual strain part, a technique for applying a current and heating a bent part of a press formed product to remove residual strain is known (see, for example, Japanese Unexamined Patent Application Publication No. H07-303919). 
     SUMMARY 
     There has been a problem that it is difficult to transfer heat to an end part of a steel plate punched in punching processing and residual strain cannot be completely removed or when an end part of a steel plate is heated to such an extent that residual strain can be removed, areas other than the end part are excessively heated, thereby causing a change in the hardness of a part of the steel plate. 
     The present disclosure provides a method for processing a steel plate capable of removing residual strain at a trim edge thereof without causing overheating in areas of the steel plate other than the trim edge. 
     In a first example aspect of the present disclosure, a method for processing a steel plate includes: punching a steel plate as a workpiece fixed to a punching die by a punch; making a first heating electrode provided on a punch holder for supporting the punch and a second heating electrode provided on the punching die face each other by maintaining the punch in a state in which the punch penetrates the steel plate and heating a part of the steel plate including a trim edge punched in the punching; and pulling out the punch from the punching die. By disposing the heating electrodes on the punch holder and at a position facing the punch holder in the punching machine, and applying a current to the heating electrodes while the punch is penetrating the steel plate, the trim edge can be sufficiently ended. By heating the part of the steel plate including the trim edge, it is possible to appropriately remove residual strain and avoid overheating of areas other than the part of the steel plate including the trim edge. 
     The above processing method is effective if it further includes reducing heat generated in the heating and forming a stretch flange at the trim edge. When the stretch flange is formed on the steel plate in a state where the heat is reduced, damage to a flange die can be reduced. 
     Further, the pulling-out may be started based on a detected temperature of the trim edge. If the punch is pulled out when the temperature reaches an appropriate temperature, the residual strain can be removed appropriately, and softening and hardening of the steel plate can be avoided. Furthermore, relative positions of the punch holder and the punching die are preferably adjusted so that the first heating electrode and the second heating electrode face each other at a bottom dead center of the punch. Such an adjustment eliminates the need for complicated position control of the punch. 
     Further, each of the heating electrodes may be a coil electrode for generating an induced electromotive force in the steel plate to carry out the heating. The heating in this case includes heating the part of the steel plate without bringing the electrode into contact with a surface of the steel plate. When the steel plate is heated using an induced electromotive force in this manner, damage to the electrodes can be reduced, because the steel plate can be heated without bringing the electrodes into contact with the surface of the steel plate. When induction heating is used in this way, the trim edge can be efficiently heated by the edge effect. Moreover, when an insulator part is provided on a peripheral part of the coil electrode, in the above heating, the insulator part may be brought into contact with the surface of the steel plate to heat the above part of the steel plate. When the insulator part is brought into contact with the surface of the steel plate, it is possible to stably heat the steel plate while preventing damage to the coil electrode. 
     In a second example aspect of the present disclosure, a punching machine includes: a punch holder including a first heating electrode and configured to hold a punch for punching a steel plate as a workpiece; a punching die including a second heating electrode, the steel plate is being fixed to the punching die; and a current applying control unit configured to apply a current to the first heating electrode and the second heating electrode so that a part of the steel plate including a trim edge punched by the punch is heated when the first heating electrode and the second heating electrode face each other while the punch is penetrating the steel plate. By disposing the heating electrodes on the punch holder and at a position facing the punch holder in the punching machine, and applying a current to the heating electrodes while the punch is penetrating the steel plate, the trim edge can be sufficiently ended. By heating the part of the steel plate including the trim edge, it is possible to appropriately remove residual strain and avoid overheating of areas other than the part of the steel plate including the trim edge. 
     In a third example aspect of the present disclosure, a method for processing a steel plate includes: punching a steel plate as a workpiece fixed to a punching die by a punch; and heating a part of the steel plate including a trim edge punched in the punching by a first heating electrode provided on a fixing jig for fixing the steel plate to the punching die and a second heating electrode provided on the punching die. 
     In a fourth example aspect of the present disclosure, a punching machine includes: a punching die including a second heating electrode, a steel plate as a workpiece is being fixed to the punching die; a fixing jig including a first heating electrode and configured to fix the steel plate to the punching die; and an current applying control unit configured to apply a current to the first heating electrode and the second heating electrode so that a part of the steel plate including a trim edge punched by the punch is heated. According to the method for processing a steel plate in the third example aspect and the punching machine in the fourth example aspect, the heating electrodes are disposed on the fixing jig of the steel plate and at a position facing the fixing jig in the punching machine and then the trim edge is heated. Since the part of the steel plate including the trim edge of the steel plate as a workpiece can be heated, it is possible to appropriately remove residual strain and avoid overheating of areas other than the part of the steel plate including the trim edge. 
     According to the present disclosure, it is possible to provide a method for processing a steel plate capable of removing residual strain at a trim edge thereof without causing overheating in areas of the steel plate other than the trim edge. 
     The above and other objects, features and advantages of the present disclosure will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram schematically showing steps of a processing method according to an embodiment; 
         FIG. 2  is a partial cross-sectional view showing a state in which heating electrodes are brought into contact with a steel plate in a heating step; 
         FIG. 3  is a partial cross-sectional view showing a relationship between the heating electrodes and the steel plate when another punching machine is used; 
         FIG. 4  illustrates an example of a formed product; 
         FIG. 5  is a partial cross-sectional view showing a relationship between the heating electrodes and the steel plate when still another punching machine is used; and 
         FIG. 6  illustrates an example of a formed product. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic diagram schematically showing steps of a processing method according to this embodiment. In the processing method described below, a hole is punched through a steel plate as a workpiece, and a peripheral edge of the hole is deformed to form a stretch flange. As shown in  FIG. 1 , broadly speaking, the processing method includes a punching step of punching a steel plate, a heating step of heating a part of the steel plate including a trim edge, a pulling-out step of pulling out a punch, a cooling step of reducing the heat generated in the heating step, and a stretch flange step of forming a stretch flange at the trim edge. 
     The punching step includes punching a steel plate  100  fixed to a punching die by a punch  711  that moves forward and backward together with a support shaft  710  of an upper die. As will be described in detail later, the heating step is a step of pressing a punch holder  712  holding the punch  711  against the steel plate  100  and heating a peripheral edge of the hole  110  formed in the punching step. The pulling-out step is a step of pulling out the punch  711  from the punching die after the heating is ended. Note that as shown in the drawing, the area heated in the heating step is a heating area  111  including a trim edge of the hole  110 , which area is a part of the steel plate  100 . 
     The cooling step is a step of reducing the heat in the heating area  111  heated in the heating step. Specifically, the steel plate  100  is left for a certain time in a room temperature environment. The stretch flange step is a step of inserting a flange die  900  into the hole  110  and plastically deforming a peripheral edge part of the hole  110  to thereby form a stretch flange  113 . 
     Residual strain generated at the peripheral edge part of the hole  110  in the punching step is removed in the heating step. After the cooling, the steel plate  100  is subjected to the stretch flange step. When the stretch flange is formed on the steel plate  100  in a cooled state, it is possible to reduce the damage to the flange die  900  more than when the stretch flange is formed on the steel plate  100  in a heated state. In particular, in this embodiment, since the steel plate  100  can be sufficiently heated up to the trim edge, the residual strain can be satisfactorily removed. This will be described later. Further, since the heating step can be performed between the punching step and the pulling-out step, an independent heating step of placing the steel plate  100  on a heating apparatus and heating it, which has been necessary in related art, can be omitted. This can shorten the time taken for a series of processing. 
       FIG. 2  is a partial cross-sectional view of the heating electrodes brought into contact with the steel plate  100  in the heating step. Specifically,  FIG. 2  schematically shows a partial cross section of the punching machine  701  and the steel plate  100  including the central axis of the hole  110 . In the heating step, by maintaining the punch  711  in a state of penetrating the steel plate  100 , the first heating electrode  713  and the second heating electrode  723 , which are a pair of heating electrodes, sandwich the steel plate  100 , and a current is applied to these heating electrodes to thereby heat the steel plate  100 . Specifically, an electrode surface of the first heating electrode  713  is brought into contact with one surface side of the steel plate  100 , and an electrode surface of the second heating electrode  723  is brought into contact with another surface side of the steel plate  100 , and then a current is applied to the steel plate  100 . 
     The heating temperature at this time is adjusted in such a way that a trim edge  112  becomes 200° C. or higher and lower than the Ac1 point. The residual strain can be appropriately removed when the heating is within this temperature range. In particular, when the steel plate  100  is heated to the Ac1 point or higher, the steel plate  100  undergoes austenite transformation. Thus, the steel plate  100  softens when air-cooled or hardens when rapidly cooled by running water or the like, and then formability in the stretch flange step decreases. Therefore, it is preferable to keep the heating temperature below the Ac1 point. 
     The punching machine  701  includes, as an upper die, the punch  711 , a punch holder  712  including a first heating electrode  713  and detachably supporting the punch  711 , and a support shaft  710  that is integrated with the punch holder  712  and moves the attached punch  711  back and forth. Since the first heating electrode  713  is provided on the punch holder  712 , the first heating electrode  713  is located substantially near the base of the punch  711 . The punching machine  701  further includes, as a lower die, a second heating electrode  723  and a punching die  720  to which the steel plate  100  is fixed. The punching die  720  includes a die hole  721  for retracting the punch  711  and a punched piece at the time of punching. The first heating electrode  713  and the second heating electrode  723  are disposed so as to face each other with the punch  711  penetrating the steel plate  100  and to be brought into contact with the respective surfaces of the steel plate  100 . 
     The punching machine  701  further includes, as a part of the control mechanism, a current applying control unit that applies a current to the first heating electrode  713  and the second heating electrode  723  when the first heating electrode  713  and the second heating electrode  723  are brought into contact with the steel plate  100  and face each other. When a current is applied, the heating area  111  including the trim edge  112  shown in  FIG. 1  is heated. When the heating area  111  is heated by such an arrangement of the pair of heating electrodes, the trim edge  112  can be sufficiently heated, and the residual strain concentrated on the peripheral part of the trim edge  112  can be satisfactorily removed. Further, since the heating area  111  is a part of the entire steel plate  100 , electric power for heating unnecessary areas can be reduced, and overheating that causes softening and hardening can be avoided. 
     The first heating electrode  713  may be composed of a plurality of electrodes arranged apart from each other around the punch  711  instead of being an annular electrode surrounding the punch  711  as shown in the drawing. Likewise, the second heating electrode  723  may composed of a plurality of electrodes arranged apart from each other around the die hole  721  instead of being an annular electrode surrounding the die hole  721  as shown in the drawing. In any of these cases, the first heating electrode  713  and the second heating electrode  723  are provided in the vicinity of the punch  711  or the die hole  721  so that the trim edge  112  of the hole  110  to be formed can be sufficiently heated. 
     The relative positions of the punch holder  712  and the punching die  720  are adjusted in such a way that the first heating electrode  713  and the second heating electrode  723  face each other and are brought into contact with the steel plate  100  at the bottom dead center, which is the lowest end where the punching punch  711  reaches. For example, prior to using the punching machine  701 , a user adjusts the relative positions of the punch holder  712  and the punching die  720  by correcting an initial position of the support shaft  710  in the vertical direction according to the thickness of the steel plate  100 . Such an adjustment eliminates the need for complicated position control of the punch  711 . 
     A heating time in the heating step may be a preset time or a time until the trim edge  112  reaches a preset temperature. In the latter case, for example, a temperature sensor may be provided between wall surfaces of the second heating electrode  723  and the die hole  721  in the punching die  720  to detect the temperature. The pulling-out step is started after such a heating time has elapsed. When the punch  711  is pulled-out when the temperature reaches an appropriate temperature, the residual strain can be removed appropriately, and the softening and hardening of the steel plate  100  can be avoided. 
       FIG. 3  is a partial cross-sectional view showing a relationship between heating electrodes and the steel plate  100  when another punching machine  703  is used. Like  FIG. 2 ,  FIG. 3  is a cross-sectional view showing one side of the punching machine  703  and the steel plate  100  including the central axis of the hole  110 . 
     The pair of heating electrodes shown in the drawing is a coil electrode that applies an alternating current to generate an induced electromotive force in the steel plate  100  to thereby heat the steel plate  100 . The pair of heating electrodes is composed of a first heating coil  733  and a second heating coil  743 . The first heating coil  733  is surrounded by a first support  735  that is an insulator, and the second heating coil  743  is surrounded by a second support  745  that is an insulator. The heating step is carried out by maintaining the punch  731  in a state in which the punch  731  penetrates the steel plate  100  so that the first heating coil  733  and the second heating coil  743  face each other with the steel plate  100  interposed therebetween and then applying a current to these heating coils. 
     During the heating, the first support  735  and the second support  745  are brought into contact with the surfaces of the steel plate  100 . As a result, the distance between the first heating coil  733  and the second heating coil  743  becomes stable, which makes it easy to control the temperature. Further, since the first heating coil  733  and the second heating coil  743  are not brought into direct contact with the surfaces of the steel plate  100 , damage to the electrodes can be reduced. Note that the temperature range for the heating is the same as that for the punching machine  701 . 
     The punching machine  703  includes, as an upper die, the punch  731 , a punch holder  732  including the first heating coil  733  supported by the first support  735  and detachably supporting the punch  731 , and a support shaft  730  that is integrated with the punch holder  732  and moves the attached punch  731  back and forth. The punching machine  703  further includes, as a lower die, a punching die  740  including the second heating coil  743  supported by the second support  745 . The steel plate  100  is fixed to the punching die  740 . The punching die  740  includes a die hole  741  for retracting the punch  731  and a punched piece at the time of punching. The first heating coil  733  and the second heating coil  743  are disposed so as to face each other while the punch  731  is penetrating the steel plate  100  and to be brought into contact with the respective surfaces of the steel plate  100 . 
     The punching machine  703  further includes, as a part of the control mechanism, a current applying control unit that applies a current to the first heating coil  733  and the second heating coil  743  when the first heating electrode  733  and the second heating electrode  743  are brought into contact with the steel plate  100  and face each other. When a current is applied, the heating area  111  including the trim edge  112  shown in  FIG. 1  is heated. When the heating area  111  is heated by such an arrangement of the pair of heating coils, the trim edge  112  can be sufficiently heated, and the residual strain concentrated on the peripheral part of the trim edge  112  can be satisfactorily removed in a manner similar to the punching machine  701 . 
     The relative positions of the punch holder  732  and the punching die  740  are adjusted in such a way that the first support  735  is brought into contact with the steel plate  100  at the bottom dead center, which is the lowest end where the punching punch  731  reaches. A heating time in the heating step may be adjusted in a manner similar to that for the punching machine  701 . 
     An example of a formed product formed by the above-described processing method will be described.  FIG. 4  illustrates an FR lower arm  200  used for a vehicle suspension as an example of the formed product. As shown in an enlarged view of a part surrounded by a dotted line, a bush press-fitting part  210  of the FR lower arm  200  is formed by the above-described processing method. 
       FIG. 5  is a partial cross-sectional view showing a relationship between heating electrodes and the steel plate  100  when still another punching machine  705  is used. Unlike the punching machines  701  and  703 , the punching machine  705  is not for punching a hole and instead is for cutting off an unnecessary part of the steel plate  100 . 
     The punching machine  705  includes a punch  751  as an upper die. The punching machine  705  further includes, as a lower die, a punching die  760  including a second heating electrode  763 . The steel plate  100  is fixed to the punching die  760 . The steel plate  100  is fixed to the punching die  760  by a fixing jig  752  in which a first heating electrode  753  is embedded. When the steel plate  100  is fixed by the fixing jig  752 , the first heating electrode  753  and the second heating electrode  763  are brought into contact with the respective surfaces of the steel plate  100  and face each other. 
     Further, the punching machine  705  includes, as a part of the control mechanism, a current applying control unit that applies a current to the first heating electrode and the second heating electrode so as to heat a part of the steel plate  100  including a trim edge punched by the punch  751 . When a current is applied, a heating area including the trim edge is heated. When the heating area is heated by such an arrangement of the pair of heating electrodes, the trim edge can be sufficiently heated, and the residual strain concentrated on the peripheral part of the trim edge can be satisfactorily removed. Further, since the heating area is a part of the entire steel plate  100 , electric power for heating unnecessary areas can be reduced, and overheating that causes softening and hardening can be avoided. 
     In the processing method in this case, the order of the heating step and the pulling-out step corresponding to  FIG. 1  does not matter. That is, the processing method may only need to include the punching step of punching the steel plate  100  as a workpiece fixed to the punching die  760  by the punch  751  and the heating step of heating, by the first heating electrode  753  and the second heating electrode  763 , the part of the steel plate  100  including the trim edge punched in the punching step. Further, since the heating step can be performed while the steel plate  100  is fixed to the punching die  760 , an independent heating step of placing the steel plate  100  on a heating apparatus and heating it, which has been necessary in related art, can be omitted. This can shorten the time taken for a series of processing. 
     An example of a formed product formed by such a processing method will be described.  FIG. 6  illustrates an A pillar lower  300  used for a window column of a vehicle as an example of the formed product. In the stretch flange step, a flange die is pressed against the trim edge, from which an unnecessary part has been cut off, to form a stretch flange. A stretch flange forming part  310  of the A pillar lower  300  is formed in this way. 
     From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.