Patent Publication Number: US-9833828-B2

Title: Hot-press deep-drawing forming method and hot-press deep-drawing forming method apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2014-057652 filed on Mar. 20, 2014, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a hot-press deep-drawing forming technology that allows obtaining a high-strength product through deep-draw forming of a steel plate, in a state where the steel plate is heated at or above a transformation temperature, followed by die quenching of the formed steel plate. 
     2. Related Art 
     A steel plate, as a workpiece, is worked by draw-forming in order to form a product, having a three-dimensional shape through press-working of the steel plate. Products that are worked by forming include products that have: a main body having a U-shaped cross-sectional shape and made up of a front wall and side walls that are contiguous with the front wall; and a flange that is integral with the main body. 
     A press die for drawing a product having such a three-dimensional shape has: a die provided with a forming surface corresponding to the outer surface of the main body; a punch provided with a forming surface corresponding to the inner face of the product; and a blank holder that presses the edges of the workpiece, i.e. the steel plate, to suppress thereby the occurrence of wrinkles in the steel plate. Japanese Unexamined Patent Application Publication (JP-A) No. H2-205210 discloses a press die having a movable punch for intermediate drawing, and having upper and lower blank holders, wherein drawing of the central portion of the workpiece, and reverse-drawing of the outside of the workpiece, are performed through a single working operation of a press ram, JP-A No. H8-90094 discloses a drawn product forming apparatus in which a stepped drawn product is formed over one stroke. This apparatus has a lower die provided with a vertically movable blank holder, a punch that: can advance and retreat inward of the upper blank holder, and a vertically movable lower blank holder that clamps a thin plate together with the upper blank holder, 
     JP-A No. 2011-50972 discloses a hot-press forming method. In this forming method, one stroke from forming start to forming termination is set to obey different forming conditions in initial-stage forming, middle-stage forming and final-stage forming, using a hot-press die that has a punch, a die and a die cushion, so that, as a result, wrinkles that occur halfway during forming are removed at the final-stage forming. JP-A No. 2010-69535 discloses a forming method that involves press quenching using a press apparatus having a lower forming tool, a vertically movable upper forming tool, and a blank holder, wherein a partially non-quenched region is formed at the edge region of a press-quenched steel plate. 
     JP-A No. 2005-297042 discloses a deep drawing method in hot forming. In this deep drawing method, a steel plate is heated at or below the melting point, and is formed at or above the transformation temperature, and the clearance between a blank holder surface and a die face is set to be equal to or smaller than the clearance of a gap, greater than, the thickness of the steel plate, and for which there occur wrinkles derived from contact of a same surface of the steel plate. 
     The above publications JP-A No. 2011-50972 and JP-A No. 2010-69535 both involve a method of flattening wrinkles at the final stage of forming, while allowing for the occurrence of wrinkles daring the forming process. However, there are parts difficult to form that give rise to problems such as excessive wrinkling during the forming process, with unacceptable wrinkles, or occurrence of wrinkles that cannot be fully flattened at the final stage of forming, with non-uniform contact between a die punch and a die. In cold working, by contrast, step drawing is performed in some instances in addition to that by a blank holder mechanism (for example, JP-A No. H2-205210). In hot press dies, however, there are sites where the workpiece continues to contact the die during the step drawing or the step performed by the blank holder mechanism. This may lead to unintended ongoing removal of heat from the workpiece, that gives rise to impaired ductility of the workpiece and to forming defects such as breakage in the formed product. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to make it possible to work a hot-press deep drawing product, using a steel plate as a workpiece, and without occurrence of forming defects such as wrinkles and breakage, in difficult-to-form parts that cannot be formed in accordance with the methods in the above citations. 
     An aspect of the present invention provides a hot-press deep-drawing forming method of forming a product that has a main body of U-shaped cross-sectional shape provided with a front wall and a side wall, and a flange that is contiguous with the main body, through press-working of a workpiece made of a steel plate and heated at or above a transformation temperature, the method including: a pre-forming die movement step of causing a die, having a built-in cooling mechanism and provided with an outer-face forming surface that forms an outer surf ace of the product, as well as a blank, holder and a pre-forming die that clamp a scrap section of the workpiece, to relatively perform an approach movement towards a punch, having a built-in cooling mechanism and provided with an inner-face forming surface that, forms an inner face of the product, and with a bent corner against which the scrap section abuts, the approach movement being performed until the scrap section abuts the bent corner; and a final forming step of stopping the movement of the scrap section that is clamped by the blank holder and the pre-forming die, when the die moves towards the punch after the scrap section has abutted the bent corner. 
     Another aspect of the present invention provides a hot-press deep-drawing forming apparatus for forming a product that has a main body of U-shaped cross-sectional shape provided with a front wall and a side wall, and a flange that is contiguous with, the main body, through press-working of a workpiece made of a steel plate and heated at or above a transformation temperature, the apparatus including: a punch, having a built-in cooling mechanism, and provided with an inner-face forming surface that forms an inner face of the product, and with a bent, corner against which a scrap section of the workpiece abuts; a die, having a built-in cooling mechanism, provided with an outer-face forming surface that forms an outer surface of the product, and performing an approach-separation movement relatively to the punch; a blank holder against which the scrap section abuts; a pre-forming die that clamps the scrap section together with the blank holder; and a double action mechanism that causes the blank holder and the pre-forming die to move, together with the die, until the scrap section clamped by the blank holder and the pre-forming die abuts the bent corner, and that stops the movement of the blank holder and of the pre-forming die when the die approaches the punch after the scrap section has abutted the bent corner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a front-view diagram illustrating an example of a product that, is formed by deep-draw forming,  FIG. 1B  is a left side-view diagram of  FIG. 1A , and  FIG. 1C  is a plan-view diagram of  FIG. 1A ; 
         FIG. 2A  is a front-view diagram illustrating another example of a product,  FIG. 2B  is a left side-view diagram of  FIG. 2A ; and  FIG. 2C  is a plan-view diagram of  FIG. 2A ; 
         FIG. 3A  is a perspective-view diagram illustrating yet another example of a product, and  FIG. 3B  is a longitudinal cross-sectional diagram of  FIG. 3A ; 
         FIG. 4  is a cross-sectional diagram illustrating an initial stage of forming in a hot-press deep-drawing forming apparatus in an implementation; 
         FIG. 5  is a cross-sectional diagram illustrating the hot-press deep-drawing forming apparatus of  FIG. 4  in a state where forming has progressed from that in  FIG. 4 ; 
         FIG. 6  is a cross-sectional diagram illustrating the hot-press deep-drawing forming apparatus of  FIG. 4  in a state where a pre-forming die movement process is over; 
         FIG. 7  is a cross-sectional diagram illustrating the hot-press deep-drawing forming apparatus of  FIG. 4  in a state where a final forming process is over; 
         FIG. 8  is a cross-sectional diagram illustrating an initial stage of forming in a hot-press deep-drawing forming apparatus in another implementation; 
         FIG. 9  is a cross-sectional diagram illustrating the hot-press deep-drawing forming apparatus of  FIG. 8  in a state where a final forming process is over; and 
         FIG. 10  is a cross-sectional diagram depicting a hot-press deep-drawing forming apparatus that is illustrated as a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     Implementations of the present invention will be explained next in detail with reference to accompanying drawings. A product  10   a  illustrated in  FIG. 1  has a main body  15  provided with a front wall  11  that extends in a longitudinal direction and with side walls  13 ,  14  that are contiguous with the front wall  11 , on both sides in the width direction of the front wall  11 , by way of respective arc-shaped bends  12 , as illustrated in FIGS,  1 A and  1 B. The width dimensions of both side walls  13 ,  14  are substantially identical. The reverse side of the main body  15  with respect to front wall  11  constitutes an opening end, with the side walls  13 ,  14  being slightly tilted outwards facing the opening end. The width dimension of the opening end is slightly larger than the width dimension of the front wall  11 , and the transversal cross-sectional shape of the main body  15  is U-shaped. The flanges  16 ,  17 , that are integral with the main body  15 , are contiguous with the side walls  13 ,  14  by way of respective arc-shaped bends  18 , and are parallel to the front wall  11 . Further, the product  10   a  does not extend completely straight in the longitudinal direction but is bent, at a portion of a bend  19 , to an overall dogleg shape as illustrated in  FIG. 1C . 
     The entire shape of a product  10   b  illustrated in  FIG. 2  is substantially identical to that of the product  10   a  illustrated in  FIG. 1 . The product  10   b  differs from the product  10   a  in that herein the width dimension of the side wall  13  is larger than the width dimension of the side wall  14 . 
     A product  10   c  illustrated in  FIG. 3  has a main body  24  that is provided with a circular front wall  21 , and a cylindrical side wall  23  that is contiguous with the outer periphery of the front wall  21  via a bend  22 . The reverse side of the main body  24  with respect to the front wall  21  constitutes an opening end. The cylindrical side wall  23  is slightly tilted outwards facing the opening end. A flange  25  is integral with the main body  24 . The flange  25  is contiguous with the side wall  23 , by way of an arc-shaped bead  26 , and is parallel to the front wall  21 . 
     In the products  10   a  to  10   c  above, a steel plate as a workpiece undergoes plastic working to a three-dimensional shape, by hot-press deep drawing work, and is then quenched i.e. subjected to a quenching process. 
       FIG. 4  to  FIG. 7  are cross-sectional diagrams illustrating a hot-press deep-drawing forming apparatus  30  for forming the product  10   a  illustrated in  FIG. 1 , through deep drawing of a workpiece W. The workpiece W that is worked to the product  10   a  has a cuboid shape and includes a product section T that encompasses the main body  15  and the side walls  13 ,  14 , and a scrap section S that is contiguous with the outside of the product section T. The workpiece W made of a steel plate is heated beforehand at the austenitizing transformation temperature, and is conveyed to the forming apparatus  30  illustrated in the figure. 
     The forming apparatus  30  has a lower die  31  made up of a punch. The lower die  31  has an inner-face forming surface  32  that forms the inner face of the product  10   a , The inner-face forming surface  32  has an inner-face forming surface  32   a  that forms the inner face of the thee front wall  11 , inner-face forming surfaces  32   b  that form the inner faces of the bends  12 , and inner-face forming surfaces  32   c  that form the inner faces of the side walls  13 ,  14 , and further has inner-face forming surfaces  32   d  that form the inner faces of the bends  18 , and inner-face forming surfaces  32   e  that forms the inner faces of the flanges  16 ,  17 . The scrap section S of the work piece W abuts against bent corners  33  that are provided on both sides of the lower die  31 . The cross-sectional shape of the bent corners  33  is arcuate, such that the ends of the bent corners  33  are contiguous with respective lower die side faces  34  that extend in the vertical direction, A base  35  is provided at the lower end of the lower die  31 . The lower die  31  is fixed to a bolster, i.e. a platform, not shown in the figures, of the base  35 . 
     A cooling system  36  through which a cooling medium circulates is built into the lower die  31 , for the purpose of quenching the workpiece W, by cooling, once press-working of the workpiece W is over. A coolant, as a cooling medium, is supplied into the cooling system  36  by a pump, not shown, such that the lower die  31  is cooled down to a predetermined quench temperature by the cooling mechanism that is configured by the cooling system  36 . 
     The forming apparatus  30  has an upper die  41  made up of a die. The upper die  41  is disposed above the lower die  31  that is made up of a punch, and can move so as to come close to and draw apart from the lower die  31 , i.e. is vertically movable. The upper die  41  has an outer-face forming surface  42  that forms the outer surface of the product  10   a . The outer-face forming surface  42  has an outer-face forming surface  42   a  that forms the outer surface of the front wall  11 , outer-face forming surfaces  42   b  that form the outer surfaces of the bends  12 , and outer-face forming surfaces  42   c  that form the outer surfaces of the side walls  13 ,  14 , and further has outer-face forming surfaces  42   d  that form, the outer surfaces of the bends  18 , and outer-face forming surfaces  42   e  that form the outer surface the flanges  16 ,  17 . The outer-face forming surfaces  42   e  are contiguous with upper die side faces  44  that extend in the vertical direction. A base  45  is provided at the upper end of the upper die  41 , such that the upper die  41  is attached to a vertical movement mechanism, not shown, at a portion of the base  45 . 
     A cooling system  46  through which a cooling medium circulates is built into the upper die  41 , for the purpose of quenching the workpiece W, by cooling, once press-working of the workpiece W is over. A coolant, as a cooling medium, is supplied into the cooling system  46  by a pump, not shown, such that the upper die  41  is cooled down to a predetermined quench temperature by the cooling mechanism that is configured by the cooling system  46 . 
     Blank holders  51  that abut the scrap section S of the workpiece W are disposed at the base  35  of the lower die  31 , on both sloes of the lower die  31 . The blank holders  51  are mounted on the lower die by holder driving members  52  that run through the base  35 . Bottoming blocks  53  that abut the bottom faces of the blank holders  51  are attached to the base  35 , in order to restrict the descent position of the blank holders  51 . The bottoming blocks  53  may be attached to the bottom faces of the blank holders  51 . 
     Pre-forming dies  61  are provided above the blank holders  51  such that the scrap section S of the workpiece W is clamped between the blank holders  51  and the pre-forming dies  61 . The pre-forming dies  61  are mounted on the base  45  or the upper die  41  by way of elastic members  62 , for instance rubber, coil springs or the like. By virtue of the elastic force of the elastic members  62 , the scrap section S is brought to state of being pinched between the blank holders  51  and the pre-forming dies  61 , i.e. a clamped state. Clearance blocks  63  are attached to the bottom faces of the pre-forming dies  61 , in order to provide a clearance corresponding to the thickness of the scrap section S between the pre-forming dies  61  and the blank holders  51 . The clearance blocks  63  may be attached to the top faces of the blank holders  51 . A corner  64  is formed at the bottom of each pre-forming die  61  at a portion facing the upper die  41 . 
     The holder driving members  52  constitute a movable pressure source that exerts a clamping force on the scrap section S. The elastic members  62  constitute a pressure source that exerts a clamping force on the scrap section S, via the pre-forming dies  61 . A double action mechanism  65  is configured by the holder driving members  52 , the bottoming blocks  53  and the elastic members  62 . The double action mechanism  65  clamps the scrap section S between the blank holders  51  and the pre-forming dies  61 , during deep-draw forming at an early stage of work in which the upper die  41  is caused to approach the lower die  31 , and then brings both the blank holders  51  and the pre-forming dies  61  closer to the lower die  31 , until the scrap section S abuts the bent corners  33  of the lower die  31 . The occurrence of wrinkles in the workpiece W at the time of deep drawing work is suppressed thanks to this approach movement. As a result of the approach movement of the pre-forming dies  61  towards the lower die  31 , the blank holders  51  abut the bottoming blocks  53  when the scrap section S abuts the bent corners  33 . The descent of the blank holders  51  and the pre-forming dies  61 , i.e. the approach movement thereof towards the lower die  31 , is arrested thereby, and the movement stops. 
     In a state where the movement of the blank holders  51  and the pre-forming dies  61  has stopped, the upper die  41  moves then closer to the lower die  31 , whereupon a portion of the product  10   a  becomes formed by the lower die  31  and the upper die  41 , without the scrap section S coming into contact with the bent corners  33  or the lower die side faces  34  of the lower die  31 . The elastic members  62  contract in response to the movement of the upper die  41 , since the movement of the blank holders  51  and the pre-forming dies  61  at this time is stopped. With the blank holders  51  in a stopped state, heat is not transmitted from the scrap section S to the lower die  31  and so forth, and excessive heat removal is thus prevented. 
     In the hot-press deep drawing work by the forming apparatus  30 , thus, the blank holders  51  and the pre-forming dies  61  function as blank holders upon a predetermined initial forming stroke, such that heat removal of the workpiece W is suppressed after the blank: holder fraction is stopped. As a result, it becomes possible to form the workpiece W to the product  10   a  of predetermined shape, without occurrence of forming defects, upon hot-press deep drawing work of a steel plate, as the workpiece, to a three-dimensional shape such as the one illustrated in  FIG. 1 . 
     An explanation follows next, with reference to  FIG. 4  to  FIG. 7 , on a forming method that involves deep-draw forming of the workpiece W using the forming apparatus  30 . 
     The workpiece W, in a state of having been heated beforehand at or above the transformation temperature, is conveyed onto the lower die  31 . At this time, the upper die  41  is at an upper-limit position retracted from the lower die  31 . In this state, the upper die  41  is driven and is brought thereby closer to the lower die  31 , whereupon the scrap section S of the workpiece W becomes clamped by the blank holders  51  and the pre-forming dies  61 .  FIG. 4  illustrates a forming initial stage wherein as a result of the movement of the upper die  41  and the pre-forming dies  61 , the scrap section S of the workpiece W is clamped by the blank holders  51  and the pre-forming dies  61 , whereby the scrap section S is folded downward, of the inner-face forming surface  32   a  of the lower die  31 , and the outer-face forming surfaces  42   d  of the upper die  41  abut the workpiece W. 
     When the upper die  41  approaches the lower die  31 , i.e. descends, the outer-face forming surfaces  42   d  of the upper die  41  abut the workpiece W, whereby the side walls  13 ,  14  become formed accompanying this descent, as illustrated in  FIG. 5 . At this time, the blank holders  51  and the pre-forming dies  61  as well descend together with the upper die  41 . 
       FIG. 6  illustrates a state wherein the scrap section S abuts the bent corners  33  of the lower die  31  as a result of the descent of the upper die  41  together with the blank holders  51  and the pre-forming dies  61 . At this time, the blank holders  51  abut the bottoming blocks  53 , as illustrated in  FIG. 6 , and the descent of the blank holders  51  and the pre-forming dies  61  is stopped. The process of descent of the upper die  41  until the stop of the descent of the blank holders  51  and the pre-forming dies  61  constitutes a pre-forming die movement process. In this pre-forming die movement process the scrap section S is clamped, and hence occurrence of wrinkles in the workpiece W is suppressed. In a state where the scrap section S abuts the bent corners  33  of the lower die  31  only a small portion of the scrap section S comes in contact with the lower die  31 , since the bent corners  33  have an arcuate cross-sectional shape. 
     The process where the upper die  41  approaches the lower die  31  down to a descent limit position, from a state in which one scrap section S abuts the bent corners  33  and the descent of the blank holders  51  and the pre-forming dies  61  is stopped, constitutes herein a final forming process. In this final forming process, the movement of the scrap section S that is clamped by the blank holders  51  and the pre-forming dies  61  in a stopped state is discontinued, such that, when the upper die  41  moves down to the descent limit position, the product  10   a  having the flanges  16 ,  17  and the main body  15  made up of the front wall  11  and the side walls  13 ,  14  undergoes deep drawing work by the product section T of the workpiece W, as illustrated in  FIG. 7 . 
     In the final forming process, the outer-face forming surfaces  42   d  of the upper die  41  abut the outer surface of the workpiece W, and move towards the lower die  31 . Hence, the inner-face forming surfaces  32   c  of the lower die  31  and the outer-face forming surfaces  42   c  of the upper die  41  move gradually so as to come closer to a portion, in the product  10   a , that corresponds to the side walls  13 ,  14 . The final forming process is over when the flanges  16 ,  17  of the product  10   a  are clamped between the inner-face forming surfaces  32   e  of the lower die  31  and the outer-face forming surfaces  42   e  of the upper die  41 . At this point in time, the entirety of the product section T in the workpiece W is in contact with the inner-face forming surface  32  of the lower die  31  and the outer-face forming surface  42  of the upper die  41 . The product section T is then quenched by undergoing a heat removal treatment. 
     In the final forming process, as illustrated in  FIG. 7 , only part of the scrap section S is in contact with the bent corners  33 , in that the scrap section S wraps around the bent corners  33  of the lower die without coming in contact with the lower die side faces  34 . Therefore, the final forming process is performed in a state of enhanced formability, without excessive removal of heat from the workpiece W. The product  10   a  after forming is then subjected to quenching while in contact with the lower die  31  and the upper die  41 . The product.  10   a  having a three-dimensional shape such its the one illustrated in  FIG. 1  can be formed as a result with good yield and free of forming defects. The scrap section S of the workpiece W after quenching is cut off at a portion of a boundary L between the scrap section S arid the product section T. The product  10   a  of three-dimensional shape illustrated in  FIG. 1  is thus worked as the product section T. The product  10   b  illustrated in  FIG. 2  and the product  10   c  illustrated in  FIG. 3  are worked similarly. 
     In the forming apparatus  30  illustrated in  FIG. 4  to  FIG. 7 , both the blank holders  51  and the pre-forming dies  61  are brought closer to the lower die  31  until the scrap section S abuts the bent corners  33  of the lower die  31 , and in the final forming process, the double action mechanism  65  for stopping the blank holders  51  and the pre-forming dies  61  is configured by the holder driving members  52 , the bottoming blocks  53  and the elastic members  62 . 
     The pre-forming dies  61  need not necessarily be provided in the upper die  41  via the elastic members  62 ; alternatively, the ore-forming dies  61  may be attached to respective slide dies that are provided on the upper die  41  side, so that, as a result, the pre-forming dies  61  may similarly undergo a double action. 
       FIG. 8  and  FIG. 9  illustrate a variation of the forming apparatus  30  in which the pre-forming dies  61  are caused, to move vertically, in synchrony with the blank holders  51 , by a forming die driving member, i.e. a vertical movement mechanism. In  FIG. 8  and  FIG. 9 , those members shared with the members that make up the forming apparatus  30  described above will be denoted by identical reference symbols. 
       FIG. 8  illustrates a state, corresponding to  FIG. 4 , in which the pre-forming dies  61  have descended, by a predetermined distance, together with the upper die  41 .  FIG. 9  illustrates a state where the final forming process is over, with the upper die  41  closest to the lower die  31 . As illustrated in  FIG. 8  and  FIG. 9 , slide dies  41   a  are mounted on the upper die  41  so as to be vertically movable. The forming apparatus  30  has a double action press structure. The pre-forming dies  61  are attached to the slide dies  41   a.  Except for the pre-forming dies  61  being attached to the slide dies  41   a , other features are identical to those of the forming apparatus  30  described above. 
     In this forming apparatus  30 , the scrap section S of the workpiece W is clamped, by the blank holders  51  and the pre-forming dies  61 ; the pre-forming dies  61  are thereafter caused to descend, by the slide dies  41   a , towards the lower die  31 , in synchrony with the blank holders  51 . 
     The descent of the pre-forming dies  61  is stopped by the slide dies  41   a  after the scrap section S abuts the bent corners  33 . The descent of the blank holders  51  as well is stopped at this time by the holder driving members  52 . A double action operation on the pre-forming dies  61  may be performed thus by resorting to the slide dies  41   a.    
       FIG. 10  is a cross-sectional diagram illustrating a hot-press deep-drawing forming apparatus  30   a  as a comparative example. In this comparative example, the scrap section S is clamped between the blank holders  51  and clamping surfaces  71  that are provided on the upper die  41 ; formation of the scrap section S proceeds thereupon until the upper die  41  moves down to the descent limit position. Accordingly, the scrap section S is brought to a state of being pressed against the bent corners  33  and the lower die side faces  34 , as illustrated in  FIG. 10 , and hence the lower die  31  removes heat from, the workpiece W before the final forming process. Concerns arise as a result, in the forming apparatus  30   a  illustrated, in  FIG. 10 , of forming defects such as wrinkles or breakage in the formed product, and reduced forming yield. 
     In the various forming apparatuses  30  described above, the lower die  31  constitutes a punch that forms the inner face of the product, and the upper die  41  constitutes a die that forms the outer surface of the product. Alternatively, hot-press deep drawing work can be similarly performed on a product having a three-dimensional shape, with good yield and while suppressing the occurrence of wrinkles, using the upper die as the punch and the lower die as the die. 
     The present invention is not limited to the above implementations, and may accommodate various modifications without departing from the gist of the invention. Products that can be worked using the forming apparatus  30  are not limited to those illustrated in  FIG. 1  to  FIG. 3 , and products of various shapes can undergo deep drawing work if the products involve a significant degree of drawing. In the forming apparatus  30  illustrated in the figures, the punch is the lower die  31  and the die is the upper die  41 , but the top-bottom configuration may be reversed, and the upper die may serve as the punch and the lower die may serve as the die. Further, the punch may be caused to move vertically, instead of the die moving vertically. Also, both the die and the punch may be set to move vertically, so long as a relative approach-separation movement relationship between the die and the punch is obeyed.