Patent Publication Number: US-9833826-B2

Title: Press forming method

Description:
FIELD 
     The present invention relates to a press forming method that forms a shrink flange by performing press forming on a metal sheet. 
     BACKGROUND 
     When a flange portion is formed on a metal sheet by press forming along a projecting outer edge a part of which projects outward, there are cases in which shrinkage deformation arises in an end portion of the projecting portion of the flange portion. This is referred to as shrink-flanging. In the case of minor shrink-flanging, its influence is limited to only a phenomenon of an increase in sheet thickness. However, when the amount of shrink-flanging increases, wrinkles are formed in the course of press forming and the wrinkles remain after press forming. 
     The wrinkles are undesirable because they can cause shape defects of press formed products and wastage of dies. In particular, when the shrinkage is intense and large wrinkles are formed, the wrinkles further cause cracks of press forming on the metal sheet. Various methods have been developed that avoid the forming of wrinkles caused by such a shrink flange. For example, Patent Literature 1 discloses a method that facilitates compressive forming of a shrink flange portion by providing through-holes in the shrink flange portion in advance. Patent Literature 2 discloses a method of preventing wrinkles by clamping, at least at an early phase of press forming, a part of a press formed product, on which shrink-flanging is performed, at near its end portion with a U-shaped block and performing press forming afterward. Patent Literature 3 discloses a method of dispersing deformation by elaborating the shape of the end portion of a bending tool. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Laid-open Patent Publication No. 2007-253173 
     Patent Literature 2: Japanese Laid-open Patent Publication No. 07-039954 
     Patent Literature 3: Japanese Laid-open Patent Publication No. 2010-201486 
     SUMMARY 
     Technical Problem 
     In the method of providing through-holes in the flange portion disclosed in Patent Literature 1, however, the external appearance, strength, and sealing property of the final product are affected because non-uniform preliminary forming is performed on a metal sheet, and when the blank material is a surface-treated steel sheet (galvanized steel sheet), rust-prevention is further affected. Hence, the region to which the method can be applied is limited. In the method disclosed in Patent Literature 2, the structure of the tool of press forming to restrain the periphery of the shrink flange portion is complicated, and it is disadvantageous in terms of the fabrication of the tool and the cost of maintenance. In the method disclosed in Patent Literature 3, the elaboration in the complicated shape of the tool is required. Because it further appears that the end portion of the bending tool does not act effectively unless the bending angle of the metal sheet at the time of press forming is close to 90 degrees, it is disadvantageous in that it is not applicable when the bending angle of the metal sheet is small. 
     An object of the present invention is to provide a press forming method that can fundamentally solve the problem of potential wrinkles in shrink-flanging without any adverse effects on final products. 
     Solution to Problem 
     A press forming method according to the present invention is a press forming method of pressing and forming a formed part including: a top portion having a projecting outer edge, a part of the projecting outer edge projecting outward; and a flange portion that formed by bending along the projecting outer edge of the top portion, and includes: a first forming step of forming, at a region of a blank material in which the flange portion is to be formed, a preformed part including a vertical wall portion to be a part of the flange portion and a trough portion that is bent outward from the vertical wall portion and recessed on a side of the top portion; and a second forming step of forming the flange portion by bending a region including the trough portion of the preformed part formed in the first forming step. 
     In the above-described press forming method according to the present invention, the first forming step clamps a region of the blank material to be the top portion with a pad and a first die and forms a region of the blank material to be the flange portion with a first punch, and the second forming step clamps a region of the preformed part to be the top portion with the pad and a second die and performs forming with a second punch that lies along a shape including the trough portion of the preformed part. 
     Advantageous Effects of Invention 
     According to the present invention, the problem of potential wrinkles in shrink-flanging can be fundamentally solved without any adverse effects on the final products. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a diagram for explaining a first forming process in a press forming method according to one embodiment of the invention. 
         FIG. 1B  is a diagram for explaining a preformed part by the first forming process in the press forming method in the one embodiment of the invention. 
         FIG. 1C  is a diagram for explaining a second forming process in the press forming method in the one embodiment of the invention. 
         FIG. 1D  is a diagram for explaining a target shape formed by the second forming process in the press forming method in the one embodiment of the invention. 
         FIG. 2  is a diagram for explaining a formed part formed by the press forming method in the one embodiment of the invention. 
         FIG. 3  is a diagram for explaining a preformed part formed by the first forming process in the press forming method in the one embodiment of the invention. 
         FIG. 4  is a diagram for explaining a first punch used in the first forming process of the press forming method in the one embodiment of the invention. 
         FIG. 5  is a diagram for explaining a second punch used in the second forming process of the press forming method in the one embodiment of the invention. 
         FIG. 6  is a contour diagram illustrating an increasing rate of sheet thickness in the second forming process of the press forming method in the one embodiment of the invention. 
         FIG. 7  is a contour diagram illustrating the increasing rate of sheet thickness when forming is performed by a conventional press forming method. 
         FIG. 8  is a diagram for explaining a second punch used in a second forming process of a press forming method according to another embodiment of the invention. 
         FIG. 9  is a diagram for explaining a formed part in examples of the invention. 
         FIG. 10  is a diagram for explaining a first punch in the examples of the invention. 
         FIG. 11  is a diagram for explaining a second punch in the example of the invention. 
         FIG. 12  is a diagram for explaining a second punch in the example of the invention. 
         FIG. 13  is a chart for explaining the effects in the examples of the invention. 
         FIG. 14  is a diagram for explaining a first punch used in a first forming process of a press forming method according to another embodiment of the invention. 
         FIG. 15  is a diagram for explaining a first punch used in a first forming process of a press forming method according to yet another embodiment of the invention. 
         FIG. 16  is a diagram for explaining a first punch used in a first forming process of a press forming method according to still another embodiment of the invention. 
         FIG. 17  is a diagram for explaining a second punch used in a second forming process of the press forming method in the other embodiment of the invention. 
         FIG. 18  is a contour diagram illustrating the increasing rate of sheet thickness in the second forming process of the press forming method in the one embodiment for a narrow-width blank material of the invention. 
         FIG. 19  is a contour diagram illustrating the increasing rate of sheet thickness by the conventional press forming method for a narrow-width blank material. 
         FIG. 20  is a contour diagram illustrating the increasing rate of sheet thickness in the second forming process of the press forming method in the one embodiment of the invention for a narrow-width blank material with an increased flange height. 
         FIG. 21  is a contour diagram illustrating the increasing rate of sheet thickness by the conventional press forming method for a narrow-width blank material with an increased flange height. 
         FIG. 22A  is a diagram for explaining a mechanism of a press forming method according to the invention. 
         FIG. 22B  is a diagram for explaining the mechanism of the press forming method in the invention. 
         FIG. 23  is a diagram for explaining the mechanism of the press forming method in the invention. 
         FIG. 24A  is a diagram for explaining the mechanism of the press forming method in the invention. 
         FIG. 24B  is a diagram for explaining the mechanism of the press forming method in the invention. 
         FIG. 25  is a diagram for explaining the mechanism of the press forming method in the invention. 
         FIG. 26  is a diagram for explaining the mechanism of the press forming method in the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to the accompanying drawings, the following describes in detail a press forming method according to exemplary embodiments of the present invention. Note that the embodiments are not intended to limit the scope of the invention. 
     The inventor has earnestly investigated a solution of fundamentally alleviating the focusing of shrinkage on an end portion of a projecting bend of a flange portion in shrink-flanging. As a consequence, the inventor has conceived that, if the shrinkage and stretch arise simultaneously and are offset to each other at the end portion of the projecting bend of the flange portion at the time of press forming the flange portion, neither large shrinkage deformation would arise at the end portion of the projecting bend nor wrinkles would be formed at that region. The inventor has then examined a press forming method in which the shrinkage and stretch would arise simultaneously at the end portion of the projecting bend of the flange portion. The following describes the details of examination based on  FIGS. 22A to 26 . 
       FIG. 22A  is a diagram illustrating a first blank  50  of a flat sheet shape. A broken line indicates a first bend line  53  to form a first flange portion  51  (see  FIG. 22B ), and a bold solid line at the center indicates a first slit  55  made to the sheet. When such a first blank  50  is bent along the first bend line  53  and the first flange portion  51  is formed, as illustrated in  FIG. 22B , the portions of the first slit  55  in the first flange portion  51  overlap. Thus, when the first slit  55  is not present on the first blank and the sheet is not split, in the first flange portion  51 , shrinkage arises in the region indicated by hatched lines in  FIG. 23 , and when the shrinkage is not absorbed by the increase in sheet thickness, wrinkles arise. This is the shrink-flanging. 
       FIG. 24A  is a diagram illustrating a second blank  57  in which a rectangular sheet is formed in a trough shape at its center. A broken line indicates a second bend line  61  to form a second flange portion  59 , and a bold solid line at the center indicates a second slit  63  made to the sheet. When such a second blank  57  is bent along the second bend line  61  and the second flange portion  59  is formed, as illustrated in  FIG. 24B , a part of the blank opens at the center of the second flange portion  59 . Thus, when the second slit  63  is not present on the second blank and the sheet is not split, in the second flange portion  59 , a stretch arises in the region indicated by hatched lines in  FIG. 25 , and when the stretch is large, a crack arises. This is the stretch-flanging. 
     As described above, as illustrated in  FIG. 23 , when the first blank  50  of a flat sheet shape is bent along the projecting first bend line  53  having a part of its outer edge projecting outward, and the first flange portion  51  is formed, the shrinkage arises in the bend end portion of the first flange portion  51 . Furthermore, as illustrated in  FIG. 25 , when the second blank  57  of a trough shape is bent along the second bend line  61  crossing the trough and the second flange portion  59  is formed along the trough, the stretch arises in the bend end portion of the second flange portion  59 . 
     Consequently, by simultaneously causing shrinkage and stretch on the same portion of such a flange portion during the press forming, the shrinkage and stretch are offset. To do this, the flange portion only needs to be formed by bending along a bend line that provides the two characteristics of the first bend line  53  that projects outward as illustrated in  FIG. 23  and the second bend line  61  that lies along the trough shape illustrated in  FIG. 25 . 
     To perform such press forming, a preliminary intermediate shape (preformed shape) by which the bend line having the two characteristics is achieved only needs to be formed at a stage prior to the press forming of the flange portion of a target shape.  FIG. 26  is a diagram illustrating one example of such a preformed shape. This preformed shape  65  is in a shape including a top portion  69 , a vertical wall portion  71 , and a trough portion  73 . The top portion  69  includes a projecting outer edge  67  a part of which projects outward. The vertical wall portion  71  is formed by bending along the projecting outer edge  67  of the top portion  69 , and constitutes a part of the flange portion. The trough portion  73  is bent outward from the vertical wall portion  71  and is recessed on the top portion  69  side. In the preformed shape  65  illustrated in  FIG. 26 , a third bend line  75  formed on the vertical wall portion  71  is the bend line that provides the foregoing two characteristics. That is, because the preformed shape  65  projects outward when viewed from above, the third bend line  75  is in the same shape as that of the first bend line  53  in  FIG. 23 . Furthermore, because the preformed shape  65  is in a trough shape when viewed from the front, the third bend line  75  is in the same shape as that of the second bend line  61  in  FIG. 25 . 
     When the preformed shape  65  is formed and, as indicated by arrows in  FIG. 26 , the trough portion  73  is formed along the third bend line  75  of the vertical wall portion  71  that has appeared on the preformed shape  65 , in the X portion at the end of the center of the trough portion  73 , the shrinkage illustrated in  FIG. 23  and the stretch illustrated in  FIG. 25  arise at the same time. As a result, the shrinkage and the stretch are offset, and wrinkles attributable to the shrinkage and cracks attributable to the stretch do not arise. While the shrinkage arises at the center (the projecting portion of the projecting shape) of the vertical wall portion  71  when the preformed shape  65  is formed, because the drooping distance from the top portion  69  is short at that region, the shrinkage is not severe and is not a matter. The present invention is based on the foregoing findings, and specifically, is composed of the following constituents. 
     A press forming method according to one embodiment of the invention is a press forming method in which a formed part  1  illustrated in  FIG. 2  is press-formed. The formed part  1  includes a top portion  5  that has a projecting outer edge  3  a part of which projects outward and a flange portion  7  that is formed by bending along the projecting outer edge  3  of the top portion  5 . 
     The press forming method in the present embodiment includes a first forming process S 1  and a second forming process S 2 . In the first forming process S 1  ( FIGS. 1A and 1B ), at a region of a blank material  9  in which the flange portion  7  is formed, a preformed part  15  (see  FIGS. 1B and 3 ) is formed including a vertical wall portion  11  that is to be a part of the flange portion  7 , and a trough portion  13  that is bent outward from the vertical wall portion  11  and recessed downward. In the second forming process S 2  ( FIGS. 10 and 1D ), a second punch  35  that lies along the shape of the preformed part  15  including the trough portion  13 , which was formed in the first forming process S 1 , performs bending-forming on the region including the trough portion  13  along a boundary line  19  (see  FIG. 3 ) with the vertical wall portion  11 , and forms the flange portion  7  (see  FIG. 1D ). In the following description, the formed part  1  that is the target shape of the press forming method in the embodiment, the first forming process S 1 , and the second forming process S 2  will be described in detail. 
     Press Formed Part 
     The press formed part  1  that is the target shape of press forming in the embodiment, as illustrated in  FIG. 2 , includes the top portion  5  that has the projecting outer edge  3  a part of which projects outward and the flange portion  7  that is formed by bending along the projecting outer edge  3  of the top portion  5 . In the formed part  1  of such a shape, shrinkage is focused on a bend end portion  21  of the flange portion  7  and wrinkles are likely to form in that region. 
     First Forming Process 
     The first forming process S 1  in the embodiment is a process in which the preformed part  15  (see  FIG. 3 ) is formed. The preformed part  15  includes, at the region of the blank material  9  in which the flange portion  7  is formed, the vertical wall portion  11  that is to be a part of the flange portion  7  and the trough portion  13  that is bent outward from the vertical wall portion  11  and recessed on the top portion  5  side. 
     In the press forming in the first forming process S 1 , as illustrated in  FIG. 1A , a first die  23  that is a lower die, a first punch  17  that comes down from above the die, and a pad  25  that holds down the blank material  9  are used. 
     The first punch  17 , as illustrated in  FIG. 4 , includes a flat portion  27 , a vertical-wall forming portion  29 , and a trough forming portion  31 . The flat portion  27  is positioned at the region corresponding to the top portion  5  of the formed part  1 . The vertical-wall forming portion  29  forms the vertical wall portion  11  extending downward along the projecting outer edge  3  of the preformed part  15 . The trough forming portion  31  extends from the vertical-wall forming portion  29  in the horizontal direction and forms the trough portion  13  for which the upside is recessed. 
     The first die  23  has a shape corresponding to the shapes of the respective forming portions of the first punch  17 . It is preferable that the pressing force of the pad  25  pressing the blank material  9  to the first die  23  be a pressure strong enough to cause no deformation on the top portion  5  at the time of forming by moving down the first punch  17 . 
     The first forming process S 1  will be described more specifically. In the first forming process S 1 , as illustrated in  FIG. 1A , in a state of the first die  23  and the pad  25  clamping the blank material  9 , the first punch  17  moves down toward the first die  23 . When the first punch  17  moves down, the center of the trough forming portion  31  of the first punch  17  comes into contact with the blank material  9  first. When the first punch  17  further moves down, the forming of the trough portion  13  and the forming of the vertical wall portion  11  are simultaneously performed in sequence from the center of the blank material  9 . 
     As in the foregoing, by the first forming process S 1 , the trough portion  13  is formed, and on the vertical wall portion  11 , the boundary line  19  with the trough portion  13  is formed (see  FIG. 13 ). The boundary line  19  has the same characteristics as that of the third bend line  75  that is illustrated in  FIG. 26 , that is, the characteristics that cause the shrinkage and stretch to arise simultaneously in the bend end portion  21  of the flange portion  7 . 
     Second Forming Process 
     In the second forming process S 2 , as illustrated in  FIG. 1C , a second die  33  and the pad  25  clamp the preformed part  15  that was formed in the first forming process S 1 , and the second punch  35  bends the region including the trough portion  13  downward along the boundary line  19  (see  FIG. 3 ) and forms the flange portion  7 . 
     The second punch  35  used in the second forming process S 2 , as illustrated in  FIG. 5 , has the vertical-wall forming portion  29  that lies along the vertical wall portion  11  formed in the first forming process S 1 . The second die  33  ( FIG. 1C ) has a vertical wall portion in the same shape as that of the flange portion  7  that is the target. 
     When the second punch  35  as illustrated in  FIG. 5  moves down along the vertical wall portion  11  formed in the first forming process S 1 , the shape including the trough portion  13  is bent downward in a vertical direction from the boundary line  19  with the vertical wall portion  11  and formed, and the target shape is formed as illustrated in  FIG. 1D . 
     In the second forming process S 2 , the shape including the trough portion  13  formed in the first forming process S 1  is bent downward along the boundary line  19  and formed. At this time, both the shrinkage and stretch act on the lower end portion at the center of the flange portion  7  but are offset to each other, and hence large shrinkage never arises, let alone wrinkles, by this bending-forming. 
       FIG. 6  is a contour diagram (distribution map) illustrating the distribution of sheet thickness after the second forming process S 2 . As illustrated in  FIG. 6 , sheet-thickness increased regions are dispersed in a wide area, and even in a region of the highest increasing rate of sheet thickness, the increasing rate of sheet thickness was 67%. This means that the action of offsetting the shrinkage and stretch reduces the maximum value of the increasing rate of sheet thickness and reliably prevents wrinkles from arising. Note that the increase in sheet thickness arises at the bend end portion  21  ( FIG. 2 ) of the flange portion  7  even by the method of the invention. This is because the shrinkage and stretch arising in that region do not necessarily correspond to each other perfectly. 
       FIG. 7  is a contour diagram illustrating the distribution of sheet thickness when press forming was performed by a conventional press forming method in which shrink-flanging was performed in a single process. Comparison of  FIG. 7  with  FIG. 6  tells that, in the conventional method ( FIG. 7 ), the regions in which changes in sheet thickness arise are not as dispersed in a wide area of the flange portion  7  as illustrated in  FIG. 6  (the invention), and are focused on two places at the center. The maximum increasing rate of sheet thickness in the conventional method illustrated in  FIG. 7  is 196%, and is greater than 67% of the invention illustrated in  FIG. 6 . 
     As in the foregoing, in the present embodiment, as illustrated in  FIG. 3 , in the first forming process S 1 , at the region of the blank material  9  in which the flange portion  7  is formed, the preformed part  15  is formed that includes the vertical wall portion  11  that is to be a part of the flange portion  7  and the trough portion  13  that is bent outward from the vertical wall portion  11  and recessed on the top portion  5  side. Then, in the second forming process S 2 , the part of the preformed part  15  including the trough portion  13  formed in the first forming process S 1  is formed by bending along the boundary line  19  with the vertical wall portion  11  and the flange portion  7  of the formed part  1  of the final shape is formed. Thus, in the second forming process, the shrinkage and stretch are offset at the bend end portion  21  of the flange portion  7 , and the shrink-flanging can be performed without forming large shrinkage. Consequently, the formed part  1  excellent in shape accuracy can be manufactured easily. 
     The second punch, as illustrated in  FIG. 8 , may have the trough forming portion  31  that lies along the trough portion  13  in addition to the vertical-wall forming portion  29  that lies along the vertical wall portion  11  of the preformed part  15  formed in the first forming process S 1 . When this second punch  36  moves down along the vertical wall portion  11  formed in the first forming process S 1 , the second punch  36  comes into contact with the shape including the trough portion  13 . When the second punch  36  moves further down, the shape including the trough portion  13  is bent downward in the vertical direction from the boundary line  19  with the vertical wall portion  11  and formed, and the target shape is formed. 
     Examples 
     To verify the effects of the invention, the conventional method and the method of the invention have been verified by analyzing by the finite element method. The software used in the analysis was LS-DYNA version 971 by Livermore Software Technology Corporation (LSTC), and a dynamic explicit method solver was used. As the material to be processed, a high-strength steel sheet of a sheet thickness of 1.2 mm and a tensile strength of 590 MPa class was assumed.  FIG. 9  is a diagram illustrating the shape of a formed part to be the target. Table 1 is a table indicating the dimensions and such of the various portions of the formed part illustrated in  FIG. 9 . 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 W 
                 L 
                 H 
                 θ 
                 φ 
                 R 1   
                 R 2   
               
               
                 (mm) 
                 (mm) 
                 (mm) 
                 (deg.) 
                 (deg.) 
                 (mm) 
                 (mm) 
               
               
                   
               
             
            
               
                 150 
                 100 
                 50 
                 140 
                 90 
                 30 
                 5 
               
               
                   
               
            
           
         
       
     
       FIG. 10  is a diagram illustrating the first punch used in the first forming process of the invention.  FIGS. 11 and 12  are diagrams illustrating the second punch used in the second forming process. Table 2 is the tables indicating the dimensions of the various portions of the first punch and the second punch. The situation in which the second punch illustrated in  FIG. 11  was used is defined as an inventive example 1 and the situation in which the second punch illustrated in  FIG. 12  was used is defined as an inventive example 2. The pressing force by the respective pads was 196 MN, and the press forming speed was 2 m/sec. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Wp 
                 Lp 
                 Ha 
                 Hb 
                 W1 
                 W2 
                 L1 
                 θ1 
                 θ2 
                 φ1 
                 R 
                 Rp 
                 Rt 
                 Rb 
                 Rd 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (deg.) 
                 (deg.) 
                 (deg.) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 &lt;INVENTIVE EXAMPLE 1&gt; 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1st 
                 170 
                 110 
                 0 
                 12.7 
                 40 
                 90 
                 — 
                 140 
                 140 
                 90 
                 30 
                 5 
                 30 
                 60 
                 5 
               
               
                 Punch 
               
               
                 2nd 
                 170 
                 110 
                 100 
                 100 
                 — 
                 — 
                 180 
                 140 
                 180 
                 90 
                 30 
                 5 
                 — 
                 — 
                 50 
               
               
                 Punch 
               
            
           
           
               
            
               
                 &lt;INVENTIVE EXAMPLE 2&gt; 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1st 
                 170 
                 110 
                 0 
                 12.7 
                 40 
                 90 
                 — 
                 140 
                 140 
                 90 
                 30 
                 5 
                 30 
                 60 
                 5 
               
               
                 Punch 
               
               
                 2nd 
                 170 
                 110 
                 100 
                 112.7 
                 40 
                 90 
                 180 
                 140 
                 180 
                 90 
                 30 
                 5 
                 30 
                 60 
                 5 
               
               
                 Punch 
               
               
                   
               
            
           
         
       
     
       FIG. 13  is a chart illustrating maximum increasing rates of sheet thickness at the bottom dead point of press forming when different press forming methods were performed. In  FIG. 13 , the conventional example is the press forming method in which shrink-flanging is performed in a single process and the comparative example is the press forming method in which the shrink-flanging is performed in a single process by using a punch of the same shape as that of the second punch  36  illustrated in  FIG. 12 . As illustrated in  FIG. 13 , the maximum increasing rates of sheet thickness were, in contrast to 196% in the conventional example and 87% in the comparative example, 67% in the inventive example 1 and 59% in the inventive example 2. Thus, in accordance with the press forming method of the invention, as compared with the conventional example and the comparative example, it has been verified that the maximum increasing rate of sheet thickness is reduced. This means that wrinkles are effectively prevented from arising by the shrink-flanging of the press forming method of the invention. As for the inclination angle of the trough portion  13 , it only needs to be set such that the deformation becomes the smallest considering the offset of the shrinkage and stretch that arise at the bend end portion of the flange portion in relation to the projecting portion of the flange portion to be formed. 
     In the above-described embodiment, the situation in which the top portion  5  of the formed part was flat has been exemplified. However, the top portion  5  of the formed part formed by the press forming method of the invention is not necessary to be flat. For example, the top portion  5  may be of a recessed shape having inclined surfaces inclined downward toward the center, or in reverse, the top portion  5  may be of a projecting shape having inclined surfaces inclined upward toward the center. 
     As illustrated in  FIG. 14 , a top-portion forming portion  39  of a first punch  37  when the top portion  5  is of a recessed shape is of a recessed shape composed of inclined surfaces that are inclined downward toward the center, and it is desirable that an inclined angle θ 3  of the trough forming portion  31  be smaller than an inclined angle θ 2  when the top portion  5  is flat. As illustrated in  FIG. 15 , a top-portion forming portion  43  of a first punch  41  when the top portion  5  is of an upward projecting shape is of a projecting shape composed of inclined surfaces that are inclined upward toward the center, and it is desirable that an inclined angle θ 4  of the trough forming portion  31  be greater than the inclined angle θ 2  when the top portion  5  is flat. 
     In the above-described embodiment, in the preformed part  15 , the trough portion  13  is formed at a part of the blank material  9  in the width direction. However, it may be formed extending over the entire width of the blank material  9 .  FIG. 16  is a diagram illustrating a first punch  18  when the trough portion  13  of the preformed part  15  is formed extending over the entire width of the blank material  9 .  FIG. 17  is a diagram illustrating a second punch  35  in this case. When the trough portion  13  is formed extending over the entire width of the blank material  9 , it is preferably applied to the blank material  9  of a narrow width because it is difficult to apply it to the blank material  9  of a wide width. 
       FIG. 18  is a contour diagram illustrating the result of analysis the same as that of the above-described examples performed on the press forming using the first punch  18  illustrated in  FIG. 16  and the second punch  35  illustrated in  FIG. 17 .  FIG. 19  is a contour diagram illustrating the analysis result of a conventional example. When the trough portion  13  was formed extending over the entire width of the blank material  9  of a narrow width, the increasing rate of sheet thickness arising at the bend end portion  21  of the flange portion  7  was 20% as illustrated in  FIG. 18  in accordance with the press forming method of the invention while it was 34% as illustrated in  FIG. 19  in accordance with the conventional example. As just described, it has been verified that, even when the trough portion  13  is formed extending over the entire width of the blank material  9 , as the same as that when the trough portion  13  is formed at a part of the width of the blank material  9 , the increasing rate of sheet thickness at the bend end portion  21  of the flange portion  7  is reduced. 
     Note that sheet-thickness increased regions arose at both ends of a base portion  22  of the flange portion  7  (both end portions of the boundary between the top portion  5  and the flange portion  7 ). This is because the width of the blank material  9  was narrower than the width of the blank material  9  in which the trough portion  13  was formed in a part of the width of the blank material  9  in the above-described embodiment, and both ends of the base portion  22  of the flange portion  7  were easy to deform when the shrink-flanging was performed, resulting in the increase in sheet thickness at those regions. That is, the stress acting on the bend end portion  21  of the flange portion  7  acted on the regions that would be easy to deform, resulting in the deformation (increase in sheet thickness). It is presumable that this tendency increases as the height of the flange portion  7  increases. Consequently, the analyses have been performed on situations in which the height of the flange portion  7  was increased to 30 mm that was 5 mm higher than that of the examples in  FIGS. 18 and 19  (flange height of 25 mm). 
       FIG. 20  is a contour diagram illustrating the analysis result of the press forming method of the invention when the height of the flange portion  7  is 30 mm, and  FIG. 21  is a contour diagram illustrating the analysis result of the conventional example. As illustrated in  FIGS. 20 and 21 , the increasing rates of sheet thickness arising at the bend end portion  21  of the flange portion  7  in the inventive example and the conventional example were both 14%. On the other hand, the increasing rate of sheet thickness arising at both ends of the base portion  22  was 37% in the inventive example and 86% in the conventional example. As in the foregoing, it has been verified that, when the shrink-flanging is performed on the blank material  9  of a narrow width, according to the inventive example, even though the sheet-thickness increased regions arise on both ends of base portion  22  of the flange portion  7 , the increasing rate of sheet thickness at those regions is reduced. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to the processing of forming a metal sheet into a shrink flange by press forming. This makes it possible to fundamentally solve the problem of potential wrinkles in shrink-flanging without any adverse effects on the final products. 
     REFERENCE SIGNS LIST 
     
         
         
           
             S 1  first forming process 
             S 2  second forming process 
               1  press formed part 
               3  projecting outer edge 
               5  top portion 
               7  flange portion 
               9  blank material 
               11  vertical wall portion 
               13  trough portion 
               15  preformed part 
               17  first punch 
               19  boundary line 
               21  bend end portion 
               22  both ends of base portion 
               23  first die 
               25  pad 
               27  flat portion 
               29  vertical-wall forming portion 
               31  trough forming portion 
               33  second die 
               35  second punch 
               37  first punch 
               39  top-portion forming portion 
               41  first punch 
               43  top-portion forming portion 
               50  first blank 
               51  first flange portion 
               53  first bend line 
               55  first slit 
               57  second blank 
               59  second flange portion 
               61  second bend line 
               63  second slit 
               65  preformed shape 
               67  projecting outer edge 
               69  top portion 
               71  vertical wall portion 
               73  trough portion 
               75  third bend line