Patent Publication Number: US-2018043413-A1

Title: Press-forming die

Description:
CROSS-REFERENCE 
     This application claims priority to Japanese patent application no. 2016-158812 filed on Aug. 12, 2016, the contents of which are fully incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a press-forming die for forming a stretch flange along an edge of an inward-facing corner of a metal plate. 
     BACKGROUND ART 
     In recent years, aluminum alloy sheets have come to be used in automotive parts to reduce the weight of the chassis. When a flange is to be formed on an automotive part, e.g., along an opening for holding a glass window of a sunroof, generally speaking, a longitudinal-wall-shaped flange that extends around a rectangular hole is press-formed according to a rectangular burring process. The flange is formed by first cutting or punching a rectangular hole in an aluminum alloy sheet, and then bending the peripheral edge portion (hole edge part) of the rectangular hole in a punch-advance direction by pushing a rectangular punch, which is larger than the rectangular hole, through the rectangular hole. 
     However, this known flange-forming technique forms inferior stretch flanges on aluminum alloy sheets as compared to stretch flanges that are formed on mild (low carbon) steel sheets using this known technique. Therefore, cracks have tended to occur when a curved stretch flange is formed at a corner of an aluminum alloy sheet using this known technique. For example, although uncomplicated (i.e. straight) flanges can be reliably formed along the straight edges of the rectangular hole by simply bending the straight edges in the advancing direction of the punch, when stretch flanges are formed at the corners, they must be bent (curved) while also being stretched in a direction perpendicular to the advancing direction of the punch. Consequently, to avoid cracks in the curved stretch flanges of aluminum alloy sheets, design freedoms for the corner radius, the flange length, etc. at the corners have been restricted in the past. 
     In view of this problem, various techniques have been studied for preventing cracks in the stretch flange of an aluminum alloy sheet during press forming. For example, Japanese Laid-open Patent Publication 2015-139783 discloses a method that comprises: a specifying step that specifies the position of the flange, which is to be deformed by stretching it beyond a preset amount, as a maximum stretch-flange deformed part; and a pre-deforming step, which forms a pre-deformed part that is deformed in a sheet-thickness direction relative to both sides or one side of the specified maximum stretch-flange deformed part. Thereafter, a flange-forming step, which is the final forming step, is performed after the formation of the pre-deformed part. 
     SUMMARY OF THE INVENTION 
     Nevertheless, the above-mentioned JP 2015-139783 describes a method in which the pre-deformed part should be greatly deformed on a sheet-edge side in the pre-deforming step, and the amount of increase in a cross-sectional line length in the pre-deforming step should be set to a value that is less than the amount of increase in a cross-sectional line length of the press-formed article. However, a specific method for forming the pre-deformed part is not disclosed in JP 2015-139783. In addition, multiple steps, such as the pre-deforming step and the final forming step, are necessary in this known method, which is disadvantageous from the viewpoint of equipment costs and productivity. Furthermore, because an aluminum alloy sheet is more difficult to press-form than a steel sheet is, the above-described known method is problematic in that the desired forming process cannot be suitably performed. 
     It is an object of the present teachings to provide a press-forming die and a press-forming method that are capable of press-forming, in one step, a high-quality stretch flange having an inward-facing corner flange and side flanges on both sides thereof. 
     In view of this background information, disclosed herein is a shape for an improved press-forming die, by which the inward-facing corner flange, which is created in conjunction with the forming of the stretch flange, and the side flanges on both sides of the inward-facing corner flange can be press-formed in one step. Also disclosed herein is a preferred relationship between the shoulder radius of punch shoulders and a blank (e.g., an aluminum alloy or steel sheet, which will be referred to as a “blank sheet” herein). 
     In another aspect of the present teachings, a press-forming die is disclosed that is capable of press forming a blank sheet having an inward-facing corner to form a corner flange, which rises from an inner edge of the inward-facing corner, and first and second side flanges, which respectively extend from opposite sides of the corner flange. The press-forming die preferably comprises: 
     a die having a die corner shaped to form the corner flange and first and second die sides shaped to respectively form the first and second side flanges; and 
     a punch having a punch corner and first and second punch sides that respectively cooperate with the die corner and the first and second die sides to form the corner flange, the first side flange and the second side flange; 
     wherein the punch corner has a corner groove extending along a portion at which leading surfaces of the punch and a side surface of the punch intersect in a stroke direction of the punch; and 
     the corner groove is shaped such that its groove width decreases in a direction moving away from the leading surfaces in the stroke direction and converges at a punch corner center on the side surface. 
     Because the corner groove is provided in the punch corner of the press forming die, when the punch moves (advances) relative to the die, the first and second side flanges are first press-formed by the first and second punch sides moving past the first and second die sides. Because punch shoulders of the corner groove are shaped such that the groove width decreases the direction moving away from the leading surfaces in the stroke direction and the groove width converges (becomes zero) at the punch corner center, the corner flange is initially press-formed starting from its outer sides that are adjacent to the first and second side flanges. That is, on both sides of the center of the inward-facing corner (i.e. the blank corner center), both ends of the punch shoulders first make contact with the blank sheet, and the range of the punch shoulders that makes contact with the blank sheet gradually increases towards the blank corner center as the press forming progresses. 
     Accordingly, when the punch advances while stretching the inner edge of the inward-facing corner of the blank sheet and both sides thereof, the portion of the blank sheet at the blank corner center is initially not press-formed. Subsequently, as the groove width of the corner groove that is contacting and press-forming the inward-facing corner decreases, the press-forming range of the punch increases inwardly towards the blank center such that the blank corner center is press-formed last. In this sequence of the manufacturing process, the inward-facing corner of the blank sheet rises into the corner flange shape in a satisfactory manner and the inward-facing corner is bent in the moving (advancing) direction of the punch initially starting from the side flange sides until ultimately reaching the blank corner center. 
     As a result, localization of strain is suppressed (reduced or minimized) and cracks in the stretch flange can be prevented. As a result, a flange (in particular, a curved flange) that rises from the inner edge of the inward-facing corner can be press-formed in one step and, moreover, a high quality product can be manufactured with high productivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view that shows the schematic structures of the principal parts of a press-forming die according to a first embodiment. 
         FIG. 2  shows two states of the principle parts of the press-forming die according to the first embodiment in an enlarged view; in the upper image, the shape of a blank sheet is shown before being press-formed by the press-forming die and in the lower image, the shape of the blank sheet after it has been press-formed is shown. 
         FIG. 3  is a plan view that shows the overall shape of the blank sheet prior to being press-formed by the press-forming die according to the first embodiment. 
         FIG. 4  is a plan view that shows the principal parts of the press-forming die according to the first embodiment in an enlarged view, in which the shape of the blank sheet before the press-forming (in solid lines) and the shape of the blank sheet after the press-forming (in chain lines and bold lines) are also shown. 
         FIG. 5  is an oblique view that shows the structure of principal parts of the punch of the press-forming die according to the first embodiment. 
         FIG. 6  includes a plan view (upper image), an oblique view (middle image), and a side view (lower image) of the principal parts of the punch, including a punch corner, according to the first embodiment. 
         FIG. 7  is a plan view that shows, according to the first embodiment, the contour shape of the principal parts along a leading surface of the punch and shows the change in the contour shape at a cross section parallel to the leading surface. 
         FIG. 8  is an oblique view that shows the schematic structures of the principal parts of a press-forming die according to a second embodiment. 
         FIG. 9  is an oblique view that shows the structure of the principal parts of the punch according to the second embodiment. 
         FIG. 10  is an oblique view of the principal parts that shows an example of another shape of a punch according to the first and second embodiments. 
         FIG. 11  is an oblique view that shows the schematic structure of the principal parts of a press-forming die according to a third embodiment. 
         FIG. 12  is an oblique view that shows the structure of the principal parts of the punch according to the third embodiment. 
         FIG. 13  includes a plan view (upper image), an oblique view (middle image), and a side view (lower image) that show the principal parts of the punch, including the punch corner, according to the third embodiment. 
         FIG. 14  is an oblique view that shows the schematic structure of the principal parts of a press-forming die according to a fourth embodiment. 
         FIG. 15  includes a plan view (upper image), an oblique view (middle image), and a side view (lower image) of the principal parts of the punch, including the punch corner, according to the fourth embodiment. 
         FIG. 16  contains five enlarged oblique views of the principle parts of the punch and the blank sheet and shows a series of states from the start of the press-forming to the completion of the press-forming. 
         FIG. 17  shows enlarged cross-sectional views of the principal parts according to a working example, in which a series of press-forming states of the blank sheet at a corner-center position and at two round end positions of the punch are shown in accordance with the advancing movement (stroke) of the punch. 
         FIG. 18  is an enlarged plan view for explaining, according to a working example, the flow of metal when the stretch flange of the blank sheet is being press-formed, in comparison with a conventional technique. 
         FIG. 19  is an enlarged plan view for explaining a comparison of a working example to a conventional technique with regard to the expansion of the line length when the stretch flange of the blank sheet is press-formed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In one embodiment of the above-described press-forming die, a shoulder radius Rgs of groove shoulders, which are portions along which the corner groove and the leading surfaces intersect, and a shoulder radius Rps of punch shoulders, which are portions along which the corner groove and the leading surfaces intersect with the side surface, preferably satisfy the relationships Rps≦Rgs and Rgs≦30 mm. In such an embodiment, while the press-forming range from the side flanges towards the corner flange is widening (increasing) during the press-forming operation, the effect of suppressing (reducing) an increase in strain at both ends of the corner groove also increases. 
     In another embodiment of the present teachings, the positions at which the above-mentioned punch shoulders and the groove shoulders intersect on the leading surfaces are preferably at the boundaries of the punch corner with the adjacent punch sides on the side surface of the punch, or on the outer sides thereof (with reference to the blank corner center). In such an embodiment, by suitably selecting the shoulder radius Rgs, a groove depth D, and the like, the effect of suppressing (reducing, minimizing) localization of strain increases. It is noted that the boundaries of the punch corner with the adjacent punch sides include the round end positions, which constitute both ends of the punch corner, and positions in the vicinity thereof. 
     The ratio D/W of the corner groove, which is the ratio of the groove depth D in the stroke direction to the groove width W at the leading surfaces, is preferably larger than 0.25. In such an embodiment, localization of strain is suppressed (reduced, minimized) and the effect of reducing maximum principal strain is further increased. 
     In another embodiment of the present teachings, the corner groove preferably has a shape of a pair of opposing inclined surfaces. For example, the corner groove can have a V-groove shape as will be described below in further detail. 
     The present press-forming die can be utilized to bend various kinds of metal sheets as the blank sheet provided with the inward-facing corner according to the present teachings. In particular, superior or optimal results can be obtained when bending an aluminum alloy sheet or a steel sheet to press-form one or more corner (curved) stretch flanges. 
     First Embodiment 
     A first embodiment of a press-forming die  1  according to the present teachings will now be explained with reference to the drawings. 
     As shown in  FIGS. 1-2 , the press-forming die  1  includes a die  2  and a punch  3 . A blank (referred to herein as “blank sheet”)  4 , e.g., a metal sheet made, e.g., of aluminum alloy or steel, having an inward-facing corner  41  is disposed between the die  2  and the punch  3 . As shown in the lower image of  FIG. 2 , a corner flange Fc, which rises from an inner edge  43  at the inward-facing corner  41 , and a pair of (first and second) side flanges Fs extending from both sides of the corner flange Fc are press-formed. The side flanges Fs rise from the inner edge  43  along a pair of (first and second) side parts  42  extending from both sides of the inward-facing corner  41 . The side flanges Fs together with the corner flange Fc form a flange F. 
     The die  2  has a round die corner  21 , which follows along the round inward-facing corner  41 . A pair of (first and second) straight die sides  22  extends from the die corner  21  and follow along the pair of straight side parts  42 . In addition, the punch  3  has a punch corner  31  and a pair of (first and second) punch sides  32 , which cooperate (interact) with the die corner  21  and the pair of (first and second) die sides  22 , respectively. When the punch  3  of the press-forming die  1  moves (advances) upwardly relative to the die  2 , with the direction perpendicular to the sheet surface of the blank sheet  4  serving as the stroke direction X (i.e., the up-down direction in the figures), the inward-facing corner  41  and the adjacent side parts  42  of the blank sheet  4  are press-formed into the desired flange shape. Although each of the side parts  42  is straight-shaped in the present embodiment, they may have another shape. For example, at least a portion of the side parts  42  may have a curved-side shape. This applies likewise to the die sides  22  and the punch sides  32 . 
     It is noted that the press-forming die  1  forms the flange F, which has at least one corner flange Fc, on the blank sheet  4 , and the blank sheet  4  (prior to press-forming) has at least one inward-facing corner  41 .  FIGS. 3-4  show one representative, non-limiting example of the blank sheet  4 , and  FIGS. 1-2  show the portion of the forming die  1  that forms ¼ th  of the overall shape of the press-formed blank sheet  4  (e.g., the shape demarcated by the dotted lines in  FIGS. 3-4 ). Prior to press-forming, the blank sheet  4  has four inward-facing corners  41 , all of the same round shape, at the four corners of a substantially rectangular hole (through hole)  4 A, which passes through a central part of the blank sheet  4 . 
     As shown in  FIG. 2 , the punch  3  has a corner groove  6  located in at least the round punch corner  31  where leading surfaces  33  (i.e., upper surfaces in  FIG. 2 ) intersect a side surface  34  of the punch  3  in the stroke direction X. A pair of (first and second) straight sides  35  is formed at edges of the leading surfaces  33  on both sides of the punch corner  31 . The portions of the punch  3 , along which the side surface  34  intersects the corner groove  6  and the straight sides  35  of the leading surfaces  33 , serve as punch shoulders  3 S. As shown in  FIG. 5 , the corner groove  6  has a groove width that decreases in the direction moving away (downward in the figures) from the leading surfaces  33  in the stroke direction X and has a shape (e.g., a wedge or V shape) that converges at a punch corner center C 1 . On the side surface  34 , the two (lateral) ends of the round punch corner  31  respectively extend to the flat punch sides  32 . The two boundary positions of the round punch corner  31  with the adjacent flat punch sides  32  will be referred to herein as round ends C 2 , C 3 . The particular shape of the corner groove  6  according to this embodiment of the present teachings is described in more detail below. 
     In  FIG. 1 , the block-shaped die  2  has a shape that is substantially similar to that of the rectangular hole  4 A (e.g., refer to  FIG. 3 ), which has four round corners within the blank sheet  4 . That is, the die  2  has a rectangular tubular hole  2 A, the perimeter of which is larger than that of the rectangular hole  4 A. The die corner  21  has an inward-facing corner that constitutes the portion of the tube wall of the rectangular tubular hole  2 A that follows along the inward-facing corner  41  of the blank sheet  4 . Prior to press-forming, the perimeter of the rectangular hole  4 A of the blank sheet  4  is located inside of the perimeter of the rectangular tubular hole  2 A in plan view. 
     In addition, the block-shaped punch  3  is sized to slide into the rectangular tubular hole  2 A and is movable relative to the rectangular tubular hole  2 A in the stroke direction X. The punch corner  31  of the punch  3  has an outward-facing corner shape, and is disposed opposite the die corner  21 . The punch sides  32  are disposed so as to oppose the die sides  22 . The die shoulders  2 S on the die sides  22  on both sides of the die corner  21  respectively cooperate with the punch shoulders  3 S. The shoulder radius of the die shoulders  2 S is constant, as will be further discussed below. 
     A blank holder  5 , on which the blank sheet  4  is mounted, is provided downward of the die  2  and is vertically movable. Prior to the press-forming, as shown in  FIG. 1 , the blank sheet  4  is interposed between the die  2  and the blank holder  5 . A peripheral edge-portion of the rectangular hole  4 A protrudes inward from the inner side of the die  2  by a prescribed width (discussed below) and is located upward of the punch  3 . It is noted that the punch  3  has a rectangular-block shape that is complementary to the interior shape of the rectangular tubular hole  2 A of the die  2 , and the portion of the punch  3  having one of the punch corners  31  is shown in  FIG. 1 . 
     As shown in  FIGS. 3-4 , the contours of the die corner  21  and the die sides  22  extending therefrom (e.g., indicated by broken lines in  FIG. 3  and  FIG. 4 ) respectively follow the inward-facing corner  41  and the side parts  42  of the blank sheet  4 . During the press-forming of the blank sheet  4 , the inward-facing corner  41  and the side parts  42  will bend along the positions of the contours of the die corner  21  and the die sides  22 , thereby forming the corner flange Fc and the side flanges Fs, which rise from the bent positions that serve as the inner edge  43 . As shown in  FIG. 4 , the flange lengths Ls at both ends of the inward-facing corner  41  and the flange lengths Ls at the side parts  42  are equal, and the flange length Lc at the blank corner center  41 C is greater than or equal to the flange length Ls (i.e., Ls≦Lc). In one embodiment of the present teachings, the flange length Ls may be calculated, as a function of the flange length Lc at the blank corner center  41 C, according to the equation: 
         Ls =( Lc/ 2)×√{square root over (√2)}.
 
     The corner flange Fc and the side flanges Fs (e.g., refer to the lower image in  FIG. 2 ) rise in the stroke direction X from plane ab, which includes an a direction and a b direction that mutually intersect and are the directions of two intersecting sides of the blank sheet  4 . The rectangular flange F, which is composed of the corner flange Fc and the straight-sided side-flanges Fs, can be formed at each corner of a rectangular opening in, for example, an opening for glass window of an automobile sunroof, or the like. 
     As was noted above, the blank sheet  4  optionally may be composed of, for example, an aluminum alloy sheet or a steel sheet, such as a mild (low carbon) steel sheet. Aluminum alloy sheets are more effective for reducing weight than steel sheets. In addition, because aluminum alloy sheets are generally inferior to mild steel sheets in terms of stretch flange formability, the effect produced by using the press-forming techniques of the present teachings on aluminum alloy sheets is relatively large. Accordingly, a flange F having a relatively large height can be press-formed on an aluminum alloy sheet while suppressing (minimizing) strain. The die  2  and the punch  3  of the press-forming die  1  can be composed (made) of, for example, any well-known steel material typically used for casting metal dies. 
     As shown in  FIGS. 5-6 , the punch  3  has a V-groove-shaped corner groove  6 , which is composed of a pair of inclined surfaces  61 ,  62  defined in the punch corner  31 . Groove shoulders  6 S are formed along the portions of the leading surfaces  33  at which the inclined surfaces  61 ,  62  and the leading surfaces  33  respectively intersect. The groove width W of the corner groove  6  is largest at the leading surfaces  33 . Referring to  FIG. 7 , the corner groove  6  extends along a diagonal line that passes through the punch corner center C 1 , and the corner groove  6  has a shape such that the groove width W, in a cross section parallel to the leading surfaces  33 , becomes smaller moving away (downward in the figures) from the leading surfaces  33  and such that the two inclined surfaces  61 ,  62  approach one another. The pair of inclined surfaces  61 ,  62  ultimately unite (converge) along the diagonal line that intersects the punch corner center C 1 , and the corner groove  6  converges (disappears) into the outward-facing corner (round) shape of the lower portion of the punch corner  31 . A groove-valley (groove-nadir)  6 V is formed along the portion at which the two inclined surfaces  61 ,  62  of the corner groove  6  intersect one another, i.e. along the diagonal line. 
     In the up-down direction between the cross section at which the corner groove  6  disappears into the end surface on the side opposite the leading surfaces  33  and the bottom surface of the punch  3 , the punch  3  has a rounded rectangular outer shape (see e.g.,  FIG. 5 ) that is similar (complementary) to the rectangular tubular hole  2 A of the die  2  and, in particular, has the outward-facing round corner shape that is complementary to the die corner  21 . The portions of the punch  3 , along which the corner groove  6  and the leading surfaces  33  intersect the side surface  34 , are curved portions of the punch shoulders  3 S. The shoulder radius Rps of the punch shoulders  3 S is constant here. 
     The shoulder radius Rps of the punch shoulders  3 S is preferably set such that the relationship t≦Rps≦Ls is satisfied, in which t is the sheet thickness of the blank sheet  4 , and Ls is the flange length at both ends of the inward-facing corner  41  (see  FIG. 4 ). By setting the shoulder radius Rps to a length between the sheet thickness t of the blank sheet  4  or greater and the flange length Ls or less, the blank sheet  4  can be bent (perpendicularly or substantially perpendicularly) in the moving (advancing) direction of the punch  3  such that the inner peripheral hole edge portion of the blank sheet  4  rises into the flange shape in a satisfactory manner and the inward-facing corner  41  moves (flows) towards the blank corner center  41 C. Therefore, the relationship t&lt;Rps&lt;Ls is preferably satisfied. In addition, the smaller the shoulder radius Rps, the more that the rise (bent) position nears the die  2 , and the press-forming can proceed while causing the sheet material to flow toward the blank corner center  41 C. 
     In addition, the shoulder radius Rgs of the groove shoulders  6 S, which are the portions along which the corner groove  6  and the leading surfaces  33  intersect, and the shoulder radius Rps preferably satisfy the relationship Rps≦Rgs. In addition, it is preferable that Rgs≦30 mm. In such an embodiment, as the press-forming range widens from the side flanges Fs towards (and ultimately to) the corner flange Fc during the press-forming operation, the effect of suppressing (reducing or minimizing) an increase in strain in the blank sheet  4  proximal to both ends of the corner groove  6  increases. In the present embodiment, for example, the shoulder radius Rps of the punch shoulders  3 S, the shoulder radius Rgs of the groove shoulders  6 S, and a valley radius Rgv of the groove-valley  6 V are all equal (i.e., Rps=Rgs=Rgv). 
     In addition, the leading surfaces  33 , along which the punch shoulders  3 S and the groove shoulders  6 S intersect, are preferably positioned at, or on the outer side of, the boundaries between the punch corner  31  and the punch sides  32  on the side surface  34 . In this regard, the boundaries include the round ends C 2 , C 3 , which are the boundary positions, and positions in the vicinity thereof. That is, connecting parts S 1 , S 2  (see  FIGS. 5-6 ), which connect the punch shoulders  3 S to the groove shoulders  6 S, may be located at the positions of the round ends C 2 , C 3  of both ends of the punch corner  31  or in the vicinities thereof (not shown), or may be located on the outer sides of the round ends C 2 , C 3  as shown in  FIGS. 5-6 . In such embodiments, when press-forming starts, it is possible to suppress (reduce or minimize) the occurrence of localized strain in the blank sheet  4  proximal to the connecting parts S 1 , S 2 . 
     As was noted above, the connecting parts S 1 , S 2  of the groove shoulders  6 S are positioned outwardly of the round ends C 2 , C 3  in the present embodiment. Thus, as shown in  FIG. 6 , the groove width W of the corner groove  6  at the leading surfaces  33  is larger than the maximum (uppermost) width W 0  of the punch corner  31 . Here, the maximum (uppermost) width W 0  of the punch corner  31  is the straight-line distance between lines that respectively project the round ends C 2 , C 3  (i.e. both ends of the punch corner  31 ) to the leading surfaces  33 . 
     In the corner groove  6  as shown in  FIG. 6 , the ratio D/W of the groove depth D in the stroke direction X to the groove width W at the leading surfaces  33  is preferably larger than 0.25. The larger (deeper) the groove depth D, the more that localization of strain is suppressed and the effect of reducing the maximum principal strain is increased, while the punch  3  is moving, in the direction from the side flanges Fs towards the blank corner center  41 C. However, when the groove depth D becomes large, the stroke length needed to form the flange becomes large, and consequently the size of the press-forming die  1  increases. Therefore, for example, the ratio D/W should be set such that it is smaller than 1.9 and more preferably less than 0.9. In the present embodiment, for example, the groove depth D is set to larger than 50% of the groove width W at the leading surfaces  33 . In this case, for example, the angle α that the pair of inclined surfaces  61 ,  62  of the corner groove  6  respectively form with the adjacent leading surfaces  33  (see the lowermost image in  FIG. 6 ) is set in the range of 30°-75° and preferably in the range of 30°-60°. 
     Second Embodiment 
     The first embodiment described an example in which the shoulder radius Rps of the punch shoulders  3 S and the shoulder radius Rgs of the groove shoulders  6 S are the same size. However,  FIGS. 8-9  show a second embodiment of the present teachings, in which the relationship Rps&lt;Rgs is preferably set, such that the punch shoulders  3 S of the punch sides  32  connect more smoothly (less acutely) with the punch shoulders  3 S of the punch corner  31 . The occurrence of local strain can thereby be further suppressed (reduced or minimized) in the blank sheet  4  proximal to the connecting parts S 1 , S 2  at the start of press-forming. In the range of Rgs≦30 mm, the larger the shoulder radius Rgs of the groove shoulders  6 S, the greater the effect of suppressing (reducing or minimizing) strain. However, if Rgs exceeds 20 mm, then this effect no longer changes significantly. Accordingly, the range Rgs≦20 mm should preferably be set where appropriate. 
     Other structural elements are the same as in the first embodiment and are assigned the same symbols and reference numbers, and explanations thereof are therefore omitted. This applies likewise to the subsequent embodiments disclosed in this specification. It is noted that, in the first and second embodiments, the corner groove  6  may be formed in the leading surfaces  33  of the punch  3  continuously with a not-shown punch corner  31  that is located on the side opposite the punch corner  31  in the diagonal direction. This applies likewise to the two corner grooves  6  of the punch  3  in the other diagonal direction. 
     In addition, as another exemplary shape shown in  FIG. 10 , a discrete corner groove  6  may be disposed at each punch corner  31  of the punch  3 , and may be formed independently of the other punch corners  31 . In this case, the corner groove  6  can be made into a shape in which, for example, the two inclined surfaces  61 ,  62  oppose each other and, on the side opposite the punch corner  31 , a vertical wall  63  connects the two inclined surfaces  61 ,  62 . 
     Third Embodiment 
     The first embodiment described an example in which the groove depth D of the corner groove  6  in the stroke direction X is larger than 50% of the groove width W at the leading surfaces  33 . However,  FIGS. 11-13  show a third embodiment of the present teachings, in which the groove depth D may of course be set such that it is smaller than 50% of the groove width W. In this example, the groove width W at the leading surfaces  33  is still larger than the maximum (uppermost) width W 0  of the punch corner  31 , the same as in the first embodiment. 
     Fourth Embodiment 
     Alternatively,  FIGS. 14-15  show a fourth embodiment of the present teachings, in which the corner groove  6  can also be set (designed) such that the groove depth D is smaller than 50% of the groove width W and, furthermore, such that the groove width W at the leading surfaces  33  is smaller than the maximum (uppermost) width W 0  of the punch corner  31 . In either case, as described above, the ratio D/W is set larger than 0.25. However, if the groove width W at the leading surfaces  33  is set smaller than the maximum (uppermost) width W 0  of the punch corner  31 , then local strain tends to occur in the blank sheet  4  proximal to the connecting parts S 1 , S 2  at the start of the press-forming. Therefore, to avoid this, there is a risk that design freedoms with regard to the shoulder radius Rps of the punch shoulders  3 S, the groove depth D of the corner groove  6 , and the like will be restricted. In addition, if the groove width W is markedly larger than the width W 0  of the punch corner  31 , then the stroke length needed to form the flange will become large, thereby increasing the size of the press-forming die  1 . Accordingly, the ratio W/W 0  of the groove width W at the leading surfaces  33  to the maximum (uppermost) width W 0  of the punch corner  31  should preferably be set such that it falls within the range of 0.9-1.5. 
     Next, a representative, non-limiting method of press-forming a blank sheet  4  using a press-forming die  1  according to the first embodiment will be explained. 
     As shown in  FIG. 1 , the blank sheet  4  is held interposed between the die  2  and the blank holder  5 , and the punch  3  is disposed inward of the die  2 . Prior to the press-forming, as shown in the upper image in  FIG. 2 , the punch  3  is disposed downward of the blank sheet  4 . When the punch  3  is caused to rise (advance) in the up-down direction (i.e. in the stroke direction X) by using a punch drive (not shown), the punch  3  moves (advances) upward relative to the die  2 . When the punch  3  further rises, the leading surfaces  33  and the punch shoulders  3 S first contact the peripheral edge portion of the rectangular hole  4 A of the blank sheet  4 , which is disposed in the moving direction. 
     As shown in the lower image in  FIG. 2 , as the punch  3  further rises, the punch shoulders  3 S expand the rectangular hole  4 A of the blank sheet  4 . Attendant therewith, the peripheral edge portion of the rectangular hole  4 A of the blank sheet  4  is bent along the die shoulders  2 S, thereby press-forming the flange F that rises to a prescribed height from the inner edge  43 . The corner flange Fc of the flange F is formed at the inward-facing corner  41  by the die corner  21  and the punch corner  31 , and the straight side flanges Fs, which continue from the corner flange Fc, are formed by the die sides  22  and the punch sides  32 . It is noted that, in  FIG. 2 , depictions of the die  2  and the blank holder  5  are omitted for the sake of clarity. 
     In this representative, non-limiting embodiment of the present teachings, as shown by arrows in  FIG. 4 , the corner flange Fc rises such that it follows along the corner shape of the die corner  21  (e.g., corner radius R C Die =70 mm) while widening in the circumferential direction. That is, after the press-forming, the line length of the flange increases in the inward-facing corner  41  of the blank sheet  4 , which is to become the corner flange Fc, and thereby the stretch flange is formed. In case an inner-end-edge part of the inward-facing corner  41  prior to the press-forming (e.g., corner radius R C blank sheet =70 mm) is to be press-formed into a stretch flange, it has been confirmed that the smaller the corner radius Rc of the corner flange Fc prior to the press-forming and the larger the flange length L, the more the strain increases. It is noted that the corner radius R C blank sheet  of the blank sheet  4  is preferably at least substantially equal to the corner radius R C Die  of the die corner  21 . 
     On the other hand, if the present press-forming method is applied to, for example, an opening for an automobile sunroof or the like, the corner flange Fc formed at the opening is preferably smaller than the corner radius R and larger than the flange length L. Consequently, there is a concern that cracks will be created by press-forming the stretch flange, and consequently it is required that strain produced by press-forming the stretch flange be maximally reduced in the corner flange Fc. This is regarded as important, especially for an aluminum alloy sheet that has poor stretch flange forming properties. 
     To address this potential problem, the punch  3  shown in  FIG. 2  has the corner groove  6  in the punch corner  31 . The punch shoulders  3 S between the two ends of the punch corner  31  do not make contact with the blank sheet  4  during the initial stages of the press-forming. Because the groove width W of the corner groove  6  shrinks (at the level adjacent to the blank sheet  4 ) as the press-forming progresses and both ends (edges) of the corner groove  6  gradually approach one another, stretching at the blank corner center  41 C can be suppressed (reduced). By setting the shoulder radius Rps of the punch shoulders  3 S in the above-prescribed range, the side flanges Fs rise quickly; furthermore, the effect on the corner flange Fc is minimized by adjusting the shoulder radius Rgs of the groove shoulders  6 S, the groove width W, the groove depth D, etc. As a result, when the corner flange Fc is being press-formed while the die corner  21  and the punch corner  31  cooperate to bend the peripheral edge portion of the rectangular hole  4 A of the blank sheet  4 , localization of strain at the blank corner center  41 C can be suppressed (reduced or minimized). 
     WORKING EXAMPLES 
     Next, in working examples in which the corner flanges Fc were press-formed at the four inward-facing corners  41  of the blank sheet  4  using a press-forming die  1  having the above configuration, the effect of suppressing (reducing or minimizing) the generation of strain in the corner flanges Fc was evaluated. 
     Working Examples 1-23 
     The blank sheet  4  was an aluminum alloy sheet (a 6000-series aluminum alloy) having a thickness of 1.2 mm. The corner radius Rc of the corner flanges Fc after press-forming was 70 mm, and the flange length Lc after press-forming was 16 mm at the blank corner center  41 C. The flange length Ls at both ends of the inward-facing corner  41 , which are the round ends C 2 , C 3 , and at the side parts  42  was 11.3 mm (i.e., 8√{square root over ( )}2 mm=11.3 mm). As shown in Table 1, in the punch  3 , the shoulder radius Rps of the punch shoulders  3 S and the shoulder radius Rgs of the groove shoulders  6 S were varied in the range of 2-10 mm while setting the shoulder radius Rps equal to the shoulder radius Rgs in all examples. It is noted that the valley radius Rgv of the groove-valley part  6 V was set equal to the shoulder radius Rps. In addition, the ratio W/W 0  of the groove width W at the leading surfaces  33  of the corner groove  6  to the maximum (uppermost) width W 0  of the punch corner  31  was set in the range of 1-1.5. The incline angle α of the inclined surfaces  61 ,  62  (see bottom image in  FIG. 6 ) was varied in the range of 30°-75°. 
     According to the press forming method shown in  FIG. 2 , the blank sheet  4  was held interposed between the die  2  and the blank holder  5 , the punch  3  was raised (advanced), the flange F was formed along the entire peripheral edge portion of the rectangular hole  4 A, and the change in the maximum principal strain during the press-forming step was examined. Based on a press-forming simulation using the finite element method, the maximum principal strain was derived from the strain distribution that occurred in various portions of the corner flange Fc attendant with the rise of the punch  3 , and the maximum value at the completion of the press-forming was designated as the maximum principal strain ε 1 . The results are provided in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Maximum 
               
               
                   
                   
                   
                   
                   
                   
                 Principal 
               
               
                 Working Example 
                 Rps 
                 Rgs 
                 α 
                 D/W 
                 W/W0 
                 Strain 
               
               
                 Comparative Example 
                 (mm) 
                 (mm) 
                 (°) 
                 (—) 
                 (—) 
                 ε1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Comparative Example 1 
                 10 
                 — 
                 — 
                 — 
                 — 
                 0.233 
               
               
                 Working Example 1 
                 2 
                 2 
                 30 
                 0.29 
                 1.25 
                 0.210 
               
               
                 Working Example 2 
                 2 
                 2 
                 45 
                 0.50 
                 1.25 
                 0.193 
               
               
                 Working Example 3 
                 2 
                 2 
                 60 
                 0.87 
                 1.25 
                 0.177 
               
               
                 Working Example 4 
                 4 
                 4 
                 45 
                 0.50 
                 1.25 
                 0.203 
               
               
                 Working Example 5 
                 4 
                 4 
                 60 
                 0.87 
                 1.25 
                 0.190 
               
               
                 Working Example 6 
                 6 
                 6 
                 45 
                 0.50 
                 1.25 
                 0.210 
               
               
                 Working Example 7 
                 6 
                 6 
                 60 
                 0.87 
                 1.25 
                 0.198 
               
               
                 Working Example 8 
                 10 
                 10 
                 60 
                 0.87 
                 1.25 
                 0.207 
               
               
                 Working Example 9 
                 10 
                 10 
                 75 
                 1.87 
                 1.25 
                 0.196 
               
               
                 Comparative Example 2 
                 14 
                 14 
                 45 
                 0.50 
                 1.25 
                 0.245 
               
               
                 Working Example 10 
                 2 
                 2 
                 30 
                 0.29 
                 1 
                 0.210 
               
               
                 Working Example 11 
                 4 
                 4 
                 45 
                 0.50 
                 1 
                 0.203 
               
               
                 Working Example 12 
                 6 
                 6 
                 45 
                 0.50 
                 1 
                 0.210 
               
               
                 Working Example 13 
                 6 
                 6 
                 60 
                 0.87 
                 1 
                 0.196 
               
               
                 Working Example 14 
                 6 
                 6 
                 75 
                 1.87 
                 1 
                 0.181 
               
               
                 Working Example 15 
                 10 
                 10 
                 45 
                 0.50 
                 1 
                 0.224 
               
               
                 Working Example 16 
                 10 
                 10 
                 60 
                 0.87 
                 1 
                 0.206 
               
               
                 Comparative Example 3 
                 14 
                 14 
                 30 
                 0.29 
                 1 
                 0.240 
               
               
                 Working Example 17 
                 2 
                 2 
                 30 
                 0.29 
                 1.5 
                 0.210 
               
               
                 Working Example 18 
                 2 
                 2 
                 45 
                 0.50 
                 1.5 
                 0.193 
               
               
                 Working Example 19 
                 2 
                 2 
                 60 
                 0.87 
                 1.5 
                 0.178 
               
               
                 Working Example 20 
                 6 
                 6 
                 45 
                 0.50 
                 1.5 
                 0.211 
               
               
                 Working Example 21 
                 6 
                 6 
                 60 
                 0.87 
                 1.5 
                 0.199 
               
               
                 Working Example 22 
                 10 
                 10 
                 60 
                 0.87 
                 1.5 
                 0.207 
               
               
                 Working Example 23 
                 10 
                 10 
                 75 
                 1.87 
                 1.5 
                 0.197 
               
               
                 Comparative Example 4 
                 14 
                 14 
                 60 
                 0.87 
                 1.5 
                 0.244 
               
               
                   
               
            
           
         
       
     
     Comparative Example 1 
     For the purpose of comparison, the blank sheet  4  was press-formed using the same method as in the above-described working examples 1-23, except that the shape of the punch  3  was modified. More specifically, the punch for comparison had the same shape as that of the punch  3  of the first embodiment, except that it had no corner groove  6 . The configuration of the press-forming die  1 , except that of the punch, was the same, the maximum principal strain ε 1  at the time that the flange F was formed in the blank sheet  4  was derived, and the results are provided in Table 1. 
     Comparative Examples 2-4 
     For the purpose of comparison, the shape of the punch  3  of working example 1 was modified such that the shoulder radius Rps of the punch shoulders  3 S was 14 mm, and the blank sheet  4  was press-formed according to the same method. In this case, the relationships shoulder radius Rps=shoulder radius Rgs and valley radius Rgv=shoulder radius Rps were set, and the ratio W/W 0  of the groove width W to the width W 0  of the punch corner  31  was set in the range of 1-1.5. Likewise, the maximum principal strain ε 1  at the time that the flange F was formed on the blank sheet  4  was derived, and the results are provided in Table 1. 
     As shown in Table 1, in the comparative example 1 in which the punch  3  did not have the corner groove  6 , the maximum principal strain ε 1  was 0.233. In contrast, in working examples 1-23, in which the punch  3  had the corner groove  6 , the maximum principal strain ε 1  was smaller, namely in the range of 0.177-0.224. In addition, in the comparative examples 2-4, in which the shoulder radius Rps of the punch shoulders  3 S was larger than the flange length Ls, the maximum principal strain ε 1  was 0.240-0.245 and larger than that in comparative example 1. Based on the above results, it was determined that, when the relationship t≦Rps≦Ls is satisfied for the shoulder radius Rps of the punch shoulders  3 S with respect to the sheet thickness t of the blank sheet  4  and the flange length Ls at both ends of the inward-facing corner  41 , satisfactory results can be obtained. 
     Working Examples 24-27 
     As shown in Table 2, the shape of the punch  3  of working example 2 was varied such that the shoulder radius Rgs of the groove shoulders was in the range of 2-30 mm. In working examples 24-27, the relationship valley radius Rgv=shoulder radius Rps was maintained. The blank sheet  4  was press-formed according to the same method as described above for working examples 1-23, the maximum principal strain ε 1  at the time when the flange F was formed on the blank sheet  4  was derived, and the results are provided in Table 2. 
     Working Examples 28-38 
     As shown in Table 2, the shape of the punch  3  of working example 1 was varied such that the shoulder radius Rps of the punch shoulders  3 S was in the range of 2-10 mm and the ratio W/W 0  of the groove width W to the width W 0  of the punch corner  31  was set to 0.9 or 1.1. In working examples 28-38, the relationships shoulder radii Rps=Rgs and valley radius Rgv=shoulder radius Rps were maintained. The press-forming of the blank sheet  4  was performed according to the same method as described above for working examples 1-23, the maximum principal strain ε 1  at the time that the flange F was formed on the blank sheet  4  was derived, and the results are provided in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Maximum 
               
               
                   
                   
                   
                   
                   
                   
                 Principal 
               
               
                   
                 Rps 
                 Rgs 
                 α 
                 D/W 
                 W/W0 
                 Strain 
               
               
                 Working Example 
                 (mm) 
                 (mm) 
                 (°) 
                 (—) 
                 (—) 
                 ε1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Working Example 24 
                 2 
                 2 
                 45 
                 0.5 
                 1.25 
                 0.193 
               
               
                 Working Example 25 
                 2 
                 10 
                 45 
                 0.5 
                 1.25 
                 0.193 
               
               
                 Working Example 26 
                 2 
                 20 
                 45 
                 0.5 
                 1.25 
                 0.193 
               
               
                 Working Example 27 
                 2 
                 30 
                 45 
                 0.5 
                 1.25 
                 0.193 
               
               
                 Working Example 28 
                 4 
                 4 
                 30 
                 0.29 
                 0.9 
                 0.218 
               
               
                 Working Example 29 
                 6 
                 6 
                 30 
                 0.29 
                 0.9 
                 0.224 
               
               
                 Working Example 30 
                 6 
                 6 
                 45 
                 0.50 
                 0.9 
                 0.208 
               
               
                 Working Example 31 
                 10 
                 10 
                 60 
                 0.87 
                 0.9 
                 0.206 
               
               
                 Working Example 32 
                 2 
                 2 
                 30 
                 0.29 
                 1.1 
                 0.209 
               
               
                 Working Example 33 
                 2 
                 2 
                 45 
                 0.50 
                 1.1 
                 0.194 
               
               
                 Working Example 34 
                 4 
                 4 
                 45 
                 0.50 
                 1.1 
                 0.203 
               
               
                 Working Example 35 
                 4 
                 4 
                 60 
                 0.87 
                 1.1 
                 0.188 
               
               
                 Working Example 36 
                 6 
                 6 
                 45 
                 0.50 
                 1.1 
                 0.209 
               
               
                 Working Example 37 
                 6 
                 6 
                 60 
                 0.87 
                 1.1 
                 0.197 
               
               
                 Working Example 38 
                 10 
                 10 
                 60 
                 0.87 
                 1.1 
                 0.207 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, even in the examples in which the relationship of the shoulder radius Rgs of the groove shoulders  6 S with respect to the shoulder radius Rps of the punch shoulders  3 S was Rps≦Rgs, and Rgs≦30 mm, equivalent results could be obtained. In addition, even if the ratio W/W 0  was 0.9 or 1.1, equivalent results could be obtained. 
     In  FIG. 16 , a process of forming the corner flange Fc using the punch  3  of working example 2 is shown as one example. Prior to the press-forming (i.e., stroke length=0 mm), which is shown in the uppermost image in  FIG. 16 , the corner groove  6  is positioned such that the leading surfaces  33  of the punch  3  are exposed through the rectangular hole  4 A of the blank sheet  4  with the blank corner center  41 C of the inward-facing corner  41  disposed (interposed) therebetween. From this state, when the punch  3  rises or advances (i.e., stroke length=16 mm), the punch shoulders  3 S first make contact with the blank sheet  4  from the punch sides  32  at both ends of the corner groove  6 , that is, at the connecting parts S 1 , S 2 , at which contact is made with the groove shoulders  6 S. Thereby, the side flanges Fs progressively rise at the side parts  42 . 
     The punch shoulders  3 S are connected to the straight sides  35  of the leading surfaces  33  and first make contact with the blank sheet  4  at the punch sides  32 , which are positions where strain is relatively small. In this case, because the shoulder radius Rps of the punch shoulders  3 S is set comparatively small within a range larger than the sheet thickness t, the side flanges Fs rise quickly. In addition, because the groove width W of the corner groove  6  is larger than the width W 0 , the punch shoulders  3 S of the punch corner  31  do not make contact with the inward-facing corner  41  of the blank sheet  4  at the start of the press-forming. Consequently, while stretching is suppressed (minimized or even prevented) at the inward-facing corner  41 , the press-forming range migrates inwardly towards the blank corner center  41 C of the inward-facing corner  41 . 
     When the punch  3  further rises (i.e., stroke length=48 mm, 80 mm), the groove width W of the corner groove  6  (as viewed at the level of the blank sheet  4 ) progressively becomes smaller. As a result, the range of contact of the blank sheet  4  with the punch shoulders  3 S widens (increases or converges) towards the inward-facing corner  41 , and the corner flange Fc progressively rises. When the corner flange Fc rises to the position at which the lower ends of the punch shoulders  3 S are exposed at the blank corner center  41 C (i.e., stroke length=122 mm), the press-forming of the corner flange Fc is complete. 
       FIG. 17  schematically shows the press-forming process using the punch  3  of working example 2 by comparing cross sections taken at the punch corner center C 1  of the punch corner  31  and at the round ends C 2 , C 3  on both sides thereof. When the press-forming starts, which is shown in the two uppermost images in  FIG. 17  (i.e., stroke length=0 mm, 16 mm), the press-forming of the side flanges Fs starts along the not-shown punch sides  32 . Because the groove width W (at the leading surfaces  33 ) of the corner groove  6  is larger than the maximum width W 0 , the punch shoulders  3 S do not extend to the round ends C 2 , C 3  of the punch corner  31 . Thus, as the punch  3  rises further (i.e., stroke length=48 mm) and the press-forming of the blank sheet  4  begins at the round ends C 2 , C 3 , the punch  3  and the blank sheet  4  are still spaced apart at the blank corner center  41 C. 
     After the corner flange Fc has been completely raised at the round ends C 2 , C 3  (i.e., stroke length=80 mm), the V-shaped shoulders  3 S of the punch  3  approach the blank sheet  4  at the blank corner center  41 C. Subsequently, as the punch  3  rises further (i.e., stroke length=122 mm), the V-shaped punch shoulders  3 S raise the blank sheet  4  at the inward-facing corner  41 , and thereby the press-forming of the corner flange Fc is complete. 
     Thus, because the punch corner  31  of the punch  3  has the corner groove  6 , which varies in size in the stroke direction X (i.e. it narrows or tapers downwardly in the figures), it becomes possible to control the timing at which contact is made with the peripheral edge portion of the blank sheet  4 , as well as the timing at which the peripheral edge portion of the blank sheet  4  rises and assumes the flange shape. Specifically, the punch shoulders  3 S first raise the side flanges Fs on the outer side of the punch corner  31 ; next, the round ends C 2 , C 3 , which are the boundaries of the punch sides  32 , and the vicinities thereof contact the blank sheet  4 , and the range of contact widens (increases) toward the blank corner center  41 C. In conventional punch configurations that do not have the corner groove  6  (e.g., refer to the left image in  FIG. 18 ), while the round shape of the inward-facing corner  41  of the blank sheet  4  along the inner edge  43  is changing from radius Ro to radius R (at the conclusion of the press-forming), some of the sheet (metal) material flows outwardly (away from the blank corner center  41 C), and it is believed that this outward flow caused an increase in the line length of the portion near along the blank corner center  41 C (e.g., refer to the curved arrow in the left image in  FIG. 19 ). 
     In contrast, because the punch  3  according to the present teachings has the corner groove  6  (e.g., refer to the right image in  FIG. 18 ), a timing difference is created in the flange forming process such that the corner flange Fc forms after the side flanges Fs. In addition, because the side flanges Fs rise quickly, it becomes possible for the sheet material to flow inwardly (toward the blank corner center  41 C), and it is believed that this inward flow prevents an increase in the line length of the blank corner center  41 C (e.g., refer to the shorter curved arrow the right image in  FIG. 19 ). 
     Although the above-described embodiments explained representative examples of the present teachings in which four of the corner flanges Fc are formed along a rectangular hole having four inward-facing corners  41 , the press forming die  1  may be designed to press form at least one inward-facing corner  41 . In such embodiments, the blank sheet  4  is not limited to having a hole shape opening such as a rectangular hole. Instead, the opening may have, e.g., one or two or more of the inward-facing corners  41 , each having a shape that is curved in the longitudinal direction. 
     In addition, although the above-described embodiments explained representative examples of forming, at the inward-facing corner provided in the glass opening for an automobile sunroof, the corner flange Fc that rises from the inner edge of the inward-facing corner, the present teachings are not limited thereto. The press forming dies  1  and the press-forming methods, which use the press forming dies  1 , of the present teachings can be readily adapted to an opening provided in any of a variety of parts for a vehicle or for other than a vehicle. 
     Additional embodiments of the present teachings include, but are not limited to: 
     1. A press-forming die ( 1 ) capable of forming, on a blank sheet ( 4 ) having an inward-facing corner ( 41 ), a corner flange (Fc), which rises from an inner edge ( 43 ) of the inward-facing corner ( 41 ), and a pair of side flanges (Fs), which extend from both sides of the corner flange (Fc), the press-forming die ( 1 ) comprising: 
     a die ( 2 ) having a die corner ( 21 ), which follows along the inward-facing corner ( 41 ) of the blank sheet ( 4 ), and a pair of die sides ( 22 ), which follows along a pair of side parts ( 42 ) of the blank sheet ( 4 ); and 
     a punch ( 3 ) having a punch corner ( 31 ) and a pair of punch sides ( 32 ) that respectively cooperate with the die corner ( 21 ) and the pair of die sides ( 22 ); 
     wherein the punch corner ( 31 ) has a corner groove ( 6 ) extending along a portion at which leading surfaces ( 33 ) and a side surface ( 34 ) intersect in a stroke direction (X) of the punch ( 3 ); 
     the corner groove ( 6 ) has a shape such that its groove width (W) decreases in a direction moving away from the leading surfaces ( 33 ) in the stroke direction (X) and converges at a punch corner center (C 1 ); and 
     a shoulder radius (Rps) of punch shoulders ( 35 ), which are portions along which the corner groove ( 6 ) and the leading surfaces ( 33 ) intersect with the side surface ( 34 ), a sheet thickness (t) of the blank sheet ( 4 ), and a flange length (Ls) of the blank sheet ( 4 ) at both ends of the inward-facing corner ( 41 ) satisfy the relationship t≦Rps≦Ls. 
     2. The press-forming die ( 1 ) according to the above embodiment 1, wherein a shoulder radius (Rgs) of groove shoulders ( 6 S), which are portions along which the corner groove ( 6 ) and the leading surfaces ( 33 ) intersect, and the shoulder radius (Rps) satisfy the relationships Rps≦Rgs and Rgs≦30 mm. 
     3. The press-forming die ( 1 ) according to the above embodiment 1 or 2, wherein the positions on the leading surfaces ( 33 ), at which the punch shoulders ( 35 ) and the groove shoulders ( 6 S) intersect, are boundaries on the side surface ( 34 ) between the punch corner ( 31 ) and the punch sides ( 32 ), or outer sides thereof. 
     4. The press-forming die ( 1 ) according to any one of the above embodiments 1-3, wherein a ratio D/W of a groove depth (D) of the corner groove ( 6 ) in the stroke direction (X) to a groove width (W) of the corner groove ( 6 ) at the leading surfaces ( 33 ) is larger than 0.25. 
     5. The press-forming die ( 1 ) according to any one of the above embodiments 1-4, wherein the corner groove ( 6 ) has a pair of inclined surfaces ( 61 ,  62 ) that surround the blank corner center ( 41 C). 
     6. The press-forming die ( 1 ) according to any one of the above embodiments 1-5, wherein the blank sheet ( 4 ) is composed of an aluminum alloy sheet or a steel sheet. 
     Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved press-forming dies as well as methods of manufacturing and using the same. 
     Moreover, combinations of features and steps disclosed in the above detailed description, as well as in the experimental examples, may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     EXPLANATION OF THE REFERENCE NUMBERS 
     
         
           1  Press forming die 
           2  Die 
           21  Die corner 
           2 S Die shoulder 
           3  Punch 
           31  Punch corner 
           3 S Punch shoulder 
           4  Blank sheet 
           41  Inward-facing corner 
           5  Blank holder 
           6  Corner groove 
           6 S Groove shoulder 
         Fc Corner flange 
         Fs Side flange