Patent Publication Number: US-2023150002-A1

Title: Press forming method and press formed product

Description:
FIELD 
     The present invention relates to a press forming method and a press formed product. In particular, the present invention relates to a press forming method for a press formed product and the press formed product, the press formed product having a top portion, a side wall portion continuous from the top portion through a punch shoulder portion, and a flange portion continuous from the side wall portion through a die shoulder portion, the press formed product including a curved portion that is concave and curved in the top view. 
     BACKGROUND 
     Press forming is a manufacturing method by which metal parts can be manufactured at low cost and in a short time, and is used to manufacture many automotive parts. In recent years, higher-strength metal sheets have been used in automotive parts to improve collision safety of the automobile and realize weight reduction of the automotive body. Main problems in press forming high-strength metal sheets include a fracture due to reduced elongation and wrinkles due to increased yield strength. 
     For example, in press forming to create, as a target shape, a press formed product  101  having a concave curved side wall portion  107  in the top view as illustrated in  FIG.  9   , a flange portion  111  in a curved portion  113  is pulled in the circumferential direction, to cause a fracture easily. In addition, deformation, which is shrinkage in the circumferential direction as the reaction force, occurs at a top portion  103  and a punch shoulder portion  105  in the curved portion  113 , to cause wrinkles easily. This deformation is called stretch flange deformation. Thus, preventing occurrence of a fracture and wrinkles in such stretch flange deformation is important in press forming the press formed product  101 . 
     As a technique for preventing a fracture and wrinkles of a press formed product that is concave and curved in the top view, Patent Literature 1, for example, discloses a method for press forming, from a metallic sheet, L-shaped parts having a top portion and a side wall portion that is connected to the top portion through a bent portion having a portion curved in an arc shape and that has a flange portion on the opposite side of the bent portion. According to the press forming method, a portion of the metallic sheet corresponding to the top portion is pressurized by a pad, an end of a portion of the metallic sheet corresponding to a lower side of the L shape of the L-shaped part is allowed to slide (in-plane movement), and the portion corresponding to the lower side of the L shape is drawn into the side wall portion side to form the side wall portion and the flange portion, thereby enabling prevention of a fracture in the flange portion and wrinkles in the top portion. 
     In addition, Patent Literature 2 discloses a press forming method for press forming a part having a hat shaped or U-shaped sectional shape and having a curved portion curved along the longitudinal direction and straight side portions connecting to both ends of the curved portion. According to the press forming method, a fracture due to stretch flange deformation can be prevented by causing a material movement to mitigate tensile deformation in the circumferential direction that occurs in the flange portion of the curved portion. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 5168429 
     Patent Literature 2: Japanese Patent No. 6028956 
     SUMMARY 
     Technical Problem 
     In the technique disclosed in Patent Literature 1, as illustrated in  FIG.  10   , the movement directions of a material of a blank (metal sheet) flowing out from the top portion  103  to the flange portion  111  side do not coincide with the direction (circumferential dotted line) in which the material is pulled by the stretch flange deformation. When the material movements of the blank are decomposed into vectors in two directions as illustrated in  FIG.  11   , the movements (solid black arrows in the figure) from the end sides of the curved portion  113  toward the center are effective in preventing a fracture of the flange portion  111  due to stretch flange deformation, while the movements (black broken-line arrows in the figure) from the top portion  103  toward the side wall portion  107  do not contribute to preventing stretch flange deformation. Furthermore, even for the material movements toward the center of the curved portion  113 , there is concern that material movements (white painted arrows in the figure) near the center of the curved portion  113  may induce wrinkles near the top portion  103  and the punch shoulder portion  105 . 
     Although the technique disclosed in Patent Literature 1 prevents wrinkles in the top portion by pressurizing the portion of the metallic sheet corresponding to the top portion with the pad, a higher-strength steel sheet requires a greater load of the pad that pressurizes wrinkles, and there is concern that a pressure generating device such as a gas cylinder installed inside a die may expand to immense size. As a result, problems arise in that space to install the pad may not be provided in the die or upsizing the die increase costs. Also, when the technique of Patent Literature 1 is applied to press forming of the press formed product  101  illustrated in  FIG.  9   , wrinkles cannot be prevented at the punch shoulder portion  105  because the punch shoulder portion  105  cannot be pressurized with the pad. 
     Furthermore, according to the techniques disclosed in Patent Literatures 1 and 2, in a press formed product in which a bead shape needs to be given to the top portion  103  in the curved portion  113 , for example, the material in the portion corresponding to the top portion  103  in the curved portion  113  illustrated in  FIG.  11    may fail to move to the flange portion  111  where stretch flange deformation occurs, so that it is difficult to prevent wrinkles. 
     The present invention is made in view of the above problems, and an object of the present invention is to provide a press forming method and a press formed product, the press forming method by which, in the press formed product that has a top portion, a side wall portion, and a flange portion and that is concave and curved in the top view, a fracture can be prevented in the flange portion where stretch flange deformation occurs and wrinkles can be prevented in the top portion and the punch shoulder portion on the flange portion side. 
     Solution to Problem 
     A press forming method according to a first aspect of the present invention includes: press forming a press formed product including a top portion, a side wall portion continuous from the top portion through a punch shoulder portion, and a flange portion continuous from the side wall portion through a die shoulder portion, the press formed product including a curved portion that is concave and curved in a top view; and increasing a bending radius of the punch shoulder portion in the curved portion from a center of a curve toward an end side of the curve. 
     A bending radius of the die shoulder portion in the curved portion may be decreased from the center of the curve toward the end side of the curve. 
     A minimum bending radius of the die shoulder portion may be made smaller than a minimum bending radius of the punch shoulder portion. 
     A rotational motion restraining shape portion that restrains rotational motion of a blank during a press forming process may be formed on the top portion on the end side of the curve. 
     A flange width of the flange portion in the curved portion may be made wider in the center of the curve than on the end side of the curve. 
     A blank to be used for press forming of the press formed product may be a metal sheet having a tensile strength of 440 MPa grade or higher to 1600 MPa grade or lower. 
     A press forming method according to a second aspect of the present invention includes press forming an intermediate formed product to have a target shape, the intermediate formed product being a press formed product press formed by the press forming method according to the first aspect of the present invention, wherein the intermediate formed product has a bending radius of the punch shoulder portion on the end side of the curved portion larger than the target shape does. 
     A press formed product according to the present invention includes: a top portion; a side wall portion continuous from the top portion through a punch shoulder portion; a flange portion continuous from the side wall portion through a die shoulder portion; and a curved portion that is concave and curved in a top view, wherein a bending radius of the punch shoulder portion in the curved portion is increased from a center of a curve toward an end side of the curve. 
     A bending radius of the die shoulder portion in the curved portion may be decreased from the center of the curve toward the end side of the curve. 
     A minimum bending radius of the die shoulder portion may be smaller than a minimum bending radius of the punch shoulder portion. 
     A rotational motion restraining shape portion that restrains rotational motion of a blank during a press forming process may be formed on the top portion on the end side of the curve. 
     A width of the flange portion in the curved portion may be wider in the center of the curve than on the end side of the curve. 
     The press formed product may be obtained by press forming a metal sheet having a tensile strength of 440 MPa grade or higher to 1600 MPa grade or lower. 
     Advantageous Effects of Invention 
     According to the present invention, a material can be moved from the top portion on the end sides of the curve toward the flange portion in the center of the curve, which can prevent a fracture in the flange portion of the curved portion and can also prevent wrinkles in the top portion and the punch shoulder portion on the flange portion side of the curved portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a press formed product to be formed in a press forming method according to an embodiment of the present invention ((a) perspective view, (b) top view). 
         FIG.  2    is a diagram explaining a reason why a fracture can be prevented by the press forming method according to the embodiment of the present invention ((a) top view, (b) sectional view of a curve on the end side, (c) sectional view of the center of the curve). 
         FIG.  3    is a diagram illustrating an example of a press formed product to be formed in a press forming method according to another aspect of the embodiment of the present invention ((a) perspective view, (b) top view). 
         FIG.  4    is a diagram explaining a reason why wrinkles can be prevented by the press forming method according to the another aspect of the embodiment of the present invention ((a) top view, (b) sectional view of a curve on the end side, (c) sectional view in the center of the curve). 
         FIG.  5    is a diagram illustrating an example of a press formed product that is to be formed in the press forming method according to the embodiment of the present invention, and the shape of which is formed by a side wall portion formed on a blank end to restrain rotational motion in the horizontal plane parallel to the top portion. 
         FIG.  6    is a diagram illustrating another example of the press formed product that is to be formed in the press forming method according to the embodiment of the present invention and the shape of which is formed by a bead to restrain rotational motion in the horizontal plane parallel to the top portion. 
         FIG.  7    is a diagram illustrating an example of a press formed product that is to be formed in the press forming method according to the embodiment of the present invention and that has a wider flange width of a flange portion in the center of the curve. 
         FIG.  8    is a diagram explaining a reason why a fracture can be prevented in the press formed product having a wider flange of the flange portion in the center of the curve by the press forming method according to the embodiment of the present invention. 
         FIG.  9    is a diagram explaining a fracture and wrinkles that occur in press forming of a press formed product having a portion that is concave and curved in the top view. 
         FIG.  10    is a diagram explaining a mechanism by which a fracture occurs in the process of press forming a press formed product that is concave and curved in the top view. 
         FIG.  11    is a diagram explaining material movements in press forming of a press formed product that is concave and curved in the top view. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A press forming method and a press formed product according to an embodiment of the present invention will be described below with reference to  FIG.  1    to  FIG.  8   . 
     As illustrated in  FIG.  1    as an example, the press forming method and the press formed product according to the present embodiment press form a press formed product  1  that has a top portion  3 , a side wall portion  7  continuous from the top portion  3  through a punch shoulder portion  5 , and a flange portion  11  continuous from the side wall portion  7  through a die shoulder portion  9 , and that includes a curved portion  13  that is concave and curved in the top view and straight portions  15  extending straight from both ends (broken lines in  FIG.  1   ) of a curve in the curved portion  13 . The punch shoulder portion  5  in the curved portion  13  has a bending radius that increases from the center of a curve toward the end sides of the curve. 
     Reasons why both a fracture of the flange portion  11  and wrinkles of the top portion  3  and the punch shoulder portion  5  in the curved portion  13  of the press formed product  1  can be prevented in the press forming method and the press formed product according to the present embodiment will be described with reference to  FIG.  2    that schematically illustrates material movements during a press forming process. The broken lines in  FIG.  2    indicate both ends of the curve (boundaries between the curved portion  13  and the straight portions  15 ). 
     The punch shoulder portion  5  has a bending radius that increases from the center (B-B′ section) of the curve toward the end side (A-A′ section) of the curve in such a manner that a bending radius R p,1  at the end of the curve ( FIG.  2 ( b ) ) is larger than a bending radius R p,2  in the center of the curve ( FIG.  2 ( c ) ). Thus, as indicated by the black arrows in  FIG.  2   , the material easily moves along the ridge direction (perpendicular to the paper) from the end sides toward the center of the curve of the punch shoulder portion  5 , and is drawn into the flange portion  11  side in the center of the curve. In contrast, the center of the curve of the punch shoulder portion  5  has a smaller bending radius than the end sides of the curve, so the material is less likely to move in the ridge direction during the press forming process. 
     Thus, during the press forming process of the press formed product  1 , the material can be moved from the top portion  3  on the end side of the curve toward the flange portion  11  in the center of the curve, which can prevent a fracture of the flange portion  11  in the curved portion  13  and can also prevent wrinkles in the top portion  3  and the punch shoulder portion  5  of the curved portion  13 . 
     In the press forming method according to the present embodiment, the minimum bending radius of the die shoulder portion  9  is preferably smaller than the minimum bending radius of the punch shoulder portion  5 . The minimum bending radius of the die shoulder portion  9  is the smallest bending radius of the die shoulder portion  9  in the curved portion  13 . In the press formed product  1 , the minimum bending radius of the die shoulder portion  9  is R d  because the bending radius R d  of the die shoulder portion  9  is constant from the center of the curve to the end sides of the curve, as illustrated in  FIG.  2   . Meanwhile, the minimum bending radius of the punch shoulder portion  5  is the smallest bending radius of the punch shoulder portion  5  in the curved portion  13 . In the press formed product  1 , the minimum bending radius of the punch shoulder portion  5  is the bending radius R p,2  in the center of the curve because the bending radius of the punch shoulder portion  5  increases from the center of the curve toward the end sides of the curve, as illustrated in  FIG.  2   . In this manner, by making the minimum bending radius (=R d ) of the die shoulder portion  9  smaller than the minimum bending radius (=R p,2 ) of the punch shoulder portion  5 , the material movements at the die shoulder portion  9  can be restrained during the press forming process; accordingly the material can be drawn strongly from the top portion  3  side to the flange portion  11  side during the press forming process. As a result, a fracture of the flange portion  11  and wrinkles of the top portion  3  and the punch shoulder portion  5  can be further prevented in the center of the curve. 
     The press forming method and the press formed product according to the present embodiment is such that the bending radius of the punch shoulder portion  5  in the curved portion  13  is increased from the center of the curve toward the ends of the curve, as in the press formed product  1  illustrated in  FIG.  1   . The press forming method and the press formed product according to another aspect of the present embodiment may be such that the bending radius of a punch shoulder portion  25  in a curved portion  33  is increased from the center of the curve toward the end sides of the curve, and that the bending radius of a die shoulder portion  29  in the curved portion  33  is decreased from the center of the curve toward the end sides of the curve in the curved portion  33 , as in a press formed product  21  illustrated in  FIG.  3    as an example. 
     The operation and effect of decreasing the bending radius of the die shoulder portion  29  from the center of the curve toward the end sides of the curve in the curved portion  33  will be described with reference to  FIG.  4    that schematically illustrates material movements during a press forming process of the press formed product  21 . The broken lines in  FIG.  4    indicate both ends of the curve (boundaries between the curved portion  33  and straight portions  35 ). 
     The die shoulder portion  29  has a bending radius R d,1  on the end side (A-A′ section) of the curve ( FIG.  4 ( b ) ) that is smaller than a bending radius R d,2  in the center (B-B′ section) of the curve ( FIG.  4 ( c ) ). This facilitates material movements toward a curved concave portion (D illustrated in  FIG.  4 ( a ) , the root of a flange portion  31  joining to a side wall portion  27  in the curve) in stretch flange deformation, and also inhibits a material movement that pulls in the circumferential direction of the curve in the flange portion  31 , thereby enabling prevention of a flange fracture. 
     As a result, a fracture of the flange portion  31  in the curved portion  33  can be further prevented during the press forming process of the press formed product  21 . 
     In the press forming method and the press formed product according to another aspect of the present embodiment, the minimum bending radius of the die shoulder portion  29  in the curved portion  33  is preferably smaller than the minimum bending radius of the punch shoulder portion  25 . The minimum bending radius of the die shoulder portion  29  is the smallest bending radius of the die shoulder portion  29  in the curved portion  33 , and is the bending radius R d,1  on the end side of the curve, as illustrated in  FIG.  4 ( b ) . Meanwhile, the minimum bending radius of the punch shoulder portion  25  is the smallest bending radius of the punch shoulder portion  25  in the curved portion  33 , and is the bending radius R p,2  in the center of the curve, as illustrated in  FIG.  4 ( c ) . In this manner, by making the minimum bending radius (=R d,1 , of the die shoulder portion  29  smaller than the minimum bending radius (=R p,2 ) of the punch shoulder portion  25 , the bending radius R p,1  on the end side of the curve of the punch shoulder portion  25  is even larger than the minimum bending radius R p,2  (in the center of the curve) of the punch shoulder portion  25 , so that the material can be drawn strongly from a top portion  23  side to the flange portion  31  side during the press forming process. As a result, a fracture of the flange portion  31  and wrinkles of the top portion  23  and the punch shoulder portion  25  can be further prevented in the curved portion  33 . 
     In the above description, the bending radius of the punch shoulder portion  5  has been changed as illustrated in  FIG.  1   , and the bending radii of the punch shoulder portion  25  and the die shoulder portion  29  have also been changed as illustrated in  FIG.  3   . The press forming method and the press formed product according to the present invention may form a rotational motion restraining shape portion  57  in a top portion  43  on the end sides of a curved portion  53  and straight portions  55 , the rotational motion restraining shape portion  57  restraining the rotational motion of the blank in the horizontal plane parallel to the top portion  43  during the press forming process, like a press formed product  41  illustrated in  FIG.  5   . 
     The rotational motion restraining shape portion  57  is a bent shape formed between a side wall portion  59  and the top portion  43 , the side wall portion  59  continuing to the opposite side of a side wall portion  47  of the straight portions  55 . A punch shoulder portion  45  is formed so that the bending radius increases from the center of the curve toward the end sides, like the punch shoulder portion  5  of the press formed product  1  described above. 
     In this manner, forming the rotational motion restraining shape portion  57  during the press forming process enables the material to be moved from the top portion  43  of the end sides of the curve and the straight portions  55  to a flange portion  51  in the center of the curve through the punch shoulder portion  45 , while the rotational motion of the blank in the horizontal plane parallel to the top portion  43  is restrained during the press forming process. In addition, wrinkles of the top portion  43  and the punch shoulder portion  45  can be sufficiently prevented in the curved portion  53 . 
     The present invention is not limited to the rotational motion restraining shape portion  57  having the shape illustrated in  FIG.  5   , and a shape may be possible that can restrain the rotational motion of the blank in the horizontal plane parallel to the top portion  43  during the press forming process, like a rotational motion restraining shape portion  63  having a bead shape formed on the top portion  43  of a press formed product  61  in  FIG.  6   . In a case where a bead is formed on the top portion  43 , the shape of the bead is not limited to being concave like the rotational motion restraining shape portion  63 , but may also be convex. 
     Although the rotational motion restraining shape portion  57  illustrated in  FIG.  5    and the rotational motion restraining shape portion  63  illustrated in  FIG.  6    are formed from the ends of the curved portion  53  to the straight portions  55 , this does not exclude forming a rotational motion restraining shape portion only at the ends of the curved portion  53  or only at the straight portions  55  for the position where the rotational motion restraining shape portion is formed. 
     Furthermore, the press forming method and the press formed product according to the present invention may have a wider flange width of a flange portion  81  in the center of a curved portion  83  than on the end sides of the curve, as in a press formed product  71  illustrated in  FIG.  7   , in addition to the bending radius of the punch shoulder portion and the die shoulder portion described above. 
     The operation and effect of press forming the press formed product  71  illustrated in  FIG.  7    are as follows. For example, the press formed product  1  described above is obtained by press forming a blank  91  having such a shape as the shape of the press formed product  1  when press formed, as illustrated in  FIG.  8 ( a ) . In this case, a flange equivalent portion  93  in the blank  91  becomes the flange portion  11  ( FIG.  1   ) of the press formed product  1 . 
     In contrast, the press formed product  71  illustrated in  FIG.  7    is press formed using a blank  95  with increased excess metal (hatched area in the figure) in a flange equivalent portion  97  corresponding to the flange portion  81 , as illustrated in  FIG.  8 ( b ) . 
     When the blank  95  is used to press form the press formed product  71 , the material of the flange portion  81  in the curved portion  83  is less stretchable, so that the material insufficient to form the flange portion  81  is drawn in from a top portion  73  side through a punch shoulder portion  75  and a side wall portion  77 . As a result, there is more material toward the center of the curve in the curved portion  83 , and a fracture of the flange portion  81  can be further prevented. 
     When the flange width of the flange portion  81  in the curved portion  83  is made wider in the center of the curve than on the end sides of the curve, as in the press formed product  71  illustrated in  FIG.  7   , the maximum width of the flange in the center of the curve is preferably 1.1 to 1.5 times the flange width on the end sides. 
     If the flange width in the center of the curve is less than 1.1 times the flange width on the end sides, the force that draws the material from the top portion  73  to the flange portion  81  side during the press forming process does not increase much. If the flange width in the center of the curve is more than 1.5 times the flange width on the end sides, the flange width of the flange portion  81  is too wide and becomes an obstacle when joined to other parts, so the flange portion  81  is cut off to make the flange width narrower in a downstream step, which increases the number of work steps and further reduces the yield rate. 
     The press forming method and the press formed product according to the embodiment of the present invention are intended to form the press formed product  1  having the straight portions  15  extending from both ends of the curve of the curved portion  13 , as illustrated in  FIG.  1   , for example. The present invention may press form a press formed product having only a curved portion or a press formed product having a straight portion extending from only one end of the curve, and it does not matter whether there is a straight portion. 
     In the above description, the bending radius of the punch shoulder portion is increased from the center of the curve toward both end sides, but may be increased from the center of the curve toward either end side. 
     Similarly, the curvature radius of the curve of the die shoulder portion is decreased from the center of the curve to both end sides, but may be decreased toward either end side. 
     The above description has illustrated by example the specific aspects of the present invention on the basis of the embodiment of the present invention. The present invention also includes, for example, the press formed product  1  illustrated in  FIG.  1    as an intermediate formed product, which is then press formed into a target shape. The press formed product  1  serving as the intermediate formed product should have a bending radius of the punch shoulder portion  5  on the end sides of the curved portion  13  larger than the target shape does. In other words, the present invention includes a press forming method made up of two steps: a step of press forming the press formed product  1 , which is an intermediate formed product; and a step of press forming the press formed product  1  into a press formed product having a target shape. 
     Even if a fracture of the flange portion  111  and wrinkles of the top portion  103  and the punch shoulder portion  105  occur in the curved portion  113  of the press formed product  101  when the press formed product  101  with the target shape of the bending radius of the conventional punch shoulder portion  105  illustrated in  FIG.  9    is press formed at a single step, according to the press forming method of the present invention, the press formed product  101  having a target shape can be obtained while a fracture of the flange portion  111  and wrinkles of the top portion  103  and the punch shoulder portion  105  are prevented in the curved portion  113 . 
     Likewise, the present invention includes a press forming method in which, for example, the press formed product  21  illustrated in  FIG.  3    as an intermediate formed product and the intermediate formed product is press formed into the press formed product  101  ( FIG.  9   ) having a target shape, the press formed product  21 , which is an intermediate formed product, has a bending radius of the die shoulder portion  29  in the center of the curved portion  33  larger than the target shape has. In other words, the present invention includes a press forming method made up of two steps: a step of press forming the press formed product  21 , which is an intermediate formed product; and a step of press forming the press formed product  21  into the press formed product  101  having a target shape. 
     In this case also, a fracture of the flange portion  111  and wrinkles of the top portion  103  and the punch shoulder portion  105  still occur in the curved portion  113  when the press formed product  101  with the target shape of the bending radius of a conventional die shoulder portion  109  is formed at a single step. According to the present invention, however, the press formed product  101  having a target shape can be obtained while a fracture of the flange portion  111  and wrinkles of the top portion  103  and the punch shoulder portion  105  are further prevented in the curved portion  113 . 
     The press forming method and the press formed products according to the present invention have no particular restrictions on the type of metallic sheet used as the blank material, but can be preferably applied when a metal sheet having low elongation is used. Specifically, a metal sheet having a tensile strength of 440 MPa grade or higher to 1600 MPa grade or lower and a sheet thickness of 0.5 mm or higher to 3.6 mm or lower is preferably used. 
     A metal sheet having a tensile strength of less than 440 MPa grade is less likely to have a fracture due to stretch flange deformation because of its high elongation, so there is little advantage in using the present invention. However, if a part shape is difficult to press form, using the present invention is preferable even if the metal sheet has a tensile strength of less than 440 MPa grade. Although there is no particular upper limit to the tensile strength, a metal sheet exceeding the 1600 MPa grade has poor elongation and is prone to a fracture in the punch shoulder portion and the die shoulder portion, making press forming difficult. 
     The press forming method and the press formed product according to the present invention can also prevent a fracture of metal sheets due to stretch flange deformation in automotive parts having L-, T-, Y-, or S-shaped portions that are curved in the top view. As specific examples, the present invention can preferably be applied to cases where an A-pillar lower having an L-shaped part, a B-pillar having a T-shaped part, a rear side member having an S-shaped part, and the like. 
     EXAMPLES 
     Specific press forming experiments have been conducted, and the operation and effect of the press forming method according to the present invention will be described below. 
     In the press forming experiments, steel sheets having the material properties illustrated in Table 1 were used as blanks, the press formed product  1  ( FIG.  1   ), the press formed product  21  ( FIG.  3   ), the press formed product  41  ( FIG.  5   ), and the press formed product  61  ( FIG.  6   ) illustrated in the embodiment describe above were used as press forming objects, and went through crash forming, to provide invention examples. 
     The radius of the curve of the curved portion in the height center of the side wall portion of each press formed product was 153 mm, the smallest bending radius of the punch shoulder portion in the curved portion was 7 mm, the smallest bending radius of the die shoulder portion was 6 mm, a side wall height of the side wall portion in the press forming direction was 60 mm. The flange width of the flange portion was 30 mm in the center of the curve and 25 mm on the end sides of the curve for the press formed product  1 , and was 10 mm in the center of the curve and 25 mm on the end sides of the curve for the press formed products  21 ,  41 , and  61 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Sheet 
                 Yield 
                 Tensile 
                   
               
               
                   
                 thickness/mm 
                 strength/MPa 
                 strength/MPa 
                 Stretch/% 
               
               
                   
                   
               
             
            
               
                   
                 1.6 
                 880 
                 1210 
                 13 
               
               
                   
                   
               
            
           
         
       
     
     Tables 2 and 3 illustrate the bending radius R p  of the punch shoulder portion and the bending radius R d  of the die shoulder portion in the press formed products used as press forming objects in the press forming experiments. The ratios of the bending radius R p  of the punch shoulder portion illustrated in Tables 2 and 3 are the ratios of the largest bending radius to the smallest bending radius in the ridge direction of the punch shoulder portion. Likewise, the ratios of the bending radius R d  of the die shoulder portion illustrated in Tables 2 and 3 are the ratios of the smallest bending radius to the largest bending radius in the ridge direction of the die shoulder portion. 
     Furthermore, in the press forming experiments, as a comparison object, the press formed product  101  was set to Conventional Example 1, the press formed product  101  being press formed by holding the portion corresponding to the top portion  103  of the press formed product  101  illustrated in  FIG.  9    with a pad while allowing rotational motion of the blank in the horizontal plane parallel to the top portion  103  by following the method disclosed in the Patent Literature 1. 
     In Conventional Example 1, the curvature radius of the curve of the curved portion  113 , the bending radius of the die shoulder portion  109 , and the side wall height of the side wall portion  107  were the same as for the press formed product according to the invention examples. In Conventional Example 1, the smallest bending radius of the punch shoulder portion  105  was set constant (=7 mm) in the ridge direction. 
     Then, a fracture and wrinkles were evaluated in each of the press formed products according to the invention examples and the conventional examples. To evaluate a fracture, a sheet thickness reduction rate was calculated by dividing the difference between the sheet thickness of the blank and the sheet thickness of the tip (for example, part C illustrated in  FIG.  2   ) of the flange portion at the bottom of the concave portion in the curved portion by the sheet thickness of the blank. A smaller value was evaluated to be the better for fracture prevention. To evaluate wrinkles, sensory evaluation of the top portion and the punch shoulder portion in the curved portion was performed visually. A “X” was given when there were significant wrinkles, a “Δ” when there were minute wrinkles that can be visually confirmed but is acceptable in terms of part performance, and a “◯” when wrinkles could not be visually confirmed. Tables 2 and 3 illustrate the results of a fracture and wrinkles evaluated for each press formed product. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Conventional 
                 Invention 
                 Invention 
                 Invention 
                 Invention 
                 Invention 
                 Invention 
               
               
                   
                 Example 1 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Details 
                 Rotational 
                 Bending radius 
                 ← 
                 ← 
                 Bending radius R p   
                 ← 
                 ← 
               
               
                   
                 motion of 
                 R p  of the 
                   
                   
                 of the punch 
               
               
                   
                 the blank 
                 punch shoulder 
                   
                   
                 shoulder portion 
               
               
                   
                 was allowed 
                 portion was 
                   
                   
                 and the bending 
               
               
                   
                 during 
                 increasing in 
                   
                   
                 radius R d  of the 
               
               
                   
                 press 
                 the curved 
                   
                   
                 die shoulder 
               
               
                   
                 forming 
                 portion 
                   
                   
                 portion were 
               
               
                   
                   
                   
                   
                   
                 increasing in the 
               
               
                   
                   
                   
                   
                   
                 curved portion 
               
               
                 Ratio of the 
                 1.0 
                 1.1 
                 1.5 
                 2.0 
                 1.5 
                 ← 
                 ← 
               
               
                 bending radius 
               
               
                 R p  of the punch 
               
               
                 shoulder portion 
               
               
                 Ratio of the 
                 1.0 
                 ← 
                 ← 
                 ← 
                 0.9 
                 0.75 
                 0.5 
               
               
                 bending radius 
               
               
                 R d  of the die 
               
               
                 shoulder portion 
               
               
                 Fracture 
                 18 
                 17   
                 14   
                 12   
                 13 
                 12 
                 10 
               
               
                 evaluation 
               
               
                 (sheet thickness 
               
               
                 reduction rate 
               
               
                 (%)) 
               
               
                 Sensory 
                 Δ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 evaluation for 
               
               
                 wrinkles 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 No. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Invention 
                 Invention 
                 Invention 
                 Invention 
                 Invention 
                 Comparative 
               
               
                   
                 Example 7 
                 Example 8 
                 Example 9 
                 Example 10 
                 Example 11 
                 Example 1 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Details 
                 Bending radius R p   
                 ← 
                 Excess metal 
                 Excess metal 
                 Excess metal 
                 Bending 
               
               
                   
                 of the punch 
                   
                 was given to 
                 was given to 
                 was given to 
                 radius R p  of 
               
               
                   
                 shoulder portion 
                   
                 the blank of 
                 the blank of 
                 the blank of 
                 the punch 
               
               
                   
                 and the bending 
                   
                 Invention 
                 Invention 
                 Invention 
                 shoulder 
               
               
                   
                 radius R d  of the 
                   
                 Example 2 
                 Example 5 
                 Example 7 + 
                 portion in 
               
               
                   
                 die shoulder 
                   
                   
                   
                 rotational 
                 the entire 
               
               
                   
                 portion were 
                   
                   
                   
                 motion of 
                 part was 
               
               
                   
                 increasing in the 
                   
                   
                   
                 the blank 
                 uniformly 
               
               
                   
                 curved portion + 
                   
                   
                   
                 was 
                 increased 
               
               
                   
                 rotational motion 
                   
                   
                   
                 restrained 
                 (1.5 times) 
               
               
                   
                 of the blank was 
               
               
                   
                 restrained 
               
               
                 Ratio of the 
                 1.5 
                 ← 
                 ← 
                 ← 
                 ← 
                 1.0 
               
               
                 bending radius R p   
               
               
                 of the punch 
               
               
                 shoulder portion 
               
               
                 Ratio of the 
                 0.75 
                 ← 
                 1.0 
                 0.75 
                 ← 
                 1.0 
               
               
                 bending radius R d   
               
               
                 of the die 
               
               
                 shoulder portion 
               
               
                 Fracture 
                 14 
                 14 
                 9 
                 7 
                 9 
                 9 
               
               
                 evaluation 
               
               
                 (sheet thickness 
               
               
                 reduction rate 
               
               
                 (%)) 
               
               
                 Sensory evaluation 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 for wrinkles 
               
               
                   
               
            
           
         
       
     
     In Conventional Example 1, the sheet thickness reduction rate was as large as 18% and minute wrinkles occurred. 
     In Invention Example 1, the press formed product  1  was press formed with the bending radius R p  of the punch shoulder portion  5  increased at a ratio of 1.1 from the center of the curve toward the end sides of the curve. As illustrated in Table 2, the sheet thickness reduction rate was 17%, which was less than that of Conventional Example 1, and no wrinkles were observed. 
     In Invention Example 2, the ratio of the bending radius R p  of the punch shoulder portion  5  was set at 1.5, which was larger than that of Invention Example 1. As illustrated in Table 2, the sheet thickness reduction rate was 14%, which was less than that of Invention Example 1, and no wrinkles was observed. 
     In Invention Example 3, the press formed product  1  was press formed with the bending radius R p  of the punch shoulder portion  5  increased from Invention Example 2 at a ratio of 2.0. As illustrated in Table 2, the sheet thickness reduction rate was 12%, which was much less than that of Invention Example 2, and no wrinkles were observed. 
     In Invention Example 4, the press formed product  21  was pressed with the bending radius R p  of the punch shoulder portion  25  increased at a ratio of 1.5 from the center of the curve toward the end sides of the curve and the bending radius R d  of the die shoulder portion  29  decreased at a ratio of 0.9 from the center of the curve toward the end sides of the curve. As illustrated in Table 2, the sheet thickness reduction rate was 13%, which was less than that of Invention Example 2 the ratio of the bending radius R p  of the punch shoulder portion  25  of which was the same, and no wrinkles were observed. 
     In Invention Example 5, the press formed product  21  was press formed with the bending radius R d  of the die shoulder portion  29  decreased from Invention Example 4 at a ratio of 0.75. As illustrated in Table 2, the sheet thickness reduction rate was 12%, which was less than that of Invention Example 4, and no wrinkles were observed. 
     In Invention Example 6, the ratio of the bending radius R d  of the die shoulder portion  29  was set at 0.5, which was much less than that of Invention Example 5. As illustrated in Table 2, the sheet thickness reduction rate was 10%, which was less than that of Invention Example 5, and no wrinkles were observed. 
     In Invention Example 7, the press formed product  41  was press formed with the bending radius R p  of the punch shoulder portion  45  set at a ratio of 1.5, the bending radius of a die shoulder portion  49  set at a ratio of 0.75, and the rotational motion restraining shape portion  57  formed between the top portion  43  and the side wall portion  59 . As illustrated in Table 3, the sheet thickness reduction rate was 14%, which was less than that of Conventional Example 1, and no wrinkles were observed. 
     In Invention Example 8, the press formed product  61  was press formed with the ratios of the bending radius R p  of the punch shoulder portion  45  and the bending radius of the die shoulder portion  49  set equal to those of Invention Example 7, and the rotational motion restraining shape portion  63  having a bead shape formed from the end sides of the curve to the straight portions in the top portion  43 . As illustrated in Table 3, the sheet thickness reduction rate was 14%, which was less than that of Conventional Example 1, and no wrinkles were observed. 
     In Invention Example 9, the press formed product  71  was press formed using the blank  91  having a shape in which excess metal was given to the flange equivalent portion  93 , with the bending radius of the punch shoulder portion  75  set at a ratio of 1.5 and the flange width of the flange portion  81  in the center of the curved portion  83  set at 1.5 times the flange width (=25 mm) on the end sides of the curve. As illustrated in Table 3, the sheet thickness reduction rate was reduced to 9%, which was good, and no wrinkles were observed. 
     In Invention Example 10, the press formed product  21  was press formed using the blank  91  having a shape in which excess metal was given to the flange equivalent portion  93 , with the bending radius of the punch shoulder portion  5  set at a ratio of 1.5 and the bending radius R d  of the die shoulder portion set at a ratio of 0.75, as in Invention Example 5. As illustrated in Table 3, the sheet thickness reduction rate was 7%, which was much less than that of Invention Example 9, and no wrinkles were observed. 
     In Invention Example 11, the press formed product  21  was press formed using the blank  91  having a shape in which excess metal was given to the flange equivalent portion  93 , with the bending radius of the punch shoulder portion  5  set at a ratio of 1.5, the bending radius R d  of the die shoulder portion set at a ratio of 0.75, as in Invention Example 7, and the rotational motion restraining shape portion  57  formed. As illustrated in Table 3, the sheet thickness reduction rate was reduced to 9%, which was good, and no wrinkles were observed. 
     An example of the press formed product  101  in which the bending radius R p  of the punch shoulder portion  105  was uniformly increased as compared with that of Conventional Example 1 is illustrated in Table 3 as Comparative Example 1. In Comparative Example 1, the bending radius of the ridge of the entire punch shoulder portion  105  in Conventional Example 1 was increased by 1.5 times to a constant 10.5 mm. As a result, the sheet thickness reduction rate was 9%, which was good, but significant wrinkles problematically occurred. 
     It has been demonstrated that the press forming method and the press formed products can prevent a fracture of the flange portion in the curved portion and prevent wrinkles of the top portion and the punch shoulder portion in the curved portion. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, a press forming method and a press formed product can be provided, the press forming method by which, in the press formed product that has a top portion, a side wall portion, and a flange portion and that is concave and curved in the top view, a fracture can be prevented in the flange portion where stretch flange deformation occurs and wrinkles can be prevented in the top portion and the punch shoulder portion on the flange portion side. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Press formed product 
               3  Top portion 
               5  Punch shoulder portion 
               7  Side wall portion 
               9  Die shoulder portion 
               11  Flange portion 
               13  Curved portion 
               15  Straight portion 
               21  Press formed product 
               23  Top portion 
               25  Punch shoulder portion 
               27  Side wall portion 
               29  Die shoulder portion 
               31  Flange portion 
               33  Curved portion 
               35  Straight portion 
               41  Press formed product 
               43  Top portion 
               45  Punch shoulder portion 
               47  Side wall portion 
               49  Die shoulder portion 
               51  Flange portion 
               53  Curved portion 
               55  Straight portion 
               57  Rotational motion restraining shape portion 
               59  Side wall portion 
               61  Press formed product 
               63  Rotational motion restraining shape portion 
               71  Press formed product 
               73  Top portion 
               75  Punch shoulder portion 
               77  Side wall portion 
               79  Die shoulder portion 
               81  Flange portion 
               83  Curved portion 
               85  Straight portion 
               91  Blank 
               93  Flange equivalent portion 
               95  Blank 
               97  Flange equivalent portion 
               101  Press formed product 
               103  Top portion 
               105  Punch shoulder portion 
               107  Side wall portion 
               109  Die shoulder portion 
               111  Flange portion 
               113  Curved portion 
               115  Straight portion