Press die designing method using an index value obtained from two stress gradients in sheet thickness direction and gradient of surface stress distribution in direction

A method for manufacturing a press die for press forming a metal sheet by bending deformation thereby forming a bending ridge line extending in a direction away from a sheared surface of the metal sheet, the method comprising:

• evaluating, at a bending position on an end face of a sheared surface of a metal sheet, two surface stress distributions among stress distributions occurring in a vicinity of the sheared surface of the metal sheet due to press forming during forming simulation, the gradients of the two surface stress distributions including:
• a gradient of a surface stress distribution in a sheet thickness direction Δσthickness;
• and
• a gradient of a surface stress distribution in a direction parallel to the direction of the bending ridge line on a bending outer surface of the metal sheet Δσin-plane;
• calculating an index value Δσcombine according to one of the following Equations (2)-(5):
• Δσcombine=(Δσthickness+Δσin-plane)/2;
• (2)
• Δσcombine=√((Δσthickness)2+(Δσin-plane)2);
• (3)
• Δσcombine=Δσthickness+Δσin-plane;
• or (4)
• Δσcombine=(a×Δσthickness+b×Δσin-plane), (5)
• wherein “a” and “b” are weighting factors, and a>b;
• measuring a deformation limit strain εlimit by subjecting the end face to two or more tests in which stress gradients occurring during deformation are different, wherein the deformation limit strain εlimit is defined as a maximum principal strain or a maximum equivalent plastic strain on the sheared surface;
• calculating a deformation limit relationship between the index value Δσcombine and the deformation limit strain εlimit according to the following Equation (6):
• εlimit=A×Δσcombine+B, (6)
• wherein “A” and “B” are material constants, and B≥0;
• and
• manufacturing a press die having a press die shape designed to minimize the principal strain or equivalent plastic strain on the sheared surface during press forming according to the deformation limit relationship, thereby suppressing occurrence of a crack on the end face of the metal sheet.