52 巻 (2011) 604 号 p. 569-573
To improve the stretch flangeability of ultra high strength steel sheets having a low ductility, flanging with a punch having gradual contact was developed. The tensile stress on the corner of the sheet flange was reduced by using angle of inclination of the punch. Although the stretch flanging ratio decreased as the angle and width of the punch inclination increased, the fracture on the corner of the sheet flange appeared at the limiting stretch flanging ratio. The large camber on the upper surface of the sheet after flanging with the punch was observed. The optimum angle and width of the punch inclination for the prevention of the occurrence of fracture were calculated using finite element simulation under a limited punch stroke. The limited flange length of the sheet with the optimum punch showed an increase of 30% for the 980 MPa grade ultra high strength steel sheet.