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To meet the requirements for the weight reduction of automobiles, high-tensile-strength steel sheets have been applied extensively to the manufacture of auto parts. But their relatively lower press formability has obstructed their further application, and it is strongly demanded that the stretch flange ability of such steel sheets be increased, and cracks that occur when stretching edges during the press forming of such steel sheets be suppressed. Thus the authors have carried out a study to improve the stretch flange ability of high-tensile-strength steel sheets by optimizing the punching method and obtained the following conclusions. Work hardening on a pierced edge is mainly caused by deformation by the punch motion while shearing the material. By attaching a hump of certain shape at the bottom of the punch, the ductile fracture in the material caused by the punch corner is accelerated by the hump's effect to increase stress triaxiality, which reduces the extent of total deformation through shearing, thereby improving the stretch flange ability of the material.

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