Improvement of Stretch Flangeability of High-Tensile-Strength Steel Sheets by Piercing under Tension Using Humped Bottom Punch

To meet the requirements for 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 been obstructing their further application, and so it is recommended that the stretch flangeability of such steel sheets be increased, and cracks that occur when stretching the edges during the press forming of such steel sheets be suppressed. Thus the authors have conducted a study to improve the stretch flangeability of high-tensile-strength steel sheets by optimizing the punching method and obtained the following conclusion.

Work hardening beneath the pierced edge is mainly caused by deformation due to the punch motion while it is shearing the material. By attaching a hump of a certain shape at the bottom of the punch, ductile fracture in the material adjacent to the punch corner is accelerated by the hump's effect to increase stress triaxiality, which reduces the extent of total deformation which takes place during the shearing process, thereby improving the stretch flangeability of the material.