2014 Volume 55 Issue 638 Pages 248-253
To meet the needs for weight reduction and improvement in the crash worthiness of automobiles, it is necessary to improve the press formability of high tensile strength steel sheets. The authors have developed a piercing method that improves the stretch flange ability of steel sheets, in which steel sheets are pierced under tension using a punch with a protrusion at its bottom. The method reduces straining at the pierced edge by causing early crack growth during piercing. A study of the effect of protrusion height on the pierced edge profile, work hardening, and stretch flange ability was conducted, and the following findings were obtained. 1) As protrusion height increases, stretch flange ability increases, until it exceeds a certain height after which it decreases. 2) The change in stretch flange-ability owing to the protrusion height change can be attributed to the change in the amount of strain at the surface of the pierced edges, and in the inner part of the edge. The change in strain distribution can be associated with the change in the size of the burnish zone and to rollover. 3) The change in stretch flange ability owing to the change in protrusion height can be explained by the crack growths, based on the strain distribution in each case speculated, using fracture mechanics.