Abstract
In this study, a piercing method using a large chamfered die is proposed for the improvement in the hole expansion ratio. To validate this method, high-strength steels pierced by a large chamfered die were investigated. The result shows that holes pierced by the large chamfered die present a much higher hole expansion ratio than holes pierced by a conventional die, whereas a small chamfered die does not improve the hole expansion ratio. Additionally, as long as the chamfer value is above a specific value, the effect of the large chamfered die on the hole expansion ratio is independent of the chamfer value and the clearance between the punch and die. To clarify the mechanism of this improvement, the hardness close to the pierced surface and the material deformation process were analyzed. The hardness measurement indicated a decrease in the extent of work hardening when the hole was pierced by the large chamfered die. This contributes to the improvement in the hole expansion ratio. The decrease in the extent of work hardening is caused by material deformation without contact between the material and die, which leads to the suppression of shear deformation around the die edge.
