Elimination of springback of high-strength steel sheet by using additional bending with counter punch

This study proposes a new technique to eliminate the springback of high-strength steel (HSS) sheets in the U-bending process. In this technique, the bottom plate is additionally bent with a counter punch at the final stage of the U-bending process. In an experiment, a flat-headed punch with a shallow hollow on the punch head was employed in combination with a counter punch. The U-bending process consists of four sequential steps: (1) clamping of a sheet between a punch and a counter punch, (2) U-bending under a constant clamping force, (3) bottom pushing-up by the counter punch, and (4) final unloading process. From the experiment on a 980Y HSS sheet, an appropriate combination of the clamping force and the final bottom pushing-up force that eliminates springback entirely was found. To investigate the mechanism of the reduction in springback in this process, an FE simulation of this process was conducted using the Yoshida-Uemori model, an advanced kinematic hardening model. The simulation result showed that the major mechanism of springback elimination is the “spring-go” induced by the release of the negative bending moment generated by bottom pushing-up.