A novel technology to eliminate U-bending springback of high strength steel sheet by using additional bending with counter punch

The aim of this paper is to present a new technology to eliminate springback of HSS sheets in U-bending process, where the bottom plate is additionally bent with a counter punch at the final stage of U-bending process. The U-bending process consists of four steps starting with clamping of a sheet between a punch and a counter punch, then U-bending with constant clamping force followed by bottom pushing-up by the counter punch, and final unloading process. From the experiment on 980Y HSS sheet, an appropriate combination of the clamping force and the final bottom pushing-up force that eliminates springback entirely was found. From the FE simulation of the process, it was found that the major mechanism of the springback elimination is 'spring-go' by releasing negative bending moment which has been generated by bottom pushing-up. (C) 2014 Published by Elsevier Ltd.