Double-action bending for eliminating springback in hat-shaped bending of advanced high-strength steel sheet

This paper presents a novel technology called ‘double-action bending (DAB)’ to eliminate springback in hat-shape forming of advanced high-strength steel sheets (AHSS). It consists of four steps: (1) clamping of a sheet between an inner punch and a counter punch, (2) U-bending under a constant clamping force, (3) pushing-up the sheet by the counter punch from the bottom end, and (4) bending of the flange by an outer punch. A great advantage of this technology over other hat-bending processes, such as draw bending and crash forming, is that perfect hat-shaped channels are easily obtained for any type of AHSS just by determining two controllable process parameters, namely, the bottom pushing-up force and outer-punch stroke in flange bending. An accurate finite element (FE) simulation allows us to determine these process parameters prior to experimental tryout. The validation for this process was shown by performing the hat-shaped DAB experiments on a dual-phase 980-MPa AHSS and the corresponding FE simulation.