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High strength steel (HSS) have several advantages such as lightweight and high shock absorption. In sheet metal forming of HSS, twist springback occurs remarkably, which results in the low product quality. In addition, major defects such as wrinkling and tearing easily occur due to the low formability. Blank holder force (BHF) and blank shape have a direct influence on the twist springback, the wrinkling and the tearing. Variable BHF (VBHF) that the BHF varies through the stroke is valid for the twist springback reduction. In this paper, we propose a novel method to evaluate the twist springback, in which both the torsion angle of top of the product surface and the angle of left/light flange are evaluated. Based on the evaluation, a design optimization to determine the VBHF trajectory and blank shape is performed under several design constraints. The numerical simulation is so intensive that a sequential approximate optimization using radial basis function network is adopted to determine the optimal VBHF trajectory and the blank shape. Thorough the numerical simulation, the validity of proposed approach is examined.

Design optimization of variable blank holder force trajectory and blank shape for S-rail shaped product using a novel springback evaluation