Deformation of advanced high strength steels during V-die bending

V-die bend testing is a required qualification test for the automotive industry, since it mimics large-scale press-brake forming operations. The deformation behaviour of a DP, CP and QP steel, all with the same strength and nominal thickness, was assessed during V-die bend tests using punches with different radii coupled with digital image correlation for surface strain measurements. The strain hardening rate, as measured from tensile tests, significantly influences strain localization, the strain levels at the bending spine, and springback. The QP980 steel showed a high strain hardening rate that is sustained up to high strain levels. This superior hardening behaviour is directly related to the extended strain-induced martensitic transformation (TRIP) response and to the high mechanical mismatch between the constituent phases in this steel. Consequently, strain concentration, tangential strain at the bending spine, and springback angle are reduced for this steel. The strain hardening rate of the DP980 steel was very high at yielding, due to its characteristic composite effect, but decreased rapidly with deformation. As a result, this steel showed higher strain localization and tangential strain at the bending spine at the end of the tests. The CP980 steel exhibited a high yield strength, which compromised its strain hardening capability and resulted in high springback angles. However, the TRIP effect contributed to the improvement of the strain hardening rate after yielding, resulting in lower strain localization and tangential strain at the end of the tests in comparison with the DP980 steel. Based on the experimental results, the QP980 steel is the most suitable material for V-die forming, as it exhibits lower strain concentration at the bending spine that prevents crack formation and offers superior springback control.