Understanding cutting-induced reduction in sheet metal formability: An in-situ investigation of microstructural effects

Abstract

Cracking during forming processes near shear-cut edges has been a major issue for the forming of advanced high strength steel sheets. The current understanding is limited by the complexity of the problem, which involves complex microstructures, stress states, and strain paths. In this work, to overcome these challenges, we have systematically investigated two quenching & partitioning (QP) steel grades, focusing on the microstructural evolution during cutting and forming, using various advanced characterization techniques, including novel in-situ scanning electron microscopy and synchrotron X-ray diffraction tests. We have investigated various factors including cutting-induced roughness, sub-surface damage, austenite stability, and phase constitution. These investigations shed light on why some higher strength QP steels can exhibit better cut-edge failure resistance compared to lower strength QP grades. More specifically, it was observed that the presence of the soft ferrite phase plays a critical role in heterogeneity of micro-strain evolution, surface roughness development, damage nucleation, and the overall cut-edge behavior. In contrast, a higher strength QP steel with the larger proportion of tempered martensite and bainite has exhibited more favorable deformation characteristics following shearing. This finding thus provides a key microstructural design guideline that can be applied to various other advanced high strength steels, for alleviating the edge cracking phenomenon during forming operations.