Enhanced stretch flangeability and crack propagation behavior of an 1100 MPa grade TRIP-aided bainitic ferrite steel

Abstract

In order to face the challenges of lightweight, safety, and emission reduction in the automotive industry, transformation-induced plasticity aided bainitic ferrite (TBF) steel was fabricated by controlling the size and shape of bainitic ferrite and compared with quenching and partitioning (Q&P) steels. The effects of bainite transformation on the mechanical properties, microvoid formation, and crack propagation were analyzed by dilatometry measurement and microstructure characterization. At lower isothermal bainite transformation (IBT) temperature, the lathy bainitic matrix can prevent crack formation and limit the crack propagation inside its laths, in favor of improving the strength and stretch flangeability simultaneously. In addition, granular bainite, coarse martensite, and austenite (M-A) islands would increase the risk of void formation at higher IBT temperatures. These voids provided the initial sites for crack formation, which was also responsible for the reduction of the hole expansion ratio (HER). In conclusion, the outstanding properties were achieved in the T-375 samples with the HER of 33.5%, the total elongation of 16.9%, the tensile strength of 1191 MPa, and the yield strength of 934 MPa.