Impact of Thermal Conditions on Predicted Formability of TRIP Steels

To improve vehicle fuel economy and improve safety, automakers have focussed on substituting low-carbon steel for advanced high-strength steel (AHSS) alloys. A common grade of AHSS is the transformation-induced plasticity (TRIP) steels, which undergo a phase transformation from austenite to martensitic that enhances the ductility and strength. This work employs a phenomenological constitutive model for TRIP 800 steel to study the thermomechanical behaviour over large strains. This formulation is implemented into the recently developed fully coupled thermomechanical Marciniak–Kuczynski (MK) framework by Connolly et al. (2018). These are employed to analyze formability of a TRIP 800 alloy under a range of thermal conditions expected in realistic stamping operations. This work demonstrates that control of the initial blank temperature, die temperature, and die conduction coefficient can produce improvements in uniaxial and plane strain formability of up to 44% and 41% relative to room temperature formability. In contrast, the presented study shows that poor control of these parameters can reduce uniaxial and plane strain formability by up to 35% and 41%. Additionally, the conditions for optimal formability are shown to be strain path dependent, suggesting that tightly coupling component, process, and die design could improve final component designs.