State-of-the-Science of High Manganese TWIP Steels for Automotive Applications

Recent trends in automotive industry towards improved passenger safety and reduced weight have led to a great interest in AHSS (Advanced High Strength Steel), and DP, TRIP, CP, MA and high-Mn TWIP (TWinning Induced Plasticity) steels are particularly promising due to their superior toughness and ductility. The properties of low SFE (Stacking Fault Energy) austenitic high Mn FeMnC steel exhibiting twinning-induced plasticity have recently been analyzed in detail. It is argued that although the mechanical properties of TRIP and TWIP steels are often assumed to be solely due to effects related to straininduced transformation and deformation twinning, respectively, other mechanisms may also play an essential role such as point-defect cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing, e.g. in the case of TWIP steel. At low strain rates, the plastic deformation of TWIP steels is often controlled by the movement of very few well-defined localized deformation bands. The formation and propagation of these Portevin-LeChatelier (PLC) bands lead to serrated stress-strain curves, exhibiting a small negative strain rate sensitivity.

The present contribution offers a critical analysis of the mechanical properties of high-Mn TWIP steels and focuses on their potential as automotive materials. In addition, the challenges related to the production and applications of high-Mn TWIP steels are discussed. The new insights in the properties of TWIP steels result from the use of new experimental techniques combining high sensitivity infrared thermo-graphic imaging and optical in situ strain analysis. Finally the importance of the use of TEM (Transmission Electron Microscopy) in understanding the development of deformation microstructures in TWIP steel is also illustrated.