Development of an Advanced Ultrahigh Strength TRIP Steel and Evaluation of Its Unique Strain Hardening Behavior

A novel ferrite-53% austenite medium-Mn transformation induced plasticity (TRIP) steel with extraordinary combination of 1420 MPa tensile strength and 27% elongation was produced by a simple designed thermomechanical treatment. The deformation and strain hardening behavior of this material were studied in detail using field emission scanning electron microscopy and electron backscatter diffraction examinations of different strained specimens. Exclusive strain hardening behavior observed in this new TRIP steel was found to be related to the severe grain refinement to less than 200 nm and austenite grain disintegration within the first 1% of applied strain. Moreover, it was shown that the deformation of highly unstable retained austenite benefits the mechanical properties, since it could promote the severe grain refinement and subsequent uniform strain distribution within the microstructure of the TRIP steel. Finally, based on the strain hardening behavior analysis a new and simple measurable criterion was introduced as a reliable replacement of traditional austenite stability parameter or the K value.