Tailoring microstructure evolution and austenite stability of TRIP steels by Rare-Earth micro-alloying

The microstructure evolution and austenite stability of Rare Earth (RE)-alloyed Transformation Induced Plasticity (TRIP) steels subjected to intercritical annealing and isothermal bainitic transformation have been investigated for superior strength-ductility performance. Structure morphology, austenitizing temperature and phase proportion during thermomechanical process were carefully compared, confirming that minor RE additions indeed refined the microstructure, and affected the thermodynamics of austenite and bainite transformation. Austenitizing ranges of TRIP steels were lifted with increasing RE content then stabilized when exceeding 360 ppm; the intercritical austenite reduced but dissolved more C-Mn atoms, hence the bainite transformation in RE-alloyed TRIP steels was significantly inhibited, which elevated the retained austenite quantity and thermal stability simultaneously. Therein, more austenite nucleated from the structure fragments with RE-refined lamellar morphologies and strictly grew into acicular counterparts or aggregated into clusters. The aggregated austenite, composed of several subgrains with [011¯] coaxial relations or Σ3 60°〈111〉 misorientation but coherent interfaces, were engulfed or being swallowed up by coalescing ferrites in sluggish growth, guaranteeing the higher mechanical stability than intergranular austenite. In this study, the product of strength and elongation of TRIP steels after optimized RE-alloying was increased by 38% as compared to that without RE additions. The micro-alloying effect of RE elements in TRIP steels highlights the innovations of new-generation automotive steels. [Display omitted] •The quantity and stability of retained austenite were simultaneously improved by Rare Earth (RE) micro-alloying, guaranteeing the superior strength-ductility balance of TRIP steels.•RE additions lift the austenitizing range of TRIP steels and stabilize the intercritical austenite then significantly inhibit its bainite transformation.•Static recrystallization of ferrites lags behind the austenite nucleation as the solute-drag effect of RE atoms, promoting the formation of aggregated austenite in RE-alloyed TRIP steels.•Subgrains of aggregated austenite usually keep [011¯] coaxial relation or Σ3 60° misorientation but coherent interfaces, for minimizing the Gibbs free energy, with the benefit of higher mechanical stability.