Tailoring austenite stability and mechanical behaviors of IQ&P steel via prior bainite formation

Abstract

The prior carbide-free bainite (CFB) was introduced into a low carbon IQ&P steel via an isothermal bainitic holding (IBH) process, named as IBQ&P steel. It is essential to investigate the influence of prior CFB formation on microstructural evolution and mechanical properties, especially focused on austenite stability and mechanical behaviors. It reveals that the influence of prior CFB on austenite stability changed from deteriorated effect into mechanical stabilization with increasing IBH time, whilst the location of prior CFB expanded from the region adjacent to interface of constituent phases into the interior of parent austenite. Meanwhile, the amount of bainite with morphology of lath and blocky significantly increased accompanied by improvement of microstructural refinement, while the fraction of initial martensite with lath or twin structure decreased with existence of abundant tempered dislocation and ε-carbide. As compared to IQ&P steel, the carbon content of retained austenite in IBQ&P steel was improved with elevated stability, ascribing to the retention of high carbon austenite in the exterior of parent austenite and refinement of microstructure. The practical carbon content of retained austenite was much larger than the predicted results merely considering carbon partitioning, while nearly close to that of NPLE/PLE transition line of γ to α transformation including synergistic partitioning of C, Mn and Si elements. Moreover, the high carbon retained austenite in IBQ&P steel gave rise to TRIP effect occurred at large strain with expanded range. Finally, the introduction of prior CFB significantly improved the combination of strength and ductility with optimal PSE value exceeding 30 GPa%, which was obtained by properly tailoring the decomposition and retention of metastable austenite.