An EBSD Study on the Stability of Retained Austenite in Low-Si TRIP Steels During Uniaxial Tension

Two transformation-induced plasticity steels, with and without Nb-Ti additions, were subjected to uniaxial tension after simulated thermo-mechanical processing. Electron back-scattering diffraction, with phase segmentation implemented during post-processing, was used to investigate the stability of retained austenite (RA) by analyzing its size, morphology, and neighboring phases. The rate of RA transformation to martensite was higher in the Nb-Ti-containing steel than in the base steel as most RA was co-located between bainitic ferrite with parallel arrangement of laths in the former steel. Depending on the location of RA and the developed stress state, its stability in tension declines in the following order: RA in bainite > RA at polygonal ferrite/bainite interfaces > RA embedded in polygonal ferrite grains and at polygonal ferrite triple junctions or grain boundaries. Fine grains of RA may be less stable than their coarser counterparts if they are located in unfavorable stress regions of the microstructure.

Automated summary

The study found that the rate of transformation of retained austenite to martensite is faster in the Nb-Ti-containing steel compared to the base steel, mainly due to the co-location of most retained austenite with bainitic ferrite in the former. The stability of retained austenite in tension decreases in the order of: RA in bainite > RA at polygonal ferrite/bainite interfaces > RA embedded in polygonal ferrite grains and at polygonal ferrite triple junctions or grain boundaries. Fine grains of retained austenite may be less stable if located in unfavorable stress regions of the microstructure.