Low carbon bainitic steel processed by ausforming: Heterogeneous microstructure and mechanical properties

Thermodynamic stability of austenite is a primary factor affecting mechanical properties of low-carbon bainitic steels. In this work, ausforming was applied to a low-carbon steel, in which additional dislocations and defects were induced in austenite prior to isothermal transformation of bainite at a relatively low temperature. Hence, the stability of the retained austenite was improved, and the formation of fresh martensite during the secondary stage of cooling was suppressed. Results of microstructure characterizations by electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and dilatometry were examined. The correlations between the process parameters and developed heterogeneous microstructures were established. It was found that the volume fraction of retained austenite was greatly promoted along with the reductions of fresh martensite and bainitic ferrite. Tensile properties of the bainitic steel were enhanced due to the occurred heterogeneous microstructure and fair balance of its phase constituents. The significantly increased strengths were mainly attributed to microstructure refinement as well as the enrichment of geometrically necessary dislocations (GNDs) within the bainitic ferrite and retained austenite. Furthermore, a large fraction of retained austenite showed a significant impact on the
enhancement of both uniform elongation and strength, benefitting from the transformation-induced plasticity
(TRIP) effect. The presence of microcracks and tiny dimples with thick tearing edges in the vicinity of the inclusions in the fractographs were directly related to the refined microstructure, which certainly provided a
remarkable ability to reduce the formation of macro/micro-cracks and prolong the post-necking.