Bainitic stabilization of austenite in low alloy sheet steels

The beneficial role that transformation plasticity plays in the uniform ductility of microalloyed sheet steels has been firmly established. Recent work supporting this role has demonstrated that effective stabilization of retained austenite can be achieved by employing an isothermal bainitic transformation after intercritical annealing. The result is a 'triple-phase' ferrite/bainite/austenite microstructure. This paper further explores the relationship between austenite stability and mechanical properties. The results obtained in this study emphasize the important role of austenite stability in the enhancement of uniform ductility in triple-phase steels. Superior properties are obtained with only 8% austenite, compared with 20% austenite, when the austenite stability is sufficiently increased toward the optimum. The results further indicate that, in addition to carbon enrichment, particle size refinement is an important contributor to enhanced stability.