Constitutive flow behaviour of austenite at low temperatures and its influence on bainite transformation characteristics of ausformed medium-carbon steel

Abstract In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low temperature ausforming in the bay between ferrite and bainite C-curves at 500 °C in order to impart high dislocation densities in the austenite prior to phase transformation. Two experimental high-silicon, medium carbon steels were suitably designed and processed for this study, with one steel containing small additions of 0.3Mo and 0.03Nb. Flow stress measurements were made using single-hit compression tests in the temperature range 300–900 °C in steps of 100 °C at different strain rates in the range 0.1–10 s − 1 on a Gleeble simulator. Samples ausformed at 500 °C were isothermally held for 1 h at different transformation temperatures in the range of 300–400 °C to complete the bainitic transformation. Influence of strain induced bainite transformation on flow stress was obvious at 0.01 s-1, particularly at 300 and 400 °C. Despite enhanced nucleation in fine-grained steel B containing Nb + Mo, growth of bainite sheaves was much slower. Dilatation behaviour was comparable for the two steels at