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 <350 °C, but at higher temperatures, the effect of Nb + Mo on slower transformation kinetics was obvious. The microstructure of both steels showed extremely fine bainitic ferrite below 325 °C, but at higher temperatures, coarse bainite with M/A constituents and extensive martensite formed in steels without or with Nb + Mo constituents. A correlation between hardness data and retained austenite contents has been established in both the steels. The paper presents the first account of the flow stress and transformation behaviour including the influence of Nb + Mo alloying and the details concerning the carbon-enriched austenite retained at room temperature and hardness variation as a function of isothermal holding temperature