An experimental and mathematical study on the evolution of ultrafine ferrite structure during isothermal deformation of metastable austenite

Heavy deformation of metastable austenite at intercritical temperature is known to develop 'ultrafine ferrite-grain' (ULFG) structure and provide grain boundary strengthening. Systematic thermomechanical simulation was conducted in Gleeble®3500 by deforming the samples isothermally at two intercritical temperatures: 810 °C (~40 °C below Ae3 and ~160 °C above Ar3) and 710 °C (~140 °C below Ae3 and ~60 °C above Ar3) to identify the critical conditions for the formation of ULFG structure in a low carbon microalloyed steel. Both single-pass and multi-pass deformations with varying equivalent total strain level were considered in order to provide a solution towards the development of ULFG structure upon industrial rolling. Microstructure evolution suggested that multi-pass intercritical deformation can produce uniform distribution of ultrafine ferrite grains (grain size ≤ 2 µm) as a combined effect of static- and dynamic strain-induced transformations (SSIT and DSIT) and continuous dynamic recrystallization (CDRX). Based on the microstructural evidences and strain analysis following Militzer-Brechet model, a descriptive model has been proposed discussing the mechanism of grain refinement during isothermal intercritical deformation.