Dynamic Transformation of Austenite to Ferrite in Low Carbon Steel

Abstract

The design of thermomechanical processing schedules to control microstructures requires the knowledge of the austenite-to-ferrite transformation start temperature (Ar3). In this industrial process, during deformation, the temperature usually decreases continuously. Thus, two new methods to determine the Ar3, based on continuous cooling compression (CCC) and continuous cooling torsion (CCT), have been developed. While the former is applicable for low strains only, the latter can be used for low and high strain processes. The aim of this investigation was to determine the effect of deformation in the single phase austenite and two phase austenite plus ferrite region on the transformation and dynamic transformation behaviour of austenite-to-ferrite. CCC tests were carried out on a low carbon steel and the influence of strain was examined. As expected, deformation in the single phase austenite region increased the kinetics of the austenite-to-ferrite transformation, raising the Ar3. The faster kinetics leads to a finer polygonal ferrite grain size after transformation. Straining in the two phase region causes strain concentration on the softer ferrite and, consequently, recrystallization of this phase. Deforming close to the Ar3 maximizes the strain effect on dynamically transformed ferrite. © 1994, The Iron and Steel Institute of Japan. All rights reserved.