Flow Softening-based Formation of Widmanstatten Ferrite in a 0.06%C Steel Deformed Above the Ae3

Compression tests were carried out at a strain rate of 1 s(-1) on a 0.06%C-0.3%Mn-0.01%Si steel over two temperature ranges: i) 920 degrees C to 980 degrees C, and ii) 500 to 750 degrees C. Optical and scanning electron microscopy images indicated that significant volume fractions of Widmanstatten ferrite were formed dynamically above the Ae(3) temperature. The ferrite plates coalesced into polygonal grains during straining. The double differentiation method was applied to the stress-strain curves, providing average values for the dynamic transformation (DT) and dynamic recrystallization (DRX) critical strains of 0.12 and 0.20, respectively. These results are interpreted in terms of the flow softening-based transformation model by calculating both the driving forces promoting the transformation as well as the energy barriers that oppose it. The model predicts the temperature range over which DT can occur as well as the observed critical strains.