Effect of undercooling of austenite on strain induced ferrite transformation behavior

Hot compression test of up to 70% reduction was performed to examine the effect of undercooling (DeltaT=Ae(3)-Ar-3) of austenite on strain induced ferrite (SIF) transformation behavior. The undercooling of austenite was controlled by applying various cooling rates from the austenitization temperature to the deformation temperature. In order to examine ferrite formation, the flow curve was measured and compared with the calculated flow stress of austenite. Fine ferrite grains with a size of 2 mum were observed within prior austenite grains. Increasing DeltaT was an effective way to increase the amount of SIF, when the applied reduction was relatively small. With heavy deformation, the amount of SIF was nearly the same regardless of DeltaT, while ferrite grains became finer and equiaxed by dynamic continuous recrystallization after transformation. It was also found that the amount of reduction for onset of ferrite formation within austenite grain was reduced with increasing DeltaT. The decrease of flow stress was observed compared with that of austenite, which was mostly from ferrite formation during the deformation.