Determination of dynamic ferrite transformation during deformation in austenite

The dynamic ferrite transformation during heavy deformation has been studied to obtain the ultrafine-grained structure in steel. In this study, the flow stress was measured at various temperatures with up to 1.2 of true strain and constant strain rate (10/s) by Gleeble 1500 in order to determine dynamic ferrite transformation kinetics quantitatively. The microstructural change was investigated as dynamic transformation of ferrite was propagated in order to determine the critical strain and the volume transformed. The flow stress was partly attributed by the flow stress of austenite and that of ferrite. Therefore, the fraction of dynamic ferrite was determined as follows; X-f = (sigma(m) - sigma(a))/(sigma(f) - sigma(a)) Where, X-f is the dynamic ferrite fraction, sigma(m) is the measured flow stress, sigma(a) is the estimated flow stress of austenite and sigma(f) is the estimated flow stress of ferrite. The transformation behavior was determined from the flow stress analysis and was compared with the experimental ones. The fine ferrite grains with the average size of 1-2 mum, which were transformed dynamically, were observed within prior austenite grain. With increasing deformation, the dynamic ferrite was formed differently. First, film-shaped ferrite (ferrite-I) along austenite grain boundaries was observed. Second, the shape of ferrite was changed from film-shaped to polygonal. Third, very fine ferrite with the size of 1-2 mum (ferrite-II) was observed inside austenite grain. The critical strain for dynamic ferrite was defined as the onset strain for the formation of the fine ferrite inside austenite-grain. It was also confirmed that high cooling rate to deformation temperature shifted the critical strain for dynamic ferrite to lower strain due to increasing austenite undercooling.