Effect of strain path on dynamic strain-induced transformation in a microalloyed steel

Dynamic strain-induced transformation (DSIT) in a low-carbon microalloyed steel was studied by hot cyclic torsion to understand the interactions between DSIT and strain path reversals, and the subsequent microstructure evolution when subjected to continuous cooling. The critical strain for DSIT (εc,DSIT) can be determined by analysing the dynamic softening of the flow stress–strain curves. When deformed to the same total accumulative strain of 2.0, deformation with a small strain amplitude in each pass and multiple strain path reversals led to the suppression of DSIT compared to the extensive DSIT ferrite produced by deformation with a large strain amplitude and a single strain reversal. The results reveal that the amplitude of monotonic strain, not the total accumulative strain, in relation to εc,DSIT determines the occurrence of DSIT. The suppression or promotion of DIST can be attributed mainly to the increment of the austenite grain boundary area associated with deformation, especially the development of serration and bulging, and, to a lesser extent, to the generation of high-angle boundaries by austenite grain subdivision. The evolution of these planar defects, which are believed to be the primary ferrite nucleation sites, is strongly influenced by strain path changes, and lead to significantly different DSIT behaviours. DSIT ferrite also showed very limited coarsening after continuous cooling as the ongoing deformation produces further nucleation sites in the austenite matrix and causes orientation variation of the DSIT ferrite inherited from austenite parent grains. Based on these observations, it is believed that the transformation mechanisms for DSIT are essentially the same as the reconstructive mechanism during static phase transformations.