Formation of ultrafine ferrite by strain-induced dynamic transformation in plain low carbon steel

Strain-induced dynamic transformation (SIDT) is of great advantage to obtaining ultrafine-grained ferrite in low carbon steels partly by early impingement of ferrite nuclei which are very rapidly and concurrently formed due to the deformation and partly by random crystallographic orientation distribution of ferrite grains. The SIDT fraction increases with the increase of strain. There is, however, a critical strain under which no SIDT occurs. In order to apply this refining mechanism to actual production lines, it is necessary to reduce the critical stain and enhance the kinetics of SIDT. It has been known that refining prior austenite grain is the most effective for this purpose. The influences of deformation temperature and strain rate on SIDT were also examined. Because SIDT is a kind of softening mechanism of strained austenite, it competes with dynamic recrystallization (DRX) at below Ae3 temperature. Comparing the critical strains of SIDT and DRX, SIDT is predominant softening mechanism, which enables us to utilize it for grain refinement. This provides an important clue to overcome the limitation of conventional thermomechanical control process (TMCP) in grain refinement area.