Important factors to obtain homogeneous and ultrafine ferrite–pearlite microstructure in low carbon steel

The microstructural evolutions during compressions at different temperatures and strains were investigated using a Gleeble-1500 test machine for a low carbon steel, with and without the formation of pre-eutectoid ferrite. The physical processes that occurred during heavy deformation were discussed by observing the microstructure and analyzing the true stress–true strain responses. The important factors to obtain the equiaxed, homogeneous and ultrafine ferrite, include the initial microstructure before deformation, the deformation temperature and the true strain imposed. The refinement mechanisms for attaining the ultrafine-grained ferrite were discussed in terms of the strain-assisted transformation (SAT) of austenite to ferrite and the dynamic recrystallization of ferrite. The adequate ferritic microstructure has been obtained by deforming the undercooling austenite with a sufficient strain in a reasonable temperature range. The SAT should preferentially be considered in obtaining the ultrafine ferrite because it occurs more easily than the dynamic recrystallization of ferrite. Decreasing the recovery rate of ferrite and increasing the defect concentration in austenite is beneficial to the formation of ultrafine ferrite.