Microstructural evolution and mechanical properties of dual phase steel produced by strip casting

In the present study, an as-cast strip of dual phase steel (Fe-0.16C-0.3Si-0.81Mn-0.03Ti wt%) was successfully produced by twin-roll strip casting (TRSC) process. The as-cast strip was subjected to two different processing routes. The microstructure of the as-cast strip, cold-rolled sheet and annealed sheet was studied and related to mechanical properties of the annealed sheet. It was observed that the as-cast strip microstructure consisted of grain boundary ferrite, Widmanstatten ferrite, pearlite and intragranular ferrite, and the solidification structure was characterized by dendritic structure without the equiaxed center zone. The microstructure consisted of ferrite, martensite and non-recrystallization (N-R) region was obtained in the annealed sheet when the as-cast strip was directly cold rolled and annealed at 740-780 °C for 3 min. However, a typical dual phase microstructure of ferrite-martensite was obtained in the annealed sheet when the martensitic structure in the as-cast strip was subjected to cold rolling and annealing at 740-780 °C for 3 min. The dual phase microstructure in the annealed sheet having martensite fraction of ~ 51-77% was obtained, leading to tensile strength of ~ 851-914 MPa and corresponding total elongation of ~ 16-11%. Tempering treatment reduced martensite hardness, leading to lower tensile strength (~ 702-518 MPa) and corresponding higher total elongation (~ 19.8-30%) compared to non-tempered steel.