Effect of thermomechanical processing on the microstructure of Si-Mn TRIP steel

High strength and ductility of the transformation-induced plasticity (TRIP) steels are attributed to strain-induced martensitic transformation of the retained austenite in the multiphase microstructure (ferrite and bainite). Thermomechanical processing (TMP) was performed on Si-Mn bulk TRIP steel using a computer controlled torsion machine with different rolling schedules, by varying the deformation in the austenitic recrystallization region and non-recrystallization region, as well as the isothermal hold temperature for ferrite transformation. It was found that the choice of applied strain and temperature of deformation prior to transformation had a strong impact on transformation kinetics, final multiphase (ferrite, bainite and retained austenite) structure characteristics and mechanical properties of TRIP steel. The volume fraction of retained austenite was measured by both X-ray diffraction and image analysis method. The retained austenite volume fraction varied with strain in the non-recrystallization region. The increase of percentage reduction in the non-recrystallization region results in a greater amount of elongated austenite grains and deformation bands in the interior of the grains consequently very fine ferrite structures were formed. Furthermore, the dynamically recrystallized austenite structure tends to retain more austenite at room temperature.