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 straininducedmartensitic transformation of the retained austenite in the multiphase microstructure (ferrite andbainite). Thermomechanical processing (TMP) was performed on Si-Mn bulk TRIP steel using a computercontrolled torsion machine with different rolling schedules, by varying the deformation in the austeniticrecrystallization region and non-recrystallization region, as well as the isothermal hold temperature for ferritetransformation. It was found that the choice of applied strain and temperature of deformation prior totransformation had a strong impact on transformation kinetics, final multiphase (ferrite, bainite and retainedaustenite) structure characteristics and mechanical properties of TRIP steel. The volume fraction of retainedaustenite was measured by both X-ray diffraction and image analysis method. The retained austenite volumefraction varied with strain in the non-recrystallization region. The increase of percentage reduction in the nonrecrystallizationregion results in a greater amount of elongated austenite grains and deformation bands inthe interior of the grains consequently very fine ferrite structures were formed. Furthermore, the dynamicallyrecrystallized austenite structure tends to retain more austenite at room temperature