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

Abstract

High strength and ductility of the transformation-induced plasticity (TRIP) steels are attributed to straininduced
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 nonrecrystallization
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.