Transformations During Intercritical Annealing and Their Implications for Microstructure and Mechanical Properties of Medium Mn Transformation-Induced Plasticity Steel in Continuous Annealing Line

Phase transformation during intercritical annealing (IA) and the relationship between microstructure and mechanical properties of the intercritical annealed 0.2C-5MnTRIP steels were investigated using a combined method of dilatometry, scanning electron microscope, transmission electron microscope, x-ray diffraction, and tensile testing. With the increase in IA temperature and holding time, the fraction of reversed austenite increases while the chemical stability of reversed austenite decreases, leading to the transformation of reversed austenite to martensite in the cooling stage. IA temperature is found to have a greater influence on the stability of austenite than IA time. The microstructures mainly comprised of ferrite and retained austenite, with granular carbide and martensite also observed in some specimens. The carbide precipitates formed during hot rolling remain after cold rolling and annealing process and gradually dissolve at higher IA temperature or with longer IA time. The optimal IA parameter, i.e., 650 °C for 15 min, contributes to a good product of tensile strength and elongation (997 MPa × 29.25%) due to an excellent TRIP effect. The present work gives a clear insight in controlling the mechanical properties of medium Mn TRIP steel in the continuous annealing line by a reasonable design of the IA process.