Carbide Dissolution and Austenite Growth in the Intercritical Annealing of Fe–C–Mn Dual Phase Steels

The dissolution of carbide, and the growth and shrinkage of austenite in the intercritical annealing of Fe–C–Mn dual phase steels are followed, using SEM and optical microscopy and discussed on the basis of a local equilibrium growth model. It is found experimentally that there are several stages in the austenite formation process, and that prior subcritical annealing can have a profound influence on the kinetics of austenite formation at intercritical temperatures. This latter effect is attributed to manganese concentration in cementite on sub critical annealing.The intercritical evolution of austenite fraction was examined for a 1.5 wt % Mn steel at 740°C, for a number of initial microstructures, and correlated with austenite morphology and carbide microchemistry.These results are of value in determining the design of composition and heat treatment for dual phase steels, either for continuous annealing or for batch processes.