Effect of Microstructure before Intercritical Annealing on Mechanical Properties of 5% Mn Steel

Recently, 5% Mn steel has been focused on as one of the promising candidates for third generation AHSS by showing an excellent TS (Tensile Strength)-El (Elongation) relationship. The excellent TS-El relationship is brought about by a large volume fraction of retained austenite through the enrichment of austenite stabilizing elements such as C and Mn in retained austenite. The effect of the microstructure of mother hot band on the changes in microstructure and mechanical properties was compared with the intercritical annealing time in this study. The steel containing about 10% of retained austenite in a mother hot band exhibited a higher volume fraction of retained austenite and higher strength after intercritical annealing. On the other hand, the steel which did not contain retained austenite in a mother hot band exhibited excellent TS-El combination. The difference of work hardening behavior in these steels was analyzed and thought to be brought about by the difference of transformation behavior during deformation determined by the stability of retained austenite affected by Mn concentration. Although the precipitation of cementite was intended to effectively act as a nucleus of reverted austenite formation and to accelerate its formation, this affirmative result was not obtained. Since the volume fraction of cementite in a short annealing time is nearly the same in all hot rolling conditions, the higher volume fraction in hot band did not act to increase retained austenite during intercritical annealing.