Initial Microstructure Effect on Intercritical Annealing Behavior and Mechanical Properties in Medium Mn Steels

Abstract

The present work reports the effect of different initial microstructures on intercritical annealing behavior and mechanical properties in Fe-0.14C-7Mn-1Si (wt.%) medium Mn steel. Five different initial microstructures were produced by hot-rolling, cold-rolling and cold-rolling followed by austenitization at 820 °C, 860 °C and 900 °C. The specimen austenitized at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitization temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at early stage of intercritical annealing, but retards the transformation kinetics afterwards. In addition, excellent hole expansion properties were observed in specimens with lath-type austenite, which is attributed to the morphological difference of austenite.