Abstract: Often, the addition of more than 4 wt.% Mn to medium-Mn steels is necessary to enhance the thermal stability of intercritical austenite for achieving sufficient amounts of retained austenite (RA) at room temperature. In this paper, a medium-Mn steel with Mn content as low as 2.7 wt.% was designed via alloying with a small amount of Al, and the microstructure and mechanical properties of the steel, subjected to intercritical annealing (IA) at 745 °C for different times followed by oil quenching, were investigated. Results show that the volume fraction of RA increases first and then decreases with IA time, with the maximum of 0.36 obtained at IA time of 50 min. It is demonstrated that Al addition slows down the interface migration and growth kinetics of reverted austenite via retarding C diffusion in ferrite during IA, which, hence, decreases the amount and size of the reverted austenite and partitions more C and Mn into it. This suggests that Al plays a favorable role in enhancing the thermal stability of reverted austenite and increasing the amount of austenite retained at room temperature. Due to the presence of large amounts of RA and the strong transformation-induced plasticity effect generated during plastic deformation, the steel exhibits persistent high strain hardening and superior mechanical properties, comparable to those of reported medium-Mn steels containing higher Mn content. The present result offers a new insight into the role of Al in adjusting microstructure-property relationships and opens a promising way for designing low-cost, high performance medium-Mn steels with low Mn content for industrial applications.

Key words: Medium-Mn steel, Transformation-induced plasticity, Transformation kinetics, Austenite thermal stability, Mechanical properties