Enhancing the ductility and yield strength of 2.7Mn steel via two-step partitioning heat treatment

Fresh martensite (FM) is often present in medium-Mn steels, especially when containing lower Mn content, due to the insufficient thermal stability of reverted austenite; this FM is brittle, largely deteriorating the ductility. In this paper, large ductility and high yield strength are achieved in an Al/Si-added medium-Mn steel containing 2.7Mn via a two-step partitioning heat treatment, i.e. intercritical annealing (IA) followed by low-temperature partitioning (LTP). We show that, during the IA, C and Mn atoms partition from the pre-quenched martensite to reverted austenite; Al addition reduces the size of reverted austenite and promotes C and Mn enrichment in the reverted austenite by decelerating its growth kinetics. This enables the reverted austenite more thermally stabilized, thereby reducing the amount of FM and increasing the amount and mechanical stability of retained austenite (RA) at room temperature. During the LTP, accompanied with the recovery of dislocations and the suppression of carbide precipitation by Al and Si, C atoms further partition from FM to RA, which enables the RA more mechanically stabilized and thereby sustains the high strain hardening to larger strains. Simultaneously, the FM becomes less hard and less brittle due to C atoms depletion and dislocations recovery, alleviating the stress/strain localization and favoring the uniform plastic deformation. Furthermore, the decrease in mobile dislocation density that is accompanied with the recovery of dislocations is believed to be mainly responsible for the enhanced yield strength of the steel. The present results indicate that the synergetic effects of the primary element partitioning (promoted by Al) during IA, which increases the thermal stability of reverted austenite, and the secondary element partitioning (enhanced by Al and Si) and as well dislocation recovery during LTP, which increases the mechanical stability of RA and the uniformity of plastic deformation, significantly enhance both the ductility and yield strength of medium-Mn steel with low Mn content.