Modulation of δ-ferrite formation and austenite stability in Al-added medium-Mn steels and its role in the structure–property relationships

The present work investigates the influence of Al content on δ-ferrite formation and its volume fraction, resulting in different austenite stability, microstructure evolution, corresponding mechanical behaviour and underlying deformation mechanisms for three Fe-8Mn-XAl-1Si-0.2C steels with Al contents of 1.23 wt.% (1.25Al steel), 3.5 wt.% (3.5Al steel) and 5.1 wt.% (5Al steel), respectively. Steel ingots are casted and processed through hot deformation, entailing hot forging, hot rolling and subsequent intercritical annealing at various temperatures. It is observed that Al addition promotes the formation of δ-ferrite that enhances the mechanical stability of austenite. Diffusion simulation using TC-DICTRA supports that preferential partitioning of Mn and C from δ-ferrite to austenite leads to enhanced mechanical stability of austenite. Both maximum (k ∼ 0.49) and minimum (k ∼ 7.30) austenite stability, where obtained in 5Al and 1.25Al steels, respectively, followed by inferior mechanical properties. This occurs due to the presence of a higher δ-ferrite (∼ 65%) and no δ-ferrite fraction in 5Al and 1.25Al steels, respectively. However, achieving an intermediate δ-ferrite content of approximately 34% results in optimal stability of austenite (k ∼ 2.50) in 3.5Al steel that results in an optimal microstructure comprising both intercritical α-ferrite and δ-ferrite, as well as excellent tensile properties as represented by an ultimate tensile strength of 825 MPa with ∼18% elongation.