Microstructure, plastic flow and fracture behavior of ferrite-austenite duplex low density medium Mn steel

This study addresses the microstructure, plastic flow and fracture behavior of a laboratory Al-containing low density Fe-0.3C-6.7Mn-5.9Al medium Mn steel, intercritically annealed at either 850 °C or 900 °C, after cold rolling. Both resulting materials present an unordinary bimodal duplex microstructure made of coarse δ-ferrite grains and fine-grained (FG) {α-ferrite + retained austenite} regions. Tensile tests at room temperature and at −50 °C and Charpy impact tests showed that for both materials, each microstructural region presents its own fracture mechanisms and a specific ductile-to-brittle transition. A transition from brittle cleavage to ductile fracture is observed for coarse δ-ferrite grains, while FG regions present a transition from interfacial fracture to ductile fracture. The overall impact behavior seems to be driven by that of δ-ferrite grains.