Evolution of the microstructure and mechanical properties of an austenite-ferrite Fe-Mn-Al-C steel

The microstructural evolution and mechanical properties of an austenite-ferrite duplex structure steel were investigated after solution treatment. The steel exhibited an optimal mechanical combination of tensile strength and ductility after being solution treated at 1000 °C for 0.5 h and then water quenched, and considerable product of strength and ductility was ascribed to the three-stage strain hardening behavior. Research on the phase transformation, microhardness change and three-dimensional grain growth characteristics of both ferrite and austenite was performed to bridge the relationship between microstructural evolution and performance improvement. The volume fraction, grain size and microhardness depended on the solution temperature, which resulted in a transition from stable to metastable behavior of austenite phase in the range 900-1200 °C. By contrast, the δ-ferrite phase was formed and stablized in high temperature. A faster growth rate of austenite grains than ferrite grains in the normal direction (ND) led to the stuck and separation of banded-structure δ-ferrite grains, which improved the microstructural homogeneity and thus enhanced the plastic deformation ability. The distinct fracture behavior under different processes was also discussed in the article.