Effects of Solution Temperature on Microstructure and Properties of Fe–Mn–Al–C Ferrite-Based Steel

Abstract

The effects of solution temperature of 700–950 °C on microstructure and properties of Fe–8Mn–6Al–0.2C ferrite-based steel were investigated. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were used to study the morphology and composition of microstructure. The tensile test at room temperature was used to detect mechanical properties. The results showed that the as-hot-rolled steel exhibited a complex microstructure, consisting of banded ferrite, martensite and small amount of retained austenite. With the increase of solution temperature, δ-ferrite solidified from the liquid phase and existed during subsequent heat treatment at all the temperatures, while the other phases varied from M₅C₂, α-ferrite, austenite and martensite. At 900 °C, dual phase microstructure with ferrite and island of austenite was obtained. With increasing solution temperature, the ultimate tensile strength (UTS) decreased at first and then increased after reaching the minimum at 850 °C, while the total elongation (TE) increased firstly and declined after the peak value of 44.5% at 850 °C. Therefore, the as-hot-rolled experimental steel heat treated at 900 °C had an excellent combination of tensile strength and elongation, with UTS of 846.4 MPa, TE of 32%, and UTS × TE of 27.1 GPa%. The remarkable mechanical properties, both strength and elongation, were attributed to the relative fraction and stability of austenite. And the partition of elements, especially for Mn, had great effect on the austenite stability.