Superior High-Temperature Properties of Low-Density Steel Enabled by a Tunable Hierarchical Microstructure

The tensile properties, microstructure characteristics, and fracture mechanisms at 500 degrees C of Fe-Mn-Al-C-Mo-V steel processed by hot rolling (HR) and warm rolling (WR) are investigated. The HR steel obtained by rolling 1150 degrees C, consisting mainly of equiaxed austenite and banded ferrite, exhibits a tensile strength of 695 MPa, accompanied by an elongation of approximate to 45.5%. In contrast, the WR steel processed by rolling at 750 degrees C shows a hierarchical microstructure that consists mainly of lamellar ferrite/austenite + fine carbides, rendering a significantly increased tensile strength to 1040 MPa and an excellent elongation to approximate to 20.0%. This is mainly attributed to the remarkable refinement of ferrite and austenite in the WR steel, strengthening finely dispersed carbides, and obtaining numerous low-angle grain boundaries in the matrix. The tensile fracture morphology is a ductile failure in both types of steel. The fracture of HR steel is attributed to the cracks enabled by the austenite localized slip deformation in the austenite and the fracture of the precipitates; the failure of the WR steel is mainly ascribed to the cracking along the lamellar structure due to the kappa-carbides pinning lamellar ferrite phase boundary leading to high strain localization.