A Comprehensive Exploration of the Relationship between Microstructure Optimization and Strength Enhancement in Low-Density 5Al-5Mn Steel

The low-density medium-Mn steel is widely studied and applied in the automobile and construction machinery due to the low costs and high strength-ductility. Adding lightweight elements, such as aluminum, is considered an efficient way to reduce the density of the steels. A novel 5Al-5Mn-1.5Si-0.3C (wt%) low-density and high-strength delta-ferrite/martensite (delta-F/M) steel was designed in this study. The study indicated that the designed steel annealed at 1080 degrees C was characterized by an excellent combination of tensile strength of 1246 MPa and density of 7.24 g/cm3. Microscopic characterization shows that the higher prior-austenite volume fraction (i.e., martensite plus retained austenite) significantly increases the tensile strength, and the strip-like martensite and retained austenite (M&RA) mixture benefits elongation. High martensite fraction owns higher origin geometrically necessary dislocations, contributing to better work-hardening behaviors. Concurrently, the synergistic presence of M&RA mixtures' volume fraction and morphology enhances their capability to absorb stress and obstruct crack propagation, significantly improving mechanical performance. The extended strength formula, accounting for the contribution of the M&RA mixture, is consistent with the quantitative agreement observed in experimental results. These insights provide a valuable technological reference for the knowledge-based design and prediction of the mechanical properties of low-density and high-strength steel.