Three lightweight steels with densities between 7.3 and 7.5 g/cm3 met property goals for third generation advanced high strength steels. Plates were cast, homogenized, and hot rolled to produce a prior austenite grain size of 50 μm. These steels consisted of microstructures of δ-ferrite, bainitic ferrite, and retained austenite. First principles calculations were used to determine defect structures and phase stability. The compositions of the alloys were formulated using phase equilibrium modeling and stacking fault energy calculations. The composition in weight percentage of the alloys ranged from 0.1 to 0.2% C, 13.5 to 13.9% Mn, 3.4 to 4.5% Al, and 1.3 to 2.1% Si. This range of compositions produced alloys with calculated stacking fault energies between 26 mJ/m2 and 35 mJ/m2. The alloys exhibited transformation induced plasticity during tensile testing with tensile strengths between 865 and 974 MPa with necking strains up to 39%.