<p>The strength-ductility trade-off in low-Mn lightweight steels is a significant challenge due to the low thermal stability of austenite and the presence of δ-ferrite. Two types of low-Mn lightweight steels containing V and NbVMo microalloying elements were developed by warm rolling. Among these, NbVMo steel demonstrated superior properties, achieving a tensile strength of ~1.2 GPa and a product of strength and elongation exceeding 45 GPa%. In-depth mechanism analysis by atom probe tomography and quasi-in-situ electron backscatter diffraction revealed that different microalloying compositions influence the mechanical properties by strengthening δ-ferrite, refining retained austenite and homogenizing matrix strain. In NbVMo steel, δ-ferrite strengthening is attributed to the synergistic effects of (V, Mo, Cr, Nb)C composite precipitation, fine NbC and MoC precipitates, and the solid solution strengthening of Mo. These mechanisms collectively contribute to a higher yield strength and δ-ferrite microhardness compared to V steel. Consequently, δ-ferrite and the surrounding matrix in NbVMo steel exhibit coordinated elongation during deformation, enhancing the ductility. The improved microstructural and strain uniformity in NbVMo steel mitigates stress concentration effects on δ-ferrite deformation and serves as a barrier that delays the transformation of retained austenite. In contrast, the retained austenite in V steel exhibits a blocky morphology with larger grain sizes, resulting in lower stability. Combined with localized stress concentrations due to non-uniform strain distribution, this leads to premature transformation of retained austenite to alleviate stress, ultimately impairing elongation and the continuity of strain hardening. Furthermore, the precipitation mechanisms of (V, Mo, Cr, Nb)C composite precipitates are elucidated.</p>