The Fe–Mn–Al–C duplex lightweight steels have received considerable attention from the automotive industry because of their low density and good mechanical performance. However, their poor cold-rollability limits their applications. This study proposes a thermomechanical treatment that enhances the cold-rollability of duplex lightweight steels via deformation-induced ferritic transformation (DIFT). DIFT activated by warm rolling destroys the initial band structure and causes a partial grain refinement, thereby causing mechanical strengthening without the loss of ductility. Subsequent annealing enabled cold rolling, while the microstructure formed by DIFT was observed to remain. The steels fabricated using the proposed method exhibited more cold-rollability compared with that exhibited by the conventionally hot-rolled samples. Such an improvement can be attributed to the increased austenitic stability and the resultant inhibition of strain-induced martensitic transformation via DIFT.