κ-Carbide in lightweight steel has been studied. Its thermal stability, crystal morphology, orientation relationship, degree of lattice misfit and mechanical properties have been measured experimentally. The mechanisms for the microstructural evolution of κ-carbide are considered based on the crystal structure, lattice misfit, element diffusivity and solute partition. The hard γ/κ grain and ductile α+κ grain provide an opportunity to develop lightweight steels with a desirable combination of mechanical properties.