Manganese partitioning and mechanical properties of dual phase steels have been examined. The manganese partitioning coefficient increases to a limit with increasing isothermal holding time, and the final equilibrium partitioning coefficient decreases with increasing annealing temperature. The combination of diffusion equations in austenite and ferrite plus mass balance and austenite growth can give the manganese distribution in austenite and ferrite respectively, and the manganese partitioning coefficient at any isothermal holding time. The introduction of rolling during annealing or at the end of annealing can strengthen dual phase steels by elongating austenite and introducing internal stresses and dislocations, the magnitude of which depends on finishing rolling temperature and the timing of rolling during annealing. The tensile properties of as-rolled dual phase steels are strongly influenced by finishing rolling temperature and martensite volume fraction because of internal stresses and dislocation density, whereas those of post-rolling annealed steels depend on only martensite volume percentage. To model the heat-treatment for producing dual phase steels (“in-situ” dual phase steel), a steel-steel composite is produced by incorporating high strength steel-wires into ductile steel sheets through hot-rolling (“artificial” dual phase steel). The properties of the reinforcing wire, matrix steels, interfacial strength between wire and matrix steels and steel-steel composite are affected by the surface condition of wire, finishing rolling temperature, preform holding time at rolling temperature before rolling, and heat-treatment after fabrication of composites. Mileiko’s theory can predict the relationship between the steel-steel composite strength and the volume fraction of wire. But for dual phase steels, Mileiko’s theory can be applied only when martensite volume fraction is over 30% because the residual stress, high dislocation density and carbide are produced at low temperature, at which low martensite volume fraction is obtained. Continuous wire composites can simulate the as-rolled dual phase steel when the reinforcement content is over 30%, but discontinuous wire composites can not simulate the post-rolling annealed dual phase steel.