The effect of Nb and Cu on the microstructure and hardness of quenching–partitioning–tempering (Q–P–T) dual-phase steels was investigated. Nanostructured martensite and retained austenite were obtained when the steels were treated with a Q–P–T process. The hardness, volume fraction of retained austenite and final carbon content in austenite of the Nb + Cu-containing steel were higher than the Nb-containing steel. This was attributed to the promotion of Nb precipitation by the addition of Cu.