Influence of combined Cu and Nb addition on the quenched microstructure and precipitation during tempering in ultra-low carbon steels

► Cu and NbCN precipitates are distributed separately during tempering. ► Cu addition slightly delays the coarsening of NbCN. ► Nb addition enhances the precipitation and coarsening of Cu particles. ► Strengthening of NbCN overwhelms the strength reduction due to accelerated coarsening. Three ultra-low carbon steels with different additions of Cu and Nb are studied to figure out the influence of combined addition of Cu and Nb on the microstructure, hardness and precipitation behavior during tempering. TEM observation shows that the Cu and NbCN particles are distributed separately. The increase of Nb content significantly promotes the Cu precipitation and also accelerates coarsening. The addition of Cu slightly delays the coarsening of NbCN precipitates. The calculation shows that the strength increment due to the precipitation of NbCN is significantly higher than the strength reduction caused by the accelerated coarsening of Cu particles due to the combined addition of two elements.