The effect of different Nb additions on the mechanical properties and microstructure evolution of grain boundary allotriomorphic ferrite(F_(GBA)) / granular bainite(B_G) air cooling bainitic steels was investigated.The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa,respectively with the addition of 0.02wt%Nb.The steel acquires superior strength and toughness with the addition of 0.06wt%Nb.The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in F_(GBA)/B_G steels.The refinement effect of granular bainite is improved with the increase of Nb content. The effect of different Nb additions on the mechanical properties and microstructure evolution of grain boundary allotriomorphic ferrite (F_ (GBA)) / granular bainite (B_G) air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02 wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06 wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increase volume fraction of granular bainite in F_ (GBA) / B_G steels. The refinement effect of granular bainite is improved with the increase of Nb content.