Abstract: The effects of different heating temperatures and different Nb contents on the austenitizing process and grain size of eutectoid rail steel were studied.The formula of solid solution product of NbC equilibrium solid solution in austenite was calculated.The distribution position and morphology of insoluble second phase containing Nb were analyzed by SEM and EDS.The results show that the effect of Nb content on austenite grain size is significant and increases linearly below 1150 ℃, while the effect of Nb content on austenite grain size is not significant within 1250 ℃.The formula of solid solubility product of NbC equilibrium solid solution in austenite is modified.There are differences in the position and morphology of Nb containing insoluble phase at different heating temperatures.Below 1150 ℃, NbC is mostly distributed at the austenite grain boundary, and the morphology is irregular long strip.At a heating temperatre of 1250 ℃, a small amount of NbC+trace TiC particles are distributed at the austenite grain boundary, most of them are distributed in the crystal, and the morphology is mostly regular diamond or square.