Abstract: On the basis of the chemical composition of HR3C steel, the mass fractions of niobium element were adjusted to 0.5%, 0.8% and 1.1%, respectively. The ingot samples with the three components were obtained by melting, and then subjected to hot rolling, homogenization annealing at 1 200℃, solid solution at 1 200℃, and aging at 750℃ for different times (0-2 000 h). The effects of niobium content on the microstructure, grain size and hardness of the test steel were studied. The results show that increasing the niobium content could increase the content of undissolved MX phase in the solid solution test steel and reduce the grain size. During aging at 750℃, increasing the niobium content could promote the precipitation and coarsening of σ phase at grain boundaries and inside grains, and improve the hardness. When aging for 2 000 h, the niobium content had little effect on the size and distribution of grain boundary precipitates, but had some influence on the size and shape of intragranular precipitates; the test steel containing 1.1wt% niobium had a relatively small number of strip-like intragranular precipitates.