Abstract: The effect of niobium on the phase transformation law of Si-Mn spring steel was studied by thermal simulation experiment, and the deformation-induced precipitation behavior of NbC was analyzed.The results show that pearlite transformation is delayed;and the minimum cooling rate of martensite transformation is reduced from 5 ℃·s^-1 to 3 ℃·s^-1.The pearlite lamellar spacing is refined, and the cementite lamellae become thinner, irregular, bending and discrete.Deformation-induced precipitation NbC locates in the ferrite lamellae, pearlite pellet borders and dislocations when the Nb-bearing spring steel was deformed at 850 ℃, and NbC is approximately spherical shape with a diameter of 10 to 15 nm.