Abstract: Equilibrium precipitated phases in switch K2 spring steel in the temperature range of 400 to 1600℃ and the solubilities of Nb and V in austenite were calculated by using the thermodynamic software Thermo-Calc to investigate the effect of alloying element content on the equilibrium precipitated phases.Then,the effect of Nb content on the precipitated phases was analyzed by carbon extraction replica and transmission electron microscopy(TEM).It is shown that the main equilibrium precipitated phases in switch K2 spring steel are MX,M₇C₃,M₃C₂ and AlN,and the stability of MX phase can be significantly improved by adding Nb.TEM results reveal that the precipitated phases are spherical or disc-shaped,and their sizes range from several nanometers to more than 100nm.With the content of Nb increasing,the proportion of fine particles which precipitated from low-temperature phase ferrite increases dramatically,and the proportion of undissolved carbonitride also increases,overall the average particles size is refined.The large precipitates are Nb-rich carbides,while the small ones are V-rich carbides,which is in accordance with the results of thermodynamic calculations.