Abstract: The SKS51 steel was subjected to rapid spheroidizing annealing treatment (including two stages of austenitizing and isothermal spheroidizing) based on the principle of divorced eutectoid transformation. The effects of the austenitizing temperature (750, 780 ℃), austenitizing holding time (10, 20 min) and isothermal spheroidizing temperature (650, 680, 700 ℃) on the microstructure and hardness were studied. The results show that under the test conditions, by increasing the austenitizing temperature or prolonging the austenitizing holding time, the flaky carbides in the steel increased in number, the spheroidizing effect became deteriorating, and the hardness increased. After austenitizing at 750 ℃ for 10 min, with the increase of the isothermal spheroidizing temperature, the content and size of spherical carbides in the SKS51 steel increased, and the hardness decreased. After 750 ℃×10 min+700 ℃×2 h rapid spheroidizing annealing, the SKS51 steel had the largest number of spherical carbides and the lowest hardness.