Abstract: The effect of calcium treatment on the precipitation of TiN secondary particles was studied using optical microscope and scanning electron microscope.The results showed that,after calcium treatment,TiN nucleated on the interface of CaS particles,forming composite precipitation.There was a large amount of coarse TiN-CaS particles,and the proportion of composite precipitation particles above 1 μm was up to 63%.Without the calcium treatment,TiN nucleated independently,the size of the precipitation particles decreased significantly,and the second phase particles above 1 μm decreased to 42%.The mechanism of nucleation and precipitation of TiN at CaS interface had been further discussed.CaS came from the reaction of Ca and S or complex reaction of CaO,Al₂O₃ and S in molten steel.The structure of the high melting temperature phases of CaS and TiN were cubic crystal with low lattice mismatch degree and low interface energy,which promoted TiN nucleation at its interface in advance.