Abstract: X-ray fluorescence、X-ray diffraction、SEM and EDS were used to study the phase composition and microstructure of the 260 t ladle furnace refining slag of a steel mill, and the thermodynamic software FactSage was used to study the precipitation process of each precipitated phase in the cooling process of the LF refining slag.The results indicate that the main phases of the LF refining slag are calcium aluminum phase and calcium silicon phase, and also a small amount of Ca(OH)2 and MgO. Calcium aluminum phase appears as different shapes based on whether it contents sulphur or not; calcium silicon phase mainly appears as round or irregular oval shape, and in accumulation state; free MgO phase has irregular shape, mainly distributed around the calcium silicon phase. In the slow cooling process of the LF refining slag, the final precipitated phases are calcium aluminate contenting MgO(Ca3MgAl4O10),CaO,a small amount of Ca2(Al,Fe)2O5 and γ-C2S, and the precipitating temperatures of each phase are respectively 1 280 ℃,1 100 ℃,1 100 ℃ and 850 ℃.

Key words: LF refining slag, phase composition, microstructure, cooling process