Abstract: In order to improve the cleanliness of molten steel, control the morphology of inclusions and find a suitable calcium treatment process technology, the thermodynamic calculation of calcium treatment of aluminum-killed steel Q690 was carried out by using FactSage software. The effects of composition and temperature of molten steel on calcium treatment were analyzed. The calcium treatment model of Q690 steel was established, and the minimum calcium feeding amount required for complete liquid modification of inclusions in Q690 steel was obtained. The influencing factors of calcium yield were analyzed, and the calcium yield increased with the decrease of T.O content in steel before calcium treatment. The calcium yield was inversely proportional to the amount of wire feeding, meaning that the lower the feeding amount, the higher the calcium yield. Considering the castability and inclusion removal efficiency of Q690 steel, in order to reduce the risk of nozzle clogging during the casting process and avoid the complete liquidization of inclusions, the appropriate calcium addition amount of Q690 steel was determined to be 25.85-26.92 g/t. The results of industrial test show that after RH calcium treatment, the mass fraction of T.O can be reduced to less than 5×10^-6, and in billet it can be reduced to below 6×10^-6. After calcium treatment, the number density and area fraction of inclusions in the steel remained unchanged, and a large amount of inclusions with high CaS content precipitated in the steel during casting, as a result, the number density and area fraction of inclusions in continuous casting billet increase sharply, the number density of inclusions is about 6-8 per mm², the area fraction is about (150-230)×10^-6, the maximum size of inclusions detected in continuous casting billet is between 10-20 μm, and the cleanliness meets the quality requirements of high quality steel grades.

Key words: Q690 steel, cleanliness, calcium treatment, total Ca content, inclusion