Abstract: In order to improve the cleanliness of silicon-killed steel Q235B and find a suitable refining slag system, thermodynamic calculations and laboratory experiments were carried out to determine the appropriate composition of refining slag and reduce the basicity of the original process slag system.The basicity of the optimized slag system was controlled between 1.70 and 2.0, and the mass fraction of Al₂O₃ was controlled between 7.0% and 10.0%. The optimized slag system were successfully used in industrial tests.The results showed that after optimizing the slag system, the mass fraction of T.O in Q235B silicon killed steel after LF refining in the industrial test furnace could be reduced to 42×10^-6 on average, with a minimum of 26×10^-6.The average mass fraction of  T.O in the slab can be reduced to less than 35×10^-6. The maximum diameter of the inclusions in the tundish can be controlled below 20 μm,the number density of inclusions in the rolled material can be controlled below 15/mm², and the area fraction of inclusions in the rolled material can be controlled below  35×10^-6. The grade of inclusions in steel was significantly reduced and the cleanliness of molten steel was improved.

Key words: Q235B, refining slag system, total oxygen content, inclusions, silicon killed steel