Effect on cleanliness of molten steel with different refining slag systems for low alloy ship plate

To study the effect of refining slag on the compositions of molten steel and inclusions, the reaction between ship plate steel and different slag systems (slag A-CaO/Al2O3: 1.0, SiO2: 5 mass-% and slag B-CaO/Al2O3: 1.5, SiO2: 9 mass-%) was investigated by laboratory-scale high-temperature equilibrium experiments. Results showed that the desulphurisation capacity of the two slags was very similar, and the average sulphur content for both was 0.002 mass-% in steel, but the deoxidation capacity of slag B was slightly higher than slag A. The amount of inclusions <5 μm was more in steel balanced with slag B than in that balanced with slag A. To generate inclusions smaller than 5 μm, and spherical liquid CaO–MgO–Al2O3–SiO2 inclusions, and to decrease the T[O] (total oxygen) content in steel, refining slag composition should be: CaO/Al2O3 ∼ 1.5, and SiO2 ∼ 9 mass-%. Slag system optimisation can reduce or even eliminate calcium treatment during production. T[O] content in the slab could be controlled to 15–21 ppm, with typical micro-inclusions of ≤10 μm, and CaO–Al2O3–MgO and CaO–Al2O3–CaS systems in molten steel.