Effect of different refining slag systems on the cleanliness of molten steel for carbon structural steel

Abstract

The effect of two refining slags (A: 1.0-1.8 CaO/Al2O3 ratio, 11-20 wt% SiO2; and B: 1.0-1.2 CaO/SiO2 ratio, 25-35 wt% Al2O3) on the inclusion morphology of a carbon structural steel (Q235B) was studied by controlling the balance between the steel and the refining slag at 1600°C in a Si-Mo resistance laboratory furnace. The results showed that most of the inclusions in the molten steel balanced with slag A were formed by a lowmelting CaO-MgO-Al2O3-SiO2 system with a total oxygen content (T[O]) being fixed at 6-13 ppm. The inclusions in liquid steel balanced with the slag B were mostly formed by the CaO-MnO-Al2O3-SiO2 system, with T[O] controlled to 19-26 ppm. The overall size of the inclusions under balance conditions was higher in the steel reacted with the slag B than that reacted with slag A, with the latter showing more inclusions smaller than 3μm. In order to generate a large number of spherical CaO-MgO-Al2O3-SiO2 inclusions smaller than 3 m while decreasing the T[O] in the resulting steel, the composition of slag A should be used. This composition allowed improving the cleanliness of the slab, and is also advantageous for the castability of the molten steel.