Optimizing RH refining process to maximize cleanliness of low-carbon low-silicon Al killed steel

In the LD–RH–CC process for producing low-carbon low-silicon steel, the cleanliness level of the slab was studied by optimizing the compositions of the refining slag and oxygen blowing flow in trial heats through judicious control. In order to realize RH without oxygen blowing, the oxygen content of molten steel should reach 475–525 ppm before RH degassing to guarantee that the free oxygen is in the range of 100–150 ppm after the carbon–oxygen reaction. When the slag compositions were controlled to CaO = 41–50 wt-%, Al₂O₃= 29–36 wt-%, SiO₂= 5–11 wt-% and MgO = 6–10 wt-% in the RH treatment without feeding calcium, the total oxygen content in the plate was 18 ppm, and the nitrogen content decreased to 17 ppm in the improved process. The micro-inclusions in the slab mainly consisted of MgO–Al₂O₃–(MnS) and Al₂O₃–(MnS), and the particle size was mainly concentrated below 3 μm. The average total amount of large electrolytic inclusions in the slab was 1.384 mg kg steel⁻¹. The large inclusions were mainly SiO₂, Al₂O₃, SiO₂–Al₂O₃–(K₂O), and MgO–Al₂O₃.