<p>In this study, the effects of varying K₂O contents on the viscosity and melting properties of CaO-SiO₂-Al₂O₃-MgO-based refining slags for spring steel were systematically investigated. The results revealed that as the K₂O content in the refining slags increased from 0 to 10 wt pct, the viscosity of the slags progressively decreased. This was attributed to the breakdown of the complex silicate network structure, which predominates in the CaO-SiO₂-3 wt pct Al₂O₃-7 wt pct MgO-based refining slags, facilitated by the free oxygen ions (O²⁻) produced through the ionization of K₂O. When the K₂O content exceeded 7.5 wt pct, the impact of K₂O on reducing slag viscosity gradually reached saturation. At the same time, the melting properties of the slags were significantly enhanced, with the flow temperature gradually decreasing from 1367 °C to 1292 °C. The primary phase of the refining slags transitioned from the high-melting-point phase CaSiO₃ to the lower-melting-point phase Ca₂MgSiO₇, accompanied by the formation of the low-melting-point phase KAlSi₃O₈. Notably, when the K₂O content reached 10 wt pct, the formation of the high-melting-point phase Ca₃MgSi₂O₈ increased the liquidus temperature. In addition, when 7.5 wt pct K₂O was added, the results of the slag–metal reaction experiment demonstrated a significant improvement in inclusion removal efficiency. Based on the above results, this study designed a novel refining slag with the optimal K₂O content of 7.5 wt pct suitable for Si-killed spring steel, exhibiting excellent physicochemical properties.</p>