Abstract: With total oxygen and nitrogen content analysis, X-ray fluorescence, scanning electronic microscope (SEM), and energy dispersive X-ray spectroscopy (EDS), the total oxygen mass fractions, components of LF refining slag, and shapes, sizes, and components of inclusions in billets of spring steel produced by “100 t EAF→LF→VD→CC” process in a domestic steel factory were analyzed. And based on this, thermodynamic calculation was also carried out by using FactSage software to optimize present refining slag. The results showed that the main composition of refining slag after optimization was below: w(CaO)=36 %－44 %，w(SiO2)=36 %－44 %，w(Al2O3)＜10 %，w(MgO)=9 %－11 % and basicity was around 0.8－1.2. Applying this new refining slag in industrial tests, inclusion plasticity was promoted and the percentage of inclusions which enter the lowmeltingpoint area increases from 12.5 % to 75 %. Besides, the average size of inclusions decreases to 1.48 μm and the inclusions whose sizes were over 2.5 μm had not been observed.

Key words: spring steel, LF refining slag, inclusion plasticity