Effect of Refining Slag on the Composition and Inclusions of Bismuth‐Containing Free‐Cutting Steel

Refining slag has practical significance for the production of bismuth‐containing free‐cutting steel. This study investigates the effects of slag basicity R (1.0–2.5) and w (Al 2 O 3 ) (10%–30%) on the composition and inclusions of steel through laboratory thermal experiments and thermodynamic calculations. The results demonstrate that elevating either R or w (Al 2 O 3 ) effectively reduces the T.[O] (total oxygen content), while L s (sulfur distribution coefficient) is enhanced by higher R but weakened by increasing w (Al 2 O 3 ). The Bi recovery rate ( Y Bi ) correlates positively with the slag layer blocking index (∑ slag ), reaching 5.08% at R is 1.0 and w (Al 2 O 3 ) is 30% due to suppressed volatilization. Enhancing R from 1.0 to 2.5 reduces the inclusion area fraction from 1.73% to 1.24%, and decreases the average diameter of inclusions from 3.82 μm to 2.89 μm. Increasing w (Al 2 O 3 ) reduces the average diameter of inclusions from 4.62 μm to 3.41 μm but increases the area fraction of inclusions from 1.58% to 2.61%. There are four forms of Bi existing in steel, i.e., isolated; partially encapsulated by MnS; completely encapsulated by MnS; and connected between two parts of MnS. The median aspect ratio of inclusions in the furnace with a basicity of 1.0 and w (Al 2 O 3 ) of 30% is 1.525, indicating good spheroidization.