Inclusion Control in Calcium Treated Steels

Calcium alloys are widely used for controlling the composition and morphology of nonmetallic inclusions (oxides and sulfides). By fine-tuning the calcium treatment, producing very clean steels with good fatigue limits is possible, as is improved machinability steels covering a wide range of machining speeds from carbide tool machining to ceramic tool machining. When producing such steels the steelmaker faces a double problem: adjusting the quantity of calcium to the inclusions to be modified, and forecasting the partition of calcium between oxides and sulfides. To help the steelmaker, two developments were achieved. First was a thermodynamical model predicting the partition of calcium between oxides and sulfides and the compositions of oxides. Second was the analysis of inclusions extracted from steel by nonaqueous electrolyte dissolution to check the model and to provide inclusion ratios like (Ca/AI)_oxide and Ca_sulf to be correlated with steelmaking practice or a machinability index.