Thermodynamic Simulations and Industrial Trials Applied to Inclusion Control of SAE 9254 Si-Mn Killed Steel

Applying SAE 9254 steel to springs manufacture requires precise control of non-metallic inclusions. The effects of operational parameters on steel cleanliness such as the amount of BOF slag carry over as well as addition of graphite and ladle Furnace slag basicity have been evaluated using Chemical EQuilibrium Calculations for the Steel Industry (CEQCSI^®) software. Slag basicity is expected to strongly influence steel inclusions. Accordingly, industrial tests were carried out for characterization of inclusions in steel samples, collected at the beginning, middle, and end steps of Ladle Furnace refining treatment and in billet and wire rod products. Inclusion features such as quantity, size distribution, and chemical composition were assessed by SEM (Scanning Electron Microscopy) using EDS (Dispersive Energy Spectroscopy) and ASCAT (Automated Steel Cleanliness Analysis Tool). The results showed a decrease in the amount of inclusions by up to 60 pct and an increase from 72 to 98 pct of inclusions of less than 10 µm in the wire rod due to the increased basicity. Also under increased slag basicity, the inclusions composition remained in the low-temperature liquidus region, an important characteristic related to the best deformability of the inclusions. The amount and average size of the major sulfides S-Mn and S-Mn-Cr in the wire rod decreases with decreasing sulfur content in the steel.