INCLUSION CONTROL IN A 16-MN STEEL USING A COMBINED RARE-EARTH AND CALCIUM TREATMENT

Two treatment methods, namely Ca-Si injection into the ladle and rare earth (RE) additions to the mold, were investigated for their effectiveness in controlling the sulphide inclusions in a 16 Mn steel. Ca desulphurization did not completely remove all of the MnS inclusions and the RE inclusion shape modification treatment increased the amount of inclusions compared to using only a Ca treatment. A combined RE plus Ca treatment gave the best control of inclusion shape, amount, and distribution. The combination treatment also provides a relatively homogeneous distribution of Mn in the banded (ferrite + pearlite) structure of hot rolled sheet. A criterion for effective inclusion control is established whereby [RE/S]effective is equal to (or slightly higher than) 3. [RE/S]effective is given by the formula [RE/S]effective, = [RE/S] + 1.875 [Ca/Si] where RE, Ca, and S are the compositions of rare earth, calcium, and sulphur in weight percent in the steel sheet.