Formation and evolution of Spinel in aluminum killed calcium treated linepipe steels

Magnesium aluminate (Spinel) inclusions have been observed to cause hook crack in line pipe steels. These inclusions are typically formed during liquid steel processing at the ladle metallurgy furnace (LMF) and subsequently modified by calcium to form less harmful liquid inclusions. Unfortunately, spinel inclusions are still sometimes observed in samples taken from the tundish and coil after calcium treatment.;The formation and evolution of spinel inclusions during liquid steel processing at two industrial mini mills was investigated by analysis of lollipop samples taken at different stages of the steelmaking process. The effect of reoxidation on the formation of spinels after calcium treatment was also investigated by thermodynamic calculations and laboratory experiments.;The results showed that spinel inclusions are first formed in liquid steel after desulfurization and by the reaction between dissolved aluminum in liquid steel and MgO in the ladle slag. These inclusions were modified by calcium to both liquid and solid calcium aluminates. Also observed after calcium treatment, was the formation of MgO rich and CaS inclusions. During liquid steel transfer to the tundish, evidence of reoxidation and ability of CaS to act as a source of calcium to modify inclusions formed during reoxidation was observed. Analysis of the results from the laboratory experiment showed that with sufficient magnesium in steel (about 4ppm) spinel inclusions can be formed during liquid steel reoxidation. The results also showed that spinel inclusions observed after reoxidation of calcium treated steel are larger in size than those observed prior to calcium treatment. The modification of spinels by calcium and reformation after liquid steel reoxidation was verified by thermodynamic calculations.