Characteristics of Spinel Inclusions Formed after Reoxidation of Calcium Treated Aluminum Killed Steel Obinna Adaba 1 , Pallava Kaushik 2 , Ronald J. O’Malley 1 , Simon N. Lekakh 1 , Von L. Richards 1 , Erik Mantel 2 , Randy Hall 2 , Eric J.Ellis 2 1 Missouri University of Science and Technology Materials Science & Engineering Dept. 1400 N Bishop Rolla, MO 65409 2 ArcelorMittal Global R&D 3001 East Columbus Drive East Chicago, IN-46312 ABSTRACT Reoxidation of liquid steel during transfer from the ladle to tundish affects the inclusion characteristics and is harmful to process stability, mechanical properties, and the overall quality of the cast product. A thermodynamic model was developed and a laboratory vacuum induction furnace was employed to simulate the reoxidation of aluminum killed calcium treated steels. Samples were collected at different times and analyzed via automated SEM/EDS. A method for representing the size distribution of inclusions using the population density function (PDF), was applied to study the effect of reoxidation on the inclusion characteristics. The results show that spinel inclusions can be produced after reoxidation if there is sufficient magnesium in solution. A comparison of the size distributions of spinel inclusion populations showed that spinel inclusions produced after reoxidation are larger than those formed prior to reoxidation and calcium treatment. INTRODUCTION Reoxidation of liquid steel during the last stages of steel production affects the inclusion characteristics and is harmful to process stability, mechanical properties, and the overall quality of the cast product [13] . In aluminum killed steels, alumina inclusions produced after liquid steel deoxidation clog submerged entry nozzles (SEN) and their irregular shape acts as preferential sites for crack initiation. To prevent these harmful effects, calcium is typically added at the end of ladle metallurgy to modify them to liquid spherical calcium aluminates. Liquid steel reoxidation produces new alumina inclusions which in the absence of calcium, retain their harmful qualities and negates the benefit of calcium treatment. In steels with sufficient magnesium in solution, spinel inclusions can also be formed after reoxidation [12, 13] . These inclusions have been observed to cause hook cracks in line pipe steels [9] . The objective of this research was to investigate the formation and characteristics of spinel inclusions after liquid steel reoxidation. Formation of Spinels in Ca-treated Reoxidized Steel Results from studies conducted in two industrial mini mills showed ladle slag as the source of magnesium for spinel formation in liquid steel [8] . Spinel inclusions were observed after the desulfurization process where the steel was continuously mixed with the MgO saturated slag using argon gas. Eq. 1 and Eq. 2 describe the two step mechanism for the formation of spinels in liquid steel. At the end of the ladle metallurgy process, the spinel inclusions were modified by calcium to liquid calcium - magnesium aluminates. As mentioned earlier, these modified inclusions are less harmful to the casting process and steel properties. Also observed was the formation of CaS inclusions that resulted from over treatment. The reaction mechanism for the calcium modification of spinels is given by Eq. 3 and shows the possibility of magnesium re- dissolution into steel.