Thermodynamic Modification Aspects by Calcium of Nonmetallic Inclusions in Low-Carbon Aluminum-Killed Steels

Low-carbon aluminum-killed steels constitute the main group of structural steels produced in Russia and worldwide. These steels are used in the key sectors of the economy such as construction, automotive industry, mining and transportation of minerals. The steel melt deoxidation by aluminum leads to the formation of nonmetallic inclusions, which can significantly affect the quality of rolled products and reduce the manufacturability due to clogging of submerged entry nozzles, metering nozzles, and ladle sliding gates during the continuous casting. For example, steel contamination by nonmetallic inclusions may result in rejection due to surface defects, reduced yield of cast slabs, increased corrosion wear rates, defects detected by ultrasonic inspection, etc. Due to a particular shape, size and state of aggregation, nonmetallic inclusions based on aluminum deoxidation products are difficult to remove from the steel melt. An effective way to reduce the steel contamination of these inclusions is modifying their compositions to the liquid state of aggregation with calcium, which requires careful preparation of the molten slag and metal. The study describes in detail the main thermodynamic features of this process. Using an IF-steel example, the target range calculation of the calcium contents ensures the modification of the inclusions to the liquid state depending on the aluminum concentration in the steel melt. The limiting sulfur concentrations in the metal melt that prevent the formation of refractory sulfide shells on oxide nonmetallic inclusions have been calculated depending on the aluminum and calcium contents.