Modification of Non-metallic Inclusions in Steel by Calcium Treatment: A Review

The calcium treatment was the most popular and effective method to modify inclusions in the molten steel. For the modification of oxide inclusions, the partial liquid inclusion was suggested to avoid the nozzle clogging caused by solid inclusions, as well as the formation of hardly removed large liquid inclusions in the Al-killed Ca-treated steel. During the calcium treatment process, the added Ca firstly modified Al2O3 inclusions to liquid CaO-Al2O3 for T.Ca< T.O in steel, while the composition of liquid CaO-Al2O3 was little influenced by the T.Ca/T.O ratio for T.Ca> T.O in steel. Meanwhile, the CaS/(CaO+ Al2O3) in inclusions increased with a higher (T.Ca-T.O)/T.S ratio in steel. For the sulfide inclusion modification, the calcium treatment retarded the precipitation of linear sulfides along the grain boundary of the steel matrix and lower the deformability of CaS during the rolling process due to the higher hardness of CaS. The length of sulfides increased with a higher T.Ca content, while it decreased with lower T.O and T.S contents in steel. Influence factors of the calcium yield of the calcium treatment were summarized to steadily increase the calcium yield, including steel composition, temperature, feeding speed, etc. An empirical equation between the [Ca], T.Ca, T.O, T.S, and [Al] was fitted to predict the [Ca] content in steel. During solidification and cooling processes of Al-killed Ca-treated steels, inclusions transformed from liquid CaO-Al2O3-MgO in the molten steel to Al2O3-MgO-CaS in the solid steel due to the equilibrium change. Moreover, the focus on the calcium treatment in the future was proposed.