Effect of Residual Element Tin on the Inclusion Characteristics and the Interaction between Lanthanum and Tin in Interstitial Free Steel

Recycling of Scrap will lead to the continuous accumulation of residual element Tin (Sn) in steel production. The influence of Sn on interstitial-free (IF) steel and the formation of Sn compounds were investigated by experiments. The types of inclusions in IF steel with Sn is TiN, Al2O3, MnS and Ti2S3, which are consistent with those in IF steel. After adding Sn from 0% to 0.5%, the starting solidification temperature of molten steel decreases, resulting in the increase of inclusion number density. The average size of inclusions decreases from 2.39 μm to 1.89 μm. After adding 0.065%La, the inclusions containing Sn, such as La-Sn, La-Sn-O and La-Sn-Ti, generate in IF steel with 0.1%Sn or 0.5%Sn. With the increase of Sn content in IF steel containing 0.065%La, the number density of inclusions decreases and the number of large size inclusions containing Sn (≥10 μm) increases from 121 inclusions to 284 inclusions. Furthermore, the addition of Sn and La refines the macrostructure of ingots. Thermodynamic calculation shows that La2Sn and La5Sn4 inclusions can form during the solidification process in sample with 0.1%Sn and 0.065%La. And in sample with 0.5%Sn and 0.065%La, La2Sn and La5Sn4 inclusions can form directly in the molten steel. The generation of inclusions containing Sn can change the existing state of Sn and is beneficial in inhibiting the grain boundary segregation of Sn. The experiment lays a theoretical foundation for the use of La to minimize the harmful effects of Sn in IF steel in the future.