Understanding the Behavior of Ti in Ultralow-C Liquid Steel

Interstitial-free (IF) steel is used for automotive outer panel applications. In order to ensure high deformability, extremely low C and N contents are required. Such low C and N contents are obtained during the RH process via vacuum degassing. Therefore, it becomes ultralow-C (ULC) steel. Additional capture of these interstitial elements is carried out by adding Ti into the liquid steel. Since Ti has an affinity to O, its addition follows Al deoxidation. During Al deoxidation, a considerable amount of alumina is generated, followed by decreasing soluble O content in the steel. The generated alumina is floated and removed to the RH slag by the circulation of the steel in the RH vessel. Therefore, the Ti addition aims to increase dissolved Ti content in the steel, which later captures C and N. However, the ULC steelmaking process often undergoes troubles including severe nozzle clogging during continuous casting1 and surface defects after cold rolling.2 These troubles deteriorate as Ti content in the steel increases.1,3 It has been still unclear why Ti addition induces such troubles. Some have suspected non-metallic inclusion evolution that contains Ti.4,5 However, the majority of the inclusions observed in the Ti-ULC steel are alumina, along with some portion of Ti content in the alumina.4,5 A complex oxide compound Al2TiO5 is often mentioned,6 and it appears in a number of inclusion stability diagrams.6,7 However, neither its existence nor its consequence to the troubles was clarified. In this article, the behavior of Ti in ULC steel is discussed by reviewing available research results including the present authors’ recent results.1,8–15 Most discussions will focus on coupled interaction between Ti and O in the steel.