Study on the precipitation and growth behaviour of TiN in IF steel during electric arc furnace smelting

Abstract

The development of electric arc furnace (EAF) short process smelting technology for interstitial-free (IF) steel has emerged as a focal point within the industry. The nitrogen (N) content in the steel melt from EAF short process smelting is typically around 50 ppm, significantly exceeding the typical control level for IF steel, which is around 20 ppm. Excessive N content can lead to the precipitation of large titanium nitride (TiN) particles during the continuous casting of IF steel, impacting the quality and properties of the final product. Currently, a substantial amount of theoretical research has been conducted on TiN precipitation. However, there is a lack of studies on the precipitation behaviour and control measures of TiN in high-N IF steel. On the other hand, theoretical calculations of TiN precipitation lack experimental validation. Considering the aforementioned issues, this paper investigated the precipitation and growth behaviour of TiN in high-N IF steel using micro-segregation and growth kinetics equations. Then, the precipitation sites of TiN in the steel sample with a cooling rate of 3.0 K·s⁻¹ were in-situ observed using a high-temperature-confocal laser scanning microscope, and the theoretical calculation results were validated based on the size statistics of the precipitated phase obtained from scanning electron microscopy (SEM)-energy dispersive spectroscopy analysis. The research results indicated that during the solidification process of the steel melt, due to the segregation of solute atoms, TiN in high-N IF steel precipitates prematurely in the mushy zone when the solid fraction exceeds 0.77. After the temperature of high-N IF steel dropped below the solidus temperature, TiN preferentially precipitated at the austenite grain boundaries. SEM images revealed that the total proportion of TiN precipitates within the size range of 1.6 to 2.5 μm in the steel sample with a cooling rate of 3.0 K·s⁻¹ was 82.5%, which was in good agreement with the results from growth kinetics calculations, validating the reliability of the theoretical calculations in predicting the size of TiN in high-N IF steel. Based on kinetic calculations, it is evident that reducing the N content in steel, thereby decreasing the growth driving force for TiN, should be the primary measure to reduce the size of TiN. Additionally, moderately increasing the cooling rate can also suppress the size of TiN in high-N IF steel, but as the cooling rate increases, its effect on inhibiting the size of TiN in high-N IF steel diminishes.