Influence of ferroalloy impurities and ferroalloy addition sequence on ultra low carbon (ULC) steel cleanliness after RH treatment

摘要

The present work outlines the statistical analysis of industrial data collected for a few hundred heats from an integrated steel plant. The main purpose was to study the influence of impurities in the ferroalloys FeTi (FeTi70 and FeTi35) and FeP on steel cleanliness. Therefore, two steel grades [ultra low carbon (ULC) high Mn and ULC low Mn] to which these ferroalloys are added were chosen for the study. The number of Al based inclusions analysed with pulse discrimination analysis-optical emission spectroscopy was taken as a measure of steel cleanliness and compared using box plot analysis and through a t test for different grades of FeTi additions. The ferroalloy parameters as well as other major process parameters during secondary metallurgy were also correlated to the inclusion number density. The statistical analysis was supplemented with elemental (ferroalloy) recovery calculations based on the industrial data and FactSage, the equilibrium Fe-Al-P-0 diagram and the inclusion extraction analysis. On the basis of this study, it is proposed that (1) FeTi70 is a comparatively cleaner ferroalloy than FeTi35 and (2) for FeP in ULC low carbon steel, the best addition practice to maintain a balance between P recovery and steel cleanliness is towards the end of decarburisation and before the addition of Al blocks.