Inclusion Population Evolution in Ti-alloyed Al-killed Steel during Secondary Steelmaking Process

摘要

This paper presents a new approach towards the evolution of non-metallic inclusion (NMI) populations in Ti-alloyed Al-killed steels, based on an extensive inclusion analysis campaign at Tata Steel Europe, Umuiden Works. Automated SEM techniques were used to characterize the inclusion populations in 120 steel samples taken from nine heats out of two casting series of this steel grade. As NMI in Ti-alloyed Al killed steels are overwhelmingly dominated by chemically simple AI_2O_3, most of the process relevant infor mation lies in the analysis of particle size distribution during the secondary steelmaking process. The pop ulation density function (PDF) concept was applied, for the first time, to the characterization of inclusion size distributions sampled from secondary steelmaking practice. Two size distribution forms predominate in the entire dataset: i) Lognormal size distributions associated with active nucleation and growth of alu mina (deoxidation and reoxidation), indicating net transfer of matter between NMI and solutes in liquid steel and ii) Power-law size distributions, associated with an inclusion population in chemical equilibrium with the melt and subject to collision/breakup processes controlling the distributions. Based on inclusion PDF observations, it is found that the size distribution of alumina inclusions suspended in steel melt, after equilibration and effective float out of large inclusions, tends to approach a Reference Distribution of power law type function (f(r) = a · r～(-3.5)) that appears to be a fundamental feature of the alumina-steel system. This Reference Distribution can guide efforts to improve and engineer inclusion populations for a better con trolled steel product.