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, IJmuiden 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 Alkilled steels are overwhelmingly dominated by chemically simple Al2O3, most of the process relevant information lies in the analysis of particle size distribution during the secondary steelmaking process. The population 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 alumina (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 powerlaw 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 controlled steel product.KEY WORDS: non-metallic inclusion; inclusion population density function; inclusion size distribution; secondary steelmaking.

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Non-metallic inclusions have always been the active subject of steelmaking research to improve the steel cleanliness and to develop the so-called oxide metallurgy technology. Inclusions in molten steel form and grow by the sequence of nucleation, chemical and physical growth and removal. Thus, the size distribution of inclusions evolves continuously with time in molten steel, and significant changes in the steel conditions are reflected in the inclusion size distribution as well as in the inclusion chemistry. This study aims to provide a new approach to interpret the inclusion size distributions. The concept of the Population Density Function (PDF) is introduced to objectively represent a given inclusion size distribution. Several possible applications of PDF analysis are presented to demonstrate the advantages of the utilization of the PDF for understanding the inclusion formation mechanism during the steelmaking process. Several ambitious ideas to utilize the PDF for inclusion size control are also presented.

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Calcium hexaluminate (CaO·6Al2O3–CA6) is usually associated as the product reaction between alumina and CA2 (CaO·2Al2O3) in cement‐bonded refractory castables. However, some investigations related to the Al2O3–CaO–MgO ternary system have indicated that CA6 could be generated by additional routes, involving two other high‐alumina phases: CaMg2Al16O27 (CM2A8) and Ca2Mg2Al28O46 (C2M2A14). Considering the lack of conclusive studies on this subject in the refractories field, the present study addresses an in‐depth microstructural evolution analysis of high‐alumina castables containing in situ or preformed spinel (MgAl2O4) in order to check the actual CA6 development steps in the presence of an MgO‐containing phase. By scanning electron microscopy (SEM) and thermodynamic calculations, it was observed that CA6 formation took place indeed as a result of the decomposition reaction of CM2A8, which was firstly generated due to the interaction between spinel and Al2O3–CaO–Na2O–SiO2 liquid. Although, the results confirmed this complex CA6 formation route regardless of the spinel incorporation method (pre‐formed grains addition or in situ reaction), the CA6 crystals distribution after the thermal treatment was entirely affected by the previous spinel grains location. Those different microstructural profiles could be the conclusive aspects to explain the poorer slag resistance of preformed spinel‐containing castables when in contact with steel ladle slags.

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The degree of supersaturation is a factor that influences the Al 2 O 3 inclusion characteristics in steel. The influence of the addition of a large amount of Al in the molten steel on the formation, growth and morphology of Al 2 O 3 inclusions was investigated by laboratory scale experiments. Consecutive steel samples were taken during the deoxidation process and subjected to chemical analysis (ICP-AES), automated image analysis (AIA) and scanning electron microscopy (SEM) assessment with respect to the extracted inclusions. The characterisation and quantification of Al 2 O 3 particles show different growth processes, leading to variations in particle size distribution as well as in the morphology.

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