In this article, the effect of CaS formation on the evolution of Al 2 O 3 -CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al 2 O 3 -CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid ␦ -front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid ␦ -front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.

A sessile-drop study was carried out on Al 2 O 3 -10 pct C refractory substrates in contact with molten iron to investigate possible chemical reactions in the system and to determine the influence of carbon and the role, if any, played by the presence of molten iron that can act both as a reducing agent and as a metallic solvent. These investigations were carried out at 1823 K (1550°C) in argon atmosphere for times ranging between 15 minutes and 3 hours. We report the occurrence of chemical reactions in the Al 2 O 3 -10 pct C/Fe system, associated generation of CO gas, and carbon pickup by molten iron. Video images of the iron droplet started to show minor deviations after 30 minutes of contact followed by intense activity in the form of fine aluminum oxide whiskers emanating from the droplet and on the refractory substrate. The interfacial region also changed significantly over time, and the formation of small quantities of iron aluminide intermetallics was recorded after 30 minutes as a reaction product in the interfacial region. These chemical reactions also caused extensive penetration of molten iron into the refractory substrate. This study has shown that alumina cannot be treated as chemically inert at steelmaking temperatures when both carbon and molten iron are present simultaneously. These findings point to an additional reaction pathway during steelmaking that could have significant implications for refractory degradation and contamination of steel with reaction products.

Experiments using ahot-stage confocal scanning laser microscope (CSLM) havebeen carried out to observe phase transformations in two steels: Si-killedr esulfurized Fe-0.38w tp ct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite coloniesa nd subsequently in the ferrite phase. The measured advance rates of the g /pearlite front were roughly twicet hoseo ft he g / a front and both interfaces were found to be curved. The g /pearlite migration ratew as found to be in qualitativeagreement with publishedrate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstä tten ferrite laths precipitate as distinct colonies from the existing allotriomorphicf errite phase but then also at MnS precipitates. The electron backscatterdiffraction (EBSD) analysis showed that all of the laths in ap articular colony exhibit similar orientation to onea nother but as lightly different orientation than the parent allotriomorph, supporting as ympathetic nucleation mechanism.T he growth rate of the laths was found to vary widelywithin arange of 1.5 to 11 m m/s. The ferrite formation is finally halted by impingement with other advancing fronts. Ther esults are presented in ap henomologicald iscussion, with some quantitativea nalysis of the transformation kinetics.