A novel laboratory technique for introducing inclusion to liquid steel
Previous studies in steel cleanness have been hampered by limited control of inclusions’ phase and composition. This limited control is often a result of studying inclusion formation and development using industrial steel melts and the inclusions therein as starting material. This is an extremely complex situation and not always readily amenable to analysis. A new experimental laboratory technique has been developed that allows addition of inclusions of known, size, phase and composition to be added to liquid steel. These inclusions can then be tracked and changes in morphology with time can be assessed for different melt conditions. The technique involves the formation of a steel-oxide compact, where high-purity inclusions are sandwiched in steel foil at high pressure. This compact is added to a steel melt of controlled and known composition. The use of this sandwiching technique overcomes problems of passing small particles (inclusions <50 μm) through the gas-melt interface and inclusion agglomeration. Details of the technique are presented and discussed and results applying this new technique in a reactivity study using cerium oxide as a synthetic inclusion in steel are given.
Inclusion reactivity: morphology and composition changes of spinel (MgAl2O4) in steel
Due to an ever increasing demand for cleaner, high quality steels, there is an increasing push for steelmakers to lower the non-metallic inclusion contents of the steel they produce. Understanding inclusion reactivity in steel is key in producing high quality steels at high production rates. Our current knowledge in controlling and predicting inclusion development during liquid steel processing is limited. In this study, spinel (MgAl2O4) inclusions of close to stoichiometric MgO.Al2O3 and known size distribution where added to the liquid steel bath prior to assessing their reactivity. The pO2 of the gas phase was controlled to 10-13 atm throughout the experiment. The inclusions were then tracked for changes in size, morphology and composition with time in an aluminium killed steel at 1600 C. It was found that the inclusions were evenly distributed through the melt. There was little change in the average size of the spinel inclusions during the reaction. However, the composition of the inclusions did change. It was found that the Mg:Al mass% ratio of the inclusions changed immediately from 0.5 to 0.08. Other inclusions found in the melt were alumina and complex sulphide inclusions. Of these it was found that the proportion of the alumina inclusions increased with time.
Thermodynamics on the formation of spinel nonmetallic inclusion in liquid steel
The nonmetallic spinel inclusion, MgOzAl2O3, has a high melting point and produces an undeformed C-type inclusion in steel products. Therefore, it is very harmful in the production of high grade wire, spring, and bearing steels. Thus, it is necessary to develop a methodology by which we can predict the occurrence of such defects. The thermodynamics of the formation of spinel nonmetallic inclusions during co-deoxidation with aluminum and magnesium is discussed in the present work. Previous literature thermodynamic values for strong deoxidizers such as aluminum, magnesium, and calcium are not in good agreement with the observed results and measurements. This discrepancy between predicted and measured results is due to an incomplete expression of the activities of dissolved oxygen and deoxidizers in current literature. Namely, only first-order interaction parameters are available despite the very strong interactions among the dissolved elements. In the present work, the deoxidation equilibrium with magnesium in liquid iron was studied in order to evaluate the activities of oxygen and dissolved magnesium and, include in the thermodynamic description, the firstand second-order interaction parameters, including the cross-product terms. A dolomite crucible was used to enhance the magnesium content in liquid iron in the present experiment on thermodynamics of spinel formation with reference to the study on desulfurization with a dolomite crucible undertaken by Ototani et al.[1] The assessment of aluminum-oxygen equilibrium in liquid iron reported by other investigators was also reviewed to develop values for the second-order interaction parameters. Finally, thermodynamic data on calcium deoxidation equilibrium were obtained. The iron sample was melted by a high frequency induction furnace. Iron of about 90 grams was charged into a crucible. The added weight of a deoxidizer such as magnesium and calcium was 1 mass pct of the weight of iron. The experimental procedure is briefly shown as follows. After the iron melted under a gas flow of a mixture of Ar-50 vol pct H2, hydrogen was impinged onto the melt surface to reduce dissolved oxygen content until it was less