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The nonmetallic spinel inclusion, MgO⅐Al 2 O 3 , 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 first-and 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 H 2 , hydrogen was impinged onto the melt surface to reduce dissolved oxygen content until it was less

Spinel (MgO · Al 2 O 3 ) is known as one of the most harmful non-metallic inclusions in steel. However, the technology to avoid spinel formation has not yet been established due to lack of thermodynamic information on the spinel itself. In the present work, the free energies of formation of MgO The activity of stoichiometric MgO · Al 2 O 3 exhibits negative deviation from ideality in the spinel solid solution. Phase stability diagram of spinel formation in liquid steel was assessed as a function of aluminum, magnesium and oxygen contents in iron at 1 873 K. The predicted phase boundaries of MgO · Al 2 O 3 , MgO and Al 2 O 3 agreed well with previous observations by one of the present authors.KEY WORDS: Spinel; non-metallic inclusion; free energy of formation; steelmaking; activity.constituents in steelmaking slags 24) was used. MgO and Al 2 O 3 are the component oxides of spinel, and their free energies of formation are necessary to determine DG°f ,MgO · Al 2 O 3 and the activity in the spinel solid solution. Reliable data on the free energy of Al 2 O 3 formation have been reported in the literature. [6][7][8][9]24) On the other hand, the reported value of DG°f ,MgO shows discrepancy with the order among DG°f of other alkaline earth oxides such as CaO, SrO and BaO. 25,26) Therefore, DG°f ,MgO is also directly measured in the present work. Experimental Principle Free Energy of Formation of MgOFor the measurement of DG°f ,MgO , experiments were conducted by equilibrating liquid copper containing graphite in a MgO crucible with CO-Ar atmosphere. The following equilibrium reaction should be established:The free energy change of Eq. (1), DG°1, can be related to its equilibrium constant, K 1 :Where, a is the activity of each component relative to the pure substance, and P CO denotes the partial pressure of CO (atm). Since the activities of MgO and carbon are unity due to the use of a pure MgO crucible and a pure graphite block, Eq. (3) can be simply derived from Eq. (2) Hultgren et al. 30) and Nayeb-Hashemi and Clark.31)The literature data on g Mg are summarized in Fig. 1(A). The agreement of the activity coefficient among literature values at the high concentration range of Mg seems to be excellent. On the other hand, there are some discrepancies in the low concentration range of Mg. Since the accuracy of g Mg at low concentration range is the most important for the precise determination of DG°1, the present authors 32) have assessed the thermodynamic properties of liquid Cu-Mg alloy with mass spectrometry. Their values are also plotted in Fig. 1(A). It is seen from Fig. 1(A) that the values of RT ln g Mg evaluated by the authors agree well with the values observed by Garg et al. and recommended by Hultgren et al. Therefore, the values of RT ln g Mg given by the authors have been employed in the present work. It has also been reported that the temperature dependence of RT ln g Mg is very small as shown in Fig. 1(A). 32) Therefore, the value of K 1 can be determined from the equilibrium concentration of Mg in l...

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