The latest publications regarding the development of technology to control inclusion compositions focusing on MgO·Al2O3 spinel inclusions were summarized in this review article. The problems caused by spinel inclusions, which affect practice as well as products were shown. The formation mechanism of MgO·Al2O3 spinel inclusions is secondly explained thermodynamically from the view points of chemistries of molten steels and slag compositions. Furthermore, crystallization behaviour of spinel was introduced. Countermeasures conducted in practices and laboratories were shown along with some problems still left that should be solved in the future.

https://www.jstage.jst.go.jp/article/isijinternational/50/10/50_10_1333/_pdf

Control of MgO·Al2O3 Spinel Inclusions in Stainless Steels

For relattively dilute solutions of oxygen and a deoxidizer M (M=Al, Cr, Ca, Mg, ...) in molten Fe, a thermodynamic model is proposed wherein the dissolved species are M*O associates as well as unassociated M and O atoms. At higher metal concentrations, a small amount of M
 2*O associates also form. Experimental deoxidation equilibria for 15 deoxidizers M are quantitatively reproduced at all temperatures, with only a constant, temperature-independent and composition-independent empirical parameter for the Gibbs energy of formation of each associate. The deoxidation behavior of Mg, Ca, and Ba is elucidated for the first time. The parameters are stored in a database, which can be used to predict complex deoxidation equilibria in multicomponent steels.

A thermodynamic model for deoxidation equilibria in steel

The current study performed thermadynamic calculation, laboratory experiments, and industrial trials for the formation and modification of MgO-Al2O3 spinel inclusions in alloy steels. The stability Mg-Al-O diagram was obtained using the thermodymanic study. The resulting MgO-Al2O3-CaO inclusions from MgO-Al2O3 spinel inclusions after the calcium treatment were spherical, and > 5 μm MgO-Al2O3-CaO inclusions have a two-layer structure: an outside CaO-Al2O3 layer and a MgO-Al2O3 core. The modification of > 5 μm MgO·Al2O3 spinel inclusions by calcium treatment includes two steps: (1) reducing MgO in the inclusion into the dissolved magnesium by the dissolved calcium in the steel and (2) generating a liquid xCaO·yAl2O3 layer at the outside of the spinel inclusion. For <2 μm MgO·Al2O3 spinel inclusions, they can possibly be modified into a xCaO·yAl2O3 inclusion by reducing all MgO component in the spinel inclusions with the added calcium.

Formation and Modification of MgO·Al2O3-Based Inclusions in Alloy Steels

A thermodynamic equilibrium between the Fe-16Cr melts and the CaO-Al2O3-MgO slags at 1823 K as well as the morphology of inclusions was investigated to understand the formation behavior of the MgO-Al2O3 spinel-type inclusions in ferritic stainless steel. The calculated and observed activities of magnesium in Fe-16Cr melts are qualitatively in good agreement with each other, while those of aluminum in steel melts exhibit some discrepancies with scatters. In the composition of molten steel investigated in this study, the log (X
 MgO/X
 Al
 2O3) of the inclusions linearly increases by increasing the log [a
 Mg/a
 Al
 2
 ·a
 O
 2
 ] with the slope close to unity. In addition, the relationship between the log (X
 MgO/X
 Al
 2O3) of the inclusions and the log (a
 MgO/a
 Al
 2O3) of the slags exhibits the linear correlation with the slope close to unity. The compositions of the inclusions are relatively close to those of the slags, viz. the MgO-rich magnesia-spinel solid solutions were formed in the steel melts equilibrated with the highly basic slags saturated by CaO or MgO. The spinel inclusions nearly saturated by MgO were observed in the steel melts equilibrated with the slags doubly saturated by MgO and MgAl2O4. The spinel and the Al2O3-rich alumina-spinel solid solutions were formed in the steel melts equilibrated with the slags saturated by MgAl2O4 and MgAl2O4-CaAl2O4 phases, respectively. The apparent modification reaction of MgO to the magnesium aluminate inclusions in steel melts equilibrated with the highly basic slags would be constituted by the following reaction steps: (1) diffusion of aluminum from bulk to the metal/MgO interface, (2) oxidation of the aluminum to the Al3+ ions at the metal/intermediate layer interface, (3) diffusion of Al3+ ions and electrons through the intermediate layer, and (4) magnesium aluminate (MgAl2O4 spinel, for example) formation by the ionic reaction.

Effect of CaO-Al2O3-MgO slags on the formation of MgO-Al2O3 inclusions in ferritic stainless steel

This study investigates the evolution of inclusions as a result of Al and Ti additions to molten Fe at 1 873 K with the objective of elucidating the transient stages of inclusion formation during ladle processing of IF steel melts. The effects of order of addition, time after addition, Al/Ti ratio and oxygen content are evaluated through an experimental approach that involves de-oxidation and sampling inside a vacuum induction furnace under conditions where the total oxygen concentration of the melt and samples are maintained constant. Each sample is analyzed for chemistry and resulting inclusion characteristics (size, morphology and chemistry). All experiments were carried out under the thermodynamic condition that Al2O3 was the only stable inclusion. The following results were found. Firstly, the equilibration time of Al was found to be faster than that of Ti under the present experimental conditions and as a result Al2O3 forms initially when Al and Ti were simultaneously added. The addition of Ti results in the formation of oxide inclusions containing Ti up to 20 mol% as a result of local Al depletion and Ti-supersaturation. This was enhanced in terms of the fraction of Ti containing inclusions as the Ti content was increased. Ultimately, in all samples, the thermodynamically stable inclusion Al2O3 was predominant within 5 min, but the morphology of these final inclusions were of polygonal shape rather than the spherical inclusions that formed immediately after Al de-oxidation. This modification in shape could have consequences on clogging, during post ladle teeming and pouring, in continuous casting as the tendency for agglomeration would be expected to increase.

The Transient Stages of Inclusion Evolution During Al and/or Ti Additions to Molten Iron

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

Thermodynamics on the Formation of Spinel Nonmetallic Inclusion in Liquid Steel

https://www.jstage.jst.go.jp/article/tetsutohagane1955/83/12/83_12_773/_pdf

Assessment of Al Deoxidation Equilibrium in Liquid Iron

Thermodynamics on the Formation of Non-metallic Inclusion of Spinel (MgO·Al2O3) in Liquid Steel

https://www.jstage.jst.go.jp/article/tetsutohagane1955/83/10/83_10_623/_pdf

Deoxidation Equilibrium of Calcium in Liquid Iron

The formation conditions of spinel non-metallic inclusion during deoxidation with aluminium in secondary refining furnace with MgO lining were discussed in the present work. The thermodynamic data of deoxidation equilibrium with magnesium, calcium and aluminum in liquid iron were determined by the authors. The activities of oxygen and dissolves deoxidizer were evaluated in the the modynamic description by the first and second order interaction parameters including the cross product terms.

Hiroyasu Itoh

Papers

Thermodynamics on the Formation of Spinel Nonmetallic Inclusion in Liquid Steel

Assessment of Al Deoxidation Equilibrium in Liquid Iron

Thermodynamics on the Formation of Non-metallic Inclusion of Spinel (MgO·Al2O3) in Liquid Steel

Deoxidation Equilibrium of Calcium in Liquid Iron

Gibbs free energy and conditions of spinel (MgO·Al2O3) formation in liquid steel