The thermodynamic data on interaction parameters of various elements in iron are regarded as an essential basis of steelmaking technology. Several experimental techniques for determining the interaction parameters between elements in molten iron have been developed so far. This article introduces the evaporation technique and the chemical equilibrium technique out of them. The former typified by a) Knudsen-mass spectrometry and b) Gas transpiration method is a straightforward way to obtain the interaction parameters because the activity of elements bears a proportional relationship to their vapor pressure. This technique is effective against the elements with high vapor pressure in molten iron. On the other hand, the latter is often carried out under the equilibrium state of the reaction that simulates the chemial reaction in refining process etc. to acquire practical values for steelmaking. c) Distribution coefficient method, d) Equilibrium measurement of chemical reaction, and e) Equilibrium measurement of oxidation via Ag are explained as a chemical equilibrium technique. The theory and characteristics of each method are described in this article.
EMF (Electro-Motive Force) method using oxygen sensor has been widely applied for determining activity coefficient and interaction parameters between alloying elements through soluble oxygen activity in molten solute metal. This method is effective particularly for evaluating activity coefficient of a deoxidizing element in molten metal through oxygen potential measurement, where chemical equilibrium can be achieved between solid oxide consisting of the deoxidizing element, the deoxidizing element and oxygen soluble in molten metal. In this paper, evaluating procedure and several key points are reviewed in terms of EMF method to determine thermodynamic parameters, particularly activity coefficient and interaction parameters, of alloying elements in molten iron-based metal.
Thermodynamic data including interaction coefficients necessary for molten Fe-Cr-Ni-Mo system alloys have not been sufficient to calculate terminal compositions in equilibrium as a result of some reaction. Particularly, the coefficients in relation with Mo are insufficient so that the corresponding coefficients have to be neglected usually. In this status, eAlMo = 0.007 was estimated by regularity according to atomic number after rearranging the other coefficients related to the activity coefficients for Al according to periodical table. Applying the value for the stability diagram of MgO / MgO·Al2O3 / Al2O3 previously reported by the author, the boundaries did not vary so much.
Subsequently, interaction coefficients related to the activity coefficients for Si, used as deoxidant the same as Al, were also rearranged with periodical table resulting in finding less clear periodic relation than Al. In particular, the values of eSiTi so far reported have significant deviation between the researchers leading to the difficulty in prediction about the terminal compositions in equilibrium when considering some reactions. Furthermore, the value of eSiMo = 0.11 recently proposed could describe the lower attained oxygen content in SUS316L containing 2 mass% Mo than that of SUS304 stainless steels. Finally, it is truly expected that the interaction coefficients related to Mo is made clear in the near future.
In recycling a lot of low grade ferrous scraps, we cannot remove tramp elements, such as copper and tin, which are inevitably dissolved in molten iron. Accordingly, the thermodynamic data between such tramp elements and alloying elements in molten iron are necessary to know the influence of them on the property of steel. In this work, the interaction between tin and M(M: Mo, B, Ni, Ti or Nb) in iron has been investigated by using immiscibility between Fe and Ag phases at 1873 K. The distribution ratio of Sn between Fe and Ag, LSn = xSn(in Ag)/xSn(in Fe), increases with increasing Mo, B, Ti or Nb content and decreases with an increase of Ni content. That is to say, it is found that the interaction coefficients of Mo, B, Ti or Nb on Sn are positive and that of Ni on Sn has a negative value. The interaction coefficients of M on Sn are estimated from the activity coefficients of M in Fe and Sn, and those values are compared with the experimental ones. It is found that the experimentally measured interaction coefficients of Pb and Nb are much different from the estimated values. It is important to measure the interaction coefficient of M on Sn experimentally to know the effect of M on Sn activity accurately.
Efficient and sophisticated refining process for highly alloyed steel is required in order to meet the highest-priority demand for physical and chemical properties of such steel products. In the present study, Al deoxidation equilibrium of the molten Fe-Mn system for which the demand has been continuously increasing nowadays was measured in the Mn composition range from 10 to 30 mass% at 1873 K. Equilibrated oxide phase varied depending on Al concentration; MnAl2O4 or Al2O3 was equilibrated at relatively low or high concentration range, respectively. The interaction parameter between Al and O, and equilibrium constant of Al2O3 dissolution reaction estimated from results of Al2O3-equilibrated experiments did not show the significant Mn concentration dependence of the Fe-Mn melt.
The interaction parameter between Al and Sn in molten iron containing high Al was measured using a chemical equilibrium technique. A molten Fe-Al alloy was equilibrated with a molten Ag-Sn alloy in an Al2O3 crucible at 1823 K. The interaction parameter of Sn and Al in Fe-Al alloys was measured to be εAlSn=–3.18 while varying the concentration of Al in a range from 0.01-0.17 in mole fraction. The influence of Sn on the deoxidation equilibrium of Al and O was estimated using the above interaction parameter. The results indicate that under a constant Al concentration, a contamination of several percent of Sn slightly raises the O concentration in high Al steel.
The interaction parameter between Mo and Al in molten Fe-Al-Mo alloy was measured using a chemical equilibrium technique. A molten Fe-Al-Mo alloy was equilibrated with molten Ag in an Al2O3 crucible at 1823 K. The activity coefficient of Mo in Fe-Al alloys was measured as
εMoAl = –6.4 ± 0.8 at 1823 K (0 < XMo < 0.03)
Using the above interaction parameter, the influence of Mo on the deoxidation equilibrium of Al and O was estimated. The results indicate that the existence of Mo affected little on the equilibrium concentration of O in an Al-containing steel.
Rare earth metals are generally used as a deoxidizer of molten steel due to its high reactivity with oxygen content in the steel. To produce high-grade steel by efficient steel refining, information on thermodynamic interaction between these rare earth metals and major alloying elements in molten steel are important, although these information are very limited. In the present study, activity coefficient of Zr element in molten iron-based alloy was experimentally determined through oxygen potential in gas / molten Fe-Zr alloy / ZrO2-containing molten slag / solid ZrO2 multi-phase equilibrium, measured by an solid electrolyte oxygen sensor. Solid Nb / liquid Nb2O5 phase equilibrium was used to determine the referenced oxygen potential. The ZrO2-containing liquid slag was provided on the liquid Fe-Zr alloy to achieve good wettability with the solid electrolyte oxygen sensor, so that stable electromotive force (EMF) measurement of the oxygen sensor was enabled. The slag composition was selected so as to be equilibrated with the ZrO2 cubic solid phase at the treated temperature. A quite low value of Zr activity coefficient in the molten iron was evaluated, which indicated highly attractive interaction between Fe and Zr in the liquid phase. In addition, the oxygen potential in molten Fe-Zr-Cr alloy in equilibrium with solid ZrO2 and ZrO2-containing slag was evaluated to determine the effect of Cr addition on dissolved O activity in molten Fe-Zr alloy.
A study has been carried out to evaluate the equilibrium phase relations of molten Si–Fe, Si–Ni, and Si–Fe–Cr alloys saturated with either silicon carbide (SiC) or graphite. The measured carbon solubilities at 2073 K were 0.19-6.6 mol% for Si– (24.1–70.1) mol%Fe, 0.061–5.2 mol% for Si– (30.0–85.0) mol%Ni, and 1.1–3.9 mol% for Si– (50-x) mol%Fe – x mol%Cr (x=10.4–40.1) alloys, respectively. Quasi-chemical model, which assumes carbon atoms to be introduced into interstitial sites of Si-Fe, Si-Ni, and Si-Fe-Cr solvents and obstruct the bonding between solvent atoms, was adopted to evaluate the activity coefficient of carbon in each alloy, and its estimation reproduced fairly well the tendency of measured carbon solublities. On the other hand, estimation using the sub-regular solution model often overestimated the carbon solubilities. It was thus found that the carbon behaviors in silicon-transition metal alloys can be well described by using the quasi-chemical model.
Expansion of the system for thermodynamic parameters in liquid iron is expected for the improvement in iron & steelmaking processes. The development of their calculation method is one of the issues for its realization. In the present work, we proposed a calculation method for an activity coefficient of solute in infinite dilute liquid iron to element i, γοi, based on the surface tension of binary liquid Fe alloys. It was found that the estimated values by our proposed method agree with recommended literature data.
The estimation of interaction parameters in liquid iron is strongly demanded due to the difficulty of their measurements and its time consuming for enormous combinations of target solute elements in liquid iron. Therefore, several estimation models have been developed so far. In this study, the interaction parameters between metal elements and/or metalloid elements in liquid Fe are estimated by neural network computation in order to improve the estimation accuracy. The input parameters used in the neural network computation are assessed by lateral inhibition learning. The estimation results by nerural network computation with the assessed parameters reasonably agree with the recommended values in the literature.
Although thermodynamic interaction between titanium and silicon in molten iron is greatly important to precisely control Si-Ti complex deoxidation of liquid steel and titanium inclusions formation in the steelmaking process, the reported values of the interaction parameter between titanium and silicon in molten iron show large discrepancies. In the present work, the equilibration of molten Fe-Ti-Si alloy with both Ti3O5 and Ti2O3, which provides the controlled activity of titanium, was performed at 1873 K and the interaction parameter was determined.