A REACTION MODEL TO SIMULATE COMPOSITION CHANGE OF MOLD FLUX DURING CONTINUOUS CASTING OF HIGH Al STEEL
Min-Su Kim1, and Youn-Bae Kang1
1Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, Rep. of Korea
Keywords: High Al steel, Multi-component kinetic model, Continuous casting
Abstract
In order to find countermeasures against degradation of mold flux properties in high Al steel continuous casting, it is important to understand the reaction between Al-containing steel and CaO-SiO2-based molten flux. In the present authorsβ research group, the reaction rate and mechanism governing the reaction have been experimentally investigated. Based on those observed mechanism, a slag-metal reaction model was developed in order to interpret the reaction and to predict similar reactions between high Al steel and mold flux used for continuous casting process. The model considers both thermodynamic and kinetic information, as well as rate-controlling step observed in the authorsβ previous investigation. Thermodynamic information for chemical reaction was interpreted by ChemApp coupled with FactSage thermodynamic database. This is connected to mass flux equations for all relevant components in the system. Flux viscosity change and its effect on the mass flux are also taken into account. The reaction model developed in the present study showed good reproduction of the experimental data. Some possible applications are also shown.
Introduction
During continuous casting of high Al steel grades, drastic composition changes in the mold flux occur such as Al2O3 accumulation and loss of SiO2 [1]. It leads to increase of flux melting temperature and change of the crystalline phase from cuspidine (3CaOΒ·2SiO2Β·CaF2) to calcium aluminate. Therefore, estimation of composition changes in mold flux during steel-flux reactions is very important in order to predict mold flux performance. From this practical point of view, development of a multi-component kinetic model is of importance to predict composition evolutions in the conventional mold flux and to design a new mold flux composition.
For accurate prediction of composition changes in the liquid steel and flux, a kinetic model should be developed based on the reaction mechanism of the steel-flux system. From the previous experimental investigations done by the present authors [2 - 4], the reaction mechanism between high Al steels and CaO-SiO2-type fluxes have been intensively investigated. Some important conclusions are summarized here:
β At low Al concentration ([%Al]0 β€ 1.8), the Al2O3 formation reaction with SiO2 reduction is mainly controlled by mass transfer of Al in liquid steel.
β At high Al concentration ([%Al]0 β₯ 4.8), the rate controlling step changes into the mass transfer in the flux phase but retardation degree of the reaction rate depends on the type of aluminate formed in the vicinity of the steel-flux interface.
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
β When initial MgO content in the flux was low, CaAl4O7 layer was formed near the steel- flux interface at high Al concentration ([%Al]0 = 5.2), resulting retardation of mass transfer in liquid flux. When initial MgO content in the flux was high, however, Al2O3
accumulation in the flux occurred rapidly at high Al concentration ([%Al]0 = 5.2) with formation of many numbers of MgAl2O4 particles in the liquid flux.
Compared to a typical slag-metal system, mass transport phenomena in the liquid flux is quite complex in the high Al steel and CaO-SiO2-type mold flux system. During the reaction, chemical composition of the flux keeps varying. This changes physico-chemical properties of the flux, and consequently affect kinetic aspect. The existing kinetic models [5 - 7] for prediction of Al2O3 accumulation in the flux, however, do not explicitly consider the mass transfer in slag or flux phase in terms of viscosity changes and aluminate formation. Also, some of those [6, 7] are based on a simple steel-slag system such as Fe-Al and CaO-SiO2-Al2O3
system. In the present study, a multi-component kinetic model was developed based on the reaction mechanism elucidated by the present authors [2-4] in order to describe steel-flux reactions in high Al steel and CaO-SiO2-type flux system.
Development of a new multi-component kinetic model
Fig 1 shows the basic concepts and the calculation procedure of the present kinetic model. Each phase is assumed to have a reaction layer in order to perform equilibrium calculation between the reaction layers under local equilibrium assumption at the steel-flux interface. Once concentrations in each reaction layer was obtained from equilibrium calculations, then mass transfer in each phase was calculated by using flux equations. For the mass transfer in the liquid flux, the calculated mass flux was distributed into the flux boundary layer and the flux bulk layer. The concentration in the flux boundary layer was utilized in order to calculate flux viscosity which alters the mass transfer coefficients in the liquid flux. This concept is similar to an approaches as long as the reaction zones were defined for the equilibrium calculations [5] or mass flux equations of various moving species were employed [6,7]. However, the present approach is distinguished from the previous approaches [5-7] that 1) actual reaction mechanism is taken into account, 2) varying chemical composition of the flux during the reaction is taken into account in the evaluation of mass transport coefficient, 3) experimentally determined mass transport coefficient [2] was used. The details for each calculation module will be followed with some assumptions and equations.
Figure 1. Model concept and calculation sequence of the present multi-component kinetic model
Equilibrium calculations
In the present kinetic model, two independent equilibrium calculations were performed for 1) steel-flux reaction at the interface, and 2) aluminate formation in the flux boundary layer. It was assumed that chemical reactions between steel and flux only occur between steel reaction layer and flux reaction layer. In the flux boundary layer, formation of MgAl2O4, CaAl2O4, and CaAl4O7 in liquid flux is considered in equilibrium calculation and those calcium aluminates are assumed to be additional resistances of mass transfer in the liquid flux. Thermodynamic calculation was conducted by ChemApp [8]. Thermodynamic database required for the calculation was extracted from commercial database in FactSage [9, 10].
Mass transfer in liquid steel
Flux density of an element M in the liquid steel was calculated by using a simple flux equation:
π½M= πMπ(πΆMππ’ππβ πΆMππ₯π) (1)
where πMπ, πΆMππ’ππ, and πΆMππ₯π are mass transfer coefficient of the element M in liquid steel, concentration of the M in steel bulk layer, and concentration of the M in steel reaction layer, respectively. πMπ was calculated from the mass transfer coefficient of Al experimentally determined in the previous study [3] by considering diffusivity of Al and M in liquid steel:
πMπ = πAlπ π·π·Mπ π‘
Alπ π‘ (2)
Mass transfer in liquid flux
Similar to the flux density equation in liquid steel, the flux equation for the components in liquid flux could be written by using the number of moles of a component MxOy as:
π½Mπ₯Oπ¦= πMππ£π₯Oπ¦(πΆMππ₯ππ₯Oπ¦β πΆMππ’πππ₯Oπ¦) (3)
where πMππ£π₯Oπ¦, πΆMππ’πππ₯Oπ¦, and πΆMππ₯ππ₯Oπ¦ are overall mass transfer coefficient of the component MxOy in liquid flux, concentration of the MxOy in flux bulk layer, and concentration of the MxOy in flux reaction layer, respectively. From the previous kinetic investigation in the high Al steel and CaO- SiO2-type flux system [4], it was found that formation of calcium aluminate layers near the steel- flux interface interfered mass transfer in liquid flux. Therefore, formation and growth of calcium aluminates and its effect on the overall reaction rate are considered in πMππ£π₯Oπ¦ to describe drastic changes in mass transfer phenomena in liquid flux:
1 πMπ₯Oπ¦ππ£ = π 1
Mπ₯Oπ¦CAπ₯ + π 1
Mπ₯Oπ¦π (4)
where πMCAπ₯π₯Oπ¦ and πMπ π₯Oπ¦ indicate virtual mass transfer coefficient of the component MxOy
through calcium aluminate layer and that in the liquid flux, respectively. Based on the amount of calcium aluminates from the equilibrium calculation in the flux boundary layer, thickness of the calcium aluminate layer was calculated. πMCAπ₯π₯Oπ¦ is defined to be inversely proportional to the thickness of the calcium aluminate layer, resulting decrease in mass flux with increase in the thickness of the calcium aluminate layer.
In order to consider the effect of flux viscosity change during the steel-flux reaction on mass transfer in the liquid flux, πMsπ₯Oπ¦ is defined as a function of flux viscosity by using the following relationship between diffusivity and viscosity:
πMπ π₯Oπ¦= πMπ ,0π₯Oπ¦ππ0π ππ π (5)
where ππ π, π0π π, and πMs,0π₯Oπ¦ are flux viscosity in the flux boundary layer at the current timestep, initial flux viscosity in the flux boundary layer, and the mass transfer coefficient of MxOy when the flux viscosity in the boundary layer was π0π π, respectively. Similar to πMπ, πMs,0π₯Oπ¦ is obtained from optimized initial mass transfer coefficient of πAls,02O3 by using the diffusivity of MxOy and Al2O3.
Flux composition and viscosity in the flux boundary layer
Once the amount of component MxOy to be transferred from the flux reaction layer to the flux bulk layer was calculated, it was distributed into the flux boundary layer and the flux bulk layer in order to obtain the composition in the flux boundary layer. For the simplicity, it was assumed that the concentration in the flux boundary layer can be set to a value between the concentration in the flux reaction layer and that in the flux bulk layer. Once the compositions in the flux boundary layer was obtained, then flux viscosity was calculated and utilized for modification of mass transfer coefficients in liquid flux.
Results and discussion Calculation results compared with experimental data
Calculation results at various initial steel and flux compositions are compared with the experimental data obtained in the previous studies [2, 4], and a few examples are show in Fig. 2.
Even though some discrepancies between the calculation results and the experimental data were observed in the flux compositions, most of the concentration changes in liquid steel and flux are well-predicted from the present kinetic model.
Figure 2. Calculated composition evolutions (solid lines) in (a) liquid steel and (b) liquid flux under 4.8 mass pct of [%Al]0 condition with experimental data[4] (solid and open symbols)
Depending on the initial Al concentration in liquid steel, the effect of flux viscosity change on the overall reaction rate shows considerable differences as can be seen in Fig. 3. When initial Al concentration in the liquid steel was low, the composition evolutions in liquid flux were almost similar regardless of consideration of flux viscosity for the calculation of mass transfer coefficients in liquid flux, indicating steel phase mass transfer control. At high Al condition, however, the present kinetic model predicted faster Al2O3 accumulation in the liquid flux when changes of mass transfer coefficients with respect to flux viscosity was not considered, which shows dominant mass transfer in flux phase. Therefore, it can be concluded that the present kinetic model is compatible with the reaction mechanism experimentally elucidated in the previous studies.
Figure 3. Calculated composition evolutions of Al2O3, SiO2, and Na2O in liquid flux at (a) low Al concentration ([%Al]0 = 1.7) and (b) high Al concentration ([%Al]0 = 4.8) with different calculation methods for mass transfer coefficients in liquid flux.
Application of the present kinetic model to continuous casting process
Based on the multi-component kinetic model developed in the present study, development of a continuous casting model was possible by simple modifications of the present kinetic model such as adding incoming and outgoing flux in the flux bulk layer, and constant concentration in the steel bulk layer. In order to check the accuracy of the present continuous casting model, several calculations based on the casting parameter from literatures [6, 11] were compared with the pilot casting data available from the same literatures, showing good prediction on the composition changes in the mold flux. In addition to that, mold flux viscosity change, one of the important
mold flux properties for successful casting, could be calculated using the composition changes obtained from the present continuous casting model.
Conclusion
A new multi-component kinetic model for high Al steel and CaO-SiO2-type flux system was developed based on the reaction mechanism experimentally elucidated in the previous investigations [2, 3, 4]. The present kinetic model consists of a reaction layer in each phase and a boundary layer in the liquid flux, which enables simple calculations of local equilibrium calculation and flux density equation separately. Multi-component equilibrium calculation was performed by ChemApp [8] with thermodynamic database from FactSage [8, 9]. Also, flux viscosity changes and formation of calcium aluminate layer in the boundary layer are considered in the model for better description of mass transfer in the liquid flux. Generally, the calculation results are in good agreement with the experimental data obtained in the present study. Rate controlling step transition from mass transfer of Al in liquid steel to mass transfer in liquid flux was successfully predicted as initial Al concentration in liquid steel increased. Application of the present kinetic model also shows good calculation results compared with pilot casting data available from literature [6, 11].
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