고 망간 고 알루미늄 강과 라임 실리카 기반 몰드 플럭스 간의 화학 반응에 관한 반응 메커니즘 연구 및 반응론적 해석

Abstract

During a continuous casting process, high Al content in the liquid steel causes rapid reaction between a high Mn-high Al steel and a CaO-SiO2-type mold flux. Due to the strong affinity of oxygen, Al in the liquid steel can react with weak oxides in the liquid flux (mainly SiO2 in CaO-SiO2-type mold flux) resulting Al2O3 formation with reduction of those weak oxides, which is so-called ``Al2O3 accumulation in the mold flux''. This Al2O3 accumulation changes the mold flux system from CaO-SiO2-based glassy liquid with low melting temperature to CaO-Al2O3-based sluggish liquid with high melting temperature. The primary crystalline phase also changes from facet cuspidine (3CaO 2SiO2 CaF2) dispersed in the liquid matrix to needle-like calcium aluminates grown as a tight layer at the solidifying steel shell-glassy flux interface. Consequently, Al2O3 accumulation is very detrimental to mold flux performance in terms of lubrication and heat transfer.

A simple countermeasure is prevention or retardation of Al2O3 accumulation in CaO-SiO2-type mold flux to maintain the initial flux properties. To do that, reaction mechanisms between high Mn-high Al steel and CaO-SiO2-type mold flux should be understood and the dominant factors for Al2O3 accumulation should be carefully controlled. Even though many kinetic studies in Al-containing steel and CaO-SiO2-Al2O3-based slag systems have been conducted under different Al concentration in the liquid steel and SiO2 concentration in the slag, the reaction mechanism is not clearly elucidated yet. Also, prediction of mold flux chemistry changes during a continuous casting process by using a kinetic model is of importance to design a new mold flux composition and reduce the number of on-site trials in the continuous casting machine. Some kinetic models for the continuous casting process are available from literature but they are not based on either a multi-component steel-flux system or experimentally-determined reaction mechanisms.

Therefore, experimental investigations on reaction mechanism between high Mn-high Al steel and CaO-SiO2-type mold flux have been conducted under various steel and flux chemistries. The dominant reaction in the present steel-flux system was Al2O3 formation reaction with SiO2 reduction while Na2O reduction by Al as well as formation and reduction of FeO and MnO are relatively small compared with SiO2 reduction by Al. The rate controlling step of the Al2O3 accumulation reaction with SiO2 reduction was determined to be mass transfer of Al in liquid steel at low Al concentration ([%Al]0 ≤ 1.8 mass pct). At high Al concentration in liquid steel ([%Al]0 ≥ 4.8 mass pct), different reaction rates of Al2O3 formation were observed depending on different aluminate formed in the liquid flux. At low MgO concentration in the liquid flux (0 - 1.78 mass pct), the overall reactions were remarkably retarded with formation of CaAl4O7 layers near the steel-flux interface. On the other hand, fast Al2O3 accumulation was observed in liquid flux with high MgO concentration (5 - 15 mass pct). Those differences are determined to be closely related to the formation mechanism of different aluminates by the steel-flux reaction.

Based on the reaction mechanisms elucidated in the present study, a new multi-component kinetic model, so-called ``diffusion-coupled reaction zone model'' was developed by considering convenient expansion to a higher-order multi-component steel-flux system and better description of mass transfer in liquid flux in terms of viscosity changes and aluminates formation. In the present kinetic model, local equilibrium calculation and flux density calculation could be conducted separately which makes model calculation simple and straightforward. Multi-component equilibrium calculation was performed by ChemApp linked in the main C code with aid of thermodynamic database extracted from FactSage. For better description of mass transfer in the liquid flux, flux viscosity changes in the flux boundary layer was considered by modifying the mass transfer coefficients in liquid flux. Also, formation of calcium aluminates in the flux boundary layer was regarded as additional resistance for mass transfer in liquid flux to describe retardation of overall reaction rate in low MgO flux. Generally, the calculation results are in good agreement with the experimental data obtained in the present study. It could be concluded that consideration of viscosity changes and calcium aluminate layer formation is very important to describe composition evolution in high Mn-high Al steel and CaO-SiO2-type mold flux system by using a multi-component kinetic model.

A continuous casting model was developed by simple modifications in the present multi-component kinetic model such as adding incoming and outgoing mold flux in the flux bulk layer, and constant concentration in the steel bulk layer. Accuracy of the present continuous casting model was verified by comparison of calculation results with the pilot plant data from literatures. From the calculation results with different variables such as [%Al]0, mold flux pool depth, and mold flux consumption rate, it was concluded that dilution of Al2O3 contents in the mold flux in terms of large quantity (mold flux pool depth) and refreshing speed (mold flux consumption rate) are important factors to achieve retardation of Al2O3 accumulation in the mold flux during high Mn-high Al steel casting.