Isothermal and Non-isothermal Crystallization Kinetics of Mold Fluxes used in Continuous Casting of Steel: A Review

Casting powders or mold fluxes, as they are more commonly known, are used in the continuous casting of steel to prevent the steel shell from sticking to the copper mold. The powders first melt and create a pool of liquid flux above the liquid steel in the mold, and then the liquid mold fluxes penetrate into the gap between water-cooled copper mold and steel shell, where crystallization of solid phases takes place as the temperatures gradually drop. It is important to understand the crystallization behavior of these mold fluxes used in the continuous casting of steel because the crystalline phase fraction in the slag films plays a crucial role in determining the horizontal heat flux during the casting process. In this work, the existing literature on the crystallization kinetics of conventional and fluoride-free mold fluxes used in the continuous casting of steel has been reviewed. The review has been divided into two main sections viz. the isothermal crystallization kinetics and non-isothermal crystallization kinetics. Under each of these sections, three of the most widely used techniques for studying the crystallization kinetics have been included viz. thermoanalytical techniques such as differential scanning calorimetry/differential thermal analysis (DSC/DTA), the single and double hot thermocouple technique (SHTT and DHTT), and the confocal scanning laser microscopy (CSLM). For each of these techniques, the available literature related to the crystallization kinetics of mold fluxes has been summarized thereby encompassing a wide range of investigations comprising of both conventional and fluoride-free fluxes. Summaries have been included after each section with critical comments and insights by the authors. Finally, the relative merits and demerits of these methods vis-à-vis their application in studying the crystallization kinetics of mold fluxes have been discussed.

Effects of Li2O on Structure and Viscosity of CaO-Al2O3-Based Mold Fluxes

Since CaO-Al2O3-based mold fluxes are one of the most important mold flux systems in metallurgic processes, it is important to explore their structure characteristics and viscosity. Molecular dynamics simulation is performed to study the effect of w(CaO)/w(Al2O3) ratio on both the structural and viscosity properties of CaO-Al2O3-based mold fluxes. A systematic analysis of the structure and thermodynamics on CaO-Al2O3-based mold fluxes is carried out, and it is well known that the viscosity of mold fluxes is related to the structure. The results show that the formation of stable structures of Si-O in the mold fluxes was beneficial to reduce the probability of structural interconnection, degree of polymerization, and viscosity of the molten slag. In the cationic structure, the contents of Ca-O-Al and Ca-O-Si are more stable, the interconnection of the Ca-O-Al and Ca-O-Si network weakens, and the viscosity decreases. The tetrahedra [AlO4] and [SiO4] have similar structures, but they exhibit different thermodynamic and physical properties. Viscosity test shows that CaO/Al2O3 = 0.88–2 continuously increased, when the cosolvent content Li2O = 1%–4%, CaO-Al2O3-based mold flux viscosity decreased, the degree of network structure polymerization decreased, and the complex structure depolymerized. Increasing the water content in the cosolvent is beneficial to reduce the viscosity of the crystallizer.

Effects of BaO and B2O3 on the Absorption of Ti Inclusions for High Titanium Steel

In order to study the effect of BaO or B2O3 on the absorption of Ti inclusions, the effects of mold fluxes with different contents of BaO (0~15%) or B2O3 (0~15%) on the mass transfer coefficients of TiO2 or TiN were studied with the rotating cylinder method. The experimental results show that with the addition of BaO in the mold flux, the mass transfer coefficient of TiO2 increases from 4.58 × 10−4 m/s to 6.08 × 10−4 m/s, that of TiN increases from 3.09 × 10−4 m/s to 4.41 × 10−4 m/s, 2CaO·MgO·2SiO2 is transformed into BaO·2CaO·MgO·2SiO2, and the Ti inclusions combine with CaO to form CaTiO3. With the addition of B2O3 in the mold flux, the mass transfer coefficient of TiO2 increases from 4.58 × 10−4 m/s to 7.46 × 10−4 m/s, that of TiN increases from 3.09 × 10−4 m/s to 5.50 × 10−4 m/s, CaO and B2O3 combine to 2CaO·B2O3, and Ti inclusions exist in the form of TiO2. During the experiment, TiN will be transformed into titanium oxide.