The role of mould fluxes in continuous casting is reviewed. The main functions of the flux are described and the importance of the flux in providing lubrication and the right level of heat transfer between the shell and the mould is described. The major problems of longitudinal, transverse and star cracking, gas and slag entrapment and deep oscillation marks in the steel product and 'sticker breakouts' are analysed. The important physical properties of the flux in combating these various problems are identified. Physical property data for the fluxes are given.

Performance and properties of mould fluxes

Values of the viscosities of liquid metals are important in the prediction of fluid flow in many metallurgical manufacturing processes This paper describes a number of methods used to measure the viscosity of liquid metals, including capillary, oscillating vessel, rotational bob or crucible, oscillating plate, draining vessel, levitation using the damping of surface oscillations and acoustic methods A number of models used to estimate viscosity for elements, the temperature dependence of viscosity, and viscosity of multicomponent systems are also given, including the Andrade equation, Arrhenius equation, Hildebrand's free volume theory, Chhabra models, Moelwyn-Hughes model and thermodynamic models The scatter of data available in the literature are highlighted by comparing two reviews of data for elements

https://iopscience.iop.org/article/10.1088/0957-0233/16/2/005/pdf

The measurement of viscosity of alloys—a review of methods, data and models

Synopsis The objective of this work was to provide process engineers with values of the physical properties of various slag systems involved in high-temperature processes. Software that calculates the thermophysical properties of slags from chemical composition is available on the www.pyrometallurgy.co.za website. This paper outlines the principles underlying the various models available in the program. The software calculates the following properties of crystalline, glassy, and liquid slags (where appropriate) as a function of temperature: heat capacity, enthalpy, density, viscosity, thermal conductivity, electrical conductivity, and surface tension. We hope, in the future, to update the program to (i) add new models as they become available, (ii) remove any ‘bugs’ discovered in existing programs, and (iii) provide guidance on the limitations of individual programs.

/pdf/estimating-the-physical-properties-of-slags-3npq5d6xpe.pdf

Estimating the physical properties of slags

More than 90% of the world's steel is produced using the continuous casting process, a method that has seen enormous advances over the last forty years. Mould fluxes play an important part in this process. These fluxes contain fluorides, which can volatilize at operational temperatures polluting both the plant air and cooling water. Airborne fluoride could potentially be a health and safety issue. Waterborne fluoride forms hydrofluoric acid (HF), which can cause plant corrosion, and may lead to contamination of watercourses necessitating water treatment schemes. This adds to production costs and may present potential environmental hazards. These concerns could be reduced or eliminated by removing fluoride from mould fluxes.The present study examines the effect of different fluxing agents upon key mould flux properties. When substituting fluorides for alternative fluxing agents the key design properties of the fluoride-containing flux must be replicated; namely, (i) flux viscosity at 1300°C, (ii) break temperature and (iii) percentage of crystallinity in the solid slag layer. This is to ensure ‘optimal casting’ where operational problems, such as sticker breakouts and defects such as longitudinal cracking, are minimized. In addition, the quality of the steel should not be affected by the substitution. Therefore, any substitute/additive or combination of additives would have to possess the capacity to replicate the effects that fluorine has on mould flux behaviour.This study focuses on B2O3 and Na2O as alternative substitutes for CaF2 in billet fluxes. The new flux has been successfully tested in a plant trial on a continuous casting plant.

http://www.jstage.jst.go.jp/article/isijinternational/45/7/45_7_1051/_pdf

Development of Fluoride-Free Fluxes for Billet Casting

In this paper, titanium-bearing blast furnace slags (CaO–SiO2–TiO2) produced at Panzhihua Iron and Steel Company (P. R. China) is used as the base material to develop fluoride-free (F-free) mold powders to improve the heat transfer between the mold and the strand. Effects of the binary basicity (CaO/SiO2), TiO2, Na2O, Li2O, MgO, MnO and B2O3 on the melting temperature, viscosity and heat flux of F-free mold powders are investigated. The laboratory results indicate that 1) the melting temperature and the viscosity of the F-free powder decrease, as expected, with increasing the content of Li2O, B2O3 and Na2O respectively, but the lowest viscosity is achieved with 6.0 mass% TiO2; 2) the heat flux of the F-free slag film with 1.0–6.0 mass% TiO2 is close to that of a conventional mold slag film with 2.0–10.0 mass% F; 3) the effect of basicity of the F-free powder on the heat flux is the same as the powder bearing fluoride; 4) the heat flux changes significantly with more than 8.0 mass% Na2O and about 4.0 mass% MnO, whereas the effects of Li2O and B2O3 in the F-free powder on heat flux are not significant. The suitable range of main components of the F-free powder with TiO2 is proposed for casting peritectic-grade-steel slabs. The industrial trials of peritectic steel casting, using the proposed F-free flux, reveals a good surface quality of the slab, and well-controlled heat transfer at the continuous casting mold by the F-free powder with the precipitated crystalline phase being perovskite (CaTiO3) instead of cuspidine in the conventional mold slags that contain fluoride.

Development of fluoride-free mold powders for peritectic steel slab casting

The performance of mould fluxes in providing the optimum levels of lubrication and horizontal heat flux is relatively simple since they rely on only three properties, the viscosity and break temperature of the flux and the crystalline fraction developed in the slag film. These parameters can be readily calculated according to the mould dimensions, casting conditions and steel grade. It is only when the flux is used to combat other problems, such as those arising from turbulent metal flow (e.g. slag entrapment) that flux behaviour becomes complex and more difficult to understand. Some recent developments may help to combat problems associated with turbulent metal flow and free the flux to carry out its principal duties of providing optimum lubrication and horizontal heat flux.

The Role of Mould Fluxes in Continuous Casting-So Simple Yet So Complex

Non-metallic inclusions composed of ZrO 2 , Al 2 O 3 , MgO and MgAl 2 O 4 are associated with problems during the continuous casting of steels and so it is desirable that such particles dissolve completely if they appear in the slag. The dissolution behaviour of particles of these oxides in a fluorine-free slag containing 1.5 wt% B 2 O 3 , was studied in situ using Confocal Scanning Laser Microscopy (CSLM). The effects of particle type, initial size, and slag temperature were investigated. Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) was performed to chemically analyze quenched samples, in order to identify any surface reaction or diffusion layers formed. Analytical prediction models were compared to the experimental data to relate the kinetics to possible rate-limiting steps. Thermodynamic solubility limits for use in the model were determined using commercial CALPHAD based software. The dissolution rates of Al 2 O 3 , MgO and MgAl 2 O 4 were found to be comparable to one another whereas the dissolution rate of ZrO 2 , is four times slower. The surface reaction appears to be controlling the rate of dissolution, with the activation energy for ZrO 2 being 128.8 kJ/mol and for MgAl 2 O 4 being 77.8 kJ/mol. This implies that the removal of ZrO 2 particles by dissolution in this type of slag is not feasible for the typical residence times expected, and strategies that prevent the incorporation of these particles should be used.

Dissolution of ZrO2, Al2O3, MgO and MgAl2O4 particles in a B2O3 containing commercial fluoride-free mould slag

A "round robin" project has been carried out to detennine the accuracy and reliability of various models used to estimate the viscosities of industrial slags. The 21 participants were divided into measurement suppliers and modelling participants. Reference materials were issued to the measurement suppliers to check the validity of the viscosity data supplied. A data base of viscosity-temperature-composition data was established for eight different categories of slags. Chemical compositions were issued to the different modelling participants and the predicted viscosities were compared with calculated values. For mould fluxes the global % errors associated with the various models were: modified lida (25), Riboud (30), Gupta (35), NPL (37). For non-fluoride slag systems, the Ling Zhang, KTH and modified Iida models all perfonned well.

A. B. Fox

Papers

The Role of Mould Fluxes in Continuous Casting-So Simple Yet So Complex

Performance and properties of mould fluxes

Development of Fluoride-Free Fluxes for Billet Casting

Dissolution of ZrO2, Al2O3, MgO and MgAl2O4 particles in a B2O3 containing commercial fluoride-free mould slag

‘Round robin’ project on the estimation of slag viscosities