INVESTIGATION OF IN-MOLD ELECTROMAGNETIC STIRRING PROCESS IN CONTINUOUS CASTING MOLD

Abstract

The genesis of most of the surface and subsurface defects in continuously cast products owe primarily to the fluid flow and solidification of liquid steel within the mold and hence the phenomena happening inside the mold is the key decider for quality of the cast product. Electromagnetic stirring in the mold, also called in-mold electromagnetic stirring, makes the liquid steel to move around and control the solidification process. In-mold electromagnetic stirring has become one of the most promising technique to alter the fluid flow in the mold for subsequently reduction of inclusions, blowholes, cracks etc. by improving the solidification characteristics inside the mold. Stirring inside mold promotes mixing of liquid steel near the solidification front and tries to regenerate the liquid characteristics of the steel. Stirring also helps in breaking down the columnar dendritic grains to form fine equiaxed grain structure. In a nutshell, in-mold electromagnetic stirring mainly improves the surface quality, mainly by reducing surface defects, ensuring the sub-surface cleanness and promoting columnar to equiaxed grain transition. The stirring phenomenon by electromagnetic field is carried out through an inductor, which generates a traveling or alternating magnetic field. When this alternating magnetic field is applied to a liquid or a solid conductor, it induces eddy current. Magnetic field and eddy current together give rise to the electromagnetic force called Lorentz force. In general, this Lorentz force is rotary in nature and if the conductor is liquid, it is set to move accordingly. The stirring of molten metal in the mold is controlled by the characteristics of the applied magnetic field, which is further controlled by the characteristics of the stirrer and the alternating current supplied to the stirrer. The optimum stirring conditions for a caster depends on the material to be cast, dimensions of the mold, and casting conditions. Before optimizing the stirring condition, it is necessary to understand their effect on the cast product or on the fluid flow and solidification behaviour. In the wake of wide use of electromagnetic stirring technology in the continuous casting of steel billets and blooms, numerous investigations have been carried out by conducting plant trials, laboratory experiments or by using different numerical modelling tools to explore the influence of various process parameters. Most of the previous numerical investigations have been performed by only simulating fluid flow i.e. without considering solidification, despite the fact that solidification decently amend the liquid steel flow behaviour. Though coupling of viii time varying electromagnetic field with the fluid flow model enhances the complexity of the model, it has been considered as such a field closely resembles to the industrial conditions. The secondary flow caused by electromagnetic stirrer has been reported to increase the turbulence at the meniscus or recirculation above stirrer position. This could increase the level fluctuation at meniscus, which has been overlooked by the researchers in the past. Based on objectives and method of generation of magnetic field, two different models have been developed. The aim is to analyse the influence of various process parameters of electromagnetic stirrer on the fluid flow, solidification, and mold level fluctuation. The constituent models such as solidification model, magnetohydrodynamic model, and model developed for electromagnetic field generation have been validated against the previously reported results of other authors. The investigation is compiled with the prediction of fluid flow velocity field, tangential velocity, axial velocity, liquid fraction of steel, solid shell thickness, magnetic flux density distribution, and steel-flux interface level fluctuation. Result shows that electromagnetic stirrer generates two types of flow, primary flow having horizontal rotary motion and secondary flow having two recirculation zones (above and below the stirrer centre) in the axial direction. The rotary flow has maximum velocity at the mold periphery and zero velocity at the mold centre. It has been observed that stirring homogenizes the steel that lead to delay in start of solid shell formation or a break in the solid shell formed at the mold wall. The solidification of steel found to dampen the flow velocity generated by the stirrer and the application of time-varying magnetic field produces asymmetric flow behaviour and solidification in the mold. The first model uses the homogeneous magnetic field in the stirrer region, where magnetic flux density and magnetic field frequency are the key process parameters. It has been observed that the tangential velocity of liquid steel increases with the increase in magnetic flux density and magnetic field frequency. The liquid fraction at the mold centre decreases and width of gap in the solid shell formed at the mold wall increases when magnetic flux density and magnetic field frequency are individually increased. While dealing with without EMS case, insignificant change in liquid fraction and solid shell formation have been obtained for low magnetic flux density and/or low magnetic field frequency, which represents the insufficient stirring intensity produced by stirrer. On shifting the stirring position down the mold, it was observed that both length and diameter of stirring decreases because of solid shell formation ix and decrease in liquid fraction. The tangential velocity of the liquid steel is increased initially, and then decreases when stirring position was shifted down the mold. For second model, a separate model has been developed to generate the electromagnetic field based on the current intensity and frequency applied to the stirrer. It has been observed that magnetic flux density distribution increases significantly with the increase in current intensity. On the other hand, with increase in frequency, the value of magnetic flux density initially increases and then decreases. The tangential velocity coupled with axial velocity of the flow field show significant increase in its value with the increase in frequency. The solid shell thickness predicted at the mold wall shows marginal increase in delay in initial start of solid shell formation with the increase of both current intensity and frequency. Width of the stirrer is the initial design characteristics to be considered before installation of the setup. Increase in stirrer width results into increase in tangential velocity and axial velocity above the stirrer centre and decrease in axial velocity beneath the stirrer centre. Due to flow of electric current in a conducting media, heat is generated because of finite conductivity of the media. The heating due to these resistive losses is called Joule heating. The Joule heat calculated is found to be confined at the mold periphery and in the stirrer region, while the total Joule heat generated is less than 1% of the total heat removed from the steel during solidification. The mold level fluctuation because of electromagnetic stirring has been tracked using volume of fluid method. The geo-reconstruction scheme has been used to explicitly discretize advection equations of volume of fluid model. The result shows that steel-flux interface level fluctuation increases with current intensity, frequency, stirrer width and density of mold flux. However, at 10 Hz and 20 Hz frequency, the instability of the interface wave exceeds the critical value calculated from the top surface standing wave criteria. The emulsification of flux into steel has also been observed at 20 Hz frequency. It has been noticed that maximum rise in interface level is at the mold corners and maximum slump near the submerged entry nozzle wall.