Figure 1. Continuous casting process
The cooling of molten steel in the mold is the primary cooling zone, where a solidified shell is formed near the walls of the mold. This solid shell should have adequate thickness at the exit of mold to withstand the ferrostatic pressure of liquid metal present inside it. The quality of the strand also gets impaired by the presence of nonmetallic inclusions and segregation of solute elements. The minimization of such irregularities can be achieved by altering the flow of fluid inside the mold. The liquid steel is stirred inside to generate heat transfer and helps breaking down the columnar dendritic solidification.
The stirring effect can be introduced by introducing external electromagnetic field inside the mold. When a magnetic field is applied to a liquid or solid conductor, eddy currents are induced. This produces an electromagnetic force called Lorentz force, which is generally rotary in nature. The strong rotation of liquid steel inside the mold caused by the electromagnetic stirring improves the surface quality by reducing the surface defects and ensures sub-surface cleanness. Stirring in the specific region of the mold is generally used to change the flow pattern and achieve the regeneration of liquid characters and also to form equiaxed grain structure.
The effect of stirring phenomenon in continuous casting on its metallurgical effectiveness for improving the quality of steel billets and slabs has been developed for many years. Many experimental and numerical investigations have been carried out to explore the effect of electromagnetic stirrer (EMS) on continuous casting of steel. Mramor et al. developed an advanced meshless method to investigate the effect of electromagnetic stirring and observed that the EMS stirs the flow by the application of an alternating magnetic field which improves the quality of the strand, reduction of defects at the surface and subsurface, boosts the solidification and reduces breakouts at the exit of the mold. Maurya and Jha developed a coupled three-dimensional flow and solidification model to study the effect of position of electromagnetic stirrer and reported that vertical recirculation zones are formed above and below the EMS and as the position of the EMS was lowered the length and diameter of vortices diminishes. A numerical investigation of Ren et al. studied the interaction of the flow and impinging jet coming from a submerged entry nozzle (SEN) in the caster, due to the rotational movement made by the mold EMS, it shows that the electromagnetic force seemed to circinate at the position EMS in the round bloom and a dominating swirl motion along azimuthal direction of the horizontal plane is observed. Javurek et al. compared numerical solutions of flow and temperature of continuous casting subjected to electromagnetic stirring with semi-empirical solutions available in literature and concluded that the numerical solutions overpredicted the velocities. Yu and Zhu used a combined 3-D finite element-finite volume magnetohydrodynamic (MHD) model to study continuous casting process under the influence of rotating electromagnetic force led to the formation of two pairs of recirculation zones. It is also observed that the stirring effect makes inclusions to float on the top of the mold. The relation between the stirring current and rotational velocity of the molten metal have been analyzed by Yang et al. It is concluded that the rotational velocity increases with increase in the current intensity and the number of the vortices formed is also relative. Cho et al. carried nail board dipping experimental test and compared with large eddy current simulations coupled with Lagrangian discrete phase method to study the surface velocity and level fluctuations in continuous casting process by employing electromagnetic field. It is reported that electromagnetic braking increased surface level stability. A transient 3D model of a slab-type continuous casting with electromagnetic braking and stirring near the meniscus separately is investigated by Takatani. It is reported that the electromagnetic stirrer near the meniscus removes bubbles and inclusions as the flow velocity is low but turbulence is high. Otake et al. investigated the effects of double-axis electromagnetic stirrer on continuous casting process which is comprised of a rotating and a vertical electromagnetic stirrer using unsteady hydrodynamic analysis and found that there is a balance between rotational and vertical electromagnetic stirrer of optimum intensity that is to be maintained to stabilize the free surface. Sivak et al. suggested that an optimum linear velocity by the action of the stator of electromagnetic stirrer is required and reported that a maximum linear velocity range of 0.5-1.0 m/s provide for an efficient stirring. Barna et al. observed that optimum velocities near the solidifying shell are favored to improve the solidification by the conversion of columnar dendritic to equiaxed grain structure.
Based on review of literature the effect of direction and position of the electromagnetic stirrer on the quality of continuous casting needs to be explored. Therefore, a two-dimensional transient magnetohydrodynamic model of continuous casting of steel using ANSYS-FLUENT software is developed to analyze the effect of direction and position of the electromagnetic stirrer on the flow and solidification characteristics with and without electromagnetic stirrer in this investigation.