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.