Entry Flows
Nozzle blockages might soon be done away with
say, R Sambasivam and NU Girase, while
offering a model solution
The design of a bifurcated Submerged
Entry Nozzle (SEN) plays a critical role in
improving slab quality in continuous
casting. The flow pattern within the
submerged entry nozzle and at the ports exit affects
the cast steel quality to a great extent. The
phenomenon of nozzle clogging is also closely related
to the flow behaviour in the submerged entry nozzle.
The problem of SEN clogging is an important issue
related to the quality of slabs in continuous casting of
aluminium-killed low-carbon steels. Clogging of the
nozzle causes premature termination of casting,
reduces casting sequences and lowers the caster
productivity. It is caused by the deposition of solid
micro-inclusions (principally Al2O3, but also TiO2,
ZrO2, rare earth oxides and CaS) present in the liquid
steel. During casting there is a gradual build-up of
alumina inclusions on the walls of the SEN. This
build-up detaches periodically because of the shearing
action of the flowing liquid steel and enters the
mould, thereby increasing the level of harmful macroinclusions
in the cast products. These macroinclusions
have a bearing on the quality of the cold
rolled sheets and are frequently the cause of slivers in
the rolled product. Moreover, the mould level
fluctuates due to any sudden dislodgment of the builtup
macro-inclusions from the nozzle walls. This
mould level fluctuations cause slag entrapment,
thereby deteriorating the quality of the cast products.
Thus, it is imperative to study the flow pattern within
the nozzle and at the port to analyse the inclusion
build-up and clogging tendency of the SEN.
In order to understand the effect of SEN design
on the flow in the mould and associated
phenomenon, primarily water models of the SEN and
its mould have been used in the past. The water
models give a qualitative picture of the flow within the
SEN and the mould and can be used to infer the
general flow characteristics.
To quantify different flow parameters and study
the effect of argon gas bubbles on the flow in the
mould, many numerical studies have been performed
on the complex geometry . Due to the complexities
involved in the phenomena of clogging, it is a task of
tall order to quantify nozzle clogging with SEN design
directly. Since there is a strong linkage amongst
clogging, flow within the nozzle and the design of
SEN, analysing the flow pattern within the SEN for
different nozzle geometries can yield a qualitative
picture of clogging tendency. Thus, the analysis of
SEN design in this perspective assumes greater
significance.
SEN clogging
The phenomena of clogging is two-fold:
* Transport of the inclusions to the submerged
entry nozzle wall
* Adhesion of inclusions to the wall.
The transport of the inclusions from the turbulent
stream of liquid steel to the nozzle wall is intimately
related to the flow characteristics and design of the
nozzle. Clogging takes place preferentially at the
locations where stagnation and separation of the flow
occur within the SEN. Stagnation of the flow leads to
very low velocity of the liquid steel and separation of
the flow keeps the fluid entrapped in the zone. Thus,
both stagnation and separation force the inclusions to
stay near the refractory wall of the SEN for a longer
duration, thereby aiding clogging. Moreover, SEN
samples collected from the caster show severe clogging
near the bottom wall of the SEN and this reinforces
the intimate relationship between clogging and flow
within the SEN. Thus, the objective of designing a
SEN with reduced tendency to clog can be directly
....CONTD
