During the 1970s,
in all major steel making units,
a progressive replacement of
stopper rods by sliding gate in
ladles, took place. The
development was largely due to
increased activities in ladles
during steel making, resulting
in longer holding time in
ladles. It will be interesting
to note that the concept of
slide gate system was originally
patented in 1885 by David D.
Lewis of USA. The idea was
however not utilised till 1960s.
The ladle nozzle
filling mass prevents erosion
and corrosion of slide plates,
the most vulnerable component of
slide gate refractories and the
ladle nozzle, from the very high
impact while tapping of steel in
ladle and decarbonization by the
highly oxidised tapped steel. It
does seal the slide gate
refractories from the molten
bath till the casting of
steel, starts in ingots or
tundishes, preventing any
leakage or breakout; any
sticking of solidified steel on
slide surface and subsequent
peeling of slide surface. It
also safeguards the slide plates
and collector nozzle from
thermal shocks arising out of
the situation leading to radical
cracks; breaking of bore edge;
and peeling of slide surface.
Any leakage or breakout may also
damage the electrical &
mechanical assemblies. It may be
noted that these factors are
interrelated and affect the wear
progress through a
multiplication effect.
The above factors
make the material design for
ladle nozzle filling mass very
crucial and has led to radical
changes in design in order to
obtain the desired safety and at
the same time ensure a free
opening for casting of steel The
material needs to sinter at
steel making temperature forming
a hard layer on the ladle nozzle
filling mass. As the erosion of
well block and the ladle nozzle
progresses, the sintered layer
of nozzle filling mass
also erodes, exposing fresh
layer to sinter. While it should
sinter at steel making
temperature, it should not fuse
as the fused mass may cause
problem during opening of slide
gate. Such situation leads to
forced opening by lancing of
oxygen, which is highly
undesirable.
Slight expansion
is a desired property of the
material, as negative PLC
(permanent linear change) leads
to gap formation.
The most common
material being used for the
purpose is silica, for the
ladles with the operating
temperature to a maximum of 16500C.
For a higher operating
temperature chromite is used as
base material. For the ladles to
be used for secondary steel
making (ladle metallurgy) with a
long holding time, zircon or
high alumina is used as base
material.
..................................................................................................................
