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Aluminium is the most
effective of the deoxidants
commonly employed in steel
making. Because of the small
amounts of residual oxygen
and aluminium in steels, the
value of the equilibrium
constant of the deoxidation
reaction with aluminium
found experimentally, are
rather divergent. Of the
three deoxidants Mn, Si
and Al, most often used for
deoxidation in steel making,
aluminium is the strongest.
For instance, the residual
concentration of oxygen in
steel in equilibrium with
0.1% of Mn, Si and Al are
0.175%, 0.021% and 0.00028%
at 16000C and
0.12%, 0.089% and 0.00013%
at 15000C
respectively. Aluminium also
prevails over Mn & Si for
deoxidation in certain
grades of steel, where Mn &
Si are undesired elements.
Aluminium
is obtained from alumina (Al2O3)
through Bayer's process
using an electrolysis
method. Finishing or
adjusting composition of
aluminium is done in melting
or holding furnaces. The
melting point of Aluminium
is 6600C and its
specific gravity is 2.70.
Aluminium,
on being used alone for
obtaining low concentration
of dissolved oxygen, may
cause formation of solid
alumina, if the reaction of
formation of FeO.Al2O3
has not enough time to
proceed. The formation of
alumina also causes the
problem of choking of
tundish nozzles, while
casting. However, alumina
formation of aluminium
deoxidised heats can be
prevented by way of
injecting Ca-bearing alloys
and that aluminium killed (deoxidised)
steel grades can be smoothly
cast.
Aluminium
can be added to the molten
steel in ladle in various
ways, namely, as alloy; as
Al bullet through shooter;
by Al powder injection; and
by Al wire feeding. Of all
the above methods, the
Al-wire feeding is the most
accepted method as it
restricts generation of
fumes & splashes and related
hazards.
With
the wire feeding method,
aluminium wires can be
introduced continuously in
the ladle, the centre riser
or the tundish during
continuous casting by means
of a feeding machine. The
feeding rate can be varied
depending on the bath volume
by means of a regulating
machine. |
