You...dont sell slag. so....slag has no economic value in
itself. Its importance lies not in the cost at which, if at all, it sells,
but…by the ROLEs it plays in steel making.
Its importance
is underestimated/ never gets understood by all. But it is important for those
who just start into REAL secondary steelmaking process. That is, only
during our formative years as secondary steel makers do we slowly….very
slowly…. understand the crucial role of slag. that understanding is absolutely necessary for good performance of secondary steel making process metallurgists to make good heats CONSISTENTLY & ALWAYS.
Slag must
perform a couple of functions SUCCESSFULLY
in steel making.
DEOXIDATION:
A good slag
promotes good de-O.
In
general, secondary slag is reducing in nature. Necessary to remove oxygen from
liquid steel, which will have lower solubility as the steel solidifies. Unless
sufficiently removed, it will lead to poor quality castings through blowhole
formations.
This
de-O is done using deoxidisers like C, Si, Mn, Al., in different combinations.
Oxygen, from steel, forms corresponding oxides and various compound oxides too,
mostly in the form of inclusions.
Other
than oxides, there will be sulphides, nitrides too. Almost all of these inclusions should be removed from steel as much
as possible. Here’s where the term CLEAN STEEL figures in. loosely used in
steel making contexts, it seems to mean different qualities to different
people.
Unless
good amount of inclusions is reduced in steel bath, castability is hampered.
Unless
the liquid steel is cast smoothly in the caster and rolled neatly in the mills,
it’s not good steel. Castability becomes an issue in high speed/quality
casting.
DESULPHURISATION:
A good slag
promotes de-S.
De-S
follows, de-O. de-S is necessary to have good grade of steel, through better
mechanical properties. MnS/ FeS layers formation along the grain boundaries will
be reduced during casting/rolling. This gives better mechanical properties.
Good slag
has good amount of free Ca-ions to remove S from FeS. And this Ca is expected
to come from lime, rather than from Ca-cored wires. Having sufficient Ca-ions needs
controlled slag.
One must
bear in mind that, being surface active elements, O and S vie with each other
to combine with Ca, in the most reactive zone of the steel bath- that is
slag-metal interface. So a good steel making needs a very good control over
this slag-metal interface/reaction zone.
SINK:
A good slag acts
as a sink to the inclusions.
Slag
must be prepared in such a way to accommodate the upcoming inclusions from
bath. It should act as a buffer for inclusions. To act as a buffer, it needs to
be tailor-made to have perfect combination of various oxides that impart the
right viscosity.
As many
may know, thicker slag is tooooo viscous
to permit the upcoming inclusions to enter into it, while thinner slag is tooooo fluidic to RETAIN inclusions
within itself. Any amount of purging post calcium treatment would not produce
good results.
Purging would only be able to float
inclusions to the top layers of the steel, but not into the thicker slag-phase.
In thinner slag, inclusions would be floated into the thinner-slag-phase but
would be drawn/forced back into the steel-phase again as inclusions do not get
trapped in the thinner-slag-phase.
If this function of slag fails,
then, be ready to get choked suddenly in between the casting sequences, throwing
the shop floor into dizzy logistics.
curiously, synthetic slag acts only as this sink, NOT as a desulphurising medium.
BURIED-ARCING:
A good slag
buries the arc, thereby reducing arc flare. Reduced arc flare would
lead to reduced refractory damages, giving better ladle life (esp, in slag
lining area).
Buried arc
further conserves energy by reducing radiation losses. Much of the heat
produced by arc is trapped by slag and gets directed into the heat-bath.
ELECTRODE SPALLING:
A good slag
reduces electrode spalling by reducing harsh, direct arc being
established between the hard-steel surface and the electrode tip. This open arc
seems to generate a lot of vibrations that may or may not reach the resonance
frequencies of the electrodes. Slag contains gases, esp CO/CO2, entrapped; that
acts like a cushion to accommodate the vibrations that accompany arcing.
Thicker,
dry slag deflects the arc away.
Thinner,
wet slag may not bury the arc sufficiently and, may permit direct arcing too.
CARBON PICKUP:
A good slag
reduces carbon pickup into the metal from electrode. This may be
attributed to thermionic emissions, mechanical erosion of electrode into the
metal due to physically uncovered, direct arcing.
This will
become more visible during ULC grades making.
HYDROGEN/NITROGEN PICKUP:
A good slag
reduces hydrogen and nitrogen pickup by providing a good barrier
between atmosphere and the steel-bath-top. Thinner slag permits pickup. So it
has to be paid attention.
In some
applications, permitted N and H levels will have a limited threshold values.
OTHERs:
A good
slag permits easier deslagging as it does not cling hard to the ladle inner
linings, due to optimal fluidity.
A good
slag permits easier penetration of Al/Ca-cored wires to pierce through into the
metal-bath.
A good
slag is less reactive towards the ladle refractory linings. You will have
better ladle lives.
A good
slag permits easier carbon pickups during pet-coke or coke breeze additions. You
can make Hi-C grades easier in LF units.
It’s
true that good steel making is only a good slag making.
To put
it exactly…… if you donot know how to PRACTICALLY make a good slag, then it’s an unfortunate fact that,….you DO NOT
KNOW steel making.
Happy steel making!