Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to methods of adding reactive
metals to steels being continuously cast and particularly to
methods for adding reactive metals such as aluminum, titanium,
zirconium and rare earths that ordinarily react in steel to
produce inclusions that plug the tundish nozzles of continuous
casting machines in a manner that such plugging is eliminated.
Farrel and Hilty showed in their paper presented at
the 1971 Electric Furnace Conference that the oxides and/or
oxysulfides of certain elements frequently added to steel for
the purpose of deoxidizing or desulfurizing steel will be
deposited in the tundish nozzle as the steel flows from the
tundish into the continuous casting mold. These deposits can
and frequently do form a plug in the nozzle and co~pletely stop
the flow of steel. The elements that Farrel and Hilty investi-
gated that would cause tundish nozzle plugging were: aluminum~
titanium, zirconium, rare earths (REs) and, in some instances,
siliconO The problem oE plugging of nozzles when casting
steels containing aluminum has been largely overcome by the use
of stopper rods in the tundish to control the flow of steel and
large nozzles that are not readily clogged by precipitation of
alumina. However, this is an imperfect solution and the
reaction products deposited in the bores of the tundish nozzles
are frequently required to be removed by mechanical or chemical
means durlng the continuous casting of steel. The solution to
the tundish nozzle plugging problem applied to aluminum is,
however, less than satisfactory a~s a solutio~ to the problem of
nozzle plugging due to additions of titanium, zirconium, or REs
because ~hese other materials have been shown to have a yreater
tendency to plug nozzles than aluminum.
Since titanium, zirconium and REs all have a hiqher
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affinity for oxygen and sulfux than aluminum, the cleanness of
continuously cast steel can be enhanced with the proper amounts
of these elements. Since steel cleanness can be related to
improved steel ductility or ability to stre-tch under load, it
is desirable to produce steels as clean as possible.
The presen-t invention provides a method of adding
reactive metals to steel being continuously cast to elimina-te
the problem of tundish nozzle plugging comprising the steps of:
(a) continuously casting molten steel through a
tundish nozzle;
(b) introducing a reactive metal into the molten
steel in a fully divided form and at a point and rate into the
tundish nozzle such that said reactive metal is substan-tially
unreacted in said nozzle and high melting oxides and/or oxy-
sulfides of the reactive metal are either not formed or do not
come in contact with -the bore of the nozzle in amounts sufficient
to cause plugging in -the nozzle before passing through the
nozzle; and
(c) continuously casting the molten steel in a
continuous casting system.
I have discovered that the addition of reactive alloys
such as titanium, aluminum, zirconium and REs, entrained in an
inert gas and inserted either below the tundish nozzle into the
snorkel or down the tundish stopper rod results in the addition
of these metals in such a manner that they do not have time to
react with the oxygen and sulfur in the steel to form the tundish
nozzle plugging products of reaction found by Hilty in his
research. Further, I have discovered that even if some of these
solid reaction products are formed that -their mel-ting point can
be reduced by fluxes so that the melting point of -the resulting
mix-ture or compound is lower -than the temperature of the steel
going -through the nozzle. Hilty has shown that inclusions -that
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are liquid going through the tundish nozzle do not resul-t in
nozzle plugging.
Guthrie has shown that when metal partlcles, such as
alloy additions, are added into steel that the firs-t thing tha-t
happens is -that the cold particle extracts enough hea-t from the
molten steel that surrounds it, that that steel immediately
solidifies into a complete shell around the cold metal particle.
There is a finite time necessary for the shell around the cold
particle to remelt and on]y then does the alloy particle begin
to go into solution or melt, and a further finite tlme is
necessary to have the alloy go completely into solution in the
steel. Once in solution these alloys react with the impurities
in the steel such as oxygen and sulfur and
2a.
form the solid reaction products that are responsible for
tundish nozzle blockage.
I have discovered that the times necessary for most
of the alloys added with the methods of this invention such as
through the tundish stopper rod to dissolve the steel shell
formed when they are first added, to go into solution and react
with the impurities in the steel to form tundish nozzle
blocking inclusion are long enough to prevent tundish nozzle
blocking inclusions from forming~ Further, if the alloys can
be added through a port in the snorkel entrained in the inert
gases used at this location for stirring, neither the reactive
metals nor their reaction products with impurities of the steel
ever comes in contact with the tundish nozzle.
I have further discovered that if the solution time
of the reactive alloy added entrained with the gases coming
through the tundish stopper rod is so short and their reaction
rate with the impurities in the steel is so swift that nozzle
blocking inclusion call form in the bore of the tundish nozzle,
that such inclusions can be prevented from attaching to the
bore of the tundish nozzle if their melting point i5 reduced hy
adding a suitable flux simultaneously with the reactive
alloy. As an example, if a low melting point alloy of REs were
added according to this invention and the alloy went into
solution swiftly and reacted swiftly with the impurities to
form solid inclusions capable of nozzle blockage while the
steel was still in the noæzle, one of the compounds formed
would be rare earth oxide~ (RE2O3~, whose melting point exceeds
3000F. It is possible to lower the melting point of the RE2O3
so formed by reacting it with a flux such as cryolite,
Na2AlF6- One such compound formed when cryolite and RE2O3 are
reacted together is Na2(RE)F4 whose melting point is less than
1900F, far below the temperature of the steel golng through
the tundish nozzle (2800-2900F). Cryolite is not the only
compound capable of fluxing RE2O3, however, most of the
compounds capable of fluxing R~2O3 are halide salts of one kind
or another.
I have discovered that the flux necessary to form a
compound with khe reaction products from the addition of
reactive metal~ which is fluid at the temperature of the steel
going through the tundish nozzle can be introduced into the
system by at least two methods. The first means is to coat the
alloy particle with the flux. This could be accomplished by
dissolving the flux in a fluid such as water and then the alloy
to be coated can then be immersed in a concentrated solution oE
the flux and subsequently dried leaving a coating of the flux
on the particle. A simpler procedure would be to make an
intimate mixture of fine particles of the alloy and fine flux
particles so that formation of low melting point compounds can
be formed with the reaction products of the alloy and the flux.
I have further discovered that in instances where
continuously cast steel must have the ultimate in cleanness
that it may be preferable at times to add the reactive metals
such as REs either in the ladle or in the tundish to allow a
greater time for the 1Otation of the reaction products rom
the steel. However~ the steel going through the tundish nozzle
will contain RE oxides or oxysulfides that can precipitate in
the bore of the noæzle and stop the flow of steel through the
nozzle. In such instances r a flux such as cryolite can be
entrained in the stream of inert gas being injected down the
tundish stopper rod. A portion of this flux~ when dissolved in
~2~
the steel just prior to its entry into the tundish nozzle~ wil1
contact the high melting point compounds previously precipi-
tated on the nozz].e to reduce their me].ting point below the
temperature of the steel going through the ~undish nozzle thus
dissolving the deposits from the bore of the nozzle.
In the foregoing specification I have set out certain
preferred practices and embodiments oE my invention, however,
it will be understood that this invention may be otherwise
embodied within the scope of the following claims.
