Note: Descriptions are shown in the official language in which they were submitted.
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BACKGRO~ND OF THE INVENTION
This invention relates to mold powders employed ln lngot
steel production. More specifically, the present invention
discloses a casting powder to be employed in bottom pour steel
molds which has the unique ability to act as both a bottom pour
flux and a hot topping compound.
The use of bottom pour process to produce ingot steel has
enjoyed substantial recent success with millions of tons of
steel each year produced with this process. In order for the
process to work effectively, fluxes must be added on the surface
of the molten steel as it begins to enter the mold. These
fluxes are crucial both to prevent reoxidation through a
complete covering of the rising steel and to insulate the steel
and prevent premature solidification and skulling.
Presently two separate casting powders are applied in the
production of each bottom poured ingot. First, a sealed bag of
bottom pour flux is suspended in the mold approximately six to
eighteen inches above the inlet for the molten steel. The
bottom pour flux consists of chemical compounds which melt and
spread rapidly across the surface of the molten steel. The
molten slag coating the surface of the steel acts to create the
correct menisus shape and prevents oxidation of steel surface.
Additionally, the molten slag insulates the surface of the
molten steel to slow solidification, and spreads a thin coat of
homogenous glass between the mold and the molten steel to allow
constant heat transfer and solidification and thus lessen
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th~rmally induced stresses and resultant cracking. Further the
flux absorbs impurities 6uch as deoxidation and reoxidat~on
products and refractory particles. The bag containing the
bottom pour flux burns upon the introduction of the molten steel
into the mold thus automatically releasing the flux.
Although traditional bottom pour fluxes are crucial for
efficient production of steel ingots they are not sufficient.
In order to assure the surface quality of the ingots and
maximizing yield by avoiding ~pipe~ (i.e. shrinkage and
segregation), an additional layer of insulative material must be
added immediately after the molten steel has filled the mold and
entered ~hot top~ region. This material is referred to as ~hot
topping compound.~ Without the addition of hot topping
compound, the molten steel would freeze in the hot top, thus not
providing liquid steel to feed the shrinkage cavity (i.e. pipe)
formed due to ingot solidification. The result would be to
discard an entire segment of the steel ingot causing reduction
in yield.
However, the application of hot topping compounds is not
without its detractions. Adding hot topping compound is
cumbersome, labor intensive, environmentally disruptive, and may
contaminate the ingot steel.
Personnel, who could be better utilized elsewhere, must be
stationed on the pouring platform above the molds to distribute
the hot topping compound bags onto the molten steel. In
addition to the cost of personnel, this process has two serious
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drawbacks. First, the pouring of the fine grained hot topping
compound some two to ten feet onto the top of the powdery flux
layer generates extensive clouds of environmentally harmful
dust and smoke. Second, the addition of hot topping compound
has been associated with a condition known as "core of debris."
Core of debris occurs when the chilling effect of the hot
topping compound causes steel to solidify around refractory
inclusions which then sink into and contaminate the steel
ingots.
In light of the foregoing, it is a primary object of the
present disclosure to create one casting powder which provides
the benefits of both a bottom pour flux and a hot topping
compound.
It is a further object to provide a one-step casting powder
which is automatically dispensed through the bag suspension-burn
method or a board presently used to dispense bottom pour fluxes.
It is an additional object to provide a one-step casting
powder which is economic to produce and use, entails little
~nvironmental risk, and does not contribute to ingot contamination.
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SUMMARY OF THE INVENTION
The present invention is directed to casting powders
for use in ingot steel production through bottom pour process.
Instead of the bottom pour flux and the hot topping compound
presently employed, a single casting powder is substituted which
provides the benefits of both the prior products.
A compound known as '`expandable graphite" is used in
place of a portion of the carbon component of a standard bottom
pour flux. The resulting mixture provides all the insulative
and protective benefits of standard bottom pour fluxes as well
as the full insulative requirements of hot topping compounds.
The powder is cleanly and automatically dispersed upon
introduction of molten steel into the steel ingot molds. It
eliminates the noxious by-products of hot topping compound and
its potentially contaminating effect of "core of debris", and
does not require the labour input demanded for application of
hot topping compound.
More paxticularly, in accordance with a first aspect
of the invention there is provided a method for increased
efficiency of protecting a steelmaker's mold and a steel ingot
as bottom poured into the mold, including protecting against
excessive piping of the steel which tends to occur when the
molten steel cools too rapidly in the mold comprising,
introducing into tbe mold a predetermined quantity of
a casting powder, comprising chemical components combined to
produce a bottom pour flux, and including an expandable graphite
as at least 4.0~ by weight of the composition of said casting
powder,
pouring molten steel into said mold and covered with
said casting powder to cause a substantial portion of said
casting powder to rise to the vicinity of the top of said mold,
thinly coating the side walls of said mold as it rises, and
expanding into a thick insulative blanket on top of the molten
steel of a sufficient insulative quality to avoid need for a hot
topping compound limiting the steel's rate of cooling and
thereby minimizing the piping of the ingot:
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wherein said insulative quality is achieved through
use of an expandable graphite that expands to 100-300 times its
volume and comprises 4.0 to 12.0% by weight of the composition
of said casting powder.
In accordance with a second aspect of the invention
there is provided a casting powder employed in a mold for bottom
pour steel ingot production comprising,
a mixture of chemical components to produce a bottom
pour flux which coats and protects the top and ~ides of molten
steel as it is introduced into the mold, including an expandable
graphite component as at least 4.0~ by weight of the composition
of said mixture, and
which expands to form a thick insulative blanket on
top of the molten steel of sufficient insulative quality to
avoid need for a hot topping compound, limiting the steel's rate
of cooling and thereby minimizing piping of the ingot;
wherein said insulative quality is achieved through
use of an expandable graphite that expands 100-300 times its
volume and comprises 4.0 to 12.0~ by weight of the composition
of said mixture.
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_~ DETAILED DESCRIPTION~OF THE INVENTION
The present invent~on pro~ides a mixture of chemlcal
components which function a8 a unlque casting powder or bottom
pour ingot steel production. The present invention combines the
crucial properties of both bottom pour fluxes and hot ~opping
compounds into a single, easily dispensed, composition.
Bottom pour fluxes ~or powders) presently employed are
required to have specific qualities for covering and protecting
molten steel rising in an ingot mold. These include: molten
slag layer to completely coat the molten steel, to insulate,
maintain a proper surface shape, protect against oxidation, and
absorb deoxidation and reoxidation products: and an ability to
form a thin layer of homogenous glass between the molten steel
and the side walls of the mold so as to insulate, reduce
thermally induced ætresses and thus decrease cracking.
15 To this end, a composition of a traditional bottom pour flux
may comprise the following:
ConstituentPercentage (~) Range by Weight
Silica (SiO2) 30.0 - 35.0%
Aluminum Oxide (A12O3) 15.0 - 17.0
Calcium Oxide (CaO)6.5 - 8.0
Iron Oxide (Fe23)4.0 - 6.0
Alkali Oxide 5.5 - 8.0
Carbon (C) 5.0 - 27.0
Hot topping compounds have only one primary purpose: to
provlde a thlck Insulatlve bla k- on top of the molt n steel to
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l~duce the heat loss from the top to avold ~pipe.~ Plpe i8 a
condition which occurs when there ls no molten steel to feed the
shrinkage cav~ty formed due to ingot solidification. Due to
the expansion of the steel while molten, this d~screpancy leaves
the sides too high in respect to the core. Thus, without proper
insulation, the center of the steel ingot will solidify in a
sunken position or with severe imperfections -- creating an
entire segment of the ingot which must be excised and discarded.
To accomplish the necessary insulation, a wide variety of
compositions have been utilized. Typical ranges are as follows:
Constituent Percentage (~) Range by Weight
Silica (si2)
AluminUm oxide (A123) 25-75
Calcium Oxide (CaO) 0-2
Iron Oxide (Fe2O3) 0-4
Sodium Oxide (Na2O) 0-2
Potassium Oxide (R2O) 0-3
Carbon (C) 0-15
Magnesium Oxide (MgO) 5-60
Chloride (cl) 0-5
Aluminum (Al) 0-40
Aluminum Nitride (AlN) 0-4
Any attempt to comblne the properties of bottom pour flux
and hot topping compound i8 confronted wlth a paradox -- how to
provide a viscous coating material on the molten steel as it
rises in the mold and also provide a highly insulative blanket
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in the upper (or "hot top") region of the steel ingot mold. The
present method and powder accomplish this through use of
"expandable graphite."
Expandable graphite is produced through treatment of high
grade natural crystalline graphite through oxidation or
electrolysis by various oxidizing materials. It is commercially
available in a number of grades from graphite suppliers.
When expandable graphite is heated rapidly it expands along
the C-axis of the crystal to a magnitude of 40 to 300 times its
original size.
By substituting expandable graphite for a portion of the
carbon component usually employed in bottom pour fluxes, an
entirely new and unique casting powder is provided. The
composition of this casting powder is as follows:
ConstituentPercentage (~) Range by Weight
Silica (SiO2) 30.0 - 35.0%
Aluminum Oxide (A12O3)15.0 - 17.0
Calcium Oxide (CaO) 6.5 - 8.0
Iron Oxide (Fe2O3) 4.0 - 6.0
Sodium or Potassium
Oxide ((na,K)2O) 5.5 - 8.0
Total Carbon (C) 5.0 - 27.0
Expandable graphite4.0 - 12.0
In all other respects, the present powder is prepared in
the same manner as standard bottom pour flux.
Expandable graphite has different expansion rates according
to its quality. It is desired to use expandable graphite that
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expands between 100 and 300 times its volume within the
percentage weight range provided above. It is believed that
ideally an expansion of 200 to 250 times its volume at a
percentage weight of 6.0 to 8.0% should be employed.
Due to the affinity of the expandable graphite particles for
one another, a highly expanded layer is produced which is as
heat resistant and chemical resistant as standard graphite. The
result is a thick insulative blanket which functions very well
in place of hot topping compound.
However, the casting powder has a viscosity nearly identical
to that of standard bottom pour flux (i.e. at 1500C, bottom
pour flux has a viscosity of approximately 50-200 poise, the
present invention produces a flux with a viscosity of 50-200
poise). Moreover, under pressure the present powder produces
a unique quality compression product having anistrophy. This
results in a substrate perfectly suitable to properly coat
between the molten steel and the side walls of the ingot mold
during pouring. Thus, the casting powder produced provides
superior results as both a bottom pour flux and a hot topping
compound.
The casting powder is dispersed in the same manner as
standard bottom pour flux. It is placed in a combustible
container or bag, such as paper bag with grommet reinforcements,
and suspended six to eighteen inches above the bottom of the
ingot mold. It can also be preformed into a board and placed at
the bottom of the mold. The incoming molten steel consumes the
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contalner or dissolv~s the board causing the release of the
powder. The powder then rapidly spreads across the surface of
the incoming molten steel. This is a clean, automatic process
which requires little human input and supervision.
The benefits of the present invention are realized through
the elimination of hot topping compound. Personnel are no
longer required to be stationed above the molds to apply the
insulative material. Atmospheric dust from the hot topping
compound and ~core of debris~ are also eliminated.
Additionally, no smoke products are produced whatsoever.
Despite the somewhat higher cost of substituting expandable
graphite for standard graphite, the elimination of hot topping
compound and the considerable cost savings in application
provide a considerable overall cost savings.
While particular embodiments of the present invention have
been disclosed herein, it is not intended to limit the invention
to such a disclosure and changes and modifications may be
incorporated and embodied within the scope of the following
claims.
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