Insulative coating for refractory bodies

An insulative coating for the thermal protection of ceramic refractory bodies, such as submerged pouring nozzles, and like pieces used in continuous casting of molten steel and other metals. The coating is prepared as a slurry having a preferred composition comprising (by weight %) fused silica grains (30-85%); ceramic fibers (0-10%); binders (0-7%); frits (0-40%); and water (15-30%). A refractory body, which preferably has a previously applied anti-oxidation glaze thereon, is employed at either ambient temperature or preheated to a temperature within the range of about 70.degree.-120.degree. C., and dipped into the slurry composition for a controlled time period of between 5 to 60 seconds to achieve a desired coating thickness of between about 1 to 6 mm. The coated refractory body is then dried and ready for service. Pouring nozzles coated with the insulative compositon may be used in a cold start-up continuous casting mode without the need for preheating.

BACKGROUND OF THE INVENTION 
The present invention relates generally to refractory components used in 
casting molten metal and, more particularly, to thermal insulative 
coatings for protecting such refractory components and to enhance their 
performance while prolonging their service lives. 
The insulative coating of the present invention is particularly suited for 
use on a variety of refractory bodies, such as submerged pouring nozzles, 
ladle to tundish shroud tubes and like consumable components used in the 
continuous casting of metals, such as molten steel. Heretofore, in order 
to protect submerged pouring nozzles, shroud tubes and the like from the 
thermal shock experienced during start-up of a continuous casting run, it 
has been common practice to prepare the nozzle in some manner in order to 
minimize the thermal shock caused by a cold nozzle start-up. One common 
practice has been to preheat the pouring nozzles prior to casting. Another 
common expedient to protect against the thermal shock of start-up has been 
the application of a ceramic fiber-impregnated insulative paper product 
around the outside of the pouring nozzle. The paper product, while 
somewhat effective in minimizing thermal shock and resultant cracking 
problems, is expensive due to the cutting, wrapping and wiring required to 
size and properly fit the paper sheet around the exterior surface of the 
refractory nozzle. Of course, the paper coating is not applied to the 
interior cavities of the nozzle. 
In addition to the thermal shock problems, prior pouring nozzles employed 
in continuous casting operations also experience bridging or freezing of 
metal between the submerged nozzle and the adjacent walls of the water 
cooled continuous casting mold. The aforementioned preheating or paper 
wrapping operations tend to minimize such objectionable bridging, however, 
all problems are not solved. For example, in the case of the wrapped 
nozzle, there is an internal build-up of frozen metal due to the fact that 
the interior of the nozzle has no insulative protection. 
The present invention solves many of the time-consuming and expensive 
problems heretofore encountered in the preparation and use of submerged 
pouring nozzles and like components used in continuous steel casting 
operations. The present invention provides an insulative coating for such 
pouring nozzles and the like which eliminates the need to preheat the 
nozzle. The coating of the present invention prevents undesirable thermal 
shock in the nozzle body during start-up and also eliminates unwanted 
bridging or freezing between the nozzle and the mold during casting. The 
insulative coating of the invention also covers the interior bore of the 
pouring nozzle to provide insulation therewithin which prevents unwanted 
build-up of frozen metal within the nozzle bore during casting. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention provides a thermal insulative coating 
for a refractory body exposed to molten metals, such as submerged pouring 
nozzles used in the continuous casting of steel. The insulative coating is 
preferably applied to both the exterior and interior surfaces of the 
refractory body as a slurry, preferably by dipping. A preferred slurry 
coating composition of the invention consists essentially of: 
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Ingredient Weight % 
______________________________________ 
Fused silica grains 30-85% 
Ceramic fibers 0-l0% 
Water l5-30% 
Binders 0-7% 
Frits 0-40% 
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The coefficient of thermal expansion of the coating can be altered to 
simulate that of the refractory substrate by substituting one or more 
ceramic materials for the fused silica grains in the above preferred 
slurry composition. Suitable substitute ceramic materials include alumina 
powder, zirconia powder, mullite powder and alumina bubbles. 
In a preferred form of the invention, a continuous casting nozzle or like 
component, of a conventional refractory material such as alumina graphite, 
for example, is dip coated with the above fused silica refractory slurry 
composition at a controlled time of between about 5 to 60 seconds. The 
refractory body is preferably glazed and may be at ambient temperature or 
preheated to a temperature of between about 70.degree.-120.degree. C. 
prior to dipping to obtain an insulative coating thickness of between 
about 1 to 6 mm. A preferred coating thickness is about 3 mm. After an 
appropriate drying time, the coated refractory body is ready for use. The 
coating has a relatively hard surface in the dried state and is suitable 
to withstand the rough handling expected in a mill environment. 
DETAILED DESCRIPTION OF THE INVENTION 
A slurry of the insulative coating composition is prepared by first forming 
a ceramic slip, preferably consisting essentially of a major portion of 
fused silica (SiO.sub.2) grains (30-85% by weight) plus water (15-30% by 
weight). The fused silica grains, preferably obtained by an atomized 
process, are in a fine powder state, having a particle size preferably 
less than 100 mesh sieve size. The SiO.sub.2 particles are completely 
deflocculated in the slip prior to making the slurry. The fine particle 
size of the fused silica powder results in a very fast reaction so as to 
achieve the desired rapid deflocculation in the slip. The apparent 
specific gravity (ASG) of the slip should be controlled within a range of 
about 1.700 to 2.000 grams per cubic centimeter. The viscosity should also 
preferably be controlled between a range of about 0.3 to 0.5 pa second, 
using an LV model Brookfield viscometer with a #1 spindle at 12 rpm. Other 
ceramic materials may be partially or wholly substituted to dilute to 
replace the fused silica ingredient, such materials which may be used 
include: alumina powder, zirconia powder, mullite powder and alumina 
bubbles. Fused silica has very low coefficients of thermal expansion and 
thermal conductivity and, hence, is an excellent material in most coating 
applications. In some applications, however, it may be desirable to 
increase the thermal expansion coefficient of the coating or other 
properties to more closely match that of the refractory body substrate. In 
such instances, the above-mentioned diluting ceramic powders may be 
employed in the slurry in a manner well known to those skilled in the art. 
The coating composition of the present invention also preferably includes 
an addition of up to 10% by weight of ceramic fibers which are added to 
the slurry mixture. A preferred ceramic fiber which may be used is a 
commercially available and relatively inexpensive, alumina-silica fiber, 
having a typical diameter of between 1-5 microns and a length of between 
about 1-10 millimeters. The chemical composition, the size and/or the 
specific type of ceramic fibers are not, in themselves, considered 
critical to the invention, but the presence of the ceramic fibers is 
important in order to increase the green strength of the coating. Other 
materials which may be substituted for the alumina-silica ceramic fibers, 
are materials such as, zirconia fibers, titania fibers, silicon carbide 
and alumina fibers. Commercially available alumina bubbles and zirconia 
bubbles are also suitable substitutes for the ceramic fibers and these not 
only impart additional strength to the green coating but also improve the 
thermal insulation characteristics of the coating due to the trapped air 
spaces contained within their hollow shells. 
As stated above, the slurry is prepared by mixing the fine fused silica 
powder with water to obtain complete deflocculation, or dispersion of the 
slip. The ceramic fibers are then added. Good results are obtained by 
using a paddle-type mixer, such as a Zyklos mixer. Binders in an amount of 
up to about 7% by weight are also preferably added to the slurry. Binders 
such as Glass H, sodium silicate, or acrylic resins may be employed to 
impart hardenability, i.e., hardness and toughness, to the coating. If too 
much binder is added to the coating composition, there is an undesirable 
decrease in the refractoriness of the product. 
The slurry composition also preferably contains a glass-forming frit 
material to impart some degree of pyroplasticity to the coating during 
high temperature use. Frits are well known, and are mixtures of oxide 
materials having glassy phases which undergo softening at specific 
temperature ranges and serve to fill any shrinkage cracks which might form 
in the coating due to sintering as the temperature increases. 
After the slurry coating has been applied to the surface of the refractory 
body and has been dried, the coated refractory can be handled in a normal 
manner without damaging the coating. In the dried state a preferred 
coating composition consists essentially of: 
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Ingredient Weight % 
______________________________________ 
Fused Silica* about 45 to l00% 
Ceramic fibers 0 to about l0% 
Binders 0 to about 7% 
Frits 0 to about 40% 
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EXAMPLE NO. 1 
Five conventional alumina graphite refractory pouring tubes of the type 
used in continuous steel casting were dipped in a slurry of the following 
composition: 72% by weight fused silica grains less than 100 mesh, 5% by 
weight alumina-silica ceramic fibers and 23% by weight water. The tubes 
were first given a conventional antioxidation glaze and then preheated to 
a temperature in the range of 70.degree. to 120.degree. C. The table below 
shows the effect of preheated temperature versus immersion time on coating 
thickness. 
TABLE 
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Coating Thickness 
Immersion Time (seconds): 
Tube Temperature 
10 sec. 20 sec. 30 sec. 
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l00.degree. C. 
2.0 mm 2.5-3.5 mm 
4.0-5.0 mm 
6O.degree. C. 
N/A 1.0 mm 2.0 mm 
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The data in the above table indicate that higher tube temperature or 
increased dipping time results in a greater coating thickness. From trial 
tests it is observed that the thickness of the coating should preferably 
be between about 1 to 6 mm in order to be effective in preventing thermal 
shock and/or bridging between the nozzle and the mold sidewalls. The 
coating thickness is preferably about 3 mm, which, from the above table, 
is obtained when the tube is at a preheated temperature of about 
100.degree. C. and immersed in the slurry for a time period of about 20 
seconds. The coating thickness is also controlled by the viscosity and 
water content of the slurry with greater thickness being obtained with 
increasing viscosity, at constant time and temperature. Dipping a tube at 
ambient temperature is also possible and is one of the preferred methods 
included within the scope of the present invention. 
Of course, it will occur to those skilled in the art that methods other 
than dipping may be employed to apply the slurry coating to the refractory 
piece. Such alternative methods include spraying, brushing or casting of 
the slurry. The dipping method of applying the slurry is particularly 
suitable for coating refractory tubes, such as submerged pouring nozzles, 
since the inside surfaces of the tube bore may be coated along with the 
outside when the part is immersed in the slurry. The coating when applied 
to the inside of the bore provides a valuable insulation layer during cold 
start-up which prevents metal freezing within the nozzle bore and the 
resultant undesirable flow restrictions. 
EXAMPLE NO. 2 
Twenty-four conventional alumina graphite refractory submerged pouring 
nozzles were given an anti-oxidation glaze and then dip coated in a slurry 
prepared in accordance with the composition set forth in Example No. 1. 
The coating was 3 mm in thickness and after drying consisted essentially 
of about 93% by weight fused silica and about 7% by weight of 
alumina-silica ceramic fibers. Both exterior and interior surfaces of the 
nozzles were coated. The twenty-four nozzles were tested at a steel mill 
where freezing or bridging of steel between the pouring tube and water 
cooled continuous casting mold is a frequent problem. The casting trials 
with the twenty-four nozzles showed no freezing or bridging in any of the 
test nozzles coated in accordance with the invention. 
EXAMPLE NO. 3 
An additional fourteen conventional alumina graphite refractory submerged 
pouring nozzles were dip coated, both interior and exterior, with the same 
composition coating as the previously described test nozzles of Example 
Nos. 1 and 2. These pieces were tested to observe the effect of cold 
starting on a continuous casting nozzle. Normally, such conventional 
nozzles are preheated in the range of between 1,000.degree.-2,000.degree. 
C. to prevent thermal shock damage (cracking) to the nozzles. The fourteen 
coated test nozzles were each subjected to a cold casting start-up (no 
preheat) and all survived with no indication of damage due to thermal 
shock. In addition, there was no observable build-up of frozen metal 
within the internal diameter of any of these fourteen test pieces. No 
external oxidation of the carbon in the alumina graphite nozzle body was 
evident in any of the test pieces, further indicating the advantages of 
the coating of the present invention. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of the invention which is to be given the 
full breadth of the appended claims and any and all equivalents thereof.