Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention concerns a coating of thermall~ insulating material for
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k~ a continuous strip casting machine such as, for example, the Hazelett strip
caster.
With this continuous casting process liquid metals e.g. aluminum,
copper, zinc, steel and alloys of these metals are cast in strip form. Two
moving endless strips which are usually made of steel, are guided over
supporting rolls and deflection rolls in such a way that they form a para-
llel gap into which the liquid metal is cast. The heat of solidification
is conducted away by the steel strips which are cooled with water on the
other side.
In order to control the rate of removal of the heat, the steel strips
are provided with a coating. This coating prevents the steel strips from
warping due to overheating when they are in contact with the liquid metal.
At the same time, heat transfer during the solidification is controlled in
such a way that, in particular in the production of aluminum strips, metall-
urgically sound strips of cast metal are produced.
The coatings on the steel strips must simultaneously satisfy a series
of conditions difficult to fulfil. The coatings must have heat resistance,
resistance to high temperature and thermal shock, good adhesion to the steel
strip, the ability to be uniformly deposited, long life wear resistance and
flexibility in order to avoid cracks forming when the strip is in service.
Some of these requirements are met by a number of known coatings. For ex-
ample organic resins such as polyvinylpyrrolidon (PVP) or silicones are
sometimes used, mixed with suitable lubricating and insulating materials.
Many of the known coatings have however a pronounced disadvantage in that
they abrade the solidifying strip. Furthermore, since the organic resins
are heat resistant only up to temperatures far below the temperature of the
liquid melt, their lifetime is limited. They are therefore often prevented
from coming into direct contact with the liquid metal by means of finely
dispersed materials. This increases the lifetime but adds the d:isadvantage
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of transfer of the finely dispersed materials to the cast strip.
In order to avoid these difficulties a coati-ng of ceramic materials,
deposited by means of thermal spraying, in particular plasma spraying, has
been developed. This ceramic coating has the decisivle advantage of having
resistance to high temperature and also complete freedom from wear. Its
lifetime is very long and therefore, in contrast to the known coatings it is
considered a permanent coating.
This coating has a certain porosity which can lead to the coating
taking up moisture from the surrounding atmosphere. The atmosphere can have
a high moisture content as large quantities of water circulate around the sides
of the caster. These pores, together with a certain roughness in the surface
of the coating lead to the liquid metal that flows between the steel strips
wetting the surface of the coating non-~miformly. rrhis Ls particularly so
when the metal is alumLnum. The non-unlEorm wetting produces surfaced Elaws
on the cast strip generally known as segregation scars.
This invention relates to a method of use of a sealing layer for a
porous coating of ceramic materials, said coating being applied to strip molds
in a contlnuous casting operation, wherein said sealing layer consists of an
organic resin or at least of one fine particulate inorganic substance or of a
colloidal lubricating agent, wherein saLd sealing layer precludes said coating
from taking up moisture and thereby forming water vapor on contact with liquid
metal.
The invention seeks to improve the above described wetting behaviour
by sealing the pores or by providing the coating with another layer of material.
Thus, the present invention provides a sealing layer for a porous coating of
ceramic materials, which is applied to strip moulds of a continuous casting
unit, which sealing layer consists of an organic resin or at least of one
fine particulate inorganic substance or of a colloidal lubricating agent.
By sealing the pores with an organic resin such as polyphenylene
sulphide (PPS), polybenzimidicarbazole (PBI), polyvinylpyrrolidone (PVP), poly-
imides, silicone resins or at least one fine particulate substance such as
graphite, zirconium silicate, hexagonal boron nitride, talc, ceramically bonded
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aluminum, or by the applicatîon of a thin protective layer of, for example,
Fiberfrax QF/50 of carborundum, or by working in a lubricant such as graphite,
magnesium hydroxide or electrophoretically applied silicon dioxide, the uptake
of moisture is prevented and the formation of water vapour on contact with
the liquid metal avoided. At the same time the wetting nature of the liquid
metal on the coating is changed so that a smooth clean surface is obtained
on the cast strip.
The relationships between the pores in the coating, humidity,
sealing agents and wetting or surface finish of the cast strip have not yet
been
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completely explained. It was found in trials however that the ~aterials
used prevent segregation from forming and improve the surface of the cast
strip.
The sealing agents adhere in the pores of a ceramic coating extremely
well and lead, even after partially burning away, to a degree of wear in
contacting the strip, which does not impair the quality of the product. Use-
fully the thickness of the sealing layer (with the e~ception of Fiberfrax~
should not exceed the average depth of irregularity which makes up the rough-
ness in the ceramic coating i.e. a depth of 1~20 microns. The sealing layer
should definitely not constitute a complete covering layer over the ceramic
coating.
The invention will be further described with reference to the accomp-
anying drawings in which:
Figure 1 is a diagram of the continuou9 casting proces9, and
Figure 2 illustrates a strip mould having a sealing layer according
to the invention.
In Figure 1 two moving endless steel strips 1 are guided over support- -
ing and deflecting roller 2 to form a parallel-sided space 3. Liquid metal
4 is flowed into the space 3 where it solidifies. To assist solidification
the strips 1 are cooled with water from jets 5.
Figure 2 shows an enlarged section of a strip mould 6, which has a
cerc~mic coating 7 on it, in particular a coating depo9ited by plasma 9praying,
the pores 8 of which have been sealed or blocked up, whilst the closed pores
9 or those away from the surface are not sealed.
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