Note: Descriptions are shown in the official language in which they were submitted.
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Process for preheating a nozzle
The invention relates to a process for preheating a no-
ale, in particular a nozzle for feeding metal between two
bands, belt-like tracks or the like of a mound for a con-
tenuous casting machine with moving mound walls.
Machines for continuous casting in particular ferrous and nonferrous metals have been developed and namely such
that the mound features continuously moving walls. These
include machines where the metal to be cast is poured
10 between two moving steel bands or belts, in particular the
well known Hazelitt strip casters e.g. according to the US
patent 2,640,235.
Also known are other machines in which the cast product
obtains its shape by means of mound halves joined together
15 from a pair of endless belts like caterpillar tracks.
At the pouring end the mound halves face each other and
move thus over a specific distance in which they form the
actual mound. The mound halves then separate and after a
short time return to the pouring or casting end.
20 With, in particular, machines featuring caterpillar track
type mounds for casting relatively thin strips e.g. strips
only 20 mm thick and less, it is the feeder nozzle for the
metal which presents the greatest problems. One of the
main reasons for this is that there are few materials
25 which are able to withstand the high temperatures of the
metal flowing through it. Graphite for example is one of
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the few materials able to meet the necessary require-
mints. On the other hand graphite has the disadvantage of
high thermal conductivity, as a result of which the heat
is conducted away from the melt so quickly that the metal
has a tendency to solidify in the nozzle.
In any case the parts of the feeder nozzle which come into
contact with the molten metal must be made of a refractory
material which e.g. when casting aluminum comprises a
mixture of 30 % diatomaceous earth (almost pure silica in
10 the form of microscopic cells), 30 % long asbestos fires,
20 % sodium silicate (dry weight) and 20 % chalk (to form
calcium silicate.
For casting steel one can use in particular ZrO2 for the
nozzle material, or ZrSiO4 containing various amounts of
15 ZrO2; for cost saving purposes the nozzle can also be made
of two materials. The ZrO2 used is a stabilized, fine con-
amid material with very accurately controlled porosity.
Such a structure is able to accommodate the change in volt
use occurring in the Zr2 in such a way that problems of
20 cracking are avoided.
Although the above feeder nozzles made of a refractory ma-
tonal feature good thermal barrier properties and low
heat capacity, their basic disadvantage is that, because
of lack of homogeneity in terms of chemical composition
25 and mechanical properties, the material employed exhibits
an up-take of moisture, irreversible changes in chemical
composition on heating up to the operating temperature and
a related further embrittlement or lower mechanical
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strength and - connected with this - normally permits the
nozzle to be used only once. In spite of the above men-
toned low thermal capacity and poor thermal conductivity
of the known ceramic material when the molten metal is
poured for the first time unto the nozzle, the nozzle is
exposed to extreme thermal shock with the result that the
sudden thermal stress created inside the nozzle can cause
it to break apart or at least to crack. A further problem
is the so called freezing-up of the nozzle as the metal to
10 be cast comes into contact with the cold inner surface of
the nozzle.
In order to avoid such thermal shock or freezing up of the
nozzle it has been proposed e.g. in the German patent pub-
ligation DE-05 28 16 500 that the whole nozzle should come
15 prose a plurality of hollow sections-lying adjacent to
each other held together in a metal holder and made of a
material which resists melting and is heat resistant -
acting as outlet nozzles, and such that channels for heat-
in the device are provided parallel to the channels
20 through which the melt passes. With the aid of such a
heating facility it is possible to heat the nozzle up to
the necessary melt pouring temperature before casting
starts. This can be done so slowly that no thermal shock
occurs or thermal stresses are created. On the other hand
25 such a nozzle is complicated in its make up and in par-
shackler the heating mechanism has a disturbing influence
during normal operation i.e. after the start-up phase is
past.
The object of the present invention is therefore to dove-
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lop a much simpler possibility for preheating a nozzle to
the desired temperature before the actual start of feeding
the metal to be cast.
This object is achieved by way of the invention in that,
before the actual metal to be cast is passed through the
nozzle, at least one substance at a temperature other than
of the said metal is passed through the nozzle.
This extremely simple idea is not only very inexpensive
and uncomplicated, but enables conventional nozzles to be
10 used and eliminates the need for additional heating fact-
titles.
This substance / these substances should, according to the
invention, be at a temperature higher than that of the me-
tat to be cast. This process has the advantage that only a
15 small amount of preheating substance is required and also
the time of contact between substance and nozzle can be
kept short. However, this requires considerable experience
with regard to heat transfer between substance and nozzle
material.
20 Preference should be given therefore to a substance or
substances which is / are at a lower temperature than the
metal to be cast. This prevents the nozzle being heated to
too high a temperature - which would also cause the metal
to be overheated and make additional cooling in the mound
25 necessary.
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The substance to be passed through the nozzle before the
metal which is to be cast is preferably another metal. As
a rule an inexpensive metal is suitable for this purpose.
Before introduction into the nozzle this metal can be
heated in a special container to the desired temperature,
be this above or below that of the metal to be cast.
Preferred, however, is a metal of lower density than the
metal to be cast. This way the nozzle and in particular
its side walls which limit the width of the strip are not
10 suddenly subjected to excessive pressure at the start of
casting. If for example a ferrous metal - in particular
steel - is to be cast, then aluminum may be employed as
it is less dense than steel and its oxide skin prevents
the metal penetrating slits or the like. However, this
15 could lead to contamination of the metal to be cast - in
this case steel - or at least the first of the metal to be
cast. For this reason it is proposed, in accordance with
the invention, when casting a ferrous metal to allow an-
other ferrous metal e.g. cast iron (grew cast iron) to be
20 passed through the nozzle before the actual casting be-
gins.
It is also within the scope of the invention for a pour-
amity of the same ox different substances at different
temperatures to be passed first through the nozzle. By way
25 of example the following steps in the process should be
mentioned: The nozzle is heated to a temperature of about
200 - 250C with hot air. Then molten aluminum at a them-
portray of about 760C is introduced. This is followed by
grew cast iron at a temperature of about 1200 C. After
30 this the steel which is to be cast can then be introduced
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without problem into the nozzle.
To this end, and within the scope of the invention, the
tundish upstream of the nozzle features a plurality of
clambers for the various substances or metals. In order
that the metal which is to be cast is not unnecessarily
brought into contact with the lower temperature materials
a plurality of separate tundishes can be provided upstream
of the nozzle.
This process makes it possible, and in a simple manner,
10 also for already fitted nozzles at the casting machine to
be heated to the desired temperature prior to casting, and
so to employ for the nozzle a material which is relatively
sensitive to thermal shock with the processes used up to
now.
