Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~337205
The invention relates to a method for manufacturing an
immersion nozzle for metal melts, particularly steel melts, from a
starting mixture containing aluminium oxide and boron nitride with
the use of a fluxing agent, a bonding agent and optionally further
conventional additives by preparing the starting mixture with
liquid, shaping, drying and optionally firing the shaped body.
Immersion nozzles are used in the processing of metal
melts, particularly in the continuous casting of steel. DE-C-
3003046 discloses a ceramic composition based on aluminium oxide
or zirconium oxide or silicon oxide, carbon and bonding agents for
manufacturing such immersion nozzles, which contains 5 to 15 wt.%
calcium silicon and/or ferrosilicon and/or boron nitride. Due to
the boron, nitride a reduction of the melting point of the solid
oxide suspensions present in the steel, e.g. alumina particles of
1 to 30mm is supposed to be achieved, so that these become liquid
and do not adhere to the refractory wall. A mixture of different
refractory materials is however not proposed in this DE-C-3003046
and further nothing is said about the grain size of the corundum
(aluminium oxide) used. Additionally, DE-A-3439954 discloses a
refractory wear part for pouring liquid melts, which is
manufactured from a mixture of A1203, graphite, a fluxing agent
combination and a carbon-containing bonding agent and a metallic
powder and in which the fired wear part is wholly or partially
rough glazed and optionally subsequently fired in an oxidising
atmosphere. However, in the manufacture of this refractory wear
part, only aluminium oxide is used as the refractory material and
no boron nitride is used. Immersion nozzles, in which a layer of
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boron nitride is applied to the surfaces which come into contact
with the steel, are disclosed in DE-C-3401999.
The problem referred to above of the adhesion of oxide
components contained in the steel melt is particularly present
with steel melts when using immersion nozzles, whereby a clogging
of the immersion nozzle may result in an interruption in the
pouring of the metal melt. It is absolutely important to make the
pouring time as long as possible for cost reasons. A further
problem with immersion nozzles is their resistance to changes in
temperature which should be sufficiently high in order to avoid
the formation of cracks and a premature replacement of the
immersion nozzles.
It has now been found that in immersion nozzles as a
result of the use of fine grained aluminium oxide in conjunction
with fluxing agents and boron nitride such a growth of oxide slag
constituents of the metal melt can be avoided, but that when using
fine aluminium oxide the resistance to changes in temperature is
reduced. If aluminium oxide is used for the manufacture of
immersion nozzles it was previously used with grain sizes of up to
0.5mm. When using such coarse grained aluminium oxide there was
however always a deposition of oxide components, particularly of
aluminium oxide from the steel.
It is thus the objection of the present invention to
provide an immersion nozzle which not only exhibits a reduced
tendency to deposits or clogging but which also has a sufficiently
high resistance to changes in temperature.
According to an aspect of the present invention, there
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3 1 337 20S
is provided a method of manufacturing an immersion nozzle of the
type described above, which comprises:
[A] preparing a starting mixture using a liquid,
the said starting mixture containing:
(a) finely grained aluminium oxide having a
maximum grain size of 250~m,
(b) a refractory material which (i) is other
than aluminium oxide and (ii) has a lesser
tendency of formation of deposits than
aluminium oxide when used in the metal melt,
(c) boron nitride,
(d) a fluxing agent, and
(e) a bonding agent,
[B] shaping the starting mixture;
[C] drying the shaped mixture; and
[D] where required, firing the dried mixture.
Two major characteristic features of the method
according to the present invention are:
a) the combination of aluminium oxide and the other
refractory material having a lesser tendency to form deposits than
aluminium oxide, and
b) the use of finely grained aluminium oxide.
Preferably the aluminium oxide has a maximum grain size
of 150~m, more preferably 90~m and particularly preferably 44~m.
The smaller is the maximum, grain size of the aluminium oxide
used, the smaller is the tendency to adhesion of oxide components
from the metal melt but on the other hand if the structure is too
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finely grained, the resistance to changes in temperature tends to
become worse.
The other refractory material which has a lesser
tendency to form deposits than aluminium oxide, may thus have a
coarser grain size than the aluminium oxide used, i.e. preferably
with a minimum grain size of 44~m, more preferably with a minimum
grain size of 90~m and particularly preferably with a minimum
grain size of 150~m, whereby the maximum grain size of these other
refractory materials is determined only by technical requirements,
i.e. the maximum grain size extends up to the usual values of
l.Omm.
The effect of the combination of finer grained aluminium
oxide and coarser grained other refractory materials is that the
resistance to changes in temperature of the finished immersion
nozzle can be improved so that only the problem of the clogging of
the immersion nozzle but also the problem of the good resistance
to changes in temperature is achieved which is at least as good as
in an immersion nozzle manufactured with a coarse grained alumina.
The aluminium oxide used in the method in accordance
with the invention can be a conventional material used for
refractory purposes, e.g. appropriately fine grained fused
corundum or tabular alumina.
Examples of the other refractory or oxide materials
which have a lesser tendency of deposits formation than aluminium
oxide, are mullite, silicon carbide, silicon nitride, vitreous
silica, broken porcelain and fused lime. These other refractory
materials can in each case be used individually or as a mixture
1337205
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together with the aluminium oxide.
The total amount of the fine grain aluminium oxide and
the other refractory material is not particularly critical and is
in a range of common use. Usually it is from about 30 to about 80
wt.%, preferably from about 40 to about 60 wt.% with respect to
the stating material.
The weight ratio of aluminium oxide to the other
refractory materials is not very critical, and in a preferred
embodiment, it is 30:70 to 70:30. This results on the one hand in
the good refractory characteristics of aluminium oxide and on the
other hand an adequate resistance to changes in temperature is
achieved by use of the other refractory material. The grain size
of the other additives normally ranges up to a maximum grain size
of l.Omm, preferably 0.5mm, i.e. their grain size corresponds to
the usual grain sizes used in the manufacture of such immersion
nozzles.
In a further preferred embodiment graphite and/or
elemental silicon can be added as a further additive to the
starting mixture. Flakey graphite is advantageously used as the
graphite and the elemental silicon is commonly used in finely
divided form, i.e. with a maximum grain size of 0.2mm. Due to the
use of flakey graphite expensive boron nitride can be saved.
Elemental silicon serves to strengthen and to protect the carbon
compound against oxidation.
When manufacturing the immersion nozzles in accordance
with the invention at least one fluxing agent is used. This can
be selected from fluxing agents commonly used in the manufacture
1337205
of immersion nozzles. This can be boron-free or boron-containing
fluxing agents, examples of which are glass frits, feldspars,
boric acid, borax etc.
In a preferred embodiment a fluxing agent combination
comprising a glass frit and an alkaline feldspar are used as the
flowing agents. The glass frit used can be either a boron-
containing glass frit, e.g. a glass frit with an approximate oxide
composition of 30% SiO2, 6% A1203, 4% Fe203, 8% CaO, 1% MgO, 0.4%
Mn304, 25% Na20, 2% P205, 3.2% BaO and 20% B203. Furthermore, it
is also possible to use so-called glass frits, e.g. with an
approximate oxide composition of 66% SiO2, 23% A1203, 5% CaO, 4%
ZnO and 2% LiO2. When using such boron-free glass frits a boron-
containing fluxing agent, as were referred to above, is
advantageously also used.
In the manufacture of the immersion nozzles in
accordance with the invention a conventional temporary bonding
agent is also used. In an advantageous embodiment a resin,
particularly a synthetic resin in the form of a novolak or a resol
resin or a conventional pitch is used as the temporary bonding
agent, whereby synthetic resin and pitch can also be used as a
mixture. When using a curable synthetic resin a suitable
hardener, e.g. hexamethylenetetramine, can be added in the amount
necessary to harden the synthetic resin added, for instance when
using novolaks. When using resol resins a hardener additive is
normally not necessary. The temporary bonding agent is decomposed
during the firing step.
The boron nitride used in the manufacture of the
1337205
immersion nozzles in accordance with the invention may be
hexagonal boron nitride commonly used in fine grain form, i.e.
with a grain size not more than lOO~m.
The amount of the boron nitride used, with respect to
the dry starting mixture, is usually in the range of 5 to 30 and
advantageously in the range of 8 to 25 wt.%. The amounts of
optionally added elemental silicon are in the usual ranges of 3 to
8 wt.% with respect to the dry starting mixture and the amounts of
graphite in the range of 0 to 20 wt.%.
The fluxing agents are used in amounts between 2 and 12
wt.%, with respect to the dry starting mixture.
The temporary bonding agent is generally used in amounts
of 5 to 20 wt.%, with the respect to the dry starting mixture.
The invention will be described in more detail by way of
the following examples.
Example 1
A starting mixture of 12 parts by weight fused corundum
with a maximum grain size of 120~m, 14.5 parts by weight tabular
alumina with a maximum grain size of 44~m, 21 parts by weight
comminuted vitreous silica with a grain size of 0.09 to 0.5mm, 4
parts by weight feldspar, 1.5 parts by weight borax, 2.5 parts by
weight of a boron-containing glass frit with a B203 content of
20%, 15 parts by weight flakey graphite, 10 parts by weight boron
nitride with a maximum grain size of lOO~m and 5 parts by weight
finely divided silicon with a maximum grain size of 75~m was
thoroughly premixed in a mixer. Subsequently 14.5 parts by weight
of a resol resin solution were added. The resol resin solution
8 133720S
was sufficient as the mixing liquid.
After thorough mixing an immersion nozzle was moulded
from the mixture, dried for 4 hours at 120C and subsequently
hardened for 6 hours at 180C.
This nozzle was subsequently slowly heated in a reducing
atmosphere and fired for 4h at 1000C.
In use, the immersion nozzle exhibited only a very small
tendency to adhesion of oxide components to the surfaces coming
into contact with liquid steel and had an adequately good
resistance to changes in temperature.
Examples 2 to 5
The procedure of Example 1 was repeated with the
starting components given in the following table, whereby
immersion nozzles were produced which also had only a small
tendency to the accumulation of slag components from a steel melt
and had a sufficiently high resistance to changes in temperature.
` 9 1337205
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