Immersed metallurgical pouring nozzles

An immersed metallurgical pouring nozzle, such as a submerged entry nozzle for pouring molten steel, comprises a body of refractory material, such as graphite alumina, which defines a flow passage. An annular member of refractory material, such as zirconia with a very low or no-content of carbon, whose erosion resistance is higher than that of the body of the nozzle is wholly encapsulated in the material of the body of the nozzle.

1. FIELD OF THE INVENTION 
The present invention relates to immersed metallurgical pouring nozzles, 
that is to say pouring nozzles of which a portion, typically the 
downstream end, is immersed in a pool of molten metal, in use. The 
invention is particularly concerned with so-called submerged entry nozzles 
(SENs) for pouring molten steel, that is to say pouring nozzles which 
conduct molten steel from a tundish or other metallurgical vessel into a 
mould, typically a continuous casting mould from which the solidified 
metal is continuously withdrawn. The invention does, however, relate to 
other types of pouring nozzles, such as so-called ladle shrouds for 
conducting molten steel from a metallurgical vessel into a tundish, whose 
downstream end is also submerged, in use, in molten metal. 
2. DESCRIPTION OF THE PRIOR ART 
When continuously casting steel, molten steel is continuously introduced 
into the open upper end of the mould through an SEN whose lower end is 
submerged in the metal in the mould. The surface of the steel in the mould 
is thus exposed to the air and is thus subject to reoxidation. In order to 
prevent this and to minimise the heat loss from the exposed surface, the 
surface of the molten steel is typically covered by a layer of insulating 
powder comprising a combination of fluxing agents or glasses together with 
carbon, silica and alumina. The powder melts into a glassy layer which 
shields and insulates the molten steel surface and tends to be drawn down 
between the molten steel and the sides of the water-cooled mould and thus 
to act as a lubricant. However, this molten glassy layer has a highly 
aggressive and corrosive tendency with respect to the material of the SEN. 
The outer surface of the SEN tends to be rapidly eroded away by the glassy 
layer at the slag line, that is to say at the region at which the SEN 
passes through the surface of the molten steel and glass, and it is this 
erosion which limits the service life of the SEN and necessitates its 
being replaced relatively frequently. 
SENs for casting steel are typically made of a mixture of alumina and 
graphite. The graphite is added to impart thermal shock resistance to the 
alumina because it will be appreciated that at the commencement of 
operation, even if the SEN is preheated, as is common, a relatively cold 
SEN is contacted by molten steel at a temperature of ca. 1550.degree. C. 
which represents a very substantial thermal shock. Pure alumina would tend 
to crack when subjected to this thermal shock but graphite has a high 
coefficient of thermal conductivity and thus tends to accelerate the 
dissipation of thermal gradients and also has considerable lubricant 
characteristics and thus permits slight relative movement of the 
constituent alumina particles of an SEN without cracking occurring. 
However, the presence of the graphite in the alumina reduces the resistance 
to erosion by the glassy layer at the slag line by its influence on the 
bonding matrix. Accordingly the graphite content need be as high as 
possible to produce one of the necessary characteristics of SENs, namely 
thermal shock resistance, and as low as possible to achieve the other 
necessary characteristic, namely resistance to erosion at the slag line. 
The construction and composition of all SENs thus necessarily constitutes 
a compromise between these two conflicting requirements. 
Various different constructions of SEN have been proposed and used in an 
attempt to minimise these problems and certain of these are illustrated 
schematically in FIGS. 1a-1d. 
FIG. 1a shows a simple SEN which is of uniform alumina graphite 
construction with its lower end immersed in a pool of molten steel 2 on 
which a glassy protective layer 4 of molten mould powder floats. As may be 
seen, the body 6 of the SEN is eroded very substantially at the slag line 
and the rate of wear or erosion is typically 7 to 10 mm per hour. The 
composition of such nozzles includes 40 to 65%, typically 51%, by weight 
Al.sub.2 O.sub.3 and 20 to 35%, typically 31%, by weight C and has a bulk 
density of 2.20 to 2.65, typically 2.40 g/ml. 
The modified SEN shown in FIG. 1b includes an annular body 8 of zirconia 
graphite which is copressed with the alumina graphite and affords the 
external surface of the SEN in the region of the slag line. The alumina 
graphite has the same composition as that set forth above and the zirconia 
graphite has a composition including 65 to 82%, typically 74%, by weight 
ZrO.sub.2 and 17 to 25%, typically 20%, by weight C and a bulk density of 
3.20 to 3.60, typically 3.60, g/ml. In this construction, the rate of 
erosion can be reduced to typically 1.5 to 3.5 mm per hour and whilst this 
represents a substantial improvement the rate of erosion is still 
substantial. The reason for this is that the zirconia graphite insert 
necessarily includes a significant graphite content in order that it has 
the necessary thermal shock resistance and this graphite content renders 
the bonding matrix of the insert subject to substantial rates of erosion 
at the slag line. 
The further modified construction shown in FIG. 1c is very similar but in 
this case the entire lower portion of the SEN is made of zirconia graphite 
whose composition is the same as that set forth above. The performance and 
disadvantages of this construction are the same as those of the 
construction of FIG. 1b. 
FIG. 1d represents a different approach in which a preformed, high 
temperature fired annular sleeve of sintered zirconia is secured by 
refractory cement to the external surface, in the region of the slag line, 
of an SEN of otherwise conventional shape. The zirconia sleeve has a very 
high erosion resistance, whereby the erosion is reduced to typically 0.2 
to 0.5 mm per hour, but due to the absence of graphite its thermal shock 
resistance is lower which means that in practice this construction is 
unacceptable due to the possibility of thermal shock failure of the sleeve 
and/or its refractory cement connection to the SEN, especially if the 
preheating conditions are not accurately controlled. 
Accordingly it is an object of the present invention to provide an immersed 
metallurgical pouring nozzle, particularly an SEN for pouring steel, which 
avoids the problems referred to above and which in particular has a 
reduced tendency to erosion at the slag line but which nevertheless is not 
subject to thermal shock failure. 
SUMMARY OF THE INVENTION 
According to the present invention an immersed metallurgical pouring 
nozzle, particularly an SEN, of the type comprising a body of refractory 
material which defines a flow passage and an annular member of refractory 
material whose erosion resistance is higher than that of the body of the 
nozzle is characterised in that the annular member is wholly encapsulated 
in the material of the body of the nozzle. 
The body of the nozzle may be made of a single refractory material e.g. 
alumina graphite. 
In a modified embodiment, the body of the nozzle comprises an upper portion 
of refractory material and a lower portion of refractory material in which 
the annular member is encapsulated and whose erosion resistance is greater 
than that of the upper portion but less than that of the annular member. 
In a further modified embodiment the annular portion of the body of the 
nozzle which is situated outside the annular member is made of a 
refractory material whose erosion resistance is greater than that of the 
remainder of the body of the nozzle but is less than that of the annular 
member. 
Thus the nozzle in accordance with the invention is provided with a band or 
annular member of erosion resistant material, as in the known 
constructions, but differs from the known constructions in that the 
erosion resistant material does not constitute a part of the outer surface 
of the nozzle but is surrounded by a layer of the material constituting 
the body of the nozzle. 
In the known nozzles, at the beginning of pouring, the molten metal and 
erosive glass layer come directly into contact with the erosion resistant 
material which is thus subjected to a substantial temperature gradient and 
thermal shock and must be constructed to resist this. However, in the 
nozzle in accordance with the present invention, the molten metal and 
erosive glass layer do not initially come into direct contact with the 
erosion resistant material but instead contact the material of the body of 
the nozzle inside and outside it which means that the temperature gradient 
and thus the thermal shock to the erosion resistant material is subjected 
are substantially reduced. This means that the erosion resistant material 
need no longer represent the same compromise between thermal shock 
resistance and erosion resistance, or at least not to the same extent as 
previously, and thus that it may have a lower graphite content, preferably 
0 to 10% and more preferably 6% or less, than was previously possible 
whilst still having adequate resistance to the reduced thermal shock to 
which it is subjected. Its erosion resistance may thus be substantially 
higher than was previously possible. The covering layer of the material of 
the body of the nozzle is rapidly eroded at the slag line but by the time 
the molten glass layer contacts the erosion resistant material it has 
already substantially reached the temperature of the molten metal and is 
not then subjected to a further substantial thermal shock. 
Further features of the invention will be apparent from the following 
description of two specific embodiments of the invention which is given 
with reference to FIG. 2 of the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The SEN shown in FIG. 2 comprises a tubular body 6 which defines a central 
flow passage 7 and is for the most part made of pressed alumina graphite, 
whose composition is the same as that described in connection with FIGS. 
1a-1d. Wholly encapsulated within the body at its lower end, that is to 
say in the vicinity of the slag line, i.e. where the nozzle will pass 
through the layer of molten mould powder when it is in use, is an annular 
member 8 of substantially higher erosion resistance, e.g. carbon bonded 
zirconia, optionally with a low content of graphite. The annular portion 
11 of the body 6 outside the insert 8 comprises a layer of zirconia 
graphite and this will be subject to the same compromise as regards carbon 
content as was discussed in connection with FIGS. 1a-1d and will therefore 
have the same composition as described in connection with FIG. 1c. 
At the commencement of the operation, the annular member 8 is not directly 
contacted by molten steel or mould powder but is initially protected and 
insulated by the surrounding alumina graphite material of the annular 
portion 11. It is therefore subjected to a substantially reduced thermal 
shock which it can adequately resist with only a low graphite content. The 
outer layer of alumina graphite is rapidly eroded at the slag line, at a 
rate which is slower than that at which the alumina graphite of the 
remainder of the body would be eroded until the slag contacts the insert 
8, whereafter the rate of erosion is further reduced, typically to less 
than 1 mm per hour. 
The erosion resistant insert 8 may be a unitary, self-supporting member 
which is copressed with the alumina graphite of the nozzle body. It is 
preferred that the insert comprises carbon bonded zirconia comprising 85 
to 92%, typically 88%, by weight ZrO.sub.2 and 2 to 10%, typically 6%, by 
weight C and has a bulk density of 3.9 to 4.4, typically 4.1, g/ml. 
Alternatively, the insert may be presintered and incorporated into the 
nozzle body during its manufacture. In this event the insert will 
preferably contain 87 to 97%, typically 95.5%, by weight ZrO.sub.2 and 
will have a bulk density of 4.1 to 4.6, typically 4.3, g/ml. However, the 
fact that the insert is not exposed to the atmosphere and is wholly 
supported by the material of the nozzle, opens up the possibility of the 
insert 8 being carbon and graphite free and in powder or partially 
presintered form in the as supplied state and then subsequently densifying 
and fully sintering under the action of the heat of the molten metal as 
the nozzle is first used. In this event the insert may comprise 84 to 94%, 
typically 92%, by weight ZrO.sub.2 and will have a bulk density of 3.9 to 
4.3, typically 4.0 g/ml. The material thus initially has a high thermal 
shock resistance which changes progressively to a high erosion resistance 
as sintering proceeds. If densification and sintering of the erosion 
resistant insert occurs in situ, this will be associated with a reduction 
in volume but this can be readily accommodated by providing a layer of 
compressible, refractory material, e.g. ceramic fibres adjacent the inner 
surface of the insert 8. 
The service life of a nozzle as shown in FIG. 1a is sufficient to enable 
only one ladle of molten or even less to be poured before replacement is 
necessary due to slag line erosion. Nozzles as shown in FIGS. 1b and 1c 
have an increased service life sufficient to pour, typically, four ladles 
of molten steel. However, the nozzle in accordance with the invention as 
shown in FIG. 2 is found to have a significantly improved service life 
sufficient to pour, typically, ten ladles. 
It will be appreciated that, as an alternative to alumina graphite, the 
nozzle body 6 may be made of any material suitable for the purpose, such 
as fused silica, and that the erosion resistant insert 8 may comprise 
materials other than zirconia, e.g. magnesia or even alumina with a lower 
graphite content than the nozzle body. The invention has been described 
principally in connection with nozzles for pouring steel but it is equally 
applicable to nozzles for pouring non-ferrous metals, such as aluminium, 
where similar nozzle erosion problems arise. 
Obviously, numerous modifications and variations of the present invention 
are possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.