Note: Descriptions are shown in the official language in which they were submitted.
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Improvements in or relating to refractory products
This invention relates to improvements in or relating to refractory
products and, more particularly, to improvements in refractory products used
in the handling of molten metals to increase reliability under high
temperature operating conditions.
Metal teeming, and in particular the casting of steel usually begins with
the metal being melted and transferred to a vessel, e.g, a ladle or tundish.
Refractory devices are required, amongst other things, for the regulation of
the
flow of the molten metal exiting from a nozzle mounted in the bottom of the
vessel. In the casting of steel, this is typically applied through an opening
in
the base of the vessel via nozzles and shrouds into a water-cooled mould.
Refractory devices such as sub-entry shrouds and pouring nozzles are often at
least partly submerged for long periods of time in the molten metal during the
metal teeming process and are therefore subject to high temperatures and
stresses during the effective lifetime of the device.
In a typical teeming process, metal is melted in a furnace, transferred
first to a ladle and then to a tundish from which it flows in a controlled
manner into a cooled mould. A flow control valve is provided in the tundish
comprising a flow control stopper rod selectively engageable with an outlet
nozzle seat. The stopper would normally be raised off the seat by a certain
amount to achieve a particular rate of flow of molten metal through the valve
to ultimately cast a product in a mould.
The teeming apparatus would usually include a pouring nozzle or a
shroud located beneath the flow control valve either of which may be
immersed in melt as the casting operation proceeds.
In an exchange nozzle casting mechanism, the exchange pouring nozzle
or shroud is supported beneath a stopper upper nozzle and stationary plate
assembly which is used for sealing off the flow of molten metal above the
pouring nozzle or shroud to allow the pouring nozzle or shroud to be changed
during the teeming process.
CONPIRMATION COPY
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EP-A-O 346 378 describes the development of a monotube
configuration and compares that to a two part plate and tube assembly
generally known and used within an exchange nozzle casting mechanism as
described above. The pouring tube element combines a body of high thermal
shock resistance and corrosion resistance with a sliding plate surface able to
form a tight closure against the stationary components of the mechanism.
The sliding plate surface also incorporates a hard edge to permit cutting
through any metal skin which may form during the casting operation and
which may restrict free movement of the exchange monotube during the
replacement procedure.
An important advantage of the monotube configuration over the
original fired plate and tube or cast plate and tube assemblies was the
elimination of generally horizontal joints connecting the internal casting
bore
of the tube with the external atmosphere, thereby eliminating the risk of air
ingress or metal leakage across this joint region.
As casting conditions have become more severe and service life
requirements of refractory products increased, new demands have been
placed on the monotube elements of an exchange of an exchange nozzle
casting mechanism.
In meeting these demands alternative compositions for the pouring
tube element have been developed making it possible to maintain the plate
surface and cutting edge configuration whilst providing improved corrosion
and erosion resistance. These improved materials for the pouring tube
element of a monotube do however exhibit different thermo-mechanical
properties from the original materials as shown in the following table:
MONOTUBE POURING TUBE ELEMENT COMPOSITIONS
CONVENTIONAL HIGH CORROSION RESISTANCE
A1203% 64
18 Si02% 6
35 28 C% 24
8 Zr02% 6
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4 SiC%
2.38 Bulk density g/ml 2.6
0.35 Thermal Expansion% 0-1000 0.52
In operation, it has been shown that whilst the overall criteria for
performance improvement has been met there is an increased risk that
thermo mechanical stresses arising at the outset of casting can cause an
external micro-crack fracture at the section change between the head and
body portions of the pouring tube. In many instances, this micro-crack
feature is contained by the inherent integrity of the ceramic body. This
results in no operational problem, but in extreme cases it is possible for the
external micro-crack fracture to propagate across the ceramic wall of the tube
to the inner bore. This allows either air ingress or metal leakage, both of
which cause termination of the cast and possible damage to the exchange
nozzle casting mechanism.
Studies of the behaviour of the conventional metallic can and pouring
tube element showed that the metallic can, essential to provide the accurate
geometry required for a precise fit into the exchange nozzle casting
mechanism could also act to transfer heat from the pouring element into the
cooled mechanical mechanism, thereby increasing the thermal gradient at
this critical point. Additionally at the temperatures experienced during
preheat prior to cast start up the lower region of the can would reach a
temperature of approx 900 C at which the relatively mild steel from which it
is
formed loses its rigidity and ceases to provide the desirable structural
support
below the section change.
A further development of the monotube concept is shown in US 5 866
022 which describes the assembly of a co-pressed, mixed material tube
element, as described by EP A 0 346 378 adapted to the desired operational
configuration by use of castable materials directly infilling the void between
the outer surface of the tube and the inner surface of the metallic support
element.
Whilst this design concept has shown benefits in terms of reduced
incidence of microcrack formation causing in service failures, examination of
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used pieces shows that a risk remains that a crack will propagate from the
angle between the tube and plate sections of the co-pressed tube element, as
shown in Fig 5. This behaviour is not of such severe consequence as the
failures of the type illustrated in Fig 3 as it does not necessarily result in
molten steel leakage. It is however desirable to eliminate this risk.
Extensive computer simulation of the thermo-mechanical stresses
arising during preheat and start up of casting has identified the possibility
of
minimising the stresses leading to such micro crack formation and
propagation, by minimising the thermal gradient across the tubular pouring
element, providing continuing support below any section change and
optimisation of the external geometry of the tubular pouring element.
An object of this invention is to obviate or mitigate the risks of
exaggerated thermo mechanical stresses in the new generation of pouring
tube elements, and this is found to be achievable by revising both the design
of the pouring tube element and the manner in which it is contained within
the can. It will be recalled that location of the refractory within the
support
can requires care to provide the correct geometrical configuration to allow
effective operation of the exchange tube mechanism and maintain the
principle of no direct horizontal connection from the bore to the exterior
other
than the machined sliding surface.
According to one aspect of the present invention there is provided a
refractory device for use in the teeming of molten metal comprising a ceramic
body having a ceramic pouring tube element and a ceramic support element,
said support element being adapted to be received within a metallic can, and
there is provided between said elements a shock-absorbing interface zone
wherein there is provided a material the thermal properties of which are such
that it is substantially solid at ambient temperatures but becomes deformable
at the elevated temperatures experienced during metal teeming.
Thus, the interface zone provides continuity of mechanical support to
the body portion when in the substantially solid (cool ambient temperature)
condition to ensure structural integrity of the assembled refractory device,
but
deforms sufficiently to provide a buffer against sudden differential thermal
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stresses, thereby minimising the risks of micro-crack fracture through the
body portion due to thermo mechanical stresses during pre-hat and at the
start of the casting operation.
Advantageously, the material selected for use in the interface zone is
structurally solid at temperatures up to about 700 C and becomes deformable
without any appreciable chemical degradation at temperatures above about
700 C. Preferably the material providing the interface zone comprises a
pyroplastic ceramic material.
Preferably, the interface zone comprises a ceramic material such as a
paste or bonding agent or additional structural ceramic element exhibiting the
aforesaid properties.
Conveniently, the pyroplastic material is a frittable composition applied
over at least one of the co-operating assembly surfaces of the pouring tube
element and the ceramic support element.
The ceramic support element is normally fully encapsulated within the
metallic can, and fits with and around the upper part of the pouring tube
element by virtue of said ceramic support element having an internal profile
corresponding sufficiently to the external profile of the pouring tube.
Conveniently, the respective profiles are such as to provide corresponding
interference fit surfaces or otherwise matching, e.g. tapering surfaces to
facilitate assembly, and in-fill or insertion of the required shock-absorbing
interface zone material.
The ceramic support element may be pre-formed from a ceramic
material of low thermal conductivity, or formed in situ by a suitable casting
operation of a type familiar to those in this art.
The refractory device may be otherwise finished as is known in the art
to suit its intended purpose, e.g. with regard to provision of flat surfaces
and
outlet nozzles etc.
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Embodiments of the invention will now be described with reference to
the accompanying drawings in which:
Figure 1 is a cross-sectional view of a two-part plate and tube
configuration in accordance with prior art;
Figure 2 is a cross-sectional view of a prior art monotube configuration;
Figure 3 is a cross-sectional view of a monotube configuration showing
a stress micro-crack fracture of the type minimised by the present invention;
Figure 4 is a cross-sectional view of a refractory device according to a
second aspect of the present invention.
Figure 5 is a diagram showing crack mark observed during service
trials of such a configuration.
Figure 6 is a cross-sectional view of a refractory device according to one
aspect of the present invention; and
Referring now to the figures, there is shown in Figures 1-3 cross-
sectional views of prior art refractory devices including the two-part plate
and
tube assembly known generally in the prior art and the early monotube
configuration discussed above.
Figure 6 is a cross-sectional view of a refractory product according to
one aspect of the present invention. This shows a refractory pouring device
having a ceramic pouring tube element 10 such as for example of a pouring
nozzle or sub entry shroud. The pouring tube element is supported in a
metallic can
11, which maintains the desired geometrical configuration of the tube for
mechanical
integrity of the pouring mechanism. A low thermal conductivity ceramic support
element 12 is encapsulated within the metallic can, and fits with and around
the
upper part of the pouring tube element, by virtue of said ceramic support
element
having an internal profile corresponding sufficiently to the external profile
of the
pouring tube. Here, a stepped shoulder, interference fit arrangement is
illustrated.
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The low thermal conductivity of the ceramic support element reduces
heat losses from the pouring tube during metal teeming thereby minimising
the differential thermal stresses experienced by the pouring tube which could
lead to propagation of stress micro-crack features.
A shock absorbing interface zone 13 is formed between the low
conductivity ceramic support element 12 and the pouring tube element 10.
The zone is formed in accordance with one aspect of the invention by a layer
of pyroplastic ceramic cement, the properties of which are chosen to provide
optimum mechanical strength in temperatures below about 700 C to support
the pouring tube during preheating operations and manipulation. The
cement has a degree of pyroplasticity at elevated temperatures encountered
during use of the pouring tube in the metal teeming process to absorb any
residual differential stresses, which may be created during this process.
By way of example, the pyroplastic ceramic cement may be formed from an
alumina-silicate mixture with an addition of fluxing agents to generate the
pyroplastic behaviour. A typical analysis of said pyroplastic cement being
alumina
20%, silica 54%, potassium oxide 6%, boric oxide 12% and sodium oxide 8%. Such
a composition will provide for progressive melting from about 700 C to impart
plasticity to the layer.
Figure 4 illustrates a further embodiment of the present invention
wherein the pouring tube element 20 is coated with a pyroplastic surface
layer 24 on its upper region to provide the desired low temperature rigidity
and high temperature malleability. The coated tube is then encapsulated
within the metallic can by a ceramic concrete 22, which provides mechanical
support to the pouring tube during the teeming process. Furthermore, the
ceramic support element reduces heat losses from the pouring tube during
metal teeming thereby minimising the differential thermal stresses experienced
by
the pouring tube which lead to propagation of stress microcrack features.
In use of either of the refractory device described above, the pouring
tube is mounted beneath the orifice of a vessel (not shown). Molten metal is
poured through the pouring tube for example into a water-cooled mould (not
shown). During the metal casting process, the external temperature of the
pouring tube rises typically to between 700 C and 900 C. At temperatures up
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to about 700 C, the pyroplastic interface zone (13; 24) between the pouring
tube element (10; 20) and the ceramic element (12; 22) encapsulated in the
metallic can remains solid and provides structural continuity and additional
mechanical support to the pouring tube. Thereby, structural integrity of the
refractory device is provided for e.g. during handling for transport purposes,
and initially during assembly into a pouring mechanism and pre-heat. At
temperatures above about 700 C however, at which differential thermal
stresses between the pouring tube and the support therefor in the metallic
can would have previously possibly caused a stress micro-crack fracture of
the pouring tube, the pyroplastic interface zone becomes deformable, thereby
minimising differential thermal stresses experienced by the pouring tube in
the region supported by the metallic can. Therefore, in this way the
possibility of micro-crack fracture through the refractory device and failure
thereof is obviated or mitigated. Thus, the present invention results in an
improved refractory device that has better reliability and is less prone to
damage from differential stress micro-crack features.
