The present invention finds particular use, for example, in the continuous casting of molten steel. In the continuous casting of molten metal it is common practice to use a shroud to enclose molten metal as it flows from one vessel to another so as to reduce contamination of the metal with undesirable materials such as air, dust or other foundry contaminants. For example, in the casting of steel it is common practice to employ a shroud for the molten steel as it passes from a ladle to a tundish.

As is well known in the art, the"ladle"is a vessel used for the primary storage and feeding of the molten steel, and the"tundish"is a vessel, fed by the ladle, for holding the molten steel as it is being fed to a mould or die. The provision of a"ladle shroud" for feeding the molten steel from the ladle to the tundish is extremely important when it is desired to minimise the introduction of impurities, particularly air, into the steel. For example, in the absence of such shrouding, molten steel picks up oxygen and nitrogen from the atmosphere, and this leads to the presence of undesirable impurities in the produced cast steel. Impurities of this type generally produce undesirable effects in the steel, for example, a reduction in the tensile and impact strength of the steel.

The conventional ladle shroud employed in prior art processes for the manufacture of cast steel generally comprises a vertical tubular body through which the molten steel may flow. The upper part of the tubular body of the shroud is adapted to abut a nozzle located in the base of the ladle (generally referred to as a"ladle collector nozzle") for the delivery of molten steel. The lower part of the tubular body of the shroud delivers molten steel to the tundish. As is also well known in the art, the ladle collector nozzle can be opened or closed to the passage of the molten steel by means of a moveable slide gate located in the ladle. The abutment between the ladle collector nozzle and the ladle shroud must be capable of being connected and disconnected

repeatedly during service to accommodate ladle changes during the continuous manufacture of cast steel.

The abutment between the ladle collector nozzle and the ladle shroud can have a variety of different forms, for example, the ladle collector nozzle and the ladle shroud can be provided with complementarily fitting conical-shaped adaptor portions. In another type of system, the ladle collector nozzle can be provided, around its outlet, with a flat circular surface, eg a flange, that abuts a corresponding flat surface (eg a flange or a spigot) at the inlet to the ladle shroud.

In practice it is found virtually impossible to achieve a long lasting gas-tight fit between the abutting surfaces of the ladle shroud and the nozzle, even when the abutting surfaces are carefully machined to provide a good fit. The main problems encountered are as follows: 1. The abutting surfaces can become contaminated with steel sculls during repeated use, and this in turn can impair the mating together of the surfaces and hence their ability to reduce ingress of air and other contaminants. Steel "sculls"comprise solidified steel, often contaminated with oxide or nitride, that can form as a deposit on surfaces which have been in contact with molten steel.

2. Steel sculls formed on the surfaces have to be removed from time to time, eg by oxygen lancing, and this procedure can cause damage to the abutting surfaces.

3. If the abutting surfaces are designed to fit together too closely they can become difficult or impossible to separate when the ladle changes are required. This problem is most acute when the abutting surfaces are non- planar abutting pairs, for example when they are in the form of a complementary conical abutting pair.

4. The flow of molten metal through the ladle shroud creates a partial vacuum therein, thus increasing the tendency of air and other contaminants to penetrate through any gaps in the abutment between the ladle collector nozzle and the ladle shroud.

There has been a number of attempts to solve the problem reduce of ingress of air at the abutment between the ladle collector nozzle and the ladle shroud. For example, a fibre gasket has been applied between the ladle shroud and the collector nozzle. Argon has been supplied above the ladle shroud to reduce as far as possible the amount of air in the gas sucked in at the abutment. Argon has also been supplied into the gap between the ladle shroud and the ladle collector nozzle, for example through a porous material or via slots inserted horizontally or vertically into a tapered portion of the ladle shroud. Combinations of all these techniques are frequently used in foundry practice. These techniques have not, however, been entirely successful.

Japanese laid open publication 06-114529 relates to an annular gasket for use in a double walled stalk tube, the gasket providing a seal between an upper end of a first tube and a flange at the top end of a second tube, the second tube extending into the first tube. The gasket comprises a lamination of annular angled steel plates between each pair of which is sandwiched a packing of expandable graphite. The inner and outer steel plates may be provided with a layer of asbestos.

It is an object of the present invention to provide a gasket adapted for reducing or eliminating ingress of air or other contaminants at an abutment between a source of molten metal, and a receptor for said molten metal, especially between a ladle collector nozzle and a ladle shroud. It is another object of the present invention to provide a process for sealing the abutment (i. e. the interface) between a ladle collector nozzle and a ladle shroud in the continuous casting of steel.

Accordingly the present invention provides a gasket for forming a seal for an abutment between an inlet of a receptor for receiving a flow of molten metal and an outlet nozzle from a source of molten metal, said gasket comprising an expandable graphite sealing composition, wherein said gasket has at least one of (i) a single outlet nozzle sealing surface consisting of said expandable graphite sealing composition which in use contacts the outlet nozzle and (ii) a single inlet receptor sealing surface, also consisting of said expandable graphite sealing composition which in use contacts said receptor inlet.

Although other outlet nozzle and inlet receptor sealing surfaces not consisting of said expandable graphite composition can be provided, said sealing surface (s) of said expandable graphite composition are preferably the only sealing surfaces.

Preferably, said sealing surface (s) are homogeneous.

As used herein"single"means only one and not more than one. It will be understood that the gasket disclosed in JP-06-114529 differs from that of the present invention in that the former comprises a plurality of discrete expandable graphite sealing surfaces separated by steel plates.

The present invention also resides in a method of forming the gasket, comprising preparing an expandable graphite sealing composition and forming said sealing composition into a desired shape, said shape forming step being selected from :-.

(i) feeding said composition into a mould followed by curing using heat or a catalyst; (ii) compressing said composition in a die; and (iii) rolling said composition into a sheet followed by cutting or pressing said gasket therefrom.

The receptor for receiving the flow of molten metal is preferably a ladle shroud.

The outlet nozzle from a source of molten metal is preferably a ladle collector nozzle.

The gasket of the present invention can be a preformed article, or it can be formed in situ, for example by the application of a suitable paste or liquid comprising the expandable graphite sealing composition to one or both of the surfaces which form the abutment.

The gasket can have any desired shape consistent with the function it has to perform, i. e. to form a seal between (1) an inlet of a receptor for molten metal and (2) an outlet of a source of molten metal. Generally speaking the majority of pipes, conduits, shrouds and nozzles employed for conducting molten metal in metal foundries have circular cross section, and the abutment surfaces (eg flanges) of inlet and outlet pipes for the molten metal are frequently of circular (annular) shape. However, the shape can be, for example, rectangular, square, hexagonal or elliptical. The abutment surfaces or flanges between such pipes can be for example planar, tapered (eg conical or pyramidal), or curved (eg spherically curved). The gasket of the present invention is preferably adapted to fit closely around the abutment surface of the inlet of the receptor.

The abutment surface of the inlet of the receptor can be, for example, a circular flange, in which case the gasket preferably has an annular shape matching the flange.

Preferably the abutment surface at the inlet to the receptor is an internal conical surface having its wider diameter at the upper end of the inlet, and the abutment surface at the outlet from the source of molten metal is an external conical surface having its narrow diameter at the lower end of the outlet. Under these circumstances, the conical angle on the inlet to the receptor can be the same as, or larger than, or smaller than the conical angle on the outlet of the source of molten metal. The gasket employed preferably matches the conical profile of the inlet to the receptor.

Alternatively, the shape of the gasket can be such as to fit closely against the mating surface of the outlet for delivering the molten metal. Thus for example it can take the form of a conical sleeve which can be positioned on a conical nozzle of a source of molten metal (eg a ladle collector nozzle).

The gasket comprises an expandable graphite sealing composition. Expandable graphite is a well-known material comprising, on the molecular scale, layers of two- dimensional planes of graphite (i. e. carbon atoms in lamellar hexagonal orientation), and interstitial layers of water molecules and/or hydroxyl groups. When the material is heated to high temperature, the water molecules and/or hydroxyl groups are converted to steam at high pressure. The steam forces the graphite layers to separate and become irreversibly convoluted into a lower density, but mechanically strong, form of graphite having changed molecular structure. Preferably, the expandable graphite sealing composition comprises randomly orientated layers of expandable graphite. Random orientation of the layers promotes even expansion in all directions and hence better sealing. In addition, alignment of the layers may result in channels for air ingress in the sealing composition being formed, thereby reducing the efficiency of the seal.

For the continuous casting of steel it is preferred to change the gasket of the present invention each time a ladle change occurs, thus ensuring an optimum seal each time a fresh batch of molten steel is delivered through the ladle shroud. Under these circumstances, when a fresh gasket is placed in the inlet of the ladle shroud, it will gradually be heated to a temperature close to the temperature of the shroud inlet.

Preferably the temperature at which the gasket undergoes intumescent expansion is slightly higher than the temperature of the material forming the shroud inlet so that when the hot ladle connector nozzle connects to the shroud, intumescence of the gasket rapidly follows.

It is preferred to employ the gasket of the present invention in a shaped and self- supporting form. Although it would be possible to manufacture a suitable gasket from pure expandable graphite, eg by compressing the material in a suitable die, it is preferred include at least a binder, eg, at least 1 weight % of binder, to provide additional mechanical strength to the gasket.

A preferred composition for forming a gasket comprises (1) 60 to 80 weight % of a refractory material (2) 12 to 18 weight % of a binder material and (3) 2 to 18 weight % of expandable graphite.

The refractory material is suitably any powdered refractory material that does not deleteriously affect the other components of the composition. Examples are powdered graphite, powdered carbon, mica and clay. Powdered graphite is preferred.

The binder is suitably any substance capable of binding together the refractory material and the inorganic intumescent material to form a self-supporting gasket.

Examples of suitable binders are thermoplastic polymers, thermosetting resin compositions and catalytically curable resin compositions.

Examples of suitable binder materials are thermoplastic, thermosetting or catalytically curable polyester resins (eg of the type used in glass fibre reinforced polyester resins), epoxy resins, alkyd resins, acrylic resins or phenol-formaldehyde resins. Preferably the binder is a catalytically curable resin or a thermosetting resin.

The gasket may be formed from the composition, for example, by moulding means, eg, by feeding the composition to a suitable mould and then curing by the action of heat or a suitable catalyst; or by rolling the composition into a sheet and cutting or pressing shaped gaskets therefrom. If the binder material is a thermoplastic or a thermosetting plastic, the composition can be, for example, extruded or injection moulded to form shaped gaskets.

In a preferred embodiment of the present invention the expandable graphite sealing composition is preferably incorporated into a ceramic paper profile. Thus, the invention further resides in a method of forming a gasket comprising forming a shaped support and applying an expandable graphite sealing composition to said support.

The ceramic paper profile preferably has a shape adapted to fit accurately into, or onto, one of a pair of abutments between which it is desired to form a sealed connection. Ceramic paper is well known in the foundry art as a material stable at high

temperature, and formed from ceramic fibres woven or bonded together. The ceramic paper can be shaped into any desired profile, for example, by cutting and bonding suitable sheets of the paper, by moulding and curing compositions containing ceramic fibres, or by forming (eg vacuum forming) shaped articles from a slurry of the ceramic fibre in a manner similar to that employed in conventional paper making processes.

The ceramic fibres employed in the ceramic paper are preferably any inorganic fibres having a melting point or softening temperature above 700° C. Examples of suitable fibres are aluminosilicate fibre, carbon fibre, and silica fibre.

The expandable graphite sealing composition can be incorporated in the ceramic paper profile using conventional methods well known on the art. For example a paper profile can be formed having a suitable recess into which either a pre-cured expandable graphite composition may be inserted; or a gasket may be formed in situ in the recess either by application of a curable composition and then curing to form the final product, or by coating a flowable composition onto the surface of the profile followed by drying.

It will be readily appreciated that the expandable graphite sealing composition may be incorporated on either or both sides of the ceramic paper profile (i. e. either or both of the internal and external surfaces of a frusto-conical shaped profile).

As a further alternative the expandable graphite sealing composition may be incorporated onto a support which burns off (or is otherwise removed) at the intended operating temperature of the gasket. For example, the support may be a (wood pulp) paper profile. Thus, in use, the gasket essentially consists of only the expandable graphite sealing composition. Such an arrangement avoids the possibility of air ingress through the support (eg. ceramic fibres) or through any interface between the support and the expandable graphite sealing composition.

The present invention can, if desired, be used in combination with other known methods for reducing the ingress of air or other contaminants into molten metals using, for example, the techniques hereinbefore described. For example, argon or other inert gas may be fed to the abutment between the source of molten metal and the receptor for the molten metal. An example of this type of system is the use or a jacket on the head of a ladle shroud through which argon gas is passed and directed inwardly toward the abutment of the ladle shroud and the ladle collector nozzle.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which Figures 1 and 2 represent a gasket in accordance with the invention, in elevation view and plan view respectively, Figure 3 depicts another gasket in accordance with the present invention, incorporating a ceramic fibre support, Figure 4 depicts an"exploded"view of an assembly of a ladle shroud and a ladle collector nozzle separated by the gasket depicted in Figure 3, Figure 5 shows yet another gasket in accordance with the present invention, and Figure 6 is a comparative graph of pressure against time for an expandable graphite sealing composition at elevated temperature and for an expandable vermiculite sheet.

Figures 1 and 2 represent a gasket 1 having a frusto-conical shape with a wider diameter circular cross section upper portion 2 and a narrower diameter circular cross section lower portion 3. The gasket is open at the top and bottom. The gasket was manufactured by injection moulding an expandable graphite sealing composition prepared by blending together an intumescent (expandable) graphite powder (10 wt%), an epoxy resin binder (eg Araldite) (15 wt%) and powdered graphite (75 wt%). It will be understood that the outer frusto-conical surface 4a of the gasket defines a single homogeneous receptor inlet sealing surface and the inner frusto-conical surface 4b defines a single homogeneous outlet nozzle sealing surface. The gasket can be used by dropping it into the mouth of the receptor inlet before engaging the outlet nozzle with the receptor inlet. Alternatively, the gasket can be fitted as a sleeve onto the outlet nozzle before bringing the part together.

Figure 3 represents a gasket similar to that of figures 1 and 2 but additionally comprising a ceramic paper profile 6 supporting the moulded expandable graphite sealing composition 5. The ceramic paper profile 6 comprises a frusto-conical portion 7 and a flange portion 9. The ceramic paper profile 6 was made by vacuum forming a slurry of aluminosilicate fibres in a known manner. The moulded expandable graphite sealing composition 5 is mounted within the ceramic paper profile 6 towards its end of narrower diameter and is a tight fit with the wall 11 of the ceramic paper profile 6.

Figure 4 depicts, in"exploded"view for the purpose of greater clarity, an assembly of a ladle collector nozzle 13, a ladle shroud 15 and the gasket depicted in Figure 3. In practice the ceramic paper profile 6 supporting the moulded expandable graphite composition 5 is dropped into the mouth 17 of the ladle shroud so that the flange 9 contacts the head surface 19 of the ladle shroud 15. The ladle shroud 15 containing the gasket is then brought up towards the ladle connector nozzle 13 so that the conical abutment surface 21 enters the opening 23 of the gasket and contacts the inner frusto-conical sealing surface 4b. It will be noted that in this embodiment, there is no direct contact between the ladle shroud 15 and the outer frusto-conical surface 4a of the gasket. Molten steel is then transferred from the ladle (not shown) through the ladle connector nozzle 13 and through the ladle shroud 15 into a tundish (not shown). The heat from the molten steel causes the gasket to intumesce and so form a gas tight seal between the two abutting surfaces 21 and 25 so protecting the steel stream from ingress of air or other contaminants.

Figure 5 depicts another gasket in accordance with the present invention. The gasket comprises a ceramic paper support 30 having a frusto-conical region 32 and a flange 34 at its end of wider diameter. The ceramic paper support 30 is of the same composition and was formed in the same manner as the support 6 described with reference to Figure 3. Each of the inner and outer frusto-conical surfaces of the ceramic paper support carries a substantially homogeneous layer 36,38 of expandable graphite sealing composition.

The layers 36,38 were formed by dipping the support 30 into a container of the expandable graphite sealing composition (composition: water and rheological control agents such as low molecular weight anionic surfactants like sodium lauryl sulphate in combination with high molecular weight heteropolysaccharides such as xanthum gum 20wt%, latex emulsion 25wt%, graphite 40wt%, expandable graphite lOwt% and boric acid anti-oxidant 5wt%). The rheology of the latex mix was controlled by varying the proportions of the rheological agents so that after dipping, an even layer of 4 mm thickness was formed on the inner and outer surfaces of the support 30. The gasket was then dried for 30 minutes at 120°C.

In use for sealing a ladle shroud and a ladle collector nozzle, the expandable graphite sealing composition layer 36 on the inner surface of the ceramic paper support

30 defines a single collector nozzle sealing surface, and the expandable graphite sealing composition layer 38 on the outer surface of the ceramic paper support 30 defines a single ladle shroud sealing surface.

The gasket so formed was tested by measuring the average nitrogen pick-up of molten steel over many trials for a ladle shroud-collector nozzle arrangement in which argon was passed between the ladle shroud and the collector nozzle through a porous material. Average nitrogen pick up was found to be 5.3 ppm compared to 9-10 ppm in the absence of the gasket, representing an improvement of about 45%.

As a further test, the intumescent properties of expandable graphite were compared with expandable vermiculite. A 4 mm thick cured and dried latex sheet comprising 25wt% expandable graphite, boric acid anti-oxidant 10wt%, graphite 50% and latex binder 15wt% was sandwiched between the plates of a pressure transducer which was then placed in a furnace at 600°C (approximating actual operating temperature of the seal between a ladle shroud and nozzle collector). The experiment was repeated using a 25wt% latex sheet of expandable vermiculite and the increase in pressure over time was plotted (Figure 6). As can be seen from Figure 6, in the case of the expanded graphite sheet (series 1), the increase in pressure due to vaporisation of the intercalated water is significant and maintained, since the confined graphite sheets fold over on themselves and prevent gas escape, thereby forming a very efficient gas seal. In contrast, the vermiculite planes within a flake of vermiculite largely retain their parallel aspect during vaporisation of internal water, which therefore escapes more easily, resulting in a lower (and not sustained) increase in pressure on heating (series 2).

Vermiculite therefore forms a much less efficient gas seal.