Continuous casting moulds

[57] ABSTRACT

A continuous casting machine includes a tundish having an outlet defined by a body of refractory material, a mould of a material having high thermal conductivity and defining a mould passage arranged with its inlet end in communication with the outlet of the tundish and an apertured plate of silicon nitride positioned between the refractory material and the mould and held in compression by a metal body which surrounds the perimeter of the plate. The metal body is mounted in good heat transfer relationship with the mould.

Claims, 3 Drawing Figures

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Patented Aug. 14, 1973

3,752,218

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3,752,218

CONTINUOUS CASTING MOULDS

SUMMARY OF THE INVENTION

This application is a continuation-in-part of application Ser. No. 20,981, filed Mar. 19, 1970 now abandoned.

This invention relates to continuous casting machines which comprise a tundish for receiving a quantity of molten metal to be cast with a refractory outlet of the tundish in communication with the inlet of an openended continuous casting mould.

Continuous casting consists essentially of allowing molten metal stored in a tundish to flow through a mould and for the metal to be cooled sufficiently in the mould for the casting leaving the mould to retain the cross sectional shape of the mould. It is necessary therefore for the mould to be cooled in order to extract sufficient heat from the molten metal to cause at least the outer surface of the metal to solidify and form a tough skin as it passes through the mould.

To ensure that the required amount of heat is withdrawn from the metal as it passes through the mould, the mould body must be of a material having a good thermal conductivity and copper is a suitable material. A liquid coolant is also provided. The refractory material which defines the outlet of the tundish and directs the molten metal to the inlet of the mould is at a very high temperature during the operation of casting and the temperature of the mould is necessarily much lower. Consequently, if the refractory material abuts directly against the cooler mould, the refractory material at the junction with the mould material is cooled by the mould. The molten metal tends to penetrate into the pores of this cooled part of the refractory material adjacent the mould and also tends to stick to the surface of the refractory material. This may result in damage being caused to the refractory material or the casting may stick sufficiently tightly to the refractory material to prevent satisfactory withdrawal of the casting from the mould.

It is an object of the present invention to provide a continuous casting machine in which these difficulties are overcome.

According to the present invention a continuous casting machine comprises a tundish for receiving a quantity of molten metal to be cast, the tundish having an outlet defined by a body of refractory material, a continuous casting mould of a material having a high thermal conductivity and with a mould passage arranged with its inlet end in communication with the outlet of the tundish and a body of silicon nitride in the form of an apertured plate with the cross sectional dimensions of the aperture being not greater than the corresponding cross sectional dimensions of the mould passage, held in compression by a metal body surrounding the perimeter of the plate, the metal body being mounted in good heat transfer relation with the mould and the body of silicon nitride being sealed to the body of the refractory material and the mould so as to prevent leakage of molten metal therebetween.

Silicon nitride is substantially non-porous and has a smooth surface finish and the body of silicon nitride serves as a transition material between the refractory material and the mould material. Consequently there is little tendency for the molten metal to stick to the silicon nitrdde. Silicon nitride is also able to withstand, without damage, sudden increases in temperature as is the case when casting commences and the molten metal first flows from the tundish to the mould. Silicon nitride does not disolve in liquid steel and the material is particularly suitable for use in continuous casting machines for casting steel.

It has been found that for optimum results it is necessary for the body of silicon nitride to be subjected to compression forces acting inwardly from the perimeter of the body. This is to prevent cracking of the silicon nitride due to differential expansion between the centre and outside of the body which would otherwise occur due to the radial temperature gradient which exists across the body. These compression forces are conveniently provided by a metal body which surrounds the perimeter of the body of silicon nitride. To provide the compression forces the metal body is initially shrunk on to the body of silicon nitride. However when the casting machine is in use, and the temperature of the metal body rises, the body expands to a greater extent than the silicon nitride thus tending to lower the compression forces acting on the silicon nitride. To prevent this from occuring the metal body which surrounds the perimeter of the silicon nitride body is mounted in good heat transfer relationship with the mould so that the metal body is cooled sufficiently to prevent it from deforming plastically when the machine is in use and in this way inwardly extending compression forces are applied to the silicon nitride through out the casting operation.

Preferably the silicon nitride plate is urged into engagement with the mould by the metal body and the metal body is bolted to an end face of the mould.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawing in which:

FIG. 1 is a sectional side elevation showing part of a continuous casting machine;

FIG. 2 is a sectional side elevation showing in more detail some of the components of a continuous casting machine; and

FIG. 3 is a sectional side elevation of an alternative embodiment of the invention to that illustrated in FIG. 2.

Referring now to FIG. 1, a continuous casting mould A made from a material of high thermal conductivity such as copper has a mould passage B extending therethrough and arranged with its longitudinal axis substantially horizontal. Molten metal to be cast is stored in a tundish, not shown, and leaves the tundish through a refractory nozzle C made usually of fire clay. A sleeve D of silicon nitride is positioned between the outlet end of the nozzle C and the inlet end of the mould passage B. At the inlet end of the mould A the passage B is enlarged to receive an end portion of the sleeve of silicon nitride. The other end of the sleeve abuts against the outlet end of the nozzle C. In the arrangement shown in FIG. 1 the inner surface of the sleeve D is aligned with the surface of the mould passage and in fact the silicon nitride defines part of the mould surface at the inlet end of the mould. When casting, the molten metal passes through the nozzle and the silicon nitride sleeve and into the mould. The mould is at a much lower temperature than that of the nozzle C and the silicon nitride acts as a transition piece between the two components. The temperature of the silicon nitride is such that freezing of the molten metal commences part-way

3,752,218 along the axial length of the sleeve. As the silicon nitride is substantially non-porous the molten metal does not tend to penetrate into the silicon nitride and the casting consistently breaks away from the silicon nitride and this results in a good surface finish on the casting.

Referring now to FIG. 2, a continuous casting mould, particularly suitable for casting steel, comprises a tubular copper sleeve 1 surrounded by but spaced from a steel jacket 2, but alternatively the mould may be made entirely of copper. Liquid coolant is arranged to flow between the sleeve and the jacket in order to cool the mould. The sleeve 1 defines the mould passage 3.

At the inlet end of the mould passage there is provided a body 4 of silicon nitride. The body is in the form of an apertured plate and it surrounds the inlet end of the passage with a portion 5 of the body projecting inwardly of the mould passage substantially normal to the longitudinal axis of the passage. The plate 4 is mounted in an aperture 6 in a steel block 7. The relative dimensions of the aperture in the block 7 and the outside dimensions of the plate 4 are such that the block is shrunk onto the plate so that the plate 4 is held in compression by the block 7. The block is bolted to the steel jacket 2 by a plurality of bolts 8 and it engages with a flange 9 at the end of the copper sleeve and in this way there is good thermal contact between the block and the jacket and the sleeve. It is necessary to ensure that the block is in good thermal contact with the sleeve and the jacket otherwise when the apparatus is in use the steel block will expand relative to the silicon nitride thereby reducing the compression forces applied to the body of silicon nitride by the block. The plate 4 is also urged into engagement with the mould by the metal block 7 and this seals the silicon nitride to the mould to prevent leakage of molten metal therebetween.

A refractory feed tube 10 which constitutes the outlet nozzle from the tundish 11 is sealed by means of conventional refractory cement to the body 4 to prevent leakage of molten metal between the refractory material and the silicon nitride. The refractory feed tube is also sealed with conventional refractory cement to the refractory lining 12 around an opening 13 in the wall of the tundish.

The cross sectional dimensions of the aperature in the plate 4 are not greater than the corresponding cross sectional dimensions of the mould passage 3, and preferably the cross sectional dimensions of the aperture are less than those of the mould passage so that a portion of the plate 4 extending inwardly of the mould passage serves as a break ring from which the casting consistently breaks away and this results in a good surface finish on the casting. The form of break ring which is preferred is such that the cross sectional dimensions of the aperture on the side of the plate adjacent the mould are larger than those on the side of the plate adjacent the refractory feed tube 10.

In the embodiment of the invention illustrated in FIG. 3 the body of silicon nitride 15 positioned at the inlet end of the mould 16 is in two parts 15A and 15B. The part 15B which is nearest to the mould is in the form of a ring, the inner surface which is substantially aligned with the mould passage 7 and this part of the silicon nitride ring serves to define part of the mould passage. The part 15A is also of ring form but has a portion which projects inwardly normal to the longitudinal axis of the mould passage. The refractory feed tube 17 to the mould abuts against the part ISA of the body of silicon nitride.

In a preferred arrangement the two parts 15A and 15B of silicon nitride are held in compression by a metal plate 18 surrounding both parts and held in good thermal contact with the cooled mould 16.