The present invention concerns casting metals in an ingot mould with the use of a so-called hot-top positioned on th.e mould to reduce pipe-formation in the solidified ingot.

Such a ethod is described in U.S. patent specification No. 3766965. In this U.S. patent specification olten metal is pou- red into an ingot mould having a hot-top. After a shell or lay- er of solidified metal has been formed the hot-top is moved me- chanically so as to provide a gap between the inner wall of the hot-top and the shell. This gap acts as insulation to prevent the upper portion of the ingot from cooling too rapidly.

Accordingly in one aspect the present invention consists in a method of casting metal ingots comprising teeming the molten metal in an ingot mould with a hot-top, the hot-top having a metal inner wall which is thin co pared to the thickness of the mould wall and which is surrounded by a heat insulating barrier, the resulting thermal expansion of the hot-top forming a gap between the inner wall of the hot-top and the shell of solidified metal on the ingot without external ma- nipulation.

In a second aspect the invention consists in a hot-top for use with an ingot mould in the casting of metal ingots, the hot-top comprising a hollow body having a base adapted to be positioned at the upper end of an ingot mould and comprising an inner wall of heat conductive metal surrounded by a heat insulating barrier.

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In a third aspect the invention consists in the combination of a hot-top as set forth hereinbefore with an ingot mould, where- in the thickness of the inner wall of the hot-top is chosen in relation to the capacity of the ingot mould in such a manner as to prevent melting or fusion of the inner wall during casting molten material in the mould, and wherein the inner wall is thin compared to the thickness of the mould wall.

In a fourth aspect the invention consists in an ingot mould comprising a hot-top, the hot-top having an inner wall which is thin compared to-the thickness of the mould wall and is made of heat conductive material having a higher melting point than the material of the mould wall, said inner wall being surrounded by a heat insulating barrier and being positioned inside the upper part of the ingot mould, the said heat insulating barrier being formed between said inner wall and the wall of the said upper part of the ingot mould.

In order that the present invention may be more readily under- stood, various embodiments thereof will be described by way of example and with reference to the accompanying drawings wherein :

Figure 1 is a cross section through an ingot mould and an asso¬ ciated hot-top according to a first embodiment of the invention, figure 2 is a cross section through an ingot mould with a hot- top according to another embodiment of the invention, figure 3 is a cross section through an ingot mould with incor- porated hot-top according to still another embodiment of the invention.

Figure 1 shows an ingot mould 1, a hot-top 2 and a cover plate 3 and is divided into two by a line A so that two stages of the casting process can be shown. The left hand side of the figure shows the mould just after molten metal 4 has been poured into the mould. The molten metal will normally be steel or iron, though non-ferrous metals may also be cast in a similar manner.

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The hot-top 2 has a metal inner wall 5 having a thickness which is small compared to that of the mould walls. The inner wall 5 is surrounded by a heat barrier 6 made from a (heat resistant) ceramic fibre material. It is essential that the thickness of the inner wall 5 be chosen in relation to the capacity of the mould 1 such that the wall will not melt or become fused with the poured molten metal. The inner wall 5 should have a melting point which is not more than 300 C below the casting tempera- ture of the molten metal. The inner wall 5 may be 10 to 30 mm thick. A suitable metal is cast iron but of course the material of the inner v/all will depend on the metal being cast. As shown in figure 1 the hot-top tapers from its base to its open top which is closed by the top cover 3. This taper is not, however, essential.

An additional heat barrier 9 is provided around the lower edge of the inner wall 5 to prevent heat transfer between the inner wall and the ingot mould. This additional heat barrier may not always be needed.

The hot-top is surrounded by a metal head case 10 provided with lifting lugs by means of which the hot-top and head case can be mounted on or removed from the ingot mould.

The metal may be poured either uphill or downhill. In the lat¬ ter case the cover plate 3, which may be as described in U.K. patent specification No. 1,496,348, is placed on the hot-top after pouring.

As previously mentioned the left-hand side of figure 1 shows the molten metal immediately after teeming. Shortly after the poured metal contacts the inner wall 5 a thin outer layer or shell of solidified metal is formed on the poured metal due to heat being conducted away from the molten metal by the mould 1 and the inner wall 5 of the hot-top.

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The heat absorbed by the inner wall 5 causes the hot-top to expand. However, because of its lower mass with respect to the mould 1 the hot-top 2 will expand substantially more rapidly than the mould 1. Thus the hot-top is moved off the ingot with- out external manipulation to form a gap 7. Because of the above mentioned difference in masses the gap 7 between the hot-top 2 and the ingot will be formed more rapidly and will be larger than the gap between the ingot and the mould.

The metal is then allöwed to cool until it is solid. This is the Situation shown in the right hand side of figure 1. The hot-top 2 and the cover 3 re ain in position throughout the en- tire cooling process. The gap 7, together with the insulation provided by the heat barrier 6, ensure that the upper portion of the ingot does not cool too rapidly with respect to the re- mainder of the ingot. Furthermore, there is no need to intro- duce exothermic material into the gap 7 during cooling of the cast metal because of the thermal insulation provided by the gap 7 and heat barrier 6.

It will be appreciated that the hot-top described is reusable and also avoids the necessity of Special Provision being made to move the hot-top mechanically during the casting Operation.

Several modifications of the hot-top described above are possi- ble. Thus in the case where the metall inner wall of the hot- top is made from a number of sections, it may be advantageous to position means for causing extra thermal expansion of the hot-top when it is heated during teeming. Thus the sections of the hot-top may be separated by Strips of a material having a high degree of thermal expansion such as copper, or a bimetallic substance. Furthermore, the inner surface of the hot-top may be coated with a heat reflective or a parting agent.

Figure 2 shows a hot-top positioned inside the upper part of an ingot mould 21. An inner wall part 51 rests on a shoulder

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the mould wall and is surrounded by a heat insulating barrier 61 corresponding to heat barrier 6 described above. The outer wall of the hot-top is constituted by the wall of the ingot mould itself, which leads to a simplified structure of the co - bination. A heat barrier ^" 91 can be inserted between the lower edge of wall 41 and the shoulder of the ingot mould. The wall part 51 of the hot-top is made of a material such as steel hav¬ ing a higher melting point than the material of the input mould itself which is generally cast-iron.

Figure 3 shows an embodiment si ilar to that of figure 2 where¬ in an inner wall part 52 has a base part 53 which is incorpora- ted in the wall of the ingot mould 22. In this case the inner wall of the hot-top is maintained at its lower edge in a given Position with respect to the ingot mould, but its upper edge can expand to achieve the desired effect. A heat barrier 62 is again inserted between the ingot mould wall and the inner wall part 52.

The embodiment of figure 3 provides an ingot mould in which the hot-top wall is incorporated at the time of making the ingot mould itself and does therefore not require an additional ma- nufacturing step.

It has been discovered that with the ingot mould and casting method described above the amount of piping on the final ingot is low and the contamination of the ingot is often negligible so that it can be rolled i mediately on being stripped from the mould.