Method and device for regulating the level of liquid metal in a

Details Method and device for regulating the level of liquid metal in a Method and device for regulating the level of liquid metal in a

1995-10-23

1997-02-25

Hail, III, Joseph J.

Metal founding

Process

With measuring, testing, inspecting, or condition determination

1641513, 1644505, B22D 1118, B22D 1116

Patent

active

056051883

DESCRIPTION:

BRIEF SUMMARY
CROSS-REFERENCE TO RELATED APPLICATION

This application is A 371 OF PCT/FR94/00292, filled Mar. 17, 1994.


FIELD OF THE INVENTION

The invention relates to the field of the continuous casting of metals, especially steel. More precisely, it relates to the regulating of the level of the liquid metal present in a continuous casting mold.
In an installation for a continuous casting of steel, the liquid metal which flows out of the pouring ladle firstly passes via an intermediate vessel, called a tundish. One of the roles of the tundish is to direct the liquid metal toward the single oscillating mold or, more generally, the multiple oscillating molds of the continuous casting machine, in which molds the ferro-metallurgical products (slabs, blooms or billets) start to solidify. Above each mold, the metal flows out from the tundish via an outlet orifice and thus forms a casting stream which penetrates the mold by passing through the meniscus, that is to say the surface of the liquid metal present in the mold. On its travel between the tundish and the mold, the casting stream is confined in a tube made of refractory material, called a casting nozzle. The upper end of the nozzle is fixed to the bottom of the tundish, while its lower end passes through the meniscus and dips into the liquid metal. The functions of the nozzle are to protect the stream of liquid metal from being oxidized by the atmosphere, to prevent the stream, as it passes through the meniscus, from entraining with it part of the covering slag which covers the meniscus, which entrainment would cause the cleanness of the cast product to deteriorate, and finally to force the flow of liquid metal in the mold to adopt a configuration favorable to satisfactory solidification of the product. For this reason, its lower end may include a multiplicity of lateral orifices (or slots), each directed toward one or other of the faces of the mold.
One of the essential parameters in obtaining a sound product is the stability of the level of the meniscus in the mold. If this stability is not satisfactorily ensured, solidification of the product takes place under excessively variable conditions. It is thus possible to end up with a solidified thickness of the product which is locally too small, hence a risk of tears of varying magnitude in the solidified skin. At best, the end product is of poor surface quality; at worst, liquid metal can flow out through the tears (a phenomenon called "breakout") and cause a halt to the casting and serious damage to the machine. The mean level of the meniscus is determined by the flow rate of steel flowing out of the tundish and by the speed at which the solidifying product is extracted from the mold. The flow rate of liquid steel penetrating the mold is generally regulated by a refractory stopper rod, the conical tip of which closes off to a greater or lesser extent the outlet orifice of the tundish. Even though it is desired to keep this flow rate to a constant value, it is necessary to vary the position of the tip of the stopper rod in order to take into account steady or abrupt changes in the other casting parameters. These changes may, for example, be a variation in the height of the metal in the tundish, the progressive wear of the slots in the nozzle, or their blockage by nonmetallic inclusions, or their sudden unblocking if these inclusions become dislodged from the walls. In order to regulate the level of liquid metal in the mold satisfactorily, it is essential to use an automatic system which controls the position of the stopper rod. It moves the latter depending on the results of a comparison between the desired level of the meniscus and that actually measured. This level measurement is normally carried out by means of a single inductive or optical sensor. It supplies an electrical signal which, after processing, is used to control the position of the stopper rod.
It is in the case of the continuous casting of slabs that the problem of regulating the level of the meniscus is most complicated. The reason for this is that

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