Method for the casting of metallurgical products on a continuous-casting plant comprising a tundish

Abstract

Method for the casting of metallic products on a continuous-casting plant comprising a tundish coated on the inside with a refractory material, characterized in that metal plates are fastened to the surface of the said refractory material prior to the filling of the said tundish with the liquid metal to be cast.

Description

[0001] The invention relates to the continuous casting of metals and, more particularly, to the continuous casting of steel, especially in the form of thin strips.

[0002] Liquid steel is conventionally prepared by converter from liquid melt or by electric furnace from scrap iron. Its exact composition is then set in a ladle. Finally, it is cast on a continuous-casting machine. The latter comprises a tundish, which is a vessel coated internally with refractories and receiving the liquid steel flowing via the bottom of the ladle through a refractory tube. The liquid metal subsequently flows via the bottom of the tundish into one or more bottomless moulds having vigorously cooled walls, where the solidification of the product is initiated. The tundish forms a reserve of liquid metal which makes it possible to maintain a mould feed rate at the desired value at all the stages of casting, especially at the start and at the end of the emptying of the ladle, and also during the periods when a change of casting ladle takes place. This makes it possible to carry out the emptying of a plurality of consecutive ladles, without casting being interrupted (sequential casting). In other words, the tundish serves as a “buffer” vessel between the ladle and the mould or moulds.

[0003] Usually, the first cast tonnes of steel which have served to fill the initially empty tundish have an inclusionary cleanliness which is appreciably lower than the following ones. This is attributed to the conjunction of two phenomena:

[0004] on the one hand, the fact that, during the first moments of filling, the liquid steel flows in the open air, the end of the refractory tube not yet being submerged in the liquid steel;

[0005] on the other hand, chemical reactions between the deoxidizing elements contained in the liquid steel and some components of the surface of the refractories coating the tundish; the inclusions occurring as a result of these reactions are concentrated in the metal serving for filling the tundish.

[0006] The first cause can be remedied to some extent by covering the tundish with a hood and by injecting an inert gas, such as argon, under the hood, so as to make the atmosphere at the start of casting non-oxidizing.

[0007] By contrast, there is no satisfactory means of remedying the second cause, apart from coating the tundish with refractories which are highly resistant to chemical attacks and which are inevitably costly and not always optimal in thermal terms.

[0008] The non-metallic inclusions formed during the filling of the tundish are found largely in the surface layers of the first cast tonnes of product. If the product is a conventional slab of great thickness (for example of the order of 200 mm), it may be acceptable to eliminate these surface layers by a grinding of the surface of the slab or of the few slabs affected by this defect.

[0009] By contrast, when the product is a thin strip with a thickness of a few mm, capable especially of being cast on a casting plant between two internally cooled rolls rotating in opposite directions, such grinding would be absolutely prohibitive in economic terms. It would apply to too large a product surface and would therefore be unduly long and would remove an excessive quantity of material in relation to the quantity of steel cast. Moreover, it would require an unwinding of the strip before the cold treatments of the latter.

[0010] The object of the invention is to afford to the users of continuous casting of metals, especially of steel, and, in particular, of plants for the casting of thin strips, an efficient and economical means for appreciably improving the cleanliness of the first tonnes cast.

[0011] To achieve this, the subject of the invention is a method for the casting of metallic products on a continuous-casting plant comprising a tundish coated on the inside with a refractory material, characterized in that metal plates are fastened to the surface of the said refractory material prior to the filling of the said tundish with the liquid metal to be cast.

[0012] According to one variant of the invention, at least some of the said plates are fastened before an operation for the pre-heating of the tundish.

[0013] According to another variant of the invention, the said plates are fastened after an operation for the pre-heating of the tundish.

[0014] A final pre-heating of the said plates may be carried out by means of an induction device.

[0015] The said plates preferably have a composition corresponding to that of the cast metal.

[0016] In a preferred example of use, the continuous-casting plant is a plant for the continuous casting of thin strips.

[0017] As will have been understood, the invention involves essentially covering the tundish on the inside, before the start of casting, with a set of metallic plates preventing direct contact between the first tonnes of metal poured into the tundish and the new refractories of the tundish. The contacts between the liquid metal and the refractories are thus essentially delayed until the fusion of the plates. This occurs preferably at a moment when the tundish is filled completely, with metal flows corresponding to a stable flow rate.

[0018] The invention will be understood better from a reading of the following description.

[0019] According to the prior art, the tundish is coated on the inside with refractory materials based on oxides (essentially lime, magnesium oxide, alumina and silica) present in variable proportions. The choice of the composition of these refractories is governed by considerations of refractoriness, of chemical compatibility with the steel to be cast and of cost. These refractories may be in worked or castable form. The refractories of the tundish are then subsequently heated by burners in order to dry them and to give them, before casting, a high temperature limiting the thermal losses of the liquid metal when the latter is introduced into the tundish.

[0020] According to the invention, metal plates intended for covering at least the largest part of the inner coating of the tundish are also prepared.

[0021] These plates may be fastened separately to the bottom and the side walls of the tundish by means such as nails, cramps, etc. At least some of these plates may also be assembled together before being fastened to the refractories of the tundish.

[0022] According to a first variant of the invention, the plates are fastened to the refractories of the tundish when these are still cold. After this fastening of the plates, the tundish is sent to the pre-heating station, so that the flames of the burners come into contact no longer directly with the refractories, but with the plates. This may result in local fusions of the plates in the region of the impacts of the flames. However, these fusions affect only limited zones and do not impair the essential protective capacities of the plates. During the pre-heating, the plates will expand: the fastening means must take this expansion into account so as to prevent the plates from buckling unduly. For example, if nails are used, substantial play may be provided, cold, between the nails and the contours of the orifices made in the plates for receiving them.

[0023] Another variant of the invention involves installing the plates in the already at least essentially preheated tundish, if plate fastening devices protected against the radiation from the refractories of the tundish can be used. The pre-heating of the plates is subsequently carried out by burners or by an induction device which makes it possible to reach the desired temperature, of the order of 1 100-1 200° C., in a few minutes.

[0024] These two methods may also be combined by installing some of the plates before the start of pre-heating and the remainder at a subsequent stage.

[0025] During the covering of the tundish with the plates, the zones surrounding each of the nozzles via which the liquid metal flows during casting must be left free so that the stopper controlling the flow of the liquid metal via the said nozzle can perform its function.

[0026] The material of the plates is selected such that, after its fusion, the components which it contains do not modify the composition of the cast metal in a way which puts the latter outside the composition limits sought for the products to be cast. Optimally, the plates have a composition according to that of the said products to be cast. Advantageously, they consist of portions of strips which have been rejected after their casting (with regard to the instance of a plant for the casting of thin strips) or their rolling, for example heads and tails of coils.

[0027] The thickness of the plates is preferably selected such that their fusion is completed only after the filling of the tundish, and preferably after the start of casting, hence at a moment when the flows within the tundish have reached their normal rate. At this moment, generalized direct contact between the cast liquid metal and the refractories is possible, without causing an excessive pollution of the metal. To be precise, the inclusions arising from the first metal/refractory reactions will be distributed in a continuously flowing stream of metal, instead of being concentrated in the metal serving for the first filling of the tundish, as in the prior art which the invention aims to improve.

[0028] As stated, the invention has a preferred use in the plants for the direct casting of thin strips, for which, in general, the tundishes are of relatively small size, that is to say with a high surface ratio of refractories to volume of metal contained.

[0029] The casting of steel is a preferred example of the use of the invention, but it goes without saying that this may be used for the continuous casting of other metals and alloys.

Claims

1. Method for the casting of metallic products on a continuous-casting plant comprising a tundish coated on the inside with a refractory material, characterized in that metal plates are fastened to the surface of the said refractory material prior to the filling of the said tundish with the liquid metal to be cast.

2. Method according to claim 1, characterized in that at least some of the said plates are fastened before an operation for the pre-heating of the tundish.

3. Method according to claim 1, characterized in that the said plates are fastened after an operation for the pre-heating of the tundish.

4. Method according to one of claims 1 to 3, characterized in that a final pre-heating of the said plates is carried out by means of an induction device.

5. Method according to one of claims 1 to 4, characterized in that the said plates have a composition corresponding to that of the cast metal.

6. Method according to one of claims 1 to 5, characterized in that the continuous-casting plant is a plant for the continuous casting of thin strips.