GB2265564A

GB2265564A - Tundish cover layer containing flux ingredients and expandable graphite

Info

Publication number: GB2265564A
Application number: GB9305366A
Authority: GB
Inventors: Ronald Barker
Original assignee: Foseco International Ltd
Current assignee: Foseco International Ltd
Priority date: 1992-03-31
Filing date: 1993-03-16
Publication date: 1993-10-06
Anticipated expiration: 2013-03-16
Also published as: GB2265564B; GB9305366D0; GB9206946D0

Abstract

A powder for use as a covering layer for steel in a tundish comprises flux ingredients which melt in contact with the molten steel and expandable graphite which expands from the heat of the molten steel to give a thermal insulation. The flux ingredients may be calcium fluoride, calcium aluminate, non-expandable graphite, Al2O3, GaO, MgO or SiO2. Perlite may also be added. The expandable graphite may be acid-treated graphite that can expand under heat to a volume up to 300 times its original volume. This cover layer better meets the conflicting requirements of melting to provide a liquid-liquid interface for removal of unwanted inclusions from the molten steel and maintaining a solid covering layer to provide effective thermal insulation.

Description

TUNDISH COVER LAYER This invention relates to a powder used as a covering layer for molten steel in a tundish.

In the continuous casting of steel a tundish is used as an intermediate vessel between a ladle and a mould to provide a reservoir of molten metal, and to distribute the molten steel to the mould. In recent times steelmakers have investigated the tundish, not only as a reservoir provider and distributor, but also as a vessel in which non-metallic oxide inclusions for example, solid alumina and liquid calcium aluminates and slag carried over from the ladle, can be removed from the molten steel.

It is normal practice to apply to the surface of the molten metal in the tundish an insulating, refractory powder to reduce heat loss and, by providing a physical barrier between the molten metal surface and the atmosphere, to reduce oxidation and nitrogen contamination of the molten metal. These insulating powder covers may be calcined rice hulls which are predominantly silica of very low bulk density or, more conventionally, aluminosilicate or sometimes basic refractory compositions. With all of the established materials for this application, it is necessary for them to remain unmelted if good thermal insulation properties are to be maintained. Should melting occur, then the rate of heat loss to the atmosphere increases and there is a resultant drop in metal temperature in the tundish. Products which remain unmelted are usually referred to as insulating or inert tundish covers.

However, it has become accepted that refractory powder covers which remain essentially unmelted or only partially melted are not effective in absorbing non-metallic inclusions already present in the molten metal. In order to be effective in absorbing non-metallic inclusions from the molten metal, the cover material must be both molten and have a suitable chemical composition. Such covers are usually referred to as active tundish covers.

Consequently, in order to achieve the aim of producing "clean" steel in the tundish, steelmakers have started to use flux compositions containing components such as silica, calcium oxide, alumina, magnesium oxide and calcium fluoride as tundish covers.

For example, Japanese Unexamined Patent Publication No. 60-258406 describes the use as a tundish cover of a flux composition containing 3% by weight carbon, 5 - 15% by weight silica, 5 - 20% by weight alumina, 30 - 60% by weight calcium oxide, 5 - 20% magnesium oxide and 10 - 40% by weight calcium fluoride.

However, such cover layers do not fully meet the sometimes conflicting requirements of providing good insulation properties and simultaneously providing a flux layer that can effectively remove unwanted inclusions from the steel while preventing re-oxidation or other chemical contamination.

The problem essentially appears to be the need to provide a covering layer that melts to provide an interface with the molten steel, as such a liquid liquid interface is needed for effective inclusion removal, whereas a solid covering layer is needed to provide effective thermal insulation.

In order to resolve this conflict of aims, it has been known in the art to use two separate covering layers. A first layer, in direct contact with the molten steel, is the flux layer which melts and removes inclusions. A second layer of different composition is placed over the first layer, the second layer providing the desired insulation by remaining in solid form This solution is satisfactory provided that the length of time of casting steel via the tundish is not too long. However, in modern casting practice, continuous casts of up to 20 hours or even longer are sometimes required. Over such a long period of time, the first covering layer may need to be replenished but this is not a practicable proposition because of the presence of the second, insulating, covering layer.

It can be seen, therefore, that there remains a need for an improved covering layer that can better meet the various demands on it.

Accordingly, the invention provides a powder for use as a covering layer for molten steel in a tundish, the powder comprising flux ingredients which melt in contact with the molten steel and expandable graphite which expands from the heat of the molten steel to give a thermal insulation.

The expandable graphite is preferably present in an amount of at least 0.5% by weight and preferably from 5 to 20% by weight of the total weight of the powder and is preferably substantially uniformly distributed throughout the powder. This distribution is readily achieved by normal powder mixing techniques.

The expandable graphite, e.g. acid-treated graphite, is a readily-available commercial product.

Typically, it can expand under heat to a volume that may be as much as two or three hundred times its original volume and in this condition it imparts excellent insulation properties to mixtures in which it is contained.

Although not wishing to be limited by any particular theory, it is believed that the effectiveness of the powder of the invention is achieved by the distributed and expanded graphite slowing down the melting of the powder through the thickness of the covering layer. Thus, that portion of the layer in immediate direct contact with the molten steel melts and any expanded graphite in that melted sub-layer may dissolve in that sub-layer. The presence of the graphite in the regions immediately above the melted sub-layer has an insulating effect that not only fulfills the basic requirement of preventing premature cooling of the steel, but it also slows down the progress of melting of the constituents of the covering layer through its thickness.Thus, the molten sublayer will only gradually extend through towards the top of the layer while it continues to carry out its primary function of inclusion removal, prevention of re-oxidation and the like.

The flux ingredients of the covering layer may be any suitable materials such as those conventionally used. Metallic oxides, e.g. oxides of silicon, aluminium, calcium and magnesium, calcium aluminate, calcium fluoride, non-expandable graphite, may all be incorporated in varying amounts.

Thus, typical compositions of the invention may be formulated as detailed below.

Parts by weight expandable graphite 5 to 20 magnesium oxide 0 to 40 calcium fluoride 5 to 40 alumina 10 to 60 total carbon up to 25 silica 0 to 25 Calcium oxide 20 to 70 The calcium oxide content of the flux composition may be provided by the use of materials such as lime chippings, limestone or calcined dolomitic lime and the magnesium oxide content may be provided by materials such as dead burnt magnesite or calcined dolomitic lime. The alumina, which is included as a fluxing agent to lower the melting point of the flux composition, is preferably added in the form of bauxite or cement fondu, the latter particularly being used in mixtures which are additionally required to contain calcium oxide. Perlite, which has a relatively low density compared with the other raw materials used to produce the flux composition, may also be included in the compositions.It has the effect of reducing the overall density of the composition and improving the thermal insulation properties of the composition in use. Perlite will also provide or contribute to the silica content of the composition and also provides a minor amount of alumina.

Other materials may optionally be included, e.g. other fluxes such as soda ash.

In silica bearing formulations it is preferred that the CaO:Si02 ratio in the powder formulation be at least 0.6:1 by weight and silica-free formulations may also be used, if desired, i.e. in which the only silica would be in the form of contamination in the various raw materials used. Minor amounts of other impurities, e.g. sodium oxide and iron oxide, may also be present from the raw materials used.

The compositions used as the basis of the covering powder may also be as described in our U.S.

Patent No. 5028257. This describes a flux composition which contains more magnesium oxide than has hitherto conventionally been used, the composition containing from 22 - 35% by weight of magnesium oxide and having a weight ratio of CaO to MgO of 0.6 to 2.5:1. Such a composition may, therefore, be formulated with at least 0.5 and especially from 5 - 20 parts by weight of expandable graphite for use in the present invention.

In use as a tundish cover, the powder is applied to the surface of the molten steel in the tundish at the beginning of the casting operation, to form a layer, for example, about 20mm thick. During casting, as subsequent heats of steel are cast, further amounts of the powder may be added to maintain adequate cover.

Claims

1. A powder for use as a covering layer for steel in a tundish, the powder comprising flux ingredients which melt in contact with the molten steel and expandable graphite which expands from the heat of the molten steel to give a thermal insulation.

2. A powder according to Claim 1, in which the expandable graphite is present in an amount of at least 0.58 by weight of the total weight of the powder.

3. A powder according to Claim 1 or 2, in which the expandable graphite is present in an amount of from 5 to 20% by weight of the total weight of the powder.

4. A powder according to Claim 1, 2, or 3, in which the expandable graphite is substantially uniformly distributed throughout the powder.

5. A powder according to any one of the preceding claims, in which the expandable graphite is an acidtreated graphite that can expand under heat to a volume up to three hundred times its original volume.

6. A powder according to any one of the preceding claims, in which the flux ingredients are one or more of calcium fluoride, calcium aluminate, non-expandable graphite and oxides of aluminium, calcium, magnesium and silicon.

7. A powder according to any one of the preceding claims, in which the composition of the powder is as follows: Parts by weight expandable graphite 5 to 20 magnesium oxide 0 to 40 calcium fluoride 5 to 40 alumina 10 to 60 total carbon up to 25 silica 0 to 25 calcium oxide 20 to 70

8. A powder according to any one of the preceding claims, which contains silica and calcium oxide and in which the Ca0:Si02 ratio is from 0.6:1 to 2.5:1 by weight.

9. A powder according to any one of the preceding claims, in which the powder composition excluding the expandable graphite contains from 22 to 30% by weight of magnesium oxide and has a CaO:MgO ratio of 0.6 to 2.5:1 by weight.

10. A powder according to Claim 1, substantially as hereinbefore described.