GB2080505A - Composite Refractory Slabs for Lining Tundishes - Google Patents

Abstract

A tundish for continuous casting of steel, having an outer metal casing and, adjacent the casing, a permanent refractory lining is provided with an inner, expendable lining comprising refractory, heat-insulating slabs having an inward-facing, erosion resistant layer and a more heat- insulating backing layer, in use the erosion resistant layer protecting the backing layer from attack by molten steel and the heat-insulating of the backing layer limiting the temperature at the outer face of the slabs to a maximum of 700 DEG C at the end of one hour of continuous casting. The backing layer had a density of 0.8 g/cm<3> and is molded from an aqueous slurry comprising 83% silica flour, 6.3% calcium silicate fibre, 3.1% phenol formaldehyde resin and 1.3% urea formaldehyde resin, and the facing layer had a density of 1.6 gm/cm<3> and is molded integrally with the backing layer from an aqueous slurry comprising 89.3% magnesium oxide 5.4% calcium silicate fibre, 2% phenol formaldehyde resin 2.5% boric acid and 0.8% of a flocculating agent.

Description

SPECIFICATION Tundishes This invention relates to lined tundishes, slabs for use in lining tundishes and a method of making the slabs.

In the continuous casting of metals, e.g. steel, molten metal is cast into a continuous casting mould via an intermediate vessel which acts as a constant head reservoir and is called a tundish.

The tundish has an open mouth and a metal floor and sidewalls and one or more outlet nozzles set in the floor. To protect the metal floor and walls of the tundish from the effects of molten metal it is usual to line the interior of the tundish with a relatively permanent lining, often made of bricks.

The tundish may additionally be provided with an inner, expendable lining of refractory, heatinsulating slabs. This is described in British patent specification 1364665 and is highly advantageous.

Expendable inner lining slabs for tundishes have to meet various distinct requirements and certain of these conflict to a greater or lesser extent. In particular, whilst it is desirable for the slabs to be highly heat-insulating, it is also desirable for them to have substantial resistance to erosion by molten metal and slag. However, any change in the nature of the slab that improves the heat-insulating properties of the slab is usually associated with a reduction in the density of the slab and any reduction in the density of slab tends also to result in the slab having reduced erosion resistance.

Specification 1364665 mentioned above indicates that the molten metal contacting surface of the expendable lining may be coated with a protective refractory dressing. However, this presents manufacturing difficulties and such a dressing is incapable of providing good erosion resistance throughout the lengthy casting runs increasingly used in modern practice in the continuous casting of steel.

German laid-open patent specification 2745271 describes a way of obtaining a useful compromise as regards heat-insulating and erosion resistance properties and this involves incorporating a proportion of particulate carbonaceous matter in the slabs. Whilst improved slabs can be made in this way, there is the limitation that increasing the proportion of the carbonaceous matter above a certain level to improve the erosion resistance can adversely affect the heat-insulation properties to an unacceptable extent.

According to the present invention a tundish, for use in the continuous casting of steel and having an outer metal casing, and adjacent the casing, a permanent lining of refractory material, is provided with an inner, expendable lining comprising refractory, heat-insulating slabs having an inward-facing, erosion resistant layer and a more heat-insulating backing layer, in use the erosion resistant layer protecting the backing layer from attack by the molten steel in the tundish and the heat-insulation provided by the backing layer limiting the temperature at the outer face of the slabs to a maximum of 7000C at the end of one hour of continuous casting.

By use of a tundish according to the invention it is possible to limit loss of heat from the steel to a sufficiently low level and yet have an inner, expendable lining able to last satisfactorily for the duration of a lengthy casting run and without a tendency to contaminate the steel.

In accordance with the invention it has also been found that slabs meeting.all of a number of particular requirements are especially satisfactory for the inner, expendable lining of a tundish of the invention.

A slab according to the invention for use in the inner, expendable lining of a tundish of the invention has an erosion resistant facing layer (to face molten steel in the tundish) having a density of 1.2 to 1.8 g/cm3 and comprising 60 to 95% by weight in total of one or more particulate refractory materials chosen from chromite, alumina, zirconia, high alumina aluminosilicates and magnesium oxide, 0.5 to 15% by weight of refractory fibre, 0 to 3% by weight of organic fibre, 0 to 20% by weight of particulate carbonaceous matter and a binder, the total amount of any organic matter not exceeding 4.5% by weight and the total amount of any inorganic binder not exceeding 20% by weight, and, secured to the facing layer, a backing layer having a density of 0.3 to 1.0 g/cm3 and comprising 60 to 95% by weight of particulate refractory material, 0 to 30% by weight of refractory fibre, 0 to 10% by weight of organic fibre and a binder, the amount of any organic binder not exceeding 10% by weight and the amount of any inorganic binder not exceeding 10% by weight.

The relatively high density of the facing layer and the specified composition, especially the substantial percentage of the specified particulate refractory material(s), enable very good erosion resistance properties to be obtained but a complete slab having the density and composition specified for the facing layer of the present invention would generally be regarded as providing insufficient heat-insulation properties for an inner, expendable lining slab for a tundish.

However, by virtue of the presence of the backing layer in the slabs of the invention, good heatinsulating properties can be imparted to the slabs and without having any adverse effect on their erosion resistance.

The good heat-insulating properties imparted to the slabs by the backing layer are associated with the relatively low density of the backing layer. A complete slab having a low density such as 0.6 g/cm3 would have inadequate erosion resistance even if it contained one or more of the refractory materials specified for the facing layer of the slabs of the invention.

The facing layer protects the backing layer from direct, and potentially erosive, contact with molten steel in the tundish and, as the facing layer does itself have some heat-insulating properties, it means that even the part of the backing layer nearest to the molten steel is subjected to a temperature substantially less than that of the molten steel. The fact that the backing layer is protected in these ways is advantageous in that it means that the requirements regarding refrectoriness are less stringent than for a layer in contact with molten steel and there is not a requirement for erosion resistance. As a consequence of this, certain relatively cheap materials can be used in the backing layer that would be unacceptable in the facing layer.

Examples of suitable particulate refractory materials for the backing layer include silica, e.g.

silica flour, and refractory silicates, which may be simple silicates e.g. olivine or complex silicates such as aluminosilicates. Suitable aluminosilicates may be minerals or reclaimed materials e.g. fire clay grog. Lightweight particulate refractory materials may be used e.g.

expanded fire clay grog, diatomaceous earth, fly ash floaters, expanded perlite and calcined rice husks. Preferably any refractory fibre in the backing layer is calcium silicate fibre. The backing layer preferably contains organic fibre, e.g. paper.

Preferably any organic binder in the backing is starch and/or one or more resins such as phenolformaldehyde and urea-formaldehyde resins. Any inorganic binder in the backing layer is preferably colloidal silica sol, colloidal alumina sol, a phosphate binder, a hydraulic or refractory cement or a binding clay such as ball clay or bentonite.

Any particulate carbonaceous matter in the facing layer is preferably coke, graphite, carbon electrode scrap, pitch, tar, carbon black, charcoal powder or anthracite dust.

The binder in the facing layer may be organic and/or inorganic and suitable binders include those mentioned above in connection with the backing layer. It is generally preferred that the amount of binder in the facing layer should be less than that in the backing layer. A sintering aid, e.g.

boric acid, may be included in the facing layer to increase further the ability of that layer to withstand erosion and high temperatures.

The slabs of the invention are preferably made by providing a layer comprising the ingredients for the backing layer in a permeable mould, supplying an aqueous slurry of the ingredients for the facing layer over the layer in the mould, dewatering the slurry through the layer in the mould and heating the composite slab thus formed to dry it and to harden the binder. The layer in the mould comprising the ingredients for the backing layer may be formed in the mould by dewatering in the mould an aqueous slurry of the ingredients for the backing layer or the backing layer may be a preformed coherent layer, e.g. a slab, put into the mould.A flocculating agent may be included in the slurry or slurries to enhance the stability of the slurry and a pH adjusting additive, e.g. boric acid, may be included in the slurry if the other ingredients of the slurry are such as to render adjustment of the pH of the slurry desirable.

The invention is illustrated by the following Example.

Example The following ingredients in the percentages (by weight) specified were formed into a first aqueous slurry: Ingredient silica flour 83 calcium silicate fibre 6.3 paper fibres 6.3 phenol-formaldehyde resin 3.1 urea-formaldehyde resin 1.3 The slurry was dewatered in a permeable mould.

The following ingredients in the percentages (by weight) specified were formed into a second aqeuous slurry: Ingredient magnesium oxide 89.3 calcium silicate fibre 5.4 phenol-formaldehyde resin 2 boric acid 2.5 flocculating agent 0.8 The second slurry was introduced into the mould over, and also at the sides of, the layer obtained by dewatering the first slurry and the second slurry was dewatered through the previously deposited layer. The matter in the mould was then removed as a damp two layer slab, having matter from the second slurry on top of and at the sides of the layer from the first slurry, and this slab was then heated to dry it and to harden the binder.The two layers adhered well and the first layer deposited i.e. the backing layer had a density of 0.8 g/cm3 and a thickness of 1 5 mm and the other layer i.e. the facing layer had a density of 1.6 g/cm3 and a thickness of 1 5 mm.

A tundish having a permanent lining of refractory material adjacent its metal casing was provided with an inner, expendable lining comprising slabs as described above, with the facing layer facing into the tundish, and the expendable lining performed well in the continuous casting of steel, providing good heatXi insulation and having sufficient erosion resistance to survive satisfactorily a continuous casting run lasting 3.5 hours.

A heat-insulation test was performed on one of the slabs and showed that if the temperature at the front of the facing layer was maintained at 1 5200C the temperature at the back of the backing layer would not rise above 6000C at the end of one hour.

Claims (5)

Claims

1. A tundish, for use in the continuous casting of steel, having an outer metal casing, a permanent lining of refractory material adjacent the casing and an inner, expendable lining comprising refractory heat-insulating slabs having an inward-facing, erosion resistant layer and a more heat-insulating backing layer, in use the erosion resistant layer protecting the backing layer from attack by the molten steel in the tundish and the heat-insulation provided by the backing layer limiting the temperature at the outer face of the slabs to a maximum of 7000C at the end of one hour of continuous casting.

2. A refractory, heat-insulating slab, for use in the inner, expendable lining of a tundish according to claim 1, having an erosion resistant facing layer with a density of 1.2 to 1.8 g/cm3 and comprising 60 to 95% by weight in total of one or more particulate refractory materials chosen from chromite, alumina, zirconia, high alumina alumino-silicates and magnesium oxide, 0.5 to 15% by weight of refractory fibre, 0 to 3% by weight of organic fibre, 0 to 20% by weight of particulate carbonaceous matter and a binder, the total amount of any organic matter not exceeding 4.5% by weight and the total amount of any inorganic binder not exceeding 20% by weight, and, secured to the facing layer, a backing layer having a density of 0.3 to 1.0 g/cm3 and comprising 60 to 95% by weight of particulate refractory material. 0 to 30% by weight of refractory fibre, 0 to 10% by weight of organic fibre and a binder, the amount of any organic binder not exceeding 10% by weight and the amount of any inorganic binder not exceeding 10% by weight.

3. A slab according to claim 2 in which the particulate refractory material of the backing layer comprises silica or a refractory silicate.

4. A slab according to claim 2 or claim 3 in which the backing layer contains organic fibre.

5. A slab according to any of claims 2 to 4 in which the backing layer contains more binder than the facing layer.