GB2087055A - Collapsible Coverboards - Google Patents

Abstract

An insulating board for covering the top of a tundish or other vessel, at the start of continuous casting or other operations involving molten metal, combines the functions of a lid and an insulating powder. The board is bonded by a thermally-degradable binder and so disintegrates upon breakdown of the bond when radiantly heated by the molten metal as the metal fills the tundish. The disintegrated board forms an insulating powder which floats on the surface of the molten metal and preferably sinters to form an insulating crust. The board remains intact long enough to confine any splash which occurs as the tundish or other vessel is filled.

Description

SPECIFICATION Collapsible Coverboards The invention relates to the handling of molten metals and is particularly of relevance to the continuous casting of metals, especially the continuous casting of steel.

In the continuous casting of steel, molten steel is poured from a ladle via a tundish into the tops of one or more water-cooled open-bottomed moulds. Strands of solidified steel emerge at a steady speed from the bottoms of the moulds.

The tundish provides a head of metal which smooths out any fluctuations in the rate of flow of metal from the ladle. In the case of two or more moulds, the tundish also serves to distribute the metal among the moulds.

Tundishes are usually in the form of a metal shell lined with refractory material. This lining protects the shell from attack bt the molten steel and also provides a certain degree of thermal insulation of the metal, but considerable heat losses can occur as the molten steel flows continuously through the tundish, so that further precautions are desirable or even necessary, in order to minimise heat losses.

Two procedures are commonly used for minimising heat losses through the walls and floor of tundishes, namely the "hot" practice and the "cold" practice. The hot practice involves preheating the refractory lining to 10000C-- 1 5000C (typically 1 2000C), before pouring the molten metal into the tundish. Preheating typically takes 1212 hours and involves considerable energy costs. In the cold practice, the refractory-lined tundish is provided with a disposable inner lining consisting of insulating slabs with a low thermal capacity. The only preheating which is then required is preheating of the tundish outlet nozzles, which typically takes 1 5-30 minutes.The cold practice eliminates most of the cost and delay associated with hot practice preheating and also cuts down heat losses more effectively.

It is also necessary to minimise heat loss from the exposed top surface of the molten steel in the tundish and this is conventionally done, in both the hot and cold practices, by means of a layer of insulating powder which floats on the molten steel.

The tundish has one or more outlet nozzles which deliver the molten steel from the tundish to the mould or moulds. At the start of casting, the required head of steel is built up in the tundish by temporarily closing the nozzles. When the metal first enters the tundish and while the head is building up, there is a danger that molten metal might splash up from the exposed inner surface of the tundish and possibly injure any operators who are nearby. Metal splashes can also damage equipment, electric cables and any material placed near the tundish for use during casting. To eliminate the risk of injury and damage from these causes, the tundish is usually provided with a lid of cast iron or steel which is provided with a refractory lining which protects the lid from splashes and from the heat radiated by the melt.

Such lids are used in both the hot practice and the cold practice. The capital costs of these lids are high and maintenance of the lids is also expensive labour must be kept available to burn away solidified splashes from the refractory linings of the lids and the linings must be replaced periodically.

The invention is directed to improving upon conventional continuous casting practice, by eliminating the insulating powder and the costly lid and providing instead an insulating board which combines the functions of the powder and the lid.

According to one aspect of this invention, a collapsible coverboard is provided, for use in or on a vessel, the coverboard serving to contain splashing as molten metal is poured into the vessel and to reduce heat loss from the metal in the vessel, wherein the coverboard comprises at least one particulate refractory thermallyinsulating material bonded together by means of a thermally-degradable bonding agent, wherein the bonding agent is selected so that heat radiated from the metal causes the coverboard to disintegrate into an insulating powder which falls on the molten metal surface, as and/or after pouring takes place.

According to another aspect of the invention, a method of pouring molten metal into a vessel is provided, which comprises assembling one or more coverboards in or on the vessel above the intended maximum height of the molten metal therein, the one or more coverboards overlying the open top of the vessel except where the pouring is to occur, and pouring the molten metal into the vessel the one or more coverboards being formulated and positioned so as to contain splashing and to undergo thermal degradation into an insulating powder which falls on to the molten metal surface.

The invention also consists in a method of making collapsible coverboards and in a metallurgical vessel fitted with one or more or such collapsible coverboards.

In carrying out continuous casting by the improved practice which this invention makes possible, one or more of the insulating boards according to the invention are placed across the top of the tundish so that they are generally horizontal and are located above the highest level to which the head of molten steel will rise in the tundish. They may lie freely on top of the tundish or, if the tundish ;tapers downwardly, they may be wedged between opposite walls of the tundish.

The number of boards needed for one tundish is typically from 2 to 6, depending on the size of the tundish. The boards are arranged so as to leave at least one gap between them, to accommodate the stream of molten steel as it is poured from the ladle into the tundish.

The boards are made of a material which disintegrates upon being heated to a temperature which, in the normal case where the metal is steel, is well below the temperature of the molten steel. The temperature at which disintegration occurs therefore is preferably about 450or 5oooC. When steel is initially poured into the empty tundish, the boards act as a lid and confine any splash within the tundish. As the tundish fills, the heat radiated by the molten metal raises the temperature of the boards until they reach their disintegration temperature and collapse, becoming a powder which falls on to the surface of the molten steel and forms an insulating layer.

A disintegration temperature of about 450or 5000C is preferred and the bonding agent is therefore preferably selected accordingly, because it allows the boards to remain intact long enough to confine all the initial splash, without remaining intact so long that excessive heat is lost from the surface of the melt before the powder layer forms.

According to the invention, the bonding agent is a thermally-degradable substance, e.g. a urea or phenol/formaldehyde resin or cereal flour or mixture of these binders.

A preferred feature of the invention is the inclusion of a fibrous carbonaceous material which substantially burns out as the board disintegrates.

The preferred composition of the boards, by weight, is: Paper Pulp 2.510% Binder 2.5%7.5% Kieselguhr 1 0%-30% Vermiculite 1 0%-30% Refractory material, e.g. firebrick grog Balance The preferred method of manufacture is to slurry the mix with water and then vacuum-form the slurry into boards of the desired size, which are then dried and, in the preferred case where the bonding agent comprises a heat-curable resin, stoved to cure the bonding agent or binder.

Injection moulding or pressing could be used instead of vacuum forming.

The fibrous carbonaceous material facilitates handling of the boards during manufacture, by providing green strength in the unstoved state, and also gives resilience to the finished product.

Any type of carbonaceous fibre material which burns out at a low enough temperature maybe used; burnout of the fibre must be complete at the desired disintegration temperature of about 450 500 C. If the fibre content is less than 2.5%, the board will normally be too brittle. If the fibre content is greater than 10%, the board will normally be too fibrous and lack rigidity.

The binder (also referred to as the bonding agent) provides coherence and strength and it must form a thermally-degradable bond which breaks down at the desired disintegration temperature; the binders previously discussed are all suitable. In general, binder contents of less than 2.5% make the material difficult to bond; binder contents greater than 7.5% give an excessively firm bond which is slow to break down and thus delays disintegration.

The kieselguhr assists in providing a lower density and lowers the thermal conductivity of the powder layer formed by the disintegrated board.

The indicated range of 10% to 30% is normally preferred, because kieselguhr contents less than 10% do not sufficiently lower the density of the material; kieselguhr contents greater than 30% make the powder layer too sticky and slag-like.

Other materials with low density and low thermal conductivity can be used in addition to or instead of kieselguhr, e.g. diatomaceous earth or expanded fire-clay grog.

Vermiculite is desirably included because it exfoliates, i.e. it swells upon heating, and thus ensures total collapse of the board. Any other material which exfoliates or otherwise expands on heating at comparatively low temperatures, may be used, e.g. unexpanded perlite, but vermiculite is particularly suitable because of its behaviour in the powder layer after disintegration. Because the melting point of vermiculite is lower than the temperature of molten steel, the vermiculite becomes molten at the interface between the powder layer and the steel and becomes semimolten throughout the remainder of the powder layer, sintering the powder layer to form a coherent crust. The sintering of the powder layer prevents dust formation and also prevents the powder from becoming entrained in the molten steel.Vermiculite contents less than 109/0 are usually inadequate to ensure total collapse of the board. Vermiculite contents greater than 30% are generally not used, because they give excessive exfoliation, so that the crust becomes too voluminous.

The firebrick grog provides refractoriness. Any type of refractory material which provides adequate insulation and is compatible with the method of manufacture may be used. For example, the refractory material may comprise one or more of an alumina silicate grog, silica, alumina and magnesia.

The period of time for which the boards are required to remain intact is typically from 45 seconds to 5 minutes, depending on the size and rate of fill of the tundish or other vessel. The period may be adjusted by altering the proportions of the ingredients in the mix, while preferably remaining within the limits given above. Altering the proportions of the ingredients affects the overall thermal capacity of the board, so that it takes a longer or shorter time to reach its disintegration temperature.

The boards of the invention are suitable for both the hot practice and the cold practice.

However, when used for the cold practice, they offer the additional advantage that by prepositioning they can act as a wedge between the tops of opposite sidewall slabs as a safeguard against such slabs caving in at the start of casting.

These slabs are usually secured by metal clips or by wooden or metal cross-pieces as a safeguard against collapse; the clips and cross-pieces may not be necessary if the boards of the invention are wedged between the slabs.

The invention has been described in terms of the continuous casting of steel, but may also be applied to the continuous casting of other metals.

When the invention is applied to the casting of metals whose casting temperature is significantly different from the casting temperature of steel, the composition of the board should be altered to give the appropriate disintegration temperature.

This can be achieved usually by choosing a bonding agent with the appropriate disintegration temperature and/or by incorporating a material which exfoliates or otherwise expands at the appropriate temperature.

The boards of the invention are also useful for other metallurgical vessels for which a combination of splash protection and insulation is required. For instance, when molten metal is carried in a ladle from a furnace to a location at which the metal is to be used, one or more boards can be placed across the top of the ladle to prevent splashing during transportation, and the boards then form an insulating powder to reduce heat loss until the metal is poured from the ladle.

Claims (24)

Claims

1. A collapsible coverboard for use in or on a vessel, the coverboard serving to contain splashing as molten metal is poured into the vessel and to reduce heat loss from the metal in the vessel, wherein the coverboard comprises at least one particulate refractory thermally insulating material bonded together by means of a thermally-degradable bonding agent, wherein the bonding agent is selected so that heat radiated from the metal causes the coverboard to disintegrate into an insulating powder which falls on to the molten metal surface, as and/or after pouring takes place.

2. A coverboard according to claim 1, wherein the bonding agent is a thermally-degradable synthetic resin.

3. A coverboard according to claim 1 or 2, wherein the bonding agent comprises one or more of a urea/formaldehyde resin, a phenol/formaldehyde resin and cereal flour.

4. A coverboard according to claim 1,2 or 3, wherein the bonding agent is selected so that disintegration occurs at a temperature in the range from 4500 to 5000 C.

5. A coverboard according to any preceding claim, which includes a fibrous carbonaceous material which substantially burns out as the board disintegrates.

6. A coverboard according to claim 5, wherein the carbonaceous material comprises paper pulp.

7. A coverboard according to any preceding claim, which includes at least one material which swells upon heating and so facilitates the thermal disintegration.

8. A coverboard according to claim 7, which includes vermiculite.

9. A coverboard according to any preceding claim which includes at least one of kieselguhr, aiatomaceous earth and expanded fireclay grog.

1 0. A coverboard according to any preceding claim, wherein the refractory material comprises one or more of alumina silicate grog, silica, alumina and magnesia.

11. A coverboard according to any preceding claim, which is formulated so that the insulating powder sinters to a coherent crust on the molten metal.

12. A coverboard.according to any preceding claim, which remains intact for a time from 45 seconds to 5 minutes in the presence of molten steel, before thermal disintegration begins.

1 3. A coverboard according to any preceding claim, which comprises, by weight: Paper pulp 2.5% to 10% bonding agent 2.5% to 7.5% kieselguhr 10% to 30% vermiculite 10% to 30% refractory material balance

1 4. A coverboard according to claim 1, substantially as hereinbefore described.

1 5. A method of manufacture of a collapsible cover board as defined in any preceding claim, which comprises slurrying with water a mixture of the board ingredients, forming the slurry into boards and then drying the boards.

1 6. A method according to claim 15, wherein the bonding agent comprises a heat-curable synthetic resin and the dried boards are stoved to cure the resin.

17. A method according to claim 1 5 or 16, wherein the slurry is shaped into boards by vacuum-forming, injection-moulding or pressing.

18. A method according to claim 15, substantially as hereinbefore described.

19. A collapsible coverboard, when made by a method according to any of claims 1 5 to 18.

20. A method of pouring molten metal into a vessel, which comprises assembling one or more coverboards in or on to the vessel above the intended maximum height of the molten metal therein, the one or more coverboards overlying the open top of the vessel except where the pouring is to occur, and pouring the molten metal into the vessel, the one or more coverboards being formulated and positioned so as to contain splashing and to undergo thermal degradation into an insulating powder which falls on to the molten metal surface.

21. A method according to claim 20, wherein the one or more coverboards are formulated so as to form a sintered coherent crust on the molten metal surface.

22. A method according to claim 20 or 21, wherein the coverboard(s) is/are as defined in any of claims 1 to 14 and 19.

23. A metallurgical vessel, whenever fitted with one or more collapsible coverboards in accordance with any of claims 1 to 14 and 19.

24. A metallurgical vessel in accordance with claim 23, comprising a tundish for use in the continuous casting of steel.