GB2331262A

GB2331262A - A ceramic pouring tube

Info

Publication number: GB2331262A
Application number: GB9724279A
Authority: GB
Inventors: Brendan Mortimer Moriarty
Original assignee: Vesuvius Crucible Co
Current assignee: Vesuvius Crucible Co
Priority date: 1997-11-17
Filing date: 1997-11-17
Publication date: 1999-05-19
Also published as: GB9724279D0

Abstract

A ceramic pouring tube for molten metal, the pouring tube comprising a bore and a core 1 disposed at least partially within the bore, the core being arranged such that the core radiates heat to the pouring tube when a flame or hot gas is directed through the bore. The tube, which may be of metal mesh, is provided with a plurality of metal mesh fins 3 which are welded to and extend along the length of the tube 2 and increase the effective surface surface area of the core 1 from which heat may be radiated. Two of the fins 3 abut the inner surface defining the bore of the pouring tube in use.

Description

A CERAMIC POURING TUBE 2331262 The present invention relates to ceramic

pouring tubes which may be used in the steel industry. In particular, it relates to a core, which may be used to accelerate the preheating of such tubes prior to molten metal being poured through a bore of the tube.

The present invention also relates to a process for preheating such tubes.

is In the steel-making industry, molten steel is poured through ceramic pouring tubes, for example into a continuous casting mould. In view of the high temperature of molten steel, it has been common practice to preheat the pouring tubes before the molten steel is passed therethrough in order to reduce the incidence of thermal shock to the tubes. Thermal shock results in cracks developing in the tubes. In some cases preheating also ensures that the surface of the insides of the tubes is carbon-free, thus inhibiting alumina build up from aluminium contained in the molten steel. Preheating can be achieved, for instance, by directing a flame or hot gas, such as burnt natural gas, butane, propane, town's gas, blast furnace gas, with either air or oxygen, through one or more ports and into the bore of the pouring tube shortly before it is used. The flame or hot gas may be provided by, for example, a gas burner.

We have now developed a ceramic pouring tube which can be preheated in a reduced time and/or heated to a higher temperature for the same burner heat input.

- 2 Accordingly, in a first aspect the present invention provides a ceramic pouring tube for molten metal, the pouring tube comprising a bore and a core disposed at least partially within the bore, the core being arranged such that the core radiates heat to the pouring tube when a flame or hot gas is directed through the bore.

In a second aspect the present invention provides a core for use in the preheating of a ceramic pouring tube having a bore, the core being adapted to be arranged within the bore, whereby, in use, a flame or hot gas directed through the bore heats the core and the core radiates heat to the ceramic pouring tube.

is In a third aspect the present invention provides process for preheating a ceramic pouring tube having bore to reduce the incidence of thermal shock when molten metal is poured through a bore thereof, the process comprising the step of directing a heating means into the bore of the pouring tube, wherein the bore contains a core which is heated by the heating means and radiates heat to the pouring tube.

The heating means preferably comprises a flame or a hot gas or a combination thereof.

The present invention also provides a process for the continuous casting of a metal which comprises pouring molten metal through a ceramic pouring tube, which has been preheated by a process as herein described, into a continuous casting mould and withdrawing solidified metal therefrom.

In all three aspects of the present invention the 3 - core preferably comprises a metal, a ceramic or a combustible material or any combination thereof. For the avoidance of doubt the term metal as used herein is intended to cover any metallic material including single metallic elements and alloys.

The ceramic pouring tube according to the present invention is preferably used in casting moulds for the continuous casting of steel into, for example, slabs, billets, sheets or blooms and may be, for example, a subentry nozzle or a sub-entry shroud.

In the prior art the ceramic pouring tubes are preheated by convection and radiation from the hot gas produced by a flame from a gas burner directed through the bores of the tubes at a gas temperature of typically from about 1000 to about 20000C, more typically from about 1300 to about 18000C. It has been found that there is a poor transfer of heat from the hot gas to the ceramic material making up the tube. In the present invention the core within the bore of the tube easily heats up from the gas or the flame. The core then radiates heat to the inside of the ceramic pouring tube. This can reduce the time taken to heat the pouring tube and/or allow the pouring tube to achieve a higher temperature for the same burner heat input. A core comprising a combustible material would also burn and consequently radiate extra heat to the inside of the ceramic pouring tube.

The process of the present invention can be used, for example, to preheat pouring tubes to a temperature of from about 500 to about 13000C. It will be appreciated that parts of the pouring tube may be preheated to different temperatures. For instance one part of the pouring tube may need to be heated to about 5000C whereas another part may need to be heated to about 12000C. It has been found that the process according to the present invention can reduce the time to preheat a pouring tube by up to, for example, more than about 20%, and often more than about 50% compared with the prior art process in which no radiant core is used. The time required to preheat the pouring tube will depend, of course, not only on the core design and temperature of the hot gas or flame, but also on the design of the tube itself such as the size and shape thereof.

In the ceramic pouring tube according to the present invention it is preferable that all or substantially all of the outer surface of the core is spaced apart from the inner surface of the pouring tube defining the bore. This means that most or substantially all of the heat transfer from the core to the pouring tube is by radiation. It will be appreciated, however, that some heat transfer may additionally occur by conduction and/or convection. It will also be appreciated that the flame or hot gas directed through the bore of the pouring tube will generally pass through the bore of the core and also through the space between the outer surface of the core and the inner surface of the pouring tube.

The ceramic pouring tube may be of conventional construction. Such tubes are well known to those skilled in the art and are described, for example, US-A-4,836,508, EP-A-293,830 and EP-A-302,215. The ceramic pouring tube typically comprises a carbon bonded ceramic material, such as alumina-graphite, zirconia-graphite, magnesite-graphite and zirconia mullite. Alumina- silicates, boron carbide, boron nitride, silicon carbide and other refractory materials may also be used.

A metal core is preferably fashioned from sheet material or metal mesh. The metal mesh may be expanded, woven or punched mesh. A suitable mesh size is from about 0.5 to about 15mm. The particular mesh size depends upon the scale of the application. The thickness of the metal sheet comprising the core is preferably less than about 1.5mm, more preferably less than about 0.5mm. The metal is preferably a mild steel or a hightemperature metal or alloy including but not limited to stainless steel, a refractory metal or alloy, and nickel alloys, such as Inconels.

A ceramic core preferably comprises a refractory material such as an oxide or a carbide. Suitable examples include silica, alumina, alumina-silicates, boron carbide and silicon carbide and mixtures thereof. The ceramic core may comprise, for example, fibre, a bundle of two or more fibres in the form of rope, or a cast ceramic tube of wall section typically of from about 0.2 to about 15mm, depending on the material used. The tube may also be perforated and/or have fins attached.

A suitable ceramic core according to the present invention may be formed by rolling ceramic fibre paper, having a thickness of from about 0.5 to about Imm, around a former rod. An appropriate cement or adhesive such as calcium silicide, for example can then be applied to produce a tubular core having a wall thickness of, for example, from 2 to 3mm. Other processes to produce the ceramic core may also be used, for example vacuum forming ceramic fibre shapes.

is A core comprising a combustible material may comprise a cellulose material such as paper and/or cardboard. A mixture of ceramic fibres and cellulose may advantageously be used. The combustible material may further comprise an oxidizing agent to facilitate combustion in applications where, for example, there is a lack of oxygen. The core may comprise a combustible material provided as a layer on the surface of the bore of the pouring tube, in which case a metal or ceramic core spaced apart from the surface of the bore may optionally also be included.

A core may consist of only the metal, ceramic or combustible material, or may comprise any combination thereof. For example the combustible material may be attached to or provided in the form of a combustible layer on a metal or ceramic core as herein described to augment the core's temperature during the preheating treatment. This ensures that the temperature of the core is raised quickly to the gas temperature at the start of the preheating process and may help to maintain ignition of the bore flame further into the pouring tube.

Whilst the core will generally be in the form of a tube having a substantially circular cross-section, it will be appreciated that it may be of any shape so long as it is able to be heated by the flame or gas and able to radiate heat to the ceramic pouring tube. Accordingly, the core could have, for example, an oval or square cross-section. Alternatively, the core may have an X-section with an optional spiral twist up the bore, or may simply take the form of a spiral twisted strip. It may also take the form of a random loose wire mesh or combustible material simply packed into the bore.

If the bore of the pouring tube is of straight and generally cylindrical shape, the core may be rigid. If the bore is curved or of complex shape, the core may be flexible. A flexible core may comprise, for example, a ceramic fibre rope. Generally it will be of cylindrical shape having a diameter of less than that of the bore of the pouring tube so that it is spaced therefrom to allow heat transfer by radiation. The diameter of such a core may be, for example, from is 0.40 to 0.95, preferably 0.50 to 0.85, times the diameter of the inside of the pouring tube.

The core desirably has a shape which maximises heat transfer from the heated gas to the core. Various heat exchange geometries, individually or in combination, may be used to improve heat transfer, including but not limited to fins, slots, holes and/or turbulence initiators. For example a core such as a tubular core may advantageously include outwardly projecting fins arranged on an outer surface thereof. These fins ensure that the main body of the core is spaced apart from the inside surface of the pouring tube. Accordingly, when the core is heated by a hot gas or flame, the core can subsequently radiate heat to the inside of the pouring tube. The fins may be made of the same material as that of the core or a different material. Thus, for example, a metal mesh core may have metal, metal mesh or ceramic fins. The spacing fins preferably extend along substantially the entire length of the core, and increase its effective 8 - diameter to one which is slightly less than the inside diameter of the pouring tube, for example from 0.20 to 0.90, preferably from 0.30 to 0.80, times the diameter of the inside of the pouring tube. The fins may be integrally formed with the core or, alternatively, can be attached by, for example, welding, chemical adhesion or a suitable mechanical fastening means. The core may contain, for example, from 3 to 12 fins, preferably from 3 to 8 fins. The fins are generally arranged equidistantly around the circumference of the core. It will be appreciated that if fins are provided they result in at least a portion, and preferably substantially all, of the outer surface of the core being spaced apart from the surface of the bore of the tube. Generally two or more spacing fins will abut the inner surface of the poring tube in use. One or more inwardly projecting fins may be arranged on an inner surface of the core. The fins increase the effective surface area of the core from which heat may be radiated.

The outer surface of the core may also be spaced apart from the surface of the bore of the pouring tube by the use of, for example, one or more centralising wires. In this case, if fins are provided they may be spaced from the surface of the bore or one or more of the fins may abut the said surface. Alternatively, the core may simply be suspended by a wire, for example, in the bore of the pouring tube.

The core may have a size and/or shape which alters along the length of the pouring tube in order to enhance the heating effect at certain parts of the tube, for example at the end of the tube farthest from the flame or hot gas inlet. This provides variable enhancement at different parts of the pouring tube owing to the difference in the distance from the burners and/or the increased heat loss from the core. In order to achieve this object, the core may have a varying diameter and/or a change in the number and size of fins, if present, along the length thereof. If the core is in the form of a metal mesh, then the arrangement and size of mesh holes can also be used to enhance the heating effect at certain parts of the pouring tube.

The surfaces of the metal or ceramic core may be treated to increase the emissivity thereof to enhance radiative transfer of heat. Suitable treatments include accelerated oxidation, anodising, metal or ceramic spraying and surface roughening.

The core is preferably inserted into the bore of the pouring tube after the pouring tube has been manufactured, although the ceramic pouring tube may be manufactured with the core as an integral part.

In order to preheat the pouring tube, a flame or hot gas may be directed into the bore of the pouring tube in the usual manner, for example through the end of the pouring tube or through a port positioned close to the end. An insulated shroud may be placed between the means providing the flame or hot gas and the port of the pouring tube. This maximises the amount of combustion that is complete before the flame enters the pouring tube.

After the pouring tube has been preheated, the core will generally be removed. Removal may be accomplished either by simply pulling the core out of the bore or by flushing the core out of the tube with the flow of molten metal. The method chosen depends on the material comprising the core. For example, low melting point metals and friable ceramics can be flushed; although pulling the core out of the bore permits the core to be re-used if desired.

The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a core in accordance the present invention; and is Figure 2 is a graph of nozzle temperature against time which will be described with reference to Example 1 below.

In Figure 1, the core 1 comprises a metal mesh tube 2 with a plurality of metal mesh fins 3. The fins 3 are welded to and extend along the length of the tube 2 and increase the effective surface area of the core 1 from which heat may be radiated. Two of the fins 3 abut the inner surface defining the bore of a pouring tube (not shown) in use. This results in the outer surface of the tube 2 being spaced apart from the inner surface of the pouring tube. Abutment with the inner surface of the pouring tube is preferably on curved surfaces of the fins 3 to limit scratching of the glaze on the bore of the pouring tube. When the core 1 is heated by a hot gas or flame, the tube 2 and those parts of the fins 3 not in contact with the pouring tube radiate heat to the inner surface thereof. Some heat transfer will also occur by conduction through those parts of the fins 3 which are in contact with the pouring tube, and by convection and radiation from the hot gas or flame through the holes of the metal mesh.

The present invention will now be described further with reference to the following Example.

Example

A sub-entry nozzle for a continuous casting mould was positioned on a stand with a gas burner positioned at each port for heating the nozzle. The nozzle had a number of drilled holes along the length of a wall portion thereof into which thermocouples were inserted. An expanded metal mesh sheet of mild steel was rolled into a tube and tack-welded at the required diameter. Eight metal mesh fins of mild steel were tack-welded to the thus formed tubular core. The change in temperature with respect to time was measured for the case where no core was provided in the bore of the nozzle and for the case where the above described core was suspended by a wire in the bore, with the fins abutting the inner surface of the nozzle defining the bore. The same gas flow to the burners was used in both cases. The results are presented in Figure 2, which shows a graph of nozzle temperature against time. It is clear that a higher nozzle temperature may be achieved for the same burner heat-input when a core according to the present invention is disposed in the bore of the nozzle.

12

Claims

CLAIMS is 1. A ceramic pouring tube for molten metal, the pouring tube

comprising a bore and a core disposed at least partially within the bore, the core being arranged such that the core radiates heat to the pouring tube when a flame or hot gas is directed through the bore.
2. A ceramic pouring tube as claimed in claim 1, wherein the core comprises a metal, ceramic or combustible material or any combination thereof.
3. A ceramic pouring tube according to claim 1 or claim 2, wherein the core has an outer surface and the pouring tube has an inner surface defining the bore, and wherein all or substantially all of the outer surface of the core is spaced apart from the inner surface of the pouring tube.
4. A ceramic pouring tube according to any one of claims 1 to 3, wherein the core comprises a metal or ceramic mesh.
5. A ceramic pouring tube according to any one of claims 1 to 4, wherein the core has a substantially cylindrical shape.
6. A ceramic pouring tube according to any one of claims 1 to 5, wherein the core has an inner surface and an outer surface whereby a wall is defined, the wall having a thickness of less than 3 mm.
7. A ceramic pouring tube according to any one of claims 1 to 6, wherein the core comprises one or more 13 mechanical heat exchangers.
8. A ceramic pouring tube according to claim 7, wherein the one or more mechanical heat exchangers core comprise(s) fins, slots, 1oles and/or turbulence initiators.
9. A ceramic pouring tube according to claim 2, wherein the core comprses a combustible material comprising a cellulose material.
10. A ceramic pouring tube according to claim 2 or claim 9, wherein the combustible material further comprises an oxidizing agent to facilitate combustion.

is
11. A ceramic pouring tube according to claim 2, claim 9 or claim 10, wherein the core is provided as a layer on part or all of an inner surface of the pouring tube defining the bore.
12. A ceramic pouring tube according to any one of claims 2 to 8, wherein the core is formed from stainless steel, mild steel, a refractory metal or alloy, or an Inconel alloy.
13. A ceramic pouring tube according to any one of claims 1 to 12, wherein an outer surface of the core is treated to increase the emissivity.
14. A ceramic pouring tube according to any one of claims 1 to 13, wherein the core is flexible.
15. A core for use in the preheating of a ceramic pouring tube having a bore, the core being adapted to be arranged within the bore, whereby, in use, a flame C_ 1 14 - or hot gas directed through the bore heats the core and the core radiates heat to the ceramic pouring tube.
16. A core according to claim 15, wherein all or substantially all of an outer surface of the core is spaced apart from an inner surface of the pouring tube defining the bore.
17. A core according to claim 15 or claim 16, wherein the core comprises one or more mechanical heat exchangers.
18. A process for preheating a ceramic pouring tube having a bore, the process comprising the step of directing a heating means into the bore of the pouring tube, wherein the bore contains a core which is heated by the heating means and radiates heat to the pouring tube.
19. A process according to claim 18, further comprising the step of removing the core from the bare after the preheating step.
20. A process for the continuous casting of a metal, the process comprising the steps of preheating a ceramic pouring tube by a process as claimed in claim 18 or claim 19, and then pouring molten metal through the preheated pouring tube into a continuous casting mould and withdrawing solidified metal therefrom.
21. A ceramic pouring tube for molten metal, the pouring tube having an inner surface defining a bore, wherein a core is disposed at least partially within the bore, the core comprising a sheet of metal mesh in is the shape of a tube and having on an outer surface thereof one or more fins, the core being arranged such that all or substantially all of the outer surface thereof is spaced apart from the innr surface of the pouring tube, and the core being further disposed relative to the bore such that the core radiates heat to the pouring tube when a flame or hot gas is directed through the bore.
22. A core for use in the preheating of a ceramic pouring tube having a bore, the core comprising a sheet of metal mesh in the shape of a tube and having on an outer surface thereof one or more fins, wherein the core is adapted to be arranged within the bore is whereby, in use, a flame or hot gas directed through the bore heats the core and the core radiates heat to the ceramic pouring tube.
23. A core for use in the preheating of a ceramic pouring tube substantially as hereinbefore described with reference to or as illustrated in Figure 1.
24. A core for use in the preheating of a ceramic pouring tube substantially as hereinbefore described with reference to Example 1.
25. A ceramic pouring tube for molten metal substantially as hereinbefore described with reference to Example 1.
26. A process for preheating a ceramic pouring tube substantially as hereinbefore described with reference to Example 1.