GB2347886A - Apparatus for removing superheat from liquid metal using a distributor - Google Patents

Abstract

An apparatus for removing superheat from liquid metal as it passes from a holding vessel to a casting mould comprises a distributor section which extends into the upstream end of a cooling section of the apparatus and is adapted to direct liquid metal onto a surface of the cooling section. By distributing the metal stream directly onto the surface of the cooling section, the tendency of a thick and substantial skull to form within the flow is substantially reduced. It is preferred if the cooling section is substantially cylindrical, in which case it is also preferred if the distributor directs metal onto an inner surface of the cylinder with some radial component of velocity. Thus, liquid metal can impinge transversely on a cooling section surface. The cooling section is suitably water cooled. In this case, it is preferred if the water flow passages are linked to a source of compressed air. In emergency, the water flow can be shut off and the compressed air released into the cooling system thereby purging it of water. It has been found that this arrangement of the casting apparatus is highly effective in providing a reproducible superheat removal.

Description

APPARATUS FOR REMOVING SUPERHEAT FROM LIQUID METAL The present invention relates to apparatus removing superheat from liquid metal, particularly steel. It is particularly useful in continuous casting applications.

For many applications, it is known to be advantageous to cast metals in the viscous phase, i. e. at a temperature within the solidification range of the metal in question. Through this method, it is possible to obtain fine homogenous equiaxed structures in which segregation in the mould is minimised. This can minimise undesirable features such as centre-line segregation and reduce susceptibility to inter-columnar cracking.

In order to achieve such casting, at least some super heat must be removed from the liquid metal between the tundish and the mould. EP 0269180 discloses such a system, in which molten steel is deflected towards a refractory material by a distributor, following which the liquid steel flows over a water-cooled region and thence into a mould. EP 269180 discloses the importance that the liquid metal is delivered first to a refractory inlet section without substantial cooling, followed by a distinct cooling section of, for example, water-cooled copper.

It has been found by the applicant that the major factor in achieving thermal transfer from the liquid metal to the cooler is not in fact the characteristics of the water flow or of the copper surface. It is rather the characteristics of a"skull"of solidified metal which tends to form on the inner layer of the cooling section. As it cools and shrinks, the skull tends to detach from the inner surface of the cooling section, thereby reducing heat transfer and insulating the liquid metal flow. Therefore, improved operation of the cooling section can be obtained if the design thereof is such as to minimise the skull growth and profile.

The present invention provides apparatus for removing superheat from liquid metal as it passes from a holding vessel to a casting mould, the apparatus comprising a distributor section which extends into the upstream end of a cooling section of the apparatus and is adapted to direct liquid metal onto a surface of the cooling section.

By distributing the metal stream directly onto the surface of the cooling section, the tendency of a thick and substantial skull to form within the flow of EP 269180 is substantially reduced. The cooling section is however able to withstand the liquid metal impact, probably due to the thinner skull which nevertheless develops and the materials used.

It is preferred if a cooling section is substantially cylindrical, in which case it is also preferred if the distributor directs metal onto an inner surface of the cylinder with some radial component of velocity. Thus, liquid metal can impinge transversely on a cooling section surface. Ideally, the flow at this point is not horizontal, as this would involve a significant loss of momentum and energy resulting in poorer heat transfer and increased skull formation.

The cooling section is suitably water cooled. In this case, it is preferred if the water flow passages are linked to a source of compresse air. In emergency, the water flow can be shut off and the compresse air released into the cooling system thereby purging it of water. This will prevent the water from boiling and possibly causing pressure build-up or explosion.

It has been found that this arrangement of the casting apparatus is highly effective in providing a reproducible superheat removal. In particular, the apparatus tends to remove a set proportion of the superheat, rather than achieve a constant temperature reduction. It is common for the superheat within the tundish to be variable, in which case the removal of a proportion rather than a pre-set amount will tend automatically to reduce significantly variation in the temperature of the metal supplie to the casting mould.

This is particularly advantageous in continuous casting where the properties of the resultant steel cast are dependent on the position of the "sump". The sump is the point of final solidification at the centre of the cast section, which is some distance below the base of the mould. The position of the sump depends in turn on the degree of superheat present in the steel, together with other factors. Thus, a less variable superheat equates directly to a less variable sump position, allowing better machine optimisation to obtain more stable properties of the steel.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying Figures, in which; Figure 1 is a vertical cross-section through a tundish fitted with an embodiment of the present invention ; Figure 2 shows the embodiment in more detail ; Figure 3 illustrates the known submerged entry nozzle for use in continuous casting equipment; Figure 4 illustrates the tundish nozzle of the embodiment of the present invention; Figures 5 and 6 show vertical and horizontal sections through the tundish nozzle of Figure 4, Figure 5 being a section on V-V of Figure 6; Figures 7 and 8 show vertical and horizontal sections through an alternative tundish nozzle, Figure 8 being a section on VII-VII of Figure 7; and Figures 9 and 10 show the nozzle of Figures 7 and 8 in place.

Referring to Figure 1, this shows a tundish 10 lined with refractories 12 at the base of which is formed an opening 14. A stopper rod 16 is movable vertically so as to open and close the opening 14. Thus, steel 18 within the tundish 10 can be delivered vertically downwardly through the opening 14 under control of the stopper rod 16.

A casting mould 20 is placed below the opening 14 for receipt of molten steel. The casting mould can be any suitable casting mould, for example a continuous casting arrangement.

A heat exchanger 22 is fitted to the tundish 10 immediately below the opening 14, above the casting mould 20. This heat exchanger serves to remove"super heat", i. e. heat above and beyond the liquidus temperature of the steei. Thus steel delivered to the casting mould will be closer to its solidification point, thus obtaining the casting advantages set out above.

The heat exchanger is shown in Figure 1 and, in more detail, in Figure 2. Referring to both Figures, it can be seen that the heat exchanger 22 is supplie with liquid steel by a refractory flow passage 24 in which a vertical flow 26 of steel is deflected by a distribution member 28 to an outward flow 30,32. After exiting the refractory flow passage 24 in a direction with a significant radial component, the molten steel impinges on the interior face of a tubular copper member 34. This copper member is constructed of a toughened copper grade (e. g. Elbrodur') in order to assist in withstanding the direct molten steel impingement. Other grades of copper or other metals are however capable of exhibiting adequate properties. The copper is surrounded by a helix 36 (in this case a four start helix) of water cooling channes. The helix 36 is supplie with water by a main inlet 38, and conducts water helically around the copper heat exchange surface to a main outlet 40. The water is then cooled and recycle.

An exit port 42 is provided below the cylindrical copper cooling region 34. This terminates at an outlet 44 which typically comprises openings directed sidewardly in order to improve the casting characteristics of the stream. A downward opening arrangement is also possible.

The entire assembly is held together and supported in a body portion 46. This is in turn supported on the underside of the tundish 10 by attachments 48.

Figure 3 shows, for comparison, a standard continuous casting submerged entry nozzle as would be fitted to the underside of the tundish.

It simply comprises a cylindrical body 50 defining an inlet opening 52 at the top end thereof, leading to two laterally directed openings 54,56. The bottom end of the tube is closed with a distribution block 58.

Figure 4 shows the refractory outlet from the tundish for use in the present invention. It is attached directly to the upper and lower well blocks 60,62, and comprises the vertical flow section 24 leading to the distribution block 28. Steel thus exits with strongly directed components through openings 64, 66.

Figures 5 and 6 show the nozzle of Figure 4 in still greater detail.

Figure 6 shows a vertical view, illustrating five outlets 64,66, distributed around the circumferential base of the distribution block 28. In this embodiment, each outlet encompasses an angle of 47 , with a 25 gap between each. The size of the outlets is clearly a balance between achieving a uniform radial flow and in providing sufficient material between the outlets to support the distribution block 28. In general, using standard refractories, and angle of between 35 and 50 is advisable. Similarly, the number of openings is a balance between uniformity of flow and mechanical support. Figure 6 illustrates five such openings, but four, six or other numbers would also be adequate.

Figures 7 and 8 show an alternative nozzle to that of Figures 5 and 6.

This is a relatively simplified design, in which the well block and distributor nozzle pieces are combined into a single item which would mate directly with the tundish stopper rod. The principe is unchanged relative to Figures 4-6, but this design allows assembly sequences to be brought into line with standard cast operation and eliminates various joints, assembly operations, refractory weight and costs. As the unit is a direct replacement, suitable dimensions are generally similar.

The similarity of the two designs means that similar reference numerals have been used. In practice, the design of Figures 7 and 8 can be inserted directly as a replacement for the embodiment of Figure 4, as illustrated in Figures 9 and 10.

There are many variations and augmentations that can be made to the above described embodiments, withoutdeparting fromthe present invention.

Many relate to improvements in ease of use and safety of operation. For example, thermocouples can be embedded into the copper hot face, preferably in pairs mapping the radial and axial temperature distribution within the unit. The thermocouple pairs can be arranged such that one is closer to the hot face than the other, so that an estimate of localise heat flux can be achieved as well as measurement of absolute temperature. The information provided by this can be used to check equipment performance and confirm that operation is within predetermined safe limits. On-line control systems could take advantage of such data.

As illustrated in Figures 1 and 2, the heat exchange core can possess a very slight taper from top to bottom. This serves to improve heat removal by matching the shrinkage profile of the skull, and also to facilitate skull removal after casting. As illustrated, the top is slightly wider than the bottom. It is likely that a similar effect would be provided by the reverse arrangement.

The water cooling system can be provided with an emergency air purge. This requires the provision of high pressure air, and the ability to switch the water input 38 to the high pressure air source rather than the water source. This enables total evacuation of water from the heat exchanger should a situation arise in which mixing of liquid steel and water might take place. This type of safety feature is not normally required in continuous casting technology as any water leak would be into a wet area, i. e. remote from liquid steel. In the present embodiment such an air purge arrangement overcomes the risk of water becoming trapped in small confined areas.

The refractory linkages between the heat exchanger and the tundish can be varied in generally known fashions. For example, whilst a stopper rod is illustrated, other valves and/or sliding gates can be included. A backup sliding gate or other emergency cut-off device may be included immediately below the stopper rod to shut off steel flow if difficulties were encountered with the stopper rod 16.

A standard submerged entry nozzle 42 suitably adapted to fit the heat exchanger has been set out above. However, the precise means of delivering the molten steel to the casting mould will depend on the circumstances.

The present invention therefore provides a particularly compact arrangement which is able to remove superheat from molten steel immediately prior to casting in a controlled and reproducible manner. The device alleviates the formation of a"skull"through the novel flow pattern.

Although the above description has been given in relation to molten steel, the device could of course be used to remove superheat from other molten metals.

Claims (7)

1. CLAIMS 1. Apparatus for removing superheat from liquid metal as it passes into a casting mould, comprising a cooling section and a distributor section, the distributor section extending into the upstream end of the cooling section and being adapted to direct liquid metal onto a surface of the cooling section.
2. 2. Apparatus according to claim 1 wherein the cooling section is substantially cylindrical.
3. 3. Apparatus according to claim 2 in which the distributor directs metal onto an inner surface of the cylinder with a radial component of velocity.
4. 4. Apparatus according to claim 3 in which the liquid metal flow onto the inner surface of the cylinder is not horizontal.
5. 5. Apparatus according to any preceding claim in which the cooling section is water cooled.
6. 6. Apparatus according to claim 5 in which the water flow passages are linked to a source of compresse air.
7. 7. Apparatus for removing superheat from liquid metal substantially as described herein with reference to and/or as illustrated in the accompanying figures.