GB2230719A - Controlling deposition of particles from molten metals - Google Patents

Abstract

For controlling deposition or removal of undissolved particles from molten metal, eg. alumina from steel, the metal is flowed through a duct or nozzle device (20c) which has a refractory body defining a duct passage (21). The wall of the duct passage is configured to provide at intervals therealong pockets (32) and also, optionally, inwardly projecting ribs (30). The wall configuration is such that as the molten metal flows through the duct or nozzle device (20c) a plurality of eddies (26) is formed and the undissolved particles become entrapped therein and thus are removed from the main flow of metal through the device. <IMAGE>

Description

"CONTROLLING DEPOSITION OF PARTICLES FROM MOLTEN METALS" The present invention relates to controlling deposition of particles from molten metals.

More particularly, this invention concerns a device and a method useful for controllably removing undissolved particles from molten metals so as to reduce the amount of such particles therein.

Thanks to the invention, a solidified metal product such as a casting or metal stock for processing e.g. by rolling can be obtained, which has a reduced content of undesirable inclusions.

In the practice of this invention, the particles are deposited in a controlled manner while a molten metal is flowing along a duct or nozzle. Hitherto, undissolved particles have had a tendency to deposit on the wall of a duct leading e.g. to a mould such as a continuous caster mould. Gradual accretion proceeds to a stage where either the duct becomes entirely or significantly occluded, or parts of the deposit break off and are swept away in the molten metal, to become substantially-sized inclusions in the metal product. The invention has been devised to combat these especial problems encountered when a molten metal is flowing through a duct.

The undissolved particles may for example comprise non-metallic components such as metal oxides.

More particularly, but not exclusively, the invention aims to remove particles oe alumina from molten iron or steel. Alumina is particularly prone to adhere to the walls of the refractory ducts or nozzles commonly used in pouring melts in the iron and steel industry.

Alumina build up is especially troublesome when pouring deoxidised, aluminium-killed ferrous metal.

The invention is not limited to use with ferrous metals, however. It is also applicable to the removal of undissolved particles such as oxides from non-ferrous metals.

According to the present invention, there is provided a device for controlling the deposition of, and thereby removing, undissolved particles such as oxides from a flowing molten metal, comprising a refractory body defining a duct passage the wall of which, at intervals therealong, is so configured as to provide pockets therein and/or protrusions projecting into the passage, the wall configuration being such as to create a plurality of locations in which, in use, eddies form and wherein said particles collect.

The invention also provides a method of controlling the deposition of, and thereby removing, undissolved particles such as oxides from a flowing molten metal, wherein the metal is passed along a duct passage configured at intervals therealong to create localised particle-collecting regions wherein eddies are developed adjacent a wall defining the duct passage.

The method may be practised using the device as defined in the last preceding paragraph.

Various wall configurations will be mentioned and described hereinafter. Devices according to the invention can comprise an array of recesses only, an array of protrusions only or an array of associated recesses and protrusions.

Without being bound by hypothesis, it appears that undissolved particles - e.g. alumina in molten steel or iron - migrate outwardly away from the axis of the flowing stream, perhaps because their natural flow velocity and their density differ from those of the flowing metal. They can then become entrapped in the eddies. Particles circulating in the eddies may then coalesce with other particles from the stream and may ultimately adhere to the duct in known locations, namely on the surfaces of the recesses andZor protrusions. It appears that the accumulated deposit is either sufficiently strongly adhered to surfaces of the recesses and/or protrusions, or is so protected by them from the flowing metal, that its detachment by the flowing metal and its intrainment therein are unlikely to happen.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings, in which: Fig. 1 diagrammatically illustrates a ladle furnished with a duct device according to the invention; Fig. 2 diagrammatically illustrates a tundish or degassing vessel furnished with a duct device according to the invention; Fig. 3 is a schematic, longitudinal section through a duct forming a first embodiment of the invention; Fig. Lt is a schematic, longitudinal section through a duct forming a second embodiment of the invention; Fig. 5 is a schematic, longitudinal section through a duct forming a third embodiment of the invention; and Fig. 6 is an end view of the embodiment of Fig. 5.

It is common practice, especially in the iron and steel industry, to pour molten metal from a supply vessel to a receiver along a duct system. Often, the molten metal contains undissolved particles such as oxides, more particularly alumina, which can deposit on a wall of the duct system and create the aforementioned problems. To overcome these problems, this invention provides a specially designed duct or nozzle device which is described hereinafter.

The device can, for example, be fitted to a bottom pour supply vessel such as ladle 10 or tundish 12 shown in Figs. 1 and 2, in flow communication with pouring opening 14 thereof. Flow through the opening 14 may be controlled, if required, by valve means 15 or stoppers 16. The duct device 20 of this invention can comprise a discharge or collector nozzle 17 of valve means 15. Alternatively, it can be incorporated as an extended or submerged pour tube 18.

In each of the several embodiments 20a-20c shown in Figs. 3 to 6, the duct device comprises a generally tubular body of refractory material.

The refractory may be a pressed and fired material or a cementitious castable as is known in the art.

Duct device 20a of Fig. 3 has a central flow passage 21 of cylindrical form extending between inlet and outlet ends 22, 23 of the device. The inner wall 24 defining the passage 21 is configured at intervals therealong to provide pockets 25 wherein eddies 26 will form when molten metal is teemed through the device. As shown in Fig. 3, duct device 20a has three pockets each in the form of an encircling annular recess in the inner wall 24. The recesses 25 are of rectangular form, although other forms such as square would be permissible.

Duct device 20b of Fig. 4 has protrusions 28 projecting inwardly from its inner wall 24 at spaced intervals along the passage 21. In effect, adjacent pairs of protrusions 28 define pockets therebetween wherein eddies 26 as shown will form in use.

The protrusions 28 can be integral with the wall 24 of the duct device 20b or may be attached thereto e.g. by sintering or ceramic bonding when the device is fired.

As shown in Fig. 4, the protrusions 28 are in locations staggered along the passage 21 and no protrusion is directly opposed to another. Each protrusion is in the form of a radial rib which extends around part only of the circumference of the passage 21. The ribs may for instance each extend around about one-half of the circumference, their lengths being more or less than half the circumference, as desired.

In the duct device of Fig. II, the circumferential location of each rib 28 is non-coincident with the circumferential locations of the two ribs nearest adjacent thereto. The ribs 28, where they extend around less than half the circumference of the passage, may be so arranged as envisaged in Fig.

4 that they are disposed alternately to one side and the other of a central longitudinal plane through the passage 21. In essence, then, the ribs are circumferentially located such that the middle of each is disposed 1800 from the middles of the two adjacent ribs in the array thereof.

If the device of Fig. 14 is viewed end on, the central lumen therein will be a closed or open circle depending on whether the ribs extend around more than half the circumference and overlap, or less than half the circumference. An open circle lumen may have the appearance illustrated by the solid lines in the centre of Fig. 6.

A modified device akin to device 20b of Fig.

4 could have the ribs, which extend around less than 1800, placed directly i.e. diametrically opposite one another, so that opposed pairs of ribs are in a common plane. Another modified device could have annular ribs, i.e. ribs which each extend all round the periphery of the passage 21.

Alternatively, modified devices akin to device 20b of Fig. 4 could have ribs 28 whose circumferential extents are appreciably less than half the circumference of the passage. By way of example, the ribs could have circumferential extents about equal to one third or one quarter of the said circumference, the ribs being located such that the middles thereof are 1200 or 900 from their neighbours. These ribs could be longitudinally staggered, or sets of three or four ribs could be disposed in common planes.

Duct devices having inward protrusions or ribs 28 as discussed above in use can serve to create (i) a metal flow rate profile wherein the flow rate increases towards the centre or axis of the duct passage, and (ii) particle-collecting eddies between the protrusions.

Duct devices having inward protrusions or ribs can also feature recesses in the walls thereof, such recesses e.g. being located downstream of an associated protrusion. Fig. 5 illustrates such a device 20c.

Device 20c has sets of associated ribs 30 and recesses 32. Each associated rib and recess down stream thereof has a circumferential extent of less than 1800. The sets of ribs and recesses are centred along two lines 1800 apart and they are staggered alternately to either side of central longitudinal plane 34 through the passage, see Fig. 6. No rib is thus diametrically opposed to another rib, as will be seen. Viewed end-on, see Fig. 6, the duct device will be found to have a central lumen of open circular form.

Again in use of this device a flow rate profile will be established wherein the metal flow rate increases towards the centre or axis of the duct passage, and particle-collecting eddies 26 will form downstream of each protrusion or rib 30 and in the associated pocket or recess 32.

As just described, the ribs and recesses extend circumferentially over less than 1800, but this is not an essential requirement. By way of example, this embodiment could be modified by making the ribs and recesses extend around 1200 or 900 or thereabouts, the ribs and recesses being disposed relative to one another as described hereinbefore in relation to modified devices of the Fig. 4 type.

Another modified device embodying the invention could comprise pockets or recesses and no ribs, wherein unlike the Fig. 3 embodiment, the recesses do not extend through 3600. Such a modified device can be visualised by reference to Fig. 5, the ribs shown being regarded as omitted.

In such a modified device, the recesses could have any convenient circumferential extent by way of example, each could have a circumferential extent of about 1800, 1200 or 900.

Any convenient number of ribs and/or recesses can be incorporated in duct devices according to the invention, the numbers appearing in the accompanying drawings being merely illustrative.

Duct devices according to the invention can be made by a lost moulding technique. They can be made by pressing refractory material around shaped cores which replicate (negatively) the desired internal profile, and then firing the shaped material. Alternatively, they can be made from cementitious castable material cast around such cores. The cores are made of a material consumable by solvents or by combustion when subjected to firing temperatures. One suitable material, by way of example, is polystyrene.

As indicated hereinbefore, in use of the devices according to the invention eddies 26 will form in the recesses or downstream of the ribs.

Undissolved particles will be caught up in the eddies and may coalesce and ultimately deposit on the duct wall clear of the main flow of molten metal. Accumulated deposits will adhere well to the duct wall and are unlikely to become detached therefrom thanks to the protective effect of the ribs or recesses which shield the deposit from the main melt flow, and which may also serve as keying means to assist attachment of the accumulated deposits to the duct walls.

Claims (20)

CLAIMS:

1. A device for controlling the deposition of, and therby for -removing, undissolved particles such as oxides from a flowing molten metal, comprising a refractory body defining a duct passage the wall of which, at intervals therealong, is so configured as to provide pockets therein and/or protrusions projecting into the passage, the wall configuration being such as to create a plurality of locations in which, in use, eddies form and wherein said particles collect.

2. A device according to claim 1, wherein the duct wall has a plurality of recesses therein defining the said pockets.

3. A device according to claim 2, wherein the recesses are annular.

4. A device according to claim 2 or claim 3, wherein the recesses are of square or rectangular form.

5. A device according to claim 1, wherein the duct wall has a plurality of protrusions thereon projecting radially inwardly of the passage, the protrusions in use serving to create a metal flow rate profile wherein the flow rate increases towards the middle of the duct, and eddies downstream of the protrusions.

6. A device according to claim 5, wherein the protrusions comprise annular ribs encircling the duct passage.

7. A device according to claim 1 or claim 5, wherein the protrusions each comprise a radial rib, which extends around part only of the circumference of the duct passage.

8. A device according to claim 7, wherein the circumferential location of each radial rib is non-coincident with the circumferential locations of the nearest adjacent ribs to either side thereof.

9. A device according to claim 8, wherein the circumferential locations of the ribs are such that, when the passage is viewed end-on, they are seen to overlap circumferentially and thereby form a passage central portion of circular form.

10. A device according to claim 8, wherein each rib has a circumferential extent such that it extends around less than half the passage wall, and the ribs are located alternately to one side and the other of a central longitudinal plane through the duct passage.

11. A device according to any of claims 6 to 10, wherein the duct wall has a recess therein associated with a rib.

12. A device according to any of claims 6 to 10, wherein the duct wall has a recess therein associated with and adjacent each of the ribs.

13. A device according to claim 11 or claim 12, wherein the duct passage has an entry end and an outlet end, and the said recess is located downstream of the associated rib.

14. A device for controlling the deposition of, and thereby for removing undissolved particles from a flowing molten metal, substantially as herein described with reference to and as shown in Figs. 3 to 5 of the accompanying drawings.

15. A device according to any of claims 1 to 14, which comprises a collector nozzle, an extended or submerged pour tube or a shroud tube.

16. A molten metal supply vessel having a pouring opening with a device according to any of claims 1 to 15 in flow communication therewith.

17. A method of making a device according to any of claims 1 to 15, wherein a refractory material is moulded or cast about a core which is a negative replica of the wall configuration, the said core being made of a material which is combustible or soluble in a solvent to enable its removal after the moulding or casting has achieved a shape-retaining state.

18. A method of controlling the deposition of, and thereby removing, undissolved particles such as oxides from a flowing molten metal, wherein the metal is passed along a duct passage configured at intervals therealong to create localised particle-collecting regions wherein eddies are developed adjacent a wall defining the duct passage.

19. A method of controlling the deposition of, and thereby removing, undissolved particles such as oxides from a flowing molten metal, wherein the metal is passed through a device according to any of claims 1 to 15.

20. A method according to claim 18 or claim 19, wherein the particles are removed while molten metal such as steel is teemed from a supply vessel to a receiving vessel or mould.