GB2149699A - Method and apparatus for avoiding vortexing in a bottom pour vessel - Google Patents

Abstract

The formation of a vortex in a molten metal bath over the pour opening (30) from a teeming vessel is eliminated by the injection of fine, inert gas bubbles into the bath to form about the pour opening (30) a dense curtain (56) of flowing gas that converges toward the surface of the bath. For this purpose, a wellblock (22) includes permeable refractory rods (36) spaced around the pour opening with their lower inlet ends in a gas manifold (40) and their upper discharge ends (38) at the bottom of the teeming vessel. An alternative to the rods (36) is an annular member. <IMAGE>

Description

SPECIFICATION Method and apparatus for avoiding vortexing in a bottom pour vessel The invention relates to the avoidance of vortexing in the teeming of molten metal from a bottom pour vessel, such as a ladle or tundish of a continuous caster.

In the teeming of metal from such vessels through pour openings disposed in the bottom thereof, the flow circulation pattern that develops includes a swirling motion which increases in intensity as the metal approaches the pour opening thereby establishing a vortex at the opening. As the level of the molten bath in the vessel drops with continued pouring, the vortex core may be caused to extend the full height of the bath. This problem is most acute in tundishes from which metal is teemed into the mold of a continuous caster, since the bath depth in such vessels is shallow, being rarely greater than thirty-three inches (0.84m), andvortexing commonly occurs at depths as great as twenty-one inches (0.53m).

Vortexing is to be avoided in teeming operations, especially those associated with continuous caster installations, because of its potential for inducing floating scum material, such as particulate slag or non-metallic products of deoxidation or reoxidation of the metal, into the caster mold, leading to degradation of product purity and the possibility of metal imperfections adjacent the skin of the strand product leaving the mold that can result in rupture of the skin and breakout of the still-molten material occupying the region of the strand interiorly of the skin. Moreover, under conditions of severe vortexing, as when the hollow vortex core extends from the free surface of the bath to the discharge end of the pour passage to the mold, air can be aspirated through the core resulting in undesirable reoxidation of the metal teemed to the mold.

In the past, avoidance of the harmful effects of vortexing in tundishes that supply molten metal to continuous casters has been achieved by insuring that the tundish has a continuously available supply of metal from ladles feeding the tundish which will prevent the tundish bath level from falling below the level at which vortexing becomes a problem. Should an obstruction occur in the supply of metal from the ladle, however, as might happen as a result of a malfunction in the ladle pouring operation or in the unavailability of a fresh ladle from which to feed the tundish, it has been necessary either to terminate caster operation or to tolerate a product of reduced purity.The problem of vortexing that occurs in the pouring from ladles into the ingot molds, on the other hand, must similarly be accommodated by either tolerating a product of reduced quality or by terminating pouring from the ladle before vortexing occurs.

All of the above solutions to the problem are costly since they result either in a product of reduced commercial value or in a reduction in the yield of the metal-producing operation.

Recently, there have been developed for insertion in the teeming vessel pour opening, devices, such as nozzle inserts containing a fluted pour passage, intending to prevent vortexing in the metal bath.

Such devices, a typical one of which is shown and described in United States Patent Specification No.

4,079,868, are not totally dispositive of the concerned problem because, although effective to prevent vortexing over short periods of metal pouring, the devices are subject to flowing metal rapidly eroding the material oftheflute-defining ribs of the nozzle thereby rendering the devices ineffective to produce their desired function. Such apparatus is incapable of use in a tundish for supplying metal to the mold of a continuous caster where pouring is conducted unabated over several hours.

It is to the alleviation or elimination of the aforementioned problems, therefore, to which the present invention is directed.

According to one aspect of the invention, there is provided a method of alleviating the occurrence of vortexing during the pouring of molten metal through the opening of a bottom pour vessel comprising the steps of injecting a gaseous fluid from about the vessel opening and directing said gaseous fluid in counterflow relation to the flow of molten metal along a convergent flow path to form a tapering gas curtain in superposed, surrounding relation to the inlet of said opening. The gaseous fluid is an inert gas, preferably argon, orthe like, and is injected into the body of molten metal within the vessel at the rate of from about 0.2 to about 0.5 scfm (944 x 10-7 to 2361 x 10-7 m3/sec) in the form of fine gas bubbles.Injection is such as to establish a generally hollow conical or frusto-conical gas curtain surrounding the vessel pour opening, the initial angle of inclination of the sides of the curtain being a nominal thirty degrees with respect to the horizontal at its origin and gradually decreasing as the surface of the bath is approached. The effect of the gas curtain is to establish a flow circulation pattern in the molten metal bath, and extending to the surface thereof, which is in opposition to the vortexproducing, inward swirl that is the general tendency of the metal flowing toward the discharge passage.

Thus, the inward swirling motion of the metal over the metal flow passage is destroyed and, concomitantly, the danger of establishing a vortex avoided.

According to another aspect of the invention, apparatus useful in the practice of the above inventive method is provided in the form of wellblock apparatus for use at the opening of a bottom pour vessel for teeming molten metal comprising a body of low permeability refractory material for disposition in the bottom of said vessel and formed to contain an axial through-opening defining the vessel pour opening; a refractory material of higher permeability than said low permeability material annularly arranged in said body of low permeability material in concentrically spaced relation to said vessel pour opening and defining a passage through said low permeability material for the supply of gaseous fluid to the interior of said vessel; means for supplying gaseous fluid to said passage; and means at the discharge end of said gaseous fluid flow passage effective to direct the flow of gaseous fluid injected into said vessel convergently in the direction of the free surface of the body of molten metal in said vessel.

For a better understanding of the invention, its operating advantages and the specific objectives obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred embodiment thereof. In these drawings: Figure l is a vertical sectional representation of a molten metal teeming vessel incorporating the present invention; Figure2 is a more detailed vertical enlarged sectional view of the wellblock apparatus of Figure 1; Figure 3 is a plan view of the wellblock apparatus of Figure 2; and Figure 4 is a plan view of an alternative form of wellblock apparatus incorporating the invention In Figure 1 of the drawings, there is illustrated a bottom pour teeming vessel, here shown as tundish 10, containing a body 12 of molten metal for pouring to a receiver (not shown) that may be the mold of a continuous caster.The tundish 10 comprises a metal shell 14 in which the bottom 16 and sides 18 thereof are lined with a refractory lining 20. The lining 20 in the bottom 16 of tundish 10 contains a wellblock apparatus 22 constructed according to the invention and set in an opening 24 provided in the vessel lining.

The wellblock apparatus 22 comprises a block 26 of refractory material that is set in the opening 24 by a layer of refractory cement 28. Preferably, the block 26 is formed of a castabie refractory consisting of tabular alumina aggregate and high-strength, highpurity calcium-aluminate cement with a bulk density of about 170 pounds per cubic foot to impart the apparatus with good strength and resistance to chemical and mechanical attack from ferrous and non-ferrous metals and slag at service temperatures up to about 3300"F. The block 26 illustrated in the Figures is square in cross-section but other shapes, such as rectangular, circular or elliptical, may also be employed.The pour opening 30 through the block 26 is axially disposed having an upper portion 32 formed of generally arcuate surface that converges into the tapered wall 34 of the pour opening 30. The line of tangency between the wall 34 and portion 32 defines the inlet of the pour opening and is indicated by the diameter Din Figure 2.

According to the invention, the wellblock apparatus 22 incorporates means for injecting inert gas into the bath of molten metal within the vessel 10 that produces a beneficial alteration of the fluid flow conditions in the region of the bath above the well block pour opening 30 thereby resulting in the prevention or contol of vortexing of the molten bath.

In the embodiment of the invention illustrated in Figures 2 and 3 of the drawing, the inert gas injection means comprises a plurality of upstanding, circumferentially spaced rods 36 embedded in the block 26 and disposed in concentrically spaced relation about the pour opening 30. The discharge ends of the rods 36, indicated as 38 in the Figures, are inclined at an angle from about twenty degrees to about forth degrees with respect to the horizontal, with an angle of about thirty degrees being most preferred. The rods 36 may be formed by a dry pressing process of a refractory material having a higher permeability than that of the material that forms the block 26.

Material, such as magnesite, alumina, fireclay, mullite, zircon and zirconia, may be suitable for use in the formation of the rods although selection of the material will depend on the physical and metallurgical properties that may be required in a given application, as well as the length of service life desired. The material for fabrication of the rods 36 must be sufficiently gas-permeable as will produce a stream or streams of very fine gas bubbles on entering the molten bath. The material, therefore, should have an apparent porosity of from twentyfive to thirty percent.

In order to supply inert gas, preferably argon, to the gas passages defined by the rods 36, an annular manifold, indicated generally at 40, is provided. The manifold 40 consists of a plurality of annular, metal plates 42,44,46 and 48 forming the top. bottom and sides of the manifold. The top plate 42 is apertured at circumferentially spaced locations to supportingly receive the lower ends of the rods 36. An annular mounting plate 50 disposed within the manifold and vertically spaced from the top plate 42 forms a base upon which the lower ends of the rods 36 abut.

Openings 52 in the plate 50, axially aligned with the ends of the respective rods 36, permit passage of the gas from the interior of the manifold 40 into the flow passages defined by the rods. A supply tube 54 attached at one end to the manifold side plate 46 in communication with the manifold interior and connected at its other end to a gas source exteriorly of the vessel 10 provides a controllable supply of gas to the manifold 40.

In operation, a tundish 10 with the wellblock apparatus 22 according to the invention installed in its bottom lining and with the pour opening 30 of the wellblock closed by either a sliding gate valve or a stopper rod, both of which are well-known in the art and, accordingly not shown in the drawings, is filled with molten metal. When the pour opening 30 is opened to effect flow of metal from the tundish 10, inert gas supply to the manifold 40 is commenced.

Gas should be supplied to the manifold at flow rates of about 0.2 to 0.5 scfm (944 x 10-7 to 2361 x 10-7m3/sec) in order to effectively prevent vortexing of the molten bath as hereinafter explained. Gas flows greater than about 0.5 scfm (2361 x 10-7m3/ sec) are to be avoided since excessive gas injection is apt to produce turbulence in the bath that tends to emulsify scum or slag on the free surface of the bath thereby facilitating the transfer of these undesirable impurities with the metal passed through the pour opening. The gas exits the discharge ends 38 of the rods 36 along inclined flow paths thereby to establish an essentially hollow, conical gas curtain, indicated as 56 in Figure 1,formed of fine gas bubbles directed toward the free surface of the bath 12.1 In flowing toward the bath surface, the gas in the curtain 56 provides a quiescent, stable, dense bubble plume tending to induce in the metal flow to the pour opening 30 a flow, indicated by arrows 58 in Figure 1, in the opposite direction and countering the forces that establish the vortex in the molten bath, thereby controlling or eliminating the vortex and its undesirable consequences.

In order to achieve the most desirable performance of the invention, it has been determined that the optimum circle for location of the rods 36 about the pour opening 30 should have a diameter of 1.75 to 2.5 times the inlet diameter, D, of the pour opening. Additionally, a gas curtain 56 of optimum characteristics will be achieved if the effective diameter of the respective rods 36 at their discharge ends is equal to from about .10 to about .15 times the diameter of the aforementioned circles on which the rods are located.

An alternative form of the invention is illustrated in Figure 4 of the drawings where like numerals are employed to designate like elements of the embodiment of Figure 3. In the embodiment of Figure 4, the rods 36 of the Figure 3 embodiment are replaced by an annular cylindrical insert 60 formed of refractory material which may be the same material as that suggested above for the formation of the rods 36. It will be appreciated that a vertical sectional view of the Figure 4 embodiment is identical to that of Figure 2 with the exception that the rods 36 are replaced by the insert 60. It should further be appreciated that the upper end of the insert 60 that defines the discharge end of the annular gas passage formed by the insert is inclined to a comparabie extent as the inclination of the discharge ends 38 of the respective rods 36 as described above.

The invention further contemplates, where the physical dimensions of the well block are insufficient for embedding the rods 36 or insert 60 together with the associated manifold structure 40 within the block 26, disposing the gas-injecting elements of the invention in the vessel lining 20 in surrounding relation to the block.

Accordingly, there is provided by the present invention an effective method and apparatus for reducing or eliminating the problem ofvortexing of the molten metal bath above the vessel pour opening and of removing the attendant danger of transferring with the poured metal particulate impurities or aspirated air from the vessel interior into the receiving mold. Unlike comparable apparatus of the prior art, the apparatus of the invention is capable of performing its intended function over extended periods of operation thereby rendering it particularly useful for application in continuous casting installations.

Additionally, the creation of an inert gas curtain in the manner described through which the molten metal must pass prior to entering the vessel pour opening provides a secondary benefit of serving to remove suspended non-metallic inclusions from the metal passed from the teeming vessel. This supplementary cleansing action further insures the quality of the product of the teeming operation and is enhanced by the fact that the direction of the injected inert gas is away from the vessel pour opening thereby removing the danger present where gas is injected into the pour opening or its immediate vicinity of aspirating impurities from the vessel into the teemed metal flow stream.

Claims (18)

1. A method of alleviating the occurrence of vortexing during the pouring of molten metal through the opening of a bottom pour vessel comprising the steps of injecting a gaseous fluid into the molten metal from about the vessel opening and directing said gaseous fluid in counterflow relation to the flow of molten metal along a convergent flow path to form a tapering gas curtain in superposed, surrounding relation to the inlet of said opening.

2. A method as claimed in claim 1 in which the injected gaseous fluid produces a flow of fine gas bubbles.

3. A method as claimed in claim 1 or claim 2 in which the injected gaseous fluid is an inert gas.

4. A method as claimed in claim 3 in which said inert gas is argon.

5. A method as claimed in claim 3 or claim 4 in which said inert gas is injected at the rate of 0.2 to 0.5 scfm (944 x 10-7to 2361 x 10-7m3/sec).

6. A method as claimed in claim 5 in which the gaseous fluid flow path has an initial angle of inclination with respect to the horizontal of from 20 to 40 degrees.

7. A method as claimed in claim 6 in which the diameter of the base of said conical gas curtain is 1.75 to 2.50 times the inlet diameter of said vessel opening.

8. A method as claimed in claim 7 in which said gas curtain has an effective annular width at its base of .10 to .15 times the base diameter of said curtain.

9. Wellblock apparatus for use at the opening of a bottom pour vessel forteeming molten metal comprising a body of low permeability refractory material for disposition in the bottom of said vessel and formed to contain an axial through-opening defining the vessel pour opening, a refractory material of higher permeability than said low permeability material annularly arranged in said body of low permeability material in concentrically spaced relation to said vessel pour opening and defining a passage through said low permeability material for the supply of gaseous fluid to the interior of said vessel, means for supplying gaseous fluid to said passage, and means at the discharge end of said gaseous fluid flow passage effective to direct the flow of gaseous fluid injected into said vessel convergently in the direction of the free surface of the body of molten metal in said vessel.

10. Wellblock apparatus as claimed in claim 9 in which the annular passage formed by said higher permeability refractory material has a diameter of 1.75 to 2.50 times the diameter of said vessel pour opening.

11. Wellblock apparatus as claimed in claim 9 or claim 10 in which the effective width of said annular passage is .10 to .15 times the diameter of said annular passage.

12. Wellblock apparatus as claimed in claim 9,10 or 11 in which said higher permeability refractory material has an apparent porosity of 25 to 30 percent.

13. Well block apparatus as claimed in any of claims 9 to 12 in which the discharge end of said annular passage is defined by an inclination of the discharge end surface of said higher permeability refractory material, said inclination being 20 to 40 degrees with respect to the horizontal.

14. Well block apparatus as claimed in any one of claims 9 to 13 in which said higher permability refractory material is in the form of a hollow cylindrical body in concentrically spaced relation to said vessel pour opening.

15. Wellblock apparatus as claimed in any one of claims 9 to 13 in which said higher permeability material is in the form of a plurality of circumferentially spaced cylindrical rods concentrically spaced about said vessel pour opening.

16. Wellblock apparatus as claimed in claim 14 or claim 15 including means embedded in said low permeability refractory material forming an annular manifold concentrically spaced from said vessel pour opening, said manifold including means for securing said high permeability refractory material in fluid communication with the manifold interior, and means for attaching said gaseous fluid supply means to said manifold.

17. A method of alleviating the occurrence of vortexing substantially as hereinbefore described with reference to the accompanying drawings.

18. Wellblock apparatus, constructed and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.