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(11) | EP 0 175 132 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Fluid flow control structure for tundish |
| (57) An elongated tundish (10) for the continuous strand casting of molten metal has a
plurality of outlet openings (22), (23), (24), (25) arranged in a row extending in
the longitudinal direction of the tundish. There is an elongated dam (30) extending
in said longitudinal direction between the row of outlet openings and the location
of impact (28) on the tundish bottom of a ladle nozzle stream of molten metal. The
dam provides uniform residence times in the tundish for molten metal exiting the tundish
at inner and outer outlet openings in the row of openings. Fluid flow control structure
(39) is located between the dam and the tundish side wall adjacent the row of outlet
openings to avoid a dead zone in that region. |
[0001] The present invention relates generally to apparatus employed in the continuous strand casting of molten metal, such as steel, and more particularly to a tundish which contains structure for controlling the flow of molten metal therein.
[0002] In continuous strand casting, molten metal is poured from a ladle into a tundish having a multiplicity of outlet openings from which exit a multiplicity of molten metal strands each directed into a mould through which the strand moves, and the strand is solidified as it moves through the mould. It is desirable that each of the strands exiting from the tundish be of substantially uniform quality and composition with a minimum of inclusion-type impurities. This requires that the molten metal exiting through each of the outlet openings in th tundish be subjected to substantially the same amount of mixing action in the tundish, have essentially the same residence time in the tundish, and be subjected to a sufficient amount of slagging action to remove inclusion impurities from the molten metal to the extent desired.
[0003] Inclusion impurities are removed in a tundish by floating a slag cover on top of the molten metal in the tundish and subjecting the molten metal to a period of contact with the slag cover during which the molten metal is not itself undergoing a mixing action. This can be accomplished by providing so-called "plug flow" to the molten metal, as will be explained below in more detail.
[0004] A certain amount of mixing action is desirable, before the molten metal undergoes plug flow to the outlet openings, because this contributes to uniformity in the composition of the metal exiting through the various outlet openings in the tundish.
[0005] It is also desirable to minimise the locations in a tundish where there are dead zones of molten metal, that is, locations where there is neither a mixing action nor a plug flow. Dead zones are undesirable because the molten metal at those locations undergoes little or no mixing, and dead zones can result in the formation in the tundish of skulls (solidified volumes of metal).
[0006] A tundish of the general type which the present invention is intended to improve is elongated and comprises a pair of side walls disposed substantially in the longitudinal direction of the tundish and a pair of opposite end walls each extending in a lateral direction between the side walls. The tundish has a bottom and a substantially open tundish top. The tundish bottom has a plurality of molten metal outlet openings all of which are aligned in a row extending longitudinally between the tundish end walls. In one kind of tundish, there are a pair of inner outlet openings and a pair of outer outlet openings, and all of the openings in the row of outlet openings are generally equally spaced from each other.
[0007] Molten metal is directed from a ladle through a ladle nozzle toward the tundish bottom at a ladle nozzle stream impact location laterally spaced from the row of outlet openings and disposed between the end walls at a substantial distance from each end wall, typically midway therebetween. Molten metal impinges against the ladle bottom at that location and flows from there along the ladle bottom to other areas of the tundish.
[0008] Molten metal exits as strands from all of the outlet openings in the tundish bottom and passes into the solidification moulds. However, in a tundish of the type described above, the quality of molten metal exiting from the inner pair of outlet openings differs from the quality of molten metal exiting from the outer pair of outlet openings, and this is undesirable. In addition, there is a substantial dead zone volume for the molten metal when employing a tundish of the type described above. Another drawback is that the stream quality for the inner strands is poor in that it displays significant "roping", a form of turbulence in the stream. Roping is undesirable because a stream with roping has more surface area exposed to the surrounding atmosphere than a stream without roping, thereby increasing the stream's susceptibility to oxidation and rendering the molten metal "dirtier" which is undesirable.
[0009] The defects described in the preceding paragraph are due to the fact that there is a short circuiting of molten metal to the inner outlet openings and that there is a relatively low volume fraction of plug flow to the inner outlet openings. Plug flow refers to molten metal (or fluid) which flows as a plug from a location where it has undergone mixing to the outlet opening. This is flow as in a pipe. A volume of molten metal undergoing ideal plug flow does not undergo mixing or have turbulence within itself. As a result, inclusions can be removed from that volume of metal into a slag cover atop the bath of molten metal in the tundish. Iri a volume of molten metal undergoing mixing action within itself, this cannot occur.
[0010] Because the plug flow volume to the inner outlet openings is relatively low, both in an absolute sense and in comparison to the plug flow volume to the outer pair of outlet openings, the slagging out of inclusions from molten metal exiting through the inner pair of outlet openings is both less than desirable and less than occurs in the molten metal exiting thorugh the outer pair of outlet openings.
[0011] The drawbacks and defects associated with a tundish of the type described above are greatly reduced or, in some cases, eliminated in a tundish constructed in accordance with the present invention.
[0012] In accordance with the present invention, the tundish is provided with an elongated dam extending upwardly from the tundish bottom between the ladle nozzle stream impact location and the row of outlet openings, the dam extending longitudinally between the end walls of the tundish, and the dam having a pair of opposite ends each spaced from a respective end wall of the tundish.
[0014] The elongated dam substantially equalises the residence time in the tundish of molten metal exiting through the inner outlet openings with the residence time of the molten metal exiting through the outer outlet openings. The elongated dam prevents short circuiting of molten metal to the inner outlet openings and increases the residence time in the tundish of molten metal exiting through the inner outlet openings, thereby increasing the floating out of inclusion impurities from the molten metal to a slag layer atop the molten metal in the tundish. The elongated dam also substantially increases the plug flow volume fraction of molten metal exiting the tundish through the inner outlet openings, thereby contributing to the floating out of inclusion impurities.
[0015] The parameters of the elongated dam may be controlled in accordance with the present invention to optimise the flow characteristics of the molten metal exiting the tundish through both the inner and outer outlet openings.
[0016] Additional flow control structure, associated with the elongated dam, may be provided to reduce the dead zone volume fraction of molten metal in the tundish.
[0017] The elongated dam also eliminates roping in the stream of molten metal exiting from the inner outlet openings.
[0018] Other features and advantages are inherent in the structure claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying diagrammatic drawings.
FIGURE I is a diagrammatic plan view of a tundish with fluid flow control structure in accordance with an embodiment of the present invention;
FIGURE 2 is an enlarged fragmentary sectional view taken along line 2--2 in Figure I;
FIGURE 2a is a fragmentary sectional view showing a variation of some of the structure shown in Figure 2;
FIGURE 3 is a sectional view taken along line 3--3 in Figure I;
FIGURE 4 is a diagrammatic plan view of another embodiment in accordance with the present invention;
FIGURE 5 is a sectional view taken along line 5--5 in Figure 4;
FIGURE 6 is a reduced, diagrammatic plan view of a further embodiment in accordance with the present invention; and
FIGURE 7 is a reduced, diagrammatic plan view of still another embodiment in accordance with the present invention.
[0019] Referring initially to Figures 1-3, indicated generally at 10 is an embodiment of a tundish with fluid flow control structure in accordance with an embodiment of the present invention. Tundish 10 is elongated and comprises a pair of side walls 11, 12 disposed substantially in the longitudinal direction of the tundish and a pair of opposite end walls 17, 18 each extending in a lateral direction between side walls I I, 12.
[0020] Side wall 12 comprises a centre side wall portion 13 and a pair of end portions 14, 15 converging from tundish end walls 17, 18, respectively, toward centre portion 13 of side wall 12. Side wall 12 defines what is essentially a delta-shaped tundish portion.
[0021] Tundish 10 also comprises a bottom 19 and an open top 20 (Figure 3). Tundish bottom 19 has two pairs of molten metal outlet openings, an inner pair 22, 23 and an outer pair 24, 25. All of the outlet openings 22-25 are aligned in a row extending longitudinally between end walls 17, 18. Openings 22-25 all have substantially the same diameter.
[0022] Illustrated in Figure 3 at 27 is a nozzle or spout from a ladle for containing molten metal which is directed through spout 27 toward a ladle nozzle stream impact location 28 on tundish bottom 19 (Figure I). Impact location 28 is normally laterally spaced from the row of outlet openings 23-25 and is disposed between end walls 17, -18 (or the lateral extension thereof) a substantial distance from each end wall.
[0023] An elongated dam 30 extends upwardly from tundish bottom 19 between ladle nozzle stream impact location 28 and the row of outlet openings 23-25. Dam 30 extends longitudinally between end walls 17, 18 and has a pair of opposite ends 34, 35 each spaced from a respective end wall 17, 18. Dam 30 also comprises a top surface 31 and a pair of dam side walls 32, 33 each facing a respective tundish side wall I I, 12.
[0024] As shown in Figure 3, dam 30 extends upwardly from tundish bottom 19 to a height which is substantially uniform from one dam end 34 to the other dam end 35.
[0025] All of the tundish walls 11, 12 and 17, 18 extend upwardly from tundish bottom 19 to a predetermined, uniform height. Some examples of the relative dimensions of dam 30 and certain parts of tundish 10 are described below.
[0026] Dam 30 extends upwardly from tundish bottom 19 to a maximum height substantially less than one-half the predetermined height of the tundish walls. Preferably the dam's maximum height is about 25-35% of the height of the tundish walls.
[0027] Each of the two inner outlet openings 22, 23 is longitudinally spaced from the other and each is located on a respective opposite side of the longitudinal mid-point of tundish 10 at substantially the same distance therefrom as the other inner outlet opening. Dam 30 has a length greater than the distance between the respective centre lines of inner outlet openings 22, 23 but substantially less than the distance between the outer pair of outlet openings 24, 25. Preferably, dam 39 has a length not substantially greater than the distance between the longitudinally outermost points 51, 52 on inner outlet openings 22, 23.
[0028] The distance, from centre to centre, between a respective outer outlet opening 24 or 25 and the closest inner outlet opening 22 or 23 is substantially the same as the distance between the pair of inner outlet openings 22, 23. The length of dam 30 is substantially less than 150% of the distance between adjacent openings 24, 22 or 22, 23 or 23, 25. Preferably dam 30 has a length about 120-135% of the distance between the centre lines of inner outlet openings 22, 23.
[0029] The width of dam 30 is substantially less than the distance between the centre line of the dam and the centre line of the row of outlet openings 22-25. The row of outlet openings is disposed between dam 30 and side wall I I. The maximum distance, in a lateral direction, between the centre line of the row of outlet openings and side wall II is less than the maximum distance, in the lateral direction, between the centre line of the row of outlet openings and other side wall 12. In the embodiment of Figures 1-3, the maximum distance to other side wall 12 would be at centre portion 13 of wall 12. The distance between the centre line of dam 30 and the centre line of the row of outlet openings 22-25 is less than the distance, in a lateral direction, between the centre line of the row of outlet openings and side wall II.
[0030] There is a central area 38 on tundish bottom 19 defined substantially by side wall I and dam 30 at opposite sides of the area and by inner outlet openings 22, 23 at opposite ends of area 38. Absent flow control structure of the type about to be described, this area will constitute a zone with substantially no flow of molten metal, which is undesirable. Therefore, a substantial portion of central area 38 preferably is occupied by a monolithic flow control structure 39 which extends upwardly from tundish bottom 19, abuts against both dam 39 and side wall II, and is spaced from each of the inner outlet openings 22, 23. Flow control structure 39 renders the dead flow volume of molten metal adjacent tundish bottom 19 at area 38. In the embodiment illustrated in Figures 1-3, monolithic flow control structure 39 comprises an inner layer 40 adjacent dam 30, an intermediate layer 41 adjacent inner layer 40, and an outer layer 42 adjacent side wall I I.
[0031] As noted above, dam 30 has a maximum height substantially less than the height of side wall I I. Monolithic flow control structure 39 has a height which increases progressively from side wall I to dam 30, but the maximum height of flow control structure 39 is less than the height of dam 39 anywhere, which also makes it less than the height of dam 30 at any location on dam 39 where structure 39 abuts dam 30. In the embodiment illustrated in Figures 1-3, the height of flow control structure 39 increases progressively in a stepped fashion from side wall I I to dam 30, but the height may also increase progressively along a slope from side wall I to dam 30.
[0032] Monolithic flow control structure 39 has a dimension, in the longitudinal direction of tundish 10, which increases progressively from side wall I to dam 30. In the embodiment illustrated in Figures 1-3, the increase in dimension is in discrete stages from outer layer 42 to inner layer 49 but monolithic structure 39 could also have side walls diverging in straight lines from tundish side wall I I to dam 30.
[0033] In another embodiment, central area 38 may be substantially occupied by a monolithic flow control structure having unchanging dimensions in all directions (e.g. a block) rather than progressively changing in height and in the longitudinal direction as does flow control structure 39. Alternatively, the flow control structure at central area 38 may have a constant height with changing dimensions in the longitudinal direction, or vice versa.
[0034] Monolithic flow control structure 39 buttresses dam 39 against fluid pressure exerted against side 33 of dam 30 as well as reducing dead zone volume in area 38 of the tundish bottom.
[0035] Dam 30 equalises (a) the residence time in the tundish of molten metal exiting through inner outlet openings 22, 23 with (b) the residence time of molten metal exiting through outer outlet openings 24, 25. This avoids short circuiting of molten metal to inner outlet openings 22, 23 and the disadvantages associated therewith. Such short circuiting would occur in a tundish 10 without dam 30. Dam 30 also substantially increases the residence time of molten metal exiting tundish 10 through inner outlet openings 22, 23, compared to a tundish without a dam 30, thereby increasing the floating out of inclusion impurities from the molten metal to a slag layer atop molten metal in tundish 10.
[0036] Dam 30 substantially increases the plug flow volume fraction of molten metal exiting tundish 10 through inner outlet openings 22, 23, compared to a tundish without a dam 30, thereby contributing to said floating out of inclusion impurities.
[0037] In the embodiment of Figures 1-3, located on bottom 19 of tundish 10 is a pouring pad 43 disposed between dam 30 and side wall 12, at centre portion 13 thereof. Pouring pad 43 absorbs the impact of the pouring stream issuing from ladle nozzle 27 and prevents wear on tundish bottom 19.
[0038] A variation of the embodiment illustrated in Figure 2 is shown in Figure 2a wherein the dam 30a tapers from its bottom toward its top, e.g. a decrease in width of about 25% from dam bottom to dam top. In this embodiment, flow control structure 39 includes an inner layer 40a having a bevelled surface 60a abutting the adjacent sloped surface 32a of tapered dam 30a to accommodate the taper. Similarly, there is a pouring pad 43a haing a bevelled end 61a abutting adjacent sloped surface 33a of dam 30a to accommodate the taper. The taper on dam 30a and the abutting bevelled surfaces 60a and 61a cooperate to form a keying structure which holds dam 30a in place and better prevents it from being floated out of place by the action of molten metal in the tundish.
[0039] Figures 4-5 illustrate another embodiment in which the tundish is essentially identical to tundish 10 in the embodiment of Figures 1-3. The principal difference between the embodiment illustrated in Figures 4-5 and that illustrated in Figures 1-3 resides in the dam 44 in the embodiment of Figures 4-5. Dam 44 has two top surface portions 45, 46 converging toward the centre of the dam to define a V-shaped dam. Otherwise, the dam is essentially the same as dam 30 in the embodiment of Figures 1-3.
[0040] The embodiments of tundish illustrated in Figures 1-3 and Figures 4-5 have a so-called delta shape in plan view. Other embodiments of a tundish in accordance with the present invention may employ a rectangular shape in plan view (tundish 47 in Figure 6) or they may have a T-shape in plan view (tundish 48 in Figure 7) wherein the ladle nozzle stream impact location 28 is in an appendage 49 constituting a part of tundish 48.
[0041] In all of the embodiments, tundish 10, the various dams and the other flow control structure are composed of refractory material. Tundish 10 has an exterior shell 21 (Figure 2) composed of steel.
[0042] Set forth below are tables comparing the fluid flow characteristics of tundishes employing fluid flow control structure in accordance with the present invention and of a tundish without such fluid flow control structure. The data reflected by the Tables were obtained on laboratory-scale models which are approximately one-third the size of a commercial-size tundish employed in commercial steel-making practices. The fluid employed in obtaining the results was water rather than molten steel, but the results obtained would be applicable to molten steel because the kinematic viscosities for (I) water and (2) molten steel are comparable.
[0043] Table I reflects data obtained with a tundish 10 having a dam 30 and monolithic flow control structure at tundish central area 38. The data reflected in Table I pertains to an embodiment wherein the monolithic flow control structure in dead zone area 38 differs some from that illustrated at 39 in the embodiments of Figures 1-3 in that the monolithic flow control structure reflected by the data in Table I does not have progressively increasing height and width but has a uniform height and a uniform width throughout.
[0044] Table II reflects data obtained employing a tundish 10 with elongated dam 30, but without monolithic flow control structure at area 38. Table III reflects data obtained employing the embodiment of Figures 4-5 utilising a dam 44 having a V-shaped top surface and without monolithic flow control structure at central area 38. Table IV reflects data obtained with a tundish 10 employing no dam or other flow control structure.
[0045] The approximate dimensions, in millimetres, of the scale model of tundish 10 are the same for all four Tables and are set forth below:
[0046] The approximate dimensions, in millimetres, of scale model dam 30 employed in connection with the data obtained in Tables I and II and other relevant dimensions are set forth below:
[0047] The approximate dimensions, in millimetres, of the scale model monolithic flow control structure at area 38, employed in connection with the data obtained in Table and other dimensions relevant to the embodiment reflected in Table I ae set forth below:
[0048] The approximate dimensions, in millimetres, of scale model V-shaped dam 44 employed in connection with the data obtained in Table III and other relevant dimensions are the same as for dam 30 given above except as set forth below:
[0049] Other parameters and conditions relevant to the data reflected by Tables I-IV are set forth below:
[0050] In the Tables, residence times, in seconds, were determined by adding a dye to the fluid (water) directed into the tundish. Minimum residence (Min. Res.) time indicates the time required for the dye to appear in the tundish exit strands after the injection of dye into the ladle stream from ladle nozzle 27. Peak residence (Peak Res.) time indicates the time when the highest level of colour appeared at the tundish exit strands. Both peak and minimum residence times are important in that they share the highest direct correlation with the degree of inclusion float-out in molten steel systems. In the Tables, "submergence depth" indicates the depth to which ladle nozzle 27 extended below the level of the liquid in the tundish, said level being indicated at 50 in Figures 3 and 5. "Vol. Frac." is volume fraction.
[0051] The data in Table IV indicate that, with no flow control structure in the tundish, there is a significant amount of short circuiting of fluid to the inner strands, as indicated by the minimum residence timesfor the inner and outer strands. In addition, the peak and mean residence times also show large differences between the inner and outer strands. These differences in flow conditions, if permitted in actual practice, could lead to substantial differences in quality for steel obtained from the inner and outer strands.
[0052] There is almost no plug flow volume for the inner strands and a small fraction for the outer strands in Table IV. Plug flow volume is extremely important for inclusion float out. There is significant dead volume for both the inner and outer strands.
[0053] Adjustment of nozzle depth causes little difference in flow conditions for the outer strands, but does slightly affect the amount of dead volume obtained for the inner strands in Table IV. In the latter case, the amount of dead volume appears to be highest at intermediate submergence levels.
[0054] Referring now to Table III, which relates to the V-shaped longitudinal dam, the use of this flow control device reduces the amount of short circuiting to the inner strands, lowers the difference in residence times between the inner and outer strands, and increases the amount of plug flow volume for the tundish, compared to the case. where no flow control devices are used (Table IV). The amount of dead volume obtained is slightly higher than when no control devices are used, and the minimum residence time within the tundish is significantly increased. Overall, these factors should produce better utilisation of the entire tundish volume and improved steel quality and consistency from strand to strand.
[0055] There appears to be little variation in flow residence times or in plug, mixed, and dead volumes due to changing nozzle submergence in Table III. Stream quality from each strand was quite good with this embodiment.
[0056] With regard to Table II (straight longitudinal dam only), the residence time data indicate that this configuration leads to nearly even behaviour between the inner and outer strands. The respective minimum, peak, and mean residence times for the inner and outer strands are all very close. Compared to the differences in residence times between inner and outer strands obtained with the Vshaped longitudinal dam (Table III), the results for the straight dam appear to be better. The minimum residence times for the inner strands are much greater than those obtained for the tundish with no flow control devices (see Table IV).
[0057] Variation in the nozzle depth had a significant impact on the residence times and volume fractions associated with plug, mixed, and dead zone in Table II. Intermediate submergence of the inlet nozzle leads to the least dead volume, but also decreases the minimum residence time. Shallow or deep submergence tend to give similar residence time and volume fraction results. Overall, the use of this longitudinal dam results in more even behaviour between inner and outer strands and in improved residence times and tundish volume utilisation compared to the embodiments reflected by Tables lll and IV. The even behaviour between the inner and outer strands should produce significant metallurgical benefits.
[0058] Certain areas of the tundish of Table II are relatively "dead" with respect to flow. This is reflected by the volume fractions shown in Table II, which indicate that in most cases significant dead volume exists for both the inner and outer strands. Dye tracer studies (where dye was placed directly in various regions of the tundish) were conducted to more clearly define the dead areas that exist in the tundish when a straight longitudinal dam is employed. These tests resulted in the identification of central dead zone area 38. In addition the areas adjacent to the very ends of the tundish at 17, 18 are also very slow flow regions.
[0059] The residence time and flow volume fractions obtained for the embodiment with a straight longitudinal dam and a bricked-in central dead zone area 38 are reported in Table I. Compared to the embodiment reflected by Table II, the central dead zone area in the front of the tundish (i.e. at 38) is greatly reduced. In fact, the region between the two inner outlet openings 22, 23 is a very active flow zone. The liquid is rapidly swept out of this region of the tundish into the highly turbulent impact zone of the ladle stream. The whole central zone of the tundish is a very well mixed region, ideal for tundish additions.
[0060] Some dead volume exists in the tundish near its ends 17, 18, but the total amount of dead volume associated with this configuration of flow control devices (Table I) is generally less than that obtained when just employing the straight longitudinal dam (Table II). In addition, for most cases, it is less than that obtained when no flow control devices are used.
[0061] The residence time data in Table I indicates that the flow behaviour between the inner and outer strands is very similar for all levels of nozzle submergence.
[0062] The use of a straight longitudinal dam with bricked in dead area 38 or similar flow control devices (as at 39 in Figures 1-3) appears to give the best overall flow behaviour of any of the configurations tested, and produces favourable metallurgical performance for all tundish.
[0063] Tests are conducted to determine the effect on the flow behaviour in the tundish of using a shorter or longer straight longitudinal dam. In the first case, the dam was lengthened by about 101 mm to a length of 812 mm, and, in the second case, it was shortened by a similar amount to 6)0 mm. The height and placement of the dams corresponded to that used previously. The results indicate that substantially lengthening or shortening the dam in the embodiment of Figure I does not appear desirable. Lengthening the dam leadsto more dead volume and shorter overall residence times, and shortening the dam leads to greater differences in flow behaviour between the inner and outer strands. In the one-third scale model of the embodiment of Figure I, dam 30 is about 711 mm in length and can be in the range of about 669-752 mm, for example.
[0064] Tests were also conducted to determine how the flow behaviour would be affected if the ladle nozzle stream impact location 28 was varied either towards tundish side wall 12, or towards dam 30. In these tests, impact location 28 was shifted on the scale model either about 38 mm toward the dam or a similar distance toward side wall 12 from the normal location shown in Figure 1, which is about 89 mm from side wall centre portion 13. The results indicate that bringing the ladle stream closer to the dam reduces the dead volume in the tundish, but also increases the difference in residence times between the inner and outer strands. In addition, the volume fraction for plug flow is increased when the inlet stream is shifted toward the dam. Shifting the inlet stream toward the side wall 12 can cause the inlet flow to reach the outer strands before the inner strands.
[0065] These results indicate that the placement of the impact location for the ladle stream can affect the flow behaviour in the tundish. This effect can be good or bad depending on the conditions existing in the tundish. For example, if it is found that the ends of the tundish are cooler than the central regions, then adjustment of the impact location toward side wall 12 should allow warmer metal to reach the tundish ends, and perhaps, reduce skull build up in the cool spots. Other operational considerations may warrant placement of the impact location closer to the dam. ,
[0066] Two tests were conducted to determine the qualitative effect of shutting off either an inner or an outer strand. For these tests, the usual fluid level (212 mm) was maintained, and dye was injected after one outlet opening was plugged. The whole tundish volume remains very active when an inner outlet opening 22 or 23 is plugged. In contrast, when an outer outlet opening 24 or 25 is plugged, the region around the plugged opening is essentially dead to fluid flow. Such behaviour will likely lead to skull formation in this dead zone of the tundish. Therefore, it is more desirable to plug an inner opening than an outer opening.
1. An elongated tundish for the continuous strand casting of molten metal, said tundish
comprising:
a pair of side walls disposed substantially in the longitudinal direction of said tundish and a pair of opposite end walls each extending in a lateral direction between said side walls;
a tundish bottom and a substantially open tundish top;
said tundish bottom comprising a plurality of inner molten metal outlet openings and an outer pair of outlet openings all of which are aligned in a row extending longitudinally between said end walls;
said tundish bottom having a ladle nozzle stream impact location laterally spaced from said row of outlet openings and disposed between said end walls at a substantial distance from each end wall;
and an elongated dam extending upwardly from said tundish bottom between said ladle nozzle stream impact location and said row of outlet openings;
said dam extending longitudinally between said end walls and having a pair of opposite ends each spaced from a respective end wall.
2. A tundish as claimed in claim I wherein:
said elongated dam extends upwardly from the tundish bottom to a height which is substantially uniform from one end of the dam to the other.
3. A tundish as claimed in claim I or claim 2 wherein:
said tundish walls extend upwardly from the tundish bottom to a predetermined height;
and said dam extends upwardly from the tundish bottom to a maximum height substantially less than one-half said predetermined height of the tundish walls.
4. A tundish as claimed in claim 3 wherein:
said dam's maximum height is about 25-35% of said predetermined height of the tundish walls.
5. A tundish as claimed in any one of the preceding claims wherein:
said inner outlet openings comprise a pair of longitudinally spaced openings each located on a respective opposite side of the longitudinal mid-point of the tundish at substantially the same distance therefrom as the other inner outlet opening;
and said dam has a length greater than the distance between the respective centre lines of said inner outlet openings but substantially less than the distance between the outer pair of outlet openings.
6. A tundish as claimed in claim 5 wherein:
said dam has a length not substantially greater than the distance between the longitudinally outermost points on said inner outlet openings.
7. A tundish as claimed in claim 5 wherein:
the distance between a respective outer outlet opening and the inner outlet opening closest thereto is substantially the same as the distance between said pair of inner outlet openings;
and the length of said dam is substantially less than 150% of the distance between adjacent openings.
8. A tundish as claimed in claim 5 wherein:
said dam has a length about 120-135% of the distance between said centre lines of said inner outlet openings.
9. A tundish as claimed in any one of the preceding claims wherein:
the width of said dam is substantially less than the distance between the longitudinal centre line of the dam and the centre line of said row of outlet openings.
10. A tundish as claimed in any one of the preceding claims wherein:
said row of outlet openings is disposed between said dam and one of said side walls;
the maximum distance, in a lateral direction, between the centre line of said row of outlet openings and said one side wall being less than the maximum distance, in a lateral direction, between the centre line of said row of outlet openings and the other side wall.
11. A tundish as claimed in claim 10 wherein:
the distance between the centre line of the dam and the centre line of said row of outlet openings is less than the distance, in a lateral direction, between the centre line of said row of outlet openings and said one side wall.
12. A tundish as claimed in any one of the preceding claims wherein:
said elongated dam comprises means for substantially equalising (a) one or more of the minimum, peak and mean residence times in said tundish of molten metal exiting through said inner outlet openings with (b) one or more of the minimum, peak and mean residence times of molten metal exiting through said outer outlet openings.
13. A tundish as claimed in any one of the preceding claims wherein:
said elongated dam comprises flow control means for substantially increasing the minimum and peak residence times of molten metal exiting said tundish through said inner outlet openings, compared to a tundish without internal flow control structure, to increase the floating out of inclusion impurities from said molten metal to a slag layer atop said molten metal in said tundish.
14. A tundish as claimed in any one of the preceding claims wherein:
said elongated dam comprises flow control means for substantially increasing the plug flow volume fraction of molten metal exiting said tundish through said inner outlet openings, compared to a tundish without internal flow control structure, to increase the floating out of inclusion impurities from said molten metal to a slag layer atop said molten metal in said tundish.