Apparatus and method for containing inert gas around molten metal stream

A molten metal stream descends from a ladle through a nozzle and then a shroud into a molten metal bath in a tundish. Structure is provided to contain a ring of inert gas around the nozzle immediately adjacent the junction of the nozzle and the shroud and to prevent outside air from entering the shroud at the junction.

BACKGROUND OF THE INVENTION 
The present invention relates generally to methods and apparatuses for 
handling a stream of molten metal and more particularly to methods and 
apparatuses for preventing the entry into the molten metal stream of gas 
from the surrounding atmosphere. 
In conventional casting operations, a vertically descending stream of 
molten metal (e.g., molten steel) flows from an upper container such as a 
ladle to a lower container such as the tundish of a continuous casting 
apparatus. The stream typically flows through a vertically disposed nozzle 
having an upper end communicating with a bottom outlet from the ladle and 
a lower end disposed above the top surface of a molten metal bath in the 
lower container, e.g., the tundish. In the absence of protective measures, 
that portion of the molten metal stream between the lower end of the 
nozzle and the top of the molten metal bath is exposed to the outside 
atmosphere surrounding the stream, e.g., air. In such a case, air can be 
absorbed into the stream which is undesirable because it introduces oxygen 
and nitrogen as impurities into the molten metal. To prevent this from 
occurring, it has been conventional to enclose that part of the descending 
metal stream, below the lower end of the nozzle, within a vertically 
disposed, tubular shroud having a lower end submerged within the molten 
metal bath in the lower container. 
The shroud is aligned with the nozzle and has an upper portion which 
surrounds and removably engages the lower portion of the nozzle at a 
junction of the two. The interior of the tubular shroud typically has a 
cross-sectional area (or diameter) greater than the cross-sectional area 
(or diameter) of the nozzle's interior. Because the shroud has a larger 
interior cross-section than the nozzle, a descending molten metal stream 
which fills the entire interior cross-section of the nozzle will not fill 
the entire interior cross-section of the shroud. As a result, the molten 
metal stream descending from the nozzle through the interior of the shroud 
will create a partial vacuum in the shroud. There is a seam where the 
upper portion of the shroud removably engages the lower portion of the 
nozzle at their junction, and the partial vacuum created within the 
interior of the shroud has a tendency to aspirate outside air from the 
atmosphere surrounding the shroud and the nozzle into the interior of the 
shroud through the seam at the junction. This is undesirable because it 
will introduce oxygen and nitrogen into the molten metal stream. 
Attempts have been made in the past to prevent outside air from being 
aspirated into the interior of the shroud, but none of these attempts has 
been sufficiently successful. For example, in one attempt, an annular 
clearance was provided between the upper portion of the shroud and the 
lower portion of the nozzle, at the location of the seam, and an inert 
gas, such as argon, was continuously introduced into the clearance to 
exclude air from entering the clearance. The inert gas was drawn from the 
clearance into the shroud by the partial vacuum in the shroud, and this 
necessitated replenishment of the inert gas in the clearance. 
A problem with the expedient described above was the occurrence of eddy 
currents in the inert gas in the clearance. This allowed the periodic 
escape of the inert gas from the clearance to the atmosphere and the entry 
of outside air into the clearance. This air was drawn from the clearance 
into the interior of the shroud where it could mix with the descending 
stream of molten metal. In addition, the escape of the inert gas to the 
atmosphere was wasteful and necessitated too great a replenishment of the 
inert gas in the clearance. 
SUMMARY OF THE INVENTION 
The drawbacks and defects in the attempts described above are overcome by a 
method and apparatus in accordance with the present invention. In addition 
to providing an inert gas in the clearance between the upper portion of 
the shroud and the lower portion of the nozzle, the present invention 
provides a ring of inert gas around the nozzle above the clearance and 
immediately adjacent the junction. This ring of inert gas communicates 
with the gas in the clearance, and the pressure of the inert gas in the 
ring is maintained over the pressure of the atmosphere outside the shroud 
and the nozzle. This is accomplished by maintaining the ring of inert gas 
within an enclosure surrounding the nozzle adjacent the shroud's upper 
portion. The enclosure comprises structure for preventing the entry, into 
the clearance, of gas from the atmosphere around the shroud and the 
nozzle. This prevents outside air from entering the shroud at the 
junction. 
Another feature of the present invention is the provision on the nozzle of 
structure defining a fixed, unvarying, vertical reference level for use in 
mounting the gas containment enclosure around the nozzle. 
Other features and advantages are inherent in the method and apparatus 
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.

DETAILED DESCRIPTION 
Referring initially to FIG. 1, there is shown an upper container or ladle 
20 having a bottom opening 21 closed by a gate 22. Gate 22 is of 
conventional construction and may be opened to allow the flow of molten 
metal, such as molten steel, from within ladle 20 outwardly through bottom 
opening 21 into a nozzle 24 communicating with a shroud 25 having a lower 
end 26 located below the top surface 27 of a bath 28 contained within a 
lower container such as a tundish 29 of a continuous casting apparatus of 
conventional construction. 
In operation, a stream of molten metal descends from ladle 20 through 
nozzle 24 and shroud 25, which is vertically aligned with nozzle 24, into 
tundish 29. Both nozzle 24 and shroud 25 are tubular and both are 
vertically disposed. 
Referring now to FIGS. 2-3, nozzle 24 includes a lower portion 31 
comprising an outer metal jacket 32 surrounding an inner lining 33 
composed of refractory material unspaced from jacket 32. Nozzle 24 also 
includes an upper portion 35 comprising an upward extension 36 of 
refractory lining 33 and a metal jacket 37 surrounding and spaced from 
refractory lining extension 36. The space between metal jacket 37 and 
refractory lining extension 36 is filled with refractory mortar 39 (FIG. 
3), which bonds jacket 37 to lining extension 36 and fixes refractory 
lining 33 relative to jacket 37, forming a single, continuous piece 
comprising jacket 37 and lining 33. 
With reference to FIGS. 2 and 5, shroud 25 comprises an outer metal jacket 
40 surrounding an inner refractory lining 41 unspaced from jacket 40. 
Refractory lining 41 has a lower, unjacketed extension 42 terminating at 
the shroud's lower end 26. 
Referring again to FIGS. 2 and 3, the nozzle's lower portion 31 has a top 
part 44, a bottom part 45 and an intermediate part 46 between top part 44 
and bottom part 45. Bottom part 45 terminates at a nozzle lower end 47. As 
shown in FIGS. 2 and 5, shroud 25 has an annular upper portion 48 
surrounding bottom part 45 of nozzle lower portion 31. Shroud upper 
portion 48 removably engages nozzle lower portion 31 at their junction, 
and this will be described in more detail below. 
Located adjacent the shroud's annular upper portion 48 is an interior ledge 
49 upon which is seated an annular pad or gasket 50 sandwiched between 
ledge 49 and lower end 47 of nozzle 24 when the nozzle's lower portion 31 
is engaged by the shroud's annular upper portion 48 as shown in FIG. 2. 
There is an annular clearance 52 between the shroud's upper portion 48 and 
the surrounded bottom part 45 of the nozzle's lower portion 31 (FIG. 2). 
The junction, between shroud upper portion 48 and nozzle lower portion 31, 
is defined by annular clearance 52 and the space occupied by annular pad 
50. Pad 50 is typically composed of graphite, but it may be composed of 
other suitable material which will perform the function of a gasket while 
withstanding the temperatures which prevail when molten metal descends 
through the nozzle and the shroud. 
As shown in FIG. 2, the interior cross-section or diameter of shroud 25, 
below lower end 47 of nozzle 24, is greater than the interior 
cross-section or diameter of nozzle 24. As a result, a descending stream 
of molten metal which entirely fills the interior cross-section of nozzle 
24 will not entirely fill the interior cross-section of shroud 25, and a 
partial vacuum will be created within the interior of shroud 25 by the 
descent of the molten metal stream therethrough. In the absence of some 
preventive expedient, the partial vacuum created within the interior of 
shroud 25 will aspirate outside air, from the atmosphere surrounding 
nozzle 24 and shroud 25, into the interior of shroud 25 through annular 
clearance 52 between the shroud's annular upper portion 48 and bottom part 
45 of nozzle lower portion 31. This would be undesirable because it would 
allow the introduction of oxygen and nitrogen into the molten metal stream 
descending through the interior of shroud 25. 
As noted above, annular graphite gasket 50 is sandwiched between lower end 
47 of nozzle bottom part 45 and ledge 49 adjacent the shroud's upper 
annular portion 48, but the presence of gasket 50 will not entirely 
prevent the aspiration of outside air into the interior of shroud 25. 
Gasket 50 is not leakproof. Gas contained within clearance 52 can be 
aspirated past annular gasket 50 into the interior of shroud 25. 
Moreover, even if the cross-sectional area (diameter) of shroud 25 were the 
same as that of nozzle 24, the downward movement of the stream of molten 
metal from nozzle 24 into shroud 25 would still create an aspirating 
effect at the seam between the bottom part 45 of nozzle lower portion 31 
and upper annular portion 48 of shroud 25 where the latter engages the 
former. 
The shroud's annular upper portion 48 has an interior recess 53 
communicating with the inner end of a channel 54 having an outer end 
communicating with a coupling 55 for connecting channel 54 with a source 
of inert gas. Recess 53 communicates with clearance 52 located between the 
shroud's annular upper portion 48 and the bottom part 45 of the nozzle's 
lower portion 31. Recess 53 extends all the way around clearance 52. 
Channel 54 constitutes a gas passageway, and the inlet to channel 54 
communicates with the exterior of the shroud's upper annular portion 48, 
when coupling 55 is disconnected from an external source of inert gas. 
When inert gas is introduced through coupling 55 it flows through channel 
54 and recess 53 into clearance 52 and impedes the entry of outside air 
into clearance 52. However, the introduction and replenishment of inert 
gas into clearance 52 would not entirely prevent outside air from entering 
the interior of shroud 25 through clearance 52. Eddy currents in the inert 
gas within clearance 52 allow the escape of inert gas into the outside 
atmosphere and the entry of air from the outside atmosphere into clearance 
52 from where the air can be aspirated into the interior of shroud 25. 
In accordance with the present invention, structure is provided to contain 
the inert gas within clearance 52 and to prevent outside air from entering 
clearance 52. More particularly, surrounding intermediate part 46 of 
nozzle lower portion 31 is a gas containment ring 58 (FIG. 2). As shown in 
FIGS. 2, 7 and 8, ring 58 is composed of metal and comprises an inner wall 
portion 59 integral with an upper wall portion 60 integral with an outer 
wall portion 61 integral with a skirt 62 which extends downwardly and 
outwardly relative to outer wall portion 61. Outer wall portion 61 on gas 
containment ring 58 extends between the ring's upper wall portion 60 and 
the shroud's annular upper portion 48. Inner wall portion 59 on gas 
containment ring 58 comprises structure for frictionally engaging metal 
jacket 32 at intermediate part 46 of nozzle lower portion 31. 
Referring to FIGS. 2 and 5, metal jacket 40 on shroud 25 has a horizontally 
disposed top portion 63 which, together with wall portions 59-61 on ring 
58, define a gas containment structure having an interior 67 communicating 
with clearance 52. Shroud jacket top portion 63 is integral with an 
exterior arcuate shoulder portion 64 integral with a vertical portion 65 
on which coupling 55 is mounted. 
When gas containment ring 58 is located in the position illustrated in FIG. 
2, the ring's inner wall portion 59 engages metal jacket 32 on nozzle 
lower portion 31, and the ring's skirt 62, which extends downwardly and 
outwardly relative to interior 67 of the gas containment structure, 
tangentially engages arcuate shoulder porton 64 of the shroud's metal 
jacket to effect a seal. As shown in FIG. 2, the gas containment structure 
is devoid of any opening to the outside atmosphere around nozzle 24. Gas 
containment ring 58 is assembled in place around intermediate part 46 of 
nozzle lower portion 31 by sliding the ring upwardly from below the 
nozzle. 
Ring 58 is positioned at a fixed vertical reference level on the nozzle, 
employing structure now to be described with reference to FIGS. 2-4. As 
noted above, nozzle upper portion 36 has a metal jacket 37 which in turn 
has a bottom part 38 to which is attached a metal ring 70 at a weldment 76 
(FIG. 4), for example. Metal jacket 32 on nozzle lower portion 31 has an 
outwardly extending peripheral flange 34 located at top part 44 of the 
nozzle lower portion. Metal ring 70 defines an inwardly extending 
peripheral ledge 71 at the bottom part 38 of metal jacket 37. Ledge 71 
comprises structure for seating peripheral flange 34 to support nozzle 
lower portion 31 and upward extension 36 of refractory lining 33. Ledge 71 
defines a fixed, vertical, reference level for flange 34. 
Metal ring 70 has a bottom surface 75 defining another fixed, vertical 
reference level on nozzle 24. When gas containment ring 58 is slid 
upwardly around intermediate part 46 of nozzle lower portion 31, the top 
surface on the ring's upper wall portion 60 abuts against bottom surface 
75 of metal ring 70. In this manner, the vertical position of gas 
containment ring 58 is fixed in relation to nozzle 24. 
After gas containment ring 58 has been assembled in place, around 
intermediate part 46 of nozzle lower portion 31 (FIG. 2), the shroud's 
upper annular portion 48 is engaged around the bottom part 45 of nozzle 
lower portion 31 by sliding the shroud's upper annular portion upwardly 
into telescoping relation with the bottom part of the nozzle's lower 
portion. Shroud 25 is held in engagement with nozzle 24 by a manipulator 
arm 77 having a pair of parallel, notched ears, only one of which is shown 
at 79 in FIG. 1, each engaging a respective pin 78 extending outwardly 
from vertical portion 65 of metal jacket 40 on shroud 25. This type of 
arrangement is described in greater detail in Rellis, et al., U.S. Pat. 
No. 4,747,584, and the disclosure thereof is incorporated herein by 
reference. 
The arrangement described in the preceding paragraph mounts shroud 25 for 
pivotal movement, relative to nozzle 24, about a horizontal axis defined 
by pins 78. Clearance 52 accommodates the movement described in the 
preceding sentence. This movement may be initiated by currents or 
disturbances in bath 28 acting upon lower extension 42 of the shroud's 
refractory lining 41. Skirt 62 on gas containment ring 58 comprises 
structure for wiping arcuate shoulder portion 64 on the shroud's metal 
jacket 40, during pivotal movement of the shroud, for maintaining the seal 
for the inert gas ring around nozzle lower portion 31. The curvature on 
arcuate shoulder portion 64 facilitates maintenance of the seal by the 
wiping action of skirt 62 during pivotal movement of the shroud. 
Metal ring 70, in addition to providing fixed, vertical, reference levels 
for ring 58 and flange 34 on the nozzle's lower portion 31, also functions 
to reinforce the nozzle. 
Lower end 47 of nozzle 24 cannot be used as a fixed, vertical, reference 
level because end 47 can erode away during service. Similarly, the tip of 
metal jacket 32 on nozzle lower portion 31 adjacent the nozzle's lower end 
47 cannot be used as a fixed, vertical, reference level because the metal 
jacket tip is subject to bending during usage. There is a need for a 
vertical reference level higher up on the nozzle than bottom part 45 of 
the nozzle's lower portion 31. A fixed, vertical, reference level on the 
nozzle itself is desirable in situations where, as here, a seal is 
effected on the nozzle itself. In apparatus of the type employed by the 
present invention, a seal is desirable at the location where the nozzle's 
lower portion 31 is engaged by the shroud's upper portion 48. 
Metal jacket 37, to which reference ring 70 is attached, is in turn 
attached to a vertically fixed part 72 of gate 22. The embodiment of gate 
22 illustrated in FIG. 1 is a reciprocating gate in which vertically fixed 
part 72 reciprocates horizontally back and forth, with nozzle 24 and 
shroud 25, under the urging of a pneumatic piston and cylinder arrangement 
indicated at 73 in FIG. 1. Gate part 72 is mounted for reciprocating 
movement relative to lower gate part 74 which is cut away at appropriate 
locations (not shown) to accommodate reciprocating movement of the 
nozzle's upper portion 35. As noted above, the gate shown in FIGS. 1 and 3 
is of the reciprocating type. A rotary type gate could also be employed. 
However, whatever type of gate is employed, nozzle 24 and shroud 25 
typically move together with the gate. 
There are also arrangements in which nozzle 24 would be stationary relative 
to a movable gate element. All of the gate constructions described above 
are conventional and commercially available and do not constitute a part 
of the present invention. 
In essence, gas containment ring 58 cooperates with the intermediate part 
of the nozzle's lower portion 31 and with the top of the shroud to provide 
a ring of inert gas around the nozzle, above and in communication with 
clearance 52, immediately adjacent the junction of the nozzle and the 
shroud. Preferably, the pressure of inert gas in the ring is maintained 
above the pressure of the atmosphere outside nozzle 24 and shroud 25, 
typically at least about 10% greater than atmospheric pressure. The inert 
gas is introduced directly into clearance 52 and enters the interior 67 of 
the gas containment ring by virtue of the communication between interior 
67 and clearance 52. 
Gas containment ring 58 is usable for more than one heat. However, 
typically, it is replaced after every heat. 
Referring now to FIG. 9, there is illustrated another embodiment of 
structure for containing gas within clearance 52 and preventing the entry 
into clearance 52 of outside air from the surrounding atmosphere. More 
particularly, surrounding the nozzle's intermediate part 46 is an annular, 
gas-sealing element or ring 80 composed of refractory material. Refractory 
ring 80 has a bottom portion 81 abutting the top portion 63 of the 
shroud's metal jacket 40, from above. Refractory ring 80 also has a top 
portion 82 abutting reference ring 70 from below and an inside portion 83 
frictionally engaging metal jacket 32 of nozzle lower portion 31 at the 
latter's intermediate part 46. Bottom surface 75 of metal ring 70 defines 
a fixed, vertical, reference level for positioning refractory ring 80 
around the nozzle's lower portion at intermediate part 46 thereof. 
As shown in FIG. 2, clearance 52 is upwardly inclined and thus has a 
vertical component. Being vertically inclined, clearance 52 has upper and 
lower ends. In the embodiment of FIG. 2, the upper end of clearance 52 
communicates with the interior 67 of gas containment ring 58. In the 
embodiment of FIG. 9, the lower portion 81 of refractory ring 80 comprises 
structure for closing the upper end 84 of clearance 52. In this manner, 
refractory ring 80 prevents the entry into clearance 52 of outside air 
from the atmosphere around shroud 25 and nozzle 24. 
Refractory ring 80 is composed of a material such as silica fiber. Ring 80 
is slid up into frictional engagement around intermediate part 46 of 
nozzle lower portion 31, in the same manner as metal ring 58. Refractory 
ring 80 is preferably replaced after each heat. 
The metal jackets and refractory linings of nozzle 24 and shroud 25 are 
composed of materials conventionally used for those purposes. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be obvious to those skilled in the art.