Stopper rod with an improved gas distribution

The stopper rod for regulating the flow of a liquid has a body traversed by an axial channel and having at one end a porous nose fed with gas from the channel. The nose has a free space in the form of a hemisphere-shaped slot or lattice network communicating with the channel for delivery of gas thereto. Preferably, a plurality of spaced-apart bridges of solid material connect the two faces of the free space in order to increase the strength of the porous nose.

BACKGROUND OF THE INVENTION 
The invention concerns a stopper rod for regulating the flow of a liquid, 
having a porous part fed with gas. 
Stopper rods are frequently used in industry for opening and closing an 
orifice of a receptacle containing a liquid such as molten steel in a 
metallurgical vessel. By movement of the stopper rod from the orifice of 
the receptacle, the flow rate of the liquid is regulated. In some cases, 
these flow regulating stopper rods have appropriate internal channels and 
/or porous portions which make it possible to blow a treating gas into the 
liquid contained in the receptacle. Thus, the use of a stopper rod for 
controlling the flow of a molten steel emerging from a tundish into a 
water-cooled, continuous-casting mold is well known. In addition, it is 
conventional practice to introduce an inert gas, generally argon, into the 
molten metal through the stopper rod. The purpose of the argon is to 
eliminate unwanted inclusions contained in the molten steel. Another 
purpose is to reduce the deposits of alumina that occur in the casting 
elements, particularly when casting aluminum killed steel. Finally, the 
injection of argon makes it possible to avoid the development of a vacuum 
inside of the casting elements. Such a vacuum is capable of causing an 
aspiration of air through the porous refractory casting elements which, in 
turn, causes harmful oxidation of the molten metal. 
According to one known technique, the inert gas is injected by means of an 
axial channel that passes through the stopper rod and exits at the end 
thereof. A further known stopper rod has a separate, porous stopper or 
plug sealed in the refractory material at the end of the axial channel of 
the stopper rod for emitting the inert gas to the molten metal. In the 
case of the first-mentioned technique, the hole at the end of the stopper 
rod has a substantial diameter, on the order of 2-3 mm. Consequently, a 
back flow of molten metal can occur through this orifice in the case where 
the pressure of the inert gas is interrupted for any reason. Furthermore, 
the gas injection is localized at one point and induces large bubbles that 
are less effective for eliminating the impurities contained in the metal. 
The second solution mentioned above makes it possible to produce small 
bubbles distributed on the surface of the porous stopper, however, there 
is the risk of unsealing of this stopper which leads to a back flow of 
molten metal within the axial channel of the stopper rod. 
U.S. Pat. No. 4,791,978 to Mark K. Fishler, and owned by assignee of the 
present invention, discloses a stopper rod having a porous nose 
isostatically co-pressed at the same time as the body. The porous nose has 
a composition similar to that of the body, but its permeability is much 
higher. The copressing makes it possible to avoid the risk of losing the 
porous nose. 
Nevertheless, in the stopper rod according to U.S. Pat. No. 4,791,978, the 
internal surface of the end of the axial channel of the stopper rod is 
relatively small. In addition, the thickness of the porous material which 
must be traversed by the inert gas is substantial, e.g., on the order of 
40 mm, for a stopper rod of current dimensions. These characteristics lead 
to a limitation of the inert gas flow rate obtained at elevated 
temperatures. Thus, the maximum flow rate of argon obtained at 
1500.degree. C. is about 6-8 Nl/min. for a molten metal counterpressure of 
2.8 bar. This flow rate is insufficient in some cases and also the 
relatively substantial counterpressure that is necessary is dangerous for 
the axial channel, the connection of the stopper rod and the gas feed 
piping. The principal risk is the bursting of the nose during the casting, 
the catastrophic consequence of which would be loss of control of the 
molten metal flow. The second problem is a high risk of leakage in the gas 
connections, leading to the inefficacy of gas flow through the porous 
nose. 
Given that a high resistance to erosion by steel is necessary, all the 
attempts made to increase the permeability of the porous material 
constituting the nose failed because an increase in the number of pores 
and/or an increase in their size result in an inacceptable reduction in 
the erosion resistance of the porous material. 
The present invention provides a remedy to these shortcomings of the prior 
art. 
An object of the invention is to create a stopper rod for regulating the 
flow of a liquid that preserves the advantages of stopper rods of the 
prior art, while permitting an increase in the flow rate of gas. The 
improved flow rate is obtained at a lower inert gas pressure than 
heretofore possible. 
SUMMARY OF THE INVENTION 
This result is obtained in accordance with the invention due to the fact 
that the porous nose has a free space into which the inert gas in 
introduced. 
This free space has the effect of bringing the gas close to the outside 
surface of the stopper rod. Consequently, the flow rate of gas is 
increased for a same gas counterpressure value because the thickness of 
material to be traversed is decreased. The free space is preferably a slit 
that can be continuous or discontinuous. Preferably, bridges of material 
connect the two faces of the slits. These bridges avoid the loss of the 
outer part of the porous nose in the case where erosion due to the steel 
would reach the slit. According to the invention, the free space has a 
surface greater than the inside surface of the porous nose. Due to this 
characteristic, the contact surface between the inert gas and the porous 
material is increased. The passage cross section offered this gas is thus 
increased. Consequently, the flow rate of the gas is increased for the 
same value of the counterpressure. 
According to one further embodiment, the space left free in the porous 
stopper is comprised of a network of channels or a mesh network provided 
in the porous material. It is obvious that the configuration of the free 
space is not limited to these examples, but can be chosen freely as a 
function of the needs of the user and the application. 
Finally, in some applications, the thickness of the porous material 
separating the outer surface of the stopper of the free space is chosen so 
as to define at least a preferential zone of blowing. In other words, the 
distance that separates the free space, e.g., the slit, from the outer 
surface of the porous nose is not constant. It can be less in a given zone 
in order to obtain a greater flow rate of gas in this zone, the passage of 
the gas being facilitated by the decrease in the thickness of the wall to 
be traversed. Preferably, the free space is contained entirely in the 
porous stopper in order to avoid the development of fragile zones at the 
level of the interface between the porous nose and the body of the stopper 
rod.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a cross sectional view of a stopper rod designed to regulate 
the casting of a liquid, for example, molten steel according to known 
prior art. The prior art stopper rod is comprised of a non-porous 
refractory body 2 of general cylindrical form and a porous refractory nose 
4 located at a lower end of the body 2. The body is traversed by a 
longitudinally extending axial channel 6. The entrance end of the axial 
channel 6 is threaded to fit a connection for feeding the stopper rod with 
an inert gas, for example, argon. The bottom end of channel 6 extends into 
the porous nose 4, to facilitate the introduction of inert gas thereto. 
The nose 4, for example, has a hemispherical or ogival form. It is found 
that in this embodiment the gas exchange surface area at the end of the 
axial channel 6 and the porous nose is reduced relative to the surface 
area on the exterior surface of the porous nose. Furthermore, the 
thickness of the porous nose 4 that is to be traversed by the inert gas is 
relatively substantial, of the order of 40 mm in a current commercial 
embodiment. Consequently, for a stopper rod of this type, the argon flow 
rate obtained at a temperature of 1500.degree. C. does not exceed 8 Nl/min 
for a counterpressure created by the molten steel of 2.8 bar. Although 
such a gas flow rate is adequate in some cases, it is dangerous to work 
with such a high counterpressure in the stopper rod and in the gas feed 
system, as explained previously. In effect, there are risks of leakage in 
the area of the connection between the stopper rod and the gas pipes, as 
well as the risk of bursting of the refractory stopper rod. 
FIG. 2 shows a cross-sectional view of a stopper rod according to the 
invention, having a gas distribution opening or free space formed in the 
porous refractory material of the nose 4. The embodiment of FIG. 2 is 
distinguished from the stopper rod of the prior art, shown in FIG. 1, by 
the fact that it has the gas distribution manifold defined by free space 
8, formed in the porous material comprising the nose 4. In the example 
shown, the free space 8 is comprised of a substantially continuous open 
slot of essentially hemispherical or ogival shape and which is essentially 
concentric with to the outer surface 9 of the porous nose 4. It is noted 
that a plurality of spaced apart bridges 10 of refractory material connect 
the two edges of the slot. These bridges 10 strengthen the structure and 
prevent the loss of the outer part of the porous nose 4 when erosion due 
to the steel reaches the slot of free space 8. 
It should be noted that the junction of interface 11 between the porous 
refractory material of the nose 4 and the non-porous refractory material 
of the stopper rod body 2 is, preferably, not traversed by the slot of 
free space 8 in order to avoid weakening this critical interface zone in 
which stresses are present by reason of the slightly different nature of 
the materials in contact. 
In the exemplary embodiment shown in FIG. 2, the apex of the slot of free 
space 8 is tangent to the lower part of the axial channel 6 so as to 
communicate therewith in the area of the apex. Thus, the free space 8 is 
fed with inert gas directly through the end of channel 6, as well as 
through a number of radial passages 12 communicating with the axial 
channel 6 and the slot of the free space 8 that permit feeding of inert 
gas to the upper end of the slot of the free space. It is not necessary, 
however, that the slot be tangential to the end of the axial channel 6. In 
one modified embodiment, this slot could be included entirely in the 
material constituting the porous nose and, thus, spaced from the end of 
the axial channel 6. 
The above-described free space 8 provides several advantages. The free 
space 8 makes it possible to reduce the thickness of the porous material 
to be traversed by the gas. In addition, due to the fact that the surface 
area of the free space slot is greater than the surface of the interface 
between the axial channel 6 and the lower part of the porous nose 4, the 
area of the exchange surface is increased. These two factors contribute to 
increasing the flow rate of inert gas for the same counterpressure value. 
FIG. 3 shows a further presently preferred embodiment of the invention in 
which the gas distribution free space, instead of being comprised of an 
essentially continuous slot, is in the form of cup-shaped lattice network 
designated 108. The free space of network 108 is obtained by means of a 
net-like array of wax wire, which is eliminated by melting and 
vaporization during firing. In the embodiment of FIG. 3, the surface area 
of the free space of network 108 is smaller than in the preceding example 
of FIG. 2. Consequently, the exchange surface with the central passage 6 
is also smaller than in the previous example. The advantage, however, that 
resides in introducing the inert gas close to the outer surface of the 
porous nose is preserved so that the flow rate of the gas is increased 
relative to the prior art stopper rod for an identical value of the 
counterpressure of the molten metal. 
FIG. 4 shows another modified form of the present invention, in which the 
gas distribution free space is formed in an outwardly skewed manner to 
define a preferential blowing zone 15. The thickness of the porous 
material to be traversed by the gas in this embodiment is not constant, 
but, rather, is sharply diminished in the zone 15 where one wishes to 
obtain a preferential blowing. In this manner, the inert gas within the 
free space will more readily traverse the diminished thickness or porous 
material in zone 15 to provide a greater volume of gas flow in that zone. 
It is thus apparent that the shape of the gas distribution free space is 
not limited to the several examples described herein, but the skilled 
artisan can adapt the shape of the free space as a function of his 
particular needs. 
The manufacture of the stopper rod shown in FIGS. 2-4 will now be 
explained. A molding form 16 is placed on a mandrel (see FIG. 5); its 
shape corresponds to the shape of the gas distribution free space 8 that 
one wishes to obtain, for example, a continuous slot (FIG. 2) or a mesh 
network (FIG. 3) or any other form desired. This molding form is made of 
expendable material, of a known type, such as wax, that will be eliminated 
by melting and vaporization in a subsequent high temperature firing stage 
of the process. The molding form 16 also has a plurality of holes 20 
formed therein which make possible the formation of the solid bridge areas 
10, referred to above. In addition, centering rods 14 also of an 
expendable material, such as wax, assure the positioning of the molding 
form 16 on the pressing mandrel. Then, in a classic known manner, the 
pressing mandrel with the molding form 16 positioned thereon is placed in 
a pressing envelope that is filled first with non-porous refractory 
materials of the stopper rod body and then porous refractory materials of 
the nose 4. After isostatic pressing, the stopper rod is heated 
moderately, or dried, and then fired in a furnace. During the heating 
and/or firing operation, the molding form of wax is eliminated, leaving 
vacant the empty space defining the free space 8 or the network 108 
desired as well as the radial channels 12 for the passage of the gas. 
EXAMPLE 
A stopper rod according to the invention, having a slot forming a gas 
distribution free space 8 of the type shown in FIG. 2, was produced. With 
this stopper rod, an increase in the argon flow rate up to 20 Nl/min. was 
measured at a steel temperature of 1500.degree. C. and a counterpressure 
less than 2 bar. For comparison purposes, a stopper rod of the prior art 
of identical dimensions, at a temperature of 1500.degree. C., was not 
capable of reaching an argon flow rate more than 8 Nl/min., a molten metal 
counterpressure of 2.8 bar. This comparison consequently shows that the 
invention makes it possible to increase the argon flow rate by a factor of 
2 or more. 
Although the invention was described principally with reference to a 
stopper rod, it is equally applicable to any casting of a fluid, comprised 
of a porous part and a nonporous part. It can also be advantageously 
applied to a pouring nozzle having a porous sleeve on the inside bore for 
the injection of a gas.