FIELD OF THE INVENTION

[0001 ] The present invention generally relates to stoppers for controlling the flow of molten metal out of an outlet of a metallurgical vessel towards a mould or a casting tool. In particular, it concerns stoppers equipped with an integrated temperature measurement device allowing the measurement of the instantaneous temperature of a metal melt at proximity of the outlet.

BACKGROUND OF THE INVENTION

[0002] As illustrated in Figure 1 , in continuous metal casting processes, a ladle (21 ) is positioned a few meters above a tundish (22) and delivers molten metal to the tundish through a ladle shroud in the form of a long tube leading from the ladle into the tundish. The tundish is disposed between the ladle and a casting tool (23) or mould which is fed with molten metal from the tundish through an outlet in the form of an inner nozzle (22d) located inside the tundish in fluid communication with a pouring nozzle located outside and below the tundish and supplying the molten metal to be formed. The flowrate of the molten metal flowing through the inner nozzle can be controlled by a stopper (1 ) positioned vertically above the inner nozzle, and comprising a nose tip mating the geometry of an inlet of the inner nozzle. By moving the stopper up and down the vertical distance of the stopper nose tip to the inlet of the inner nozzle can be varied, thus controlling the flowrate of molten metal from zero, if the nose tip is in mating contact with the inner nozzle, to a maximum flowrate when the nose tip is positioned sufficiently far away from the inner nozzle to not influence the flow rate anymore. In some embodiments, stoppers only control the position of the stopper between an opening and closing positions, and variation of the flowrate between these two positions is controlled by other means.

[0003] Continuous temperature measurement in the tundish is not a mandatory feature and many plants are using spot temperature measurement (2 to 3 measurements per hour). However, it is advantageous when the temperature of the molten metal in the tundish is monitored continuously. In early installations, temperature sensors were immersed in the molten metal. The temperature of the molten metal in the tundish, however, is not and needs not be homogeneous. The most critical temperature of the molten metal is adjacent to the inner nozzle, since from there the molten metal is cast directly into a mould or tool. For this reason, it has been proposed to include a temperature sensor positioned in an inner cavity of a stopper, in the general region of the nose tip, to capture the temperature of the molten metal close to the inner nozzle. DE291 231 1 and WO20151 04241 describe systems for measuring the temperature of an inner surface located at a closed end of an inner cavity of an elongated refractory body, which can be a stopper. The closed end is near a tip of the stopper. Although a great improvement over the early installations, such devices for measuring temperature have a great drawback in that, as shown in Figure 7(a), the response time of such devices to a temperature variation of the molten metal is too long. In a series of tests discussed more in details in the next sections, which results are illustrated in Figure 7, the time required by such prior art stopper immersed at 958 mm in a molten metal for detecting within ± 2°C a 10°C variation in temperature of the molten metal. The detection time for the prior art stopper (a) tested was 390 s (= 6.5 min), which is far too long to satisfactorily monitor the temperature of the molten metal at proximity of the inner nozzle. Another way to evaluate the response time, usually used in automation, is the time to reach 63% and/or 95% of the target temperature variation (1 0°C variation for instance). No matter the method used to measure the response time, the results are comparable. [0004] Because any variation of temperature occurring at the outer surface is transmitted to the inner surface mostly by conduction across a strip separating the inner from the outer surfaces, reduction of the response time of the device for measuring temperature of the inner zone to a temperature variation of the molten metal in contact with the outer zone can be achieved by reduction of the thickness of the strip and/or use of a material of higher conductivity in the strip. In a stopper as illustrated in Figure 7(a) and described in DE291231 1 (fig.1 and 2) or in DE19752548, including a thermocouple positioned at the closed end of an inner cavity centred on the longitudinal axis of the stopper, the thickness of refractory material separating the inner surface from the outer surface is very large and cannot be reduced substantially. Indeed, increasing the diameter of the inner cavity would yield issues of insufficient mechanical resistance of the stopper.

[0005] US5361 825 proposed to offset the inner cavity with respect to the longitudinal axis, X1 , as illustrated schematically in Figure 7(b). This solution permits a substantial reduction of the thickness, t, separating the inner from the outer surfaces, but the reduction in response time of the device is disappointing, with a reduction of only 12% from 390 s for a centred inner cavity down to 344 s for an offset inner cavity (compare Figure 7(a) and (b)). Furthermore, the offset design proposed in US5361 825 results in a bulkier stopper, more difficult to handle because heavier and not axisymmetric, and requiring more refractory material for its production.

[0006] JP2009090323 describes a stopper comprising a through-hole extending normal to the longitudinal axis of the stopper for streamlining the flow of molten metal around the stopper and reducing the formation of vortices, responsible for inclusion of impurities from the slag being dragged down into the cast slab, when the depth of the molten metal in a tundish is low. No measurement of temperature of the molten metal is described in this document.

[0007] There remains a need for a device suitable for detecting a temperature variation of the molten metal at proximity of an inner nozzle of a tundish in a shorter time than is possible to date. The present invention proposes a solution for satisfying such need with a stopper having a particular geometry allowing a substantial reduction of the response rate to a temperature variation of a molten metal compared with prior art stoppers. These and other advantages of the present invention are presented in continuation.

SUMMARY OF THE INVENTION

[0008] The present invention is defined by the attached independent claims. The dependent claims define preferred embodiments. In particular, the present invention concerns a refractory stopper for controlling the flow of molten metal out of a metallurgical vessel, said refractory stopper comprising:

(a) an elongated body comprising a bulk made of a refractory material and defined by a peripheral wall, the elongated body extending over a length, L, along a longitudinal axis, X1 , from a distal end to a nose tip, and

(b) a device for measuring the temperature at an inner zone located within the bulk of the elongated body, the inner zone being separated from an outer zone located outside of the bulk of the elongated body by a strip of material of lowest thickness, t, wherein the temperature of the inner zone is representative of an instantaneous temperature at the outer zone,

[0009] The stopper of the present invention is characterized in that, the elongated body comprises a through hole, extending through the elongated body along a transverse axis, X2, transverse, preferably normal to the longitudinal axis, X1 , over a length, D, and defined by a through-hole wall extending from a perimeter of a first opening to a perimeter of a second opening, said first and second openings being located at the peripheral wall, and in that, the outer zone belongs to the through hole wall and is separated from the nose tip by a distance, M, lower than two third of the length, L, of the elongated body, M < 2/3 L, preferably lower than one third of the length, L, M < ½ L It is preferred that the transverse axis, X2, be normal to X1 .

[0010] In a preferred embodiment, the through hole is symmetrical with respect to (a) a plane (X1 , X2), and/or (b) a plane normal to X2 and including X1 , and/or (c) a plane normal to X1 and including X2, and/or (d) the axis X2. Such symmetries reduce the risk of malfunctioning because of an inaccurate orientation of the stopper with respect to the longitudinal axis (in rotation), during mounting of the stopper.

[001 1 ] Molten metal flowing or stagnating in the through-hole must not freeze. The through- hole therefore has dimensions requirements. For example, the through-hole preferably satisfies one or more of the following dimensional requirements.

• the through hole has a cross-section normal to X2, having an area of at least 900 mm ² along the whole length, D, and/or • the through hole has a height, H, measured parallel to the longitudinal axis, X1 , between the outer zone and an opposite portion of the through hole wall, comprised between 1 and 12% of the length, L, of the elongated body, 0.01 L < H < 0.12 L; and/or

• the through hole has a width, W, measured along a direction normal to both longitudinal and transverse axes, X1 &X2, comprised between, ½ wx and 3 wx, preferably between wx and 2 wx, wherein wx is a width of the inner zone measured along the same direction as W.

[0012] The first and second openings may have a cross-section selected from rectangular, elliptical, or a combination of a rectangular portion and an elliptical portion. Such cross-sectional geometries are preferably maintained along the whole length, D, of the through-hole, though not necessarily with constant dimensions. Each of the inner and outer zones preferably comprises a portion parallel to one another, which is preferably planar or comprises a single or double curvature.

[0013] In a particularly preferred embodiment, the outer zone forms a protrusion jutting out of a surface of the through hole wall. For example, the protrusion can be in the shape of a reversed hollow bell, with an opening facing towards the distal end of the elongated body, and an opposite closed end protruding out of a surface of the through hole and extending parallel to the longitudinal axis, X1 . In particular, the protrusion may have:

• a width, d, comprised between 50% and 1 00% of a width, W, of the through hole, ½ W < d < W, preferably between 60 and 90% of the width, W, 0.6 W < d < 0.9 W, wherein d and W are measured along a same direction normal to both longitudinal and transverse axes, X1 &X2, and/or

• a height, h, comprised between 10% and 75% of a height, H, between the outer zone and an opposite portion of the through hole wall, 0.1 H < h < 0.75 H, preferably between 25 and 60% of the height, H, 0.25 H < h < 0.6 H, wherein h and H are measured along a same direction parallel to the longitudinal axis, X1 .

[0014] The device for measuring the temperature at an inner zone can be any device, it can be an optical pyrometer or a thermocouple. The elongated body preferably comprises an inner cavity extending from an open end, opening at the distal end of the elongated body to a closed end forming the inner zone. The optical pyrometer is preferably located adjacent to the open end of the inner cavity, in optical communication with the inner zone formed by the closed end. A projection of the closed end of the inner cavity forming the inner zone onto a plane normal to the longitudinal axis, X1 , has an area, A3i, preferably comprised between 4 and 75% of the area, A1 w, of a cross-section of peripheral wall of the elongated body normal to the longitudinal axis, X1 , at the level of the intersection between the inner zone and the longitudinal axis, X1 , 0.04 A1 w < A3i < 0.75 A1 w. The area, A3i is more preferably comprised between 4 and 50% of the area, A1 w, 0.04 A1 w < A3i < 0.5 A1 w.

[0015] In a preferred embodiment, the inner zone and outer zone are part of an insert cemented to the bulk of the refractory body.

[0016] In order to shorten the response time of the device, the strip of material separating the inner zone from the outer zone may be made of a second refractory material different from a first refractory material forming at least 70% in volume of the elongated body. The second refractory material has a thermal conductivity higher than the first refractory material. [0017] For mechanically reinforcing the stopper at the level of the through-hole, the refractory body may form a bulge at the level of the through hole, such that a cross-section normal to the central axis, X1 , at the level of the through hole has a larger perimeter than at any other level of the elongated body within a distance of 2/3 L from the nose.

[0018] In a preferred embodiment, the stopper further comprises a channel comprising an inlet connectable to a source of inert gas, preferably argon, extending along a portion of the elongated body and comprising an outlet at the peripheral wall of the elongated body, located adjacent to the through hole, but not at the through hole wall and at a distance from the nose tip comprised between 1 00 mm and L I 2. The refractory stopper is equipped with a pressure measuring device for measuring the pressure in the gas line, and with a controller adapted for recording the pressure measured in the gas line as a function of the time at a constant gas flow rate, and for correlating said pressure to the depth of immersion, G, of the outlet when the refractory stopper is immersed in a metal melt.

[0019] A refractory stopper for controlling the flow of molten metal out of a metallurgical vessel, can comprise: · an elongated body comprising an outer surface made of a refractory material extending over a length, L, along a longitudinal axis, X1 , from a distal end to a nose, a gas line comprising an inlet connectable to a source of inert gas, preferably argon, extending along a portion of the elongated body and comprising an outlet at the outer surface of the elongated body, located at a distance of at least 30 mm, preferably at least 1 00 mm from the nose tip, and of not more than L / 2, preferably not more than L/3, more preferably, not more than 300 mm from the nose tip, and

• a pressure measuring device for measuring the pressure in the gas line, and

• a controller configured for maintaining a constant gas flow rate in the gas line, recording the pressure measured in the gas line, and for correlating said measured pressure to the depth of immersion, G, of the outlet when the refractory stopper is immersed in a metal melt. The controller is also configured for identifying the presence of slag at the level of the outlet, based on the pressure measured in the gas line. This is particularly useful during the draining of the metallurgical vessel.

BRIEF DESCRIPTION OF THE FIGU RES.

[0020] Various embodiments of the present invention are illustrated in the attached Figures. Figure 1 : shows a metallurgic installation comprising a ladle, a tundish and a casting tool, a stopper controls the flowrate of metal through the inner nozzle.

Figure 2: shows several embodiments of stoppers according to the present invention. Figure 3: shows cut views of a stopper according to an embodiment of the present invention. Figure 4: . shows cut views of a stopper according to an alternative embodiment of the present invention.

Figure 5: schematically shows the various dimensions used to define a stopper according to the present invention.

Figure 6: schematically shows the various dimensions used to define an alternative stopper according to the present invention

Figure 7: plots the detection times of a variation of 10°C of the temperature of a molten metal with stoppers of different geometries, (a) & (b) prior art stoppers, and (c) to (e) stoppers according to the present invention.

Figure 8: illustrates a stopper comprising an inert gas line for measuring the filling level of a tundish.

DETAILED DESCRIPTION OF THE INVENTION

[0021 ] As can be seen in Figure 2 a refractory stopper (1 ) according to the present invention, for controlling the flow of molten metal out of a metallurgical vessel, comprises:

(a) an elongated body comprising a bulk (1 b) made of a refractory material and defined by a peripheral wall (1 w), the elongated body extending over a length, L, along a longitudinal axis, X1 , from a distal end (1 d) to a nose tip (1 n), and

(b) a device (5) for measuring the temperature at an inner zone (3i) located within the bulk of the elongated body, the inner zone being separated from an outer zone (3o) located outside of the bulk of the elongated body by a strip of material of lowest thickness, t; the the temperature of the inner zone must be representative of an instantaneous temperature at the outer zone.

[0022] The gist of the present invention is to reduce the thickness of the strip separating the inner and outer surfaces by including in the elongated body a through hole (2), extending through the elongated body along a transverse axis, X2, transverse, preferably normal to the longitudinal axis, X1 , over a length, D. As illustrated in Figures 2 to 6, the through hole is defined by a through-hole wall extending from a perimeter of a first opening to a perimeter of a second opening.

[0023] The peripheral wall is defined herein as the wall defining the outer contour of the stopper. It is generally substantially cylindrical over a major portion of the length, L, of the stopper (excluding the nose tip and, optionally, a bulging portion reinforcing the portion comprising the through-hole). [0024] The through-hole wall is the wall defining the through-hole over its length, D. The through-hole wall is bounded at each end of the through-hole by the perimeters of a first and second openings. The perimeters can easily be defined by overlapping a cut view of a stopper with a through-hole with a cut view of the same stopper without through-hole. The perimeter of the openings is simply defined by the points wherein the two cut views start differing from one another.

[0025] Since, when dipped into molten metal, both peripheral wall and through-hole wall of a stopper are in contact with molten metal, they form together an outer surface of the nozzle. By contrast, an inner surface, inner wall, or inner cavity are defined as surfaces, walls, or cavities which are designed to never be in contact with molten metal. This is the case of the inner zone (3i), which is formed by an inner surface.

[0026] The outer zone (3o) belongs to the through hole wall. It therefore forms a portion of the outer surface and, in use, is wetted by molten metal, as illustrated by the dot-shaded area in Figure 3, representing molten metal (1 0). As illustrated in Figures 5 and 6, the outer zone is separated from the nose tip by a distance, M, lower than two third of the length, L, of the elongated body, M < 2/3 L, preferably lower than one third of the length, L, M < ½ L. This limitation has two reasons. First, it ensures that the melt temperature is measured relatively close to the inner nozzle (22d). Second, the through-hole weakens resistance of the stopper (In particular in flexion). The stopper is exposed to forces, due to its repetitive up and down movements about the inner nozzle and to the flow of molten metal flowing past its outer surface and nose tip. In standard stoppers, the highest bending moment is measured at the top of the stopper. In the stopper according to the present invention, the larger the distance, M, separating the through-hole is located from the nose tip, the higher the bending moment on the portion of the stopper comprising the through-hole is important, thus increasing the risk of failure.

[0027] The geometry of the through-hole is not particularly restricted as long as it ensures that (a) molten metal can either flow therethrough or stagnate therein without freezing, and (b) a good conductive heat exchange can be established with the inner zone (3i). In a preferred embodiment, it is preferred that the transverse axis, X2, along which the through-hole extends be normal to X1 . It is also preferred that the through hole be symmetrical with respect to (a) a plane (X1 , X2), and/or (b) a plane normal to X2 and including X1 , and/or (c) a plane normal to X1 and including X2, and/or (d) the axis X2. The reasons for these specific geometries is that it is difficult to predict the flow trajectories of the molten metal through the through-hole. This is particularly true in case the orientation of the opening (rotation about the longitudinal axis, X1 ) is not controlled. In such conditions, an asymmetrical design of the through-hole could lead to uneven flows of molten metal depending on the orientation it was mounted at, and on the instantaneous conditions of the molten metal bath.

[0028] The first and second openings (2a) may have a cross-section selected from rectangular, elliptical, or a combination of a rectangular portion and an elliptical portion. Preferably, the through-hole has one of the foregoing cross-sections along the whole length, D, of the through hole. The inner and outer zones preferably comprise a portion parallel to one another. For example, the parallel portions can for example be planar as illustrated in Figures 2 and 7(f), or it may comprise a single curvature as illustrated in Figures 3 and 5, or a double curvature as illustrated in Figures 4, 6, and 7(g). This ensures a homogeneous heat transfer by conduction across the strip separating the outer zone from the inner zone.

[0029] Figure 7 compares for same testing conditions and for different geometries of similar stoppers of same dimensions and materials, the response time of an optical pyrometer measuring the temperature of the closed end of an inner cavity (4) forming the inner zone (3i), to a 1 0°C-variation of temperature of molten metal contacting an outer zone. The times recorded correspond to a measurement by the pyrometer of a value within ± 2°C from the new temperature of the molten metal. The stoppers (a) and (b) are prior art designs discussed in the Background Art section supra. The shortest response time recorded for the prior art stoppers was 344°C. The stopper (c) is according to the present invention. The through-hole is cylindrical and the inner zone forms a half sphere. The response time recorded for stopper (c) was 203 s, which is a little above half of the response time recorded for stopper (a). Stopper (d) and (e) yielded similar response times of 1 70 s. In stopper (d) the outer zone is a flat surface, and the inner zone is a half spherical surface. In stopper (e) the outer zone is a half-cylinder and the inner zone is a flat surface. Stopper (f) comprises parallel flat surfaces forming both inner and outer zones, yielding a response time of 126 s. Finally, Stopper (g) comprises a reversed bell shaped protrusion, with a half spherical inner zone nested in a substantially half spherical outer zone. Stopper (g) yielded a response time of 53 s, about 14% of the time recorded with prior art stopper (a). It can be seen that the response times decrease with increasing areas of strips wherein the inner and outer zones extend parallel to one another. The protrusion offers a 3D-portion with parallel inner and outer zones. When a flat surface faces a curved surface, as in stoppers (d) and (e), the response time increases. And when the inner surface and outer surface form diverging curved surfaces, longer response times are recorded, as in stopper (c). It should be noted that, although yielding the worst response time among the stoppers according to the present invention, with a response time of 203 s, stopper (c) is substantially more responsive than any of prior art stoppers (a) and (b) having response times of 390 s and 344 s, respectively.

[0030] Freezing of the metal present in the through-hole must be avoided, lest the temperatures measured at the inner zone (3i) would cease to be representative of the temperature of the molten metal at proximity of the inner nozzle. For this reason, the through-hole must have dimensions sufficiently large to prevent such freezing to occur. In particular, the through hole may have a cross-section normal to X2, having an area of at least 900 mm ² along the whole length, D. Alternatively or concomitantly, the through hole may have a height, H, measured parallel to the longitudinal axis, X1 , between the outer zone and an opposite portion of the through hole wall, comprised between 1 and 12% of the length, L, of the elongated body, 0.01 L < H < 0.12 L. Similarly, the through hole may have a width, W, measured along a direction normal to both longitudinal and transverse axes, X1 &X2, comprised between, ½ wx and 3 wx, preferably between wx and 2 wx, wherein wx is a width of the inner zone measured along the same direction as W.

[0031 ] In a preferred embodiment, the outer zone forms a protrusion jutting out of a surface of the through hole wall. This embodiment, illustrated in Figures 2(b)-(e), 4, 6, and 7(g), increases the area of both outer and inner zone, for a constant cross-sectional area normal to the longitudinal axis, X1 . The outer zone defined by the exposed sides of the protrusion is surrounded by and contacts molten metal at all sides. The heat transfer towards the inner zone through the strip of the refractory material is therefore enhanced.

[0032] The protrusion can be in the shape of a reversed hollow bell, with an opening facing towards the distal end of the elongated body, and an opposite closed end protruding out of a surface of the through hole wall and extending parallel to the longitudinal axis, X1 . This embodiment is illustrated in Figure 2(b)-(e), 4, 6, and 7(g). The advantage is that the inner surface can be the closed end of an inner cavity forming the inner side of the reversed bell, and is surrounded over 360° by molten metal. Care must be taken with this design to let enough clearance between the protrusion and sidewalls of the through-hole wall, to prevent any metal from freezing in dead zones surrounded by refractory material.

[0033] Alternatively, the protrusion can extend like a wave over the whole width, W, of the through-hole, as illustrated in Figure 5. This design reduces the risk of formation of dead zones where molten metal can freeze. As in Figure 5, the protrusion may form a half cylinder like a (reversed) Romanic nave of a church, thus defining parallel inner and outer zones having the concentric half-cylindrical geometries. [0034] Other geometries than a reversed bell or a wave as described supra are possible. Though not essential, in general, a certain degree of symmetry is preferred, in particular if the rotational orientation of the openings with respect of the longitudinal axis, X1 , is not controlled explicitly. The following dimensions are indicative for avoiding problems of freezing of molten metal in the through-hole. They can be taken individually or combined in any manner desired.

[0035] The protrusion may have a width, d, comprised between 50% and 100% of the width, W, of the through hole, ½ W < d < W. The width is preferably comprised between 60 and 90% of the width, W, 0.6 W < d < 0.9 W, wherein d and W are measured along a same direction normal to both longitudinal and transverse axes, X1 &X2. Alternatively, or concomitantly, the protrusion may have a height, h, comprised between 1 0% and 75% of a height, H, between the outer zone and an opposite portion of the through hole wall, 0.1 H < h < 0.75 H, preferably between 25 and 60% of the height, H, 0.25 H < h < 0.6 H, wherein h and H are measured along a same direction parallel to the longitudinal axis, X1 .

[0036] The device (5) for measuring the temperature is preferably selected from an optical pyrometer or a thermocouple. The optical pyrometer and, in the case of a thermocouple, the controller of the thermocouple are positioned outside the tundish, preferably at or near the distal end of the stopper or at or near the mechanical system controlling the vertical up and down motions of the stopper. In case of an optical pyrometer, the inner zone must necessarily be in optical communication with the pyrometer. There must be an unobstructed straight line or cone between the pyrometer and the whole area of the inner zone. As illustrated in Figures 2, 5(a) and 6(a) this can be achieved by providing the elongated body with an inner cavity (4) extending from an open end, opening at the distal end of the elongated body to a closed end forming the inner zone (3i). The optical pyrometer can thus be located adjacent to the open end of the inner cavity (4), in optical communication with the inner zone (3i) formed by the closed end. [0037] A thermocouple is formed by two dissimilar conductor wires joined to one another at one end to form an electric circuit, each conductor wire forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. In one embodiment, the conductor wires can be embedded in the bulk of the elongated body and joined to one another at a location adjacent to the outer zone. The voltage to temperature converter can be positioned anywhere outside the tundish. The inner zone would then be formed by the refractory material contacting the two wires where they join. This solution has the defect that in case a wire breaks (for any reason) or the two wires come out of contact, there is no way of fixing the problem and getting an information on the temperature of the molten melt. Furthermore, embedding a thermocouple in the bulk of the refractory material exposes the thermocouple to the severe pressures and temperatures used to produce the elongated body, which are likely to damage the thermocouple.

[0038] For this reason, it is preferred that, even when a thermocouple is used, the elongated body comprises an inner cavity (4) as described supra in reference of optical pyrometers. This way, a thermocouple can be introduced into the inner cavity after complete production of the elongated body, with the joined wires positioned against the closed end of the inner cavity forming the inner zone. If a thermocouple breaks for any reason, it can be replaced easily without interrupting metal casting operations. Since the conductor cables are flexible, it is not essential that the inner cavity extends along a straight line. In practice, it is preferred that the inner cavity extends along a straight line, preferably along the longitudinal axis, X1 , because (a) it is simpler to produce, and (b) a thermocouple can easily and rapidly be positioned against the inner zone by coupling the wire to a relatively rigid rod, which can easily be inserted into the inner cavity forcing the joined wires to contact the inner zone.

[0039] The inner zone must be sufficiently large to be representative of the temperature of the molten metal contacting the outer zone. The maximum dimension of the inner zone is limited by the mechanical properties of the elongated body comprising an inner cavity, which decrease with increasing cross-section of the inner cavity and, consequently, of the inner zone. A projection of the closed end of the inner cavity forming the inner zone onto a plane normal to the longitudinal axis, X1 , may have an area, A3i, comprised between 4 and 75% of the area, A1 w, of a cross- section of peripheral wall of the elongated body normal to the longitudinal axis, X1 , at the level of the intersection between the inner zone and the longitudinal axis, X1 : 0.04 A1 w < A3i < 0.75 A1 w (cf. Figures 5(e) and 6(e)). The ratio A3i / A1 w is preferably comprised between 4 and 50%: 0.04 A1 w < A3i < 0.5 A1 w.

[0040] As illustrated in Figure 2, the inner zone (3i) and outer zone (3o) can be part of an insert (3c) cemented to the bulk of the refractory body. The insert may form a protrusion, as illustrated in Figure 2(b) to (e) with all the advantages discussed supra in reference of protrusions. Alternatively, the insert may be flush with the through-hole and form no protrusion, as illustrated in Figure 2(a). The use of an insert is advantageous for several reasons. First, the geometry of the inner and outer zones can be better controlled in a process comprising the following steps:

(a) an insert is produced separately, affording much freedom in the geometry of the inner and outer zones;

(b) an elongated body is produced comprising a through-hole (2) and a through cavity extending parallel to the longitudinal axis, X1 , and opening at the distal end and at the through-hole wall and,

(c) cementing the insert to the opening of the through cavity at the through-hole wall, to form an inner cavity (4) comprising an inner zone.

[0041 ] A second advantage is that the insert can be made of a refractory material having higher thermal conductivity than the refractory material forming the bulk of the elongated body.

[0042] A drawback of the use of an insert is of course that this process requires several moulds and steps, including assembly steps. A stopper according to the present invention can also be produced in a single step, yielding inner and outer zones forming an integral part of the elongated body. The inner cavity (4) is formed with a closed end forming the inner zone by inserting in the tool an inner rod-shaped core extending along the longitudinal axis, X1 , and the through-hole is formed by inserting a core, transverse to the longitudinal axis, X1 , and at a short distance from an end of the inner rod-shaped core, thus defining the strip separating the inner from the outer zones. Different refractory materials can be locally used. This process is simpler but affords less freedom in the design of the inner and outer zones and through-hole.

[0043] Regardless of whether an insert is used or not, it is advantageous to use a refractory material for the strip separating the inner zone from the outer zone having a high thermal conductivity. The rest of the bulk of the elongated body, constituting at least 70% in volume of the elongated body can be selected for the mechanical and physio-chemical properties required by a stopper, regardless of the thermal conductivity, whilst the refractory material of the insert can be optimized for thermal conductivity. A strip having higher thermal conductivity yields a quicker response to variations in temperature of the molten metal around the outer zone. [0044] As discussed earlier, the through-hole may weaken mechanically the elongated body. In particular, the compression strength of the elongated body is decreased by the presence of the through-hole. As illustrated in Figure 2(e), to compensate the lower compression properties of a stopper according to the present invention, the refractory body may form a bulge (1 bw) at the level of the through hole, such that a cross-section normal to the central axis, X1 , at the level of the through hole has a larger perimeter than at any other level of the elongated body within a distance of 2/3L from the nose. This simple solution allows the through-hole to be flanked on either side by additional material, thus locally reinforcing the elongated body.

[0045] As illustrated in Figures 2(c) to (e) and 8, many stoppers are equipped with a blanket gas line (7n) extending from the distal end of the elongated body, where it is coupled to a source of pressurized inert gas, typically argon, and extending along the bulk of the elongated body until an outlet located at or adjacent to the nose tip. Argon can thus be injected from the nose tip into the molten metal flowing out through the inner nozzle (22d) into a casting tool. This prevents inter alia, ingress of air at a position of low pressure, due to a Venturi effect created by molten metal flowing into the gap formed between the nose tip and inlet of the inner nozzle. [0046] Molten metal in a tundish comprises an upper layer of slag floating at its surface. Slag is useful for thermally insulating the molten metal, protecting it from any contact with atmospheric air, and for drawing any impurities out of the molten metal. As molten metal flows out into the casting tool through the inner nozzle, the level of molten metal drops. If all the molten metal has flown out of the tundish, slag will reach the inner nozzle and be cast into the casting tool, wasting a whole portion of slab production. It is important to prevent the level of molten metal to lower below a certain depth and, above all, to prevent slag from flowing through the inner nozzle into the casting tool.

[0047] WO20050421 83 proposed to provide a gas line (7n) for injecting an inert gas under pressure to the tip of the nose with a restriction to build up some pressure, and the pressure downstream from the restriction is monitored. The slag has a density very different from the density of molten metal. When slag flows into the inner nozzle when the tundish is close to be empty, the pressure varies suddenly and the stopper can be actuated to close the inner nozzle. This system has the major drawback that by the time slag has reached the outlet of the argon line at the nose tip, it is already too late, as slag has already flown through the inner nozzle and into the casting tool. Furthermore, because of the molten metal flowing into the gap formed between the nose tip and the inner nozzle, large pressure variations occur varying constantly with the movements of the stopper and with the flow rate of the molten metal. The system proposed in WO20050421 83 can only serve to detect very large variations of pressure, caused, as described in the document, by slag reaching the nose tip. No prevention is possible with this system.

[0048] In the present invention illustrated in Figure 8, it is proposed to provide a stopper comprising: an elongated body comprising an outer surface made of a refractory material extending over a length, L, along a longitudinal axis, X1 , from a distal end to a nose, a gas line (7w) comprising an inlet connectable to a source of inert gas (7), preferably argon, extending along a portion of the elongated body and comprising an outlet at the outer surface of the elongated body, located at a distance of at least 30 mm, preferably at least 1 00 mm from the nose tip, and of not more than L / 2, preferably not more than L/3, more preferably, not more than 300 mm from the nose tip, and a pressure measuring device for measuring the pressure in the gas line, and a controller (7c), such as a programmable logic controller (PLC), configured for maintaining a constant gas flow rate in the gas line, recording the pressure measured in the gas line, and for correlating said measured pressure to the depth of immersion, G, of the outlet when the refractory stopper is immersed in a metal melt and, optionally, for correlating said measured pressure to the presence of slag at the level of the outlet.

[0049] By locating the outlet of the gas line at least at 1 00 mm from the nose tip, the pressure of the molten metal at the level of the outlet varies moderately, as it is remote from the Venturi channel formed by the gap between the nose tip and the inlet of the inner nozzle. This has the advantage that the controller (e.g., a PLC) ensures a constant flow rate of the gas and can correlate the pressure measured in the gas line as a function of the hydrostatic pressure (sometimes called ferrostatic pressure), which itself depends on the immersion depth of the outlet. The controller can therefore determine from the pressure variations recorded in the gas line, the depth of immersion, G, of the gas line outlet and, since the distance of the nose tip and of course, of the gas line outlet from the inlet of the inner nozzle is controlled by the controller, the controller can determine instantaneously the filling level of the molten metal in the tundish. This way, instructions can be given to a ladle to replenish the tundish or, if a ladle is not available, then to close the inner nozzle with the stopper when the filling level drops below a predefined reference value.

[0050] As described in WO20050421 83, when slag reaches the outlet of the gas line, a substantial pressure variation is recorded because of the difference in densities between molten metal and slag. The system of WO20050421 83 relies solely on the slag reaching the outlet of the gas line located at the nose tip, when it is too late. The present system determines the instantaneous filling level of the tundish and leaves plenty of time for an operator or a computer to take appropriate measures to ensure that casting is not interrupted. In case of failure of the system, however, because, for example, of pressure fluctuations more important than the system can cope with, when slag reaches the gas line outlet, the system will detect a major pressure variation, as described in WO20050421 83. The advantage, however, is here that since the outlet of the gas line is at least at 1 00 mm from the nose tip, when slag reaches the gas line outlet, there is still time to react before the slag reaches the inner nozzle.

[0051 ] In a preferred embodiment, the stopper comprising a through hole described above is provided with a gas line (7w) as illustrated in Figures 2(d) and (e) and 8 for determining the filling level of the tundish. Accordingly, the stopper of the present invention preferably comprises a channel (7w) comprising an inlet connectable to a source of inert gas (7), preferably argon, extending along a portion of the elongated body and comprising an outlet at the peripheral wall of the elongated body, located adjacent to the through hole, but not at the through hole wall and at a distance from the nose tip comprised between 1 00 mm and L / 2. In this embodiment, the refractory stopper further comprises a pressure measuring device (7p) for measuring the pressure in the gas line, and a controller (7c) (such as a PLC or equivalent calculator) maintaining a constant flow rate of gas in the gas line, and configured for correlating said pressure to the depth of immersion, G, of the outlet when the refractory stopper is immersed in a metal melt. This same controller may also run a real time algorithm on the measured pressure to detect the interface between slag and liquid steel when passing in front of the outlet, based on the densities difference between metal melt and slag This feature is particularly useful when draining the metallurgical vessel.

[0052] A stopper according to the present invention may also comprise a blanket gas line (7n) as illustrated in Figures 2(c) to (e) and 8, comprising an inlet connectable to a source of inert gas (7), preferably argon, extending along the elongated body and comprising an outlet at the nose tip. The blanket gas line (7n) is used for forming a blanket of inert gas in the molten metal flowing out through the inner nozzle.

# Feature

1 b Bulk

1 bw bulging portion

1 d distal end

1 n nose tip

1 w peripheral wall

1 elongated stopper

2 a through hole openings

2 through hole

3 c insert

3 i inner zone

3 o outer zone

3 p Protrusion

4 Inner cavity

5 device for measuring temperature of inner zone

7 c Controller (e.g. PLC)

7 n argon line at nose tip

7 Ρ pressure measuring device

7 w argon line at peripheral wall (away of nose tip)

7 source of inert gas

10 molten metal

21 Ladle

22 d inner nozzle

22 Tundish

23 Mould