FIELD OF THE INVENTION

[0001 ] The present invention generally relates to stoppers for controlling the flow of molten metal out of a dispensing outlet of a metallurgic vessel towards a mould or a casting tool. In particular, it concerns stoppers equipped with an integrated pressure measurement device allowing the determination of the position of slag and of the instantaneous molten metal filling level, Gt, of a metallurgic vessel at proximity of the dispensing 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 a dispensing 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.

[0003] 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 the dispensing outlet of the metallurgic vessel. A stopper is a slender element formed by 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 the nose tip, By moving the stopper up and down the vertical distance, Gn, 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 (Gn = 0), 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 nose tip between an opening and closing positions, and variation of the flowrate between these two positions is controlled by other means.

[0004] Many stoppers are equipped with a blanket gas line (7n) extending from a 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 a blanket 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 dispensing outlet (22o) of 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. [0005] Molten metal (10) in a tundish comprises an upper layer of slag (1 0s) 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. Figure 7 compares the level, Gt, of the liquid bath in a tundish with the level, GL, in two successive ladles, L1 , L2, replenishing the tundish, . As can be seen in Figure 7, as molten metal flows out into the casting tool through the dispensing outlet of the inner nozzle, the level, Gt, of molten metal in the tundish is maintained substantially constant by new molten metal being poured into the tundish by a ladle (21 ). When a ladle is empty, it is removed and replaced by a full ladle. During the switching time of ladles, the tundish is not replenished anymore and the level of molten metal in the tundish drops as metal flows out. The switching time is normally very short, and the drop of the level of metal in the tundish is not substantial. In case of problems with a change of ladle or towards the end of a casting operation, the level of molten metal in the tundish may drop to lower levels as metal flows out. When all the molten metal has flown out of the tundish, the slag layer of thickness, Gs, reaches the inner nozzle and is cast into the casting tool, wasting a whole portion of metal (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.

[0006] WO20050421 83 proposed to provide a gas line (7n) for injecting an inert gas under pressure to the nose tip 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. For example, when molten steel has a density of about 7.2, a slag can have a density of the order of 3. 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 blanket outlet of the argon line at the nose tip, it is already too late, as slag has already flown through the dispensing outlet of 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 WO2005042183 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.

[0007] To date, if the slag must not reach the tool, casting is generally interrupted when the tundish still contains considerable amounts of molten metal as a safety margin, thus increasing the production costs. There remains a need for a device suitable for detecting the slag preventively to avoid contamination of the metal parts being cast. Monitoring of the level of liquid contained in the tundish is also desired to give information to an operator or to a controller for taking appropriate actions. 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 metallurgic installation comprising:

(A) A metallurgic vessel, typically a tundish, for containing a liquid bath comprising a layer of slag floating on top of molten metal, and for dispensing the molten metal out of a dispensing outlet located at a bottom floor of the metallurgic vessel;

(B) A refractory stopper for controlling the flow of molten metal out of the dispensing outlet of the metallurgic vessel, said refractory 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 tip, and is mounted over the dispensing outlet with the longitudinal axis, X1 , substantially vertical, such that the nose tip can move from an open position allowing molten metal to flow through the dispensing outlet to a sealed position wherein the molten metal cannot flow through the dispensing outlet,

• a gas line comprising a gas inlet connectable to a source of inert gas, preferably argon, extending along a portion of the elongated body and comprising a gas outlet at the outer surface of the elongated body, located at a distance, Ln, measured along the longitudinal axis, X1 , of at least 30 mm, preferably at least 100 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,

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

• a controller configured for: o maintaining a constant gas flow rate in the gas line, o recording the pressure measured in the gas line, and o correlating said measured pressure to the presence or not of slag at the level of the gas outlet.

[0009] In a preferred embodiment, the correlation of the measured pressure to the presence or not of slag at the level of the gas outlet is based on the value of the secondary derivative, d ² P / dG ² , of the pressure to the distance, G, between the gas outlet and an upper surface of the liquid bath in which the gas outlet is immersed. The controller is preferably configured for moving the stopper to the sealed position when slag is detected at the level of the gas outlet.

[0010] In an alternative embodiment, the controller is also configured for correlating said measured pressure to a level, Gt, of the liquid bath, wherein Gt = G + Go; with G is the distance of the gas outlet to an upper surface of the liquid bath, and Go is the distance of the gas outlet to the bottom floor. The controller can be configured for moving the stopper to the sealed position when the level, Gt, drops below a predetermined threshold value, G_lim. If the metallurgic vessel is being replenished by a ladle provided with a gate controlling a flowrate of molten metal out of the ladle and into the metallurgic vessel, the controller can be configured for instructing the slide gate to increase the flow rate out of the ladle to raise the level, Gt, in the metallurgic vessel when the level, Gt, drops below a predetermined threshold value, GJim.

[001 1 ] The gas flow rate in the gas line can be maintained substantially constant by providing a flow valve in the gas line and a flowmeter, and in response to a measurement of the flow rate through the gas line the controller can vary the flow valve opening. [0012] The stopper can comprise a blanket gas line extending from the distal end of the elongated body, where it is coupled to a source of pressurized inert gas, and extending along the bulk of the elongated body until a blanket outlet located at or adjacent to the nose tip.

[0013] The present invention also concerns a stopper as defined supra, for controlling the flow of molten metal out of the metallurgic vessel of a metallurgic installation as discussed above, said refractory 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 tip, and suitable for being partially immersed including the nose tip with the longitudinal axis, X1 , substantially vertical, in a liquid bath comprising a layer of slag floating on top of molten metal and contained in the metallurgic vessel, a gas line comprising a gas inlet connectable to a source of inert gas, preferably argon, extending along a portion of the elongated body and comprising a gas outlet at the outer surface of the elongated body, located at a distance, Ln, measured along the longitudinal axis, X1 , of at least 30 mm, preferably at least 100 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, a pressure measuring device for measuring the pressure, P, in the gas line, and a controller configured for: o maintaining a constant gas flow rate in the gas line, o recording the pressure measured in the gas line, and o correlating said measured pressure to the presence or not of slag at the level of the gas outlet. [0014] In a preferred embodiment, the elongated body of the refractory stopper 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. A device for measuring the temperature at an inner zone located within the bulk of the elongated body is provided. The inner zone is 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. 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. The gas outlet preferably opens at the through hole wall or is separated from the through hole wall by a distance of not more than 50 mm.

BRIEF DESCRIPTION OF THE FIGU RES.

[0015] 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 a cut view of a stopper according to the present invention partially immersed in a liquid bath in a tundish.

Figure 3: shows plots of the pressure, P, first derivative, dP / dG, and second derivative d ² P / dG ² as a function of the distance, G, between the gas outlet and the surface of the liquid bath.

Figure 4: shows plots of G and Gi as a function of time, t, wherein i = t, m, s, and o, with varying distance, Go.

Figure 5: shows plots of G and Gi as a function of pressure, P, wherein i = t and o, with varying distance, Go.

Figure 6: shows several embodiments of stoppers according to the present invention.

Figure 7: shows an example of a portion of a casting sequence, with two ladles, L1 , L2, replenishing a tundish to maintain the level, Gt, of liquid in the tundish substantially constant throughout the casting operation. Figure 8: shows a stopper according to the present invention at four different levels, Gt, of the liquid bath. Reference to Figure 8(b) to (d) can be found in Figure 4.

DETAILED DESCRIPTION OF THE INVENTION [0016] 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 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 tip, and suitable for being partially immersed including the nose tip with the longitudinal axis, X1 , substantially vertical, in a liquid bath comprising a layer of slag (10s) of thickness, Gs, floating on top of molten metal (10) and contained in the metallurgic vessel (22),

[0017] According to the present invention the stopper also comprises a gas line (7w) comprising a gas inlet connectable to a source of inert gas (7), preferably argon, extending along a portion of the elongated body and comprising a gas outlet (7o) at the outer surface of the elongated body. The gas outlet (7o) is located at a distance, Ln, from the nose tip measured along the longitudinal axis, X1 , of at least 30 mm, preferably at least 1 00 mm, more preferably at least 150 mm, most preferably at least 200 mm, and of not more than L / 2, preferably not more than L/3, more preferably, not more than 300 mm from the nose tip. The stopper also comprises: a pressure measuring device (7p) for measuring the pressure, P, in the gas line, and a controller (7c) configured for:

• maintaining a substantially constant gas flow rate in the gas line (7w),

• recording the pressure measured in the gas line, and

• correlating said measured pressure to the presence or not of slag at the level of the gas outlet. [0018] The stopper of the present invention is used in a metallurgic installation comprising: (A) a metallurgic vessel (22), typically a tundish, for containing a liquid bath comprising a layer of slag (1 0s) floating on top of molten metal (1 0), and for dispensing the molten metal out of a dispensing outlet (22o) located in an inner nozzle (22d) at a bottom floor of the metallurgic vessel; and (B) a refractory stopper (1 ) as defined above, for controlling the flow of molten metal out of the dispensing outlet of the metallurgic vessel.

[0019] The gist of the present invention is to locate the gas outlet (7o) of the gas line at least at 30 mm, preferably at least at 100 mm-200 mm from the nose tip. The pressure of the molten metal at the level of the gas outlet (7o) varies regularly, with relatively little noise, as it is remote from the Venturi channel formed by the gap between the nose tip and the dispensing outlet (22o) of the inner nozzle. This has the advantage that the controller (e.g., a PLC) ensures a constant flow rate of the gas with little perturbations and noise 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, G, of the gas outlet (7o). SLAG DETECTION

[0020] Figure 3 shows a correlation between the distance of the gas outlet (7o) to the top surface of the liquid bath for a fixed position, Go, of the stopper, as a function of

• the pressure, P, measured in the gas line at constant flow rate (long dashed line),

• the first derivative, dP / dG of the pressure, P, with respect to the distance G, (short dashed line) and

• the second derivative, d ² P / dG ² of the pressure, P, with respect to the distance, G, (continuous line).

[0021 ] It can be seen in Figure 2, that the pressure, P, varies linearly with the immersion depth, G, of the gas outlet (7o). When the level, Gm, of metal melt, of density 7.2 for steel, drops below the level of the gas outlet, the gas outlet is in contact with slag, of lower density, of the order of 3. The slope of the G(P)-curve therefore decreases until both P and G reach the 0-value (the system is calibrated to measure a pressure P = 0 when the gas outlet faces air). By calibrating a given system (temperature, type of molten metal, slag composition, and tundish dimensions) the measurement of the pressure required to blow inert gas into the liquid bath at constant flow rate gives an instant reading:

• of the passage of slag (1 0s) before the gas outlet, indicated by a change of the slope of the G(P)-curve, and

• of the actual depth, G, of immersion of the gas outlet (7o), by correlating the measured value of P to the corresponding value of G; The depth of immersion, G, is simply the distance of the gas outlet to the top surface of the liquid bath (including the slag). By adding the distance, Go, of the gas outlet to the bottom floor of the tundish to the depth, G, of immersion of the outlet, the instant level, Gt, of the liquid bath can be determined (cf. Figure 5).

[0022] In the graph of Figure 3, the slag layer (1 0s) has a thickness of 30 mm, corresponding to the change of slope of the G(P)-curve. The P-signal measured can be noisy, because of gas leaks in the gas line (7w), or of turbulences at and around the gas outlet (7o), etc. Depending on the level of noise in the signal a change of slope (even for a slope change of factor 2.4 for steel density of 7.2 and slag density of 3) is not necessarily straightforward and conclusive to identify. For this reason, it is preferred to monitor the second derivative, d ² P / dG ² , which shows a sharp peak when the gas outlet (7o) is reached by the interface / interphase between molten metal and slag (cf. solid line in Figure 3). The d ² P / dG ² -peak is normally higher than the level of noise of the signal, even for relatively high levels of noise. By monitoring the second derivative, d ² P / dG ² , a clear signal that the slag has reached the level of the gas outlet can be unambiguously detected, and actions can be taken. [0023] In a preferred embodiment, the controller (7c) gives an optical or acoustic warning when the presence of slag is detected at the level of the gas outlet (7o) to warn an operator that actions are required to prevent slag from flowing out into the mould through the dispensing outlet (22o). For example, the operator can move the stopper to the sealed position or increase the flow rate of molten metal flowing from a ladle (21 ) into the tundish by opening wider the slide gate controlling the flow out of the ladle. Alternatively, or additionally, the controller may be configured for automatically moving the stopper to the sealed position when slag is detected at the level of the gas outlet (7o).

[0024] The present invention provides a stopper able to detect when the slag (1 0s) reaches a given level, Go, in the tundish. It is advantageous over prior art solutions, such as the one described in WO20050421 83, in that since the gas outlet (7o) is at a distance, Ln, of at least 30 mm, preferably at least 100 to 200 mm from the nose tip,

• detection of the presence of slag at the level of the gas outlet provides enough time to react such as to prevent any slag from flowing out through the dispensing outlet, and

• the flows in the liquid bath at and around the gas outlet are much more streamlined than at the level of the nose tip, thus allowing a cleaner signal of the pressure in the gas line than in the prior art.

The present invention also allows the instant depth monitoring of the liquid bath in the tundish. INSTANT DEPTH MONITORING

[0025] Figure 4 illustrates an example of how the level, Gt, of the liquid bath can be determined from the value of the immersion depth, G, of the gas outlet determined by the measurement of the pressure, P, as explained supra, together with the distance, Go, of the gas outlet from the tundish bottom: Gt = G + Go (cf. also Figure 2). If the stopper moves up and down, the value of the distance, Go, varies accordingly, and must be taken into account for the determination of the level, Gt. Since the movements of the stopper are controlled by the controller and the measured values of the pressure are correlated to the corresponding values of the immersion depth, G, the controller can easily determine the instant value of Gt, as a function of time, as shown in Figure 4. The distance, Go, of the gas outlet to the bottom floor of the tundish is equal to Gos, when the stopper seals the dispensing outlet (22o) of the tundish, i.e., when the distance, Gn, of the nose tip to the sealed position equals 0 (Go (sealed position) = Gos and corresponds to Gn = 0).

[0026] Figure 5 shows an example of correlation between the pressure, P, in the gas line (7w) and the immersion depth, G, of the gas outlet (7o) as the stopper moves, with varying values of the distance, Go, of the gas outlet to the bottom floor of the tundish. Note that Figure 3 is a plot corresponding to a fixed position, Go, of the stopper. It can be seen in Figure 5 that the value of G varies with the movements of the stopper (Go). The sum of G and Go, yields the level, Gt, of the liquid bath, and the variation of Gt as metal flows out of the tundish, with respect to P is constant, i.e., dGt / dP = constant, independently of the position of the stopper.

[0027] After calibration of a given system, with a simple measurement of the pressure, P, in the gas channel, the controller (7c) can thus determine the instant level, Gt, of the liquid bath at any time (cf. Figures 4 and 5). There are other systems for determining the instant level, Gt, of the liquid bath, but (a) not all installations are equipped with such systems, and (b) said systems are independent of the stopper and its movements. The present invention therefore provides an alternative solution for the determination of the instant level, Gt, of the liquid bath, with data which are directly related to the stopper and its movements, thus allowing rapid reactions triggered by a single controller, without interfaces with other control systems.

[0028] For example, the controller (7c) can be configured for sending an optical and/or acoustic warning signal when the level, Gt, drops below a predetermined threshold value, GJim. Alternatively or additionally, the controller can be configured for moving the stopper to the sealed position when the level, Gt, drops below the predetermined threshold value, GJim. The measurement is instantaneous, the data treatment is simple and rapid, and the command can be given immediately by the controller to shut the dispensing outlet (22o) by moving the stopper all the way down.

[0029] In another example, when the level, Gt, drops below the predetermined threshold value, GJim, the controller can be configured for instructing the slide gate (21 g) of the ladle (21 ) to increase the flow rate out of the ladle to raise the level, Gt, in the metallurgic vessel. By increasing the flow rate out of the ladle to values higher than the flow rate out of the tundish, the level, Gt, of the liquid bath in the tundish can raise above GJim.

STOPPER CONFIGURATIONS [0030] Figures 2 and 6 illustrate some examples of stoppers according to the present invention. As shown in Figure 2, a stopper has an elongated body extending over a length, L, along a longitudinal axis, X1 , from a distal end to a nose tip and ,in use, is partially immersed including the nose tip with the longitudinal axis, X1 , substantially vertical, in a liquid bath comprising a layer of slag floating on top of molten metal and contained in the metallurgic vessel. The gas line (7w) comprises a gas inlet connectable / connected to a source of inert gas (7), preferably argon. The gas line penetrates into the elongated body and extends along a portion of the elongated body and comprises a gas outlet (7o) at the outer surface of the elongated body. In order to yield reliable measurements of the pressure in the gas line, it must be as gas tight as possible over the whole length thereof. Seals or gaskets (7s) can be used for sealing the portion of gas line penetrating into the refractory body (cf. Figure 6). The gas outlet is located at a distance, Ln, measured along the longitudinal axis, X1 , 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. [0031 ] The stopper is coupled to a controller (7c). The controller is configured for maintaining a constant gas flow rate in the gas line. For example, the gas line can be equipped with a flow valve (7v) and with a flowmeter (7f) for measuring the flow rate in the gas line downstream of the flow valve. With a closed loop control system, the controller can ensure that the flowrate in the gas line (7w) remains substantially constant by varying the opening of the valve in response to the measurements of the flowrate by the flowmeter (7f). For example, a proportional-integral- derivative (PI D) controller readily available on the market can be used to this purpose

[0032] A pressure measuring device (7p) is provided on the gas line for measuring the pressure, P, in the gas line. The pressure measuring device can be any type of pressure gauge suitable for measuring a gas pressure in a pipe. The present invention is not restricted to any particular type and model of such pressure gauge. The pressure measuring device is coupled to the controller (7c) for recording the values of the pressure, P, measured therewith.

[0033] The controller is configured for correlating the measured pressure values to the presence or not of slag at the level of the gas outlet, as described above. In particular, the controller can determine the second derivative, d ² P / dG ² , of the pressure variations with respect to the immersion depth, G, of the gas inlet, and determine whether a peak extending over a predetermined threshold value is present or not (cf. Figure 3, solid line). The controller can also correlate the pressure values to the level, Gt, of the liquid bath in the tundish as described supra, in relation with Figures 4&5.

[0034] In a preferred embodiment illustrated in Figure 6, the stopper comprises a through hole (2), extending through the elongated body along a transverse axis, X2, transverse, preferably normal to the longitudinal axis, X1 , 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 (2a) being located at the peripheral wall. The stopper also preferably comprises 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, wherein the temperature of the inner zone is representative of an instantaneous temperature at the outer zone. The outer zone belongs to the through hole wall and is separated from the nose tip by a distance, M, lower than one half of the length, L, of the elongated body, M < ½ L, preferably lower than one third of the length, L, M < ½ L. The outer zone is generally the portion of through-hole wall located closest to the distal end of the elongated body. With this embodiment, it is possible to measure the temperature of the molten metal close to the dispensing outlet (22o) of the tundish, with a response time to temperature variation much shorter than with existing systems.

[0035] The gas outlet (7o) of the gas line preferably opens at the through hole wall (cf. Figure 6(b)) or is separated from the through hole wall by a distance of not more than 50 mm (cf. Figure 6(a)&(c)-(e). This location is preferred because the turbulence at and around the through-hoe are generally of lower intensity than close to the nose tip and, at the same time, is at an optimal distance for warning of the passage of slag at the level of the gas outlet and still give time to an operator or to the controller to act in order to prevent any slag from flowing out through the dispensing outlet (22o). [0036] As shown in Figure 6(c)&(e), the outer zone forms a protrusion (3p) jutting out of a surface of the through hole wall. The protrusion is preferably 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 . The area of the outer zone (3o) can thus be increased for a more efficient heat measurement.

[0037] As shown in Figure 6(d)&(e) the refractory body can forms 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 2/3 L from the nose. The bulge can reinforce mechanically the zone weakened by the through-hole.

[0038] A stopper according to the present invention may also comprise a blanket gas line (7n) as illustrated in Figures 2 and 6(c)&(e). The blanket gas line (7n) comprising an inlet connectable to a source of inert gas (7), preferably argon, extending along the elongated body and comprising a blanket 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.

APPLICATIONS OF THE STOPPER

[0039] As illustrated in Figure 7, the level, Gt, of the liquid bath in a tundish should remain substantially constant throughout a whole continuous casting operation, with a balance between the molten metal flowing from a ladle into the tundish and the molten metal flowing from the tundish into the mould. This is the mass balance problem well known to all students of determining the level of water in a bath tub with an open tap and an open drain. The stopper of the present invention can identify when there is a dysfunction in the flowrates from the ladle or from the tundish, resulting in an abnormal evolution of the level, Gt, of the liquid bath in the tundish. Such problems are, however, quite rare. The stopper of the present invention is particularly advantageous in non-stationary stages of a casting operation, namely, on the one hand, the replacement of an empty ladle by a full ladle and, on the other hand, at the end of a casting session, when the tundish is emptied.

[0040] As shown in Figure 7, when a ladle, L1 , is empty, it must be replaced by a new ladle, L2, full of molten metal. This operation is very swift, and the resulting drop of the level, Gt, in the tundish is generally very moderate. All sorts of problems, however, can occur during this swapping operation: the new ladle may not be in place, the coupling of a ladle shroud to the ladle gate can be laborious, the gate may not work properly, etc. Any of these problems would prolong the time during which the tundish is not replenished, leading to a decrease of the level, Gt, of the liquid bath which can reach critical levels in case of long delays before the next ladle is operational again. In such events, the stopper of the present invention can prevent the accidental flow of slag into the mould, thus saving a whole portion of the production from waste. An alarm can warn that slag has reached the level, Go, vis-a-vis the gas outlet (7o), leaving time to an operator or to the controller to take the adequate actions to prevent slag from flowing through the dispensing outlet (22o).

[0041 ] At the end of a casting operation, the last ladle, L2, is not replaced by a new one, and the tundish is emptied for the last stage of the casting operation. Absent a reliable indication of the instant level of the Gs-thick slag layer in the tundish as it is being emptied, it is common practice to take a large security margin of say 1 0 Gs or more, and stop casting too early, thus wasting a large amount -several tons- of molten metal. The present invention allows the reduction of the security margin, emptying the tundish to lower predetermined threshold level values, GJim, and thus pushing the production further.

[0042] Referring to the graph of Figure 3, when the tundish is being emptied, the position of the stopper can preferably be set to a given value corresponding to a distance, Go, of the gas outlet to the tundish bottom floor, to reduce noise in the pressure measurements. When the stopper identifies that the slag level has reached the gas outlet (7o), the operator or controller know that it remains a level, Gm, of molten metal equal to Go (Gm = Go), corresponding to the situation illustrated in Figure 8(b) and indicated in Figure 4 with the label 8(b). If the predetermined threshold value, GJim, is lower than Gt = Go + Gs, at point 8(b) of Figure 4, the only way to proceed with control using the correlation of whether the slag has reached the gas outlet is to lower the plunger, to reduce, Go. With the correlation of whether the slag has reached the level of the gas outlet, the predetermined threshold value, GJim = Go + Gs, wherein Go can be reduced up to a certain point, higher than at position Gos, because as the plunger lowers, the flow rate decreases accordingly, until becoming too weak to produce an acceptable part.

[0043] By monitoring the level, Gt, as a function of time as described supra, the predetermined threshold value can be lowered to values as low as, GJim = Go, which is lower by a value, Gs, than the lowest value of GJim possible with the sole detection of the passage of the slag at the level of the gas outlet.

[0044] At a first position of Go, as illustrated in Figure 8(c), and at point 8(c) in Figure 4, the decrease of the level, Gt, can be monitored, until Gt = Go. The level, Gt, of the liquid bath cannot be monitored anymore, when Gt < Go, as the gas outlet would then be facing open air and no correlation between pressure and depth would be possible anymore. As shown in Figure 8(d) and at point 8(d) in Figure 4, the stopper can be lowered to a lower value of Go, corresponding to the predetermined threshold value, GJim. The flow of molten metal can continue at a lower rate (because the stopper is closer to the sealed position, Gos) until the level of the bath reaches Gm = Go as shown in Figure 8(d) and in point 8(d) of Figure 4, or even lower to

Gt = GJim = Go. At this stage, a warning signal can be emitted (acoustic and / or optical) and the controller is preferably configured for lowering the stopper to its sealed position, Gos, to stop the flow of liquid out of the dispensing outlet (22o).

[0045] Considering that a 1 00 mm deep bath of molten steel weighs 720 kg / m ² , it is clear that the stopper of the present solution allows several tons of steel to be saved from waste per casting operation, with a fool proof system, economical to implement, and highly reliable.

# 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 f Flowmeter

7 n gas line at nose tip

7 o Outlet of gas line 7w

7 P pressure measuring device

7 S Seal to gas line

7 v Valve controlling the flow rate in gas line

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

7 source of inert gas

10 molten metal

21 Ladle

21 g Gate valve of ladle

22 d inner nozzle

22 o Tundish outlet

22 Tundish

23 Mould

G Distance of gas line outlet 7o to molten metal

G L Level of liquid bath in ladle

G_ lim Predetermined threshold value

G n Position of nose tip wrt sealing position

G m Level of molten metal

G o Distance of the gas outlet to bottom floor

G os Go at sealed position of the stopper

G s Thickness of slag layer

L n Distance of gas line outlet 7o to nose tip

L 1 Ladle #1

L 2 Ladle #2