METHOD FOR DETECTING SLAG FLOW

Abstract: This invention relates to a method for exactly detecting the inflow of slag into a molten steel stream when a molten steel is poured into an intermediate vessel such as a ladle or a tundish from a refining vessel such as a refining furnace or a ladle through a tap hole or a nozzle, by supplying an inert gas into the molten steel flow inside the nozzle from the side of the tap hole or the nozzle and detecting the inflow of slag on the basis of changes in the flow rate of the inert gas absorbed into the molten steel stream and a back pressure.

Description: The present invention relates to a process for detecting outflow of slag with high accuracy on pouring a molten steel. Slag is ordinarily likely to flow out into a stream of a molten steel in the final stage of discharging molten steel from a refining furnace such as a converter to a ladle through a molten steel-discharging opening, or in the final stage of pouring molten steel from a ladle to an intermediate vessel, such as a tundish, through a nozzle. If the slag is discharged into the molten steel, alloying components such as Aℓ, Fe-Mn and Fe-Si added thereto are taken into the slag, and production costs rise due to a reduction in yield of such alloying components. Further, since the molten steel is oxidised with the slag discharged, the purity of the steel deteriorates, so that the quality of steel products is adversely affected. For this reason, it is an extremely important priority to reduce the outflow of the slag into the molten steel to a minimum, and various measures have formerly been adapted for this purpose. A conventional technique for detecting the outflow of the slag into the molten steel stream, is to use visual judgement. Methods for detecting the slag entering the outflow stream, particularly, from the ladle to the tundish, include for instance, Japanese Patent Application publication No. 57-112,963 disclosing a process for measuring vibrations, Japanese Patent Application publication No. 53-53,521 disclosing a process for measuring the impedance, Japanese Patent Application publication Nos. 60-3,955 and 60-3,956 disclosing a process for measuring microwaves, and Japanese Patent Application publication No. 61-262,454 disclosing a process for measuring the internal pressure of a nozzle. However, the above-mentioned conventional techniques have the following problems. That is, the above visual judgement depends for its accuracy on the individual involved. Human reaction time is comparatively slow and such a method is inapplicable to cases where the steel is poured through a sealed passage and as such is not visible to an outside observer. The vibration-measuring process, the impedance-measuring process, and the microwave-measuring process require that a measuring sensor is brought into proximity with extracted steam. Thus, problems exist with respect to maintenance and operability. Furthermore, the apparatus must be detrimentally increased in size, and it becomes expensive. In the nozzle internal pressure-measuring process, a pressure-measuring hole is liable to be blocked by the molten steel or slag on measuring a negative pressure inside a long nozzle. Thus, the pressure cannot be detected in many cases. Moreover, since the change of pressure inside the nozzle is extremely small, at about 19613 Pa (0.02 kgf/cm²) when the poured melt steam is changed from the molten steel to the slag, it is difficult to detect the change accurately. In addition, since pressure loss can be large depending upon the shape of the pressure-measuring hole, it may become impossible to detect the pressure change due to slag discharge. Thus, pressure cannot be detected accurately. Furthermore, the internal pressure of the nozzle detected by this method is measured by press fitting a long nozzle to a nozzle of the ladle, pouring the inside of the nozzle through blowing an inert gas upon a press-fitted portion via an inert gas-blowing pipe, and measuring the static pressure (negative pressure) inside the nozzle. Although the inclusion amount of the inert gas into the molten steel stream differs from the inclusion amount of the inert gas owing to the discharging slag (the amount of the gas sucked through the press-fitted portion), the inert gas is sucked through the press-fitted portion so that the internal pressure may be constant irrespective of the flow-down kinetic energy of the flowing material. Therefore, since the internal pressure inside the nozzle is maintained at almost the same level, this process has a shortcoming in that discharge of the slag cannot be stably or accurately detected. An example, of the above, may be found in JP-A 61-30615. This concerns a process, which includes introducing a bubbling gas into molten material, as it flows through a passage, formed in the bottom of a converter and communicating with a tap hole. Pressure and/or flow rate of the bubbling gas may be measured to detect variation related to the amount of slag in the metal being tapped. Upon detection of a pressure variation, the converter is turned towards its original angular position. An embodiment of the present invention overcomes the above-mentioned problems, and provides a process capable of detecting outflow of slag into a molten steel stream with high accuracy. According to the present invention there is provided a a method for detecting outflow of slag, when refined molten steel stored in a first vessel is poured into a second vessel through a feed pipe, which method comprises feeding inert gas into a stream of the molten steel in the feed pipe through a gas feed hole provided in the wall of the feed pipe, characterised in that a reduction in flow rate of the inert gas sucked into the feed pipe and/or an increase in back pressure of the inert gas sucked into the feed pipe is/are used to determine the presence of slag in the feed pipe. In the present invention, the first vessel means a refining vessel such as a ladle, and the second vessel means a refining furnace such as a converter or an intermediate vessel such as a ladle or a tundish. The feed pipe means a steel-discharging hole or a nozzle. Accordingly, molten steel-pouring systems to which the process of the present invention is applicable include the case where the molten steel is poured from a refining furnace to a ladle through the molten discharging hole and the case where the molten steel is poured from a ladle into a tundish through the nozzle. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- Fig. 1 is a schematic view of a preferable control system for use in effecting the invention process; Fig. 2 is a diagram showing the relationship between back pressure and flow rate of the gas entering the stream; Figs. 3a and 3b show diagrams illustrating changes in the flow rate and back pressure of Ar gas in the final stages of the pouring operation; Fig. 4 is a diagram showing the relationship between the mixed amount of slag in the ladle and an increase in a clogging index of the nozzle for the tundish; Fig. 5 is a characteristic view showing the distribution of the extent of slag contamination in the ladle when the melt is poured into the tundish according to the invention process; Fig. 6 is a characteristic view illustrating the distribution of the mixed amount of the slag in the ladle when the melt is poured into the tundish using conventional means; Fig. 7 is a diagram illustrating the relationship between the amount of the poured melt and the nozzle-clogging index per tundish nozzle. Fig. 1 is a diagrammatical view showing a preferred control system for effecting the process of the present invention, which illustrates a case where outflow of slag is to be detected when molten steel is poured from a ladle to a tundish in continuous steel casting. In Fig. 1, reference numerals 1 and 2 are the ladle and a ladle nozzle fitted to the bottom of the ladle, respectively. Reference numeral 3 is the tundish into which the molten steel 5 held in the ladle 1 is poured through a long nozzle 4. The slag 6 floats on the upper surface of the molten steel 5. Reference numeral 7 shows a gas supply hole provided through a side face of the ladle nozzle 2. An inert gas is fed through the gas supply pipe 8. Reference numerals 9 and 10 are a flow meter, and a pressure gauge, respectively, which are both attached to the gas supply pipe 8. Measurement signals are fed to a slag detector 11 from them. As the case may be, it may be that a constant pressure controller 12 is provided for the gas supply pipe 8. A slag outflow detector is constituted as follows. When molten steel 5 in the ladle 1 is to be poured into a tundish 3, inert gas is supplied to the ladle nozzle 2 through the gas supply hole 7. At that time, whether the slag flows out into a stream of the molten steel or not is determined by detecting reduction in the flow rate of the inert gas sucked into the stream of the molten steel inside the nozzle. Depending upon this determination, the outflow of slag into the tundish can effectively be controlled by stopping the pouring of the molten steel from the ladle through operating a stopper or sliding nozzle not shown. Since the molten steel must not be oxidized again, the inert gas used is preferably Ar gas. The gas supply hole 7 may be provided at a peripheral side of the long nozzle 5. The combination of the ladle 1 with the tundish 3 has been explained in the above construction. However, when a refining furnace such as a converter is combined with a ladle, a gas supply hole 7 is provided in a steel discharge hole of the refining furnace. Next, the principle of the process for detecting the outflow of the slag according to the present invention will be explained below. In general, a water stream blown though a nozzle is mixed with a medium to be driven at a throat in the case of a water-ejecting pump, and its kinetic energy is given to the medium. Then, a speed head is converted to a pressure head by a diffuser to produce suction forces. In a similar manner, suction forces are generated in a gas flow hole provided in the pipe (for instance, a steel discharge hole, a ladle nozzle, a long nozzle or the like), when the molten steel stream passes inside the pipe. The magnitude of suction forces varies depending upon the diameter and the shape of the gas flow path or the pipe, and is greatly influenced by the kinetic energy of the discharging stream. Therefore, since the density differs between the molten steel stream and the slag stream, their suction forces naturally differ. Thus, as shown in Fig. 1, when the molten steel 5 is poured from the ladle nozzle 2, inert gas is fed through the gas feed opening 7, reduction in the flow rate of the inert gas sucked into the molten steel stream and/or increase in the back pressure are individually or simultaneously measured. Thereby, the outflow rate of the slag into the molten steel stream can be detected based on changes in the flow rate and the back pressure, that is, changes in the suction forces. Here, the magnitude of the kinetic energy of the poured melt stream depends upon the head level of the molten steel 5 inside the ladle 1, the open area of the ladle nozzle 2, and the density of the poured molten steel stream. Therefore, when the poured stream changes from molten steel to slag, the density of the poured stream changes significantly. (While the density of the molten steel is about 7000Kg/m³, that of the slag is about 2500Kg/m³.) Accordingly, the kinetic energy of the poured stream changes likewise. However, when the inert gas is fed into the ladle nozzle 2, the gas is drawn into the outgoing melt stream by the kinetic energy possessed by it. The inert gas inside the gas supply hole 7 and further in the gas feed pipe 8 are sucked into the ladle nozzle 2. Since the suction forces depend upon the kinetic energy possessed by the poured melt stream at that time, the suction forces change significantly when the poured melt stream changes from molten steel to slag. Therefore, the outflow of the slag can be detected by continuously measuring the flow rate and the back pressure of the inert gas flowing inside the gas supply pipe 8. Fig. 2 illustrates the relationship between the back pressure and the flow rate of the inert gas sucked in when the gas is fed into the ladle nozzle 2 through the gas supply hole 7. It is seen that the relationship Q=α √(P+1) , in which P and Q denote the back pressure (kgf/cm²) and the flow rate ( ℓ /min), respectively. As is understood from this figure, when the back pressure P of the inert gas fed is not more than 1 kgf/cm² as the atmosphere pressure, the flow rate changes by a significant amount F₁ for slight change P₁ in the back pressure. Therefore, it is preferable to measure the flow rate Q in this case. On the other hand, when the back pressure P is more than 98,000 Pa (1 kgf/cm²), the flow rate Q changes by only a small amount F₂ even when the back pressure changes by as much as P₂. It is possible to enhance measuring accuracy when measurements of the flow rate Q and the back pressure P are combined. When the back pressure P of the gas is kept constant by attaching the pressure-maintaining unit 12, reduction in the flow rate Q is increased during outflow of slag, and thus detecting accuracy increases. As mentioned above, according to the slag-detecting process of the present invention, since the inert gas is positively fed into the discharging stream through the gas supply hole 7, a problem formerly encountered in that the gas flow path is clogged with metal, is substantially avoided, and the outflow of slag may be accurately detected. In an experiment, while Ar gas was fed into a ladle nozzle at a flow rate of 15 ℓ/min under a back pressure of 9,800 Pa (0.1 kgf/cm²), molten steel was poured from a ladle having a volume of 230 tonnes to a tundish by using a long nozzle. Changes in the flow rate and the back pressures at that time were measured. Their measurement results in a pouring final stage are shown in Figs. 3a and 3b, respectively. Judgements were also visually effected at the same time. As shown in Fig. 3a, the flow rate of Ar gas began to change 6 seconds before the visual judgement, and changed to 13 ℓ/min 4 seconds before the visual judgement. Thereafter, the flow rate was conspicuously lowered to reach 3 ℓ/min at the time of the visual judgement. On the other hand, as shown in Fig. 3b, the back pressure was 9800 Pa (0.1 kgf/cm²) with respect to the atmospheric pressure 4 seconds before the visual judgement, and increased to 29400 Pa (0.3 kgf/cm²) at the time of the visual judgement. From this, it may be inferred that slag began to flow at the point of time when the Ar gas flow rate became smaller by 2 ℓ/min than the initial flow rate, that is, 4 second before the visual judgement. Therefore, if the pouring of the molten steel from the ladle is stopped at this point of time by operating a stopper or a sliding nozzle for the ladle, outflow of the slag can be significantly reduced. The decision criterion of the outflow of slag may be set appropriately depending upon operation conditions. Next, the state of clogging of the tundish was examined by using the above slag outflow-detecting process. Results are shown in Fig. 4. In this figure, the abscissa shows the amount (kg) of the ladle slag entering the tundish from the ladle per charge, and the ordinate shows the increase (cm².min/tonne) in the clogging index of the tundish nozzle. The clogging index of the tundish is based upon an open area of the nozzle capable of feeding 1 ton of the molten steel per one minute. The greater the clogging index, the more conspicuous the clogging of the nozzle. As is clear from Fig. 4, the smaller the amount of the ladle slag entering the tundish from the ladle, the smaller the nozzle-clogging index. Particularly, when the slag contamination level from the ladle is not more than 100 kg, the nozzle-clogging index is almost zero. Therefore, when the slag contamination level from the ladle is set at not more than 100 kg, the molten steel can continuously be poured without leading to any clogging. Fig. 5 is a characteristic diagram showing the distribution of the slag contamination level from the ladle when the molten steel was poured into the tundish according to the process of the present invention. In the experiment, the number of times the tundish was changed, "n" was 50. The average contamination level "X" of the slag from the ladle was 50.3 kg per charge, and the standard deviation "σ" was 24.1 kg. For comparison purposes, Fig. 6 shows the distribution of the contamination level of the slag from the ladle according to a conventional process. At that time, the number of times the ladle was charged "n", was 75 charges. The average contamination level "X" of slag from the ladle was 203.9 kg, and the standard deviation "σ" was 56.5 kg. As is clear from the above results, the contamination level of slag from the ladle was reduced to about one third of that in the conventional case by using the invention process. Fig. 7 is a characteristic diagram showing the relationship between the amount of melt poured per one tundish nozzle and the nozzle-clogging index. As may be inferred from Fig. 7, the nozzle-clogging index increased with increases in the amount of melt involved. Particularly, when it was 500 tonne/nozzle or more, the extent of clogging of the nozzle became significant. However, when the process according to the present invention was employed, almost no clogging of the nozzle was observed despite increasing the amount of the melt. From the above, it is seen that according to the present invention, in which the amount of slag entering the tundish through the nozzle is suppressed to not more than 100 kg per charge, the melt can continuously be poured at 500 tonne/nozzle without clogging the nozzle. According to the present invention, since the outflow of the slag from the refining furnace or the ladle can be detected at an early stage, the amount of the slag flowing out into the ladle or the tundish can be reduced, and the following effects can be obtained. 1. Yield of A or an alloyed iron such as Fe-Mn or Fe-Si added into the ladle is increased. 2. Since the amount of molten steel oxidized with the slag can be reduced, cleanliness of the molten steel can be improved. 3. The cost of a refractory material can be reduced by increasing the amount of the molten steel continuously poured per one nozzle. 4. Since clogging of the nozzle can be prevented, molten steel can continuously be poured with high efficiency.

Claims: A method for detecting outflow of slag, when refined molten steel stored in a first vessel is poured into a second vessel through a feed pipe, which method comprises feeding inert gas into a stream of the molten steel in the feed pipe through a gas feed hole provided in the wall of the feed pipe, characterised in that a reduction in flow rate of the inert gas sucked into the feed pipe and/or an increase in back pressure of the inert gas sucked into the feed pipe is/are used to determine the presence of slag in the feed pipe.

Inventors: HAMAGAMI KAZUHISA, MORIOKA NOBUHIKO, OGURA SHIGERU, HAMAGAMI, KAZUHISA, MORIOKA, NOBUHIKO, OGURA, SHIGERU
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1989
Main classification: C21C5
IPC codes: C21C7, B22D11, B22D37, C21C5, B22D2