METHOD AND APPARATUS FOR DEGASSING MOLTEN METAL UTILIZING RH METHOD

Abstract: A method and apparatus for vacuum degassing performs blow of gas into molten metal so as to form substantially uniform distribution of bubble over the path area defined by an induction pipe. In order to achieve this, discharge nozzles are separated into at least two groups. The gas pressure to be discharged through the discharge nozzles in different groups is controlled independently of the other so that gas blown through first group of nozzle forms bubble at around the central portion of the induction pipe. The pressure of the gas blown through the other second group of nozzle is adjusted for forming the bubble in the vicinity of the periphery of the induction pipe.

Description: The present invention relates generally to a technique for the degassing of molten metal to produce high purity metal products, such as extra low carbon steel. More specifically, the invention relates to a method and apparatus for degassing molten metal utilizing the RH method. Recently, the requirement for high quality metal has substantially increased. Some kinds of metal require vacuum degassing treatment for removing impurities to improve quality. To respond to such a requirement, it is desired to develop a method and apparatus which allows high efficiency in the degassing treatment and a lower tapping temperature in the converter to lower the production cost. It is well known to introduce molten metal in a ladle into a vacuum chamber where it is subjected to vacuum to remove carbon and other impurities. For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa 51-71202, published on June 19, 1976, discloses a vacuum degassing treatment for molten metal utilizing the apparatus set forth above. The disclosed apparatus has an induction pipe immersed into the molten metal in the ladle for introducing the molten metal into the vacuum chamber from the ladle, and a return pipe to return the molten metal from the vacuum chamber to the ladle. The induction pipe and the return pipe are oriented in spaced apart relationship to each other so that the molten metal in the vacuum chamber flows from the discharge outlet of the induction pipe to the return inlet of the return pipe in the vacuum chamber. By this construction, the molten metal can be maintained for a longer period in the vacuum chamber to remove a greater amount of impurities. Such a method is known as the "Ruhrstahl Hausen (RH) method". In such a case, the efficiency of the removal of impurities is variable depending upon the circulation rate of the molten metal in the vacuum chamber and the surface area of the molten metal in direct contact with the vacuum atmosphere in the vacuum chamber. In the RH method, an inert gas, such as Ar gas, is blown into the induction pipe for circulating the molten metal in the ladle into the degassing chamber. The inert gas bubbles serve not only to circulate the molten metal in the degassing or vacuum chamber but also to provide a greater surface area in the chamber by bubbling at the surface. Therefore the use of such gas blow technology in the RH method results in a more efficient impurity removal. As will be naturally appreciated, in order to increase the circulation rate of the molten metal, it is important to increase the gas flow rate through the induction pipe. To increase the gas circulation rate, it is necessary to provide a wider path area in the induction pipe connecting the ladle and the vacuum chamber. However, the path area of the induction pipe is limited by the internal diameter of the vacuum chamber. Therefore, in order to expand the path area, it becomes necessary to expand the diameter of the vacuum chamber, which involves the re-construction of other associated facilities. On the other hand, in the usual degassing process, the pressure of the gas blown into the molten metal is limited to be lower than or equal to 9.9 kg/cm². Such relatively low pressure gas is blown into the molten metal and introduced into the vacuum chamber via an induction pipe of 3 to 5 mm diameter. In practice, 8 to 20 gas discharge pipes or nozzles are circumferentially arranged around the ladle for blowing the gas. However, bubbles are formed only around the periphery of the induction pipe whereas the central zone of the induction pipe is not in a bubbled condition. Such flow of bubbles in the induction pipe will be hereafter referred to as "peripheral bubble flow". Peripheral bubble flow is produced in the conventional art and is not at all effective to improve the efficiency of impurity removal to the degree required or expected. In order to overcome this drawback of the prior art, Japanese Patent First Publication (Tokkai) Showa 51-6103, published on January 19, 1976, discloses an apparatus which directs the blown inert gas to the vicinity of the central axis of the induction pipe. This was successfully introduced for the purpose of forming bubbles at the central portion of the induction pipe and thus improving impurity removal efficiency. Such flow of bubbles will be hereafter referred to as "center bubble flow". However, this still has some drawbacks. For instance, since the gas nozzle has to be introduced into the molten metal in the ladle, it is subjected to substantial heat and this shortens the life of the nozzle. Furthermore, since the nozzle is subjected to substantial heat, replacement of the nozzle for repairing is difficult and requires an unacceptably long period. In addition, such center bubble type flow tends to cause temperature fluctuation at the peripheries of the vacuum chamber and the induction pipe thus causing spalling of the refractory material therein. Therefore, it is an object of the present invention to provide a method and apparatus which may solve the problems encountered in the conventional art. Another object of the invention is to provide a method and apparatus for removing impurities including carbon for producing extra low carbon metal, which can reasonably improve the efficiency of the degassing treatment. In order to achieve the aforementioned and other objects and advantages, the method and apparatus for vacuum degassing, according to the invention, blow gas into the molten metal so as to form a substantially uniform distribution of bubbles over the path area defined by the induction pipe. In order to achieve this, the discharge nozzles are separated into at least two groups. The gas pressure to be discharged through the discharge nozzles in different groups is controlled independently of the other so that gas blown through the first group of nozzles forms bubbles at around the central portion of the induction pipe. The pressure of the gas blown through the second group of nozzles is adjusted so as to form bubbles in the vicinity of the periphery of the induction pipe. GB-A-920 874 describes a vacuum degassing technique for molten metal wherein a degassing chamber connected to a vacuum is located above a container for the molten metal. First and second vertical paths are provided for supplying molten metal from the container to the degassing chamber and for returning molten metal from the degassing chamber to the container. Gas discharging means are provided to introduce inert gas into the first path at at least two positions at different levels. Although different quantities of gas may be introduced at these positions, there is no suggestion to control the gas pressures independently to obtain a uniform distribution of gas across the path. According to one aspect of the present invention, there is provided a method for blowing gas into molten metal in an apparatus for degassing molten metal which includes a container containing a molten metal, a degassing chamber arranged above the container and connected to a vacuum source, a first path for supplying the molten metal from the container to the degassing chamber, a second path for returning the molten metal from the degassing chamber to the container, and first and second gas discharge nozzles connected to the first path upstream of the degassing chamber for discharging gas into the molten metal, said method comprising the steps of: reducing the pressure in said degassing chamber by means of said vacuum source; introducing a high pressure gas into said first path through said first gas discharge nozzles, and introducing a low pressure gas into said first path through said second gas discharge nozzles characterised in that the pressures of said high pressure gas and said low pressure gas are controlled independently of each other so as to establish a uniform distribution of gas across said first path. The high pressure gas and the low pressure gas may be discharged transversely to the flow direction of the molten metal at the positions where the high pressure gas and the low pressure gas are introduced. Preferably, the method further comprises the step of connecting the degassing chamber with the container by first and second vertical paths, the first vertical path being arranged upstream of the degassing chamber for introducing molten metal from the container to the degassing chamber and the second vertical path being arranged downstream of the degassing chamber for recirculating the molten metal from the degassing chamber to the container. In this case the high and low pressure gas discharge nozzles are arranged to discharge the high and low pressure gases in substantially horizontal directions. In a preferred embodiment, the high and low pressure gases to be discharged into the molten metal comprise inert gas. Further the inert gas preferably contains a given amount of oxygen. On the other hand, the pressure of the high pressure gas is variable depending upon the distance the pressurized gas has to reach since the high pressure gas is intended to be blown into the central portion of the first vertical path. Therefore, the pressure of the high pressure gas may be variable depending upon the diameter of the first path. In a generalized consideration, the gas pressure of the high pressure gas can be illustrated by: P = K x d i x D j where P is the gas pressure (kg/cm²); K is in a range of 0.15 to 0.80 i is 3.8/d + 1 j is 2/3 d is the discharge nozzle diameter (mm); and D is the diameter of the first path. In a preferred embodiment, the pressure of the high pressure gas may be in a range of from about 30 kg/cm² to about 50 kg/cm² and the pressure of the low pressure gas may be in a range of from the static pressure of the molten metal to 9.9 kg/cm². According to another aspect of the present invention there is provided an apparatus for degassing molten metal comprising: a container for containing molten metal; a degassing chamber arranged above said container and connected to a vacuum source for maintaining a vacuum in the container; a first path for supplying said molten metal from said container to said degassing chamber; a second path for returning said molten metal from said degassing chamber to said container; a high pressure gas discharging means for discharging a high pressure gas into said molten metal in said first path, a low pressure gas discharging means for discharging a low pressure gas into said molten metal in said first path, and means provided to control the pressures of said high pressure gas and said low pressure gas independently of each other characterised in that said high pressure gas discharging means has a smaller gas flow area than that of said low pressure gas discharging means. The high pressure gas discharging means has a smaller gas flow area than that of the low pressure gas discharging means so as to achieve uniformity of gas flow rate in the gas discharged from the high and low pressure gas discharging means. In the preferred construction, the high pressure gas discharging means comprises a plurality of first nozzles arranged in circumferential alignment and the low pressure gas discharging means comprises a plurality of second nozzles in circumferential alignment. The first and second nozzles are vertically arranged at mutually different elevations. More specifically, the first nozzles circumferentially arranged in alignment are oriented at a lower elevation than that of the second nozzles circumferentially arranged in alignment. The first and second nozzles may have axes extending substantially transverse to the axis of the first path. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of a preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a sectional view of a preferred embodiment of an RH degassing apparatus according to the present invention; Fig. 2 is an enlarged section of the major part of the RH degassing apparatus of Fig. 1; Figs. 3a and 3b are illustrations of water-utilizing models showing the relationship between bubble formation and the pressure of the gas being discharged; Figs. 4(a) and 4(b) are graphs showing the variation of the molten metal circulation rate in the vacuum chamber in relation to the blown gas flow rate; Fig. 5 is an explanatory illustration of a portion around the discharge outlet of a discharge nozzle in a degassing apparatus; Fig. 6 is a graph showing the variation of the molten metal flow rate relative to the gas flow rate; when argon and an argon/oxyen mixture are used, and Fig. 7 is a graph showing the variation of the size of the mushroom formed around the discharge outlet of the discharge nozzle with gas flow rate when the gases used in Fig. 6 are used. Referring now to the drawings, and particularly to Figs. 1 and 2, the preferred embodiment of an RH degassing apparatus according to the invention, has a molten metal container 10, such as a molten metal ladle, containing molten metal 12, and a degassing apparatus 14 which defines a degassing chamber 16 therein. The degassing chamber 16 is connected to a vacuum pressure source (not shown) to be maintained at a vacuum atmosphere. As can be seen from Fig. 1, the degassing apparatus 14 is located above the container 10. In the shown construction, the degassing apparatus 14 has an induction (or suction) pipe section 18 and a return pipe section 20. The induction pipe section 18 and the return pipe section 20 are integrally formed with the body of the degassing apparatus 14 and communicate with the degassing chamber 16 at their upper ends. The lower ends of the induction pipe section 18 and the return pipe section 20 are immersed into the molten metal 12 in the container 10. Therefore, the molten metal container 10 and the degassing chamber 14 are connected to each other via the induction pipe section 18 and the return pipe section 20. The molten metal 12 in the container 10 rises up to the degassing chamber 16 via the induction pipe section 18 and recirculates into the container 10 via the return pipe section 20. A plurality of high pressure gas discharge nozzles 22 and low pressure gas discharge nozzles 24 are provided in induction path 26 defined by the induction pipe section 18. The axes of the nozzles 22 and 24 at least at the gas discharge outlets are directed substantially transversely to the axis of the induction pipe section 18. The high pressure gas discharge nozzles 22 are connected to a high pressure gas source (not shown). Similarly, the low pressure gas discharge nozzles 24 are connected to a low pressure gas source (not shown). As shown, the high and low pressure nozzles 22 and 24 are located at vertically different height positions, as particularly shown in Fig. 2. In the shown embodiment, the high pressure gas discharge nozzles 22 are vertically located at a lower elevation than the low pressure gas discharge nozzles 24 and are circumferentially arranged with given angular intervals. Similarly, the low pressure gas discharge nozzles 24 are circumferentially arranged with given angular intervals at a higher elevation than the high pressure gas discharge nozzles. As can be seen from Fig. 2, the internal diameter of the high pressure gas discharge nozzles 22 is smaller than that of the low pressure gas discharge nozzles 24. The ratio of the diameters of the high pressure gas discharge nozzles 22 and the low pressure gas nozzles 24 is determined in view of the ratio of the gas pressure to be discharged therethrough so that the gas flow rates at the central portion and the circumferential portion of the induction pipe section 18 become even to obtain uniformity of bubble generation at the respective portions. Though the shown embodiment employs an arrangement where the high and low pressure gas discharge nozzles 22 and 24 are vertically offset, it may be possible to arrange for both the high pressure gas discharge nozzles and the low pressure gas discharge nozzles to be at the same vertical height. Furthermore, although in the shown embodiment, the high pressure gas discharge nozzles are arranged in circumferential alignment at a lower elevation and the low pressure gas discharge nozzles are arranged in circumferential alignment at a higher elevation, it may be possible to arrange for both of the high and low gas pressure discharge nozzles to be at different elevations in a circumferentially alternative arrangement. However, the shown arrangement of nozzles is preferred in view of the avoidance of interference of the high and low pressure gas discharged through the high and low pressure gas discharge nozzles which may otherwise occur. Concerning the gas flow rate q in the induction pipe section 18, this is proportional to the discharge nozzle diameter d and the discharge gas pressure P. Therefore, the gas flow rate can be calculated by: q = b x d i x P On the other hand, the distance Lh reached by the gas is proportional to the velocity of the gas. The distance Lh can be illustrated by: Since the high pressure gas has to reach the central portion of the induction pipe section 18, the distance Lh can be illustrated by: Lh = D/2 On the other hand, the pressure of the high pressure gas is variable depending upon the distance the pressurized gas has to reach since the high pressure gas is intended to be blown into the central portion of the first path. Therefore, the pressure of the high pressure gas may be variable depending upon the diameter of the first path. In a generalized consideration, the gas pressure of the high pressure gas can be illustrated by: P = K x d -(i -2) x D 1/j where D is the diameter of the induction pipe section 18; and K is 0.15 to 0.8 and is a coefficient obtained through experiments. In order to determine the optimum values of K, i and j, experiments have been performed. As a result, it has been found that the optimum performance can be obtained in the following condition: K : 0.15 to 0.8 i : 3.8/d + 2 j : 2/3 In order to confirm the effect of the gas flow for increasing the molten metal flow rate into the degassing chamber 16, models shown in Figs. 3a and 3b were prepared. The model of Fig. 3a is designed for discharging low pressure gas and the model of Fig. 3b is designed for discharging high pressure gas. Experiments were performed utilizing water as the liquid. As can be seen from Figs. 3a and 3b, depending upon the pressure of the gas being discharged, the radial distance at which bubbles are formed varies. In Figs. 3a and 3b, arrows show the water flow velocity. Based on the results obtained from the experiments set forth above, the gas pressure to be discharged through the high pressure gas discharge nozzles 22 is set in a range of from 30 to 50 kg/cm² and the gas pressure to be discharged through the low pressure gas discharge nozzles 24 is set in a range of from 0 to 9.9 kg/cm² for an induction pipe section 18 having an internal diameter of from 40 to 50 cm. In a practical degassing operation, the gas flow rate through the high pressure gas discharge nozzles 22 is set at a value of about 1000 Nl/min and the gas flow rate through the low pressure gas discharge nozzles 24 is set in a range of from 1500 to 2000 Nl/min. With the construction set forth above, the molten metal flow rate in the degassing chamber was increased by about 90% in comparison with that of a conventional apparatus and the period for the degassing treatment could be shortened by about 50%. Here, in the aforementioned degassing process, Ar gas has been conventionally used as the inert gas to be blown into the induction path. Discharging of the gas through the gas discharge nozzles 22 and 24 tends to cool the molten metal around the discharge outlet thereof. As a result of cooling the molten metal, a mushroom 30 tends to grow around the discharge outlet of the nozzle (Figure 5). When the mushroom 30 grows around the discharge outlet of the nozzle, the pressurized gas has to flow through small diameter openings 32. Since the internal diameter of the openings 32 is substantially small and constitutes a resistance to gas flow, the discharge pressure of the gas may be lowered to such an extent that it is insufficient to reach the central portion of the induction path even when high pressure gas is supplied. In order to prevent the mushroom from being grown around the discharge outlet of the gas discharge nozzles, a preferred embodiment of the degassing apparatus employs an inert gas, such as Ar gas containing a given amount e.g. in a range of from 0.1 to 20.0%, preferably in a range of from 1.0 to 5.0%, of O₂. Fig. 7 shows the diameter of mushrooms formed during gas blowing. As can be seen from Fig. 7, when Ar gas not containing O 2 is used, the diameter of the mushroom increases as the gas flow rate increases. On the other hand, when the inert gas contains O 2 , the diameter of the mushroom can be held substantially smaller. For confirming the performance of the degassing apparatus set forth above, a degassing operation was performed for molten metal in a ladle having a volume of 250 tons using varying gas flow rate. Experiments were performed using Ar gas containing no O 2 gas and Ar gas with 5% of O 2 to check the molten metal flow rate in an induction pipe section having an internal diameter of 550 mm. The molten metal flow rate to the degassing chamber 16 relatively to the gas flow rate was substantially constant at a gas flow rate greater than 1500 Nl/min when pure Ar gas was used. On the other hand, in case of the O 2 containing Ar gas, the molten metal flow rate increased with increasing gas flow rate even at a gas flow rate greater than 1500 Nl/min, as shown in Fig. 6. This confirms that O 2 contained in the inert gas is effective for suppressing the growing of mushrooms around the discharge outlets of the gas discharge nozzles.

Claims: A method for blowing gas into molten metal in an apparatus for degassing molten metal which includes a container (10) containing a molten metal, a degassing chamber (16) arranged above the container and connected to a vacuum source, a first path (18) (26) for supplying the molten metal from the container to the degassing chamber, a second path (20) for returning the molten metal from the degassing chamber to the container, and first and second gas discharge nozzles (22,24) connected to the first path upstream of the degassing chamber for discharging gas into the molten metal, said method comprising the steps of: reducing the pressure in said degassing chamber by means of said vacuum source; introducing a high pressure gas into said first path through said first gas discharge nozzles (22), and introducing a low pressure gas into said first path through said second gas discharge nozzles characterised in that the pressures of said high pressure gas and said low pressure gas are controlled independently of each other so as to establish a uniform distribution of gas across said first path. A method according to claim 1, wherein said high pressure gas and said low pressure gas are discharged transversely to the flow direction of said molten metal at the position where said high pressure gas and said low pressure gas are introduced into the molten metal. A method according to claim 1 or 2, wherein said high and low pressure gases comprise inert gas. A method according to claim 3, wherein said high and low pressure gases additionally contain oxygen. A method according to any preceding claim, wherein the pressure of said high pressure gas is in a range defined by: P = K x d -(i -2) x D 1/j where P is the gas pressure (kg/cm²); K is in a range of 0.15 to 0.80 i is 3.8/d + 2 j is 2/3 d is the discharge nozzle diameter D is the diameter of the first path A method according to any preceding claim, wherein the pressure of said high pressure gas is in the range of from 30 kg/cm² to 50 kg/cm² and the pressure of said low pressure gas is in a range of from the static pressure of the molten metal to 9.9 kg/cm². A method according to any preceding claim wherein said first gas discharge nozzles define a smaller gas flow area than said second gas discharge nozzles. An apparatus for degassing molten metal comprising: a container (10) for containing molten metal; a degassing chamber (16) arranged above said container and connected to a vacuum source for maintaining a vacuum in the container; a first path (18) (26) for supplying said molten metal from said container to said degassing chamber; a second path (20) for returning said molten metal from said degassing chamber to said container; a high pressure gas discharging means (22) for discharging a high pressure gas into said molten metal in said first path, a low pressure gas discharging means (24) for discharging a low pressure gas into said molten metal in said first path, and means to control the pressures of said high pressure gas and said low pressure gas independently of each other characterised in that said high pressure gas discharging means (22) has a smaller gas flow area than that of said low pressure gas discharging means (24). An apparatus as claimed in claim 8, wherein said high pressure gas discharging means comprises a plurality of first nozzles arranged in circumferential alignment and said low pressure gas discharging means comprises a plurality of second nozzles in circumferential alignment. An apparatus as claimed in claim 9, wherein said first and second nozzles are vertically arranged at mutually different elevations. An apparatus as claimed in claim 10, wherein said first nozzles are located at a lower elevation than said second nozzles. An apparatus as claimed in claim 9, 10 or 11, wherein said first and second nozzles have axes extending substantially transversely to the axis of said first path. An apparatus as claimed in any one of claims 8 to 12, wherein the apparatus comprises supplies of high pressure gas and low pressure gas and said high pressure and low pressure gases comprise inert gas. An apparatus as claimed in claim 13, wherein said high and low pressure gases contain oxygen. An apparatus as claimed in any one of claims 8 to 14, wherein the apparatus comprises supplies of high pressure gas and low pressure gas, the pressure of said high pressure gas is in a range of from 30 kg/cm² to about 50 kg/cm², and the pressure of said low pressure is in a range of from the static pressure of the molten metal to 9.9 kg/cm². An apparatus as claimed in claim 9 or any one of claims 10 to 15 when appendant to claim 9, adapted so that the pressure of said high pressure gas is in a range defined by: P = K x d -(i -2) x D 1/j where P is the gas pressure (kg/cm²); K is in a range of 0.15 to 0.80 i is 3.8/d + 2 j is 2/3 d is the discharge nozzle diameter (mm); and D is the diameter of the first path

Inventors: IKEDA, RYUICHI KAWASAKI STEEL CORPORATION, KOJIMA, SHINJI KAWASAKI STEEL CORPORATION, OHMIYA, SHIGERU KAWASAKI STEEL CORPORATION, OKUDA, HARUJI KAWASAKI STEEL CORPORATION, TAKAHASHI, KIYOSHI KAWASAKI STEEL CORPORATION, TAKASHIBA, NOBUYOSHI KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C21C7
IPC codes: C21C7, C22B9, F27D3