Abstract:
A multi-function lance for insertion into a vacuum degassing chamber, includes a top-blown oxygen lance provided as a path for gaseous oxygen. A nozzle is provided at a downstream end of the oxygen path used for blowing oxygen onto a molten metal. A water-cooled jacket surrounds the outer periphery of the top-blown oxygen lance. At least a set of paths for a fluid fuel and a gas for burning the fuel is positioned in a multi-function lance, namely in the water-cooled jacket, and a combustion burner is provided at a downstream end of the path. By using the multi-function lance according to the present invention, not only a safer and shorter preheating operation is enabled, but also oxygen lancing, temperature elevation of molten steel, and heating of the vacuum degassing apparatus itself are smoothly realized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-function lance for use in a vacuum degassing chamber. The invention also relates to a technique for continuously performing, by using such a lance either intermittently or continuously, the multiple operations of preheating a vacuum degassing apparatus (generally comprising a vacuum degassing chamber, an evacuation means, a ladle, etc.) for vacuum refining, composition control and the like, of a molten steel; elevating the temperature of molten steel maintained inside the apparatus; blowing gaseous oxygen for decarburization and refining the molten steel, and blowing a powdered desulfurizing agent or the like for composition control of the molten steel. 
     2. Description of The Related Art 
     Recently, the molten steel output from a conversion furnace or an electric furnace is often subjected to further refining (denoted as secondary refining) in order to produce a high quality steel. For such secondary refining, a prevailing method comprises blowing oxygen onto the molten steel maintained inside the vacuum degassing apparatus (which is often referred to hereinafter as “a degassing chamber”) to perform decarburization. However, in such decarburization refining, the temperature of the molten steel can be lowered too greatly when the chamber is insufficiently preheated, or smooth operation can be disrupted due to the adhesion of a large quantity of raw metal to the inner wall of the degassing chamber. 
     Consequently, various countermeasures have been studied and implemented, such as preheating the degassing chamber itself, elevating the temperature of the molten steel, etc. Recently, in JP-A-Hei6-73431 (the term “JP-A” as used herein signifies “an unexamined published Japanese patent application”) there was proposed a vacuum degassing apparatus known as a so-called “complex lance  1 ” as shown in present FIG. 4, comprising an oxygen blowing portion having a throat portion  15  provided in its axial core, a downwardly extending portion  16  connected to the lower side thereof, and a gaseous fuel supply hole  17  opening into the downwardly extending portion  16 . This vacuum degassing apparatus is intended to achieve greatly favorable effects of blowing oxygen to the molten steel, heating the molten steel by burning gaseous fuel with oxygen, and preventing raw metal from adhering to the degassing chamber, etc., by using only one complex lance  1 . 
     However, according to the structure of the complex lance disclosed in JP-A-Hei6-73431, a gaseous fuel is simply blown out from the nozzle portion and mixed with oxygen. Consequently, the calorific value provided thereby was found insufficient as a practical matter to preheat the inside of the degassing chamber. Furthermore, because the structure is extremely simple, the following problems were expected to occur in practice: 
     (1) The structure is effective only when spontaneous ignition occurs upon mixing the oxygen and gaseous fuel, and the temperature therefore cannot be raised once the temperature inside the degassing chamber has been lowered. Moreover, the reliability is very poor. 
     (2) Even in case spontaneous ignition occurs, if for any reason the burning gas becomes extinguished during the operation, there remains a danger of causing an explosion due to the fuel gas filling inside the chamber. 
     (3) Thus, from the viewpoint of safety, the necessary preheating operation of the degassing chamber takes such a long time that productivity is decreased. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the above-described problems by providing a multi-function lance for a vacuum degassing apparatus, that is capable of not only performing a preheating operation in a safer manner and in a shorter amount of time, but also blowing oxygen, elevating the temperature of molten steel, heating the vacuum degassing apparatus itself, and refining and controlling the composition of the molten steel by feeding powdered desulfurizing agents and the like, all using the same lance. 
     In order to achieve this object, the present inventors conducted extensive studies, and as a result developed a multi-function lance for use in a vacuum degassing chamber comprising a water-cooled cylindrical top-blown oxygen lance being equipped with a path for gaseous oxygen, and a nozzle provided at the downstream end of the oxygen path and used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of the top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being positioned in the multi-function lance; and a combustion burners provided at the downstream end of said paths. 
     In accordance with another aspect of the present invention, there is provided a multi-function lance for vacuum degassing chamber as above, wherein at least one of said burners is equipped with an internally provided ignition plug. 
     According to still another aspect of the present invention, there is provided a multi-function lance for a vacuum degassing chamber as above, wherein an aperture having a transparent plate is provided axially centrally of the top-blown oxygen lance, and a sensor is provided that detects the flame via the aperture. 
     According to a further aspect of the present invention, here is provided a multi-function lance for a vacuum egassing chamber wherein a lance for blowing a powder is provided in parallel with the burner as shown in FIG. 5, or, instead of the burners, one or a plurality of lances for blowing a powder, are provided between said top-blown oxygen lance and the water-cooling jacket as shown in FIG.  6 . 
     The present invention also provides a method for using a multi-function lance in a vacuum degassing chamber, comprising actuating a combustion burner disposed in a vacuum degassing chamber comprising a top-blown oxygen lance equipped with a gaseous oxygen path formed with a nozzle provided at the downstream end of said oxygen path used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of the top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being defined between the top-blown oxygen lance and the water-cooled outer jacket; a combustion burners provided at the downstream end of said paths; and a sensor provided as such that it detects the flame via an aperture provided in a transparent plate at the axial center of the top-blown oxygen lance; and supplying a small amount of oxygen to the inside of a top-blown oxygen lance connected to said aperture so as to elevate the temperature of the portion of the flame which is being detected by said sensor. 
     The invention also includes a method for using a multi-function lance in a vacuum degassing chamber, comprising operating a combustion burner associated with a vacuum degassing chamber that comprises a water-cooled cylindrical top-blown oxygen lance equipped with a gaseous oxygen path formed with a nozzle provided at the downstream end of said oxygen path used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of said top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being positioned in the multi-function lance; and a combustion burners provided at the downstream end of said paths; wherein a powder is supplied to the inside of the top-blown oxygen lance to blow the powder onto the molten metal in the vacuum degassing chamber from the lance. 
     Thus, the multi-function lance of the present invention prevents the fluid fuel from being extinguished during operation, and enables heating a degassing chamber and blowing of oxygen in a safer and far more stable manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is an axial sectional view of an embodiment of the multi-function lance according to the present invention; 
     FIG.  1 ( b ) is a plan view of the FIG.  1 ( a ) embodiment; 
     FIG. 2 shows a preheating stage of a degassing tank using the multi-function lance according to the present invention; 
     FIG. 3 shows a decarburizing refining stage of a molten steel using the multi-function lance according to the present invention; 
     FIG. 4 shows a vertical cross section view of a prior art complex lance; 
     FIG.  5 ( a ) is an axial sectional view, along line A—A of FIG.  5 ( b ), of a multi-function lance according to another embodiment of the present invention, provided with a lance specifically used for blowing a powder in parallel with the burner, and a section plate  29  for establishing a defined path for circulation of cooling water; 
     FIG.  5 ( b ) is a plan view of the FIG.  5 ( a ) embodiment; 
     FIG.  6 ( a ) is an axial sectional view of a multi-function lance according to yet another embodiment of the present invention, provided with a lance specifically used for blowing a powder in the place of the burner; 
     FIG.  6 ( b ) is a plan view of the FIG.  6 ( a ) embodiment; and 
     FIG. 7 shows a stage of blowing powder onto molten steel in case powder is supplied to an upward oxygen blowing lance. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A fuller explanation of the present invention will now be given with reference to the accompanying drawings, which illustrate preferred but non-limiting examples of the present device and method. In the drawings, the following list associates the depicted reference numerals with the associated component: 
       1  Multi-function lance 
       2  Nozzle 
       3  Path for gaseous oxygen 
       4  Inflammable transparent plate 
       4 ′ Aperture 
       5  Sensor 
       6  Combustion burner 
       7  Path for fluid fuel 
       8  Path for a combustion gas (e.g., air) 
       9  Ignition plug 
       10  Vacuum degassing chamber (degassing chamber) 
       11  Slag pot 
       12  Ladle 
       13  Molten steel 
       14  Alloy hopper 
       15  Throat portion 
       16  Downwardly extending portion 
       17  Fuel gas supplying portion 
       18  Water-cooled cylindrical top-blown oxygen lance 
       19  Water-cooling jacket 
       20  Inner tube 
       21  Outer tube 
       22  Fluid fuel 
       23  Combustion gas (e.g., oxygen) 
       25  Gaseous oxygen 
       26  Cooling water 
       27  Melt 
       28  Powder 
       29  Section plate for cooling water 
       30  Powder compression feeding tank 
       31  Gaseous nitrogen 
       32  Compression-feeding powder 
     An embodiment according to the present invention is described below with reference to drawings for a case comprising four burners. 
     Referring to FIG.  1 ( a ) and FIG.  1 ( b ), a multi-function lance  1  for a vacuum degassing apparatus according to the present invention is based on water-cooled cylindrical top-blown oxygen lance  18  having a path for gaseous oxygen  3  and a nozzle  2 , which supply oxygen to be blown against the molten steel. The top-blown oxygen lance  18  is further surrounded by a water-cooling jacket  19  and a section plate for cooling water  29  is positioned in the multi-function lance, namely in water-cooling jacket  19 . A plurality of pipes are provided between the water-cooling outer jacket  19  and the top-blown oxygen lance  18  to provide paths  7  for a fluid fuel  22  and paths  8  for a combustion gas (e.g., oxygen)  23 . The downstream end of the pipes include built-in combustion burners  6  (referred simply hereinafter as “burners  6 ”). The burners  6  are generally provided in a double-tube structure, such that a fluid fuel (e.g., LPG)  22  flows through the inner tube  20  and a combustion gas  23  for the fuel flows through the outer tube  21 . An important aspect of the present invention is that an ignition plug  9  (at the downstream end of the inner tube  20 ) is attached to at least one of the plurality of burners  6 , so that a spark is generated to ignite the gas mixture comprising the fluid fuel  22  and the combustion gas  23 . In this manner, extinguishing during the operation of the burner  6  can be prevented from occurring. 
     In the present invention, to further increase the function of preventing extinguishing, a non-flammable transparent plate  4  made of quartz glass or the like is attached to the upstream end (i.e., the side opposite to the nozzle on the concentric axis) of the cylinder used as the path for gaseous oxygen  3 , so that the interior may be observed therethrough, and a sensor  5  (e.g., an ultraviolet detector) is provided for detecting the flame. If no flame is detected by the sensor  5 , the supply of the fluid fuel  22  and the combustion gas  23  to the burner  6  is halted, and a signal is sent to purge with an inert gas such as N 2 . Furthermore, a small amount of auxiliary oxygen for aiding combustion is simultaneously supplied to nozzle  2  together with the supply of the fluid fuel  22  so as to maintain the high temperature by stabilizing the flame in the vicinity of the nozzle  2  at a predetermined temperature. In this manner, false alarms from the sensor  5  are prevented. 
     The method for using the multi-function lance  1  according to the present invention in a vacuum degassing chamber  10  is described below. The inner and outer pipes (i.e., the paths  7  and  8  for the fluid fuel and the combustion gas, respectively) of the burner  6  are first purged with an inert gas such as gaseous N 2  for a predetermined duration of time, and, after supplying a fluid fuel  22  and a combustion gas  23  to the burner equipped with an ignition plug, the fuel is ignited by generating a spark. After confirming the ignition of the fuel by using the detection sensor  5  for the flame, or after the passage of a predetermined duration of time, the fluid fuel  22  and the combustion gas  23  are supplied to the other burner  6  to start combustion. After a passage of a predetermined duration of time, the combustion flame is monitored with the sensor  5 . If the flame is detected, the supply of the fluid fuel and the like is continued, but if the flame is not detected, the flame is extinguished and the inside of the burner  6  is purged with an inert gas such as gaseous N 2 . Simultaneously with the supply of the fluid fuel  22  to the burner  6 , a small amount of oxygen is supplied to the nozzle  2  to maintain the monitored flame at a high temperature, thereby preventing the malfunction of the sensor  5  from occurring. As a matter of course, if combustion failure occurs, the auxiliary oxygen gas for aiding combustion is stopped at the time the extinction is detected, and gaseous N 2  is supplied instead. 
     On the other hand, when aluminum is added to the molten steel  13  inside the vacuum degassing chamber  10  to elevate the temperature, or in case refining such as decarburization and the like is conducted, gaseous oxygen is blown from the nozzle  2  to accelerate the oxidation reaction. In such a case, no heating by the burner  6  of the (vacuum) degassing chamber  10  or the molten steel  13  is performed. Instead, a predetermined amount of inert gas such as gaseous N 2  is supplied to the burner to avoid clogging of the front end of the burner due to splashes and the like. 
     In case of performing composition control such as desulfurization by adding a powdered desulfurizing agent, etc., to the molten steel  13  inside the vacuum degassing chamber  10 , the powder is blown from an oxygen lance or a specifically provided lance. If a specific lance is used, the heating of the degassing chamber  10  using the burner  6  and the blowing of oxygen from the nozzle  2  is halted, and, instead, an inert gas such as gaseous N 2  (nitrogen) is supplied to the nozzle  2  and burner  6  in a predetermined quantity to avoid clogging of the front end of each lance due to splashes and the like. 
     EXAMPLE 1 
     A multi-function lance  1  according to the present invention was applied to a vacuum degassing apparatus  10  of a RH type. Since a large quantity of raw metal was first found to be adhered to the inner wall surfaces of the degassing chamber  10  used in vacuum refining, the raw metal was removed while operation was suspended, and, at the same time, the degassing chamber  10  was preheated for the next operation. After placing a vessel  11  (denoted as “a slag pot”) for receiving the melt  27  (e.g., raw metal, slag, etc.) at the lower side of the degassing chamber  10  as shown in FIG. 2, a multi-function lance  1  according to the present invention was inserted from the upper side of the degassing chamber  10  and positioned. Then, in accordance with the heating method of the degassing chamber  10  as described above, the burner  6  of the multi-function lance  1  was used to preheat the degassing chamber  10 . The vacuum degassing tank  10  is a RH type vacuum degassing tank capable of processing 180 tons of molten steel, and the preheating of the chamber was conducted for 5 hours in total by using gaseous propane as the fluid fuel, which was supplied at a flow rate of 60 Nm3/hr for 4 hours and at 134 Nm3/hr for one hour. During this process the temperature of the inner wall of the chamber was raised from 1045° C. to 1400° C. As a result, the removal of the raw metal and the preheating were smoothly achieved without suffering any extinction. The time necessary for the preheating was only about 70% of the time necessary in case of using a conventional lance  1  as shown in FIG.  4 . 
     Then, by using the preheated vacuum degassing chamber  10 , the decarburization smelting of a molten steel  13  was carried out. The molten steel  13  was fed into the ladle  12 , and the lower portion of the degassing chamber  10  was immersed therein. Thus, smelting was conducted by circulating the molten steel  13  under vacuum between the ladle  12  and the degassing chamber  10 . In this case, as shown in FIG. 3, the multi-function lance  1  according to the present invention was inserted from the upper side of the degassing chamber  10  and positioned, such that gaseous oxygen  25  was blown under predetermined conditions to the molten steel  13  via the nozzle  2 . The concentration of carbon (C) incorporated in the molten steel in the ladle ( 12 ) immediately after output from the conversion furnace was 496 ppm. Thus, by using the lance according to the present invention in an RH type vacuum degassing chamber ( 10 ), the molten steel was subjected to oxygen lancing refining under vacuum at an oxygen flow rate of 20 Nm3/min for a predetermined duration of time. After 23 minutes, the carbon concentration of the molten steel was found to be decreased to the region of super-low carbon content of 20 ppm. As a result, it was found that the decarburization of the molten steel  13  was conducted smoothly in a manner comparable to a case using a conventional lance  1  of FIG.  4 . 
     EXAMPLE 2 
     Referring to FIG. 7, an example of blowing a desulfurizing agent based on CaO using a multi-function lance according to the present invention is described below. 
     The powder was lanced under conditions as follows: 
     Amount of desulfurizing agent blown—6.7 kg/ton of molten steel 
     blowing rate—100 to 126 kg/min 
     blowing duration—15 to 18 minutes 
     Flow rate of carrier gas—3.0 Nm3/min 
     As a result, the content of sulfur was reduced to 15 ppm or less. 
     As described above, a safer preheating, decarburization refinig, or desulfurization operation, or any combination of these operations, can all be achieved using the same lance according to the invention, and in a shorter time. 
     While the invention has been described in detail by making reference to specific examples, it should be understood that various changes and modifications can be made without departing from the scope and the spirit of the present invention.