JP4938246B2 - Method for refining molten metal under reduced pressure and top blowing lance for refining - Google Patents

Description

  The present invention relates to a molten metal refining method in which an oxygen-containing gas is blown from an upper blowing lance toward a molten metal under a reduced pressure lower than atmospheric pressure, and the molten metal is refined by oxidation. It is about Lance.

  As a refining method for refining molten steel under a reduced pressure lower than atmospheric pressure, refining using RH vacuum degassing equipment, DH vacuum degassing equipment, VOD equipment or the like is widely known. In such refining under reduced pressure, as one of the methods for rapidly decarburizing the molten steel, oxygen gas is blown upward from the upper blowing lance having a Laval nozzle at the tip thereof toward the molten steel surface. A method of refining is known (see, for example, Patent Document 1).

  In decarburization refining by top blowing acid under reduced pressure, various technical developments have been conducted for the purpose of improving refining efficiency such as decarburization efficiency. From the results of these technical developments, In order to achieve this, it has been found that the development regarding the acid delivery conditions and the shape of the Laval nozzle (nozzle design) is extremely important. In other words, in order to increase the efficiency of decarburization refining, it is effective to increase the dynamic pressure on the molten steel surface of the flow of oxygen gas from the top blowing lance (referred to as “oxygen jet”). As a means to increase the oxygen jet energy, increase the energy of the oxygen jet itself, or reduce the distance between the top of the top lance and the molten steel surface (referred to as “lance height”). Measures such as suppressing jet attenuation have been implemented.

  For example, in Patent Document 2, in order to efficiently supply oxygen and decarburize, a Laval nozzle having an atmospheric pressure at the time of designing an acid feeding laval nozzle as an upper limit of a fluctuation range of atmospheric pressure in a tank during acid feeding refining is disclosed. There has been proposed a method for reducing the arrival loss of oxygen gas due to the attenuation of an oxygen jet under an atmosphere having a pressure lower than 13.3 kPa (100 torr). Patent Document 3 discloses that the lance height is in the range of 1 to 5 m based on the nozzle back pressure at the time of designing the top blowing lance (also referred to as “design secondary pressure”) and the actual nozzle back pressure at the time of operation. In addition, Patent Document 4 describes the shape of the Laval nozzle, the flow rate of the oxygen gas, the degree of vacuum in the tank at the end of acid delivery, and the lance height, using the ultimate pressure of the oxygen gas on the molten steel bath surface as an index. A method of adjusting the thickness has been proposed.

  However, although excessive deliberation of these measures improves the decarburization reaction, a large amount of iron scattering occurs due to an increase in the dynamic pressure of oxygen gas, and adhesion of metal to the equipment occurs, which greatly increases the operation. In addition, the decarburization rate in the extremely low carbon region is remarkably reduced by picking up the carbon concentration due to the remelting of the adherent metal having a high carbon concentration, or the component standard of the ultra low carbon steel is set. A big problem such as coming off occurs.

  In recent years, combined with the increase in steel types that require a smelting method using vacuum degassing equipment to adjust the composition of molten steel, in order to reduce the smelting cost of molten steel, converters and RH vacuum degassing. There is a need to improve production efficiency in the melting process that is consistent with the equipment. In addition, the carbon concentration in the molten steel at the end of decarburization refining in the converter is reduced for the purpose of reducing the iron oxide concentration in the slag generated by converter refining and reducing the oxygen concentration in the molten steel after converter refining. The operation of making the concentration higher than the conventional one is performed, and the load of the decarburization process in the RH degassing refining tends to increase.

  Therefore, when the molten steel is decarburized by top blowing in the RH vacuum degassing equipment, the decarburization time is extended as compared with the conventional method, thereby changing the atmospheric pressure in the vacuum tank during the decarburization process. The decarburization process is started in a relatively high pressure atmosphere that exceeds 13.3 kPa (100 torr) in some cases, and the decarburization process is terminated at a low atmospheric pressure in the vicinity of 1.3 kPa (10 torr). As a result, the pressure fluctuation range of the atmosphere during the decarburization process is extremely large. In addition, when the supply amount of oxygen gas (hereinafter referred to as “acid feed rate”) is increased for the purpose of shortening the decarburization processing time, the energy of the oxygen jet on the molten steel bath surface increases, It will cause operational hindrance such as adhesion of bullion.

  In order to cope with this, development of an acid-feeding decarburization technique that is high-speed and has little metal adhesion even under an atmosphere of a wide pressure range has become an urgent task. In the case of verifying 4), none of the methods has reached a fundamental solution.

  On the other hand, various measures using a burner lance for raising the temperature and melting the metal are proposed separately from the decarburization treatment as a countermeasure against the metal adhesion. For example, Patent Document 4 mentioned above proposes that two lances are used, the decarburization function and the temperature raising / dissolving function are made independent, and the temperature raising using the secondary combustion is performed simultaneously with the decarburizing process. Has been.

  Further, Patent Document 5 discloses a lance in which only oxygen gas is blown at the time of decarburization and fuel is mixed with oxygen gas at the time of melting and heating and used as a burner. Proposed. However, this method does not simultaneously perform decarburization and temperature rise / metal dissolution. Even if fuel is mixed at the time of decarburization, the amount of oxygen gas supplied for decarburization is usually large, and the flow rate is high near the nozzle, so that the combustion of fuel is difficult to occur, and even if burned, It is unstable and it is difficult to burn and raise the temperature near the lance.

  Further, Patent Document 6 proposes an upper blowing lance provided with liquid fuel and combustion gas passages on the outer periphery of the oxygen supply passage, and provided with a combustion burner at the outlet of these passages. However, the decarburization refining of molten steel and the temperature rise and metal melting are not performed simultaneously.

As described above, the conventional measures for adhesion of bullion are basically independent of the decarburization refining process and the temperature raising / dissolution process. At present, no suppression method has been proposed.
JP-A-55-125220 JP-A-57-137415 JP-A-9-146545 Japanese Patent Laid-Open No. 2-77518 JP-A-6-73433 JP 2000-328134 A

  The present invention has been made in view of such circumstances, and in high-efficiency and high-speed in oxidizing and refining the molten metal by blowing an oxygen-containing gas from the top blowing lance toward the molten metal under a reduced pressure lower than the atmospheric pressure. Furthermore, it is an object of the present invention to provide a molten metal refining method capable of reducing the adhesion of metal to equipment, and a refining top blowing lance used at that time.

  The present inventors have conducted intensive research to solve the above problems. As a result, at the time of top blowing oxygen delivery, a flame or combustion zone is formed around the oxygen jet injected from the Laval nozzle so as to surround the oxygen jet. At the same time, it was found that iron scattering could be reduced.

The present invention has been made on the basis of the above findings, and the method for refining molten metal under reduced pressure according to the first invention uses an upper blowing lance having a Laval nozzle installed at the tip thereof, and is lower than atmospheric pressure. When the molten metal is oxidatively refined by blowing oxygen gas from the Laval nozzle toward the molten metal under a pressure atmosphere, the throat diameter and the number of the throat diameter of the Laval nozzle around the Laval nozzle are as follows. Three or more sub-holes satisfying the formula (1) and having an inclination angle equal to or less than the value obtained by adding 10 ° to the maximum inclination angle of the Laval nozzle are installed at the tip of the upper blowing lance, by blowing an oxygen-containing gas, around the jet portion of the oxygen jet blown from the Laval nozzle, the atmosphere around surrounding the oxygen jet temperature It is characterized in that to form a flame or combustion zone to increase. In Equation (1), Dt is the throat diameter of the Laval nozzle, Dtt is the throat diameter of the subhole, N is the number of subholes installed, and α is an arbitrary coefficient larger than 3.

Refining method of molten metals under reduced pressure according to the second aspect, in the first shot bright, the supply amount of the oxygen-containing gas supplied from the auxiliary hole is 1/3 or less of the oxygen gas supply amount supplied from the Laval nozzle It is characterized by being.

According to a third aspect of the present invention, there is provided a method for refining molten metal under reduced pressure. In the first or second aspect , the molten metal is molten steel, and the oxidative refining is decarburization refining for removing carbon in the molten steel. It is characterized by being.

According to a fourth aspect of the present invention, there is provided the method for refining a molten metal under reduced pressure according to any one of the first to third aspects, wherein the oxidative refining starts refining when the atmospheric pressure is 40 kPa or less. Refining is terminated when the pressure is less than 3 kPa.

An upper blowing lance for refining molten metal under reduced pressure according to a fifth aspect of the present invention has a Laval nozzle installed at the tip thereof, and blows oxygen gas from the Laval nozzle toward the molten metal in an atmosphere at a pressure lower than atmospheric pressure. An upper blow lance for refining molten metal by oxidation refining, wherein the upper blow lance has the throat diameter and the number of installations in relation to the throat diameter of the Laval nozzle around the Laval nozzle. Three or more sub-holes that are satisfied and whose inclination is equal to or less than the value obtained by adding 10 ° to the maximum inclination of the Laval nozzle are installed at the tip of the top blowing lance, and oxygen-containing gas is introduced from the sub-holes. by blowing, around the jet portion of the oxygen jet blown from the Laval nozzle and raise the temperature of the atmosphere around surrounding the oxygen jet It is characterized in that the flame or combustion zone is configured to be formed.

  According to the present invention, when oxygen-containing gas is blown from the top blowing lance toward the molten metal under reduced pressure lower than atmospheric pressure to oxidize and refine the molten metal, the oxygen-containing gas is blown from the Laval nozzle around the oxygen-containing gas. Since the flame or combustion zone is formed so as to surround the contained gas, the temperature around the oxygen jet becomes high, and the density of the atmospheric gas around the oxygen jet is reduced. As a result, the amount of the atmospheric gas entrained in the oxygen jet is reduced. Less, the dynamic pressure of the oxygen jet increases. As a result, even if the lance height is increased, a high dynamic pressure can be obtained, the reaction efficiency of oxygen is increased and the reaction rate is promoted, and at the same time the lance height can be increased. Gold adhesion is reduced. Furthermore, the temperature in the refining vessel rises due to the flame or combustion zone that is formed, and adhesion of metal to the refining vessel is suppressed. As a result, it is possible to significantly reduce the refining cost of the molten metal under reduced pressure, and to bring about an industrially beneficial effect.

  Hereinafter, the present invention will be specifically described.

  The present invention relates to a technique for oxidizing and refining a molten metal by blowing an oxygen-containing gas such as oxygen gas, air or oxygen-enriched air from an upper blowing lance toward the molten metal in an atmosphere reduced to a pressure lower than atmospheric pressure. As an example of the refining equipment under reduced pressure, the RH vacuum degassing equipment widely used in the refining of molten steel is used as an example. Also, as the oxidative refining performed by blowing oxygen-containing gas, the RH vacuum degassing is used. We analyzed the decarburization treatment of molten steel performed in the facility as an example.

  First, the RH vacuum degassing facility will be described. FIG. 1 shows a schematic cross-sectional view of an RH vacuum degassing facility used in carrying out the refining method according to the present invention.

  As shown in FIG. 1, the RH vacuum degassing equipment 1 includes a vacuum tank 5 composed of an upper tank 6 and a lower tank 7, an ascending side dip pipe 8 and a descending side dip pipe 9 provided at the lower part of the lower tank 7. The upper tank 6 is provided with a duct 11 connected to an exhaust device (not shown), a raw material inlet 12, and an upper blowing lance 13 that is movable in the vertical direction inside the vacuum tank 5, The ascending-side dip tube 8 is provided with a reflux gas blowing tube 10. From the reflux gas blowing tube 10, Ar gas is blown into the rising side immersion tube 8 as the reflux gas.

  A Laval nozzle for blowing oxygen-containing gas into the vacuum chamber 5 is installed at the tip of the upper blowing lance 13. A schematic sectional view of this Laval nozzle is shown in FIG. As shown in FIG. 2, the Laval nozzle 16 has a shape composed of two cones, ie, a section where the cross section is reduced and an area where the cross section is enlarged. In the Laval nozzle 16, the reduced portion is the throttle portion 17 and the enlarged portion is the skirt. A portion that transitions from the portion 19 and the narrowed portion 17 to the skirt portion 19 and is the narrowest portion is called a throat 18. The oxygen-containing gas that has passed through the upper blowing lance 13 passes through the throttle portion 17, the throat 18, and the skirt portion 19 in order, and is jetted as a supersonic or subsonic jet. In FIG. 2, Dt is the throat diameter, De is the exit diameter, and the spread angle θ of the skirt portion 19 is usually 10 ° or less.

  In the Laval nozzle 16 shown in FIG. 2, the constricted portion 17 and the skirt portion 19 are conical, but the constricted portion 17 and the skirt portion 19 do not need to be conical in the Laval nozzle 16, and the inner diameter changes in a curved manner. The throttle portion 17 may be a straight cylindrical shape having the same inner diameter as the throat 18. When the throttle part 17 and the skirt part 19 are configured by curved surfaces whose inner diameter changes in a curved manner, an ideal flow velocity distribution can be obtained as the Laval nozzle 16, but it is extremely difficult to process the nozzle. When 17 is a straight cylindrical shape, it is slightly dissociated from the ideal flow velocity distribution, but there is no problem in use, and the machining of the nozzle becomes extremely easy. In the present invention, all these divergent nozzles are referred to as Laval nozzles 16.

  In the RH vacuum degassing facility 1 configured as described above, first, the ladle 2 for storing the molten steel 3 is conveyed directly under the vacuum tank 5, and the ladle 2 is raised by an elevating device (not shown), The ascending side dip tube 8 and the descending side dip tube 9 are immersed in the molten steel 3 accommodated in the pan 2. Next, Ar gas is blown into the ascending-side dip tube 8 from the reflux gas blowing tube 10 as a reflux gas, and the inside of the vacuum chamber 5 is evacuated by an exhaust device connected to the duct 11. Depressurize the inside. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 accommodated in the ladle 2 ascends the rising side dip tube 8 together with Ar gas blown from the reflux gas blowing tube 10 and flows into the vacuum chamber 5. Then, a flow returning to the ladle 2 via the descending side dip tube 9, that is, a so-called recirculation is formed, and RH vacuum degassing is performed.

  During this RH vacuum degassing refining, oxygen gas is blown and supplied as an oxygen-containing gas from the top blowing lance 13 toward the molten steel 3 inside the vacuum chamber 5 to decarburize the molten steel 3. Since it is necessary to increase the oxygen concentration of the molten steel 3 in the decarburization reaction, it is preferable that the molten steel 3 be in an undeoxidized or semi-deoxidized state before the decarburization treatment is started. Inside the ladle 2, slag adhering to the ladle 2 and slag generated in the refining of the previous process such as a converter or an electric furnace are partially mixed, and the molten steel 3 is covered as a slag 4.

  In the decarburization refining under reduced pressure (also referred to as “vacuum decarburization refining”), the present inventors can perform the decarburization processing at high speed and reduce adhesion of metal to the equipment. In order to develop a refining method capable of refining, the attenuation behavior of the top blowing acid jet under reduced pressure was investigated in detail, and the influence of the nozzle shape and atmospheric pressure on the dynamic pressure attenuation of the oxygen jet injected from the Laval nozzle 16 was investigated. Clarified. The survey results will be described below.

  As a result of measuring the flow velocity of the oxygen jet, it was quantitatively understood that the entrainment of the atmospheric gas was greatly suppressed under reduced pressure, and as a result, the flow velocity of the oxygen jet increased. Here, in order to improve the decarbonation efficiency, it is important to increase the dynamic pressure of the jet on the bath surface and efficiently supply oxygen to the steel bath. Since the dynamic pressure is a function of speed and density, in addition to the speed fluctuations under reduced pressure as described above, the density also fluctuates significantly, and the behavior of the dynamic pressure becomes extremely complicated. Therefore, the influence of the dynamic pressure fluctuation on the decarburization reaction efficiency is extremely large, and the control of the dynamic pressure is important.

  As a result of further investigation, it was quantitatively clarified that the dynamic pressure greatly increases as the atmospheric pressure decreases. In addition, it has been clarified that these dynamic pressures increase with an increase in the atmospheric temperature of the oxygen jet under reduced pressure, that is, the attenuation of the oxygen jet is suppressed by increasing the atmospheric temperature.

  Here, in order to maintain the decarbonation efficiency at a high level, it is effective to reduce the lance height (distance between the top lance tip and the molten steel surface) for the purpose of securing dynamic pressure. In addition, the dynamic pressure inevitably increases when high-speed acid feeding is intended. However, when the dynamic pressure is increased, the decarboxylation efficiency increases and the scatter of the metal becomes intense, and the adhesion of the metal to the surface of the top blowing lance 13 and the inner wall of the vacuum chamber 5 increases. In addition, the degree of vacuum at the beginning of acid delivery is low, and the degree of vacuum increases during acid delivery.Therefore, the fluctuation in the degree of vacuum during acid delivery decarburization is extremely large. It is very difficult to control the dynamic pressure such as high dynamic pressure at the end of acid. Therefore, in order to maintain the decarbonation efficiency, the dynamic pressure has to be increased, and the scattering of bullion has been an unavoidable problem.

  Therefore, the present inventors have studied and studied to suppress the adhesion of the metal while maintaining a high dynamic pressure under reduced pressure. As a result, a sub-hole was installed around the laval nozzle 16 of the upper blowing lance 13. It has been found that high dynamic pressure can be obtained while suppressing the adhesion of metal by performing decarburization refining while blowing oxygen-containing gas from the sub-hole using the blow lance 13. FIG. 3 shows a schematic cross-sectional view of the upper blowing lance 13 according to the present invention having this auxiliary hole. Note that the Laval nozzle 16 that supplies an oxygen jet to the sub-hole 20 is also referred to as a main hole.

  As shown in FIG. 3, the upper blowing lance 13 according to the present invention includes a cylindrical lance body 14 and a lance nozzle 15 connected to the lower end of the lance body 14 by welding or the like, and The lance body 14 is composed of four types of concentric steel pipes consisting of an outer pipe 21, an intermediate pipe 22, an inner pipe 23 and an innermost pipe 24, that is, a quadruple pipe. The Laval nozzle 16 is installed as a main hole facing the vertically downward direction, and a plurality of sub-holes 20 are installed around the Laval nozzle 16.

  The gap between the outer tube 21 and the middle tube 22 and the gap between the middle tube 22 and the inner tube 23 serve as a cooling water flow path for cooling the upper blowing lance 13. The gap between the inner tube 23 and the innermost tube 24 serves as a supply flow path for the oxygen-containing gas to the sub-hole 20, and the gap between the inner tube 23 and the innermost tube 24 from the upper end of the upper blowing lance 13. The supplied oxygen-containing gas passes through the gap between the inner tube 23 and the innermost tube 24, and is ejected from the auxiliary hole 20 into the vacuum chamber 5. Further, the inside of the innermost tube 24 serves as a supply flow path for the oxygen-containing gas to the Laval nozzle 16, and the oxygen-containing gas supplied from the upper end of the upper blowing lance 13 to the inside of the innermost tube 24 is the innermost tube. It passes through the inside of the tube 24 and is ejected from the Laval nozzle 16 into the vacuum chamber 5.

  In FIG. 3, the upper blowing lance 13 has a quadruple tube structure, but has a triple tube structure of an outer tube 21, an intermediate tube 22, and an inner tube 23, and the inside of the inner tube 23 contains oxygen into the Laval nozzle 16 and the sub-hole 20. A gas supply channel may be used. However, in this case, the oxygen-containing gas sent through the inner pipe 23 is injected from the Laval nozzle 16 and the sub-hole 20 at a ratio corresponding to the cross-sectional area of each nozzle discharge hole of the Laval nozzle 16 and the sub-hole 20. Accordingly, the independent flow path shown in FIG. 3 is preferable to the dual flow path in that the supply amount of the oxygen-containing gas can be arbitrarily changed between the Laval nozzle 16 and the auxiliary hole 20.

  The sub-hole 20 is for supplying an oxygen-containing gas for burning the CO gas generated by the decarburization reaction. In order to stably burn the CO gas, the total of the nozzle discharge hole cross-sectional areas of the sub-holes 20 is used. The value is made smaller than the nozzle discharge hole cross-sectional area of the Laval nozzle 16, that is, when the throat diameter of the sub-hole 20 is Dtt and the number of the sub-holes 20 is N, the following equation (1) It is preferable to arrange the sub holes 20 so as to satisfy the above. When the supply path for the oxygen-containing gas supplied to the Laval nozzle 16 and the sub-hole 20 is also used, the flow rate ratio flowing out from each nozzle is determined, which is particularly important. Here, in the equation (1), Dt is the throat diameter of the Laval nozzle 16, Dtt is the throat diameter of the sub-hole 20, N is the number of sub-holes installed, and α is an arbitrary coefficient larger than 1. In FIG. 3, the sub-hole 20 is a straight nozzle, and in this case, the inner diameter may be adopted as the throat diameter Dtt of the sub-hole 20.

  As described above, conventionally, in order to increase the dynamic pressure of the oxygen jet, it is necessary to reduce the lance height, but in that case, adhesion of the metal to the top blowing lance 13 becomes a problem. On the other hand, when oxygen gas is supplied as oxygen containing from the sub-hole 20 using the top blowing lance 13 of the present invention, CO gas in the atmospheric gas generated by the decarburization reaction by the Laval nozzle 16 and the sub-hole 20 are supplied. Reacts with the oxygen gas to form a flame or a combustion zone around the jet part of the oxygen jet. For this reason, the temperature around the oxygen jet is partially increased, and the density of the atmospheric gas around the jet is reduced, so that the amount of atmospheric gas entrained in the oxygen jet is reduced, that is, the oxygen jet is attenuated by the atmospheric gas. Is suppressed, and the dynamic pressure of the oxygen jet increases. As a result, high dynamic pressure can be obtained even if the lance height is increased. Further, since the CO gas generated by the decarburization reaction is used, no fuel gas is required. When the combustion zone is not closely attached to the periphery of the oxygen jet, the above effect is remarkably reduced.

  In this way, in the present invention, the lance height can be increased while maintaining the dynamic pressure of the oxygen jet, so that the operation with the increased lance height is possible. Gold adhesion can be reduced. In order to obtain this effect, it is necessary to generate a combustion zone around the ejection portion of the oxygen jet ejected from the Laval nozzle 16, so that a plurality of sub-holes 20 are required around the Laval nozzle 16. This effect can be obtained if three or more sub-holes 20 are provided, but the more sub-holes 20 are better in order to wrap the Laval nozzle 16 in the combustion zone. From this point of view, the effect increases when the number of sub-holes 20 is 5 or more, and 8 or more are more preferable.

  At this time, from the viewpoint of stably generating the combustion zone, the amount of acid sent from the sub-hole 20 is preferably smaller than the amount of acid sent from the Laval nozzle 16, and is preferably 1/3 or less of the amount of acid sent from the Laval nozzle 16. . When the oxygen-containing gas supply flow path is shared by the Laval nozzle 16 and the sub-hole 20, the sub-hole 20 may be designed and installed with the constant α in the equation (1) set to 3 or more.

Moreover, it is desirable to arrange the sub holes 20 evenly around the Laval nozzle 16. Moreover, although the subhole 20 should just be perpendicularly downward, you may provide an angle. However, if the angle is excessively widened, the combustion zone diffuses to the surroundings, and the combustion zone moves away from the periphery of the oxygen jet injected from the Laval nozzle 16, and the effect of suppressing the attenuation of the jet is reduced. There are inconveniences such as direct contact with the side walls and increased wear of refractories. For this reason, it is desirable that the inclination angle θ _{0 of the} sub-hole 20 is smaller than a value obtained by adding 10 ° to the maximum inclination angle of the Laval nozzle 16 (maximum inclination angle of the Laval nozzle 16 + 10 °). The inclination angle θ ₀ is preferably as small as possible, and is preferably equal to or less than the inclination angle of the Laval nozzle 16. In FIG. 3, the inclination angle of the Laval nozzle 16 is 0 °. The Laval nozzle 16 mainly has a single hole, but the same effect can be obtained even when it is made porous. The sub-hole 20 is preferably a straight nozzle in order to stabilize combustion.

  In the present invention, the adhesion of the metal to the top blowing lance 13 is reduced by increasing the lance height, but the scattering of the metal is increased by increasing the dynamic pressure of the oxygen jet. However, when the combustion zone is formed by the sub-hole 20, the side wall temperature of the vacuum chamber 5 rises, and the effect of suppressing the adhesion of metal to the side wall of the vacuum chamber 5 can be obtained at the same time. In addition, since the height of the lance can be increased in the present invention, a wide range of metal suppression effect inside the vacuum chamber 5 can be obtained, and it is not necessary to use a lance for secondary combustion separately.

  The atmospheric pressure at the end of the decarburization process may be 13.3 kPa (100 torr) or more, but when the energy of the oxygen jet increases to 13.3 kPa (100 torr) or less, particularly 9.3 kPa (70 torr) or less. The decarburization efficiency is high, and therefore, the atmospheric pressure at the end of the decarburization process is preferably 13.3 kPa (100 torr) or less. In addition, even when the atmospheric pressure at the start of the decarburization process is 13.3 kPa (100 torr) or less, the decarburization process can be performed, but the dynamic pressure becomes too high and the scattering of the bullion becomes intense, so 13.3 kPa (100 torr) The above is preferable. However, if the atmospheric pressure is too high and the fluctuation range of the atmospheric pressure is too large, a sufficient effect cannot be obtained over the entire decarburization treatment. Therefore, the atmospheric pressure is preferably 40.0 kPa (300 torr) or less. In addition, the carbon concentration of the molten steel 3 decreases with the progress of oxygen blowing, and the decarbonation efficiency decreases as with the converter decarburization at atmospheric pressure. The speed may be reduced. As a result, the oxygen efficiency is improved, and a rapid increase in dynamic pressure can be suppressed even when the atmospheric pressure is low from the middle to the end of oxygen blowing.

  As described above, according to the present invention, when the molten steel 3 is vacuum decarburized and refined by blowing oxygen gas from the top blowing lance 13 toward the molten steel 3 under a reduced pressure lower than the atmospheric pressure, the molten steel 3 is sprayed from the Laval nozzle 16. Since a flame or a combustion zone is formed around the oxygen gas jet so as to surround the oxygen jet, the temperature around the oxygen jet becomes high, and the density of the ambient gas around the oxygen jet is reduced. The amount of atmospheric gas involved in the jet is reduced and the oxygen dynamic pressure is increased. Thereby, even if the lance height is increased, a high dynamic pressure is obtained, the reaction efficiency of oxygen is increased and the reaction rate is promoted, and at the same time the lance height can be increased. Can reduce the adhesion of metal to the vacuum chamber, and the combustion zone heats up the inside of the vacuum chamber 5 to suppress the adhesion of metal to the vacuum chamber 5, greatly reducing the cost of vacuum decarburization and refining. It becomes possible to do.

  In the above description, the acid feed from the sub-hole 20 is used to form the combustion zone. However, the present invention is not limited to this, and the main point is the supersonic oxygen jet from the main hole (Laval nozzle 16). Therefore, a combustion zone may be provided around the main hole, and therefore, a combustion burner may be provided around the main hole and burned simultaneously with decarburization. Further, the supersonic jet may be injected by injecting fuel from the nozzle inner wall of the main hole. In this case, in order to form a combustion zone around the jet, it is necessary to hold the flame, and it is necessary to provide a flame holding groove on the inner wall of the Laval nozzle. However, as described above, the use of the above-described method in which a combustion zone is obtained only by oxygen gas from the sub-hole 20 eliminates the need for fuel, so that the facilities are simplified and the maximum merit can be obtained.

  Examples of the present invention are shown below together with conventional examples. First, about 360 tons of hot metal was charged into an upper bottom blown combined blowing smelting converter in which oxygen gas was blown up and stirring gas was blown at the bottom, and decarburization blowing was mainly performed. The end of converter decarburization blowing was the time when the carbon concentration in the molten steel reached 0.05 to 0.07 mass%, and the molten steel temperature at the end of decarburization blowing was targeted at 1650 ° C.

Next, after the molten steel obtained by converter refining is discharged from the converter, it is transported to the RH vacuum degassing facility shown in FIG. 1 in an undeoxidized state, and is about 0.18 Nm ³ from the laval nozzle of the top blowing lance. / Min · t oxygen gas was supplied to the molten steel surface in the vacuum chamber, and at the same time, vacuum decarburization refining was performed while supplying a total of 0.04 Nm ³ / min · t oxygen gas from the sub-holes. In this vacuum decarburization refining, the amount of deposited metal and the decarbonation efficiency were investigated. The amount of adhesion metal was evaluated for the adhesion metal to the top blowing lance. The survey results are shown in Table 1. In the column of metal adhesion in Table 1, ◎ indicates that the amount of adhesion is small, and × indicates that the amount of adhesion is large.

  As shown in Table 1, in the examples of the present invention, the decarbonation efficiency was as high as 68% or more, and the adhesion of the metal was very small. On the other hand, in the conventional example in which no sub-hole is installed, although the adhesion of the base metal is reduced when the lance height is increased, the decarbonation efficiency is lowered. Although the efficiency is high, the adhesion of bullion has increased and both cannot be satisfied.

It is a schematic sectional drawing of the RH vacuum degassing equipment used when implementing the refining method by this invention. It is a schematic sectional drawing of the Laval nozzle installed in the front-end | tip of an upper blowing lance. It is a schematic sectional drawing of the top blowing lance which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 RH vacuum degassing equipment 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 6 Upper tank 7 Lower tank 8 Rising side immersion pipe 9 Lowering side immersion pipe 10 Recirculation gas blowing pipe 11 Duct 12 Raw material inlet 13 Upper blowing lance 14 Lance Body 15 Lance nozzle 16 Laval nozzle 17 Throttle part 18 Throat 19 Skirt part 20 Sub-hole 21 Outer pipe 22 Middle pipe 23 Inner pipe 24 Innermost pipe

Claims (5)

When an oxygen gas is blown from the Laval nozzle toward the molten metal in an atmosphere having a pressure lower than atmospheric pressure using an upper blowing lance having a Laval nozzle at its tip, the molten metal is oxidized and refined around the Laval nozzle. , sub its throat diameter and installation number thereof relative to the throat diameter of the Laval nozzle satisfies the following formula (1), the inclination is 3 or more or less maximum inclination angle to plus 10 ° value of the Laval nozzle A hole is opened at the tip of the upper blowing lance, and an oxygen-containing gas is blown through the auxiliary hole, and the surrounding atmosphere surrounding the oxygen jet is surrounded by the blowing portion of the oxygen jet blown from the Laval nozzle . A method for refining molten metal under reduced pressure, characterized by forming a flame or a combustion zone for increasing temperature .
Dt ² = Dtt ² × N × α (1)
In Equation (1), Dt is the throat diameter of the Laval nozzle, Dtt is the throat diameter of the subhole, N is the number of subholes installed, and α is an arbitrary coefficient larger than 3. 2. The method for refining molten metal under reduced pressure according to claim 1, wherein the supply amount of the oxygen-containing gas supplied from the sub-hole is 1/3 or less of the supply amount of oxygen gas supplied from the Laval nozzle. . The method for refining molten metal under reduced pressure according to claim 1 or 2, wherein the molten metal is molten steel, and the oxidative refining is decarburization refining for removing carbon in the molten steel. . The oxidation refining starts refining when the ambient pressure is below 40 kPa, characterized in that the ambient pressure is terminated refining when: 13.3 kPa, any one of claims 1 to 3 A method for refining molten metal under reduced pressure as described in 1. A top blowing lance for refining, in which a laval nozzle is installed at the tip thereof, and oxygen gas is blown from the laval nozzle toward the molten metal in an atmosphere at a pressure lower than atmospheric pressure to oxidize and refine the molten metal. In the lance, the throat diameter and the number of the throat diameter of the Laval nozzle around the Laval nozzle satisfy the following formula (1), and the inclination angle is less than the value obtained by adding 10 ° to the maximum inclination angle of the Laval nozzle. There are three or more sub-holes opened at the tip of the upper blowing lance, and by blowing an oxygen-containing gas from the sub-holes, the oxygen jet is blown from the Laval nozzle around the jet portion of the oxygen jet. characterized in that the flame or combustion zone to increase the temperature of the ambient atmosphere surrounding the oxygen jet is configured to be formed To, lance on for refining molten metal in vacuo.
Dt ² = Dtt ² × N × α (1)
In Equation (1), Dt is the throat diameter of the Laval nozzle, Dtt is the throat diameter of the subhole, N is the number of subholes installed, and α is an arbitrary coefficient larger than 3.

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