JP4470673B2 - Vacuum decarburization refining method for molten steel - Google Patents

Description

  The present invention relates to a vacuum decarburization refining method for decarburizing and refining molten steel by blowing an oxygen-containing gas toward molten steel from a laval nozzle at the tip of an upper blowing lance under a reduced pressure lower than atmospheric pressure.

  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 method of quickly decarburizing the molten steel, oxygen gas is blown upward from the upper blowing lance having a Laval nozzle installed at the tip thereof toward the molten steel surface. A method of refining is known (see, for example, Patent Document 1).

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

  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.

Several means have been proposed for accelerating the decarburization reaction while solving the problems caused by the adhered metal. For example, in Patent Document 2, an atmosphere having a pressure lower than 100 torr (13.3 kPa) is obtained by using a Laval nozzle in which the atmospheric pressure at the time of designing the Laval nozzle is the upper limit of the fluctuation range of the atmospheric pressure in the tank during acid refining. A method of reducing the arrival loss of oxygen gas due to the attenuation of the oxygen jet below has been proposed. 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. Further, in Patent Document 4, the shape of the Laval nozzle, the oxygen gas flow rate, the degree of vacuum in the tank at the end of the acid delivery, and the lance height are measured 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.
JP 55-125220 A Japanese Examined Patent Publication No. 61-57886 Japanese Patent No. 3293023 Japanese Patent No. 2667007

  By the way, in recent years, combined with the increase in steel types that require a smelting method using vacuum degassing equipment to adjust the components of molten steel, the converter and RH vacuum degassing are aimed at reducing the smelting cost of molten steel. 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 increasing the concentration is performed as compared with the conventional case, 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 such that the width increases and, in some cases, exceeds 100 torr (13.3 kPa), and the decarburization process is terminated at a low atmospheric pressure in the vicinity of 10 torr (1.3 kPa). 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 obstacles such as adhesion of bullion. In order to cope with this, there has been an urgent need to develop an acid-feeding decarburization technology that is fast and has little metal adhesion even under a wide range of pressures.

  However, when the prior art is verified from this point of view, in Patent Document 2, it is a region where the fluctuation range of the assumed atmospheric pressure is small, and the range of the design atmospheric pressure of the Laval nozzle is narrow. When pressure varies, it becomes difficult to design an optimal Laval nozzle. In addition, in the case of carrying out the acid feeding treatment at a high atmospheric pressure exceeding 100 torr (13.3 kPa), it is doubtful whether the fluctuation range of the operating atmospheric pressure becomes extremely large and the high reaction efficiency is maintained. In this case, it is unclear about the optimization with the adhesion of bullion. Furthermore, since the damping behavior of the oxygen jet under reduced pressure is still unclear, the examination of the oxygen surface bathing pressure control is insufficient.

  In Patent Document 3, the behavior of the oxygen jet under reduced pressure is not affected by the atmospheric pressure, and the influence of the atmospheric pressure is not sufficiently taken into account, which is not appropriate. Moreover, in patent document 4, since the atmospheric pressure which is one of the conditions to control is not the thing of the arbitrary time points during acid sending but the atmospheric pressure at the time of completion | finish of acid feeding, As mentioned above, the atmospheric pressure at the time of operation is the same. When the fluctuation range is large, it is difficult to take into account the effects of atmospheric pressure fluctuations, and it is difficult to say that the refining reaction is sufficiently controlled. In addition, judging from the examples, the assumed upper blowing lance is designed at a relatively low atmospheric pressure of 50 torr (6.7 kPa), and therefore, a high atmospheric pressure of 100 torr (13.3 kPa) or more is obtained. Such large atmospheric pressure fluctuations are not assumed.

  The present invention has been made in view of such circumstances, and when the oxygen-containing gas is sprayed from the top blowing lance toward the molten steel under a reduced pressure lower than the atmospheric pressure, when the molten steel is vacuum decarburized and refined, fluctuations in the atmospheric pressure are performed. This solves the problem that the fluctuation of the bath surface dynamic pressure of the top-blowing oxygen jet generated by the air hinders high-efficiency decarburization and destabilization of molten steel, and is highly efficient and fast. It aims at providing the vacuum decarburization refining method of the molten steel which can reduce the metal adhesion to an installation.

  The present inventors have conducted intensive research to solve the above problems. As a result, high efficiency and high speed can be achieved by appropriately controlling the oxygen delivery conditions such as the lance height of the top blowing lance and the acid delivery speed according to the atmospheric pressure in the tank that changes every moment during the top blowing acid. It was found that vacuum decarburization refining is possible under efficient and stable operation with less metal adhesion to equipment. Hereinafter, the examination results will be described.

  When supplying an oxygen-containing gas such as oxygen gas or oxygen gas diluted with Ar gas to the molten steel bath surface using an upper blowing lance, the oxygen-containing gas is usually passed through a Laval nozzle arranged at the tip of the upper blowing lance. Supplied. This is because the oxygen-containing gas jet (hereinafter referred to as “oxygen jet” including all oxygen gas jets) injected from the Laval nozzle is supersonic or subsonic, and the reaction efficiency between oxygen gas and molten steel is high. Because it becomes. The design of the Laval nozzle is performed based on the supply amount of the oxygen-containing gas from the top blowing lance, that is, the supply speed (F). In order to obtain a supersonic or subsonic oxygen jet, the Laval nozzle throat diameter (Dt) and outlet diameter (De) is designed in an optimum shape.

That is, the design of the Laval nozzle is such that the oxygen-containing gas supply rate Fh (Nm ³ / hr) per hole of the Laval nozzle obtained from the oxygen-containing gas supply rate F (Nm ³ / hr) from the top blowing lance and the throat diameter Dt ( mm) to determine the nozzle back pressure Po (kPa) by the following equation (1), or the oxygen-containing gas supply speed Fh (Nm ³ / hr) per nozzle nozzle and the nozzle back pressure Po (kPa) From the following formula (1), the throat diameter Dt (mm) is determined. From the throat diameter Dt (mm) thus determined and the nozzle back pressure Po (kPa), and the design atmospheric pressure Pe (kPa), (2), the outlet diameter De (mm) of the Laval nozzle is determined. When oxygen gas is supplied alone as the oxygen-containing gas, the supply rate F of the oxygen-containing gas coincides with the acid feed rate F.

  Here, the supply rate Fh of the oxygen-containing gas per hole of the Laval nozzle is determined by the ratio of the cross-sectional area of the throat diameter Dt of each Laval nozzle to the total cross-sectional area of the throat diameter Dt of the Laval nozzle and the supply rate F of the oxygen-containing gas. In general, when a plurality of Laval nozzles are installed, the throat diameter Dt of each Laval nozzle is substantially the same, so the supply rate F of the oxygen-containing gas is the number of installed Laval nozzles. It can be obtained by dividing. The design atmospheric pressure Pe is an atmospheric pressure outside the Laval nozzle. In the case of vacuum decarburization refining, the design atmospheric pressure Pe is arbitrarily selected from the atmospheric pressure in the tank under reduced pressure. In the case of vacuum decarburization refining, since the atmospheric pressure in the tank fluctuates, the design atmospheric pressure Pe cannot be determined as a constant value, and therefore it is arbitrarily selected from the atmospheric pressure in the tank under reduced pressure. Become. However, as will be described later, there is a preferable range even though it is arbitrarily selected. These design atmospheric pressure Pe and nozzle back pressure Po are displayed in absolute pressure. That is, the pressure is 101.3 kPa (1 atm) when the pressure is the same as the atmospheric pressure.

  The jet flow velocity Vo (m / sec) at the Laval nozzle outlet of the oxygen jet injected from the Laval nozzle having this shape can be obtained by the following equation (3). The expressions (1), (2), and (3) are relational expressions that are established in the Laval nozzle, and are well-known expressions that are used when designing the Laval nozzle. In the Laval nozzle, when the atmospheric pressure Pea in the tank under reduced pressure coincides with the designed atmospheric pressure Pe, the oxygen jet ejected from the Laval nozzle expands appropriately to reach the maximum flow velocity, while the atmosphere in the tank As the pressure Pea deviates from the design atmospheric pressure Pe, the oxygen jet becomes overexpanded or underexpanded, and the jet velocity of the oxygen jet decreases.

  When an oxygen-containing gas is supplied in a reduced pressure atmosphere using an upper blow lance equipped with a Laval nozzle designed in this way, the condition affecting the oxygen jet is the lance height (the tip of the upper blow lance And the molten steel surface), the atmospheric pressure Pea in the tank, the shape of the Laval nozzle, the nozzle back pressure Po, and the like. When one of these changes, the action of the oxygen jet changes greatly. In addition, the behavior of oxygen jet under reduced pressure is completely different from the behavior under air, and information that has been cultivated in converters and the like in the past cannot be used.

  As a result of detailed investigations on the relationship between the shape of the Laval nozzle and the supply conditions of the oxygen-containing gas, the decarburization behavior and the molten steel scattering behavior, the present inventors include the scale of the equipment, the atmospheric pressure, and the supply speed of the oxygen-containing gas. The behavior of the top-blowing supersonic oxygen jet under reduced pressure under all conditions was clarified, and the relationship between the ultimate pressure of the oxygen jet on the molten steel surface, the decarburization behavior and the molten steel scattering behavior could be clarified.

  Therefore, the action due to the change in these conditions is determined by the parameter P which is an indicator of the ultimate pressure at the molten steel surface on the central axis of the injected oxygen jet. In experimentally determining the parameter P, first, the flow velocity V of the oxygen jet on the molten steel bath surface was experimentally determined. This is because as the flow velocity V at the molten steel bath surface of the oxygen jet increases, the ultimate pressure at the molten steel surface increases, and the parameter P is proportional to the flow velocity V at the molten steel bath surface of the oxygen jet. The experiment was performed using Laval nozzles having various outlet diameters De (mm) and throat diameters Dt (mm), changing the lance height, atmospheric pressure Pea (kPa) and nozzle back pressure Po (kPa). The flow velocity V (m / sec) at the molten steel surface of the supersonic oxygen jet below was measured. As a result, the following equation (4) was derived as a regression equation of the flow velocity V (m / sec) on the molten steel surface of the oxygen jet. However, Vo in the equation (4) is the flow velocity (m / sec) of the oxygen jet at the Laval nozzle outlet calculated by the equation (3), Pea is the atmospheric pressure (kPa), Lh is the lance height (mm), De Is the outlet diameter (mm) of the Laval nozzle.

  As a regression equation of the parameter P (−), which is an index of the ultimate pressure on the molten steel surface, from the flow velocity V (m / second) of the oxygen jet on the molten steel surface determined by the equation (4) and the atmospheric pressure Pea (kPa). The following equation (5) was derived.

  When the parameter P, which is an indicator of the ultimate pressure, is smaller than 0.5, it has been found that the oxygen jet does not sufficiently reach the bath surface and the decarbonation efficiency is greatly reduced. On the other hand, when the parameter P is larger than 2.0, it has been found that the oxygen jet strikes the molten steel bath surface too much, causing problems such as promoting molten steel scattering. Therefore, by controlling the supply conditions of the oxygen-containing gas so that the parameter P falls within the range of 0.5 to 2.0, an appropriate ultimate pressure can be obtained in both decarburization and iron scattering. It was found that the range of 0.8 to 1.7 is a more preferable range. That is, by controlling the oxygen-containing gas supply conditions based on the parameter P corresponding to the pressure reached by the oxygen-containing gas to the molten steel surface in consideration of the atmospheric pressure Pea and the scale factor of the equipment, Despite this, the knowledge that vacuum decarburization refining with high efficiency and less molten steel can be achieved was obtained. In addition, since the optimum condition may vary depending on the molten steel condition, adjusting the supply condition so that the fluctuation range of the parameter P is within 0.5 within the range of the parameter P may further increase the effect. I understood. Although it is preferable to adjust the supply conditions continuously, there is no problem even if it is performed stepwise.

  The present invention has been made based on the above examination results, and the method for vacuum decarburization and refining of molten steel according to the first invention uses an upper blowing lance having a rubber nozzle installed at the tip thereof, and has an atmosphere lower than atmospheric pressure. When decarburizing and refining by blowing an oxygen-containing gas toward the molten steel below, the throat diameter (Dt) and outlet diameter (De) of the Laval nozzle and the per Laval nozzle hole under the condition that the pressure of the atmosphere is 40 kPa or less A Laval nozzle having a shape in which the supply rate of the oxygen-containing gas (Fh), the nozzle back pressure (Po), and the design atmospheric pressure (Pe) satisfies the above formulas (1) and (2) is used. The lance height of the top blowing lance and the oxygen-containing gas in accordance with the atmospheric pressure so that the parameter P calculated from the equations (3), (4) and (5) is 0.5 to 2.0. Supply speed It is characterized in that for adjusting at least one of.

  The vacuum decarburization refining method for molten steel according to the second invention is characterized in that, in the first invention, the atmospheric pressure (Pea) at the start of the blowing of the oxygen-containing gas from the upper blowing lance is 13.3 kPa or more. It is what.

  The method for vacuum decarburizing and refining molten steel according to the third invention is the first or second invention, wherein the design atmospheric pressure (Pe) is 13.3 kPa or more and oxygen-containing gas is blown from an upper blowing lance. It is characterized by being in a range not exceeding the maximum pressure of the atmosphere.

  The method for vacuum decarburization and refining of molten steel according to the fourth invention is the method according to any one of the first to third inventions, wherein the atmospheric pressure (Pea) at the end of the blowing of the oxygen-containing gas from the upper blowing lance is 6.7 kPa. It is characterized by the above.

  The method for vacuum decarburizing and refining molten steel according to the fifth aspect of the present invention is the method according to any one of the first to fourth aspects, wherein the carbon concentration in the molten steel at the start of blowing the oxygen-containing gas from the top blowing lance is 0.04. It is characterized by being not less than mass%.

  According to the present invention, when decarburizing and refining molten steel by blowing an oxygen-containing gas toward molten steel from an upper blowing lance in which a Laval nozzle is disposed under a reduced pressure lower than atmospheric pressure, the dynamic pressure of the oxygen jet on the molten steel surface Since the lance height of the top blowing lance or the supply rate of the oxygen-containing gas is adjusted in accordance with the atmospheric pressure so that the parameter P, which is an index of the above, falls within the range of 0.5 to 2.0, a wide range of atmospheric pressures can be obtained. Even in the fluctuating range, the dynamic pressure of the oxygen jet on the molten steel surface is optimized, high decarbonation efficiency is maintained, and at the same time, the scattering of bullion can be suppressed and the adhesion of bullion to equipment can be reduced. Thus, a significant reduction in the decarburization and refining cost of the molten steel under reduced pressure is achieved, resulting in an industrially beneficial effect.

  Hereinafter, the present invention will be specifically described. The hot metal discharged from the blast furnace is received in a hot metal holding / conveying vessel such as a hot metal ladle or torpedo car, and transferred to a converter for decarburization and refining in the next step. During the conveyance, the hot metal can be subjected to hot metal pretreatment such as dephosphorization or desulfurization. This hot metal is decarburized and refined in a converter, and the resulting molten steel is discharged from the converter into a ladle. The molten steel is then used in a RH vacuum degassing facility, a DH vacuum degassing facility, or a VOD facility. Oxygen-containing gas such as oxygen gas, air, oxygen-enriched air, oxygen gas diluted with Ar gas is sprayed from the top blowing lance toward the molten steel in an atmosphere depressurized from atmospheric pressure, and the molten steel is decarburized and refined. It transports to the vacuum degassing equipment which can be performed, and the vacuum decarburization refining which concerns on this invention is implemented. In this case, the molten steel to be used is not limited to the molten steel obtained by decarburizing and refining the molten iron discharged from the blast furnace, and may be molten steel obtained by refining iron scrap or the like in an electric furnace.

  Typical equipment for vacuum degassing equipment for processing molten steel is RH vacuum degassing equipment. Hereinafter, RH vacuum degassing equipment is used as vacuum degassing equipment, oxygen gas is used as oxygen-containing gas, and air blown up. The present invention will be described with an example of acid decarburizing and refining. First, the RH vacuum degassing facility will be described. FIG. 1 shows a schematic cross-sectional view of an RH vacuum degassing facility used when carrying out the vacuum decarburization refining method according to the present invention, and FIG. 2 shows a schematic enlarged cross-sectional view of the upper blow lance shown in FIG. 3 is a schematic cross-sectional view of a Laval nozzle installed at the tip of the upper blowing lance shown in FIG.

  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 distance Lh between the tip of the top blowing lance 13 and the molten metal surface of the molten steel 3 inside the vacuum chamber 5 is referred to as a lance height.

  As shown in FIG. 2, the upper blow lance 13 is composed of a cylindrical lance main body 14 and a lance nozzle 15 connected to the lower end of the lance main body 14 by welding or the like. The outer tube 20, the inner tube 21, and the inner tube 22 are composed of three concentric steel pipes, that is, a triple pipe, and the center hole of the copper lance nozzle 15 has a central hole facing vertically downward. A Laval nozzle 16 is installed. The gap between the outer tube 20 and the middle tube 21 and the gap between the middle tube 21 and the inner tube 22 serve as a cooling water flow path for cooling the upper blowing lance 13. The inside is an oxygen gas supply flow path to the Laval nozzle 16, and the oxygen gas supplied from the upper end of the upper blowing lance 13 into the inner tube 22 passes through the inner tube 22, and from the Laval nozzle 16 to the vacuum chamber. 5 is ejected into the interior.

  As shown in FIG. 3, the Laval nozzle 16 has a shape composed of two cones, ie, a portion whose cross section is reduced and a portion where the cross section is enlarged. In the Laval nozzle 16, the reduction portion is a throttle portion 17 and the enlargement portion is a 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 gas that has passed through the inside of the lance main body 14 passes through the throttle portion 17, the throat 18, and the skirt portion 19 in order, and is injected as a supersonic or subsonic jet. In FIG. 3, 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. 3, the constricted portion 17 and the skirt portion 19 are conical bodies. However, as the Laval nozzle 16, the constricted portion 17 and the skirt portion 19 do not need to be conical bodies, 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 exhausted 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 immersion 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. Inside the ladle 2 is partially mixed with slag 4 generated in the refining of the previous process such as a converter or an electric furnace to cover the surface of the molten steel 3.

During this RH vacuum degassing refining, oxygen gas is blown toward the molten steel 3 inside the vacuum chamber 5 from the top blowing lance 13 and supplied, and the molten steel 3 is vacuum decarburized. The supply amount of oxygen gas, that is, the acid feed rate F (Nm ³ / hr) is the difference between the carbon concentration of the molten steel 3 before the vacuum decarburization treatment and the target carbon concentration at the end of the vacuum decarburization, the amount of molten steel, It can be determined empirically from the charcoal processing time and the like, for example, an acid feed rate capable of supplying an oxygen gas of 0.1 to 0.2 Nm ³ / min per ton of molten steel. In this case, if the acid feed rate F (Nm ³ / hr) is determined, the nozzle back pressure Po (kPa) is uniquely determined by the above-described equation (1) as long as the same top blowing lance 13 is used.

The Laval nozzle 16 disposed in the upper blowing lance 13 has a throat diameter Dt (mm) and an outlet diameter De (mm). The acid feed rate Fh (Nm ³ / hr) per Laval nozzle hole (in this case, the nozzle hole is 1 Since it is the same, it is the same as the acid feed rate F), the nozzle back pressure Po (kPa), and the design atmospheric pressure Pe (kPa). It is a thing. In this case, the design atmospheric pressure Pe is set to a value equivalent to the atmospheric pressure Pea inside the vacuum chamber 5 during the period in which oxygen gas is supplied from the top blowing lance 13. In particular, in the recent vacuum decarburization refining in which the top blowing acid decarburization is performed when the atmospheric pressure Pea in the vacuum chamber 5 is 13.3 kPa (100 torr) or more, the design atmospheric pressure Pe is set to 13.3 kPa (100 torr). It is preferable that the pressure is within a range not exceeding the maximum pressure of the atmospheric pressure Pea when the acid is sent from the top blowing lance 13. By setting the design atmospheric pressure Pe in this manner, the oxygen jet is appropriately expanded in a relatively high region of the atmospheric pressure Pea where the carbon concentration of the molten steel 3 is high and the decarburization speed needs to be increased. In addition, the decarburization reaction efficiency is improved, and at the same time, it becomes possible to suppress an increase in the dynamic pressure of the oxygen jet due to insufficient expansion in the end region of the acid feed decarburization where the atmospheric pressure Pea is low. It is possible to efficiently prevent bullion scattering.

  Using the upper blowing lance 13 provided with the Laval nozzle 16 designed in this way, the parameter P calculated by the above-described equations (3) to (5) is in the range of 0.5 to 2.0. The lance height Lh or the acid feed rate F is preferably set in accordance with the atmospheric pressure Pea in the vacuum chamber that changes every moment during the top blowing acid decarburization so that it is preferably in the range of 0.8 to 1.7. change. When the acid feed rate F is changed, the nozzle back pressure Po naturally changes as is apparent from the equation (1). Further, since the operation becomes more stable as the fluctuation range of the parameter P is smaller, it is preferable to change the lance height Lh or the acid feed rate F so that the fluctuation range of the parameter P becomes 0.5 or less. When the parameter P can be adjusted within a predetermined range only by changing the lance height Lh, it is not necessary to change the acid feed rate F. The inventors of the present invention have confirmed that in most cases, it can be dealt with only by changing the lance height Lh. When carrying out vacuum decarburization refining with a significantly different acid feed rate F, an upper blow lance 13 provided with a Laval nozzle 16 having a throat diameter Dt and an outlet diameter De corresponding to the acid feed rate is prepared. It is also possible to deal with this by exchanging the upper blowing lance 13.

  Thus, in the method of the present invention, the parameter P, which is an index of the dynamic pressure of the oxygen jet on the molten steel surface, is controlled within the range of 0.5 to 2.0 according to the atmospheric pressure Pea of the vacuum chamber 5. Even if the atmospheric pressure Pea in the vacuum chamber 5 changes significantly, the dynamic pressure of the oxygen jet on the molten steel surface is controlled to an appropriate range, and high decarbonation efficiency is maintained, and at the same time, metal scatter is suppressed. As a result, it is possible to reduce adhesion of metal to the equipment.

  If the atmospheric pressure Pea of the vacuum chamber 5 is increased, the circulating flow of the molten steel 3 is hindered and smooth vacuum decarburization treatment cannot be performed. Therefore, the atmospheric pressure Pea during vacuum decarburization refining is 40.0 kPa (300 torr) or less. There is a need to. On the other hand, if the atmospheric pressure Pea during vacuum decarburization refining becomes too low, the adhesion of metal to the inner wall of the vacuum tank 5 by the oxygen jet from the top blowing lance 13 increases rapidly, so that during the top blowing acid decarburization The atmospheric pressure Pea is preferably 6.7 kPa (50 torr) or more. That is, the atmospheric pressure Pea during the top blowing acid decarburization is preferably in the range of 6.7 to 40.0 kPa. Further, from the viewpoint of shortening the entire processing time in the RH vacuum degassing facility 1, the acid sent from the top blowing lance 13 is started from a relatively high stage where the atmospheric pressure Pea of the vacuum chamber 5 is 13.3 kPa (100 torr) or more. It is preferable to start. By doing in this way, even if the time spent for the vacuum decarburization refining by the top blowing acid is equivalent, the time to start the top blowing acid decarburization is shortened, and the whole of the RH vacuum degassing refining is reduced. Processing time can be shortened. In vacuum decarburization refining by top blowing oxygen blowing, the atmospheric pressure Pea decreases with the lapse of treatment time, so the atmospheric pressure Pea at the start of acid feeding is usually the highest, and the atmospheric pressure at the end of the acid feeding Pea is lowest.

  Furthermore, when the carbon concentration of the molten steel 3 is 0.04% by mass or more, the effect of the acid feed rate on the decarburization, that is, the acid feed conditions, is large. When it is at least%, the effect of the method of the present invention increases. When the carbon concentration of the molten steel 3 before the start of acid feeding is 0.05% by mass or more, the effect is even greater. In addition, since the decarburization reaction proceeds as the oxygen concentration of the molten steel 3 increases, it is preferable that the molten steel 3 be in an undeoxidized state before the decarburization treatment is started. Further, when oxygen gas diluted with Ar gas instead of oxygen gas is used as the oxygen-containing gas, the supply amount (supply rate F) of the diluted oxygen gas is used in place of the acid feed rate F and the Laval nozzle 16 is used. By designing the method, the method of the present invention can be implemented in accordance with the above description.

  In addition, although the said description demonstrated as an example the vacuum decarburization refining of the molten steel 3 in the RH vacuum degassing equipment 1, this invention is not restricted to the said description, The top blowing acid vacuum using another degassing equipment. The same effect can be obtained also in decarburization refining. Moreover, although the example in which only the center hole is installed as the upper blowing lance 13 has been described, a porous upper blowing lance having a plurality of nozzles may be used. Further, it may be combined with a technique such as refining agent projection.

  Examples of the present invention are shown below together with comparative examples. First, about 320 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 set at the time when the carbon concentration in the molten steel reached 0.04 to 0.12% by mass, and the molten steel temperature at the end of decarburization blowing was set to 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 approximately 0.14 to 0.00 from the top blowing lance. 18 Nm ³ / min · t of oxygen gas was supplied to the molten steel surface in the vacuum chamber, and vacuum decarburization refining was carried out by changing the lance height of the top blowing lance. The Laval nozzle used was mainly designed using the above-mentioned formulas (1) and (2) with a design atmospheric pressure Pe of 20.0 kPa, and designed atmospheric pressures of 10.7 kPa, 24.0 kPa, and 33.3 kPa. A Laval nozzle designed with Pe was also used. The carbon concentration of the molten steel at the end of the top blowing acid was about 0.02 to 0.06% by mass. Then, based on the lance height, the acid feed speed, the atmospheric pressure, and the Laval nozzle shape, the value of the parameter P during the acid feed decarburization refining is obtained using the above-described formulas (3) to (5), and the obtained parameter P And the relationship between the amount of metal in the top lance and decarbonation efficiency. The operating conditions and survey results are shown in Table 1. Here, the decarbonation efficiency is the ratio of oxygen gas that contributes to the decarburization reaction in the supplied oxygen gas, and is expressed as a percentage.

  As shown in Table 1, in the test Nos. 1 to 8 in which the parameter P is in the range of the present invention, the adhesion of the bare metal is small, and the decarbonation efficiency can be a high value of about 70% or more. . On the other hand, in the test Nos. 9 to 13 in which the parameter P is out of the scope of the present invention, the decarbonation efficiency is lower or the adhesion of the metal is higher than in the tests No. 1 to 8. Was confirmed.

It is a schematic sectional drawing of the RH vacuum degassing equipment used when implementing the refining method by this invention. It is a general | schematic expanded sectional view of the upper blowing lance shown in FIG. It is a schematic sectional drawing of the Laval nozzle installed in the front-end | tip of the upper blowing lance shown in FIG.

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 Outer pipe 21 Middle pipe 22 Inner pipe

Claims (5)

When decarburizing and refining by blowing an oxygen-containing gas toward molten steel in an atmosphere at a pressure lower than atmospheric pressure using an upper blowing lance having a Laval nozzle installed at the tip, the pressure of the atmosphere is 40 kPa or less. The throat diameter (Dt) and outlet diameter (De) of the Laval nozzle, the oxygen-containing gas supply rate (Fh) per Laval nozzle hole, the nozzle back pressure (Po), and the design atmospheric pressure (Pe) Using a Laval nozzle having a shape that satisfies the following equations (1) and (2), the parameter P calculated from the following equations (3), (4), and (5) is 0.5-2. A method for vacuum decarburization refining of molten steel, wherein at least one of a lance height of an upper blowing lance and a supply speed of an oxygen-containing gas is adjusted so as to be zero.

However, in the formulas (1) to (5), each symbol represents the following.
Fh: Supply rate of oxygen-containing gas per hole of Laval nozzle (Nm ³ / hr)
Dt: Laval nozzle throat diameter (mm)
De: Laval nozzle outlet diameter (mm)
Po: Nozzle back pressure (kPa)
Pe: Design atmosphere pressure (kPa)
Vo: Oxygen jet flow velocity at the outlet of the Laval nozzle (m / sec)
V: Flow velocity of oxygen jet on molten steel surface (m / sec)
Pea: Atmospheric pressure (kPa)
Lh: Lance height of top blow lance (mm)
P: Parameter (-)   The method for vacuum decarburization refining of molten steel according to claim 1, wherein an atmospheric pressure (Pea) at the start of blowing of the oxygen-containing gas from the upper blowing lance is 13.3 kPa or more.   The design atmosphere pressure (Pe) is set to a range that is not less than 13.3 kPa and does not exceed the maximum pressure of the atmosphere when the oxygen-containing gas is blown from the top blowing lance. The method for vacuum decarburization refining of molten steel as described.   The vacuum of molten steel according to any one of claims 1 to 3, wherein an atmospheric pressure (Pea) at the end of the blowing of the oxygen-containing gas from the upper blowing lance is set to 6.7 kPa or more. Decarburization refining method.   The molten steel according to any one of claims 1 to 4, wherein a carbon concentration in the molten steel at the start of blowing of the oxygen-containing gas from the upper blowing lance is 0.04 mass% or more. Vacuum decarburization refining method.

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