JP2020019984A - Refining method of molten steel under reduced pressure - Google Patents

Abstract

To refine a molten steel efficiently by controlling a residence time of a powder in flame in a case where the molten steel is refined by blowing the powder onto a molten steel bath surface together with a carrier gas through a top blowing lance that forms the flame below a tip under an atmosphere at a pressure lower than an atmospheric pressure.SOLUTION: The present invention provides a refining method of the molten steel under a reduced pressure in which a refining powder ejected together with a jet of carrier gas from the tip of the top blowing lance is heated by the flame formed at a tip of a top blowing lance 13 provided in a vacuum chamber 5 of a vacuum degassing apparatus 1, and a heated powder is blown on the molten steel bath surface in the vacuum chamber. The residence time of the powder in the flame is made 0.02 seconds or more.SELECTED DRAWING: Figure 1

Description

  The present invention provides a method for refining molten steel by blowing powder onto a molten steel bath together with a carrier gas through an upper blowing lance that forms a flame below the tip under an atmosphere at a pressure lower than the atmospheric pressure. The present invention relates to a method for refining molten steel.

  In recent years, the use of steel materials has been diversified, and they are often used under more severe environments than before. Along with this, the demands on the mechanical properties and the like of steel products have become stricter than ever. Under these circumstances, manganese-containing low-carbon steels that combine high strength and high workability have been developed for the purpose of increasing the strength, weight, and cost of structures. Widely used in various fields such as steel sheets. Here, the manganese-containing low carbon steel refers to a steel having a carbon concentration of 0.05% by mass or less and a manganese concentration of 0.2% by mass or more.

  By the way, in the steelmaking process, manganese ore, high-carbon ferromanganese and the like are used as inexpensive manganese sources used for adjusting the manganese concentration in the molten steel. When smelting the above manganese-containing low-carbon steel, when decarburizing and refining hot metal in a converter, manganese ore is introduced into the converter as a manganese source to reduce manganese ore, At the time of tapping, high-carbon ferromanganese is added to molten steel to increase the manganese concentration in the molten steel to a predetermined concentration while suppressing the cost of adjusting the manganese component (for example, Patent Document 1). See).

  However, when these inexpensive manganese sources are used, the carbon concentration in molten steel cannot be sufficiently reduced by decarburization refining in a converter in order to reduce manganese ore, or high Due to the carbon contained in the carbon ferromanganese, the carbon concentration in the molten steel after tapping from the converter increases. As a result, if there is a possibility that the carbon concentration in the molten steel may exceed the allowable range for manganese-containing low-carbon steel, it is necessary to perform a separate treatment (refining) to remove carbon from the molten steel after tapping from the converter. Becomes

  As a method for efficiently removing (decarburizing) the carbon in the molten steel after the steel is removed from the converter, a vacuum degassing device such as an RH vacuum degassing device is used, and the molten steel is exposed to an atmosphere under reduced pressure. Decarburization method using the reaction of dissolved oxygen (oxygen dissolved in molten steel) and carbon in molten steel contained in undeoxidized molten steel, and oxygen source such as oxygen gas to molten steel under reduced pressure Is known, and carbon in molten steel is oxidized with an oxygen source to decarbonize. These decarburization methods under reduced pressure are called "vacuum decarburization refining" as opposed to decarburization refining in a converter performed under atmospheric pressure.

  In order to remove carbon introduced by an inexpensive manganese source by vacuum decarburization refining, for example, in Patent Document 2, high-carbon ferromanganese is put into molten steel in an initial stage of vacuum decarburization refining in a vacuum degassing facility. Patent Document 3 discloses a method for melting ultra-low carbon steel in a vacuum degassing facility, in which a high-carbon ferromagnetic steel is used for a period up to 20% of the processing time of vacuum decarburization refining. A method of introducing manganese has been proposed.

  However, in vacuum decarburization refining of molten steel containing a large amount of manganese, oxygen reacts not only with carbon in the molten steel, but also with manganese in the molten steel, so oxidized loss of the added manganese occurs and the manganese yield decreases. I do. This also makes it difficult to accurately control the manganese content in the molten steel.

  In addition, regarding a suitable oxygen source and a decarburization reaction promoting method used in vacuum decarburization refining, for example, in Japanese Patent Application Laid-Open No. H10-209, solid oxygen such as a mill scale is put into a vacuum chamber to thereby oxidize manganese. Patent Literature 5 proposes a method of controlling the concentration of carbon in molten steel and the temperature of molten steel at the end of decarburization refining in a converter by vacuum degassing. A method has been proposed in which manganese ore is added to a device to vacuum decarburize molten steel.

  Further, in Patent Document 6, when molten steel is subjected to vacuum decarburization and refining with an RH vacuum degassing device, MnO powder or manganese ore powder is blown upward together with a carrier gas toward the molten steel surface in the vacuum chamber. A method of vacuum decarburization refining has been proposed. In Patent Document 7, manganese ore powder is blown into a molten steel in a vacuum tank of an RH vacuum degassing apparatus together with a carrier gas through a nozzle provided on a side wall of the vacuum tank. A vacuum decarburization refining method has been proposed in which molten steel is decarburized by oxygen in manganese ore and the manganese concentration in the molten steel is increased.

  On the other hand, with the increasing added value of steel materials and the expansion of uses, the demand for improved material properties is increasing. One of the means to meet this demand is to purify steel, specifically, Low sulfurization is taking place.

  When smelting low-sulfur steel, desulfurization is generally performed at the hot metal stage with a high desulfurization reaction efficiency. However, low-sulfur steel with a sulfur content of 0.0024 mass% or less or sulfur content of 0% or less is used. In ultra-low sulfur steel of 0.0010% by mass or less, it is difficult to sufficiently reduce the sulfur concentration to the target concentration only by the desulfurization treatment in the hot metal stage, and therefore, the sulfur content is 0.0024% by mass or less. In the case of low-sulfur steels and ultra-low-sulfur steels with a sulfur content of 0.0010% by mass or less, not only desulfurization treatment at the hot metal stage but also desulfurization treatment of molten steel after tapping from the converter is performed. .

  The method of performing desulfurization treatment on the molten steel after tapping from the converter is, for example, a method of injecting a desulfurizing agent into the molten steel in the ladle, adding the desulfurizing agent to the molten steel in the ladle, and then adding the desulfurizing agent to the molten steel. Conventionally, various methods have been proposed, such as a method of stirring a mixture. However, in these methods, a new process (desulfurization process) is added during the period from the output from the converter to the treatment in the vacuum degassing equipment, which lowers the molten steel temperature, increases the production cost, and further increases the production cost. This leads to lower productivity. In order to solve these problems, attempts have been made to consolidate and simplify the secondary refining process by providing a desulfurization function to the vacuum degassing equipment.

  For example, Patent Document 8 discloses a method of desulfurizing molten steel by spraying a CaO-based desulfurizing agent together with a carrier gas from a top blowing lance onto a molten steel bath surface in a vacuum chamber as a desulfurization method using vacuum degassing equipment. Proposed.

  However, during refining in a vacuum degassing facility, when manganese ore for smelting manganese-containing low-carbon steel or oxide powder such as CaO-based desulfurizing agent for desulfurization treatment is blown from the upper blowing lance, The molten steel temperature is lowered by the sensible heat, latent heat and decomposition heat required for the thermal decomposition of the oxide powder to be added by spraying. In addition, spraying powder along with a carrier gas onto molten steel via an upper blowing lance is also referred to as “projection”.

  As a method of compensating for the decrease in the molten steel temperature, a method of increasing the molten steel temperature in the previous process of the vacuum degassing equipment, or adding metal aluminum to the molten steel during refining in the vacuum degassing equipment, and setting a heat of combustion of aluminum. For example, a method of raising the temperature of molten steel has been carried out. However, the method of raising the temperature of molten steel in the pre-process of the vacuum degassing facility causes large wear of refractories in the pre-process, resulting in a cost increase. In addition, the method of adding metal aluminum in a vacuum degassing facility to raise the temperature has a disadvantage such as a decrease in cleanliness of molten steel or an increase in auxiliary material costs due to the generated aluminum oxide. .

  Therefore, a method of projecting an oxide powder while suppressing a decrease in molten steel temperature has been proposed. For example, Patent Document 9 proposes a method of projecting an oxide powder such as manganese ore onto a molten steel bath surface while heating it with a flame of a burner provided at the tip of an upper blowing lance. Further, Patent Document 10 discloses that when a CaO-based desulfurizing agent is projected from an upper blowing lance to desulfurize molten steel, a fuel gas and an oxygen gas are ejected from the upper blowing lance and a flame of a burner is provided below a tip of the upper blowing lance. A method has been proposed in which a CaO-based desulfurizing agent is heated and melted by the flame to reach a molten steel bath surface.

JP-A-4-88114 JP-A-2-47215 JP-A-1-301815 JP-A-58-73715 JP-A-63-293109 JP-A-5-239526 JP-A-1-92312 JP-A-5-31231 International Publication No. 2013/137292 JP 2012-172213 A

  Heating powder such as manganese ore and CaO-based desulfurizing agent into the molten steel in a burner flame formed at the tip of the upper blowing lance, thereby increasing the reaction speed and simultaneously increasing the molten steel temperature In the refining method, the residence time of the projected powder in the flame affects not only the yield of the manganese ore and the desulfurization efficiency of the CaO-based desulfurizing agent, but also the heating efficiency performed through the powder. I do. That is, when the residence time of the powder projected through the upper blowing lance in the flame is not controlled, the effect of heating and projecting the powder with the flame of the burner cannot be sufficiently obtained.

  However, the optimal powder projection conditions for heating powder such as manganese ore and CaO-based desulfurizing agent in a flame to reach the molten steel and raise the temperature of the molten steel, including Patent Documents 9 and 10, are still unclear. Has not been.

  The present invention has been made in view of the above circumstances, and an object thereof is to form a powder together with a carrier gas through an upper blowing lance which forms a flame below a tip under an atmosphere having a pressure lower than the atmospheric pressure. In refining molten steel by spraying it onto the molten steel bath surface, by controlling the residence time of the powder projected through the top blowing lance in the flame, the molten steel can be efficiently refined under reduced pressure. To provide a method for refining molten steel.

The gist of the present invention for solving the above problems is as follows.
[1] A flame formed at the tip of an upper blowing lance installed in a vacuum tank of a vacuum degassing facility, which heats a powder for refining, which is jetted from the tip of the upper blowing lance together with a jet of carrier gas. A method for refining molten steel under reduced pressure, wherein the heated powder is sprayed on a molten steel bath surface in a vacuum chamber,
A method for refining molten steel under reduced pressure, wherein a residence time of the powder in the flame is 0.02 seconds or more.
[2] The method for refining molten steel under reduced pressure according to the above [1], wherein the residence time is calculated from the following equation (1) using equation (5).

Here, t is the powder residence time in the flame (s), u _p is the rate of the powder after leaving the top-blown lance (m / s), a _p is the powder acceleration ( m ^{/ s} 2), x is the distance from the top lance tip to the molten steel bath surface (m), _{F D} occurs in the flow direction of the powder drag (N), _{u g} exited from the top lance The gas velocity (m / s) of the immediately following jet flow, g is the gravitational acceleration (m / s ² ), ρ _p is the density of the powder (kg / m ³ ), and ρ _g is from the top blowing lance. The gas density (kg / m ³ ) of the jet flow immediately after, μ _g is the gas viscosity (Pa · s) of the jet flow immediately after leaving the top blowing lance, d _p is the diameter (m) of the powder, C _d is the drag coefficient (-) of the powder, and Re is the Reynolds number (-).
[3] As the upper blowing lance, a fuel injection nozzle having a powder injection nozzle for injecting the powder with the carrier gas and a fuel injection nozzle opening formed on the outer peripheral side of the powder injection nozzle is used to inject fuel. One or more fuel injection nozzles for injecting an oxidizing gas for fuel combustion having an oxidizing gas injection nozzle opening formed on the outer peripheral side of the powder injection nozzle. The method for refining molten steel under reduced pressure according to the above [1] or [2], wherein an upper blowing lance having an oxidizing gas injection nozzle is used.
[4] The residence time of the molten steel in the vacuum tank of the RH vacuum degassing device, calculated using the following formula (6), using an RH vacuum degassing device as the vacuum degassing device, is 5 seconds or more. The method for refining molten steel under reduced pressure according to any one of the above [1] to [3], wherein the molten steel in the ladle is recirculated to a vacuum tank.

Here, τ is the residence time (s) of the molten steel in the vacuum chamber, ρ _M is the density of the molten steel (kg / m ³ ), and _Sv is the vacuum in the horizontal section at a height of 100 mm from the floor of the vacuum chamber. The area (m ² ) of the empty part in the tank, g is the gravitational acceleration (m / s ² ), Q is the annular flow rate of the molten steel (ton / min), and Sd is the empty part in the horizontal cross section of the lower end of the dip pipe. (M ² ) and H _m are the height (m) of the molten steel in a vacuum bath with the bath surface stationary.
[5] The powder according to any one of [1] to [4], wherein the powder is one or more of manganese ore, manganese ferroalloy, and CaO-based desulfurizing agent. A method for refining molten steel under reduced pressure as described in (1).
[6] The molten steel under reduced pressure according to any of [1] to [5], wherein the pressure of the atmosphere in the vacuum chamber is 2.7 kPa or more and 13.3 kPa or less. Refining method.

  According to the present invention, under reduced pressure atmosphere, the powder for refining is heated by a flame formed at the tip of the upper blowing lance, and when the heated powder is projected on molten steel, Since the residence time is 0.02 seconds or more, the powder to be projected can be efficiently heated and the molten steel can be efficiently refined. As a result, the refining reaction is promoted, and a high heating efficiency is obtained, contributing to a reduction in the refining cost.

FIG. 1 is a schematic vertical sectional view of an example of an RH vacuum degassing apparatus used when carrying out the present invention. It is an outline longitudinal section of the tip part of an example of the upper blowing lance used.

  Hereinafter, the present invention will be described specifically. Vacuum degassing equipment that can use the method of refining molten steel under reduced pressure according to the present invention includes RH vacuum degassing equipment, DH vacuum degassing equipment, VAD furnace, VOD furnace, and the like. The most typical one is an RH vacuum degasser. Therefore, the present invention will be described by way of an example in which the present invention is implemented using an RH vacuum degassing apparatus.

  FIG. 1 shows a schematic longitudinal sectional view of an example of an RH vacuum degassing apparatus used when carrying out the present invention. In FIG. 1, reference numeral 1 denotes an RH vacuum degassing apparatus, 2 denotes a ladle, 3 denotes molten steel, 4 denotes slag, 5 denotes a vacuum tank, 6 denotes an upper tank, 7 denotes a lower tank, 8 denotes an ascending-side immersion pipe, and 9 denotes a immersion pipe. Descending side immersion pipe, 10 is a reflux gas blowing pipe, 11 is a duct, 12 is a raw material inlet, 13 is an upper blowing lance, and the vacuum tank 5 is composed of an upper tank 6 and a lower tank 7, and The upper blowing lance 13 is installed above the vacuum chamber 5 and can move up and down inside the vacuum chamber 5.

  In the RH vacuum degassing apparatus 1, the ladle 2 containing the molten steel 3 is raised by an elevating device (not shown), and the ascending immersion pipe 8 and the descending immersion pipe 9 are immersed in the molten steel 3 in the ladle. . Then, while circulating gas is blown into the rising side immersion pipe 8 from the circulating gas blowing pipe 10, the inside of the vacuum tank 5 is evacuated by an exhaust device (not shown) connected to the duct 11, and the vacuum tank is evacuated. The inside of 5 is depressurized. When the inside of the vacuum chamber 5 is decompressed, the molten steel 3 in the ladle rises along with the reflux gas and rises the rising-side immersion pipe 8 by the gas lift effect of the reflux gas blown from the reflux gas blow-in pipe 10. The gas flows into the tank 5 and then returns to the ladle 2 via the descending immersion pipe 9 to form a so-called reflux, and is subjected to RH vacuum degassing refining.

The upper blowing lance 13 is provided with a powder flow path for supplying one or more kinds of powders such as manganese ore, manganese-based iron alloy, and CaO-based desulfurizing agent together with a carrier gas as powder for refining, , An oxidizing gas passage for supplying an oxidizing gas (oxygen-containing gas) for burning the hydrocarbon gas as a fuel, and cooling the upper blowing lance 13. Pipe structure having a cooling water supply channel and a drain channel independently for each other. The manganese-based ferroalloys include high-carbon ferromanganese and medium-carbon ferromanganese, and the CaO-based desulfurizing agents include quicklime (CaO) alone, and quicklime containing fluorite (CaF ₂ ) or alumina (Al ₂ O _3). ) In a range of 30% by mass or less (including pre-melt).

  FIG. 2 shows a schematic longitudinal sectional view of a tip portion of an example of the upper blowing lance 13 to be used. As shown in FIG. 2, the upper blowing lance 13 is provided with a powder injection nozzle 21 for injecting the powder for refining together with the ejection flow of the carrier gas at a substantially central position of the tip portion. Further, one or more fuel injection nozzles 23 for injecting fuel having a fuel injection nozzle opening 24 on the outer peripheral side of the powder injection nozzle 21 and the outer peripheral side of the powder injection nozzle 21 are oxidized. One or more oxidizing gas injection nozzles 25 having an oxidizing gas injection nozzle opening 26 for injecting an oxidizing gas for fuel combustion. Note that a pilot burner, a spark plug, or the like for igniting the fuel may be provided at the tip of the upper blowing lance 13 to facilitate the ignition of the fuel.

  The powder injection nozzle 21 has a Laval structure in which the cross-sectional area gradually widens toward the powder injection nozzle opening 22, and the powder for refining is applied to the molten steel 3 circulating inside the vacuum chamber 5. Can be injected together with the carrier gas. Further, it is also possible to inject only gas (for example, oxygen gas) without injecting powder. However, the powder injection nozzle 21 is not limited to the Laval structure, and may be a straight shape.

  The fuel injection nozzle 23 is configured to inject, for example, a hydrocarbon gas as fuel for forming a flame. The oxidizing gas injection nozzle 25 is configured to inject an oxidizing gas for burning the fuel by mixing with the fuel injected from the fuel injection nozzle 23. Oxygen gas is generally used as the oxidizing gas, but an oxygen-containing gas having an oxygen gas content equal to or higher than air can also be used as the oxidizing gas. Here, an example in which a hydrocarbon-based gas is used as the fuel will be described, but a liquid fuel such as light oil or heavy oil can also be used as the fuel.

  A cooling water supply passage 27 and a cooling water drainage passage 28 for cooling the upper blowing lance 13 are provided on the outer peripheral side of the upper blowing lance 13. The cooling water supply channel 27 and the cooling water drainage channel 28 have an annular shape when viewed from the lower side in the vertical direction, and the cooling water is inverted and circulated at the tip of the upper blowing lance 13.

  The upper blowing lance 13 used is configured as described above, and the fuel (hydrocarbon-based gas) injected through the fuel injection nozzle 23 is oxidized by the oxidizing gas injection nozzle 25. Burning is performed by gas (oxygen gas, oxygen-enriched air, air, etc.), and a burner flame is formed below the tip of the upper blowing lance 13.

  The upper blowing lance 13 is connected to a hopper (not shown) storing powders of manganese ore, manganese ferroalloy, CaO desulfurizing agent, etc., and these powders are blown up together with the carrier gas. The powder is supplied to the lance 13 and injected from the powder injection nozzle 21 at the tip of the upper blowing lance 13. An inert gas such as an argon gas or a nitrogen gas is usually used as a powder transfer gas. However, when performing vacuum decarburization refining of the molten steel 3 as in the case of melting manganese-containing low carbon steel. In the above, an oxidizing gas can be used as a carrier gas. Since the fuel injection nozzle 23 and the oxidizing gas injection nozzle 25 are arranged on the outer periphery of the powder injection nozzle 21, the burner flame formed below the tip of the upper blowing lance 13 with respect to the jet flow from the powder injection nozzle 21. The influence is reduced, and the scattering of the powder injected together with the jet flow is suppressed.

  The upper blowing lance 13 is connected to a fuel supply pipe (not shown) and an oxidizing gas supply pipe (not shown). From the fuel supply pipe, carbonized fuel such as propane gas or natural gas is supplied as fuel. A hydrogen-based gas is supplied to the upper blowing lance 13, and an oxidizing gas for burning the hydrocarbon-based gas is supplied to the upper blowing lance 13 from the oxidizing gas supply pipe. Is injected from a fuel injection nozzle 23 provided at the tip of the upper blowing lance 13, and the oxidizing gas is injected from an oxidizing gas injection nozzle 25 provided at the tip of the upper blowing lance 13. .

  Using the RH vacuum degassing apparatus 1 configured as described above, a flame is formed by the combustion of a fuel (hydrocarbon-based gas) below the tip of the upper blowing lance 13, and the powder projected from the upper blowing lance 13 by this flame Is projected onto the bath surface of the molten steel 3 which recirculates the vacuum tank 5 while heating. At that time, control is performed so that the residence time of the powder in the flame is 0.02 seconds or more. The lance height of the upper blowing lance 13 during powder projection (the distance from the molten steel surface to the tip of the lance when the bath surface in the vacuum chamber is stationary) is preferably 1.0 to 7.0 m. If the lance height is less than 1.0 m, the fuel injection nozzle 23 and the oxidizing gas injection nozzle 25 may be blocked by the splash of the molten steel 3, while if the lance height exceeds 7.0 m, the suction into the duct 11. The amount of powder discharged from the vacuum chamber 5 together with the exhaust gas to be discharged increases, and the yield of powder addition decreases.

  The residence time of the powder in the flame is determined by the length of the flame and the speed of the powder as it passes through the flame. However, in order to accurately obtain the residence time of the powder in the flame, it is preferable to use the following equations (1) to (5).

Here, t is the powder residence time in the flame (s), _{u p} is the powder rate of (m / s), _{a p} is the powder of the acceleration (m ^{/ s} 2), x is , the distance from the top lance tip to the molten steel bath surface (m), F _D occurs in the flow direction of the powder drag (N), u _g is the gas velocity of the jet stream immediately after exiting from the top lance ( m / s), g is the gravitational acceleration (m / s ² ), ρ _p is the density of the powder (kg / m ³ ), and ρ _g is the gas density of the jet immediately after leaving the top blowing lance ( kg / m ³ ), μ _g are the gas viscosity (Pa · s) of the jet flow immediately after leaving the upper blowing lance, d _p is the diameter of the powder (m), and C _d is the drag coefficient of the powder. (−) And Re are the number of Reynolds nozzles (−).

  If the pressure of the atmosphere inside the vacuum chamber 5 is excessively reduced, the amount of powder discharged from the vacuum chamber 5 together with the exhaust gas sucked into the duct 11 increases. It is preferable that the pressure of the atmosphere inside the vacuum chamber 5 be set to 2.7 kPa or more and 13.3 kPa or less.

  When the RH vacuum degassing apparatus 1 is used as the vacuum degassing equipment, the RH vacuum is calculated using the following equation (6) in order to heat the powder to the molten steel 3 with high efficiency. It is preferable that the molten steel 3 stored in the ladle 2 be returned to the vacuum tank 5 so that the residence time of the molten steel 3 in the vacuum tank 5 of the degassing device 1 is 5 seconds or more. However, if the staying time is too long, the processing time of the vacuum degassing refining becomes long, and there is a possibility that the temperature of the molten steel 3 may decrease. Therefore, the staying time is desirably 2 minutes or less.

Here, τ is the residence time (s) of the molten steel in the vacuum chamber, ρ _M is the density of the molten steel (kg / m ³ ), and _Sv is the vacuum in the horizontal section at a height of 100 mm from the floor of the vacuum chamber. The area (m ² ) of the empty part in the tank, g is the gravitational acceleration (m / s ² ), Q is the annular flow rate of the molten steel (ton / min), and Sd is the empty part in the horizontal cross section of the lower end of the dip pipe. (M ² ) and H _m are the height (m) of the molten steel in a vacuum bath with the bath surface stationary.

  When the RH vacuum degassing apparatus 1 is used as the vacuum degassing equipment, the distance x (m) from the tip of the upper blowing lance to the molten steel bath surface in the equation (1) is calculated from the following equation (9) as ( It can be obtained by equation (11).

Here, H _L is the distance from the insole of the vacuum chamber to the lance tip (m), H _m is the molten steel height in the vacuum chamber (m), h is the surface of molten steel vacuum chamber from a ladle of molten steel in the surface distance to (m), h _a is the distance from the lower end surface of the dip tube to the insole of the vacuum chamber (m), h _d is immersion depth of the molten steel dip tube (m), P ₁ is larger Atmospheric pressure (Pa), P ₂ is the pressure (Pa) of the atmosphere in the vacuum chamber, ρ _S is the density of the slag (kg / m ³ ), g is the gravitational acceleration (m / s ² ), and h _s is the slag the thickness of the (m), the [rho _M, a molten steel density (kg / ^{m 3).} The “lay” of the vacuum chamber is the surface position of the refractory constituting the bottom of the vacuum chamber 5.

  Hereinafter, the present invention is applied to smelting of manganese-containing low-carbon steel, low-sulfur steel (sulfur content: 0.0024% by mass or less) and extremely low-sulfur steel (sulfur content: 0.0010% by mass or less). An example will be described. First, a method for melting manganese-containing low carbon steel will be described.

  Hot metal from a blast furnace is received in a holding vessel or a transport vessel such as a hot metal pot or torpedo car, and the received hot metal is transported to a converter where decarburization and refining is performed. Usually, in the middle of this transportation, the hot metal is subjected to hot metal pretreatment such as desulfurization treatment and dephosphorization treatment, and in the present invention, hot metal pretreatment is necessary from the viewpoint of the component standard of the manganese-containing low carbon steel. If not, it is preferable to perform a hot metal pretreatment, particularly a dephosphorization treatment.

  This is because manganese ore is added as an inexpensive manganese source during decarburization refining in converters when smelting manganese-containing low carbon steel, and when dephosphorization is not performed, During the decarburization refining, it is necessary to promote the dephosphorization reaction simultaneously with the decarburization reaction, and for that purpose, it is necessary to add a large amount of a CaO-based solvent into the converter. As a result, the amount of slag increases, the amount of manganese distributed to the slag increases, and the yield of manganese decreases.

  The transported hot metal is charged into the converter, and then manganese ore is added as a manganese source into the converter, and if necessary, a small amount of CaO-based solvent such as quicklime is added, and oxygen gas is blown upward. And / or decarburization and refining by bottom blowing to obtain molten steel having a predetermined component composition. Thereafter, the molten steel is discharged to the ladle 2 without adding a deoxidizing agent such as metallic aluminum or ferrosilicon to the molten steel, that is, while keeping the molten steel in an undeoxidized state. However, at that time, a predetermined amount of inexpensive manganese-based ferroalloys such as high-carbon ferromanganese may be added.

  In addition, in the decarburization refining in the converter, as described above, an inexpensive manganese source such as manganese ore or high-carbon ferromanganese is used, so the carbon concentration in the molten steel is inevitably high. After the manganese concentration is adjusted, the carbon concentration in the molten steel is preferably suppressed to 0.2% by mass or less. When the carbon concentration in the molten steel exceeds 0.2% by mass, the vacuum decarburization refining time in the RH vacuum degassing apparatus 1 in the next step becomes longer, which not only reduces the productivity but also extends the vacuum decarburization refining time. It is necessary to increase the molten steel temperature during tapping in order to compensate for the accompanying decrease in the molten steel temperature, which leads to a decrease in iron yield and an increase in refractory wear due to an increase in refractory wear, which is not preferable. .

  The molten steel 3 discharged from the converter is conveyed to the RH vacuum degassing device 1. In the RH vacuum degassing apparatus 1, the molten steel 3 in an undeoxidized state is circulated between the ladle 2 and the vacuum tank 5. Since the molten steel 3 is in a non-deoxidized state, when the molten steel 3 is exposed to an atmosphere under reduced pressure in a vacuum chamber, carbon in the molten steel reacts with dissolved oxygen in the molten steel (C + O = CO) to remove the vacuum. Charcoal refining proceeds. When the recirculation of molten steel 3 is started, manganese ore is projected from upper lance 13 using argon gas as a carrier gas. Before and after the projection of the manganese ore, a hydrocarbon-based gas and an oxidizing gas are injected from the upper blowing lance 13 to form a flame below the tip of the upper blowing lance 13. The manganese ore is heated by the heat of the flame and projected onto the molten steel bath surface.

  The manganese ore projected on the molten steel bath surface is reduced by the carbon in the molten steel, increasing the manganese concentration in the molten steel and decreasing the carbon concentration in the molten steel. That is, the manganese ore functions not only as a manganese source for adjusting the molten steel component, but also as an oxygen source for the decarburization reaction of the molten steel 3.

  When a flame is formed below the tip of the upper blowing lance 13 and the manganese ore is projected from the upper blowing lance 13, the lance height of the upper blowing lance 13 (from the molten steel surface to the lance tip when the bath surface is stationary). Distance) is preferably 1.0 to 7.0 m, and the residence time in the flame calculated from the expression (1) by the expression (5) and the expression (9) by the expression (11) is 0.02 seconds. As described above, a flame is formed at the tip of the upper blowing lance 13 and manganese ore is projected. By projecting in this manner, the manganese ore can be efficiently heated and added to the molten steel 3 without scattering.

  As a result, the temperature drop of the molten steel 3 due to the addition of manganese ore can be suppressed, and since manganese ore is efficiently added to molten steel 3, reduction of manganese ore, which is an inexpensive manganese source, is promoted. The manganese yield is improved, and the manufacturing cost of manganese-containing low carbon steel is reduced.

  If the manganese concentration in the molten steel does not satisfy the standard only by adding manganese ore according to the manganese concentration specification of the manganese-containing low carbon steel, before adding the manganese ore, high carbon ferromanganese (carbon content; about 7% by mass) may be projected while being heated by a flame via the upper blowing lance 13. Alternatively, a powder obtained by mixing high-carbon ferromanganese and manganese ore may be projected while being heated by a flame via the upper blowing lance 13. The particle size of the powder is preferably 3 mm or less from the viewpoint of increasing the reaction efficiency.

  After performing vacuum decarburization refining for a predetermined time, when the carbon concentration in the molten steel reaches the range of the component standard value, a strong deoxidizing agent such as metallic aluminum is added to the molten steel 3 from the raw material inlet 12 to dissolve in the molten steel. The oxygen concentration is reduced (deoxidation treatment), and the vacuum decarburization refining is completed. If the temperature of the molten steel after the completion of the vacuum decarburization refining is lower than the temperature required from the next step, for example, the continuous casting step, metallic aluminum is further added to the molten steel 3 from the raw material charging port 12 and the top is blown. Oxygen gas may be blown from the lance 13 to the molten steel bath surface to burn aluminum in the molten steel, thereby increasing the temperature of the molten steel.

  The molten steel 3 deoxidized by the addition of the strong deoxidizer then continues refluxing for several more minutes. When the manganese concentration of the molten steel 3 is less than the standard value, metal manganese or low carbon ferromanganese is charged into the molten steel 3 from the raw material inlet 12 into the reflux to adjust the manganese concentration of the molten steel 3. Further, during this reflux, if necessary, a component adjuster such as aluminum, silicon, nickel, chromium, copper, niobium, or titanium is charged into the molten steel 3 from the raw material inlet 12 to bring the molten steel component into a predetermined composition range. After the adjustment, the inside of the vacuum chamber 5 is returned to the atmospheric pressure, and the vacuum degassing and refining is completed.

  Next, a method for melting low-sulfur steel and ultra-low-sulfur steel will be described.

  Hot metal from a blast furnace is received in a holding vessel or a transport vessel such as a hot metal pot or torpedo car, and the received hot metal is transported to a converter where decarburization and refining is performed. In the course of this transportation, the hot metal is subjected to a desulfurization treatment of the hot metal pretreatment. The dephosphorization treatment of the hot metal pretreatment is performed when it is necessary to perform the phosphorus concentration specification of the low sulfur steel and the ultra low sulfur steel to be melted, but other than that, it is not necessary to carry out.

  Charge the transferred hot metal into the converter, and then, if necessary, add manganese ore as a manganese source into the converter, and further add a small amount of CaO-based solvent such as quicklime as needed, Oxygen gas is blown up and / or blown down and decarburized and refined to obtain molten steel having a predetermined composition. Thereafter, without adding a deoxidizing agent such as metallic aluminum or ferrosilicon to the molten steel, that is, the molten steel is discharged to the ladle 2 in a non-deoxidized state. However, at that time, a predetermined amount of inexpensive manganese-based ferroalloys such as high-carbon ferromanganese may be added.

  The molten steel 3 discharged from the converter is conveyed to the RH vacuum degassing device 1. The molten steel 3 which has been transported to the RH vacuum degassing apparatus 1 is subjected to vacuum decarburization refining by blowing oxygen gas from the top blowing lance 13 to the molten steel 3 as necessary, and Adjust the carbon concentration of. When the carbon concentration in the molten steel reaches the component standard, a strong deoxidizing agent such as metallic aluminum is added to the molten steel 3 from the raw material inlet 12 to perform a deoxidation treatment to reduce the dissolved oxygen concentration in the molten steel. End vacuum decarburization refining.

  However, if the carbon concentration standard of the low-sulfur steel and the ultra-low-sulfur steel to be smelted is at a level that can be smelted without vacuum decarburization refining, vacuum decarburization refining is not performed. When vacuum decarburization refining is not performed, the molten steel 3 does not need to be in a non-deoxidized state. When the molten steel 3 is discharged from the converter to the ladle 2, the molten steel flow during the tapping is The molten steel may be deoxidized by adding aluminum. At that time, a medium solvent containing quicklime or CaO may be added to the tapping flow in addition to metallic aluminum. After tapping the molten steel 3 into the ladle 2, a slag modifier such as metallic aluminum is added to the slag on the molten steel to reduce iron oxides such as FeO and manganese oxides such as MnO in the slag, It is preferable to convey to the RH vacuum degassing device 1.

  When the temperature of the molten steel after the completion of the vacuum decarburization refining is lower than the temperature required from the next step, for example, a continuous casting step, metallic aluminum is further added to the molten steel 3 from the raw material inlet 12 and Oxygen gas may be blown from the blowing lance 13 onto the molten steel bath surface to burn aluminum in the molten steel to increase the temperature of the molten steel. When the undeoxidized molten steel 3 is subjected to vacuum decarburization refining, the manganese ore is projected from the upper blowing lance 13 while heating the manganese ore with a flame in the same manner as in the above-described manganese-containing low carbon steel smelting method. Is also good.

  Thereafter, a CaO-based desulfurizing agent is injected from the upper blowing lance 13 into the molten steel 3 which has been deoxidized with a strong deoxidizing agent such as metallic aluminum, and the CaO-based desulfurizing agent is heated by a flame formed at the tip of the upper blowing lance 13. Then, it is projected on a molten steel bath surface to perform a desulfurization treatment.

  When a flame is formed below the tip of the upper blowing lance 13 and the CaO-based desulfurizing agent is projected from the upper blowing lance 13, the lance height of the upper blowing lance 13 (from the molten steel surface when the bath surface is stationary, the lance tip Distance) is preferably 1.0 to 7.0 m, and the flame residence time calculated from the equations (1) to (5) and (9) to (11) is 0.1. A flame is formed at the tip of the upper blowing lance 13 so as to be 02 seconds or longer, and a CaO-based desulfurizing agent is projected. By projecting in this manner, the CaO-based desulfurizing agent can be efficiently heated and added to the molten steel 3 without scattering.

  As a result, the temperature drop of the molten steel 3 due to the addition of the CaO-based desulfurizing agent can be suppressed, and the heated CaO-based desulfurizing agent is efficiently added to the molten steel 3, so that the desulfurization reaction is promoted and high. Desulfurization rate can be obtained.

As the CaO-based desulfurizing agent to be added, quicklime (CaO) alone, a mixture (including premelt) obtained by adding and mixing fluorite (CaF ₂ ) and alumina (Al ₂ O ₃ ) to quicklime in a range of 30% by mass or less. Etc. can be used. The particle size of the CaO-based desulfurizing agent is preferably 3 mm or less from the viewpoint of increasing the reaction efficiency.

  When the sulfur concentration of the molten steel 3 has decreased to a predetermined value or less, the projection of the CaO-based desulfurizing agent from the upper blowing lance 13 is stopped, and the desulfurization process is terminated. Thereafter, the molten steel 3 is circulated for several minutes, and a component adjuster such as aluminum, silicon, nickel, chromium, copper, niobium, and titanium is supplied from the raw material inlet 12 to the molten steel 3 as needed. The molten steel component is charged to adjust the composition to a predetermined composition range. Thereafter, the inside of the vacuum chamber 5 is returned to the atmospheric pressure, and the vacuum degassing refining is completed.

  As described above, according to the present invention, the powder for refining is heated with a flame formed at the tip of the upper blowing lance under a reduced pressure atmosphere, and when the heated powder is projected on the molten steel, Since the residence time of the body in the flame is 0.02 seconds or more, the powder to be projected can be efficiently heated, and the molten steel can be efficiently refined with the powder to be projected. As a result, the refining reaction is promoted, and high heat transfer efficiency is obtained, so that the refining cost can be reduced.

  Although the above description has been made with reference to the example using the RH vacuum degassing apparatus, even when using other vacuum degassing equipment such as a DH vacuum degassing apparatus or a VOD furnace, the manganese-containing gas is removed by following the above method. Low carbon steel, low sulfur steel, extremely low sulfur steel and the like can be melted.

[Example 1]
This is an example in which the present invention is implemented in an RH vacuum degassing apparatus. 300 tons of undeoxidized molten steel in the ladle, which has been discharged from the converter, is vacuum decarburized and refined using the RH vacuum degassing device shown in FIG. 1 to produce manganese-containing low carbon steel. The test was performed. The molten steel component at the time of tapping from the converter had a carbon concentration of 0.03 to 0.04 mass% and a manganese concentration of 0.07 to 0.08 mass%. The concentration of dissolved oxygen in the molten steel upon arrival at the RH vacuum degassing device was 0.04 to 0.07% by mass.

  The height of the lance of the upper blowing lance inserted from the upper part of the vacuum tank of the RH vacuum degassing device is fixed at 5 to 7 m, and during the vacuum decarburization refining, LNG as fuel and oxidation for fuel combustion are supplied from the upper blowing lance. Oxygen gas was supplied as a gas to generate a burner flame at the tip of the upper blowing lance. At the same time, 5 kg / molten steel-ton (addition amount: 1.5 ton) was injected with manganese ore at a rate of 200 kg / min using argon gas as a carrier gas, and the heat transfer rate to the molten steel and the manganese yield were evaluated. . The residence time of the powder in the flame was calculated using the equations (1) to (5) and the equations (9) to (11).

The flow rate of LNG from the upper blowing lance was 500 Nm ³ / h, the flow rate of oxygen gas for fuel combustion was 1100 Nm ³ / h, and the flow rate of argon gas for carrier gas was 250 to 500 Nm ³ / h. In all tests, the pressure of the atmosphere in the vacuum chamber during powder projection was 6.7 kPa, and the flow rate of the argon gas for circulation was 3000 NL / min. Here, “N” in the unit “Nm ³ ” and the unit “NL” means the volume when the gas is in a standard state at 0 ° C. and 1 atm.

Table 1 shows the test results. The heat transfer rates shown in Table 1 were calculated using the following equation (12).
Heat transfer rate (%) = [heat input to molten steel (cal) / total heat of burner combustion (cal)] x 100 ... (12)
Here, the amount of heat input to the molten steel is the amount of heat (cal) that has reached the molten steel out of the total amount of heat generated by the burner combustion, and the total amount of heat (cal) of the burner combustion is the amount of heat generated by the fuel (cal / Nm ^3). ) And the flow rate (Nm ³ ) of the fuel used during the treatment.

  Table 1 shows the following.

  As the residence time of the manganese ore in the flame increased, the heat rate increased. This is considered to be because the temperature of the manganese ore as the heat transfer medium increased with the increase in the residence time in the flame. In addition, the manganese yield increased with the increase of the manganese ore residence time in the flame. This is considered to be because the temperature of the manganese ore was increased with the increase in the residence time in the flame, and the melting of the manganese ore was promoted. As a result, the reduction rate of the manganese ore was increased.

[Example 2]
This is an example in which the present invention is implemented in an RH vacuum degassing apparatus. Desulfurization treatment was performed on 300 tons of deoxidized molten steel in a ladle using a RH vacuum degassing apparatus shown in FIG. 1. The molten steel composition before the RH vacuum degassing refining has a carbon concentration of 0.08 to 0.10 mass%, a silicon concentration of 0.1 to 0.2 mass%, and an aluminum concentration of 0.020 to 0.035 mass. %, The sulfur concentration was 0.0032 to 0.0033% by mass, and the molten steel temperature was 1600 to 1650 ° C.

  Before the desulfurization treatment, the temperature of the molten steel in the ladle was measured as necessary, and it was confirmed whether the required molten steel temperature was secured before the desulfurizing agent was added. The temperature required at this time is a temperature determined for each processing apparatus and processing condition in consideration of a temperature reduction due to the lapse of the scheduled processing time and a temperature reduction due to the addition of the desulfurizing agent. When the temperature was insufficient, metallic aluminum was added to the molten steel from the raw material charging port, oxygen gas was blown from the upper blowing lance, and the temperature of the molten steel was increased by burning the metallic aluminum.

Then, metal aluminum for the purpose of deoxidation and component adjustment is added to the molten steel from the material input port, and after the addition, the lance height of the upper blowing lance inserted from the upper part of the vacuum chamber is fixed at 5 to 7 m, and the upper blowing is performed. LNG as fuel and oxygen gas as oxidizing gas for fuel combustion were supplied from the lance to generate a burner flame at the tip of the upper blowing lance. At the same time, as a carrier gas, argon gas, a pre-melt the desulfurizing agent in _CaO-Al 2 _{O 3} at an addition rate of 200 kg / min, 5 kg / molten steel -ton (amount: 1.5 tons) projected to perform desulfurization treatment It was evaluated whether low-sulfur steel (sulfur content: 0.0024% by mass or less) can be efficiently melted. The residence time of the powder in the flame was calculated using the equations (1) to (5) and the equations (9) to (11).

The flow rate of LNG from the upper blowing lance was 500 Nm ³ / h, the flow rate of oxygen gas for fuel combustion was 1100 Nm ³ / h, and the flow rate of argon gas for carrier gas was 250 to 500 Nm ³ / h. In all tests, the pressure of the atmosphere in the vacuum chamber during powder projection was 6.7 kPa, and the flow rate of the argon gas for circulation was 3000 NL / min.

  Table 2 shows the test results. The heat transfer rates shown in Table 2 were calculated using the above equation (12). The desulfurization rate is a ratio (percentage) of the difference between the sulfur concentration in molten steel before desulfurization and the sulfur concentration in molten steel after desulfurization to the sulfur concentration in molten steel before desulfurization. In the column of “Evaluation of desulfurization” in Table 2, the test in which the sulfur concentration in the molten steel after the desulfurization treatment is 0.0024% by mass or less is “○”, that is, the test is passed, and the test in which the sulfur concentration exceeds 0.0024% by mass is “X”, that is, Indicated as failed.

  Table 2 shows the following.

As the residence time of the CaO-Al ₂ O ₃ premelt desulfurizing agent in the flame increased, the heat rate increased. This is with an increase of the flame retention time is believed that the temperature of the pre-melt the desulfurizing agent in the CaO-Al ₂ O ₃ is a heat transfer medium is due to increase.

Further, by projecting a pre-melt the desulfurizing agent in _CaO-Al 2 _{O 3,} low硫鋼in all tests; was possible melting of (sulfur content 0.0024 mass% or less). However, in the test where the residence time in the flame was 0.02 seconds or more, the desulfurization rate was high.

It is considered that the reason why the desulfurization rate was high in the test in which the residence time in the flame was 0.02 seconds or more was that the melting of the desulfurizing agent was promoted by heating the desulfurizing agent to a high temperature. This is because the temperature of the pre-melt desulfurizing agent of CaO-Al ₂ O _{3 as} the heat transfer medium increased with the increase of the residence time in the flame, the temperature of the desulfurizing agent increased, and the melting of the pre-melt desulfurizing agent was promoted. It is considered to be.

DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 6 Upper tank 7 Lower tank 8 Upside immersion pipe 9 Downside immersion pipe 10 Gas flow pipe for reflux 11 Duct 12 Material inlet 13 Upper blowing lance 21 Powder Body injection nozzle 22 Powder injection nozzle opening 23 Fuel injection nozzle 24 Fuel injection nozzle opening 25 Oxidizing gas injection nozzle 26 Oxidizing gas injection nozzle opening 27 Cooling water supply channel 28 Cooling water drainage channel

Claims (6)

The flame formed at the tip of the upper lance installed in the vacuum chamber of the vacuum degassing equipment heats the powder for refining, which is ejected from the tip of the upper lance together with the jet of carrier gas, and heats the powder. Spraying the powder to a molten steel bath surface in a vacuum chamber, a method of refining molten steel under reduced pressure,
A method for refining molten steel under reduced pressure, wherein a residence time of the powder in the flame is 0.02 seconds or more. The method for refining molten steel under reduced pressure according to claim 1, wherein the residence time is calculated using the following equations (1) to (5).

Here, t is the powder residence time in the flame (s), u _p is the rate of the powder after leaving the top-blown lance (m / s), a _p is the powder acceleration ( m ^{/ s} 2), x is the distance from the top lance tip to the molten steel bath surface (m), _{F D} occurs in the flow direction of the powder drag (N), _{u g} exited from the top lance The gas velocity (m / s) of the immediately following jet flow, g is the gravitational acceleration (m / s ² ), ρ _p is the density of the powder (kg / m ³ ), and ρ _g is from the top blowing lance. The gas density (kg / m ³ ) of the jet flow immediately after, μ _g is the gas viscosity (Pa · s) of the jet flow immediately after leaving the top blowing lance, d _p is the diameter (m) of the powder, C _d is the drag coefficient (-) of the powder, and Re is the Reynolds number (-). As the upper blowing lance,
A powder injection nozzle for injecting the powder with a carrier gas,
One or more fuel injection nozzles for injecting fuel, having a fuel injection nozzle opening formed on the outer peripheral side of the powder injection nozzle;
One or more oxidizing gas injection nozzles for injecting oxidizing gas for fuel combustion, having an oxidizing gas injection nozzle opening formed on the outer peripheral side of the powder injection nozzle,
The method for refining molten steel under reduced pressure according to claim 1 or 2, wherein an upper-blown lance having: An RH vacuum degassing device is used as the vacuum degassing device, and the residence time of the molten steel in the vacuum tank of the RH vacuum degassing device, calculated using the following equation (6), is set to 5 seconds or more. The method for refining molten steel under reduced pressure according to any one of claims 1 to 3, wherein the molten steel in the ladle is returned to the vacuum tank.

Here, τ is the residence time (s) of the molten steel in the vacuum chamber, ρ _M is the density of the molten steel (kg / m ³ ), and _Sv is the vacuum in the horizontal section at a height of 100 mm from the floor of the vacuum chamber. The area (m ² ) of the empty part in the tank, g is the gravitational acceleration (m / s ² ), Q is the annular flow rate of the molten steel (ton / min), and Sd is the empty part in the horizontal cross section of the lower end of the dip pipe. (M ² ) and H _m are the height (m) of the molten steel in a vacuum bath with the bath surface stationary.   5. The powder according to claim 1, wherein the powder is one or more of manganese ore, manganese ferroalloy, and CaO desulfurizing agent. 6. Of refining molten steel under reduced pressure.   The method for refining molten steel under reduced pressure according to any one of claims 1 to 5, wherein the pressure of the atmosphere in the vacuum chamber is not less than 2.7 kPa and not more than 13.3 kPa.

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