JP2000063930A - Method for refining molten low sulfur steel containing little nitrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refining method of molten low sulfur steel containing little nitrogen, with which the increase of consumption of steam, etc., supplied into a booster, etc., used for reduction of the pressure and the lowering of temp. of the molten steel are prevented and the absorption of the nitrogen is restrained. SOLUTION: In the refining method of the molten steel for executing decarburization and degassing-refining by reducing the pressure in an immersion tube 13, while blowing inert gas from the bottom part 18 of a ladle 11 by dipping one piece of the immersion tube 13 into the molten steel 12 in the ladle 11, after executing the decarburization and the degassing-refining, fuel and oxygen-containing gas are supplied into the immersion tube 13 to raise the inner pressure in the immersion tube 13 higher than the atmospheric pressure while burning the fuel, and flux is added into the immersion tube 13 to execute the desulfurizing-refining of the molten steel 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining low-sulfur molten steel with a small amount of nitrogen absorption (nitrogen pickup) using a vacuum refining device for decarburization and degassing refining.

[0002]

2. Description of the Related Art As a method for lowering the carbon, nitrogen or hydrogen concentration of molten steel, RH and D utilizing vacuum (reduced pressure)
H and VOD are widely used. However, although these methods can reduce carbon and nitrogen in the molten steel to a certain extent, it is difficult to produce the molten steel that has been sufficiently reduced. Therefore, as a method of increasing the refining efficiency under reduced pressure to produce molten steel such as ultra-low carbon, low hydrogen and low sulfur, for example, in JP-A-1-92314, during decarburization and degassing refining, A cylindrical dip tube having an inner diameter of 0.5 or less of the ladle inner diameter is used, and the dip tube is evacuated to reduce the pressure.
Blow inert gas from the bottom of the ladle to stir molten steel,
During desulfurization (refining), an inert gas or desulfurizing agent is supplied into molten steel from the tuyere or lance at the bottom of the ladle by using a cylindrical dipping tube having an inner diameter larger than 0.5 of the ladle. A ladle refining method has been proposed. In addition, JP-A-7
The following refining method is described in JP-A-278639. That is, the inner diameter D _{1 of the} dipping tube / the inner diameter D of the ladle is set to 0.
The immersion pipe adjusted to 5 to 0.8 is immersed in molten steel, the interior of the immersion pipe is decompressed and the molten steel is sucked up, and an inert gas is blown from the bottom of the ladle to mix Al or A while stirring the molten steel.
lAlloy iron was added to carry out blowing acid to heat up, and then desulfurization refining was carried out by adding a flux for desulfurization in the dipping pipe in a depressurized state, and then the dipping pipe was subjected to multiple pressure to atmospheric pressure to dip the dipping pipe. The immersion depth is less than 0.5 m, and stirring is performed with an inert gas. Since this refining can raise the temperature of the molten steel by blowing acid, it guarantees the temperature of the molten steel that decreases with the passage of time required for desulfurization refining, etc., and the refining furnace (converter)
There is an advantage that the amount of cold iron source (scrap) used in the above can be increased.

[0003]

However, in the refining method described in JP-A-1-92314, the conditions of the dipping pipe used for decarburization and degassing and the dipping pipe used for desulfurization refining are different. Different types of dip tubes are required.
Therefore, it is difficult to continue desulfurization refining using the decarburized and degassed refining dip pipes as they are, and because different dip pipes are used for each, the refractory of the dip pipe is consumed. And the cost of refractories increases. Further, even in desulfurization refining, since the pressure is reduced to a high degree of vacuum in the vicinity of 2 torr, the consumption of steam, electric power, etc. supplied to the booster and the like increases. Further, the heat of the desulfurization refining lowers the temperature of the molten steel, which causes problems such as metal clogging, nozzle clogging, lowering of casting yield and stopping of casting in the subsequent steps such as continuous casting.
Further, in the refining method described in Japanese Patent Application Laid-Open No. 7-278639, since Al or Al alloy iron is added and the temperature is raised by performing blowing acid, the slag is excessively oxidized to inhibit or increase the desulfurization reaction. In the case of heat, the refractory may be worn due to local heating. Further, in this refining, the pressure is reduced to a high vacuum degree of 100 torr or less to enhance the agitation of the molten steel to desulfurize, so that the consumption of steam, electric power, etc. supplied to the booster and the like increases, and the heat release of the desulfurization refining melts the molten steel. Causing a temperature drop,
As in the refining method described in Japanese Patent Application Laid-Open No. 1-92314, there are problems such as attaching metal, clogging of nozzle, lowering of casting yield, and stopping of casting. Further, after the desulfurization refining is completed, the inside of the dip pipe is subjected to multiple pressure to atmospheric pressure, and then the dipping depth of the dip pipe is set to less than 0.5 m to stir the molten steel with an inert gas. However, there is a problem in that nitrogen absorption (nitrogen pickup) occurs in the molten steel due to the outside air (air) infiltrating from the above, which makes it difficult to produce the molten steel having a low nitrogen content.

The present invention has been made in view of the above circumstances, and prevents an increase in consumption of steam or the like supplied to a booster or the like used for depressurization and a decrease in temperature of molten steel, and suppresses nitrogen absorption of molten steel to reduce the amount of nitrogen. An object is to provide a refining method for low-sulfur molten steel.

[0005]

A method according to the above-mentioned object.
The method for refining low-sulfur molten steel containing less nitrogen is described by immersing a single dip tube in molten steel in a ladle and depressurizing the dip tube while depressurizing the dipping tube while blowing an inert gas from the bottom of the ladle. In a method for refining molten steel for performing charcoal and degassing refining, after performing the decarburization and degassing refining, supplying a fuel and an oxygen-containing gas into the immersion pipe, while burning the fuel, the internal pressure in the immersion pipe Is higher than atmospheric pressure, and a flux is added to the immersion pipe to desulfurize and refine the molten steel.

The refining method for low-sulfur molten steel containing a small amount of nitrogen according to a second aspect is the refining method for a low-sulfur molten steel containing a small amount of nitrogen according to the first aspect, wherein the CaO content of the flux added to the dip pipe is 50 to 50%. It is set to 99% by weight. here,
If the CaO content of the flux added to the immersion pipe is less than 50% by weight, the content of CaO in the flux will be insufficient and the desulfurization of molten steel will be poor. On the other hand, Ca of flux
When the O content exceeds 99% by weight, the purity of CaO is high and the price of the flux is high, so that the desulfurization and refining cost is increased.

According to a third aspect of the present invention, there is provided a method for refining low-sulfur molten steel containing a small amount of nitrogen. Having a total molten steel surface area in the pan of 0.1-0.7.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, referring to the attached drawings, an embodiment in which the present invention is embodied will be described to provide an understanding of the present invention. FIG. 1 is a cross-sectional view of a reduced pressure refining apparatus 10 applied to a method for refining low-sulfur molten steel containing less nitrogen according to an embodiment of the present invention. First, the vacuum refining device 10
A ladle 11 made of steel and lined with a refractory (not shown), a single dip tube 13 and a vacuum tank 14 for dipping in the molten steel 12 in the ladle 11, and an inside of the dip tube 13 and the vacuum tank 14 Exhaust duct 15 connected to an ejector for exhausting and reducing the pressure
In addition, the dipping tube 13 is provided with a storage hopper 16 and an addition chute 17 for adding iron alloy or the like. Further, a porous plug 19 for blowing an inert gas into the ladle 11 is provided on the bottom portion 18 of the ladle 11, and the flow of the molten steel 12 shown by the arrow is formed by the blowing of the inert gas. Further, a lance 20 for mixing and burning fuel and oxygen-containing gas is provided above the vacuum chamber 14 so that the lance 20 can be freely moved up and down. Incidentally, 12a is a molten metal surface of the molten steel 12 in the dip tube 13, and 13a and 14a are flanges for joining the dip tube 13 and the vacuum chamber 14 by fastening means such as bolts and nuts.

Next, a method for refining low-sulfur molten steel containing a small amount of nitrogen, which is applied with the reduced pressure refining apparatus 10, will be described. Ladle 1
A converter, which is an example of a refining furnace (not shown in FIG. 1), receives 150 tons of molten steel 12 that has been decarburized and refined to a carbon concentration of 100 to 600 ppm, and argon gas, which is an example of an inert gas, is melted from the porous plug 19. 0.6 to 15 in 12
While blowing NL / (min./ton of molten steel), this molten steel 12
The immersion tube 13 is immersed in the inside of the immersion tube 13 and the vacuum chamber 14.
The inside was decompressed to 0.1 to 100 torr for decarburization and degassing refining. The conditions for the ladle 11 and the dipping pipe 13 are as follows: the inner surface area (S ₂ ) of the molten steel dipping portion of the dipping pipe 13
0.1 to 0.7 of the total surface area of molten steel (S ₁ ) in the ladle 11
I chose This is because when (S ₂ ) / (S ₁ ) is smaller than 0.1, the bubble activation surface (the emission surface of the argon gas bubbles) formed by the argon gas blown into the dip tube 13 becomes narrow and decarburization occurs. And the promotion of desulfurization refining performed thereafter is hindered. On the other hand, if (S ₂ ) / (S ₁ ) is larger than 0.7, workability such as sampling is hindered. For this reason, more preferable results can be obtained by setting (S ₂ ) / (S ₁ ) to 0.15 to 0.65. Then, by the argon gas supplied from the porous plug 19, as shown in FIG.
As indicated by the arrow in the middle, the molten steel 12 in the ladle 11 is agitated, and the decarburization reaction is promoted by the bubble activation surface of the argon gas expanded in the molten metal surface 12a of the molten steel 12 of the dip tube 13 by the reduced pressure atmosphere. Dehydrogenation (degassing) is performed. In this decarburization and degassing refining, the carbon concentration in the molten steel 12 is 3 to 10 pp.
m and the hydrogen concentration reached 1 to 5 ppm.

After finishing the decarburization and degassing refining, the amount of argon gas from the porous plug 19 is changed to 0.6-1.
While supplying 0 NL / (min / mol steel ton), the pressure inside the dip pipe 13 and the vacuum tank 14 is doubled to the atmospheric pressure, and at the same time, the lance 2
After 0 is lowered to a predetermined position, propane gas, which is an example of fuel, is supplied at 15 to 150 Nm ³ / hour and oxygen as an oxygen-containing gas is supplied at 60 to 500 Nm ³ / hour, and the mixture is mixed and combustion is started. And 5 CaO from the storage hopper 16
Flux for desulfurization containing 0 to 99% by weight of 1 to 5 k
g / (ton of molten steel) is cut out and added into the dipping pipe 13 from the addition chute 17. For this flux, by adding 1 to 50% by weight of fluorspar, alumina or the like to CaO, or by using Mg or a Mg alloy in addition to the above-mentioned flux, dissolution or softening is promoted and the desulfurization reaction is improved. preferable. The added flux is on the molten metal surface 12a.
At that time, so-called desulfurization refining is performed in which the flux in the molten steel 12 is supplemented with sulfur in the molten steel 12 by contacting the molten steel 12 stirred by the flow indicated by the arrow in FIG.

By the way, the insides of the desulfurization and refining dip pipe 13 and the vacuum chamber 14 are isolated from the outside air.
Actually, outside air (air) invades through a gap formed in an exhaust system such as a gas cooler or an ejector including the exhaust duct 15, so that the nitrogen concentration in the immersion pipe 13 and the vacuum tank 14 becomes high, and the nitrogen in the air is increased. Comes into contact with the slag and the molten metal surface 12a. As a result, the molten steel 12 absorbs nitrogen and causes nitrogen pickup. In order to suppress this nitrogen absorption, as described above, propane gas and oxygen are supplied from the lance 20 in the immersion pipe 13 and the vacuum tank 14 to cause mixed combustion, so that the internal pressures of the immersion pipe 13 and the vacuum tank 14 are increased. 1 to 200m above atmospheric pressure
Increase mH ₂ O. By making the internal pressures of the immersion pipe 13 and the vacuum chamber 14 higher than the atmospheric pressure, the air invading the inside is suppressed and the nitrogen concentration in the atmosphere can be kept low, so that the nitrogen pickup of the molten steel 12 can be prevented. In addition, since propane gas and oxygen are supplied and burned in the immersion pipe 13 and the vacuum tank 14, the insides of the immersion pipe 13 and the vacuum tank 14 can be sufficiently kept warm, and the temperature drop of the molten steel 12 can be prevented. . By preventing the temperature drop of the molten steel 12,
The desulfurization reaction between the slag and the molten steel 12 can also be promoted. Here, when the inside of the dip pipe 13 and the vacuum tank 14 is higher than the atmospheric pressure by less than 1 mmH ₂ O, the internal pressure becomes too low and the outside air enters from the brick joints of the dip tube 13 and the vacuum tank 14 to the molten steel 12. Nitrogen pickup occurs due to nitrogen absorption. On the other hand, the internal pressure of the dip tube 13 and the vacuum chamber 14 is 200 mmH above atmospheric pressure.
_If it exceeds ₂ O and becomes higher, the flame generated from the lance 20 is hard to reach the molten metal surface 12a, and the heat-adhesion efficiency is deteriorated. Further, when the discharge pressure of the lance 20 is increased, the refractory in the dip tube 13 and the vacuum chamber 14 is hit by the flame, which causes a problem such as damage to the refractory. For this reason, the internal pressure is more preferably higher than the atmospheric pressure by 10 to 100 mmH ₂ O.

[0012]

EXAMPLE Next, an example of a refining method for low-sulfur molten steel containing a small amount of nitrogen will be described. Use a converter to increase the carbon concentration to 3
While dipping 5 NL / (min. Molten steel ton) of argon gas from the porous plug 19 into the ladle 11 containing 150 ton of molten steel 12 decarburized to 00 ppm, the dipping pipe 13
Further, after decompressing and degassing refining by depressurizing the inside of the vacuum tank 14, desulfurization refining was carried out while burning fuel continuously. Then, the nitrogen (N) and sulfur (S) concentrations of the molten steel were investigated.

As shown in Table 1, first, in Example 1, the inner surface area (S ₂ / S ₁ ) of the molten steel dipping portion of the dipping tube 13 was 0.5 with respect to the total molten steel surface area in the ladle 11. Immersion tube 1
3 was immersed and propane gas was supplied at 112 Nm ³ / hr and oxygen was supplied at 280 Nm ³ / hr to burn the dip pipe 13 while maintaining the internal pressure at 10 mmH ₂ O higher than the atmospheric pressure.
Quicklime powder (98% by weight of CaO is an example of flux.
2 kg / ton of molten steel was added, and desulfurization refining was performed for 6 minutes. As a result, a good value was obtained for the sulfur concentration of 19 ppm, a low-sulfur molten steel having a nitrogen concentration of 15 ppm was obtained without any nitrogen pickup of the molten steel 12, and the overall evaluation was an excellent result (◯). Further, in Example 2, the immersion pipe 1 in which the inner surface area (S ₂ / S ₁ ) of the molten steel immersion portion of the immersion pipe 13 with respect to the total surface area of the molten steel in the ladle 11 was 0.3.
3 was immersed and propane gas was supplied at 168 Nm ³ / hour and oxygen was supplied at 420 Nm ³ / hour to burn the inside of the immersion pipe 13 at an internal pressure 100 mmH ₂ O higher than the atmospheric pressure, while CaO is contained at 60% by weight ( 5 kg / ton of molten steel was added to the quicklime powder containing the remainder (fluorite) and desulfurization refining was performed for 6 minutes. As a result, a good value was obtained with a sulfur concentration of 15 ppm, a low-sulfur molten steel with a nitrogen concentration of 10 ppm was obtained without any nitrogen pickup of the molten steel 12, and the overall evaluation was an excellent result (◯).

[0014]

[Table 1]

On the other hand, in the comparative example, the immersion pipe 13 whose inner surface area (S ₂ / S ₁ ) of the molten steel immersion portion of the immersion pipe 13 is 0.3 with respect to the total surface area of the molten steel in the ladle 11 is immersed. Then
When the fuel and oxygen-containing gas are not supplied and the pressure is restored to atmospheric pressure, then 5 kg of quicklime powder containing 60% by weight of CaO (the balance is fluorite) / ton of molten steel is added and desulfurization refining is performed for 6 minutes. The sulfur concentration was 18 ppm, which was a substantially good value. However, nitrogen pickup was generated in the molten steel 12 and the nitrogen concentration was as high as 30 ppm, and low-nitrogen molten steel could not be obtained, which is poor as a comprehensive evaluation (×).
It became a result.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment,
All changes in conditions without departing from the gist are within the scope of the present invention. For example, before the desulfurization and refining of molten steel, A
1, the flux may be added after adding a deoxidizing agent such as Al or Al alloy, and the method of adding the flux may be batch, divided or continuous. Further, in addition to the lance, it is possible to use a multifunctional lance or a combustion burner that mixes and sprays fuel such as gas or liquid with powder of flux. Further, as a condition for decarburizing and refining, it is possible to obtain a low-sulfur molten steel with a small amount of nitrogen by performing the desulfurization refining using a low-carbon molten steel that is not decarburized to an extremely low carbon (carbon concentration of 10 ppm or less). . Regarding the method of multiple pressure in the immersion pipe and the vacuum tank, after stopping the supply of steam to the booster, for example, fuel supplied from the lance, combustion gas such as oxygen-containing gas, and argon gas blown from the bottom of the ladle. Therefore, the internal pressure may be increased, and a nitrogen-free gas such as CO ₂ gas or an inert gas may be blown.

[0017]

According to the refining method of low-sulfur molten steel with low nitrogen content according to claims 1 to 3, a single dip pipe is immersed in the molten steel in the ladle, and an inert gas is blown from the bottom of the ladle. In the refining method of molten steel for decarburizing and degassing refining by reducing the pressure in the dip tube, after decarburizing and degassing refining, supplying fuel and oxygen-containing gas into the dip tube while burning the fuel Since the internal pressure in the dip pipe is raised above atmospheric pressure and flux is added to the dip pipe to perform desulfurization and refining of molten steel, the same dip pipe can be used to reduce the amount of steam supplied to boosters, etc. and prevent the wear of refractories. It is possible to produce a low-nitrogen low-sulfur molten steel by preventing the temperature of the molten steel from lowering and suppressing nitrogen pickup due to nitrogen absorption.

Particularly, in the refining method of low-sulfur molten steel containing a small amount of nitrogen according to the second aspect, the C of the flux added in the dip pipe is
Since the aO content is 50 to 99% by weight, stable desulfurization is possible, and molten steel with low sulfur can be surely produced.

In the method for refining low-sulfur molten steel with a small amount of nitrogen according to claim 3, the internal surface area of the molten steel dipping portion of the dipping pipe has a desulfurization reaction of 0.1 to 0.7 of the total molten steel surface area in the ladle. Therefore, molten steel with low sulfur can be stably produced.

[Brief description of drawings]

FIG. 1 is a sectional view of a reduced pressure refining apparatus 10 applied to a method for refining molten steel with low nitrogen and low sulfur according to an embodiment of the present invention.

[Explanation of symbols]

10 Decompression refining equipment 11 Ladle 12 Molten steel 12a Hot water surface 13 Immersion pipe 13a Flange 14 Vacuum tank 14a Flange 15 Exhaust duct 16 Storage hopper 17 Addition chute 18 Bottom 19 Porous plug 20 Lance

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Claims (3)

[Claims] 1. A single dip tube is immersed in molten steel in a ladle, and while blowing an inert gas from the bottom of the ladle,
In a method for refining molten steel in which the inside of the immersion pipe is decompressed to perform decarburization and degassing refining, after performing the decarburization and degassing refining, a fuel and an oxygen-containing gas are supplied into the immersion pipe to supply the fuel. A method for refining low-sulfur molten steel with a small amount of nitrogen, characterized by increasing the internal pressure in the immersion pipe above atmospheric pressure while burning it and adding a flux to the immersion pipe to desulfurize and refine the molten steel. 2. The C of the flux added in the dip tube
The method for refining low-sulfur molten steel containing a small amount of nitrogen according to claim 1, wherein the content of aO is 50 to 99% by weight. 3. The low nitrogen content low according to claim 1 or 2, wherein the internal surface area of the molten steel immersion portion of the immersion pipe has 0.1 to 0.7 of the total molten steel surface area in the ladle. Refining method of molten sulfur steel.