JP3252726B2 - Vacuum refining method for molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum refining method for degassing and desulfurizing molten steel.

[0002]

2. Description of the Related Art As a method of vacuum refining molten steel, a method of degassing a molten steel in a ladle under a vacuum atmosphere like a VOD method, and a method of removing a dip tube inside a ladle like a RH method and a DH method. And smelting by reducing the pressure in the vacuum tank and the immersion tube.

[0003] In the VOD method, the free board cannot be made large, so that it is impossible to avoid the problem of molten steel overflow. Therefore, it is difficult to increase the stirring power of the molten steel, and the vacuum refining capacity is limited accordingly.

In the DH method, it is necessary to immerse one dipping tube provided at the lower part of the vacuum tank in molten steel in a ladle and repeatedly raise and lower either the vacuum tank or the ladle. Therefore, a large-scale lifting device is required, and the equipment cost increases.

In the RH method, two dip tubes provided at the lower part of a vacuum chamber are immersed in molten steel in a ladle, and the molten steel is circulated by blowing a reflux gas from one of the dip tubes. Stirring of molten steel and slag cannot be performed.

[0006] In order to solve the above problems, the following refining methods different from the conventional DH or RH method have been proposed.

[0007] 1) Japanese Patent Application Laid-Open No. Hei 3-6317 discloses that a snorkel (immersion pipe) having a large inner diameter is immersed in a molten metal, and the inside of the snorkel is depressurized to suck up the molten metal. A ladle refining method in which Ar gas is blown from the whole area to degas is shown. In this method, the molten metal forms an upward flow along the inner peripheral wall of the snorkel by bubbling Ar gas along the wall. At the center of the snorkel, a descending amount corresponding to the rising amount is generated and circulation of the molten metal is performed. The above publication discloses the relationship between the volume W1 of the molten metal in the portion surrounded by the inner wall of the snorkel and its lower extension surface, the bottom surface of the ladle and the molten metal bath surface in the snorkel, and the total volume Wo of the molten metal in the ladle. It is described that W1 / Wo ≧ 0.4 is preferable.

2) Japanese Patent Application Laid-Open No. 52-52109 discloses that a molten metal is sucked into a cylindrical body (immersion tube) immersed in molten steel, and a part of the lower end of the peripheral wall of the cylindrical body (with a central angle of 120 °). )), A refining method is shown in which a gas is blown from above to form an ascending flow, and a descending flow is formed on the opposite side to stir and mix the entire treated molten metal.

3) JP-A-5-217748 discloses the above 2)
The technique is basically the same as the method described in
The ratio (D / H) between the inner diameter (D) of the lower end immersion part of the (immersion tube) and the suction height (H) of molten steel from the lower end of the immersion part is 1 or more, and the inert gas injection point Is set in the range of 60 to 270 degrees with respect to the central angle of the cylindrical container.

[0010]

However, the conventional methods 1) to 3) have the following problems.

In the conventional method 1), gas is blown from below the snorkel (immersion pipe). However, the arrangement of the stirring gas tuyere for stirring and circulation of molten steel is not disclosed at all. Inferring from FIG. 3 of the publication, it can be considered that the stirring gas tuyeres are arranged all around the snorkel. However, if the stirring gas is blown from the entire circumference of the immersion pipe, the mixing of the molten steel is insufficient and the refining characteristics cannot be sufficiently improved.

The conventional method 2) is the same as the above-mentioned method 1) in that gas is blown from the lower portion of the cylindrical body (immersion tube), but the tuyeres are unevenly installed to sufficiently secure a downward flow area. This is trying to secure the amount of molten steel circulated. In this method, the amount of molten steel sucked into the cylinder is increased as much as possible.
(The molten steel inhalation rate is 15% or more.) The conditions are given for the purpose of shortening the uniform mixing time and promoting the uniform reaction of the bath and the early uniform dissolution at the time of metal addition. Therefore, the optimal conditions for promoting the degassing reaction have not been clarified. Although the arrangement of the stirring gas tuyere is described as being within the range of the central angle of 120 °, there should be optimal conditions for the arrangement of the stirring gas tuyere depending on the shape and dimensions of the ladle and immersion tube. I do not consider this point at all.

The conventional method 3) is characterized in that the tuyere for injecting gas from the lower part of the cylindrical vessel (immersion pipe) is unevenly installed to secure a sufficient downward flow area to secure the molten steel circulation amount. Same as conventional method 2). in this way,
Focusing on the condition that the ratio (D / H) of the inner diameter D of the lower end immersion part in the vacuum chamber to the molten steel suction height H should be 1.0 or more, the optimum immersion pipe inner diameter for the ladle shape is considered. There is no consideration. The above D / H is merely specified in order to satisfy the essential requirement of increasing the ratio of the circulation flow rate to the ladle to the total circulation amount of the molten steel. Therefore, this D / H
Conditions are not optimal conditions for the size of the immersion tube when focusing on the degassing reaction. In the method 3), the injection point of the inert gas is set to 60 to 270
° range. However, as described above, the optimum central angle of gas injection should depend on the shape of the ladle and the dip tube. As with the invention 2), the invention 3) does not consider this point at all.

An object of the present invention is to clarify the optimum conditions for gas injection, which have not been sufficiently elucidated, in the smelting treatment of molten steel using a ladle and a dip tube capable of reducing pressure. An object of the present invention is to provide a vacuum refining method for molten steel suitable for rapidly melting low hydrogen steel, extremely low nitrogen steel, extremely low sulfur steel, and high clean steel.

[0015]

The gist of the present invention resides in the following vacuum refining method.

A dip tube having an inner diameter D satisfying the following formula and having a submerged tuyere for gas injection on the inner wall at the lower end within a range satisfying the following formula was used, and the dip tube was immersed in molten steel in a ladle. Then, the molten steel is sucked into the dip tube by reducing the pressure in the dip tube, and the molten steel is introduced into the molten steel from the above-mentioned dip tuyere.
A vacuum refining method for molten steel, characterized by blowing a gas at a rate of 0.004 to 0.03 Nm ³ / min per ton . 0.5 ≦ D / Do ≦ 0.8 ・ ・ ・ ・ ・ ・ 100 × (D / Do) + 85 ≦ θ ≦ 100 × (D / Do) +145 ・ ・ ・ ・ However, D / Do is The ratio between the inner diameter D of the immersion pipe and the inner diameter Do of the ladle, θ, is the central angle (deg) of the immersion pipe representing the installation range of the gas injection immersion tuyere. Hereinafter, in the present invention, the unit of angle “°”
Alternatively, “degree” is represented by “deg”.

[0017]

FIG. 1 and FIG. 2 schematically show an example of an apparatus for carrying out the method of the present invention.
The method of the present invention will be described based on these figures.

FIG. 1 (a) is a longitudinal sectional view schematically showing a main part of the apparatus, and shows a state in which a vacuum pumping is performed and a normal operation is performed. FIG. 1 (b) is a schematic view of a horizontal section taken along the line of the immersion tuyere of FIG. 1 (a). The apparatus comprises a ladle 1 for accommodating molten steel 4, a tubular immersion tube 2 having a lower end opened and having a plurality of immersion tuyeres 3 on the inner wall at the lower end for gas injection. An upper blowing tuyere 5 for supplying an oxidizing gas is provided on an upper peripheral wall of the cylindrical immersion pipe 2. The upper part of the immersion pipe 2 has the same structure as that of FIG. 2 (however, there is no upper lance 6), but is omitted in FIG.

In FIG. 1, reference symbol D denotes the inner diameter of the cylindrical immersion pipe 2, Do denotes the inner diameter of the ladle 1, that is, the inner diameter at the surface position of the molten steel 4 during steady operation, and θ denotes the immersion tuyere 3 of the immersion pipe 2. The central angle representing the installation range, Δθ is the angle between the tuyeres when a plurality of immersion tuyeres 3 are installed, and h is the distance from the position of the immersion tuyeres 3 to the surface of the molten steel sucked up.

FIG. 2 is a schematic longitudinal sectional view showing an example of an apparatus provided with a vertically movable upper blowing lance 6 for supplying an oxidizing gas, instead of the upper blowing tuyere 5.

In the method of the present invention, molten steel is subjected to vacuum refining by the following method using the apparatus having the structure shown in FIG. 1 or FIG.

After the molten steel 4 treated in a converter or the like is tapped into a ladle 1, the cylindrical immersion pipe 2 is immersed in the molten steel 4, and the interior of the immersion pipe 2 is evacuated to a reduced pressure to reduce the molten steel 4 to the immersion pipe 2. Suck up inside. At this time, the suction height of molten steel (h in FIGS. 1 and 2)
It is desirable to set it to about 1000 to 2000 mm.

Next, a gas (preferably an inert gas such as Ar) for stirring the molten steel 4 is blown from the immersion tuyere 3 provided on the inner wall at the lower end portion of the immersion tube 2, while the gas is supplied to the immersion tube 2. From the port 7 (not shown in FIG. 1), alloy iron, flux or the like is added to the molten steel 4 in the immersion pipe 2 as needed. When the temperature of the molten steel 4 is to be raised, an oxidizing gas is supplied from the upper blowing tuyere 5 or the immersion tuyere 3 or the upper blowing lance 6.

Desirable conditions in the above method are as follows.

There is no particular restriction on the horizontal cross-sectional shape of the cylindrical immersion tube 2, but considering that the ladle is usually circular,
It is desirable to have a circular shape as shown in FIG.

A plurality of immersion tuyeres 3 are arranged with a tuyere angle (Δθ) shown in FIG. 1 (b). Δθ range is 5
It is desirable to set it to 30 deg.

The gas blown from each immersion tuyere 3 must be dispersed evenly in the ascending flow area of the molten steel 4 in the immersion pipe 2. When the tuyere angle (Δθ) is less than 5 °, the gas blown from the adjacent immersion tuyere 3 is united, and the efficiency of generating the upward flow decreases. On the other hand, if the angle is larger than 30 deg, a region where the distribution of gas in the molten steel 4 is locally coarse occurs between the immersion tuyere 3 and the efficiency of generating the ascending flow is locally reduced.

The desirable range of the installation position of the immersion tuyere 3 in the vertical direction of the immersion tube 2 is 50 mm from the lower end of the immersion tube 2.
~ 500 mm. If it is less than 50 mm from the lower end, the refractory may fall off in the early stage of operation or the immersion pipe 2
Because of the danger of erosion, the immersion tuyere 3 may be damaged, making operation impossible. These problems can be avoided if the position of the immersion tuyere 3 is set higher than the lower end of the tubular immersion tube 2 by 50 mm or more. On the other hand, 500m from the lower end
If it exceeds m, the immersion tuyere 3 will be too close to the surface of the molten steel 4 in the immersion pipe 2, so that the splash in the pipe will increase and the ingot will increase. In such a case, the frequency of the slashing operation during non-operation increases, which causes problems such as a decrease in productivity and a rise in labor costs due to securing of work personnel.

A desirable range of the flow rate of the stirring gas blown from the immersion tuyere 3 for stirring the molten steel is 0.004 to 0.03 Nm ³ / min per ton of the molten steel to be treated.

The flow rate of the stirring gas for effectively circulating the molten steel was investigated, and the following findings were obtained regarding the above range.

By setting the flow rate of the stirring gas to 0.004 Nm ³ / (min · ton) or more, it is possible to sufficiently stir the inside of the cylindrical immersion tube, and to circulate the molten steel in the tube and the molten steel in the ladle. Is also significantly promoted. In addition, since the moving speed of the substance to the free surface facing the bubble interface or the vacuum atmosphere of the degassing species component in the molten steel can be sufficiently secured, the degassing efficiency can be increased.

However, the flow rate of the stirring gas is 0.03 Nm ³ / (min · to
If the value exceeds n), the splash in the immersion tube increases, and there is a problem in stable operation due to sticking of metal in the tube and suction of metal into the exhaust system. Further, the load of the vacuum pump increases, the pressure in the immersion tube does not decrease (the degree of vacuum cannot be increased), and the equilibrium concentration of the gas to be removed in the molten steel does not decrease. Therefore, the degassing speed may be reduced.

Since the ultimate pressure in the immersion tube greatly affects the degassing speed, it is important to reduce the pressure (increase the degree of vacuum). The ultimate pressure is determined by a balance between the flow rate of the blown gas and the exhaust capacity of the vacuum pump. The ultimate pressure at the time of the degassing process is preferably 100 Torr or less, more preferably 5 Torr or less.

Based on the results of the study under the above preconditions, the above formulas and the conditions of the formulas in the method of the present invention will be described.

Ratio of inner diameter D of immersion tube 2 to inner diameter Do of ladle 1
D / Do must be within the range of the following formula.

0.5 ≦ D / Do ≦ 0.8 When the inner diameter D of the immersion tube 2 is small, the reaction interface area effectively contributing to the degassing reaction inside the immersion tube 2 decreases. As a result of various examinations of the inner diameter conditions of the cylindrical immersion pipe 2 necessary for securing an effective reaction interface area, it was found that when the ratio D / Do was smaller than 0.5, the degassing rate was significantly reduced.
Therefore, D / Do needs to be 0.5 or more. On the other hand, since the immersion pipe 2 is covered with a refractory, its inner diameter D cannot be increased to the inner diameter of the ladle 1. Considering that it is necessary to secure a clearance between the outer surface of the cylindrical immersion pipe 2 and the inner surface of the ladle 1 in operation.
The upper limit of D / Do is 0.8.

Further, in the method of the present invention, it is necessary to satisfy the following equation simultaneously with the above equation.

100 × (D / Do) + 85 ≦ θ ≦ 100 × (D / Do) +145 where θ is the central angle of the immersion tube 2 representing the installation range of the gas-blowing immersion tuyere 3 ( deg) (see Fig. 1 (b)).

If the above center angle θ is not appropriate, there may be cases where degassing, desulfurization and cleaning are not sufficient even if the above equation is satisfied.

FIG. 3 is a diagram showing the influence of D / Do and central angle θ on the dehydrogenation rate constant. As shown in FIG. 3, in order to set the dehydrogenation rate constant (K _H ) of molten steel to 0.2 min ⁻¹ or more, it is necessary to satisfy the above and the above expressions at the same time. However, the dehydrogenation rate constant K _H is a value obtained from the following equation.

K _H = ln ([H] o / [H] e) / T [H] o: Hydrogen concentration in molten steel before treatment (ppm) [H] e: Hydrogen concentration in molten steel after treatment (ppm) T: treatment time (min) FIG. 4 is a diagram showing the influence of D / Do and central angle θ on the desulfurization rate constant. As shown in FIG. 4, in order to set the desulfurization rate constant (K _S ) of molten steel to 0.2 min ⁻¹ or more, it is necessary to simultaneously satisfy the above expression and the expression. However, the desulfurization rate constant K _S is a value obtained from the following equation.

K _S = ln ([S] o / [S] e) / T [S] o: Sulfur concentration in molten steel before treatment (ppm) [S] e: Sulfur concentration in molten steel after treatment (ppm) T: processing time (min) where K _H is 0.2 min ⁻¹ or more and K _S is 0.2 min ⁻¹
The above criteria were established because the refining limits for dehydrogenation and deflow by the current RH method and gas bubbling method under atmospheric pressure are 0.2 min ^-1 , respectively. To get it.

FIG. 5 is a diagram showing the influence of D / Do and central angle θ on the decarburization rate constant. As shown in the figure, if the above and the above expressions are simultaneously satisfied, the decarburization rate constant (K _C ) of the molten steel can be set to 0.3 min ⁻¹ or more. However, the decarburization rate constant K _C is a value obtained from the following equation.

K _C = ln ([C] o / [C] e) / T [C] o: concentration of carbon in molten steel before treatment (ppm) [C] e: concentration of carbon in molten steel after treatment (ppm) T: Processing time (min) FIG. 6 is a diagram showing the influence of D / Do and central angle θ on the attained nitrogen concentration by the denitrification processing. As is clear from the figure, if the above and the above expressions are simultaneously satisfied, the ultimate nitrogen concentration of the molten steel can be reduced to 20 ppm or less.

When the central angle θ is less than 100 × (D / Do) +85, the rising flow area of the molten steel in the immersion pipe is smaller than the falling flow area,
The descending flow velocity of the molten steel in the descending flow area is remarkably reduced, and the speed of replacing and circulating the molten steel in the immersion pipe with the molten steel in the ladle is reduced. Further, as the inner diameter D of the immersion pipe increases, the horizontal reach distance of the stirring gas becomes relatively shorter, and the ascending flow area of the molten steel relatively decreases. Therefore, as shown in FIGS. 3 and 4, it is necessary to increase the lower limit value of θ as D / Do increases.

On the other hand, if the central angle θ exceeds 100 × (D / Do) +145, the upflow area of the molten steel in the immersion pipe is larger than the downflow area, and the downflow area becomes relatively small, contrary to the above. Due to the interference with the upward flow of molten steel in the upward flow region, the downward flow of molten steel does not sufficiently enter the ladle. Further, as the inner diameter D of the immersion pipe increases, the upward flow area of the molten steel relatively decreases as described above. Therefore, as shown in FIGS. 3 and 4, it is necessary to increase the upper limit value of θ as D / Do increases.

As described above, in the method of the present invention, a dehydrogenation rate constant and a desulfurization rate constant of 0.2 min ^-1 or more can be achieved, the efficiency of decarburization and denitrification can be increased, and the vacuum refining time can be shortened. it can. Also, as shown in the examples described later, when the temperature of the molten steel is increased by blowing an oxidizing gas, good cleanliness is obtained in which the total oxygen and T. [O] in the molten steel after the treatment are less than 20 ppm. be able to.

[0048]

【Example】

(Test 1) Ladle (Do = 4) using the apparatus shown in FIG.
The immersion pipe 2 is immersed in 250 tons of molten steel housed in the m), and the inside is evacuated to an inner wall near the lower end of the immersion pipe (distance from the lower end to the immersion tuyere = 300 mm). ×
Ar gas is 2,500 from 12 immersion tuyeres (Δθ = 9 to 21 deg).
The molten steel was gas-stirred by blowing at N l / min, and the pressure (degree of vacuum) in the immersion pipe during processing was controlled to 2 Torr, and h = 1.8 m
The dehydrogenation treatment was carried out for 9 to 11 minutes while maintaining the temperature. Table 1 shows the processing conditions and processing results. The flow rate of the stirring gas with respect to the amount of the molten steel to be processed is 0.01 Nm ³ / (min · ton).

[0049]

[Table 1]

As shown in Table 1, under the condition that D / Do is 0.7, the optimum range of θ obtained by the above equation is 155 to 215 deg.
In Test Nos. 2 to 4, which are inside, a high dehydrogenation rate constant of 0.2 min ^-1 or more was obtained, but the dehydrogenation rate constant was less than 0.15 min ^-1 at θ other than the above range.

Under the condition that D / Do is 0.4, the optimum range of θ is
Although it is 125 to 185 deg, the dehydrogenation rate constant was as low as 0.12 min ^-1 or less even in this range as in Test Nos. 8 and 9.

From the above, it can be seen that in order to set the dehydrogenation rate constant to 0.2 min ^-1 or more, it is necessary to control D / Do to 0.5 or more and to control the range of θ so as to satisfy the above expression.

(Test 2) Deoxidation is performed at the time of tapping, and quick lime is put into tapping to add (% CaO) / (% Al in slag.
₂ O ₃ ) was adjusted to 1.2 to 2.0, and the [S] concentration in molten steel before vacuum treatment was set to 40 to 50 ppm.

Using an apparatus having the structure shown in FIG. 1 as in Test 1, the immersion tube was immersed in 250 tons of molten steel housed in a ladle (Do = 4 m), and the immersion tube was evacuated to a vacuum. Ar gas is blown at 2500 Nl / min from an immersion tuyere with an inner diameter of 3 mm x 12 pieces (Δθ = 8 to 20 deg) provided on the inner wall near the lower end of the tub (distance from the lower end to the immersion tuyere = 300 mm). Was subjected to gas agitation, and a desulfurization treatment was performed for 9 to 10 minutes while controlling the pressure (degree of vacuum) in the immersion tube during the treatment at 10 Torr. h is
Maintained at 1.8 m. In addition, the stirring gas flow rate with respect to the total amount of molten steel to be treated is 0.01 Nm ³ / (min · ton). Table 2 shows the test conditions and results.

[0055]

[Table 2]

As shown in Table 2, under the condition that D / Do is 0.6, Test No. 12 which is within the optimal range of θ (145 to 205 deg)
At ~ 14, a high desulfurization rate constant of 0.2 min ^-1 or more was obtained, but at θ excluding the above range, the desulfurization rate constant was less than 0.15 min ^-1 .

Under the condition that D / Do is 0.3, the optimum range of θ is 11
Test Nos. 18 and 19 within 5 to 175 deg are within this range.
However, the desulfurization rate constant was as low as 0.15 min ^-1 or less.

From the above, it can be seen that in order to make the desulfurization rate constant 0.2 min ^-1 or more, it is necessary to control D / Do to 0.5 or more and to control the range of θ so as to satisfy the above equation.

(Test 3) Using the apparatus having the configuration shown in FIG.
An immersion pipe is immersed in 250 tons of molten steel stored in a ladle (Do = 4m), and the inner wall near the lower end of the immersion pipe (distance from the lower end to the immersion tuyere = 300 mm) is evacuated. 3mm x 12 (Δθ = 9-20deg) immersion tuyere
Ar gas was blown at 2500 Nl / min, and gas agitation was performed for 5 minutes while adding 1.6 kg of metallic aluminum per ton of molten steel into the immersion tube. Ultimate vacuum is 5
0 Torr, h = 1.7 m. A sample of molten steel before the heating treatment was collected and analyzed for T. [O].

Thereafter, while the gas stirring of the molten steel is continued,
A heating treatment was performed in which oxygen gas 0.2 Nm ³ / (min · ton) was blown from the top blowing lance onto the molten steel surface for 5 minutes. At this time, the lance height was 3 m.

The temperature of the molten steel before the temperature increase was 1580 to 1620 ° C., and the temperature increase in the temperature increase of each charge was about 40 ° C. In addition,
The stirring gas flow rate with respect to the total amount of molten steel is 0.01 Nm ³ / (min
n).

Further, after the supply of oxygen gas was stopped, gas stirring was continued for 3 minutes, and a sample of the molten steel after the heating treatment was taken to analyze T. [O]. Table 3 shows test conditions and results.
Shown in

[0063]

[Table 3]

As shown in Table 3, under the condition that D / Do is 0.65, Test No. 22 which is within the optimal range of θ (150 to 210 deg)
~ 24, good cleanliness of molten steel having a T. [O] after the temperature rise of less than 20 ppm was obtained, but when θ was not within the above range, T. [O] after the temperature rise was 50 ppm. Above, the cleanliness of the molten steel deteriorated.

Under the condition that D / Do is 0.4, the optimum range of θ is 125
175175 °, but even in Test Nos. 28 and 29 within this range, T. [O] after treatment exceeds 50 ppm, and the cleanliness of molten steel is not good.

From the above, T. [O] after the temperature raising treatment was reduced to 20 ppm.
It can be seen that in order to make the ratio less than 0.5, it is necessary to control the range of θ to the above-defined range after setting D / Do to 0.5 or more.

(Test 4) A 250-ton molten steel blown in a converter was deoxidized and discharged into a ladle (Do = 4 m). At the time of tapping, 1 kg of metallic aluminum per ton of molten steel was used as a deoxidizing agent. Thereafter, using a device having the configuration shown in FIG. 1, the immersion tube is immersed in molten steel in a ladle, and the interior of the immersion tube is evacuated to a vacuum and the vicinity of the lower end of the immersion tube (distance from lower end to immersion tuyere = 30).
0 mm) from the inner immersion tuyere with an inner diameter of 3 mm × 12
Gas was blown in at 2500 Nl / min, and the molten steel was agitated for decarburization. The ultimate vacuum in the decarburization treatment was 1 Torr and h = 1.8 m. The stirring gas flow rate with respect to the total amount of molten steel treated is 0.01 Nm ³ / (min · ton). The carbon concentration of the molten steel before treatment is 380-420 ppm, and the oxygen concentration is 480-
It was 530 ppm, and decarburization treatment was performed for 9 to 11 minutes in each heat. Table 4 shows the results.

[0068]

[Table 4]

As shown in Table 4, under the condition that D / Do is 0.65, the optimum range of θ is 150 to 210 deg. Within this range, a high decarburization rate constant was obtained. Outside this range, the decarburization rate constant does not reach 0.30 min ^-1 . On the other hand, D / Do
In the case of 0.35, the decarburization rate constant is extremely small even within the optimal range of θ of 150 to 210 deg.

(Test 5) A 250-ton molten steel blown to a low carbon concentration region in a converter was deoxidized and removed into a ladle (Do = 4 m). At the time of tapping, 1 kg of metallic aluminum per ton of molten steel was used as a deoxidizing agent. Then, using a device having the configuration shown in FIG. 1, the immersion tube is immersed in molten steel in a ladle, and the inside of the immersion tube is evacuated to a vacuum and the vicinity of the lower end of the immersion tube (distance from lower end to immersion tuyere = 300). Ar gas was blown in at 2500 Nl / min from a submerged tuyere having an inner diameter of 3 mm x 12 provided on the inner wall of (mm) and the molten steel was agitated for denitrification. The ultimate vacuum degree in the denitrification treatment was 1 Torr and h = 1.8 m. In addition, the stirring gas flow rate with respect to the total amount of molten steel is 0.01 Nm ³ / (min ・ ton)
Becomes The nitrogen concentration of the molten steel before treatment is 36-41 ppm,
Each heat was subjected to a denitrification treatment for 9 to 13 minutes. Table 5 shows the results.

[0071]

[Table 5]

As is clear from Table 5, when D / Do is 0.75, when θ is in the optimum range of 160 to 220 deg, 20 ppm
The ultimate nitrogen concentration is as follows. On the other hand, when D / Do is 0.4, the attained nitrogen concentration is high even when θ is in the optimal range of 125 to 185 deg.

From these results, if the formula and the conditions of the formula defined by the present invention are satisfied, [N] can be increased to 20 pp in 12 minutes or less.
It can be seen that extremely low nitrogen steel of m or less can be obtained.

[0074]

According to the method of the present invention, the time required for the vacuum refining of molten steel can be shortened, and the ultimate concentration of [H], [S], [C] and [N] to be removed can be significantly reduced. It is possible to melt clean steel.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a configuration example of an apparatus for carrying out the method of the present invention, in which (a) is a longitudinal sectional view of a main part showing a situation during steady operation, and (b) is (a). 3) is a schematic view of a horizontal cross section taken along the line of the submerged tuyere.

FIG. 2 is a schematic longitudinal sectional view showing another example of the configuration of an apparatus for performing the method of the present invention.

FIG. 3 is a diagram showing the influence of D / Do and central angle θ on the dehydrogenation rate constant.

FIG. 4 is a diagram showing the influence of D / Do and central angle θ on the desulfurization rate constant.

FIG. 5 is a diagram showing the influence of D / Do and central angle θ on the decarburization rate constant.

FIG. 6 is a diagram showing the influence of D / Do and central angle θ on the attained nitrogen concentration during the denitrification treatment.

[Explanation of symbols]

1: Ladle, 2: Immersion tube, 3: Immersion tuyere, 4: Molten steel, 5: Top blowing tuyere, 6: Top blowing lance, 7: Input port, D: Inner diameter of dipping tube, Do: Ladle of ladle Inner diameter,
θ: central angle representing the installation range of the immersion tuyere, Δθ: angle between the tuyere of the immersion tuyere, h: distance from the immersion tuyere to the surface of the molten steel sucked up

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C21C 7/10

Claims (1)

(57) [Claims] 1. An immersion tube having an inner diameter D satisfying the following formula and having a immersion tuyere for gas injection on the inner wall at the lower end in a range satisfying the following formula: After immersion, the molten steel is sucked into the dip tube by reducing the pressure in the dip tube, and the molten steel is introduced into the molten steel from the above-mentioned immersion tuyere.
A vacuum refining method for molten steel, characterized by blowing a gas at a rate of 0.004 to 0.03 Nm ³ / min per ton . 0.5 ≦ D / Do ≦ 0.8 ・ ・ ・ ・ ・ ・ 100 × (D / Do) + 85 ≦ θ ≦ 100 × (D / Do) +145 ・ ・ ・ ・ However, D / Do: The ratio of the inner diameter D of the immersion tube to the inner diameter Do of the ladle. Θ: Central angle (deg) of the immersion tube, which indicates the installation range of the immersion tuyere for gas injection.