JPH08109411A - Vacuum desulfurization refining of molten steel - Google Patents

Abstract

PURPOSE: To provide a melting method, by which extra-low sulfur and extra- clean treatment is executed without blowing CaO-containing flux and the heat loss is restricted to the min., at the time of melting an HIC resistant steel. CONSTITUTION: A single leg cylindrical immersion tube 14 is dipped into the molten steel 12 in a ladle 10 under the conditions of the undermentioned (i)-(iii) to execute a vacuum degassing treatment. (i) The composition of a ladle slag satisfies the following condition. 0.8<= (%CaO)/(%Al2 O3 ) <=2.5, (%SiO2 ) <=10 and (%FeO)+(%MnO) <=2.0. (ii) The stirring power per unit cross-sectional area shown in the following equation satisfies: ε/A=[4.18.Q.T.In(1+ρ.g.h/ Po )]-/(W.π.D1 <2> /4) and ε/A(watt/ton/m<2> )>=15, wherein, Q is flow rate of the stirring gas (Nm<3> /min), T is molten steel temp. (K), ρ is molten steel density (kg/cm<3> ), (g) is gravity acceleration (m/s<2> ), (h) is blowing depth of the stirring gas (m), Po is atmospheric pressure in a vacuum vessel (Pa), W is treated molten metal quantity (ton) and D1 is inner diameter of the immersion tube (m). (iii) Ratio (D1 /D2 ) of the inner diameter D1 of the immersion tube to the inner diameter D2 of the ladle is >=0.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum desulfurization refining method for molten steel, and more particularly to a vacuum desulfurization refining method which enables efficient treatment without providing a long blowing lance in a vacuum tank.

[0002]

2. Description of the Related Art Today, the characteristics required for steel materials are becoming more severe, and many improvements are being made in terms of quality and manufacturing costs. In particular, there is a demand for an inexpensive manufacturing method of low sulfide steel with S ≦ 10 ppm, and many proposals have been made so far.

Japanese Unexamined Patent Publication No. 56-98415 discloses, "2 to 8 kg / T of lime is added to molten steel in a tapping steel from a converter to a ladle, and 0.05 to 0.05% of Al is added to a ladle surface slag layer after tapping. 0.40kg / T added,
After degassing, Ar gas was passed through the top blowing lance soaked in molten steel under the condition of 0.006 to 0.009 Nm ³ / min.
More than a minute to desulfurize the molten steel, and then add Ca to the molten steel.
The steel manufacturing method is characterized by adding a Ca substance corresponding to 0.125 to 0.500 kg / T.

In Japanese Patent Laid-Open No. 56-9317, "a CaO-containing flux is blown into a molten steel which has been previously deoxidized in a ladle by a carrier gas for deoxidation and desulfurization, and then the molten steel is continuously treated.
We propose a method for producing low-oxygen, low-sulfur steel with controlled sulfide morphology, characterized by controlling the sulfide morphology in molten steel by injecting a Ca alloy.

By the way, in the above, although the degassing treatment is applied to reduce the hydrogen concentration in the molten steel, since the desulfurization treatment is performed after that, the water content in the slag increases the hydrogen concentration in the molten steel. Will occur.

In addition, in the above, the injected Ca
Hydrogen pick-up from the O-containing flux also becomes a problem. Then, in order to solve these problems, the following conventional techniques have been proposed.

In Japanese Patent Laid-Open No. 58-3913, "A CaO-containing flux is blown into a molten steel which has been previously deoxidized by a carrier gas for deoxidation and desulfurization, and then a Ca alloy is blown into the molten steel to determine the sulfide form in the molten steel. In the method of controlling low oxygen and low sulfur steel, the step of blowing the CaO-containing flux and the Ca
We propose a method for producing low-hydrogen steel with controlled sulfide morphology, which is characterized by performing vacuum degassing treatment during the step of injecting alloy, and reducing the hydrogen concentration increased in the step of injecting CaO-containing flux. Therefore, the vacuum degassing process is performed before the step of injecting the Ca alloy.

However, in the above, it is necessary to use different refining devices for the blowing process and the vacuum degassing process. In such a complex treatment, there is a temperature drop of molten steel in each process, and in order to compensate for this temperature, it is necessary to raise the blowing stop temperature significantly compared with ordinary materials, and the treatment process becomes complicated and the cycle time is extended. Due to the process bottleneck, for example, the fact that continuous casting cannot be performed, the productivity and the yield are lowered, which makes it unsuitable for large-scale processing. Therefore, the following proposals have been made to satisfy ultra-low sulfur, ultra-clean, Ca addition, and ultra-low hydrogen in a single process.

In Japanese Patent Laid-Open No. 58-22320, "In a vacuum degassing tank, a method of injecting a processing agent into a molten steel in a ladle combined with a vacuum degassing tank with a carrier gas such as an inert gas and depressurizing Depressurize and blow the desulfurizing agent into the ladle, and then stop adding the desulfurizing agent while keeping the vacuum degassing tank in a vacuum state and only blow the inert gas, and then in the vacuum degassing tank. A ladle refining method characterized by injecting a Ca alloy or a Ca compound after the pressure is restored to atmospheric pressure has been proposed.

In Japanese Patent Laid-Open No. 4-99812, "Melted steel pre-deoxidized in a ladle is CaO 1.5-4.0.1 at 0.1-1 Torr.
Inject 5 kg / TS, then CaSi 0.3 to 0.7 kg / T at atmospheric pressure
A method for producing a steel having excellent HIC resistance characteristics, which is characterized by controlling the concentration of [Ca], [Al], and [O] in the S steel and the molten steel has been proposed.

[0011]

However, the following problems also occur in the above-mentioned conventional method and method. In the conventional method, ultra low sulfur, ultra clean,
It is said that it becomes possible to add Ca and satisfy extremely low hydrogen. However, since it is assumed that CaO-containing flux is blown in, a CaO flux blowing lance with a long lifting stroke is required in the vacuum chamber as in the example, and the equipment cost is enormous. Become. Further, since the lance length is long, the possibility of breakage of the lance is high, and since the stroke is large, it takes much labor to replace the lance, and stable operation is difficult. In addition, the temperature drop during the injection of CaO-containing flux is extremely large, and even in a single process such as the conventional method, the heat loss becomes large. Alternatively, Al heating in ladle refining and Al heating by heat of Al oxidation reaction by oxygen supply and arc heating.
Plasma heating etc. will be required.

When the blowing temperature of the converter is increased, the melting loss of the refractory material of the converter is increased, the cost of the refractory material is increased due to the shortening of the furnace life, and the cost of the refractory material construction is increased due to the increase in the frequency of furnace construction.

In the case of heat compensation in ladle refining, if the heat compensation is carried out in the same process as described above, the treated molten steel stays in the same process for a long time, and the cycle time is prolonged and the number of castings is increased continuously. This leads to reduced productivity and yield, making it unsuitable for large-scale processing.

If heat compensation is performed in a process different from the above process, a separate process for raising the temperature is required, resulting in a double investment in equipment cost. Even in the conventional method, blowing CaO-containing flux is the same, and it is necessary to solve the same problems as described above.

Therefore, an object of the present invention is to produce an ultra-low sulfur content without melting CaO-containing flux in a single process in melting ultra-low-sulfur steel or HIC-resistant steel.
It is to propose a melting method that performs extremely clean treatment and minimizes heat loss. More specifically, an object of the present invention is to provide an economical and efficient vacuum desulfurization refining method for smelting [S] ≤ 10 ppm HIC resistant steel.

[0016]

In order to solve the above-mentioned problems, the inventors of the present invention did not blow CaO-containing flux in a single process, and thus ultra-low sulfur, ultra-clean (including Ca and ultra-low hydrogen were included). )), And earnestly studied a melting method that minimizes heat loss.

As a result, in order to obtain extremely low sulfur and extremely clean without blowing CaO-containing flux, an appropriate gas stirring force is set under an appropriate slag composition and an immersion pipe diameter is made appropriate. I found a need.

Here, the gist of the present invention is that after immersing a tubular dipping tube consisting of one leg in molten steel in a ladle,
When the molten steel is sucked up into the dip tube by depressurizing the dip tube, and the stirring gas is blown from the tuyere provided in the lower part of the ladle below the projection surface of the dip tube or the inner wall of the dip tube, (i) the ladle slag composition is 0.8 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 2.5 (% SiO ₂ ) ≦ 10 (% FeO) + (% MnO) ≦ 2.0 where (% CaO): slag Medium CaO concentration (wt%), (% Al ₂ O
₃ ): Al ₂ O ₃ concentration in slag (% by weight), (% SiO ₂ ): In slag
SiO ₂ concentration (wt%), (% FeO): FeO concentration in slag (wt%
%), (% MnO): MnO concentration in slag (% by weight) (ii) Further, stirring power ε / A per unit cross-sectional area shown in the following formula
(watt / ton / m ² ) satisfies the condition of 15 or more, and ε / A = {4.18 ・ Q ・ T ・ In (1 + ρ ・ g ・ h / Po)} / (W ・
π · D ₁ ^2/4) where, Q: agitation gas flow rate ^{(Nm 3 / min), T} : temperature of molten steel
(K), ρ: Molten steel density (kg / m ³ ), g: Gravitational acceleration (m /
s ² ), h: Stirring gas injection depth (m), Po: Vacuum chamber atmosphere pressure (Pa), W: Processed molten steel amount (ton), D ₁ : Immersion pipe inner diameter (m) and (iii) Immersion the ratio D ₁ / D ₂ of the inner tube diameter D ₁ and the ladle inner diameter D ₂ is a vacuum desulfurization refining process of the molten steel characterized by satisfying the above 0.5.

[0019]

Next, the operation of the present invention will be briefly described with reference to the accompanying drawings, and then the reason why the processing conditions are limited in the present invention as described above will be described in detail together with its operation. FIG. 1 of the accompanying drawings is an explanatory view of an example of a combination of a one-legged cylindrical dipping tube and a ladle for carrying out the method according to the present invention, in which a converter (not shown) changes to a ladle 10. The molten steel 12 that was tapped is usually 16
The temperature is about 20 to 1670 ° C, and the one-legged dipping tube 14 is dipped in this. In the illustrated example, the dipping pipe 14 is provided with an oxidizing gas top blowing lance 16, an alloy charging port 18, and an exhaust port 20 connected to a vacuum device (not shown) as appropriate.

The molten steel 12 in the ladle is provided with a porous plug or a through-hole plug 24 at the bottom of the ladle to blow an inert gas into the molten steel. Alternatively, an inert gas blowing lance (not shown) may be immersed in the projection plane of the dip tube 14.

First, according to the present invention, the molten steel contained in the ladle 10 is used.
A cylindrical dip tube 14 consisting of one leg is dipped in 12 and this dip tube
Evacuate the inside of 14 and insert the porous plug 24 or blowing lance (not shown) from the lower part of the ladle below the projection surface of the dipping tube.
It is a method of blowing an inert gas through the. Blowing of this inert gas such as Ar gas is preferably continued from the start to the end of ladle refining, and any vacuum treatment in the present invention is carried out under the blowing of this inert gas. Here, the reason for defining the melting conditions as described above in the present invention will be explained.

First, in order to obtain an ultra-low-sulfur steel without injecting CaO-containing flux, it is essential to control the slag composition within the optimum range. This is because when the CaO-containing flux is blown, a desulfurization reaction (transit reaction) in the process of the flux floating in molten steel can be expected.
This is because it is necessary to desulfurize only by the slag-metal reaction (permanent reaction) when the contained flux is not blown.

The optimum conditions of the slag composition in the present invention are 0.8 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 2.5, preferably 0.9 ≦
(% CaO) / (% Al ₂ O ₃ ) ≦ 2.0 and (% SiO ₂ ) ≦ 10, preferably (% SiO ₂ ) ≦ 8 and (% FeO) + (% MnO) ≦ 2.0, preferably , (% FeO) + (% MnO) ≦ 1.0.

With the slag composition other than this, according to the experiments of the present inventors, it was not possible to eliminate the charge of [S]> 10 ppm no matter how the conditions of vacuum degree and stirring force were changed. In the desulfurization reaction, it is important to increase the S-containing capacity (sulfide capacity) of the slag and to reduce the oxygen potential of the slag.

As described above, 0.8 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 2.5 and (% SiO ₂ ) ≦ 10 are the limiting conditions for increasing the sulfide capacity. FeO) + (% MnO) ≦ 2.0 is a limiting condition for reducing the oxygen potential of slag. The limiting condition for (% FeO) + (% MnO) is 2% or less, but 1% or less is more desirable.

In order to adjust the slag composition as described above, CaO is added to the ladle slag at the time of tapping the ladle or thereafter.
Further performs the insertion of Al ₂ O ₃ or CaO -Al ₂ O ₃ based flux necessary. In this case, the molten steel temperature is not substantially lowered even by adding the flux.

Although the frequency of [S] <10 ppm is increased by controlling the slag composition within the above range, there are still cases where extremely low sulfur ([S] <10 ppm) cannot be obtained. It turns out that the cause is the lack of stirring power of molten steel,
Therefore, in the present invention, the stirring power of the ladle bottom blow is further optimized.

When desulfurization is performed by bubbling under vacuum using a ladle flux without blowing CaO-containing flux, the desulfurization rate of the slag metal cannot be expected because the transitional reaction due to the blown powder cannot be expected as described above. It was devised to correspond to the stirring force, and the desulfurization ability was evaluated by the stirring power ε / A (watt / ton / m ² ) per unit cross-sectional area shown in the following formula. ε / A = {4.18 ・ Q ・ T ・ In (1 + ρ ・ g ・ h / Po)} / (W ・
π · D ₁ ^2/4) where, Q, T, ρ, g , h, Po, W, S 1 is as already mentioned.

This stirring power ε / A is adjusted to a critical value of 15 watt / ton / m ²
If it is made larger than the above, the flux in the immersion pipe is finely dispersed in the molten steel even without powder blowing, and the dispersed flux particles bring about the same effect as the blown powder,
It exhibits desulfurization and cleaning capabilities. This ε / A
The upper limit of is not specified, but from the practical point of view, 100watt / to
n / m ² or less.

Here, the smaller the inner diameter D ₁ of the immersion pipe, the larger the index ε / A of the stirring power, but the amount of slag that effectively contributes to the desulfurization and cleaning treatment inside the immersion pipe decreases accordingly. .

Therefore, as a result of various studies on the conditions of the inner diameter of the immersion pipe required to secure the effective slag amount, the inner diameter of the immersion pipe D ₁
It was found that when the ratio D ₁ / D ₂ between the ladle inner diameter D ₂ and the ladle inner diameter D ₂ is less than 0.5, the desulfurization and cleaning ability decreases. D ₁ / D ₂ is 0.5
It is necessary to be above, and it is desirable to be 0.6 or above. Also, since the dip pipe is made of refractory, it is not possible to increase the inner diameter of the ladle. Operationally, considering the clearance between the dip tube and ladle, D ₁
The upper limit of / D ₂ is 0.8.

By performing the treatment as described above, it is possible to perform the treatment without CaO-containing flux blowing. Therefore, the two treatments of "CaO-containing flux blowing + bubbling" as in the above-mentioned prior art can be performed by "Bubbling". It can be integrated into one process.

Here, of the composition in the ladle slag ((% Ca
O), (% Al ₂ O ₃ ), and (% SiO ₂ ) are the outflow amount of converter slag to the ladle, the amount of flux added at the time of tapping, the amount of flux added after tapping, or in the above dipping pipe. It can be adjusted by the amount of flux added to the.

In addition, (% FeO) and (%
To reduce MnO), add a mixture of metal Al and a foaming agent (for example, CaCO ₃ ) to molten steel during or after tapping, and stir the slag with the foaming agent while slag with metal Al.
You may accelerate the reduction of FeO and MnO 2.

On the other hand, during or after tapping, a stirring gas is flowed from the bottom of the ladle to stir the ladle slag.
It is desirable to promote slag formation. Because, in the method according to the present invention, the snorkel covered with the refractory material, that is, 1
Since the main leg immersion pipe is immersed in the molten steel, the stirring of the slag inside the immersion pipe is strong, but the stirring of the slag outside the immersion pipe is relatively smaller than that inside.

This stirring gas can be added from a porous plug or a through-hole plug provided at the bottom of the ladle. Further, the bubbling lance may be immersed in the molten steel in the ladle and stirred during or after tapping.

Further, after the steel is taken out of the converter and before or after the desulfurization treatment in the ladle, the temperature of the molten steel in the ladle is compensated by raising the temperature of the molten steel by supplying oxygen or by electric heating to measure the decrease in the temperature of the steel taken out of the converter. May be.

Further, in order to obtain "extremely low hydrogen", in addition to the above conditions, it is desirable to perform the treatment at a vacuum degree of 2 Torr or less. For steel grades that require Ca treatment and require extremely low hydrogen, the following melting procedure is desirable.

For example, simultaneous desulfurization / dehydrogenation treatment may be performed at a vacuum degree of 2 Torr or less, and then Ca treatment may be performed. Alternatively, desulfurization may be performed under a low vacuum degree of 100 Torr or less, followed by dehydrogenation at 2 Torr or less, and then Ca treatment.

The atmospheric pressure for the Ca treatment is usually atmospheric pressure, but may be slightly increased or reduced pressure. However, when the Ca-containing substance is added under reduced pressure, it is preferable that the atmospheric pressure is 300 Torr or higher, considering that the vapor pressure of Ca is high and vaporization is easy.

Further, in the case of melting a steel type that requires Ca treatment, the Ca-containing substance to be added may be Ca-containing material, such as Ca-Si, Fe-Ca, Ca-Al and Ca. -Ni, Fe-
An alloy such as Ca-Ni may be used. Also, a mixture of a metal powder containing Ca and another metal powder, or
You may use what was pressure-formed after mixing. Also these
CaO, Al ₂ O ₃ , CaF ₂ , CaO-Al ₂ O ₃ , CaO-Ca
_{F 2, CaO-Al 2 O} 3 -CaF 2 flux may be mixed and used.

The effect of mixing and adding these fluxes is described in JP-A-54-37019, JP-B-59-22765, and JP-A-SHO-
This is as described in Japanese Patent Laid-Open Nos. 64-75622 and 3-47910.

At that time, the purity of Ca in the Ca-containing substance must be 50% or less, preferably 40% or less. Since Ca is highly reactive, if the purity is too high, the reaction at the time of addition becomes vigorous, causing a problem of splash generation. On the other hand, if the purity is too low, the amount of metal components other than Ca increases,
The basic unit of the Ca-containing substance, which is necessary when adding the pure Ca, becomes too large.

The S concentration in the molten steel before Ca treatment is preferably 10 ppm or less. This is because if it is more than this, the formation of CaS-based inclusions cannot be sufficiently suppressed even if the addition rate of the Ca-containing substance is reduced.

The amount of Ca added or the basic unit of Ca (both are Ca
It should be 1.0 kg / T or less.
5 kg / T or less is desirable. The reason is that if the Ca addition amount becomes excessive, the Al ₂ O ₃ -based inclusions that should react will decrease or disappear even if the Ca addition rate is reduced, and Ca in the CaO-Al ₂ O ₃ -based inclusions will decrease.
This is because the O 2 concentration becomes too high. Furthermore, Ca
This is because if the added amount is too large, the formation of CaS-based inclusions cannot be sufficiently suppressed.

On the other hand, the added amount of Ca or the basic unit of Ca (both in terms of pure Ca) must be 0.1 kg / T or more. The reason is that if the amount of Ca added is too low, the Al ₂ O ₃ -based inclusions do not become CaO-Al ₂ O ₃ -based inclusions sufficiently, and the alumina cluster-based inclusions remain. .
Further, if the amount of Ca added is too low, MnS which has a bad influence on the HIC resistance performance in the region where Mn is concentrated due to center segregation during casting.
Is generated.

[0047]

Example In this example, using the apparatus shown in FIG. 1, a one-legged dip pipe was immersed in molten steel at 1640 ° C. stored in a 250-ton ladle, and the bottom of the ladle was evacuated in vacuum. Argon gas was blown from the blow plug at 2 Nm ³ / min to stir the molten steel for desulfurization.

The sulfur concentration in the molten steel before the treatment in this example is 20
The hydrogen concentration in the molten steel before processing was 5 to 15 ppm. In the slag with quicklime added during tapping (% CaO) / (% Al ₂ O ₃ ).
Was adjusted to 0.5-9. It was visually observed that when (% CaO) / (% Al ₂ O ₃ ) exceeds 2.5, slag slag formation is significantly reduced.

Further, the (% SiO ₂ ) concentration in the ladle slag is set to 1 by adjusting the outflow amount of the converter slag to the ladle during tapping.
Adjusted to ~ 25%. At the time of tapping, a metal Al-containing substance is added to perform deoxidation tapping, and reduction by this Al ((%
FeO) and (% MnO) have been reduced, but if the added Al at the time of tapping is not sufficient due to the large outflow of converter slag, after tapping
A mixture of (Al + CaCO ₃ ) was added on the slag.

When the metal Al-containing substance is added to the molten steel in the ladle and the molten steel is heated by supplying oxygen, the concentration in the slag (% Al ₂ O ₃ ) increases, so Additional quicklime was added to control (% CaO) / (% Al ₂ O ₃ ).

Desulfurization time by gas stirring under vacuum is 7 to 15
Minutes The results are shown graphically in Figures 3-5. First, Fig. 2 shows the CaO-Al ₂ O ₃ -SiO ₂ 3 state diagram, which is outside the range of (% CaO) / (% Al ₂ O ₃ ) ＝ 0.8〜2.5 or
It can be seen that desulfurization does not proceed easily when (% SiO ₂ )> 10.

The effect of lower oxides is shown in FIG.
When + (% MnO) is 2% or less, desulfurization proceeds and [S] <10ppm is obtained. Further, if (% FeO) + (% MnO) is reduced to 1% or less, [S] <5ppm is obtained. You can see that

FIG. 4 shows the influence of the stirring force. It can be seen that the effect of promoting desulfurization is large when ε / A is 15 or more. Fig. 5 shows the effect of the immersion pipe diameter. It can be seen that the effect of promoting desulfurization is large when D ₁ / D ₂ is 0.5 or more.

After desulfurization treatment of Example 1, [S] <10 ppm
0.8 kg per ton of molten steel with Ca-Si (Ca content 30%) in molten steel of the steel type that requires Ca treatment (sour-resistant line pipe material)
After that, Ca treatment was performed. As a result, all of the obtained steel materials achieved [H] <1 ppm in the steel and satisfied the NACE conditions.
The NACE test conditions are as follows, and the presence or absence of cracks is adjusted.

NACE test conditions: Solution: 5% NaCl + 0.8% CH ₃ COOH Temperature: 24 ± 2.8 ° C. pH: Max. 4.5 hours: 96 hours H ₂ S concentration: H ₂ S saturated H ₂ S flow rate: 100-200 cc / min Test piece: Width 20 mm x Length 100 mm x (WF-2mm) Surface condition: # 320 Paper finish End surface coating: Not possible

[0056]

EFFECTS OF THE INVENTION According to the present invention, extremely low-sulfur steel with [S] ≤ 10 ppm can be melted inexpensively and efficiently, and if Ca is also used, hydrogen-induced cracking does not occur and Contributes to stabilization of steel quality.

[Brief description of drawings]

1 is a schematic view of an apparatus for carrying out the method according to the invention.

FIG. 2 is a CaO—Al ₂ O ₃ —SiO ₂ 3 element phase diagram.

FIG. 3 is a graph showing a relationship between [S] after treatment and (% FeO) + (% MnO) in slag.

FIG. 4 is a graph showing the relationship between stirring force and post-treatment [S].

FIG. 5 is a graph showing the effect of immersion pipe diameter on [S].

Claims (1)

[Claims] 1. A tubular dip tube consisting of one leg is immersed in molten steel in a ladle, and then the molten steel is sucked up into the dipping tube by depressurizing the inside of the dipping tube, and the lower part of the ladle is below the projection surface of the dipping tube. Alternatively, in the vacuum desulfurization refining method of molten steel in which stirring gas is blown from the tuyere provided on the inner wall of the dipping pipe, the ladle slag composition, stirring, and the conditions of the dipping pipe and ladle shape that satisfy the following conditions (i) to (iii) A method characterized by treating molten steel below. (i) Ladle slag composition: 0.8 ≤ (% CaO) / (% Al ₂ O ₃ ) ≤ 2.5 (% SiO ₂ ) ≤ 10 (% FeO) + (% MnO) ≤ 2.0 where (% CaO): CaO concentration in slag (wt%), (% Al ₂ O ₃ ): Al ₂ O ₃ concentration in slag (wt%), (% SiO ₂ ): SiO ₂ concentration in slag (wt%), (% FeO): FeO concentration in slag (wt%), (% MnO): MnO concentration in slag (wt%) (ii) Stirring power ε / A (watt / ton / m ² ) per unit cross-sectional area
15 or more: ε / A = {4.18 ・ Q ・ T ・ In (1 + ρ ・ g ・ h / Po)} / (W ・
π · D ₁ ^2/4) where, Q: agitation gas flow rate ^{(Nm 3 / min), T} : temperature of molten steel
(K), ρ: Molten steel density (kg / m ³ ), g: Gravitational acceleration (m / s ² ), h: Stirred gas injection depth (m), Po: Vacuum chamber atmosphere pressure (P
a), W: the processing amount of molten steel (ton), D _1: dip tube inside diameter (m) (iii) a dip tube inner diameter D ₁ and the ladle inner diameter D ₂ and the ratio D ₁ / D ₂ is 0.5 or more.