JPH07278639A - Method for refining extra-low sulfur steel - Google Patents

Abstract

PURPOSE:To obtain an extra-low sulfur steel while combining vacuum heat-raising and desulfurizing treatment, by supplying oxidizing gas while adding Al, at the time of refining molten steel by dipping a single leg type immersion tube into the molten steel in a ladle and evacuating the immersion tube to suck up the molten steel. CONSTITUTION:The single leg type immersion tube 30 is dipped into the molten steel 12 in the ladle 10 and the inner part of the tube is evacuated. At the same time, an inert gas is blown through a blowing lance 22 or a porous plug 24 from the lower part of the ladle 10 under the plane of projection of the immersion tube 30 to execute the stirring treatment. At that time, the ratio of the inner diameter D1 of the immersion tube/the inner diameter D2 of the ladle is made to be 0.5-0.8. Successively, the Al is added into the molten steel 12 in the immersion tube 30 under vacuum collectively or in division or continuously. Together with this addition, the oxygen or the gas containing the oxygen is supplied from a top blowing tuyere 16 to execute the heat- raising and the desulfurizing refining to the molten steel 12. Thereafter, the pressure in the immersion tube 30 is returned back to the atmospheric pressure and the dipping depth D3 of the immersion tube 30 is kept within 0.5m and the gas stirring treatment with the inert gas is continued.

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 extremely low sulfur steel having excellent cleanliness, and more particularly to a method for refining extremely low sulfur steel by vacuum desulfurization treatment in combination with vacuum heat treatment.

[0002]

2. Description of the Related Art In the production of ultra-low-sulfur steel, a gas is usually stirred under atmospheric pressure with a desulfurization flux placed on the molten steel in a ladle, or powder for desulfurization is carried out while stirring the gas. The method of injecting into molten steel is performed. In this case, a ladle lid may be used to avoid air oxidation of molten steel or increase in nitrogen concentration due to contact with air in the atmosphere.

According to these desulfurization treatments, the gas is agitated or the powder is blown under the atmospheric pressure, so that the agitation force is not sufficient, and therefore the desulfurization rate cannot be increased so much. Therefore, when trying to reduce the sulfur concentration in molten steel to an extremely low sulfur range, such as 10 ppm or less,
The processing time is long, and the molten steel temperature drops during that time. Then, since the molten steel temperature is low this time, the reaction rate is further slowed down, and by repeating this, desulfurization cannot be achieved to a desired degree.

Thus, in the above conventional desulfurization method,
Since the temperature drop during desulfurization is large, it is necessary to raise the tapping temperature prior to desulfurization to achieve extremely low sulfurization. However, if the tapping temperature is raised too much, the melting loss of the refractory material of the converter will increase, and the life of the converter furnace will be shortened. Further, in order to raise the tapping temperature, it is necessary to reduce the addition amount of scrap, which is a cold material, in the converter, and to perform the blow-down operation in the blowing of the converter. Each of these causes an increase in the hot metal ratio and a decrease in the iron yield, leading to a decrease in productivity and an increase in cost. Therefore, in order not to raise the converter tapping temperature excessively, it is necessary to heat molten steel in ladle refining.

By the way, as a method for heating molten steel under atmospheric pressure, an inert gas is blown from the bottom of a ladle to stir the molten steel, and while the molten steel is immersed in the dipping pipe, an oxidizing reactant is added and oxygen gas is blown. A method of heating molten steel has been proposed. Further, as a molten steel heating method under vacuum, a method has been proposed in which oxygen gas is introduced into molten steel in a vacuum tank and an inert gas is blown from the bottom of the vacuum tank to stir.

[0006] For example, in JP-A-63-266017,
"By immersing a large dip pipe in the ladle in the ladle and supplying the exothermic agent and oxygen gas to raise the temperature of the molten steel, before and after this temperature rise, the slag-making agent is added and powder injection is performed to perform the powder injection. A method for performing heat-up refining of molten steel "has been proposed.

In Japanese Patent Laid-Open No. 4-253213, "One-legged dip pipe is immersed in molten steel in a ladle, the dip pipe is evacuated and a stirring gas is blown to supply oxygen gas. A method for heating molten steel in a ladle has been proposed.

On the other hand, as a desulfurization method in a ladle, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-100216 discloses that "a dipping pipe is immersed in molten steel in a ladle, and an exothermic agent is added to the dipping pipe from above. With the heating of the propellant acid, in the lower surface area of the dip tube projection,
In addition, there is proposed a ladle refining method for molten steel, characterized in that a desulfurizing agent is blown below the hot spot formation region associated with the blowing acid. This is because the heating of molten steel and desulfurization are performed at the same time, and there is an advantage that the total processing time can be shortened.

Japanese Unexamined Patent Publication (Kokai) No. 1-92314 proposes "a method of immersing a one-legged dip tube in molten steel in a ladle, evacuating the dip tube to desulfurize while blowing a stirring gas". In addition, a method has also been proposed in which the entire ladle containing molten steel is taken into a vacuum chamber, and the interior of the vacuum chamber is evacuated and decompressed, and the ultra-low sulfur steel is refined by gas agitation or gas agitation and desulfurization agent injection.

[0010]

However, the above-mentioned conventional method has the following problems. In JP-A-63-266017, as shown in the example, the molten steel is heated by supplying oxygen gas under atmospheric pressure, so that a large amount of lower oxide is produced, which is a disadvantageous condition for desulfurization. .

Japanese Patent Laid-Open No. 4-253213 discloses heating of molten steel under vacuum, which has extremely high utilization efficiency of an exothermic reaction agent and suppresses formation of lower oxides. Although it is disclosed that the purpose is heating and the cleanliness is improved, nothing is said about the point of producing extra low sulfur steel.

On the other hand, in Japanese Patent Laid-Open No. 1-100216, a desulfurizing agent is blown below the hot spot forming area where the temperature of the propellant acid is raised, so that a transient desulfurization reaction of the transient tree occurs during the process in which the desulfurizing agent rises in molten steel. Although it occurs, this desulfurizing agent reaches the hot spot formation area on the surface of molten steel and then undergoes re-sulfurization at the hot spot, which is in a peroxide state, resulting in extremely low overall desulfurization efficiency. On the other hand, a big problem arises.

Japanese Patent Laid-Open No. 1-92314 discloses that the desulfurization rate is improved as compared with the case where the same treatment is performed under atmospheric pressure. For example, in the case of melting ultra-low sulfur steel having a sulfur concentration of 5 ppm or less, Even in the treatment with high desulfurization efficiency,
The processing time is extended and the temperature drop during the processing becomes large. In the case of temperature compensation, the above-mentioned problems similarly occur in heating the molten steel ladle by raising the temperature of the steel output from the converter or by blowing oxygen gas under atmospheric pressure.

As described above, even in the prior art, some ladle refining methods for ultra-low sulfur steel have been proposed, but none of them satisfy both ultra-low sulfurization and cleanliness, and S
A more economical refining method of ultra-low-sulfur clean steel with ≤10ppm and cleanliness index of 1.0 or less has not yet been developed. The “cleanliness index” is a value obtained by microscopically examining a sample after rolling and using the number of large inclusions per unit area as an index.

An object of the present invention is to provide an efficient and inexpensive refining method for extra-low sulfur steel excellent in cleanability. More specifically, an object of the present invention is to refine ultra-low sulfur steel having a cleanliness index of 1.0 or less and S ≦ 10 ppm, which can be more efficiently and inexpensively performed by more effectively performing desulfurization treatment under vacuum. Is to provide a method.

[0016]

Means for Solving the Problems The inventors of the present invention have made various studies in order to solve the above problems, and have completed the present invention by obtaining the following knowledge. Considering the economic efficiency of the furnace, the ratio D of the inner diameter of the immersion pipe to the inner diameter of the ladle
It ₁ / D ₂ to be from 0.5 to 0.8 is advantageous. Under such conditions, it is advantageous to perform desulfurization after heating. When the temperature rise of molten steel and desulfurization are combined, if the temperature is raised under atmospheric pressure, the desulfurization efficiency will decrease and the cleanliness of the steel will deteriorate. When desulfurization treatment is performed, unexpectedly S ≦ 10ppm, especially S ≦ 5ppm can be achieved in a short time and the cleanliness can be secured. Continue slag bubbling after desulfurization to improve slag and minimize re-sulfurization.

Here, the gist of the present invention is that a cylindrical dip tube consisting of one leg is dipped in the molten steel contained in a ladle, and the dip tube is evacuated to suck the molten steel into the dip tube. In the refining method of extremely low-sulfur steel in which an inert gas is blown from the lower part of the ladle below the projection surface of the dip tube in the state, the ratio D ₁ / D _{2 of the} inner diameter D _{1 of the} dip tube and the inner diameter D ₂ of the ladle is 0.5. 0.8 or more
Determine the inner diameter of the immersion pipe to be the following value, add Al to the molten steel sucked into the immersion pipe all at once or in divided or continuous manner, and heat the molten steel by supplying an oxidizing gas to the molten steel. The method for refining ultra-low-sulfur steel is characterized in that the molten steel is desulfurized and refined, the inside of the immersion pipe is restored to atmospheric pressure, and the immersion depth of the immersion pipe is within 0.5 m to carry out gas agitation treatment. .

According to a preferred embodiment of the present invention, after the desulfurization refining, the pressure in the dip pipe is restored to atmospheric pressure, and then the metal Ca-containing substance is sprayed, injected, added by wire-shaped material or molten steel. The molten steel may be supplied by means such as pushing from above.

The oxidizing gas and Al may be supplied to the molten steel through an upper blowing lance, an injection lance, a submerged tuyere or an upper blowing tuyere installed inside the submerged pipe, and there is no particular limitation.

Similarly, the desulfurizing powder during the desulfurization treatment is
It may be added from an upper blowing lance or an injection lance installed in the dipping tube, a dipping tuyere inside the dipping tube, or an upper blowing tuyere, and there is no particular limitation.

According to another preferred embodiment of the present invention, the hydrogen concentration in the molten steel may be reduced by subjecting the dip tube to vacuum refining treatment under the condition of 100 Torr or less after heating the molten steel. According to still another preferred embodiment of the present invention, the hydrogen concentration in the molten steel may be reduced by subjecting the dip pipe to a vacuum refining treatment under the condition of 100 Torr or less after desulfurizing the molten steel. According to still another preferred embodiment of the present invention, the gas agitation treatment may be performed before heating the molten steel to atmospheric pressure in the dip tube and the dip depth of the dip tube to within 0.5 m.

[0022]

Next, the operation of the present invention will be briefly described with reference to the accompanying drawings, and the reason why each processing condition is limited in the present invention as described above will be described in detail together with its operation. In FIG. 1 of the accompanying drawings, the molten steel 12 tapped from the converter (not shown) to the ladle 10 is usually about 1620 to 1670 ° C., but the dipping pipe 14 is immersed therein. 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 that is appropriately connected to a vacuum device (not shown).

A blowing lance 22 is immersed in the molten steel 12 in the ladle within the projected surface of the dip tube 14, and an inert gas is blown into the molten steel. Alternatively, a porous plug 24 may be provided at the bottom of the ladle.

First, according to the present invention, the molten steel contained in the ladle 10
A cylindrical dip tube 14 consisting of one leg is dipped in 12 and this dip tube
This is a method in which the inside of 14 is evacuated and an inert gas is blown from the lower part inside the ladle below the projection surface of the dipping tube through the blowing lance 22 or the porous plug 24. 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.

At this time, the inner diameter D _{1 of} the dip tube and the inner diameter D _{2 of the} ladle
The reason for defining the inner diameter of the dip tube such that the ratio D ₁ / D ₂ of the above is 0.5 or more and 0.8 or less is as follows. The larger the area in the dip tube in which the desulfurization reaction occurs, the larger the reaction interface area in the dip tube and the higher the desulfurization reaction rate. However,
If the inner diameter of the immersion pipe is increased more than necessary, the melting rate of the immersion pipe is increased, which causes problems such as increasing the frequency of refractory repairs and shortening the life of the immersion pipe.

The ratio of the inner diameter of the dipping pipe to the inner diameter of the ladle D ₁ / D ₂
The relationship between (A) desulfurization rate and (B) immersion pipe repair frequency index was investigated using molten steel contained in a 250-ton ladle. as a result,
As an optimum range for making both (A) and (B) at a satisfactory level at the same time, select the ratio D ₁ / D ₂ of the inner diameter D ₁ of the immersion pipe and the inner diameter D _{2 of the} ladle that is 0.5 or more and 0.8 or less. I did.
It is preferably 0.6 to 0.75.

Next, Al is added to molten steel under vacuum in a batch, divided or continuously, and at the same time, an oxidizing gas, that is, an oxygen gas or an oxygen-containing gas, is usually applied to the top blowing lance or injection or the inside of the dipping pipe. After the molten steel is heated by being supplied to the molten steel from the installed dipping tuyere or upper blowing tuyere, the molten steel is subsequently desulfurized and refined under vacuum. In the illustrated example, an oxidizing gas top blowing tuyere is provided.

The amounts of Al and oxidizing gas introduced at this time are not limited, but the purpose thereof is to compensate for the temperature decrease during the subsequent desulfurization, and therefore, the molten steel temperature is usually 1620 to 1700.
It suffices to add an amount necessary and sufficient to raise the temperature to about 0 ° C. Al is preferably added in powder form.

In the present invention, since the heating is performed under vacuum,
Compared with conventional heating under atmospheric pressure, lower oxides (FeO, MnO
Etc.) is suppressed. If the desulfurization treatment is performed after heating the molten steel, inclusions suspended in the molten steel such as alumina generated by the reaction between Al and oxygen during the heating of the molten steel are adsorbed and separated by the desulfurizing agent or desulfurization slag used in the desulfurization treatment. It is possible to separate and remove it from the molten steel. However, if desulfurization is performed first and then the molten steel is heated, some of the alumina inclusions generated during the heating are adsorbed by the slag and can be separated and removed, but most of them remain in the molten steel. , Deteriorates the cleanliness of steel. Therefore, it was decided to perform desulfurization treatment after heating the molten steel in this way. Also,
As a result, even when the desulfurization treatment time is very long, such as when melting extremely low-sulfur steel, it is possible to perform desulfurization treatment without raising the converter steel output temperature.

The desulfurizing agent used in the present invention is CaO ₂ -CaF.
A conventional one such as a system, CaO-Al ₂ O ₃ system, CaO-Al ₂ O ₃ -CaF ₂ system can be used. As a result, in the present invention, desulfurization up to S ≦ 10 ppm is possible, and the molten steel temperature at the end of desulfurization is 1590 to 1620.
It can be set to about ° C.

As shown in JP-A-4-253213, the heating of molten steel under a vacuum suppresses the formation of lower oxides during heating than under atmospheric pressure, thus ensuring the cleanliness of the steel. At the same time, the subsequent desulfurization treatment becomes advantageous.

The molten steel desulfurization treatment is performed under vacuum because the desulfurization efficiency is higher than that under atmospheric pressure, particularly in that the stirring power is increased, as shown in JP-A-1-92314. After desulfurization, the inside of the dip pipe 14 is restored to atmospheric pressure, and the inert gas is blown from the blowing lance 22 or the porous slag 24 under the atmospheric pressure and with the dipping depth D ₃ of the dipping pipe 14 being 0.5 m or less. The gas stirring process is performed by continuing the mixing. The reason for this is as follows.

In order to accelerate the desulfurization reaction, it is necessary to reduce the oxygen potential at the slag metal interface, but for that purpose, it is necessary to reduce the amount of lower oxides in the slag. Further, the low-grade oxide in the slag causes re-oxidation of molten steel and deteriorates the cleanliness of molten steel, so it is necessary to reduce this as much as possible.

When the desulfurization treatment is performed in a state where the inside of the dip tube is under vacuum, the slag inside the dip tube is well stirred, but the slag outside the dip tube is not sufficiently stirred. Even when the inside of the dip tube is treated at atmospheric pressure, if the dip depth of the dip tube is large, the stirring of the slag outside the dip tube is still insufficient.

Therefore, the present inventors investigated the influence of the immersion depth of the immersion pipe on the concentration of lower oxides in the slag by setting the atmospheric pressure inside the immersion pipe. As a result, when the immersion depth was more than 0.5 m, there was a difference in the lower oxide concentration in the slag inside and outside the immersion pipe, and within 0.5 m, the difference in concentration was significantly small. This is because the rising flow of molten steel generated by the blowing of the inert gas becomes a horizontal flow toward the inner wall of the immersion pipe on the surface of the bath in the immersion pipe, and when the immersion depth of the immersion pipe exceeds 0.5 m, the molten steel flow descends completely on the inner wall of the immersion pipe. In contrast, the horizontal molten steel flow does not change to a complete downward flow within the immersion depth of 0.5 m, but forms a molten steel flow outside the immersion pipe, thus allowing slag stirring. This is because

According to a preferred embodiment of the present invention, the pressure in the dip tube is restored and the molten steel is subjected to the gas stirring treatment, or after the gas stirring treatment is performed, the molten steel is, for example, Ca--Si, Fe--Ca, Fe--Ca--Ni,
The metal Ca-containing substance such as Ca-Al may be supplied by means such as top blowing, injection, addition of wire or pushing from the top of the molten steel, which can be processed under the same equipment under vacuum and atmospheric pressure. This is because the HIC resistant steel is smelted by making the most of the features of this refining method.

As a result, even in HIC-resistant steel which requires the addition of metallic Ca for controlling the inclusion morphology, Ca yield is not reduced as in RH degassing, that is, evaporation loss is reduced, and The process can be completed.

At the time of vacuum desulfurization treatment, the desulfurizing agent powder is added from the upper blowing lance or the injection lance installed in the dipping pipe, or the submerged tuyere or the upper blowing tuyere inside the dipping pipe. This is to improve the rate.

When the desulfurization process is performed at a vacuum degree of 100 Torr or less, the gas is blown in under a vacuum, so that the higher the pressure inside the dipping pipe (the worse the vacuum degree), the weaker the stirring force of the blown gas becomes. Also, since the equilibrium hydrogen concentration increases, the dehydrogenation rate significantly decreases, which is disadvantageous for dehydrogenation. Therefore, in the present invention, the dehydrogenation process is performed at a vacuum degree of 100 Torr or less.

After the desulfurization treatment, the treatment at a vacuum degree of 100 Torr or less is because it may not be possible to obtain a sufficient hydrogen concentration in the molten steel by the dehydrogenation reaction that occurs simultaneously with the desulfurization treatment under vacuum. This is because it is necessary to perform dehydrogenation treatment. The vacuum degree is set to 100 Torr or less for the same reason as above.

In a modified example of the present invention, the gas agitation treatment may be carried out before the heat treatment at atmospheric pressure and the immersion depth of the immersion tube is within 0.5 m. This is because if the low-grade oxide in the slag before the treatment is very high, the subsequent treatment alone may not be able to fully perform the reforming of the slag on the outside of the dip tube, which is a countermeasure. Next, specific working effects of the present invention will be described with reference to examples.

[0042]

【Example】

Example 1 In this example, the apparatus shown in FIG.
Immerse a one-legged dip tube in molten steel at 1620 ° C stored in a ladle, and evacuate the dip tube to 100 Torr or less, and then evacuate argon gas from the bottom blown porous plug of the ladle to 3 Nm ³ / min.
It was blown in and the molten steel was stirred with gas. While the gas was being stirred in this manner, the molten steel was subjected to a heat treatment to raise the temperature to 1670 ° C. under vacuum, and then desulfurization was performed under a vacuum of 100 Torr or less.

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. After desulfurization, gas agitation was performed with the immersion depth of the immersion tube set to within 0.5 m. In the present example, for the heating of the molten steel, Al powder was added to the molten steel from the alloy charging port, and oxygen gas was supplied to the molten steel from the oxidizing gas top blowing lance as the oxidizing gas.

Before heating the molten steel, Al is added in an amount of 160 to 80 depending on the amount of heating.
When 0 kg was added and oxygen gas flow rate was 50 Nm ³ / min and oxygen gas was supplied for 2 to 10 minutes with each charge, the rate of temperature rise was about 8 ° C./min for each charge.

The inner diameter of the ladle was kept constant at 4 m, and the low hydrogen ultra-low sulfur steel was melted under the conditions of the inner diameter of the dipping pipe being 1.5, 1.8, 2, 2.5, 3, 3.5 m. Also, as desulfurization flux
10 kg / T of a desulfurizing agent consisting of 85% by weight of CaO and 15% by weight of CaF ₂ was added after heating, through the alloy charging port of the dipping tube. The results are shown in Table 1.
Are shown together.

As shown in Table 1, the desulfurization time under vacuum at 100 Torr or less is 13 to 15 minutes, and D ₁ / D ₂ of 0.5 or more is desirable as the desulfurization condition. The dehydrogenation behavior during desulfurization under vacuum is shown in Table 2. D ₁ / D ₂ is
It can be seen that 0.5 or more is preferable as the dehydrogenation condition.

The immersion pipe life and the immersion pipe repair frequency are D ₁ /
Table 3 shows the indexed values based on the case of D ₂ = 0.375. From Table 3, it can be seen that when D ₁ / D ₂ exceeds 0.8, the life of the immersion pipe and the repair frequency remarkably increase.

(Example 2) After the desulfurization in Example 1 and stirring the gas, 0.8 kg of Ca-Si was added per ton of molten steel, and Ca treatment was carried out. As a result, the obtained steel has the following NA
The CE condition was satisfied.

NACE 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 saturation 〃 flow rate: 100-200 cc / min.

Comparative Example 1 In this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625, which is almost the same as No. 4 in Table 1, but
The heating and desulfurization operations were performed simultaneously. The results are summarized in Table 4.

Comparative Example 2 In this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625, which is almost the same as No. 4 in Table 1, but
The heating was performed under atmospheric pressure, and then the pressure was reduced to perform desulfurization under a high vacuum of 100 Torr or less. The results are also summarized in Table 4. For reference, the results are also shown for an example in which only the vacuum temperature rise was performed.

(Comparative Example 3) Also in this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625 which is almost the same as No. 4 in Table 1,
The gas stirring under atmospheric pressure after desulfurization was omitted. The results are summarized in Table 4.

(Comparative Example 4) Also in this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625, which is almost the same as No. 4 in Table 1, but
Desulfurization was performed prior to heating. The results are summarized in Table 4.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

[0057]

[Table 4]

[0058]

As described above, by using the refining method for ultra-low sulfur steel according to the present invention, the desulfurization rate under vacuum can be stabilized at a high level and the lower oxide concentration of the entire ladle slag can be reduced. As a result, it was possible to reduce the sulfur concentration level obtained with ultra-low sulfur steel, and at the same time suppress wear of refractory materials. Further, by controlling the degree of vacuum during vacuum processing, low hydrogen steel can be refined at the same time.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of the installation situation of a ladle and a dip tube for carrying out the present invention.

Claims (2)

[Claims] 1. A cylindrical dip tube consisting of one leg is dipped in the molten steel contained in a ladle, and the dip tube is evacuated to suck the molten steel into the dip tube. In the refining method of ultra-low-sulfur steel, which is performed while injecting an inert gas from the lower part of the pot and performing a gas stirring process,
_The inner diameter of the immersion pipe is determined so that the ratio D ₁ / D _{2 of 1} to the ladle inner diameter D ₂ is 0.5 or more and 0.8 or less, and Al is added to the molten steel sucked in the immersion pipe all at once or in divided or continuous additions. In addition, the molten steel is heated by supplying an oxidizing gas to the molten steel, then the molten steel is desulfurized and refined, and then the inside of the immersion pipe is restored to atmospheric pressure, and the immersion depth of the immersion pipe is adjusted.
A refining method for ultra-low-sulfur steel, characterized in that the gas agitation treatment is continued by blowing the above-mentioned inert gas into the ladle from the lower part within 0.5 m. 2. A refining method for extremely low-sulfur steel, which comprises, after the desulfurization refining according to claim 1, restoring the pressure in the dip pipe to an atmospheric pressure state and then supplying the metal Ca-containing substance to the molten steel.