JP5343308B2 - Desulfurization method for molten steel - Google Patents

Description

  The present invention relates to a method for desulfurizing molten steel using an RH vacuum degassing apparatus. Specifically, the present invention is not intended for so-called ultra-low sulfur steel having a target S concentration of several ppm level, but has a target S concentration of several tens of ppm. The present invention relates to a method for desulfurizing molten steel to prevent the upper limit of the target S concentration in low-sulfur steel or higher level steel.

  As a secondary refining method for desulfurizing molten steel after it has been discharged from the converter, a ladle refining facility called LF (Ladle Furnace) capable of arc heating and slag refining is used, and a large amount of desulfurization flux is supplied to the molten steel. A method of stirring the molten steel while heating it (see Patent Document 1, for example), and using an RH vacuum degassing apparatus, the desulfurization flux is introduced into the molten steel in the vacuum degassing tank of the RH vacuum degassing apparatus. The desulfurization method is generally used (see, for example, Patent Document 2 and Patent Document 3). In any of these methods, the composition of the desulfurization flux and its input method are devised so as to be suitable for the production of ultra-low sulfur steel having a target S concentration represented by a line pipe material of several ppm level.

In the production of such ultra-low sulfur steel, even if the power cost increases due to arc heating in LF, a special desulfurization flux containing CaF ₂ or Ca is used for the desulfurization treatment. Even if the desulfurization flux cost increases, or due to the use of such a special desulfurization flux, the refractory in the ladle or the vacuum degassing tank is melted, thereby increasing the refractory cost. However, there is no problem because the steel price is traded according to the price.

  On the other hand, in low-sulfur steels with a target S concentration of several tens of ppm level and general ordinary steels of more than that, desulfurization treatment in the secondary refining process is especially achieved by reducing the S concentration in the hot metal pretreatment stage. It was made to become below the upper limit of target S concentration, without implementing. This is because the desulfurization treatment in secondary refining cannot compensate for the above-mentioned cost increase in low-sulfur steel and ordinary steel.

However, in recent years, in view of environmental regulations for the slag, and it has become a trend that does not use fluorite fluorine-containing substances to medium solvent in BOF decarburization refining, the demand for CO ₂ emissions reduction, scrap iron As a result of an increase in operations that use a large amount of steel in converters, the lower limit of the target S concentration has occurred in these low-sulfur steels and plain steels. This is due to the inevitable mixing of S from scrap iron with poor quality and, at the same time, the desulfurization ability of converter slag due to the fact that fluorite is no longer used as the solvent solvent in the converter. Together.
JP 2005-179762 A Japanese Patent Application Laid-Open No. 2003-342631 Japanese Patent Laid-Open No. 11-6209

  As described above, when the S content at the time of steel leaving the converter is out of the upper limit of the target S concentration in ordinary steel or low-sulfur steel, there is no effective means for relieving this. For example, if such a charge is desulfurized with LF, the manufacturing process up to the continuous casting process will be disturbed, which will hinder continuous casting in the continuous casting process. For this reason, the cost is remarkably increased and it is not economically viable.

  On the other hand, in the present situation where most of ordinary steel and low-sulfurized steel are processed with an RH vacuum degassing apparatus for the purpose of reducing inclusions and adjusting alloy components, desulfurization during RH degassing is not possible. There is no worry of disturbing the manufacturing process. However, in the RH vacuum degassing apparatus, using a desulfurization flux having a high desulfurization capability such as Patent Document 2 and Patent Document 3 causes a problem of increasing the desulfurization flux cost and the refractory cost.

  In addition, if the amount of relief of the target S concentration of ordinary steel or low-sulfur steel as described above is remedied, it can be sufficiently dealt with by reducing the amount of flux used for desulfurization. When the desulfurization treatment is performed with the flux for use, a part of the desulfurization flux may adhere to the side wall of the vacuum degassing tank or to the ladle wall, which may not contribute to the desulfurization reaction. Variations occur. As a result, a case where the target S concentration exceeds the upper limit cannot be solved.

  The present invention has been made in view of the above circumstances, and the object of the present invention is when the S content of ordinary steel or low-sulfurized steel deviates from the upper limit of the target S concentration at the time of converter steelmaking, or the target If the upper limit of the S concentration is just below the limit of the target S concentration by resulfurization, the manufacturing process up to the continuous casting process is not disturbed, and the increase in manufacturing cost is suppressed and stable. The present invention is to provide an effective method for desulfurizing molten steel that can reduce the S content below the target upper limit value.

In order to solve the above-mentioned problems, the present invention takes out molten steel obtained by decarburizing and refining hot metal from a converter to a ladle, and performs reduction treatment of slag present on the bath surface of molten steel in the ladle. Then, when secondary refining of molten steel with RH vacuum degassing equipment, Al was introduced into the molten steel with RH vacuum degassing equipment, the molten steel was deoxidized, and then the upper blow lance provided in the RH vacuum degassing equipment The desulfurization flux is heated and melted by spraying desulfurization flux onto the molten steel bath surface in the vacuum degassing tank, and the desulfurization flux heats and melts the mixture of CaO powder and Al ₂ O ₃ powder. And obtained by pulverizing after solidifying , 48 to 58 mass% of CaO, 42 to 52 mass% of Al ₂ O _3, and a premelt flux for desulfurization that does not contain CaF ₂ , A method for desulfurization of molten steel is provided. At that time, it is preferable to put MgO into the molten steel in the vacuum degassing tank before spraying the desulfurization premelt flux from the top blowing lance.

  According to the present invention, when the S content of ordinary steel or low-sulfurized steel deviates from the upper limit of the target S concentration at the time of converter steelmaking, or just below the upper limit of the target S concentration, When there is a risk of exceeding the upper limit, desulfurization is performed using RH vacuum degassing equipment that can treat most of ordinary steel and low-sulfur steel, so that desulfurization should be performed without disturbing the manufacturing process. Can do.

Further, according to the present invention, as the desulfurization flux, a premelt flux containing 48 to 58 mass% of CaO, 42 to 52 mass% of Al ₂ O ₃ and not containing CaF ₂ is used, that is, CaF ₂ ( Since fluorite is not used, the desulfurization flux cost is low, and there is no fear of melting the refractory in the vacuum degassing tank. In addition, since it is a pre-melt flux, it is harder than quick lime, it is difficult to pulverize during transportation, and when it is blown into the vacuum degassing tank through the top blowing lance, there is little carry over to the vacuum exhaust system, Not only can it penetrate into molten steel without waste, but once it penetrates into molten steel, it melts quickly and absorbs the deoxidation product of Al introduced in the RH vacuum degasser for deoxidation of molten steel, making it ideal for desulfurization A molten phase of CaO · Al ₂ O ₃ (CaO corresponds to 34 mass%, Al ₂ O ₃ corresponds to 66 mass%) is generated, and this quickly desulfurizes the molten steel.

Furthermore, since it is a pre-melt flux, the latent heat of melting is small, and the temperature drop of molten steel is small compared to a flux obtained by simply mixing CaO and Al ₂ O ₃ or a sintered flux. And the temperature drop of the molten steel is small, because it is inexpensive because it does not contain that and CaF ₂ and metal Ca there is no fear of the refractory erosion for not containing CaF _2, a relatively large amount of desulfurization Therefore, an excellent effect that the desulfurization reaction efficiency is stabilized can be obtained.

  In addition, MgO is introduced into the molten steel in the vacuum degassing tank before the desulfurization premelt flux is sprayed toward the molten steel, so that this MgO is taken along with the molten steel flow from the downcomer of the RH vacuum degasser. The molten steel enters the molten steel in the pan, and floats to form a high melting point barrier layer between the molten steel and the slag present on the bath surface of the molten steel in the ladle. This barrier layer can prevent reoxidation of the molten steel due to oxidizing components such as FeO and MnO in the slag present in the ladle, can maintain an atmosphere with a low oxygen potential suitable for desulfurization, and can also be resulfurized. Can be prevented.

  Hereinafter, preferred embodiments for concretely implementing the present invention will be described.

  In the present invention, when producing normal steel or low-sulfur steel by decarburizing and refining hot metal in a converter, the S content of the molten steel component at the end of the converter decarburization retreats from the upper limit of the target S concentration. In the case where the upper limit of the target S concentration is exceeded, and there is a possibility that the upper limit of the target S concentration may be deviated depending on the resulfurization, the molten steel is desulfurized as follows, and the S content of the molten steel is set to the target S. Decrease to below the upper limit of concentration.

  That is, when the S concentration of the molten steel component analysis value at the end of converter decarburization retreats or is likely to deviate from the upper limit of the target S concentration, the converter performs a special treatment on the molten steel. The resulting molten steel is discharged from the converter to the ladle as planned, and a strong deoxidizer such as Al is added to the slag that flows out of the converter into the ladle at the end or after the start of the steel discharge. Add slag to reduce the slag, then transport the molten steel to the RH vacuum degassing device, put Al into the molten steel during RH degassing treatment, deoxidize the molten steel, and then install it in the vacuum degassing tank The desulfurization flux is sprayed from the top blowing lance toward the molten steel bath surface in the vacuum degassing tank to desulfurize the molten steel.

  Here, the low sulfur steel is a steel having a target S concentration of several tens of ppm, and the ordinary steel is a target S concentration equal to or higher than the target S concentration of the low sulfur steel, and the upper limit is about 0.03 mass%. About steel. However, even in ordinary steel, the target value of the S concentration varies depending on the specifications, and the upper limit value is various, such as 0.010 mass% and 0.015 mass%.

  The hot metal used in converter decarburization refining is not particularly limited, but is preferably one in which S and P are reduced by hot metal pretreatment. In particular, if S is reduced by the hot metal desulfurization treatment, even if the S content of the molten steel obtained by converter decarburization refining exceeds the target upper limit, the excess amount is small, and desulfurization by the RH vacuum degassing device This is because the load can be reduced. There are no particular restrictions on the type of converter that performs decarburization and refining, and there are no restrictions on the top blowing converter, the top bottom blowing converter with the inert gas bottom blowing stirring method, and both the top blowing lance and bottom blowing tuyere are used for molten steel. Either an oxygen top-bottom converter or a bottom-blowing converter for supplying oxygen may be used.

  The steels targeted by the present invention are low-sulfur steel whose upper limit of the target S concentration is several tens of ppm, and ordinary steel that can tolerate even higher S concentrations. An extremely low sulfur steel having an upper limit of the target S concentration of several ppm is not preferable because it is often difficult to sufficiently desulfurize in the present invention.

In the present invention, a strong deoxidizer such as Al is reduced to the slag on the surface of the molten steel that has flowed into the ladle when steel is discharged from the converter in order to reliably perform desulfurization in the RH vacuum degassing apparatus. It is necessary to add slag and reduce slag. As the slag reducing agent, Al ash (also referred to as “aluminum dross”) is preferable because of its low cost. Al ash is a mixture of metal Al containing 30 to 50 mass% of metal Al and Al ₂ O _3, and also contains other components.

  As a standard of slag reduction, it is desirable that the total content of FeO and MnO in the slag is about 5 mass% or less. If the total content of FeO and MnO in the slag is greater than 5 mass%, reoxidation of the molten steel by the slag occurs during or after the desulfurization treatment, thereby inhibiting the desulfurization reaction or causing resulfurization. Cause. On the other hand, to reduce to less than 2 mass%, a large amount of reducing agent must be used, which is not economical. When the molten steel is removed from the converter, the molten steel may be roughly deoxidized with Si, Mn, Al, etc., as in normal refining.

In the RH vacuum degassing apparatus, during the RH degassing treatment of molten steel, Al is introduced into the molten steel in the vacuum degassing tank to deoxidize the molten steel, and then the vacuum degassing is performed from an upper blow lance provided in the vacuum degassing tank. Add desulfurization flux by spraying toward the molten steel bath surface in the gas tank. Here, the amount of Al input into the molten steel depends on the target component composition (varies depending on the steel type) of each steel type, and thus cannot be unconditionally determined. However, in a normal aluminum killed steel, the total Al is 0.050. Since .about.0.100 mass% is often the target composition, the molten steel components before the RH degassing treatment are analyzed, and the deficiency may be added accordingly. The total.Al is the total value of Al dissolved in the molten steel and Al existing in the molten steel in the form of oxides such as Al ₂ O ₃ , Adding is also called “projection”.

  The top blow lance for projection is known to be inserted into the vacuum degassing tank vertically from above or obliquely inserted from the side wall of the vacuum degassing tank. From the viewpoint of reducing the carry-over of the flux, it is preferably inserted vertically.

As a flux for desulfurization, a mixture of CaO powder and Al ₂ O ₃ powder is heated, melted, solidified and then pulverized, and contains 48 to 58 mass% CaO and 42 to 52 mass% Al ₂ O ₃ And a premelt flux not containing CaF ₂ is used. If a simple mixture of CaO powder and Al ₂ O ₃ powder is used as the desulfurization flux, pulverization (particularly CaO pulverization) is likely to occur during the conveyance of the flux. It is easy to be sucked into the exhaust system of the vacuum degassing tank before it reaches (so-called “carry over”), and the dissolution after entering the molten steel is slow and desulfurization does not proceed rapidly, but CaO and Al ₂ O All of these problems can be solved by premelting ₃ . The desulfurization flux does not contain CaF ₂ , but this is necessary to prevent the refractory from being melted as much as possible and to reduce the desulfurization flux cost.

48～58Mass% of CaO in the desulfurization flux, reason why the Al ₂ O ₃ and 42～52Mass%, when desulfurizing flux into the molten steel invades, Al ₂ O in the molten steel is deoxidized product _This is because a liquid phase having a CaO · Al ₂ O ₃ composition optimum for desulfurization by absorbing ₃ is easily generated. Or CaO is less than 48Mass%, when Al ₂ O ₃ is greater than 52Mass% are poor desulfurization performance is not preferable. Also, if CaO is greater than 58mass%, Al ₂ when O ₃ is less than 42Mass% has a high melting point of the desulfurization flux, and desulfurization delay does not dissolve quickly be penetrated in the molten steel, It is not preferable.

The particle size of the premelt flux is preferably 90% or more in terms of mass ratio with a particle size of less than 1 mm, desirably less than 150 μm, from the viewpoint of reaction efficiency. On the other hand, from the viewpoint of reducing carryover, it is desirable that the amount of fine powder is small. Therefore, the particle size of less than 10 μm is preferably less than 10% by mass, and the particle size of less than 50 μm is less than 10%. preferable. In the desulfurization flux, SiO ₂ up to 5 mass% is acceptable as an impurity. If there is more SiO ₂ than this, the desulfurization ability decreases, which is not preferable.

  Furthermore, in order to maintain the oxygen potential of the molten steel at the time of desulfurization at a low level, and to effectively prevent desulfurization after desulfurization, before adding the premelt flux for desulfurization by spraying with a RH vacuum degasser, a vacuum is applied. It is preferable to put MgO into the molten steel in the degassing tank. As a method of charging MgO, either a method of charging from an auxiliary material charging chute provided at the upper part of the vacuum degassing tank or a method of projecting from an upper blowing lance that projects a desulfurization flux may be used. The former method is preferably employed from the viewpoint that carry-over is reduced and a massive magnesia clinker can be used.

  This MgO is accompanied by the molten steel flow from the downcomer of the RH vacuum degassing device, enters the molten steel in the ladle, floats in the molten steel, and between the slag present on the bath surface of the molten steel in the ladle and the molten steel A barrier layer having a high melting point is formed. This barrier layer can prevent reoxidation of the molten steel due to oxidizing components such as FeO and MnO in the ladle slag, can maintain a low oxygen potential atmosphere suitable for desulfurization, and can also prevent resulfurization. In addition to magnesia clinker, magnesia-based refractory waste can be used as the MgO source. A preferable input amount of MgO for exhibiting the above effect is 1 kg (hereinafter referred to as “kg / t”) or more, more preferably 1.5 kg / t or more per ton of molten steel. However, since a temperature drop of the molten steel is caused when a large amount is added, the upper limit is preferably 5 kg / t, more preferably 3 kg / t.

  According to the present invention configured as described above, the upper limit of the target S concentration is deviated during converter steelmaking, or the upper limit of the target S concentration is deviated from the upper limit of the target S concentration. It is possible to effectively desulfurize ordinary steel and low-sulfur steel with fear. In addition, in recent years, most of ordinary steel and low-sulfur steel have been treated with RH vacuum degassing equipment for the purpose of reducing inclusions and adjusting alloy components, so desulfurization treatment has been added to RH degassing refining. Even if it implements, there is no worry that the manufacturing process will be disturbed.

[Invention Example 1]
The present invention was applied to the refining of low-carbon aluminum killed low-sulfur steel with an upper limit of the target S concentration of 0.0024 mass%. In producing the low-sulfur steel, the hot metal used was desulfurized hot metal, but it was difficult to stably achieve the target S concentration judging from the S content of the hot metal. The present invention was applied.

  The desulfurized hot metal was decarburized and refined in a converter to obtain about 300 tons of molten steel, and the molten steel was taken out into a ladle. The S content of the molten steel in the ladle was 0.0026 mass%. Aluminum ash (metal Al content: 30 mass%) is added to the slag in the ladle to reduce the slag, and the total content of FeO and MnO in the slag is 2.8 mass%. It was conveyed to the gas device.

After starting the recirculation of the molten steel with the RH vacuum degassing apparatus, Al was introduced into the molten steel in the vacuum degassing tank for deoxidation treatment. After the deoxidation treatment, a premelt flux for desulfurization (particle size: 10 to 250 μm) of CaO: 51 mass%, Al ₂ O ₃ : 45 mass%, SiO ₂ : 1 mass%, Ar gas as a carrier gas, and vacuum degassing tank It projected toward the molten steel bath surface in a vacuum degassing tank through the upper blowing lance inserted from the upper part. The pressure in the vacuum degassing tank at the time of projection is 2.6 to 3.9 kPa (20 to 30 torr), the projection speed is about 80 kg / min (0.27 kg / min · t), and the carrier gas flow rate is 8 Nm ³ / Projection was performed for 10 minutes as min (0.027 Nm ³ / min · t). The projected premelt flux for desulfurization was 750 kg (2.5 kg / t). The S content in the molten steel after the desulfurization treatment was 0.0019 mass%, and the desulfurization rate was 26.9%.

  After the desulfurization treatment, an alloy material necessary for the molten steel was added and final adjustment of the molten steel components was performed, and then the RH degassing treatment was finished. Next, the molten steel was conveyed to a continuous casting facility and continuously cast, and a representative molten steel sample was collected from the tundish during continuous casting. As a result of analyzing the S concentration of this representative molten steel sample, the S concentration was 0.0021 mass%, which satisfied the upper limit of the target S concentration.

  The same treatment as above was performed for 9 charges of low-carbon aluminum killed low-sulfur steel with an upper limit of the target S concentration of 0.0024 mass%. As a result, the average basic unit of the premelt flux for desulfurization was 2.2 kg / t, the average desulfurization rate was 26.1%, and the variation in desulfurization rate was ± 6.5%, and the S content was adjusted in a narrow range. We were able to.

[Invention Example 2]
In the same manner as in Example 1 of the present invention, the present invention was applied to refining a low-carbon aluminum killed low-sulfur steel having an upper limit of the target S concentration of 0.0024 mass%. In producing the low-sulfur steel, the hot metal used was desulfurized hot metal, but it was difficult to stably achieve the target S concentration judging from the S content of the hot metal. The present invention was applied.

  The desulfurized hot metal was decarburized and refined in a converter to obtain about 300 tons of molten steel, and the molten steel was taken out into a ladle. The S content of the molten steel in the ladle was 0.0027 mass%. Aluminum ash (metal Al content: 30 mass%) is added to the slag in the ladle to reduce the slag, and the total content of FeO and MnO in the slag is 3.0 mass%. It was conveyed to the gas device.

After starting the recirculation of the molten steel with the RH vacuum degassing apparatus, Al was introduced into the molten steel in the vacuum degassing tank for deoxidation treatment. When 2 minutes had elapsed from the introduction of Al, 540 kg (1.8 kg / t) of magnesia clinker was added onto the molten steel bath surface in the vacuum degassing tank from the auxiliary material charging chute located above the vacuum degassing tank. Then after two minutes, after which magnesia clinker to the molten steel bath surface on the vacuum degassing vessel is no longer _{present, CaO: 51mass%, Al 2} O 3: 45mass%, SiO 2: 1mass% of desulfurization premelt flux (A particle size of 10 to 250 μm) was projected toward the molten steel bath surface in the vacuum degassing tank through an upper blowing lance inserted from above the vacuum degassing tank using Ar gas as a carrier gas. The pressure in the vacuum degassing tank at the time of projection is 2.6 to 3.9 kPa (20 to 30 torr), the projection speed is about 75 kg / min (0.25 kg / min · t), and the carrier gas flow rate is 7.5 Nm. ³ was projected /min(0.025Nm ³ / min · t) as a 10 min. The projected pre-melt flux for desulfurization was 720 kg (2.4 kg / t). The S content in the molten steel after the desulfurization treatment was 0.0020 mass%, and the desulfurization rate was 25.9%.

  After the desulfurization treatment, an alloy material necessary for the molten steel was added and final adjustment of the molten steel components was performed, and then the RH degassing treatment was finished. Next, the molten steel was conveyed to a continuous casting facility and continuously cast, and a representative molten steel sample was collected from the tundish during continuous casting. As a result of analyzing the S concentration of this representative molten steel sample, the S concentration was 0.0020 mass%, which satisfied the upper limit of the target S concentration.

  The same treatment as above was performed for 9 charges of low-carbon aluminum killed low-sulfur steel with an upper limit of the target S concentration of 0.0024 mass%. As a result, the average unit of premelt flux for desulfurization was 2.2 kg / t, the average desulfurization rate was 25.9%, and the variation in desulfurization rate was ± 2.5%, and the S content was adjusted within a narrow range. We were able to.

[Comparative example]
As in Examples 1 and 2 of the present invention, the hot metal used in producing the low-sulfur steel of low-carbon aluminum killed with an upper limit of the target S concentration of 0.0024 mass% is the hot metal subjected to desulfurization treatment. Since it was difficult to stably achieve the target S concentration as judged from the S content of desulfurization, desulfurization treatment was performed by an RH vacuum degassing apparatus using a conventional desulfurization flux containing CaF _2. I decided to do it.

  The desulfurized hot metal was decarburized and refined in a converter to obtain about 300 tons of molten steel, and the molten steel was taken out into a ladle. The S content of the molten steel in the ladle was 0.0026 mass%. Aluminum ash (metal Al content: 30 mass%) is added to the slag in the ladle to reduce the slag to a total content of FeO and MnO in the slag of 2.9 mass%. It was conveyed to the gas device.

After starting the recirculation of the molten steel with the RH vacuum degassing apparatus, Al was introduced into the molten steel in the vacuum degassing tank for deoxidation treatment. When 2 minutes had elapsed since the introduction of Al, 550 kg (1.83 kg / t) of magnesia clinker was added onto the molten steel bath surface in the vacuum degassing tank from the auxiliary material charging chute located above the vacuum degassing tank. Then after two minutes, after which magnesia clinker is not on the molten steel bath surface in the vacuum degassing _{vessel, CaO: 57mass%, CaF 2} : 39mass%, SiO 2: 2mass% of desulfurization premelt flux (grain 10 to 250 μm in diameter) was projected toward the molten steel bath surface in the vacuum degassing tank through an upper blowing lance inserted from above the vacuum degassing tank using Ar gas as a carrier gas. The pressure in the vacuum degassing tank at the time of projection is 2.6 to 3.9 kPa (20 to 30 torr), the projection speed is about 75 kg / min (0.25 kg / min · t), and the carrier gas flow rate is 7.5 Nm. ^The projection was performed for 7 minutes at ³ / min (0.025 Nm ³ / min · t). The projected premelt flux for desulfurization was 540 kg (1.8 kg / t). The S content in the molten steel after the desulfurization treatment was 0.0019 mass%, and the desulfurization rate was 26.9%.

  After the desulfurization treatment, an alloy material necessary for the molten steel was added and final adjustment of the molten steel components was performed, and then the RH degassing treatment was finished. Next, the molten steel was conveyed to a continuous casting facility and continuously cast, and a representative molten steel sample was collected from the tundish during continuous casting. As a result of analyzing the S concentration of this representative molten steel sample, the S concentration was 0.0020 mass%, which satisfied the upper limit of the target S concentration.

  The same treatment as described above was performed for 15 charges of low-sulfur aluminum-killed low-sulfur steel whose upper limit of the target S concentration was 0.0024 mass%. As a result, the average basic unit of the premelt flux for desulfurization was 1.5 kg / t and the average desulfurization rate was 25.5%, but the variation in the desulfurization rate was ± 15.1%. The content varied greatly.

Claims (3)

The molten steel obtained by decarburizing and refining the hot metal is discharged from the converter to the ladle, and after reducing the slag present on the bath surface of the molten steel in the ladle, the molten steel is removed with an RH vacuum degasser. In the next refining, after dehydrating the molten steel by introducing Al into the molten steel with an RH vacuum degassing device , the mixture of CaO powder and Al ₂ O ₃ powder is heated and melted and solidified as a desulfurization flux. After using pulverization treatment, 48 to 58 mass% of CaO, 42 to 52 mass% of Al ₂ O ₃ , and using only a pre-melt flux for desulfurization that does not contain CaF ₂ are provided in the RH vacuum degassing apparatus. over lance it was, toward the molten steel bath surface in the vacuum degassing vessel, wherein characterized the desulfurization pre-melt flux to desulfurize the molten steel by blowing Turkey, desulfurization process of the molten steel.   2. The desulfurization method for molten steel according to claim 1, wherein MgO is introduced into the molten steel in a vacuum degassing tank before the premelt flux for desulfurization is sprayed from the upper blowing lance. The particle size of the pre-melt flux for desulfurization is 90% or more when the particle size is less than 1 mm, and less than 10% when the particle size is less than 10 μm. Item 3. A method for desulfurizing molten steel according to Item 2.