Method for desulfurizing molten steel

Abstract

To provide a desulfurization method of molten metal with excellent desulfurization ability.SOLUTION: In a desulfurization method of molten metal using an electric furnace, the electric furnace is equipped with a first electrode 1 in contact with the molten metal 3 and a second electrode 2 above a slag 4 on the molten metal 3. The second electrode 2 is apart from the slag 4 on a static bath surface 5. The liquid phase fraction of the slag 4 at 2000°C is 80% or more. In the desulfurization method of molten metal, a direct current is applied with the first electrode 1 as the negative electrode and the second electrode 2 as the positive electrode, with the surface of the molten metal covered with the slag 4 in an arc spot circle 6 whose center is the point where the vertical line passing through the central axis of the second electrode 2 intersects the surface of the molten metal and whose radius R is determined using the radius r of the electrode and a slag-second electrode distance d.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for desulfurizing molten metal, and more particularly to a method for desulfurizing molten metal using an electric furnace. [Background technology]

[0002] In general, in refining using an electric furnace, after melting the raw metal, molten slag for desulfurization is formed on top of the molten metal, and the S in the molten metal, for example, molten steel, is transferred to the slag for desulfurization, a process known as slag desulfurization. However, in conventional electric furnaces, there was a limit to how much the slag removal rate could be increased with normal slag desulfurization, and there was an issue of resulfurization from the slag remaining in the furnace.

[0003] In a DC electric furnace having an upper electrode and a bottom electrode in contact with the molten metal, the upper electrode is usually energized as a negative electrode and the bottom electrode as a positive electrode. This type of polarity (hereinafter referred to as positive polarity energization) is believed to provide good thermal efficiency when thermoelectrons strike the molten metal (slag) surface and good arc stability.

[0004] Patent Document 1 discloses a DC electric furnace steelmaking method, which is characterized in that, when producing molten steel by melting an iron-containing cold material using a DC electric furnace having an upper electrode and a bottom electrode, in a first step the iron-containing cold material is melted, and then in a second step the molten steel is desulfurized by using the upper electrode as the positive electrode and the bottom electrode as the negative electrode. By making the upper electrode the positive electrode (reverse polarity current) in the second step, it is said that the transfer of S to slag is significantly promoted, and also the vaporization desulfurization (desulfurization in which S in the liquid phase is oxidized by oxygen in the atmosphere and transferred to the gas phase) is significantly promoted.

[0005] Patent Document 2 discloses a method for desulfurizing and refining molten iron in a refining vessel, which is characterized by introducing plasma gas into a plasma torch, directly irradiating the surface of the molten iron with a plasma arc so that the oxygen concentration in the plasma airflow is 1% by volume or more and 100% by volume or less, and directly carrying out desulfurization by oxidation and vaporization from the molten iron with oxygen dissociated by the plasma arc. This utilizes the direct oxidation and vaporization desulfurization reaction from the metal caused by the oxygen in the plasma airflow.

[0006] Patent Document 3 discloses a method for desulfurizing molten iron, which comprises the steps of: adding a desulfurizing agent to desulfurize molten iron in a first step; leaving all or part of the desulfurization slag produced in the first step that covers the surface of the molten iron in a second step; and irradiating the top surface of the slag with hydrogen gas or argon gas containing 1% or more by volume of hydrogen gas as a plasma gas. When a gas containing hydrogen is blown onto the sulfur-containing slag after the desulfurization treatment, the oxygen partial pressure of the atmosphere is reduced, which reduces the oxygen activity of the molten iron, promoting the flux desulfurization reaction and at the same time providing an extremely high vaporization desulfurization capacity.

[0007] Patent Document 4 discloses a method for desulfurizing molten metal using a DC power source, in which an electrode in contact with the molten metal is a negative electrode and an electrode in contact only with the molten slag is a positive electrode, and a potential difference is applied between the molten slag and the molten metal through the electrodes, characterized in that the potential difference between the electrodes is changed according to the S concentration in the molten metal so that the potential difference is larger during periods when the S concentration is relatively low than during periods when the S concentration is relatively high. When a potential difference is applied between the molten slag and molten metal so that the potential on the molten metal side is lower than that on the slag side, the application of an electrochemical potential changes the equilibrium constant of the desulfurization reaction, and the equilibrium S concentration decreases, so that the desulfurization reaction between the slag and metal progresses. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 11-293327 [Patent Document 2] JP 2011-140684 A [Patent Document 3] JP 2009-249667 A [Patent Document 4] JP 2020-180319 A Summary of the Invention [Problem to be solved by the invention]

[0009] The methods described in Patent Documents 2 and 3 require the preparation of plasma gas, which increases the equipment and processing costs. The use of hydrogen or argon poses the problem of gas costs. The method described in Patent Document 4 advances the desulfurization reaction between the slag and metal by applying a potential difference between the molten slag and the molten metal so that the potential on the molten metal side is lower than that on the slag side, but does not use vaporization desulfurization from the slag.

[0010] Patent Document 1 discloses a method in which the transfer of S to slag is significantly promoted and vaporization desulfurization (S in the liquid phase is oxidized by oxygen in the atmosphere and transferred to the gas phase) is significantly promoted by making the upper electrode a positive electrode (reverse polarity current passing). On the other hand, there is a demand for further improvement in desulfurization capacity. An object of the present invention is to provide a method for desulfurizing molten metal having excellent desulfurization capacity. [Means for solving the problem]

[0011] That is, the gist of the present invention is as follows. [1] In a method for desulfurization of molten metal using an electric furnace, The electric furnace includes a first electrode in contact with the molten metal and a second electrode above the slag on the molten metal, and the second electrode and the slag are spaced apart from each other at a stationary bath surface; The liquid phase rate of the slag at 2000°C is 80% or more, In a state where the molten metal surface within a circle having a radius R calculated by the formula (1) and a center point where a vertical line passing through the central axis of the second electrode intersects with the molten metal surface is covered with the slag, Applying a direct current to the first electrode as a negative electrode and the second electrode as a positive electrode. A method for desulfurizing molten metal, comprising: R = r + d × tan 30° (1) Here, r is the radius of the electrode, and d is the distance between the slug and the second electrode. [2] The method for desulfurizing molten metal according to [1], characterized in that the distance between the second electrode and the slag surface is 50 mm or more and 700 mm or less. [3] The method for desulfurizing molten metal according to [1] or [2], characterized in that the sulfur concentration (% by mass) in the slag is at least 1 time the sulfur concentration (% by mass) in the molten metal. [4] The method for desulfurizing molten metal according to any one of [1] to [3], wherein the liquid phase rate of the slag is 80% or more at the temperature of the molten metal. [5] The method for desulfurizing molten metal according to any one of [1] to [4], wherein a ratio of the direct current to the mass of the molten metal in the furnace is 0.6 kA / t or more. Effect of the Invention

[0012] The effect of vaporization desulfurization has eliminated the issue of resulfurization from the slag. Because vaporization is promoted by passing electricity, there is no longer a need to spray gases such as oxygen or hydrogen, and the associated issues have also been eliminated. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing the relationship between electrodes, molten metal, and slag in an electric furnace. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] <Electric furnace> The electric furnaces covered by the present invention are assumed to be DC arc electric furnaces and DC LF (ladle furnaces).

[0015] <Reverse polarity current, open arc> A DC electric furnace will be described with reference to FIG. 1. Hereinafter, the electrode in contact with the molten metal 3 will be referred to as the first electrode 1, and the electrode disposed above the slag 4 on the molten metal 3 will be referred to as the second electrode 2. The second electrode may have a tip immersed in the slag or a tip spaced apart from the slag. In the present invention, as described later, the second electrode 2 and the slag 4 are spaced apart from each other on the stationary bath surface. The number of second electrodes 2 in the electric furnace may be one as shown in FIG. 1, or two or more second electrodes 2.

[0016] In a DC electric furnace having a second electrode 2 (upper electrode) and a first electrode 1 (furnace bottom electrode) in contact with the molten metal, the second electrode 2 is usually used as the negative electrode and the first electrode 1 is used as the positive electrode (positive polarity current). In contrast, in the desulfurization method of the present invention, the first electrode 1 in contact with the molten metal 3 is used as the negative electrode and the second electrode 2 above the slag 4 is used as the positive electrode (reverse polarity current), which can promote the desulfurization reaction between the molten metal 3 and the slag 4 (see formula (2) below) and the vaporization desulfurization reaction between the slag and the gas phase (see formulas (3) and (4) below). In particular, the vaporization desulfurization reaction is remarkable when current is applied in a so-called open arc, in which the second electrode 2 and the slag 4 are separated from each other. In the present invention, the second electrode 2 and the slag 4 are separated from each other at the stationary bath surface 5 (see FIG. 1). S(metal)+2e - →S 2- (slag) (Desulfurization reaction between molten metal and slag) (2) S 2- (slag) → S2(gas) + 2e - (Slag-gas phase vaporization desulfurization reaction 1) (3) S 2- (slag)+2O 2- (slag) → SO2(gas) + 6e - (Slag-gas phase vaporization desulfurization reaction 2) (4)

[0017] <Slag coverage of arc spot - liquid phase ratio> As described above, desulfurization can be promoted by using the second electrode 2 above the slag 4 as the positive electrode (reverse polarity current application). Furthermore, in order to obtain the desulfurization promotion effect by current application, the arc spot needs to be covered with liquid slag.

[0018] In the present invention, the composition of the slag 4 formed on the molten metal 3 generally contains CaO-SiO2-Al2O3, and further contains MgO, MnO, FeO, and CaF2.

[0019] In desulfurization refining, the higher the CaO content, the better from the viewpoint of increasing the sulfide capacity of the slag. On the other hand, when the CaO content is high, the melting point of the slag is high, and sufficient molten slag is not generated even at the arc spot. In particular, when trying to obtain an electrochemical desulfurization promotion effect as in the present invention, it was found that if there is insufficient molten slag, the expected effect cannot be obtained and vaporization desulfurization does not progress.

[0020] The slag temperature at the arc spot is approximately 2000°C. The liquid phase ratio of the slag at 2000°C was used as an index representing the melting state of the slag at the arc spot. The liquid phase ratio of the slag at 2000°C can be obtained by experimental methods such as differential thermal analysis, or by numerical calculations using thermodynamic calculation software.

[0021] The slag composition was changed in various ways, and the liquid phase ratio at 2000°C for each slag composition was determined. As a result of experimental investigation, it was found that the liquid phase ratio at 2000°C must be 80% or more in order to obtain the desulfurization promotion effect by applying current. In the experimental evaluation, the liquid phase ratio was calculated using the thermodynamic calculation software FactSage (registered trademark) 7.3. Therefore, in the present invention, it is specified that the liquid phase rate of the slag 4 at 2000° C. is 80% or more.

[0022] The liquid phase ratio at 2000℃ can be adjusted by the slag components. In the case of steel refining, this is done by adding a CaO source, MgO source, SiO2 source, Al2O3 source, and adjusting the FeO concentration. Additives that can be used include quicklime, slaked lime, dolomite, bauxite, waste bricks, and silica stone. FeO is produced by spraying oxygen gas. FeO can be reduced by the reaction between the slag and the [C] in the molten iron, or by adding a reducing agent to the slag. Examples of reducing agents include carbon sources such as coal powder, metal Si sources such as FeSi, and metal Al sources such as Al dross. In terms of slag composition (mass ratio), CaO / SiO2 is called C / S, and CaO / Al2O3 is called C / A. In general, within the range of C / S≧1, the lower the C / S, the higher the liquid phase ratio at 2000℃. In addition, in the range of C / A≧0.6, the lower the C / A, the higher the liquid phase ratio at 2000°C or the higher the liquid phase ratio can be maintained at 2000°C. In addition, the liquid phase ratio at 2000°C can be increased by increasing the FeO concentration or CaF2 concentration. For example, in a slag having a mass ratio of CaO=70.1%, SiO2=9.4%, Al2O3=14.7%, MgO=3.5%, MnO=1.0%, FeO=1.2%, and CaF2=0.1%, the liquid phase ratio at 2000°C calculated using the thermodynamic calculation software FactSage (registered trademark) 7.3 was 80%, and a liquid phase ratio of 80% or more was achieved.

[0023] <Area covered with slag around the arc spot> As described above, in order to obtain the desulfurization promotion effect by energizing, the arc spot formed by the arc heating by the second electrode 2 needs to be covered with liquid slag.

[0024] The horizontal reference position of the second electrode 2 is the central axis of the cylindrical electrode. It is said that the spread angle of the DC arc occurs in the range of 15-30° from the electrode. Therefore, as shown in Figure 1, the "electrode radius" = r, the "distance between the slag and the second electrode" = d, R = r + d × tan 30° (1) The relationship between the state of slag covering a circle (hereinafter referred to as "arc spot circle 6") with radius R and the promotion state of vaporization desulfurization was investigated. As a result, it was clarified that electrochemical vaporization desulfurization is promoted when the arc spot circle 6 is covered with molten slag. The slag coverage state of the arc spot circle 6 can be confirmed, for example, by taking a thermographic image using the difference in emissivity between molten iron and slag. Here, "slag-second electrode distance" = d means the distance between the second electrode and the slag at the stationary bath surface. The distance d can be evaluated as follows.

[0025] The electrode height is controlled by measuring the slag surface position and then adjusting the electrode tip position. The slag surface position can be measured optically using microwaves or obtained from images from a furnace monitoring camera. The electrode tip position can be calculated based on the position of the electrode arm when it passes through the electrode hole in the furnace cover.

[0026] In proceeding with electrochemical desulfurization by vaporization, it is sufficient to at least cover the arc generation point (arc spot circle 6) on the molten metal surface side with molten slag, and no adverse effects are observed even if bare molten metal occurs at a position away from the electrode.

[0027] Based on the above results, in the present invention, the molten metal is desulfurized in a state where the molten metal surface is covered with slag within a circle having a radius R calculated by formula (1) and centered at the point where a vertical line passing through the central axis of the second electrode 2 intersects with the molten metal surface. In order to cover the arc spot circle with molten slag, it is necessary to generate liquid slag by controlling the slag composition and to select flow conditions that prevent the liquid slag from being removed from the arc spot circle. Specifically, when gas is injected above the slag or into the molten metal for the purpose of stirring or refining, the slag may be removed by the gas flow. As a countermeasure, it is possible to select an injection position where the injected gas does not come into contact with the arc spot circle, or to set an upper limit on the gas flow rate according to the slag thickness so that the molten metal surface is not exposed even if the gas comes into contact with the arc spot circle.

[0028] <Electrode height> In the present invention, the effect of the present invention can be achieved as long as there is a distance between the second electrode 2 and the slag 4 on the stationary bath surface. Furthermore, by appropriately setting the electrode height, the desulfurization treatment of the present invention can be carried out more effectively. It is preferable to set the lower limit of the electrode-slag distance d to 50 mm. If the electrode-slag distance d is small, the arc spread becomes small and the reaction area where vaporization desulfurization occurs becomes small, which adversely affects the desulfurization rate when the S concentration in the slag is low.

[0029] On the other hand, if the electrode-slag distance d is large, the arc length becomes too long, and the heat load on the furnace wall, etc. due to radiation from the arc becomes high, making it impossible to perform desulfurization for a long period of time and ensuring a sufficient desulfurization width. It is preferable to set the upper limit of the electrode-slag distance d to 700 mm.

[0030] <Ratio of S in slag (S) to S in metal> In general desulfurization using slag, S in the metal is removed by taking it into the slag and concentrating it, and then separating the slag from the metal, so the ratio of (S) in the slag to [S] in the metal must be high. On the other hand, in the desulfurization method of the present invention, the vaporization desulfurization effect is exerted, so that S in the metal is once passed through the slag but is finally removed into the gas phase, so the ratio of (S) in the slag to [S] in the metal may be low. However, if the ratio of (S) in the slag to [S] in the metal is less than 1, the movement of S from the metal to the slag is slower than the movement of S from the slag to the gas phase, which leads to restrictions on the vaporization desulfurization range and extension of the processing time. Therefore, the ratio of (S) in the slag to [S] in the metal ((S) / [S]) should be 1 or more by mass, preferably 2 or more, and more preferably 5 or more. To increase this index ((S) / [S]), methods such as adding a CaO source to the slag, adding deoxidizing elements such as Si and Al to the molten metal, and raising the temperature can be used. The slag composition should preferably be CaO / (Al2O3+2SiO2)≧0.6 in mass ratio.

[0031] <Liquid phase ratio of slag at molten metal temperature> Vaporization desulfurization from the slag to the gas phase occurs at the arc spot. On the other hand, the desulfurization reaction from the metal to the slag can occur not only at the slag-metal interface directly below the arc spot, but throughout the entire slag-metal interface in the furnace. Therefore, if the liquid phase ratio of the slag is high in areas other than directly below the arc spot, the S absorbed into the slag at a position away from the arc spot can more easily move (by advection or diffusion) to the arc spot, allowing for more efficient desulfurization. The temperature of the slag at the slag-metal interface away from the arc spot can be considered to be the same as the temperature of the molten metal.

[0032] Therefore, the relationship between the liquid phase rate of the slag at the temperature of the molten metal and the desulfurization efficiency was evaluated. As a result, it was found that the desulfurization efficiency of the present invention can be further promoted if the liquid phase rate of the slag at the temperature of the molten metal is 80% or more. The liquid phase rate at the temperature of the molten metal can be adjusted by changing the slag composition. Specifically, the liquid phase rate at a predetermined molten metal temperature can be increased by the same specific guideline as the specific guideline for increasing the liquid phase rate at 2000°C. In addition to adjusting the slag composition, it can also be controlled by changing the temperature of the molten metal. The liquid phase rate of the slag at the temperature of the molten metal can be obtained by an experimental method such as differential thermal analysis or by a numerical calculation using a thermodynamic calculation software. For example, when the molten metal temperature is 1550℃, and the slag composition is CaO=62.6%, SiO2=11.4%, Al2O3=19.1%, MgO=4.5%, MnO=1.2%, FeO=1.1%, CaF2=0.1% by mass, the liquid phase ratio calculated using the thermodynamic calculation software FactSage (registered trademark) 7.3 is 83%, which is more than 80%. The liquid phase ratio of the slag with the above composition at 2000℃ is 100%, which is more than 80%.

[0033] <Electrical condition: current> Electrochemical reactions occur with the movement of electrons, and the greater the current, the greater the effect. In the present invention, the current (kA / t) per mass of molten metal in the furnace is 0.6 kA / t or more, preferably 0.8 kA / t or more, and more preferably 1.0 kA / t or more. If the current is made large, the voltage must be reduced, and the above-mentioned minimum arc length cannot be ensured. The upper limit for a typical electric furnace is about 10 kA / t. EXAMPLES

[0034] <Common conditions> The DC arc electric furnace used was equipped with a first electrode 1 in contact with the molten metal 3 and a second electrode 2 above the slag 4 on the molten metal 3. The molten metal 3 was molten steel, and the amount of molten steel was 95 to 110 t / ch. The molten steel components before desulfurization treatment were [S]: 150 ppm. This was adjusted by mixing the iron source and adding FeS after burn-through.

[0035] After the melting of the raw materials was completed, the second electrode 2 and the slag 4 were spaced apart on the stationary bath surface, and a direct current was applied to the first electrode 1 as the negative electrode and the second electrode 2 as the positive electrode (reverse polarity current). Only in some levels, the first electrode 1 was the positive electrode and the second electrode 2 was the negative electrode (positive polarity current).

[0036] The electric current of the electric furnace during the desulfurization treatment was 100 kA, the treatment time was 20 minutes, and the electrode diameter of the second electrode 2 was 700 mm (28 inches).

[0037] <Individual conditions> The amount of slag was 90-100 kg / t, and CaO-SiO2-Al2O3-MgO-MnO-FeO slag was formed. The slag composition was changed in various ways. In order to change the liquid phase rate of the slag at each temperature while maintaining the desulfurization ability of the slag, the slag components were adjusted in the range of CaO / (Al2O3+2SiO2):C / (A+2S):0.6-2.2 in mass ratio. In the present invention examples 7 and 8 in Table 1, C / (A+2S) was adjusted to be low within the above range, and only in the present invention example 9, C / (A+2S)=0.5 was set to a low basicity slag composition. The molten metal temperature during the desulfurization treatment was adjusted to the temperature shown in Table 1. After analyzing the components of the slag generated at each level, the liquid phase rate of the slag at 2000°C and the liquid phase rate of the slag at the molten metal temperature were calculated using the thermodynamic calculation software FactSage (registered trademark) 7.3.

[0038] The distance d between the second electrode 2 and the surface of the slag 4 was varied. The slag surface was based on the stationary bath surface during desulfurization. The distance d was evaluated by the method described above.

[0039] <Vaporization desulfurization index> The total amount of S before desulfurization was calculated by multiplying the analytical values ​​of slag (S) and metal [S] before desulfurization by the amount of slag and the amount of molten steel, and then adding them up. The total amount of S after desulfurization was calculated by multiplying the analytical values ​​of slag (S) and metal [S] after desulfurization by the amount of slag and the amount of molten steel, and then adding them up. The reduction in the total amount of S in the slag and metal before and after desulfurization is evaluated as the amount of vaporized desulfurization. The amount of slag used in the above calculation was the average of the values ​​obtained by CaO balance and Si balance. The value obtained by dividing the amount of vaporized desulfurization by the amount of molten iron (value converted to metal [S] (expressed in ppm by mass)) is used to define the "vaporized desulfurization index". The value of the "vaporized desulfurization index" was scored according to the following criteria. In the following scoring system, ☆ to △ were considered good, and × was considered bad. ☆ : 25ppm or more ◎: 20 ppm or more and less than 25 ppm ○: 15ppm or more and less than 20ppm △: 10 ppm or more and less than 15 ppm ×: Less than 10 ppm

[0040] <Arc spot slag cover status> R was calculated using the above formula (1), and an evaluation was made as to whether or not a circle (arc spot circle 6) with a radius of R and centered at the point where a vertical line passing through the central axis of the second electrode 2 intersects with the molten metal surface was covered with slag 4 (slag coverage condition). The slag coverage condition was confirmed by taking a thermographic image using the difference in emissivity between the molten iron and the slag.

[0041] The test results are shown in Table 1. Values ​​and items outside the range of the present invention are underlined.

[0042] [Table 1]

[0043] Regarding the polarity of the electrodes, only Comparative Example 1 employs positive polarity energization (energization is performed with the first electrode 1 as the positive electrode), while all the others employ reverse polarity energization (energization is performed with the second electrode 2 as the positive electrode).

[0044] Regarding the state of slag coverage of the arc spot, in Comparative Example 3, Ar gas was blown as a stirring gas from a manipulator inserted through the slag door toward the inside of the arc spot circle, thereby realizing a state in which the arc spot circle 6 was not covered by the slag 4. This result is indicated as "X" in Table 1. In Invention Examples 1 to 11 and Comparative Examples 1 and 2, the Ar gas was blown from the manipulator toward a position between the arc spot circle and the slag door so that the position where it was blown was outside the arc spot circle, thereby realizing a state in which the arc spot circle 6 was covered by the slag 4. This result is indicated as "O" in Table 1.

[0045] In all of Examples 1 to 11 of the present invention, reverse polarity current was applied (current was applied to the second electrode 2 as the positive electrode), the liquid phase rate of the slag 4 was 80% or more at 2000°C, the arc spot circle 6 was covered by the slag 4, and as a result, the vaporization desulfurization indexes were all good.

[0046] In Comparative Example 1, the electrode current was positive, in Comparative Example 2, the liquid phase rate of the slag at 2000°C was less than 80%, and in Comparative Example 3, the slag coverage of the arc spot was poor, and all of the vaporization desulfurization indices were poor. [Explanation of symbols]

[0047] 1 1st electrode 2 2nd electrode 3. Molten Metal 4. Slug 5 Stationary bath surface 6 Arc Spot Circle

Claims

1. In a method for desulfurizing molten steel using an electric furnace, The electric furnace includes a first electrode in contact with molten steel and a second electrode above the slag on the molten steel, and the second electrode and the slag are spaced apart from each other at a stationary bath surface; The liquid phase rate of the slag at 2000°C is 80% or more, In a state where the molten steel surface within a circle having a radius R calculated by the formula (1) and a center at the point where a vertical line passing through the central axis of the second electrode intersects with the molten steel surface is covered with the slag, Applying a direct current to the first electrode as a negative electrode and the second electrode as a positive electrode. A method for desulfurizing molten steel, comprising: R=r+d×tan30° (1) Here, r is the radius of the electrode, and d is the distance between the slug and the second electrode.

2. 2. The method for desulfurizing molten steel according to claim 1, wherein the distance between the second electrode and the surface of the slag is 50 mm or more and 700 mm or less.

3. 3. The method for desulfurizing molten steel according to claim 1, wherein the sulfur concentration (% by mass) in the slag is at least one time the sulfur concentration (% by mass) in the molten steel.

4. The method for desulfurizing molten steel according to any one of claims 1 to 3, characterized in that the liquid phase rate of the slag is 80% or more at the temperature of the molten steel.

5. The method for desulfurizing molten steel according to any one of claims 1 to 4, characterized in that a ratio of the direct current to a mass of molten steel in the furnace is 0.6 kA / t or more.

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