Method for dephosphorizing molten iron

Abstract

The present invention proposes a method which is for dephosphorizing molten iron and which makes it possible to stably produce steel containing a small amount of phosphorus. Proposed is a method for dephosphorizing molten iron, said method comprising: obtaining dephosphorized molten iron by, while blowing one of a hydrogen gas and a hydrocarbon gas or a mixed gas thereof into molten iron held in a container, supplying a slag material and an oxygen source so that the molten iron is dephosphorized; and separating, from the dephosphorized molten iron, slag floating on the surface of the dephosphorized molten iron after the dephosphorylation. The oxygen source is an oxygen gas. One of the hydrogen gas and the hydrocarbon gas or the mixed gas thereof is supplied in an amount in the range of less than 0.040 Nm3 / (min·t) per mass of the molten iron and per unit time on a hydrogen gas basis. After the slag is separated, the dephosphorized molten iron is optionally deoxidized with a deoxidizer.

Description

Method for dephosphorizing molten iron

[0001] The present invention relates to a method for dephosphorizing molten iron to produce steel products with a low phosphorus concentration, and more particularly to a method for dephosphorizing molten iron obtained by melting a cold iron source. In this specification, "molten iron" refers to molten metal mainly composed of Fe, including molten pig iron and molten steel. The unit of mass "t" means 1000 kg. The "N" prefixed to a unit representing the volume of gas refers to gas under standard conditions, which are a temperature of 0°C and a pressure of 101,325 Pa. The symbol [M] indicates that element M is dissolved in molten iron, and the symbol (R) indicates that a substance of chemical formula R is contained in slag.

[0002] In recent years, from the perspective of preventing global warming, the steel industry has also been reducing its consumption of fossil fuels and reducing CO ２ Technological developments are underway to reduce the amount of CO2 generated. In conventional integrated steelworks, iron ore is reduced with carbon to produce molten pig iron. To produce this molten pig iron, a carbon source of about 500 kg is required per ton of molten pig iron for the reduction of iron ore, etc. On the other hand, when molten steel is produced using a cold iron source such as iron scrap as a raw material, the carbon source required for the reduction of iron ore is not required, and only the amount of energy sufficient to melt the cold iron source is required. Therefore, CO2 ２ This will enable a significant reduction in emissions.

[0003] When scrap iron is used as the cold iron source, the phosphorus concentration of the molten iron after melting is lower than that of blast furnace hot metal. On the other hand, when reduced iron is used as the cold iron source, the phosphorus concentration of the molten iron after melting is almost the same as that of blast furnace hot metal. Therefore, dephosphorization refining is required to bring the phosphorus concentration within the product specification range. The amount of scrap circulating on the market is limited, and the use of reduced iron is expected to increase in the future. In that case, it is thought that the dephosphorization load during the molten iron refining stage will increase.

[0004] When producing low-phosphorus steel from blast furnace hot metal, the dephosphorization reaction is thermodynamically more favorable at lower temperatures, so that so-called hot metal dephosphorization, in which dephosphorization is performed at the hot metal stage, is widely practiced. See, for example, Patent Document 1. On the other hand, when producing low-phosphorus steel by blending a high amount of cold iron source, the mainstream method is to melt the cold iron source in an electric furnace such as an arc furnace and produce low-carbon molten iron without going through the hot metal process.

[0005] A method for dephosphorizing molten iron at the molten steel stage is described, for example, in Patent Document 2. In the method described in Patent Document 2, oxidizing molten slag used in refining the molten iron in a refining furnace, quicklime, and a quicklime slag accelerator are first added to undeoxidized molten steel having an oxygen concentration of 400 mass ppm or more. The molten steel is then stirred to dephosphorize it. In addition, Patent Document 3 describes a method for suppressing slag foaming during refining by using hydrogen gas or hydrocarbon gas as a stirring gas in the molten iron.

[0006] Japanese Patent Laid-Open No. 53-134715 Japanese Patent Laid-Open No. 60-021315 Japanese Patent Laid-Open No. 2001-107125

[0007] However, the above-mentioned conventional techniques still have the following problems to be solved. ２ To reduce emissions, further increases in the use ratio of cold iron sources are expected. As a result, the carbon concentration in molten iron after melting will be low, reaching the same level as molten steel. When dephosphorization is performed using a method similar to that described in Patent Document 1, which involves dephosphorization of molten pig iron, which has a higher carbon concentration than molten steel, the proportion of oxygen dissolved in the molten iron among the supplied oxygen sources increases significantly, resulting in increases in the oxygen concentration in the molten iron and the iron oxide concentration in the slag. These oxygen sources react with the carbon in the furnace body refractory, causing wear of the furnace body refractory. Wear of the furnace body refractory shortens the furnace body life and increases the frequency of furnace body repairs, making it difficult to ensure a stable supply of crude steel. Hereinafter, a state in which the oxygen concentration in molten iron exceeds 700 mass ppm and the iron oxide concentration in the slag exceeds 30 mass%, at which significant wear of the furnace body refractory occurs, is referred to as a "peroxidation" state.

[0008] In the method described in Patent Document 2, after dephosphorization by refining in a refining furnace, undeoxidized molten steel with an oxygen concentration of 400 ppm by mass or more is tapped. After tapping, oxidizing refining furnace slag or the like is added and Ar gas is supplied to stir the molten steel, thereby further dephosphorizing the molten steel to a low phosphorus concentration. Therefore, the supply of an oxygen source is not required. However, if the oxygen concentration in the molten steel before the start of the dephosphorization process is low and the phosphorus concentration is high, it is difficult to dephosphorize the molten steel to a low phosphorus concentration without the supply of an oxygen source.

[0009] The method described in Patent Document 3 describes an example in which a converter is assumed as the refining vessel for carrying out the treatment. It is said that the method can also be applied to refining vessels other than converters (such as for molten iron pretreatment and secondary refining). In addition, the flow rate of hydrogen gas or hydrocarbon gas used as a stirring gas in molten iron is 0.04 Nm3 for hydrogen gas flow rate. ３ However, the molten iron pretreatment is carried out in a torpedo car, and the secondary refining is carried out in a ladle. These have a smaller refining vessel volume and a lower freeboard height than converters. Therefore, the stirring gas flow rate is set to 0.04 Nm ３ If the stirring rate is set to 1 / (min·t) or more, there is a risk that the molten iron will flow out of the furnace due to strong stirring.

[0010] An object of the present invention is to solve the above problems and to provide a technology for performing dephosphorization treatment while suppressing the outflow of molten iron out of the furnace and avoiding the molten iron from becoming overoxidized.

[0011] A first method for dephosphorizing molten iron according to the present invention, which advantageously solves the above-mentioned problems, comprises: dephosphorizing the molten iron held in a vessel while supplying a slag former and an oxygen source to obtain dephosphorized molten iron, while injecting either hydrogen gas or a hydrocarbon gas, or a mixed gas thereof, into the molten iron; and separating slag floating on the surface of the dephosphorized molten iron after the dephosphorization from the dephosphorized molten iron, wherein oxygen gas is used as the oxygen source, and the hydrogen gas or the hydrocarbon gas, or the mixed gas, is supplied in an amount of 0.040 Nm3 in terms of hydrogen gas per mass of molten iron per unit time. ３ / (min·t), and optionally, after separating the slag, the dephosphorized molten iron is deoxidized with a deoxidizer.

[0012] The first method for dephosphorizing molten iron according to the present invention further comprises the steps of: (a) supplying the oxygen gas at a flow rate of 0.10 Nm3 per unit time per mass of molten iron; ３ / (min・t) or more 1.50Nm ３ (b) the hydrogen gas and / or the hydrocarbon gas or the mixture thereof is supplied in a range of 0.0017 Nm3 / (min·t) or less per unit time per mass of molten iron in terms of hydrogen gas; ３ / (min・t) or more 0.040Nm ３ More preferable solutions include (a) supplying an amount of molten iron in a range of less than 1 / (min·t), (b) setting the oxygen concentration in the molten iron during and after the dephosphorization treatment to 700 ppm by mass or less and setting the T.Fe concentration in the slag to 10% by mass or more and 30% by mass or less, (c) setting the oxygen concentration in the molten iron during and after the dephosphorization treatment to 700 ppm by mass or less and setting the T.Fe concentration in the slag to 10% by mass or more and 30% by mass or less, (d) setting the phosphorus concentration in the molten iron before the dephosphorization treatment to 0.020% by mass or more, (e) the molten iron before the dephosphorization treatment is obtained by melting a cold iron source, (f) the cold iron source contains reduced iron, and (g) the vessel is a ladle.

[0013] Furthermore, a more preferable solution to the second problem of the present invention is that in any of the above first methods for dephosphorizing molten iron, a cold iron source is melted in a melting furnace to obtain molten iron, and then, when the molten iron is tapped from the melting furnace into the container, slag that has flowed into the container together with the molten iron is separated from the molten iron, prior to the dephosphorization treatment.

[0014] According to the present invention, the above-mentioned method makes it possible to prevent the molten iron from leaking out of the furnace and to avoid an overoxidized state, while also enabling dephosphorization treatment to a phosphorus concentration below a predetermined value.

[0015] 1 is a schematic vertical cross-sectional view showing the configuration of an apparatus suitable for application to a method for dephosphorizing molten iron according to an embodiment of the present invention; 2 is a graph showing the relationship between the T.Fe concentration in slag and the wear index of a furnace body refractory;

[0016] The following is a detailed description of embodiments of the present invention. The following embodiments are intended to exemplify equipment and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope defined in the claims.

[0017] FIG. 1 shows the configuration of an apparatus suitable for use in the molten iron dephosphorization method according to this embodiment. A ladle 1 is covered with a furnace lid 2. A top-blowing gas supply lance 3 is inserted through an upper hole in the furnace lid 2. In the example shown in FIG. 1, oxygen gas and a slag former 4 are sprayed from the lance 3. The ladle 1 contains molten iron 7 tapped from a melting furnace. Dephosphorization slag 5 is formed on the molten iron 7 by the slag from the melting furnace that was mixed in when the molten iron was tapped from the melting furnace and the slag former. In the example shown in FIG. 1, stirring gas 6 is supplied from a bottom-blowing gas supply plug 8.

[0018] The inventors performed dephosphorization treatment on molten iron 7, the initial carbon content of which had been adjusted, by supplying a slag former and an oxygen source while supplying hydrogen gas, hydrocarbon gas, or a mixture thereof as stirring gas 6. In this case, the inventors considered that if the supply amount of the oxygen source was adjusted in accordance with the increase in the oxygen concentration in the molten iron, dephosphorization treatment could be performed while avoiding an overoxidation state. Furthermore, the inventors considered that by setting an upper limit on the flow rate of the stirring gas, i.e., hydrogen gas, dephosphorization treatment could be performed while preventing the molten iron from leaking out of the furnace.

[0019] The initial phosphorus concentration varies depending on the type of iron source used as the cold iron source. The cold iron source refers to scrap or reduced iron. Scrap is an iron source that has been dephosphorized and then commercialized. Because reduced iron is not typically dephosphorized during production, the phosphorus content varies depending on the grade of the raw iron ore. As the proportion of reduced iron used as a cold iron source increases in the future, the burden of dephosphorization after melting is expected to increase. For example, if reduced iron containing 17% gangue by mass and 71% iron by mass and 0.12% phosphorus by mass is melted, the phosphorus concentration converted to metal content is 0.16% by mass. Therefore, it is preferable to set the upper limit of the initial phosphorus concentration to 0.16% by mass. Furthermore, if the phosphorus concentration is less than 0.02% by mass, it is similar to the phosphorus concentration after melting and tapping only scrap in an electric furnace. In such cases, the dephosphorization treatment of this embodiment is not required, and therefore it is preferable to set the lower limit of the phosphorus concentration before treatment to 0.02% by mass.

[0020] In order to promote the dephosphorization reaction in the dephosphorization treatment of molten iron, for example, oxygen gas is supplied as an oxygen source. For example, as shown in FIG. 1, it is preferable to spray oxygen gas onto the molten iron from a lance 3. The oxygen source is a gas supply of 0.10 Nm3 per unit time per mass of molten iron. ３ / (min・t) or more 1.50Nm ３ It is preferable to supply oxygen gas in the range of 0.1 / (min·t) or less. This allows phosphorus in the molten iron 7 to be oxidized and removed into the slag 5. If the supply amount of oxygen gas is less than the lower limit, the dephosphorization reaction may not proceed, or the reaction rate may be too slow, prolonging the treatment time and making the treatment uneconomical. If the supply amount of oxygen gas exceeds the upper limit, the amount of oxidation may be too large, leading to concerns about an overoxidized state. More preferably, the lower limit is 0.20 Nm ３ / (min·t), and more preferably the upper limit is 1.00 Nm ３ / (min·t), and more preferably, the upper limit is 0.50 Nm ３ / (min·t).

[0021] Stirring gas is supplied to promote the dephosphorization reaction during the dephosphorization process of molten iron. As mentioned above, there is a concern that the dephosphorization process may become overoxidized during and after the process. Therefore, to avoid this overoxidation, reducing gases, either hydrogen gas or hydrocarbon gas, or a mixture of these gases, are supplied as the stirring gas. The hydrogen and carbon produced by thermal decomposition of the supplied hydrogen or hydrocarbon gas react with the oxygen in the molten iron and the iron oxide in the slag, thereby promoting the deoxidation reaction. Furthermore, the injection of hydrocarbon gas produces several times the amount of gas produced by thermal decomposition, providing several times the stirring power compared to the same flow rate of inert gas. This accelerates the dephosphorization reaction, making this a beneficial method for both dephosphorization and avoiding overoxidation. Furthermore, because the slag composition (excluding iron oxide) remains unchanged, excessive increases in slag volume and lime requirements are suppressed.

[0022] Strengthening the stirring promotes the slag-metal reaction, making it possible to dephosphorize to a low phosphorus concentration. On the other hand, because the treatment is performed outside the melting furnace, the internal volume of the refining vessel is small, and the freeboard height, i.e., the distance from the surface of the molten iron to the top of the vessel containing the molten iron, is lower than in a converter, making it easier for the molten iron to flow out of the furnace. In order to prevent the molten iron from flowing out of the furnace, the stirring gas flow rate must be kept low, and the amount of stirring gas supplied is 0.040 Nm3 per unit time per mass of molten iron, converted into hydrogen gas. ３ Preferably, the lower limit is 0.010 Nm / (min·t) in terms of hydrogen gas. ３ / (min·t).

[0023] Furthermore, since a large amount of gangue, such as silicon oxide and aluminum oxide, is generated when melting reduced iron, it is not preferable to perform dephosphorization treatment directly in the melting process, i.e., in the melting furnace. By removing the slag after tapping from the melting furnace and performing dephosphorization treatment in a ladle or the like, it is possible to prevent an increase in slag volume due to the influence of gangue contained in the reduced iron.

[0024] Hereinafter, an embodiment of the present invention will be specifically described. In the first step, an electric steelmaking furnace, for example, is used as a melting furnace, and an iron source is melted and heated by electric energy. In this case, the iron source may be not only a solid iron source (cold iron source) such as scrap or reduced iron, but also molten iron melted in a separate process. Furthermore, the thermal energy supplied for melting the solid iron source and heating the iron source is not limited to electric energy, and metal combustion heat or the like may be used as a supplement. The fact that these energies are renewable energy has the advantage of reducing CO ２ This is desirable from the perspective of reducing emissions.

[0025] In the second step, the molten metal is tapped into a container such as a ladle and the slag is removed. Slag removal can be performed using a slag dragger or, if the ladle has insufficient freeboard height, the furnace body can be tilted before tapping from the electric furnace and the molten metal can be tapped after the slag has been removed.

[0026] In the third step, the molten iron contained in the ladle is dephosphorized. The amount of slag former added is adjusted so that the slag basicity, defined as the ratio of calcium oxide concentration to silicon oxide concentration by mass, is approximately 3.0. The slag former used as the calcium oxide source may be any of quicklime, calcium carbonate, and calcium hydroxide. The slag former may be supplied by spraying it together with the oxygen source or by injection with a carrier gas. It may also be added from a hopper above the furnace.

[0027] In the dephosphorization treatment, oxygen gas, for example, is supplied from a top-blowing lance as an oxygen source. The oxygen source used in this embodiment may be gaseous pure oxygen gas or air, or solid iron ore containing oxygen. For the reasons mentioned above, the flow rate of oxygen gas per unit mass of molten iron is set to 0.20 Nm ３ / (min・t) or more 1.50Nm ３ / (min t) or less. Spitting behavior varies depending on the ladle freeboard height and the shape of the top-blowing lance nozzle, so it is preferable to fine-tune the oxygen supply rate and lance height. Extending the skirt portion of the furnace cover can also be effective in order to ensure an apparent freeboard height. When oxygen gas is supplied, the molten iron temperature rises due to the heat of the oxidation reaction, so there is no problem with adding a solid oxygen source such as iron oxide to adjust the molten iron temperature. Alternatively, an oxygen-containing gas obtained by diluting oxygen gas with an inert gas may be used as the oxygen source.

[0028] The relationship between the total iron (T.Fe) concentration (mass%) in the slag and the furnace body refractory wear index (-) is shown in Figure 2. The furnace body refractory wear index is an index that quantitatively indicates the depth of wear of the furnace body refractory due to treatment. The furnace body refractory wear index was normalized to 1 when the average T.Fe concentration in the slag in the converter in the blast furnace-converter process is 20 mass%. To suppress the furnace body refractory wear index to 2 or less, it is preferable to set the upper limit of the T.Fe concentration in the slag to 30 mass%. Furthermore, a slag composition optimal for the dephosphorization reaction is achieved when the T.Fe concentration in the slag is around 20 mass%. At this point, the equilibrium phosphorus concentration is lowest. However, because the dephosphorization ability significantly decreases when the T.Fe concentration in the slag is less than 10 mass%, it is preferable to set the lower limit to 10 mass%. The upper limit of the oxygen concentration in molten steel is preferably set to 700 ppm by mass, based on the equilibrium value with the T.Fe concentration in the slag at the molten steel temperature, due to the iron-iron oxide equilibrium. On the other hand, from the viewpoint of dephosphorization treatment, the oxygen concentration in molten iron is preferably set to 300 ppm by mass or more.

[0029] At the start of dephosphorization, a hydrogen-atom-containing gas consisting of hydrogen gas, hydrocarbon gas, or a mixture thereof is supplied into the molten iron. This hydrogen-atom-containing gas may be supplied from an injection lance or by installing a porous plug or the like at the bottom of the ladle. The hydrogen-atom-containing gas causes a deoxidation reaction of the oxygen dissolved in the molten iron, thereby preventing excessive oxygen from dissolving in the molten iron. Furthermore, the supplied hydrogen-atom-containing gas and the steam gas generated by the deoxidation reaction also have the effect of promoting stirring of the molten iron.

[0030] In the fourth step, the slag floating on the surface of the dephosphorized molten iron is separated from the dephosphorized molten iron. For example, the vessel containing the dephosphorized molten iron, such as a ladle, is tilted, and the slag floating on the surface of the dephosphorized molten iron is scraped off using a slag dragger or the like. Immediately after the dephosphorization process, the dephosphorized portion of the phosphorus contained in the molten iron prior to the dephosphorization process has migrated into the slag. Therefore, by separating the dephosphorized slag from the dephosphorized molten iron, it is possible to prevent the re-transfer of phosphorus from the slag to the molten iron, a phenomenon known as rephosphorization, even if the dephosphorized molten iron is deoxidized in the subsequent deoxidation process. From the perspective of preventing rephosphorization, it is preferable to remove the slag so that as little dephosphorized slag as possible remains on the surface of the molten iron. However, if the slag removal is carried out until the surface of the molten iron is completely exposed, the iron yield may decrease and the temperature of the molten iron may drop significantly, so the degree of slag removal may be adjusted according to the required level of phosphorus concentration in the product.

[0031] In the fifth step, after separating slag from the dephosphorized molten iron, the dephosphorized molten iron is deoxidized with a deoxidizer. This deoxidization is performed within the period from separating slag from the dephosphorized molten iron to casting the molten iron. For example, deoxidization may be performed by adding a deoxidizer to the ladle containing the molten iron immediately after slag removal, or by transferring the ladle containing the molten iron to a refining facility for the next refining process after slag removal and adding a deoxidizer during the refining process. Specifically, if the next process is a vacuum degassing process using an RH-type vacuum degassing facility, deoxidization may be performed by adding a deoxidizer during the vacuum degassing process. Here, the timing of adding the deoxidizer during the vacuum degassing process is not particularly limited. A deoxidizer may be added at the beginning of the vacuum degassing process to deoxidize the molten iron, followed by refluxing the molten iron after deoxidation, a process known as a killed treatment. Alternatively, a rimming treatment may be performed in the first half of the vacuum degassing process, in which the molten iron is refluxed without adding a deoxidizer and decarburized by supplying oxygen as needed during this period, followed by adding a deoxidizer in the second half of the process and then performing a killed treatment. The next step is not limited to treatment in an RH-type vacuum degassing system; it may also be treatment in a VOD system or a ladle furnace (LF). The timing of adding the deoxidizer during treatment in these systems is not particularly limited, as in the case of the vacuum degassing process in the RH-type vacuum degassing system described above. Furthermore, commonly used deoxidizers, such as metallic aluminum, metallic silicon, ferrosilicon, and silicon manganese, may be used as the deoxidizer.

[0032] Scrap and reduced iron were charged as cold iron sources into a 150-ton electric furnace and melted. The initial phosphorus and carbon concentrations after melting varied depending on the cold iron source. To adjust the initial carbon concentration and facilitate melting, carbonaceous material was also charged along with the cold iron source. After melting, the molten iron was separated from the slag when and after pouring from the electric furnace into the ladle. Dephosphorization was performed using equipment such as that shown in Figure 1, with oxygen gas supplied from a top lance and a slag former added while stirring gas was supplied from a bottom-blowing gas supply plug at the bottom. The stirring gas used was argon gas, hydrogen gas, hydrocarbon gas, or a mixture of hydrogen and hydrocarbon gases. After dephosphorization, the slag on the ladle surface was removed, followed by vacuum degassing in an RH reflux system. An Al-containing deoxidizing material was added, and other components were adjusted. Table 1 summarizes the test conditions and evaluation results. The dephosphorization treatment time was 20 minutes. A post-treatment phosphorus concentration [P]f in the molten iron of 0.020 mass% or less was regarded as excellent dephosphorization and marked with a "good" symbol. A post-treatment phosphorus concentration [P]f in the molten iron of more than 0.020 mass% and 0.025 mass% or less was marked with a "fair" symbol. The evaluation of "peroxidation" was marked with a "good" symbol when the post-treatment oxygen concentration [O]f in the molten iron was 700 mass ppm or less and the post-treatment T.Fe concentration (T.Fe)f of the slag was 30 mass% or less. Otherwise, a "x" symbol was marked in the evaluation column.

[0033] In Test Nos. 1 to 8, the initial carbon concentration [C]i in the molten iron before dephosphorization treatment was fixed at 0.05 mass% and the initial oxygen concentration [O]i was fixed at 80 mass ppm. In all of Test Nos. 1 to 8, the hydrogen gas equivalent supply rate was 0.040 Nm ３ / (min·t), it was possible to prevent the molten metal from flowing out of the furnace.

[0034] Test Nos. 1 to 6 were performed using hydrogen (H ２) gas was used. In Tests No. 1 to 6, the phosphorus concentration [P]i in the molten iron before dephosphorization, the hydrogen gas flow rate, and the oxygen supply rate were changed for dephosphorization. In Test No. 2, the pre-treatment phosphorus concentration [P]i was higher than in Test No. 1, and the oxygen supply rate was increased accordingly. However, the post-treatment phosphorus concentration [P]f was reduced to 0.020 mass% or less without causing an overoxidation state. In Tests No. 3 and 4, the pre-treatment phosphorus concentration [P]i was even higher than in Test No. 2, and the hydrogen gas supply rate was increased to strengthen the stirring of the molten iron. As a result, the post-treatment phosphorus concentration [P]f was reduced to 0.020 mass% or less, and dephosphorization was possible without causing an overoxidation state.

[0035] In Tests No. 5 and 6, the phosphorus concentration before treatment was the same as in Test No. 1, but the hydrogen gas flow rate and oxygen flow rate were increased, respectively. In Test No. 5, the iron oxide in the slag effectively contributed to the dephosphorization treatment due to the strengthened stirring, resulting in a decrease in the phosphorus concentration and T.Fe concentration in the slag after treatment. In Test No. 6, the hydrogen gas flow rate was reduced and the oxygen flow rate was increased compared to Test No. 5. As a result, the dephosphorization treatment was sufficiently promoted and an overoxidation state was avoided.

[0036] In Tests No. 7 and 8, the gas species used as the stirring gas was changed to methane (CH4) and a mixed gas of 50 vol% hydrogen and 50 vol% methane, respectively, and the dephosphorization treatment was performed by reducing the unit flow rate of the stirring gas. The conditions were the same as in Test No. 6, except that the pre-treatment phosphorus concentration [P]i was 0.075 mass% and the unit flow rate was different. In Tests No. 7 and 8, in which a gas containing methane was injected as the stirring gas, dephosphorization was achieved to a lower phosphorus concentration than in Test No. 6, in which hydrogen gas was injected, and a significant effect of avoiding a peroxidation state was achieved.

[0037] In Test No. 9, the stirring gas was argon, an inert gas, and both the oxygen in the molten iron and the T.Fe concentration in the slag were in an excessively peroxidized state, resulting in a phosphorus concentration after treatment that was worse than that of Test No. 1. In Test No. 10, the flow rate of hydrogen gas was excessive, causing the molten iron to leak out.

[0038]

[0039] The method for dephosphorizing molten iron according to the present invention can stably produce low-phosphorus steel without dissolving excessive oxygen, while preventing the molten metal from flowing out of the furnace, and even when the phosphorus concentration or the amount of slag increases during melting. In particular, low-phosphorus steel can be stably produced even when reduced iron produced using a reducing agent with reduced carbon emissions is blended. ２ This contributes to reduction and is useful in industry.

[0040] REFERENCE SIGNS LIST 1 Ladle 2 Furnace cover 3 Lance (for top-blowing gas supply) 4 Oxygen gas and slag former 5 Slag 6 Stirring gas 7 Molten iron 8 Plug for bottom-blowing gas supply

Claims

1. A method for dephosphorizing molten iron held in a vessel by supplying a slag former and an oxygen source while injecting either hydrogen gas or a hydrocarbon gas, or a mixture thereof, into the molten iron to obtain dephosphorized molten iron, and separating slag floating on the surface of the dephosphorized molten iron after the dephosphorization treatment from the dephosphorized molten iron, wherein oxygen gas is used as the oxygen source, and the hydrogen gas or the hydrocarbon gas, or the mixture thereof, is supplied at a rate of 0.040 Nm3 per mass of molten iron per unit time in terms of hydrogen gas. ３ / (min·t), and optionally, after separating the slag, deoxidizing the dephosphorized molten iron with a deoxidizer.

2. A method for dephosphorizing molten iron as set forth in claim 1, wherein, prior to the dephosphorization treatment, a cold iron source is melted in a melting furnace to obtain molten iron, and when the molten iron is tapped from the melting furnace into the container, slag that has flowed into the container together with the molten iron is separated from the molten iron.

Images