Method for dephosphorization of molten metal

Abstract

Provided is a technique for performing a dephosphorization treatment while avoiding a peroxide state. In performing a dephosphorization treatment of molten metal held in a container while supplying a slag-making material and an oxygen source to the molten metal and blowing a gas into the molten metal to stir the molten metal, a value QO2 / (QB[C]i) is obtained by dividing, by a pre-treatment carbon concentration [C]i (mass%) in the molten metal, the ratio of a supply quantity QO2(Nm3 / t) per unit mass molten metal as oxygen gas in the oxygen source to a supply amount QB(Nm3 / t) per unit mass molten metal of the gas supplied for stirring the molten metal, and the value is adjusted, on a volume basis in a standard state, to be in a range of at least 20(1 / mass%) and no more than 4.2×105(1 / mass%).

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. A typical refining process for molten iron in an electric furnace involves charging raw materials including the cold iron source, then supplying electrical energy to melt the cold iron source to obtain low-carbon molten iron. Then, an oxygen source and a dephosphorization flux are supplied to the molten iron, thereby reducing the carbon concentration in the molten iron and the phosphorus concentration.

[0005] As mentioned above, it is expected that the need for technology to dephosphorize molten iron obtained with a high content of reduced iron will increase in the future, but the following issues will become apparent when this happens. Currently, most of the reduced iron in circulation is reduced with hydrocarbons, and therefore contains approximately 1% by mass of carbon. However, in the future, it will be necessary to use more CO ２ Due to the need to reduce emissions, reduced iron produced using reduced carbon reducing agents such as hydrogen is expected to become mainstream. This reduced iron may contain almost no carbon. In this case, the carbon concentration in the molten iron produced by dissolving the reduced iron will decrease and the phosphorus concentration will increase. Because the melting point of molten iron increases with decreasing carbon concentration, dephosphorization of low-carbon, high-phosphorus molten iron is unavoidable under higher temperatures, which are unfavorable for the dephosphorization reaction. Therefore, a dephosphorization method for molten iron that satisfies these conditions is required. Furthermore, reduced iron contains gangue components such as silicon oxide and aluminum oxide contained in the iron ore used as a raw material, generating a large amount of slag during melting. Therefore, this point must be taken into consideration when developing a dephosphorization method for molten iron.

[0006] 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 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 ppm by mass or more. The molten steel is then stirred to dephosphorize the molten steel. Another method for refining molten steel simultaneously with the supply of oxygen gas at the molten steel stage is described, for example, in Patent Document 3. In the method described in Patent Document 3, a space above the molten steel surface contained in a ladle is sealed with a sealing lid, and a combustible and oxygen or air are supplied into the space to form a combustion zone directly above the molten steel surface. Flux is then supplied to perform heating and refining.

[0007] JP-A-53-134715 JP-A-60-021315 JP-A-62-136515

[0008] 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 the molten iron after melting will be low, reaching the same level as that of molten steel. When the molten iron is dephosphorized using a method similar to that described in Patent Document 1, which is a dephosphorization method for molten pig iron, which has a higher carbon concentration than molten steel, the proportion of oxygen contributing to the decarburization reaction among the supplied oxygen sources will be low. As a result, the proportion of oxygen contributing to the oxidation of the molten iron and its dissolution into the molten iron increases, resulting in increases in the iron oxide concentration in the slag and the oxygen concentration in the molten iron. This reaction with carbon in the furnace body refractory leads to 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. This makes it difficult to ensure stable crude steel production. Hereinafter, a state in which the oxygen concentration in the 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.

[0009] Furthermore, an increase in the iron oxide concentration in the slag reduces the lime concentration in the slag, thereby reducing the dephosphorization ability of the slag. This makes it difficult to dephosphorize to a low phosphorus concentration, making it difficult to produce ultra-low phosphorus steel. Although the introduction of an oxygen source is necessary to promote the dephosphorization reaction, which is an oxidation reaction, an excessive supply of the oxygen source makes it difficult to dephosphorize to a low phosphorus concentration. Therefore, an appropriate amount of oxygen source must be supplied. In this regard, the method described in Patent Document 2 is a process for further dephosphorization to a low phosphorus concentration after dephosphorization by refining in a refining furnace, and the phosphorus concentration before the start of the process is low. Therefore, additional supply of an oxygen source is not required. In contrast, if the phosphorus concentration before the start of the dephosphorization process is high, it is difficult to dephosphorize to a low phosphorus concentration without additional supply of an oxygen source. There are very few methods for performing refining in the molten steel stage while simultaneously supplying an oxygen source under atmospheric conditions. The technology described in Patent Document 3 supplies oxygen gas to the molten steel in the ladle, but its purpose is to heat the slag and molten steel. Although it is stated that dephosphorization is possible, the surface of the slag and molten steel is in a reducing atmosphere due to unburned reducing gas, and is not intended to promote the dephosphorization reaction.

[0010] An object of the present invention is to solve the above problems and to provide a technology for carrying out dephosphorization treatment while avoiding an overoxidized state.

[0011] A first method for dephosphorizing molten iron according to the present invention, which advantageously solves the above-mentioned problems, comprises the steps of: supplying a slag former and an oxygen source to molten iron held in a vessel; and injecting a gas into the molten iron to stir the molten iron; and determining a supply amount Q of the gas per unit mass of the molten iron to stir the molten iron on a volumetric basis under standard conditions. Ｂ [Nm ３ / t], the supply amount Q per unit mass of molten iron as oxygen gas in the oxygen source Ｏ２ [Nm ３ / t] by the carbon concentration [C]i (mass%) before treatment in the molten iron, Q Ｏ２ / (Q Ｂ [C]i) is 20 (1 / mass%) or more and 4.2 × 10 ５ The content is adjusted to be in the range of 1 / mass % or less.

[0012] Furthermore, in the first method for dephosphorizing molten iron according to the present invention, (a) the carbon concentration in the molten iron before the treatment is set to 1.0 mass % or less, (b) the phosphorus concentration in the molten iron before the dephosphorization treatment is set to 0.020 mass % or more, (c) the molten iron before the dephosphorization treatment is obtained by melting a cold iron source, and (d) the cold iron source contains reduced iron, etc., are more preferable solutions.

[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, it is possible to dephosphorize molten iron held in a vessel outside a melting furnace to a low phosphorus concentration. Furthermore, by suppressing the increase in the amount of oxygen present in the slag and molten steel, it is possible to perform the treatment while suppressing wear on the furnace body refractories.

[0015] 1 is a graph showing the relationship between the T.Fe concentration in the slag and the wear index of the furnace body refractory material, and FIG.

[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] When dephosphorizing molten iron with a carbon concentration of 2% by mass or less, i.e., molten steel, the carbon concentration is low, unlike molten pig iron. Therefore, the iron oxide concentration in the slag and the oxygen concentration in the molten iron easily increase, accelerating wear of the furnace body refractories. In addition, this leads to a deterioration in the dephosphorization ability of the slag. As a countermeasure, the ratio of the oxygen gas supplied as the oxygen source to the gas supplied to stir the molten iron divided by the carbon concentration [C]i (mass%) in the molten iron before treatment is calculated as Q. Ｏ２ / (Q Ｂ [C]i) (1 / mass%) was used as an index. Ｂ is the amount of gas supplied per unit mass of molten iron to stir the molten iron during dephosphorization (Nm ３ / t). Also, Q Ｏ２ is the amount of oxygen gas supplied per unit mass of molten iron in the oxygen source supplied during dephosphorization (Nm ３ / t). In this embodiment, this index Q Ｏ２ / (Q Ｂ [C]i) is 20 (1 / mass%) or more and 4.2 × 10 ５ The content is adjusted to be in the range of 1 / mass % or less.

[0019] Indicator Q Ｏ２ / (Q ＢIn adjusting the carbon concentration [C]i in the molten iron before dephosphorization, the supply rate of the oxygen source can be set to a predetermined value or less, thereby reducing the amount of oxygen that contributes to the increase in the iron oxide concentration in the slag and the oxygen concentration in the molten iron but does not contribute to the dephosphorization reaction. Furthermore, by setting the supply rate of the gas supplied for stirring to a predetermined value or more, the mass transfer of carbon in the molten iron can be promoted. This can suppress a local increase in the oxygen concentration in the molten iron and promote the reduction of the slag. Therefore, it is possible to suppress the increase in the iron oxide concentration in the slag. In addition, the mass transfer of phosphorus in the molten iron can be promoted, thereby improving the dephosphorization reaction rate and completing the dephosphorization process before excessive oxidation of the molten iron. Furthermore, by increasing the carbon concentration [C]i in the molten iron before dephosphorization, the oxygen concentration in the molten steel can be reduced, thereby increasing the amount of reduction in the slag. Therefore, it is effective to set the supply rate of the oxygen source to a predetermined value or less and to set the supply rate of the gas supplied for stirring and the carbon concentration in the molten iron to a predetermined value or more.

[0020] However, if the supply rate of the oxygen source is reduced below a predetermined value, there is a concern that the oxygen source required for the dephosphorization reaction will be insufficient, resulting in poor dephosphorization. Furthermore, if an excessive amount of gas is supplied for stirring, the amount of metal scattered increases. In this case, since the freeboard of a refining furnace other than an electric furnace is small, metal may be scattered outside the furnace, resulting in increased costs due to iron loss. Furthermore, if the carbon concentration [C]i in the molten iron before dephosphorization is high, excessive reduction can lead to poor dephosphorization and an extended decarburization time, resulting in increased processing time. Therefore, it is necessary to maintain the supply rate of the oxygen source at or above a predetermined value, and to maintain the supply rate of gas supplied for stirring and the carbon concentration in the molten iron at or below predetermined values.

[0021] The index Q is set using these parameters. Ｏ２ / (Q Ｂ [C]i) is 4.2 × 10 ５(1 / mass%) or less. This upper limit was calculated taking into consideration the following conditions: (1) A large amount of dephosphorization was performed, and the amount of iron oxide generated in the slag and the amount of oxygen required for decarburization were taken into consideration. (2) The amount of stirring gas estimated from the minimum stirring power required to promote the dephosphorization reaction was taken into consideration. (3) The amount of oxygen required for decarburization when the carbon concentration before treatment is relatively low was taken into consideration. Preferably, the index Q Ｏ２ / (Q Ｂ The upper limit of [C]i) is 10,000 (1 / mass%), and more preferably, the upper limit is 1,000 (1 / mass%).

[0022] On the other hand, this indicator Q Ｏ２ / (Q Ｂ If [C]i) is low, the oxygen source required to promote the dephosphorization reaction will be insufficient, and the necessary stirring force will not be secured sufficiently, so it is necessary to set it above a certain level. Therefore, the lower limit was set to 20. This lower limit was calculated taking into consideration the following conditions: (1) A small amount of dephosphorization, the minimum amount of iron oxide generated in the slag, and the amount of oxygen required for decarburization were taken into consideration. (2) The amount of stirring gas estimated from the maximum stirring power operable in the refining ladle outside the melting furnace was taken into consideration. (3) The amount of oxygen required for decarburization when the carbon concentration before treatment is relatively high was taken into consideration. Preferably, the index Q Ｏ２ / (Q Ｂ The lower limit of [C]i) is 30 (1 / mass %), and more preferably 40 (1 / mass %).

[0023] In order to adjust the above index within a predetermined range, the carbon concentration [C]i in the molten iron before dephosphorization is preferably set to an upper limit of 1.0 mass% and a lower limit of 0.001 mass%, more preferably 0.50 mass% and 0.01 mass%, respectively.

[0024] In order to adjust the above index within a predetermined value range, the supply amount Q of oxygen gas in the oxygen source per unit mass of molten iron is Ｏ２ 4.5 Nm ３ / t or more 20Nm ３ In order to adjust the above index to a predetermined value range, the supply amount Q of gas supplied to stir the molten iron per unit mass of molten iron is Ｂto 0.0040 Nm ３ / t or more 0.35Nm ３ The range shall be less than / t.

[0025] 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. Reduced iron is not usually dephosphorized during production, so the phosphorus content varies depending on the grade of the raw iron ore. In the future, the proportion of reduced iron used as a cold iron source will increase, and the burden of dephosphorization after melting is expected to increase. When the phosphorus concentration is less than 0.020% by mass, the phosphorus concentration is approximately the same as 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, so it is preferable to set the lower limit of the phosphorus concentration before treatment to 0.02% by mass.

[0026] 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.

[0027] 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 similar. If the ladle's freeboard height is insufficient, the furnace body can be tilted before tapping from the electric furnace, and the molten metal can be tapped after the slag has drained. Since large amounts of gangue, such as silicon oxide and aluminum oxide, are generated when melting reduced iron, dephosphorization in a ladle or similar can prevent an increase in slag volume due to the gangue contained in the reduced iron.

[0028] 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.

[0029] In the dephosphorization treatment, for example, oxygen gas is supplied from a top-blown 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. The flow rate of the oxygen gas from the oxygen source is determined by the above-mentioned index Q Ｏ２ / (Q Ｂ The oxygen supply rate is adjusted to be 0.20 Nm3 per unit mass of molten iron per unit time as oxygen gas. ３ / (min・t) or more 1.50Nm ３ It is preferable to set the oxygen flow rate within a range of 1 / (min·t) or less. Since the occurrence of spitting varies depending on the freeboard height of the ladle and the shape of the nozzle of the top lance, it is preferable to fine-tune the oxygen flow rate and lance height. Extending the skirt portion of the furnace cover can also be effective in order to ensure the apparent freeboard height. When oxygen gas is supplied, the temperature of the molten iron rises due to the heat of the oxidation reaction, so there is no problem in 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.

[0030] 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, the oxygen concentration in molten iron is preferably set to 300 ppm by mass or more. By keeping the iron oxide concentration in the slag low, it is possible to suppress losses due to oxidation of molten iron and maintain a high yield of molten iron.

[0031] When the dephosphorization process begins, stirring gas is supplied to the molten iron. This stirring gas can be supplied from an injection lance or by installing a porous plug at the bottom of the ladle. The stirring gas promotes the reaction between the slag and the molten iron, thereby accelerating the dephosphorization reaction.

[0032] 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 rephosphorization, i.e., phosphorus re-migration from the slag into the molten iron, even if the dephosphorized molten iron is deoxidized in the subsequent deoxidation process. From the viewpoint 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.

[0033] 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.

[0034] Scrap and reduced iron were charged as cold iron sources and melted in a 200-ton electric furnace. A carbon source was added during melting of the cold iron sources to adjust the carbon concentration in the molten iron after tapping. The initial phosphorus concentration after melting varied depending on the type and amount of cold iron sources used and the progress of the dephosphorization reaction in the electric furnace. After melting the cold iron sources in the electric furnace, the molten iron was poured into a ladle. After tapping, dephosphorization was performed by adding a slag former while supplying oxygen gas from a top lance and argon gas as a stirring gas from a bottom gas supply plug at the bottom, as shown in Figure 1. After dephosphorization, the slag on the surface of the molten iron in the ladle was removed, and the molten iron was heated by electrodes to increase the temperature. The molten iron was then vacuum decarburized using the VOD method and other composition adjustments were made by adding alloys. Table 1 shows the test conditions and evaluation results. The dephosphorization 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 in dephosphorization and was marked with a symbol "Good." The evaluation of "peroxidation" was marked with a symbol "Good" when the post-treatment oxygen concentration [O]f in the molten iron was 700 mass ppm or less and the post-treatment total iron concentration (T.Fe)f in the slag was 30 mass% or less. Other cases were marked with a symbol "Poor."

[0035] In Tests Nos. 1 to 3, the pre-treatment phosphorus concentration [P]i in the molten iron was changed to 0.020, 0.040, and 0.060 mass%, respectively, while the oxygen supply rate, stirring gas flow rate, and other conditions were kept constant. Dephosphorization progressed to a lower phosphorus concentration under conditions with a lower pre-treatment phosphorus concentration. Furthermore, the amount of oxygen required for dephosphorization until the phosphorus concentration reached 0.020 mass% or less differed depending on the pre-treatment phosphorus concentration. Under conditions with a lower pre-treatment phosphorus concentration, less oxygen was required for dephosphorization, resulting in higher oxygen concentrations in the molten iron and higher T.Fe concentrations in the slag after treatment.

[0036] In Tests Nos. 4, 5, 8 and 9, similarly to Treatment No. 2, the phosphorus concentration [P]i before treatment was 0.040 mass%, and the oxygen supply amount, the stirring gas amount, and the pre-treatment [C]i were used as the index Q Ｏ２ / (Q Ｂ [C]i) was changed within the allowable range. Ｏ２ / (Q ＢAs the [C]i) becomes higher, oxidation progresses, and the oxygen concentration [O]f in the molten iron and the T.Fe concentration (T.Fe)f in the slag after treatment become higher. Ｏ２ / (Q Ｂ The low [C]i) suppressed the progression of oxidation, resulting in low oxygen concentrations in the molten iron [O]f and low T.Fe concentrations in the slag (T.Fe)f after treatment. The equilibrium phosphorus concentration was lowest when the T.Fe concentration in the slag was 20 mass%, and was high in other cases. Therefore, compared to Test No. 2, in which the T.Fe concentration in the slag (T.Fe)f after treatment was 20 mass%, Test Nos. 4, 5, 8, and 9 had higher post-treatment phosphorus concentrations [P]f.

[0037] Test Nos. 6 and 7 show comparative examples. Ｏ２ / (Q Ｂ When [C]i) was out of the allowable range, one or both of the overoxidation state and the poor dephosphorization became a problem. Ｏ２ / (Q Ｂ [C]i) exceeded the upper limit, and oxidation progressed to an overoxidized state. Ｏ２ / (Q Ｂ [C]i) was below the lower limit, and the progress of oxidation was suppressed. In both cases, the T.Fe concentration in the slag was significantly outside the range in which the equilibrium phosphorus concentration was low, resulting in poor dephosphorization.

[0038]

[0039] The method for dephosphorizing molten iron according to the present invention can stably produce low-phosphorus steel without dissolving excessive oxygen and even when the phosphorus concentration or slag amount increases. 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 Top-blowing gas supply lance 4 Oxygen gas and slag former 5 Slag 6 Stirring gas 7 Molten iron 8 Bottom-blowing gas supply plug

Claims

1. When dephosphorizing molten iron held in a vessel while supplying a slag former and an oxygen source to the molten iron and injecting gas into the molten iron to stir the molten iron, the amount of gas supplied per unit mass of molten iron, Q, is determined based on the volume of the standard condition. Ｂ (Nm ３ / t) as oxygen gas in the oxygen source per unit mass of molten iron Q Ｏ２ (Nm ３ / t) by the carbon concentration [C]i (mass%) before treatment in the molten iron, Q Ｏ２ / (Q Ｂ [C]i) is 20 (1 / mass%) or more and 4.2 × 10 ５ (1 / mass%) or less.

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.

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