Molten iron dephosphorization method

Abstract

The present invention provides a technology for performing a dephosphorization process in which both dephosphorization ability and solidification inhibition of molten iron are ensured. This molten iron dephosphorization method is for performing a dephosphorization process on molten iron held in a container while supplying a slag-making material and an oxygen source to the molten iron and agitating the same by blowing an agitation gas into the molten iron, and is characterized in that: the temperature of the molten iron at the start of the dephosphorization process is 1650-1750°C; the basicity which is the mass ratio of CaO with respect to SiO2 contained in slag generated by melting the slag-making material in the molten iron is 4.0-6.0; and a supply gas rate ratio which, based on volume in a standard state, is the ratio of the per-unit time supply amount of oxygen in the form of a gas in the oxygen source with respect to the per-unit time supply amount of the agitation gas supplied for agitating the molten iron is adjusted.

Description

Method for dephosphorizing molten iron

[0001] The present invention relates to a method for dephosphorizing molten iron to produce steel products with low phosphorus concentrations, particularly a method for dephosphorizing molten iron obtained by melting a cold iron source. In this specification, "molten iron" refers to molten metal primarily composed of Fe, including molten pig iron and molten steel. Furthermore, "x to y" representing a range of values ​​means x to y, including the boundary value. The mass unit "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 scrap) is used as the cold iron source, the phosphorus concentration of the melted molten iron 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 melted molten iron 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 in the market is limited, and the use rate of reduced iron is expected to increase in the future. In this case, it is thought that the dephosphorization load in the molten iron refining stage will increase. Furthermore, since the carbon concentration of the molten iron obtained by melting the cold iron source is lower than that of hot metal, its melting temperature increases, increasing the possibility of solidification during the dephosphorization process. Conventional dephosphorization technologies are mainly targeted at molten iron with a low melting temperature or molten steel with a low phosphorus concentration. Specifically, technologies such as those described in Patent Documents 1 to 3 listed below are known.

[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 ２Due to the need to reduce emissions, reduced iron produced using reduced carbon reducing agents 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, and generates 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. To prevent solidification of molten iron during dephosphorization by increasing the melting point, it is conceivable to apply an electric current to electrodes to heat the molten iron. However, the high iron oxide concentration in the slag during dephosphorization would severely damage the electrodes, which are made of carbon. Therefore, by heating the molten iron in the melting furnace before tapping and then tapping it, the pre-dephosphorization temperature can be raised to a high level, preventing the molten iron from solidifying due to a temperature drop during the process. However, because the dephosphorization reaction is exothermic, treating the molten iron at a high temperature would likely inhibit the reaction, making it impossible to dephosphorize the molten iron to a low phosphorus concentration. In other words, the conventional techniques had the problem of reduced dephosphorization performance when dephosphorization was performed at a high temperature to prevent solidification.

[0009] In addition, in the future, CO ２ To reduce emissions, further increases in the usage 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 then reacts 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's lifespan and increases the frequency of furnace body repairs. This makes it difficult to ensure a stable crude steel production volume. Hereinafter, a state in which the oxygen concentration in the molten iron exceeds 700 mass ppm and the iron oxide concentration in the slag (T.Fe concentration in the slag) exceeds 30 mass %, at which the wear of the furnace body refractory material becomes significant, will be referred to as a "peroxidation" state.

[0010] When dephosphorization is performed on molten steel after melting a cold iron source using a method similar to that described in Patent Document 1, the proportion of oxygen contributing to the decarburization reaction in the supplied oxygen is low, resulting in an increase in the proportion of oxygen contributing to the oxidation of the molten iron and its dissolution into the molten iron, resulting in an increase in the iron oxide concentration in the slag and the oxygen concentration in the molten iron. Therefore, the technique described in Patent Document 1 has the problem that the iron oxide concentration in the slag and the oxygen concentration in the molten iron increase, resulting in a decrease in the lime concentration in the slag and a decrease in the dephosphorization ability of the slag, making it difficult to dephosphorize to a low phosphorus concentration and making it difficult to produce ultra-low phosphorus steel. Furthermore, the technique described in Patent Document 1 requires the introduction of an oxygen source to promote the dephosphorization reaction, which is an oxidation reaction. However, supplying an excessive amount of 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 process is low. Therefore, additional supply of an oxygen source is not required. On the other hand, if the phosphorus concentration before the dephosphorization process is high, it is difficult to dephosphorize the molten steel 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 pressure. 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 said to be capable of dephosphorization, the surfaces of the slag and molten steel are in a reducing atmosphere due to unburned reducing gas, and it is not intended to promote the dephosphorization reaction. In other words, the conventional technology had the problem of accelerating wear of the furnace body refractory due to the overoxidation state and reducing the dephosphorization capacity.

[0011] On the other hand, a method for accelerating the dephosphorization reaction and removing phosphorus to a low concentration is to carry out dephosphorization treatment using high-basicity slag. ２The higher the mass ratio of CaO to slag (the ratio of CaO to slag), the more the dephosphorization reaction is accelerated, enabling dephosphorization to a low phosphorus concentration. However, using a high-basicity slag composition increases the melting point of the slag and increases the amount of undissolved slag. Because slag contributes to the dephosphorization reaction by dissolving, an increase in undissolved slag leads to a deterioration in the dephosphorization reaction. In the method of Patent Document 2, oxidizing molten slag used in refining molten iron in a refining furnace is used to dissolve the slag, and a quicklime slag accelerator is added along with quicklime. However, recent environmental regulations have limited the amount of quicklime slag accelerator used, so it cannot be actively used. In other words, when using high-basicity slag for dephosphorization, the prior art had the problem of poor slag melting and reduced dephosphorization performance.

[0012] In order to solve the above-mentioned problems in the prior art, an object of the present invention is to provide a method for dephosphorizing molten iron that can simultaneously inhibit solidification of molten iron and ensure dephosphorization capacity, and also to suppress wear of the furnace body refractories.

[0013] A first method for dephosphorizing molten iron according to the present invention, which advantageously solves the above-mentioned problems, is a method for dephosphorizing molten iron held in a vessel, in which a slag former and an oxygen source are supplied to the molten iron while a stirring gas is blown into the molten iron, the method comprising: setting the temperature of the molten iron at 1650°C to 1750°C at the start of the dephosphorization treatment; and removing SiO contained in slag produced by dissolving the slag former in the molten iron. ２ a basicity, which is the mass ratio of CaO to CaO, of 4.0 to 6.0; and a gas supply rate ratio, which is the ratio of the amount of oxygen gas in the oxygen source supplied per unit time to the amount of the stirring gas supplied per unit time for stirring the molten iron, is in the range of 1 to 100 on a volume basis under standard conditions.

[0014] Furthermore, in the first molten iron dephosphorization method according to the present invention, (a) the carbon concentration in the molten iron before the dephosphorization treatment is set to 1.0 mass% or less, (b) the oxygen concentration in the molten iron during and after the dephosphorization treatment is set to 700 mass ppm or less and the T.Fe concentration in the slag is set to 30 mass% or less, (c) the molten iron before the dephosphorization treatment is obtained by melting a cold iron source, (d) the cold iron source contains reduced iron, and (e) the vessel is a ladle.

[0015] Furthermore, a more preferable solution to the second molten iron dephosphorization method according to the present invention is any of the above first molten iron dephosphorization methods, in which, prior to the dephosphorization treatment of the 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.

[0016] According to the present invention, it is possible to dephosphorize high-temperature molten iron held in a vessel outside a melting furnace to a low phosphorus concentration while suppressing solidification of the molten iron. Furthermore, by suppressing an increase in the amount of oxygen present in the slag and molten steel, it is possible to perform the treatment while suppressing wear of the furnace body refractories.

[0017] 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.

[0018] 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.

[0019] 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 and the slag former, which is mixed in when the molten iron is tapped from the melting furnace. That is, the slag 5 is generated by dissolving the slag former in the molten iron 7. In the example shown in FIG. 1, a stirring gas 6, an inert gas such as Ar gas, is supplied from a bottom-blowing gas supply plug 8. In this manner, the molten iron held in a vessel such as a ladle is dephosphorized while the slag former and oxygen source are supplied to the molten iron, and the stirring gas is injected into the molten iron to stir the molten iron.

[0020] The high-temperature treatment, oxygen source supply, and use of high-basicity slag in the dephosphorization method of molten iron according to this embodiment will be described below. Here, the basicity of the slag refers to the amount of SiO contained in the slag produced by dissolving a slag former in molten iron. ２ High basicity slag refers to slag that has a basicity of 4.0 to 6.0.

[0021] <High-Temperature Treatment of Molten Iron and Supply of Oxygen Source> First, the temperature of molten iron in the dephosphorization method of this embodiment will be described. Specifically, the dephosphorization method of this embodiment involves supplying a slag former and an oxygen source to molten iron held in a vessel while stirring the molten iron by injecting a stirring gas into the molten iron. The temperature of the molten iron is set to 1650°C to 1750°C. In this dephosphorization method of this embodiment, the temperature of the molten iron (hereinafter also referred to as the treatment start temperature) is set to a high temperature of 1650°C or higher. This prevents the molten iron from solidifying due to a temperature drop during the treatment. Furthermore, the application of heat to the slag with a high melting point facilitates the melting of the slag. Molten steel with a low carbon content is easily oxidized, and the iron oxide concentration in the slag is likely to be high. The high iron oxide concentration in the slag lowers the melting point of the slag, facilitating its melting. Therefore, compared with the case of molten pig iron with a high carbon content, in the case of molten steel, the slag is more likely to dissolve even when dephosphorization is performed with a high basicity slag composition, as described below.

[0022] For this reason, when dephosphorizing molten iron to a low phosphorus concentration using high-basicity slag, in addition to supplying the oxygen source, high-temperature treatment is effective. However, if the starting temperature for dephosphorizing molten iron is excessively high, the dephosphorization capacity will decrease, making it impossible to dephosphorize to a low phosphorus concentration. Therefore, the starting temperature for dephosphorization is preferably 1750°C or lower. That is, the starting temperature for dephosphorization in this embodiment is in the range of 1650°C to 1750°C. Starting temperatures below 1650°C are undesirable because they may cause solidification of the molten iron. On the other hand, starting temperatures above 1750°C are undesirable because they may cause a decrease in dephosphorization capacity, as mentioned above.

[0023] Here, the treatment start temperature of the molten iron is set in the range of 1650°C to 1750°C, while the carbon concentration in the molten iron before dephosphorization is set to 1.0 mass % or less.

[0024] In this way, by supplying an oxygen source and raising the temperature of the molten iron, it is possible to avoid the deterioration of slag melting that occurs when dephosphorization treatment is performed using high basicity slag.

[0025] <High basicity slag> Next, the high basicity slag used in the dephosphorization method of molten iron according to this embodiment will be described. That is, the dephosphorization method of molten iron according to this embodiment is carried out by dissolving a slag former in molten iron to remove SiO 2 contained in the slag. ２ The slag produced by dissolving a slag former in molten iron has a basicity in the range of 4.0 to 6.0, which is the mass ratio of CaO to molten iron. High basicity slag is a type of slag produced by dissolving a slag former in molten iron.

[0026] Here, the dephosphorization reaction of molten iron is an exothermic reaction. Therefore, dephosphorization of molten iron at high temperatures inhibits the progress of the dephosphorization reaction, creating extremely unfavorable conditions for the dephosphorization of molten iron. Therefore, in the dephosphorization method of molten iron according to this embodiment, even when dephosphorizing molten iron at high temperatures, the slag generated by dissolving the slag in the molten iron is dissolved in the molten iron so that the basicity of the slag is 4.0 to 6.0 in order to enhance the dephosphorization ability of the slag. Then, the molten iron can be treated with the high-basicity slag generated by dissolving the slag in the molten iron. The dephosphorization method of molten iron according to this embodiment dissolves the slag in the molten iron so that the basicity of the slag generated by dissolving the slag in the molten iron falls within this range, thereby enabling dephosphorization to a low phosphorus concentration. As described above, in the dephosphorization method of molten iron according to this embodiment, the basicity of the slag used for dephosphorization of molten iron is in the range of 4.0 to 6.0. A slag basicity of less than 4.0 is undesirable because the dephosphorization of molten iron by the slag is insufficient. On the other hand, a slag basicity of more than 6.0 is undesirable because the melting point of the slag is reduced. In other words, the dephosphorization method of molten iron according to this embodiment uses slag with high basicity, which allows the dephosphorization of molten iron to be performed while preventing a decrease in dephosphorization ability due to high-temperature treatment.

[0027] As described above, according to the present embodiment, by setting the temperature of the molten iron before treatment at 1650°C to 1750°C, setting the basicity of the slag at 4.0 to 6.0, and treating the molten iron while supplying oxygen, it is possible to prevent solidification of the molten iron and to prevent a decrease in dephosphorization capacity. In other words, the method for dephosphorizing molten iron according to the present embodiment can simultaneously inhibit solidification of the molten iron and ensure dephosphorization capacity.

[0028] <Oxygen Supply Rate / Stirring Gas Flow Rate (Supply Gas Rate Ratio)> Next, the dephosphorization method of molten iron according to this embodiment is characterized in that the supply gas rate ratio, which is the ratio of the supply amount of oxygen gas in the oxygen source per unit time to the supply amount of the stirring gas per unit time supplied to stir the molten iron, is set in the range of 1 to 100 on a volumetric basis under standard conditions. Here, when dephosphorizing molten iron having a carbon concentration of 1.0 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 stirring gas flow rate (Nm m ), which is the supply amount of the stirring gas per unit time supplied to stir the molten iron on a volumetric basis under standard conditions, is set to 100. ３ / (min t)) is the oxygen supply rate (Nm ３ / (min·t)) (hereinafter also referred to as the “supply gas velocity ratio”) was used as an index. In this embodiment, this index is adjusted to be in the range of 1 to 100.

[0029] In adjusting the gas flow rate ratio (the index) within a predetermined range, the supply rate of the oxygen source per unit time 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, the supply rate of the stirring gas supplied for stirring per unit time can be set to a predetermined value or more, thereby promoting the mass transfer of carbon in the molten iron. This can suppress local increases 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 the molten iron becomes excessively oxidized. Therefore, it is effective to set the supply rate of the oxygen source per unit time to a predetermined value or less and to set the supply rate of the stirring gas supplied for stirring to a predetermined value or more.

[0030] Specifically, it is effective to set the feed gas velocity ratio, which serves as the index, to 1 to 100. If the feed gas velocity ratio is less than 1, it is not preferable because a sufficient oxygen source for promoting the dephosphorization reaction cannot be secured. On the other hand, if the feed gas velocity ratio exceeds 100, the oxygen source becomes excessive, which is also not preferable.

[0031] In order to adjust the above index to a predetermined value range, the supply amount per unit time (oxygen supply rate) of oxygen gas in the oxygen source is set to 0.1 to 10 Nm ３ / (min·t), preferably 0.2 to 2.0 Nm ３ In order to adjust the above index to a predetermined value range, the supply amount per unit time of the stirring gas supplied to stir the molten iron (stirring gas flow rate) is set to a range of 0.001 to 0.10 Nm ３ / (min·t), preferably 0.002 to 0.06 Nm ３ / (min·t) range.

[0032] As described above, the method for dephosphorizing molten iron according to this embodiment can suppress wear of the furnace body refractory material due to an overoxidation state and also prevent a decrease in dephosphorization ability by setting the supply gas velocity ratio, which is the ratio of the supply amount per unit time of oxygen gas in the oxygen source to the supply amount per unit time of stirring gas supplied for stirring molten iron, to 1 to 100 on a volume basis under standard conditions.

[0033] The steps according to an embodiment of the present invention will be specifically described below. In the first step, an electric steelmaking furnace is used as the melting furnace, for example, 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 makes it possible to reduce CO ２ This is preferable from the viewpoint of reducing emissions. Here, the temperature may be increased again at the end of the second step described below or before proceeding to the third step described below, and then the molten iron may be dephosphorized.

[0034] 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.

[0035] In the third step, the molten iron contained in the ladle is subjected to dephosphorization. Here, the molten iron needs to be heated before being dephosphorized. This heating can be carried out in an electric furnace, or it can be carried out by electrode heating after removing slag from the ladle after tapping from the electric furnace. When adding the slag former, the ratio of (calcium oxide: CaO concentration) / (silicon oxide: SiO ２The amount of slag former added should be adjusted so that the slag basicity, defined as the ratio of slag concentration to slag, is approximately 4.0 to 6.0. The slag former used as the calcium oxide source may be quicklime, calcium carbonate, or calcium hydroxide. The slag former may be sprayed together with the oxygen source or injected together with a carrier gas. It may also be added from a hopper above the furnace.

[0036] In the dephosphorization treatment, oxygen gas, for example, 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 as the oxygen source is adjusted so that the above index (feed gas velocity ratio) falls within a predetermined range. The oxygen supply rate is 0.10 to 3.0 Nm3 as oxygen gas per unit mass of molten iron per unit time. ３ / (min·t), preferably 0.15 to 2 Nm ３ / (min·t) range is preferable. Spitting behavior differs depending on the ladle freeboard height and the shape of the top lance nozzle, so 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 an 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.

[0037] Figure 2 shows the relationship between the total iron (T.Fe) concentration (mass%) in the slag and the furnace body refractory wear index (-). The furnace body refractory wear index is an index that quantitatively indicates the depth of wear on the furnace body refractory caused by the dephosphorization of molten iron. The furnace body refractory wear index was normalized to 1.0 when the average T.Fe concentration in the slag in the converter in the blast furnace-converter process is 20 mass%. As shown in Figure 2, the upper limit of the T.Fe concentration in the slag is preferably 30 mass% so that the furnace body refractory wear index can be suppressed to 2.0 or less. Furthermore, when the T.Fe concentration in the slag is around 20 mass%, the slag has an optimal slag composition for the dephosphorization reaction. At this time, the equilibrium phosphorus concentration is lowest. However, if the T.Fe concentration in the slag is 20 mass%, the slag has a low T.Fe concentration. When the Fe concentration is less than 10 mass%, the dephosphorization ability is significantly reduced, so the lower limit of the T.Fe concentration in the slag is preferably set to 10 mass%. The upper limit of the oxygen concentration in the molten iron is preferably set to 700 mass ppm, based on the equilibrium value with the T.Fe concentration in the slag at the molten steel temperature, based on the iron-iron oxide dissolution equilibrium. On the other hand, from the viewpoint of dephosphorization treatment, the oxygen concentration in the molten iron is preferably set to 300 mass ppm or more.

[0038] In the method for dephosphorizing molten iron according to this embodiment, a stirring gas is supplied into the molten iron at the start of the dephosphorization process. This stirring gas may be supplied from an injection lance or from a porous plug or the like installed at the bottom of the ladle. The stirring gas promotes the reaction between the slag and the molten iron, thereby accelerating the dephosphorization reaction.

[0039] In the fourth step, the slag floating on the surface of the dephosphorized molten iron is separated from the dephosphorized molten iron. For example, a vessel such as a ladle containing the dephosphorized molten iron 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 treatment, the dephosphorized portion of the phosphorus contained in the molten iron prior to the dephosphorization treatment 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, i.e., so-called rephosphorization, even if the dephosphorized molten iron is deoxidized in the subsequent deoxidation step. 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.

[0040] 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.

[0041] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0042] In the examples and comparative examples, the dephosphorization process was carried out as follows. Scrap and reduced iron were charged as cold iron sources into a 200-ton electric furnace and melted. The initial phosphorus concentration after melting varied depending on the type and amount of cold iron source used and the progress of the dephosphorization reaction in the electric furnace. After melting in the electric furnace, the molten iron was poured into a ladle. The molten iron was heated in the electric furnace so that the temperature after pouring was 1650°C or higher. After pouring into the ladle, dephosphorization was carried out by adding a slag former while supplying oxygen gas from a top-blowing gas supply lance and argon gas as a stirring gas from a bottom-blowing gas supply plug, 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. Vacuum decarburization was then carried out by the VOD method, and other components were adjusted by adding alloys.

[0043] The results of Test Nos. 1 to 10 (inventive examples) are shown in Table 1, and the results of Test Nos. 11 to 16 (comparative examples) are shown in Table 2. The dephosphorization treatment time was 10 to 60 minutes. A post-treatment phosphorus concentration [P]f in the molten iron of 0.020 mass% or less (hereinafter also referred to as the "target phosphorus concentration") was evaluated as excellent dephosphorization and marked with a "good" 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 in the slag was 30 mass% or less. Otherwise, the evaluation was marked with a "bad" symbol.

[0044] (Tests Nos. 1 to 3) Molten iron was dephosphorized using the dephosphorization method of the present embodiment. In Tests Nos. 1 to 3, the initial phosphorus concentration (initial condition) was changed to 0.02, 0.04, and 0.06 mass%, respectively, while the other conditions were the same. Other conditions included a stirring gas flow rate of 0.060 Nm ３ / (min t) and the oxygen supply was 0.23 Nm ３ By carrying out the treatment at an oxygen supply rate of 1 / (min·t), the ratio of the supply rate of the oxygen source to the supply rate of the stirring gas supplied to stir the molten iron (supply rate ratio) was set to 4.0, which is less than 100. In addition, the treatment start temperature was set to 1650°C, and the slag basicity was set to 4.0.

[0045] In Tests 1 to 3, the initial phosphorus concentrations were different, and therefore the amount of oxygen required for dephosphorization until the phosphorus concentration reached 0.020% by mass or less varied. Conditions with higher initial phosphorus concentrations required more oxygen. Consequently, the oxygen concentration in the molten iron and the total iron content in the slag at the end point were high. In the region where the total iron content in the slag was 20% by mass or more, the dephosphorization ability of the slag decreased as the total iron content in the slag increased, resulting in a high phosphorus concentration at the end point.

[0046] (Tests Nos. 4 and 5) Molten iron was dephosphorized using the dephosphorization method of the present embodiment. In Tests Nos. 4 and 5, the initial phosphorus concentration was set to 0.04 mass%, and the feed rate ratio was varied within a range of 100 or less by changing the stirring gas and oxygen supply rates. The other conditions of the treatment start temperature and slag basicity were the same as those in Tests Nos. 1 to 3. As the feed rate ratio increased, oxidation progressed, and the oxygen concentration in the molten iron and the T.Fe concentration in the slag at the end point increased. As in Tests Nos. 1 to 3, the T.Fe concentration in the slag was high in the region of 20 mass% or more, and therefore, the phosphorus concentration at the end point increased as oxidation progressed.

[0047] (Test No. 6) Molten iron was dephosphorized using the dephosphorization method of the present embodiment. In Test No. 6, the conditions were the same as those in Test No. 1, except that the initial phosphorus concentration was 0.04 mass% and the treatment start temperature was 1675°C. When the temperature drop during treatment is large, the treatment start temperature needs to be set high to avoid solidification due to the temperature drop during treatment. However, a high treatment temperature is unfavorable to the dephosphorization reaction, which is an exothermic reaction, and dephosphorization to a low phosphorus concentration was impossible. Therefore, the end point phosphorus concentration was higher than in Test No. 1, where the treatment temperature was lower.

[0048] (Test No. 7) Molten iron was dephosphorized using the dephosphorization method of the present embodiment. Test No. 7 was conducted under the same conditions as Test No. 1, except that the initial phosphorus concentration was 0.04 mass% and the slag basicity was 4.5. By increasing the slag basicity, the dephosphorization reaction proceeds to a low phosphorus concentration when the slag is completely dissolved. However, when the slag basicity is increased, the melting point of the slag becomes high, making dissolution difficult. Under these conditions, the slag was dephosphorized to a lower phosphorus concentration than in Test No. 1 because as much of the slag as possible was dissolved. However, because the slag was not completely dissolved, the dephosphorization reaction only proceeded slightly.

[0049] (Tests Nos. 8 to 10) Molten iron was dephosphorized using the dephosphorization method of this embodiment. In Tests Nos. 8, 9, and 10, the slag basicities were set to 5.0, 5.5, and 6.0, respectively. In Tests Nos. 8, 9, and 10, the treatment start temperatures were set to 1690°C, 1720°C, and 1750°C, and the stirring gas and oxygen supply rates were changed to vary the supply rate ratio within a range of 100 or less. Under these conditions, the treatment start temperature was high and a large amount of slag dissolved, so the phosphorus concentration after treatment was reduced to less than half of the phosphorus concentration before treatment.

[0050] (Tests Nos. 11 to 14) Molten iron was dephosphorized using the dephosphorization method of this embodiment. In Tests Nos. 11 to 14, the treatment start temperature or slag basicity was outside the range of the present invention, resulting in poor dephosphorization. Furthermore, even when other influencing factors were changed, treatment was impossible, as in Test No. 11, or dephosphorization to the target phosphorus concentration was not achieved, confirming that the dephosphorization process itself was not successful.

[0051] (Tests Nos. 15 and 16) Molten iron was dephosphorized using the dephosphorization method of the present embodiment. In Tests Nos. 15 and 16, although the treatment start temperature and slag basicity were within the ranges of the present invention, the supply rate ratio was outside the ranges of the present invention, resulting in a shortage or excess of the oxygen source for dephosphorization, which resulted in a low lime concentration in the slag and made it impossible to dephosphorize to the target phosphorus concentration.

[0052]

[0053]

[0054] As can be seen from Tables 1 and 2, in Tests Nos. 1 to 10, by setting the treatment start temperature, slag basicity, and feed rate ratio within the ranges of the present invention, an overoxidation state was avoided and dephosphorization to the target phosphorus concentration was achieved.

[0055] On the other hand, in Tests Nos. 11 to 16, it was confirmed that an overoxidation state could not be avoided and / or dephosphorization to the target phosphorus concentration could not be achieved because any of the treatment start temperature, slag basicity, and feed rate ratio was outside the range of the present invention. Furthermore, when the treatment temperature was low, as in Test No. 11, the molten iron solidified due to a temperature drop during treatment, and treatment became impossible (shown as "treatment impossible" in Table 2).

[0056] As described above, the dephosphorization method of molten iron according to the present invention can simultaneously inhibit solidification of molten steel and ensure dephosphorization capability, and also can suppress wear of the furnace body refractory due to an overoxidized state. Furthermore, by suppressing an increase in the amount of oxygen in the molten steel, the addition of aluminum becomes unnecessary, and aluminum oxide can be prevented from remaining in the molten steel, thereby preventing deterioration of the properties of the molten steel.

[0057] 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. ２Furthermore, with the shift from the current blast furnace process to a molten steel manufacturing process using a melting furnace such as an electric furnace, the present invention is extremely useful in treating molten iron with a high phosphorus concentration when a cold iron source is used.

[0058] 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. A method for dephosphorizing molten iron, which comprises supplying a slag former and an oxygen source to molten iron held in a vessel and injecting a stirring gas into the molten iron to stir the molten iron, wherein the temperature of the molten iron at the start of the dephosphorization treatment is 1650°C to 1750°C, and SiO contained in slag produced by dissolving the slag former in the molten iron is 1650°C to 1750°C. ２ a basicity, which is the mass ratio of CaO to CaO, of 4.0 to 6.0, and a gas supply rate ratio, which is the ratio of the amount of oxygen gas in the oxygen source supplied per unit time to the amount of the stirring gas supplied per unit time for stirring the molten iron, is in the range of 1 to 100 on a volume basis under standard conditions.

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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