Pre-treatment method for molten iron

Abstract

To provide a preliminary treatment method of molten iron capable of suppressing occurrence of a base metal sticking trouble and performing economically advantageous refining.SOLUTION: A preliminary treatment method of molten iron includes a first blowing step in which a gas oxygen source is supplied to molten iron in a converter type refining furnace from a top blow lance for desiliconization and dephosphorization of the molten iron, a slag discharge step in which at least a part of the slag produced in the first blowing step is discharged from the converter type refining furnace, and a second blowing step in which, after the slag discharge step, a CaO-based solvent is added to the converter type refining furnace and a gas oxygen source is supplied from the top blow lance for dephosphorization and decarbonization of the residual molten iron. In the second blowing step, when the gas oxygen source is supplied from the top blow lance to the molten iron bath surface, the blowing is carried out so that an oxygen collision pressure PC obtained by a predetermined relation becomes 58800 Pa or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steelmaking method using a converter or a converter-type smelting furnace, and to a pretreatment method for molten iron therefor, including desiliconization, dephosphorization, and decarburization of the molten iron. In the following description, the unit of mass "t" is 10 3 This represents kg. Furthermore, the "N" prefix attached to the unit of gas volume represents the volume at standard conditions, which are defined as 0°C and 101325 Pa. In this specification, "x~y" representing a numerical range means x or greater and y or less, including boundary values. [Background technology]

[0002] In conventional pretreatment methods for molten iron, which include desiliconization, dephosphorization, and decarburization processes, a gaseous oxygen source is supplied from an upper-blowing lance to perform the desiliconization, dephosphorization, and decarburization treatments of the molten iron. To efficiently perform the desiliconization, dephosphorization, and decarburization treatments, treatment methods are employed that vary three values: the acid supply rate, the lance height, and the bottom-blowing gas flow rate.

[0003] For example, Patent Document 1 discloses a pretreatment method for molten iron in which desilicate treatment and dephosphorization treatment of molten iron are performed consecutively using a converter-type smelting furnace, with an intermediate slag discharge step in between. It is stated that a sufficient amount of slag can be quickly discharged from the furnace in the slag discharge step after the desilicate treatment, and that sufficient dephosphorization treatment can be performed in the subsequent dephosphorization treatment in terms of cost and quality. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2014-159632 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, the above-mentioned conventional technology had the following problems. In the technology disclosed in Patent Document 1, specific numerical values of the acid feeding rate, lance height, and bottom blowing gas flow rate have not been established. Therefore, in the dephosphorization and decarburization treatment process, the quality of the dephosphorization treatment has been improved. On the other hand, the molten metal ejected and generated during blowing may adhere to and fall on the hood portion of the converter equipment. And there was room for improvement in reducing the operating cost for removing the molten metal.

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a pretreatment method for hot metal that suppresses the occurrence of troubles such as the adhesion and fall of molten metal generated during blowing on the hood portion of the converter equipment and enables economically advantageous refining.

Means for Solving the Problems

[0007] The pretreatment method for hot metal according to the present invention that advantageously solves the above problems includes a first blowing step of supplying a gaseous oxygen source from an upper blowing lance to the hot metal in a converter-type refining furnace to perform desiliconization and dephosphorization treatment on the hot metal, a slag discharging step of discharging at least a part of the slag generated in the first blowing step from the converter-type refining furnace, and after the slag discharging step, adding a CaO-based solvent to the converter-type refining furnace and supplying a gaseous oxygen source from the upper blowing lance to perform dephosphorization and decarburization treatment on the remaining hot metal. The pretreatment method for hot metal includes a second blowing step, and in the second blowing step, when supplying the gaseous oxygen source from the upper blowing lance to the hot metal bath surface, the oxygen impact pressure P obtained by the following formulas (1) to (3) C is blown so as to be 58800 Pa or less. P C =ρ·V 2 / 2 (1) V / (V0 - V loss ) = d e / (2·C·Z) (2) C = 0.016 + 18600 / (P0 - P e ) (3) Here, P C : Oxygen impact pressure on the bath surface (Pa), ρ: Gas density of the gaseous oxygen source (1.43 kg / m 3 ), V: Central flow velocity of the gaseous oxygen source (m / s), V0: Nozzle exit jet velocity (m / s), V loss : Flow velocity loss due to improper expansion (m / s), d e : Nozzle exit diameter (m), C: Constant, Z: Lance height (m), P0: Nozzle absolute pressure (Pa), P e : Atmospheric pressure (Pa) represents.

[0008] In addition, in the hot metal pretreatment method according to the present invention, in the second blowing step, regarding the supply of the gaseous oxygen source, when the cumulative oxygen supply amount from the start of blowing is less than 30% of the total oxygen supply amount, the oxygen impact pressure P C is in the range of 10800 - 11800 Pa, and when the cumulative oxygen supply amount is in the range of 30% or more and less than 70% of the total oxygen supply amount, the oxygen impact pressure P C is in the range of 14700 - 15700 Pa, and in the range where the remaining cumulative oxygen supply amount is 70% or more of the total oxygen supply amount, it is more preferable to set the oxygen impact pressure P C in the range of 49000 - 50000 Pa. [Effects of the Invention]

[0009] According to the present invention, since the oxygen impact pressure during oxygen blowing is properly controlled, the occurrence of troubles such as the adhesion and dropping of the ingot generated during blowing to the hood part of the converter equipment is suppressed, and an economically advantageous pretreatment of hot metal can be performed. [Brief Description of the Drawings]

[0010] [Figure 1] (A) - (E) are schematic diagrams showing the hot metal pretreatment method according to an embodiment of the present invention in the order of steps. [Figure 2] It is a pattern diagram of the blowing of the present invention. [Figure 3] It is a pattern diagram of blowing outside the scope of the present invention. [Figure 4] It is a graph showing the comparison of the number of troubles caused by ingots between the method of the present invention and outside the scope of the present invention. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described in detail below. The following embodiments are illustrative of equipment and methods for realizing the technical idea of ​​the present invention, and do not limit the configuration to those described below. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0012] Figures 1(A) to 1(E) show a schematic diagram of a pretreatment method for molten iron according to one embodiment of the present invention, in order of steps. In this embodiment, first, new molten iron 9a is received from the charging ladle 14 into the converter-type refining furnace 1 (Figure 1(A)), and a first blowing step (Figure 1(B)) is performed in which oxygen-containing gas 12 as a gaseous oxygen source is supplied to the molten iron 9 in the converter-type refining furnace 1 from the top blowing lance 2 to desilicate and dephosphorize the molten iron 9. Next, a slag removal step (Figure 1(C)) is performed in which at least a portion of the slag 10 generated in the first blowing step is discharged from the converter-type refining furnace 1. Next, after the slag removal step, a CaO-based solvent is added into the converter-type refining furnace 1, and a second blowing step (Figure 1(D)) is performed in which oxygen-containing gas 12 as a gaseous oxygen source is supplied from the top blowing lance 2 to dephosphorize and decarburize the remaining molten iron 9. Next, the molten iron 9, after the blowing process is complete, is tapped out. (Figure 1(E))

[0013] In this embodiment, the oxygen impingement pressure P during oxygen blowing in the second blowing step is C This is managed to control the scattering of metal during smelting. The oxygen impact pressure from the upper blowing lance 2 can be calculated by the following equations (1) to (3). P C =ρ·V 2 / twenty one) V / (V0-V loss )=d e (2·C·Z) (2) C = 0.016 + 18600 / (P0 - P e ) (3) Here, P C : Oxygen impingement pressure (Pa) on the bath surface, ρ: Gas density of the gaseous oxygen source (1.43 kg / m³)3 ), V: Central flow velocity of the gaseous oxygen source (m / s) V0: Nozzle outlet flow velocity (m / s) V loss : Flow velocity loss due to improper expansion (m / s) d e : Nozzle outlet diameter (m), C: Constant, Z: Lance height (m) P0: Nozzle absolute pressure (Pa) P e Atmospheric pressure (Pa) It represents.

[0014] In the second blowing process, the amount of acid supplied per unit time is increased to improve dephosphorization efficiency, and the height of the upper blowing lance is lowered to increase the oxygen impingement pressure P. C A blowing process is performed to increase the oxygen impingement pressure P. C If the oxygen impact pressure exceeds 58,800 Pa, a large amount of iron particles will be scattered from the surface of the molten iron 9 by the oxygen jet. There is a risk that these iron particles will accumulate on the hood of the converter equipment, becoming attached to the base metal, or that they will fall and damage surrounding equipment, causing frequent problems. Therefore, the oxygen impact pressure P of the oxygen-containing gas 12 blown onto the molten iron from the upper blowing lance 2 in the second blowing process is important. C It will be limited to a maximum of 58,800 Pa.

[0015] On the other hand, if the oxygen impact pressure is lowered too much, the dephosphorization efficiency decreases, leading to problems such as an extended processing time. Therefore, the oxygen impact pressure P during the second blowing process is important. C It is preferable that the pressure be at least 9800 Pa or higher.

[0016] The pretreatment method for molten iron according to this embodiment will be described in detail. The pretreatment method for molten iron according to this embodiment using a converter-type smelting furnace 1 is carried out in the following order, as shown in the flow chart of Figure 1: (A) molten iron charging process, (B) first blowing process, (C) slag discharge process, (D) second blowing process, and (E) molten metal tapping process. In particular, it is preferable to repeatedly perform these processes in the same furnace, as this enables efficient pretreatment of molten iron.

[0017] [Hot metal charging process] First, the molten iron charging process (A) will be explained. In this process (A), it is preferable to charge new molten iron 9a from the charging ladle 14 while leaving the slag 17 (hereinafter simply referred to as "dephosphorization and decarburization slag") 17 generated in the previous pretreatment of molten iron after the second blowing process in the converter-type smelting furnace 1. On the other hand, dephosphorization and decarburization slag often adheres to the furnace wall of the converter-type smelting furnace 1 after the previous pretreatment of molten iron. In addition, molten iron 9a is charged after charging a cold iron source 11 such as iron scrap 11 before charging. The cold iron source 11 to be charged in advance may be iron scrap as specified in the "Unified Standards for Inspection and Acceptance of Iron Scrap" of the Japan Iron Source Association, or it may be iron-based, such as directly reduced iron or cold iron.

[0018] Here, the slag 17 after the previous dephosphorization and decarburization treatment, which is left in the refining furnace 1 in preparation for the next refining, or which adheres to the furnace wall, is used to adjust the slag basicity during the subsequent first blowing process. The basicity of this dephosphorization and decarburization slag, that is, the concentration ratio of (CaO) to (SiO2) in the slag (CaO) / (SiO2) (hereinafter simply referred to as "basicity") on a mass basis, is preferably 1.2 or higher, and more preferably 1.4 or higher. The reason for this is that if the basicity of the dephosphorization and decarburization slag 17 at the end of the previous second blowing process is less than 1.2, leaving this slag in place may not be sufficient for adjusting the basicity in the first blowing process. Therefore, it becomes necessary to add a large amount of lime-based solvent during the desilicate treatment in the first blowing process. There is no particular upper limit on the basicity of the dephosphorization and decarburization slag mentioned above. Normally, the basicity of slag produced during the dephosphorization and decarburization of molten iron is kept below approximately 3.0. Therefore, there is no need to increase the basicity further.

[0019] Furthermore, the amount of slag 17 remaining in the furnace after the previous dephosphorization and decarburization treatment is preferably 30% by mass or more of the amount of slag generated during the previous dephosphorization and decarburization treatment of the molten iron, in order to effectively adjust the basicity. It is more preferable to have 60% by mass or more. In this embodiment, if the entire amount of slag remaining in the furnace after tapping the molten iron through the second blowing process is used for the desiliconization treatment of the new molten iron, it is even more effective in adjusting the basicity during the desiliconization treatment. Moreover, if this method is carried out continuously, the pre-treatment slag discharged will be homogeneous, consisting only of slag after the first blowing process. Therefore, the pre-treatment slag will not be mixed with the dephosphorization and decarburization slag, which has a high basicity. As a result, problems such as slag expansion due to hydration reactions and alkali leaching will not occur. Thus, this embodiment is extremely effective in terms of utilizing the slag as a material.

[0020] Furthermore, the slag after the dephosphorization and decarburization treatment described above has relatively high basicity, a relatively high melting point, and is relatively low temperature (around 1350°C or below), resulting in low fluidity. Therefore, even if a cold iron source is charged onto this dephosphorization and decarburization slag, the cold iron source will not be enveloped by the slag, preventing delayed melting. This eliminates the need for inefficient operations in terms of heat and mass balance, such as adding a large amount of coolant to solidify the slag, as is done when so-called decarburization slag is left in the furnace. In addition, due to the above characteristics, this dephosphorization and decarburization slag is rich in solid phase and has low fluidity, and therefore contains a large amount of fine metallic iron within its structure. For this reason, even after solidifying the slag, crushing it, and then undergoing magnetic separation, it still contains more than 10% by mass of metallic iron. Conventionally, this slag was discharged outside the system. According to this embodiment, the slag after the dephosphorization and decarburization treatment can be carried over to the next refining treatment, so that most of the metallic iron in the slag after the dephosphorization and decarburization treatment can be recovered into the molten iron, thus reducing iron loss. On the other hand, the slag after the first blowing process has relatively high fluidity, so the metallic iron in the slag tends to become coarser. Therefore, in such slag, there is little metallic iron remaining in the slag that is not recovered after the slag crushing and magnetic separation treatment. Thus, in this embodiment, iron loss into the slag can be reduced throughout the entire pretreatment of the molten iron.

[0021] [First blowing process] Next, the first blowing process (B) shown in Figure 1(B) will be described. This process (B) involves raising the converter-type smelting furnace 1 upright and supplying oxygen-containing gas 12 as a gaseous oxygen source to the molten iron 9 via the acid supply lance 2 to perform desiliconization and dephosphorization. In this first blowing process, the Si source and the lime-based flux contained in the hopper can be added to the converter-type smelting furnace 1 via chutes. In addition, carbon materials that serve as heat sources, Si sources, or iron oxide that serve as oxygen sources can also be added in the same manner. As an oxygen source for the desiliconization and dephosphorization process, from the viewpoint of dissolving a large amount of cold iron source 11, it is preferable to use only oxygen-containing gas 12 without using iron oxide, which has a large heat absorption capacity. It is especially preferable to use pure oxygen. It is preferable to blow in bottom-blowing gas 13 from the bottom-blowing tuyeres 3 and stir.

[0022] In this desiliconization process, Si contained in the molten iron 9a, or Si contained in the Si source and cold iron source 11 that migrate into the molten iron upon melting, reacts with the oxygen source (Si + O2 → SiO2) to desiliconize. This helps to improve the efficiency of the dephosphorization reaction in the subsequent second blowing process. In addition, P contained in the molten iron 9a is simultaneously dephosphorized. In this first blowing process, oxidation heat of silicon and phosphorus is generated, and this oxidation heat raises the molten metal temperature, promoting the melting of the cold iron source 11 in the molten iron. The composition of the slag in this first blowing process is determined by considering the amount and estimated composition of the slag 17 from the previous dephosphorization and decarburization treatment that is left in the furnace beforehand, and the amount of SiO2 produced by the above reaction.

[0023] In other words, it is preferable to adjust the basicity of the slag during the first blowing process to between 0.8 and 1.5. The reason for this is that if the basicity of the slag during the first blowing process is less than 0.8, depending on the [Si] (mass%) concentration in the molten iron, a phenomenon of rephosphorization may occur due to a decrease in the dephosphorization ability of the slag 17 after dephosphorization and decarburization treatment, that is, an increase in the [P] concentration in the molten iron. On the other hand, if this slag basicity is greater than 1.5, the solid phase ratio increases due to the increase in unslagled CaO, which worsens the fluidity of the slag during the first blowing process, and it may become impossible to remove the slag. The preferred upper limit for the basicity of the slag is around 1.2.

[0024] Next, it is preferable to adjust the basicity of the slag at the end of the first blowing process to be between 0.5 and 1.5. If the basicity of the slag at this stage is less than 0.5, the viscosity of the slag will be high, and there is a risk that a good slag removal rate cannot be secured in the next slag removal process. Therefore, it is preferable that the basicity of the slag at the end of the first blowing process be 0.5 or higher, and more preferably 0.8 or higher. Furthermore, if the basicity of the slag at this stage is higher than 1.5, the fluidity of the slag will decrease, which can lead to problems such as a small amount of slag being removed in the next slag removal process or difficulty in controlling the amount of slag removed. This is also inefficient in terms of reducing the amount of lime-based solvents used. Therefore, it is preferable that the basicity of the slag at the end of the first blowing process be 1.5 or lower, and more preferably 1.2 or lower. Furthermore, to adjust the basicity, lime-based fluxes such as quicklime, limestone, and dolomite can be used, as well as steelmaking slags selected from decarburized slag, derinsed slag, and ladle slag.

[0025] The molten metal temperature at the end of the first blowing process is preferably adjusted to between 1240°C and 1400°C. This is because temperatures above 1400°C cause rephosphorization from the slag 17 remaining in the furnace after dephosphorization and decarburization, leading to an increase in the [P] concentration in the molten iron. This increases the dephosphorization load in subsequent processes, making them inefficient. Furthermore, it becomes necessary to increase the magnesia (MgO) concentration in the slag to prevent wear of the magnesia carbon brick lining, resulting in higher costs. More preferably, the temperature should be 1350°C or lower. On the other hand, temperatures below 1240°C reduce the fluidity of the slag, leading to problems such as a reduced amount of slag discharged during the next slag discharge process or difficulty in controlling the amount of slag discharged. In addition, the melting rate of the scrap decreases. More preferably, the temperature should be 1260°C or higher.

[0026] Furthermore, the molten metal temperature at this stage must be controlled in order to efficiently perform dephosphorization in the subsequent second blowing process. For example, if the molten metal temperature at the end of the first blowing process is kept below 1350°C, the amount of coolant such as iron ore added for temperature control in the second blowing process can be significantly reduced. It should also be noted that when desiliconization / dephosphorization and dephosphorization / decarburization processes are performed consecutively in the same furnace, adding scrap before the dephosphorization / decarburization process is difficult in terms of working time. Additionally, the cold iron sources that can be added from the furnace during processing are either expensive, well-grained materials or limited in quantity, such as ingots generated within the steelworks. Therefore, it is difficult to use large quantities of cold iron sources on a regular basis during the dephosphorization / decarburization process. In practice, the number of types of auxiliary materials that can be used in the furnace input device is also limited. In such cases, cold iron sources may not be added from the furnace. In short, conventional coolants used in dephosphorization and decarburization processes are limited to iron oxides such as iron ore, and it is generally not possible to fully utilize inexpensive cold iron sources such as scrap.

[0027] The molten metal temperature after the first blowing process may be measured using thermocouples or similar devices, or it may be estimated from the heat balance. In the heat balance estimation method, coefficients can be adjusted, and variables can be added or removed depending on the individual equipment and operating conditions. Note that if there is undissolved scrap after desiliconization and dephosphorization, the measured value tends to be slightly higher than the estimated value, but this is within the margin of error and does not affect its use.

[0028] The additives may also contain carbon. This carbon can be derived from carbon materials such as coke or earthy graphite, or from carbonized materials such as the aforementioned Si carbon. As a fluxing agent, auxiliary materials such as quicklime, light-calcined dolomite, and magnesia clinker can be used. In addition, slags such as derinsing slag, decarburizing slag, and ladle slag can be used as a source of calcium oxide or magnesium oxide. As an example of an inexpensive auxiliary material, calcium or magnesium carbon oxides or hydroxides may be used. However, since these have a large heat absorption capacity, it is preferable to distinguish them from other fluxing agents and adjust the estimated heat balance when using them in large quantities.

[0029] Ferrosilicon can be used as the Si source to be added. Alternatively, it is preferable to use cheaper Si carbide briquettes or waste refractories mainly composed of Si carbide. It is not necessary to use only this Si source as a heat source; other heat sources such as carbon materials may be used in combination as long as productivity does not decrease. The carbon material should be added in a manner that predicts the amount of decarburization, etc., so that the [C] concentration in the molten iron at the end of the first blowing process is 3.3 mass% or more. This is because if it is less than 3.3 mass%, there will be insufficient heat in the subsequent dephosphorization and decarburization processes. At the same time, the carburization rate on the surface of the cold iron source such as scrap will decrease, leading to a decrease in the melting rate of the cold iron source.

[0030] As will be explained in detail later, in order to improve the slag removal efficiency of the slag 10 after the first blowing process, it is preferable to induce appropriate foaming in the slag within the converter-type smelting furnace 1. To achieve this, it is effective to increase the generation rate of CO gas produced by the reaction between carbon in the molten iron and iron oxide in the slag. Therefore, in order to obtain a stable slag removal rate in the next slag removal process, it is preferable to supply more than the stoichiometric amount of oxygen necessary to oxidize the Si in the molten iron and the added Si source.

[0031] The oxygen supply to the molten iron during the first blowing process is 2 Nm³ of the amount stoichiometrically required for desiliconization. 3 / t-molten iron or higher, more preferably 4Nm 3 It is preferable to add an amount of 1 / t-molten iron or more. In this embodiment, it is preferable to perform such acid supply to reduce the [Si] concentration in the molten iron to 0.2 mass% or less at the end of the first blowing process. More preferably, it is 0.1 mass% or less, and even more preferably 0.05 mass% or less. This makes it possible to maintain the foaming state and good slag discharge even when slag is discharged after the first blowing process. At the same time, it becomes possible to suppress rephosphorization from slag to molten iron. For oxygen blowing for desiliconization treatment, the top blowing acid supply rate is 1-2 Nm 3 / (min·t-molten iron) is preferred. The upper blowing lance height Z is preferably 2.2 to 3.3 m. The oxygen impingement pressure P obtained from the above relational equation. C It is preferable to keep the pressure below 10800 Pa. At the same time, the injection velocity of the bottom-blown gas should be 0.02 to 0.2 Nm 3 We have confirmed that the above effect can be obtained when the molten iron is approximately / (min·t-molten iron).

[0032] Furthermore, it is preferable to control the [Si] concentration in the molten iron in combination with the control of the basicity of the slag and the control of the molten metal temperature described above. By doing so, even if the second blowing process is carried out with the entire amount of slag remaining in the furnace after the dephosphorization and decarburization treatment from the previous process, the lime content in the slag after dephosphorization and decarburization treatment can be effectively utilized without causing rephosphorization. In addition, by combining the control of the [Si] concentration in the molten iron, the basicity of the slag, and the molten metal temperature, and by leaving the slag remaining in the furnace after the dephosphorization and decarburization treatment from the previous process, the (P2O5) concentration in the slag can be increased. This also promotes slag foaming. In particular, the (P2O5) in the slag has the effect of lowering the surface tension of the slag, promoting the reaction with the molten iron and the dispersion of fine bubbles. For this reason, it is thought that even at relatively low iron oxide concentrations such as a total iron concentration (T.Fe) of about 10 mass%, slag foaming can be maintained and slag discharge performance can be kept good.

[0033] [Slag removal process] Next, the slag removal process (C) shown in Figure 1(C) will be described. In the molten iron pretreatment method according to this embodiment, the slag removal process is performed after the first blowing process described above. That is, a slag removal process is performed to discharge the low-basicity post-treatment slag containing a large amount of SiO2 generated during the first blowing process from the converter-type refining furnace 1. Discharging the post-treatment slag 10 is effective in reducing the amount of lime-based flux used to adjust the slag basicity to an appropriate level in the next process, the second blowing process. In addition, in this embodiment, the desiliconization and dephosphorization process of new molten iron is performed while a large amount of dephosphorization and decarburization slag generated during the previous molten iron pretreatment remains in the furnace. In this case, the desiliconization and dephosphorization process is performed in a way that prevents rephosphorization from the slag to the molten iron, so the (P2O5) concentration in the slag becomes higher than in conventional methods. If a large amount of slag remains after processing, the amount of (P2O5) in the furnace slag during the subsequent second blowing process will become excessive, reducing the dephosphorization effect. Therefore, this is also important in preventing that from happening.

[0034] A characteristic feature of the pretreatment method for molten iron according to this embodiment is that when the aforementioned treatments (A) to (E) are repeatedly and continuously performed, caution is required because if the slag discharge after the first blowing process is insufficient, the accumulation of (P2O5) in the furnace will progress. This is because if the amount of (P2O5) in the furnace slag becomes too large during the dephosphorization and decarburization treatment stage, the efficiency of the dephosphorization reaction decreases due to the increase in the (P2O5) concentration in the slag, and the [P] concentration in the molten iron after treatment increases. In addition, the amount of lime-based flux required for the dephosphorization reaction increases.

[0035] Therefore, in this embodiment, the slag discharge rate after the first blowing process is preferably at least 40% by mass, and more preferably 60% by mass or more. Here, the slag discharge rate (mass%) is defined as the percentage of the value obtained by dividing the mass of discharged slag by the mass of slag in the furnace at the end of the first blowing process. If the slag discharge rate is less than 40% by mass, the amount of lime-based solvent used in the subsequent dephosphorization process will increase as described above. Furthermore, if the amount of slag increases as a result, it becomes impossible to suppress slag forming, and slag ejection from the furnace opening may occur during the dephosphorization process, potentially leading to operational problems due to slag ejection.

[0036] Thus, in the molten iron pretreatment method according to this embodiment, it is preferable that the molten metal temperature at the end of the first blowing process be between 1240°C and 1400°C so that the basicity of the slag at the end of the first blowing process is within the range of 0.5 to 1.5. Furthermore, by optimizing the oxygen intensity to promote slag forming, good slag fluidity and gas holdup can be ensured. Then, a good slag removal rate can be obtained simply by tilting the furnace body after the end of the first blowing process to discharge the slag from the furnace opening. When the tilting angle of the furnace body is adjusted to discharge the slag without causing the molten iron to flow out, a certain amount of slag must remain in the furnace. However, the volume ratio of the slag being formed is about 1 / 10, and its bulk density is lower than its true density. Therefore, the amount of slag remaining in the furnace can be controlled to be low. Furthermore, if the slag forming subsides, it can lead to a decrease in the slag removal rate. Therefore, it is preferable that the time from the end of the first blowing process to the start of tilting the furnace body for slag removal be within 4 minutes.

[0037] In the slag removal process, if the basicity of the slag at the end of the first blowing process is less than 0.5, the viscosity of the slag will increase, and it may not be possible to ensure a good slag removal rate. On the other hand, if the basicity of the slag at the end of the first blowing process exceeds 1.5, solid phase slag will be generated, reducing the fluidity of the slag and potentially lowering the slag removal rate. Thus, from the viewpoint of ensuring slag removal properties and a good slag removal rate, it is preferable to set the basicity of the slag to around 0.5 to 1.5. However, from the viewpoint of preventing rephosphorization from the slag in the first blowing process and reducing the amount of lime-based solvent used, it is more preferable to adjust the basicity of the slag to a range of 0.8 to 1.2.

[0038] Furthermore, slag forming progresses through the reaction of total iron (T.Fe) in the slag at the end of the first blowing process during the slag discharge stage, i.e., fine CO bubbles generated by the reaction between iron oxide and carbon-containing iron particles suspended in the molten iron or slag. To verify the appropriate range of (T.Fe) concentration, separate investigations revealed that slag forming was insufficient when the total iron (T.Fe) was less than 5% by mass. As a result, the driving force for slag discharge by tilting the converter was small, making sufficient discharge difficult. On the other hand, when the total iron (T.Fe) exceeded 25% by mass, the generation of CO bubbles in the slag progressed rapidly, and a bumping phenomenon was observed, forcing the interruption of slag discharge work. Thus, it is preferable to set the appropriate range of total iron (T.Fe) in the slag at the end of the first blowing process, i.e., during the slag discharge process, to 5-25% by mass.

[0039] Furthermore, in the slag removal process of this slag removal step, if the slag temperature at the end of the first blowing step is low, for example, below 1240°C, it leads to an increase in slag viscosity due to the formation of solid phase slag and an increase in the viscosity of liquid phase slag. This reduces the fluidity of the slag and leads to a decrease in the slag removal rate. Therefore, it is preferable to adjust the cold iron source unit consumption according to the initial conditions of the molten iron used, as well as at least one of the heat source addition amounts such as Si carbide or ferrosilicon and the oxygen unit consumption. By setting the molten metal temperature at the end of the first blowing step to 1240°C or higher, the slag temperature will also be 1240°C or higher. More preferably, the molten metal temperature should be 1260°C or higher.

[0040] However, if all of the slag generated after the first blowing process is removed, the laxation of the lime-based flux added in the subsequent dephosphorization and decarburization process will be delayed. This will also inhibit the dephosphorization reaction. To counteract this, fluorite can be added to promote laxation. However, as mentioned above, this would limit the uses of the slag and hinder its utilization. Another method to promote laxation is to add iron oxide such as iron ore. However, this method involves significant heat loss due to the endothermic decomposition reaction of iron oxide. Therefore, it is not advisable as it reduces the amount of heat available for melting the cold iron source.

[0041] Therefore, in order to promote slag formation of the lime-based flux without using fluorite or iron oxide in the dephosphorization and decarburization stage, it is preferable to leave a suitable amount of slag 10 with a preferred composition and temperature in the furnace after the first blowing stage. It is effective to promote slag formation by utilizing the SiO2 and iron oxide in the slag. When discharging the slag after the first blowing stage, it is preferable to adjust the tilt angle of the furnace body so that 4 to 20 kg / t of molten iron slag after the first blowing stage remains in the furnace. A suitable slag discharge rate of 40% by mass or more, more preferably 60% by mass or more, can be maintained. This allows for efficient promotion of the dephosphorization reaction without using iron oxide in the second blowing stage. Furthermore, the reaction heat due to the endothermic decomposition of iron oxide can be indirectly utilized as heat for melting the cold iron source in the first blowing stage. In this regard, if the residual slag after the first blowing process is less than 4 kg / t-molten iron, it becomes necessary to use iron oxide in the subsequent second blowing process to promote slag formation of the lime-based flux. On the other hand, if this exceeds 20 kg / t-molten iron, the amount of lime-based flux used may increase, or the operation of the second blowing process may be hindered.

[0042] [Second blowing process] Next, the second blowing process (D) shown in Figure 1(D) will be explained. After the slag discharge process (C), a lime-based flux is added to the molten iron 9 remaining in the same converter-type smelting furnace 1, and oxygen blowing is performed to provide a gaseous oxygen source and dephosphorize and decarburize the molten iron 9. In this second blowing process, it is preferable to use only oxygen-containing gas 12 from the oxygen supply lance 2 as the oxygen source to reduce heat loss. It is preferable to blow in bottom-blowing gas 13 from the bottom-blowing tuyeres 3 and stir. [P] in the molten iron is oxidized by the oxygen in the supplied oxygen source to become phosphorus oxide (P2O5). This phosphorus oxide is stably incorporated into the slag produced by the slag formation of the lime-based flux, and the dephosphorization of the molten iron proceeds. To efficiently carry out the dephosphorization reaction, it is preferable to add a lime-based flux so that the basicity of the slag after the second blowing step (slag 17 after dephosphorization and decarburization treatment during the preliminary treatment) is between 1.2 and 3.0, and to carry out the second blowing step so that the molten metal temperature after the completion of the second blowing step is between 1280°C and 1360°C by acid supply. More preferably, the basicity of the slag is 1.4 or higher.

[0043] If the slag basicity of the slag 17 produced during the second blowing process after dephosphorization and decarburization is less than 1.2, or if the molten metal temperature exceeds 1360°C, the slag's dephosphorization ability decreases, and the [P] concentration in the molten iron after treatment may not be sufficiently reduced. On the other hand, if the slag basicity exceeds 3.0, slag formation of the lime-based flux becomes difficult, reducing the utilization efficiency of the lime-based flux and potentially increasing costs. Even if the molten metal temperature is below 1280°C, slag formation of the lime-based flux also becomes difficult, reducing dephosphorization ability and potentially leading to insufficient heat during the subsequent decarburization process. Therefore, to ensure sufficient heat during the decarburization process, it is preferable to set the molten metal temperature after the completion of the second blowing process to between 1280°C and 1360°C. In addition, it is preferable to adjust the amount of oxygen used and the amount of carbon added in the first and second blowing processes so that the carbon concentration in the molten iron at the end of the second blowing process is 2.5% by mass or more.

[0044] When operating according to this embodiment, the [Si] concentration, [P] concentration, and molten metal temperature of the molten iron change, resulting in cases where the molten metal temperature is low at the end of the first blowing process, or where the [P] concentration of the molten iron is high, resulting in a large dephosphorization load. In such cases, it is effective to spray powdered lime or a lime source such as calcium carbonate onto the molten metal surface using oxygen gas or an inert gas from an acid supply lance or a separately installed lance to promote the dissolution of lime in the second blowing process. In the region where the top-blowing oxygen irradiates the molten metal surface, a high temperature of approximately 2000°C occurs due to direct decarburization reactions and iron oxidation, and the addition of powdered lime to this region promotes melting.

[0045] In the pretreatment method for molten iron according to this embodiment, SiO2-containing slag that contributes to the melting of lime is discharged after the first blowing process, making early dissolution by projecting a powdered lime source effective. Furthermore, this method utilizes the heat of oxidation of Si in the molten iron to promote the melting of the cold iron source. Therefore, higher temperatures are preferable for the melting rate of scrap in the molten iron. However, high-temperature treatment is rather disadvantageous for preventing rephosphorization and promoting the dephosphorization reaction during the second blowing process. Therefore, in this embodiment, it is preferable to simultaneously blow powdered iron oxide onto the area where the above-blown oxygen is injected, thereby locally cooling only the reaction area through the decomposition reaction (endothermic reaction) of iron oxide. This makes it possible to promote dephosphorization or suppress rephosphorization under macroscopically high-temperature conditions. Here, the auxiliary raw materials containing lime and calcium carbonate may be not only individual materials but also recycled materials such as slag generated during converter decarburization blowing. Furthermore, in addition to iron ore itself, recycled materials such as rolling scale, sintered ore powder, and dust collection can also be used for iron oxide.

[0046] In this embodiment, in order to prevent the oxygen jet from scattering iron particles, the oxygen impact pressure P in the second blowing process is C The blowing process is carried out so that the pressure is 58800 Pa or less. On the other hand, from the viewpoint of dephosphorization efficiency, the oxygen impact pressure P during the second blowing process is CIt is preferable that the oxygen impingement pressure P be at least 9800 Pa or higher. As described above, in the second blowing process, the slag remaining after the first blowing process initially forms, and as the lime-based flux is added, the forming subsides, and the oxygen jet of the upper blowing lance reaches the molten iron more significantly. For example, in the range where the ratio of the cumulative amount of acid supplied to the total amount of acid supplied in the second blowing process is less than 30%, the oxygen impingement pressure P C The oxygen impingement pressure P is set to be in the range of 10800 to 11800 Pa, and in the range where the cumulative acid delivery is 30% or more but less than 70% of the total acid delivery, C The oxygen impingement pressure P is set to be in the range of 14700 to 15700 Pa, and the remaining cumulative acid delivery is 70% or more of the total acid delivery. C It is preferable to set the pressure in the range of 49,000 to 50,000 Pa. Within this range, the occurrence of problems such as metal generated during blowing adhering to and falling onto the hood of the converter equipment is suppressed, and economically advantageous refining is possible. In addition, it is possible to process the molten iron at a lower and more stable [P] concentration than conventional methods. Furthermore, the acid supply rate in the second blowing process should be 2.2 to 2.6 Nm 3 A rate of / (min / t-molten iron) is preferred. The bottom-blowing gas flow rate should be 0.04~0.15 Nm³. 3 (min / t-molten iron) is preferred.

[0047] [Hot water supply process] Next, the tapping process (E) shown in Figure 1(E) will be described. In this process (E), when the [P] concentration in the molten iron has decreased to a predetermined value after the second blowing process described above, the converter-type smelting furnace 1 is tilted toward the side where the tapping port is installed, and the molten iron in the converter-type smelting furnace is tapped into a molten metal holding container (not shown). It is preferable that the predetermined [P] concentration be 0.030 mass% or less. In the tapping process, it is preferable to leave 30 mass% or more of the dephosphorized and decarburized slag with a slag basicity of 1.2 or higher in the container.

[0048] As described above, in the pretreatment method for molten iron according to this embodiment, the oxygen impact pressure P in oxygen blowing is CBy properly controlling this process, it is possible to suppress the occurrence of problems such as metal generated during smelting adhering to and falling onto the hood of the converter equipment, thereby realizing a pre-treatment of molten iron that is economically advantageous for refining.

[0049] Furthermore, in this embodiment, the amount of lime-based flux added during the second blowing process can be kept to a minimum. This contributes to reducing the unit cost of auxiliary raw materials for the entire molten iron refining process. [Examples]

[0050] (Example 1) Figure 2 shows the blowing pattern according to the present invention. Figure 2(a) is the blowing pattern of the first blowing process, and Figure 2(b) is the blowing pattern of the second blowing process. Table 1 summarizes the values ​​of various blowing parameters at the blowing stage (ratio of cumulative acid supply to total acid supply) according to this example of the invention. In addition, the metallurgical properties and metal deposition have been evaluated and listed. The evaluation criteria are as follows. (1) Metallurgical properties (compared to conventional examples) The following evaluation was performed based on whether the [P] concentration in the molten iron was worse than in conventional cases. ○: Lower than the [P] concentration in molten iron in conventional examples. △: Equivalent to or lower than the P concentration in molten iron in conventional examples. ×: Higher than the [P] concentration in molten iron in conventional examples. (2) With base metal (compared to conventional examples) The following evaluation was conducted based on whether or not metal adhered to the hood portion of the converter equipment. Small: No metal attached to the hood, no problems caused by the metal. Medium: Has a metal hood attached, but no problems caused by the metal. Large: Has a metal hood, and there are problems with the metal.

[0051] Figure 3 shows the blowing pattern for a conventional example. Figure 3(a) shows the blowing pattern for the first blowing process, and Figure 3(b) shows the blowing pattern for the second blowing process. Table 2 summarizes the values ​​of various blowing parameters for this conventional example at different blowing stages (ratio of cumulative acid supply to total acid supply). In addition, the metallurgical properties and metal deposition have been evaluated and listed.

[0052] [Table 1]

[0053] [Table 2]

[0054] Figure 4 compares the frequency of metal adhesion problems in the hoods of converter equipment in the inventive example and the conventional example. In Figure 4, the frequency of metal adhesion problems in the inventive example is standardized to 1 for comparison. In the conventional example, metal adhesion problems occurred 14 times more frequently than in the inventive example.

[0055] (Example 2) Tables 3 and 4 show various blowing conditions, along with evaluations of metallurgical properties and metal deposition. The results of operations under these blowing conditions are summarized below. In Tables 3 and 4, the cumulative acid supply / total acid supply (%) for Steps 1-4 and 5-8 are the same as in Tables 1 and 2. Processes No. 1 and 2 are examples of the invention in which the oxygen impact pressure in the second blowing process was at a low level within a suitable range, and operation was possible without metal adhesion. The [P] concentration in the molten iron was the same as or lower than conventional levels. Process No. 3 is an inventive example in which the maximum value of the oxygen impact pressure in the second blowing process was increased within a suitable range compared to processes No. 1 and 2. Although there was some hood metal attached, there were no metal-related problems. The [P] concentration in the molten iron was lower than conventional methods. Processes No. 4 and 5 have the second blowing process within a suitable range, and the oxygen impact pressure P of the first blowing process is within a suitable range. C This is an example of an invention where excessive forming occurred due to an excessively high pressure. There were cases with a hood base metal, but no base metal problems occurred. The [P] concentration in the molten iron was the same as or lower than conventional methods. Process No. 6 involves the oxygen impact pressure P in the second blowing process. C This is a comparative example where the [P] concentration was too high, leading to frequent problems due to metal adhesion. The [P] concentration in the molten iron was higher than conventional methods. Process No. 7 is an inventive example in which the second blowing process was within a suitable range, there was no hood metal buildup, and the pretreatment of molten iron was operated in the most economical way. The [P] concentration in the molten iron was lower than conventional methods. Process No. 8 involves the oxygen impingement pressure P in the initial stage (Step 5) and final stage (Step 8) of the second blowing process. C This is an example of a low-risk invention. There is no hood metal attached, and the [P] concentration in the molten iron is the same as or lower than conventional methods. Process No. 9 is the oxygen impact pressure P in the initial stage (Step 5) of the second blowing process. C The oxygen impingement pressure P is low in the final stage (Step 8). C This is an example of an invention where the [P] concentration was high and scattering of iron particles was observed. Although there were cases with a hood base metal, there were no base metal problems. The [P] concentration in the molten iron was the same as or lower than conventional methods. Process No. 10 involves the oxygen impact pressure P in Steps 5-7 of the second blowing process. C This invention resulted in a low phosphorus content, low dephosphorization efficiency, and extended processing time. There was no hood with a metal base, and the [P] concentration in the molten iron was the same as or lower than conventional methods.

[0056] [Table 3]

[0057] [Table 4] [Explanation of Symbols]

[0058] 1. Converter-type smelting furnace 2. Acid supply lance (upward blowing lance) 3 Bottom-blown tuyere 4. Hot water outlet 9. Molten iron 9a Hot metal 10 (After desiliconization and dephosphorization, and after the first blowing process) Slag 11 Cold Iron Source 12. Oxygen-containing gas (gaseous oxygen source) 13 Bottom-blowing gas 14 Charging pot 17. Slag after dephosphorization and decarburization (after the second blowing process)

Claims

[Claim 1] The first blowing process involves supplying a gaseous oxygen source to the molten iron in a converter-type smelting furnace from an upper blowing lance to desilicate and dephosphorize the molten iron, A slag discharge step in which at least a portion of the slag generated in the first blowing step is discharged from the converter-type refining furnace, A method for pretreatment of molten iron, comprising: a second blowing step in which, after the slag removal step, a CaO-based solvent is added to the converter-type smelting furnace and a gaseous oxygen source is supplied from the upper blowing lance to dephosphorize and decarburize the remaining molten iron; In the second blowing process, when supplying a gaseous oxygen source from the upper blowing lance to the molten iron bath surface, the oxygen impingement pressure P is determined by the following equations (1) to (3). Ｃ A method for pre-treating molten iron, comprising: blowing the iron to a pressure of 58,800 Pa or less; setting the oxygen impact pressure PC in the range of 10,800 to 11,800 Pa when the cumulative amount of acid delivered from the start of blowing is less than 30% of the total amount of acid delivered; setting the oxygen impact pressure PC in the range of 14,700 to 15,700 Pa when the cumulative amount of acid delivered is 30% or more but less than 70% of the total amount of acid delivered; and setting the oxygen impact pressure PC in the range of 49,000 to 50,000 Pa when the remaining cumulative amount of acid delivered is 70% or more of the total amount of acid delivered. P Ｃ =ρ・V ２ / 2 (1) V / (V) ０ -V ｌｏｓｓ )=d ｅ / (2・C・Z) (2) C=0.016+18600 / (P ０ -P ｅ ) (3) Here, P Ｃ : Oxygen impact pressure on the bath surface (Pa), ρ: Gas density of the gaseous oxygen source (1.43 kg / m³) ３ ), V: Central flow velocity of the gaseous oxygen source (m / s), V ０ : Nozzle outlet jet velocity (m / s), V ｌｏｓｓ : Flow velocity loss due to improper expansion (m / s), d ｅ : Nozzle outlet diameter (m), C: constant, Z: Lance height (m) P ０ : Nozzle absolute pressure (Pa), P ｅ :Ambient pressure(Pa) It represents.

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