Molten iron refining method, refining equipment, and steel manufacturing method

Abstract

To provide a technology that can determine an amount of solvent material required for a dephosphorization treatment based on clear criteria.SOLUTION: A method includes: a desiliconization treatment performed by charging molten iron tapped from a blast furnace into a converter-type of vessel where slags after dephosphorization treatment remain; a desiliconization step of adjusting at the end of the desiliconization step the molten iron with a predetermined Si concentration and a predetermined molten iron temperature, and the slags with a predetermined basicity; an intermediate slag removal step of retaining a portion of the molten iron and the slags after desiliconization treatment in the vessel, and discharging a predetermined amount of slags; a dephosphorization step in which the molten iron and slags desiliconized and remained in the vessel are added with a lime-based solvent, and oxygen is blown thereinto for dephosphorization of the molten iron; and a tapping step of tapping while leaving a portion of the slags with a predetermined basicity in the vessel after dephosphorization treatment. The method is for refining molten iron in which each step is repeated in sequence, in the dephosphorization step, a vibration of an oxygen supply lance is measured, and the amount of solvent used for the dephosphorization treatment is controlled according to a measured vibration value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a molten iron refining process, particularly to a molten iron refining method, a refining apparatus, and a steel manufacturing method that perform both desiliconization and dephosphorization in the same converter-type vessel. Here, molten iron includes molten pig iron tapped from a blast furnace, molten pig iron in which a cold iron source has been dissolved, molten pig iron melted by heating, molten pig iron such as molten steel, and molten pig iron from which a portion of the carbon has been removed by refining, as well as mixtures thereof. [Background technology]

[0002] Before molten pig iron is decarburized and refined in a converter, it is common for the molten iron to undergo pretreatment to remove Si and P. This pretreatment is carried out for the purposes of reducing the amount of flux used in refining, increasing the purity of the molten steel, preventing overoxidation during converter operation to improve manganese yield, and reducing the amount of slag produced by refining. Various pretreatment methods have been proposed, including those for the subsequent decarburization process.

[0003] During pretreatment, refining slag is generated. This slag should not be discarded but rather utilized for various purposes. Traditionally, fluorite (CaF2) was used as a fluorine source to enhance the dephosphorization reaction efficiency during dephosphorization. However, depending on the application of the slag, it is necessary to prevent fluorine and other substances from leaching out. Therefore, pretreatment methods without using fluorite as a fluorine source have been investigated. Furthermore, in recent years, the steel industry has been required to reduce greenhouse gas emissions. Therefore, refining methods that reduce the use of blast furnace hot metal, which requires a large amount of energy to reduce iron oxide and emits a large amount of CO2, while increasing the use of cold iron sources such as iron scrap, have been investigated. Against this background, recent pretreatment methods have tended to increase the use of cold iron sources while improving refining methods.

[0004] One pretreatment method for desiliconizing and dephosphorizing molten pig iron involves adding a refining agent (flux agent) such as quicklime to the molten pig iron, along with a solid oxygen source such as gaseous oxygen or iron oxide, to remove the silicon and phosphorus from the molten pig iron and turn them into slag. The vessels used for pretreatment include torpedo cars, blast furnace ladles, and other transport vessels, as well as converter-type vessels (refining furnaces). To handle large amounts of scrap, it is advantageous to use converter-type vessels with a large furnace volume.

[0005] For example, Patent Document 1 discloses a method for pre-treating molten pig iron in the processes of desiliconization, dephosphorization, and decarburization, which efficiently reduces the P concentration by suppressing the amount of flux used, while ensuring a heat source for scrap melting, thereby improving iron yield. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-167015 [Non-patent literature]

[0007] [Non-Patent Document 1] Yuichi Uchida, Naotaka Sasaki, Yuji Miki, Iron and Steel, Vol. 102 (2016), No. 12, pp. 31-37 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the above-mentioned conventional techniques have the following problems. That is, in the technology disclosed in Patent Document 1, since intermediate slag removal is involved, it is difficult to determine the slag basicity during blowing, and it is difficult to predict the optimum basicity for dephosphorization treatment and the required amount of solvent for that purpose. As a result, it is not possible to sufficiently reduce the solvent consumption for dephosphorization treatment.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for refining molten iron, a refining apparatus, and a method for manufacturing steel products, which are capable of determining the amount of solvent required for dephosphorization treatment based on clear criteria without measuring the components during blowing. [Means for solving the problem]

[0010] The inventors discovered that during the dephosphorization process, the oxygen supply lance is not immersed in the molten metal, but it vibrates in contact with the slag that has formed and formed into slag. They also discovered that once the slag has formed and settled, it no longer comes into contact with the oxygen supply lance, and the vibration of the oxygen supply lance stops. The present invention was developed based on these observations.

[0011] The method for refining molten iron according to the present invention, which advantageously solves the above problems, comprises a desiliconization step in which molten iron tapped from a blast furnace is charged into a converter-type vessel and subjected to desiliconization treatment, and at the end of the desiliconization treatment, the molten iron has a predetermined Si concentration and a predetermined molten temperature, and slag has a predetermined basicity; an intermediate slag removal step in which the molten iron and a portion of the slag after the desiliconization treatment are left in the vessel, and a predetermined amount of slag is discharged; and a final desiliconization step in which a lime-based solvent is added to the molten iron and slag after the desiliconization treatment and the slag remaining in the vessel, and oxygen is blown into the slag. This method for refining molten iron includes a dephosphorization step in which phosphorus is treated, and a tapping step in which a portion of the dephosphorized slag of a predetermined basicity remains in the vessel and the molten iron is tapped, and the desiliconization step, intermediate slag removal step, dephosphorization step, and tapping step are sequentially repeated. In the desiliconization step, the molten iron tapped from a blast furnace is charged into the vessel in which the dephosphorized slag remains, and in the dephosphorization step, the vibration of an oxygen supply lance for blowing oxygen is measured and the amount of the solvent used is controlled in accordance with the measured vibration value of the oxygen supply lance. Here, "basicity" refers to the concentration ratio of (CaO) to (SiO2) in the slag on a mass basis (the same applies hereinafter).

[0012] The molten iron refining apparatus of the present invention, which advantageously solves the above-mentioned problems, is an apparatus for refining molten iron used in the above-mentioned method, and is characterized by comprising a converter-type vessel, a means for adding a lime-based solvent, an oxygen supply lance, a vibration measuring device for measuring the vibration of the oxygen supply lance, and a control means for controlling the amount of solvent added in accordance with the measured vibration value of the oxygen supply lance.

[0013] The method for producing a steel material according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that the molten iron refined by the above-mentioned method is subjected to secondary refining, including vacuum treatment as necessary, to adjust the composition, and then cast into a steel material, which is then rolled to produce a steel material. [Effects of the Invention]

[0014] The method and apparatus for refining molten iron according to the present invention enable the amount of flux required for dephosphorization to be determined based on clear criteria without measuring the composition during the blowing process. In particular, the vibration of the oxygen supply lance is measured during the dephosphorization process, changes in the state of the slag are grasped in a timely manner, and the amount of lime-based flux required for dephosphorization is determined. This eliminates the need to add excess flux. Furthermore, leaving slag of a predetermined basicity in the vessel after dephosphorization leads to a reduction in the unit consumption of auxiliary materials throughout the entire refining process. Furthermore, by measuring the vibration of the oxygen supply lance, it is possible to determine when slag foaming has subsided, thereby suppressing slag leakage during tapping. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1(a) is a schematic vertical cross-sectional view of a converter-type vessel according to one embodiment of the present invention, and FIG. 1(b) is a schematic top view of a vibration measuring device. [Figure 2] 1(A) to 1(E) are schematic diagrams showing the order of steps in a method for refining molten iron according to one embodiment of the present invention. [Figure 3] FIG. 2 is a FeO—CaO—SiO ternary phase diagram showing the composition of slag in the dephosphorization step of the method for refining molten iron according to the embodiment. [Figure 4]1 is a graph showing the relationship between the amount of lime-based solvent added during dephosphorization treatment and the P concentration below the furnace, comparing an example of the present invention with a conventional example. [Figure 5] 1 is a graph showing the unit consumption of lime-based solvent added during dephosphorization treatment, comparing an example of the present invention with a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0016] The following describes in detail the embodiments of the present invention. The following embodiments are merely examples of devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope defined in the claims.

[0017] FIG. 1(a) shows a refining apparatus suitable for use in a method for refining molten iron according to one embodiment of the present invention. This refining apparatus is configured as a converter-type refining furnace 1, which is a converter-type vessel. In this refining furnace 1, top blowing of oxygen is performed by spraying oxygen-containing gas 12 toward molten iron 9 from the tip of a top-blowing oxygen supply lance 2 that can be raised and lowered. The oxygen-containing gas 12 can be pure oxygen, an inert gas such as Ar gas or nitrogen gas, or a mixture of oxygen gas and a diluent gas such as carbon dioxide. Industrial pure oxygen is preferred. Bottom blowing of gas is performed using a bottom-blowing tuyeres 3 installed at the bottom of the refining furnace 1. The bottom-blown gas 13 is typically an oxygen-containing gas or an inert gas such as Ar gas or nitrogen gas. Additionally, a gas that functions to enhance the agitation of the molten iron 9 and promote the dissolution of the cold iron source by being injected into the molten iron 9, or a gas that functions to inject a solvent into the molten iron together with a carrier gas, may also be used. It is also possible to inject an endothermic gas, such as a hydrocarbon, to protect the bottom-blowing tuyere 3. Reference numeral 4 in the drawing denotes a tapping port for tapping the molten iron 9 after refining.

[0018] In this embodiment, two or more converter-type refining furnaces 1 are used to refining molten iron, and at least one of the converter-type refining furnaces 1 is used for pre-treatment, i.e., desiliconization and dephosphorization. The remaining converter-type refining furnace 1 is used for decarburization of the molten iron that has been pre-treated by applying this embodiment. For example, it is preferable to perform pre-treatment in the converter-type refining furnace 1 for pre-treatment, and then transfer the pre-treated molten iron 9 to a converter-type refining furnace for decarburization and refining.

[0019] In this embodiment, the oxygen supply lance 2 further includes a vibration measurement device 20 for measuring vibrations, a hopper 7 and a chute 8 for adding the lime-based solvent 16, and a control device 21 for controlling the amount of the lime-based solvent 16 added based on the vibration measurement values. Figure 1(b) shows a schematic top view of an example of the vibration measurement device 20. Two uniaxial acceleration pickup sensors 20a for measuring vibrations are preferably used. The vibration detection directions 20b of the sensors are aligned in a horizontal plane, preferably perpendicular to each other, to measure the horizontal vibration of the oxygen supply lance 2. Alternatively, a triaxial acceleration pickup may be used. The vibration measurement values ​​may be vibration acceleration, vibration acceleration level, vibration level, displacement amplitude, or velocity amplitude. Vibration intensity at specific frequencies may also be measured using vibration spectrum analysis. Converting vibration acceleration to potential is simple and preferable. For example, a predetermined maximum potential value can be linked to the maximum value of past vibration measurements, and zero potential can be set as the maximum value of constant vibration as noise.

[0020] The control device 21 is composed of a sequencer or a computer. The control device 21 may include a control unit, a memory unit, an operation unit, a display unit, a communication unit, etc., and each unit may be connected via a bus. The control unit is exemplified by a computer composed of a central processing unit (CPU), a random access memory (RAM), etc. In response to operations on the operation unit, the CPU of the control unit reads system programs and various processing programs stored in the memory unit, for example, in a memory area that stores programs in the memory unit. The CPU then loads the programs in the RAM's work area, executes various processes according to the loaded programs, and realizes each function of the control means. The CPU also receives signals and data from other components via the bus and sends control signals and commands. In addition, the CPU receives information from external sensors such as the vibration measuring device 20 and sends and receives control signals to the lime-based solvent adding means via the communication unit. The communication unit is configured to communicate with other devices and equipment via wired or wireless communication.

[0021] The storage unit is configured with a non-volatile semiconductor memory such as an SSD (Solid State Drive) or a hard disk (HDD: Hard Disk Drive), etc. The storage unit may also include a removable flash memory, etc. The storage unit stores various programs, including programs for executing various processes in the control unit, parameters required for executing processes by the programs, and data such as processing results. The various programs stored in the storage unit are stored in the form of computer-readable program code, and the control unit sequentially executes operations in accordance with the program code.

[0022] The communication unit includes a LAN (Local Area Network) adapter, modem, wireless communication device, etc., and controls transmission and reception between the vibration measuring device 20 and the adding means connected by the communication means. The communication unit may include a communication interface such as a network card. The communication unit is capable of transmitting and receiving various data to and from external devices. For example, information detected by the vibration measuring device 20 is input to the control device 21 via this communication unit. The operation unit includes a keyboard with cursor keys, numeric input keys, and various function keys, a mouse, a pointing device such as a touch panel, and a stick controller, and outputs command signals input by key operation, mouse operation, etc. to the control unit.

[0023] The display unit may be, for example, a monitor such as a liquid crystal display (LCD), a CRT, or an organic light-emitting diode (LED). It displays information detected by the vibration measurement device 20 and the operating status of the adding means, allowing the vibration state of the oxygen supply lance 2 and the addition status of the lime-based solvent to be monitored. Based on the vibration measurement value displayed on the display unit, the operator can send instructions to the adding means, such as to stop adding the lime-based solvent. It is preferable to determine that foaming has subsided when the vibration measurement value falls below a predetermined value or when this state continues for a predetermined period of time. This operation can also be automatically controlled. The display unit can also display information about the subsidence of foaming and instructions to stop adding the lime-based solvent, in order to provide guidance to the operator.

[0024] The method for refining molten iron according to this embodiment using a converter-type refining furnace 1 is carried out in the following order, as shown in the flow chart of Fig. 2: (A) molten iron charging step, (B) desiliconization step, (C) intermediate slag removal step, (D) dephosphorization step, and (E) tapping step. In particular, by repeatedly carrying out these steps in the same furnace, efficient preliminary treatment of molten iron becomes possible. The molten iron charging step and the desiliconization step are collectively referred to as the desiliconization step.

[0025] [Hot metal charging step] First, the molten iron charging step (A) will be described. In this step (A), new molten iron 9a is charged from a charging ladle 14 while leaving behind slag 17 generated after the dephosphorization treatment in the previous pretreatment of molten iron (hereinafter simply referred to as "dephosphorization slag") in the converter-type refining furnace 1. Alternatively, a cold iron source 11 such as iron scrap is charged before the molten iron is charged, and then the molten iron 9a is charged. The cold iron source 11 charged in advance may be iron scrap as specified in the "Unified Standard for Iron Scrap Inspection" of the Japan Iron and Steel Association, or may be a material mainly composed of iron, such as direct reduced iron or cold iron.

[0026] The slag 17 left in the refining furnace 1 after the previous dephosphorization treatment is used to adjust the slag basicity during the subsequent desiliconization treatment. The basicity of the slag after the dephosphorization treatment, i.e., the ratio of the CaO concentration to the SiO2 concentration in the slag (CaO) / (SiO2) (hereinafter simply referred to as "basicity"), is preferably 1.2 or greater, more preferably 1.4 or greater. This is because if the basicity of the slag after the previous dephosphorization treatment is less than 1.2, leaving the slag behind may be insufficient for adjusting the basicity during the subsequent desiliconization treatment. This requires the addition of a large amount of lime-based solvent during the desiliconization treatment. There is no upper limit to the basicity of the slag after the dephosphorization treatment. Typically, the basicity of the slag used in the dephosphorization treatment of molten iron is set to approximately 3.0 or less. Therefore, there is no need to increase the basicity beyond this.

[0027] Furthermore, to effectively adjust the basicity, the amount of post-dephosphorization slag 17 remaining in the furnace is preferably 30 mass% or more of the amount of slag generated during the previous dephosphorization of molten iron. More preferably, it is 60 mass% or more. In this embodiment, if the entire amount of slag remaining in the furnace after the dephosphorization process is used for the desiliconization of new molten iron, basicity adjustment during the desiliconization process is even more effective. Furthermore, if this method is continuously performed, the discharged pretreatment slag will be homogeneous, consisting only of the slag at the end of the desiliconization process (hereinafter referred to as "post-desiliconization slag"). Therefore, the pretreatment slag will not be mixed with the post-dephosphorization slag, which has a high basicity. Therefore, problems such as slag expansion due to hydration reactions and alkali elution do not occur. Therefore, this embodiment is extremely effective in utilizing slag as a material.

[0028] The dephosphorized slag has a relatively high basicity, a relatively high melting point, and a relatively low temperature (below approximately 1350°C), resulting in low fluidity. Therefore, even if a cold iron source is charged onto the dephosphorized slag, the cold iron source is not enveloped in the slag, delaying its melting. This eliminates the need for inefficient operations in terms of heat and material balance, such as adding a large amount of coolant to solidify the so-called decarburized slag remaining in the furnace. Furthermore, due to the above characteristics, the dephosphorized slag is rich in solid phase and has low fluidity, resulting in a large amount of fine metallic iron in its structure. Therefore, even after the slag is solidified, crushed, and magnetically separated, it still contains approximately 10 mass% or more of metallic iron. Conventionally, this slag has been discharged outside the system. According to this embodiment, the dephosphorized slag can be carried over to the next refining process, allowing most of the metallic iron in the dephosphorized slag to be recovered in the molten iron, thereby reducing iron loss. On the other hand, the fluidity of the slag after desiliconization treatment is relatively high, so the metallic iron in the slag tends to coarsen. Therefore, in such slag, there is little metallic iron that remains in the slag after pulverization and magnetic separation. Therefore, in this embodiment, iron loss in the slag can be reduced throughout the entire refining process of molten iron.

[0029] [Desiliconization step] Next, the desiliconization step (B) shown in FIG. 2(B) will be described. In this step (B), the converter-type refining furnace 1 is placed upright, and oxygen-containing gas 12 is supplied to the molten iron 9 via the oxygen supply lance 2 to perform desiliconization. In this desiliconization process, a silicon source 15 contained in a hopper 5 and a lime-based solvent 16 contained in a hopper 7 are added to the converter-type refining furnace 1 via a chute 6 and a chute 8, respectively. Other materials, such as carbonaceous material serving as a heat source, silicon source, and iron oxide serving as an oxygen source, are also added. 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. Pure oxygen is particularly preferable.

[0030] During this desiliconization process, Si contained in the molten iron 9a or the Si source 15 and the cold iron source 11 that transfer to the molten iron during melting reacts with the oxygen source (Si + O2 → SiO2) to be desiliconized. This helps to improve the reaction efficiency of the subsequent dephosphorization process. During this desiliconization process, oxidation heat is generated, which increases the temperature of the molten iron and promotes the dissolution of the cold iron source 11 in the molten iron. The composition of the slag during this desiliconization process is determined by taking into account the amount and estimated composition of the slag 17 from the previous dephosphorization process that was left in the furnace, as well as the amount of SiO2 produced by the above reaction.

[0031] In other words, the basicity of the slag during the desiliconization treatment is preferably adjusted to 0.8 or more and 1.5 or less. This is because if the slag basicity during the desiliconization treatment is less than 0.8, depending on the [Si] (mass%) concentration in the molten iron, the dephosphorization ability of the slag 17 after the dephosphorization treatment decreases, resulting in a phenomenon of rephosphorization, i.e., an increase in the [P] concentration in the molten iron. On the other hand, if the slag basicity is greater than 1.5, the solid fraction increases due to an increase in undissolved CaO, which reduces the fluidity of the slag 17 after the dephosphorization treatment, and this may make it impossible to remove the slag. The preferred upper limit of the slag basicity is approximately 1.2.

[0032] Next, the basicity of the slag at the end of the desiliconization process is preferably adjusted to between 0.5 and 1.5. If the basicity of the slag (post-desiliconization slag) at this stage is less than 0.5, rephosphorization from the previous dephosphorization slag 17 remaining in the furnace will increase the [P] concentration in the molten iron, increasing the dephosphorization load in subsequent processes and resulting in inefficient dephosphorization. Therefore, the basicity of the post-desiliconization slag at the end of the desiliconization process is preferably 0.5 or more, more preferably 0.8 or more. Furthermore, if the slag basicity at this stage is higher than 1.5, the fluidity of the slag will decrease, resulting in problems such as reduced slag discharge volume during the next intermediate slag discharge or difficulty in controlling the discharge volume. This is also inefficient in terms of reducing the amount of lime-based solvent. Therefore, the slag basicity at the end of the desiliconization process is preferably 1.5 or less, more preferably 1.2 or less. To adjust the basicity, lime-based solvents such as quicklime, limestone, and dolomite, as well as steelmaking slag selected from decarburization slag, dephosphorization slag, and ladle slag, can be used as solvents.

[0033] The molten metal temperature at the end of the desiliconization process is preferably adjusted to between 1240°C and 1400°C. This is because temperatures above 1400°C cause rephosphorization from the dephosphorization slag remaining in the furnace, resulting in an increase in the [P] concentration in the molten iron. This increases the dephosphorization load in subsequent processes, making the process inefficient. Furthermore, it is necessary to increase the magnesia (MgO) concentration in the slag to prevent wear on the lining magnesia-carbon bricks, which increases costs. A more preferable temperature is 1350°C or lower. On the other hand, temperatures below 1240°C reduce the fluidity of the slag, resulting in reduced or difficult-to-control slag discharge during the subsequent intermediate slag discharge process. Additionally, the scrap melting rate decreases. A more preferable temperature is 1260°C or higher.

[0034] Furthermore, the molten metal temperature at this stage must be controlled to ensure efficient dephosphorization in the subsequent dephosphorization process. For example, if the molten metal temperature at the end of the desiliconization process is kept below 1350°C, the amount of coolant, such as iron ore, added for temperature control during the dephosphorization process can be significantly reduced. However, when desiliconization and dephosphorization processes are performed consecutively in the same furnace, adding scrap before the dephosphorization process is difficult due to the time constraints. Furthermore, the cold iron source that can be added from the furnace during the process is often expensive, granulated, or limited in quantity, such as ingots generated in the steelworks. Therefore, it is difficult to consistently use a large amount of cold iron source during the dephosphorization process. In practice, the number of types of auxiliary materials that can be used in the furnace top-feeding device is also limited. In such cases, the cold iron source is sometimes not added from the furnace. In short, the coolant used in the dephosphorization process has traditionally been limited to iron oxide, such as iron ore, and inexpensive cold iron sources, such as scrap, cannot be fully utilized.

[0035] Generally, it is relatively easy to increase the amount of inexpensive cold iron source material used in the desiliconization process. This allows the molten metal temperature at the end of the desiliconization process to be kept below 1350°C. This significantly reduces the amount of iron oxide used in the dephosphorization process. As a result, the large amount of heat absorbed by the decomposition reaction of iron oxide can be reduced, and this heat can be allocated to melting the cold iron source material in the desiliconization process. Note that if the molten metal temperature at the end of the desiliconization process is too low, the cold iron source material may remain unmelted. However, the remaining cold iron source material can be retained in the furnace along with the molten iron and melted until the next dephosphorization process. In other words, as long as the cold iron source material is completely melted by the end of the dephosphorization process, there will be no operational problems.

[0036] The molten metal temperature after desiliconization may be measured using a thermocouple or estimated from the heat balance. When estimating from the heat balance, coefficients can be adjusted and variables can be added or deleted according to the individual equipment and operating conditions. If there is residual scrap after desiliconization, the measured value tends to be slightly higher than the estimated value, but this is within the margin of error and does not affect the use of the value.

[0037] Here, the Si oxide content is the sum of that contained in the hot metal 9a, the cold iron source 11, and additives. The [Si] concentration in the hot metal 9a is determined by rapid analysis of a sample taken from the hot metal before each charge. However, a substitute method, such as calculation using other analytical values, such as the composition of the hot metal from the blast furnace, may also be used. The Si concentration in the various cold iron sources 11 is determined by, for example, the analytical value of a representative sample from each lot. Cold iron is often stable at a concentration similar to that of hot metal. Although the Si concentration in scrap varies depending on the source, it is generally stable at less than 1 / 10 of that in pig iron, so it can be used as a representative value or can be ignored.

[0038] The additives may also include the following Si-containing substances that can be converted into the oxides. Si-containing substances that exist in an oxidizable form include non-oxide Si-containing substances. These include those that are present as iron silicide, metallic Si, Si carbide, Si nitride, or other silicides. Typical additives include ferrosilicon and powder containing approximately 60 mass% Si carbide formed into briquettes (hereinafter referred to as "Si carbide briquettes"). The ferrosilicon analysis method described in JIS G 1312-1~3:2011 is an example of an analytical method for non-oxide Si in additives. Other methods for estimating Si include a combination of analysis of Si and other contained elements and analysis of compounds by X-ray diffraction. Examples of analytical methods for contained elements include total silicon analysis, acid-soluble silicon analysis, total carbon analysis, total oxygen analysis, total nitrogen analysis, thermogravimetric analysis, and carbon analysis by combustion with controlled temperature history.

[0039] The additives also contain carbon. Examples of carbon include carbonaceous materials such as coke or amorphous graphite, as well as carbon in carbides such as the aforementioned silicon carbide. As for the flux, auxiliary materials such as quicklime, lightly burned dolomite, and magnesia clinker are used. Other slags, such as dephosphorization slag, decarburization slag, and ladle slag, can also be used as calcium oxide or magnesium oxide sources. Examples of inexpensive auxiliary materials include calcium or magnesium carbonates and hydroxides. However, because these have large heat absorption capacities, when used in large quantities, it is preferable to distinguish them from other fluxes and revise the estimated heat balance.

[0040] Next, in this embodiment, the cold iron source consumption rate is adjusted. However, to melt a large amount of cold iron source in a short time, an appropriate amount of a Si source with a high calorific value may be used as a heat source. In this case, the Si consumption rate (not oxide) added during charging or desiliconization is preferably in the range of 2 to 10 kg / t of molten iron per total mass of the charged molten pig iron 9a and cold iron source 11. A more preferred range is 4 to 8 kg / t of molten iron. In this regard, adding Si at a rate exceeding 10 kg / t of molten iron results in excessive SiO2 generation during desiliconization. Furthermore, even if desiliconization is performed while leaving all of the slag from the previous dephosphorization process in the furnace, a large amount of calcium oxide source must be added to adjust the basicity. Furthermore, the amount of slag in the furnace increases, which may increase refining costs. Furthermore, if the Si consumption rate is less than 2 kg / t of molten iron, the heat generated by the oxidation reaction of Si is small and is not effective in melting the cold iron source. Furthermore, the amount of silicic acid produced during the desiliconization process decreases, resulting in a large amount of highly basic post-dephosphorization slag remaining in the furnace. In this case, the basicity of the post-desiliconization slag becomes high, which hinders subsequent slag removal and may require the addition of a SiO2 source, which is wasteful in terms of both heat and resources. In this regard, a silicon consumption rate in the range of 4 to 8 kg / t-molten iron is preferable in terms of both adjusting the basicity of the post-desiliconization slag and ensuring a heat source for melting the cold iron source.

[0041] Ferrosilicon can be used as the silicon source. It is also preferable to use cheaper silicon carbide briquettes or waste refractories containing silicon carbide as the main component. It is not necessary to use this silicon source alone; other heat sources, such as carbonaceous materials, may be used in combination as long as productivity is not reduced. The carbonaceous materials are preferably added based on the estimated amount of decarburization, so that the carbon concentration in the molten iron at the end of the desiliconization treatment is 3.3 mass% or higher. A carbon concentration of less than 3.3 mass% would result in an insufficient heat source for the subsequent dephosphorization and decarburization processes. This also reduces the carburization rate on the surface of the cold iron source, such as scrap, which leads to a decrease in the melting rate of the cold iron source.

[0042] In this desiliconization process, the temperature of the molten iron at the end of the desiliconization process is controlled within an appropriate range, and silicon is used as a heat source. Therefore, even if a large amount of cold iron source is used at a unit cost of 100 to 250 kg / t of molten iron per total mass (t) of molten pig iron and cold iron source, productivity does not decrease and refining costs do not increase. Furthermore, melting of the cold iron source and preliminary refining of the molten iron can be carried out efficiently. However, if the unit cost of the cold iron source exceeds 250 kg / t of molten iron, an additional heat source is required, which may increase costs, or the blowing time may be extended, which may decrease productivity. Furthermore, due to limitations on the cold iron source charging equipment, further increasing the amount used is inefficient.

[0043] As will be described in detail later, in order to improve the slag removal efficiency of the desiliconization slag 10, it is preferable to cause moderate foaming of the slag in the converter-type refining furnace 1. To achieve this, it is effective to increase the rate of CO gas generation generated 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 subsequent intermediate slag removal process, it is preferable to supply oxygen in an amount greater than the stoichiometric amount required to oxidize the Si in the molten iron and the added Si source.

[0044] The oxygen consumption rate for the molten iron during desiliconization is 2 Nm3 / min plus the amount required for stoichiometric desiliconization. 3 / t-molten iron or more, preferably 4Nm 3It is preferable to use an amount of oxygen that is equal to or greater than 1 / t-molten iron. In this embodiment, it is preferable to use oxygen supply in this manner to make the [Si] concentration in the molten iron at the end of the desiliconization treatment 0.2 mass% or less. More preferably, it is set to 0.1 mass% or less, and even more preferably, it is set to 0.05 mass% or less. This makes it possible to maintain a foamed state and maintain good slag removal performance even when removing slag after the desiliconization treatment. At the same time, it becomes possible to suppress re-phosphorization from the slag to the molten iron. In the oxygen blowing for the desiliconization treatment, the top blown oxygen supply rate is 1 to 2 Nm 3 / (min·t-molten iron) is preferable. At the same time, the bottom gas injection rate is 0.02 to 0.2 Nm 3 It has been confirmed that the above effect can be obtained when the temperature is about / (min·t-molten iron).

[0045] It is preferable to combine the control of the [Si] concentration in the molten iron with the control of the slag basicity and the molten iron temperature. This allows for the effective utilization of the lime in the dephosphorized slag, even when the entire dephosphorized slag from the previous process is left in the furnace. Furthermore, by combining the control of the [Si] concentration, slag basicity, and molten iron temperature and leaving the dephosphorized slag in the furnace, the (P2O5) concentration in the slag can be increased. This also promotes slag foaming. In particular, the (P2O5) in the slag reduces the surface tension of the slag, promoting its reaction with the molten iron and the dispersion of fine bubbles. Therefore, it is believed that slag foaming and good slag removal can be maintained even at relatively low iron oxide concentrations, such as a total iron concentration of approximately 10 mass% (T.Fe).

[0046] [Intermediate slag removal process] Next, the intermediate slag removal step (C) shown in FIG. 2(C) will be described. In the method for refining molten iron according to this embodiment, a slag removal process is performed after the desiliconization step described above. That is, an intermediate slag removal process is performed in which low-basicity desiliconization slag containing a large amount of SiO2 generated during the desiliconization process is discharged from the converter-type refining furnace 1. The intermediate slag removal of the desiliconization slag 10 is effective in reducing the amount of lime-based solvent used to adjust the slag basicity to an appropriate level in the subsequent dephosphorization process. Furthermore, in this embodiment, a large amount of dephosphorization slag generated during the previous molten iron refining process is left in the furnace while a new molten pig iron is desiliconized. In this case, the desiliconization process is performed to prevent rephosphorization from the slag to the molten iron, resulting in a higher (P2O5) concentration in the desiliconization slag than in conventional processes. If a large amount of slag is left after desiliconization, the amount of (P2O5) in the slag in the furnace will become excessive in the subsequent dephosphorization process, reducing the dephosphorization effect.

[0047] A characteristic feature of the molten iron refining method according to this embodiment is that when the above-described processes (A) to (E) are repeatedly and continuously carried out, care must be taken to prevent the accumulation of (P2O5) in the furnace if the slag is not sufficiently discharged after the desiliconization process. This is because if the amount of (P2O5) in the slag in the furnace becomes too large during the dephosphorization process, the increase in the (P2O5) concentration in the slag reduces the efficiency of the dephosphorization reaction, resulting in an increase in the [P] concentration in the molten iron after the process. In addition, the amount of lime-based solvent required for the dephosphorization reaction increases.

[0048] Therefore, in this embodiment, the slag removal rate after desiliconization is preferably at least 40 mass% or more, more preferably 60 mass% or more. Here, the slag removal rate (mass%) is defined as the percentage obtained by dividing the mass of the discharged slag by the mass of the slag in the furnace at the end of the desiliconization treatment. If the slag removal rate is less than 40 mass%, the amount of lime-based solvent used in the subsequent dephosphorization treatment will increase, as described above. Furthermore, if the amount of slag increases as a result, slag foaming cannot be suppressed, and slag ejection from the furnace throat during the dephosphorization treatment may occur, causing operational problems.

[0049] As described above, in the method for refining molten iron according to this embodiment, it is preferable to set the molten iron temperature at the end of the desiliconization treatment to 1240°C or higher and 1400°C or lower so that the slag basicity at the end of the desiliconization treatment is within the range of 0.5 to 1.5. Furthermore, by optimizing the oxygen consumption rate and promoting slag foaming, good slag fluidity and gas holdup can be ensured. Furthermore, a good slag removal rate can be achieved simply by tilting the furnace body after the desiliconization treatment to allow the slag to flow out of the furnace throat. When the tilting angle of the furnace body is adjusted to allow the slag to flow out without causing the molten iron to flow out, a certain amount of slag must remain in the furnace. However, the volume ratio of the foamed slag is approximately 1 / 10, and its bulk density is lower than its true specific gravity. Therefore, the amount of slag remaining in the furnace can be controlled to a low level. If the foaming of the slag subsides, the rate of slag removal will decrease, so it is preferable to keep the time from the end of the desiliconization treatment to the start of tilting the furnace body for slag removal within 4 minutes.

[0050] The basicity of the slag at the end of the desiliconization treatment, which is required in this intermediate slag removal step, is less than 0.5. If the basicity of the slag at the end of the desiliconization treatment is less than 0.5, the viscosity of the slag will increase, and a satisfactory slag removal rate may not be achieved. On the other hand, if the basicity of the slag at the end of the desiliconization treatment exceeds 1.5, solid slag will form, reducing the fluidity of the slag and decreasing the slag removal rate. Thus, from the viewpoint of ensuring the slag removal property and the slag removal rate, it is preferable to set the slag basicity to approximately 0.5 to 1.5. However, from the viewpoint of preventing rephosphorization of the slag in the desiliconization treatment step and reducing the amount of lime-based solvent used, it is more preferable to adjust the slag basicity to the range of 0.8 to 1.2.

[0051] In addition, slag foaming occurs due to the formation of fine CO bubbles generated by the reaction of the total iron (T.Fe) in the desiliconization slag (i.e., iron oxide) with the molten iron or carbon-containing iron particles suspended in the slag during the intermediate slag removal process. To verify the optimum T.Fe concentration range, we conducted separate studies and found that slag foaming was insufficient when T.Fe was less than 5 mass%. This resulted in a weak driving force for slag removal by tilting the converter, making sufficient removal difficult. On the other hand, when T.Fe was greater than 25 mass%, CO bubbles rapidly formed in the slag, resulting in bumping, which forced the slag removal process to be suspended. Thus, the optimum T.Fe range in the slag at the end of the desiliconization process, i.e., during the slag removal process, is preferably between 5 and 25 mass%. The boundaries of these two ranges are indicated by dashed lines in the FeO-CaO-SiO2 ternary phase diagram in Figure 3.

[0052] Furthermore, if the slag temperature at the end of the desiliconization process is low (for example, below 1240°C), the formation of solid slag increases the slag viscosity, and the liquid slag viscosity also increases. This reduces the fluidity of the slag, resulting in a lower slag removal rate. Therefore, it is preferable to adjust the cold iron source consumption rate based on the initial conditions of the molten iron used, as well as at least one of the amount of heat source added, such as silicon carbide or ferrosilicon, and the oxygen consumption rate. By setting the molten iron temperature at the end of the desiliconization process to 1240°C or higher, the slag temperature will also be 1240°C or higher. A molten iron temperature of 1260°C or higher is more preferable.

[0053] However, if all of the slag generated after the desiliconization treatment is removed, the slag formation of the newly added lime-based solvent in the next dephosphorization process will be delayed, hindering the dephosphorization reaction. To address this issue, fluorite can be added to promote slag formation. However, as mentioned above, this restricts the uses of the slag and inhibits its utilization. Another method is to add iron oxide, such as iron ore, to promote slag formation. However, this method results in a large heat loss due to the endothermic decomposition reaction of the iron oxide. This is not advisable, as it reduces the amount of heat available for melting the cold iron source.

[0054] Therefore, to promote the slag formation of the lime-based solvent without using fluorite or iron oxide in the dephosphorization stage, it is preferable to leave an appropriate amount of post-desiliconization slag with the desired composition and temperature in the furnace. It is effective to promote slag formation by utilizing the SiO2 and iron oxide contained in the slag. Furthermore, when discharging the post-desiliconization slag, it is preferable to adjust the tilt angle of the furnace body so that 4 to 20 kg / t of post-desiliconization slag remains in the furnace. A suitable desiliconization slag discharge rate of 40 mass% or more, and more preferably 60 mass% or more, can be maintained. This allows the dephosphorization reaction to be efficiently promoted without using iron oxide in the dephosphorization stage. Furthermore, the endothermic reaction heat generated by the decomposition of iron oxide can be indirectly utilized as heat for melting the cold iron source in the desiliconization process. In this regard, if the amount of post-desiliconization slag remaining is less than 4 kg / t of molten iron, it is necessary to use iron oxide in the subsequent dephosphorization step to promote the slag formation of the lime-based solvent. On the other hand, if this exceeds 20 kg / t-molten iron, the amount of lime-based solvent used may increase and the operation of the dephosphorization treatment may be hindered.

[0055] [Dephosphorization process] Next, the dephosphorization step (D) shown in Figure 2(D) will be described. After the intermediate slag removal step (C), a lime-based flux is added to the molten iron 9 remaining in the same converter-type refining furnace 1, and oxygen is blown in to provide the oxygen, thereby dephosphorizing the molten iron 9. To reduce heat loss, it is preferable to use only oxygen-containing gas 12 from the oxygen supply lance 2 as the oxygen source in this dephosphorization step. [P] in the molten iron is oxidized by oxygen in the supplied oxygen source to form phosphate oxide (PO). This phosphate oxide is stably incorporated into the slag produced by the slag formation of the lime-based flux, thereby promoting the dephosphorization of the molten iron. To efficiently promote the dephosphorization reaction, it is preferable to add a lime-based solvent so that the basicity of the slag after the dephosphorization treatment (slag 17 after the dephosphorization treatment during the refining treatment) is 1.2 or more and 3.0 or less, and to perform the dephosphorization treatment by supplying oxygen so that the temperature of the molten metal after the dephosphorization treatment is 1280°C or more and 1360°C or less. More preferably, the basicity of the slag is 1.4 or more.

[0056] If the slag basicity of the post-dephosphorization slag 17 generated during the dephosphorization treatment is less than 1.2 or the molten metal temperature exceeds 1360°C, the dephosphorization ability of the slag may be reduced, and the [P] concentration in the molten iron after the treatment may not be sufficiently reduced. On the other hand, if the slag basicity exceeds 3.0, it becomes difficult to form a slag from the lime-based flux, which reduces the utilization efficiency of the lime-based flux and may increase costs. If the molten metal temperature is less than 1280°C, it also becomes difficult to form a slag from the lime-based flux, reducing the dephosphorization ability and potentially resulting in insufficient heat for the subsequent decarburization treatment. To ensure sufficient heat for the decarburization treatment, it is preferable to set the molten metal temperature after the dephosphorization treatment to between 1280°C and 1360°C. At the same time, it is preferable to adjust the amount of oxygen used and the amount of carbon added during the desiliconization and dephosphorization treatments so that the carbon concentration in the molten iron after the dephosphorization treatment is 2.8 mass% or higher.

[0057] During the operation according to this embodiment, the [Si] concentration, [P] concentration, and temperature of the molten iron may change, resulting in a low molten iron temperature at the end of the desiliconization treatment, or a high [P] concentration in the molten iron, resulting in a large dephosphorization load. In such cases, to promote lime dissolution during the dephosphorization process, it is effective to spray powdered lime or a lime source such as calcium carbonate onto the molten metal surface using oxygen gas or inert gas from an oxygen supply lance or a separately installed lance. In the region where the molten metal surface is irradiated with top-blown oxygen, the temperature rises to approximately 2000°C due to direct decarburization and iron oxidation. Adding a powdered lime source to this region promotes melting.

[0058] In the molten iron refining method according to this embodiment, SiO2-containing slag, which contributes to the melting of lime, is discharged after desiliconization, making early melting by blasting 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, operation at a higher temperature is preferable to increase the melting rate of scrap in the molten iron. However, high temperatures are disadvantageous in preventing rephosphorization during desiliconization and promoting dephosphorization. Therefore, in this embodiment, powdered iron oxide is preferably top-blown into the region where the top-blown oxygen is injected, thereby locally cooling only the reaction region through the decomposition reaction (endothermic reaction) of the iron oxide. This makes it possible to suppress dephosphorization or rephosphorization even under macroscopically high temperature conditions. Here, the auxiliary materials containing lime and calcium carbonate may be used alone or recycled materials such as slag generated during converter decarburization blowing. Furthermore, the iron oxide may be iron ore or other raw materials, or recycled materials such as rolling scale, sintered ore powder, and collected dust may be used.

[0059] In this embodiment, the dephosphorization process is performed while measuring the vibration of the oxygen supply lance 2 using a vibration measuring device 20 installed above the oxygen supply lance 2. As described above, the slag after intermediate slag removal is subjected to the dephosphorization process while maintaining foaming. The oxygen supply lance 2 does not contact the molten metal, but it does contact the foamed slag. This causes vibration in the oxygen supply lance 2. As the basicity of the slag increases during the dephosphorization process due to the addition of a lime-based solvent 16, the foaming subsides and the slag no longer comes into contact with the oxygen supply lance. Therefore, when the measured vibration value of the oxygen supply lance, for example, the vibration acceleration, falls below a predetermined value, the addition of the lime-based solvent is stopped by the control means.

[0060] For example, the dotted line in the FeO-CaO-SiO2 ternary phase diagram in Figure 3 indicates the slag composition with a 20 mass% FeO concentration. The arrows in Figure 3 indicate the change in slag basicity during dephosphorization. The hatched area in Figure 3 indicates the 2CaO·SiO2 saturation region described in Non-Patent Document 1. Non-Patent Document 1 states that a phosphorus-rich phase precipitates in this region. The slag basicity after intermediate slag removal is approximately 0.8, and its melting point is approximately 1350°C. When the slag basicity increases to approximately 1.3 (e.g., the open circle (○) in Figure 3) by adding a lime-based solvent, the hatched region where the phosphorus-rich phase precipitates is reached. This region where the phosphorus-rich phase precipitates has a melting point above 1400°C, where the high-melting-point 2CaO·SiO2 precipitates as a solid phase. The molten metal temperature during dephosphorization is controlled below 1400°C to accelerate the dephosphorization reaction. This change in the state of the slag, i.e., the cessation of foaming, can be grasped by dynamically measuring the vibration of the oxygen supply lance 2. This means that the composition of slag with a basicity of 1.2 to 1.3 can be grasped with greater accuracy than the conventional calculation of basicity obtained from material balance. This eliminates the need to add excess lime-based solvent to slag with a basicity exceeding 1.5, which is suitable for dephosphorization treatment. Furthermore, because the molten metal can be tapped only after confirming that foaming has quenched, accidents such as slag leaking from the furnace throat during tapping can be prevented.

[0061] [Tapping process] Next, the tapping step (E) shown in Fig. 2(E) will be described. In this step (E), when the [P] concentration in the molten iron has decreased to a predetermined value after the dephosphorization step, the converter-type refining furnace 1 is tilted to the side where the tapping port is installed, and the molten iron in the converter-type refining furnace is tapped into a molten metal holding vessel (not shown). The predetermined [P] concentration is preferably 0.030 mass% or less. In the tapping step, it is preferable to leave 30 mass% or more of the slag after the dephosphorization treatment, which has a slag basicity of 1.2 or more, remaining in the vessel.

[0062] As described above, in the method for refining molten iron according to this embodiment, after the molten iron is dephosphorized and tapped, at least a portion of the dephosphorized slag remaining in the furnace is left undischarged. New molten iron is then charged into the furnace and desiliconized, resulting in a continuous process. Therefore, most of the slag discharged from the converter-type refining furnace (pretreatment furnace) is desiliconized slag, thereby reducing the contamination of dephosphorized slag with a relatively high basicity. This is to avoid the limitations on slag applications due to the risk of alkali elution and expansion due to hydration reactions if dephosphorized slag is present. In particular, a method in which no dephosphorized slag is discharged at all avoids these problems, simplifies slag treatment, and enables the use of dephosphorized slag in high-value-added applications. This effect is particularly significant when the pretreatment of molten iron is performed repeatedly and continuously using a single converter, as in this embodiment.

[0063] In addition, in this embodiment, the amount of lime-based solvent added during the dephosphorization treatment can be minimized, which contributes to reducing the unit consumption of auxiliary materials in the entire smelting process of molten iron.

[0064] [Steel manufacturing method] Another embodiment of the present invention is a method for producing steel, in which molten iron refined by the above-described method is subjected to secondary refining, including vacuum treatment as necessary, to adjust the composition, and then cast into a steel material, which is then rolled to produce a steel product. For secondary refining, RH-type vacuum treatment, stirring by bubbles, mechanical stirring, electromagnetic stirring, ladle refining, or the like can be used. For casting, ingot casting and blooming, conventional continuous casting, thin slab continuous casting, or the like can be used. Examples of steel products after rolling include hot-rolled steel plates, thick plates, shaped steel, steel bars, and wire rods. [Example]

[0065] The molten iron refining process was carried out using the molten iron refining apparatus and vibration measuring device 20 having the configuration shown in Figure 1. The converter-type refining furnace 1 had a capacity of 400 t, and the oxygen supply rate of the oxygen supply lance 2 was a maximum of 600 Nm 3 / min. The molten iron refining process was operated within the above-mentioned preferred ranges, except for the dephosphorization process. The inventive example and the conventional example were compared assuming the same molten iron [P] concentration, molten metal temperature, and other conditions before dephosphorization, as well as the oxygen supply rate, lime-based solvent addition rate, and other conditions during dephosphorization, and the target [P] concentration and target molten metal temperature after dephosphorization.

[0066] In the example of the invention, a vibration measuring device 20 was installed above the oxygen supply lance 2, and the horizontal vibration of the oxygen supply lance 2 was measured. The measured vibration acceleration was converted to voltage. The maximum voltage was set to the maximum value of the vibration acceleration measured in the past, and the maximum value of constant vibration (noise) was set to zero. When the voltage measured as a vibration value exceeded zero, it was determined that the oxygen supply lance was vibrating, that is, that the foamed slag was in contact with the oxygen supply lance. Then, the addition of lime-based solvent was continued. When the voltage measured as a vibration value remained at zero for 10 seconds, it was determined that the foaming of the slag had subsided, and the addition of lime-based solvent was stopped.

[0067] In the conventional example, the amount of slag remaining after intermediate slag removal was estimated, and the amount of lime-based solvent added was determined from the material balance so that the slag basicity would be within the appropriate range after dephosphorization. In neither the inventive example nor the conventional example, the slag basicity was measured during the dephosphorization process.

[0068] The dephosphorization results for the inventive and conventional examples are shown in Figures 4 and 5. Figure 4 shows the relationship between the amount of lime-based flux added per treatment (kg / ch) and the [P] concentration in the molten iron after tapping (below-furnace [P] concentration) (mass%) for the inventive and conventional examples. For the inventive example, open circles (◯) are plotted, and the linear regression approximation line is shown with a dashed line. For the conventional example, crosses are plotted, and the linear regression approximation line is shown with a dotted line. Figure 5 shows the consumption rate of lime-based flux added during dephosphorization per ton of molten iron for the inventive and conventional examples. The bars in Figure 5 represent the average consumption rate, and the error bars indicate the range of variation. As a result, the inventive example maintained a narrow variation in the below-furnace [P] concentration, and compared to the conventional example, the consumption rate of lime-based flux during dephosphorization was reduced by approximately 1.4 kg / t of molten iron. There was no significant difference in the average value of the [P] concentration below the furnace between the inventive example and the conventional example.

[0069] In this specification, the unit of mass, "t", is 10 3 kg. The symbol "N" attached to the unit of gas volume represents the value at standard conditions of 0°C and 101,325 Pa. The symbol [M] indicates that element M is dissolved in molten pig iron or iron. The symbol (R) indicates that the compound with chemical formula R is contained in the slag. [Industrial Applicability]

[0070] According to the molten iron refining method, refining apparatus, and steel manufacturing method of the present invention, the amount of lime-based flux added is controlled by measuring the vibration of the oxygen supply lance during dephosphorization, eliminating the need to add excessive flux. This reduces the risk of slag spillage during tapping. This improves the overall productivity of the molten iron refining process and is industrially useful. In this invention, the amount of lime-based solvent used during dephosphorization is controlled by utilizing the relationship between the vibration of the oxygen supply lance and the slag basicity. This method is also suitable for applications where changes in vibration can be correlated with abnormal phenomena such as the end point of the reaction or slopping. [Explanation of symbols]

[0071] 1 Converter-type refining furnace 2 Oxygen supply lance 3 Bottom-blown tuyere 4. Tap 5, 7 Hopper 6, 8 shots 9. Molten Iron 9a Hot metal 10 Slag after desiliconization 11 Cold Iron Source 12 Oxygen-containing gases 13 Bottom blown gas 14 Charging pot 15 Si source 16 Lime-based media 17 Slag after dephosphorization 20 Vibration measuring device 20a Acceleration pickup sensor 20b Detection direction 21 Control device

Claims

1. a desiliconization process in which molten iron tapped from a blast furnace is charged into a converter-type vessel and desiliconized, and upon completion of the desiliconization process, the molten iron has a predetermined silicon concentration and a predetermined molten temperature, and slag has a predetermined basicity; an intermediate slag removal step in which the molten iron and a portion of the slag after the desiliconization treatment are left in the vessel and a predetermined amount of slag is discharged; a dephosphorization step of dephosphorizing the molten iron by adding a lime-based solvent to the desiliconized molten iron and the slag remaining in the vessel and blowing oxygen into the molten iron; a tapping step of tapping the molten metal while leaving a portion of the slag having a predetermined basicity after the dephosphorization treatment in the vessel; A method for refining molten iron, comprising sequentially repeating a desiliconization step, an intermediate slag removal step, a dephosphorization step, and a tapping step, In the desiliconization step, the molten iron tapped from a blast furnace is charged into the vessel in which the slag remaining after the dephosphorization treatment is left, In the dephosphorization process, vibration of an oxygen supply lance for blowing oxygen is measured, and when the vibration measurement value of the oxygen supply lance becomes equal to or less than a predetermined value, control is performed to stop the addition of the solvent.

2. In the tapping step, 30 mass% or more of the slag after the dephosphorization treatment, in which the slag has a basicity of 1.2 or more, is allowed to remain in the vessel; In the desiliconization step, at the end of the desiliconization treatment, the Si concentration is 0.2 mass% or less, the molten iron temperature is 1240 ° C. or more and 1400 ° C. or less, and the slag basicity is 0.5 or more and 1.5 or less, 2. The method for refining molten iron according to claim 1, wherein in the intermediate slag removal step, 40 mass% or more of the slag after the desiliconization treatment is removed from the furnace.

3. A molten iron refining apparatus used in the method according to claim 1 or 2, A molten iron refining apparatus comprising: a converter-type vessel; a means for adding a lime-based solvent; an oxygen supply lance; a vibration measuring device for measuring the vibration of the oxygen supply lance; and a control means for controlling the amount of solvent added in accordance with the measured vibration value of the oxygen supply lance.

4. A method for producing a steel material, comprising the steps of: subjecting molten iron refined by the method according to claim 1 or 2 to secondary refining, including vacuum treatment as necessary, to adjust its composition; casting the molten iron into a steel material; and rolling the steel material to produce a steel material.

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