JP7754076B2 - Molten iron refining method, refining equipment, and steel manufacturing method - Google Patents

Description

The present invention relates to a molten iron refining process, particularly a molten iron refining method, refining equipment, and 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 some of the carbon has been removed by refining, as well as mixtures thereof.

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

During pretreatment, refining slag is generated. This slag should not be discarded but should be utilized for various purposes. Conventionally, fluorite (CaF ₂ ), a fluorine source, has been used during dephosphorization to enhance the efficiency of the dephosphorization reaction. 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, 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 proportion of blast furnace hot metal, which requires a large amount of energy to reduce iron oxide and emits a large amount of CO ₂ , while increasing the proportion of cold iron sources, such as iron scrap, have been investigated. Against this background, recent pretreatment methods have tended to increase the proportion of cold iron sources used while improving refining methods.

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

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

JP 2013-167015 A

Yuichi Uchida, Naotaka Sasaki, Yuji Miki, Iron and Steel, Vol. 102 (2016), No. 12, pp. 31-37

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 treatment. As a result, it is not possible to sufficiently reduce the solvent consumption for dephosphorization treatment.

The present invention was made in consideration of the above circumstances, and its purpose is to provide a molten iron refining method, refining equipment, and steel manufacturing method that can determine the amount of solvent required during dephosphorization based on clear criteria without measuring the components during blowing.

The inventors discovered that during the dephosphorization process, the oxygen supply lance is not immersed in the molten metal, but does come into contact with the slag that has turned into slag and formed, causing it to vibrate. They also discovered that once the slag has turned into slag and settled, it no longer comes into contact with the oxygen supply lance, causing the lance to stop vibrating. The present invention was developed based on the results of these observations.

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 slag of a predetermined basicity after the dephosphorization treatment is left 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, and is characterized in that in the desiliconization step, the molten iron tapped from a blast furnace is charged into the vessel in which the dephosphorization treatment 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).

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 flux, 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 flux added in accordance with the measured vibration value of the oxygen supply lance.

The steel manufacturing method of the present invention, which advantageously solves the above-mentioned problems, is characterized by the fact 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 the steel material.

The molten iron refining method and refining apparatus of the present invention enable the amount of solvent required for dephosphorization to be determined based on clear criteria, without measuring the components during the blowing process. In particular, by measuring the vibration of the oxygen supply lance during the dephosphorization process, changes in the state of the slag can be grasped in a timely manner, and the amount of lime-based solvent required for dephosphorization can be determined, eliminating the need to add excess solvent. Furthermore, by leaving slag of a specified basicity in the vessel after dephosphorization, this leads to a reduction in the unit consumption of auxiliary materials throughout the entire refining process. In addition, by measuring the vibration of the oxygen supply lance, it is possible to determine when slag foaming has subsided, thereby suppressing slag leakage when the molten metal is tapped.

1A is a schematic vertical cross-sectional view of a converter-type vessel according to one embodiment of the present invention, and FIG. 1B is a schematic top view of a vibration measuring device. 1A to 1E are schematic diagrams showing a method for refining molten iron according to one embodiment of the present invention in the order of steps. FIG. 1 is a FeO—CaO—SiO ₂ ternary phase diagram showing the composition of slag in the dephosphorization step of the method for refining molten iron according to the embodiment. 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. 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.

The following describes in detail the embodiments of the present invention. Furthermore, the following embodiments are intended to exemplify devices and methods that embody the technical concepts of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concepts of the present invention can be modified in various ways within the technical scope set forth in the claims.

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 consisting of 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 gas, 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, it may be a gas that, when injected into molten iron 9, enhances the agitation of the molten iron 9 and promotes the dissolution of the cold iron source, or a gas that can inject a solvent into the molten iron together with a carrier gas. It is also possible to inject an endothermic gas, such as a hydrocarbon, to protect the bottom-blowing tuyere 3. Note that reference numeral 4 in the figure denotes a tapping port for tapping the refined molten iron 9.

In this embodiment, two or more converter-type refining furnaces 1 are used to refine molten iron, with at least one of these converter-type refining furnaces 1 being 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 a converter-type refining furnace 1 for pre-treatment, and then transfer the molten iron 9 after pre-treatment to a converter-type refining furnace for decarburization and refining.

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 value. 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 value may be vibration acceleration, vibration acceleration level, vibration level, displacement amplitude, or velocity amplitude. The intensity of vibration at a specific frequency 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 to the maximum value of constant vibration as noise.

The control device 21 is composed of a sequencer or computer. The control device 21 may include a control unit, memory unit, operation unit, display unit, 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), random access memory (RAM), etc. In response to operation of 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 within the memory unit. The CPU then loads the programs in the RAM's working area, executes various processes in accordance with 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. Additionally, the CPU receives information from external sensors such as the vibration measurement device 20 and sends and receives control signals to the lime-based solvent addition means via the communication unit. The communication unit is configured to communicate with other devices and equipment via wired or wireless communication.

The storage unit is composed of non-volatile semiconductor memory such as a solid state drive (SSD) or a hard disk drive (HDD). The storage unit may also include removable flash memory. 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.

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 via 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, or a stick controller, and outputs command signals input by key operation, mouse operation, etc. to the control unit.

Examples of the display unit include monitors such as a liquid crystal display (LCD), CRT, or organic light-emitting diode (LED). It displays information detected by the vibration measurement device 20 and the operating status of the dosing means, allowing for monitoring of the vibration state of the oxygen supply lance 2 and the addition status of the lime-based solvent. Based on the vibration measurement value displayed on the display unit, the operator can send instructions to the dosing 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 foaming subsiding and instructions to stop adding the lime-based solvent, in order to provide guidance to the operator.

The molten iron refining method according to this embodiment, which uses a converter-type refining furnace 1, is carried out in the following order, as shown in the flow chart of Figure 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 performing these steps in the same furnace, efficient preliminary treatment of molten iron is possible. The molten iron charging step and desiliconization step are collectively referred to as the desiliconization step.

[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 dephosphorization 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 containing iron as a main component, such as direct reduced iron or cold iron.

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 CaO to SiO ₂ (CaO)/SiO ₂ (hereinafter simply referred to as "basicity") by mass, 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 17 is less than 1.2, leaving the slag behind may be insufficient for adjusting the basicity during the subsequent desiliconization treatment. Therefore, a large amount of lime-based solvent must be added during the desiliconization treatment. The upper limit of the basicity of the slag after the dephosphorization treatment is not particularly limited. 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 value.

Furthermore, to effectively adjust 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. A value of 60 mass% or more is more preferable. 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 performed continuously, 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 highly basic post-dephosphorization slag. This also prevents problems such as slag expansion due to hydration reactions and alkali elution. Therefore, this embodiment is extremely effective in utilizing slag as a material.

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, which occurs when the slag is left 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 within its structure. Therefore, even after the slag is solidified, crushed, and magnetically separated, it still contains approximately 10% or more metallic iron. Conventionally, this slag was 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 molten iron, thereby reducing iron loss. On the other hand, since the fluidity of slag after desiliconization is relatively high, the metallic iron in the slag tends to coarsen. Therefore, this type of slag contains 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 smelting process of molten iron.

[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.

During this desiliconization process, Si contained in the molten pig iron 9a or Si contained in the Si source 15 and the cold iron source 11 that transfers to the molten iron during melting reacts with the oxygen source (Si + O ₂ → SiO ₂ ) 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 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 SiO ₂ produced by the above reaction.

In other words, it is preferable to adjust the slag basicity during desiliconization to between 0.8 and 1.5. This is because if the slag basicity during desiliconization is less than 0.8, depending on the [Si] (mass%) concentration in the molten iron, the dephosphorization ability of the slag 17 after dephosphorization may decrease, resulting in re-phosphorization, 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 phase fraction increases due to an increase in undissolved CaO, which reduces the fluidity of the slag 17 after dephosphorization, making it impossible to remove the slag. The preferred upper limit of slag basicity is approximately 1.2.

Next, it is preferable to adjust the basicity of the slag at the end of the desiliconization process 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 higher, more preferably 0.8 or higher. Furthermore, if the slag basicity at this stage is higher than 1.5, the fluidity of the slag will decrease, resulting in 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 lower, more preferably 1.2 or lower. 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.

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, which is inefficient. Furthermore, it is necessary to increase the magnesia (MgO) concentration in the slag to prevent wear on the magnesia carbon brick lining, which increases costs. A temperature of 1350°C or lower is more preferable. On the other hand, temperatures below 1240°C reduce the fluidity of the slag, resulting in problems such as reduced slag discharge volume during the next intermediate slag discharge or difficulty in controlling the discharge volume. Additionally, the scrap melting rate decreases. A temperature of 1260°C or higher is more preferable.

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 1,350°C, the amount of coolant, such as iron ore, added to regulate the temperature 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 material, or limited in quantity, such as ingots generated within the steelworks. Therefore, it is difficult to consistently use large amounts 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, have not been fully utilized.

Generally, it is relatively easy to increase the amount of inexpensive cold iron source used in the desiliconization stage. This allows the molten metal temperature at the end of the desiliconization process to be kept below 1,350°C. This makes it possible to significantly reduce the amount of iron oxide used in the dephosphorization stage. 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 during the desiliconization process. Note that if the molten metal temperature at the end of the desiliconization process is too low, some of the cold iron source may remain unmelted. However, the remaining cold iron source can be retained in the furnace along with the molten iron and allowed to continue melting until the next dephosphorization stage. In other words, as long as the cold iron source is completely melted by the time the dephosphorization process is completed, there will be no operational problems.

The molten metal temperature after desiliconization can 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 removed depending on the individual equipment and operating conditions. If there is residual scrap after desiliconization, the measured value will tend to be slightly higher than the estimated value, but this is within the margin of error and does not affect its use.

Here, the amount of Si that becomes oxide is the sum of that contained in the molten pig iron 9a, the cold iron source 11, and additives. The [Si] concentration in the molten pig iron 9a is determined by rapid analysis of a sample taken from the molten pig iron before each charge. However, a substitute method may be used, such as calculation using other analytical values, such as the composition of the blast furnace's tapped pig iron. Furthermore, the Si concentration in the various cold iron sources 11 is determined by, for example, the analytical value of a representative sample for each lot. Cold pig iron is often stable at a concentration similar to that of molten pig iron. Furthermore, although the Si concentration in scrap varies depending on the source, it is stable on average at a concentration of less than one-tenth that of pig iron, so this can be used as a representative value or can be ignored.

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

Additives also contain carbon. This carbon can come from carbonaceous materials such as coke or amorphous graphite, as well as carbon in carbides such as the aforementioned silicon carbide. Solvents can also be used as auxiliary materials, such as quicklime, lightly burned dolomite, and magnesia clinker. Slags such as dephosphorization slag, decarburization slag, and ladle slag can also be used as sources of calcium oxide or magnesium oxide. Examples of inexpensive auxiliary materials include calcium or magnesium carbonates and hydroxides. However, because these have a large heat absorption capacity, when used in large quantities, it is advisable to distinguish them from other solvents and revise the estimated heat balance.

Next, in this embodiment, the cold iron source consumption rate is adjusted. However, in order to melt a large amount of cold iron source in a short period of time, an appropriate amount of a Si source with a high calorific value may be used as a heat source. In this case, the non-oxide Si consumption rate added during charging or desiliconization treatment is preferably in the range of 2 to 10 kg/t-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-molten iron. In this regard, adding Si at a rate exceeding 10 kg/t-molten iron results in excessive _SiO2 generation during desiliconization treatment. Furthermore, even if desiliconization treatment is performed while leaving all of the slag from the previous dephosphorization treatment in the furnace, it becomes necessary to add a large amount of calcium oxide source for basicity adjustment. 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-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 decreases during the desiliconization process, resulting in a large amount of highly basic dephosphorized slag remaining in the furnace. In this case, the basicity of the desiliconized slag increases, hindering subsequent slag removal and potentially necessitating 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 desiliconized slag and ensuring a heat source for melting the cold iron source.

Ferrosilicon can be used as the silicon source. It is also preferable to use cheaper silicon carbide briquettes or waste refractories primarily composed of silicon carbide. 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 a predicted amount of decarburization, so that the carbon concentration in the molten iron at the end of the desiliconization process 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. Furthermore, the carburization rate on the surface of the cold iron source, such as scrap, decreases, leading to a slower melting rate of the cold iron source.

In this way, in the desiliconization process of this embodiment, the molten metal temperature 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, there is no decrease in productivity or increase in refining costs. Furthermore, melting of the cold iron source and preliminary refining of the molten iron can be carried out efficiently. However, if the cold iron source unit cost exceeds 250 kg/t of molten iron, an additional heat source will be required, which may increase costs, or the blowing time may be extended, which may reduce productivity. Furthermore, due to constraints on the cold iron source charging equipment, further increasing the amount used is not efficient.

As will be explained in detail later, in order to improve the slag removal performance of the slag 10 after desiliconization, 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 at which CO gas is 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.

The oxygen consumption rate supplied to the molten iron during desiliconization is preferably 2 Nm ³ /t-molten iron or more, more preferably 4 Nm ³ /t-molten iron or more, added to the amount stoichiometrically required for desiliconization. In this embodiment, such oxygen supply is preferably performed to maintain the [Si] concentration in the molten iron at the end of the desiliconization process at 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 ensure good slag removal performance even when removing slag after desiliconization. At the same time, it is possible to suppress re-phosphorization from the slag to the molten iron. In oxygen blowing for desiliconization, the top-blowing oxygen supply rate is preferably 1 to 2 Nm ^{3 /(min·t-molten iron). At the same time, it has been confirmed that the above-mentioned effects can be obtained when the bottom-blowing gas injection rate is approximately 0.02 to 0.2 Nm 3 /} (min·t-molten iron).

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 effective utilization of the lime content in the dephosphorized slag, even when the molten iron is desiliconized while leaving the entire amount of dephosphorized slag 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 P ₂ O ₅ concentration in the slag can be increased. This also promotes slag foaming. In particular, the P ₂ O ₅ in the slag has the effect of reducing 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 can be maintained and slag removal can be performed satisfactorily even at a relatively low iron oxide concentration, such as a total iron concentration (T.Fe) of approximately 10% by mass.

[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 of the slag from the slag into 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 the desiliconization treatment, the amount of (P ₂ O ₅ ) in the slag in the furnace in the subsequent dephosphorization step will become excessive, reducing the dephosphorization effect. Therefore, it is important to prevent this.

A characteristic feature of the method for refining molten iron according to this embodiment is that when the above-described processes (A) to (E) are repeatedly and continuously carried out, if the slag is not sufficiently discharged after the desiliconization treatment, care must be taken to prevent the accumulation of (P ₂ O ₅ ) in the furnace. This is because if the amount of (P ₂ O ₅ ) in the slag in the furnace becomes too large during the dephosphorization treatment stage, the increase in the (P ₂ O ₅ ) concentration in the slag reduces the efficiency of the dephosphorization reaction, resulting in an increase in the [P] concentration in the molten iron after treatment. In addition, the amount of lime-based solvent required for the dephosphorization reaction increases.

Therefore, in this embodiment, the slag discharge rate after desiliconization treatment is preferably at least 40 mass% or more, and more preferably 60 mass% or more. Here, the slag discharge 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 discharge rate is less than 40 mass%, the amount of lime-based solvent used in the subsequent dephosphorization treatment will increase, as mentioned above. Furthermore, if the amount of slag increases as a result, slag foaming cannot be suppressed, and slag will eject from the furnace throat during the dephosphorization treatment, which may cause operational problems.

As described above, in the molten iron refining method according to this embodiment, it is preferable to set the molten metal temperature at the end of the desiliconization process to 1240°C or higher and 1400°C or lower so that the slag basicity at the end of the desiliconization process is within the range of 0.5 to 1.5. Furthermore, optimizing the oxygen consumption rate to promote slag foaming ensures good slag fluidity and gas holdup. Furthermore, simply tilting the furnace body after the desiliconization process to allow the slag to flow out of the furnace throat can achieve a good slag removal rate. When adjusting the tilting angle of the furnace body 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. Furthermore, if the slag foaming subsides, the slag removal rate will decrease, so it is preferable to keep the time from the end of desiliconization treatment to the start of tilting the furnace body to remove the slag within 4 minutes.

The basicity of the slag at the end of the desiliconization process, required in this intermediate slag removal step, is less than 0.5. If the slag basicity at the end of the desiliconization process is less than 0.5, the slag will become too viscous, and a satisfactory slag removal rate may not be achieved. On the other hand, if the slag basicity exceeds 1.5, solid slag may form, reducing the fluidity of the slag and resulting in a low slag removal rate. From the perspective of ensuring slag removal and a satisfactory slag removal rate, it is therefore preferable to set the slag basicity to approximately 0.5 to 1.5. However, from the perspective of preventing rephosphorization of the slag during the desiliconization process 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.

Furthermore, 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 during the intermediate slag removal process, i.e., iron oxide, with the molten iron or carbon-containing iron particles suspended in the slag. To verify the appropriate 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 optimal T.Fe range in the slag at the end of the desiliconization process, i.e., during the slag removal process, is preferably set to 5-25 mass%. In the FeO-CaO-SiO ₂ ternary phase diagram of FIG. 3, the boundaries are indicated by two-dot chain lines.

Furthermore, if the slag temperature at the end of the desiliconization process in this intermediate slag removal step is low, for example, below 1240°C, the formation of solid slag will increase the viscosity of the slag and the viscosity of the liquid slag will increase. This will then reduce the fluidity of the slag and result 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 adjust at least one of the amount of heat source added, such as silicon carbide or ferrosilicon, and the oxygen consumption rate. Furthermore, by ensuring that the molten metal temperature at the end of the desiliconization process is 1240°C or higher, the slag temperature will also be 1240°C or higher. It is more preferable to set the molten metal temperature to 1260°C or higher.

However, if all of the slag produced after desiliconization 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 limits 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.

Therefore, to promote the slag formation of the lime-based solvent without using fluorite or iron oxide in the dephosphorization process, 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 utilize the _SiO₂ and iron oxide contained in the slag to promote slag formation. 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, 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 process. 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 process 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.

[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. In order to reduce heat loss, it is preferable to use only the 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 phosphorus _oxide ( _P2O5 ). This phosphorus 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 (the dephosphorization slag 17 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.

If the slag basicity of the post-dephosphorization slag 17 generated during this dephosphorization process is less than 1.2 or the molten metal temperature exceeds 1360°C, the slag's dephosphorization ability may be reduced, and the [P] concentration in the molten iron after the process 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. Even 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 process. To ensure sufficient heat for the decarburization process, it is preferable to set the molten metal temperature after the dephosphorization process 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 processes so that the carbon concentration in the molten iron at the end of the dephosphorization process is 2.8 mass% or higher.

When performing operations according to this embodiment, the [Si] concentration, [P] concentration, and molten metal temperature of the molten iron may change, resulting in cases where the molten metal temperature at the end of the desiliconization process is low, or where the [P] concentration of the molten iron is high, 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 area where the top-blown oxygen is irradiated onto the molten metal surface, direct decarburization reactions and iron oxidation occur, raising the temperature to approximately 2000°C. Adding a powdered lime source to this area promotes melting.

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.

In this embodiment, the dephosphorization process is carried out 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. Although the oxygen supply lance 2 does not come into contact with the molten metal, it does come into contact with 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 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 lime-based solvent is stopped by the control means.

For example, the dotted line in the FeO-CaO- _SiO2 ternary phase diagram in Figure 3 indicates the slag composition with a FeO concentration of 20 mass%. The arrows in Figure 3 indicate the transition 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-concentrated 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 addition of a lime-based solvent increases the slag basicity to approximately 1.3 (e.g., the open circle (○) in Figure 3), the hatched region where the phosphorus-concentrated phase precipitates is reached. This region where the phosphorus-concentrated phase precipitates has a melting point of 1400°C or higher, where the high-melting-point 2CaO· _SiO2 precipitates as a solid phase. The temperature of the molten metal during dephosphorization is controlled to 1,400°C or below to promote 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 approximately 1.5, which is suitable for dephosphorization. Furthermore, because the molten metal can be tapped only after confirming that foaming has cessated, accidents such as slag leakage from the furnace throat during tapping can be prevented.

[Potting 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 toward 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, in the vessel.

As explained above, in the molten iron refining method 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 and desiliconized, resulting in a continuous process. Therefore, most of the slag discharged from the converter-type refining furnace (pretreatment furnace) is desiliconized slag, reducing the contamination of dephosphorized slag, which has a relatively high basicity. This is to avoid the risk of slag contamination, which could lead to alkali elution or expansion due to hydration, thereby limiting the uses of the slag. In particular, a method in which no dephosphorized slag is discharged at all avoids these problems, simplifies slag treatment, and enables application to 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.

In addition, this embodiment allows the amount of lime-based solvent added during dephosphorization to be kept to a minimum, contributing to a reduction in the unit consumption of auxiliary materials throughout the entire molten iron refining process.

[Steel manufacturing method]
Another embodiment of the present invention is a method for producing steel material, 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 material. For secondary refining, RH vacuum treatment, stirring by bubbles, mechanical stirring, electromagnetic stirring, ladle refining, or the like can be used. For casting, ingot making/blooming methods, conventional continuous casting methods, thin slab continuous casting methods, or the like can be used. Examples of steel materials after rolling include hot-rolled steel plates, thick plates, shaped steel, steel bars, and wire rods.

Molten iron was refining using the molten iron refining apparatus and vibration measuring device 20 configured as shown in Figure 1. The converter-type refining furnace 1 had a capacity of 400 t, and the oxygen flow rate of the oxygen flow lance 2 was a maximum of 600 ^Nm3 /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 that the molten iron [P] concentration, molten metal temperature, and other conditions before dephosphorization, the oxygen flow rate, the lime-based solvent addition rate, and other conditions during dephosphorization, the target [P] concentration, and the target molten metal temperature after dephosphorization were the same.

In the invention example, 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 be the maximum value of the vibration acceleration measured in the past, and the maximum constant vibration (noise) value 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. The addition of lime-based solvent was then continued. When the voltage measured as a vibration value remained at zero for 10 seconds, it was determined that the slag foaming had subsided, and the addition of lime-based solvent was stopped.

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.

The dephosphorization results for the inventive example and the conventional example are shown in Figures 4 and 5. Figure 4 shows the relationship between the amount of lime-based solvent added per treatment (kg/ch) and the [P] concentration in the molten iron after tapping ([P] concentration below the furnace) (mass %) for the inventive example and the conventional example. 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 solvent added during dephosphorization per ton of molten iron for the inventive example and the conventional example. 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 variance in the [P] concentration below the furnace, reducing the consumption rate of lime-based solvent during dephosphorization by approximately 1.4 kg/ton of molten iron compared to the conventional example. There was no significant difference in the average under-furnace [P] concentration between the invention example and the conventional example.

In this specification, the mass unit "t" represents ¹⁰ 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 molten iron. The symbol (R) indicates that a compound of chemical formula R is contained in slag.

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.

1 Converter-type refining furnace 2 Oxidizing lance 3 Bottom-blowing tuyeres 4 Tap holes 5, 7 Hoppers 6, 8 Chute 9 Molten iron 9a Molten pig iron 10 Desiliconized slag 11 Cold iron source 12 Oxygen-containing gas 13 Bottom-blowing gas 14 Charging ladle 15 Silicon source 16 Lime-based solvent 17 Dephosphorized slag 20 Vibration measuring device 20a Acceleration pickup sensor 20b Detection direction 21 Control device

Claims (4)

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. 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. 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. 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.