TWI683000B - Method for dephosphorizing molten iron - Google Patents

Abstract

A method for dephosphorizing molten iron according to one embodiment of the present invention comprises: measuring an amount of Si in an initial composition of the molten iron; adding a first dephosphorizing agent into the molten iron; and dephosphorization blowing the molten iron, in which powdery second dephosphorizing agent is added into the molten iron during the dephosphorization blowing, and adding the second dephosphorizing agent starts after a point of time at which the amount of Si in the molten iron is decreased to be 0.10 mass% or less by the dephosphorization blowing and before a point of time at which an amount of oxygen blown into the molten iron becomes 3.0 Nm ³/ t.

Description

Dephosphorization method of melting milling

The invention relates to a dephosphorization method for melt milling. This case claims priority based on Japan’s Japanese Patent Application No. 2018-040784, which was filed in Japan on March 7, 2018, and the contents are cited here.

Background of the invention Because the P (phosphorus) contained in the milling mill has a bad influence on the various characteristics of the steel such as strength, toughness and elongation, it must be removed as much as possible during the steel refining stage.

The step of removing P from melt milling is called dephosphorization refining. This dephosphorization refining uses a dephosphorization agent of a compound containing CaO (or CaCO ₃ ) as a main component. The dephosphorization agent is used in dephosphorization refining to help dephosphorization by generating slag with dephosphorization ability. The more the dephosphorization agent is used, the more the slag basicity will increase, and the more the slag dephosphorization ability can be improved. However, the greater the amount of dephosphorization agent used, the greater the amount of slag. Slag has a heavy environmental load, and its treatment will increase refining costs. Therefore, it is desirable to reduce the amount of dephosphorization agent used by optimizing the dephosphorization step.

Patent Document 1 discloses a dephosphorization method that can reduce the amount of slag blasting and can make the [P] concentration in melt milling to be 0.020% or less. In this dephosphorization method, gas milling is carried out from the melting mill where the first dephosphorizing agent, which is CaO, is added from the converter furnace, and the oxygen-containing gas is blown on top to produce a cover slag (cover slag). After the pre-dephosphorization, the oxygen-containing gas is used as the carrier gas, and the second dephosphorization agent, that is, the CaO-containing dephosphorization agent, is sprayed to the melt milling. However, in Patent Document 1, the reduction of the amount of slag is not a problem, nor is the method disclosed.

Patent Document 2 discloses a converter refining method, which can enjoy the advantages of dephosphorization refining and decarburization refining in the same converter, and can stably melt extremely low phosphorus steel with particularly strict P specifications. In this converter refining method, after the first dephosphorization refining and subsequent slag removal, before the decarburization refining, a flux is added to perform the second dephosphorization refining, after which the slag is removed, and then the decarburization refining is performed In this way, the P concentration in the molten steel after decarburization refining can be sufficiently reduced to a very low P steel grade. However, Patent Document 2 does not discuss the improvement of dephosphorization efficiency in each dephosphorization refining, nor does it disclose the method. Patent Document 3 discloses a method of using fluxes that do not contain F sources such as CaF ₂ in the dephosphorization treatment performed as a pretreatment for melt milling to perform efficient dephosphorization of melt milling. This method is a manufacturing method of low-phosphorus smelting milling, which is characterized in that, in the dephosphorization treatment performed as a smelting milling pretreatment, an oxygen source is supplied before adding a flux of CaO source to the smelting milling, thereby increasing the oxidation in the slag in advance Iron concentration, and then add the flux of CaO source. However, the technique of Patent Document 3 must make the flux into a bulk and add it in small amounts at a time. That is, since the technology of the patent document intermittently adds flux, it is impossible to stabilize the basicity of the slag. In addition, in the technique of Patent Document 3, the Si concentration of the melt milling must be 0.10 wt% or less before starting the dephosphorization process, so the desiliconization process of the melt milling is required before the dephosphorization process, which results in poor manufacturing efficiency.

Prior technical literature Patent Literature Patent Document 1: Japanese Patent Laid-Open No. 2001-64713 Patent Document 2: Japanese Patent Laid-Open No. 2011-144415 Patent Document 3: Japanese Patent Application Publication No. 2002-309310

Summary of the invention Problems to be solved by invention In order to fully dephosphorize melt milling without increasing the amount of dephosphorizing agent used and the amount of slag generation, the invention provides a demilling agent with high slagging rate and excellent dephosphorization efficiency of melt milling Phosphorus method is the subject.

Means for Solving the Problems The gist of the invention is as follows. (1) The dephosphorization method of melt-milling according to an aspect of the present invention includes the steps of: measuring the Si content in the initial component of the melt-milling; step of putting the first dephosphorization agent in the melt-milling; and By blowing oxygen into the melting mill to dephosphorize and blow the molten mill; in the dephosphorizing and blowing step, a powdered second dephosphorizing agent is further put into the molten mill, In addition, it is set to start the introduction of the second dephosphorization agent in the following period: after the time point at which the Si content of the melt milling becomes 0.10 mass% or less by the dephosphorization blowing, and after the Before the amount of oxygen blown into the mill becomes 3.0 Nm ³ /t. (2) In the dephosphorization method of melt milling as described in (1) above, the ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the melt milling can also be set to 0.60~ 2.00. (3) In the dephosphorization method of melt milling as described in (1) or (2) above, the input amount of the second dephosphorization agent may also be controlled so that the slag at the end of the dephosphorization refining step of the melt milling The loading alkalinity becomes 1.3 to 4.0. (4) In the dephosphorization method of melting milling according to any one of (1) to (3) above, one or both of the first dephosphorization agent and the second dephosphorization agent may be the following It contains at least one selected from the group consisting of quicklime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary refining slag containing CaO, and CaO, CaCO ₃ and CaF ₂ The total content in terms of CaO equivalent is 30 to 100% by mass. (5) In the method of dephosphorization by melting milling according to any one of (1) to (4) above, the first dephosphorization agent may be a bulk dephosphorization agent. (6) In the dephosphorization method of melt milling according to any one of (1) to (5) above, a carrier gas may also be used to blow the second dephosphorization agent into the melt mill, and the carrier gas system is selected from More than one gas in the group consisting of Ar, N ₂ , CO ₂ and O ₂ . (7) In the dephosphorization method of melt-milling according to any one of (1) to (6) above, the Si content in the initial component of the melt-milling may be 0.25% by mass or more.

Invention effect According to the present invention, it is possible to provide a dephosphorization method for smelting and milling that can sufficiently dephosphorize molten milling without increasing the amount of slag because the slagging rate of the dephosphorizing agent is high and the dephosphorization efficiency of the molten milling is excellent.

Forms for Carrying Out the Invention The present inventors have repeatedly studied the improvement of the dephosphorization capability of dephosphorization refining (that is, the improvement of the dephosphorization efficiency of dephosphorization refining) without increasing the amount of slag. Specifically, the present inventors explored a method that can increase the slagging rate of the dephosphorizing agent put into melting milling during dephosphorization refining. In addition, in this embodiment, slagging of the dephosphorizing agent means that CaO and/or CaCO _{3 of the} dephosphorizing agent is melted into slag. In this embodiment, the slagization rate is defined as the following value: the alkalinity after dephosphorization (the value obtained by measuring the alkalinity of the slag taken after dephosphorization blowing) is divided by the loading alkalinity (assumed melting The Si of the milling and the additives all become SiO ₂ , and the CaO (or CaCO ₃ ) of the dephosphorization agent input becomes the value obtained by melting the slag alkalinity of CaO). On the other hand, "actually loaded alkalinity" refers to the value obtained by measuring the slag composition, that is, the actual performance value. The slagging rate is an index showing the degree of melting of CaO and/or CaCO ₃ in the dephosphorization agent. The basicity of the slag is the ratio of the amount of molten CaO in the slag to the amount of molten SiO ₂ and is calculated by the following formula A. Slag basicity = amount of molten CaO in slag/ amount of molten SiO ₂ in slag: formula A

The dephosphorizing agent refers to a compound mainly composed of CaO (or CaCO ₃ etc.). CaCO ₃ contained in the dephosphorization agent will be decomposed into CaO and CO ₂ within a short time due to the heat energy of melting milling. Examples of dephosphorization agents include: quicklime, limestone and dolomite lime and other auxiliary materials, converter slag and secondary refining slag, etc., containing CaO, etc., and mixtures thereof. Due to the CaO contained in the dephosphorization agent (including CaO from CaCO ₃ etc., the same applies hereinafter), the following chemical reactions occur in the dephosphorization blowing (blowing oxygen into the milling mill for dephosphorization). 2[P]+5(FeO)→(P ₂ O ₅ )+5[Fe]: Formula B (P ₂ O ₅ )+3(CaO)→(3CaO·P ₂ O ₅ ): Formula C is described in Formula The chemical formulas enclosed in square brackets in B and Formula C are the chemical formulas of the components in melt milling, and the chemical formulas enclosed in parentheses are the chemical formulas of the components in slag. In the dephosphorization blowing, first of all, as shown in formula B, [P], that is, P in melt milling, is oxidized to P ₂ O ₅ by (FeO), that is, FeO in the slag. Then, as shown in formula C, the P ₂ O ₅ is fixed by (CaO), which is molten CaO in the slag, and a stabilized compound (3CaO·P ₂ O ₅ ) is generated. As shown in Formula B and Formula C, (CaO) is very important for dephosphorization.

As a means to increase the amount of molten CaO in the slag, it may be considered to increase the input amount of dephosphorization agent for melt milling. However, increasing the amount of dephosphorization agent will increase the amount of slag and increase the environmental load of dephosphorization refining. In order to suppress the amount of dephosphorization agent input and increase the amount of molten CaO in the slag, it is necessary to increase the slag rate of the dephosphorization agent input as much as possible. The higher the slagization rate, the higher the ratio of the amount of molten CaO in the slag to the input amount of the dephosphorization agent, and the dephosphorization can be performed efficiently.

The inventors conducted repeated investigations and found that the first dephosphorization agent was put into melting milling before the start of dephosphorization blowing, and the second dephosphorization agent was put in after a certain period of time after the start of dephosphorization blowing In the case of melt milling, the slagging rate of the dephosphorization agent will be significantly improved. We speculate that this phenomenon is caused by the following mechanism.

Fig. 1 is a state diagram of the CaO-SiO ₂ -Fe _t O ternary system. The CaO of the dephosphorization agent is initially located at the lower left of the ternary state diagram of FIG. 1. Since this CaO has a high melting point, it exists in an unmelted state in the slag. However, if dephosphorization blowing is performed during dephosphorization refining, Si in the milling process will be oxidized, and the SiO ₂ concentration in the slag will increase. As the concentration of SiO ₂ increases, CaO of the dephosphorization agent will become a ternary compound of CaO-SiO ₂ -Fe _t O. That is, CaO of the dephosphorizing agent will move from the lower left to the central part of FIG. 1 along the arrow. The melting point of the compound located in the center of FIG. 1 is lower than that of CaO. Therefore, as the concentration of SiO ₂ rises, CaO of the dephosphorization agent becomes easily melted.

At the time point when dephosphorization blowing starts, SiO _{2 is} hardly present in the slag. Therefore, at the time when the dephosphorization blowing starts, the first dephosphorization agent hardly melts, and if the second dephosphorization agent is added at this stage, it is presumed that the first dephosphorization agent will not be sufficiently melted. After that, even as the dephosphorization blowing progresses and the amount of SiO ₂ in the slag increases, the unmelted material of the first dephosphorization agent will remain in the slag, thereby reducing the slagging rate. On the other hand, when the second dephosphorization agent is not added until SiO _{2 is} sufficiently produced, the second and second dephosphorization agents are added in a state where the slagging of the first dephosphorization agent has proceeded. Both phosphorus agents can be fully slagged.

The dephosphorization method of melt milling according to the present embodiment based on the technical ideas described above includes the following steps: a step of measuring the Si content of the melt mill, a step of putting the first dephosphorization agent in the melt mill, and a demilling of the melt mill In the step of phosphorus blowing, in the step of dephosphorization blowing, the second dephosphorizing agent is further put into melting milling, and the start time of the second dephosphorizing agent is set to the following period: Si content from melting milling From dephosphorization blowing, it becomes 0.10 mass% or less until the amount of oxygen blown into the milling mill becomes 3.0 Nm ³ /t. Hereinafter, the dephosphorization method of melt milling in this embodiment will be described in detail.

The dephosphorization method of melt milling in this embodiment first measures the Si content of the initial component of melt milling. The measured value of the Si content of the initial component of smelting milling is necessary to determine the input time point of the second dephosphorization agent. In addition, the measured value of the Si content of the initial component of the milling mill can also be used to determine the input amount of the first dephosphorization agent. In order to estimate the alkalinity of the slag at the end of the dephosphorization blowing described later, the content of elements other than Si in melt milling can also be measured. In addition, the initial composition of melting milling refers to the melting milling composition before dephosphorization and blowing. The measurement of the Si content of melt milling can be carried out after the melt milling is installed in the furnace, or it can be carried out before the furnace is installed. In addition, when melt-milling is in a solidified state (that is, in the form of milling iron), of course, it does not prevent the implementation of the above measurement. The Si content in the initial component of melt milling is not particularly limited, but it is preferably set to 0.25% by mass or more. As described above, as the concentration of SiO ₂ rises during dephosphorization blowing, CaO of the dephosphorization agent becomes easily melted. That is, in the dephosphorization method of melting milling of the present embodiment, from the viewpoint of further promoting slag formation, it is desirable to increase the SiO ₂ concentration in the slag. Therefore, the initial component of melting milling before the start of dephosphorization and blowing preferably contains a certain amount of Si. Therefore, the Si content of the melt-milling before blowing oxygen is preferably at least 0.25% by mass. The Si content in the initial component of melt milling may also be set to 0.27 mass% or more, 0.30 mass% or more, or 0.32 mass% or more.

Next, the first dephosphorization agent is added during melt milling. The form and input amount of the first dephosphorization agent are not particularly limited, and can be appropriately set according to the target values of the melt milling component and the steel component. From the standpoint of preventing input loss, the form of the first dephosphorization agent is preferably massive.

Also, it is desirable to control the input amount of the first dephosphorization agent so that the ratio of the CaO equivalent of the first dephosphorization agent to the Si equivalent of the Si content in the initial component of the milling mill (that is, the CaO equivalent of the first dephosphorization agent/ The SiO ₂ equivalent of the Si content of the initial component of melt milling is 0.60 to 2.00. The CaO equivalent of the dephosphorizing agent assumes the CaO content of the dephosphorizing agent when all the Ca in the dephosphorizing agent is formed into CaO. Si content of the initial ingredients melt milled in SiO ₂ equivalent, based milling of the Si melt are assumed to be the amount of SiO ₂ SiO ₂ of Time. When the above conditions are satisfied and the first dephosphorization agent is added, the slag basicity is about 0.60 to 2.00 at the time when the dephosphorization blowing progresses and the Si in melt milling has substantially become SiO ₂ .

By setting the ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the milling mill to 0.60 or more, dephosphorization can be performed at a high level. This is because sufficient supply of molten CaO into the slag improves the dephosphorization ability of the slag. On the other hand, by making the ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the milling mill below 2.00, a high CaO slagging rate can be maintained, and a higher dephosphorization can be maintained Phosphorus efficiency. The ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the milling mill is more preferably 0.80 or more, 0.85 or more, or 0.90 or more. Furthermore, the ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the milling mill is more preferably 1.50 or less, 1.20 or less, 1.15 or less, or 1.10 or less.

In addition, when additional Si is added to the melting mill before the start of dephosphorization blowing, or when the first dephosphorization agent contains Si, etc., the SiO ₂ source of the slag is not limited to the case of melting milling. The amount of Si should also be taken into consideration when investing the amount. For example, in order to reduce the amount of dephosphorization agent used, when slag generated by dephosphorization refining is used in other dephosphorization refining, sources of SiO ₂ from melting milling will be generated. At this time, it is sufficient to include Si from other than milling in "SiO ₂ equivalent of Si content in the initial component of milling". That is, as long as the loading amount of the first dephosphorization agent and other additives is controlled so that the Si oxidation reaction in the dephosphorization blowing is completed, the estimated value of the alkalinity of the slag according to the loading amount will be 0.60 to 2.00 .

Next, melt-milling is carried out for dephosphorization and blowing. The dephosphorization blowing is carried out by blowing oxygen into the milling. In this dephosphorization blowing, the second dephosphorization agent is put into melt milling. In addition, in this dephosphorization blowing, the time point at which the second dephosphorization agent is started to be controlled is controlled as described below.

It is set to start to input the second dephosphorization agent to the melt milling during the following period: after the time point at which the Si content of the melt milling becomes 0.10 mass% or less by dephosphorization blowing, and to the melt milling blow after this time point The amount of oxygen input was 3.0 Nm ³ /t. Here, "Nm ³ /t" is the amount of oxygen injected (Nm ³ ) per ton of milling. Fig. 2 shows the period when the second dephosphorization agent is put into the melting mill. Fig. 2 is a graph schematically showing the situation in which the Si content of molten milling decreases as oxygen is gradually blown in during dephosphorization blowing. The vertical axis of the graph represents the Si content (mass %) of molten milling, and the horizontal axis represents the blowing of molten milling The amount of oxygen (Nm ³ /t). The hatched area in FIG. 2 is the time when the second dephosphorization agent is put into the melt milling in the dephosphorization method of the melt milling in this embodiment.

The time when the second dephosphorization agent is put into the melt milling must be set at the time when the Si content of the melt milling is sufficiently reduced by dephosphorization blowing, that is, the time when the Si content of the melt milling is reduced to less than 0.10% by mass After the point. When the second dephosphorization agent is started before the Si content of the milling mill is sufficiently reduced, the SiO ₂ in the slag is insufficient, and the second dephosphorization agent is added before the first dephosphorization agent is fully melted, which will hinder the dephosphorization The melting of the phosphorous agent reduces the slagging rate and damages the dephosphorization efficiency. The time when the second dephosphorization agent is put into melting milling may also be set after the time point when the Si content of melting milling is reduced to 0.05% by mass or less or 0.01% by mass or less.

On the other hand, it is necessary to start to add the second dephosphorizing agent to the melt milling within the following period: after the Si content of the above melt milling is reduced to 0.10% by mass or less, oxygen blowing is continued, and the blowing amount becomes 3.0 Nm ³ /t period. If the input of the second dephosphorization agent for melting and milling is delayed, the dephosphorization efficiency will be impaired. It can be presumed that the dephosphorization blowing is performed in a state where molten CaO in the slag is insufficient, and the molten CaO in the slag has the effect of stabilizing unstable P ₂ O ₅ . We should begin to melt into the second milling during dephosphorization agent in the following: decrease the Si content of the melt milled to 0.10 mass% or less of the blowing time until 2.5Nm ³ /t,2.0Nm ³ / t or 1.5Nm ³ / t oxygen. In addition, when the Si content of melt milling is reduced to less than 0.10% by mass, according to the general method, the initial composition of melt milling, the composition and input amount of additives such as the first dephosphorization agent, and the blowing of oxygen into the melt milling To estimate. In addition, although it is generally difficult, if the change in Si content of melt milling during dephosphorization blowing can be measured immediately, the measurement of the Si content of the initial component of melt milling can be omitted.

The input amount of the second dephosphorization agent is not particularly limited, and can be appropriately set according to the target values of the melt milling component and the steel component. The form of the second dephosphorization agent is powder. By making the second dephosphorization agent powdery, the carrier gas that can be blown from the spray gun can be passed through, and the second dephosphorization agent can be continuously put into the melting mill. By continuously feeding the second dephosphorization agent, the rapid change of the slag basicity can be suppressed. By stabilizing the basicity of the slag, the rapid slag blistering phenomenon can be prevented and the operation can be stabilized. In addition, by making the second dephosphorization agent powdery, the slagging rate of the second dephosphorization agent can be improved, and the composition of the slag can be easily controlled. At this time, the powdery second dephosphorization agent is preferably sprayed on the milling mill using a carrier gas.

The input amount of the second dephosphorization agent should be controlled so that the alkalinity of the slag charged at the end of the step of dephosphorization and refining by melting milling becomes 1.3 to 4.0. The input amount of the second dephosphorization agent is more preferably controlled so that the alkalinity of the slag charged at the end of the dephosphorization and refining step of melting milling becomes 1.3 to 3.0. A linear relationship can be observed between the loading alkalinity of the slag after the second dephosphorization agent is input, and the alkalinity and dephosphorization rate of the slag after dephosphorization. The greater the loading alkalinity, the more the slag basicity and dephosphorization rate tend to increase after dephosphorization. When the input amount of the second dephosphorization agent is controlled so that the basicity of the slag charged at the end of the dephosphorization refining step is 1.3 or more, the dephosphorization ability of the slag can be further improved and the demilling of the slag can be carried out at a high level phosphorus. The input amount of the second dephosphorization agent can also be controlled so that the alkalinity of the slag charged at the end of the dephosphorization refining step is 1.5 or more, 2.0 or more or 2.5 or more. On the other hand, when the input amount of the second dephosphorization agent is controlled so that the basicity of the slag charged at the end of the dephosphorization refining step is 4.0 or less, the increase in the amount of slag can be suppressed, and a higher dephosphorization efficiency can be maintained. It can further reduce the environmental load of the dephosphorization step. The input amount of the second dephosphorization agent can also be controlled so that the alkalinity of the slag charged at the end of the dephosphorization refining step is 3.5 or less, 3.0 or less, 2.8 or 2.3 or less. In addition, the input amount of the second dephosphorization agent with a basicity of 1.3~4.0 at the end of the step of dephosphorization and refining of melting milling may be based on the initial composition and second dephosphorization of melting milling according to the general method. The components of the agent, the components of the additives such as the first dephosphorization agent and the input amount, and the amount of oxygen blown into the melt mill are estimated. Although the optimal input amount of the second dephosphorization agent will be affected by the above-mentioned conditions, it is usually 1.0~5.0t under general conditions.

The types of the first dephosphorization agent and the second dephosphorization agent are not particularly limited as long as the alkalinity of the slag can be controlled as described above. As one or both of the first dephosphorization agent and the second dephosphorization agent, for example, one selected from the group consisting of quicklime, limestone, calcium ferrite, dolomite lime, and converter slag or One or more of the secondary refining slag containing CaO, and the total content of CaO, CaCO ₃ and CaF ₂ in terms of CaO equivalent is 30 to 100% by mass.

As long as the above-mentioned requirements are satisfied, the dephosphorization method of melting milling of this embodiment may include additional steps. For example, smelting and milling after dephosphorization may be further provided for decarburization refining. The decarburization refining may be carried out continuously in the furnace after dephosphorization refining or in a furnace different from the furnace after dephosphorization refining Implementation.

In addition, the apparatus for implementing the dephosphorization method of melt milling of this embodiment is not particularly limited. As far as the present inventors know, for example, the top-bottom converter 1 illustrated in FIG. 3 is preferably used to carry out the dephosphorization method of the smelting-milling of this embodiment. The top-bottom converter 1 has Spray gun into which the second dephosphorizing agent 5 is blown. When the top and bottom blowing converter 1 is used for dephosphorization refining of melting and milling, the second dephosphorizing agent 5 should be put directly under and near the spray gun 4 for blowing the top blowing oxygen 6 into the melting and milling. The area immediately below and near the spray gun 4 becomes a very high temperature area due to the heat of oxidation such as Si and C during melting and milling, that is, the combustion point 7. By putting the second dephosphorization agent 5 in this area, the second dephosphorization agent 5 can be melted more efficiently. In addition, although FIG. 3 illustrates an embodiment in which the second dephosphorizing agent 5 is blown using the spray gun 4 that can blow the top blowing oxygen 6, the top and bottom blowing converter 1 may also be provided to blow the second Spray gun for dephosphorization agent 5. In addition, by using a top-bottom blowing converter and dephosphorization refining, slag removal and decarburization refining in the same converter, the overall refining time can be shortened, the amount of dephosphorization agent used can be further reduced, and the heat during refining can be reduced loss.

The type of carrier gas used when blowing the powdery second dephosphorization agent into the milling mill is also not particularly limited, and for example, one or more kinds selected from the group consisting of Ar, N ₂ , CO ₂ and O ₂ can be used Gas, as a carrier gas. In consideration of cost and equipment stability, the second dephosphorizing agent is preferably blown in using N ₂ gas. In melt milling in the dephosphorization and blowing stage, since the melt milling has a high content of C that prevents N from being incorporated, even if N _{2 is} used to blow the second dephosphorization agent into the melt mill, it is incorporated into the melt mill The amount of N in is also negligible. Examples

In the dephosphorization step, the powdery second dephosphorization agent is blown in at any of the following stages. Hereinafter, the time when the Si content of melt milling becomes 0.10% by mass or less due to dephosphorization refining is referred to as "when de-Si is completed." Stage 1 (Comparative Example): 40 seconds from the completion of the removal of Si Stage 2 (Comparative Example): Stage 20 (before the completion of the removal of Si) Stage 3 (invention example): Stage 4 (invention) immediately after the completion of the removal of Si Example): After 20 seconds from the completion of the removal of Si, in the fourth stage above, the amount of oxygen blown into the mill after the completion of the removal of Si was 3.0 Nm ³ /t.

In addition, the operating conditions other than the time point at which the second dephosphorizing agent is blown are as follows. • The form of the first dephosphorization agent: a mixture of massive quick lime, limestone and converter slag (CaO equivalent of 25% by mass or more) • The form of the second dephosphorization agent: powdered quicklime • The CaO equivalent of the first dephosphorization agent The ratio of SiO ₂ equivalent to the Si content in the initial composition of the milling milling: in the range of 0.60 to 2.00 ・Carrier gas type: N ₂

[Table 1]

[Table 2]

[table 3]

[Table 4]

The experimental results are shown in Tables 1 to 4 and Figures 4 to 6. The numerical values shown in Table 1 to Table 4 are the experimental results of the first stage to the fourth stage. The "Block CaO" column and the "Powder CaO" column show the respective input amounts of the first dephosphorization agent and the second dephosphorization agent. "Initial component Si content" is the Si content in the initial component of melting milling (the component before dephosphorization and blowing begins). Figures 4 to 6 are graphs of the data shown in Tables 1 to 4.

The horizontal axis of the graph shown in FIG. 4 is the basicity of the loading alkali (assuming that both the Si of the milling mill and the additive become SiO ₂ , and the Ca of the first dephosphorization agent and the second dephosphorization agent are both molten slag. Of alkalinity), the vertical axis is the dephosphorization rate. The dephosphorization rate is obtained by reducing the P content caused by dephosphorization blowing (from the measured value of P content of melt milling before dephosphorization blowing minus the measured value of P content of melt milling after dephosphorization blowing Value) divided by the measured value of the P content of melt milling before dephosphorization and blowing, that is, the value calculated by the following formula. Dephosphorization rate = ([P]i-[P]f)/[P]i In the above formula, [P]i is the measured value of the P content of the milling before dephosphorization blowing, [P]f is the dephosphorization Determination of the P content of melt milling after blowing.

The horizontal axis of the graph shown in FIG. 5 is the loading alkalinity, and the vertical axis is the alkalinity of the slag after dephosphorization (a value obtained by measuring the alkalinity of the slag taken after the dephosphorization blowing is completed). Fig. 6 is a graph showing the average slagging rate in the first to fourth stages. As mentioned above, the value of the slag basicity after dephosphorization divided by the basicity of the load is the slagging rate, and the average value of the slagging rates of the first stage to the fourth stage is taken as the first stage to the fourth The average slagging rate of each stage.

As shown in FIG. 4, the dephosphorization rate of the invention example is significantly improved compared to the comparative example using the conventional dephosphorization method. In addition, as shown in FIGS. 5 and 6, the slagging rate of the invention example is significantly improved compared to the comparative example using the conventional dephosphorization method.

Industrial availability In the dephosphorization method of melt milling of the present invention, the dephosphorization agent has a high slagging rate and excellent dephosphorization efficiency of melt milling. Therefore, the dephosphorization method of smelting and milling of the present invention can fully dephosphorize smelting and milling without increasing the amount of slag, so that high-grade steel with a low P content can be manufactured in a low environmental load. For the reasons described above, the present invention has extremely high industrial applicability.

1... Converter 2...melt milling 3...Slag 4...Spray gun 5...Second dephosphorization agent 6...top blowing oxygen 7...Burning point

Fig. 1 is the state diagram of CaO-SiO ₂ -Fe _t O ternary system. FIG. 2 is a schematic diagram showing the change in Si content in smelting and milling in the dephosphorization refining and the input period of the second dephosphorization agent. FIG. 3 is a diagram showing an example of the input place of the second dephosphorization agent. FIG. 4 is a graph showing the relationship between the input time point of the second dephosphorization agent and the dephosphorization rate. FIG. 5 is a graph showing the relationship between the input time point of the second dephosphorization agent and the slagging rate. 6 is a graph showing the relationship between the input time point of the second dephosphorization agent and the slagging rate.

Claims (7)

A method of dephosphorization of melt milling is used to dephosphorize melt milling; the method is characterized by the following steps: a step of determining the Si content in the initial composition of the aforementioned melt milling, and putting the first dephosphorization into the aforementioned melt milling The step of phosphorous agent, and the step of dephosphorizing and blowing the molten mill by blowing oxygen into the melting mill; the second dephosphorizing agent in powder form in the step of dephosphorizing blowing Put into the melt milling, and set to start to put the second dephosphorization agent in the following period: after the dephosphorization blowing, the Si content of the melt milling becomes 0.10 mass% or less, and after the Before the time point, the amount of oxygen blown into the milling mill before 3.0 Nm ³ /t. According to the dephosphorization method of melt-milling of claim 1, the ratio of the CaO equivalent of the first dephosphorization agent to the SiO ₂ equivalent of the Si content in the initial component of the melt-milling is set to 0.60 to 2.00. The dephosphorization method of melt milling according to claim 1 or 2, which controls the input amount of the second dephosphorization agent so that the alkalinity of the slag charged at the end of the dephosphorization refining step of the melt milling becomes 1.3 to 4.0 . The dephosphorization method of smelting and milling according to claim 1 or 2, wherein one or both of the first dephosphorization agent and the second dephosphorization agent are the following substances: selected from the group consisting of quicklime, limestone and calcium Ferrite, dolomite lime, and one or more of converter slag or secondary refining slag containing CaO, and the total content of CaO, CaCO _3, and CaF ₂ in terms of CaO equivalent is 30 to 100% by mass. According to the dephosphorization method of smelting and milling according to claim 1 or 2, the first dephosphorization agent is a bulk dephosphorization agent. The dephosphorization method of melt milling according to claim 1 or 2, which uses a carrier gas to blow the aforementioned second dephosphorization agent into the aforementioned melt milling, and the carrier gas system is selected from the group consisting of Ar, N ₂ , CO ₂ and O ₂ More than one gas in the group. The dephosphorization method of melt-milling according to claim 1 or 2, wherein the Si content in the initial component of the melt-milling is 0.25% by mass or more.

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