WO2019172195A1 - Dephosphorization method for molten iron - Google Patents

Abstract

A dephosphorization method for molten iron according to an embodiment of the present invention comprises a step for measuring the amount of Si contained in an initial component of the molten iron, a step for injecting a first dephosphorization agent into the molten iron, and a step for subjecting the molten iron to dephosphorization blowing. During the dephosphorization blowing step a powdery second dephosphorization agent also is injected into the molten iron, and the injection of the second dephosphorization agent is started after the Si content in the molten iron has been brought to 0.10 mass% or less by the dephosphorization blowing and before the amount of oxygen blown into the molten iron reaches 3.0 Nm3/t.

Description

How to remove hot metal

The present invention relates to a hot metal dephosphorization method.
This application claims priority on March 7, 2018 based on Japanese Patent Application No. 2018-040784 filed in Japan, the contents of which are incorporated herein by reference.

P (phosphorus) contained in the hot metal adversely affects various properties of the steel, such as strength, toughness, and elongation. Therefore, it is necessary to remove this as much as possible in the steel refining stage.

The process of removing P from the hot metal is called dephosphorization refining. In this dephosphorization refining, a dephosphorizing agent which is a compound mainly composed of CaO (or CaCO ₃ ) is used. The dephosphorizing agent contributes to dephosphorization by generating slag having dephosphorization ability in dephosphorization. The greater the amount of dephosphorization agent used, the higher the basicity of the slag and the better the dephosphorization ability of the slag. However, if the amount of dephosphorizing agent used is large, the amount of slag also increases. Slag has a high environmental impact and its treatment increases refining costs. Therefore, it is desired to reduce the amount of dephosphorization agent by making the dephosphorization process more efficient.

Patent Document 1 discloses a hot metal dephosphorization method capable of reducing the spitting amount and reducing the [P] concentration in the hot metal to 0.020% or less. In this dephosphorization method, the hot metal charged with CaO-containing material as the first dephosphorizing agent is gas-stirred, and an oxygen-containing gas is blown up to generate cover slag to form the hot metal. After the preliminary dephosphorization, a CaO-containing dephosphorizing agent, which is a second dephosphorizing agent, is further sprayed onto the molten iron using an oxygen-containing gas as a carrier gas. However, in Patent Document 1, there is no problem of reducing the amount of slag, and there is no disclosure of the method.

Patent Document 2 describes a converter that can stably melt even extremely strict P-standard extremely low phosphorus steel while enjoying the advantages of dephosphorization and decarburization refining in the same converter. A refining method is disclosed. In this converter refining method, after the first dephosphorization refining and the subsequent slag removal, before the decarburization refining, the flux is added to perform the second dephosphorization refining, and then the slag removal is performed. Furthermore, by performing decarburization refining after that, the P concentration in the molten steel after completion of decarburization refining can be sufficiently reduced to an extremely low P steel level. However, Patent Document 2 does not discuss improving the dephosphorization efficiency in each dephosphorization and does not disclose the method.
Patent Document 3 discloses a method of performing hot metal preliminary dephosphorization using a medium solvent that does not contain an F source such as CaF ₂ in the dephosphorization process performed as the hot metal preliminary process. This method is a method for producing low phosphorus hot metal, and in the dephosphorization process performed as a hot metal pretreatment, iron oxide in slag is supplied by supplying an oxygen source before adding a solvent as a CaO source to the hot metal. It is said that the concentration is increased, and then a solvent as a CaO source is added. However, in the technique of Patent Document 3, it is essential to make the medium solvent into a lump and add it in small amounts. That is, in the technique of the patent document, since the solvent is intermittently added, the basicity of the slag cannot be stabilized. Furthermore, in the technique of Patent Document 3, since it is essential that the Si concentration of the hot metal be 0.10 wt% or less before the dephosphorization process is started, the desiliconization process of the hot metal is essential prior to the dephosphorization process. Thus, production efficiency is poor.

Japanese Unexamined Patent Publication No. 2001-64713 Japanese Unexamined Patent Publication No. 2011-144415 Japanese Unexamined Patent Publication No. 2002-309310

In order to sufficiently remove phosphorus from hot metal without increasing the amount of dephosphorization agent used and the amount of slag generated, the present invention has a high dephosphorization rate and has high dephosphorization efficiency of hot metal. It is an object to provide a method.

The gist of the present invention is as follows.
(1) The hot metal dephosphorization method according to one aspect of the present invention includes a step of measuring an Si content in an initial component of the hot metal, a step of introducing a first dephosphorizing agent into the hot metal, and oxygen in the hot metal. A step of dephosphorizing and blowing the molten iron by blowing, and in the step of dephosphorizing and blowing, a powdery second dephosphorizing agent is added to the molten iron, and the second dephosphorization is performed. After the time when the Si content of the hot metal was reduced to 0.10% by mass or less by the dephosphorization blowing, the amount of oxygen blown into the hot metal after the time was 3 .Before setting to 0 Nm ³ / t.
(2) In the hot metal dephosphorization method described in (1) above, the ratio of the CaO equivalent of the first dephosphorizing agent to the SiO ₂ equivalent of the Si content in the initial component of the hot metal is 0. .60 to 2.00 may be used.
(3) In the hot metal dephosphorization method according to the above (1) or (2), the amount of the second dephosphorizing agent used is the slag charge base at the end of the step of dephosphorizing the hot metal. The degree may be controlled to be 1.3 to 4.0.
(4) In the hot metal dephosphorization method according to any one of the above (1) to (3), one or both of the first dephosphorizing agent and the second dephosphorizing agent is used as quick lime, limestone, calcium. Including one or more selected from ferrite, dolomite-based lime, and converter slag or secondary refining slag containing CaO, with CaO equivalents of CaO, CaCO ₃ , and CaF ₂ The total content may be 30 to 100% by mass.
(5) In the hot metal dephosphorization method according to any one of (1) to (4) above, the first dephosphorizing agent may be a bulk dephosphorizing agent.
(6) In the hot metal dephosphorization method according to any one of (1) to (5), the second dephosphorizing agent is selected from the group consisting of Ar, N ₂ , CO ₂ , and O _2. The hot metal may be blown into the hot metal using one or more kinds of carrier gas.
(7) In the hot metal dephosphorization method according to any one of the above (1) to (6), the Si content in the initial component of the hot metal may be 0.25% by mass or more.

ADVANTAGE OF THE INVENTION According to this invention, since the hatching rate of a dephosphorizing agent is high and it is excellent in the dephosphorization efficiency of hot metal, the hot metal dephosphorization method which can fully dephosphorize hot metal, without increasing the amount of slag is provided. be able to.

FIG. 4 is a CaO—SiO ₂ —Fe _t O ternary phase diagram. It is the schematic which shows the change of Si content of hot metal in dephosphorization refining, and the injection | throwing-in time of a 2nd dephosphorizing agent. It is a figure which shows an example of the injection location of a 2nd dephosphorizing agent. It is a graph which shows the relationship between the injection | pouring timing of a 2nd dephosphorization agent, and a dephosphorization rate. It is a graph which shows the relationship between the injection | throwing-in timing of a 2nd dephosphorization agent, and a hatching rate. It is a graph which shows the relationship between the injection | throwing-in timing of a 2nd dephosphorization agent, and a hatching rate.

The present inventors have repeatedly studied to improve the dephosphorization ability of dephosphorization without increasing the amount of slag (that is, to increase the dephosphorization efficiency of dephosphorization). Specifically, the present inventors examined means for improving the hatching rate of the dephosphorizing agent introduced into the hot metal during dephosphorization refining. In this embodiment, hatching of the dephosphorizing agent means that the dephosphorizing agent CaO and / or CaCO ₃ is melted to form slag. In this embodiment, the hatching rate is the basicity after dephosphorization (the value obtained by measuring the basicity of slag collected after dephosphorization), the basic charge (hot metal and additives) This is a value defined as a value divided by the basicity of slag when it is assumed that all of Si of SiO ₂ becomes SiO ₂ and all of the added dephosphorization agent CaO (or CaCO ₃ ) becomes molten CaO. On the other hand, the “mounting basicity” is a value obtained by measuring a slag component, that is, an actual value. The hatching rate is an index indicating the degree of melting of CaO and / or CaCO _{3 in} the dephosphorizing agent. The basicity of slag is the ratio of the amount of molten CaO and the amount of molten SiO ₂ in the slag, and is calculated by the following formula A.
Basicity of slag = Amount of molten CaO in slag / Amount of molten SiO _{2 in} slag: Formula A

The dephosphorizing agent is a compound containing CaO (or CaCO ₃ or the like) as a main component. CaCO ₃ contained in the dephosphorizing agent is decomposed into CaO and CO ₂ within a short time by the heat of the hot metal. As an example of the dephosphorizing agent, there are auxiliary materials such as quick lime, limestone, and dolomite lime, converter slag, secondary refining slag, and the like containing CaO and the like, and mixtures thereof. CaO contained in the dephosphorizing agent (including CaO derived from CaCO ₃ or the like, the same applies hereinafter) causes the following chemical reaction during dephosphorization blowing (blowing oxygen into the molten iron 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
The chemical formulas enclosed in square brackets described in the formulas B and C are chemical formulas of the components in the hot metal, and the chemical formulas enclosed in parentheses are chemical formulas of the components in the slag. In the dephosphorization blowing, first, as shown in the formula B, [P], that is, P in the molten iron is oxidized to (FeO), that is, FeO in the slag, to become P ₂ O ₅ . Next, as shown in Formula C, this P ₂ O ₅ is fixed to (CaO), that 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 for increasing the amount of molten CaO in the slag, it is conceivable to increase the amount of dephosphorization agent introduced into the hot metal. However, an increase in the amount of dephosphorization agent causes an increase in the amount of slag and increases the environmental load of dephosphorization. In order to increase the amount of molten CaO in the slag while suppressing the amount of dephosphorization agent added, it is necessary to increase the hatching rate of the dephosphorizer to be added as much as possible. The higher the hatching rate, the higher the ratio of the amount of molten CaO in the slag to the input amount of the dephosphorizing agent, and the dephosphorization can be carried out with high efficiency.

As a result of repeated studies, the present inventors put the first dephosphorizing agent into the hot metal before the start of dephosphorization blowing, and after a certain period of time after the start of dephosphorization blowing, the second dephosphorizing agent was added. It has been found that the hatching rate of the dephosphorizing agent is remarkably improved when it is put into the hot metal. This phenomenon is presumed to be caused by the following mechanism.

FIG. 1 is a CaO—SiO ₂ —Fe _t O ternary phase diagram. The dephosphorizing agent CaO is initially CaO located in the lower left part of the ternary phase diagram of FIG. Since CaO has a relatively high melting point, it exists in an unmelted state in the slag. However, when dephosphorization blowing is performed in the dephosphorization, Si in the hot metal is oxidized and the SiO ₂ concentration in the slag increases. As the SiO ₂ concentration increases, the dephosphorizing agent CaO becomes a ternary compound of CaO—SiO ₂ —Fe _t O. That is, the dephosphorizing agent CaO moves from the lower left part of FIG. 1 to the central part along the arrow. The melting point of the compound located in the center of FIG. 1 is lower than the melting point of CaO. Therefore, as the SiO ₂ concentration increases, the dephosphorizing agent CaO is easily melted.

At the start of dephosphorization blowing, there is almost no SiO ₂ in the slag. Therefore, the first dephosphorizing agent is hardly melted at the start of dephosphorization blowing, and if the second dephosphorizing agent is added at this stage, it is estimated that the first dephosphorizing agent does not melt sufficiently. . Then, even if the amount of SiO ₂ in the slag increases with the progress of dephosphorization blowing, it is considered that the unmelted material of the first dephosphorizing agent remains in the slag and the hatching rate decreases. On the other hand, when the second dephosphorizing agent is put on standby until the SiO ₂ is sufficiently formed, the second dephosphorizing agent is introduced in a state where the hatching of the first dephosphorizing agent has progressed. And the second dephosphorizing agent is believed to be fully hatched.

The hot metal dephosphorization method according to the present embodiment based on the technical idea described above includes a step of measuring the Si content of hot metal, a step of adding a first dephosphorizing agent to the hot metal, and dephosphorizing the hot metal. In the step of dephosphorizing and blowing, the second dephosphorizing agent is added to the molten iron, and the second dephosphorizing agent is started to be added at a Si content of 0. The time is from 10% by mass or less until the amount of oxygen blown into the hot metal reaches 3.0 Nm ³ / t. Hereinafter, the hot metal dephosphorization method according to this embodiment will be described in detail.

In the hot metal dephosphorization method according to this embodiment, first, the Si content of the initial component of the hot metal is measured. The measured value of the Si content of the hot metal initial component is required to determine the timing of the second dephosphorizing agent. Moreover, the measured value of the Si content of the initial component of the hot metal may be used to determine the input amount of the first dephosphorizing agent. In order to estimate the basicity of the slag at the end of dephosphorization blowing described later, the content of elements other than Si in the hot metal may be measured. The initial component of hot metal means the component of hot metal before dephosphorization blowing. The measurement of the Si content of the hot metal may be performed after the hot metal is charged into the furnace or before that. In addition, when the molten iron is in a solidified state (that is, when it is in the form of pig iron), it is naturally not hindered to perform the above measurement.
The Si content in the hot metal initial component is not particularly limited, but is preferably 0.25% by mass or more. As described above, as the SiO ₂ concentration increases during the dephosphorization blowing, the dephosphorizing agent CaO is easily melted. That is, in the hot metal dephosphorization method according to this embodiment, it is preferable to increase the SiO ₂ concentration in the slag from the viewpoint of further promoting hatching. Therefore, it is preferable that a certain amount of Si is contained in the hot metal initial component before the start of dephosphorization blowing. Therefore, it is preferable that the Si content of the hot metal before blowing oxygen is 0.25% by mass or more. It is good also considering Si content in the initial component of hot metal as 0.27 mass% or more, 0.30 mass% or more, or 0.32 mass% or more.

Next, the first dephosphorizing agent is added to the hot metal. The form and the input amount of the first dephosphorizing agent are not particularly limited, and can be appropriately set according to the hot metal component, the target value of the steel component, and the like. From the viewpoint of preventing input loss, the first dephosphorizing agent is preferably in the form of a lump.

The amount of the first dephosphorizing agent introduced is the ratio of the CaO equivalent of the first dephosphorizing agent to the Si equivalent of the Si content in the initial component of the hot metal (that is, the CaO equivalent of the first dephosphorizing agent / It is preferable that the initial content of the hot metal is controlled so that the Si content (SiO ₂ equivalent) of the hot metal is 0.60 to 2.00. The CaO equivalent of the dephosphorizing agent is the CaO content of the dephosphorizing agent when it is assumed that all the Ca in the dephosphorizing agent forms CaO. The SiO ₂ equivalent of the Si content in the initial component of the hot metal is the amount of SiO ₂ when it is assumed that all of the Si in the hot metal has become SiO ₂ . When the first dephosphorizing agent is added while satisfying the above conditions, the basicity of the slag is approximately 0.60 ~ when the Si in the hot metal is substantially all SiO ₂ due to the progress of dephosphorization blowing. 2.00.

By setting the ratio of the CaO equivalent of the first dephosphorizing agent and the SiO ₂ equivalent of the Si content in the initial component of the hot metal to 0.60 or more, the dephosphorization can be carried out at a high level. This is considered to be because molten CaO is sufficiently supplied into the slag and the dephosphorization ability of the slag can be improved. On the other hand, by setting the ratio of the CaO equivalent of the first dephosphorizing agent and the SiO ₂ equivalent of the Si content in the initial component of the hot metal to 2.00 or less, the hatching rate of CaO is kept high, and the dephosphorization efficiency. Can be kept higher. The ratio of the CaO equivalent of the first dephosphorizing agent to the SiO ₂ equivalent of the Si content in the initial component of the hot metal is more preferably 0.80 or more, 0.85 or more, or 0.90 or more. The ratio between the CaO equivalent of the first dephosphorizing agent and the SiO ₂ equivalent of the Si content in the initial component of the hot metal is more preferably 1.50 or less, 1.20 or less, 1.15 or less, or 1.10. It is as follows.

In addition, when Si is separately added to the hot metal before the start of dephosphorization and when the first dephosphorizing agent contains Si, when the slag SiO ₂ source is not limited to hot metal, it is derived from other than hot metal Si to be considered should also be taken into account when determining the input of the first dephosphorizing agent. For example, in order to reduce the amount of dephosphorization agent used, when slag generated by dephosphorization is recycled in another dephosphorization, a SiO ₂ source derived from other than hot metal is generated. In this case, Si derived from other than hot metal may be included in the “SiO ₂ equivalent of Si content in the initial component of hot metal”. That is, the first dephosphorizing agent and other additives so that the estimated basicity based on the charged amount of slag at the end of the Si oxidation reaction in dephosphorization blowing is 0.60 to 2.00. It is sufficient that the amount of charging is controlled.

Next, dephosphorizing and blowing the hot metal. Dephosphorization is carried out by blowing oxygen into the hot metal. In this dephosphorization blowing, the second dephosphorizing agent is charged into the hot metal. In this dephosphorization blowing, the timing of starting the addition of the second dephosphorizing agent is controlled as described below.

After starting the introduction of the second dephosphorizing agent into the hot metal, when the Si content of the hot metal was reduced to 0.10% by mass or less by dephosphorization, the amount of oxygen injected into the hot metal after this point Is set to 3.0 Nm ³ / t. Here, “Nm ³ / t” is an oxygen blowing amount (Nm ³ ) per 1 ton of hot metal. FIG. 2 shows the start timing of introducing the second dephosphorizing agent into the hot metal. FIG. 2 is a graph schematically showing how the Si content of hot metal decreases as the oxygen blowing in dephosphorization blowing proceeds, and the vertical axis of the graph represents the Si content (mass%) of hot metal. The horizontal axis indicates the amount of oxygen blown into the hot metal (Nm ³ / t). The hatched area in FIG. 2 is the start timing of introducing the second dephosphorizing agent into the hot metal in the hot metal dephosphorization method according to the present embodiment.

The second dephosphorizing agent starts to be introduced into the hot metal after the time when the Si content of the hot metal is sufficiently reduced by dephosphorization blowing, that is, after the time when the Si content of the hot metal is reduced to 0.10% by mass or less. Need to be done. When charging of the second dephosphorizing agent is started before the Si content of the hot metal sufficiently decreases, the second dephosphorizing agent is insufficient before the first dephosphorizing agent is sufficiently melted due to insufficient SiO _{2 in} the slag. Therefore, the melting of the dephosphorizing agent is hindered, the hatching rate is lowered, and the dephosphorization efficiency is impaired. You may make the start time of injection | pouring of the 2nd dephosphorization agent into hot metal after the time when Si content of hot metal fell to 0.05 mass% or less, or 0.01 mass% or less.

On the other hand, when the second dephosphorizing agent was introduced into the hot metal, oxygen was continuously blown in after the time when the Si content of the hot metal was lowered to 0.10% by mass or less. Must start before ³ / t. When the introduction of the second dephosphorizing agent into the hot metal is delayed, the dephosphorization efficiency is impaired. This is presumably because dephosphorization blowing proceeds in a state where molten CaO in the slag having a function of stabilizing unstable P ₂ O ₅ is insufficient. Preferably, introduction of the second dephosphorization agent into molten iron, or 2.5Nm ³ /t,2.0Nm ³ / t, the oxygen from the time the Si content of the molten iron drops below 0.10 wt% 1 .5Nm ³ / t Start until insufflation. In addition, when the Si content of the hot metal was reduced to 0.10% by mass or less, the initial components of the hot metal, the components and input amounts of additives such as the first dephosphorizing agent, and the oxygen to the hot metal were It can be estimated from the amount of air blown. Although it is generally difficult, if it is possible to measure the change in the Si content of the hot metal during dephosphorization blowing in real time, the measurement of the Si content of the initial component of the hot metal may be omitted. it can.

The input amount of the second dephosphorizing agent is not particularly limited, and can be appropriately set according to the hot metal component, the target value of the steel component, and the like. The form of the second dephosphorizing agent is powdery. By making the second dephosphorizing agent into powder, the second dephosphorizing agent can be continuously fed into the hot metal via the carrier gas blown from the lance. It is possible to suppress a rapid change in slag basicity by continuously adding the second dephosphorizing agent. By stabilizing the slag basicity, it is possible to prevent the occurrence of a sudden slag forming phenomenon and to stabilize the operation. Furthermore, by making the second dephosphorizing agent powdery, the hatching rate of the second dephosphorizing agent can be improved and the composition of the slag can be easily controlled. In this case, it is preferable that the powdery second dephosphorizing agent is blown into the molten iron using a carrier gas.

The input amount of the second dephosphorizing agent is preferably controlled so that the basicity of the slag at the end of the step of dephosphorizing the hot metal is 1.3 to 4.0. More preferably, the input amount of the second dephosphorizing agent is controlled such that the basicity of the slag at the end of the step of dephosphorizing the molten iron is 1.3 to 3.0.
There is a linear relationship between the basicity of the slag after the second dephosphorization agent and the basicity and dephosphorization rate of the slag after dephosphorization. As the charging basicity is larger, the slag basicity and the dephosphorization rate after dephosphorization tend to be improved. When the amount of the second dephosphorization agent is controlled so that the basicity of the slag at the end of the dephosphorization process is 1.3 or more, the dephosphorization ability of the slag is further improved, and the molten iron is removed. Phosphorus can be implemented at a high level. The input amount of the second dephosphorizing agent may be controlled so that the basicity of the slag at the end of the dephosphorizing 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 dephosphorizing agent is controlled so that the slag charge basicity at the end of the dephosphorization process is 4.0 or less, the increase in the slag amount is suppressed and the dephosphorization efficiency is reduced. Can be kept higher and the environmental load of the dephosphorization process can be further reduced. The amount of the second dephosphorizing agent is controlled so that the basicity of the slag at the end of the dephosphorizing process is 3.5 or less, 3.0 or less, 2.8 or less, or 2.3 or less. May be. Note that the amount of the second dephosphorizing agent at which the basicity of the slag is 1.3 to 4.0 at the end of the process of dephosphorizing the hot metal is determined by the usual method, the initial component of the hot metal, It can be estimated from the components of the two dephosphorizing agents, the components and amounts of additives such as the first dephosphorizing agent, and the amount of oxygen blown into the hot metal. The optimum amount of the second dephosphorizing agent is influenced by the above-mentioned various conditions, but is considered to be 1.0 to 5.0 t under normal conditions.

The type of the first dephosphorizing agent and the second dephosphorizing agent is not particularly limited as long as the basicity of the slag can be controlled as described above. For example, quick lime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary refining slag containing at least one selected from those containing CaO, CaO, CaCO ₃ , and CaF ₂ The total content of CaO in an amount of 30 to 100% by mass can be used as one or both of the first dephosphorizing agent and the second dephosphorizing agent.

As long as the above-mentioned requirements are satisfied, the hot metal dephosphorization method according to the present embodiment may include an additional step. For example, the hot metal after dephosphorization may be further subjected to decarburization refining, and even if this decarburization refining is continuously performed in the furnace where dephosphorization refining is performed, it is different from the furnace where dephosphorization refining was performed. It may be carried out in a furnace.

Also, the apparatus for carrying out the hot metal dephosphorization method according to the present embodiment is not particularly limited. From the knowledge of the present inventors, for example, an upper-bottom blowing converter 1 having a lance for blowing a powdery second dephosphorizing agent 5 using a carrier gas, as exemplified in FIG. It is preferable for carrying out the hot metal dephosphorization method according to the present invention. When performing dephosphorization refining of hot metal using the top-bottom blowing converter 1, it is preferable to add the second dephosphorizing agent 5 directly below and in the vicinity of the lance 4 for blowing up the blown oxygen 6 into the hot metal. Immediately below and in the vicinity of the lance 4 is a region where the temperature is very high due to oxidation heat of Si, C, etc. in the hot metal, that is, a fire point 7. By introducing the second dephosphorizing agent 5 into this region, the second dephosphorizing agent 5 can be melted more efficiently. Although FIG. 3 shows an embodiment in which the second dephosphorizing agent 5 is blown using the lance 4 for blowing the top blowing oxygen 6, another lance for blowing the second dephosphorizing agent 5 is used as the upper bottom. It may be provided in the blow converter 1. In addition, converter refining using a top-bottom blowing converter and dephosphorizing, slag removal, and decarburization refining in the same converter reduces the total refining time and further uses the dephosphorizing agent. It is possible to reduce heat loss in refining.

The type of carrier gas used when blowing the powdery second dephosphorizing agent into the hot metal is not particularly limited. For example, one or more gases selected from the group consisting of Ar, N ₂ , CO ₂ , and O ₂ are used. It can be used as a carrier gas. Considering the cost, the stability of the equipment, etc., it is considered preferable to blow the second dephosphorizing agent using N ₂ gas. In the hot metal at the stage of dephosphorization, since the content of C having a function of preventing N uptake of hot metal is high, even if the second dephosphorizing agent is blown into the hot metal with N ₂ , It is considered that the amount of N taken in is small enough to be ignored.

In the dephosphorization step, the powdery second dephosphorization agent was blown in one of the following stages. Hereinafter, the time when the Si content of the hot metal is reduced to 0.10% by mass or less by dephosphorization blowing is referred to as “when de-Si is completed”.
First stage (comparative example): 40 seconds before completion of desiliconization Second stage (comparative example): 20 seconds before completion of desiliconization Third stage (invention example): Fourth stage (invention example) immediately after completion of desiliconization: 20 seconds after the completion of desiliconization The above-mentioned fourth stage is before the amount of oxygen blown into the hot metal after the completion of desiliconization is set to 3.0 Nm ³ / t.

The operating conditions other than the timing of blowing the second dephosphorizing agent are as follows.
-Form of the first dephosphorizing agent: a mixture of massive lime, limestone and converter slag (CaO equivalent 25% by mass or more)
The form of the second dephosphorizing agent: the ratio of the CaO equivalent of the powdered quicklime and the first dephosphorizing agent to the SiO ₂ equivalent of the Si content in the initial component of the hot metal: 0.60 to 2.00 Within range / carrier gas type: N ₂

The experimental results are shown in Tables 1 to 4 and FIGS. The values shown in Tables 1 to 4 are experimental results in the first to fourth stages. The “lumped CaO” column and the “powder CaO” column indicate the input amounts of the first dephosphorizing agent and the second dephosphorizing agent, respectively. The “initial component Si content” is the Si content in the hot metal initial component (the component before the start of dephosphorization blowing). 4 to 6 are graphs of the data shown in Tables 1 to 4. FIG.

The horizontal axis of the graph shown in FIG. 4 shows the basicity of charging (the molten iron and the additive Si are all SiO ₂ , and the first and second dephosphorizing agents Ca are all molten CaO. Slag basicity), and the vertical axis represents the dephosphorization rate. Dephosphorization rate is the amount of decrease in P content by dephosphorization (subtracting the measured value of P content of hot metal after dephosphorization from the measured value of P content of hot metal before dephosphorization) (Value) divided by the measured value of the P content of the hot metal before dephosphorization, that is, a calculated value by the following equation.
Dephosphorization rate = ([P] i− [P] f) / [P] i
In the above formula, [P] i is a measurement value of the P content of the hot metal before dephosphorization blowing, and [P] f is a measurement value of the P content of the hot metal after dephosphorization blowing.

The horizontal axis of the graph shown in FIG. 5 is the charging basicity, and the vertical axis is the slag basicity after dephosphorization (value obtained by measuring the basicity of slag collected after dephosphorization). is there. FIG. 6 is a graph showing the average hatching rate in the first to fourth stages. As described above, the value obtained by dividing the slag basicity after dephosphorization by the charged basicity is the hatching rate, and the average hatching rate in each of the first to fourth stages is expressed as the first to fourth stages. The average hatching rate for each of the four stages is used.

As shown in FIG. 4, the dephosphorization rate of the inventive example was remarkably improved as compared with the comparative example obtained by the conventional dephosphorization method. Further, as shown in FIGS. 5 and 6, the hatching rate of the inventive example was remarkably improved as compared with the comparative example by the conventional dephosphorization method.

In the hot metal dephosphorization method according to the present invention, the hatching rate of the dephosphorizing agent is high and the hot metal dephosphorization efficiency is excellent. Therefore, the hot metal dephosphorization method according to the present invention can sufficiently dephosphorize the hot metal without increasing the amount of slag, so that high-grade steel with a low P content can be produced with a low environmental load. For the reasons described above, the present invention has extremely high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Converter 2 Hot metal 3 Slag 4 Lance 5 Second dephosphorizing agent 6 Top blowing oxygen 7 Fire point

Claims (7)

A method for dephosphorizing hot metal, the step of measuring the Si content in the initial component of the hot metal,
Adding a first dephosphorizing agent to the hot metal;
Dephosphorizing and blowing the hot metal by blowing oxygen into the hot metal;
With
During the dephosphorizing and blowing step, a powdered second dephosphorizing agent is added to the molten iron,
The start of charging the second dephosphorizing agent is performed after the time when the Si content of the hot metal is reduced to 0.10% by mass or less by the dephosphorization blowing, and the oxygen is added to the hot metal after the time. A hot metal dephosphorization method, wherein the amount before blowing is set to 3.0 Nm ³ / t. The ratio between the CaO equivalent of the first dephosphorizing agent and the SiO ₂ equivalent of the Si content in the initial component of the hot metal is 0.60 to 2.00. The hot metal dephosphorization method as described. The input amount of the second dephosphorizing agent is controlled so that the basicity of the slag at the end of the step of dephosphorizing the molten iron is 1.3 to 4.0. Item 3. The method for dephosphorizing hot metal according to Item 1 or 2. One or both of the first dephosphorizing agent and the second dephosphorizing agent are selected from quick lime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary slag containing CaO. The total content of CaO, CaCO ₃ , and CaF _{2 in} an equivalent amount of CaO is 30 to 100% by mass. The hot metal dephosphorization method according to claim 1. The method for dephosphorizing hot metal according to any one of claims 1 to 4, wherein the first dephosphorizing agent is a bulk dephosphorizing agent. The second dephosphorizing agent is blown into the molten iron using a carrier gas that is one or more selected from the group consisting of Ar, N ₂ , CO ₂ , and O ₂ . The hot metal dephosphorization method according to any one of the preceding claims. The hot metal dephosphorization method according to any one of claims 1 to 6, wherein an Si content in the initial component of the hot metal is 0.25% by mass or more.

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