Title of invention: Method for dephosphorizing hot metal
Technical field
[0001]
　The present invention relates to a method for dephosphorizing hot metal.
　The present application claims priority based on Japanese Patent Application No. 2018-040784 filed in Japan on March 7, 2018, and the content thereof is incorporated herein.
Background technology
[0002]
　Since P (phosphorus) contained in the hot metal adversely affects various properties of the steel such as strength, toughness, and elongation, it is necessary to remove it as much as possible in the refining stage of steel.
[0003]
　The step of removing P from hot metal is called dephosphorization refining, and in this dephosphorization refining , a dephosphorizing agent which is a compound containing CaO (or CaCO 3 ) as a main component is used. The dephosphorizing agent contributes to dephosphorization by producing slag having a dephosphorizing ability in dephosphorization refining. The greater the amount of the dephosphorizing agent used, the higher the basicity of the slag and the higher the dephosphorizing 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 load, and its treatment increases refining costs. Therefore, it is desired to reduce the amount of the dephosphorizing agent used by increasing the efficiency of the dephosphorizing step.
[0004]
　Patent Document 1 discloses a hot metal dephosphorization method capable of reducing the amount of spitting and making the [P] concentration in the hot metal 0.020% or less. In this dephosphorization method, the first dephosphorizing agent CaO-containing material gas stirs the hot metal charged from the converter furnace, and the oxygen-containing gas is blown upward to form cover slag to generate hot metal of the hot metal. After the preliminary dephosphorization, the hot metal is further sprayed with a CaO-containing dephosphorizing agent which is a second dephosphorizing agent by using an oxygen-containing gas as a carrier gas. However, in Patent Document 1, there is no problem in reducing the amount of slag, and there is no disclosure of the method.
[0005]
　Patent Document 2 discloses a converter which can enjoy stable smelting of extremely low phosphorus steel with a particularly strict P standard while enjoying the advantages of performing dephosphorization refining 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. By further performing decarburization refining after that, it is said that the P concentration in the molten steel after the 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 refining process, and does not disclose the method.
　Patent Document 3 discloses a method for efficiently performing preliminary hot metal dephosphorization using a solvent that does not contain an F source such as CaF 2 in the dephosphorization treatment that is performed as the hot metal pretreatment . This method is a method for producing low phosphorus hot metal, and in the dephosphorization treatment performed as a hot metal pretreatment, by supplying an oxygen source before adding a solvent solvent which is a CaO source to the iron oxide in the slag. It is said to be characterized in that the concentration is increased and then a solvent medium which is a CaO source is added. However, in the technique of Patent Document 3, it is indispensable to make the solvent medium lumpy and add this little by little. That is, since the medium solvent is intermittently added in the technology of the patent document, the basicity of the slag cannot be stabilized. Further, in the technique of Patent Document 3, it is essential that the Si concentration of the hot metal be 0.10 wt% or less before the start of the dephosphorization process, so that the desiliconization process of the hot metal is essential before the dephosphorization process. Therefore, the manufacturing efficiency is poor.
Prior art documents
Patent literature
[0006]
Patent Document 1: Japanese Unexamined
Patent Publication No. 2001-64713 Patent Document 2: Japanese Unexamined
Patent Publication No. 2011-144415 Japanese Patent Unexamined Publication No. 2002-309310
Summary of the invention
Problems to be Solved by the Invention
[0007]
　The present invention, in order to sufficiently dephosphorize hot metal without increasing the amount of dephosphorizing agent used and the amount of slag generated, the dephosphorizing agent has a high degree of slag formation, and the hot metal dephosphorization is excellent in the dephosphorization efficiency of hot metal. The challenge is to provide a method.
Means for solving the problems
[0008]
　The gist of the present invention is as follows.
(1) A method for dephosphorizing hot metal according to one aspect of the present invention includes a step of measuring a 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 pig iron by blowing a powder of the second dephosphorizing agent into the molten pig iron during the step of dephosphorizing and blowing the molten iron. After the start of the introduction of the agent, the Si content of the hot metal is set to 0.10 mass% or less by the dephosphorization blowing, and the amount of the oxygen blown into the hot metal after the time is 3 Before setting to 0.0 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. It may be set to 0.60 to 2.00.
(3) In the method for dephosphorizing hot metal according to the above (1) or (2), the amount of the second dephosphorizing agent added is such that the slag charging base at the end of the step of dephosphorizing and refining the hot metal is completed. The degree may be controlled to be 1.3 to 4.0.
(4) In the method for dephosphorizing hot metal according to any one of (1) to (3) above, one or both of the first dephosphorizing agent and the second dephosphorizing agent is calcium oxide, limestone or calcium. Ferrite, dolomite-based lime, and converter slag or secondary refining slag containing one or more selected from those containing CaO, wherein CaO, CaCO 3 , and CaF 2The total content of CaO in 30% by weight may be 30 to 100% by mass.
(5) In the method for dephosphorizing hot metal according to any one of (1) to (4), the first dephosphorizing agent may be a bulk dephosphorizing agent.
(6) In the method for dephosphorizing hot metal according to any one of (1) to (5) above, 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 by using at least one carrier gas.
(7) In the method for dephosphorizing hot metal according to any one of (1) to (6), the Si content in the initial component of the hot metal may be 0.25% by mass or more.
Effect of the invention
[0009]
　According to the present invention, since the slag removal rate of the dephosphorizing agent is high and the dephosphorization efficiency of the hot metal is excellent, there is provided a hot metal dephosphorization method capable of sufficiently dephosphorizing the hot metal without increasing the amount of slag. be able to.
Brief description of the drawings
[0010]
FIG. 1 is a CaO—SiO 2 —Fe t O ternary phase diagram.
FIG. 2 is a schematic diagram showing changes in the Si content of hot metal in dephosphorization refining and the timing of adding the second dephosphorizing agent.
FIG. 3 is a view showing an example of a location where a second dephosphorizing agent is charged.
FIG. 4 is a graph showing the relationship between the timing of introducing the second dephosphorization agent and the dephosphorization rate.
FIG. 5 is a graph showing the relationship between the timing of introducing the second dephosphorization agent and the rate of slag formation.
FIG. 6 is a graph showing the relationship between the charging timing of the second dephosphorizing agent and the slagification rate.
MODE FOR CARRYING OUT THE INVENTION
[0011]
　The present inventors have conducted extensive studies on improving the dephosphorization ability of dephosphorization refining without increasing the amount of slag (that is, increasing the dephosphorization efficiency of dephosphorization refining). Specifically, the present inventors have studied means for improving the slagging rate of the dephosphorizing agent added to the hot metal during dephosphorization refining. In the present embodiment, the slag formation of the dephosphorizing agent means that CaO and/or CaCO 3 of the dephosphorizing agent melts and becomes slag. In the present embodiment, the slag formation ratio means the basicity after dephosphorization (the value obtained by measuring the basicity of the slag collected after the completion of dephosphorization blowing) as the charging basicity (hot metal and additives). Is a value defined as a value obtained by dividing Si by SiO 2 and CaO (or CaCO 3 ) of the dephosphorization agent charged into molten CaO, and dividing by the slag basicity. On the other hand, the "mounting basicity" is a value obtained by measuring the slag component, that is, an actual value. The slagification rate is an index showing 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
[0012]
　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 in a short time by the heat of the hot metal . Examples of the dephosphorizing agent include secondary materials such as quick lime, limestone, and dolomite lime, converter slag and secondary refining slag containing CaO and the like, and mixtures thereof. CaO contained in the dephosphorizing agent ( including CaO derived from CaCO 3, etc., the same applies hereinafter) causes the following chemical reaction during dephosphorization blowing (blowing oxygen into the hot metal 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
　Described in Formulas B and C. The chemical formulas enclosed in square brackets are the chemical formulas of the components in the hot metal, and the chemical formulas enclosed in parentheses are the chemical formulas of the components in the slag. In the dephosphorization blowing, as shown in the equation B, [P], that is, P in the hot metal is oxidized by (FeO), that is, FeO in the slag to P 2It becomes O 5 . Next, as shown in Formula C, this P 2 O 5 is fixed to (CaO), that is, molten CaO in the slag to form a stabilized compound (3CaO·P 2 O 5 ). .. As shown in Formula B and Formula C, (CaO) is very important for dephosphorization.
[0013]
　As a means for increasing the amount of molten CaO in the slag, it is conceivable to increase the amount of dephosphorization agent added to the hot metal. However, an increase in the amount of the dephosphorization agent causes an increase in the amount of slag, which increases the environmental load of dephosphorization refining. In order to increase the amount of molten CaO in the slag while suppressing the input amount of the dephosphorizing agent, it is necessary to increase the slagging ratio of the dephosphorizing agent to be input as much as possible. The higher the slag formation rate, the higher the ratio of the molten CaO amount in the slag to the input amount of the dephosphorization agent, and the dephosphorization can be performed with high efficiency.
[0014]
　As a result of repeated studies, the inventors of the present invention introduced the first dephosphorizing agent into the hot metal before the start of the dephosphorizing blowing, and after a certain period of time from the start of the dephosphorizing blowing, the second dephosphorizing agent was added. It was found that the slagging ratio of the dephosphorizing agent is remarkably improved when it is added to the hot metal. It is presumed that this phenomenon was caused by the following mechanism.
[0015]
　FIG. 1 is a phase diagram of the ternary system CaO—SiO 2 —Fe t O. The dephosphorizing agent CaO is initially CaO located at the lower left of the ternary phase diagram of FIG. Since CaO has a relatively high melting point, it exists in the slag in an unmelted state. However, when dephosphorization is performed in the dephosphorization refining, Si in the hot metal is oxidized and the SiO 2 concentration in the slag increases. SiO 2 with increasing concentration, CaO dephosphorization agents CaO-SiO 2 -Fe t the ternary compound of the O. That is, the dephosphorizing agent CaO moves from the lower left portion of FIG. 1 to the central portion 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 increases, CaO of the dephosphorizing agent becomes easier to melt.
[0016]
　At the start of dephosphorization blowing , almost no SiO 2 was present in the slag. Therefore, it is presumed that the first dephosphorizing agent is hardly melted at the start of dephosphorization blowing, and if the second dephosphorizing agent is added at this stage, the first dephosphorizing agent is not sufficiently melted. .. After that, 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 slagging rate decreases. On the other hand, if the introduction of the second dephosphorizing agent is awaited until sufficient SiO 2 is produced, the second dephosphorizing agent is introduced in a state where the first dephosphorizing agent has been slagged. It is believed that both and the second dephosphorizing agent are fully slagged.
[0017]
　The hot metal dephosphorization method according to the present embodiment based on the above-described technical idea includes a step of measuring the Si content of the hot metal, a step of adding the first dephosphorizing agent to the hot metal, and a dephosphorization blowing of the hot metal. In the step of dephosphorizing and blowing, the second dephosphorizing agent is further charged into the hot metal, and the start time of the second dephosphorizing agent is adjusted so that the Si content of the hot metal is 0. It is set from the time when it becomes 10 mass% or less to the time when the amount of oxygen blown into the hot metal becomes 3.0 Nm 3 /t. Hereinafter, the method for dephosphorizing hot metal according to this embodiment will be described in detail.
[0018]
　In the hot metal dephosphorization method according to the present embodiment, first, the Si content of the initial component of the hot metal is measured. A measurement of the Si content of the initial component of the hot metal is needed to determine the dosing timing of the second dephosphorizing agent. Further, the measured value of the Si content of the initial component of the hot metal may be used for determining the input amount of the first dephosphorizing agent. The content of elements other than Si in the hot metal may be measured in order to estimate the basicity of the slag at the end of dephosphorization blowing described later. The initial component of the hot metal means the component of the hot metal before dephosphorization and blowing. The measurement of the Si content or the like of the hot metal may be carried out after charging the hot metal into the furnace or before it. Further, it is of course not hindered to carry out the above-mentioned measurement when the hot metal is in a solidified state (that is, when it is in the form of pig iron).
　The Si content in the initial component of the hot metal is not particularly limited, but it is preferably 0.25 mass% or more. As described above, CaO of the dephosphorizing agent is likely to melt as the SiO 2 concentration increases during dephosphorizing blowing . That is, in the hot metal dephosphorization method according to the present embodiment, it is preferable to increase the SiO 2 concentration in the slag from the viewpoint of further promoting slag formation . Therefore, it is preferable that the initial component of the hot metal before the start of the dephosphorization blowing contains a certain amount of Si. Therefore, the Si content of the hot metal before blowing oxygen is preferably 0.25 mass% or more. The Si content in the initial component of the hot metal may be 0.27% by mass or more, 0.30% by mass or more, or 0.32% by mass or more.
[0019]
　Next, the first dephosphorizing agent is added to the hot metal. The form and the amount of the first dephosphorizing agent are not particularly limited, and can be appropriately set according to the target values ​​of the hot metal composition and the steel composition. From the viewpoint of preventing the input loss, the first dephosphorizing agent is preferably in the form of lumps.
[0020]
　The amount of the first dephosphorizing agent added 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, 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 of the initial component (SiO 2 equivalent) is 0.60 to 2.00. The CaO equivalent of the dephosphorizing agent is the CaO content of the dephosphorizing agent, assuming that all Ca in the dephosphorizing agent forms CaO. SiO the Si content in the initial component of the hot metal 2 is an equal amount, Si hot metal all SiO 2 SiO assuming that became 2 is the amount. When the first dephosphorizing agent is added and the above conditions are satisfied, the basicity of the slag is about 0.60 to about 60 when Si in the hot metal becomes substantially SiO 2 due to the progress of dephosphorization blowing. It becomes 2.00.
[0021]
　Dephosphorization can be carried out at a high level by setting the ratio of the CaO equivalent of the first dephosphorizing agent and the Si content of the initial component of the hot metal to the SiO 2 equivalent of 0.60 or more. It is considered that this is because molten CaO can be sufficiently supplied into the slag to improve the dephosphorization ability of the slag. On the other hand, by setting the ratio of the CaO equivalent of the first dephosphorizing agent and the Si content of the initial component of the hot metal to the SiO 2 equivalent of 2.00 or less, the slagging ratio of CaO is kept high and the dephosphorization efficiency is improved. Can be kept higher. The ratio of the CaO equivalent of the first dephosphorizing agent and the Si content of the initial component of the hot metal to the SiO 2 equivalent is more preferably 0.80 or more, 0.85 or more, or 0.90 or more. 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 1.50 or less, 1.20 or less, 1.15 or less, or 1.10. It is the following.
[0022]
　In addition, if the SiO 2 source of the slag is not limited to hot metal, such as when Si is added to the hot metal before the start of dephosphorization blowing, and when the first dephosphorizing agent contains Si, it is derived from sources other than hot metal. The amount of Si to be added should also be taken into consideration when determining the input amount of the first dephosphorization agent. For example, when the slag generated by dephosphorization refining is recycled for use in another dephosphorization refining in order to reduce the amount of dephosphorization agent used, a SiO 2 source derived from other than hot metal is generated. In this case, Si derived from other than the hot metal may be included in the “ SiO 2 equivalent of the Si content in the initial component of the hot metal ”. That is, the first dephosphorizing agent and other additives are added so that the estimated value of the basicity of the slag at the end of the Si oxidation reaction in the dephosphorization blowing is 0.60 to 2.00 based on the charging amount. It suffices that the charging amount of is controlled.
[0023]
　Next, the hot metal is dephosphorized and blown. Dephosphorization blowing is carried out by blowing oxygen into the hot metal. In this dephosphorization blowing, the second dephosphorizing agent is added to the hot metal. In this dephosphorization blowing, the timing of starting the introduction of the second dephosphorization agent is controlled as described below.
[0024]
　After the start of the introduction of the second dephosphorizing agent into the hot metal, the time when the Si content of the hot metal was adjusted to 0.10 mass% or less by dephosphorization blowing, and the amount of oxygen blown into the hot metal after this time. Is 3.0 Nm 3 /t. Here, “Nm 3 /t” is the amount of oxygen blown in per ton of hot metal (Nm 3 ). Fig. 2 shows the start timing of adding the second dephosphorizing agent to the hot metal. FIG. 2 is a graph schematically showing how the Si content of the hot metal decreases as the oxygen blowing in the dephosphorization blowing progresses, and the vertical axis of the graph represents the Si content (mass %) of the hot metal. The horizontal axis represents the amount of oxygen blown into the hot metal (Nm 3 /t). The hatched region in FIG. 2 is the start timing of the introduction of the second dephosphorizing agent into the hot metal in the hot metal dephosphorizing method according to the present embodiment.
[0025]
　The start time of adding the second dephosphorizing agent to the hot metal is after the time when the Si content of the hot metal is sufficiently reduced by the dephosphorization blowing, that is, after the time when the Si content of the hot metal is reduced to 0.10 mass% or less. Needs to be done. When the introduction of the second dephosphorizing agent is started before the Si content of the hot metal is sufficiently reduced, the SiO 2 in the slag becomes insufficient, and the second dephosphorizing agent is sufficiently melted before the first dephosphorizing agent is sufficiently melted. As a result, the melting of the dephosphorization agent is hindered, the slag formation rate is reduced, and the dephosphorization efficiency is impaired. The start time of adding the second dephosphorizing agent to the hot metal may be after the time when the Si content of the hot metal has decreased to 0.05% by mass or less, or 0.01% by mass or less.
[0026]
　On the other hand, when the second dephosphorizing agent was added to the hot metal, the blowing of oxygen was continued after the Si content of the hot metal decreased to 0.10 mass% or less, and the blowing amount was 3.0 Nm. It must start by the time it reaches 3 /t. If the introduction of the second dephosphorizing agent into the hot metal is delayed, the dephosphorization efficiency will be impaired. It is presumed that this is because dephosphorization blowing proceeds in a state where molten CaO in the slag, which has a function of stabilizing unstable P 2 O 5 , is insufficient. Preferably, introduction of the second dephosphorization agent into molten iron, 2.5 Nm oxygen from the time the Si content of the molten iron drops below 0.10 wt% 3 /T,2.0Nm 3 or / t, 1 It is started before the blowing of 0.5 Nm 3 /t. In addition, when the Si content of the hot metal is reduced to 0.10% by mass or less, the initial components of the hot metal, the components and input amounts of the additives such as the first dephosphorizing agent, and the oxygen content of the hot metal are measured by a conventional method. Can be estimated from the injection amount of It is generally difficult to omit the measurement of the Si content of the initial component of the hot metal if it is possible to measure the change in the Si content of the hot metal during dephosphorization blowing in real time. it can.
[0027]
　The amount of the second dephosphorizing agent added is not particularly limited, and can be appropriately set according to the hot metal composition, the target value of the steel composition, and the like. The form of the second dephosphorizing agent is powder. By powdering the second dephosphorizing agent, the second dephosphorizing agent can be continuously introduced into the hot metal through the carrier gas sprayed from the lance. By continuously feeding the second dephosphorizing agent, it becomes possible to suppress a sudden change in the slag basicity. By stabilizing the slag basicity, it is possible to prevent a sudden slag foaming phenomenon from occurring and stabilize the operation. Further, by powdering the second dephosphorizing agent, the slagging ratio of the second dephosphorizing agent can be improved and the composition of the slag can be easily controlled. In this case, the powdery second dephosphorizing agent is preferably blown into the hot metal by using a carrier gas.
[0028]
　The amount of the second dephosphorizing agent added is preferably controlled so that the basicity of charging the slag at the end of the step of dephosphorizing and refining the hot metal is 1.3 to 4.0. More preferably, the amount of the second dephosphorizing agent added is controlled such that the basicity of charging the slag at the end of the step of dephosphorizing and refining the hot metal is 1.3 to 3.0.
　There is a linear relationship between the basicity of the slag charged after the second dephosphorizing agent is added and the basicity and the dephosphorization rate after the dephosphorization. The greater the charging basicity, the higher the slag basicity and the dephosphorization rate after dephosphorization. When the input amount of the second dephosphorizing agent is controlled so that the basicity of slag charging at the end of the dephosphorization refining process is 1.3 or more, the dephosphorization ability of the slag is further improved and the demolition of the hot metal is performed. Phosphorus can be implemented at a high level. The amount of the second dephosphorizing agent added may be controlled so that the basicity of charging the slag 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 charging amount of the second dephosphorizing agent is controlled so that the charging basicity of the slag at the end of the dephosphorization refining step 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 dephosphorization agent is controlled so that the basicity of charging the slag at the end of the dephosphorization refining process is 3.5 or less, 3.0 or less, 2.8 or less, or 2.3 or less. May be done. The amount of the second dephosphorizing agent at which the charging basicity of the slag is 1.3 to 4.0 at the end of the step of dephosphorizing and refining the hot metal is carried out by the usual method. It can be estimated from the components of the second dephosphorization agent, the components and addition amounts of additives such as the first dephosphorization agent, and the amount of oxygen blown into the hot metal. The optimum amount of the second dephosphorizing agent is influenced by the various conditions described above, but it is considered that it is often 1.0 to 5.0 t under normal conditions.
[0029]
　The types of the first dephosphorizing agent and the second dephosphorizing agent are not particularly limited as long as the basicity of the slag can be controlled as described above. For example, quicklime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary refining slag containing one or more selected from those containing CaO, such as CaO, CaCO 3 , and CaF 2. The total content of CaO in 30 to 100% by mass can be used as one or both of the first dephosphorizing agent and the second dephosphorizing agent.
[0030]
　As long as the above requirements are met, the hot metal dephosphorization method according to this embodiment may include additional steps. For example, the hot metal after dephosphorization may be further subjected to decarburization refining, and even if this decarburization refining is continuously carried out in the furnace where dephosphorization refining was carried out, it was different from the furnace where dephosphorization refining was carried out. It may be carried out in a furnace.
[0031]
　Further, the apparatus for carrying out the hot metal dephosphorization method according to the present embodiment is not particularly limited. The present inventors have found that, for example, the upper-bottom blowing converter 1 having a lance for blowing the powdery second dephosphorizing agent 5 by using a carrier gas is exemplified in FIG. It is preferable to carry out the hot metal dephosphorization method according to the present invention. When performing dephosphorization refining of the hot metal using the upper-bottom blowing converter 1, it is preferable to introduce the second dephosphorizing agent 5 immediately below the lance 4 for injecting the upper-blown oxygen 6 into the hot metal and in the vicinity thereof. Immediately below and in the vicinity of the lance 4 is a region where the temperature is extremely high due to the heat of oxidation of Si and C in the hot metal, that is, the fire point 7. By introducing the second dephosphorizing agent 5 into this region, the second dephosphorizing agent 5 can be more efficiently melted. Although FIG. 3 shows an embodiment in which the second dephosphorizing agent 5 is blown by using the lance 4 for blowing the upper blowing oxygen 6, another lance for blowing the second dephosphorizing agent 5 is used as the upper bottom. It may be provided in the blowing converter 1. In addition, by using a top-and-bottom blowing converter and deoxidizing and refining, removing slag, and decarburizing and refining in the same converter, the overall refining time is shortened and the amount of dephosphorizing agent used is further reduced. It is possible to further reduce the heat loss in refining.
[0032]
　The type of carrier gas used when blowing the powdery second dephosphorizing agent into the hot metal is not particularly limited, and for example, one or more gases selected from the group consisting of Ar, N 2 , CO 2 , and O 2 may be used. It can be used as a carrier gas. Considering the cost and the stability of the equipment, it is considered preferable to blow the second dephosphorizing agent using N 2 gas. The molten iron phase of dephosphorization blowing, because of the high content of C, which has a function to prevent the N uptake of hot metal, N 2 is also a blow secondary dephosphorization agent using as went into the hot metal, hot metal It is considered that the amount of N taken in by is negligibly small.
Example
[0033]
　In the dephosphorization step, the powdery second dephosphorization agent was blown in at any of the following stages. Hereinafter, the time point when the Si content of the hot metal becomes 0.10 mass% or less by dephosphorization blowing is referred to as "at the completion of Si removal".
First stage (comparative example): 40 seconds before completion of Si removal
Second stage (comparative example): 20 seconds before completion of Si removal
Third stage (Invention example): Immediately after completion of Si removal
Fourth stage (Invention example): 20 seconds after the completion of Si removal
　, the above-described fourth step is before the amount of oxygen blown into the hot metal after the completion of Si removal is set to 3.0 Nm 3 /t.
[0034]
　The operating conditions other than the timing of blowing the second dephosphorizing agent are as follows.
-Form of first dephosphorizing agent: lumpy mixture of quick lime, limestone and converter slag (CaO equivalent 25 mass% or more)
-Second form of dephosphorizing agent: powdery quick lime
-first dephosphorizing agent Ratio of CaO equivalent to SiO 2 equivalent of Si content in the initial component of hot metal : within the range of 0.60 to 2.00
・Carrier gas type: N 2
[0035]
[table 1]

[0036]
[Table 2]

[0037]
[Table 3]

[0038]
[Table 4]

[0039]
　The experimental results are shown in Tables 1 to 4 and FIGS. 4 to 6. The values ​​shown in Tables 1 to 4 are the experimental results in each of the first to fourth stages. The "lump CaO" column and the "powder CaO" column show 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 initial component of hot metal (the component before the start of dephosphorization blowing). 4 to 6 are graphs of the data shown in Tables 1 to 4.
[0040]
　The horizontal axis of the graph shown in FIG. 4 is the basicity of the charge (Si of the hot metal and the additive Si are all SiO 2 , and the charged first dephosphorizing agent and the second dephosphorizing agent Ca are all molten CaO. It is the basicity of the slag assuming that) and the vertical axis is the dephosphorization rate. Dephosphorization rate refers to the amount of decrease in P content by dephosphorization blowing (the measured value of P content of hot metal before dephosphorization blowing minus the measured value of P content of hot metal after dephosphorization blowing) Value) is divided by the measured value of the P content of the hot metal before dephosphorization 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 a measured value of P content in the hot metal before dephosphorization blowing, and [P]f is It is a measured value of P content of the hot metal after dephosphorization blowing.
[0041]
　The horizontal axis of the graph shown in FIG. 5 is the charging basicity, and the vertical axis is the slag basicity after dephosphorization (the value obtained by measuring the basicity of the slag collected after the completion of dephosphorization blowing). is there. FIG. 6 is a graph showing average slagging rates in the first to fourth stages. As described above, the value obtained by dividing the slag basicity after dephosphorization by the charging basicity is the slag formation rate, and the average value of the slag formation rate in each of the first to fourth stages is the The average slagging rate for each of the four levels is used.
[0042]
　As shown in FIG. 4, the dephosphorization rate of the invention example was remarkably improved as compared with the comparative example by the conventional dephosphorization method. Further, as shown in FIGS. 5 and 6, the slagging ratio of the invention example was remarkably improved as compared with the comparative example by the conventional dephosphorization method.
Industrial availability
[0043]
　In the hot metal dephosphorization method according to the present invention, the dephosphorizing agent has a high degree of slag formation, and the hot metal dephosphorization efficiency is excellent. Therefore, the method for dephosphorizing hot metal according to the present invention can sufficiently dephosphorize 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.
Explanation of symbols
[0044]
1 Converter
2 Hot Metal
3 Slag
4 Lance
5 Second Dephosphorizer
6 Top Blown Oxygen
7 Fire Point
The scope of the claims
[Claim 1]
　A method for dephosphorizing hot metal,
　the step of measuring the Si content in the initial component of the
　hot metal, the step of introducing a first dephosphorizing agent into the
　hot metal, and the oxygen is blown into the hot metal to remove the hot metal. A step of performing phosphorus blowing,
and
　during the step of dephosphorizing blowing, further adding a powdery second dephosphorizing agent to the hot metal,
　starting the introduction of the second dephosphorizing agent of the hot metal After the time when the Si content is reduced to 0.10 mass% or less by the dephosphorization blowing, and before the amount of the oxygen blown into the hot metal after the time is 3.0 Nm 3 /t. A
method for dephosphorizing hot metal, which comprises:
[Claim 2]
The ratio 　of CaO equivalent of the first dephosphorizing agent to SiO 2 equivalent of the Si content in the initial component of the hot metal is set to 0.60 to 2.00. Method for dephosphorizing hot metal described.
[Claim 3]
　The amount of the second dephosphorizing agent added is controlled so that the basicity of charging the slag at the end of the step of dephosphorizing and refining the hot metal is 1.3 to 4.0. Item 3. A method for dephosphorizing hot metal according to Item 1 or 2.
[Claim 4]
　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 refining slag containing CaO. Any one of claims 1 to 3, characterized in that the total content of CaO, CaCO 3 , and CaF 2 in CaO equivalent is 30 to 100% by mass. The method for dephosphorizing hot metal according to 1).
[Claim 5]
　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.
[Claim 6]
　The second dephosphorizing agent is blown into the hot metal by using a carrier gas which is at least one selected from the group consisting of Ar, N 2 , CO 2 , and O 2 . The method for dephosphorizing hot metal according to any one of claims.
[Claim 7]
　The method for dephosphorizing hot metal according to any one of claims 1 to 6, wherein the Si content in the initial component of the hot metal is 0.25% by mass or more.