Molten iron pretreatment method

Abstract

To provide a molten iron pretreatment method by which the compatibility between dephosphorization and suppression of yield loss can be stably realized.SOLUTION: In a molten iron pretreatment method using a converter-type vessel, a temperature of molten iron after a dephosphorization treatment is estimated based on an input amount of auxiliary raw materials other than solid acid, and at least one of input of the solid acid, adjustment of a lance height, and adjustment of an oxygen flow rate is performed according to the estimated temperature of the molten iron, thereby performing the blowing while controlling an FeO concentration in slag after the dephosphorization treatment and a temperature of the molten iron to approach a set value and a set temperature, respectively.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention particularly relates to a method for pre-treating hot metal that achieves both high phosphorus removal performance in phosphorus removal treatment and suppression of iron yield loss due to intermediate slag discharge.

Background Art

[0002] In the steelmaking process in ironmaking, impurities such as C, Si, P, and S in hot metal obtained by reducing iron ore in a blast furnace are removed in the refining process to obtain molten steel, and then slabs and the like are manufactured by casting the molten steel in the casting process. In the refining process, impurities are removed from hot metal as slag or gas through redox reactions. In recent years, pre-treatment of hot metal using a converter has been carried out for the purpose of reducing the amount of slag generated or reducing the amount of auxiliary raw materials used. In the pre-treatment of hot metal, desiliconization and dephosphorization treatments are performed in the converter. In the dephosphorization treatment, P in the hot metal is removed by the reaction of the following formula (1), and slag containing P2O5 is generated. On the other hand, if the slag containing P2O5 generated in the dephosphorization treatment is present during the subsequent decarburization treatment, as the temperature of the hot metal increases due to the decarburization treatment, P2O5 in the slag decomposes and P returns to the hot metal again. 2[P]+5(FeO)→(P2O5)+5[Fe] ···(1) Here, [ ] indicates that it is in molten steel, and ( ) indicates that it is in slag.

[0003] To avoid rephosphorization, it is necessary to react P2O5 in the slag with CaO to prevent decomposition (fix it in the slag), and it is effective to add a CaO source such as quicklime and limestone during the decarburization treatment. However, this leads to an increase in the amount of CaO source used and the amount of slag generated. Therefore, in order to minimize the use of the CaO source, after performing desiliconization and dephosphorization treatments and before performing the decarburization treatment, slag discharge (intermediate slag discharge) of the slag containing P2O5 generated in the desiliconization and dephosphorization treatments is generally carried out.

[0004] Furthermore, if the amount of P removed after the dephosphorization treatment is insufficient, additional CaO source needs to be added for further removal of P during the decarburization treatment. Therefore, in order to reduce the usage amount of the CaO source, it is also important to control the temperature of the hot metal during the pretreatment of the hot metal that affects the dephosphorization reaction. Since the dephosphorization reaction is thermodynamically more likely to proceed at lower temperatures, by adjusting the input amount of auxiliary raw materials and other factors to make the temperature of the hot metal during the dephosphorization treatment suitable for dephosphorization, the dephosphorization reaction can proceed efficiently. And by making the temperature of the hot metal after treatment reach about 1300 - 1350 °C to further promote the dephosphorization reaction, the P concentration in the hot metal can be lowered, that is, the P₂O₅ concentration in the slag can be increased, and the amount of P₂O₅ discharged out of the furnace through the intermediate slag discharge can be increased, reducing the amount of P carried over to the decarburization treatment, and thus the usage amount of the CaO source added in the decarburization treatment can be reduced.

[0005] Also, as can be seen from formula (1), since the dephosphorization reaction is affected by the amount of FeO present in the slag, it is necessary to control the production amount of FeO or the supply amount of the iron oxide source by adjusting the supply rate of oxygen by the top blowing lance or bottom blowing, etc., to ensure the FeO concentration in the slag. On the other hand, since the slag is discharged out of the system during the intermediate slag discharge, if the FeO concentration is too high, the amount of Fe discharged out of the system will increase, which will cause a reduction in the yield of iron. Therefore, in the dephosphorization treatment, it is necessary to lower the P concentration in the hot metal by adjusting the temperature of the hot metal after treatment and the FeO concentration in the slag, and at the same time, prevent the FeO concentration in the slag from becoming too high.

[0006] As a technique for achieving both dephosphorization and suppression of yield loss, Patent Document 1 discloses a method of promoting the production of FeO by adjusting the height of the top blowing lance and the oxygen gas flow rate at the initial stage of blowing, and promoting the reduction of FeO by adjusting the height of the top blowing lance and the oxygen gas flow rate at the final stage of blowing.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] In order to reduce the amount of CaO source used in the decarburization treatment, it is also important to increase the amount of slag discharged in the intermediate slag tapping (slag tapping property) as much as possible. For this purpose, it is effective to form slag by CO gas generated by the decarburization reaction between oxygen supplied in the dephosphorization treatment and C in the hot metal, and increase the slag volume. On the other hand, when excessive foaming occurs, the slag may overflow from the converter mouth during blowing, or the slag may scatter onto the front-of-furnace operating floor during slag tapping. Therefore, it is necessary to end the dephosphorization treatment when the formed slag reaches a predetermined height set in advance in the converter. Therefore, since the dephosphorization treatment time changes depending on the timing of foaming, the dephosphorization amount and the FeO concentration in the slag may vary, leading to poor dephosphorization and yield loss.

[0009] In the method described in Patent Document 1, the end timing of the dephosphorization treatment is unclear, and it is considered that when excessive foaming occurs and the end of the dephosphorization treatment is early, the adjustment to the target value cannot be sufficiently made. In particular, regarding the FeO concentration in the slag, since it is controlled to increase once and then decrease at the end of blowing, when the end of the dephosphorization treatment is early, the FeO concentration in the slag cannot be sufficiently controlled.

[0010] In view of the above problems, an object of the present invention is to provide a hot metal pretreatment method capable of stably achieving both dephosphorization property and suppression of yield loss.

Means for Solving the Problems

[0011] The inventors have found that there is a correlation between the FeO concentration in the slag and the end timing of the dephosphorization treatment, and have found that by controlling the FeO concentration in the slag to a certain value, fluctuations in the end timing of the dephosphorization treatment by forming can be suppressed. Here, as a method for controlling the FeO concentration, a method of controlling the generation of FeO by adjusting the oxygen supply conditions by top blowing or bottom blowing in a converter can be mentioned. However, when a solid acid is added as a cooling material to lower the temperature of the hot metal and promote the dephosphorization reaction, it is necessary to consider that the FeO concentration in the slag increases due to the addition of the solid acid.

[0012] Therefore, the inventors have specifically estimated the hot metal temperature at the end of the dephosphorization treatment to determine whether to add a solid acid, and by controlling the FeO concentration according to the determination result, it has been found that fluctuations in the FeO concentration in the slag and the end timing of the dephosphorization treatment can be suppressed. More specifically, when it is estimated that the hot metal temperature after the dephosphorization treatment is below a predetermined temperature, it is not possible to add a solid acid, so the FeO concentration is controlled by adjusting the oxygen supply conditions from the top blowing lance. On the other hand, when it is estimated that the hot metal temperature after the dephosphorization treatment is higher than a predetermined temperature, a solid acid is added to lower the hot metal temperature and promote the dephosphorization reaction. Paying attention to the relationship between the added amount of the solid acid and the FeO concentration, in addition to temperature adjustment, by adjusting the added amount of the solid acid so that the FeO concentration does not rise excessively, the fluctuation of the FeO concentration is made smaller than that in the control of the FeO concentration only by top and bottom blowing.

[0013] The present invention is as follows. [1] A method for preliminary treatment of hot metal using a converter-type vessel, estimating the hot metal temperature after the dephosphorization treatment based on the input amount of auxiliary raw materials other than the solid acid, and performing at least one of the addition of a solid acid, adjustment of the lance height, and adjustment of the oxygen flow rate according to the estimated hot metal temperature, so as to control the FeO concentration in the slag and the hot metal temperature after the dephosphorization treatment to approach the set value and the set temperature respectively, and performing blowing. A method for preliminary treatment of hot metal, characterized by this. [2] When the estimated molten iron temperature is higher than the set temperature, calculate the input amount of solid acid to make the molten iron temperature the set temperature, and adjust at least one of the input amount of solid acid, the lance height, and the oxygen flow rate so that the FeO concentration in the slag after the dephosphorization treatment estimated by the input of solid acid at the calculated input amount approaches the set value. When the estimated molten iron temperature is equal to or lower than the set temperature, at least one of adjusting the lance height and the oxygen flow rate is performed. The method for pretreatment of molten iron according to [1] above. [3] When the FeO concentration in the slag after the dephosphorization treatment estimated by the input of solid acid at the calculated input amount is smaller than the set value by a predetermined value, input solid acid at the calculated input amount, and at least one of adjusting the lance height and the oxygen flow rate. When the FeO concentration in the slag after the dephosphorization treatment estimated by the input of solid acid at the calculated input amount is larger than the set value by a predetermined value, adjust to input solid acid in an amount smaller than the calculated input amount of solid acid. The method for pretreatment of molten iron according to [2] above. [4] When the estimated molten iron temperature is equal to or lower than the set temperature, the lance height is adjusted based on the relationship between the previously prepared change amount of the lance height and the change amount of the FeO concentration in the slag. The method for pretreatment of molten iron according to [2] or [3] above.

Effect of the Invention

[0014] According to the present invention, it is possible to provide a method for pretreatment of molten iron that can stably achieve both dephosphorization properties and suppression of yield loss.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining a refining process in a converter according to this embodiment. Hereinafter, the refining process in the converter will be described with reference to FIG. 1.

[0017] First, in the de-Si·de-P (hereinafter simply referred to as de-P for simplicity of explanation) process for mainly removing Si and P in hot metal, a CaO source such as quicklime is charged into the hot metal as a sub-material, and an oxygen gas is blown by inserting an upper lance from the mouth of the converter, and at the same time, a bottom gas is blown from the bottom of the furnace to stir the hot metal to perform the de-P process. At this time, if necessary, a solid acid is charged as a sub-material to lower the temperature of the hot metal. The solid acid to be charged at this time is not particularly limited as long as it can lower the temperature of the hot metal, but basically, it is mainly composed of iron oxide. Specifically, sinter powder, iron ore, etc. can be mentioned. By the de-P process, slag containing CaO is generated, and the P removed in the de-P process is incorporated into this slag as P2O5. At this time, when the slag component is adjusted to a forming range, the slag forms (foams) as the de-P process progresses, the slag volume increases, and the slag surface in the converter rises.

[0018] Then, when the height of the slag surface reaches a predetermined value set in advance, the blowing of oxygen is stopped to end the dephosphorization treatment, the converter is tilted to perform intermediate slag removal, and a part of the slag is discharged out of the system. Thereafter, auxiliary materials are charged again, oxygen gas is blown from the top blowing lance, and bottom blowing gas is blown from the furnace bottom to stir the hot metal and perform the decarburization treatment. Then, the decarburization treatment is ended at the timing when the specified C concentration and molten steel temperature are reached, and the molten steel is tapped. In order to reduce the amount of the CaO source charged in the dephosphorization treatment, the hot metal may be charged into the converter in the next charge while leaving the slag (decarbonized slag) remaining in the furnace after tapping, and the hot recycle of the slag may be performed. Further, the decarbonized slag may be cooled and crushed, etc., and charged as decarbonized slag in the dephosphorization treatment in a separate charge, and the cold recycle of the slag may be performed. Thereby, the auxiliary materials newly charged can be saved.

[0019] Next, the correlation between the treatment conditions in the dephosphorization treatment and the FeO concentration in the slag will be described. As described above, the treatment time in the dephosphorization treatment is the time from the start of blowing oxygen from the top blowing lance until the slag surface reaches a predetermined height.

[0020] Here, when considering the factors affecting the treatment time in the dephosphorization treatment, it was found that the higher the FeO concentration in the slag, the shorter the blowing time (treatment time) tends to be. This is considered to be due to the change in physical properties such as the viscosity of the slag due to the FeO concentration, the change in the reactivity of the decarburization reaction due to FeO and C in the hot metal, and the change in the amount of slag affecting the formation of the slag. Therefore, if, as in the method described in Patent Document 1, the FeO concentration is once increased for dephosphorization and then decreased to the target value, the slag may form when the FeO concentration is increased, and the dephosphorization treatment may have to be terminated halfway.

[0021] The FeO concentration in the slag varies not only based on the oxidation of Fe in the hot metal by upward blowing but also according to the input per unit of solid acid introduced into the furnace. That is, as the input per unit of solid acid increases, the FeO concentration becomes higher. Solid acid is used as a coolant to lower the hot metal temperature to ensure dephosphorization performance. On the other hand, if the hot metal temperature drops too much, the slagging property of the auxiliary raw materials deteriorates, so there is a limit to its usage amount. Therefore, for the FeO that cannot be adjusted by the input of solid acid, the height of the upward blowing lance or the oxygen flow rate is changed to vary the production amount of FeO, thereby adjusting the FeO concentration in the slag.

[0022] From the above results, in order to stably achieve both dephosphorization performance and suppression of yield loss, it is important to set the hot metal temperature to a temperature suitable for the dephosphorization reaction and control the FeO concentration in the slag to be a constant value to reduce the variation in the treatment time. The target FeO concentration in the slag is approximately 15 - 35 mass%, but an appropriate concentration can be determined based on the relationship between the treatment time (degree of reaction progress) and the slag discharge property. An example is shown below.

[0023] According to the experiments conducted by the inventors, the intermediate slag discharge rate showed a tendency to gradually decrease as the FeO concentration decreased, and decreased rapidly around 25 mass%. Along with this, the out-of-system discharge amount of P changed in tendency around 25 mass% of the FeO concentration, similar to the intermediate slag discharge rate. On the other hand, regarding the out-of-system discharge amount of iron content (T.Fe) due to intermediate slag discharge, it increased proportionally to the increase in the FeO concentration even in the region where the change in the intermediate slag discharge rate was small. From the above results, in this embodiment, considering both dephosphorization performance and suppression of yield loss, the FeO concentration in the slag is set at a target set concentration of 25%, and the FeO concentration in the slag is controlled.

[0024] Next, a specific method for controlling the FeO concentration is shown below. Figure 3 is a diagram showing the processing flow for determining the processing conditions in the dephosphorization treatment. Hereinafter, a method for determining the processing conditions for controlling to the target FeO concentration will be described with reference to Figure 3.

[0025] First, measure the amount of hot metal charged into the converter, the amount of scrap, the Si concentration in the hot metal, and the hot metal temperature. Next, since it is necessary to adjust the basicity (C / S) of the slag so that it has high dephosphorization properties and a viscosity suitable for forming, determine the input amount of the auxiliary raw material (excluding solid acid) that is the CaO source so that the targeted C / S is achieved. First, regarding the SiO2 concentration in the slag, calculate the total amount of SiO2 in the slag by converting it to the amount of SiO2 when all the Si contained in the hot metal, scrap, and hot metal charged into the converter is oxidized. When performing hot recycling or cold recycling as described above, calculate the total amount of SiO2 by adding the SiO2 content in the decarburized slag. Then, based on the calculated total amount of SiO2, determine the input amount of the CaO source so that C / S becomes 0.8 to 1.2. Also, determine the input amount of auxiliary raw materials other than the CaO source, such as the MgO source input for refractory protection, as other auxiliary raw materials excluding solid acid.

[0026] Subsequently, using the input amount of the auxiliary raw material excluding solid acid determined by the above method, calculate the hot metal temperature at the end of the dephosphorization treatment when no solid acid is input. In this embodiment, the temperature change from the hot metal temperature before treatment when the height of the top lance and the oxygen flow rate blown from the top are set to predetermined reference values is calculated based on the specific heat calculation, the heat of dissolution, and the reaction heat due to the oxidation-reduction reaction to estimate the hot metal temperature at the end of the treatment. Specifically, using the following formula (2) of the estimated formula for the hot metal temperature at the end of the treatment, estimate the hot metal temperature at the end of the treatment with the input amount of sinter powder (solid acid) being 0. In this embodiment, an example of calculating the hot metal temperature is shown assuming that sinter powder is used as the solid acid, quicklime is used as the CaO source, limestone, and raw dolomite that also serves as the MgO source are input, and [Si] in formula (2) represents the Si concentration in the hot metal before dephosphorization (before desiliconization treatment). Also, the coefficients of each parameter in formula (2) are values obtained by multiple regression based on the temperature measurement results during operation. When the input auxiliary raw material changes, the parameters of formula (2) may be set according to the input auxiliary raw material, and each coefficient may be obtained by multiple regression based on the operation results. Temperature of molten iron at the end of treatment (°C) = Temperature of molten iron before treatment (°C) - {Scrap (t) × 1.0 + Sintered powder (t) × 2.8 + Quicklime (t) × 0.6 + Limestone (t) × 2.3 + Raw dolomite (t) × 2.3 + Cold pig iron (t) × 0.6} ÷ Quantity of molten iron (t) × 100 + [Si] × 0.0013 ···(2)

[0027] Next, the temperature of the molten iron at the end of treatment estimated using Equation (2) with the input amount of sintered powder set to 0 is compared with the set temperature of the molten iron temperature suitable for dephosphorization. Here, since the dephosphorization reaction is thermodynamically more likely to proceed at lower temperatures, it is set to 1350°C or lower as the temperature suitable for dephosphorization. Also, if the molten iron temperature is too low, it will be difficult for the auxiliary raw materials to melt and the slag properties will change, so it is preferably set to 1300°C or higher. In this embodiment, the target temperature of the molten iron temperature at the end of treatment will be described as 1300°C.

[0028] It is determined whether or not the temperature of the molten iron at the end of treatment estimated using Equation (2) with the input amount of sintered powder set to 0 is 1300°C or lower. If the estimated temperature of the molten iron at the end of treatment is 1300°C or lower, since the molten iron temperature cannot be lowered further, the iron oxide source for cooling (solid acid) cannot be added. Here, the FeO concentration in the slag in the state where the solid acid is not added is grasped in advance based on past operation results. In this embodiment, the FeO concentration in the slag is about 15 mass%.

[0029] As described above, in this embodiment, the target FeO concentration in the slag is set to 25% by mass. Therefore, when the solid acid cannot be charged, the FeO concentration in the slag will be insufficient, so the height of the top-blown lance or the oxygen flow rate will be changed to increase the FeO concentration. Fig. 2 shows the relationship between the change amount of the height of the top-blown lance and the change amount of the FeO concentration in the slag. As shown in Fig. 2, as the height of the top-blown lance increases, it approaches soft blowing, so the FeO concentration in the slag tends to increase. Therefore, the relationship between the lance height and the change amount of the FeO concentration is obtained in advance, the increase amount of the lance height is calculated so that the FeO concentration approaches 25% by mass, and the lance height for blowing is set. When the calculated lance height exceeds the upper limit in terms of equipment or operation, it is set to the upper limit lance height.

[0030] On the other hand, when the estimated molten iron temperature at the end of the treatment is higher than 1300 °C, the input amount of solid acid required to set the molten iron temperature at the end of the treatment to 1300 °C is calculated using the above formula (2). In this embodiment, the left side of formula (2) is set to 1300 °C, and the input amount of sinter powder is calculated as an example of solid acid.

[0031] Subsequently, based on the relationship between the input amount of solid acid obtained in advance and the FeO concentration in the slag from the input amount of solid acid calculated using formula (2), the FeO concentration in the slag is estimated. When the estimated FeO concentration is around 25% by mass, since it is not necessary to adjust the input amount of solid acid and the height of the top-blown lance, etc., the setting of the treatment conditions is terminated. Note that the vicinity of 25% by mass means a range of ±5% around 25% by mass. On the other hand, when the estimated FeO concentration deviates from the vicinity of 25% by mass, the adjustment method differs depending on whether the estimated FeO concentration is greater or less than the vicinity of 25% by mass.

[0032] When the estimated FeO concentration is higher than around 25% by mass, in order to reduce the input amount of solid acid, within the range acceptable for P removal (1300°C to 1350°C in this embodiment) on the left side of Equation (2), adjust the input amount of solid acid so that the FeO concentration approaches around 25% by mass. Specifically, calculate the input amount of solid acid at which the FeO concentration becomes around 25% by mass from the relationship between the input amount of solid acid and the FeO concentration in the slag described above. If the calculated tapping temperature of the hot metal at the end of the process is 1350°C or lower when put into the right side of Equation (2), adjust the input amount of solid acid to the calculated input amount. If it exceeds 1350°C, adjust the input amount of solid acid to the amount calculated with the left side of Equation (2) set to 1350°C.

[0033] On the other hand, when the estimated FeO concentration is lower than around 25% by mass, similar to the case where the calculated tapping temperature of the hot metal at the end of the process with the solid acid estimated to be 0 is 1300°C or lower, calculate the increase in lance height so that the FeO concentration approaches 25% by mass from the relationship between the change in lance height and the change in FeO concentration as shown in Figure 2, and set the lance height for blowing. If the calculated lance height exceeds the upper limit in terms of equipment or operation, set it to the upper limit lance height.

[0034] After determining the input amount of auxiliary materials (mainly CaO source) excluding solid acid, the height of the top-blowing lance, and, if necessary, the input amount of solid acid according to the above procedure, charge the auxiliary materials with the determined input amount, and adjust the height of the top-blowing lance as necessary to start blowing. Since the hot metal temperature is controlled to a temperature suitable for the P removal reaction and the FeO concentration is controlled to the target concentration as described above, it is possible to reduce the variation in the treatment time determined by forming, and stably achieve both P removability and suppression of yield loss.

[0035] In this embodiment, when the solid acid is charged and the estimated FeO concentration is less than around 25% by mass, or when the estimated tapping temperature of the molten iron is 1300°C or lower at the end of the treatment, the lance height is adjusted. However, the oxygen flow rate may be adjusted instead. In this case, similar to FIG. 2, the relationship between the oxygen flow rate and the change in FeO concentration is estimated in advance, and the oxygen flow rate may be reduced to perform soft blowing instead of raising the top blowing lance. Also, when the solid acid is charged and the estimated FeO concentration is greater than around 25% by mass, the target tapping temperature of the molten iron is increased and the input amount of the solid acid is recalculated. However, the top blowing lance may be lowered or the oxygen flow rate may be increased to approach hard blowing.

Example

[0036] Next, examples of the present invention will be described. These conditions are one example of the conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this example. The present invention can be implemented by various means without departing from the gist of the present invention and achieving the object of the present invention.

[0037] After performing pre-desiliconization treatment on the molten iron, using a converter, dephosphorization treatment, intermediate slag removal, and decarburization treatment were performed on 400 t of molten iron. Specifically, in the dephosphorization treatment, the input amount of the CaO source was calculated so that the C / S after treatment became 1.1. Further, within the range where the tapping temperature of the molten iron after treatment was 1300 to 1350°C and the FeO concentration in the slag after treatment was targeted at 25% by mass, the input amount of the solid acid and the lance height were calculated. Based on the calculation results, 400 t of molten iron was charged into the converter, and the CaO source and the solid acid were charged in the calculated amounts, and the top blowing lance was adjusted to the calculated height. Then, the oxygen flow rate was fixed at 60 kNm 3 / h and oxygen was blown to start the blowing process. After the start of the blowing process, when the height of the slag surface reached a position 3000 mm from the mouth of the converter, the blowing process was stopped, and the furnace body was tilted to perform intermediate slag removal. The experiment was conducted for 16 charges as an inventive example according to the above procedure.

[0038] On the one hand, for comparison, without setting a target value for the FeO concentration in the slag after treatment, the input amount of the CaO source was calculated so that the C / S after treatment became 1.1. Furthermore, the input amount of solid acid and the lance height were also calculated so that the temperature of the hot metal after treatment was in the range of 1300 - 1350 °C. The same was also carried out when 400 t of hot metal was charged into the converter, and then the CaO source and solid acid were charged with the calculated input amounts, and the top-blowing lance was adjusted to the calculated height. Then, the oxygen flow rate was fixed at 60 kNm 3 / h during the stable flow period for oxygen injection, and the blowing was started. After the start of the blowing, when the height of the slag surface reached the position 3000 mm from the mouth of the converter, the blowing was stopped, and the furnace body was tilted to perform intermediate slag removal. The experiment was carried out for 36 charges as a comparative example according to the above procedure.

[0039] As an evaluation method, regarding the P removal property, the unit consumption of P discharged out of the system (P discharged out of the system unit consumption) by intermediate slag removal was measured for each charge. The measurement results are shown in Figure 4. Figure 4 shows a histogram indicating the frequency of each interval with the P discharged out of the system unit consumption divided into sections of 0.05 kg / t, and the horizontal axis shows the value obtained by taking the difference based on the median value of the P discharged out of the system unit consumption of the inventive example. Also, regarding the suppression of yield loss, the unit consumption of FeO discharged out of the system (iron component discharged out of the system unit consumption) by intermediate slag removal was measured for each charge. The measurement results are shown in Figure 5. Figure 5 shows a histogram indicating the frequency of each interval with the iron component discharged out of the system unit consumption divided into sections of 1 kg / t, and the horizontal axis shows the value obtained by taking the difference based on the median value of the iron component discharged out of the system unit consumption of the inventive example.

[0040] As can be seen from the results of Figure 4 and Figure 5, in terms of P removal property and suppression of yield loss, on average, both the inventive example and the comparative example were equivalent, but the inventive example had less variation and could more stably achieve both P removal property and suppression of yield loss.

Explanation of Signs

[0041] 1 Converter 2 Top-blowing lance

Claims

1. A method for preliminary treatment of hot metal using a converter-type vessel, comprising: estimating the temperature of the hot metal after P removal based on the input amount of auxiliary raw materials other than solid acid; performing at least one of the input of solid acid, the adjustment of lance height, and the adjustment of oxygen flow rate according to the estimated hot metal temperature, so as to control the FeO concentration in the slag and the hot metal temperature after P removal to approach the set value and the set temperature respectively, and performing blowing. This is a method for preliminary treatment of hot metal.

2. When the estimated hot metal temperature is higher than the set temperature, calculate the input amount of solid acid to make the hot metal temperature reach the set temperature, and adjust the input amount of solid acid so that the FeO concentration in the slag after P removal estimated by the input of solid acid at the calculated input amount approaches the set value, or perform at least one of the adjustment of lance height and the adjustment of oxygen flow rate; When the estimated hot metal temperature is equal to or lower than the set temperature, perform at least one of the adjustment of lance height and the adjustment of oxygen flow rate. The method for preliminary treatment of hot metal according to Claim 1 is characterized by this.

3. When the FeO concentration in the slag after P removal estimated by the input of solid acid at the calculated input amount is smaller than the set value by a predetermined value, input solid acid at the calculated input amount, and perform at least one of the adjustment of lance height and the adjustment of oxygen flow rate; When the FeO concentration in the slag after P removal estimated by the input of solid acid at the calculated input amount is larger than the set value by a predetermined value, adjust to input solid acid in an amount less than the calculated input amount of solid acid. The method for preliminary treatment of hot metal according to Claim 2 is characterized by this.

4. When the estimated hot metal temperature is equal to or lower than the set temperature, adjust the lance height based on the relationship between the previously prepared change amount of lance height and the change amount of FeO concentration in the slag. The method for preliminary treatment of hot metal according to Claim 2 or 3 is characterized by this.

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