Method for dephosphorizing molten iron

Abstract

To provide a method for dephosphorizing a molten iron, capable of increasing a basicity of a slag in second dephosphorization treatment and performing good slag removal by foaming the slag, in a MURC3blow in which dephosphorization treatment and an intermediate slag removal are performed twice.SOLUTION: The present invention relates to a method for dephosphorizing a molten iron, wherein: in a first step (first dephosphorization treatment), a charged amount of a CaO source 15 is adjusted so that a basicity, which is a mass concentration ratio of CaO and SiO2 in a slag after dephosphorization treatment, is in a range of 1.0 to 1.4; and in a third step (second dephosphorization treatment), a powder (a CaO-iron oxide-mixed powder 16) obtained by mixing CaO and iron oxide is continuously supplied from a top blowing lance 2 together with oxygen at any time during an oxygen top blowing period, while ensuring 60 to 100% of the total top blowing time, and a powder supply amount is adjusted so that the basicity of the slag 12 after the dephosphorization treatment is in a range of 1.5 to 1.8.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for dephosphorizing hot metal using a top-bottom blown converter.

Background Art

[0002] In an integrated hot metal steelmaking process, impurities contained in hot metal tapped from a blast furnace are removed by refining molten steel in the steelmaking process. As a molten steel refining apparatus, a top-bottom blown converter is used, and hot metal is refined by blowing oxygen from an upper lance at the upper part of the converter and blowing bottom blowing gas from the bottom of the converter. In top-bottom blown converter blowing, Si in the charged hot metal is oxidized and removed, and then a dephosphorization reaction of hot metal proceeds with slag formed in the converter. Thereafter, the dephosphorization refining efficiency can be increased by separating the slag with a high phosphorus concentration from the hot metal. Further thereafter, decarburization blowing is performed for the purpose of removing phosphorus remaining in the hot metal and decarburizing / heating.

[0003] As a method for producing low phosphorus steel in a conventional converter blowing method using a top-bottom blown converter and performing dephosphorization treatment, slag removal, and decarburization blowing in the same converter, there is a method in which the slag is left in the converter after dephosphorization treatment, the molten metal is tapped out of the furnace, and then the molten metal is charged again into the converter from which the slag has been discharged. Specifically, LD-ORP (for example, Non-Patent Document 1) and SRP (Non-Patent Document 2) are known. Although it is advantageous in that the slag after dephosphorization treatment is completely removed and decarburization blowing is performed, it takes a long time for blowing, and heat loss increases due to tapping during the process.

[0004] As a method of discharging the slag after dephosphorization without discharging the molten metal out of the furnace, the MURC method (Multi-Refining Converter) is known (for example, Non-Patent Document 3). In this MURC method, during the dephosphorization process, slag is formed up to a height suitable for discharging slag from the converter mouth, and a part of the slag with a high phosphorus concentration is discharged out of the furnace from the converter mouth by tilting the converter (intermediate slag discharge), and new refining materials are added to the converter that has left the hot metal with a low phosphorus concentration to perform further dephosphorization and decarburization (decarburization blowing). In this method, while it is possible to avoid an extension of the refining time and an increase in heat loss due to tapping, not all of the slag present in the converter can be removed. When the temperature of the molten metal rises during decarburization blowing, rephosphorization from the remaining slag containing a large amount of P2O5 into the molten metal occurs, which becomes a factor in increasing the P concentration in the molten steel after the end of decarburization blowing.

[0005] As a method for improving the MURC method, there is known a method in which the dephosphorization treatment is carried out in two stages. After the first dephosphorization treatment, the first intermediate slag discharge is performed, then the second dephosphorization treatment is carried out, and then the second intermediate slag discharge is performed, and then decarburization blowing is carried out (Patent Documents 1 and 2). It is said that by this method, even in the case of extra-low phosphorus steel, it can be stably produced by the MURC method, which is excellent in productivity and economy. Hereinafter, as in Patent Documents 1 and 2, a blowing method in which two dephosphorization treatments, two intermediate slag discharges, and subsequent decarburization blowing are carried out in one converter is referred to as "MURC3blow".

[0006] In Patent Document 1, the slag basicity (CaO / SiO2 mass ratio) at the end of the first dephosphorization treatment is 1.6 to 1.9 in the examples, and the slag basicity at the end of the second dephosphorization treatment is 1.4 to 2.0 in the examples. In the second dephosphorization treatment, a CaO-based flux and a SiO2-based flux are added.

[0007] In Patent Document 2, a CaO source is added so that the slag basicity at the end of the first dephosphorization treatment is 0.8 to 1.3, and in the second dephosphorization treatment, the CaO source is not added or is 5 kg / ton or less so that the slag basicity is in the range of 0.8 to 1.3.

[0008] Patent Document 3 discloses a method for dephosphorizing hot metal using a top-bottom blown converter, wherein the charging basicity during dephosphorization is set to 1.4 to 2.0, calcium ferrite is supplied in lumps at 4 kg or more per ton of hot metal, and CaO is supplied in powder form in the range of 2 to 4 kg per ton of hot metal by spraying it into the hot metal together with oxygen through an upper blowing lance. Thereby, a method for dephosphorizing hot metal that improves the iron yield and has high dephosphorization efficiency without using halides typified by fluorite is provided.

[0009] Patent Document 4 discloses a method for dephosphorizing hot metal by adding a refining agent containing 90 mass% or more of quicklime, iron oxide, and calcium ferrite into a furnace using a top-bottom blown converter. The addition of the refining agent containing calcium ferrite is carried out by a method of charging particles with a particle size of 5 to 50 mm into the furnace from above the converter. Thereby, without using fluorite, while suppressing the unit consumption of calcium ferrite, low phosphorus content can be achieved with a small unit consumption of quicklime.

[0010] Patent Document 5 discloses a method for dephosphorizing hot metal in a top-bottom blown converter, wherein when spraying a mixed powder containing CaO powder, Al2O3 powder, and Fe2O3 powder from an upper oxygen lance, the mixing ratio of Al2O3 powder and Fe2O3 powder in the mixed powder is specified, and the carrier gas addition rate and addition rate are specified for blowing.

[0011] Patent Document 6 discloses a converter blowing method for producing molten steel with a C concentration of 0.3 mass% or more. In this method, hot metal with an Si concentration of 0.15 mass% or less and a P concentration of 0.09 mass% or more is charged into a top-bottom blown converter, and a mixed powder of CaO powder and Fe2O3 powder with a mass ratio of CaO to Fe2O3 of 9:1 to 6:4 is blown upward together with oxygen gas at a predetermined flow rate. During the upward blowing, the upward blowing amount and supply rate of CaO in the mixed powder are within a predetermined range, and the upward blowing of the mixed powder is terminated while the C concentration in the molten iron is 2.5 mass% or more. After the upward blowing of the mixed powder is completed, the upward blowing of oxygen gas is continued. In the examples, it is described that hot metal with an Si concentration reduced to 0.05 mass% by desiliconization treatment is charged into the top-bottom blown converter, and it is clear that the desiliconization treatment and dephosphorization / decarbonization treatment of the hot metal are carried out in separate refining vessels.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0013]

Non-Patent Document 1

Non-Patent Document 2

[0014] As described above, in order to produce extra-low phosphorus steel by the MURC method, it is important to absorb a large amount of P in the hot metal into the slag during the dephosphorization treatment and to remove a large amount of P-containing slag by intermediate slag removal. A process (MURC3blow) that performs the dephosphorization treatment and intermediate slag removal twice has been developed. Also in MURC3blow, in order to enhance the dephosphorization ability of the slag, it is necessary to increase the basicity of the slag during the dephosphorization treatment. However, when the basicity of the slag is increased, the (FeO) concentration and the liquid phase ratio of the slag decrease, the slag does not form, and slag removal becomes extremely difficult. Further, if a large amount of slag is discharged during the first slag removal, the amount of slag becomes extremely small during the second dephosphorization treatment and the second slag removal, making slag removal difficult. Therefore, in order to minimize the carry-over P amount to the decarburization blowing, the amount of the first slag removal must be suppressed.

[0015] An object of the present invention is to provide a method for dephosphorizing hot metal that can increase the basicity of the slag in the second dephosphorization treatment without suppressing the amount of the first slag removal in MURC3blow in which the dephosphorization treatment and the intermediate slag removal are performed twice, and can form the slag and perform good slag removal without causing a decrease in the (FeO) concentration and the liquid phase ratio of the slag. [Means for Solving the Problems]

[0016] That is, the gist of the present invention is as follows. [1] When performing dephosphorization treatment of hot metal using a top-bottom blown converter, a first step of charging hot metal and a CaO source into the converter and performing dephosphorization treatment by oxygen blowing; a second step of tilting the converter after the first step and discharging slag while leaving hot metal in the furnace; a third step of performing dephosphorization treatment by oxygen blowing after the second step; a fourth step of tilting the converter after the third step and discharging slag while leaving hot metal in the furnace; and a fifth step of performing decarburization treatment by oxygen blowing after the fourth step. In the first step, the input amount of the CaO source is adjusted so that the basicity, which is the mass concentration ratio of CaO and SiO2 in the slag after dephosphorization treatment, is in the range of 1.0 to 1.4. In the third step, a powder (hereinafter referred to as "CaO iron oxide mixed powder") in which CaO and iron oxide are mixed with oxygen is supplied from an upper blowing lance at an arbitrary time within the oxygen upper blowing period in the third step while ensuring 60 to 100% of the total oxygen upper blowing time in the third step, and the powder supply amount is adjusted so that the basicity of the slag after dephosphorization treatment is in the range of 1.5 to 1.8. A method for dephosphorizing hot metal, characterized in that. [2] The CaO iron oxide mixed powder supplied in the third step has a total blending ratio of CaO and iron oxide of 90% by mass or more and a blending ratio of iron oxide of 40 to 80% by mass. When the CaO iron oxide mixed powder is supplied by upper blowing, the supply rate of the pure iron oxide in the CaO iron oxide mixed powder is 1.2 to 3.2 kg / t / min. The method for dephosphorizing hot metal according to [1], characterized in that.

Effect of the Invention

[0017] According to the present invention, in MURC3blow in which dephosphorization treatment and intermediate slag discharge are performed twice, without suppressing the amount of slag discharged in the first time, the basicity of the slag can be increased in the second dephosphorization treatment, and without causing a decrease in the (FeO) concentration and liquid phase ratio of the slag, slag can be formed and good slag discharge can be performed. Therefore, it is possible to melt low-phosphorus steel without tapping the molten metal outside the furnace during refining using a top-bottom blown converter.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0019] When performing dephosphorization treatment of hot metal using a top-bottom blown converter, the present invention includes a first step of charging hot metal and a CaO source into the converter and performing dephosphorization treatment (the first dephosphorization treatment) by oxygen blowing; a second step (the first slag removal) of tilting the converter after the first step and removing slag while leaving hot metal in the furnace; a third step of performing dephosphorization treatment (the second dephosphorization treatment) by oxygen blowing after the second step; a fourth step (the second slag removal) of tilting the converter after the third step and removing slag while leaving hot metal in the furnace; and a fifth step of performing decarburization treatment by oxygen blowing after the fourth step. The present invention is directed to a method (MURC3blow) for dephosphorizing hot metal.

[0020] Hereinafter, the CaO / SiO2 mass concentration ratio of the slag is also referred to as the slag basicity.

[0021] As shown in Fig. 1, the top-bottom blown converter has a converter 1, an upper blowing lance 2, and a bottom blowing tuyere 3. Fig. 1(A1) shows the initial stage of the first step (the first dephosphorization treatment). Molten iron 11 is contained in the converter. While adding a massive CaO source 15, oxygen gas 13 is blown from the upper blowing lance 2, and bottom blowing gas 14 is supplied from the bottom blowing tuyere 3. Fig. 1(A2) shows the end stage of the first step, where slag 12 is forming near the furnace mouth 4. Fig. 1(B) shows the second step (the first slag removal), where the converter 1 is tilted to remove the slag 12 in the converter into the slag removal ladle 6. Fig. 1(C) shows the third step (the second dephosphorization treatment), where slag 12 is forming near the furnace mouth 4. Fig. 1(D) shows the fourth step (the second slag removal), where the converter 1 is tilted to remove the slag 12 in the converter into the slag removal ladle 6. Fig. 1(E) shows the fifth step (decarbonization treatment). After the fifth step is completed, the converter is tilted to the side opposite to that during slag removal, and the molten steel is tapped into a ladle through the tapping hole 5 (not shown), thus completing the refining using the converter.

[0022] In the first step of the present invention, the hot metal charged into the converter is the hot metal that has not been desiliconized after being tapped from the blast furnace. In that case, the Si content in the charged hot metal is 0.4% or more.

[0023] Regarding the above problems in MURC3blow, even on the premise of removing a large amount of slag in the first slag removal, as a method capable of increasing the dephosphorization ability in the second dephosphorization treatment and the slag removal amount in the second slag removal, an upper blowing technique has been developed in which CaO iron oxide mixed powder is blown into the converter together with upper blowing oxygen in the second dephosphorization treatment. In Fig. 2, (A) is a schematic diagram when the CaO iron oxide mixed powder is not blown, and (B) is a schematic diagram when the CaO iron oxide mixed powder is blown.

[0024] Here, in order to ensure the slag discharge amount with a small amount of slag, it is effective to sufficiently form the slag and increase the forming height of the slag. In order to sufficiently form the slag, first, it is effective to ensure the amount of liquid-phase slag, and second, to promote the generation of CO gas at the slag / metal interface ((Equation 1)). In Equation (1), (FeO) means FeO in the slag. (FeO) + C = CO↑ + Fe ···(1)

[0025] In order to satisfy these first and second conditions, it is effective to increase the slag (FeO) concentration. On the other hand, in order to ensure the dephosphorization ability in the dephosphorization process, it is effective to increase the slag basicity. However, if the slag basicity is increased, the (FeO) concentration of the slag will decrease. Therefore, it becomes difficult to ensure the above first (ensuring the amount of liquid-phase slag) and second (promoting CO gas generation) conditions, and the slag cannot be formed. Also, despite increasing the slag basicity, the dephosphorization ability itself deteriorates due to the decrease in the slag (FeO) concentration.

[0026] On the other hand, FeO generated by oxidizing molten iron with oxygen blown from the top-blown lance and iron oxide supplied by the input of solid oxygen sources such as iron ore and sinter powder can also increase the slag (FeO) concentration by being contained in the slag. However, as long as these are in contact with the molten metal as simple iron oxide, they are easily reduced by the contained C in the hot metal before combining with the slag, so the efficiency of increasing the slag (FeO) concentration is poor (see Figure 2(A)).

[0027] As shown in Figure 2(B), by blowing and supplying the CaO iron oxide mixed powder 16 from the top-blown lance 2 into the molten iron 11 together with the oxygen gas 13, it is considered that a CaO-FeO melt 22 is generated at the hot spot where the temperature is high. Then, since the activity of FeO decreases by becoming the CaO-FeO melt 22, the C reduction reaction of FeO in the melt during the movement of the CaO-FeO melt 22 from the hot spot to the slag 12 is suppressed, and it is assumed that the supply efficiency of FeO to the slag 12 is greatly improved.

[0028] Therefore, in the present invention, in the second dephosphorization treatment of the third step, as shown in FIGS. 1(C) and 2(B), it was conceived to blow and supply a mixed powder of CaO and iron oxide (CaO iron oxide mixed powder 16) from the top blowing lance 2 into the molten iron 11 together with the oxygen gas 13. Further, by continuously supplying iron oxide (FeO source) as a mixed powder of CaO and iron oxide to the slag 12, it becomes possible to maintain a high level of the slag (FeO) concentration throughout the dephosphorization treatment. As a result, due to the above-described effects of the first (ensuring the amount of liquid-phase slag) and the second (promoting CO gas generation), as shown in FIG. 2(B), the slag (forming) 23 can be formed even with a small amount of slag, and as a result, the slag discharge can be promoted, and the dephosphorization ability can also be improved.

[0029] Hereinafter, the present invention will be described in detail.

[0030] <<First Embodiment>> <<First Step (First Dephosphorization Treatment)>> In the first step (FIGS. 1(A1) and (A2)), the input amount of the CaO source 15 is adjusted so that the basicity, which is the mass concentration ratio of CaO and SiO2 in the slag 12 after the dephosphorization treatment, is in the range of 1.0 to 1.4.

[0031] In order to melt low-phosphorus steel, it is necessary to perform a certain degree of dephosphorization even in the first dephosphorization treatment, and the slag basicity needs to be at least 1.0 or more. On the other hand, if the slag basicity is increased too much, it becomes difficult for the slag 12 to form and the slag discharge property deteriorates, so the upper limit is set to 1.4.

[0032] <<Second Step (First Slag Discharge)>> (FIG. 1(B)) In the present embodiment, as a result of suppressing the slag basicity in the first step to 1.4 or less, the slag forming during the dephosphorization treatment in the first step is good. Therefore, since the first slag discharge in the second step is performed in a state where the slag 12 has formed, sufficient slag discharge can be performed.

[0033] <<Third Step (Second Dephosphorization Treatment)>> In the third step of the first embodiment, as shown in Fig. 1(C), a powder (CaO-iron oxide mixed powder) in which CaO and iron oxide are mixed with oxygen gas 13 is continuously supplied from the top-blowing lance 2 at an arbitrary time within the oxygen top-blowing period in the third step, while securing a time of 60 to 100% of the total oxygen top-blowing time in the third step, and the powder supply amount is adjusted so that the basicity of the slag after the dephosphorization treatment is in the range of 1.5 to 1.8.

[0034] By supplying a powder (CaO-iron oxide mixed powder 16) in which CaO and iron oxide are mixed with oxygen gas 13 from the top-blowing lance 2 to the molten iron 11, at the ignition point where the oxygen gas 13 collides with the molten iron 11, the supplied CaO-iron oxide mixed powder 16 melts to generate a CaO-FeO melt, and FeO can be stably supplied to the slag 12. Compared with the case of supplying only oxygen gas without mixing powder with the oxygen gas or introducing a solid oxygen source into the molten iron, the supply efficiency of FeO to the slag 12 is significantly improved. Here, the iron oxide refers to either Fe2O3, FeO, or a mixture of both. Also, although the particle size of the mixed powder used is not particularly specified, it is preferably less than 1 mm from the viewpoints of powder acceleration during top-blowing and reaction stability after landing.

[0035] Also, at an arbitrary time within the oxygen top-blowing period in the third step, the CaO-iron oxide mixed powder 16 is continuously supplied while securing a time of 60 to 100% of the total oxygen top-blowing time. If the CaO-iron oxide mixed powder 16 is not continuously supplied over a long period, FeO in the slag will be continuously reduced by the molten iron C from moment to moment, and the (FeO) concentration will decrease. If the powder supply period in the third step is less than 60% of the total oxygen top-blowing time, it is impossible to maintain the slag (FeO) concentration and satisfy the formability, slag dischargeability, and dephosphorization ability. Therefore, the range is set to 60% to 100% of the total oxygen top-blowing time in the third step.

[0036] Furthermore, adjust the powder supply amount so that the basicity of the slag after the dephosphorization treatment in the third step is in the range of 1.5 to 1.8. To ensure the dephosphorization ability in the third step, it is necessary to increase the slag basicity to a certain extent, with 1.5 as the lower limit. On the other hand, if the slag basicity is too high, even if iron oxide is supplied, the slag (FeO) concentration will not increase, and forming and slag removal will become difficult. Therefore, the upper limit of the slag basicity in the third step is set to 1.8. In the present invention, by supplying the CaO iron oxide mixed powder 16 together with the oxygen gas 13 from the top blowing lance 2 to the molten iron 11, as described above, FeO can be stably supplied to the slag 12. As a result, the above-mentioned conditions of the first (ensuring the amount of liquid-phase slag) and the second (promoting the generation of CO gas) are ensured. Despite the high slag basicity of 1.5 to 1.8, the forming ability of the slag 12 is ensured, and good slag removal can be performed in the second slag removal in the next fourth step.

[0037] The basicity of the slag after the second dephosphorization treatment in the third step is estimated from the slag composition after the first step based on the dissolution rate of the auxiliary materials, the amount of slag removal in the second step, etc., and preliminarily calculated from the powder supply amount in the third step to determine the operating conditions. As an example of the method for estimating the dissolution rate of the auxiliary materials, a method of experimentally collecting the slag after the first step, analyzing the components, and grasping in advance the ratio of the amount of CaO charged into the converter that dissolved during the first step can be mentioned. Also, for the amount of slag removal in the second step, for example, a method of grasping in advance the relationship between the furnace tilting angle and the amount of slag removal during slag removal and controlling the amount of slag removal at the furnace tilting angle can be mentioned.

[0038] 《Fourth Step (Second Slag Removal)》 In this embodiment, despite the high slag basicity of 1.5 to 1.8 in the dephosphorization treatment in the third step, the forming ability of the slag is ensured. Therefore, after the dephosphorization treatment, the slag forms well, and good slag removal can be performed in the second slag removal in the fourth step (Figure 1(D)).

[0039] 《Intermediate Slag Removal Rate (Total of the First and Second Slag Removals)》 In the present invention, the quality of the intermediate slag removal is determined by the sum of the first slag removal and the second slag removal (the intermediate slag removal rate shown in the following formula (2)). If the intermediate slag removal rate is 70% or more, it can be determined that the effect of the invention has been obtained, and if it is 80% or more, it can be determined that the effect of the invention has been obtained more remarkably.

Number

[0040] The derivation of the above formula (2) will be described. Hereinafter, the basicity CaO / SiO2 (mass ratio) will also be referred to as C / S. The CaO / SiO2 (mass ratio) (C / S B2 ) of the slag after blowing in the fifth step is given by the following formula. C / S B2 =(CaO B1 ×(1 - intermediate slag removal rate)+CaO B2 ×slag conversion rate α) / (SiO 2B1 ×(1 - intermediate slag removal rate)+SiO 2B2 ) By transforming the above formula, the above formula (2) is obtained. For the slag conversion rate α, based on the analysis results of the decarburization operation, α = 0.8 is adopted. In addition, the SiO2 [t] generated up to the third step means the total of SiO2 generated by oxidizing Si in the hot metal and SiO2 from the SiO2 source input as a subsidiary raw material.

[0041] 《Second Embodiment》 In the second embodiment, preferably, the CaO iron oxide mixed powder supplied in the third step has a total blending ratio of CaO and iron oxide of 90% by mass or more and an iron oxide blending ratio of 40 to 80% by mass. When the CaO iron oxide mixed powder is supplied upward, the supply rate of pure iron oxide in the CaO iron oxide mixed powder is 1.2 to 3.2 kg / t / min. Here, the supply rate is the supply amount (kg) per minute per ton of hot metal.

[0042] In the CaO iron oxide mixed powder supplied in the third step, when the blending ratio of iron oxide is less than 40% by mass, as a result, the melting point of the CaO-FeO melt becomes high, making it difficult to form the melt at the flame point, so that the intended effect cannot be fully obtained. On the other hand, when the blending ratio of iron oxide exceeds 80% by mass, the blending ratio of CaO relatively decreases. In addition to the fact that the effect of suppressing C reduction due to the reduction of the FeO activity by CaO cannot be fully obtained, the dephosphorization efficiency also decreases. Therefore, preferably, the blending ratio of iron oxide in the CaO iron oxide mixed powder is 40 to 80% by mass.

[0043] The reason for setting the total blending ratio of CaO and iron oxide in the CaO iron oxide mixed powder to 90% by mass or more is that the invention effect will be reduced if the proportion of these components contributing to dephosphorization is small, and the variation of the invention effect will be caused by impurity components.

[0044] From the viewpoints of the dephosphorization ability of the slag and ensuring the liquid phase ratio, the slag (FeO) concentration in the third step is desirably in the range of approximately 20 to 30% by mass. If the supply rate of pure iron oxide in the CaO iron oxide mixed powder is too slow, the C reduction rate of the slag FeO will exceed the FeO supply rate, and the slag (FeO) concentration cannot be increased to the preferred range. Therefore, preferably, the lower limit of the supply rate of pure iron oxide in the CaO iron oxide mixed powder is set to 1.2 kg / t / min. On the other hand, if the supply rate of pure iron oxide in the CaO iron oxide mixed powder is too fast, the (FeO) concentration in the slag will increase excessively, the viscosity of the slag will decrease, and conversely, the forming property will decrease. Therefore, the upper limit is set to 3.2 kg / t / min.

Example

[0045] Using a top-bottom blown converter with a hot metal charge of 350 t, the hot metal was dephosphorized.

[0046] The charged hot metal composition was [C]: 4.0 - 4.4 mass%, [Si]: 0.3 - 0.7 mass%, [Mn]: 0.2 - 0.3 mass%, [P]: 0.12 - 0.15 mass%. The hot metal [P] concentration for each example is shown in Table 1.

[0047] For the top blowing acid supply of the top-bottom blown converter, the following conditions were used. The acid supply amount for each example is shown in Table 1. Acid supply rate: 40000 Nm 3 / h Acid supply amount: First dephosphorization treatment (first step): 10.0 - 12.0 Nm 3 / t Second dephosphorization treatment (third step): 4.0 - 6.0 Nm 3 / t

[0048] The hot metal temperature was as follows. After the first dephosphorization treatment (first step): 1300 - 1350 °C After the second dephosphorization treatment (third step): 1350 - 1400 °C

[0049] Regarding the powder mixture of CaO and iron oxide (CaO-iron oxide mixed powder) supplied from the top blowing lance together with oxygen in the third step, iron ore (Fe2O3 content 93%) was used as the iron oxide source and quicklime (CaO content 100%) was used as the CaO source, and the particle size distribution was under 1 mm. The total mixing ratio of CaO and iron oxide in the CaO-iron oxide mixed powder was adjusted to 95%.

[0050] In Table 1, for each example, the mixing ratio of iron oxide in the CaO-iron oxide mixed powder in the third step is listed in the "iron oxide mixing ratio" column, the supply time of the CaO-iron oxide mixed powder is listed in the "powder supply time" column, the ratio of the supply time of the CaO-iron oxide mixed powder to the total oxygen top blowing time in the third step is listed in the "powder supply time ratio" column, and the supply rate of the pure iron oxide in the CaO-iron oxide mixed powder is listed in the "iron oxide supply rate" column.

[0051]

Table 1

[0052] As a method for confirming the effects of the invention, regarding the slag component analysis, slag was collected after the first, third, and fifth steps, and the slag components were analyzed by chemical analysis. The basicity of the slag at the end of the first and third steps is described in the "Slag Basicity" column of Table 1. Regarding the intermediate slag discharge rate (total of the second and fourth steps), from the CaO and SiO2 concentrations in the slag after decarburization blowing (after the fifth step), the input amounts of the CaO source and the generated amount of the SiO2 source in the first and third steps, and the input amounts of the CaO source and the SiO2 source in the fifth step, the evaluation was performed based on the intermediate slag discharge rate calculated using the mass balance formula of the above formula (2), and it is shown in the "Intermediate Slag Discharge Rate" column of Table 1. It was judged that the effects of the invention were obtained when the intermediate slag discharge rate was 70% or more, and it was judged that the effects of the invention were more significantly obtained when it was 80% or more.

[0053] Regarding the dephosphorization rate, it was judged that the effects of the invention were obtained when the dephosphorization rate of the following formula (3) was 70% or more, and it was judged that the effects of the invention were more significantly obtained when it was 80% or more. Dephosphorization rate = 100×([P] in hot metal before the first step - [P] after the third step) / [P] in hot metal ··· (3)

[0054] Examples 1 (No. 1 to 3) in Table 1 obtained the effects of the first embodiment of the present invention. Since all the requirements of the first embodiment were satisfied, both the intermediate slag discharge rate and the dephosphorization rate were good, and the effects of the invention were obtained.

[0055] Examples 2 (No. 4 to 6) in Table 1 particularly significantly obtained the effects of the second embodiment of the present invention. Since all the requirements of the first and second embodiments were satisfied, both the intermediate slag discharge rate and the dephosphorization rate were very good, and the effects of the invention were particularly significantly obtained.

[0056] The comparative examples (No. 7 to 13) in Table 1 did not meet the requirements of the present invention.

[0057] In No.7, the CaO iron oxide mixed powder was not blown from above in the third step, and the slag basicity was adjusted with lump lime. Although the slag basicity in the third step satisfied the scope of the present invention, both the intermediate slag removal rate and the dephosphorization rate resulted in poor outcomes.

[0058] In No.8, since the slag basicity after the treatment in the first step was high, the dephosphorization was good, but the intermediate slag removal rate resulted in a poor outcome. In No.9, since the slag basicity after the treatment in the first step was low, the intermediate slag removal rate was good, but the dephosphorization resulted in a poor outcome.

[0059] In No.10, since the slag basicity after the treatment in the third step was low, the intermediate slag removal rate was good, but the dephosphorization resulted in a poor outcome. In No.11, since the slag basicity after the treatment in the third step was high, the dephosphorization was good, but the intermediate slag removal rate resulted in a poor outcome.

[0060] In No.12, the supply time of the CaO iron oxide mixed powder in the third step was short, so the powder supply time ratio was outside the lower limit of the scope of the present invention, and all of dephosphorization, slag forming, and slag removal resulted in poor outcomes.

Explanation of Signs

[0061] 1 Converter 2 Top blowing lance 3 Bottom blowing tuyere 4 Furnace mouth 5 Tapping hole 6 Slag removal ladle 11 Molten iron 12 Slag 13 Oxygen gas 14 Bottom blowing gas 15 CaO source 16 CaO iron oxide mixed powder 21 FeO 22 CaO - FeO melt 23 Slag (forming)

Claims

1. When performing dephosphorization treatment of hot metal using a top-bottom blown converter, a first step of charging hot metal and a CaO source into the converter and performing dephosphorization treatment by oxygen blowing; a second step of tilting the converter after the first step and discharging slag while leaving hot metal in the furnace; a third step of performing dephosphorization treatment by oxygen blowing after the second step; a fourth step of tilting the converter after the third step and discharging slag while leaving hot metal in the furnace; and a fifth step of performing decarburization treatment by oxygen blowing after the fourth step, and In the first step, the input amount of the CaO source is adjusted so that the basicity, which is the mass concentration ratio of CaO and SiO in the slag after the dephosphorization treatment, falls within the range of 1.0 to 1.4, ２ and in the third step, a powder obtained by mixing CaO and iron oxide with oxygen (hereinafter referred to as "CaO-iron oxide mixed powder") is continuously supplied from an upper blowing lance at an arbitrary time within the oxygen upper blowing period in the third step while ensuring a time of 60 to 100% of the total oxygen upper blowing time in the third step, and the powder supply amount is adjusted so that the basicity of the slag after dephosphorization treatment is in the range of 1.5 to 1.

8. A method for dephosphorizing hot metal, characterized by this.

2. The CaO-iron oxide mixed powder supplied in the third step has a total blending ratio of CaO and iron oxide of 90% by mass or more, and a blending ratio of iron oxide of 40 to 80% by mass. When the CaO-iron oxide mixed powder is supplied by upper blowing, the supply rate of pure iron oxide in the CaO-iron oxide mixed powder is 1.2 to 3.2 kg / t / min. The method for dephosphorizing hot metal according to Claim 1, characterized by this.

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